Large area glass coating

Surface and Coatings Technology 112 (1999) 358–365

Large area glass coating G. Bra¨uer * Leybold Systems GmbH, Wilhelm-Rohn-Straße 25, D-63450 Hanau, Germany

Abstract Since the end of the seventies vacuum coating technology for the deposition of optical thin films on large area glass substrates has enjoyed a steady growth. Nowadays, the main applications are low emissivity and solar control thin film systems on architectural glass as well as transparent electrodes for flat panel displays. Future markets like electrochromic or anti-reflective coatings appear on the horizon; the latter covering a wide range of products like picture frames, show-cases, shop windows, and all kinds of data and TV screens. The paper gives an introduction into the above-mentioned applications and outlines the present state of sputtering technology for large area glass coating. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Flat panel displays; Glass coatings; Low emission coatings; Magnetron sputtering; Optical coatings

1. Introduction The fascination of glass as a construction material for various applications is founded on its outstanding properties. High volume manufacturing and processing of glass is possible in a simple and inexpensive way. Glass is extremely stable against environmental attacks and highly scratch resistant. Its high transparency for the visible spectrum permits a clear neutral colour and thus realistic view into the outside world. All these features have led to an intensive use of glass in the field of architecture and transportation. Furthermore, glass is one of the most important materials in modern information and communication technologies. The drastically growing markets for TV screens and data displays need glass with a variety of coatings on its surface. Glass is not perfect with regard to a lot of applications, in particular for architectural use. On the one hand, its low reflection in the far infrared (room temperature radiation) causes undesired losses of thermal energy which is needed to heat buildings in colder climate regions. On the other hand, its high transmission in the near infrared (solar radiation) increases the energy necessary for cooling of buildings in hot climate zones. In the visible part of the electromagnetic spectrum a glass pane reflects 8.4% of the impinging light (4.2% per side). This reflection in some cases is rather disturbing; * Tel: +49 6181 34 1794; Fax: +49 6181 34 1850.

in optical lens systems consisting of many individual elements it is inadmissible. Carefully designed coatings on glass nowadays can overcome all these drawbacks. Since the end of the seventies vacuum coating technologies for the deposition of optical thin films on large area glass substrates have enjoyed a steady growth. The current worldwide installed annual production capacity of industrial vacuum coating systems for architectural and automotive glass amounts to approximately 120 million square metres, it has doubled since 1992. The main applications today are found in the fields of so-called low emissivity and solar control coatings. Since the end of the eighties a new and fast growing market for glass coating has developed: the flat panel display industry. Transparent and conductive ITO layers of highest quality are the basis for liquid crystal display manufacturing. New opportunities and challenges appear on the horizon, like electrochromic thin films or layers used for solar cells. All of these applications require fast and stable deposition of metal and metal oxide/metal nitride layers with high reproducibility and excellent uniformity on large area substrates (size up to 3.2 m×6 m). Table 1 summarizes important compound materials for glass coating. Due to their high plasma density and uniform plasma distribution in the longitudinal direction, sputter magnetrons provide deposition rates reasonable for economic

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 73 7 - 3

G. Bra¨uer / Surface and Coatings Technology 112 (1999) 358–365 Table 1 The most important metal compounds for large area glass coating Material

Index of refractiona, n (550 nm)

SiO 2 Si N 3 4 SnO 2 ZnO In O –SnO 2 3 2 TiO 2

1.46 1.95–2.05 1.95–2.05 1.95–2.05 1.95–2.05 2.35–2.55

a In most cases the index of refraction depends on preparation conditions.

industrial production as well as good coating uniformities up to widths of nearly 4 m. Therefore, magnetron sputtering has become the leading process for large area coating. However, its applicability to the deposition of insulating materials up to now has suffered from some serious drawbacks. The development of long term stable high rate magnetron sputter processes for the reactive deposition of materials like SiO , Si N or TiO has 2 3 4 2 been a challenging task throughout the past decade. Today, twin magnetron arrangements ( TwinMagA) powered by medium frequency are seen as the road into the future for high performance coatings on glass.

