Factors Affecting Adhesion of Lithographic Materials - ACS Publications

26 Factors Affecting Adhesion of Lithographic Materials

Downloaded by UNIV OF ROCHESTER on August 31, 2017 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch026

K. L. MITTAL IBM Data Systems Division, E. Fishkill, Hopewell Jct., NY 12533

The lithographic technique is used extensively for the fabrication of semiconductor devices and large scale integrated circuits by planar technology. The present and future demands for extremely small pattern dimensions put more stringent requirements on all the materials and processes involved in the lithographic technique. Typical processing steps used for delineating patterns using resists are shown pictorially in Figure 1. Figure 1 shows line pattern generation in SiO insulator film but the same steps are also used for making patterns in metal and alloy films. It should be noted that the cleaning and preparation (for example, use of adhesion promoters) of the substrate (e.g. SiO ) and application of a resist layer precede the process of exposure to radiation. Different resists are sensitive to different sources of radiation: UV light, X-ray, E-beam, ion beam or laser. If the resist i s sensitive to UV light, then i t is termed a photoresist and the technique i s called photolithography. In the case of E-beam lithography, the resist is sensitive to E-beam and so on. 2

2

The resist c o m p o s i t i o n consists essentially o f three components: polymeric resin, photoactive compound or sensitizer, and solvent. Sometimes a surfactant is also a d d e d t o e n h a n c e its wettability characteristics. T h e r e are two kinds o f resists: n e g a t i v e and positive. I n the case of negative resists, t h e p o l y m e r i c resi n gets crosslinked o n e x p o s u r e t o radiation, t h e r e b y rendering it insoluble in the developer, w h e r e a s a positive resist o n e x p o s u r e t o radiation u n d e r g o e s c h e m i c a l transformation that increases its solubility in s u b s e q u e n t developing solutions. It is t h e differential solubility o f the e x p o s e d and the u n e x p o s e d r e g i o n s that allows t h e d e v e l o p m e n t of high-resolution chemically resista n t patterns. E a r l y m a t e r i a l s w h i c h showed c h a n g e s i n s o l u b i l i t y u p o n UV e x posure were: F i s h g l u e , a s p h a l t , s u g a r s , g e l a t i n s s e n s i t i z e d w i t h

0-8412-0485-3/79/47-095-371$05.25/0 ©

1979 A m e r i c a n C h e m i c a l Society

Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

DURABILITY O F M A C R O M O L E C U L A R

372


EXPOSING RADIATION EXPOSURE

MATERIALS

IRRADIATED REGION RESIST THIN FILM SUBSTRATE

POSITIVE DEVELOPING RESIST / \

NEGATIVE RESIST

RESIST

RESIST

ETCHING A N D STRIPPING

Annual Review of Materials Science

Figure 1.

Pattern delineation sequences for positive and negative resists (I)



26.

MiTTAL

Adhesion

of Lithographic

Materials

373

d i c h r o m a t e s . I n 1953, Kodak i n t r o d u c e d t h e c o m m e r c i a l l y a v a i l a b l e n e g a t i v e p h o t o r e s i s t s . P o s i t i v e p h o t o r e s i s t s were developed a t a l a t e r d a t e , and t h e E-beam, X - r a y o r l a s e r - r e s i s t s became a v a i l a b l e r e l a t i v e l y r e c e n t l y . The l a t e s t d e v e l o p m e n t s i n r e s i s t m a t e r i a l s and p r o c e s s e s and f u t u r e r e q u i r e m e n t s a r e d i s c u s s e d i n r e f e r e n c e s 1, 2, 3. The p u r p o s e o f t h e r e s i s t i s t o p r o t