Photobleaching Chemistry of Polymers Containing Anthracenes - ACS

Chapter 20

Downloaded by UCSF LIB CKM RSCS MGMT on September 3, 2014 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch020

Photobleaching Chemistry of Polymers Containing Anthracenes James R. Sheats Hewlett Packard Laboratories, Palo Alto, CA 94304

We describe a dye system that is particularly useful for resolution enhancement in optical lithography because of the possibility of complete separation of dye and resist exposures, and because of its utility in the deep UV spectral region. Substantial variations in reactivity occur in connection with the substituents of the anthracene nucleus, the type of polymer matrix, the presence of sensitizers, and binding of the anthracene to the polymer backbone. The best results are obtained with small alkyl substituents and an external sensitizer. Variations are also observed in deep UV behavior. Novel reactions occurring in the absence of oxygen are described; these are intensity and wavelength dependent and presumably involve highly excited states. It is probable that these reactions lead to the crosslinking of some anthracene-containing polymers.

Despite the steadily shrinking dimensions of VLSI circuits, which are now well into the submicron regime for advanced production, optical lithography continues to be the patterning method of choice (1). Although electron beam direct writing can produce extremely small features, it is unlikely that its throughput will allow it to be competitive in high volume production. The excellent intrinsic resolution of x-ray lithography must be considered along with the cost of synchrotron sources and the formidable problems of mask making. For these reasons optical methods will be vigorously pursued as long as they are viable. There are in general two aspects to resolution in optical lithography: the imaging system and the resist. Current activities involving shorter wavelength and higher numerical aperture (N.A.) imaging are described in ref 1. Single-layer resist materials have also been improved considerably in recent years (2); however it is not easy to satisfy all the requirements in a single material; thus multilayer 0097-6156/89/0412-0332$06.00/0 o 1989 American Chemical Society

In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Photobleaching Chemistry ofPolymers Containing Anthracenes 333

approaches, despite the disadvantages o f a greater number of processing steps, may play a role i n wringing the m a x i m u m possible performance f r o m a given exposure t o o l (3).


T w o general types of multilayer process may be distinguished, depending o n whether they use oxygen reactive i o n etching ( R I E ) (4) or o p t i c a l exposure (5-11) to transfer the pattern into the resist. O p t i c a l pattern transfer may suffer from some l i m i t a t i o n due to substrate reflections, but has a n advantage i n the simplicity of the equipment c o m p a r e d to R I E . T h i s paper w i l l describe the chemistry underlying one such process, w h i c h we have called P h o t o c h e m i c a l Image E n h a n c e m e n t (9,11) because it uses photobleaching to i m p r o v e the image incident o n the resist. It is related to, but distinct from, other photobleachable dye processes such as Contrast E n h a n c e d Lithography ( C E L ) (6) and B u i l t o n M a s k ( B O M ) (8,12). It involves no inherent throughput penalty (as does C E L ) , and the processing is simpler than the P o r t a b l e C o n f o r m a b l e M a s k ( P C M ) that has been successfully used i n p r o d u c t i o n (5); it is quite sub stantially simpler than R I E - b a s e d methods. A resolution o f 0.5 μ τ η w i t h between 1 a n d 2 μτη total depth of focus has b e e n demonstrated using 436 n m , 0.42 N . A . imaging (13); similar results were also obtained by Hargreaves, et a l . w i t h i-line exposure (14). >

A schematic o f P I E is given i n refs. 9 a n d 11. Briefly, a layer o f polymer containing a dye that is photobleachable by the imaging r a d i a t i o n is a p p l i e d over the resist. Image-wise exposure o f the dye creates a latent image o f dye concen tration (while not affecting the resist), and this image is transferred to the resist by a flood exposure at a wavelength that is strongly absorbed by the dye and to w h i c h the resist is sensitive; the dye is nonreactive during the flood exposure. T h e image quality is enhanced by the exponential dependence o f transmittance o n dye con centration arising f r o m the coupling o f Beer's l a w to the photobleaching reaction: regions o f high [dye] transmit m u c h less light than those w i t h l o w e r [dye]. M o r e extensive theoretical analysis is given i n refs. 9, 13, a n d 15. C E L similarly uses a dye over the resist, but the dye and resist are simultaneously exposed i n the imaging instrument. T h e o p t i m u m contrast enhancement is thus only transiently obtained. T h e chemistry involves the photooxidation o f 9,10-substituted anthracenes, w h i c h must be embedded i n a p o l y m e r of high oxygen permeability. Previous publications have described the p r e l i m i n a r y applications to lithography (13) and the issue o f oxygen permeability (15); the present report concentrates o n photo bleaching kinetics, sensitizer concentration effects, and deep U V exposure char acteristics. D a t a concerning the effect o f having the anthracene b o u n d to a hydrocarbon p o l y m e r c h a i n are presented and c o m p a r e d to observations o n the photochemistry o f anthracene/polymer mixtures under nitrogen, w i t h various wavelengths and sensitizers. EXPERIMENTAL T h e apparatus for recording photobleaching has b e e n described i n previous pub lications (10,15,16). Briefly, a n argon i o n laser (364,476,514nm, o r a l l visible lines) or a H e - C d laser (442 nm) w i t h greatly expanded b e a m i l l u m i n a t e d the sample (fused silica wafer) w h i l e its spectrum was recorded by a n H P 8 4 5 0 A spectrometer.


