Design, Simulation And Optimization Of New Micro Gas Sensor

With improvements in science and society, the species and application range of inflammable, explosive, poisonous, harmful gases have been increased continuously, gas sensors that play a role in monitoring and controlling the harmful gases have been broadly applied, and micro gas sensors based on MEMS technology are widely studied by domestic and overseas scholars, being one of the hotspots in gas sensor.In this paper, a series of research is carried out around new micro gas sensor. To start with, three new types micro-hotplates of gas sensors are designed, then the thermal fields and magnetic fields of them are simulated, optimized and compared with the help of finite element analysis software ANSYS, finally the thermal fields of SnO₂ micro gas sensors based on the three micro-hotplates are analyzed and compared. The main work is as follows:1. Design, simulation and optimization of type I micro-hotplate Type I micro-hotplate structure of gas sensor is designed, its base shape is cylindrical body with cross section area of 1.5 mm2 and thickness of 300 μm. Through the simulation and optimization, the results show that when the thickness of front SiO₂, Si and rear SiO₂ in the substrate of the micro-hotplate are 10 μm, 120 μm, 170 μm respectively, the heating electrode and signal electrode width are both 10 μm, the center area of the micro-hotplate can obtain high temperature of 352.26 oC and temperature difference of 0.1oC. Moreover, the magnetic flux density value lies in the minimum numerical range in color contour and the minimum is 9.84×10-10 T in the center area, which can be ignored when compared with the maximum value of 383.066 T located in the heating electrode, it can effectively reduce the influence of magnetic field on the measuring signal.2. Design, simulation and optimization of type II micro-hotplate The size of type II micro-hotplate is 1mm×1.5mm×300μm, then the micro-hotplate is optimized by using software ANSYS. The results show that when the substrate is optimized as silicon cup structure, silicon layer of thermal conductivity and protective layer of SiO₂ are sandwiched between the insulating layer of SiO₂ and silicon base, and the heating electrode width and spacing are 50 μm and 60 μm, the measuring electrode width is 50 μm, the temperature and difference in temperature are 364.76 oC and 0.3oC in the center of the micro-hotplate. The minimum value of magnetic flux density is 3.48×10-9 T and the effect can be ignored compared with the maximum value of 1×10-4 T, meeting the requirements of high and uniform temperature and low magnetic induction intensity in the center of the micro-hotplate.3. Design, simulation and optimization of type III micro-hotplate The third type micro-hotplate is designed, and its size is 1mm×1.5mm×300μm, the substrate of the micro-hotplate is designed as silicon cup structure. The optimized results are as follows: the heating electrode width and spacing are 50 μm and 100 μm, and the measuring electrode width is 50 μm, the temperature in the center of the micro-hotplate is 361.77 oC and varies in 1oC, the minimum value of magnetic flux density is 1.38×10-12 T and can be ignored compared with the maximum value of 1.66×10-4 T, successfully reducing the interference on the measurement circuit.4. Comparison of the three micro-hotplates The temperature distribution and the magnetic field distribution of the three types of micro-hotplates are compared, it turned out that with the same power consumption, type II possesses the highest temperature value of 364.76 oC and varies in 0.3oC in the center area, followed by type III that is 361.77 oC and temperature difference is about 1oC, the lowest temperature value of 352.26 oC is type I, its temperature difference is 0.1oC. The magnetic induction intensity values of the three micro-hotplates all lie in the minimum numerical range in color contour, type III is the smallest, followed by Type I and II, the values of III I II are 1.38×10-12 T, 9.84×10-10 T and 3.48×10-9 T.5. The thermal field analysis of SnO₂ micro gas sensors The core of gas sensor is gas sensing material, SnO₂ is selected as gas sensing material in this paper because it is the most widely used, then SnO₂ micro gas sensors based on the three micro-hotplates are researched about their thermal fields. With the same power consumption, temperatures from type I to type III are about 290 oC, 310 oC, 292 oC in the SnO₂ work area of the three gas sensors, and the corresponding temperature differences are 0.1oC, 0.1oC, 0.6oC, respectively, so it’s obvious that the order from high to low in temperature by turns is II, III, I.