Rüdiger Arens-Fischer

Formation of porous silicon on patterned substrates

Abstract Application of porous silicon in device structures requires the formation of micron-size porous areas. Therefore, selective area anodization on photolithographically patterned p-doped substrates was investigated. As shown in this work, porosity and layer thickness vary from the edge to the middle of the structures. This inhomogeneity strongly depends on the doping level of the substrate and the lateral size of the structure. When organic photoresists are used, an anisotropic undercutting of up to several 10 μm occurs at the edge of the structures. This can largely be reduced by using thermally treated Si 3 N 4 deposited by plasma-enhanced chemical vapour deposition as a masking layer. In this case an isotropic undercutting of the masking layer is observed permitting fabrication of porous silicon structures in the μm range by photolithography.

Optical Interference Filters Made of Porous Silicon

Porous silicon (PS) layers can easily be formed by an electrochemical etch process using a mixture of hydrofluoric acid (HF) and ethanol. The microstructure and porosity of the layers depend on the HF concentration, the doping level of the substrate and the current density applied during the etch process. Changing the current density during the etch process will result in a well defined layer structure consisting of layers with different porosities. Each single layer can be treated as an effective medium exhibiting a refractive index depending mainly on the porosity of the layer. Using reflectance measurements we have investigated the dependence of the refractive index of PS layers on the formation current density for different substrates. In addition the etch rate was determined by thickness measurements with an electron microscope. Based on these results various kinds of optical interference filters were studied. We have formed samples consisting of discrete single layers with different porosities (e.g. Bragg reflectors) as well as samples with continuous variation of the refractive index (rugate filters). Combining these PS filters with standard photolithography steps, microoptical devices such as spectral sensitive photodiodes can be realized.