A circular electrostatic zipping actuator for the application of a MEMS tunable capacitor

Abstract

Micromechanical circuits such as MEMS switches, tunable capacitors (varactors) or resonators in general have lower loss and consume less power than their CMOS counterparts and have seen an increase of applications in high-value communication systems as well as low-cost commercial communication networks. Significant advances have been made in the areas of MEMS switches. However, MEMS resonators that operate in GHz range, have high quality factor and are highly tunable are still under active pursue. In this thesis, we study the design of a tunable capacitor that can be integrated with a resonant cavity to form a tunable electromagnetic cavity resonator. The design, fabrication, modeling and testing of a proof-of-concept MEMS tunable capacitor are presented. The tunable capacitor consists of a circular fulcrum that acts as a pivot for a thin silicon plate. The outer plate is an electrostatic, circular zipping actuator that bends the center plate through the fulcrum. By doing so, it opens the gap of the capacitor, which is formed by two smooth surfaces, one being the center plate, that are initially separated by a dielectric layer. The design is enabled mainly by the deep reactive ion etching and anodic bonding microfabrication techniques.

(cont.) The structure of the device is modeled using both numerical methods with Matlab boundary value problem (BVP) and finite element analysis with ANSYS. The Matlab results match well with the ANSYS results for the before pull-in and the zip-in actuation stages. The Matlab model is used to perform parametric design studies. Two types of assembly methods are used to construct the final devices: wafer-level and die-level. Depending on how they are assembled, the devices operate in different actuation stages. A laser interferometer system is used to measure the center displacement of the plate and an impedance analyzer is used to measure the capacitance change. Testing results are comparable with the ANSYS simulations. Based on the lessons learned from the proof-of-concept tunable capacitor, a design of the electromagnetic cavity resonator with an integrated tunable capacitor is proposed.