ICMCTF2000 Session G5: Large-area Production Coatings for Webs/Plasma Cleaning and Pretreatment of Large Surfaces

The reactive magnetron sputtering of optical coatings, such as low-E coatings, onto glass for automotive and architectural applications is a well-established technique. However, these coatings can be prone to damage by abrasive particles encountered during service. Consequently, it is often necessary to use either a double-glazed product, where the coating is on an internal surface, or a laminated product, where the coated surface is encapsulated. Improving the durability of the functional film would remove the need to protect the film, and offer the potential to develop lighter, cheaper, less complex products.

To address the above problem, a study has been carried out in which films of alumina, titania and silica have been deposited and characterised in terms of structure, hardness, adhesion and wear resistance. The coatings were deposited by both DC and pulsed DC reactive magnetron sputtering. Using a range of different power delivery systems, it was possible to vary a number of deposition parameters, including pulse frequency (over the range 20-350kHz); duty factor (1-0.5); and reverse voltage (10-20%). Coating properties were analysed using nanoindentation, scratch adhesion and pin-on-disc wear testing techniques. Generally, the pulsed DC coatings exhibited a marked superiority over the DC coatings in terms of their structures and properties. For example, the critical loads of titania films deposited by pulsed DC were found to be some 30% higher than the values obtained for similar DC films. Optimum operating conditions for improved film properties have been identified through a systematic study of the pulsed DC deposition process

SiO₂ is widely used as a coating material due to its interesting properties like low refractive index, relative high hardness, high resistance against abrasion wear and good barrier properties. The presented paper gives a comparison of microstructure and layer properties between SiO₂ coatings deposited by reactive pulsed magnetron sputtering (PMS) and by hollow cathode activated EB evaporation (HAD). The specific advantages and drawbacks of both process types in respect of microstructure, layer properties and last but not least productivity of the coating process are discussed.

SiO₂ coatings deposited by HAD process at high deposition rates of about 600 nm/s have a relative low hardness of about 1,7-4,1 GPa, a Young's Modulus between 15 and 42 GPa and a low intrinsic stress. These results correspond to the comparable low packing density of the coatings. A substantial advantage of the HAD process is the relative low specific thermal load of the substrate which allows the coating of temperature sensitive plastic materials with some micrometers thick abrasion resistant layers. The Taber Abraser test shows remarkable good results for SiO₂ coatings on plastic substrates. Probably, the porosity is a reason for a favorable abrasion mechanism, leading to a higher dispersion of crack energy. The liability of cracking under mechanical load can be further reduced by introduction of hexamethyldisiloxane (HMDSO) in the plasma activated deposition process.

In contrast the SiO₂ coatings deposited by pulsed magnetron sputtering at deposition rates of about 5 nm/s have a much higher hardness of 8 GPa, a Young's Modulus of 70 GPa and more glass like properties. The high density of the coatings is the reason for the excellent environmental stability in the case of thin layers in optical applications. According to the high energy of condensing particles a higher specific thermal load of substrate and higher intrinsic stresses in the coatings have to be taken into account.

For some important oxide film materials - widely used especially in optical applications - conventional magnetron sputtering suffers from low deposition rates and difficult process controlling. The recently developed technique of Reactive Gas Flow Sputtering (RGFS) is able to circumvent both the rate and the stability problem owing to it's unique principle of operation. An additional advantage of RGFS is it's excellent target utilization of >60%.

For the large area deposition of high quality oxide films (Al₂O₃, TiO₂, ZrO₂, ITO, ...) a new generation of linear RGFS sputter sources has been developed. A target length of 1000 mm (39.4'') has been realized and further upscale of the process is easily attainable. Above an input power threshold which depends on material and geometry of the targets the hollow cathode glow discharge becomes distributed uniformly over about 90% of the length of the sputter source. A smooth decrease of the discharge occurs only at the ends of the target gap due to an increased loss of charge carriers. The homogeneity of the film thickness is mainly determined by the proper design of the gas flow system. A contamination of the films by residual gases can be effectively suppressed by an optimized gas flow guiding and an appropriate heat control of the housing and the surrounding of the sputter source. Due to the low argon content of the layers (<0.01%) the density and the refractive index of the oxide films are close to the values of the corresponding bulk materials. Indium tin oxide (ITO) films with high electrical conductivity (10...150 Ω per square) and excellent transparency have been deposited even at low substrate temperatures (T<80 deg. Celsius).