Novel methods to measure residual stresses in thin films

The present work summarizes the novel experimental methods recently developed at the Hong Kong University of Science and Technology to evaluate residual stresses in thin films. These methods include the indentation fracture technique, the micro-rotating-structure indicator and the microbridge test. The indentation fracture technique is based on a semi-empirical formula, which shows that the ratio of the indentation load to the cubic of the square root of the indentation-induced crack length is linearly proportional to the reciprocal of the square root of the crack length, the magnitude of the residual stress, and the film thickness. The micro-rotating-structure indicators were designed and fabricated with both silicon nitride and polysilicon thin films deposited on silicon wafers. The results indicate that the micro-rotating-structures have the ability to measure spatially and locally a large range of residual tensile or compressive stresses in thin films with appropriate sensitivities. The deflection-load formula for the microbridge test was derived with considering the substrate deformation and given in a closed-form. The microbridge test has the capability to evaluate simultaneously the Young's modulus, residual stress and bending strength of thin films. Two versions of the microbridge testing method were developed, respectively, for single-layer and bilayer thin films. In a similar manner, it should be straightforward to extend the bilayer version to multilayer thin films.