(331d) Fabrication Of Nanoporous Silicon Optical Filters Via Galvanic Displacement Processes In Water-In-Oil Microemulsions

Nanoporous silicon optical filters are promising for applications in optical and sensing devices. Optically flat thin films of nanoporous silicon are typically fabricated by electrochemical anodization etching processes. Multiple reflections of white light at the top and bottom of the porous silicon layer create a Fabry-Perot fringe pattern. When chemical and biological substances are captured into the pores, the effective optical refractive index of the porous silicon layer is modified, and a corresponding shift in the interference pattern can be observed, which serves as the basis for sensor applications.

Recently, metal-assisted electroless etching processes have received considerable interest as a convenient and cost-effective approach to fabricate porous silicon and silicon nanowires from crystalline silicon substrates. This strategy is based on metal-induced excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. However, the lack of control on the metal particle size makes it difficult to fabricate the uniform and homogenous porous silicon layers. Therefore, the Fabry-Perot fringe pattern has not been previously observed on porous silicon layers fabricated by the metal-assisted electroless etching process.

Here we show that porous silicon Fabry-Perot filters can be fabricated by metal-assisted electroless etching processes via galvanic displacement processes in water-in-oil microemulsions. The water-in-oil microemulsions are employed to control the size of the Au clusters, which in turn controls the size of the pores fabricated by the Au-assisted electroless etching method. The thin porous silicon layers produces Fabry-Perot fringe patterns upon white light illumination. By tuning the microemulsion parameters and the etching time, various interference fringe patterns have been obtained.