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Some sources o f error are discussed i n ref. 15; i n addition, it should be noted that these exposures were started w i t h a m a n u a l shutter w h i l e simultaneously pressing the "start" button o f the spectrometer, w h i c h leads to timing uncertainty of around +1Λ sec. 248 n m i r r a d i a t i o n was f r o m a L u m o n i c s excimer laser, w i t h a L a s e r P r e c i s i o n Rj-7200 energy meter for dosimetry. Pulse lengths are n o m i n a l l y 35 nsec (manufacturer's specification). Oxygen was excluded by a cell w i t h fused silica windows a n d V i t o n o-rings, and a flow of N 2 (from l i q . N 2 boil-off) at a few cc/sec. ( A few tests w i t h A r ([O2] ~ 0.4 p p m by Spectra-Gases assay) gave the same results.) D o s e measurement for the cw laser is estimated to be uncertain by as m u c h as + / - 1 0 - 2 0 % due to spatial positioning uncertainty, although the photodiodes were calibrated by ferrioxalate actinometry (17,18). T h e data i n Figures 4,5 a n d 7 were obtained w i t h the laser setup described i n ref. 16, w h i c h is m o r e accurate ( + / - 3 % ) . T h e exposure i n F i g u r e 6 used a n H g l a m p w i t h bandpass filter for 365 n m (10 n m F W H M ) ; dose accuracy is similar to the spectrophotometer system. T h e f i l m i n F i g u r e 8 was exposed i n the gas cell and then r e m o v e d for spectral analysis. T h e dose quoted for the all-lines exposure is somewhat less accurate than the others, because a n approximate average photodiode responsivity was used; thus probably ~ + /-25%. H y d r o c a r b o n polymers were purchased f r o m A l d r i c h C h e m i c a l C o . , and siloxanes f r o m Petrarch Systems. T h e k e t o c o u m a r i n K c 4 5 0 , f r o m K o d a k , is 3,3 -carbonylbis(7-diethylaminocoumarin). Diphenylanthracene ( D P A ) , dimethylanthracene ( D M A ) and eosin were purchased f r o m A l d r i c h ; l,2-bis(10(trimethylsiloxy)-9-anthryl)ethane ( D S A E ) was p r o v i d e d by Professor H . - D . B e c k e r o f the U n i v e r s i t y o f G o t e b o r g , Sweden. T h e three copolymers were p r o v i d e d by D r . J.S. Hargreaves o f H e w l e t t P a c k a r d C o . , w h o has published syn thetic procedures elsewhere (19). T h e y are abbreviated as follows: 1:2 P ( M A M M A : P M D S M A ) , 1:2 P ( V D P A : P M D S M A ) , and 1:2 P ( V P A : P M D S M A ) ; where MAMMA = (10-methyl-9-anthryl)methyl methacrylate; VDPA vinyldiphenylanthracene, or 9-(p-ethenylphenyl)-10-phenylanthracene; VPA = 9-vinylphenylanthracene and P M D S M A = 3-methacryloxypropylpentamethyldisiloxane. A l l chemicals were used as received; films were spun f r o m chlorobenzene solution. T h e films were i n general not b a k e d since b a k i n g caused no apparent difference i n photochemical behavior; the films i n Figures 4-7 received a 9 0 ° C , 30 m i n . o v e n bake. F u r t h e r details are given i n ref. 15. ,

