Resonance Frequency and Vibration Mode Modification of Piezoelectric MEMS Ultrasonic Sensors on Buckled Diaphragm Structures for High Sensitivity and High Resolution Measurement

Vibration modes and resonance frequencies of piezoelectric MEMS ultrasonic sensors with buckled diaphragm structures were investigated from the viewpoint of high resolution and ghost-suppressive measurement techniques using multiple frequencies. The resonance frequency was modified through inverse piezoelectric stress, and the buckling introduced to improve sensitivity also varied with the stress. The inverse piezoelectric stress was generated by applying an external voltage to the tuning electrode of the sensor in the range of 0 V to 10 V. The buckling deflection was accordingly varied from
$\mathbf{12}\ \boldsymbol{\mu} \mathbf{m}$
to
$\mathbf{6}\ \boldsymbol{\mu} \mathbf{m}$
, and the resonance frequency was varied in the range between 37 kHz and 102 kHz. As the result of the stress control, a multiple frequency technique that effectively improves the directivity and the resolution was achieved by combining 84 kHz and 37 kHz frequencies with tuning voltages of 0 V and 8 V, respectively. The buckling profiles and vibration modes of the sensor diaphragm at these tuning voltages were also well suitable for high-sensitivity measurements.