Silicon hotplates for metal oxide gas sensor elements

Abstract

 Low-power-consumption metal oxide gas sensors and gas sensor arrays can be produced by combining micromachining and thin-film technologies. In the present paper the state-of-the art in this field is reviewed. In the first part the problem of thermal losses from a heated metal oxide film is addressed and the necessity of miniaturisation of gas sensing devices is pointed out. The thermal properties of realized silicon hotplates are compared and analysed with respect to thermal equivalent circuit models. In addition, the results of accelerated thermal ageing tests are presented. These latter results demonstrate that micromachined heater elements are likely to exhibit device lifetimes of the order of 30 years when operated at membrane temperatures around 400 °C. In the second part of the paper attention is drawn to novel methods of gas detection which are enabled by employing stacks of different thin film materials on micromachined hotplates. In this context, recent results on temperature-and field-effect modulated gas detection experiments are discussed.