Intensity Dependence in Polymer Photochemistry - ACS Symposium

Chapter 19

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Intensity Dependence i n Polymer Photochemistry James R. Sheats and John S. Hargreaves Hewlett-Packard Laboratories, Palo Alto, CA 94304

Intensity dependent bleaching of poly(methylmethacrylate) doped with acridine or diphenyl butadiene and an acridine-containing methacrylate copolymer, has been observed using excimer lasers (XeF, KrF). Threshold-like irreversible bleaching was seen with the copolymer; more gradual transient bleaching with the mixtures. The implications for submicron microlithography are discussed.

Despite frequent predictions of its imminent demise, optical lithography continues to be a prime contender in the current integrated circuit fabrication arena; there are now several plausible candidates for a viable half-micron optical lithography (1). There are two fronts on which advances are being made. The capabilities of imaging tools are being extended further into the ultraviolet, as a consequence of advances in lens design (2,3) and the introduction of excimer lasers as illumination sources (2). Second, novel image processing schemes have been introduced to enhance image quality beyond what is available from the lens (4-7). One way to increase resolution is by use of a photobleachable chemical system (applied as a thin polymer film on top of a conventional resist) whose response to light is strongly nonlinear (4-8). As the image from the lens traverses this medium, higher doses (or intensities) are attenuated to a lesser degree than lower doses (or intensities), and so the sharpness ("contrast") of the image is increased before it is incident on the resist. The ultimate limit of such an approach is a "threshold detector": a system resulting in full exposure of the resist receiving incident doses (or intensities) above some threshold value, and no exposure for incident doses (or intensities) below that value. Although in this manner one should obtain very high resolution (i.e., be able to print vertical-walled resist features of some small width), linewidth control may be inadequate: the features may not have the width they were intended to have (7). This is because the relative intensities at the nominal 0097-6156/87/0346-0224506.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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f e a t u r e edges f o r f e a t u r e s o f v a r i o u s s i z e s and w i t h v a r i o u s n e i g h bors a r e not t h e same, and so a s p e c i f i c t h r e s h o l d w i l l not c o r r e s pond to the l i n e edge f o r a l l f e a t u r e s (9). One p o s s i b l e way to deal w i t h t h i s problem i s t o b i a s the mask: depending on the s i z e o f the f e a t u r e and what i s next to i t , the mask d i m e n s i o n may be changed so t h a t the p r i n t e d f e a t u r e w i l l have the c o r r e c t s i z e . At f i r s t s i g h t t h i s would appear to i m p l y a p r o h i b i t i v e l y c o m p l i c a t e d i t e r a t i v e c a l c u l a t i o n , s i m i l a r to what has been d i s c u s s e d i n c o n n e c t i o n w i t h t h e e l e c t r o n beam p r o x i m i t y e f f e c t (10). However, the l a t e r a l d i f f r a c t i o n s p r e a d t h a t l e a d s t o the edge i n t e n s i t y v a r i a t i o n s i n o p t i c a l l i t h o g r a p h y i s r e l a t i v e l y s h o r t r a n g e d , and c o r r e c t i o n s f o r f e a t u r e s q u i t e c l o s e t o the d i f f r a c t i o n l i m i t may not r e q u i r e c o n s i d e r a t i o n o f more than the a d j a c e n t f e a t u r e , nor any i t e r a t i v e a d j u s t m e n t . Thus i t may be p o s s i b l e t o d e v i s e a mask b i a s i n g p r o t o c o l t h a t does not r e q u