Synthesis of New Metal-Free Diazonium Salts and Their Applications

Chapter 19

Synthesis of New Metal-Free Diazonium Salts and Their Applications to Microlithography 1

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 20, 2016 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch019

Shou-ichi Uchino, Michiaki Hashimoto , and Takao Iwayanagi Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan

New metal-free diazonium salts have been synthesized and applied to microlithography as photobleachable dyes for a contrast enhancing layer (CEL), negative working photoactive compounds, and photoacid generators. 4-N, N-dimethylaminobenzenediazonium trifluoromethanesulfonate (D1) is the most suitable photobleachable dye. Upon i-line (365nm) exposure, 0.4-µm line-and-space patterns were obtained using a positive resist in conjunction with the D1-CEL. D1 also shows good properties as a negative working sensitizer for a two-layer resist system formed by means of a "doping" process, in which the diazonium salt is distributed in the top and bottom layers. 4-Methoxybenzenediazonium trifluoromethanesulfonate can be used as a photoacid generator for acid catalyzed cross-linking. Submicron resolution can be achieved with high sensitivity. I n recent years, demands on m i c r o l i t h o g r a p h i c techniques have become much more severe along w i t h t h e reduced s i z e s o f semiconductor devices. Various techniques t o enhance t h e performance o f m i c r o l i t h o g r a p h i c resist systems have been developed. Contrast enhanced l i t h o g r a p h y (CEL) i s one technique which improves r e s i s t r e s o l u t i o n (1,2). The CEL process involves spin-coating a CEL l a y e r containing a photobleachable dye on top o f a conventional p h o t o r e s i s t . This CEL l a y e r i s opaque before exposure, but during exposure, t h e most h i g h l y exposed regions bleach f i r s t w h i l e the l e a s t exposed regions bleach later. Therefore, optical images that a r e degraded by t h e lens system o f t h e exposure apparatus a r e sharpened by passing through t h e CEL l a y e r . The photobleachable dyes used i n c o n t r a s t enhancing m a t e r i a l s can be c l a s s i f i e d i n t o four c a t e g o r i e s : n i t r o n e s ( 1 , 2), p o l y s i l a n e s ( 3 ) , s t y r y l p y r i d i n i u m s ( 4 ) , and diazonium s a l t s (5 9). r

1

Current address: Yamazaki Works, Hitachi Chemical Company Ltd., Hitachi, Ibaraki 317, Japan 0097-6156/89AM12-0319$06.00/0 © 1989 American Chemical Society

Reichmanis et al.; Polymers in Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

320

POLYMERS IN MICROLITHOGRAPHY


The relationship between resist contrast and optical p r o p e r t i e s ( e x t i n c t i o n c o e f f i c i e n t and b l e a c h i n g quantum y i e l d ) of photobleachable dyes was p r e v i o u s l y described by a simple model (9), and w a t e r - s o l u b l e diazonium s a l t s with good o p t i c a l p r o p e r t i e s were developed for the CEL process (9). Diazonium salts are also useful as a photosensitive m a t e r i a l i n a photobleachable two-layer r e s i s t system based on a "doping process"(10). High-resolution resist patterns were obtained u s i n g this two-layer resist scheme and an i-line reduction projection aligner. As mentioned above, the conventional diazonium s a l t s have good optical properties as CEL dyes and negative working s e n s i t i z e r s for the two-layer r e s i s t system. However, almost all diazonium salts are stabilized with metal-containing compounds such as z i n c c h l o r i d e , t e t r a f l u o r o b o r a t e , hexafluoroantimonate, hexafluoroarsenate, or hexafluorophosphate, which may not be d e s i r a b l e i n semiconductor f a b r i c a t i o n because of p o t e n t i a l device contamination. To a l l e v i a t e the p o t e n t i a l problem, new m e t a l - f r e e m a t e r i a l s have been sought for. In this paper we report on the use of trifluoromethanesulfonates (Table 1) of 4-N, N-dimethylaminobenzenediazonium (Dl) and 4-methoxybenzene-diazonium (D2) as CEL dyes, negative working s e n s i t i z e r s , and photoacid generators for chemical a m p l i f i c a t i o n r e s i s t systems(11).

