Novolac Based Positive Electron Beam Resist Containing a Polymeric

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 15, 2018 | https://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch014

14 Novolac Based Positive Electron Beam Resist Containing a Polymeric Dissolution Inhibitor Preparation and Exposure Characteristics HIROSHI SHIRAISHI, ASAO ISOBE1, FUMIO MURAI, and SABURO NONOGAKI Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan A novolac-based positive electron beam resist has been investigated for use in direct device fabrication. The resist is a composite system consisting of an alkaline soluble novolac resin, and poly(2-methylpentene-1 sulfone)(PMPS) which serves as a dissolution inhibitor. Upon exposure to an electron beam, PMPS decomposes and loses its dissolution inhibiting ability. One difficulty with the use of PMPS as a dissolution inhibitor is that film uniformity is not always sufficient, because of phase separation in the spin coating. It was however found that most of novolac or phenolic resins containing PMPS can form homogeneous films when isoamyl acetate is used as a coating solvent. A cresol novolac resin has been synthesized which exhibits a much greater dissolution-inhibiting effect than in various commercially available novolac or phenolic resins. Using this resin, a positive electron beam resist was prepared and its exposure characteristics were examined. A tetramethylammonium hydroxide aqueous solution was used as the developer. The sensitivity reaches 3x10-6 C/cm without post-exposure baking. It was found that the sensitivity to double exposure was much higher than that to single exposure with the same total dose. A similar phenomenon was also found for PMPS film. The mechanisms for these phenomena are discussed. Electron beam lithography for direct device fabrication requires high performance electron beam resists having high sensitivity, resolution and especially dry etching resistance. Various kinds of negative electron beam resists showing excellent dry etching resistance have been developed for the purpose of direct device fabrication (1) (2) (3). 2

1

Current address: Yamazaki Works, Hitachi Chemical Co., Ltd., Hitachi, Ibaraki 317, Japan 0097-6156/ 84/ 0242-0167506.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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However c o n v e n t i o n a l p o s i t i v e e l e c t r o n beam r e s i s t s l i k e PMMA(4) o r PBS(JL) do n o t h a v e e x c e l l e n t d r y e t c h i n g r e s i s t a n c e . The electron beam s e n s i t i v i t i e s o f these p o s i t i v e r e s i s t s primarily r e s u l t f r o m radiâtion-induced d e g r a d a t i o n o f polymer main c h a i n s . I f the main c h a i n bonding f o r c e o f these polymers i s weakened i n order t o improve s e n s i t i v i t i e s , the dry etching resistances o f these polymers w i l l decrease. I n such c a s e s , sensitivity t o e l e c t r o n beam e x p o s u r e and d r y e t c h i n g r e s i s t a n c e are i n a t r a d e - o f f r e l a t i o n s h i p . Rai and Shepherd(6.) h a v e showed t h a t p o l y s t y r e n e h a s good sputtering resistance. I t i s w e l l known t h a t t h e i n c o r p o r a t i o n o f phenyl groups i n t o r e s i s t polymers o r t h e use o f aromatic polymers such as polystyrene o r n o v o l a c s enhances t h e d r y e t c h i n g r e s i s t a n c e o f the polymer. Conventional photoresists which c o n s i s t ofa l k a l i - s o l u b l e novolac resins and p h o t o a c t i v e d i s s o l u t i o n i n h i b i t o r s Q . ) have excellent dry etching resistance. H a t z a k i s and h i s c o l l e a g u e s ( & ) have i n v e s t i g a t e d s u c h p o s i t i v e p h o t o r e s i s t s as p o s i t i v e e l e c t r o n beam r e s i s t s . However, s e n s i t i v i t y t o e l e c t r o n beam e x p o s u r e was n o t so g o o d ( 2 x l 0 ~ C/cm* a t 20 k V ) . Bowden and h i s coworkers(£) p r o p o s e d a new t y p e o f p o s i t i v e e l e c t r o n beam r e s i s t w h i c h c o n s i s t s o f a n a l k a l i - s o l u b l e n o v o l a c and polymeric dissolution inhibitor. The p o s i t i v e w o r k i n g mechanism o f t h i s new t y p e p o s i t i v e r e s i s t ( NPR ) i s s i m i l a r t o that f o r the conventional p o s i t i v e photoresist(10). I t was a l s o found that poly(2-methylpentene-l s u l f o n e ) ( PMPS ) i s good as a polymeric d i s s o l u t i o n i n h i b i t o r f o r NPR(IO). I n a d d i t i o n , i t was clarified that one o f t h e d i f f i c u l t i e s with NPR i s phase separation i n the r e s i s t films(10)(11). In this p a p e r we r e p o r t on t h e s y n t h e s i s o f n o v o l a c r e s i n s suitable t o NPR. A l o o k i s a l s o t a k e n a t t h e anomalous 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 NPR. 5

