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Electrode Integrated Wafer-Level Packaging for Out-of-Plane MEMS Devices
Keywords: Wafer-level packaging, RF MEMS, Resonator
This paper presents an innovative way to integrate electrodes into a wafer-level packaging for out-of-plane actuated MEMS devices. Resonators operating in out-of-plane direction have the advantage of large transduction areas on the top and bottom surfaces, resulting smaller motional-impedances (Rx). However, the reported out-of-plane resonators, such as results from Demirci and Nguyen in JMEMS2006, and Wang and Nguyen in JMEMS2000, share the same shortcomings. The requirement of additional thin and long electrodes, would add large electrode resistance to device¡¯s Rx. It is troublesome for applications that require matching impedance of 50§Ù. Furthermore; the out-of-plane electrodes can possibly be damaged during packaging processes. This work provides a novel solution for out-of-plane devices with a stable package and low impedance electrodes. The integration allows reduction of parasitic resistance by 180X. The packaging is based on the wafer-level epitaxy-silicon encapsulation technology developed at Stanford University. The key feature of this work is the out-of-plane electrode is grown with the packaging material, epi-silicon in this case, and is electrically separated from the packaging. The electrode is defined by the enclosed trenches in the epi-silicon layer. Also, the electrode is anchored to the device layer to provide stability. The over-hanging electrode is incorporated in the epitaxy-silicon layer and can capacitively sense and actuate out-of-plane devices. Furthermore, the resistance of electrode is calculated to be less than 1§Ù. A simple rotational ¡°see-saw¡± mode resonator is used to demonstrate the out-of-plane transduction. The SEM pictures of fabricated resonators and top electrodes in the package were taken. The measured resonant frequency is 194KHz with quality factor of 970. The COMSOL simulation showed the resonant frequency of 201KHz. The resonant frequency is varying with the applied VDC due to spring-softening effect. The dependency was recorded and the fitting curve extrapolates the frequency at zero bias to be 199KHz.
Kuan-Lin Chen, Research Assistant
Stanford University
Stanford, CA

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