Design and fabrication of a high performance resonant MEMS temperature sensor

Abstract

This paper presents a high performance MEMS temperature sensor comprised of a double-ended-tuning-fork (DETF) resonator and strain-amplifying beam structure. The temperature detection is based on the ‘thermal strain induced frequency variations’ of the DETF resonator. The major source of thermal strain leading to the frequency shifts is the difference in thermal expansion coefficients of the substrate and the device layers of the fabricated structures. By selecting the substrate as glass and the device layers as single crystal silicon, i.e. materials with different thermal expansion coefficients, the tines of the resonators are exposed to axial load with the changing temperature, which causes a change in the resonance frequency of the resonators. This resonance frequency shift can be related with the changing temperature by taking the thermal strain relations into consideration, which enables utilization of the resonator as a highly sensitive temperature sensor. The resonators used in this study have been fabricated by utilizing the advanced MEMS process that incorporates the simple silicon-on-glass process with the wafer level vacuum packaging Torunbalci et al (2015 J. Microelectromech. Syst. 24 556–64). The fabricated resonators have been tested in a temperature-controlled oven between −20 °C and 60 °C, and the results of two distinct designs are compared to be able to observe the effectiveness of the strain amplifying beam. Measurement results show that the design with the strain amplifying beam increases the temperature coefficient of frequency of the resonators by 33 times when compared to the one-end free DETF resonators. Minimum detectable temperature variations observed by the resonators used in this study is 0.0011 °C. This kind of very high resolution temperature sensing can be achieved by integrating this MEMS temperature sensor with any type of physical MEMS sensor where its fabrication process includes different materials for the substrate and sensor structures.