18 Sensitized Photodegradation of Polymethylmethacrylate MINORU TSUDA and SETSUKO OIKAWA—Laboratory of Physical Chemistry, Pharmaceutical Sciences, Chiba University, Chiba 260, Japan YOHICHI NAKAMURA, HIDEO NAGATA, AKIRA YOKOTA, and HISASHI NAKANE—Tokyo Ohka Kogyo Co., Ltd., Samukawa, Kanagawa 253-01, Japan TOSHIRO TSUMORI and YASUAKI NAKANE—Sony Corporation, Semiconductor Development Division, Atsugi, Kanagawa 243, Japan
Poly(methylmethacrylate), PMMA, is a well-known degradable polymer in the radiation chemistry of macromolecule (1). Hatzkis reported that PMMA is an excellent resist material usable in the microfabrication technology for manufacturing the microelectronic devices where X-rays and electron beams are used as radiation sources (2). There is an empirical rule proposed by Millar, Wall and Charlesby which is useful for the prediction of the radiation effect of polymers, i . e . , crosslinkable or degradable (1); where vinyl polymers in which there are two side chains attached to a single carbon (namely,-CH -CR R -) degrade while those with a single side chain or no side chain (namely, -CH -CR H- or -CH -CH -) crosslinks. However, considerable exceptions are known to this rule (1). Recently, the authors demonstrated theoretically that there is a fundamental difference between degradable and crosslinkable polymers, which is due to the chemical or electronic structures of polymers themselves (3). The radiation effects of polymers are governed by the reaction in the excited states without exceptions. Millar-Wall-Charlesby's empirical rule is exact only when the shape of the adiabatic potential curve in the ground state reflects those in the excited states. Typical examples illustrating this for degradable and crosslinkable polymers are shown in Fig. 1 and Fig. 2, respectively (4). We find that the activation energies of the main chain cleavage reactions in the excited states are very small in the former and quite large in the latter. On the other hand, it is easily shown by the same procedure that the C-H bond is very weak in the excited states and a polymer radical which crosslinks (4) will be formed especially readily in the latter case. If the newly proposed theory is applicable in the case of PMMA, the polymer should be degraded by the irradiation of UV light because PMMA has a weak absorption band near the 230 nm wavelength region (Fig. 5). Lin demonstrated that PMMA is degradable under the irradiation of Deep UV (200V350 nm light) giving a high-quality resist image (5). Since the deep UV lithography 2
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0-8412-0540-X/80/47-121-281$05.00/0 © 1980 American Chemical Society
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Journal of Polymer Science, Polymer Chemistry Edition
Figure 1. Adiabatic potential curves in the main chain scission of a model com pound of poly(isobutylene): 2,2-, 4,4-tetramethylpentane (4). &E (=0.61eV), AE (=0.35eV), and AE (=2.05eV) are the activation energies of the main chain scission in the lowest singlet excited state (Sj), the lowest triplet state (T ), and the ground state, respectively. ai
Tj
o
t
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Figure 2. Adiabatic potential curves in the main chain scission of a model com pound of polyethylene: ethane (Φ) Alg; (O) Alu; « ) A2g; A2u; (A) Eg; (Δ ) Eu; ( j singlet; ( ) triplet (4)