e c t t h e d e s i r e d a r e a s f r o m t h e a t t a c k o f t h e c h e m i c a l e t c h a n t s w h i l e t h e exposed p o r t i o n s o f t h e s u b s t r a t e a r e being d i s s o l v e d , and t h e importance o f the a d h e s i o n of the r e s i s t m a t e r i a l t o t h e s u b s t r a t e i s q u i t e obvious. Lack o r inadequency o f adhesion culminates i n u n d e r c u t t i n g which l i m i t s t h e . p a t t e r n r e s o l u t i o n and consequently t h e d e v i c e y i e l d . I d e a l l y , one w o u l d l i k e t o h a v e p a t t e r n s w i t h v e r t i c a l w a l l s a n d t h e phenomenon of u n d e r c u t t i n g r e s u l t s i n s l a n t e d p a t t e r n s . Also during the developing stage, the r e s i s t patterns should adhere v e r y w e l l t o t h e s u b s t r a t e , o t h e r w i s e t h e y w i l l be l i f t e d o f f b y t h e a c t i o n o f t h e d e v e l o p e r . So i t i s q u i t e c l e a r t h a t t h e adhesion o f the r e s i s t i s important both during the developing and t h e e t c h i n g s t e p s . I t should be noted t h a t i t i s t h e wet c h e m i c a l e t c h i n g p r o c e s s w h i c h causes concern about a d h e s i o n o f r e s i s t s , and tjjere i s an i n c r e a s i n g i n t e r e s t i n g o i n g t o t h e d r y e t c h i n g o r RIE process which a l l e v i a t e s such concern, b u t i t r e q u i r e s s p e c i a l r e s i s t s w h i c h do n o t d e g r a d e when e x p o s e d t o R I E . I n any c a s e , a l l o f t h e p r o c e s s i n g s t e p s and t h e m a t e r i a l s ( r e s i s t , d e v e l o p i n g medium, e t c h a n t ) c a n i n f l u e n c e t h e a d h e s i o n of a r e s i s t and t h e e f f e c t s o f these on t h e r e s i s t a d h e s i o n w i l l be a n a l y z e d . A s most o f t h e w o r k h a s b e e n done u s i n g p h o t o l i t h o graphy i n making p a t t e r n s i n the S i C ^ f i l m , I w i l l concentrate i n t h i s paper on t h e f a c t o r s a f f e c t i n g adhesion o f p h o t o r e s i s t s o n SiC>2 s u r f a c e s . What i s P h o t o r e s i s t A d h e s i o n a n d How i s i t M o n i t o r e d ? B e f o r e d e f i n i n g and d i s c u s s i n g p h o t o r e s i s t a d h e s i o n , i t i s i n order t o d i s c u s s b r i e f l y t h e concept o f a d h e s i o n o f f i l m s and c o a t i n g s a s p r a c t i c e d i n o t h e r a r e a s o f human e n d e a v o r . The w o r d " a d h e s i o n " s i m p l y s i g n i f i e s s t i c k i n g t o g e t h e r o f two s i m i l a r o r d i s s i m i l a r materials. I n the case o f t h i n f i l m s o r t h i c k deposits (e.g., e l e c t r o d e p o s i t s ) o n e i s more c o n c e r n e d w i t h what h a s b e e n t e r m e d " p r a c t i c a l adhesion" (4,5). P r a c t i c a l adhesion represents the f o r c e s o r t h e work r e q u i r e d t o e f f e c t s e p a r a t i o n o f t h e a d h e r i n g p h a s e s a n d d e p e n d s upon t h e i n t e r m o l e c u l a r i n t e r a c t i o n s a t t h e i n t e r f a c e (termed a s " b a s i c " o r "fundamental" a d h e s i o n ) and " o t h e r factors". These " o t h e r f a c t o r s " i n c l u d e s t r e s s e s i n t h e f i l m , s i t e s o f e a s y f r a c t u r e mode, a n d t h e method o f a p p l y i n g e x t e r n a l s t r e s s e s t o d i s r u p t the a d h e r i n g system. There a r e a l e g i o n o f methods f o r m e a s u r i n g p r a c t i c a l a d h e s i o n o f t h i n f i l m s ( 6 ) a n d thick deposits (7). * Reactive

