High Frequency Thermally Actuated Single Crystalline Silicon Micromechanical Resonators with Piezoresistive Readout

Over the past decades there has been a great deal of research on developing high frequency micromechanical resonators. As the two most common and conventional MEMS resonators, piezoelectric and electrostatic resonators have been at the center of attention despite having some drawbacks. Piezoelectric resonators provide low impedances that make them compatible with other low impedance electronic components, however they have low quality factors and complicated fabrication processes. In case of electrostatic resonators, they have higher quality factors but the need for smaller transductions gaps complicates their fabrication process and causes squeezed film damping in Air. In addition, the operation of both these resonators deteriorates at higher frequencies. In this presented research, thermally actuated resonators with piezoresistive readout have been developed. It has been shown that not only do such resonators require a simple fabrication process, but also their performance improves at higher frequencies by scaling down all the dimensions of the structure. In addition, due to the internal thermo-electro-mechanical interactions, these active resonators can turn some of the consumed electronic power back into the mechanical structure and compensate for the mechanical losses. Therefore, such resonators can provide self-Q-enhancement and self-sustained-oscillation without the need for any electronic circuitry. In this research these facts have been shown both experimentally and theoretically. In addition, in order to further simplify the fabrication process of such structures, a new controlled batch fabrication method for fabricating silicon nanowires has been developed. This unique fabrication process has been utilized to fabricate high frequency, low power thermal-piezoresistive resonators. Finally, a new thermal-piezoresistive resonant structure has been developed that can operate inside liquid. This resonant structure can be utilized as an ultra sensitive biomedical mass sensor

This content has been downloaded from IOPscience. Please scroll down to see the full text. Detection of sub-ppm traces of aqueous heavy-metal ions using micro-electro-mechanical beam resonators Detection of sub-ppm traces of aqueous heavy-metal ions using

Abstract Capacitive silicon micro-mechanical resonators have been utilized in this work as ultra-sensitive mass sensors for the detection of trace amounts of copper ions in water samples. The approach is based on the reduction of aqueous metal ions by the silicon in a resonant structure and consequently deposition of a very thin metal layer on the resonator surface changing its resonant frequency. Measurements demonstrate successful detection of sub-ppm concentrations of copper(II) ions in water. Relatively large frequency shifts (hundreds of ppm) have been measured for resonators exposed to copper concentrations as low as 4 μM (0.26 ppm). An analytical model for the resonant frequency of the resulting complex beams has been derived and used to calculate the thickness of the deposited copper layer based on the measured frequency shifts. The model shows that the measured frequency shifts correspond to only a few atomic layers of copper (as thin as ∼7Å) deposited on the resonator surfaces. This corresponds to a mass sensitivity of more than 4000 Hz μg −1 cm −2 which is much larger than the highest mass sensitivities measured for quartz crystal microbalances

Localized thermal oxidation for frequency trimming and temperature compensation of micromechanical resonators

This work demonstrates electronically controllable frequency trimming and temperature compensation of individual single crystalline silicon thermal-piezoresistive resonators via localized self-induced thermal oxidation. Frequency trimming as high as ~3.7 % has been demonstrated using this technique for a 53MHz resonator. At the same time, the formed oxide layer using this technique can significantly suppress the temperature coefficient of frequency (TCF) for such resonators. TCF values as low as 0.2 ppm/ºC have been demonstrated for resonators with initial TCF of-37ppm/ºC

Microelectromechanical disk resonators for direct detection of liquid-phase analytes

a b s t r a c t This paper presents preliminary measurement results for real-time detection of biomolecules using rotational-mode MEMS resonant structures and capability of such to directly and specifically measure concentration of thiol-terminated DNA molecules in liquid. Thin film piezoelectric disk resonators with quality factors (Q) as high as ∼100 in aqueous solutions have been fabricated and utilized as direct biomolecular detectors that can address the problem of low Q for MEMS resonators when in direct contact with liquid. To adsorb thiol-terminated molecules, a gold layer is deposited on the top resonator surface. A gradual frequency shift of ∼10 kHz (3800 ppm) was recorded in real-time while forming monolayers of mercaptohexanol in aqueous solution, demonstrating the potential of such structures as highly sensitive biosensors. Over and above detection of target single-stranded-DNA (ssDNA) sequences using the disk resonators (with mass sensitivities as high as 19.3 ppm cm 2 /ng (65 Hz cm 2 /ng) in aqueous solution), the response of such devices has been characterized using different concentrations of thiol-terminated DNA molecules. For one order of magnitude change in concentration of functionalizing thiol-terminated-ssDNA solution, ∼2X difference in measured frequency shifts of the disk resonators was observed