The effect of release-etch holes on the electromechanical behaviour of MEMS structures

When wet etches are used to release a large microstructural element, it is common practice to distribute small holes in the microstructure to promote under-etching and release, and for reduction of squeezed-film damping. In this paper, we demonstrate that while the mechanical properties are strongly affected by the presence of etch holes, because of fringing-field effects at the edges of the etched holes, the electrostatic-force behaviour of structural elements with an array of small etched holes is nearly that of an unetched element. This leads to an "effective medium" approximation for the mechanical material that can then be used with standard electrostatic actuation models to give good predictions of electromechanical behaviour. Comparison with experimental device behaviour shows the validity of this approach.