i o n etching.



374


Figure 2.

MATERIALS

Etched thin film line profiles: (A) without; (B) with normal; and (C) with heavy undercutting

wwv

Y

V t

\VA\W

s\\ s

\\

American Society for Testing and Materials

Figure 3. Schematic of a patterned SiO /silicon specimen after being subjected to the undercutting test and stripped of photoresist. The parameters of equation for K are illustrated. (8) t

u



26.

MiTTAL

Adhesion

of Lithographic

Materials

375

However, i n t h e c a s e o f r e s i s t s t h e a d h e s i o n o r r a t h e r t h e l a c k o f i t i s expressed i n terms o f " f l o a t i n g images" observed d u r i n g d e v e l o p i n g and " u n d e r c u t t i n g " observed d u r i n g c h e m i c a l e t c h i n g a s shown i n F i g u r e 2. I t s h o u l d b e n o t e d t h a t t h e r e i s always a n a t u r a l degree o f undercut as the etchant penetrates i n a l l d i r e c t i o n s , b u t the l a c k o f adhesion exaggerates the undercutting. The u n d e r c u t t i n g i s c a u s e d b y t h e p e n e t r a t i o n o f t h e etchant along the s u b s t r a t e - r e s i s t i n t e r f a c e . So i n t h e c a s e o f p h o t o r e s i s t a d h e s i o n , one i s p r i m a r i l y c o n c e r n ed w i t h t h e e v e n t s a t t h e p h o t o r e s i s t - s u b s t r a t e i n t e r f a c e a n d how t h e s e a r e a f f e c t e d b y t h e d e v e l o p e r a n d t h e e t c h a n t . The r e l a t i o n s h i p b e t w e e n t h e c o n v e n t i o n a l p r a c t i c a l a d h e s i o n ( i n t e r m s o f p e e l , p u l l , s h e a r , s c r a t c h ) a n d f l o a t i n g images and/or u n d e r c u t t i n g b e h a v i o r has n o t been i n v e s t i g a t e d . I t i s q u i t e p o s s i b l e t h a t the p h o t o r e s i s t has v e r y poor p r a c t i c a l a d h e s i o n (as d e f i n e d above) b u t i s q u i t e a c c e p t a b l e i n terms o f r e s i s t a n c e t o e t c h a n t p e n e t r a t i o n . C o n v e r s e l y , t h e r e c o u l d be v e r y h i g h p r a c t i c a l a d h e s i o n but extremely poor r e s i s t a n c e t o the etchant. T h i s w i l l become c l e a r e r when t h e r o l e o f t h e h e x a m e t h y l d i s i l a z a n e (HMDS) a s a n a d h e s i o n p r o m o t e r f o r p h o t o r e s i s t s i s discussed. Apropos o f m o n i t o r i n g adhesion o f p h o t o r e s i s t s , there are three techniques discussed i nt h el i t e r a t u r e . The e x t e n t o f u n d e r c u t t i n g has b e e n u s e d f o r a l o n g t i m e *to m o n i t o r a d h e s i o n o f p h o t o r e s i s t s on a r e l a t i v e b a s i s ; b u t r e c e n t l y (8,9) t h i s t e c h n i q u e h a s b e e n u s e d to d e s c r i b e the a d h e s i o n b e h a v i o r i n terms o f u n d e r c u t t i n g c o n s t a n t , K . The t e c h n i q u e i s b a s e d u p o n t h e u s e o f a f l u o r i d e - c o n t a i n i n g enchant which exaggerates the r a t e o f u n d e r c u t t i n g o f p h o t o r e s i s t on Si(>2 s u r f a c e s . A n e m p i r i c a l e q u a t i o n = r In (x+l)/y-y i s g i v e n w h i c h a p p r o x i m a t e l y d e s c r i b e s t h e shape o ? t h e u n d e r t u t o x i d e e d g e s , t h u s a l l o w i n g a n u m e r i c a l measure o f t h e r e l a t i v e a d h e s i o n i n t e r m s o f K . The v a r i o u s p a r a m e t e r s u s e d i n t h e a b o v e e q u a t i o n a r e shown i n f i g u r e 3; r i s t h e r a t e o f o x i d e u n d e r c u t . The l a r g e v a l u e s f o r the u n d e r c u t t i n g constant correspond t o poor photor e s i s t / S i C ^ adhesion. Some o f t h e r e s u l t s f r o m t h i s s t u d y a r e shown i n Table I . A n o t h e r t e c h n i q u e d e s c r i b e d b y K l e m a n d L u s s o w (10) i s based upon the i o n m i g r a t i o n p r i n c i p l e . Figure 4 i s a schematic o f t h e e x p e r i m e n t a l s e t u p . T e s t s a m p l e s c o n s i s t o f SiO« s u b t r a t e s o v e r c o a t e d w i t h p h o t o r e s i s t i n w h i c h windows t o t h e S i 0 2 are developed out. I t should be p o i n t e d o u t t h a t t h i s technique i s n o t r e s t r i c t e d t o SiO« s u b s t r a t e , r a t h e r t h e t e c h n i q u e i s applicable to a l l d i e l e c t r i c or insulating substrates. Individual t e s t s i t e s o n a w a f e r c o n s i s t o f two i d e n t i c a l windows s e p a r a t e d by a p h o t o r e s i s t l a n d o f known w i d t h . A c a l i b r a t e d amount o f i o n i c s o l u t i o n d e p o s i t e d i n e a c h window r e s u l t s i n d i r e c t c o n t a c t w i t h the S i C ^ / p h o t o r e s i s t i n t e r f a c e as i n t h e s u b s t r a t e e t c h i n g process. A n y e l e c t r o n i c f l o w b e t w e e n t h e two s o l u t i o n s c a u s e d g


376


T a b l e I . V a l u e s o f 1/K

Obtained

Specimen

i n Undercutting Tests

HMDS Relative

KTFR/Aged S i 0


KTFR/New S i 0

-

2

2

WAYCOAT IC/Aged S i 0

AZ-1350B/Aged S i 0

2

2

I d e a l Specimen

+

0.29

-

0.18

+

0.23

-

0.20

+

0.27

-

0.16

'

0.28 1

^

^Based on t h e o r e t i c a l specimen having p e r f e c t i n which l a t e r a l distance

1/K Adhesion

0.25

+ 1

MATERIALS

3

photoresist

undercut equals thickness

of S i 0

adhesion, 2

etched

away (^0.4 ym i n a t e s t o f 1-min d u r a t i o n ) .

American Society for Testing and Materials


MiTTAL

26.

