Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With irradiation or illumination
Reexamination Certificate
2001-10-26
2003-08-05
Phasge, Arun S. (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With irradiation or illumination
C205S656000
Reexamination Certificate
active
06602397
ABSTRACT:
The invention concerns a method for production of a filter.
This type of method is known from WO 99/05344. Fine filter pores are etched with this method in an n- or p-doped silicon blank, which is connected as an anode or a cathode. A first side of the blank is then dipped in an etching solution in which a counterelectrode is arranged to generate an etching current. At the same time, a second side of the silicon blank opposite the first side is exposed to light so that minority charge carriers are created in the blank. The blank therefore forms a “working electrode” that is electrochemically perforated as a function of its doping, the etching current density and the chemical composition of the etching liquid.
Ultrafine filters can be produced with this method, whose structural dimensions, i.e., pore diameter and pore spacing, lie in the micrometer range (&mgr;m) to the nanometer range (nm). If the structural dimensions are smaller than about 100 nanometers, however, the microstructure of the filter can become porous or sponge-like.
One problem here is that the etching process occurs not only on the “pore bottom,” but also across the pores, which can lead to a situation in which the pore cross sections are not kept precisely constant over the pore length and, in addition, adjacent pores are connected to each other. Such “cross-etchings” occur, in particular, when minority charge carriers or “defect electrons” reach the pore walls of the blank being processed. The defect electrons are “holes” in the valence band of the blank atoms and behave like positively charged particles.
Another method for production of ultrafine filters by electrochemical etching is described in U.S. Pat. No. 5,139,624, in which it is stated that the pore diameter depends on the etching current and the dopant concentration and the etching solution concentration.
U.S. Pat. No. 5,348,627 describes a similar etching process in which the blank is exposed to light to generate holes in the valence band, during which the light intensity can be alternately varied in order to vary the etching rate. A broadband light source or monochromatic light source is then used.
DE 4 202 454 C1 describes a similar method for the production of a filter, geared toward generating pores, whose cross sections are not constant but have cavity-like expansions. Such cross-sectional expansions are achieved by changing the current density in the substrate wafer, in which the cross-sectional enlargement is achieved by increasing the illumination intensity.
EP 0 296 348 A1 also describes an etching method in which electrolysis is initiated by illuminating the silicon element from the back and the illumination is kept constant or varied in time in order to control the etching current by the formation of minority charge carriers. The etching current is then a function of the light impinging on the silicon element and primarily determines the hole width.
SUMMARY OF THE INVENTION
The objective of the present invention is to improve a method, as mentioned at the outset, so that a roughly equivalent stipulated pore diameter can be maintained over the entire length of the filter pores.
The basic principle of the invention consists of reducing the generation rate of minority charge carriers that are formed during the etching process in the blank being etched as a function of the etching progress, i.e., with increasing pore depth.
The blank employed here consists of an n- or p-doped etchable semiconductor material, like silicon, GaAs, etc., which is connected according to the doping as an anode or a cathode and dipped with at least one first side into an etching liquid in which a counterelectrode is present. Activation energy is supplied to the blank during the etching process, for example, by light exposure or by heat. Minority charge carriers or defect electrons are generated by the activation of the individual atoms of the blank, in which the generation rate increases with the activation energy supplied per unit time and with the supplied activation power. If the blank is connected as the anode, for example, i.e., positively charged, the positively charged defect electrons or holes migrate to the outside of the blank to the side facing the negatively charged counterelectrode, i.e., the cathode.
The electric field that can be established in the blank is then “bent” by the smallest recesses or irregularities in the planar surface of the blank to their tips or bottoms, in which the defect electrons follow the electric field, which means that etching primarily occurs on the bottoms or tips of the recesses and fine pores are therefore formed.
It turns out that with increasing etching progress, i.e., the increasing pore depths, the defect electrons not only migrate to the “pore bottoms,” where the etching progress is supposed to primarily occur, but also to the pore walls, which would lead to the enlargement of the pore diameter. This “cross-etching” can be avoided on the pore walls according to the invention if the activation power fed to the blank or the activation energy fed per unit time is reduced as a function of the etching progress and the increasing pore depth. Very fine pores with roughly constant cross sections can therefore be produced, whose diameters can lie in the range from a few hundred micrometers to one nanometer.
It is known from the prior art that the radiation intensity can be increased to increase the pore cross section, but it thus far has not been recognized that the activation energy supplied to the blank must be reduced with increasing pore depth for etching pores with constant cross sections. In contrast to the prior art, an assignable pore diameter can be kept roughly constant over the entire pore length with the invention.
If the activation energy is supplied by light exposure, the side of the blank opposite the side dipped into the etching liquid is preferably exposed. The supplied activation energy can be achieved, for example, by sequential connection and disconnection of the light source or by a gradual reduction of the light intensity, i.e., by dimming the light source. A light source with a broad frequency spectrum or a monochromatic light source can be used, whose frequency is adjusted to the blank material.
According to a modification of the invention, “start recesses” for the pores being etched are created before the electrochemical etching of the blank on its first side, the spacing and the arrangement of which correspond to the desired pore spacing of the pore distribution. The start recesses, for example, can be pre-etched using a hole mask or produced with laser beams so that a filter with a very uniform micro- or pore structure can be produced.
The dopant concentration of the blank can lie between 10
16
and 10
19
cm
−3
or higher. It is also possible to dope the blank with different types of atoms and optionally different dopant concentrations. The individual dopants have characteristic activation energies and can therefore be activated individually, i.e., when monochromatic light is used. By changing the wavelength of the light, one dopant after another will “respond.” One preferably begins with the highest doped or most highly concentrated dopant and proceeds in diminishing progress steps to the more weakly concentrated dopant in order to reduce in stages the generation of defect electrons. It can be prescribed as a support that the dopant concentration diminish in the etching direction, which is achieved by allowing the dopant to diffuse from the first side during doping.
According to a modification of the invention, it can be prescribed that specified regions of the blank be only relatively weakly doped or undoped and are therefore hardly attacked or are not attacked by the etchant at all during the etching process. Because of this, unetched “stiffening ribs” can be produced in the blank that increase the mechanical strength of the filter being produced and improve handling.
According to another modification of the invention, a magnetic field is generated during the etching process directed esse
NFT Nanofiltertechnik Gesellschaft mit beschrankter Haftung
Phasge Arun S,.
Senniger Powers Leavitt & Roedel
LandOfFree
Method for producing a filter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for producing a filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for producing a filter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3116015