Residual stress in stabilized zirconia thin films prepared by r.f. magnetron sputtering

Abstract

Calcia-stabilized zirconia thin films were deposited by r.f. magnetron sputtering onto 2 inch 〈111〉 silicon substrates. Three samples of cubic phase, with thicknesses 0.36, 0.72 and 1.08 μm were deposited under the same operating conditions, and both thickness uniformity and surface roughness were controlled accurately by profilometry. The considerable residual stress field present in the samples was studied by two very different techniques: measurement of the substrate curvature by profilometry, and measurement of the thin film lattice strain by X-ray diffraction (XRD). The adopted XRD parallel beam geometry permitted also study of the texture developed; the main orientation was such that the angle between [111] directions of Si and CaO—ZrO2 was about 13°, with a fraction of crystallites grown along [111]Si∥[111]ZrO2. Residual stress values as high as 3 GPa were measured but, as is typical of many highly textured physically vapour deposited films, the sin2ψ method gave different stress values along different crystallographic directions and the trend of interplanar distances dhkl vs. sin2ψ was not always linear. The main reason for this behaviour is the anistropy of the thin film elastic constants, which has a strong effect particularly in highly textured materials. XRD data were evaluated following a recently proposed procedure which takes into account elastic anisotropy and thin film preferred orientations. The residual stress measured by XRD methods was systematically higher than that obtained from curvature measurements; this difference is discussed with reference to the hypotheses of the two methods and the nature of the stress in the film/substrate composite system.