i r e an e l a b o r a t e c a l c u l a t i o n , and t h i s would make a t h r e s h o l d d e t e c t o r l i t h o g r a p h y system a p o t e n t i a l l y v i a b l e r o u t e to h i g h r e s o l u t i o n w i t h adequate linewidth control. R e s o l u t i o n may s t i l l be i n c r e a s e d by use o f a p h o t o b l e a c h a b l e dye t h a t i s n o t a t h r e s h o l d d e t e c t o r ( 4 , 6 , 7 ) . While the u l t i m a t e r e s o l u t i o n o f such a p r o c e s s may be l e s s than t h a t o f the t h r e s h o l d d e t e c t o r , c o n s i d e r a b l e g a i n s have been demonstrated ( 4 , 6 , 7 ) . For m o d e r a t e l y h i g h r e s o l u t i o n ( e . g . , 0.5 ym f e a t u r e s u s i n g a l e n s o f 0.42 n u m e r i c a l a p e r t u r e a t 365nm), computer s i m u l a t i o n s have shown t h a t l i n e w i d t h v a r i a t i o n s due to f e a t u r e - d e p e n d e n t image v a r i a t i o n s can be reduced to the l e v e l s r e q u i r e d i n m a n u f a c t u r i n g (+10% o f nominal)(7). T h i s p r e d i c t i o n i s made f o r a p h o t o c h e m i c a l system t h a t undergoes " l i n e a r " p h o t o b l e a c h i n g (the r a t e o f r e a c t i o n i s l i n e a r i n both i n t e n s i t y and c o n c e n t r a t i o n , though the t r a n s m i t t a n c e i s a n o n l i n e a r f u n c t i o n o f d o s e ) , b u t w i t h the i m p o r t a n t p r o p e r t y t h a t the r e s i s t i s u n a f f e c t e d by the dye e x p o s u r e , and the dye i s unchanged d u r i n g the r e s i s t exposure (through t h e dye l a t e n t i m a g e ) . The same s i m u l a t i o n model s u g g e s t s t h a t a p r o c e s s i n which t h e dye and r e s i s t p h o t o c h e m i s t r y o c c u r s i m u l t a n e o u s l y (4) w i l l have c o n s i d e r a b l y poorer l i n e w i d t h c o n t r o l . Thus i f a p h o t o b l e a c h i n g system t h a t i s not a t h r e s h o l d d e t e c t o r i s t o be u s e f u l , i t a p p e a r s t h a t i t s h o u l d p r o v i d e s e p a r a t i o n o f dye b l e a c h i n g and r e s i s t e x p o s u r e ; t h e r e s i s t s h o u l d see an unchanging image p r o f i l e o f the b e s t c o n t r a s t t h a t the dye system can p r o v i d e . Up t o now, most p r o p o s a l s f o r p h o t o b l e a c h i n g image enhancement have r e l i e d on l i n e a r p h o t o c h e m i s t r y , i n which the t r a n s m i t t a n c e i s a f u n c t i o n o n l y o f t o t a l d o s e , and not on the r a t e a t which t h a t dose i s d e l i v e r e d . The k i n e t i c s o f such l i n e a r p h o t o c h e m i s t r y a r e w e l l u n d e r s t o o d and have been d e s c r i b e d a n a l y t i c a l l y ( 2 8 ) . The exposure depends s o l e l y on a s i n g l e parameter which i s the p r o d u c t o f e x t i n c t i o n c o e f f i c i e n t , quantum y i e l d , i n t e n s i t y , and t i m e . No i n c r e a s e i n c o n t r a s t can be o b t a i n e d by changing e x t i n c t i o n c o e f f i c i e n t o r quantum y i e l d , s i n c e t h i s m e r e l y s c a l e s t h e d o s e . Cont r a s t can be i n c r e a s e d o n l y by i n c r e a s i n g the i n i t i a l o p t i c a l d e n s i t y , which i n c r e a s e s the dose r e q u i r e m e n t . Only w i t h n o n l i n e a r ( i n t e n s i t y dependent) p h o t o c h e m i s t r y can one o b t a i n s t e e p e r b l e a c h ing curves at a s p e c i f i e d o p t i c a l d e n s i t y . I n t e n s i t y dependent p h o t o c h e m i s t r y c o u l d be u s e f u l i n a t l e a s t two d i s t i n c t "modes". An o p t i c a l l y b i s t a b l e d e v i c e (11) i s an e s s e n t i a l l y p e r f e c t t h r e s h o l d d e t e c t o r f o r the i n c i d e n t l i g h t

