Room-temperature synthesis of a-SiO2 thin films by UV-assisted ozonolysis of a polymer precursor
Abstract
A room-temperature synthesis route for thin films of amorphous silica (a-SiO2) based on irradiation of a silicon-containing polymer by UV light in pure O2 atmosphere has been developed. The chemical conversion of spin-coated films of poly(pentamethyldisilylstyrene) (pPMDSS) to silicon oxycarbide and finally to amorphous silica is achieved by UV-assisted ozonolysis. The conversion process has been followed by Fourier transform infrared spectroscopy (FTIR), ellipsometry, and X-ray photoelectron (XPS) and Auger electron spectroscopies (AES). The control of the irradiation time allows for control of the chemical composition of the converted films ranging from that of a silicon oxycarbide for short exposure times to that of a-SiO2 after 18 h of exposure. The surface composition of the fully converted films obtained by XPS is characterized by an atomic ratio O/Si = 2.00 ± 0.07. Auger electron depth profiles reveal a uniform chemical composition of the a-SiO2 films with a residual carbon content in the bulk of the films below 1%. Converted a-SiO2 films of thicknesses up to 150 nm were achieved. Ellipsometry shows that the conversion of the films in a-SiO2 is accompanied by a progressive decrease of the film thickness and evolution of the refractive index to an asymptotic value of 1.44. The film surface of the converted films probed by optical microscopy over large areas and by atomic force microscopy (AFM) does not show any cracks and is atomically flat with a RMS roughness below 0.4 nm.