Manja Krüger

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.

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.

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.

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.

Molybdenum alloys for high temperature applications in air

Abstract Molybdenum base silicide alloys exhibit promising oxidation resistance in addition to the inherent high temperature strength of refractory metals. However, alloys with sufficient oxidation resistance are effectively brittle up to temperatures above 816°C (1500°F). Recent progress in alloy and process development, utilising a PM manufacturing route with mechanical alloying as a crucial step, has allowed significant improvement of both oxidation resistance and mechanical properties via micro-alloying additions including nano-dispersed second phase oxide particles.

Synthesis and characterization of lamellar and fibre-reinforced NiAl-Mo and NiAl-Cr

Directionally solidified (DS) alloys of the eutectic systems NiAl-10Mo and NiAl-34Cr (at.%) are potential candidates for high-temperature structural applications. Here, these alloys were first arc-melted and drop-cast. Thereafter, they were directionally solidified (DS) at growth rates of 20 and 80 mm/h while rotating at a fixed rotation speed of 60 revolutions per minute. Specimens of the DS alloys were tested in three-point-bending and uniaxial compression to obtain mechanical properties, including the ductile to brittle transition temperature (DBTT). For the NiAl-Cr system DBTT was found to be around 300 °C. Microstructural observations revealed that in the section perpendicular to the growth direction a uniform distribution of fibres was observed. The expected decrease of the fibre diameter with increasing growth rate was not observed. Instead, the fibre diameter slightly increased with increasing crystal growth rates. First compression tests were performed to get insights into the creep behaviour of these fibre-reinforced microstructures.

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.