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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i n t e n s i t y ; I n s t a b i l i t y w i t h p e r f e c t l y v e r t i c a l t r a n s i t i o n s has been o b s e r v e d i n a p h o t o c h e m i c a l system ( 1 2 ) . Thus i t s h o u l d be p o s s i b l e t o c o n s t r u c t an i n t e n s i t y dependent photochemical system t h a t would s e r v e as a t h r e s h o l d d e t e c t o r f o r use i n o p t i c a l l i t h o g r a p h y . Alt e r n a t i v e l y , i n t e n s i t y dependent mechanisms might be d e t e r m i n e d t h a t would p r o v i d e s t e e p e r b l e a c h i n g c u r v e s than o r d i n a r y l i n e a r p h o t o bleaching f o r a given i n i t i a l absorbance. The c l a s s i f i c a t i o n o f b l e a c h i n g b e h a v i o r as " t h r e s h o l d d e t e c t o r " or " n o n - t h r e s h o l d d e t e c t o r " i s o f c o u r s e somewhat a r b i t r a r y ; we s h a l l d e f i n e a t h r e s h o l d d e t e c t o r as a system whose t r a n s m i t t a n c e r i s e s from near i t s i n i t i a l v a l u e t o a v a l u e a t l e a s t ten times h i g h e r w i t h an i n c r e a s e o f i n t e n s i t y o r dose o f 10% o r l e s s . An example o f a p o t e n t i a l l y u s e f u l n o n l i n e a r system t h a t i s not a t h r e s h o l d d e t e c t o r i s t r a n s i e n t photobleaching, or a b s o r p t i o n s a t u r a t i o n (13). Regions o f a f i l m e x h i b i t i n g t r a n s i e n t photobleaching t h a t are subjected to high i n c i d e n t i n t e n s i t i e s w i l l q u i c k l y become b l e a c h e d , and w i l l then remain a t a c o n s t a n t t r a n s m i t t a n c e as l o n g as the i r r a d i a t i n g s o u r c e i s p r e s e n t . Regions t h a t e x p e r i e n c e low i n c i d e n t i n t e n s i t i e s w i l l remain a t a c o n s t a n t low transmittance. Thus the u n d e r l y i n g r e s i s t w i l l be s u b j e c t to a temporally unchanging image, u n l i k e the case i n which the b l e a c h a b l e f i l m i s i r r e v e r s i b l y b l e a c h e d w i t h i n t e n s i t y independent p h o t o chemistry (4). The f u l l c o n t r a s t o f the b l e a c h a b l e f i l m i s r e t a i n e d throughout the exposure o f t h e r e s i s t , and t h e r e i s no r e s t r i c t i o n t h a t the exposure o f the r e s i s t must r e q u i r e a s m a l l e r dose than t h a t which b l e a c h e s t h e d y e . Photochemical (or p h o t o p h y s i c a l ) r e a c t i o n s may be e i t h e r r e v e r s i b l e or i r r e v e r s i b l e . I f they a r e r e v e r s i b l e , the system r e t u r n s t o i t s i n i t i a l t r a n s m i t t a n c e as soon as the l i g h t i s t u r n e d o f f . An i r r e v e r s i b l e r e a c t i o n causes a permanent change i n the t r a n s m i t t a n c e . In p r i n c i p l e , t h e r e i s no c o n n e c t i o n w i t h whether a r e a c t i o n i s r e v e r s i b l e or not and whether i t i s a t h r e s h o l d d e t e c t o r o r n o t . In t h e e x p e r i m e n t s to be d e s c r i b e d , however, we have o b s e r v e d r e v e r s i b l e ( t r a n s i e n t ) b l e a c h i n g w i t h a smooth (but s t i l l s t e e p ) c h a r a c t e r i s t i c , and i r r e v e r s i b l e b l e a c h i n g ( i n a d i f f e r e n t p h y s i c a l system) with t h r e s h o l d - l i k e c h a r a c t e r i s t i c s . The i n t e n s i t i e s f o r the two c a s e s a r e q u i t e d i f f e r e n t , but both a r e i n the range such t h a t p u l s e d l a s e r s a r e needed; we have used excimer l a s e r s o p e r a t i n g a t 350nm (XeF) and 248nm ( K r F ) . Experimental A c r i d i n e and d i p h e n y l b u t a d i e n e were o b t a i n e d from A l d r i c h ; p o l y (methylmethacrylate)(PMMA) from KTI C h e m i c a l s (as a 9 wt.% s o l u t i o n in chlorobenzene). S o l u t i o n s f o r s p i n - c a s t i n g were p r e p a r e d as follows: 1 ml o f 1M a c r i d i n e ( i n 2 - e t h o x y e t h y l a c e t a t e ) i n 9 ml o f 9 wt.% PMMA (thus 16 wt.% a c r i d i n e i n f i l m ) ; 0.3 g d i p h e n y l b u t a d i e n e i n 7 ml o f 9% PMMA s o l u t i o n (thus 30 wt.% diphenylbutadiene) (because t h i s p r e p a r a t i o n was near i t s s o l u b i l i t y l i m i t , t h e r e i s c o n s i d e r a b l e u n c e r t a i n t y i n the c o m p o s i t i o n ) . F i l m s were p r e p a r e d by s p i n - c a s t i n g a t 3000 rpm f o r 30 s e c . on fused s i l i c a s u b s t r a t e s ( s i m i l a r d i m e n s i o n s t o c o n v e n t i o n a l 3 i n c h d i a m e t e r s i l i c o n w a f e r s ) : the f i l m s were not baked. Films of poly [2-(9- a c r i d y l ) e t h y l methacrylate-co-methyl methacrylate] (herein


