Michael Kohler
Etching in Microsystem Technology Translated by Antje Wiegand
Weinheim New York Chichester Brisbane Singapore Toronto
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This Page Intentionally Left Blank
Michael Kohler
Etching in Microsystem Technology
mWILEY-VCH
Michael Kohler
Etching in Microsystem Technology Translated by Antje Wiegand
Weinheim New York Chichester Brisbane Singapore Toronto
Dr. Michael Kohler Institut fur Physikalische Hochtechnologie e. V., Jena Helmholtzweg 4, D-07743 Jena Germany This book was carefully produced. Nevertheless, author and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No. applied for
A CIP catalogue record for this book is available from the British Library Deutsche Bibliothek Cataloguing-in-PublicationData: Kohler, Michael: Etching in microsystem technology / Michael Kohler. Trans]. by Antje Wiegand. - Weinheim ; New York ; Chichester ; Brisbane ; Singapore ;Toronto : Wiley-VCH, 1999 ISBN 3-527-29561-5
0 WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany), 1999
Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form -by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition: Mittenveger Werksatz GmbH, D-68723 Plankstadt Printing: Strauss Offsetdruck GmbH, D-69509 Morlenbach Bookbinding: J. Schaffer GmbH & Co. KG, D-67269 Griinstadt Printed in the Federal Republic of Germany
Preface
Microcomponents and microdevices in the form of electronic chips are very common nowadays in everyday life. They are of decisive importance in computers, but also in other devices. They are found in science and technology, in trade and industry, at official departments, at schools and in vocational training, that means in all parts of public, economic and private life. For some time, however, microcomponents have been used not only for electronic devices. Miniaturized data processors, sensors and actuators of all kinds, even complete microsystems are developed and gain more and more applications. The specific functions of all of these minute devices depend strongly on the selection and combination of the materials they are built of, i.e., the chemical and physical solid-state properties of these materials and their styling. Normally this styling is performed by lithographic etching processes. Hence these etching processes take up a key position in microfabrication. The microtechnical etching of functional patterns is a typical interdisciplinary area. The actual dissolution of the material is connected as a rule with a change of matter and is therefore of chemical nature. The chemical action during etching in liquid media can be described by the methods of coordination chemistry, electrochemistry, and surface chemistry. In dry-etching procedures plasma physical, plasma chemical, and photochemical processes prevail. The change of matter in either kind of etching processes is accompanied by some physical processes that concern, e.g., fluid and gas dynamics, as well as solid state physics. The purpose of etching is a microtechnical pattern. Special devices are used for the microlithographic process and its control, the functional principles of these are dealt with by engineering. Chemistry, physics and engineering science supply jointly the basis for microtechnical etching processes. This book is an introduction to the essential microlithographic etching methods. Its purpose is the presentation of the characteristics and the area of use of the respective etching processes. The basic scientific principles of significant processes are dealt with, and their importance for the respective microtechnical etching process and its product, i.e. the pattern or structure in the microdevice, is explained. The joint discussion of physical-chemical and microtechnical aspects should strengthen the understanding of the methods, their advantages and possible applications and their specific characteristics.
VI
Preface
The book is devided in two sections. The wet and dry etching processes are presented in a general section. A kind of catalogue of etching bath compositions, etching instructions and parameters is given in a second, more special section. This list should enhance the comprehension of the general section and also give an overview of essential data for the practical microtechnical training as well as for microtechnical research and development. The book is intended for engineers, technicians and natural scientists, who work in the area of microtechniques and deal with microtechnical etching methods. The book also is addressed to students, preferentially those of physics, engineering or chemistry, as their fields will be shaped more and more by the application of microsystems in years to come, and therefore specific microtechnical solutions and new developments in microsystems technology will be in demand. Here, the author likes to express his gratitude to all colleagues, who contributed to the book by cooperation, with discussions and hinds. Particularly, this thank is directed to H. Dintner, A. Lerm, G. Mayer, T. Schulz, M. Sossna and A. Wiegand. P. Pertsch, W. Pilz and G. Kohler have contributed with critical discussions to the manuscript and are gratefully acknowledged. For the support by SEM images the authors thanks to E Jahn. The main thank is directed to Antje Wiegand, who not only cooperated for a longtime in many aspects of micro etching, but also supported the preparation of the german manuscript and, finally, made possible this english edition of the book by her translation, which in this form would not have been possible without her deep insight into microlithographic technology. The publisher and namely B. Bock are greatly acknowledged for all support in the preparation of manuscript for press.
Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V VII
Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XI
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XV
1
2 Distinctive Features of Microtechnical Etching . . . . . . . . . . . . . . . 4 2.1 Etching as a Fashioning Method . . . . . . . . . . . . . . . . . . . . . 4 2.1.1 Limits of Additive Microtechnical Pattern Generation . . . . . . . 6 2.1.2 Subtractive Pattern Generation . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Etch Rate and Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.1 Etch Rate and Time Request . . . . . . . . . . . . . . . . . . . . . . . . 9 9 2.2.2 The Etching Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Transport Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.4 Process Velocities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.
.................... Edge Geometry and Roughness . . . . . . . . . . . . . . . . . . . . . . Deviations from Ideal Geometry . . . . . . . . . . . . . . . . . . . . . Edge Geometry in Isotropic Etching . . . . . . . . . . . . . . . . . . . Fabrication of Low Slope Angles by Isotropic Etching . . . . . . . Edge Geometries in Anisotropic Etching . . . . . . . . . . . . . . . . Isotropic and Anisotropic Etching
14
18 18 18 19 21
2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 Fabrication of Low Slope Angles by Partially Anisotropic Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23 24
.....................
26
2.6
Monitoring of Etching Processes
VIII
Contents
........................ 29 Etching at the Interface Solid-Liquid . . . . . . . . . . . . . . . . . . 29 Preparation of the Surface . . . . . . . . . . . . . . . . . . . . . . . . . 30 30 Surface Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Digital Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Etching of Dielectric Materials . . . . . . . . . . . . . . . . . . . . . . 35 Wet Etching by Physical Dissolution . . . . . . . . . . . . . . . . . . . 35 Wet-Chemical Etching of Non-Metals . . . . . . . . . . . . . . . . . . 37 Etching of Metals and Semiconductors . . . . . . . . . . . . . . . . . 41 Outer-Currentless Etching . . . . . . . . . . . . . . . . . . . . . . . . . 41 Selectivity in Outer-Currentless Etching . . . . . . . . . . . . . . . . 53 Etching of Multilayer Systems Forming Local Elements . . . . . . 60 Geometry-Dependent Etch Rates . . . . . . . . . . . . . . . . . . . . 62 Geometry-Dependent Passivation . . . . . . . . . . . . . . . . . . . . 69 Electrochemical Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Photochemical Wet Etching. . . . . . . . . . . . . . . . . . . . . . . . . 79 PhotoelectrochemicalEtching . . . . . . . . . . . . . . . . . . . . . . . 80 Crystallographic Etching . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Chemical Wet Etching of Monocrystalline Surfaces . . . . . . . . . 84 Anisotropic Etching of Monocrystalline Metals . . . . . . . . . . . . 87 88 Anisotropic Etching of Silicon . . . . . . . . . . . . . . . . . . . . . . .
3 Wet-Chemical Etching Methods 3.1 3.2 3.2.1 3.2.2 3.2.3 3.3 3.3.1 3.3.2
3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.6 3.4.7 3.4.8 3.5 3.5.1 3.5.2 3.5.3 3.5.4 Anisotropic Electrochemical and Photoelectrochemical Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.5.5 Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3S.6 Anisotropic Etching of Compound Semiconductors . . . . . . . . . 103 3.6 3.6.1 3.6.2 3.6.3
Preparation of Free-Standing Micropatterns . . . . . . . . . . . . . . Surface Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . Bulk Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous Silicon as Sacrificial Material . . . . . . . . . . . . . . . . . . .
4 Dry-Etching Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
105 105 107 109 111
4.1
Removal at the Interface Solid-Gas
...................
111
4.2 4.2.1 4.2.2 4.2.3 4.2.4
Plasma-Free Etching in the Gas Phase . . . . . . . . . . . . . . . . . Plasma-Free Dry Etching with Reactive Gases . . . . . . . . . . . . Photo-Assisted Dry Etching with Reactive Gases . . . . . . . . . . Directly-Writing Micropatterning by Laser Scanning Etching . . . Electron-Beam-AssistedVapour Etching . . . . . . . . . . . . . . . .
116 116 118 119 120
122 4.3 Plasma Etching Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Material Removal by Reactions with Plasma Species . . . . . . . . 122
Contents
Ix
4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9 4.3.10 4.3.11
Plasma Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Plasma Etching in the Barrel Reactor . . . . . . . . . . . . . . . . . . 127 Plasma Etching in the Down-Stream Reactor . . . . . . . . . . . . . 128 Plasma Etching in the Planar-Plate Reactor . . . . . . . . . . . . . . 129 Magnetic-Field-BiassedPlasma Etching . . . . . . . . . . . . . . . . . 130 Plasma Etching at Low Pressure and High Ion Density . . . . . . . 130 Forming of Etch Structures in Plasma Etching . . . . . . . . . . . . 131 Geometry Influence on Plasma Etching . . . . . . . . . . . . . . . . . 131 Plasma Jet Etching (PJE) . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Applications of Plasma Etching . . . . . . . . . . . . . . . . . . . . . . 133
4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6
Etchig Methods with Energized Particles . . . . . . . . . . . . . . . . 137 Sputter-Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Reactive Ion Etching (RIE) . . . . . . . . . . . . . . . . . . . . . . . . 144 Magnetic-Field-Enhanced Reactive Ion Etching (MERIE) . . . . 150 Ion Beam Etching (IBE) . . . . . . . . . . . . . . . . . . . . . . . . . . 150 Reactive Ion Beam Etching (RIBE) . . . . . . . . . . . . . . . . . . . 155 Magnetic-Field-Enhanced Reactive Ion Beam Etching 156 (MERIBE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemically-Assisted Ion Beam Etching (CAIBE) . . . . . . . . . . 156 Reactive Etching with Excitation from Several Sources . . . . . . . 156 Electron-Beam-Supported Reactive Ion Etching (EBRE) . . . . . 157 Focussed Ion Beam Etching (FIB) . . . . . . . . . . . . . . . . . . . . 159 Nanoparticle Beam Etching (NPBE) . . . . . . . . . . . . . . . . . . . 160 Formation of the Structure Sidewall Geometry in Ion Beam 161 Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Material Defects in Etching with Energized Particles . . . . . . . . 168 Application of Etching Methods with Energized Particles . . . . . 169
4.4.7 4.4.8 4.4.9 4.4.10 4.4.11 4.4.12 4.4.13 4.4.14
5 Microforming by Etching of Locally Changed Material . . . . . . . . . 173 5.1 Principle of Forming by Locally Changed Material . . . . . . . . . 173 173 5.2 Inorganic Resists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 5.4 5.5 5.6
Etching of Photosensitive Glasses . . . . . . . . . . . . . . . . . . . . Etching of Photo-Damaged Areas . . . . . . . . . . . . . . . . . . . . Etching of Areas Damaged by Ion Beams . . . . . . . . . . . . . . . Particle Trace Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 ChosenRecipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1
Explaining the Collection of Recipes
..................
174 175 176 176
179 179
X 6.2
Contents
Collection of Recipes.
Ag Al Al(l-4 (Al,Ga)As NOS ( h . 5 p (Al,Ga,In)P (Al,In)As AlInN Ind' AlN a 2 0 3
AsSG (Arsenosilicate Glass) Au Bi BSG (Borosilicate Glass) C (amorphous) C (Diamond)
............................ GaP GaSb Ge Ge,Si,.,
Pb,Zrx'Ii,.,O3(PZT) PSG (Phosphosilicate Glass) Pt
Hf HgTe
RuOZ
InAs (In ,Ga)N InN InP InSb (InSn) (In,,Sn,)O In2Te3 KTiOPO4(KTP)
(C;H,[0,N7F7C1,Br])-Polymere LiA102 CdS
LiGa02 LiNb03 Mg Mo MoSi2
Fe (Fe,C) (Fe,Ni) GaAs (Ga,In)As (Gao.sIno.5P GaN (Ga, Gd)z03
181
Nb NbN Ni (Ni,Cr) NiMnSb
Sb Si Sic Si3N4 Si02 Si,O,N, Sn Sn02 Ta TkN Ta205 TaSi2 Tao.nSio.~N Te Ti TiN Ti02
V W wo3
WSi2 krBa2Cu307.,(YBCO) Zn ZnO ZnS ZnSe
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
345 361
Symbols
area, electrode area - etching area - total area - anodic electrode area - cathodic electrode area - chemical activity - chemical activity of particle of kind i -
- percentage area of coverage - base, ligand - width of pattern
concentration - concentration in the interior of the solution - concentration at the surface - initial concentration -
diffusion coefficient precipitate - thickness of diffusion layer - apparent thickness of diffusion layer - limiting diffusion layer thickness -
potential - electric field strength - normal potential - bias field strength - floating potential - ion energy - electron, negative elementary charge
-
-
Faraday constant
film thickness, height of structure etchdepth ~ t c h / p a s s - material removal up to passivation -
XI1
Symbols
I I+ L
-
L l i i+ i
io
- current anodic current cathodic current - cathodic partial current on large areas in outer-currentless case - current density - anodic current density - cathodic current denstiy - cathodic partial current denstiy on large areas in outer-currentless case
equilibrium constant - complex forming equilibrium constant - solubility product - chronopotentiometric constant - constant - Boltzmann constant
K KB KL K+
-
L 1 1"
-
M
-
k k
M w
m m m, mi
uncharged ligand - lenght, width of pattern - undercutting metal or semiconductor - molar mass, atomic weight - mass - stoichiometric factor - electron mass - ion mass
NO n n
- Avogadro constant - counting variable - mol number
OM
-
pe PR-
- Peclet number - polymer molecular residue (polymer trunk, polymer radical)
oxidant
- gas constant R R-, R - radical r - etch rate - gross etch rate rB - average etch rate rd - electrochemical etch rate re1 - etch rate of an auxiliary rh - layer rH - horizontal etch rate
Symbols
rm rn rP rres rn
- etch rate of mask - etch rate dependent on total area - penetration rate (etch rate of interface) - removal rate of material not to be etched - radius of tube reactor
S&
- etching selectivity
T
-
Te t fetch
t” to V V VO ve
absolute temperature electron temperature time etching time overetching time etching time up to complete removal of film
volume, plasma volume velocity - gas flow rate in inlet region of an apparatus - velocity of energized particles -
pattern distance - width of sidewall
W Wf
-
X-
- monovalent acid anion, monovalent negative ligand - local coordinate - stoichiometric factor
X X
Y Y
-
Z
-
Z 2
z, Z.
ligand, uncharged - stoichiometric factor acid radical ion - stoichiometric factor - electrochemical valency - anodic electrochemical valency - cathodic electrochemical valency
am
- interface angle, slope angle - slope angle during preferred etching - slope angle of etching mask
P
- reactivity factor in plasma etching
Y
- degree of anisotropy
E EO
-
ci aIF
potential zero-current potential
XI11
XIV
Symbols
Y Vi
- frequency - stoichiometric factor of particle of kind i
e
-
density (specific mass)
2
-
sum
z z
to
- transition time life time of reactive plasma species transition time in outer-currentless state
-
Abbreviations
ARDE AsSG
- Aspect-Ratio-Dependent Etching - Arsenosilicate glass
CAIBE
-
EBRE ECM ECR EDTA EMM ERIBE
-
FIB
-
HF
- High frequency
IBE IBAE IT0
- Ion beam etching - Ion-beam-assisted etching - Indium tin oxide
JEM
-
KTP
- Potassium titanyl phosphate, potassium titanate phosphate
LPCVD
-
Chemical-assisted ion beam etching
Electron-beam-supported reactive ion etching - Electrochemical machining - Electron cyclotron resonance - Ethylene diamine tetraacetic acid - Electrochemical micromachining - Electron-beam-enhanced reactive ion beam etching Focussed ion beam etching
Jet electrochemical micromachining
Low pressure chemical vapour deposition
M - Metal, semiconductor - Magnetic-field-enhanced ion etching MIE MERIBE - Magnetic-field-enhanced reactive ion beam etching MERIE - Magnetic-field enhanced reactive ion etching NA NPBE
- Numeric aperture - Nanoparticle beam etching
OM
- Oxidant
XVI
Abbreviations
m,
- Electron mass
pa
- Adsorbed product
Pd PE PEC PJE PMMA PSG PZLT PZT
- Desorbed product - Plasma etching - Photoelectrochemical etching - Plasma jet etching - Polymethyl methacrylate - Phosphosilicate glass - Lead lanthanum zirconate titanate - Lead zirconyl titanate, lead zirconate titanate
R RIBE RIE RNE
- Radical
rf
-
SCE SECM SF
- Saturated calomel electrode - Scanning electrochemical microscope - Surface
Ten
- Energized particle - Thermalized reactive particle
Tr
Reactive ion beam etching Reactive ion etching Reactive neutral gas etching Radio frequency (= HF - High frequency)
UME
uv
- Ultramicroelectrode - Ultraviolett
YBCO
- Yttrium barium cuprate
1 Introduction
Microtechnical patterning methods deal with the preparation and application of components that cannot be prepared by classical mechanical methods. The precise designing of various materials is a quite essential prerequisite for fabricating microdevices. Micropatterning of substrates and films microdevices are composed of, is therefore an elementary process of physical microtechnique. While cutting methods are essential in precision-mechanical designing they do not play a role in microtechniques. Instead, etching processes are the most essential methods for designing micropatterns. Etching processes in combination with electron beam lithography and, especially, with photolithography have become the central tools of microproduction. Along with a large group of standard methods, in which lithographic masks are used for patterning, a lot of special methods have been developed, partly using masks, partly working without them. The subject of this book are lithographic etching processes. For a long time etching processes have played an important role in microcrystallographic material analysis. They were developed in semiconductor technology for characterizing semiconductor solid materials. The etching media were optimized to make visible morphological characteristics, e.g., crystallite structure, lattice disturbances or doping boundaries. Some of the methods are used today, either in their original or in a modified form for micropatterning by means of lithography. Those etching methods that serve exclusively to make the texture of solids visible are not considered here. Although application of etching techniques in microelectronics and microsystem technology was established only in recent years, the combined effect of etching technique and resist technique has been used for a long time. Such aggressive natural liquids like lactic acid, citric acid or acetic acid were probably already used for the treatment of materials in prehistoric times. But only after preparing highly corrosive strong mineral acids (hydrochloric acid, sulfuric acid, nitric acid) by Arabian and European alchimists, systematic treatment of surfaces by etching processes had been possible. During the Middle Ages such etching processes were applied mostly in the field of art and craftwork. Etching of metal surfaces that were partly covered with resins was used largely in the late Middle Ages to adorn arms and armour. The pattern, engraved in the resin, could be transferred permanently into the metal by the
2
1 Introduction
etching process. This kind of metal etching developed into an important means of plastic arts. A metal plate, mostly hammered copper, was covered with a film of varnish. In this film a pattern was engraved by means of a needle. This engraved pattern had to be a negative of the picture, later to be printed. By the following etching process the pattern was transferred into the metal plate in the form of small etched grooves. The width and depth of these grooves depended on the choice of composition of the etch bath or etch paste. After removing the varnish film and applying paint the picture was transferred in a last working step to a paper or another material. Already the Old Masters like Durer and Rembrandt used this technique of etching. In opposition to all cutting and engraving methods, the etching technique allowed the fabrication of agreeable patterns without rims, swarf or burrs. With the etching processes in craftwork and in fine arts, where resins are used as resistant films against the etching media, i.e. a resist, we already find the method of the primary pattern generation in the resist and the pattern transfer by local dissolution of a metal which are the essential fabrication steps of the modern microlithographic etching technique. After the discovery of hydrofluoric acid by Scheele in 1771 the etching of a variety of materials became possible. Hydrofluoric acid was soon used for etching patterns into glass. This was the first time that an inert non-metallic material was designed by an etching process. A further decisive impuls for the development of the etching method was the discovery that some resins and bitumen hardened under the influence of sunlight (Senebier 1792). This method was used by Nikpce for the storage of pictures (Heliographie, Nikpce 1522). The discoveries of Senebier and Nikpce were applied in manufacturing printing plates of stone. Hence the name lithography for the method from the greek word lithos for stone. Transferring photographic images onto stone plates enabled a convenient amplification by printing. A disadvantage was that those images could be transferred in black and white only. Gradation was achieved in 1936 using printing plates with small rasters. The ratio of the width of the raster dots to the etched spaces determines the grey level. The general principle in all lithographic methods consists in producing a relief by etching through an etch resistant mask. This mask is patterned by engraving a resin or by exposing a so-called photoresist. Thus a twodimensional shape is determined, its transferring by etching produces the three-dimensional structure, i.e. the relief, in the underlying material. This method has been used on industrial scale since the middle of the 20th century. Already before microscopically small elements had to be fabricated, the etching method proved to be a convenient method for precise shaping of threedimensional metallic components, especially in the case of workpieces of complex structure. The industrial etching techniques were called chemical milling, or photochemical milling if the etch mask pattern was generated by exposure to light of short wavelength. The wet chemical etching process has established itself as a special field of the lithographic etching technique in the production of printed circuit boards and integrated solid-state circuitry. Etch-
1 Introduction
3
ing processes developed in microelectronics are now used and adopted in microsystem technology. The rapid growth in the variety of the microdevices demands high reliability and standardization of the devices and great flexibility in technology. Nowadays etching procedures are applied for a very large amount of materials and a great variety of material combinations in the field of microsystem technology. The majority of metals and semiconductors as well as many alloys and non-metallic compounds are treated by etching processes. The measures of the produced patterns reach from some millimeters to few micrometers in many micromechanical devices. In microelectronis usually structures in the sub-micrometer range (0.5 to 0.3 pm) are produced. By combining electron beam lithography with dry-etching methods structures of less than 0.1 pm, even down to the 10-nm-level can be generated. These structures, however, were used up to now mainly in research only, especially for investigating electronic quantum effects. Because of the character of the material removal, etching processes are suitable also for extreme miniaturization. With mechanical removal of material, bits of the material are seperated. Although these can be very small, they are made up of a lot of atoms or molecules. Whereas in an etching process molecule by molecule or atom by atom is seperated from the solid surface. Hence etching processes are of high resolution according to their character. Only with reaching atomic or molecular dimensions, they get to their theoretical border. Therefore etching processes are suited for treating the smallest conceivable solids.
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2 Distinctive Features of Microtechnical Etching
2.1 Etching as a Fashioning Method As practically in all physically working technologies, the working steps in microtechology start on a surface. In contrast to precision mechanics working with curved and typically cylindrical surfaces, microtechnology is concerned with flat surfaces mainly. This is explained by the following facts. The building up and removal of material in microtechniques is done by processes reacting on a large area simultaneuosly, but homogenuously on flat surfaces only, whereas in precision mechanics a special tool works on a single local point at one time. Furthermore, a quite even surface of the solid to be worked, is the precondition for the generation of patterns in the etch-resistant mask by a reproducing lithographic method. Another important difference to cutting and remodelling procedures concerns the character of the process. Even in producing swarf, the removed material is the same in its material properties as the starting material. In etching procedures the removing proceeds by the transition of single atoms or molecules or of small clusters with less than 100 atoms from the solid into a mobile phase. The transition is connected with a material change, at least with the phase transition, mostly, however, with chemical changes. The transport of material from a solid through a phase boundary into a mobile phase is the central characteristics of all etching processes. Patterns in microtechniques are generated by means of masks mostly. For that purpose a masking layer is prepared on the surface of the work piece, which is called substrate subsequently. The thickness of this masking film lies between some hundred nanometers and a few micrometers. Preferentially, a thickness about 1 pm is used. The masking layer is patterned by microlithographic methods, i.e., it is removed at certain parts and preserved in others. The geometric measures of these patterns in the masking layer correspond to the measures of the devices being produced in the substrate. They are in the order of magnitude from millimeters down to sub-micrometers. According to the kind of pattern transfer one has to distinguish photolithography (using visible light and a photosensitive resist as masking layer), W-lithography (using W-light with the resist), X-ray and electron beam lithography (using X-rays or electrons of high energy, respectively, with a special resist as masking
6
2 Distinctive Features of Microtechnical Etching
layer). The transfer of the original pattern into the masking layer is feasible on flat surfaces only with most lithographic methods. Thus the tolerance level of flatness is less than 1 ym for ULSI-circuits in photolithography.
2.2.1 Limits of Additive Microtechnical Pattern Generation Patterning is also possible by local deposition of material on a substrate surface. This technique is called additive patterning. Limiting preconditions exist for additive patterning. If - as usual in microtechniques - the material layers are deposited on the whole substrate area, the additive pattern generation needs masking as well. E.g., a metal can be deposited locally by a microgalvanic method into openings patterned in an insulating layer on a conductive substrate. In this case the preconditions are electrically conductive and insulating materials, respectively. The so called lift-off technique is another additive method. A substrate is covered with a mask pattern reverse to the desired functional pattern. The functional film is deposited on this pattern. Subsequently the mask material is dissolved in an appropriate medium and the adherent functional film is lifted off. Only the unmasked areas of the patterns stay covered with the functional material. One precondition in this process is that the side slopes of the mask patterns are not covered with the functional material so that the dissolving medium can get in contact with and attack the mask material. Another is a not too large area to be lifted, because otherwise the dissolution process takes a long time or comes to a standstill. The geometries are dependent on the actual materials used. Furthermore the functional material is not dissolved and chippings of it contaminate the solution, eventually disturbing following processing. Because of the limiting conditions of the additive processes, they play only a minor role in microlithographic patterning.
2.1.2 Subtractive Pattern Generation Processes in which parts of a complete functional material are removed are called subtractive methods. Subtractive patterning is feasible by a probe technique removing the material directly at a local spot, or by using a mask that protects the areas not to be removed from the general attack of the etching medium. This mask is generated normally using data recording. The data record can either be used to generate the pattern directly in the etch resistant mask material, the photoresist or electron beam resist, on the respective substrate by means of a so-called photolithographic pattern generator or an electron beam exposure system, or by transferring the pattern from a photolithographic mask, generated by the said methods, in a photolithographic exposure step
2.1 Etching as a Fashioning Method
7
with an aligner instrument to the mask material, the photoresist. In both cases the exposed pattern has to be “developed”, i.e., either the exposed or the unexposed part of the pattern will be dissolved, leading to a positive or negative image of the pattern of the applied photolithographic mask, or of the data of the generated pattern, respectively. The development procedure is dependent on the character of the used resist. In the subtractive working mask processes, the mask produced in the primary pattern transfer step covers all parts of the surface of functional material, which shall remain unchanged. All methods of subtractive pattern generation in a functional material by means of a micropatterned etch-resistant mask material belong to the microtechnical etching methods. In principle all materials can be patterned by etching processes. Hence microtechnical etching, in opposition to additive pattern generation, is a universally appliable method. The choice of the respective etching process has to be done, however, considering the chemical properties of the materials to be removed and all the other materials not to be removed, but lying open to the etching medium. The etching process not only determines the rate of dissolution of the respective materials, but also affects the geometries, i.e., certain deviations from the pattern of the etch-resistant mask. The rate and selectivity of the dissolution as well as the geometries produced during progressing etching are the essential criteria in choosing the etching procedure. Etching processes are devided into two major groups, the so-called wetetching and dry-etching processes (fig. 2-1). The processes of the two groups differ in the mobile phase acting as etching medium, i.e., the phase in which the particles from the solid are transferred into and removed from the surface. In wet etching processes the detaching of the material is done by its interaction with a liquid, the “etching bath”. The interaction is accomplished by redox and solvation processes. If the oxidation proceeds without an outer source of current, we speak of outer-currentless etching processes. We also know electrochemical, photochemical and photoelectrochemical etching processes with either a source of current, or supported by light or with both. Wet chemical etching processes are classified also by the etch grooves they produce. Generally wet etchants work isotropically, but there are as well wet chemical etchants with crystallographic preferences that work anisotropically. These are used for shaping monocrystalline materials. In dry-etching methods the material is transferred into the gaseous phase. Along with the etching in reactive vapours, plasma and ion etching processes are of importance. Etching processes working with accelerated ions are subdevided into sputter etching, reactive ion etching and various ion beam etching techniques. Beside the accelerated ions, other sources for activating etching processes in the gaseous phase are used, e.g. electron beams or light. A seperate branch of the microtechnical etching evolved with micromechanics. The term micromechanics is used with two different meanings. On the one hand it stands for the field of miniaturized devices and on the other it includes all those methods that are convenient for patterning and shaping in the microrange with depths that are deeper than those in thin film technology,
8
2 Distinctive Features of Microtechnical Etching
LITHOGRAPHIC ETCHING
I
I WET ETCHING]
DRYETCHING
I
LASERBEAM
BEAM ETCHING IONETCHING REACTIV
CHEMICAL ASSISTED ION BEAM ETCHING
REACTIV ION BEAM ETCHING
Fig. 2-1. Overview over the most important classes of microlithographic etching processes
i.e. deeper than a few micrometers, and especially, for fabricating flexible microstructures. Also in this field of micromechanics etching techniques are of decisive importance. They dominate over miniaturized cutting methods. The etching methods of micromechanics are distinguished by the etched material and the etch depth as surface and bulk (substrate) micromechanics. All etching processes have some criteria in common, that are independent of the material to be etched, the kind of etching medium and the application. The important parameters etching rate and selectivity, the form determining parameters degree of anisotropy and degree of sloping of sidewalls decide on the efficiency and quality of an etching process. These parameters shall be introduced in the following, before dealing with the respective groups of methods.
2.2 Etch Rate and Selectivity
9
2.2 Etch Rate and Selectivity 2.2.1 Etch Rate and Time Request Microtechnical etching has to be realized in a technically reasonable time. The various steps of a process should proceed rapidly ensuring a short time required for the fabrication of the whole component. Thus the etching steps should demand times comparable to the other process steps. The times of the photoresist steps have time intervals of some seconds to a minute. With wet chemical processes, this magnitude can be met by choosing a convenient etching medium and etching conditions as long as thin films are to be etched (thickness about 1 pm or less). With thicker films and principally in dry etching processes there are fundamental physical and chemical limits. Processes at the phase boundary and transport processes from and to the phase boundary cannot proceed with arbitrary velocity. Hence the etching of film and substrate materials with thicknesses of more than 10 pm, as e.g. in deep etching of substrates with etching depth of 10 pm to 1 mm, costs more time. Also dry etching processes are of longer duration, as their etching rates often do not exceed 1to 10 d s , sometimes they are even less than 1 d s . Thus the etching af a 1 pm thick film needs about 3 to 30 minutes. With thin films of a few nanometers thickness, etching times much less than a minute can be achieved. However, ensuring good controllability and reproducibility of the etching process the etching time must not be too short, as the starting and the end phase of etching are especially prone to interferences. The quality of an etching process is difficult to control1 if the actual etching process is of short duration in comparison to its initial and end phase, e.g. the removal of contaminating films, the immersion and taking out of the substrate or the switching on and off of a plasma influence the beginning and the end. Because of this etching times of about 1 minute are to be preferred.
2.2.2 The Etching Process In contrast to high precision and ultra high precision mechanics in microtechnical etching the material is transferred in form of single atoms or molecules from the solid into the liquid or gaseous phase (mobile phases). Each etching process, therefore, is a process in which material is transferred through a phase boundary. Because of the removal of single atoms or molecules in the elementary steps, very high accuracies can be achieved. The elementary process of etching becomes limiting only on the molecular or atomic scale for the accuracy of patterning. The central physical quantity for subtractive patterning in a material to be treated microtechnically is the etch rate r. It is the ratio of the actual etched material thickness hetchand actual etching time tetch.
10
2 Distinctive Features of Microtechnical Etching
The etch rate also can be given as an average (medium) etch rate r, for the etching of the whole film of the thickness h, where the end of the etching process is defined by the disappearing of the film after the etching time to (2)
rm= h/t,
The temporary etch rate r(t) deviates often to a great extent from the medium etch rate r,. it is determined by the differential quotient of film thickness and time. r(t)
= dh/
dt
(3)
The transfer through the phase boundary is understood as a pure physical process only, if atoms or molecules are detached directly by a mechanical impuls. That, however, is the case only in detaching solid material by an exclusively mechanical effect of particles of high energy (sputter effect - see section 4.4.1). But also in this case, as in all other cases, the detachment of the atomic or molecular particles from the surface is connected with a change in the interaction of the particles within the surface, i.e. their mutual acting cohesive and chemical bonds. Except for the sputter effect, the chemical component dominates nearly all etching processes in the actual step of phase transition. During etching as well in liquid as in gaseous media, the particles of the mobile phase interact with those on the surface causing them to leave the solid. In the molecular or atomic environment the neighbouring atoms or molecules of the detached surface particles are substituted by particles of the mobile phase or the interaction among the surface particles is minimized transferring the surface particle or its reaction product by thermal activation spontaneously into the mobile phase. In wet chemical etching processes the change in the interaction in the neighbourhood of the particle is accompanied by a chemical change, e.g. the building up of a shell of solvent molecules (solvation), or of ligands (complexing), the transition of charge (redox reaction), or a combination of these processes.
2.2.3 Transport Processes The transport of the particles from the interface into the interior of the mobile phase is of physical nature. Either it is caused as an oriented movement by momentum transfer (etching processes in vacuum) or by brownian movement in a concentration gradient (diffusion). With momentum transfer by energized particles the rate of the etching process is independent of the velocity of the movement of the single particle from the surface. The etching rate is determined in this case by the number of impacting energized particles and the number of detached particles of the solid per impacting particle.
2.2 Etch Rate and Selectivity
11
Transport by diffusion is the quicker the greater the difference of concentration per unit of length (concentration gradient), i.e. diffusion-controlled etching processes possess etching rates that are the higher the greater the molecular mobility of the particles to be transported, the higher their concentration on the surface of the solid, the lower their concentration in the interior of the mobile phase, and the shorter the distance between the solid surface and the interior of the mobile phase (diffusion distance). The transport process can be accelerated by a spontaneous or forced movement in the mobile phase (convection). This accelerating effect is caused by a relative motion of the solid surface to the near-surface area of the fluid phase that increases the concentration gradient and thus improves diffusion. In special cases transporting is supported by a field, e.g. electrical migration. In all dissolution processes in a liquid medium, but also in etching processes in a reactive plasma the reaction at the phase boundary is preceded by a transport process of the reactants from the mobile phase. The atoms and molecules, necessary for the phase transition, have to move from the interior of the liquid phase to the solid surface where they react. If the passage of energized particles to the surface, of which particles have to be sputtered off, is considered as a transport process as well, it applies for all etching processes that the actual phase transition step is preceded by an transporting step to the surface. Thus results the general division in three steps of the etching process: 1. Transport of the reacting particles from the interior of the mobile phase to the solid surface 2. Transition at the solid surface 3. Transport of the detached particles from the solid surface into the interior of the mobile phase.
2.2.4 Process Velocities As the above mentioned three general steps of the etching process are succeeding steps, the velocity of the etching process is determined by the slowest step. Thus we differentiate between transport-controlled and interfacecontrolled etching processes. The transport control1 can be caused by the transport of the reactants to the solid surface or the transport of the reaction products off the solid surface. Diffusion-controlled etching processes play a decisive role in many wet etching processes. The transport control1 by diffusion is described by the 1. Fickian Law:
dc/ dt
=D
dc/ dx
(4)
The transport of matter is given by the change of concentration in time (dc/ dt). This change is the quicker the higher the specific diffusion constant (D) and the local concentration gradient ( d d d x ) . Under most hydrodynamic conditions a diffusion layer of a characteristic thickness is formed in the vicinity of
12
2 Distinctive Features of Microtechnical Etching
the solid surface. In this layer the transport is determined by diffusion, exclusively. Outside the layer, convection contributes largely to the material transport. Under stationary conditions, i.e. with a constant flow of matter in the diffusion layer, the concentration gradient is temporally and locally constant. In this case the concentration gradient equals the quotient of the difference of the concentration in the interior of the solution C, and the concentration at the interface CsFand the thickness of the diffusion layer d: d d dx = (G -
d
(5)
If the transport of reactants to the surface is the controlling step, a higher etching rate is achieved by increasing the the concentration of the limiting species in the solution. If the rate is controlled by the diffusion of the reaction products from the surface, the concentration of those should be small in the interior of the solution. Equation (5) shows that the transport process can be accelerated efficiently by reducing the thickness of the diffusion layer, as the concentration gradient increases with decreasing diffusion layer thickness at the same concentration difference and with that the diffusion rate (eq.(4)). The thickness of the diffusion layer depends on the viscosity of the solution and the convection in the solution. With increasing convection the diffusion layer thickness is reduced. Therefore, transport-controlled etching processes are accelerated by moving the substrate in the eching medium, by stirring the etching medium, by ultrasonic treatment and other convection intensifying measures. Working with a rapidly spinning substrate and spraying the etch medium through nozzles on it is an effective method. According to the Levic equation the diffusion layer thickness at a rotating disc decreases in dependence on the square root of the spinning velocity cr):
Without any forced convection the diffusion layer thicknesses are between about 50 and 500 pm, typically'. These values result from the spontaneous convection in liquids caused by slight temperature and density gradients. In open etching baths these gradients occur because of the cooling by evaporation at the liquid surface. Etching processes are in the most cases exothermal processes and thus the dissolution itsself contributes to the formation of minor temperature gradients responsible for spontaneous convection. In dissolution processes the density of the solution near to the dissolving surface is changed because of the changing in chemical composition. The occuring density difference contributes distinctly to the spontaneous convection and therefore diminishes the diffusion layer thickness. Convections caused by the reaction process can diminish the diffusion layer thickness to
' K. Vetter (1962)
2.2 Etch Rate and Selectivity
13
values smaller than 100 pm. A n essential further decrease in the diffusion layer thickness and thus considerably intensifying the wet chemical etching process can be achieved by an externally forced convection. In strongly moved liquids or with rapidly spinning substrates diffusion layer thicknesses less than 10-5 pm can be realized. In this way transport-controlled etching processes can be accelerated by 1 to 2 orders of magnitude. The chain of effects that the convective processes take on the rates of transport-controlled etching processes is illustrated by the following scheme: Convection + Decreasing the diffusion layer thickness + Increasing the concentration gradient + Increasing the diffusion velocity + Increasing the etching rate The rates of etching processes with the rate controlled by the phase transition, are determined by the respective interfacial process (e.g. redox reaction or complexing reaction). Whereas the rate control of a transport step has a general characteristicfor all etching processes, the rate control of such etching processes, that are controlled by an interface reaction, is decisively dependent on the character of the respective interface process. In wet chemical etching these are, e.g., swelling, solvation or complexing processes, in dry etching, e.g., plasma chemical and impulse-induced reactions on surfaces, as well as reactions that form desorbable species. These specific processes are dealt with in sections 3 and 4.
Etch Rate Ratios As etching processes at work pieces are carried out on two or more materials and, as a rule, in the presence of a mask material, not only the absolute etch rates of the materials are of importance. Rather of greater importance for the quality of an etching process are the ech rates of the materials that should not be dissolved in a certain etching step. The feasibility of an etching step or the choice of an alternative method depends on the ratio of the etch rates of the different materials. If no etching method is found with the needed etch rate ratios for patterning a single material in combination with other materials, it can be necessary to modify the material combination of the system for realizing a certain microtechnical element. In some cases auxiliary films or auxiliary patterns are included in order to avoid or at least minimize an undesirable material loss. Selectivity Etching media attacking only one of the components of a given material combination would be technologically very convenient for patterning. Such an etching medium is called a specifically reacting etching medium, it can be a wet chemical etching bath or a reactive plasma. In reality a specific etching medium, even under subtly prepared chemical conditions, does not react on one material only. It reacts more or less selectively on different materials. A really specific attack, however, is not necessary in most cases. The selectivity for patterning by chemically etching has to
14
2 Dlstinctive Features of Microtechnical Etching
be seen in connection with the chemical properties of the other materials in the device. So it is possible that in fabricating a certain device a generally nonspecific medium affects only one of the materials in the device as the others are resistant to it. In another case, it is feasible that a comparably specific medium for a certain material is not selective if a related material belongs to the device to be patterned. The etching selectivity Set& can be expressed by the quotient of the etch rate r of the material to be patterned and the etch rate rres of the material not to be etched:
The specifity of the etching of a material is a decisive criterion for the general development of etching media. In fabricating concrete devices it is required additionally that in each manufacturing step the used etching medium is selective to all materials in the device not to be etched, but lying open to the etchant in that step. The suitability of etchants for selective structuring is to be assessed not only by their chemical properties but also by some specifics of microsystem technology. Underlying materials, e.g., can be effectively covered by overlying layers and therefore, be protected against the attack of the etchant. Tiny pores in such layers, however, that would not affect the functioning of the designed device, could be able to cancel the protecting effect. The properties of thin film materials differ from those of the bulk so that the etching rates of thin films are not the same rates measured for bulk materials. Etching rates and with them etching rate ratios are dependent on the morphology of the material (grain boundaries, grain size, and texture) as well as on the geometry of the structure elements ( size, position)( see sectios 3.4.4, 3.4.5 and 4.3.9). Interaction among the thin films not only affect the etching rate ratios but their individual absolute etching rates, e.g. by galvanic effects (see section 3.4.3). In general, however, it can be concluded that these mutual influences can be understood, and therefore, it is possible to find convenient etching procedures for successful microstructuring complicated compositions of materials in one microdevice, too.
2.3 Isotropic and Anisotropic Etching Etching methods are devided into two classes according to their etching velocities in the directions in space: isotropic and anisotropic etching methods. With isotropic etching methods there is no preference of any space direction, i.e., the etch rate is the same in all directions. In anisotropic etching methods, certain directions are preferentially etched. In this case etching rates depend on the directions. Anisotropic etching can be caused either by the dependence
2.3 Isotropic and Anisotropic Etching
15
of surface processes on the direction, or by a preferential motion of reactive particles in the mobile phase. The most important case of anisotropical etching due to the dependence on direction of surface processes is the crystallographic etching, especially of monocrystalline material (see section 3.5). Anisotropic etching caused by the preference of the direction of motion of particles in the mobile phase is used in ion etching methods (section 4.4). The velocity of etching processes, which is controlled by the movement of thermally activated molecules or atoms in the mobile phase, is of the same value in all directions. The movement of atoms and molecules near the surface are normally controlled by diffusion. In isotropic media having no preference in their orientation in space, no preferred direction of the moving particles exists. Therefore, most of the transport-controlled etching processes in liquid media are isotropical processes. Isotropic etching causes the so-called “undercutting”, i.e., material under the etch-resistent mask rim is etched. Ideally isotropic etching processes react in all directions with the same velocity. The horizontal etch rate rHworking under the mask edge, therefore, is as large as the etch rate r in normal direction: rH = r (isotropic etching) The undercutting 1, is equal to the etching depth hetchin this case: 1, = hetch (isotropic etching)
(9)
Isotropical and anisotropical etching differ in the shape of the etching groves formed under the opening of an etching mask. With ideal isotropic etching behaviour, a quarter of a cylinder mantle is developed along the mask edge as the new surface (fig. 2-2), at an inner corner of a mask opening an eighth of a sphere is formed. Curved surfaces with locally varied radii of curvature emerge as in progressing etching through seperated, but neighboured mask openings due to the undercutting effect the etched openings get in contact and then overlap. In isotropic etching the shapes of the emerging surfaces become more intricate as the film in a mask opening is completely etched off and the etched edge forms a borderline to the underlying material. From this moment the concentration distribution changes and with it the local pattern of transport processes. The edges of the etched structure steepen (see section 2.4). Isotropic etching does not allow to prepare at the same time arbitrarily deep and narrow slits. Because of undercutting the mask edges, an isotropic etchant is able only to pattern slits with a maximum depth of half their width. The ratio of depth to width (aspect ratio) is not greater than 0.5. This boundary condition is relatively uncritical as long as the film thicknesses are essentially smaller than the lateral measures of the pattern. Many microtechnical
16
2 Distinctive Features of Microtechnical Etching
Fig. 2-2. Forming of a round sidewall profile in isotropic etching of a functional film
components nowadays require, however, structures with lateral measures that are smaller than their depth (aspect ratio >1). For such applications anisotropic etching methods are necessary. Thus practically exclusively dry etching processes with a certain degree of anisotropy are applied in the fabrication of very and ultra large-scale integrated circuits. On the other hand, the progressing of an etching front under another nonetching film is of practical advantage for some applications. So a functional film can be patterned by selective isotropic etching of a so-called sacrificial film lying under the film to be patterned. Such sacrificial layer techniques are an essential precondition for preparing free-standing microstructures as they are needed in micromechanical devices with flexible structure elements. Beside isotropic etching sacrificial layers, also the etching of substrate material is used for the fabrication of flexible micromechanic structures (see section 3.6). By ideal anisotropic etching in the stricter sense etching with an ignorable etching effect in lateral direction is understood, i.e. the etching front moves in one direction only, preferentially in normal direction to the substrate surface into the material. In this case vertical edges are formed directly under the mask edges (fig. 2-3). The two-dimensional pattern of the etch-resistant mask is transferred into the material to be etched as a relief, in which all lateral measures of the mask are preserved. Theoretically, the structures that remain after etching, can be much higher than wide, i.e. arbitrary aspect ratios are feasible. Actually, aspect ratios greater than 100have been achieved by special methods, as e.g., anisotropic crystallographic silicon etching (see section 3.5). The anisotropy of etching is described quantitatively by the degree of anisotropy y. If the velocity r, of lateral undercutting under a mask edge is equal to the velocity r in normal direction, then the degree of anisotropy is equal to 0. If r, tends to the value 0, then y tends to 1.
2.3 Isotropic and Anisotropic Etching
17
functional layer
v ........................
Fig. 2-3. Generation of vertical sidewalls in anisotropic etching through a window in a mask (crossection)
y
=
1- r,/r
y = l
........................
t=tl
t=t2
(for r,
(10)
(if ru >r)
(11)
According to (5) the absolute undercutting 1, can be estimated for partially anisotropic etching processes (O
18
2 Distinctive Features of Microtechnical Etching
2.4 Edge Geometry and Roughness 2.4.1 Deviations from Ideal Geometry Etching processes provide smooth or ideally round and steep edges in exceptional cases only. Fluctuations or disturbances in the solid, the etching bath, or the processing cause local deviations in the formed structure from the theoretical measures. To such deviations are counted, errors in the mask geometry, as well as grain bounderies, distributions of crystallite size, and crystal defects in the etching material. Other deviations arise because of fluctuations in the etching medium, e.g., due to locally varied convection, due to particles and other stochastic influences, and, possibly, due to spontaneous processes of forming structures, as hydrodynamic vortices or oscillating chemical processes. Real etching structures, therefore, possess very seldom ideally smooth edges. They are characterized by a finite roughness and real edge geometry. The edge roughness is a measure for the local fluctuation of the width of lithographic patterns. It is determined by the mean deviation of the local edge position from the middle position of the edge. Edge roughnesses can be caused by local differences in the inner structure of the material, as the position or the density of crystal defects or the density and the position of grain boundaries in crystalline or amorphous materials. Such local differences in the material affect the velocity of surface processes at the respective reaction site. The geometry of a flank formed during an etching process under the edge of the mask is determined by the global specifics of the etching method as well as by the local properties of the material. The latter affects the flank geometry in a like manner as the edge roughness. Local deviations in the flank geometry are reproduced directly in the edge roughness.
2.4.2 Flank Geometry in Isotropic Etching Any lithographically generated structure is marked by the shape of its flanks. In the simplest case the cross section of a flank can approximately be described by a straight line. The slope of the straight line and, therefore, of the flank, is given by the slope angle. Lies the foot point of the slope directly under the mask edge, the slope angle is determined by the etching depth h(etch) and the undercutting l(u): tan a = hetch/lu (straight slope, no undercutting) However, no straight, but curved flank cross sections, mostly circular ones, arise during isotropic etching. The circle line is always vertical to the mask layer. The radius of the circle grows with progressing etching time.
2.4 Edge Geometry and Roughness
19
Fig. 24. Raising of the sidewall profile during undercutting of a functional film under the mask edge in isotropic etching. The successive lines demonstrate the sidewall profile at increasing etching times in arbitrary units.
The geometry of the flanks can be defined to a certain degree in isotropic etching. Upright flanks can be achieved by overetching. The radius of the isotropically etched circular etching profile is equal to the etch depth and thus grows with etching time. The growing of the curvature radius of isotropically etched flanks continues under the mask edge, if the film thickness is etched through and the underlying material is not attacked (fig. 2-4). As the contact line of the flank and mask is preserved, the slope of the flank gets more and more vertical in isotropic etching as well. The flank width w(f) obeys the following idealized equation: wf = hetch (hetch etch depth, h film thickness) The etching depth is also characterized by the ratio of the etching time to, till etching of the film is complete, to the overetching time t, Therefore, the flank width can be estimated by the overetching-time
[
wf = h (1 +
2)-
JZ;
+
I);(
Is the edge slope a approximated by a straight line from the foot point to the contact point with the mask, the resultant slope angle a is given by:
a
=
arctan(hetCh/wf)
(16)
2.4.3 Fabrication of Low Slope Angles by Isotropic Etching Low slopes can be produced by isotropic etching using an auxiliary layer over the functional film that is to be etched (fig. 2-5). This auxiliary layer etches isotropically and with a higher etch rate in the respective etching medium than the functional film2.By the gradual dissolution of the auxiliary layer more and more film areas get free to the etching bath. It results a nearly straight edge profile with a small bevelling angle. The quicker the etching of the auxiliary layer, the smaller gets the angle (fig. 2-6). The slope angle p is determined by the ratio of the etch rate of the etching film r and the auxiliary film r,:
J.J.Kelly und G.J.Koel (1978)
20
2 Distinctive Features of Microtechnical Etching Mask
- Auxiliary layer Functional layer Substrat
Mask Auxiliary layer Functional layer, partially etched 'Substrat
-
Mask Auxiliary layer . Functional layer with tilted flank 'Substrat
+
b. Fabrication of a sloped sidewall using an auxiliary layer with a higher etch rate: (a) schematically; (b) sloped sidewall of a thin aluminium film, etched with an increased removal at the interface metaYphotoresist
Fig. 2 5 a
13 = arctan(r/rh)
(17)
The price to pay for this possibility of adjusting the slope angle in isotropic etching is undercutting, which means a deviation in measures. The maximum aspect ratio diminishes to values smaller than 1/2 with the decreasing angle p. In the simplest case the interface between the etching mask and the material of the functional film works as the fast etching area. In this case one cannot speak of an etch rate in the stricter sense of the word. The etching medium penetrates in the interface and produces a gap, wherein the etching bath attacks the film surface unimpeded. The bevelled edge of a structure gets the lower the higher the penetration rate rp compared to the etch rate ro. For adjusting the slope angle aIFthe following equation is valid, approximately: tan (aIF)= rdr,
(18)
2.4 Edge Geometry and Roughness
21
fig. 2-6. Adjusting the slope angle of the functional film by choosing the etch rate of the auxiliary film
This angle could be set in wide limits, e.g., when etching Si02by choosing the temperature and composition of the etching bath3. Low slopes can be attained also in wet etching, if the films possess high mechanical stress that probably causes high etch rates in the interface area4.
2.4.4
Flank Geometries in Anisotropic Etching
According to the material and the running of the etching process, various flank geometries can be created by anisotropic etching. Whereas in crystallographic etching the crystal structure and the cutting direction of the substrate determine the shaping, more possibilities of shaping etching grooves exist in etching with directed particle beams (ion etching). Setting defined slope angles in anisotropic etching, is performed by convenient etch rate ratios of the etching and the masking films. Ideally vertical edges are produced by complete selectivity, independent of the slope in the mask material. In case of finite etch rate ratios, the slope of the mask is projected into the etching film. The greater the difference in the etch rates, the steeper gets the angle of the etching film slope in comparison to that of the mask slope (fig. 2-7). The opposite case, a smaller angle in the etching film G.I.Parisi et al. (1977) K.Kato, T.Wada (1991)
22
2 Dlstinctive Features of Microtechnical Etching
Fig. 2-7. Forming of a vertical structure flank in ideally isotropic etching (SEMpicture of a polymer film)
than in the mask, is achieved, if the etching rate of the mask film is higher. For that, a thicker masking film than etching film is necessary. The slope angle a of the etching film compared with that of the masking film a, is, therefore, directly proportional to the etch rate ratio of the etch rate r functional (etching) film to the rate r, of the mask material: tan a
=
tan a,
- r/r,
(19)
Adjusting the etching rate ratios and the shaping of the flank geometries in dry etching procesess is strongly dependent on the gas pressure, the surface temperature and the process running (compare section 4.4.11.). This transferring of slope angles is of interest for generating threedimensional (3D) structures. These are prepared with the help of 3D-mask structures that are generated by the method of variable doses, applied especially in microoptics5.With the same etch rates for the mask and the functional material 3D-structures of the mask are transferred 1:l into the functional material. Is the rate of the mask material higher than the rate of the functional material then the etched geometry is compressed in normal direction, with the opposite ratio the resultant structure is elongated in comparison to the mask.
E.B. Kley et al. (1995)
2.4 Edge Geometry and Roughness
23
2.4.5 Setting the Flank Geometry by Partial Anisotropic Etching Whereas with ideal anisotropic and ideal isotropic etching the slope angle can be controlled by the etch rate ratios of the mask and the etching material, various flank profiles can be generated in partial anisotropic etching even with a very high mask stability. However, in this methods not always straight slope lines are produced. According to conducting the process, when partially undercutting the mask edges, a variety of aspect ratios, slope angles and curvature is attained. In general the undercutting increases with decreasing degree of anisotropy in the etching method. The degree of anisotropy, however, is not a constant in many partially anisotropically conducted processes, it depends, e.g., on the aspect ratio, the structure size and the etching state. The preparation of profiles with very low slope angles is possible using special etching masks. Such masks possess a gap between the mask and the etching material with only a limited opening for the gaseous etching medium to get to the surface (fig. 2-8). Thus ideally graduated etch rates are obtained, as directly at the opening high reaction rates exist, that decrease with increasing distance from the opening6. With this method etching profiles with slope angles smaller than 1" can be prepared. The preparation of additional separate masks is necessary.
Fig. 28. Fabrication of structure sidewalls with extremely small slope angles by isotropic etching in the gas phase using a micromechanically prepared slit mask
A.Bertz, S.Schubert,Th.Werner (1994)
24
2 Distinctive Features of Microtechnical Etching
2.5
Accuracy
The advancing miniaturizing of structure elements necessitates an increasing accuracy in transferring the patterns. In the ideal case the designed data should be transferred absolutely precisely via the lithographic steps into the functional material. In reality, however, there are practically in every process step deviations that generate deviations in the measure of the structures. Principally, deviations in measures are possible in both directions, i.e., slits as well as bars can broaden or narrow. Undercutting under the mask edge in etching processes always means deviations in one direction. Slits grow broader and bars more narrow. Isotropic etching processes that are always accompanied by undercutting, therefore, always show deviations in this direction. In addition to the undercutting during the etching-through time of the functional film, a further deviation in the same direction is caused by the necessary minimal overetching times (e.g. compensating local differences of the etch rate and thickness deviations of the material). As the lateral etch rate that determines the undercutting under the mask in wet etching processes often increases in the final phase, the undercutting under the mask edge is in many cases significantly broader than the thickness of the etched functional film. Along with undercutting the other process steps lying between designing in form of a logical record and the completing of the etched structure in the functional film,also contribute to deviations in measure. The way from design to finished device runs over the following steps in general: 0 0 0 0 0 0
designing the device laying-out the individual lihographic layers preparing a prototype (mask, reticle) copying the prototype in a masking film (e.g. resist mask) developing and stabilizing the resist mask transferring the pattern of the mask into the functional material (etching)
Due to the small dimensions of microlithographic structures tiny absolute alterations of edges mean considerable relative errors that influence the functioning of the device. The isotropic undercutting, e.g., under both edges of a structure in a 1 ym thick film means a 2 pm deviation in the width of the structure. If in such a case at a 5 pm wide mask structure further alterations in the same direction occur, e.g. an additional undercutting of 0.5 pm because of necessary over-etching times, a line width of only 2 pm is the result. (5pm-2.(1pm+OSpm)), i.e., theloss inlinewidthis60%. Knowing the systematic deviations of the dimensions of the individual process steps allows for compensating this deviations to an essential degree by a certain change in the measures of the relevant structures in the data records of the lay-outs of the respective photolithographic layers. Such compensations are imperative, if the demands for accuracy of the parameters of the device are high. But only some of the deviations, e.g. undercutting, are foreseeable
2.5 Accuracy
25
concerning their directions and approximate values, and thus can be compensated. The extent of this compensation is limited by the kind and measures of the structure itself, as each edge in a mask for a slit cannot be shifted more than the half of the slit width, unless the slit mask opening would disappear and the slit would not be open. Besides, a minimum width is required to obtain regular etching. In the resist technique as well as in etching, local deviations occur beside the global deviations, e.g. due to the so-called proximity effect in electron beam lithography and photolithography, or topology dependent etch rates. Systematic deviations, that are dependent on topology, i.e., on the local geometrical circumstances, can be taken into account for the mask lay-out (see section 3.4.6). As the deviations are dependent on position and structure size, the compensation for each individual structure has to be calculated. Often inadequate local compensations are carried out because of under- or overestimating the local effects. Distinct deviations are accepted in deliberate undercutting by prolonging the etching time. In micromechanics, e.g., large areas of a functional film are undercut as flexible structures are prepared by means of so-called sacrificial films, that are completely etched between the functional film pattern and the substrate (see 3.6). Neighbouring functional film patterns of larger dimensions that should not be free-standing are accordingly undercut. If this undercutting is disturbing, the sacrificial film has to be patterned beforehand.
Table 2-1. Sources for Deviations in Etching, Illustrated at a Photolithographically and Wet-Chemically Patterned Film Process
Source of Deviation
film deposition
film composition exactness of film thickness homogeneity deviations in mask pattern resist thickness resist thickness homogeneity resist sensitivity (batch, age) light dose, dose distribution resist developer (concentration, temperature, convection) etchant concentration etchant temperature etchant convection adhesion of resist mask degree of anisotropy of etching adhesion of film on underlayer etching time control, over-etching time
lithographic prototype (mask) resist mask
Etching
26
2 Distinctive Features of Microtechnical Etching
As structures of exact measures or at least exactly reproducible measures are required, anisotropically etching processes are searched for, that reproduce the mask geometry most exactly. Dry-etching processes, especially ion beam etching produce the least deviations from the resist mask. They possess low lateral etch rates and thus allow structures of great aspect ratios to be prepared, i.e., structures that are as high as they are broad or even higher than broad. Besides they are preferentially used for structures that are smaller than 5 pm. The observance of certain requirements of accuracy of measures increase considerably the technical expenditure in lithographic procedures. Already when designing a device, but especially when choosing the technology steps, the requirements of accuracy of measures that would be necessary for the planned functions of the microtechnical device, should be considered. For complex devices coordinating accuracy requirements and the compatibility of technological steps is a central problem of planning and developing technology.
2.6 Monitoring of Etching Processes Monitoring of etching processes is important for scientific as well as for technological reasons. The progressing of etching is monitored in time to get kinetic data characterizing the process and the influence of process and material parameters on etching rates. The chief purpose of monitoring is to determine the etch depth and to minimize the attack of the etchant on materials not to be etched. As the etch depths in microtechniques lie in the nanometer and micrometer range, in special cases of micromechanics in the submillimeter range as well, microscopic or special chemically differentiating methods for monitoring must be applied. The progressing of etching in time must be well known, if a material in a patterning process is not to be etched completely, i.e., is not to be etched to the interface of the underlying material. The etching has to be stopped after a certain time, which demands the knowledge of the exact etch rate. The etching time fetch is given by the material thickness detchto be etched and the etch rate r. tetch = detch/r
(20)
If the etch rate can be estimated only by a limited accuracy and the start and final phase of etching are not reproducible, it is better not to end the etching after a pre-determined time, but monitor the etching progression, which is done in standardized technical procedures as well. Thus ill controlled influences on the etching behaviuor, e.g. immersion and passivating films in wet etching processes, can be considered. In dry-etching processes the turning on of plasma and inhomogeneous surface compositions can be taken into account. Monitoring of etching processes is indispensable, if the etchant is not selective enough against the underlying material. The exact stopping of an etching
2.6 Monitoring of Etching Processes
27
process is especially important, if sensitive thin films, that shall be preserved, lie under the etching material. Monitoring is more important in dry-etching processes than in wet etching, as dry etching processes, in general, work less selectively. There are principally two different ways of monitoring etching. Either the etching is controlled by an in-situ measurement of the film thickness, or the etching products are detected giving clues to their origin. The in-situ control of film thickness is well suited for measuring optical transparent films, as the thickness can be monitored by interference or ellipsometric measurements, if the etching material and the underlying material have different refractive indices or an reflective interface. With these optical methods thicknesses of a few or even only one nanometer can be detected. Electrically conducting materials on isolating underground can be monitored during etching by conductivity measurements. The necessity of electrically contacting the material and the ill chance that the contact is off before the material is completely etched are disadvantages of the method. The etching of non-transparent materials can be monitored by characterizing their products. In plasma etching processes the light of emission of the plasma itself can be used for detection. Each species in the reaction chamber is exposed to collisions in the plasma. Therefore, excited molecules or fragments of them exist of the etching products also, and their optical emission lines can be observed in the spectrum of the plasma. The appearing of new emission lines during the plasma process indicates that another material is etched. The disappearing of the lines indicates the etching being complete for a certain material. Thus emission spectroscopy can be applied for controlling the end point of an etching process. Is the individuell emission of the etch products too weak, fluorescence can be excited by irradiation with intensive light, e.g. by laser induced fluorescence measurements (LIF). So it is possible to control such etch products as well. As an alternative to optical-spectroscopical methods, mass-spectroscopic methods are frequently used for end point control. They have the disadvantage in comparison to the former method detecting only longer-living species, because the components of the plasma have to be differentially removed out of the reactor to the mass spectrometre and besides very reactive species disappear by decomposing spontaneously or by collisions with the reactor walls. The advantage of mass spectroscopy for end point detection is the greater accuracy, as the species are more clearly defined by the masskharge ratio than by their emission lines. Besides, very low concentrations can be detected by mass spectroscopy, so that the end point of etching processes can be indicated very accurately. Also in wet etching processes end point control1 is possible by detecting etching products. Beside absorption spectroscopy electrochemical methods are used for monitoring. These methods, however, are not so sensitive as spectroscopy in the plasma. In wet etching processes, because of higher selectivity or at least better known etching rates, a highly sensitive end point control is dispensable in many cases.
28
2 Distinctive Features of Microtechnical Etching
Determining Etching Rates Monitoring transparent films during etching by a method measuring the film thickness, as e .g. interference spectroscopy or ellipsometry, thickness and time are registered together, so not only the brutto etch rate but the variation of the etch rate during the whole process is determined. If the brutto etch rates rBcannot be determined exactly by these monitoring methods, they are estimated afterwards by the etched thickness and the etching time fetch
The estimation of the etch rate is reduced to measuring time and film thickness. The accuracy of this estimation is dependent on the accuracy of measuring the etch depth. Mostly this measurement is performed at structure edges prepared by an etch mask that covers part of the etching film. After removing the mask, the height of the etch step is measured by a mechanical profilometer, an optical profilometer, an atomic force microscope or an electron microscope. According to equation (20) the etch rate can be estimated by the time required for the complete etching of a film. In this case the film thickness must be known and the time is to be measured. Principally the etched mass can be determined by weighing. As the etched films are less than 0.1% of the mass of the substrate, such a gravimetric method, even with a comparatively high absolute accuracy, has a considerable relative error, so it is rarely used. An especially simple method for estimating etch rates is posssible for electrochemical etching. According to the Faraday laws (see section 3.4.1 and 3.4.6) the absolute etched mass of a material in electrochemical etching is equivalent to the charge. The quantity of the material etched per time unit corresponds to a current, the etch rate is clearly characterized by the respective current density (currendarea). As the electrical entities are measureable without difficulty, the etch rate is estimated without difficulty, if the density and the electrochemical valency of the material are known.
3 Wet-Chemical Etching Methods
3.1 Etching at the Interface Solid-Liquid All microlithographical etching methods etching at the interface solid-liquid are summerized under the term “wet chemical etching”. Wet chemical etching methods for patterning were applied in classic lithographic methods long before microtechniques existed, e.g. especially in printing techniques and fabrication of circuit boards. Nowadays they play a key role in microtechniques. Wet etching methods are distinguished from dry etching methods by essentially greater selectivity. This selectivity is due to the specific interactions between the components of the liquid and the solid, determining the reaction rate and whether a dissolution reaction takes place at all. Wet chemical etching, using photolithographic masks, is applied not only in thin film techniques. Flat objects with thicknesses of some 10 pm up to a few millimeters are wet etched to get three-dimensional patterns. Etching procedures for several materials of microtechnical importance were developped in connection with the so-called PCM-technique (photochemical machining or photochemical milling)’. This technique is not a real photochemical etching technique, the term stands for the use of photoresist techniques for shaping three-dimensional components with the help of wet chemical etching. In contrast, there are special methods using light for initiating or accelerating the etching process itself, meaning a real photochemical process. When dissolving a solid material in a liquid, the components of the solid are transferred into the liquid phase. For this the binding forces between the particles of the solid have to be overcome. The components of the solid are changed into soluble chemical compounds, the particles of which are transported by diffusion and convection off the surface into the interior of the solution. The interactions between the particles of the solid are substituted by interactions between the particles of the solid and particles of the liquid. In the simplest case the solvent molecules themselves form a shell, the solvate shell around the dissolved particle. The thus solvated particles are well mobile by diffusion in the solvent, i.e. in the liquid phase.
’ A.E Bogenschiitz et al. (1975); D.M.Allen et al. (1986); D.M.Allen (1987)
30
3 Wet-ChemicalEtching Methods
In most etching processes water is used as the solvent. The solvate shell, built up around the dissolving particles is a hydrate shell in this case. If the material is of molecular structure, etching by physical dissolution is possible (see section 3.3.1). Beside physical dissolution of molecular-built up materials chemical dissolving methods exist as the solid material is exposed to a chemical reaction at the interface (see section 3.3.2). In the case of metals and semiconductors the phase transition is accompanied by an electron transfer, as the metals cannot be transferred as atoms but as ions only into solution. The etching of metals and semiconductors is an electrochemical process, the partial steps obey the laws of electrochemistry (see section 3.4.). Metals and semiconductors, often do not dissolve as naked ions, but as complexes. In these complexes smaller molecules or ions (ligands) form a chemically bound primary shell around the central atom. This complex is then solvated, e.g. by water. Ligands are added to liquid etchants. Especially for etching metals, they are an essential component of the etching bath. Only in some special cases ligands come from surface films of the etching solid. Wet etching can proceed very effectively, as the necessary reactants are present in high concentrations at the solid surface. By high concentrations of hydroxide or hydrogen ions (extreme pH-values) many etching processes can be made very efficient in aqueous solutions. With varying the concentration of rate determining components, but also by other influences like temperature, viscosity and convection of the liquid, the etch rate can be adapted in a wide range. The three last mentioned factors are unspecific parameters influencing all etching rates of a system in the same direction, in contrast to the specifically reacting components of an etchant. In wet etching procedures the etching products are enriched in the etchant during the process, while the concentration of the reactants is diminishing. The consequence is a decrease in the etch rate. It is possible then that the etching process cannot go on with satisfying homogeneity. Sometimes etch rate ratios change with the consumption of the etch bath, i.e. selectivities are changed. The adjustment of well defined conditions considering the maximum accumulation of etching products is an essential pre-condition for reproducible microlithographic etch results.
3.2 Preparation of the Surface 3.2.1
Surface Condition
The condition of the surface of the material to be etched determines the etching manner essentially. Already in conventional technologies the surface condition is decisive for the process and the process results of surface processes, such as e.g. soldering and glueing. That counts especially for microprocesses, as with them the surface structures and coating layers geometrically and still
3.2 Preparation of the Surface
31
more by their chemical properties cannot be neglected in comparison to the material to be structured. The influence of surface contaminations, particles or other disturbing impurities are various. At best the disturbances show higher or equal etch rates compared to the material to be patterned. In that case they are easily etched and demand only a prolonging of etching time according to their thickness. Less favourable is the situation, if the surface impurities have an essentially smaller etch rate than the respective etching material demanding a stronger prolonged etch time with bad consequences as larger undercutting, the attack of components of the layer system not to be etched, variations in line width and a greater roughness of edges. Fatal are surface contaminations that cannot be etched at all by the etching system for the respective material. As a contineous film, such contaminations can prevent etching totally. As a local spot, they act as etching mask for the material to be patterned and unwanted residues remain in open areas of the mask pattern. This effect occurs especially in anisotropic dry etching processes because of their high resolution, which is due to the fact that no lateral etching takes place and therefore no undercutting. Therefore, the removal of passivating layers, spots or even small particles is very important, particulary in anistropical but selective dry etch methods, which are very sensitive to surface contamination. The surface condition of practically all materials during etching is different from the condition at the normal atmosphere. The surface composition under atmospheric conditions is determined by the interactions of the material with the reactive components of the atmosphere. These components are oxygen and the atmospheric water vapour, that are omnipresent, and therefore, we are used to them, but by their chemical nature they are very reactive and react with many substances already at room temperature. These reactions, as is well known, can lead to advancing corrosion or to the build up of passivating films. Also carbon dioxide, contained in pure air by 0.035 % only, reacts with many materials in neutral or low acid environment forming carbonates. Especially aggressive are some gases of the normal air, the concentration of which, however, depends strongly on the quality of the air, as e.g. sulphur dioxide, hydrochloric acid gas, ammonia, nitrose gases, ozone or hydrogen sulphide. The latter is always found in the air of a room, if people are present. Hydrogen sulphide forms contaminations of sulphides on the surface of many metals. Many sulphides are sparingly soluble and are not removed in simple rinsing processes or by cleaning in non-oxidizing acids. Acidically or basically reacting gases, occuring in low concentrations in polluted air, are soluble in water and accumulate, therefore, in surface films of physisorbed water on solids. They react in the presence of water with many substances forming salts and complexes, but oxides and hydroxides also. Coating films are formed by corrosion as well as by passivation on the surface hampering the reaction of the etchant. The thickness and composition of these coating layers depend on the conditions of their origination, as duration of storing at the free atmosphere, temperature and humidity. Besides low concentrations of impurities, as particles or salt residues of preceding process
32
3 Wet-Chemical Etching Methods
steps, can influence the forming of coating films. Traces of salts, but also of surfactants can react as catalysts, so that very small amounts of impurities in the course of time influence the forming of coating films over larger areas. Special contaminations of surfaces occur in vacuum processes. The evaporation rates of sparingly volatile materials also increase with decreasing pressure. At typical pressures of to lo-’ torr, common in film deposition and dry etching processes, sparingly volatile compositions possess a considerable evaporation rate. Therefore oil films from reactor surfaces get as oil vapor into the gas phase and can condensate on the substrates. Beside the actual coating films, other contaminations occur by improper storing, transport or handling. Particle sedimentation is minimized by consequent handling under cleanroom conditions. Fat and salt depositions have their origin in the human skin and can be avoided by not touching the surface with naked hands. Contaminations by droplets of body liquids, e.g. saliva, containing beside organic components aggressive anions like chlorides and sulphates, can be suppressed by using a face mask.
3.2.2
Cleaning
Cleaning processes are not always necessary before a microtechnical etching step. The necessity of a cleaning step depends on the kind of the respective material, on the used etchant, and essentially on the possible contamination of the surface in preceding steps as well as by the conditions of transport and storage. Loose sticking particles can be removed with particle-free compressed air or a clean inert gas. Faster sticking particles and loose coating films can be removed by mechanical cleaning, such as high pressure liquid methods (preferentially with water) or by scrubbing. In case of extreme pollution or corrosion, grinding or polishing steps can be necessary. Mechanical cleaning steps are useful, if fast sticking particle contaminations exist or thicker surface layers are unevenly spread across the surface. Volatile components as well as physisorbed or chemisorbed water can be removed by thermal treatment. By such a thermal step before film deposition the sticking of the film material, also of masking films, can be improved decisively. Organic components, e.g. residues of polymers or films of grease, are insoluble in water solutions. Adding detergents (tensides) helps to remove organic particles or films. If that does not work, organic contaminations can be treated in organic solvents, which is a gentle method for underlying inorganic layers. For organic polymer films or substrates lying open to the cleaning bath, the choice of the solvent must be done carefully, in order to avoid an attack of the polymer surface. Cleaning with organic solvents has the disadvantage that often small amounts of the impurities to be dissolved stay at the surface either by preferential molecular interaction with surface atoms or simply as residues by evaporation of the solvent. Such residual pollutions can be minimized using cascades of cleaning baths, as the residual concentrations
3.2 Preparation of the Surface
33
within the cascade diminish from bath to bath according to the volume ratios of the liquid, sticking to the substrate, and the bath volume. Treating many substrates in the same cascade a considerable effect of spreading the pollutions can occur. With a cascade, concentrations of specifically absorbed molecules are scarcely reduced, thus a hundred per cent cleaning of any organic pollutants off the surface is not possible in this way. Complete removing of organic residues is successful with methods breaking down the organic molecules chemically. By their nature such cleaning steps are aggressive, and therefore hazardous for underlying materials. With strongly oxidizing agents contaminating organic films are decomposed forming the small inorganic molecules water, carbon monoxide and carbon dioxide. Strongly sulphuric acidic chromate solutions decompose grease and oil very effectively, and were often used in the past. Chromates, however, are poisonous to humans and environment and should not be used any longer for cleaning purposes. In their place mixtures of inorganic acids, preferentially sulphuric acid, and hydrogen peroxide are used. By combining chemical and mechanical methods the cleaning effect is rather efficient. Not too thick coating layers can be removed efficiently by ion beam or plasma procedures. By the impact of high energetic particles (atoms or ions) with kinetic energies of some dozens, hundreds or more electron volts on the surface to be cleaned, chemical bonds are broken unspecifically and small molecular fragments, atoms or ions are transferred from the solid surface into the gas phase. The “sputter effect”, applied as well in microtechnical etching processes (see section 4.4.1), is used in these cleaning methods. The unspecific removal affects the underlying material as well, so it is necessary that the damage is tolerable to the surface or the surface areas of the lattice of the solid. Oxygen plasma decomposes comparatively selectively organic components and is, therefore, well-suited for removing oil and grease from surfaces. Metals and semiconductors, however, form oxidic coating films, that can be thicker and chemically more resistant than coating films formed under normal conditions. In the presence of carbon and nitrogen, nitrides or carbides that are removed only with great difficulty can be formed under the influence of energized particles. For removing such layers physical sputtering processes are convenient. Enhancing sputter rates is achieved by adding substances to the plasma which set free chlorine or fluorine, thus volatile chlorides, oxichlorides or fluorides are formed. The chemical and physical processes in the etching of contaminating films are principally the same as those when etching microtechnical films of the same composition. In contrast to actual etching processes, cleaning processes are to be carried out often without any knowledge of the chemical composition of the substances that have to be removed or of their varying thickness across the surface. In general contamination films are thinner than functional films, often there are only sub-molecular layers. When removing such thin films from essentially thicker functional films a certain etch loss of the under-
34
3 Wet-Chemical Etching Methods
lying film is tolerable. The permissibility of the application of an etchant as a cleaning medium depends on the allowable thickness deviation of the functional layer and the product of the etch rate of the functional layer and the necessary reaction time of the cleaning medium. When choosing a cleaning procedure, the variety of possible pollutants should be considered. The more universal the cleaning mechanism of the pocedure the more effective is the removal of the various contaminations. But also a functional film material is attacked the sooner. On the other hand a selective cleaning medium can be without any effect if the contaminants possess other properties than expected. The use of universally acting and therefore especially aggressive media becomes more difficult with the increasing complexity of microtechnical devices. As a rule the number of different material components of such a device increase with complexity. Also during a fabrication process the number of materials increase and with them the sensitivity against cleaning processes. Efficient cleaning steps become more complicated, more expensive and decrease the yield more and more. Therefore, avoiding contaminations gets higher and higher priority compared to cleaning processes. Thus avoiding contaminations is an essential motive for extreme automation in microtechnical manufacturing lines.
