Reception tuning of gas-sensor microsystems by selective coatings

Selectively permeable SiO2 and Al2O3 coatings have been developed to customize the selectivity and improve the stability of SnO2 conductivity sensors of array microsystems for the detection of organic gases in air. The membranes have been prepared by chemical vapour deposition in thicknesses of 1-70 nm. While Al2O3 is obtained by pyrolysis of Al(OC3H7)3, SiO2 membranes are produced from Si(C6H5)(OC2H5)3 with ion-beam assistance at room temperature. Thereby, initially considerable amounts of carbon are incorporated into the bulk of the membrane, which are afterwards removed by heating to 350 °C, the operation temperature of the sensors. Tests of the coated sensors with air containing water, methane, propane, benzene and toluene show the intended selectivity change with respect to the naked sensors. The results are interpreted in terms of size-selective transport through the membrane and selective adsorption of aromatics. Up to 20 nm thickness, the membranes cause enhanced selectivity for small molecules such as water and methane accompanied by propane discrimination and a preferred toluene detection. Signal reduction due to the coating is less than a factor of 10. The response time is increased up to 250 s only for low concentrations.