High-Q COMS-compatible poly-silicon germanium electrostatic RF MEMS resonators

This thesis describes the design, fabrication, and operation of two polycrystalline silicon-germanium (poly-SiGe) electrostatic CMOS-compatible RF MEMS microresonators capable of high frequency operation and high mechanical quality factors: the bulk longitudinal resonator (BLR) and the radial bulk annular resonator (RBAR). The annular design of the RBAR makes it scalable (i.e., the average radius can be scaled up or down without changing the frequency), enabling it to decouple frequency and insertion loss. The RBAR was the first such MEMS microresonator design to realize this possibility. The majority of other MEMS contour-mode microresonators decoupling frequency and insertion loss employ a similar annular design. The operation of the BLR and RBAR are analyzed mathematically, and equivalent circuits are developed from first principles for their unique mode shapes.; Poly-SiGe is the technology that enables the BLR and RBAR to be fabricated directly on top of modern foundry CMOS. Poly-SiGe is shown to be a material capable of high quality factors at VHF frequencies (61,100 at 20 kHz [6], 54,000 at 47 MHz [8], and 3,500 at 205 MHz [54]). The Young's Modulus and strain gradient of in-situ boron doped (p-type) poly-SiGe are determined to be E ≈ 155 GPa and 4 x 10 -4 mum-1, respectively, for poly-Si0.35Ge 0.65. A model is proposed and verified for the peroxide etch mechanism of poly-SiGe. The experimental data used to verify the peroxide etch mechanism is used to suggest optimal deposition conditions for poly-SiGe where nano-gaps are needed. The peroxide etch rate of in-situ boron doped (p-type) poly-Ge is also determined. The stress drift of low temperature oxides is studied as well, and a mechanism for this drift is developed and supported by experimental evidence.; Results from the successful operation of a fully CMOS-compatible, poly-SiGe BLR and two poly-SiGe RBARs are examined in the final chapters. Their successful operation at VHF frequencies with high quality factors validates: electrostatic operation at such frequencies, poly-SiGe as a viable material for RF MEMS, and the theory and analysis behind the resonators. Finally, one of the RBARs exhibits a large rejection for its Q and resonant frequency.