3.2.3
Digital Etching
The forming of coating films, if it is reproducibly controlled, can be applied in microlithographic etching processes. These coating layers must not lead to rapid passivation. Porous capping layers and slowly built up coating films, that are easily soluble in another etching bath, can be tolerated. During certain etching processes the forming of such coating layers can be put up with. The layers are removed in a second etching bath of a different composition. By changing from one etching bath to the other the initial state of the surface is reproduced. This method is applied, e.g., in alkaline etching of NiCrO, or NiCrSiO, films’. As in each etching cycle a certain constant amount of the film thickness is etched, but possibly many cycles have to be run to structure a material completely, the etch depth can be exactly defined by the number of etching cycles. Therefore the term “digital etching” was introduced for such a method in GaAs etching’. Nearly without any problem digital etching is done with single layers. The change between two corrosive media is essentially more critical in two- and manyfold film systems as the selectivity is not ensured as the materials not to be etched must be resistant against both etchants.
A.Wiegand et a1 G.C.DeSalvoet al. (1996)
3.3 Etching of Dielectric Materials
35
3.3 Etching of Dielectric Materials 3.3.1 Wet Etching by Physical Dissolution Several classes of binding forces can occur in solids. The properties of the solid influencing decisively the etching behaviour depend on the density and strength of chemial bonds and on their topology. In materials built up of molecules, beside strong intramolecular binding forces - mostly covalent bonds, weak intermolecular bonds occur. The most frequently occuring intermolecular forces are unspecific interactions of the electron sheath (van der Waals forces). Dipole-dipole interactions and hydrogen bonds stabilize molecular solids in addition to the van der Waals forces. The topology of strong chemical bonds reaches from quasi-zero-dimensionalstructures with small molecules to three-dimensional structures with polymer solids where between all next neighbours very stable interactions exist, as e.g. in Si02or salt-like ionic crystals. In between, there are approximately linear structures, in the case of solids linear high-polymers and approximately two-dimensional structures in the case of layer structures, as they occur in some monocrystalline materials consisting of more components. Branched polymers and partly interlinked polymers form bonding structures that can be understood as a so to speak brokendimensional or fractal binding topology. The etching of solids without a change of the chemical bonds within the molecules can be interpreted as a physical dissolution process. In such processes the weak intermolecular interactions are cancelled by the etching bath and substituted by the interactions with solvate molecules, whereas the stronger intramolecular forces (covalent, coordinative, metallic and ionic bonds) are not destroyed. Purely physically reacting wet-etching processes are applicable only to molecular built-up materials with low dimensionality of topology. Such molecular solids also can possess various kinds of inner structures, e.g. mono- or polycrystalline or glass-like structures. Whereas crystalline films are formed above all by low molecular materials deposited by moderate rates, rapidly deposited materials or such consisting of larger linear or branched molecules preferentially form a glass-like matrix.
Dissolution of Low Molecular Materials The Patterning of low molecular functional materials is up to now of minor significance in applied microtechniques. Films of organic molecules are used, e.g., in the form of dye layers, deposited by sublimation. Recently, molecular films are applied for investigating electron transfers at micropatterned ultrathin molecular barriers (among other methods by single-electron tunneling) or spectroscopic examinations. The molecular films are precipitated by socalled self-assembling or Langmuir-Blodgett (LB) techniques. The molecular structure of these materials is mono-molecular or arranged in molecular layers, so it is comparable to that of liquid crystals, holding an intermediate place
36
3 Wet-ChemicalEtching Methods
between crystalline and amorphous solids. These films can be patterned using solvents for dissolution, the properties of solvation of which is adapted to the class of compounds concerned. Good quality of micropatterning, i.e. low edge roughness and little undercutting, can be achieved only by dry-etching processes for these films. More complicated than the choice of an applicable solvent is the deposition and patterning of an convenient etching mask, as the solvents in the commercial photoresists are aggressive against interesting organic molecular films, because of their similar properties. The same counts for removing the etching mask. This so-called stripping has to be selective against the functional film. Therefore the resist mask has to be adapted to the necessary process media. Possibly a negative resist based on alkyl esters (aliphatic, aprotic polymer) can be used in place of the usual positive resist based on phenolic resins (aromatic, protic polymer). Besides, the developing and stripping processes must be modified according to the properties of the functional molecular films. If in this way selectivity cannot be reached, auxiliary masks could be used, e.g. water-soluble polymers (polyvinyl pyrrolidone, polyvinyl alcohol and others) or metal films, as long as their preparation and removal do not destroy the organic functional films. Dissolution of Linear and Interlinked Polymers
The patterning of organic polymer films considerably gained in importance in recent years. Such films are applied not only as isolating layers in electrical devices, they are also used as elements with dielectric functions, as sensor films, as special materials in optics or as organic conductors. As a rule, organic polymer functional films consist of unlinked molecular chains that can be brought into solution physically. In case of interlinked polymer chains physical dissolution processes are not applicable, and a decomposing wetchemical-etching process or a reactive dry-etching process must be used. Dissolution processes of polymers differ from etching processes of low molecular solids. The polymer chains cannot be set free from the solid into the solution by a single dissolving step. Only in the course of the dissolving process the molecular chains by and by gain mobility till they can leave the solid surface and diffuse into the interior of the liquid phase. An essential cause for this gradual dissolution of long molecular chains is their entanglement in the solid state. All amorphous polymers have statistically entangled molecular chains in the matrix. In consequence only a small section of one polymer molecule lies in the surface and can react with the solvating molecules and get mobile by solvation. Only by the solvation of other molecules in neighbouring sections the solvent can reach the deeper lying parts of a molecular chain. During the dissolution process a near-surface area is formed that can be described neither as a completely mobile liquid nor as a rigid solid. In this gellike zone, the extent of which is correlated to the length of the polymer molecule, the mobile solvated parts of the polymer chain stick out into the solution, while their opposite ends are still anchored in the solid film. In the
3.3 Etching of Dielectric Materials
37
course of time the the out-sticking mobile parts of the molecules grow longer and the fast sticking parts shorten until the whole molecule is freed from the surface. Beside the formation of the gel-like surface layer during the dissolution process, the diffusion of small solvent molecules into the solid polymer is possible, thus increasing the volume and mobilizing the polymer parts in the interior of the solid. Such swelling processes play a role in the dissolution of branched and partly interlinked polymer molecules, as the mobility of their molecular chains is reduced by covalent bonds. Etching processes that stand somewhat closer to the real wet-chemicaletching processes are those in which weaker, but specific interacting forces have to be broken. To such bonds within a molecular solid matrix hydrogen bonds and dipole-dipole interactions between parts of molecules are counted. These interactions have to be overcome in particular in patterning polymers with polar and protic functional groups. The dissolution of molecular films with such weakly interlinking bonds succeeds by simple solvation, where as a rule a protic solvent is required if a polymer with protic functional groups is to be dissolved. The interlinking by hydrogen bonds is broken when dissolving such polymers, but the back bone of the macromolecular chain bonded covalently is not destroyed.
Dissolution of Salt-Like Materials Salt-like materials (alkali halides) play a role as windows in infrared spectroscopy. Salts built up of ions and well solvated by water can be etched in water. Normally, devices built of such materials have to be stored under exclusion of moisture, i.e. in an atmosphere of a very low partial pressure of water vapour. For removing resist masks no alkaline but only organic removers (strippers) can be used.
3.3.2 Wet-Chemical Etching of Non-Metals There is no exclusively physical dissolution process for the most materials, that transfers molecules or atoms out of the solid into the liquid phase. Wet etching in most cases is only possible if the dissolution is accompanied by a chemical change of the material. Nearly all microtechnically relevant materials can be patterned by a chemical reaction in a liquid. Wet chemical procedures can mostly be carried out rather specifically for a material concerned, so that a great variety of material combinations can be patterned by selective wet-chemical etching. As an alternative to wet-chemical procedures dry-etching methods are used for especially inert materials or for high demands for anisotropy and accuracy in the measure of the microlithographic structures (see section 4). By wet-chemical etching, all methods shall be understood that react at the interface solidiquid really changing their chemical bonds. Beside these
38
3 Wet-ChemicalEtching Methods
wet-chemical etching processes in the stricter sense, physical dissolution processes of molecular materials in a liquid medium without changing bonds (see section 3.3.1.) are subsumed. Being an important special case of wet-chemical patterning, etching of metals and semiconductors is dealt with in a seperate section (section 3.4). Etching by Acid-Base Reactions Dissolution processes in aqueous solutions are based on the property of water to dissociate in hydrogen and hydroxyl ions. The concentrations of both kinds of ions (denoted by the pH-value) can be regulated in a wide range (from l@14 to 10' moV1). By a buffer system their concentration can be stabilized against shifts in concentration caused by the etching reaction. Acid-base reactions are convenient for patterning processes if the material to be etched reacts with hydrogen or hydroxyl ions. The etching at low pHvalues is especially of interest for microstructuring oxides and salts, as they often can be dissolved in strongly acidic solutions by releasing cations. Such etching processes are applicable with oxides and hydroxides of metals and semiconductors.
+ 2y H+ X MZy'"++ y Hz0 M(OH), + X H+* M"+ + X HzO M,O,
(22) (23)
M = metal or semiconductor (e.g. Cu, Si) The cations are solvated by water as a strong polar solvent and can diffuse rapidly into the interior of the solution. Thereby the equilibria (22) and (23) are shifted to the products. Are the etch rates controlled by the surface reactions, they can be influenced by the choice of the pH-value quantitatively. In analogy the reactions of salts proceed in weak acids, if those have to be patterned as a film material or be removed.
+
M,Z, + y-n H++ y H,Z X MY'"/"Z = acid anion (e.g. acetate)
(24)
In these reactions the crystalline oxides and salts are dissolved off their surfaces. The reaction rate is often controlled by the surface process. The microand nanomorphology of the crystals determines the etch rate decisively, because of the size of the specific surface. Acid-base reactions are also of interest for micropatterning organic polymers, if they are functionalized with protic or protonizable groups. With OHgroups or carboxylic groups functionalized polymers possess acidic properties and are, therefore, soluble at high pH-values.
3.3 Etching of Dielectric Materials
PR-OH S PR-0-
+ H+
39 (25)
PR-OH + OH-+ PR-0- + H20 PR= polymer molecule residue For example phenolic resins, polymers with carboxylic or sulphonic acid groups, but also polymer alcohols can be dissolved in alkaline solutions. If the density of OH-groups is great enough, these compositions are soluble in neutral aqueous solutions. Alkaline functionalized polymers can be etched in acidic solutions. To these polymers belong above all such polymers that contain nitrogen in reduced form, as e.g. amines, pyrines, pyrimidines, imides, and imidazoles. Because of the free electron pair at the nitrogen atom, these groups possess a high affinity to protons forming ions that are easily hydrated. PR-NHz+ H + + PR-NH3’
(27)
The solubility and the dissolution velocity of these alkaline functionalized polymers depend on the one hand on their molecular weight and on the other hand on the basicity of the functional groups and their density in the molecule. Beside entirely alkaline or acidic functionalized polymers also polymers with a mixture of both functions, i.e. amphoteric organic polymers are of interest in microtechniques. Copolymers as well as polypeptides are counted to this group. The latter will gain in importance especially in developments of microbiotechnology, as e.g. carriers for miniaturized substance libraries or for affinity tests, in microenzyme reactors or biosensors. Due to their amphoteric character these polymers are soluble in solutions of a wide pH-range, and can be patterned, therefore, in various etchants.
Non-Oxidizing Etching by Forming Complexes At very high pH-values some metals and semiconductors form stable hydroxocomplexes (e.g. silicon dioxide forms a hexahydroxocomplex) that are easily solvated by water due to their ionic nature, and accordingly etch with high rates. SiOz + 2 HzO + 2 OH- + [Si(OH),I2-
(28)
The forming of hydroxocomplexes is a special case of non-oxidizing etching. Instead of hydroxide ions acidic anions, as e.g., chloride and fluoride, or neutral molecules react as ligands.
M,O,
+ z Zn-+ y H,O + x [MZz](ZJ”x-2n) + 2y OH-
(29)
40
3 Wet-ChemicalEtching Methods
M,O,
+ z L + y H 2 0+ [ML,]2y'x++ 2y OH-
(30)
A typical example for non-oxidizing dissolution in microtechiques is the etching of Si02 in HF-containing etchants. '' Complexing is not restricted to dissolution of oxidic materials. Also salt-like films can be dissolved by complexing agents. It is possible that the anions of a slightly soluble salt react as ligands themselves and form a soluble complex. In this way films of copper(1) chloride are etchable in neutral KC1-solutions. CUCl + 2 c1-+ [cUc13]*-
(31)
The complexing of metal ions is described in connection with the patterning of metals (see section 3.4.1). Oxidative Etching The oxidative dissolution of conducting inorganic compounds is like the etching of metals and alloys a complex electrochemical process (see section 3.4). Some organic compounds, especially interlinked polymers cannot be microlithographically etched neither by physical dissolution processes nor by acidbase reactions. If dry-etching processes are not to be applied, the only possibilty for such materials is the use of a strong oxidizing etchant that decomposes the polymer structure. Such an oxidative decomposition as a rule decomposes the organic molecule completely, i.e. the end products in such a oxidative etching of polymer hydrocarbons are carbon dioxide and water. As etchants, strongly oxidizing liquids or solutions of strongly oxidizing agents are used. Preferentially used are concentrated sulphuric acid, mixtures of sulphuric acid and hydrogen peroxide, solutions of peroxodisulphate, or acidic solutions of oxoanions with metalls in high oxidation states, as e.g. chromates or manganates. Oxidizing solutions of salts possess mostly minor etch rates than concentrated sulphuric acid or strongly acidic hydrogen peroxide solutions. But they are easier to handle and supply generally better structure edges. Besides, the sticking of the resist mask is less critical in these solutions. There are special recipes for etching and cleaning solutions used in removing organic layers and contaminations. Such solutions consist of organic solvents and strongly oxidizing agents, a mixture reacting efficaciously. Their disadvantage is their instability, as the organic solvent is also decomposed by the oxidizing agent. For safety reasons these mixtures should only be applied with a great water content to minimize the reaction rate of decomposition and the connected heat and gas evolution. Particularly it is necessary to warn against mixing alcohols with acidic nitrates, salpetric acid or other oxidizing agents, as explosive compounds are formed. lo
Ch.Ch. Mai und J.C. Looney (1966)
3.4 Etching of Metals and Semiconductors
41
Enzymatic Etching Processes Many processes that otherwise would proceed only very slowly, can be accelerated by catalysts. Especially enzymes can so strongly accelerate reactions between solid films and appropriate solvents that they can be used for processes in microtechniques. Principally many materials can be structured with the help of enzymes. Nature presents a spectrum of bio-catalysts effecting reactions that reach from dissolving redoxprocesses on metals and their compounds to the selective decomposition of organic-synthetic or biogenic polymers". The essential advantage in using enzymes is their high specifity, that can be adapted for etching processes. By enzymatic structuring the distinction of binding states during dissolution processes of related types of organic substances is posssible that could not be patterned by conventional etching techniques. Enzymes work in physiological environment, i.e. under mild conditions that are very soft in comparison to other etching media. The etching temperatures lie in the range of room temperature or a little above it, the pHvalues lie in the neutral region or deviate only slightly from pH 7. Enzymatic media are environment-friendly and no hazard to health and safety at work. Enzymes are predestined for patterning biogene, modified biogene and bio-analogous polymers. For example, gelatine layers (collagene) can be decomposed very gently by proteolytic enzymes (proteases) without any attack against other layer system components, also after synthetic interlinking. The structure quality is comparable to other microtechnical etching processes. The edge roughness is less than lpm, so that a line width of a few micrometers can be patterned. The use of enzymatic etching processes is still in its infancy. It is to be expected that with increasing importance of microsystem technologies in chemistry, biochemistry, molecular biology and medicine more and more organic polymers and biopolymers will be integrated in microdevices, and with it the importance of highly selective biochemical methods will increase.
3.4 Etching of Metals and Semiconductors 3.4.1 Outer-CurrentlessEtching Partial Processes The outer-currentless etching processes are the most essential wet-chemical methods for etching metals and semiconductors. In these processes the materials, elements or alloys, are transferred into an ionic state. The oxidation reaction of the metal or semiconductor M is the central step of the respective etching process:
'' E. Ermantraut et al. (1996)
42
3 Wet-Chemical Etching Methods
M"' + n e(anodic partial process) The number n of the liberated electrons e-.is equal to the charge of the newly formed metal ions. This partial process is an anodic electrode reaction proceeding on any metal surface immersed in an electrolyte. The process ceases if the electrons cannot be carried away. In such a case an electrode potential is formed opposing the further formation of metal ions. The negative potential increases in magnitude with the number of released metal ions till the electrode potential is so high that no more metal ions can change into solution. If this negative potential is reduced by an outer-current, the formation of metal ions can proceed. Then etching is performed by the outer current source, so-called electrochemical etching (see section 3.4.6). Without an outer current source the dissolution process can go on if the electrode potential is changed into the opposite direction by a second chemical redox process. This principle is used in the outer-currentless etching of metals and semiconductors. The electrons set free in the anodic partial process are transferrred to an oxidizing agent OM:
OM + n e - e OM". (cathodic partial process)
(33)
The following brutto-reaction results (anodic and cathodic partial processes):
M
+ OM * M"+ + OM"-
(34)
As the etching materials are conductive, the electrons need not to be transferred directly from the metal atom to the oxidizing agent. Instead, two separate electrode processes proceed (fig. 3-9). Both electrode reactions can take place with totally different partners and supply different reaction products. Then the cathodic and anodic partial processes have only the electrode potential in common. If the sum of the electrons released in the anodic process is equal to that consumed in the cathodic process, the electrode potential remains constant. A n etching process takes place if both partial processes can proceed at the spontaneously set electrode potential. The etch rate of a metal is unambiguously determined by the intensity of the anodic partial pressure (equation(35)). As the metal in wet-chemical etching is dissolved in form of its ions, the etch rate corresponds to the number of metal ions produced at the electrode surface per time unit. It is, therefore, proportional to the interchanged anodic partial current I+ over the surface A+ or the area-independent partial current density i+: r
- i+
i+ = I+lA+
(35) (36)
3.4 Etching of Metals and Semiconductors
43
a n d c partlal
I
7:c
0 current potenhal
, charactensucs
of cathdcal partla1 process
outer-Current potential
a
E.
E current potential characteristicof the anodic partlal process
Fig. 3-9 Current-potential curve in outer-currentlessetching. Adjusting of the etching potential epsilon null by superposinganodic and cathodic partial currents. The etching potential adjusts itself so that both electrochemical partial processes have oppositely equal intensities. (a) Low etch rate, (b) high etch rate
I
current potent~al charactensucs of cathcdlc pmal
/I
b EO
E
The connection between the anodic partial current and the etched quantity of material is explained by the relationship of charge described by the Faraday law. The product of the anodic partial current I+ and the etching time t is accordingly equal to the product of material quantity n (quantity of ions), the valency of the ions z (charge) and the Faraday equivalent F expressing the charge of a mole of electrons.
I,. t = n - z , - F (37) The etch rate of a material can be estimated by this relationship. The quantity of matter is the quotient of mass m and atom mass M of the etching material, where the mass can be expressed by the volume V and the density e of the material. n=mlM=g-VlM By inserting (38) in the Faraday equation one gets: i, .A,. t
= g
.
(&)a
-
z+ F
(38) (39)
The etching film thickness h is the quotient of volume V and the etching (i.e. the anodic effective) surface A+: h = VIA,
(40)
44
3 Wet-Chemical Etching Methods
The etching rate r can be expressed as a function of an electrochemical term and a material term:
- F)) - ( M / e) = h / t r = (i+ / F) - (M / (z+ Q) (i+ / (z,
and
The molecular weight, the density and the valency of the ions are materialspecific quantities. As the Faraday equivalent is a constant value, the anodic current density remains as the only variable quantity. Its value is dependent on the standard electrode potential of the electrode reaction and the electrode potential. Etching processes are chemical processes that proceed considerably far from the chemical equilibrium. The electrode potential is dependent on the standard potentials of the participating electrode reactions, the dissolution activities of the participating species and the temperature. The equilibrium of the electrode reaction E~ is described by the Nernst equation, in which is included as a further parameter the universal gas constant R:
Eois the normal potential of the electrode reaction, ai are the activities of the participating species (activity is the product of concentration and activity coefficient), and vi is the stoichiometric coefficient, the value of which describes the respective stoichiometric factor of the participating species and the sign of which depends on the direction of the electron current (a positive sign means that the respective species reacts in the oxidizing direction of the equilibrium, a negative sign stands for the reducing direction of the equilibrium). The cathodic partial process obeys the same laws as the anodic partial process. It only differs from the anodic partial process by the other characteristic normal potentials and the other species that determine the equilibrium constant of the electrode reaction. If a voltage is applied to an electrode a current can set in. The current for the metal dissolution is the higher the higher the potential. It is valid for outer-currentless etching that a uniform electrode potential exists for all electrode processes proceeding at the electrode surface. This uniform potential is set by the interplay of anodic and cathodic partial processes. The potential adjusts itself just so that the anodic partial current I+ and the cathodic partial current I. become equal: 11,
I = 111.
(44)
This current correlation is valid for all outer-currentless etching processes. If the etching surface is uniform, then the anodic and cathodic surface areas are equally large and the partial current densities also: with A+ = A also li+l= 1i.l is valid
(45)
3.4 Etching of Metals and Semiconductors
45
These facts are charaterized by the position of the outer-currentless etching potential (rest potential) by superposition of the current-potential curves of the anodic and cathodic partial processes at accurately that value at which the partial currents of both branches are equal in magnitude (fig.9). Do more materials with various chemical and electrochemical properties take part, very different current densities arise in the partial processes although there is no outer current. These are responsable for local and temporal differing etch rates and a varying distribution of etch products during etching. They are caused by local currents (see section 3.4.3).
Forming of Complex Compounds The actual electrode reactions are responsible for the electrochemical conditions of the metal dissolution. Only in a few etching baths the metal is dissolved as a simple solvato-complex. In most etching processes etching media are used that contain molecules or ions binding as ligands coordinatively to the ions that should be dissolved. In coordinative compounds (complex compounds), the binding electron pair for the bond comes from the ligand. The ligands are included in the potential determining anodic reaction. Beside uncharged ligands (Y) anionic ligands (X-) are used in etching methods. In the electrode reaction the according complex is formed instead of the metal ion, e.g.: M+4Ya[MY4]++e-
(46)
Metal ions form in general complexes according to the following equation: M+
+ m Y a [My,]'
(47)
The greater the number of ligands in the complex the more the complex formation is dependent on the ligand concentration. The stoichiometric factor of the ligands appears as exponent m in the complex forming equation. The constant of complex formation KB determines the ratio of the equilibrium concentration of the participating species (central ion, ligand and complex).
If anions react as ligands the formed complex can be negatively charged, e.g.: M
+ 2 X- <==> [MXJ + e-
(49)
As the ligands in a coordinative bond transfer electron density to the metal ion the forming of the complex compound shifts the electrode potential to lower values. The electrode potential decreases with increasing concentration of ligands. In the case of equilibrium the shift of the electrode potential is quantitatively described by the Nernst equation, e.g., for the reaction equation (49) as follows:
46
3 Wet-Chemical Etching Methods E~
= E,+[
R - T1 (z+ F)] - h ( a
)
(50)
The ligands as reaction partners are on the side of the educts (reduced state). Therefore they appear in the denomitator of the equilibrium constant. An increase of the concentration causes a decrease in potential. As the number of ligands in the potential-forming reaction stands in the exponent of the constant of redox equilibrium, especially in multi-ligand complexes the potential of the anodic partial process is shifted drastically to more negative potentials if the ligand concentration is increased. The shift of the anodic current-potential curve to lower potentials by adding ligands can cause a higher etching rate. This effect occurs always, if in the range of potential shift by the addition of ligands the intensity of the cathodic partial process is determined by the electrode potential. The choice of the ligands depends substantially on the properties of the etching metal and its ions. As the metal ions practically always perform as strong Lewis acids ligands are convenient that react as strong Lewis bases. In aqeous solutions acid residue ions of inorganic and organic acids are applied advantageously. A n especially suitable reaction partner of many etching processes is the chloride ion C1-, that performs as a moderately strong Lewis base forming stable chlorocomplexes with many metals that form not too hard cations and furthering in this way the wet chemical etching. In aqueous solutions the ligands have to compete with hydroxide ions and water that form with metal ions in many cases slightly soluble hydroxides or oxides. The oxoion 02and the hydroxide ion OH- perform as hard Lewis bases12, i.e., their electron sheath is scarcely polarizable. Such hard Lewis bases interact preferentially with hard Lewis acids that many metal ions are, as their electron sheath are little polarizable too. These metal ions therefore form preferentially hydroxides and oxides that build a three-dimensional network of strongly polar bonds. The bonds very often have a strong salt-like ionic character. These substances form coating layers on the metal surface that impede the further etching (see below about disturbing passivation). While ions of low charge number are solvatized by water or various other ligands, ions of higher oxidation states perform due to their compact electron sheath as very hard Lewis acids and form stable passivating coating layers, as e.g. is given in the following table (3-2). In very high oxidation states some metals form oxoanions due to the strong interaction between the metal ions and the oxygen from the water. These anions can be considered as especially stable complex compounds, oxygen performing as ligand. Such oxoanions as a rule are soluble again. The transpassive dissolution process is due to their formation, as e.g. in the case of chromium as Cr(VI) in the chromate ion. For dissolving oxide and hydroxide films that cannot be dissolved simply by a change in pH-value, the 0x0- or hydroxide ions have to be exchanged by l2
Hard and Soft Acids and Bases R.G.Pearson 1969
3.4 Etching of Metals and Semiconductors
47
Table 3-2. Passivating Microtechnically Important Semiconductors and Metals (selection) Material -
S Cr
Oxidation state
Passivating oxide or hydroxide
+4
SiOs Cr203 ?iO* Ni00H,Ni203 A100H,A1203 CU20
~
+3
Ti
+4
Ni Al cu
+3 +3 +1
ligands that form a soluble species with the central ion. As oxygen is distinguished by a high electronegativity, OH- and 02' belong to the hardest Lewis bases at all. For the chemical decomposition of very slightly soluble oxides and hydroxides containing central ions in higher oxidation states and being, therefore, hard Lewis bases themselves the most other ligands are bad competitors in ligand-exchanging processes. Therefore, the spectrum of soluble complex compounds that could possibly be formed in the etching medium is limited. There are, therefore, only two possibilities for very hard metal ions. Either complexing is accomplished by the hard hydroxide ions themselves or oxygencontaining chelate ligands, or the only still harder ligand is to be used that is available for wet-chemical etching, the fluoride ion F.The fluoride ion is due to the extremely high electronegativity of the fluor atom a very hard Lewis base that can compete with oxygen in oxides and hydroxides and, therefore, is able to dissolve oxidic coating films, functional films and substrate material. In the dissolution reaction the respective fluorocomplexes are formed. For example glass and quartz are etched in this way, and films of titanium that cannot be etched with strongly oxidizing solutions are dissolved in hydrofluoric acid solutions forming the water soluble complex (T@6)2-:
Ti + 6 HF+ [?iF6I2-+ 2 H+ + 2 H2f
(51)
Ammonia, amines and aromatic nitrogen heterocycles can be applied in etching processes as they are comparably hard Lewis bases. E.g, ammonia and amines are added to etching media for copper forming stable Cu2' complexes. By that means the patterning of copper in the neutral pH-range gets possible. Dissolution processes can be accelerated strongly by forming complex compounds with high complex forming constants. For metal ions with polarizable electron sheath ligands are available that are weaker Lewis bases, i.e. ligands that possess easily polarizable electron sheath. Such ligands are used with heavier metal ions in lower oxidation states. Favourite additions of ligands to etching baths for metals that form soft cations are the higher halides Br-
48
3 Wet-ChemicalEtching Methods
and J-. Also pseudohalides as cyanide, CN- that possesses extraordinary complexing properties could be used, but because of its toxicity is of minor importance in practical use of etching baths. Especially stable complex compounds are formed if a ligand can add electron pairs over several atoms for coordinative bonds to a single atom. This ability is especially possessed by acid residue anions of multi-valent carboxilic acids due to the electrons of the carboxylic oxygen atoms, by multi-valent alcohols, hydroxylic acids and multi-valent phenols due to the neighbouring oxygen atoms. Besides amino groups and heteroaromatic nitrogen atoms as well as other electron-rich functional groups of organic compounds can function as electron pair donors. Therefore, acetic acid, perchloroacetic acid, citric acid, tartaric acid, succinic acid and their salts, but also EDTA (ethylene diamine tetra acetic acid) and phenolic aromates like o-hydrochinone, pyrocatechol and gallates are used as additions to etching baths. As the geometric and electronic properties of the ligands and metal ions together determine the complex forming constant, the respective ligands react very selectively in many cases of metal dissolution. Therefore, the selectivity of an etching process can be strongly influenced by the choice of the ligands. Sometimes the complex formation has to compete with the formation of coating films. Such coating films are often of passivating character. Beside coating films of some or many molecular layers also monomolecular or atomar layers influence the etching process. E.g., the silicon surface is largely saturated by hydrogen atoms in alkaline or fluoride-containing etching solutions. By substitution of the surface hydrogen by the nucleophilic ligands OHor F, soluble fluoro- or hydroxocomplexes are formed 13.
Obstructing of Etching by Passivation Passivity has the electrochemical consequence that the anodic current density does not go on growing with increasing potential but falls to very low values, possibly to zero. That means passivation hinders the material to be etched. A typical current-potential curve of a passivating metal is given in fig. 3-10. The non-monotony of the current-potential curve is caused by the formation of a coating film on the metal surface, obstructing the transition of metal ions into solution. The formation of such a passivating film can proceed without an outer current as well as outer-currentless etching. The velocity of film formation depends on the outer-currentless electrode potential which is determined by the intensity and the normal potential of the anodic and cathodic partial processes. Some passivating metals can be dissolved at potentials above the passivation range by forming compounds in higher oxidation states. Such a behaviour is typical for chromium, dissolving in the active range as Cr(II), passivating as Cr(III), and again dissolving in the transpassive range as Cr(VI). l3
M.Sc.V.Costa-Kieling(1992)
49
3.4 Etching of Metals and Semiconductors
anodic parnoi Plocess activedissolution
I
0
c
_________-
/
I -\,’ , ’,
transpassive dissolultion
I
c
/I’
/ ,
\
passive region
I I
cmodic pama1 prmess
I I I
I
Tafd behoviour
1
diffusioA control
E Fig. 3-10. Current-potential curve of an electrode with passivating properties. Transpassive dissolution does not take part with all materials.
Most passivating films in aqueous solutions consist of oxides, hydroxides or oxihydrates. Also salt-like coating films can possess passivating properties. A very efficacious passivation occurs if passivating films are free of any pores, i.e. the metal surface is protected against a further attack of the etching medium. The formation of passivating films implies a species that is slightly soluble in the etch medium or at least is more rapidly formed than dissolved. Often competing reactions proceed simultaneously. In general coating layers (D) arise by reactions of ions of the film material (M) with species of the solution (X-). This reaction is an equilibrium reaction as a rule: M+ + X - S D ./,
(52)
For equilibrium reactions the law of masses is valid in which the concentrations of the participating species are connected by the solubility product KL:
KL is reversely proportional to the equilibrium constant of the film formation. High KL-valuesindicate little tendency to film formation, low values of KL mean that film formation occurs at low metal ion concentrations already. The metal ion concentration depends strongly on the dissolution rate of the metal. The transgression process through the electrode surface enlarges the metal ion density in the near-surface solution. Diffusion into the interior of the solution diminishes the metal ion density. In reaction-controlled dissolution processes the metal ion density is low near the electrode surface. Increasing the etch rate leads in this case only to a
50
3 Wet-Chemical Etching M e t h o b
proportional increase of the metal ion concentration in the near-surface solution. Is the metal dissolution limited by the transport of complexing species to the surface the increased dissolution potential leads to an enhanced transgression process permanently increasing the metal ion concentration thus after a certain time the solubility product for coating film formation is surpassed. With partial transport control an enhanced anodic transgression process is noticeable by an over-proportional increase in the stationary metal ion concentration in the near-electrode solution. Decreasing, e.g., the ligand concentration at the electrode from 20% to 10% of the solution concentration, enhances the anodic partial process up to 9/8 (i.e. by 12.5%) due to the higher diffusion velocity in the greater concentration gradient. Because of the reduced ligand concentration the metal ion concentration increases by multiples, e.g. by the factor 16 for a complex with four ligands: K B
=
[ML,]+/([M’] [L14)= [ML,] +/(16.[M’] * ([L]/2),)
(54)
Therefore, it is possible that already with partial anodic transport control the solubility product of substances forming passivating films is overrun although due to complex forming under equilibrium conditions a stationary concentration of metal ions in the near surface solution could have been expected. In aqueous solutions is the pH-value for the dissolution process and possible passivation processes of special importance, because the equilibrium of forming oxides, hydroxides or oxihydrates is dependent on the H+concentration. Thus increasing the pH-value in the presence of metal ions M+ or their aquo-complexes shifts the equilibrium to precipitation, e.g. for monovalent metal ions:
+
M’ + H 2 0 MOH 1 H+ = [M’] / [H+] with Khydroxide
(55)
or
, + 2 M ’ + H ~ O ~ M J~ O2 H ’ with I(oxide’ = [M’] / [H’] Setting of the pH-value in an etching solution is possible, however, often it shifts due to certain reactions during the etching process. Consumption of the etching bath increases the pH-value if in the cathodic partial process protons are consumed. That is always the case if the hydroxide ions themselves react as oxidizing agent (“acid etching” of metals) or if the oxidizing agent is an oxocompound (e.g., persulphate, hydrogen peroxide, hypochlorite, perchlorate, bromate, chromate, permanganate). If in the etching process hydroxocomplexes are formed, the pH-value causes the reaction to proceed in the opposite direction. The tendency of passivation increases with decreasing pH-value, as, e.g., in the case of tetrahydroxo-complexes of a monovalent metal:
3.4 Etching of Metals and Semiconductors
51
or 2 [M(OH)4I3-+ 6 H+ S M20 J + 7 H20 with I(oxide” = ([[M(OH)4I3-]* [HfI3) The decrease in the pH-value is due to the consumption of hydroxide ions in the anodic partial process. Undesired passivations can lead to tremendous trouble in wet-etching processes. Therefore one must be careful that critical concentrations for the formation of obstructing coating films cannot be reached. As high etching rates because of productivity are desirable, conditions for diffusion control of the etch rate are chosen. Is this diffusion controlled by the transport of ligands or solvating molecules from the interior of the solution to the surface, passivation can take place as parameters change. The passivation risk increases by the following factors: 0
0
0 0 0 0 0
decrease in ligand concentration in the etching solution (e.g. by consumption or aging of the etch solution) decrease in the diffusive off-transport of metal ions or complexes by an increase in the metal ion concentration in the etch solution (consumption of etch solution) decrease in the diffusive off-transport of metal ions or complexes by diminishing convection in the solution falling below the necessary etching temperature increase of the pH-value of the electrolyte by consumption or aging of the etch solution (except in forming hydroxocomplexes) decrease of the pH-value in case of forming hydroxocomplexes due to consumption or aging increase of the cathodic partial current (e.g. by relative enlargement of the effective cathodic surface in comparison to the effective anodic surface, see section 3.4.5).
Solubility products are material specific constants that decisively determine the tendency for passivation of metals, especially those of oxides, hydroxides and oxihydrates. Passivation can possibly be retarded in time if the anodic partial process is transport-controlled (see section 3.4.2). Passivation of metals in microtechniques is by no means only a disturbing factor, but is used to etch selectively certain metals (see section 3.4.2). Combining Cathodic and Anodic Partial Processes As oxidizing agents often oxocomplexes of metal ions or peroxoions with a non-metal central atom are used in etching processes. These compounds are distinguished by a high redox-potential and good solubility. Therefore, etching
52
3 Wet-ChemicalEtching Methods
solutions can be composed with them that etch with rather high rates and are not rapidly exhausted. The reduction of these oxocomplexes is, however, in several cases kinetically retarded, i.e., inspite of high concentrations of the oxidizing agent at the metal surface and low redox-potentials of the anodic partial process the cathodic partial process procedes only slowly. More effective than with the metal surface these oxocompounds react with metal ions or metal complexes that can change into a higher oxidation state, whereas the metal ions or complexes in their oxidized form take part in the actual partial reaction. Therefore, additions of metal ions or complexes that can occur in at least two oxidation states act catalytically in etching media and increase the etch rate. If metals are etched that form themselves ions in two or more oxidation states, these ions can react with the oxidizing agent forming themselves the oxidizing agent for the cathodic electrode reaction. Then the redox-process at the electrode is accompanied by at least one redox-process in the near-surface solution, e.g.: MeM++e(anodic partial process, proceeds at the metal surface)
+
(59)
+
M+ OM e M ~ + OM(redox-process in the near-surface solution)
+
M2+ e- & M+ (cathodic partial process, proceeds at the metal surface)
(61)
With such a reaction mechanism not only ligands that complex the metal in the higher oxidation state react accelerating, acceleration is also possible by ligands that enable the formation of complexes of the metal in the lower oxidation state. If only by this complex formation the dissolution of the metal in the lower oxidation state becomes possible, small additions of ligands can enhance the etch rate considerably. Ligands possess, if they are liberated in the oxidation reaction of the complex with the metal ion in the lower oxidation state to an uncomplexed metal ion in the higher oxidation state, a real catalytic effect even with concentrations much lower than that of the oxidizing agent. As metals in the lower oxidation state are softer Lewis acids than those in the higher oxidation state, often such ligands are catalytic that are weaker Lewis bases as those normally used for etching. These ligands, however, must not undergo rapid oxidation themselves as they would consume the oxidizing agent decreasing its concentration in the etching bath.
3.4 Etching of Metals and Semiconductors
53
Geometries in Isotropic Wet-Chemical Etching Isotropic wet-chemical etching processes do not allow arbitrary aspect ratios in etching groove structures. Because of isotropy both edges of the groove are widend at least by the measure of the depth to be etched. Therefore the the groove is at least twice as wide as deep, i.e. the obtainable aspect ratio for groove structures is 0.5 maximum. The conditions are different for preparing embossed structures. Principally small line structures can also be produced by wet etching processes. It is an unfounded prejudice that only dry etching processes can be applied for preparing sub-micrometre and nanometre structures. If the undercutting under the mask edges is thoroughly controlled by an exact time control, very small patterns can be achieved also with larger etching depth. Thus Si-columns of 45 nm diametre prepared by dry-etching were reduced to only 10nm diametre by a following wet-etch process in aqueous HF-solution14. Also the fabrication of styli for tunnel microscopy by wet-chemical or electrochemical etching shows that by wet-etching processes very small structure elements with high aspect ratios can be formed reliably.
3.4.2 Selectivity in Outer-Currentless Etching As a rule, microsystems consisting of several metallic layers have to be patterned by etching the respective layers selectively to each other, i.e., that for each metal an etching bath must be found with a sufficiently high anodic partial current density for an acceptable etch rate while the anodic current densities of the other metals are very small or zero. Selectivity can be achieved very easily by choosing an oxidizing agent with a suitable standard potential. The tendency of a metal to yield electrons and turn into the ionic state depends on its position in the periodic table of the elements. The elements with a small number of electrons in the outer shell yield electrons more easily than elements with a higher number of outer electrons. The tendency to yield electrons increases also with the periode number, i.e., heavy elements of the same main or auxiliary group yield the electrons more easily than the lighter ones of the same group. All metals can be arranged in a series according to their redox-properties. This order is in accordance with the potential that an electrode of the respective material assumes in an electrolyte referred to a standardized electrode. Therefore this arrangement is termed “electrochemical series”.
l4
P.B. Fischer et al. (1993)
54
3 Wet-Chemical Etching Methods
Electrochemical series of chosen metals (normal potential at room temperature in aqueous solution)15: -2.38 V -1.71 V -0.76 V -0.56 V 4.52 V -0.44 V m2+ -0.34 v -0.34 v 1n/1n3+ co/co2+ -0.28 V Ni/Ni2+ -0.25 V
Sn/Sn2+ -0.14 V Pb/Pb2+ -0.13 V Fe/Fe3+ -0.04v 0.20 v Sb/Sb3+ Bi/Bi3+ 0.20 v As/As3+ 0.30 V 0.34 V cu/cu2+ 0,80 v A&%+ Au/Au3+ 1.50 V
Mg/Mg2+ Al/A13+ Zn/Zn2+ Cr/Cf+ Ga/Ga3+ Fe/Fe2+
Elements standing at the upper or left side of the eletrochemical series (less noble or base elements) dissolve at lower potentials than those standing more to the right or farther below (noble elements). Therefore the former can be etched at lower potentials at which the latter are not attacked. However, it is a precondition that the oxidizing agent works in the potential range the metal is etched in, i.e. the metal to be etched possesses a finite anodic current density whereas the other metal does not. In this way less noble metals can be etched selectively in the presence of more noble metals. The characteristic slope of the anodic current-potential curve for such an etch behaviour is shown in fig.3-11. The oxidizing agents can be arranged in a series due to their standard potenI I
I I I
/
I I I
metal I,/
I
/
/
/
,/'
./-4
I '
metal 2
/
__-metal 2 is not attacked
------/
/
/
(\'
metalll Is etched selectlveiy
oxidizing agent E
Fig. 3-ll. Current-potential curves of two metals (anodic branches) and an oxidant (cathodic branch) in an etching solution that only dissolves the less noble metal l5
J. D'Ans und E. Lax (193), 1251; H.-D. Jakubke und H. Jeschkeit (1987), 1059
3.4 Etching of Metals and Semiconductors
55
Electrochemical series of selected oxidizing agents (normal potential at room temperature in aqueous solution)2: H+/H2 ov ClO,-/Cl1.34 V Cu2+/Cu+ 0.167 V Cr20?-/C$+ 1.36 V Ce4+/Ce3+ 1.44 V [Fe(CN),]?[Fe(CN),]" 0.466 V 0.535 V C103-/Cl12n1.45 V Fe3+/Fe2+ 0.771 V M n 0 J M n 2 + 1.52V Br2/13rH202/H20 1.78 V 1.065 V IOJr 1.085 V s20g%20722.18 v tial as well. To the frequently used oxidizing agents in etching solutions belong metal ions of higher valence states, oxoanions and complex ions. Does the oxidizing agent react also at higher potentials, both metals are etched (fig.3-12).The ratio of the etch rates depends on several factors: density and electrochemical valency, as well as normal potentials of the metals, transport or potential control of the electrochemical partial processes, existence of a galvanic contact between the metals. Etch rates can be very different if the metals are not in galvanic contact and the cathodic partial process proceeds with potential control or if with galvanic contact the normal potentials are far apart. In these cases the less noble metal etches with a higher etch rate as the more noble metal and a certain selectivity can be achieved. The etch rates of both metals are not very different if the cathodic partial process is diffusion controlled, i.e., its intensity is independent of the etch potential and both metals are not in galvanic contact. Galvanic contact means the formation of an galvanic element which as a rule enhances the etch rate of the less noble component as long as any passivation does not occur (see section 3.4.3). I
I
I
I
oxidizing agent
I
,
I
E Fig. 3-12. Current-potential curves of two metals (anodic branches) and an oxidant (cathodic branch) in an etching solution that etches both metals
56
3 Wet-Chemical Etching Methods
Selectivity by Passivation More noble metals can be etched in the presence of less noble metals if the less noble metal is passivated in the potential range in which the noble metal is etched. For passivation a thin coating film is formed on the metal surface that inhibits the charge transition. Passivation occurs at a characteristic potential, the passivation potential. At this potential the species arise that cause the formation of coating films. The coating film is a transport barrier for metal cations in direction from the metal into solution, i.e. for the anodic charge transport. For the selective etching by means of passivation of a less noble material component an oxidizing agent must be chosen that allows the etch potential of the more noble metal to be adjusted to a potential range in which it is etched with a sufficiently high etch rate and the less noble metal is passivated. The current-potential curve for the anodic partial processes of a less noble passivating metal and a more noble metal that is actively etched is shown together with the current-potential curve of the cathodic process in fig.3-13. Also by passivation a mutual selectivity of etching of two metals can be attained. While the less noble metal is etched at a lower potential the more noble metal is not attacked due to its higher normal potential in the first etching solution. The second etching solution causes passivation of the less noble metal and etches the more noble.
I
;,'
metall I) I
I I
1 ,I
-
-
I
O
I
// metal2
1 1 1
/i
: I
I
'
/
/j I
j
,-
/ /
jI/
metal 1 wsrjva+es /
metal 2 is etched selectiveh/
/
oxidizing agent E Fig. 3-13. Current-potential curves for anodic and cathodic partial processes of an etching solution that selectively dissolves the nobler metal 2 while the less noble metal 1 passivates
3.4 Etching of Metals and Semiconductors
57
'Ikansport Control during the Passivation Process The passivation of a metal1 that should not or only very slightly be attacked during the etching of another metal can proceed without any transgression of metal ions into solution. The metal ions form in this case no solvatocomplexes but immediately the undissolvable species forming the passivating film. In some cases, however, competes the formation of passivating films with the dissolution process or the formation of non-passivating films. The latter are formed, e.g., of precipitates of anodically liberated metal ions on the electrode surface by surpassing the solubility product. Passivation is only reached if the ligand concentration in the near-surface solution falls below a critical value caused by the dissolution process. After immersion of the passivating metal into the etch solution the competing dissolution process can still be intensive as the ligand concentration in the near-electrode solution is as high as in the interior of the solution at the beginning. The local ligand concentration decreases, however, by complex formation. Thus a concentration gradient is built up near the surface, in which the metal ions reach the metal surface by diffusion. This gradient diminishes in the next process phase by impoverishment of the solution with respect to the ligands. By further decrease in the ligand concentration the critical concentration is reached at which the formation of the passivating layer sets in. At the point of passivation the diffusion zone into the interior of the solution attains a thickness that is termed boundary diffusion layer thickness. The boundary diffusion layer thickness is not determined by convection, but only by diffusion of the ligands, their concentration in the solution and their consumption at the metal surface. It is smaller than the diffusion layer thickness caused by convection. The electrode potential passes in such a kind of passivation a transient range. The potential of the transient range is determined by the active dissolution of the material, i.e., by the progress of the anodic partial process. At the end of the transient range the passive potential is reached over a rapid potential increase (fig.3-14). It is possible that several consecutive redox processes have to proceed until passivation sets in. That is the case if several potential forming electrode processes can occur, e.g., if the metal forms ions in several valence states. In such cases several consecutive transient potentials are observed where the respective transient potential is characteristic for the respective anodic process. The life time of the transient states is determined by the chemical rate constants, by the concentration and - as far as it is decisive for the transport-controlled partial steps - by the diffusion rates. The transition time to between immersion of the material in the electrolyte and passivation increases with increasing ligand concentration in the etching solution and decreases with increasing anodic partial current density. The progress in time can be described by the method of chronopotentiometry, if during passivation a nearly constant (i.e., in first approximation in this range potential-independent) cathodic partial current flows. This condition is fulfilled in the frequently occuring case that the cathodic partial process is controlled by diffusion as well.
58
3 Wet-ChemicalEtching Methods
1 I
passivated
electrode .__y"..._..".._...I. ." ..... ................. I
I I
".........._.."I
i'steep potmtlal increase m e l e c d e process 1 before passivation
."..........
_I
"."_,,."................. ..-..........-. ..
c..
the cows. of formation of a passlvamg top layer
,I ,,,,,,.,.,.~.
slight mcrease of potential due
achve dissolution
to the decrease of ligand wncenbation UI the near-
electrode solution
t
Fig. 3-14. The progression of the outer-currentless potential E~ during spontaneous passivation (schematically) of a metal in an etching solution. tocharacterizes the outercurrentless transition time.
This transport-controlled passivation process is described by the chronopotentiometric equation:
K, is the chronopotentiometric constant that is characteristic for the dissolution process, the present ligand concentration and its mobility in the solution. A n increase in the ligand concentration in general means a shift of the complex formation equilibrium in the direction of the complex and with it a preferential dissolution. This is marked by an increased chronopotentiometric constant. The anodic partial current density i, is in the outer-currentless case and with ignoring local currents equal to the cathodic partial current density i- which only depends on the intensity of this partial process. During the transition time material of the passivating metal is disssolved. The dissolved quantity of material can be estimated by combining Faradays law and the chronopotentiometric equation: i,
=
e - (V/ M ) - z
F/(A+.t )
(63)
The thickness that is dissolved until passivation is reached can be estimated in approximation by: hetch,pass
and
= VIA,
i, = I i. I t = To
(64)
3.4 Etching of Metals and Semiconductors
59
Equation (61) shows that with enhanced cathodic partial current density i. inspite of increased anodic partial current density and, therefore, increased etching rate of the passivating material during the transition time the etched material quantity diminishes. This behaviour is caused by the indirect, squared dependency of the transition time on the anodic current density. The intensity of the cathodic partial process is often determined by the concentration of the oxidizing agent in the solution. Enhancing the concentration of the oxidizing agent leads according to the chronopotentiometric equation to an accelerated passivation. Due to the squared ratio small concentration changes effect a greater change in the transition time. Vice versa a decreasing oxidizing agent concentration caused by solution consumption or aging means possibly a significant increase of transition times and thus a retarded passivation. A decreasing concentration of oxidizing agent, e.g. by thinnig effects or consumption of the oxidizing agent during the etching of several substrates in the same etching solution, is critical for the selectivity of etching processes. In such cases not only the etch rate of the metal to be patterned decreases, at the same time passivation of the material to be protected can be retarded or totally prevented, i.e. the etch solution does not react selectively any longer. In the case of transport limitation in the cathodic partial process, the bath convection affects the transition time, too. The stronger the convection of the bath, the more intensive the cathodic partial process and the sooner passivation occurs. With increasing temperature most chemical processes are accelerated. The tendency to passivation, however, decreases with increasing temperature, as a rule. Hence, temperature is a further critical parameter in selective etching baths that use the passivation of a component of a thin film system. A deviation in bath temperatures can disturb passivation and prevent selective etching.
Photochemical Influence on Passivation Beside the electrochemical parameters light can influence passivation, the anodic or cathodic charge transgression through the passivating film or the dissolution of the passivating film. A light-induced shift of electrode potentials occurs preferentially with semiconductors as these because of the smaller medium electron mobility form space charge zones leading to deformation of the band edges by electrode processes at the interfaces to the electrolyte. As by the absorption of light electrons are lifted from the valence into the conducting band, the exposure of semiconductor surfaces improves the formation of cations. This is used in photochemical etching (see section 3.4.8) for dissolving layers. But the photoinduced shift of the electrode potential effects also the forming and dissolution of passivating films. Such, e.g., n-doped silicon passivates more rapidly under light exposure while at p-doped passivated silicon the etching process is
60
3 Wet-ChemicalEtching Methods
reactivated16.The local activation of material surfaces by light can be used for direct, photochemically induced micropatterning without an etch-resistant mask. The activating light is projected through a convenient mask or is focussed as a beam on the sample surface immersed in the etching solution. At the exposed areas the passivating film is dissolved and then the underlying material is etched. As the passivation is preserved in the non-exposed surface areas the exposed areas are etched selectively. Many metal oxides and hydroxides are compound semiconductors, the conductivity and redox behaviour of which are changed by the absorption of visible or UV-light. According to the electrode material and, therefore, according to the composition of the passivating films the effect of the light can be either activating or passivating. As light absorption can lead to charge seperation in near-surface areas of the solid, the exposure relevant to etch or passivation behaviour enhances the electrode potential of the anodic processes. By the assistance of light absorption, metals and semiconductors with relatively high normal potentials can be etched with weak oxidizing agents. Light absorption, however, can also shift the outer-currentless electrode potential from the active into the passive range and cause passivation. By exposure of passivated surfaces the transpassive range can be reached.
3.4.3 Etching of Multilayer Systems Forming Local Elements Electrode processes can proceed on all conducting materials. Therefore, if etching multilayer systems, it is necessary to consider the electrochemical conditions for the material to be patterned in dependence on the other materials. In systems in which several materials are open to the etch solution a local current can flow from one material to the other if these materials are in electrical contact. In metallic multilayer systems the several layers are in general in electrical contact. Outer-currentless etching under such conditions only means that the substrate as a whole is currentless to the outside. But an anodic electrode process can be dominating on one of the materials and the cathodic on another as long as the currents are mutually compensated across the substrate. The most important aspects of the local element formation in outer-currentless etching shall be discussed in the following by a system of two materials. As a rule the etch rate of a metal is changed by local element formation, if the dissolution rate is potential-dependent and one of the partial processes takes place on a second material being in electrical contact. The partial process on the second material can be exclusively cathodic. But, it is also possible that on an uppermost cathodic material anodic processes proceed at the same time. The intensity of the respective partial processes is determined by the current density-potential characteristic of the single processes and the common electrode potential. l6
R.Vol3 (1992)
3.4 Etching ofMetals and Semiconductors
61
The conditions are most simple if on the second material merely cathodic partial processes advance. This is generally the case if a less noble metal is etched in the presence of a nobler metal. Then the potential in the etch solution lies in such a range that the baser metal is etched, but the nobler metal is not attacked. Both metals have the common electrode potential at which the oxidizing agent in the solution is reduced. Hence, the cathodic partial process proceeds on the nobler metal which is not patterned. For compensation, electrons must be supplied. These are liberated by a respective increase of the intensity of the anodic partial process, i.e., the formation of cations of the less noble metal. The local element is formed solely by the electron stream from the etching material to the cathodic active material. The etching process is so much intensified as electrons flow off (fig.3-15). Often the surface area ratios determine the increase of the etch rate by formation of local elements (see section 3.4.4). The function of the nobler metal can possibly be assumed by a passivated less noble metal. Passivated materials are not anodically active, but can be cathodically. This behaviour is caused by the properties of the passivating film that frequently obstructs only one direction of the charge transgression, i.e., possesses diode properties. A little more complicated is the situation if anodic and cathodic processes proceed at both materials. By forming the common electrode potential, the anodic process intensity is enhanced at the material with the originally lower etch potential and decreased at the material with the originally higher electrode potential. The opposite is the case for cathodic partial processes as far as they are potential-dependent in the concerning potential range. Due to the common electrode potential the anodic and cathodic partial current densities approximate. Normally the cathodic partial processes proceed at all electrodes with nearly the same intensity because their potential is determined essentially by the redox properties of the solution system. Therefore the cathodic partial currents and their influence on the dissolution rates can be estimated by the
I
opencircuit etch potential without local element
element formation
Fig. 3-35. Current-potential curve for increased etch rate by forming local cells with steady anodic partial process
cathodic process without local element
62
3 Wet-Chemical Etching Methodr
area ratios. But also cathodic partial processes can be dependent on the electrode material, e.g., if the reaction overpotentials are determined by the surface properties of the cathodic active material. Under such conditions it can occur that the cathodic partial process proceeds on one material exclusively, but the anodic processes on both. Cathodic overpotentials caused by the material arise, e.g., by setting free gases in the cathodic partial process. In microsystem technology typical cathodic processes liberating gases are, e.g., the etching in acidic or strongly alkaline solutions liberating hydrogen or forming nitrose gases in salpetric acidic etch solutions. Local Electrochemical Potentials In microtechnical systems, considerable potential gradients can develop beside locally equal mixed potentials during formation of mixed potentials in an etching solution. That is always the case if local currents flow over conducting elements with increased electrical resistance. Resistance layers are technical function layers in respective devices, the surface of which lies open to the electrolyte in certain etching steps. Electrically conducting layers with an increased resistance are to be expected in the end phase of etching thin metal layers, as soon as the etching layer is very thin or partially oxidized. Lateral potential gradients are formed in that case for only a short time. High resistances occur regularly at semiconducting materials where they can lead to considerable local etch differences or local passivations. The magnitude of local potential differences depends on the resistance of the conducting elements and on the magnitude of the local current. Such process-induced potential gradients, therefore, are determined by running the process (etch rates) and the solid state properties (area resistance) as well as the technical pattern topology (site and order of resistance patterns). Local potential differences effect the position-dependent etch times for respective structure elements. They can lead to locally different undercutting. In extreme cases staggered passivation at different sites of the substrate are caused by potential gradients due to local element formation.
3.4.4 Geometry-Dependent Etch Rates Under given etch conditions like temperature, bath convection and etching bath concentrations, the etch rate is constant if the considered areas are large and the edges of the areas can be neglected. The etch rate for a small structure element, however, can depend on its size and environment. That is the case if the etch rates are controlled by transport processes in the etching bath and the characteristic length for the material transport is not any longer negligibly small in comparison to the structure dimensions. Such conditions are not an exception in wet-chemical etching processes for preparing small structures. For productivity reasons high etch rates are desired, hence microlithographic etching processes often are limited by transport processes. The dimensions of
63
3.4 Etching of Metals and Semiconductors
the aimed-at patterns are as a rule so small that they are comparable to or even smaller than the diffusion layer thickness so that it cannot be neglected in comparison to the structure dimensions and geometry-dependent etch rates are observed. Reaction-controlled etching processes differ from transport-controlled etching processes by the distribution of educts and reaction products of the electrode processes in the near-surface solution. With reaction-controlled etch rates the concentration of educts and products is nearly the same at the solid surface and in the interior of the solution (fig. 3-16, on the left). The educts are brought to the electrode so rapidly that their consumption in the surface reaction is negligible compared to the total concentration. Also the products are transported rapidly off the electrode that their concentration stays so low at the surface that they do not effect the electrode processes. If the consumption of educts and the generation of products by the electrode processes cannot be compensated completely by transport processes, a concentration gradient is formed within the so-called diffusion layer in the near-surface solution. The diffusion layer is described by the thickness d, within which the transport of substances proceeds diffusion-controlled. In mixed reaction-diffusion control the velocity of surface processes decreases because of moderate deviations of educt and product concentrations at the surface and in the interior of the solution (fig. 3-16, centre). Proceed the surface processes more rapidly than the transport processes, the educts are consumed immediately at the surface and the product concentration can assume very high values (fig. 3-16, on
C
C
Id Educt
C(O,E]
C
Rcduct
d X
reaction controlled
C(0,E)
C(0,El
d
d
X
mixed reaction and diffusion controlled
X
diffusion controlled
Fig. 3-16. Representation of the concentration of educts and products in an etching process in dependence on the position x within the diffusion layer of thickness d. 1st case “reaction control”: no concentration gradients; 2nd case ‘‘mixed reaction-diffusioncontrol: small concerntration gradients in the diffusion layer; 3rd case “diffusioncontrol” strong concentration gradients in the diffusion layer
64
3 Wet-Chemical Etching Methods
C(0.E)
E(0.E) C
C
X
X
reaction rote controlled by me transpolt of ducts 1e.g.llgands)to me solid surface
d
d
d
mixedcontrol of the etching rate by dinuslon of educls and products
X
control of me etch rate by tronsport of piducts from me surface into solution
Fig. 3-17.Three cases for the distribution of the concentration of edicts and products in an etching process in dependence on the position x within the diffusion layer d with transport-controlled etch rate
the right). The latter effect sometimes leads to the fact, that at high etch rates also in the outer-currentless case the solubility products of easily soluble salts are overstepped and precipitates are formed on the etched surface. If the transport by diffusion in the diffusion layer determines the etch rate, the process is transport-controlled. Transport control by diffusion can occur in the anodic as well as in the cathodic partial process. Under certain conditions both partial processes can be transport-controlled. As a rule, however, only one of the processes is transport-limited. Mostly, educts and products are not transport-limited to the same measure. There are educt-limited transportcontrolled etch processes (fig. 3-17, on the left), product-limited etch processes (fig. 3-17, on the right) and mixed educt-product-limited etching processes (fig. 3-17, centre). Frequent cases of limitation are the transport of ligands for the formation of soluble complexes from the anodically liberated metal ions (transport control of educts in the anodic partial process) or the transport of oxidizing agent molecules (transport of educts in the cathodic partial process).
Size-Dependent Etch Rates (Size Effect) If an etch process is transport-controlled, the etch rate of a small structure element is the higher the smaller the area to edge ratio, i.e. the smaller the structure element. Whereas the transport of educts to the substrate surface and of products off the substrate surface into the interior of the solution in the case of extended area elements is essentially perpendicular to the subtrate surface, the transport at the edges proceeds in a half space (fig. 3-18). The expanse of this half space depends on the diffusion layer thickness. The diffusion layer is the volume element that is in the immediate neighbourhood of the subsrate surface during the etch process and in which the material transport is not determined by convection but by diffusion only. The prerequisite for diffusion is a concentration gradient. These gradients arise by consumption and libera-
3.4 Etching of Metals and Semiconductors
65
Fig. 3-18. Edge effect for transport-controlledetch rates. Material transport, mainly vertically to the substrate surface for large structures and mainly from a half space for small structures
tion of chemical subtances by electrode processes. The shape of the diffusion layer shows the distribution of concentration gradients in the space of the near-electrode solution. At extended areas and with local constant diffusion layer thickness the diffusion layer has a nearly prismatic shape. Its outer border to the solution interior is an area that is parallel to the substrate surface. With flat substrates the boundary of the diffusion layer is a plane. In the edge range on the contrary the outer boundary of the diffusion layer is curved, as the concentration gradients also are built up in the lateral directions. Straight edges possess a diffusion half space in shape of a quarter cylinder. At rectangular corners of structures arise half spaces in form of the eights of a sphere. In case of very small structure elements the lateral measures (width b) of which can be neglected compared to the diffusion layer thickness d possess a diffusion half space in shape of a hemisphere. The thickness of the diffusion layer in the case of free convection was estimated to be in the range of 0.05 to 0.2 mm, i.e., it is large in comparison to micropatterns. The diffusion layer diminishes if stronger convections occur. Such convections can be caused by the etching process itself. Mostly in etching metal films, ions with a high atom mass are released forming compounds of high specific weight. The near-surface solution enriched with etching products has, therefore, a high density so that it sinks to the bottom because of gravity. This sinking causes a considerable local convection that can reduce the thickness of the diffusion layer. Furthermore the diffusion layer thickness can be minimized by moving the substrate, stimng the etch solution or by spraying the etch solution onto the substrate. By these means the etch process is accelerated. Each change in convection effects a change in the diffusion layer and the shape of the diffusion half space influencing the etch rate of the surface area. The stronger the convection, the smaller the area elements that are effected by a size-dependent etch rate. The increase of the etch rate with decreasing area size in transportcontrolled etching processes can be estimated simply. For very small areas (b<
66
3 Wet-Chemical Etching Methods
The etch rate proportional to the anodic partial current density i+(i+.=.I/b2)is linearly dependent on the solution concentration of the rate-detemning species Q, its valency z+ and its diffusion constant D: r-i+=4-z+.F-c,.D/b
(68)
Are structure width b and diffusion layer thickness d comparable, so approximately an area in the edge range increased by the apparent diffusion layer thickness d’ (d = d’) can be assumed as the diffusion-effectivearea. The following equations are valid for the etch rate r in dependence on the etch rate ro of extended areas and the structure width b: r
=
r,
(b
+2
d’)/b (for lines)
(69)
respectively r
= r,
. (b + 2 - d’)2/b2 (for squares)
(70)
As an example for structure-size-dependent etching of a metal layer the velocity of etching copper in dependence on structure size in a chloride ions containing etch solution is given in fig. 3-19. In microtechniques mostly not single structures are prepared, but arrays of them. Then not only the size of one element is to be considered for the etch rate, but also the distance between the structure elements. If the size of structure elements is small compared to the double apparent diffusion layer thickness (b< 2.d’), the percentage area of coverage by the pattern, B, is decisive for the enhancement of the etch rate of transport-controlled processes. The etch rate depends on the ratio of the total area &, to the sum of area elements of the structures to be etched A: r = (ro/ B) = ro . Ages/ A , (for b < 2 . d’) Under these conditions the exact size of the diffusion layer thickness is irrelevant for the etch rate. It is only determined by the area ratios on the substrate. For small and tightly packed structures the lay-out determines the etch rate in such a case. Changes in the area ratios, as they occur from lay-out to lay-out, influence the etch rate. If on one and the same substrate chips with a different percentage area of coverage of the materials to be etched are present different etch rates are possible from chip to chip. The sizes of the chips (mm-range) are mostly much greater than the diffusion layer thickness, the sizes of the structure elements are distinctly smaller. Possibly local rate differences can be balanced in the lay-out, e.g., by blind structures or the division of larger areas in smaller ones, as far as such measures can be tolerated considering the function of the patterns. Corrections of size for compensating the undercutting (broader lines in the mask) must be different from lay-out to lay-out or from chip to chip.
67
3.4 Etching of Metals and Semiconductors
r [nm/min]
Fig. 3-19, Dependence of the etch rate r on the structure width 1 in etching of copper (square structure elements)
0.4
0.8
1.2
1.6
2
For transport-dependent etching behaviour of microstructures it is important whether the transport control occurs in the cathodic or anodic partial process. If the transport control concerns the cathodic partial process only and all structure elements are in electrical contact, as it is the case with a contineous metal layer, then the electrode potential determines the common etch rate. In this case the etch rates can be different from substrate to substrate, but on one substrate structures of all sizes and different coverages have the same etch rate. The common etch rate is determined by the total intensity of the cathodic partial process and the global anodic electrode potential. Are the structure elements electrically isolated, e.g., by a former patterning step, so the above mentioned dependencies are valid (equ.69-71). More complicated is the situation with transport control in the anodic partial process. In this case the edge ranges of area elements are etched more rapidly than areas lying further in the centre of the substrate. Consequently the inhomogeneity of etching does not only cause differences in etching time among different substrates, chips and structures with different structure width or coverages, but also structures of the same size and shape etch inhomogeneously. With smaller structures the etching progress from rim to centre is more rapid in relation to the structure size than with larger structures. Because of the increased etch rates in the edge range with anodic transport control undercutting sets in especially early. Hence, high deviations in structure sizes are to be expected by isotropic undercutting. The undercutting is especially aggravating as the anodic active areas of the structure sides under the mask are mostly very small in comparison to the lateral sizes of the areas to be etched. Due to transport control the etching process under the mask edges proceeds very rapidly. So the undercutting can be a multiple of the film thickness. With anodic transport control the position of the substrate in the etching bath (inclination to the effect of gravity) and the movement of the substrate have influence on the local etch rates. Differences in the locally active convection cause locally dependent regional etch rates ro, that effect the local etch rates r.
68
3 Wet-Chemical Etching Methods
Area Ratio-Dependent Etch rates (Local Element Effect) Changes in etch rates are observed independently of the absolute area size, if local currents occur between different materials (see section 3.4.3). In such cases the ratio of the partaking electrochemically active areas is important only the cathodic partial beside their absolute size. If on one area (Ametal2) process proceeds, but on the other (Ametal the cathodic and anodic partial processes then the electrons flow from the anodic active area to the solely cathodic active area, the more the greater the cathodic total activity of this area. If the intensity of the anodic partial process on one area (hence the etch rate) is only dependent on the potential, and the intensity of the cathodic partial process is transport-controlled, so the cathodic active area A determines the etch rate directly: r
=
ro.A./A+
(72)
with
and
At equal anodic current density, small etching area elements, being in contact with large solely cathodic active area elements, possess much greater anodic current densities and hence etch rates than isolated anodic active areas (fig. 3-20). If both partial processes are potential-dependent the common mixed potential determines the intensity of the partial processes, while the cathodic partial currents on metal 1and metal 2 are divided according to their area ratio. The enhancement of the etch rate by formation of local elements can be immense if in the end phase of the etching process the small flanks under the mask edge form the last surface area of the etching material and large areas of the nobler material lye open to the electrolyte on which the cathodic partial process proceeds. The result are high etch rates under the mask edges and hence large undercutting. Such extreme undercutting can be avoided working with etch solutions that are not potential-dependent in the anodic partial process, but are controlled by transport, too. That is why polishing etching solutions are advantageously used in micropatterning. These solutions produce moderate differences in the etch rate creating less undercutting inspite of galvanic contact''.
" J.J.
Kelly und C.H. de Minjer (1975); J.J. Kelly und G.J. Koel (1979)
3.4 Etching of Metals and Semiconductors
69
Fig. 3-20. Increase of the anodic partial current density when forming local cells of a small etching area with a larger mainly cathodic area
3.4.5 Geometry-Dependent Passivation Passivation of a metal surface is a surface process in which a coating layer is formed preventing or at least minimizing the anodic transgression of metal ions into solution. Passivation is used in microtechnical etching processes to make a certain material insensitive to the attack of an etch solution that etches another material selectively to the former (see section 3.4.2). Passivation is like all other chemical reactions characterized by a reaction velocity. This passivation rate can be determined by the velocity of the chemical reaction on the surface. As the chemical reactions determining the etch rate can be dependent on the transport conditions in the near-surface solution and hence on the geometry of the structures to be patterned, so the transport processes necessary for the passivation of a metal or semiconductor surface can be ratecontrolling. Whereas the diffusion layer thickness depends on outer factors like convection and stays constant during the process, a characteristic layer thickness develops between times in transport-controlled passivation processes. This is the layer thickness of a transient diffusion or deficiency zone formed at the point when the concentration at the solid surface of the species that promotes the non-passivating anodic process has sunk under a critical value. This characteristic thickness is termed diffusion boundary layer thickness d,. Passivation starts as soon as in the impoverishing process the diffusion boundary layer thickness is reached (fig. 3-21). Passivation sets in the sooner the
70
3 Wet-Chemical Etching Methods
dg
X
d
Fig. 3-2l.Three time stages for the concentration distribution of complexing species, the impoverishment of which leading to passivation of an anodic reacting electrode. Reaching the concentration C = 0 at the electrode the impoverished front has advanced to the depth d,. Passivation sets in because d, < d (diffusion layer thickness)
smaller d,. Reaches the diffusion zone the dimension of the diffusion layer thickness d passivation does not occur because the concentration gradients level off (fig. 3-22) The concentrations of dissolved species that partake in a possible passivation reaction decide whether a passivating film is formed. In analogy to transport-controlled etching processes the intensity of the cathodic partial processes responsible for passivation depends on the geometry (structure size and coverage) of the structure elements to be passivated if the cathodic partial process is transport-controlled. Small areas passivate under these conditions quicker than large areas. With a low density of small structures (small coverage) passivation within a total area arises more rapidly than by a higher density (higher coverage). The term coverage is related to the electrochemical area, i. e. the area that is not covered by a mask. The shortening of transition times (equ. 63 in section 3.4.2) for transportdependent outer-currentless passivation of an area A, can be estimated considering the apparent diffusion layer thickness d’ in relation to the width b of the passivating strutures in the chronopotentiometric basic equation. For the anodic current density i+ in the transition phase the following is valid (according to equations (35) and (44)):
Due to diffusion in the edge range an apparently enlarged cathodic active area is available:
3.4 Etching of Metals and Semiconductors
71
d X
Fig. 3-22. Five time stages (1-5) for the concentration distribution of complexing species, after immersion of an anodic reacting electrode. The concentration C does not decrease to 0, as the impoverished front reaches the depth d, beforehand. Passivation does not sat in, because d, > d (diffusion layer thickness)
AI= A+ - (2-d’+b)/b (for lines) A,
= A+
- (2.d’+b)2/b2 (for squares)
(76) (7)
The cathodic current that effects passivation is derived by the cathodic current density i0observed at large areas:
- A+ . (2.d’+b)/b (for lines) - A, = i0- A+ - (2-d’+b)2/b2(for squares)
I.I = iI. Al=
(78)
I-, = i.,
(79)
An indirect dependence on the apparently enlarged cathodically active area
results for the transition times of lines (to,)and squares (toq): tOl =
(K+ /
(I
i.,
tq =
K+2/
(I
i-,, l2 (@.d’+b)A~)~) (for squares)
I - (2*d’+b)/b) (for lines)
-
(80) (81)
Because of the squared dependence of the transition time on the anodic current density, the transition times are reduced dramatically with diminishing structure width b in comparison to the diffusion layer thickness d’. In analogy the layer losses during transition time are decreased. Reducing the transition time is also possible forming galvanic contacts with a predominantly or exclusively cathodically active area of a second (nobler)
72
3 Wet-ChemicalEtching Methods
material. For an increase in the anodic partial density i, leading to passivation the same laws are valid as for enhancig the etch rate by local currents (see secunder the influence of tion 3.4.3). The outer-currentless transition time (tog) local currents to exclusively cathodically active areas of a second metal depends on the area ratio of both metals:
Hence, changing the area ratios leads to various transition times. In critical passivation steps the concrete lay-out independently of the layer material and etch solution can decide the feasibility of a selective etch step. The dissolution of the passivating material can be different due to locally different transition times, i.e. there are local differencies in selectivity.
3.4.6 Electrochemical Etching In an electrochemical etching process the substrate to be etched is connected to a current source as an anode immersed in an electrolyte. The electrode potential is brought to a value at which atoms of the etching material contineously pass the surface as ions. The released electrons flow off through the outer circuit (fig. 3-23). Whereas in outer-currentless etching an oxidizing agent in the etch solution reacts with the electrons keeping the electrode potential in the necessary range, in electrochemical etching the electrode potential is controlled by an outer current source (current providing device, potentiostate or galvanostate). By the free choice of the potential of the substrate (working electrode) the etch rate and necessarily the selectivities of the materials can be better
substrate to be etched
Fig. 3-23. Set-up of a work station for electrochemical etching under potential control (schematically)
3.4 Etching of Metals and Semiconductors
73
tuned than in outer-currentless etching. The dissolving species for the different materials must be contained in the electrolyte. As the metal or semiconductor has to be dissolved as ions or as complex compounds, the ligand concentration and the pH-value have to be chosen that all cations can be dissolved rapidly and the formation of obstructing coating layers especially passivating layers is avoided. Formation of coating layers comprises the possibility of very inhomogeneous etching, e.g. this would exclude the application in microtechniques. The etch rate relin electrochemical etching is determined by the outer current density. The quantitative correlation of etch rate, current density and material constants M, z and e is given by equation (42), as in outercurrentless etching. The outer current I stands for the anodic partial current, because the outer current and the anodic current are of equal amount in electrochemical etching. With equal outer current the etch rate is the higher the smaller the etching area A:
In electrochemical etching the etching material has to be connected by an electrical contact with the current source. That is achieved by a contact area provided in the rim area of the substrate. This area is not immersed in the etching solution, as the current source is connected to this area. For anodic current flow the etching material must be electrically conducting. It must not possess a too great electrical resistivity lest essential potential differences occur that cause variating etch rates or prevent etching at all. To achieve undisturbed etching all structure elements must stay in electrical contact till the end of the etching process. This condition is always met if the etching material is deposited on a continuous conducting material, that is not etched under the chosen conditions. Does an isolating layer lie beneath the etching material so necessarily contact disruptions arise if after etching isolated area elements shall remain. As soon as the contact is disrupted during etching of the areas between these elements, the electrochemical etching of the isolated regions stops. To avoid such a topological effect conducting paths or layers of an etch resistant material should provide electrical contact throughout the whole process. Possibly these auxiliary structures can be removed afterwards by selective outer-currentless etching. Disruptions of the anodic current also occur if edges of free areas are preferentially etched, isolating, e.g., areas in the centre of windows. Such etching is observed if the velocity of the etching process is controlled by the transport of ligands to the surface and the reaction products into the interior of the solution. Hence, transport-controlled processes are inconvenient for electrochemically etching films on isolating surfaces. If necessary it is possible to apply an etching process which is controlled by the electrode potential. The disadvantage of a process without any at least partial transport control is the strong influence of surface properties (grain boundaries, roughness, defects) on the etching behaviour, causing inhomogeneous etching.
