A Switchable High-Performance RF-MEMS Resonator with Flexible Frequency Generations

Abstract

Resonators with multi-frequency generations at the device-level are highly desired in the future multiband, reconfigurable, and compact wireless communications. In this work, a switchable radio frequency micro-electro-mechanical system (RF-MEMS) resonator with multiple electrodes is presented. The resonator is designed to operate at the whispering gallery modes (WGMs). Simultaneous excitations of the second to seventh modes with high Q values are implemented within a single device using a pair of electrodes for driving and sensing. For effective multi-frequency excitations, the electrode span angle is optimized. The frequencies of the 37 μm and 18 μm-radius resonators range from 53 MHz to 176 MHz and 112 MHz to 366 MHz, respectively. The Q values in each mode are over 10 4 . Moreover, with the multi-electrode configurations, the specific mode can be enhanced with other modes suppressed. A more than 6 dB improvement of the spectrum peak is realized and the high Q values are maintained. A comprehensive theory is built up to clarify the driving/sensing principles under different electrode configurations. Furthermore, the air damping is found to have a significant effect on Q values for resonator with high stiffness vibrating in all the WGMs. The Q values in vacuum have at least two times improvement. The high-performance switchable resonator could dramatically reduce the power consumption, simplify the processing circuits, and occupy less footprint, which has great potential applications in future advanced RF front end systems.Future wireless communications, like 5 th generation (5 G) 1,2 , Internet of Thing (IOT) 3,4 , and tactile Internet 5 , urge for radio frequency (RF) front end transceivers with high operating frequencies, pronounced tunability and reconfigurability, reduced hardware redundancy and less power consumption 6 . Radio frequency micro-electro-mechanical system (RF-MEMS) technologies are emerging as an enabling solution to address the fast-growing demands of the advanced wireless communications 7-9 . MEMS resonators with remarkable characteristics of high f × Q products, high-level IC compatibility, small footprint, and low power consumption have great potential in communication applications, which are regarded as the key element to constitute the future RF front end transceivers 10-14 . Mechanically coupling high-Q MEMS resonators, MEMS filters with ultra-narrow passbands less than 0.1% and high stopband rejection over 50 dB have been demonstrated 15 , which are beneficial for the direct channel selection. Meanwhile, using MEMS resonators as frequency selection components, MEMS oscillators with low phase noise of −138 dBc/Hz @ 1 kHz 16 and excellent long-term frequency stability of ±0.1 ppm 17 have been implemented. The miniaturized MEMS oscillators are regarded as competitive alternatives of traditional off chip devices such as quartz crystals and surface acoustic wave (SAW) resonators 18 .Multi-frequency devices are highly desired to meet the requirements of wider frequency coverage 19 . Although...

Dominant Loss Mechanisms of Whispering Gallery Mode RF-MEMS Resonators with Wide Frequency Coverage

“…The disk resonator is surrounded by eight uniformly distributed electrodes at the circumference. Utilizing the one-pair of electrode configuration, i.e., two opposite electrodes, the 2nd to 7th WGMs can be simultaneously excited [22]. The supporting stem is located at the center of the disk, corresponding to the nodal region of the disk; thus, the anchor loss can be minimized [21].…”

“…A small angle cannot provide sufficient electromechanical couplings for low-order modes, while the large one will suppress the high-order modes. Therefore, a traded-off value of 34 • was employed [22]. The three-layer self-aligned process was adopted to batch fabricate the 37 µm and 18 µm-radius WGM resonators [28].…”

“…where h is the thickness of the disk, ω n is the angular resonance frequency of the nth mode, and X(r,θ) is the displacement of an arbitrary point in the disk, which can be expressed as [22]:…”

“…In addition, the standing waves formed at the disk periphery are beneficial for setting multiple electrodes to implement multi-mode excitations [20]. In [21,22], WGM based switchable disk resonators have been demonstrated. Distinguished from traditional single-frequency BAW resonators, multiple modes can be simultaneously excited to achieve wide frequency coverages, and the Q values retain over 10 4 for each mode at atmosphere.…”

“…However, for multi-mode WGM resonators with wide frequency coverages, the dominant loss mechanisms of each mode can be different and have not been reported yet; thus, the relevant loss sources should be fully taken into account. In the previous work [22], the energy loss sources for the multi-mode WGM resonators have been preliminarily investigated in different experimental conditions; however, the theoretical models were not built up, although the theoretical analyses are essential for quantitatively evaluating each loss mechanism.…”