Experimental PMPS P r e p a r a t i o n : PMPS was p r e p a r e d using t h e same p r o c e d u r e described b y Bowden and h i s c o w o r k e r s ( 1 2 ) . 2 - M e t h y l p e n t e n e - l was refluxed over l i t h i u m aluminum h y d o r i d e f o r two h o u r s , and t h e n distilled. 50ml o f d i s t i l l e d 2 - m e t h y l p e n t e n e - l was t r a n s f e r r e d into a 300ml r e a c t i o n v e s s e l a t t a c h e d t o a vacuum l i n e c o n t a i n i n g a m a g n e t i c s t i r r e r t i p , and t h e n d e g a s s e d . The v e s s e l was c o o l e d t o -90 °C and t h e n c h a r g e d w i t h 100ml o f l i q u i d s u l f u r d i o x i d e dried through a phosphor p e n t o x i d e column. 3g o f t - b u t y l h y d r o p e r o x i d e was t r a n s f e r r e d i n t o t h e v e s s e l by means o f vacuum distillation. The m i x t u r e was warmed s l o w l y t o -50°C t h r o u g h magnetic stirring, and k e p t a t t h a t temperature f o r 5 hours. After evacuation o f excess sulfur dioxide, t h e c o n t e n t s were dissolved in 1000ml o f a c e t o n e and t h e p o l y m e r was then precipitated i n t o m e t h a n o l . The p o l y m e r was f i n a l l y d r i e d f o r 48 h o u r s a t 30 °C.

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


14.

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Novolac Based Positive Electron Beam Resist

169

N o v o l a c P r e p a r a t i o n ; N o v o l a c r e s i n s were p r e p a r e d by an a c i d catalyzed condensation o f m- and p - c r e s o l s w i t h f o r m a l d e h y d e . A three necked flask with a distillation reflux condenser, t h e r m o m e t e r and mechanical stirrer was c h a r g e d w i t h mand p-cresols, f o r m a l d e h y d e aqueous s o l u t i o n , and c o n e , h y d r o c h l o r i c acid as a catalyst. The f l a s k was immersed i n an o i l b a t h and heated t o 90 °C and k e p t f o r 2 h o u r s w h i l e u n d e r g o i n g s t i r r i n g . After the f l a s k was c o o l e d t o room t e m p e r a t u r e on s t a n d i n g , t h e supernatant layer of the contents was removed by décantation. Then t h e v o l a t i l e components were e l i m i n a t e d by d i s t i l l a t i o n u n d e r a n i t r o g e n gas f l o w and s l o w h e a t i n g t o 175°C. The m o l t e n c o n t e n t was p o u r e d i n t o a s t a i n l e s s s t e e l t r a y t o c o o l . C h a r a c t e r i s t i c s : The s t r u c t u r e o f t h e o b t a i n e d PMPS was confirmed by measurements of infrared s p e c t r a , and nuclear magnetic resonance s p e c t r a using a D i g i l a b FTS-20C/D F o u r i e r t r a n s f o r m infrared spectrometer, and a H i t a c h i R-24 NMR spectrometer, respectively. The molecular weight and molecular weight distribution o f t h e n o v o l a c o r p h e n o l i c r e s i n s were d e t e r m i n e d by gel permeation chromatography using a H i t a c h i 635 liquid chromatography system. Dissolution R a t e M e a s u r e m e n t s : Sample f i l m s were spun onto s i l i c o n w a f e r s f r o m i s o a m y l a c e t a t e s o l u t i o n s o f sample r e s i n s o r sample r e s i n and PMPS. The f i l m s were immersed i n an aqueous s o l u t i o n of tetramethylammonium h y d r o x i d e . D i s s o l u t i o n r a t e s were obtained from a p l o t o f the measured r e s i d u a l f i l m t h i c k n e s s against the immersion time. The f i l m t h i c k n e s s was m e a s u r e d w i t h an i n t e r f e r o m e t e r . Resist P r e p a r a t i o n : R e s i s t s o l u t i o n s were p r e p a r e d by d i s s o l v i n g novolac r e s i n s and PMPS i n i s o a m y l a c e t a t e . The s o l u t i o n s were t h e n f i l t e r e d t h r o u g h an 0.2 um T e f l o n f i l t e r . Sensitivity Measurements: R e s i s t f i l m s were spun o n t o s i l i c o n wafers. The f i l m s were t h e n p r e b a k e d and e x p o s e d i n a m o d i f i e d Hitachi e l e c t r o n m i c r o s c o p e t o an u n d e f l e c t e d , n e a r l y c o l l i m a t e d e l e c t r o n beam a t an a c c e l e r a t i o n v o l t a g e o f 15kV. A f t e r exposure the resist f i l m s were d e v e l o p e d i n an aqueous s o l u t i o n o f tetramethylammonium h y d r o x i d e . S e n s i t i v i t y c u r v e s were o b t a i n e d by p l o t t i n g f i l m t h i c k n e s s a g a i n s t i n c i d e n t d o s e . Patterning of t h e R e s i s t : R e s i s t s were d e l i n e a t e d w i t h a v e c t o r scanning type, shaped e l e c t r o n - b e a m d r a f t i n g m a c h i n e , s p e c i a l l y designed by the H i t a c h i C e n t r a l Research Laboratory. The a c c e l e r a t i o n v o l t a g e was 30kV. R e s u l t s and

