Chapter 21 I m p o r t a n c e of the Interface C o n d i t i o n
Polymers for High Technology Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/04/18. For personal use only.
u p o n Photoresist Image A d h e s i o n i n M i c r o e l e c t r o n i c Device F a b r i c a t i o n 2
J. N.Helbert1and N. C. Sana
1Motorola Bipolar Technology Center, Mesa, AZ 85202 Semiconductor Research and Development Laboratory, Motorola SPS, Phoenix, AZ 85008
2
Microelectronic device fabrication currently relies primarily upon photoresist processing for integrated circuit pattern delineation. Adhesion of polymeric photoresist patterns, especially those of micron and submicron dimensions, to the required fabrication substrates is of paramount importance. Photoresist image adhesion problems encountered in device fabrication have been solved by chemical interfacial treatments. Current new trends in microelectronic adhesion technology w i l l be described and discussed with emphasis upon the chemical nature of the interface involved as determined by ESCA.
Integrated circuit pattern image adhesion is one of the most important polymeric resist performance parameters(1,2). Resist lithographic design and performance are important, but i f the resist is not process compatible in an adhesion sense, i t is of academic interest only. Fortunately, this parameter can usually be satisifed by substrate interfacial chemical treatments without changing the chemical nature of the photoresists themselves. Since positive photoresists, mixtures of novolak resins and diazoquinone photoactive dissolution inhibitors, are the most plagued by problems with image adhesion (see Figure 1), we w i l l focus a l l discussion upon treatments which solve adhesion problems for this generic class of resist materials· Surface chemical changes created by these surface chemical reactions w i l l be monitored by Electron Spectroscopy for Chemical Analysis (ESCA)O), a very powerful spectroscopic technique for investigating surface compositions extending from 1-20 monolayers in depth from the surface. When the spectrometer is equipped with angle resolution capability, i t also offers a means of non-destructive depth profiling of the upper 50A of substrate layers. 0097-6156/87/0346-0250$06.00/0 © 1987 American Chemical Society
21.
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Microelectronic
Device
Fabrication
251
EXPERIMENTAL Wafer T e s t i n g The r e s i s t s u s e d i n t h i s w o r k a r e a l l c o n v e n t i o n a l p o s i t i v e photoresists. They a r e a l l p r o p r i e t a r y f o r m u l a t i o n s , b u t g e n e r i c a l l y t h e y a r e composed o f a m i x t u r e o f (1) n o v o l a k r e s i n s , (2) photoactive components of the d i a z o q u i n o n e t y p e , (3) l e v e l i n g a g e n t s and/or s u r f a c t a n t s , and (4) g l y c o l - b a s e d s p i n n i n g s o l v e n t s . The s p e c i f i c r e s i s t s t e s t e d w e r e P o l y c h r o m e 1 2 9 , H u n t 2 0 4 , K T I - I I , D y n a c h e m OFPR 8 0 0 , a n d AZ 1 3 5 0 . They were a l l a p p l i e d by c o n v e n t i o n a l s p i n n i n g t e c h n i q u e by e i t h e r an S i l i c o n V a l l e y G r o u p (SVG) t r a c k , o r by Headway o r S o l i t e c m a n u a l s p i n n e r s y s t e m s . The c o m m e r c i a l l y a v a i l a b l e a d h e s i o n p r o m o t e r s ( P e t r a r c h S y s t e m s I n c . ) t e s t e d w e r e a p p l i e d a s d i l u t e s o l u t i o n s 0 . 