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i s expected t o be a n e a r - f u t u r e technique f o r the p r o d u c t i o n o f t h e m i c r o e l e c t r o n i c d e v i c e s ( 6 ) , t h e development o f polymer mater i a l s w h i c h h a v e e x c e l l e n t p r o p e r t i e s a s a Deep UV r e s i s t i s a n i m p o r t a n t p r o b l e m . A l t h o u g h PMMA g i v e s a h i g h - q u a l i t y r e s i s t image, i t s d i s a d v a n t a g e from the p r a c t i c a l v i e w p o i n t i s i t s v e r y l o w s e n s i t i v i t y t o Deep UV l i g h t ( t h e s l o w r e a c t i o n r a t e o f t h e p h o t o d e g r a d a t i o n ) and i t s weakness a s a r e s i s t i v e m a t e r i a l i n t h e plasma e t c h i n g which i s used i n the m i c r o f a b r i c a t i o n process. G e n e r a l l y s p e a k i n g , i t i s n o t e a s y t o f i n d a new p o l y m e r i c m a t e r i a l w h i c h g i v e s a h i g h - q u a l i t y image. S o , t h e a c c e r e l a t i o n o f t h e r e a c t i o n r a t e o f t h e p h o t o d e g r a d a t i o n ( t h e phenomenon i s c a l l ed s e n s i t i z a t i o n ) o f PMMA i s p r e f e r a b l e f o r t h e p r e s e n t p u r p o s e . A s u c c e s s f u l example o f s e n s i t i z a t i o n i s found i n t h e c a s e o f p o l y ( v i n y l cinnamate) ( 7 ) . We r e c e n t l y f o u n d t h a t some a r o m a t i c compounds a c c e r e l a t e t h e d e g r a d a t i o n r e a c t i o n o f PMMA u n d e r t h e Deep UV l i g h t i r r a d i a t i o n . The s e n s i t i v i t y o f t h e s e n s i t i z e d PMMA i s 4 t i m e s l a r g e r t h a n t h a t o f u n s e n s i t i z e d PMMA, when t h e s e n s i t i z e d m a t e r i a l was u s e d a s a p o s i t i v e t y p e Deep UV r e s i s t f o r t h e f o r m a t i o n o f m i c r o i m a g e b y t h e p r a c t i c a l a l i g n e r w h i c h l o a d s a Xe-Hg s h o r t a r c lamp n e w l y d e v e l o p e d f o r t h e Deep UV l i t h o g r a p h y ( 8 ) . We c o n f i r m e d t h a t t h e m a i n c h a i n d e g r a d a t i o n o f PMMA i s a c c e r e l a t e d b y t h e s e n i s t i z a t i o n . The measurement o f t h e s p e c t r a l d e p e n d e n c e o f t h e a b s o l u t e quantum y i e l d s p e c t r a o f PMMA s e n s i t i z e d b y some o f t h e n o v e l s e n s i t i z e r s r e v e a l e d t h a t t h e quantum y i e l d i t s e l f i n c r e a s e s e v e n i n t h e wavel e n g t h r e g i o n w h e r e PMMA i t s e l f a b s o r b s l i g h t a n d h a s i t s i n t r i n sic sensitivity. T h i s p a p e r p r e s e n t s t h e s e new f i n d i n g s f r o m t h e experimental point o f view. EXPERIMENTAL Measurement o f S e n s i t i v i t y PMMA i n e t h y l e n e g l y c o l m o n o e t h y l e t h e r m o n o a c e t a t e (7 wt%) was c o a t e d i n 0.5 ym t h i c k n e s s b y t h e s p i n n i n g method o n a t h e r m a l l y o x i d i z e d s i l i c o n w a f e r . The c o a t e d p o l y m e r was d r i e d a t 80°C f o r 30 m i n ( p r e b a k i n g ) , a n d t h e n i r r a d i a t e d s t e p w i s e a t t h e r e g u l a r t i m e i n t e r v a l s b y a u l t r a h i g h - p r e s s u r e Hg s h o r t a r c lamp (100 W) a t t h e d i s t a n c e o f 11 cm. The i r r a d i a t e d p o l y m e r c o a t i n g was i m mersed i n the d e v e l o p e r ( m i x t u r e o f e t h y l a c e t a t e 1 volume and i s o a m y l a c e t a t e 9 v o l u m e s ) a t 25°C f o r 1 m i n ( d e v e l o p m e n t ) a n d l i n s e d . The r e l a t i v e s e n s i t i v i t y o f p o l y m e r was d e f i n e d a s t h e r e c i p r o c a l of the l e a s t i r r a d i a t i o n time r e q u i r e d f o r the d i s s o l u t i o n o f t h e p o l y m e r w h e r e more t h a n 90 % o f t h e o r i g i n a l t h i c k n e s s was m a i n t a i n e d a t t h e n o n - i r r a d i a t e d p a r t o f t h e c o a t i n g . (See F o r m u l a ( 3 . a)) The t i m e i n t e r v a l was a j u s t e d i n o r d e r t o o b t a i n t h e same s i g n i f i c a n t f i g u r e s . The s e n s i t i z e d p o l y m e r s o l u t i o n was p r e p a r e d by a d d i n g t h e s e n s i t i z e r (10 w t % o f t h e p o l y m e r ) t o t h e PMMA e t h y l e n e g l y c o l m o n o e t h y l e t h e r monoacetate s o l u t i o n . S i m i l a r measurements w e r e a l s o made u s i n g a Xe-Hg s h o r t a r c lamp (500 W) f o r Deep UV l i t h o g r a p h y .
