Phenolic Resin-Based Negative Electron-Beam Resists

Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan. Phenolic resin-based negative resists such as MRS. (deep UV negative resist) ...
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Chapter 7 Phenolic Resin-Based Negative Electron-Beam Resists

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H. Shiraishi, N. Hayashi, T. Ueno, O. Suga, F. Murai, and S. Nonogaki Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan

Phenolic resin-based negative resists such as MRS (deep UV negative resist) do not swell aqueous alkaline developers. An attempt has been made to clarify the non-swelling dissolution mechanism of the resist containing poly-p-vinylphenol as a matrix following exposure to electron beam irradiation. The following results have been obtained: (1) electron beam exposure causes an increase in molecular weight of the poly-p-vinylphenol matrix, (2) the dissolution rate of poly-p-vinylphenol in the developer decreases with increasing molecular weight, and (3) there is no fractional dissolution during the development of the resist. It is concluded that non-swelling development, non-fractional dissolution in the proceeds by an etching-type mechanism rather than the fractional dissolution mechanism that occurs with most organic-laser developers. Many reports have been published on negative electron-beam resists. Most of these resists utilize radiation-induced gel-formation as the insolubilzation reaction. However, a major problem with these resists, is that their resolution is limited by swelling which is induced by the developer during development. Novolac- or phenolic resin-based resists usually show no pattern deformation induced by swelling during development in aqueous alkaline solution. Examples of such resists are naphthoquinonediazide/novolac positive photoresists, novolac-based positive electron-beam resist (NPR) (1), and azide/phenolic negative deep-UV resist (MRS) (2). Iwayanagi et al.(2) reported that the development of MRS proceeds in the same manner as the etching process. This resist, consisting of a deep-UV sensitive azide and phenolic resis matrix, is also sensitive to electron-beams. This paper deals with the development mechanism of non-swelling MRS and its electron-beam exposure characteristics.

0097-6156/87/0346-0077$06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

78 Experimental

Materials. The r e s i s t used i n t h i s study c o n s i s t e d of poly(pv i n y l p h e n o l ) as t h e p h e n o l i c r e s i n m a t r i x and 3 , 3 - d i a z i d o d i p h e n y l sulfone as t h e s e n s i t i z e r . T h e c o m p o s i t i o n was e s s e n t i a l l y the same a s t h a t o f R D - 2 0 0 0 N ( t r a d e name o f H i t a c h i C h e m i c a l C o . s MRS). Two k i n d s o f p o l y ( p - v i n y l p h e n o l ) were u s e d . One was c o m m e r c i a l l y a v a i l a b l e f r o m M a r u z e n O i l C o . , u n d e r t h e name o f R E S I N M , a n d t h e o t h e r was s y n t h e s i z e d i n o u r l a b o r a t o r y . The l a t t e r p o l y m e r , m o s t l y w i t h h i g h m o l e c u l a r w e i g h t , was p r e p a r e d b y h y d r o l y z i n g poly(p-acetoxystyrene) with a hydrazine(3). The poly(p-acetoxys t l y r e n e ) was o b t a i n e d b y p o l y m e r i z a t i o n o f t h e monomer u s i n g 2 , 2 ' azobisisobutyronitrile a s an i n i t i a t o r . The a l k a l i n e d e v e l o p e r s u s e d were aqueous tetramethylammoniumhydroxide (TMA) solutions. Isoamylacetate was u s e d as t h e o r g a n i c s o l v e n t developer. 1

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1

Characteristics. E l e c t r o n - b e a m exposure e x p e r i m e n t s were c a r r i e d o u t by u s i n g a p r o t o t l y p e H L - 6 0 0 H i t a c h i E l e c t r o n - B e a m L i t h o g r a p h y System which i s a vector scanning type variable-shaped electronbeam m a c h i n e . T h e a c c e l e r a t i o n v o l t a g e was 30 k V . Resist films w e r e f o r m e d on s i l i c o n w a f e r s b y s p i n - c o a t i n g a n d p r e b a k e d a t 80 C f o r 20 m i n b e f o r e e x p o s u r e . The molecular weight distributions were measured by gel p e r m e a t i o n c h r o m a t o g r a p h y (GPC) w i t h a H i t a c h i 635 l i q u i d c h r o m a t o graphy slystem equipped w i t h Gelpack A150, A140, a n d A120 GPC columns ( H i t a c h i Chemical C o . ) . T h e GPC s o l v e n t was tetrahydrofuran. F i l m t h i c k n e s s was d e t e r m i n e d w i t h a n A l p h a S t e p 200 ( T e n c o r ) profilometer. The d i s s o l u t i o n r a t e o f t h e s a m p l e f i l m s d u r i n g development was m e a s u r e d b y l a s e r - i n t e r f e r o m e t r y ( 4 ) . A 5 mW H e - N e l a s e r was used as the monitering light source and a s i l i c o n photodiode c o n n e c t e d t o a c h a r t r e c o r d e r was u s e d a s t h e s i g n a l detector. Results

