On The Theory Of Novel Piezoresistive MEMS Temperature Sensor

Temperature is one of basic physical quantities. With the development of society, many areas, like in scientific research, industrial and agricultural production and daily life, are involved in temperature measurement and temperature sensors. Compared to traditional sensors, MEMS sensors have small size, low power consumption, high sensitivity, ease of integration with IC; MEMS-based temperature sensor has become a new direction in recent years. In this thesis, two novel MEMS temperature sensors based on Piezoresistive effect of silicon are designed:(1) Piezoresistive MEMS temperature sensor based on double-layer thin film structure, (2) Piezoresistive MEMS temperature sensor based on composite micro-beam structure.Temperature sensors based on double-layer thin film consist of Piezoresistive Wheatstone Bridge which is on the surface of silicon-based micro-bridge and temperature-sensitive polymer thin-film deposited on its surface. Temperature sensitive polymer thin film will expand when the temperature changes, and bring the deformation of silicon-based micro-bridge, which will make the value of Piezoresistive change. Thus, the temperature-voltage conversion can be achieved by Wheatstone bridge. In this thesis, the theoretical model of double-layer-thin-film temperature sensors is created and analyzed based on the thin plate theory in Elasticity and thermoelasticity. Then, the equivalent temperature load of sensor is proposed, the bending differential equation about double layer film is founded, and the coefficient of flexural stiffness is modified. Finally, the output expression of sensor is deduced. The results show that the sensor output and the measured temperature have a linear relationship. Besides, the relationship between the structure size and performance is analyzed based on this model, and the results indicate that the more thick of temperature-sensitive polymer thin-film and the more thin of silicon-based micro-bridge, the higher sensitivity.Temperature sensors based on composite micro-beam structure consist of doubly-layer composite micro-beam made up two different materials and Piezoresistive Wheatstone Bridge under its surface. When changing the temperature, the micro-beam is bending because of thermal stress accused by different coefficients of thermal expansion of two materials, which will convert into output voltage by Piezoresistive Bridge on the surface of silicon-based micro-bridge. Therefore, the temperature is measured. In this thesis, the stress distribution on the surface of silicon-based micro-beam is analyzed, then, the output expression of sensor is deduced. The results show that the output and the environment temperature have a linear relationship. Besides, the relationship between the structure size and performance is analyzed based on this model, and the results indicate that the more thin material and the greater different coefficients of thermal expansion between two materials, the higher sensitivity.Finally, the structure sizes of two different temperature sensors are preliminary design based on theoretical models above.