Diaphragm deflection of silicon interferometer structures used as pressure sensors

The influence of isotropic pressure on the mechanical properties of silicon diaphragms used in fiber-optical pressure sensors has been investigated by comparing experimental sensor output data with a simple theoretical treatment based on an analytical method and with a more rigorous numerical treatment based on the finite-element method (FEM). The sensor structures are interferometers prepared in silicon by wafer bonding. This method allows cavities to be prepared with accurately controlled dimensions, bounded on one side by a silicon diaphragm. Optical interference occurs at the cavity when the devices are illuminated by light. The interference can be influenced by an external pressure perturbation such that light flux reflected by the sensor body becomes a measure of the applied pressure. Three different structures with different diaphragm dimensions have been investigated. It is found that the analytical treatment can be used for rough estimates of sensor properties only for structures with thin diaphragms and for pressures up to about 2 bar. For accurate predictions and for thicker diaphragms, where the entire sensor body may be influenced by pressure, numerical methods must be used.