Chapter 14 A New High-Sensitivity, Water-Developable Negative Photoresist 2
2
Anders Hult1,Otto Skolling1,Sven Göthe, and Ulla Mellström 1Departmentof Polymer Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden AB Wilh. Becker, Box 2041, S-195 92 Märsta, Sweden 2
Polymers based on methylacrylamidoglycolate methylether (MAGME) have been synthesized and used as negative tone photoresists. MAGME containing polymers can undergo acid-catalyzed crosslinking by a selfcondensation reaction. P-Toluene sulfonic acid, a UV-deblockable sulfonic acid and a triphenylsulfonium salt have been used as catalysts. Acid-catalyzed crosslinking is another example of chemical amplification in photoresist sysems. These MAGME-polymers exhibit high sensivity but a limited line-width resolution. They are soluble in harmless solvents like water and alcohols.
Much recent research has been focused on the development of new generation resist materials which possess improved sensitivity and enhanced resolution (1). An interesting approach to improve sensitivity involves the phenomenon of chemical amplification (2). This strategy has been demonstrated successfully for resist materials that undergo either acid catalyst hydrolysis (3) or polymerization (4). The key to these processes is photogeneration of strong acids. This can be achieved either by the use of onium salts (5) or latent UV-deblockable sulfonic acids (6). Both catalysts also generate a substantial amount of free radicals which may or may not interfere in the reaction. This paper will discuss another approach to chemical amplification, namely acid catalyzed self-condensation of acrylic polymers. These materials, based on methyl acrylamidoglycolate methylether, generate negative-tone images and consist of a water (or alcohol) soluble polymer which can undergo acid catalyzed crosslinking. 0097-6156/87/0346-0162$06.00/0 © 1987 American Chemical Society
14.
HULT ET A L .
High-Sensitivity,
Water-Developable
Negative
Photoresist
Experimental M e t h y l a c r y l a m i d o g l y c o l a t e m e t h y l e t h e r (MAGME) ( A m e r i c a n C y a n a m i d ) w a s f i l t e r e d w h i l e warm a n d r e c r y s t a l l i z e d f r o m x y l e n e (mp 7 0 - 7 3 ° C ) . A l l o t h e r monomers w e r e f r e e d f r o m i n h i b i t o r o n a n a l u m i n i u m o x i d e c o l u m n . p - T r i m e t h y l s i l y l s t y r e n e was s y n t h e s i z e d from p - c h l o r o s t y r e n e u s i n g a G r i g n a r d r e a c t i o n and c h l o r o t r i m e t h y l s i l a n e . A z o b i s i s o b u t y r o n i t r i l e ( A I B N ) was u s e d as f r e e - r a d i c a l i n i t i a t o r i n a l l p o l y m e r i z a t i o n s and c a r b o n t e t r a b r o m i d e as c h a i n - t r a n s f e r a g e n t . P o l y m e r i z a t i o n s were c a r r i e d o u t a t 60°C i n a 5 0 : 5 0 m i x t u r e o f t o l u e n e a n d b u t a n o l u n d e r a n i t r o g e n a t m o s p h e r e . T h e r e a c t i o n was c a r r i e d o u t f o r 3,5 h and the formed polymer p r e c i p i t a t e d i n c o l d d i e t h y l e t h e r . I t was t h e n r e d i s s o l v e d a n d r e p r e c i p t a t e d p r i o r t o f u r t h e r u s e . once more IR s p e c t r a w e r e r e c o r d e d o n a P e r k i n E l m e r 1710 F T I R a n d NMR o n a 2 0 0 MHz B r u k e r WP 2 0 0 . F T I R was u s e d t o determine the co-polymer composition. p-Toluene s u l f o n i c a c i d (I), a UV-deblockable sulfonic acid (II) and t r i p h e n y l s u l f o n i u m h e x a f l u o r o a n t i m o n a t e ( I I I ) were used as a c i d c a t a l y s t s i n t h e c r o s s l i n k i n g r e a c t i o n s ( F i g u r e 1 ) . F i l m s 20μπι t h i c k w e r e c a s t e d o n g l a s s p l a t e s f r o m m e t h y l ethylketone solutions; t h i n f i l m s (Slym) were s p i n - c o a t e d on s i l i c o n wafers from a cyclohexanone s o l u t i o n of the polymer. I r r a d i a t i o n s w e r e p e r f o r m e d w i t h an O r i e l 82410 1000 W i l l u m inator. Results
and D i s c u s s i o n
MAGME i s a m u l t i f u n c t i o n a l a c r y l i c monomer ( F i g u r e 2 ) . I t i s e a s i l y p o l y m e r i z e d b y a f r e e - r a d i c a l m e c h a n i s m a n d c a n be co^p o l y m e r i z e d w i t h s e v e r a l v i n y l monomers ( 7 ) . We h a v e p r e p a r e d a v a r i e t y o f M A G M E - c o n t a i n i n g p o l y m e r s ( T a b l e 1) a n d s t u d i e d t h e i r a b i l i t y t o undergo 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 . M o l e c u l a r w e i g h t a n d 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 was c o n t r o l l e d b y a d d i t i o n o f c a r b o n t e t r a b r o m i d e w h i c h a c t s as a c h a i n t r a n s f e r agent. MAGME p o l y m e r s c a n e i t h e r be c r o s s l i n k e d i n a s e l f c o n d e n s a t i o n r e a c t i o n o r w i t h a p o l y o l ( S c h e m e 1 ) . T h e p o l y o l c a n e i t h e r be blended o r c o - p o l y m e r i z e d w i t h MAGME. A n example of such monomer i s 2 - h y d r o x y e t h y l m e t h a c r y l a t e . I n t h i s p a p e r we w i l l o n l y d i s c u s s s e l f c o n d e n s a t i o n of MAGME-polymers.
