Development of an integrated resonant MEMS temperature sensor

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2016
Köse, Talha
This thesis presents the design, fabrication and characterization of a high performance, integrated, resonant MEMS temperature sensor, and temperature compensation of a capacitive MEMS accelerometer. Two different double-ended-tuning-fork (DETF) type resonator designs are developed and characterized for temperature sensing. The strain-amplifying beam structure is added to the DETF resonators in order to enhance thermal strain induced on the DETF tines due to the different thermal expansion coefficients of the substrate and the sensor structure. Two different supplementary temperature sensor designs have been demonstrated in order to compare the effectiveness of the strain-amplifying beam, i.e., the doubly-clamped DETF resonators, and one-end-free DETF resonators. Two DETF resonators with different geometric properties have been used in the temperature sensor designs in order to verify analytical equations representing the operation of DETF resonators as temperature sensors. Finite Element Analysis simulations (modal and thermo-mechanical) have been conducted for each of the temperature sensor designs. To realize real-time data acquisition from the temperature sensors, a read-out circuit which sustains self-resonance of the DETF resonators has been designed and demonstrated with the circuit simulations. The co-fabrication of the MEMS temperature sensors and MEMS accelerometer have been demonstrated by on-chip integration of the devices. This integration enables precise temperature measurements by removing the thermal lag between the temperature sensor and the MEMS accelerometer to be temperature-compensated. After fabrication, characterization tests have pointed out a good vacuum environment (pressure inside the sensor dies in range of 10-100 mTorr) which corresponds to quality factors in the order of 10000. The system-level temperature tests (from -20 °C to 60 °C) are done while the accelerometer and two temperature sensors were simultaneously operated in closed-loop. Temperature coefficient of frequency values (TCf) of 730 ppm/K and 636 ppm/K have been achieved with the proposed temperature design, which reveals an improvement in sensitivity (TCf) up to 2.4 times when compared to doubly-clamped DETF resonators, and an improvement up to 35 times improvement when compared to the one-end-free DETF resonators. Minimum detectable temperature attained with the proposed temperature sensor designs has been reported as 1.1 mK. The temperature compensation of MEMS accelerometer has resulted in an improvement up to 830 times by reducing the temperature dependence of the accelerometer output from 1164 µg/°C to 1.4 µg/°C. The noise performances of the MEMS accelerometers used in this study points out bias instability of 10 µg up to integration time of 100 seconds, and the velocity random walk of 13 µg/√Hz.

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Citation Formats
T. Köse, “Development of an integrated resonant MEMS temperature sensor,” M.S. - Master of Science, Middle East Technical University, 2016.