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Porous silicon based multilayer structures for optical sensors have been simulated, fabricated and tested. The properties of optical sensors using porous silicon multilayer can be adjusted by appropriate substrate material, morphology, process parameters in the pore formation process and by surface treatment (thermal oxidation). Heavily and lightly doped p-doped substrates have been used to realize porous silicon layers with different morphology, porosity (30-80%), pore size (mesoporous range) and specific surface area (200-700 m2/cm3). Thermal oxidation stabilizes the surface and results in hydrophilic surfaces for effective adsorption of liquid analytes. Oxidation reduces the porosity and the pore size but improves the wetting behavior of liquid analytes in the porous volume. Different multilayer structures using native and oxidized porous silicon and corresponding concepts of optical sensor systems have been proved for aqueous and organic analytes. Sensors using small pore size (2-4 nm) and high porosity (70-80%) have been realized and characterized. A simple, low cost optical sensor system based on multilayer, a tunable light source and a detector has been realized.