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Mesurement o f S p e c t r a l S e n s i t i v i t y The p o l y m e r c o a t i n g p r e p a r e d a s d e s c r i b e d a b o v e was i r r a d i a t ed s t e p w i s e a t t h e r e g u l a r i n t e r v a l s b y t h e m o n o c h r o m a t i c l i g h t o b t a i n e d b y t h e monochrometer m o u n t i n g t h e 200 n m - b l a z e g r a t i n g and 5 kW X e s h o r t a r c lamp (9) ( F i g . 3 ) . L i g h t e m i t t e d f r o m t h e Xe lamp was i n t r o d u c e d t o t h e s p e c t r o s c o p e u s i n g a r e f l e c t i v e grating. The s p e c t r o g r a m o f t h e s e n s i t i v i t y o f t h e m o n o c h r o m a t i c l i g h t i s o b t a i n e d when t h e r e s i s t c o a t i n g was i r r a d i a t e d a t t h e s a m p l e h o l d e r . The r e l a t i v e s e n s i t i v i t i e s o v e r a w i d e r a n g e o f t h e w a v e l e n g t h a r e o b t a i n e d a t o n c e b y t h i s method a s shown i n F i g . 4. The i n t e n s i t y o f t h e m o n o c h r o m a t i c l i g h t was m e a s u r e d u s i n g a vacuum t h e r m o c o u p l e w i t h a q u a r t z window ( 1 0 ) . The s p e c t r a l s e n s i t i v i t y was c o r r e c t e d b y u s i n g t h e i n t e n s i t y d a t a shown i n F i g . 6. Measurement o f Quantum Y i e l d The number o f p h o t o n s , i n c i d e n t t o 1 c m s u r f a c e o f t h e p o l y mer c o a t i n g , o f 2537 Â l i g h t f r o m t h e l o w p r e s s u r e Hg l o n g a r c lamp was m e a s u r e d b y a c h e m i c a l a c t i n o m e t e r ( 1 1 ) , w h e r e a s o l u t i o n o f K F e ( C 0 z . ) 3 H 0 i n a c e l l w i t h a q u a r t z window was i r r a d i a t e d i n p l a c e o f a p o l y m e r c o a t i n g s a m p l e . The l e a s t number o f p h o t o n s r e q u i r e d f o r t h e d i s s o l u t i o n o f t h e PMMA c o a t i n g when immersed i n t h e d e v e l o p e r was 1.457«ΙΟ"" * e i n s t e i n / c m . I t was f o u n d f r o m t h e a b s o r p t i o n s n e c t r u m t h a t 1.1 % o f t h e i n c i d e n t p h o t o n s w e r e a b s o r b e d a t 2537 A b y a PMMA f i l m o f 0.5 ym t h i c k n e s s ( F i g . 5 ) . The 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 a n d t h e a v e r a g e m o l e c u l a r w e i g h t o f t h e c o a t e d p o l y m e r w h i c h was i r r a d i a t ed f o r t h e l e a s t i r r a d i a t i o n t i m e r e q u i r e d f o r t h e d i s s o l u t i o n o f p o l y m e r c o a t i n g i n t h e d e v e l o p e r w e r e measured b y g e l - p e r m e a t i o n chromatography ( F i g . 7 ) . From n o t i n g t h e c h a n g e s o f 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 s and t h e a v e r a g e m o l e c u l a r w e i g h t b e f o r e a n d a f t e r UV l i g h t i r r a d i a t i o n , we t e n t a t i v e l y p o s t u l a t e t h e r e a c t i o n mechanism a s f o l l o w s : 2
e
3
2
2
1
R
ÇH3
-c CH -t I c0 2
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hv.
