Chapter 46 Patterning of Fine Via Holes in Polyimide by an Oxygen Reactive Ion Etching Method Hiroshi Suzuki, Hiroyoshi Sekine, Shigeru Koibuchi, Hidetaka Sato, and Daisuke Makino Yamazaki Works, Hitachi Chemical Company, L t d . , 4-13-1 Higashi-cho, Hitachi-shi, Ibaraki 317, Japan
A new silicon-containing positive photoresist having 0 RIE (reactive ion etching) resistance was selected as an etching mask for the polyimide dry etch. The RIE selectivity of this resist to polyimide is more than five. By using this resist, 2 µm x 2 µm via holes are formed in 2 µm thick polyimide film. After the RIE treatment the resist width and thickness decreased, and the removal rates in horizontal and vertical direction are 5 and 10 nm/min., respectively. After the RIE treatment, a lawn-like scum was observed in a bottom of via hole. The amount of scum generated in silicon containing polyimide patterning is much more than in conventional polyimide patterning. However, the scum and the resist layer could be stripped by treatment with HF/NH F solution and a successive phenolic type resist stripper. 2
4
With the i n c r e a s e o f the degree o f i n t e g r a t i o n o f m i c r o c i r c u i t s , the m u l t i l e v e l i n t e r c o n n e c t t e c h n o l o g y becomes i n e v i t a b l e f o r f u t u r e VLSI manufacture. Polyimide e x h i b i t s s u p e r i o r p l a n a r i t y over stepped s t r u c t u r e s and i s expected t o be one o f the most p r o m i s i n g m a t e r i a l s f o r the d i e l e c t r i c i n s u l a t i o n o f V L S I ' s . However, s i n c e the s m a l l e s t v i a h o l e s so f a r a c h i e v e d by wet e t c h i n g i s 3 um (1 ) , the f o r m a t i o n o f f i n e v i a h o l e s by a d r y e t c h p r o c e s s i s needed f o r t h e a p p l i c a t i o n o f p o l y i m i d e t o VLSI h a v i n g f i n e metal w i r i n g . U s u a l l y , d r y e t c h i n g o f p o l y i m i d e i s performed by RIE w i t h 0 , CF^, or t h e i r mixtures as an e t c h a n t gas, utilizing positive photoresist ( 2 ) , m e t a l s such as aluminum ( 3 ) , s p i n - o n - g l a s s (4), or S i N (5) as an e t c h i n g mask. However, i n the former c a s e , i t i s d i f f i c u l t t o d e f i n e a f i n e v i a - h o l e as s m a l l as 2 pm o r l e s s because the r e s i s t t h i c k n e s s must be two o r more t i m e s t h a t o f the p o l y i m i d e as a result o f the e q u a l e t c h i n g r a t e s between p h o t o r e s i s t and polyimide. In the l a t t e r c a s e , though t h e f i n e p a t t e r n can be o b t a i n e d the a d d i t i o n a l p a t t e r n t r a n s f o r m a t i o n from t h e p h o t o r e s i s t to the masking l a y e r i s n e c e s s a r y . In t h i s paper we r e p o r t t h e r e s u l t s o f our s t u d i e s on t h e f i n e 2
0097-6156/87/0346-0547506.00/0 © 1987 A m e r i c a n C h e m i c a l Society
American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036
POLYMERS FOR HIGH T E C H N O L O G Y
548 p a t t e r n i n g o f p o l y i m i d e by the 0 RIE p h o t o r e s i s t as the masking m a t e r i a l . 2
method u s i n g s i l i c o n - c o n t a i n i n g
