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 d ₃₃ and d ₃₁ of Sc _x Al _1-x N thin films directly deposited onto silicon wafers, as well the radio frequency (RF) electrical characteristics of Sc _0.35 Al _0.65 N bulk acoustic wave (BAW) resonators at around 2 GHz, and found a maximum value for d ₃₃ of 28 pC/N and a maximum -d ₃₁ of 13 pm/V at 40% scandium concentration. In BAW resonators that use Sc _0.35 Al _0.65 N as a piezoelectric film, the electromechanical coupling coefficient k ² (=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.