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r e f e r r e d t o as "PAMA") were s i m i l a r l y p r e p a r e d and baked f o r 30 m i n . a t 100°C. T h i c k n e s s e s were e s t i m a t e d from a b s o r p t i o n s p e c t r a . The s y n t h e t i c r o u t e t o a c r i d i n e - c o n t a i n i n g polymers i s o u t l i n e d i n Scheme I. 9 - M e t h y l a c r i d i n e (II) was p r e p a r e d v i a 9 - e t h y l malonate a c r i d i n e by the method o f Campbell e t a l ( 1 4 ) , from 9 - c h l o r o a c r i d i n e ( I , Kodak), (mp. 117-8°C from l i g r o i n ) . 9-(3hydroxyethyl) a c r i d i n e (III) was s y n t h e s i z e d v i a the c o n d e n s a t i o n o f formaldehyde w i t h II (15) (mp. 154-6°C, from e t h a n o l - w a t e r ) . Methac r y l o y l c h l o r i d e , f r e s h l y d i s t i l l e d , was r e a c t e d w i t h III i n d r y THF w i t h p y r i d i n e as base to g i v e IV, 2 - ( 9 - a c r i d y l ) e t h y l m e t h a c r y l a t e , p u r i f i e d by chromatography w i t h s i l i c a g e l / t o l u e n e t o g i v e r e d gum (IR 5.85 ym e s t e r c a r b o n y l s t r e t c h ) . The monomer was p o l y m e r i z e d w i t h m e t h y l m e t h a c r y l a t e u s i n g AIBN in xylene. T y p i c a l l y , 5 g. of methylmethacrylate, 0.5 g. o f a c r i d i n e monomer, 0.05 g . o f AIBN were d i s s o l v e d i n 5 ml o f x y l e n e i n a polymerization tube. A f t e r 3 freeze-pump-thaw c y c l e s t o remove oxygen the tubes were s e a l e d and p l a c e d i n an oven a t 60°C o v e r night. The p l l y m e r s were then p r e c i p i t a t e d i n t o methanol s e v e r a l times and t h o r o u g h l y d r i e d under vacuum. The mole f r a c t i o n o f a c r i d i n e p r e s e n t i n the polymers was d e t e r m i n e d by UV s p e c t r o s c o p y u s i n g a c r i d i n e i t s e l f as a s t a n d a r d . The s p e c t r a o f one o f the polymers i n s o l u t i o n and i n a f i l m a r e shown i n F i g u r e s 1 and 2; the near-UV spectrum o f the polymer i n t h e f i l m i s v e r y s i m i l a r t o that in s o l u t i o n . Polymers w i t h a c r i d i n e mole r a t i o s ( r e l a t i v e t o methyl m e t h a c r y l a t e monomer) o f 0.023 (#1) and 0.070 (#2) were o b t a i n e d from feed r a t i o s o f 0.043 and 0.144 r e s p e c t i v e l y . Excimer l a s e r p h o t o l y s i s e x p e r i m e n t s were done w i t h two d i f f e r e n t setups. The f i r s t a p p a r a t u s was i d e n t i c a l t o t h a t used i n r e f e r e n c e 5. A Lambda-Physik l a s e r (nominal 10 nsec p u l s e s ) was t h e s o u r c e ; t h e beam, a f t e r p a s s i n g t h r o u g h a 12mm d i a m e t e r a p e r t u r e , was s p l i t by a f u s e d s i l i c a p l a t e t o p r o v i d e a r e f e r e n c e beam. The main beam was f o c u s e d by a 10cm f o c a l l e n g t h l e n s , and t h e sample was p l a c e d a f t e r the f o c u s . A 3.00mm d i a m e t e r a p e r t u r e was p l a c e d i m m e d i a t e l y i n f r o n t o f t h e sample w a f e r ; t h i s a p e r t u r e was a t t h e c e n t e r o f the beam. The i n t e n s i t y a t t h e sample was v a r i e d o v e r about a f a c t o r o f 50 by moving i t w i t h r e s p e c t t o t h e f o c u s (from 6 t o 45 cm); f o r s t i l l lower i n t e n s i t i e s t h e beam g o i n g i n t o t h e l e n s was o b t a i n e d from the r e f l e c t i o n from a f u s e d s i l i c a p l a t e . T r a n s m i t t a n c e was measured by U V - s e n s i t i v e p h o t o d i o d e s (EG&G) c o n n e c t e d to a s t o r a g e o s c i l l o s c o p e . I n c i d e n t energy was measured by a p h o t o d i o d e t o which a p o r t i o n o f t h e i n c i d e n t beam was d i r e c t e d , and t h i s p h o t o d i o d e was c a l i b r a t e d by a S c i e n t e c h c a l o r i m e t e r - t y p e power meter (which c o u l d be used r e l i a b l y o n l y i n the a v e r a g e - p o w e r mode due t o thermal d r i f t ) , which measured the energy t r a n s m i t t e d t h r o u g h the 3.00mm a p e r t u r e f o r a g i v e n p h o t o d i o d e s i g n a l . To measure t r a n s m i t t a n c e , the two p h o t o d i o d e s i g n a l s were measured both w i t h and w i t h o u t the sample i n p l a c e f o r each p o s i t i o n i n g o f t h e 3mm a p e r t u r e w i t h r e s p e c t t o t h e l e n s . The second s e t u p used a Lumonics l a s e r w i t h an e l e c t r o d e s e t t h a t y i e l d e d nominal 35 nsec p u l s e s . The beam t r a n s m i t t e d by a 98% r e f l e c t i n g d i e l e c t r i c m i r r o r p r o v i d e d the r e f e r e n c e ; both i n c i d e n t energy and t r a n s m i t t a n c e were measured w i t h a dual head L a s e r Precision p y r o e l e c t r i c joulemeter. The sample p o s i t i o n was moved w i t h r e s p e c t t o the f o c a l p l a n e o f a 475mm f o c a l l e n g t h l e n s (between l e n s and f o c u s ) f o r s m a l l v a r i a t i o n s i n i n t e n s i t y , and