3 - 7% b y w e i g h t i n a c e t o n e or x y l e n e , o r as f o r h e x a m e t h y l d i s i l a z a n e (HMDS), e i t h e r as a l i q u i d or vapor (Imtec S t a r 2000). ESCA A n a l y s i s ESCA m e a s u r e m e n t s w e r e c a r r i e d o u t i n a P H I M o d e l 5 3 0 0 ESCA s p e c t r o m e t e r u s i n g m a g n e s i u m s - a l p h a X - r a y s . The b a s e p r e s s u r e i n t h e a n a l y z i n g c h a m b e r was 1 X 10 t o r r or b e t t e r d u r i n g a n a l y s i s . In the PHI M o d e l 5300 s y s t e m , t h e v a r i a b l e t a k e - o f f a n g l e measurements a r e p e r f o r m e d by r o t a t i n g t h e s a m p l e o n a n a x i s t h r o u g h t h e s a m p l e surface. The m e a s u r e m e n t s c a n be made f r o m g r a z i n g a n g l e s o f 3 to 10 past normal. RESULTS AND D I S C U S S I O N Single Crystal S i l i c o n The r e a s o n f o r s t u d y i n g t h i s s u b s t r a t e i s one o f e a s e o f ESCA a n a l y s i s and d e t e c t i o n s e n s i t i v i t y c o n s i d e r a t i o n s . A s u b s t r a t e was n e e d e d , where d e t e c t i o n o f c a r b o n and s i l i c o n - c o n t a i n i n g s u r f a c e s p e c i e s w o u l d n o t be o v e r - s h a d o w e d by s i g n a l s f r o m t h e b u l k s p e c i e s , e v e n a t l o w a n g l e s . Of c o u r s e , t h i s s u b s t r a t e c o n t a i n s a n a t i v e o x i d e l a y e r l e s s t h a n 20A t h i c k , b u t t h i s s u r f a c e i s s i m i l a r t o t h a t o f other s i l i c o n d i e l e c t r i c s u b s t r a t e s d i f f e r i n g only i n growth temperature. A l t h o u g h the i n t e g r a t e d c i r c u i t (IC) i s b u i l t upon a s i n g l e c r y s t a l s i l i c o n w a f e r , t h i s s u b s t r a t e and i n t e r f a c e a r e u s u a l l y n o t s e e n b y t h e p a t t e r n i n g p h o t o r e s i s t i n new MOS f a b r i c a t i o n p r o c e s s e s ; h o w e v e r , t h i s may n o t be t h e c a s e f o r o l d e r p r o d u c t process flows. Before determining surface c o n d i t i o n treatment e f f e c t s , the s u r f a c e s e n s i t i v i t y enhancement o f a n g l e r e s o l v e d ESCA s u r f a c e a n a l y s i s i s demonstrated i n F i g . 2. The f i g u r e c l e a r l y shows t h e 0 , S i , and C s u r f a c e c o n c e n t r a t i o n s to be d r a m a t i c a l l y d i f f e r e n t t h a n the r e s p e c t i v e b u l k c o n c e n t r a t i o n s . The s i l i c o n c o n c e n t r a t i o n i n t h e t o p s u r f a c e l a y e r m e a s u r e d a t 5 ° ESCA t a k e - o f f a n g l e , f o r e x a m p l e , i s a p p r o x i m a t e l y 2 . 5 X l e s s t h a n t h a t o b t a i n e d a t a 75 take-off angle, w h i c h r e p r e s e n t s a n a p p r o x i m a t e d e p t h o f 50A f r o m t h e s u r f a c e . M o s t of the s u r f a c e d a t a p r e s e n t e d h e r e i s at the low t a k e - o f f a n g l e o f 5 to c o n c e n t r a t e p r i m a r i l y upon the wafer s u r f a c e c o n d i t i o n i n the f i r s t 2 - 3 a t o m i c l a y e r s , a n d how i t a f f e c t s p o l y m e r i c p h o t o r e s i s t adhesion.
POLYMERS FOR HIGH T E C H N O L O G Y
252
ADHESION
JU
U " '
±J
H >ν UN
GOOD POOR LOSS OF ADHESION Figure
1.
O p t i c a l micrograph image·
of " l i f t e d "
photoresist
Take-Off Ancle
Figure
2.
ESCA p r o f i l e
data
f o r Y58 b l a n k
wafer.
test
21.
