Process for producing superconducting thin films

Abstract: A process for producing a superconducting thin film on a surface of a substrate of the present invention comprises the steps of: sputtering a target made of a group IIIa metal and/or an oxide thereof, a group IIa metal and/or an oxide thereof and copper and/or an oxide thereof with an ion beam or a neutral beam to allow the sputtered particles from the target to be deposited on the substrate; and impinging an oxygen neutral beam on the substrate.

Description: The present invention relates to a process for producing a superconducting thin film on a surface of a substrate, using a sputtering method. From the "Journal of Vacuum Science and Technology A, Volume 3, No. 6, Second Series, November-December 1985, New York, U.S.A., pp 2702-2703, S. Scaglione et al," a method for forming oxide films by reactive dual ion beam sputtering is known in which an ion beam from a first ion source is directed onto a target of a material to be sputtered. The deposition material selected were SiO₂, Y₂O₃, Al₂O₃, which were deposited on glass, quartz or silicon substrates. The sample deposited is directly irradiated with a oxygen ion beam from a second ion source to form an oxide thin film and to improve adhesion property, density, composition and optical properties of the thin film. In the document "Physical Review Letters, Volume 58, No. 9, 2 March 1987, U.S., New York, pp 908-910, M.K. Wu et al," a Y-Ba-Cu-O compound superconductor showing a stable and reproducable superconductivity transition between 80 and 93 K is described. Superconducting materials hold much promise for use in various cryoelectronic devices such as Josephson devices and SQUID (superconducting quantum interference device) sensors. However, conventional superconducting materials having been processed into oxide ceramics by the sintering process which is practically ineffective for the production of superconducting thin films. From JP-A-61-194786, a method as mentioned at the beginning of the description is known in which a superconductor thin film is formed by heat treatment of a polycrystalline thin-film consisting of BaPb 1-x Bi x O₃ deposited onto a substrate through a sputtering method. It is the object of the present invention to provide a new, improved method for producing a superconducting thin film. The method according to the present invention is characterized by sputtering a target made of a group IIIa metal and/or an oxide thereof, a group IIA metal and/or an oxide thereof and copper and/or an oxide thereof with an ion beam or a neutral beam to allow particles sputtered from the target to be deposited on the substrate; and impinging an oxygen neutral beam on the substrate to thereby form a thin film of a IIa-IIIa-Cu oxide. When an ion beam or a neutral beam is permitted to impinge on the target, it makes elastic collision with the structural atoms and molecules in the surface of the target. As a result, these atoms and molecules are sputtered and the sputtered particles are deposited on the substrate. Since an oxygen neutral beam is being permitted to impinge on the substrate, it reacts with the deposited particles to form a superconducting thin film made of a IIIa-IIa-Cu oxide on the surface of the substrate. Fig. 1 shows schematically the interior of a thin-film forming apparatus that may be used to implement the method of the present invention; and Fig. 2 shows a cross section of a target that may be used in the apparatus shown in Fig. 1. An embodiment of the present invention is described hereinafter with reference to the accompanying drawing. Fig. 1 shows schematically the interior of a thin-film forming apparatus that is suitable for use in implementing the method of the present invention. An inert gas such as Argon is ionized and extracted with an extracting electrode system in a sputtering ion source 1 from which the resulting ion beam (IB) is emitted. The ion beam (IB) is neutralized by means of a neutralizing filament 2 that is mode of a suitable metal such as tantalum or tungsten and which is optionally used if a target 3 to be described later in this specification is made of in insulating material. The target 3 is sputtered with the neutralized beam, which is hereinafter sometimes referred to as a neutral beam (NB). If the target 3 is made of an insulating material, static charges will build up as a result of impingement of an ion beam and if they exceed a certain limit, insulation breakdown occurs and the target will be disrupted. Neutralization of the ion beam (IB) is effective for the purpose of preventing such troubles and is not particularly needed if the target 3 is made of a metal. The target 3 may be a solid target that is composed of a group IIIa metal and/or an oxide thereof, a group IIa metal and/or an oxide thereof and