Experimental Synthesis of diazonium salts. The d e t a i l s of the s y n t h e s i s of 4-N, N-dimethylaminobenzenediazonium trifluoromethanesulfonate (Dl) are described below. N, N-Dimethyl-p-phenylenediamine (27.7 g, 0.2 mole) was d i s s o l v e d i n a c e t i c a c i d (150 ml). T r i f l u o r o m e t h a n e s u l f o n i c a c i d (30.6 g, 0.2 mole) was added to the s o l u t i o n i n an atmosphere of nitrogen. The s o l u t i o n was kept at room temperature while i s o p e n t y l n i t r i t e (26 g, 0.22 mole) was added dropwise. The s o l u t i o n was allowed to react for half an hour. After completion of diazotization, tetrahydrofuran (400 ml) was added to the s o l u t i o n and the precipitated Dl (55 g) was filtered. Dl (20 g) was r e c r y s t a l l i z e d from acetone (90 m l ) / t e t r a h y d r o f u r a n (260 ml). P r e c i p i t a t e d Dl c r y s t a l s (14 g) were f i l t e r e d and d r i e d . The m e l t i n g point of Dl i s 127t and the maximum a b s o r p t i o n i n water appears at 376 nm. 4-Methoxybenzenediazonium t r i f l u o r o m e t h a n e sulfonate (D2) was synthesized s i m i l a r l y .

Lithographic

evaluation.

CEL dye. A CEL s o l u t i o n was obtained by d i s s o l v i n g p o l y v i n y l p y r r o l i d o n e ) (PVP) (7 g) and D l (5.8 g) i n 50 wt% aqueous acetic acid. (87.2 g). The CEL l a y e r was spin-coated onto a photoresist, RI-7000P ( H i t a c h i Chemical C o . ) , and baked at 80T for 20 minutes. Exposure was performed with an in-house i - l i n e reduction projection aligner. The r e s i s t was developed i n a 2. 38 wt% tetramethylammonium hydroxide aqueous s o l u t i o n . The f i l m thickness was measured with an A l p h a - s t e p 200 (Tencor)