HELBERT A N D SAHA
Microelectronic
Device
Fabrication
253
The ESCA r e s u l t s do not i n d i c a t e a d r a m a t i c change i n C / S i r a t i o s f o l l o w i n g HMDS t r e a t m e n t , l i q u i d or vapor phase. O/Si r a t i o s of HMDS t r e a t e d samples, p a r t i c u l a r l y a f t e r vapor phase treatment, do i n d i c a t e a d e t e c t a b l e l o w e r i n g o f the r e l a t i v e oxygen s u r f a c e c o n c e n t r a t i o n compared t o b l a n k w a f e r s , c o n s i s t e n t w i t h d e h y d r a t i o n which i s known t o a l s o occur w i t h HMDS treatment Q ) . F i g u r e 3a shows a t y p i c a l h i g h r e s o l u t i o n C I s ESCA spectrum r e c o r d e d f o r a b l a n k s i l i c o n wafer (Y58) a t 5° t a k e - o f f a n g l e . D e c o n v o l u t i o n a f t e r background s u b t r a c t i o n , g i v e s t h r e e w e l l d e f i n e d peaks a t 285.5+/- 0.1 eV, 287.0 and 289.7 eV. The peak a t 285.5 eV i s a s s i g n e d t o a -CH (hydrocarbon) s p e c i e s , the 287.1 eV peak t o -CH 0 ( e . g . a l c o h o l o r h y d r o p e r o x i d e ) s p e c i e s , and the 289.7 eV peak to a -CO^- ( c a r b o x y l i c a c i d o r e s t e r ) adsorbed s p e c i e s . These carbon compounds are found on n e a r l y a l l s u b s t r a t e s , and a r e thought t o be adsorbed from the p r o c e s s i n g ambient. F i g s . 3 (b & c ) show C I s ESCA s p e c t r a from HMDS (SVG)/Y58 and HMDS (*2000) t r e a t e d Y58 s i l i c o n w a f e r s . I n t e r e s t i n g l y , the C I s s p e c t r a f o r the HMDS (*2000)/Y58 sample c o n s i s t m a i n l y o f a s i n g l e peak w i t h o n l y a s m a l l s h o u l d e r a t h i g h e r b i n d i n g energy ( B E ) . The BE of the main C I s peak i s 284.5 eV, w h i c h i s 1.0 eV lower than t h e adsorbed hydrocarbon peak i n the b l a n k w a f e r s . T h i s peak i s a t t r i b u t e d t o the "0H^ s u r f a c e groups r e s u l t i n g from the HMDS t r e a t m e n t , c o v a l e n t l y anchored t o the s u r f a c e . We f a v o r c o v a l e n t bond f o r m a t i o n j ^ b e c a u s e o v e r n i g h t s t o r a g e o f the sample i n the h i g h vacuum ( 2 χ 10 t o r r ) o f the ESCA a n a l y z e r chamber d i d not reduce the C I s peak i n t e n s i t y . T h i s would not have o c c u r r e d i f the s i g n a l was due t o j u s t adsorbed HMDS o r the dimer ( C H - ^ S i - O , w h i c h u s u a l l y forms due t o s u r f a c e h y d r o l y s i s ; the s i g n a l would not have been as s t a b l e , the o b s e r v a t i o n f o r some c o n t r o l s u b s t r a t e s w i t h no s u r f a c e treatments. The C I s s p e c t r a l changes ( F i g s . 3a and 3c) s t r o n g l y suggest t h a t the vapor phase HMDS treatment c l e a n s the wafer v e r y e f f i c i e n t l y by removing a l l t h r e e types o f adsorbed carbon compounds, f o l l o w e d by the s p r e a d i n g o f the methyl b l a n k e t from the s u r f a c e HMDS r e a c t i o n a c r o s s the s u b s t r a t e . S i n c e the samples were handled i n a i r and s p e c t r a recorded a t l e a s t 24 hours a f t e r vapor p r i m i n g , t h i s p r o c e s