K.L. Phan

Narrow Bandwidth Single-Resonator MEMS Tuning Fork Filter

We present a solution for a fourth-order, narrow-bandwidth filter comprising of a single silicon tuning fork resonator driven using one electrode only. Voltage controlled electrical spring tuning is employed to match the primary and secondary modes of the resonator to achieve filter response. A narrow bandwidth single resonator MEMS tuning fork filter is demonstrated with a center frequency of 1.2866 MHz, a 3 dB-bandwidth of 0.0085% and a 1.5 dB ripple.

A piezo-resistive resonant MEMS amplifier

A MEMS resonator using electrostatic to piezo-resistive transduction is demonstrated to be capable of simultaneous signal filtering and amplification. The mechanical resonance serves as a high Q electrical filter, while the piezo-resistive readout allows for signal amplification. Amplification factors up to 4.6 dB and Q values up to 60,000 are obtained for a 15 MHz resonator.

56 MHZ piezoresistive micromechanical oscillator

A fully functional oscillator has been developed, based on a resonator with an electrostatic-to-piezoresistive transduction. Both resonator and amplifier IC have been processed on a SOI wafer with identical SOI layer thickness of 1.5 μm. The resonator is a bulk-acoustic ‘dogbone’ design, for which an extended electrical model is presented. At an oscillation frequency of 56.1 MHz the oscillator consumes 6.1 mW and reaches a phase noise of −102 dBc/Hz at 1 kHz offset from carrier.

Failure analysis of a thin-film nitride MEMS package.

In this paper, the failure mechanism of a thin-film nitride MEMS package is studied by an integrated test structure. The cause of the failure is investigated by advanced characterization techniques and accelerated tests on the packaging material. From the research, we can conclude that PECVD silicon nitride is a proper sealing material for thin-film packaging because of its good sealing property. However, outgassing of this material, at elevated temperature, remains the main concern for the reliability.

Mechanical phase inversion for coupled lamé mode resonator array filters

This paper reports a novel mechanism for mechanical phase inversion in MEMS filters to overcome the effects of capacitive parasitics without using external electrical phase inverter circuits. A 29.6 MHz, 0.05% bandwidth and 44.4 MHz, 0.1% bandwidth Lame mode square-plate resonator filters are designed and fabricated to experimentally validate the feasibility of electrically or mechanically coupled silicon microfabricated resonator filters. The mechanical phase inversion mechanism and reported internal electrical phase inversion mechanism [1] enhances the design flexibility for MEMS resonator based filters and the principle can be extended to filters based on other resonator topologies.

Internal electrical phase inversion for FF-beam resonator arrays and tuning fork filters

This paper reports on a novel MEMS electrical phase inverter resonator filter driving using differential DC bias. The internal electrical phase inverter mechanism is demonstrated by a clamped-clamped (CC) beam resonator and wire coupled free-free (FF) beam resonator arrays with in-phase or differential DC-bias. The filter performance is then demonstrated by an anchor coupled double-ended tuning fork (DETF) resonator and is realized in a silicon microfabrication process. The filter bandwidth is voltage tunable employing the electrical spring softening effect and the minimum tunable bandwidth is set by the energy coupling between neighbouring modes.

Spontaneous mechanical oscillation of a DC driven single crystal

A micrometre-scale device that exploits the piezoresistive characteristics of silicon acts like an engine, converting heat into mechanical work in one mode of operation, and, in another, like a refrigerator, suppressing mechanical fluctuations.There is a large interest to decrease the size of mechanical oscillators since this can lead to miniaturization of timing and frequency referencing devices, but also because of the potential of small mechanical oscillators as extremely sensitive sensors. Here we show that a single crystal silicon resonator structure spontaneously starts to oscillate when driven by a constant direct current (DC). The mechanical oscillation is sustained by an electrothermomechanical feedback effect in a nanobeam, which operates as a mechanical displacement amplifier. The displacement of the resonator mass is amplified, because it modulates the resistive heating power in the nanobeam via the piezoresistive effect, which results in a temperature variation that causes a thermal expansion feedback-force from the nanobeam on the resonator mass. This self-amplification effect can occur in almost any conducting material, but is particularly effective when the current density and mechanical stress are concentrated in beams of nano-scale dimensions.

Piezoresistive ring-shaped MEMS resonator

We have previously reported a new class of MEMS resonators based on the piezoresistive readout principle and extensional vibration mode. Those devices suffer from unwanted mode-coupling at large vibration amplitudes, giving rise to problems such as beat patterns in the time signal. In this paper, we present a novel type of piezoresistive resonator that has a shape of a ring and operates in a flexural in-plane mode shape. The new resonators can be operated at larger excitation forces without having the mode-coupling problem, as compared to the conventional piezoresistive resonators.

Methods to correct for creep in elastomer-based sensors

Elastomer materials such as PDMS and SU-8 have been used as mechanical springs in sensors and actuators. A disadvantage of elastomer-based structures is that they slightly creep with time, especially for low frequency and static applications, which causes hysteresis up to a few percents in the sensor signal. In this paper, by thoroughly studying the creep process and model, we propose some novel signal processing methods to correct for the creep in real time. The methods have been successfully applied in an elastomer-based magnetoresistive accelerometer, which shows that the error due to creep has been reduced from 3% down to 0.08%.

A Piezo-resistive, Temperature Compensated, MEMS-Based Frequency Synthesizer

This paper describes a frequency synthesizer based on a MEMS resonator. Uniquely, the piezo-resistive properties of silicon are exploited to read out the resonator, resulting in low impedance levels at resonance frequencies up to several 100 MHz. A 55 MHz MEMS oscillator with a phase noise of −128 dBc/Hz @ 1 kHz offset and a −140 dBc/Hz noise floor has been realized. The oscillator is combined with a programmable PLL to realize a complete frequency synthesizer that can generate output frequencies ranging from 25 MHz to 200 MHz. It achieves ±20 ppm frequency accuracy over temperatures ranging from −20°C to +85°C, and draws 15 mA from a 2.5 V supply at an output frequency of 25 MHz.