Ruediger Arens-Fischer

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.

Porous silicon as a substrate material for potentiometric biosensors

For the first time porous silicon has been investigated for the purpose of application as a substrate material for potentiometric biosensors operating in aqueous solutions. Porous silicon was prepared from differently doped silicon substrates by a standard anodic etching process. After oxidation, penicillinase, an enzyme sensitive to penicillin, was bound to the porous structure by physical adsorption. To characterize the electrochemical properties of the so build up penicillin biosensor, capacitance - voltage (C - V) measurements were performed on these field-effect structures.

Investigation and design of optical properties of porosity superlattices

Abstract We have investigated the optical properties of porosity superlattices and complex multilayer systems. Type II superlattices reveal a more complex layer structure than expected from the substrate doping levels. Type I layer systems have been used to form highly reflective layer systems and Fabry-Perot filters.

Analysis of the depth homogeneity of p-PS by reflectance measurements

Abstract We have investigated changes in the etch rate of p-PS with increasing etching time as well as changes of the porosity of buried layers with depth. These effects can be attributed to the influence of chemical etching and variations in the electrolyte composition with depth. To study these changes, first the porosities of layers above and below layers with different thicknesses were determined by a fit of the reflectance spectra of these layer systems using the effective medium theory. Secondly we have measured oscillations in the reflectance during the formation of PS layers caused by the increasing layer thickness. Using these experimental results we are able to give a functional description of the changes in the optical thickness with depth. In addition, the influence of the chemical etching and changes of the HF concentration on the optical thickness can be estimated. As a result a method for changing the current with depth can be given, which can be used to minimize porosity gradients.

Investigation of different oxidation processes for porous silicon studied by spectroscopic ellipsometry

Abstract In this paper we investigate the oxidation of porous silicon by O3, H2O2, and, for comparison, in normal air. Such an oxidation may serve as passivation for porous silicon in applications in order to prevent devices from degradation. The changes in the dielectric function caused by this oxidation was monitored by spectroscopic ellipsometry. Application of both H2O2 and O3 resulted in a significant lowering of the values of the imaginary part of the dielectric function as expected when oxidizing the inner surfaces of these layers. For a multilayer structure we show that ozone treatment of this structure indeed passivates that sample against further oxidation in air as studied over an extended period of time (3 months).

Formation of Porous Silicon Filter Structures with Different Properties on Small Areas

Porous silicon (PS) layer systems have a broad range of possible applications. An advantage is the good control of the refractive index and the etch rate of the layers by the applied current density and the time respectively. For micro-optical devices you need patterned PS. For some optical devices it is not sufficient to have only one filter but it is necessary to form filters with different properties on a small area. We applied a method (M. Frank, U.B. Schallenberg, N. Kaiser, and W. Buß, in Conference on Miniaturized Systems with Microoptics and Micromechanics, edited by M.E. Moamedi, L.J. Hornbeck, and K.S.J. Pister (SPIE, San Jose, 1997), SPIE Proceedings Series 3008, p. 265) to PS which fits this goal by the following steps: fabrication of the desired reflectors below each other and partial removal of upper reflectors with reactive ion etching (RIE). The technological aspects of patterning PS after the fabrication are an important topic of this work. Problems are discussed in detail and solutions are given.

Fabrication of laterally displaced porous silicon filters

Abstract Porous silicon superlattices have been used to manufacture laterally displaced dielectric filters with different optical properties on one substrate. Two different fabrication processes for two-colour microfilter arrays are presented. Both methods overcome the problem of non-uniform optical properties of the well-known procedure where two filter stacks are grown one upon another, with subsequent partial removal of the upper filter by reactive ion etching. The novel methods give uniform optical properties of the two filter areas, profiting from the main property of the formation process of porous silicon: the formation of porous silicon occurs only at the porous silicon–substrate interface.

Electrical Control of the Reflectance of Porous Silicon Layers

In this paper we demonstrate the filling of porous silicon (PS) layers with liquid crystals (LCs) in order to control the reflectance electrically. The preparation of PS and the choice of the right group of LCs will be presented. Especially an oxidation of PS is necessary so that the methods and parameters of oxidation will also be discussed. As a first result the increasing and decreasing of the thickness oscillations in the reflectance as a function of the applied voltage can be observed.