Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With irradiation or illumination
Reexamination Certificate
1999-06-19
2002-07-02
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With irradiation or illumination
C205S656000
Reexamination Certificate
active
06413408
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a layer with a porous area for use, for example, in an interference filter. The invention further relates to a process of manufacturing such a layer.
Porous silicon (PS) is known which, because of its compatibility with the highly developed Si-microelectronics and because it is easy and inexpensive to manufacture, is a promising material for use as sensors (M. Thust et al., Meas. Sci. Technol. 6, (1995), as well as Y. Duvault-Herrera et al., colloid. Surf., 50,197, (1990) and, because of its electroluminescence, is suitable for applications in the area of display technology (P. Steiner et al., Appl. Phys. Lett., 62(21), 2700, (1993). Furthermore, porous silicon is known to be used in connection with color-sensitive photo detectors (M. Krüger et al., EMRS 96, Thin Solid films) or passive reflection filters.
The manufacture of layer systems of PS has been demonstrated (M. G. Berger et al., J. Phys. D: Appl. Phys. 27, 1333, (1994), DE P 43 19 413.3 or M. G. Berger et al., Thin Solid Films, 255, 313, (1995)). These layer systems exhibit for example, a color-selective reflectivity depending on the manufacturing parameters. Furthermore, the structured manufacturing of PS is known, whereby areas with different spectral behavior an be made (M. Krüger et al., EMRS 95, Thin Solid films).
Specifically, porous silicon consists of a foam-like skeleton of silicon crystallites, which include pores. The size of the crystallites and of the pores varies, depending on the doping and the manufacturing parameters, between some nanometers and some micrometers. If the wave length of the light is much greater than the structures in the PS, the PS appears to the light to be a homogeneous material (“effective medium”) and its properties can therefore be described by an effective refraction index (W. Theiss: The Use of Effective Medium Theories in Optical Spectroscopy, in Festkörperprobleme/Advances in Solid State Physics, Volume 33, page 149, Vieweg, Braunschweig/Wiesbaden), which depends on the refraction indices of the silicon crystallite, an oxide possibly present on the surface of the crystallite and of the material in the pores. Consequently, interference filters can be constructed from various porous layers, which extend parallel to one another and have different optical properties. The various layers are constructed parallel to one another and have, within the respective layer, a constant layer thickness normal to the layer surface. It is however disadvantageous that for each of the different spectral characteristics, a separate filter must be made in separate manufacturing steps.
It is also disadvantageous that, in the known methods, the manufacture of adjacent filters with different characteristcs can be achieved only by photolithographic steps, or respectively, by separate etching of the filters with particular characteristics.
It is therefore the object of the invention ti provide a layer including a porous layer and an interference filter including such a layer as well as a process of manufacturing such an interference filter wherein a simplified establishment of interference filter functions of porous silicon with laterally gradually variable reflection and transmission characteristics is achieved.
SUMMARY OF THE INVENTION
In an interference filter having a layer with an area consisting of a porous material extending from the surface of the layer to the interior, the dimensions of the porous layer area in a direction normal to the layer surface have different values to provide for varying reflection or, respectively, transmission characteristics.
It has been found that, based on the well known manufacture of layer systems of porous silicon, a method could be provided wherein a lateral change of the reflection and transmission capabilities is achieved.
In particular, a layer system with a well-defined laterally variable spectral characteristic is manufactured thereby in a single process step. In this process, a porous layer area is so formed that the porous layer thickness assumes different values within this layer. In this way with such a porous area adjustable characteristics within this single layer can be achieved depending on desired boundary conditions. It is no longer necessary to manufacture several components individually which have different characteristics and which are then combined.
Furthermore, it may be advantageous to provide within the porous area several different porosity values in order to provide for individually desired characteristics in a controllable manner. Also, the degree of porosity may be established laterally for example in a continuous way. The area furthermore may have several partial layer areas with different degrees of porosity.
For a simplified manufacture, it may be advantageous to form the porous layer and/or the partial layer areas such that they are wedge-shaped. Preferably Al, GaAs, or SiGe are suitable materials, but most advantageous is the silicon which is used in many ways in microelectronics.
It has been realized that it is advantageous to establish during the manufacture of such a layer, upon etching for making the material porous, a gradient with respect to a physical value which corresponds to the etching velocity of such an etching procedure. Alternatively, or additionally, a value can be selected which corresponds to the porosity of the material. As physical value, preferably the electric field is employed and, the temperature is used. The material of the substrate or the doping of the material may be employed. The values may be employed individually or in combination together.
It is advantageous to utilize the electrodes provided for the electrochemical etching for the formation of the field gradient. In this case, a first electrode may be arranged in the electrolyte disposed above the surface to be etched, whereas a second electrode is disposed on the side of the substrate remote from this surface. Furthermore, such an electrode may be arranged in a tilted fashion so as to form a field gradient. Furthermore, the electrode or electrodes may be in the form of a net and may include a mesh structure to form gradients wherein the mesh openings are increasingly narrower in the direction of the gradient.
The layer system according to the invention can be made for example by current flow with a lateral gradual change of the reflection or respectively, transmission characteristic utilizing the temperature dependency of the etching process. As a result of the temperature dependency, the etching rate or, respectively, the porosity changes when the temperature of the electrolyte/substrate changes. Consequently, temperature gradients in the electrolyte or in the substrate can change locally the etching rate and, as a result, also the optical properties of a filter.
Alternatively, the layer system according the invention can be made with a laterally gradual change of the reflection and or, respectively, transmission characteristics utilizing a changed anode or, respectively, cathode arrangement. As a result of the dependency of the etching process on the field strength between the anode and the cathode, the etching rate or, respectively, porosity is changed between the electrodes. Consequently, field strength gradients may change the etching rate between the electrodes and consequently also the optical properties of a filter.
The invention however is not limited to such processes. In accordance with the invention, alternative processes are possible wherein another value, which affects the etching process, is used to achieve a gradual change of the porosity. For example, the doping of the substrate material before the etching may be so selected that there is a lateral gradient in the substrate.
The invention is explained below in greater detail on the basis of figures and embodiments.
REFERENCES:
patent: 4283259 (1981-08-01), Melcher et al.
patent: 4303482 (1981-12-01), Buhne et al.
patent: 4622114 (1986-11-01), Glass et al.
patent: 5218472 (1993-06-01), Josefowicz et al.
patent: 5338415 (1994-08-
Arens-Fischer Rüdiger
Berger Michael
Hilbrich Stefan
Krüger Michael
Lang Walter
Bach Klaus J.
Bell Bruce F.
Forschungszentrum Jülieh GmbH
Nicolas Wesley A.
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