Structure and Behavior of Materials for Micromechanical Devices

The mechanical behavior of thin film microfabricated devices depends in large part on the material microstructure, which in turn depends on the conditions under which the films are deposited. As a result of material sensitivities to various deposition parameters, new techniques have been developed for thorough characterization and monitoring of film properties such as modulus, residual stress, microactuator force generation, and fatigue. Residual stresses in films, which can lead to fracture, buckling, or curling of surface micromachined devices and can influence microactuator behavior, vary considerably with processing conditions. A study of asdeposited in situ phosphorus doped polycrystalline silicon shows a strong correlation between microstructure and residual stress. The gradient in residual stress through the film thickness was determined by measuring the substrate's curvature, then etching away a thin layer of the film with a plasma etch, and repeating the process until the entire film was removed. These measurements were then compared with crosssectional transmission electron micrographs of the films, an example of which is shown in Fig. 1. While the overall stress is compressive, crystallization occurring near the substrate results in a tensile stress component, inducing a stress gradient.