Photochemical and Photophysical Studies on Chemically Amplified Resists

Journal of Photopolymer Science and Technology Volume 5, Number 1(1992) 35 - 46

PHOTOCHEMICAL AND PHOTOPHYSICAL STUDIES ON CHEMICALLY AMPLIFIED RESISTS

NIGEL P. HACKER, DONALD C. HOFER and KEVIN M. WELSH IBM Research Division, Almaden Research Center, San Jose, California 95120-6099, U.S.A.

Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, S-

and 4-phenylthiobiphenyl isomers by an in-cage reaction, whereas diphenylsulfide and other aromatic photoproducts are formed by cage-escape reactions with solvent. The cage and escape reactions both generate acid, which is the primary initiator for many chemically amplified resists. Photo-CIDNP has been used to characterize the cage and escape reactivity of radical intermediates formed from

photolysis of triphenylsulfonium salts. In addition nanosecond laser flash photolysis studies have found key intermediates in the direct and triplet sensitized photolysis of these salts. The photophysics of a number of aromatic polymers was examined to understand how the polymer participates in the photoinitiation

process. The polymer fluorescence was quenched by sulfonium salts in solution by a dynamic mechanism, whereas in polymer films the quenching was by a static mechanism. Fluorescence lifetimes for the polymers, estimated from the

quenching plots in solution, were relatively short, 4-S nsec and the values agreed well with those obtained by time-resolved spectroscopy.

1. Introduction

A large number of chemically amplified resists have been designed around systems containing phenolic or substituted phenolic resins and a photo-acid generator. Among the resins that have been used are poly(vinylphenol), substituted poly(vinylphenols), e.g. poly[4-[ (tert-butoxycarb- onyl)oxy] styrene] (poly-TBOC), Novolac, 'NovoBOC', copolymers or blends of these mater- ials. l' 2 Among the photo-acid initiators used are the 'onium' salts, particularly the triarylsulfonium salts, N-imidoyl esters, pyrogallol esters, nitrobenzyl esters, x-sulfonyloxyketones and

Received May 11, 1992 Accepted June 11, 1992 35 J. Photopolym. Sci. Technol., Vol. 5, No. 1, 1992

tris-(trichloromethyl)triazines,l, 3-7 Mechanistic studies on these systems have been quite limited, although more recently the importance of understanding the performance chemically amplified resists has led to more detailed studies by a number of groups. We have been interested in the photochemistry of onium salts and from analysis of the photoproducts for "in-cage versus cage-escape" reactivity, many new reaction pathways for the photodecomposition of these photo- acid initiators have been discovered (Scheme I). For example, triphenylsulfonium salts can undergo heterolysis, homolysis, triplet energy transfer and electron transfer under different photolysis conditions which affects both the "cage : escape" ratios and also the nature of the cage and escape reaction products.

Recent studies on chemically amplified resists have found that not only direct photolysis of the photo-acid initiator is important, but also that initiation from the excited state of the polymer resin can occur. For example, photolysis of the acid generator 1,2,3-tris(methanesulfonyloxy) benzene (TMSB) in polymer matrices gave a quantum yield of 20. 4 However because TMSB absorbed less than 1% of the incident light, it was concluded that the excited state of the polymer initiated the formation of acid from PM SB and this gave a more reasonable quantum yield of about 0.2. For triphenylsulfonium salts in substituted poly(styrene) resins, the photo-acid initiator absorbs about 50 % of the incident light at 5 wt % loading and the quantum yield for acid formation does not reveal whether the excited state of the polymer is involved in the generation of acid. However "cage : escape" ratios for the sulfide photoproducts reveal that a dual photoinitiation process occurs in which both the excited state of the polymer and the photoinitiator can generate acid.

The above studies reveal that the photochemistry and photophysics of both the photoinitiator and the polymer play important roles in acid generation. I n addition, while the analysis for sulfide photoproducts, in addition to acid, has proven useful in the elucidation of photochemical mechanisms, there has been no direct observation of the proposed intermediates by photophysical techniques. In this paper we will discuss some of our recent studies on triarylsulfonium salt photochemistry using photo-CIDNP (chemically induced dynamic nuclear polarization) and nanosecond laser flash photolysis techniques. Also the photophysical properties of some substituted phenolic polymers will be described to determine how the polymer excited state participates in the generation of acid.