Adhesion

of Lithographic

377

Materials

by i o n i c m i g r a t i o n t h r o u g h t h e r e s i s t , t h e S i 0 o r a l o n g t h e i n t e r f a c e i s d e t e c t e d b y c o n t a c t i n g t h e s e s o l u t i o n s w i t h t h e two p l a t i n u m p r o b e s i n the t e s t c i r c u i t and a p p l y i n g a p o t e n t i a l a c r o s s them. I n d e p e n d e n t e x p e r i m e n t s c o n f i r m e d (11) t h a t t h e f l o w was a l o n g t h e i n t e r f a c e a n d n o t t h r o u g h t h e r e s i s t l a y e r . The t i m e i n t e r v a l b e t w e e n d e p o s i t i o n o f i o n i c s o l u t i o n t o d e t e c t i o n o f c u r r e n t ( 1 microamp t h r e s h o l d ) i s i n d i c a t i v e o f i o n i c m i g r a t i o n t i m e a c r o s s t h e r e s i s t b a r r i e r . The t i m e required for a s o l u t i o n of s p e c i f i e d composition t o penetrate a f i x e d d i s t a n c e , t , a l o n g t h e i n t e r f a c e would be a measure o f t h e s t r e n g t h o f i n t e r a c t i o n between t h e s u b s t r a t e ( S i C O and t h e p h o t o r e s i s t l a y e r . The l a r g e r t h e t , t h e h i g h e r t h e a d h e s i o n , a n d t h u s t h i s t e c h n i q u e o f f e r s a method f o r c o m p a r i n g a d h e s i o n f o r d i f f e r e i p h o t o r e s i s t s and d i e l e c t r i c s u b s t r a t e s . The t h i r d t e c h n i q u e d e s c r i b e d b y Yanazawa e t a l , (12) i s b a s e d u p o n c a l c u l a t i n g t h e t h e r m o d y n a m i c w o r k o f a d h e s i o n ; w\>, b e t w e e n the d r y p h o t o r e s i s t and t h e s u b s t r a t e ( S i 0 , S i ^ N ^ w i t h and w i t h o u t v a r i o u s s u r f a c e t r e a t m e n t s ) a n d W^ b a s e d upon t h e p e n e t r a t i o n o f t h e l i q u i d , e.g., w a t e r , a s shown i n F i g u r e 5. They u s e d water a s the l i q u i d because they used p o s i t i v e p h o t o r e s i s t s i n t h e i r s t u d y a n d a n aqueous medium i s u s e d a s t h e d e v e l o p e r f o r such r e s i s t s . B a s e d upon t h e c o n c e p t o f W i n t h e d r y a n d w e t s t a | e , they d e f i n e d wet a d h e s i o n f a c t o r , f w e t as f" = A ( £^*V(dry) * Subsequently they c o r r e l a t e d f with N w h i c h s i g n i f i e s t h e number o f p a t t e r n s l i f t e d . The e x p e r i m e n t i n v o l v e d m a k i n g p h o t o r e s i s t (+ve) p a t t e r n s a n d c o u n t i n g how many w e r e l i f t e d o f f a f t e r u l t r a s o n i c a g i t a t i o n i n w a t e r . T h e i r r e s u l t s a r e shown i n ^ F i g u r e s 6 a n d 7. F i g u r e 7 shows t h e c o r r e l a t i o n b e t w e e n f and a d h e s i o n o f photor e s i s t on a v a r i e t y o f s u b s t r a t e m a t e r i a l s . A l t h o u g h t h e y h a v e made a number o f a s s u m p t i o n s i n c a l c u l a t i n g W^'s b u t s t i l l t h e i r r e s u l t s a r e i n t e r e s t i n g . I t w i l l b e f u r t h e r i n t e r e s t i n g t o apply t h i s approach i n the case o f other developing media a s w e l l as e t c h a n t s .


2

2

w

we

w e t

What i s D e w e t t i n g a n d what F a c t o r s

Influence i t ?

A n o t h e r r e l a t e d phenomena t o a d h e s i o n o f p h o t o r e s i s t s i s t h a t o f d e wetting. The d e w e t t i n g s i g n i f i e s r e t r a c t i o n o f a u n i f o r m l y spread l a y e r o f p h o t o r e s i s t . I t s h o u l d b e added t h a t q u i t e o f t e n d e w e t t i n g and non-wetting are used synonymously, b u t t h e r e i s a f u n d a m e n t a l d i f f e r e n c e b e t w e e n t h e two. N o n - w e t t i n g means t h a t the S i 0 s u r f a c e i s not wetted anytime by the p h o t o r e s i s t , whereas d e w e t t i n g means t h a t t h e p h o t o r e s i s t no l o n g e r s p r e a d s ( o n the t o t a l s u r f a c e o r on a s e l e c t e d s p o t ) where i t spread e a r l i e r . 2



378


MATERIALS

Figure 5.

Schematic of adhesion tension in photoresist-liquid-substrate system (12)

Figure 6.

Wet adhesion test for positive working photoresist and Si0 system (12)

2

or SigN^



26.

MiTTAL

Adhesion

of Lithographic

Adhesion 0 : Good X : Poor

Substrates

Si0

p o 2

2

Si0 Poly Si

As Grown

0

Etchea (HF aq.)

X

Converted (HMDS)

0

• As Deposited

5

2

•

S13N4 Al

379

Materials

1

1 1

1

1

1

1

1 1

1

1 1

X

Converted (HMDS)

0

As Deposited

0

Converted (HMDS)

0

As Deposited

X

Converted (HMDS)

0

As Evaporated

0

1 1

1

1

1 1 1 1 0.5

•

1 1.0

fwet

1.5

Hitachi Ltd.

Figure 7.

Obtained i

wct

values for various materials common to silicon integrated circuit processes (12)


380


MATERIALS

For complete w e t t i n g (or contact a n g l e , 9 , t o be z e r o ) i t i s axiomatic i n surface chemistry that y, £ y wiiere y and y r e p r e s e n t t h e s u r f a c e f r e e e n e r g i e s o r t h e l i q u i d and s o l i d surface respectively. I f t h i s c o n d i t i o n i s not f u l f i l l e d , then t h e r e w i l l be a p o s i t i v e 0 . T h i s i s a thermodynamic r e q u i r e m e n t and o n e m u s t a l s o c o n s i d e r t h e k i n e t i c a s p e c t s o f s p r e a d i n g . E v e n i f t h i s c o n d i t i o n i s s a t i s f i e d , t h e l i q u i d may n o t s p r e a d t o f o r m 0 = 0 ° b e c a u s e t h e l i q u i d i s t o o v i s c o u s and a d e q u a t e t i m e was n o t a l l o w e d t o l e t i t s p r e a d o n t h e s o l i d . So t h e v i s c o s i t y o f and s p i n n i n g speed f o r p h o t o r e s i s t s o l u t i o n s a r e quite important. l

