High-Q X Band Aluminum Nitride Combined Overtone Resonators

We experimentally demonstrate a new class of micro-electro-mechanical system (MEMS) resonators, aluminum nitride (AlN) Combined Overtone Resonators (CORs). AlN CORs utilizing a combination of the $2^{{n}{d}}$ (A2) and $3^{{r}{d}}$ (A3) order asymmetrical Lamb-wave overtone at 8.8 GHz are built using one-micron aluminum nitride plates fabricated with a simple 2-mask process. Mechanical quality factor Q m and loaded quality factor Qi as high as 1100 and 750 are achieved (corresponding to $f^{\star}Q_{m}$=9.7e12 and $f^{\star}$Qr-6.6e12), being the highest reported for piezoelectric MEMS resonators operating in X band and higher bands. Those devices show near 50 $\Omega$ termination impedances (56 $\Omega$), low motional resistance R m values (20 $\Omega$) and an electromechanical-coupling coefficient k ² of 0.3%. Frequency tuning capability by varying pitch size is also demonstrated. Low- cost X band narrow band filters and oscillators can be easily built using such a new resonator technology with relaxed photolithography requirements and simple fabrication processes.