Micromechanical extensional wine -glass mode ring resonators for wireless communications.

Abstract

This dissertation investigates new designs and techniques to improve the performance of vibrating micromechanical signal processors for wireless communication applications, primarily focusing on ring-type resonators. The thesis discusses the theoretical design, simulation, fabrication, and characterization of such devices, with a purpose of providing a new path towards further lowering the motional impedance of micromechanical resonators. Vibrating polysilicon micromechanical ring resonators, utilizing a unique extensional wine-glass mode shape to achieve lower impedance than previous UHF resonators, have been demonstrated at frequencies as high as 1.2GHz with a <italic>Q</italic> of 3,700, and 1.52GHz with a <italic>Q</italic> of 2,800. The 1.2-GHz resonator exhibits a measured motional resistance of 1MO with a dc-bias voltage of 20V, which is 2.2x lower than measured on radial mode disk counterparts at the same frequency. The extensional wine-glass resonator, which integrates the advantages of the extensional vibration mode, nonintrusive side supports and a ring geometry to achieve high frequency, high <italic> Q</italic>, and impedance tailoring capability, presents itself as another attractive candidate for miniaturization of transceivers. This dissertation also develops techniques to eliminate undesired resonance modes and thus, solves one of the most disturbing issues with ring-type resonators. Through a combination of suspension-derived damping, electrostatic force tailoring, and sense electrode current cancellation, spurious modes at 406.7, 418, and 419 MHz, normally observed near the resonance of a 415-MHz extensional wine-glass mode resonator have been successfully suppressed, as have all spurious modes within at least a 50% bandwidth region around the resonant frequency. These improvements pave the way for the use of these devices in micromechanical oscillator and filter circuits targeted for future wireless transceivers. A 217-MHz filter structure, composed of two mechanically-coupled ring resonators with solid dielectric gaps, has been investigated to prove the design concept including a notched coupling to achieve a narrow passband as small as 0.02% percent bandwidth. The fully differential drive/sense technique is employed in the filter measurement, where a large stopband rejection of 25dB is measured due to the significant reduction of severe parasitic feedthrough currents. This result verifies the efficacy of differential micromechanical filter design.