Sensitive, selective, and analytical improvements to a porous silicon gas sensor

Conductometric porous silicon gas sensors consisting of a sensitive surface layer which is conducive to the rapid and reversible transduction of sub-ppm levels of analyte gas have been developed. Several new fabrication and testing methods allow the detection of a number of analytes including CO (<5 ppm), NOx (<1 ppm), SO2 (<1 ppm), and NH3 (500 ppb). We outline a progression of fabrication techniques, including an HCl cleaning process, which allow the formation of much more efficient porous silicon sensors. Selectivity and enhanced sensitivity are developed using electroless metal deposition to form a gold or tin oxide nanostructured framework interacting with the nanopore-coated microporous surface. The ability to monitor sensor response in the presence of external noise sources is increased with the introduction of an FFT filtering technique. These studies present the first detection of CO with a porous silicon sensor as well as a considerable improvement in the sensitivity to NH3. It is suggested that a diffusion-based model can be used to parameterize the response of the sensors. We demonstrate several applications of these sensors to the monitoring of gas mixtures as exemplified by the NH3/NOx system. The potential for these sensors in arrayed configurations with integrated CMOS devices is considered.