19. UCHINOETAK

Metal-Free Diazonium Salts

321


profilometer. S p e c t r o s c o p i c measurements were performed on a H i t a c h i 340 UV spectrophotometer and a H i t a c h i 260-10 IR spectrophotometer. Negative two-layer resist. A c r e s o l novolac r e s i n (Alnovol PN-430) was spin-coated on a s i l i c o n wafer and baked a t 80T, f o r one minute on a hot plate. The s i l i c o n wafer was set on a s p i n c o a t e r and a p h o t o s e n s i t i v e s o l u t i o n c o n s i s t i n g o f D l (3 wt%), PVP (5 wt%), a c e t i c a c i d (46 wt%), and water (46 wt%) was deposited so as t o c o n t a c t t h e p h e n o l i c r e s i n f i l m on the s i l i c o n wafer. The s o l u t i o n on the p h e n o l i c r e s i n f i l m stood for 2 minutes. The r e s i d u a l s o l u t i o n on t h e p h e n o l i c r e s i n f i l m was spun t o form a Dl-PVP top l a y e r and baked a t 8 0 t f o r two minutes. By t h i s process, a two-layer r e s i s t was formed (10). The r e s i s t was exposed w i t h an in-house i - l i n e r e d u c t i o n p r o j e c t i o n a l i g n e r o r a 600-W Xe-Hg lamp (Cannard Hanovia) i n c o n j u n c t i o n w i t h a UVD2 band pass f i l t e r (Toshiba g l a s s Co.). A f t e r removing t h e top l a y e r by r i n s i n g i n water, t h e r e s i s t was developed w i t h a 2. 38 wt% tetramethylammonium hydroxide aqueous solution. Photoacid generator. D l (4 wt%J was mixed w i t h p o l y ( g l y c i d y l methacrylate) (PGMA) (20 wt%J i n e t h y l c e l l o s o l v e acetate. The mixture was spin-coated on a s i l i c o n wafer and baked a t 80T f o r 1 minute. Exposure was performed w i t h a 600-W Xe-Hg lamp i n c o n j u n c t i o n w i t h a UVD2 f i l t e r . The r e s i s t was developed i n a mixture o f methyl e t h y l ketone t o ethanol (7/1 w/w). In another a p p l i c a t i o n , t h e diazonium s a l t (Dl o r D2) (2.5 wt%) was d i s s o l v e d i n a mixture o f cyclohexanone and a c e t i c a c i d c o n t a i n i n g 12.5 wt% poly(4-hydroxystyrene) and 2.5 wt% Methylone r e s i n (GE 75108). The r e s i s t was exposed w i t h a 600-W Xe-Hg lamp through a 313-nm i n t e r f e r e n c e f i l t e r . A f t e r exposure t h e r e s i s t was baked a t 8 0 t f o r 3 minutes and developed i n a 1 wt% tetramethylammonium hydroxide aqueous s o l u t i o n . Results and discussion Synthesis of diazonium salts. The c o n v e n t i o n a l d i a z o t i z a t i o n of amino compounds i s performed as d e s c r i b e d below. An amino compound i s d i s s o l v e d i n a c i d i c water, then aqueous sodium n i t r i t e i s added dropwise t o t h e s o l u t i o n , w h i l e t h e s o l u t i o n temperature i s kept below Ot. After completion o f d i a z o t i z a t i o n , a diazonium i o n s t a b i l i z e r such as z i n c c h l o r i d e , sodium t e t r a f l u o r o b o r a t e , o r sodium hexafluorophosphate i s added to t h e r e a c t i o n s o l u t i o n . Therefore, diazonium s a l t s t h a t a r e s y n t h e s i z e d by the c o n v e n t i o n a l process c o n t a i n metal ions t h a t may contaminate semiconductor devices. I n a d d i t i o n , because some diazonium s a l t s a r e extremely s o l u b l e i n water, i t i s o f t e n d i f f i c u l t t o i s o l a t e them from the aqueous r e a c t i o n s o l u t i o n . To avoid the above mentioned problems, diazonium s a l t s were synthesized by d i a z o t i z i n g amino compounds w i t h isopentyl n i t r i t e i n a c e t i c a c i d c o n t a i n i n g a s t r o n g a c i d such as trifluoromethanesulfonic acid. After completion o f d i a z o t i z a t i o n , t h e r e a c t i o n s o l u t i o n was poured i n t o an o r g a n i c solvent. The p r e c i p i t a t e d c r y s t a l s were f i l t e r e d and d r i e d .


322


T h i s process makes i t p o s s i b l e t o remove metal i o n s from diazonium s a l t s and a l s o provides easy i s o l a t i o n of diazonium s a l t s t h a t are h i g h l y s o l u b l e i n water.

the the

Thermal stability of Dl. The thermal s t a b i l i t y of the D l i n 50 wt% a c e t i c a c i d aqueous s o l u t i o n was evaluated by k i n e t i c analysis. The thermal decomposition of diazonium s a l t s i n an aqueous s o l u t i o n i s a f i r s t - o r d e r r e a c t i o n . The thermal decomposition constants of D l at 25, 75, and 90t are 9. 1x10" , 2. 9x10" , 1. 3x10" /hr, r e s p e c t i v e l y . The A r r h e n i u s p l o t of D l i n a c e t i c a c i d s o l u t i o n i s shown i n F i g u r e 1. The a c t i v a t i o n energy of D l (23. 0 kcal/mol) and the decomposition r a t e constant a t 5 t were obtained from F i g u r e 1. T h i s decomposition r a t e constant (k =3. 7x10" /hr) i n d i c a t e s t h a t 1% of the Dl i n the a c e t i c a c i d aqueous s o l u t i o n w i l l decompose i n about 3 years at 5Î. 6


8

1

7

6T:

Photoproduct of Dl in solid state. Major photoproduct from D l in the solid s t a t e was isolated by silica g e l column chromatography and analyzed by IR and NMR spectroscopy (Figures 2 and 3). NMR spectrum shows t h a t a photoproduct of Dl has the a b s o r p t i o n band of the N, N-dimethylamino group (2.86 ppm) and the symmetric a b s o r p t i o n band of aromatic protons (6.57-7. 10 ppm). The s t r o n g IR a b s o r p t i o n of the s u l f o n y l group (1210 and 1420 cm ) can be seen i n F i g u r e 3. From NMR and IR spectra, the main photoproduct of the Dl was identified as t r i f l u o r o m e t h a n e s u l f o n i c a c i d 4-N, N-dimethylaminophenyl ester. -1

kpplication to a CEL dye. The UV s p e c t r a of D l i n a PVP m a t r i x before and a f t e r b l e a c h i n g are shown i n F i g u r e 4. The a b s o r p t i o n maximum of D l l i e s at 376 nm. The photobleaching curve of the Dl-CEL l a y e r c o n s i s t i n g of Dl and PVP i s shown i n F i g u r e 5. The t r a n s m i t t a n c e of the unbleached l a y e r i s l e s s than 1% and t h a t of the completely bleached l a y e r i s about 90%. Contrast and r e s o l u t i o n of the p h o t o r e s i s t are expected t o be g r e a t l y improved w i t h the use o f the Dl-CEL. The exposure curves of the p o s i t i v e p h o t o r e s i s t H i t a c h i Chemical RI-7000P w i t h and without the Dl-CEL are shown i n F i g u r e 6. The r e s i s t c o n t r a s t w i t h the Dl-CEL(y=3. 9) i s three times higher than t h a t without the Dl-CEL(y=l. 3). Scanning e l e c t r o n microphotographs of submicron r e s i s t p a t t e r n s p r i n t e d w i t h and without the Dl-CEL are shown i n F i g u r e 7. The photographs show t h a t the CEL l a y e r c o n t a i n i n g Dl enables imaging of 0.4-μπι l i n e - s p a c e p a t t e r n s on an i - l i n e r e d u c t i o n projection aligner. Application to the two-layer resist system. Photobleachable r e s i s t systems t h a t have a s t r o n g a b s o r p t i o n before exposure and t h a t bleach completely upon UV exposure a l l e v i a t e the l i g h t r e f l e c t i o n from the substrate. A photobleachable r e s i s t system formed by means of the "doping process" was reported i n our previous paper(9). T h i s r e s i s t system c o n s i s t s of two l a y e r s i n which a diazonium s a l t i s d i s t r i b u t e d i n both the top and bottom l a y e r s . When exposed t o i - l i n e , the diazonium s a l t i n



19. UCHINO ET AI*

323


Table i . Newly synthesized diazonium salts structure of Methylene used in this experiment

NAME

and

chemical

of Dl thermal decomposition solution.

in a 50

CHEMICAL STRUCTURE λιτβχ (nm)

Dl

D2

H

3COh0-N2SC3CF

3

Ç-CH -CH=CH 2

Êmax xlO*

376

3.37

312

2.37

2

•(CH OH)m 2

m = 1 -3

METHYLONE

ΔΕ=210 Kcai/mol

-10

I ι

2.5 Figure 1. Arrhenius plot wt% acetic acid aqueous

ι

ι

ι

I

ao

ι

ι

_

ι

ι

1 1

as



1.0 RELATIVE INTENSITY Ρ en c il

I

li

:>


324

8.0

SO

40

2.0

0

CHEMICAL SHIFT (ppm)

Figure

2. NMR spectrum

of the main photoproduct

from DL

100 ρ

£80 -

WAVENUMBER (cm* ) 1

Figure

3. IR spectrum

of the main photoproduct

from DL



19.

