4 Compositional Analysis of a Terpolymer Photoresist by Raman Spectroscopy F . J . P U R C E L L and E . R U S S A V A G E Spex Industries, Inc., Edison, N J 08820
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E . R E I C H M A N I S and C . W. W I L K I N S , J R . Bell Laboratories, Murray Hill, N J 07974
The trend towards miniaturization in microstructure fabri cation has created a demand for improved methods of production. The preceding paper (1) detailed one of the areas of research in this area, development of a deep UV-degradable photo-resist and presented a likely candidate, poly (methyl methacrylate-co-3oximino-2-butanone methacrylate-co-methacrylonitrile) (P(M-OM-CN)).
While a l l of the results to date are encouraging, the exact com position of the terpolymer samples tested has been unknown. That information should be obtained in order to take full advan tage of this resist. These polymers are especially resistant to the standard methods of analysis. Elemental analysis can be plagued by inaccuracies that arise from difficulties in deter mining low percentages of nitrogen and from residual solvent or monomers present in the polymers. UV spectrophotometry is useless because only the 3-oximino-2-butanone moiety yields a distinct UV spectrum. Pmr (proton magnetic resonance) spec troscopy has problems with overlapping absorptions. Only methyl methacrylate and the α-keto-oxime methacrylate have distinguisha ble resonance peaks, and only the methyl methacrylate can be in tegrated accurately. Thus, pmr can give no information about the methacrylonitrile and merely a rough estimate of the ratio for the other two. A possible answer to the problem may be found in C-13 nmr. However, this technique is rather time 0097-6156/82/0184-0045$05.00/0 © 1982 American Chemical Society Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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consuming due t o t h e i n h e r e n t l y weak carbonyl and n i t r i l e s i g n a l s caused by t h e i r long r e l a x a t i o n times. Infrared spectroscopy has problems w i t h overlapping as w e l l as weak absorptions. F o r t u n a t e l y , each homopolymer does have a d i s t i n c t Raman-active band i n a d d i t i o n t o a band common t o a l l components which can serve as an i n t e r n a l standard. Thus, Raman spectroscopy provides a simple, nondestructive, and absolute method f o r the determination o f composition o f P(M-OM-CN). Before s t a r t i n g t h e main p o r t i o n o f the d i s c u s s i o n , the Raman e f f e c t (2^,3.) w i l l be b r i e f l y described. "When monochromatic r a d i a t i o n o f frequency v impinges on a sample, a small p o r t i o n o f the l i g h t i s s c a t t e r e d . Most o f t h i s i s s c a t t e r e d e l a s t i c a l l y ; t h a t i s , i t has t h e same frequency as t h e i n c i d e n t l i g h t and i s known as Rayleigh s c a t t e r i n g . A much smaller percentage i s a l s o s c a t t e r e d i n e l a s t i c a l l y with a frequency equal to = vo + Av , where A v i s a Raman s h i f t or Raman frequency. For molecular systems, Av^ i s g e n e r a l l y a s s o c i a t e d with r o t a t i o n a l , v i b r a t i o n a l o r e l e c t r o n i c t r a n s i t i o n s though only v i b r a t i o n a l modes w i l l be discussed i n t h i s paper. T r a n s i t i o n s from t h e ground s t a t e t o a v i b r a t i o n a l l y e x c i t e d s t a t e are c a l l e d Stokes l i n e s and those o r i g i n a t i n g i n an e x c i t e d s t a t e are anti-Stokes bands. This i s shown g r a p h i c a l l y i n F i g u r e 1. Normally, only the Stokes spectrum i s recorded because t h e i n t e n s i t y o f t h e anti-Stokes spectrum i s dependent on the populat i o n o f t h e e x c i t e d s t a t e s which f o l l o w s t h e normal Boltzman d i s t r i b u t i o n . Thus, at room temperature, except f o r bands very c l o s e t o the e x c i t i n g l i n e corresponding t o the l o w e s t - l y i n g e x c i t e d s t a t e s , the i n t e n s i t y o f anti-Stokes l i n e s i s g r e a t l y reduced. The i n t e n s i t y o f Rayleigh