A device for forming a compound oxide superconducting thin film

Abstract: There is disclosed a device for forming a thin film on a substrate (1) by irradiating a target (3) of a compound oxide superconducting material with a laser beam and evaporating on the substrate (1) a thin film corresponding to a composition of the target (3) in an oxygen ambient atmosphere by laser evaporation, a scanning optical system (10, 12) for causing the laser beam to scan being disposed in an optical path of the laser beam.

Description: Background of the Invention (Field of the Invention) This invention relates to a device for forming a compound oxide superconducting thin film, specifically to a device for forming a compound oxide superconducting thin film by laser evaporation. (Prior Art) Superconductivity phenomenon, which is said to be a phase transition of electrons, has been recognized as a phenomenon observed only at very low temperatures whose realization requires liquid helium. But in 1986 Bednorz, Muller et al. discovered (La,Ba) 2 CuO 4 , which exhibits superconductivity at 30 K. In 1987 Chu et al. discovered YBa 2 Cu 3 O y having a superconductivity critical temperature Tc in the order of 90 K was discovered. In 1988 Maeda et al. discovered the so-called Bi-system compound oxide superconducting material having a critical temperature higher than 100 K. These compound oxide superconducting materials can exhibit superconductivity phenomenon even by cooling by inexpensive liquid nitrogen, and the potential of the practical applicability of superconduction technology has been noted. Compound oxide superconducting materials having the above-described high critical temperatures were originally prepared in sintered bodies by powder metallurgy. But the sintered bodies could not exhibit preferable characteristics, especially critical current density, etc. Recently processes for forming superconducting materials in thin films have been dominantly studied. Usually compound oxide superconducting thin films are formed on SrTiO 3 mono crystal substrates, MgO mono crystal substrates, etc. by various kinds of evaporation, such as vacuum evaporation, sputtering, etc. The compound oxide thin film formed by various kinds of conventionally known evaporation do not generally exhibit sufficient superconductivity as formed. To make them useful superconducting thin films, it is necessary to post-anneal the thus-formed compound oxide thin films. That is, it is generally known that compound oxide superconducting materials exhibit high oxygen non-stoichiometry, and also that the lower non-stoichiometry they have, the higher superconductivity they exhibit. In the conventional superconducting thin film, the post-annealing is performed for the purpose of supplementing oxygen due to the oxygen non-stoichiometry. But it is known that when a thin film formed on a substrate is annealed, the substrate material is diffused in the thin film, and in the region near the substrate, the quality of the superconducting thin film is much degraded. The thus-treated thin film is usable only at the portion near the surface for experimental purposes, but is not usable for practical purposes, such as production of various devices, etc. Under these circumstances, laser evaporation, which has not been so far noted, is suddenly noted. Laser evaporation comprises irradiating a target with a high power laser beam, evaporating material of the target, and depositing the same on a substrate. The heating of the target, and that of the substrate can be controlled independently of each other. It is not always necessary to make the ambient atmosphere highly vacuum, It is possible to increase the film forming speed by selecting suitable conditions. In addition, when a compound target is used, little composition deflection takes places between a composition of the target and that of the thin film. Thus, laser evaporation is considered suitable to form compound oxide superconducting materials in thin films. It has been confirmed that the compound oxide superconducting films formed by laser evaporation without the above-described post-annealing exhibit useful superconductive characteristics. Laser evaporation is expected to be dominant in processes for forming oxide superconducting films. In the above-described laser evaporation (see Nuclear Instruments and methods in Physics Research B, 39 , 640-643 (1989)), when a laser beam irradiates a target, a blaze-like thing which is called plume is generated. This is a collectivity of active materials generated from the surface of the target by the irradiation of the laser beam and it is deposited on a substrate, so that a thin film is formed. But a sectional area to be contributed to the film formation by the plume is generally smaller than an area of a substrate. A thin film is formed by laser evaporation only on a part of the film forming surface of the substrate. On a usually used substrate having a size of about 20 mm x 20 mm, a thin film which can exhibit effective superconductive characteristics is formed in about 10 mm-diameter at most, and such small thin film is not practical. Summary of the Invention