Measurement and Analysis of Radiation Effects in Devices and ICS

IEEE Nuclear and Space Radiation Effects Conference Short Course “Measurement and Analysis of Radiation Effects in Devic...

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IEEE Nuclear and Space Radiation Effects Conference Short Course “Measurement and Analysis of Radiation Effects in Devices and ICS”

July 13,1992 Hyatt Regency Hotel New Orleans, Louisiana Sponsored by IEEE NPSS Radiation Effects Committe Cosponsored

by

Defense Nuclear Agency Jet propulsion Laboratory Sandia National Laboratories

Nuclear

29th International and Space Radiation Effects Conference

SHORT COURSE

Measurement

and Analysis of Radiation Effects in Devices and ICS

New Orleans, Louisiana July 13, 1992

@ 1999 IEEE

Table of Contents

Foreword Biographies

.............................................................................................................. ii ... ............................................................................................................ 111

Chapter 1 Practical Dosirnetry for Radiation Hardness Testing Klaus G. Kerris, Harry Diamond Laboratories

.................................... I 1-50

Chapter 2 Total-Dose Radiation Effects (From the Perspective of the Experirnentalist) ............................................. II 1-87 Peter S. Winokur, Sandia National Laboratories

Chapter 3 Measurement of Single Event Phenomena in Devices Fred W. Sexton, Sandia National Laboratories

and ICS .................. III 1-55

Chapter 4 Displacement Damage: Mechanisms and Measurements Geoffrey P. Summers, Naval Research Laboratory

.......................... IV 1-58

Foreword

Over the last several years, many advances have been made in our understanding of the mechanisms controlling semiconductor device response in radiation environments. Key to this understanding has been the development of new and innovative measurement and analysis techniques that are providing a more detailed picture of the interaction of energetic photons and particles with semiconductor materials. Because many of these techniques are relatively new, I thought it would be appropriate this year to focus on the experimental aspects of radiation effects. With this in mind, the theme of this year’s Short Course is “Measurement and Analysis of Radiation Effects in Devices and ICS. ” Before we can understand how to properly measure an effect, we must fuxt lay a foundation in the underlying mechanisms that control device response, We will build on this foundation with detailed descriptions of the measurement techniques that are available and how they are properly used. Our goals for this short course are to: ●

describe the mechanisms controlling device response in the use-environment

●

describe how device response is measured

●

describe how data from radiation effects measurements

is analyzed

Of course, any measurements we take are dependent on how well we have characterized the radiation environment itself. We begin this year’s short course with a session on practical dosimetry. We follow that with three presentations specific to the response of devices to the natural space environment; that is, total-dose radiation effects, single-event phenomen~ and displacement damage effects. I hope you will fmd the information presented here useful and timely as you pursue your chosen field of research. I want to express my gratitude to the short course presenters, who have labored with this work for the last year. Their diligence is evident in the quality of the following session notes. I also want to thank Mayrant Simons of RTI for giving me the opportunity to organize this year’s short course. It’s been a rewarding experience. Finally, thanks to Lew Cohn of DNA for his able guidance of this work through the classitlcation review process, and LtCol, Al Constantine of DNA for his assistance in printing this notebook,

Fred W. Sexton Sandia National Laboratones Albuquerque, NM

ii

Biographies

Klaus G. Kerris Klaus G. Kerris received his B.A. and M.A. in Physics from UCLA in 1957 and 1959, respectively. Currently, Klaus is a physicist in the Simulation Technology Branch of the Han-y Diamond Laboratories. At HDL, Klaus has been in charge of dosimetry and radiation physics at the Aurora Pulsed Radiation Facility since 1971. In addition to his work at Aurora and the other hardness testing facilities at HDL, Klaus manages several ongoing Army R&D programs. He is active in dosimetry standards development within the ASTM, and is a member of IEEE. Peter S. Winokur Peter S. Winokur received his B.S. degree in Physics from The Cooper Union in 1968, and M.S. and Ph.D. degrees in Physics from the University of Maryland in 1971 and 1974, respectively. From 1968 to 1983 he was a research physicist at the Harry Diamond Laboratories in Washington, D.C. He joined Sandia National Laboratories in 1983 and worked on techniques to characterize and improve the radiation hardness of CMOS technologies. In 1987, he became supervisor of the Radiation Technology and Assurance Division, with responsibilities for radiation physics and hardness assurance activities in support of microelectronics used in space and defense applications. Peter has authored over fifty scientific publications dealing mainly with ionizing radiation effects in semiconductor devices and circuits. He has actively participated in IEEE activities throughout his professional career, having served as Technical Program Chairman for the IEEE Nuclear and Space Radiation Effects Conference in 1989 and for the IEEE Interface Specialists Conference in 1985. He is presently the Vice Chairman of the IEEE/NPSS Radiation Effects Committee. Peter is a member of the American Physical Society and is an IEEE Fellow. Fred W. Sexton Fred W. Sexton received his B.S. and M.S. degrees in Electrical Engineering from the University of Arkansas in 1976 and 1978, respectively. In 1978 he joined Sandia National Laboratories as a Member of Technical Staff and has worked in the areas of silicon solar cell development, IC technology development, IC manufacturing, and radiation effects. He has worked in radiation effects in microelectronic devices for ten years, and has more than 38 technical publications to his credit His current role is project leader for the Single Event Phenomena program at Sandia. He has served as session chairman for the Nuclear and Space Radiation Effects Conference and has been a technical reviewer for both the NSREC and HEART conferences. He won the 1984 NSREC and 1985 and 1990 HEART Outstanding Paper Awards and is a member of IEEE. Geoffrey P. Summers Geoffrey P. Summers is head of the Displacement Damage Effects Section in the Radiation Effects Branch at the Naval Research Laboratory, and chairman of the Department of Physics at

...

111

the University of Maryland, Baltimore County. He received his B. A., M.A. and D. Phil. degrees in physics at Oxford University in England in 1965, 1967, and 1969, respectively. He joined NRL in 1985 after being chairman of the Department of Physics at Oklahoma State University for six years, where his research involved the optical and electrical effects of radiation- and thermochemically-induced devices in oxides. His recent research involves the properties and effects of radiation-induced defects in semiconductor, superconductor, and dielectric materials, micro-electronic and opto-electronic devices, and most recently, biological systems. He has co-authored more than 70 scientific journal articles. Dr. Summers is a member of the IEEE and the American Association of Physics Teachers.

iv

Practical Dosimetry for Radiation Hardness Testing

Klaus G. Kerris Harry Diamond Laboratories

This work was supported in part by the Defense Nuclear Agency.

Contents

1.

Introduction

2.

Dosimetry

3.

I-2 I-3

Theory

I-3 I-3 I-4 I-5 I-5 I-5

2.1

Definitions 2.1.1 Absorbed Dose 2.1.2 Absorbed Dose Rate 2.1.3 Particle Fluence 2.1.4 Energy Fluence 2.1.5 Linear Energy Transfer (LET)

2.2

Radiation Which Produces Primarily Ionization 2.2.1 Charged Particle Equilibrium 2.2.2 Equilibrium Absorbed Dose 2.2.3 Bragg-Gray Cavity Theory 2.2.4 Non-equilibrium Dose Deposition. Dose Enhancement.

I-6 I-6 I-8 I-9 1-10

2.3

Radiation Which Produces Primarily Displacements

1-10

Practical

1-12

Dosimetry

3.1

Measurement of Ionizing Radiation Dose 3.1.1 TLDs 3.1.2 Calorimeters 3.1.3 Dyed Plastic Dosimeters 3.1.4 PIN Diodes

1-12 1-12 1-18 1-19 1-21

3.2

Measurement of Ionizing Radiation Dose Rate 3.2.1 Silicon PIN Diodes 3.2.2 Compton Diodes 3.2.3 Scintillator-photodetectors

1-21 I-22 I-23 I-24

3.3

Neutron Dosimetry

I-26

3.4

Dosimetry for Single Event Upset Testing 3.4.1 Heavy Ion Dosimetry 3.4.2 Proton Dosimetry

I-28 I-28 I-29

References

1-31

Appendix A. Quick Reference Handbook of Practical Dosimetry Techniques

I-36

Appendix B. Commercial Sources of Dosimeters and Dosimetry Instrumentation

1-50

I-1

1.

Introduction

This short course session deals with radiation dosimetry from the point of view of the experinuentalist. To that end, this session, after a brief introduction, is divided into two parts: a brief, but quite necessary review of dosimetry theory, followed by a section on practical dosimehy. This latter section should emble the experimentalist to evaluate the quality of dosimetry data furnished him by others, as well as enabling him to do his own dosimetry by guiding him to the appropriate references. I have endeavored to include references to ASTM and other standards wherever appropriate since adhenmce to such standards is an increasingly important requirement in most hardness testing situations. Finally I have provided an appendix that summarizes all the important information from the body of this session on single, handbook-style pages for quick reference, plus a list of commercial sources in the USA for dosimeters and dosimetry instrumentation. Dosimetry is the study of how dose is deposited in matter. Dose is the energy deposited by radiation per unit mass of material in a small volume of interest. The central problem of dosimet~ is that it is in general quite impossible to measure the energy deposited in a region of interest by any direct means. The problem that practical dosimetry must solve, therefore, has two parts: first we measure a quantity that we can measure more or less directly, and secondly, we deduce from this measurement the quantity that we are really interested in. Specifically then, practical dosimetry is almost always a distinctly twestep

process:

1. Measurement of the dose deposited in a reference material in a measuring device; that is, in a dosimeter. 2. Deduction of the dose in the material of the region of interest (that is, the device under test) from the dose as measured by the dosimeter. In the section on practical dosimetry I will always endeavor to give practical guidance on how to accomplish both of these steps, first by describing techniques for making accurate and precise dose or dose-rate measurements with a dosimeter, and then by indiuting how to configure the measurement process so that the dose in the test deviee can be dedueed from the dosimeter dose with a minimum of error. Radiation hardness testers often think of dosimetry as being an art that lies somewhere between light and darkness; as something in the Twilight Zone. I hop to demystify dosimetry in this short course session; to show that, to the contrary, dosimetry is really an exact science. However, practitioners of dosimetry should also be aware of William’s Law of Dosimetry, as quoted by Bill McI-aughlin The man with one dosimeter thinks he knows the dose, the man with two has only a ballpark estimate and begins to wonder, and the man with three realizes he doesn’t know, but can make a better judgement than the other two. [1]

I-2

2. 2.1

Dosimetry

Theory

Definitions

Since I promised to present dosimetry as one of the exact seienees, it is neeessary to define the basic cmcepts rigorously. By and large I will follow here the approved nomenclature of the ICRU [2] and the ASTM [3], with some allowances for common usage as evideneed in the literature. 2.1.1 Absorbed Dose: The mean energy absorbed per unit mass of irradiated material at the point of interest, P. If AE~ is the mean energy imparted by ionizing radiation to matter of mass Am, then: D= AE@rn. (1) As is evident from Figure 1, if AE~ is the energy of ionizing radiation entering the mass element Am and ~ is the energy leaving it then ~ = AEE - ~.

Figure 1. Definition of Absorbed Dose.

Absorbed dose is also frequently called dose or total dose in the literature. The customary unit of dose is the rad: 1 rad = 100 erg/g The S1 unit of dose is the gray (Gy): 1 Gy = 1 J/kg lGy=100rads Although the use of the gray has been strong] y advocated by the various standards organizations and Government agencies, the rad seems still to be used in actual practice by a large majority of radiation effects experiment.m at this time. Since the absorbed dose depends on the material of interest, the speeific material should always be referenced in parentheses right after the name of the unit; e.g., md(Si), Gy(GaAs), etc. The absorbed dose, as defined in Equation 1, is a macroscopic quantity; that is, it can only be defined for small but non-zero values of Am. If we imagine the mass Am surrounding the point P

I-3

to start at a fairly large value and consider the quotient AE~Am as Am is reduced, we will generally find that this quotient slowly increases and then rmches a constant value. It is at this constant plateau that D is defined by Equation 1. If we now imagine Am to be reduced still more, we will find that AE~Arn will begin to vary statistically about D, with the variation increasing as Am is reduced. For very small Am, AED/Arn will be zero in the majority of cases, but when different from zero it can he orders of magnitude larger than D. [4] This stochastic nature of microdosimetry can become important in the radiation hardness testing of microelectronic circuits containing very small cinxit elements. [51 2.1.2 Absorbed Dose Rate: The time rate of change of the absorbed dose. the increment of absorbed dose in the time interval d~ then: D=dD/dt

If we let dD be

(2)

Absorbed dose mte is often also simply called dose rate in the literature.

D

Figure 2. Definition of Absorbed Dose Rate. Note that the absorbed dose rate, D, is a function of time. Although radioisotope sources such as Co~ and Cslsv irradiators have such a slowly varying dose rate that it can be assumed to be constant for the duration of a hardness test, the absorbed dose rate delivered by pulsed radiation sources such as flash x-ray generatom and LINACS changes very rapidly with time as illustrated in Figure 2. In this case, the term ‘dose rate’ is generally taken to mean the absorbed dose rate at the peak of the pulse, D~= in Figure 2. An important consequence of Figure 2 is the relationship D.m/D=Hm/A

(3)

The implication of Equation 3 is that if one has an uncalibrated dose rate detector of some kind, and if one also knows the absorbed dose at the location of the dose rate detector (by measurement with a dosimeter) then the @ dose rate is given by

14

Dm== D . (~m/A)

(4)

(I+~,X might be measured in amperes, for example, and A in coulombs.)

It is evident that the

quantity (A/~z) has dimensions of time, and we can consider ten= (A/HH) to be the effective pulse width of the radiation pulse. Note that Equation 4 is exact, whereas the frequently used approximation D~= = D / FWHM is not, since the full-width-at-at-half-maximum, general not equal totem as defined above. 2.1.3 area.

Particle Fluence:

The number of particles incident on a sphere of unit cross-sectional (cm-~

a=dNldA 2.1.4

Energy Fluence:

FWHM, is in

(a

The mdiant energy incident on a sphere of unit cross-sectional (MeV-cm-Z)

w=dEldA

area. (Q

2.1.5 Linear Energy Transt’er (LET): LET is the important dosimetric parameter used in the study of single-event upsets (SEU). The strict textbok definition of LET is energy per unit path length transferred to matter by a charged particle. [6] The units of LET are energy per unit distance; commonly, MeV/pm for heavy particles. (This quantity is what we used to call dE/dx back in nuclear physics class.) The S1 unit of L~ would be J/m; it is seldom used. However, the term LET is defined in a variety of different ways in the SEU literature, so it is important to ascertain which definition is meant. The vtious commonly used definitions are listed below; [71 it is evident that they are very similar Mass stopping power, (dWdx)/p Energy deposited per unit path length hnwr Charge Deposition (LCD) Electron-hole pairs p unit path length

(MeV-cmVmg) (MeV/pm)

(m (8) (9) (lo)

(@pm) (pm)-l

A related quantity which is often seen in the literature ix ‘Effective LET’ for a thin detector

LET*

where (3refers to the angle of incidence of the energetic particle.

I-5

= L~-sec

6

(11)

2.2

Radiation

Which

Produces

Primarily

Ionization

Radiations which produce primarily ionization are photons (x-rays, gamma rays, bremsstrahlung) and charged particles. Heavy charged particles are primarily of interest for SEUS which will be treated later; in this section the only charged particles which will be considered are electrons. 2.2.1

Charged

Particle

Equilibrium

An important concept which it is necessary to understand when considering photon dosimetry is that all of the energy imparted to matter by ionizing radiation is deposited by electrons. Photons do no impart energy directly to matter (i.e., to atoms). Photons can only transfer energy to electrons, which can in tum transfer their energy to atoms. This is illustrated in Figure 3 which shows in diagrammatic form all of the interactions which can transfer energy between photons, electrons, and matter. [8] Note that there is no physical process which transfers energy from photons directly to matter. Energy deposition by photons is therefore always a two-step process: ● An energetic photon imparts some energy to an electron, ● The electron may depsit part or all of its kinetic energy to matter.

Photoelectric Absorption I

Y

Compton Scattering

Fluorescence

Photoelectrons

EXCITATION ENERGY OF ATOMS

4

KINETIC

‘“%”

Ionization Delta Rays ➤ (Knock-on Electrons)

ELECTRONS

*

Figure 3. The Interaction of Photons and Electrons with Matter

I-6

Pair Production Annihilation

o

3J.J-=E!!E+

Auger Elw.ons~

o

Annihilation in Flight Bremsstrahhuw

REST ENERGY OF PAIRS

This leads to the most important concept of photon dosimetry: charged particle equilibrium, or CPE. Most real-world dosimetry problems are so complicated that it is not possible to formulate them using a simple mathematical model. However, there is one special case which is not only easily manageable theoretically, but which is applicable to a large class of important practical dosimetry problems. This is the case of charged particle equilibrium. Assume that the incident radiation consists only of photons and electrons, and that all secondary particles are either photons or electrons. This is true for the photon energies used in most radiation hardness testing situations. Recall the definition of absorbed dose: (1)

D = AE#rn. Using the concepts introduced in Figure 1 we can rewrite this equation:

(12)

D = (AE~ - AE#Mn If all the dose deposited in Am is from photons and electrons, then ~=

AE~y) - A%(y) +@(e)

(13)

- ~(e)

where the symbols (y) and (e) refer to photons and electrons respective y. The condition of Charged Particle Equilibrium (CPE) exists (by definition) when the total energy carried out of the mass element Am by electrons is equal to the energy canied into it by electrom, that is

Substituting Equation 14 into Equation 13 yields: [for CPE]

(1s)

The physical meaning of charged particle equilibrium is easy to visualize in this case. Consider a thick slab of some homogeneous material with a beam of photons incident on it, see Figure 4. We see that an element of volume, Am, Imated near the front face of this slab would have more electrons scattered out of it than are w.attered into it. Hence there is a net loss of energy carried by eleclrons and charged particle equilibrium does not exist. As we imagine volume elements Am farther into the material slab we eventually reach a depth (Wprofimakly eqti to the range of the most energetic secondary electron) where charged particle equilibrium is achieved. This minimum thickness of material requiml to achieve charged particle equilibrium is commonly called the “equilibrium thickness”. (The equilibrium thickness depends

I-7

on the nature of the material and on the energy spectrum of the radiation. ) The condition of charged particle equilibrium applies for this, and greater, material thicknesses.

b

e Photon Beam

\ No CPE here

CPE exists here

>

Figure 4. Illustration of Charged Particle Equilibrium (CPE).

We can now define the equilibrium absorbed dose by combining equations(1)

[for CPE]

- AEL(Y)] / Am Dq = [AEE(Y) 2.2.2

Equilibrium

Absorbed

and ( 15): (16)

Dose

AEL(Y)is related to AE~(y) by the photon absorption equation:

AEL(Y) = A%(y)exp[-(!.%h)

~ ‘x]

( 17)

where pe~/p is the mass energy absorption coefficient of the material of interest, p is its density, and Ax is its thickness. Since the quantity approximation:

in square brackets

is usually quite small, we can use the Taylor

series

exp[-x] = 1- x+ . ..

(18)

AEL(Y) = AEE(Y) - AEE(Y) (p../p) P ~

(19)

Equation (17) then becomes

Using the two relations

p= Aml AAAx

where

element, and

WY= AE~(y) I M

where YY is the photon energy fluence

I-8

AA is the area of the volume

(MeV/cm~, equation ( 19) can be rewritten (20)

AE~(y) - AE~(y) = !PY(P~~p) Am Combining equations (16) and (20) gives us the important result that [for CPE]

D eq = Wy (1%1/P)

(21)

This implies that if we have two different materials, 1 and 2, exposed to the same photon energy fluence, WY,and if (and only if) charged particle equilibrium exists, then the equilibrium

doses in

the two materials are related by

D(J1) 1 D.q(2) = (P.tI/P)I

/

(P,U1P)2 [for CPEI

(22)

Equation 22 shows why the concept of equilibrium dose is important to practical dosimetry. For if we cart measure the equilibrium dose in some reference material, 2, then we can calculate what the equilibrium dose in some other material of interest, 1, would be in the same radiation field by using Equation 22. If the measurement in material 2 is not a measurement of equilibrium dose, then we can say nothing about the dose in material 1. It is therefore crucial to be able to measure the equilibrium dose in a reference material. 2.2.3

Bragg-Gray

Cavity

Theory

From the foregoing discussion of equilibrium dose it is clear that in order to measure equilibrium dose with a detector (a TLD, for example) that detector must be surrounded with an equilibrium thickness of the same material as the detector. Since it is frequently impractical to make this equilibrium jacket of the same material as the detector, we need an expression which relates the dose measured by the detector, D~et, to the equilibrium dose in the wall material, DW.ll. This relationship is described by Bragg-Gray cavit y theory. [9] Two limiting cases are easy to describe. Let Td~t be the detector thickness and TWall be the equilibrium thickness of the wall material which depends on the photon energy, ~. Case 1: At higher photon energies where Td,t << TWdl:

Dwall = [&et(Ey)

1 %. II(EY)l‘det

(23)

where

~ is the photon energy, Rd~tis the range of electrons of energy ~ in the detector,

and

RWdlis the range of electrons of energy EY in the wall material. Electron range tables are found in standard references. [10, 11]

I-9

Case 2: At lower photon energies where Tdet > TWdl: If CPE exists in the wall material, then CPE exists in the detectaor material as well since Td~t > TWdl, and the dose in a reference material can be found from the detector dose by equation 22:

where

Dr.f =

[(P.n/P)r.fi

(kr@)det

is the mass

(~enlp)detl

-’w

(24)

‘det

absorption

coefficient for photons

of energy EYin the detector material, and

(P~n/P)Al is the mass energy absorption Coefficient for PhOtOnS of energy EYin the wall material. Tables of pe,/p are found in standard references. [12, 13]

Although neither of these two limiting cases is usually achievable in practice, it is nevertheless possible in many cases to configure an experiment so that either Equation 23 or Equation 24 is applicable. These situations are described in section 3 on practical dosimetry. 2.2.4

Non-equilibrium

Dose Deposition

and Interface

Dose

Enhancement

One final dose related phenomenon must be discussed here which is of great importance to the process of deducing the absorbed dose in a region of interest in a device from an equilibrium dose measurement made by a dosimeter. This is the phenomenon of interface absorbed dose enhancement (also frequently called interface dose enhancement, or simply dose enhancement). Consider a layered structure of two dissimilar materials, say silicon and gold, each several equilibrium thicknesses thick, exposed to a beam of photon radiation. Applying the equilibrium dose relationship, Equation 22, to this situation, we would expect the dose in the gold layer to be higher than the dose in the silicon layer by the ratio of the respective mass energy absorption coefficients, and we would expect there to be a discontinuity in the dose at the interface. However, Equation 22 does not include the effects of non-equilibrium electron transport across the interface. When this is properly taken into account, we find that the dose in the Si layer near the interface is higher than predicted by Equation 22, while the dose in the gold layer near the interface is lower than predicted by Equation 22. The ratio of the actual dose at a point in a material to the equilibrium dose in that material is called the Dose Enhancement Factor, FDE. The dose enhancement factor is of importance in semiconductor device testing because the presence of high atomic number materials near an active region of a device can make it very difficult to relate the actual dose in that region to the dose measured by a dosimeter which measures equilibrium dose. [14, 15, 16, 17, 18, 19] In a typical semiconductor

device most of the interfaces are between layers of dissimilar materials

1-10

which are much thinner than an equilibrium thickness. In that case all electron transport is nonequilibrium, and the energy deposited at or near an interface can come from electrons which originated in materials which may be several layers removed from the interface in question. Calculating the dose in such a device from the equilibrium dose as measured by a dcxsimeter is very difficult; however, the section on practical dosimetry gives some procedures which may be employed to minimize dose enhancement effects and the attendant dosimetry uncertainties.

2.3

Radiation

which

produces

primarily

displacements

Radiations which prduce displacement damage are neutrons and heavy charged particles. Heavy charged particles are primarily of interest for SEUS which will be treated later; in this section the only particles which will be considered are neutrons. The quantity of interest in neutron hardness testing is the neutron particle fluence, or more specifically, the 1 MeV equivalent neutron fluence. The unit of neutron fluence which is commonly employed is rdcmz. The 1 MeV equivalent neutron fluence is that neutron fluence which produces the same amount of displacement damage in Si as if all neutrons had an energy of 1 MeV. Expressing the neutron fluence in terms of 1 MeV equivalent neutron fluence is a convenient way to remove the effects of variations in the neutron spectrum which occur among neutron sources. While neutrons are usually accompanied by ionizing gamma radiation (prcxked by n,y r=tions in air and sun-ounding materials), their effects can be considered separately, and neutron and gamma dosimetry can be done independently. The theory which applies to the gamma component has already been covered in section 2,2. The measurement of 1 MeV equivalent neutron fluence is covered in the following section 3 on practical dosimetry.

1-11

3. 3.1 3.1.1

Practical

Dosimetry

Measurement

of Ionizing

Thermoluminescent

Radiation

Dosimeters

Dose (TLDs)

By far the most widely used dosimeter in radiation hardness testing is the TLD. Reasons for this are not hard to find. TLDs have a dose range from 10-2- 1@ rad (10-4 - 1(P Gy), they are small, passive, inexpensive, relatively easy to use and evaluate, retain dose information for a long time following expmure, and can be configured to have an energy response similar to that of Si in most applications. Many materials are available for thermoluminescence dosimetry; however, the favorites for radiation hardness testing are Lithium Fluoride (LiF), Manganese activated Calci urn Fluoride (CaF2:Mn), and Dysprosium activated Calcium Fluoride (CaF2:Dy). Dosimeters are available as powder, chips made from pol ycrystalline material, and discs consisting of very fine powder uniformly dispersed throughout a polytetrafluoroethylene (PTFE, Teflon) matrix. A commonly used size of chip is 3.2 mm x 3.2 mm x 0.9 mm (0, 125” x 0.125” x 0.035”). A commonly used size of Teflon dosimeter is a disc, 6 mm in diameter and 0.4 mm thick.

I

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ti7 eV ,,

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LUMINESCENCE 1 CENTER u-

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(a) Band Diagram CONDUCTION

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/&zii&%i%zz4

(C) Readout

(b) Irradiation Figure 5. The Thermoluminescence

1-12

Process in LiF TLD Material

All thermoluminescent materials consist of a crystalline insulator with added dopants which introduce stable electron traps into the forbidden band gap. Ionizing radiation creates electrons and holes which are trapped by stable traps in the band gap. The density of filled traps is proportional to the dose absorbed by the material. Subsequent heating of the material empties the electron traps, allowing electrons from F-centers to recombine with free holes at luminescence centers, emitting light. The integrated light output is proportional to the density of filled traps, and therefore to the absorbed dose in the TLD material. This process is illustrated in the energy band diagram in Figure 5 for LiF [20]; the process is similar for all TLD phosphors. To make an equilibrium dose measurement, the TLD must be enclosed in an equilibrium capsule of the appropriate material and thickness when it is exposed. A general method for determining equilibrium capsule thickness is given in ASTM Standard E665 [21]. Readout of TLDs is accomplished by an instrument consisting of a heater, optical system and photomultiplier detector to measure the light emitted by the TLD during a predetermined heating cycle, and an integrating picoammeter which can measure the current or the charge from the photomultiplier. The total integrated charge from the photomultiplier during part or all of the heating cycle is usually related to the absorbed dose in the dosimeter. TLD readout instruments are available from a number of manufacturers. Detailed procedures for the use of TLDs in radiation hardness testing are given in ASTM Standard E668. [22] General guidance for relating the dose as measured by a TLD to the absorbed dose in another material is provided in ASTM Standard E666. [23] Detailed procedures for the use of TLDs in specific radiation hardness testing situations follow. Consider a CaFz TLD surrounded by an equilibrium then become: Case 1: High ~,

TC,F2

<<

thickness of Al. Equations 22, 23, and 24

TAI

Dsi / ‘Al

DAI

= (p../p)sj/ (p../p)Al

1 ‘CaF2

= ‘CaF2

(25) (26)

/ ‘Al

Combinjng equations (25) and (26) gives Dsi

f DcaF2

‘Si f ‘CaF2

= [( Wen/P)sj

= (~enip)sil

In both cases, all mass energy absorption photon energy, ~.

coefficients

1-13

1 (PeII/P)Al]

(Pen/p)CaF2

“ [RCaF2

1 RAI1

(23

(28)

and electron ranges are evaluated at the

Figure 6 is a plot of equations 27 and 28 as a function of the monoenergetic photon energy, EY. The Al equilibrium shield and a 0.9-mm-thick (0.035 in.) CaFz dosimeter are indicated schematically in the upper right corner of the diagram. Recall that the equilibrium thickness is approximately equal to the range of the most energetic secondary electron. The energy of the most energetic secondary electron is approximately Ee = 0.75 EYfor EY= 1 MeV. This relationship, as well as the range of electrons in CaF2, is shown graphically near the bottom of the figure. This indicates that for photon energies ~ >800 keV, Tc~Fz < TAI, and equation 27 (the dotted curve) applies.

For photon energies ~ <800

keV, TC~~z > TA1, and equation

28 (the solid curve)

applies. Note, however, that the solid curve and the dotted curve are equal for EY>400 keV. Therefore, for a dosimeter consisting of a 0.9-mm-thick CaFg TLD in an Al equilibrium shield, equation 28 may be used for &photon energies, EY. This property is used in the following two specific applications for TLDs in CoGoand flash x-ray testing.

2.0

f ,.:.:.:,..:.:,.,.,..:...:.: .,.,:,...:.:,..,.: ..’r’’’’’’~$~.~-r ..........:.:.: .,.:, .....-.... .............. ““’”’’”’’’’’’’’’~ i k’E.w.p.m.~w:JW,ym :: “““’“““ ‘:j

‘\[ \

,.5

.. . .w" ...."..m..."...r ....2......x" .... .... . ““’””’-H: ..."".".." ....".."...... rimMMEMg .".."..." . ....m.."." ........."... ..."."...T.... ......"....." \

DSi =

(Wenb)si

. fkaF2

Wa.F2i

~ \

[

RAI ‘~-””--....z-DcaF2 (Wenb)Al 1.0 -'''"-'"`-"'"--"`''"'"-"'`'"""'--"`------"`"``"`-`-`""``""-""""~-`-""`--`----~~~~'-'''‘ E \ $ 0.5 -.,.,,_. DSi = DCaF2

O.O1O

(~en/p)Si (Pen/P)CaF2

-—.-— -.— ~ I I I I 200 300 500 600 1000 keV [ 100 , Energy of Most Energetic Secondary Electron i , f t

100

Photon

Energy,

keV

10,000

1000

Figure 6. Photon Energy Response of an [Al I CaF2:Mn I Al] Dosimeter

Note that figure 6 is plotted for monoenergetic

photons.

When the source spectrum, VY, contains

appreciable photon energies <300 keV, all p~~/p must be replaced by (p../p), integrated over the photon spectrum. That is,

which is ~en/p

*

—J

Y(E) [pen(E) /p] dE

(Penfp) = — !

Y(E) di

1-14

,

(29)

and the ratio in equation 28 is replaced by

(Ven/P)Si (krrip)cl~

I = !

Application

!P(E) [~n(E)/p]si W(E) [~n(E)/p]~

dE dE

(30)

.

of TLDs in C060 Testing

Radiation hardness testing of electronic devices using Co@ ii-radiators is ide.rdly suited for the application of thermoluminescence dosimetry since at the energy of the 1.17 MeV and 1.33 MeV COW gammas the mass energy absorption coefficients of Si, Si02, CaF2, and Al are all such that equation 27 can be used to calculate absorbed dose in the test device provided that certain precautions are taken to minimize dose enhancement effects. Procedure 1. Use CaF2:Mn or CaF2:Dy TLDs. (Protect from fluorescent light or sun light) 2. Enclose the TLD in an Al capsule of wall thickness >2.2 mm. If the above two prcwedures are followed, the Si (or Si02) equilibrium calculated from the measured &Fz TLD dose using Equation 28 as follows: ‘(@Si

or Si02 = [( Wen/p)si or si~

j (~~#f)Al]

D=

dose can be

(28)

where the (pe~p) are taken at 1.2S MeV.

The actual Dsi or DsiQ in a microelectronic device is approximately equal to the equilibrium dose as given by Equation 25 if dose enhancement following procedures:

effects are reduced by the

3. Low-energy dose enhancement effects are reduced by sumounding the dosimeter and the device under test with a filter Imx consisting of an outer layer of between 1.5 and 2,0 mm (= 0.063 in.) of Pb and an imer layer of between 0.7 and 1.0 mm (= 0.030 in.) of Al. This reduces the low-energy scattered gamma ray @mponent of most C@ irradiators. 4. High-energy dose enhancement effects are reduced by orienting the device under test so that the plane of the semiconductor chip is perpendicular to the incident radiation with higher atomic numlxr material layers toward the incident radiation, if possible. The use of procedures 3. and 4. will reduce the dose enhancement factor, FDB to between 0.9 and 1.2. Dose enhancement effects will therefore contribute an error of no more than 20% if Equation 2S is used. [24, 25, 261

1-15

Application

of TLDs

in Flash

X-Ray

Testing

The way TLDs are used in the bremsstrahlung environment produced by flash x-ray generators is similar to their application in CoGotesting. Several additional restrictions and limitations apply as described below. Procedure 1. Use 0.9-mm-thick CaF2: Mn or CaFz:Dy TLDs. (Protect from fluorescent light or sun light.) 2. Enclose the TLD in an Al equilibrium capsule. The appropriate equilibrium thickness of the capsule depends on the source spectrum and needs to be determined for each individual flash x-ray generator by using the methods of ASTM Standard E665. 3. If the above two procedures are followed, the Si (or SiOz) equilibrium be calculated from the measured CaF2 TLD dose using Equation 28:

‘Si

Case a.

(or Si02) / ‘CaF2

Gamma

= (~en/~JSi

Effect Simulators:

(28)

(or Si@) / (~en/~)CaF2

Ee z 8 MeV

In this case the photon spectrum contains no appreciable Equation

dose can

components

below =300 keV.

28 is applied with (pen/p) evaluated at the average photon energy hv,v~ of the

photon spectrum Y?Y. 4a. The actual Dsi or Dsio2 in a microelectronic device is approximately

equal to the

equilibrium dose as given by Equation 28 provided that there is no significant contribution to the photon spectrum below = 300 keV. Case b.

X-Ray

Effect

Simulators:

E. = 1 - 1.5 MeV, hvavg = 100 keV

For these flash x-ray generators whose photon spectrum contains appreciable photon energies < keV, the ratio in equation 28 above must be replaced by the ratio of integrals in equation 30. Note:

It is necessary to know the photon source spectrum,

VY, of the flash x-ray

generator to do the calculations required by Step 3. above. 4b. Calculation of the actual Dsi or Dsio2 in a micrmlectronic

device is a much more

difficult dosimetry problem than Case 1 above because dose enhancement now plays an important part in determining the dose in a region of interest in a semiconductor device. The dose in the region of interest of the device under test must now be calculated from the Si

1-16

equilibrium dose from the TLD using a coupled electron-photon transport code such as the Integrated Tiger Series of Monte-Carlo transport codes (ITS) [27] or a discrete ordinates code such as CEPXS/ONELD [28] or ONETRAN. [29] Note: In all cases, if the TLD is not made of 100% TLD phosphor but consists of TLD powder in a Teflon matrix, extra care must be taken in modeling the TLD structure properly. [30] Application

of TLDs in LINAC

Testing

The electron beam from a high-energy (12-60 MeV) linear electron accelerator (LINAC) is often used to simulate a gamma-ray photon environment, particularly for microelectronic piece parts testing. Since the primary incident radiation in this case is electrons, not photons, the equilibrium dose considerations discussed in Section 2 do not appl y. However, provided care is taken that the material immediate y surrounding the region of interest does not appreciable y perturb the incident electron beam (a condition somewhat analogous to CPE for photons), expressions corresponding to Equations 21 and 22 for photons can be developed for electrons: Again

consider

two different

materials,

1 and 2, exposed to the same electron

(e-”cm-2), and let SCO1/pbe the mass collision stopping power (MeV”cm2/g)/e-.

and Procedure

fluence,

Q.,

Then

D = ~~ (SCO[/p)

(31)

D(1) / D(2) = (sco,/p) ~ / (sco~lp)~

(32)

[31 ]

1. Requirement on LINAC electron beam energy: 12 MeVs Ee s 60 MeV. Ensure that there is no dark current, and that the LINAC is properly tuned. 2. Use CaF2:Mn or CaF2:Dy TLDs. (Protect from fluorescent light or sun light.) 3. Place the TLD adjacent to the device under test, not in front of it. It is necessary to have uniform irradiation over the device under test and the TLD. 4. Ensure that there are no high-Z materials near the device under test. Either:

5a. Wrap the TLD in 25 pm (= 0.001 in.) Al foil. 6a. The dose in the region of interest, Z, of the device under test can be calculated from the measured CaF2 TLD dose using Equation 32 as follows:

Dz = [(ScoI/P)z / (S.OI/P)CaF2]

‘CaF2

(33)

The error in Dz when using this procedure will bes 5% plus the uncertainty inherent in the TLD measurement. @:

5b. Surround the TLD with a material structure which matches the packaging of the device under test as nearly as possible.

1-17

6b. The dose in the region of interest, Z, of the device under test can be calculated from the measured CaFz TLD dose using the same Equation 33 as above. The error in Dz when using this procedure will be< 2% plus the uncertainty inherent in the TLD measurement.

3.1.2

Calorimeters

Calorimeters make use of the fact that when a mass of material, Am, absorbs a quantity of heat energy, AE, the resulting temperature rise is given by:

AT = ( I/cp)”

AlZ/Am

(34)

where CP is the specific heat at constant pressure. Since A&Am = D by Equation 1, we see that D = CP”AT

(35)

If the specific heat of a particular material is known, and provided that all the energy deposited by ionizing radiation goes into heat and not chemical changes or atomic displacements, we can determine dose by simply measuring the temperature rise of the reference material. In practice, then, a calorimeter used to measure absorbed dose consists of a smatl mass, Am, of some reference material; to which some type of temperature sensor is attached. Advantages of calorimeters are that they offer a wide choice of reference materials in which to measure dose as opposed to the very limited selection of available thermoluminescent phosphors, and their response is independent of the absorbed dose rate. Disadvantages of calorimeters are: ● Calorimeters are not commercial y available (as TLDs and dye films are). They must be carefully designed for each specific application to have the proper dose range, and to avoid thermal losses and other thermal defects.

●

Calorimeters are not passive detectors. Each calorimeter requires an instrumentation channel

protected from electromagnetic interference, capable of measuring signals in the microvolt range. ● The practical dose range of calorimeters is more restricted than that of TLDs. The lower level of detectability is constrained by temperature measurement techniques. The lower limit of detectability of a Si calorimeter with an Iron-Constantan thermocouple is =l@ rad(Si). If a thermistor is used instead of a thermocouple, then doses as low as =100 rad(Si) could be measurable with this calorimeter. The upper limit of detectability is quite high, since it is principally constrained by the ability of the calorimeter to survive thermomechanical damage.

1-18

The principal application for calorimeters is dosimetry at large flash x-ray facilities with fairly lowenergy photon spectra, typically those with accelerating voltages of 1.5 MV or less. Two types of calorimeter are frequently referred to: the “dose” calorimeter and the “fluence” calorimeter. A “dose” calorimeter consists of an absorber which is thin enough so that the energy deposition is uniform throughout its depth. The absorber is surrounded by enough material to establish charged particle equilibrium as discussed in Section 2. Such a calorimeter measures equilibrium dose. The “fluence” calorimeter derives its name from the fact that it measures not dose, but the incident photon energy fluence, @Y(MeV/cm~). This is accomplished by making the absorber thick enough to absorb practically all of the incident photon energy and by defining the active area of the absorber with a collimator. Such a calorimeter is frequently used to measure photon energy fluence for very low photon energy spectra, typically on accelerators with an accelerating voltage of =250 kV or less.

3.1.3

Dyed Plastic

Dosimeters:

Radiochromic

Dye Films and Dyed PMMA

The operation of all dyed plastic dosimeters depends on the spectrophotometric measurement of the color change in plastic films or plates which contain organic dyes. Because dyed plastic dosimeters have typical dose ranges of l@s D <107 rad (101 - I@ Gy), they are one of the most widely used routine means for measuring dose in radiation processing applications. They are not widely used for making individual dose measurements in radiation hardness testing. [32] Dyed plastic dosimetry materials are available in a variety of physical forms: 1. Radiochromic Films - Radiochromic Films are thin, nearly colorless plastic films which become colored when exposed to radiation. Absorbed dose is a non-linear function of the change in optical density of the film. Evaluation of these films requires a dedicated radiochromic film densitometer or a spectrophotometer. Available radiochromic film types include: ●

●

●

Aminotriphenylmethane dye in solid solution in Nylon 50pm (0.002 in.) Film Thickness: 10’$-107 rad (102 - l@ Gy) Absorbed Dose Range: 601 nm Absorption Band: Malachite green dye in pol ychlorostyrene film 630 nm Absorption Band: Coated film (GAFchromic) Film Thickness: Absorbed Dose Range:

100 pm (0.004 in.) 1(P - l@ rad (101 - l@ Gy)

Radiochromic dye films are commercial y available in 1 cm x 1 cm squares for individual dose measurements, and in larger sheets which are particularly useful for two-dimensional dose mapping of radiation fields at flash x-ray facilities and in C@ gamma-ray irradiators.

1-19

2. Dyed PMMA - Polymethylmethacrylate (PMMA) which contains certain dyes darkens when it is exposed to radiation. Dyed PMMA is evaluated using a spectrophotometer in the same way as the radiochromic films described above. Dyed PMMA dosimeters available from UKAEA Harwell Laboratory include: ●

Red Perspex 4034 Thickness:

3 mm

Absorbed Dose Range: Absorption Band: ●

Amber Perspex 3042 Thickness:

3 mm

Absorbed Dose Range Absorption Band: ●

GAMMACHROME Thickness:

5S10s - 5“106 rad (5-50 kGy) 640 nm

2“105- 2“106 rad (2-20 kGy) 603 and 651 nm

YR

Absorbed Dose Range: Absorption Band:

3 mm 1“104 - 3“10s rad (0.1-3 kGy) 530 nm

The above dyed PMMA dosimeters are commercially individual, hermetical y sealed pouches. Procedure

for Use of Radiochromic

available as 30 x 11 x 3 mm blocks in

Film

1. Protect film from ultraviolet radiation (fluorescent light, sun light). 2. Ambient relative humidity (RH) must be controlled during irradiation and at least 24 hours before irradiation. Typical RH for storage of radiochromic film is 50%. Precision of dose measurements is adversely affected by permitting the RH to deviate substantially from this value. & 15% at RH = 109?o Precision: *2%at RH=30-75~o + 15c710 at RH = 90?lo. 3. To make an equilibrium dose measurement, the film must be sandwiched between two blocks of material of the appropriate composition and thickness. For Co@ irradiations, 2.2 mm thick Al blocks are appropriate if it is desired to measure Si equilibrium dose. Note: Conditions 1-3 can be adequately satisfied if the film with its equilibrium blocks is heat sealed in a polyethylene-aluminum-Mylar trilaminate pouch. If the above three procedures are followed, the Si (or SiOQ) equilibrium

dose for

irradiations made in a Com environment can be calculated from the measured film dosimeter

1-20

dose using Equations 22 and 23 as follows: ‘Ieq)

Si or Si02 = [( Wen/P)Si or Si02 /

(~eJp)All “[Rfl[~/

RAII

Dfilm

(36)

where all ( pe~p) and R are taken at 1.25 MeV. Applicable ASTM standards for measuring dose with dyed plastic systems are E1275 [33] for radiochromic film, El 276 [34] for dyed PMMA, and E1400 [35] for calibration. 3.1.4

PIN

Diodes

Silicon PIN diodes are used primarily as dose-rate detectors, as discussed in the following section. However, the dose-rate as a function of time from a calibrated Si PIN diode can be integrated to give the dose. This technique is used primarily for measuring Si device dose in 10-keV x-ray testers. The similarity in construction between a Si microelectronic circuit and a Si PIN diode thus makes it possible to measure dose directly without the need for dose enhancement corrections which would otherwise be very important at these low energies.

3.2

Measurement

of Ionizing

Radiation

Dose Rate

A dose-rate detector is any device that produces an electrical signal that is proportional to the absorbed dose rate as a function of time. In order to satisfy this requirement, the dose rate detector must meet all of these three criteria ● Energy Response - The sensitive region of the detector must have the same photon energy response function as the reference material over the photon energy range of interest. For example, to be useful as a device that measures rad(Si)/s in a flash x-ray environment, the sensitive region of the detector must be either Si, or a material that has the same energy response function (to within a multiplicative constant) as Si over the photon spectral range of that flash x–ray generator. This is important because the photon energy spectrum is, in general, not constant as a function of time during the radiation pulse.

c Time Response - The overall time response of the detector plus its associated electronics must be at least as fast as the highest frequency Fourier component of the radiation pulse. ●

Utzearily - The output of the detector must be linear with absorbed dose rate up to the

maximum dose rate of the radiation pulse. If any one of these three criteria is not met, then the detector output will not be proportional to the radiation dose rate over the entire radiation pulse; that is, the detector output pulse will be distorted relative to the radiation pulse shape.

1-21

3.2.1

Silicon

PIN

Diodes

A silicon PIN diode absorbed dose rate detector consists of associated circuitry to measure the primary photocurrent, depletion region produce electron-hole pairs. The electrons produce a current pulse which is proportional to the absorbed is illustrated schematically in figure 7.

Intrinsic region,

a reverse-biased PIN diode with the IPP. Photons passing through the are swept out by the bias field and dose rate as a function of time. This

J**”

~L-J

N region j +-, w j-\\ ;,+:+ & + Photon ..+ @eam&+”

P region

+

v(t) ‘-

L-d

RL

+V.

Figure 7. Silicon PIN Junction Detector (Schematic)

As always, to measure equilibrium absorbed dose rate the sensitive region of the detector must be surrounded by an equilibrium thickness of the appropriate material; commercial Si PIN diodes that are designed expressly for dosimetry are supplied with an integral Al equilibrium shield. ● Energy Response - Since the sensitive region of this detector is Si, its energy response is perfectly matched to that of Si for all energies.

Time Response - The time response of a PIN diode depends on the junction area and the depletion depth. The step function response of a very large PIN diode (junction area = 1 cmz, depletion layer thickness = 250 pm) is 7.0 ns, and it has a frequency response of 47.7 MHz. Smaller PIN diodes would be faster; very small PIN diodes (junction area = 0.00025 mm2, ●

depletion layer thickness = 5 pm) can have a time response of <1 ns. [36] ● Linearity - The sensitivity of a Si PIN diode (A / rad.s- 1) is directly proportional to the depletion volume. The dose rate response of a reverse-biased PIN diode will be linear as long as the output voltage does not exceed one-half the bias voltage; that is, I ~PR~ < V~i~~2. Linear

operation up to 10s rad(Si)/s is achievable with commercially available Si PIN diodes. Linear operation in excess of 1011 rad(Si)/s has been reported with extremely small Si PIN junctions. [36] The primary advantages of Si PIN diodes as dose-rate detectors is their small size and the fact that their energy response is perfectly matched to that of Si at all photon energies. Their primary drawback is their high sensitivity which limits their usefulness to dose ratess 10s rad(Si)/s.

I-22

3.2.2

Compton

diodes

A typical Compton diode consists of a medium-Z electron emitter (plastic) and a high-Z electron collector (tungsten) contained in a shielded enclosure as shown in Figure 8. Photons passing through the emitter region produce electrons, primarily by Compton scattering, which are stopped in the high-Z collector. The Compton electron current is proportional to the absorbed dose rate in the emitter region. The tapered construction provides a good impedance match to the coaxial cable for good high-frequency time response. I-

Electron Emitter z

Photon Wl#lnlllllll

Electron Collector

Coaxial Connector

]Ililll.

Aluminum Housing

Figure 8. Compton Diode Detector (Schematic) Compton diodes are operated unbiased. Compton electron collection from the insulator region is minimized by the use of a low-density insulator material. [371 c Energy Response - Because this detector depends on Compton scattering for its operation, it is usable only at energies for which the Compton effect is the predominant interaction of photons with matter; that is, hv >200 keV. In this spectral region the photon energy response of plastic is quite similar to that of Si. Therefore, Compton diode detectors can be used to measure dose rate at flash x-ray generators that have an accelerating potential >8 MV. “ Time Response - The frequency response of a Compton diode in a well matched 50-ohm system has been reported to be in excess of 1 GHz; this implies a time response of -= 1 ns. + Linearity - Linear operation of Compton diodes has been demonstrated

for dose rates up

to 3“1013 rad/s. [38, 39] The primary advantages of Compton diode detectors are their low photon sensitivity which makes them usable at very high dose rates. Care must be taken, however, that the cable response does not compete with the Compton diode signal. The disadvantage of Compton diodes is that they are limited to high photon energies. Compton diodes are normally calibrated by exposing them at a known dose rate in a calibrated C040 irradiator. If the source calibration is known in terms of rad(Si)/s, then the sensitivity, S, (A-s-l), of the Compton diode is found by dividing its output signal (A) in the COGOsource by the known dose rate.

I-23

3.2.3

Scintillator-photodetectors

Scintillator-photodetectors consist of a plastic scintillator optically coupled to a photodetector as shown schematically in Figure 9. Photons passing through the plastic scintillator produce a pulse of visible light of an intensity which is proportional to the absorbed dose rate in the scintillator material. The photodetector, typically a vacuum photodiode, produces an electrical current pulse which is proportional to the light intensity. Vacuum Photodiode

Figure 9. Scintillator-photodetector ●

(Schematic)

Energy Response - The photon energy response of plastic is well matched to that of Si

for photon energies hv >200 keV. Therefore, scintillator-photodetectors can be used to measure dose rate at flash x-ray generators that have an accelerating potential z 8 MV. “ Time Response - The time response of the overall detector depends both on the luminescence decay time of the plastic scintillator material and on the time response of the photodiode and its associated circuitry. The luminescence decay time constant, z, is around 2 ns for the fastest plastic scintillator materials. (The light intensity from a plastic scintillator in response to a delta function radiation pulse decays as e-t’~.) The circuit time constant of a vacuum photodiode in a low-capacitance, 50-ohm system is typically about 0.3 ns. Thus the overall detector response is determined by the scintillator decay time constant. Linearity - The linearity of the overall detector depends on the linearity of the plastic scintillator material and on the linearity of the photodiode and its associated circuitry, Linearity of the plastic scintillator is ultimately limited by depletion of luminescence centers as well as by transient radiation induced darkening of the plastic material. A very thin scintillator (= 0.5 mm) can be operated linearly up to 1011 rad(Si)/s. Linearity of a planar vacuum photodiode is determined by space-charge limited current flow in the diode which depends on photodiode dimensions and bias voltage. The Langmuir-Child space-charge limited current in a planar vacuum diode is given by: ●

where

and

IL= (4/9) V03j2 eO(2e/m)l/2 A / dz

(A)

VOis the bias voltage

(v)

(4/9) SO(2e/m) l/z = 2.33.106

(mksu)

A is the electrode area d is the electrode spacing

(m2) (m).

I-24

(37)

The maximum linear current is (38)

I max = 1/2 IL

It should be clear from Equation 37 that the dynamic range of a vacuum photodiode detector is maximized by operating it at the highest pssible bias voltage, since the maximum linear current is proportional to V0312. The primary advantage of scintillator-photodetectors is their large output signal (as high as 800 V in a 50-S2 system for V. = 5 kV). This makes it easy to discriminate against radiation induced cable signals as well as electromagnetically coupled noise. The disadvantage of scintillatorphotcxlet.ectom is their relative] y large size and the requirement for a high bias voltage. Scintillator-photodetectors are normally not calibrated. Instead, the absorlxd dose, D, is measunxl at the location of the detector with a TLD for example. Given the detector signal, I(t), and the total charge from the detector, Q, the absorbed dose cate as a function of time is found by application of Equation 4, as follows: D(t) =

~m

“

(39)

I(t) / Q

Cherenkov-photodetectors. If high-purity quartz is used instead of a plastic scintillator material, the phohxiiode will detect Cherenkov radiation emitted by the quartz. Such a detector has been reported to be linear up to 3-1013 rad/s, with a time responses

I-25

1 ns. [37, 38]

3.3

Neutron

Dosimetry

The quantity of interest for neutron hardness testing is the 1 MeV equivalent

neutron fluence,

‘lMeV; ‘hat ‘s> ‘hat ‘eutron ‘lUence ‘hich Produces ‘he ‘ame damage in ‘i as if all neutrons ‘ad an energy of 1 MeV. Units of neutron fluence are n/cmz.

In normal practice the neutron fluence above 3 MeV, @>q~ev, is determined by measuring the activation of pressed sulfur powder pellets. [40, 41] The dominant reaction, which has a threshold energy of 3 MeV is SSZ (n,p) Psz. PS2 decays with the emission of a 1.71 MeV beta ray with a half life of 14.29 days. The 1 MeV equivalent fluence is then calculated from the measured fluence above 3 MeV by multiplying it by the 1 MeV equivalent/> 3 MeV fluence ratio, (@~Mev/@>~Mev). This ratio, usually called the 1 MeV / Sulfur ratio is neutron spectrum dependent and must be determined for each reactor and irradiation condition as described under Calibration, below. (@~Mev/@>~Mev) Is often defined in terms of the hardness parameter,

HP= (@~Mev/@>~~~ev),

and the spectral index, S1 = (@>10kev/@>~Mev), as follows:

(@lMeV/@>3

M.V)

=

HP “ S1 = (@~Mev/@>10kev)”(mlo

(35)

k. V/@>3 MeV)

The sulfur used in this technique is usually employed in the form of pressed powder pellets. Common sizes of sulfur pellets are 1 in. diameter x 0.250 in. thick, 0.250 in. diameter x 0.250thick, or 0.250 in. diameter x 0.125 in. thick. It is also possible to pack sulfur powder in an Al capsule. Sulfur pellets are reusable if sufficient time elapses between uses. It is recommended that irradiated sulfur pellets be allowed to decay for one year before reuse. Procedure 1. Irradiate the sulfur pellet. If the reactor spectrum contains an appreciable thermal neutron fluence, @t~,ASTM Standard E265 recommends shielding the pellets with 1 mm of Cd. 2. Wait at least 8 hours to allow some of the competing reactif ns to decay. Waiting for 8 hours results in a measured fluence error of < 29?0. Two Count the sulfur pellet in a proportional counter for 0,000 counts. 3. against proportional counters are normally used in anti-coincidence to discriminate background.

4. Calculate the neutron fluence >3 MeV, 0>3 ~ev, using the sensitivity

of the

sulfur pellet m found by calibration (see below). 5. Calculate the 1 MeV equivalent fluence, @~Mev, by multiplying Sulfur ratio, (@~Mev/@>~~,v), as determined by calibration (See below).

I-26

by the 1 MeV /

Calibration 1. Expose the sulfur pellet to a calibrated

C~52 neutron

source whose spedral

neutron fluence, @n(E), is weIl known. 2. Count the sulfur pellet using the procedure described above. 3. Calculate the sensitivity of the sulfur pellet to neutrons of E >3 MeV. 4. Determine the spectral neutron fluence of the reactor configuration of interest by using an activation foil set. [42, 43] 5. Calculate the 1 MeV equivalent fhence, @IM.IJ. for this re~~r

and experimental sF~.

[4

6. Measure the neutron fluence above 3 MeV, 0>3 ~ev, at this location using sulfur pellets and the sensitivity determined in step 2. 7. Using the results of steps 5 and 6, calculate the ratio (@lM.v/@>3 Mev). This is the 1 MeV / Sulfur ratio required in step 5 of the procedure above. In mixed neutron-gamma fields the neutron fluence and the gamma dose can be measured independently using sulfur pellets for neutron fluence as described above and CaF2 TLDs for gamma dose as discussed in Section 3.1.1. Since the 1 MeV / Sulfur catio is spxtrum dependent, it is important to exercise a certain amount of quality control over the reactor spectrum so that sulfur dosimetry results remain valid. [4~

I-27

3.4

Dosimetry

for

Single

Event

Upset

Testing

Depending on the effects to be simulated, single event upset (SEU) experiments maybe done either with energetic heavy ions such as Fe, or with energetic protons. Different dosimetry techniques are employed in these two cases. 3.4.1

Heavy

Ion

Dosimetry

Heavy ions provide good simulation for single event upsets at geosynchronous altitudes because SEUS there are produced primarily by cosmic rays, since geosynchronous orbits are outside the trapped proton belts. Cosmic ray particles and heavy, high-LET ions produce a high density of ionization along the particle track, producing potential y one SEU per incident particle. The goal of heavy ion SEU testing is therefore to determine the effect produced by a single incident particle. Heavy ions are produced by tandem van de Graaff generators and by cyclotrons. Fe ions are popular for SEU testing because the cosmic ray spectrum (intensity vs. Z) has a peak for Fe. Ions of some of the noble gases (Ar, Kr) are also used for SEU testing. Important parameters which it is necessary to measure for heavy ion SEU testing are: “ Particle type and energy (which, in turn, determine LET). Particle flux and uniformity, ~x,y). . Beam purity (there should be no wall scattering and no contamination).

●

A typical dosimetry setup for heavy ion SEU dosimetry is shown in Figure 10.

Vacuum Chamber

r Cyclotron

~ DUT H .0001“ Stint.