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WAVELENGTH (nm) F i g u r e 1. UV spectrum o f PAMA #2, 0 . 1 8 5 g / l i t e r i n c h l o r o b e n z e n e (1 cm p a t h ) . X =360nm, absorbance 0 . 9 8 4 . e(monomer)=8.70xl03 1/mol cm. m

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d i e l e c t r i c f i l t e r s (CVI L a s e r C o r p . ) were used i n the main beam ( b e f o r e the l e n s ) f o r l a r g e r c h a n g e s . The same 3.00mm a p e r t u r e was p l a c e d d i r e c t l y i n f r o n t o f t h e sample as b e f o r e , t o o b t a i n a r e g i o n o f u n i f o r m and known energy d e n s i t y . I n t e n s i t y i s d e f i n e d here as t h e energy d e n s i t y ( f l u e n c e ) d i v i d e d by the nominal ( i . e . , m a n u f a c t u r e r ' s l i t e r a t u r e ) p u l s e length. I t s h o u l d be kept i n mind t h a t the temporal p u l s e shape i s not g a u s s i a n , and may d i f f e r from one l a s e r t o a n o t h e r ; the sample i s a c t u a l l y s u b j e c t t o a range o f i n t e n s i t i e s d u r i n g the p u l s e , and the peak i s g r e a t e r than the v a l u e on the g r a p h . Results Reversible (transient) Photobleaching. R e v e r s i b l e b l e a c h i n g was o b s e r v e d o n l y a t 350nm (XeF l a s e r ) . F i g u r e 3 shows the t r a n s m i t t a n c e o f s i n g l e 10 nsec p u l s e s through a f i l m o f acridine/PMMA (~1.2ym). Each p o i n t i s t h e a v e r a g e o f s e v e r a l ( u s u a l l y about 5) p u l s e s (but the same s p o t on the s a m p l e ) ; t h e v a r i a t i o n o f t h e averaged data i s not g r e a t e r than the p u l s e - t o - p u l s e v a r i a t i o n from the l a s e r ( t y p i c a l l y +10-15% o r l e s s ) , and t h e r e i s never any t r e n d from f i r s t t o l a s t p u l s e . T h u s , the b l e a c h i n g i s t r a n s i e n t o n l y (though see below f o r much l a r g e r d o s e s ) : by the time t h e next p u l s e i s d e l i v e r e d ( i n a sequence a t n o m i n a l l y c o n s t a n t i n c i d e n t i n t e n s i t y ) , the sample t r a n s m i t t a n c e has r e t u r n e d t o i t s original value. ( T h i s t i m e was t y p i c a l l y s e v e r a l s e c o n d s ; a more d e t a i l e d p r o b i n g o f the time dependence o f t h e decay o f b l e a c h i n g was not p o s s i b l e . ) The d a t a i n F i g u r e 3 were o b t a i n e d by u s i n g a new s p o t f o r each new i n c i d e n t i n t e n s i t y . U s i n g the same s p o t f o r more than one i n c i d e n t i n t e n s i t y gave t h e same r e s u l t s . However, a f t e r accumul a t i n g a l a r g e dose (always g r e a t e r than 100 m J / c m ) , i r r e v e r s i b l e changes were sometimes s e e n ; t h e e f f e c t was m o s t l y n o t i c e d f o r i n t e n s i t i e s o f 500 kW/cm o r more. These i r r e v e r s i b l e e f f e c t s were not q u a n t i t a t i v e l y i n v e s t i g a t e d . To be v e r y s a f e , t h e r e f o r e , each s p o t used f o r F i g u r e 3 was s u b j e c t t o no more than about 10 p u l s e s , a l t h o u g h i t was demonstrated i n one s e r i e s o f e x p o s u r e s t h a t 75 p u l s e s o f 100-200 kW/cm c o u l d be g i v e n t o a s i n g l e s p o t w i t h o u t o b s e r v a b l e damage. F i g u r e 4 shows s i m i l a r d a t a f o r d i p h e n y l b u t a d i e n e . Solubility l i m i t a t i o n s made i t d i f f i c u l t t o get a l a r g e o p t i c a l d e n s i t y , and s h o t - t o - s h o t f l u c t u a t i o n s i n the l a s e r o u t p u t were more s e v e r e f o r t h i s experiment. The same b l e a c h i n g e f f e c t i s c l e a r l y e v i d e n t , however, and i t o c c u r s a t s i g n i f i c a n t l y lower i n t e n s i t y and w i t h s u b s t a n t i a l l y less sharpness. I r r e v e r s i b l e damage was a g a i n e v i d e n t a t i n t e n s i t i e s h i g h e r than t h o s e shown i n F i g u r e 4 . 2