copper and/or an oxide thereof, which is prepared by mixing the powders of these components at desired molar ratios and calcining the mix at 1,000 - 1,200 °C. Alternatively, a composite target may be prepared by the following procedures: the powders of a group IIIa metal and/or a oxide thereof and a group IIa metal and/or a oxide thereof are mixed at desired molar ratios; the mix is calcined at elevated temperatures as in the preparation of the solid target; rectangular chips are formed from the calcined product; and a plurality of those chips are disposed on a plate made of copper and/or an oxide thereof in such a way that the area covered by the chips assumes a predetermined proportion of the copper end/or copper oxide plate. Fig. 2 shows another modification of the composite target 3, in which a plate 31 made of copper and/or an oxide thereof is overlaid with chips 32 made of a group IIIa metal and/or an oxide thereof and chips 33 made of a group IIa metal and/or an oxide thereof, thereby forming a composite target in which a predetermined proportion of the surface area of the plate is assumed by the chips 32 and 33. The apparatus shown in Fig. 1 also includes substrates 4 typically made of zirconia or sapphire, on the surface of which sputtered particles S emitted from the target 3 sputtered with the ion beam (IB) or neutral beam (NB) are to be deposited. The surface of the substarets 4 also provides an area where an oxygen neutral bean (OB) to be described later in this specification will impinge. An oxygen ion beam is extrated from an extracting electrode system in an oxygen ion source 5. This beam is neutralized to an oxygen neutral beam (OB) with electrons emitted from a neutralizing filament 6 typically made of tantalum or tungsten. The substrates 4 are supported by a substrate holder 7 which optionally has a built-in heater for boating the substrates 4. The apparatus shown in Fig. 1 has a gas oulet 8 through which vacuum is to be drawn from a film-forming compartment 9 in the apparatus. The ion beam (IB) with an energy of approximately 500 - 2,000 eV that has been extracted from the ion souce 1 enters the film-forming compartment 9 where it is converted to a neutral beam (NB) by neutralization with electrons from the filament 2, the neutral beam then sputtering the target 3 made of a group IIIa metal and/or an oxide thereof, a group IIa metal and/or an oxide thereof, and copper and/or an oxide thereof. The sputtered particles S emitted from the target 3 are deposited on the substrates 4. Simultaneously or alternately with the emission of the ion beam (IB), an oxygen ion beam having an energy of approximately 50 - 500 eV is extracted, optionally intermittently, from the oxygen ion source 5 and enters the film-forming compartment 9, where it is converted to an oxygen neutral beam (OB) by neutralization with electrons from the filament 6, the neutral beam then impinging on the substrates 4. As a result, the sputtered particles S react with the oxygen neutral beam (OB) to form a superconducting thin film on the surface of the substrate which is made of an oxide of the IIIa-IIa-Cu system. The following examples were conducted by the present inventors for the purpose of further illustrating the present invention but are in no way to be taken as limitting. EXAMPLE 1 A solid target 3 with a diameter of 75 mmΦ was prepared by mixing Y₂O₃, BaO and CuO powders at a molar ratio of 3:6:4 and calcining the mix in an electric furnace at 1,000 °C for 2 hours. Substrates 4 made of square sapphire (18 x 18 mm) were attached to a substrate holder 7 and heated at 700 °C while the holder was rotating. Using the target 3 and the substrates 4, a film-forming experiment was conducted under the following conditions: Ar⁺ ion beam as sputtering ion source 1; 1 keV x 50 mA as beam energy of the Ar⁺ ion beam; neutralizing filament 2 supplied with 5V voltage and 5A current; O⁺ ion beam as oxygen ion source 5; 100 eV x 1 mA as beam energy of the O⁺ ion beam; and neutralizing filament 6 supplied with 3V voltage and 3A current. As a result, a superconducting thin film of 1 µm thick that was made of Y-Ba-Cu oxide was formed on the surface of the sapphire substrates 4. Gold electrodes were formed on this super conducting film by sputtering and the electrical resistance across this film was measured in liquid nitrogen. Zero resistance was confirmed. EXAMPLE 2 A composite target 3 was prepared by the following method: Y₂O₃ and BaO powders were mixed at a molar ratio of 1:2 and calcined in an electric furnace at 1,000 °C for 2 hours; the calcined product was cut into rectangular chips (4 x 6 mm) which were placed on a copper plate so as to cover 60 % of its area. Substrates 4 made of circular zirconia (20 mm Φ) were attached to a substrate holder 7 and heated at 700 °C while the holder