Porous SiC as an ammonia sensor

Ammonia gas (NH/sub 3/) detection is widely used, from air conditioning to searching for life on Mars, and in many situations there is an increasing demand for cheap and reliable NH/sub 3/ sensors. When used as the dielectric in a capacitive sensing arrangement, porous SiC has been found to be extremely sensitive to the presence of NH/sub 3/ gas. The exact sensing method is still not clear, but NH/sub 3/ levels lower than /spl sim/0.5 ppm could be detected. We report the fabrication and preliminary characterisation of NH/sub 3/ sensors based on porous SiC. SiC is a very durable material and should be good for sensors in harsh environments. So far, the only NH/sub 3/ sensors using SiC have been FET based, and the SiC was not porous. In our devices, SiC was deposited by PECVD on standard p-type single-crystal Si and was then made porous by electrochemical etching in 73% HF using anodisation current-densities of 1-50 mA/cm/sup 2/. Preliminary data is given for our devices response to NH/sub 3/ in the range 0-10 ppm NH/sub 3/ in dry N/sub 2/ carrier gas, as well as the response to relative humidity between 10% RH and 90% RH.