Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Coating
Reexamination Certificate
1995-03-28
2001-01-09
King, Roy V. (Department: 1762)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Coating
C505S480000, C505S731000, C427S062000, C117S108000
Reexamination Certificate
active
06172008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing oxide thin films having excellent flatness and high crystallinity, and more specifically to a reactive co-evaporation process particularly for preparing thin films of oxide superconductor materials and oxide insulator or dielectric materials, which have clean and smooth surfaces, high crystallinity and excellent superconducting or dielectric properties without any heat treatment after deposition.
2. Description of Related Art
Oxide superconductors have been found to have higher critical temperatures than those of metal superconductors, and therefore considered to have good possibility of practical application. 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 100K.
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 having a thickness of a few nanometers to some hundreds micrometers. It is considered to be preferable to utilize various deposition methods, such as sputtering methods, laser ablation methods, MBE (Molecular Beam Epitaxy) methods and reactive co-evaporation methods for forming oxide superconductor thin films. In particular, it is possible to form an oxide superconductor thin film by depositing atomic layers layer by layer through utilizing a MBE method and a reactive co-evaporation method. Additionally, in-situ observation during and between depositing thin film is possible so that a high quality oxide superconductor thin film can be obtained by the MBE method and reactive co-evaporation method.
Insulator thin films are also necessary to fabricate superconducting devices and superconducting integrated circuits. Oxide dielectrics such as SrTiO
3
, MgO, etc. are preferably used for insulator thin films combined with the oxide superconductor. In particular, SrTiO
3
has a layered crystal structure similar to that of the oxide superconductor so that it is possible to accurately control qualities and thickness of its thin films by depositing atomic layers layer by layer through utilizing a MBE method and a reactive co-evaporation method.
In order to deposit an oxide superconductor thin film and an oxide dielectric thin film on a substrate by the MBE method and the reactive co-evaporation method, constituent elements of the oxide excluding oxygen are supplied as molecular beams towards the substrate by using Knudsen's cell (abbreviated to K cell hereinafter) type molecular beam sources. In addition, an oxidizing gas such as O
2
including O
3
, NO
2
or N
2
O is supplied near the substrate so that the molecular beams are oxidized so as to form the oxide thin film on the substrate.
In general, when a thin film is deposited by the MBE method and the reactive co-evaporation method, a pressure of deposition atmosphere is reduced as low as possible so as to prevent contamination in the process. Namely, vacuum level of the deposition atmosphere is increased as high as possible.
However, in case of an oxide thin film, a above distinctive process in which an oxidizing gas is supplied near the substrate during deposition of the oxide thin film is employed. It is also preferable, even in this case, to reduce the pressure in the vicinity of the substrate as low as possible so as to prevent contamination of impurities into the oxide thin film.
For this purpose, in a prior art, the pressure in the vicinity of the substrate has been adjusted to 1×10
−5
Torr during the deposition. However, it may be sometimes difficult to cause sufficient oxidation near a surface of the substrate.
In order to prevent diffusion of constituent elements of the substrate or a lower layer into a growing thin film, it is preferable to reduce a substrate temperature during deposition of the thin film as low as possible. However, oxidation does not sufficiently progress at the low substrate temperature. In addition, enough migration of atoms deposited on the substrate does not occur at the low substrate temperature so that a surface of the thin film becomes uneven.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a process for preparing an oxide thin film having clean and smooth surfaces with high crystallinity and excellent properties at a low substrate temperature under a low pressure (high vacuum level), which has overcome the above mentioned defects of the conventional ones.
Another object of the present invention is to provide a process for preparing an oxide dielectric thin film, 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 film formed of an oxide material on a substrate by using an apparatus comprising a vacuum chamber in which an oxidizing gas of O
2
including O
3
can be supplied near the substrate so that pressure around the substrate can be increased while maintaining high vacuum around an evaporation source and Knudsen cell evaporation sources arranged in the vacuum chamber wherein the substrate is heated, molecular beam of constituent atoms of the oxide excluding oxygen are supplied from the K cell evaporation sources, an oxidizing gas is locally supplied to the vicinity of the substrate and a growing thin film is illuminated by ultraviolet.
According to the present invention, a growing thin film is illuminated by ultraviolet which promotes reactions near a surface of the growing thin film and migration of deposited atoms. Therefore, a high quality oxide film of high crystallinity, having a smooth surface and excellent properties can be obtained even at a low substrate temperature and under a low pressure in the vicinity of the substrate. For example, a high quality c-axis orientated Y
1
Ba
2
Cu
3
O
7−x
oxide superconductor thin film can be deposited even at a substrate temperature of 630 to 670° C. and under a pressure of 5×10
−7
to 5×10
−6
Torr in the vicinity of the substrate. In a prior art, a c-axis orientated Y
1
Ba
2
Cu
3
O
7−x
oxide superconductor thin film has been deposited at a substrate temperature of not lower than 700° C. and under a pressure of not lower than 1×10
−5
Torr in the vicinity of the substrate.
In case of a SrTiO
3
oxide dielectric film, it can be deposited even at a substrate temperature of 330 to 500° C. and under a pressure of 5×10
−7
to 5×10
−6
Torr in the vicinity of the substrate. In a prior art, a SrTiO
3
oxide dielectric film has been deposited at a substrate temperature of not lower than 500° C. and under a pressure of not lower than 1×10
−5
Torr in the vicinity of the substrate.
The lower pressure can reduce contaminants, in particular hydrocarbonates or metal carbides, deposited on or within the oxide thin film prepared by the process in accordance with the present invention.
In accordance with the present invention, the ultraviolet preferably has a wavelength of 150 to 300 nanometers. These wavelength are suitable to promote reactions near a surface of a growing thin film and migration of deposited atoms.
In addition, the substrate can be formed of an insulating substrate, preferably an oxide single crystal substrate such as MgO, SrTiO
3
, CdNdAlO
4
, etc. These substrate materials are very effective in forming or growing a crystalline film having a high degree of crystallinity.
However, it is possible to deposit an oxide thin film on an oxide superconductor layer with little inter diffusion between them, in accordance with the present invention. For example, a SrTiO
3
thin film can be deposited on a Y
1
Ba
2
Cu
3
O
7−x
oxide superconductor layer so as to prevent inter diffusion b
Kerins John C.
King Roy V.
Miles & Stockbridge P.C.
Sumitomo Electric Industries Ltd.
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