74
3 Wet-ChemicalEtching Methods
Also in electrochemical etching photolithographic masks can be applied. The anodic etching proceeeds in the windows of these masks. For the anodic etching the same laws as for the anodic partial process of outer-currentless etching are valid. This kind of electrochemical etching was termed throughmask electrochemical machining. For the according microtechnique the term through-mask emm (through-mask electrochemical micromachining) is used. In difference to simple EMM photolithographically patterned masks are applied instead of another tool (see below The technique can be used preparing holes in foils, e.g. for microfluidics To achieve a homogeneous etch depth within the windows as well as across the whole substrate area the anodic current density has to be spread homogeneously”. Beside the shape and distribution of the structures on the substrate the flow conditions in the electrolyte and the position of the substrate in the electrolyte are of influence on the current density distribution. Electrochemical etching is especially of advantage if the material cannot or can only be etched under extreme conditions in an outer-currentless etch process, e.g., films of noble metals like the much used platinum. Convenient anodic etching behaviour of platinum is already achieved in 3-molar hydrochloric acid. To attain a relatively high etching rate and a sufficient selectivity against other materials, e.g. titanium, an electrochemical pulse method (frequency in the kHz-range) is useful. The potential slope in time (form of pulse) can be optimized according to the material. Such an electrochemical pulse method has been applied successfully for etching rhodium in the presence of titanium2’.
i’;
Electrochemical Machining (ECM) As an alternative to electrochemical etching through masks the anodic patterning of conductive workpieces in ECM is carried out with counter electrodes of appropriate shapes (fig. 3-24). The counter electrode as the “tool” is brought into direct neighbourhood of the surface to be patterned. This technique was proposed by W. Gussef in 1929, demonstrated for the first time by C.F. Burgess in 1941, and used in industry since the fifties. It was applied especially to form hard metallic materials like steels or carbides. In analogy to classical mechanical methods of surface treatment, electrochemical polishing, drilling, and milling were developed’*. The method was introduced to microtechnical etching under the term electrochemical micromachining ( E m z 3 ) . The mechanisms of ECM have been not yet clarified in detail. The method is based on the fact that only in the narrow gap between the tool connected as cathod and the surface element to be etched high current densities exist, but M.Datta (1995) E.Rosset and D.Landolt (1989); A.C.West et al. (1992) R.P. Frankenthal und D.H. Eaton (1976) 22 W. Gussef (1929); vgl. auch M. Hiermaier (1990) C. van Osenbruggen und C. De Regt (1985) l9
*’
3.4 Etching of Metals and Semiconductors
75
Fig. 3-24. Set-up and connection of tool and work piece in electrochemical micromachining (EMM)
not in the farther neighbourhood. The necessarily high local etch rate differences must be garanteed by the local electrode potentials as well as by the local conditions in the electrolyte in the gap and on the surface of the workpiece. In ECM a very high current flows through the electrolyte, so that also at high ion concentrations in the electrolyte considerable ohmic drops in potential arise between working electrode and counter electrode. The potential drops are the greater the greater the distance between working and counter electrode, i.e., high currents flow on those surface parts that lie in the area of the narrow gap between the electrodes, hence the etch rates there are high, whereas the more distant surface areas due to the solution resistance etch more slowly. Also, the electrolyte can be so composed that etch resistent coating layers are formed in correspondence to the differences in the ohmic potential drop in the electrolyte. In the areas not to be etched, these layers cause an extremely small etch rate while in the gap areas etching proceeds rapidly at the higher potential. Convenient conditions for such processes can be achieved with transpassively etching materials like chromium and chromium containing steels. With adequate anodic polarisation the transpassive potential is reached in the gap and chromium is dissolved as Cr(VI), whereas the neighbouring surface parts stay passive as chromium forms a dense coating of Cr(II1) at the slightly lower electrode potential. In some cases certain milieu conditions are provoked by the cooperation of the anodic partial process at the workpiece and the cathodic partial process at the tool that allow rapid anodical etching, assisted by the low potential drop in the electrolyte within the small electrode distance in the gap. Especially, a change in the pH-value by the cathodic partial process can possibly accelerate the anodic dissolution. Additionally, the heat liberated by the electrochemical reaction in the gap contributes to enhancing the etch rate. Beside conducting salts, and complexing agents forming soluble coordinative compounds with the etching metals, oxidizing agents can be added to the electrolyte. Such an addition may increase the etch rate considerablyB.
24
J.A. McGeough (1974)
76
3 Wet-Chemical Etching Methods
Structure details in the sub-millimeter range are produced in EMM. This range is difficult to access as well by precision mechanics (cm- to mm-range, i.e. from larger dimensions) as by lithographic methods (ym-range, i.e. from smaller dimensions). EMM is predestined for forming metals in this range of size, because metals due to their polycrystalline structure cannot be patterned by anisotropic crystallographic deep-etching methods like those used for etching monocrystalline silicon substrates. The gap widths are typically about 10 ym, i.e., they are distinctly smaller than the pattern details. For minimizing etching at the side slopes passivating processes are used or the tool is coated at the sidewalls (fig. 3-25). The electrolyte is pressed with high pressure (about 1MPa) through the ECM fluid cell. The mostly very good conducting electrolyte and the small gap in the working area allow high current densities. At current densities about 1A/cm2,rates in the range of some 100 n m / s are typically obtained (see table below). In some cases current densities of more than 100A/cm2can be applied. Thus etch rates of several W m i n (about lo4 to 105nm/s) can be obtained, more than a thousend times the normal etch rates in wet-chemical outer-currentless etching.
tool
micromachining with nonpasslating electrolyte
/---micromachiningwith passivatingelectroiyte micromachining with passivating electroiyte
Fig. 3-25. Forming of grooves in electrochemical micromachining (EMM) with nonpassivating and passivating electrolytes using a sidewall passivating tool
3.4 Etching of Metals and Semiconductors
77
Table 3-3. Theoretical Etch Rates in Anodical Etching of Metals (at a current density of 1A/cm2,by A.E.DeBam and D.A.Oliver (1968)) Metal
Valency
Etch rate ( n d s )
Aluminium Beryllium Chromium Cobalt Iron Copper Manganese Molybdenum
3 2 6 3 3 2 2 3 4 2 3 3 1 4 3 3 5 6 2 2
340 250 125 230 245 370 380 245 325 350 230 360 1086 310 365 290 175 160 475 840
Nickel Niobium Silver silicon Titanium Vanadium Tungsten Zinc Tin
Electrochemical Etching with Nanoprobes (SECM-Etching) A special ECM technique uses an electrochemical micro- or nanoprobe for the generation of etch grooves with measures in the range of 10 to 1pm. As such a probe the scanning electrochemical microscope (SECM) developed for surface characterization is applied. By mounting the workpiece on a computer-aidedxy-table with a piezodrive, it can be positioned very precisely under the probe electrode. The accuracy lies in the range of a few nanometers. The conducting probe has a very small radius of curvature, e.g., a ultramicroprobe with a radius of 2 pm is used. The probe as the tool is positioned in height to the workpiece with the aid of a piezodrive working in z-direction. In contrast to the ECM-technique the probe is connected as anode (fig. 3-26). At this electrode a strongly corrosive species is produced by oxidizing a substance diluted in the electrolyte. Because of the extremely narrow gap the corrosive species rapidly diffuses to the opposite surface causing there the local etching of the material to be patterned. The concentration of the reactive species very rapidly decreases in the direct neighbourhood of the nanoprobe, because the volume between the nanoprobe and the workpiece is very small compared to the total volume of the electrolyte. Therefore, the reaction of the corrosive species can be neglected already in a short distance from the nanoprobe. A separate element in greater distance to probe and substrate func-
78
n
3 Wet-ChemicalEtching Metho&
7
Fig. 3-26. Anodic connection of the tool in EMM using special tools, especially applied for nanoprobe technique
tions as cathodic counter electrode. Etching grooves of a few micrometers were produced in that way in the compound semiconductors GaAs, Gap, CdTe and (HgCd)TeZ. Principally pattern sizes much smaller than 1 pm should be feasible by probe-initiated patterning processes. With better conducting materials like metals or very highly doped semiconductors the electrochemical generation of ligands at the probe is more useful for the local intensifying of the etching process than the formation of the oxidizing agent.
Jet Electrochemical Micromachining The jet electrochemical micromachining (JEM) is a special form of electrochemical micropatterning. Under high pressure a fine beam of the electrolyte is forced through a nozzle on to the surface to be patterned. The workpiece is anodically polarised. According to the immersion of the workpiece the counter electrode can be positioned either in the surrounding electrolyte or in the fluid channel of the jet. Because of the high velocity of the liquid and hence the thin diffusion layer thicknesses the etch process proceeds very rapidly in the range of the impacting fluid jet whereas it is neglegible in the farther surroundings. A small distance between nozzle and substrate surface is required and achieved by mechanical positioning of the jet in normal direction to the substrate. The topology is atterned by moving a substrate table that is adjustable in x- and y-direction . Jet electrochemical etching has proved its worth for fabricating sub-mm holes in metal substrates, e.g. in tungsten foils. Etch depth of 50 to 500 pm and diameters of 0.4 to 0.lmm were produced”.
zr
D. Mandler und A.J. Bard (1990) M. Datta et al. (1989) 27 S.-J. Jaw et al. (1994 und 1995) 25
26
3.4 Etching of Metals and Semiconductors
t
powei. supply
111:
cathode
u nozzle
79
electrolytejet
\ I/
b+++++++++++ I (anode)
I X-Y -stage
Fig. 3-27. Set-up of electrochemical beam etching
3.4.7 Photochemical Wet Etching In photochemical etching the outer-currentless process of dissolution of conducting materials is assisted by light. The method is used preferentially for patterning semiconductors. With them the charge carrier density in nearsurface areas can be enhanced essentially by light exposure. Besides, the conducting and valency band are lifted energetically. As well the anodical as the cathodical partial processes can be enhanced by this means. The increase of the etch rate is dependent on the intensity and the wavelength of the exposure light. Defect electrons (holes) in the lower band left by absorption of light support the release of cations. At the same time the probability of electron acception by an oxidizing agent in the solution from the conducting band raised energetically near the surface is increased. Hence the photochemical assistance of etching processes can be well applied to outer-currentless etching. Special importance belongs to photoelectrochemical etching (see section 3.4.8). In outer-currentless photo-aided etching etch solutions are used with compositions that are related to respective etch solutions working in the dark. Thus for etching silicon, fluoride containing electrolytes forming the soluble complex ion (SiF,)” are applied2*.At surfaces of compound semiconductors the exposure of certain areas leads to enhanced etch rates at the non-exposed surface areas. This phenomenon is due to the shift of the electrochemical stationary potential to higher values by exposure. The shift is explained by the intensification of the cathodic partial process in the exposed areas. This process is plausible by the photo-induced raise of electrons into the conducting band and their following transfer to the oxidizing agent. The generated defect electrons migrate from the exposed to the dark areas of the electrode inducing an increase in the electrode potential. Etching GaAs in KOH under partial laser exposure resulted in an etch rate increase of at least 600 times the etch V. Svorcik and V. Rybka (1991)
80
3 Wet-ChemicalEtching Methods
rate of unexposed G ~ A sWith ~ ~ focused . laser beams the factor 1,000,000 was observed in the same system3'. Photochemical etching can be used for direct pattern generation without a lithographic mask. The intensive exposure with a focused laser beam allows, e.g., the wet-chemical fabrication of holes in semiconductors like GaAs with aspect ratios of about one hundred31. Lines also can be etched. Thus optical lattices can be etched in IIINsemiconductors, if a lattice is projected by a HeNe- or Ar-laser (543.5 nm and 488 nm respectively) directly on the substrate surface immersed in the etching bath. By projecting plane patterns of special shapes even more complex forms can be structured directly3'. Principally substrates with curved planes could be photoelectrochemically etched by direct projection exposure. Prerequisite would be an optically sharp exposure of the pattern geometry and the homogeneous illumination of the etching area with light of sufficiently high intensity. Laser-assisted etching in corrosive etching baths can be applied to metals, too. The obtained rate differences between non-irradiated and irradiated areas can be more than the factor 1O00, where the increase of rate occurs in a relatively narrow interval of the power density of the laser radiation33. So structures can be written directly. Especially passivatable materials show high rate differences between irradiated and non-irradiated areas. The passive areas are activated by irradiation. This laser-assisted method is applied to easily passivating materials like steel, chromium, titanium and cobalt, but to copper as well. It is assumed that the dissolution is accelerated rather by thermal than by photochemical processes34.
3.4.8 Photoelectrochemical Etching (PEC) In photoelectrochemical etching the anodic process of dissolution of conducting materials is assisted by light. Like the photochemical wet-etching processes this method is applied for patterning semiconductors. Photoelectrochemical etching like photochemical etching uses light to assist the dissolution process, but like electrochemical etching it does not need an oxidizing agent in the electrolyte as the etching is achieved by anodic connection to an outer current (fig. 3-28). The use of light for increasing etch rates has been well known since the nineteen twenties and has been used in classic methods for treating metallic workpieces by anodic etching. The metals aluminium, copper and its alloys, J. Van de Ven and H.J.P. Nabben (1991) M. Datta, L.T. Romankiw (1989); Y. Tsao, D.J. Ehrlich (1983) 31 M. Datta, L.T. Romankiw (1989); D.V. Podlesnik et al. (1984) 32 R. Matz and J. Meiler (1990) 33 R. Nowak et a1 (1994); R. Nowak and S. Metev (1996a) )4 R. Nowak and S. Metev (1996b,c) 29
3o
3.4 Etching of Metals and Semiconductors
I oxidizing agent in the etching solution
~
etching solution with complexing species
light radiation during etching
81
I substrate polarised as anode
~
wet etching
Fig. 3-28. Overview over wet chemical etching methods, classified according to the use of oxidants, anodic polarization and light
gold, silver and steel are electrochemically etched supported by light, if satisfactory roughnesses (about 1pm for 100pm etching depth) shall be obtained with sufficient etch rates (about 3nm/s to lOOOnm/s) at high concentrations of oxidizing and complexing substances in the etching baths (up to 10%). Workpieces of materials that are worked on and shaped more difficultly like titanium, zirconium, niobium, tungsten, rhenium, rhodium, iridium, platinum and tantalum are etched with this method35.Experiences, made with this classical methods in photelectrochemical etching, considerable etching depth are nowadays used in microsystem technology for structuring an extended material spectrum. E.g., photoelectrochemical etching of silicon and germanium was investigated in an early stage of microelectronics d e ~ e l o p m e n t ~ ~ . In current microtechniques photoelectrochemical etching is applied to patterning semiconductors, preferentially compound semiconductors. The method is of importance for semiconductors, as they possess a lower conductivity in comparison to metals and a considerable band gap between the valence and conduction band. Semiconductors in an electrolyte show bending of band edges near the surface. The cause of this phenomenon are the changed electronic conditions at the interface solidelectrolyte due to the formation of the electrochemical double layer. The cations that passed to the solution side of the phase boundary left electrons back. These electrons are transported into the interior of the semiconductor by an outer anodic potential generating a potential gradient into the inner of the semiconductor (fig. 3-29A). 3s
36
D.M. Allen (1990) A. Uhlir Jr. (1956)
l
82
3 Wet-Chemical Etching Methods SEMICONDUCTOR
El
ETCHING ELECTROLME
2
LEVEL
I
VALENCY BAND
IB A
X
SEMICONDUCTOR
ETCHING ELECTROLWE
.*.-
FERMI
m
M K IB VALENCY BAND B X
B
Fig. 3-29. Photoelectrochemical etching: band bending in etching semiconductors without (A) and with (B) beam-induced electronic excitation (H. Gerischer 1988)
By absorbing light charge carriers are transported from the lower electronrich band into a higher electron-deficient band. Hence, the conductivity is increased as well by the defect electrons (holes) in the lower band as by the increased number of electrons in the upper band. Additionally the tendency of forming ions is enhanced by the existence of more defect electrons in the lower band, i.e. the anodic transition probability for ions through the solid surface is increased. The defect electrons formed by the light-induced charge separation migrate under the conditions of anodic dissolution to the electrode surface. There, they assist the formation of cations M+ that pass into the liquid phase where they react with a ligand B (electron pair donor) forming a soluble species. The electrons are transported by the electric field into the inner of the semiconductor. Due to the light-assisted charge separation an increased anodic dissolution proceeds at the same electrode potential under irradiation (fig. 3-29B). Photoelectrochemical etching processes have been worked out for many semiconducting materials. Silicon” as well as compound semiconductors like GaAs, AlGaAs, GaSb, Id‘, InAs, and Sic are etched photoelectrochemi-
ally^^. The irradiation of the semicondutor surface cannot be accomplished from the side of the solid because it is not transparent. Instead, the electrolyte is 37 38
R.Voss et a1 (1991) H.F. Hsieh et a1 (1993); E.K. Probst et a1 (1993); D. Harries et a1 (199); J. S. Shor et a1 (1992); J:S. Shor andA.D. Kurtz. (1994); R. Khare et a1 (1993) J. Van de Ven and H.J.P. Nabben (1990)
3.4 Etching of Metals and Semiconductors
83
irradiated. Hence, it is necessary that the absorption of the electrolyte is low, on one hand not to dim the light and on the other hand not to heat the electrolyte unnecessarily. As the etch rate increases with the intensity of the irradiating light, intensively emitting light sources are used. To avoid inhomogeneities in the local etch rates it is of great importance to irradiate the substrate surface homogeneously. The wavelength of the irradiating light has to be adapted, so that the energy of the light quanta is at least as high as the breadth of the band gap of the etching material. Quanta of lower energy cannot provoke the charge separation in the near-surface areas of the semiconductor and are therefore without effect on the etching process. These demands are met by laser sources. Firstly, they emit a very intensive radiation. Secondly, they can be very well guided optically and hence supply a homogeneous irradiation of the area to be etched. Thirdly, lasers supply monochromatic light and are available with different wavelength to be chosen according to the band gap. Beside the actual semiconductors, metals with semiconducting coating films are etched photoelectrochemically as well. These coating films often possess passivating properties, i.e., they impede the anodic charge transiton. Whereas the Fermi level in metals is a constant energy level up to the metal surface the coating film shows the band gap characteristic for semiconductors. In some cases two or more coating films are formed on a metal surface possessing different semiconductor properties. Like bulk or film semiconductors,coating film semiconductorsas well show a bend of valence and conduction band near the interface to the electrolyte (fig. 3-3OA). By the action of light the conduction and valence band in this case are also raised and the charge transition through the coating film is favoured (fig. 3-30B). This improvement of the charge transition means an intensification of the electrochemical partial process and hence an increase of the etch rate at a given potential. Photoelectrochemical etching of metals with semiconducting coating films is of importance in microtechnical etching as well for less noble metals forming stable, strongly passivating oxidic coating films like titanium, as for more noble metals forming less strongly passivating oxidic or saltlike coating films like copper. As light can be easily focused or formed to light stamps by apertures, photoelectrochemical etching like photochemical etching can be used for microtechnical patterning without the use of a lithographic mask. Fabricating structures in InP and GaAs with local laser irradiation in diluted salpetric acid, hydrofluoric acid and sulphuric acidic hydrogen peroxide solution etch rates up to 5 O d s and up to 1 0 0 d s were achieved, re~pectively~~.
39
M.N. Ruberto et a1 (1991)
84
3 Wet-ChemicalEtching Methods
Fig. 3-30. Photoelectrochemical etching: band bending in etching metals with semiconducting coating layers without (A) and with (B)
3.5
Crystallographic Etching
3.5.1 Chemical Wet-Etching of Monocrystalline Surfaces Wet-chemical etching processes are isotropical as far as such processes dominate that do not prefer any direction, like all diffusive transport processes. The etching material itself can induce preferential etching of certain directions if its inner structure is not isotropical and the etch rate is determined completely or partly by this anisotropic structure of the solid. Such an influence of the solid structure on the etching process is always possible if a partial reaction of the etching process proceeding at the solid surface is rate determining. Even without any current flowing, etching processes at metals and semiconductors are as a rule connected with electrochemical, i.e. anodic and cathodic partial processes (see section 3.4). The character of the etching process, i.e., whether isotropic or crystallographicallyanisotropic etching proceeds, can be determined by the choice of the etching medium and the appropriate concentration of etching bath components. Crystallographic etching has as precondition, that the anodic partial process proceeds with surface control, that means that the amount of ligands at the solid surface must be so high that the transition reaction is rate determining. Diminishing the ligand concentration in the solution and hence the ligand supply at the interface, the transport of ligands from the solution, i.e. their diffusion, can become rate determining and the anisotropic etching process turns isotropic.
3.5 Crystallographic Etching
85
Anisotropic wet-etching processes and the shape of micropatterned solids and pattern elements are dependent on the atomic or molecular structure of the solid. The struture of the solid can be classified according to its state of order in three groups having relevance to microtechniques: A solids with building units (atoms or molecules) that show no preference of direction in their order (glass-like and other amorphous materials) B solids consisting of a single monocrystal (monocrystalline materials) C solids built up of numerous monocrystals with different direction in space (polycrystalline materials) Between the types A and C exist as transition forms partly crystalline and grannular built up amorphous materials. Poly- and partly crystalline materials again can be built up of crystallites, the orientation of which is evenly distributed. However, it is possible that some orientations are more frequent than others. The last case is very probable for film materials grown on a surface, i.e. for materials deposited by evaporation, sputter techniques or CVD. In ideally amorphous materials all orientations occur with all components. Such materials cannot be etched crystallographically anisotropically. The other types B, C and the transition formes A/C can show an etching behaviour with globally or locally preferred orientations. Poly- or partially crystalline materials where all orientations are present show no global differences in etch rates, but locally considerable differences can occur. By this phenomenon locally inhomogeneous etching is caused, especially with materials of column shaped or otherwise significant grain boundaries. By different etch rates at different monocrystalline surfaces or by different etch rates of the grains or grain boundary areas, the material structures, that are not necessarily obvious, can be made visible at the surface by etching as micro- or nanotopographies. By the etching process local hollows develop in areas of high etch rates and local peaks in areas of low etch rates. This behaviour is used for years for characterizing materials. A lot of etching baths were developed especially for crystallograhic characterization of semiconducting materials and components. According to the etch behaviour due to local composition (doping) or structure these etching baths cause differing material removal so that the local crystal properties are recorded as a relief in the surface that can be viewed by a light microscope'"'. For etching microdevices such a locally inhomogenous etching behaviour is undesirable. Whereas the anisotropic removal of monocrystalline material is of great importance in microtechniques, especially for micromechanics. By choosing a certain crystallographic plane as the surface plane of a substrate, defined geometric structures can be prepared by anisotropic wet-chemical etching in a suitable etching medium. In this way it is possible to generate other than the spherical or cylindrical surface areas occurring in isotropical etching. Crystallographic etching P.J. Holmes (1962); A X Bogenschutz (1967)
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allows to prepare wet-chemically flat planes and sharp edges, that can be declined or shifted to the edges of the etching mask. The position of the structure edges is determined by the etch rate ratio of the crystallographic planes and the relative position of the mask edges to the crystallographic directions. The achievable geometries depend on the crystallographic orientation of the surface which lies at first open to the etching medium. The precondition for crystallographic etching are etching solutions in which the different crystallographic planes possess different etch rates. Generally, planes with high densities of atoms have low and such with low atom densities high etching rates. But probably, the distribution of electron orbitals in the crystal plane plays a significant role beside the absolute density of atoms in the surface. It is decisive for chemical elementary processes taking place at the surface, especially the addition or separation of groups that impede a removal of the surface, like oxides, salts or hydrides. Furthermore it influences the attack of complexing agents effecting the removal of metal ions from the solid surface forming soluble coordination compounds. The position and geometry of unoccupied surface orbitals is of importance for the reaction probability with sterically special chelate ligands. The crystal structure and the choice of the cutting defining the surface of the microtechnical substrate determine the achievable geometry of the patterns. By crystallographic wet-chemical etching the planes with the smallest etch rates are preserved. The geometry developed by etching through a mask window is determined by the angle between the substrate surface and the first crystallographic plane with the lower etch rate. The geometry in the depth results from the choice of the mask window and the crystal cut. The structure of the crystal lattice determines the etch rates as well as the feasible geometries. Therefore it is important to know which type of crystal lattice shall be etched. For microtechniques especially cubic crystal lattices are of importance. E.g., silicon and GaAs form cubic crystal lattices. Crystalline quartz which beside quartz glass is important as well in microtechniques can occur as cubic (P-crystobalite), trigonal (a -quartz), hexagonal (P-tridymite, p-quartz), rhombic (a-tridymite) or tetragonal (a-cristobalit) lattice. The exact geometry of crystallographically anisotropically etched substrates depends not only on the crystal structure. Recent investigations have shown that in dependence on the used etching media (concentration and temperature) deviations of several degrees to the ideal angles can occur41. For optimizing geometries of three-dimensionally etched monocrystalline microstructures as they are used in micromechanics and other fields, a series of experimental test methods as well as etch simulation programmes have been developed. As a rule, the etch rates in the different crystallographic directions are estimated empirically or, if already available, are gathered from tabulated values. The mask geometries can be optimized with the help of the known etch rates and the choice of the adequate crystal orientation in the 4'
I. Stoev-1996 I)
3.5 Crystallographic Etching
87
wafer plane using a computer programme that simulates the process of etching. Such simulations cut back possible variants of the mask lay-out and facilitate the choice of mask geometries. Small deviations in the angles of the initial geometries (cutting angle of the wafers, alignment of the mask edges) and deviations in the etch rate ratios can cause considerable deviations in the geometries practically etched compared to the computed shape, especially if complex geometries or even such with convex comers are considered. Hence, in practice an empirical optimization follows the computing ascertaining the best mask geometry for the respective three-dimensional structure to be fabricated.
3.5.2 Anisotropic Etching of Monocrystalline Metals All monocrystalline materials can be etched anisotropically.This possibility is used for long in material science to make visible textures in the material. Also in etching metals low indicated crystallographic planes are preserved. These properties can be used for anisotropical micromachining of monocrystalline metallic materials. In contrast to etching silicon other planes than the 111-planes can be the most resistant against etching baths. Beside the velocity of the etch removal in normal direction to the crystallographic unique surface the velocity of the removal from edges plays an essential role. Decisive for the etching behaviour at differently indicated planes is the position of electron orbitals that can interact in the anodically formed ions as free orbitals with the free electron pairs of ligands of the solution. At platinum monocrystals the (110)-planes are preserved preferentially in electrochemically etching in aqua regia. In this example (111)-planes are more slowly attacked in normal direction than the (110)-planes, but from the edges they are more rapidly removed4’. In contrast to semiconductors and piezoelectrically active dielectric monocrystals or ceramics, monocrystalline metals are rarely used in microtechniques up to mow. Anisotropic etching was performed, e.g., in connection with electrochemical studies concerning the behaviour of metals. For patterning the anisotropic etching is sometimes used with poly- or partly crystalline films that are deposited with a certain morphology, i.e. the preferential orientation of grain boundaries and crystallites. So e.g., the preferred etching of grain boundaries can produce steep structure edges at column-shaped deposited material. The pattern edges formed by this kind of etching reflect the morphology of the film. As a rule the steep flanks are payed for by edge roughness in the range of lateral grain boundary areas and by inhomogeneous etching in time because the etching on the open areas proceeds in the grain boundaries while the etching in the inner of the grains stays back. 42
R.Caracciolo und L.D.Schmidt (1983)
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3 Wet-ChemicalEtching Methods
3.5.3 Anisotropic Etching of Silicon Anisotropic etching of monocrystalline silicon advanced in importance in micromechanics in recent years. This importance is more due to its mechanical and thermal than to its semiconducting properties. But due to its application in microelectronics silicon is available in high quality and comparably unexpensive, besides it is easily treated by well-introduced microlithographic methods. Silicon is used as function material as well as support and sacrificial material. By means of anisotropic etching three-dimensional shapes with exact measures are produced. These shapes are square and rectangular holes in the substrates, etch grooves with the shape of the frustum of a pyramid, or microchannels with triangular or trapezoidal crossection, which find a variety of applications in science and technology. Micromachined silicon is used nowadays for a series of mechanical sensors, e.g. for the atomic force microscopy in pressure sensors and as mass articles in accelerating sensors for air bags in automobiles. Micromechanically etched silicon is also applied in thermal sensors, in microfluidic devices like pumps and valves, in chemical sensors and in capillary devices for electrophoresis and chr~matography~~. Currently it is under development for molecular biological, biochemical and microbiological applications4. Silicon has a cubic face-centred crystal structure (diamond lattice). The consequence of the highly symmetric structure are an identical order of atoms and hence of lattice planes when rotating the crystal. Therefore only three cutting orientations of low indication are relevant in micromachining, the (111)direction, the (100)-direction and the (110)-direction. The (010)- and the (001)-directions are equivalent to the (100)-direction, as are the (101)- and (011)-directions to the (110)-direction (fig. 3-31). In silicon micromechanics mostly polished substrates of the orientations (100) and (110) are used.
Etching Media and Mask Techniques The wet-chemical anisotropical silicon etching is used as a standard technique in many places nowadays. In its character it is comparable to wet-chemical etching of metals, as it is an outer-currentless etching process. In the anodic partial process silicon passes as Si(1V) into solution. The electrons released in the solid are transferred in the cathodic partial process to an oxidizing agent of the etching solution. In most media the protons of water serve as oxidizing agent forming gaseous hydrogen in the reaction. This process is feasible, because the redox potential of the reductive hydrogen formation lies higher than the redoxpotential of the silicon electrode. In this respect silicon behaves like a less noble metal. In some etching media special oxidizing agents are S. Biittgenbach (1991);A. Maw et al. (1990,1993),K. Seiler (1993), D.J. Harison et al. (1993), C.S. Effenhauser et al. (1993), 44 Northrup et al. (1993,1995); J.M. Kohler et al. (1995), A. Schober et al. (1995) 43
3.5 Crystallographic Etching
89
t110)
Fig. 3-31. Position of low indicated crystallographicplanes in cubic crystals, important for chemical wet etching of silicon
added, e.g. to minimize the intensity of the formation of hydrogen gas bubbles. Diminishing of gas bubbles is especially important in the preparation of sensitive micromechanical structures and thin film membranes. Besides redox mediators can be added, e.g. metal ions possessing several oxidation states. Important in choosing additions is the consideration of their solubility products, that can even be exceeded for relatively well soluble salts, as by the frequently applied alkali hydroxides the counter cation (mostly K+, Na+or Li+) is present in high concentration. Hence, often two or more kinds of alkaline ions are used in one medium. Silicon is dissolved forming silicon(1V)-compounds. Because of the relatively low electronegativityof silicon (1.74), the electron density at the silicon central atom is very close to the ionic state Si4+.Soluble species formed during etching can be understood as coordination compounds. This assumption is in accordance with the fact that the coordinative bond of hard, i.e. little polarizable, ligands to silicon atoms of the surface are evidently an essential step for the passing of silicon from the solid into the etch solution. Is the concentration of such ligands too low, the reaction of the silicon surface atoms with water forming slightly soluble silicon dioxide is predominant. Only if the formation of soluble complexes can compete with the formation of the surface oxide, etch removal takes place. Hence, silicon is only dissolved in media with high concentrations of such ligands. Beside F used in isotropically etching media, only electron-rich electronegative elements like nitrogen and oxygen can serve as hard donors for the formation of soluble silicon complexes. The free electron pairs of both kinds of atoms take part in the coordinative bond. The simplest species to comply with this request to a reactive ligand for the silicon atom is the OH--ion. However, it is necessitated in very high concentrations to
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3 Wet-ChemicalEtching Methods
perform a significant etch removal. Therefore, extremely concentrated alkaline solutions are applied frequently for anisotropical silicon etching. The compatibiliy of ligands forming soluble complexes and species forming surface oxides is not only determined by electronic properties but as well by steric properties of the ligands. The complex forming probability is essentially enhanced by two or more hard donor atoms belonging to one ligand, the chelate ligand, and thus being restricted in their movability to each other. Therefore, soluble compounds containing several electron-rich oxygen atoms are especially effective as etching component. In chelate ligands nitrogen of aminogroups can compete inspite of its lower electronegativity compared to oxygen with the formation of surface oxides. So to some silicon etching solutions two- or multivalent amines are added. Anisotropic etching of silicon is based on the distinctly decreased etch rate of (111)-planes compared to (110), (100) and higher indicated planes in different etching solutions45.As origin of the differences in etch rates the geometries of orbitals of the atoms lying in the surface plane and the formation and reaction rates of surface complexes as well as the stability of intermediary oxide films SiO, are discussed&.While in concentrated mixtures of salpetric acid and hydrofluoric acid4' polishing isotropical etching is observed, with low acid concentration partial anisotropic etching occurs. Especially distinct is the anisotropic etching in alkaline etch solutions. The rate differences vary with etch bath composition and temperature by 100 up to 10oO times. Very good etching results are obtained in heated concentrated lithium, sodium or potassium hydroxide solution (see also section 5, etch instruction for silicon). Etch rate ratios Si(llO)/Si(lll) of up to 5500 can be achieved, if cesium hydroxide solutions are used as etching batha. To obtain satisfying etch rates for etching through the whole substrate thickness in acceptable times (several hours), very high alkali concentrations (20 to 30 % mostly) at elevated temperatures (50 to 80 "C)are applied. Under such conditions the etch bath due to its general corrosivity is a critical medium concerning health and safety. Hence special etching containers are used. Besides the loss of liquid would be tremendous with heated open bath containers during the hours of etching so that the alkali concentration would increase continuously by evaporation of water. Thus generally closed vessels are used with - condensation facilities like cooling spirales or reflux condensers. The traditional material for laboratory devices, glass, is inconvenient as container material for strongly alkaline solutions as glass itself is attacked rather rapidly forming silicate complexes which on the one hand destroys the vessel and at the other has the etching bath aged more rapidly. Thus beside quartz glass devices, such of refined steel and special ceramics are used. ~
45
46 47
~
R.M.Finne and D.L. Klein (1967); D.B. Lee (1969); K.E. Bean (1978) E.D. Palik et al. (1985); H. Seidel et al. (1990) H. Robbins und B. Schwartz(1959) und (1961) L.D. Clark et al. (1988)
3.5 Crystallographic Etching
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A grave disadvantage of alkaline etching media is their quick dissolution of resist masks based on novolaks. Hence, this advanced group of positive photoresist is not applicable for anisotropic etching. But other organic photoresists as well are badly suited as masks in strongly alkaline media. Therefore, auxiliary films have to be used as etching masks with low solubility in the silicon etch solution, that can be patterned by means of standard resists. S O 2 that is readily patterned with positive resists has a distinct dissolution rate in strong alkaline solutions. Thus silicon dioxide films prepared by thermal oxidation of the silicon surface or CVD can be used only for little and middle etching depths, as they can simply be produced only in thicknesses of 100nm up to one or a few microns. For etching depth of 100 ym the silicon dioxide film has to be at least 1ym thick, i.e. for the depth of a 4”-wafer the Si02-film should exceed 5 pm in thickness. Such thick mask films mean considerable efforts in film deposition on the one hand and on the other a deviation in mask measures due to undercutting during wet etching of the mask film and hence minimizing the accuracy of pattern transfer into silicon. More convenient as masks for anisotropically patterning silicon in strongly alkaline media are silicon nitride films. Their own patterning is possibly more complicated if their properties demand a dry-etching process or an auxilary mask like Si02 for etching in hot agressive media like H3P04.But they have the advantage of being extremely resistant against hot concentrated alkaline solutions. Hence, silicon nitride films of a few hundred nanometer thickness are sufficient as a mask for etching through the depth of a silicon wafer (e.g. 500 pm). A typical process for silicon deep-etching, e.g. the fabrication of through-holes in 4-inch wafers or freestanding membranes, consists in the following steps: 1. Deposition of the actual mask film (Si3N4) 2. Deposition of the secondary auxiliary mask film (SOz) 3. Preparation of the primary lithographic resist mask 4. Pattern transfer from the resist mask into the Si02-film 5. Removal of the resist mask 6. Pattern transfer from the secondary auxiliary etch mask (SOz) into the deep-etch mask (Si3N4) 7. Deep-etching 8. Removal of the mask film
To avoid this complex process, other etching baths were searched for that allow the use of thinner SiOz mask films. Si02 is resistant against aqueous solutions of amines and a complexing agent, e.g. an applied bath composition is ethlendiamine with pyrocatechol as complexing agent (EDP)49.Further bath compositions contain alkylammonium hydroxides or organic bases, e .g. aqueous solutions of isopropanol and hydrazine’’. The multi-valent phenols
49
so
R.M. Finne und D.L. Klein (1967) D.B. Lee (1969)
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3 Wet-ChemicalEtching Methoak
(pyrocatechol, pyrogallol, gallate51) support the silicon removal by forming chelates of Si(1V). Like the OH--ions in inorganic strongly alkaline solutions, the phenolic O H - g r o ~ p of s ~pyrocatechol ~ and the according phenolate ions respectively, or the alcoholic OH-groups are hard donors and hence suitable ligands for the central ion Si4+being a hard Lewis acid. These ligands are also well suited to occupy the free valences of the silicon atom. Thus three pyrocatechol molecules as bivalent ligands occupy three edges of an octaeder in the six-fold coordination sphere of Si4+.The disadvantage of organic complexing agents is their easy oxidizability which makes the etching solutions very sensitive against oxidizing agents. Hence it is necessary to avoid contact to air lest the etching bath is destroyed. This low chemical stability and the toxicity of the amine vapours restrict widespread use of such bath compositions. In technological development the use of alkaline etching solutions is preferred to those of amine or phenol basis.
Influence of Bath Composition and Temperature on Etch Rates In etching baths of ethylenediamine/pyrocatechol/water the etch rate increases with the concentration of pyrocatechol in the lower concentration range. From a certain concentration onwards the etch rate does not increase further. Thus the maximum etch rate of 8.3 nm/s in ethylendiamine/watermixtures of 68:32 is reached at 14% pyrocatechol and 110 C. In changing the water content the etch rate passes a maximum. E.g., at a pyrocatechol concentration of 3gA the maximum etch rate of 6.4nm/s is observed at a mole fraction of water of 0.6 for a temperature of 100 C53.The degrees of anisotropy are 17 for the etch rate ratio of (100)- to (111)-planes and 10 for (110)- to (111)-planes. The etch rates can be strongly increased by the addition of other substances. Thus e.g., pyrazine catalizes the etching process. Also p-chinone is a catalyst for the etching reaction. This influence is rather critical as pyrocatechol is oxidized to o-chinone already by oxygen from the air and o-chinone is readily changed to p-chinone. Hence, very strongly increased etch rates can occur in aged etching baths, which makes process control difficult or impossible. Temperature influences the etch rates very strongly. The activation energies of the etching reaction estimated from lateral etch rates in EDP-solutions are dependent on the angle between the pattern edges of the etch mask and the low indicated crystallographic planes of the solid. They range from 24kJ/mol (0.25eV) for (100)-planes to 53 kJ/mole (0.55eV) for (111)-planes. These differences in the activation energies result in low etch rates at low temperatures but very high selectivities in the different crystallographic directions, so that high degrees of anisotropy can be achieved. A temperature rise increases the ~~
H. Linde und L. Austin (1992) 52 S.A. Campbell et al. (1993) 53 H. Lijwe et al. (1990); R.M. Finne and D.L. Klein (1967): 51
3.5 Crystallographic Etching
93
etching rates, but decreases the selectivity of etching in the crystallographic directions. Extrapolating the temperature dependence of the etch rates to higher temperatures indicates approximately the same etch rates for all directions at about 400 C, as the plots of the Arrhenius functions intersect in this range54. Geometries The geometries that can be fabricated by anisotropic Si-etching are determined by the choice of the mask geometry and the crystallographic cutting of the substrate, i.e. the crystallographicplane in the substrate surface. In microsystem technology the (100)- and (110)-planes are used preferentially as wafer surfaces. (111)-wafers used in microelectronics are not suitable for anisotropic etching as the slowest etching crystal plane lies in the surface. Are the etching masks positioned arbitrarily to the crystallographic orientations of the substrate, irregular structures develop with rough egdes mostly. In the flanks of the etch grooves (111)-planes are predominating, beside some smaller areas of higher indicated planes. The latter vary rapidly in shape and size because of their high etch rates. In the result of such etching processes only very imprecise patterns are preparable. Undercutting under the etching masks is tremendous. With all anisotropic etching operations where the mask edges are aligned according to the crystallographic directions, the etching results in etch grooves belonging to the crystal geometry and bounded by slowly etching (111)-planes and possibly an additional plane lying parallel to the wafer surface. An appropriate orientation of the mask edges is parrallel to the (111)-planes. Is this alignment very exact the undercutting under the mask edges is nearly completely subdued. At (100)-wafers the intersection lines of two (111)-planes with the surface are vertical to each other. Both (111)-planes intersect the wafer surface in an angle of 54.7 degrees. Square mask patterns aligned parallel to the (111)planes form therefore a frustum of a pyramid the sidewalls of which are formed by (111)-planes (fig. 3-32). Etching proceeds with a high etch rate of the (100)-planes until the (111)-sidewallsmeet. At reaching this squared pyramidal shape of the etch groove the (100)-planehas vanished. The low etch rate of the four (111)-planes allows the etch grooves only to grow very little. Hence, the size of the pyramid does not increase with overetching times, it is nearly exclusively determined by the size of the squared window in the etch mask. Etching (100)-substrates through rectangular mask windows creates grooves with trapezoidal crossections (fig. 3-33) resulting in such with triangular crossections in proceeding etching as the bottom (100)-plane vanishes. Because of their profile such grooves are called “V-grooves”. 54
H. Seidel et al. (1990)
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3 Wet-ChemicalEtching Methods
Fig. 3-32. Forming of a crystallographic etching structure in a monocrystalline Si-chip of the orientation (100) with (100)- and (111)-planes, schematically: A: the thick-lined cuboid stands for a silicon chip, the cube of weak lines shows the orientation of the crystallographic unit cell. The edges of the triangularly bordered (111)-planes are demonstrated by dotted lines. B: A square mask parallel to the intersecting lines of the (111)-planes with the substrate plane result in pyramidal etching grooves (thick lines), the sidewalls of which are formed by (111)-planes
m (100)-plane
Si( 100)-substratwith rectangular mask window
trench with
(100)-plane
trapezoidal cross section
prepared V-groove after crystallographic etch stop due to the comulete disappearance of Si (100) planes
Fig. 3-33. Crystallographicetching in Si(100): Forming of an etching groove with trapezoidal crossection and a V-groove
Does the etch mask deviate in shape or alignment from a rectangular, exactly positioned to the (111)-planes, so at first irregular and rough etch slopes develop between small (111)-areas off the corners of the etch mask. The (111)-areas grow as they etch much slower than the surrounding other planes and the mask is accordingly undercut to an immense extent possibly. In the end the (111)-planes form a regular etch groove, enlarged in comparison to the mask openings (fig. 3-34). The undercutting is the greater the more the
3.5 Crystallographic Etching
95
Fig. 3-34. Crytallographic etching in Si-(lOO) with a square etching mask that is rotated to the (111)planes: forming of a crystallographically oriented pyramidal etch groove with strongly undercut mask (schematically)
mask edges deviate from parallelity to the (111)-planes. As well with square as with circular mask windows develop pyramidal shaped etch grooves, if the etching process is carried out to the crystallographic etch stop, i.e. the intersection of the (111)-planes (fig. 3-35,3-36). The resulting undercutting is desirable for certain micromechanical applications, e .g. for getting freestanding membranes of the mask material. The surface of (110)-wafers is intersected by two families of (111)-planes in normal direction (fig. 3-37). Both families of (111)-planes include an angle of 70.53 (109.47) degrees. If etching masks with windows in form of parallelograms are prepared on both wafer surfaces, aligned identically to each other and parallel to the (111)-planes standing vertical to the wafer surface, so through channels with vertical walls are etched as etching proceeds from both
Fig. 3-35. Crytallographic etching in Si-(lOO) with a square etching mask that is rotated to the (111)-planes: forming of a crystallographically oriented pyramidal etch groove with strongly undercut mask (SEM picture)
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3 Wet-ChemicalEtching Methods
Fig. 3-36. Crystallographic etching in Si-(100) with a circular etching mask: forming of a crystallographically oriented pyramidal etch groove with strongly undercut mask (SEM picture)
A
B
Fig. 3-37. Position of the slowly etching (111)-planes in a monocrystalline Si-chip of the orientation (110), schematically: A: the thick-lined cuboid stands for a silicon chip, the cube of weak lines shows the orientation of the crystallographic unit cell. B: The edges of the triangularly bordered (111)-planes which are vertical to the (110)surface are demonstrated by dotted lines.
sides (fig. 3-38). If etching proceeds only through a window from one wafer side sloped sidewalls of 35.26 degrees develop in the acute comers of the parallelograms. The changes in the lateral etch rate in dependence on the mask alignment to the substrate orientation can be determined by test masks consisting in lines that are staggered by a small angle to each other. Plotting the lateral etch rate over the staggering angle instructive etch rate diagrammes are received, that reflect the symmetry of the crystal structure. In the (110)-plane two mirror planes stand vertically to each other, in the (100)-plane four mirror planes include an angle of 45 degrees. Surrounding the (010)-direction high etch rates occur changing significantly in dependence on the angle. The very low etch rates of the (111)-planes in concentrated alkali solutions are achieved only by exact alignment of the mask windows on (100)- and (110)-wafers, respectively, to the (111)-direction. Only with small deviations the lateral etching increases very strongly. Lateral undercutting is doubled with a deviation in the alignment angle in both directions of 15 angle minutes. The undercutting rate increases
3.5 Crystallographic Etching
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Fig. 3-38. Example of wet chemically fabricated structures with extremely high aspect ratios, etched in a Si(ll0)-wafer
from 0.05 nm/s with exact alignment to 0.43 nm/s with a deviation of 2 degrees for etching in 32 % KOH at 44 C55. At real monocrystalline substrates the geometries deviate from the expected values, especially the angles between slowly etching planes deviate in some cases by several degrees. These deviations are different for different etch bath compositions. In more complex etch solutions somewhat greater deviations were observed than in KOH- or hydrazine solutions (only 2 compon e n t ~ )This ~ ~ . fact emphasizes the necessity of individual optimization of the mask patterns and the etching processes to obtain exact geometries with anisotropic silicon etching.
Etch-StopTechniques and Doping Influence To terminate the etching process in a certain depth substrates can be used that possess a layer at which the etching process comes almost to a standstill. These etch-stop materials have etch rates in the respective etch bath that are essentially smaller than those of the material to be patterned. Is such an etch-stop material not available, the etching process has to be stopped after a certain time, i.e. the etch depth can be controlled by a time routine only. The small etch rate ratios of (111)-planes to the other crystal planes are the basis for the crystallographical etch stop. Anisotropc silicon etch processes subside, when the etch groove is etched so far that it is bounded by (111)planes only. This kind of etch stop is possible only with the few geomtries that the crystal structure itself supplies. For etch-stops in other pattern geometries, etch rate differences have to be achieved by the choice of adequate materials. The etch rates of semiconductor materials depend strongly on the respective doping materials. For silicon p-doping materials are of greater influence. So a silicon etching process can be terminated by a change in dopant kind and 55
56
H. Seidel et al. (1990) I. Stoev (1996)
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concentration. Such an etch-stop is similar in principal to using a special etchstop material. A special trait of crystallographic silicon etching is, however, that relatively small dopant amounts decrease the etch rate very efficiently. Doping with boron with a concentration of more than 5-1019 atoms/cm3lowers the silicon etch rate considerably5'. With a concentration of Id0 boron atoms/cm3 the etch rate is only 1%of the undoped silicon material. As boron with three outer-electrons possesses one electron less than silicon, being a p-dopant, the etch-stop is called p+-etch-stop.Etch-stop layers of boron-doped silicon can be epitaxially grown and built into a silicon layer package. Hence, etchable and etch-stop layers can be combined in the desired sequence without disrupting the monocrystalline character. An alternative method for creating etch-stops is the implantation of boron atoms. By bombarding the monocrystalline silicon surface with energized boron atoms the necessary concentrations for an etch stop are obtained in the silicon solid. Boron-doped surface layers can be made into membrane layers etching off the undoped silicon from the backside. As the penetration depth of the particles is strongly dependent on the kinetic energy, such an etch stop layer can be deposited in different depth beneath the surface by varying the energy of the bombarding particles. In such a way so-called buried etch-stop layers are fabricated. The near-surface material parts are etched with a high rate because of their low dopant concentration, whereas the layer beneath these parts is rich with dopant atoms having a very low etch rate, thus etching stops in this layer. Doping with other foreign atoms also can decrease the etch rate. With germanium or phosphorus as dopants a decrease in etch rate sets in only at very high dopant concentrations, and is much weaker than in the case of boron5'. If instead of p-doped areas n-doped areas shall remain an electrochemical etch-stop method is available. Is a p/n-junction with a potential of 0.6 to 1V anodically polarized, the p-doped material can be etched electrochemically while the n-doped material is not attacked.
3.5.4 Anisotropic Electrochemical and Photoelectrochemical Etching In analogy to other electrochemical etching methods, mono-crystalline metals or semiconductors can be etched anisotropically electrochemically in appropriate etch solutions without the necessity of using a tool as in micromachining. As in outer-currentless anisotropic etching the crystallo-graphic structure of the solid determines the achievable geometries. In contrast to the electrochemical etching of metals, space charge zones play additionally an essen57 58
H. Seidel et al. (1990) H. Seidel et al. (1990)
3.5 Crystallographic Etching
99
tial role in the electrochemical etching process of semiconductors. Space charge zones are responsible for the formation of deep etch grooves with extreme aspect ratios in anodically etching of semiconductors at sufficiently high current densities. Especially the technique of photoelectrochemical etching is sensitive to crystallographic conditions. Etch rate differences occur'in dependence on the crystal orientation under otherwise equal conditions with many materials and diverse etching solutions59.By illumination the probability of the charge transition through the solid surface into the electrolyte is influenced, as electrons of the valence band are raised into the conduction band by absorbing light in the upper atomic layers of the solid. The surface process controls the overall reaction rate which is an important prerequisite for anisotropic etching. The method of fabrication of small structures with high aspect ratios by anodic etching of monocrystalline material with and without light assistance was at first investigated systematically with silicon and proposed for fabricating capacitive micro-devices. The apparative assembly equals the other electro- and photoelectrochemical methods (see section 3.4.6 and 3.4.8). However the etching surface need not be illuminated directly. The method is related to the preparation of porous silicon (see section 3.4.9). Diluted hydrofluoric (2.5 %) acid is used as electrolyte for etching silicon. In such an etching process, e.g., the front and back side of an n-doped silicon wafer is exposed by a tungsten lamp. Deep holes and grooves arise under prepatterned windows in (100)-silicon substrates at current densities below 30 mA/cm3.Whereas with low dopant concentrations and low electrode potentials nearly cylindrical holes with extreme aspect ratios (e.g. 42 pm deep, diameter 0.6 pm, aspect ratio 70) are obtained, with increasing potential and increasing dopant concentration branching etch structures are observed with secondary etch channels standing preferentially perpendicular to the (111)and (100)-planes. In analogy to the anodic preparation of porous silicon origination of stuctures with extreme aspect ratios are probably due to the enhanced charge carrier concentration in the bottom region of the etch grooves and channels. In contrast to the preparation of porous silicon the position of each channel can be exactly defined by prepatterning by means of lithographic masks@.' The preparation of microtechnical etch patterns with extremely great aspect ratios is accomplished also with other semiconductor materials by anisotropic photoelectrochemical etching. E.g., hole structures were prepared in GaAs by etching in sulphuric acidic hydrogen peroxide solution6'.
For InP see P.A. Kohl et al. (1991) V. Lehmann and H. FOll (1990); S.S. Cahill et al. (1993); V. Lehmann et al. (1991) 61 J.van de Ven und H.J.P. Nabben (1990)
59
6o
100
3 Wet-ChemicalEtching Methoh
3.5.5
Porous Silicon
Beside simple etching of silicon films and monocrystalline bulk silicon, electrochemical and photoelectrochemical etching of silicon won in importance, especially for the preparation of porous silicon62.Porous silicon is an interesting material for the preparation of light emitting diodesa, the fabrication of micro-thermal devices@and for micromechanical preparations. The etching process for preparing porous silicon is basically different from isotropic and anisotropic wet-etch processes. With the first mentioned group of etching processes the preparation of porous silicon shares the lithographic definition of a part of the substrate surface. From this part the pores are generated in the silicon, i.e. in contrast to the conventional etching method the material is partly removed only and a porous solid is left back in the treated substrate area. The generated pores are not built up and spread isotropically, they possess preferential directions. The most essential preferential direction is perpendicular to the substrate surface. It is determined by the electrical field on the on hand and by the reaction direction of the etchant on the other. Secondary preferential directions are determined by the crystal structure. The spreading of etch pores takes place vertically to the (100)-planes. The transition from fine nanoporous material, material with larger pores to real vertically oriented holes can be determined by the choice of etchant, potential and dopant concentration6’. Photoelectrochemical Preparation of Porous Silicon Porous silicon is produced by the common effect of an etching bath, an electrochemical charge flow and the reaction of light. The monocrystalline substrate is connected as anode. The solid surface being immersed in the electrolyte is illuminated. Hydrofluoric acid containing etching media with an addition of detergents are used. The etch rate, the shape of the pores, and their size distribution are determined by the electrochemical conditions (potential and silicon conductivity) on the one hand and on the other by the composition of the etching bath. The mass removal in electrochemical fabrication of porous silicon increases linearly by the product of the concentration of complexing ions and the converted electrochemical power. The porosity, however, possesses a maximum in dependence on this product@. The mechanism of the pore development depends probably on the formation of the space-charge region near the solid surface. It is assumed that the
63
P. Steiner et al. (1993), W. Lang et al. (1993); R.L. Smith (1995); W. Lang (1995) A. Richter et al. (1991):
A. Drost et al. (1995); V. Lehmann und H. FOll(1990) 66 L.T. Canham (1990); G. Di Francia and A. Salerno (199 )
3.5 Crystallographic Etching
101
electrochemical transition process proceeds preferentially in the deepest parts of the pores. The positive charge carriers of the solid (holes) migrate in the electrical field to the nearest electrode parts at the borderline of the pores, while the electrical potential in the intermediate space to the farther surface parts decreases only slightly, so that the anodic partial current density in the outer areas is low as well as the related local charge flow. This mechanism has self-biassing character. The reduction of the local anodic current densities by forming micro- or nanolocal space charge regions during etching feeds back positively. The deeper a pore the more adequate are the electronic conditions for the anodic charge transition at the bottom of the pore. The longer and more fissured a small silicon bridge between the pores is the less is its charge flow in its interior. The positive feed back as basis for this model explains the origin of the pores very well. The very small initial differences in the local removal rate intensify dramatically during the process. The character of the dissolution process is typical for a spontaneous structure forming process far from thermodynamic equilibrium. A certain insight in the geometric conditions of the pore formation and the influence of material and process parameters are given by investigations of photoelectrochemical fabrication of trenches by means of mask patterns. In the range of optimum doping and moderately high potentials the formation of deep holes is observed that are well defined in shape and position. Partially they possess extremely high aspect ratios (far greater than 10). Also deep single holes of very small diameter can be produced. The diameter increases with increasing anodic current density. By increasing the potential the formation of side pores branching off the main pore is initiated. This branching process leads to more and more branched-off side pores. Whereas with only a few side pores the pore pattern shows an orientation to the crystallographic lattice, the further formation of pores leads to a pore network of a highly fractal character, to the space structure of porous silicon. The dopant concentration influences the structure of the pores as well. With low dopant concentrations a selection process takes place in which the depth and the diameter of the greater pores grow on the expense of smaller neighbouring ores. High dopant concentrations further the formation of pore branching6 P. The formation of porous silicon can be applied to very small lithographic structures if the monocrystalline silicon is treated by a convenient ion bombardment. By intensive bombardment with highly energized W-ions (0.1 to 0.175 MeV) an extensive amorphization of silicon is achieved. Using a metal mask or a focussed ion beam the amorphous range can be locally bounded. The silicon, in this way made amorphous, is inert to removal in a following anodic pore etching process and can be recrystallized by baking after the electrolytic pore formation in the non-exposed neighbouring parts of the substrate. Is the monocrystalline silicon, however, exposed to a bombardment of noble gas atoms of low energy (e.g. Ar with 30 to 50 eV) only single defects 67
V. Lehmann und H. FOll(1990)
102
3 Wet-ChemicalEtching Methoak
are produced at the surface. Such damaged Si-surfaces form essentially more rapidly denser pores in electrolytic etching than undamaged areasa.
Selective Etching of Porous Silicon By an appropriate mask technique, e.g., using thin silicon nitride films, the porous silicon can be produced in well-defined parts of a silicon surface. Time and nature of the fabrication process for porous silicon determine the kind of the pore order and the depth of the porous region. Thereby threedimensionally defined regions can be prepared that are used in consecutive microtechnical working steps. On the surface of nanoporous silicon, thin films and stacks of thin films can be deposited and micropatterned, so that the fabrication of nearly any microtechnical element is possible. By selective removal of the porous silicon freestanding mechanically movable or thermally isolated thin film structures can be prepared. Silicon carbide, polysilicon or gold are applicable as mask material. Nanoporous silicon is etched with high selectivity against bulk silicon. The large surface is an excellent precondition for rapid etching. Interfacecontrolled dissolution processes proceed much quicker than with massive material. Besides, in nanopores there are many areas of the monocrystal that have other orientations than the (111)-orientation, and furthermore a lot of edges that promote the etch attack. Hence, no etch-stop by (111)-planes can occur. These properties make the porous silicon an interesting material for sacrificial techniques necessary especially in micromechanics (see also section 3.6.3). According to the preparation process of the porous silicon thin sacrificial areas (only a few pm) or deep areas can be produced. Only 1% KOHsolution supplies sufficiently high etch rates at room t e m p e r a t ~ r e ~ ~ . The etch rates of massive silicon are under these conditions negligible. The following table shows the extreme rate differences in concentrated KOHsolution, a typical silicon deep-etch bath, for porous silicon, silicon and other microtechnical materials at an elevated bath temperature. Table 3-4. Etch Rates of Porous Silicon and other Si-Containing Microtechnical Materials in Comparison (40% KOH-Solution, 60°C)
Material
Formula
Etch Rate
LPCVD-nitride thermal oxide (100)-silicon porous silicon
si3N
0.0004 n m / s 0.02 nm/s 5.6 nm/s >150 n m / s
69
SiOz Si Si
S.P. Duttagupta et al. (1995) W. Lang et al. (1993) und (1995)
3.5 Crystallographic Etching
103
The sacrificial technique using porous silicon stands at the border line between surface and bulk micromachining. Porous silicon is a material that is generated from the surface and possibly forms a layer only, but it is formed by the substrate material and can comprise the thickness range of the substrate.
3.5.6 Anisotropic Etching of Compound Semiconductors Crystallographic etching plays also a role for patterning compound semiconductors, especially IIW-materials. In contrast to silicon, strongly orientationdependent etch rates are found in acidic etching media, e.g., for InP in concentrated hydrochloric acid or in hydrochloric or acetic acid solutions of hydrogenperoxide. The etch rates in <100>-direction are essentially higher than in
-direction as in the case of sili~on’~. According to the compound character the curve of the etch rate over the crystal orientation is less symmetrical than for silicon. The distribution of the etch rates are more complicated as the crystal symmetry of the cubic lattice is reduced in comparison with silicon due to the fact that the lattice places of the elementary cell are occupied by different kinds of atoms. Compared to the homogeneous monocrystalline material the number of identical crystal planes is smaller, i.e. more different cutting planes are available. There is only one mirror axis in the polar diagram of Id‘-patterning, e.g., in methyl bromide. The reduced geometry in cubic lattices is the cause for the fact that (1’11)- and (11’1)-planes possess equal etch rates which are very small in comparison to etch rates of the (1’11’)- and the (11’1’)-planesthat also have equal etch rates. The shapes of the anisotropically prepared etch grooves are more varied in compound semiconductors than in silicon, as the crystal structures are more complex. Beside the well-known shapes of the cubic crystals there are combinations of flat and rounded areas. These are due to partial anisotropic etching processes in which on the one hand crystallographic planes are developed, e.g., the slowly etching (111)-plane, on the other hand occur areas in the same etching process the shape of which is produced by isotropic etching of various crystallographic planes that differ only little or not at all in their etch rate and hence curved boundary areas are formed (fig. 3-39). For the complicated etch rate ratios in compound semiconductors, electronic, i.e. bonding properties of the different atoms in the crystal are discussed. In the -direction of A(III)/B(V)-semiconductors, planes of trivalent (“A”) and pentavalent (“B”) atoms lie alternating over each other. In other directions both kinds of atoms lie in one plane. The geometry of the orbitals of the atoms lying in the surface plane are according to the respective crystal orientation more or less sensitive to nucleophilic or electrophilic attack, which affects the intensity of the anodic partial process (nucleophilic O ’
F. Decker et al. (1984); P. Rosch (1992)
104
3 Wet-Chemical Etching Methods
(11'0)
(11'1)
Sulfuric acid, 104 Potassium iodide; I&; (IlO)-SubStrat
HCW (11'0)
Substrat
11'1)
Bromine m
mnhaoolel InF' (lIO>Substrat
phosphoric Acid, Hydrogen Peroxide/ GaAs (1lO)Substrat
Fig. 3-39. m i c a 1 forms of etch grooves formed by compound semiconductors (GaAs and IuP in various etching solutions, I? Rotsch 1992)
.2,
attack of the ligands or of the cathodic partial process (electrophilicattack of the oxidizing agent) . Anisotropic etching of compound semiconductors is of importance for the fabrication of optoelectronic devices. It is used for the definition of reflector edges, for chip separation and the preparation of microchannels. Like monocrystalline silicon, compound semiconductors can be anodically etched to form micro- or nanoporous materials. Pore formation was observed, e.g., treating InP in hydrochloric acid72,GaP in sulphuric and GaAs in hydrochloric Treating Si-doped GaAs(100) in 0.1 molar hydrochloric acid, fissured pores are obtained with typical pore width in the sub-pm range at an anodic potential of 6 V (vs SCE). Preparations of broken edges show that these pores possess a preferential orientation perpendicular to the surface.
'' H. Lijwe et al. (1990) N.G. Ferreira et al. (1995) B.H. Erne et al. (1995) 74 P. Schmuki et al. (1996) 72
73
3.6 Preparation of Free-Standing Micropatterns
105
3.6 Preparation of Free-Standing Micropatterns 3.6.1 Surface Micromachining Free-standing micropatterns have a variety of applications in various devices in microsystem technology. Free-standing elements are needed where micropatterns have to be flexible or thermally isolated from the surroundings. Various kinds of miniaturized cantilevers, springs etc. are applied in many mechanical sensors and actuators. Free-standing patterns are of interest if small masses, thermal capacities, thermal conductivities or double-sided contacts of thin films with gaseous or liquid surroundings are necessary, as e.g., in the case of some thermal and chemical sensor components. All these applications require the technique of removing a sacrificial material to prepare free-standing patterns. The subsequently free-standing material is deposited as a film on a substrate surface in most cases. If the sacrificial material is deposited in form of a thin film and is etched from the front side of the substrate, the process is called surface micromachining. With this technique bridges and cantilevers can be prepared in an over-lying film by etching isotropically in the sacrificial film and undercutting the sides of the overlying material. The etchant must possess a lateral etching component (fig3-40). Besides, it must be selective lest the free-standing material acting as mask material is attacked. The broader the pattern that is to become free-standing the longer lasts the undercutting process. Principally isotropical dry-etch processes can be used for preparing free-standing patterns. But a very high selectivity and an acceptable rate are necessary. Typically free-standing patterns are prepared as follows: 1. Deposition of the sacrificial layer 2. Deposition of the layer of the later free-standing material (functional layer) 3. Preparing an etch mask for the functional layer (This mask contains the lateral geometries of the free-standing elements, e.g., bridges or cantilevers.) 4. Etching of the functional layer 5. Removal of the etch mask of the functional layer 6. Etching and time-controlled over-etching of the sacrificial layer (The overetch time is determined by the width of the functional pattern and the lateral etch rate.) Free-standing micropatterns prepared by surface micromachining are available in the whole lithographically possible size range. The material spectrum reaches from silicon and silicon dioxide to metals and even to polymers (fig3-41). Microbridges have been prepared with thicknesses smaller than 0.1 pm up to a few micrometres and with widths down to the ~ub-pm-range~~. The 75
J.M. Kohler (1992)
106
3 Wet-ChemicalEtching Methods
Fig. 3-40.Surface micro machining: Example for forming free-standing thin film structures by selective isotropical etching of an sacrificial film
Fig. 3-41. Free-standing thin film tongues of titanium (SEM-picture)
shape of free-standing patterns, especially of cantilevers that are fastened only on one side, is determined beside the mask measures by possible gradients in tension in the film materials. Mostly the tension acts vertically to the substrate surface causing bending in normal direction. More rare are lateral distortions. These bendings are undesirable in most cases. But for some mechanical actuators or sensors they are produced purposely and used for signal transducing.
3.6 Preparation of Free-Standing Micropatterns
107
The preparation of flexible structures by wet etching is made difficult by sticking of the structures to the substrate surface after removing the sacrificial layer. This sticking is caused by adhesion forces being as a rule stronger than the mechanical resetting forces. The contact of the flexible micropattern with the substrate surface is mediated by the rinsing agent after the sacrificial layer etching. The liquid residues gather in the capillary gap between the freestanding element and the substrate. During drying of the last liquid volume, the surface tension of the liquid can attract the flexible structures so near to the substrate surface that the flexible structure touches the substrate and the adhesive forces between both solids gain effect. Sticking can be averted by the surroundings during the drying process. As a rule the possibility of the phase transformation solid-gaseous is chosen. Sacrificial layers can be removed, e.g., in an isotropically and selectively etching vapour or plasma, so that the wet chemical removal is avoided. In the more customary wet chemical sacrificial layer etching the rinsing liquid can be substituted by such a liquid that solidifies, if necessary by cooling, and sublimes upon heating. Sublimation is aided by a moderate vacuum. By this the capillary gap is emptied without the influence of capillary forces typical for liquids. In certain cases, the undesired adhesion of free standing structures can be avoided by additional beams fixing the movable structural elements during the last solvent evaporation step. This supporting beams can be cut by focused ion beams76.
3.6.2 Bulk Micromachining As sacrificial material, parts of the substrate itself can be removed instead of a specially deposited layer. In this case greater etch depth in the substrate material are produced. This method is called bulk micromachining in contrast to surface micromachining. The etching of greater depth in the substrate or the complete through-etching of the substrate is also called deep-etching (see also section 3.5.3). If the substrate is etched isotropically the free-standing structure can be etched from the front side as in surface micromachining (fig 3-42). Only as soon as the structure is completely undercut an anisotropically etching medium can be applied. In this way free-standing structures can be prepared over grooves with smooth side walls and the typical angle of 54.7 degrees using (lOO)-Si-wafers. With (110)- Si-wafers, even vertical sidewalls can be achieved under free-standing elements (fig. 3-43). Frequently (lOO)-Si-wafers are used for micromechanical preparations. If anisotropically etching free-standing elements from the front side, convex corners in the mask pattern are a prerequisite. Hence, bridges cannot be undercut in this method as (111)-planes are formed stopping the undercutting of the mask. Cantilevers can be etched, because the etching can proceed from the convex corners. 76 J .
H. Daniel et al. (1997)
108
3 Wet-ChemicalEtching Methods
Lithographic mask at front and back side
,cantilever at front side ' in the front side mask
window in the back side mask for deep etching free standing Result after bulk etching: hole in th substrate
frame of bulk material
Fig. 3-42.Fabrication of freestanding thin film structures by etching a substrate as sacrificial material (schematically)
fig. 3-43. Bulk micromachining: Forming of a free-standing microstructure by at first selective isotropical and subsequently selective anisotropic etching of substrate material (Si(ll0))
In bulk micromachining the substrate can be etched from the back side to accomplish free-standing patterns. The mask windows on the back side are positioned opposite the site of the structure on the fi-ont side that is to be freeetched. In the mask windows the whole substrate thickness is removed. Thus a window is etched in the substrate that is covered by the free-standing element only. With this technique unpatterned, i.e. uniform thin film membranes can be produced. Such membranes are widely used, e.g., as carriers for masks in X-ray lithography, micromechanical, microthermal or other microdevices (fig.3-44).
3.6 Preparation of Free-Standing Micropatterns
109
Fig. 3-44. Backside of a thin film sensor (thermoelectric IR-radiation sensor with with deep-etched window in a silicon chip SiOz/Si3NJSiO2-membrane)
3.6.3 Porous Silicon as Sacrificial Material Porous Silicon is used very advantageously as sacrificial material in micromachiningn. The porous silicon is prepared by an electrochemical or photoelectrochemical etching process (see section 3.5.6). Because of its greatly enhanced etch rate in comparison to massive silicon the region to be removed can be limited without observing a strict time regime in etching. Also more complex geometries in the sacrificial areas can be prepared independently of the shape of the free-standing element to be prepared, i.e. the use of porous silicon allows other geometric shapes than customary bulk and surface micromachining. The working steps for preparing free-standing structures using porous silicon are as follows (fig. 3-45):
1. Fabrication of the etch mask with the lateral shape of the porous silicon regions, with consideration of lateral expansion of the porous regions under the mask edges in dependence on etch depth, crystal orientation and running the process 2. Preparing the porous silicon by electro- or photoelectrochemical etching 3. Removing of the primary mask layer (The porous silicon lies open in the etched areas on the substrate surface.) 4. Deposition of the functional layer covering porous and massive silicon 5. Etching of the functional layer with an appropriate etch mask, possibly deposition and patterning of further functional layers 71
W. Lang et al. (1993) & (1995); R.L. Smith (1995)
110
3 Wet-ChemicalEtching Methods
free
-
Fig. 3-45. Working steps for fabricating free-standing thin film structures using porous silicon as selectively etchable sacrificial material
6. Dissolving of the porous silicon, i.e. the free-etching of the functional structures
Because of the high selectivity of the etching of porous silicon to massive silicon thin free-standing structures of massive silicon can be prepared without any auxiliary masks, e.g., bridges of 0.5 pm thickness over 80 pm deep gaps7'.
'* W. Lang et al. (1993) & (1994); R.L. Smith (1995)
4 Dry-Etching Methods
4.1 Removal at the Interface Solid-Gas All etching methods in which material is removed from the solid surface directly into the gas phase, are subsumed under the term “dry-etching method”, to which belong plasma chemical etching, sputter etching and ion beam etching. In this sense also the thermally activated removal of materials from surfaces by electrons or laser beams or simple heating can be added. In dry-etching as well as in wet-etching processes, the transition of solid material through a phase boundary is the decisive, characteristic process step. For this purpose the material to be removed has to be changed in a physical or chemical process into single atoms or molecules, radicals and clusters of a few atoms that can desorb from the surface getting into the mobile phase. In contrast to the liquid phase the particle densities and hence the concentrations of reactive chemical components are much less in the gas phase, especially at low pressures as they are necessary for sputter and ion beam etching processes. Instead, there are more efficient transport mechanisms in the gas phase. Whereas in wet-chemical etching at least in the near-surface area, a diffusion step does take place, the etching particles from the gas phase can be brought to the surface by a directed movement. The only precondition is that the directed movement can be produced in such a distance from the surface that the particles can reach the surface without loosing their direction characteristics by collisions with other particles. Another advantage of a dry-etching process is the possibility to accelerate the particles very strongly in the gas volume at reduced pressure. Such accelerated particles can get such high kinetic energies that exceed the energies of chemical bonds by far. The mechanical momentum transfer can become the decisive component in the etching process in the gaseous phase. The achievable energies are dependent on the free paths of the particles and hence are essentially determined by the pressure in the gas volume. The gas volume offers a third essential peculiarity compared to the liquid phase: In the gas phase plasmas with high proportions of extremely reactive species can be formed by coupling energy into the plasma from the outside. Whereas in the liquid phase electronically excited particles quickly relax or react due to the high particle density and the constant impact processes, a
112
4 Dry-Etching Methods
considerable part of the present particles can exist in form of very reactive radicals, electronically or vibrationally excited particles in the gas phase. The increase of the medium particle energy by a single vibrational level (10 to 50 kJ/mol) only is an essential quantity for decreasing the activation energies. Electronically excited particles possess a molar energy elevated in comparison to their ground state by about 150 to 400 kJ/mol, the range of chemical binding energies of many substances. Hence such particles possess a much higher reaction probability on contact with the solid surface than respective nonactivated atoms or molecules. That explains that inspite of the low particle density in the gas volume compared to the liquid phase with coupling in an appropriate power, high reaction rates and acceptable etching rates are achievable. Two principally different mechanisms are available for the removal reaction at the interface solid-gas. Both react in many etching processes at the same time. By the first, the particles can be transferred into the gaseous phase by an mechanical impulse (sputter effect, see section 4.4.1). Subsequently they must be removed by convection from the gas volume to avoid redeposition on the substrate surface. That is accomplished by evacuating the recipient. By the other mechanism, the particles are changed into volatile species that transit into the gaseous phase. Also in this case the reaction product has to be removed from the gas volume. The contact with the surface does not cause redeposition in any case, but with a certain probability surface reactions take place that form the reactants or slightly volatile compounds, that deposit on the surface or at the side walls of the etched patterns. Such secondary deposition processes are the more probable the higher the concentration of the reaction products in the gaseous phase, in many dry-etch processes. With the increase in their concentration the concentration of the etch-reactive species or its preliminary stages is lowered. Thus a contiuous supply of reactive components and a gas flow through the recipient is necessary. Beside in mechanical induced, i.e., sputter processes, the desorbabilityof the etching products is decisive for the success of a dry-etching process. Desorbability is especially important for purely chemical dry-etching processes. The general demands on the reactive etching gas are the following: 1. To obtain desorbable species, compounds must be available in the gas volume which the material to be removed forms a chemical species of high vapour pressure with. 2. The etching gas or the mixture of etching gases must be so reactive that the material can react in technologically adequate times. Frequently it is necessary to remove surface films that had been formed in preliminary processes or at the atmosphere, e.g. oxidic coating films on metals or semiconductors. As the metals or semiconductors of a functional film etching in a certain etching gas frequently supply the the same etch products as their oxides or hydroxides, many etching processes are able to remove these coating films automatically. The rates of coating layer removal, however,
4.1 Removal at the Interface Solid-Gas
113
can be much less than those of the functional films. If necessary such coating films must be removed in a seperate etching step with a specific gas composition. 3. Side products that are difficult to desorb must not be formed, because they would enrich at the surface and mask the parts of the surface to be removed impeding the etch attack in this areas. In most of the patterning processes high selectivity to other materials that lie also open during the etching process is desired. That is especially important for the material lying under the etching material, as it always gets open to the etch reactant in the end phase of the process. In many etching processes a further requirement on the etching gas is a certain selectivity in direction (anisotroPY) * The desorbability of etching products is determined by its chemical nature. High desorbability is correlated as a rule with high vapour pressures and low boiling temperatures. Especially low molecular weight species possess high desorbability and vapour pressures, if their intermolecular forces are comparably low. These properties are found with hydrides and some oxides and halides of non-metals. For microtechniques, however, beside polymers and carbon, semiconductors and metals as well as their compounds and alloys are of importance. Metal and semiconductor oxides, salts of oxoacids, also higher chalkogenides and binary compounds of metals and semimetals of the IV., V., and VI.main group are highly molecular and hence difficultly desorb. In the case of metals and semiconductors, especially, hydrides, organyles, and halides, partly oxohalides also have low boiling points. Hence, these classes of substances are the most important products in dry-etching processes. Because of the competitive situation of oxygen, contained in our atmosphere, preferentially elementarily, and in the reactive medium water and many other compounds, fluor plays a like important role in dry etching as in wet etching. Also in dry etching is fluor the only element that can displace oxygen in stable compounds with hard cationic components. Furthermore fluor forms the lightest monomolecular binary compounds beside the hydrides. Whereas halides in general belong to the easily desorbable substances, fluorides of hard cationic components desorb still better than the other halides in most cases. The higher halides play as etching gases a role for such materials consisting of elements that possess slightly to somewhat stronger polarizable oxidation states. Important easily desorbable compounds formed by materials interesting for microtechnical applications are listed in section 6. In dry etching the fluor is preferred in form of fluor-substituted lowmolecular aliphates. The most used fluoro-hydrocarbons are CF,, CHF, and GF,. The carbon contained in the etching gases can be eliminated either in form of fluor-containing radicals or its condensation products (fluorsubstituted higher aliphates) . The presence of carbon, however, can cause the formation of organic polymers, carbides or diamond-like depositions. Such secondary surface films contaminate the etching material and form, especially in ion-assisted etching, masking layers, bringing the etching process to a
114
4 Dry-Etching Methods
standstill. Such depositions are sometimes technologically desired to impede lateral etching. But they disturb in most cases. Disturbing secondary contaminations by carbon compounds can be reduced by appropriate additions to the etching gas. By addition of hydrogen to the etching gas the fluorine atoms can be substituted by hydrogen atoms forming smaller molecules as side products. If oxygen is tolerated by the etching material, oxygen or water can be of assistance forming CO or C 0 2 from the carbon-containing etching gas. As these are gaseous at high pressures and low temperatures, they can be pumped off very easily. To avoid depositions, sulphur compounds of fluorine, especially SF6,can be applied instead of fluorinated hydrocarbons. Sulphur in elementary form evaporates under normal pressure at 444.6"C and hence is better to desorb than carbon. In hydrogen-containing atmosphere H2S can be formed (Bp. 60.75 "C). Also sulphur fluorides with increased su1phur:fluorine ratio are easily desorbable. (Bp.: SF, -40.4"C, FSSF -15"C, SSF, -10.6"C, F5SSF5 29.25 "C, FSSF339 0C)79.In the presence of oxygen, desorbing sulphur oxides are formed if they do not react with the etching material forming salts. In some cases chlorides of microtechnical materials (e.g. Al) are better desorbable than fluorides. For some metals the boiling points of halogenides decrease with increasing periode. In such cases chlorine and chlorinated hydrocarbons as well as bromine- and iodine-substituted hydrocarbons are of interest as etching gases. A complicating factor is, that the boiling points of the respective halgenohydrocarbons that can act as halogen donator (Bp CH2C1240.2 "C, CH2Br295 "C, CH2J2181 "C)" rise with increasing molecular weight. Halogenoalkanes and other volatile or gaseous molecules rich in chlorine like NC13, BC13and SiC1, are used in dry-etching techniques as chlorine donator. Highly reactive are elementary halogens, interhalogens and rare gas halides. All these compounds have the advantage of being strong oxidizing agents. Compounds like ClF3 and XeF3 are preferentially applied as etching gases in chemical etching processes without plasma enhancing". Both substances are extremely good fluorine donators. For only slightly etchable alloys and for especially gentle preparation, reductive plasmas with hydrogen or alkane atmospheres are applied to form hydrides or alkyles of the material to be removedg2. Organic polmers like all hydrocarbons can be etched easily in oxygencontaining atmospheres. As well the gaseous carbon oxides CO an C 0 2as the water formed by the hydrogen make a quick transition into the gas phase and are pumped off. Hence, all organic photoresists, among them the mostly applied resists on novolak basis, but also those in the UV-and the electron beam technique preferred resists on methacrylate basis are easily etched in an
79
8o
A.F. Hollemann und E. Wiberg (1985), 491 H. Bayer (1968), 106 Y. Saito et al. (1991) VJ. Law et al. (1991)
4.1 Removal at the Interface Solid-Gas
115
oxygen atmosphere. Also nitrogen contained in several materials, e.g. polyimides, in form of amino-, imino- or nitro- groups is removed without problems, as it is gaseous or easily sublimed in elementary or reduced form (NH,) as well as in oxidic form (N20, NO, NOz,N205). In the following sections (4.2 to 4.4) the most important dry-etching methods are described. Also some less customary methods are presented. The character of the removal process and the kind of the formation of the desorbing species are the basis for the organization. This organization is different from other systematic arrangements that classify the methods according to the kind of reactor used. In opposition to such a classification the chosen organization has the advantage to classify the processes according to their molecular mechanisms. Hence, the customary classification used for wet-etching processes is applied to dry-etching processes as well, so that both can be considered in the same way. At first the chemically activated processes are dealt with. Among them the thermally and the photochemically activated processes are of greater importance (section 4.2). The following chapter (4.3) deals with electronically generated plasmas (“cold plasmas”). In these plasmas reactive plasma particles are the decisive reaction partners for the etching process, i.e.the inner character of these particles is crucial not the especially high kinetic energy. To this process group belong all plasmachemical etching processes in the stricter sense including the so-called down stream etching. In a third chapter (4.4) those methods are summarized, in which the particles with high kinetic energy are the decisive reaction partners for velocity and quality of the etching process. The energized particles are also generated in cold plasmas, where the plasma-chemical reaction step, as far as necessary for the method, must be aided by the impact of quick particles on the solid surface to be removed. To the methods, the etching with energized particles is essential for, belong e.g. sputter etching and the related ion milling, reactive ion etching (RIE), ion beam etching (IBE), reactive ion beam etching (RIBE) and the respective magnetron-biased variants of the methods. As for many aspects ion etching and ion beam etching are typical, both variants are dealt with as example for analogous methods (e.g. etching with energized neutrals or fast radicals). As a special variant etching with energized clusters and other fast multi-atomic particles with sizes in the nanometer range are counted to this group of methods (section 4.4.11).