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Scheme I. Product formation from direct and triplet sensitized photolysis of triphenylsulfonium salts.

2. Photo-CIDNP Studies on Triarylsulfonium Salts 9

Nuclear Magnetic Resonance (NMR) spectra are recorded as absorption spectra of thermally equilibrated spin populations in the lowest Zeeman levels. However radicals can be generated where the upper or lower Zeeman levels have nonequilibrium spin populations and this can result in an enhanced NMR absorption or an NMR emission. This technique, Chemically Induced Dynamic Nuclear Polarization (CIDNP), is a powerful tool for determining the multiplicity and the cage (or escape) reactivity of radical pair intermediates. 10 Photo-CIDNP is an NM R technique where UV light is directed to the sample in the spectrometer cavity and generates radical intermediates. The observation of CIDNP effects requires the involvement of free radical pairs even though polarized signals may not be observed for both radicals. Kaptein developed a formalism for the net polarization (FN) of NM R signals for radical intermediates:

TN = µ*i*eg*ai ...... (1)

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Figure I. Photo-CIDNP spectra from direct Figure II. Photo-CIDNP spectra from triplet photolysis of triphenylsulfonium salt in CD3CN. sensitized photolysis of triphenylsulfonium salt in Spectra collected (A) before and (B) during (CD3)2C = 0. Spectra collected (A) before, (B) photolysis. during and (C) after photolysis. where ii is negative (-) for a singlet radical pair precursor and positive ( + ) for a triplet radical pair, a is positive for geminate (cage) products and negative for scavenging (escape) products, L1gis the sign of (gi - g) where gi is the g-factor of the radical containing the observed nucleus and g is the g-factor of the other radical in the pair and ai is the sign of the hyperfine splitting constant for the observed nucleus. t t The g-factors and ai are can be measured experimentally and thus the net polarization observed for a particular NM R signal depends on multiplicity of the radical and whether the radical is an in-cage or cage-escape intermediate.

Figures I and II show the photo-CIDNP spectra collected from photolysis of triphenylsulfonium triflate in CD3CN and (CD3)2C = 4 respectively. Figure I shows the NM R spectra obtained before and during (inset) photolysis. The only signals detected are a very weak emission for benzene and a weak signal for diphenylsulfide. The latter signal remains in the spectrum recorded after photolysis and is therefore a photoproduct, not an enhanced absorption. The emissive polarization for benzene implies the intermediacy of phenyl radical which escapes from the precursor radical pair of intermediates and reacts with solvent to form benzene-d1 . Unfortunately polarizations for phenylthiobiphenyl or diphenylsulfide were not detected. These are products from reactions of the other expected intermediate, diphenylsulf nyl radical cation, with phenyl radical (cage reaction) or solvent (escape reaction). However the observation of CIDNP can

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Figure III. Photo-CIDNP spectra from anthracene sensitized photolysis of triphenylsulfonium salts in CD3CN. Spectra collected (A) before, (B) during and (C) after photolysis. only occur if a radical pair of intermediates is formed and thus the detection of an emission for benzene-di necessarily implies formation the corresponding diphenylsulfinyl radical cation intermediate. Thus for the benzene emission: FN is negative, E is negative as benzene is an escape product, Og is negative (2.0024-2.0074) and a; is positive, 17.4 G. Substituting the above values into Equation 1 gives a negative value for µ, i.e. the benzene emission arises from a singlet radical pair. From photoproduct studies we found the in-cage recombination was the major reaction pathway and that cage-escape reactions were a minor process for direct photolysis of sulfonium salts in acetonitrile (Scheme I). 12 Furthermore evidence was found for both heterolytic (phenyl cation) and homolytic (phenyl radical) photoproducts. It was concluded that the phenyl radical intermediates were formed by an electron transfer reaction from the phenyl cation intermediates generated from the singlet excited state of the salt:

Thus the weak emission for benzene-di in the photo-CIDNP experiments arises from the above in-cage reaction.

