Abstract

Thin films, irrespective of the techniques used to deposit them, are invariably under a state of residual stress which can, under appropriate conditions, lead to failure. These stresses arise from a variety of different factors including differential grain growth, thermal expansion mismatch and non-equilibrium incorporation of defects. Developments in fracture mechanics indicate that only certain failure modes can occur and that they are dependent on particular combinations of film geometry, film stress, film thickness and fracture resistance. In particular, films fracture or decohere when a figure of merit, the cracking number, appropriate to the pertinent geometry, exceeds a critical value. Since, the two parameters usually unknown are the film stress and the decohesion fracture energy, we have been developing techniques for their measurement. To measure film stress, we have been using the optical techniques of piezo-spectroscopy. An advantage of these is that the information is not restricted to measuring the average strain, as are wafer bending measurements, but can also be used to measure strains from local regions identifiable under an optical microscope or sampled with a laser probe. In order to measure the decohesion energy, a new technique has been developed at UCSB based on a thin-film lithography in which a thin "stressor" film, under very high residual tension, is applied to a narrow strip of the film being tested. When a critical stressor film thickness is reached, the driving force for decohering the film/substrate interface exceeds the interface fracture energy. Thus, through a series of measurements with stressor films of different thicknesses the decohesion energy can be obtained.

© 1995 Optical Society of America