s c a t t e r i n g i s on the order o f 10" times t h e i n t e n s i t y o f t h e i n c i d e n t l i g h t , and the Raman i n t e n s i t i e s are at l e a s t 1 0 " l e s s than t h a t of the Rayleigh s c a t t e r . Thus, t h e Raman e f f e c t i s obviously a weak phenomenon which r e q u i r e s a high i n t e n s i t y monochromatic e x c i t a t i o n source (a l a s e r ) and a high d i s p e r s i o n spectrometer w i t h e x c e l l e n t s t r a y - l i g h t c h a r a c t e r i s t i c s t o observe i t . The b a s i c mechanism o f the Raman e f f e c t i s energy t r a n s f e r . An i n c i d e n t photon perturbs t h e molecule e i t h e r g i v i n g up energy or accepting energy from t h e molecule. Quantum mechanic a l l y , the i n c i d e n t photon i s a n n i h i l a t e d and a new photon o f lower or greater energy i s created a f t e r i n t e r a c t i o n with t h e molecule. Concomittant with t h i s process i s t h e c r e a t i o n o r d e s t r u c t i o n o f a quantum o f v i b r a t i o n a l energy. The s e l e c t i o n r u l e f o r Raman spectroscopy r e q u i r e s a change i n the induced d i p o l e moment or p o l a r i z a b i l i t y o f the molecule, and so i t i s a complementary technique t o i n f r a r e d which r e quires a change i n t h e permanent d i p o l e moment. F o r molecules having a center o f i n v e r s i o n , a l l Raman-active bands are i n f r a r e d i n a c t i v e and v i c e v e r s a . As t h e symmetry o f the molecule i s lowered, the coincidences between Raman-active and i n f r a r e d -
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Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
PURCELL E T AL.
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V«4 V«3 V »2
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V»0
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V-0 Laser Figure 1.
Stokes
Rayleigh
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anti-Stokes
Energy level diagram depicting vibrational Stokes, Rayleigh, and antiStokes transitions.
American Chemical Society Library 1155 16th St. N. w. Washington, 0. C. 20036 Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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a c t i v e bands i n c r e a s e . However, because o f t h e d i f f e r e n t s e l e c t i o n r u l e s , v a r i o u s f u n c t i o n a l groups have d i f f e r i n g i n t e n s i t i e s i n t h e two techniques. For example, water which i s a strong absorber i n the i r i s a very weak Raman s c a t t e r e r . In g e n e r a l , one can expect good Raman s i g n a l s from l a r g e , deformaable groups such as S, I ~ , unsaturated groups, C = 0, and C = N. Some o f the information a v a i l a b l e from Raman spectroscopy i s q u a l i t a t i v e ; that i s , i t t e l l s which f u n c t i o n a l groups are present as determined from t h e c h a r a c t e r i s t i c group frequencies. I t may a l s o be q u a n t i t a t i v e , supplying information as t o t h e amount o f a p a r t i c u l a r substance present i n a sample. T h i s may be on an absolute b a s i s by means o f an i n t e r n a l standard as i n the present case, or more normally, with the help o f c a l i b r a t i o n curves. The information may a l s o be s t r u c t u r a l . A g r o u p - t h e o r e t i c a l a n a l y s i s o f t h e data can provide a breakdown of the number o f v i b r a t i o n s according t o symmetry groups f o r various p o s s i b l e geometries o f the molecule which, when compared to t h e experimental data, may e l i m i n a t e a l l but one o f the p o s s i b l e geometries. Chain l e n g t h can be c a l c u l a t e d from t h e f r e quency o f the l o n g i t u d i n a l a c o u s t i c mode i n polymer l a m e l l a e . A d d i t i o n a l l y , conformational information o f l a r g e molecules may be obtained. Since the l a t e 1960 s a few papers have demonstrated comp o s i t i o n a l a n a l y s i s o f v a r i o u s polymer systems by Raman s p e c t r o s copy. For example, Boerio and Yuann (k) developed a method o f a n a l y s i s f