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In contrast Figure IIB shows a strong enhanced absorption for benzene-d1 in the photo-CIDNP collected during photolysis of triphenylsulfonium triflate in (CD3)2C = 0. The values for E, eg and al are the same as above and therefore the change in sign for rN from negative to positive must be because µ has also changed sign. Thus as t is positive, the benzene-d1 is formed via phenyl radical from a triplet radical pair precursor. This agrees well with previous photoproduct studies which found that acetone (ET 80 kcal mole1) photosensitized the decomposition of triphenylsulfonium triflate (ET 74 kcal mol&1) to give only the escape products Biphenyl sulfide and benzene via the triplet diphenylsulfinyl radical cation - phenyl radical pair (Scheme I). 12'13 As with the direct photolysis, no polarizations were detected from diphenylsulfide although diphenylsulfinyl radical cation is implicated as an intermediate because CIDNP can only be observed from radical pairs.

Photo-CIDNP spectra were obtained from photolysis of triphenylsulfonium triflate in the presence of polycyclic aromatic hydrocarbons (PAl-I). PAH's absorb strongly in the UV and are good excited state electron donors. It is known that photolysis of a PAH in the presence of an onium salt initiates an electron transfer reaction to give triphenylsulfur radical which rapidly decomposes to phenyl radical and diphenylsulfide. Acid is generated by phenylation of the PAl-I + ' by phenyl radical, a cage reaction or by reaction of PAH+' with solvent, an escape reaction. 14

[PAII]*-+ Phi S + X------> PAH +' Phi S' X------> PAH +' Ph2S Ph' X------> PAH-Ph + Ph2S + PhH + I I + X-

Figure I I I B shows the spectrum collected during photolysis of anthracene in the presence of triphenylsulfonium triflate. In this spectrum all the anthracene resonances are broadened and there is a strong emission for benzene-d1, an escape photoproduct. Kaptein's formalism predicts an emission for benzene if it is formed from a singlet radical pair, implicating an electron transfer reaction from the singlet excited state of anthracene.

3. Nanosecond Laser Flash Photolysis of Triarylsulfonium Salts 9

Flash photolysis of triphenylsulfonium triflate in acetonitrile gave a broad relatively long-lived transient which absorbed at Amax = 465 nm (Figure IV). The lifetime of this transient depended on the polarity of the solvent and changed from 11 ,usec in acetonitrile to 32 ,cosecin ethanol. This transient was not affected by the addition of cumene, a radical scavenger. From these observations it was concluded that the 465 nm transient was a cation and not a radical. There

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Figure W. Absorption spectrum of transient detected from direct photolysis triphenyis ulfomum salts in

CH3CN. are three transient cationic species which could potentially form during photodecomposition of triphenylsulfonium salts: phenyl cation, protonated phenylthiobiphenyl and protonated diphenylsulfide. Phenyl cation is should absorb below 300 nm and is a very reactive species which should only be detectable on the picosecond time-scale in solution. Similarly protonated diphenylsulfide should also absorb below 300 nm, c.f. triphenylsulfonium cation, Amax = 250 nm. The most likely intermediate for the 465 nm transient is protonated phenylthiobiphenyl which has an extended delocalized it-system and is the direct precursor for the major phatoproduct, 2-phenylthiobiphenyl.

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Figure V. Absorption spectrum of transient detected from triplet sensitized photolysis triphenylsulfonium salts in (C113)2C = 0.

Photolysis of triphenylsulfonium triflate in acetone gives two long-lived transient absorptions at 340 nm and 750 nm (Figure V). The lifetimes for both absorptions were approximately 20 ~csec and are assigned to diphenylsulfinyl radical cation. Similar absorptions have been reported for diphenylsulfinyl radical cation from photolysis of diphenylsulfide in the presence of a phenanthrolium salt. Phenyl radical, Amax = 240 nm, t 5 was not detected as the flash photolysis equipment lacks sensitivity for absorptions less 300 nm. These results also agree well with the photoproduct and photo-CIDNP studies. The photoproduct studies indicate that acetone sensi- tizes the triplet excited state of the opium salt and gives radical intermediates which exhibit only escape reactivity. The photo-CIDNP spectrum in acetone-d6 shows an enhanced absorption for benzene-d1 , also implicating an escape reaction from a radical pair. Finally transient absorbances due to diphenylsulfinyl radical cation, indicating a radical reaction, are detected in the flash photolysis experiments using acetone solvent.

