Electron impact reactions of triphenylsulfonium salt resist sensitizers in the solid state

Abstract

To elucidate the mechanism of electron beam exposure of triphenylsulfonium salt sensitized resists, films of triphenylsulfonium hexafluoroantimonate or triflate in various polymers and in the pure solid state were exposed to pulses of electrons. The composition of the resulting pulses of gaseous products was analyzed by Fourier transform mass spectrometry. Components of product mixtures were identified by using both electron impact and chemical ionization techniques. The main volatile products of triphenylsulfonium under electron impact are diphenyl sulfide, benzene, and biphenyl. The yield of benzene is much larger from triphenylsulfonium salts dissolved in polymer than in the pure solid state. This implies rapid hydrogen-abstraction reactions by the intermediate phenyl radicals from matrices with available aliphatic hydrogens. The product distributions observed from several polymer solvents, and their dependence on electron energy, show few other rapid reactions between the matrix and the primary products. No volatile products arising from the anions were observed. This indicates that acidic products formed in the electron stimulated decomposition reactions of triphenylsulfonium salts are retained dissolved in the polymer under high vacuum conditions. Thus under electron beam exposure, as in deep-UV exposure, it is the subsequent acid-catalyzed reactions of the resin that are responsible for the high sensitivity of onium–salt resists.