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Photochemical and Photophysical Studies on Chemically Amplified Resists

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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 substi- tuted phenolic polymers will be described to determine how the polymer excited state participates in the generation of acid. 36 J. Photopolym. Sci. Technol., Vol.5, No. 1, 1992 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 multi- plicity 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) 37 J. Photopolym. Sci. Technol., Vol. 5, No. 1, 1992 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 38 J. Photopolym. Sci. Technol., Vol.5, No. 1, 1992 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 inter- mediate. 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 inter- mediates 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. 39 J. Photopolym. Sci. Technol., Vol.5, No. 1, 1992 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
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