s must be c o n s i d e r e d t o be a v e r y good s u r f a c e s t a b i l i z i n g treatment. Furthermore, t h e l i q u i d phase r e a c t i o n o f HMDS from the wafer t r a c k c o u l d not remove the adsorbed s p e c i e s as c o m p l e t e l y , nor was i t capable o f r e n d e r i n g the s u b s t r a t e r e s i s t a n t to r e a d s o r p t i o n o f carbonaceous c o n t a m i n a t i o n . F i g u r e s 4 ( a - c ) show t h r e e ESCA S i 2p s p e c t r a r e s u l t i n g from the (a) Y58 b l a n k , (b) HMDS (SVG t r a c k ) and ( c ) HMDS (*2000)/Y58 w a f e r , r e s p e c t i v e l y . A l l s p e c t r a were r e c o r d e d a t 5° t a k e - o f f a n g l e . These s p e c t r a c l e a r l y i n d i c a t e the e v o l u t i o n o f a new peak between t h e e l e m e n t a l s i l i c o n and the S i 0 peaks. The growth i s v e r y pronounced i n the case o f the HMDS (*2000)/Y58 t r e a t e d wafer. The new S i 2p peak, a r i s i n g from HMDS treatment o f the w a f e r , i s c e n t e r e d a t 101.8 eV (see F i g u r e s 4b and 4 c ) . The o t h e r f i v e peaks p r e s e n t i n t h e spectrum o f the b l a n k wafer are a s s i g n e d t o S i , SiO, S i 0^ and SiO^ species (4). The new peak a t 101.8 eV i s a s s i g n e d t o (CH^)^ S i - 0 type o f S i s p e c i e s formed on the s u r f a c e due t o the HMDS r e a c t i o n . The presence o f t h i s new S i 2p peak i s c o n s i s t e n t w i t h the e a r l i e r i n t e r p r e t a t i o n o f the C I s ESCA d a t a . 2
9
254
POLYMERS FOR HIGH T E C H N O L O G Y
CORRECTED BE VALUES C1s
4599
PEAK
BE
SPECIES
284.eV
3449
-CH
3
-CH (ADSORBED) X
285.3 ΓΓΓ 2299 h
- C - O -
287.1
I
(ADSORBED)
I
1150 h
-C=0
288.6
I
ο (ADSORBED) 295.0
291.3
287.5
283.8
280.0
BINDING ENERGY, eV 4571 CORRECTED BE VALUES C1s BE
SPECIES
284.5 eV
-CH
3
285.3
-CH (ADSORBED)
287.1
- C - O -
X
I
I (ADSORBED) 288.6
-C=0
I
ο (ADSORBED) 295.0
291.3
287.5
283.8
280.0
BINDING ENERGY, eV 3661
CORRECTED BE VALUES C1s
2746 h -
BE
SPECIES
284.5 eV uT 1831 r-
285.3 287.1
915 r-
-CH
3
—CH (ADSORBED) X
I - c - o -
I (ADSORBED)
291.3
287.5
283.8
280.0
BINDING ENERGY, eV Figure (b)
3.
C Is
(a)
C Is
ESCA s p e c t r a
ESCA s p e c t r a
silicon
wafer.
silicon
wafer.
(c)
for
C Is
for
blank
SVG c o n v e n t i o n a l spectra
for
Star
s i l i c o n wafer liquid 2000
(Y58).
HMDS-treated
vapor
HMDS-treated
21.
HELBERT AND SAHA
Microelectronic
Device
255
Fabrication
CORRECTED BE VALUES Si2p PEAK
BE
1
99.2 eV
SPECIES
Sl°
99.8 100.2 SiO
x
Si0
2
101.4 103.4
108.0
105.2
102.5
99.7
97.0
BINDING ENERGY, eV CORRECTED BE VALUES Si2p PEAK
BE
SPECIES
99.2 eV 99.8 100.2 SiOv 101.4 101.8
(CH )3
I
3
Si
I
Ο I 103.4 105.2
102.5
-SiSI02
99.7
BINDING ENERGY, eV CORRECTED BE VALUES Si2p PEAK
BE
SPECIES
99.2 eV Si 99.8 100.2 SiO
x
101.4 101.8
(CH )3 3
Si
I
ο
I
105.2
102.5 BINDING ENERGY, eV
99.7
—Si-
97.0 103.4
I
Si0
Figure 4. ( a ) S i 2p E S C A s p e c t r a f r o m t h e u n t r e a t e d b l a n k Y 5 8 wafer. ( b ) S i 2 p E S C A s p e c t r a f r o m HMDS ( l i q u i d ) - t r e a t e d Y 5 8 w a f e r o n a SVG t r a c k . ( c ) S i 2p E S C A s p e c t r a f r o m S t a r 2 0 0 0 v a p o r primed Y58 w a f e r .