Particle ................................. Beam

,

Collimated Si SB Detect

1111. n I

““”””””””””L Photodetector

Figure 10. Dosimetry for Heavy Ion SEU Testing

The LET of the incident particles is measured by a collimated Si surface-barrier detector which is moved into the beam in place of the device under test (DUT). The particle flux is measured

1-23

continuously photodetector.

by a very thin (0.0001 [46]

in. or 2.5 pm thick) scintillator

foil coupled

to a

The principle of operation of the Si surface barrier detector is illustrated in Figure 11. The detector is fully depleted; therefore, all electrons and holes are collected. If the preamplifier has a long time constant, the puk height out of the prearnp will be proptional to the total collected charge. Since the number of electron-hole pairs collected is proportional to the energy deposited along the particle track, the pulse height out of the pm.amp will be proportional to the energy lost in the detector. In Si, the energy required to produce one electron-hole pair is 3.6 eV; the energy loss required to produce 1 pC of collected charge is 22.5 MeV. In GaAs, the energy required to prmluce one electron-hole pair is 4.8 eV; the energy loss required to produce 1 pC of charge is 30.0 MeV.

Au Si Al

+

Figure 11. Si Surface Barrier Detector Used to Measure LE’f of Heavy Ions

3.4.2

Proton

Dosimetry

Protons provide good simulation for single event upsets in near earth orbits because SEUS there are produced primarily by protons from the trapped radiation belts. Energetic protons produce SEUS in devices by causing nuclear reactions (Sin (p, p’s) Mg~, for example) along their tracks. Protons produce approximate y one nuclear reaction for every l@ protons traversing a typical microelectronic circuit. The goal of proton SEU testing is therefore primaril y the determination of the rate of SEUS produced per incident proton, [471 Protons in the energy range of 20-150 MeV are produced by cyclotrons. from cyclotrons are = 1(Y - l@ p / cmzs.

T ypiczd proton fluxes

Important parameters which it is necessmy to measure for proton SEU testing are: c Particle flux. Particle energy, ~ (but this is not as important as for heavy ions). “ Energy resolution, AEJEP (especially important if a degraded beam is used).

●

A typical dosimetry setup for proton SEU dosimetry is shown in Figure 12. The particle flux is measured continuous y by a secondary emission monitor (SEM) which is calibrated by a Faraday cup which can be moved into the beam in place of the device under test (DUT). The proton

I-29

energy, EP, and energy resolution, AEP/EP, are measured by a NaI crystal spectrometer.

Vacuum Chamber

r

Cyclotron

Proton .................................. Beam

-

..r..3 !

[48]

Collimator

I

0.001“

5 cm thick Graphite Faraday Cup

Al foil SEM

I ..............

I

$

stainless Steel Zxit Window

II~

1111.

.

f 0.001’ !“”---!%.. ground plane

).001“

,“.,,-,. “,...

.

.

.

.

.

.

.

.

.

.

.“.

-.”,”

.

.

. .

BmKW!l

Figure 12. Dosimetry for Proton SEU Testing

1-30

. .

. .

. .. .. . . . .

t

...

●

DUT

References [1] W. L. Mcbughlin andothers, Francis, Philadelphia (1989).

Dosimetry

for Radiation

Processing,

Taylor&

[2] International Commission on Radiation Uti@ad MemuremenW, ``Radiation Quantities and Units”, ICRU Report 33, ICRU, 7910 Woodmont Ave., Bethesda, MD 20814, USA, (1980). [3] ASTM Standard E170, “Standard Terminology Relating to Radiation Measurements and Dosimetry”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually). [4] H. H. Rossi, “Microscopic Energy Distribution in Irradiated Matter”, Chapter 2 in F. H. Attix and others, Radiation Dosimetry, 2nd cd., volume I: Fundamentals, editors, Academic Press, New York (1968). [5] E. A. Burke, and others, “Gamma-Induced NS-28 (No. 6), 4068 (Dec. 1981).

Noise in CCDS”, IEEE Trans. Nut. Sci.,

[6] F. H. Attix, Introduction to Radiological Physics page 179 ff., John Wiley& Sons, New York ( 1986).

and

Radiation

Dosimetry,

[7] P. J. McNulty, “Predicting Single Event Phenomena in Natural Space Environments”, IEEE Nuclear and Space Radiation Effects Conference Short Course, Reno, NV, July 1990. [8] J. H. Hubbell, “Photon Cross Sections, Attenuation Coefficients, and Energy Absorption Coefficients From 10 keV to 100 GeV”, U. S. Department of Commerce, National Bureau of Standards, Handbook NSRDS-NBS 29 (1%9). [9] T. E. Burlin, “Cavity-Chamber Theory”, Chapter 8 in Radiation Dosimetry, 2nd cd., volume, I: Fundamentals, F. H. Attix and others, ed., Academic Press, New York (1968). [10] M. J. Berger and S. M. Seltzer, “Tables of Energy Losses and Ranges of Electrons and Positrons”, National Aeronautics and Space Administration, Report NASA SP-3012 ( 1964). [11] International Commission on Radiation Units and Measurements, “Stopping Powers for Electrons and Positrons”, ICRU Report 37, ICRU, 7910 Woodmont Ave., Bethesda, MD 20814, USA, (1984). [12] R. D. Evans, “X-Ray and Gamma-Ray Interactions”, Chapter 3 in Radiation F. H. Attix and others, editors, Dosimetry, 2nd cd., volume I: Fundamentals, Academic Press, New York (1968).

1-31

[13] J. H. Hubbell, “Photon Mass Attenuation and Energy-absorption to 20 MeV”, Int. J. Appl. Radiat. Isot., 33, 1269-1290 ( 1982).

Coefficients from 1 keV

[14] D. M. Fleetwood, et al., “Comparison of Enhanced Device Response and Predicted X-Ray Dose Enhancement Effects on MOS Oxides”, IEEE Trans. Nut. Sci., NS-35 (No. 6), 1265 (Dec. 1988). [15] E. A. Burke, J. C. Garth, “An Algorithm for Energy Deposition at Interfaces”, IEEE Trans. Nut. Sci., NS-23 (No. 6), 1838 (Dec. 1976). [16] W. L. Chadsey, “X-ray Dose Enhancement”, 1591 (Dec. 1978).

IEEE Trans. Nut. Sci., NS-25 (No. 6),

[17] J. C. Garth, W. L. Chadsey, R. L. Sheppard, “Monte Carlo Analysis of Dose Profiles Near Photon Irradiated Material Interfaces”, IEEE Trans. Nut. Sci., NS-22 (No. 6), 2562 (Dec. 1975). [18] J. A. Wall, E. A. Burke, “Gamma Dose Distribution at and Near the Interface of Different Materials”, IEEE Trans. Nut. Sci., NS-17 (No. 6), 305 (Dec. 1970). [19] J. C. Garth, “An Algorithm for Calculating Dose Profiles in Multi-Layered Devices Using a Personal Computer”, IEEE Trans. Nut. Sci., NS-33 (No. 6), 1266 (Dec. 1986). [20] Klaus Becker, Solid State Dosimetry, Cleveland (1973).

Chapter 2, “Thermoluminescence”,

CRC Press,

[21] ASTM Standard E665, “Standard Practice for Determining Absorbed Dose Versus Depth in Materials Exposed to the X-Ray Output of Flash X-Ray Machines”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually). [22] ASTM Standard E668, “Standard Practice for the Application of ThermoluminescenceDosimetry (TLD) Systems for Determining Absorbed Dose in Radiation-Hardness Testing of Electronic Devices”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anuall y). [23] ASTM Standard E666, “Standard Practice for Calculating Absorbed Dose From Gamma or X Radiation”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually).

I-32

[24] ASTM Standard E1249, “Standard Practice for Minimizing IMimetry EITors in Radiation Hardness Testing of Silicon Electronic Devices Using CO-60 Sources”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia PA 19103-11S7, USA, (215) 299-5400, (published anually), [2~ ASTM Standad E1250, “Standard Method for Application of Ionization Chambers to Assess the Low Energy Gamma Component of Cobalt-60 In-adiatms Used in RadiationHardness Testing of Silicon Electronic Devices”, in Annual Book of ASTM Standan@ volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-11&7, USA, (215) 299-5400, (published anually). [261 E. A. Burke, and others, “The Direct M~urement of Dose Enhancement in Gamma Test Facilities”, IEEE Trans. Nut. Sci., NS-36 (No. 6), 1S90 (Dec. 19S9) [271 J. A. Halbleib and T. A. Melhom, “ITS: The Integrated TIGER Series of Coupled Electron/Photon Monte Carlo Transport Cedes”, Sandia Report SANDS4-0573, Sandia National Laboratories, Albuquerque, NM (1%4). [28] L. J. brence, and others, “Results Guide to CEPXS/ONELD: A One-Dimensional Coupled Electron-Photon Dixrete Ordinates Cede Package, Version 1.0”, Sandia Report SAND89-2211, Sandia National Laboratories, Albuquerque, NM S7185 (1990). [29] T, R. Hill, “ONETRAN A Discrete Ordimtes Finite Element Code for the Solution of the One-Dimensional Muhigroup Transport Equation”, Los Alamos Report LA-5!W0-MS, Las Alarnos National IAomtory (1975). [30] K. G. Kerris, S. G. Gorbics, F. H. Attix, “The Energy Dependence of CaF2:Mn/Teflon Thermoluminescent Dosimeters”, IEEE Trans. Nut. Sci., NS-37 (No. 6), 1752 (Dee 1990). [31] D. E. Beutler, L. J. Lorence, and D. B. Brown, “Dosimet~ in LINAC Electron-BEnvironments”, IEEE Trans. Nut. Sci., NS-38 (No. 6), 1171 (Dec. 1991). [32] W. L. McLaughlin and others, Dosimetry Taylor & Francis, Philadelphia (19S9).

for Radiation

Processing,

page 163, ff.,

[33] ASTM Standard E1275, “Standard Practice for Use of a Radiochromic Film Dosimetry System”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia PA 19103-1187, USA, (215) 299-5400, (published anually).

I-33

[34] ASTM Standard E1276, “Standard Practice for Use of a Polymethylmethacrylate Dosimetry System”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-54M), (published anually). [35] ASTM Standard El@O, “Standard Practice for Characterization and Performance of a High-Dose Gamma Radiation Dosimetry Calibration Laboratory”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually). [36] J. Blackburn and K. Kerris, “Measurement of Dose Rate in the Pinched HIFX Photon Beam Using a Small-Volume Silicon PIN Detector”, HDL Internal Report (Dee 1991). [37] G. A. Carlson, T. W. L. Sanford, and B. A. Davis, “A Solid Dielectric Compton Diode for Measuring Short Radiation Pulsewidths”, Rev. Sci. Instr., 61, 3447(1990). [38] T. W. L. Sanford, and others, “Characterization of Flash y-Ray Detectors That Operate in the Trad/s Range”, Nut. Inst. & Meth. in Phys. Res., A294, 313-327 (1990) [39] T. W. L. Sanford, and others, “Production and Measurement of Flash X-Ray Dose Rates in Excess of 1013 rad(CaFz)/s”, IEEE Trans. Nut. Sci., NS-38 (No. 6), 1195 (Dec. 1991) [40] ASTM Standard E261, “Standard Practice for Determining Neutron Fluence Rate, Fluence, and Spectra by Radioactivation Techniques”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually). [41] ASTM Standard E265, “Standard Test Method for Measuring Reaction Rates and FastNeutron Fluences by Radioactivation of Sulfur-32”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually). [42] ASTM Standard E720, “Standard Guide for Selection of a Set of Neutron-Activation for Determining Neutron Spectra Used in Radiation-Hardness Testing of Electronics”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103-1187, USA, (215) 299-5400, (published anually).

I-34

Foils

[43] ASTM Standard E721, “Standard Method for Determining Neutron Energy Spectra With Neutron-Activation Foils for Radiation-Hardness Testing of Electronics”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, Philadelphia% PA 19103-11S7, USA, (21S) 299-5400, (published anually). [44] ASTM Standard E722, “Standard Practice for Characterizing Neutron Energy Fluence Spectra in Terms of an Equivalent Monoenergetic Neutron Fluence for Radiation-Hardness Testing of Electronics”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Stint, Philadelphia, PA 19103-1187, USA, (21S) 2S9-54M, (published amlally). [4Sl ASTM Standard E844, “Standard Guide for Sensor Set Design and Irradiation for Ractor Surveillance, E 706( IIC)”, in Annual Book of ASTM Standards, volume 12.02: Nuclear(II), Solar, and Geothermal Energy, American Society for Testing and Materials, 1916 Race Street, l%iladelphi~ PA 191~-1 187, USA, (215) 299-5400, (published anually). [461 W. A. Kolasinski, and others, “Simulation of Cosmic-Ray Induced Soft Errors and Latchup in Integrated-Circuit Computer Memories”, IEEE Trans. Nut. Sci., NS-26 (No. 6), 50s7 (Dec. 1979). [471 Edward Petersen, “Soft Emors Due to Protons in the Radiation Belt”, IEEE Trans. Nut. Sci,, NS-28 (No. 6), 3981 (Dec. 1981). [48] K. M. Murray, W. J. Stapor, and C. Castaneda, “Calibrated Charged Particle Radiation System with Precision Dosimetric Measurement and Control”, Nut. InsL and Meth. in Phys. Res., A281, 616-621 (1989).

I-35

Appendix Quick

Reference

Handbook

A

of Practical

I-36

Dosimetry

Techniques

Quick Reference Handbook of Practical Dosimetry Technique!

IEEE Nuclear and Space Radiation Effects Conference Tutorial Shorl Course - Dosimetry Session ew Orleans, LA, July 1992

Definitions of Dosimetry Terms Absorbed Dose

The mean energy absorbed

per unit mass.

D= AE/Am Units: 1 rad = 100 erg/g,

Absorbed Dose Rate

1 Gy = 1 J/kg = 100 rad.

The time rate of change of absorbed dose. .

D=dD/dt Units of absorbed dose rate are Gy/s or radk.

Particle Fluence

The number of particles incident on a sphere of unit cross-sectional area.

@=dN/dA The units of particle fluence are particles/

Energy Fluence

cm.2.

The radiant energy incident on a sphere of unit cross-sectional area.

Y=dE/dA The units of energy fiuence are MeV / cm p.

Flux

The time rate of change of fluence.

@=d@/dt

cm-2s-’

~=dW/dt

MeV/cm2s B

1

LET

Mass stopping power, (dE/dx) /p Energy deposited

per unit path length

Linear Charge Deposition Electron-hole

Applicable ASTM Standards

(LCD)

pairs per unit path length

MeV cm p /mg MeV/pm PC I pm pm ‘1.

E170 ASTM, 1916 Race St., Philadelphia,

I-37

PA 19103-1187,

(215) 299-5400

Quick Reference Handbook of Practical Dosimetry Technique$

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session !WOrleansr LA, July 1992

Equilibrium Dose Theory Photons

Equilibrium thickness

y

1111111111111111111111[ lllllllll[.

y

llllllllllllllllllllll[lllllllllll.

Therefore: Bragg-Gay Cavity Theory:

y

~ Material 3, T ~ << T2

❑lEemP)2

1111111111111111111111[ 111111111,.

Material 2

Therefore:

Electrons

Material 2

D1/D3S [(~en/p)l

/(~en/~)2]

(’3 /R2)

-et Scol /p be the mass collision stopping power, where superscripts pri and sec refer to the energy of primary and secondary electrons.

I

I

If Material 1 is thin to primary electrons, and Materials 2 and 3 are thick to I

),1 ),1 D2/D3=

Therefore:

Applicable ASTM Standards

[(s::;/P)2/(s::;/P)31

E666 ASTM, 1916 Race St., Philadelphia,

I-38

PA 19103-1187,

(215) 299-5400

Quick Reference Handbook of Practical Dosimetry Technique:

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session ?WOrleans, LA, July 1992

Application of TLDs in CO-60 Testing Applicability

Absorbed

Precautions

Use and Key Issues

Cobalt-60

Type of Radiation:

Gamma Radiation

10 mrad(Si) -3 E5 rad(Si) I

Dose Range:

Protect bare TLD from ultraviolet radiation (fluorescent light and sunlight.) Correct for fadina.

Dosimeter:

“.:’”:, ’;::..: .?$$ .,:::.:; ,,:,:W:X ;: ..:.: ,. , ~:: *%:.;{ ~:j ‘W;t%W.~#~w +: :!::::::::, :!’*:::,,.::: :.:.,:.*::: !,,~:, > ::,:fiti, >:: ,*,~.; ;;; ?:*:: :,.::,, w,,, ,:;, ;, ;:,i:.~,j:: ~: X’W!* .:, n,:, W:W

CaF2 :Mn or CaF2 :Dy TLD

2.2 mm wall thickness Aluminum capsule =

Dsi(eq) = [(~enlp)si/ (~~n/p)&F2] DCaF2 Experimental

Arrangement: Device under test

Filter Box: ;:g:

;

Cobalt-60 Radiation Dosimeter

D~ (SK)2) = [(~en/~)si(si02)

Applicable ASTM Standards

E668

E666

ASTM, 1916 Race St., Philadelphia,

❑

w! jj;~w,;

i (~en/~)CaF2]

El 249 PA 19103-1187,

DCaF2

El 250 (215) 299-5400 1

I-39

IEEE Nuclear and S~ace Radiation Effects Conference Tutorial Short Course - Dosimetry Session sw Orleans, LA, July 1992

Quick Reference Handbook of Practical Dosimetry Techniaue:

Application of TLDs in Flash X-Ray Testing Applicability

Type of Radiation: Absorbed

Precautions

Flash x-ray bremsstrahlung 10 mrad(Si) -3 E5 rad(Si)

Dose Range:

Protect bare TLD from ultraviolet (fluorescent light and sunlight.) Correct for fadirm.

radiation I

Use and Key Issues

Aluminum capsule must be of a wall thickness appropriate to the flash x-ray bremsstrahlung spectrum. Determine the correct equilibrium thickness using ASTM Standard E665.

Accelerating Voltage 210

Dsl = DCaF2

MV: (Wen/P)Si (Wen/P)CaF2

Accelerating Voltage = 1 -1.5

MV:

Ds, = ~Y(E) [~en(E)/pJs, dE DcaF2 ,fY(E) [!-%n(E)/p]c.~2dE

Applicable ASTM Standards

[

E668

E666

ASTM, 1916 Race St., Philadelphia,

E665

PA 19103-1187,

II

(215) 299-5400 4

1-40

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetrv Session Iew Orleans, LA, July 1992 -

Quick Reference Handbook of Practical Dosimet~ Techniaue:

Application of TLDs in LINAC Testing Applicability

Type of Radiation: Absorbed

Precautions

Use and Key Issues

12-60

Dose Range:

MeV electrons

10 mrad(Si)

-3 E5 rad(Si)

Protect bare TLD from ultraviolet radiation (fluorescent light and sunlight.) Correct for fadinq.

Dosimeter: CaFq :Mn or CaF~ :Dy TLD

.,...

w ~“”” ““””~’ 0.001” Al foil ~ or match packaging of DUT (better)

Experimental

Arrangement:

Uniform LINAC Electron Beam lllllllllllllllllllllllllllllllllllllllllllllllllllllllll lllllllllllllllllllllllllllilllllllllllllllllllllllllllll

Device under test I

1111]111

I

llllllllllllllllllllllllllllllllllllllllllllllIllllllllll lll1lllllllllllllllllllllllllllllllllllllilllllllllllllll

..:,;:::.:.

II

...

IF 11111119

I

Region of interest, Z

1 ~j$

Jjllll,

,::::

111111]’

M D;j”i

Place dosimeter adjacent to DUT, not in front of it!

Dosimeter

Dz =

Applicable ASTM Standards

[(scoI/P)z

/

(Scol/f)CaFz]

Low-Z Materials

DCaFp

E668 ASTM, 1916 Race St,, Philadelphia,

1-41

PA 19103-1187,

(215) 299-5400

Quick Reference Handbook of Practical Dosimetry Technique:

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session ?WOrleans, LA, July 1992

Application of Radiochromic Film in CO-60 Testing Applicability

Type of Radiation: Absorbed

Precautions

Cobalt-60

Radiation

1 krad(Si) -10

Dose Range:

Protect bare film from fluorescent Control relative humidity:

Use and Key Issues

Gamma

Mrad(Si)

light and sunlight.

30%s

RHs 75?40.

Dosimeter: [

Radiochromic film

Experimental

Hermetically sealed pouch

Arrangement: Device under test I

Filter Box: ::::;

::

/

1

~

Plane of device normal to

High-Z Iayers toward source

Cobalt-60

incident 111(11111111 11111111111111111111111111111111111111111111111~

[

Radiation Dosimeter

radiation

& ;*. D~~ P

F Dsi = [ (~en/p)si

Applicable ASTM Standards

E1275

/{~en/p)Al

E1276

ASTM, 1916 Race St., Philadelphia,

I-42

1 [ R~lm/RA1

I ‘film

E1400

PA 19103-1187,

(215) 299-5400

I

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session New Orleans, LA, July 1992

Quick Reference Handbook of Practical Dosimetry Technique!

Application of Calorimeters in Flash X-Ray Testing Accelerating Voltaae = 1 .0-1.5

Applicability

Type of Radiation: Absorbed

Precautions

Dose Range:

MV

= 100 keV average X-Radiation and very low energy X-Radiation >100 rad(Si) I

Design to minimize thermal losses from sensitive element. I

Use and Key Issues

Dose

Calorimeter:

Primary Application: Measurement of silicon equilibrium dose at moderate-energy (E e =1 .0-1.5 MeV) flash x-ray generators. Si Equilibrium Shield

~

Si Calorimeter

Element

Si Equilibrium Shield

Thermocouple

)--r 12.1

D = CPAT

rad(Si)

Fluence Calorimeter: Primary Application: Measurement of photon energy fluence at very low energy x-ray facilities. A Area of aperture = A

II

Thickness of calorimeter element is chosen to absorb all of the incident photon energy.

m

—

High-Z Collimator Au Calorimeter mass. m

Element

Thermocouple

E Y = cPm AT/A ~

MeV/cm2 —

I-43

Quick Reference Handbook of Practical Dosimetry Techniques

IEEE Nuclear and S~ace Radiation Effects Conference Tutorial Short Course - Dosimetrv Session ew Orleans, LA, July 1992 -

Application of Si PIN Diodes in Flash X-Ray Testing Applicability

X-rays, Bremsstrahlung

Type of Radiation:

s 1 E9 rad(Si) /s

Dose Rate Range:

Precautions

Operate so that output signal never exceeds 0.5 V ~i~s.

Use and Key Issues

Simple

Circuit:

Disadvantage: Bias voltage appears on signal cable. Cable response can interfere with PIN diode signal. Blocking Capacitor

PIN Diode

vout

4 Isolation Resistor ‘~

()

=

C

RL

‘T

=

V:as =

Preferred

Circuit:

I PIN Diode

A

No bias voltage on signal cable.

Separate bias voltage cable v

1

Isolation Resistor

.C

I V;as

T

vout

RL

=

Electrically the two circuits are equivalent, In both circuits, the capacitor C, must be sized so that it can supply all of the charge appearing at the load resistor without appreciably reducing the bias voltage applied to the PIN diode.

1.e.: C V~iaS>> ~(Vout/R~ ) dt

I-44

Quick Reference Handbook of Practical Dosimetry Technique!

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session 3W Orieans, LA, July 1992

Application of Compton Diodes in Flash X-Ray Testing Accelerating Voltaqe z 8 MV

Applicability

= 1 MeV average Gamma Radiation

Type of Radiation: Dose Rate Range:

Precautions

s 3 E13 rad(Si) /s

Arrange the measurement so that the cable Compton current signal is much smaller than the Compton diode siqnal.

Use and Key Issues

I

n Collector

Photon

Coaxial Connector

111111111111111111111 11111.

Beam

‘Ill

Aluminum Housing

Simple

Circuit:

Compton Diode

?7v0ut

Circuit Compton Diode

=

=

with

cable

Signal

current

compensation: BALUN

vout

= Twisting the signal cable together with an identical dummy cable can provide cable signal compensation in areas where the cable is subjected to high gamma dose rates.

—

I-45

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session >WOrleansl LA, July 1992

Quick Reference Handbook of Practical Dosimetry Technique:

Application of Scintillator-Photodetectors Accelerating

Applicability

Voltaae28

Type of Radiation:

in FXR Testing

MV

= 1 MeV average Gamma Radiation s 1 El 1 rad(Si) /s

Dose Rate Range:

Precautions

3perate so that the output signal, Vout / R ~ , never exceeds me half of the Child- Lagmuir space-charge limited current 1, = (4/9) v~~ &O(2e/m)”2A

Use and Key Issues

Simple

/d 2

Circuit:

Disadvantage: Bias voltage appears on signal cable. Cable response can interfere with PIN diode signal.

•1

Blocking Capacitor

Scintillator

vout Isolation Resistor

T

C ‘T

‘b

RL

*

Preferred Stint.

Circuit:

Photodiode No bias voltage on signal cable

n u

Separate bias voltage cable =

.C

Resistor

I

‘;as

-i-

Electrically the two circuits are equivalent. In both arcuits, the capacitor, so that it can supply all of the charge appearing at the load resistor without appreciably reducing the bias voltage applied to the PIN diode,

C,must be sized

1.e.: .C VbiaS>> ~(Vout/RL ) dt

I-45

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session ew Orleans, LA, July 1992

Quick Reference Handbook of Practical Dosimetry Techniaue:

Neutron Dosimetry at Reactor Facilities Applicability

Type of Radiation:

Precautions

Wait at least 8 hours after exposure before counting.

Fission Neutrons Approximate Energy: 1 MeV

Let used sulfur pellet decay for 1 year before reusing.

Use and Key Issues

Dosimeter: [W: ~,,, ~ :! :::.::.:: ;,5 :,;.<,:::,: ,: .: %,fit.::: w,<,,. ,, , :%>, it.l: :; x,*:,, ,, +::;j,~:: l~:w,x .:,, .:...@ ....

Sulfur Pellet Enclose in 1 mm ~W cadmium shield if thermal neutron flux dominates. 1

Determination

of Neutron

Fluence:

Let 01 ~ev be the 1 MeV equivalent

neutron fluence

Let B be the beta count rate, cpm Let S be the sulfur sensitivity, cDs ~~v/ cpm (Determined by Cf-252 calibration.)

Let R be the 1 MeV Equivalent/ Sulfur Ratio (Determinedfor each reactor spectrum by measurementwith threshold foil set.) Then

Applicable ASTM Standards

E261

@l~ev=

E265

BxSXR

E720

ASTM, 1916 Race St., Philadelphia,

I-47

E721 PA 19103-1187,

E722

E844

(215) 299-5400

I

Quick Reference Handbook of Practical Dosimetry Technique

IEEE Nuclear and Space Radiation Effects Conference Tutorial Short Course - Dosimetry Session ew Orleans, LA, July 1992

Heavy Ion Dosimetry for SEU Testing Applicability

Heavy Ions (Ar, Fe, Kr)

Type of Radiation: Parameters of Interest:

Precautions

LH,

Particle Flux: $ (x)y)

Itis important to monitor beam purity as well, to ensure that there is no wall scattering and no contamination by other ion species.

Use and Kev Issues LET:

LET of the particles is measured by a Si surface barrier detector that can be moved into the beam in place of the device under test (DUT). With a long time constant preamp, pulse height is proportional to total collected charge. For Si, the energy loss required to produce 1 PC of collected charge is 22.5 MeV.

Vacuum Chamber

11 cyclotron

# DUT El .0001“

I

Stint.

+1 Heav Ion I................. ...............................{l.... ...BeamY.................l

Collimated Si SB Detect. II

I IA

! Photo~ector ,

$1

Particle flux is measured in real time by a transmission detector consisting of an extremely thin scintillator coupled to a photodetector. A counter counts individual particles as they pass through the scintillator foil.

—

I-48

Quick Reference Handbook of PracticA Dosimetry Technique:

IEEE Nuclear and .%ace Radiation Effects Conference Tutorial Short Course - Dosimetw Session New Orleans, LA, July 1992 -

Proton Dosimetry for SEU Testing Applicability

Protons,

Type of Radiation:

20-150

MeV.

Proton Flux: $ ~, E ~, AEP /E ~.

Parameters of Interest:

Use and KeV Issues

Proton flux is measured in real time by a thin foil secondary emission monitor (SEM) which is calibrated by a Faraday cup which can be moved into the proton beam. Vacuum Chamber

h-I

El Cyclotron

Proton ................................... .................... [ t Beam

H

0,001”

.—.

.... 4

Collimator

0.001“

1

5 cm thick Graphite Faraday Cup

Al foil SEM

I [ ::;l:s~ steel ~ Exit Window

..(ll .1.

Al ,———.. .--–+-—----” ground plane

*

t–--’------t-

I

Eim+lbh

E: AEIE:

Proton energy, Ep , and energy resolution, AEP /Ep , are measured by a Nal crystal spectrometer which can be moved into the proton beam in place of the device under test (DUT).

—

I-49

Appendix

B

Commercial Sources of Dosimeters and Dosimetry Instrumentation in the USA TLDs and TLD Readers Solon Technologies, Inc., 6801 Cochran Rd., Solon, OH 44139 Teledyne-Isotopes, 50 Van Buren Ave., Westwood, NJ 07675 Victoreen Instrument Co. Dyed Plastic

Dosimeters

and Readout

(216) 248-7400 (201) 664-7070

Instrumentation

Radiochromic Dye Film and Readers, GAFchrom”cfihn: Far West Technology, Inc., 330-D South Kellogg, Goleta, CA 93017

(805) 964-3615

Dyed PA4MA: Harwell Laboratory, Irradiation and Dosimetry Service, Chemist~ Div., Bldg. 10,30, OxfordshireOX110RA, UK Tel: Abingdon (0235) 24141 Ext. 5704 or 4232 Spectrophotometers: Bausch & Lomb PIN

Diodes Quantrad Corporation,

Compton

19900 S. Normandie Ave., Torrance, CA W502

(213) 538-9800

Diodes

SAIC, Las Vegas, NV Team Specialty Products, 11030 Cochiti Rd. SE, Albuquerque,

NM 87123 (505) 291-0182

Scintillators National Diagnostics, 198 Route 206 South, Somerville, NJ 08876 Nuclear Enterprises America, 23 Madison Rd., Fairfield, NJ 07006 Vacuum

Photodiodes

Hamamatsu Coqm-ation, 420 South Ave., Middlesex, NJ 08846 ITT Electro-Optical Prod. Div., 3700 E, Pontiac St., FL Wayne, IN 46803

1-50

(219) 423-4341

Total-Dose Radiation Effects (From the Perspective of the Experimentalist)

Peter S. Winokur Sandia National Laboratories Albuquerque, NM 87185

This work was supported by the US. Department of Energy through contract number DE-AC04-76DPO0789

TOTAL-DOSE R4DIATION EFFECTS (From The Perspective Of The Experimentalist) Peter S. Wmokur Sandia National Lalnmatories Albuquerque,

NM 87185

1.0 Introduction 1.1 Goal 1.2 Scope 2.0 Review of Total-Dose ionization Effects 2.1 Basic Mechanisms 2.2 Effects on MOS Devices and Circuits 2.3 Important Measurement Parameters 3.0 Measurement and Analysis Techniques 3.1 Test Structure and Discrete Device Measurements 3.2 integrated Circuit Measurements 3.3 ASTM, MILITARY, and DNA Test Standards 4.0 Critical Measurement Variables 4.1 Dose 4.2 Time 4.3 Dose Rate 4.4 Bias (Electric field) 4.5 Temperature 4.6 Energy 5.0 Future Trends 5.1 QML Testing Requirements 5.2 Wafer-Level Testing 6.0 Summary

1.0 INTRODUCTION 1.1 Goal Many applications require integrated circuits (Ks) that are tolerant to high levels of ionizing or total-dose radiation. nis radiation Can result from nuclear weapo~ nuclear reactor, large accelerator (e.g., Superconducting Super Collider) or natural space environments. ln addition, many techniques used to fabricate today’s complex VLSI circuits expose the circuits to ionizing radiation during the process sequence [1]. Whatever its origin, ionizing radiation can cause significant damage to integrated-circuit materials [2]. This damage can lead to circuit performance degradation, logic upset, and even catastrophic circuit failure.

II-1

Important goals of the radiation-effects community include the fabrication of radiation-hardened ICS and the assurance that ICS will operate in severe radiation environments. Clearly, radiation testing of test structures and product microcircuits plays an important role in accomplishing these goals. The focus of this short course segment will be on how to perform total-dose radiation tests and on how to analyze the acquired data. The student should leave this course with insight into the improved design of experiments based on a first principles understanding of the radiation response of siliconbased devices to ionizing radiation. The student should leave with a better understanding of which device parameters need to be measured what tools are available to make these measurements, and how to design the experiment so results are easily extended or extrapolated to radiation environments of interest, e.g., space or nuclear reactor.

1.2 Scope The short course segment will begin with an overview of ionization effects in siliconbased devices. The discussion will be focused on CMOS technology, and often radiationhardened CMOS technology; some testing issues associated with advanced bipolar technologies will be addressed. After reviewing the basic interaction of ionizing radiation with an MOS structure, the effects of ionizing radiation on simple devices and integrated circuits will be summarized. This brief survey will provide the framework for the rest of the short course segment and should serve several purposes. First, it should help identify For example, parasitic leakage currents what device parameters need to be measured. (see Section 2.2.2) may dominate the radiation response of a commercial CMOS technology and must be characterized. On the other hand, parasitic leakage currents are generally not an issue for radiation-hardened CMOS technology that utilizes “hardened” field-oxides or guardbands for isolation. Second, an understanding of basic mechanisms can be an aid in making useful and relevant measurements. For example, it has been shown that the radiation response of CMOS devices is controlled by the growth and

annealing of radiation-induced defects, and consequently, device response varies with time. If the device parameter being measured changes with time, that needs to be taken into account. Otherwise, it will be difficult to compare measurements taken at different dose rates (which is often the case) or to use irradiations at a given dose rate to reliably predict how an integrated circuit will perform in differing radiation environments. Finally, basic mechanisms can serve as a useful check on the correctness of data. Does the data make sense in light of our understanding of the physical processes governing device response? If not, have we discovered a new effect, perhaps one that is technology specific, or do we simply have bad data? A comprehensive review of measurement and analysis techniques for characterizing the response of CMOS devices to ionizing radiation will be provided. The test structures needed, advantages and disadvantages, and analysis routines required for implementation of each technique will be described. Some techniques readily support radiation testing for production and/or hardness assurance, while others primarily find their application in basic research. An important aspect of this short course segment will be a discussion of the basic physics underlying each technique and its associated analysis. An understanding II-2

of this physics is essential to the correct design of experiments, and is often the starting point in the development of generic test procedures and guidelines used by the radiation effects community, e.g., ASTM standards. Much of the discussion will be focused on discrete device measurements, although there is a separate section on IC measurements. Throughout the text, several examples of test structure-to-IC correlation will hopefully bridge the gap from measurements on discrete devices to measurements on integrated circuits.

The short course segment will also focus on critical measurement variables (or test conditions) that must be specified when determining the radiation response of CMOS devices to ionizing radiation. Critical variables include applied bias, temperature, energy of incident radiation, dose, dose rate, and time (e.g., between irradiation and test). Each of these variables can affect the measured radiation response and need to be carefully chosen, controlled, and documented. For example, should static or dynamic bias be applied during irradiations to characterize part response? How are space-charge or doseenhancement effects are accounted for? Should irradiations always be performed at the system dose rate? Can 10-keV x rays be used instead of l-MeV Cow gamma rays to simulate space radiation effects? A discussion of each critical measurement variable will include a discussion of literature results, practical considerations and tradeoffs for testing, and a testing recommendation. In a final brief section, some future trends in total-dose radiation testing will be discussed. These trends include an increased reliance on test structures and wafer-level testing as part of new qualification initiatives like the Qualified Manufacturers List. The short course will focus on components,

2.0 REVIEW OF TOTAL-DOSE

and not subsystem

IONIZATION

and system test issues.

EFFEC1’S

2.1 Basic Mechanisms

When SiOz metal-oxide-semiconductor

(MOS) structures

are exposed to ionizing

radiation, electron-hole pairs are generated along the track of the incident particle. (Total-dose radiation effects are illustrated in Fig. 1 with the use of an MOS band In general, some fraction of these electron-hole pairs will recombine, but that diagram.)

fraction is a complicated function of the material, the kind of radiatio~ and the applied field. Experimentally the “yield of hole-electron pairs that escape recombination can be determined from a threshold-voltage shift measured at liquid nitrogen temperature and, in Si02, is consistent with an electron-hole pair-creation energy of =17 eV [3-5]. That is, ionizing radiation generates electron-hole pairs in Si02 at the rate of one pair per = 17 eV of energy deposited by the irradiation. Following the initial creation process, the radiation-generated electrons and holes transport under the applied electric field. For positive

bias, the electrons

are swept to the gate and collected

in picosecond,

while the

holes undergo a “stochastic” transport over to the SiOz/Si interface [6]. “Stochastic” transport involves hole motion via polaron hopping between localized sites randomly II-3

When the holes arrive at the SiOz/Si interface a certain percentage are trapped. This percentage strongly depends onprocessing. Uncommercial oxides, it can be greater than 50 percent, while for oxides that receive special processing to decrease their sensitivity to radiatio~ it can be a few percent or even lower. distributed

in the SiOz.

DEEPHOLE

TRAPPINO AT TW B1/S102 INTERFACE

ELECTRON-HOLE PAIRS -NERATED BY IONIZINQ RAOIATION

[

INTERFACE TRAPS MSULTING FROM INTERACTION OF NOLE8

POLY-SI TRANSPORT H@pl)m OF NOLE8 rHRouGH LOCAUZED STATEB IN +

WLK

Figure 1. Band diagram illustrating physical processes governing the response of MOS devices to total-dose ionizing radiation. (After F. B. McLean et al., Ref. [2]) Most of the positively-charged holes are trapped within 7.5 nm of the SiOz/Si interface and generally anneal with time. The most widely used model for predicting hole anneal is the first-order tunneling model of McLean [7]. This model assumes that the dominant charge loss mechanism for irradiated MOS devices is recombination between trapped holes and electrons that tunnel from the silicon substrate. Holes trapped within the first 3.0 nm recombine in the first minute with electrons that tumel from the Si. The model accounts for many features of experimentally observed hole anneal, including electric field effects. ln more recent work, McWhorter et al. [8] have derived a new model for predicting hole anneal that, in addition to tunneling, includes a thermal emission charge loss mechanism. The thermal emission model assumes the ameal of radiation trapped holes results from the thermal emission of holes from traps in the oxide to the valence band of the oxide. This combined tunneling/thermal emission model, illustrated in Fig. 2, is consistent with a wide range of experimental data in the literature which examines the effect of temperature, as well as electric field, on hole ameal [9-12].

Both tunneling and thermal emissionmodels are also consistentwith a wide bodyof experimental data that suggests the anneal of holes proceeds with log(t) [13-17]. In the tumeling formalism, log(t) behavior results because the probability that an electron will tunnel to a trapped hole depends exponentially on the distance from the trap to the SiOz/Si interface. The maximum rate of tunneling occurs at a tunneling front, x~ (shown in Fig. 2). To first order, all holes closer to the interface than x~ have annealed and all

holes farther away have not. In a thermal emission formalism, log(t) behavior results because the probability that a hole will be excited to the valence band depends exponentially on the energy of the trap relative to the SiOz valence band. Similar to the II-4

tunneling front, the anneal is characterized by a thermal emission front, $. (shown in Fig. 2), and to first order, all holes with energies less than +~have annealed and all holes with energies greater have not.

1

xm=&In[d]

r-i

mmw EMfSNU#

I

POLF SI

,,...,,..,.-...

SI(3Z Figure 2. Band diagram illustrating tunneling and thermal-emission charge loss mechanisms for anneal of trapped holes. Tunneling, ~, and thermal emission, ~, charge-loss fronts are shown, (After Ref. [E])

In addition to hole trapping and anneal at the SiOJSi interface, there is the buildup of radiation-induced interface traps [18]. Many models have been proposed to describe the origin and nature of these traps at the interface that have energies in the Si band gap. Some models suggest that radiation-induced interface traps result from (1) hole trapping at the interface followed by electron injection [19], (2) hydrogen interacting at the SiOz/Si interface that is liberated during irradiation in either bulk [20,21] or interracial regions in the oxide [22], and (3) stress [23]. Radiation-generated interface traps can have either a positive or negative charge depending on whether they are donor or acceptor states, and their charge occupancy depends on the applied bias or band bending at the SiOJSi interface. Specifically, a donor trap level is in a neutral charge state when it is below the Fermi level, and becomes positive by donating (giving up) an electron when it moves above the Fermi level. An acceptor trap level is in a neutral charge state when it is above the Fermi level, and becomes negative by accepting an electron when it moves below the Fermi level. When a voltage is applied to the gate of an MOS device, the interface trap levels move up or down (along with the valence and conduction bands) relative to the Fermi level. The charge state of the interface trap changes when it crosses the Fermi level. Under all conditions, the interface trap is in its more positive charge state when it is above the Fermi level. Many radiation studies have demonstrated that interface traps contributing to rt-channel threshold-voltage shifts are predominant tly negatively charged and acceptorlike, while those contributing to p-channel threshold-voltage shifts are predominantly positively charged and donor-like. In recent years [24], the P~ center (a trivalent silicon II-5

atom bonded to three other silicon atoms) has been identified as a single, amphotenc defect responsible for the interface traps. As the Fermi level moves from the valence band toward rnidgap, the positively charged P~ center accepts an electron and becomes

paramagnetic and neutral (donor-like trap). When the Fermi toward the conduction band edge, the neutral P~ center picks and becomes diamagnetic and negatively charged (acceptor-like paramagnetic or neutral, i.e., there is net zero charge in the Fermi level is at rnidgap.

level moves from rnidgap up an additional electron trap). The pb centers are interface traps, when the

Most experimental observations suggest that interface traps build up over time following irradiation. This growth can take place from seconds to thousands of seconds or more at room temperature [15,25-33]. An example illustrating a time-dependent buildup of interface traps is shown in Fig. 3. The figure shows normalized high-frequency capacitance-voltage (C-V) traces of an aluminum-gate dry-oxide MOS capacitor both before, and for a series of times from 0.04 to 400 s following pulsed electron-beam irradiation. Immediately following irradiation at 0.04 s, the C-V curve is simply shifted along the negative voltage axis with no discernible change in its shape relative to the preirradiation trace. The next few curves from 0.4 to 4 s show a rigid shift back in the positive voltage direction, but with little change in shape--this recovery is largely due to the final stages of hole transport and annealing of trapped holes at the SiOJSi interface. However, between 4 and 40 s following irradiatio~ a significant stretchout or increase in distortion of the C-V traces becomes evident. This stretchout becomes even greater at 400 s. The increased distortion with time in the C-V traces is attributed to a timedependent,

“delayed’ buildup

of radiation-induced

interface

traps.

1.0 0.8 4.00

s

0.6 0.4 0.2

0.0 -6

I

I

I

I

I

-5

-4

-3

-2

-1

n-TYPE c1OO> DOSE= 200 krad (SI02] I I

0

1

2

3

APPLIED VOLTAGE (V) Normalized 1-MHz CHF-V curves of an MOS Al-gate capacitor several times following pulsed electron beam irradiation. (After Ref. [26])

II-6

at

2.2 Effects on MOS Devices and Circuits As discussed in the previous sectio~ the pnma~ effect of ionizing radiation on MOS structures is to produce electron-hole pairs in the Si02 layer. The radiation-generated electrons rapidly exit the material following irradiation while the radiation-generated holes transport and eventually become trapped in interracial regions. lt is important to note that almost all models for radiation-induced interface traps begin with the transport and/or trapping of radiation-generated holes [18]. Electrons and holes that escape the initial recombination process can produce photocurrents and space charge effects in MOS devices and circuits. This short course segment will focus on (1) space charge effects as opposed to photocurrents and (2) long-term or semi-permanent effects associated with For example, semi-permanent effects resulting from hole trapping and space charge. anneal or interface-trap buildup and anneal will be discussed. However, short-term recovery effects, taking place at times less than 1 second and mairdy associated with hole transport will not be discussed. Semi-permanent effects on MOS devices and circuits caused by the buildup of space charge in the SiOz layer fall into several categories. Q Voltage

offsets or shifts

●

Induced

parasitic

●

Speed (mobility)

leakage currents degradation.

Each of these categories of effects will be discussed in more detail below.

22.1

Voltage offsets or shifts

The total-dose response of n- and p-channel MOS transistors to ionizing radiatio~ illustrated in Fig. 4, is due to trapping of holes in the oxide and the buildup of interface traps [34]. In general, the effect of radiation-generated charge, Ap, on the thresholdvoltage shift, AVth, of a transistor is given by

AV,~ = (-l/COX) ~Ap(x)(x/tOX)@ o

(1)

where tOXis the oxide thickness, COXis the oxide capacitance, and x is measured from the gate-SiOz interface. From Eq. (1) it can be seen that positive charge, i.e., trapped holes, will cause a negative shift in the threshold voltage of a device, while negative charge will cause a positive shift in the threshold voltage. In general, the initial response of an MOS transistor to radiation is a negative shift in the threshold voltage due to the buildup of trapped holes. For sufficiently large amounts of trapped positive charge, the n-channel device may be turned on even for zero applied gate bias. In this case, the device is said to have gone into “depletion mode.” When strongly into depletio~ the n-channel device

ceases to function because it cannot be switched from the

“on” to the

“off” state

N-CNANNEL TRANSISTOR

3

OXIDE . TRAPPED

t

I !

INTERFACE

THRESHOLD VOLTAGE

(v)

1

o -1

1

102

,.3

1

,.4

TOTAL IONIZING

I

,.5

*I

I

-

,~06\ ,.7

DOSE [rad (Sl)l

-2 -3 P-CHANNEL TRANSISTOR -4

Figure 4. Threshold voltage versus dose for irradiated n- and p-channel transistors. (After Ref. [34]) --it is always on!

At some later time, the threshold

voltage may “turn around’ and start to shift in the This recovery is unique to n-channel transistors and can be attributed positive direction. to negatively charged (acceptor-like) interface traps building up at a higher rate than holes. It is possible for the threshold voltage of an n-channel transistor to increase to values above its preirradiation value following irradiation, a condition termed “rebound” [9] or “superrecovery” [35]. This occurs when most of the trapped holes are annealed leaving primarily the negative charge contribution of the interface traps [9,35]. During “rebound” the threshold voltage overshoots its preirradiation value, causing the shift at long times to be opposite to that observed at short times after irradiation. For the p-channel transistor, the buildup of positively-charged trapped holes and positivelycharged (donor-like) intetiace traps causes the threshold voltage to continually become If magnitude of the p-channel threshold becomes greater than V~~, the more negative. supply voltage, the p-channel device ceases to function because it cannot be switched from the “off’ to the “on” state--it is always off!

The changes in threshold voltage for n- and p-channel transistors described above will affect current-voltage (I-V) curves in either the subthreshold or saturation regions of device operation [2,36]. Figure 5 shows a series of subthreshold I~s-V~~ curves for an n-channel transistor recorded at prerad and total doses of 30, 100, 300, and 1000 krad(SiOJ [37]. Irradiations were performed with 10 V between gate and source, i.e., v = 10 V, using parts from Sandia’s 4/3-pm technology [38,39]. Note that, for these deG~ces, the current value associated with Vt~ is -1 PA With increasing irradiation, I-V curves are seen to shift to more negative voltages due to the buildup of trapped positive

charge. There is also an increase in source to drain current at V~~ = O V, i.e., the “off’ mode. The “off’ mode current or leakage on this transistor is sub-picoamp at prerad, and increases to - 1 nA at 100 krad(SiOz) and to - 1 PA at 1 Mrad(SiOz). II-8

10”2 ‘DS =1OV

—

10-4

I ~~ (A)

1 Inv

10-6 10

1 M:aP

-lo

10

-12

411-’4J- 1 lW

iz!ir

1th

1 o-e

I

I

I

I

I

I

-5=4-3-2-101234 GATE VOLTAGE (V)

I

I

5

Figure 5. Subthreshold current-voltage curves for an MOS transistor before irradiation and at four different radiation levels. The gate bias during irradiation was 10 v. Currents corresponding to threshold (It~), inversion (li~v), and midgap (1~~) are marked for each curve. (After Ref. [37]) Shifts in threshold-voltage have important effects on the operation of integrated circuits. Clearly, threshold-voltage shifts can lead to functional failure of the integrated circuit when n-channel transistor threshold voltages become less than O volts and/or

p-channel transistor threshold voltages becomes greater (in magnitude) than V~~. In addition, “rebound” reduces n-channel transistor drive, thereby slowing down the integrated circuit. “Rebound” has been observed to cause IC failure [9]. Shifts in p-channel transistors also reduce drive and lead to a degradation in speed or loss of TTL comparability. Finally, increased “off’ mode transistor leakage will be reflected by an increase in “standby” power supply current for an IC. Figure 6 plots power supply current, I DD) during irradiation and anneal for an 8-bit microprocessor from the Sandia 4/3-pm technology [38]. The horizontal axis is in units of logarithmic time, with times corresponding to the end of the IN and 107 rad(SiOJ irradiations marked. During the irradiation to 107 rad(SiOz), increases in power supply current follow increases in n-channel subthreshold current at V~9 = O V (shown in Fig 5). At 107 rad(Si02), IDDis - 0.7mA. During the 125 “C anneal, the n-channel threshold voltage recovers quickly, subthreshold current at zero volts decreases, and IDD decreases sharply. In Fig. 7, n-channel transistor leakage is correlated with integrated circuit power supply current through a series of irradiations and anneals [40]. A one-to-one correlation is seen with postirradiation ameal points falling on the same curve as traced during increasing dose. The magnitude of this relation depends on the number and size of n-channel transistors which are at zero gate bias at any given state of the circuit. Its slope is approximately unity as expected. An equation [40] which describes this relationship is I DD

= weffIDS(v~S

II-9

=

O) + 1,=

(2)

where W,ff is the combined represents the contribution discussed in the next section.

width of n-channel of parasitic leakage

transistors at zero gate bias and IP,r paths, e.g., “field’ transistors, that are

1.0 0.9 0.0

IRRADIATION ~

0.7 0,6 1Do (mA)

0.5

=Ilv

0.4

AT VDO

0.3 0.2 0.1 0,0 PRE

0.1 TIME

106 R.ADS 1.0

107RADS 10.0

100.0

(hrs)

Figure 6. Static power supply current for an 8-bit microprocessor irradiation to 107 rad(SiOJ and anneal at 125”C. (After Ref. [39].) 10

-3

during

r LOT Q2403A LOW NOT

IDD(A) =11 V C3VDD

❑ SN7697

V SN759B HIGHN

OT

O SN7686

A SN7590 ● POSTANNEAL

-0 ,0

-6 ,0

@

= OV

IDs (A)@

I

I

I

I

I I 1O-Q ~a.1210.1110-101Q

.7 ,0

I -6 ,0 .5

power supply current for 2k SRAMS versus n-channel leakage for a series of irradiations and anneals. (After Ref. [40].)

Figure

‘7.

Static

transistor

2.202 Induced parasitic leakage currents In commercial CMOS technologies, the dominant effect of ionizing radiation is usually charge buildup in isolation or “field’’-oxide regions. A cross-section of a modern recessed field-oxide structure that provides device-to-device isolation is shown in Fig. 8 II-lo

(bottom) [2]. [The particular type of isolation pictured in Fig. 8 (bottom) is called LOCOS, which stands for local oxidation of silicon. A bird’s beak structure is formed during growth of a thick SiOz layer, typically 400 to 1000 u during which the channel region is masked by a nitride layer that prevents its oxidation.] During irradiatio~ there is positive charge buildup and the formation of radiation-induced current leakage paths in the bird’s beak regions of the device--the buildup of trapped holes inverts the underlying p-type silicon to form a parasitic leakage path between source and draiq as illustrated in Fig. 8 (top). In essence, a field-oxide transistor (i.e., a FOX.FET) has been “turned on” and remains so at zero volts, leading to increased standby current in the device. This is illustrated in Fig. 9 for a set of I-V curves obtained on a commercial n-channel transistor [29]. Before irradiation, 1~~ at zero volts is approximately 1 pA. Following only a 2 krad(SiOJ irradiation at 200 rad(SiOz)/s, a parasitic leakage path is created and the current at zero volts rises to -500 pA. At 10 krad(SiOJ, a leakage current at zero volts greater than 1 PA is observed! The transistor I-V characteristics are clearly dominated by the parasitic leakage component. As was the case for increased current due to thresholdvoltage shifts, described above and given by Eq. (2), increased transistor leakage leads to increased power supply current for the integrated circuit. The turn-on of a parasitic parallel transistor near the bird’s beak region of the lateral oxide isolation leads to excessive current in the integrated circuit and is nearly always the primary failure mode in modern commercial CMOS technology [29].

\

k----------

-----J

EWF:T Field Oxide Region

Bird’s Beak

Gate Oxide

/

Posltlve Trapped Charge Induced Current Leakage

Path Figure 8. (Top) Parasitic leakage path between source and drain caused by inversion of p-type Si beneath thick field oxide. (Bottom) Cross-section showing LOCOS isolation used in commercial CMOS technology and radiation-induced leakage paths. (After F. B. McLean et al., Ref. [2])

As seen in the above example, parasitic leakage paths can be “turned on” at doses as This occurs because, to first order, the threshold-voltage shift low as several krad(SiOJ. from trapped holes is proportional to tOXZ[41-43]. A thickness-squared dependence is expected because (1) the number of radiation-generated holes is linear with tOXand (2) AV~h,the first moment of the trapped-hole distribution as given by Eq. (l), is linear with 11-11

However, shifts infield oxide structures are never as large as the tOXzlaw might s;ggest because (1) much of the radiation-generated charge recombines at the small electric fields (typically - 1~ to 105 V/cm) present in the micron-thick field oxide [3-5] and (2) the threshold-voltage shift tends to saturate due to trap filling and/or field reversal in the thick oxide (see Section 4.1). Because increased standby currents can lead to circuit failure, radiation hardened CMOS technologies employ “guardbanded” or “hardened field-oxide” structures to eliminate this leakage path. For these technologies, total-dose radiation hardness is then dominated by the thin gate oxide, and hardness levels t.

of 10 Mrad(Si)

can be achieved

[38,39,44,45].

10-2

1

1jo.4

1-

10.6 I ~~

r

OKI-TT N-CHANNEL vGg=5 v

(A) 10-6

200 rnd (S1)/s

!

10-10

10-12

t 10.14

I .5

I

I

-4

-3

1

I -2

-1

I

I

I

I

1

I

0

1

2

3

4

5

GATE VOLTAGE (V)

Figure 9. Subthreshold current-voltage curves following CO-60 irradiation for OKI n-channel transistors irradiated with 5-V bias at 200 rad(SiOz)/s, Large increases in current at zero volts are caused by parasitic field-oxide leakage. (After Ref. [29])

Parasitic field-oxide leakage can also be a significant concern for the hardness of bipolar integrated circuits. The major advance in bipolar technology has been due, in part, to the introduction of “recessed oxides [46].” The recessed oxide, illustrated in Fig. 10, is a field oxide which extends from the surface into the silicon as deep as the active components. This oxide provides lateral dielectric isolation, acts as a diffusion stop, and Thus, recessed oxides allow much smaller feature size, minimizes junction capacitances. increased packing density, and higher speed. However, when irradiated, several parasitic leakage paths can be formed including buried layer to buried layer channeling, collector to emitter channeling on walled emitters, and increased sidewall current [47]. The increased current associated with inversion of these parasitic MOS field transistors can lead to circuit failure as low as 10 krad(Si) [46-48].

II-12

COUECTOR TO EMITTER 2 ~ANNELING ON WALLED

INCREASED SIDEWAU 3 CU~ENTs

EMITTERS Y

7

P BASE N EPITAXIAL

N+ BURIED LAYER

N+ BURIEDLAYER P SUBSTRATE BURIEDIAYER TO BURIED LAYER CHANNELING

Figure 10. Typical cross section of recessed field-oxide bipolar structure utilizing walled emitters. Radiation-induced parasitic leakage paths are shown. (After Ref. [46])

22.3 Speed (mobility) degradation Interface traps seriously affect the performance of MOS devices and circuits. They play an important role in controlling the threshold voltage, channel mobility, transconductance, and surface recombination velocity of irradiated transistors. These changes in basic device properties then result in parametric changes in the operation of silicon integrated circuits, e.g., timing, speed, and drive. Several effects of interface traps on devices and circuits are illustrated below.