2. Applications of large area glass coating 2.1. ITO coatings for liquid crystal displays (LCD’s) One of the most important and most critical process steps in manufacturing LCD’s is the deposition of ITO as a transparent electrode. The major concerns for ITO deposition are as follows: (1) low specific resistivity (<150 mV cm), (2) high uniformity across the substrate (Dd/d<±5%, d: film thickness), (3) low particle contamination, (4) low manufacturing costs. Magnetron sputtering using ceramic ITO targets of high density is the preferred deposition technology today. For achievement of lowest resistivities, the glass substrates have to be heated to temperatures up to 380 °C prior to deposition. Furthermore, film defects resulting from radiation damage by bombardment of negatively charged ions during the sputtering process can be minimized by a decrease of the discharge voltage of the cathode, thus leading to a further improvement of conductivity. By use of strong permanent magnets, a discharge voltage of approximately −240 V and a specific resistivity of 120 mV cm have been realized. However, for coating on colour filters substrate temperatures of more than 200 °C are not applicable due to the thermal sensitivity of the filter materials. In order to obtain lowest sheet resistivities even for this temper-

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ature, a further reduction of the magnetron discharge voltage is necessary. Besides this, the necessity to optimize production costs calls for high utilization of the expensive ITO targets. ( The target utilization of a standard magnetron is between 20 and 25%.) Both requirements have led to the development of the so-called MoveMag cathode driven by a mixture of DC and RF power [1]. The permanent magnet set of the MoveMag cathode creates a double race track which is continuously shifted across the target surface, resulting in a wide erosion groove and a material utilization of more than 40%. The combination of DC and RF power results in a decrease of discharge voltage to values below 100 V. Fig. 1 shows specific resistivity, dynamic deposition rate and DC voltage for different DC/RF power mixtures. The optimum resistivity value of 150 mV cm at a substrate temperature of 200 °C is found for an RF portion of 50%. Table 2 contains all relevant process parameters and thin film properties for ITO deposition with the DC/RF MoveMag. To maintain the quality of the ITO layer for the entire life time of the display, a diffusion barrier consisting of SiO has to be applied between the glass substrate and 2 the ITO film. In general, the SiO is deposited by means 2 of RF sputtering or with the TwinMagA described below. Fig. 2 sketches a vertical inline system for deposition of SiO and ITO layers for LCD applications. Such 2 systems today are available for coating widths ranging from 500 to 1600 mm. To double the throughput, the coater can be equipped with sputter cathodes on both sides. In this case, substrates are mounted on both legs of a U-shaped carrier. Prior to deposition, the carriers pass a centre heating system which heats the substrates on both sides simultaneously. The productivity depends on the coating width and the number of cathodes. For a width of 830 mm and 6 cathodes on each side (2 for Table 2 Process data and results for ITO deposition with the DC/RF MoveMag Target dimensions Target composition Coating width Substrate material Substrate temperature Cathode power DC voltage Dynamic deposition rate Mean static deposition rate Film thickness Specific resistivity Transmission at 550 nm Resistivity uniformity Target utilization

810 mm×200 mm 90 wt% In O , 10 wt% SnO 2 3 2 550 mm Float glass 200 °C 2 kW DC/1 kW RF 115 V 18 nm×m/min 1.5 nm s−1 150 nm 150 mV cm 97.8% ±5% >40%

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Fig. 1. Specific resistivity, dynamic deposition rate and DC voltage for a DC/RF driven MoveMag cathode as a function of RF/DC power mixture [1].