v

g


S

v

V

I f the c o n d i t i o n f o r s e t t i n g i s t h e o r e t i c a l l y s a t i s f i e d , then the prime cause of non-wetting i s the presence of contaminant l a y e r s or p a r t i c l e s (these l e a d to non-wetting i n spots) which render the s u r f a c e n o n w e t t a b l e by r e d u c i n g i t s s u r f a c e f r e e e n e r g y . However, the main causes o f d e w e t t i n g a r e : ( i ) A u t o p h o b i c nature of the s u r f a c e . A u t o p h o b i c i t y means t h a t t h e l i q u i d d o e s n o t s p r e a d o n i t s own l a y e r a n d i s c a u s e d b y t h e p r e s e n c e o f a l a y e r whose s u r f a c e f r e e e n e r g y i s l e s s t h a n t h e s u r f a c e f r e e energy of the spreading l i q u i d . S u c h l a y e r s c o u l d b e due t o c e r t a i n r e a c t i o n products of the l i q u i d w i t h the s o l i d surface o r i n t h e c a s e o f two c o m p o n e n t s s p r e a d i n g l i q u i d , o n e o f t h e c o m p o n e n t s may s e l e c t i v e l y a d s o r b a t t h e s o l i d s u r f a c e g i v i n g r i s e to t h e s e r e l a t i v e l y low s u r f a c e f r e e energy l a y e r s , (ii) S t r e s s e s i n the s o l i d i f i e d f i l m or l a y e r . The i n t e r n a l s t r e s s e s (caused by e v a p o r a t i n g s o l v e n t ) i n t h e p h o t o r e s i s t f i l m can be a source of dewetting. A s a c o r o l l a r y t o t h i s d i s c u s s i o n one s h o u l d be a b l e to e l i m i n a t e t h e problem o f n o n - w e t t i n g and/or d e w e t t i n g by a p r o p e r c h o i c e o f p h o t o r e s i s t components. B e f o r e c l o s i n g t h i s s e c t i o n , i t i s p r o p e r and i m p o r t a n t t o d i s c u s s t h e r e l a t i o n s h i p b e t w e e n w e t t i n g and a d h e s i o n o f p h o t o resists. T h e a d h e s i o n o f a l a y e r d e p e n d s u p o n t h e q u a l i t y and q u a n t i t y of i t s intimacy w i t h the s u b s t r a t e . Q u a l i t y r e f e r s to the n a t u r e of bonding (van der Waals) a c i d - b a s e , electrostatic) and q u a n t i t y s i g n i f i e s t h e a r e a o f i n t i m a t e c o n t a c t . Complete w e t t i n g (0=0°) s i m p l y s i g n i f i e s t h e i n c r e a s e i n t h e a r e a o f i n t i m a c y ; however, i f the q u a l i t y of bonding i s poor then the net r e s u l t would be poor a d h e s i o n . On t h e o t h e r h a n d , i f t h e r e i s no w e t t i n g , t h e r e i s no a d h e s i o n . In other words, wetting i s necessary b u t n o t s u f f i c i e n t f o r good a d h e s i o n . So t h r e e c a s e s a r e possible: ( i ) p o o r w e t t i n g , p o o r a d h e s i o n ; ( i i ) good w e t t i n g , p o o r a d h e s i o n ; ( i i i ) good w e t t i n g , good a d h e s i o n . R e c e n t l y i t has been emphasized t h a t the v a n der Waals)and the a c i d - b a s e a r e the only i n t e r a c t i o n s which are important i n adhesion (13). I t s h o u l d be added t h a t a d h e s i o n i s used h e r e t o r e p r e s e n t i n t e r f a c i a l i n t e r a c t i o n s , and i f t h e i n t e r f a c e i s s u s c e p t i b l e t o t h e a t t a c k o f t h e e t c h a n t , t h e n i t may v e r y w e l l r e s u l t i n t o p o o r photoresist adhesion.


26.