This invention relates to a device for forming a thin film by laser evaporation. The object of this invention is to provide a device for forming a thin film, especially a thin film of a compound oxide superconducting material, on a substrate having a larger film forming surface. This object is met by providing a device for forming a thin film on a substrate by irradiating a target with a laser beam and depositing on the substrate a thin film corresponding to the composition of the target. The device includes a scanning optical system for causing the laser beam to scan linearly a target disposed in an optical path of the laser beam and means for moving the substrate perpendicular to the scanning direction The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the claims will become apparent to those skilled in the art from this detailed description. Brief Description of the Drawings FIG. 1 is a pictorial view of an arrangement of the device for forming a superconducting thin film according to a first embodiment of this invention; FIG. 2 is a view of an arrangement of a substrate transfer device used in the device of FIG. 1: FIG. 3 is a plan view of a test-formed large area-thin film; and Description of the Preferred Embodiment The device according to this invention is a device for forming a compound oxide superconducting thin film by laser evaporation and is characterized by comprising a scanning optical system such as a rotary polygon mirror and a usually used unit as a film forming device. That is, the device according to this invention has realized that a laser beam is caused to scan by, e.g., a rotary polygon mirror, so as to form over a larger area a superconducting thin film, and film formation over such layer area can not be formed by a fixed laser beam. But in the case that a scanning direction of a laser beam by the scanning optical system by a laser beam is one dimension, the substrate is transfered in a direction perpendicularly to the direction of scanning of the laser beam to form a film on a substrate. Thus a superconducting thin film can be formed over a required area. This invention is applicable to all thin film forming using laser evaporation, and the superconducting thin films to which this invention is especially effectively applied are oxide superconducting materials including La-Ba-Cu, Y-Ba-Cu, Bi- and Tl-system oxides, which are formed on SrTiO 3 or MgO single-crystal substrate, etc. FIG. 1 is a pictorial view of an arrangement of the device for forming a thin film according to a first embodiment of this invention. The interior of the film forming chamber is shown in a sectional view in Fig. 1. As shown in FIG. 1, this device mainly comprises a film forming chamber 8 which houses a substrate holder 2 for holding a substrate 1 and a target holder 4 for holding a target 3, and has an exhaust port 5, an oxygen supply port 6 and a laser beam entrance window 7; and laser irradiating means including a laser 11 which has an optical system 10 and generates a laser beam, and a rotary polygon mirror 12. The substrate holder 2 is set on a translating device 20 for the substrate holder 2, which will be explained later, and is movable back and forth with respect to the sheet of FIG. 1. The oxygen supply port 6 is connected to a nozzle 6a so that oxygen gas is blown near to a film forming surface of the substrate 1. The optical system 10 converges a laser beam emitted from the laser 11 or expand a beam radius of the laser beam to supplement its output power. Optical system 10 is designed suitably to meet necessary film forming requirements. FIG. 2 shows a structure example of the transfer device 20 for the substrate holder 2 shown in Fig. 1. In FIG. 2, the transfer device 20 is laid on a side with a film forming surface of the substrate faced this way. As shown in FIG. 2, the transfer device 20 mainly comprises a frame 21 having a predetermined sectional shape, and a plurality of pairs of guide rollers 22 with the rollers of each pair opposed to each other. The guide rollers 22 are arranged so as to engage in a pair of guide grooves 2a cut in both side walls of the plate-shaped substrate holder 2. Some of the guide rollers 22 are forcedly driven by driving means 24. The driving means 24 may be reversible to thereby reverse a transfer direction. In the frame 21, there is provided a heater 23. The heater 23 comprises a plurality of rod-shaped heating bodies 23a arranged perpendicular to a transfer direction. Each of the rod-shaped heating bodies 23a can be individually controlled, so that temperature distributions can be formed in a transfer direction of the substrate holder 2. The transfer device 20 of the above-described structure is used as follows. The substrate holder 2 with a substrate 1 set on is inserted into the frame 21 so that the guide rollers 22 are engaged into a pair of guide grooves 2a. The frame 21 is placed as it is at a set position in the film forming chamber of FIG. 1. The driving means 24 is actuated to transfer the substrate 2 linearly. At this time, the heater 23 is turned on to heat the substrate 1 up to a required temperature. For accurate