In the fabrication of high-speed Ti:LiNbO3 optical waveguide modulators, the sputtering deposition of 1 mu m-thick-order SiO2 films on LiNbO3 substrates and the subsequent O-2 atmosphere annealing are key processes. However, large numbers of pinholes appear in the films after the annealing process limiting the fabrication yield. Here, we demonstrate that such pinholes are eliminated in films deposited on heated LiNbO3 substrates. In addition to the common effect of increased film adhesive strength and density, the reduction in H2O impurities in LiNbO3 and SiO2 by substrate heating is considered to be the reason for this phenomenon.