Measurement of longitudinal piezoelectric coefficient of film with scanning-modulated interferometer

A scanning-modulated interferometer has been designed to measure the effective d₃₃ coefficient of piezoelectric film by monitoring the modality of surface vibration of an active electrode area together with its passive surroundings in response to the applied ac voltage. By introducing a slow modulation to an ordinary interferometer, accurate measurement of the maximum intensity change corresponding to the small vibration at the most sensitive quarter-wavelength point can be achieved by measuring the envelope amplitude of the modulated vibration signal. The environment disturbance can only affect the position at which the envelope peak occurs but cannot change its amplitude value. Therefore, very reliable measurement results can be achieved. The modulation technique enables the simultaneous measurement of the vibration signal V_out and the reference signal V_p-p, thus a real time calibration can be realized. Since the small displacement is proportional to the ratio of V_out and V_p-p, the testing result is neither susceptible to the roughness or reflectance change of the sample surface, nor to the instability of the laser intensity. Taking this advantage, it is possible to realize automatic scanning of the sample surface for measuring the vibration modality, while maintaining the measurement accuracy. The d₃₃ value of piezoelectric thick films made by composite coating processing was characterized by using this scanning-modulated interferometer. Typically, films with thicknesses ranging from 3 to 7 μm have effective d₃₃ value of more than 100 pm/V.