Discussion

Coating s o l v e n t s were i n v e s t i g a t e d f o r c o m p o s i t e s y s t e m s o f PMPS and v a r i o u s c o m m e r c i a l l y a v a i l a b l e p h e n o l i c o r n o v o l a c r e s i n s . I t was f o u n d t h a t most o f n o v o l a c o r p h e n o l i c r e s i n s c o n t a i n i n g PMPS can f o r m homogeneous f i l m s when i s o a m y l a c e t a t e i s u s e d as a coating solvent.



170

PMPS has b e e n shown e l s e w h e r e t o be v a p o r d e v e l o p a b l e , w h i c h means t h a t the p o l y m e r i s v a p o r i z e d and removed d u r i n g e l e c t r o n beam e x p o s u r e ( 1 3 ) . The v a p o r d e v e l o p m e n t c h a r a c t e r i s t i c s o f PMPS are shown h e r e i n F i g u r e 1. PMPS was n o t c o m p l e t e l y removed by a d o s e o f 2 x l 0 " C/cm a t 15kV. T h e r e f o r e , a r e s i n s u i t a b l e t o NPR should show a large s o l u b i l i t y increase with a decrease i n i t s concentration i n NPR. To s e l e c t a r e s i n s u i t a b l e f o r N P R , we d e f i n e d p a r a m e t e r R i o as Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 15, 2018 | https://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch014

5

2

R10 = T o / T i o

where To and T10 are r e s p e c t i v e l y the d i s s o l u t i o n r a t e s of the r e s i n w i t h o u t and w i t h 10 wt.% o f PMPS t o t h e r e s i n . A l a r g e RJO value i n d i c a t e s the l a r g e d i s s o l u t i o n i n h i b i t i n g e f f e c t of PMPS. We m e a s u r e d RJO v a l u e s f o r v a r i o u s c o m m e r c i a l l y a v a i l a b l e p h e n o l i c and novolac r e s i n s , together w i t h those f o r s y n t h e s i z e d novolac resins. The r e s u l t s a r e summarized i n T a b l e I .

T a b l e I . I n v e s t i g a t e d r e s i n s and Resin Resin M R e s i n MB A l n o v o l PN430 R e s i t o p PSF2803 R e s i t o p PSF2807 R e s i t o p XPS4800B Penol Novolak m-High C r e s o l N o v o l a k m-Cresol Novolak H i t a n o l HP-607N Sample R e s i n - 1 Sample R e s i n - 3

t h e i r Rio

values.

Origin M a r u z e n O i l Co., L t d . M a r u z e n O i l Co., L t d . Hoechst Japan L t d . G u n e i C h e m i c a l I n d u s t r y Co. , L t d . G u n e i C h e m i c a l I n d u s t r y Co. , L t d . G u n e i C h e m i c a l I n d u s t r y Co. , L t d . S h i n k o T e c h . R e s e a r c h Co., L t d . S h i n k o T e c h . R e s e a r c h Co., L t d . S h i n k o T e c h . R e s e a r c h Co., L t d . H i t a c h i C h e m i c a l Co., L t d . T h i s work T h i s work

