Piezoelectric coefficients measured by picosecond ultrasonics

Abstract

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.