Selection of Photoresist Many k i n d s o f s i l i c o n - c o n t a i n i n g p h o t o r e s i s t s have been r e p o r t e d ( 6 11) t o have b o t h 0 RIE r e s i s t a n c e and h i g h r e s o l u t i o n . Among them a positive photoresist called ASTRO(1^1), a m i x t u r e o f conventional p o s i t i v e p h o t o r e s i s t and a s i l i c o n r e s i n d e v e l o p e d by H i t a c h i f o r the imaging l a y e r o f b i - l e v e l r e s i s t system, was s e l e c t e d as an e t c h i n g mask i n t h i s experiment f o r the f o l l o w i n g reasons. First its p o s i t i v e tone imaging i s more a p p r o p r i a t e t o f o r m i n g v i a h o l e o p e n i n g than n e g a t i v e p h o t o r e s i s t . A l s o , the same development p r o c e s s as the c o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t can be a p p l i e d t o ASTRO. S i n c e the wettability o f ASTRO t o p o l y i m i d e was not sufficient, a slight modification was made t o ASTRO, and the r e s u l t i n g material is d e s i g n a t e d as X-8000K2. 2
P a t t e r n i n g o f the
Photoresist
Exposure c h a r a c t e r i s t i c s of X-8000K2 and conventional positive photoresist OFPR-800 (Tokyo Ohka Co.) are shown i n F i g u r e 1. X-8000K2 shows the comparable s e n s i t i v i t y t o c o n v e n t i o n a l one. Line and space and v i a h o l e p a t t e r n s o f X-8000K2 a r e shown i n F i g u r e 2. 0.8 um l i n e and space p a t t e r n s and 1.2 um v i a h o l e p a t t e r n s were obtained. 0
2
RIE
Selectivity
The dependence o f the e t c h i n g s e l e c t i v i t y o f X-8000K2 r e l a t i v e t o PIQ (a p o l y i m i d e type r e s i n from H i t a c h i Chemical Co.,) on 0 RIE c o n d i t i o n s was examined. RIE power, 0 p r e s s u r e , f l o w r a t e and the d i s t a n c e between the e l e c t r o d e s were s e l e c t e d as the parameters to determine the RIE c o n d i t i o n . For t h e s e parameters, v a l u e s o f 100 W, 10 m T o r r , 10 SCCM and 6 cm were s e l e c t e d as a s t a n d a r d c o n d i t i o n . The changes i n the e t c h i n g r a t e s are shown i n F i g u r e 3a, i n which one parameter i s v a r i e d and the r e m a i n i n g t h r e e parameters are f i x e d . E t c h i n g s e l e c t i v i t y v a r i e s from 5 t o 15 w i t h i n the range o f parameters i n v e s t i g a t e d as shown i n F i g u r e 3b. When the RIE power i n c r e a s e s the e t c h i n g s e l e c t i v i t y d e c r e a s e s and i t i n c r e a s e s when the 0 pressure decreases. The optimum RIE c o n d i t i o n determined from the s e l e c t i v i t y and the p a t t e r n p r o f i l e i s shown i n T a b l e I . The e t c h i n g s e l e c t i v i t y o f X-8000K2 under t h i s c o n d i t i o n i s about 10. 2
2
2
Table
I.
Optimum RIE
Equipment Power Flow r a t e Pressure D i s t a n c e between
electrodes
Conditions CSE-1110 (ULVAC, JAPAN) 100 W 10 SCCM 10 mTorr 3 cm
46.
SUZUKI ET A L .
Patterning
of Fine Via Holes in
Polyimide
549
ίο-
Ο
: Μ
X-8000 K2
π conventional positive photoresist
υ
•J 0.5
ι
0.1
100 Exposure Time Lamp
(eec.)
500 W Xe-Hg lamp 1.65 mW/cm2 at 365 nm
Development
I tuner β ion 23*C/60eec. by X-8000 K2 developer.
Film Thickness : 1.2pm
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 X-8000K2.
F i g u r e 2. R e s i s t p a t t e r n s o f X-8000K2 p r i n t e d by a RA-501 (g l i n e ) (a) 0 . 8 p i L & S, (b) 1.2pm v i a - h o l e .
POLYMERS FOR HIGH T E C H N O L O G Y
550
5
r
— O — PoweKW) —o—
0 flow(SCCM) 2
-—apressure(Torr) ···-O— - distance between electrodes (cm)
Figure of
3.
(a)
X-8000K2.
Etching
—ι
1
1
50
1 00
150
1
1
1
5 3
rate
10
~
1
1
1
5
10 6
30
of X-8000K2.
9
1
20 π
50
r-
100
(b) E t c h i n g
selectivity
46.
SUZUKI ET AL.
