MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY

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MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY

R. Osgood, Jr

To cite this version:

R. Osgood, Jr. MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEM- ISTRY. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-133-C5-138. �10.1051/jphyscol:1983522�.

�jpa-00223103�

JOURNAL DE PHYSIQUE

Colloque C5, supplement au nD1O, Tome 44, octobre 1983 page C5-133

M A T E R I A L D E P O S I T I O N A N D R E M O V A L U S I N G L A S E R - I N I T I A T E D C H E M I S T R Y

R.M. Osgood, Jr.

Departments o f EZectricaZ Engineering and Applied Physics, CoZwnbia University, NY, NY 10027, U.S.A.

~ 6 s u m ; - L ' u t i l i s a t i o n du r a y o n l a s e r pour c o n t r a l e r d e s r 6 a c t i o n s chimiques 5 g r a n d e e t p e t i t e 6 c h e l l e e s t d i s c u t k e . T r o i s a p p l i c a t i o n s s p d c i f i q u e s s o n t donne'es comme exemple.

A b s t r a c t - The u s e of l a s e r l i g h t t o c o n t r o l l a r g e and s m a l l s c a l e c h e m i c a l r e a c t i o n s i s d i s c u s s e d . T h r e e s p e c i f i c a p p l i c a t i o n s a r e g i v e n a s examples.

I

-

INTRODUCTION

I n t h e l a s t few y e a r s , i t h a s become i n c r e a s i n g l y a p p a r e n t t h a t r a p i d h e a t i n g w i t h l a s e r l i g h t i s a powerful t e c h n i q u e f o r forming new and unexpected m a t e r i a l s o r m a t e r i a l s p r o p e r t i e s . L a s e r t r a n s i e n t a n n e a l i n g h a s been u s e d t o produce

s e m i c o n d u c t o r s w i t h doping l e v e l s e x c e e d i n g t h e s o l i d s o l u b i l i t i e s a t t h e a n n e a l i n g t e m p e r a t u r e / I / . New m e t a l - g l a s s e s have been formed by t r a n s i e n t h e a t i n g of mixed f i l m s of m e t a l and s e m i c o n d u c t o r s . P i c o s e c o n d l a s e r p u l s e s have been u s e d t o s w i t c h a semiconductor r e p e a t e d l y between i t s amorphous and c r y s t a l l i n e s t a t e s . The r e s u l t s have g i v e n new i n s i g h t i n t o m a t e r i a l s p r o p e r t i e s and s u g g e s t e d a h o s t of new p r o c e s s i n g o p t i o n s f o r p r e p a r i n g e l e c t r o n i c m a t e r i a l s and components. L a s e r a n n e a l i n g h a s a c h i e v e d t h e s e r e s u l t s by c o n t r o l l i n g o n l y one p a r a m e t e r of m a t e r i a l s p r e p a r a t i o n , namely, t h e r a t e of energy i n p u t o r o u t p u t . However, i t i s a l s o p o s s i b l e t o c o n t r o l t h e f l o w of m a t e r i a l s t o and from a s u r f a c e by u s i n g l a s e r r a d i a t i o n t o i n i t i a t e and d r i v e a s p e c i f i c c h e m i c a l r e a c t i o n 121.

L a s e r c h e m i s t r y i s a r e s e a r c h f i e l d which a n t e d a t e s any e x t e n s i v e r e s e a r c h i n l a s e r a n n e a l i n g ; however, it i s o n l y r e c e n t l y t h a t l a s e r s have been used t o c o n t r o l i n t e r f a c e r e a c t i o n s and a c h i e v e r e s u l t s a n a l o g o u s t o l a s e r a n n e a l i n g . L a s e r c h e m i s t r y c a n b e used t o d e m o n s t r a t e a much w i d e r r a n g e of p r o c e s s i n g o p t i o n s t h a n s i m p l e h e a t i n g / 2 , 3 , 4 / ; m a t e r i a l s c a n n o t o n l y b e m o d i f i e d , b u t m e t a l s , i n s u l a t o r s , and s e m i c o n d u c t o r s c a n b e d e p o s i t e d o r removed ( e t c h e d ) . Small amounts of atoms can b e i n c o r p o r a t e d i n t o a s o l i d s u r f a c e by s i n g l e - s t e p photochemical doping. L a s e r c h e m i s t r y can b e b a s e d on r e s o n a n t o r non-thermal phenomena and i n t h e s e c a s e s , t h e p r o c e s s i n g h a s t h e a d v a n t a g e of k e e p i n g t h e s u b s t r a t e a t room t e m p e r a t u r e . Because t h e e x t e n t of t h e r e a c t i o n zone c a n b e v a r i e d by changing t h e s i z e o f t h e laser-beam, b o t h l o c a l and l a r g e - a r e a r e a c t i o n s c a n b e o b t a i n e d . S i n c e t h e mecha- nisms of l a s e r i n t e r f a c e r e a c t i o n s c a n b e c o n s i d e r a b l y d i f f e r e n t from t h e u s u a l homogeneous o r inhomogeneous r e a c t i o n s , new "nonequilibrium" m a t e r i a l s c a n b e grown.

