Andras Kovacs

Optoelectrical Detection System Using Porous Silicon-Based Optical Multilayers

Porous silicon-based multilayer structures for optical sensors have been simulated, fabricated and tested. The properties of optical sensors using porous silicon multilayers 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 LED-based illumination system providing discrete wavelengths (RGB) and a wide band detector has been realized and tested.

Surface Micromachining Process for C-Si as Active Material

We have investigated a new type of surface micro-machining process. Using porous Si as sacrifical layer very thin, well defined and reliable free-standing crystalline Si (c-Si) structures have been fabricated. Excellent thickness control has been achieved in the thickness range 5 µm to 0.4 µm. The thickness is mainly controlled by junction depth xj and surface doping concentration Ns. Additionally, a proper choice of current density allows a fine tuning of the pattern thickness during the formation process of the sacrifical layer (porous Si). Typical accuracy of the resulting thickness to target thickness is about 63 nm. This process combines the advantages of bulk-micromachining (use of c-Si as active material) and surface micromachining (smaller devices, integration in standard CMOS processes, any shape). As porous Si can be used also as active layer, e.g. for humidity sensing or thermal isolation, porous Si as a sacrificial layer will lead to multifunctional devices without complicating the fabrication process too much.