temperature control, it is preferable to make the substrate holder 2 of a metal or ceramics, which has good heat conductivity. In the device for forming a thin film, which has the above-described structure, the film forming operation is explained as follows. A substrate 1 and a target 3 are set on the substrate holder 2 and a target holder 4, and then the film forming chamber 8 is degassed. Subsequently, with the rotation the rotary polygon mirror 12, the laser 11 is actuated to irradiate a laser beam onto the target 3. The transfer device 20 is moved during the irradiation of the laser beam, so that plume 9 generated from the target 3 radiates all over the substrate. During the film forming, oxygen gas is blown from the nozzle 6a onto a film forming surface of the substrate 1. By this operation a uniform thin film can be formed on the entire surface of the substrate 1. The inventors actually formed Y-B-C-O system thin film using the same device as shown in FIGs. 1 and 2. As the laser beam source an Ar-F pulse laser oscillator having an output energy density of 2 J/cm 2 on the surface of the target and 10 Hz frequency was used. As the target, a sintered body having a size of 15cm x 3cm x 1cm and made of YBa 2 Cu 3 O x was used. As the substrate, 2 x 3 = 15 sheets of MgO(100) having a size of 5 cm x 5 cm substrates were located as shown in FIG. 3. The distance between the substrate and the target was set to be 5 cm. The interior of the film forming chanber was kept vacuum at 26,6 Pa (200 mTorr). Oxygen was blown onto the substrate from the nozzle at the rate of 30 mℓ(cc)/min. The film forming temperature was 700 °C. The polygon mirror had 12 planes. The rotation speed was 5 rpm. The thin film forming was conducted for 50 minutes under these conditions. On Positions (a) to (f) in FIG. 3, the thickness of the film (nm (Angstrom: Å)), the critical temperature Tc (K), the critical current density Jc (A/cm 2 ) were measured. The following is the resultant data. Positions Thickness nm (Å) Tc(K) Jc(A/cm 2 ) a 8000 (800) 89 5.0 x 10 6 b 7800 (780) 88 4.0 x 10 6 c 8200 (820) 89 5.5 x 10 6 d 7800 (780) 88 4.8 x 10 6 e 7900 (790) 87 4.2 x 10 6 f 8200 (820) 89 5.2 x 10 6 It was found that by using this embodiment a homogeneous, good compound oxide superconducting thin film can be formed over even an area as large as about 15x10cm. This invention is not limited to the above-described embodiment and cover various modifications. To give an example, the optical system for causing a laser beam to scan is not limited to the optical system of the type in which a reflecting mirror is movable, but instead an optical system of the type in which the optical refractive index is variable may be used. For example, electrooptic crystals, such as LiNbO 3 , can vary the refractive index by an electric field. It is possible to cause a laser beam to transmit through these crystal having electrodes so that the laser beam is caused to scan in the above-mentioned directions X, Y by applying a voltage to the electrodes. The deflection of a laser beam in the direction X can be performed by moving the laser beam source 11, because the scanning in the direction X is slower than in the direction Y. In short, the device according to this invention is characterized in that a substrate for a thin film to be formed on is moved at a low speed in the direction X while causing the laser beam to scan in the direction Y by an optical system at a relatively high speed. These characteristics may be practiced by any means. The superconducting thin film formed by laser evaporation has little diffusion of a substrate material into a superconducting thin film and has accordingly high quality, because laser evaporation can omit post-annealing from the thin film forming process. The oxide superconducting thin film formed by the device according to this invention is suitably usable in various devices, such as a string of thin film devices, superconducting quantum interference devices (SQUIDs), etc.

Claims: A laser device for evaporating material from a target and depositing it in form of a compound oxide superconducting thin film on a substrate, including means for linearly scanning the laser beam over the target and characterized by transfer means for moving the substrate perpendicular to the scanning direction. A device according to claim 1, which comprises means for supplying oxygen to the surface of the substrate. A device according to claim 1, wherein the laser is pulsed. A process for the deposition of compound oxide superconducting thin films on a substrate comprising the steps of: linearly scanning a laser beam over a target and linearly moving the substrate in a direction perpendicular to the scanning direction.

Inventors: ITOZAKI HIDEO, NAKANISHI HIDENORI, TANAKA SABURO, YAZU SHUJI, ITOZAKI, HIDEO, NAKANISHI, HIDENORI, TANAKA, SABURO, YAZU, SHUJI, ITOZAKI, HIDEO, C/O ITAMI WORKS OF SUMITOMO, NAKANISHI, HIDENORI, C/O ITAMI WORKS OF SUMITOMO, TANAKA, SABURO, C/O ITAMI WORKS OF SUMITOMO, YAZU, SHUJI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1991
Main classification: H01L39
IPC codes: C01B13, C30B29, C23C14, H01L39, C01G1