When interface traps are generated by irradiation, the shape of the current-voltage characteristics is changed. As the voltage is swept, interface traps empty or fill, and modify the the electric field by requiring more (or less) charges on the gate to create a given surface field in the semiconductor. Typical changes that occur in the subthreshold region for n-channel transistors are shown in Fig. 5 [37,49]. In general, these characteristics (for both n- and p-channel transistors) are seen to shift to the left a direct consequence of the buildup of positive oxide-trapped charge, and decrease in slope. The decrease in subthreshold slope can be directly related to (and is a measure of) the radiation-induced buildup of interface traps (see Section 3.1.3). Subthreshold slope determines how the transistor turns on and off--a decreased slope means that a larger swing in gate voltage is required to bring the transistor into strong inversion. Therefore, radiation-induced interface traps reduce the switching speed of MOSFETS. In the saturation region, a decrease in slope is also observed and represents a decrease in transconductance g~ of the device, which is defined (for constant V~J as (3)

II-13

where IDs is saturated drain current, and VGs and VDs are the gate and drain voltages, respectively. In addition, as I-V traces are shifted to more negative voltages and stretched out, output drives of n- and p-channel transistors change. For the n-charnel device shown in Fig. 5, the output drive, IDN, at the supply voltage VDD has increased during irradiation--the reduction is drive caused by interface traps is more than offset by the increase in drive resulting from the net negative threshold-voltage shift. For a p-channel transistor undergoing a similar radiation-induced shift, the output drive IDp will decrease. Significant decreases in output drive can result in a reduction in speed, as well as loss of TTL compatibility. A very important effect of the buildup of interface traps is mobility degradation. Initial work [50,5 1] suggested that reductions in mobility were due to increased lattice and Coulomb scattering by charged interface traps, and that the average surface mobility was proportional to I/Nit, where Nit{cm-z} is the areal density of interface traps. Following the earlier work of Sun and Plummer [52] and Galloway et al. [53,54], Sexton and Schwank [40] showed that mobility degradation can be fitted (over a wide range of experimental conditions) by the empirical relationship (4)

p = 1%/(1 + a(~it))

where POis the pre-irradiation value of mobility and a = (8 Y2) x 10-13cmz. The data of Sexton and Schwank [40], showing mobility degradation following irradiations of n- and pchannel transistors under all bias conditions, is plotted in Fig. 11. Galloway et al. [53,54] have used this relationship between mobility and interface traps as a basis for a simple model to separate the effects of oxide-trapped and interface-trap charge on MOSFET I-V characteristics. From first principles, radiation-induced decreases in mobility lead to reductions in subthreshold slope, g~, transistor drive, circuit speed, etc. i 100

80

NORMALIZED MOBILITY

60 HIGH

Nltl Not C) N.ON

Nit’ Not ● N - ON

AN-OFF

AN-OFF

❑

■

40

20

VP

0

LOW

o

p-OFF

-ON,

P-OFF

7P-ON 4

I

I

I

8

12

16

AN,t (1011 cm-2) Figure 11. Normalized effective channel interface- trap density. (After Ref. [40])

II-14

mobility

plotted

as a function

of

surface recombination velocity [5457]. Surface recombination velocity, SO,is proportional to density of interface traps, Nit, and the square root of the product of the hole and electron capture cross sections, u. and The generation

of interface

traps also increases

UP:

(5)

so = l/z(unuP)lt2vTkTNit,

where v is the thermal velocity and k is Boltzmann’s constant [18,36]. An important effect (in addition to parasitic leakage, see Section 2.2.2) of total ionizing dose in bipolar semiconductor devices is the generation of interface traps and the subsequent increase in the charge recombination rate at the interface of the emitter and Si02 passivation [58]. This increased surface recombination velocity requires an increased base current to maintain equilibrium. The reduction in gain is given by A(l/~)

= AIB/Ic,

where ~ is the DC gain, Ic is the collector current, and &is currents.

(6) the sum of all surface base

A natural extension of the effect of interface traps on transistors is their effect on functional ICS. One example of that effect is shown in Fig. 12, in which the change in “read’ timing is plotted for a 2k SRAM [29]. Read time, AtRD,is a measure of the time between a read request to the memory and valid data out. The timing is seen to degrade (increase in magnitude) with ionizing radiation. This degradation in timing is caused by threshold-voltage shifts and reduced n- and p-channel mobility, which are directly related to the buildup of radiation-induced interface traps. A second example of the effect of interface traps on ICS is given by Saks et al. [59]; this work demonstrated an excellent correlation between the increase in dark current and the increase in interface traps in irradiated CCDS. 20.0 0

SANDIA SA3001

24.0

LOT G0250A

0

0,09 rad (Sl)/a

20.0 o AtRD(ns) @o

VDD=l

16.0 -

OV

12.0 0 0.0 0 4.0 0 0.0 PHE

Figure 12. Change in “read” timing, Atm, versus dose for 2k SRAMS irradiated with 1O-V bias at 0.09 rad(Si)/s. (After Ref. [29])

II-15

2JL4 Failure

dose for ICS

In order to quantitatively measure the “total dose hardness” of an integrated circuit, it is necessary to define some criterion for failure. Many studies in the past [29,60-63] have defined failure “functionally” or “catastrophically,” i.e., the IC simply ceases to function. However, it is well understood that an IC may fail its circuit or system application long before it experiences a “functional” failure. For example, an IC has effectively failed if it cannot drive signal lines rapidly or if it draws too much current from available power sources. In previous work [29,63], failure was defined “parametrically,” such that an IC fails if one of its parameters, e.g., static power supply current, timing, or TTL input level, exceeds a preset specification following irradiation. The use of “parametric” failure criteria allows a more realistic measure of “total-dose hardness” and provides important information to the design, technology, or system engineer on how to improve IC performance in the intended radiation environment. In the discussions associated with Figs. 6 and 12, parametric failure could have been defined in terms of an increase in power supply current, N~~, or an increase in read timing, At~~. For purposes of illustration, we could have defined an SRAM failure as static currents greater than 0.5 rnA m timing changes greater than 20 m [29,63].

23 Important Measurement

Parameters

With an understanding of total-dose basic mechanisms and of the effects of totalionizing dose on devices and circuits, it is useful to identify some key measurement parameters. That is, what do we need to measure to successfully characterize the totaldose radiation response of devices and ICS? This characterization may support research studies to further elaborate mechanisms underlying the radiation response, device characterization to identify and correct specific failure mechanisms, production efforts to improve the hardness of a technology, or hardness assurance activities to verify that ICS meet radiation specifications as part of QPL and QML qualification schemes [64]. In general, both test structures and ICS will need to be characterized before and after irradiation. The most widely used and relevant of these test structures are n- and p-channel MOSFETS and simple capacitor structures. MOSFETS are the basic building blocks of CMOS technology, and provide significant insight into the radiation response of ICS. That is why a great deal of the discussion so far has focused on the transistor structure. The following parameters can be, and are often, measured on MOSFETS and/or capacitors. This list is not completely inclusive of all measurement parameters

that mightbe of interest. ●

v~h{v}, threshold vohage

●

VOt{V}, threshold voltage due to oxide-trapped charge

c Vit{V}j threshold

voltage due to interface traps

11-16

c NOt{cm-z}, areal density of oxide-trapped charge ●

Nit {cm-z}, areal density of interface-trap

charge

c Dit {cm-aeV- 1}, distribution of interface traps in the Si band gap ●

p{crnW-%-1}, normalized effective channel mobility

●

IDN{A}, n-charnel output drive

●

IDP{A}, p-channel output drive

Ideally, electrical characterization measurements on CMOS integrated performed before irradiation and after each exposure include the following [65] Functionality supply voltage ●

circuits

testing at maximum and minimum operating frequencies, minimum

. Shmoo plot of maximum operating frequenq,

fu

{MHz}, versus supply voltage

●

Access and delay times

●

IDD{A}, power supply current

●

Leakage currents in tri-state outputs and all inputs; output drive currents

●

VIH{V}, input high and VIL{V}, input low voltage.

For bipolar devices, the DC gain, ~, is often measured as well as several leakage paths including buried-layer-to-buried-layer (BLBL) and collector-to-emitter (C-E).

3.0 MEASUREMENT

AND ANALYSIS TECHMQUES

This section will outline many of the tools available to the experimentalist for characterizing the response of CMOS devices to ionizing radiation. The test structures needed, advantages and disadvantages, and analysis routines required to implement each technique will be described. Some of techniques readily support radiation testing for production and/or hardness assurance, while others primarily find their application in basic research. AII important aspect of this section will be a discussion of the basic physics underlying each technique and its associated analysis. An understanding of this physics was often the starting point in the development of the measurement technique itself. For example, by understanding the properties of radiation-induced interface traps, we can design measurement techniques to detect them and to distinguish them from other types of defects. In additio~ an understanding of the physics is essential to the correct design of

11-17

experiments, and supports the development of generic test procedures and guidelines used by the radiation effects community, e.g., ASTM standards. Techniques and tools to measure each of the device and circuit parameters identified in Section 2.3 will now be discussed.

3.1 Test Structure and Discrete Device Measurements 3.1.1 Threshold voltage, linear and saturated

The threshold or “turn on” voltage of a MOSFET is perhaps its most basic parameter. Threshold occurs when sufficient voltage is applied to the gate of a MOSFET to invert the semiconductor surface. If a small drain voltage is applied, a current will flow from the source to the drain through the conducting channel. Thus the channel acts as a resistance, and the drain current IDs is proportional to the drain voltage VDs. This is the “linear” region of device operation. As the drain voltage increases it eventually reaches a point at which the channel “pinches off” and the drain current saturates at a constant value. This is Threshold voltage measurements can be the “saturation” region of device operation. made in either the linear or saturation regions of device operation. When making a threshold voltage measurement, temperature should be controlled with specified limits, typically * 5°C. In the linear region the drain-source current can be approximated by IDs(lin) ~ (pCOXW/L)(V~s - VtJVDs

for VDs < c (V~s - VtJ,

(7)

where W is the width of the device, and L is its length [36]. For these measurements, the drain-source current of the MOSFET under test is measured at several values of gate voltage for a fixed drain voltage; VDs is typically 0.1 V. A linear plot is made of IDs versus Vcs, and the tangent to the curve with the maximum slope is extrapolated to the gatevoltage axis [66]. This intercept is the threshold voltage for the drain voltage and measurements, leakage temperature of the test. In making linear threshold-voltage currents should be subtracted from the drain current. An example of a IDs-VGs cume for which the above analysis has been applied is given in Fig. 13. The channel mobility for the MOSFET can be determined using this technique by equating the measured slope to PCOXW/L, in general, COX,W, and L are known. In the saturation region the drain current IDs is given by I~s(sat) = (m@&W/L)(VGs

- Vth)’,

(8)

where m is a function of doping concentration and approaches 1/2 at low dopings [36,67]. From Eq. (8), the saturated threshold voltage is given as the zero current intercept on a plot of the square root of the drain current versus gate voltage. A typical set of ~ 1~~-V~s curves for an n-channel transistor is shown in Fig. 14. These curves were taken before II-18

irradiation and after irradiations of 30, 100, 300, and 1000 krad(Si02) with an applied bias of 5 V. For this data set, VDs was 5 V and a least-squares-fit was applied to currents measured in a range from 16 to 100 @ The J ID-VWcurves are seen to shift to more negative values with the buildup of positive charge. In additio~ a decrease in slope, i.e., (mpCOXW/L)l@, is observed following the 1000-krad(SiOJ irradiation. This decreased slope corresponds to the buildup of radiation-induced interface traps and an associated decrease in mobility, p. This measure of slope is seldom used to determine absolute values of channel mobility because the prefactor m is not well knowq but it can be used as a measure of relative changes in mobility. As was the case for linear threshold-voltage measurements, any leakage current should be subtracted from the drain current. Recently, concerns have been expressed concerning power dissipation in the MOSFET during saturated threshold voltage measurements. Increases in device temperature caused by power dissipation may need to be accounted for. The temperature sensitivity of the threshold voltage may be as large as -5 mV/°C, or more [36,66]. 30

z 3

20

_L4 10

0 Gate Vottage, VGS(V’) Figure 13. Current-voltage curve for an n-channel used to determine “linear” threshold voltage.

3.1.2 Threshold-voltage AVOhand AVit

MOSFET illustrating

analysis

shift due to oxide trapped charge and interface traps,

Several techniques have been developed to divide radiation-induced thresholdvoltage shifts into components due to oxide-trapped charge, AVOt,and interface traps, AVit, i.e., AVth = A1’O~+ Avi~.

(9)

These are called “charge separation” techniques. As discussed in Section 2.1, oxidetrapped charge and interface-trap charge are the two primary radiation-induced defects that govern the total-dose radiation response of MOS devices. It has been shown that l-I-19

each of these defects needs to be measured and controlled if integrated circuits are to survive in differing radiation environments [38]. In this sectio~ approaches available to perform the separation denoted in Eq. (9) will be described. Initial discussions will focus on the transistor structure. Later discussions will be extended to capacitors, the simplest MOS structure, as methods to independently measure the density of interface traps, Nit, are reviewed. 10

~ z 3 Q “m _t

8 –

VDS =5V

6 -

300

4

#’

#’

2 I

0 -1

,’ ,’

,’

,’ ,’ ,’ ,’

,’

,’,’,~~ ~ # f’1’ #’ ,’ 1’ ,’ ,’ 1’ ,’ ,’ ,’ I If’ ,’ ,’

0

1

I 2

Gate Voltage, vGsM

Figure 14. Square root of current versus gate voltage curves taken before irradiation and following irradiations of 30, 100, 300, and 1000 krad(Si02). Extrapolations of ~ID-VK, curves to zero current provides a measure of the ‘saturated” threshold voltage. The most widely used “charge separation”

technique for separating the effects of radiation-induced oxide-trapped and interface-trap charge on transistor performance is the “midgap” method developed by Winokur and McWhorter [37,49]. The method provides a simple, straightfonvard analysis without requiring sophisticated measurements. The “midgap” method is illustrated in Fig. 5, which shows a series of subthreshold-current characteristics for an n-channel transistor recorded at prerad and total doses of 30, 100, 300, and 1000 krad(SiOJ. At each total dose, the current levels corresponding to specific Si surface potentials were calculated using known equations that describe current in the subthreshold region as a function of Si surface potential [36,68]. We have marked the current values corresponding to threshold, inversio% and midgap (denoted as Ith, IinV,and Im~ in Fig, 5). Inversion and midgap currents correspond to Si surface potentials (or “band-bending”) of $~ and 24*, where 4B is the bulk potential; it is important to note that the Si surface potential corresponding to the measured threshold on the transistor is generally greater than 2c#J~.The value of Ik~ is simply the current value associated with Vt~, i.e., threshold voltage determined in the saturation regime (see Section 3.1.1). The value of mobility used in these equations was determined from plots of the square root of drain-source current versus gate voltage for saturated current in range from 20 to 100@ However, the analysis of the subthreshold curves was not very sensitive to the absolute magnitude of the mobility, i.e., a variation in mobility by an order of magnitude would 11-20

only change results (i.e., AVOtand AVit) by six percent. Due to leakage currents and basic measurement limitations, it was not possible to measure currents in the range (< 10-12 A). In order to indicate the position of I~a, subthresholdcorresponding to 1~~ current curves were extrapolated to lower current levels shown as the dashed part of the subthreshold characteristic in Fig. 5. In making this extrapolation, the subthreshold curve is extended using the slope at the lowest current levels that were at least an order of magnitude greater than any leakage current. This slope was felt to be the most representative for the part of the Si band gap covered by the extrapolation. When performing the separation denoted in Eq. (9), it is assumed that interface traps in the upper half of the Si band gap are predominantly acceptors, while those in the lower half of the band gap are predominantly donors. (See Section 2.1 for a discussion or the donor/acceptor nature of interface traps.) Following this reasoning, the voltage shift in subthreshold-current curves at midgap yields AVOt,since at this position of the band gap neither acceptor or donor interface states are charged and the shift is due entirely to trapped oxide charge [9,69-71], Similarly, the increase (between post- and pre-irradiation subthreshold curves) in “stretchout” of the subthreshold-current curves along the voltage axis from midgap to threshold yields AVit, the shift caused by occupied interface traps in the Si band gap between rnidgap and threshold. That k, AVOt = AVi~ =

Wng)pollt

[v(Ith)-v(Im.)]@

- ‘(hg)pre

(lo)

- [V(Ith)-V(Iw)]P,.

(11)

For n-channel transistors, the interface traps between midgap and threshold are acceptors in the upper half of the band gap and produce a positive shift, while for p-channel transistors interface traps between rnidgap and threshold are donors in the lower half of the band gap and produce a negative shift. Figure 15 presents the results of the “midgap” method applied to the subthresholdcurrent curves shown in Fig. 5. For each of the curves, the threshold-voltage shift and its contributions horn oxide-trapped charge and interface traps are plotted. Although the net threshold-voltage shift is only -1 V following a 1 Mrad(SiOz) irradiation with 10 V applied between gate and source, the contributions from trapped-oxide charge and interface traps are --4.5 and - +3.5 V, respectively. The relatively small net thresholdvoltage shift is obtained by compensating considerably larger shifts due to trapped-oxide circuits that will charge and interface traps. In order to fabricate radiation-hardened and defense operate in differing radiation environments, i.e., space, accelerator, applications, both AVOtand AVit need to be controlled and reduced. “Hardness” must be defined as the reduction of both types of radiation-induced charge, and not simply as the reduction in the net threshold-voltage shift of the transistor [38]. A second charge separation technique is the dual-transistor method developed by Fleetwood [72]. The dual-transistor method requires n- and p-charnel transistors with identically processed oxides, preferably on the same chip or wafer to best match oxide radiation response. These transistors are then irradiated under identical conditions at the 11-21

same oxide electric field. Based on literature studies, it is assumed that (1) AVOkis approximately equal for n- and p-channel transistors [37,73] (2) interface traps predominantly are charged negatively for n-channel transistors and positively for p-channels [9,37,49,69-71], (3) the post irradiation mobility, p, is related to the preirradiation mobility, PO,by an equation of the Sun-Plummer form [40,52-54], i.e., p = P~/(1 + a(ANit)) = PJ(I + a*(AVik))(see Section 2.2.3), and finally (4) a“ is taken to be the same for n- and p-channel transistors. The parameter a“, which should remain constant during irradiation and/or anneal, serves as a self-consistency check on the analysis. This dual-transistor method essentially combines the single-transistor “midgap” method and mobility models (see Section 2.2.3 and Ref. [53]) to determine AVOtand AVik by measuring threshold voltage in the saturation region for n- and p-channel transistors, i.e., extrapolated from linear least-squares fits of J l~~-VWplots (see Section 3. 1.1). 5 4 3 2 VOLTAGE

1

SHIFT

0

(v)

-1 .-

-2 -3 AVOt

-4 -5 1/11111111, PRE

105

RADIATION

I>IIIIIU

d 109

107

LEVEL [rad(S102)]

Figure 15. Net threshold-voltage shift and contributions to that shift due to interface traps and oxide-trapped charge calculated from the subthreshold currentvoltage curves of Fig. 5 via the “midgap” charge separation technique of Winokur and McWhorter. (After Ref. [49]) The advantage The dual-transistor

of this technique is that it does not rely on low current measurements. technique can be applied at currents two to five orders of magnitude

above those required for typical rnidgap, subthreshold slope, or charge pumping analysis. (As discussed in the next section, midgap, subthreshold-slope and charge-pumping techniques require measurements from sub-picoamps to tens of nanoamps.) Low-current measurements are difficult to perform at high speed, e.g., following a short pulse of radiatio~ or when parasitic leakage currents are present. To illustrate the usefulness of the dual-transistor technique, Fig. 16A shows a series of subthreshold I-V curves as a function of dose and elevated temperature biased annealing for an n-channel transistor with a 32-rim gate oxide made in Sandia’s CMOS IHA process [74]. Clearly, for irradiations of ’70, 150, and 500 krad(Si02), there is a considerable parasitic leakage component. In order to apply the midgap method, this leakage needs to be subtracted or

II-22

the assumption

made that 1-2 decades of “straight” I-V curve above the leakage level is enough to allow reasonable estimates of AVOtand AVit. (A)

N-ch

,..4

ton= 33

nm

,~-e z Ii

=

10.8 600 10O”C Anneal ,..12

-4

-3

-2

.1

1

0

3

2

v~~(v)

1.2

TRAN319TOn MIOOAP

● DUAL ■

1.0

“-T ,“”’;~ /’ / (a’=1.25

* 0.11)

—loo l/a.

“c— lJv

0.4

H1216AIWI0 to, - 33 nm

0.2

IA

0.0 o

I I 100

I

I

I

200

300

400

DOSE &rad (S102)]

1 I SOO*’

I

1.03.010301

I 10

ANNEALTIME(h)

Figure 16. (A) Subthreshold I-V curves as a function of dose and elevated-temperature biased annealing for an n-channel transistor with a 32-rim gate oxide made in Sandia’s CMOS 111A process. (B) Comparisons of AVit estimated with midgap and dual-transistor analysis. (After Ref. [72])

In Fig. 16B, comparisons of AVikfor these devices are estimated with midgap and dual-transistor analysis. To try to minimize errors in analysis caused by leakage, midgap values of AVit are derived from current measurements in the range 0.2 to 1.0 x1O-TA. Still for doses from 50 to 200 krad(Si), values of AVit obtained by the midgap method are = 4-times higher than values obtained with the dual-transistor method. After l-hr anneal at 1000 C, as well as preirradiation, the parasitic leakage is reduced to ~ 10-11 A. For these cases, both methods agree well. It is interesting to note that the value of AVit obtained using the dual-transistor analysis increases linearly with dose, in agreement with results on devices with similarly processed gate oxides but no parasitic leakage [75]. This dependence is in strong contrast to the nonlinearity suggested by rnidgap analysis. For the data in Fig. 16B, a“ was well behaved with a value of 1,25 *0.11, indicating the assumptions of the dual-transistor method remained valid during the irradiation and anneal sequence. Finally, for the corresponding p-charnel transistors that did not show II-23

high leakage, estimates of AVOt and AVit obtained with both methods differ by less than 10%. Clearly, the dual-transistor method can provide more accurate estimates of AVOt and AVit for devices with high leakage than midgap, subthreshold slope, or charge pumping techniques. In a related study, Galloway et al. [53,54] have presented a simple model for separating the effects of radiation-induced oxide-trapped and interface-trap charge on MOSFET I-V characteristics. In a manner similar to that of Sun and Plummer [52], the radiation-induced increase in interface traps ANit is related to the mobility degradation of the MOSFET following irradiation by ~ = PO/(l + aNit)(see Section 2.2.3 and Fig. 11). In this expression POand P are nobilities pre- and post-irradiation and are determined from current-voltage (I-V) curves in the linear region of MOSFET operation. The parameter, a, can be adjusted to reflect process variations; however, a value of (8x2)x1O-19cmz has been used to fit data of Sexton and Schwank [40] and Galloway et al. [53]. Finally, charge separation can be performed by determining the threshold shift due to interface traps, AVitJ from measured interface-trap distributions. These shifts can be within several percent of those determined by the “midgap” method described above. The relationship between AVit and several different interface-trap variables is AVi~ = AQi~/COx= qANi~/COx= (q/COx) j ADitd@,

(12)

where Qit is the areal density of interface-trap charge. Once AVit is determined, then AVO~= AVk~- AVit and the charge separation is complete. In applying Eq. (12), it is important to only count interface traps that contribute at threshold. ADit is integrated over the appropriate part of the band gap to obtain the total number of interface traps that contribute to AVit at the threshold condition. In Fig. 17, this approach is illustrated for an n-channel transistor -- ~it is integrated from ~ to 2& (i.e., the difference in surface potential between threshold and midgap) to get the areal density of interface traps that contribute to AVit at threshold. Techniques for measuring interface-trap distributions are now discussed.

3.1.3 Measuring

interface traps, Di~{cm-zeV-l} and Ni~{cm-z}

Many techniques have been developed over the years to measure the density of interface traps at the oxide-semiconductor interface. It is not the purpose of this section to discuss these procedures in any great detail, since one can be referred to many excellent texts and an extensive literature on this subject (see for example, Ref. [18]). Rather the intent is to outline the qualitative nature of these techniques, the information that is available, advantages and disadvantages, and the ease of application for research and/or production support.

Most of the techniques to determine the densities and energy distributions of interface traps are designed for use on the MOS capacitor. The MOS capacitor has the advantage of being the simplest test structure available for characterizing the properties II-24

of the oxide-semiconductor interface, and it is well suited to the study of ionizing radiation effects. Its disadvantage is that it is several thousand times larger in area than the MOS transistor used in actual integrated circuit fabrication. Concerns over geomet~ effects, stress, and areal inhomogeneities always need to be addressed in the interpretation and extension of MOS capacitor results to the MOSFET. In this section, techniques for determining g interface traps using capacitors will be reviewed. This will be followed by a discussion of methods for determining interface traps using MOS transistors. Since the primary focus of this section is radiation-induced interface traps, the section will end with a discussion of lateral charge nonuniformities (LNUs). LNUS are often present in the overall response of MOS devices to ionizing radiation, and produce effects in MOS capacitors and transistors that can be incorrectly interpreted as arising from interface traps. vh(P-cJl) <0

10’

VH(N+h)>O

“c q g- 10’ a10’

10’

I

4.2

I I I I 0

I

I I I 1 I

0.2 0.4

0.6 0.0

Surface Potential, #~ (’v) Figure 17. Schematic diagram illustrating technique for determining AVit from a measured interface-trap distribution. For an n-channel transistor, ADit is integrated

from fp~ to 2@~ (i.e., the difference in surface potential between threshold and midgap) to get the areal density of interface traps that contribute to AVit at threshold.

3.1.3.1 Ca Dacitor techniques

There are two basic classes of capacitor measurements in use today--capacitance methods

and conductance

methods.

Both capacitance

and conductance

measurements

contain similar information about the interface traps. The capacitance methods are easy to implement and provide quick results. The investments are low, and the evaluation theory is simple and transparent. However, it is not possible to determine capture cross sections and time constants or to distinguish between the acceptor and donor nature of the interface traps. In addition, data interpretation can be misleading since distortions in the curves may result from LNUS as opposed to interface trap phenomena (see Section 3.1.4). The conductance method, on the other hand, can give the most accurate results II-25

especially for MOS devices with relatively low interface-trap densities. It is possible to determine capture cross sections and time constants, and to distinguish between donor or acceptor traps. However, the measurement and evaluation effort is high and time consuming. There are several different techniques that utilize capacitance measurements for determining interface-trap density. In general, interface traps stretch out and distort the C-V curves of MOS capacitors and, at low measurement frequencies, change the magnitude of the measured capacitance at a given surface potential. Distortion of the C-V curve leads to a broader voltage range over which the capacitance is changing. The distortion is measured as (1) an increase from a preirradiation curve or (2) deviations from an ideal, theoretical curve which assumes no interface traps. The measured distortion is then interpreted in terms of an interface-trap density across the Si band gap. In the final analysis, all measurement techniques (for capacitors or transistors) depend on fundamental properties of the interface traps, i.e., time constant response versus surface potential and temperature, donor/acceptor nature or charge occupancy relative to the Fermi level, and spatial location relative to the interface. As a result, several of these techniques measure the frequency response of the traps as a function of (1) applied AC and DC signals and (2) temperature. The most widely used capacitor techniques are the Stretchout, Terman, Kuhn or Berglund, and Gray-Brown techniques. A brief description of these, as well as several other less widely used, techniques follows. The stretchout method [76,77] is the simplest technique for measuring increases in radiation-induced interface traps over a specific region of the silicon band gap as a function oi dose. The “stretchout” of a high frequency C-V curve is defined as the voltage spread of the C-V curve taken at two different capacitances (or surface potentials). The increase in interface traps, ANit{cm-z}, corresponding to increased C-V stretchout at two different doses, AV,O, is then given by ANit = C~~(AV,,/q), where COXis the oxide capacitance per unit area and q is the electronic charge. In a typical case, stretchout is measured between capacitances corresponding to surface potentials at flatband and strong inversion. For moderate doping densities, these surface potentials represent an energy range (in the Si band gap) of some 0.5 -0.6 eV. Figure 18 replots the data from Fig. 3, which showed a series of normalized C-V traces for an n-substrate capacitor taken at prerad and times from 0.04 to 400 s following pulsed 200 krad(Si02) electron-beam irradiation. On the C-V curves, normalized capacitance values corresponding to flatband and strong-inversion have been marked and denoted as cfb/cox and Ci~v/COX$ respectively. The capacitances corresponding to specific Si surface potentials were calculated based on the measured values of oxide thickness and doping density (determined from the ratio of capacitance in strong inversion to accumulation). At 400s following irradiation, the stretchout (between flatband and inversion) V~Ois 1.9 V, while at prerad, V,Ois 0.6 V. The increase in the interface-trap density at 400s following this 200 krad(Si02) irradiation is 3.9x1011cm-z. The details of the calculation follow ANit{cm-z} = COX(AV,O/@= &&OAV,O/(tOXq) ANit{cm-z} = (3.86) (8.85X10-li)(l.9-0.6) /(7.l4XlO-S)(l.6XlO-lg) II-26

(13)

ANit{crn-z} =3.9x1011, where e is the dielectric constant for Si02 and SOis the permittivity of vacuum. Using this technique, the interface-trap density Nit at 400s following irradiation is 5.7x1OU cm-z.

_.J!i”$%3r .—— ——

0.6

r

0.4

YJ I I

:

z ~

0.2

_>rl

––{c~v’~x

-v,~+

I I

0 z .5

-4

n-TYPE <100> DOSE= 200 krad (S102)

.3

I

I

-2

-1

APPLIED Figure

18.

ox =71,4nm

- ‘–

—

I

0.0 -6

t

VOLTAGE

0

I

I

1

2

3

(V)

The stretchout technique for measuring interface traps.

(After Refs.

[76,77]) Note that midgap method applied to transistors in Section 3.1.2 has a similar technical basis as the stretchout method applied here to capacitors. By applying the midgap method to capacitors, Winokur et al. [37] compared the radiation responses of

MOS capacitors and transistors fabricated on the same wafer. A good correlation was observed between p-substrate capacitors and n-channel transistors irradiated at 10 V, as well as between n-substrate capacitors and p-channel transistors irradiated at OV. These correlations were verified for samples having large variations in the amount of radiationinduced trapped holes and interface traps. An excellent correlation was also observed between n-channel capacitors and n-substrate transistors irradiated under positive bias [37]. The Terman method [78] is in essence, a refinement of the stretchout method that yields the actual distributio~ Dit(~~)$of interface traps in the Si band gap. In the Terman technique, the capacitance of an MOS capacitor is measured at a high frequency (typically 1 MHz) as a function of bias voltage at a fixed temperature. The interface traps that are measured do not respond to the 1 MHz AC signal, but do respond to the more slowly varying DC bias. The interface traps act as fixed charge, and the resulting shift of the C-V curve along the voltage axis at a given capacitance is a measure of both the oxide-trapped and interface-trap charge densities at the corresponding surface potential. It is important to note that, regardless of the interface-trap level density, the high frequency capacitance of an MOS capacitor will be the same as an ideal one without interface traps. Corresponding to each value of C~~, there will be a unique value of surface potential& By measuring the shift at two different capacitances (or equivalently two different surface II-27

1,0

O.n 4.00 c

0.6

0.4

0.2

n-lYPE <100> DOSE= 200 krad (Si02)

0.0 -6

I

I

I

I

I

-5

-4

-3

-2

.1

APPLIED

0

VOLTAGE

1

1

1

2

(V)

Figure 19. The Terman technique for measuring interface traps.

5 2

(After Ref. [78])

1000”C (Ox= 71,4 nm n-TYPE

. F ; al N ~ w u n-

3

z200

<100> krad (S102)

1012

\

\\J::’

6

1 ~

J

2

4a 400ma 40m8

/PRE-RAD

,011 5

-0.6

VALENCEJ-0.8 BAND EDGE

‘0.4

SURFACE

-0.2

POTENTIAL,@S

0

092

(V)

density versus surface potential, calculated using the Figure 20. Interface-trap Terman technique, for the C-V traces shown in Fig. 19. (After Ref. [26]) potentials)

and differentiating, Dit(+s)

an interface trap density can be calculated, and is given by

= (1/q)~Qit/W

=

(Vq)[(Q,-QJ/(&4Jl

(14)

The Terman technique is illustrated in Figure 19, where shifts AVI and AVZare shown at two different surface potentials. (Note: The contribution of oxide-trapped charge is the same at every surface potential and therefore cancels out when the difference is taken in Eq. (14). ) In measuring radiation-induced interface traps, a comparison of the experimental C-V trace following irradiation can be made with either (1) an ideal C-V trace or (2) an experimental preirradiation C-V trace. The advantage of using the latter is that it eliminates uncertainties in the technique associated with calculating the ideal C-V II-28

curve--these uncertainties arise from assumptions concerning the Si doping density and are assumed not to change with radiation. The region of the band gap accessible with this technique is limited in general by the condition that interface traps must not respond to the test frequency, and in inversion by the flatness of the C-V trace, i.e., the derivative in Eq. (14) is large and becomes difficult to calculate toward inversion. Using a l-MHz test signal, the Terman technique probes the middle range of the Si band gap, i.e., ~0.3 eV about midgap. Figure 20 shows density of interface traps versus surface potential determined by the Terman method for the C-V traces given in Figure 19. The interfacetrap density is continuous throughout the Si forbidden energy gap, with a “U-shaped distribution that monotonically increases with energy toward the band edges from a minimum near midgap [79-85]. The peak near the conduction band, seen on distributions calculated from C-V traces taken at 40 and 400s, may simply be an artifact of the analysis; as mentioned above, the “flatness” of the C-V trace in this region makes graphical differentiation on this part of the C-V trace questionable. Integrating the interface-trap distribution for the C-V trace recorded at 400 s from flatband to inversion (surface potentials of O to --0.5 V) yields an interface-trap density of -6x1011 cm-z, in agreement with the stretchout analysis above. Berglund [86] showed that interface-trap densities can be determined by comparing theoretical and measured low frequency C-V curves. Kuhn [87] substituted a quasistatic C-V measurement for the low frequency AC capacitance. It is based on a measurement of the MOS charging current i(t) in response to a linear voltage ramp V(t), so that the charging current is directly proportional to the incremental MOS capacitance. [If the voltage ramp is given by V(t) = VI+ at, and if i(t) = C(t)dV(t)/dt, then it follows that i(t) = &(t), i.e., the charging current is proportional to capacitance.] In both of these approaches, it is assumed that the interface traps respond to both the slowly varying AC measurement signal and the applied DC bias. Interface traps result in an interface-trap capacitance in parallel with the silicon space charge capacitance; this additional capacitance term, Cit, determined by the difference between ideal and experimental lowfrequency C-V traces, is related to an interface-trap density by the equation, DiJ&) = CiJ~,)/q. More specifically, the measured low-frequency MOS capacitance CLF can be written l/c~F = I/COX + l/(Cs. + Cit),

(15)

where COX,C,c, and Cit are oxide, ideal silicon space charge, and interface-trap capacitances, respectively. Equation (15) yields the following expression for interface-trap density Dit(~~) = I/q{ [C~F/(l-(C~F/COX))] - C,C}.

(16)

Figure 21 shows measured (open circles) and calculated or ideal (solid line) lowfrequency capacitance curves as a function of surface potential. The difference between measured and calculated capacitance at a given surface potential is a measure of the interface-trap density at that surface potential. As expected, there is strong agreement in accumulation and inversion where the capacitance due to the silicon space charge region II-29

dominates. In order touse Eq. (16) tocalculate Dit, itisfirst necessa~to obtain the relationship between surface potential & and the applied voltage VGs. This mapping is accomplished (to within an additive constant) by integrating the measured C~F curve from a voltage corresponding to strong accumulation to one corresponding to inversion [87], i.e., inv &(vGs) = j [l-&@Gs)/COX]dVGs acc

+ &,

(17)

where @,Ois an additive constant. The resulting integration should provide a value close to that of the Si band gap, i.e., 1.1 V.

o . . .

.

. .

“AC from Interface traps .

1 -6

1

I

I

1

1

-5-4-3-2-1012 Applled Voltage (V)

1

1

3

The quasistatic technique for measuring interface traps compares Figure 21. measured (open circles) and calculated (solid line) low-frequency capacitance curves. The difference between measured and calculated capacitance at a given surface potential is a measure of the interface-trap density at that surface potential. (After Ref. [87])

As illustrated in the quasistatic technique, the measured capacitance is a strong function of measurement frequency when interface traps are present. Capacitance is seen to increase at a given applied bias (surface potential) as the measurement frequency is lowered. At a given AC signal measurement frequency, only those interface traps with time constants greater than or equal to the measurement frequency can respond. As the measurement frequency is lowered, more interface traps can respond resulting in an increasing ~t and overall capacitance. The advantages of low-frequency techniques are: (1) They permit measurement of surface potential and interface-trap density over a larger part of the energy gap than high frequency techniques. (2) They provide an order of magnitude greater sensitivity than high frequenq techniques. (3) It is possible to use a single sample to probe the entire band gap. (4) They provide a direct test for the presence of gross lateral nonuniformities (see Section 3.1.4). A basic disadvantage of 11-30

these techniques is that they are time consuming (> 30 min per sample), and therefore cannot be used to provide quick turnaround for a large number of samples, or for measurements on quick-annealing devices. Quasistatic measurements maybe combined with high frequency C-V measurements to obtain the interface-trap density in a restricted energy range, as discussed by Castagne and Vapaille [88]. This technique, sometimes referred to as the “high-low” method, eliminates the need for an ideal C-V curve, and therefore uncertainties associated with silicon doping profiles. The approach recommends itself most strongly when interface trap information in the energy range corresponding to inversion is not required. Because the contribution of lateral charge nonuniformities is independent of measurement frequency [26,76], the technique provides a strong test for the presence of LNUS (see Section 3.1.4). In this technique, the capacitance difference between quasistatic and 1MHz C-V traces is taken as a measure of Dit in the central part of the band gap. Specifically, Dik = Cit/q = [CLFCOx/(COx-CLF)]- [C~~COX/(COX-C~~)],

(18)

where CLF is the “low-frequency” capacitance obtained from the quasistatic curve and CHF is the “high-frequency” capacitance obtained from the l-MHz C-V trace. There are several techniques that determine interface-trap densities by variations in The most widely used of these techniques for studying radiation-induced temperature. interface traps is the Gray-Brown technique [89]. In most applications of this technique, the sample temperature is varied from room temperature to liquid nitrogen temperature (77 K). By varying the temperature, the Fermi level in the semiconductor is shifted and the occupancy of interface traps change. It is generally assumed that the sample is kept in thermal equilibrium as the temperature is varied. The changed occupancy of interface traps is simply measured as a change in flatband voltage between high frequency C-V The range of band gap accessible by this traces taken at the different temperatures. technique is very small, being determined by the variation of the Fermi level within the temperature limits dictated by oxide instability at high temperatures and impurity By reducing the temperature, interface-trap time deionization at low temperatures. constants are increased and the range of energy probed is extended towards the majority carrier band edges (some 0.2 to 0.3 eV below the conduction band for n-type samples). The Gray-Brown technique is illustrated in Fig. 22, where the voltage difference (called the “Gray-Brown shift”) at flatband between l-MHz C-V traces taken at room (solid) and liquid nitrogen temperature (dashed) is used to calculate an interface-trap density of 8.7x1011 cm-z. The band diagram illustrates the relative positions of the Fermi level at 77 and 300 K and shows acceptor-like interface traps that are uncovered as the temperature is lowered. The additional negative charge contribution at 77 K from the acceptors shifts the C-V trace toward more positive voltages. In the Jenq technique [27,77,90], interface traps are measured at liquid nitrogen The sample is initially biased into accumulation at room temperature to temperature. charge interface traps with majority carriers. It is cooled to 77 ~ and a C-V trace is taken II-3 1

—

300 K

_-77K — Ec .. . “--.. EF (77 K) P = WEF (300K) 7

n-type

GRAY-BROWN

—Ev 4“ ,9--* b

-6

-6

-4 -2 0 2 APPLIED VOLTAGE (V)

4

6

22. Gray-Brown shift between C-V traces taken at 77 and 300 K following 1 Mrad(SiO.J irradiation. (After Ref. [26])

Figure a

(in the dark) by sweeping the bias from accumulation to deep depletion. The sample is then held in deep depletion, illuminated with visible light to charge interface traps with minority carriers, and a C-V trace is retaken by sweeping the bias from inversion to

accumulation. The voltage shift between these 77 K C-V traces near the inversion point, and parallel to an ideal deep depletion 77-K trace, is a measure of interface traps that are =0.2 eV from both the conduction and valence band, i.e., in the mid 0.7 eV of the Si band gap. Finally, Bluzer et al. [91] introduced a low-temperature technique that measures interface traps closer to the band edges and at higher resolution. His technique uses the transfer inefficiency at 77 K of an 800-gate charge-coupled device (CCD) to measure interface traps 0.1 to 0.2 eV from the valence and conduction band edges and provides a resolution of 5xl@ cm- ZeV-1. The AC conductance technique yields the most detailed and accurate information about interface traps. The small signal differential AC conductance of an MOS diode is essentially due to the exchange of charge between interface traps and the bulk semiconductor [92-96]. If this conductance is measured as a function of bias and frequency, information concerning the density, distribution, time-constants, and capture cross section of the interface traps present can be obtained. Figure 23A represents a comparison of the measured and theoretical equivalent parallel conductance, (GP/u), versus frequency for an MOS capacitor irradiated to 1 Mrad(Si02). The measurement is made in depletion at a fixed DC voltage. The equivalent parallel conductance is the conductance associated with interface traps filling and emptying in response to a small AC voltage. Since the measurement is made with the DC bias constant, the conductance measurement is probing interface traps at a specific energy in the band gap. The density of interface traps is determined from the peak of the conductance-versus-frequency curve. The time constant of the interface traps being probed is determined from the frequency at which the peak conductance signal occurs. For a given conductance-versus-frequency curve as shown in Fig. 23A the density and time constant of interface traps is determined II-32

for one surface potential (determined by the DC bias at which the measurement was made). In order to map out the density of interface traps throughout the band gap, the conductance-versus-frequency measurement must be repeated at various biases, as shown in Fig. 23B. It can be seen in Fig. 23B that the frequency at which the peak conductance signal occurs depends on the surface potential, and that interface traps near midgap respond more slowly than those near the band edges. Analysis of the conductance curves is straightfonvard only when the capacitor is biased in depletio~ so to examine the upper and lower half of the band gap, both n- and p-type capacitors must be used. eo

(A)

I —

40

I

THEO~V

o

T

FROM

CONDUOTAN02

PnRW~NCY ● 2AK

OF

I

RxMnlM6NT

40

I

———

36

———

.

CONDUC7AN06

30 26

20 16 10

v

6 n

f

(HZ)

30

(B)

~;

:;

-

@,= *.-

.107

“4e

0 z

2’ 18

at ‘o F w

A

30 -a u

,~ 12

@ n

V3

-

I ox I

o 102

,0s

I ,06

I ,04

TYPE SAMPLE = 460 A I 10 0

ti ,0

7

f (HZ)

Figure 23. (A) Conductance-versus-frequency curve at a fixed DC bias for an MOS capacitor irradiated to 1 Mrad(SiOJ. (B) Conductance-versus-frequency at various surface potentials; surface potential is in units of eV, with q$=O being flatband. (After Ref. [96])

measurements provide a much more sensitive means of detecting the presence of fast interface traps than do capacitance measurements. The drawbacks of this technique are that a large number of measurements are required to achieve such detailed results, and only the portion of the energy gap between midgap and weak inversion can be As mentioned above, probing a wider range of the band gap conveniently probed. requires both n- and p-type samples. Ziegler [95], and later McWhorter et al. [96], use the Conductance

II-33

dispersion peaks, to dispersion randomly

of the conductance signal, i.e., the width of the conductance-versus-frequency distinguish between the donor and acceptor characterof interface traps. This has been related [97-99] to surface potential fluctuations generated (in part) by distributed charges at the interface.

3.1.3Q Transistor Traditionally,

techniques

of interface-trap measurements, the operating characteristics of transistors and the inferred. Recent work has shown a positive correlation between of capacitors and transistors [37,64,100,101], but questions in that response due to geometry (e.g. stress) still remain separate techniques for measuring interface traps on transistors

the MOS capacitor

density. Based on these performance of ICS are the radiation response concerning differences [23,101,102]. Therefore, are valuable.

has been used for measurements

The easiest and most widely used technique for measuring interface traps in transistors is the subthreshold technique. In this technique, interface traps are determined from changes in the subthreshold slope or “swing”, S. The subthreshold swing is a measure of the change in gate voltage necessary to reduce transistor current by one decade. For samples where S is relatively constant throughout the subthreshold regio~ the change in mean interface-trap density following an irradiation, ADitj is given by

ADit = [c../k~(lo)](sD~

- SDJ,

(19)

where SDZand SD~ are subthreshold swings measured at two different radiation levels D2 and D 1, respectively [9,36,103,104]. For as-processed samples, the subthreshold slope is given by =kTln(lO)[l + CD/COX],where CD is the depletion layer capacitance [36]. The theoretical limit for subthreshold slope is kTln(lO) or -60 mV/decade at room temperature. For samples with large interface-trap distributions in which S is not constant throughout the Si band gap, the simple application of this approach is questionable, Winokur et al. [37] extended this technique to calculate interface-trap distributions for the case of large interface-trap distributions that are not uniform throughout the Si band gap. In recent years, charge pumping has been widely used in the radiation effects community for measuring interface traps [37,100,105- 112]. Although the measurement is straightforward, the associated analysis of data is somewhat complex. A simplified circuit (A) and principles of operation (B) for charge-pumping measurements is shown in Fig. 24 [111]. In the charge pumping method, the source and drain junctions of a MOSFET are reverse biased or grounded and a voltage signal is used to repetitively switch the charnel of the transistor from inversion to accumulation. When the surface under the gate is inverted, interface traps capture charge from the inversion layer. Alternately, when the region is accumulated, the charge in the interface traps is emitted and recombines with majority carriers in the bulk silicon. As the gate is repetitively pulsed, a charge pumpeddensity Nit by the current, ICP, is measured which is related to the interface-trap relationship II-34

I CP =

q%Nit

= q%<

‘it>

(20)

‘A,

where f is the voltage pulse frequenq, & is the area under the gate which is accumulated and inverted, c Dit > is the mean interface trap density averaged over the energy levels swept through by the Fermi level during the measurement, and A+, is the total sweep of surface potential. In essence, the charge pumping current arises from the interface trap recombination process--interface traps initially filling and then recombining with majority carriers. Many variations of this technique exist [100,105-112] by merely changing the magnitude and transition times of the voltage signal used to pulse the gate area from accumulation to inversion. To integrate across most of the band gap, the voltage pulse must be of sufficient magnitude to cause the channel of the transistor to accumulate and to invert, and the transition times must be sufficiently fast to minimize the loss of charge due to carrier emission during the transition. For a typical square wave signal with a 100-ns transition time, < Dit > represents an average value of interface traps ranging from 0.4 eV above and below the Fermi level. Due to the manufacturing process, the MOSFET gate area & used in Eq. (20) can be different from its drawn dimensions. Processing effects that can cause this discrepancy include lithographic steps and high temperature treatments such as anneals and junction diffusions. Usually, & is taken as the width of the transistor times the metallurgical charnel length, i.e., the distance between the the junctions formed by the source and drain with the channel. (A) i

(B)

i

Accumulation

●

b

Inversion

o

A simplified circuit (A) and principles of operation 24. pumping measurements. (After Ref. [111])

Figure

(B) for charge-

One advantage of the charge-pumping method is its sensitivity--densities as low as 1(P cm-zeV-l can be measured on transistors of 100-pmz charnel area at a frequency of 100 kHz. Another advantage is that charge-pumping measurements are not prone to errors due to charge lateral nonuniformities (LNUS) [100,109] (see Section 3.1.4). This makes the technique especially useful for low temperature studies ( < 200 K), where radiation-generated holes are somewhat immobile and often produce LNUs. A series of charge pumping curves taken on 50- x 1.2-~m p-charnel transistors is shown in Fig. 25 II-35

[112]. These curves were measured on the wafer level at prerad and following 10-keV x-ray irradiations of 300, 1000, and 3000 krad(SiOz). The value of ADit following the 3 Mrad(SiOz) irradiation was - 1011traps/cm- ZeV-l. The charge pumping current reaches a steady plateau, a strong indication that the technique is working correctly and that the AC pulse amplitude is sufficiently large. 10-6 t ox 10”7

v=

= 25 = 5V

nm

300 krad 1 Mrad 3 Mrad

10-8 10-g

10-’0 10-” 10-’2 -8

-7

-6

-5

-4

‘offset Figure

25.

krad(SiOJ, [112])

-3

-2

-1

0

1

(v)

Charge pumping current, ICP, for n-channel transistors irradiated to 300 1 Mrad(Si02), and 3 Mrad(SiOz) using 10-keV x rays. (After Ref.

Table I below summarizes capacitor and transistor techniques for measuring For each technique, the table includes: (1) its interface traps and their properties. sensitivity, (2) the region of the band gap probed by the technique, (3) its application in either research, production, and/or hardness assurance, and (4) whether its analysis is complicated by the presence of LNUS.

3.1.4 Lateral charge nonuniformities (LNUS)

It has long been known that interface traps and laterally nonuniform charge (LNUS) produce apparently similar abnormalities or distortions in C-V and I-V curves [26,28,76,87,88,90,113]. This is especially troubling in a radiation-effects scenario, where following irradiation a great deal of charge is created, transported, and eventually trapped at interfaces. The most widely accepted model of a laterally nonuniform MOS capacitor is a parallel combination of noninteracting small capacitors, each of which can be considered to be uniform over a small characteristic area. Differences in trapping for individual capacitors arises during the device fabrication. As Brews and Lopez point out, this model will not provide an accurate representation for lateral nonuniformities of extremely small dimensions. The theoretical work of Chang [115] provides an approach toward defining an approximate flatband-voltage distribution to describe the laterally nonuniform charge distribution. Hughes and King [76] have applied this technique in II-36

Table I.

Summary Traps.

Techniaue

Capacitor Stretchout Terrnan Quasistatic High-Low Gray Brown Jenq Conductance

of Capacitor

and Transistor

Techniques For Measuring Interface

Sensitivity

Region of Band gap

(1010

Probed (eV)

Application: Research, Production, Hardness Assurance

-1 -2-5 -1 -1 -0.1

0.5-0.6 0.6 0.6 0.3-0.4 0.2-0.3 0.7 0.6

R,P,HA R,P,HA R R R R R

Y Y Y N N N N

-1-2

0.6

R,P,HA

Y

-0.1

0.6-0.8

R,P,HA

N

-1-2 -1-2

~~-2ev-1)

Complicated by LNUS

Tranxirtor

Subthreshold Slope Charge Pumping

analyzing the effect of LNUS on irradiated MOS capacitors with different processing histories. Recent work by Frietag et al. [116] suggests that LNUS also may arise from a non-uniform distribution of dose deposition in the oxide layer. These authors found the relative standard deviation for the deposited dose greater for thin oxides, for 10-keV x rays as opposed to C060, and at low doses. Fortunately, there are several techniques that can be used to distinguish between LNUS and interface traps. These techniques essentially verify that the distortions observed in MOS characteristics are caused by electronic states or traps whose fundamental properties (or dependencies) identify them as interface traps. One of the properties of interface traps, whether process- or radiation-induced, is that they must be located within one or two atomic bond distances (=0.5 nrn) from the silicon lattice so that electrons and holes in the silicon conduction and valence bands can readily make quantum mechanical transitions into and out of these interface traps. It has been shown that the transition rate to a band increases exponentially with decreasing energy depth of the interface trap level measured from the band edge [92,96,99, 117]. The time constant (inverse of the transition rate) versus surface potential measured for an as-processed MOS capacitor with a steam grown oxide on <100> Si is shown in Fig. 26 (circles and lines). At room temperature, interface traps located near the silicon midgap have a transition rate of =100 transitions per second or a time constant of =0.01 s, while interface traps near the band edges can respond in microseconds. A similar time constant versus surface potential plot is observed (triangles) for a dry grown oxide on <100> Si following irradiation [96]. Based on the data in Fig. 23, the mean time for emission of a trapped electron and hole is apparently not altered by exposure to ionizing radiation. The transition rate to a band is also known to depend exponentially on the inverse of the absolute temperature, decreasing as the absolute temperature decreases [92,116]. At II-37

lOOK, interface traps located near the silicon rnidgap have a transition rate of =1O-1E transitions per second or a time constant of =101s s, while interface traps near the band edges can only respond in a million seconds. Electrons and holes are essentially “frozenThis transition rate is at the basis of many of the experimental in” at low temperatures. capacitance-voltage (C-V) and conductance-voltage (G-V) techniques that are used to measure the density and location of interface traps. ,@.1

Bz-

UHALP OF BANDOAP n.m

LOWER NALP OP DANDOAP P - TVPE-=1OO>

/’\ -t

McWHORTER

d [93.]A

300 K

‘\

TIME CONSTANT (SECONDS)

“; /’ I’A /-’ A

A

A /\

A A

A

[ “-.1G

A

-12.5-404112 d+s-+B)

It /kT

Figure 26. Variation of time constant versus surface potential for as-processed (circles) and radiation-induced (triangles) interface traps. (After Refs. [92] & [96])

The second

fundamental

property

of these traps is that the net charge residing in

them can either be positive, neutral, or negative. Based on their allowable charge states, interface traps are classified as either donors or acceptors (see Section 2.1). As a final point regarding the properties of interface traps, a distinction is sometimes made in the literature between fast and slow interface traps, i.e., between interface traps that can exchange charge with the semiconductor rapidly or slowly, respectively. These slow traps are generally thought to be located spatially more distant from the interface. The defect may appear either as a fixed charge in the oxide or as a trap with a long time constant for response. Boesch and Taylor [99] measured time constants of slow traps in thick field oxide structures and found the same qualitative dependence (as for fast traps) on surface potential, i.e., r =exp(-b+~). Using the time-resolved response of the interface traps to step-function surface potential changes, they reported time constants for slow states well in excess of 1 s. In this discussion, as well as most of the literature, data is interpreted as ansing solely from fast interface traps. The first technique to distinguish between interface traps and LNUS depends on the fact that interface traps have a characteristic time constant response (see Fig. 26), while nonuniform oxide-trapped charge does not (i.e., it has an essentially infinite time constant) [26,1 13]. By measuring C-V cumes in a frequency range typically between a few Hz to 1 MHz, a frequency dispersion in the C-V traces in the depletion regime can be observed due to the va~ing response of interface traps. This frequency dispersion is in II-38

addition to, or measured relative to, the frequency dispersion of the silicon space charge capacitance for a capacitor having the same semiconductor doping profile. As the measurement frequency is lowered more interface traps can respond, and the net capacitance at a given applied bias (surface potential) increases due to the increasing contribution of an interface-trap capacitance in parallel with the silicon space-charge capacitance. On the other hand, a frequenq independent stretch-out relative to the ideal curves would indicate the presence of lateral nonuniformities. The second technique utilizes a freeze-in of carriers in the interface traps at liquid nitrogen temperature [26,28, 113, 115]. This technique involves measuring a highfrequency C-V trace at both room temperature and 77 K. If interface traps are responsible for distortion in the room-temperature C-V trace, the distortion should disappear in the 77-K C-V trace because interface traps cannot respond to the applied DC bias. This is observed in Fig. 22, where l-MHz C-V traces are measured at room and liquid nitrogen temperature. Clearly, the liquid nitrogen trace is considerably sharper (i.e., less distorted). However, distortion in room temperature C-V traces resulting from lateral nonuniformities should remain at 77 K The approach is illustrated in Fig. 27, where C-V traces taken at 300 and 77 K are shown before and after room-temperature high-field bias stressing of an MOS capacitor with an unstable thermal oxide. This is clearly a case where there is a buildup of lateral nonuniformities, with negligible interface-trap production.

1.0

SI02 O.e

AFTER ❑IAS STRESS

/ ~

1000”C DRY OXIDE

/

—300K

0.6

--77K

0.4

0.2 0.0 -12

-10

-8

-6

-4

-2

0

2

4

6

APPLIED VOLTAGE (V)

Gray-Brown measurements on an unstable MOS capacitor exhibiting lateral nonuniformities after high-field bias stressing at room temperature for several hours, (After Ref. [26]) The Castagne-Vapaille or high-low method [48] (described in Section 3.1.3) provides an excellent test for the presence of lateral charge nonuniforrnities. This method uses the difference in capacitance between quasistatic and l-MHz C-V traces to calculate the

number of interface traps in the energy range from accumulation to weak inversion. If distortions in high- and low-frequency C-V traces are due to frequency-independent II-39

lateral nonutifotities, thecalmlation should field noresultant intetiace-trap demi~. In essence, this technique compares the interface-trap densities calculated by Terman and Kuhn techniques. Any discrepancies in this comparison indicate the presence of lateral nonuniformities. The Kuhn or quasistatic technique [87] also has the advantage of providing a direct test for the presence of lateral nonuniformities in MOS structures. In the technique, an integration of the quasi-static C-V curve from strong accumulation toward inversion (see Eq. (17)) yields the surface potential variation for the particular doping density of the semiconductor. If the integral is larger than expected (=1.1 V), the presence of lateral nonuniformities is assumed. Finally, charge pumping [48,100,105-112] and conductance [92-96] methods should not be sensitive to the presence of LNUS. These techniques rely directly upon the filling and emptying of interface traps during the measurement sequence. If LNUS due to fixed oxide charge are present, they will not contribute to either the charge pumping current or to any conductance signal.

