Microfabrication of coupled fluid–structure systems with applications in acoustic sensing

A fabrication process for manufacturing coupled fluid–structure microsystems is described. This process allows production of a novel class of trapped-fluid acoustic sensors. The systems consist of wafer-thick fluid chambers bounded by arbitrarily shaped membranes. A stacked film structure of silicon nitride and boron doped polysilicon is used to reduce residual stresses in the membranes. Tin–gold fluxless solder bonding is used to fabricate capacitive structures which allow electrical sensing of membrane vibrations. An acoustic sensor fabricated using this process is demonstrated. An equivalent acoustic circuit model for the system is described, and models the low-frequency system response accurately. Experimental measurements of sensitivity, noise density, and linearity are presented.