2

2

Irreversible Bleaching. I r r e v e r s i b l e b l e a c h i n g was o b s e r v e d a t both 350nm and 248nm, but t h e most i n t e r e s t i n g e f f e c t s from the l i t h o g r a p h i c v i e w p o i n t were o b t a i n e d a t 248nm. F i g u r e 5 shows t h e t r a n s m i t t a n c e o f KrF l a s e r p u l s e s (35nsec) vs i n t e g r a t e d d o s e , f o r v a r i o u s i n t e n s i t i e s i n c i d e n t on a sample o f PAMA #2 whose i n i t i a l t r a n s m i t t a n c e (based on t h e r a t i o o f e x t i n c t i o n c o e f f i c i e n t s a t 248 and 351nm measured i n a much t h i n n e r f i l m ) , was about 10-25 (~4.2 \im t h i c k ) . T h e r e a r e a c t u a l l y two " t h r e s h o l d s " e v i d e n t i n t h i s graph. For the h i g h e r i n t e n s i t i e s , the t r a n s m i t t a n c e f a r a g i v e n


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WAVELENGTH (nm) F i g u r e 2 . UV spectrum o f PAMA #2 f i l m on q u a r t z . Assuming t h e same e x t i n c t i o n c o e f f i c i e n t a t 360nm as i n c h l o r o b e n z e n e s o l u t i o n , and t h e d e n s i t y o f PMMA ( 1 . 2 g/cm.3, Polymer Handbook), t h e a c r i d i n e monomer u n i t c o n c e n t r a t i o n i s 0.73M, and t h e f i l m t h i c k n e s s 0.22 ym.

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s

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100 150 200 250 300 350 400 450 500

INTENSITY (kW/cm*) F i g u r e 3 . T r a n s m i t t a n c e o f s i n g l e XeF l a s e r p u l s e s ( l O n s e c nominal FWHM) v s . i n t e n s i t y (kW/cm ) f o r a c r i d i n e i n PMMA ( 1 . 1 M , 1.2ym t h i c k ) on q u a r t z . Each p o i n t i s from t h e average o f a p p r o x i m a t e l y 5 p u l s e s , d e l i v e r e d t o t h e same s p o t on t h e sample. 2


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Intensity Dependence in Polymer Photochemistry - ACS Symposium

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