116
4 Dry-Etching Methodr
4.2 Plasma-Free Etching in the Gas Phase 4.2.1 Plasma-Free Dry-Etching with Reactive Gases Reactive gases are used as “dry” etchants for microtechnical etching processes. The kinetic boundary conditions, however differ immensely from etching in the liquid phase. At normal pressure the particle density in the gas volume is about a thousandth of that in the liquid phase, so that even with high molar fractions the volume concentration of a etching species in the gas phase is comparatively small. For many materials, etching processes in the gas phase with technologically acceptable rates can be achieved only by the participation of energized atoms, molecules and ions or highly reactive particles (e.g. radicals). Higher densities of such particles can be produced in the cold plasmas (section 4.3). For some microtechnically relevant materials there are etching processes that have acceptable etch rates without plasma or accelerated ions. Precondition for this is a high reaction probability between the material to be etched and the etching gas. Indispensable is the formation of products rapidly desorbing off the surface into the gas phase. If necessary, the surface to be etched is heated to obtain sufficiently high etch rates. The reaction rates are frequently directly dependent on the concentration of the etching species in the gas volume. Hence the pressures must not be too small to offer sufficient reaction partners. As result of the reaction, products are desired that already under normal conditions are gaseous or can evaporate at a moderate temperature increase or a soft vacuum. Etching with reactive gases is performed in a vacuum reactor provided with a gas system and a heatable sample table (fig. 4-46). The decreased pressure ensures sufficiently high desorption rates for the formed reaction products getting off the surface. The choice of the pressure is a compromise between the supply of reactants in the gas phase according to the desire for high concentrations and hence high pressures and the decrease of boiling and sublimation temperature of the desorbing species by means of a low total pressure. The surface reactions can be accelerated and the desorption process thermally assisted by heating the substrate table. In some cases analytical facilities like mass spectrometers are attached to the reactor for process control1 or investigation~~~. Etching in reactive vapours is applied beside for micropatterning for removing surface films. It is possible to remove native oxide films off silicon surfaces in water-free HF-vapours without attacking the silicon itselfw. After such a process the binding sites of the cleaned substrate surface are saturated with fluorine atoms which can be split off by UV-exposure.
ffl
Y. Saito et al. (1991) N. Miki et al. (1990)
4.2 Plasma-Free Etching in the Gas Phase
117
spectrometry probe for gas analysls
-b
vacuumsystem
I
Normally a mask is used in etching with reactive gases that in complete analogy to wet-etching processes prevents the attack of surface parts by the corrosive atmosphere. Because of the high aggressivity of the etching gases these masks must be especially stable. In 1966 vapour etching of silicon was proposed by P.J.Holmes and J.E.Sne11E5.The observed etch rates were comparable to etch rates in hydrofluoric acid. For plasma-free dry-etching of metals and semiconductors etching gases must be used that supply the elements for forming desorbable products being at the same time strong oxidizing agents. These demands are satisfied by three classes of halogen compounds: 0 0
0
elementary halides, interhalides and rare gas halides.
Among the halogen fluorides ClF3is the strongest oxidizing and fluorizing agent (Bp.11.75 O C ) % . It is well suited for etching elementary silicon, whereas SO2,Al and stainless steel are not attacked. It possesses an essentially higher vapour pressure (p2,, c= 1140 t01-r)~’than XeF2 (subl. 120 C, pu c= 4.6 torr) also used for vapour etching. Also XeF6(p25 = 28 torr) and XeOF4(p25 C= 4.6 torr)@possess vapour pressures suitable for etching.
Etching with Reactive Gases Using Catalytic Masks The use of catalytically reacting masks is a special case of microlithographic etching with reactive gases. At high temperatures the solubility of certain film materials in others is considerably enhanced. Under such conditions a mates P.J. Holmes und J.E. Snell (1966) 86
A.E Holleman und E. Wiberg (1985), 417 Y.Saito et al. (1991) Y.Saito et al. (1991)
118
4 Dry-Etching Methods
rial to be patterned can diffuse into a coating layer. If the dissolved material is consumed at the surface by a chemical reaction with components of the gas phase, a concentration gradient in the coating film is generated causing the removal of the underlying material. If the coating material catalizes the surface reaction, the removal rate can be essentially greater than without the coating film. This is lithographically used covering areas to be removed with the coating material, while the others stay open. Hence, with such catalytic coating film masks negative patterning processes are performed. This method is used in the iron-catalyzed reductive high-temperature micropatterning of diamondm.
4.2.2 Photo-Assisted Dry-Etching with Reactive Gases Instead of purely thermal activation, etching with reactive gases can be initiated and accelerated by light. Such a mechanism is used in photo-assisted dryetching. Light can activate chemical reactions in two ways. Light absorption can cause directly electronically excited states of molecules on the surface or the released heat causes as a rule an increase of all reaction rates and hence the rates of desorption of the etch products off the surface. Conventional light sources with high radiation intensity (halogen lamps or mercury high-pressure lamps) as well as lasers are applied in photo-assisted etching in the gas phase. In contrast to plasma etching methods in photo-assisted vapour etching the especially reactive etching species are generated only in the cone of the light used for etching and not in the whole gas volume. In many cases the formation of active species proceeds due to special absorption conditions of the light and the adsorption properties of the etching gases only on the surface of the solid to be etched. As with simple chemichal etching in the gas phase the use or generation of very reactive gases is a precondition for the realization of technically sufficiently high etch rates in photo-assisted etching. Hence preferentially halogens and their compounds are used for etching metals, semiconductors and their alloys and compounds. Beside the halogens themselves multiple or perhalogenated hydrocarbons are used from which halogen radicals can be formed. For this formation photolysis with light of short wavelength is necessary. High intensities of such short wavelengths are obtained with excimer lasers as light source. E.g., silicon can be etched exposed to a krypton fluoride laser (248 nm emission wavelength) in monochloro-pentafluoroethan. With pulse frequencies of 0.1 kHz, etch rates about 20 n m / s are obtainedg0.A volume etch rate of about 4 - lo4 pm3/s was obtained with a Nd:YAG-laser (532 nm) with a pulse power density of 32 h4W/cm2 in etching silicong*.In V.G. Ralchenko et al. (1993) S.D. Russell und D.A. Sexton (1990) 91 A. Schumacher et al. (1996) 89
4.2 Plasma-Free Etching in the Gas Phase
119
principle, the etching with reactive gases can be supported by electromagnetical radiation of much higher energy, too. R. Inanami et al. studied the etching of Si wafers in a CF, atmosphere using synchrotron radiation. They found etch depths up to about 1micron in 0.16 torr CF, ,if a negative bias potential was applied on the silicon%.
4.2.3 Directly Writing Micropatterning by Laser Scanning Etching By means of lasers high light intensities can be focused on very small areas. Thus etching processes can be activated very efficiently. By focusing the laser beam, microstructurescan be produced directly. Is the laser beam deflected in one direction lines can be written and areas can be put together by lines. Hence laser-assisted etching can be used as a directly writing lithography without the use of etching masks. For directly writing laser etching a vacuum chamber is used that is connected to a gas supply and a vacuum system as in simple vapour etching. Additionally an exposing facility and a mechanical positioning device are needed. The activating laser beam can be switched on or off by an aperture. This beam blanking is necessary to produce exposed and unexposed areas. The laser beam can be deflected in x- and y-direction, so each surface element can be exposed to the beam. By precise positioning in z-direction the laser beam is focused at the surface to realize a high resolution and a high energy density (fig. 4-47). The scope of deflection of the laser beam in x- and y-direction is too small to write over the whole substrate area, therefore the substrate table can be moved in these directions, too. The functional material is transferred into the gas volume, locally limited by the reaction of the activated gas under the laser beam. The formed etching products are transported by the vacuum system out of the gas volume. Extremely high etch rates can be achieved by the method. Laser-assisted directly-writing silicon 3d-micromachining was carried out in chlorine with volume etch rates of 0.1 mm3/s at scan rates of 7.5 mm/s and a resolution of 15 pm. At lower scan rates and using objectives of high numeric aperture (NA = 0.5) sub-pm-structuresdown to 0.2 pm were written93. A simple alternative to reactive laser-assisted etching is the thermal laser ablation. Instead of a reactive etching process a simple thermal evaporation or sublimation process takes place. At appropriate power densities of the laser radiation high surface temperatures are produced being able to evaporize slightly evaporizable materials like silicon, The method has the advantage of not needing a special gas supply equipment. The generated structures, however, possess rough edges and the high condensation tendency of the primary ablation products is disadvantageous, too. As no chemical changes take place
92
R. Inanami et al. (1998)
93
T.M. Bloomstein and D.J. Ehriich (1993); A. Schumacher et al. (1996)
120
4 Dry-Etching Methods
Fig. 4-47. Device for directly writing laser supported etching (principle)
into volatile products as a rule in laser ablation, the removed material condensates in the surroundings of the ablation area. The consequence are disturbances by this material at the side walls of the etched patterns. Such disturbances cannot be tolerated in microlithographic multi-layer processes. Whereas laser ablation is not used for microlithographic structuring, it is a frequently applied tool for subsequent operations at individual rnicropatterns. Especially laser ablation is used for cutting through conducting tracks (laser trimming), i.e., for changing the geometry of single thin film resistors and hence exactly determining resistance values in special electrical circuits.
4.2.4 Electron-Beam-AssistedVapour Etching A special variant of etching in plasma-free vapours is the electron-beamassisted etching. Similar to photo-assisted etching the surface is activated by an additional energy source. Instead of photons electrons with kinetic energies above the chemical binding energies activate the reaction. In its mode of operation the electron-assisted etching is related to ion etching techniques. As with those the interaction of the energized electrons with the solid surface is largely a chemically unspecific process. The reaction is initiated by the ionizing effect of fast electrons and the input of high mechanical energy into the solid. Furthermore electronic effects like the release of secondary electrons
4.2 Plasma-Free Etching in the Gas Phase
121
and the relaxation of electron sheaths of the target materials having specific chemical reactions as consequence, as e.g., fragmentation and crosslinking reactions, contribute to the activation of the reaction. Because electron beams can be focused very well it is possible to write directly with them. The necessary equipment is in its structure similar to electron beam exposure devices, applied in fabricating microlithographic masks. The beam source is an electron-optical column, the exposure chamber with the substrate table is performed as an etching reactor. The energies of the electrons are several hundred to several ten thousand electron volts. These energies are a multiple of chemical binding energies. Frequently electron beam energies of 20 to 30 keV are used. By these even electrons are struck off the inner electron shells of the atoms with such high energies that they can ionize neighbouring atoms. These ionization processes start cascades of consecutive ionizations spreading into the solid near the impacting electron beam. Thus an excited area is formed that possesses a multiple diameter of the primary electron beam diameter. These area is the greater the higher the energy of the electron beam and the lower the medium atom mass of the film to be etched. For getting a high reaction rate it is important that spontaneously desorbing chemical species are formed out of the target material by excitation with the electron beam. Therefore it is necessary that molecules are provided in the gas volume that rapidly react with the activated surface forming such desorbable species. Electron beams can be focused to spots of a few nanometers diameter. Thus principally, lateral resolutions smaller than the light wave length are achievable which in extreme cases include only a few or even single molecules. Such high resolutions are only possible under very specific conditions. For direct patterning in the 10-nm range electron beam-assisted etching techniques with focussed low-energized electrons (10 eV to 1keV) are developed where the excited area is so small that the desired resolution can be achieved94.As the electron beam is focussed only a comparably short time is available for the reaction because of productivity reasons. If an area of 0.1 pm2is activated the electron beam can only stay for 1 ps on one area element for writing on a whole wafer area of 50 cm2within about 14 hours. That the electron beam can get through the equipment and the substrate is saved from contaminations and the heated electrode from corrosive chemical processes, a vacuum of 10-6 torr is necessary. Hence, it is nearly impossible to etch thicker films in acceptable times with this method. So the method cannot be used for mass production. For very small structures and thin films the method is rather elegantly as one does not need a mask which would mean as a rule deviations in measure. The equipment has to be furnished with a vacuum system and a gas supply system for the etching gases. According to the principle of electron microscopes the beam is deflected in a magnetic field thus scanning the sample. Relatively large areas of the sample (mm-range) can be scanned without mov94
H.P. Gillis et al. (1992)
122
4 Dry-Etching Methods
ing the substrate table. By electronic beam blanking the exposure of areas is avoided that are scanned but shall not be excited.
4.3 Plasma-Etching Methods 4.3.1 Material Removal by Reactions with Plasma Species The majority of dry-etching methods uses plasmas or particles from plasmas for generating micropatterns. For this purpose so-called cold plasmas are applied. In these plasmas the ion and electron formation is not caused by thermal, but by electronic excitation. The composition of these plasmas is different from that of thermally activated plasmas. Also cold plasmas are distinguished by a high concentration of extremely reactive components. So they are much better suited for etching processes than non-activated gases. They contain ions, activated atoms, molecules and radicals, i.e. moleclues or parts of molecules with unpaired electrons (fig. 4-48). Besides particular reactive molecules in the ground state are formed, which under normal conditions occur only in low concentrations like, e.g., ozone in an oxygen plasma. Cations of more electronegative elements or compounds are able to take over electrons from neutral particles transferring those into radical cations. Anions of more electropositive elements transfer their electrons easily to neu-
II
molecules in gases
electrons anions
Fig. 4-48. Particles in plasma (schematical overview)
I
4.3 Plasma-Etching Methods
123
tral particles forming radical anions of those which can undergo further reactions. By recombinations of electrons and cations in the plasma arise molecules in electronically excited states. The electronic desactivation of these states creates light quanta that cause the charateristic glow of the plasmas. The emission of the plasma is not a spectral continuum, but thin lines are emitted as the light quanta originate from electronic and vibrational quantum states of single particles of certain kinds. These lines can be used to spectroscopically characterize the plasma. The reaction activity of particles in the excited state is essentially higher than in the ground state, as the activation energies are lower. If the electronic excited state is accompanied by vibrational excitation, the activation barrier is lowered still further. Besides the vibrational excitation relaxes slowly in comparison with solids or liquids, as only by impacts with neighbouring particles or emission of photons energy can be delivered. Immediately after relaxation to the electronic ground state the molecules are in a vibrational excited state. The enhanced vibrational excitation corresponds to equilibrium temperatures of 100 to lo00 K above room temperature, hence chemical reactions are very likely in this state. The most important group of reactive particles in a plasma are radicals. They are formed by collision processes with electrons or ions from molecules by homolytic bond cleavage:
Due to their unpaired electrons, radicals possess a strong tendency to forming new bonds. There is the possibility of recombination by forming an electron pair, or forming an anion by abstraction of an electron generating a radical cation of the reaction partner, or delivering an electron forming a cation and generating a radical anion of the reaction partner. Under normal conditions these highly rective species cannot exist or only in very tiny amounts, as formed radicals react by impacts with less active particles and the radical chain reaction subsides by recombination events very quickly. In plasmas of adequate gases, however, radicals can be generated in high concentrations. Plasma etching in the stricter sense means removal by reactive or thermalized particles. Thermalized particles are atoms, molecules or radicals that possess a translational energy differing only slightly from the medium kinetic energy of gas particles at room temperature. As these are neutral in opposition to ions accelerated by an electrical field, simple plasma etching is called reactive neutral gas etching (FWE). Plasma etching and several types of plasma-free etching with reactive vapors have such thermalized particles as etching species in common (see section 4.2). They distinct both these groups from etching methods with ion etching techniques (sputter etching, ion etching and ion beam etching, see section 4.4). Especially atomic radicals of oxygen and halogens are extremely reactive species. They are able to abstract efficiently electrons from or form strongly polar bonds with electropositive materials like metals and semiconductors,
.
124
4 Dry-Etching Methods
but also hydrogen, carbon, sulphur and other non-metals. In pure halogen or oxygen plasmas the reactive species are formed from the elements.
+ energy from an energized particle + 2 0. C12 + energy from an energized particle +-2 C1.
O2
(85)
(86)
Radicals are generated from molecules in the plasma by asymmetric homolytic bond cleavage, e.g., from halogenomethanes like chloromethane (methyl chloride):
+
+
CH3C1 energy from an energized particle + CH3. C1.
(87)
or trifluoromethane: CHF,
+ energy from an energized particle + CHF,. + F.
(88)
Beside the atomic radicals molecule radicals are formed. Trifluoromethyl radicals are formed also by symmetric cleavage of hexafluoroethane:
GF6+ energy from an energized particle + 2 CF,.
(89)
If two radicals meet, they can recombine to molecules. By collisions of moleculs with radicals new radicals can be formed in the plasma. By thermal movement the radicals from the plasma reach the surface of the material to be patterned and react there directly or by intermediates to desorbable species. Thus e.g., on plasma etching silicon or silicon-containing materials with fluorine-containg etching gases at first fluorine-substituted surfaces are formed. Only by binding of fluorine at the fourth valency of silicon the desorbing tetrafluorosilane (SiF4)is formed. In an analogous way the decomposition of hydrocarbons proceeds over intermediates in an oxygen plasma. At first alcoholic, aldehydic and acidic groups are formed at the surface before the carbon skeleton is decomposed by releasing C 0 2 or CO. Hydrogen passes as water into the gas volume. In dependence on the reaction mechanism of etching, intermediates on the solid surface occur beside the gaseous products. Well investigated is the formation of chemical surface structures in etching organic films of novolaks in plasmas containing oxygen and fluorine compounds. Whereas on unetched surfaces only C-H-, C-C- and C-O-bonds were identified, additionally alcoholic, carbonylic, carboxylic and ester groups were found during etch removal in the oxygen plasma. During etching in fluorine-containing etchin gases (e.g. SF6)mono-, di- and tri-fluorinated carbon atoms were detected9F.
95
0.Joubert et al. (1990)
4.3 Plasma-Etching Methods
125
Using alkyl radicals as etching species avoids an oxidizing atmosphere in plasma etching. The alkanes themselves can be applied as etching gases. The active alkyl radicals are formed in the plasma by molecule fragmentation. For example GaAs in a methane plasma is attacked preferentially by CH,'. The etch rates, however, are low (0.1 to 0.2 nm/s)". Even hydrocarbons can be decomposed by reductive plasmas. Polyimide films, e.g., can be etched in a hydrogen plasma. The rates are comparatively low (0.5 nm/s)".
4.3.2 Plasma Generation The generating of cold plasmas needs a vacuum for providing a sufficiently long free path of the charged particles to ensure such a high acceleration in the electric field that their kinetic energy is higher than the ionization energy of the gas particles. The usual pressures for plasma etching range from 30 to 300 Pa. The necessary electric field for accelerating the electrons so much that they can ionize atoms or molecules on impact, depends on the vacuum. The lower the pressure the smaller are the field strengths for sufficiently high electron energies. For keeping the plasma, constantly electric energy has to be fed into the gas volume. This is simply done by a high-frequency generator (HF-transmitter) the energy of which is introduced by electrodes into the plasma. Among the HF-sources with frequencies in the range of about 0.1 to 100 MHz, especially frequently such with 13.56 MHz are used. Beside these frequencies those of microwaves (several GHz) are applied which provide very high power densities. For high-rate etching processes several HF-sources can be used in one apparatus. Usefull is the application of a HF-source of medium power (0.1 to 0.5 kW) combined with a microwave source of high power (1 to 5 kW). The higher the power, the higher the density of the plasma, Le., the ratio of ionized particles to neutral particles. With increasing plasma density the number of radicals increases, too, i.e., the number of particles with unpaired electrons. The total concentration of radicals is determined by the plasma density and the reactor pressure (fig. 4-49). Investigations of etch rates in dependence on the plasma frequency in a wide range have shown that an etch rate maximum occurs in dependence on the frequency. This etch rate maximum for etching silicon with chlorine occurs at a pressure of 0.3 torr and a power density of 18 W/cm2at 400 kHz. The etch rate sinks from this maximum of 11nm/s by changing the frequency over 1up to 30 MHz by more than an order of magnitude to 0.3 rids. Probably the energy of ions falling into this range from about 0.5 kV to less than 0.1 kV is responsible for the decreasing etch rate9*. With electrically conductive substrates or functional films the alternating fields induce alternating currents. At high powers these alternating currents 96 V.J.
Law et al. (1991) V.J. Law et al. (1991) 98 R.H. Bruce and A.R. Reinberg (1996)
97
126
4 Dry-Etching Methods low plasma density
high plasma density 0
0
low Diessure
0
0 atom radical OD
OD
OD
0
OD OD
OD
OD
0
0
Fig. 4-49. Molecules and radicals at high and low pressures as well as high and low plasma densities (schematically)
can be considerable. The consequence is heating by Joule heat released by the current flow. Such heat generation can be desired to enhance the etch rate by a high working temperature. This is the case, e.g., in plasma ashing for removing resist masks. Often the thermal stress on substrates and functional films cannot be tolerated. Then an electrical shielding has to keep the surroundings of the substrate free of the electrical field. This is accomplished by a perforated electrode or a wire mesh acting as Faraday cage. The plasma distinguished by the light emitting zone, burns only out of this wire mesh leaving the inner part of it dark. The decay times of the electronically excited species causing the luminescence are in the nanosecond range and hence much shorter than the life times of radicals and the diffusion times of particles. Through the meshs of the wire the radicals get to the substrate surface. Radicals have a lifetime of seconds so that the concentration of the radicals is practically the same within the Faraday cage and in the glowing plasma. As the etch rate is determined beside by temperature nearly exclusively by the radical concentration, the reaction velocity is high enough to achieve high etch rates inspite of shielding the electrical field. The power or more accurately the power density electrically coupled into the plasma, the frequency the plasma is generated by and especially the atmospheric composition of the inlet as affect the quantitative composition of the plasma and hence the etch rate 9F. 99
R.H. Bruce and A.R. Reinberg (1996); A.M. Wrobel et al. (1988)
4.3 Plasma-Etching Methods
127
4.3.3 Plasma Etching in the Barrel Reactor Reactors for plasma etching differ in the kind of the electrode placement. The classic arrangement is the barrel reactor. In this arrangement the plasma is generated in a barrel-shaped recipient. In the case of a dielectric wall (e.g. glass) the electrodes can be attached on the outside (fig. 4-50). The substrates to be etched are placed in the inner of the plasma, so that the particles of the plasma can unimpeded reach their surface. They are in the dark zone into which the radicals as reactive particles diffuse, whereas in the outer part of the cylinder, seperated by a conductive shield, the plasma burns. The etching gas is introduced in this outer part. If reactors made of conducting wall materials, such as steel, are used, the electrodes must be placed in the interior of the reactor. A disadvantage is that the electrode material can be attacked in the process. That can lead to redeposition of material from the electrodes on the substrate causing disturbances in the etching process or in the function of the devices fabricated. Furthermore such an attack causes in the course of time the corrosion of the electrode material spoiling the functioning of the electrodes. If the vacuum part of the plasma reactor is made of dielectric material, such as glass, the electrodes can be attached to the outside as the electric field goes through the dielectric wall. Beside saving the electrodes from corrosion such a reactor has the advantage of a smaller even surface because of less inner installations, which avoids chemical and particulate contaminations of the substrate. Besides the cleaning of the interior of the reactor is much more easily done. The cylindrical configuration of the plasma reactor is not specific for micromachining, but is used in many technical plasma applications, among these are surface film depositions and cleaning of large work pieces. Inhomogeneities in the distribution of reactive plasma components can be minimized by the convenient arrangement of gas inlet and electrodes on one hand and the placing of the substrates on the other.
light ernimng plasma electrodes
Foraday cage gas SUPW dark space containingfree radicals substrate stage
Fig. 4-50. Barrel reactor for plasma etching (principle)
vacuum sy3er-n
128
4 Dry-Etching Methods
4.3.4 Plasma Etching in the Down-Stream Reactor A special case of plasma etching is verified in the so-called down-stream reactor. In such a reactor the etching gas is let in from above through a perforated shielding electrode into the main part of the reactor streaming vertically against the substrates (fig. 4-51). The exciting energy is coupled in, e.g., as microwave by means of a hollow conductor through a dielectric wall into the upper part of the vacuum reactor, where the etching gas is let in, too. The glowing plasma is formed between the dielectric wall and the perforated electrode. The radicals being formed in this plasma get with the gas stream into the gas volume underneath and react as etching medium for the substrate. The etch rate is controlled by the concentration of reactive radicals. The spatial separation of plasma generation and etching allows a better monitoring of the etching process. For enhancing the etching rate heated substrate tables are in use. The typical pressure in down-stream methods lies in the range of 1torr (133 Pa). The method is used for stripping of photoresists and other organic materials. At working temperatures between 150 and 200°C in oxygen-containing or pure oxygen plasmas etch rates of 2.5 to 17.5 n d s are achieved@ ' .'
' \
dark space with high
la,
Sh. Fujimura (1991)
dielectrical wall (e.g.glass)
metall gauze for micro wave confinement --D vacuum pump
4.3 Plasma-Etching Methods
129
4.3.5 Plasma-Etching in the Planar-Plate Reactor Planar plate reactors are charcterized by a vacuum chamber in which two parallel electrodes for plasma generation are arranged. They are distinguished by an especially homogeneous field distribution and hence well controllable plasma conditions. The plasma sources work in the MHz-range, mostly. As a rule only one substrate is directly placed on one of the electrodes (fig. 4-52). That means, however, that these reactors are less productive than barrel reactors in which batches of several dozen substrates can be etched simultaneously. Hence planar reactors are favoured in research. In industry they can be used for small numbers of substrates, if the substrate exchange is accelerated by vacuum locks. Planar reactors are preferentially applied for reactive ion etching (RIE, see section 4.4.2). A RIE-reactor can principally work in the plasma etching mode. In that case the smaller electrode on which the substrate lies is earthed and the HF-power is coupled in through the counter electrode. Under these conditions a sputter effect cannot practically occur, and alone the radicals determine the etching process. Working at pressures about 1torr the free path length of the particles in the gas volume is shortened. Thus under normal excitation conditions energized ions do not appear and hence the surface to be etched is only attacked by thermalized particles.
I,
gas
I%
i7
HF-generator
SUPPlV
I
vacuum system
Fig. 452. Planar plate reactor for plasma etching (principle)
130
4 Dry-Etching Methods
4.3.6 Magnetic-Field-Enhanced Plasma Etching In normal HF-plasmas the electrons move on straight trajectories. The number of possible collisions between electrons and gas particles is determined by the distance between the electrodes, the pressure in the reactor and the electron density. If higher plasma densities are desired, the path of the electrons through the gas volume has to be elongated. Thus the number of collisions of electrons with gas particles can be increased decisively. An effective change of the path of the electrons is achieved by coupling in magnetic fields. The magnetic fields compel the charged particles on spiralshaped trajectories. Hence, especially the dwell time of the electrons in the plasma is considerably increased. As a result essentially increased plasma densities are obtained. By using magnetic-field-assisted plasma sources (magnetrons) the etch rate is raised by increasing the ion and radical density in the gas volume. The disadvantage of magnetron-assisted plasma etching is the limited homogeneity of the plasma density. Due to the shape of the magnetic fields the distribution of ions and radicals in the plasma is relatively inhomogeneous. Thus radial maxima of the plasma density and hence of the etching rates arise. They can be corrected only a little by moving the etching substrates. Magnetic-field-assisted plasma etching cannot be applied if high homogeneity of removal is demanded"'.
4.3.7 Plasma Etching at Low Pressure and High Ion Density Plasma etching at low pressure and high ion density is a current special case of reactive dry etching with excitation from several sources. The process stands in its characteristic between the conventional plasma etching and reactive ion etching. In spite of the low pressure range and the reactor setup in two parts, which is the characteristic of the method putting it near to ion etching methods, it is, however, to count to plasma etching due to the kind of reacting of the etching species. The main part of the energy applied for plasma generation (typically 80 to 90%) come from the microwave excitation. The microwave energy is fed in through a hollow conductor into the plasma space of the reactor. There, an increase in the ion density is achieved by an additional magnetic field that acts as a source with electron-cyclotron resonance. The electrons forced on an orbit cause an essential densifying of the plasma. Thus the plasma is in effect a high-power microwave ECR-discharge. Additionally to this discharge a medium HF-power (10 to 20 % of the applied energy) is fed into the plasma. The substrate itself is connected as working electrode. Thus the plasma is simultaneously excited in two different frequency ranges (>lo7 Hz and >lo%). As a result plasmas with high ion density are obtained (e.g. lo1' ions/cm3at 1kW microwave power) '02.
ICn
D. Dane et al. (1992) J.W. Lee et al. (1996)
4.3 Plasma-EtchingMethods
131
Because of the comparable low pressure for plasma processes (about 1mtorr) the build-up of an electric field is to be expected at the working electrode. In this field ions from the highly dense plasma are extracted and accelerated to the substrate. This effect contributes in plasma etching at low pressure and high ion density to the high plasma density and causes high etch rates by the action of ions with energies above the sputter threshold.
4.3.8 Forming of Etch Structures in Plasma Etching As the etch removal in plasma etching is preferentially caused by the action of radicals the kinetic behaviour of these determines the spatial distribution of the etch rate. In contrast to the accelerated electrons and ions the radicals only possess a low kinetic energy. Their movement in the gas volume proceeds with velocities that correspond to the thermal energy at room temperature or little above it. The higher kinetic energies transferred by collisions are rapidly reduced in the plasma by collisions with less energetic particles and the wall. As thermalized particles, the radicals possess no preferential direction of movement in the plasma. For their reaction with the substrate material only their contact with the substrate surface, not, however, the direction from where the contact is reached is important. The etching process, therefore, is independent of the orientation of the surface to be etched within the plasma space, i.e. the etch rate is the same in all spatial directions. In this respect the plasma etching process is an isotropic process. Anisotropy is possible only like in wet-etching processes by the selective removal in a crystallographicallypreferred direction. In general all materials are removed in plasma etching in all directions with the same rate. The formed etching structures are isotropical, i.e., they form under the structure edges of the etching mask a section of a cylinder surface and at corners fragments of spherical surfaces in complete analogy to wet-chemical isotropic etching.
4.3.9 Geometry Influence on Plasma Etching The supply of reactive radicals as well as of particles in the ground state that can react with reactive centres on the substrate surface determines decisively the etch rate. As the medium free path length in plasma etching compared to the reactor measures is small, the reactive particles essentially move by diffusion in the neighbourhood of the surfaces. Their local concentration is decisively determined by their consumption in reactions at the surface and the surface-near gas volume. Hence, the area of the etched substrate influences the local etching gas composition and with it the etch rate. The etch rate decreases with the number of substrates loaded into the reactor (loading effect)lo3.As concentration differences in the components of the etch gas and '03
C.J. Mogab (1977)
132
4 Dry-Etching Methods
the formed reaction products occur within the reactor, etch rates can be dependent on the configuration of the reactor and the position of the loaded substrateslM.The etch rate in plasma reactors is often reciprocally linearly dependent on the number of wafers in multi-wafer etch equipments. In the same way the etch rate sinks with increasing proportion of the etching area AB to the total area of the wafers A, i.e. in dependence on the percentage area of coverage of the wafers A$A. The dependence of the etch rate r, on the total etching area n.AB can be described by the following equation, that was confirmed by etching silicon in a CF4/02-plasma'05:
p y
= reactivity coefficient = stoichiometry coefficient
= life time of reactive species G = rate of generation of energetic electrons No = Avogadros constant M = molecular weight of the solid to be etched V = volume of plasma
z
The etch rate can be related to the idealized etch rate roat very small etching areas in dependence on the number of the substrates n. With respect to the total inner surface A,, at which the plasma can react with the rate k, , to the area to be etched As of a substrate and to the etch rate of the material to be etched ks one gets approximately''? r,,
= ro
- l/(l+ n) - (A,-k,)/(&-k,)
(91)
As the concentration of the reactive species is not ideally the same in the total reactor volume the etch rate in a plasma reactor is also dependent on the place. Transport processes cause concentration gradients in the reactor. The concentration gradient is determined by reaction and diffusion rates and the dimensionless Peclet-number Pe. Pe is determined in the case of a cylindrical reactor by the average velocity of the gas at the gas inlet vo,the radius roof the reactor and the diffusion coefficient D of the gas'07. Pe
= v,.r,,/D
(92)
Phenomena as the loading effect are observed also with other than cylindrical reactors. Also in the planar reactor the etch rate rises as a rule with the decreasing area to be etched. This effect causes an increased etch rate in latA.G. Nagy (1984) C.J. Mogab (1977); C.J. Mogab und H.J. Levinstein (1980) '06 K. Schade et al. (1990) '07 E.C. Stassinos, H.H. Lee (1990) '04 '05
4.3 Plasma-Etching Metho&
133
era1 direction if the film to be structured is completely etched through. Hence mask edges are more than proportionally undercut during the over-etching phase.
4.3.10 Plasma Jet Etching (PJE) By generating a very thin plasma beam (plasma jet) very high local etch rates can be achieved. For that purpose the working gas is pressed through a jet (nozzle) that is arranged in one of the HF-electrodes. The plasma jet is formed in this jet and can be directed on a workpiece that is placed before the jet. Because of the high plasma density and the very quick outlet of the reactive species by the jet, etch rates up to 2 p d s are reached in the centre of the plasma jet. The etching efficiency is aided by the self-biasing effect. In the narrow gap between the jet orifice and the workpiece (typically 1 to 3 mm) an electrical field is formed that is characterized by a strong potential decrease in both directions to the electrodes (jet and workpiece). Thus a high field strength (up to several 1OOV) arises in front of the surface of the workpiece. In this field ions of the plasma are accelerated so that as in reactive ion etching (see section 4.4.2) an additional sputter-aided removal results. The etch rate remarkably falls along the radius to the circumference of the plasma jet. Hence holes with sloping round sidewalls are produced under a fix standingjet. Jet etching can also be applied using lithographicalmasks providing a satisfying patterning qualitylW.
4.3.l.l
Applications of Plasma Etching
Plasma etching processes are applied where dry-etching processes have advantages to wet-etching processes, and if no anisotropy is required. They are preferred if wet-etching processes only supply low rates, or if especially agressive, poisonous or otherwise dangerous etchants would have to be used. Metals can be etched with plasma methods if they form volatile products. Aluminium as important material for the fabrication of conducting paths in microelectronics and mirrors in micromachining, e.g. forms the relatively volatile chloride AlC13(subl. 182.7"C)'09.It can be etched in chlorine plasmas or in plasmas of chlorine containing compounds of low molecular weights'l'. Silicon as microtechnically especially important material and its compounds are preferentially etched in fluorine containing plasmas (concerning the etch gases and methods see also the catalogue of etching methods in part 6 ) . L. Bardos et al. (1990) A.E Holleman und E. Wiberg (1985), 875 'lo D.W. Hess (1981)
'08
'09
134
4 Dry-Etching Methods
Beside CF, and SF6also SiF, can be used for plasma etching of siliconlll.For Si and SO2,it was shown that the etch rate is determined by the concentration of atomic fluorine in the gas atmosphere. The following dependencies of the etch rates r were estimated which otherwise still depend on the absolute temperature only112. rsi = (0.485 +_ 0.3) . lo-', -
fl-nF- e''.108 eVkT n m l S
rSio2= (1.02 f 0.08) - l@"*
0nF - e"~163eVkTnm/~
(93) (94)
GaAs can be patterned as well in halogen plasmas as in reductive hydrogenalkane plasmas. The etch rates are relatively low (up to 0.03 nm/s for ethane and up to 0.2 n m / s for methane)l13. Because of the chemical selectivity of plasma etching processes some monocrystalline materials can also be etched crystallographically in a plasma. Thus (111)- and (110)- planes were prepared in GaAs in in C12-and Br2-plasmas"4. Inorganic dielectrica can be etched if volatile compounds are formed. However, many dielectric materials on oxide basis (glasses, ceramics) especially oxides of less noble metals are thermodynamically much more stable than metals. Hence for forming volatile compounds essentially higher activation energies have to be introduced. The removal rates are accordingly lower as a rule. As well other dielectrica on the basis of nitrides and carbides are etched relatively slowly only in plasmas because of their thermodynamical stability. Higher rates can be achieved with sputter and beam etching methods (see section 4.4.), as in these methods by mechanical activation the chemical reaction barrier is more easily overcome. The especially important dielectric material Si02and related materials like glass are etched in chlorine containing plasmas because of the high volatility of SiF4. Especially advantageously CF4/02and NFdAr plasmas are applied. With these plasmas good pattern qualities and high selectivities to other materials like GaAs and InP can be achieved"'. As organic dielectrica synthetic hydrocarbon polmers are used in microtechniques. These are preferredly etched in oxygen plasmas. The achievable rates are so high that the method can be applied for fast etching of thin polymer films but also for patterning thicker layers in acceptable times1l6.Partially rates of more than 100 nm/s are achieved. Atomic oxygen in the electronic ground state is the actual reactive species. Because of their high electronegativity and their radical character free oxygen atoms abstract very effectively hydrogen atoms from the hydrocarbons, forming hydrocarbon radicals:
'" H. Boyd und M.S. Tang (1979) D.L. H a m et al. (1981) V.J. Law et al. (1991) 'I4 D.E. Ibbotson et al. (1983) 115 VM. Donnelly et al. (1984) I.S. Goldstein and F.Kalk (1981) 'I2
4.3 Plasma-Etching Methods
R-H + 0.+ R
+ OH.
135 (95)
The hydrocarbon radical R. is very reactive to oxygen in the ground state (triplett oxygen), so that an oxidative decomposition over further radical intermediates in form of radical chain reactions takes place. In this chain reaction alkoxyradicals are formed and at last the volatile low molecular products CO, C02, and H20. For cleaving the C-C-bond a reaction way is discussed that leads over radicals and addition of oxygen molecules to peroxides and ketones117,118:
-c-c-+ 0 2 +-co2-c-
(96)
CO2-C- + RH + -C(OOH)-C- + R
(97)
-C(OOH)-C- + -C-(O')-C- + OH.
(98)
+ -c.
-c-(O.)-C- + -co-
(99)
The decomposition reaction in the plasma is temperature-dependent. At higher temperatures the etch rate rises more quickly which is brought into connection with the rate control by the actual chemical reaction. At low temperatures a low apparent activation energy exists which speaks for a rate control of the material transport in the surface layer"'. Apparent activation energies observed with different materials and under various conditions differ considerably. The values span from 0.08 eV (7.7 kJ/mole) etching plasmapolymerized tetrafluorethylene in an oxygenplasma to 0.58 eV (55.7 kJ/mole) etching polyimides in an oxygen plasma and 0.64 eV (61.5 kJ/mole) etching photoresists in an oxygen downstream microwave plasma'20. The etch rate in an oxygen plasma can be enhanced strongly by adding a fluorine containg etching gas, e.g. CF4.The formed fluoro radicals abstract still more efficiently than oxygen atomic hydrogen and facilitate thereby the oxidative decomposition by the oxygen plasma. At higher concentrations of Fcontaining etching gases the etch rate decreases again, because free valences at the surface of the hydrocarbons are more and more occupied by fluorine atoms and these fluoro-alkyl groups have an essentially lower etch rate than the respective unsubstituted groups. The maximum of the etch rate is found typically in the range between 10% CF, (for polyimides or aromatic polymers) and 40% CF, (for a1iphates)"l. Because of the competition between
'I7 'I9
ED. Egitto et al. (1990); S.J. Moss et al. (1983) ED. Egitto et al. (1990) S. 332 I. Eggert and W. Abraham (1989); 0.Joubert et al. (1989) ED. Egitto et al. (1990) S.R.Cain et al. (1987);V. Vukanovic et al. (1987); A.M. Wrobel et al. (1987, 1988)
136
4 Dry-Etching Methou3
the formation of fluoride-rich surfaces and the increase of the etch removal by flurine-induced H-abstraction the reaction conditions affect the situation of the maximum etch rate. The maxima of the etch rates are shifted in dependence on the gas composition as well as by the total pressure. The highest etch rates are found in a relatively narrow concentration range. That means that at high rates the process reacts very sensitive to small changes in the etching conditions. The increase of the etch rate by adding fluorine containing gases depends strongly on the nature of the gas. CF, reacts stronger than CHF3 in etching polyimide, the latter essentially stronger than CF2C12122. The etch rate of organic polymers depends under otherwise same conditions on the chemical composition of the polymerslu. In general the etch rate increases with decreasing C:H-ratio and increasing oxygen proportion in the polymer. Aromatic polymers have as a rule lower etch rates than aliphatic polymers. Polymers rich with hydroxy groups etch more rapidly than unsubstituted aliphates. The table below gives an overview over typical etch rates in dependence on the chemical nature of the polymer. Plasma etching of organic polymers is often used for removing organic photoresist masks (stripping). As in plasma etching the hydrocarbon chains are chemically decomposed, covalently networked materials not removable in organic solvents or alkaline removers can be removed, too. Therefore, plasma stripping is applied if thermal or photochemically hardened (crosslinked) resists have to be removed. Such resist layers are used in multi-layer processes or as especially stable resist masks, e.g., if a stability of the etch mask is necessary in beam etching processes. It is possible that non-crosslinked resist masks can suffer crosslinking in beam etching processes, especially at the surface. As Table 4-5. Etching Rates of Organic Polymers in CF.J02-Plasmas(according to L.A. Pederson 1982) No.
Iu
Material
?LPe
etch rate nds
glassy carbon polydivinyl benzene polystyrene AZ photoresists polyvinylidene fluoride polyimide polyvinylolacton methacrylate polymer cellulose
pure carbon interlinked aromatic polymer linear aromatic polymer linear alkyl-substitutedpolymer halogen-substitutedaliphatic polymer nitrogen- and oxygen-containg polymer oxygen-containingaliphatic polymer acrylate (oxygen-rich aliphatic polymer) carbohydrate
0.7 1 1.1 2 2.2 >4 4 . 3 41.7
ED. Egitto et al. (1990) L. Eggert et al. (1988)
< 0.1
4.4 Etchig Methods with Energized Particles
137
these films as a rule are not dissolvable in organic or alkaline strippers, plasma stripping in oxygen atmosphere is preferred to remove such resist masks124. The components hydrogen and carbon from silicon containing polymers (e.g. silicones, siloxanes etc.) are changed by etching in oxygen plasmas into gaseous or volatile products (like CO, C02, H20). the organically bound silicon reacts, however, forming the extremely difficultly to evaporize SO2. Hence these polymers form in the oxygen plasma a thin surface film essentially consisting of Si02.This film restricts the access of oxygen radicals to the volume elements of the polymer beneath this film that cannot be removed, consequentlylZ. Therefore Si-containingpolymers can serve as etching masks for Si-free organic polymers in oxygen plasma etching as well as in oxygenRIE and -RIBE (see sections 4.4.2. and 4.4.5.).The complete plasma removal of Si-containing polymers is possible in mixtures of oxygen- and fluorinedonating etch gases like CF,.
4.4 Etchig Methods with Energized Particles 4.4.1
Sputter-Etching
The Sputter-Effect Sputter-etching is a process in the gas phase that can be performed practically with all materials126.In sputter-etchingU7atoms or clusters are knocked out of the solid and brought into the gas phase by mechanical momentum of fast ions or neutral particles. The kinetic ener ies of these ions are typically in the range between 0.1 and 1keV (about 10f to lo8J/mole). These energies exceed the typical binding energies of the solid by a hundred to thousand times. If the energy of the single ions is too low, particles cannot be knocked out of the surface. The sputter-effect does not occur. Also a high ion density cannot induce the sputter-effect if the ions do not possess the necessary minimum energy. Does an energized ion impact on a solid surface, it transfers its kinetic energy to the atoms of the solid. By interaction of the atoms of the solid the mechanical momentum is transferred within 10 to 100 femtoseconds to adjacent atoms, and hence distributed to a group of atoms in the near-surface layer. The momentum transfer does not proceed in the impact direction of the sputtering ion only, but by momentum transfer in the lattice in others as well, also back to the surface. The direct momentum exchange causes a corresponding deviation from the equilibrium site of the atoms in the lattice of the
* M.A. Harney et al. (1989) M.A. Hartney et al. (1989); H. Namatsu (1989) Davidse (1969) A.N. Broers (1965); C.M. Melliar-Smith (1976); R. Wechsung and W. Brauer (1975)
'~5
126 P.D.
12'
138
4 Dry-Etching Methods
solid. As soon as the atoms in the surface gain mechanical energies that surpass their binding forces the atoms move out of the solid and into the gas volume. With this transfer material is removed from the solid. Low collision probability assumed, the released atoms move into the inner of the gas volume and are transported out of the recipient with the gas stream. The minimum kinetic energy that impacting particles must possess to release particles from the solid is called sputter-threshold. As the sputter-threshold is dependent on the binding energy of the particles in the solid, different materials possess different sputter-thresholds. In most cases the highest sputter-yields do not occur with vertically impacting energized particles (90"), but with those impacting under an angle between 50" and 90". For monocrystalline cubic materials the optimum sputter-angle is 60".At this angle the kinetic energy of the sputtering particles is transferred to the atoms of the target, thus the maximum number of target atoms pass into the gas volume (fig. 4-53). Whole groups of atoms among which the bonds are preserved can turn in the same direction off the surface and tear off the surface at sufficiently high energies. In such cases clusters (elementary solids) or molecules, radicals or groups of molecules (molecular solids) pass from the surface into the gas volume. The atoms, molecules or clusters primarily tearing off can possess high kinetic energies. Relaxation in the solid transfers the kinetic energy of the impacting particle of the atoms in vibrational energy. Hence at the site of the impact exists an ensemble of atoms with highly excited vibrational states. Thus the transition probability for the atoms into the gas volume is increased. Single atoms, radicals, or molecules can overcome the binding energy to the surface and pass with relatively low kinetic energies into the gas volume. The concentration of highly vibrational excited states at the impact site means a local high temperature of the solid. The impact site is a microscopic "hot spot". The vibrational energy, however, is rapidly transferred to deeper parts of the solid, and the atoms at the impact site relax to lower vibrational levels. The velocity of momentum transfer among the atoms lies in the range of oscillation velocities (oscillation period in the range of about 10-13s).By this means the temperature of the hot spots is quickly balanced. In all, about 3/4 of the mechanical energy of the impacting energized particles is changed into heat even in efficient sputter-processes. perpendicular impact
declined impact direction
& Higher depth
Lower depth
of effect of energetic particles
Fig. 453. Dissipation of released energy of bombarding energized particles on solid surfaces:The more the incidence deviates from normal direction the smaller is the depth where the energy is deposited.
4.4 Etchig Methods with Energized Particles
139
If the density and the frequency of the impacting particles is very high, the areas of the hot spots superimpose. Then the heat released by the particle impact cannot be transferred sufficiently fast into the interior of the solid. Under these conditions the sputter-process causes not only local but global heating of the solid surface. For the technically requisite removal rates the ion density is practically always chosen so high that the medium surface temperature is significantly increased. The sputter-heat is transported in the substrate vertically to the surface. As a rule thin functional films with thicknesses of several 100nm up to 1pm do not limit the heat transport. In contrast, the carrier substrates with typical thicknesses of several 100 pm function as heat sinks or thermal isolation. Hence, the thermal conductivity of the substrate material determines very decisively the thermal conditions during ion etching processes. By far more restricted is the heat transport in the gas volume as, at pressures common in sputter- and other ion etching methods, practically no heat transport takes place by convection in the gas phase, but nearly exclusively by radiation which is far less efficient as the heat conduction through the solid. Small gaps between substrate and table act frequently as very efficient heat flow barrier. To avoid too high surface temperatures, the substrates have to be thermally contacted, i.e. they are connected by a thin film of an appropriate contacting material to a cooled substrate table, or the backsides of the substrates are rinsed with a cooling medium. In microtechniques heat removal is especially critical if sputter-processes or other heat developing vacuum processes are carried out on thin free-standing membranes, the backsides of which cannot be cooled by a direct mechanical contact with the substrate table. As such membranes possess only small thermal capacities and the lateral heat transport to the bulk areas is limited because of their small crossections, the surfaces of the membranes are heated during etching very much causing extreme mechanical tensions that can lead to tearing off the micromechanical elements, or undesirable chemical phase-changing processes. The generated temperature at the surface can be influenced by the sputter-power or by the gas volume conditions, especially the pressure or the gas composition.
Generating Energized Ions in a Sputter-Reactor Energized ions are generated either by a separate ion source and extraction of the ions by an electrode (the typical case in ion etching, section 4.3.3),or directly in the etch reactor. The most common arrangement is the parallelplate reactor (fig. 4-54).This reactor consists of a vacuum chamber (recipient) with vacuum and gas supply system, an energy source (emitter), and two electrodes, the smaller of which serves as substrate stage. At pressures about 0.1 to 1 Pa a plasma can be ignited in the recipient by coupling a HF-power via the electrodes. At these pressures and a sufficiently high electrical amplitude free electrons and positive ions are generated by cascades of collisions in the plasma. The charged particles follow in their movement the electrical alternating field. The oscillating amplitudes of the ions are
140
4 Dry-Etching Methods
gas SUPPW
I
-
t working electrode
relatively low, those of the electrons very high. The cause is the great charge:mass ratio of the electrons (ca.100.000 times larger than the charge:mass ratio of the ions). The consequence is that electrons even of not too great a HF-power reach the walls and electrodes and get discharged. This
Fig. 455. Forming of potentials in the HF-induced plasma due to the different mobility of electrons (-) and ions (+)
4.4 Etchig Methoak with Energized Particles
141
discharge entails excessive positively charged ions in the inner of the gas volume (fig. 4-55). In the inner of the gleaming plasma continuously new electrons and ions are formed. Whereas the electrons are rapidly extracted by the alternating field, the ions migrate essentially more slowly to the wall. In the plasma due to the frequent collisions particles arise in electronically excited states relaxing by spontaneous emission of light quanta. The consequence is the gleaming plasma in the central area between the electrodes that is seperated by the dark spaces from the electrodes. At the electrodes an excess of electrons arises or at best electroneutrality is achieved if the electrode is grounded. The consequence is a voltage drop from the r i m s of the gleaming plasma across the dark spaces or “plasma sheet” to the electrodes. If the electrodes are isolated against the outside, this voltage drop can be measured as “self-biasing”, “bias voltage” or “float potential”. This potential E depends only on the electron temperature T, and the particle mass of the ions mi. Besides, the elementary charge e and the electron mass m, are in the equation? E = - (kaTJ(2-e)
1n(mi/(2.3-m))
(100)
These spontaneously formed potentials can amount to several hundred volt or even kilovolt. The occurring bias field strengths Eb depend theoretically strongly on the area ratio Al and A2 of both electrodes:
This dependence is used in sputter-etching for generating high field strengths in the gas volume above the etching substrate while at the same time the sputter-effect at the wall and the counter-electrode stays small. Thereby the sputter-threshold at the working electrode is exceeded by a multiple, but at the counter-electrode it is by far not reached and hence the undesired material removal does not occur. However, in respective experiments a smaller dependence on the area ratio was found than was expected according to equation (101). In many sputter equipments self-biasing is additionally biased by a dcpotential that is superimposed to the HF-signal. In the electrical field of the dark spaces, positively charged ions are accelerated to the negatively charged electrodes. The maximum electron energies correspond to the field strength of the dark spaces. In reality the medium ion energy lies under the maximum value, as also in the dark spaces collisions of accelerated ions and thermalized particles (mainly neutral particles) are still possible. In these collisions the ions loose their energy and change their direction more or less according to the geometric conditions and the mass ratios. The velocities of the ions are essentially higher than those of the thermalized particles. Wheras the velocity of thermalized argon atoms with M = 40 g/ mole, e.g. at room temperature (300 K) with A.J. van Roosmalen et al. (1989)
142
4 Dry-Etching Metho&
v
=
YqmiiT)
VAr(thenna exit)
= 250
is equal to 250 d s , the energy of ions of the mass mi on leaving the gleaming part of the plasma must be equivalent to the electron temperature T,, that lies e.g., for 2 eV at 23,000 K and the argon ion velocity is witha9: v=d equal to 2.2 km/s.
m
After having been accelerated in the area of the plasma sheet, the velocity of the ions is determined exclusively by the electrical field that is passed by the ions. To the ion energy Ei of 1 kV corresponds a velocity of energized particles v,: v,
=
q@&cJ
This is for argon ions equal to 49 km/s, i.e., about 200 times the velocity of thermalized particles. With low pressures in the plasma the number of collisions is small. The plasma sheets are large. The ions reach the substrate surface with a high energy. The impact direction is practically always vertical to the substrate, as the electrical field independent of the orientation of the substrate in space is formed vertical from the substrate surface into the space. As a result of the vertical ion impact the etch removal is nearly ideally anisotropic. At higher pressures and hence higher particle densities the free path of the ions is short. The plasma sheets are narrow, the gleaming plasma is larger. Hence many collisions take place in the gas volume and the ion energies are low (fig. 4-56). If the free paths become shorter, the field lines are distorted and the ions receive a broader distribution of directions, i.e. the etch removal is not any longer ideally anisotropical.
SputtePRate The sputter-rate depends as well on the plasma parameters as on the material parameters. High sputter-rates are achieved in case of 0 0 0 0
high ion current densities high ion energies effective momentum transfer from ions to solid particles (optimum sputter angle) low binding energies of the particles in the solid
In general the ion current density increases with the electric power. More important than the power are the gas volume conditions. High ion current L29
A.J. van Roosmalen et al. (1989)
4.4 Etchig Methods with Energized Particles
143
Fig. 4-56. Changing of width of gleaming plasma and adjacent dark spaces in sputter etching for low, medium and high pressure. Schematical picture of the reactor with electrodes and plasma (left side) and potential curve (right side)
densities require sufficiently high concentrations of ionizable particles in the gas volume. Furthermore a considerable part of these particles has to be actually ionized. At high pressures a high concentration of ionizable particles is present, but because of the frequent collisions the particles relax thermally very rapidly (“thermalizing”) and collision cascades break off. Therefore at high pressures as a rule, high ionization degrees are not achievable, but high ion current densities are obtained in a medium pressure range. Because of the reduced number of collisions at low pressures, energized patricles are more slowly retarded than at higher pressures. In general, at low pressures highly energized particles of lower density and at higher pressures less energized particles of higher densities are produced. Hence, with a simple HF-generation the product of ion current density and medium ion energy and therefore the sputter-rate is maximized in a medium pressure range (about 1 Pa). The sputter-rate is material-dependent in that way that for high binding energies of the atoms, molecules and molecular fragments, respectively, more mechanical energy per released particle is necessary for the impacting particles as for low binding energies. The sputter-rate is approximately reversely proportional to the sublimation heat of the material to be etched. For unspecific sputter-etching the used gas must be chemically inert. By ionization and formation of radicals very reactive states are achieved that are able to cause undesirable reactions with a variety of materials, even with such inert substances like molecular nitrogen (e.g. formation of etch resistant nitrides).
144
4 Dry-Etching Methods
The possibility of undesired chemical reactions with the sputter-gas is avoided by using noble gases that are atomic and extremely inert. Argon is preferred as sputter-gas, because it is the most frequent of the noble gases in the normal atmosphere (99 % of the noble gases) and hence is easily to produce and inexpensively available. Besides its atom mass of 40 g/mole corresponds with the atom masses of many microtechnically relevant elements. Thus those can be sputtered efficiently with argon. Argon is also used as inert gas in other microtechnical vacuum processes, e.g. deposition processes (sputter deposition). Besides argon is the inert component in various reactive gas mixtures (see section 4.4.2 to 4.4.10). By the combination of inert argon and reactive gas components the ratio of sputter-removal to reactive removal by specific chemical reactions can be varied in a wide range. Apart from the influence of the sublimation energy, sputter-etching is practically non-selective. Chemical rate and equlibrium constants that differ by orders of magnitude and facilitate selective removal in wet-etching, vapour etching and plasma-etching processes do not play a role in sputter-etching. Hence, sputter-etching is used if selectivity is of no importance or an unspecific removal is requisite, e.g., for etching through stacks of different materials. In contrast to wet and plasma etching, microstructures can be transferred with very accurate measures by sputter-etching. Besides vertical walls can be prepared due to the vertical impact and hence, patterns with high aspect ratios. Sputter-etching is used for a number of materials that are chemically very inert so that sufficiently high etch rates cannot be achieved with wet- and plasma-etching methods. To this number belong the noble metals Pt, Ir, Rh, Pd and resistant binary compounds like carbides, borides, nitrides and some oxides. A special problem in sputter-etching is the etch-resistant mask, as this due to the low selectivity of the method is also etched. Materials with high sublimation energies like Si02 are well suited. Frequently however, for very thin films masks are used that have an higher etch rate than the functional film to be etched. The necessary thickness of the etch mask has to be determined according to the conditions of the etching process. With a respective high removal of mask material the geometry and its change during the etching process has to be considered in optimizing the etching conditions.
4.4.2 Reactive Ion Etching (ME) Instead of noble gases reactive gases can be used as atmosphere in sputteretching. In the FUE-process cations are produced from reactive gases, that are accelerated with high energy to the substrate and as well can react chemically with the substrate material. From the reactive gas ions and reactive neutral particles are formed that support the etching process. Ions as well as neutral particles of high kinetic energy are formed also in the periphery of the plasma. They get their energy by collision processes with accelerated ions or are formed of the ions by collisions with charge transfer. Radicals and other reac-
4.4 Etchig Methods with Energized Particles
145
tive species get to the surface by diffusion as in sputter-etching. Thus RIE combines the characteristic properties of the sputter-plasma (particles of high kinetic energy) with those of plasma etching (highly reactive thermalized particles). Choosing adequate etching gases and excitation conditions, the specific advantages of plasma etching (high selectivity) and of sputter-etching (anisotropic removal) can be combined in the RIE process. The ME-plasma can be generated as in sputter-etching in a planar reactor'30.The generation of the thermalized and the energized particles proceeds in the same non-partitioned plasma space. Hence, the forming and decomposing rates of all types of particles are strongly interdependent. The change of characteristic parameters like HF-power, HF-frequency, total gas flow, total pressure, partial pressures, superimposed DC-signal, electrode distance influences the concentration distribution and energy distribution of all kinds of active species, that an independent setting of inner plasma parameters (particle concentration and particle energies) is not feasible. Parameter changes affect differently the various kinds of particles and their concentrations and energy distribution, so that by the choice of the outer conditions at least a certain influence is viable. The composition of the etching gas, the power density of the plasma (related to the active etching area), the substrate temperature and the total pressure are the important parameters for adjusting the etch rates and the percentage of anisotropy.
Planar-Plate Reactor for WE-Processes The construction of the planar plate reactor in principle is completely in accordance with that of the sputter-etch reactor. Only the choice of the materials for the built-in equipment and the gas supply equipment have to be adapted to the applied reactive gases. This is especially important for the uncovered parts of the electrodes, that are exceedingly exposed to the attack of reactive and energized particles. As the concentrations of reactive gas components on one hand and of the reaction products on the other determine the removal rates the gas supply in the reactor has to be oriented to facilitate a most homogeneous concentration distribution of the etching gas and the products for achieving the same etch rate over the total area of the substrates, i.e. homogeneous etching over the whole area. In contrast to plasma etching the formation of a considerable bias potential to the substrate is required in reactive ion etching. In this field the ions from the plasma shall be accelerated to energies that are far above the sputter-threshold of the etching material. Hence, in analogy to mere sputter-etchingthe substrate stage is connected as working electrode (fig. 54). Its area should be smaller than the counter-electrode for achieving high acceleration potentials. As a rule planar reactors can be used for sputter-etching, plasma etching and reactive ion etching. The operation differs only in the kind of the used gases, the pressure in the reactor and the connection of the electrodes: J.A. Bondur (1976)
146
4 Dry-Etching Methods
Tab. 46. Modes of Operation of Parallel Plate Reactors for Dry-Etching Mode of operation
Sputter-etching
Plasma etching
Reactive ion etching
Substrate electrode Counter electrode Etching gas Pressure range
HF-electrode mass inert gas (Ar) ca. 0.1-5 Pa
mass HF-electrode reactive gas ca. 10-100 Pa
HF-electrode mass reactive gas ca.0.2-10 Pa
Whereas the total power is virtually determined by the HF-amplitudes, the removal conditions in RIE can be influenced by a superimposed dc-voltage (outer bias). By this voltage the electrical field can be enhanced in front of the working electrode. In this field the electrons are accelerated to higher energies. The ion energy is limited by collisions in the gas volume. At too high particle densities (higher working pressures) the ions loose their energy by repeated collisions and cannot reach the requisite velocities for a good sputter-efficiency. At low pressures the absolute ion density is lower, but it is high in relation to the total number of particles providing a strongly increased sputter-yield and hence higher etch rates. If the radicals of the plasma shall contribute to the etch removal to achieve a chemically controlled selectivity, the pressure must not be too low. The contribution of the radicals to the etch removal depends directly on the concentration. The share of radicals in the total particle number can be enhanced by a high bias. At low pressures, however, the radical concentrations are low at high plasma densities (high relative share of radicals), because the total particle density is low.
Etch Rates and Anisotropy In reactive ion etching two different mechanisms contribute to the etch removal: 1. Etching by impact of energized particles (mainly ions) 2. Etching with thermalized highly reactive particles (mainly radicals) In contrast to mere sputter-etching, reactive components can enhance the etch rate in ion etching also with the first mechanism, e.g. if the impacting ions react with the substrate material forming volatile species or if surface species are released that had been formed beforehand by specific reactions with components of the plasma. Etching with energized particles means that the direction of movement is the preferred direction of etch removal. Hence reactive ion etching is counted to the anisotropic etching processes. As energized particles impact preferentially vertically to the substrate surface, the preferred removal proceeds in vertical direction. Inclining the substrate (working elec-
4.4 Etchig Methods with Energized Particles
147
trode) to the counter-electrode changes hardly this direction as the ions follow the lines of force of the field, that are bent according to the tilt of the substrate standing nearly vertically on the substrate surface despite its incline to the counter-electrode. Because of that, anisotropically etched more gradually sloped sidewalls cannot be prepared by RIE simply by inclining the substrate. The second mechanism corresponds to the conditions in plasma etching. Removal is performed by thermal activation. The particles diffuse from all directions to the surface and hence are available for reaction on all surface elements with the same probability. This etch mechanism effects isotropical removal. The mechanism is temperature dependent as a rule, i.e. thermal activation barriers determine the reaction rate. The medium temperature of the surface is determined by the sum of energy input by the energized ions and the reaction heats released by all surface processes. The common occurrence of isotropic and anisotropic etching is a fundamental property of reactive ion etching. By the choice of the plasma conditions the ratio of anisotropic to isotropic etching can be influenced. Whereas by high HF-amplitudes as well the ion as the radical density is increased, an increase in pressure causes a shift of ionic to radical etching. At high pressures (above ca. 10 Pa) the free path lentghs are short and the ion energies and hence the sputter yields low. The anisotropic component of etching is weak. The radical density can be high under the same conditions. The isotropic etching dominates. Chemical selectivity is better in this case. At low pressures (ca. 1 Pa and less) the free path length is large. Ions are accelerated over greater distances in the electrical field of the plasma without loosing their energy in collision processes. They obtain a high energy and the sputter-yield is high. As a rule anisotropic etching is strong. The concentration of radicals in the gas volume is low, and hence isotropical etching weak. At low pressures the anisotropic mechanism is predominant. Chemical selectivity cannot be achieved. A high degree of anisotropy and high selectivity at the same time can only be achieved in the case that desorbable particles are exclusively formed if the impact of energized particles and a chemically specific reaction step work together. This can only be achieved if on the one hand, the sputter-threshold is high and hence not reached in the process and on the other, removal processes by thermalized particles without impact of energized particles can be neglected. Such conditions are very rarely obtained in praxi. However, impact-free removal processes can be reduced by an intensive substrate cooling. Two models for anisotropic selective removal can be proposed: A) Chemical-assisted etching with energized particles
Ten+ OF + Pa Pa
+ T, + Pd
148
4 Dry-Etching Methods
B) By energized particles supported etching with reactive gas or plasma
T,
+ OF+
Pa
(107)
(Tenenergized particle; TTthermalized particle, Pa adsorbed product, Pd desorbed product) By a skilful choice of gas composition, pressure and power, the whole range between unspecific, anisotropic sputter-etching and specific, isotropic plasma etching can be performed by reactive ion etching. In this way selectivity and degree of anisotropy are freely but not independently of each other adjustable. For higher selectivity diminished anisotropy must be accepted. With a high degree in anisotropy the etch removal cannot be very selective. The best possibility to influence the intensity of isotropical removal independently of pressure is by choosing the surface temperature. Whereas anisotropy produced by impact of energized particles is relatively independent of temperature isotropical etching can be enhanced by temperature increase and nearly suppressed by an effective cooling. By intensive cooling high degrees of anisotropy can be achieved in the higher pressure range of ME which is important for high rate processes. The smallest undercutting was found in cryogenic RIE of silicon in a SF,-plasma at -120 "Cand of polyimide in an oxygen plasma at -100 "C, re~pectively'~'.An efficient method for profile control in RIE is the specifically used side wall passivation, i.e. the deposition of material at the walls of the etch groove (see section 4.4.12.). To achieve high selectivity, high anisotropy, high mask stabilities and high etch rates for etching metal films, there is a tendency to apply low pressures and highest plasma densities. The necessary etch reactors work with pressures in the range of 1 to 20 mtorr (0.13 to 2.6 Pa), at 0.5 Pa, prefer en ti all^'^^.
Etching Gases The choice of the etching gas depends on the material to be removed. In principle, the etching gas - as in vapour and plasma etching - has to facilitate the formation of volatile compounds of the respective material. Hence in etching metals, semiconductors and their alloys and compounds halogens, interhalogens, halogeno-hydrocarbons and other halogen compounds are preferredly used. For RIE of organic materials oxygen is the appropriate etching gas as in plasma etching. An increase in the etch rate and a reduction of secondary depositions is performed by a suitable mixture of etching gases. Beside F-radicals for H-abstraction, reducing additives like hydrogen play a role. 13' 13'
K. Murakami et al. (1993); M. Takinami et al. (1992) P. Burggraaf (1994); J. Givens et al. (1994)
4.4 Etchig Methods with Energized Particles
149
Geometry-Dependent Etch Rates In analogy to the loading effect in plasma etching the etch rate in RIE can depend on the area ratio of the area to be etched. This is the case if the transport of the reactive species to the surface gets rate-controlling. Structure size-dependent etch rates are observed mainly in etching structures with high aspect ratios. Because of this dependency the etching behaviour is called aspect ratio-dependent etching (ARDE).The etch rate decreases with increasing etch depth if small holes or grooves are etched. Narrow structures etch more slowly than wider ones, and the etch rate decreases with progressing etching time, i.e. with growing depth of the etch grooves. This phenomenon is called RIE-lag. The decrease of the etch rate with the structure width is determined by the local particle trajectories in the holes and grooves. For the trajectories of the charged particles the local field distribution is essential beside the pressure. With increasing aspect ratio of narrow grooves or holes and increasing pressure an increasing deflection of the energized particles occurs. The angular distribution of the energized particles determined by reactor pressure, field conditions in the reactor, local field distribution in the immediate neighbourhood of the etching surface is the central factor of influence for the RIE-lag’33. ARDE is more distinct at higher pressures than at low pressures. Hence the effect increases from microwave etching ( 4 0 mtorr), to RIE (10 to 100 mtorr) to chemical plasma etching (0.1 to 10 torr)l’. At least in plasma etching the collision frequency with increasing pressure and hence the frequency of contacts between reactive particles and the sidewalls play an essential role beside the direction distribution of fast particles ( “hot” molecules, radicals or ions). The frequency of wall contacts of the impacting energized particles increases with decreasing structure width and increasing structure depth13’. The wall contact of energized particles also determines the shape of the structure walls. In narrow structures reflexions can occur at the etching material but also at the edges of the etch resistant mask’36.Dramatic minimizing of the etch rate with increasing etch depth was observed in cryogenic RIE-deepetching of silicon for micromechanical structures. The etch rate of 5 pm-wide structures decreased and showed in the depth of 30 pm only 1/10of the initial value13’. The geometries of the side walls can be simulated with good approximation by computer programme^'^^.
Gottscho et al. (1992); H. Jansen et al. (1997) K. Nojiri et al. (1989), vgl. auch R.A. Gottscho et al. (1992) Y.H. Lee und Z.H. Zhou (1991), s. z.B. auchA.D.Bailey et al. (1995) J.W. Cobum und H.F. Winters (1989), E.S.G. Shaqfeh und Ch.W. Jurgensen (1989) 13’ M. Esashi et al. (1995) 1)8 J. Pelka et al. (1989); Y.-J.T. Lii and J. JomC (1990)
u3 R.A. 134
”’
150
4
4.4.3
Magnetic-Field-Enhanced Reactive Ion Etching (MERIE)
Dry-Etching Methods
In analogy to plasma etchin the plasma density in RIE can be enhanced by the aid of an magnetic field". In MERIE the magnetic field serves mainly for generating a high density of reactive ions. Besides the radical concentration is increased at the same time and hence the total plasma density. In the simplest case the working electrode is a magnetron electrodelm. Magnetic field enhancement provides a considerable increase in the etch rate at the same HF-bias power. In MERIE of GaAs with silicontetrachloride etch rates up to 25 n m l s were performed at 2 mtorr (0.26 Pa)141.InP was etched in MERIE with rates up to 2 nm/s in reducing atm~sphere'~~. In MERIE of photoresists using an additional magnetic field, etch rate increases by the factor 2.5 were observed. The etch rate enhancement is possibly material-dependent and can be used in this case for increasing the etch rate ratio. Thus by using a magnetic field the etch rate as well as the selectivity in 0,-RIE of silylized photoresists (DESIRE-process) was improved'43.
4.4.4
Ion Beam Etching (IBE)
Ion beam etching is a special kind of sputter etching (section 4.4.1). As in sputtering also in ion beam etching removal is achieved by the impact of energized ions or energized neutral particles on the solid surface. It differs from sputter-etching by the spacial seperation of ion generation and etching in different parts of the reactor. This functional separation facilitates the variation of the etching conditions, as the plasma generation and the extraction and the acceleration of the ions can be adjusted to a great extent independently. The reactor pressure is considerably lower than in sputter-etching (fig. 4-57). Ion beam etching can be performed in wide ranges of particle density and particle energy. The basic separation between plasma generation and etching is achieved by an electronical decoupling of both parts of the gas volume. By an ion source a high density of ions is first produced. The ions are extracted by an acceleration voltage from the source into the actual reactor. The acceleration voltage for generation and acceleration of ions is not generated by a field opposite to the substrate as in the case of a planar plate reactor, but by a further electrode (grid). That allows the more exact setting of the kinetic energies of the ions (fig. 4-58). Thus it is possible to define more accurately the conditions of etching by energized particles. After extracting the ions out of the plasma generation space, they can get into a nearly field-free space to which by choice a H. Okano et al. (1982) H. Okano et al. (1982) 14' M. Meyyappan et al. (1992) '41 J. Singh (1991) 143 H.J. Dijkstra (1992) 139
'40
151
4.4 Etchig Methods with Energized Particles 1
d8 1 torr
1d6 particle density
10 mtorr
0 7 - 3 1 14
10
0.1 mtorr
id2
1 ddtorr
10
10 0.01
0.1
1
10
100
1 000
10000
particle energy [ev
Fig. 4-57. Ranges of particle density and particle energy in essential dry etching processes cathode [electronsource]
gas inlet
Onode permanent magnet
-----
shledllng mesh
acceleration electrode neutralisation electrode
subsirate table
Fig. 458. Ion beam etching reactor (principle)
I
vacuum system
DC-field can be applied for retarding or accelerating the ions. Another electrode facilitates the neutralization of the ions without decreasing the density of energized particles. Thus the substrate can be bombarded with ions or energized neutral particles. Beside electronic decoupling, the chemical conditions in the plasma source and in the reaction zone can be made different by using separate gas supply systems to the respective parts of the reactor. By a separate gas outlet from
.
152
4 Dry-Etching Methoh
the plasma source plasma components that should not reach the substrate can be held back. This separation is of importance in reactive ion beam etching (FUBE, see section 4.4.5) and in chemically-assisted ion beam etching (CAIBE, see section 4.4.7).
Ion Sources For the generation of ions a variety of different kinds of sources is known, that are applied in microelectronics and micromechanics. For dry-etching processes the following types are preferred? Kaufman source
The Kaufman source is a hot cathodic source. A wire electrode consisting of an inert, temperature-stable material is heated by an electric current. Tungsten or tungsten-rhenium alloys are used because of their extremely high melting points. For creating a large reactive surface, the electrodes are shaped as hair pin or spiral. In the discharge chamber of the Kaufman source an anode is fixed in shape of a cylindrical metal sheet. The electric field between the heated cathode and the anode extracts the spontaneously emitted electrons from the surroundings of the cathode. Their kinetic energy is so high that by collisions with gas atoms in the discharge chamber cations are formed. The density of the ions is frequently enhanced by additional magnetic fields. The ions are extracted by a cathodic voltage out of the discharge chamber. For this purpoose grids are used consisting of a material rather stable against the sputter effect (e.g. carbon or molybdenum). At the same time these grids serve as shields against electrons. The extraction potentials are in general one to several k e y High-frequency sources (HF-sources)
Charge carriers can be generated in a gas volume without primary electron emission from a special electrode or an arc discharge. As in plasma etching (see section 4.3.) an ion beam can be formed from a source in which an alternating HF-field from a HF- or a microwave source (see below) is capacitively coupled in by flat electrodes (e.g. metal plates) or inductively by coils. The advantage of such arrangements is the fact that the electrical or electromagnetical functional elements need not be mounted in the discharge space itself. Instead the alternating fields can be generated outside the ionization chamber and coupled in through a dielectric wall into the discharge chamber. Alternatively microwaves can be coupled in by hollow conductors. At frequencies in the MHz-range the cyclotron resonance frequency of ions is reached that allows an efficient biasing with HF-energies. The efficiency of the biasing electromagnetic alternating field can be increased by an additional magnetic field. The ion beam is as in the Kaufman sources extracted from the discharge chamber by an extraction grid or hole. 144
H. Frey (1992)
'
4.4 Etchig Methods with Energized Particles
153
Electron Cyclotron Resonance (ECR-)Sources
The ECR-sources are a special case of HF-sources. In difference to conventional HF-sources, microwave frequencies are used. By the three to four orders of magnitude higher frequency, resonance with the moving electrons of the plasma is possible. Thereby high power densities can be resonantly coupled into the plasma. The resonance frequency of electrons is much higher than the resonance frequency of ions due to their much lower mass-to-charge ratio. Because of the high plasma densities ECR-sources are widely used recently. They allow the generation of a high cation density in the excitation space from where high ion currents can be extracted. Thus high ion densities and hence high etch rates can be achieved on comparably large substrates.