One o f t h e m o s t c o m m o n l y u s e d a c i d c a t a l y s t s i n o r g a n i c r e a c t i o n s i s p - t o l u e n s u l f o n i c a c i d ( P T S A ) . T h i s a c i d was u s e d t o e v a l u a t e t h e p o s s i b i l i t y o f s e l f c o n d e n s a t i o n o f MA GME>- p o l y m e r s . T h i c k ( 2 0 um) f i l m s w e r e c o a t e d o n g l a s s p l a t e s a n d c u r e d i n a n oven a t d i f f e r e n t t e m p e r a t u r e s and c u r i n g t i m e s . D a t a i n T a b l e 2 show t h e t i m e r e q u i r e d a t l o w e s t p o s s i b l e c u r i n g t e m p e r a t u r e . I n t h e c a s e o f PTSA i t was d i f f i c u l t t o a c h i e v e c u r i n g a t t e m p e r a t u r e s b e l o w 120°C. T h i s , and the f a c t t h a t i t took n e a r l y
POLYMERS FOR HIGH TECHNOLOGY
C H
F i g u r e 1. sulfonium
CO
Ο Η II ι (grC-Ç-0-S^>-CH3
3^®-S0 H 3
®-S SbF +
I PTSA, I I l a t e n t s u l f o n i c a c i d , hexafluoroantimonate.
(9) (a)(b) ?CH H C=ÇH (f)Ç=0 (c)Ç-NH-C(d)
triphenyl-
3
ppm a) 127.69 b) 129.24 c) 164.66 d) 7791 e) 52.63 f) 1 6 7 4 6 a) 56.49
2
Ο
III
6
ÔCHo (e) 3
CDCU
f c
200
150
Figure
100
2.
13
50
C NMR o f MAGME monomer.
0 ppm
14.
HULT ET A L .
High-Sensitivity,
Water-Developable
165
Negative Photoresist
TABLE 1
% CBn, Conver sion % (w/w)
Polymer MAGME MAGME-MMA (26:74) MAGME-MMA (48:52) MAGME-BA (39:61) MAGME-ERA (46:54) MAGME-STY MAGME-TMSiSTY (50:50)
Mn
Mw/Mn
Tg C
4
1.93
78
33
7.4 χ 10*
2.00
80
0.8
73
2.1 χ 10*
2.06
0.6
24
4.9 χ 10*
1.89
0.6
34
3.3 χ 10*
1.86
0.6
18
4.4 χ 10*
1.66
97
35
2.0 χ 10*
2.90
-100
0.6
7
0.6
0.6
2.5
χ 10
A l l c o p o l y m e r i z a t i o n s were c a r r i e d o u t w i t h 50:50 m i x t u r e s o f the two c o - m o n o m e r s . M M A - m e t h y l m e t h a c r y l a t e , B A - n - b u t y l a c r y l a t e , EHA - e t h y l h e x y l a c r y l a t e , STY - s t y r e n e , T M S i S T Y - p - t r i m e t h y l s i l y l styrene. TABLE 2 Crosslinking v i a self-condensation polymers
Polymer
MAGME MAGMEMMA (48:52) MAGMEMMA (2:98) MAGMEMMA (2:98) MAGMEBA (39:61) MAGMETMSiSTY (50:50)
Film thick n e s s (μ)
Cata lyst
Cone, (w/w)
1
III
10
20
I
20
III
5
20
II
5
20
I
1
III
o f poly-MAGME and i t s
Irradi ation time (sec) 3
Curing time (min)
100
2
120
20
4
100
5
1
100
5
120
20
100
2
0.3
0.3
10
Curing temp ( c)
co
3
POLYMERS FOR HIGH T E C H N O L O G Y
166
0
«
0=C0CH,
I
,
3
-C-NH-CH-OCH,
n
il
0=C0CH. 0 .
