(98f) Thermal Analysis of SiC Microhotplates and Gas Sensors

Chemical microsensors using microhotplates have attracted a great interest in recent years because of tremendous demand and growth in the chemical sensor market. The demand for chemical microsensors comes from new legal regulations on environmental air pollutions such as automobile exhausting gases NOx, CO, and increasing health awareness in the population of industrialized countries. Microfabricated gas sensors have advantages of micro size, fast response, and low power consumption. Microhotplate and metal oxide as sensing element are key factors for practical applications of microhotplate-based gas sensors. Current metal and polysilicon based microhotplates can not reach a temperature above 500 oC. Oxidation of metal and polysilicon heating elements at high temperatures also induces increasing power consumption and reliability problems.

SiC is well known for its combination of excellent mechanical, electrical, and chemical properties, which include high mechanical strength, wide bandgap, high breakdown voltage, high thermal conductivity, resistance to oxidation and creep, and chemical inertness to oxidation. It is a very promising compound semiconductor for MicroElectroMechanical Systems (MEMS) and microelectronics for high temperature and extremely harsh environment applications.

This work reports our effort to use MEMS software CoventorWare to do thermal simulation of silicon carbide (SiC) microhotplates to guide the design of SiC microhotplate based gas sensors for harsh environment (high temperature, high corrosion, high oxidation) applications. SiC has excellent oxidation resistance at high temperatures comparing with metal and polysilicon. Heavily doped SiC thin films deposited by low pressure chemical vapor deposition (LPCVD) are used for microhotplate heating element. Multiple simulations have been done to explore the effect of microhotplate geometry and supporting membrane on the maximum temperature and the temperature distribution. The preliminary experiment and simulation results indicate that SiC microhotplate can reach a temperature above 600 oC. Further simulation will be performed to determine the effect of changing the insulating material from silicon nitride to, for example, silicon dioxide, as well as altering the overall size of the hotplate, the thicknesses of each layer, and the doping concentration of the silicon carbide. The collective purpose of these simulations will be to optimize the power consumption maximum temperature and the temperature uniformity across the hotplate, making it ideal for use in gas sensors. Microfabrication of SiC microhotplates and gas sensors for sensing CO, NOx, and NH3 are in progress and will be characterized. The tested results of the SiC microhotplates will be compared with the simulated results and the characterization of gas sensors will also be reported.