BLEACHING KINETICS Figures 1-3 show deep U V transmittance (248 o r 260 n m ) for several different anthracene derivatives inpoly(phenylsilsesquioxane) ( P P S Q ) , w i t h and without an external triplet sensitizer (Kc450, or "Kc"). It is clear that there are substantial differences i n the bleaching contrast. B y "bleaching contrast" is meant the ratio R = [ ( D / D ) / E / E ) ] , where D refers to transmitted dose, Ε to incident dose, and the points 1 a n d 2 are two points relatively close to each other o n the curve (E /E - 1 . 1 ) . W e have argued elsewhere (15) that this is the most appropriate figure of merit w i t h w h i c h to evaluate pho tobleachable materials for image enhancement (including for C E L and B O M ) ; the larger R is, the better. R w i l l vary along the bleaching curve, and the greatest image resolution enhancement should be obtained i f the m a x i m u m o f R occurs approximately at the n o m i n a l line edge. C

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Photobleaching Chemistry of Polymers Containing Anthracenes 335


0

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I N C I D E N T E N E R G Y (mJ/cm2)

Figure 1. Transmittance Τ (260 nm) vs. incident energy Ε (364 or 442 nm) foi DMA/PPSQ films as indicated (film irradiated at 442 nm contains 0.4 wt.% Kc450)

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Figure 2. Τ (248 nm) vs. Ε (442 or 476 nm) for 10 wt.% DMA/PPSQ/0.4% Kc450 and 25% DPA/PPSQ/0.075% Kc450 as indicated.


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0.350 0. 315

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Figure 3. T vs. E for films with (a) 28% DSAE/PPSQ/4.8% Kc450, 140 μπι/cm , 442 nm, T , O D ° = 3 . 4 ; (b) 29.1% DSAE/PPSQ/0.146% Kc450, 160 μπι/cm , 442 nm, T « , OD°=2.84; and (c) 40% DPA/PDPS, 6.46 mW/cm , 364 nm, T , O D ° = 3 . 1 5 . The energy scale for (b) is 10x greater than shown, and for (c) it is 32X greater. 2

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INCIDENT E N E R G Y AT 364 nm (mJ/cm ) 2

Figure 4. Τ vs. Ε for 1:2 P(MAMMA:PMDSMA), 200 mW/cm , 364 nm. Solid line: calculation for second order kinetics.



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Photobleaching Chemistry of Polymers Containing Anthracenes 337

"0

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INCIDENT E N E R G Y AT 364 nm ( m J / c m ) 2

Figure 5. Τ vs. Ε for 1:2 P(VDPA:PMDSMA), 100 mW/cm , 364 nm. Solid line: calculation for second order kinetics.

too 90 \— CD 80 Ζ Ζ = 70

> 1, equation 1 reduces to that o f a simple first order reaction (such as C E L materials are usually assumed to follow (6)). If β Α < < 1, the reaction becomes second order i n A . I n a similar manner, the sensitized reaction varies between zero order and first order. F o r the anthracene loadings required b y t h e P I E process (13,15), A is close to 1 M , so β > >1 is required for first order unsensitized kinetics. A l t h o u g h i n solution, β for D M A is - 5 0 0 , and - 2 5 for D P A (20), we have found β =3 for D M A / Ρ Ε Μ Α , a n d β = 1 for D P A / P B M A Thus although the c h e m i c a l trends are i n the same direction i n the p o l y m e r as i n solution, the numbers are quite different, indicating a substantial


340


restrictive effect of the p o l y m e r o n the reaction. T h a t is, we can conclude that steric effects, o r "microviscosity" effects w i l l have a substantial effect o n the reaction kinetics. A l t h o u g h , as mentioned, the m o d e l does not quantitatively describe D P A / P E M A (which is similar to D P A / P P S Q ) , the discrepancy is not large, and the behavior is approximately second order; likewise that for D P A / P E M A (or D P A / P P S Q ) w i t h K c 4 5 0 is essentially first order (13). Such effects of free v o l u m e a n d microscopic m o b i l i t y have b e e n discussed some time ago b y Somersall, et. a l (21) and by others (22,23). T h e trends seen here, such as the greater R o f D M A relative to D P A , and the approximate equality o f D M A (unsensitized) to D P A (sensitized), are thus consistent w i t h expectation. D S A E has substantial absorbance at 442 n m (0.143 at 436 n m i n 0.68 μιτι thickness, 29.1 wt.%) (cf. ref. 15), so the l o w [Kc] case i n F i g u r e 3 c a n be considered as essentially unsensitized. It is noteworthy that D S A E behaves m o r e l i k e D P A than D M A , even though its substituents are aliphatic. I n solution (20), β is determined by electronic structure: disubstituted systems react more efficiently than monosubstituted ones, and aliphatic substituents m o r e than aro matic ones. I n polymers, however, the hindering effect of the p o l y m e r lends greater importance to steric characteristics. It seems plausible, then, that the greater bulk, or l o w e r conformational mobility, o f D S A E causes it to react less efficiently than D M A , even though b o t h are aliphatic. Its contrast is still better than D P A .