L a s e r chemical p r o c e s s i n g c a n b e s u b d i v i d e d a c c o r d i n g t o t h e p h a s e of t h e medium b e i n g c h e m i c a l l y a c t i v a t e d . T a b l e I shows t h i s d i v i s i o n . N o t i c e t h a t l a s e r photochemical r e a c t i o n s have been d e m o n s t r a t e d i n v i r t u a l l y a l l forms of c h e m i c a l l y a c t i v e medium. Each m a t e r i a l p h a s e h a s i t s p a r t i c u l a r a d v a n t a g e s . A g a s e o u s medium i s d r y and c a n b e flowed s i m p l y o v e r t h e s u b s t r a t e . Thus, many of t h e c a p a b i l i t i e s of g a s p r o c e s s i n g a r e s i m i l a r t o t h o s e f o r CVD o r plasma p r o c e s s i n g . A l i q u i d medium i s u s e f u l b e c a u s e i o n - c h e m i s t r y p r o v i d e s a p a r t i c u l a r l y powerful approach t o producing s e l e c t i v e d i s s o l u t i o n e f f e c t s f o r e t c h i n g o r c a r r y i n g n o b l e m e t a l s s u c h a s g o l d and p l a t i n u m . Molecular s u r f a c e l a y e r s p r o v i d e a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983522

C5-134 JOURNAL DE PHYSIQUE

h i g h - d e n s i t y chemical medium s u c h a s a l i q u i d o r s o l i d , b u t may b e r e a d i l y formed u s i n g a g a s ambient.

TABLE I CHEMICAL MEDIUM FOR LASER CHEMICAL PROCESSING

PHASE

-

Gas

Liquid

EXAMPLES

Thermochemical deposition of silicona Etching of silicon by clZb and sioZd Deposition

Photoetching of 111-lve compounds Deposition of Au f

Solid ~tching of polymersg

Adsorbed Layers Surface Nucleation of Cd Atoms h Photodeposition of A1 i

a) ALLEN S.D., BASS

M.,

Appl. Phys. Lett. 76 (1981) 431.

E W I C H D.J., OSGOOD R.M., DEUTSCH T.F., Appl. Phys. Lett. 39 (1981) 957;

BAUERLE P., Appl. Phys. 28 (1982) 267.

b)

EHRLICH D.J., OSGOOD R . M ~ DEUTSCH T .F., Appl. Phys . ^Lett. 2 1018 (1982) c) CHUANG T.J.,

3.

Vac. Sci. Technol. 21 (1982) 800.

d) SOLANK1 R., BOYES P.V., COLLINS C.J., Appl. Phys. Lett. K(1982) 1048.

e) See, for example, OSGOOD R.M., SANCHEZ A., EHRLICH D.J., and DANEU V., Appl. Phys. Lett. 40 (1982) 391 and references cited therein.

f) VON GUTFELD R.J., ACOSTA

R . E . ,

ROMANKIN L.T., IBM 3. Res Develop. 26 (1982) 136.

g) SHRINAVASON

R.

and WAYNE-BANTOR V., Appl. Phys. Lett. 61 (1982) 576.

h) EHRLIGH D.J., OSGOOD R.M., DEUTSCH T.F., Appl. Phys. Lett. 2 (1981) 946.

i) D.J. Ehrlich, R.M. Osgood, T.F. Deutsch, J. Vac. Sci. Technol. 2, ⁽¹⁹⁸²⁾

23.

