SiOx thin films were prepared via a PECVD system at a substrate temperature below 70 degrees C using ethyltrimethylsilane (ETMS). The refractive index (n) of SiOx increased with increasing carbon concentration in the film, which is related to the stoichiometry of SiOx determined by both Si-O-Si stretching absorption frequency and XPS analysis. No notable Si-OH FTIR absorption was observed in ETMS Sig,. FTIR carbon free SiO2 him, which showed a bulk-like refractive index, was prepared with a deposition rate of 0.16 nm/s. The external magnetic field and Ar inert gas strongly influence SiOx physical properties. Such effects may be due to the enhancement of the excited atomic oxygen and Ar in situ monitored by optical emission spectroscopy (OES).