c

F i g u r e 3 shows that a second order system ( D P A / P D P S ) is essentially just a "linear image transfer agent"; at this initial absorbance it gives neither enhancement nor degradation o f contrast. T h i s places a lower l i m i t o n the k i n e t i c order that one can utilize i n a process such as P I E . It is worthwhile to consider, however, that such a linear system c o u l d still have value i n P I E (although it w o u l d be worthless for C E L ) . O n e might image w i t h the K r F laser stepper at 248 n m (where available resists have undesirably strong unbleachable absorbance (3)), and then transfer the pattern at 260-265 n m , where the anthracene absorbance is highest and where some potential resists have relatively lower absorbance (24,25). (Exposure of the resist during imaging w o u l d be avoided by adjusting its sensitivity relative to that o f the anthracene.) Figures 4 and 5 present the kinetic behavior of two copolymers of an anthracene-containing m o n o m e r (methacrylate or styryl) w i t h a siloxanecontaining methacrylate; the anthracene moiety is related to D M A and D P A respectively. A l t h o u g h the transmittance at 364 n m rather t h a n 248 o r 260 is recorded, c o m p a r i s o n to the data o f Figures 1-3 is made possible by the theoretical curves, w h i c h are the best possible fits for a second-order kinetic law (26). It can be seen that there is a major deviation; the bleaching proceeds m o r e slowly than second order, and gets progressively slower as the reaction proceeds. T h e deviation is m o r e p r o n o u n c e d for the phenyl-substituted case than for the methyl. It is virtually impossible to completely bleach either f i l m at 364 n m . T h i s is p a r a l l e l e d by a n insolubilization process (14,19), as shown i n F i g u r e 6. T h e appearance of fully insoluble m a t e r i a l is at approximately the same dose as that at w h i c h the deviation from expected kinetics (i.e., for D M A ) begins to be noticeable. T h e deviation can not be related to oxygen permeability since reciprocity failure due to insufficient oxygen is not observed for intensities 2.5x higher.


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Photobleaching Chemistry ofPolymers Containing Anthracenes 341


It might b e thought that the insolubilization is caused b y anthracene d i m e r i z a t i o n (27,28), although D P A has never b e e n observed to d i m e r i z e (29) due to steric hindrance f r o m the non-planar uhenyl rings. H o w e v e r , it was found that a f i l m o f P ( V P A : P M D S M A ) w i t h ~ l O ^ M K c 4 5 0 , extensively irradiated at 436 n m , also becomes insoluble. T h i s reaction is extremely unlikely to be dimerization, since it has b e e n w e l l verified i n solution phase studies that d i m e r i z a t i o n proceeds v i a the singlet state only (28-33); even for dianthrylethanes (where the excited triplet is h e l d i n close proximity to a ground state partner for its entire lifetime), d i m e r i zation is found only for the singlet except for some carbonyl-substituted species (30). A n o t h e r possible explanation is that singlet O 2 somehow leads to crosslinking. T h e reactions of O2 have b e e n extensively studied (34), a n d do not appear relevant to these copolymers. T h e only functionality that c o u l d conceivably react w i t h singlet O 2 is a v i n y l c h a i n termination, w h i c h c o u l d produce a hydroperoxide that might then participate i n crosslinking. H o w e v e r , i n a study of free radical p o l y m e r i z e d P M M A (35), the m a x i m u m fraction o f polymer chains w i t h v i n y l ends was found to be 0.36, for b u l k polymerized material; i n benzene solution the fraction was 0-3%. T h i s result, plus the fact that the insolubilization occurs immediately during photolysis at r o o m temperature, makes it very u n l i k e l y that such hydrop eroxides are involved. PHOTOREACTIONS U N D E R INERT GAS I n a n attempt to clarify these phenomena, mixtures of anthracenes a n d polymers were irradiated under N or A r , b o t h at 364 n m (direct excitation) and w i t h triplet sensitization, using the blue lines o f an A r laser as w e l l as 514 n m . 1:2 P ( M A M M A : P M D S M A ) was also irradiated at 364 n m (Figure 7). Some data for D M A / Ρ Ε Μ Α irradiated at 364 n m are given i n ref. 15, where it is shown that bleaching does occur i n the absence o f O2 (transmittance rises f r o m 0.63 to 0.80 w i t h ~ 5 J / c m , for a 13.6 w t . % film); the bleaching is similar to that o f the copo lymer. D P A also undergoes such bleaching, albeit w i t h substantially lower effi ciency. T h e effect can also be seen w i t h triplet sensitization: a D M A / Ρ Ε Μ Α f i l m w i t h eosin, irradiated w i t h a l l lines o f the A r laser, is slowly bleached (Figure 8). H o w e v e r , w h e n 514 n m alone is used, no reaction can be detected after 8 k J / c m (Figure 8). T h e excitation of eosin triplets is demonstrated by the fact that 514 n m i r r a d i a t i o n i n air very quickly (with a few h u n d r e d m J / c m ) bleaches the D M A . T h e triplet energy of eosin is high enough to be transferred to anthracene (eosin is 43 k c a l / m o l (36), a n d anthracene 42 (37)). T h e absence o f r e a c t i o n (under N ) at 514 n m demonstrates that eosin itself has no effect. Therefore the bleaching reaction must proceed through the anthracene triplet. T h e bleaching is accompa n i e d by a change i n polymer molecular weight. 2