was rotating. Using the traget 3 and the substrates 4, a film-forming experiment was conducted under the following conditions: Ar⁺ ion beam as sputtering ion source 1; 1 keV x 50 mA as beam energy of the Ar⁺ ion beam; neutralizing filament 2 supplied with 5V voltage and 5A current; O⁺ ion beam as oxygen ion source 5; 100 eV x 1 mA as beam energy of the O⁺ ion beam; and neutralizing filament 6 supplied with 3V voltage and 3A current. As a result, a superconducting thin film of 1 µm thick that was made of a Y-Ba-Cu oxide was formed on the surface of the zirconia substrates 4. Gold electrodes were formed on this superconducting film by sputtering and the electrical resistance across this film was measured in liquid nitrogen. Zero resistance was confirmed. In the EXAMPLEs 1 and 2, the superconducting thin films were formed by depositing sputtered particles on the heated substrates 4. Needless to say, films may be formed without heating the substrates, with the formed films being optionally subjected to annealing. Besides Ar+, the ion beams may consist of other tonic species such as Kr and Xe. It should also be stressed that the neutralizing filaments 2 and 6 may be designed as a common filament that neutralizes either the sputtering ion beam or oxygen ion beam as required. As will be apparent from the foregoing description, the process of the present invention has the advantege that it enables the formation of a superconducting thin film from a IIIa-IIa-Cu oxide. The present invention will not be limited to the embodiment and the examples described above. For instance, the substrate 4 may be in wire form which can be provided with a coating of IIIa-IIa-Cu oxide by the method of the present invention. The resulting product can be wound into a coil form which is suitable for use as a superconducting coil.

Claims: A process of producing a superconducting thin film on a surface of a substrate, using a sputtering method, characterized by sputtering a target made of a group IIIa metal and/or an oxide thereof, a group IIa metal and/or an oxide thereof, and copper and/or an oxide thereof with an ion beam or a neutral beam to allow particules sputtered from the target to be deposited on the substrate (4); and impinging an oxygen neutral beam on the substrate, to thereby form a thin film of a IIa-IIIa copper oxide. The process of Claim 1, wherein said step of sputtering a target with ion beam or a neutral beam and said step of impinging an oxygen neutral beam on the substrate are conducted simultaneously with each other. The process of Claim 1, wherein said step of sputtering a target with ion beam or a neutral beam and said step of impinging an oxygen neutral beam on the substrate are conducted alternately with each other. The process of Claim 1, further comprising the step of heating the substrate while the sputtered particles from the target are deposited on the substrate. The process of Claim 1, wherein prior to sputtering the target, an ion beam is neutralized and prior to impinging on the substrate, an oxygen ion beam is neutralized to provide the neutral beam and the oxygen neutral beam. The process of Claim 1, further comprising the step of forming the target, said step having the steps of: mixing at desired molar ratios powders of a group IIIa metal and/or an oxide thereof, a group IIa metal and/or an oxide thereof and copper and/or an oxide thereof; and calcining the mix at a high temperature to thereby form a solid target. The process of Claim 1, further comprising the step of forming the target, said step having the steps of: mixing at desired molar ratios powders of a group IIIa metal and/or an oxide thereof and a group IIa metal and/or an oxide thereof; calcining the mix at high temperature; forming rectangular chip from the calcined product; and disposing the chip on a plate made of copper and/or an oxide thereof so that the area covered by the chip assumes a predetermined proportion of the copper and/or copper oxide plate to thereby form a composite target. The process of Claim 1, further comprising the step of forming the target, said step having the steps of: preparing a chip from powders of a group IIIa metal and/or an oxide thereof; preparing a chip from powders of a group IIa metal and/or an oxide thereof; and disposing the chips on a plate made of copper and/or an oxide thereof so that the area covered by the chips assumes a predetermined proportion of the copper and/or copper oxide plate to thereby form a composite target.

Inventors: KAMIJO EIJI C O NISSIN ELECTRI, NOGAWA SHUICHI C O NISSIN ELEC, KAMIJO, EIJI C/O NISSIN ELECTRIC CO., LTD., NOGAWA, SHUICHI C/O NISSIN ELECTRIC CO., LTD.
Applicants: NISSIN ELECTRIC CO LTD, NISSIN ELECTRIC COMPANY, LIMITED
Application year: 1988
Main classification: H01L39
IPC codes: H01B13, C23C14, H01L39, H01B12, C30B29