-O I
2
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il"
2
c1=o
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tCH
3
CH
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\CH
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2
c=o I1
1
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3
-Ç-CH
-Ç-CH J
J
m
0I
>CH
3
/
U s i n g t h e m o l e c u l a r w e i g h t change a n d t h e number o f p h o t o n s r e q u i r e d f o r t h e c h a n g e , t h e a b s o l u t e quantum y i e l d a t 2537 A o f PMMA was o b t a i n e d w h e r e t h e v a l u e o f 0.9 was u s e d a s t h e d e n s i t y o f PMMA f i l m . Once t h e a b s o l u t e quantum y i e l d a t 2537 Â was o b t a i n e d , t h e a b s o l u t e quantum y i e l d s o v e r a l l t h e s p e c t r a l r a n g e can b e c a l c u l a t e d f r o m t h e r e l a t i v e d a t a o f t h e s p e c t r a l s e n s i t i -
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Figure 3. Optical system of instrument of spectral sensitivity measurement: (L) Xe arc lamp; (Ml) concave mirror; (M2, M3) mirror; (S) shutter; (SI) slit; (G) concave reflective grating
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125 250 500 1000 2000 4000 3000 16000 COUNT
PMMA + p - t e r t Butyl Benzoic A c i d 125 250 500 1000 2000 4000 8000 16000 COUNT
200
2 , 4 - d i - t e r t Butyl Phenol 62 125 250 500 1000 2000 4000 8000 COUNT
200
PMMA+ 2 , 4 , 6 - t r i - t e r t Butyl Phenol
WAVELENGTH (nm) Figure 4. Spectral sensitivity data of PMMA and sensitized PMMA obtained by the equipment shown in Figure 3: (A) PMMA; (B) PMMA + ^-text-butyl benzoic acid; (C) PMMA + 2,4-di-tert-butylphenol; (D) PMMA + 2,4,6-tri-tert-butylphenol
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v i t y a s w i l l b e shown l a t e r ( F i g . 8). F o r t h e s e n s i t i z e d p o l y m e r c o a t i n g , t h e same method i s a p p l i c a b l e . V a r i a b l e s a r e t h e photon a b s o r p t i o n d a t a i n t h e % s c a l e w h i c h depends upon t h e a b s o r p t i o n spectrum o f t h e s e n s i t i z e r , and t h e l e a s t i r r a d i a t i o n time r e q u i r ed f o r d i s s o l u t i o n o f p o l y m e r c o a t i n g i n t h e d e v e l o p e r . The a b s o r p t i o n s p e c t r a o f s e n s i t i z e r s w e r e m e a s u r e d i n PMMA f i l m o f 0.5 ym t h i c k n e s s a n d was a l w a y s 0.5 ym a s m e a s u r e d b y TALYSTEP ( T a y l e r -Hobson, E n g l a n d ) . The d e n s i t y o f t h e f i l m was assumed t o b e 0.9 in a l l cases. RESULTS AND DISCUSSIONS D e f i n i t i o n s o f S e n s i t i v i t y a n d Quantum Y i e l d S e n s i t i v i t y , S, o f t h e t h i n r e s i s t s f i l m i s d e f i n e d a s t h e number, N, o f a s p e c i f i c r e a c t i o n o c c u r e d i n one p o l y m e r m o l e c u l e , w h i c h forms t h e f i l m , by one photon i r r a d i a t e d on a u n i t s u r f a c e (cm ). I n t h i s c a s e quantum y i e l d , φ, i s d e f i n e d a s t h e number, N, o f a s p e c i f i c r e a c t i o n o c c u r e d i n one p o l y m e r m o l e c u l e b y one p h o t o n a b s o r b e d p e r one p o l y m e r m o l e c u l e ; n a m e l y , Φ i s t h e number Ν o f a s p e c i f i c r e a c t i o n c a u s e d b y one p h o t o n a b s o r b e d : 2
S = dN ( r e a c t i o n / m o l e c u l e ) / d E
2
i r r a ( L
(photon/cm )
(l.a)
φ = dN ( r e a c t i o n / m o l e c u l e ) / d E (photon/molecule) = dN / d E (reaction/photon) (l.b) a b s o r b #
a b s o r b #
d E
d E
irrad.