2
POLYMERS FOR HIGH T E C H N O L O G Y
256
Si0
2
Substrates
S i n c e SiO~ s u b s t r a t e s appear f r e q u e n t l y d u r i n g IC f a b r i c a t i o n , t h e a d h e s i o n t e s t r e s u l t s f o r t h i s s u b s t r a t e a r e i m p o r t a n t . Four types o f o x i d e s have been e x t e n s i v e l y t e s t e d . They a r e ( 1 ) t h e r m a l o x i d e grown at 1100 C, ( 2 ) s o f t e r o x i d e s processed by c o n v e n t i o n a l s p i n - o n - g l a s s t e c h n o l o g y , ( 3 ) phosphorus doped l i q u i d phase c h e m i c a l vapor d e p o s i t i o n (LPCVD) o x i d e , and ( 4 ) low temperature (200°C) plasma d e p o s i t e d o x i d e (PEO). Adhesion has been a c h i e v e d on these o x i d e s through a v a r i e t y o f p r o c e s s e s . C o n v e n t i o n a l l i q u i d phase a p p l i c a t i o n o f HMDS, however, was not adequate f o r the l a t t e r t h r e e s u b s t r a t e s l i s t e d above. However, i t d i d p r o v i d e adequate p h o t o r e s i s t adhesion f o r thermal o x i d e s . F o r the l a s t t h r e e s u b s t r a t e s , a double a d h e s i o n promoter process was needed and developed. T h i s process has been i n c o r p o r a t e d into actual device f a b r i c a t i o n processing. The double promoter process i n v o l v e s the s u c c e s s i v e a p p l i c a t i o n of l i q u i d promoter s o l u t i o n s o f v i n y l t r i c h l o r o s i l a n e (VTS) and 3 - c h l o r o p r o p g l t r i m e t h o x y s i l a n e , f o l l o w e d by s u c c e s s i v e cure c y c l e s i n dry a t 90 C b e f o r e p h o t o r e s i s t a p p l i c a t i o n . L a t e r , the s u c c e s s f u l but somewhat complex double promoter process was r e p l a c e d by the "vapor phase" HMDS p r o c e s s i n the S t a r 2000. Then s u p e r i o r r e s i s t image adhesion was o b t a i n e d on a l l f o u r o x i d e s u b s t r a t e s w i t h a l l the p h o t o r e s i s t s t e s t e d . What a r e the mechanisms o f these two s u c c e s s f u l promoter processes? To answer t h i s q u e s t i o n , ESCA a n a l y s i s was employed. ESCA survey scans recorded f o r thermal o x i d e s u r f a c e s i n d i c a t e the presence o f S i , C and 0. A g a i n , the s i m p l e e l e m e n t a l c o m p o s i t i o n s do not show any d r a m a t i c change due t o the s u r f a c e c h e m i c a l m o d i f i c a t i o n s by the HMDS t r e a t m e n t , except f o r the decrease i n O/Si r a t i o . B u t , t h e ESCA d a t a do show t h a t the c h e m i c a l n a t u r e o f these elements changes s i g n i f i c a n t l y , i . e . , C/0 c o n t a i n i n g m o l e c u l a r i m p u r i t i e s a r e b e i n g r e p l a c e d by a c o v a l e n t l y bonded Si/O/C c o n t a i n i n g s t a b i l i z a t i o n l a y e r . As seen f o r Y58 w a f e r s , ESCA s p e c t r a r e s u l t i n g from S i 2p t r a n s i t i o n s f o r HMDS t r e a t e d o x i d e wafers a l s o d e p i c t the e v o l u t i o n o f a new peak a t 101.8 eV. A g a i n , t h i s peak i s most prominent i n the S t a r 2000 HMDS t r e a t e d o x i d e sample, and i s a s s i g n e d t o the S i 2p peak p r e s e n t on the o x i d e s u r f a c e due t o (CH^)^ Si-0- surface species. When a c