and D i s c u s s i o n

I n t h e r e s i s t s c o n t a i n i n g p h e n o l i c r e s i n and b i s a z i d e , electronbeam e x p o s u r e r e s u l t e d i n t h e p r o d u c t i o n o f p r i m a r l y a m i n e s and increased the m o l e c u l a r weight of the phenolic resin (5). A t y p i c a l e x a m p l e i s shown i n F i g . 1. I n t h i s f i g u r e , peaks 1 and 2 are bisazide and t h e poly(p-vinylphenol) matrix, respectively. Peak 3 is a primary amine p r o d u c e d from the decomposition of bisazide. The p r i m a r y a m i n e d o e s n o t a f f e c t t h e s o l u b i l i t y o f t h e resist (6). Since high-molecular-weight p h e n o l i c r e s i n s are less a l k a l i n e - s o l u b l e than low-molecular-weight resins (7), it follows t h a t t h e r e d u c e d s o l u b i l i t y must be due t o t h e i n c r e a s e i n m o l e c u lar weight. The change i n molecular weight distribution of p h e n o l i c r e s i n c o n t a i n e d i n MRS f o l l o w i n g e l e c t r o n - b e a m e x p o s u r e is shown i n F i g . 2 . T h e m o l e c u l a r w e i g h t o f t h e r e s i n i n c r e a s e d as exposure proceeded. I n a p r e v i o u s p a p e r ( 8 ) , we c o n c l u d e d t h a t t h e p h o t o - i n d u c e d decomposition of azide i n the p h e n o l i c r e s i n m a t r i x c a u s e s a n i n c r e a s e i n t h e r e s i n ' s m o l e c u l a r w e i g h t due t o crosslinking. This scheme is the same m e c h a n i s m as that in the i n s o l u b i l i z a t i o n of 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 . T h e r e f o r e , as

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g . 1. M o l e c u l a r w e i g h t d i s t r i b u t i o n s o f MRS b e f o r e a n d a f t e r electron-beam exposure. Dose: 50 u C / c m . P e a k s 1, 2 , 3 a r e a z i d e , p o l y ( p - v i n y l p h e n o l ) , and p r i m a r y a m i n e , r e s p e c t i v e l y . 2

10" MOLECULAR

10

3

WEIGHT

F i g . 2. M o l e c u l a r weight d i s t r i b u t i o n s of poly(p-vinylphenol) m a t r i x i n e l e c t r o n - b e a m - e x p o s e d MRS f i l m s as a f u n c t i o n o f e l e c t r o n dose.

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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p r e d i c t e d i n s u c h c a s e s , h i g h s e n s i t i v i t y c a n be e x p e c t e d when h i g h e r m o l e c u l a r w e i g h t p h e n o l i c r e s i n i s u s e d as t h e m a t r i x .

a

F i g u r e 3 shows t h e e l e c t o n - b e a m exposure characteristics of two MRS s a m p l e s using high molecular weight poly(p-vinylphenol) (Mw=7 .4X1Ο * ) and standard molecular weight poly( p-vinylphenol) ( M w = 6 . 7 x l 0 ^ ) as m a t r i c e s . Azide content i n e a c h r e s i s t was 1 6 . 7 % by weight. The d e v e l o p e r s were aqueous a l k a l i n e s o l u t i o n s . The development of each resist was stopped immediately after the u n e x p o s e d a r e a s were d e s s o l v e d by the d e v e l o p e r . The i n c r e a s e i n sensitivity shown i n F i g . 3 i s s m a l l e r t h a n e x p e c t e d given the assumption that the sensitivity is proportional to the weight average m o l e c u l a r weight of the r e s i s t . Some p a t t e r n deformation d u e t o s w e l l i n g was s e e n i n t h e c a s e o f h i g h m o l e c u l a r w e i g h t M R S . In c o n v i e n t i o n a l n e g a t i v e electron-beam r e s i s t s , the g e l - p o i n t dose is inversely p r o p o r t i o n a l to the weight average m o l e c u l a r weight (9). In order to determine the g e l - p o i n t s o f t h e s e MRS s a m p l e s , t h e r e s i s t f i l m s were d e v e l o p e d w i t h o r g a n i c s o l v e n t . The e x p o s u r e c h a r a c t e r i s t i c s t h u s o b t a i n e d a r e shown i n F i g . 4 . G e l - p o i n t s were 2.3 yC/cm f o r t h e h i g h m o l e c u l a r w e i g h t M R S , a n d 70 y C / c m f o r t h e s t a n d a r d m o l e c u l a r w e i g h t MRS.

