MEMS resonators have been very promising in replacing conventional resonators used in portable wireless applications. They have the merits of small size, high quality factor (Q), low power consumption, etc. However, packaging for MEMS devices is a big challenge. Because of its delicate structure, a MEMS resonator needs to be protected inside of a pristine environment. Also, air-damping will greatly reduce the quality factor of resonator and thus a low-pressure environment is also essential.
We presents the design, fabrication, and measurement of a 207 MHz, half-wave mode resonator that is encapsulated in the epitaxy-silicon encapsulation technique. It is the world highest frequency fully encapsulated resonator reported.
The design of half-wave thickness shear mode resonator is illustrated in Fig. 1. The main resonator beam is consisted of three layers, a 100 nm polysilicon layer, a 100 nm silicon nitride layer, and a 3 µm single-crystal silicon layer. As shown in Fig. 1, the polysilicon is served as input and output electrodes and silicon nitride layer is used as high-K dielectric to enhance the transduction.
The fabrication process is based on a combination of two previous works. [1, 2] The detail process flow is shown in Fig. 2 and Fig. 3 shows the fabricated resonator. A differential measurement technique, inspired by Rantakari’s work [3], was performed in order to alleviate the impact from parasitic capacitance. A RF signal from the network analyzer first goes into a splitter to create in-phase and out-of-phase signals, represented by +RFin and –RFin, respectively. In order to induce resonance of resonator, a VDC was added into the +RFin. Two identical resonators were driven by +RFin and –RFin. While +RFin was driving the resonator, the pure AC –RFin was driving the dummy resonator, which serves as the parasitic capacitance. The outputs from both structures were then mixed together, where the out-of-phase current from the parasitic capacitance cancels out the parasitic component in the signal from resonator. The result is plotted in Fig. 4, showing the resonant frequency of 207 MHz and quality factor of 6,400. This is the highest frequency resonator packaged in a vacuum packaging to date.
A fully-encapsulated thickness shear mode resonator was successfully fabricated and presented. In the future, we hope to demonstrate on-chip high performance tunable filters and filter banks.
Contact:
Kuan-Lin Chen
Fig. 1
Fig. 2
Fig. 3
Fig. 4
