Study On The MEMS Processes Of Micro Hotplate Gas Sensors

Gas sensors are widely used in the detection of combustible gas, toxic gas and atmospheric composition. The micro hot plate gas sensor based on the MEMS process becomes current research hotspots in the field of gas sensor for the characteristics of low power consumption, small volume and easiness to integrate. Most MEMS gas sensors use platinum as the heating wire, and use the back-side bulk silicon etching process to suspend the micro hot plate. Tungsten, which has stable property, can reduce the cost to replace platinum as the heating wire. The back-side bulk silicon etching process has the problem of fabricating micro hot plate of large size and high power consumption. Therefore, the author chooses platinum and tungsten as the heating wire, and uses the front-side bulk silicon etching process to study the MEMS process of micro hot plate.First of all, according to the design requirements of the micro hot plate and available processes in the MEMS center in Dalian university of technology, the author designs the layer structure and plane structure of the device. Masks of the micro hot plate gas sensors based on four inch wafer are drawn using Mentor graphics. The masks consist of three layers which are heating wire layer, etching window layer and gas sensitive electrode layer. Each micro hot plate has four supporting arms. The serpentine heating wire was adopted with a width of 7 microns. The micro hot plate center area is 100?m*100?m.Second, the author starts to fabricate micro hot plate according to the existing equipment conditions and MEMS processes, including thermal oxidation isolation process, PECVD silicon nitride supporting layer, platinum and tungsten sputter process, PECVD silicon dioxide dielectric protecting layer, exposing etching windows, fabricating gas sensitive electrode, incising silicon wafer, front-side bulk silicon etching process, ink jet printing gas sensitive material and encapsulation. Considering the release of residual stress to enhance the structure stability of the micro hot plate, the silicon oxide and silicon nitride deposition thickness and relative proportion are adjusted. In order to improve the tungsten heating wire quality and graphical integrity, RF magnetron sputtering and lift-off process are adopted. Because the alkaline solution under heating condition corrodes tungsten, the solution is modified by adding the silicic acid and ammonium persulfate to reduce the PH value. Thus the tungsten can be protected in the front-side bulk silicon etching process. In a word, the author completes the whole processes of platinum and tungsten micro hot plate.Finally, the author tests the chip after encapsulation, including heating wire resistance conformance test, resistance temperature characteristic test, stability test, power consumption test, thermal response time test and gas sensitive test. The test results show that the micro hot plate gas sensor, which has advantages of high integration, low power consumption, fast heat response and high sensitivity, can be fabricated by front-side bulk silicon etching process. The device using platinum heating wire has good thermal stability characteristic under 300 degree centigrade, high yield in the whole processes because of the excellent corrosion resistant properties. The resistivity of the sputtered tungsten heating wire is significantly higher than the bulk material and the thermal stability characteristic is not ideal, by reason that the sputtered tungsten film is in the metastable state. The stable tungsten wire can be gained by high temperature thermal annealing after sputtering or changing the depositing process of tungsten, which is supposed to obtain tungsten micro hot plate gas sensor with excellent performance.