Polyimide
Patterning
of Fine Via Holes in
551
Polyimide
Patterning
Samples f o r polyimide patterning evaluations were p r e p a r e d as follows. PIQ and a s i l o x a n e - c o n t a i n i n g p o l y i m i d e PIX were spun on a s i l i c o n wafer f o l l o w e d by b a k i n g a t 100°C f o r 30 minutes, 200°C f o r 30 minutes and 350°C f o r 60 minutes s u c c e s s i v e l y i n a c o n v e c t i o n oven to g e t a 2 um t h i c k f i l m . Then X-8000K2 was c o a t e d and prebaked a t 90° C f o r 20 minute. X-8000K2 f i l m w i t h a t h i c k n e s s o f 1.2 pm was exposed u t i l i z i n g a H i t a c h i RA501 5:1 g - l i n e r e d u c t i o n p r o j e c t i o n printer. Exposed f i l m was developed a t 23°C f o r 60 seconds by u s i n g a tetramethylanmonium h y d r o x i d e s o l u t i o n as a d e v e l o p e r , f o l l o w e d by p o s t b a k i n g a t 130°C f o r 20 m i n u t e s . E t c h i n g o f PIQ and PIX f i l m s was performed by a ULVAC CSE-1110 RIE equipped w i t h 250 mm diameter electrode. F i g u r e 4 shows a 2 χ 2 pm v i a h o l e p a t t e r n formed i n a PIX f i l m under optimum RIE c o n d i t i o n s shown i n T a b l e I . I n t h i s photograph the r e s i s t has n o t been s t r i p p e d . The t a p e r angle o f t h e v i a h o l e i s about 90° . N o r m a l l y , t a p e r a n g l e s r e q u i r e d t o p r e v e n t wire o p e n i n g and t o m a i n t a i n h i g h d e n s i t y w i r i n g a r e around 70° . The dependence o f t h e t a p e r a n g l e on the parameters mentioned above was examined, but i t c o u l d n o t be reduced t o 70° w i t h i n the e v a l u a t i o n range shown i n F i g u r e 3. A f t e r t h e RIE treatment, a l a w n - l i k e scum was observed i n t h e bottom o f v i a h o l e s as shown i n F i g u r e 4. The g e n e r a t i o n o f scum i s enhanced w i t h the l o n g e r e t c h i n g p e r i o d as c l e a r l y shown i n t h e photographs o f F i g u r e 5. The change o f r e s i s t p a t t e r n s i z e a l s o o c c u r r e d as a r e s u l t o f t h e RIE treatment. The r e s i s t s u r f a c e became rough and t h e r e s i s t w i d t h and t h i c k n e s s d e c r e a s e d . The r e s i s t e r r o s i o n r a t e s i n the h o r i z o n t a l and v e r t i c a l d i r e c t i o n s a r e 5 and 10 nm/min, r e s p e c t i v e l y . I n a optimum e t c h i n g time o f 24 minutes f o r 2 pm thick polyimide, the r e s i s t diminished about 120 nm in a horizontal direction. The scum shown i n F i g u r e s 4 and 5 i s thought t o be produced by the r e a c t i o n o f oxygen and s i l i c o n c o n t a i n e d i n b o t h p o l y i m i d e PIX and X-8000K2. In o r d e r t o c o n f i r m t h i s presumption, RIE e x p e r i m e n t s were performed f o r the v a r i o u s c o m b i n a t i o n s o f p h o t o r e s i s t and p o l y i m i d e , i . e . : (a) X-8000K2 (1.2 pm t h i c k ) on PIQ (2 pm t h i c k ) (b) X-8000K2(1.2 pm t h i c k ) on a S i wafer (c) conventional positive p h o t o r e s i s t (2 pm t h i c k ) on PIQ (0.5 pm t h i c k ) (d) c o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t (2 pm t h i c k ) on PIX (0.5 pm t h i c k ) . The r e s u l t s a f t e r RIE a r e shown i n F i g u r e 6. As can be seen i n F i g u r e 6a, when PIQ was p a t t e r n e d u s i n g X-8000K2 as an mask scum was a l s o g e n e r a t e d but i n o n l y s l i g h t amounts compared t o PIX. When the X-8000K2 p a t t e r n was formed d i r e c t l y on a S i wafer, and was t r e a t e d by 0 RIE, no scum was o b s e r v e d as shown i n F i g u r e 6b. The absence o f scum i s a l s o observed f o r sample 6c, i n which c o n v e n t i o n a l p h o t o r e s i s t was formed on p o l y i m i d e which c o n t a i n s no s i l i c o n atoms. However, i n t h e case o f t h e s i l i c o n c o n t a i n i n g p o l y i m i d e , c o n s i d e r a b l e scum appeared. These r e s u l t s a r e summarized i n T a b l e I I . 2
The d i f f e r e n c e o f the amount o f scum among t h e samples i s c l e a r l y e x p l a i n e d assuming the scum t o be s i l i c o n o x i d e produced by the r e a c t i o n o f oxygen and s i l i c o n atom. The d i f f e r e n c e o f the amount o f scum i n F i g u r e s 6a and 6c comes from the d i f f e r e n t concentration of silicon contained i n X-8000K2 and c o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t . The amount o f scum observed i n F i g u r e 5 i s the
5
5
2
POLYMERS FOR HIGH T E C H N O L O G Y
F i g u r e 4. V i a - h o l e p a t t e r n formed i n PIX f i l m under RIE c o n d i t i o n , 2pm v i a - h o l e (X-8000K2 1.2pm t h i c k , PIX 2pm t h i c k ) .
