7 Effects of Developer Concentration on Linewidth Control in Positive Photoresists Tom Batchelder West Coast Technology Center, GCA Corporation, Sunnyvale, CA 94086
As lµ/m and submicron processes move from laboratory to preproduction development, the focus changes from "capability" to "repeatability" and finally to "controllability and throughput". It is toward improvements in these last two categories as they pertain to photoresist process that this paper will address. In order to restrict the scope of discussion i t will be assumed that in addition to smaller geometries on VLSI chips, that several other process design constraints will become standard practice for either technical or economic reasons, discussed below. The first of these constraints is "in-line process" for a l l imaging steps. This leaves considerable latitude for the details of baking resist coatings and development techniques but includes the c r i t i c a l limitation that development time must be compatible with exposure throughput at the camera. As the VLSI era matures the pressures of automation will both increase the cost of equipment and demand smooth flow of material from resist coating through development of the image. If processes can be designed to meet this limitation a vital ancilary benefit will be reduced exposure to particulate contamination. The second constraint is that on c r i t i c a l mask levels nearly monochromatic exposure will be required in order to obtain high quality images. The requisite reduction in photon flux will lead to camera throughputs in the neighborhood of 60 wafers per hour. Tight registration limitations may further limit this number but i t will be used as a reasonable throughput upper limit for VLSI wafers in next four to five years. Nearly monochromatic exposure emphasizes a second problem even as i t creates increased image quality, namely: increased standing waves at the image edge. There are two techniques for minimizing this problem: multistep baking (2) of resist (before and after image creation) and the utilization of substrate treatment which renders it non-reflective(3). It is not clear which of these techniques will give both high reliability and lowest process cost. The reason for this is that several other variables, which may differ in a 0097-6156/ 85/ 0290-0108$06.00/ 0 © 1985 American Chemical Society
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g i v e n IC m a n u f a c t u r i n g p r o c e s s must be c o n s i d e r e d , namely: s t e p h e i g h t t o be p r i n t e d o v e r , s u b s t r a t e r e f l e c t i v i t y and roughness, minimum f e a t u r e s i z e t o be p r i n t e d and d e s i r e d camera t h r o u g h p u t . In a l l but the most s e v e r e c a s e s , i t i s l i k e l y t h a t a p p r o p r i a t e adjustment o f h o t p l a t e bake p r o c e s s e s b e f o r e and a f t e r imaging w i l l be s u f f i c i e n t t o c r e a t e r e l i a b l e p r o c e s s down t o about 1/im minimum feature sizes. I n a d d i t i o n f u l l u t i l i z a t i o n of p r o c e s s c o n t r o l c a p a b i l i t i e s o f automated i n - l i n e w a f e r development t e c h n i q u e s must be made. Thus, the above c o n s t r a i n t s l i m i t t o t a l development time a v a i l a b l e from as much as 2 minutes w i t h b a t c h d e v e l o p m e n t ( J ) t o development p r o c e s s e s of about 30 t o 60 seconds. I f the #e(6) v a l u e i s used as a measure o f p r o c e s s q u a l i t y (which i s o n l y p a r t i a l l y the case as d i s c u s s e d b e l o w ) , i t can be seen ( F i g u r e 1) t h a t d i l u t e d e v e l o p e r g i v e s the b e s t performance. However, the p e n a l t y i s t h a t r e q u i r e d exposure time and thus camera throughput d e c r e a s e s ( F i g u r e 2 ) , I n a d d i t i o n , s i n c e the dose r e q u i r e d t o o b t a i n b r e a k t h r o u g h (Eo) i s not o n l y i n c r e a s e d but has a s t e e p s l o p e f o r s h o r t development t i m e , l i n e w i d t h c o n t r o l might be e x p e c t e d t o degrade as w e l l . T h i s , i n f a c t , has been our e x p e r i e n c e i n p r a c t i c e . In o r d e r t o o b t a i n e q u i v a l e n t l i n e w i d t h c o n t r o l w i t h M i c r o p o s i t 35105) (3.5:1 and 5:1 d i l u t i o n ) , the t o t a l s p r a y develop time had t o be i n c r e a s e d from 30 t o 60 seconds. Theory and
Experiment
L i n e w i d t h over w a f e r t o p o l o g y ("steps") i s a f f e c t e d by t h r e e separate p h y s i c a l e f f e c t s : bulk e f f e c t (thickness v a r i a t i o n at a s t e p edge), i n t e r f e r e n c e e f f e c t s and u n d e s i r e d r e f l e c t i o n from the s t e p o r o t h e r nearby f e a t u r e . Only the b u l k e f f e c t and i n t e r f e r e n c e (or s t a n d i n g wave) e f f e c t a r e amenable t o c l e a r m a t h e m a t i c a l t r e a t ment. The i n t e r f e r e n c e e f f e c t can be m i n i m i z e d by a n t i r e f l e c t i v e f i l m s under the r e s i s t c o a t i n g o r i t s r e s u l t s can be m i n i m i z e d by p o s t exposure bake which s l i g h t l y r e d i s t r i b u t e s the s o l u b i l i t y i n h i b i t o r a t the image edge and/or i n h i b i t s development a t t h e image edge due t o c r o s s l i n k i n g . I n any c a s e , r e f l e c t i o n s from t h e sub s t r a t e o r the s t e p edge cannot be e l i m i n a t e d by development p r o c e s s , d e v e l o p e r c o n c e n t r a t i o n o r p h o t o r e s i s t c o n t r a s t enhancement. These phenomena a r e o p t i c a l problems and must be d e a l t w i t h as p a r t o f the exposure p r o c e s s . On the o t h e r hand, the b u l k e f f e c t i s d i r e c t l y r e l a t e d t o development p r o c e s s and r e s i s t c o n t r a s t . I t has been found t h a t ( J O , f o r submicron i m a g i n g , i t i s the dominant cause o f l i n e w i d t h v a r i a t i o n (0.5/jm v s . 0.2jLim) o v e r s t e p s on aluminum sub strates. Thus, w h i l e r e f l e c t i o n e f f e c t s must be c a r e f u l l y managed t h e r e i s s t i l l s u b s t a n t i a l b e n e f i t t o be g a i n e d by o p t i m i z i n g development p r o c e s s , I f Θ, the edge w a l l a n g l e i s c o n s t a n t f o r the f u l l r e s i s t t h i c k n e s s , i t can be shown t h a t Q ) : (1)
tan θ
= K Zofle 1 +
