Resonant Frequency Tuning of Piezoelectric Ultrasonic Microsensors by Bias Voltage Application to Extra Top-Electrodes on PZT Diaphragms

Tunable ultrasonic microsensors have been fabricated using piezoelectric lead-zirconate-titanate (PZT) thin films on micromachined diaphragm structures with inner- and outer-top electrodes. The inner electrode is used for detecting ultrasonic signal through piezoelectric effect induced by a sound pressure and the outer electrode is used for stress control through converse-piezoelectric effect by applying external dc voltage. Precise fabrication processes have been developed to control buckling of the diaphragms, and two kinds of sensors having flat and buckled diaphragms have been fabricated selectively. The flat and buckled diaphragm sensors have shown reversed characteristics in the voltage-induced frequency shift and have exhibited tunability of + 90% and -50%, respectively, under +/- 8 V application to 1 mu m-thick PZT films.