A highly sensitive methane sensor with nickel alloy microheater on micromachined Si substrate

A highly sensitive methane sensor on micromachined Si substrate (10 Omega-cm, 400 mu m, p < 100 >) operating at relatively low temperature regime (similar to 200 degrees C) is reported in this paper. A nickel alloy (DilverP1), having high yield stress similar to 680 MPa and low thermal conductivity similar to 17.5 W/m/degrees C, based microheater with appreciable temperature distribution uniformity and low power consumption (similar to 140 mW at optimum operating temperature) was designed and fabricated for the purpose. Meander shaped heater structure for a device size of 4 mm x 4 mm with a membrane dimension of 2 mm x 2 mm (with an active area of 1.5 mm x 1.5 mm) was investigated as a prototype. The temperature uniformity in the membrane region was further improved with the incorporation of a thin (50 mu m) Si membrane in place of normally reported SiO2/Si3N4 composite membrane. Thermal stress development during sensor fabrication (owing to high temperature processing steps involved in sensing layer formation) was avoided by incorporation of a low temperature chemical deposition technique of ZnO. MEMS based sensor (nano structured ZnO as sensing layer) with a noble metal catalytic contact Pd-Ag (70%) was tested for five different methane concentrations (e.g. 0.01%, 0.05%, 0.1%, 0.5% and 1.0%) in the temperature range of 125-225 degrees C with N-2 as the carrier gas. The response magnitude, response time and recovery time were studied in detail. The MEMS based sensor showed similar to 97% response magnitude at an optimum operating temperature of 200 degrees C with similar to 47 s response time at 1% CH4 in N-2. Moreover, the sensor offered appreciable response at even much lower temperature (150 degrees C) also. (C) 2012 Elsevier Ltd. All rights reserved.