Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Producing halogen – containing superconductor
Reexamination Certificate
1999-12-29
2002-07-30
Kunemund, Robert (Department: 1765)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Producing halogen , containing superconductor
C505S123000, C505S729000, C505S731000, C117S089000, C117S090000, C117S091000, C117S102000, C117S105000
Reexamination Certificate
active
06426320
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to methods of making superconductors having epitaxial layers.
Superconductors are used in a variety of applications. Often, the mechanical integrity of a superconductor can be enhanced by forming a multilayer article that includes a layer of superconductor material and a substrate layer, but the use of a substrate can present certain complications.
Chemical species within the substrate may be able to diffuse into the layer of superconductor material, potentially resulting in a loss of superconductivity. Moreover, the coefficients of thermal expansion as well as the crystallographic spacing and orientation of the substrate and the superconductor layer can be different, causing the article to peel apart during use.
To minimize these complications, a buffer layer can be disposed between the substrate and the superconductor layer. The buffer layer should reduce diffusion of chemical species between the substrate and the superconductor layer, and the buffer layer should have a thermal coefficient of expansion that is about the same as both the substrate and the superconductor layer. In addition, the buffer layer should provide a good crystallographic match between the substrate and the superconductor.
One approach to controlling the crystallographic properties of a layer has been to use epitaxy. An epitaxial layer is a layer of material that is grown on the surface of a substrate such that the crystallographic orientation of the layer of material is determined by the lattice structure of the substrate. By epitaxy is also meant materials with ordered surfaces whether formed by conventional epitaxy or graphoepitaxy. Epitaxial layers have been grown using physical vapor deposition (PVD), chemical vapor deposition (CVD) and sputtering techniques.
Typically, PVD involves the evaporation of a solid material and transfer of the vapor to the substrate surface in a diffuse gas beam in which only a small portion of the total amount of evaporated solid may reach the substrate surface. Thus, the material usage efficiencies obtained with PVD can be low. In addition, PVD is usually performed at a chamber pressure of at most about 1×10
−4
torr, so the flux of evaporated solid at the substrate surface can be small, resulting in low epitaxial layer growth rates.
In CVD, one or more reactant gases within the chamber adsorb to the substrate surface and react to form the epitaxial layer with product gases desorbing from the substrate surface. Generally, the reactant gases reach the substrate surface by convection and diffusion, so the *material usage efficiencies can be low. Furthermore, CVD is typically conducted at a chamber pressure of at least about 0.1 torr, and, to grow epitaxial layers at these elevated pressures, relatively high substrate temperatures are usually used. Thus, the selection of substrate materials used in CVD can be limited.
Sputtering methods of growing epitaxial layers can also be limited by the aforementioned considerations.
When using PVD, CVD or a sputtering technique, the quality of the epitaxial layer can depend upon the chemical nature of the substrate surface during layer growth. For example, contaminants present at the substrate surface can interfere with epitaxial layer growth. In addition, native oxides present at the substrate surface can help or hinder epitaxial layer growth. Further, PVD, CVD and sputtering methods can be ineffective at providing control of the chemical nature of the substrate surface during layer growth, so the epitaxial layers formed by these techniques can be of poor quality.
SUMMARY OF THE INVENTION
The invention features a low vacuum vapor deposition process for producing a superconductor article with an epitaxial layer. The epitaxial layer is disposed on a substrate that can be of non-identical composition.
In one aspect the invention features a method that includes the steps of placing a textured or crystallographically oriented target surface of a substrate typically including contaminant materials, in a low vacuum environment, and heating the target surface (substrate surface) to a temperature which is greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment under otherwise comparable conditions. A layer-forming stream, including an inert carrier gas and a dispersion of a first species (layer forming gas), which is a chemical component of the desired epitaxial layer, is directed at a positive velocity greater than about 1 m/sec toward the substrate surface through the low vacuum environment. A second species (conditioning gas) is provided in the low vacuum environment, directed toward the substrate surface at a velocity substantially similar to the velocity of the layer-forming stream, and reacted with one or more of the species present in the substrate surface, for example, a contaminant material. This reaction conditions the substrate surface and promotes nucleation of the epitaxial layer. A desired material chemically comprising the first layer forming gas is deposited from the stream onto the substrate surface to form an epitaxial layer. A layer of an oxide superconductor is then deposited on the epitaxial layer.
As used herein, “layer forming gas” refers to a gas that can adsorb to a surface and become a component of an epitaxial layer. A layer forming gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like.
As used herein, “conditioning gas” refers to a gas that can interact with a surface to remove surface contaminants, remove undesired native oxides present at the substrate surface or form desired native oxides or other components at the substrate surface. A conditioning gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like.
In another aspect, the invention features a method of making a superconductor. The method includes growing, at a chamber pressure of at least about 1×10
−3
torr, an epitaxial buffer layer on a substrate having a temperature that is about the same as a PVD epitaxial growth threshold temperature for a chamber pressure of at most about 1×10
−4
torr. The method also includes depositing a superconductor material or a precursor of a superconductor material on the epitaxial buffer layer.
A “precursor of a superconductor material” as used herein refers to a material which can undergo subsequent treatment to become a superconductor. For example, subsequent to heating a particular temperature, a precursor of a superconductor material can become a superconductor material.
For a given epitaxial layer, the “PVD epitaxial growth threshold temperature for a chamber pressure of at most about 1×10
−4
torr” refers to the minimum substrate temperature that can be used to grow the epitaxial layer at a chamber pressure of at most about 1×10
−4
torr, typically at most about 1×10
−5
torr, using a PVD with a diffuse gas beam.
As used herein, a “diffuse gas beam” refers to a gas beam in which less than about 50% of any layer forming gas in the gas beam is incident at the substrate surface.
In still another aspect, the invention features a method of making a superconductor. The method includes growing an epitaxial buffer layer on a substrate surface at a rate of at least about 50 Angstroms per second in a vacuum chamber having a pressure of at least about 1×10
−3
torr. The method also includes depositing a layer of superconductor material or a precursor of a superconductor material on the epitaxial buffer layer.
In a further aspect, the invention features a method of making a superconductor. The method includes growing an epitaxial buffer layer on a substrate surface by exposing the substrate surface to a gas beam having a layer forming gas, wherein at least about 75% of the layer forming gas in the gas beam is incident at the substrate surface. The method also includes depositing a layer of superco
Buczek David M.
Fritzemeier Leslie G.
American Superconductors Corporation
Fish & Richardson P.C.
Kunemund Robert
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