Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication Systems

We present results on the development of a novel micromachined parallel-plate tunable capacitor with a wide tuning range. Different from conventional two-parallel-plate tunable capacitors, this novel tunable capacitor consists of one suspended top plate and two fixed bottom plates. One of the two fixed plates and the top plate form a variable capacitor, where as the other fixed plate and the top plate are used to provide electrostatic actuation for capacitance tuning. For the fabricated prototype tunable capacitors, a maximum controllable tuning range of 69.8% has been achieved experimentally, exceeding the theoretical tuning range limit (50%) of conventional two-parallel-plate tunable capacitors. This novel tunable capacitor also exhibits very low return loss (< 0.6dB between 45 MHz and 10GHz). Its fabrication process is completely compatible with the existing standard IC (integrated circuit) fabrication technology, which makes it useful in integrated wireless communication systems. Introduction In recent years, MEMS (microelectromechanical systems) technology has begun to be used in wireless communication systems to improve performance of existing devices based on structures or operational principles. Tunable RF components such as voltage-controlled oscillators (VCO) and tunable filters have been developed using micromachining technology. Young, et al reported a VCO [1] that contains a micromachined three-dimensional coil inductor [2] and a two-parallel-plate tunable capacitor with a tuning range of 16% [3]. Both passive elements are micromachined on silicon substrates and thus can be integrated with active circuit components using modified IC fabrication processes. In the past few years, as an important tunable RF component, tunable capacitors based on MEMS technology are under active development [4-6]. Compared with solid-state varactors, MEMS tunable capacitors have the advantages of lower loss and potentially greater tuning range. The interconnection loss and noise can also be less than those of using off-chip RF components. Among all the MEMS tunable capacitors developed to date, parallel-plate configuration (using electrostatic actuation) is the most commonly used. A parallel-plate tunable capacitor can be fabricated easily using surface micromachining techniques. However, the theoretical tuning range of such capacitors is limited to 50% by the pull-in effect. The actual achieved tuning range is often much smaller than the theoretical value due to parasitic capacitance (e.g. a measured tuning range of 16% was reported in [3]). Various broadband communication applications require a wide tuning range. Recently, much effort has been made to improve the tuning capability of MEMS tunable capacitors. Dec, et al [4] uses a three-parallel-plate configuration (two suspended plates and one fixed plate on the substrate) to compensate the pull-in effect and obtain a tuning range of 87%. The fabrication process requires two layers of structural materials and two layers of sacrificial materials. Yao, et al [5] reported a tunable capacitor with a tuning range of 200%. It is based on lateral comb structures (instead of parallel plate) etched by deep reactive ion etching (DRIE) on SOI (silicon-on-insulator) substrate. Feng, et al [6] used a thermal actuator in their tunable capacitor and achieved a tuning range of 270%. The disadvantage of thermal actuators is that their response speed is generally slower than that of electrostatic actuators. We report the development of a novel MEMS electrostatically tunable capacitor. This new design keeps the simplicity of conventional two-parallel-plate configuration, while providing a much wider tuning range. Only one structural layer and one sacrificial layer are used. The measured tuning range achieved was 69.8%, exceeding the 50% tuning range limit on conventional two-parallel-plate tunable capacitors. The capacitor operates under a wide frequency range (45 MHz to 10GHz) with signal loss below 0.6 dB. The new tunable capacitor is fabricated using standard surface micromachining process and can be monolithically integrated with other RF (radio frequency) devices and IC circuits if necessary.