Fabrication of porous silicon-based optical sensors using metal-assisted chemical etching

Porous silicon (PSi) is a versatile nanomaterial which has been utilized in several applications such as optical switching, drug delivery and sensors since its discovery in 1991. This material has been extensively investigated as an optical sensor due to its high surface area, high sensitivity and variety of optical transduction possibilities, e.g. changes in uorescence or reectance (interferometric sensing). It can be produced by electrochemical etching, stain etching, and metal-assisted chemical etching. For fabricating highly reective PSi layers suitable for interferometric sensing PSi was exclusively produced by electrochemical etching – to the best of our knowledge. In an electrochemical etching process a silicon (Si) substrate is xed in an electrochemical etching cell, immersed in a uoride containing etching solution and a current is applied. In the electrochemical reaction the Si acts as anode whereas a platinum electrode represents the cathode. Depending on a variety of parameters such as the doping of the Si, the composition of the etching solution and the applied current density different porous structures can be generated. Thereby the current density plays a major role in the resulting pore size and was successfully exploited for controlling the pore diameter in the porous layer