Gas Diffusion in Porous Silicon Sensors

Nanopore covered microporous silicon conductometric gas sensors have been produced via electrochemical etching and standard microfabrication techniques. Reversible and sensitive gas sensors working at room temperature have been fabricated. Sensing of NH$_{3}$, NO$_{x}$ and PH$_{3}$ at or below the ppm level have been observed. The porous surface has been modified using selective coatings including electroless tin, gold, nickel and copper solutions to increase the response to NO$_{x}$, NH$_{3}$, PH$_{3}$ respectively. The diffusion of the analyte species has been investigated in'the nanopore and micropore regimes by numerical analysis. Comparing the response time of the hybrid porous sensor surface with numerical diffusion calculations on the pores, it has been observed that Knudsen diffusion time scales dominate the sensor response. A transduction model is proposed based on nanopore limited gas diffusion and the experimental response and recovery data.