Fused silica and Si-O-x coatings are of interest for use under high flux conditions of laser light. Si-O-x coatings are sputter deposited from silicon and fused quartz targets using planar magnetrons operated in the r.f. mode with a variable working-gas mixture of Argon-Oxygen. A series of coatings are prepared on optically flat, fused quartz substrates. Analysis of surface curvature reveals the deposition process conditions that minimize residual stress. Compressive stress levels that exceed 1.5 GPa can be reduced to less than 0.4 GPa for an optimum working-gas pressure. Characterization using Rutherford backscattering and X-ray photoelectron spectroscopy indicates that both the fused quartz and silicon targets can be used to sputter deposit coatings with the chemical bonding features of the fused-quartz substrate material.