3.1.5 Measuring

oxide-trapped

charge, NO~{cm-z}

Most often oxide-trapped charge is inferred by measuring voltage shift due to oxide~trapped charge and equating

ANO,{cm-2} = COXAVOt/q= (-l/q)

the threshold-

~Ap(x)(x/tOX)dx, o

or flatband-

(21)

In making this equality, it is assumed that all the oxide-trapped charge is located at the i.e., “x” is set equal to tOXin Eq. (21). This is a reasonable Si02/Si interface, approximation for gate oxides with thicknesses =0 nm. For state-of-the-art gate oxides with thicknesses approaching 15 nm, it becomes necessary to account for the fact that

positively trapped holes are located within 7.5 nm of the interface. The assumption that all radiation-generated charge, &(x), is trapped exactly at the interface leads to an underestimate for ANOt. For situations in which interface traps do not significantly affect device characteristics, parallel shifts in C-V or I-V curves are excellent measures of AVot, and hence ANOt. Such parallel shifts are often observed for commercial devices at low total doses and moderate to high dose rates [29,63] and sometimes observed even for radiationhardened devices at doses of megarads or greater [43,44]. However, when interface traps become significant, charge-separation techniques (see Section 3.1.2) must be used to extract AVOt. The most straightfonvard and popular of these is the “midgap” method in which the voltage shift in subthreshold-current or C-V curves at midgap is taken as AVOt, since at this position of the band gap it is assumed neither acceptor or donor interface traps are charged and the shift is due entirely to oxide-trapped charge [9,69-71]. 11-40

In more recent years, thermally-stimulated-current (TSC) measurements have been used to infer the density and energy distribution of radiation-induced trapped holes in MOS devices [118-127]. As illustrated in Fig. 28A, a TSC measurement is made by applying a bias across the insulator of an MOS capacitor and recording the current as the sample temperature is raised, typically from room temperature to 350-400”C. TSC measurements are generally made on capacitors irradiated under positive bias in which trapped holes build up in the vicinity of the SiOz/Si interface. During a TSC measurement on an irradiated device, trapped holes are thermally excited to the valence band and transport to the gate electrode when a negative TSC bias is applied (see Fig. 28B). [Please note the bias applied during TSC is not the same as the bias applied during irradiation; in most studies, the irradiation bias is positive and the TSC bias is negative.] Because the gate-to-substrate potential is fixed, negative charge must flow from the substrate through the ammeter to the gate in response to the transporting holes. This is the measured TSC. As discussed in the literature, TSC is not sensitive to interface traps [123,126] and is small when performed under positive bias [126]. In addition, the applied TSC bias must be sufficiently large to overcome any space-charge effects associated with the trapped holes. Space charge effects can reduce or even reverse the local electric field leading to a reduction in the measured TSC and an underestimation of the density of trapped

holes [125,126].

(A)

‘B) P

‘robe

~

THERM4L

Cap ‘GS<

0 ;F Om.

PoLY-

w ln[AT2t] — q

>

..- ... ..

SI

SI

Heater

SIO* (A) Schematic diagram of TSC measurement system that includes a heater or oven, picoammeter, and voltage suPply. (B) Band diagram showing trapped holes are thermally excited to the valence band and transport to the gate electrode during a negative-bias TSC measurement. Figure

28.

In a TSC measurement,

radiation-generated

oxide-trapped

QTSC= J (Ipt - Ipre)dt

charge is given by

(22)

where IPO,t is the TSC for an irradiated device and lP,, is the background current measured during an identical control run on an unirradiated device. Typical TSC curves, II-41

measured at -60 V on 350-nm soft oxides irradiated to 20 krad(Si02) under positive bias, are seen in Fig. 29A. TSC levels range from hundreds of femtoamps at prerad to tens of picoamps postirradiation. To reliably perform such measurements and maintain lowbackground noise at elevated temperature, special attention must be paid to sample mounting and to circuit parasitic [126]. The simplified rate equation governing the depopulation of trapped holes is dnJdt

= -~uexp[-AE/kT(t)],

(23)

where nk is the occupied trap density, v is the “attempt-to-escape” frequency, AE is the activation energy of the detrapping process, and T is the temperature in Kelvins which is a function of time. For high-field TSC using a linear temperature ramp, Simons et al. [119] have shown that the TSC spectra is, to a good approximatio~ a direct image of the ener~ distribution of occupied traps. The net TSC spectrum derived from data in Fig. 29A is shown in Fig. 29B. The lower x-axis is temperature, while the upper x-axis is energy (eV) I (A)

I

I

I

1

I

‘w 10

porn-lad

~ u E ~

1

s o

P~md 0.1 TBCBIW. -6OV 1 50

n ---ni. t

0

1

I

100

1s0

I

I 2W

Uo

1

300

3W

Tamparature ~C)

(B)

161

01 0

Energy (oV) 1.45

1.2 I

1.75

2.1

I

I

1

1

1

I

50

100

150

I

200

I

260

1 \

300

1

350

Temporatur. ~C)

Figure 29. (A) Pre- and post-irradiation TSC measurements performed at -60 V on a 350-nm soft oxide irradiated to 20 krad(Si02) at a bias of +30 V. (B) Net TSC spectra, derived from the data in (A), and plotted on a linear scale to emphasize

peaks at = 50”C (1.2 eV) and = 225°C (1.7 eV). (After Ref. [126])

II-42

relative to the oxide valence band. For this sample, there are peaks in the trapped-hole density at - 1.2 eV (50”C) and -1.7 eV (225”C). The advantage of TSC is that it provides trapped holes in the oxide. That is

a

direct measure of the total number of

QTSC = (&AVhOl~ - @Nh, where

~h

is the radiation-induced

trapped-hole

measured on a capacitor following irradiation charge and can be written as

(24)

It is important to note AVOt is a measure of the net oxide-trapped density.

Qot- q(~h - W),

(25)

where AN, is the number density of trapped electrons. Many studies have indicated the presence of electrons in the interracial region [9,10,125-130]. These electrons evidently are injected from the Si during the positive-bias irradiation and “compensate” the trapped holes. They are believed to be associated with the trapped hole complex [70,131-134] and, as such, are trapped very close to the Si02/Si interface. Since these electrons move only a short distance during the negative bias TSC (i-e., they are excited into the oxide conduction band and transport <7.5 m to the Si02/Si interface), they do not contribute significantly to the negative bias TSC. From Eqs. (24) and (25), the approximate electron trap density in MOS capacitors is given by [127]

AN, -

(1/q)(@sc

-

Qot).

(26)

3Q Integrated Circuit Measurements The purpose of this section is to discuss the tests used to characterize the parametric degradation and/or functional failure of ICS that are exposed to total ionizing dose. IC tests consist of either real time functional testing while the ICS are irradiated or functional and parametric tests performed before and after cumulative irradiation steps. The IC is usually biased during irradiation, and may be in a dynamic operating mode or in a static bias condition. During irradiation, parts need to be tested at different biases to ensure that both gate- and field-oxide leakages are monitored under worst-case conditions or that conservative measures are provided for timing degradation. In any case, functional tests require well-defined input patterns (e.g., checkerboard) that place the IC in a “known” state and an appropriate set of test vectors (i.e., inputs and outputs) to interrogate the device, i.e., the test program. Functional testing can be performed using a comparison test (between the device under test and a shielded standard) or a dynamic in-situ test. The effectiveness of each test varies widely, based on the accuracy and calibration of the test equipment and the designed-in testability of the part. It is often not possible to test 100% of the active elements or paths in a comple~ state-of-the-art IC [135]. Standard parametric measurements include power supply current, maximum functional frequency, access and delay times, and input/output voltages. For both parametric and functional II-43

testing, failure criteria or specifications must be clearly defined and often depend on system applications. Listed below is a brief description of measurements routinely made in the electrical evaluation of complex, state-of-the-art ICS.

32.1

Functionality

testing at maximum operating frequency

Maximum operating frequency f~Ax{MHz} is measured by monitoring circuit response to a series of test vectors or cell operations at increasing clock speeds. Outputs are checked for correctness in response to these test vectors that exercise critical paths or key cell operations, e.g., multiplication. A binary search is often performed to zero in on f~w. Testing to determine the maximum operating frequency should be performed at the minimum supply voltage, which represents the worst-case for transistor drive. Figure 30 shows a measurement of maximum operating frequency fM~ during irradiation and anneal for an 8-bit microprocessor fabricated in Sandia’s 4/3-pm technology [39]. During the irradiation to 107 rad(SiOJ, decreases in n- and p-channel drives due to thresholdvoltage shifts and mobility degradation lead to a reduction in the maximum operating frequency. At 107 rad(SiOJ, f~fi has decreased from its preirradiation value of 14 MHz to below 8 MHz. During the 125 “C anneal, p-channel drive is restored and fis seen to increase. For the intended system application, the specification was 4 MHz; therefore, these parts met the specification during the irradiation and anneal sequence. As opposed to

dynamic circuitry, static circuits should operate down to DC. 14.0 12.0 FREQUENCY (MHz) ‘T ‘DD

= 9“

10.0 8.0

1=

6.0

IRRADIATION ~

-

ANNEAL 125°C~

4.0 2.0 00

~

qoti RA#S PRE

1.0

0.1

i~7 10.0

RA~s 100.0

TIME (hrs)

Figure

30.

irradiation

microprocessor to 107 rad(SiOz) and anneal at 125‘C. After Ref, [39].) Maximum

operating

frequency

for

an g-bit

during

3X2 Shmoo plot of access time versus supply voltage Full characterization of ICS requires Shmoo plots. A Shmoo plot is a twoof the IC as two dimensional plot of the pass/fail nature of some characteristic II-44

independent parameters are varied simultaneously. An example might be to measure the effect on propagation delay as the basic clock rate or the supply voltage are varied. Figure 31 is a Shmoo plot of access time (i.e., the time between a read request signal to a

memory and valid data back) versus power supply voltage VDD for a 2k-by-8 radiation hardened SRAM. The plot indicates the SFL4M has a minimum access time of 29 m at supply voltage of 5.0 V. Shmoo plotting is often used to determine sensitivity the nominal and assess the timing characteristics of ICS. Maximum frequenq against variations in power supply voltage (VDD) and temperature gives valuable information about internal timing and critical paths through the device. 6

Ilr I

2k-by-8

SRAM

o P~ssod

I

u

I

I

F~llod

- 1111111

- 1111111 -

111111110 - 111111111 %11111111111 ~ 111111111111 11111111111111

4 20

Figure 31.

25

30

35 40 45 ACCESS TIME (ns)

50

Shmoo plot of access time versus power supply

by-8 radiation-hardened

55

60

voltage

VDD for a 2k-

SRAM.

3.2.3 Timing--access and delay times Timing parameters on an IC can be very complex and are clearly defined in the product specification [136] for an IC. Timing parameters include the time to fetch an instruction, to read or write to a memory locatio~ or to address 1/0. As an example, consider the “read” timing in an SRAM. “Read” time is essentially an access time, i.e., a measure of the time between a “read” request signal to the memory and valid data back. In Fig. 12 (see Section 2.2.3), the change in “read” time is plotted versus dose at a dose rate of 0.09 rad(Si)/s [29]. The initial “read” time of the memory was 61 m. The read “time” is degraded by 27 ns at 400 krad(Si). This timing parameter is a complicated function of n- and p-channel nobilities, drive, and bias states. Timing degradation is caused by the buildup of interface traps which degrade n- and p-channel nobilities and produce threshold-voltage shifts (negative for the p-channel and positive for the n-charnel at low dose rates) that lead to reduced drive, and consequently, slow the IC down; in addition, timing degradation can result from oxide-trapped charge buildup that leads to reduced drive in p-channel devices.

II-45

Perhaps the simplest timing measurement is propagation delay, the time it takes a signal to transit through a simple circuit element, e.g., an inverter pair. It is often convenient to speak of “inverter pair delay” that includes the delay for one “low-going” transition and one “high-going transition.” In a practical sense, propagation delay of a signal through a pair of inverters can be measured by subtracting the propagation delay through a string of five pairs of inverters from that through a ten pair string, and dividing by five. This removes constants such as the delay due to input and output buffers. To gain some insight into the effect of radiation on propagation delay, consider the effect of bias applied to the inverter pair during irradiation. Clearly, the bias condition of the first and second stage of the inverter pair changes with input bias (see Table II). The propagation delay for an input transition from low to high, tP~~, depends on the drive capability of the first stage n-channel transistor and the second stage p-channel transistor; similarly tP~~, an input transition from high to low, depends on the drive capability of the first stage p-channel transistor and the second stage n-channel transistor. For an input “low” bias during irradiation, tP~~ is affected by the radiation response of the “off” n-channel in the first stage and the “off’ p-channel in the second stage. For the same bias condition during irradiatio~ tpHL is affected by the “on” p-channel in the first stage and the “on” n-channel in the second stage. (Essentially, n-channel transistors “pull down” a node from V~~ to ground, while p-channels “pull up” a node from ground to V~~.) The change in propagation delays, tpLHand tPHL,during an irradiation to I@ rad(SiOz) and a biased anneal at 100”C for 42 hrs, is shown in Fig. 32; both irradiation and biased anneal were performed under “low” input bias. The increase in tpLHduring irradiation is due to decreased drive in the “off’ lst-stage n-channel transistor (caused by mobility degradation) and decreased drive in the “off’ 2nd-stage p-channel transistor (caused by a negative threshold-voltage shift) [40]. The increase of tPHLduring anneal occurs when the 2ndstage “on” n-channel transistor (1) “rebounds” above its preirradiation value thereby reducing its drive and (2) suffers further mobility degradation as interface traps continue to build at elevated temperature. Also seen in Fig. 32 are SPICE simulations for tpLHand tPHLduring irradiation and anneal. Inputs to the simulation included changes in threshold voltage for transistors in each bias condition and changes in mobility as function of totaldose and anneal using the Sun-Plummer [52] relationship of Eq. (4). These two measures of transistor performance fully accounted fOr tpLHand tpHL during irradiation and aMeal. There is excellent agreement (within 15%) in Fig. 32 between measured and simulated values for the delay pairs. Table II. Integrated

Circuit Timing and Transistor

TEST CC)NDITION t~LH tPHL RADIATION

Input “low” Input “high’

Bias Condition

n-channel p-channel

2nd Stage p-channel n-channel

n-off p-on n-on p-off

n-on p-off n-off p-on

~

BIAS

II-46

[40]

7.0 6.0

O A MEASURED , A SIMULATED

5.0 4.0 PROPAGATION DELAY (nsec)

PHL 3.0 PLH

2.0

PRE

0.1 TIME (houre)

Figure 32. Propagation delay, tpLH and tpn, for an inverter pair during irradiation and biased anneal. Measured values are compared to SPICE predictions, agreeing to within 15%. (After Ref. [40])

3.2.4 Power supply current, I~~{A} The power supply current I~~{A} is simply measured by putting an ammeter in series

between V~~ and ground. This is a DC parametric test. For state-of-the-art CMOS, 1~~ is typically 50 to 500 pA prior to irradiation. An example of static leakage current during irradiation to 107 rad(Si02) and anneal at 125“C for an 8-bit microprocessor was shown in Fig. 6 [39]. As seen in the figure, leakage current for the microprocessor increases to This increase is primarily due to increased n-channel nearly 1 mA during irradiation. transistor leakage at VGs = O V due to negative threshold-voltage shifts during irradiation. The relationship between static power supply em-rent and n-charnel transistor leakage was shown in Fig. 7. To insure worst-case leakage, power supply current should also be measured using the complement of the pattern stored during irradiation. If a “checkerboard” data pattern was read into a memory for a static-bias exposure, then an “inverse checkerboard’ should

be written into memoryprior to measuring the leakage current.

The reason is that n-channel transistors irradiated in the “on” condition often (but not always) suffer the largest threshold-voltage shifts, and draw the largest current when placed in the “off” condition, i.e., VGs = O V. If the data pattern stored during irradiation is maintained during measurement, the main current contribution is from n-channel transistors irradiated in the “off” condition. Since these “off” n-channels generally don’t experience threshold-voltage shifts as large as the “on” n-charnel transistors, IC leakage may not be worst case. The data in Fig. 33 shows standby ~rrent for a 64k SRAM irradiated with solid “ones,” and tested with either the solid “ones” or its complement pattern [45]. As showq the complement data pattern was found to produce standby currents of 2 PA and 72 @ after total doses of 2x1W and 1x1OErad(SiOz), respectively. By compariso~ the II-47

noncomplement pattern (i.e. solid ones) produced currents of only 2 PA and 4 PA at the same doses and test conditions. For this pa~ the complement pattern applied during test clearly resulted in the maximum radiation-induced leakage. 10-4 \

1 ●

g + z w a m ~ 1.-5

Complement

o Solld Ones

* m a z a $

Pre-Rad

104

105

106

TOTAL DOSE pad (S102)l Figure 33. Standby and noncomplements

32.5

current of a 64k SRAM versus total dose for the complements test patterns. (After Ref. [45])

Output drive currents, I~~{A} and I~P{A}

A final measure of integrated circuit performance is the drive current which the output buffers car source or sink. For TIZ compatibility, output drive currents of CMOS ICS are typically measured 0.5 V off either power supply rail. That is, for output drive into a high load, IDP, the output pin is held at VDD-0.5 V and the p-channel source current measured. Similarly, for output drive into a low load, IDN, the output pin is held at V~~ + 0.5 V and the n-channel sink current measured. Changes in output drive current as a function of total dose and post-irradiation biased anneal are shown in Fig. 34 [40]. Both IDN and IDP decrease in absolute magnitude during irradiation. During irradiation, the drive of both n- and p-channel transistors decrease due to a reduction in mobility caused by the buildup of interface traps. During the biased anneal, IDN continues to decrease while IDP recovers slightly. This occurs because the p-channel transistor recovers drive due to annealing of oxide-trapped charge, while the n-channel drive decreases more dramatically as Vt~ rebounds above its pre-irradiation value.

3.2.6 Other testing considerations

Today’s VLSI circuit testers consist of a central control processor that generates timing and control signals, and is capable of sensing outputs. State-of-the-art testers are capable of operating at frequencies of 200 MHz. Basically, the tester sends out a series of signals to the IC (i.e., inputs or test vectors) and measures IC response. If the measured II-48

10

5 OUTWT DRIVE (mA)

1o

BIASED AN NEAL~ T = 100”C

RADIATION ~ 1

0.1

1

-5

Figure 34. Output drive currents, IDN and IDP, as a function post-irradiation biased anneal. (After Ref. [39])

of total dose and

response varies from the expeeted value, the IC fails the test. Signals/sensors are often referred to as power supply units, PSUS, and parametric measurement units, PMUS. As a general practice, periodic calibration procedures must be applied to eliminate skew and drift inaccuracies in these testers. Also, control samples should be used as an additional safeguard against tester errors. As stated above, it is often not possible to test 100% of the active elements or paths in a complex, state-of-the-art IC. The enormous number of possible sequential combinations makes exhaustive testing impractical. Intel estimates it would take > lN s (or 1157 days) to test a 80386 exercising eve~ active element through every possible path assuming that the tester is operated continuously at the device rated frequeney [135]; that is 24 hours/day, 365 days/year! The challenge for test engineers is to design test strategies that achieve the necessary degree of screening with confidence at a reasonable cost and time. Designing VLSI circuits for testability is the most efficient way to reduce the relative costs of assuring high chip reliability in radiation environments.

3.3 ASTM, MILITARY, and DNA Test Standards

The reader should be aware that many test standards exist for the electrical characterization and radiation testing of semiconductor devices. The American Society for Testing and Materials (ASTM) has an F-1.l 1 Electronics Subcommittee on Hardness and Quality Assurance that meets three times a year to develop, revise, and ratify test standards for use by the electronics indust~. These standards are readily available and published annually [138]. A listing of test standards relevant to total-dose testing that are presently being considered by the F-1.11 Subcommittee are given in Table III. Several of these ASTM standards have already been discussed in this text. The test standard that governs total-dose testing of microelectronics in the DoD is MIL-STD-883D, Method 1019.4, Ionizing Radiation (Total Dose) Test Procedure [139].

II-49

Table III.

ASTM F-1 Electronics Standard in Process

Subcommittee

on Hardness

and Quality--Test

Number

Titl~

F-616

Std Test Method for Measuring MOSFET Drain Leakage Current

F-996

Test Method for Separating Total Dose Induced MOSFET Threshold Voltage Shift into Components due to Oxide Trapped Holes and Interface States Using the Subthreshold Technique

11A48

Std Test Method for Determining MOSFETS by Charge Pumping

F-1096

Method for Measuring MOSFET Saturated Threshold Voltage

11A49

Std Method for Use of an X-Ray Tester in Radiation Hardness Testing of Microelectronic Devices

11A50

Std Method for the Radiation Testing of Microprocessors

the Mean Interface Trap Density of

This test procedure defines the requirements for testing packaged semiconductor integrated circuits for ionizing radiation (total-dose) effects from a COGOgamma ray source. The latest revision of this procedure provides an accelerated aging test for estimating ionizing radiation effects on devices in low-dose-rate environments, e.g., space. This test procedure provides specific guidance on how testing is to be performed. For example, it specifies bias and temperature conditions, allowable dose rates, time between irradiation and test, etc. Many of these issues are key to successful and repeatable measurements and will be discussed in the next section. Finally, testing guidelines are sometimes provided to contractors by government agencies that sponsor work in radiation-hardened microelectronics, e.g., DNA and USASDC [140]. These guidelines are intended to standardize testing so useful comparisons can be made between product supplied by different vendors. DNA/USASDC guidelines originated during the DNA/VHSIC Phase I program and have been improved upon under the USASDC SAT 8.1 hardened VLSI technology development and PMA-A1104 programs [141]. These guidelines are all based o% or are generally consistent with, MIL-STD-883D, “Test Methods and Procedures for Microelectronics,” except in specific circumstances where deviations are permitted.

4.0 CRITICAL MEASUREMENT

VARIABLES

section, some of the tools and techniques used to measure the In the previous response of CMOS devices and circuits to total-dose ionizing radiation were discussed. Prior to that, a review of basic mechanisms outlined the physical processes that actually 11-50

control the radiation response, e.g., charge generation; hole transport, trapping, and anneal; and interface-trap growth and anneal. Many of these processes were seen to be time-dependent, and many depend on external variables like the applied electric field, sample temperature, and energy of the radiation source. These external variables, termed “critical” measurement variables, need to be specified to properly characterize the radiation response of the device or to properly interpret the results of a test. “Critical” measurement variables also have important testing implications and are the basis of many hardness assurance and prediction schemes. In this section, each of these “critical” measurement variables will be explored in greater detail. Initially some data will be presented to illustrate how the variable affects the radiation response, and then testing implications will be summarized. Critical measurement ●

Dose

●

Dose rate

●

Bias (Electric field)

●

Temperature

●

Energy

variables include

4.1 Dose Many variables affect the dependence

of oxide-trapped charge and interface traps on radiation dose. These variables include the field applied during irradiatio~ the range of dose studied, and the details of sample processing. (The details of field dependence will be described later in Section 4.4. In this section, for purposes of companso~ data is primarily given for irradiations under positive gate bias with resultant fields -1 MV/cm.) At positive fields, most studies suggest that (at low to moderate doses) oxide-trapped charge builds linearly with dose [13, 14,75, 142,143], while interface traps have been reported to build linearly [35,64,75] or sublinearly with dose (usually Dzls) [27,28,99,103,144-147]. Another generally observed feature of the total-dose radiation response of MOS devices is a tendency of the negative radiation-induced threshold-voltage shift, AVth, to

saturate at high doses [13,148]. The total-dose level at which saturation becomes observable may va~ considerably with device processing and irradiation conditions; saturation may occur at doses s 10 krad(Si02) for commercial devices and above 1 Mrad(SiOz) for radiation-hardened devices. Figure 35 shows AVkh,AVOt,and AVit as a function of dose as measured on a Texas Instruments radiation-hardened MOSFET with a II-51

27.4 -nm gate oxide irradiated with a 10-keV source [148]. Also shown are the initial slopes (i.e., shift/unit dose {V/rad(Si02)} ) describing the buildup of AVOtand AVit. AVth saturates at a maximum negative shift of -0.49 V and then shifts slightly positive. For these samples, it is evident that the “hard’ saturation of AVth is partially a result of the near compensation of AVOtand AVit, and partially a result of saturation in the AVOt buildup. At doses greater than 1 Mrad(Si02), the rate of the AVOt buildup has significantly decreased below its initial rate. Clearly, processes are acting to severely limit AVOt at high dose. Note that Avit is, itself, showing evidence of saturation in these samples, although many studies in the literature [18,28] suggest interface traps do not saturate at doses in excess of 20 Mrad! 1.0

~ m ~ z W 8 : o >

0.5 TI 27.4 nm OXIDE 2MV/cm 0.0

0.5 \ \ -1.0 1 .1.5

0

2

4

a

6

10

12

14

GATE OXIDE DOSE [Mrnd (S102)]

AVth, and its components AVOkand AVit, as a function of dose for a Texas Instruments radiation-hardened MOSFET with a 27.4-rim gate oxide irradiated with a 10-keV source. (After Ref. [148]) Figure

35.

Figure 36 shows a series of I-V curves taken at prerad and doses of 2, 5, and 10 Mrad(SiOz) for a transistor fabricated in a silicon-on-insulator (S01) technology [149]. The SOI transistor has a 20-nm-thick gate oxide, and was irradiated with 5 V on the gate undergoes a nearly parallel using a 10-keV x-ray source. The I-V characteristic - 0.6-V shift following the 10-Mrad(SiOz) exposure. Based on the preirradiation and 10-Mrad(SiOz) I-V curves, one might assume this technology is radiation-hardened to doses in excess of 10 Mrad(Si02). However, the 2 and 5 Mrad(Si02) I-V curves tell a different story. These curves indicate the presence of a sidewall-oxide leakage path that “turns on” at intermediate doses in the range from 1 to 5 Mrad(SiOz). This leakage path results from the buildup of positive oxide-trapped charge in the sidewall oxide that ihverts the underlying p-type silicon, thus creating a channel between the source and drain of the device. At higher doses, this leakage path is “turned off” as negatively-charged interface traps build up and compensate the oxide-trapped charge. This behavior is analagous to that shown in Fig. 16A for a CMOS field oxide [72,150]. Testing Implicah”ons:

Because saturation II-52

effects are observed for AVth and AVOt,

10-3 10-4 10”5 10”6 ~ *

10”7

-n

10”’

5 Mrad~ 2 Mrad

N-CNANNEL

10”0 10 -lo 200 krtd (S102)/mh

10 -11 .-

-4

-3

-2

-1

0

1 GATE VOLTAGE (V)

2

3

4

Figure 36. I-V curves taken at prerad and doses of 2, 5, and 10 Mrad(SiOJ for a transistor fabricated in a silicon-on-insulator (S01) technology. The SOI transistor has a 20-nm-thick gate oxide, and was irradiated with 5 V on the gate and using a 10-keV

x-ray

source.

(After

Ref. [149])

efforts to characterize the total-dose radiation response of a semiconductor device should involve a series of irradiations performed at increasing levels, e.g., 0.2, 0.5, 1.0, 2.0, 5.0, etc. times the specified radiation level at which the device is required to operate [139]. These step-stressed measurements can be used to determine a failure dose for an IC, as well as the margin a particular device has with respect to its specification. Because of saturation effects, it would be a mistake to irradiate a radiation-hardened IC to 10 Mrad(SiOz) and attempt to infer its response at 500 krad(SiOz) or 1 Mrad(SiOz). If saturation has occurred at the higher dose, parametric degradation at lower doses maybe greater than expected. Depending on the technology and operating conditions, saturation may be due to: trap filling, large space charge effects in the oxide to the point of field reversal, or recombination of the trapped holes with radiation-generated electrons moving through the trapped hole distribution. Most experiments to characterize the total-dose radiation response of devices are designed to avoid large space charge effects in the oxide. In addition, field or isolation oxides may initially invert and create a leakage path at a given dose, only to recover at a higher dose as interface traps build and compensate oxidetrapped charge [150]. Once agaiq initial characterization of a technology must involve step-stressed irradiations.

4Q Time

In Fig. 37, the threshold voltage shift due to interface traps, AVit, is plotted as a function of postirradiation anneal time for n-channel transistors with 60-nm gate oxides [75]. Irradiation and anneal bias is 6 V. “Zero” on the time axis is taken to be the beginning of each of the respective irradiation periods. Data are shown for LINA~ x-ray, and CSISTirradiations to a total dose of 100 krad(Si02), followed by biased anneal. Dose II-53

rates range horn 6 x 1(Y to 0.05 rad(SiOJ/s. It is most important to notice here that the buildup of interface traps with postirradiation annealing time is independent of the radiation source employed and dose rate, and the results all fall on a cmnrnon “defectgrowth” curve. For example, whether devices were exposed to two IJNAC pulses, each of 8 ps duratioq and annealed for one weelq or whether devices were exposed to the same total dose over the course of a week, to first order, the same number of interface traps are measured [75]. 1.2

x-ray 0.9

(50-5000

rad/s)

A Vlt (v)

●

●

o~ .

\ A

~/’T

Ca-137 (0.16 rad/s)

0.3

C8-137 (0.05 rad/s)

LINAC (6 x 109 rad/s) I

0 -1

10

10°

I

10’

1

1

102

I

103

I

I

104 106 106

107

TIME (80C) Figure 37. Threshold-voltage shift due to interface traps for n-channel transistors with 60-nm gate oxides versus postirradiation annealing time for varying dose rate exposures to 100 krad(SiOJ. Irradiation and anneal bias was 6 V. (After Ref. [75]) In Fig, 38, the threshold voltage shift due to oxide-trapped charge, AV~, is plotted as a function of postirradiation anneal time for the transistors of Fig. 39 [75]. Note tha~ with the exception of the small, “short-lived tails (regions in which AVOtfalls slightly below the straight line, as shown most clearly for the LINAC data), the values of AVd all fall on a straight line that represents linear response with logarithmic time. The slight deviations from this response at short times after exposure are a result of the detailed impulse response of MOS devices, and are due simply to the fact that the total charge in each irradiation is not deposited at the same time. (This behavior for trapped-hole annealing has been characterized extensively in previous work via linear-response analysis [13, 17,29,142,143]). To be able to plot points on a single “transient-annealing” curve (dashed line in the figure), one must be 1-2 decades beyond that time in which (at least the greatest fraction of) the dose is deposited so that differences in annealing time for different units of trapped charge are no longer significant. Once agai~ to first order, it doesn’t matter if devices are irradiated at a high dose rate and annealed for one week or irradiated to the same equivalent dose over a longer period of time. AU the data falls on a common “defect-annealing” cume called the “transient-annealing cume” which is well described by the equation

-AVO= (-ti(t)+ n-54

C)/Y.,

(27)

where -AVOis the transient annealing curve per unit dose, TOis the total dose used to obtain the transient annealing curve, A is the magnitude of the slope of the transient annealing curve, and C is the intercept at the end of a specified irradiation In Fig. 38, -yO is 100 krad(SiOz), A is 0.07 V, and C is 1.25 V at 1 s. Once the impulse response or transient-annealing curve for an MOS device has been determined, the radiation response at a given dose rate AV(t) is just its convolution with the dose rate, d~/d~ i.e., t AV(t) = j [d~(~)/dt]AVO(t-~) dr. o

(28)

Taken together, the results of Figs. 37 and 38 strongly suggest that, over a wide range of dose rates and measuring times, there are no true dose rate effects on MOS device postirradiation response. As long as the time is appropriately normalized, bias and temperature are maintained roughly constant, and corrections are made for doseenhancement and electron- hole recombination effects, MOS postirradiation response at a given total dose can be described with simple defect-growth and transient-annealing curves. The same general trends are observed for p-, as well as n-channel transistors, and for irradiations in either the “on” or “off’ state [75]. Or

I -0.4

= -Aln(l)+C

-llvo

X-ray

t

L

A = 0.07 V C=l.25V

.0.8

Avot (v) -1.2

●9

\A .

-1.6

I -2.0

!1

10

LINAC I

I

I

I

I

I

10°

10’

102

103

104

105

I 7

106

10”

TIME (see) charge for n-channel Figure 38. Threshold-voltage shift due to oxide-trapped transistors with 60- nm gate oxides versus postirradiation annealing time for varying dose rate exposures to 100 krad(SiOJ. Irradiation and anneal bias was 6 V. (After Ref. [75])

39 shows the static power supply current 1~~ for 2k static RAMs irradiated with 1O-V bias to 500 krad(Si) at dose rates of 200 rad(Si)/s and 2 rad(Si)/s [29]. A 1O-V static bias was applied during the irradiation. 1~~ was measured immediately following irradiation and at logarithmic times between irradiation and test while under 1O-V (solid) or O-V (open) bias. Following the 200-rad(Si)/s irradiatio~ the devices under 1O-V bias anneal 579Z0in the first hour, while the devices under O-V bias ameal 31%. The decrease Figure

II-55

in static power supply current follows the anneal of oxide-trapped charge. Following the 2-rad(Si)/s irradiation, there is not much annealing. The reason is that the irradiation takes 69 hrs, and there is little additional amealing in the next l-hr interval. 70.0

I

I

I I

11111

I

I

I

I

-8

60.0

I

11111

I

I

SANDIA SA3001 BIAS DURING ANNEAL

50.0

lDD(~A)

I 1111

9

200 rad (Sl)/S

0

40.0

,A1O

d“

@

VDD= 1 IV 30,0

OAO

●

20.0

o

● 2 rad (S1)/s

10.0

AA~~

0.1

I I I I ,,,,,

V

o

-

0.0

V

o

● ~,

A

,,,,,#

, 9A

,9,,,,;

1.0 10.0 TIME BETWEEN IRRADIATION AND TEST (hr)

100.0

Figure 39. Static power supply current IDD for 2k static RAMs irradiated with 10V bias to 500 krad(Si) at dose rates of 200 and 2 rad(Si)\s. (After ref. [29])

Method 1019.4 (see Section 3.3), which governs total-dose testing in the DoD permits testing at dose rates from 50 to 300 rad(Si)/s and anytime within the first hour following irradiation. Very different measures of the hardness of a part can be obtained by variations in allowable dose rates and times between irradiation and test. For example, compare the measured response following a l-Mrad(Si) irradiation performed (1) at 50 rad(Si)/s with 1 hour between irradiation and test and (2) at 300 rad(Si) /s and made immediately following irradiation. There is a difference of nearly 6 hours in those two test scenarios between the start of the irradiations and when testing is actually performed! Based on the data in Fig. 39, the radiation response may vary significantly depending on the time scales for defect growth and annealing. Because measured device parameters are changing with time, it is important to specify “when” the measurement is being made. “When” includes the time it takes to perform the irradiation and the time between irradiation and test. Remember, the clock begins at the start of the irradiation! During irradiation, generation and annealing processes take place simultaneously. This is reflected in the convolution integral given by Eq. (28). Testing Implications:

A fundamental understanding of the time-dependent nature of oxide-trapped charge annealing and interface-trap buildup should provide a self-consistency check on measurements. If a 100-krad(Si02) irradiation is performed at high-, moderate-, and lowdose rates, and if measurements are made immediately following irradiation, then AVOt should be largest following the high-dose rate irradiation. If it is not, then check the Also, an understanding of defect growth and dosimetry or measurement apparatus. annealing provides the technical basis for acceleration schemes that relate radiation response measured in the laboratory (at 1019.4 like dose rates) to that measured in II-56

environments of interest, e.g., space, accelerator, or weapon. Such acceleration prediction schemes are an important element in present hardness assurance programs.

or

4.3 Dose Rate

When performing total-dose radiation-effects testing, it is important to specify the dose rate of the incident radiation. MIL-STD-883D, Method 1019.4 suggests testing in a range of dose rates from 50 to 300 rad(Si)/s [151]. Clearly, laboratory dose rates differ by several orders of magnitude from dose rates typically encountered in either space or weapons environments. As will be discussed below, recent work has demonstrated that failure dose for an IC is a complicated function of dose rate [29,35]. The challenge in testing parts for use in weapon or space scenarios is then to predict their response in differing environments based on measurements performed in the laboratory. Laboratory measurements have the advantage of being more controllable, practical, and cost For example, performing a measurement at a space-like dose rate (i.e., effective. -1 mrad(Si)/s) might take months to years, is simply not practical for qualifying parts, and is fraught with pitfalls. Johnston [35] studied the failure dose of a commercial NMOS microprocessor as a function of dose rate. The failure criteria he defined for the circuit was AVt~ = i-O.45 V; the active input threshold was determined by stepping an input pin through small voltage steps until the device turned on. A plot of the dependence of the circuit failure level on dose rate is shown in Fig. 40, along with predictions from empirical models describing the buildup and annealing of oxide-trapped charge and interface traps. Note that a peak in the failure-dose versus dose-rate curve results because there is a change in failure mode [29,35]. At high dose rates, failure is defined by AVt~ = -0.45 V and is caused by the buildup of oxide-trapped charge, while at low dose rates, failure is defined by AVt~ = + 0.45 V and is caused by the buildup of interface traps. The peak in the dose rate response in Fig. 40 is due to nearly exact cancellation (or compensation) of the two competing mechanisms that contribute to the shift in threshold voltage. For a single transistor, the peak could be many orders of magnitude above the base level. In similar work, Winokur et al. [29] observed a peak in the failure-dose-versus-doserate curve when radiation-hardened 2k SRAMS were irradiated at dose rates ranging from 0.09 to 200 rad(Si)/s. For these SRAMS, failure was defined “parametrically” as static leakage currents greater than 1 PA or timing increases greater than 6 m. Once agai~ the “peak” in the failure-dose-versus-dose-rate curve for these 2k SRAMs occurs because of a change in failure mechanism i.e., from failure caused by timing changes at lower dose rates to failure caused by increases in static leakage current at higher dose rates. To gain some insight into the physical mechanisms responsible for the change in failure mode, transistors on the same die as the IC were irradiated. In Fig. 41, threshold-voltage shifts for n-charnel transistors are shown following irradiations at varying dose rates. At a dose rate of 200 rad(Si)/s, the threshold-voltage shift steadily decreases with dose and is -0.4 V at 1 Mrad(Si). At 0.23 rad(Si)/s, the threshold-voltage shift steadily increases or “rebounds” with dose and is +0.7 V at 1 Mrad(Si). Data for the 0.05 rad(Si)/s irradiation II-57

is only available to 500 krad(Si), but indicates more “rebound” at equivalent doses than the 0.23 rad(Si)/s irradiation. The behavior of the threshold-voltage shift at dose rates of 20 and 2 rad(Si)/s is intermediate between what is observed at 200 and 0.23 rad(Si)/s. 120

100 FAILURE DUE TO POSITIVE THRESHOLD aHIFT

80 FAILURE LEVEL

Y

60

(krad (S1))

●

~ ~ MOOELCALCULATION I MEAWRED

FAILURE

I , FAILURE CRITERIA:

LEVELS AV,h

B

= * 0.45V

I l

●

40 20 I

1

,.-l

,.O

I

0 10

.3

,.-2

1

■

,.l

I

I

,.2

,.3

,.4

DOSE RATE (red (S1)/s)

Figure 40. Dependence of circuit failure microprocessors. (After Ref. [35]) 0.8 “

1 I I I G0239AIW7

0.6 ‘GS

THRESHOLDVOLTAGE SHIFT

0.4

AVth(V)

0-2

level on dose rate for commercial

I

1111]

I

I I 11111

I

NMOS

I 1111

I

I

I

I I 1111

0.23

=1OV 0.O5 rad (S1)/s

2

0.0 -0.2

20 .0.4

200 I

-0.6 ,04

I I I 11111 35 ,05

I

I I 1 11111 35 106

I

DOSE (red (Sl))

Figure 41. Threshold-voltage shifts versus dose at varying dose rates for n-channel transistors irradiated with 1O-V bias applied between gate and substrate. (After Ref. [29]) In Fig. 42, the contributions to the net-threshold-voltage shift due to oxide-trapped and interface-trap charge are shown for the devices of Fig. 41. The oxide-trapped charge

component, AVOt,steadily decreases as the dose rate decreases. This is consistent with the fact that lower dose-rate irradiations are longer and consequently the oxidtirapped charge (i.e., trapped holes in the vicinity of the Si02/Si interface) has more time to anneal II-58

[13-17,75,142,143], As discussed in Section 2.1, annealing of oxide-trapped charge may result from the injection of electrons from the Si, from thermal detrapping, or some combination of injection and detrapping. The interface-trap charge componen~ AVit, is steadily increasing as the dose rate is decreasing. This results from the longer times associated with lower dose-rate irradiations which supports a very long-term delayed buildup of interface traps [15,25-33,63,75]. As the dose rate is lowered, there are fewer oxide-trapped charges and more interface traps. This results in the net threshold being more positive at any given total dose as the dose rate is lowered. Therefore, in a low-dose rate environment, the net threshold-voltage shift for these transistors would be positive, largely controlled by AVit, and cause increases in timing in 2k SR4MS. In a high dose rate environment, the net threshold-voltage shift for these transistors would be negative, largely controlled by AVOk,and cause increases in static leakage in 2k SRAMS. VOLTAGE SHIFTS (V)

2.0

1 -

~

I

I

I

I I I II

I

I

I II

Ill

I

I

I

I I I II’

;gO r-d (S1)/sl

1.5 ‘A2

‘“N

1.0

0.23 0.05

:8

0.5 0.O

G0239AIW7 N-CHANNEL vGS = 10V

-0.5

AVot -1.0

a I

-1.5 ,.4

I I 35

I 11111

I

1

I I 11111

35

,05

,06

I

I

I

I 1111

3

DOSE (rad (S1))

Figure 42. Contributions to the net threshold-voltage shift from oxide-trapped, AVOt, and interface-trap charge, AVit, for the irradiations shown in Fig. 34. (After

Ref. [29]) Figure 43 shows leakage current versus dose for 16k SRAMS irradiated at dose rates of 100, 1800, and 1(F rad(SiOz)/s [150]. These SRAMS were fabricated in Sandia’s CMOS 111A process [74]. For the 1800 and 1(P rad(SiOz)/s irradiations, a large and rapid increase in IDD with dose is observed at doses of 50 to 100 krad(Si02). However, no significant increase in leakage current is observed for the 100 rad(Si02)/s irradiations until doses approaching 1 Mrad(Si02). Using test transistors, the authors demonstrated that the large increases in leakage current at the higher dose rates were caused by the The longer times associated with the 100 turn-on of a parasitic field-oxide transistor. rad(Si)/s irradiation allowed for sufllcient amealing of oxide-trapped charge and buildup of interface-trap charge in the field oxide to prevent the turn-on of the parasitic leakage path. If failure for these 16k SR4MS is “parametrically” defined as leakage currents in excess of a given level, e.g., 1 ~ then the failure dose would increase as the dose rate decreased. At the higher dose rates failure results from field-oxide parasitic leakage, while at lower dose rates failure results from normal gate-oxide leakage measured at II-59

V~~ = OVorfiom ''rebound' caused bythebuildup ofintetiace above, there is a change in failure mode as the dose rate varies.

traps. Like the examples

‘O-’ ~ ,~.

z

106rnd/~ ~

a.

,~-3

_~

,.-4 \ L~

1833 rod

●

100 radla

/

t

‘“-’w

01 =31nm

,“.e

PRE

‘

●

1

0.1 Dose,

10

Mrad(SK12)

Figure 43. Leakage current IDD versus dose for 16k SRAMS irradiated of 100, 1800, and 10G rad(SiOJ/s. (After Ref. [150])

at dose rates

Figure 44 illustrates how the failure dose of three commercial devices depends on the dose rate of the irradiation [152]. The “parametric” failure dose was defined as static power current in excess of 100 PA for OKI 81C55 2k static RAMs and HarrisHM65044k Static RAMs, and 1 PA for SGS 4007 inverters, though the trends illustrated in the figure are independent of this exact definition. For these commercial devices, the failure dose either remains constant or improves as the dose rate is lowered [29,143,152,153]. A similar trend is observed for transistors built in a specially softened process at Sandia (G1916A/W33) [29,153]. Finally, Fig. 45 plots the change in reciprocal gain (A1/~) versus total dose at two different dose rates and for two different bipolar processes [154]. The two processes are denoted as the solid and dashed lines. At 200 krad(SiOz), there is approximately an order of magnitude more gain degradation at the lower dose rate. There are several important testing implications arising from Testing Implications: the strong dependence of CMOS and bipolar device response on dose rate illustrated in Figs. 40-45. The most important is that a test performed at an intermediate laboratory dose rate can give a different result than a test performed at a higher or lower dose rate; and to make matters worse, failure mechanisms at different dose rates may differ! As the Johnston data in Fig. 40 shows, testing at an intermediate dose rate can give a very misleading measure of the actual failure dose of an IC in its intended use environment. For the microprocessors that Johnston studied, testing at -1 rad(Si)/s led to a failure dose 1 to 2 orders of magnitude higher than what might be expected in a lower-dose-rate space environment. For the 16k SRAMS shown in Fig. 43, testing at a laborato~ dose rate of 100 rad(SiOz/s would have given no indication of the field-oxide related leakage that the SRAMS would experience in a strategic environment. For the bipolar devices shown in Fig. 45, the gain 13was more degraded following low-dose-rate irradiations. This 11-60

—

1

1019 r G1916AIW33 . OKI

. SGS 007 6s04 F+. !

z A T

.

0.01

0.001

—

. :sm

100

1000

Rate ~:d (SIO~ );:]

Figure 44. Failure dose versus dose rate for three types of commercial MOS devices and a specially softened Sandia device (G1916A/W33). (After Refs. [152,153])

10-’

10-2[

10-3

■

0

1.1 rad/8

d

< [

10”41

#

.“1

a

1

“pr*=f#** q. . -----n nm

11

4

radls

.300 ,

n # 1 n

II

10°

10s

a

a

I

m 1 1

{[

107

rad (S102) Figure 45. Changes in reciprocal gain (A1/@ versus total dose at two different dose rates and for two different bipolar processes. (After Ref. [154])

would need to be accounted for if measurements are confined to intermediate dose rates normally used in the laboratory. Sometimes, testing at laboratory dose rates provides a consemative estimate of the failure dose, as was seen in Fig. 44 for commercial CMOS devices. When an experiment is designed to support the qualification of an IC for use in a spedic radiation environment, variations in failure dose with dose rate need to be taken into account. Method 1019.4, the latest revision of MIL-STD-883D, provides an accelerated aging test for estimating low dose rate ionizing radiation effects on MOS devices [139,152,153]. This test was designed to account for differences in device response at varying dose rates. Clearly, the time-dependent nature of the buildup and anneal of II-61

oxide-trapped charge and interface traps discussed in the previous section is the reason for these dose rate dependencies. A second testing implication of the dependence of CMOS device response on dose rate is the need to measure components of the threshold-voltage shif~ AVOtand AVit, in addition to AVth itself. AVt~ is simply the algebraic sum of AVOtand AVitPand for n-channel transistors these components can cancel each other (see Section 2.2. 1). Just measuring AVth can be misleading if you are interested in radiation hardness. In general, AVOt dominates the threshold voltage and IC response at high dose rates, while AVit can dominate at low dose rates. Therefore, each should be independently measured and controlled for optimum radiation tolerance in differing environments [37,38]. Some may say that testing at the system dose rate is the best approach.

Clearly, it is always an option to the experimenter. However, irradiations performed at extremely high or low dose rates are difficult to execute, and often expensive and impractical. For example, a low-dose-rate irradiation performed over several months requires constant monitoring to ensure that bias is maintained and that temperature is adequately controlled. The dose rate of an irradiation must be specified!

4.4 Bias (Electric field)

The radiation response of CMOS devices depends on electric field in a fairly The electric field plays an important role in the initial complicated manner. recombination process; hole transport, trapping, and anneal; and the interface-trap buildup. Historically, maximum threshold shifts and leakage resulting from oxide-trapped charge were measured at positive gate bias for n-channel transistors and zero gate bias for However, as technologies scale down and employ thinner gate p-channel transistors. oxides, several studies suggest different prescriptions for what constitutes worst-case static bias [155-160]. In Fig. 46, AVhhfor n-channel transistors with 32-rim gate oxides is shown during a 200-krad(SiOz) irradiation and subsequent anneal [157]. Irradiations and anneals were performed with several combinations of 6-V “on” and O-V “off” bias conditions. For this technology, the largest shifts during irradiation are observed for samples with zero gate bias, VGs = O V! (Note that during irradiation AVth is negative and dominated by oxide-trapped charge.) This result seems surprising at first, but can be explained by examining the effect of oxide electric field EOXon (1) the number of holes that escape the initial recombination process and (2) the number of holes that are trapped at the SiOz/Si interface [157]. In terms of recombinatio~ the higher the field, the greater the yield of radiation-generated holes [4,161,162]. Due to the semiconductor work function difference between the gate and substrate, i.e, @w =1.1 V for these devices, there is still a field of EOX=0.34 MV/cm at zero applied gate bias, and an associated yield of -70% [3-5,163-166]. There is certainly more yield at positive bias, but a substantial number of holes still escape the initial recombination process with zero volts on the gate. On the other hand, the number of holes that are trapped at the SiOz/Si interface is II-62

determined byan E-ha hole-trapping cross-section [4,22,161-163,167,168]. Therefore, there is more trappingat zero gate bias than appositive gate bias for the devices of Fig.46. The combined effect of recombination and trapping is agreater shift at zero applied bias. Foradvanced technolo@es, EOXmsociated tiththe work fintiion difference will increase as the gate oxide thickness decreases, i.e., EOX=$ W/tOX,leading to even greater yields. It is also seen in Fig. 46 that the largest shifts during ameal are observed under positive bias. During anneal AVt~ is positive and dominated by interface-trap charge. This occurs during postirradiation anneal when more interface traps buildup and more holes are removed under positive bias.

1.0 0.8

k

RAD~ANNEAL@”c)

E

0.6 0.4 0.2 0.0

=0.2 -0.4 0 .01

0.1

1.0

10

100

1000

10000

TIME (hours)

Figure 46. Threshold-voltage shifts for n-channel transistors with 32-rim gate oxides shown during a 200-krad(Si02) irradiation and subsequent 25°C anneal. (After Ref. [157])

In Fig. 47, threshold-voltage shifts for n-charnel transistors are plotted for steadystate static bias and AC-bias irradiations [159]. Note that the 50-kHz irradiation does not lead to values of AVt~ that lie between the O-V and 1O-V values, as one might initially expect. Instead, AVt~ is more positive for the 50-kHz irradiation than for either of the steady-state irradiations ! For the AC-bias irradiation the solid cmwe represents a model prediction based on the steady-state irradiations at O and 10 V [159]. This result is typical

of n-channelresponsefor technologieswith a lot of interface-trapbuildup [155,156,159]. The larger positive radiation-induced increase.

shifts under AC bias occur because hole annealing is enhanced charge neutralization [159,160], while interface traps continue

by to

Testing Implications: The most important test issue dealing with the applied bias or field is what constitutes “worst-case” bias during irradiation and ameal. As the figures above illustrate, this is pretty tricky business. Fortunately, an understanding of the basic physics underlying the MOS radiation response provides insight and explanation to It’s also clear that advanced technologies will need to be seemingly surprising results. tested at different biases to ensure that both gate- and field-oxide leakages are monitored

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1

0.5 0.0

c

1

-o‘.5 ——— —w. -1

CO-60

.0 t

19,6 krad (Sl~ )/ 48 nm oxide

mln I

.,~~

“ 0.0

0.2

0.4

0.6

0.8

1.0

1.2

DOSE [Mrad (S102fl Threshold-voltage shifts for n-channel transistors plotted for steadyFigure 47. For the AC-bias state static bias (O- and 1O-V) and AC-bias irradiations. irradiation, the solid curve represents a prediction based on the steady-state irradiations at O and 10 V. (After Ref. [159])

under worst-case conditions. As with many of the variables applied during irradiations needs to be specified.

already

discussed,

the bias

Also, irradiations need to be performed to avoid space-charge effects that may lead to sample debiasing. If sufficient positive charge is trapped in the oxide, internal fields associated with space charge can lead to regions of zero field or even field reversal! This is important since the oxide electric field affects the yield and transport of radiationgenerated holes, and the holes in turn play a key role in interface-trap generation. To minimize space charge effects, radiation-induced voltage shifts must be held to a small fraction of the applied voltage across the oxide. Clearly, at higher fields space-charge effects are reduced. Finally, Method 1019.4 prescribes that ICS be placed in conductive foam during transfer from the irradiation source to a remote tester and back again for further irradiation [139]. This procedure is intended to minimize annealing or other timedependent effects between irradiation and test. As seen in Fig. 39, the annealing reduced when O V, as opposed to 10 V, is applied to the gate.

rate

is

4.5 Temperature Many of the physical processes that govern the radiation response of MOS devices In general, temperature affects the time scale for many of these depend on temperature. processes, often speeding things up at elevated temperature and slowing them down at For example, several investigators have studied the effect of sample lower temperature. temperature on the time-dependent interface-trap buildup [25,27,28, 109,169- 172]. The

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temperature dependence is illustrated in Fig. 48, where an integrated interface-trap density is shown as a function of time following an 800-krad(Si) LINAC irradiation, The irradiation was performed on Al-gate wet-oxide capacitors with an applied field of 4 MV/cm and at temperatures of 273, 295, 323, and 373 K. The major effect of an increase in temperature appears to be a sharp reduction in the time scale for the buildup. For example, the time to reach saturation is = 2000s at 295 ~ but only = 2 s at 373 K It is interesting to note that the final or saturation value of interface traps does not vary with temperature by more than 20%, suggesting that temperature does not affect the number of traps created by the radiation. Also, the slopes of the Nit versus log(t) curves plotted in the figure do not vary and their shapes do not change significantly with temperature--the curves are simply translated along the log(t) axis by a change in temperature. An activation energy of 0.8 eV for interface-trap buildup was determined from the data. Nearly identical results were obsemed by Saks et al. [109] for Si-gate dryoxide transistors following pulsed LINAC irradiation. In that work, interface-trap densities were measured (using charge pumping) as a function of time for temperatures ranging from 283 to 355 K An activation energy of 0.83 eV, in close agreement with Ref. [28], was obtained from the data. Most studies demonstrate that the time-dependent buildup of interface traps is strongly temperature activated, and completely inhibited (on any re~onable time scale) at temperatures below 100 K 3 ‘ LINAC IRRADIATION WET OXIDE Eox= 4 MV/cm

1

0

1 10-1

101 102 100 TIME AFTER PULSE (S)

103

Figure

48. Interface- trap density for wet -oxide capacitors as a function of time following a 800-krad(SiOJ LINAC irradiation with 4 MV/cm applied across the oxide at temperatures of 273, 295, 323, and 373 K. (After Ref. [28])

Boesch [77] reports in thick field oxides that the generation rate of interface traps is the same at 77 and 295 ~ i.e., 4x1OTtraps/(cmz-eV) per rad. This result suggests that the buildup of prompt interface traps, which dominate in thick field oxide structures, is not temperature activated. This is consistent with the idea that prompt interface traps simply result from direct interaction of radiation at the Si02/Si interface, and therefore their generation is immediate and has no explicit field or temperature dependence.

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interface traps has generally not been observed at normal temperatures [9,25,28,109, 147,169,171], except for samples with very high densities of ( > 10IZ/cmZ) of interface traps [172]. Significant annealing appears to occur only for temperatures > 100”C [9,25,28, 147,169,172,173] and to be very sensitive to how the oxide was processed. Sabnis [25] and Fleetwood and Dressendorfer [173] both report an activation energy of 1.4 *0.3 eV for the removal of interface traps at elevated temperature.

Annealing operating

of radiation-induced

Several models proposed for the anneal of radiation-induced trapped holes are based on the thermal emission of holes from traps which are distributed in energy in the oxide [8-13,104] (see Section 2.1). The annealing process is often characterized by a wide

distribution of activation energies around 1-3 eV [120-122,127,175] (see Fig. 29B). In early work, Messenger et al. found that 90910of the radiation-induced AVth in MOSFETS was removed at 16 h at 150°C [176]. The technique of isochronal annealing (comtantduration annealing intervals at progressively higher temperatures) was employed by Zaininger, who found that major recovery effects took place between 100° and 300*C [177]. As discussed in Section 3.1.5, a technique that provides information on the energetic of thermal hole annealing is thermally stimulated current (TSC) [120-127]. The measured TSC provides a mapping of the density of trapped holes versus energy in the SiOz band gap [119]. Testing Implications: Because time scales for the annealing of trapped holes and the buildup of interface traps depend on temperature, the temperature during irradiation and anneal needs to be controlled. The new proposed MIL-STD-883D, Method 1019.4, suggests testing at an ambient temperature of 240Ci-60 C. Many fully charged research irradiators, e.g., Nordion’s Gammacell 220, that permit irradiations at dose rates of -300 rad(Si)/scan significantly raise the sample temperature tens of degrees during the These sources need a forced air supply to maintain course of a megarad irradiation. sample temperature with allowable limits. In general, it is a good practice to monitor sample temperature using a thermocouple to rule out any unwanted temperature increases during irradiations.

