Sensitivity of piezoelectric MEMS ultrasonic sensors using sol-gel PZT films prepared through various pyrolysis temperatures

Sensitivity of piezoelectric ultrasonic microsensors was investigated with sol-gel derived piezoelectric lead-zirconate-titanate (PZT) films under various pyrolysis temperatures. The sensors were fabricated on thin diaphragm structures, whose buckling deflection strongly affects the sensitivity and is controlled through stress of the PZT film. The pyrolysis temperature in the sol-gel process was varied in the range from 250°C to 400°C, and the prepared PZT films on 2-inch wafers were evaluated in crystalline orientation and the stress. The sensor structures were then fabricated on the wafers, and the buckling deflection and the sensitivity of the sensors were evaluated. The stress of the films decreased with increasing the pyrolysis temperature. The preferred crystalline orientation switched from (111) to (100) between the pyrolysis temperatures below and at 400°C. The sensors with the 400°C-pyrolyzed PZT showed much larger buckling deflection and much higher sensitivity than those with PZT pyrolyzed at the lower temperatures.