Adhesion

MITTAL

of Lithographic

381

Materials

E f f e c t s o f P r o c e s s i n g Steps and M a t e r i a l s on the Adhesion o f a Resist Material. Effect of Substrate. I n t h e a r e a o f c o n v e n t i o n a l a d h e s i o n , some c o r r e l a t i o n has been found between t h e j o i n t s t r e n g t h and t h e s u r f a c e e n e r g e t i c s o f t h e s u b s t r a t e m a t e r i a l ( 1 4 ) . The s u r f a c e e n e r g e t i c s has been expressed i n terms o f w e t t i n g o f t h e s u b s t r a t e by t h e l i q u i d a d h e s i v e a n d i n many c a s e s s i m p l y b y t h e c o n t a c t a n g l e o f w a t e r , 0^ Q , o n t h e s u b s t r a t e . S i m i l a r a t t e m p t s h a v e b e e n made t o correlate2water w e t t a b i l i t y o f variously treated SiO surfaces and t h e a d h e s i o n o f p h o t o r e s i s t s i n t e r m s o f u n d e r c u t . Thermal S i 0 i s t h e commonly u s e d s u b s t r a t e s o we w i l l c o n c e n t r a t e o n t h i s material. S i 0 s u r f a c e h a s been c h a r a c t e r i z e d by water c o n t a c t a n g l e (15,16,17,18,19) a n d i t s w a t e r w e t t a b i l i t y depends upon i t s method o f p r e p a r a t i o n , s t o r a g e c o n d i t i o n s , r e l a t i v e h u m i d i t y , e t c . , and c a n b e f u r t h e r m o d i f i e d b y v a r i o u s t r e a t m e n t s . (See F i g u r e 8 and T a b l e I I ) . A l s o i t h a s b e e n shown t h a t w a t e r w e t t a b i l i t y o f S i 0 depends upon i t s t h i c k n e s s a n d a f t e r a c e r t a i n t h i c k n e s s i t h a s no e f f e c t (See F i g u r e 9 ) . B e r g h (16) f o u n d t h a t t h e l o w e r t h e 0 ^, t h e l o w e r was t h e a d h e s i o n o f KPR*; h o w e v e r , L u s s o w ( 1 8 ; 2 c o n c l u d e d t h a t n o t o n l y t h e 0jj but a l s o the surface s t r u c t u r e o f S i 0 i s important i n d i c t a ting 2 t h e a d h e s i o n b e h a v i o r o f a p h o t o r e s i s t . He d e d u c e d t h r e e types o f S i 0 s u r f a c e s t r u c t u r e s from w e t t a b i l i t y changes caused by v a r i o u s s u r f a c e t r e a t m e n t s a s shown i n F i g u r e 1 0 . The w e t t a b i l i t y o f these s u r f a c e s a s measured by t h e c o n t a c t a n g l e o f w a t e r d r o p l e t s increases i n the order: s i l o x a n e < s i l a n o l < hydrated form whereas negative photoresist adhesion i s i n the reverse order. According t o L u s s o w ' s w o r k , t h e a d h e s i o n o f KTFR* i s most s e n s i t i v e t o the s u r f a c e composition and o n l y s a t i s f a c t o r y on t h e s i l o x a n e t y p e , w h e r e a s KPR* a d h e r e s w e l l o n s i l a n o l s u r f a c e s t o o . Hyd *ated s u r f a c e s y i e l d p o o r a d h e s i o n w i t h b o t h r e s i s t s . KMER* coatings are r e l a t i v e l y unaffected by the surface c o n s t i t u t i o n . I t can be concluded from t h i s d i s c u s s i o n t h a t adsorbed water on the S i 0 s u r f a c e i s u n d e s i r a b l e f o r p h o t o r e s i s t adhesion. I n order t o a v o i d the unpleasant e f f e c t o f the adsorbed water, c o u p l i n g a g e n t s o r a d h e s i o n p r o m o t e r s (HMDS, s i l a n e s , e t c . ) a r e commonly u s e d . T h e s e c o u p l i n g a g e n t s r e a c t s o a s t o f o r m a s i l o x a n e bond w i t h t h e S i 0 s u r f a c e a n d t h e o r g a n i c m o i e t y ( f o r e x a m p l e , - N H , -CH~) i s t u r n e d o u t w a r d s a s shown i n F i g u r e 1 1 . Once t h e S i 0 s u r f a c e i s c o v e r e d w i t h a d h e s i o n promoter l a y e r s , t h e i n t e r a c t i o n s between these o r g a n i c l a y e r s and t h e p h o t o r e s i s t s h o u l d d i c t a t e i t s a d h e s i o n b e h a v i o r . I t i s i n t e r e s t i n g t o n o t e t h a t t h e HMDS i s t h e w o r k h o r s e o f t h e p h o t o r e s i s t i n d u s t r y , b u t i t c a n n o t b e recommended a s a n a d h e s i o n promoter f o r promoting adhesion i n the mechanical ( p e e l , p u l l , e t c . ) s e n s e , b e c a u s e a f t e r r e a c t i o n w i t h S i 0 , t h e HMDS l e a v e s a b e d o f -CH3 g r o u p s o n t h e o u t s i d e w h i c h a r e q u i t e i n e r t c h e m i c a l l y ?


2

2

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resists.




382

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I

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15

30

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120

MATERIALS

Minutes of Boiling Journal of the Electrochemical Society

Figure 8.

Water contact angle on boiled Si0

2

T a b l e I I C o n t a c t A n g l e s o f Water on S i After

(15)

A f t e r O x i d a t i o n and

Rinsing

Type o f O x i d e

1050> °C-0xide 650°C - O x i d e

0 After Oxidation

0 After Rinse

Average Value

Range

Average Value

Range

26-36

24

18-32

Good

32

KPR Adherence

0-8

3

0-8

Poor

C0 -0xide

21

11-38

4

3-12

Poor

RPS-Oxide

32

19-38

7

5-12

Poor

2

3

After Rinse

Journal of the Electrochemical Society


MiTTAL

Adhesion

of Lithographic


T

Materials

1

383

1

r

Oxide Thickness (A) Applied Physics Letters

Figure 9.

Cos 6 as a function of the oxide thickness: oxide grown at room temperature; (A), oxide grown at 600°C; (O)no oxide (19)

H

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(c) Journal of the Electrochemical Society

Figure 10. Schematic of thermal Si0 surface structures: (a) Type 1, with adsorbed molecular water, referred to as hydrated; (b) Type 2, containing silanol groups with no adsorbed molecular water; (c) Type 3, predominantly siloxane structure, no adsorbed molecular water (18) 2



384


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+

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Figure 11. Schematic of: (Top) reaction of alkoxysilanes with a highly hydroxylated Si0 ; (Bottom) reaction of hexamethyldisilazane (HMDS) with hydroxylated SiOg. R group contains —NH , —CH —CH =CH, etc. moieties. 2

2

Sy

2


26.

