Ark-Chew Wong

VHF free-free beam high-Q micromechanical resonators

Free-free-beam flexural-mode micromechanical resonators utilizing nonintrusive supports to achieve measured Qs as high as 8400 at VHF frequencies from 30 to 90 MHz are demonstrated in a polysilicon surface micromachining technology. The microresonators feature torsional-mode support springs that effectively isolate the resonator beam from its anchors via quarter-wavelength impedance transformations, minimizing anchor dissipation and allowing these resonators to achieve high-Q with high stiffness in the VHF frequency range. The free-free-beam micromechanical resonators of this paper are shown to have an order of magnitude higher Q than clamped-clamped-beam versions with comparable stiffnesses.

Frequency trimming and Q-factor enhancement of micromechanical resonators via localized filament annealing

A batch-compatible, post-fabrication annealing technique based upon filament-like heating of microstructures is demonstrated as an effective means for trimming the resonance frequencies (f/sub 0/s) and increasing the quality factors (Qs) of surface-micromachined, polysilicon, mechanical resonators. Although the technique is straightforward, involving the mere application of a suitable voltage between the anchors of a micromechanical resonator, it provides a substantial range of adjustment, with frequency trims of over 2.7% and Q increases of up to 600%, depending upon resonator fabrication history. By pulsing the anneal voltage waveforms, controlled frequency trims of less than 16 ppm per trial are achievable.

Parallel-resonator HF micromechanical bandpass filters

High frequency, fourth-order, micromechanical bandpass filters, with tunable frequency and bandwidth, and filter Qs in the thousands, are demonstrated in a polysilicon surface micromachining technology. These filters utilize a parallel-resonator architecture, in which properly phased outputs from two or more micromechanical resonators are combined to yield a desired filter spectrum. Design formulas are given for Butterworth, Chebyshev, and Bessel filters, and each of these filter types are demonstrated with center frequencies close to 14.5 MHz and filter Qs ranging from 830 to 1600.

Tunable, switchable, high-Q VHF microelectromechanical bandpass filters

Recent attempts to reduce the cost and size of wireless transceivers feature higher levels of transistor integration in alternative architectures to reduce the need for the off-chip, high-Q passives used in present-day super-heterodyne transceivers. Unfortunately, removal of off-chip passives often comes at the cost of increased power consumption in circuits preceding and including the analog-to-digital converter (ADC), which must have higher dynamic ranges to avoid desensitization caused by larger adjacent channel interferers. A selectivity (Q) versus power trade-off is seen here. This device makes possible a paradigm-shifting transceiver architecture that, rather than eliminate high-and passive components, attempts to maximize their role with the intention of harnessing the above Q versus power trade-off.

A Bonded-Micro-Platform Technology for Modular Merging of RF MEMS and Transistor Circuits

A technology has been demonstrated that uses compression bonding to modularly combine platform-supported jumechanical filters with integrated BiCMOS transistor circuits while attempting to preserve the Q of mounted resonators. In this process, jumechanical devices are first fabricated onto SOI platforms, which are then released (together with devices) and compression bonded onto a transistor circuit wafer, making electrical connections at the bonds. Prior to bonding, while mounted on unreleased platforms, 6 MHz and 40 MHz clamped-clamped beam jumechanical resonators exhibit Q’s of 2,000 and 300, respectively. After release and bonding to the circuit wafer, the Q’s are degraded to 520 and 120, respectively. Poor bonding quality is identified as a likely reason for the observed Q reductions.

Micromechanical mixer and filters

A device comprised of interlinked micromechanical resonators with capacitive mixer transducers has been demonstrated to perform both low-loss frequency translation (i.e. mixing) and highly selective filtering of applied electrical input signals. In particular, successful down-conversion of radio frequency (RF) signals from 40-200 MHz and subsequent filtering at a 27 MHz intermediate frequency (IF) with less than 15 dB of combined mixing conversion and filter insertion loss is demonstrated using this single, micromechanical device. Mixing conversion gain/loss is shown to depend upon a ratio of local oscillator amplitude and applied bias voltages. A single-sideband (SSB) noise figure of 18 dB is achieved.