Direct determination of photoresist composition changes during UV exposure

Abstract

Exposure of photoresists to ultraviolet light results in outgassing of species that have the potential to contaminate surrounding optical surfaces. Of particular concern are silicon-containing products which cannot be cleaned and permanently detune optical coatings. Collection and identification of those species and quantification of the amounts formed is a difficult analytical problem because of the number and variety of products. We describe a general methodology for determining acidolytic decomposition pathways and absolute elemental composition changes induced in photoresists during exposure. Two silicon-containing 193 nm resists that differ in the mode of attachment of trimethylsilyl to the polymer have been investigated. Elemental abundances are measured in post-apply baked, exposed and post-expose baked films by secondary ion mass spectrometry (SIMS), thus probing volatile product formation from all photochemical and thermal decomposition pathways. Complementary data on primary thermal acidolytic deprotection pathways during post-exposure bake are obtained by mass spectrometry, enabling SIMS elemental abundance changes to be interpreted. The results show that decomposition of both the polymer protecting groups through room temperature acidolysis and the photoacid generator by photolysis lead to volatile product formation during exposure. Silicon bound through oxygen is acid labile while silicon bound through carbon is not, resulting in very low to no silicon outgassing from the latter polymer. Sulfur-containing products formed from PAGs outgas in significant amounts from the photoresists investigated, supporting recent mass spectrometric observations of sulfur outgassing by R. Kunz and coworkers.