Structural Strain of Piezoelectric Films on MEMS Diaphragm Structures and Its Suppression through Fabrication Process for High Conversion Efficiency

A structural strain of piezoelectric material and the strain suppression through modifying the fabrication process were discussed from the viewpoint of high piezoelectric conversion efficiency of the statically buckled diaphragm structures. The conversion efficiency of a vibrating piezoelectric diaphragms is enhanced by a static buckling of the diaphragm, whereas a too large buckling decreased the efficiency due to a too large tensile strain of the piezoelectric PZT film when the diaphragm buckles in the fabrication process. In this study, modified fabrication processes were proposed so that the PZT film is deposited on the already buckled diaphragm structure, and preparation condition of the bottom electrode of platinum/titanium films were also modified to reduce the residual tensile stress of the films to enhance the buckling deflection. The conversion efficiency improvement by using these process modifications was estimated through numerical evaluation of the buckling deflection. The analytical results showed that the process modifications suppressed the additional tensile strain and much enhanced the buckling deflection, and the efficiency improvement by 4.6 times was expected.