Analysis of vapor deposition processes
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AND
J. J. POVEROMO
A g e n e r a l f o r m u l a t i o n is p r e s e n t e d for d e s c r i b i n g the net r a t e of v a p o r d e p o s i t i o n f r o m a m o l t e n s o u r c e onto an i n i t i a l l y c o l d c o n d e n s i n g s u r f a c e . In the f o r m u l a t i o n a l l o w a n c e is m a d e for diffusion and m o l e c u l a r t r a n s p o r t and the r e s u l t a n t equations a r e shown to r e duce to the c o r r e c t e x p r e s s i o n for m o l e c u l a r d i s t i l l a t i o n at low r e s i d u a l p r e s s u r e s . In the s t a t e m e n t of the p r o b l e m a l l o w a n c e is a l s o m a d e for p o s s i b l e heat t r a n s f e r effects when the l a t e n t h e a t of d e p o s i t i o n is not d i s s i p a t e d r a p i d l y enough. Under t h e s e conditions the diffusional o r m o l e c u l a r t r a n s p o r t p r o b l e m has to be s o l v e d t o g e t h e r with the a p p r o p r i a t e t r a n s i e n t h e a t conduction e q u a t i o n s w r i t t e n for the d e p o s i t and the b a c k i n g p l a t e . The c o m puted r e s u l t s w e r e shown to a g r e e r e a s o n a b l y well with p r e v i o u s l y r e p o r t e d m e a s u r e m e n t s on v a p o r d e p o s i t i o n , and the f o r m u l a t i o n given h e r e m a y thus be u s e d for identifying the v a r i o u s a s y m p t o t i c r e g i m e s of v a p o r d e p o s i t i o n p r o c e s s e s .
T H E d e p o s i t i o n of m e t a l v a p o r s onto c o o l e d s u r f a c e s p l a y s an i m p o r t a n t r o l e both in m a t e r i a l s f i n i s h i n g ope r a t i o n s and in s e p a r a t i o n p r o c e s s e s . A c o m m o n f e a t u r e of m o s t of t h e s e o p e r a t i o n s is that t h e r e e x i s t s a m o l t e n m e t a l pool which a c t s a s the s o u r c e of the v a p o r , in the p r o x i m i t y of an i n i t i a l l y cold s u r f a c e (or continuo u s l y c o o l e d s u r f a c e ) onto which the v a p o r is d e p o s i t e d . Such an a r r a n g e m e n t is s k e t c h e d in Fig. 1. In its gene r a l f o r m v a p o r d e p o s i t i o n is a c o m p l e x p r o c e s s , the i n d i v i d u a l c o m p o n e n t s of which m a y i n c l u d e : a) diffusion and h e a t t r a n s f e r in the liquid pool (source); b) diffusion o r s e l f - d i f f u s i o n in the gas s p a c e b e tween the s o u r c e and the sink; c) a l t e r n a t i v e l y , the m o v e m e n t of m o l e c u l a r b e a m s in v a c u u m b e t w e e n the s o u r c e and the sink; d) h e a t t r a n s f e r at the s i n k s u r f a c e . As it is r e a d i l y s e e n f r o m the f o r e g o i n g , the s i t u a tion m a y be p a r t i c u l a r l y c o m p l i c a t e d b y the fact that m o l e c u l a r and continuum p h e n o m e n a m a y o c c u r both in p a r a l l e l and c o n s e c u t i v e l y . By f a r the g r e a t e s t p r a c t i c a l a p p l i c a t i o n of v a p o r d e p o s i t i o n t e c h n i q u e s is the f o r m a t i o n of c o a t
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