Residual stress influences on the sensitivity of ultrasonic sensor having composite membrane structure

Stress and sensitivity of arrayed ultrasonic sensors utilizing piezoelectric thin film (lead–zirconate–titanate film: Pb(Zr0.52Ti0.48)O3) having composite membrane structure are demonstrated. Due to different thermal and elastic characteristics of each constitutive layer, a subsequent residual stress and deflection is generated on the resultant composite membrane. We present the influence of residual stress on the mechanical behaviors and sensitivities of a Si-based integrated sensor device. The design of sensor structure and the fabrication process especially relating to thermal treatment have significant effects on the stress state of the composite membrane, and the relationships among the stress states, deflections of membrane and sensitivities of sensor devices are considered. Tensile stress induced on the membrane consequently leads to sensitivity deterioration, while compressive stress improves sensitivity. Sensitivities of each sensor structure are expressed as the ratio of output voltage (V) to the acoustic pressure (P), and the variations of those are discussed relating to the residual stress states, PZT film properties and mechanical behaviors of each composite membrane.