L a s e r c h e m i c a l p r o c e s s i n g may a l s o be c l a s s i f i e d a c c o r d i n g t o t h e s i z e of t h e r e a c t i o n zone. F o r example, excimer l a s e r s / 5 / have r e c e n t l y been used t o d e p o s i t i n s u l a t o r s o r t o e t c h GaAs and s i l i c o n o v e r a n a r e a comparable t o t h a t of a

semiconductor w a f e r . T h i s t y p e of l a s e r c h e m i s t r y p r o v i d e s t e c h n i q u e s which become a n a l t e r n a t i v e t o t h e u s u a l p l a n a r p r o c e s s i n g t e c h n i q u e s f o r semiconductor f a b r i c a - t i o n ; e . g . , vacuum d e p o s i t i o n and plasma e t c h i n g . The r e l a t i v e l y h i g h a v e r a g e power a v a i l a b l e from excimer l a s e r s i s s u f f i c i e n t t o p r o c e s s a n e n t i r e w a f e r . I n g e n e r a l , t h i s t y p e of p r o c e s s i n g r e q u i r e s a mask f o r p a t t e r n d e l i n e a t i o n . On t h e o t h e r hand, by u s i n g a f o c u s s e d , cw l a s e r i t is p o s s i b l e t o form submicrometer r e a c t i o n zones, and t h e r e b y produce a h i g h l y l o c a l , l a s e r c h e m i s t r y . T h i s t y p e of l a s e r p r o c e s s i n g h a s been c a l l e d d i r e c t w r i t i n g / 3 , 6 / , s i n c e d e p o s i t e d o r e t c h e d

s t r u c t u r e s can be produced without a mask and, i n most c a s e s , i n a s i n g l e s t e p . The a p p l i c a t i o n s f o r d i r e c t w r i t i n g a r e g e n e r a l l y t h o s e which r e q u i r e a d i s c r e - t i o n a r y o r custom type of s e r i a l p r o c e s s i n g over a s e l e c t e d r e g i o n of a s u r f a c e ; e.g. mask-metallization r e p a i r o r d i s c r e t i o n a r y i n t e r c o n n e c t i o n s . F i n a l l y , l a s e r chemical p r o c e s s i n g can a l s o accomplish m a t e r i a l growth and e t c h i n g i n ways which a r e unique t o a c o h e r e n t o p t i c a l s o u r c e . Recently u l t r a h i g h - r e s o l u t i o n GaAs g r a t i n g s have been made v i a a maskless technique which r e l i e s on t h e i n t e r f e r e n c e of two l a s e r beams i n a chemical medium. S i m i l a r l y , a s e r i e s of experiments h a s shown t h a t m a t e r i a l s grown w i t h p o l a r i z e d l i g h t have a c o h e r e n t l y arranged m i c r o s t r u c t u r e . The former technique h a s d i r e c t a p p l i c a t i o n s t o f a b r i c a t i n g v a r i o u s e l e c t r o o p t i c a l components. The l a t t e r o f f e r s t h e e v e n t u a l promise of producing m a t e r i a l s whose e l e c t r i c a l and s t r u c t u r a l p r o p e r t i e s a r e c o n t r o l l e d by l a s e r l i g h t .

I n t h e remainder of t h i s review, I would l i k e t o d e s c r i b e b r i e f l y t h r e e examples of l a s e r chemical p r o c e s s i n g . The examples a r e chosen s o t h a t they each i l l u s t r a t e one of t h e g e n e r i c approaches t o l a s e r p r o c e s s i n g given i n Table 11. More ex- t e n s i v e reviews of l a s e r chemical e f f e c t s have been given i n t h e reviews l i s t e d i n t h e r e f e r e n c e s / 2 , 3 , 4 , 5 , 7 / .

TABLE I1

GENERIC APPLICATIONS OF LASER CHEMICAL PROCESSING

TYPE

- CHARACTERISTICS EXAMPLES

P l a n a r P r o c e s s i n g Large-area, i . e . -10cm 2 D e p o s i t i o n of i n s u l a t o r s

