EUV photofragmentation and oxidation of a polyarylene – Sulfonium resist: XPS and NEXAFS study

Abstract

Secondary electrons play a vital role in Extreme Ultraviolet Lithography (EUVL). However, the mechanisms and chemical reactions after photon absorption in this high energy region only recently have been studied in more detail. The present work is an attempt to obtain more information about the photofragmentation process when EUV radiation is used in polymers. EUV irradiation on a non-chemically amplified resist (non-CAR), poly[methyl (4-(phenyl thio) - phenyl) – sulfonium trifluoromethanesulfonate] (PAS), was studied using monochromatic synchrotron radiation at 103.5 eV of excitation energy. XPS and NEXAFS spectroscopy were used to follow the surface chemical changes after the samples were irradiated and oxidized. The results showed the fast desorption of the sensitive sulfonium triflate with important rearrangements that kept the sulfur contents and polymer backbone with minor changes. This was confirmed by the detection of new SC and SS signals after irradiation. New carbonyl signals were also detected and it was also observed that the aromatic rings resisted the irradiation of EUV photons. The photofragmentation process showed a different mechanism in relation with similar resists that we have studied in the past. Angle resolved-NEXAFS results and semiempirical calculations showed that the PAS film adopted a random configuration in relation to the Si(100) plane. It may allow a strong interaction after EUV irradiation between the excited species or radicals formed and the polymer chains favoring rearrangement reactions. The present investigation shows the need to understand the photochemical mechanisms under EUV photon excitations which are different to UV light. This knowledgement may lead to prepare better resist formulations for EUVL. Based on our finding a simple mechanism was proposed.