Fast growing protrusions from epitaxial semiconductor surfaces
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MACROSCOPICdefects
a r e often found p r o t r u d i n g f r o m the s u r f a c e of epitaxial s e m i c o n d u c t o r l a y e r s . 1-3 It has b e e n known for some time that such defects h i n d e r s u b s e q u e n t device f a b r i c a t i o n p r o c e d u r e s . This p r o b l e m is e s p e c i a l l y c r i t i c a l d u r i n g m a s k i n g , where the defects s e v e r e l y a b r a d e and s o m e t i m e s b r e a k the m a s k . 4 They will a l s o p r e v e n t p r o p e r m a s k p o s i t i o n i n g , r e s u l t i n g in a s i g n i f i c a n t loss of r e s o l u tion. Until r e c e n t l y the r e s p o n s e s e e m s g e n e r a l l y to have b e e n to t o l e r a t e the defects. However, with the i n t r o d u c t i o n of m e t a l m a s k s a n d with m o r e s t r i n g e n t m a s k i n g r e q u i r e m e n t s n e c e s s i t a t e d by f u r t h e r device m i n i a t u r i z a t i o n , this i s no l o n g e r f e a s i b l e . T h i s p a p e r d i s c u s s e s an u n u s u a l c l a s s of s u r f a c e defects which a p p e a r d o m e - s h a p e d and faceted, a n d have a r e l a t i v e l y high growth r a t e . B e c a u s e of t h e i r l a r g e s i z e , they a r e p a r t i c u l a r l y d a m a g i n g to m a s k i n t e g r i t y and a l i g n m e n t . F o r p e r s p e c t i v e it i s i n s t r u c t i v e to c o n t r a s t these dome defects with other, b e t t e r known s t r u c t u r e s : A s c h e m a t i c r e p r e s e n t a t i o n of the r e l a t i v e s i z e s and shapes of s e l e c t e d epitaxial o v e r g r o w t h s i s shown in Fig. 1. The t r i p y r a m i d , Fig. l(a), i s intended to be r e p r e s e n t a t i v e of m a n y w e l l - k n o w n types of s u r f a c e defects. The defects in this c l a s s a r e r e l a t i v e l y low and b r o a d ; t h e i r height is of the o r d e r of magnitude of the epitaxial l a y e r t h i c k n e s s . T h i s may be cont r a s t e d with the s y n t h e t i c a l l y induced V a p o r - L i q u i d Solid (VLS) o v e r g r o w t h , Fig. l(c), of W a g n e r and E l l i s . s We will have cause to r e f e r to this s t r u c t u r e l a t e r so a b r i e f review of its growth m e c h a n i s m is in o r d e r h e r e : When a dot of gold o r some o t h e r m e t a l is deposited on a s e m i c o n d u c t o r s u b s t r a t e wafer, the m e t a l alloys with the s e m i c o n d u c t o r and local m e l t i n g o c c u r s d u r i n g v a p o r growth. Since the g a s e o u s r e a c t a n t s have a h i g h e r s t i c k i n g coefficient on the liquid than on the solid, the liquid will a b s o r b the r e a c t a n t at a g r e a t e r rate. However, the liquid will not s u p p o r t m o r e than a c e r t a i n c o n c e n t r a t i o n of s e m i c o n d u c t o r so that growth p r o c e e d s by s o l i d i f i c a t i o n on the lower i n t e r f a c e . A f t e r cooling, a two phase r e g i o n with a J. M. GREEN is Research Staff Member, IBM Thomas J. Watson Research Center, Y
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