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T h e most probable explanation o f these results is found u p o n examining the absorption spectrum of the first triplet state T o f anthracene, w h i c h is strong (e > 4 x l 0 1 / m o l cm) and m a x i m i z e d i n the blue ( - 4 2 5 - 450 n m , depending o n substituent) (38,39). Its absorbance at 514 n m is negligible. T h u s the unexpected bleaching very likely results from absorbance by Τχ to produce a highly excited triplet, f r o m w h i c h novel photochemistry may w e l l occur. ( B l u e light is present directly during sensitized irradiation; with 364 n m i r r a d i a t i o n the T excitation may be either by "trivial" (emission-absorption) energy transfer or by a Forster-Dexter x

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mechanism.) T h e nature of this reaction is u n k n o w n , but it appears to be a relatively specific, b i m o l e c u l a r reaction from a highly excited (hence short-lived) state; the state is b o u n d (39) and so simple dissociation (as seen elsewhere (40)) is r u l e d out. E n e r g y transfer f r o m T is k n o w n for a variety o f cases (41), a n d a rearrangement reaction f r o m a highly excited triplet i n the pleiadene family (possibly T7) was observed i n a 77K rigid glass (42). T h e latter reaction was observed only v i a direct two-photon excitation. B i m o l e c u l a r reactions of upper excited states are quite rare (41). I n the present case, where B i r k s (43) shows the state as ~ T , electron transfer or hydrogen a t o m abstraction by anthracene are plausible candidates for the pro cess; the formation o f radicals is likely. 2


7

A n anthracene radical adjacent to another anthracene c o u l d couple (44) to produce crosslinked m a t e r i a l i n the case o f the copolymers; the concentration i n the 1:2 copolymer is high enough for such juxtaposition to b e c o m m o n . T h e res onance stabilization of the anthryl radical should decrease the rate o f alternative decay pathways. A c c o r d i n g to the picture o f the photooxidation kinetics as described above, such crosslinking should further restrict the conformational m o b i l i t y o f the reacting anthracene rings and thus hinder the reaction (effectively reducing β ) . A t the 100-300 m J / c m dose at w h i c h crosslinking is found (Figure 6), F i g u r e 7 shows that a few % o f the anthracenes have b e e n bleached i n the non-oxidative reaction, w h i c h is enough to crosslink chains o f average molecular weight -5x1ο . H o w e v e r , 1 J / c m (365 n m ) does not cause crosslinking o f 1:2 P ( M A M M A : P M D S M A ) under N . -200 J / c m (+/-50%) at 355-375 n m under N produces thorough crosslinking (which is not reversed by b a k i n g at 105°C for 50 m i n . , a n d is therefore not dimerization (45)); the dose requirements for this reaction have not b e e n m o r e precisely determined. Thus, although 0 is not r e q u i r e d for crosslinking, i t greatly enhances it, possibly b y forming peroxide crosslinks. 2

