Piezoelectric coefficients measured by picosecond ultrasonics

The needs for materials in thin films characterization are increasing with the raising interest for Micro-Electro-Mechanical-Systems. Picosecond ultrasonics (PU), a non-contact and non-destructive technique, has been largely developed to answer the mechanical metrology requirements of thin films technology, particularly in the field of Bulk Acoustic Wave (BAW) resonators. The PU technique uses a pulsed laser source to excite and detect longitudinal acoustic waves at very high frequencies (100 GHz to 1 THz). Basically, the technique enables longitudinal sound velocity measurements and thickness control. It has been shown that the insertion of a wavelength tunable laser source improves the accuracy of the measurements. Moreover, this particular configuration enriches the measurable parameters of thin films (acoustic attenuation, temperature coefficients, transverse sound velocity). Here, using a particular BAW configuration and electrical probes, we demonstrate that the PU technique is able to measure the d₃₃ piezoelectric coefficient and the stiffness variation induced by an applied dc voltage. These measurements demonstrate that the frequency drift of a BAW resonator respect to a dc voltage is mainly due to the stiffness variation of the piezoelectric layer.