Diaphragm stretching and contact problems for microelectromechanical pressure transducers

Characteristics of microelectromechanical pressure transducers are analyzed to improve the understanding of response and provide the basis for credible designs. General results are applicable to devices irrespective of size, but the focus is on transducers with overall dimensions which are typically hundreds of microns. The principal component is a polysilicon diaphragm with piezoelectric elements fabricated over a silicon substrate. With a sufficiently small gap, one-sided overload protection is provided by the substrate. The creation of a bridge over the opposite side facilitates the design of a differential pressure transducer with two-sided overload protection. Such bridges are constructed by the so-called LIGA process. In this, a few hundred angstroms of metal are sputtered on to a substrate to form a plating base, coated by a thick photoresist layer (hundreds of microns), and exposed to synchrotron radiation using an appropriate mask. After developing, the cavities are filled by electrodeposition and remaining photoresist is removed. The metal structure is continuously bonded but the necessary gap to realize the bridge can be made by incorporating a sacrificial layer (e.g., polyimide) between a portion of the metal and the plating base.; An analysis has been made of the mechanical interaction of the diaphragm with the metal overpressure bridge. A finite element code was developed for axisymmetric problems and assessed flexure, stretching and contact. The commercial code ANSYS which does not include stretching for such problems, was used for analyzing square diaphragms. Parametric results are presented for transducer design, including capacitive output for signal verification.; In addition to diaphragm analysis, mechanical stress and deflections are determined in the elastic bridge. Since the material properties of the bridge and substrate are generally quite different, thermal stresses and deflections have been computed for uniform temperature changes.; Secondary supporting studies have also been carried out. Two-dimensional beam (panel) models were analyzed with stretching effects. A code was developed using the shooting method and applied to cases with uniform pressure as well as electrostatic loading. Also, the related problem of determination of residual strains in electrodeposited layers was addressed. Diagnostic micromechanical structures incorporating buckling elements were analyzed by theoretical and finite element methods.