A combined experimental and theoretical study on electron induced fragmentation of methyl acetate, a model compound for side chain fragmentation and decarboxylation as pathways to main chain scission of polymethyl methacrylate as EUV lithography resist material

Abstract

With extreme ultraviolet lithography (EUVL) being established alongside the conventional deep UVL in high-volume semiconductor manufacturing processes comes a transition from the use of non-ionizing radiation to the use of ionizing radiation in the lithographic process. Correspondingly, the chemistry in the solubility switching of the resist materials changes from photochemistry to electron-induced chemistry and may thus resemble the resist chemistry induced in electron beam lithography rather than the photochemistry governing deep ultraviolet lithography. This in turn calls for rethinking of the resist formulations, a better understanding of the respective electron induced chemistry, and eventually tailoring it to provide high performance EUVL formulations. In the current study, we take a step in this direction and revisit electron induced fragmentation of methyl acetate as the simplest model compound for the functional side group of polymethyl methacrylate (PMMA), a high-performance main chain scission resist material in electron beam lithography. Appearance energies for individual fragmentation reactions in dissociative ionization (DI) in the gas phase are determined, and quantum chemical calculations are conducted to elucidate the underlying reactions. The results are discussed in context to previous work on dissociative ionization and dissociative electron attachment (DEA) of methyl acetate, and quantum chemical calculations are used to explore the thermo-chemistry of decarboxylation as a path to main chain scission of PMMA through both DI and DEA when this resist material is exposed to EUV radiation.