Plusmatron Sources As in Kaufman sources ionization in plasmatron sources is done between a heated cathode and an anode. In contrast to the Kaufman source the ions are generated by an arc discharge. A spatially bounded plasma of very high density, a so-called plasma bubble, is formed between the electrodes. Through a drill-hole in the conical anode an outer electrical field can affect the plasma bubble and extract cations from the plasma. From these ions the ion beam is formed by an appropriate arrangement of further electrodes. Held Emission Sources Between a pin with a very small point radius (<1pm) and a drilled-through electrode positioned very closely to the point of the pin, extremely high field strengths can be produced at moderate voltages. Thus gas atoms adsorbed at the top surface can be ionized forming a source for cations. Instead of a fast pin electrode a tiny droplet of a molten metal of extremely low vapour pressure can serve as electrode. Magnetron Sources As magnetron sources in IBE, sources are understood that are assisted in enhancing the ion density near the target surface by an additional magnetic field. They do not serve like the other sources for IBE for the primary generation of the plasma, but only for enhancing the etch effect of the ion beam generated in another source.
Etching with Chemically Inert Energized Ion Beams Etching with inert ions corresponds to the conditions in simple sputteretching. In contrast to sputter-etching, the ion current density and the ion energy in ion beam reactors can be adjusted independently of each other and in a relatively narrow distribution range. Thus it is feasible to obtain considerable etching rates already shortly above the sputter-threshold, by setting a high ion current density. Thus selective etching of a material with a low sput-
154
4 Dry-Etching Methoa's
ter-threshold against a material with a high sputter-threshold is possible. The etch rates of different materials are different, but they differ only by one to two orders of magnitude (see table 4-7). For etching with an inert ion beam argon is preferentially used as etching gas as in sputter-etching in a parallel plate reactor. Table 4-7. Etch rates in IBE with Ar-ions ; ion energy: lkeV, ion current density: lmA/cm2, pressure: 0.05mtorr
Material
Chemical symbol
Etch rate
Reference
Aluminium oxide
A1203
0.2 n d s
Chromium Chromium Titanium
Cr Cr
0.33 n m / s 0.33-0.67 n d s 0.33 nm/s
R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E. Lee (1984) C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) C.-M. Melliar-Smith (1976) C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E. Lee (1984) C.-M. Melliar-Smith (1976) R.E. Lee (1984) R.E. Lee (1984) C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E.Lee (1984) C.-M. Melliar-Smith (1976) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) C.-M. Melliar-Smith (1976) R.E. Lee (1984) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976) R.E. Lee (1984) C.-M. Melliar-Smith (1976) R.E. Lee (1984) R.E. Lee (1984)/ C.-M. Melliar-Smith (1976)
Ti
Vanadinium Manganese Niobium
V Mn
Nb
0.37 n d s 0.45 nm/s 0.5 n d s
Zirkonium
Zr
0.53 nm/s
Iron
Fe
0.53 nm/s
Silicon Silicon Photoresist KTFR Silicon dioxide Silicon dioxide Molybdenum
Si Si (Hydrocarbon) Si02 Si02 Mo
0.63 nm/s 0.6-1.25 nm/~ 0.65 n m / s 0.67 n m / s 0.47-1.11 nm/s 0.67 nm/s
Aluminium AlUminiUm Photoresist AZ 1350 Lithium niobate Iron(11)-oxide Electron resist PMMA Gold Gold Silver Gallium arsenide
A1
0.74 nm/s 0.75-1.2 nm/~ (Hydrocarbon) 1 nm/s
Al
Limo3 1.1 n m / s FeO 1.1 n m / s (Hydrocarbon) 1.4 n m / s Au Au Ag GaAs
2.7 nm/s 2.7-3.6 nm/~ 3.3 n d s 4.3 nm/s
4.4 Etchig Methods with Energized Particles
155
Etching with Energized Neutral Particles As with several materials the charge condition of the impacting particles affects the kind of the proceeding process of atoms and molecules, beams of neutral particles are preferred to ion beams in certain cases. E.g., if organic polymers are etched, they can interlink or fracture under the influence of the charge of energized particles. The achievable changes in selectivity by the choice of the charge condition of the particle beam are generally small. In the simplest case beams of noble gas ions (preferentially argon) are neutralized for etching as neutral particles. As well other gases from which an ion beam is formed in the ion source can serve as etching beam of neutral particles.
4.4.5 Reactive Ion Beam Etching (RIBE) In comparison to etching with beams of inert particles an essential increase in the etch rate results if reactive gases serve as source for the ion beam. In that case the particles extracted from the source act not only as carrier for the kinetic energy, but react with the material to be etched forming volatile compounds that desorb easily from the surface. Beside the increase in the etch rate the selectivity of the process is improved. RIBE and IBE show a similar relation as RIE and sputter etching in inert plasmas. In the principal construction the RIBE-reactor is like other ion beam etching reactors. In contrast to those it possesses beside the gas supply for the inert carrier gas (argon) another gas supply system for reactive gases. The choice of the material for the reactor equipment is more demanding for reactive gases than in the case of reactors in which only inert gases are used. As well the reactor walls as the electrodes must be corrosion-resistant against the etching gases and their products formed in the plasma. The attack of the material and the fine electrode arrangements is really considerable. Especially the choice of the extraction and neutralization electrodes that are expoesd to the continuous bombardment of reactive ions has to be tuned to the used gases to accomplish justifiable service life of the equipment. To reactive ion etching in the broader sense etching with a surplus of reactive gas molecules is counted, where the gas molecules are let into the reactor through the source. Under adequate conditions of the gas flow by diffusion, i.e. thermal movement, these gas molecules can get to the substrate surface, and contribute to an acceleration in etching by forming volatile products. The process corresponds to the conditions in CAIBE (see section 4.4.7). Beside the neutral gas molecules and ions also radicals and other reactive species can be formed from the etching gas diffusing also in the etching chamber. The latter particles can contribute together with the beams of energized particles to enhancing the etching rates .The chemical conditions in removing are related to those in RIE. The thermalized radicals cause a significant etch rate in lateral direction, i.e. they increase the etch removal, but decrease the degree of anisotropy. In contrast to reactive ion etching the portion of energized parti-
156
4 Dry-Etching Methods
cles, their energy and flow densities and the portion of molecules and radicals can be essentially more specifically set by choosing the gas flows, pressures, the ion energy and the ion current density in the reactor.
4.4.6 Magnetic-Field-Enhanced Reactive Ion Beam Etching (MERIBE) Also in RIBE the density of reactive ions and hence the etch rate can be enhanced by magnetic fields. The method is termed magnetic-field-enhanced ion beam etching (MERIBE). Whereas the simple structure of a planar plate reactor has only limited space for arranging magnetic fields, the magnetic field can be placed at several positions in the RIBE reactor. A magnetic field in the ion source, as it is frequently used in ECR-sources, is of advantage. The electrons forced to spiral trajectories produce by enhancing the plasma density an increase in the ion density. Thus essentially higher ion currents can be extracted from the source into the reactor. Additional magnetic fields in the etch reactor further increase the etch rate. Using an ECR-source and an additional magnetic field for focusing the ion current, etch rates of 7 nm/s were accomplished etching Si in chlorine with about 1% oxygen'45.Especially for etching substrates of larger diameters an inhomogeneous magnetic field causes a strong inhomogeneity in the etch rate. The magnetic field can be made more homogeneous if in the substrate position additional field coiles are arranged. Under these conditions the lines of strength of the magnetic field are vertical to the substrate surface and across the whole diameter homogeneously di~tributed'~~.
4.4.7 Chemically-Assisted Ion Beam Etching (CAIBE) Whereas in ion beam etching the reactive gas particles are formed in the ion source to which the gas is let in, the reactive gas components are let directly into the etch reactor in the process of chemically-assisted ion beam etching (CAIBE). Thus the generation of ion beams is separated from the supply of reactive particles. The reactive gas molecules diffuse to the surface of the substrate, but are restricted from the source. Hence the attack of the source material and of the other reactor equipment is minimized. In principle two mechanisms of increased removal by assisting reactive gases are feasible. These mechanisms correspond to the cases given in the equations (91) to (94). In contrast to reactive ion etching both mechanisms can be adjusted in reactive ion beam etching by the guidance of the energized 145
146
D. Dane et al. (1992) Ch.Takahasi und S.Matsuo (1994)
4.4 Etchig Methoak with Energized Particles
157
particles on the one hand and the thermalized particles on the other. In case (A) at first a reaction or physisorption of the separately introduced reactive gas at the solid surface takes place and subsequently the reaction due to the impact of a particle of well defined energy from the ion or neutral particle beam forming volatile products:
(A) 1.Thermal gas molecule solid surface intermediate 2. intermediate + energized particle S etch product
*
+
In the other case (B) primarily excited surface states are formed by the impact of an energized particle that subsequently reacts with the reactive gas component to volatile products.
(B) 1. energized particle + solid surface excited surface state 2. excited surface state + thermal gas molecule s desorbed etch products
*
As “hot spots” after impact of energized particles on surfaces rapidly abate and reactive surface states after impact of energized particles relax, the second mechanism is the more improbable. Especially efficient effects of the reactive gas in CAIBE can be expected if, assisted by the reactive component, a high density of chemisorbates is formed on the surface. Thus high sputteryields are realized by the impacting beam particles desorbing volatile particles which contain the atoms of the material to be etched.
4.4.8 Reactive Etching with Excitation from Several Sources For achieving high etch rates in dry-etching processes, a high energy density must be coupled into the plasma. Because of the very different dynamic properties of atoms and ions on the one hand and of eletrons at the other hand, but also because of the very much differing life times of active species, it is important in what frequency range and in which position energy and where additional magnetic fields are coupled into the plasma. A consequent continuation of the concept of densifying the plasma by additional electrodes and magnetic fields is the combination of sources for high frequency alternating fields working in different frequency ranges.
Microwave-AssistedReactive Ion Etching An essential enhancement of the plasma density is accomplished by combining HF-sources and microwave sources. Microwaves have the advantage that they need not be generated in the reactor, but can be introduced by wave guides, e.g. hollow conductors, into the reactor. Through a dielectric material the microwaves can be conveniently coupled in the desired zone of the plasma.
158
4 Dry-Etching Methods
Beside microwave-enhanced plasma etching (see section 4.3) microwaves are also applied for anisotropic dry etching in reactive ion etch processes using HF-sources with powers of 0.1 to 1kW. The frequencies of both sources differ by two orders of magnitude. Whereas for the HF-generation sources of 13.56 MHz are used, the preferentially used microwave sources are at 2.45 GHz. In some arrangements the HF-source in the plasma is assisted by the microwave guide. However, it turned out that high etch rates can be achieved if high energy densities are coupled in by means of the microwave. Then the energy of the HF-power need not be very high. The HF-source is already efficient if only 10 to 20% of the total power are put in at the low frequency. Thus the HF-excitation may be understood as assistance to the microwave excitation. In this respect this multi-source method should be more correctly termed HFassisted microwave etching.
4.4.9 Electron-Beam-Supported Reactive Ion Etching (EBRE) Beam etching processes can be supported in the same way by extraction and acceleration of electrons as by energized ions or atoms. The advantage of electron-supported dry etching consists in the much lower attack of the substrate in the case of energized electrons compared to ions. In an arrangement similar to such for an CAIBE-process, an ECR-source is used from which by a positive electrical field negative electrons instead of Ar+-ions are extracted into the etching chamber. Above the substrate the etching gas is let in (fig. 4-59). In EBRE of GaAs in presence of chlorine as reactive component the etch rate was increased by more than an order of magnitude in comparison to RIBE without electron beam upp port'^'. Satisfying removal rates can be achieved already using low-energized electrons (1 to 15 eV). In contrast to ion etching methods such low-energized electrons cause practically no damage of the substrate material’&. A directly writing, i.e. maskless, microlithography is possible if the primary patterning by the electron beam and the subsequent plasma etching proceed A preconone after the other in time (“in-situ electron beam ~atterning”)’~’. dition for this method is that a film covers the material to be etched and that this film changes its resistance to the attack of the etching plasma drastically. The film reacts like an electron resist. The speciality of the process is that in consecutive etching as well the exposed upper film as the functional film are etched with high rates, while the unexposed areas rest stable. This process is physically not to be understood as electron beam supported etching. However, it is related to it. By the process patterns in GaAs were structured, changing at first its surface by an oxygen plasma process (ECR) into a thin oxide film and then writing with the electron beam the pattern, followed by H. Watanabe und S. Matsui (1993) H.P. Gillis et al. (1996) 149 N. Takado et al. (1992) 14’
4.4 Etchig Methods with Energized Particles
159
U GAS
U
Fig. 459. Reactor for electron beam assisted etching (principle)
VACUUMSVSlEM
reactive ion etching in chlorine atmosphere. Thus the exposed oxide and the underlying GaAs were etched selectively to the unexposed areas.
4.4.10 Focused Ion Beam Etching (FIB) Ion beams can be focused and optically guided like light (photon beams) or electron beams. That can be used to let ion beams react only on a spatially limited area. With ion beams due to their extremely short wavelength, essentially smaller foci than with light or electron beams can be accomplished. The particle densities in the small foci, however, are limited by the electrostatical repulsion of the ions. Are these focused ion beams applied for etching thin film materials, directly writing of lithographic patterns can be accomplished by accordingly guiding the beam. The basic construction features of the exposing device and the features of the method are analogous to those of direct writing by electron-beam-assisted etching in the gas phase (see section 4.2.4). In the place of an electron-optical a respective ion-optical system is necessary, the construction of which must correspond to the essentially greater mass-charge ratio of the ions. As in electron-assisted etching, also in FIB a vacuum system and a device for mechanical alignment are necessary (fig. 4-60) The FIB-method is used for lithograhic operations on small areas and single substrate treatment, as e.g. for repairing lithographic masks. Generally the pure sputter-effect of the impacting focused particles on the surface is used for material removal'''. The ion energies are as a rule by one or two orders of magnitude higher than for IBE and are typically between 10 and 100 keV, rarely <1 k e y J. Melngailis et al. (1986); P.J. Heard et al. (1985)
lM
160
4 Dry-Etching Methoak
- -I .
r msvstem
-stage
Fig. 460. Reactor with optical column for beam shaping in directly writing etching with focused ion beams (focused ion beam etching, FIB-etching, principle)
Higher etch rates can be reached in the FIB-process by reactive atmospheres. In this case the reactor has to be equipped with a supply system for reactive gases. With chemically assisted etching (Cl,) of Si, Al and GaAs with focused ion beams (Ga', 30 keV) submicrometer structures with high aspect ratios (>5) could be directly written151.The removal per incident ion depends on its energy and the composition of the gas atmosphere. In the following table such removal yields are shown for several methods. Low ion energies are applied if radiation damage must be avoided, e.g. in the lithographic fabrication of sensitive structures in monocrystalline materials. In this case strongly reactive etching gases are used, but the removal process has to be activated by ion beams. This method (ion-beam-assisted etching IBAE) can be performed advantageously with focussed ion beams. Thus a writing etching is possible under comparatively gentle conditions. The method was used for etching GaAs with Ga+-ions in Cl,-atrno~phere'~~.
4.4.ll Nanoparticle Beam Etching (NPBE) Nanoparticle beam etching is a special etching method basically related to ion beam etching methods. The mechanical momentum transfer from an energized particle to a substrate is applied to carry material from the substrate into the gas phase. Whereas in IBE ionized atoms or molecules of a few atoms are used, the beams in NPBE consist of nanoparticles of high kinetic energy. The typical diameter of the particles is about 5 nm, i.e. the particles contain about lo4 atoms.
152
R.J. Young et al. (1993) T. Kosugi et al. (1991)
4.4 Etchig Methods with Energized Particles
161
Table 4-8. Particle yield per Energized Ion in Etching with Focused Ion Beams in Reactive Gas Atmosphere for Some Chosen Materials (K. Gamo, S. Namba 1990(a) and R.J.Young et al. 1993 (b)
Target material
Kind of ion
Energy
Reactive gas
Al Al
Ar Ga Ga Ga Ga Ga Ar Ga Ga Ar Ga Ga Ar Ar,Xe Ar Ga Ar,Xe
0.5 keV 35 keV 30 keV 30 keV 30 keV 35 keV 0.5 keV 30 keV 35 keV 0.5 keV 35 keV 30 keV 0.5 keV 50 keV 0.5 keV 30 keV 50 keV
Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Chlorine Xenon fluoride Chlorine Chlorine Xenon fluoride
A1 Au GaAs GaAs GaAs InP InP InP Si Si Si Si3N4 SiOz SiOz SiOz
Pressure
particle yield
Ref.
64 20 mtorr
7 ca. 5 10-26 50 20 35 32 80 80 9 21 4 9 0.4 1.4 27
a a b b a,b a a b a a a b a a a b a
5 torr 20 mtorr 20 mtorr 20 mtorr 20 mtorr 20 mtorr
The nanoparticles receive their energy in total analogy to ion beams by charging and acceleration in the electric field. At particle energies of 100 keV impact craters are formed of a few nanometers. The size of these craters is still much smaller than the sizes of current microtechnical structures. In the case of sub-micrometer patterns the size of the craters could be perceptible in edge roughnesses. The material of the nanoparticles contains reaction partners that form volatile products with the target material. Thus the method can be used as reactive nanoparticle beam etchinglS3.
4.4.12
Formation of the Structure Sidewall Geometry in Ion Beam Etching
Angle Dependence of the Etch Rate The interaction of the energized particles with the substrate surface is independent of the kind of ion generation. Hence, an angle dependence occurs as well in ion beam etching as in sputter-etching (see section 4.4.1). The maxi153
J. Gspann (1995)
162
4 Dry-Etching Methoa3
numbers indicate the relotive density of impacting ions on areas of different inclination
Fig. 461. Density of the impacting particles in beam etching in dependence on the surface inclination to the impact direction of the energized particles
mum removal at an angle of 60”observed at cubic crystals in ion beam etching with inert ions can be diminished or shifted to slightly other angles in the case of reactive plasmas. The angle dependency of sputter yields provides changed etch rates on the total substrate area for substrates that are inclined to the ion beam. In the case of substrate surfaces raised in relief the locally varying inclination of the surface causes different sputter yields, and consequently differing etch rates. Improving the sputter yield by inclining the substrate to the vertical ion beam is accompanied by a decrease in the ion density on the surface due to geometric relations. The greater the incline the lower is the area density of the impacting beams (fig. 4-61). Profile Formation (Benching, Bowing, Facetting) The direction of impact of the energized particles is dependent on the orientation of the substrate in the reactor in ion beam etching, but not in sputter etching and reactive ion etching where due to the bias voltage the ions are vertically accelerated to the surface and hence impact vertically. The arbitrary orientation of the substrate in ion beam etching allows to produce certain profiles of structure sidewalls and undercutting angles under the mask edges. Especially it is possible to produce etch grooves with sidewalls that are parallel to each other, but inclined to the substrate plane (fig. 4-62). A characteristic of sputter etching and ion beam etching is the effect of the individual ion trajectories on the shape of the generated structures. Thus particle reflections at the sidewalls of the mask and the etching structure occur. They are noticeable as the incident beam has a preferential direction. At steep edges those particles are reflected with a negligible energy loss and impact on other surfaces. There they cause an increased etch removal according to their angle of impact, as they come as an addition to the normal impact enhancing the density of energized particles. Hence often trenches are observed along etched edges, that is why the phenomenon is called “trenching” (fig. 4-63). If at more declined etched edges of narrow mask openings the particles are reflected to the opposite wall the result is an increased lateral removal produc-
4.4 Etchig Methods with Energized Particles
Fig. 462. Fabricating vertical and sloped side walls in ion beam etching
Fig. 463. Origination of the trenching effect along steep structure edges by increase of the effective density of impacting ions by reflection of the energized particles in ion beam etching
163
164
4 Dry-Etching Methods
Fig. 4-64. Origination of the bowing effect by reflection of energized particles at sloped mask edges
ing bowing sidewalls (fig. 4-64). The bowing effect can be enhanced by an isotropical etching component. Besides it is possible that in deep etch grooves by locally formed surface potentials the etch removal into the depth is diminished contributing to an inreased lateral removal, i.e. to the bowing effect'54. Sometimes small areas are formed at the structure edges which are so inclined that the sputter rate is reduced. Energized particles are reflected at these areas. The neighbouring areas, however, are etched. Thus the areas with the critical inclination angle are enlarged. The areas grow into a contiuous smooth plane that is inclined to the substrae plane forming a facette (facetting) .
Redeposition and Sidewall Passivation Species of the gas volume are in principle able to react by adsorption to the substrate surface. Such an adsorption takes place if on contact of the particles with the surface, products are formed that cannot easliy desorb. In sputter etching clusters or atoms of the removed material can simply recondensate on the surface. In reactive plasmas deposition processes can proceed on the surface instead of removal processes. Under certain etching conditions a thermally controlled deposition process can compete with the etching process which is initiated by the impact of energized particles: 1. Particles of the gas room + surface + film deposition 2. Surface energized particles + film removal
+
154
J.W. Coburn and H.F. Winters (1989)
4.4 Etchig Methoh with Energized Particles
165
In such a case the density of the incident energized particles determines whether deposition or removal of material is locally predominant. At high densities the surface material is removed, at low densities a layer is deposited. Areas vertical to the preferential direction of the ion beams, i.e. as a rule being parallel to the substrate surface, get the highest particle density in the process. With increasing inclination angle of the areas the effective particle density is diminished. Vertical walls practically get no energized particles. At a certain angle the velocity of the removal is the same as the velocity of the redeposition from the gas phase. At vertical walls redeposition is predominant. (fig. 4-65,4-66). Such a sidewall redeposition can be desired, if it forms a cover against undercutting and bowing, and in this way allows the formation of vertical sidewalls. But, such a protective effect is not always observed. In many cases, side-wall depositions are porous and do not inihibt the attack of side wall by radicals. In case of protective sidewalls, the quantitative ratio between the thermally controlled sidewall deposition and the removal by energized particles determine the shape of the structure sidewall^'^^. Sidewall depositions often occur in etching of e.g. silicon, silicondioxide, or other silicon-containing films or metal films, if fluorinated hydrocarbons are used as etching gases. Among others the radicals CF,, CF3,C2F4and GF5are formed in the plasma. These completely fluorine-substituted alkylradicals are preferentially adsorbed at solid surfaces and can there recombine to longerchained carbon skeletons that form a coating. These adsorption and chemical condensation processes are also possible, if not so pronounced, with other halogenoalkane radicals like chlorohydrocarbons also used for etching siliconcontaining layers, but aluminium and other materials as well'56. Frequently the chemical nature of the secondarily deposited material is totally different to the original film material. Hence, the etch rates of both substances can considerably differ.Thus undesired new structure elements can be formed disturbing the consecutive process steps. These disturbing sidewall depositions either have to be avoided by appropriate etch conditions or have to be etched afterwards in a secondary etch step. Undesired redepositions can als be formed on a structure-free and maskless surface. Redeposited small particles on the surface that possess very low removal rates can prevent etching in the underlying part of the substrate material and can form condensation nuclei for the deposition of more disturbing masking. As a result column shaped relicts of the etching material stay on the surface. If the redepositions are rather densly distributed forming columns of small diameters on the whole surface, the surface looks like grown over by grass. This effect is diminished by a certain proportion of isotropical etching, as by undercutting the small masked spots are again and again dislodged. The etching mask, films that are not to be removed in the actual process step, and the electrodes are material sources for redepositions. As these matelIis lIi6
J. Pelka et d. (1989); B. Vasquez et al. (1989) D.L. Flamm et al. (1984)
166
4 Dry-Etching Methods
etch mask IresL)
I functlonal laver to be etched
1
3
4
Fig. 4-65. Formation of sidewall depositions in the range of the mask edge and functional structure edge in dry etching: 1 etching mask; 2 and 3 material depositions at the etching edge; 4 sidewall film after the etching process; 5 embossed side-wall structure after removing of the etch mask (principle)
Fig. 4-66. Vertical sidewall deposition from ME-preparation, visible as narrow ridge in the sidewall region released by selective etching of the etch mask (principle)
4.4 Etchig Methoa3 with Energized Particles
167
rials shall not be attacked by the etching plasma or ion beams, respectively, the etching gas mixture is so composed that only non-volatile compounds are formed on these surfaces. On impact of energized particles, however, a certain removal occurs due to the sputter-effect. The released particles can of course condensate on any surface provoking the disturbing resistant spots on the surface of the etching material. The etching gas itself is another source of material for redepositions. Carbon-containing etching gases (e.g. hydrocarbons) form diamond-like layers or carbides that etch very slightly. Nitrides can be formed in the presence of nitrogen from the air or from the etching gases. Nitrides are thermodynamically stable compounds possessing low etch rates. Residual oxygen or oxygen from water vapours can possibly form stable oxides that can act as mask in some etching processes. The best way to avoid undesired depositions is the use of etching gas compositions that only produce volatile compounds with the etching materials and do not allow any non-volatile compounds to be formed by sideproducts. Additionally, mechanical components of the reactor (e.g. electrodes) must not be more than very slightly attacked by energized particles or reactive radicals to minimize redepositing particles from those materials. Redepositions can be subdued by increasing the substrate temperature, as a higher surface temperature shifts the adsorption-desorption equilibrium in the direction of desorption. Effective is a quick removal of the etched and desorbed materials from the gas volume by a high gas flow through the reactor. In general redepositions are formed under conditions that promote anisotropy (high proportion of removal by energized particles, low removal rates of reactions with thermalized particles, low surface temperatures). Thus freTable 49. Function of Secondary Additions to the Etching Gas in Reactive DryEtching Processes Chemical Function Subdueing oscillating plasmaprocesses by adding inert gases Forming volatile products from oxidic or salt-like compounds of the material to be etched Catching of undesired radicals and unsaturated compounds halogenohydrocarbons and unsaturated species Species being able to adsorb or polymerizate as inhibitors at the surface
Effect in Etching Stabilizing the plasma and the total process
Example Argon addition, often main component
Removal of native films or technological passivating thin coating films
BCkaddition in Al and GaAs etching
Increasing of the density of etching species, decreasing of undesired redepositions
Oxygen addition for catching halogenohydrocarbon radicals and unsaturated species Hydrocarbon additions
Masking of sidewall for enhancing the degree of anisotropy
168
4 Dry-Etching Methods
quently a compromise is necessary between ideally anisotropic etching and redeposition-free etching. The relations of etch rate, anisotropy, sidewall deposition, and undesired deposition of disturbing material on the surface areas to be etched can be influenced as well by setting the physical plasma conditions (pressure, power, ion energy, plasma density) as by choosing the gas composition. The most important possibilities of controlling the etching behaviour is given in the table. Sidewall depositions from etching processes are in some cases the cause for unsatisfying long-term stability of devices. As in sidewall depositions beside the elements of the removed material also the elements of the etching gas can be bound, species sometimes occur in the sidewall depositions that can corrode the film material of the patterns in the device. A special corrosion danger stem from the halogens, that are often components of reactive gases. As the halogens in form of halide ions form complexes with many metals, they further corrosion of conducting paths and contact elements, but also of reflectors and other microcomponents. The release of halide ions from sidewall depositions easily proceeds in the presence of solvents, especially water or aqueous solutions in consecutive process steps. It is also possible that the halide ions are formed by condensed water films if the devices are not vapour-proofly sealed. A typical corrosion caused by sidewall deposition occurs at AlSi- or AlSiCuconducting paths. The main component is etched in chlorine-containing plasmas. If Si is deposited in form of Si02 at the sidewalls, chlorine is bound in this redeposition. Small amounts of released chlorine from this layer later on initiate pitting in the conducting paths. This corrosion can be avoided if the redeposition is removed in a subsequent dry-etching step. This can be done in fluorine containing plasmas or in a two-step etching process in CF, and CF4/02 plasma lS7.
4.4.U Material Defects in Etching with Energized Particles Each dry-etching process leaves the material with a surface the properties of which are influenced by the etching process. To these influences count on one hand the chemical reactions of species on the surface that impair the microtechnical devices and the technology of their fabrication and on the other hand the energetic reactions of the energized particles the kinetic energy of which is a multiple of that of chemical bonds causing changes in the nearsurface layers of the exposed solid. Nearly all etching processes are accompanied by processes that are able to cause residues on the surface. These residues can be in the most favourable case single adsorbed atoms in worse cases continuous films of some atomic or molecular layers. The composition and the amount of the material in these residue layers depends on the etched material, the used etching gases, the Is’
K. Sakuma et al. (1994)
4.4 Etchig M e t h o h with Energized Particles
169
running of the process, and possibly on secondary materials like electrodes, etching masks or reactor walls. The following substances are frequently found in residue layers: 0
0
0 0
Residues of the etched material in incomplete etching processes Compounds of the surface material with etching gas components (oxides in the presence of oxygen or water, halides from fluorine-, chlorine- or bromine-containing gases, hydrides) Adsorbed hydrocarbons (from the polymers of the etching masks, or the oils of the vacuum system ) Carbides or nitrides, that form by the reaction of the plasma, especially the reaction of energized particles with nitrogen- or carbon-containing molecules in the gas volume
As far as the components of these surface layers are formed from elements that are not contained in the etched film system, but come from the reactor atmosphere or the reactor equipment, they may be called surface contaminations. Some of these residues consist of undesired and inert side products of the dry-etching process, that possess a low vapour pressure and hence are slower or not at all removed. Such residues already cause problems during the etching process by undesirable masking. Apart from these extreme cases, the residue thickness is only a few monolayers, i.e. it is of the order of 0.1 to 1nm. The contamination films are often not continuous, but have holes or are of an insular structure. Impacting energized particles can cause damages in the material beneath the residual layers. On the one hand the energized particles can deeply penetrate into the solid getting built into the lattice thus changing the elementary composition of the material. On the other hand the energy of the impact of the particle can cause disturbances in the lattice, i.e. it is partially dissipated in the solid instead of removing the material. The disturbed area reaches to a depth of several lOnm in the usual beam etching methods depending on the respective ion energies. Either the energized particles themselves or secondarily activated particles penetrate to this depth. Hydrogen, as a very light element, can even penetrate several microns into the solid. Various reactive plasma etching techniques or wet chemical etching methods are used for removing undesirable surface films after dry etching. For this purpose combined steps must be applied because of the different chemical properties of the residues (acidic or alkaline, oxidizable). Some methods for cleaning silicon surfaces are given in table 4-10.
4.4.14 Application of Etching Methods with Energized Particles In principle all materials can be patterned by ion etching techniques. Hence the ion etching techniques are a universal type of patterning. The choice of the actual etching method, the etching gas and the exact parameters depends
170
4 Dry-Etching Methods
Table 4-10. Cleaning Methods after Dry Etching Processes (for Silicon Surfaces) Method
Ref.
Oxygen plasma etching with following wet etching in a hot mixture of sulphuric and salpetric acid Oxygen plasma etching with following wet etching in a hot mixture of sulphuric and salpetric acid and etching in HF-solution Oxygen plasma etching with following wet etching in a hot mixture of sulphuric and salpetric acid and etching in a chromium etchant and following cleaning in hydrochloric acidic hydrogen peroxide solution Reductive cleaning in hydrogen plasma with high pressure and high flow rate Oxygen plasma etching or RIE with following etching in buffered HF-solution, wet etching in a hot mixture of sulphuric and salpetric acid and RCA-cleaning (ammonia-containinghydrogen peroxide solution and following hydrochloric acidic hydrogen peroxide solution) Oxygen plasma etching with following wet etching in a chromium etchant and following cleaning in a hot mixture of sulphuric and salpetric acid and RCAcleaning (ammonia-containinghydrogen peroxide solution and following hydrochloric acidic hydrogen peroxide solution)
S.J. Fonash (1990) after X.-C. Mu et al. (1985) S.J. Fonash (1990) after X.-C. Mu et al. (1985) S.J. Fonash (1990) after X.-C. Mu et al. (1985
S.J. Fonash (1990) after J.P. Simko et al. (1991) S.J. Fonash (1990) J.P. Gambino et al. (1990)
S.J. Fonash (1990) J.P. Gambino et al. (1990)
not only on the material to be etched, but also on the requirements for rate, selectivity, homogeneity of the removal and the avoiding of radiation damage in the sublayers. Ion etching processes are used for generating small patterns and such with high aspect ratios in metals. Furthermore metals, alloys, and metal compounds that cannot be etched or only with a small rate in wet-chemical processes are patterned by means of ion etching processes. Thus noble metals and their compounds as well as passivating metals are preferentially patterned with ion etching processes. Steep sidewalls, i. e. high aspect ratios, were prepared with reactive ion etching in silicon by the simultaneous supply of fluorine or other halogens in the plasma. By dexterily choosing the composition the silicon of the sidewalls was substituted by fluorine. The fluorinated surfaces are practically not attacked by thermalized plasma species. Whereas the combination of ion bombardment and supply of radicals of higher halogens (Bro,Jo)removes silicon effe~tively'~~.
'51
C.J. Mogab und H.J. Levinstein (1980)
4.4
Etchig Methods with Energized Particles
171
Beside silicon the other microtechnically relevant semiconductors are frequently patterned by dry-etching processes, RIE and RIBE preferentially. Many reactive dry etching processes were also worked out for compound semiconductors (IIW- and IINI-semiconductors). For preparing structures with extremely high aspect ratios (microchannel plates) in GaAs and Si RIBE-, CAIBE- and magnetron-enhanced RIE- (MIE-) processes were compared. It was shown that CAIBE with C12as etching gas produces very steep sidewalls, i. e. high aspect ratios, in GaAs with an etch rate of 3 n d s . Higher etch rates (8 n m / s ) are found for GaAs with RIE. In MIE with Si rates of 3 n m / s and less, steep sidewalls were observed'59. Most of microtechnically important inorganic dielectrica are such compounds that are chemically inert and simultaneously possess high evaporation and decomposition temperatures. To this group of compounds count many metal1 oxides as well as carbides and nitrides. These substances frequently cannot at all be patterned wet chemically. Hence ion etching methods have to be used always. For all kinds of organic polymers reactive ion etching processes with oxygen-containingplasmas are applied. As in plasma etching the formation of gaseous and volatile products (CO, C 0 2 , water and low-molecular hydrocarbons) is very efficient. According to the method the oxygen is supplied as energized ion (O,'), as reactive radical (mostly 0') or as molecular reaction partner in the ground state (02).
159
G.L. Snider (1994)
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5 Microforming by Etching of Locally Changed Material
5.1 Principle of Forming by Locally Changed Material A special technique of fabricating microstructures is the selective dissolution of chemically changed material from an unchanged matrix. This method is in contrast to all above mentioned methods (sections 3.1 to 4.4)a maskless process. In a first process step the material is locally changed, that in a second step it can be dissolved without attacking the neighbouring unchanged material. This principle is the basis for photoresist techniques. In that case the chemical change is a photochemical reaction that changes the solubility properties of the polymer, e.g., crosslinking (negative photoresist process) or forming carboxyl groups in an additive of the resin making the polymer dissolvable in alkaline media (positive photoresist process), Photoresist techniques, however, are not counted to the etching methods and shall not be dealt with here. Besides there are other optical methods in which after a local pretreatment the local solubility is changed and this change is used for producing small structures, e.g., in the maskless patterning of photosensitive metal and alloy films or glasses (see section 4.5.2. and 4.5.3). Beside light other radiations can be used for the local modification of microtechnical material. Among these, energized particles have got importance in microtechnical patterning.
5.2 Inorganic Resists In classic resist techniques organic polymers are used as main component of lithographic masks additionally photoreactive substances can be contained. Beside this organic materials that change their solubility properties under the influence of light or other radiations there are inorganic materials that react sensitively to light changing their solubility properties. These materials can be applied as lithographic resists and are termed “inorganic resists”. According to their composition they are comparable to the function layers, not to the organic resists. Hence the developing process is a wet-chemical etching process of the inorganic material and shall be shortly dealt with here.
174
5 Microforming by Etching of Locally Changed Material
A group of alloy films or film stacks, respectively, changing efficiently their solubility when exposed to light, consists of silver and compounds or alloys of silver with various elements, preferentially such of the main groups IV to VI of the periodic table of the elements, which are generated on semiconducting layers (sensitizing). Especially suitable are films of GeSez,on the surface of which a very thin film of AgzSeis formed by immersion in a silver ion-containing solution. Upon exposition to W-radiation of short wavelength silver is formed. If the surface is thereafter treated with diluted aqua regia (a mixture of hydrochloric and salpetric acid), the metallic silver in the exposed areas is diluted whereas the silver in the unexposed areas remains bound as Ag2Se. In a second step the GeSez film is etched in an aqueous isopropylic solution of dimethyloammonia in those areas where the silver had been removed. The sensitivity of this process, too low for applications, could be essentially increased by exposing to excimer lasers in the deep W-range (249 nm)'@. The term resist is not limited to materials that are sensitive to electromagnetic radiation. Resist techniques are also necessary for particle lithographic patterning, as e.g., electron or ion beam lithography. In most cases organic resists are applied in these applications, too. But for special applications inorganic resists are used. E.g., tungsten trioxide (WO,) is used as negative ion beam resist for fabricating field emitter diodes in FIB-technque 161. The developing process is the dissolution of the unexposed areas of the tungsten trioxide film in 0.01 moVl NaOH-solution, the etch rate being 7 nm/s. The exposed WO, can be reduced by baking and used as functional element, e.g., as emitter or collector electrode.
5.3 Etching of Photosensitive Glasses Beside the metallic, there are glass-like materials that change their properties under the influence of radiation increasing dramatically their solubility in an appropriate etchant. To this group count the photosensitive glasses. These glasses possess a composition that disintegrates if heated. Thus phases are formed with essentially higher etch rates than their unexposed surroundings. If cerium in form of Ce(II1) and silver in form of Ag(1) is added to such glasses, the decomposition process can be accelerated by silver nuclei forming by the action of light: Ag'
+ Ce3++ W-Licht +.
Ag,
+ Ce4+
(109)
By the electron transfer, initiated by light, from the reduced form of the cerium ion, silver nuclei are formed. This process is anologous to the genera-
16'
K.J. Polasko et al. (1984) Y. Gotoh et al. (1994)
5.4 Etching of Photo-Damaged Areas
175
tion of latent images by exposition in photography. In a consecutive baking process decomposition nuclei are formed at the silver nuclei. In this way volumes of higher removal rates are formed only in the exposed areas of the glass. Convenient are lithium containing glasses that contain sodium and potassium as well. Lithiummetasilicate forms an extra phase in the triple system SiO2/Al20JLi2O,which possesses a high etch rate in HF. If such glasses contain redox active metals like cerium and antimony (typical concentrations <0.01%), they can be used as photostructurable materials. After exposing, the primarily photochemically formed crystallization nuclei are transformed in a primary baking step at 500 to 550°C into more stable nuclei and subseqently into crystals at 550 to 600"C. With photostructurable glass a great variety of three-dimensional geometries can be realized. The areas to be removed have only to be exposed with a sufficiently high dose of light. It must only be guaranteed that the etchant is sensitive to the exposed and recrystallized surface areas only. Thus patterns with high aspect ratios can be produced. The lateral resolution is limited by the edge roughness which is dependent on the size of the recrystallized areas ical width of etched structures are in and hence on the density of nuclei. the range of several 10 micrometers16 Typ .
5.4 Etching of Photo-Damaged Areas Monocrystalline solids can also be changed by the action of light so that increased etch rates in comparison to the adjacent areas are realized. Monocrystals of a single chemical element as they are represented by Si-wafers, however, are not chemically changed in a comparable way as microphases are formed when irradiating or baking photosensitive glasses, but disturbances in the lattice arrangement can increasedly occur by the interaction of light. The consequence is that in the demolition zone the crystallographic planes normally possessing low etch rates in anisotropically reacting etchants are disturbed and hence more sensitive to the etching attack. By laser irradiation of Si-wafers the lattice is changed that much that (111)-planes lying in the irradiation areas do not function as etch stop in anisotropic etching. Thus also (111)oriented wafers can be etched anisotropically as the (111)-planeslying parallel to the surface as well as the layers underneath can be removed by the etchant as far as the demolition zone reaches into the depth. For this irradiation highly energized lasers are applied. The laser light is absorbed in a partial volume and melts the material in its core area. This area is surrounded by an area of greater damage, i. e. the area which is more easily etched'63.
16*
163
D. Hiilsenberg (1992) M. Alavi et al. (1992)
176
5 Microforming by Etching of Locally Changed Material
5.5 Etching of Areas Damaged by Ion Beams In analogy to the interaction of light at solid surfaces the lattice can be damaged by beams of energized ions provoking an increased etch rate of the impacted area in comparison to the surrounding areas. The inrease of the etch rate depends on the material and the etchant as well as on the reaction time, the intensity and the energy of the ion beams. In contrast to the damage by laser light the damage by ion beams is not primarily caused by the total power density of the impacting particles, but each single particle creates a very small locally damaged area. The superposition of this damaged areas yields the total damage of the surface. The heating of the substrate by the change of kinetic energy in thermal energy has only a supporting character. Energized particles with typical energies in the range of 1to 100 keV form an area of damage with a depth of a few nanometers up to one micrometer. In general the damage zone of light elements is deeper than that of heavier elements.
5.6 Particle Trace Etching Instead by a focused beam of ions of medium energy, patterns can be generated by means of single energized particles. It is necessary that the energy of a single particle impacting the solid surface is high enough to cause a damage zone along its path which can be wet-chemically removed. By the etching along a damage zone a channel is etched along the trace of the particle. Hence the term particle trace etching. To get a sufficiently high penetration of the particles in the target solid the kinetic energies must be in the range above 100 k e y Ions with energies up to the GeV-range are applied. As the chemical binding energies are in the range of a few eV such an energized particle on its path through the solid can give off 100.000 to 100.000.OOO times the energy that is necessary for breaking a chemical bond. In this way ion-damaged channels with a depth up to about 100 pm were produced with energized particles penetrating about 500.000 atom layers. For generating particle traces by means of energized atomic nuclei four different types of sources are availablelW: 1. Nuclear reactors: By a high flow density of thermal neutrons energized fission products can be generated in a converter foil of fissile material (e.g. u5U). These fission products react on the particle trace-sensitive material. The penetration depth of these particles are 10 to 20 pm. 2. Fissile material: Nucleids releasing a particles or heavier fission particles on spontaneous disintegration can be used. Preferentially such nucleids B.E. Fischer and R. Spohr (1988 a and b)
5.6 Particle Trace Etching
177
are applied the half-life value of which is sufficiently high to store the material some weeks to months, but is low enough for an intensive decay and hence a high radiation intensity. '"Cf is mostly applied. This nucleid decays spontaneously with a half-life value of 2.2 years emitting mainly a particles with an energy of 6,112 MeV 165. The material can be recovered from fuel rods of atomic reactors and deposited as thin films on handy supports. 3. Accelerators: By means of particle accelerators ions with uniform mass and nuclear charge can well-definedly brought to the same kinetic energy. It is a broad spectrum of particle energies possible so that penetration depth from nanometers to millimeters are realizable. Partical accelerators offer excellent preconditions for experiments for generating particle traces. The method is demanding and expensive. 4. Ion beam microprobes: These sources are a special kind of accelerators. The particle beam in these devices is focussed and can be deflected with high speed by electrodes or magnetic fields. Thus the energized ions can be positioned like the electrons in an electron microscope. Hence ion beam microprobes are suitable for directly-writing lithographic pattern generation. The greatest penetration depths of such devices are about 20 pm. The lateral resolution of currently 0.5 pm should be reduced to 1Onm in the future. The particle trace causes a zone of damage with three areas in the target material. An inner core, in which strong radiation damage occurs, i.e. a strong change of the target material is caused, has a diameter of only 10 nm. By interaction of the energized particles with the atoms of this zone a diluted area is generated. This diluted area is bounded to the exterior by a narrow zone of densified material. This whole core area is surrounded by a rim region of about 0.05 to 0.5 pm thickness. It represents the zone of damage by the energized electrons that are set free in the core zone. Most inorganic solids change their etching behaviour only in the core zone. Very narrow holes result from etching of a single particle trace. These holes, however, are not ideally cylindrical because the etch rate in radial direction off the axis of the particle trace does not sharply, but continuously change. Therefore conical etching grooves develop during the etching process. the selectivity, i. e. the etch rate ratio of glasses is in the range of 2 to 100. In some crystalline materials like mica selectivities of up to 100,OOOwere found. Especially in organic materials strong solubility changes are also found in the outer damage zone. Naturally, such materials show high selectivities (up to 100,OOO) that are sensitive to electron beams'&. By means of energized nanoparticles a variety of shapes and etching structures can be produced. Traces of single particles can be etched according to the selectivity of the etchant forming cylinders, cones, conical segments,
166
G. Friedlander und J.W. Kennedy (1962) B.E. Fischer and R. Spohr (1988 a)
178
5 Microforming by Etching of Locally Changed Material
rounded cones and spherical segments. Monocrystalline materials can form according to the crystal cut pyramidical, rhombical or hexagonal structures. By superposing single traces, mask structures can be patterned. A specialty of nuclear trace etching technique is the generation of two or more stepped etching structures. These steps are possible by successive irradiation with energized particles of different energies, i.e. of different penetration capability. Choosing different masks for the various irradiation steps three-dimensional complex structures can be pr~duced’~’.
16’
B.E. Fischer and R. Spohr (1988 a and b)
6 Chosen Recipes
6.1 Explaining the Collection of Recipes In the following part of the book chosen etching methods are compiled. The choice comprises a broad material spectrum: semiconductors, metals, alloys, glasses and polymers. However, it does not claim completeness neither concerning the materials nor the methods. Materials and methods are given as examples to introduce the reader to the practice of microtechnical etching. This collection of procedures shall add to the general part of the book (especially section 3 and 4) etching methods of certain materials. Secondly, it should support the practical worker in choosing the etching method and make easier the access to current literature. As in the various sources differing units are given, the dates were made uniform for better comparability. Because of rounding-up the dates deviate a little from the originally given values. Concentrations of etching baths are not always given in usual units in the references and can only be reconstructed by means of multiple procedures. As an uniform and in chemistry universally used unit, the concentration is given here in mole per litre (mole/l = M) and millimole per litre (mmole/l = mM), from which the quantities for the etchants can be easily deduced. A compromise was necessary for giving the parameters of dry-etching processes. The composition of etching gases is given in volume percent. For the flow rate, standard cubic centimeters per minute (sccm) was chosen as this unit is dominantly used in the scientific literature. The pressure is given in torr, as this is still predominant, but in many papers concerning ion beam etching and reactive ion etching pascal (Pa) is used. For small pressures millitorr (mtorr) is given. The conversion factor is 1 torr
=
133 Pa, 1 Pa
= 7.5 mtorr.
In dry-etching methods the power data are usually given in watt (W) or kilowatt (kW)). For etching processes the power density (powedarea) is the real decisive parameter. However in many references only the power of the dry etching facility is given without any relation to an area. In such cases the power density cannot be deduced. Frequences of the sources are given in kilo-
180
6 Chosen Recipes
hertz (kHz), megahertz ( M H z ) , or gigahertz (GHz),rf stands for radio frequency and ECR for electron cyclotron resonance. The etch rate is given in nm/s for wet- and dry-etching processes, the often used unit pdmin can be easily deduced (factor 0.06).
6.2 Collection of Recipes
182
6.2 Collection of Recipes
Ag-Si lver Wet etching Readily soluble reaction products:
Ag(1) is soluble in form of Ag+ and in form of complexes like [Ag(NH3)2]+or [Ag(CN),]-
Etchant 1:
Thiourea- iron nitrate- ammonium fluoride solution2)
Concentrations:
Fe(N03)3
0.3 moVl
CS(M2)2
1.4 moYl 0.8 moVl
rn
Temperature: Etch rate: Remarks:
50°C 5 nm/s Etchant lasts only 1day
Etchant 2:
Salpetric acid2)
Concentration: Remarks:
E.g. semi-concentrated Rapid removal, not for small structures
Etchant 3:
Iodine-potassium iodide solution3)
Composition:
KI I2
0.5 moVl 0.09 moVl
Etch rate:
ca. 300-1000 nm/s
Etchant 4:
Ammonia-methanolic hydrogen peroxide solution4)
Composition:
NH3 1.4 moVl H202 1.5moVl in ca. 67 Vol% methanol in water ca. 100 nm/s
Etch rate:
6.2 Collection of Recipes
Dry etching Volatile compounds (at high temperatures) : AgI AgBr AgCl
Bp. 1504°C Bp. 1533°C 1' Bp. 1550°C
Dry-etching method:
Ion beam etching with argon
Pressure: Ion energy: Ion current density:
0.3 mtorr 1 keV 0.85 &cm2 0"
Angle of incidence of ions: Etch rate: References:
5 nm/s
')AX Holleman and E.Wiberg (1985) 2)In-houseprescription PTI Jena (1983) 3)R.Glang and L.V.Gregor (1970) 4)F.Okamato (1974) 5)E.G.Spencer and €?H.Schmidt(1971)
183
184
6.2 Collection of Recipes
Al -Aluminium Wet etching Readily soluble reaction products:
Al(II1) is soluble as aquo-com lex [Al(H20),]3f or as fluoro-complex [AlF6l3''
Etchant 1:
Phosphoric acid-salpetric acid solution')
Concentrations:
H3P04 11.8 moVl €€NO3 0.6moVl 20°C 1.2 n d s 35°C 17 n m / s
Temperature: Etch rate: Temperature: Etch rate:
P
Etchant 2:
Phosphoric acid-salpetric acid-hydrofluoric acid solution3)
Concentrations:
H3P04 10.7 moVl €€NO3 1.04 mom 50°C 12 n d s
Temperature: Etch rate:
Etchant 3:
Phosphoric acid-salpetric acid-acetic acid solution3)
Concentrations:
H3P04 HNo3 CH3COOH 0.5 n m / s
Etch rate:
11.8 moVl 0.6 moVl 1.4 moVl
Dry etching Volatile compounds:
Remarks:
AlC13 subl. 182.7 "C') AU3r3 subl. 255 "C') MI3 subl. 381 "C') For dry etching chlorine containing etching gases are preferentially used (e.g. CC4 , SiC14,BC13, C12or mixtures of them).4-7)
6.2 Collection of Recipes
1. Dry-etching method
Plasma etching in CCh5)
Gas composition: Pressure: Plasma conditions:
100% ccl, 67 Pa Parallel-plate reactor; 13.5 Mhz 25 mm electrode distance, 204 mm electrode diameter 100w 100°C 2.5 n m l s (of the native oxide: 0.02 nmls) 150°C 6 nmls (of the native oxide: 0.06 nmls) In dry etching a lag in removal occurs (lag phase) due to the native surface oxide
Power: Substrate temperature: Etch rate: Substrate temperature: Etch rate: Remarks:
2. Dry-etching method Plasma etching in BC1,6) Gas composition: Pressure: Plasma conditions: Power: Temperature: Etch rate: Remarks:
100% ccl, 13 Pa Parallel-plate reactor; 13.5 Mhz 25 mm electrode distance, 204 mm electrode diameter 0.3 Wkm2 50°C 0.9 n m l s (of the native oxide: 0.Olnmls) In dry etching a lag in removal occurs (lag phase). Due to the native surface oxide
3. Dry-etching method Plasma etching C12/BCl&HClJHe8) Gas composition: 9 YOC12 ; 9 % BC13; 4 %CHC13;78 % He Flow rate: Pressure: Plasma conditions: Power: Etch rate:
324 sccm 1 torr Parallel-plate reactor; 13.5 M H Z 0.9 Wkm2 13 n m l s
4. Dry-etching method
Reactive ion etching in BC13/CHJC12 9,
Gas composition: Flow rate: Pressure:
ca. 65 % BC13; ca. 32%C12;ca. 3 %CH, 22.6 -31.6 sccm 10-40 mtorr
185
186
6.2 Collection of Recipes
Reactor:
Temperature: Etch rate:
Parallel-plate reactor; electrode distance 70 mm electrode diameter 152 mm 20°C 4 nrds
5. Dry-etching method Magnetic-field-enhanced RIE in C12/H2plasma'') Gas composition: Flow rate: Pressure: Power: Etch rate:
30 % H2 ; 70 %Clz 47 sccm 0,l torr rf 0.56 Wlcm2 17 n m / s
6. Dry-etching method Sputter-etching with Ar-ions") Flow rate: Electrode potential: Power: Surface temperature: Etch rate:
36 sccm 1.5 kV (parallel-plate reactor) 1.6 Wlc 190°C 0.2 n m / s
7. Dry-etching method Ion beam etching in Ar12) Pressure: Ion energy: Ion current density: Ion incidence angle: Etch rate: Ion incidence angle: Etch rate:
0.1 torr 0.5 keV lIllfdCn12 0" 0.7 n m / s 45" 1.3 nm/s
8. Dry-etching method: Reactive ion beam etching with Cl;3) Etch gas: Pressure: Ion energy: Ion current density: Energy density: Etch rate: Remarks:
Pure chlorine approx. 0.1 mtorr 0.6 keV 0.4 mA/cm2 0.24 W/cm2 1.25 nm/s The method supplies nearly vertical side walls (82") in thicker layers (10 pm)
6.2 Collection of Recipes
References:
')AX Holleman and E.Wiberg (1985) 2)H.Beneking (1991) 3)S.Buttgenbach (1991), 104 4)P.M.Schaible (1978) ')K. Tokunaga and D.W.Hess (1980) 6)D.W. Hess (1989) ')Widmann et al. (1988) "R.H. Bruce and G.P.Malafsky (1983) 9)J.W.Lutze et al. (1990) ")H. Okano et al. (1982) "'R.T.C. Tsui (1967) 12)S.Somekh (1976) 13)P.Surbled et al. (1997)
187
188
6.2 Collection of Recipes
AI(Ti) - Aluminium with Titanium Additions Wet etching Readily soluble reaction products:
Al(II1) is soluble as aquo-com lex [Al(H20),I3+ or as fluoro-complex [AlF,] 3- 1p X(1V) in strong acids as [Ti(OH)2]2 + , [Ti(OH),]+and derived complex ions, among them F as preferred ligand')
Etchant 1:
Hydrochloric acid-salpetric acid solution')
Concentrations:
HC1 5 moVl HN03 0.5moM 400-800 nm/s
Etch rate:
Dry etching Volatile compounds:
Remarks: References:
AlC13 subl. 182.7 "C') AlBr, subl. 255 "C') Al13 subl. 381 "C') XC14 Bp. 136.45 "C') XBr4 Bp. 233.45 "C') TiF4 subl. 284 "C') 'Ii5 4 Bp. 377 "C') For dry etching chlorine containing etching gases are preferentially used. "A.F. Holleman and E.Wiberg (1985) 2, R.J. Ryan et al. (1970)
6.2 Collection of Recipes
189
(Al,Ga)As - (Aluminium, Gallium) Arsenide Wet etching Readily soluble reaction products:
Al(II1) is soluble as aquo-com lex [Al(H20),I3+ or as fluoro-complex [A1F,I3- gallium is soluble as Ga3+(in acids) or as gallate (Ga(OH),, in alkalies)'), arsenic is soluble as As(II1)-salt, chloro-complex, as As(V) in arsenic acid')
Etchant 1:
Ammonia-hydrogen peroxide s ~ l u t i o n ~ ' ~ )
Concentrations:
1.2 mom H202 0.14 mom Room temperature 4,2-12,3 nm/s NH3 0.4moVl H202 ca. 0.2 moVl Room temperature ca. 8 nm/s for (~0.28Gaa.72)AS The etch rate is strongly dependent on convection in the etchant.
Temperature: Etch rate: Concentrations: Temperature: Etch rate: Remarks:
'[
NJ33
Etchant 2:
Sulfuric acidic hydrogen peroxide solution2)
Etch rate:
4,5 nm/s (Room temperature)
Etchant 3:
Citric acidic hydrogen peroxide solution4)
Concentrations:
Citric acid 2,3 M; H2 0 21 M Room temperature 4 nm/s for (A&.3Ga,,7)As (Without stirring) The etchant produces smooth surfaces. In opposition to ammonia-containing etchants, photoresists (especially of the 1400 series) are rather stable in the etchant.
Temperature: Etch rate: Remarks:
190
6.2 Collection of Recipes
Wet-etching method 4:
Photoelectrochemical etching in diluted Salpetric acid')
Electrolyte:
HN03 : H 2 0 = 20:l 0,2 W/cm2(150 W Halogen lamp) Room temperature 6.7 nm/s for (Alo.3Gao.7)As
Light: Temperature: Etch rate:
Dry etching Volatile compounds:
AlC13 AIBr3 MI3 GaC1,
ASH^
less volatile compounds:
AsFS AsF~ AsC13 AsBr3 GaC12 GaN
subl. 182.7 "C') subl. 255 "C') subl. 381 "C') Bp. 201.3 "C ') Bp. -543°C 6, Bp . -52.9"C 6, Bp. 63 "C 6, Bp. 130.4 "C 6, Bp. 221 "C 6 , Bp. 535 "C ') sub1.>800"C7)
Dry-etching method 1: Reactive ion beam etching with hydrochloric acid gas8) Gas composition: HC1 in Ar Flow rate: Ion current density: Plasma conditions: Power: Ion source: Etch rate:
3 sccm 0-200 pA/qcm 2,5 * lo4 ton50 W Kaufman type 0.5-3.3 n d s for 0-60 % Al
Dry-etching method 2:
Reactive Ion beam etching with chlorine')
Gas composition: Flow rate: Ion current density: Plasma conditions: Power: Ion source: Etch rate:
C12in Ar 5 sccm C12 0-50 pA/cm2 2,5 * 10-4ton 50 W Kaufman-type 2-3.2 n d s for 0-60 mol% Al
6.2
Collection of Recipes
191
Dry-etching method 3: RFECR-Etching in CH&€,/Ar Plasmas') Gas composition: Ar: 56 YO; CI&: 11YO; Ht:33 YO Flow rate: Pressure: Microwave energy: Rf-energy: Etch rate: References:
45 sccm 3 mtorr
1kW 150W (13.56 M H Z ) 2.5 nm/s "A.F. Hollemann and E.Wiberg (1985) 2)T.Wipiejewski and K.J. Ebeling (1993) 3)N.Chand and R.F. Karlicek Jr. (1993) 4)G.C.DeSalvo et al. (1992); C. Juang et al. (1990) "Th. Fink and R.M. Osgood Jr. (1993) @J.D'Ans and E. Lax (1943), 218 ')J. D'Ans and E.Lax (1943), 231 8)J.D. Skidmore et al. (1993) 9)S.J.Pearton et al. (1996)
192
6.2 Collection of Recipes
Alo.5Gao.5P - Aluminium Gallium Phosphide Wet etching Readily soluble reaction products:
Remarks:
Etchant 1: Concentrations: Temperature: Etch rate:
Etchant 2: Concentrations: Temperature: Etch rate:
Al(III) is soluble as aquo-com lex [Al(H20)6]3+ or as fluoro-complex [U6l3Gallium as Ga3+(in acids) or as gallate (Ga(OH)i, in alkalies)’), The etch rates increase in etchants 1-4 with increasing Al-content x in compositions Al,Ga,.,P. The concentration dependence of the etch rate is less pronounced in oxidizing etchants (Bromomethanol, Hydrochloric acid-Salpetric acid mixtures) than in hydrogen halide solutions2).
P
Methanolic bromine solution’) Br2 (1%ig in Methanol) Room temperature 15 n d s
Hydrofluoric acid’) HF (49 YOig) Room temperature 5nds
Etchant 3:
Hypophosphorous acid’)
Concentrations: Temperature: Etch rate:
H3P02(95 %is) Room temperature 0.42 n d s
Etchant 4:
Phosphoric acid’)
Concentrations: Temperature: Etch rate:
H3P04(95 %is) Room temperature 0.13 nm/s
6.2 Collection of Recipes
Dry etching Volatile compounds:
AlC13 AlBr3 MI3 GaC13 GaCl, GaN PF3 PH3 PF5 PC15
subl. 182.7 "C1) subl. 255 OC1) subl. 381 "C1) Bp. 201.3 "C ') Bp. 535 "C') subl .>80O0C3) Bp. -101°C" Bp. -88"C? Bp. -75°C'" Bp. 62
"c)
1. Dry-etching method: Etching in reductive plasmas of high densitf) Gas composition: Flow rate: Ion density: Plasma conditions: Power : Etch rate:
18YOCH,; 27 YOH2; 55 "LO Ar 45 sccm ca. iOi1/cm3 1.5 mtorr 150 W (rf 13.56 M H z ) ; 0.8 kW (microwave 2.45 GHz) 3.7 nm/s
2. Dry-etching method: RUMicrowave Etching in IBr plasma6) Gas composition: Flow rate: Plasma conditions: Power: Etch rate: References:
50% IBr; 50% Ar 8 sccm 1.5 mtorr 150 W (rf 13.56 MHz); 1kW (microwave 2.45 GHz) about 5.5 nm/s (up to an Al content of 60 %) ')AX Holleman and E.Wiberg (1985) *)J.W.Lee et al. (1996): J. Electrochem. SOC143'1 (1996)' L1 3)J.D'Ans and E.Lax (1943)' 2311 4)J.D'Ans and E.Lax (1943)' 251 "J.W. Lee et al. (1996) @J.Hong et al. (1996)
193
194
6.2 Collection of Recipes
(AI,Ga,In)P - (Aluminium, Gallium, Indium) Phosphide Wet etching Readily soluble compounds:
M(II1) is soluble as aquocomplex [A1(H@)6]3fOr as fluorocomplex [A1F6I3-') gallium as Ga3+(in acids) or as gallate (Ga(OH),, in alkalies)'), In(II1) as aquocomplex [In(H20)6]3+or as fluorocomplex [InF6I3-.')
Etchant 1:
Hot sulfuric acid2)
Concentrations:
concentrated 60°C 2.9 d s for &.2Gao.31no.5P 9.7 nm/s for A6.35G%.l5h.5P 70°C 5.3 nm/s for &.2Gao.31no.5P 17-1d s for [email protected]&.5P
Temperature: Etch rate: Temperature:
Etchant 2: Concentrations: Temperature: Etch rate:
Hydrochloric a ~ i d ~ ' ~ ) 13 M 25°C 10.2 n d s for A&,2Gao.31n,,5P 38.3 d s for Alo.3sGao.151~.5P
Dry etching Volatile compounds:
MC13 mr3 MI3 GaC13 GaC12 GaN InBr3 InC13 PF3 PH3 PF5 PC15
subl. 182.7 "C') subl. 255 "C') subl. 381 "C') Bp. 201.3 "C ') Bp. 535 "C1) subl. >800"C?) subl. 371 "C1) subl. 418 "C') BP. -101~5) Bp. -88"C5) Bp. -75"C5) Bp. 62 "C5)
6.2 Collection of Recipes
1. Dry-etching method: Reactive ion etching in SiCldC€L,/Ar6) Gas composition: Flow rate: Pressure: Plasma conditions: Power: Temperature: Etch rate: References:
Ar: 50 Vol% ; C&: 15 Vol% ; Sic&:35 Vol% 36 sccm 7.6 mtorr Parallel-plate reactor; 13.5 MHz 100 w 60°C 2.5 nm/s ') A.F. Hollemann and E. Wiberg (1985) 2)T.R.Stewart and D.P. Bour (1992) 3)seealso J.R. Lothian et al. (1992) 4)J.D'Ans and E. Lax (1943)' 231 ')J. D'Ans and E. Lax (1943), 251 "C.V.J.M. Chang and J.C.N. Rijpers (1994)
195
196
6.2 Collection of Recipes
(Al,In)As - (Aluminium, Indium) Arsenide Wet etching Readily soluble reaction products:
M(III) is soluble as aquocom lex [M(H2O),I3+ or as fluorocomplex [AF,] 3- 1p In(III) as aquocomplex [In(H2O),I3+or as fluorocomplex [InF6I3-'), As as As(II1)-salts, or chlorocomplexes, or as As(V) in arsenic acid')
Etchant 1:
Citric acidic hydrogen peroxide solution2)
Concentrations: Temperature: Etch rate:
0.4 M H202; 2.5 M CJ3807 Room temperature 0.34 nm/s for &.481n,,52A~
Dry etching Volatile and moderately Volatile compounds:
Alc13 mr3 MI3 InBr3 InC13 ASH, ASFS AsF~ AsC13 AsBr3
subl. 182.7 "C') subl. 255 "C') subl. 381 "C') subl. 371 "C') subl. 418 "C') Bp. -54.8"C 3, Bp. -52.9"C 3, Bp. 63 "C 3, Bp. 130.4 "C 3, Bp. 221 "C 3,
1. Dry-etching method: RIE in C12-plasma4) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
33% Ar; 67% C12 15-35 sccm 50 mtorr 0.8 Wkm2 Parallel-plate reactor,(13.56 MHz), Electrode distance 7 cm 2.9 nm/s
6.2 Collection of Recipes
2. Dry-etching method: RIE in SiC14-plasma4) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
33% Ar; 67% SIC4 15-35 sccm 50 mtorr 0.8 W/cm2 Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm 1.3 nm/s
3. Dry-etching method: RFECR-Etching in CH4/H2/ArPlasmas') Gas composition: Ar: 56%; C&: 11%; H2: 33 'YO Flow rate: Pressure: Microwave energy: Rf-energy: Etch rate: References:
45 sccm 3 mtorr 1 kW 150W (13.56 MHz) 2.5 n m / s "A.F. Hollemann and E. Wiberg (1985) 2)G.C.DeSalvo et al. (1992) 3)J. D'Ans and E. Lax (1943), 218 4)S.J. Pearton et al. (1990) %.J. Pearton et al. (1996)
197
198
6.2 Collection of Recipes
(Al, In) N - (Aluminium, Indium) Nitride Wet etching Readily soluble reaction products:
Al(II1) is soluble as aquocom lex [Al (H20)6]3+ or as fluorocomplex [M6l3.) In(II1) as aquocomplex [In(H20)6]3+ or as fluorocomplex .l)
Wet etchant:
Etching in alkaline media')
Composition:
Photoresist developer AZ400K with KOHaddition 20°C 2.5 nm/s for A&uIno.75N 20°C 30 nm/s for Alo.71~.3N
Temperature: Etch rate: Temperature: Etch rate:
P
Dry etching Volatile or moderately Volatile compounds:
AlC13 AlBr, MI3 InBr3 InC13
subl. 182.7 "C') subl. 255 OC') subl. 381 OC') subl. 371 OC') subl. 418 "C')
1. Dry-etching method: ECR-etching in Clz/Hz/Cl?L,/Ar -plasma3) Gas composition: 26 YO C12; 40 YO H2; 8 YO C K ; 26 Yo Ar Flow rate: Plasma conditions: Power: Temperature: Etch rate: References:
38 sccm 1mtorr 850 W (microwave plasma); +150 W (rf 13.56 M H Z ) 30°C 2 nm/s ')AX Hollemann and E. Wiberg (1985) *)C.B.Vartuli et al. (1996) 3)R.J.Shul et al. (1996);
6.2 Collection of Recipes
199
Alo.51no.5P - (Aluminium, Indium) Phosphide Wet etching Readily soluble compounds:
Al(III) is soluble as aquocom lex Al or as fluorocomplex [A1F6I3-. ) In(III) [ (as aquocomplex In HzO)6]3+ or as fluorocomplex [1n~,13-.1
P
F (
Etchant 1:
Hot Sulphuric acid2)
Concentration: Temperature: Etch rate:
concentrated 70°C 37,3 n m l s
Etchant 2:
Hydrochloric acid (1 :1)2*3)
Concentration: Temperature: Etch rate:
13 moVl 25°C 47.8 n m l s
Dry etching Volatile or moderately volatile compounds:
NC13 mr3 MI3 GaC13 GaClZ GaN PF3 PH3 PF5 PC15
subl. 182.7"C') subl. 255 "C') subl. 381 "C') Bp. 201.3 "C ') Bp. 535 "C') sub1.>800"(?) BP. -101~5) Bp. -88"C5) Bp. -75"C5) Bp. 62 "C5)
1. Dry-etching method: Reactive ion etching in SiCldCwAr6) Gas composition: Ar:50 Vol% ; CI&: 15 Vol% ; Sic&:35 Vol% Flow rate: Plasma conditions: Power: Etch rate:
36 scan Parallel-plate reactor; 13.5 MHz; 7.6 mtorr loo w 2.5 nm/s (60°C)
200
6.2 Collection of Recipes
2. Dry-etching method: Etching in reductive plasmas of high density7) Gas composition: 18% C&; 27% H2; 55% Ar Flow rate: Ion density: Plasma conditions: Power: Etch rate: References:
45 sccm ca. 1011/cm3 1.5 mtorr 150 W (rf 13.56 MHz); 1kW (microwave 2.45 GHz) 3.7 nm/s ')A.EHollemann and E. Wiberg (1985) 2)T.R.Stewart and D.P. Bour (1992) 3)seeJ.R. Lothian et al. (1992) 4)J.D'Ans and E. Lax (1943), 231 5)J.D'Ans and E. Lax (1943), 251 "C.V.J.M. Chang and J.C.N. Rijpers (1994) 7)J.W. Lee et al. (1996)
6.2 Collection of Recipes
201
AlN -Aluminium Nitride Wet etching Readily soluble reaction products:
Al(II1) is soluble as aquocom lex [A1(H20)6]3t or as fluorocomplex [A1F6I3-. )
P
1. Wet etching method: Etching in KOH containing resist developer ') AZ 400 K developer solution Etchant: Temperature: Etch rate: Temperature: Etch rate: Temperature: Etch rate: Further wet etching methods:
50°C 12 nm/s (without annealing) 90°C 7 nm/s (film annealed at 700°C) 90°C 1.5 nm/s (film annealed at 1100°C) For wet etching of AlN e.g. the following solutions are applied3): - a mixture of equal parts of glycerol, salpetric acid and hydrofluoric acid (for AlN doped with 1YONi) - 0.1 to 1 moVl NaOH-solution (for AlN doped with CaCJ - Hydrochloric acid - Sulphuric acid
Dry etching Moderately Volatile compounds:
AlC13 AlBr3 MI3
subl. 182.7"C') subl. 255 OC') subl. 381 "C')
1. Dry-etching method: ECR-etching in CH4/H2/Ar-plasma4) 17 YOC€&;50 % H2; 33 % Ar Gas composition: Flow rate: Plasma conditions: Power: TemDerature: Etch rate:
30 sccm 1.5 mtorr 1 kW ECW 450 W (rf 13.56 MHz) 23°C 3 nm/s
202
6.2 Collection of Recipes
2. Dry-etching method: ECR-etching in Cl,/Ar -plasma4) 33 % C12; 67 % Ar Gas composition: Flow rate: Plasma conditions: Power: Temperature: Etch rate: References:
15 sccm 1.5 mtorr 1kW (ECR); 450 W (rf 13.56 MHz) 23°C 2.7 d s ')AX Holleman and E. Wiberg (1985) "C.B. Vartuli et al. (1996a) 3)C.-D.Young and J.-G. Duh (1995) 4)C.B.Vartuli et al. (1996b)
6.2 Collection of Recipes
203
A1203-Aluminium Oxide Wet etching Readily soluble reaction products:
Al(II1) in ionic form, e.g., as A13+,AlFi, AlF2-
Etchant 1:
Heated phosporic acid')
Concentration: Temperature: Etch rate:
14.61 M H3P04 55 "C 0.53 d s 14.61 M 50 "C 0.47 nm/s 10.0 M 50 "C 0.38 n d s 4.8 M H3P04 50 "C 0.27 nm/s 14.61 M 41 "C 0.22 nm/s
Concentration: Temperature: Etch rate: Concentration: Temperature: Etch rate: Concentration: Temperature: Etch rate: Concentration: Temperature: Etch rate:
Dry etching Volatile or moderately volatile compounds:
AlF3 Al2C16 AlBr3 d J 3
subl. 1272°C*) subl. 182.7 "C 2, Bp. 255°C 3, Bp. 385,4)"C3,
1. Dry-etching method: Laser etching with CF, 4, Gas composition: Energy source:
CF4 XeC1-Laser, 308 nm
204
6.2 Collection of Recipes
2. Dry-etching method: Reactive ion etching in a Cl,/Ar-mixture') Gas composition: 71 Val% Ar; 29 Val% C12 Plasma conditions: Power: Source: Substrate Temperature Etch rate: Substrate Temperature: Etch rate:
Tmtorr, -750 V bias W XeC1-Laser, 308 nm 20°C 5 nm/s 250°C 15 nm/s
3. Dry-etching method: Reactive ion beam etching in CH2F2or CH3F6, Gas composition: Plasma conditions: Power: Ion energy: Ion current density: Remarks:
Etch rate:
100% CH2F2or CH3F 0.2 mtorr W 0.8 kV 0.6 mA/cm2(30"Angle of incidence) By adding CHF3 , the etch rate of photoresists can be lowered, and even material deposition on the resist mask from the gas phase is achievable. No removal of photoresist is achieved with 20 % CHF3-additionin C H P or with 40 YOCHF3addition in CH2F2The etch rate loss of the Al2O3 is only ca. 10-15 % . 1nm/s
4. Dry-etching method: Etching by bombardment with inert ions7) Gas composition: Pressure: Power: Temperature: Etch rate: References:
Ar 11mtorr 100 W/ 1.6W/cm2;rf 1,5 kV 190°C 0.03-0.08 n m / ~ ')B. Zhou and W.F. Ramirez (1996) ')AX Hollemann and E. Wiberg (1985) 3)J.D'Ans and E. Lax (1943), 214 4)N.Heinan et al. (1980) 5)D.Bauerle (1986) @T.Kawabe et al. (1991) ')R.T.C . Tsui (1967)
6.2 Collection of Recipes
205
AsSG (As203,Si02)- Arsenosilicate Glass Wet etching Readily soluble compounds:
As(II1)-Salts, Chlorocomplexes, as As(V) in arsenic acid Si(1V) in form of complexes, e.g. in strongly alkaline media as [Si(OH)6]2-or in Fcontaining media as [SiF6l2Etchants for structuring Si02 , are also suited for etching of AsSG. The etch rates of AsSG exceed as a rule the Si02-etchrate by a multiple.