o r copolymers o f g l y c i d y l methacrylate w i t h methyl methacrylate and styrene. Sloane and Bramston-Cook (5) a n a l yzed t h e terpolymer system poly(methyl methacrylate-cobutadiene-co-styrene). The composition o f copolymers o f styrene-ethylene dimethacrylate and styrene-divinylbenzene was determined by Stokr ert a l ( 6 ) . F i n a l l y , Water (7) demonstrated that Raman spectroscopy c o u l d determine t h e amount o f r e s i d u a l monomer i n poly (methyl methacrylate) t o the 1% l e v e l . This was subsequently lowered t o l e s s than 0.1$ ( 8 ) . In s p i t e o f i t s many advantages, the p o t e n t i a l o f Raman spectroscopy f o r the a n a l y s i s o f polymer systems has never been f u l l y e x p l o i t e d . f
Experimental A l l Raman s p e c t r a were recorded from samples i n c a p i l l a r y tubes w i t h a SPEX RAMALOG Raman system c o n s i s t i n g o f a Model ll*03 Double Monochromator, a Model 1^59 I l l u m i n a t o r , and a lU60 LASERMATE tunable f i l t e r . The 5lh.5 nm l i n e o f a Spectra-Physics Model 16U-08 argon-ion l a s e r s u p p l i e d 0 . l 6 t o 0.2 W o f power at t h e sample. The d e t e c t i o n system c o n s i s t e d o f a cooled RCA 03103*+ GaAs phot omul t i p l i e r tube and a SPEX DPC2 d i g i t a l photon-counting u n i t . The spectrometer was cont r o l l e d , and a l l data manipulations were performed by t h e SPEX SC32 SCAMP microprocessor data system. The s p e c t r a f o r the
Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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compositional a n a l y s i s were run w i t h a k cm" s p e c t r a l bandpass, and an i n t e g r a t i o n time o f 10 seconds (to optimize the S/N ratio). Only the peaks o f i n t e r e s t were scanned i n order t o reduce the time o f a n a l y s i s t o about 30 minutes per sample. 1
Results and D i s c u s s i o n Figure 2 shows survey Raman s p e c t r a o f the homopolymers, poly(methyl methacrylate) (PMMA), poly(3-oximino-2-butannone methacrylate)(POM), and p o l y ( m e t h a c r y l o n i t r i l e ) ( P M A N ) , and one terpolymer (P(M-OM-CN)) with a S/N r a t i o o f about 1 0 : 1 . Each of the polymers has a band s p e c i f i c t o t h a t polymer: 812 Acm-1 ( v (C-O-C) f o r PMMA), 1 6 2 2 Acm-1 (v (C=N) f o r POM), and 2237 Acm-1(v (C=N) f o r PMAN). A d d i t i o n a l l y , there i s an asymmetric C-H bending mode a t 1^53 Acm"l, common t o a l l three homopolymers, which serves as an i n t e r n a l standard. These bands are i n d i c a t e d by arrows i n F i g u r e 2 . A broad f l u o r e s c e n c e background i s evident, but i t can be reduced t o acceptable l e v e l s by exposure t o h i g h l a s e r power f o r 10-30 minutes, depending on the sample. R e s i d u a l background f l u o r e s c e n c e may be due t o t h e oximino chromophore i t s e l f . Figure 3 d e p i c t s an example o f a c t u a l data f o r a 7 5 : 1 5 : 1 0 terpolymer with a S/N r a t i o o f about 5 0 : 1 . s
s
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s
The compositional a n a l y s i s was performed by f i r s t n o r m a l i z i n g the s p e c t r a with r e s p e c t t o t h e i n t e g r a t e d i n t e n s i t i e s o f the i n t e r n a l standards o f POM and PMAN t o t h a t found f o r PMMA. Once normalized, s c a l i n g f a c t o r s f o r the three components were e s t a b l i s h e d by t a k i n g t h e r a t i o o f normalized i n t e n s i t i e s o f the 1622 (POM) and 2237 (PMAN) Acm" bands t o t h a t o f t h e 812 (PMMA) Acm"l band. The i n t e g r a t e d areas o f t h e s c a l e d i n d i c a t o r bands f o r a terpolymer were summed t o give t h e t o t a l area f o r a l l three components. The weight percent o f each component was then obtained by d i v i d i n g t h e area o f t h e i n d i c a t o r band f o r each polymer by t h e t o t a l area. The same procedure was a p p l i e d t o the copolymers. Table 1 gives a comparison o f Raman and pmr r e s u l t s f o r a s e r i e s o f copolymers. In the pmr data o f F i g u r e U, t h e CH absorption o f t h e polymer backbone a t
