MEMS resonant temperature sensing with variable coupling stiffness and improved sensitivity

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2020-10-23
Şimşek, Ertuğ
This thesis presents the design, modelling, fabrication and characterization of a resonance based MEMS temperature sensor with improved sensitivity. The temperature sensor is composed of an electrostatically coupled double ended-tuning fork (DETF) MEMS resonator. The sensor utilizes the thermal expansion coefficient difference of the materials to detect the temperature change. The design consists of 2 resonator tines where each of them has two capacitive plates on each side. The capacitive plates facing each other on the inner side of the resonators are used for the electrostatic coupling, which is the crucial point of the study. The negative electrostatic coupling stiffness generated between these tines enables mode-ordering. By mode-ordering, the sensor can be operated closer to pull-in in order to achieve higher sensitivity for the out-of-phase mode. The outside capacitive plates are used for actuation and sensing, whose mechanisms are explained with the equations. The analytical model is presented with the thermo-electro-mechanical equations for the mode shapes and their corresponding natural frequencies. The model is verified by the Finite Element Analysis by comparing the resonance frequencies of the modes of interests. In FEM analysis, the effects of the electrostatic coupling are shown with the parametric sweep for the various proof mass voltage configurations with the thermal expansion physics node included. Having the thermal expansion, the effect of the temperature increase is shown in the modal analysis as the frequency shift in the mode of interests. The characterization tests are performed in a vacuum environment. The quality factor for the out-of-phase mode is about 25500 at a pressure of around 0.15mTorr. Resonance frequencies for the mode of interests of the resonator are close to the analytical model and the FEM simulation results. The effect of the electrostatic softening effect is investigated for the same, and the opposite sign proof mass voltage configurations. The frequency change for the opposite sign proof mass configuration from 6V to 20V is 3063Hz and 666Hz for out-of-phase and in-phase, respectively. For the same proof mass configuration, the resonance frequency shifts 579Hz. The out-of-phase mode and the in-phase mode frequency changes for the temperature increase from 25°C to 65°C are 1078Hz and 988 Hz for the VPM1=20V and VPM2=-20V. For the same proof mass configuration (VPM1=20V and VPM2=20V), the frequency change is 986Hz, still lower than the inphase mode frequency of the opposite sign proof mass voltage configuration but very close as expected. The overall sensitivities are obtained using the maxima of the frequency response plots. For the opposite sign proof mass configuration, out-ofphase mode and in-phase mode temperature sensitivity are increased from 24.4Hz/K to 26.5Hz/K and 24Hz/K to 24.4Hz/K, respectively, with the voltage increase from 8V to 20V. This study shows that the temperature sensitivity can be increased by varying coupling stiffness by adjusting the proof mass voltages close to pull-in voltage and mode-ordering.

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Citation Formats
E. Şimşek, “MEMS resonant temperature sensing with variable coupling stiffness and improved sensitivity,” M.S. - Master of Science, Middle East Technical University, 2020.