This paper aims to design and simulate the surface-micromachining micro tunable capacitor for parameters optimization. This work also creates an equivalent circuit model of micro tunable capacitor and proceeds relative electromechanical analysis, including the distribution of field and charge, resonant frequency and pull-in voltage analysis. This micro tunable capacitor is constructed by one suspended top metal plate and two stationary bottom metal plates (one is signal electrode and the other one is bias electrode). By driving electrostatic force, the gap between top and bottom electrodes will be changed and results in a variation of capacitance. To increase the tuning range, the micro tunable capacitor with two different gap space will be presented in this research. High frequency analysis, equivalent circuit analysis and electromechanical dynamic analysis are using Ansoft HFSS, Agilent ADS and the IntelliSuite software respectively. Through these simulation and analysis, it is possible to obtain the optimized specification of micro tunable capacitor. The quality factor (Q) and the pull-in voltage extracted by simulation software well match to the measured results; thus, the function of the analysis method and equivalent model adopted in this thesis can be demonstrated.

The passive devices used in the wireless communication system ¡]including resistor, capacitor and inductor¡^usually need high quality factor and low power dissipation characteristics. This thesis aims to develop a micro tunable capacitor with high-quality-factor and wide-tuning-range using surface micromachining. In contrast with conventional low-tuning-rate parallel-plate tunable capacitors, this research presents a concave structure and eight-suspending-beams layout design of the top electrode to enhance the elastic rigidity and tuning rate. In addition, this study appropriately decreases the thickness of top electrode, the tuning rate of such device can be improved to 65~2100%. On the other hand, in order to substantially increase quality factor, this thesis adopted the glass substrate ¡]Corning 7740¡^to reduce the power dissipation of high frequency operating signal. The optimized quality factor of this work is approximately equal to 41 under 2.4 GHz operation frequency. The material of sacrificial layer and top electrode adopted in this dissertation is aluminum and gold respectively. To avoid any breakage of the vertical supporting beams during releasing process, this research appropriately increases the width of vertical supporting beams, however, keep the thickness of the suspending part of top electrode for the maintenance of high quality factor and low driving voltage.