Research On Air Mass Flow Sensor Chip With High Accuracy

Gas flow measurement plays a very important role in many fields such as industrial process control, automobile, aeronautics and astronautics, and so on. Compared with traditional volumetric air flow sensor, the hot-film air flow sensor develops very fast due to its advantages on accuracy of measurement, response time and reliability. At present, the field in which the hot-film air flow sensor is widely used is the automobile electronic-control fuel injection system. The automobile electronic-control fuel injection system can control the amount of the fuel injected to the engine, according to the engine’s air inflow measured by the hot-film air flow sensor. The automobile with the electronic-control fuel injection system has better fuel efficiency. In this article, the basic principle and working characteristics of the hot-film air flow sensor are studied and a kind of silicon machined air flow sensor based on the principle of thermal distribution effect which can compensate the changes of the environment temperature is developed. The sensor can be used in the automobile electronic-control fuel injection system. The process flow of fabricating the sensor is designed and the air flow sensor has been fabricated according to the process flow. The detailed contents are as follows:(1) Analyze the basic principle of the hot-film air flow sensor and the mass flow measurement principle by using the theories of heat transfer. Analyze two different types of measurement circuit and study the relationship between the mass flow and the electrical signal of the sensor’s heater resistance. Analyze the advantages and disadvantages of the constant power heating circuit and the constant temperature difference heating circuit on measuring the flow rate; Study the principle of sensor’s environment temperature compensation. Develop a kind of silicon machined air flow sensor based on the principle of thermal distribution effect which can compensate the changes of the environment temperature and improve the sensor’s performance when measuring low flow or the flow with a small change; (2) Design the parameters of the hot-film air flow sensor by using finite element analysis software to carry out thermal analysis and flow field analysis. Design the resistance value of the resistor and the layout of the chip. Study the influence of the thickness of the silicon nitride film and the suspended film structure on the sensor’s accuracy of measurement and power dissipation;(3) Design the process flow of the hot-film air flow sensor and the process parameters, and fabricate the sensor chip by MEMS technology.