R10

3.6 18.0 11.1 9.5 21.5 3.4 9.6 14.5 16.8 6.7 28.3 >45

As can be s e e n i n T a b l e I . one s y n t h e s i z e d n o v o l a c r e s i n ( Sample R e s i n - 3 ) showed a v e r y l a r g e R i o v a l u e compared w i t h other r e s i n s . I t was f o u n d t h a t R i o i s s t r o n g l y d e p e n d e n t on t h e r a t i o o f m- t o p - c r e s o l c o n c e n t r a t i o n s i n t h e r e s i n s . D i s s o l u t i o n rates are shown as a f u n c t i o n o f PMPS c o n t e n t f o r t h i s r e s i n i n F i g u r e 2. Exposure c h a r a c t e r i s t i c s f o r NPR with use of the c r e s o l novolac a r e shown i n F i g u r e 3. T h i s NPR c o n t a i n s 12% PMPS a g a i s t the resin weight. Bowden and h i s coworkers showed that post-exposure baking increases the sensitivity of NPR(IO). However, w i t h t h e p r e s e n t NPR, t h e p o s t - e x p o s u r e b a k i n g does n o t remarkably a f f e c t s e n s i t i v i t y . An example o f t h e f i n e p a t t e r n f o r this NPR as obtained by e l e c t r o n beam d e l i n e a t i o n i s shown i n F i g u r e 4.



14.

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171

2

DOSE (C/cm ) F i g u r e 1.

Exposure c h a r a c t e r i s t i c s o f PMPS p r e p a r e d b y v a p o r d e v e l o p m e n t : ( a ) a t 15kV; ( b ) a t 30kV.

F i g u r e 2.

Dissolution rate PMPS c o n t e n t .

o f r e s i n f i l m , shown as f u n c t i o n o f



172


F i g u r e 3.

F i g u r e 4.

Exposure c h a r a c t e r i s t i c s 15kV.

SEM

o f NPR.

photomicrograph of

voltage:

30kV; Dose: 8 x l 0 ~

NPR 6

Acceleration

pattern.

voltage:

Acceleration

2

C/cm .



14.

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173


It was a l s o f o u n d t h a t t h e s e n s i t i v i t y t o d o u b l e e x p o s u r e i s much h i g h e r t h a n t h a t t o s i n g l e e x p o s u r e w i t h t h e same t o t a l d o s e . This d o u b l e e x p o s u r e e f f e c t i s shown i n F i g u r e 5. The d e g r e e o f sensitivity enhancement i s r e p r e s e n t e d by t h e e q u i v a l e n t s i n g l e dose w i t h w h i c h t h e e x p o s e d a r e a w o u l d show t h e same s o l u b i l i t y as for t h e d o u b l y e x p o s e d a r e a . S e n s i t i v i t y enhancement depends on the t i m e i n t e r v a l between t h e f i r s t and s e c o n d e x p o s u r e , and a l s o the dose fraction ratio, as c a n be seen i n F i g u r e 5. The sensitivity enhancement peak i s r e a c h e d a t a f i r s t dose f r a c t i o n of 0.25. A s i m i l a r d o u b l e e x p o s u r e e f f e c t was f o u n d f o r PMPS f i l m , as F i g u r e 6 shows. I n t h i s f i g u r e , t h e enhancement i s e x p r e s s e d by the equivalent s i n g l e dose w i t h w h i c h t h e e x p o s e d f i l m t h i c k n e s s w o u l d become t h e same as f o r a d o u b l y e x p o s e d a r e a . I n t h i s c a s e , sensitivity enhancement does n o t depend on the time i n t e r v a l between t h e f i r s t and second exposure, but the sensitivity enhancement r a p i d l y r e a c h e s a maximum i n t h e r e g i o n o f t h e l o w f i r s t dose f r a c t i o n , whereby i t s l o w l y f a l l s . I t was f o u n d t h a t b o t h s e n s i t i v i t y and d o u b l e e x p o s u r e e f f e c t are r e d u c e d w i t h m i x i n g o f a s m a l l amount o f r a d i c a l s c a v e n g e r , l,l-diphenyl-2-picrylhydrazyl ( DPPH ) , i n t o t h e PMPS. This indicates t h a t r a d i c a l r e a c t i o n s a r e i n v o l v e d i n the mechanism o f d o u b l e e x p o s u r e s e n s i t i v i t y enhancement. The d o u b l e e x p o s u r e e f f e c t c a n be e x p l a i n e d on t h e b a s i s o f a few a s s u m p t i o n s . We h a v e assumed t h a t p u l s e d e l e c t r o n beam irradiations p r o d u c e PMPS r a d i c a l s i n PMPS o r NPR f i l m s . We a l s o assumed t h a t t h e i n i t i a l amount o f PMPS r a d i c a l s i s p r o p o r t i o n a l to the exposure d o s e , and t h a t t h e c o n c e n t r a t i o n , C., d e c r e a s e s w i t h b o t h f i r s t - o r d e r and s e c o n d - o r d e r r e a c t i o n s . The f i r s t - o r d e r r e a c t i o n may be t h e end o f z i p p i n g o r s t a b i l i z a t i o n by c h a i n transfer, and t h e s e c o n d - o r d e r r e a c t i o n may be a r a d i c a l - r a d i c a l recombination reaction. E q u a t i o n 1 c a n be o b t a i n e d s u c h t h a t dC ki

