Design and optimisation of a high-temperature silicon micro-hotplate for nanoporous palladium pellistors

The conventional design of the heater in a silicon micro-hotplate employ a simple meandering resistive track to form a square element. We show that this heater structure produces an uneven thermal profile characterised by a central hot spot with a significant variation in temperature of some 50 8C across the plate at an average temperature of 500 8C. Four novel micro-heater designs are reported here and fabricated on hotplates with an active area that ranges from (200 £ 200) mm to (570 £ 570) mm in order to vary systematically the ratio of membrane to heater length from a value of 5.0–2.7, respectively. All the designs have been simulated using a 3D electro-thermo-mechanical finite element model and results agree well with thermal profiles taken using an infrared microscope. One of the designs, referred to here as ‘drive-wheel’ structure, performs best and reduces the lateral variation in temperature to only ^10 8C. The different resistive micro-heaters have been calibrated with the lowest power consumption being 50 mW at 500 8C, which is well below the power consumption of any commercial pellistor; the maximum temperature before rupture being 870 8C. The micro-hotplates were electrochemically coated with a 20 nm thick mesoporous palladium catalyst and the pellistors’ response tested to 2.5% methane in air. The micro-heaters were observed to be stable for a period of 1000 h and should provide a good platform for exploitation in commercial catalytic pellistors. q 2002 Elsevier Science Ltd. All rights reserved.