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1

2

2

A s shown i n F i g . 3 a n d 4 , t h e e x p o s u r e c h a r a c t e r i s t i c s o f h i g h m o l e c u l a r w e i g h t MRS a r e s i m i l a r when d e v e l o p e d i n d i f f e r e n t devel­ opers. I n a d d i t i o n , p a t t e r n s i n h i g h m o l e c u l a r w e i g h t MRS d e v e l o p ­ ed w i t h o r g a n i c s o l v e n t a r e d e f o r m e d as a r e s u l t o f s w e l l i n g . To elucidate this phenomenon, the r e l a t i o n s h i p between d i s s o l u t i o n r a t e i n a l k a l i n e d e v e l o p e r and t h e m o l e c u l a r w e i g h t o f t h e p o l y m e r was i n v e s t i g a t e d over a wide range of m o l e c u l a r w e i g h t . The r e s u l t s are summarized i n F i g . 5. The d i s s o l u t i o n r a t e o f p o l y ( p vinylphenol) in the alkaline solution decreased rapidly with i n c r e a s i n g m o l e c u l a r w e i g h t up t o 10 * . However, the decrease tend­ ed t o s a t u r a t e a f t e r the m o l e c u l a r weight exceeded 10 . This is t h e r e a s o n t h e h i g h m o l e c u l a r w e i g h t MRS was n o t as s e n s i t i v e as e x p e c t e d when d e v e l o p e d i n a l k a l i n e d e v e l o p e r . 1

4

On t h e c o n t r a r y , t h e s t a n d a r d m o l e c u l a r w e i g h t MRS e x h i b i t s a large difference i n e x p o s u r e c h a r a c t e r i s t i c s between F i g . 3 and 4 . In order to i n v e s t i g a t e this difference, the m o l e c u l a r weight of poly(p-vinylphenol) i n t h e e x p o s e d MRS f i l m was m e a s u r e d a s a f u n c ­ t i o n of electron-beam dose. T h e r e s u l t s a r e shown i n F i g . 6 . The m o l e c u l a r weight of the m a t r i x - r e s i n i n c r e a s e d a f t e r the electronbeam d o s e e x c e e d e d 5 μ C / c m . W i t h i n t h e d o s e r a n g e o f 5 t o 20 yC/cm , the m o l e c u l a r weight ranged f r o m 7 x 1 0 ^ t o 1χ1θ{ . This m o l e c u l a r weight range corresponds to the r a p i d l y changing range of t h e d i s s o l u t i o n r a t e i n F i g . 5. T h i s d i s s o l u t i o n r a t e change gave t h e s t a n d a r d m o l e c u l a r w e i g h t MRS much h i g h e r s e n s i t i v i t y i n a l k a ­ l i n e d e v e l o p e r comared w i t h the g e l - p o i n t dose. 2

2

Figure 7 shows SEM p h o t o g r a p h s of the s t a n d a r d MRS fine p a t t e r n s d e l i n e a t e d by e l e c t r o n - b e a m s . A s shown i n t h e f i g u r e , no s w e l l i n g occured d u r i n g the development o f MRS h a v i n g a s t a n d a r d molecular weight matrix, because the d i s s o l u t i o n process is an e t c h i n g - l i k e p r o c e s s , i . e , the r e s i s t f i l m d i s s o l v e s from the s u r ­ f a c e g r a d u a l l y w i t h o u t any f r a c t i o n a l d i s s o l u t i o n . We h a v e p r e v i ­ o u s l y r e p o r t e d a s i m i l a r n o n - f r a c t i o n a l development process (10). In order to confirm the non-fractional dissolution, molecular w e i g h t d i s t r i b u t i o n s were measured for electron-beam-exposed MRS

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

SHIRAISHI ET AL.

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F i g . 3 . E x p o s u r e c h a r a c t e r i s t i c s o f MRS o b t a i n e d b y a q u e o u s a l k a l i n e development. o m o l e c u l a r w e i g h t MRS ( M w = 7 A x l O ) . · : s t a n d a r d m o l e c u l a r w e i g h t MRS ( M w = 6 . 7 x l 0 ) . k

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F i g . 4 . E x p o s u r e c h a r a c t e r i s t i c s o f MRS o b t a i n e d b y o r g a n i c solvent development. ο : h i g h m o l e c u l a r we i g h t MRS ( M w = 7 . 4 x l O ) . · : s t a n d a r d m o l e c u l a r w e i g h t MRS ( M w ^ o , 7 x l 0 ) . l +

3

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g . 5. D i s s o l u t i o n r a t e o f p o l y ( p - v i n y l p h e n o l ) as a f u n c t i o n molecular weight. 0 : 1.0% t e t r a m e t h y l a m m o n i u m h y d r o x i d e ( T M A ) aqueous s o l u t i o n , α : 1.2% TMA a q u e o u s s o l u t i o n .

