Influence of top electrodes to vibration modes in impulse responses of MEMS piezoelectric diaphragms for ultrasonic microsensors

Vibration modes of MEMS diaphragms were investigated in piezoelectric ultrasonic microsensors from the viewpoint of how their top electrodes affect the vibrations caused by an airborne ultrasound pulse. The top electrode of the sensors is divided into two parts for ultrasonic sensing and resonant frequency tuning to utilize them for a high resolution measurement technique which uses two resonant frequencies. The vibration modes of the sensor diaphragms having the divided electrode with various thickness were evaluated using scanning laser Doppler vibrometry. The top electrode strongly affected the vibration mode even in the thickness around 10 nm and two vibration peaks of the diaphragms having the top electrode were found to be useful for the high resolution measurement.