o n v e n t i o n a l l y a p p l i e d VTS s o l u t i o n i s used, a d h e s i o n i s i n c r e a s e d t o S i 0 s u b s t r a t e s , C 5 ) and no t r a c e o f CI remains a t t h e s u r f a c e . ESCA r e s u l t s f o r VTS t r e a t e d PSG o x i d e s show r e p r o d u c i b l e i n c r e a s e s i n carbon c o n c e n t r a t i o n as expected from -Si-O-Si-iCH^CH^) surface r e a c t i o n products. Consistent with t h i s hypothesis, broadening o f the ESCA S i 2p peak a t lower BE i s a l s o observed. The broadened S i 2p spectrum can be s i m u l a t e d by two G a u s s i a n curves w i t h 1.8 eV f u l l w i d t h a t h a l f maximum (FWHM) c e n t e r e d a t 103.0 and 103.7 BE. The 103.7 eV peak would be the same as t h a t observed f o r t h e b l a n k w i t h the second peak a t 103.0 eV a t t r i b u t e d t o the S i product of the VTS s u r f a c e r e a c t i o n . ESCA r e s u l t s f o r double promoted o x i d e s a r e l e s s d e f i n i t i v e than those p r e v i o u s l y d e s c r i b e d , b u t d e f i n i t e l y show d i f f e r e n c e s i n t h e S i 2p s p e c t r a , hence, i n d i c a t i n g t h a t a g a i n s u r f a c e changes a r e o c c u r r i n g due t o the i r r e v e r s i b l e s u r f a c e c h e m i c a l r e a c t i o n s . A t the 2
21.
HELBERT A N D S A H A
Microelectronic
Device
257
Fabrication
same time, these o x i d e s u r f a c e s a r e b e i n g a t l e a s t p a r t i a l l y c l e a n e d of C/0 c o n t a i n i n g atmospheric contaminants. HMDS T r e a t e d S u b s t r a t e Comparison S e v e r a l e n l i g h t e n i n g comparisons can be made from the d a t a o f T a b l e I . F i r s t an e s t i m a t e o f the vapor phase HMDS s u r f a c e r e a c t i o n e f f i c i e n c y v s t h a t o f the l i q u i d phase can be o b t a i n e d . The f e a t u r e s of the vapor phase and l i q u i d phase treatments a r e found i n T a b l e s I I and I I I . From T a b l e I , t h e v a l u e s f o r Y58 s u b s t r a t e s a r e 0.22 and 0.11, r e s p e c t i v e l y , thus i n d i c a t i n g an approximate 100% g r e a t e r e f f i c i e n c y towards s i l a n o l c o n v e r s i o n t o t r i m e t h y l s i l y l l a b e l l e d r e a c t i o n product f o r the vapor t r e a t m e n t . An approximate 30% g a i n i s o b t a i n e d f o r the o x i d e s u b s t r a t e comparison. T a b l e I . Comparison o f HMDS Coverage on Y-58 and Oxide S u r f a c e s
METHOD
VAPOR PHASE
SURFACES
Y-58
AREA RATIO [(CH3)C-SI-0-]/SI02
0.22 +_ 0.02
C/SI
RATIO
TOTAL
[Cis PEAK #1/Si2p PEAK #€] C / S I RATIO
3.2 ± 0.2
0.8 ±. 0.1
HMDS (*2000)
0.13
3-4
0.8
Y-58
0.11
2.9
1.0
OXIDE
0.10
« 5
1.1
Y-58
NA
1.1
OXIDE
NA
1.1
OXIDE
LIQUID PHASE HMDS (SVG)
BLANK
Next, i t i s i n t e r e s t i n g t o l o o k a t the r e s u l t s i n the f o u r t h column o f the t a b l e . Here, t h e t h e o r e t i c a l C l s / S i 2p r a t i o s h o u l d be 3, t h e s t o i c h i o m e t r y o f the l a b e l l i n g t r i m e t h y l s i l y l group. The v a l u e s found f o r Y58 s u b s t r a t e s a r e 3.2 and 2.9, w h i l e l a r g e r v a l u e s between 3-5 were observed f o r the o x i d e s u b s t r a t e s . The l a t t e r l a r g e
POLYMERS FOR HIGH T E C H N O L O G Y
258
Table I I .