F i g . 6. M o l e c u l a r weight of p o l y ( p - v i n y l phenol) a function of electron-beam dose.

of

i n e x p o s e d MRS a

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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7.

F i g . 7. SEM p h o t o g r a p h s o f MRS p a t t e r n s d u r i n g a l k a l i n e development. Patterns: 0 . 4 pm l i n e a n d s p a c e . D o s e : 40 y C / c m . D e v e l o p e r : 0 . 7 2 w t . % TMA s o l u t i o n . 2

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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f i l m s b e f o r e and a f t e r development. T h e r e s u l t s a r e shown i n F i g . 8. T h e e x p o s u r e d o s e was 15 u C / c m , a n d d e v e l o p m e n t was s t o p p e d s o a s t o l e a v e 50% o f t h e i n i t i a l f i l m thickness remaining i n the exposed areas. T h e m o l e c u l a r w e i g h t d i s t r i b u t i o n o f MRS b e f o r e d e v e l o p m e n t was a l m o s t t h e same a s t h a t a f t e r h a l f - d e v e l o p m e n t . It i s c o n c l u d e d t h a t t h e r e i s no e v i d e n c e o f f r a c t i o n a l d i s s o l u t i o n i n which lower molecular weight components are dissolved preferen­ tially. 2

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Conclusion Electron-beam exposure characteristics of the negative deep-UV resist (MRS) w e r e investigated when t h e r e s i s t was u s e d a s a p h e n o l i c r e s i n - b a s e d n e g a t i v e e l e c t r o n beam r e s i s t . In p a r t i c u l a r , the development p r o c e s s was s t u d i e d i n a n a q u e o u s a l k a l i n e s o l u t i o n by measuring t h e d i s s o l u t i o n r a t e o f p h e n o l i c r e s i n and t h e m o l e c u ­ lar weight distributions of electron-beam-exposed resist films. The m o l e c u l a r w e i g h t o f t h e r e s i n m a t r i x i n c r e a s e d c o n t i n o u s l y as exposure proceeded. The d i s s o l u t i o n r a t e of the resin matrix decreased r a p i d l y with increasing molecular weight i n the molecu­ l a r w e i g h t r a n g e b e l o w ϊΦ , a n d t h e d e c r e a s e t e n d e d t o s a t u r a t e when m o l e c u l a r w e i g h t e x c e e d e d 10 * . When a s t a n d a r d molecular w e i g h t p h e n o l i c r e s i n m a t r i x and a l k a l i n e d e v e l o p e r were u s e d , no f r a c t i o n a l d i s s o l u t i o n was s e e n t o o c c u r d u r i n g t h e d e v e l o p m e n t o f the r e s i s t . T h i s development process corresponded t o an e t c h i n g ­ l i k e development of the r e s i s t . 1

(a)

POLYSTYRENE EQUIVALENT MOLECULAR WEIGHT Fig. 8. Molecular weight d i s t r i b u t i o n s of electron-beam-exposed MRS r e s i s t . D o s e : 15 y C / c m . (a) b e f o r e development, (b) a f t e r h a l f d e v e l o p m e n t (50% r e m a i n i n g ) . 2

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(2)

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(3)

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(4)

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(5)

H. Shiraishi, T. Ueno, O. Suga, and S. Nonogaki, ACS Symp. Series 266 "Materials for Microlithography", Ed. by L. F. Thompson, C. G. Willson, and J. M. Frecht, 423-434 (1984).

(6)

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(7)

S. Nonogaki, M. Hashimoto, T. Iwayanagi, and H. Shiraishi, Proc. SPIE 539, Advances in Resist Technology and Processing II, 189-193 (1985).

(8)

M. Hashimoto, T. Iwayanagi, H. Shiraishi, and S. Nonogaki, Technical Papers, SPE Regional Technical Conference "Photopolymers: Principles, Processes and Materials", pp. 11-33, Ellenville, New York, Oct. 28-30 (1985).

(9)

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(10) Y. Hatano, H. Shiraishi, Y. Taniguchi, S. Horigome, S. Nonogaki, and K. Naraoka, Proc. Symp. Electron and Ion Beam Sci. Technol. 8th Int. Conf., 332-340 (1978). RECEIVED May 5, 1987

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.