Requires masking o r and m e t a l s

p a t t e r n e d l a s e r beam

High a v e r a g e power l a s e r Etching of GaAs and S i Etching of Polymers

S e r i a l o r D i r e c t - Processing of a Writing of conducting

Write P r o c e s s micrometer-scale p i x e l l i n k s

Low-average-power l a s e r

Maskless Etching of v i a s i n

One-s t e p S i l i c o n and GaAs wafers

Modulated doping Coherent P r o c e s s i n g R e l i e s on t h e coherence Etching of high

of l a s e r l i g h t r e s o l u t i o n g r a t i n g s i n GaAs

I1 - PHOTON-ASSISTED PLANAR PROCESSING-ETCHING OF GaAs

Dry-etching i s a g e n e r a l term f o r f a b r i c a t i o n of h i g h - r e s o l u t i o n s t r u c t u r e s without using aqueous s o l u t i o n s . Dry-etching i s now done by u s i n g charged p a r t i c l e s from a gas d i s c h a r g e o r an i o n s o u r c e . I n t h e c a s e of t h e compound semiconductors, t h i s form of e t c h i n g i s sometimes u n s a t i s f a c t o r y because of s u r f a c e damage due t o c h a r g e - p a r t i c l e i n c o r p o r a t i o n . We have r e c e n t l y developed an approach t o dry-etching G a A s which u s e s p h o t o d i s s o c i a t i o n of CH3Br, CF3Br, e t c . , t o produce f r e e r a d i c a l s which remove t h e s u r f a c e m e t a l l o i d s 181.

The approach used i s t o p l a c e t h e GaAs i n a sample c e l l c o n t a i n i n g t h e u n d i s s o c i a t e d e t c h i n g gas. The gas i s t h e n i r r a d i a t e d w i t h t h e pulsed output from an ArF

excimer l a s e r . The e t c h i n g parameters and t h e e t c h i n g r a t e s a r e summarized i n

C5-136 JOURNAL DE PHYSIQUE

F i g . 1. To o b t a i n t h e r a p i d e t c h i n g rates shown i n t h e f i g u r e , i t i s n e c e s s a r y a l s o t o i l l u m i n a t e t h e s u r f a c e w i t h l a s e r r a d i a t i o n . We b e l i e v e t h a t t h i s s u r f a c e e x p o s u r e removes t h e r e a c t i o n p r o d u c t s , such a s Ga(CF3)3, which form a condensed f i l m on t h e semiconductor s u r f a c e . The removal mechanism i s a t t r i b u t a b l e t o a combination of t h e r m a l and photochemical d e s o r p t i o n .

EXPOSURE TIME Imbn.1

F i g . 1

-

E t c h d e p t h i n GaAs a s a f u n c t i o n of e x p o s u r e t i m e .

The r e a c t i o n r a t e may b e monitored by p h y s i c a l l y measuring t h e e t c h d e p t h v i a a m e c h a n i c a l s t y l u s . An a d d i t i o n a l approach r e l i e s on t h e f a c t t h a t t h e uv l i g h t p h o t o d i s s o c i a t e s t h e Ga m o l e c u l a r p r o d u c t s and t h e r e b y produces e x c i t e d Ga atoms;

t h e s e atoms t h e n e m i t a t t h e g r e e n g a l l i u m r e s o n a n c e l i n e . Thus, one c a n probe t h e e t c h i n g by simply o b s e r v i n g t h e f l u o r e s c i n g g a l l i u m atoms.

We a r e c u r r e n t l y comparing t h e e t c h i n g r a t e u s i n g v a r i o u s , s t a n d a r d and p e r f l u o r i n a t e d m e t h y l - h a l i d e s .

III

-

DIRECT WRITING - LASER WRITING OF CONDUCTING LINES

I n many a r e a s of I . C . f a b r i c a t i o n , i t would b e d e s i r a b l e t o w r i t e , o n a d i s c r r t i o n a r y b a s i s , m i c r o m e t e r - s c a l e c o n d u c t i n g l i n e s . During t h e l a s t few y e a r s . we have shown t h a t m e t a l l i n e s c a n b e w r i t t e n by u s i n g u l t r a v i o l e t p h o t o d i s s o c i a t i o n of m e t a l - a l k y l v a p o r . Thus f a r , most of t h i s work h a s been d i r e c t e d toward u n d e r s t a n d i n g t h e l i m i t s on t h e r e s o l u t i o n of t h e p r o c e s s . I n t h i s c o n n e c t i o n , t h e a u t h o r and D. E h r l i c h / 9 / have r e c e n t l y shown t h a t l i n e s a s narrow a s 0.5 pm can b e produced by u s i n g a 257-nm l a s e r beam f o c u s e d w i t h a l e n s w i t h a n u m e r i c a l a p e r t u r e of 0.5.

