Silicon dioxide thin films have been prepared at room temperature by remote plasma-enhanced chemical vapor deposition in a downstream reactor by using Si(CH3)(3)Cl as a volatile precursor and a microwave electron cyclotron resonance external source. Experiments are done at constant pressure by changing,the relative amount of Ar species R in the plasma gas. The aim was to obtain thin films with low density and, therefore, low refractive index. Characterization of the species of the plasma is carried out by optical emission spectroscopy. The changes of the plasma conditions are correlated with the growing rate and microstructure of the films, the latter determined by atomic force microscopy and infrared spectroscopy. It is found that the growing rate of the films decreases and their roughness increases as R increases. The optical properties Of SiO2 thin films are analyzed by optical ellipsometry. A decreasein the refractive index is found for the films grown with high values of R. The possible routes for activation of the precursor and the formation of the SiO2 thin films are discussed. (C) 2003 American Vacuum Society.