Polarization Dependence in the Carbon K‑Edge Photofragmentation of MAPDST Photoresist: An Experimental and Theoretical Study

The use of tunable soft X-rays from synchrotron radiation (SR) opens the possibility of inducing selective chemical bond scission because of its high localization in a chemical bond. The selective fragmentation of a potential extreme ultraviolet resist, poly­(4-(methacryloyloxy) phenyldimethylsulfoniumtriflate) (MAPDST), was examined using inner shell-polarized SR excitation. Selective bond dissociation processes were studied using a combination of carbon K-edge excitation, angle-resolved irradiation, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Detailed theoretical calculations carried out with the FEFF9 modeling program allowed the interpretation of all the observed experimental features. NEXAFS results indicated that the aromatic group of the polymer lies parallel to the substrate surface. FEFF9 theoretical calculations confirmed the origin of the splitting of the main C 1s → πCC * resonances observed. The transition C 1s → παCC * (285.3 eV) can be associated with the four internal carbons of the aromatic ring. The transition C 1s → πβCC * (286.9 eV) was assigned to the carbon atoms attached to the oxygen and sulfur atoms. According to the theoretical calculations, the origin of the splitting is due to the different absolute energy of C 1s. The results showed a strong selective dissociation effect when the excitation energy was tuned to the C 1s → παCC * transition and the electric field vector of the photon was perpendicular to the substrate plane (grazing angle). On the contrary, other transitions were in general less affected. When the SR irradiation angle changed from grazing to normal incidence, the intensity of the C 1s → πCC * transitions was almost unaffected by 285.3 eV photons. The experimental results suggest that site-specific core excitation combined with the direction of the electric field vector of the incidence SR can efficiently control the localization of the photon energy to produce selective bond dissociation in MAPDST thin films. The results presented here can also be useful to guide new processing lithographic methods for extreme ultraviolet lithography using the polarization properties of light in ordered polymeric thin films.