S. Hilbrich

Dielectric filters made of PS: advanced performance by oxidation and new layer structures

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. For the formation of PS dielectric filters a detailed calibration of the etch rates and refractive indices is required. The effective dielectric function of PS was determined for different substrate doping levels as a function of the anodization current density by fitting reflectance spectra. Based on these results a number of different dielectric filters were realized. For device applications a thermal oxidation step is necessary to reduce aging effects which occur as a result of the native oxidation of PS. In addition, thermal oxidation results in a qualitatively improve filter performance due to a reduced absorption in the PS layers. Therefore the dielectric functions of PS oxidized in dry O 2 at temperatures up to 950 °C were determined. A continuous variation of the porosity and hence the refractive index with depth was used to realize so-called rugate filters. This type of interference filter allows the design of structures with more complex reflectance or transmittance characteristics than structures consisting of discrete single layers.

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.

Color-sensitive photodetector based on porous silicon superlattices

Color-sensitivity of Si photodiodes was achieved by integrating porous silicon (PS) Bragg reflectors and Fabry–Perot filters. The PS was formed in the p+-type part of the p+n junction which required illumination of the samples during anodization. The optimal illumination power density turned out to be a compromise: high power densities are necessary to enable high anodization current densities, but this results in a degraded filter performance. The PS layers had no significant influence on the electrical characteristics of the photodiodes, but as expected they strongly modified the spectral response. The results are in good agreement with the reflectance spectra of the filters.

Suppression of ageing effects in porous silicon interference filters

Interference filters made from porous silicon can be a useful alternative to conventional dielectric filters because of the fast and cheap fabrication and the compatibility to conventional silicon technology. However, so far the main disadvantages of these structures were ageing effects due to the oxidation of porous silicon in ambient air. In this paper we demonstrate that the problem can be solved by a thermal pre-oxidation of porous silicon. This treatment allows the use of these filters not only at room temperature, but even at high temperatures up to 600°C. Moreover, the pre-oxidation reduces the absorption in the blue and UV which is necessary for future applications in this spectral range. The complex refractive index of the pre-oxidised porous silicon is determined by numerical simulations of reflectance measurements.

The application of porous silicon interference filters in optical sensors

Abstract Optical filters for the infrared to the UV spectral range made of porous silicon are produced and tuned by modulating the current density during the etching process. Using such filters in an optical sensor it is possible to do quantitative absorption spectroscopy without a spectrometer, as will be shown.

Color-Sensitive Si-Photodiode Using Porous Silicon Interference Filters

A new method for the fabrication of color-sensitive Si-photodiodes is presented. Color sensitivity was achieved by using porous silicon multilayer stacks which act as interference filters if the formation parameters are controlled carefully. These filters were integrated in the upper, p+-type part of a p+n-junction. As expected, the spectral response of the photodiodes was determined by the transmission spectra of the filters, while the porous silicon had no significant influence on the electrical characteristics. The great advantage of this method over conventional ones is that it makes very cheap, fast filter fabrication requiring no expensive deposition process possible.

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.