Etchant 1:
Hydrofluoric acidammonium fluoride solution')
Concentrations:
m 3 . 3 . moVl HF 3 moVl 24 "C 10 nm/s for non-densified AsSG-films 2.5 n m / s for densified AsSG-films ) N€&F 10 moVl HF 2.4 moVl 2, 1.7 n m / s for 2 mol % As203in Si02 2.3 n m / s for 7.5 mol % As203in Si02
Temperature: Etch rate: Concentrations: Etch rate:
Dry etching Volatile compounds:
Remarks: References:
AsH3 Bp. -54.8"C 3, AsF, Bp. -52.9"C3) AsF3 Bp. 63 "C 3, AsC13 Bp. 130.4 "C 3, AsBr3 Bp. 221 "C 3, SiH, Bp. -111.6"C 4, SiF4 Bp. -95.7 "C 4, Si2& Bp. -15 "C 4, SiHC13 Bp. 31.7 "C 4, Sic& Bp. 56.7 "C 4, Si20Cl6 Bp. 135.5 "C 4, Si2C16 Bp. 147 "C 4, For reactive dry etching fluoride-containing etching gases are preferentially used. ')H. Proschke et al. (1993) 2)M.Ghezzo and D.M. Brown (1973) 3)J. D'Ans and E. Lax (1943), 218 ')J. D'Ans and E. Lax (1943), 261
206
6.2 Collection of Recipes
AU - Gold Wet etching Readily soluble reaction products:
Gold is soluble in form of complexes in oxidation stages (I) and (11), e.g. as [AuC12]-or[AuCl4I2-* Au(II1) in strongly alkaline media as aureate [Au(OH)~]')
Etchant 1:
Iodine-potassium iodide-Solution2)
Concentrations:
I20.09 mom KI 0.6 mom 8-15 d~
Etch rate:
Dry etching: Volatile compounds: CI*-
Slightly volatile Au2C16(stable under increased pressure) and Au2Br;)
1. Dry-etching method: Etching in chlorine plasma 3, Gas composition: Plasma conditions: Etch rate:
C12 0.04 torr 2 nm/s (bei 180°C)
2. Dry-etching method: Reactive ion etching in CF4/CC14-Plasma4, Gas composition: 47 YO CF4; 53 YoCCb Flow rate: Plasma conditions: Power: Etch rate:
36 sccm 150 mtorr 350 W 1.5 nm/s
3. Dry-etching method: Etching by bombardment with inert ions') Gas composition: Pressure: Power: Temperature: Etch rate: References:
Ar 11mtorr 100 W/ 1.6 W/cm2 (HF); 1,5 kV 190°C 0.3-0.6 d~ "A.F. Holleman and E. Wiberg (1985) 2)H.Beneking (1991); S. Buttgenbach (1991) 3)D.L.Flamm et al. (1984) 4)R.M.Ranade et al. (1993) 5)R.T.C.Tsui (1967)
6.2 Collection of Recipes
207
Bi - Bismuth Wet etching Readily soluble reaction products:
Bi(II1) is soluble in form of hydroxocomplexes, e.g. Bi6(OH)'2+ or Big(OH)2;+, BiC1, and BSr, are readily soluble as well; chelating organic acids, especially citric acid, enhance dissolution ')
Etchant 1: Concentrations:
Citric acidic peroxodisulfate solution2)
Temperature: Etch rate: Remarks:
(NHJ2S208 0.48 moVl Citric acid 0.57 moVl Fe(N03), 0.025 moVl Room temperature 8.3 nm/s As the organic acid and the peroxoanion undergo a slow redoxreaction, the etchant must be replaced after a few hours.
Dry etching Volatile and slightly volatile compounds:
Remarks: References:
BiH3 Bp. 22°C) BiF5 Bp. 230 "C') BiC13 Bp. 441 "C') BiBr, Bp. 462 "C') Reactive dry etching is feasible with fluorinecontaining etching gases. "A.F. Holleman and E.Wiberg (1985) 2)M.Kohler, A. Lerm, A. Wiegand (1983a) Etchant for Bismuth and/or Antimon Bogenschiitz (1967) 4)J. D'Ans and E. Lax (1943), 269
208
6.2 Collection of Recipes
BSG (B203,SiO,)
- Borosilicate Glass
Wet etching Readily soluble compounds:
Si(1V) in form of complexes, e.g. in strongly alkaline media as [Si(OH)6]2'or in Fcontaining media as [siF6l2-;boron is easily soluble as borate. Etchants for SiOz are also usable for etching BSG.
1. Etchant:
Diluted HF-solution')
Composition: Etch rate:
16 M HF 10 n d s for 5 % B2O3 300 n d s for 30% B2O3
2. Wet-etching method: Salpetric acidic hydrofluoric acid etchant (((BHF")2) Composition: Etch rate:
2.4 M HF;10 M 0.7 nm/s for 5 YOB2O3 0.6 nm/s for 30 YOB2O3
Dry etching Volatile compounds:
Remarks: References:
Sib Bp. -111.6"C 3, SiF, Bp. -95.7 "C 3, Si2& Bp. -15 "C 3, SiHC13 Bp. 31.7 "C 3, Sic& Bp. 56.7 "C 3, Si20Cl6 Bp. 135.5 "C 3, Si2C16 Bp. 147 "C 3, BF3 Bp. - 101 "C!) B2& Bp. -92.5 "C!) BCl3 Bp. 7.6 "C? BBr3 Bp. 90.1 "C" Reactive dry etching is carried out in fluorinecontaining etching gases. ')W. Kern and R.C. Hein (1970) 2)A.S.Tenney and M. Ghezzo (1973) 3)J. D'Ans and E. Lax (1943)' 261 ,)J. D'Ans and E. Lax (1943)' 222
6.2 Collection of Recipes
C - Amorphous Carbon Dry etching Forming of volatile reaction products:
In reactive etching gases, containing oxygen, carbon is liberated as CO or C02.
1. Dry-etching method: Reactive ion etching in C1,IBClJHBrl Ar-plasma') Gas composition: Self-bias voltage: Temperature: Etch rate: References:
Cl2/BC1JHBr/Ar -370 V 160°C 0.17 d s K.Y. Hur et al. (1994)
209
210
6.2 Collection of Recipes
C - Diamond Dry etching Forming of volatile reaction products:
In reactive etching gases, containing oxygen, carbon is liberated as CO or C 0 2 (oxidative etching). At high temperatures and in the presence of catalysts carbon reacts with hydrogen forming gaseous methane (reductive etching). Diamond changes at about 600°C into graphite the sublimation Temperature of which is about 3700 "C 2).
Dry-etching method 1: Sputter-etching with Ar' Plasma conditions: Ion energy: Ion current density: Etch rate:
')
8 ptorr 10 kV 1.3 mA/cm2 4 nm/s (at a beam angle of 20")
Dry-etching method 2: Laser etching with 022) Plasma conditions: Source: Average laser power: Pulse energie: Single pulse power density: Removal per single pulse: Average etch rate:
8 ptorr KrF-Laser (20 Hz repetition rate) 70 W/cm2 3.5 J/cm2(20 ns pulse) 175 h4J/cm2 140 nm 2800 nm/s (at a beam angle of 45")
Dry-etching method 3: Metal-catalysed high-temperature etching) Conditions of the gas volume: Substrate temperature:
H2
950°C
Metal film: Etch rate:
0.1-1p.m Fe 133 nm/s
Metal film: Etch rate:
0.2-11 pm Ni 4.5 nm/s
Metal film: Etch rate: Mechanism:
0.2-11 p.m Pt 0.1 nm/s The carbon of the diamond dissolves in the metal, diffuses to the surface reacting there with the hydrogen to methane.
6.2 Collection of Recipes
References:
')H. Saitoh et al. (1996) *)D.-G.Lee et al. (1994) 3)VG.Ralchenko et al. (1993)
211
212
6.2 Collection of Recipes
(C,H,[O,N,F,CI,Br]) - Organic Polymers General Considerations Because of the variety of organic polymers, that differ not only by their elementary composition, but also by such properties as isomerism, average molecular weight, distribution of the molecular weight (dispersion), degree of branching etc., our list of etching methods can never be complete with respect to all special materials. Hence some general suggestions are given here together with some typical materials as examples for the spectrum of organic polymers.
Wet etching Readily soluble reaction products: According to the chemical composition of the polymer, organic solvents with apolar or polar, aliphatic or aromatic, aprotic or protic character are available for physical dissolution. If the polymer film is prepared by spin-coating of a photoresist, it can be re-dissolved as a rule in the solvent of the respective resist or a solvent of similar composition, provided that the polymer film had not undergone chemical changes during the microtechnical process, causing, e.g., a decrease of solubility by interlinking of polymer chains. Polymers with acidic functional groups (e.g. sulphonic acids, phenols) are frequently dissolved in aqeuous or alcoholic solutions at an increased pH-value, polymers with alkaline functional groups (e.g. amines, amides, imides, pyrindines, imidazoles, anilines) are accordingly dissolved in protic solutions at low pH. The hydrocarbon skeleton can be disintegrated chemically under strongly oxidizing conditions, e.g. by Car0 acid or chromium sulphuric acid. The structuring quality of the latter agents is in general very poor, hence they are normally used for the complete removal of organic films and for cleaning surfaces, respectively.
6.2 Collection of Recipes
213
Dry etching Volatile compounds: Polymers composed of the elements C,O,N,H form under appropriate conditions in the gasroom (oxidizing atmosphere) preferentially gaseous compounds: CO Bp. -191.5 "C C02 Subl. -78.5 "C H2O Kp. 100°C N2 Kp. -195.8"C NH3 Kp. -33.4"C N202 Kp. -151.8"C N20 Kp. -88.5"C NO2 Kp. 21.15 "C1) However, by an inadequate running of the process, non-volatile compounds can be formed on the surface protecting the underlying material from the attack of the reactive gases, ions or plasmas. Such species are, e.g., elementary carbon C (subl. only at 3370°C, 127 bar), especially in diamond or diamond-like modifications, and also polycyan (CN), (decomposition above 800°C in Dicyan GN2,Bp. -21.2"C)'). Dry etching methods disintegrate chemically the carbon skeleton of the polymers. As carbon is non-volatile itself, preferentially oxygen-containing plasmas, oxygen ions or oxygen-containinggases are used as reactive components together with other energized particles in chemically-enhanced beam etching processes. ')AX Holleman and E. Wiberg (1985) References:
214
6.2 Collection of Recipes
Dry etching of organic polymers: Synopsis of materials containing (C,H,[0,N ,S,F,Cl,Br]) Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References: Material: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Etch rate:
Cellulose') Plasma etching 8 % 02/92 % CF, 15mVmin 0.55 torr 0.2 kW rf, 13.56 M H z ll.7nds ') L. .A.Pederson (1982) Epoxy resin Reactive ion etching of epoxy resin (Spurr)') 0 2
10 mtorr 0.28 W/cm2; 13.56 M H z 3.5 nm/s Microwavelrf-etching of Epoxy resin DER566A8d) 75 % 02/25 % CF, 70 sccm 0.15 torr Parallel-plate reactor 0.26 kW 25°C 22 nm/s ') 1.S.Goldstein and EKalk (1981) 2, A.M.Wrobe1 et al. (1988) Novolak Plasma etching') 0 2
0.2 Pa 33 w 70°C 0.3 nm/s Low pressure plasma etching) 0 2
4 mtorr 1 kW 20 nm/s
6.2 Collection of Recipes
References:
215
')L.Eggert and W.Abraham (1989) R. Hsiao et al. (1997) Photoresist Plasma etching of photoresist KTFR') ')
Material: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Temperature: Etch rate: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Etch rate: Etching method: Gas composition: Pressure: Etch rate:
0 2
1 torr Down stream 100°C 2.5 nm/s Microwave etching of photoresist AZ 1376) 0 2
4.5 torr 160°C 17 n m / s Microwave etching of photoresist AZ 5214 E3) 0 2
20 sccm 3 mtorr ECR I 1,5 kW microwave 13.3 nm/s Reactive ion etching of photoresist AZ 24504) 0 2
20 mtorr Parallel-plate reactor 40°C 8 nm/s Plasma etching of photoresist Kodak 7475) 0 2
1 torr Parallel-plate reactor 100°C 2.3 n m / s Microwave etching of photoresist HPR 204 6, SF, 0.fmtorr 0.9 kW; Ion: 180 eV; 750pA/cm2 1.25 n d s Microwave etching of photoresist HPR 2047) 0 2
20 mtorr 0.55 nm/s
216
6.2 Collection of Recipes
References:
S.M.Irving (1968) B.Robinson and S.A.Shivashankar (1984) 3, S .W. Pang et al. (1992) 4, B.R.Soller et al. (1984) ') A. Szekeres et al. (1981) 6, 0.Joubert et al. (1990) ') B. Charlet and L.Peccoud (1984) Polyamid Nylon 66l) Microwave/rf-etching 70 % 02/30 %CF4 70 sccm 0.14 torr Parallel-plate reactor 0.21 kW 25°C 11 nm/s ') A.M. Wrobel et al. (1988) Polycarbonat Lexan Plasma etching') 8 % 02/92 % c F 4 15dmin 0.55 torr 0.2 kW rf; 13.56 MHz 1.2 nm/s Microwave/rf-etching) 80 % 02/20 %CF4 70 sccm 0.25 tonParallel-plate reactor 0.23 kW 25°C 22.5 nm/s ') L.A.Pederson et al. (1982) 2, A.M.Wrobe1 et al. (1988) Polyester Mylar') Microwave- / rf-Etching 80 % 02/20 % CF4 70 sccm 0.14 torr Parallel-plate reactor 0.21 kW 25°C 10 nm/s ')
2,
Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate:
6.2 Collection of Recipes
References:
217
Material:
A.M.Wrobe1 et al. (1988) Polyethylen Plasma etching') 79 YO02/21 % CF4 72 sccm 0.35 torr 0.3 kW rf 16 nm/s ') S.R.Cain et al. (1987) Polyimid
Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate:
Plasma etching of Polyimid Kapton') 61 YO02/39 % CF4 72 sccm 0.35 torr 0.3 kW rf 27.5 nm/s
Etching method:
Microwave/rf-etching of Polyimid Kapton DuPon t2) 89 '7002/11 % CF4 70 sccm 0.27 torr Parallel-plate reactor 0.4kW 25°C 6.7nds Microwave etching of Polyimid Kapton3) 20 % CFJ80 Yo 0 2 : 21.7 d s 12% CFJ88% 0 2 6.7 nm/s Magnetic field-enhanced reactive ion etching4)
Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References:
Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Etch rate: Gas composition: Etch rate: Etching method: Gas composition: Pressure: Etch rate:
')
0 2
50 mtorr 42 n d s Microwave/rf-etchin2)
Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate:
20 sccm 0.5 mtorr ECR; 1,5KW Microwave+ 300 W rf 22 nm/s
Etching method: Gas composition:
Plasma etching6) 90% 02/ 10% C F 4
0 2
218
6.2 Collection of Recipes
Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: Remarks:
0.5 torr Parallel-plate reactor 85°C 33 n d s Reactive ion etching') 90% 0 2 1 10% SF, 250 mtorr Parallel-plate reactor 80°C 17 n m / s Microwave plasma etching*) 93 YO 0 2 1 7 YO CF4 0.7 torr Down stream 100°C 97 n d s Microwave plasma etching') 76 YO0214 YO Ad20 YO CF4 0.3 torr Down stream, 58 W 1.2 n m / s Plasma etching of Polyimid DuPont P125661°) 0 2
0.1 torr 0.34 W/cm2 < 50°C 1.9 nm/s Reductive plasma etching of Polyimid DuPont PI2566lO) H2 0.1 torr 0.34 W/cm2 < 50°C 0.5 n m / s Deep-temperature reactive ion etching of Polyimid Kapton H") 0 2
30 mtorr 2 WIcm2 -100°C 12 n m / s It results a very strong anisotropic etching.
6.2 Collection of Recipes
References:
Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: dc-bias-Spannung: Microwavenpower: Etch rate: References: Material: Etching method: Gas composition: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition:
"S.R. Cain et al. (1987) 2)A.M.Wrobel et al. (1988) 3)F.D.Egitto et al. (1990) 4)J.T.C.Yeh et al. (1984) 5)W.H.Juan and S.W. Pang (1994) 6)T.Yogi et al. (1984) "G. Turban and M. Rapeaux (1983) *)B.Robinson and S.A. Shivashankar (1984) 9)V.Vujanovic et al. (1988) ")F.Y. Robb (1984) ")K. Murakami et al. (1993) Poly isopren') Plasma etching 68 % 02/32 % CF4 72 sccm 0.35 torr 0.3 kW rf 26 nm/s ') S.R. Cain et al. (1987) Polymethylglutarimide') ECR-/microwave etching in 02-Plasma 0 2
90 sccm 30 mtorr 150V 0-15OW 9 nm/s
"S.J. Pearton et al. (1991 b) Polymethylmethacrylatl) Plasma etching 0 2
0.2 torr 40 W 92°C 0.67 nm/s ') L. Eggert and W. Abraham (1989) Polystyren Plasma etching 8 YO02/92 YOCF,
219
220
6.2 Collection of Recipes
Gas flow rate: Pressure: Reactor: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References:
15dmin 0.55 torr 0.2 kW rf, 13.56 M H Z 1.1d s "L.A. Pederson (1982) Polyvinylalcoho155/121) Plasma etching 0 2
150 mVh 70 mtorr 38 W 26°C 0.75 d s ') L. Eggert et al. (1988) Polyvinylbenzal Plasma etching 0 2
180 mVh 80 mtorr 55 w 26°C 0.3 d s L. Eggert et al. (1988) Polyvinylcarbazol') Plasma etching ')
0 2
180 mVh 80 mtorr 28.5 W 26°C 0.13 n m l s Plasma etching" 8 %02/92 %CF, 15 ml/min 0.55 torr 0.2 kW rf; 13.56 h4Hz 1n m l s ')L. Eggert et al. (1988) 2)L.A.Pederson (1982)
6.2
Material:
Polyvinylchloride') Plasma etching
Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate:
150 mVh 70 mtorr 55 w 26°C 0.9 n m l s
References:
')
Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Temperature: Etch rate: References:
Collection of Recipes
0 2
L.Eggert et al. (1988)
Polyvinylformal') Plasma etching 0 2
150 mVh 70 mtorr 55 w 26°C 0.9 n m l s
Material:
L. Eggert et al. (1988) Polyvinylidenfluoride')
Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate:
Plasma etching 8 %02/92 %CF, 15mVmin 0.55 torr 0.2 kW rf; 13.56 MHz 2.1 n m l s
References:
L.A. Pederson (1982) Polyvinylolacton') Plasma etching 8 YO02/92 % CF, 15 mllmin 0.55 torr 0.2 kW rf; 13.56 M H z
Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate:
')
')
4 nmls "L.A. Pederson (1982) Polyvinylpyrrolidon K90') Plasma etching 0 2
150 ml/h
221
222
6.2 Collection of Recipes
Pressure: Reactor: Temperature: Etch rate: References: Material: Etching method: Gas composition: Gas flow rate: Pressure: Reactor: Etch rate: References:
70 mtorr 55 w 26°C 0.3 nm/s ') L. Eggert et al. (1988)
Polytrialkoxysilylnorbornene') Reactive ion etching 92 YO0218 YOCHF3 50 sccm 0.3 torr 0.3 kW 30 n d s (Si contents between 0.7 and 4.4YO) ') L.A. Pederson (1982)
6.2 Collection of Recipes
223
CdS - Cadmium Sulfide Wet etching Readily soluble reaction products:
Cd(I1) forms soluble coordination compounds, Cd(OH)2 is soluble in acids. CdS is difficult to dissolve and hence can only be dissolved by oxidation of the sulphur.')
Dry etching Volatile compounds:
Cd(CHJ2 Bp. 106°C') Cd12 Bp. 796°C') H2S Bp. -60.3"C') SF4 Bp. -40.4 "C1) so2 Bp. -10°C') SF2 Bp. 39 "C') so3 Bp. 44.5"C') SC12 Bp. 59.6 "C') Bp. 57°C (bei 0.22 torr)') S2Br2
Dry-etching method:
ECR-RIE in reductive plasma Arm(/cH4))2)
Gas composition: Flow rate: Plasma conditions: Power: Source: Dc-bias-voItage: Etch rate:
C 6 17 Val% ; C& 57 Val% ;Ar 26 Val% 30 sccm 1 mtorr 150 W (Microwave power) ECR with additional HF-power 13.56 M H z - 250 V 0.67 nmls "A.F. Holleman and E. Wiberg (1985) 2)S.J. Pearton and E Ren (1993)
References:
224
6.2 Collection of Recipes
CdTe - Cadmium Telluride Wet etching Readily soluble reaction products:
Cd(I1) forms soluble coordination compounds, Cd(OH)z is soluble in acids.. CdS is difficult to dissolve and hence can only be dissolved by oxidation of the sulphur.') Te(1V) is soluble as Te4+in strong acids and as Te0:- in strongly alkaline solutions'), as Te(I1) in form of chelates in tartaric acidic solutions; Te02can be dissolved in different multifunctional organic acids.')
1. Wet-etching method: Etching in hydrobromic acidic iodinepotassium iodide solution. 2, Composition: Etch rate:
4.15 g KI and 0.5 g Iz in 12.5ml HBr (No concentration given for HBr) 50 nm/s
Dry etching Volatile compounds:
Cd(CH3)z Bp. 106°C') CdIz Bp. 796°C') HzTe Bp. -2.3 "C')
1. Dry-etching method: ECR-RIE in reductive plasma (Arm2(/cH4))3) Gas composition: Flow rate: Plasma conditions: Power: Source: dc-bias-Spannung: Etch rate:
CH, 17%; CH457%; Ar 26% 30 sccm 1mtorr 150 W (Microwave power) ECR with additional HF-power 13.56 MHz -250 V 0.3 n d s
References:
"A.F. Holleman and E. Wiberg (1985) ')P. W. Leech et al. (1990) 3)S.J. Pearton and E Ren (1993)
6.2 Collection of Recipes
225
(Co, Cr) - Cobalt Chromium Wet etching Readily soluble reaction products:
Cobalt forms as Co(I1) and especially as Co(II1) a variety of soluble complexes. Cr(II1) is soluble in form of coordination compounds.
Etchant:
Hydrochloric acidic iron(III)-solution')
Concentrations:
FeC13 1.2 M; HC14 M
Dry etching Slightly volatile compounds:
CoClz CrC13
Volatile compounds:
Cr02C12 Bp. 117"C3) Cr(CO)f5 Bp. 151°C 3, C T ( N O ~- )9H20 ~ Bp. 125.5"C3)
References:
')PTI-in-house instruction (1985) "5. D'Ans and E. Lax (1943), 237 3)J. D'Ans and E. Lax (1943)' 227
Bp. 1O5O0C2) subl. 130O0C3)
226
6.2 Collection of Recipes
(Co, Nb, Zr) - Cobalt Niobium Zirconium Wet etching Readily soluble reaction products:
Cobalt forms as Co(I1) and especially as Co(II1) a variety of soluble complexes. Nb(V) is soluble as fluoride NbF,') Zirconium forms as Zr(1V) allone or together with metals in the valence state two fluoro complexes')
Dry etching Moderately volatile compounds:
CoCl2 NbF, NbCl, Zr(B€€,), ZrC14 ZrBr4 ZrI, ZrF,
Dry-etching method:
Ion beam etching with Ar 3, 100% Ar
Gas composition: Ion energy: Ion current density: Etch rate: References:
Bp. 1050"C2) Bp. 229°C') Bp. 247.4 "C') Bp. 123°C') subl. 331°C') subl. 357°C') subl. 431°C') subl. 903°C')
1 kV 0.17 mA/cm2 0.2 n m / s (at vertical ion incidence) 0.3 n m / s (at an angle of incidence of ions of 50")
')A.E Holleman, E. Wiberg (1985) 2)J. D'Ans and E.Lax (1943) ''O.J. Winmers et al. (1990)
6.2 Collection of Recipes
227
Co,Si - Cobalt Silicide Wet etching Readily soluble reaction products:
Cobalt forms as Co(I1) and especially as Co(II1) a variety of soluble complexes.') Si(IV) is soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),I2-or in F-containing media as [SiF6l2-
1.Wet-etching method: Etching in hydrofluoric acid2) Etchant composition: HF:3 moM Etch rate:
0.6 nm/s
2. Wet-etching method: Etching in hydrochloric acidic hydrofluoric acid') Etchant composition: Etch rate:
HF: 1 mom; pH: 0 0.4 nm/s
Dry etching Slightly volatile compounds: Volatile compounds:
CoCl, Bp. 1050"C3)
Dry-etching method:
Reactive Ion etching in Clz-Plasma4)
Gas composition: Reactor: Etch rate:
100% Cl' Parallel reactor; 13.56 MHz, - 0,4 kV bias 5 n m / s (bei 250°C) ')A.E Holleman and E. Wiberg (1985) ,)M.R. Baklanov et al. (1996) 3)J. D'Ans and E. Lax (1943) 4)F.Fracassi et al. (1996)
References:
SiH, SiF4 Si2& SiHC13 Sic& Si20C& Si2C16
Bp. Bp. Bp. Bp. Bp. Bp. Bp.
-111.6"C3) -95.7 "C3) -15 "C3) 31.7 'C3) 56.7 "C3) 135.5"C3) 147 "C3)
228
6.2 Collection of Recipes
Cr - Chromium Wet etching Readily soluble reaction products:
Cr(I1) in form of coordination compounds or Cr(VI) in form of chromates
Etchant 1:
Etch orange')
Concentrations: Etch rate:
(NH,J2Ce(N03)60.3 moM; HC1040.5 moM As Ce(1V)-salt Ce2 (NH,&(so4)3 is frequently used. ca. l n d s
Etchant 2:
Alkaline hexacyanoferrate(II1)-solution
Concentrations: Temperature: Etch rate:
K3Fe(CN), 0.76 moM; NaOH 3 mom 50°C ca. 1 nmJs
2,
Dry etching Volatile compounds:
Cr02C12 Bp. 117"C3) Cr(C0)6 Bp. 151"C3) C T ( N O ~- )9H20 ~ Bp. 125.5"C3)
Dry-etching method:
Reactive ion etching in 02/C12-Plasma4)
Etch rate: Selectivity to Novolak: Selectivity to trimethylsilylsubstituted PMMA:
0.14 n d s 0.3 4.25
References:
')€'TI-in-house instruction (1985); see also A.R. Janus (1972) 2)S.Buttgenbach (1991) 3)J.D'Ans and E. Lax (1943), 227 4)A.E.Novembre et al. (1993); vgl. S. Tedesco et a1 (1990)
6.2 Collection of Recipes
229
Cu - Copper Wet etching Readily soluble reaction products:
Cu(1) in form of halogeno- and pseudohalogenocomplexes [CuX2]-, [CuX3I2-l) and in strongly alkaline media as / CU( OH) ~ ] - ~ ) ; Cu(I1) as Cu2+and its complexes'
Etchant 1 :
Hydrochloric acidic Iron(II1)-chloride solution
Concentrations: Remarks:
HC13 moM, FeC130.5 moM Strong undercutting occurs.
Etchant 2:
Ammonia hypochlorite solution
Concentrations:
NH3 0.67 mom, NaOCl ca. 0.7 moVl (NH&C03 2.6 moVl 100 n d s
Etch rate:
Etchant 3:
Sulphuric acidic potassium dichromate solution2)
Concentrations: Temperature: Etch rate:
H2S041.3 moM; &Cr207 0.63 moM 50°C 100 nm/s
Etchant 4:
Hydrochloric acidic CuC12/KC1-solution3)
Concentrations:
3.5 moM CuC12 0.5 M HC1; 0.5 M KCI 12 n m / s According to the chosen concentrations and flow rates of the etchant, wall angles between 25 and 86 degrees were achieved.
Etch rate: Remarks:
Dry etching Moderately volatile compounds:
CuC1, CuBr,
Bp. 655°C) Bp. 900°C)
230
6.2 Collection of Recipes
1. Dry-etching method: Sputter-etching with Ar-ions 5, Gas composition: Ar Pressure: Power: Temperature: Etch rate:
11mtorr 100 Wl 1.6 Wlcm2; rf 1,5 kV 190°C 0.3-0.6 n d s
2. Dry-etching method: Ion beam etching with Argon 6, Ion energy: Ion current density: Etch rate:
0.5 keV
1&cm2 0.75 n m l s
3. Dry-etching method: Reactive ion etching in C12/Arplasma7) Gas composition: Pressure: Flow rate: Rf power: Etch rate:
Ar: 97 % ; Cl2: 3 % 7.5 .. 37.5 mtorr 150 sccm 150 W or 250 W (100 kHz or 13.56 MHz) up to 17 n m l s
Remarks:
0.2 pm lines were produced. ')A.E Holleman and E. Wiberg (1985) 2)PTI-In-houseprescription (1985) 3)M.Georgiadou and R. Alkire (1993 a and b) 4)J. D'Ans and E. Lax (1943), 239 "R.T.C. Tsui (1967) 6)I? Gloersen (1976) 7)M.Markert et al. (1997)
References:
6.2 Collection of Recipes
231
Fe I (Fe, C) - Iron ( and Steel) Wet etching Readily soluble reaction products:
Fe(I1) and Fe(II1) in form of coordination compounds (CN-, C1-)
Etchant 1:
Iron(II1)-chloride - hydrochloric acid solution for stainless steel AISI 316l) FeC13: 3.2 mom; HC1: 0.04 mom
Concentrations: Temperature: Etch rate: Temperature: Etch rate: Temperature: Etch rate:
30 "C 45 n m / s 40 "C 67 n d s 50 "C 105 nm/s
Dry etching Volatile compounds:
References:
Fe(C0)s Bp. 105"C2) FeC13*6H20 Bp. 218 "C2) Bp. 319"C2) FeC13 "D.M. Allen and M.-L. Li (1988) "5. D'Ans and E. Lax (1943)' 229
232
6.2 Collection of Recipes
(Fe, Ni) - Iron Nickel Wet etching Readily soluble reaction products:
Fe(I1) and Fe(II1) in form of coordination compounds (CN-, Cl-) Ni (11) in form of Ni2+and its soluble complexes, nickel forms dense passivating layers at normal atmosphere, containing Ni(II1). Its dissolution is fesible in acidic media and in the warmth and by addition of complexing ligands as F, C1- or NH3
Etchant 1:
Citric acidic peroxodisulphate etchant. ')
Concentrations:
Etch rate:
(NH4)2S2080.9 moVl 0.03 moVl Citric acid 0.25 moVl HNo3 0.3 moVl 3nds
Etchant 2:
Iron(111)-chloride solution*)
Temperature: Etch rate:
3-54 "C 200-420 n d s
w
Dry etching Volatile compounds:
Fe(CO)5 Bp. 105"C3) FeC13 6H20Bp. 218"C3) FeC13 Bp. 319"C3) Ni(C0)4 Bp. -25°C)
Dry-etching method:
Reactive ion beam etching with Ar/O;)
Remarks:
The etchant is selective to Titanium. ')€TI-In-house prescription (1985) 2)R.J.Ryan et al. (1970) 3)J. D'Ans and E. Lax (1943), 229 4)J. D'Ans and E. Lax (1943), 249 5)R.W. Dennison (1980)
References:
6.2 Collection of Recipes
233
GaAs - Gallium Arsenide Wet etching Readily soluble reaction products:
Gallium as Ga3+(in acids) or as Gallate (Ga(OH);, in alkalies)'), Arsenic as As(II1)-salts, chlorocomplexes, as As(V) in Arsenic acid')
Wet-etching method 1: Etching in sulphuric acidic hydrogenperoxide solution (Caro acid)2) Concentrations: Temperature: Etch rate:
H2S04 4moVl H202 1.8molA 40°C 300-500 n m / s
Wet-etching method 2: Etching in alkaline hydrogenperoxide solution 2, Concentrations: Temperature: Etch rate: Remarks:
NaOH0.24 moVl H2020.17 mom 5°C 1.7 n m / s In ammonia hydrogen peroxide solution strong anisotropic etching is ob~erved.~)
Wet-etching method 3: Etching in citric acidic hydrogen peroxide solution4) Concentrations: Temperature: Etch rate: Remarks:
Citric acid 2.4 moVl H202 1.4 moVl 18°C 3 nm/s The etchant possesses a selectivity of 10 compared to AlGaAs.
Wet-etching method 4: Photoelectrochemical etching in diluted salpetric acid') Electrolyte: Concentrations: Light: Etch rate:
HN03:H 2 0 = 20:l strongly diluted (1/20) 0,2 W/cm2(150 W Halogenlampe) 8,3 nm/s
234
6.2 Collection of Recipes
Wet-etching method 5: Etching in sulphuric acidic bromate solution6) Electrolyte: Etch rate: Remarks:
H2S048 mom KBr03 0.25 moVl 670 d s (rotating substrate, 2250 U/min) Etchant for extremly high etch rates The roughness of the achieved surface changes in depedence on the sulphuric acid concentration.
Wet-etching method 6: Etching in hydrochloric and acetic acidic hydrogen peroxide solution7) Electrolyte: Etch rate:
H202 1.1 moVl HC1 0.4 moVl CH3COOH 14 moVl 4.5 d s
Dry etching Moderately volatile compounds:
(32%
GaC13 GaC12
ASH^
AsFS AsF, AsC13 AsBr3
Bp.43"C') Bp. 201.3 "C1) Bp. 535 OC1) Bp. -543°C ') Bp. -52.9"C) Bp. 63 "C? Bp. 130.4"c8) Bp. 221 "C?
1. Dry-etching method: Reactive ion etching in Sic& Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
Sic& 12 sccm Parallel-plate reactor; 13.5 MHz; 15 mtorr 15 W 5.5 d s (60"C)9'
Remarks:
Adding chlorine and enhancing the power density etch rates of more than 40 nm/s can be reached (detailed deDendencies in")).
6.2 Collection of Recipes
2. Dry-etching method: Reactive Ion etching in SiCLJCHJAr”) Gas composition: Ar:50 YO; Cfi: 10 YO; Sic&:40 % Flow rate: Plasma conditions: Power: Etch rate:
36 sccm Parallel-plate reactor; 13.5 M H z ; 7.6 mtorr 100 w 2.7 nm/s (60°C)
3. Dry-etching method: Magnetic field-enhanced RIE in SiC4l2) Gas composition: Ar: 50 YO; Cfi: 10 % ; SiCl,: 40 % Flow rate: Plasma conditions: Power: Etch rate:
15 sccm Additional magnetic field: 125 G, 13.5 M H z ; 2-15 mtorr 0.08-0.5 Wkm’ ca. 10-20 nm/s (60°C)
4.Dry-etching method: Reactive ion etching in C123) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
ClZ 40 sccm Parallel-plate reactor; 13.5 MHz; 85 mtorr 25-100 W 20-40 nm/s (45°C)
5. Dry-etching method: Reactive ion etching in BC13-containing P1asmasl4) Gas composition: Ar: 65 YO;BC13:20%; Clz: 15 Yo Flow rate: Plasma conditions: Etch rate:
40 sccm Parallel-plate reactor; 13.56 M H z ; 15 mtorr 10-20 n d s (10°C)
6. Dry-etching method: Crystallographic etching in bromine plasma15) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
Br2 30 sccm Parallel-plate reactor; 0.3 torr; 14 MHz; 30 W GaAs (100): ca. 1 p d s (100°C)
7. Dry-etching method: CAIBE in C12/Ar16) Gas composition: Flow rate: Source:
C12 8 sccm Clz Kaufman-Source (Ar+-beam: 0.2 mA/ cm2,0.5kV)
235
236
6.2 Collection of Recipes
Plasma conditions: Power: Etch rate:
Parallel-plate reactor; 13.5 M H z ; 7.6 mtorr 100w 3-4 nm/s (110°C)
8. Dry-etching method: Laser etching in dimethyl zink atmosphere") Gas composition: Source: Plasma conditions: Power pro 2.5-p.mspot: Laserinducede Temperature: Etch rate: Remarks:
Zn(CH3)2 Ar-laser 514 nm 10 torr 110 mW 550°C 27 n d s V-shaped etch grooves with clean sidewalls and without any depositions are achieved.
Dry-etching method 9: RFECR-Etching in C m 2 / A rPlasmas") Gas composition: Ar: 56 YO; C K : 11YO; H2: 33 YO Flow rate: Pressure: Microwave energy: Rf-energy: Etch rate: References:
45 sccm 3 mtorr 1kW 150 W (13.56 MHz) 10 n d s "A.F. Hollemann and E. Wiberg (1985) 2)H.Beneking (1991) 3)S.H.Jones and D.K. Walker (1990) 4)C.Juang et al. (1990) "Th.Fink and R.M. Osgood, Jr. (1993) 6)P.Rotsch (1992) "J.R. Flemish and K.A. Jones (1993) '5. D'Ans and E. Lax (1943), 229 9)S.K. Murad et al.(1993) ")A. Camacho and D.V. Morgan (1994) "'C.V.J.M. Chang and J.C.N.Rijpers (1994); for RIE of GaAs in hydrogenhydrocarbon plasmas see also G.Franz (1990), for RIE in halogenohydrocarbonplasmas see S .J . Pearton et al. (1990) 12)M.Meyyappan et al. (1992) 13)seeA. Camacho and D.V. Morgan (1994)
6.2 Collection of Recipes
237
14)K.J.Nordheden et al. (1993); see also H. Takenaka et al. (1994) and for the effect of additional microwave power S.W. Pang and K.K. KO (1992) 15)D.E.Ibbotson et al. (1983) 16)G.L.Snider et al. (1994); especially for profile forming see also W.J. Grande et al. (1990) "'T.J. Licata and R. Scarmozzino (1991) '*)S.J. Pearton et al. (1996)
238
6.2 Collection of Recipes
(Ga, In)As - Gallium Indium Arsenid Wet etching Readily soluble reaction products:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH),, in alkalies)'), In(II1) is soluble as aquocomplex [In(H20),]3+ or as Fluorocomplex [IIIF,]~- .') Arsen as As(II1)-Salts, Chlorocomplexes, as As(V) in Arsenacid ')
Etchant 1:
Sulphuric acidic hydrogen peroxide solution (Caro acid) *)
Concentrations:
H2SO4 0.2 mom H202 0.09moVl 25 "C 0.8 nm/s
Temperature: Etch rate: or Concentrations: Temperature: Etch rate:
H2S04 H202 25°C 42 nm/s
Etchant 2:
Citric acidic hydrogen peroxide solution 3,
Concentrations:
H202 3moVl Cd-1807 1.7moVl 2.4 nm/s (Gh.471n,,53As;Room temperature)
Etch rate:
1.7 moVl 0.74 moVl
Dry etching Volatile compounds:
Ga2I-b GaC13 GaC12 GaN
ASH^
AsF~ AsF~ AsC13 AsBr, InBr3 InC13
Bp. -63°C') Bp. 201.3 "C') Bp. 535 "C') sub1.>800"@ Bp. -54.8'6) Bp. -52.9"C5) Bp. 63 " 6 ) Bp. 130.4"C5) Bp. 221 " 6 ) subl. 371 "C1) subl. 418 "C')
6.2 Collection of Recipes
1. Dry-etching method: RIE in C12-Plasma6, Gas composition: 33 'YOAr;67 YOC4 Flow rate: Plasma conditions: Power: Source: Remarks:
Etch rate:
15-35 sccm 50 mtorr 0.8 Wkm2 Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm Smoother surfaces and less rough etch edges can be achieved with reduced power density (e.g. 0.3 W/cm2), if a strongly decreased etch rate is acceptable.' 3 nmls
2. Dry-etching method: RIE in SiCl,-Plasma Gas composition: 33% Ar;67% Sic& Flow rate: Plasma conditions: Power: Source: Remarks: Etch rate: References:
6,
15-35 sccm 50 mtorr 0.8 Wkm2 Parallel-plate reactor, (13.56 M H z ) , Electrode distance 7 cm (see above) 2.3 n m l s
"A.F. Holleman and E. Wiberg (1985) 2)A.F.Bogenschutz (1967) 3)G.C.DeSalvo et al. (1992) 4)J. D'Ans and E. Lax (1943), 231 5)J. D'Ans and E. Lax (1943), 229 %.J. Pearton et al. (1990)
239
240
6.2 Collection of Recipes
G*.,ln,.,P
- Gallium Indium Phosphide
Wet etching Easily soluble reaction products:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH),, in alkalies)'), In(II1) is soluble as aquocomplex [In(H20)6]3+ or as Huorocomplex [InF6I3-.l)
Etchant 1:
Hot sulphuric acid *) Concentrated 60°C
Concentrations: Temperature: Etch rate: Temperature: Etch rate:
Etchant 2: Concentrations: Temperature: Etch rate: Concentration: Temperature: Etch rate: Selectivity (Etch rate factor in comparison to GaAs): Concentration: Temperature: Etch rate: Selectivity (Etch rate factor in comparison to GaAs):
Etchant 3: Concentrations: Etch rate:
0,25 nm/s 70°C 0,63 nm/s
Hydrochloric acid 6.5 moVl 25°C 0.3 nm/s ') 4.5 moVl 23°C 0.3 nm/s ca. 0.7 3, 7.1 moVl 23°C 0.65 nm/s ca. 3 3))
Hydrochloric and acetic acidic hydrogen peroxide solution4) H202 0.2 moVl HC1 0.47 moM CH3COOH 16 moVl 1.6 n d s
6.2 Collection of Recipes
241
Dry etching Volatile and moderately volatile compounds:
Ga2H6 GaC13 GaC1, InBr3 InC13 PF3 PH3 PF5 PC15
Bp. -63°C') Bp. 201.3 "C') Bp. 535 "C') subl. 371 "C') subl. 418 "C1) Bp. -101"C6' Bp. -88"C6) Bp. -75°C) Bp. 62 "C)
1.Dry-etchingmethod: Etching in reductive plasmas of high density7) Gas composition: 4.5 Yo C&; 40 YOH2; 55.5 % Ar Flow rate: Ion density: Plasma conditions: Power:
Etch rate:
45 sccm ca. iOi1/cm3 1.5 mtorr 150 W (rf 13.56 MHz); 1kW (microwave 2.45 GHz) 3.7 n m / s
2.Dry-etching method: Etching in BCl&-plasma Gas composition: 75 YOBC13; 25 % N2 Plasma conditions: Power:
Temperature: Etch rate: References:
of high density')
1mtorr (rf 13.56 MHz); - 145 V self-bias 1 kW (microwave 2.45 GHz) 100°C 33 n m / s ')A.E Holleman und E. Wiberg (1985) 2)T.R. Stewart und D.P. Bour (1992) 3)H.Ito und T. Ishibashi (1995) 4)J.R.Flemish und K.A. Jones (1993) 5)J.D'Ans und E. Lax (1943)' 264 6)J.D'Ans und E. Lax (1943)' 231 7)J.W.Leeet al. (1996) 8)ERen et al. (1996)
242
6.2 Collection of Recipes
GaN - Gallium Nitride Wet etching Readily soluble compounds:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH);, in alkalies)'),
Etchant 1:
Hot phosphoric acid 2,
Concentrations: Etch rate:
85 %ige H3P04(200°C) 18 nm/s
2. Wet-etching method: Photoelectrochemical etching in Tartaric acid and ethylene glycol3) Concentrations: pH: Current density: Irradation: Etch rate:
3 % tartaric acid in 1:l water: ethylenglycol 7 (buffered with NH,) 2 mA/cm2 Hg lamp (365/405 nm: 60/150 mW/cm2 0.9 n m / s
Dry etching Volatile and moderately volatile compounds:
Ga2& GaCl, GaC12 GaN
Bp.-63"C1) Bp. 201.3 OC1) Bp. 535 "C') subl.>800"@
1. Dry-etching method: ECR-Etching in C€€,/H2/Ar -Plasma5) Gas composition: 17 % C K ; 50 % H2;33 Yo Ar Flow rate: Plasma conditions: Power: Temperature: Etch rate:
30 sccm 1.5 mtorr 1kW ECW 450 W (rf 13.56 M H z ) 23°C 2.8 n m / s
2. Dry-etching method: ECR-Etching in C12/Ar-Plasma5) Gas composition: 33 YO C12; 67 YO Ar Flow rate: Plasma conditions: Power: Temperature: Etch rate:
15 sccm 1.5 mtorr 1 kW (ECR); 450 W (rf 13.56 MHz) 23°C 11n m l s
6.2 Collection of Recipes
243
3. Dry-etching method: ECR-Etching in ICVAr -Plasma Gas composition: Flow rate: Plasma conditions: Power: Temperature: Etch rate: References:
50% IC1; 50% Ar 8 sccm 1.5 mtorr 1 kW (ECR);250 W (rf 13.56M H z ; dc -275V) 23°C 22 nm/s ')AX Holleman and E.Wiberg (1985) ')A. Shintani and S. Minagawa (1976) 3)H.Lu et al. (1997) 4)J. D'Ans and E.Lax (1943),264 "C.B.Vartuli et a1 (1996)
244
6.2 Collection of Recipes
(Ga,G&03 Gallium Gadolinium Oxide Wet etching Readily soluble compounds:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH);, in alkalies)'), Gadolinium oxide as a lanthanoide is readily soluble in acidic solutions, but not in alkalies. Gd(II1) is easily soluble in form of the chloride GdC13 ').
Etchant 1:
HC1 solution2)
Concentrations: Temperature: Etch rate:
HC15 moVl 25°C 10 nm/s (deposition at 100°C) 1 nm/s (deposition at 535°C) The etch rate decreases with increasing Gd content.
Remarks:
Dry etching Volatile and moderately volatile compounds:
References:
GaH3 GaC13 Bp. 201.3 OC1) GaC12 Bp. 535 OC') GaN subl.>800"C3) ') A.F. Holleman and E. Wiberg (1985) 2, F. Ren et al. (1997) 3, J. D'Ans and E. Lax (1943), 264
6.2 Collection of Recipes
245
GaP - Gallium Phosphide Wet etching Readily soluble compounds:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH)c, in alkalies)'), phosphorus as P(II1) or P(V) in form of phosphites and phosphates
Etchant 1:
Sulphuric acidic bromate solution2)
Concentrations:
H2S04 7moM KBr03 0.25moyl 133 d s (rotating substrate 2250 rpm) Very rapid etch removal, roughnes decreases with increasing concentration of sulphuric acid (>2 moM)
Etch rate: Remarks:
Dry etching Volatile and moderately volatile compounds:
Gal33 GaC13 GaClz GaN PF3 PH3 PF5 PC15
Dry-etching method:
RFECR-Etching in CHJH2/ArPlasmas') Ar: 56 Yo;C&: 11YO;H2: 33 YO
Gas composition: Flow rate: Pressure: micro wave energy: rf-energy: Etch rate: References:
Bp. 201.3 O C 1 ) Bp. 535 "C1) subl.>800"C3) Bp. -101°C" Bp. - 8 8 O @ Bp. -75°C") Bp. 62 "@
45 sccm 3 mtorr 1kW 150W (13.56 MHz) 1.5 d s "A.F. Holleman and E. Wiberg (1985) ')P. Rotsch (1992) 3)J. D'Ans and E. Lax (1943), 264 4)J. D'Ans and E. Lax (1943), 231 5)S. J. Pearton et al. (1996)
246
6.2 Collection of Recipes
GaSb - Gallium Antimonide Wet etching Readily soluble reaction products:
Gallium is soluble as Ga3+(in acids) or as gallate Ga(OH);, in alkalies)'), Antimony is soluble in strongly oxidizing liquids like, e.g. HN03,forming Sb(II1) or Sb(V) as antimonous and antimonic acid, respectively; Sb(II1) is soluble in alkaline and strongly acidic media; Sb-cations form coordination compounds, e.g. , with chelating ligands of multifunctional organic acids like, e.g. citric acid or tartaric acid
Etchant 1:
Tartrate-containing hydrochloric acidic hydrogen peroxide solution2)
Concentrations: Temperature: Etch rate:
H202 0.7 moVl HC1 0.83 moVl NaK(C&O,) 0.083 moVl Room temperature 15 nm/s
Etchant 2:
Hydrofluoric acid-salpetric acid mixture 3,
Concentrations:
HF 2.6 moVl HN03 10moVl The etchant possesses a polishing effect.
Remarks:
Dry etching Volatile and moderately volatile compounds:
GaH3 GaC13 GaC1, GaN SbH3 SbClS SbFs SbCl3 SbBr, SbF3 Sb13
Bp. 201.3 "(2') Bp. 535 "C') sub1.>800"~) Bp. -17°C') Bp. 140°C') Bp. 141°C') Bp. 223°C') Bp. 288°C') Bp. 319°C') Bp. 401°C')
6.2 Collection of Recipes
247
1. Dry-etching method: RIE in ~ ~ 2 - p l a s m a 5 ) Gas composition: Flow rate: Plasma conditions: Power: Source: Temperature: Etch rate:
25% 75% H2 20 sccm 4 mtorr 0.85 W/cm2 Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm 5 40°C 3d s
2. Dry-etching method: RIE in CC12F2/02-plasma5) Gas composition: 95% CClZF2 ; 5% 0 2 Flow rate: 20 sccm Plasma conditions: 4 mtorr Power: 0.85 Wlcm' Source: Parallel-plate reactor,( 13.56 M H z ) , Electrode distance 7 cm Temperature: 5 40°C Etch rate: 0.4 nm/s 3. Dry-etching method: CAIBE in 12/Ar-plasma6) Gas composition: 12-partialpressure: i2*io-5torr Flow rate: 30 sccm Ion beam: Ar+,3 kV, 1mA/cm2; angle of incidence: 12-15' (Substrate rotating) Etch rate: 23 nmJs
4. Dry-etching method: Microwave etching in H2/C€€,/Ar-plasma7) C&: 17%;H2: 57%;Ar: 27% Gas composition: Flow rate: 30 sccm Plasma conditions: 10 mtorr Power: 300 w Etch rate: 0.22 nm/s References: ')AX Holleman and E. Wiberg (1985) "J.G. Buglass et al. (1986) ')B .A.Irving(l962) 4)J. D'Ans and E. Lax (1943), 231 %.J. Pearton et al. (1990 a);for using an additional ECR-source see also: S.J. Pearton et al. (1991 c), S.J. Pearton et al. (1990 c) 6)L.M.Bharadwaj et al. (1991) "S.J. Pearton et al. (1991 a); for using an additional ECR-source see also: S.J. Pearton et al. (1991 c)
248
6.2 Collection of Recipes
Ge - Germanium Wet etching Readily soluble reaction products:
Ge(I1)forms halogenocomplex ions: GeF3-, GeC13-;Ge(IV) is soluble in alkalies forming germanates GeO(OH)3- or in fluoridecontainin solutions forming hexafluorogermanate GeF6'- ')
Etchant 1:
Salpetric acid- hydrofluoric acid 2,
Concentrations:
HNo3 7 moVl HF 6 moVl CH3COOH 6moVl 20°C 25 n m l s
Temperature: Etch rate:
Etchant 2:
Salpetric acid-hydrofluoric acid with KJ-addition 2,
Concentrations:
HN03 9moVl HF 2.3 moVl KJ 0.15mmoVl 23°C 117 nmls
Temperature: Etch rate:
Etchant 3:
Salpetric acid- hydrofluoric acid with Hydrogen peroxide addition2)
Concentrations:
€€NO3 2.2moVl HF 1.3 moM H202 3.6moVl 23°C 117 nm/s
Temperature: Etch rate:
Etchant 4:
Salpetric acid- hydrofluoric acid with Cu-addition 2,
Concentrations:
HN03 3.1moVl HF 10 moVl Cu(N03)*0.02 moVl 23°C 20 nmls
Temperature: Etch rate:
6.2 Collection of Recipes
Dry etching Volatile compounds:
Ge& GeF4 GeHC13 GeC& GeBr,
Bp. -90"C3) subl. -35"C3) Bp. 75.2"C3) Bp. 84"C3) Bp. 183"C3)
1. Dry-etching method: Reactive ion etching in CBrF3-plasma4, Gas composition: Flow rate: Plasma conditions: Reactor: Etch rate: Remarks:
100%CBrF3
10 sccm 50 mtorr
Parallel reactor; 13.56 M H z , 0,4 kV self-bias 1.3 nm/s Preparation of 60nm-grids
2. Dry-etching method: Reactive ion etching in CF4-plasma Gas composition: Flow rate: Plasma conditions: Power: Reactor: Etch rate: Remarks:
100% CF4 100 sccm 250 mtorr 0.28 Wlcm2 Parallel reactor; 13.56 M H z 22 nm/s Very good selectivity to Si.
3. Dry-etching method: Plasma etching in CFdOz-mixture Gas composition: 95 Yo cF4; 5 Y o 0 2 Flow rate: Plasma conditions: Power: Reactor: Etch rate: Remarks:
100 sccm 250 mtorr 0.28 Wlcm' Parallel reactor; 13.56 M H z 23 nm/s Good selectivity to Si.
4. Dry-etching method: Reactive ion etching in CFzCl2-Plasma Gas composition: Flow rate: Plasma conditions: Power: Reactor: Etch rate: Remarks:
100% cF2c1, 100 sccm 100 mtorr 0.28 Wlcm' Parallel reactor; 13.56 M H z 3.3 nm/s Good selectivity to Si.
249
250
6.2 Collection of Recipes
5. Dry-etching method: Reactive ion etching in CF3Br -plasma 1' Gas composition: Flow rate: Plasma conditions: Power: Reactor: Etch rate: Remarks: References:
100% CFar 100 sccm 100 mtorr 0.28 W / m 2 Parallel reactor; 13.56 M H z 4.5 d s Good selectivity to Si. ')AX Holleman, E. Wiberg (1985) 2)A.F.Bogenschiitz (1967) 3)J. D'Ans and E. Lax (1943), 231,232 4)T.Matthies et al. (1993) "G.S. Oehrlein et al. (1991)
6.2 Collection of Recipes
251
GexSiImx- Germanium Silicide Wet etching Readily soluble reaction products:
Ge(I1) forms halogenocomplex ions: GeF;, GeC1,; Ge(1V) is soluble in alkalies forming germanates GeO(OH)3-or in fluoride-containing solutions forming hexafluorogermanat GeF2- ') Si(IV) is soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),]" or in Fcontaining media as [SiF6l2-') Etching is supported by appropriate chelating ligands, e.g., pyrocatecol, ethylene diamine, hydrazine
Dry etching Volatile compounds:
GeH4 GeF4 GeHC13 GeC& GeBr, Sib SiF, Si2& SiHC13 Sic& Si2OC16 Si2C16
Bp. -90°C2) subl. -35"C2) Bp. 75.2"C2) Bp. 84"C2) Bp. 183"C2) Bp. -111.6"C3) Bp . -95.7"C3) Bp. -15"C3' Bp. 31.7"C3) Bp. 56.7"C3) Bp. 135.5"C3) Bp. 147"C3)
1. Dry-etching method: Reactive ion etching in SiC&/C12/He plasma4) Gas composition: 50 % Sic&;37.5 YOC1,; 12.5 % He Flow rate: Power: Plasma conditions: Reactor: Etch rate:
Gas composition: Flow rate: Power:
24.6 sccm 0.13 W/cm2 10 mtorr Parallel reactor; 13.56 MHz, -7OV bias 1nm/s (x=O.l; 10% Ge) 1.2 nm/s (x=0.2; 20 % Ge) 3 nm/s (reines Germanium) 33% Sic&;33% C12;33% He 47.5 scan 0.37 Wkm2
252
6.2 Collection of Recipes
Plasma conditions: Reactor: Remarks:
10 mtorr Parallel reactor; 13.56 M H z , -411V bias Preparation of columns with 0.2 pm diameter and ca. 0.7 pm height
2. Dry-etching method: Reactive ion etching in SFd02/He-plasma4) Gas composition: Flow rate: Power: Plasma conditions: Reactor: Etch rate:
References:
40% He 24.6 sccm 0.13W/cm* 10 mtorr Parallel reactor; 13.56 M H z , -7OV bias 2.8 nm/s (x=O; pure Si) 4.7 nm/s (x=O.l; 10% Ge) 5.5 nm/s (x=0.2; 20% Ge) 5.3 nm/s (x=0.25; 25 % Ge) 2.5 nm/s (x=l; pure Germanium) ‘)A.F.Holleman,E.Wiberg (1985) 2)J.D’An~ and E.Lax (1943), 231,232 3)J.D’Ansand E.Lax (1943), 261 4)R.Cheunget al. (1993)
6.2 Collection of Recipes
253
Hf - Hafnium Wet etching Readily soluble reaction products:
Hafnium is soluble as Hf(1V) in form of halogen salts, oxohalogen salts and omplex compounds')
1. Wet-etching method: Etching in diluted HF-solution2) Temperature:
Room temperature
Dry etching Moderately volatile compounds:
HfC&
References:
')A.EHolleman, E.Wiberg (1985) ')W.Tegert (1959)
Sblp. 319°C')
254
6.2 Collection of Recipes
HgTe - Mercury Telluride Wet etching Readily soluble reaction products:
Hg(I1) forms soluble salts and coordination compounds, Te(1V) is soluble in strong acids as Te4+ and in strong alkalies as TeO:-, Te(I1) in tartaric acidic solution as chelate; Te02 is soluble in different multifunctional organic acids')
Wet etchant: Composition:
Hydrobromic acidic Iodine-Potassium iodide solution2) 4.15 g KI and 0.5 g I2in 12.5 ml HBr
Etch rate:
(no details for HBr-concentration given) 75 n m / s
Dry etching Volatile and moderately volatile compounds: References:
Hg Bp. 3573) H2Te Bp. -2.3 O C ' ) "A.F. Holleman and E. Wiberg (1985) 2)RW.Leechet al. (1990) 3)J.D'Ansand E.Lax (1943), 254
6.2 Collection of Recipes
255
lnAs - Indium Arsenide Wet etching Readily soluble reaction products:
In(II1) is soluble as aquocomplex [In(H20)6]3for as fluorocomplex [InF6I3-.') arsenic is soluble as As(II1)-salts, as chlorocomplexes, as As(V) in arsenic acid
Etchant :
Sulphuric acidic bromate solution2)
Concentrations:
H2S04 8moVl KBrO, 0.25moVl 530 nm/s (rotating substrate 2250 rpm) Rapid etch removal, roughness of the etching surface is depdendent on themlphuric acid concentration
Etch rate: Remarks:
Dry etching Volatile compounds:
InBr, InCl, ASH, AsF, AsF, AsCl, AsBr,
subl. 371°C') subl. 418°C') Bp. -54.8"C" Bp. -52.9"C') Bp. 63°C') Bp. 130.4"C') Bp. 221°C')
1. Dry-etching method: RIE in C12-plasma3) 33 % Ar;67 % Cl2 Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
15-35 S C C ~ 50 mtorr 1WICm2 Parallel-plate reactor (13.56 MHz), Electrode distance 7 cm 2 nm/s
2. Dry-etching method: RIE in SiC14-plasma3) 33 % Ar;67 % Sic4 Gas composition: Flow rate: 15-35 S C C ~ 50 mtorr Plasma conditions: 1W/cm2 Power: Parallel-plate reactor,( 13.56 MHz), Source: Electrode distance 7 cm Etch rate: 2.2 nm/s
256
6.2 Collection of Recipes
3. Dry-etching method: Microwave etching in H2/C€&/Ar -plasma5) Gas composition: C&: 17 YO;H2: 57 %; Ar: 27 Yo Flow rate: Plasma conditions: Power: Etch rate:
30 sccm 10 mtorr 300 w 0.2 n m l s
4. Dry-etching method: RIE in C3.,4H2-plasma4) Gas composition: 25 % GI&;75 70H2 Flow rate: 20 sccm Plasma conditions: 4 mtorr Power: 0.85 Wlcm2 Source: Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm Temperature: 5 40°C Etch rate:
0.5 n m l s
5 . Dry-etching method: RIE in CC12F2/02 -plasma4) Gas composition: Flow rate: Plasma conditions: Power: Source: Temperature: Etch rate: References:
95% CC12F2 ; 5% 0 2 20 sccm 4 mtorr 0.85 Wkm2 Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm 5 40°C 0.8 n m l s ')A.F. Holleman and E. Wiberg (1985) 2)€?Rotsch (1992) 3)S.J.Peartonet al. (1990 b) 4)S.J.Peartonet al. (1990 a) ')S.J.Pearton et al. (1991 a)
257
6.2 Collection of Recipes
(In,Ga)N - Indium Gallium Nitride Wet etching Readily soluble compounds:
In(II1) is soluble as aquocomplex [In(H20),I3+or as fluorocomplex [InF6I3-.') gallium as Ga (in acids) or as gallates (Ga(OH);, in alkalies)') +
Dry etching Volatile and moderately volatile compounds:
InBr, InC13 Ga2& GaCl, GaCl, GaN
subl. 371°C') subl. 418°C') Bp. -63°C') Bp. 201.3"C') Bp. 535°C') sub1.>8WC2)
1. Dry-etching method: ECR-etching in CH&-12/Ar -plasma3) Gas composition: 17 YO CI&; 50% H2; 33 % Ar
Flow rate: Plasma conditions: Power: Temperature: Etch rate:
30 sccm 1.5 mtorr 1 kW ECW 450 W (rf 13.56 M H z ) 23°C 6 nm/s
2. Dry-etching method: ECR-etching in C12/Ar -plasma3) Gas composition: 33 Yo C12; 67 % Ar Flow rate Plasma conditions: Power: Temperature: Etch rate:
15 sccm 1.5 mtorr
1kW (ECR); 450 W (rf 13.56 MHz) 23°C 8 nm/s
3. Dry-etching method: ECR-etching in ICY& -plasma3) Gas composition: 50% ICl; 50% Ar Flow rate Plasma conditions: Power: Temperature: Etch rate: References:
8 sccm
1.5 mtorr 1 kW (ECR); 250 W (rf 13.56 M H z ; dc -275 V) 23°C 12 nm/s ')AX Holleman and E. Wiberg (1985) 2)J. D'Ans and E. Lax (1943), 264 3)C.B.Vartuli et a1 (1996); see R.J.Shul et al. (1996)
258
6.2 Collection of Recipes
InN - Indium Nitride Wet etching Readily soluble compounds:
In(II1) is soluble as aquocomplex [In(H20)6]3+ or as fluorocomplex [Ig6I3-.l)
Dry etching Moderately volatile compounds:
InBr3 InC13
subl. 371°C') subl. 418°C''
1. Dry-etching method: ECR-etching in CH&12/Ar -plasma3) Gas composition: 17 % CI!&; 50 % H2; 33 % Ar Flow rate 30 sccm Plasma conditions: Power: Temperature: Etch rate:
1.5 mtorr 1kW ECFU 450 W (rf 13.56 M H z ) 23°C 10 d s
2. Dry-etching method: ECR-etching in C12/Ar -plasma3) Gas composition: Flow rate Plasma conditions: Power: Temperature: Etch rate:
33% Cl2; 67% Ar 15 sccm
1.5 mtorr 1kW (ECR); 450 W (rf 13.56 MHz) 23°C 13 d s
3. Dry-etching method: ECR-etching in ICVAr -plasma3) Gas composition: 50% IC1; 50% Ar Flow rate Plasma conditions: Power: Temperature: Etch rate: References:
8 sccm
1.5 mtorr 1kW (ECR); 250 W (rf 13.56 M H z ; dc -275 V) 23°C 19 d s ''A.F. Holleman and E. Wiberg (1985) ''5. D'Ans and E. Lax (1943), 264 3)C.B. Vartuli et al. (1996)
6.2 Collection of Recipes
259
InP - Indium Phosphide Wet etching Readily soluble reaction products:
In(III) is soluble as aquocomplex [In(H20)6]3+ or as fluorocomplex [InFgI3-.')
Etchant 1:
Acetic- and hydrobromic acidic dichromate solution2)
Concentrations:
K2Cr207 0.1 moVl HBr 3 moVl CH3COOHmoVl Room temperature 4.2 d s Particular smooth edges are obtained in etching V grooves in HBr:K2Cr207= 3:13).
Temperature: Etch rate: Remarks:
Etching method 2:
Photoelectrochemical etching of semiisolating InP (S-doped 1018cm3)in diluted hydrochloric acid 4,
Concentrations: Temperature: Irradiation: Etch rate:
HC10.54 moVl Room temperature 250 Wkm2 22 d s (0 V) (- 0.4 V) 13 d~ 32 nm/s (0.4 V)
Etchant 3:
Sulphuric acidic bromate solution5)
Concentrations:
H2S04 8moVl KBr03 0.25moVl 370 d s (rotating substrate 2250 rpm) Very rapid removal
Etch rate: Remarks:
Etchant 4:
Galactic acid - phosphoric acid hydrochloric acid solution6)
Concentrations:
HC1 HZ04 C&03 18 d s HC1 H3P04
Etch rate: Concentrations:
c3&03
1.9 moVl 4moVl 3.6moVl 1moVl 7moVl 0.9moVl
260
6.2 Collection of Recipes
Etch rate: Remarks:
24 n m / s By adding galactic acid smooth surfaces and pattern edges are recieved.
Wet-etching method 5 : Etching in hydrochloric and acetic acidic hydrogen peroxide solution7) Electrolyte: Etch rate:
H202 0.2 mom HC1 0.47 mom CH3COOH 16 moVl 3.8 n m / s
Wet-etching method 6: Anisotropic etching in sulphuric acid hydrogen peroxide and bromomethanolic solution') Etchant composition: Etchant A: Etchant B: Process:
Temperature: Etch rate (related to etchant A): Remarks:
Br2, 0.1 Vol % ,dissolved in methanol HZS04(96 %):HzO:H202(30% ) =3 :1:1 1. Etching in etchant A (etching time according to desired etch depth) 2. Etch stop by rinsing with methanol 3. Rinsing with water and drying 4. Etching (5 min) in etchant B 20°C 6.8 n m / s Very smooth V-grooves are achieved. Trapezoidal grooves have slightly arched bottoms.
Wet-etching method 7: Photoelectrochemical etching in nitric acid') Electrolyt composition: Potential (vs SCE): Irradiation: Etch rate:
HN03:2.2 moVl - 1.ov 1.9 mW (HeNe Laser, 632.8 nm) 4 nm/s
Dry etching Volatile and moderately volatile compounds:
InBr3 InC13 PF3 PH3 PF5 PC15 PC13 POC13 p406
subl. 371°C') subl. 418°C') Bp. -lO1"C'o) Bp. -88"C10) Bp. -75°C") Bp. 62°C") Bp. 74.5"C'0) Bp. 105.4°C'0) Bp. 173"C10)
6.2 Collection of Recipes
261
1. Dry-etching method: Reductive MIE in Hz/C€&-plasmal’) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
cH4: 40%; 50 sccm 40 mtorr 0.4W/cmz 9 nm/s
2. Dry-etching method: Microwave etching in Hz/C€€,/Ar -plasmau) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
C&: 17%; Hz: 57%; Ar: 27% 30 sccm 10 mtorr 300 w 0.5 nm/s
3. Dry-etching method: CAIBE in I*/& -plasma13) Gas composition: Flow rate: Ion beam: Etch rate:
1,-partial pressure: 5*10-’ torr 30 sccm Ar’,3 kV, 1.7 mA/cm2(Substrate rotating) angle of incidence: 12-15’ 22 nm/s
4. Dry-etching method: Reactive ion etching with Clz14) Gas composition: Ion energy: Ion beam: Etch rate:
c 1 2
1bis 1.5 keV 0.6 mA/cm2 2.5 bis 3.3 nm/s
5. Dry-etching method: Reactive ion etching in iodine-containing Plasmas 15) 54 Gas composition: Pressure: Bias-voltage: Temperature: Etch rate: Remarks: 5b) G ~ composition: S Pressure: Bias-voltage: Temperature: Etch rate: Remarks:
95% Ar; 5% 12 10 mtorr 0.35 kV 105°C 8.3 nm/s Partially forming “grass” 29 % H2; 68 Yo 12; 3 % CH, 15 mtorr 0.35 kV
120°C 6.25 nm/s Smooth surface
262
6.2 Collection of Recipes
6. Dry-etching method: Microwave-enhanced rf-plasma etching in Cl2/~-plasmal6) Gas composition: Flow rate: Pressure: Power: Temperature: Etch rate:
50% C12; 50% Ar 20 sccm 2 mtorr 1 kW (Microwave); 0.1 kW (rf) 20°C 60 n m l s
7. Dry-etching method: RIE in Cl2-p1asmal7) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
33 % Ar; 67 % Cl2 15-35 sccm 50 mtorr 1 wtcm2 Parallel-plate reactor, (13.56 MHz), Electrode distance 7 cm 2.1 n m l s
8. Dry-etching method: RIE in SiCb-plasma’8) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
33% Ar; 67% Sic& 15-35 S C C ~ 50 mtorr 1 Wtcm2 Parallel-plate reactor,(13.56 M H z ) , Electrode distance 7 cm 1.6 d s
9. Dry-etching method: Etching in BC1f12-plasma of high density”) Gas composition: Plasma conditions: Power: Temperature: Etch rate:
75 % BCl3; 25 % N2 1 mtorr (rf 13.56 MHz); - 145 V self-bias 1 kW (Microwave 2.45 GHz) 100°C
30 n m l s
10. Dry-etching method: W/ECR-Etching in CHd/H2/Ar/N2 ~lasrnas”) Ar: 56 Yo ; CT&: 11 Yo ;H2: 18 Yo ;N2: 15 YO Gas composition: Flow rate: Pressure: Microwave energy: Rf-energy: Etch rate:
45 sccm 3 mtorr 1kW 150W (13.56 MHz) 13 d s
6.2 Collection of Recipes
References:
263
')AX Holleman and E. Wiberg (1985) 2)A.EBogenschiitz (1967) 3)I? Boensch et a1.(1998) 4)R.Khare et al. (1993) ')P. Rotsch (1992) %. Ikossi-Anastasiou et al. (1995) 7)J.R.Flemishand K.A. Jones (1993) *)M.Kappeltand D. Bimberg (1996) 9)K.l? Quinlan (1996) '')J. D'Ans and E. Lax (1943), 264 ")J.Singh (1991) rf 709 ")S.J.Pearton et al. (1991 a) 13)L.M.Bharadwajet al. (1991) 14)seeS.J.Pearton et al. (1990 b) 15)D.C.Flanderset al. (1990) 16)K.K.Koand S.W.Pang (1995; for temperature dependence of microwave etching of InP in C12- and HC1-plasma: D.G.Lishan and E.L.Hu (1990) 17) S.J. Pearton et al. (1990 b) 18) E Ren et al. (1996) 19) S.J. Pearton et al. (1996)
264
6.2 Collection of Recipes
lnSb - Indium Antimonide Wet etching Readily soluble reaction products:
In(II1) is soluble as aquocomplex [In(H20)6]3for as fluorocomplex [InF6I3-.')Antimony is soluble in strongly oxidizing liquids Like, e.g., HN03,forming Sb(II1) or Sb(V) as antimonous and antimonic acid, respectively; Sb(II1) is soluble in alkaline and strongly acidic media; Sb-cations form coordination compounds, e.g. , with chelating ligands of multifunctional organic acids like, e.g. citric acid or tartaric acid
Etchant 1: Concentrations:
Hydrofluoric acidsalpetric acid-mixture 2): HF 13 moVl HN03 5.5moVl Polishing solution, selective for (110)- planes in comparison to (111)- and (100)-planes HF 11 moVl HN03 4.6moVl Etchant for (100)- and (110)-planes
Remarks: Concentrations: Remarks:
Etchant 2: Concentrations: Remarks:
Hydrofluoric acidic hydrogen peroxide 2): HF 4.3 moVl H202 1.5moVl Etchant for (111)-planes
Dry etching Volatile and moderately volatile compounds:
InBr3 InC13 SbH3 SbCl5 SbF5 SbC13 SbBr3 SbF3 Sb13
subl. 371°C') subl. 418°C') Bp. -17°C') Bp. 140°C') Bp. 141°C') Bp. 223°C') Bp. 288°C') Bp. 319°C') Bp. 401°C')
6.2 Collection of Recipes
265
1. Dry-etching method: RIE in C12-plasma3) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
33 % Ar; 67 % Cl2 15-35 sccm 50 mtorr 1Wkm' Parallel-plate reactor (13.56 MHz), Electrode distance 7 cm 1.8 d s
2. Dry-etching method: RIE in SiC&-plasma3) Gas composition: 33% Ar; 67% Sic& Flow rate: Plasma conditions: Power: Source: Etch rate:
15-35
SCC~
50 mtorr 1WIcm2
Parallel-plate reactor (13.56 MHz), Electrode distance 7 cm 2.7 d s
3. Dry-etching method: CAIBE in 12/Ar-plasma6) Gas composition: Flow rate: Ion beam: Etch rate:
Iz-partialpressure: 12*1O-' torr 30 sccm Ar', 3 kV, 1mA/cm2; angle of incidence: 12-15' (Substrate rotating) 23 d s
4. Dry-etching method: Microwave etching in H2/Cl&/Ar -plasma7) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
CW: %; H2: 57%;Ar: 27 1% 30 sccm 10 mtorr 300 w 0.22 d s
References:
"A.F. Holleman and E.Wiberg (1985) ')B.A. Irving(1962) 3)S.J.Pearton et al. (1990 b) 6)L.M.Bharadwaj et al. (1991) 7)S.J.Pearton et al. (1991 a)
266
6.2 Collection of Recipes
(In, Sn) - Indium Tin Wet etching Readily soluble reaction products:
In(III) is soluble as aquocomplex [III(H~O)~]~+ or as fluorocomplex [1nF6I3-.') Sn(I1) is soluble in form of salts, Sn(IV) forms with appropriate ligands L ( e.g. L= C1-or OH-) soluble complex ions of the type [SI&]" ')
1. Wet etchant:
Hydrochloric acid etchant2)
Concentrations: Temperature: Etch rate:
HC1 ca. 1.2 moVl HN03 0.55moVl 20°C ca.2 nm/s (partially oxidized (In,Sn))
2. Wet etchant:
Etch orange3)
Concentrations:
(W)$e(N03)6 HClO4
0.3 mOM 0.5 moVl
Dry etching Volatile and moderately volatile compounds:
References:
subl. 371°C') subl. 418°C') Bp. - 52°C') Bp. 114.1"C') Bp. 203.3"C') Bp. 346°C') Bp. 605°C') subl. 705°C') Bp. 853°C') snF2 "A.F. Holleman and E. Wiberg (1985) "Merck Bakers (oJ.) 3)A.Wiegand (1981-1996) InBr, InC13 SnH4 SnCL, SnBr, Sn14 SnC12 snF4
6.2 Collection of Recipes
(In,Sn,)O
267
- Indium Tin Oxide (ITO)
Wet etching Readily soluble reaction products:
Remarks:
In(III) is soluble as aquocomplex [I~(I-I~O)~]~+ or as fluorocomplex [Ifl6I3-'1. Sn(I1) is soluble in form of salts, Sn(IV) forms with appropriate ligands L ( e.g. L= C1- or OH) soluble complex ions of the type [S&]'- ') It is convenient to deposit at first a film of InSn that can be patterned microlithographically (see In,Sn). Afterwards the patterned film is oxidized either at elevated temperatures in air or oxygen or in a oxygen plasma to form ITO.
Dry etching Volatile and moderately volatile compounds:
InBr3 hC13
SnH,
SnCh SnBr, Sn14 SnC12 sfl4 sfl2
subl. 371°C') subl. 418°C') Bp. - 52°C') Bp. 114.1"C') Bp. 203.3"C') Bp. 346°C') Bp. 605°C') subl. 705°C') Bp. 853°C')
1. Dry-etching method: Reactive ion etching in aceton-oxygenplasma2) a) Gas composition:
20 '30Aceton; 20 % O2 ; 60YOAr
Flow rate: Plasma conditions: Power:
40 sccm 40 mtorr 0.25 W/cm2
Etch rate: Remarks:
0.04 nm/s
No depositions of carbon-containing byproducts Little selectivity to photoresist (ca.factor 2.5)
268
6.2 Collection of Recipes
b) Gas composition: Flow rate: Plasma conditions: Power: Etch rate: Remarks:
20% Aceton; no O2 ; 80% Ar 40 S c c m 40 mtorr 0.25 Wkm2 0.08 n m / s Deposition of carbon-containing byproduct, High selectivity to photoresist.
2. Dry-etching method: Reactive ion etching in Hydrobromic plasma3) Gas composition: Flow rate: Plasma conditions: Power: Substrate temperature: Etch rate: References:
HBr, Ar 40 S c c m
100 mtorr 225W (1,2 W/cm') 150°C 2.5 n m / s "A.F. Holleman and E. Wiberg (1985) 2 ) ~ . Saia ~ . et al. (1991) 3)L.Y.'Isou (1993)
6.2 Collection of Recipes
In,Te,
269
- Indium Telluride
Wet etching Readily soluble reaction products:
In(III) is soluble as aquocomplex [In(€1~0)~]~+ or as fluorocomplex [w613'). Te(W) in strong acids as Te4+and in strong bases as Te0;- '), Te(I1) in tartaric acidic solutions as chelate; Te02is soluble in various multifunctional organic acids2)
Wet-etching method :
Citric and acetic acidic bromine water?
