We have fabricated silicon-rich silica thin films via RF magnetron sputtering using a SiO target. Thickness evolution and microstructure change of such $SiO_x$ (1<x<2) films were investigated in terms of the substrate temperature during the deposition. Optical interference patterns obtained from a spectroscopic reflectometer were analyzed to yield refractive index dispersion profiles of the deposited film layers in the visible wavelengths. Changes of the refractive indexes were then explained in connection with changes in microstructure and composition of the silicon-rich silicon oxide films. Increase in the number of the Si-Si bonds formed inside the $SiO_x$ thin films turned out to be mainly responsible for the increase of refractive index.