A MEMS Tunable Capacitor With Dual Deformation Modes and High Tunability and Linearity

MEMS tunable capacitors have applications in tunable filters and RF circuits where high tunability and Q-factor are desired. Conventional parallel-plate tunable capacitors have a highly nonlinear capacitance-voltage (C-V) response and limited tunability of up to 50% due to fundamental limitation and structural instability. In this work, we present a novel design idea for a parallel-plate tunable capacitor that increases the tuning ratio and provides a smoother (more linear) response. The design uses two modes of deformation, rigid-body displacement of a curved moving electrode before pull-in and deforming the plate after pull-in, and exploits nonlinear structural stiffness to improve the linearity (and the tunability) of the tunable parallel-plate capacitor. The capacitor structure is designed such that when actuation voltage is applied, first the beams holding the moving electrode deform, and capacitance increase similar to conventional design up to pull-in. After the pull-in, the top electrode (which has a curved geometry) is deformed and further increases the capacitance, as the voltage increases. The design may provide an overall simulated tunability of more than 380%, and also has a more linear C-V response. The design is modeled and simulated using ANSYS coupled-field multiphysics solver and the effect of different design parameters are investigated. The simulation results show much high tunability and better linearity than conventional parallel-plate capacitors.

Novel RF MEMS tunable capacitors

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%

Novel RF MEMS tunable capacitors

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%

Reconfigurable Multiband Operation for Wireless Devices Embedding Antenna Boosters

Wireless devices such as smart meters, trackers, and sensors need connections at multiple frequency bands with low power consumption, thus requiring multiband and efficient antenna systems. At the same time, antennas should be small to easily fit in the scarce space existing in wireless devices. Small, multiband, and efficient operation is addressed here with non-resonant antenna elements, featuring volumes less than 90 mm3 for operating at 698–960 MHz as well as some bands in a higher frequency range of 1710–2690 MHz. These antenna elements are called antenna boosters, since they excite currents on the ground plane of the wireless device and do not rely on shaping complex geometric shapes to obtain multiband behavior, but rather the design of a multiband matching network. This design approach results in a simpler, easier, and faster method than creating a new antenna for every device. Since multiband operation is achieved through a matching network, frequency bands can be configured and optimized with a reconfigurable matching network. Two kinds of reconfigurable multiband architectures with antenna boosters are presented. The first one includes a digitally tunable capacitor, and the second one includes radiofrequency switches. The results show that antenna boosters with reconfigurable architectures feature multiband behavior with very small sizes, compared with other prior-art techniques.