Screen-printed nano-grain WO3 films for micro-hotplate gas sensors

By means of an adapted screen-printing technique, sensitive layers of nanopowder tungsten trioxide were deposited on silicon micromachined substrates. The thickness of the sensing layers was 5 μm and particle size was around 50 nm. Each chip contains four thin silicon nitride membranes, in the center 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 for depositing 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 (Llobet et al., 2003). The deposition method overcomes disadvantages such as low porosity and low surface area, generally associated to chemical vapor deposition or sputtering methods, and keeps power consumption low (80 mW for a working temperature of 480°C). As an example, the sensor response to ammonia, NO₂, ethanol, CO and methane was studied. The sensors were very sensitive to NO₂ vapors. The influences of the sensor operating temperature and electrode geometry were also investigated.