C + k2 C

(1)

dt where t i s t h e t i m e a f t e r p u l s e d e x p o s u r e , and k i and k2 a r e constants. I f we assume t h a t a d e c r e a s e i n PMPS r e s u l t s f r o m t h e z i p p i n g r e a c t i o n , t h e t o t a l amount o f decomposed PMPS, M, c a n be e x p r e s s e d as

(2)

where k i s a c o n s t a n t . u s i n g E q u a t i o n 1 as

T h i s i n t e g r a t i o n c a n t h e n be

calculated




174

1 2 3 A 5 6 8 8 8 8 8 8 FIRST DOSE FRACTION F i g u r e 6.

Double exposure i n t e r v a l : 30s.

effect

7 8

i n PMPS.

Exposure


time

14. SHIRAISHI ETAL. Novolac Based Positive Electron Beam 175Resist 0 M

dt kC — dC C=C dC 0

kC (C=C

1 ) dC ki C + k C 2

2


0

dC 0

k +k C 1

2

and finally, we obtain k k M = log( 1 + C ) k ^ 2

(3)

0

2

where Co is the initial PMPS radical concentration. Since we have assumed that the initial radical concentration, Co, is proportional to the dose, Equation 3 indicates that the total amount of decomposed PMPS, M, is almost linearly dependent on log( Co ) and therefore log( Dose ), provided ( k /ki )Co is much larger than unity. This relationship was also obtained experimentally, as Figure 1 shows. The doubled exposure effect follows directly from the assumption that PMPS decreases in accordance with Equation 3 in every exposure. Furthermore, asymmetry in the enhanced sensitivity with respect to the dose fraction ratio can be explained by assuming that the efficiency of PMPS radical production in the second exposure is lower than in the first exposure. The solid curve in Figure 5 illustrates an approximation resulting from the assumption that ( k /ki )Cq = 10, the initial concentration of PMPS radicals at first exposure = pC© ( 0£p£l ), and that at second exposure = (l-p)Coexp(-2p). The curve is in good agreement with the observed data. 2

2

Acknowle dgment s The authors are grateful to Kozo Mochiji and Kiyotake Naraoka for their assistance in the electron beam fabrication experiments. They also thank Takumi Ueno for useful discussions regarding the sensitivity enhancement mechanism. Literature Cited 1. S. Imamura, J. Electrochem. Soc. 1979, 126, 1628. 2. H. Shiraishi, Y. Taniguchi, S. Horigome, and S. Nonogaki, Polymer Eng. Sci. 1980, 20, 1054.



176


3. E. D. Feit and L. E. Stillwagon, Polymer Eng. Sci. 1980, 20, 1058. 4. I. Haller, M. Hatzakis, and R. Srinivasan, IBM J. Research and Development 1968, 12, 250. 5. M. J. Bowden, L. F. Thompson, and J. P. Ballantyne, J. Vacuum Sci. Tech. 1975, 12, 1294. 6. J. H. Rai and L. T. Sheperd, Preprints ACS Organic Coatings and Plastics Chemistry 1975, 35, 252. 7. J. Pacansky, Polymer Eng. Sci. 1980, 20, 1049. 8. M. Hatzakis and J. M. Shaw, Proceedings 8th Int. Conf. on Electron and Ion Beam Sci. and Tech., Electrochem. Sci. 1978, 78-5, 285. 9. L. F. Thompson, M. J. Bowden, Ε. M. Doerries, and S. R. Fahrenholtz, paper presented at Electrochem. Soc. Meeting, Seattle, Wash. 1978, May 21-26. 10. M. J. Bowden, L. F. Thompson, S. R. Fahrenholtz, and Ε. M. Doerries, J. Electrochem. Soc. 1981, 128, 1304. 11. M. J. Bowden, J. Applid Polymer Sci. 1981, 26, 1421. 12. M. J. Bowden and L. F. Thompson, J. Applied Polymer Sci. 1973, 17, 3211. 13. L. F. Thompson and M. J. Bowden, J. Electrochem. Soc. 1973, 120, 1722. RECEIVED September 2, 1983


Novolac Based Positive Electron Beam Resist Containing a Polymeric

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