A COATING THICKNESS UNIFORMITY MODEL FOR PHYSICAL VAPOR-DEPOSITION SYSTEMS - FURTHER ANALYSIS AND DEVELOPMENT

A coating thickness uniformity model, for physical vapour deposition ( PVD) in low-pressure gas. is developed further. The original model des cribes coating uniformity in terms of the Front-to-back thickness rati o, R, of coatings on thin flat substrates, where front and hack refer to surfaces facing towards rind away from the vapour source, respectiv ely. By expanding on the principles adopted for developing the origina l model, an expression for the front-face fall-off in coating thicknes s with source-to-substrate distance, s, is derived and verified experi mentally; this expression isolates the influence of the progressive di lution of vapour flux that would occur in vacuum (the psi function) fr om gas scattering effects. Thus it is shown that the psi function appr oximates to an inverse square law with s. although the level of approx imation depends on the emission characteristics of the vapour source a nd associated virtual source effects. Knowledge of the psi function al lows the original model for R to be extended and expressions for the f ollowing source-substrate configurations are presented: (1) a thin sub strate at an angle to the vapour source, (2) a thick substrate, and (3 ) two vapour sources at different values of s. (C) 1998 Elsevier Scien ce S.A.