MiTTAL

Adhesion

of Lithographic

385

Materials

(and h y d r o p h o b i c i n n a t u r e ) a n d w o u l d n o t r e a c t w i t h any a d h e s i v e o r c o a t i n g . However, i t i s t h e l a c k o f r e a c t i v i t y o r h y d r o p h o b i c i t y o f t h e s e -CH^groups w h i c h p r e v e n t s o r c u r t a i l s the p e n e t r a t i o n o f t h e etchant along t h e i n t e r f a c e w i t h t h e r e s u l t t h a t p h o t o r e s i s t a d h e s i o n i s enhanced. T h i s example i s a good i l l u s t r a t i o n o f t h e f a c t t h a t t h e f a c t o r s a f f e c t i n g m e c h a n i c a l a d h e s i o n and t h e p h o t o r e s i s t a d h e s i o n c a n be d i s c o r dant w i t h each o t h e r . So i n summary, f o r good p h o t o r e s i s t a d h e s i o n , t h e f o l l o w i n g conditions are important, ( i ) With bare S i 0 surface, the a d s o r b e d w a t e r s h o u l d b e removed ( e . g . ; b y h e a t i n g ) s o t h a t t h e p o l y m e r i c component o f t h e p h o t o r e s i s t c a n come i n i n t i m a t e contact with the surface, ( i i ) The c o n d i t i o n f o r w e t t i n g o f b a r e o r t r e a t e d S i 0 w i t h t h e l i q u i d p h o t o r e s i s t s h o u l d be s a t i s f i e d and t h e k i n e t i c s o f w e t t i n g s h o u l d b e f a v o r a b l e , ( i i i ) As S i 0 i s a c i d i c i n c h a r a c t e r , a b a s i c polymer would culminate i n b e t t e r a d h e s i o n p r o v i d e d t h e i n t e r f a c e c a n b e made r e s i s t a n t t o e t c h a n t p e n e t r a t i o n . Under c e r t a i n c o n d i t i o n s , a c i d - b a s e c o m b i n a t i o n s c a n be made r e s i s t a n t t o h y d r o l y s i s ( 2 0 ) a n d i t i s w o r t h i n v e s t i g a t i n g how t h e s e c o u l d b e p r o f i t a b l y u t i l i z e d t o i m p r o v e p h o t o r e s i s t adhesion. A l s o u s i n g Fowkes a p p r o a c h ( 2 1 ) , t h e a c i d - b a s e c h a r a c t e r s t i e s o f t h e S i 0 t r e a t e d w i t h a d h e s i o n promoters should be i n v e s t i gated. Apropos, i t w i l l be an i n t e r e s t i n g study t o c o r r e l a t e t h e s u r f a c e e n e r g e t i c s o f t h e S i 0 t r e a t e d w i t h HMDS o r s i l a n e s w i t h t h e p h o t o r e s i s t adhesion on such s u r f a c e s , ( i v ) The s o l v e n t s h o u l d have l e s s i n t e r a c t i o n w i t h S i 0 (bare o r t r e a t e d ) t h a n t h e t h e p o l y m e r i c component. T h i s i s a m p l i f i e d i n t h e f o l l o w i n g paragraphs.


2

2

2

2

2

2

E f f e c t o f Polymer and S o l v e n t . F o r good a d h e s i o n , t h e r e s h o u l d be maximum c o n t a c t b e t w e e n t h e p o l y m e r a n d t h e s u b s t r a t e a n d i f t h e solvent has greater a f f i n i t y f o r t h e surface than the polymer, t h i s would prevent t h e polymer from coming i n t o c o n t a c t w i t h t h e s u r f a c e . The c o m p a t i b i l i t y b e t w e e n p o l y m e r s a n d s o l v e n t s i s d e s c r i b e d i n t e r m s o f t h e s o l u b i l i t y p a r a m e t e r , 6, a n d f o r a g i v e n p o l y m e r a " g o o d " s o l v e n t i s t h e o n e whose 6 m a t c h e s w i t h t h a t o f t h e p o l y m e r . The 6's of v a r i o u s polymers a r e l i s t e d i n r e f e r e n c e 22, whereas 6 s f o r s o l v e n t s a r e c o m p i l e d i n numerous p l a c e s , s e e , e . g . , r e f e r e n c e 2 3 . I n a good s o l v e n t t h e p o l y m e r i s e x t e n d e d p e r m i t t i n g l a r g e degree o f c o n t a c t between t h e polymer and t h e s o l v e n t ; whereas i n a "bad" s o l v e n t t h e polymer i s t i g h t l y c o i l e d . The r o l e o f t h e s o l v e n t i s v e r y i m p o r t a n t i n t h e a d s o r p t i o n o f polymers on s o l i d s u b s t r a t e s , and t h e importance o f t h e a c i d base c o n s i d e r a t i o n s i n polymer a d s o r p t i o n from s o l v e n t s has been e m p h a s i z e d r e c e n t l y ( 2 4 ) * So f o r good a d h e s i o n , t h e p o l y m e r s h o u l d have t h e r i g h t a c i d - b a s e c h a r a c t e r i s t i c s w i t h r e s p e c t t o t h e s u b s t r a t e , a n d s h o u l d c o v e r t h e maximum a r e a o f t h e s u b s t r a t e . The s o l v e n t s h o u l d h a v e minimum i n t e r a c t i o n w i t h t h e s u b s t r a t e , s h o u l d e v a p o r a t e f a s t , and s h o u l d n o t be t r a p p e d . Furthermore, the s o l v e n t should be f r e e from any i m p u r i t y which c a n adsorb s e l e c t i v e l y o n t h e s u b s t r a t e . F o r e x a m p l e , e v e n a t r a c e amount f


