Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Coating
Reexamination Certificate
1994-10-14
2001-02-27
King, Roy V. (Department: 1762)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Coating
C505S732000, C505S729000, C117S108000, C427S062000
Reexamination Certificate
active
06194353
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing superconducting thin film, and more specifically to an improved MBE (Molecular Beam Epitaxy) process particularly for preparing superconducting thin film formed of an oxide material, which has a clean surface, high crystallinity and excellent superconducting properties.
2. Description of Related Art
Oxide superconductors have been considered to have higher critical temperatures than those of metal superconductors, and therefore to have high possibility of practical use. For example, Y—Ba—Cu—O type oxide superconductor has a critical temperature higher than 80 K and it is reported that Bi—Sr—Ca—Cu—O type oxide superconductor and Tl—Ba—Ca—Cu—O type oxide superconductor have critical temperatures higher than 100 K. The oxide superconductor has a crystalline anisotropy in superconducting characteristics, and in particular, its critical current density is maximum in a direction perpendicular to a c-axis of crystal lattice. Therefore, when the oxide superconductor is used, attention should be paid to the crystalline orientation.
In case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film. When the oxide superconductor is in the form of a thin film, the above mentioned crystalline anisotropy of superconducting characteristics becomes more remarkable. In addition, to realize high performance superconducting devices and superconducting integrated circuits, two kind of superconducting current paths through one of which superconducting current flows parallel to the substrate and through the other of which superconducting current flows perpendicular to the substrate are necessary. For example, superconducting current flows through a superconducting electrode in the direction parallel to the substrate, flows through an interlayer superconducting current path in the direction perpendicular to the substrate. Therefore, in case of applying the oxide superconductor to superconducting devices and superconducting integrated circuits, a c-axis oriented oxide superconductor thin film of which critical current density is maximum in the direction parallel to the substrate and an a-axis oriented (or b-axis orientated, abbreviated “a-axis oriented” hereinafter) oxide superconductor thin film of which critical current density is maximum in the direction perpendicular to the substrate should be prepared.
The orientation of the oxide superconductor thin film is determined by the film deposition temperature. The film deposition temperature generally means the temperature of the substrate while the oxide superconductor thin film is deposited. An a-axis oriented oxide superconductor thin film can be deposited at the substrate temperature on the order of 50 to 100° C. lower than that of c-axis oriented oxide superconductor thin film.
In order to manufacture superconducting devices and superconducting integrated circuit of multi-layer structure, it is necessary that an a-axis oriented oxide superconductor thin film is stacked on a c-axis oriented oxide superconductor thin film. When the oxide superconductor thin films are stacked, a clean upper surface and high crystallinity are required of the lower oxide superconductor thin film. If the upper surface of the lower oxide superconductor thin film is not clean and contaminants are deposited or oxide is formed on it, the stacked thin films may become physically and electrically discontinuous or an unnecessary weak link of Josephson junction may be formed at the interface. The discontinuity and weak link have adverse effect on the operations of the superconducting device and the superconducting integrated circuit so that their performance become below the determined values or do not operate at all.
The coherence length of the oxide superconductor is very short, therefore, special attention should be given to the contamination of the upper surface of the lower thin film.
In order to clean the surface of the oxide superconductor thin film, in a prior art, the oxide superconductor thin film is heated under a high vacuum so as to vaporize the contaminants. However, oxygen of the oxide superconductor crystalline is unstable and easily escapes when the oxide superconductor thin film is heated under a high vacuum. If some amount of oxygen escapes from the oxide superconductor crystalline, the critical temperature of the oxide superconductor lowers or the oxide superconductor loses its superconductivity.
Therefore, the oxide superconductor thin film which is used a lower layer of a stacked multi-layer structure is required to have a clean outermost surface, high crystallinity and excellent superconducting characteristics.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a process for preparing an oxide superconductor thin film which has a clean upper surface, high crystallinity and excellent superconducting characteristics, which has overcome the above mentioned defects of the conventional ones.
The above and other objects of the present invention are achieved in accordance with the present invention by a process for preparing a thin film formed of an oxide superconductor on a substrate by MBE characterized in that said MBE is effected under a condition that the substrate is heated and an oxidizing gas is locally supplied to the proximity of the substrate so that the pressure of the proximity of the substrate becomes 6×10
−6
to 8×10
−5
Torr at a background pressure.
In the process in accordance with the present invention, the MBE is preferably effected at a background pressure lower than 1×10
−9
Torr. This quite low background pressure means that there is no leak nor any gas generation in the vacuum chamber of the MBE apparatus. Therefore, no contaminant, for example hydrocarbons and metal carbides, is deposited on the oxide superconductor prepared by the process in accordance with the present invention.
In a preferred embodiment, the oxidizing gas is selected from the group consisting of O
2
including 5 to 70 volume percent O
3
, N
2
O and NO
2
. These gases have higher oxidizing than pure O
2
and it is favorable for preparing an oxide superconductor thin film by the MBE.
According to one preferable embodiment of the present invention, the substrate is cooled down to the room temperature after the oxide superconductor thin film was deposited, in the atmosphere in which the oxide superconductor thin film was deposited. While the substrate is cooled down, the oxide superconductor crystal system of the thin film is transformed from a tetragonal system to an orthorhombic system. At this stage, the oxide superconductor thin film should be in the oxidizing atmosphere, in order to get excellent superconducting characteristics.
In one preferred embodiment, the MBE is effected while the oxidizing gas is struck onto a deposition surface of the substrate. By this, the pressure of the oxidizing gas on the deposition surface of the substrate was further elevated.
In a preferred embodiment, the oxide superconductor is formed of a high-T
c
(high critical temperature) oxide superconductor, particularly, formed of a high-T
c
copper-oxide type compound oxide superconductor for example a Y—Ba—Cu—O type compound oxide superconductor material, a Bi—Sr—Ca—Cu—O type compound oxide superconductor material, and a Tl—Ba—Ca—Cu—O type compound oxide superconductor material.
In case of Y
1
Ba
2
Cu
3
O
7-&dgr;
oxide superconductor, the MBE is preferably effected at a substrate temperature of 650 to 730° C. by using metal yttrium, metal barium and metal copper as evaporation sources. It is more preferable that the MBE is effected at a substrate temperature of 700° C. If the substrate temperature is lower than 650° C., the obtained Y
1
Ba
2
Cu
3
O
7-&dgr;
oxide superconductor thin film constituted of polycrystals which includes c-axis orientated crysta
Kerins John C.
King Roy V.
Miles & Stockbridge P.C.
Sumitomo Electric Industries Ltd.
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