Miniaturised arrays of tin oxide gas sensors on single microhotplate substrates fabricated by micromolding in capillaries

The concept of producing one gas sensor on one microhotplate is extended towards a gas sensor array consisting of several sensors on one hotplate. Twelve miniaturised gas sensors of nanoparticulate tin oxide were integrated as an array on a single microhotplate by using soft lithography, i.e. micromolding in capillaries. Soft lithography is a microfabrication technique based on elastomeric molds of poly-dimethylsiloxane (PDMS) for pattern transfer. It is ideally suited to directly micro-shape liquid materials, including suspensions of oxidic powders. PDMS molds containing recessed microchannels could be placed directly—and removed after processing—on freestanding silicon nitride membranes of 250 nm thickness without causing damage. Tin oxide microlines were generated after applying a suspension droplet at the entrance of the microchannels and filling them by capillary force action. A single sensor was shrunk on a minimal area of 10 μm×30 μm. In gas sensitivity experiments with carbon monoxide concentrations from 100 to 2000 ppm, a maximal sensitivity to carbon monoxide of 600 ppm was obtained. The heating power required to operate the whole array is reduced by at least one order of magnituede compared to the power that is usually required to run an array of multiple (>10) micromachined gas sensors.