Fig. 2. Inline coating system for deposition of SiO /ITO on glass for LCD applications. 2

SiO , 4 for ITO), the annual throughput (24 hours×22 2 days×12 months) is approximately 400.000 m2. 2.2. Low emissivity and solar control coatings Due to its high emissivity of e=0.85, glass doesn’t reflect infrared radiation. e can be substantially reduced by any conductive coating like ITO, doped SnO or a 2

metal. Today nearly all low emissivity glazings are based on a thin silver film. To maintain the high transparency of glass in the range of the visible spectrum and to protect the silver from corrosion, additional anti-reflective and protective layers of high refractive materials (e.g. SnO , ZnO, Si N , TiO ) have to be employed. A 2 3 4 2 further protection of the silver is achieved by a so-called ‘‘blocker’’ film. A huge variety of different designs are

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G. Bra¨uer / Surface and Coatings Technology 112 (1999) 358–365 Table 3 Thermal losses by uncoated and coated double glazing units

Relative losses by thermal conduction and convection Relative losses by thermal radiation Relative total losses Value of k

Uncoated double glazing unit

Coated double glazing unit

0.33 0.67 1 3.0 W m−2 K−1

0.33 0.07 0.4 1.1 W m−2 K−1

Fig. 3. Optical properties of a silver-based low emissivity coating on architectural glass.

available on the market. One of the simplest stacks is as follows: glass–SnO (40 nm)–Ag(10 nm)–NiCrO (1.5 nm) 2 x –SnO (40 nm) 2 In the double glazing unit the coating is located on the inner glass pane at the surface opposite to the inside of the building (so-called plane III ). Fig. 3 shows the optical properties of a typical low e film system compared to those of uncoated glass. Modern silver-based low e coatings reduce the emissivity to values around e=0.04. The relevant number for practical use is the coefficient of heat transfer k ( W m−2 K−1) describing the heat losses through an area of 1 m2 at a temperature difference of 1 K. Table 3 shows relative heat losses and k values for uncoated and coated insulating glazing units. The excellent properties of vacuum deposited low e films together with the inherently low environmental impacts of vacuum deposition have caused their current domination in the field of glass coating. Nowadays the annual worldwide production of insulating glazing units amounts to 200 million square metres. The share of vacuum coated low e glass is approximately 30%. The

biggest inline coating systems process a glass pane of 3.2 m×6 m within 45 s, corresponding to an annual throughput of 8 million square metres. Solar control film systems consist of an absorbing and reflecting material (e.g. CrN , TiN , FeN ) embedx x x ded into layers of a high refractive material (e.g. SnO ). A typical system is: 2 glass–SnO (10–100 nm)–CrN (10–30 nm) 2 x –SnO (10–30 nm) 2 Within a double glazing unit the solar control coating is located on the inner side of the outer glass pane (so-called plane II ). Depending on the thickness of the first SnO layer, a lot of reflection colours like grey, 2 silver, bronze, blue or blue-green are possible. The total solar energy transmission is determined by the thickness of the metal nitride layer. Solar control coatings are characterized by the so-called ‘‘shading coefficient’’ SC. SC is given by the ratio SC=g/g , where: o (1) g=total solar energy transmission of coated glass, (2) g =total solar energy transmission of clear glass o (thickness: 3 mm). The value of g is 0.87. Typical values for shading o coefficients range from 0.2 to 0.4.

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2.3. Anti-reflective–anti-static (ARAS) coatings on cathode ray tubes (CRT’s) Wide-band anti-reflective (AR) coatings permit a reduction of glass reflection down to values of 0.1–0.3% in a wide range of the visible spectrum. The AR effect is caused by the interference of light when passing through thin optical layers with refractive indices different from the substrate. During the past decades AR coatings have continuously spread into a wide field of applications. Besides optical instruments, eye-glasses, show-cases and architectural glazings, nowadays there is a steadily increasing demand to apply such coatings on data and TV screens (CRT’s) as well as flat panel displays. Since there is also a concern to minimize the amount of electromagnetic emission through the front of the CRT, the AR coating has to be combined with a transparent and conductive layer, resulting in a so-called ARAS system. The anti-static layer also reduces the accumulation of dust on the CRT. In general, a wide-band AR coating uses multilayer stacks of alternating high and low refractive dielectric thin films. Careful selection of coating material is necessary in order to achieve the desired optical effect as well as sufficient environmental stability of the product. As low refractive material SiO (n=1.46) is employed, a 2 prominent high refractive material is TiO (n= 2 2.20–2.50). Recent progresses in magnetron sputtering technology for deposition of dielectric layers allow the economic manufacturing of high performance AR or ARAS coatings on CRT’s as well as on large flat glass substrates. A typical specification for optical and electrical properties of an ARAS coating is given as follows:

