Design and Simulation of Closed Membrane type Micro Hotplate for Gas Sensing Applications

Author : Sarjit Verma, Rajnish, Ankur Singhal and Parveen Jindal

Abstract

The paper presents the design and simulation of microhotplate (MHP), which requires 20.5 mW-62.8 mW power to create the temperature 227oC-645oC for gas sensing application has been carried out using ANSYS 10.0 and the results were verified using mathematical calculations. A Polysilicon-based bulk micro machined hotplate of size 500 m × 500 m has been designed for fabrication as a multi-layer structure on a silicon substrate with thermal silicon dioxide as the supporting membrane, followed by LPCVD silicon nitride film, sputtered Polysilicon film patterned into a double spiral geometry for heater, and PECVD oxide film for insulation of the heater. Gas sensing film (SnO2) will be deposited on the interdigitated Polysilicon electrodes formed on the PECVD oxide layer. Silicon etching for making the suspended microhotplate structure will be performed as the last stage of the process sequence. Etching away the silicon below the thermal silicon dioxide, the hotplate will remain suspended at the four corners by diagonal bridges. Thus a bulk micro machined micro hotplate structure will be realized. The change in resistivity of the sensing films that require the elevated temperature (250 oC-500 oC) will be measured with varying gas concentration. To estimate the resistance of the Polysilicon heater, a 200 nm thick Polysilicon film has been deposited by sputtering on silicon and its sheet resistance has been measured as 20 ohm/sq. We have used this value to calculate the resistance of Polysilicon resistor, which was found 3120 ohm.

Keywords: Polysilicon heater, Bulk micromachining, Micro hotplate, Gas sensor, ANSYS simulation.