Abstract
Current tunable devices such as filters, impedance matching networks and oscillators have problems that degrade their performance at high microwave frequencies. Tuning ratios and quality factors are the major problems associated with semiconductor based tuning components. This thesis presents the design, fabrication and testing of two novel RF MEMS tunable capacitors. The first tunable capacitor is designed using electrostatic repulsive-force actuators which produce an upward movement of the moving plate of a tunable capacitor. The repulsive-force actuator is free of pull-in effect and capable of reaching large displacement. Gap increasing tunable capacitors with areas of 162μm×220μm and 300μm×302μm are developed using electrostatic repulsive-force actuators. The capacitances are calculated using simulations and maximum tuning ratios of 438.5% and 230% are obtained for a parallel and inclined plate designs, respectively, with capacitance-voltage linearity of 96.28% and 95.14%, respectively, in the presence of RF voltage. The second tunable capacitor is developed using residual stress gradient based vertical comb-drive actuator. Conventional vertical comb-drive actuators need two vertical comb fingers, i.e., one for the fixed and one for the moving comb. MetalMUMPs process provides a 20μm thick nickel layer which is subject to residual stress gradient along its thickness. Using the residual stress gradient two curve-up beams are devised to bend out of plane and upward. A moving plate is connected between the middles of the curve-up beams through supporting springs and is raised above the substrate. The moving fingers are connected to opposite sides of the moving plate. The fixed comb-drive fingers are anchored to the substrate. When a voltage is applied, the moving fingers move down towards the fixed fingers. As a result, the capacitance between the moving fingers and the fixed fingers change. Prototypes are fabricated to verify the working principles of this novel actuator using the MetalMUMPs process. Tunable capacitors based on this actuator are experimentally analyzed. Quality factors of 106.9-162.7 at 0.8GHz and 42.4-51.9 at 1.24GHz are obtained over actuation voltage of 0-100V. An optimal design of the tunable capacitors achieved a tuning ratio of 194.4% at 162.5V with linearity of 97.84%
Highlights
1.1 MotivationTuning components have been integral parts of electronic circuits such as filters, impedance matching networks and oscillators used extensively in communication circuits
Recent studies showed that MEMS technology has better potential to be used as tunable components for high frequency applications by offering better performance such as tuning ratios, quality factors and linearity [4, 5, 12-14]
This thesis work presented the first attempt of implementation of MEMS tunable capacitors driven by repulsive-force electrostatic actuators
Summary
1.1 MotivationTuning components have been integral parts of electronic circuits such as filters, impedance matching networks and oscillators used extensively in communication circuits. Tunable capacitors with high quality factors and high tuning ratios are desirable for applications at higher microwave frequencies. High quality factors can be achieved by using metal-insulatormetal (MIM) capacitors with low dielectric constants and low resistivity contact metals The disadvantage of such capacitors is that they provide constant capacitance over all frequencies, i.e. capacitance tuning ratio of 1 which is non-tunable. Recent studies showed that MEMS technology has better potential to be used as tunable components for high frequency applications by offering better performance such as tuning ratios, quality factors and linearity [4, 5, 12-14]. Vertical comb drive actuators require two structural layers to fabricate vertically separated fixed fingers and moving fingers. Power losses are computed using the S-parameter results to account of the RF power dissipated in the substrate and the metal
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