=
d[l t]
( . )
0
2
a b s o r b . = (1/n) d [ l ( l - e x p ( - k c ) ) t ]
a
(2.b)
0
2
w h e r e I i s t h e number o f p h o t o n s i n c i d e n t o n t h e 1 c m s u r f a c e of t h e r e s i s t f i l m d u r i n g a u n i t t i m e a t a s p e c i f i c w a v e l e n g t h , k t h e a b s o r p t i o n c o e f f i c i e n t p e r one c h r o m o p h o r e a t t h e same wave l e n g t h a n d c t h e number o f c h r o m o p h o r e i n t h e 1 c m o f t h e r e s i s t f i l m of a definite thickness. The v a l u e ( l - e x p ( - k c ) ) i s t h e a b s o r p t i o n o f t h e r e s i s t f i l m i n t h e t r a n s m i t t a n c e s c a l e (%), η i s the number o f p o l y m e r m o l e c u l e i n t h e 1 c m r e s i s t f i l m o f a d e f i n i t e t h i c k n e s s and t i s t h e i r r a d i a t i o n time. A d e f i n i t e number o f m a i n c h a i n c l e a v a g e s s h o u l d o c c u r i n t h e s o l u b l e p a r t o f t h e f i l m when a d e v e l o p e r i s s p e c i f i e d a n d t h e l e a s t amount o f p h o t o n s r e q u i r e d f o r t h e d i s s o l u t i o n o f t h e f i l m i n t h e developer used. I f t h e d e f i n i t e number i s K, t h e f o l l o w i n g formulae are obtained; 0
2
2
J g =
K
d Ν δ
Ιο
= A t
/ dt Ο
Κ_ I At 0
(3.a)
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f d N (3.b) Io(l-exp(-kc))/n J
A t
dt "
A
t
* (l-exp(-kc) )/n
where At i s the l e a s t i r r a d i a t i o n time r e q u i r e d f o r the d i s s o l u t i o n of the i r r a d i a t e d part o f the r e s i s t f i l m i n the developer. From (3.a) and (3.b) we o b t a i n the r e l a t i o n s h i p between the sen s i t i v i t y S and the quantum y i e l d φ; namely, s
=
φ (l-exp(-kc)) η
(
Using formula (4), we o b t a i n the absolute s p e c t r a l of various r e s i s t f i l m s ( F i g . 9 ) .
4
)
sensitivities
R e l a t i v e S e n s i t i v i t y o f S e n s i t i z e d PMMA The r e l a t i v e s e n s i t i v i t y of the t h i n r e s i s t f i l m i s defined as the r e c i p r o c a l of the l e a s t i r r a d i a t i o n time required f o r the d i s s o l u t i o n o f the polyemr c o a t i n g when the l i g h t source, the i r r a d i a t i o n c o n d i t i o n s , the c o n d i t i o n s o f the development and the developer are s p e c i f i e d ; because i n t h i s case I and Κ o f the formula (3.a) become constant. Some examples of the s e n s i t i v i t i e s of the s e n s i t i z e d PMMA coatings are shown i n Table 1, where 2 , 4 , 6 - t r i - t e r t - b u t y l phenol i s most e f f e c t i v e . 0
Table 1. R e l a t i v e S e n s i t i v i t y Using Ultra-High Pressure Hg Lamp as a L i g h t Source SENSITIZER (PMMA) p - t e r t - b u t y l benzoic A c i d p - t e r t - B u t y l Benzene p - t e r t - O c t y l Phenol t e r t - B u t y l Hydroquinone 2 , 5 - D i - t e r t - B u t y l Hydroquinone 2 , 6 - D i - t e r t - B u t y l p-Cresol 2 , 4 - D i - t e r t Butyl Phenol 2 , 4 , 6 - T r i - t e r t - B u t y l Phenol 4,4'-Thio-bis(6-tert-Butyl 3-methyl Phenol) 2,2'-Methylene-bi s(4-methyl6 - t e r t - B u t y l Phenol) 2,2'-Methylene-bi s ( 4 - e t h y l 6 - t e r t - B u t y l Phenol)
R.S. 100 200 180 200 120 220 131 250 330 150 131 133
R.S.: R e l a t i v e S e n s i t i v i t y (PMMA=100)
