Investigation of 20% Scandium-doped Aluminum Nitride Films for MEMS Laterally Vibrating Resonators

This paper reports on the investigation of 1 mu m thick films of 20% Scandium-doped Aluminum Nitride (ScAlN) for the making of piezoelectric MEMS laterally vibrating resonators (LVRs). The ScAlN films, which can be sputter-deposited such as undoped Aluminum Nitride (AlN) films, were used to demonstrate high performance resonators. These devices showed quality factor (Q(s)) in excess of 1000 in air centered around 250 and 500 MHz and enhanced electromechanical coupling (k(t)(2)) in the range of 3.2-4.5%. This k(t)(2) is double the value of what has been achieved on similar resonators made out of AlN films. A 3-dB Q(s) of 1300 has been recorded both for 1-port and 2-port resonators at 250 and 500 MHz, while a maximum Q(s) of 1500 has been recorded for a 1-port resonator at 500 MHz. Along with experimental results from actual devices, this work also reports the etching characteristics of the piezoelectric material under Cl-2/BCl3 chemistry to attain high selectivity and straight sidewall with a SiO2 hard mask. More broadly, enhancement of resonators design and fabrication process, suppression of spurious modes and increase in the concentration of Sc (theoretically up to 40%) will lead to significant performance improvements for many classes of piezoelectric MEMS, especially tunable filters.