Fabrication of piezoelectric acoustic transducers built on cantilever-like diaphragm

The paper presents a technique to fabricate a cantilever like diaphragm that releases the residual stress (and also is mechanically flexible) much like a cantilever, and yet is itself a diaphragm with its four edges cramped. Using the high mechanical flexibility (i.e., extremely low Youngs Modulus) of parylene as a holding layer, we have successfully fabricated various piezoelectric acoustic transducers built on silicon nitride layer (either in cantilever form and/or freely-suspended island form) with electrodes and piezoelectric ZnO film. Since parylene has relatively low melting point (around 280 degreesC for parylene C), we deposit the parylene holding layer toward the end of the fabrication process after processing all the high temperature steps. Then after releasing the diaphragm with KOH etching, the silicon nitride is patterned from the backside with a reactive ion etcher (RIE). To demonstrate the effectiveness of a parylene-held cantilever-like diaphragm in releasing the residual stress, the devices before and after patterning the silicon nitride are compared. The acoustic outputs of the fabricated transducers have been measured with B&K 4135 microphone.