A n i m p o r t a n t c r i t e r i a f o r d i r e c t w r i t i n g t o b e p r a c t i c a l i s t h a t t h e w r i t i n g r a t e must b e s u f f i c i e n t l y f a s t t h a t u s e f u l l y l a r g e l i n e a r d i s t a n c e s c a n b e w r i t t e n . We have r e c e n t l y begun a s y s t e m a t i c measurement of t h e s e r a t e s a t s u f f i c i e n t l y h i g h g a s p r e s s u r e s t h a t r a p i d growth r a t e s c a n b e e x p e c t e d . These measurements show t h a t f o r even v e r y low l a s e r powers, e.g. -100 ~ I W , -400-600-nm/sec r a t e s c a n be o b t a i n e d . Use of h i g h e r l a s e r powers s h o u l d produce r a t e s s e v e r a l o r d e r s of magnitude f a s t e r .

Andther i m p o r t a n t r e c e n t r e s u l t h a s been t o a c h i e v e s u f f i c i e n t c o n t r o l of l a s e r p r o c e s s i n g t h a t s e v e r a l t y p e s of l a s e r w r i t i n g c a n b e u s e d t o f a b r i c a t e more complex s t r u c t u r e s t h a n a s i n g l e c o n d u c t i n g l i n e . F i g . 2 shows a m i c r o s t r u c t u r e c o n s i s t i n g of a n S i 0 2 l i n e w r i t t e n on a n S i - s u b s t r a t e / 9 / ; t h e l i n e i s t h e n c r o s s e d by a m e t a l l i n e (Cd). The SiO2 l i n e i s w r i t t e n u s i n g a spin-on s i l i c a t e p a t e r i a l o b t a i n e d from A l l i e d C o r p o r a t i o n . The unexposed s i l i c a t e i s removed u s i n g a n a l c o h o l r i n s e b e f o r e t h e m e t a l d e p o s i t i o n . The t h r e e m a t e r i a l s shown i n F i g . 2 a l l o w

one t o w r i t e MOS s t r u c t u r e w i t h a l a s e r , a r e s u l t r e c e n t l y r e p o r t e d by McWilliams, e t a l .

/ l o / .

- -

F i g . 2 - Cd l i n e d i r e c t l y w r i t t e n o v e r S i 0 2 p a t t e r n on a s i l i c o n w a f e r .

I V

-

COHERENT PROCESSING

-

LASER INTERFERROMETRIC PRODUCTION OF DIFFRACTION GRATINGS

V i s i b l e l i g h t can b e u s e d t o enhance t h e d i s s o l u t i o n r a t e of s e m i c o n d u c t o r s immersed i n c e r t a i n aquaeous m i x t u r e s , c o n t a i n i n g , t y p i c a l l y , a g e n t s f o r o x i d a t i o n and o x i d e d i s s o l u t i o n . T h i s t e c h n i q u e , which i s a form of a n o d i c o x i d a t i o n , h a s r e c e n t l y been u s e d i n c o n j u n c t i o n w i t h l a s e r r a d i a t i o n t o produce h i g h l y

l o c a l i z e d e t c h i n g of a semiconductor s u r f a c e . I n a n e f f o r t t o d e t e r m i n e t h e u l t i m a t e l i m i t a t i o n s i n t h e p r o c e s s r e s o l u t i o n , we h a v e i n v e s t i g a t e d t h e e t c h i n g of very-small-period g r a t i n g s

I l l / .

These g r a t i n g s a r e made by i n t e r f e r i n g two c o l l i m a t e d l a s e r beams i n a n e t c h i n g s o l u t i o n . By p a y i n g c a r e f u l a t t e n t i o n t o t h e e t c h i n g s o l u t i o n c h e m i s t r y and t o t h e s t a b i l i t y of t h e o p t i c a l t r a i n , we have produced g r a t i n g s w i t h 130-nm p e r i o d s on n-Type GaAs. We b e l i e v e t h a t 100-nm g r a t i n g s h a v e been produced; however, d e t e c t i o n of t h i s f i n e a g r a t i n g p e r i o d i s e x t r e m e l y d i f f i c u l t .

A wide v a r i e t y of u n u s u a l g r a t i n g p r o f i l e s c a n b e o b t a i n e d u s i n g t h i s t e c h n i q u e . For a s h a l l o w b u t f u l l y developed g r a t i n g , t h e p r o f i l e i s s c a l l o p e d ; a n example i s shown i n F i g . 3 f o r t h e c a s e of a 200-nm g r a t i n g . With o t h e r s o l u t i o n s t h e g r a t i n g p r o f i l e r e s e m b l e s a s e r i e s of narrow p e a k s ; a t i l t e d s u b s t r a t e w i l l y i e l d a g r a t i n g w i t h a b l a z e d p r o f i l e .