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Main Chain Degradation o f PMMA In order t o prove that the s e n s i t i z e r i s e f f e c t i v e i n the main chain degradation o f PMMA, the change i n the molecular weight d i s t r i b u t i o n o f PMMA f i l m before and a f t e r i r r a d i a t i o n was mea sured by GPC u s i n g t e t r a h y d r o f u r a n as a e l u t i o n developer and c a l i b l a t e d w i t h a standard sample o f p o l y s t y l e n e . The c a l i b l a t i o n method i s known t o give a r e l i a b l e r e s u l t when the e l u t i o n deve l o p e r i s t e t r a h y d r o f u r a n (12). The l e a s t dosage o f r a d i a t i o n r e quired f o r the d i s s o l u t i o n o f the s e n s i t i z e d f i l m i n the developer was used i n t h i s experiment. The s e n s i t i z e r used was 2 , 4 , 6 - t r i t e r t - b u t y l phenol. The r e s u l t s appear i n F i g . 7. I t i s c l e a r from the r e s u l t s o f Table 1 and F i g . 7 that the main chain c l e a vage r e a c t i o n i s a c c e r e l a t e d by a d d i t i o n o f s e n s i t i z e r s . The f e a t u r e o f the p r o f i l e o f the curves i n F i g . 7 i s that the i n i t i a l wider molecular weight d i s t r i b u t i o n range converges to that o f a smaller range by the i r r a d i a t i o n . These moleuclar weight d i s t r i b u t i o n changes are given i n two ways i n F i g . 7; i . e . , the l i n e a r s c a l e and the exponential s c a l e i n the molecular weight ( a b s c i s s a ) . The l i n e a r s c a l e has such a simple p h y s i c a l meaning that the area under the curves are the t o t a l weight o f the sample. The exponential s c a l e i s not as e a s i l y i n t e r p r e t e d although t h i s s c a l e i s o f t e n used. I t i s i n t e r e s t i n g that the exponential d i s t r i b u t i o n curves r e f l e c t the commonly used d i s t r i b u t i o n measure, Mw/Mn. The values of Mw/Mn are approximately equal before and a f t e r i r r a d i a t i o n (Table 2 ) , r e f l e c t i n g that these two curves are s i m i l a r . The values o f Table 2 were c a l c u l a t e d from the data o f F i g . 7. S p e c t r a l Dependence o f the Absolute Quantum Y i e l d o f the Main Chain Cleavage Reaction The raw data o f the s p e c t r a l s e n s i t i v i t i e s of the s e n s i t i z e d and u n s e n s i t i z e d PMMA, which were obtained u s i n g the equipment shown i n F i g . 3, appear i n F i g . 4. These data were c o r r e c t e d a t f i r s t by the i n t e n s i t y data shown i n F i g . 6; and then the s p e c t r a l dependence o f the quantum y i e l d was obtained by the f o l l o w i n g pro cedure: i n equation o f (3.a), I A t i s the number o f the photon quanta(per 1 cm o f the f i l m ) i r r a d i a t e d a t the r i d g e l i n e o f the mountain i n F i g . 4, because the l e a s t amount o f the photon quanta r e q u i r e d f o r the d i s s o l u t i o n o f PMMA f i l m was i r r a d i a t e d a t the ridge l i n e ; ( l - e x p ( - k c ) ) i s the absorption o f the f i l m i n a d e f i n i t e thickness (0.5 ym i n t h i s experiment) i n the transmittance s c a l e (%); η i s the number o f polymer molecule c o n t a i n i n g i n the 1 cm f i l m at the d e f i n i t e t h i c k n e s s . These values are a l l mea surable. The value Κ i s the number o f the main chain f i s s i o n o c c u r r i n g i n one polymer molecule by the i r r a d i a t i o n o f the photon quanta of I A t . The v a l u e o f Κ i s e a s i l y c a l c u l a t e d t o be about 4 from the data i n Table 2. The value o f I A t was p r e c i s e l y de termined a t 2537 A using a chemical actinometer. Using t h i s data the s p e c t r a l dependence o f the absolute quan tum y i e l d o f the main chain f i s s i o n o f PMMA was c a l c u l a t e d f o l l o w 0
2
2
0
0
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MOLECULAR WEIGHT (MW)
MOLECULAR WEIGHT (MW) /TO
4
Figure 7. Molecular weight distribution of PMMA. F(mol wt) is normalized in such a way that f Y (mol wt)à(mol wt) = 1: (Q) before exposure; (A) after exposure
Table 2. Molecular Weight Change by U V - I r r a d i a t i o n * Irradiation before after
Mw 685,000 160,000
Mn
Mw/Mn
341,000 91,400
2.01 1.74
* The l e a s t i r r a d i a t i o n r e q u i r e d f o r the d i s s o l u t i o n of polymer c o a t i n g i n the developer.
