Design,Simulation,Fabrication And Temperature Measuring Of Micro-structure Gas Sensor

MHP (Micro-heating Plate) is a very important part of MEMS micro sensor. The semiconductor gas sensor is a kind of chemical sensors, and it can work well only when the sensitive material is heated to reach a high temperature. Therefore, the property of MHP is related to whether the sensitive material can work well or not. The target of research in my thesis is to make out a kind of MHP, which has a field of uniform temperature that can enhance the selectivity and stability of gas sensor, and to make the device reach the temperature about 350~400℃. The power consumption is as low as possible at this case in order to realize the portable style. The design of MHP involves two aspects, heating design and structure design. The heating design is about MHP heating power, temperature distribution etc, so as to pursue lower power heating, uniform heating, quick heating and quick temperature change. Structure design will solve the mechanism intensity of MHP, so as to pursue high production and mechanism intensity. The difficult is how to get a compatible fabrication process between semiconductor IC fabrication process and gas sensor fabrication process. The feasibility of technique will be first considered before design work. The theory only analyzes the device behavior under ideal conditions. However, the fabrication conditions are quite complex. The behaviors are affected by many factors. Therefore, the combination of theory approximately analysis and experiment exploring will be employed. First of all, the basic structure has been set up in term of operation principle of gas sensor by fixing on the manner of heating power. A crystal silicon wafer is chosen as MHP substrate, silicon dioxide as hot insulation, and silicon nitride as electronic insulation layer between metal Pt heating strip and signal electrodes. With the all the parameters and the techniques of IC and micro mechanism, the layout and technique process have been designed. Secondly, the design is simulated by finite element analysis methods with Ansys software. The benefits of combination finite element analysis with experiments are to reduce experiment cost and period. Relative with key part of MHP design, the distribution of MHP steady state heat is simulated and analyzed. At first, corresponding physical equations have been solved with the methods of theory, and then the temperature of MHP has been analyzed, finally the model of MHP is optimized. The simulation work gives us some new ideas and experiences. As for the design of burying Pt as heating device, its fabrication process is not compatible with IC technique process because metal Pt owns high stick property. Furthermore, the sandwich structure is replaced by placing heating bar and signal electrodes on one plane. As result, the redesign micro structure gas sensor can be fabricated by IC technique process. This improvement design not only reduces the complex of fabrication process but also improve mechanism intensity. With the design optimized, the analysis has been altered relevantly. By analysis, the method of combination of SiNx and SiO2 makes the hot lost quickly to substrate, and the power consumption rises. The heating conductivity of SiNx is 14 times as that of SiO2. Instead, using SiO2 as a transition layer between heating bar and substrate, decreased technique complexity and power consumption, for owns excellent insulation performance. Given sensor material and its basic shape, heating bar is analyzed by finite element tool Ansys software. It indicates that the high temperature concentrate on center, and that is important to heat sensitive material withsensor effectively. The low and high temperature about the sensor of different thickness and two types material have been analyzed. As to crystal Si substrate, the low and high temperature is the same in long rang of thickness, because of high heating conductivity of Si. As to SiO2, the situation is obviously different for the low and high temperature. When the thickness is below 5um, the difference between low and high temperature increasingly rises, which is a vital reference to reduce power consumption. The temperature distribution can be simulated by Ansys software, but it is difficult to measure the temperature on the surface of micro zone accurately. In order to obtain a high performance of sensitive material, the micro structure sensor must be heated to a quite high temperature (above 3500C). And the traditional method is to fabricate a temperature sensor on MEME device simultaneously. First demarcate temperature sensor in constant temperature box, and then detect the real temperature of micro sensor with temperature sensor. But this method can only measure the average temperature in a certain range, and is very difficult to measure micro area. With the constant study, an idea about measuring temperature with fiber comes out, which can detect the temperature of micro area (60um×60um), and it satisfies the need of MEMS device. This is an innovation method that cannot be reported in any papers. The other work in this thesis is involved silicon material etching. Various structures can be obtained through anisotropy and isotropy etching. In the post process of technique, the thickness of devices has been reduced by etching, which makes a high degree between highest and lowest temperature. It will be useful to increase the performance of MEMS device.