4. Fluorescence Spectroscopy of Poly(4-oxystyrene) Derivatives 16

We have recently shown that the photoinitiation process in the poly(TBOC) resist occurs partly by direct photodecomposition of the triphenylsulfonium salt excited state and partially by electron transfer from the excited state of the polymer.8 There are important consequences due to this

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Table I. Photophysical parameters for poly-HOST, poly-MOST, poly-TBOC and Novolac.

'dual photoinitiation' process , not only are the absorbance properties and quantum yield of the photoinitiator important, but also the photophysical properties of the polymer need to be optimized to improve resist performance. The polymer needs to efficiently absorb the incident photon, be a good excited state donor, have a high fluorescence quantum yield and long lifetime. We have examined some of these properties for phenol-like polymers. Poly(4-hydroxystyrene) (poly-HOST), poly(4-methoxystyrene) (poly-MOST), poly-TBOC and Novolac all fluoresce upon excitation at A = 248 or 290 nm. The fluorescence maxima are in the 300-330 nm range, exhibit a minor solvent dependence and have red-shifted tails due to partial ordering or 'excimer' formation. The fluorescence is quenched by the addition of onium salt photoinitiator in both solution and films.

The effect of adding opium salt to acetonitrile solutions of poly-HOST, poly-MOST and poly- TBOC was studied. In all cases the intensity of the fluorescence decreases with added onium salt. For Stern-Volmer (dynamic) quenching of fluorescence:

b0/:b = 1 + kqT [Q]

where q:0 and :I: are the fluorescence qunatum yields in the absence and presence of quencher; k is the bimolecular quenching rate constant; z is the lifetime of the polymer and [Q] is the quencher concentration. Thus a plot of l /I versus [Q] is linear with a gradient of kqt and an intercept at 1. This the case for all three polymers and using a value of 2 x 1010 M-1 s-1 for

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Figure VI. Fluorescence decay of 310 nm emission from excitation of poly-TBOC at 250 nm in CH3CN.

the bimolecular rate constant in acetonitrile, the lifetimes of the polymers could be estimated (Table l). These lifetimes agree very well with the experimental values obtained for the model monomers and the value actually measured for poly-TBOC (Figure VI).

x In the solid films of polymer and onium salt a linear Stern-Volmer plot for I /I was not obtained. Perrin developed a model for solid state (static) quenching;

ln(Io/I) = V N [Q]

where Io and I are the fluorescence intensities in the presence and absence of quencher; V is the volume of the active sphere and [ Q ] is the quencher concentration. The radius of the active quenching sphere is calculated from the active volume.

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R= 3V 4 1

The values obtained for R (Table I) are 16-20 A and are too large for pure static quenching and may be the result of an additional, small dynamic component.

5. Conclusions

The photo-CIDNP have confirmed the intermediacy and identified the multiplicity of radical species in the direct, triplet energy sensitized and photo-induced electron transfer reactions of

triphenylsulfonium salts. Diphenylsulfinyl radical cation is directly observed from the triplet sensitized reaction by nanosecond laser flash photolysis. Photophysical studies on 4-oxystyrene

polymers confirm that the polymer backbone participates in the 'photo-acid' generation reaction.

Acknowledgements

We would like to thank our co-workers whose work is cited in the references. We are especially grateful to Dr. John Dektar and Professor Nicholas Turro for many helpful discussions.

References

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10. Turro, N. J. Modern Molecular Photochemistry, Benjamin-Cummings, Menlo Park, 1978, p. 318. 11. Kaptein, R; Adv. Free Radical Chem. 1975, 5, 381. 12. Dektar, J. L.; Hacker, N. P. J. Am. Chem. Soc. 1990, 112, 6004. 13. Dektar, J. L.; Hacker, N. P. J. Org. Chem., 1988, 53, 1833. 14. Dektar, J. L. ; Hacker, N. P. J. Photochem. Photobiol., A. Chem., 1989, 46, 233. 15. Hatton, W. G.; Hacker, N. P; Kasai, P. H. J. Chem. Soc., Chem. Commun. 1990, 227. 16. Hacker, N. P.; Welsh, K. M. Chapter in Structure-Property Relations in Polymers; Spectros- copy and Performance, Urban, M. W., Claver, C. D. Eds.; ACS Advances in Chemistry Series No. 236, American Chemical Society, Washington D. C. 1992, accepted.