Martijn Goossens

Small, low-ohmic RF MEMS switches with thin-film package

We report on small, low-ohmic RF MEMS switches with a circular membrane actuator design. A low temperature process is used to manufacture both the MEMS switch as well as its hermetic, thin-film package resulting in a very small footprint device. The hermetic seal of the package significantly increases the switch lifetime and reliability. The switches demonstrate good RF performance and high switching speeds. A comparison with other MEMS switches reveals that these MEMS switches possess a low on-resistance while occupying only a very small area on the wafer.

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.

MEMS oscillating squeeze-film pressure sensor with optoelectronic feedback

This work reports on an oscillating pressure sensor that converts pressure into frequency using the squeeze-film effect. A new aspect is the laser Doppler vibrometer (LDV) in the optoelectronic feedback loop that is used to bring the sensor element into sustained mechanical oscillation. A phase shifter and automatic gain control circuit stabilize the oscillation. The frequency stability of the pressure sensor is investigated by measuring its Allan deviation and is compared to the performance of a quartz oscillating pressure sensor. Finally, the pressure resolution of this oscillating sensor is compared to conventional pressure sensors.

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.