S t a r 2 0 0 0 V a p o r P h a s e HMDS F e a t u r e s
and
FEATURES: 1. LOW PRESSURE BAKE PREPARATION 2.150 C DRY NITROGEN CYCLED IN-SITU DEHYDRATION BAKE 3. VAPOR PHASE HMDS TREATMENT AT 150 C 4.250 WAFERS/40 MINUTES 5. LOW HMDS USE- COST REDUCTION ADHESION TESTS POSITIVE WITH: 1. THERMAL OXIDE(1100 C) 2. SILICON NITRIDE 3. SPIN ON GLASS 4. PECVD OXIDE(200C) 5. PSG OXIDE 6. POLYSILICON
Table I I I .
Conventional
Processing
Steps
1. DEHYDRATE BAKE: NITROGEN-PURGED OVEN OR HOT PLATE AT 1 ATMOSPHERE
2. AIR TRANSFER
3. LIQUID HMDS TREATMENT: NEAT OR 30% SOLUTION
4. CURE IN NITROGEN OVEN OR HOT PLATE AT 00 C, OR NO CURE JUST RESIST PREBAKE(SEE #6)
5. AIR TRANSFER
6. PHOTORESIST COAT/PREBAKE AT 90-105 C
Results
21.
H E L B E R T A N D SAHA
Microelectronic
Device
Fabrication
259
v a l u e s r e f l e c t a g r e a t e r peak d e c o n v o l u t i o n u n c e r t a i n t y f o r o b t a i n i n g t h i s r a t i o f o r h i g h e r t e m p e r a t u r e o x i d e s u r f a c e s , w h i c h have l e s s w e l l r e s o l v e d E S C A s p e c t r a a n d / o r may c o n t a i n a d d i t i o n a l c h e m i s o r b e d HMDS s p e c i e s — t h u s l e a d i n g t o h i g h e r r a t i o s . F i n a l l y , i t i s i n t e r e s t i n g to f o l l o w the t o t a l C / S i r a t i o of the l a s t c o l u m n o f T a b l e I . The o v e r a l l r e s u l t o f v a p o r t r e a t m e n t creates a s u r f a c e w i t h a 27% r e d u c e d o v e r a l l c a r b o n s u r f a c e c o n c e n t r a t i o n . I t i s a l s o f e l t t h e t r i m e t h y l s i l y l b l a n k e t c r e a t e d by t h e i n - s i t u gas p h a s e HMDS r e a c t i o n h a s a much more s t a b l e c a r b o n s u r f a c e p o p u l a t i o n , as w e l l a s , p r o d u c i n g a r e d u c e d c a r b o n s u r f a c e l a y e r c o n c e n t r a t i o n . T h i s e f f e c t combined w i t h the d e h y d r a t i o n of the s u b s t r a t e ( 1 ) , w h i c h i s w e l l known to o c c u r , c r e a t e s a s u r f a c e c o n d i t i o n o f t h e w a f e r c o n d u c i v e to low day t o day v a r i a t i o n i n w a f e r s u r f a c e c o n d i t i o n . That i s , changes i n h u m i d i t y and a m b i e n t c a r b o n i m p u r i t y c o n t a m i n a n t c o n c e n t r a t i o n s w i l l have l i t t l e i n f l u e n c e upon the wafer surface l e a d i n g to improved p h o t o r e s i s t a d h e s i o n reproducibility. Photoresist
and P r o c e s s i n g
Effects
A l t h o u g h the p h o t o r e s i s t s employed were g e n e r i c a l l y s i m i l a r i n f o r m u l a t i o n composition, they e x h i b i t e d s i g n i f i c a n t l y different a d h e s i o n image " l i f t i n g r e s u l t s . The m a j o r i t y o f t h e r e s u l t s a r e f o r standard t r a c k a p p l i e d l i q u i d promoter p r o c e s s i n g . " L i f t i n g " was o b s e r v e d f o r f i r s t g e n e r a t i o n r e s i s t s , l i k e PC 129 a n d K T I I I , o n Si(>2 s u b s t r a t e s . H u n t 204 i m a g e s h a v e l i f t e d o n o x i d e s u r f a c e s , a n d i n one c a s e , n o t e v e n d o u b l e p r o m o t e r p r o c e s s i n g c o u l d s o l v e t h e problem. The r e p r e s e n t a t i v e s e c o n d g e n e r a t i o n r e s i s t tested, O F P R - 8 0 0 , d i d n o t s u f f e r f r o m " l i f t i n g " e v e