I
3
II
W-NH-CH-N-C-{ + I CH,0-CH 3 ι
CH 0H 3
COOCH,
0
»I
0=Ç0CH,
I
0
3
f-C-NH-CH-OCH,
Scheme
1.
R-OH
, I
0=C0CH,
I
3
h-C-NH-CH-OR
R i s another Polymer
CH 0H 3
chain.
20 m i n u t e s t o c u r e t h e f i l m s i n d i c a t e d t h a t t h e s y s t e m was n o t very acid s e n s i t i v e . H o w e v e r , when t h e c a t a l y s t was c h a n g e d t o the onium s a l t and U V - i r r a d i a t e d , both t h e c u r i n g temperature a n d t i m e d e c r e a s e d . T h i s was a l s o t r u e a t v e r y l o w d o s e s (~ 1 0 m J / c m ) o f UV r a d i a t i o n . 2
T h i s i n c r e a s e d s e n s i t i v i t y i s b e l i e v e d due t o t h e f a c t t h a t t h e o n i u m s a l t p r o d u c e s a much s t r o n g e r a c i d , i n t h i s case HSbFg. A n o t h e r c o n t r i b u t i n g f a c t o r c o u l d be p a r t i c i p a t i o n o f f r e e r a d i c a l s , formed d u r i n g i r r a d i a t i o n o f t h e onium s a l t . To t e s t t h i s h y p o t h e s i s , experiments were performed w i t h a l a t e n t U V d e b l o c k a b l e s u l f o n i c a c i d . T h i s c o m p o u n d p r o d u c e s b o t h PTSA a n d f r e e r a d i c a l s when i t i s i r r a d i a t e d . A l t h o u g h t h e a c i d p r o d u c e d was P T S A , t h e c u r i n g r e s u l t w a s c o n s i s t e n t w i t h t h e r e s u l t f r o m the onium s a l t experiment. These experiments i n d i c a t e s i t i s the f r e e r a d i c a l s which a r e e f f e c t i v e i n c r o s s l i n k i n g t h e m a t r i x . H o w e v e r , i t may a l s o j u s t b e a s o l u b i l i t y e f f e c t , e . g . c a t a l y s t s I I a n d I I I may b e s i m p l y m o r e s o l u b l e i n t h e MAGMEp o l y m e r s t h a n PTSA. F u r t h e r e x p e r i m e n t a t i o n i s needed t o d e t e r mine whether i t i s a s o l u b i l i t y e f f e c t o r p a r t i c i p a t i o n o f f r e e r a d i c a l s t h a t e x p l a i n s t h e l o w s e n s i t i v i t y o f PTSA. I n t h e e x p e r i m e n t s w i t h p u r e PTSA, no i n c r e a s e i n s e n s i t i v i t y was o b s e r v e d when t h e PTSA c o n c e n t r a t i o n w a s i n c r e a s e d a b o v e 0 . 3 Î w/w. I n o r d e r t o e v a l u a t e t h e p o s s i b l e u s e o f MAGME-polymers i n r e s i s t a p p l i c a t i o n s , c r o s s l i n k i n g s t u d i e s were conducted o n t h i n f i l m s (1 um ) , s p i n n - c o a t e d o n s i l i c o n w a f e r s . P o l y - M A G M E i s w a t e r s o l u b l e a n d most o f i t s c o - p o l y m e r s a r e s o l u b l e i n a l c o h o l s ( T a b l e 3) m a k i n g t h e m a t e r i a l s r e l a t i v e l y attractive t o work w i t h from a p r o d u c t i o n p o i n t o f v i e w .
14.
HULT ET A L .
