Surface depth analysis of chemical changes in random copolymer thin films composed of hydrophilic and hydrophobic silicon-based monomers induced by plasma treatment as studied by hard X-ray photoelectron spectroscopy and neutron reflectivity measurements

Yamamoto, Katsuhiro*; Imai, Tatsuya*; Kawai, Atsuki*; Ito, Eri*; Miyazaki, Tsukasa*; Miyata, Noboru*; Yamada, Norifumi*; Seto, Hideki*; Aoki, Hiroyuki  

In this study, a silicon-based copolymer, poly(tris(trimethylsiloxy)-3-methacryloxypropylsilane)-co-poly(N,N-dimethyl acrylamide), thin film was subjected to plasma surface treatment to make its surface hydrophilic (biocompatible). Neutron reflectivity (NR) measurement of the plasma-treated thin film showed a decrease in the film thickness (etching width: similar to 20 nm) and an increase in the scattering length density (SLD) near the surface (similar to 15 nm). The region with a considerably high SLD adsorbed water (DO) from its saturated vapor, indicating its superior surface hydrophilicity. Nevertheless of the hydrophilicity, the swelling of the thin film was suppressed. Hard X-ray photoelectron spectroscopy (HAXPES) performed at various takeoff angles revealed that the thin-film surface (similar to 20 nm depth) underwent extensive oxidation. NR and HAXPES analysis quantitatively yielded the depth profiling of elemental compositions in a few tens of nm scale. Si oxidation and hydrogen elimination (probably CH groups) in the vicinity of the surface region increased the SLD and decreased the hydrophobicity. A combination of Soft X-ray photoelectron spectroscopy and NR measurements revealed the surface chemical composition and mass density. It was considered that the surface near the film was chemically composed close to SiO, forming a gel-like (three-dimensional network) structure that is hydrophilic and suppresses swelling due to moisture, indicating it can be expected to maintain stable hydrophilicity on the film surface.