Piezoelectric properties of ScAlN thin films for piezo-MEMS devices

This paper reports the piezoelectric properties of ScAlN thin films. We evaluated the piezoelectric coefficients d₃₃ and d₃₁ of Sc_xAl_1-xN thin films directly deposited onto silicon wafers, as well the radio frequency (RF) electrical characteristics of Sc_0.35Al_0.65N bulk acoustic wave (BAW) resonators at around 2 GHz, and found a maximum value for d₃₃ of 28 pC/N and a maximum -d₃₁ of 13 pm/V at 40% scandium concentration. In BAW resonators that use Sc_0.35Al_0.65N as a piezoelectric film, the electromechanical coupling coefficient k² (=15.5%) was found to be 2.6 times that of resonators with AlN films. These experimental results are in very close agreement with first-principles calculations. The large electromechanical coupling coefficient and high sound velocity of these films should make them suitable for high frequency applications.