Investigation of 20% scandium-doped aluminum nitride films for MEMS laterally vibrating resonators

This paper reports on the investigation of 1 μm 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
/BCl
₃
chemistry to attain high selectivity and straight sidewall with a SiO
₂
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.