Electrical characterisation and reliability studies of thick film gas sensor structures

The three major problems of gas sensing devices are the sensitivity to gases, the selectivity towards different gases and the electrical stability of the device. This paper describes the study of the electrical stability of a thick film SnO/sub 2/ gas sensor which has been investigated with an in-situ conductance measurement technique, in order to understand the disturbing problems of the resistance drift of the sensor material during the operation of the gas sensor. For this purpose a new test structure has been realised in screen printing technology, such that each layer in the gas sensor can be characterised and the influence of the top layer on underlying layers can be investigated. By using in-situ conductance measurements the relevant electrical properties are measured continuously during a predefined temperature profile. The in-situ technique offers both a high measurement accuracy, as well as the availability of many data points when compared to conventional off-line testing methods. As a result, a complete electrical characterisation of the gas sensor could be done very accurately.