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

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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 (