UCHINOETAL

325


200

300 400 WAVELENGTH (nm)

500

Figure 4. UV absorption spectra of a Dl-CEL layer consisting of 4-N, N-dimethylaminobenzenediazonium trifluoromethanesulfonate and PVP. The solid line is the unbleached layer, and the broken line is the completely bleached layer. 100,

DOSE (mJ/cm2)

Figure 5. Photobleaching at the i-line (365 nm).

curve

of the Dl-CEL

layer

exposed


326


with


Dl-CEL

300 DOSE (mJ/cm ) 2

Figure 6. Exposure curves of positive resist Hitachi Chemical RI-7000P with and without the Dl-CEL. The contrast (γ-value) of the Dl-CEL resist is three times of that of the non-CEL resist.

Figure 7. SENl photographs of resist patterns printed and without the Dl-CEL on commercially available (Hitachi Chemical RI-7000P).


with resist

19.

UCHINOETAL.


327

the top l a y e r bleaches t o act as a c o n t r a s t enhancing m a t e r i a l , w h i l e the diazonium s a l t i n the bottom l a y e r decomposes t o cause i n s o l u b i l i z a t i o n of the p h e n o l i c r e s i n i n the base developer.


The exposure curve of the two-layer r e s i s t based on the doping process i s shown i n F i g u r e 8. The two-layer r e s i s t system has a high c o n t r a s t and high r e s o l u t i o n c a p a b i l i t y . Submicron l i n e - a n d - s p a c e p a t t e r n s are obtained u s i n g t h i s twol a y e r r e s i s t system (Figure 9). Application to photoacid generators. Recently much a t t e n t i o n has been focused on c h e m i c a l l y a m p l i f i e d r e s i s t systems u s i n g onium s a l t s (11), halogen compounds (12), or n i t r o b e n z y l ester(13) as photoacid generators. Since the newly synthesized diazonium s a l t s have t r i f l u o r o m e t h a n e s u l f o n i c a c i d anion as the counter ion, they generate a s t r o n g p r o t o n i c a c i d upon UV exposure. Dl and poly ( g l y c i d y l mathacrylate) (PGMA) were mixed and exposed to i - l i n e . The exposure curve of the Dl-PGMA r e s i s t system i s shown i n F i g u r e 10. Here i t can be seen t h a t the Dl-PGMA r e s i s t works as a h i g h l y s e n s i t i v e negative r e s i s t . The newly synthesized diazonium s a l t s might be u s e f u l as photoacid generators f o r chemical a m p l i f i c a t i o n r e s i s t systems. However, the Dl-PGMA r e s i s t system d i d not show e x c e l l e n t image q u a l i t y because of s w e l l i n g d u r i n g the development process. Another a c i d - c a t a l y z e d c r o s s - l i n k i n g type r e s i s t system devoid of s w e l l i n g was evaluated, which c o n s i s t e d of a p h e n o l i c r e s i n , Methylone r e s i n , and the diazonium s a l t . Both D l and D2 were used. The exposure curve of the r e s i s t system c o n t a i n i n g D2 as the photoacid generator i s shown i n F i g u r e 11. The s e n s i t i v i t y and c o n t r a s t of the r e s i s t d r a m a t i c a l l y improved w i t h post exposure baking. An SEM photograph of the submicron p a t t e r n of the a c i d c a t a l y z e d r e s i s t system u s i n g D2 as the photoacid generator i s shown i n F i g u r e 12. The r e s i s t system which contains D l does not show high s e n s i t i v i t y and high contrast characteristics. It is probable that the photogenerated a c i d i s wasted by the p r o t o n a t i o n of the N, Ndimethylamino group of Dl. The d e t a i l s of the r e a c t i o n mechanism are under i n v e s t i g a t i o n .

