Research Of High Q RF-MEMS Planar Inductor

Nowadays, the application of RF-MEMS (Radio Frequency Micro-Electro-Mechanical System) technology to modern wireless communication has been gotten more and more attention in both industry and academic society. As one of the most important components of RF circuits, high Q inductors realized with RF-MEMS are the most active research subjects. In this thesis, fabrication of MEMS planar inductors on different substrates is investigated. In order to reduce substrate loss and improve inductors' Q factor (Quality Factor), a novel insulation method, forming thick OPS layer on the low resistivity silicon (LRS) substrate, is proposed. As a result, Copper inductors are successfully fabricated on modified LRS substrates with thick OPS layers and HRS substrates respectively. Very high Q factors are achieved. In the first part of this thesis, a physical model of RF-MEMS planar inductor is introduced. Based on this model, analytical formulas used for the calculation of Q factor and inductance is obtained, and the analysis of the impact factors, such as inductor size and material parameters, to the inductor Q value are further performed. In order to calculate the inductor parameters with higher accuracy, Ansoft HFSS is then used, and the obtained result is accordant well with that obtained using the physical model of MEMS planar inductor. That proves the validity of both methods. By deeply investigating the loss mechanisms, we propose to improve the Q factors of MEMS planar inductors via using modified LRS substrate with thick OPS layers or HRS substrate as low loss substrates and preparing the coils with copper material.To realize the designed planar RF-MEMS inductor, several key processes should be developed, among which are porous silicon (PS) oxidization, polyimide layer preparation and dry etching, and copper electroplating. After many experiments, we obtained the optimized preparation conditions of selective PS. Followed by drying the PS in pentane environment and oxidizing it in a two-step oxidization method, the thick selective OPS layers on LRS substrates are successfully realized. The maximal OPS thickness is 60 microns. Meanwhile, we studied the formation and dry etching of polyimide films, which is used as dielectric layer between inductor coils and underpass lead. To fabricate thick copper coils, a selective copper electroplating on seed layer with resist as structural mold is established. The main process steps include seed layer sputtering, lithography of thick photoresist, copper electroplating and seed layer removal. After the structure design and technology process, high Q copper inductors on LRS substrates with thick OPS insulation layers and HRS substrates are achieved and the microwave measurement is performed. The best copper inductor on modified LRS substrate has a Q factor of 12 at 10GHz and an inductance of 0.9nH with the resonant frequency beyond 20GHz。The Q factor of copper inductors on HRS substrate reaches 25 at 5GHz. The inductance is 2nH and the resonant frequency is 15GHz.