Design, optimisation and characterisation of Silicon Carbide based thermal flow sensors for harsh environments

Abstract

Thermoresistive and Joule heating effects in metals (e.g. platinum, nickel) and semiconductors (e.g. silicon) have been extensively utilized to develop Micro Electro-Mechanical Systems (MEMS) based thermal sensors. Due to the electronic simplicity and easy implementation using micro-fabrication and micromachining techniques, these sensors have been found in a wide range of applications such as temperature sensing, flow sensing and acceleration sening. Neverthless, their material cost, inadequate sensitivity, inflexibility and incapability to work in harsh environments hinder these sensing materials in many applications, particularly where the temperature is high. Consequently, there is a growing demand for investigating the alternative materials with high thermoresistive sensitivity for harsh environment flow sensors. This research aims to develop thermal flow sensor that has high temperature capability yet is small in size, high sensitivity, low power consumption and inexpensive through mass fabrication. To provide a systematic approach for detailed study, the knowledge of heat-transport mechanism in macro and micro-scale thermal sensors is paramount. Thus, in the chapter three of this thesis, a steady-state analytical model for describing the temperature distribution in suspended bridge type thermal sensors was developed. Parametric optimization also offered an insight on the thermal sensor characteristics. Furthermore, experiments were conducted to verify the effectiveness of both the models using a scaled-up low cost device. The developed macro-scale model was then extended to micro-scale multi-layer thermal sensor models in chapter four. This suspended type configuration consumed less power in the order of milliwatts and paved the way for developing thermal flow sensors presented in subsequent chapters. In chapter five, a new fabrication methodology was employed to fabricate SiC hot-film flow sensor on a thermally insulating glass substrate. The thermoresistive characterization of the fabricated sensor resulted in a large Temperature coefficient of resistance (TCR) of approximately -20716 ppm/K at ambient temperature (298K) and -9367 ppm/K at 443K respectively. Later, the wide dynamic range of flow characterization at ambient temperature resulted in high sensitivity which primarily motivated us to set-up and conduct high temperature flow measurement presented in chapter six. Thus, chapter six involved the development of a novel technique to characterize the hot-film flow sensor at temperatures up to 200°C. This chapter also addressed various challenging aspects in the context of harsh environments: material choice, metallization interconnects, Ohmic contact stability, measurement set-up and flow calibration. Although the developed characterization technique ensured a stable high temperature operation, the sensitivity of the sensor, on the other hand could still be improved by calorimetric flow sensing principle, which in general is very sensitive to low flow velocities. Hence, chapter seven involved the design and characterization of calorimetric flow sensor using a new material platform of SiC/glass/Si. COMSOL Finite Element Modelling (FEM) tool was initially utilized to obtain the optimized distance between the heater and pair of temperature sensors. The optimized geometrical parameters were then considered for the micro-fabrication and subsequent device characterization. Collectively, the findings from this study at large, suggest the feasibility of utilizing SiC as a sensing/heating material for high-temperature flow applications.