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i n g t h e f o r m u l a ( 3 . b ) , a n d t h e r e s u l t s a r e shown i n F i g . 8. The quantum y i e l d o f t h e s e n s i t i z e d PMMA i n c r e a s e s o v e r w i d e r a n g e o f w a v e l e n g t h when we compared i t w i t h t h a t o f PMMA i t s e l f . Absolute Spectral S e n s i t i v i t y The a b s o l u t e s e n s i t i v i t y was d e f i n e d a s t h e number o f m a i n c h a i n s c i s s i o n s o c c u r r i n g i n o n e p h o t o n m o l e c u l e when one p h o t o n was i r r a d i a t e d o n a u n i t s u r f a c e ( 1 c m ) o f t h e f i l m . I f the s p e c t r a l dependence o f t h e quantum y i e l d h a s b e e n o b t a i n e d , t h e a b s o l u t e s e n s i t i v i t y i s c a l c u l a t e d f o l l o w i n g t h e f o r m u l a ( 4 ) . The r e s u l t s o b t a i n e d a r e shown i n F i g . 9. The p r a c t i c a l s e n s i t i v i t y w i l l be the i n t e g r a l o f the product o f the absolute i n t e n s i t y o f the i r r a d i a t e d l i g h t and the a b s o l u t e s e n s i t i v i t y i n F i g . 9 over a l l the wavelength o f the spectra. 2
APPLICATION TO DEEP UV MICROLITHOGRAPHY Experiments i n t h i s s e c t i o n were c a r r i e d o u t u s i n g p r a c t i c a l e q u i p m e n t , b e c a u s e t h e e v a l u a t i o n o f t h e PMMA r e s i s t s h o u l d h a v e p r a c t i c a l meaning. F o r the p r o d u c t i o n t e s t o f t h e m i c r o - p a t t e r n s a Canon PLA-520F a l i g n e r f o r Deep UV l i t h o g r a p h y l o a d i n g a Xe-Hg s h o r t a r c lamp was u s e d ( 8 ) , a n d f o r p l a s m a e t c h i n g t e s t a Tokyo Ohka OAPM-300 p l a s m a e t c h i n g m a c h i n e was u s e d ; b o t h o p e r a t e d u n d e r practical conditions. R e l a t i v e S e n s i t i v i t y o f t h e S e n s i t i z e d PMMA R e l a t i v e s e n s i t i v i t i e s o f PMMA s e n s i t i z e d b y v a r i o u s s e n s i t i z e r s w e r e m e a s u r e d u s i n g p r a c t i c a l c o n d i t i o n s . The r e s u l t s a r e shown i n T a b l e 3. Formation o f Micro Patterns The e l e c t r o n m i c r o s c o p e images o f t h e f i n e p a t t e r n s o b t a i n e d by PMMA i t s e l f a n d PMMA s e n s i t i z e d b y 2 , 4 , 6 - t r i - t e r t - b u t y l p h e n o l on t h e s i l i c o n w a f e r a p p e a r i n F i g s . 10 a n d 1 1 , r e s p e c t i v e l y . The s l i t w i d t h o f t h e f i n e p a t t e r n s i s 0.5, 1.0 a n d 1.5 ym i n ( a ) , and t h e m a g n i f y i n g image o f t h e 0.5 ym a r e a i s shown i n ( b ) . The t h i c k n e s s o f t h e c o a t i n g i s 0.5 ym. R e s i s t i v i t y t o the Plasma E t c h i n g The c o m p a r i s o n o f t h e r e s i s t i v e p r o p e r t y i n t h e p l a s m a e t c h i n g p r o c e s s b e t w e e n PMMA i t s e l f a n d PMMA s e n s i t i z e d b y 2 , 4 , 6 - t r i t e r t - b u t y l p h e n o l i s shown i n F i g . 1 2 . F o l l o w i n g t h e e t c h i n g time, t h e t h i c k n e s s o f t h e PMMA c o a t i n g becomes t h i n n e r . The r a t e o f the decreasing o f the f i l m thickness i s p r o p o r t i o n a l t o the etchi n g t i m e i n t h e f o r m e r c a s e , b u t i t becomes v e r y s l o w i n t h e c a s e o f t h e s e n s i t i z e d PMMA. T h e r e f o r e , t h e s e n s i t i z e d PMMA i s a s u p e r i o r r e s i s t t h a n PMMA i t s e l f i n b o t h p r o p e r t i e s o f t h e s e n s i t i v i t y and t h e r e s i s t i v i t y . This f a c t i s t r u e i nthe cases o f other sensitizers. The r e a s o n f o r t h e i n c r e a s e o f t h e r e s i s t i v i t y o f t h e s e n s i -
294
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WAVELENGTH (nm) Figure 8. Spectral dependence of quantum yield: (Φ) PMMA; PMMA p-tert-butyl benzoic acid; (A) PMMA + 2,4-di-tert-butylphenol; (O) PMMA 2,4 6-tri-tert-butylphenol f
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Figure 9. Spectral sensitivities of PMMA and sensitized PMMA: (Φ) PMMA; (\T\) PMMA + p-tert-butyl benzoic acid; (A) PMMA + 2,4-di-tert-butylphenol; (O)PMMA + 2,4,6-tri-tert-butylphenol
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296
Figure 10.