F i g . 3 - P r o f i l e of GaAs g r a t i n g produced by i n t e r f e r i n g two v i s i b l e l a s e r beams.

V

-

CONCLUSION

The above p r e s e n t s a b r i e f i n t r o d u c t i o n t o t h e u s e of l a s e r i n t e r f a c e c h e m i s t r y f o r m a t e r i a l s p r o c e s s i n g . U n l i k e t h e c a s e of l a s e r a n n e a l i n g , l a s e r c h e m i s t r y

JOURNAL DE PHYSIQUE

c a n n o t o n l y change t h e c o m p o s i t i o n of a s o l i d s u r f a c e , b u t i t can a l s o c a u s e

a d d i t i o n and d e l e t i o n of m a t e r i a l t o t h e s u b s t r a t e . L a s e r p r o c e s s i n g i s i n t e r e s t i n g n o t o n l y f o r i t s immediate a p p l i c a t i o n such a s p h o t o l i t h o g r a p h i c mask r e p a i r , b u t a l s o f o r i t s l o n g t e r m impact on s u r f a c e s c i e n c e and t h e p r e p a r a t i o n of n o v e l m a t e r i a l s .

V I - ACKNOWLEDGMENT

I would l i k e t o acknowledge members of t h e Columbia M i c r o e l e c t r o n i c s S c i e n c e L a b o r a t o r y f o r t h e i r c o n t r i b u t i o n t o and s u p p o r t of t h i s work. I n a d d i t i o n , s p e c i a l t h a n k s go t o my former c o l l e a g u e s , Tom Deutsch, Dan E h r l i c h , and Don S i l v e r s m i t h a t L i n c o l n L a b o r a t o r y f o r t h e i r c o l l a b o r a t i o n o v e r t h e p r e v i o u s s e v e r a l y e a r s .

REFERENCES

(1) See, f o r example, APPLETON B.R. and CELLER G.K. ( e d s . ) L a s e r and E l e c t r o n - Beam I n t e r a c t i o n s w i t h S o l i d s , North H o l l a n d : New York (1982).

(2) OSGOOD R.M., J r . i n Ann. Rev. Phys. Chem. (1983) i n P r e s s .

(3) EHRLICH D . J . , OSGOOD R.M., DEUTSCH T.F., J . Quantum E l e c t r o n . QE-16 (1980) 1233.

( 4 ) OSGOOD R.M., J r . , BRUECK S.R.J., and SCHLOSSBERG H . , Ed. L a s e r D i a g n o s t i c s and Photochemical P r o c e s s i n g f o r Semiconductor E l e c t r o n i c s . North Holland:

New York (1983).

(5) For a b r i e f r e v i e w s e e OSGOOD R.M., J r . i n P r o c e e d i n g s of Excimer L a s e r Meeting, Lake Tahoe (1983) AIP P r e s s .

(6) OSGOOD R.M., J r . , EHRLICH D . J . , DEUTSCH T.F., SILVERSMITH D . , and SANCHEZ A., "Direct-Write L a s e r F a b r i c a t i o n C u s t o m i z a t i o n , C o r r e c t i o n and R e p a i r " , t o b e p u b l i s h e d i n P r o c e e d i n g s of Les Deux A l p e s Meeting on M i c r o e l e c t r o n i c s (1983).

( 7 ) CHUANG T . J . , J . Vac. S c i . Technol.

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(8) BREWER P . , HALLE S., OSGOOD R.M., J r . P a p e r p r e s e n t e d a t North-East R e g i o n a l Chemical S o c i e t y , H a r t f o r d ( J u n e 1 9 8 3 ) .

(9) EHRLICH D.J., OSGOOD R.M., and DEUTSCH T.F., J. Vac. S c i . Technol.

z,

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(10) MCWILLIAMS B . , HERMAN I . P . , MITLITZKY F . , and WOOD L. p a p e r p r e s e n t e d a t CLEO '83, B a l t i m o r e , Md (1983).

(11) PODLESNIK D . , GILGEN H.H., OSGOOD R.M., J r . t o b e p u b l i s h e d . See a l s o r e l a t e d a r t i c l e i n Ref. 4.