Design, optimization and fabrication of amorphous silicon tunable RF MEMS inductors and transformers

High performance inductors are playing an increasing role in modern communication systems. Despite the superior performance offered by discrete components, parasitic capacitances from bond pads, board traces and packaging leads reduce the high frequency performance and contribute to the urgency of an integrated solution. Embedded inductors have the potential for significant increase in reliability and performance of the IC. Due to the driving force of CMOS integration and low costs of silicon-based IC fabrication, these inductors lie on a low resistivity silicon substrate, which is a major source of energy loss and limits the frequency response. Therefore, the quality factor of inductors fabricated on silicon continues to be low. The research presented in this thesis investigates amorphous Si and porous Si to improve the resistivity of Si substrates and explores amorphous Si as a structural material for low temperature MEMS fabrication.; The process development for depositing thick amorphous Si films at low temperature with good uniformity and surface roughness is presented. The film is optimized for our proposed application with acceptable optoelectronic properties and mechanical stress. Porous Si is also studied as another candidate to improve the resistivity of Si substrates. A Teflon etch cell was machined to electrochemically form porous Si at room temperature and the process conditions are optimized to achieve thick films with suitable porosity for multi-layer applications.; Planar inductors are built-on undoped amorphous Si in a novel attempt to improve the isolation of RF passive devices from the low resistivity Si substrate. A 56% increase in Q was measured by incorporating a thick amorphous Si film before depositing the underpass of the inductor. Planar inductors are also built-on a porous Si and amorphous Si bilayer and showed 47% improvement in Q compared to the same structure without a porous Si layer.; Amorphous Si is proposed as a low temperature alternative to polysilicon for MEMS devices. Tunable RF MEMS inductors and transformers are fabricated on low resistivity Si in a six mask process. The design concept is based on an amorphous Si and Al bimorph coil that is suspended and warps in a controllable manner that is accurately predicted by thermomechanical simulations. The tuning of the devices is achieved by applying a DC voltage and due to joule heating the air gap can be adjusted. A tunable inductor with a 32% tuning range from 5.6 to 8.2 nH and a peak Q of 15 was measured. A transformer with a suspended coil demonstrated a 24% tuning range of the mutual coupling between two stacked windings.; The main limitation posed by post-CMOS integration is a strict thermal budget which cannot exceed a critical temperature where impurities can diffuse and materials properties can change. The research carried out in this work accommodates this temperature restriction by limiting the RF fabrication processes to 150? to facilitate system integration on silicon.