Multiphysics modelling of the fabrication and operation of a micro-pellistor device

Abstract

Downsizing efforts in gas-sensing applications lead to ever smaller active elements. Integration with data processing circuitry requires the use of CMOS compatible fabrication technology, autonomous operation poses limits on energy consumption of the elements, whereas reliable catalytic detection often needs high temperatures that may otherwise be constrained by safety considerations. Under these conditions, development of active sensor elements proves to be a growing challenge for design and fabrication.In this work we present a step-by-step study on a ≈500 J.lm diameter thermally isolated membrane element of a gas detecting microsensor device. Sensitivity is based on high temperature (≈3-400 0C) catalytic activity of a porous pellistor deposited on a multilayer SiO2/SiNx - filament heated - membrane that has to be durable enough for several thousand hours of operation, and as thin as possible to reduce heat conduction to the substrate. SiO2 membranes tend to show high residual stress that can be significantly reduced by "sandwiching" with SiNx. We have used COMSOL Multiphysics® 4.3a [I] to assist the initial product design, and evaluation of operational constrains of the multi-layer thin film. The first part involved systematic thermo-mechanical iterations, while the latter consisted of a combination of gradual static thermo-electro-mechanical simulation steps. As shown by simulating the steps of the deposition process in this work, the right combination of different techniques produces a stable 4-layer membrane with only a sub-micron deformation, and tolerable residual stresses after membrane forming (substrate removal) and during operation. Also, the pellistor filament heating power should be minimized and still reach the operating temperature of the catalyst hotspot. This design, supported by our model calculation was used to realize the device with targeted characteristics. The structure endures the distortion and thermal expansion and contraction during the heating cycles, whereas low power operation widens the range of possible applications.