Conclusion New m e t a l - f r e e diazonium s a l t s were s y n t h e s i z e d and a p p l i e d t o microlithography. A CEL l a y e r c o n s i s t i n g of D l and PVP has good o p t i c a l p r o p e r t i e s f o r i - l i n e exposure and r e s o l v e s 0.4-/mi l i n e and-space p o s i t i v e r e s i s t patterns. Dl i s a l s o a u s e f u l m a t e r i a l as a negative working s e n s i t i z e r f o r p h e n o l i c r e s i n s . 4Methoxybenzenediazonium t r i f l u o r o m e t h a n e s u l f o n a t e can be used as a photoacid generator f o r the design o f mid-UV r e s i s t system based on a c i d - c a t a l y z e d c r o s s - l i n k i n g . Submicron r e s i s t p a t t e r n s are r e s o l v e d u s i n g t h i s h i g h l y s e n s i t i v e r e s i s t system.




328

3

10

30 100 DOSE (mJ/cm )

300

2

Figure 8. Exposure curve for a negative two-layer system formed by "doping. " Exposure was performed i-line(365nm).

resist at the

Figure 9. SEUL photograph of the negative two-layer resist formed by means of "doping. " Exposure was performed at the i-line. (A) 0. 5-μm IAS, (B) 0. 6-\Lm IAS.



19.

UCHINO ET AL.

Metal-Free Diazonium Salis

Figure 10. Exposure Exposure was performed

curve of PGM at the i-line.

1

329

sensitized

with

Dl.

3 10 D0SE(mJ/cm2)

Figure 11. Exposure curve of the acid-catalyzed resist system consisting of poly(4-hydroxystyrene) D2, and Methylone. Exposure was performed using a Xe-Hg lamp equipped with a 313-nm interference filter. f



330


Figure 12. SEM. photograph of the acid-catalyzed resist system exposed at 313 nm with an exposure energy of 5 mJ/cm*.


19. UCHINO ET AL.


331

Acknowledgments The authors would l i k e t o thank Mr. Tsuneo Terasawa and Mr. T o s h i h i k o Tanaka f o r performing the o p t i c a l exposure experiments and Dr. M. Toriumi f o r performing d i s s o l u t i o n r a t e measurement u s i n g l a s e r interferometory.


Literature Cited 1. Griffing, B. F.; West, P. R., Polym. Eng. Sci., 1983, 23, 947. 2. Griffing, B. F.; West, P. R., IEEE Electron Device Lett., 1983, EDL-4, 14. 3. Hofer, D. C.; Miller, R. D.; Willson, C. G.; Neureuther, A. R., Proc. SPIE, 1984, 469, 108. 4. Yonezawa, T.; Kikuchi, H.; Hayashi, K.; Tochizawa, N.; Endo, N.; Fukuzawa, S.; Sugito, S.; Ichimura, Κ., Conf. "Photo polymers: Principles, Processes, and Materials," SPE, Mid Hudson Section, Ellenvill, New York, 1988, 183. 5. Halle, L. F . ; J. Vac. Sci. Technol., 1985, B3, 323. 6. Nakase, Μ., Proc. SPIE, 1985, 537, 60. 7. Sasago, M.; Endo, M.; Hirai, Y.; Ogawa, K.; Ishihara, T., Proc. SPIE, 1986, 631, 321. 8. Tomo, Y.; Jinbo, H.; Yamashita, Y.; Ohno, S.; Asano, T.; Nishibu, S.; Umehara, Η., Conf. "Photopolymers: Principles, Processes, and Materials," SPE, Mid Hudson Section, Ellenvill, New York, 1988, 195. 9. Ueno, T.; Uchino, S.; Iwayanagi, T.; Nonogaki, S.; Tanaka, T.; Shirai, S.; and Moriuchi, N., J. Imaging Sci., 1988, 32, 144. 10. Uchino, S.; Iwayanagi, T.; Hashimoto, Μ., Proc. SPIE, 1988, 920, 100. 11. Ito, H.; Willson, C. G., ACS Symp. Ser., 1984, 242, 11. 12. deGrandpre, M.; Graziano, K.; Thompson, S. D., Proc. SPIE, 1988, 923, 158. 13. Houlihan, F. M.; Shugard, A.; Gooden, R.; Reichmanis, Ε., Proc. SPIE, 1988, 920, 67. RECEIVED

June 14, 1989


Synthesis of New Metal-Free Diazonium Salts and Their Applications

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