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

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.