Study On The Steady Heat Transfer Of The Micro Thermal Airflow Sensor

In this dissertation, the steady heat transfer of the micro thermal airflow sensor is systematically studied. Firstly, the thin film thermal conductivity measurement method is studied. A heat transfer model of the thin film-substrate structure is built to extend the original Raman method for the thermal conductivity measurement of the submicrometer- or nanometer-scale thin films. The extended Raman method is applied to the thin films used in the micro thermal airflow sensor to obtain their thermal conductivities. Secondly, based on the obtained material thermal parameters, the conjugate conduction-convection heat transfer of the micro thermal airflow sensor is studied, a steady heat transfer model is built and the numerical analysis is performed. Based on the analysis results, the optimal design criteria are proposed to enhance the steady performance of the micro thermal airflow sensor. Thirdly, based on the proposed optimal design criteria, the operation mode of the micro thermal airflow sensor is designed, the materials of the micro thermal airflow sensor are chosen and the structure of the micro thermal airflow sensor is designed to enhance the steady performance of the micro thermal airflow sensor. Fourthly, according to the present experimental devices and techniques of the lab, the applicable fabrication processes of the micro thermal airflow sensor are chosen, and the designed micro thermal airflow sensors are fabricated with the achievements of the fabrication processes experiments. The obtained micro thermal airflow sensors are test for their steady performance, and the test results have confirmed the validity of the proposed optimal design criteria.The main contents of this dissertation are briefly stated as the followings:1. The study on the thin film thermal conductivity measurement method. The original Raman method for the film thermal conductivity measurement is modified to adapt the Gaussian laser condition. Then, a heat transfer model of the thin film-substrate structure is built to analyze the effects of the thin film thermal conductivity, substrate thermal conductivity and the film/substrate interface thermal resistance. An analytical process is applied to the heat transfer model and thin film thermal conductivity equation is obtained, which means that the Raman method is extended for the thermal conductivity measurement of the submicrometer- or nanometer-scale thin films. The extended Raman method is applied to the porous silicon film, silicon dioxide film and the silicon nitride film to obtain their thermal conductivities. This study provides accurate thermal parameters of the materials used in the micro thermal airflow sensor for its steady heat transfer model. 2. The study on the steady heat transfer model of the micro thermal airflow sensor. The conjugate conduction-convection heat transfer of the micro thermal airflow sensor is analyzed, and the steady heat transfer model of the micro thermal airflow sensor is built. In this model, not only the effect of the heat transfer between the airflow and the sensor through the convection is described, but the heat transfer within the sensor through the conduction of the sensor substrate is also taken into account. The finite difference scheme is applied to the steady heat transfer model for the numerical analysis. Based on the analysis results, the optimal design criteria including the sensing elements positioning and the heating elements positioning are proposed to enhance the steady performance of the micro thermal airflow sensor. The proposed optimal design criteria lay the theoretical foundation for the design of the micro thermal airflow sensor.3. The study on the optimal design of the micro thermal airflow sensor. The temperature balance mode for the operation of the micro thermal airflow sensor is theoretical analyzed to describe the relationship between the sensor output signal and the flow rate. The high linearity of the sensor output signal is proved by the theoretical analysis. According to the practical condition of the experimental devices and techniques in the lab, the materials used in the micro thermal airflow sensor are studied and then chosen for the enhancement of the sensor performance, including the sensor substrate material, insulation layer material, heating element material, sensing element material and the passivation layer material. The structure of the micro thermal airflow sensor is designed according to the proposed optimal design criteria.4. The study on the fabrication processes and the steady performance test of the micro thermal airflow sensor. Based on the comparison of the different MEMS fabrication processes, and combined the practical condition of the experimental devices and techniques in the lab, the experiments are made separately for the fabrication of the sensor substrate, insulation layer, heating element, sensing element and passivation layer, and then the designed micro thermal airflow sensor is fabricated. The sensors are tested for their steady performance, and the test results have confirmed the validity of the proposed optimal design criteria.