Avery important property of temperature is its ability to control the time scale of the Temperature is often used as an radiation response of semiconductor devices. “acceleration factor” to speed things up. An an example, the new proposed MII.AWD883D, Method 1019.4 [139] provides an accelerated aging test for estimating low dose rate ionizing radiation effects on devices. This test involves an irradiation to 1.5 times the specification level followed by a biased anneal at 100”C for one week [75,152,153]. (The 50 percent overexposure is required to compensate for uncertainties in defining true worst-case bias conditions, and to allow for p-channel annealing [75,153 ].) The purpose of the elevated temperature ameal is to accelerate the annealing of trapped holes and the buildup of interface traps, which both occur during low-dose-rate irradiations typical of The temperature must be chosen to accelerate the buildup of space environments. interface traps without the accompanying danger of significant interface trap annealing. To illustrate this test, in Fig. 49 the critical parameter Avit is plotted as a function of postirradiation amealing time for n-channel transistors irradiated to 300 krad(SiOJ with

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a CO-60 source. Devices are annealed either at room temperature (solid circles) or at 100”C (solid triangles). Note that 1) the value of AVit obtained from room temperature annealing measurements agrees exactly with the value of AVit obtained from a 3-week-long CS-137 exposure to the same total dose, and 2) that measurements of Avit obtained for elevated temperature annealing saturate very quickly at a level that is slightly above that achieved at the end of 4 months of room temperature postirradiation measurements. Clearly, the elevated temperature anneal provides a conservative estimate for the buildup of radiation-induced interface traps that dominate device response for many radiation-hardened technologies in low-dose-rate environments. ‘“” ~ 100DC Anneal

1 wk

0.8 -

1 yr II =-9-=

----

A

10yr

mo- .0 s

Vit ‘“6

(w

o.~ Anne a\ 0,2

–

a

102

+ 300 krad CO-60 (400 rad/s) I I

103

104

CS-137 (0.16 rad/s)

1

I

I

I

105

106

107

10*

10*

TIME (see)

AVit for n-channel transistors with 32-rim gate oxides versus Figure 49. postirradiation anneal time following CO-60 exposure to 300 krad(SiOJ for room Irradiation and anneal bias was 6 V. temperature and elevated temperature anneals. Shown for comparison is a low-dose-rate Cs- 137 exposure to 300 krad(SiOJ. (After Ref. [75]) Finally, product specifications often call for testing over the full military temperature range from -550C to + 125 ‘C. For timing measurements, worst-case is often at elevated temperature where nobilities are degraded. However, for ICS that use feedback resistors to mitigate single-event phenomen~ worst-case timing can be at low temperature, where feedback resistors fabricated in high resistivity polysilicon have their highest values [64].

4.6 Energy Many radiation sources involve irradiations with photons. Photons interact with electronic materials through the photoelectric effect, Compton scattering, and pair production [178]. In the photoelectric effec~ the incident photon is completely absorbed by an inner shell electron of the target atom, which is then emitted. For Compton scattering, the photon energy is considerably larger than the binding energy of atomic electrons. The incident photon gives up a portion of its energy to scatter an atomic II-67

electron, thereby creating an energetic Compton electro~ and the lower energy scattered photon continues to travel in the target material. Finally, in pair productio~ a photon striking a high-Z target maybe completely absorbed and cause a positron/electron pair to form; this process has a threshold energy of 1.02 MeV. Figure 50 illustrates the relative importance of these three types of photon interactions as a function of atomic number and photon energy [2]. A dashed horizontal line is drawn for silicon with an atomic number z = 14. ’20 ~ PHOTOELECTRIC EFFECT DOMINATES

&

m

3

COMPTON

o 0.01

0.1

PAIR PRODUCTION DOMINATES

SCAITERING

1.0

10

100

PHOTON ENERGY IN MeV Figure 50. Relative importance of three types of photon interactions as function of atomic number and photon energy. The dashed horizontal line is drawn for silicon with an atomic number Z = 14, (After Refs. [2, 178])

Historically, most irradiations were performed with COGOsources that emit 1.17- and 1.33-MeV gamma rays. Based on Fig. 50, the 1.25-MeV (on average) photons interact with the semiconductor device primarily through Compton scattering. The cross-section for Compton scattering is roughly proportional to the atomic number, i.e., u -Z. For 1.25-MeV photons, this dependence leads to less than a 596 variation in absorbed dose for the three layers of an MOS device consisting of Al or polysilico~ SiOz, and Si [179]. For COGOsources, dose is normally determined by irradiating CaFz thermoluminescent dosimeters and using the ratio of mass energy absorption coefficients p,./P to relate dose in the CaF2 to dose in the SiOz layer. In recent years, however, x-ray testers with typical energies of 10 keV have become popular. These testers allow for wider range of dose rate and permit irradiations on the wafer level. As seen in Fig. 50, 10-keV photons interact with semiconductor materials mainly through the photoelectric effect. The cross-section for the photoelectric effect varies as the fourth power of the atomic number, i.e., u - Z4, resulting in large differences in absorbed dose between different semiconductor materials. For x-ray testers, absorbed dose is normally determined using a Si-PIN diode. To calculate dose in the Si02 layer, a conversion factor of 1.8 is required to account for variation in the photoelectric cross section between Si and Si02, i.e., dose(Si) = 1.8[dose(SiOz)].

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Assuming dosimetry is performed correctly, there are a couple of additional radiation effects that depend on the energy of the incident radiation. These are recombination and dose enhancement effects. Let’s discuss electron-hole recombination effects first. When calculating the flatband- or threshold-voltage shifts for a given absorbed dose, the “yield” of radiation-generated holes must be known. The “yield” is a measure of the number of holes that escape the initial recombination process and are free to either transport or undergo trapping in the oxide. The “yield” depends on electric field and particle energy [3-5, 163-166]. In general, at a given electric field, lower energy particles have denser ionization tracks, more initial electron-hole recombination, and reduced hole yield. For COGO,the yield at 2-3 MV/cm is greater -9070, while for 10-keV x rays it is -70% at the same field. Recombination effects can be very important when irradiating devices fabricated in an oxide (as opposed to junction) isolation technology [46,137,180]. Typically, isolation technologies like silicon-on-oxide have thick (-0.1-1 pm) backgate and sidewall oxides. At nominal operating voltages, electric fields in these oxides are -104 V/cm and, consequently, there is a great deal of recombination, as well as potentially large differences in recombination between Cow and x-ray irradiations.

A second effect is dose enhancement. Dose enhancement occurs under photon irradiation at interfaces between materials with different atomic number. It arises from differences in the secondary electron production and transport properties of the two adjoining materials. Figure 51 illustrates the effect for an MOS structure [165]. Shown are schematics of the spatial profiles of the actual dose delivered (dashed lines) to an MOS device with (A) a thick (> 500 nrn) SiOz layer and (B) a thin (< 100 nm) SiOz layer that is typical of current technologies. For 10-keV x rays in SiOz, the average secondary electron range is -500 nm, so this distance is the criterion for thin or thick layers for purposes of dose enhancement effects. The solid straight curves in each figure represent the bulk equilibrium dose levels. Figure 51B shows that the dose in the thin SiOz layer is increased due to the flux of secondary electrons from adjoining layers with higher atomic number (Al or Si). Dose enhancement effects will become increasingly important as highZ materials, e.g., silicides, are more widely used in VLSI device fabrication. Fleetwood et al. [181] report dose enhancement factors of -2 for TaSi capacitors exposed to 10-keV x rays. However, it should not be assumed that high-Z materials will always lead to a degraded radiation response. For example, Smeltzer [182] and Kasama et al. [183] both report reduced interface-trap buildup in radiation damage studies of refractory gate metals. These “heavier” gates reduced mechanical stress at the SiOz/Si interface (leading to smaller radiation-induced interface-trap buildup) that more than offset any dose enhancement effects for these devices. There is a great deal of interest today in the use of x-ray testers. As mentioned above, these testers allow for a wider range of dose rate and permit irradiations on the wafer level. Issues of dosimetry, recombination, and dose enhancement need to be properly accounted for when using a 10-keV x-ray source as a hardness assurance tool [137, 146,161-166,181, 184]. Once these issues are addressed, i.e., the radiation-generated hole yield is known, it is generally believed that the physical mechanisms associated with oxide-trapped charge and interface-trap buildup are the same for different energy radiation sources. The only caveat is that the incident photons have Testing Implications:

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(A)

7 THICK

(B)

r S102 (>500

m)

x

THIN

S102

(<100

nm)

x

Figure 51. Schematic diagram illustrating dose enhancement effects. Shown are spatial profiles of (1) the actual dose delivered (dashed lines) to an MOS device with a (A) thick (>500 nm) Si02 layer and (B) thin (<100 nm) SiOz layer and (2) bulk equilibrium dose levels (solid straight lines). (After Ref. [165])

sufficient energy to create electron-hole pairs in the oxide, i.e., > the - 9 eV band gap energy [185- 187]. In fact, interface-trap generation has been observed using radiation sources whose energies range from 10.2 eV to greater than 200 MeV. Dose enhancement effects can also be important when making Co~ measurements. There are many different types of COGOfacilities, both air and water sources. Due to geometry or shielding, there is often a low-energy component of the radiation field that results from scatter inside the cell or from nearby walls [188,189]. This low-energy component needs to be filtered out since the CaF2 TLDs normally used to measure dose To minimize dose enhancement caused by aren’t sensitive to this low-energy radiation. low-energy, scattered radiation, the present MIL-STD-883D, Method 1019.4 requires that samples be placed inside a Pb/Al container [139]. A minimum of 1.5 mm Pb, surrounding an inner shield of at least 0.7 mm Al is required. This Pb/Al container produces an approximate charged particle equilibrium for Si and for TLDs such as CaF2. The radiation field intensity should be measured inside the Pb/Al container (1) initially, (2) when the source is changed, and (3) whenever the orientation or configuration of the source, container, or sample is changed. As a final cautio~ samples packaged with AuKovar lids are more sensitive to low-energy scattered radiation than are devices with ceramic lids [184, 188]. As might be expected, the Au-Kovar, with its high-Z content, produces a flood of secondary electrons that raise the dose in the underlying sample.

5.0 FUTtJRE TRENDS This short radiation-effects

course segment ends with a brief discussion of some future trends testing. These trends include (1) a heavy emphasis on characterization 11-70

in of

simple test structures and their correlation with integrated circuit response, which is a vital element of the Qualified Manufacturers List (QML) methodology [64,190,191] and (2) testing on the wafer level, which reduces costs associated with packaging and provides statistically more meaningful data.

5.1 QML Testing Requirements The Qualified Parts List or QPL is the existing methodology to qual@ microcircuits with radiation requirements. This methodology relies on extensive testing of product microcircuits to determine their radiation hardness levels, as well as detailed quality conformance inspections (QCI) of product to prevent delivery of product failing to meet Testing and documentation costs under QPL are expensive, and the specifications. qualification process is prohibitively long. Under the sponsorship of Rome Air Development Center and the Defense Electronics Supply Center, the U.S. Government has proposed a Qualified Manufacturers List (QML) methodology to qualify integrated The details of this methodology are circuits for high reliability and radiation hardness. defined in military specification MILI-38535 [190]. In this approach, a production line is certified on a “one-time” basis, and all product from that line is subsequently qualifled per the requirements of MIL-I-38535. The approach places a large initial burden on the manufacturer or production source, who is tasked with demonstrating that the quality of the part is “built i~” as opposed to being “tested in.” This “built-in” quality is assured by the proper control of the IC manufacturing sequence from design through assembly.

QML methodology relies heavily on the evaluation of test structures, whose response to various threats and stresses must be correlated with the response of ICS fabricated on the line. These test structures can be macrocells (e.g., lk SRAMS, random logic, buffers, etc.) on a Technology Characterization Vehicle (TCV) or capacitors from an in-line Process Monitor (PM). It is important that the test structures be designed to account for the radiation threat (total-dose, transien~ etc.), failure mode, and technology. Through the use of test structures, the manufacturer can reduce the cost of microcircuits to system users by eliminating costly QCI and screening tests. However, establishing the relationship between test structure and IC response represents a formidable technical challenge.

An exampleof a test structureto IC correlation is shown in Fig. 52. In Fig. 52, the change in the “read” time, Atm, of radiation-hardened SA3001 2k SRAMS fabricated in Sandia’s 4/3-pm technology is plotted versus AVit shifts for n-charnel test transistors on the same die. “Read” time, measured on a Fairchild Sentry tester, is the time between a “read” request signal to the memory and valid data out. The initial “read’ time of the memory was 61 ns. The memories and transistors were irradiated with 1O-V bias in a CS19Tcell at a dose rate of 0.2 rad(Si)/s. The data in Fig. 52 show a strong correlation between AVit shifts and increases in circuit timing [40]. These correlations helped form the technical basis for “rebound” tests that are presently recommended for space qualification of ICs [75,150,152,153] and suggest that AVit is an excellent monitor of totaldose radiation hardness in space applications for these devices. Also shown in the figure II-71

2k SRAM 20

~

0.20 rad(Sl)/s

,5

w n K G ‘0 5

0 0.0

0.2

0,4

0.6 O.e 1.0 AVlt (Volts)

1.2

1.4

e

Correlation between AVit measured on n-channel transistors and At~~ on 2k SRAMS irradiated with 1O-V bias at 0.2 rad(Si)/s. Total dose levels corresponding to each data point are given in parentheses. (After Ref. [64]) Figure 52.

/

0.8

m

Ssndla 4 / 3-Win Technology 500 krad(S102)

0.7

I

L

0.6

UCL

RidXbar

d-

~-

0.3

LCL 0.2

■ Non SPC

0.1

0.0

o

I

I

I

I

I

I

20

40

60

60

100

120

Consecutive

1 140

I

I

I

160

180

200

I

I

220

240

260

Lots Processed

Figure 53.

Control chart showing lot-to-lot variation of AVit over a 40 month period for more than 256 lots fabricated in Sandia’s 4/3-pm technology. Average values (Xbar) and upper and lower control limits (UCL & LCL) are shown. SPC violations are indicated by solid squares, (After Ref. [64])

are the total doses corresponding to each data point. For these samples, AVit is seen to be approximately linear with dose. Clearly AVit is a technology parameter that needs to be controlled to maintain circuit CMOS technologies in timing within acceptable limits. For many radiation-hardened dose rates <1 mrad(Si)/s), shifts in threshold voltage are space applications (i.e., dominated by AVit and accompanied by a degradation in transistor mobility, II-72

transconductance, and circuit timing. A control chart of AVik for 256 lots fabricated in Sandia’s 4/3-pm technology is given in Fig. 53. Each data point is the lot average of 1 transistor taken from 8 wafers. In order to calculate capability indices [64,192] for the dat~ it is necessary to set acceptance limits consistent with system requirements. For a 500-krad(Si) space application requiring 5-V operatioq a reasonable specification limit might be AVit < 1.2 V, which is equivalent to a change in “read” time of =30% or 17 ns. If it is assumed that the target value for this process was 0.4 V, then capability indices would be ~ = ~k = 2. These capability indices suggest that Sandia’s 4/3-pm technology is excellent in meeting the 500-krad(Si) space application. However, space applications requiring a l-Mrad(Si) capability level with these specification limits would not always be met by this process/technolo~ flow. Although SPC was not fully attempted during the manufacture of ICS in Sandia’s 4/3-pm technology, it is illustrative to apply Western Electric SPC trend rules [64,193] to the control chart in Fig. 53 and flag SPC violations, indicated by solid squares. These violations represent a non-random source of variation whose root cause needs to be identified and eliminated. Based on the data in Figs. 52 and 53, control of AVit may eventually be used to ensure the radiation hardness of the final product without the need for end-line radiation testing.

5.2 Wafer-Ixvel Testing Wafer-level radiation test systems are capable of baselining, controlling, and monitoring technology processes and parameters relevant to the radiation hardness. These systems are becoming increasingly popular and provide important feedback for process control and improvement and permit detailed studies of intrawafer, wafer-towafer, and lot-to-lot uniformity required to accurately measure process “capability.” Figure 54 is a wafer map of threshold-voltage shifts on 50- x 1.2-pm n-channel test tramistors following 3-Mrad(Si) irradiation [112]. The transistors were from Honeywell

Lot #PCMD Wafer #242-5 mean

=

-0.754v

sigma = 0.042 V

,,. .:,:,:,:,:,:,::: ..... ❑ Untested .,.....:..,,..... ,:::,.:,,::,,:,::, . . ,:, P’E::V”I-M”u’”””” // u 3U

Figure 54. Wafer map of threshold-voltage shifts on 50- x 1,2-~m n-channel test transistors following 3- Mrad(Si02) 10-keV x-ray irradiation. (After Ref. [112]) II-73

Yield and Circuit Reliability Analysis Test (YCRAT) wafers. YCRAT wafers are fabricated in Honeywell’s RICMOS 3 technology and have all the necessary test structures for parameterizing active and passive circuit elements on the same die with a delay chain and a 2k by 8 radiation-hardened SRAM. The mean of the AVth distribution shown in Fig. 54 is -0.75 V and the sigma is 0.04 V. In the figure, AVt~values are plotted using a gray scale that separates data into 6 bins, i.e., <-3U, -3U to -lU, -la to p, p to + la, + lU to +3U, and > + 3u. There is only one value of AVk~that fell outside the * 3U limits. There were also a couple of die (shown as bkznks) that the test system had difficulty measuring. The wafer-level test system used to take the data in Fig. 54 consists of a 10-keV ARACOR x-ray irradiator interfaced to an hp4062 for test structure measurements and to an hp82000 for parametric and functional IC measurements. The system permits 4-terminal DC measurements and supports rnidgap, dual-transistor, and charge pumping techniques to separate AVth into its contributions due to oxide-trap charge, AVOt,and interface-trap charge, AVit [112].

6.0 SUMMARY The goal of this short course segment was to provide insight into the improved design of experiments based on a first principles understanding of total-ionizing-dose (TID) radiation effects. A great deal of radiation effects data was presented and interpreted in light of the physical mechanisms that govern device response to TID. An understanding of the basic mechanisms helped to identify the important device parameters that needed to be measured. Many tools and techniques for characterizing the TID radiation response of discrete devices and integrated circuits were reviewed and, once agai~ a great deal of emphasis was placed on the underlying physics. In the last decade, the radiation effects community has embraced “charge separation” Charge separation techniques divide the net techniques for radiation characterization. radiation-induced threshold-voltage shift into its components due to oxide-trapped charge, AVOt,and interface traps, AVit, i.e., Avth = Avot + AVit. As discussed in Section 2.1, oxide-trapped and interface-trap charge are the two primary radiation-induced defects that govern the total-dose radiation response of MOS devices. Each of these defects needs to be measured, controlled, and reduced if integrated circuits are to survive in The most widely used charge separation technique is differing radiation environments. the “midgap” method, although the dual-transistor method provides clear advantages when device leakage is significant or high-speed measurement is required.

Interfacetraps often play an importantrole in the survivalof radiation-hardenedCMOS technologiesin space applications (i.e., dose rates < 1 mrad(Si)/s). Shifts in threshold voltage can be dominated by AVit and accompanied by a degradation in transistor mobility, transconductance, and circuit timing. Many techniques are available to study the buildup and/or annealing of radiation-induced interface traps. Historically, interface traps were measured on capacitors, the simplest MOS structure. In recent years, several transistor techniques have found wide application, the most prominent being the charge Table I summarized capacitor and transistor techniques for pumping technique. II-74

measuring interface traps and their properties. Foreach technique, the table includes(l) its sensitivity, (2) the region of the band gap probed by the technique, (3) its application in either research, production, and/or hardness assurance, and (4) whether its analysis is complicated by the presence of lateral nonuniforrnities (LNUs). For situations in which interface traps do not significantly affect device characteristics, parallel shifts in C-V or I-V curves are excellent measures of oxide-trapped charge, ANOt. Parallel shifts are often observed for commercial devices at low total doses at moderate to high dose rates, and are occasionally observed even for radiation-hardened devices at doses of megarads or greater. However, when interface traps become signitlcant, chargeseparation techniques must be used to extract information about ANOt. Thermallystimulated-current (TSC) measurement is the only technique that provides a direct measure of the total number of trapped holes in the oxide. TSC can also be used to infer the density and energy distribution of radiation-induced trapped holes in MOS devices. IC tests generally consist of functional or parametric tests performed before and after cumulative irradiation steps. In either case, functional or parametric testing, failure criteria or specifications must be clearly defined and often depend on system applications. Standard parametric measurements include power supply current maximum functional frequency, access and delay times, and input/output voltages. It is often not possible to test 100% of the active elements or paths in a comple~ state-of-the-art IC. Designing VLSI circuits for testability is the most efficient way to reduce the relative costs of assuring high chip reliability in radiation environments. Many test standards exist for the electrical characterization and radiation testing of semiconductor devices. The American Society for Testing and Materials (ASTM) has an F-1. 11 Electronics Subcommittee on Hardness and Quality Assurance that develops, revises, and ratifies test standards for use by the electronics industry. These standards are readily available and published annually. A listing of test standards relevant to total-dose testing that are presently being considered by the F-1.11 Subcommittee are given in Table III. The test standard that governs total-dose testing of microelectronics in the DoD is MIL-STD-883D, Method 1019.4, Ionizing Radiation (Total Dose) Test Procedure [139]. This test procedure provides specific guidance on how testing is to be performed. For example, it specifies bias and temperature conditions, allowable dose rates, time between irradiation and test, etc. The latest revision of this procedure provides an accelerated aging test for estimating ionizing radiation effects on devices in low-dose-rate environments, e.g., space.

The short course segment also focused on “critical”measurement variables or test conditions. These variablesneed to be specifiedto properly characterize the radiation response of a device or to interpret the results of a test. included dose, time, dose rate, applied bias, temperature, radiation. Using data in the literature, it was shown how each Many seemingly complex the measured radiation response. readily explained in light of the basic physics underlying the implications for each of these variables was summarized to

II-75

Critical variables discussed and energy of incident of these variables can affect and surprising results were radiation response. Testing support the acquisition of

accurate, controlled, and appropriately documented test results. This short course segment ended with a brief discussion of some future trends in radiation-effects testing. These trends include (1) a heavy emphasis on characterization of simple test structures and their correlation with integrated circuit response, which is a vital element of the Qualified Manufacturers List (QML) methodology and (2) testing on the wafer level, which reduces costs associated with packaging and provides more statistically meaningful

data. It is hoped

that the student came away with the belief that an understanding of TID radiation effects is essential to the correct design of experiments and is required for the development of test procedures used by the radiation-effects community. Clearly, a first principles understanding of TID radiation effects provides a strong underpinning for performing radiation effects testing.

ACKNOWLEDGMENTS

The author would like to thank Dan Fleetwood and Paul Dressendorfer of Sandia National Laboratories, and Ron Pease of the Mission Research Corporatio~ for their careful reading of this manuscript. Each of these reviewers provided valuable suggestions regarding the content and organization of the material in this short course segment. A special thank you to Daq whose up-to-date understanding (and recall) of the radiationeffects literature helped fill in some gaps that I might otherwise have overlooked. I owe Marty Shaneyfelt of Sandia National Laboratories a debt of gratitude for the use of his unpublished data. Finally, a heartfelt thank you to Fred Sexton, the Short Course Organizer, for his patience, suppor~ and helpful advice during this fulfilling endeavor.

REFERENCES [1]

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[2]

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Measurement Of Single Event Phenomena In Devices And ICS

Fred W. Sexton

Sandia National Laboratories Albuquerque, NM 87185

This work was supported by the U.S. Department of Energy through contract number DE-A C04-76DPO0 789

MEASUREMENT OF SINGLE EVENT PHENOMENA IN DEVICES AND ICS F. W. Sexton Sandia National Laboratories Radiation Technology and Assurance Department

1,0 Introduction 1.1 Motivation 1.2 Brief History 1,3 Session Contents 2.0 Description of Heavy-ion Interaction with Matter 2.1 Charge Collection 2.2 Definition of Terms 3.0 Laboratory Measurements 3.1 Units of Measure 3.2 Experiment Setup 3.3 Circuit Test Techniques 3.4 Circuit Effects 3.5 Accelerator Facilities Available for SEU Testing 3.6 Ion Microbeam Techniques 3.7 Alternative Laboratory Techniques 4.0 Error Rate Predictions 4.1 The Natural Space Environment 4.2 Basics of Error Rate Algorithms 4.3 SER for Proton Environrnents 4.4 Errors in Predicting SER 5.0 summary

1.0 INTRODUCTION

The primary goal of single-event-phenomena (SEP) experiments is to improve the understanding of the effect of high-energy ionized particles on semiconductor devices and integrated circuits (ICS) in space environments. Some experiments are designed to provide insight into the underlying physics of the interaction of high-energy particles with semiconductor material and the subsequent charge transport and collection. Others are designed to improve the understanding of device and/or “circuit failure mechanisms induced by high-energy particles. Still other experiments are designed to qualify piece-parts for a particular application. The study of SEP encompasses many diverse fields from high-energy physics to IC design and system architecture. Despite the breadth of the SEP field, a worbng knowledge can be obtained by mastery of a few concepts from these diverse areas. These concepts are key to understanding the experimental aspects of single event phenomena.

III-1

1.1 Motivation A high-energy particle strike on an integrated circuit may result in a “soft” error in which only the logic state of the circuit is changed and no permanent damage to the circuit results. This is usually referred to as a single-event-upset (SEU), Soft errors are correctable by simply reloading the correct information into memory or by restarting an algorithm in a central processing unit. However, the system overhead required to detect and correct errors may be prohibitive. In fact, if errors occur too frequently, the enor handling architecture of a spacecraft may be overwhelmed, resulting in compromised performance or even loss of mission.

In certain circuits “hard” errors are observed, where permanent damage is induced by a high-energy particle strike. Lnnonvolatile memories, for example, energy deposition in a small region of the memory dielectric can result in catastrophic failure of the dielectric. This effect is called single-event gate rupture (SEGR). Cosmic rays may also trigger high-current conditions which can result in circuit failure. Latchup in CMOS ICS (SEL), snap-back in NMOS devices, and burnout in power transistors (SEB) are examples of this effect. 1.2 Brief History of SEU SEU was f~st postulated by Wallmark and Marcus in 1962 [1], but was not observed until 1975, when Binder and Smith [2] attributed bit-flips in bipolar J-K flip-flops in a communication satellite to galactic cosmic rays, In 1978, May and Woods [3] observed upsets in dynamic random-access memories (DRAMs) caused by alpha particles from radioactive decay of material in ceramic packages, That same year, Pickel and Blandford [4] reported soft-enors in NMOS DR4Ms in space. In 1979, Guenzer and coworkers [5] reported the first observation of proton and neutron induced upset. Since that time, upsets have been observed in many satellite systems. With the increased solar activity observed since late 1989, SEP in satellite systems has been particularly intense [6]. Although the study of SEP has expanded considerably, many aspects of the field are still not well understood. The importance of SEP is increasing as device geometries shrink to ever-smaller feature sizes [7]. As devices get smaller, less charge is stored on circuit nodes, and the energy required to change the state of a node decreases. In the early ‘70s, about 50 pJ was required to switch an information node. As device geometries decreased over the next five years, the switching energy decreased an order of magnitude to 5 pJ, equivalent to only 107 electron-hole pairs. At VHSIC integration levels (roughly 5 x 1011gate-hertz), the energy representing a bit decreases to less than 0.5 pJ, or about 106 charge carriers. This implies that circuit vulnerability to upset from highly-ionizing particles will also increase as feature size decreases. High-energy particles arise from a variety of sources. The packaging material in which ICS are assembled, for example, emit radioactive byproducts from the decay of naturally occurring radioisotopes. ICS used in or near a nuclear reactor will, of course, be exposed to a wide variety of fission fragments as well as high energy neutrons. For ICS in satellites, the natural space environment is the primary source of high-energy ionizing particles.

III-2

1.3 Session Contents In this session, we will strive to give the reader a thorough introduction into the experimental We begin with a discussion of the aspects of the measurement of single-event-phenomena. interaction of heavy ions with matter and definition of terms used in the SEP field. Then we define the units of measure and outline the general test protocol for SEP measurements. We then describe in some detail the various effects that are observed in semiconductor devices and how these effects are measured. We will also describe the accelerator facilities used for SEP measurements and more recent developments in low-cost laboratory measurement techniques. Finally, we will discuss the natural space environment and how data from SEP experiments are used to predict error rates in space.

2.0 DESCRIPTION 2.1 Charge

Collection

OF HEAVY-1ON

in Semiconductor

INTERACTION

WITH MATTER

Devices

When a high-energy ion passes through a semiconductor material it loses energy by excitation and ionization of atoms along its path within the material [8]. Some of the deposited energy goes into the creation of a very dense plasma of electron-hole pairs along the path of the If the electron-hole plasma is generated in a region with an particle through the material. electric field – at a pn junction, for example – electrons and holes are separated and a current spike is observed at the circuit node as free carriers are collected as shown in Figure 1 [9]. This current spike has two components: a prompt component which occurs on the order of hundreds of picosecond after the ion strike, and a delayed component which may last hundreds of nanoseconds. The prompt current is due to charge collection in the original depletion region and the funnel region [10- 13]. The delayed component of current is due to carrier diffusion up to the depletion or fimnel edge where it is quickly collected by the junction electric field. The funnel region is caused by the extension of the junction electric field into the substrate, as shown in Figure 1. The dense electron-hole plasma is highly conductive and causes the electric field lines of the original depletion region to distort into a cylinder which follows the path of the ion strike, Charge collection is ‘enhanced in the early time frame following a

+–_ i– + /’7”

+~~

QF)

‘,)s —prompt

I CR

,

o

Diffusion

0.2

(QD+

c“)

0.4

1 Time

10

100

(ns)

Figure 1: Ion irack and cunent pulse induced by paasage of a heavy ion through a depletion region of an integmted circuit. After J. C. Pickel, [9].

III-3

heavy-ion strike by the presence of the funnel region, The funnel collapses as the free carrier concentration drops and equilibrium is re-established. 2.2 Definition of Terms As a particle passes through a material it loses energy by collisions with the electrons (electronic stopping) and nuclei (nuclear stopping) of the target material. The rate of energy loss, dE/dx, from both mechanisms is called the stopping power and is expressed in units of erg/cm, Electronic stopping dominates except for low energies near the end of range for the particle. For SEP, we normally consider the mass stopping power, l/p dE/dx, where p is the density of the target material. This is called the linear energy tiansfer (LET) and is given in units of MeV/mg/cm2 (or MeV-cm2/mg) for a given material. Figure 2 is a plot of stopping power for 2.5-MeV helium ions as a function of depth in silicon. The point of maximum stopping power is called the Bragg peak. The stopping power decreases on either side of this peak.

1.6 1.4 1.2 1.0

0.8 0.6 0.4 0.2 0.0 0

10

5 Depth

15

(&m)

Figure 2: The mass stopping power of 2.5-MeV He ions as a function of depth in silicon. Maximum stopping occurs at the Bragg peak, and decreases on either side,

The magnitude of stopping power at the Bragg peak increases and shifts to higher energies with increasing ion mass as shown in Figure 3, where we plot the stopping power of several ions in silicon for energies up to 104 MeV [14]. Maximum stopping power occurs at the Bragg peak, and decreases on either side. Therefore, the LET for a given ion is multi-valued, i.e. an ion can have the same LET at two different energies (see Figure 3). Stopping power is also dependent Experimental and theoretical on the target material. several materials have been published by Northcliffe powers

can be calculated

for silicon,

germanium,

values

of stopping

power

for most ions in

et al. [15] and Ziegler et al. [16]. Stopping

gallium

arsenide,

and many compounds

using

the TRIM code [17] on most IBM compatible personal computers. Since the primary consideration for upset in an IC is the charge deposited in the material, a second measure of stopping power is the linear charge deposition, or LCD. Energy can be converted to charge by considering that 3.6 eV are required to generate an electron-hole pair in silicon [18]. The conversion factor from LET to LCD is approximately 10-2for silicon [19]. An LET of 40 MeV-cm2/mg is therefore equivalent to about 0.4 pC/~m. For GaAs, 4.8 eV are

m-4

I

k*

c? m

c ,— n

10-1

.-

, ~o

,

~1

I&

, ~3

Energy

, ~4

, ~5

, of

(MeV)

F~ure 3: The mass stopping power for several ions as a fonction of energy. Maximum stopping power increases with ion mass, and decreases for a given ion above and Wow the Bragg peak. After R.N. Harem, et al. [14].

required to generate an electron-hole pair [18]. With a density of 5.32 g/cm3, the conversion factor from LET to LCD is about 0.018. Cosmic ray flux is isotropic and may impinge on a spacecraft from any direction. Flux is defined as the number of particles per saond impinging on a unit sphere horn all directions, and has units of #/mZ-s. (A unit sphere is used in this definition since the projection of a sphere onto a plane is independent of direction.) This deftition is illustrated in the left-hand sketch of Figure 4. Often, it is important to consider the directionality of flux, where now we define flux as the number of particles passing through the center of a sphere horn a unit solid angle (right-hand sketch of Figure 4). This measure has units of #/m2-steradian-s, and is by definition 4X smaller than the isotropic flux. Isdropk Flux

Dlreotlonal

Flux

\ 0. \

\

On Sphere FmrrI ~

Al&

Dkedons

on Surfaoa far Dlrectloflsltly

Figure 4: Isouopic and directional definitions of cosmic ray flux. After Pickel [4].

III-5

3.0 LABORATORY

MEASUREMENTS

The effect of the natural space environment on semiconductor devices is experimentally simulated using techniques that generate a dense plasma of free carriers in the target semiconductor. Accelerators, alpha-emitting radioactive isotopes, such as ‘lAm and 20gPl, and naturally fissioning radioisotopes, such as 252Cf, provide an easily controlled source of high-energy particles for SEP tests. ASTM standard F1 192-88 describes proper test methodology for SEP testing with these sources [20]. A new JEDEC standard is also being drafted which addresses SEP testing using heavy-ion accelerators [21]. A MIL-STD test method

is being developed for single-event induced gate rupture of dielectrics in power FETS [22]. Near-IR lasers can also be used to produce free carriers through band-to-band excitations. This section describes how single-event-phenomena are measured in semiconductor devices and ICS. We begin with a discussion of units of measure. We follow this with a discussion of the many types of effects observed in devices, and then discuss test techniques to measure SEP. This is followed by a discussion of beam diagnostics, and accelerator facilities available for testing. Finally, we discuss recent developments in low-cost laboratory techniques that don’t rely on accelerators. 3.1 Units of Measure The sensitivity of an IC to single event upset is expressed as the ratio of number of upsets to the total particle fluence, that is, # upsets/pa2ticles/cm2. Because this term has units of cmz, it is referred to as the error cross-section. Experimental cross-section data is normally plotted as a function of LET, for particles striking the IC normal to the surface. Two keys parameters are determined from cross section curves: the upset threshold, Lti, and the saturation cross-section, o ,,,. The upset threshold is a measure of the minimum charge required to upset the most sensitive region of the chip, while the saturation cross-section approximates the total area of the sensitive regions of the chip, These parameters are used to estimate the error rate in a spec~lc environment as discussed in section 4 of this text. In Figure 5 we show an ideal and a measured cross section curve for a hypothetical device. The ideal cross section curve (dashed line) is a step function with a well-defined threshold and saturation cross section. Below threshold no cells upset, while above threshold all cells upset. However, the upset threshold varies across a device, resulting in a non-ideal cross section as a function of LET. This is shown as the solid line in Figure 5, Measured cross section curves have a threshold set by the most sensitive cell on the chip. As LET increases, more regions of the chip upset, The measured cross section increases until all sensitive regions upset and the cross section curve approaches saturation, The upset threshold for the IC in Figure 5 is about 38 MeV-cm2/mg, while saturation cross section is about 3 x 106 cmz, As a check of the validity of the experirnen~ the saturation cross section should be approximately equal to the total sensitive area of the chip, e.g. for a CMOS K the sum of the sensitive n- and p-channel drain regions. Cross section curves are sometimes plotted as a function of linear charge deposited (LCD) with x-axis units of pC/Lm. The reader is reminded that the conversion between LET and LCD is -102 for silicon.

III-6

10+ 10-5

10-’

10-7

10+

100

25

~

50

L

75

100

125

la

LET (MeV–cm2/mg)

Figure 5: An ideal and typical measured cross-section curve showing threshold LET, Lti, and saturation cross-section, IS= The ideal cross-section curve is a step function with all circuit elements having exactly equal thresholds, while mwmred cross section curves have a gradual threshold and asymptotically approach saturation.

0v9rlayer yJst#

Figure 6: The effect of varying angle of tildence on pathlength and particle flux through a sensitive volume. LET is corrected by l/cos (3to account for incraw.d pathlength as t3 increases, To define a full cross section curve the particle LET is varied. This can be accomplished by varying the ion species or the ion energy to obtain the desired stopping power (see Figure 3). An easier way of varying the deposited charge, however, is to vary the path length of the ion through the sensitive volume by varying the angle of incidence of the ion beam relative to the device under test as shown in Figure 6, As the path length through the sensitive volume is increased, the deposited charge increases. For many, but not all, types of SEP, this is equivalent to increasing the LET. The pathlength 1’is given by 1/=— 1 Cos e

(1)

where 1 is the normal incidence length of the ion track through the sensitive volume and (3is the angle of incidence relative to the surface normal. As the pathlength increases, the amount of II I-7

charge deposited in the sensitive volume increases, which is equivalent to increasing the LET. The ej&ective LET is therefore simply given by the normal incidence LET divided by cos 9, or

‘ETeff

LETO =~

(2)

As the angle of incidence is varied the effective flux changes also. This can be seen by observing that the projection of the normal surface area, 00, into the plane perpendicular to the axis of the ion beam, u’, decreases as cos 9. Therefore, the measured fluence is also corrected by cos 9, Corrected cross section is then equal to

‘=

# Errors Fxcose

(3)

where F is the fluence measured in the plane normal to the ion beam. The amount by which angle may be varied is limited by the type of package in which the chip is mounted, since the chip may be shadowed by the edge of the package. Also, as angle is increased, the time to accumulate a certain fluence will increase since the flux through the sensitive volume decreases as cos 9. The practical upper limit to angle is about sixty degrees. The range of LET accessible with a given ion is therefore restricted to about two times its value at normal incidence. To vary the LET over a larger range, a different ion species is selected In this manner, by selecting an ion and varying the angle of incidence, a cross section curve is obtained for a given device. The sensitive volume of a device does not normally begin at the semiconductor surface. There is often a dead layer where almost no charge is collected because of high recombination rates. In addition some energy is lost in overlying insulating films and conducting layers which are used to interconnect circuits. A correction must be made to LET to account for the effects of energy loss in these overlayers before the ion reaches the sensitive volume [20]. The energy of the ion at the surface of the sensitive volume, E,, is computed from

(4) where EOis the initial ion energy at the surface of the DUT, t is the overlayer thickness, 9 is the angle of incidence as defined in Figure 6, and dE/dx is the energy loss in the overlayer materials as a function of distance. Stopping power at the surface of the sensitive volume is then calculated using energy E, and stopping power curves (see Figure 3). Except for very sensitive devices, the primary proton or neutron does not generate sufficient They can, however, dislodge an atom from a lattice site in the charge to cause upset. semiconductor device, or cause a nuclear interaction with a lattice-site atom. The resulting secondary particles, due to their higher stopping power, can cause sufficient ionization to upset a device. Details of the energy deposition mechanisms are discussed in more detail in section 4.3.

For protons or neutrons incident on a semiconductor device, we measure an error cross section in a manner similar to that for heavy ions. The error cross section is the number of upsets per incident fluence. An error cross section curve for protons and neutrons has a similar shape to that for heavy ions, with the exception that the x-axis in this cross section curve is typically given in terms of incoming particle energy. Such a curve is shown in Figure 7 for several

III-8

10-’1 ~ o

25

50

75

100

125

150

Energy (MeV)

F~ure 7: Soft error cross section for several DRAMs (open symbols) and an SRAM (solid symbol) as a function of proton energy. After McNuhy et al., [23].

DRAMs and an SRAM [23], These curves show an increasing error rate with increasing proton

energy. As beam energy increases, more charge is deposited in the sensitive volume, Curves such as this are typical of the response observed in high-energy proton or neutron beams, 3.2 Experiment

Set-Up

The experimental apparatus for SEP experiments can typically be divided into three sections: a high-energy heavy-ion source, a beam diagnostic section, and a test chamber. A schematic SEP test apparatus is shown in Figure 8. For most SEP experiments the ion source consists of an accelerator that boosts the ion energy to the MeV/nucleon range. The accelerator facilities that are used most frequently for SEP tests are discussed in more detail section 3.5. The beam diagnostic section consists of scintillators to measure beam flux and uniforrni~, and various solid state detectors to measure beam energy and LET. The test chamber provides a means of positioning the device-under-test (DUT) in front of the ion beam and accurately changing the Accelerator

Beam Dlagn-cs

Test Cttam@r

Gate Vatvea

Figure 8: Schematicof the SEP test apparatus used at Brookhaven Nationrd Laboratories twin tandem van de Graaff accelemtor. The three main sections of the apparatus are 1) the beam source, 2) rhe beam diagnostics, and 3) the device test chandw. After Zajic, et al. [24].

III-9

angle of the device relative to the beam, It also provides a means of bringing electrical signals to the device from a tester located outside the test chamber. At MeV/nucleon energies, most ion species are attenuated by packaging material and air. Therefore, parts to be tested are delidded, and the test chamber is evacuated to a vacuum of about 10-6torr, In most experiments the beam is defocused so that the entire chip is exposed with a uniform particle flux. In some specialized tests, however, the ion beam is collimated with an aperture or focussed with magnetic lenses to form a micron-sized beam. Microbeam experiments are discussed in more detail in section 3.6.

and van de Graaff accelerators are the most commonly used sources of high energy particles for measuring SEP. The basic design of a cyclotron consists of a cylindrical vacuum tank with two D-shaped electrodes situated in a perpendicular fixed magnetic field [8]. An ionized gas is injected into the center of the cyclotron and an rf field applied between the electrodes causes the ions to oscillate between the electrodes. The perpendicular magnetic field bends the ions into a circular path, whose radius increases with each oscillation. Because the ions resonate with the rf field, they complete one revolution with each rf cycle. Thus, the velocity and energy of the ions increase as they spiral out of the center of the cyclotron. Beam energy is set by the ion mass, charge state, rf frequency, magnetic field, and radius where the beam is extracted. Only ions with a given mass charge ratio satisfy the resonance condition set by the rf frequency and magnetic field. Cyclotrons

A van de Graaff accelerator works on the same principle as early electrostatic generators, where a high potential difference is established by an electrostatic belt generator [8]. A beam of ionized particles introduced at one end is accelerated through the potential difference. Beam energy is determined by the charge state of the ion and the accelerating potential, which can be varied over the full range of maximum potential. Since impurities in the gas are also accelerated, beam purity is assured by using mass analyzing magnets and in some facilities, velocity selectors, which remove all but the ion species of interest, For both types of accelerators, the ion beam is essentially monoenergetic, and the ion LET is completely determined. The ion species is selected based on the expected response of the device to be tested. For non-destructive effects like SEU, the experirnentalist may choose to fust test for threshold using an ion species with LET less than the expected threshold at normal incidence, but which can induce upset at higher angles of incidence. Alternatively, one may begin at high LET so that the saturation cross section can be determined and then use a series of lower LET ions to map out the entire cross section curve, The approach used depends to some extent on the ease of changing ion species at the facility, It normally takes several hours to change ion species as the accelerator is retuned. With the Berkeley 88-inch cyclotron, one can change ion species rapidly when using the “cocktail,” a mixture of several different ions with nearly equal mass charge ratio that are accelerated simultaneously [25]. For destructive effects, like dielectric gate rupture (SEGR), one must begin with an ion species whose stopping power is below the expected threshold. Particle LET and depth of penetration are the first-order concerns in selecting ion energy. The ion must have the desired LET for the particular experiment and sufficient energy to penetrate through the sensitive volume at the highest angle used during testing. Uncertainties in LET result if the beam becomes contaminated with other ion species, from knock-ons for

III-10

example, or accidental selection of a second ion species with the same charge to mass ratio. Another uncertainty in LET arises as the ion beam loses energy to the silicon and its LET changes, It is therefore important to consider the effects of changing LET as the particle passes through the sensitive volume. To ensure that stopping power is relatively constant as a particle passes through a sensitive region of the IC, a beam energy near the Bragg peak is normally chosen. The depth of penetration should also be much larger than the pathlength through the sensitive volume to ensure that the maximum charge collection occurs. The beam diagnostic section is key to accurate SEP experiments, since it ensures that the LET and fluence of the ion beam is well known. Scinti.llators and their associated photomultiplier tubes (PMT) measure the particle flux and accumulated fluence during an exposure. Flux is measured in the plane perpendicular to the beam axis, and fluence is simply the integrated flux during the exposure, Flux is set low enough so that the flux can be accurately measured and the emors in the DUT accurately counted, and high enough to complete an exposure in a reasonable time, typically less than five minutes. If flux is too high beam pile-up can occur. If two particles strike a scintillator in a given time, for example, the light pulse resulting from the second particle may be truncated due to saturation effects. This results in an incorrect count of fluence and an overestimate of error cross section. Similarly, the PMT electronics have a finite response time. Flux is also limited by the time the device tester requires to detect an event and reset the device. If flux is too high, a bit may flip and then re-flip before an error is detected. A rule of thumb is that the error rate during measurement should be kept to Flux is normally set from 102 to 105 /cmZ-s, while fluence is about 1 to 4 errors/second. normally 107particles or 100 events, whichever occurs first. The energy of the ion beam is measured using solid-state detectors, usually a surface barrier detector (SBD). Briefly, surface barrier detectors and their associated electronics measure the

amount of charge deposited by incoming particles in a known volume. The energy scale for the SBD is calibrated using a known monoenergetic ion, typically the alpha emission from a radioisotope. The energy speclrum for a 208-MeV chlorine beam is shown in Figure 9, where the number of particles is shown on a log scale. Note that there is a well-defined peak in energy for the incident beam.

o

If the measured particle energy differs significantly

m

lm.3

lm.1 m

21a9

=7

316,6

Wd

from that expected,

-

(Mew

F~ure 9: The measured energy spectrum for 3SC1at an energy of 212 MeV. The spectrum shows a single peak at211.7 MeV and a low-energy tail that is three orders of magnitude lower in flux.

III-11

or if more than one peak is observed, the experimentalist should work with the accelerator operators to determine the problem and retune the beam if necessary. Once the beam energy is set, the particle LET is determined from stopping power curves. The SEU test facility at Brookhaven is equipped with specialized SBDS that can measure LET directly. These detectors are simply SBDS with a thin foil of aluminum of known thickness partially covering the detector. As shown in Figure 10, two peaks result when an ion beam strikes this detector: one for that portion of the ion beam that stiikes the detector directly, and one for that portion of the beam that passes through the foil and whose energy is slightly attenuated. The separation of the peaks is directly related to energy lost as the particle passes through the foil. With a known foil thickness, one can determine the particle LET directly. Note that this measures stopping power in aluminum, and is at most a few percent in error from the stopping power in silicon. 1

1

1

,

,

1

1

I

1

o

ms

1M2

lM.1

210.9

=m5Y

=7

S16S

M.2

-d

(WV)

Figure 10: An energyspectrumtakenwitha partiallycoveredSBD. The separationof thepeaks in energy is directly proportional to the particle LET. The LET from this measurement is 11.5 MeV-cm2/mg compared to a calculated value of 11.8 from Ziegler’s curves [16]. Total dose is accumulated by devices as they are exposed to a beam of heavy ions. Because many devices that are tested for SEP, particularly MOS and CMOS lCS, are also sensitive to total dose effects, one must record the dose accumulated by devices as they are tested, For charged particles [26], dose in rads is given by:

D=l.6xlo–5

@+*

(5)

where @ is the fluence in cm-z, and l/p (dE/dx ) is the mass stopping power of the target material in MeV-cm2/mg. 3.3 Circuit Test Techniques During SEP tests, the device or IC must be exercised using either a computer controlled system or a custom circuit board which provides data and control signals to the device. In the “golden chip” approach [27], two devices are exercised in lock-step with the same vector set. Only one of the devices is exposed to the ion beam, and errors are detected by comparing the response of both chips in real time. A second approach relies on a digital tester to exercise the

III-12

device and compare its response at every clock cycle with expected data, Errors are detected when the device response differs from the expected data. This approach has been implemented using an hp82000 at BNL for SEU tests of mimoprocessors and other specialized chips [28], while a custom VLSI tester has been used by the Aerospace group for SEU tests at Berkeley [29], Custom circuit boards have been used extensively to test microprocessors [30] or whole computer systems [31]. In most testers, there is some dead time following detection of an error as the tester captures data and resets the DUT. During this time, the tester cannot detect errors, so any particles striking the device should not be counted in the total fluence. Dead time should be kept to a small fraction of the device cycle time. 3.3.1 Memories Several different test protocols can be used depending on the device and the type of test being conducted. Memories and latch circuits can be tested in either a static or dynamic mode of operation. In the static mode, the device is loaded with a specific pattern and is then exposed to a given fluence. Afterward, the memory is interrogated and the number of errors determined. The fluence is held to a sufficiently low level that the probability of a given bit being struck more than once is negligible. In dynamic tests, the memory is written with a given pattern and then cycled through a read, write, wait sequence while the device is continuously exposed to an ion beam. Errors are detected during the read phase and corrected during the write phase. The wait time can be varied to maximize vulnerability of memory elements in the circuit, 3.3.2 Complex Digital Circuits In complex ICS, it is difficult to isolate errors to specific regions of circuitry [27,28]. In a microprocessor, for example, errors occurring in the program counter can cause the microprocessor to branch into a different section of memory and begin to misinterpret instructions as data, and vice versa. Errors are then detected in each clock cycle as the microprocessor executes incorrect code. If these errors are counted, the saturation cross section for the microprocessor will be grossly overestimated. The error handling routine in the tester must be able to parse data based on the pattern of errors observed, In this example, a pattern of multiple errors would be counted as one error. The overall microprocessor vulnerability is also dependent on the program code being executed, and this varies widely horn one application to another. One could measure the error rate while the microprocessor is executing code to be used in a given application, if known. However, the device response may be dependent on the data which the microprocessor handles. A benchmark code can be used, but it is difficult, if not impossible, to extrapolate from this test

to any other application of the rhicroprocessor, Another way to estimate the microprocessor vulnerability is to measure the sensitivity of some basic circuits and then extrapolate to the full chip vulnerability. This gives a very conservative estimate of full-chip error rate, since it assumes that errors in any latch will result in a chip error.

III -13

3.3.3 Analog circuits Analog devices, such as analog-to-digital converters (ADCS) and digital-to-analog converters (DACS), are composed of both analog and digital circuits. SEP in analog circuits is a complex function of the upset mechanisms in the digital and analog circuitry of the device. In a hybrid flash ADC, Tufflinger [32] established signatures for the different error mechanisms involved. Example histograms of errors at an LET of 1.4 and 26,7 MeV-cm2/mg are shown in Figure 11 for a gain-difference amplifier in the hybrid ADC. At the lower LET, errors are tightly clustered around the expected output (denoted by the vertical dashed line), These are attributed to noise induced in the amplifier by the heavy ions. At the higher LET of 26.7 MeV-cm2/mg, an additional component which is characterized by a regularly spaced offset from the expected data is observed in the error histogram. These errors are attributed to upsets in the analog circuitry overdriving the digital circuitry in the next section of the ADC. Such analysis is essential to understanding SEP in complex analog circuits. 1100. 1000900Boo-

LET =

1100-

I I I

Carbon 1.41

1000900-

I

~

~oo

Nickel LET =

26.7

I

700-

I I I I

600500400-

I

1450

1500

1550

Digital

1600

Output

700600-

~

500-

400-

*

i-_-L-l 1400

s &

1650

1700

1750

1.900

““-?LwdL_J 1400

1450

1500

1550

Digital

Code

1600

Output

1650

1700

1750

1600

Code

Figure 11: A histogram of errors detected in a silicon ADC at two LETs, After Turflinger [32]. 3.4 Circuit Effects Device response to a heavy-ion strike depends on several factors associated with its operating parameters. The voltage, logic state for digital devices, temperature, and frequency can be varied during SEP testing, and devices respond differently to variation of these parameters depending on the particular effect being tested. In this section, we describe the major single-event effects in devices and ICS and discuss the dependence of device response on voltage, temperature, and operating fkquency. 3.4.1 Dynamic

Circuits

Dynamic circuits, such as the dynamic RAM (DRAM) cell shown in Figure 12, depend on charge storage on a circuit node for proper operation. Information is stored on capacitor Cs~ (this capacitor can bean actual capacitor included in the cell layout or simply the capacitance of the reverse-biased source junction of transistor QW plus the gate capacitance of @). When a logic ‘1‘ is stored on Cs~, Wmsistor ~ is turned on. When the cell is subsequently accessed by turning transistor Q~ on, the bit line is pulled down and the complement state of the cell is sensed on the bit line. The cell is written and refreshed through transistor QW. Charge gradually leaks off the storage node due to leakage currents in the reverse-biased source junction of QW

III -14

and the state of the bit must be refreshed periodically. More recent DWM memory cells consist of a storage capacitor and a single transistor through which the capacitor is accessed [33]. The error mechanism for upsetting a DRAM is loss of stored charge on the storage node [4]. For the NMOS 3-transistor memory cell of Figure 12, charge is stored on C~~ when the node is biased positive (i.e. a logic ‘1‘ stored on the node). An upset occurs if sufficient electrons are collected at the storage node following a heavy-ion strike to the source of QW,resulting in a ‘1‘ to ‘O’bit flip. No upset occurs if the node is struck while at a low potential (i.e. a logic ‘O’is stored on the node). Bttllne ‘-——————l

1

e*

Wflte Figure 12: Circuit schematic of a 3-tm.nsistor DRAM memory cell. Information is stored m charge on the capacitor C~, and is accessed through transistors Q~ and Q~. The charge on Cm is written and refreshed through QW.After Pickel et al, [4].

Critical charge, ~, is defined as the rnirtimum charge necessary to upset a memory bit. For a DRAM, this is the difference between the amount of charge storcxi on the node and the minimum charge that the sense amplifiers require to reliably read the stored data. The critical charge representing a bit of information on capacitor CS~ can be as small as 0.1 to 0.5 pC. This is equivalent to only 5 x 105 to 3 x 106 electrons. If the collected charge exceeds the critical charge, an upset occurs.

The amount of charge collected at a pn junction following a heavy-ion strike decreases with decreasing voltage, since both the depletion region width [18] and funnel length [11, 12] are proportional to the square root of voltage. However, to fist order, the charge stored on a node decreases linearly with voltage (Q =CV) making it easier to upset a bit. Worst-case for upset is therefore the minimum operating voltage, Leakage current in a reverse-biased junction increases with increasing temperature. Because charge is lost horn the storage node at a faster rate between refreshes, the average charge on the node decreases [4]. This results in a decreased critical charge. Worst-case for upset for DRAMs is therefore the highest specified temperature at which the IC is expected to operate. Average charge stored on Cs~ decreases with decreasing refresh rate [4]; therefore, lowering clock frequency of a DFL%M increases its sensitivity to upset. Worst-case condition for upset is the minimum clock frequency.

IU-15

3.4.2 Static Circuits A schematic diagram of a 6-transistor (6-T) memory cell is shown in Figure 13. Each series connection of an n- and p-channel transistor forms an inverter whose input is the common gate, and whose output is the common drain. When the output of each inverter is coupled to the input of the other inverter, a hi-stable memory element is formed. Feedback resistors, labeled RF in the cross-coupling links of Figure 13, are often used to decrease sensitivity to upset, but with a commensurate performance penalty [34]. As in dynamic circuits, maximum charge collection occurs at the highest potential across the pn junction. In SRAM memory cells, therefore, the off-biased drain regions are most sensitive to upset from a heavy-ion strike. When a logic ‘O’is written to node Q, the off-biased regions are the drains of transistors P 1 and N2. Current resulting from a strike to the sources of either the n- or p-channel transistors will immediately be removed through ground or V~~, respectively. In static circuits, such as a CMOS static RAM or D-latch, upset is controlled by the rate at which charge is collected. In the following discussion, we consider the upset mechanisms for a p-well based CMOS technology, where the n-channel @ansistors are formed in a p-well region and the p-channel transistors are formed in the n-type substrate or epitaxial layer. For an n-well CMOS technology, the same mechanisms apply but in regions of opposite polarity [36]. Following an ion strike to the off-biased drain of transistor P1, a current spike is observed on storage node Q of the inverter formed by transistors N1 and P 1. If charge is collected by the drain of P1 faster than it is removed by the unstruck n-channel transistor N1 (called the restoring transistor), the node voltage will rise to a diode drop above V~~ (see the lower left waveform in Figure 13). Whether an upset occurs depends on the competing processes of removing excess charge at the struck node, and changing the state of the opposing inverter of the memory cell ., “m i

T I

11

\l

time

‘m

Figure 13: Logic diagram and circuit schematic for a 6-transistor CMOS SRAM memory cell. The cross-coupled inverters form a hi-stable memory element. Feedback resistors, RF, are inserted in the cross-coupling links to reduced SEU sensitivity. Each inverter is composed of a series connected n- and p- channel transistor, The voltage transients following a heavy ion strike to the drain of PI are shown. After H.T, Weaver et al, [35]. III-16

[35]. The time required for the struck node to recover to one-half the voltage swing at the node is defined as the recovery time, tR For example, for 5 volt operation, half the voltage swing is 2.5 volts. The time required to switch the opposing inverter formed by lmmsistors N2 and P2 is defined as the decoupling time, tm (see the lower right waveform in Figure 13), In this analysis, if t~ is greater than ~, upset will occur. Upset following a strike to the drain of an n-channel transistor in a CMOS SR4M is caused by a different mechanism than for a p-channel strike [37,38]. Where the latter is caused by collection of charge to the struck node as illustrated in Figure 1, an n-channel lxansistor upsets when the column of charge spreads laterally enough to lower the potential barrier at the source. This turns on the n-channel lmnsistor and a delayed pulse is observed at the storage node as shown in Figure 14. The same concepts of recovery time and decoupling time apply to this upset mechanism. 7 P–Hit

6 -

4 3 2 -

1 t 01 ,.-3

,0-2

,.-1 Time

10°

J ,.l

(ns)

Figure 14: Simulatedncde voltage of a CMOS SRAM following a mike to the p-channel drain (“p-hit”) or the n-channel drain (“n-hit”). The upset mechanism for the p-hit is due to photocurrent collection processes, while the n-drain upset is due to diffusive turn-on of the n-channel transistor. After Axness et al., [38].