F i g u r e 5. L i n e and space p a t t e r n s o f PIX a f t e r RIE t r e a t m e n t (X-8000K2 1.2pm, PIX 2pm t h i c k ) (a) b e f o r e e t c h i n g , (b) e t c h i n g time 18min, (c) e t c h i n g time 46min.
SUZUKI ET A L .
Patterning
of Fine Via Holes in
Polyimide
F i g u r e 6. Comparison o f scum o c c u r r e n c e a f t e r RIE t r e a t m e n t , 3pm l i n e and space (a) X-8000K2 (1.2pm t h i c k ) on PIQ (^um t h i c k ) , (b) X-8000K2 (1.2pm t h i c k ) , ( c ) c o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t (2pm t h i c k ) on PIQ (0.5pm t h i c k ) , (d) c o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t (2pm t h i c k ) on PIX (0.5pm t h i c k ) .
553
POLYMERS FOR HIGH T E C H N O L O G Y
554
largest. I n t h i s case, t h e s i l i c o n o x i d e scum was g e n e r a t e d both t h e p h o t o r e s i s t and t h e s i l i c o n c o n t a i n i n g p o l y i m i d e .
Table I I .
Comparison o f Scum O c c u r r e n c e
X8000K2 (Si Containing)
Substrate
Si
Wafer
PIQ on S i Wafer
Slight
PIX
Moderate R e s i d u e
Resist
a f t e r RIE Treatment
Resist Conventional Positive Photoresist
-
None
on S i Wafer
from
Residue
None
Moderate R e s i d u e
Stripping
In o r d e r t o remove t h e r e s i s t a f t e r t h e RIE t r e a t m e n t , wafers were immersed i n p h e n o l i c - t y p e r e s i s t s t r i p p e r , b u t t h e r e s i s t as w e l l as the scum c o u l d n o t be s t r i p p e d . S i n c e a t h i n s i l i c o n o x i d e l a y e r i s formed on t h e r e s i s t s u r f a c e , and t h e c o m p o s i t i o n o f scum i s thought to be s i l i c o n o x i d e as d i s c u s s e d above, i t i s n e c e s s a r y t o remove this silicon oxide layer (scum) p r i o r t o the r e s i s t removal. T h e r e f o r e , r e s i s t s t r i p p i n g was done i n two s t e p s . In a f i r s t step the wafer was immersed i n b u f f e r e d h y d r o f l u o r i c a c i d s o l u t i o n t o remove t h e s i l i c o n o x i d e and was then t r e a t e d w i t h c o n v e n t i o n a l resist stripper. When t h e wafer was d i p p e d i n a h i g h l y c o n c e n t r a t e d h y d r o f l u o r i c a c i d s o l u t i o n o r dipped f o r a l o n g p e r i o d , p o l y i m i d e f i l m tends t o peel o f f the substrate. From t h e experiment c a r r i e d o u t f o r a various concentrations o f HF/NH^F s o l u t i o n , t h e most suitable c o n d i t i o n s were found as f o l l o w s : c o m p o s i t i o n o f b u f f e r e d h y d r o f l u oric acid HF/NH F=l/20; d i p p i n g time, £ 2 minutes a t room temperature. When t h e wafer was d i p p e d i n t o t h i s b u f f e r s o l u t i o n , t h e amount o f scum d e c r e a s e d s h a r p l y d u r i n g t h e f i r s t t e n seconds, b u t a f t e r 30 seconds t h e removal r a t e became e x t r e m e l y low and r e s i d u e remained as shown i n F i g u r e 7. However, t h e r e m a i n i n g scum and r e s i s t l a y e r were removed p e r f e c t l y i n a second s t e p by d i p p i n g i n t o c o n v e n t i o n a l r e s i s t s t r i p p e r S-502(Tokyo Ohka C o . ) , a t 1 1 0 ° C f o r 7 t o 10 minutes as shown i n F i g u r e 8. 4
