Gradual Photochemical-Induced Conversion of Liquid Polydimethylsiloxane Layers to Carbon Containing Silica Coatings by VUV Irradiation at 172 nm

Abstract

Thin liquid layers of polydimethylsiloxane (PDMS) were irradiated by VUV light under nitrogen atmosphere using a Xe(2)- excimer lamp. The irradiated layers were analyzed with infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS), showing a gradual photochemical-induced conversion of the liquid PDMS to solid SiO(2)-like coatings. IRRAS measurements revealed a smooth frequency shift of the maximal absorption band from 1111 to 1231 cm(-1) with increasing irradiation energy density caused by a gradual shift from the asymmetric Si-O stretching vibration of PDMS to the longitudinal optical (LO) mode of SiO(2). The shift was found to be dependent on the applied irradiation energy density and the O/Si ratio in the film analyzed by XPS measurements. The atomic ratio of O/Si increases from 1:1 to about 2.5:1. At the same time, the atomic ratio of C/Si decreases from 2:1 down to 1:6.5. Images taken by high resolution field emission scanning electron microscopy (FESEM) and scanning force microscopy (SFM) show a smooth surface without cracks or pores. The controllable coating properties in combination with the possibility for local irradiation using masks are promising high potential for the coating technology.