Photo-oxidative degradation of fluorinated chemisorbed monolayers studied by contact angle measurements and time-of-flight secondary ion mass spectrometry

This study investigates photo-degradation of tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13) used for antisticking surface modifiers in nanoimprint lithography by exposure to vacuum UV light (VUV; λ = 172 nm) or UV/ozone (λ = 185 and 254 nm). The degradation of FAS13 molecules chemisorbed on silicon surfaces was monitored sensitively in the contact angle measurements using diiodomethane rather than water. Photo-oxidation made the contact angles of FAS13-modified surfaces after water wash lower than those of unwashed FAS13-modified surfaces. This suggested that photo-decomposition products were left on exposed FAS13-modified surfaces and removed with water. Time-of-flight secondary ion mass spectrometry defined that the FAS13 chemisorbed monolayer was detected as a characteristic parent anion of C₈F₁₃⁻, whereas the decomposition products were detected as C₆F₁₃⁻ and C₃F₇⁻. Further exposure made the decomposition products photo-degraded to low-molecular-weight species and finally disappear from the surface. With the photo-degradation mechanism, UV/ozone exposure in an ambient condition was essentially the same as VUV exposure.