Mechanical Characterization of Thin Films by Micromechanical Techniques

Abstract

Evaluating the mechanical properties of thin films on thick substrates is tricky. Some useful techniques for mechanical property measurements on macroscopic film specimens exist, e.g., internal stress measurements by x-ray diffraction or by wafer buckling. Many conventional techniques, however, such as indentation for hardness measurement or scratch testing for evaluation of adhesion or wear resistance, yield results that are difficult or impossible to evaluate in terms of more fundamental film properties. In fact, these techniques are more practical engineering tools than tools for scientific study. One solution is to miniaturize the test probe to dimensions approaching the film thickness. The nanoindentation technique for submicrohardness measurement described elsewhere in this issue by Pharr and Oliver is a recent example of such an approach. Still, for very thin films it is difficult to separate the influence of the substrate properties from the film properties, and the indentation process is inherently too complex to be well suited for determining fundamental materials parameters. The rapidly evolving field of micromechanics offers some new possibilities in thin film characterization. Important film properties such as internal stress, elastic moduli, plastic yield limit, and fracture data can be extracted from experiments with micromachined test structures in the 10–1,000 μm size range.