Concentration:
Acetic acid 16 moVl Citric acid (saturated) + bromine water (1 part to 19 parts organic acids) Polishing solution
Remarks:
Dry etching Moderately volatile compounds: References:
I&r3 subl. 371°C') InC13 subl. 418°C') ')AX Holleman and E. Wiberg (1985) 2)B.A.Irving (1962)
270
6.2 Collection of Recipes
KTiOP04- Potassium Titanyl Phosphate (KTP) Wet etching Readily soluble reaction products:
Ti(1V) is soluble in strong acids as [Ti(OH),] '+, [Ti(OH),]+and derived com lex ions, among them F as preferred ligand
Etchant 1:
Diluted hydrochloric acid *)
Concentrations: Temperature: Etch rate:
HC1 (1+2 diluted) Room temperature 0.8 nm/s
R
Dry etching Volatile and moderately volatile compounds:
References:
TiBr4 Bp. 233.45"C') Ti4 subl. 284°C') 'Ii 54 Bp. 377°C') PF3 Bp. -101°C' PH3 Bp. -88"C?) PF5 Bp. -75°C) PCls Bp. 62°C) PC13 Bp. 74.5"C) POC13 Bp. 105.4"C) P406 Bp. 173°C) "A.F. Holleman and E. Wiberg (1985) 2)S.Wu et al. (1995)
6.2 Collection of Recipes
271
LiAIO2- Lithiumaluminat Wet etching Readily soluble reaction products:
AI(III) is soluble as aquocomplex [AI(H,O),]~+ 3- .I), or as fluorocomplex [m6] lithium as alkali metal in practically all aqueous solutions
Wet-etching method 1: Etching in phosphoric acid2) Concentrations: Etch rate:
H3P04,concentrated 0.6 n m / s (25°C)
Wet-etching method 2: Etching in hydrofluoric acid 2, Concentrations: HF,concentrated Temperature: 25 "C Etch rate:
220 n m / s for an etching time of 1 minute 62.5 n m / s as average for an etching time of 8 minutes
Wet-etching method 3: Etching in hydrochloric acidsalpetric acid2) Concentrations: Temperature: Etch rate:
HCl: 4 moVl (1 part concentrated HC1); HN03:10 mom 12 parts concentrated %03) 25°C 28 nm/s
Dry etching Moderately and slightly volatile compounds:
AlC13 mr3 MI3 Li(e1ementary) (LiC1 LiF
Dry-etching method:
Microwave plasma etching in SFdAr'?) 67 % f b ; 33 %; sF6 Parallel-plate reactor; ECR 2.45 GHz;
Gas composition: Plasma conditions: Pressure: Power: Etch rate:
subl. 182.7"C') subl. 255°C') subl. 381°C') Bp. 1372"C3) Bp. 1383°C') Bp. 1681°C'')
1.5 mtorr 45OWrfll kWECR 4 nm/s
272
6.2 Collection of Recipes
References:
''A.F. Hollemann and E. Wiberg (1985) 2)J.W.Lee et al. (1996 b) 3)J. D'Ans and E. Lax (1943), 321
6.2 Collection of Recipes
273
LiGa02- Lithium Gallate Wet etching Readily soluble reaction products:
Gallium is soluble as Ga3+(in acids) or as gallates (Ga(OH);, in alkalies)'), lithium as alkali metal in practically all aqeous solutions
Wet-etching method 1: Etching in hydrochloric acid') Concentrations: Temperature: Etch rate: Remarks:
HC1 25°C 67ds Hydrochloric acid etches lithium gallate selectively to lithium aluminate. Crystallographically selectice etching is possible in buffered HC1 solution3).
Dry etching Moderately and slightly volatile compounds:
GaH3 GaC1, Bp. 201.3"C') GaC12 Bp. 535°C') GaN sub1.>8WC1) Li (elementary) Bp. 1372°C) (LiC1 Bp. 1383°C') LiF Bp. 1681°C'))
1. Dry etching method: Microwave plasma etching in SFdA?) Gas composition: 67 YOAr;33 %; SF, Plasma conditions: Pressure: Power: Etch rate:
Parallel-plate reactor; ECR 2.45 GHz; 1.5 mtorr 450 W
4 nm/s
2. Dry-etching method: Microwave plasma etching in Clz/A?) Gas composition: 67 % Ar; 33 % C4 Plasma conditions: Pressure: Power: Etch rate:
Parallel-plate reactor; ECR 2.45 GHz; 1.5 mtorr 450 W 1.3 nm/s
274
6.2 Collection of Recipes
References:
''A.F. Hollemann and E. Wiberg (1985) 2)J.W.Lee et al. (1996 b) 3)Th.J. Kropewnicki et al. (1998) 4)J. D'Ans and E. Lax (1943), 321
6.2 Collection of Recipes
LiNb03- Lithium Niobate Wet etching Readily soluble reaction products: Etchant:
Li(1) is soluble as Li+ Nb(V) as fluoride NbF,') HF-solution
Dry-etching Volatile and moderately volatile compounds:
NbF5 Bp. 229°C') NbC15 Bp. 247.4"C2) Li (elementary) Bp. 1372"C3) LiCl Bp. 1383°C') Bp. 1693"C3) LiF
1. Dry-etching method: Reactive ion etching in CHF3-plasma4) Gas composition: Plasma conditions: Ion energy: Etch rate:
CHF, 8*10-5ton; 0.5 k e y current density: 400 pA/cm2 0.2 n m / s
2. Dry-etching method: Sputtern in Ar-plasma4) Gas composition: Ar Plasma conditions: Ion energy: Etch rate:
8 * lo-' tom; 0.5 k e y Current density: 400pA/cm2 0.13 nm/s
References:
"A.F. Holleman and E. Wiberg (1985) 2)J. D'Ans and E. Lax (1943)' 250 3)J. D'Ans and E. Lax (1943)' 241 4)S.Matsui et al. (1980)
275
276
6.2 Collection of Recipes
Mg - Magnesium Wet etching Readily soluble reaction products:
Magnesium is soluble as Mg(II), e.g. in form of halides, or nitrate')
1. Wet-etching method: Etching in diluted salpetric acid2)
Dry etching Volatile substances (at high temperatures): References:
Mg (metal) Bp. 1105°C') MgClz Bp. 1418"C3) "A.F. Holleman, E. Wiberg (1985) ')W. Tegert (1959) 3)J. D'Ans and E. Lax (1943), 241
6.2 Collection of Recipes
277
Mo - Molybdenum Wet etching Readily soluble reaction products:
Mo(VI) forms soluble molybdates as well as fluorooxocomplexes, Mo(I1) and Mo(II1) form among others soluble chlorocomplexes, molybdenum is soluble in strongly oxidizing aqueous solutions, especially in the presence of HF');
Wet-etching method 1: Sulphuric acidsalpetric acid-mixture2) Concentrations: Temperature: Etch rate: Remarks:
20% H2S04;50% HN03 17°C 2300 n m / s Photoresist is considerably attacked, hence the preparation of small lithographic structures is difficult .
Wet-etching method 2: Alkaline hexacyanoferrate-etchant3) Composition: Etch rate:
K3Fe(CN)dNaOH 30 n m / s (immersion, 150 pm Diffusions layer thickness) 70 n m / s (Immersion, 70 pm Diffusion layer thickness) 100 n d s (Immersion, 25 pm Diffusion layer thickness) 200 n m / s (Spray etching, 20 pm Diffusion layer thickness) 230 n m / s (Spray etching, 15 pm Diffusion layer thickness)
Wet-etching method 3: Alkaline hexacyanoferrate/oxalateetchant2) Concentrations: Temperature: Etch rate: Remarks:
K3Fe(CN)6 0.61 moVl Na2G04 0.02 mom NaOH 0.5moVl 18°C 80 n m / s Positive photoresist (Novolak-based) is not stable in this alkaline etchant.
278
6.2 Collection of Recipes
Wet-etching method 4: Fe(II1)-nitrate-etchant3) Temperature: Etch rate:
50°C 80 n d s
Dry etching Moderately volatile compounds:
MoF5 M0C15
Bp. 214"C1' Bp. 628°C ')
1. Dry-etching method: Reactive ion etching in CCL,/02-plasma5) Gas composition: Flow rate: Power: Etch rate:
75 % CC14:25 % ; 02: 100 sccm 350 W 2nds
2. Dry-etching method: Reactive ion etching in CFJ02-plasma6) Gas composition: Flow rate: Pressure: Power: Etch rate:
CF,: 20%; 02: 80% 100 sccm 0.2 torr 500 kW 7d s
3. Dry-etching method: Plasma etching in NF?) Gas composition: Plasma conditions: Etch rate:
N F 3 : 100Vol%o Planar etching reactor 0.08-0.25 tom 3.3 nm/s
4. Dry-etching method: Ion beam etching with Argon') Ion energy: Etch rate:
1 keV 0.7 n d s
References:
"A.F. Holleman and E. Wiberg (1985) 2)D.M.Allen et al. (1986) 3)A.F.Bogenschiitz et al. (1991) 4)B.Gorowitz and J. Saia (1984) ')Y. Kuo (1990) 6)T.F'.Chow and A.J. Steckl (1982) 7)C.S.Korman et al. (1983) "W. Laznovsky (1975)
6.2 Collection of Recipes
279
MoSi, - Molybdenum Silicide Wet etching Readily soluble reaction products:
Mo(VI) forms soluble molybdates as well as fluorooxocomplexes, Mo(I1) and Mo(II1) form among others soluble chlorocomplexes, molybdenum is soluble in strongly oxidizing aqueous solutions, especially in the presence of HF');, Si(IV) in form of complexes, e.g. in strongly alkaline media as [Si(OH),I2-or in F-containing media as [SiF6I2-
Wet etchant:
HF-solution
Dry etching Volatile and moderately volatile compounds:
MoF~ MoC15 Sib siF4 Si2& SiHC13 SiC14 Si20C& Si2C16
Bp. 214°C') Bp. 628°C') Bp. -111.6"C2) Bp. -95.7 "C2) Bp. -15 "C2) Bp. 31.7 "C2) Bp. 56.7 "C2) Bp. 135.5"C2) Bp. 147"C2)
1. Dry-etching metllod: Reactive ion etching in CF4/02-pldsma3) Gas composition: Flow rate: Power: Etch rate:
CF4: 91 Val%; 0 2 : 9 Val% 44 sccm 0.7 W/cm2 3.2 n m / s
2. Dry-etching method: Plasma etching in NF;) Gas composition: Reactor: Pressure: Etch rate:
100% Parallel-plate reactor 0.15-0.25 t o n 13 nmls
NF3:
3. Dry-etching method: Plasma etching in CC14/O2-plasrna5) CC&:50%; 0 2 : 50% Gas composition: Pressure: 170 mtorr Etch rate: 28 nmls A factor 10-40 in selectivity to Si02is achieved. Remarks:
280
6.2 Collection of Recipes
References:
"A.F. Holleman and E. Wiberg (1985) 2)J. D'Ans and E. Lax (1943), 261 3)T.P.Chow and A.J. Steckl (1982) 4)T,lChow ? and A.J. Steckl (1982); C.S. Korman et al. (1983) ')B. Gorowitz and R. Saia (1982)
6.2 Collection of Recipes
Nb - Niobium Wet etching Readily soluble reaction products: Etchant 1:
Nb(V) is soluble as fluorid NbF,')
Fluoride-containing citric acid-Salpetric
acid-peroxodisulfate-etchant2) Concentrations:
Temperature:
(Nl&)2S208 0.66 moVl 0.27 moVl Citricacid 0.11 moVl HNO, 1.43 moVl 50°C
Etchant 2:
HF-solution
rn
Dry etching Volatile compounds:
NbF, NbCl,
Bp. 229°C') Bp. 247.4"C')
1. Dry-etching method: Plasma etching in CF4/02-plasma4,5) Gas composition: Plasma conditions: Power: Etch rate: Gas composition: Plasma conditions: Power: Etch rate: References:
CF4: 90 %; 0 2 : 10 % 1 tom 25 W 0.3-0.6 n d s CF4: 80 %; 0 2 : 20 % 0.15 torr 0.32 Wkm2 0.7 n m l s "A.F. Holleman and E, Wiberg (1985) 2)IPHT-In-houseprescription 3)J. D'Ans and E. Lax (1943)' 250 4)M.Gurvitch et al. (1983) ,)A. Shoji et al. (1982)
281
282
6.2 Collection of Recipes
NbN - Niobium Nitride Wet etching Readily soluble reaction products:
Nb(V) is soluble as fluoride NbF,')
Etchant:
HF-solution
Dry etching Volatile compounds:
NbFs NbCl,
Dry-etching method:
Plasma etching in CF4/02-plasma2) CF,: 80%; 0 2 : 20%
Gas composition Plasma conditions: Power: Etch rate: References:
Bp. 229°C') Bp. 247.4"C')
0.15 t o n 0.32 Wkm2 1.4 nm/s ')J. D'Ans und E. Lax (1943), 250 2)A.Shoji et al. (1992)
6.2 Collection of Recipes
283
Ni - Nickel Wet etching Readily soluble reaction products:
Ni (11) is soluble in form of Ni2+and its soluble complexes Nickel forms in normal atmosphere a dense passivating film, containing Ni(II1). The dissolution of it is possible in acidic media at elevated temperatures as well as by addition of complexing ligands like F, C1- or NH3.
Etchant 1:
Ammoniumperoxodisulfate/ Iron( (1II)chloride solution')
Concentrations:
(NIQ2S2080.8 mom FeCI3 0.09 moVl 50°C 33 n d s Nickel is also etched in concentrated FeC13-solution without addition of peroxodisulphate with rates of 200 to 4oO n d s . ').
Temperature: Etch rate: Remarks:
Dry etching Volatile compounds: References:
Ni(CO), Bp. -25"C3) "IPHT-in-house instruction 2)R.J.Ryan et al. (1970) 3)J.D'Ans und E. Lax (1943), 249
284
6.2 Collection of Recipes
(Ni, Cr) - Nickel Chromium The following etching methods are also applicable to partially oxidized nickel chromium (Ni,Cr)O, and with small additions of silicon (Ni,Cr,Si)O,.
Wet etching Readily soluble reaction products:
Ni (11) is soluble in form of Ni2+and its soluble complexes. Nickel forms in normal atmosphere a dense passivating film containing Ni(II1). This can be dissolved in acidic media and at increased temperature, and by adding complexing ligands, like F, Cl- or NH3. Cr(II1) is soluble in form of coordination compounds
Etchant 1:
Etch orange')
Concentrations:
(NH4)2Ce(N03)6 0.3 mOVl HC10, 0.5 moVl
Etchant 2:
Alkaline hexacyanoferrate(II1)-solution
Concentrations:
&Fe(CN), 0.76 mOVl NaOH 3moVl 50°C ca. 1 nm/s The Etchant leaves a soft layer of NiO(OH), which can be removed with HCl. It may be necessary to change the etching media hexacyanoferrate(II1)- etchant and hydrochloric acid several times (see section 3.2.3). The method is in the case of very thin metal films (< 0.1 pm) conveniently applicable. It is inconvenient for thicker films.
Temperature: Etch rate: Remarks:
Dry etching Volatile compounds:
Slightly volatile compounds:
Ni(C0)4 Cr02C12 Cr(C0)6 Cr(N0&*9H20 CrC13
Bp. -25"C3' Bp. 117°C) Bp. 151°C) Bp . 125.5"C) subl. 1300°C)
*)
6.2 Collection of Recipes
285
1. Dry-etching method: Etching by bombardment with inert ions5) Gas composition: Pressure: Power: Ternperature: Etch rate: References:
Ar 11 mtorr 100 W11.6 Wlcm2;rf 1,5 kV 190°C 0.12-0.17 n d s ‘)€‘TI-in-house instruction 2)forCr: S. Buttgenbach (1991) 3)J. D’Ans and E. Lax (1943), 249 4)J. D’Ans and E. Lax (1943)’ 227 5)R.T.C.Tsui (1967)
286
6.2 Collection of Recipes
NiMnSb - Nickel Manganese Antimonide Wet etching Readily soluble reaction products:
Ni (11) is soluble in form of Ni2+and its soluble complexes. Nickel forms in normal atmosphere a dense passivating film containing Ni(II1). This can be dissolved in acidic media and at increased temperature, and by adding complexing ligands, like F,C1- or NH3.
Dry etching Volatile and slightly volatile compounds:
Ni(C0)4 NiC12 MnF2 SbH3 SbF, SbC13 SbF3
Bp. -25°C') Bp. 973"C2) Bp. > 856"C2) Bp. -17"C3) Bp. 14OoC3) Bp. 223"C3) Bp. 319"C3)
1. Dry-etching method: Reactive ion beam etching in a SFdAr plasma2) Gas composition: 50 % Ar; 50 YOSF, Pressure: Gas flow rate: Rf-power : Etch rate:
1.5 mtorr 20 sccm 450 W 21 n d s
References:
')J. D'Ans and E. Lax (1943), 249 2)J. Hong et al. (1997) 3)A.F.Hollemann and E. Wiberg (1985)
6.2 Collection of Recipes
287
Pb - Lead Wet etching Readily soluble reaction products:
Pb(I1) is soluble as nitrate in absence of precipitating anions
Etchant 1:
Etching in FeC13-solution')
Preferred temperature:
43 .. 54°C
Dry etching Slightly volatile compounds: References:
PbJ2 Bp. 872"C2) PbBr2 Bp. 914"C2) PbC12 Bp. 954 "C2) ')W. Tegert (1959) 2)J.D'Ans and E. Lax (1943)' 220
288
6.2 Collection of Recipes
PbS - Lead Sulphide Wet etching Readily soluble reaction products:
Pb(I1) is soluble as nitrate in absence of precipitating anions
1. Wet-etching method: Hydrochloric acid-salpetric acid-acetic acid mixture’) Concentrations: Remarks:
HCl 2.6 mom HNo3 2.7 moVl Acetic acid 0.4 moVl Polishing solution
Dry etching Slightly volatile compounds: References:
PbJ2 Bp. 872°C’’ PbBr’ Bp. 914°C’) PbC12 Bp. 954 “C’) ‘)B.A. Irving(1962) 2)J. D’Ans and E.Lax (1943), 220
6.2 Collection of Recipes
289
Pbo.865Lao.o,Zro.,5Ti0.3503 - Lead Lanthanum Zirconate Titanate (PET) Wet etching Readily soluble reaction products:
Pb(I1) is readily soluble in water in form of several salts like Pb(N03)2,soluble Pb(1V)-salts tend strongly to be reduced to Pb(II), e.g. Pb(CH3C02)2is readily soluble '). Lanthanum(II1) forms as hydroxide a relatively strong base and is soluble in form of various chloro- and fluorocomplexes. Zr(1V) is water-soluble as ZrOC12. 8H20, besides many complexes with sixfold coordinated Zr(1V) are known, also with organic donors like ethers and esters.') Ti(1V) is soluble in strong acids as [Ti(OH)2]2+, [Ti(OH)3]fand related complex ions, preferentially with F as ligand.')
1. Wet-etching method: HCl/HF-solution2) Composition: Remarks
HCVHF Unsatisfying pattern quality
Dry etching Volatile and moderately volatile compounds:
Slightly volatile compounds:
PbH, PbC1, Zr(B€Q4 ZrC4 ZrBr4 TiBr4 Ti4 TiJ4 Pb12 PbBr, PbC12 LaC13 LaF3 ZrF4
Bp. -13°C') Bp. ca. 15°C') Bp. 123°C') subl. 331°C') subl. 357°C') Bp. 233.45"C') subl. 284°C') Bp. 377°C') Bp. ca. 900°C') Bp. 916°C') Bp. 954°C') Bp. 1750°C') Bp. 2330°C') subl. 903°C')
290
6.2 Collection of Recipes
1. Dry-etching method: Reactive ion etching in CC12F2-plasma2) Gas composition: Pressure: Ion current density: Plasma conditions: Power: Temperature: Etch rate:
70% CCl2F2; 30% 150 mtorr CLA/cm2 125 mtorr 1W/cm2/200W 320°C 0.3 nm/s
0 2
2. Dry-etching method: CAIBE in C12-plasma2) Ion beam: Gas composition/Reactive gas: Remarks: References:
Ar C12 Essentially smoother structures are achieved than by WE. ''A.F. Holleman and E. Wiberg (1985) 2)P.F.Baude et al. (1993)
6.2 Collection of Recipes
291
PbZrxTi,-x03- Lead Zirconate Titanate (PZT) Wet etching Readily soluble reaction products:
Pb(I1) is readily soluble in water in form of several salts like Pb(N03)2,soluble Pb(1V)-salts tend strongly to be reduced to Pb(II), e.g. Pb(CH3C02)2is readily soluble'). Zr(1V) is water-soluble as ZrOC12-8H20,besides many complexes with sixfold coordinated Zr(1V) are known, also with organic donors like ethers and esters. ') %(IV) is soluble in strong acids as [%(OH),] *+, [Ti(OH),]+and related complex ions, preferentially with F as ligand. ')
Dry etching Volatile and slightly volatile compounds:
PbH, PbC1, PbI2 PbBr2 PbC12 Zr(BI&), ZrC1, ZrBr, ZrF, %Br, TlF4
TI 54
Bp. -13°C') Bp. ca. 15°C') Bp. ca. 900°C') Bp. 916°C') Bp. 954°C') Bp. 123°C') subl. 331°C') subl. 357°C') subl. 903°C') Bp. 233.45"C') subl. 284°C') Bp. 377°C')
1. Dry-etching method: Reactive ion etching in CC12F2-plasma2) Gas composition: Plasma conditions: Power: Etch rate:
70% CC12F2; 30% 125 mtorr 150 W 0.32 n m / s
References:
"A.F. Holleman and E. Wiberg (1985) ')D.l? Vijay et al. (1993)
0 2
292
6.2 Collection of Recipes
PSG - (P205,Si02)- Phosphosilicate Glass Wet etching Soluble compounds:
Si(1V) is soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH)#- or in Fcontaining media as [SiF6I2-.Etchants for SiOz are also usable for etching PSG. The etch rates of PSG exceed those of SiOz by a multiple.
1. Wet-etching method: Etching in diluted HF-solution Composition: P20,-content of the glass: Etch rate: P,O,-content of the glass: Etch rate:
')
HF 8.3 moVl 5 Yo 75 n m l s
10 Yo 400 n d s
2. Wet-etching method: Sabetric acid-fluoric acid-etchant ($etch")*) Composition: P205-contentof the glass: Etch rate: P205-contentof the glass: Etch rate:
HF HN03 5 Yo
1.2 mom 0.34moVl
8nds 10 Yo 28 nm/s
Dry etching Volatile compounds:
Bp. -111.6"C3) Bp. -95.7"C3) Bp. -15"C3' SiHC13 Bp. 31.7"C3) Bp. 56.7"C3) SiC14 SQOC16 Bp . 135.5"C3) Si2C16 Bp. 147"C3) Bp. -101°C" PF3 Bp. -88°C) PH3 Bp. -75°C) PFS PC15 Bp. 62°C" SW SiF, Si2&
6.2 Collection of Recipes
293
1. Dry-etching method: Etching in highly dense GF6-plasma5) (only 4 % PSG) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
GF6 35 sccm 4 mtorr 2700 W 20 n d s
2. Dry-etching method Etching in low-pressure - HF -vapour6) Gas composition: 75 YOHF; 25 YOH20 Pressure: Etch rate: Remarks:
Gas composition: Pressure: Etch rate: Remarks References:
6 torr 30 nm/s Selectivity to Si02:ca. factor 1OOO; the method is very selective to undoped Si02.With deficient water in the reactive vapour, the selectivity factor is 1OOO to 1oooO. The etch rates of boron doped PSG are higher by a factor of 3 to 5 than those of PSG. At higher H20-partialpressure the PSG etch rate is increased, but the selectivity to thermal Si02decreases. 23 YOHF; 77 YOH20 20 torr 1pds Selectivity to Si02: ca. factor 30 %ach M. Schulz and H. Weiss (1984) 2)W.Kern and Ch. Deckert (1978) 3)J. D'Ans and E. Lax (1943), 261 4)J. D'Ans and E. Lax (1943), 231 5)J. Givens et al. (1994) 6)H.Watanabe et al. (1995)
294
6.2 Collection of Recipes
Pt - Platinum Wet etching Readily soluble reaction products:
Pt(I1) and Pt(1V) is soluble in form of coordination compounds, e.g. with halide- and pseudohalide ions, NH3 ')
Etchant 1:
Concentrated aqua regia
Composition:
Concentrated HN03(69.2%ig): 25 Vol% ; concentrated HCl (18 %ig): 75 Vol%
Etchant 2:
Hot hydrochloric acid-Salpetric acidmixture2)
Composition:
HN03 0.69 M HCl 4.55 M 85°C ca. 1 d s
Temperature: Etch rate:
Wet etching method 3: Electrochemical pulse etching in hydrochloric acid3) Concentrations: Frequency: Temperature: Etch rate: Remarks:
3M HC1 ca. 0.5kHz Room temperature 1.6 d s Pulses are a ramp of 0.7-1.4 V (1 ms), a jump to 0.5 V, and a following ramp to -0.3 V (0.7 ms) and a jump to 0.7 V
Dry etching Volatile compounds:
PtF,
References:
"A.F. Holleman, E. Wiberg (1985) *)M.J.Rand and J.E Roberts (1974) 3)R.P.Frankenthal and D.H. Eaton (1976)
Bp. 69.1"C')
6.2 Collection of Recipes
295
Ru02- Ruthenium Dioxide Wet etching Readily soluble reaction products:
Ru(VI1) is soluble in form of perruthenates; Ru(I1) and Ru(II1) in form of diverse complex compounds')
Dry etching Volatile compounds:
Ru04 RuFs RuF40
Bp. 100°C') Bp. 227°C') Bp. 184"C2)
1. Dry-etching method: Reactive ion etching in 02/CF3CFH2plasma3) Gas composition: Plasma conditions: Power: Etch rate: References:
2.5 % CF3CFHzinO2 75 mtorr 1.57 Wlcm2 2.7 n m l s "A.F. Holleman and E. Wiberg (1985) "5. D'Ans and E. Lax (1943), 244 3)W.Pan and S.B. Desu (1994)
296
6.2 Collection of Recipes
Sb -Antimony Wet etching Readily soluble reaction products:
Sb(V)-Salts, Sb forms soluble coordination compounds with multivalent hydroxycarbonic acids, like e.g. tartaric acid and citric acid
Etchant 1:
Citric acidic peroxodisulfate solution') As the organic acid and the peroxoion suffer a slow redox reaction, the etchant has to be replaced after a few hours.
Etchant 2:
Tartaric acidic peroxodisulfate solution2)
Concentrations:
(N€&)2S208 0.18 moM Tartaric acid 0.067 moVl Room temperature ca. 5 nm/s The etchant is selective to bismuth, which forms scarcely soluble tartrate. As the organic acid and the peroxoion suffer a slow redox reaction, the Etchant has to be replaced after a few hours.
Temperature: Etch rate: Remarks:
Dry etching Volatile compounds:
References:
Bp. -18"C3) SbH3 SbC15 Bp. 105°C (unter Pre~sure)~) SbFS Bp. 149.5"C3) SbC13 Bp. 187"C3) Bp. 288"C3) SbBr3 ')M. Kohler et al. (1983a) 2)M. Kohler et al. (1983b) 3)J. D'Ans and E. Lax (1943), 217
6.2 Collection of Recipes
297
Si - Silicon Wet etching Readily soluble reaction products:
Si(1V) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),I2-or in F-containing media as [siF6I2-') Etching is supported by suitable chelate ligands e.g. pyrocatechol, ethylene diamine, hydrazine.
Etchant 1:
Salpetric acid-hydrofluoric acid-acetic acidsolution2)
Concentrations:
7 moVl HF 6 moVl CH3COOH6 moVl 23°C 2.5 nm/s HNo3 8.8 moVl HF 2.3 moVl CH3COOH 10 moVl 23°C 50 nm/s The etchant reacts to a large extent isotropically. With optimized composition and process running ideally semispherical etching grooves can be achieved by patterning with masks with small hole structures2).Silicon can also be etched in salpetric acid-fluoric acid solutions without acetic acid. In spray etching methods with rotating substrates typical etch rates are ca. 0.15 (etching of monocrystalline Si with low rpm) to 2.3 nm/s (p01y-Si)~). With an etchant composition of ca. 4.4M HN03and 15 M HF extremely high etch rates are recorded which, however, slightly differ for various crystallographic directions4): Si(ll0) 16 p d s (65°C) Si(100) 11 p d s (65°C) 8 p d s (65°C) Si(ll1)
Temperature: Etch rate: Concentrations: Temperature: Etch rate: Remarks:
HN03
Etchant 2:
Salpetric acid-fluoric acid etchant2)
Concentrations:
HN03 HF
10moM 2.3 moVl
298
6.2 Collection of Recipes
Temperature: Etch rate: Concentrations: Temperature: Etch rate: Concentrations: Temperature: Etch rate:
30°C 100 n m / s HN03 5.5moVl HF 13 mom 30°C 3.3 p d s (p-Si, 12..78 ohmcm) HN03 8.25 moVl HF 6.5 moM 30°C 3.3 p d s (n-Si, 0.05.3 ohmcm)
Etchant 3:
Perchloric acid-salpetric acid-fluoric acid etchant*)
Concentrations:
HNo3
11.5 moVl 2.2 moVl CH3COOH 1.7moM HC104 1.3 moVl 23°C 17 n m / s The etchant reacts to a great extent isotropically.
HF
Temperature: Etch rate: Remarks:
Etchant 4:
Hydrazine solution5) 64 %ige solution of hydrazine
Concentrations: Temperature: Etch rate: Remarks:
90°C Si (100): 27 n m / s Anisotropic etchant, rough surface
Etchant 5:
Potassium hydroxide solution6)
Concentrations: Temperature: Etch rate:
20 %ige solution of KOH 80°C Si(100): 24 n d s Si(ll0): 32 nm/s For calculating the etch rates rand rof Silicon with the orientations and <110> in KOH the empirical formula of H.Seide1 et al. (1990) can be used:
6.2 Collection of Recipes
299
Certain deviations in the etch rates calculated by these formulae to the given data in the diagrammes (fig. 67-71 - each curve is related to an Arrhenius plot, determined by measuring data for certain etchant concentrations) result from averaging measuring data, determined for various concentrations. Instead of KOH, solutions of other alkali hydroxides, like LiOH, NaOH, CsOH can be used. According to concentration and temperature, various etch rates and different etch rate ratios for the crystallographic directions can be adjusted. With CsOH higher selectivity to Si02can be achieved at higher concentrations (45-50 %) compared to the use of KOH') Layers of implanted B or C can be used as efficient etch stop in anisotropic alkaline etchants.
Etchant 6 :
Etching in alkaline peroxodisulfatesolution'')
Concentrations:
KOH 5 mol/l ( W ) 2 S 2 0 8 0.044 mom 80°C Si(ll0): 30 n d s NaOH 5 moVl ( W ) 2 S 2 0 8 0.09 mom 80" Si(ll0): 32 n d s
Temperature: Etch rate: Concentrations: Temperature: Etch rate:
Fig. 667. Etch rate of silicon of the orientations (lll), (100) and (110) in ethylene diamine etchant in dependence on temperature (H.Seide1 et al. 1990)
300
6.2 Collection of Recipes
1-
100000
0 80
66
80
65
70
76
W
66
W
S6
100
106
I10
mpmtun[WrmCekIU.]
ETg. 6-68. Selectivity of silicon etching of the orientations (100) and (110) in ethylene diamine etchant in dependence on temperature (H. Seidel et al. 1990)
18
8x1 3 % g 8 2s
E3 a s
tempmtun p( I -run
8s 3 % 3 % f f t Rr
98
Celalur]
ETg. 6-69. Etch rate of Si(100) in KOH-etchant in dependence on temperature (H.Seidel et al. 1990)
6.2 Collection of Recipes
301
Fig. 6-70. Etch rate of Si(ll0) in KOH-etchant in dependence on temperature (H.Seidel et al. 1990)
Fig. 67l. Selectivity of silicon etching of the orientaion (100) and (110) in KOH etchant in dependence on temperature (H.Seide1et al. 1990)
302
6.2 Collection of Recipes
Etchant 7: Concentrations: Remarks: Temperature: Etch rate: Remarks:
Etchant 8: Concentrations: Remarks: Temperature: Etch rate: Remarks:
Etchant 9: Concentrations: Remarks:
Temperature: Etch rate:
Ethylene diamine-pyrocatechol etchant") 53 YOethylene diamine; 11% pyrocatechol; 36 Yo water anisotropic etchant, smooth surfaces 85°C Si(100): 14.5 nm/s The etchant has high selectivity to Si02,hence it can be used as thin masking film for Si-deep etching.
Ethylene diamine-pyrocatechol-pyrazine etchant (type S)") 7 9 3 Vol YOethylene diamine; 0,5 Vol YO pyrazine 9 Val% pyrocatechol; 11Vol YOwater anisotropic etchant 85°C Si(100): 6 n d s Si(ll0): 7.8 n d s Si(ll1): 0,l nm/s Very high selectivity to Si02
Ammoniumhydroxide etchant13) 10 Y Ammoniumhydroxide in water Anisotropic etchant, smooth surface Especially high selectivity to Si02can be achieved using tetramethylammonium hydroxide instead of ammonium hydroxide. 90°C Si(100): 1.8 n d s
Wet-etching method 10: Electrochemical etching in KOH14) Concentrations: KOH 30% Potential (counter Hg/HgO) Remarks:
Temperature: Etch rate:
4 9v
At lower potentials the etch rate gradually decreases, at higher potentials (between -0,9 and -0.7 V) passivation sets in. The anodic etch rates can be considerably increased by raising the temperature. The passivating current inceases as well. 65 "C 7 nm/s (Si (loo), p-doped: 3-10 ohmcm, n-doped , 1-19 ohmcm)
6.2 Collection of Recipes
303
Dry etching Volatile compounds:
SiH4 SiF4 Si2& SiHC13 Sic& Si20Cl6 Si2C16
Bp. -111.6"C'5) Bp. -95.7"C15) Bp. -15"C15) Bp. 31.7"C15) Bp. 56.7"C15) Bp. 135.5"C'5) Bp. 147"C15)
1. Dry-etching method: Etching in chlorine plasma16) Gas composition: Plasma conditions: a) Plasma frequency: Etch rate: Remarks: b) Plasma frequency Etch rate: Remarks:
100% Cl2 0.3 torr 100 kHz 8 nm/s anisotropic removal 13 MHz 0.8 nm/s isotropic removal
2. Dry-etching method: Low-temperature RIE in SF6-plasma17) Gas composition: Plasma conditions: Plasma frequency Power: Substrate temperature: Etch rate: Remarks:
sF6 20 mtorr 13.56 M H z 3.2 Wlcm2 -120°C 40 n m / S Anisotropic removal, very low undercutting; by optimal oxygen addition the anisotropy and the selectivity to Si02can be improved. At high aspect ratios a considerably diminished etch rate occurs. 18).
3. Dry-etching method: Polysilicon etching in C12/~F6-plasma'9) a) Gas composition: Flow rate Plasma conditions: Power: Etch rate:
80% GF6 ;20% c12 200 sccm 0.35 torr Parallel-plate reactor rf: 0.4 kW 1.2 nm/s (p-poly-silicon) 2 nm/s (undoped poly-silicon)
304
6.2 Collection of Recipes
b) Gas composition: Flow rate Plasma conditions:
Power: Etch rate: Remarks:
20 % GF, ; 80 % Cl2 200 sccm 0.35 torr Parallel-plate reactor rf: 0.4 kW 0.5 nm/s (p-poly-silicon) 5.6 nm/s (undoped poly-silicon) P-doped poly-silicon shows up to ca. 40 Vol% an etch rate that increases linearly with increasing chlorine concentration and slower at higher chlorine concentration. Undoped poly-silicon has an etch rate maximum at 20 % chlorine.
4. Dry-etching method: Silicon etching in ClF3-vapou?') Gas composition: Flow rate Etch rate:
100% ClF3 20-100 sccm 42 n d s (3 ton-/ 120°C) 33 nm/s (10 torr/ -5°C)
5. Dry-etching method: Polysilicon microwave etching in Cl,-plasma 21) Gas composition: Plasma conditions: Power: Flow rate Etch rate: Remarks:
98,5% C12; 1,5% 3 mtorr
0 2
Parallel-plate reactor 0.7 kW Microwave power 150 sccm ECR-Reactor 8.7 nm/~(-10°C) High selectivity to Si02
6. Dry-etching method: SF6-plasma etching 22) Gas composition: Flow rate Plasma conditions: Plasma frequency Power: Etch rate:
100% SF,j 80 sccm 250 mtorr 13.56 MHz 0.5 W/cm2 12 nm/s
6.2 Collection of Recipes
7. Dry-etching method: Laser-assisted vapour etching in CClF523) Gas composition: Plasma conditions: Pulsfrequenz: excitation: Laser energy density/ pulse: Temperature: Etch rate:
100% CClF, 737 torr 100 Hz KrF-excimer laser, 248 nm 0.8 J/cm2(average power density: 80 W/cm’) 23°C 20 nm/s (0.2 nm/pulse)
8. Dry-etching method: Plasma etching in CBrF:4) Gas composition: Flow rate Plasma conditions: Power: Etch rate: Selectivity to Si02:
100% CBrF, 15 sccm 30 mtorr 150 W 1.75 nm/s approx. factor 5
305
306
6.2 Collection of Recipes
References:
"A.F. Holleman and E. Wiberg (1985) 2)A.F.Bogenschutz (1967) 3)D.L.Klein and D.J. D'Stefan (1962) 3)J.P.John and J. McDonald (1993) 4)N.Schwesinger et al. (1996) 5)W.Kern (1978) 6)H.Seidel et al. (1990) 7)L.D.Clark et al. (1988) ''A. Heuberger (1989) 9)V. Lehmann et al. (1991) ")A. Lerm et al, (1990) "'R.M. Finne and D.L. Klein (1967); see also R.Vol3 (1992) 12)H.Seidel et al. (1990); A. Reisman et al. (1979) 13)M.Asano et al. (1976) 14)R.Vol3 (1992) 15)J.D'Ans and E. Lax (1943)' 261 '@R.H.Bruce (1981) 17)M.Takinamiet al. (1992), see also K. Murakami et al. (1993) ")T. Syau et al. (1991);see also K. Murakami et al. (1993) and M. Esashi et al. (1995) 19)C.J.Mogab and H.J. Levinstein (1980) 20)Y.Saito et al. (1991) 21)D.Dane et al. (1992) 22)Y.-J.Lii et al. (1990 b) 23)S.D.Russell and D.A. Sexton (1990) 24)S.Matsuo (1980)
6.2 Collection of Recipes
307
Sic - Silicon Carbide Wet etching Readily soluble compounds:
Si(1V) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),I2-or in Fcontaining media as [SiF6I2-') Etching is supported by suitable chelate ligands e.g. pyrocatechol , ethylene diamine, hydrazine . Carbon is soluble in oxidized form as carbonate or hydrogen carbonate, also released as CO or C 0 2
Wet-etching method 1: Photoelectrochemichal etching in HF') Concentrations: Lamp: Power: Potential: Etch rate: Remarks:
HF 2,5 moM 200 W Hg (250-400 nm) on 1 cm2Flache = 0,5-0,7 W/cm2 2,2 V vs SCE n-Sic: 37 n d s p-Sic: 6,7 nm/s Essentially higher etch rates (up to. 1700nds) are achievable with intensive laser irraditation2).
Dry etching Volatile compounds:
Sib Bp. -111.6"C3) SiF, Bp. -95.7"C3) Si2& Bp. -15"C3) SiHC13 Bp. 31.7"C3) Sic& Bp. 56.7"C3) Si20C1, Bp . 135.5"C3) Si2C16 Bp. 147"C3) Carbon is released as gas in form of CO or C 0 2
Dry-etching method 1: Reactive ion etching in CHFJ02-plasma4'5) Gas composition: Plasma conditions: Power: Etch rate: Remarks
20 YO CHF3; 80 % 0 2 20 mtorr 200 w 0.7 n m l s The addition of about 10 YO H2to the etching gas are enough to avoid residues.
308
6.2 Collection of Recipes
Dry-etching method 2: Reactive ion etching in NFJ/02-plasma Gas composition: Flow rate: Plasma conditions: Power: Ion source: Remarks:
Etch rate: Remarks: Gas composition: Pressure: Gas flow rate: Power: Temperature: Etch rate:
'y6)
90%; 0 2 10% 20 sccm 20 mtorr 200 (0,4 W/cm2) m 3
Residues in form of spikes, avoided by adding H2to the plasma; alternative etching gases: CHF3or CF4or SF, highest ech rate ratio to Si with CHF3 1,4 n m / s Without oxygen in the plasma, there are no or little residues without any hydrogen addition. 88 % cHF3; 12 % 0 2 ') 1.3 torr 18 sccm 350 W 300°C 14 n m / s (p-Sic)
Dry-etching method 3: Reactive ion etching in CF4/N2/02-plasma6, Gas composition: Flow rate: Pressure: Plasma conditions: Power: Etch rate:
CF462 YO; 0 2 23 Yo ; N2 15 YO 65 sccm 190 mtorr Parallel-plate reactor, 13.56 MHz 300 w 3,7 nm/s
Dry-etching method 4: Reactive ion etching in SFd02-plasma Gas composition: Flow rate: Pressure: Power: Etch rate: Remarks
SF6 65 YO; 0 2 35 % 20 sccm 20 mtorr 200 W (0.42 W/cm2) 0.9 n m / s Residue-free etching can be achieved by adding HZ,the etch rate decreases, howeve?).
Dry-etching method 5: Reactive ion etching in CBrFJ/02-plasma9, Gas composition: CBrF325 70;O275 YO Flow rate: Pressure: Power: Etch rate:
20 sccm 50 mtorr 200 W (0.42 W/cm2) 0.7 nm/s
6.2
Collection of Recipes
309
Dry-etching method 6: Reactive ion etching in CHFd02-plasma5.9) CHF, 10 % ; 0 2 90 % Gas composition: Flow rate: Pressure: Power: Etch rate: References:
20 sccm
60 mtorr 200 W (0.42 W/cm') 0.9 nm/s "J.S. Shor and A.D. Kurtz (1994) ''J.S. Shor et al. (1992) 3)J. D'Ans and E. Lax (1943), 261 4)J.€? Li et al. (1993) '?.H.Yih and A.J. Steckl(l993) @R.Wolf and R. Helbig (1996) ')C. Richter et al. (1997) 8)W.-S.Pan and A.J. Steckl(l990) 9)P.H.Yih and A.J. Steckl(l995)
310
6.2 Collection of Recipes
Si3N4- Silicon Nitride Wet etching Readily soluble compounds:
Si(1V) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),]" or in F-containing media as [SiF6I2-')
1. Wet etchant:
Hot concentrated phosphoric acid
Concentration: Temperature: Etch rate: Remarks:
65 %ige H3P04 in water 180°C ca. 0.02 n m / s As etching mask Si02can be used').
2. Wet etchant:
HF-solution2) 26 M HF
Concentration: Temperature: Etch rate: Concentration: Temperature: Etch rate:
25°C ca. 1-2 nmJs 25 M HF 60°C 2.5 n d s
Dry etching Volatile compounds:
SiH, SiF, Si2& SiHC13 SiC1, Si20Cl6 Si2C16
Bp. -111.6"C3) Bp . -95.7"C3) Bp. -15"C3) Bp. 31.7"C3) Bp. 56.7"C3) Bp. 135.5"C3) Bp. 147"C3)
1. Dry-etching method: Etching in highly dense CHFJC02- plasma5) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
CHF3: 27 Yo ; COZ: 73 YO 126 sccm 25 mtorr 2700 W 4.2 nm/s
6.2 Collection of Recipes
311
2. Dry-etching method: Etching in CF4/H2-plasma6) Gas composition: H2: 0-20 YO; CF4: 80-100 YO Flow rate: Plasma conditions: Power: Etch rate:
100 sccm 235 mtorr 200 w 0.8 nm/s (PECVD-, LPCVD-nitride)
3. Dry-etching method: Reactive ion etching in C€€F402 - plasma’) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
CHF3: 68 % ; 0 2 : 32 ‘YO 6 sccm 30 mtorr, Parallel-plate reactor, 13.56 M H z 0.22 W/cm2 0.7 nm/s (for nanometer grooves)
4. Dry-etching method: Reactive ion etching in CF, -plasma8) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
100% CF4 200 sccm 0.3 torr, Parallel- late reactor, 13.56 MHz 1 kW (0.43 Wlcm ) 16 nm/s
Y
5. Dry-etching method: Reactive ion etching in GF6-plasma*) Gas composition: 100% GF, Flow rate: Plasma conditions: Power: Etch rate:
200 sccm 0.1 tom, Parallel- late reactor, 13.56 M H z 2 kW (0.86 Wlcm ) 1.25 nm/s
I:
6. Dry-etching method: Photochemichal stripping in ClF, - vapour’) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
90% ClF3; 10% N2 lo00 sccm 100 torr, Parallel-plate reactor 10-50 W/cm2(irradiation power at 254 nm) 0.3 nm/s (50°C) 1.3 n d s (150°C)
312
6.2 Collection of Recipes
References:
"W.V. Geldern and VE. Hauser (1967) 2)D.M.Brown et al. (1967); R. Herring and J.B. Price (1973) 3)J.D'Ans and E .Lax (1943), 261 ')J. Givens et al. (1994) 6)J.L.Lindstrom et al. (1992) 7)T.K.S.Wong and S.G. Ingram (1992); for plasma etching in fluorohydrocarbodoxygen plasmas see also R.L. Bersin (1976) ')Y. Kuo (1990 b) 9)D.C.Gray et al. (1995 a)
6.2 Collection of Recipes
313
SO2- Silicon Dioxide Wet etching Readily soluble compounds:
Si(1V) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),]’- or in F-containing media as [SiF6]’- ’)
1. Etchant:
Etching in hydrofluoric acidic ammonium fluoride solution2)
Concentrations:
NH4F
9.26 moVl HF 4.4 moVl 24°C 13.3 nm/s w F 2.8 moVl HF 1moVl 23°C 1.7 nm/s By choosing the NI&F/HF-ratio and the temperature the Si02-wallprofile could be adjusted in a wide range using a mask of negative photoresist. Especially with a high N€€,F-portion and an elevated temperature (55°C) small wall angles were observed.
Temperature: Etch rate: OF’
Concentrations: Temperature: Etch rate: Remarks:
30
26
20
TE. t! IS il 10 6
0
8
12
1s
24
33
41
w
HFconcentntlonpi]
Fig. 6-72.Dependence of the Si0,-etch rate in wet-chemical etching in hydrofluoric acid on the HF-concentration
314
6.2 Collection of Recipes
2. Etchant: Concentrations: Temperature: Etch rate:
Etching in hydrofluoric solution3) HF 4.8 Yo 25°C 0.6 nm/s
Dry etching Volatile compounds:
Sib SiF, Si2& SiHC13 SiCl, Si20Cl6 Si2C16
Bp. -111.6"C) Bp. -95.7"C'" Bp. -15°C) Bp. 31.7"C? Bp. 56.7"C? Bp. 135.5"C" Bp. 147°C"
1. Dry-etching method: Reactive ion etching in GFd02-plasma5) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
56% C2F6; 44% 0 45 sccm 0.8 ton 0.6 kW 8nds
2
2. Dry-etching method: Etching in highly dense GF6-plasma6) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
GF6 35 sccm 4 mtorr 2700 W 20 nm/s
3. Dry-etching method: Etching in CF, high-pressure plasma7v8) a) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate:
Ar 53 YO; CF4 47 % 212 sccm 2.5 torr 3.5 w/cm2 HF0.4 MHz 12 d s ' )
b) Gas composition: Plasma conditions: Power: Source: Etch rate:
CF4 3 tom 200 w HF27 MHZ 38 n d s s )
6.2 Collection of Recipes
Remarks:
315
By using a hole mask of carbon the rate can be increased to 60 d s . Selectivity to Si is small.
4. Dry-etching method: Etching in CF4/CHF3-highpressure plasma7) Gas composition: CF442 YO;Ar 46 YO; CHF3 12 YO 240 sccm Flow rate: Plasma conditions: 2.5 torr Power: 3.5 W/cm2 Source: HF 0.4 MHz Etch rate: 8 nmls Remarks: High selectivity to Si: Etch rate ratio about 18
5. Dry-etching method: Etching in CF4/C3F8-highpressure plasma’) Gas composition: CF, 6 % ;Ar 80 YO; C3F814 YO Plasma conditions: Power: Source: Etch rate: Remarks:
1.14 torr 2.8 Wlcm’ HFo.l M H Z 18 n m l s High selectivity to Si: Etch rate ratio about 20
6. Dry-etching method: Reactive ion beam etching with CF;’) Gas composition: 25 YOCF4; 75 % Ar Flow rate: Plasma conditions: Ion current density: a) Ion energy: Etch rate: Remarks: b) Ion energy: Etch rate: Remarks:
4 sccm 0.1 mtorr 0.4 mAIcm’ 1.3 keV 1 nmls Little selectivity to Si: Etch rate ratio of ca. 1.5 0.75 keV 0.75 d s Moderate selectivity to Si: Etch rate ratio of ca. 5
316
6.2 Collection of Recipes
7. Dry-etching method: Reactive ion beam etching with CqF;l) Gas composition: Plasma conditions: Ion current density: Source: Ion energy: Etch rate: Remarks:
C4FS 0.2 mtorr 0.3 mA/cm2 ECR 1 keV 1.2 nmls Very high selectivity to Si: Etch rate ratio of ca. 30
8. Dry-etching method: Magnetic field enhanced RIE with CHF3'2) Gas composition: Flow rate: Plasma conditions: Power: Etch rate: Remarks:
100% cHF3 33 sccm 50 mtorr (lo00 gauss) rf: 1.6 Wkm2
18 n d s High selectivity to Si: Etch rate ratio of ca. 9
9. Dry-etching method: ECR-assisted plasma etching with C4Fsl3) Gas composition: Flow rate: Plasma conditions: Power: Etch rate: Remarks:
100% cp, 33 sccm 3 mtorr 0.5 kW (2.45 GHz)/1kW (rf 400 kHz) 8nds High selectivity to Si: Etch rate ratio of ca. 9
10. Dry-etching method: Photochemical vapour etching with ClFJN24) Gas composition: Flow rate: Pressure: Etch rate:
90 % ClF3; 10 % N2 lo00 sccm 100 torr 0.15 d s (150°C) 0.01 d s (50°C)
11.Dry-etching method: Magnetic-field enhanced plasma etching with C&?,/N2 Gas composition: Flow rate: Pressure: Etch rate: Remarks:
80 % C814; 20 % N2 100 sccm 1 torr 17 n m l s Selectivity to Si: Factor 5
6.2 Collection of Recipes
317
12. Dry-etching method: Sputter-etching with inert ions Gas composition: Pressure: Beam conditions: Power: Temperature: Etch rate:
Ar 11 mtorr 1.5 kV rf 100W/ 1.6 W/cm2 190°C 0.2 d s
References:
"A.F. Holleman and E. Wiberg (1985) 2)A.F.Bogenschutz (1967); see also H. Proksche et al. (1992), for adjusting of wall angles see G.I. Parisi et al. (1977) 3)C.C.Mai and J.C. Looney (1966); for dissolution of Si02in HF-solutionen see also W.G. Palmer (1956) 4)J. D'Ans and E. Lax (1943)' 261 5)C.V.Macchioni (1990) 6)J. Givens et al. (1994) 7)D.L.Swithh (1984) ' ) S . Schreiter and H.-U. Poll (1992) 9)D.L.Swithh (1984) "'B.A. Heath and T.M. Mayer (1984); under similar conditions see also W. Beyer (1991) ") M. Miyamura et al. (1983) "'H. Okano et al. (1982) 13)K.Nojiri and E. Iguchi (1995) 14)D.C.Gray et al. (1995) 15)K.Schade et al. (1990) 16)R.T.C. Tsui (1967)
318
6.2 Collection of Recipes
Si,N,O,
- Silicon Oxynitride
Wet etching Readily soluble compounds:
Si(1V) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH),]'- or in F-containing media as [SiF6]'- ')
1. Etchant:
Etching in hydrofluoric acidic ammonium fluoride solution') NH'I F 10 moVl HF 1moVl
Concentrations: Temperature: Etch rate: Remarks:
ca. 25°C 0.05-0.2 nm/s The etch rate depends strongly on the ON-ratio and possible hydrogen in the film, e.g., in-built into the film by preparation from S i b or NH3in a CVD-process.
2. Etchant:
Etching in hydrofluoric acid solution2)
Concentrations: Temperature: Etch rate: Remarks:
HF 26 moVl 25°C 0.6-8 n d s The etch rate is very strongly dependent on the film composition.
Dry etching Volatile compounds:
Remarks: References:
Sib Bp. -111.6"C3) SiF, Bp. -95.7"C3' Si2& Bp. -15"C3) SiHC13 Bp. 31.7"C3) Sic& Bp. 56.7"C3) Si20Cl6 Bp . 135.5"C3) Si2C16 Bp. 147"C3) Dry etching is preferentially done in fluoridecontaining etching gases. ')T. Nozaki (1980) ''D.M. Brown et al. (1968) 3)J. D'Ans and E. Lax (1943), 261
6.2 Collection of Recipes
319
Sn -Tin Wet etching Readily soluble reaction products:
Sn(I1) is soluble in form of salts, Sn(1V) forms with appropriate ligands L ( e.g. L= C1-or OH-) soluble complex ions of the type [SnL6I2-')
1. Wet-etching method: Etching in aqueous FeC13-solution2) Temperature:
32-54 "C (preferentially)
Dry etching Volatile compounds:
References:
Sns SnC14 Sn13r4 Sn14 SnC12 SnF4
Bp. - 52°C') Bp. 114.1"C') Bp. 203.3"C') Bp. 346°C') Bp. 605°C') subl. 705°C') SnF2 Bp. 853°C'' "A.F.Holleman and E. Wiberg (1985) ')R.J . Ryan et al. (1970)
320
6.2 Collection of Recipes
SnOe- Tin Dioxide Wet etching Readily soluble reaction products:
Sn(I1) is soluble in form of salts, Sn(1V) forms with appropriate ligands L ( e.g. L= C1-orOH-) soluble complex ions of the type [SnL6]'- ')
Dry etching Volatile compounds:
Sn& SnC1, SnBr, Sn14 SnC12 SnF4 SnFz
Bp. - 52°C') Bp. 114.1"C') Bp. 203.3"C') Bp. 346°C') Bp. 605°C') subl. 705°C') Bp. 853°C"
Dry-etching method:
Reactive ion etching in Ar/CI2-plasma2)
Gas composition: Power: Source: Etch rate:
90% Ar; 10% Cl;! 0.3 kW Parallel-plate reactor 1.5 nm/s "A.F. Holleman and E. Wiberg (1985) "5. Molloy et al. (1995)
References:
6.2 Collection of Recipes
321
Ta -Tantalum Wet etching Readily soluble reaction products:
Ta(V) is soluble as fluoride TaF,')
1. Wet-etching method: Etching in hydrofluoric acid-Salpetric acid2) Composition: Temperature:
5.6moVl HF 6.4 moVl ca. 25°C HNOJ
2. Wet-etching method: Etching in alkaline hydrogenperoxide solution3) Composition: Temperature: Etch rate:
NaOH 7moVl H202 0.9moVl 90°C ca. 1.7 .. 3.3 n m / s
Dry etching Volatile compounds:
TaF5 TaC15 TaBr5
Bp. 229.5"C) Bp. 241.6"C) Bp. Ca. 320°C'"
1. Dry-etching method: Plasma etching in C12/CCL-plasma5) Gas composition: Plasma conditions: Power: Etch rate:
80% Cl2/20% CC14 0.15 torr 1.2 W/cm2 13.3 nm/s
2. Dry-etching method: Plasma etching in 02/CF4-plasma5) Gas composition: Plasma conditions: Power: Etch rate:
10% 02/90% CF, 0.15 torr 1.2 W/cm2 3.8 nm/s
3. Dry-etching method: Plasma etching in highly dense 02/CHF4 CF4-plasma6) Gas composition: 40% CHF3; 56% C F 4 4% 0,; Gas flow rate:
52 sccm
322
6.2 Collection of Recipes
Plasma conditions: Power: Etch rate:
5 mtorr 0.2 kW (rf 13.56 M H z ) 1 nm/s
+ 0.1 kW (rf 40 MHz)
4. Dry-etching method: Plasma etching in highly dense SF6-plasma6) Gas composition: Gas flow rate: Plasma conditions: Power: Etch rate: References:
100% SF, 40 sccm 5 mtorr 150W (rf 13.56 M H z ) + 50W (rf 40 M H z ) 3 nm/s ') A.F. Holleman and E. Wiberg (1985) 2, R. Glang and L.V. Gregor (1970) 3, J. Grossman and D.S.Herman (1969) 4, J. D'Ans and E. Lax (1943)' 263 ') M.Yamada et al. (1991) 6, R. Hsiao and D. Miller (1996)
6.2 Collection of Recipes
TaN -Tantalum Nitride Wet etching Readily soluble reaction products:
Ta(V) is soluble as fluoride TaF5')
1. Wet-etching method: Etching in alkaline hydrogen peroxide sohtion2) Composition: Temperature: Etch rate:
NaOH 7moVl H202 0.9moVl 90°C ca. 1.7 .. 3.3 nm/s
Dry etching Volatile and moderately volatile compounds: Remarks: References:
TaF5 Bp. 229.5"C') TaC15 Bp. 241.6"C') TaBr5 Bp. ca. 320°C') Reactive dry etching is preferentially done in fluorine-containing etching gases. ')A.E Holleman and E. Wiberg (1985) 2)J. Grossman and D.S. Herman (1969)
323
324
6.2 Collection of Recipes
Ta205-Tantalum Oxide Wet etching Readily soluble reaction products:
Ta(V) is soluble as fluoride TaFs ')
Dry etching Volatile compounds:
TaF5 TaC15 TaBrs
Dry-etching method:
Reactive ion etching in plasmas of fluorinesubstituted methanes*)
Gas composition: Flow rate: Power: Etch rate:
0.02 tom CF, 50 sccm 0.2 Wkm2 I 13.56 MHz 0.3 n m l s (20°C) 0.1 t o n CHF, 100 sccm 2,3 kW I 13.56 M H z 7 n d s (20°C) 0.1 tom CF, 100 sccm 2,3 kW I 13.56 M H z 9 nm/s (20°C) "A.F.Holleman and E. Wiberg (1985) 2)S.Seki et al. (1983); Y. Kuo (1992)
Gas composition: Flow rate: Power: Etch rate: Gas composition: Flow rate: Power: Etch rate: References:
Bp. 229.5"C') Bp. 241.6"C') Bp. ca. 320°C')
6.2 Collection of Recipes
325
TaSi, -Tantalum Silicide Wet etching Readily soluble reaction products:
Ta(V) is soluble as fluoride TaF, ') Si(1V) as SiFz'
Dry etching Volatile and moderately volatile compounds:
TaF, TaC1, TaBr, SiI-L, SiF, Si2& SiHC13 Sic& Si2OC16 Si2C16
Bp. 229.5"C') Bp. 241.6"C') Bp. ca. 320°C') Bp. -111.6"C2' Bp . -95.7"C2) Bp. -15"C2) Bp. 31.7"C2) Bp. 56.7"C2) Bp. 135.5"C2) Bp. 147"C2)
1. Dry-etching method: Reactive ion etching in SFdC12-plasma3) Gas composition: Etch rate:
75 YO SF6; 25 YO c 1 2 1.5 n m l s
2. Dry-etching method: Reactive ion etching in CF4/C12-plasma3) Gas composition: Etch rate:
90% sF6; 10% c 1 2 1.5 n d s
3. Dry-etching method: Reactive ion etching in BC13/C12-plasma4) Gas composition: Flow rate Pressure: Power: Etch rate: Remarks:
80% BCl3; 20% C12 40 sccm
10 mtorr 3 kW, 13.56 MHz 1.5 n m / s A strong loading-effect was observed, i.e., the etch rate decreases with increasing numbers of wafers in the etch reactor.
4. Dry-etching method: Etching in SF&12-plasma5) Pressure: 8 mtorr Etch rate:
2.5 n m l s
326
6.2 Collection of Recipes
5. Dry-etching method: Etching in SF4/C12-plasma5) Pressure: Etch rate: References:
23 mtorr 1.3 n d s "A.F. Holleman and E. Wiberg (1985) *)J. D'Ans and E. Lax (1943), 261 3)H.J.Mattausch et al. (1983) 4)R.W.Light, H.B. Bell (1984) 5)K.Schade et al. (1990)
6.2 Collection of Recipes
Ta0,72Si0,28N - Tantalum Silicon Nitride Wet etching Readily soluble reaction products:
Ta(V) is soluble as fluoride TaFs Si(IV) as SiF,2-
Dry etching Volatile and moderately volatile compounds:
TaFs TaCls TaBr,
Bp. 229.5"C') Bp. 241.6"C') Bp. ca. 320°C')
1. Dry-etching method: Reactive ion etching in CFJ02-plasma2) Gas composition: Flow rate: Plasma conditions: Power: Etch rate:
50 % CFd;50 % 0 2 50 sccm 0,2 tom 167 W I 13.56 M H z 2 nm/s
References:
"A.F. Holleman and E.Wiberg (1985) "G.F. McLane et al. (1994)
327
328
6.2 Collection of Recipes
Te -Tellurium Wet etching Readily soluble reaction products: Etchants:
Te(1V) is soluble in strong acids as Te4+and in strong bases as Te0;- 'I,Te(I1) in tartaric acidic solutions as chelate ;Te02in various multifunctional organic acids2) - Concentrated sulphuric acid2) - Aqua regia2) - Salpetric acid2) - Hot alkaline solutions2)
Dry etching Volatile and moderately volatile compounds:
References:
Bp. -2.3"C3) TeH, TeF, Bp. 193.8"C') Bp. 324"C3) TeC1, TeBr, Bp. 339°C') Bp. 392"C3) TeC1, Bp. 421"C3) TeBr, ')AX Holleman and E. Wiberg (1985) 2)B.A. Irving( 1962) 3)J. D'Ans and E. Lax (1943), 264
6.2 Collection of Recipes
329
Ti -Titanium Wet etching Readily soluble reaction products:
Ti(1V) is soluble in strong acids as [X(OH),] ,+, [Ti(OH)3]fand related complexions, and F as preferred ligand ')
Etchant 1:
Diluted HF-solution*) HF 0.4 moVl
Concentrations: Temperature: Etch rate: Temperature: Etch rate:
Etchant 2:
Room temperature ca. 100 nm/s 32°C ca. 200 nm/s
Concentrations:
Salpetric acid-Fluoric acid solution3) HF 2.6 moVl
Temperature: Etch rate:
€€NO3 2.2mo,/l 32°C ca. 300 nm/s
Dry etching Volatile and moderately volatile compounds:
XCL, TiBr4 TiF4 TlJ4
Bp. 136.45"C') Bp. 233.45"C') subl. 284°C') Bp. 3'77°C')
1. Dry-etching method: Etching in CF3Br-plasma4) Gas composition: Plasma conditions: Power: Source: Etch rate: Remarks:
12 YO 0,;25 % He; 63 % CF3Br 0.2-0.7 ton 80-200 W/ 40 cm Parallel-plate reactor 0.6 nm/s bei 200 W Selective etching against gold and silicon nitride is possible.
2. Dry etching method: Etching in SF6-plasmaS) Gas composition: Flow rate: Plasma conditions: Source:
100% SF, 1 cm3/s 10 Pa; torr 14 M H z
330
6.2
Collection of Recipes
Etch rate: Remarks:
5 nm/s With decreasing pressure the etch rate decreases with the square of the pressure.
3. Dry-etching method: Reactive ion etching in BC13-plasma6) Gas composition: References:
100% BC13 ''A.F. Holleman and E. Wiberg (1985) 2)H.Beneking (1991); see also Eastman Kodak (1966); R.J. Ryan et al. (1970); R. Glang , L.V.Gregor et al. (1970) 3)R.J.Ryan et al. (1970); R. Glang ,L.V. Gregor et al. (1970) 4)C.J.Mogab, T.A. Shankoff (1977) 5)R.R.Reeves et al. (1990) "J. Hollkott et al. (1995)
6.2 Collection of Recipes
331
TiN - Titanium Nitride Wet etching Readily soluble reaction products:
Ti(IV) is soluble in strong acids as [Ti(OH)2]2 + , [Ti(OH)3]+and related complexions, and F as preferred ligand ')
Dry etching Volatile and moderately volatile compounds:
TiBr4 TiF4 Ti54
Bp. 233.45"C') subl. 284°C') Bp. 377°C')
1. Dry-etching method: Reactive ion etching in CF4/02-plasma4) Gas composition: Plasma conditions: Etch rate:
CFJ02 0.1 -0.2 keV Ion energyn; Parallel-plate reactor 0.18 n m / ~- 0.3 n m / s
2. Dry-etching method: Gas composition: Power: Source: Etch rate:
Sputtern in Ar -plasma5)
Ar 1kW ECR, 2.45 GHz; bias: -50 V bis -200 V 0.1 nm/s (0.1 keV Ion) 0.23 nm/s (0.2 keV Ion)
3. Dry-etching method: Magnetic field-enhanced etching in Ar/C1,-Plasma6) Gas composition: 77 % h,23 % sF6 Plasma conditions: 150 mtorr Gas flow: 111 sccm Power: 150 W (13.56 Mhz) Magnetic field 20 Gauss Etch rate: ca. 8.3 nm/s (60°C) References: "A.F. Holleman and E. Wiberg (1985) 2)H.Beneking (1991) 3)J.D'Ans and E. Lax (1943)' 264 4)F.Fracassi et al. (1995) 5)M.E.Day and M. Delfino (1996) %E. Riley and Th.E. Clark (1991)
332
6.2 Collection of Recipes
Ti02-Titanium Dioxide Wet etching Readily soluble reaction products:
?i(IV) is soluble in strong acids as [?i(OH),] '+, [%(OH),]+and related complexions, and F as preferred ligand ')
Dry etching Moderately volatile compounds:
TBr4 Ti4
Ti5 4
Bp. 233.45"C') subl. 284°C') Bp. 377°C')
1. Dry-etching method: Reactive ion etching in CF4-plasma2) Gas composition: Plasma conditions: Power: Source: Etch rate: References:
CF4 0.12 torr 100w Parallel-plate reactor 1,5 n m l s "A.F. Holleman and E. Wiberg (1985) ''A. Matsutani et al. (1991)
6.2 Collection of Recipes
333
V - Vanadium Wet etching Readily soluble reaction products:
V(V) is soluble in strongly alkaline solutions as vanadate HVO:-; V(IV and V(II1) as fluoro- or chlorocomplexes [VCh] and [VC&I3-; V(IV) in acids as [VO(Hz0)5]2+ and in Bases as V1804z12')
Etchant:
Hydrofluoric acid
2
Dry etching Volatile compounds:
References:
VF, Bp. 48.3"C') VOF3 subl. 110°C') VOClz Bp. 152°C') VOC13 Bp. 127.2OC') VOBr, Bp. 180°C') ') A.F. Holleman and E. Wiberg (1985)
334
6.2 Collection of Recipes
W - Tungsten Wet etching Readily soluble reaction products:
W(VI) forms in strongly alkaline media wolframates WO:-, that condensate at decreased pHvalue ')
Wet-etching method 1: Etching in alkaline hexacyanoferrate(II1)solution2) Concentrations: Temperature: Etch rate: Remarks: Etch rate:
KOH 0.9 moVl Kpe(CN)6 0.15 mom 1% wetting agent (Tergitol) 25 "C 4nds The etch rates of alkaline hexacyanoferrate(II1)solutions depend considerably on the convection in the etch solution3): 16 n d s (Immersion, 150 pm Diffusion layer thickness) 35 n d s (Immersion, 70 pm Diffusion layer thickness)
40 n d s (Immersion, 25 pm Diffusion layer thickness) 85 nm/s (Spray etching, 20 pm Diffusion layer thickness) 120 nm/s (Spray etching, 15 pm Diffusion layer thickness)
Wet-etching method 2: Electrochemichal etching in alkaline Hexacyanoferrate(II1) solution3) Concentrations: Temperature: Etch rate:
KOH 0.9 moVl Kpe(CN)6 0.15 moVl 1% wetting agent (Tergitol) 25 "C 20 n d s (at 100 mA/cm2) 80 n d s (at 460 mA/cm2)
6.2
Collection of Recipes
335
Dry etching Volatile and moderately volatile compounds:
m 6
WC15 WC16
subl. 17°C') subl. 275.6"C1) subl. 346°C')
1. Dry-etching method: Etching in pulsed SF6-plasma4) Gas composition: Plasma conditions: Power: Etch rate:
sF6 0.4-4 mtorr 40 W (13.56 Mhz) permanent power + 1.5 kW (pulsed 5ms/5ms Pause) ca. 3 nm/s (0°C) ca. 5 nmJs (30°C) ca. 13 nm/s (80°C)
2. Dry-etching method: Reactive ion etching in SF,JN2-plasma5) Gas composition: Flow rate: Source: Plasma conditions: Power: Etch rate:
sF6 :50 VOl% ; N2: 50 VOl% 2 sccm ECR 1 mtorr 200 W, self bias: -70 V ca. 2 nmJs (
3. Dry-etching method: Magnetic field-enhanced etching in Ar/SF6plasma6) Gas composition: 44% &, 56% sF6 Plasma conditions: Gas flow: Power: Etch rate: Remarks:
135 mtorr 84 sccm
350 W (13.56 MHz) Magnetic field 47 Gauss ca. 9.2 nm/s (60°C) WZn can be etched with the same rate as W (1.2 nm/s) with ECR-RIE (Ar/SF,, 1 mtorr, -0,2 kV bias, 0.3 kW microwave power)7).
4. Dry-etching method: Etching in SFd02-plasma8) Gas composition: Flow rate: Source: Plasma conditions: Power: Etch rate: Power:
sF6 :90 vOl% ; 0 2 : 10 vOl% 75 sccm RF-plasma, 4.5 MHz 0.2 torr 50 W ca. 1.2 nm/s (60°C) 150 W
336
6.2 Collection of Recipes
Etch rate: Gas composition: Power: Etch rate:
ca. 5 nm/s (60°C) sF6 :90 vOl% ; 0 2 : 10 vOl% 50 w ca. 7 nm/s (150°C)
5. Dry-etching method: Plasma etching in CFd/02-plasma8) Gas composition: Flow rate: Source: Power: Plasma conditions: Etch rate: Remarks:
CF, :90V01%; 0 2 : 10V01% 75 sccm RF-Plasma, 4.5 MHz 50 w 0.2 torr ca. 1.1nm/s (60°C) ca. 3.3 nm/s (150°C) Concerning the mechanism see ref.')
6. Dry-etching method: ECR-etching in SFdCHF&Ie-plasma") (Ti-doped W) Gas composition: sF6 :6 YO; cW3:47 YO; He: 47 %
Plasma conditions: Source: Temperature: Power: Etch rate: Power: Etch rate: References:
1.2 mtorr RF-plasma, 4.5 MHz -50°C
rf: 0.34 Wlcm'; 0.2 kW Microwave 0.5 nm/s rf: 0.65 Wlcm'; 0.2 kW Microwave 0.8 nm/s
"A.F. Holleman and E. Wiberg (1985) ')W. Kern and J.M. Shaw (1971) 3)A.F.Bogenschiitz et al. (1991) 4)R.Petri et al. (1992, 1994) "C.R. Eddy Jr. et al. (1994);see also N. Mutsukara and G. lbrban (1990) %?E.Riley and Th.E. Clark (1991) 7)A.Katz et al. (1993) ')C. C. Tang and D.W. Hess (1984) ''M.C. Peignon et al. (1993) ")K. Marumoto (1994)
6.2 Collection of Recipes
W03 - Tungsten Trioxide Wet etching Readily soluble reaction products:
W(VI) forms in strongly alkaline media wolframates WO:-, that condensate at decreased pH-value ')
Wet-etching method 1: Etching in diluted NaOH-solution') Concentrations: Temperature: Etch rate:
NaOH 0.01 moyl Room temperature 7 nm/s
Dry etching Volatile and moderately volatile compounds: References:
WF6 subl. 17°C') WC15 subl. 275.6"C') WCl, subl. 346°C') ')AX Holleman and E. Wiberg (1985) 2)Y.Gotoh et al. (1994)
337
338
6.2 Collection of Recipes
WSi2-Tungsten Silicide Wet etching Readily soluble reaction products:
W(VI) forms in strongly alkaline media wolframates WO,'-, that condensate at decreased pH-value ') Si(IV) soluble in form of complexes, e.g. in strongly alkaline media as [Si(OH)6]2-or in F-containing media as [SiF6l2-')
Dry etching Volatile compounds:
subl. 17°C') subl. 275.6"C2) wcl, subl. 346°C') Bp. -111.6"C3) Sib Bp. -95.7"C3) SiF, Bp. -15"C3) Si2& SiHC13 Bp. 31.7"C3) Bp. 56.7"C3) Sic& Si20C& Bp. 135.5"C3) Si2C16 Bp. 147"C3) m 6
wc15
1. Dry-etching method: Etching in NF3/CF2C12-plasma4) Gas composition: Flow rate: Plasma conditions: Power:
: 83 Val% ; CF2C12 : 17 Val% 18 sccm 260 mtorr 200 W/0.1W/cm2(Parallel-plate reactor) NF3
2. Dry-etching method: Reactive ion etching in CF4/02-plasma5) Gas composition: Etch rate:
CF, : 75 Val% ; 0 2 : 25 Val% 7.5 d s
3. Dry-etching method: Etching in BC13/C12-plasma6) Flow rate: Plasma conditions: Power: Etch rate:
50 sccm
0.76 torr 2.4 W/cm2 7.5 d s
6.2 Collection of Recipes
References:
339
‘)A.E Holleman and E. Wiberg (1985) *)J.D’Ans and E.Lax (1943), 270 3)J.D’Ans and E.Lax (1943)’ 261 4)J.M.Parks and R.J .Jamdine (1991), rf 860 5)R.S.Benneth et al. (1981)see also K. Schade et al. (1990) 6)K. Schade et al. (1990)
340
6.2 Collection of Recipes
YBa2Cu307-x - Yttrium Barium Cuprate Wet etching Readily soluble reaction products:
Yttrium is soluble in form of Y(II1)-salts in acidic to neutral media; barium as Ba2+in various salts, but only slightly, e.g., as sulphate or carbonate; copper as Cu2' in many salts, as Cu(1) in form of halogeno or pseudohalogeno complexes or in strongly alkaline media in form of hydroxocomplexes')
Etchant 1:
Ce(IV)-salt solutions2)
Dry etching Slightly volatile compounds:
YC13 BaC12 CuC12 CuBr2
Bp. 1507°C') Bp. 1560°C') Bp. 655"C3) Bp. W C 3 )
1. Dry-etching method: Reactive ion etching in C12-plasma4) Gas composition: Plasma conditions: Power: Etch rate: Remarks:
C12: 100%; ~ O S C C ~ 1Pa; (13.56 MHz) 0.5 kW (60W/cm ) 0.22 nm/s (bias 0.65 kV) The non-reactive ion etching with argon instead of chlorine provides ca. 75 % of the etch rate under the same conditions.
2. Dry-etching method: Ion beam etching with argon4) Gas composition: Beam current Voltage Plasma conditions: Power: Etch rate: References:
Ar:100Vol%; 20 mA (2.4 mA/cm2) 0.5 kV 0.5 mPa 0.6 W/cm2 0.63 nm/s "A.F. Holleman, E. Wiberg (1985) 2)IPHTIn-houseprescription 3)J. D'Ans and E. Lax (1943), 239 4)L.Alffet al. (1992)
6.2 Collection of Recipes
341
Zn - Zinc Wet etching Readily soluble reaction products:
Zinc is in its most important oxidation state (11) readily soluble in acids as Zn2' and in form of its complexes with many ligands, like e.g. H20,C1-, NH3: OH- . Hence it can be dissolved as well in acidlc as in ammonia or strong alkaline media.')
Wet-etching method:
Etching in diluted salpetric acid2)
Etchant composition: Etch rate:
1bis 1.7 M HN03 ca. 400 nm/s
Dry etching Volatile compounds:
References:
Zn(CH3)2 Bp. 46°C') Zn(GH5)2 Bp. 118°C') Zn2Cl2 volatile at 285-350°C (unstable at room temperature)') ')AX Holleman and E. Wiberg (1985) 2)R.J. Ryan et al. (1970)
342
6.2 Collection of Recipes
ZnO - Zinc Oxide Wet etching Readily soluble reaction products:
Zinc is in its most important oxidation state (11) readily soluble in acids as Zn2+and in form of its complexes with many ligands, like e.g. H20, C1-, NH3,OH- . Hence it can be dissolved as well in acidic as in ammonia or strong alkaline media.')
Wet-etching method:
Photoelectrochemical etching in NaC12)
Light power:
40 mWlcm2 (polychromatic UV-light from a Hghigh-pressure lamp) NaCl 0.1 mom with HC1 adjusted to pH 3 1.5 n d s NaCl 0.1 moVl with NaOH adjusted to pH 12 1d s
Etchant composition: Etch rate: Etchant composition: Etch rate:
Dry etching Volatile compounds:
Slightly volatile compound:
Zn(CH3)2 Bp. 46°C') Zn(GH& Bp. 118°C') ZnZCl2 volatile at 285-350°C (unstable at room tem erature)') ZnClz Bp. 756°C
I:
1. Dry-etching method: Etching in CF2C12-Plasma3) Gas composition: Flow rate: Plasma conditions: Power: Source: Etch rate: Remarks: References:
100% CF2F2 50 sccm 240 mtorr (parallel-plate reactor) 7.23 mWlcm2 1MHZ 0.04 nm/s High selectivity to aluminium ')AX Holleman and E. Wiberg (1985) ')M.Futsuhara et al. (1996) 3)G.D.Swanson et al. (1990)
6.2
Collection of Recipes
343
ZnS - Zinc Sulfide Wet etching Readily soluble reaction products:
Zinc is in its most important oxidation state (11) readily soluble in acids as Zn2' and in form of its complexes with many ligands, like e.g. H20, Cl-, NH3, OH- . Hence it can be dissolved as well in acidic as in ammonia or strong alkaline media.')
