Investigation of Non-equibiaxial Thin Film Stress by Using Stoney Formula

Since the use of the Stoney formula only requires the measurement of the radii of curvature of specimens before and after coating the thin film, the formula has been widely used in the measurement and calculation of thin film stress. However, many assumptions were adopted in the derivation of the original Stoney formula. In particular, the state of thin film stress must be equibiaxial. In the literatures, even the thin film stress may be non-equibiaxial in some cases, the original Stoney formula was still employed to determine the thin film stress. In the past, several improvements on the assumptions of the original Stoney formula have been attempted so that the corrected formula can be applicable to both equibiaxial and non-equibiaxial states of thin film stresses. However, those improvements have not been verified experimentally. In this paper, the stress and the radii of curvature of circular disks made of PSM-1 photoelastic material and coated with the silicon dioxide (SiO2) thin film of different thicknesses were measured by photoelasticity and Fizeau interferometer, respectively. The thin film stresses determined by the two methods were then compared to assess the accuracy of the Stoney formula.