(1) reflection R<0.6% at any wavelength within 430 and 650 nm, (2) integral reflection R <0.3% within 430 and 650 nm, y (3) resistivity across the CRT <1 kV. In Fig. 4 the reflection curve of a four layer ARAS coating (CRT–ITO–SiO –TiO –SiO ) is shown. The 2 2 2 integral reflection in the wavelength range from 430 to 650 nm is less than 0.2%. The resistivity measured between contact marks located at opposite edges of the screen is approximately 900 V. The layers are highly scratch resistant and pass all relevant chemical tests. In particular they withstand temperatures of nearly 500 °C. 2.4. Electrochromic glazings Glass manufacturers all over the world put much effort on the development of glazings with variable optical and thermal properties (‘‘Variable Transmission Windows’’ or ‘‘Smart Windows’’) [2]. Among possible technical solutions electrochromism is one of the most promising principles to achieve an architectural glazing with changeable properties. Fig. 5 shows the basic design of an electrochromic window. The electrochromic effect is based on a colouration of tungsten oxide by injection of protons or other light ions. To move the ions, an electrical field has to be applied. As in LCD technology, highly conductive ITO layers serve as transparent electrodes. Sheet resistivities of much less than 10 V are necessary to realize % short switching times. The light transmission (l= 550 nm) of an electrochromic coating may vary between 80% (bleached state) and 10% (coloured state). Transmission spectra are shown in Fig. 6. Electrochromic windows nowadays are seen as the preferred architectural glazing of the next century.

Fig. 4. Reflection curve of a four layer ARAS coating (CRT–ITO–SiO –TiO –SiO ) for CRT’s. The specification R<0.6% is marked by the 2 2 2 straight line.

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Fig. 5. Operation principle of an electrochromic window.

Fig. 6. Transmission spectrum of a bleached/coloured WO layer on transparent conductive oxide coated glass [2]. 3

A large area manufacturing technology is not yet available, its development will be a challenging task for the near future.

3. Sputtering technology for deposition of insulating materials The examples shown in this paper indicate quite clearly that nearly all applications of glass coating require insulating materials. In the past, magnetron sputtering offered two alternatives for deposition of these materials.

RF powered magnetrons can be operated with dielectric compound targets, but for economic mass production on large sizes their use is prohibitive because of low deposition rates, high equipment costs (power supply and matching network), and scaling problems. DC powered magnetrons enable rather high deposition speeds, but they require conductive targets, and the poor conductive or insulating compound has to be formed in a reactive gas atmosphere. Since the reaction cannot be limited to the substrate, but also takes place on the target and all other surfaces in the vicinity of the magnetron, process stability is not sufficient and achievable deposition rates are limited by the formation of arc

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Fig. 7. Operation principle of the TwinMagA sputter system.

discharges. The main problems accompanying DC reactive sputtering of dielectrics, namely the arcing problem and the anode problem, have been discussed elsewhere [3,4]. During the past five years, thin film engineers from all over the world made substantial progress in solving the above-mentioned problems by use of bipolar pulsed single magnetrons or mid-frequency (sine wave) powered double magnetron arrangements. In both cases, the frequency is between 10 and 100 kHz. One of the results of this development work is the TwinMagA system. It consists of two identical planar magnetron cathodes mounted close to each other in the vacuum chamber and connected to the output of an MF power supply. The TwinMagA is outlined in Fig. 7. At any time, one of the magnetrons is on negative potential and acts as a sputter cathode, while the second one acts as an anode. The momentary cathode is generating secondary electrons which are accelerated towards the anode and neutralize positive surface charges having built up in insulating areas during the negative half cycle. The TwinMagA permits deposition rates 2 to 6 times higher than those obtained by conventional DC reactive

