An integrated surface micromachined convex microhotplate structure for tin oxide gas sensor array

This paper describes the fabrication process for a novel convex microhotplate (MHP) structure using surface micromachining technology. The process is used to fabricate an integrated 4×4 tin oxide gas sensor array. Surface micromachining is selected for its simplified process and CMOS compatibility. However, limited sacrificial layer thickness usually leads to higher power consumption. In this work, a convex MHP structure is developed to increase the thermal efficiency, which is critical for large dimension and low-power gas sensor array integration. Before the structure release process, 700 °C-950 °C annealing process was carried out on the 190 ×190 μm2 MHP with 2.8 μm polysilicon sacrificial layer. It is shown that higher annealing temperature leads to lower tensional stress of the MHP membrane and larger curvature of the released structure, which enables higher thermal efficiency. Among the four annealing temperatures, the 950 ° C annealed MHP has the largest curvature of 2.438 cm-1 and the highest thermal efficiency of 13 °C/mW. Experimental results showing the responses of the sensor array to different combustible gases are also illustrated in this paper. Comparison of power consumption with other designs reported in the literature illustrates the effectiveness of the proposed process.