High-Sensitivity,
Water-Developable
Negative
167
Photoresist
TABLE 3 Solubility
Solvent
of
poly-MAGME and i t s
MAGME
MAGME/MMA
Water
X
_
Methanol
X
Acetone Ethylacetate Chloroform
MAGME/ERA
MAGME/STY
_
_
_
X
X
(x)
X
X
X
X
X
X
X
X
X
X
(x)
X
X
X
X
X
X
X
X
Toluene
MAGME/BA
N-heptane χ = soluble, isopropanol
co-polymers
X
(x)
= poor
solubility,
-
= unsoluble,
= soluble
in
One a d v a n t a g e w i t h t h i n f i l m s i s t h a t t h e c o n c e n t r a t i o n o f t h e p h o t o s e n s i t i v e component c a n be i n c r e a s e d . I n c o n t r a s t t o t h e s t u d y w i t h p u r e PTSA, s e n s i t i v i t y i n c r e a s e d w i t h i n c r e a s i n g c a t a l y s t c o n c e n t r a t i o n . A s a r e s u l t , c u r i n g t i m e c o u l d be d e c r e a s e d t o 2 m i n u t e s a t 100°C. T h e l i t h o g r a p h i c b e h a v i o u r o f the MAGME-polymers i s s i m i l a r t o t h a t of o t h e r photoresist s y s t e m s b a s e d o n a c r o s s l i n k i n g m e c h a n i s m . Due t o t h e f a c t t h a t the d e v e l o p e r f o r the unexposed areas has a s t r o n g i n t e r a c t i o n w i t h t h e c r o s s l i n k e d p o l y m e r , s w e l l i n g becomes a p r o b l e m . F i g u r e 3 shows 2 μ l i n e s and 4 μ s p a c e s , w h i c h i s p r o b a b l y close to the u l t i m a t e r e s o l u t i o n of t h i s system.
Co-polymers w i t h p - t r i m e t h y l s i l y l s t y r e n e were a l s o s y n t h e s i z e d . T h i s polymer showed a good r e s i s t a n c e t o w a r d s 0 - R I E . However, r e s o l u t i o n i n t h i s r e s i s t i s a l s o c o n t r o l l e d by s w e l l i n g p r o blems i n t h e i n i t i a l d e v e l o p i n g s t e p . The MAGME-polymers c a n a l s o be t r a n s f o r m e d t o p o s i t i v e t o n e i m a g e s . T h i s i s d o n e b y e x p o s i n g the wafer t o a base a f t e r i t has been i r r a d i a t e d but b e f o r e i t has been t h e r m a l l y a c t i v a t e d ( F i g u r e 4 ) . The base ( i n o u r e x p e r i m e n t s we u s e d ammonium h y d r o x i d e ) c o n s u m e s t h e a c i d a n d f o r m s a l a t e n t c a t a l y s t i m a g e i n t h e f i l m , t h a t c a n be a c t i v a t e d by a s u b s e q u e n t f l o o d exposure. 2
Although, the r e s o l u t i o n of MAGME-polymers i s l i m i t e d t o about 2 μπι, t h e y h a v e s e v e r a l p r o p e r t i e s t h a t make t h e m a t t r a c t i v e
POLYMERS FOR HIGH T E C H N O L O G Y
Figure 3. N e g a t i v e t o n e i m a g e made f r o m p o l y - M A G M E . C o n t a c t p t i n t e d a t 254 nm, u s i n g c a t a l y s t I I I . T h e SEM s h o w s 2 μ l i n e s and 4 μ s p a c e s .
Figure
4.
Positive
tone process
for
MAGME-polymers.
14. HULT ET AL.
High-Sensitivity, Water-Developable Negative Photores
for applications like circuit board fabrication, where high resolution is not required. The MAGME-polymers have high sensitivity (~ 10raJ/cra )and they are soluble in harmless solvents which makes them attractive in a production environment. 2
Conclusion Acid-catalyzed crosslinking is another example of chemical amplification in photoresist systems. It can be achieved via selfcondens-ation of MAGME-containing polymers. Crosslinking studies with PTSA, onium salts and a latent UV-deblockable sulfonic acid, indicate that free radicals participate in the crosslinking reaction also. These MAGME-polymers exhibit high sensitivity, but in common with most negative crosslinkable resist materials, a limited resolution. They are soluble in harmless solvents like water and alcohols. Literature Cited 1. "Introduction to Microlithography", L.F. Thompson, C.G. Willson and M.J. Bowden, eds., ACS Symposium Series 219, (1983). 2. H. Ito and C.G. Willson, Polym. Eng. Sci., 1983, 23, 1012. 3. J.M.J. Fréchet, Ε. Eichler, H. Ito and C.G. Willson, Polymer, 1983, 24, 995. 4. A. Huit, S.A. MacDonald and C.G. Willson, Macromolecules, 1985, 18, 1804. 5. J.V. Crivello, "UV Curing; Science and Technology", S.P.Pappas ed., Technology Marketing Corporation, Stanford, Connec ticut, 1978, p. 23. 6. G. Berner, R. Kirchmayr, G. Rist and W. Rutsch, SME Tech nical Paper, FC 85-446, 1985. 7. Technical Bulletin, American Cyanamid. RECEIVED May 27, 1987
