Theoretical Optimization And Process Study On RF MEMS Varactors

Nowadays, the application of RF MEMS (Radio FrequencyMicro-Electro-Mechanical System) technology has attracted more and more attentionin RF communication domain. To achieve a RF VCO with wide frequency selectivityand low phase noise, special varactors with both high quality factor and large tuningrange are required. RF MEMS varactors with Electrostatic actuation have become ahot topic in academic research. This thesis presents our studies on a high tuning rangeSide Pull-down Electrodes Controlled (SPEC) MEMS varactor. It focuses on themodeling and simulation of the electrical and mechanical behaviors of the device, theeffects of parasitic components, and the key fabrication processes. Devices with highperformance have been realized and characterized.The concept and physical structure and of a SEPC MEMS varactor is introducedand FEA method is used to verify mechanical performance. It shows that SPECMEMS varactor can weaken or avoid the pull-in effect which limits the tuning rate ofconventional two-parallel-plates MEMS varactor to about 50%. In order to analyzeand optimize the structure parameters with efficiency, a numerical model with energymethod is developed. Two iterative processes are introduced to generate the trialdisplacement function and ensure its accuracy during the whole computation process.The displacement-voltage curve is obtained by energy method, and the calculationresults are verified with FEA simulations. Based on the quick and accuratecalculation, the aluminum SPEC varactors with various geometric parameters areobserved, and devices are optimized for different requirements.An electrical model of SPEC MEMS varactor is established, and the effects ofparasitic components have been analyzed and compared. Based on the model, RFperformance of the device is simulated with ADS softwareTo realize the designed SPEC MEMS varactor, several key processes have beendeveloped, including porous silicon oxidization, the etching and releasing ofpolyimide as sacrificial material, and fabrication of suspended plate. By using thesetechniques, the device has fabricated successfully.To study the influences of different substrate, devices have been fabricated onboth OPS (Oxidized Porous Silicon) substrate and HRS (High Resistivity Silicon)substrate. The measurement shows both the substrate can be used to achieve highquality factor: 30@3GHz on HRS and 45@4GHz on OPS substrate.Measurement results based the obtained SPEC MEMS varactors are presentedand discussed. The device with Al suspended plate achieve a quality factor of30@3GHz, and 42% tuning range under 50V control voltage. Another device withAl/SiON complex plate has quality factor of 18@4GHz, and 130% tuning rangeunder 40V. the device keep functional during test of 1 million actuation.