Polymer-based RF MEMS Devices
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The radio frequency micro-electro-mechanical system (RF MEMS) technology is rapidly transitioning from the research stage to commercial applications. Reducing material and fabrication costs, realizing IC compatibility, and improving the RF performance have been main focal points of many recent works. In this dissertation, we target at developing a RF MEMS methodology which realizes CMOS compatibility, cost effectiveness and high performance at the same time. The current RF MEMS devices are mostly constructed on quartz and III-V compound materials to achieve low RF losses. These approaches proved to have excellent RF performance. However these materials are expensive and unable to be integrated into CMOS circuitry. In this work, we developed a CMOS-compatible approach to construct RF MEMS components where low-resistivity silicon and plastic were used as the substrate material and dielectric layer, respectively. This method is cost-effective and presents very high RF performance. Coplanar waveguide (CPW) transmission lines were first realized using this approach. Devices were characterized from DC to 26 GHz, a low insertion loss of 3 dB/cm was achieved. Soft substrate of Kapton film was also used to build CPW lines to answer the increasing trend of flexible devices such as RFID, cell phone antenna and soft display. A very low transmission loss of 1 dB/cm was obtained. Additionally, multiple devices of various dimensional emphases were fabricated to suit different dimensional situations. A novel method of building CMOS-compatible distributed MEMS transmission line (DMTL) phase shifters was developed. A thick layer of Kapton film was bonded with low-resistivity silicon substrate to prevent RF signal leakage. Design consideration, fabrication processes and measurement results were discussed in detail. For comparison purposes, DMTL phase shifters were constructed on glass substrates as well. In order to verify the design roles and fit for different space situations, various phase shifters were fabricated using this method. Our devices showed comparable results to those built on quartz and III-V compound substrates. Tunable filters are essential elements for modern communication systems. Using the same polymer-silicon method, we built two types of tunable filters - partial-loaded short-ended filter and even-loaded open-ended filter. Design methodology, fabrication and measurement were discussed. Our tunable filter demonstrated 2 GHz tunability and 10 dB transmission loss. Failure mechanisms were examined for possible improvements. The fabrication process of the polymer-silicon approach developed for RF MEMS devices is simple, cost-effective and CMOS compatible. It virtually applies to any situation where quartz, III-V compound materials or other CMOS-incompatible materials have to be used for RF performance purposes. Thus, at the end of this dissertation, impedance tuners, digital controlled RF MEMS devices and a RF MEMS packaging technique using this polymer-silicon method were proposed for future works.