Design and Fabrication of RF-MEMS Components: Tunable HTS Resonator and Low Pull-In Voltage Capacitive Switch

將微機電技術整合在無線通訊系統中以提升系統性能的研究已經進行了數十年。而其中主要的微機電元件包含了具有高品質因子的電感、可變電容及微波開關等。在本研究中，吾人將介紹兩個將微機電元件整合在射頻電路中實際應用。 第一種應用是可調變高溫超導共振子。自從高溫超導體(High Temperature Superconductor)在1987 年問世以來，其最重要的應用便是在無線通訊上。由於高溫超導體的操作環境在其臨界溫度下時有低交流電阻的特性，故可利用來製作高品質因子的微波共振子及窄頻濾波器。然而，為了增加設計上的裕度以及對於製程與操作溫度變異的容忍性，一種有效的調變機制是需要的。除此之外，此調變機制也可在無線通訊系統中提供頻率調變的功能。本研究提供了一個可以與高溫超導體整合的微機電可變電容之設計方法與製作流程並將之實作在調變高溫超導體微波共振子之工作頻率。 本研究也將介紹以台積電點35 金氧互補式半導體製程製作之低吸附電壓懸臂梁-扭轉梁混合電容式開關。與一般懸臂梁開關不同的是本設計可提供較大的接觸面積與較低的吸附電壓。其中的扭轉梁結構，可幫助降低吸附電壓。除此之外，本開關即使在殘留應力的作用下與同面積的傳統懸臂梁開關相較仍能保持較低的吸附電壓。本研究使用的是與金氧互補式半導體製程相容之後製程，因此可以保證此微波開關可與電路相容。目前量測到最低的吸附電壓為7.5V。Employing micromachined technology in wireless communication system to improve its performance has been studied and practiced for decades. The main micromachined components include high-Q inductors, tunable capacitors, and micromachined switches, etc. In this study, we will introduce two RF MEMS applications which integrate the micromachined components with RF electric circuits. The first application is tunable high-temperature resonator. One of the most successful applications of high-temperature superconductors (HTS) is in the front-end of wireless communication systems. With the advantage of low electrical loss below critical temperature, high-Q resonators and narrow band filters have been achieved by using HTS microstrips. However, effective tuning mechanisms are required to increase the design tolerance and the device durability subjected to fabrication variations and thermal budgets, respectively. In addition, the tuning mechanisms can be applied in frequency hopping for communication systems. This study presents the design methodology and fabrication of micromachined capacitor implemented on tuning HTS resonators. The tuning technique developed in this paper can also change the resonant frequency of the resonator so that a tunable narrow band filter with high quality factor is realized. As the other application, we also present a low pull-in voltage cantilever-torsion mixed capacitive switch fabricated by TSMC 0.35um CMOS process in this study. Dissimilar to the conventional cantilever switches, the design has larger contact area and smaller pull-in voltage by combining cantilevers and torsion beams. It is verified that the torsion beams can help decrease the pull-in voltage. Besides, the residual stress can make the switch become a gap-varying switch with very small initial gap. The pull-in voltage of the switch keeps smaller than conventional cantilever switch under affection of the residual stress. A CMOS compatible post-process is utilized to guarantee the switch can be integrated with electric circuit. The measured smallest pull-in voltage of the capacitive switch is 7.5V in this paper