When a heavy ion passes through two regions of like-doping that are separated by a region of opposite doping, a high-conductivity path forms that electrically connects the similar regions, as shown in Figure 15 [37,39-42]. This “shunt effect” was first observed in layered test structures [39], but has since been observed in CMOS ICS [28,41]. If the regions are at different potentials, current flows from the high to low potential, and can cause a voltage transient which induces upset. A bipolar amplification of charge generated in the p-well region is also possible [39], since the three regions form a vertical npn bipolar transistor. However, simulations by Fu et al. [37] have indicated that any bipolar amplification would be extinguished as the n+ node voltage goes positive, limiting the amount of charge that could be transferred via this mechanism. Recovery time increases as the drive of the restoring lmnsistor decreases, which in turn is dependent on the square of the difference between gate voltage and threshold voltage. On the other hand, decoupling time is, to first order, dominated by the RC time constant of the feedback path and is weakly dependent on voltage. Therefore, as power supply voltage decreases, static circuits become more sensitive to upset as shown in Figure 16. Here, Kolasinski et al. [43]

III-17

v=

‘r ,’

Ion Tra&

Ea

- Shulll

I

n-

I

{

I -----,___––__

n+

Vm

Figure 15: Schematicrepresentation of the “shunt effect” between two regions of similar doping and different polarity. A heavy-ion strike can form a high-conductivity pti between the nsubstrate region and the n-drain region. After Hauser et al. [42]. 10-2

@ii

[

I

,,-3~HM’508-’”#=p Power

Supply Voltage

● 4.0 v

A o A

5.0 6.0 7.0 ❑ 6.0 ■ 9.0

A

I t 10-6

v v v v v

I 0,0

I

0.5

1.0

1.5

2.0

2,5

3.0

3.5

4.0

SEC ~

Figure 16:Upsetthresholddecreaseswithdecreasing~wer supplyvoltage.Datashownis fora HarrisHM6508-RHlK-bit SRAM exposed to 150-MeV Imypton ions. After Kolasinski et al. [43].

observed that upset threshold for a 1K SRAM was dependent on the square of power supply voltage. Simulation of SRAMS has indicated a linear dependence on voltage [44,45], but this is probably design dependent. Upset threshold of static circuits is relatively insensitive to temperature unless feedback resistors are used to harden against upset. Feedback resistors increase the decoupling time of the memory cell, allowing more time to remove excess charge from the siruck node. Figure 17 shows the error cross section dependence of a CMOS D-latch circuit used in the SA3300 microprocessor [34]. For this circuit, as temperature increases horn 25 to 125 ‘C, upset threshold deereases from 175 to 95 MeV-cm2/mg. In this technology, lightly-doped polysilicon is used to form the feedback resistors. Due to the negative temperature coefficient of resistance for this film, with increasing temperature the resistance decreases and the device becomes more sensitive to upset. For static latch and SRAM circuits, no dependence on frequency should be observed. However, the random logic associated with row and column decoders, and the sense amplifier

III- 18

10-2

~El

SA3300

Temperature

1 O-s

10-+ c

.-0 %

10-5

A 10+

10-7

Rm= 160 kll

O

25°C A 90°C ❑ 125°C

1 o

25

50

VDD= 4.5 v

❑ ❑

L~

100

125

t1

AA A

❑

75

n

150

t

175

200

(MeV-cm2/mg)

Figure 17: Upset threshold de-creases with increasing temperature when lightly-doped polysilicon feedback resistors are used to harden an SW to upset. This effect is due to the negative temperature coefficient of resistance of Lightlydoped polysilicon, (After Ref. [34])

circuits can become more sensitive to upset as frequency increases. Since the sensitive area of this support circuitry is orders of magnitude smaller than the memory array, errors in the support circuitry are not distinguishable from memory errors in a single test. One can differentiate between memory enors and support circuitry errors by comparing IC response in a static mode and dynamic mode test [46]. In a static mode, only the memory array is sensitive to upse~ while in a dynamic mode both the memory array and support circuitry are sensitive. The difference in error cross section between these two test modes is therefore due to support circuitry upsets In qualification tests, one should exercise the SRAM at the maximum frequency allowed by the part spedlcation, or the frequency of the part in its application. 3.4.3 Single Event Gate Rupture (SEGR)

A heavy ion passing through an insulator sometimes results in a catastrophic error due to rupturing of the gate dielectric [47-49]. This occurs under conditions of high field, such as during a write or clear operation in a nonvolatile SRAM or E2PROM. As a heavy ion passes through the capacitor formed by the gate conductor, dielectric, and silicon substrate, a highly conductive plasma path is formed through the insulator which allows the capacitor to discharge. If sufficient energy is stored on the capacitor, excessive heating during discharge can create a thermal runaway condition, Temperatures can be high enough to cause the dielectric to melt and evaporate the overlying conductive layers. The micrograph in Figure 18 is a graphic illustration of this destructive event. Two conditions must be satisfied to cause a permanent rupture in an insulating dielectric: 1) the incident ion must generate a high-conductivity path through the dielectric, and 2) the struck device must release sufficient energy along the plasma channel formed by the heavy-ion strike to destroy the device. Experimentally, the occurrence of hard errors depends on the particle LET, electric field strength across the dielectric, and the angle of the ion strike, Figure 19a shows the failure threshold voltage, Vm, as a function of inverse LET for normal incidence heavy-ion strikes on two types of dielectric stacks. The circles are for an SiOz capacitor, while the triangles denote Vm for a composite device (S~N~Si02). Vm is inversely related to LET.

III-19

Figure 18: A micrograph of an MNOS capacitor showing the physical darnage associated with a hard error after a heavy ion strike. Metal has delaminated from the dielectric and a cenual core of darnage is evident. Metal thinning has occurred out to a radius of 5 pm and indicates that the metal was heated sufficiently to evaporate a portion of the original layer. After T, F, Wrobel

[49]. Figure 19b shows the electric field dependence for the same devices as a function of inverse LET. The composite device exhibits a linear dependence of ~ on I/LET, while the oxide device shows a square root dependence. The observed dependencies have been explained in terms of charge injection under velocity-saturation conditions for the composite device, and

under constant mobility conditions for the oxide device [49]. The dependence of failure threshold voltage on incident angle is shown in Figure 20 for a composite device with Ge ions. Failure threshold increases linearly with pathlength through the dielectric, as indicated by the nearly l/cose dependence. Note that this effect is opposite that expected from the effective LET approximation. This effect has been explained in terms of the LH

w

100

(MeV–cm2/mg)

50

33

H 25

w

20

60 -

100

(MeV–cm2/mg) 25

33

50

20

10 -

50 -

2,2

nm

S102

+

35

nm

SIN,

8 -

~

04Q .3 >b 30 -

42.5

nm Si02 \

e k w 4 -

20 2 -

10 0

0.00

0.01

2.2

nm

5i02

+

35

nm

51Nx

o 0.02 1 /LET

0.03

0.04

0.05

(MeV–cm2/mg)-’

0.00

0.01

0.02 1 /LH

0.03

0.04

(MeV-cm2/mg)-’

b)

a)

Figure 19: Failure threshold voltage a) and elech-ic field b) of two dielecrnc insulators as a function of inverse LET, The circles show results for a composite dielectric of 35 nm of Si,N, and 2.2 nrn of SiOz, while the iriangles are for a 42.5-rim oxide. After T. F. Wrobel [49],

111-20

0.05

I

101 1.00

1.50

2.00

3,00

2.50

1 /cos

e

20: Dependence of failure threshold voltage on angle of incidence for a 180-Mev Ge beam. After Wrobel [ 49].

Fgure

resistance of the plasma channel in the insulator. As pathlength increases, the resistance of the plasma path increases, resuking in an apparent increase in failure threshold voltage. No temperature dependence for SEGR has been reported as of this date. 3.4.4 Single Event Burnout (SEB) Destructive failure resulting from heavy ion exposure is observed in bipolar power transistors and in power MOSFETS [50-57]. A cross section of a typical power MOSFET is shown in Figure 21, In normal operation, the MOSFET gate induces a channel between the source and drain regions. Current flows from the source to the drain near the surface, then is collected in the heavily-doped substrate. The epi-layer doping and thickness determine the onresistance of the device, while the reverse breakdown voltage determines the maximum standoff voltage of the device.

&

................... ........... ~.. .. ... ....... ... I

‘+

1

Figure 21: Cross section of a typical power MOSFET structure. Normal current flows from source to dmin at the surface, through the epi-layer into the heavily-doped substrate, and out the metal contacts on the backside of the wafer. Avalanche breakdown causes catamophic failure in the device when a heavy ion induces sufllcient current to turn on the parasitic npn transistor formed between the epi, body, and n-source. After Hold, et al. [51].

III-21

Note that a parasitic npn bipolar transistor exists between the epi-layer (collector), the p-type body (base), and the n-type source (emitter). When a heavy ion passes through this parasitic transistor, excess current is generated in the base region. Excess hole current flows toward the body contact, raising the local potential along the base-emitter junction. If sufficient current flows to raise this potential to the turn-on voltage of this junction, the base-emitter junction becomes forward biased, turning on the npn transistor. Following turn-on, the transistor can enter a second breakdown condition caused by avalanching at the epi-substrate junction. This condition has been called current-induced-avalanche (CIA) [54]. A minimum current density, J=ri,,is required to enter the CIA condition. If the external circuit can provide sufficient current, local over-heating in a portion of the device will occur, destroying the device. There is a definite threshold voltage required for burnout to occur, and this is often well below the normal breakdown voltage for the device. Fischer [52] has measured failure-threshold voltages, Vm, ranging from 22% to 90% of the rated breakdown voltage for n-channel devices from several manufacturers. He also found that p-channel devices could be operated up to their rated voltage without evidence of SEB. The immunity of p-channel devices to SEB [52,55] is thought to be due to the lower impact ionization coefficient of holes compared to electrons [18]. As particle LET increases, Vm decreases for normal incidence ions. For example, Waskiewicz et al. [55] found that Vm decreased from 190 V at an LET of 3 MeV-cm2/mg (67-MeV N ions) to 80 V at an LET of 15 MeV-cm2/mg (175-MeV Ar ions) for a 2N6766 power MOSFET. As particle LET increases, more charge is deposited in the sensitive volume of the device making it easier to initiate the avalanche breakdown mechanism. Fischer [52] found that ions with the same LET but different ranges could have markedly different Vm with the shorter range ions exhibiting a higher Vm. He explained this effect in terms of the current density along the ion track at the epi-substrate interface where secondary breakdown occurs. A higher drain voltage is required to sustain CIA when the ion does not penetrate to the epi-substrate interface. Current waveforms observed during a heavy-ion test of power MOSFETS are shown in Figure 22. When the device-under-test (DUT) is biased below failure threshold voltage, Vm, a small prompt photocuent is obse~ed which lasts for less than 10 ns. However, when the device is biased above V~ti, excess current flows in the device for times greater than 10-6 seconds and destructive burnout is observed. In performing heavy-ion tests of power transistor for burnout, it is important to provide enough current to initiate the burnout response so that it can be observed, but less than sufficient current to destroy the device. A test circuit [52] with these features is shown in Figure 23. An external capacitor, C=,,, provides current during the event. Current in the device is measured with a low-impedance current transformer (CT) probe attached to the drain, and a current-viewing-resistor in the source-power supply loop. Care must be taken to minimize inductance in the loop containing the device-under-test, the CT probe and CVR, and the capacitor, C~=t. With a low value of Ce,t, burnout is initiated but the device is not destroyed (middle curve of Figure 22).

III -22

102

I

10’ Oevice

Destroyed

~

_:

10°

10-’

10–2

Device OK ~

SEB not initiated

, ~-s

‘- lo-

,0-8

,0-7

,.-5

,0-0

Tme

,.-4

(s)

Figure 22: Current wavefoxms observed during heavy-ion tests of power MOSFETS with different values of Car After Fischer, [52]. Im /

,~

Cr 3

7

Figure 23: Test circuit for power MOSFET burnout. After Flseher, [52]. Failure-threshold measured at normal

voltage incidence

is weakly

dependent

on angle

of incidence,

with minimum

Vw

[52]. The observed dependence is probably related to the range of ions and depth of the epi-substrate junction. For very penetrating ions, no dependence on angle of incidence should be observed. In qualification tests, ions with range of penetration greater than the depth of the epi-substrate junction at normal-incidence angles should be used. The susceptibility of power transistors to SEB is weakly dependent on temperature, decreasing as temperature increases. Waskiewicz et al. [55] found a 10% increase in VH at 100”C over the room temperature failure voltage for three different device types, In work to be presented in the Single Event Phenomena session of this conference, Johnson et al. [56] found a As temperature increases the avalanche multiplication factor decreases, similar response. weakening the mechanism for secondary breakdown in bipolar transistors and power PETs. Calvel et al. [57] found that the error cross section for power MOSFETS was two orders of magnitude smaller when the devices were operated in a dynamic rather than a static bias. However, the same threshold for upset was observed under both bias conditions. They speculated that a characteristic time related to the thermal time constant of silicon is required for

III-23

self-sustained 22.

second~

breakdown.

This is consistent with the current waveforms

of Figure

An additional failure mechanism affecting both n- and p-channel power MOSFETS, similar to SEGR, has been observed by Fischer [52] and Waskiewicz [55]. Fischer found that even with low-valued external capacitances, a gate-channel rupture can be initiated well below the rated oxide breakdown voltage of the devices. He attributed this failure to the same mechanism responsible for SEGR; that is, the formation of a highly-conductive plasma in the gate dielectric through which the gate capacitance is discharged. The stored energy on the gate capacitance for these devices is sufficient to catastrophically rupture the gate oxide. For most devices, SEB is observed before gate rupture, 3.4.5 Single Event Latchup

(SEL)

Latchup is a high current condition that results from thyristor (SCR) action in 4-layer structures [58,59]. Latchup creates a low-resistance path from power supply to ground in CMOS ICS, which are vulnerable to this failure condition due to the complementary structure required for this technology, As shown in Figure 24, a pair of coupled parasitic bipolar hansistors are associated with the p-well structure. In the figure shown, a vertical npn transistor is formed from the n-type substrate, p-well, and n-channel source. A lateral pnp transistor is formed by the p-well, n-type substrate, and p-channel source. The lumped-parameter equivalent circuit is shown on the right-hand side of the figure. Latchup in SCR structures is triggered by excess current in the base of the lateral pnp. When sufficient current flows in the substrate (across Rs) the emitter base junction of the pnp transistor is forward biased and injects a large current into the p-well. This current induces a voltage drop in the p-well (across RW)which turns on the vertical npn transistor, As the npn transistor turns on, it reinforces the initial current in the substrate and a regenerative condition exists which results in high current and low resistance. A typical IV characteristic for latchup is shown in Figure 25. Note that the holding voltage for latchup is on the order of 1 volt, Latchup is

‘ss

‘DD

v~ ?

msubslmte

Figure 24: A cross section of a CMOS technology on an n-type substrate. Parasitic bipolar transistors form an SCR that is subject to latchup. The lumped-parameter equivalent circuit is shown on the right. After Dressendorfer, et al, [60].

III-24

500 1’

400 300

-

200

-

$ _!4

I 100

1

0 0

2

4

6

8

10

vD&volts)

Figure 25: A classic IV characteristic for latchup. The holding voltage is on the order of 1 volt. After Dressendorfer and Ochoa [60].

triggered on the order of hundreds of nanoseconds, and destructive burnout occurs on the order of hundreds of microseconds [61], A latchup condition can oniy be cleared by removing power from the device. The trigger current for latchup can be introduced by overvoltage stress leading to avalanche breakdown [60], the photocunent from a high-dose rate event [61] or from the local high current resulting from the passage of a cosmic ray [62-67]. Since heavy-ion induced latchup is a very localized event, in contrast to dose-rate testing for latchup, it is important that the device-under-test be exposed at a large number of angles in an attempt to exercise all possible latch-up paths. To test for latchup in internal circuitry, complex ICS must be exposed while the device is being exercised so that potential latchup paths can be struck while the node is sensitive to latchup. To test for latchup in output drivers and input protection circuits, a static test is usually sufficient, provided that the correct bias state (i.e. n-drains at low potential) can be applied to the latchup sensitive structures. Latchup of ICS is normally detected by an abrupt increase in power supply current. This can be done easily using a computer controlled programmable power supply. Alternatively, a circuit that senses excess current, as shown in Figure 26, can be used. Here, a high current condition is sensed when the voltage drop across a 1-Q resistor connected to the DUT ground exceeds a preset reference voltage on a comparator. When the high-current condition occurs, the comparator flips a latch which disables the power supply feeding the DUT. The signature of the current into the DUT can be sensed with an inductance probe connected to a digitizer. An automatic reset can be sent to the latch to power up the circuit and restart the test. The number of latchup events can be counted by connecting the latch signal from the comparator to a counter. Since latchup can be destructive to the device, current limiting techniques must be used during test Care must be taken to ensure that the power supply provides sufficient power for normal operation of the device but limits current during Iatchup. During the time required for the circuit to reset, the DUT is not susceptible to latchup. The fluence accumulated on the part during this dead time must be subtracted from the total fluence for a correct cross section calculation. An example of latchup cross-section is shown in Figure 27. a distinct threshold for upset is evident in this figure where no latchup occurs at 14 MeV-cm2/mg, but does occur at 16

III-25

30 ILF

Figure 26: A latchup test circuit which prevents device burnout during Iatchup testing. After Johnston and Baze [66].

MeV-cm2/mg and above. Latchup cross-section is calculated in the same manner as upset cross-section; that is, the number of upsets divided by the fluence to upset. If single latchup tests are performed, i.e. where the fluence to fnst latchup is recorded, a large scatter will be observed in the latchup cross section. Depending on when a latchup path is struck, a device may latchup immediately after exposure to the ion beam, or at some time after exposure. Several measurements are required with this approach to determine the average fluence to latchup, 1O-s

10-’

10-s

,o-6~ o

20

40

60

~

80

100

120

140

(MeV-.m2/mg)

Figure 27: A typical latchup cross-section curve for a lK SIUM. Measurements were made using the Berkeley 88-inch cyclotron. After Koga, et al. [67].

Latch-up sensitivity increases, that is, the threshold for upset decreases, with increasing temperature and power supply voltage [64], Therefore, SEL testing is generally done under worst-case conditions of maximum voltage and temperature. 3.4.6 Single-Event

Snapback (SES)

Snapback is a high current, low resistance condition that occurs only in n-channel transistors [68,69]. It has an IV characteristic that is similar to latchup exhibiting a negative resistance region, and a low-resistance region as shown in the right-hand graph of Figure 28. Like latchup, it is triggered by any external stimuli that injects sufficient current into the p-well to cause the

III-26

‘Ds+

4 P-

Figure 2S: The regenerative mechanism (left) and IV characteristic (right) for snapback in nchannel MOS ira.nsistors. After Ochoa et al [68].

n-source to become forward biased. Snapback initiation has been observed by avalancheinduced breakdown at the n-drain [68], by excess photocurrents generated during moderate dose-rate gamma imdiation [68], and by heavy-ion strikes to sensitive n-drain or -channel regions [69]. When excess majori~-carrier current (1) flows in the p-well, the source region of the n-channel device can be forward biased. At this point, electron cunent (2) is injected into the channel and is swept toward the drain by the electric field between the source and drain. Some of this channel current contributes to impact ionization in the drain region (3), which in turn generates an excess hole current in the p-well (4) completing the feedback mechanism. Snapback is not observed in p-channel devices because the ionization rate for holes is much lower than for electrons, and regenerative feedback is consequently much lower. A significant difference with snapback is that the holding voltage is on the order of 7 to 12 volts, much higher than the 1 volt holding voltage seen in latchup. Snapback can only be sustained when the load circuit on the n-channel device can provide sufficient holding current (see load line in Figure 28). For CMOS ICS, the load device are p-channel transistors. Since holding current is on the order of milliamps, snap back is primarily observed in output buffers and internal bus drivers.

Heavy-ion induced snapback is observed by testing for high-current conditions during exposure to a beam of high-energy heavy ions, similar to the test techniques used for Iatchup (see previous section). Local heating and subsequent burnout is a concern as in latchup, This necessitates quick detection and clearing of the snapback condition. Unlike Iatchup, the snapback condition can be cleared without removing power to the device by simply turning the n-channel transistor on. 3.5

Accelerator Facilities Available for SEP Testing

Heavy-ions have been accelerated for SEP testing at a number of facilities in the United States, CanadL and Europe. This section describes the ion species and energies available at a few of the facilities that are available to researchers. The list of facilities is by no means comprehensive, and is offered to the experimentalist as a guide of representative facilities. Please take careful note that the ion energies and ranges vary widely horn one facility to the

III -27

next. Since several SEP (e.g. SEL and SEB) are dependent on charge generation deep in the substrate, the experimentalist must ensure that the correct ion energy and range are used during SEP tests. Table 1 summarizes typical ions used for SEP testing at the Lawrence Berkeley Laboratories 88-inch cyclotron. This facility has been widely used for SEP testing by groups from JPL and Aerospace Corp. Each of these groups have dedicated test apparatus installed in separate caves on the cyclotron. At the time of this writing, these facilities are available to outside researchers only through JPL or Aerospace. However, a consortium has been established to develop a test apparatus available to outside groups [70]. Table 1 lists representative ions available at the LBL 88-inch cyclotron, As noted in section 3.2, ion energy for cyclotrons is determined by the rf frequency and magnetic field, and is limited to discrete multiples of the fundamental frequency and several harmonics. Stopping power and range are calculated at the energy shown for each specific ion, Table 2 summarizes ion species and energy available at the Brookhaven National Laboratories Twin Tandem van de Graaff accelerator. This facility has a test chamber and associated diagnostics that have been developed specifically for SEP testing. Time on this facility can be purchased by outside groups. Staff at BNL provide some support in the setup and use of the apparatus, The user must provide his own test equipment to exercise the part and detect SEP. As noted in section 3.2, for this type of accelerator, a continuous range of energies are available, up to the maximum accelerating potential of the machine. Stopping powers and LET given in Table 2 are calculated at the maximum ion energy. Table 3 lists a range of ions available at the Los Alamos National Laboratories tandem van de Graaff accelerator. Ion energy depends on charge state of the ion and accelerating potential. The maximum accelerating potential on this machine is 11 MeV. LET and range are given at the maximum energy for each ion.

Table 1:

Typical Ions Used for SEP Testing at the Berkeley 88-inch C yclotron Accelerator Ion

Energy (MeV)

LET (MeV-cm2/mg)

Range (pm)

H

1

50

0,01

>100

He

4

12

0.35

>100

o

16

428

1

>100

N

15

67

3

>100

Ne

20

90

5.6

55

Ar

40

180

15

48

Cu

65

290

32

45

Kr

86

380

41

45

Xe

136

603

63

47

Note: Ion LET and range are calculated at the energy shown in column 3.

Ill -28

Table 2:

Typical Ions Used for SEP Testing at the Brookhaven Graaff Accelerator Ion

Twin Tandem van de

Max Energy (MeV)

LET* (MeV-crn24mg)

Range* (pm)

c

12

105

1.39

202

F

19

150

3.2

133

Si

28

195

7.7

81

c1

35

210

11,5

63.1

N1

58

255

27

40.3

Br

79

285

37.3

36.4

Ag

107

“300

53.1

30.9

I

127

230

59.9

30.7

Au

197

345

82,3

27.9

* LET and range data are calculated at the ion maximum energy. Table 3:

Typical Ions Used for SEP Testing at the Los Alamos Nat’1 Laboratories tandem van de Graaff Accelerator Ion

Max Energy (MeV)

LET (MeV-cm2/mg)

Range (w)

H

1

22

0.02

>100

He

4

33

0.16

>100

Li

7

44

0.45

>100

Be

9

50

0.87

>100

c

12

65

1,94

95.7

0

16

75

3.62

64

Mg

24

85

8.34

36.6

Si

28

105

10.4

37.2

Ti

48

110

22.7

22.8

Fe

56

110

28.3

19.9

I

127

150

54.5

18.1

Au

197

150

63.8

16.7

* LET and range data are calculated at the ion maximum energy. Tables 4 and 5 summarize representative ion species and energies available at two facilities in Canada. The McMaster Accelerator Laboratory has a tandem van de Graa.ff accelerator with terminal voltage of about 10 MV. To achieve ion energies comparable to those in the true cosmic ray spectrum, GeV accelerators must be used. Relativistic beam energies are available at the Atomic Energy of Canada Limited Research Company (AECL) Tandem Accelerator Super Conducting Cyclotron (TASCC) and the Lawrence Berkeley LaboratoriesBevalac[71]. A beam line and test apparatus

HI-29

Table 4:

Representative Ions Available for SEP Testing at McMaster University Tandem van de Graaff Accelerator, Hamilton, Ontario, Canada. Ion

Typical Energy (MeV)

LET* (MeV-cm.2/mg)

Range* (J.@

H

1

m

0.02

>100

He

4

30

0.18

>100

Li

7

30

0.6

>100

c

12

70

1,85

>100

o

16

70

3.78

58.2

c1

35

125

14.2

34.4

al+

64

125

31.9

20.6

Br +

79

110

38.6

16.9

I+ Au *

127

80

44.9

12.2

197

80

47

11.3

+ May need some development work * Will require development work Representative Ions Available at the AECL TASSC Facility, Chalk River, Ontario, Canada

Table 5:

Ion

Max Energy (MeV_)

LET (MeV-crn2/mg)

Range (pm)

c

12

600

0.36

4046

N

15

700

0.49

3475

Si

28

1260

2.11

1487

cl

35

1400

3.4

1053

Cu

63

2200

10.4

592

Ge

72

2160

14

453

Br

79

220)

16.8

393

Ag

107

2550

30

280

I

127

2650

38.7

239

Au

197

2900

77.4

157

u

238

2850

98.1

133

specificallyfor SEP testing are being developed on the booster ring of the Brookhaven Alternating Gradient Synchrotron (AGS) [72]. This facility will be capable of accelerating heavy ions to GeV/nucleon energies. SEP testing with protons has been performed at several facilities in the United States. Proton energies up to 50 MeV can be achieved at the Naval Research Laboratories cyclotron, while 56-MeV protons are available at the University of California at Davis cyclotron. At Harvard University’s cyclotron, energies to 100 MeV are possible. Protons can be accelerated to 200

ILI-30

MeV at the University

At each of these facilities, lower energies can be

of Lndiana cyclonon,

achieved using degmders, 3.6

Ion Micrwbeam

Techniques

Ion microbeams have been used to accurately position a micron-sized ion beam to study the upset sensitivity in various regions of an IC. Beam sizes on the order of 1 pm have. been obtained using an apertured microbeam, where a large-area beam is collimated by passing through a micron-sized aperture in a metal foil. The beam is positioned on the IC by translating either the aperture or the IC. Using this technique, Knudson et al. [73] were able to discriminate between errors in memory cell areas and sense amplifiers in a 16K DRAM. Micron and subnticron beams have ken formed using magnetic lenses to focus the beam and electrostatic deflection plates to scan the beam across an IC [74,75]. Recently, Horn et al. [74] have used this technique to image upset-sensitive regions of a 16K SRAM, as shown in Figure 29. Circuit Mask

Upset image .. ,------—..- ..... . . .. ..... .. ——.. -—...

—

—--

Figure 29: An image of the SEU-sensitive regions of [he 16K TA670

SRAM.

A 1- pm focusscd

hewn of 24-MeV Si ions was used to generate upsc(s. After Horn el al. [74]. Microbeam techniques have also been used to direct an ion beam to a particular structure and measure the resulting charge collection following a heavy-ion strike [76-78]. This work has supported the development of a basic understanding of the time dependence of charge collection. Special device fixtunng and hi,gh-spesd digitizers are required to resolve the charge. collection Using a Hypress PSP1OOO sampling oscilloscope with process following a heavy-ion strike. 70-GHz bandwidth, Wagner et al. 177,78] has measured current transients produced by several different ion species in Si and GaAs diodes. A representative series of curves for different

substrate-doping levelsis shownin Figure30. 3.7 Alternative

Laboratory

Techniques

Recent efforts to develop inexpensive laborato~ sources for SEP testing have centered on radioisotopes or focussed laser beams. This section details the use of these newer techniques, their advantages and disadvantages.

rII-31

1,2 t

“f\k

10

Cl-cm

3 ii-cm

0.6

/

1 Q-cm

0.4

i

0,2 nn -,-

0

100

200

300 time

1 400

500

600

(pa)

Figure 30: Current transients produced by a 5 MeV alpha beam in silicon diodes with various substrate doping. After Wagner et al, [78].

3.7.1 Cf-252 Radioisotopes Californium-252 (x2Cf)is a widely used radioisotope for laboratory tests of SEP. Californium discs with up to 50 pCi activity are commercially available with thin gold coatings to prevent self-sputtering. The particle flux is attenuated in air; however, the neutron flux from these sources is not attenuated significantly in air, The dose received is <0.5 mrern/hr for a 1 ~Ci source [79], The primary danger with this material is from ingestion and subsequent chemical poisoning or cell damage due to radiation. Discs of ‘i2Cf may be safely handled by personnel wearing gloves. The disc is mounted in a vacuum chamber and the part to be testedis positioned in close proximity to the source, The nuclear characteristics of ‘2Cf are shown in Table 6. One microcurie of ‘2Cf will produce 3.5 x 104 alphas/s, 2.2 x 103 fission fragments/s, and -4 x 103 fast fission neutrons/s. Czdifomium-252 fissions into a light and heavy fragment with average energies of 79 and 103 MeV, respectively, as shown in Figure 31. The distribution of masses and energies result in a range of LETs from about 42-45 MeV-cm2/mg at the silicon surface. However, the low particle energies result in a shallow penetration depth in silicon and a rapidly changing LET with depth in the sensitive volume. This leads to a large uncertainty in Table 6: Nuclear Properties of 2s2Cf Half Life:

2.64 years (alpha decay) 85 years (spontaneous fission)

Decay Robabilities

96.91 % (dpb) 3.09% (spontaneous fission)

Alpha Energies:

5.98 Me V (0.2%) 6,08 MeV (15.2%) 6.12 MeV (81.6%)

Fission Fragment Avenge Energies:

78.7 MeV (heavy fragment) 102.5 MeV (light fragment)

Fission Fragment LET in Si:

42-45 MeV-cm2/mg

Fission Fragment range in Si:

6- 15.5pm

From J. H. Stephen et al. [79].

III-32

Mass

Number

Fragment

Kinetic

Energy (E~)

Figure 31: The mass and energy distributions from the spontaneous fissioning of “*Cf. The distributions shows two distinct peaks: one for a light fragment with average mass number of 106 and energy and 104 MeV, and one for a heavy iiagment with average mass number of 142 and energy of 79 MeV. After J. H Stephen, et al. [79].

LET. The measured cross section for a given fluence is thus a composite of the cross section at several different LETs.

Browning [80] has developed a technique where the composite cross section due to the variable LET of ‘2Cf cart be deconvolved to yield an accurate cross section curve. It is limited to parts with sufficiently thin overlayers and low threshold LETs such that upset can be produced by the fission fragments. This condition is not met in hardened circuits, and some complex circuits with multiple layers of metal and polysilicon, It is also highly suspect in SEL and SEB due to limited range irt silicon (see sections 3.4.4 and 3.4,5). In Browning’s technique, the measured cross sections are corrected to account for the LET distribution of fission fragments from ‘zCf. Figure 32 shows cross section curves for a SA3240 16K-bit SRAM with 73 I@ feedback resistors. The part was characterized using bromine ions at BNL (squares), and had a threshold LET of about 40 MeV-cm2/mg and a saturation cross section of just over 10-3 cm2. With the uncorrected ‘2Cf data, the measured threshold was 38 MeV-cm2/mg (circles). After applying the correction (diamonds) the agreement between the heavy ion data and the ‘i2Cf data is excellent. The effect of overlayers of metal and passivation glass on the LET spectrum was considered in making the correction. The applicability of this technique to other part types must still be demonstrated, but these results are very encouraging. Recently, Constantine et al. [81] and Browning et al. [82] have developed time-of-flight systems which use thin-film organic scintillators to measure the energy and velocity of each fission fragment that strikes an IC. Three parameters are required to accurately calculate the LET of a fission fragment; energy, mass, and charge state. Knowing the energy and velocity allows one to calculate mass. Although charge state is not measured, LET can be estimated fairly accurately knowing only particle mass and energy. A H-252 tester with associated instrumentation and scintillators based on this technique is now commercially available [83]. Correlation studies comparing results with heavy ion tests are underway.

III-33

L

10-3

r

N-

❑

E : ,-0 %

10-4

:

1O-s

m o

A

9=--’-’

/7 ❑

,

A

&

-

:

v

1o-a

Br

140

MeV

❑ cf–252

/(

V

Cf corrected

El 10-7 30

35

40 LH

45

50

55

60

(MeV-cm2/mg)

Figure 32: A cross section measured with ‘i~f is compared to one measured with a heavy ion accelerator. The raw ‘~f cross section curve (circles) can be corrected (iverted triangles) and compares favorably with the cross-section curve measured with bromine ions at 140 MeV (triangles). After J. S. Browning, [80].

3.7.2 Laser Induced Upset Tunable Nd:YAG dye lasers have been used [84-86] in attempts to develop low-cost laboratory sources for simulating SEU. There are two degrees of freedom with a laser beam: wavelength, which affects the absorption characteristics of the beam; and, intensity, which controls the magnitude of the electron-hole plasma within the semiconductor. Wavelength is normally chosen in the near IR range so that the beam is absorbed uniformly throughout the sensitive region. Intensity is varied to control the energy deposition per unit length in the semiconductor. A laser can be easily focussed to spot sizes on the order of the areal dimensions of sensitive regions of an IC to probe specific sensitive regions of a chip. This is difficult to do with ion beams. However, lasers can only be used to determine the threshold LET and cannot yield the full cross section cume. This is because many sensitive regions of a chip are shadowed by metallization and camot be irradiated with a laser beam. The collimated and focussed laser beam must be positioned on a transparent portion of a sensitive region to produce upset. The laser pulse width should be much less than the time for the charge collection processes after an ion strike, i.e. on the order of 100-500 ps. Pulse widths of about 50 ps are possible with commercially available Q-switched lasers. Figure 33 shows upset threshold LET for two bipolar memories that were tested using the Lawrence Berkeley Laboratories Bevalac and a Nd:YAG laser. The accelerator data is shown as discrete data points in each curve. The Nd:YAG laser test gives only a threshold for upset, which is shown as a vertical line in the curves. There is excellent agreement for the AM27S03 bipolar RAM, with a threshold of about 180 nJ/m (equivalent to an LET of about 5 MeV-cm2/mg). For the 54F189 RAM there is up to a factor of three difference in threshold between the particle data and laser data. This may be due variation in LET sensitivity across the chip or that the most sensitive region of the chip could not be illuminated with the laser beam. Difficulties with this technique arise in relating the beam energy to particle LET due to spatial and temporal differences in the structure of the free-carrier plasma generated with a laser and a heavy ion. Electron-hole pairs are created uniformly along an ion track and are collected

II-I-34

10-2

10–2

i : AM27S03 .,

8

E -3 ~ 10 z o ~ ~

● ●

‘\

54F189

~● ✎

PARTICLE OATA g o u v

●

LASER UPSET

I

PARTICLE OATA ~

●

●1 10-5 , O–B

●

8

IASER

UPS~

—

10-5 , 0–7

10-6

,0-7

,O-E

0

a

●

●

10–4

● ●

1“ ●

●

,0-6

LINE4R ENERGY DEPOSITION (J/m)

LINEAR ENERGY DEPOSITION (J/m)

Figure 33: Upset threshold determined with a dye laser and the LBL Bevalae accelerator. For the AM27S03 there is excellent agreement between thresholds measumd with the dye laser and a particle accelerator, IrI a seeond part, the 54F1 89, a very poor agreement is seen. This may be due to differences in threshold with different circuit elemen~ that were not exposed to the laser beam. After A. K. Richter, et. al. [85],

by drift in the depletion region, funneling below the depletion region, and diffusion adjacent to the funnel. In contrast, the intensity of a laser beam is attenuated exponentially with depth into the semiconductor, giving rise to a much different distribution of free carriers than a high-energy particle. High-energy uv photons are strongly absorbed near the surface. This difficulty is resolved by using near-IR photons with energies just larger than the semiconductor bandgap. Because the absorption coefficient in silicon is much smaller for near-IR photons, the electron-hole pak distribution is nearly uniform. The laser beam diameter is about 20 times larger in diameter than the plasma track from an ion strike, so plasma density, and therefore recombination, will differ signiilcantly in the early time frame, Finally, reflection from overlying metal and dielectric films, and free carrier absorption in heavily doped regions lead to uncertainties in the energy absorbed in the sensitive volume. As this technique is applied to more part types and its usefulness is demonstrated, it will be more widely used for lot acceptance testing and detailed analysis of the relative sensitivity of different regions of a device.

4.0 ERROR RATE PREDICTIONS

As mentioned in the previous section, error rate estimates are made for a given part using the experimentally determined upset threshold, Lti, and saturation cross section, cr~~.This section describes the basis for error rate calculations using these measured parameters. Critical charge is sometimes estimated using circuit analysis tools such as SPICE or transport codes, such as PISCES or SIFCOD. This subject will not be discussed in this session. 4.1 The Natural Space Environment The natural space environment is composed of three major components: very energetic galactic cosmic rays from beyond our solar system, interplanetary cosmic rays from the solar wind and solar flares, and trapped particles within the radiation belts formed by the earth’s magnetic field, In this section we describe the particle. environment which spacecraft experience in earth orbit, This includes a description of the dependence of the particle flux on energy, spacecraft altitude, inclination, and shielding. Understanding these dependencies and building accurate models of the environment experienced by spacecraft is crucial to error rate predictions.

III-35

4.1.1 Galactic Casmic Rays (GCR) As shown in Figure 34, the galactic cosmic ray (GCR) flux is highly dependent on the ion species. Its composition is roughly 859?0protons, 14~0 alpha particles, and 1% heavy nuclei, The cosmic ray flux is minimal for ion masses above nickel [87],

105 104 ~

+

103 ,0’

L1 >

10’

x 3 d

10°

la-’

o

10

20

30

40

50

60

70

80

ATOMIC MASS

Figure 34: Galactic cosmic ray particle spectrum as a function of mass. Protons and helium ions are the most abundant elements, but there is a significant number of heavier elements up to nickel, After P. Meyer et al. [87]. The energy spectra of the more abundant ions in the galactic cosmic ray spectrum is shown in Figure 35. The x-axis is given in units of MeV/nucleon to compress the graph. If we consider

an ion of helium (4 nucleons) and an ion of iron (56 nucleons) at 100 MeV/nucleon, the ions have energies of 400 MeV and 5.6 GeV, respectively, The y-axis shows directional flux per unit energy per nucleon. (This y-axis unit is convenient for differential spectra, and will be discussed further in section 4.2.) The flux for each species peaks at energies of 100 to 1000 MeV/nucleon, then tails off to energies as high as 100 GeV/nucleon. At these energies it is virtually impossible to shield circuits horn heavy ion strikes. Note also that there are four orders of magnitude difference in the peak magnitude of the intensity of iron and protons. 102 1

&=&/pr”ton’

h\q 1

x\\ \ \

““

\\

t ,.l

,02

Particle

,03

,04

Kinetic

,05

,06

Energy (MeV/nucleon)

Figure 35: The energy spectrum for the more abundant elements in the gakwtic cosmic ray flux in free space. After J. H, Adams, Jr. et. al. [88].

III -36

When considering the effect of the full range of ions on a circuit, one must take into account the charge deposited by each ion. Such an analysis is simplified if the cumulative effects of all ions in the cosmic ray spectrum can be folded together into a single curve. Such a cumulative curve is called an LET spectrum. As an example of how an LET spectrum is constructed consider the energy spectrum for galactic cosmic rays in Figure 35. The flux curve for iron, for example, is converted to a differential LET spectrum by transformirtg the energy axis to stopping power using the stopping power curve for iron (Figure 3). The curve of Figure 36a results. The peaks at either end of this curve are real, and reflect the number of particles at the minimum and maximum energy loss. When this curve is integrated over LET, the integral LET curve of Figure 36b results. This curve shows the total flux of particles with LET greater than or equtzf to a corresponding value on the

x- axis. This is equivalent to integrating from high to low energies. The differential and integral forms of LET spectra are equivalent, and their application in error rate calculations depends on the mathematical

context in which they are used. 10’

10-’>

1

Fe

L

,02

, (y

LET

10°

10,04

ld

,03

, ~4

LH

(MmV-.m2\g)

,05

(MeV-cm2\g)

b)

a)

Figure 36a) Differential and b) integral LET spectra for iron. After E. L. Petersen, et al. [89].

When all ions in the galactic cosmic ray spectrum are considered, the differential LET spectrum of Figure 37 results. The peaks in this curve are caused by the different ion species. As seen here, a large drop in particle flux occurs at 30 MeV-cm2/mg. Also shown in the figure is a straight-line approximation to the differential LET spectrum used in the Petersen approximation discussed in section 4.2. In earth orbit, the contribution of the galactic, interplanetary, and solar flare particle fluxes to the total cosmic ray flux depends on solar activity. The galactic component, for example, is affected by the screening effect “of the solar wind. Therefore, as solar activity decreases the galactic component increases. On the other hand, the interplanetary and flare components increase with solar activity, since they are primarily composed of particles originating in the sun. The change in the integral LET spectra as a function of solar activity is shown in Figure 38 for a spacecraft in geosynchronous orbit, These particular curves were calculated for 25 roils of aluminum. The lowest intensity, curve a, occurs at solar maximum excluding solar flares. This represents the absolute minimum GCR environment at geosynchronous orbit The environment

III-37

102

10’ 10° 1o–’ 10–2 10-3 10-4 10-5 10-6 ::/ 10-7 1o-a 10-9 –lo 10 10-11 10-’2 1

Petemen

Measured

spectrum

l+++

++

,.-3

Approximation

,.-1

,0-2

LET

++

,00

*

102

10’

(MeV-cm2\mg)

Figure 37: The differential LET speetrurn for all cosmic rays at solar minimum behind 0.025 inches of aluminum. After E.L. Petensen, et al. [89].

is more severe than this 1007o of the time, The environment at solar minimum, curve b, describes the environment for approximately 40% of the time. This is the pure galactic cosrrtic ray spectrum. If we add to this the interplanetary component of cosmic rays, the 90910 environment shown in curve c results. Alternatively, we say that the environment is more severe than this only 10% of the time. This curve, called Adarns’ 10% worse-case environment, is used quite frequently to represent the space environment in error rate calculations [90]. (This The worst-case spectrum has also been called Adams’ 9070 worst-case environment.) environment occurs when solar flares are also considered. The integral LET spectrum for the anomalously large solar flare of August 4, 1972 is shown in curve d of Figure 38. A flare of similar intensity can be expected once every solar cycle, or roughly every 11 years. However, a significant flare occurs about once every six months during the maximum of the solar cycle. The error rate for spacecraft in orbit for more than a few months will be dominated by solar flare activity during ‘the mission. Note that there are five orders-of-magnitude difference in the least severe and most severe environments.

L

1 O% Worst

Solar

Cae.s

Min Solar

Max

/

I -20

{ , 0–3

-4 ,.-2

,.-1

La

,00

,.l

, Oz

,03

(MeV-cm2\mg)

Figure 38: The integral LET spectra under a) solar maximum conditions, b) solar minimum conditions, c) the 107o worst-case environment, and d) for an anomalously large solar flare condition. After J. H. Adams, Jr. [90],

III-38

8

iz / / 60”

4

0

80”

30”

/

-4

/

–8

h –17

“-1~o

, ~1

,.2

, ~3

, ~4

105

,06

LET (MeV-cm2/g)

Figure 39: Integral LET speetra as a function of spaceemft orbital inclination. After J.H. Adams, Jr. [91]. The earth’s geomagnetic sphere screens out particles below a specific energy as determined by the inclination and altitude of a craft’s orbit. Near the equator the eath’s magnetic field bends

all but the most energetic ions while at the poles the cosmic ray spectrum is largely unaffected. This is illustrated by the cosmic ray flux for a 400-km circular orbit and various orbital inclinations as shown in Figure 39. At inclinations of 90°, very little shielding occurs and the spectrum approaches that of the natural environment. As the orbital inclination decreases the flux gradually decreases. The most pronounced effect occurs above 103 MeV-cm2/g, For inclinations of 40° and less, the flux above 3x 103 MeV-cm2/g (3 MeV-cm2/mg) decreases by more than three orders of magnitude, 4.1.2 The Earth’s Radiation Belts While the earth’s magnetic field moderates the low-energy components of the cosmic ray flux, it also forms regions where low energy particles are trapped by the magnetic flux, forming the radiation belts. The flux withirt the radiation belts depends directly “on the degree of solar activity, The most abundant particles are electrons and protons, however, some low-energy heavy ions are also found. The trapped particle bands are depicted in Figure 40. Here, the Mcllwain L parameter [92] is a dimensionless ratio of the earth’s radius equivalent to a constant magnetic field line in the earth’s geomagnetic sphere. This is well described by the dipole field equation R= Lcos2A

(6)

where R is the altitude in earth radii, L is the McIlwain parameter, and A is defined as the invariant latitude. Note that L = 1 is equivalent to the earth’s surface at the equator. While only protons and heavier ions have sufficient mass and energy to cause soft errors, the electron flux can contribute to total dose damage. The trapped proton distribution consists of one region extending to about 3.8 earth radii (18,000 km). The electron belts are divided into “inner” and “outer” radiation belts. The irtner belt extends to about 2,5 earth radii (9,600 km) while the outer belt begins at approximately 2.8 earth radii (11,500 km) and extends to about 12 earth radii (70,000 km). Solar flare protons extend as low as 5 earth radii, depending on the energy spectrum of the given flare, In low-earth orbit (LEO), spacecraft encounter both inner belt

III-39

Solar Flare Protons Trapped Protons Outer Zone Electrons Inner zone Electrorts

5

GEO

10

Earth Radii Figure 40: Electron and proton belts formed by the earth’s geomagnetic sphere. After Stassinopoulos, et al., [93],

electrons and protons, while in geosynchronous belt electrons are observed.

orbit (GEO), about 35,775 km, primarily outer

The proton flux as a function dipole shell is shown in Figure 41 for proton energies horn 0.1

MeV to 400 MeV. Protons with energies greater than 10 MeV occupy the region below about 3.8 earth radii, although the 2 MeV proton energies extend up to about 5.5 earth radii. Normal spacecraft shielding attenuates the proton flux with energies below 10 MeV.

.— 0

1

2

3

4

5

6

7

Oipole Shell (L)

41: Proton flux as a function of energy and McI1wain L parameter. After Stassinopoulos, et al. [93],

Figure

Abovethe Atlanticocean off the SouthAmericancoast the geomagneticspheredips toward causing a region of increased proton flux, called the South Atlantic anomaly. In Figure 42 we show the proton flux for ?. 111 l-km circular orbit at 63° inclination as a function of proton energy. The x-axis unit of time in orbit is equivalent to distance along the orbital path. A significant fraction of time in orbit intersects a region of high proton flux. Here, the proton flux for particles with energy greater than 30 MeV is 104 times more intense than at comparable altitudes over other regions of the earth. At higher altitudes the magnetic sphere is more uniform and the South Atlantic anomaly disappears. the earth

111-40

“-

O

10

20

30

40

50

60

70

80

90

100110120

Relativa Time in Orbit (rein)

Figure 42: The proton flux as a function of time for an orbit passing through the South Atlantic Anomaly. Since time in orbit is equal to distance along the orbital path, this shows a marked difference between the normal proton flux and that detected over the South Atlantic Ocean at an altitude of 1111 km. After E. L. Pekmen, [94],

Because of the relatively small stopping power for protons, very Little direct ionization of silicon occurs. Other mechanisms can be very important however. Elastic scattering of target nuclei, for example, can deposit enough energy to cause errors. Of more importance, however, is the contribution to upset from inelastic scattering events, where the incident proton reacts with the target nucleus. About one of every 105 protons will experience an inelastic collision with the target lattice. As the composite nucleus decays, it can emit alpha and garrtma particles. In addition, the daughter nucleus can recoil with enough energy to cause upset through direct ionization. The composite nucleus can also decay through a spallation reaction, where the compound nucleus breaks up into two heavy fragments, both of which recoil and deposit energy. The relative importance of each of these mechanisms is shown in Figure 43 for a 64-Mbit memory in a 63° circuhr orbit at 1111 km. For devices with sensitivities above 106 electrons, only alpha emission (labelled “a + recoils”) and spallation reactions (labelled “p + 12C + 160”) can deposit sufficient charge to cause upset. For device sensitivities down to 3 x 104 electrons, elastic recoils and p + recoils dominate. At sensitivities below about 3 x 104 electrons, direct 105 104 103 a u

2F w

RWOI18

102 10’ 10° 10-’ 10-2 [ 10’4

p+’zc+ a +

16

o

/

recoils \

,p + recoils —

— Oirect Ionization

105

10°

Ocvice Sensitivity

107

108

(#electrons)

43: The estimated error rate for a 64-Mbit memory system as a function of device sensitivity, For devices with sensitivities above 1(Yelectrons, alpha emission and spallation reactions can deposit stilcient charge to cause upset. At lower sensitivities, recoils and finally direct ionization becomes significant.. After E. L. Petersen, [94]. Figure

III-41

ionization by protons will dominate the error rates. Recall that at VHSIC levels of integration only 106 electrons represent information on a circuit node, and only a fraction of this charge must be deposited by a strike to cause upset. We therefore expect that at VHSIC levels of integration, parts will be much more sensitive to proton-induced upset. This will make advanced systems more susceptible to SEU in low-earth orbit. 4.1.3 Spacecraft

Shielding

The penetration range of cosmic rays depends on their energy. Therefore, the walls of a spacecraft attenuate the particle flux experienced by a part inside the spacecraft, The degree to which a spectrum is affected by shielding depends on the hardness of the spectrum. In Figure 44a we show the change in the worst-case solar flare environment of Figure 38 as a function of spacecraft thickness. As skin thickness increases from 0.17 g/cm2 (25 roils) to 10.8 g/cm2 (1.57 inches) the intensity of the spectrum is reduced by three orders of magnitude above 1 MeV-cm2/mg, and by five orders of magnitude above 30 MeV-cm2/mg, The effect of spacecraft thickness on the galactic cosmic ray flux is shown in Figure 44b. Here we see that there is only a small difference in the LET spectra for aluminum thicknesses in the 2 to 10 g/cm2 range (300 roils to 1.45 inch thicknesses). Only when the spacecraft walls are on the order of 50 to 100 g./cm2 thickness is appreciable shielding of this spectrum realized. Since spacecraft walls are normally about 100-250 roils thick (0.7 to 1.7 g/cm2), they can attenuate the lower-energy nuclei from a solar flare, but have very little effect on the high-energy particles of the galactic cosmic ray spectrum. Additional shielding may prove effective against soft components of the solar flare environment, but is relatively ineffective in reducing the galactic cosmic ray spectrum.

,0-3

,0-2

,0-1 LET

,00

,.l

, Oz

,03

,0-3

,0-2

,.-1

,00

,.l

,02

,03

(MeV–cm’/mg) LET (MeV–cm2/mg)

a)

b)

Figure 44: Integral LET spectra as a function of spacecraft wall thickness for a) a large solar flare, andb) the galactic cosmic ray environment. Note that while 10.8 glcm2 of Al reduces the solar flare spectrum by several orders of magnitude, the galactic cosmic ray spectrum is only

The shielding experienced by ICS within a spacecraft varies widely with position. Those near the inner surface will experience the least shielding and are therefore exposed to the greatest proton flux. The effect of various amounts of shielding on the energy spectra for trapped protons in the radiation belts is shown in Figure 45, Note that the y-axis is a linear scale in this figure. Protons with energies of 50 MeV can pass through 0.5 inch aluminum with only 60%

III -42

attenuation. At energies above 100 MeV the proton flux is reduced by about 50% with the typical shielding found in a spacecraft (shown as the heavy line in the figure) compared to the case with no shielding. Above 200 MeV, only a 4( Y?% decrease is observed. With 3 inches of shielding, the spectrum above 100 MeV is reduced an additional factor of two, while above 200 MeV the spectrum is attenuated by only 20% more than with typical shielding. Shielding, therefore, is not effective in attenuating the high energy protons, and other techniques, such as error detection and correction, and design hardening must be used.

h\ o

50

Circular

Orbit

1111 km

,0.0

63” inclinatlan

100 Proton

150

200

Energy

(MN)

250

300

350

Figure 45: The modified proton flux after passing through various thiclmesses of aluminum for a 600 nmi circular orbit as a function of proton energy. The light curves are calculated for concentric shells of increasing thickness, while the heavy curve is the calculated flux for the shielding distribution of a component in a typical location within a spacecraft. After E. L. Petersen, [94],

4.2 Basics of Error Rate algorithms The reader will, by now, appreciate that the particle envtionment which a device experiences is a complex function of spacecraft orbit, shielding, and solar activity. Fortunately, computer models have been developed over the years to perfomn single-event error rate (SER) calculations. These take into account the dependence of particle flux on orbit, shielding within the spacecraft device sensitivity, and geomeq of the sensitive volume, This section describes the basic equations and calculations performed by error rate codes. The concept of critical charge, Q, is often used to describe SEU sensitivity, where critical charge is defined as the minimum deposited charge necessary to upset a circuit. For dynamic circuits this concept is easily applied, because the deposited charge negates the stored charge at a node. However, in the case of CMOS latches and memory cells, critical charge is difficult to define since these circuits are sensitive to the rate at which charge is collected, and charge collection is modulated by the circuit response. Restated, Q in dynamic circuits is fundamentally related to the charge collection process, while in CMOS latches Q depends on the circuit response. Nevertheless, in SEU tests of CMOS latches there is a clear threshold for upset. For error rate calculations, we may then consider Q as a parameter relating threshold LET to pathlength, as discussed below, The error rate for a device is a function of the critical charge of the device, Q, the geometry of the sensitive volume (which is a function of design), and the environment to which the part

III -43

will be exposed. The minimum energy that must be deposited by a heavy-ion upset is proportional to Q and is given by the relation Ec=~x

strike to cause

(7)

Qc=22.5Qc

where q is the electronic charge, ~ is in pC, ECin MeV, and 3.6 eV is the energy required to create an electron-hole pair in silicon, Any particle that deposits energy greater than ECcan cause an upset. The energy deposited by a heavy ion depends on its pathlength in the sensitive volume and its LET, and is given by (8) AE = pLp

where L is the LET of a particle, p is its pathlength through the sensitive volume, and p is the density of silicon. Any combination of LET and pathlength that deposits energy greater than E= can result in upset. Therefore, the upset rate, NE, is the integral of the LET spectrum and the distribution of all possible paths through the sensitive volume, F(p), and is given by NE

(9)

=APJ;wrtin@@M

where AP is the average projected area of the sensitive volume, @is the integral LET spectrum, and L~inis, from (7) and (8) 22.5 QC (lo) Lfi = Pp An example distribution of pathlengths through a rectangular parallelpiped is shown in Figure 46. Note that the distribution is skewed toward the shorter pathlengths, with singularities due to the corners of the sensitive volume. The error rate determined by (9) is equivalent to determining the area under a given LET spectrum above the threshold LET. This is illustrated in Figure 47 for Adams’ 10% worst-case spectrum, As pathlength increases, more of the LET spectrum can cause upset. At normal incidence, L~ti is greatest since the pathlength, p, is shortest. For all other pathlengths L~ti is smaller, and a larger portion of the LET spectrum contributes to upset.

10°

w=

4pm

10-’

H = 0.6 pm 10-2

10-3

10-4

10-5 o

2

4

6 8 p (Am)

10

12

14

Figure 46: The differential pathlength distribution for a rectangular parallelpiped. The singularities are a resuh of comers in the volume. After J. C. Pickel, [9].

III-44

ERRORCROSS SECTION

INTEGRAL

depends

on

Q= and pathlength \ .1,

–20 L , 0–3

,0-2

)

“J ,.-1

1 Oa

,.l

,02

10’

LET (MeV–cm2/mg)

Figure 47: lle integral of measme.d cross-section against enviromnent spectrum is illustrated. Only those pmticles with sufficient LET and pathlength through the sensitive volume to deposit more charge than Q=carI cause soft- mors. The error rate is the area under the integral LET spectrum to the right of Lti for all possible combinations of Q. and patldength.

computer codes have been written to calculate the pathlength distribution and perform the integration over a user-specified environmental spectrum given a description of the sensitive volume and the critical charge. CRUP and CREME are two such codes written by groups at NRL [95,96]. Both allow selection of environments from solar minimum to a large solar flare. CREME is more powerful in that the effects of shielding are included, but it does not Several

take into account funneling. Also, orbital path can be specified, including inclination, altitude, apogee and perigee for elliptic orbits. A newer code, SPACERAD [97], is now available for use on personal computers. This code is similar to CREME, and includes the effects of funneling on the pathlength calculation. Results horn this code have been extensively compared with CREME [98]. For low upset threshold parts, a quick estimate of error rate can be made using the Petersen approximation [99]. This technique approximates the differential LET spectrum shown in Figure 37 by a power law fit, ignoring the peaks due to different ion species. This estimates the error rate in Adams’ 10% worse-case environment to be

NE=5X10–10X~

(11) th

where NEis the number of upsets per bit-day, cr,,tis expressed in units of pmz, and Lti is in units of pC/~m. This approximation is fair up to thresholds of 30 MeV-cm2/mg. Above this value a power law dependence is a poor approximation to the environment (see Figure 37), Using this relationship for parts with upset thresholds above 30 MeV-cm2/mg can cause the estimated error rate to be several orders of magnitude too high. For conservative designs where a very low error rate is required, a speciilcation is often written that allows no upset to Iqpton ions (amu=84) at any angle. As there are very few particles with mass heavier than iron (amu=56) in the cosmic ray spectrum (see Figure 34), there is a very low probability of a strike with a higher-LET particle. Since krypton has a higher LET than iron, 40 vs. 28 MeV-cm2/mg, respectively, this is a conservative rule of thumb.