Conclusions P o s i t i v e w o r k i n g s i l i c o n - c o n t a i n i n g r e s i s t w i t h 0.8 p m r e s o l u t i o n and 0 RIE s e l e c t i v i t y g r e a t e r than f i v e was used f o r f i n e v i a h o l e f o r m a t i o n i n a p o l y i m i d e f i l m by 0 RIE p r o c e s s . 2 χ 2 p m v i a h o l e s were o b t a i n e d i n 2 p m t h i c k p o l y i m i d e by 2
2
SUZUKI ET A L .
Patterning
of Fine Via Holes in
Polyimide
F i g u r e 7. L i n e and space p a t t e r n o f PIX a f t e r HF/NH^F t r e a t m e n t f o r 30 s e c . , 2μπ\ L & S.
F i g u r e 8. L & S and v i a - h o l e p a t t e r n s o f PIX a f t e r HF/NH^F t r e a t m e n t and S-502 t r e a t m e n t , (a) 2um L & S, (b) 2yjm v i a - h o l
556
POLYMERS FOR HIGH T E C H N O L O G Y
using this r e s i s t as an e t c h i n g mask. U s i n g RIE c o n d i t i o n s to achieve the etching selectivity of ten, the resist width and thickness d i m i n i s h e d a f t e r the RIE t r e a t m e n t . The r e m o v a l r a t e o f the resist in horizontal and v e r t i c a l directions was 5 and 10 nm/min., r e s p e c t i v e l y . A f t e r t h e R I E t r e a t m e n t a l a w n - l i k e scum was observed i n a bottom o f v i a h o l e . The a m o u n t o f s c u m g e n e r a t e d i n s i l i c o n - c o n t a i n i n g p o l y i m i d e p a t t e r n i n g i s much g r e a t e r t h a n c o n v e n tional polyimide patterning. H o w e v e r , t h e scum c o u l d be stripped sequentially t r e a t m e n t w i t h HF/NH^F s o l u t i o n f o l l o w e d by a p h e n o l i c type r e s i s t stripper. Acknowledgments We w i s h t o t h a n k D r . T . U e n o a n d D r . M . T o r i u m i o f H i t a c h i C e n t r a l Research L a b o r a t o r y f o r t h e i r i n v a l u a b l e a d v i c e about RIE equipment. We a l s o wish to thank T. Nishida of Hitachi Central Research Laboratory for his helpful discussion.
Literature Cited 1. Saiki, A. Polyimides; Mittal, K. L . , Ed.; Plenum Publishing Corp. 1984; Vol. 2, p 827. 2. Herndon, T. O. Kodak Interface Oct. 1979, 26. 3. Rivans, I. V. IEEE V-MIC Conf. 1984, 283. 4. Ting, C. H. IEEE V-MIC Conf. 1984, 106. 5. Samuelson, G. ACS Organic Coatings and Plastics Chemistry 1984, 43, 446 6. Hazakis, M. Proc. I n t ' l . Conf. Microlithography 1981, 386. 7. Morita, M. Jpn. J. Appl. Phy. 1983, 22, L659 8. Wilkins, C. W. J. Vac. Sci. Technol. 1984, B3, 306. 9. Reichmanis, E. SPIE "Advences in Resist Technol. and Proc." 1984, 469, 38. 10. Suzuki, M. J. Electrochem. Soc. 1983, 130, 1962. 11. Ueno, T. 4th Photo Polymer Conf. preprint 1985, 108. RECEIVED May 13, 1987