n on b l a n k w a f e r s , a l t h o u g h image e d g e " l i f t i n g " o r p o o r image e d g e a c u i t y d i d o c c u r f o r images on c o n t r o l w a f e r s . When v a p o r p r i m i n g was u s e d o n t h e S t a r 2 0 0 0 , n o " l i f t i n g " o c c u r r e d f o r any o f the r e s i s t s on t h e " l i f t i n g - s u s c e p t i b l e " substrates tested. T h i s d r a m a t i c r e s u l t m u s t be a t t r i b u t e d t o t h e process of that system. The i n - s i t u d e h y d r a t i o n b a k e o f t h a t s y s t e m i s f a r s u p e r i o r t o t h a t o f o l d e r p r o c e s s i n g . T h e r e , t h e w a f e r was (1) d e h y d r a t i o n baked i n d r y - N c o n v e c t i o n o v e n s , (2) c o o l e d i n a i r , and (3) t r a c k a d h e s i o n promoted and r e s i s t c o a t e d i n fab a r e a a m b i e n t . O b v i o u s l y , t h e w a f e r s c o u l d be r e h y d r a t e d o r s u r f a c e c o n t a m i n a t e d i n the o l d e r p r o c e s s i n g scheme. The S t a r 2 0 0 0 t r e a t e d w a f e r s , c o u l d be s t o r e d i n a m b i e n t f o r 1-3 d a y s w i t h o u t image " l i f t i n g " o c c u r r i n g , t h e r e f o r e , these processes s t a b i l i z e d the wafer surfaces. 1 1
Conclusions S u p e r i o r r e s i s t a d h e s i o n or r e s i s t a n c e to r e s i s t image " l i f t i n g " h a s b e e n a c h i e v e d by ( 1 ) c o n v e n t i o n a l l i q u i d p h a s e d o u b l e c h e m i c a l t r e a t m e n t s , (2) s i n g l e c h e m i c a l l i q u i d phase t r e a t m e n t s , and (3) v a p o r p h a s e HMDS t r e a t m e n t s . These s u c c e s s f u l b u t d i f f e r e n t adhesion
260
POLYMERS FOR HIGH TECHNOLOGY
processes have one point of commonality. They all dehydrate, clean, and chemically stabilize or passivate surfaces that require photoresist adhesion. Furthermore, the observed improved adhesion must be the result of stronger forces than simple short range van der Waals forces. Electrostatic forces extending farther than the first monolayer at the interface must be hypothesized to be occurring for these systems as well. The wafers containing micron sized resist images with properly prepared surfaces resist lifting during extensive high pressure (>30 psi) nitrogen gas dry and water rinse cycles. Furthermore, if the only forces of attraction were van der Waals, lifting would not be observed for over promoted wafers, vapor or liquid phase treated, as is experimentally observed for wafers with excessive multiple promoter treatments or vapor treatment times. Of course, acid-base forces of attraction cannot be ruled out either (6). References 1. K.L. Mittal, Solid State Technology, 89 May (1979). 2. J.N. Helbert, R.Y. Robb, B.R. Svechovsky, and N.C. Saha, Proceedings of Symposium on Surface and Colloid Science in Computer Technology, 5th International Conference on Surface and Colloid Science, in press (1985). 3. T.A. Carlson, "Photoelectron and Auger Spectroscopy," Plenum Press, New York, 1975. 4. F.J. Grunther, P.J. Grunther, R.P. Vasquez, B.F. Lewis, and J. Maserjian, J. Vac. Sci. Technol., 16 (5), 1443 (1979). 5. J.N. Helbert and H.G. Hughes, in "Adhesion Aspects of Polymeric Coatings," K.L. Mittal, editor, Plenum Press, New York, 499 (1983). 6. H. Yanazawa, Colloid Surfaces, 9, 133 (1984). RECEIVED April 8, 1987
