The Impact of Bottom Electrode Coverage Rate on Electromechanical Coupling and Quality Factor of AlN MEMS Contour Mode Resonators

This paper reports the impact of bottom electrode design on electromechanical coupling (k
²
t) and quality factor (Q) of laterally vibrating aluminum nitride (AlN) contour-mode resonators (CMRs). AlN-CMRs with various bottom electrode coverage rates ranging from 33% to 120% are fabricated at two different center frequencies of 220 MHz and 1 GHz. We have measured a total of 160 devices and the results show that bottom electrode coverage rate clearly affects both and Q, inducing shifts as much as 230% and 890%, respectively. The changes in aforementioned properties are different for the tested center frequencies. It has shown that the bottom coverage rate determines the active (or vibrating) region of the resonator, and thus directly impacting Q. In contrast, k
_t
²
increases with the bottom coverage rate, regardless to the center frequency of the resonator. Our findings indicate that a careful selection of the bottom electrode design could optimize or even improve both k
_t
²
and Q, which ultimately determines the power budget and noise level of AlN MEMS resonators.