Dry etching Volatile compounds:
Zn(CH3)2 Bp. 46°C') Zn(GH,), Bp. 118°C') Zn2C12 volatile at 285-350°C (unstable at room temperature)') H2S Bp. 40.3"C') SF4 Bp. 4.4"C') so2 Bp. -10°C') SF2 Bp. 39°C') so3 Bp .44.5"C1) sc12 Bp. 59.6"C') Bp. 57°C (bei 0.22 torr)') S2Br2
Dry-etching method:
ECR-FUE in reductive plasma (ArH2(/CH4)I2) C& 17 Val% ; CH, 57 Val% ;Ar 26 Val%
Gas composition: Flow rate: Plasma conditions: Power: Source: Dc-bias-voltage: Etch rate: References:
30 sccm 1mtorr 150 W (microwave power) ECR with additional HF-power 13.56 MHz -250 V 0.4 nm/s "A.F. Holleman and E. Wiberg (1985) ')S.J. Pearton and E Ren (1993)
344
6.2 Collection of Recipes
ZnSe - Zinc Selenide Wet etching Readily soluble reaction products:
Zinc is in its most important oxidation state (11) readily soluble in acids as Zn2+and in form of its complexes with many ligands, like e.g. H20, C1-, NH,, OH- . Hence it can be dissolved as well in acidic as in ammonia or strong alkaline media.' Se(Iv) and Se(vI) form selen acid and selenic acid as in water soluble compounds')
Dry etching Volatile compounds:
Zn(CH3)2 Bp. 46°C') Zn(GH,), Bp. 118°C') Zn2C1, volatile at 285-350°C (unstable at room temperature)') H2Se Bp. -41.3"C" Se02F2 Bp. -9°C') SeOF, Bp. 65°C') SeOC12 Bp. 178"C2)
Dry-etching method :
ECR-RIE in reductive plasma (Ar/ H2(/cH4))3'
Gas composition:
CH, 17Vol % ; CH, 57 Vol% ;Ar 26 Vol% 30 sccm 1mtorr 150 W (microwave power) ECR with additional HF-power 13.56 MHz -250 V
Flow rate: Plasma conditions: Power: Source: Dc-bias-Voltage: Etch rate: References:
0.5 nm/s
"A.F. Holleman and E. Wiberg (1985) ')J. D'Ans and E. Lax (1943), 259 3)S.J. Pearton and E Ren (1993)
References
Aita, C.R. ; Gawlak, C.J.: J.Vac. Sci. Technol. A1 (1983), 403 Alavi, M.; Schumacher, A.; Wagner, H.-J.: Proc. Micro System Technologies 92 (VDE Verlag Berlin Offenbach 1992), 227 Alff, L. ; Fischer, G.M.; Gross, R. ; Kober, E ; Beck, A. ; Husemann, K.D.; Nissel, T. : Physica C 200 (1992), 277-286 Allen, D.M.: The principles and practice of photochemical machining and photoetching (Adam Hilger, Bristoll987) Allen, D.M. Elektrolytisches Photoatzen. Manuskript (1990) Allen, D.M., Beristain, L.S. and Gillbanks, P.J. Photochemical Machining of Molybdenum. Annals of the CIRP 35 (1986) 129-132. Allen, D.M. and Li, M. Etching AISI 316 Stainless Steel with Aqueous Ferric Chloride-Hydrochloridacid Solutions. The Journal (1988) Aoki, K.; Osteryoung, J.: J. Electroanal. Chem. 122 (1981), 19 Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H.; Osteryoung, J.: J. Electroanal. Chem. 182 (1985), 218-294 Asano, M.; Cho, T.; Muraoka, H.: Electrochem. SOC.Ext. Abstr. 76-2 (1976), 911, quoted from M.Schulz und H.Weiss (1984) Bharadwaj, L.M.; Bonhomme, P.; Faure, J.; Balossier, G.; Bajpaj, R.P.: J. Vac. Sci. Technol. B9,3 (1991), 1440 Baier,V.; Lerm, A.; Volklein, E; Wiegand, A.: Patent DD 298291 (l2.12.1988/ 13.2.1992) Bailey 111,A.D., van de Sanden, M.C.M., Gregus, J.A. and Gottscho, R.A.: J.Vac. Sci.Techno1.B 13(1) (1995) 92-104. Baklanov, M.R. ; Badmaeva, I.A. ; Donaton, R.A. ; Sveshnikova, L.L. ; Storm, W.; Maex, K.: J. Electrochem. SOC.143,lO (1996), 3245 Bardos, L., Berg, S., Blom, H.-0. and Barklund, A.M.: J.Electrochem.Soc. 137 (1990) 1587-1591. Barreti,N.J:; Grange, J.D.; Sealy, B.J. ; Stephens, K.G.: J.App1. Phys. 57 (1985), 5470 Bartuch, H.; Henneberger, J.; k r m , A.; Wiegand, A.: Patent DD 160115 (22.5.1980/13.5.1987) Baude, R E ; Ye, C.; Tamagawa, T.; Polla, D.L. : J. Appl. Phys. 73,ll (1993), 7960 Bean, K.E.: IEEE Transact, ED 25, 10 (1978), 1185 Bersin, R.L.: Solid State Technology (1976), 31-36.
346
References
Bertz, A.; S. Schubert, Th. Werner (1994): Proc. Micro SystemTechnologies ‘94 (VDI-Verlag Berlin, Offenbach 1994), 331 R.S.Benneth, R.S. ; Ephrat, L.N. ;Tsai, M.Y. ; Luchese, C.J. ; Electrochem. SOC.Conf. Minneapolis (May 1981), 81-1, quoted from T.P.Chow und A.J.StecM (1984) Beyer, H.; Walter, W.: Lehrbuch der Organischen Chemie (Stuttgart 1991) Beyer, W.: Untersuchungen zum Einsatz des reaktiven Ionen- und Ionenstrahlatzens zur Strukturierung in der Halbleitertechnologie (Diss. TU Chemnitz 1991) TIB-DW4628 Bharadwaj, L.M., Bonhomme, P., Faure, J., Balossier, G. and Balpai, R.P.: . J.Vac.Sci Techno1.B 9(3) (1991) 1440-1444. Bloomstein, T.M. and Ehrlich, D.J. Laser-Chemical 3D Micromachining. Mater.Res.Soc.Sympo.Proceedings282 (1993) 165-171. Bogenschiitz, A.F. : Atzpraxis fiir Halbleiter (Miinchen 1967) Bogenschiitz, A.F. : Metalloberflache 29 (1975), 451 Bogenschiitz, A X , Knoll, A. and Mussinger, W.: Galvanotechnik 82 (1991) 1192-1196. Bondur, J.A. : J .VacSci.133(1976),1023 Boensch, P.; Wuellner, D.; Schrimpf, T.; Schlachetzki, A.; Lacmann, R.: J. Electrochem. SOC.145,4 (1998), 1273 Boyd, H.; Tang, M.S.: Solid State Technology (April 1979), 133 Broers; A.N.: Microelectron. Reliab. 4 (1965), 103 Brown, D.M. ; Gray, P.V. ;Herrmann, FK; Philipp, H.R. ;Taft , E.A. : J.Electrochem. SOC.115 (1968), 311; quoted from M.Schulz und H.Weiss (1984) Brown, D.M.; Engeler, W.E.; Garfinkel, M.; Heumann, EK.: J.Electrochem. SOC. 114 (1967), 730, quoted from M.Schulz und H.Weiss (1984) Bruce, R.H.: Solid State Technol. 24 (1981),64 Bruce, R.H. and Reinberg, A.R. Effects of exitation frequency in plasma etching. (1996) Bugless, J.G.; McLean, T.D.; Parker, D.G.: J.Electrochem. SOC.133, 12 (1986), 2565 Burgess, C.E : The Electrochemical Society (1941); quoted from A.E.DeBarr und D.A.Oliver (1968): Electrochemical Machining (Macdonald London 1968) Burggraaf , P. : Semiconductor International (August 1994), 46 Burkhart,R.W.; Silkensen, R.D.; Steving,G.;Weaver,L.R.:IBM Tech. Discl. Bull. 24-4 (1981), 2081 Buttgenbach,S.: Mikromechanik: Einfiihrung in Technologie und Anwendungen (Teubner, Stuttgart 1991) Cahill, S.S., Chu, W.; Ikeda, K.: Transducers 93 (7th internat. cod. on solid state sensors and actuators (Yokohama 1993), 250 Cain, S.R., Egitto, ED. and Emmi, E: J.Vac.Sci.Techno1.A 5 (1987) 1578-1584. Camacho, A. and Morgan, D.V.: J.Vac.Sci.Techno1.B l2(5) (1994) 2933-2940.
References
347
Campbell, S.A.; Schiffrin, D.J.; Tufton, P.J.: Journal of Electroanalytical Chemistry 344 (1993) 211-233. Canham, L.T. Appl. Phys. Lett. 57 (1990), 1046 Caracciolo, R. and Schmidt, L.D.: Journal of Electrochemical Society 130 (1983) 603-607. Chapman, B. : Glow discharge processes: Sputtering and Plasma etching (John Wiley & Sons New York-Chichester-Brisbane-Toronto-Singapore 1980) Chand, N. and Karlicek. R.F.J.: J.Electrochem.Soc. 140 (1993) 703-705. Chang; C.V.J.M. and kijpers, J.C.N. : J.Vac.Sci.Tec&ol.B’ 12(2) (1994) 536-539 Charlet, B.; Peccoud, L.:Proc. 5th Symp. on Plasma Processing (Pennington 1984), 227; quoted from M.A. Hartney et al. (1989 a) R.Cheung, R.; Zijlstrata, T.; Van der Drift, E.; Geerligs, A.H.; Verbruggen, A.H. ;Werner, K. ; Radelaar, S. : J. Vac. Sci. Technol. B 11,6 (1993), 2224 Chow, T.P.; Steckl, A.J.: J.App1. Phys. 53 (1982), 5531 Chow, T.P. and Steckl, A.J. : J.Electrochem.Soc. 131 (1984) 2325-2335. Clark, L.D., Jr., Lund, J.L. and Edell, D.J.: Solid-state Sensor and Actuator Workshop (1988) Coburn, J.W. and Winters, H.E: Appl.Phys Lett. 55(26) (1989) 2730-2732. Costa-Kieling,V.: Untersuchungen zum Atzen von Silizium in alkalischen und fluoridhaltigen Elektrolyten (Diss. TU Berlin 1993) D’Agostino, R. (Ed.): Plasma deposition, treatment and etching of polymers (Academic Press 1990), ISBN 0-12-200430-2 Dane, D., Gadgil, P., Mantei, T.D., Carlson, M.A. and Weber, M.E.: J.Vac.Sci Techno1.B lO(4) (1992) 1312-1319. Daniel, J.H. ; Moore, D.F. ;Walker, J.F., Whitney, J.T. : Microelectronic Engineering 35 (1997), 431-434 Datta, M.: J.Electrochem.Soc. 142 (1995) 3801-3805. Datta, M.; Romankiw, L.T.; Vigliotti, D.R.; von Gutfeld, R.J.: J.Electrochem. SOC.136 (1989), 2251 Datta, M. and Romankiw, L.T. : J.Electrochem.Soc. 136 (1989) 285C-292C. Davidse, P.D.: J.Electrochem.Soc. 116 (1969) 100-103. Day, M.E.; Delfino, M.: J. Electrochem. SOC.143,l (1996), 264 DeBarr, A.E., Oliver,D.A.: Electrochemical Machining (Elsevier, New York 1968) Decker, F.: J.Electrochem.Soc. 131 (1984), 1173 Dennison, R.W.; Solid State Technology (Sept. 1980), 117; quoted from M.Schulz und H.Weiss (1984) DeSalvo, G.C., Tseng, W:E and Comas, J.: J.Electrochem.Soc. 139 (1992) 831-835. DeSalvo, C.G.; Bozada, C.A.; Ebel, J.L.; Look, D.C.; Barette, J.R; Cerny, C.L.A.; Dettmer, R.W.; Gillespie, J.K.; Havasy, C.K.; Jenkins, T.J.; Nakano, K. ; Pettiford, C.I. ;Quach, T.K. ; Sewell, J.S. ;Via, G.D. : J. Electrochem. SOC.143,ll (1996), 3652 Di Francia, G. and Salerno, A.: J.Electrochem.Soc. 141 (1994) 689-690.
348
References
Dijkstra, H.J. : J.Vac.Sci Techno1.B lO(5) (1992) 2222-2229. Donnelly, V.M.; Flamm, D.L.; Dautremont-Smith, W.C.; Werder, D.J.: J .Appl .PhyS. 55(1984), 242-252 Drost,A.; Steiner,P. Moser, H. ;W.Lang,W.: Sensors and Materials 7 (1995), 111 Duttagupta, S.P., Peng, C., Fauchet, P.M., Kurinec, S.K. and Blanton, T.N.: J.Vac.Sci.Techno1.B 13 (1995) 1230-1235. Eastman Kodak Co.: Rochester N.Y.: pamphlet p-91 (1966); quoted from M.Schulz und H.Weiss (1984) Eddy, C.R., Jr., Kosakowski, J., Shirey, L.M., Dobisz, E.A., Rhee, K.W., Chu, W., Foster, K.W., Maman, C.R.K. and Peckerar, M.C.: J.Vac. Sci.Techno1.B 12(6) (1994) 3351-3355. Effenhauser, C.S., Manz, A. and Widmer, H.M.: Analytical Chemistry 65 (1993) 2637-2642. Eggert,'L.; Abraham, W.; Stiegert, S.; Hanff, R.; Kreysig, D.: Acta Polymerica 39 (1988). 376 Eggert, L: and'Abraham, W. : Acta Polymerica 40 (1989) 726-731. Ermantraut, E.; Kohler, J.M.; Schulz, T.; Wohlfart, K.: Neuartige Bader zur Erzeugung von Mikrostrukturen (submitted Patent 1996: Az 196 34 122.1-51) Erne, B.H.; VanMaekelbergh, D.; Kelly, J.J.: Adv. Mater. 7 (1995), 739 Esashi, M., Takinami, M., Wakabayashi, Y. and Minami, K.: Journal of Micromechanics and Microengineering 5 (1995) 5-10. Ferreira, N.G.; Soltz, D.; Decker, E; Cescato, L.: J.Electrochem. SOC.142 (1995), 1348 Fink, T. and Osgood, R.M., Jr.. J.Electrochem.Soc. 140 (1993) 2572-2581. Finne, R.M. and Klein, D.L.: J.Electrochem.Soc. 114 (1967) 965-970. Fischer, B.E. and Spohr, R.: Naturwissenschaften 75 (1988 a) 57-66. Fischer, B.E. and Spohr, R.: Naturwissenschaften 75 (1988 b) 117-122. Fischer, P.B., Dai, K., Chen, E. and Chou, S.Y.: J.Vac.Sci.Techno1.B 11(6) (1993) 2524-2527. Flamm,D.L., Donelly, V.M.; Ibbotson, D.E.: Basic principles of plasma etching fpr silicon devices, in: VLSI-electronics.microstructurescience 8, hrsg. von Einspruch, N.G. (Academic press, Orlando 1984),189 D.L.Flamm: J.App1. Phys. 52 (1981), 3383 Flanders, D.C., Pressman, L.D. and Pinelli, G.: J.Vac.Sci.Techno1.B 8 (1990) 199O-1993. Flemish, J.R. and Jones, K.A.: J.Electrochem.Soc. 140 (1993) 844-847. Fonash, S.J. : J.Electrochem.Soc. 137 (1990) 3885-3892. Fracassi, E; d'Agostino, R.; Lamendola, R.; Mangieri, J. :J.Vac.Sci.Techno1. A13 (1995), 335 Fracassi, E: J.Electrochem.Soc. 143,2 (1996), 701 Frankenthal, R.P. and Eaton, D.H. : J.Electrochem.Soc. 123(1976) 703-706. Franz, G. : Obefflachentechnologie mit Niederdruckplasmen, Beschichten und Strukturieren in der Mikrotechnik (2.Aufl. Springer, Berlin 1994), ISBN 0-387-57360-7
References
349
Frey, H. : Ionengestutzte Halbleitertechnologie (VDI Verlag, Dusseldorf 1992) Freyhardt, H.C. (Ed.): Silicon chemical etching (Springer New York 1982) Friedliinder, G.; Kennedy, J.W.: Lehrbuch der Kern- und Radiochemie (Munchen 1962) Anhang G Fujimura, S., Shinagawa, K., Suzuki, M.T. and Nakamura, M.: J.Vac. Sci .Techno1.B 9(2) (1991) 357-361. Futsuhara, M.; Yosihoka, K.; Ishida, Y.; Takai, 0.; Hashimoto, K.; Fujishima, A.: J.Electrochem. SOC.143,ll (1996), 3743 Gambino, J.P.; Monkowski, M.D.; Shepard, J.F.; Parks, C.C.: J. Electrochem. SOC.137 (1990), 976 Gamo, K. and Namba, S.: J.Vac.Sci.Techno1.B 8 (1990) 1927-1931. Geldern, W.V.; V.E.Hauser, YE.: J.Electrochem.Soc.ll4,8 (1967), 869 Georgiadou, M. and Alkire, R.: J.Electrochem.Soc. 140 (1993 a) 1340-1347 Georgiadou, M. and Alkire, R.: J.Electrochem.Soc. 140 (1993 b) 1348-1355. Gerischer, H.: Angewandte Chemie 100 (1988) 63-78. Ghezzo, M.; Brown, D.M.: J.Electrochem. SOC.120 (1973), 110; quoted from M. Schulz und H.Weiss (1984) Gillis, H.P., Clemons, J.L. and Chamberlain, J.P.: J.Vac.Sci.Techno1.B lO(6) (1992) 2729-2733. Gillis, H.F?; Choutov, D.A. ; Martin, K.P. ; Pearton, S.J. ; Abernathy, C.R. : J. Electrochem. SOC.143,ll (1996), L 251 Givens, J., Geissler, S., Lee, J., Cain, O., Marks, J., Keswick, F? and Cunningham, C.: J.Vac.Sci.Techno1.B 12(1) (1994) 427-432. Glang, R.; Gregor, L.V.: Handbook of Thin Film Technology (New York 1970), quoted from M. Schulz and H.Weiss (1984) Gloersen, F?: Solid State Technology (April 1976), 68,quoted from M.Schulz and H.Weiss (1984) Glesener, J.W. and Tonucci, R.J.: J.Appl.Phys. 74 (1993) 5280-5281. Goldstein, 1.S; Kalk, E: J.Vac.Sci.Techno1. 19(1981), 743-747 B.Gorowitz und R.Saia: General Electric TIS Report 82CRD249 (1982), quoted from T.P. Chow and A.J. Steckl(1984) Gotoh, Y., Inoue, K., Ohtake, T., Ueda, H., Hishida, Y., Tsuji, H. and Ishikawa, J. : Japanese Journal of Applied Physics 33 (1994) L63-L66. Gottscho, R.A.; Jurgensen, Ch.W.; Vitkavage, D.J.: J. Vac. Sci. Technol. B 10,5 (1992), 2133 Grande, W.J.; Johnson, J.E.; Tang, C.L.: J.Vac. Sci. Technol. B8,5 (1990), 1075 Gray, D.C., Butterbaugh, J.W., Hiatt, C.E, Lawing, A.S. and Sawin, H.H.: 142 (1995 a) 3919-3923. Gray, D.C., Butterbaugh, J.W., Hiatt, C.E, Lawing, A.S. and Sawin, H.H.: 142 (1995 b) 3859-3863. Grossman, J.; Heman, D.S.; J.Electrochem. SOC.116 (1969), 674; quoted from M.Schulz und H.Weiss (1984) Gspann, J. : Sensors and Actuators A 51 (1995) 37-39. Goldstein, I.S. and Kalk, E: J.Vac.Sci.Techno1. 19 (1981) 743-747.
350
References
Gurvitch, M.; Washington, M.A.; Huggins, H.A.: Appl.Phys Lett. 42(1983), 472-474 Gussef, W. (1929): British Patent 335003 Hamerton, Ph.G.: Etching and Etchers (Macmillan London 1876, republished by EP Publishing Limited British Book Cennter 1975) Harries, D. ; Kohl, P.A. ;Winnick, J. : J.Electrochem. SOC.141, 5 (1994) Harris, W.T.: Chemical Milling (Clarendon Press Oxford 1976) Harrison, D.J., Fluri, K., Seiler, K., Fan, Z., Effenhauser, C.S. and Manz, A.: Science 261 (1993) 895-897. Hartney, M.A.; Hess, D.W.; Soane, D.S.: J.Vac. Sci. Technol. B 7, 1 (1989 a), 1 Hartney, M.A., Chiang, J.N., Soane, D.S. and Hess, D.W. : Proc. SPIE 1086 (1989 b) 150-161. Hashimoto, H., Tanaka, S., Sato, K., Ishikawa, I., Kato, S. and Chubachi, N.: Japanese Journal of Applied Physics 1/32 (1993) 2543-2546. Heard, l? J. ; Cleaver, J.R.A. ;Ahmed, H. : J. Vac.Sci.Techno1.B 3 (1989, 87 Heath, B.A.; Mayer, T.M.: VLSI Electronics: Microstructure Science 8 (1984), 377 Heiman, N., Minkiewicz, Y and Chapman, B. High rate reactive ion etching 3 Si. J.Vac.Sci.Techno1. 17 (1980) 731-734. of A120and Herring,R.; Price, J.B.: Electrochem. SOC.Extend. Abstr. 73-2 (1973, 410; quoted from M.Schulz and H.Weiss (1984) Hess, D.W. Plasma etching of Aluminum. SST (1981) 189-194. Hess, D.W. ; Jensen,K.F.: Microelectronics processing: chemical engineering aspects, in: Advances inchemistry series, Vol. 221 (Washington 1989) Heuberger, A. : Mikromechanik (Springer 1989) Hiermaier, M. : Elektrochemisches Bohren, in: Moderne Aspekte der Atztechnik. VDI-TZ; Ulmer Gesprach 12 (Saulgau 1990), 112 Hollemann, A.E; Wiberg, E.: Lehrbuch der Anorganischen Chemie (de Gryuter Berlin New York 1985) Hollkott, J., Barth, R., Auge, J., Spangenberg, B., Roskos, H.G. and Kurz, H. Improved dry-etching process with amorphous carbon masks for fabrication of high-Tc submicron structures. 1nst.Phys.Conf.Ser. No 148 (1995) 831-834. Holmes,P.J. : The electrochemistry of semiconductors (Academic Press London 1962) Holmes, l?J. and Snell, J.E.: Microelectronics and Reliability 5 (1966) 337-341. Hong, J. ; Lee, J.W. ; Lambers, E.S. ; Abernathy, C.R. ; Pearton, S.J. ; Constantine, C.; Hobson, W.S.: J.Electrochem. SOC.143,ll (1996), 3656 Hong, J. ;Caballero, J.A. ; Geerts, W. ; Childress, J.R. ; Pearton, S.J. : J. Electrochem. SOC.144, 10 (1997), 3602 Hsiao, R.; Miller, D.: J. Electrochem. SOC.143, 10 (1996), 3266 Hsiao, R.; Yu, K.; Fan, L.S.; Pandhumsopom, T.; Sanitini, H.; Macdonald, S.A.; Robertson, N.: J. Electrochem. SOC.144,3 (1997), 1008 Hsieh, H.F. ;Yeh, C.C. ; Shih, H.C. :J.Electrochem. SOC.140,2 (1993), 463
References
351
Hughes,H.G.; Rand,M.J (Ed.). : Etching (The Electrochemical Society Softbond SyrnposSer., Princeton NJ 1976) Hulsenberg, D. (1992): Glas in der Mikrotechnik, in: Sitzungsberichte der Sachsischen Akademie der Wissenschaften zu Leipzig, mathematischnaturwissenschaftliche Klasse, Bd. 123. Heft 6 (Berlin 1992) Hur, K.Y., McKenna, T.P. and Kazior, T.E.: J.Vac.Sci.Techno1.B 12 (1994) 3046-3047. Ibbotson, D.E., Flamm,D.L. and Donnelly, VM.: J.Appl.Phys. 54 (1983) 5974-5981. Ikossi-Anastasiou, K., Binari, S.C., Kelner, G., Boos, J.B., Kyono, C.S., Mittereder, J. and Griffin, G.L.: J.Electrochem.Soc. 142 (1995) 3558-3564. Inanami, R.; Uchida, T.; Morita, S.: J. Electochem. SOC.143,11(1996),3754 IPHT-in-house instruction: s. A.Wiegand et al. (1981- 1996) Irving, B.A.: in “The electrochemistry of semiconductors” ed. P.J.Holmes (London&New York 1962), 256 Irving, S.M: Proc. Kodak Photoresist Seminar 2 (Rochester 1968), 26; quoted from M.A. Hartney et al. (1989 a) Jakubke, H.-P.; Jeschkeit, H.: ABC Chemie (Leipzig 1987) Jansen, H.; Boer, M. de; Wiegerink, R.; Tas, N.; Smulders, E.; Neagu, Ch.; Elwenspoek, M. : Microelectronic Engineering 35 (1997), 45-50 Janus, A.R.: J. Electrochem. SOC.119 (1972), 392; quoted from M.Schulz und H.Weiss (1984) Jaw, S.-J. ;M.Fenton, J.M.; Datta, M.: The Electrochemical Society Proceedings Vol. 94-32 (1994), 217 Jones, S.H.; Walker; D.K.: J. Electrochem. SOC.137,5 (1990), 1653 John, J.P. and McDonald, J.: 140 (1993) 2622-2625. Joubert, 0.;Pelletier, J.; Amal, Y.: J.Appl.Phys. 65(1989), 5096-5100 Joubert, O., Pelletier, J., Fiori, C. and Nguyen Tan, T.A. : J.Appl.Phys. 67(9) (1990) 4291-4296. Juan, W.H. and Pang, S.W.: J.Vac.Sci.Techno1.B 12(1) (1994) 422-426. Juang, C. ;Kuhn, K.J. ;Darling, R.B. : J. Vac. Sci. Technol. B 8,5 (1990), 1122 Kappelt, M.; Bimberg, D.: J. Electrochem. SOC.143,lO (1996), 3271 Katz, A., Feingold, A., El-Roy, A., Moriya, N., Pearton, S.J., Rusby, A., Kovalchick, J., Abernathy, C.R., Geva, M. and Lane, E.: Semicond.Sci.Techno1. 8 (1993) 1445-1450. Kawabe, T., Fuyama, M. and Narishige, S. Selective ion beam etching of A1203films. J.Electrochem.Soc. 138 (1991) 2744-2748. Kelly, J.J. and De Minjer, C.H.: J.Electrochem.Soc. 122 (1975) 931-936. Kelly, J.J. und G.J.Koe1: J.Electrochem.Soc. 125,6 (1978), 860 (6858) Kern, W.: RCA Review 39 (1978) 278-308. Kern, W.; Deckert, Ch.: Thin Film Processes (New York 1978), 417-421, quoted from M.Schulz und H.Weiss (1984) Kern, W.; Heim, R.C.: J. Electrochem. SOC.117 (1970), 562; quoted from M.Schulz and H.Weiss (1984) Kern, W. and Shaw, J.M.: J.Electrochem.Soc. 118 (1971) 1699-1704.
352
References
Khare, R., Hu, E.L., Brown, J.J. and Melendes, M.A.: J.Vac.Sci.Techno1.B ll(6) (1993)2497-2501 Klein, D.L.; D’Stefan, D.J.: J.Electrochem. SOC.109 (1962),37;quoted from M. Schulz and H.Weiss (1984) Kley, E.B.; MST 1995,4 Kline, G.R.; Lakin, K.M.: Appl. Phys. Lett. 43 (1983),750 KO, K.K. and Pang, S.W.: J.Electrochem.Soc. 142 (1995)3945-3949. Kohl, P.A., Hams, D.B. and Winnick, J.: J.Electrochem.Soc. 138 (1991) 608-614. Kohler, M.; Lerm, A.; Wiegand, A. (1983a): Atzbad fiir Wismut undoder Antimon, Patent DD 300602 (3.10.83/25.6.92) Kohler, M.; Lerm, A.; Wiegand, A. (1983b): Antimonatzbad, Patent DD 300387 (3.10.83/11.6.92) Kohler, J.M.: Micro System Technologies Conf.92 (Berlin 1992),253 Kohler, J.M.;Wiegand, A.;Lerm, A.: J. Electroanal. Chem. 213 (1986),75 Kohler, J.M. :Z. Chem. 30,3 (1990),108 Kohler, J.M.; Pechmann, R.; Schaper, A.; Schober, A.; Jovin, Th.M.; Schwienhorst, A. : Microsystem Technologies 1,4(1999,202 Korman, C.S.; Chow, T.P. and Bower; D.H.: Solid State Technology Januar (1983)115-124. Kosugi, T.; Gamo, K.; Namba, S.; Aihara, R.: J. Vac. Sci. Technol. B. 9,5 (1991,2660 Kropnewicki, Th. J.; Doolittle, W.A.; Carter-Coman, C.; Sangboem, K.; Kohl, PA.; Jokerst, N.M.; Brown, A.S.: J. Electrochem. SOC. 145, 5 (1998),L88 Kuo, Y.:J.Electrochem.Soc. 137 (1990 a) 1907-1911. Kuo, Y. : J.Electrochem.Soc. 137 (1990b) 1235-1239. Kuo, Y. : J.Electrochem.Soc. 139 (1992)579-583. Lamontagne, B.;Wrobel, A.M.; Jalbert, G.; Wertheimer, M.R.: J. Phys. D.App1. Phys. 20 (1987),844 Lang, W., Steiner, F?, Richter, A., Marusczyk, K., Weimann, G. and Sandmaier, H.: The 7th International Conference on Solid-state Sensors and Actuators (1994)202-205. Sensors and Actuators A 43 (1994)239-242 Lang, W., Steiner, P. and Sandmaier, H. Porous silicon: a novel material for microsystems. Sensors and Actuators A 51 (1995)31-36. Laznovsky, W. : Vacuum Technology Res./Dev. (August 1975), 47, quoted from M.Schulz and H.Weiss (1984) Law, VJ., Tewordt, M., Ingram, S.G. and Jones, G.A.C.: J.Vac.Sci Techno1.B 9(3) (1991)1449-1455. Lee, D.B.: Journal of Applied Physics 40 (1969)4569-4574. Lee, D., Harkness, S.D. and Singh, R.K.: Mat.Res.Soc.Symp.Proc. 339 (1994)127-132. Lee, J.W.; Pearton, S.J.; Santana, C.J.; Mileham, J.R.; Lambers, E.S.; Abernathy, C.R.; Ren, E; Hobson, W.S.: J. Electrochem. Soc 143,3(1996), 1093
References
353
Lee, J.W.; Pearton, S.J.; Abernathy, C.R.; Zavada, J.M.; Chai, B.L.H.: J. Electrochem. SOC.143,8 (1996 b) , L 169 Lee, Y.H.; Zhou, Z.H.: J. Electrochem. SOC.138,8 (1991), 2439 Leech, P.W.; Kibel, M.H.; Gwynn, P.J.: J. Electrochem. SOC.137,2 (1990), 705 Lehmann,V., Mitani, K., Feijoo, D. and Gosele, U.: J. Electrochem.Soc. 138 (1991) L3-LA. Lehmann,V. and Foll, H.: J. Electrochem.Soc. 137 (1990) 653-659. Lehmann, V.; Honlein, W.;Reisinger, H.; Spitzer, A.; Wendt, H.; Wdler, J. : Solid State Technology (November 1995), 99 Lerm, A.; Pfeiffer, R.-G.; Wiegand, A: Patent DD 300622 (26.6.1990/ 25.6.1992) Li, J.P., Yih, P.H. and Steckl, A.J.: J.Electrochem.Soc. 140 (1993) 178-182. Licata, T.J. and Scarmozzino, R.: J.Vac.Sci.Techno1.B 9 (1991) 249-254. Light,R.W.; Bell, H.B.: J.Electrochem.Soc. 131 (1984), 459 Lii, Y.-J.T. and JornC, J: J.Electrochem.Soc. 137 (1990 a) 2837-2845. Lii, Y.-J., JornC, J. , Cadien, K.C. and Schoenholtz, J.E. , Jr.:. J.Electrochem.Soc. 137 (1990b) 3633-3689. Linde, H. and Austin, L.: 139 (1992) 1170-1174. Lindstrom, J.L. , Oehrlein, G.S. and Lanford, W.A. : J.Electrochem.Soc. 139 (1992) 317-320. Lishan, D.G. ; Hu, E.L. : J.Vac. Sci. Technol. B8,6 (1990), 1951 Long, G.; Foster, L.M.: J.Am.Ceram.Soc. 42 (1959),53 Lothian, J.R., Kuo, J.M., Hobson, W.S., Lane, E., Ren, E and Pearton, S.J.: J.Vac.Sci Techno1.B lO(3) (1992) 10611065 Ldwe, H. ; Keppel, P.; Zach, D.: Halbleiteratmerfahren (Akademie-Verlag Berlin 1990) H. Lu; Z. Wu; I. Bhat: J. Electrochem. SOC.144,l (1997), L8 Lutze, J.W., Perera, A.H. and Krusius, J.P.: J.Electrochem.Soc. 137 (1990) 249-252. Macchioni, C.V.: J.Electrochem. SOC.137,8 (1990) 2595-2599. Mai, C.C. and Looney, J.C: Solid State Technology January (1966) 19-24. Mandler, D. and Bard, A.J. : J.Electrochem.Soc. 137 (1990) 2468-2472. Manos,D.M. (Ed.): Plasma etching (Academic Press 1989) ISBN 0-12469370-9 Manz, A., Graber, N. and Widmer, H.M.: Sensors and Actuators (1990) 244-248. Manz, A., Harrison, D.J., Verpoorte, E. and Widmer, H.M.: Advances in Chromatography 33 (1993) 1-66. Markert, M.; Bertz, A.; Gessner, Th.: Microelectronid Engineering 35 (1997), 333-336 Marumoto, K.; Proc. SPIE 2194 (1994), 221 Matsuo, S.: Appl.Phys.L.ett. 36 (1980) 768-770. Matsui, S.; Yamato, H.; Namba, S.: Microcircuit Engineering 80 (1980), 523 Matsuo,S.: Appl.Phys.Lett. 36,9 (1980), 768
354
References
Matsutani, A., Koyama, F. and Iga, K. : Japanese Journal of Applied Physics 30 (1991) 428-429. Matthies, T.; David, C.; Thieme, J.: J.Vac.Sci.Tehcno1. B11,5 (1993), 1873 Matz, R.; Meiler, J.: in: Moderne Aspekte der Atztechnik. VDI-TZ; Ulmer Gesprach 12 (Saulgau 1990), 74 McGeough, J.A. : Principles of electrochemical machining (Chapman and Hall, London 1974) McLane, G.F., Casas, L., Reid, J.S., Kolawa, E. and Nicolet, M.-A.: J.Vac.Sci.Technol.B 12(4) (1994) 2353-2355. Meier, D.L., Przybysz, J.X. and Kang, J.: IEEE Transactions on Magnetics 27 (1991) 3121-3124. Melliar-Smith, C.M.: J.Vac.Sci.Techno1. 13 (1976) 1008-1022. Melngailis, J. ; Musil, C.R.; Stevens, E.H. ; Utlaut, M.; Kellog, E.M. ; Post, R.T. ; Geis, M.W. ;Mountain, R.W. : Vac.Sci.Techno1.B 4 (1986), 176 Meyyappan, M., McLane, G.F., Lee, H.S., Eckart, D., Namaroff, M. and Sasserath, J.: J.Vac.Sci Techno1.B lO(3) (1992) 1215-1217. Mattausch, H.J. ; B .Hasler, B .; Beinvogel, W. : J.Vac.Sci.Technol.Bl (1983),15,; quoted fromT.l?Chow und A.J.Steckl (1984) Miki, N., Kikuyama, H., Kawanabe, I., Miyashita, M. and Ohmi, T. : IEEE Transactions on Electron Devices 37 (1990) 107-115. M.Miyamura: J.Vac.Sci.Technol.Bl (1983), 37; quoted from B. A.Heath und T.M. Mayer (1984) Mogab, C.J. : J.Electrochem.Soc. 124(1977), 1262-1268 Mogab, C.J. and Levinstein, H.J.: J.Vac.Sci.Techno1. 17 (1980) 721-729. Mogab, C.J. und Shankoff, T.A.: J.Electrochem. SOC.124 (1977), 1766 Merck Bakers: ITO-Bearbeitungsempfehlung (0.0.o.J.) Molloy, J. (1995): J. Electrochem. Soc.142,12 (1995), 4285 Mu, X.-C. ; Fonash, S.J.; Oehrlein, G.S. ; CHakravarti, S.N. ; Park, C.; Keller, J.: J. Appl. Phys. 58 (1985), 2958 Murad, S.K.; Wilkinson, C.D.W.; Wang, P.D.; Parkes, W.; Sotomayor-Torres, C.M. ; Cameron, N. : J.Vac. Sci. Technol. B 11,6 (1993), 2237 Murakami, K., Wakabayashi, Y., Minami, K. and Esashi, M.: IEEE Micro Electromechanical Syst. Proc. (1993) 65-70. Mutsukura, N. and Turban, G. : J.Electrochem.Soc. 137 (1990) 225-229. Nagy, A.G.: J.Electrochem.Soc. 131(1984), 1871-1875 Namatsu, H. : J.Electrochem.Soc. 136(1989), 2676-2680 Nojiri, K.; Iguchi, E.; Kawamura, K.; Kadota, K.: Extd. abstr. of Conf. on Solid State Devices (Tokyo 1989), 153 Nojiri, K. and Iguchi, E.: J.Vac.Sci.Techno1.B 13 (1995) 1451-1455. Nordheden, K.J. ; Ferguson, D.W. ; Smith, P.M.: J. Vac. Sci. Technol. B 11,5 (1993), 1879 Novembre, A.E. ; Mixon, D.A. ; Pierrat, Ch. ; Knurek, Ch.;Stohl, M. : SPIE 2087 (1993) 50; Nowak, R.; Metev, S.; Sepold, G.: SPIE 2207 (1994), 633 Nowak, R.; Metev, S: in: ECLAT Laser Treatment of Materials (1996 a) Nowak, R.; Metev, S: in: Proc. Micro SystemTechnologies96 (Potsdam 1996)
References
355
Nowak, R.; Metev, S: Appl. Phys. A. (1996), 133-138 Nordheden, K.J., Ferguson, D.W. and Smith, P.M. : J.Vac.Sci.Techno1.B ll(5) (1993) 1879-1883. Northrup, M.A. ; Ching, M.T. ;White, R.M. ;Watson, R.T. : Proc. of 7th internat . conf. on solid states sensors and actuators (Yokohama 1993), 924-926 Northrup, M.A. ; Gonzalez, D. ; Hadley, D.; Hills,R.F.; Landre, P.; Lehew, S. ; Saiki, R. ; Sninsky, J.J. ;Watson, R.; Watson,R. Jr. : Proc. of 8th internat. conf. on solid states sensors and actuators (Stockholm 1995), 764-767 Nozaki, T.: European Patent 0010910 (1980); quoted from M.Schulz und H. Weiss (1984) Oehrlein, G.S. ; Bestwick, T.D.; Lones, P.L. ; Jaso, M.A. ; Lindstrom, J.L. : J.Electrochem. SOC.138,5 (1991), 1443 Okamato, F.: Jpn. J. Appl. Phys. 13 (1974), 383; quoted from M. Schulz and H.Weiss (1984) Okano, H., Yamazaki, T. and Horiike, Y.: Solid State Technology (1982) 166-170. Osenbruggen, C. van; De Regt, C. : Philips Technical Review 42(1985), 22-32 E.D.Palik, V.M.Bermudez and 0.J.Glembocki: Journal of Electrochemical Society 132 (1985) 871-884. Palmer, W.G.: The Bell System Technical Journal 35 (1956) 1656-1664. Pan, W.-S. and Steckl, A.J.: J.Electrochem.Soc. 137 (1990) 214-220. Pan, W. and Desu, S.B.: J.Vac.Sci.Techno1.B 12(6) (1994) 3208-3213. Pang, S.W.; KO,K.K.: J. Vac. Sci. Technol. B 10,6 (1992), 2703 Pang, S.W., Sung, K.T. and KO, K.K.: J.Vac.Sci Techno1.B lO(3) (1992) 1118-1123. Parisi, G.I.; Haszko, S.E.; Rozgonyi, G.A.: Journal of Electrochemical Society 124 (1994) 917-921. Parks, J.M. and Jaccodine, R.J. : J.Electrochem.Soc. 138 (1991) 2736-2741. Pearson, R.G.: Survey Prog. Chem. 5 (1969), 1-52 Pearton, S.J., Hobson, W.S., Baiocchi, F.A. and Jones, K.S.: J.Electrochem.Soc. 137 (1990 a) 1924-1934. Pearton, S.J., Chakrabarti, U.K., Hobson, W.S. and Perley, A X : J.Electrochem.Soc. 137 (1990 b) 3188-3202. Pearton, S.J. ;Hobson, W.S. ;Chakrabarti, U.K. ;Derkits, Jr., G.E. ;Kinsella, A.P.: J.Electrochem. Soc.137,12 (1990 c), 3894 Pearton, S.J., Chakrabarti, U.K., Katz, A., Perley, A.P., Hobson, W.S. and Constantine, C.: J.Vac.Sci Techno1.B 9(3) (1991 a) 1421-1432. Pearton, S.J.; Ren, E; Lothian, J.R.; Fullowan, T.R.; Kopf, R.E; Chakrabarti, U.K. ;Hui, S.P.;Emerson, R.L. ;Kostelak, R.L. ;Pei, S.S. : J.Vac.Sci Techno1.B 9(5) (1991 b) 2487 Pearton, S.J. ; Chakrabarti, U.K. ; Perley, A.P.; Hobson, W.S. ; Geva, M. : J. Electrochem. Soc.137,12 (1991 c), 1432 Pearton, S.J. and Ren, E: J.Vac.Sci.Techno1.B ll(1) (1993) 15-19. Pearton, S.J. ;Lee, J.W.; Lambers, E.S. ;Abernathy, C.R.; Ren, F. ;Hobson, W.S.; Shul, R.J.: J. Ellectrochem. SOC.143,2 (1996), 752-758
356
References
Pederson, L.A.: J.Electrochem.Soc. 129 (1982) 205-208. Peignon, M.C., Cardinaud, C. and lbrban, G.: J.Electrochem.Soc. 140 (1993) 505-512. Pelka, J., Weiss, M., Hoppe, W. and Mewes, D.: J.Vac.Sci.Techno1.B 7(6) (1989) 1483- 1487. Petri, R., Kennedy, B. and Henry, D.: J.Vac.Sci.Techno1.B 12(5) (1994) 2970-2975. Podlesnik, D.V.; Gilgen, H.H.; Osgoord Jr., R.M.: Appl. Phys. Lett. 45 (1984),563 Polask0,'K.J.; Ehrlich, D.J.; Bao, J.Y.; Pease, R.F.W.; Marinero, E.E.: IEEE Electron Device Letters 5,l (1984), 24 Pouch, J.J.: Plasma properties, deposition and etching (1993) Probst, E.K., Vogt, K.W. and Kohl, PA. : J.Electrochem.Soc. 140'(1993) 3631-3635. Proksche, H., Nagorsen, G. and Ross, D.: J.Electrochem.Soc. 139 (1992) 521-524. Proksche, H., Nagorsen, G. and Ross, D.: J.Electrochem.Soc. 140 (1993) 3611-3615. F'TI-in-house instruction (1985): Interne Dokumentation des PhysikalischTechnischen Institutes Jena, Ausmg m A. Wiegand et al. (1981-1996) Quinlan, K.P.: J. Electrochem. SOC.143,9 (1996), L200 Ragoisha, G.A.; Rogach, A.L. : The Electrochemical Society Proceedings Vol. 94-32( 1994), 268 Ralchenko, V.G., Kononenko, T.V., Pimenov, S.M., Chernenko, N.V., Loubnin, E.N., Armeyev, V.Y. and Zlobin, A.Y.: Diamond and Related Materia l 2~ (1993) 904-909. Ranade, R.M., Ang, S.S. and Brown, W.D. Reactive Ion Etching of Thin Gold Films. J.Electrochem.Soc. 140 (1993) 3676-3678. Rand, M.J.; Roberts, J.E: Appl. Phys. Lett. 24 (1974), 49; quoted from M.Schulz und H.Weiss (1984) Reisman, A.; Berkenblit, M. ; Chan, S.A.; Kaufman, EB.; Green, D.C. : J. Electrochem. SOC.137, 11 (1979), 1406 Ren, E; W.S.Hobson, W.S.; Lothian, J.R.; Lopata, J.; Pearton, S.J.; Caballero, J.A. ; Cole, W.M. : J. Electrochem. SOC.143, 10 (1W6), 3394 Ren, E; Hong, J.P.; Lothian, J.R.; Cho, A.Y.: J. Electrochem. SOC.144,9 (1997), L239 Reeves, R.R.; Rutten, M.; Ramaswami, S.; Roessle, P.; Halstead, J.A.: J.Electrochem. SOC.137,ll (1990), 3517 Richter, A.; Steiner, I?; Kozlowski, E; Lang, W.: IEEE Elect. Dev. Let. 12 (1991), 691 Richter, C.; Espertshuber, K.; Wagner, C.; Eickhoff, M.; Kroetz, G.: Mat. Sci. Engin. B46 (1997), 160-163 Riley, RE. and Clark,T.E. :J.Electrochem.Soc. 138 (1991) 3008-3013. (1984) Robb, EY.: J. Electrochem. SOC.131 (1984), 1670 Robbins, H. and Schwartz, B.: Journal of the Electrochem.Soc. 106 (1959) 505-508.
References
357
Robinson, B.; Shivashankar, S.A.: Proc. 5th Symp. of Plasma Processing (Pennington 1984), 206 Rosch, I?: Untersuchungen zur ndchemischen Atzung von AIn& Verbindungen mit halogenoxidhaltigen Atzmedien (Dissertation, Berlin 1992) van Roosmalen, A.J.; Baggerman, J.A.G; Brader, S.J.H.: Dry Etching for VLSI (Plenum Press New York und London 1989) Rosset, E. und Landolt, D.: Precision Engineering 11,2 (April 1989), 79 Rossnagel, S.M.; Cuomo, J.J.; Westwood, W.D.: Handbook of plasma processing technology (Ottawa 1990), ISBN 0-8155-1220-7 Rotsch, P.: Untersuchungen zur ndchemischen Atzung von AIIIBVVerbindungen mit halogenoxidhaltigen Atzmedien (1992) Ruberto, M.N., Zhang, X., Scarmozzino, R., Willner, A.E., Podlesnik, D.V. and Osgood, R.M., Jr. : J.Electrochem.Soc. 138 (1991) 1174-1185. Russell, S.D.; Sexton, D.A.: Mat.Res.Soc.Symp.Proc.Vo1.158 (1990), 325 Ryan, R.J.; Davidson, E.B.; Hook, H.O.: Handbook of Materials and Processes for Electronics (New York 1970), quoted from M.Schulz und H. Weiss (1984) Saia, R.J., Kwasnick, R.F. and Wei, C.Y.: J.Electrochem.Soc. 138 (1991) 493-496. Saito, Y., Yamaoka, 0. and Yoshida, A.: J.Vac.Sci Techno1.B 9(5) (1991) 2503-2506. Saitoh, H., Kyuno, T., Hosoda, I. and Urao, R.: Journal of Material Science 31 (1996) 603-606. Sakuma, K., Yagi, S. and Imai, K.: Japanese Journal of Applied Physics 33 (1994) L617-L619. Schade, K.; Suchaneck, G.; Tiller, H.-J.: Plasmatechnik. Anwendung in der Elektronik (Berlin 1990) Schmuki, P.;Fraser, J. ;Vitus, C.M.; Graham, M.J. ; Isaacs, H.M. : J.Electrochem. SOC.143,lO (1996), 3316 Schnakenberg, U. : IC-ProzeSkompatible anisotrop wirkende Atzlosungen zur Herstellung integrierter Mikrosysteme in Silizium (Diss. TU Berlin, FB 12,1993) Schober, A.; Schwienhorst, A.; Kohler, J.M. ; Fuchs, M.; R. Giinther, R.; Thiirk, M. : Microsystem Technologies 1,4 (1995), 168 Schreiter, S.; Poll, H.-U.: Sensors and Actuators A 35 (1992), 137 Schwesinger, N. : Micro System Technologies Conf. (Potsdam 1996), 481 Schulz, M.; Weiss, H. (Hrsg.), unter Mitarbeit von W.Dietze, E.Doering, W.Langheinrich, A. Ludsteck, H. Mader, A.Miihlbauer, W.v.Miinch , H.Runge, L.Schleicher, M.Schnoller, M.Schulz, E.Sirt1, E.Uden und W.Zulehner: Technologie von Si, Ge und Sic; in: Landolt-Bernstein. Zahlenwerte und Funktionen aus Naturwissenschaft und Technik. Neue Serie (hrsg. von K.-H.Hellwege und 0. Madelung). Band 17. Halbleiter. Teilband c (Berlin-Heidelberg-New York Tokyo 1984)
358
References
Schumacher, A.; Alavi, M.; Schmidt, B.; Sandmaier, H.: Proc. Micro System Technologies (Potsdam 1996), 633 Schwartz, B. and Robbins, H.: Electrochem.Soc. 108 (1961) 365-372. Seidel, H., Csepregi, L., Heuberger, A. and Baumgartel, H.: J.Electrochem.Soc. 137 (1990) 3613-3626. Seidel, H., Csepregi, L., Heuberger, A. and Baumgartel, H.: J.Electrochem.Soc. 137 (1990) 3626-3632. Seiler, K., Harrison, D.J. and Manz, A.:Analytical Chemistry 65 (1993) 1481-1488. Seki, S.,Unagami, T. andTsujiyama, B.: Journal of the Electrochem.Soc. 130 (1983) 2505-2506. Shaqfeh, E.S.G.; Jurgensen, Ch.W.: J. Appl. Phys. (1989) Shintani, A.; Minagawa, S. (1976): J.Electrochem. SOC.123,5 (1976), 706 Shivaram, M.S.; C.M.Svensson, C.M.: J.Electrochem. SOC.123 (1976), 1258; quoted from M.Schulz and H.Weiss (1984) Shoji, A., Shinoki, F., Kosaka, S., Aoyagi, M. and Hayakawa, H. : Appl.Phys Lett. 41 (1982) 1097-1099. Shor, J.S., Zhang, X.G. and Osgood, R.M.: J.Electrochem.Soc. 139 (1992) 1213-1216. Shor, J.S. and Kurtz, A.D.: J.Electrochem.Soc. 141 (1994) 778-781. Shul., R.J. ; Howard, A.J. ; Pearton, S.J. ; Abernathy, C.R. ;Vartuli, C.B. : J. Electrochem. oc.143,lO (1996), 3285 Simko, J.P.; Oehrlein, G.S.; Mayer, T.M.: J. Electrochem. SOC.138 (1991), 277 Singh, J.: J.Vac.Sci Techno1.B 9(4) (1991) 1911-1919. Skidmore, J.A., Lishan, D.G., Young, D.B., Hu, E.L. and Coldren, L.A. : J.Electrochem.Soc. 140 (1993) 1802-1804. Smith, D.L.: VLSI Electronic Microstructure Science 8 (1984), 284 Smith, R.L. Applications of porous silicon to microstructure fabrication. in: Electrochemicalmicrofabrication 11, ed. Datta,M; Sheppard, K. ; Dukovic, J.O. (Pennington 1995), 281 Snider, G.L., Then, A.M., Soave, R.J. and Tasker, G.W. High aspect ratio dry etching for microchannel plates. J.Vac.Sci.Techno1.B 12(6) (1994) 3327-3331. Soller, B.R.; Shuman, R.F.; Ross, R.R.: J.Electrochem. SOC.131 (1984), 353; quoted from M.A. Hartney et al. (1989 a) Somekh, S.:J.Vac.Sci.Tehcno1. 13 (1976), 1003, quoted from M.Schulz and H.Weiss (1984) Spencer, E.G.; Schmidt, P.H.: J.Vac.Sci.Techno1. 8 (1971), 552, quoted from M.Schulz and H.Weiss (1984) Stassinos, E.C. and Lee, H.H.: J.Electrochem.Soc. 137 (1990) 291-295. Stewart, T.R. and Bour, D.P. : J.Electrochem.Soc. 139 (1992) 1217-1219. Stoev, I.: Sensors and Actuators A 51 (1996) 113-116. Surbled, P.; Dufour-Gergam, E.; Gilles, J.-P.; Grandchamp, J.-P.: J. Micromech. Microeng. 7 (1997), 104-107 Svorcik, V and Rybka, V: J.Electrochem.Soc. 138 (1991) 1947-1948.
References
359
Swanson, G.D.; Tamagawa, T.; Polla, D.L.: J.Electrochem SOC.137,9 (1990), 2982 Syau, T., Baliga, B.J. and Hamaker, R.W.: J.Electrochem.Soc. 138 (1991) 3076-3081 Szekeres,A.; Kirov, K.; Alexandrova, S.: Phys. Status SolidiA63 (1981), 371; quoted from M.A. Hartney et al. (1989 a) N.Takado, N.; Kohmoto, S.; Sugimoto, Y.; Ozaki, M.; Sugimoto, M.; Asakawa, K.: J.Vac.Sci.Techno1. B 10,6 (1992), 2711 Takahashi, C. and Matsuo, S : J.Vac.Sci.Techno1.B l2(6) (1994) 3347-3350. Takenaka, H., Oishi, Y. and Ueda, D: J.Vac.Sci.Techno1.B 12(6) (1994) 3107-3111. Takinami, M., Minami, K. and Esashi, M.: Technical Digest of the 11th Sensor Symposium (1992) 15-18. Tang, C.C. and Hess, D.W.: Journal of the Electrochem. SOC.131 (1984) 115- 120. Taylor, K.M.; Lenie, C.: J.Electrochem. SOC.107 (1960), 308 Tedesm, S. ; Pierrat, C. ;Lamure, J.M. ; Sourd, C. ;Martin, J. ; Guibert, J.C. : SPIE Conf. 1264 (1990), 144 Tegert, W.: The electrolytic and chemical polishing of metals (2. edn. Oxford 1959). quoted from M.Schulz und H.Weiss (1984) Tenney, A.S.; Ghezzo, M.: J.Electrochem. SOC.120 (1973), 1091, quoted from M.Schulz und H.Weiss (1984) Tokunaga, K. and Hess, D.W.: J.Electrochem.Soc. 127 (1980) 928-932. Tsao, Y. ; Ehrlich, D.J. Appl. Phys. Lett. 43 (1983), 146 Tsou, L.Y.: J.Electrochem.Soc. 140 (1993) 2965-2969. Tsui, R.T.C.: Solid State Technology (1967) 33-38. Turban, G.; Rapeaux, M: J.Electrochem. SOC.130 (1983), 2231; quoted from M.A.Hartney et al. (1989 a) Uhlir Jr., A. : The Bell System Technical Journal 35 (1956) 333-347. Van de Ven, J. and Nabben, H.J.P. : J.Electrochem.Soc. 137 (1990) 1604-1610. Van de Ven, J. and Nabben, H.J.P.: J.Electrochem.Soc. 138 (1991) 3401-3406. Van der Putten, A.M.T. and de Bakker, J.W.: J.Electrochem.Soc. 140 (1993) 2221-2228. Van Osenbruggen, C. and De Regt, C. Electrochemical Micromachining. Philips Technical Review 42 (1985) 22-32. VanRoosmalen, A.J.: Dry etching for VLSI (Plenum Publ. Corp. 1991) ISBN 0-306-43835-6 Vartuli, C.B.; Pearton, S.J. ; Lee, J.W.; Abernathy, C.R. ; Mackenzie, J.D. ; Zolper, J.C.; Shul, R.J.; Ren, F. (1996 a): J.Electrochem. SOC.143, 11 (1996), 3681 Vartuli, C.B.; Pearton, S.J.; MacKenzie, J.D.; Abernathy, C.R. (1996 b): J. Electrochem SOC143,lO (1996), L 246 Vasquez, B.; Tompkins, H.G.; Fejes, P.; Lee, T.Y.; Smith, L.: SPIE 1185 (1989), 148
360
References
Vetter, K. : Elektrochemische Kinetik (1962) Vijay, D.P., Desu, S.B. and Pan, W.: J.Electrochem.Soc. 140 (1993) 2635-2639. R.Voss, H.Seide1 and H.Baumgarte1: Transducers 91 Conf. (1991), 140-143. Voss, R.: Der Einflul3 von elektrochemischen Potentialen und Licht auf das anisotrope Atzen von Silizium (1992) Vossen,J.L.; Kern,W. (Ed.): Thin Film Processes (Academic Press New York 1978) Vukanovic, V.,Takacs, G.A., Matuszak, E.A., Egitto, ED., Emmi, E and Horwath, R.S.: J.Vac.Sci.Techno1.B 6 (1988)66-71. Watanabe, H., Ohnishi, S., Honma, I., Ono, H., Wilhelm, R.J. and Sophie, A.J.L.: J.Electrochem.Soc. 142 (1995)237-243. Watanabe, H. and Matsui, S.: J.Vac.Sci.Techno1.B ll(6) (1993)2288-2293. Wechsung, R. ; Brauer, W. : Vakuum-Technik 24(1975),157-166 West, A.C.; Madore, Ch.; Matlosz, Landolt, D.: J. Electrochem. SOC.139,2 (1992),499 Wiegand, A.; Lerm, A.; Sossna, M.; Kohler, J.M.: Etching Recipes. Unpublished collection of etching instructions of the Physikalisch-Technisches Institut Jena and of the Institut fiir Physikalische Hochtechnologie Jena (Jena, 1981-1996) W m e r s , O.J., Veprek-Heijman, M.G.J. and Giesbers, J.B.: J.Electrochem.Soc. 137 (1990)993-995. Wipiejewski, T. and Ebeling, K.J. : J.Electrochem.Soc. 140 (1993)2028-2033. Wolf, R.; Helbig, R.: J.Electrochem.Soc. 143,3 (1996),1037 Wong, T.K.S. and Ingram, S.G.: J.Vac.SciTechno1.B lO(6) (1992)2393-2397. Wrobel, A.M., Lamontagne, B. and Wertheimer, M.R.: Plasma Chemistry and Plasma Processing 8 (1988)315-329. WU, S., Ho, S.T., Xiong, E and Chang, R.P.H.: J.Electrochem.Soc. 142 (1995)3556-3557. Yamada, M., Nakaishi, M. and Sugishima, K.: J.Electrochem.Soc. 138 (1991) 496-499. Yeh, J.T.C. ;Grebe, K.R. ;Palmer, M.J. :J.Vac. Sci. Technol. A2 (1984),1292; quoted from M.A.Hartney et al. (1989 a) Yih, P.H. and Steckl, A.J.: J.Electrochem.Soc. 140 (1993)1813-1824. Yih, P.H. and Steckl, A.J.: J.Electrochem.Soc. 142 (1995)312-319. Yogi, T.; Saenger, K. ;Puroshotaman, S. ;Sun, C.P.:Proc. of 5th Int. Symp. on Plasma Chemistry (Pennington 1984);216;quoted from M.A.Hartney et al. (1989 a) Young, C. and Duh, J.: Journal of Material Science 30 (1995)185-195. Young, R.J., Cleaver, J.R.A. and Ahmed, H.: J.Vac.Sci.Techno1.B ll(2) (1993)234-241. Zhou, B.; Ramirez, W.F.: J.Electrochem. SOC.143,2(1996),619
Index
A 3d-micromachining 119 accelerators 177 accuracy 24 acetic acid 48 acid-base reactions 38 activated atoms 122 additions 167 additive patterning 6 AgSe 174 alchimists 1 alcoholic solutions 212 AlGaAs 82 alignment angle 96 alkaline solutions 90 alkylradicals 125 alkylammonium hydroxides 91 aluminium 184,80 aluminium gallium phosphide 192 aluminium nitride 201 aluminiumoxide 203 aluminium with titanium additions 188 (aluminium, gallium) arsenide 189 (aluminium, gallium, indium) phosphide 194 (aluminium, indium) arsenide 196 (aluminium, indium) nitride 198 (aluminium, indium) phosphide 199 amines 39 amorphization of silicon 101 amphoteric organic polymers 39 angle 86 angle dependence 161,162 angle of impact 162 anisotropic alkaline etchants 299 anisotropic etching 15,21,84,87, 88, 103 anisotropy 146 anodic current 73
anodic dissolution 82 anodicetching 74 anodic partial process 42 anodical etching 77 antimony 296 a-particles 176 ARDE 149 area of coverage 66 area ratios 66 area resistance 62 argon 144 argon addition 167 Ar-ions 154 arsenosilicate glass 205 art 1 aspect ratio 16, 53, 97, 171 auxiliaryfilms 91 auxiliary layer 19 AZ photoresists 136
B barrel reactor 127 bath composition 92 BCl3 114 BC13-addition 167 bias potential 145 bio-analogous polymers 41 bismuth 207 bond cleavage 123 boron-doped silicon 98 borosilicate glass 208 bowing effect 164 bromate 50 bulk micromachining 107, 108
C GF6 113 cadmiumsulfide 223
362
Index
cadmium telluride 224 CALBE 156 cantilevers 105,107 carbides 167, 169 carbon 209 car0 acid 212 catalytic masks 117 cathodic partial process 42 cellulose 136,214 cerium 174 CF, 113, 135 channels 176 chemical-assisted etching 147 chemical-assisted ion beam etching 156 chemical condensation 165 chemical milling 2 chemical selectivity 134 CI3F3 113 chlorine atmosphere 159 chloromethane 124 chromate 50 chromium 228 chromium sulphuric acid 212 chronopotentiometric constant 58 citric acid 48 cleaning 32 cleaning methods 170 cleaning procedure 34 CW3 114, 117 cobalt chromium 225 cobalt niobium zirconium 226 cobalt silicide 227 cold plasmas 122 collision processes 147 compensations 24,25 complex compounds 45 complexes 30,39 compound semiconductors 103,60, 81 concentration 179 concentration gradient l2,64 conductivity measurements 27 cones 177 constant of complex formation 45 contamination films 169 controllability 9 convection 11, 59 cooling 148 coordinative compounds 45 copper 80,229
corrosion 168 coverage 67 craftwork 1 cryogenic RIE 148 crystal cut 86 crystal symmetry 103 crystallographic direction 86 crystallographic etch stop 95 crystallographic etching 84, 103 crystallographic orientation 86 crystallographic plane 85,86,89 crystallographic unit cell 94 crystobalite 86 current densities 76 curvature radius 19 cylinders 177
D damage zone 176 damaged area 176 DC-field 151 deep-etched window 109 deep-etching 107 defect electrons 79 degree of anisotropy 16,92 desorbability 112, 113 detergents 32 deviations measures 24 diamond 118,210,213 diamond-like layers 167 dielectric materials 35 diffusion 11, 12 diffusion coefficient 132 diffusion constant 11 diffusion control 63 diffusion-controlledetching 11 diffusion-effectivearea 66 diffusion layer 11,63,65 diffusion layer, thickness 12, 64,65 digital etching 34 direct pattern generation 80 direct patterning 121 direct writing 119 dissolution 35 dopant 98 dopant concentration 101 down-stream reactor 128 dry-etching 111
Index
E each rate, enhancement 68 EBRE 158 ECM 74,75 ECR-discharge 130 ECR-source 153, 156 edge effect 65 edge roughness 18,87 EDP 91 EDTA 48 elctron-beam-assisted etching 120 electrical contact 73 electrical shielding 126 electrochemical etching 72, 73, 98 electrochemical machining 74 electrochemical series 53,54,55 electrode potential 44 electron beam 121 electron beam patterning 158 electron cyclotron resonance 153 electronic excitation 122 electronic excited state 123 ellipsometric measurements 27 emission of the plasma 27 EMM 74,75,76 end point control 27 energized particle 137, 176 enzymatic etching 41 enzymes 41 epoxy resin 214 etch grooves, anisotropically 103 etch rate 9, 10,28,&, 737 85, 126, 146, 150, 154 etch rate enhancement 150 etch rate, geometry-dependent 62 etch rate of silicon 299 etch rate ratios 13, 22 etch rate, size-dependent 65 etchstop 299 etch structures 131 etching area 132 etching gas 112, 148 etching process 7, 9 etching selectivity 13, 14 etching temperature 51 etching times 9 etch-stop, electrochemical 98 etch-stop layers 98 etch-stop material 98 etch-stop techniques 97
ethlendiamine 91 ethylene diamine etchant 299 excimer lasers 118 excited particles 112 excited states 138 externally forced convection 13 F facetting 164 Faraday cage 126 Faraday law 43 Fenni level 83 FIB 159 FIB-process 160 field emission sources 153 film thickness 27 fission 176 flank 19, 87 flank geometry 18,21 flexible structures 107 float potential 141 fluid cell 76 fluor 113 fluorideion 47 fluorides 113 fluorine 135 fluorine donators 114 fluorocomplexes 47 focussed ion beams 159,161 free path length 147 free-standing membranes 95 free-standing micropatterns 105 frequency ranges 130
G GaAs 82, 83, 86, 104 gallate 48, 92 gallium antimonide 246 gallium arsenide 233 gallium gadolinium oxide 244 gallium indium arsenid 238 gallium indium phosphide 240 gallium nitride 242 gallium phosphide 245 galvanic effect 69 GaP 104 GaSb 82 gelatine 41 geometry 93, 131, 161 germanium 81,248
363
364
Index
germanium silicide 251 GeSez 174 glassesceramics 134 glassy carbon 136 gleaming plasma 141 glow 123 gold 81,206 gradation 2 grain boundaries 87
H H-abstraction 136 hafnium 253 halides 113, 117 halogen radicals 118 halogenoalkanes 114 halogens 168 heat generation 126 heat removal 139 hexafhoroethane 124 HF-amplitudes 146 HF-field 152 high-frequency generator 125 high-frequency sources 152 high resolutions 121 hollow conductor 128,152 hot spot 138 hybrides 113 hydrazine 91 hydrocarbon radical 135 hydrocarbons 167, 169 hydrofluoric acid 100 hydrogen peroxide 50 hydroxide ion 46 hydroxides 38 hypochlorite 50
indium arsenide 255 indium gallium nitride 257 indium nitride 258 indium phosphide 259 indium telluride 269 indiumtin 266 indium tin oxide (ITO) 267 inorganic dielectrica 134, 171 inorganic resists 173 InP 82,83,104 in-situ measurement 27 interhalides 117 interlinked polymers 40 ion beam assisted etching 160 ion beam etching 150 ion beam microprobes 177 ionbeams 176 ion current densities 142 ion energy 142 ion generation 150 ion sources 152 ionvelocity 142 ion, energy 142 iridium 81 iron 231 iron nickel 232 isopropanol 91 isotropic etching 14, 19 isotropic wet chemical etching 53
J
E M 78 jet electrochemical etching 78 jet electrochemical micromachining 78 joule heat 126 K
I illumination 99 imidazoles 39 imides 39 impact craters 161 impact site 138 impacting particles 139 impacts 137 implantation 98 impurities 31 InAs 82 incomplete etching 169 indium antimonide 264
kapton 217 Kaufmann source 152 kinetic energy 125,137
L laser ablations 120 laser-assisted etching 80,119 laser induced fluorescence 27 laser scanning etching 119 lead 287 lead lanthanum zirconate titanate 289 leadsulphide 288 lead zirconate titanate 291
Znkx
Lewis acids 46 Lewis bases 46 lexan 216 lifetime 126 lift-off technique 6 ligand concentration 50, 51, 58 ligands 45 light 59 light-assisted charge separation 82 lithium gallate 273 lithium niobate 275 lithium aluminate 271 lithium metasilicate 175 lithography 2 loading effect 131, 149 local activation 60 local currents 62 local element 60,61 local element effect 68
M magnesium 276 magnetic field 130, 150, 156 magnetron 130 magnetron electrode 150 magnetron sources 153 mask 5,7, 74 mask edges 93,164 maskingfilm 5 mass spectroscopy 27 material defects 168 mechanical actuators 106 mechanical cleaning 32 membranes 108 mercury telluride 254 MERIBE 156 M E R E 150 meshs 126 metals 41 methacrylate polymer 136 microbridges 105 microchannel 104 microchannel plates 171 microelectrodes 65 micromachining 105 micromechanics 7 microwave excitation 128, 130 microwave source 152, 157 mixed potential 62,68 molecular films 35
molecular materials 35 molybdenum 277 molybdenum silicide 279 monitoring 26 monochloro-pentafluoroethan 118 monochromatic light 83 monocrystalline materials 85 monocrystalline metals 87 moving 65 multilayer systems 60 multi-ligand complexes 46 mylar 216
N nanoparticle beam etching 160 nanoparticles 160 nanopores 102 nanoporous materials 104 nanoporous silicon 102 nanoprobe 77 nanoprobe technique 78 NCL, 114 Nd: YAG-laser 118 negative patterning 118 Nernst equation 44 networked materials 136 neutral particles 155 neutrons 176 nickel 283 nickel chromium 284 nickel manganese antimonide 286 niobium 81,281 niobium nitride 282 nitrides 167,169 noble gases 144 non-metals 37 non-oxidizing etching 39 novolaks 91,214 nozzle 78 NPBE 160 nuclear reactors 176 nylon 216 0 o-hydrochinone 48 oil films 32 oils 169 optoelectronic devices 104 organic polymers 136,212,214 organic residues 33
365
366
Index
organic solvents 212 organyles 113 outer-currentlessetching 41, 42 overetching 19 overetching-time 19 oxidation states 52 oxidative decomposition 40 oxidative etching 40 oxides 38 oxidizing agents 55 oxidizing liquids 40 oxoanions 46 oxohalides 113 oxoion 46 oxygen addition 167 oxygen plasma 124, 134, 135 oxygen-containingchelate ligands 47
P parallel plate reactors 146 partial anisotropic etching 23 partial current density 42, 44 partially undercutting 23 particle 122 particle density 112, 151 particle energy 151 particle reflections 162 particle sedimentation 32 particle trace etching 176 particle yield 161 passivating coating layers 46 passivating film 48 passivating film, formation 49 passivation 57 passivation, geometry-dependent 69 passivation, outer-currentless 70 passivation, risk 51 passivation, tendency 50, 51 passivation, transport-controlled 69 passivation, undesired 51 passivity 48 pattern topology 62 patterning 6, 9, 173 p-chinone 92 PEC 80 perchlorate 50 percloroacetic acid 48 perhalogenated hydrocarbons 118 permanganate 50 peroxides 135
persulphate 50 phenolate 92 phenolic aromates 48 phenolic resins 39 phosphosilicate glass, PSG 292 photo-assisted dry-etching 118 photochemical etching 79,80 photochemical machining 29 photo-damaged areas 175 photoelectrochemical etching 80,82,98 photoelectrochemical fabrication 101 photoelectrochemical methods 99 photoresist 114, 215 photosensitive glasses 174 photostructurable glass 175 pH-value 50,51 piezodrive 77 planar-plate reactor 129, 140, 145 plasma 111, 126 plasma ashing 126 plasma density 125 plasma etching 122, 123, 145, 146 plasma etching, magneton-assisted 130 plasma-free dry-etching 116 plasma generation 125 plasma jet etching 133 plasma sheet 141, 142 plasma stripping 136 plasmatron sources 153 platinum 81, 87,294 polyamide 216 polycarbonate 216 polycyan 213 polydivinyl benzene 136 polyester 216 polyethylene 217 polyimide 136, 217 polyisoprene 219 polymers 36,212 polymethyl glutarimide 219 polymethyl methacrylate 219 polystyrene 136,219 polytrialkoxy silyl norbornene 222 polyvinyl benzal 220 polyvinyl carbazole 220 polyvinyl chloride 221 polyvinyl formale 221 polyvinylidene fluoride 136,221 polyvinylolactone 136, 221 polyvinyl pyrrolidone 221
Index
pore network 101 porosity 100 porous silicon 100, 102,109,110 potassium titanyl phosphate 270 potential drops 75 potential gradients 62 power 179 power density 179 preferential etching 84 pressure 143, 179 process velocities 11 profile formation 162 proteases 41 proximity effect 25 pseudohalides 48 pulse method 74 pyramid 88 pyramidal etching grooves 94 pyrazine 92 pyrimidines 39 pyrines 39 pyrocatechol 48,91,92 pyrogallol 92
Q
quartz
86
R radicals 116, 122, 124, 125, 126 rare gas halides 117 reaction control 63 reaction probability 112 reactive etching 157 reactive gases 116, 117 reactive ion beam etching 155 reactive ion etching 144, 145, 146 reactive neutral gas etching 123 reactive vapours 116, 117 recipes 179 recipient 112 redeposition 164 redox mediators 89 redox-process 52 relief 162 removal of passivating layers 31 repairing 159 reproducibility 9 residual oxygen 167 residue layers 169 resist 173
367
resistance layers 62 rhenium 81 rhodium 81 RIBE 155 RIE-lag 149 ruthenium dioxide 295 S
sacrificial film 105 sacrificial layer 16, 105, 107 sacrificial material 109 salt-like materials 37 scanning electrochemical microscope 77 SECM-Etching 77 selective cleaiing 34 selectivity 53, 145, 147 selectivity by passivation 56 selectivity, mutual 56 selectivity of silicon etching 300, 301 self-biasing 101, 141 semiconductors 41,81 SF, 114 Si 102 Si& 102 Sic 82 Si-chip, monocrystalline 94 SiCL 114 sidewall depositions 166, 168 sidewall passivation 164 sidewall profile 19 sidewall redeposition 165 sidewalls 162 silicon 81,82,86,88,297 silicon carbide 307 silicon deep-etching 91 silicon dioxide 313 silicon etching, anisotropical 90 silicon nitride 313 silicon oxynitride 318 silicones 137 siloxanes 137 silver 81, 174, 182 silver nuclei 174 SiO, 102 size effect 64 slope angles 19 soft cations 47 solubility products 64 solvate shell 30 solvent 30
368
Index
sources for deviations 25 spatial separation 150 spinning l2 spontaneous convection 12 spraying l2,65 springs 105 sputter effect 137 sputter etching 137, 140, 146 sputter rate 142 sputter reactor 139 sputter threshold 138 sputter yields 162 standard potential 53 steel 81, 231 sticking 107 stirring l2,65 stripping 36, 136 sublimation 107 sublimation heat 143 substractive patterning 6, 9 succinicacid 48 sulphonic acid 39 surface 30 surface contaminations 31 surface control 84 surface micromachining 105, 106 surface properties 73 swelling 37 synchrotron radiation 119
T tantalum 321 tantalum nitride 323 tantalum oxide 324 tantalum silicide 325 tantalum silicon nitride 327 tartaricacid 48 tellurium 328 temperature 59, 92 tetrafluorosilicon 124 thermal conductivity 139 thermal laser ablation 119 thermal stress 126 thermalizing 143 thin film membranes 108 thin film structures, freestanding 108 through-mask electrochemical micromachining 74 tin 319 tindioxide 320
titanium 81, 329 titanium dioxide 332 titanium nitride 331 tongues 106 topology 25 transient range 57 transition time 57,59,70,71 transpassive dissolution 46 transport 10, 11,63 transport control 11, 57, 64 transport of ligands 64 transport processes 132 trapezoidal crossection 88,93,94 trenching 162,163 triangular 88 tridymite 86 trifluoromethane 124 tungsten 81,334 tungsten silicide 338 tungsten trioxide 174,337
U undercut 94,95 undercutting 15,19,23,67,68,93,148 undercutting rate 96
V vanadium 333 vapour etching 117 vertical edges 21 V-groove 93,94 vibrational excitation 123 W weaker Lewis bases 47 wet chemical etching 28 working electrode 72
X XeF2 117 XeF3 114 XeF6 117
Y yttrium barium cuprate 340
Z zinc 341 zincoxide 342 zinc selenide 344 zincsulfide 343 zirconium 81
