Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
1999-11-09
2001-04-03
Beck, Shrive (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S065000, C427S126100, C427S250000, C427S255230, C427S255280, C427S444000
Reexamination Certificate
active
06210755
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and evaporation chamber for generating a continuous vapor stream containing a compound in which gallium is present in monovalent form in a vacuum coating process for vacuum coating a substrate. The invention also relates to a vacuum coating apparatus operating according to such a method and having such an evacuation chamber.
2. Description of the Prior Art
Monovalent gallium (Ga
1+
) is frequently used as a doping substance in X-ray absorber materials such as CsBr or RbBr for storage phosphors. Substrates coated therewith are needed as radiation detectors for radiographic applications, for example. The monovalent gallium is applied as GaBr, but GaBr cannot be chemically isolated. To achieve a doping despite this fact, it is suggested in U.S. Pat. No. 5,736,069 to use GaCl as the starting material, which is mixed with metallic Ga and with the X-ray absorber material CsBr. This mix is dried at 400° C., after which a monocrystal of the phosphor is grown at high temperature (e.g. 925° C.). The powder-type phosphor is obtained by milling the monocrystal, and is dispersed in a solvent (e.g. ethyl acetate) in a solution of a binder (e.g. polyethyl acrylate), and is layered onto the carrier film, for instance by pouring, rolling or sedimenting. High-quality layers cannot be produced in this way. Good X-ray absorber layers are usually produced in the context of a vacuum evaporating process; that is, the X-ray absorber material is first evaporated in an evaporating device and then settles on the carrier. The high vapor pressure of the required starting materials, namely the bivalent or trivalent gallium-halogenide compound and the gallium metal, acts to inhibit deposition of the doping substance containing monovalent gallium, which can be GaBr or GaI, by means of a vacuum coating method, since these would evaporate abruptly in the vacuum at common evaporating temperatures in the range of a few hundred degrees upon the formation of the dopant, even before the reaction of the starting materials has begun. As a result, it has not been possible to perform doping with monovalent gallium in the context of a vacuum coating, and particularly to simultaneously generate the X-ray absorber layer and to introduce the doping in the context of an evaporation process.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method that enables the generation of a continuous vapor stream containing a monovalent gallium doping substance even in a vacuum, despite the above described problems, so that a vacuum doping is possible.
This object is achieved in accordance with the invention in a method for generating a continuous vapor stream containing a compound in which gallium is present in monovalent form in the context of a vacuum coating method for vacuum coating a substrate, wherein a multivalent gallium-containing evaporation substance (i.e., an evaporation substance containing gallium in bivalent or trivalent form) is provided together with metallic gallium in an evaporating vessel or chamber that is closed on all sides and that has a vapor stream exit opening. The evaporation substance is evaporated in this evaporating chamber, and the vapor is brought into contact with the metallic gallium, causing the bivalent or trivalent gallium to be reduced to monovalent gallium, which subsequently exits in the direction of the substrate via the vapor stream exit opening.
In the inventive method, the reaction components—that is, the evaporation substance consisting of a gallium compound, particularly a gallium halogenide in which the gallium is present in bivalent or trivalent form, as well as the metallic gallium—are arranged in a closed evaporation chamber, where the evaporation of the evaporation substance and the actual reaction occur. At the chamber, only a very small vapor stream exit opening is provided, via which the vapor stream containing the doping substance (for instance the GaBr) can exit. Since the reaction substances are present in a closed chamber containing only a small opening, an abrupt evaporation is prevented, despite the temperature, since the pressure relation prevailing in the interior prevents a complete evaporation. The reaction of the vaporous evaporation substance and the metallic gallium which takes place in the closed reaction space can be sufficient to enable the generation of a continuous vapor stream containing the doping substance, which exits into the vacuum chamber and in the direction of the substrate via the exit window. The inventive method thus makes it possible to continuously generate the dopant vapor stream inside a vacuum coating apparatus in which there is a vacuum, in the context of a vacuum coating process.
The evaporation substance can be inventively arranged in the evaporation chamber on a first level, and the metallic gallium can be arranged on a second level situated over the first level, so that the evaporation substance is evaporated separately from the metallic gallium but comes into contact with it and reacts with it inside the chamber. Alternatively, the evaporation substance can be inventively mixed with the metallic gallium, so that the evaporation substance is evaporated in the metallic gallium, and an immediate reaction occurs. The reaction temperature for the reduction lies between 250° C. and 950° C., particularly between 300° C. and 900° C. The higher the temperature, the more complete the reaction; the reaction temperature at which the vaporous evaporation substance and the metallic gallium react should preferably be about 500° C. In the embodiment wherein the evaporation of the evaporation substance occurs separately from the metallic gallium, in order to prevent the substance from evaporating too rapidly, a temperature gradient from 300° C. to 500° C., particularly from 350° C. to 450° C., is inventively produced between the metallic gallium and the evaporation substance, for which purpose the evaporation substance can be inventively cooled and/or the metallic gallium can be heated. In this way it is also possible to cool the evaporation substance for lowering the vapor pressure, while allowing adjustment of the temperature in the range in which the reaction takes place.
In a further embodiment of the invention, the evaporation chamber is cooled in the region of the vapor exit opening, which likewise results in a local lowering of the pressure at that location. As a result, the expansion of particulate vapor which takes place as the vapor exits through the vapor exit opening is “weaker” in the transition into the vacuum; so that fewer particle collisions arise in this expansion; thereby allowing the vapor particles to travel more linearly in the direction of the substrate. A drifting apart and “smearing” of the vapor stream thus can be counteracted and a relatively sharper (more confined) vapor stream can be generated in the direction of the substrate. To both enable both a build-up of the required pressure relations and generation of a sufficiently sharp vapor stream jet, it is appropriate to use an evaporation chamber with a vapor stream exit opening having a diameter between 2 &mgr;m and 2 mm, particularly between 5 &mgr;m and 1.5 mm, preferably between 10 &mgr;m and 2 mm. Several such small vapor exit openings can be provided. The evaporation chamber should consist of a material that does not react with the evaporation substance and/or with the metallic gallium or which cannot be wetted by these materials, such as graphite, aluminum oxide or boric nitride. In a further embodiment of the invention, the evaporation substance can be evaporated before, during or after the evaporation of a coating material with which the substrate is being coated, particularly an X-ray absorber material. That is, the X-ray absorber material can be evaporated first followed by evaporation of the doping substance in a common evaporation apparatus. A reverse sequence of evaporation is also possible, and a simultaneous evaporation along with a coat
Fuchs Manfred
Hell Erich
Mattern Detlef
Barr Michael
Beck Shrive
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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