III-45

Alternatively, if the threshold LET for a device is above 100 MeV-cm2/mg (krypton at 66.5° angles), the SER will be extremely low since the cosmic ray flux is essentially zero above 100 MeV-cm2/mg (see Figure 38). Only high-angle strikes with long pathlengths through the sensitive volume will deposit sufficient charge to cause upset. 4.3 SER from Protons Error rate predictions for proton-induced upset are based on nuclear reaction calculations to quantify the probabilities of elastic and inelastic scattering of the incident protons as a function of energy. From these probabilities, the energy deposition in a device sensitive volume can be calculated. Then, knowing the device sensitivity and the proton spectrum in space, an error rate estimate can be made. This rather complicated approach has been greatly simplified by the Bendel one- and two-parameter models [100- 102] for proton induced upset. These semi-empirical models provide a fit to measured emor cross-section vs. proton energy using A and B parameters and eliminate the need for a detailed calculation of the nuclear reaction probabilities, Error rates are calculated by folding this analytic equation with the internal proton flux for a given orbit and spacecraft shielding. The two-parameter

Bendel equation [102] is given by

[1{

X = ~

14 l-exp(-0.18Y1/2)]4

(12)

where X is in units of 10-12upsets per proton/cm2 per bit and y_

~~

()

-—

1/2

(E -A) (13) A E is the incident proton energy. E, A, and B are in units of MeV. The A parameter correlates with the apparent upset threshold, while the ratio (B/A)14 is associated with the “limiting” or saturation cross section observed at high energies. The one parameter model has B fixed at a value of 24. For small geometry devices, the two-parameter model gives a better fit to the measured error cross-sections. 4.4 Errors in predicting SER There are four factors which lead to uncertainty in the error rate calculations as outlined in section 4.2 above. These factors are: 1) the non-abrupt threshold for upset, 2) ion species dependence which do not follow the effective LET approximation, 3) uncertainties in charge collection depth, and 4) uncertainties in the space environment. This section discusses each of these factors. A special committee has been formed within the SEP community to address these issues, A paper summarizing their findings will be presented in the Single Event Phenomena Seesion of this summer’s conference. As noted in Figure 5, measured cross section curves usually have a non-abrupt threshold. However, the single-event-upset error rate (SER) calculations described above assume a single threshold for upset for all portions of an IC. This leads to an overly conservative estimate of

III-46

error rate, since it assumes that all elements have the lowest threshold observed in the

experiments. One adjustment that is normally made to account for this effect is to define threshold for upset as the LET that is 25% of the saturation cross section, SER predictions compared to in-flight data have shown that this approach can be conservative by more than a factor of two. Recently, it has been proposed that this correction be adjusted to the 33% point [103]. Because these are simply empirical fits, it is probable that this approach is device and environment dependent. A more rigorous approach is to calculate an integral of error rates over the measured cross section curve, according to equation (9). Shoga et al. [104] found that this approach agreed to within 20% of measured upset rates for two Leasat satellites using ground-based upset measurements and particle environment measurements of the same time period from the IMP-8 satellite. Unfortunately, error rate codes that are now available do not integrate the cross section curve over the LET spectrum. One can, however, perform a manual integration by breaking the measured cross section curve into finite intervals, and then using a single threshold and a single cross section calculate an error rate for each interval. The predicted error rate is then the sum of error rates for each interval. The discussion with Figures 37 and 38 noted that the differential and integral LET spectra were calculated assuming that only particle LET was important, and that the ion species depositing charge could be neglected. In some cases, it has been demonstrated that differing amounts of charge are collected from ions with the same effective LET [14,105]. This effect leads to different values of error cross section for two ions with overlapping effective LET. As shown in Figure 48, in regions where the effective LET overlaps between ions of xenon and krypton, and krypton and copper, different values of error cross section are measured. This effect has been explained in terms of the effect of ion mass and velocity on the electron-hole plasma density generated by the passing ion [14,105,106]. As ion mass or velocity increases, the radial extent of energy deposition increases, resulting in a decreased pl&ma density. This may result in a lower recombination rate and therefore a higher charge collection rate with increasing ion mass or velocity. At present an adequate understanding of this effect has not been developed. ANSI standard F1 192-88 suggests that when an ion species dependence is observed, the data should be referenced to Z (atomic number), ion energy, and angle of incidence. For purposes of error rate calculations, data taken at normal incidence provides a worst-case error rate estimate. Another complication arises when using measured upset data to calculate error rates in the intended environment. As noted in section 4,3, the charge collection depth is a variable that enters into the error rate calculation. From upset threshold, one cannot readily determine the charge collection depth. Charge collection depth is often estimated based on knowledge of the vertical structure of the IC. However, this approach can sometimes be in error by at least a factor of two [34], leading to a large error in estimating the critical charge for upset, Recently, McNulty et al. [107] have developed a technique to directly measure the charge collection volume. The DUT is treated as a silicon detector in this approach, and the charge collected following a heavy-ion strike of known energy is measured using a charge-sensitive preamp and pulse-height ampl.i.tier. When coupled with a multi-channel analyzer, a charge

III -47

10-’ ,

o

A

(

------r 573–MeV

\,,,..,”

d Xe

,,

283–MeV

Cu

/

10-6 0

20

40

80 LET

80

100

120

140

(MeV–cm2/mg)

Figure 48: Dependence of error cross section on ion species. This effect maybe due to geometry of the collection volume, but is not well understood at present. (After Ref [46])

collection spectrum can be plotted as shown in Figure 49. In this plot, the x-axis is equivalent to charge collection or energy. The position of the peak on the x-axis is proportional to charge collection depth. This technique depends on an accurate calculation of the energy lost in overlying layers. 200

180 -

RMOS

SRAM

IDT6116V 160

140

-

1

“,

..%f“

+’..’)!: .... .. ..;.

.,-. .. ..

60 1

:. -.a.

40 20 -

3.00 Charge

Collected

“.

.,.. % ~, .. 4.00

(MeV)

Figure 49: Experiment setup and charge spectrum derived from an IC. The energy position of the peak is proportional to charge collection depth. After McNulty et al. [107].

As feature sizes shrink and devices are more closely packed, a single heavy ion may cause multiple upsets [33, 108]. This is caused by the plasma track from a single ion strike spreading to several sensitive regions as illustrated in Figure 50. This mechanism may not be detected during characterization tests at the relatively low energies and angles of incidence in an accelerator. When this mechanism is present, the error rate of a chip in actual use will be underestimated using any of the existing error rate codes. Accurate 3-dimensional transport codes are necessary to predict the relative magnitude of this effect, although some estimate could be made using a 2-dimensional code with cylindrical coordinates. Tests with very high-energy accelerators, such as the LBL Bevalac, are required to determine the cross section due to multiple upsets since they permit low angle strikes.

III -48

/

tont+lt

center XEIKX

●

Figure 50: A single sirike resulting in multiple-bit upsets. As devices are packed more closely together, the probability of this mechanism occurring increases. This can result in a large underestimate of error rate in the use-environment, After Zoutendyk et al. [33],

Finally, there are uncertainties in error rate calculations due to uncefiainties in the modeling of the space environment [109]. SER codes like CREME assume a sinusoidal variation in solar activity with 22-year periodicity. Measured solar modulation is more complicated than this, however. Depending on the solar cycle one begins with, therefore, the predicted solar activity during a given time period can be signi.tlcantly out of phase with the actual solar activity. Also, the heavy-ion spectra resulting from a solar flare has not been well-characterized. CREME uses the proton spectra as a model for the heavy-ion spectra. If the heavy-ion spectra are significantly softer than would be predicted from the proton spectra, this would explain the apparent lack of upsets due to heavy-ion induced upsets during solar flares, More accurate data on flare heavy ions has been obtained since 1988, but this data has not yet been incorporated into the error rate codes.

5.0 CONCLUSIONS As the reader wiU now appreciate, the field of SEP encompasses many diverse effects. We have attempted to give the reader and introduction into this field by describing the basics of the interaction of heavy-ions with matter and how charge is generated and collected by devices. We defined the units of measure and then described a typical test set-up for SEP experiments. Detailed explanations of the several effects observed in semiconductor devices and integrated circuits provided insight into how experiments should be conducted, and the interaction of parameter variations such as bias voltage and temperature were described, A section summarizing some accelerator facilities used for SEP testing will be useful to researchers in selecting the appropriate ion species and energy for a particular experiment, We also described alternative techniques that are being used in the laboratory in lieu of expensive accelerators, and the drawbacks to these techniques. Finally, we briefly discuss the natural space environment. We outlined the basics of error rate calculations, and uncertainties in predicting error rate in the use environment. We hope this session has given the reader a deeper appreciation for the intricacies of SEP measurement, analysis, and implications.

I-D-49

Acknowledgments I wish toexpress mygratitude to Rocky Kogaof Aerospace Corp, Jim Pickelof S-Cubed, and Peter McNulty of Clemson University for their careful review of this manuscript. Their insight and comments have been invaluable in ensuring the accuracy of this work. I also wish to thank Peter Winokur of Sandia National Laboratories for his support and encouragement during this undertaking. As always, his ideas and sense of the needs of the radiation effects community have played a major role in guiding this work.

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Displacement Damage: Mechanisms and Measurements

Geoffrey P. Summers Naval Research Laboratory Displacement Damage Effects Section Washington, D.C.

This work was supported in part by the O@ce of Naval Research

DISPLACEMENT DAMAGE: MECHANISMS AND MEASUREMENTS Geoffrey P. Summers Naval Research Laboratory Displacement damage Effects Section

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u

10.0 11.0 12.0 13.0

Introduction Background Particle-Induced Displaced Atoms Defect Identification 4,1 Electron Spin Resonance Deep Level Transient Spectroscopy 4.2 Effect of Defects on Electrical Propernes Nonionizing Energy Loss and Damage Correlation Nonionizing Energy Loss The Effect of Electric Fields 7.1 Displacement Damage Fluctuations and Fundamental Limits The Prediction of End-of-Life Response Summary and Conclusions Acknowledgements References Selected Further Reading 1.0 Introduction

Microelectronic devices in the natural space environment are subjected to constant bombardment by electrons, protons and heavy ions. Under some conditions neutrons and high intensity x-rays may also be a threat. In these notes, the effect of electrons and protons will be the major focus of attention, although mcasionally neutron effects will be briefly discussed. The interaction of a high energy charged particle with a solid causes both ionizing and nonionizing effects, both of which can be transient or permanent in nature, However, most of the kinetic energy of the incident panicle is lost to ionization and typically only about one thousandth of the energy is channeled into nonionizing events. Most of this remaining energy loss causes atoms to be displaced from their normal lattice sites, which can seriously degrade the electrical characteristics of semiconducting materials and devices. The main subject of these notes concerns the correlation of the deleterious effect of different incident particles on the electrical characteristics of semiconductor materials and devices, so that meaningful predictions can be made about device degradation in complex radiation environments. Over the years, methods have been developed to minimize the effect of displaced atoms on the performance of microelectronic devices. In order to implement these radiation hardening techniques, the basic mechanisms of particle-induced displacements need to be fully understood. Since the fmt production of semiconductor devices more than forty years ago, much progress has been made in these studies. However, any research area goes through cycles of interest and there was a great flurry of activity in displacement damage studies in the 1960s which dwindled noticeably in the subsequent decades, even though much pioneering work continued to be performed during this time. Recently interest has been rekindled by the discovery of the sensitivity

Iv-

1

of sensors and other modem devices to displacement damage effects. However, as device feature size decreases and the number of devices on a chip increases, it becomes progressively more difficult to circumvent radiation-induced device changes, even though to some extent, the very smallness of modem devices often improves their radiation tolerance. Ultimately, however, there are fundamental physical limits which prevent radiation hardening techniques from ma-king further progress. There is some evidence that these limits are now being approachixl in advanced devices, as will be shown below. One of the most interesting recent developments in experimental displacement damage studies has been the measurement of the stochastic nature of defect cascades. This development was brought about by the production of highly sensitive charge transfer devices with their multitude of nearly identical pixels. These measurements have made it possible to apply microdosimetry models to displacement effects for the fust time, thereby providing powerful predictive methods for the radiation response of focal plane arrays and other imagers used in radiation environments. These microdosimetric models have been usai successfully for many years to describe the effects of ionization, so this recent development has introduced an interesting computational parallel in the study of displacement effects. These notes will be limited mainly to what has been learned about displacement damage in devices in the last dozen years, especially the importance of nonionizing energy loss (NIEL) as a parameter for correlating the effect of electrons and protons. It will be seen that NIEL plays the same role in characterizing displacement damage that linear energy transfer (LET) plays in ionization. Those readers interested in studying the subject more generally are encouraged to review the texts and journal articles presented in the extensive bibliography assembled by Joe Srour for a short course given at the 1988 NSREC[ 1]. The bibliography at the end of the present notes contains some texts the I have found useful. It will be shown Mow how NIEL can be used to make quantitative predictions for pardcleinduced device degradation in any known radiation environment from only one or two ground test measurements, A schematic view of the overall problem is shown in Fig. 1. At the top of Fig, 1, a particular device such as a transistor, is subjected to a complex radiation environment in a particular orbit which causes its performance, e.g., its common emitter dc gain, g-radually to degrade. At the bottom of Fig. 1, a sequence of measurements and calculations is made to predict the expected degradation. An important question is obviously to determine what the minimum information necessary to make such a prediction really is, especially for a program manager confronted with cost containment issues ! It is this question that we hope to address as we go along. These notes are arranged as follows. First, in Section 2 a somewhat cursory introduction is given to the basic nuclear and solid state physics that is needed to understand the processes leading to particle-induced displaced atoms in semiconductors. The books by Kittel and Segre listed in the bibliography y can be used by those needing more information on these topics. In Section 3, a more detailed description of the formation of defects and defect cascades is given. It often seems a mystery how information about the structure of a particular defect and its affect on so in Sation 4 two experimental techniques the properties of the perfect lattice is actually obtied,

used to study defects are described. These techniques are electron spin resonance (ESR) which was developed in the 1940s after the discovery of radar made sources of microwaves readily available, and deep level transient spectroscopy (DLTS) which was developed in the 1970s. In Section 5, the effect of defects on the electrical properties of semiconductors is described. The accurate prediction of device performance in space from ground test data depends on an ability to correlate the damage effect of different particles. The subject of damage correlation is introduced in Section 6 using some recent experimental results. The parameter central to the damage correlation is nonionizing energy loss (NIEL). Details of the calculation of NIEL are given in Section 7. As device dimensions shrink, certain issue that were previously unimportant take on a new significance. One such issue is the effect of elechic fields on defect properties. A brief discussion Iv-

2

SPACE k

Radiation Environment

Degradation and Fnilure

DOviceor circuit

PrcdicWdSyatsrn Performnoce

!2

GROUND TESTS

P~odictedDavice Perforntnnce

Basic Mechanhr

Mono-energetic Particle (proton,

‘

electron)

Defect Introduction

Correlation +

(vagancim, etc.) \

Predicted Environment

!

Figure 1. The problem confronting the space engineer. In space, a device or circuit is subjected to a complex radiation environment, which leads to device degradation. The engineer has to predict this degradation from limited ground tests performed at accelerators using mono-energetic particle irradiations. of this subject is given in Section 7.1. NIEL is a parameter for predicting the mean or average

damage effect of a particular particle irradiation, However, the damage process is intrinsically a stochastic process in which a particle interaction can lead to many possible defect cascade structures, i.e., there are fluctuations in the possible stable damage produced by a single particle hit. Recently it has been possible to measure such fluctuations experimentally. The application of microdosimetry theory has allowed rapid progress to be made in describing these fluctuations quantitatively for the first time, as shown in Section 8. Section 8 also briefly introduces the subject of fundamental limits to hardening procedures. In Section 9, a method is given for predicting end-of-life performance for a device using ground test data. An actual example is described in which the expected performance of InP and Si solar cells in a series of orbits are compared. The main subjects covered in these notes are then summarized in Section 10. Finally, it should be noted that displacement damage effects continues to be a fertile area for new research, especially in areas such as low dimensional, superconducting and photonic devices made from complex new materials. Also there is much basic research still to be done applying new concepts such as fractals and percolation theory to the damage process.

2.0

Background

A quantitative understanding of the mechanisms of displacement damage produced in materials by an incident high energy charged particle requires some knowledge of both solid state and nuclear physics. It is useful in focusing on the subject to review briefly some of the basic ideas involved. Most microelectronic devices are made ii-em crystalline semicmducting materials such as silicon, germanium, gallium arsenide, and iridium phosphide, often in combination. The

Iv- 3

atoms in these crystals are arranged in a regular lattice, which has the so called diumond structure shown in Fig.2 for most of the materials of interest. In this structure, each atom is surrounded by four nearest neighbors, which for compounds such as GaAs and InP consist of the other constituent atoms. It can be appreciated that compound materials will have many more kinds of possible defects than will be found in monatomic materials such as Si.

structure of many semiconducting materials used in Each atom is surrounded by four atoms arranged at the In 111-V materials corners of a tetrahedron. such as GaAs, each atom is surrounded by four atoms of the other atomic constituent.

Figure

2.

microelectronic

The

crystal devices.

The atoms in these crystals consist of a nucleus about 10-lqcm in diameter made up of protons and neutrons. The nucleus is surrounded by elec~ons in “orbits” with diameters about 10-8 cm in diameter. An elecmon has a mass about 1/1840 that of a neutron or a proton. All charged particles interact via the long-range Coulomb force, which for similarly charged particles is repulsive, so that the protons in the nucleus are actually held together by the short-range nuclear force. When an incident charged particle approaches a nucleus, it experiences only the Coulomb force until it is very close to the nucleus. Only protons with energy >-5 MeV can approach a typical nucleus close enough to experience the nuclear force. Because of their much smaller mass, the equivalent energy for electrons is hundreds of MeV. Neutrons only interact with matter through the nuclear force, which essentially means only hard sphere collisions are possible. The interaction between an incident particle and the nucleus can be either elastic or inelastic. In an elastic interaction the kinetic energy is conserved. In inelastic interactions, some of the incident energy is used in producing nuclear reactions and the final kinetic energy of the system may be changed. These obsemations mean that the interaction between electrons in the natural space environment and satellite electronics will be only elastic Coulomb in nature, whereas naturally occurring protons will be able to cause both elastic Coulomb and nuclear interactions. For protons with energies X8 MeV, nuclear inelastic interactions are possible. The probability of a given type of interaction occurring is given by a parameter called a cross section, so the cross section for nuclear inelastic interaction is small for low energy protons and large for high energy protons. We will return to the subject of particle interactions in the discussion of nonionizing energy loss in Section 6. Iv-

4

The electrons in an atom are attracted to the positively charged nucleus by the Coulomb force which keeps them located in “orbits”. However, only certain orbits and therefore electron energies are possible, and these can be calculated by quantum mechanics. In free atoms, the energy levels are well separated and narrow. However, when atoms condense in a crystalline solid, the narrow atomic energy levels are broadened into energy bands as shown in Fig. 3. Because of the exclusion principle, not all the available electrons can go into the lowest energy band, and typically many bands ae filled. In a semiconductor at very low temperature, all the bands up to a particular energy are completely ffled. The top most ftied band is called the valence band. It is separated from the next empty band, called the conduction band, by an energy gap which is typically about an eV. This gap is often called the band gap or forbidden gap, because in the perfect crystal no electron can have an energy in this range. Many of the properties of the semiconductor are determined by the magnitude and details of this band gap. In a laser diode, for example, the band gap determines the wavelength of the emitted light.

K(L)

[060]

[1001

When

atoms

condense

are

broadened

[Ill]

Figure

3.

into a solid, the narrow electron energy levels These bands are shown as the energy differs from the parabolic versus the wavevector, g. In crystals, this dependence relationship for a free electron because of the effect of the translational symmetry. On the left is shown the band structure of Si with the familiar indirect band gap between the highest filled valence band and the lowest unfilled conduction band. On the right is shown how lattice defects or specially added dopants lead to localized donor or acceptor levels close to the conduction of valence band. After Hogarth [2].

of the atom

into

bands.

In an undoped or inrnnsic semiconductor, electrical conduction is only possible when an electron is thermally excited across the band gap. The hole left in the valence band is also able to contribute to conduction. The electrical resistivity is determined by the concentration of carriers, Nd, and by their ability to move through the lattice, i.e., their mobility, ~. Because their motion is controlled by the lattice, electrons (and holes) in semiconductors often move as though their mass

Iv-

5

were different from that of a free electron, They are said to have an efiective mass. The value of the effective mass and the scattering of the carriers by imperfections in the lattice limit the mobility of the carriers. The imperfections can be impurity atoms, lattice defects, or simply lattice vibrations (or phonons). Semiconductors used in devices are usually doped with specially chosen impurities which provide additional charge carriers to improve the conductivity. These impurities have many other effects on the properties of semiconductors, however, including having a strong effect on the rate of radiation-induced defect formation and the kind of stable defects that exist. In the band picture of the semiconductor, the dopant impurity atoms introduce a localized level into the band gap at a position close to either the conduction band for donors or close to the valence band for acceptors, as shown in Fig. 3, Donors produce n-type material and acceptors ptype. Whenever imegularities are produced in the lattice, localized energy levels can be produced in the band gap. Of particular importance are the levels introduced by defects produced as a result of radiation-induced displacements. These levels can strongly affect the electrical characteristics of the semiconductor such as the minority carrier lifetime, z, the carier concentration, Nd, and the mobility, ~.

3.0 Particle-Induced

Displaced

Atoms

In order for an incident high energy particle to produce a permanently displaced atom, enough kinetic energy must be imparted to break the chemical bonds holding it in place and to move it sufficiently far away from its original position that it does not fall back. The required energy is called the displacement threshold energy. Seitz[3] estimated that an energy equal to about four times the sublimation energy would be needed, i.e., the threshold energy would be about 25 eV for a typical material. Actual values range from a few eV up to tens of eV, although for most semiconductors of interest the range is from about 6-30 eV. Corbett and Bourgoin[4] noticed that measured values of the threshold energy appeared to scale following a power law with the reciprocal of the lattice constant. Their original figure was recently revised by Barry et al. [5], as shown in Fig.4. The reason for the scatter in the measured values of threshold energies such as those shown in Fig. 4 is an experimental one. In a typical experiment, the defect introduction rate as determined from minority carrier lifetime measurements, for example, is measured as a function of incident electron energy near the threshold. This rate will of course be small and will fall very rapidly as the threshold energy is approached. Fig,5 shows recent data of this type for GaAs[6]. The maximum kinetic energy, T~, is transferred when the incident particle makes a head-on collision with the target atom. These collisions are the ones from which the threshold is determined. For relativistic elecuon energies, T~ is given by: T~ = 2E(rn/M)(2+E/mcz)/[( l+rn/M)Z(Mcz)+2E]

(1)

where M is the mass of the target atom, m is the mass of the incident electron, c is the velocity of light and E is the incident electron energy. In order to obtain the threshold energy from such measurements, it is necessary to extrapolate to zero defect introduction rate. Alternatively the data can be fitted to a suitable relationship for the energy dependence of the interaction cross section, such as the Mckinley-Feshbach[7] model, from which the threshold energy is then extracted as a parameter. Note that at these low electron energies, essentially all the recoil energy goes into displacements. This is not the case at higher recoil energies, as we discuss below. For the example of GaAs shown in Fig. 5, the agreement with a calculation of the cross section is not very close, and the extrapolation method was used, Fig,6. Whatever method is used, the threshold energy obtained will depend on the relative orientation of the target crystal and the direction of the incident electrons because it is easier for the recoiling target atom to escape from its lattice site IV-

6

along more open crystallographic directions. energetic, the effective threshold energy will an important parameter in calculating the neuttrons) and then in estimating the number Pease[9].

1

As the incident electrons become increasing more be an angle-average value. The threshold energy is NIEL for elechons, (although not for protons or of defects produced, using a model such as Kinchin-

I

I

I

I

50

/

t

4a -

[RoI.8) 30 -

s =20 Ul”

am

15 -

10 9 87-

cd\ Cdla ~

1

,

,

,

0.15

I

I 02

0.25

0.3

Figure 4. An empirical relationship between reciprocal lattice constant and displacement threshold energy for semicondcutors reported originally by Corbett and Bourgoin[4]. Shown here is a revised version of the original figure given by Barry et.al.[5]. However, several of the values on the figure are questionable, including that for Si.

Once sufficient energy is imparted, the primary knock-on atom (PKA) is displaced from its site, moves through the lattice and comes to rest in a normally unoccupied position. The site left behind is called a vacancy and the displaced atom is called an interstitial. Together the vacancy and interstitial are called a Frenkel pair. If the PK.A has more than about twice the threshold energy, it can displace a second atom &fore coming to rest- Also, if complex defects such as divacancies are formed, a larger threshold energy can be found than for a single displacement, as shown in the data of Watkins and Corbett for Si, Fig.7. In this case, a stable divacancy requires two silicon atoms to be displaced simultaneously and the measured effective threshold energy of -40eV is approximately twice the value for a single vacancy, 21eV. For higher energy recoils, a defect cascade is produced. The disposition of defects in these cascades, and the effect of the defects in these cascades on the electrical properties of semiconductor devices, have been the focus of much discussion and are still under investigation. Iv-

7

Elec(ron microscopy 11] indicates that the size of any defect clusters produced is no larger than -30~ even by high energy recoils produced by fission neutrons. However, much of what is known about defect cascades comes from Monte Carlo sinmlations[12- 14]. These simulations can be performed using computer codes such as MARLOWE and TRIPOS, in which realistic L

,!!

“j o B

3 3 k

~ ,,@ -y -$-

01

~~1 oI ti?crd

I

1

0

I

0 &%?

,

I

L2 (!!)

Damage factor for minority carrier lifetime reduction in GaAs due to Figure 5. electron irradiation just above the displacement threshold energy. The fall-off is due to a rapid decrease in the interaction cross section with decreasing electron energy, which gives a way of determining the displacement threshold energy. After Barry et aL[6].

A linear plot of the data in Fig.5 showing how the experimental data Figure 6. is used to locate the displacement threshold. Curve B is a calculation of the energy dependence of the cross section using the McKinley -Feshbach[7] approximation of the Mott[8] cross section. The disagreement between theory and experiment may be due in part to the relatively poor nature of this approximation for high Z target atoms. After Barry et aL[6]. IV-

8

I

Figure 7. Defect V)+ in Si, showing

I

1

1 111111

I

1 1 11111

I

I

I

I

production rate of the A-center (V-O)- and the divacancy (Vdifferent threshold energies. After Corbett and Watkins[lO].

interatomic potentials and the crystal structure of the target lattice are included. Such simulations ~3] for Si predict that ~ecoils with E~e~h < E <--2 keV produce only “free” point defects. For recoils with -2< E <-12 keV, one defect cluster is formed and for recoils with E > -20 keV a tree like defect structure is formed, with terminal clusters at the end of branch-like defect regions containing point defects. Fig. 8 shows the results of Wood et al. [ 13], and Fig. 9 shows the simulated cascade for a 50 keV recoil, which indicates that the overall dimensions of the cascade are less than -100 nm. It is worth noting, however, that 2 keV Si recoils can be produced by incident electrons with energies as low as -4 MeV. The key implication of the Monte Carlo calculations is that the nature of the displacement damage produced by electrons with energies greater than a few MeV, by protons with energies greater than a few keV and by fission neutrons is similar in character and therefore that the degradation produced in the electrical characteristics of devices by these particles is expected also to be the same. Nevertheless, papers were still being published up to the mid 1980s in which this conclusion was being disputed. In particular, enhanced carrier scattering properties were assigned to clusters of defects. Calculating accurately the numlxx of defects, Nd,f, initially produced by a recoiling atom is not simple. However, approximate models have been developed such as that due to Kinchin and Pease[9]. It is frost necessary to calculate the total energy available for producing displacements, Ed~P. This is not simply the recoil energy because as the recoil energy increases a progressively larger fraction of the energy is lost to ionization 15]. We will discuss this energy partitioning process in more detail in Section 7. In the Kinchin-Pease model: Ndef- Edi~p/2E~=h

Iv-

9

(2)

Lq N

1~

“FIw”

PKARBWs

htub

OM m-

I

I

Ef

PKAWkrs

4

I

Eu

su&.asaW

— T. PKAEntf#

?

P

h

i -1-2 hv

- U-29 Mv

Figure 8. The relationship between defect cascade structure in Si and the primary knock-on atom (PKA) energy, predicted by Monte Carlo calculations. For recoil energies below -2 KeV, only point defects are produced. For higher However, the overall recoil energies, subcascade clusters of defects are formed. nature of the damage is essentially the same for all recoil energies >*2 keV. After Wood et aL[13].

The Kinchin-Pease model is only approximate and is not expected to be ve~ accurate when Ed~P is close to E~=h or very large. The constant in the denominator of Eq.(2) also has slightly different values when different approximations me used. As an example, the maximum Si recoil energy produced by a 100 MeV incident electron is about 0.77 MeV. Of this energy only about 0.1 MeV is channeled into displacements. Therefore, about 2,500 Frenkel pairs are expected to be initially produced in a tree-like cascade as shown in Fig. 9. However, about 90% of the Frenkel pairs recombine within about a minute at room temperature during a so-called “short annealing stage”, Isolated vacancies and interstitial atoms in Si are mobile at cryogenic temperatures, so the remaining “permanent” damage at room temperature consists of stable defects such as divancancies, and various arrangements of vacancies and impurities. Defect structures associated with vacancies are electrically active in Si and have been studied for many years. Only recently

have interstitial defect structures been studied, These appear to be largely inactive electrically. Defect annealing in Si has been extensively studied in the past. Some of the published results are somewhat difficult to reconcile with what is now known about defect cascade structure. For example, “short” defect annealing times measured using the recovery of transistor gain following pulsed particle irradiation, has been reported to occur much faster for -1.5 MeV electrons than for either fission or for 14 MeV (fusion) neutrons[ 16,17]. Since most of the recovery in both cases takes place in a few seconds due to the motion of isolated vacancies, it seems likely that defects in the “cluster regions”, which are not formed for low energy electrons, must also contain a long annealing component. However, the permanent defects in both cases are generally the same, so the physical reason for the different annealing results is not clear, although there is evidence that the fraction of defects initially formed that do not immediately recombine, depends on the recoil energy. One aspect that is well established, however, is that defect annealing

Iv- 10

TImlfw

aunm (DaAIL)

50

a n *a

KEV

S1

.-

U : ,~ -

\

r[PnlHu mrsT[R

< + w H n

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rminu TfmlNu WfSTfR

-240

\

-

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0

DISTANCE

Figure overall

9. Defect cascade structure dimensions of the damage.

Izo

Zdo

160

(~)

for a 50 keV Si recoil showing

the predicted

is carrier concentration, i.e., injection level, dependent [17- 19]. In particular, the annealing rate is much higher for high injection levels. If a bipolar mmsistor is irradiated, for example, this means that the recovery is much quicker following a short burst of neunons when the transistor is turned on as opposed to being off. This type of behavior is seen to occur regardless of whether the device is irradiated in the “on” or the “off’ state. A large amount of this kind of data has been obtained in the past. Sander and Gregory[20] give an interesting nomograph for estimating annealing factors for transistors following a burst of neutrons. Srour[ 1] has used this nomograph to show an example for 10 ms after a neunon burst, as shown in Fig. 10. For a transistor that was on during both irradiation and annealing, the annealing factor can k“ seen to be -1.6. The annealing factor here is defined as the ratio of the damage at a given time compared to the permanent damage. However, Fig. 10 shows that if the uansistor is off, the annealing factor is much larger, i.e., -6, indicating that much of the damage still remains after 10 ms. 4.0 Defect

Identification

It was noted in Section 3.0 that the permanent defects produced by different particles incident on Si are generally the same. It might be wondered how this is known, i.e., how are both the atomic shwcture and the positions of the different defect levels in the band gap of semiconductors determined. It is important from the viewpoint of device performance and reliability to be able to associate measured electrical effects with particular defects and to be able to detect the presence of these defects relatively easily. From a device hardening viewpoint, it is essential to be able to correlate radiation-induced device degradation with the introduction of certain defects, and then to try to suppress their formation by changing dopant types, dopant concenh-ations, device operation (e.g., injection or bias levels) and by other techniques. Many optical and electrical techniques are used to characterize the nature of defects in semiconductors. Two particularly important ones will be discussed here. They are electron spin

Iv-

11

AN NE ALINO

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0.7

,Olc

* n

lo’s

: -

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FACTOR

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#’

1,26

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ON, #

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2.s 3.0

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~ 7,0

n

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,.13

:

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s 1-

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a b u ~ A u

z

-%3.

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=

w

w 0.4

2.0

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>

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0.6

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0

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0.8

0.1

10’1

5.

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10-4 L

~

Figure 10. Nomograph for predicting the annealing factor for Si transistors with fission neutron. irradiated The example shown here, for on and off is due to Srour[l] using the method of Sander and Gregory [20]. transistors, resonance (ESR) and deep level transient spectroscopy (DLTS). Both techniques involve some complexity, but both give unique information about defect properties and behavior. 4.1 Electron

Spin

Resonance

ESR is a powerful technique for determining the microscopic, atomic structure of defects in insulators and semiconductors. It is one of the few techniques from which the actual nature of a defect can be determined at the atomic level. Most of what we know about radiation-induced defects in Si, for example, comes from pioneering ESR studies in the 1960s by Watkins and Corbett[21], In Fig. 11 is shown the structure of two of the most important radiation-induced defects as determined by Watkins and Corbett; the A-center and the divacancy. These defects are stable at room temperature and are thought to be responsible for many observed radiation effects. It will now be explained briefly how structures such as those in Fig. 11 are determined using ESR. ESR depends on the absorption of microwave power by unpaired electron spins (paramagnetism) located at the defect site. Beeause of quantum effects, only certain values of the energy of these spins are allowed when a dc magnetic field is applied as shown for example in Fig. 12 for both a one and a three electron system. Microwave power is absorbed when the microwave energy E corresponds to the energy separating these magnetically-split energy levels, i.e.,

E = hvtiC

(3)

The ESR apparatus consists of a source of microwaves such as a klystron, which are directed to the sample in suitable waveguides, Fig. 13, The sample is located in a microwave cavity at a position where the microwave field is strongest, so particular cavity shapes have to be used for unusual experimental situations, where light is required in the cavity, for example. In order to take an ESR spectrum, the external dc magnetic field is swept in either direction. A

Iv- 12

Figure 11. The atomic structure of the divacancy Watkins and Corbett from ESR measurements[21].

and the A-center

determined

by

change in the microwave signal is detected whenever the microwave frequency corresponds to an allowed transition between the split electronic levels. In order to obtain high resolution, the magnet has to be constructed so as to produce a uniform field over the whole sample. Also many kinds of microwave noise reduction techniques are used to maximize the sensitivity of the detection system.

Ho

--a 3~

d -B

—

fiHo

o

-. -P

—

-)gHo

--

----

“B

P

-Jp

Figure 12. A schematic representation of the quantization of the energy levels of the electron magnetic moment in a magnetic field. In electron spin resonance, microwave energy is absorbed and a signal is detected when transitions are induced between these energy levels. These transitions occur when the microwave is Plank’s

frequency, v, corresponds to the energy constant. After Wilmshurst[22].

Iv- 13

separation,

E = hv, where

h

Att~n~tor Phase Shifter Microwave Source Detector

I

I

1A

d

FEE —b

1

1

Cr:O#at Cavity Sample Display h

t Temperature Controller

Figure 13. A block diagram of an ESR spectrometer. of the externally applied magnetic field.

H indicates

the direction

The high resolution of the technique is due to the fact that the unpaired electrons in the defects are constrained by the charge of the neighboring nuclei to assume orbitals that have the same symmehy as the defect itself. The spherical symmetry of a free atom is therefore usually no longer pssible. As a result, the magnitude of the magnetic moment due to the unpaired electrons is effectively different when the direction of the external dc magnetic field varies with respect to the direction of the defect, which is of course fixed inside the sample. In Fig. 14 is shown the ESR spectrum of the divacancy Si with the magnetic field along the cube edge, i.e., the <100> crystallographic direction [23]. Spectra for two charge states of the vacancy are shown, called in

uKHETIC FQD (~

UmETlcFELo(Mlm

Figure 14. ESR spectra of different charge states of the divacancy in Si. The spectrum Iabelled G6 is due to the negatively charged center and the G7 spectrum is due to the positively charged center. The spectra were taken at a temperature of 20.4 K using a microwave frequency of 20 GHz. The magnetic field was along the c1OO> crystallographic direction. After Watkins and Corbett [23]. Iv- 14

the singly negatively charged divacancy in high resistivity n-type Si. The different charge states of the divacancy are detected because of the different location of the Fermi level in n- and p-type Si. To obtain further information, the direction of the dc magnetic field is changed with respect to the sample by rotating the magnet, which causes the lines in the spectrum to move as seen in Fig. 15. It can be seen that when the magnetic field is parallel to certain crystallographic directions, such as <100>, several of the spectral lines coincide, showing that the direction is a high symmetry axis of the defect. Additional information about the structure of the defect comes from small perturbations on the spectral lines due to the hyperfke interaction between the paramagnetic electrons and the central ardor surrounding nuclei.

101 I

?

I

I

q,

I

do

.!, Im q ?-mI

I (u

i

[m] o

I [al]

“[,!1] I I020M40

I

I

1,1I

wfiwmm~ t

!

I

m=ml

Figure 15 The effect of varying the direction of the external magnetic field on the position of the G6 and G7 resonance lines in the ESR spectra of the divacancy in Si. Notice the degeneracy that occurs for particular directions of the field. These plots give information about the orientation of the defect in the crystal lattice and hence the likely atomic structure of the defect. After Watkins and Corbett[23].

The ESR spectrum can be analyzed by constructing a so-called spin Hamiltonian, M, to describe the effective interaction between the magnetic field, H, and the electron spin, S, and the electron spin and the nuclear spins, I, of the neighboring atoms. In the case of the divacancy, the spectrum can be described by the spin Hamiltonian: ~

=

13H.g.S + Z I.A.S

(4)

where g and A are tensors, and S = 1/’2. There is no space here to elaborate on the interpretation of the spectra in terms of Eq. (4) and those interested further should consult the extensive literature. From such analysis, the sytietry of the defect, the nature of the surrounding atoms and hence the defect structure itself can be determined. ESR measurements can be used to obtain annealing rates,

Iv-

15

defect introduction rates, defect transformations and other important parameters. Fig. 16 shows annealing data for vacancies, for example [24]. From similar measurements, the binding energy of the divacancy has ken estimated to be >1.6 eV, while the activation energy for diffusion has been measured as -1.3 eV. ESR measurements are not trivial to make, but commercial instruments are available. Several, more complicated techniques based on ESR, such as electron-nuclear-double-resonance (ENDOR) and optically-detected-magnetic resonance (ODMR) give even more detailed information about defect structure and energy levels. The major disadvantage of ESR is that only pararnagnetic defects can be detected. Furthermore, because of certain complicating factors such as spin-lattice coupling, the measurements often have to be made at cryogenic temperatures. The equipment is therefore often suite elaborate. Nevertheless ES R can obtain information that is uniaue and . essential to our understanding of defect structure and behavior.

1000/7

Figure 16. ESR annealing kinetics 4.2 Deep

Level

m-’)

can be used to study many properties of defects as shown here for the vacancy. After Watkins[24].

Transient

including

Spectroscopy

DLTS is a powerful technique for studying defect and impurity energy levels in the forbidden band gap of semiconducto~s[25]. Unlike ESR, it does not ~ve direct h-formation about defect structure, but it does fives direct information about localized defect enerm levels. It is a transient capaci~nce techni~ue for identifying charge trapping centers in the defiietion region of Schottky diodes or p-n junctions. This capability makes it very useful in radiation studies since many devices contain such structures. For convenience, only p-n junctions will be discussed here. In DLTS, a constant reverse bias is applied across the junction causing the depletion region to widen and thereby decreasing the IV- 16

capacitance. by[26]:

For a one sided, sharp junction, the capacitance

C = [(q%N#)/(vtji

+

is related to the reverse voltage V

v -2kT/@] In

(5)

where q is the electronic charge, es is the dielecrnc constan~ Na is the carrier concentration, vbi is the junction built-in voltage and k is Boltzmann’s constant. As shown in Fig, 17, the junction width increases with reverse bias, causing the capacitance to decrease. Eq. (5) also shows that l/C is propofiional to V, and that the intercept of the plot will give the canier concentration Na

-v

o-

., Ov

B-TYPE

(b)

ZEUO

S141

ldn+w+.+

‘Ii_I-k+ (c) FOR WARO

BIA~

Figure 17. The effect of applied bias on the width of a p-n junction. As the reverse bias is increased the depletion width increases leading to a reduction in This effect is exploited in a DLTS apparatus. the junction capacitance. Any carriers emitted from traps in the depletion region are swept away by the local electric field. In a DLTS apparatus a short voltage pulse, called the fill pulse, temporarily narrows the depletion region, causes a large positive capacitance transient and allows unfilled defects in the previously depleted region to trap charge[27], as shown in Fig. 18. When the pulse shuts off, the depletion region is restored but the capacitance is reduced to a value lower than the original static value due to the space charge associated with the newly trapped majority carrier charge. As these trapped charge carriers are thermally emitted, a transient capacitance signal is produced as the capacitance finally returns to the quiescent value. The transient signal is therefore determined by the defect trapping centers at the edge of depletion layer. In an actual apparatus, the fill pulse is applied repetitively, allowing noise reduction and averaging techniques to be employed. If the fill pulse voltage is larger than the reverse bias, it drives the p-n junction briefly into forward bias causing minority carriers to be injected into the depletion region. In this way minority carrier trapping levels can be detected. Because of the sign of the space charge, trapped minority carriers cause a final decay mnsient with the opposite sign from that of majority carriers. This difference enables the signals from each kind of trap to be distinguished.

Iv-

17

.*. @

“MAJORITY CARRIER PuLSE” f-o

0:000000

t ‘\\

J m @

..

AC

I

&

QUIESCENT

\

\

/

/

REVERSE 91AS

t
a ..

\

@

( I ---- ace.

THERIAAL EMISSION 1>0

:

@

DECAY OF TRANS[EHT DUE TO

BEGINNING

y;O~RANSIENT

Figure 18. The capacitance transients induced in an n+p junction during the formation of a DLTS signal. The quiescent reverse bias at (1) is reduced by a fill pulse at (2) leading to a large positive capacitance change. When the fill pulse is removed at (3) the junction width is restored, but the capacitance is different from the quiescent value due to the space charge induced by filled majority carrier traps. value.

As these traps empty, a transient After Miller et al.[27].

restores

the capacitance

to its quiescent

In the most widely used DLTS systems, the detection system samples the decaying capacitance transient at two f~ed times after the fdl pulse and gives an output signal proportional to the difference in capacitance detected at the two sampling times, (i.e., double boxcar detection), The two sampling times determine a time constant for the decay,~, the inverse of which is said to be the rate wimbw for the detection system. It can be shown for an exponential decay that the rate window RIV is given by: l/~ = RW = ln(t~tl)/(tl

- t~

(6)

where tl and t2 are the sampling times. The peak in the DLTS signal occurs when the rate window and the thermal emission rate of the carriers are correlated, as shown in Fig. 19. The thermal emission of charge from the traps is an Arrhenius process and the emission rate eP increases exponentially with temperature as: ~ = cfPvtiN~exp(-E@T)

(7)

where UP is the capture cross section for the trap, v ~ is the thermal velocity of the charge carriers

IV- 18

and Nd is the carrier concentration.

Substituting for the temperature dependence of V~Nd leads to: c/J?

= @~)exp(-E~T)

(8)

where all tie unmeasured constants are contained in the parameter a(~).

TIME

Figure 19. In a commonly used DLTS detection system, a double boxcar repetitively samples the capacitance transient at two fixed times after the removal of the fill voltage. The difference between the two signals determines the magnitude of the DLTS signal. It can be seen how the two sampling times set a rate window which determines when a maximum DLTS signal will occur. The decay constant of the capacitance transient is changed by slowly varying the temperature of the sample. In a DLTS apparatus the rate window can also be varied. After Miller et al.[27].

Fig,20.

The output signal for a given trap and rate window varies with temperature as seen in From the variation of the peak temperature with rate window, the thermal activation

energy, E ~, of the trap can be determined as well as the value of a(a), Fig,21 shows the kind of data obtained, in this case for minority carrier traps in p-type InP irradiated with 3 MeV protons [29]. The value of E, hxates the position of the trapping level in the band gap relative to the valence band. Of particular interest in this regard is the DLTS spectrum of the divacancy as measured by Evwaraye and Sun[30] and shown in Fig.22. The peaks labelled A2 and A3 are assigned to the double- and single-negatively charged states of the divacanc y respectively. The assignment is shown explicitly in Fig. 23. The capture cross section for the traps, which can be determined directly by measuring the signal height while varying the fill pulse width, and the defect concentrations, can also be found using DLTS. These parameters have ben measured for the divacancy[30].

Iv-

19

Fig. 24 shows a schematic layout of a typical DLTS system. The reverse bias, fill pulse generator and temperature control systems are coordinated by a computer which also receives and displays the final DLTS spectrum.

c

InP

“. 21s, M

“. M “. u

O.nla 10{E, 6/s 50/s, 2ola 2oolsa 201s 1000/,, 4M/s 5m/1, 2oM/t

lr#

Died.

h=.

, .,

●l

50

Ctilpf

07:52

m

16 Maw lvW

“,015,m2

1

A

40

360

250

200

150

100

5-3

sc W u=l.5&+l&l

Figure 20. Because of the effect described in Fig.19, the DLTS spectrum of a particular defect has a peak at different temperatures depending on the setting of the rate window. The spectrum shown here is for the H4 center in p-type InP. After Walters[28]. As in the case of ESR, defect introduction rates, annealing kinetics, and transformations, and the effect of junction electric fields on defect properties are some of the other information that can be investigated using DLTS. There are also several associated techniques such as double 100

3 MeV

N,= 0 *-

Pfioton Irrad. p-type Inp

I.1X1O cm”’

= 511 O’zcm-a

1 (3-2

~\

J

~ %*

‘$

10-4

O,;jeV J

-

\

11)4

o%

106 ~

10 11

23456789

1000/T

12

(K-l)

Figure 21. The activation energy of the defect is obtained from an Arrhenius plot of the temperature of the peak position of the DLTS signal versus rate window. Here is shown such a plot for minority carrier traps EA, EB and EC in proton irradiated p-type InP. After Walters and Summers[29]. Iv- 20

Figure 22. DLTS spectrum of electron irradiated p+n junctions in Si. The peaks labelled A2 and A3 are due to the double- and single-negatively charged states of the divacancy. Al is due to the A-center. HI is due toan unknown defect. After Evwaraye and Sun[30].

~EC

~Ec

I-2) ~c-

(’2)

A2 ~ -023 c

(-J )

A3 EC_041

0.40 (-l)

~n)-----~Y+oz5

~EW

Figure 23. The energy levels assigned to different charge states of the divacancy in Si. The assignment on the left comes from ESR measurements usin~ the estimated position of the Fermi level and those on the right from fiLTS measurements. The central part of the figure refers to bands in the ir absorption of irradiated Si. After Cheng et aL[31]. Iv- 21

3“ Reverse Bias

Pulse Generator

_ Capacitance Meter

J

\ ~A~ Computer Sample

.

Temperature Controller

Figure.24.

A block diagram

of a typical

Display

k

DLTS apparatus.

DLTS (DDLTS), which can be used to separate minority carrier traps in the presence of majority traps. Minority carriers can also be injected optically using a pulsed laser system. Like ESR, DLTS is not a trivial measuremen~ but again commercial systems are available which integrate all the bias, capacitance and temperature control electronics. 5.0 Effect

of Defects

on Electrical

Properties

As we have seen in Section 4.0, radiation-induced

defect centers introduce well-defined energy levels into the band gap of semiconductors. These levels alter the electrical properties of semiconductors via several basic mechanisms. The position of the levels generally determine which mechanism is dominant, although a given level can cause several effects simultaneously. Obviously the presence of these levels changes the electrical performance of devices, leading to various radiation responses. The five basic mechanisms usually identifkd for the effect of localized defect levels on the elecrncal properties of semiconductors are illustrated in Fig. 25. The five mechanisms are: 1. Thermal generation of electron-hole pairs. 2. Recombination of electron-hole pairs. 3. Carrier trapping. 4. Dopant compensation. 5. Tunneling. 5.1 ThermaL

Generation

of Electron-Hole

Pairs

Thermal generation of electron-hole pairs can be viewed as either the simultaneous emission of a free electron from the defect level to the conduction band and of a free hole to the valence band, or as the thermal excitation of an elec~on fi-om the valence band to the defect level followed by its subsequent emission to the conduction band. The latter viewpoint is useful in emphasizing the importance of mid-gap levels to this kind of process, since the probability of the Iv- 22

excitation process depends exponentially on the energy sepwation of the defect level from the reIevant band. The generation lifetime changes as the level moves away from mid gap. Charge generation dominates over recombination processes only when the free carrier concentration is bdow the equilibrium value, as occurs most commonly in depletion regions. Displacementinduced dark current generation in sensors, and leakage currents in diodes and bipolar transistors are important

radiation

effects caused

by the generation

I

\ OENl!mATION

I ,

I

I

\ +

process.

E.

1

❑llcouol NAllom

, ,

VALENCE

TRAP?IMa

: COMPEMS4?10M

BAND

1

I

I

\ \

● ✍✍✍✍✍✍

+

effects Figure 25. Five electrical the presence of radiation-induced After Srour[l]. semiconductor.

5.2 Recombination

of Electron-Hole

that

have

localized

been identified as occurring due to defect levels in the band gap of a

Pairs

In charge recombination, a carrier of one sign is first captured at a defect center, and before re-emission, a carrier of the opposite sign is also captured, leading to recombination. The recombination can be either radiative, in which light is emitted or nonradiative, in which lattice vibrations (phonons) are emitted. Recombination is especially important in determining the minority carrier lifetime, z, and hence electrical characteristics such as bipol~ mmsistor gain and solar cell efficiency. Radiation-induced defects can reduce ~ substantially and generally carrier lifetime is the electrical parameter most sensitive to radiation damage. It should that the generation and recombination lifetimes for a material are generally different. carrier lifetime is related to the minority carrier diffusion length, L, through the diffusion

minority be noted Minority constant

D, i.e., L = 4(D ~). The recombination rate depends on several factors including the defect density, the capture cross section, the carrier concentration and the position of the defect level in the band gap. Fig. 26 shows the effect of recombination in the DLTS spectrum of p-type InP[32]. When the fill pulse is such that only majority carriers are present, the majority carrier peak H5 is detected. H5 is thought to be associated with an In vacancy center. When minority carrier electrons are injected using a forward bias fill pulse, H5 disappears and the minority carrier peak EA appears. An interpretation of this result is that the cross section for rninonty carrier trapping at the center producing H5 is much larger than for holes-, so H5 is immediately filled with electrons if they are present. Any holes that fall into the traps then recombine with the electrons so that the hole peak H5 is no longer detected. IV- 23

5.3 Carrier

Trapping

Canier trapping is the process whereby

a charge

carrier

is temporarily

captured

and then

band before recombination or any other process occurs. Carrier trapping often competes with recombination, for which trapping is the first step. Both minority and majority carriers can be trapped. Because trapping reduces the concentration of majority carriers, it

released

to the original

contributes to carrier removal and hence to reductions in conductivity. Traps with a net electical charge are also effective scattering centers that reduce the mobility. Trapping is the mechanism that is exploited in deep level transient spectroscopy. It is also important in determining transfer efficiency in charge coupled devices and many other phenomena. I

I

I

I

I

I

I

I

I

I

I

1

I

I

I

I

I

i

1

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

type lnP -1 = 80~-1

p -

200 –r

E 50 z

./ - __, - “-----

\% t

L _“\

-200 – 1

1

I

100

1

I

I

I

I

I

I

150

I

I

1

I

I

I

I

I

I

I

1

I

1

I

200 TEMPERATURE

I

1

I

250

1

I

I

I

I

I

I

I

1

300

(K}

Figure 26. An example of minority carrier recombination at the H5 defect in InP. Under minority carrier injection (V>lV), the majority carrier H5 signal in the DLTS spectrum disappears due to recombination with minority carrier already located at the trap. After McKeever et al.[32]. 5.4

Compensation

In compensation, radiation-induced defects introduce deep levels that effectively reduce the majority carrier concentration by intrwlucing carriers of the opposite sign. In radiation effects, this process is also called carrier removal and it affects device characteristics that depend on the In silicon, it is thought that irradiation majority carrier concentration, such as resistivity. eventually drives al! material intrinsic because of the location of the highly effective, amphotenc divacancy center which can compensate both p- or n-type material. Usually a dominant defect drives most materials to a particular type after prolonged irradiation. InP, for example, is ultimately driven n-type, so that irradiation first makes p-type material intrinsic, while further irradiation makes it n-type. This effect produces the curious result of apparently increasing the DLTS signal of hole centers following thermal annealing of heavily irradiated p-type InP, Long irradiation removes so many carriers that no DLTS can be observed until enough compensating defects are annealed to supply the carriers neceswuy to produce a measurable DLTS spectrum.

IV- 24

5.5 Tunneling Tunneling can occur directly from the valence band to the conduction band in a heavily doped junction in which the conduction band in the n-type region coincides in energy with the valence band of the p-type region. Such a device is called a tunnel junction, which is used, among other applications, to couple in series the sub-cells in a monolithic tandem solar cell. Tunneling is most likely in junctions of low band gap materials such as InGtis. Tunneling can be greatly enhanced by the presence of a radiation-induced defect level in the junction region, producing defect-, or trap-assisted, tunneling. The five mechanisms above show how radiation-induced defects can shoxten the minority carrier lifetime, t, and reduce both the carrier concentration, Nd and the mobility, v. For most semiconductors, z is the most sensitive to displacements and changes are observed fust in ~ as the particle fluence increases, as shown in Fig. 27. The longer the initial value of the lifetime, the smaller will be the fluence at which radiation-induced defects will become effective. This means that high gain devices that rely on minority carrier lifetime are particularly sensitive to particle irradiation. Decreases in Nd occur generally at a higher fluence than ‘t and finally the p is reduced at the highest fluences. Radiation-induced decreases in mobility occur due to increased scattering of charge caniers by defects. Charged defects are most effective because of their large scattering cross section. Such centers are often produced as carriers are removed during compensation. The dopant atoms themselves usually act to reduce the mobility, as shown in Fig. 28. 1

I

N-TYPE

FISSION

I

I

UIIJO )

r, I r,.

r, Irre

(rrO - IO nd

(rro s Ioid

I ,&

10’

1

I

SILICON (2 ohm-cm) NEUTRON8

1\

,1

,010

,011

NEUTRON

,013

1o11

FLUENCE

(n/cm’

Figure 27. The relative sensitivity of minority concentration and mobility to defect introduction. reduction due to recombination at particle-induced sensitive property. After Srour[l].

10’4

)

carrier lifetime, carrier Minority carrier lifetime defects is by far the most

Many defects can occur in several charge states, just as atoms can exist in several ionization The charge state can alter the properties of the defect, especially the annealing states. characteristics. In silicon, for example, the negatively charged vacancy (i.e., a vacancy that has trapped a single electron) is more mobile than the neutral vacancy at any particular temperature. In general, however, interstitial atoms are more mobile than vacancies in any charge state. In silicon IV- 25

the isolated interstitial is mobile even at cryogenic temperatures. Because of the difficulty in detecting radiation-induced interstitial atoms, their properties have only been investigated in any detail relatively recently. The charge state of defects can be changed in several ways. The most straightfomvard way is to change the type (n- or p-) of the material, i.e. to change the position of the Fermi level of the material, as was noticed for the divacancy in silicon in Section 4.0 above. Another method is to increase the carrier concentration either by optical excitation or by forwmd bias injection. Such

prmesses can also be viewed as raising temporarily the effective Fermi level of the material. High injection levels can also lead in several ways to enhanced defect annealing rates. First, the charge state can lead directly to increased defect mobility as described above for the 4000

,

r

Ckrmmum

1500

IOoo-

SblwOn

1400 -

100

JDO”K

-

“K

I:oa -

2s00

I 000 -

?Ooo-

800 -

I SW -

600 -

1000400 -

Soo-

200

,

,

10”

10” Doping Concenlralbon

Iola

I

(cm-’)

lo”

, 10” C-wing

1

1 Iola

1 ,Oa

Conccnlrauon (cm”])

Figure 28 The effect of dopant concentration on the mobility of electrons and holes in Si and Ge. The decrease that occurs at high dopant concentrations is due to the increased scattering probability. After Streetman[33]. vacancy in silicon. Often it is the size of the different charge states of the defect that leads to the different annealing rates. Second, non-radiative electron-hole recombination at the defect can sometimes produce enough localized vibrational energy to produce defect motion. This mechanism is thought to occur at the phosphorus-vacancy/impurity defect (the H4 center) in iridium phosphide, as shown in Fig. 29. In this example, an n+p junction in InP was injected with a current density of 6,4 mA/cm -z in a series of incremental steps[32]. The defect concentration was monitored by the height of the DLTS signal. It can be seen, however, that the H3 defect is not affected by the injection process. Other mechanisms for injection enhanced annealing have been discussed in the literature. The minority carrier lifetime is also observed to depend on the injection level in irmdiated semiconductors due to recombination effects, as shown in Fig.30. A two level model [34,35] has been used to explain this result in silicon. In this model, the lifetime fist increases with injection as a bottleneck develops due to the inability of the defects to trap the increased carrier Eventually a second level becomes effective in reducing the lifetime. In concentration. IV- 26

semiconductor laser diodes, which operate at very high injection levels, the effective minority lifetime is only tens of picosecond compared to nanoseconds in the unexcited semiconductor. In this case, the reduction in lifetime is due to nonradiative Auger recombination in which the energy of an excited canier is dissipated by increasing the energy of another canier. In one model of this process, the lifetime varies as the inverse of the square of the carrier density, so under very high injection the lifetime rapidly decreases, 1

L P-

tvPe Inp

T-l

T.2WK 1 . 6dmA

- 2S-’

Im

cm-2

h

35

1%

5%

!i ;m d

lm

20 ~

—

~

--F&

‘

,o~

u 4

Em nME(ll

10

01 Im

150 TEMPERATURE

(K)

Figure 29. Injection-enhanced annealing of the H4 defect in InP at 200 K. It is suggested that the vibrational energy released upon the nonradiative recombination of electron and holes is suftlcient to displace the defect and lead to annealing. The annealing rate increases with increasing injection level, but is not particularly sensitive to temperature. After McKeever et. al.[32].

