Screen-printed nanoparticle tin oxide films for high-yield sensor microsystems

By means of a screen-printing technique, sensitive layers of nanopowder tin oxide were deposited on silicon micromachined substrates. The thickness of the sensing layers was 5 μm and particle size was around 40 nm. Each chip contains four thin silicon nitride membranes, on the centre of which a polysilicon heating resistor, insulating layers, platinum electrodes and sensitive layer are stacked. Unlike in previously reported works, the technological procedure reported here allows the deposition of the sensing layers before the membranes have been etched. This avoids damaging the membranes during film deposition, which leads to gas sensor microsystems with an excellent fabrication yield. The deposition method overcomes disadvantages such as low porosity and low surface area, generally associated to chemical vapour deposition (CVD) or sputtering methods, and keeps power consumption low (80 mW for a working temperature of 480 °C). As an example, the sensor response to ethanol, acetone and ammonia vapours and their binary mixtures was studied. The sensors were very sensitive to ammonia vapours. The influences of the sensor operating temperature and the electrode geometry were also investigated. By using an integrated array of four microsensors operated at two different temperatures and a fuzzy ARTMAP neural network, it was possible to identify the different species measured (success rate was higher than 91%). It was also possible to determine the concentration of the samples with a success rate higher than 84%. These results confirm the viability of the technique introduced to obtain micromachined sensors and sensor microsystems suitable for battery-powered gas/vapour monitors.