Controlling stress and stress gradient during the release process in gold suspended micro-structures

In this paper it will be demonstrated that the conditions of the final release process in oxygen plasma strongly influence the shape of suspended gold micro-structures. Both single clamped and double clamped cantilever test devices have been examined. The structures have typically a thickness of 1.8 microns, and are produced by patterned gold electrodeposition above a sacrificial photoresist layer which is then removed by oxygen plasma ashing. In general, deformations are strongly reduced lowering the temperature, but the release time increases abruptly. A simple model is presented to explain the experimental observation, and from it can be deduced that yield and inelastic relaxation are critical phenomena responsible for most of the deviations from the ideal planar shape of MEMS structures. To overcome these unwanted effects, the parameters to be optimized are mainly the initial stress value, the release temperature and the vertical structural homogeneity of the gold layer.