Electron microscope images of the fine patterns obtained by (window opening pattern) (a) X 1000; (b) χ 2500
PMMA
Figure 11. Electron microscope images of the fine patterns obtained by 2,4,6-tritert-butylphenol-sensitized PMMA (window opening pattern) (a) X 1000; (b) X2500
Table 3.
R e l a t i v e S e n s i t i v i t y o f S e n s i t i z e d PMMA Using P r a c t i c a l Conditions SENSITIZER (PMMA) p - t e r t - B u t y l Benzoic A c i d 2 , 4 - d i - t e r t - B u t y l Phenol 2 , 4 , 6 - t r i - t e r t - B u t y l Phenol
100 120 200 400
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Etching Time/min.
Figure 12. Resistive property of sensitized PMMA: decrease of resist thickness by plasma (Apparatus: OAPA-300; RF: 200 W; vac: 0.55 torr; gas: CF 95% and Ο 5%; flow rate: 1.2 L/min) h
2
1.0
LU
WAVELENGTH (nm) Figure 13.
Sensitizer remaining in thin resist film after development (PMMA p-tert-butyl benzoic acid)
+
298
MODIFICATION OF POLYMERS
tized PMMA is due to the fact that the added sensitizer remains even after the development in the thin film (0.5 ym). The absorp tion spectra of the resist coating before and after development shown in Fig. 13 indicates that more than 80 % of the unchanged sensitizer remains. This data means that the resist coating scarecely swells in the development process and that fine micropatterns will be available by the sensitized PMMA resist newly developed in this research. LITERATURE CITED 1. Charlesby, Α., Atomic Radiation and Polymers, Pergamon, Oxford (1960). 2. Hatzkis, M.,J. Electrochem. Soc., (1969) 116, 1033. 3. Tsuda, M., Oikawa, S. and Suzuki, Α., Polymer Eng. Sci., (1977) 17, 390. 4. Tsuda, M. and Oikawa, S., J. Polymer Sci., Polymer Chem. Ed., (1979), in press. 5. Lin, B. J., IBM J. Res. Develop., May 213 (1976); J. Vac. Sci. Tech., (1976) No. 6, 1317. 6. Tsuda, Μ., Oikawa, S., Nakamura, Y., Nagata, Η., Yokota, A. and Nakane, Η., Photo. Sci. Eng., (1979) in press. 7. Tsuda, Μ., Bull. Chem. Soc. Japan, (1969) 42, 905, and litera tures cited therein. 8. Nakane,Y., Tsumori, T., Mifune, T . , Kira, K., Momose, K. and Kanoh, I., The 25th Spring Meeting of Japan Society of Applied Physics, 30aM2 (1978). 9. Tanaka, Η., Tsuda, M. and Nakanishi Η., J. Polymer Sci., (A-1), (1972) 10, 1729 10. Tsuda, Μ., Oikawa, S. and Miyake,R., Nippon Shashin Gakkai-shi (J. Soc. Photo. Sci. Tech. Japan) (1972) 35, 90. 11. Hatchard, C. G. and Parker, C. Α., Proc. Roy. Soc. (London), (1956) A234, 518. 12. Grubisic, Z., Rempp, P. and Bonoit, J., J. Polyemr Sci., (1967) B5, 753; Hattori, S., Hamashima, Μ., Nakahara, H. and Kamata, T., Kobunshi Ronbunshu (1977) 34, 503. RECEIVED
July
12, 1979.
