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

Abstract A highly sensitive methane sensor on micromachined Si substrate (10 Ω-cm, 400 μm, p 〈1 0 0〉) operating at relatively low temperature regime (∼200 °C) is reported in this paper. A nickel alloy (DilverP1), having high yield stress ∼680 MPa and low thermal conductivity ∼17.5 W/m/°C, based microheater with appreciable temperature distribution uniformity and low power consumption (∼140 mW at optimum operating temperature) was designed and fabricated for the purpose. Meander shaped heater structure for a device size of 4 mm × 4 mm with a membrane dimension of 2 mm × 2 mm (with an active area of 1.5 mm × 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 μm) 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 °C with N2 as the carrier gas. The response magnitude, response time and recovery time were studied in detail. The MEMS based sensor showed ∼97% response magnitude at an optimum operating temperature of 200 °C with ∼47 s response time at 1% CH4 in N2. Moreover, the sensor offered appreciable response at even much lower temperature (150 °C) also.