sputtering, together with an outstanding process long term stability (approximately 300 h of uninterrupted operation, corresponding to the life time of a target) and excellent film properties. Table 4 summarizes some deposition data for the most important optical materials. Detailed results for these compounds are given elsewhere [3,5]. Large area thin film deposition with the TwinMagA is meanwhile established in several worldwide installed production coaters for display applications, low emissivity coatings and anti-reflective coatings on glass and web substrates. In particular, the new technology is indispensable for economic manufacturing of high performance ARAS coatings on CRT’s. Fig. 8 gives a schematic view of an inline coater dedicated to the production of such coatings. As in the case of coating technology for displays, sputter magnetrons and substrate carriers are arranged in a vertical position in order to minimize particle contamination. With 1650 mm magnetrons, the usable coating width is approximately 1000 mm. The configuration of sputter sources depends on layer design and desired productivity. For the stack

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G. Bra¨uer / Surface and Coatings Technology 112 (1999) 358–365 Table 4 Deposition data of various compound materials sputtered by TwinMagA (continuous production with magnetrons up to 3750 mm) Material

Film thickness at substrate speed of 1 m min−1a (nm)

MF power for 3750 mm magnetron (kW )

Deposition rate ratio, R /R MF DC

Index of refraction, n (550 nm)

SiO 2 Si N 3 4 TiO 2 Ta O 2 5 SnO 2

50 40 50 100 100

70 70 150 60 50

6 2 6 2 2

1.46 2.05 2.65 2.18 2.11

a Rough value for static rate in nm s−1 is dynamic rate divided by 10.

Fig. 8. Inline coater A1000 for CRT’s.

glass–ITO–SiO –TiO –SiO and a total number of 6 2 2 2 magnetrons (5 TwinMagA systems for SiO /TiO and 1 2 2 conventional DC magnetron for ITO), the annual throughput (24 hours×22 days×12 months) is approximately 2 million of 17 in. or 1.5 million of 21 in. screens.

4. Conclusions Among the applications of plasma surface engineering, large area glass coating has been playing an important and continuously growing part for nearly 20 years. Besides coatings for improvement of architectural glazings, substantial new markets in the flat panel display industry have developed. During the past five years considerable progress has been made in the field of sputter technology for deposition of ITO and optical thin film systems on large scale substrates. In particular, mid-frequency driven twin magnetrons have solved the critical problems accompanying reactive sputtering of insulating compounds, thus opening the door into new applications like anti-reflective–anti-static coatings onto display screens. ITO will keep its position as one of the key materials

in the future of glass coating, for displays as well as for many other products like electrochromic glazings, which probably will find a wide use in buildings of the next century. Regarding the continuous development of deposition technology, further improvement of material utilization (e.g. ITO targets), higher deposition rates and further reduction of equipment expenses will remain as permanent challenges for the future.

References [1] C. Daube, J. Stollenwerk, Balzers Process Systems GmbH, Siemensstr. 100, D-63755 Alzenau, Germany: private communication. [2] H. Wittkopf, in: Proceedings of Glass Processing Days, Tampere, Finland, September 13–15, 1997, p. 299. [3] G. Bra¨uer, W. Dicken, J. Szczyrbowski, G. Teschner, A. Zmelty, in: Proc. 3rd International Symposium on Sputtering and Plasma Processes (ISSP ’95), Tokyo, June 8/9, 1995, p. 63. [4] J. Szczyrbowski, G. Teschner, in: Society of Vacuum Coaters, Proc. 38th Ann. Technol. Conf., 1995, p. 389. [5] J. Szczyrbowski, G. Bra¨uer, M. Ruske, G. Teschner, A. Zmelty, Some Properties of TiO Layers Prepared by Medium Frequency 2 Reactive Sputtering, in: 1st International Conference on Coatings on Glass (ICCG), Saarbru¨cken, October 27–31, 1996, to be published in J. Non-Cryst. Solids.