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Hargreaves has suggested that the insolubilization o f some closely related polymers is due t o photolytic homolysis of the endoperoxide O - O b o n d a n d sub sequent generation o f carbon-centered radicals f r o m the Ο radicals (19). T h e r e are several facts that m a k e this a n extremely unlikely explanation for the data described here; these include the quantitative insufficiency o f the m a x i m u m amount o f endoperoxide reaction obtainable w i t h a few h u n d r e d m J / c m dose (homolysis quantum y i e l d 1) despite this loss o f contrast. 2


c

S i m i l a r results have b e e n reported for D M A / P E M A (14). F i g u r e 11 shows data for D S A E / P P S Q . A l t h o u g h the same effect is present, it is quantitatively different. T h e reduction i n Τ is about a factor o f 3 for a f i l m initially bleached to Τ = 3 2 % , w h i l e for D P A about the same reduction is seen w i t h T ( i n i t i a l ) = 8%; thus it is apparently less severe for D S A E . T h e most interesting result is that D M A / Ρ Ε Μ Α , w h e n irradiated under N at 260 n m ( + / - 8 n m F W H M bandwidth) by a n H g - X e lamp, shows absolutely no change i n transmittance w i t h a dose of 100 J / c m (15). Thus the antibleaching is an intensity dependent effect that is absent at l o w intensities a n d occurs only w i t h the excimer laser (typically - 0 . 1 - 1 m J / c m i n - 3 5 nsec). 2

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R e a l - t i m e excited state m o n i t o r i n g w i l l be required to understand the m e c h a n i s m of the antibleaching reaction. It is possible that the excimer laser pulse produces such h i g h concentrations o f singlet oxygen and triplet anthracene that b i m o l e c u l a r quenching processes greatly reduce the available concentrations (39). H o w e v e r , no data are available at this time to allow a quantitative analysis. F r o m the lithographic point o f view, the important result is that l o w intensity deep U V imaging sources such as the P e r k i n - E l m e r M i c r a s c a n 1 can be used without any process degradation by this effect. T h e effectiveness o f anthracene as a P I E or C E L material w i t h K r F excimer laser sources can only be determined by further experiment, although it appears likely that quite useful enhancement factors can be gotten even i n the presence o f the degradation shown i n the figures (cf. ref. 15). CONCLUSIONS T h e photobleaching kinetics o f anthracene photooxidation have been investigated. B l e a c h i n g characteristics satisfactory for P I E (with H g g-line and i-line as w e l l as 248-265 n m deep U V imaging) and C E L (deep U V only) are observed (as l o n g as the intensity does not exceed limits i m p o s e d by oxygen per meability o f the f i l m (15)). T h e kinetics depend o n the anthracene substitution; small, m o b i l e aliphatic substituents appear to be the best, although aromatics and larger aliphatic groups also w o r k w e l l with external sensitizers. Photobleaching under inert atmosphere (with m u c h lower efficiency than under oxygen) has also b e e n investigated, a n d ascribed to sequential m u l t i p h o t o n processes; it is hypoth esized that these processes are involved also i n producing the difficulties encoun tered w h e n anthracene is b o u n d to a methacrylate or methacrylate-co-styryl p o l y m e r chain. D e e p U V stability data are presented indicating that anthracene derivatives can be fruitfully applied to deep U V P I E or C E L w i t h l o w intensity (lamp) sources; further research is necessary to assess applicability w i t h K r F laser sources, although at least some degree of utility at m i n i m u m is probable.



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Photobleaching Chemistry ofPolymers Co

INCIDENT ENERGY (476nm), m J/cm

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Figure 9. Τ (365 nm) vs. Ε (476 nm) for 25% DPA/PPSQ/Kc450, for 3 different [Kc450]: (a) 3.12%, 4.59 mW/cm , (b) 2.15%, 5.88 mW/cm , and (c) 0.356%, 53.4 mW/cm . The energy scale is correct for (a); (b) and (c) have been scaled by the [Kc450], so the true maximum dose is 580 and 3506 mJ/cm , respectively. 2

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Figure 10. Τ (248 nm) vs. Ε (248 nm, K r F laser) under N , for 25% DPA/PPSQ/0.075% Kc450, previously exposed under 0 at 476 nm to the T shown. Excimer laser pulse rep rate 25 pps, delivered in bursts of 10 or 100. 2

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346


0. 350 0.315

DSAE/PPSQ ; N *g

0.280

§

0.245

LU Ο

0.210

^

0. 175

Ι

Ε

0. 140

Ι

Ο)

0.105

—

ce 0.070

—

0.035

—

2

Photobleaching Chemistry of Polymers Containing Anthracenes - ACS

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