:

l-&Hz7!z7?&t +

-

~0

1

5

-

u

2,0.

IQ-

IN

1o11 2

5

Id’

EXCESS CiNSlfY

1

5

Iry

1

5

lb

Icm’JI

Figure 30. The effect of injection level on minority carrier lifetime in irradiated The initial increase has been associated with a bottleneck affect due to Si. occupation of all the available traps. The overall shape of the curve can be simulated with a two level model. After Curtis et al.[34]. IV- 27

6.0 Nonionizing

Energy

Loss and Damage

Correlation

It was suggested in the Introduction that one of the main problems facing an engineer when designing a system for use in a complex radiation environment is how to predict the end-of-life performance of a device or system without having to perform lengthy, difficult and often costly ground tests. A typical example of this problem is the prediction of the expected power output of a solar array after several years in a particular orbit. Also the expected degradation in Dansfer efficiency of charge coupled devices and other sensors in space has attracted considerable attention recently. A more difficult challenge is to account for the distribution of displacement damage effects (such as increased dark current noise) that will occur in the pixels of a sensor array in a space, radiation environment. As new devices and materials are introduced, this problem is becoming increasingly frequent and more difficult to solve. Often decisions on competing technologies, which commit large levels of funding, have to be made on relatively scant technical information. It is obviously important to determine how scant this information can be without the decision becoming essentially a pure guess. Probably the most obvious, direct method of approaching this problem is for the experimenter to try to simulate a complex radiation environment at a cyclotron using a series of degrading foils to obtain protons with a spectrum of energies similar to that found in space. The device would then be exposed to the required fluence in the simulated space environment. However, this is not the best approach. Apart from its complexity in an individual case, it means that different measurements have to be made for each different environment. A better approach would be for measurements of device radiation response to be made using a few mono-energetic particles and for the response in any radiation environment then to be predicted from this few data. However, for such an approach to be feasible, it is necessary to comelate the radiation effects of different particles in a systematic and reliable way. Attempts at damage correlation, especially btween electrons and protons, have been made in the past and much pioneering research has been reported over the last 30 years[36-38]. However, in the last few years the subject has been put on a firmer experimental and theoretical footing using nonionizing energy loss (NIEL) as the basis for the correlation[39], This approach will now be described. n. GnAI

,.-1

. ----

------7-/ “d’

.

●

●

,,Q’’P.

K“

,.-4

j ~ { o

‘x n+i /

,

GaA&

/’ .

/ /“’

‘/

10-~ -/’d. /;

p.lnP

/

YP-S

10-u

n.lfl -/ /

,O-qn, Curk

1[ (l~itv~

an’lammioll

.(MI-9

Figure.31. The effect of dopant (carrier) concentration on the diffusion length damage factor for n- and p-type Si, GaAs and InP. A revised version of a figure due to Yamaguchi et al.[40].

IV- 28

It is usual in radiation effects research to measure an important device characteristic as a function of incident particle fluence @in order to obtain a damage factor K for the process. The value of K depends on many factors including the material type and the carrier concentration (dopant level) [40], InFig.31 is shown the diffusion length damage factors for n- and p-type Si, GaAs and InP as a function of dopant level. Fig. 31 shows that the damage factor increases with carrier concentration for Si and GAs. This behavior is expected if the stable defects are vacancyimpurity complexes, so that the probability of defect formation increases with dopant concentration. The behavior in Si is particularly simple in that the damage factor is approximately directly proportional to the carrier concentration. Notice also that for Si and GaAs, n-type material is more susceptible to degradation than p-type. Curiously, Yamaguchi’s figure indicates that InP shows completely the opposite behavior. Not only is the minority carrier diffusion length more easily degraded in p-type InP than in n-type material, but also the damage rate decreases with camier concentration. No entirely satisfactory explanation for this behavior has yet been given and indeed there is some evidence that it may not be entirely correct. For bipolar switching transistors, the common emitter dc gain hf. is an important parameter. During particle irradiation, the gain is reduced by increases in the base current due to regeneration, recombination and surface currents caused by radiation-induced defects. Consideration of these effects leads to the Messenger-Spratt[41 ] equation:l/hf.(0) = l/hf.($) + K~

(9)

where hf~(0) is the gain at zero fluence. For the correlation method to be valid, the value of K for a particular mono-energetic particle must be related to the value for another monmenergetic particle in a calculable way. A few years ago a series of measurements were made of particle-induced gain degradation using several kinds of bipolar switching mnsistors with both n- and p-type base regions[39]. In order to study the damage effect of different particles, incident protons, deuterons, helium ions, and neutrons were used. 0.05 GAIN DEGRADATION

DUE

TO CHARGED

#

PARTICLES 2N2222A

0.04

Device

#824B

Ic - 3JmA

/

_

4 3 MeV

Deulerons

0.03

/

+

40.0 h4eV Ha !wm

0.02 +

168 WV

lie Ikms

/ ,,(-J,

~

o

2,0

1,0

3.0

PARTICLE FLUENCE ( x 10’2 cm-z)

switching transistors Figure 32. Changes in reciprocal gain of 2N2222A induced by sequential irradiations with 16.8 and 40.0 MeV He ions, and 4.3 MeV deuterons. The reductions in gain are due to particle-induced increases in base current caused by the introduction of recombination and other centers. After Summers et al.[39].

IV- 29

In the transistor experiments, the damage factors for reciprocal gain degradation were measured for various particles at many different energies. A representative example is shown in Fig.32, in which the reciprocal gain changes induced in a 2N2222A transistor by incremental fluences of 16.8 and 40.0 MeV He ions, and 4.3 MeV deuterons me shown. The geometry of these transistors is such that the incident particles lost little energy in passing through the active regions so that the damage factors are representative of a single particle energy. Consideration of Eq.(6) shows that the slopes of the lines in Fig. 32 give the damage factors. The damage factor for 16.8 MeV He ions is clearly larger, therefore, than that for 40.0 MeV He ions. Another interesting result is that the damage factors are independent of the order of the irradiation, so that the prior history of the device is not important. In many actual experiments the uansistors were frost irradiated with neutrons. A particular problem is that for a given transistor the value of h f. depends on several factors including temperature and injection level, i.e., the collector current. The temperature could be easily controlled experimentally. The variation with injection level is shown in Fig.33 for fission neutrons, for 3.7 MeV protons and for 65 MeV helium ions. It can be seen that although the magnitude of the damage factors for different particles are different, the variation with injection level is the same, i.e., the curves are all parallel. This means that the ratio of the neutron damage factor to the damage factor for another particles is independent of injection level, so that a unique correlation between damage factors could be made. Also this ratio is independent of device (i. e., material) type, because the 2N2222A and 2N2907A tmnsistors are npn and pnp respectively. Fig. 34 shows the final overall result, with the damage factor ratios for six types of transistor plotted against particle energy for protons, deuterons and helium ions. These results were then compared to calculations of the NIEL for the respective particles, again normalized by the NIEL for fission neutrons. Notice that the right and left axes are identical, so that the experimental data and the MEL calculations agree exactly with no fitted parameters.

1

I

DAMAGE FACTOR

10

1

vs COLLECTOR

‘-

-13

,

— ---

‘.

CURRENT

2N2222A 2N2907A

65 MeV Helium

Ions

10 -14 -..

3,7

MeV

Protons

~ ~ Fission

---

neutrons

,~-15

I

,()-16 ,.-6

,0-5

,()-3

,0-4

10-2

10-’

IC(A)

Figure 33. the dama~e

The effect factors for fact that ~he curves are established between the Summers

of injection level, material type and incident particle on reciprocal gain reduction in Si switching transistors. The parallel m~ans that a unique damage factor ratio can be effect of a particular particle and fission neutrons. After

et al.[39].

Iv- 30

Another, more revealing way, to view the correlation shown in Fig. 34 is to plot the damage factors directly against NIEL as shown in Fig.35. In this case the actual values of the 100

10

g la

1 – A 1

2

4

1

I

6 810

,

20 ENERGY

I

,

z

I

40 6080100

200

400

(MeV)

Figure fission

34. The damage factor ratios for protons, deuterons and He ions with neutrons, determined from data like that shown in Fig.31. The right hand axis gives the calculated values of the ratios of the nonionizing energy loss The fact that the left and right hand axes are identical (NIEL) for the particles. with no fitted parameters means that there is a direct proportionality between NIEL and the damage factors. Also the value of a damage factor for any monoenergetic particle can therefore be predicted accurately from a calculation of the NIEL and a single measurement. After Summers et al. [39]..

damage factors for 2N2222A transistors for a collector current of 30 mA are shown. For convenience, the proton energies that correspond to the FUEL values on the bottom axis are shown along the top. There are three important conclusions to be drawn from Fig.35. First, since NIEL is a calculation of the average number of defects initially produced by irradiation, the implication is that the damage factors are directly proportional to the number of defects produced, independent of the recoil energy involved in initiating the defect cascade. Second, once the damage factor for one particle and energy has been determined experimentally, the damage factors for all other particles and energies can be predicted from a calculation of the NIEL by a straightforward extrapolation of Fig.35. Third, and of particular importance for modeling displacement darnage distributions in sensor arrays, NIEL can be considered the displacement darnage equivalent of LET in ionization. This point will be discussed in more detail later. Sirnihm correlations between damage factors and FUEL have been found for other materials besides Si. Figs. 36 and 37 shows data for earner removal in GaAs[42] and minority carrier lifetime in Ge[43]. Perhaps the most surprising result, however, was found for the particleinduced depression of the transition temperature in high temperature superconductors shown in Fig.38 [44]. Again notice how the data for fission neutrons also fall on the cume. Iv- 31

DAMAGE FACTOR

m

NONIONIZING

ENERGY

DEPOSITION Im

!ml

20

10

5

4 1

2

Prolm Ensrgy [Mow 0

,~-14

–

2N2222A Ic . 30mA

~ 0 v 0 8

0 0

5 :

w :

,0-15

_

z 2

10-16

.

1 MeV

Noulrons

1

I

I

,0-3

I

11

t

1

1

,0-2

NONIONIZING

ENERGY DEPOSITION

Figure 35 The direct proportionality 2N2222A switching transistors. After

10-9

10-’

I

(MeV<m2/g)

between damage factor Summers et al. [391.

NIEL

for

I

I

I

I

and

GaAs 1(-J-1O

-

10-11

-

10-12

-

10-13

0,0

KNUDSON, er al, 1985

0,~

CAMPBELL, et al,. 19B6

o

“a 0

-

/

2Lc-Ed 10-4

NONIONIZING

10-2

,

f-Jo

102

ENERGY LOSS (MeV-cm2/g)

between carrier removal damage The direct proportionality Figure 36 and NIEL for n- and p-type GaAs devices. After Summers et al.[42]. IV- 32

factors

T

I ~~o

, ~1

Proton

Energy

(MeV)

GERMANIUM

1

Fission

Neulrons

(1 MeVEquivelenl)

J

100 I 5

I I 10301

I 1 OOICO3

Electron

Energy

(Mew A p-Ge o

,0-1

I

10 ,0-4

,0-2

,0-3

NONIONIZING

ENERGY LOSS (MeV-cm21g)

Figure 37. The direct proportionality between minority carrier lifetime factor and NIEL for Ge bipolar transistors. After Marshall et al. [43].

Heavy

Ions

kgc”

I-I+Energy (Mew

I 1 1 0.10 to

I I 100 10

damage

A

●

&

●* @ YBCO

e- Energy (MeV) .

+ (Eu,Gd)BaCuO

2n%3

w TICaBaCuOl

!

Fissionneutrons17 I

8 101

102

103

104

105

3.16

A I BiSrcacUd 4Js 106

107

108

109

NIEL (eV” cm2/g) Figure 38. The direct proportionality between the particle-induced shift in the transition temperature of high temperature cuprate superconductors and NIEL. Notice that data for fission neutrons falls on the curve. After Summers et al.[44].

Iv- 33

A word of caution should be added here. Like all general results, deviations from the linear dependence of damage factor on NIEL are expected to be found. Deviations are most likely for very small values of NIEL, such as would be found for low energy electrons close to the displacement threshold. Also deviations might & expected for interactions that produce vexy high energy recoils, for which the defect density is very high. As an example of the latter effect, evidence has been recently repofied that darnage efficiency, i.e., the fraction of the Frenkel pairs that do not rapidly recombine depends on PKA energy for neutron irradiated GMs[45]. Such a variation would clearly not lead to a linear dependence of damage effect on NIEL for these recoils. In fact what is so surprising about the results for Si, in the light of what is known from the Monte Carlo calculations of defect production and the known large fraction of Frenkel pairs that recombine almost immediately upon formation, is that the correlation with NTEL exists at all, with no consideration being given to defect cascade formation or defect annealing.

7.0 Nonionizing

Energy

Loss

Because of the importance of NIEL to the discussion above and to most of what follows, some details of the calculations will now be given. More details will be found in the references to Burke and co-workers[39,42,46]. For those only interested in using the results, tabulated values for protons and electrons incident on Si, InP, and GaAs are available for numerical calculations. In the brief discussion here, only energetic protons will be considered in any detail. The NIEL for electrons is particularly sensitive to the displacement threshold energy. However, protons involve the most complexity because of the possibility of inelastic as well as elastic nuclear interactions. The approach used by Burke is based on an method initially introduced by Simon[47], but with consideration given to imp-tarn processes not considered in the original model.

NIEL is a calculation of the rate of damage energy loss, The most straightforward approach to obtain the total NJEL is to sum the contributions from elastic and inelastic collisions, so that: NEIL = N/A(~eTe + ~lTi)

(lo)

where N is Avogadro’s number, A is the gram atomic weight, u are the total cross sections and T are the average recoil energies for elastic (e) and inelastic (i) interactions, respectively. More accurate calculations consider the angular dependence involved in the collision, as shown in Fig.38. In Fig.38 is shown the elastic interaction of an incident particle with a target nucleus in the center of mass system The NIEL is then calculated using: NIEL = (N/A)

J

L[T(@3)]T(0) d@U2 &l

(11)

where du/d.Qis the differentialcrosssectionfor a recoilindirection e, T(e) is therecoil energy, and L[T( El)] is the fraction of the recoil energy that goes into displacements 15]. The units of NEIL are eV,cmz/g. If the atomic density factor N/A is not inclu~ the calculation is equivalent to the so-called KERMA ~inetic energy ~ele.ased in matter) and the relevant units become eV.barns. The fraction L~(@] is strongly dependent on T(e) as shown in Fig.40 and has the effect of reducing the NIEL below the value it would otherwise have for high energy recoils. Fig.40 shows that the maximum average damage energy that an incident particle can generate in Si is 4.3 MeV, independent of the incident particle energy. The functional form to use for L[T(@] is still Iv- 34

somewhat uncertain. In plotting Fig.40 an approximation given by Lindhard is used for T <28 keV, whereas for T >28 keV the approximation given by Doran[48] has been used. This Proton

? \

‘?

Recoil

Atom

CENTER OF MASS SYSTEM

Figure

39.

The collision

between

an incident

proton

and a target atom in a solid.

approach seems to be a reasonable compromise for reasons that will not be discussed here. The lower limit in the integralinEq.(11) is determined by the displacement threshold energy and the upper limit murs when a direct hit between the incident particle and the target takes place. 10 “ Effect

of

Ionization

Loss

on

Damage

Energy

-1

:/’ 10ltl 2

1

1

1

1

103

1

10’ R1/c’oil

~!t~rgy

i

L

10’

~;)

Figure 40. The effect of ionization loss on the recoil energy available to cause Notice that the maximum damage displacements (i.e., the damage energy) in Si. energy in Si is about 300 keV independent of the magnitude of the energy of the recoiling target atom. The maximum damage energy possible increases with the Z of the recoiling atom, so in GaAs it is about 2 MeV. The maximum recoil energy caused by incident 8 and 200 MeV electrons are indicated.

Iv- 35

The maximum

energy Tm that can be transferred

is given by Eq.( 1) and the average

energy

tm.nsferred T,v is given by: T,v = T@l(Tfld)

-

(V/C)+

(27t2fi2V/hC2)]

(12)

The Rutherford form of the elastic differential cross sections can be used in Eq.( 11) only for proton energies c -10 MeV. When the incident proton energies are >-10 MeV, nuclear elastic interactions tmome important. These cannot be simply added to the Coulomb interactions because of quantum mechanical interference effects. It should be possible to use some suitable nuclear model to get the applicable cross sections for nuclezu elastic interaction s[49], but a safer approach is to use experimentally determined differential cross sections such as those shown in Fig.41 [50]. Fortunately nuclear elastic properties vary slowly with Z, so often data for neighboring elements can be used if they are unavailable for Si at a particular proton energy. It can be appreciated that the use of experimental data makes for an elatmrate nurnericat calculation.

lu.

u

9.0 8.0

SI I.ICON

7.0 6.0

5.0 4.0 3.0

\

2.0

1.0 (-)() .

~

0

Fi~ure.41. to”the

The

60

90

120

e~. ~. (Deg)

150

180

ratio

of the experimentally determined differential cross section for ‘nuclear elastic interactions in Si. sec~ion determined values of the differential cross sections were used in of the NIEL described in the text. After Fabrici et al. [50].

Rutherford

Experimentally the calculation

30

n

cross

Above-7 MeV,nuclearinelasticprocessesbeginto contributeto the NIEL. In these kinds of events the energy of the incoming proton is able to disrupt the target nucleus. The interaction is thought to occur in two steps called cascade and evaporation, as shown in Fig.42. The cascade process occurs more rapidly (-10-%), and as its name implies, the collision produces an intranuclear displacement cascade which results in the ejection of one or more nucleons. In the evaporation stage, which occurs later (10- 16s), additional nucleons are emitted from the excited nucleus. In both cases the recoiling atom can have a Z less than Si. The interactions produce recoil energies in the MeV range, but the resulting darnage energy is limited by the ionization losses as Although the overall mechanisms are complicated for inelastic interactions, shown in Fig.40. analytic as well as Monte Carlo approaches have been used to determine the recoil energies and IV- 36

mass numbers[51 -53]. Although the cross sections for nuclear inelastic interactions are typically athousand times smaller than those for elastic interactions, the recoil energies are typically a thousand time larger, and the overall conrnbution to the NIEL can be high. By the time the proton energy is near -100 MeV, the contribution of inelastic interactions to the total NJEL is larger than the contribution from elastic interactions, ZMcan seen in Table 1,

average

-44

~

Fast Procms

L1

S~OW ROCOSS ~ Atomic Displacements

Incident

Particle

Ev;::::Son

Inlfanuclear Cascade Process

v

9

Target Nucleus wm

?.-.9.-

1-

I

; Cascade , Nucleons I Mesons L------

1

: : I

_

-

L-

; Fra}%nts ----------

~

---

Figure 42. A schematic representation of the cascade and evaporation stages of the nuclear inelastic interaction that leads to high energy recoils and displacement damage. After Burke[54]. For electrons, the most accurate cross sections are due to Mott[8], but these are mathematically unwieldy and several useful approximations are available such as that due to McKinley and Feshbach[7]. Because of their relatively small mass, electrons generally produce low recoil energies, especially for incident energies less than a few MeV. This in turn makes the calculation of the MEL very sensitive to the cross sections used which depend closely on the value of the displacement threshold energy. It turns out that the calculation for lMev electrons is pmticularly important &cause radiation tests at this energy (and 10 Mev protons) are used to space qualify solar cells and other devices. The NIEL results we shown graphically in Fig. 43 which shows that at energies of a few MeV, protons are abut three ordersof magnitudemaredamagingthan electrons. AlSO i[ can h seen how the recoil losses to ionization cause the NIEL for protons to decrease with increasing energy even though the inelastic nuclear cross sections and the recoil energies are still increasing. However, in the case of crystals containing high Z atoms such as GaAs, the N~L for protons passes through a minimum near 100 MeV because the maximum possible damage energy is much higher [42].

Iv- 37

Table

1

I Electron

Proton

Proton or Electron

Elastic

Energy

NIEL

lNOn=lasticlTO’al lTO’a’ I NIEL

NIEL

(KeV)

I NIEL

(keV-fcmliq

1.0

61.86

--

61.06

0.0313B

1.5

42.40

_-

42.40

0.04215

2.0

32.35

--

32.35

0.05063

3.0

22.02

--

22.02

0.06359

4.0

16.74

--

16.74

0.07328

5.0

13.51

--

13,51

0.08097

6.0

11,34

--

11,34

o.oe735

7,0

9.774

0.5164

10.29

0.09272

10.0

7.091

0,7938

7.885

0.1050

15.0

4.770

1.1389

5.909

0.1183

20.0

4,037

1.3235

5.360

0.1270

30.0

3.317

1.4613

4.778

0.1379

40.0

2.857

1.4754

4.332

0,1447

50.0

2.432

1.4521

3.084

0.1494

70.0

1.773

1.3875

3.161

0.1554

1.272

1.3240

2.596

0.1602

100.0 200.0

0.5808

1.3595

1.940

0.1649

300.0

0.4160

1.4841

1.900

0.1653

400.0

0.3444

1.5633

1.90s

0.1647

500.0

0.2990

1.5723

1.071

0.1637

700.0

0.2414

1.5434

1.785

0.1616

1.4863

1.674

0.1582

1000.0

0.1882

Table 1. The calculated energy protons on Si from 1 to 1000 MeV.

dependence

of the NIEL

for electrons

and

Two additional complications occur when the NIEL is calculated for a compound contiing severdatotic conshmen~such asahghtemWramm supmonductor. Firstly, there is the problem of cmrectly accounting for the losses to ionization for the individual component atoms and secondly, there is the problem of combining the contributions of the individual atoms to obtain the total NIEL. These issues w discussed by Summers et al. [55].

7.1 The Effect of Electric Fields As discussed in Section 6, the direct proportionality found between measured damage factors and NIEL means that once the damage factor has been determined for one mono-energetic particle, the value for any other particle can be calculated without funher experiment. The question arises whether damage factors can be calculated directly for any particle for a given device without any experiment being required. The easiest case to consider seems to lx the generation current JR produc~ for example, in a depletion region of a pixel in a charge transfer device. The

IV- 38

I

1, 1 I

100

Non–Ionizing

( , , I II

,

Energy

Loss

,

for

1

,

,

\

1

Silicon

E —

protons [

I

electrons

,

0.01 1

, 1 I 1I , I 10

1

,

Energy Figure (NIEL)

43. The calculated energy dependence for protons and electrons in Si.

conventional

description

of this process

has been

1 I I 1, 1 10 (MeVY of

the

1

,

, 1I,

1 ,

1000

nonionizing

energy

loss

given by Srour et al. [56],

JR= qNiV@/K~

(13)

where q is the electronic charge, Ni is the inrnnsic carrier concentration and vd is the depletion volume. These factors me generally known in a particular case. Although the value of the damage factor Kg depends on the defect concentration, which can be estimated from the NIEL, it also depends on the device geometry in a way which is only just beginning to be fully appreciated. In particular, the generation rate can be enhanced by a large factor by electric fields in the depletion region. Although the value of the field can sometimes be calculated quite accurately from dopant profiles and applied biases, present theones relating the effective field to the enhancement factor are not entirely satisfactory. It therefore seems necessary to make at least one experimental measurement to be sure about the absolute value of the damage factor. This seems to k generally true for other kinds of devices. As average device dimensions kcome smaller, the effective electric field in active device regions will continue to grow, so that field enhancement effects are likely to beeome increasingly important. It is therefore worth reviewing these effects briefly. The two mechanisms described here are the Frenkel-Poole effect and phonon-assisted-tunneling, as shown in Fig.44 [57-60]. Fig.44 shows schematically how the electric field in the region of the defect distorts the potential barrier which determines the localized defect energy levels of the defect. The barrier is lowered in the direction of the field relative to the value that would exist without the field being present. In the Frenkel-Poole effect, the charge ca.niers in the defect escape over a barrier which is lower than usual by an amount labelled 6E1 in Fig.44. Since the probability of emission depends exponentially on the height of the barrier, any significant lowering can lead to large enhancements in the emission probability.

Iv- 39

d

Figure 44. A schematic diagram of three processes that can lead emission of a carrier from a trap or a generation center located in a field. The height of the potential barrier is reduced in the direction which leads to the Frenkel-Poole effect, phonon-assisted tunneling from the ground state. After Martin et al. [59].

to enhanced high electric of the field, or tunneling

1(-J1O

_ p-lnP H4 Center

(T=l 50K) ~

F-P (T=200K)

100

t 105

I

....

106

Electric

, 107

Field

108

(V/m)

An example of the hizh emission enhancement factors that can occur Figure.45, due to the Frenkel-Poole tunneling (PAT). The effect (F~P) and phonon-assisted calculation shown here is for the H4 center in InP at both 150 and 200 K. After et al.[61]. Messenger

Iv- 40

The one dimensional enhancement factor FF.p is given by: FF.P = exp(bE@T).

(14)

A peculiarly quantum mechanical phenomenon is the ability of a small particle to escape from a confining potential by tunneling through the barrier wall. The probability for tunneling depends on the height and width of the barrier, and the effective mass of the particle. Tunneling can occur from the ground level of the defect (pure tunneling in Fig.44) or from a higher level, which is reached by absorbing thermal energy, This latter process is called phonon-assistedtunneling (PAT). Both the Frenkel-Poole effect and phonon-assisted tunneling are temperature dependent. The elecrnc field-induced enhancement can be estimated using various approximations for the barrier shape and the defect potential [60]. In Fig. 45 is shown the calculated enhancement for emission from the H4 center in InP [61]. Fig.45 shows that at a field of 2 x 107 V/m, the emission probability y is enhanced by a factor of more than 10z over the low field value. It is appiment that certain devices will be particularly vulnerable to radiation-induced defects in regions containing high fields. The radiation-induced dark current in these high field regions will be far above what might be otherwise expected and in charge transfer devices, for example, might be expected to lead to dark current “spikes”, Such effects have actually been seen [62-64].

8.0 Displacement Damage Fluctuations and Fundamental Limits When the damage factor is measured for a conventional device such as a bipolar switching transistor, it is usual to irradiate a small sample of devices to a fairly large fluence of monoenergetic particles. The reason for the small sample size is due to the time required to measure a large number of devices, and the magnitude of the fluences necessary to make the measurements is a result of the sensitivity of the device to displacement damage. The damage produced as a result of an individual interaction is a random process, however, with a large number of possible outcomes depending on the type of interaction and the nature of the defect cascade produced. What is actually determined under normal circumstances is an average value for the damage factor due to the large number of interactions necessary to produce a measurable change in device characteristics. As device feature sizes become smaller, as the number of individual devices on a chip become larger and as the methods of detection become more sensitive, the statistical nature of the damage should become detectable and the damage fluctuations should become observable. Measurements of this kind have recently been made. In particular, the particle-induced dark current fluctuations produced in the large number of identical pixels in charge ~ansfer devices have been used to investigate the displacement damage distributions produced by protons and neutrons in Si [62-65]. Fig,46 shows for example the evolving dark current distributions measured in more than 61,000 pixels of a Si CID as the fluence of 12 MeV protons was increased from 4x1010 to 3x101 1 cm-z [65]. The very narrow distribution on the left is due to the camera response, which gives an idea of the high resolution of the measurements. The proton-induced distributions themselves show some interesting features. Notice how the disrnbutions are skewed for the lower fluences but becomes more symmetrical (Gaussian) as the fluence increases, This is because at the lower fluence, the distributions reflect the statistical spread in damage produced by a single interaction in a given pixel, (the so-called single event density function). As the numbr of interactions per pixel increases, the details of this function are smeared out to produce ultimately a The numbers such as N = 1967 above the distributions in Fig.46 Gaussian distribution. indicate the number of interactions that occur for the incident fluence involved. For 12 MeV protons these interactions are mostly due to Coulomb and nuclear elastic events.

Iv- 41

9 ++

7 % x

12 MeV PROTONS

+ 4X lo~” cm-z

8 ‘

61504

N= 1957

PIXELS

+

6 -CR. +10

5 -,

+ +

3 :

7418W

N=49~8

+

4 J

PART

lx10’’cm2

+

~$d AA

+

‘

2 :10;;::2 =

3x10’’cm2

N= 14753

::

2 -:++ + 1 :.“: + 1 o~: 0.0 2.0 ●

I

4.0

6.0

8.0

12.0

10.0

14.0

16.0

CHANGE IN DARK CURRENT lnAOCM-2) Figure 46 The dark current distributions produced in the pixels of a charge injection device (CID) as a result of incremental fluences of 12 MeV protons. The sharp peak on the left Iabelled C.R. is the camera response. The numbers above the individual distributions indicate the incident fluences and the number of Notice how the distributions are resulting interactions (e.g., N = 1967). noticeably skewed for low fluences and become more symmetrical at higher fluences. ‘After Dale et al.[65]. 14 ~

3.6x 1010 cm-z

FISSION NEUTRONS 61504 PIXELS

12 “ %

PART 5928N o x ,12, b PART 6432N

10 l,lxl

O”

cm-z

9 2,1 x 10)~ cm-z

8 “ x

4 -

5.2x

1011 cm-z N=41.

O

3.5

4.o

x

2 -

o

Oti 0.0

0,5

1,0

1.5

2.0

2.5

3.0

4.5

5.0

CHANGE IN DARK CURRENT (nAmr2) Figure 47 The dark current distributions produced in the pixels of a charge injection device (CID) as a result of incremental fluences of fission neutrons. (See Fig.44 for comparison). Notice how broad and highly skewed the distribution are in this case. The width of the distributions implies that the response of a device to a given neutron fluence can vary by a large factor due only to the stochastic nature of the damage process. After Dale et al. [65]. IV- 42

Fig. 47 shows the experimental histograms obtained following fission neuoon irradiation of the same kind of CID device and for fluences similar to the data for 12 MeV protons shown in Fig.46 [65]. It can be seen that for neutrons, the histograms remain skewed over the whole fluence range because of the different single event function and the relatively small average number of interactions, only N = 2.9 for a fluence of 3.8x101O cm-z. It should be possible to reproduce the histograms shown in Figs. 46 and 47 using Monte Carlo techniques. However, these would be time consuming and apply only to the specific case under study. Also the important physical mechanisms operative are sometimes hidden such calculations. These mechanisms need to be identified so that hardening approaches can be suggested and the fundamental radiation limits for the device can investigated. A ktter approach is to derive an analytic model. Fortunately, such a model is available as a result of the discovery of NIEL as the displacement damage equivalent of LET. The whole mathematical formalism developed successfully over the years to study fluctuations in ionizing energy deposition in microvolumes, which is related to the microscopic analog of LET, i.e., microdosimetry theory, should therefore be applicable to the displacement damage case, so long as the necessary parameter correlations can be identified. In microdosimetry theory the important parameter is the dimensionless the total relative variance, which is given by: VT=

quantity VT called

(15)

(0/~)2

where o is the standard deviation and u is the mean of a disrnbution.

The square root of VT, i.e.

(@t), is called the coefficient of variation and is often used in the presentation of data, as will be seen below. Kellerer [66] has shown how VT is obtained from the single event relative variance VI, by the formula:

VT= (Vl + 1)/N where N is the number of events.

In the displacement

(16) case, N = @crVpN~A where@ is the

fluence, o is the cross section, V is the volume, p is the density, No is Avogadro’s number and A is the gram atomic weight of the target. The single event relative variance is obtained by summing the relative variances due to fluctuations in all the parameters contributing to the displacement damage cascade and therefore to the generation of charge, resulting from a single hit. These parameters obviously include the energies of the recoils produced by the interaction, the number of atomic displacements produced, the partition of the recoil energy into ionizing and nonionizing events, defect recombination, electric field distributions in the active region, and charge generation, So far, for the micron size active volumes investigated to date, only the recoil spectrum and the electric field disrnbution have been found to be important. However, other contributions single event relative variance are expected to become important for smaller active dimensions. Table 2 shows the recoil spectrum parameters for several representative

to the device proton

energies. There is not space here to go into how the variances are combined and other details of the calculations. Thees issues have been addressed and are discussed fully in the references, Fig.48 shows fits of Eq.( 16) to experimental results such as those shown in Fig.46. The overall shape of the curves is due to the l/N-1~ dependence in the coefficient of variation, whereas the absolute magnitude is due to the contributions to the single event relative variance. The magnitude of V 1 can be extracted from the fits and compared to calculations of the recoil spectrum relative variance as shown in Table 3. The agreement between theory and experiment is generally good for elastic interactions but becomes somewhat worse when nuclear inelastic interactions are

Iv- 43

involved. However, the overall agreement, especially the accurate prediction of trends, is remarkable and gives strong support to the microdosime~ approach. It should be noted that the data discussed here are for the case in which electric field enhancement is small. However, similar TABLE

2

PROTON RECOIL SPECTRUM PARAMETERS PROTON ENERGY (MeV)

CROSS SECTION (BARNS)

12

1548

MEAN RECOIL ENERGY (MeV)

MEAN DAMAGE ENERGY (MeV)

VARIANCE OF DAMAGE ENERGY (Mew

ELASTIC REACTIONS 3,40 x 10-4 4,M H 10-4 7.77 x 10-’

857 318

22 63

4,77 x 10-6 7,71 M 10-6 1.62 K 10-s

1,76 ~ 10-4 2,13 M 10-4 2.E17x 10-4

INELASTIC REA~lONS 12 22 63

0.670 0.723 0.523

12 22 63

— — —

COMBINED

I

Table

2.

Recoil

\

REACTIONS

— — — 1

parameters

2.05 K 10-3 2.71 x 10-3 3,11 x 10-~

00765 0.111 0152

0.267 0569 1.44

for protons

I

2.09 X 10-4 3.07 ~ 10-’ 5.36 K 10-4 !

incident

(),45 0,40

2,03 x 10-5 59(3 u 10-5

1

on Si.

0

63 MeV PROTONS

d

22 Mev PROTONS

O

0,35

8,17 x 10-6

12 MeV PROTONS

—

CALCULATION

30,30 s ~ 0.25 2 ~ 0,20 a 0.15

0,10

0.05 0.0

I

I

1

1,0

2.0

1

1

3,0

40

FLLJENCE I x 1011 cm

I

5.0

I

6.0

‘1

Figure 48 The coefficient of variation (i.e. the ratio of the standard deviation to the mean) for the damage distributions measured in CIDS as a result of incremental fluences of 12, 22 and 63 MeV protons. The curves show calculations using microdosimetry formalism. This was the first application of this formalism to displacement damage distributions. Notice the very good agreement between the calculations and experiment. After Dale et al. [65].

Iv- 44

TABLE

3

PR(JTCIN

RECOIL

REUTIVE

VARIANCE: 12 MeV

FISSION

VR

22 MM

63 MeV

NEUTRON

v~

VE

—

—

CALCULATION EIASTIC

INEIASTIC

TOTAL

154

170

197

(11241

(1170)

(1034)

0.334

0.221 (1.0)

0.135 (1.0)

––

(1.0) 187

216

205

1.1

—

198

168

145

6.8

0!7

EXPERIMENTAL ltJTAL

“NUMBERS

Table current

3.

IN PARENTHESES

Comparison distributions

ARE BEFORE

of the calculated

LINDt-tARO CC) RRECTION

and measured

relative

variance

for dark

in the pixels of a Si CID.

good agreement is shown for the case involving electric field enhancement, for which the relative variances are generally higher. The microdosimetry approach would be even more appealing if the actual shape of the damage distributions could be predicted from first principles and this is now possible with considerable accuracy [67]. It is not possible here to describe in detail the method used and interested readers should look at the relevant references. For most fluences of interest, both the mean and the variance associated with the elastic interactions can be found in a straight forward way. For example, because of the large number of elastic interactions, the variance for the elastic interactions can be calculated from a product of the number of interactions and the variance associated with the probability density function for a single interaction. There will & generaLly so few inelastic interactions, however, that the intrinsic Poisson nature of the distribution cannot be ignored. Therefore an N-fold convolution of the probability density function for a single interaction is required to get the required total probability density function. The final result is obtained by a convolution of the elastic and inelastic density functions, weighted according to the Poisson distribution for the probability of N inelastic interactions occurring. The procedure is shown in Fig, 49. The mean value of the distribution cart be nonm.lized to the experimental data at one fluence, the rest can then be predicted as shown in Fig.50 The agreement here is remarkable. The availability of an analytic approach makes it possible to investigate the effect of varying device parameters on the damage distributions to be expected in a given radiation environment. For example, the effect of varying the size of the active volume is shown in Fig.51 [68]. Notice how the darnage curve moves higher as the active volume decreases in size, showing that the spread in the distribution increases markedly as the active volume decreases. This trend would be expected to be followed so long as the recoil atoms deposit all their energy in the active volume. Furthermore, the effect is due to the physical size of the device and therefore cannot be circumvented by hardening approaches.

Iv- 45

— 1.0

o

o

2.0

1.0

2.0

DAMAGE ENERGY !MeV}

DAMAGE ENERGY (MeV)

Figure 49 A calculation of the expected damage energy distribution for pixels In the left hand fi~ure, the elastic experiencing elastic and inelastic interactions. to the only case fi indicated by O. The other curves are Iabelled a;cording number of inelastic interactions. On the right, the curves for the interactions are Such weighted according to the Poisson probability for the number of hits. After calculations are comDared with experimental distributions in Fip.50. ,. Marshall et al.[67]. “ A DARK CURRENT (n+Vcm2)

o I

12

‘9

4 I

[

I

a

0°

I

I

12 MeV

I

PROTONS

4.OX It)l”lcmz

~ Ill ~

h

x ~

[

1 c

w > i= ; E

c

o 0

1.0

2.0

3!0

DAMAGE

4s3

ENERGY

5.0

60

70

(MeV)

Figure 50 A comparison of the experimentally determined dark current distributions for incremental fluences of 12 MeV protons on a CID with calculations of the type described in Fig,49. The mean of each calculated curve is fitted to the data. The excellent agreement with theory and calculation is evident. After Marshall et al. [67].

IV- 46

3.5 3.0 “ 2.5

DIMENSION OF SENSITIVE VOLUME

2.0 <

1.51.0 0.5 0.0~;;;;;6

;8910 NEUTRON FLUENCE (x 1011cm-z)

Figure

51.

on the coefficient for neutron irradiation. Notice how for a given fluence the expected distribution becomes This effect has implications relatively broader as the active volume decreases. After Dale et al.[68]. for radiation effects as device dimensions decrease. of

variation

Calculated (the

ratio

effect of

of the size of the active

the

standard

deviation

volume

to

the

mean)

The general result is of more concern than it might at frost be thought and has implications for radiation effects in individual devices as well as arrays. The essence of the result is that the degradation in a particular small device cannot be determined accurately from the average particle fluence, because the unavoidable damage fluctuations lead to a wide distribution of possible damage results. In other words the average fluence is not a good indicator of radiation response. In extreme cases, the actual energy deposited in a microvolume due to a particular average particle fluence could range over very large values, with the uncertainty getting worse as the device feature size became smaller. The implication of this result on circuits dependent on matched precision input devices is also worth noting. Even if these devices, e,g., JFETs in an oparnp, could be chosen to be identical prior to irradiation, the performance would be different following irradiation simply because of the damage fluctuations discussed above. Trying to harden such a circuit beyond the level expected from the damage fluctuations would obviously be pointless. It has been seen above how the analytic approach enables the effect of device volume to be estimated in a straightforward way. Another issue might be a comparison of the expected performance of different technologies. As an example, in Fig. 52 are shown the expectd damage

energydistributionsdepositedin a 1300~ms volumeof Si and GaAs. It can be seen that under thesecircumstances,both the mean damage energy and the variance are worse for GaAs than for Si. Also note that for a GaAs device of this size, which is relatively quite large by modem device standards, there would be an uncertainty in the response differing by almost a factor of two, due only to the intrinsic damage fluctuations.

Iv- 47

12 MeV

PROTONS

1300 pm3 VOLUME

$ $ $ Si

m

@ . 2.0x 1010 cm-2

LL w ~

aI = 2,0~10’0 cm-2

5 I.Ll m

o

1,5

1.0

0.5 DAMAGE

ENERGY

(MeV)

Figure 52 Calculated effect of material type on the expected the same size active volume and fluence of 12 MeV protons. receives more hits because of increased interaction cross resulting damage energy is higher because of relatively ionization for GaAs compared to Si. After Marshall et al. [67]. 9.0 Predicting

End-of-Life

damage energy for The GaAs volume sections, and the reduced

losses

to

Response

It was discussed in Section 5 that the linear dependence of device degradation (damage factor) on NIEL gave a way of confidently predicting the end-of-life performance of a device in a complex radiation environment, from only a few ground experiments. An example of such a prediction will now be given for the performance of InP solar cells in circular orbits out to 12,000 NM compared to the performance of Si cells in the same orbits. Such information might be requested for example by a program manager estimating the relative size of solar panels for a certain mission. Solar cells are especially vulnerable to particle damage in space bcause they are usually exposed to the full radiation environment while being protected only by a thin cover glass, ty-pically less than 12 nils thick. There is nothing special about this example, however, and the methodology can be used for any device and any particular known environment, so long as certain experimental data is available. It should be noted, however, that the actual predictions in this particular example are only preliminary and should not be taken literally. In the solar cell community, devices are usually tested by irradiating them with 1 MeV electrons and 10 MeV protons. The performances of different cells are compared by subjecting them to incremental fluences of electronsandprotonsat a suitableacceleratorfacility. In a second

step, the proton fluences are then converted to an equivalent 1 MeV electron fluence for comparison of the overall radiation response. This procedure is essentially the same as that used to test devices in fission neutron environments, in which case the reactor spectrum is converted to an equivalent 1 MeV neutron fluence for the purposes of comparing device damage factors. This was the method used, for example, to obtain the bipolar m.nsistor data discussed in Section 6. Fig.53 shows an example of the kind of data obtained for electron and proton irradiation of InP, Si and GaAs

solar

cells [69].

The properties

IV- 48

of solar

cells

are determined

primarily

by

changes in carrier lifetime in different regions of the cell, so that Fig. 53 contains information of the carrier lifetime damage factor for 1 MeV electrons and 10 MeV protons. However, extracting this information is not as simple as it might appear and it will not be discussed here [70,71]. I

20

I

I

g c .-o fu G filo 10 MeV protons

c

-do 55

-

s u

1 MeV electrons c-lnP ● LPE OaAs (data from JPL Publication 84-61) ■ GaAd(3c (from The Solar Cell Radiation Hanbook Addendum) ●

A c-Si (e data from The Solar Cell Radiation

1

1

o 1010

1011

1013

1012

Particle

Handbook)

1

1

1015

1014

1016

Fluence (cm-z)

Figure 53 Typical device radiation effects data. Here the effect of 1 MeV electrons and 10 MeV protons on solar cell efficiency is compared for Si, GaAs and InP cells. These curves contain information about the effect of irradiation on minority carrier lifetime in the devices as well as the relative vulnerability of different technologies. After Keavney et al.[69]. Nevertheless, several general observations can be made about Fig.53. First, it is immediately apparent that protons are much more damaging in general than electrons, as would be expected of course from the discussion of MEL in Section 7.0. However, the vulnerabilities of the different technologies are clearly different, since the curves are not parallel. These differences are due to a combination of material properties and device geomernes [69]. The different shapes of the curves mean that the ratio of electron fluences to proton fluences that cause a certain degradation is not the same for the different technologies. This ratio is about 3500, 1000, and 300 for Si, GAs and InP respectively, although the actual value depends on the parameter being measured, e.g., overall efficiency or open cirecuit current. This means that 10 MeV protons are about ten times more damaging to Si devices compared to 1 MeV electrons, than is the case for InP devices. The important fact so far as mcdelling is concerned is that these ratios can be related to the respective NIEL values. In Fig. 54, for example, is shown the calculated energy dependence of the NIEL

forelectronsmd protonson L@,withthe 1MeVelectronand 10MeVprotonpointsindicated. In orbit,

devices

are subjected

to fluences

of protons

and electrons

with

a spectrum

energies such as those shown in Figs. 55 and 56, and the total degradation at end-of-life is the cumulative effect of all the panicles. Consider frost the effect of the protons. The fact that the damage factor as a function of proton energy is linearly proportional to the NIEL means that the fluence cume for the mission cD(E) (obtained from a product of the annual fluence curve and the number of years for the mission) can be convoluted with the energy dependence of the NIEL, NIEL(E), including the factor ~ that relates the particular damage factor to the NLEL, to obtain the

Iv- 49

.

Protons ~/ /v NIEL 10 kfe Vprotons NIEL 1 Ikfe V Electrons

/

~ ~oo

Electrons

P“

,,/

h .A

50.01

z

100

10

1

Energy

Particle

(MeV)

The energy dependence of the nonionizing energy loss for electrons on InP showing the ~articular data for 1 MeV electrons and 10 Mev prot;ns. The ratio of the ~IEL ;or these particles enables a particular radiation environment to be specified entirely in terms of a 1 MeV electron equivalent fluence. The reasoning is similar to that used to establish a 1 MeV equivalent neutron fluence for fission neutron irradiation of Si. After Walters et al.[72].

Figure

54.

and Drotons

actual degradation expected over the course of the mission. ~ is obtained from a ground test, using data from a mono-energetic proton irradiation. It is then possible to calculate the expected change in performance using:

APerfonnance

= ~ ~ NIEL(E).@(E)dE

(17)

In actual cases, the integral will probably have to be evaluated numerically. The change in performance may be the final result required in some cases if protons are the only source of radiation damage. If the effective 10 MeV proton fluence for the mission is needed, it can be

obtained from:

0(10 MeV H+)= [1/NIEL(10 MeV H+)] ~ NIEL(E).@(E)dE

(18)

An equivalent calculation can be performed for the electron spectrum and in this case the equivalent 1 MeV electron fluence for the mission can also be calculated. The total equivalent 1 MeV electron fluence for the mission can then be obtained by converting @(10 MeV protons) to an equivalent @(l MeV electrons) using the ratio of the respective NIEL values for 1 MeV electrons and 10 MeV protons. In the example being described here, the whole set of calculations can be performed for both InP and Si solar cells in many different orbits as shown in Fig. 57. Fig.57 shows the expected changes in cell efficiency for InP and Si for both 1 and 10 year missions[72].

Iv- 50

10’” l_

1

.1

.[ .I .I

I

I

4

0’

3 J

,012

i’

L

PROTONS

//

107

.

\\\

L

\\

I

I

0.5

1

ALTITUOE

WITH

.0 Mav

E ,05

e

wllll: 234S

(THOUSANOS

10 OF NAUTICAL

1520 MILSS)

Figure 55. Typical radiation environment data used in estimating device Here is shown proton fluences as a function of altitude performance in space. Spectral information at any particular orbit is also and orbit inclination. available. The can be seen of- 1470 to in Fig. 57 is

big decrease in efilciency near 2000 N.M, is due to the location of the proton belts. It that at this altitude, the efficienc y of the S i cell has been reduced from an initial value about -2Y0. The availability of a methcxiology to generate curves such as those shown obviously extremely useful to those engaged in satellite programs. 10.0

Summary

and

Conclusions

It has been shown how incident particles such as protons can displace atoms in semiconductors and thereby introduce Icdized defect levels irtto the band gap. These defect levels affect the operation of devices made from the semiconductor by shortening the minority carrier lifetime, reducing the carrier concentration and reducing the mobility. Minority carrier lifetime is usually the most sensitive to the presence of defects. The number of defects prwluced depends on the damage energy, which is determined from the recoil kinetic energy of the target atom corrected for the losses to ionization. These losses limit the average maximum damage energy that can be produced by a particular kind of recoiling atom. The introduction of defects degrades device pefimmancein severalways,but in a particularcase a damagefactorcan be meas~ which gives the change in performance per unit fluence. Damage factors can depend on several parameters such as injection level, dopant level, temperature and so on, but these effects can usually be normalized out by taking the ratio of the damage factor to that for fission neutrons, for example. It has been shown that in many cases the damage factor as a function of particle energy is linearly dependent on the nonionizing energy loss for a large range of particles, energies and materials. The nonionizing energy loss is therefore the displacement darnage equivalent of linear energy transfer. This similarity means that just as dose can often be used to characterize ionization

Iv- 51

effects, damage energy can be used to characterize displacement effects. In addition, as the active volume of devices becomes smaller the whole formalism developed for ionization effects, i.e., microdosimetry, can be carried over to the displacement case, so long as NIEL is used in place of

/--=s

t

10E

I ..../

0.6

1

, 1 ALTITUDE

I 2

1

I

1

1

346

M+ OUSANDS

OF NAUTICAL

I

1

I

10

16

m

MILESI

Figure 56. Typical radiation environment data used in estimating device Here is shown electron fluences as a function of altitude performance in space. and orbit inclination. Spectral information for any particular orbit is also available. LET and so long as the other important parameters, such as recoil energy variances can be correctly identified and calculated. The microdosimetry approach has been investigated generated charge transfer experimentally using the particle-induced dark current distributions devices. The importance of the analytical microdosimtery approach is that the effect of various in a parameter, such as the size of the active volume and the applied electric field, can be varied systematic way, so that hardening techniques can be implemented. The other result of this methodology is that fundamental limits to the ability to circumvent radiation effects can often be identified. There is evidence already emerging that these fundamental limits are being approached in modem microcircuits and that the average fluence is not a good measure of a particular device’s radiation response.

Finally, it has been shown how the correlation of NIEL with displacement damage effects enables the effect of imadiation with a spectrum of different particles, as actually occurs in space, to be accurately calculable using a relatively small number of ground test data as input. An example comparing end-of-life efficiencies of InP and Si solar cells was described. This kind of information is essential before meaningful decisions can be made on the choice of components and technologies for future space systems.

IV- 52

,

20 \

\ \ \

\

,

,

\ \ \ \

,

/ / / .

/

/

/

/

/

\

/

/

----! \ \ \ \ \

. \

o

‘.

.

/

—1

----.

2000

4000

6000

year in orbit 10 years in orbit

8000

10000

12000

Orbital Altitude (N. M.) Figure 57. circular

Calculated performance for InP and Si solar cells as a function orbit for 1 and 10 year missions. After Walters et al.[72].

of

11.0 Acknowledgements Special thanks go to Ed Burke without whom much of the research discussed in the last few sections of these notes would not have been possible. His many insights into displacement darnage, often introduced many years ago, are only now being fully realized. Ed Burke and Mike Xapsos helped by reading an early draft of the manuscript and by making several suggestions for improvement. Rob Walters, Scott Messenger and Phil Shapiro assisted me in numerous ways during the writing of these notes. 12.0

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Iv- 53

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Iv- 55

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[57].

J. Frenkel, Phys, Rev., 54,657

(1938).

[58].

G. Vincent, A. Chantre and D. Bois, “Electric Field Effect on the Thermal Emission of Traps in Semiconductor Junctions,” J. Appl. Phys., 50,5484-5487 (1979).

IV- 56

[59].

P.A. Martin, B.G. Streetrnan and K, Hess, “Electric Field Enhanced Emission horn Non-

Coulombic Traps in Semiconductors,” J. Appl. Phys., 52,7409-7415 (1981). [60]. M.J.J. Theunissen and F.J. List, “Analysis of the Soft Reverse Characteristics of n+p Drain Diodes,” Solid State Elect., 28,417 (1985). [61].

S.R. Messenger, R.J. Walters and G.P, Summers, “Effect of Canier Concentration on the Properties of Irradiation-Induced Defects in InP grown by Metiorganic Chemical Vapor Deposition,” J. Appl. Phys., 71, (1992).

[62]. J.R. Srour and R.A. Hartrnan, “Effect of Single Neutron Interactions in Silicon Integrated Circuits,” IEEE Trans. in Nucl, Sci., 32,4195-4200 (1985). [63]. J.R. Srour, R,A. Hartrnan and K,S. Kitazaki, “Permanent Damage Produced

by Single Proton Interactions in Silicon Devices,” lEEE Trans. in Nucl. Sci., 33, 1597-1604 (1986).

[64].

P.W. Marshall, C.J. Dale, E.A. Burke, G.P. Summers and G,E. Bender, “Displacement Damage Extremes in Silicon Depletion Regions,” IEEE Trans. in Nucl. Sci., 36, 18311839 (1989).

[65]. C.J. Dale, P.W. Marshall, E.A. Burke, G.P. Summers and G,E. Bender, “The Generation Lifetime Damage Factor and its Variance in Silicon,” IEEE Trans. in Nucl. Sci,, 36, 18721881 (1989). [66].

A.M. Kellerer, “Fundamentals of Micrtiosimetry”

in The Dosimeny of Ionizing Radiation,

Vol.1, (Academic Press, New York, 1985). [67].

Displacement Damage P.W. Marshall, C.J. Dale and E.A. Burke, “Proton-Induced Distributions and Extremes in Silicon Microvolumes,” IEEE Trans. in Nucl. Sci., 37, 17761783 (1990).

[68].

C.J. Dale, P.W. Marshall and E.A. Burke, “Particle-Induced Dark Current Distributions Focal Plane An_ays,” IEEE Trans. in Nucl. Sci,, 37, 1784-1791 (1990).

in

[69], C.J. Keavney, R.J. Walters, P.J. Drevinsky, “Optimizing the Radiation Resistance of InP Solar Cells: The Effect of Dopant Density and Cell thickness,” Submitted to J. Appl. Phys. [70]. H.Y. Tada, J.R. Carter, B.E. Anspaugh and R.G, Downing, in The Solar Cell Radiation Hatiook, JPL publication 82-69 (1982). [71].

E.A. Burke, Private communication

(1992).

[72]. R.J. Walters,

S.R. Messenger, G.P. Summers, E.A. Burke and C.J. Keavney, “Space Radiation Effects in InP Solar Cells,” IEEE Trans. in Nucl. Sci., 38, 1153-1158 (1991).

13.0 Selected Further

Reading

C. Kittel, Introduction to Solid State Physics, (Wiley, New York, 1953). R.D. Evans, The Atomic Nucleus, (McGraw-Hill,

Iv- 57

New York, 1955).

F. Seitz and J.S. Kohler, in Solid State Physics, (Academic Press, New York, 1956). D.S. Billington 1957).

and J.H. Crawford,

Radiation

Vol. 2, edited by F. Seitz and D. Tumbull,

Damage

in Solids,

(Interscience,

R. Bauerlein, in Radiation Damage in Solids, edited by D.S. Billington, York, 1962).

New York,

Academic

Press, New

E. Segre, Nuclei and Particles, (Benjamin, New York, 1964). J. W. Corbett, in Solid State Physics, Suppl. 7, edited by F. Seitz and D. Tumbull Press, New York, 1966). F. Larin, Radiation Effects in Semiconductor

(Academic

Devices, (Wiley, New YorK, 1968).

J,H. Crawford and L.M. Slifkin, Editors, Point Defects in Solids, Vol. 2, (Plenum Press, New YorK, 1975). N.J. Rudie, Principles and Techniques of Radiation Hardening, Company, North Hollywood, !976).

Vols.1 -X, (Western Periodicals

V.A.J. van Lint, T.M, Flanagan, R.E. Leadon, J.A. Naber and V.C. Rogers, Mechanisms Radiation Eflects in Electronic Materials, Vol. 1, (Wiley-InterScience, New York, 1980). S.M. Sze, Physics of Semiconductor M. Lannoo and J. Bourgoin, New York, 1981),

Devices, (Wiley-Lnterscience,

of

New York, 198 1).

Point Defects in Solids I: Theoretical

Aspects, (Springer-Verlag,

J. Bourgoin and M. Lannoo, Point Defects in Solids II: Experimental New York, 1983).

Aspects, (Springer-Verlag,

J.F. Ziegler, J.P. Biersack and U. Littmark, in The Stopping and Ranges of Ions in Matter, Vol. 1, edited by J.F. Ziegler (Pergammon, New York, 1986) G.C. Messenger and M.S. Ash, The Effect of Radiation Reinhold, New York, 1986).

IV- 58

on Electronic

Systems,

(Van Nostrand