Investigation On The High-Performance Silicon-based MEMS Thermal Wind Sensors

With the pursuit of people to higher life quality,the real-time monitoring of meteorological information is becoming increasingly important.The wind sensors,as the key component for wind speed and wind direction measurement,are also receiving more attention.Especially in recent years,the rapid development of microelectromechanical systems(MEMS)technology enables the miniaturized,integrated and intelligentized wind sensors,and a large number of MEMS-based wind sensors have been proposed.Among all types of MEMS wind sensors,the wind sensor based on thermal principle has become the research focus because it has the advantages of no moving mass and high sensitivity to small flow rate.In 2000,our laboratory has started the research on MEMS thermal wind sensors,and has built a complete set of solutions for the wind sensor system.However,although the fabricated wind sensor presents high reliability,it has the problems of low sensitivity and high power consumption.These disadvantages prevent its wide use in portable equipment.Therefore,on basis of previous studies,further investigations are made to improve the sensor performance from the angle of optimizing sensor structure and packaging in this dissertation.The main work and innovations of this dissertation are described as follows:(1)Deep reactive ion etching(DRIE)technology has been used to etch the sensor chip to form insulation trenches to improve the performance of MEMS thermal wind sensor.Based on the detailed analyses for the working principle and fundamental theory of the thermal wind sensor,the dissertation proposes to fabricate insulation trenches in the chip to improve the sensor performance.According to the structural characters of sensor chip,three forms of insulation trenches have been designed.The effect of different trenches on the sensor performance has been investigated by simulation and experiment.The wind tunnel experimental results show that for the wind sensor operated in constant voltage(CV)mode,the sensitivity is improved by fabricating the arch and center trenches in the chip.When the sensor works in constant temperature difference(CTD)mode,all forms of insulation trenches can be used to reduce the power consumption.Therefore,in different application fields,various forms of insulation trenches can be combined to make the sensor have the desired performance.(2)A high sensitivity MEMS thermal wind sensor with DRIE trenches and Wheatstone full-bridge readout circuitry is designed.After introducing the DRIE trenches into the design of the wind sensor,the Wheatstone full-bridge readout circuity is further used to manufacture the thermal wind sensor.In the wind sensor with Wheatstone full-bridge design,two pairs of thermistors formed by eight temperature measuring resistors are located at different distance from the central heater,and the four thermistors in relative directions are configured to a Wheatstone full-bridges.Compared with the trational Wheatstone half-bridges formed by two thermistors and two discrete resistors,the Wheatstone full-bridge configuration has almost two times the output.The experimental results show that the sensitivity of the wind sensor can be improved by 82% with insulation trenches design and further by 226% based on the combination of insulation trenches and Wheatstone full-bridges design.(3)A new design method has been employed to enhance the performance of MEMS thermal wind sensor.Instead of improving sensitivity,this approach enhance the performance with a novel active element arrangement method.The active element with this design presents an octagon in shape.The octagon-arranged active element consists of 8 central heating resistors and 8 thermistors.These 16 resistors can be divided into two wind sensing groups.These two groups monitor wind independently,and the final measured wind speed and direction are extracted from the measurement results of the two groups.The experimental results show that for the wind speed up to 33 m/s,the octagon-shaped wind sensor has a measurement error less than ±1%,and the airflow direction over the full range of 360o is determined with a maximum error of ±1.5o.(4)A chip-level packaging scheme for MEMS thermal wind sensor is presented.In this packaging,a ceramic film is prepared on the back surface of silicon wafer by low-temperature sintering method.The preparation of the ceramic film enables the silicon sensor chip and the packaged ceramic to achieve good adhesion without using the thermal conductive adhesive,and hence avoiding the possible heat field asymmetry problem caused by the thermal conduction adhesive in the previous solutions of our laboratory.In addition,the ceramic film packaging can not only guarantee the reliability of the sensor chip,but also make the volume of the packaging ceramic significantly reduce.The volume reduction of the ceramic will suppress the effect of the packaging ceramic on the sensor performance.The experimental results show that the sensitivity of the ceramic film packaged wind sensor is only 4.7% lower than that of the wind sensor without the ceramic packaging.Compared to the nearly 70% sensitivity loss caused by large ceramic packaging board,the ceramic film packaging solution has significantly less impact on the sensor performance.The proposal of the ceramic thin film packaging scheme makes it possible to design a silicon-based MEMS thermal wind sensor with high reliability and high performance.(5)In order to block the unwanted thermal pathway caused by the low thermal conductivity epoxy resin on the front surface of the wind sensor in the traditional self-packaged method,an annular encapsulation packaging scheme is proposed.In this packaging,the low thermal conductivity adhesive is only used to encapsulate the bonding wires between the sensor chip and the external circuit,while the front surface of the sensor chip is in contact with the air.With this approach,the unwanted heat loss from the sensor to the packaging epoxy is significantly suppressed,and hence more efficient heat exchange between the sensor and the airflow is obtained via the back surface of the sensor.The experimental results show that compared with the conventional self-packaging,the annular encapsulation packaging enables the wind sensor to achieve a sensitivity improvement of 50.8%.In a word,this dissertation focuses on the improvement of structural design,preparation process and packaging method to fabricate the MEMS wind sensor with high sensitivity and high reliability.A series of optimization schemes,such as DRIE insulation design,Wheatstone full-bridge configuration,octagon-shaped active element arrangement method,ceramic film packaging and annular encapsulation packaging,have been proposed.With these designs,the silicon-based thermal wind sensor has both high reliability and high performance.