386

DURABILITY O F

MACROMOLECULAR

MATERIALS


o f w a t e r i n an o r g a n i c s o l v e n t i s e x t r e m e l y u n d e s i r a b l e a s w a t e r has g r e a t a f f i n i t y f o r t h e S i C ^ s u r f a c e . E f f e c t o f P r e - B a k e . The p u r p o s e o f t h e p r e - e x p o s u r e b a k i n g i s t o d r y t h e r e s i s t f i l m ( i . e . , t o remove t h e s o l v e n t s ) , p r e v e n t t a c k i n e s s and i m p r o v e a d h e s i o n t o t h e s u b s t r a t e . However, t h e p r e b a k i n g s t e p s h o u l d n o t d e s t r o y t h e p h o t o a c t i v e compound, so t h e p r e b a k i n g t e m p e r a t u r e and t i m e a r e i m p o r t a n t . P r e - b a k i n g can r e l i e v e the i n t e r n a l s t r e s s e s i n the r e s i s t f i l m thereby improving the adhesion of the dry r e s i s t f i l m to the s u b s t r a t e . I f the prebake i s done a t h i g h e r t h a n t h e d e s i r e d t e m p e r a t u r e , i t c o u l d l e a d t o " f o g g i n g " due t o t h e r m a l c r o s s l i n k i n g ( n e g a t i v e r e s i s t s ) o r m e l t i n g or decomposition i n the case of p o s i t i v e r e s i s t s . E f f e c t o f E x p o s u r e . The p u r p o s e o f t h e e x p o s u r e i s t o c a u s e c e r t a i n c h e m i c a l c h a n g e s i n t h e p o l y m e r r e s i n and t h i s c a u s e s d i f f e r e n t i a l s o l u b i l i t y b e h a v i o r i n t h e d e v e l o p i n g medium. F o r a n e g a t i v e r e s i s t , t h e e x p o s u r e s t e p c a u s e s c r o s s l i n k i n g and r e n d e r s t h e polymer r e s i s t i n s o l u b l e i n the developer. The l a r g e r t h e number o f c r o s s l i n k s , t h e g r e a t e r w i l l be t h e r i g i d i t y o f t h e s t r u c t u r e ; h o w e v e r , t o o h i g h r i g i d i t y may n o t be d e s i r a b l e . I f t h e p o l y m e r i s n o t p r o p e r l y exposed, i t w i l l cause u n d e s i r a b l e s w e l l i n g i n contact w i t h the d e v e l o p e r as d i s c u s s e d below. Underexposure r e s u l t s i n t o c r o s s l i n k i n g o n l y near the s u r f a c e . A l s o the e f f e c t of gaseous environment i s important. The p r e s e n c e o f o x y g e n i s c o n s i d e r e d bad a s i t p r e v e n t s c r o s s l i n k i n g by d e s t r o y i n g t h e s e n s i t i z e r , w h i c h c o u l d l e a d t o f l o a t i n g image b e h a v i o r o r t h i n n i n g o f r e s i s t p a t t e r n during developing. E f f e c t o f D e v e l o p m e n t . The phenomenon o f " f l o a t i n g i m a g e " i s a t t r i b u t e d to the i n t e r a c t i o n of the developer w i t h the polymeric r e s i n and t h e s u b s t r a t e . The d e v e l o p e r s h o u l d n o t c a u s e s w e l l i n g o f the r e s i s t p a t t e r n , as the s w e l l i n g i s r e s o l u t i o n l i m i t i n g (see Figure 12). I n the case of p o s i t i v e r e s i s t , the unexposed r e g i o n s r e m a i n q u i t e h y d r o p h o b i c and t h u s do n o t i n t e r a c t r a p i d l y w i t h t h e d e v e l o p i n g s o l v e n t and a r e n o t s u b j e c t t o s w e l l i n g . A good d e v e l o p e r s h o u l d be a p o o r s o l v e n t f o r t h e e x p o s e d (-v ^ r e s i s t , and s h o u l d p o s s e s s m i n i m a l i n t e r a c t i o n w i t h t h e s u b s t r a t e . F o r minimum s w e l l i n g , the i n i t i a l m o l e c u l a r weight of the polymer should be h i g h , t h e m o l e c u l a r w e i g h t b e t w e e n c r o s s l i n k s s h o u l d be l o w (many c r o s s l i n k s ) and t h e s o l v e n t s h o u l d be p o o r ( 2 6 ) . A l s o t h e w e t t i n g b e h a v i o r o f t h e e x p o s e d r e s i s t (-ve) p a t t e r n by t h e d e v e l o p e r s h o u l d be i m p o r t a n t . The c r i t i c a l s u r f a c e t e n s i o n o f w e t t i n g , y » o f some r e s i s t m a t e r i a l s a r e g i v e n i n T a b l e I I I a s d e t e r m i n e d by D a v i d s o n and L e v i ( 2 7 ) , b u t no a t t e m p t s have b e e n made t o c o r r e l a t e y o f t h e r e s i s t w i t h i t s f l o a t i n g image behavior. e


26.

MiTTAL

Adhesion

of Lithographic

387

Materials

Swollen -Constrained


to Substrate

Swollen - Unconstrained

Unswollen

^'-^ Where q

Factors Affecting Adhesion of Lithographic Materials - ACS Publications

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