Application of Micromachined Quartz Resonators for Pressure and Stress Sensing
Open Access
- Author:
- Goel, Nishit
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 23, 2018
- Committee Members:
- Srinivas Tadigadapa, Dissertation Advisor/Co-Advisor
Srinivas Tadigadapa, Committee Chair/Co-Chair
Mehdi Kiani, Committee Member
Siyang Zheng, Committee Member
Christopher Rahn, Outside Member - Keywords:
- Quartz
Resonator
Piezoelectric
Pressure Sensor
Stress Sensor
Metglas
Magnetrostriction - Abstract:
- In this dissertation, a study on micromachined AT-cut quartz crystal resonators for use as differential pressure and thin film stress sensing applications has been carried out. The property of quartz crystal resonator, known as “force frequency effect,” has been utilized whereby quartz resonators shift its thickness shear mode resonance frequency due to application of force/stress. A theoretical model based on first principles has also been proposed and has been modeled in a commercial finite element software to accurately predict the behavior of the quartz resonators to the external stimulus of interest. The structure of an AT-cut quartz based pressure sensor consists of an edge clamped square quartz plate that has been etched in certain regions with the help of micromachining techniques. The electrodes are placed in the etched regions or diaphragms which are resonated in their thickness shear mode resonance. A differential pressure is applied to the structure leading to stress generation in the diaphragms as hence a shift in the resonance frequency. An experimental study on varying physical dimensions such as thickness and diameter of the sensor diaphragm on sensitivity of quartz resonator based pressure sensors has been presented. The sensors have shown high sensitivity with a resolution of ~1.04 mTorr and range of operation from mTorr to >100 Torr differential pressure with high linearity. The sensors have also shown a dependence upon the face of diaphragm on which pressure is applied which relates to the structural asymmetry and boundary conditions of the device. In the second study, micromachined AT-cut quartz resonators have been utilized for sensing stresses in the thin films with a potential application for in-situ thin film stress monitoring. Mass sensitivity of quartz resonators is a well-known phenomenon and has been extensively utilized for microbalance and thickness monitoring applications. The addition of mass on a quartz resonator leads to a reduction in its thickness shear mode resonance. Hence, to compensate for this mass sensitivity for in-situ stress monitoring applications, two resonator technique has been utilized. One resonator is in a cantilever configuration which is sensitive to mass and stress whereas the other resonator is in a fixed plate configuration and hence is only sensitive to the mass. A differential measurement of the frequency shifts of the two resonators will cancel out the effect of mass and the remainder would be a measure of thin film stress. A comprehensive stress characterization study was performed by integrating the quartz cantilevers with a magnetostrictive material called Metglas® to form a unimorph structure. Magnetic fields were applied to the unimorph structure which leads to a strain generation in the Metglas® and thereby flexural bending of the unimorph structure. This bending of the quartz cantilever leads to generation of both in-plane and out of plane stresses in the structure and hence a shift in the thickness shear mode resonance frequency of the quartz resonator. Effect of cutting the micromachined quartz in the shape of a cantilever versus uncut structure on the stress sensitivity of the quartz resonator has been studied. The cut device has shown a much higher sensitivity to stresses as compared to the uncut device due to reduced flexural rigidity of the former. Due to highly anisotropic of nature of the quartz crystal, the response of quartz cantilever also depends upon the orientation of the quartz cantilever with respect to the bulk quartz crystal’s orientation. A study on two cantilevers cut at two different azimuthal angles (ψ) i.e. ψ= 0° and ψ= 90° was also carried out. The two quartz cantilevers have shown opposite shifts in their resonance frequencies when a magnetic field was applied along their respective length direction. This property can also be useful for in-situ thin film stress monitoring by doing a differential measurements of the two frequency shifts. A magnetostrictive stress sensitivity of ~ 1.17 Hz/kPa has been demonstrated for a ~ 11.6 μm thick device. In-situ stress measurements were carried on a stress sensor device consisting of one resonator in a cantilever configuration and the other resonator in an uncut configuration. Tungsten was deposited on the device by using an argon ion beam sputtering tool while monitoring the resonance frequency of the two resonators simultaneously at regular time intervals. Due to large difference in the initial resonance frequencies of the cantilevered resonator and the uncut resonator (>1-2 MHz), the differential measurement technique was not valid as the two resonators will have different mass sensitivities. Mass sensitivity of the uncut devices have been characterized experimentally and has been used to estimate the mass sensitivities of the cut devices by extrapolation. The stress sensitivities have been calculated by subtracting these mass sensitivities from experimentally observed frequency shifts during deposition of material on cut devices.