Nanogap MEM resonators on SOI

Abstract

Today's consumer electronics based on a large variety of time-keeping and frequency reference applications is based on quartz-crystal oscillators, because of their excellent performances in terms of quality factor, thermal and frequency stability. However, macroscopic size and CMOS incompatibility of quartz-crystal resonators draw a major limit on the miniaturization of wireless communication applications. For this reason, silicon microelectromechanical (MEM) resonators are considered promising candidates to replace quartz-oscillators in VLSI communication systems, due to their compactness, design flexibility and CMOS process compatibility. This work reports on the design, fabrication and characterization of MEM bulk lateral resonators with resonance frequencies in the order of tens of MHz. The need of stable, low cost and high yield processes for fabricating devices with extremely narrow (sub 100 nm) transduction gaps is a main research driver, together with the requirements of improved designs which include high quality factors and appropriate power handling. We are investigating two major designs: (1) longitudinal beam resonators and (2) novel fragmented-membrane resonators that respond to both high quality factor (Q) and low motional resistance (Rm) requirements. We propose several design optimizations for solving some of the issues which currently affect the MEM resonator performance, like the frequency stability and Q enhancement through energy loss minimization. An original fabrication process is presented, which enables the manufacturing of SOI-based, fully monocrystalline devices with 100 nm transduction gaps and aspect ratios as high as [60:1], without the need of advanced lithography techniques. We successfully validate the fabrication process on two different SOI substrates, with silicon film thicknesses of 1.5 µm and 6.25 µm. This thickness range combined to the doping level corresponds to partially depleted SOI, which can be a substrate of choice for the fabrication of future integrated hybrid MEMS-CMOS integrated circuits for communication applications. The fabricated structures are successfully characterized, demonstrating excellent resonator performance at room temperature. Q's as high as 235'000 and Rm's as low as 59 kΩ have been extracted in vacuum. Atmospheric pressure frequency response measurements of the fragmented membrane resonators show Q's of 3'600. This value is among the best reported to date, opening the possibility for atmospheric pressure applications such as mass-detection for gas sensing applications with detection done without a special package, just by direct exposure of the resonator to the environment. A novel study on temperature dependence (between 80 K and 320 K) of the fragmented membrane resonators is presented and discussed. Significant Q increase and Rm reduction are experimentally observed at cryogenic temperatures. The bulk mode resonators developed and discussed in this thesis can successfully be integrated in oscillator circuits, as we demonstrate by simulating a Pierce topology based on the calibrated equivalent circuit model of a 24.48 MHz fragmented membrane BLR. Our oscillator shows very good phase-noise performance of -142 dBc/Hz at 1 kHz offset from the carrier and the noise floor at -144 dBc/Hz, which nearly meets the GSM specifications.