Compositional Analysis of a Terpolymer Photoresist by Raman

scatter. Thus, the Raman effect is obviously a weak phenomenon ... able groups such as S, I~, unsaturated groups, C = 0, and C = N. Some of the .... M...
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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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M

M

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3

Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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V«4 V«3 V »2

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E

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V»0

V-4

V» 3 V« 2 V» I

V-0 Laser Figure 1.

Stokes

Rayleigh

G

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