Wan-Thai Hsu

High-Q UHF micromechanical radial-contour mode disk resonators

A micromechanical, laterally vibrating disk resonator, fabricated via a technology combining polysilicon surface-micromachining and metal electroplating to attain submicron lateral capacitive gaps, has been demonstrated at frequencies as high as 829 MHz and with Qs as high as 23 000 at 193 MHz. Furthermore, the resonators have been demonstrated operating in the first three radial contour modes, allowing a significant frequency increase without scaling the device, and a 193 MHz resonator has been shown operating at atmospheric pressure with a Q of 8,880, evidence that vacuum packaging is not necessary for many applications. These results represent an important step toward reaching the frequencies required by the RF front-ends in wireless transceivers. The geometric dimensions necessary to reach a given frequency are larger for this contour-mode than for the flexural-modes used by previous resonators. This, coupled with its unprecedented Q value, makes this disk resonator a choice candidate for use in the IF and RF stages of future miniaturized transceivers. Finally, a number of measurement techniques are demonstrated, including two electromechanical mixing techniques, and evaluated for their ability to measure the performance of sub-optimal (e.g., insufficiently small capacitive gap, limited dc-bias), high-frequency, high-Q micromechanical resonators under conditions where parasitic effects could otherwise mask motional output currents. [1051].

High-Q VHF micromechanical contour-mode disk resonators

A micromechanical, laterally vibrating disk resonator, fabricated via a technology that combines polysilicon surface-micromachining and metal electroplating to attain submicron lateral capacitive gaps, has been demonstrated at frequencies approaching 160 MHz with Qs as high as 9,400-the highest demonstrated to date for an on-chip resonator in this frequency range. This frequency also represents the highest to date for an electrostatically transduced micromechanical resonator and is an important step towards reaching the frequencies required by the RF front-ends in wireless transceivers. The geometric dimensions necessary to reach a given frequency are larger for this contour-mode than for the flexural-modes used by previous resonators. This, coupled with its unprecedented Q value, makes this disk resonator a choice candidate for use in the IF and RF stages of future miniaturized transceivers.

Stiffness-compensated temperature-insensitive micromechanical resonators

Polysilicon /spl mu/mechanical resonators utilizing a novel temperature-dependent electrical stiffness design technique to compensate for temperature-induced frequency shifts have been demonstrated with greatly reduced temperature coefficients (TC/sub f/s) on the order of -0.24 ppm//spl deg/C, which is 67 times smaller than exhibited by previous uncompensated resonators. With this new resonator design, the total frequency excursion over a 300 K to 380 K range has been reduced from 1,280 ppm for an uncompensated device to only 18 ppm, which for the first time, is now small enough to erase lingering concerns regarding the temperature stability of MEMS-based resonators for use in communication applications.

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.

Mechanically temperature-compensated flexural-mode micromechanical resonators

An IC-compatible, high frequency (HF), lateral micromechanical resonator supported by a mechanical structure designed to introduce stresses that counteract temperature-induced frequency shifts, has been demonstrated at 10 MHz with a much-reduced temperature coefficient of -2.5 ppm//spl deg/C and a Q of greater than 10,000. These values constitute substantial improvements over the -17 ppm//spl deg/C and 3,000 posted by previous clamped-clamped beam vertical resonators in this frequency range, and represent significant strides towards reducing the thermal dependence of micromechanical resonators, possibly to the point where such devices can be used in on-chip high-and reference oscillator applications without the need for electronic temperature compensation.

A sub-micron capacitive gap process for multiple-metal-electrode lateral micromechanical resonators

A fabrication process has been demonstrated that combines polysilicon surface micromachining, metal electroplating, and a sidewall sacrificial-spacer technique, to achieve high-aspect-ratio, submicron, lateral capacitive gaps between a micromechanical structure and its metal electrodes, without the need for advanced lithographic and etching technology. Among the devices demonstrated using this process are lateral free-free beam micromechanical resonators (Q=10,470 at 10.47 MHz), contour mode disk resonators (Q=9,400 at 156 MHz), and temperature-compensated micromechanical resonators (Q=10,317 at 13.5 MHz, with a -200 ppm frequency variation over a full 80/spl deg/C range).

Q-Optimized Lateral Free-Free Beam Micromechanical Resonators

Laterally vibrating free-free beam micromechanical resonators have been demonstrated that utilize second-mode flexural supports and optimal dc-bias application to suppress anchor dissipation and thereby attain Q’s greater than 10,000 at 10.47 MHz, while eliminating some of the key deficiencies associated with previous vertical-mode resonators. In addition to demonstrating lateral FF-beams, this work utilizes these resonators to quantify the degree to which the use of energy isolating supports actually influences the Q of this device.

Geometric stress compensation for enhanced thermal stability in micromechanical resonators

A design technique based upon competition between the thermal dependencies of geometrically tailored stresses and Youngs modulus has been demonstrated that (1) reduces the temperature coefficient (TC/sub f/) of the resonance frequencies of folded-beam micromechanical resonators by almost an order of magnitude without any additional power consumption; and (2) introduces a zero TC/sub f/ temperature at which subsequent oven controlled resonators may be biased. In particular, using this design technique, the overall frequency variation of a nickel-plated micromechanical resonator over a 27/spl deg/C to 117/spl deg/C range has been reduced from 2519 ppm to 342 ppm, and zero TC/sub f/ points have been introduced at temperatures ranging from 27/spl deg/C to 57/spl deg/C, specifiable by design. In the preparation of micromechanical devices, localized rapid thermal annealing was found to greatly increase the Q (>13000) and drift stability of the nickel resonators used in this work.

Measurement Techniques for Capacitively-Transduced VHF-to-UHF Micromechanical Resonators

An electromechanical mixing technique has been demonstrated for accurately measuring the performance of sub-optimal (e.g., insufficiently small capacitive gap, limited dc-bias), high-frequency, high-Q micromechanical resonators under conditions where parasitic effects could otherwise mask motional output currents. The technique employs the nonlinear voltage-to-force transfer function inherent in capacitive transducers, allowing an off-resonance input signal to mix down to a force at the resonance frequency and drive a resonator into vibration. Using this technique, a Q of 9,400 was measured for a non-optimal 156MHz disk resonator—3X higher than the false 3,090 obtained when parasitic feed-through is allowed to influence the frequency response.

A Resonant Temperature Sensor Based on Electrical Spring Softening

A high-resolution resonant temperature sensor based on resonance frequency shifts caused by thermally-induced changes in the electrical spring stiffness across the capacitive transducer of a 10.7 MHz µmechanical resonator has been demonstrated with a sensitivity as high as −347ppm/°C (or −3.7kHz/°C). This sensor exhibits a fairly linear temperature-to-frequency transfer function with a slope (i.e., sensitivity) that can be controlled in real-time via adjustment of a dc-bias voltage V P applied to the resonator structure. Such adaptability makes this temperature sensor useful for applications requiring wide dynamic range.