Method of and apparatus for preparing oxide superconducting film

Abstract: In order to enable formation of a smooth and dense oxide superconducting film with no clear appearance of grain boundaries in a fine structure even at a high film forming rate, a laser ablation method is employed to apply a laser beam 2 to a target 1 containing components of an oxide superconductive material and deposit particles, which are thus scattered from the target 1, on a substrate 3, while gaseous oxygen is supplied from a gaseous oxygen inlet 7 toward laser plasma 6, which is generated by the application of the laser beam 2.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of and an apparatus for preparing an oxide superconducting film using laser ablation. Description of the Background Art Formation of an oxide superconducting film by excimer laser ablation, for example, is watched with interest in the point that it is possible to implement a high-quality film. In general, a pulse laser beam is employed for such laser ablation. In an already reported method of preparing an oxide superconducting film using laser ablation, a high-quality oxide superconducting film is obtained when the laser beam is set at a repetition rate of not more than about 10 Hz so that the film forming rate is less than about 0,2 µm/min. (2000 Å/min.). When an oxide superconducting film is formed by ordinary laser ablation, the laser repetition rate is so increased as to increase the film forming rate. However, if the film forming rate is increased in excess of 0,2 µm/min. (2000 Å/min.), for example, the as-formed oxide superconducting film is obtained as an aggregate of fine crystal grains of not more than 1 µm in size, and its critical temperature and critical current density are reduced in response to clear appearance of grain boundaries. In a method of preparing an oxide superconducting film using ordinary laser ablation, it is necessary to heat a substrate to a temperature of at least 600°C, in order to facilitate crystallization of the oxide superconducting film, which is deposited on the substrate. Therefore, available materials for such a substrate are restricted to ceramic materials, single-crystalline materials of MgO, SrTiO₃ and the like, which are still unreactive with film components at such a high temperature, and it is impossible to use a substrate of a metal, for example. From patent abstracts of Japan of JP-A-63-264819 an apparatus for and likewise a method of preparing an oxide superconducting film is known. Said known apparatus comprises a laser source for applying a laser beam to the target within an atmosphere containing oxygen. From Applied Physics Letters, 55, (2), (1989), p. 197, M. Balooch et al. a similar arrangement is known wherein, in addition, the oxygen outlet is provided with a RF generator so that oxygen ions are supplied. Likewise from Applied Physics Letters, 53, (3), (1988), p. 234, S. Witanachi et al. an arrangement comparable to the afore-mentioned ones is known which additionally comprises an electrode located between the target and a heated substrate. The oxygen is supplied to the rim of the target. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method of and an apparatus for preparing an oxide superconducting film, which can obtain a smooth and dense oxide superconducting film in a fine structure with no distinct grain boundaries even in high-speed film formation. Another object of the present invention is to provide a method of preparing an oxide superconducting film, which can form an oxide superconducting film still exhibiting excellent properties on a substrate of a relatively low temperature. These objects are solved with a method in accordance with claim 1 and an apparatus in accordance with claim 5. Furthermore, according to the present invention, the gaseous oxygen which is supplied toward the laser plasma is sprayed onto a portion on the target being positioned apart from said portion which is irradiated with the laser beam 0.5 mm or 1 mm. In the present invention an electrode may be provided between the target and the substrate, thereby generating discharge plasma around the electrode. In this case the substrate may be set at a temperature of less than 600°C. When the gaseous oxygen is supplied toward the laser plasma, large quantities of atoms and/or molecules of oxygen excited species and/or oxide excited species are produced by reaction between the gaseous oxygen and the laser plasma. These excited species are essential to formation of a high-quality oxide superconducting film, and it is conceivable that a high-quality oxide superconducting film can be still implemented at a high film forming rate due to formation of large quantities of such excited species. Thus, according to the present invention, gaseous oxygen is supplied to laser plasma, whereby a smooth and dense oxide superconducting film is obtained with no distinct grain boundaries in a fine structure. Consequently, it is possible to improve superconductive properties such as a critical temperature and critical current density. The present invention has a remarkable effect particularly when the film forming rate is at least 0.2 µm/min (2000 Å/min). If the film forming rate is set at such a high value of at least 0,2 µm/min. (2000 Å/min). in ordinary laser ablation, the as-formed oxide superconducting film is obtained as an aggregate of fine crystal grains of not more than 1 µm in size, to develop distinct grain boundaries as described above. According to the present invention, on the other hand, a smooth and dense film can be formed with no distinct grain boundaries even if such a high film forming rate of at least 0,2 µm/min (2000 Å/min.) is employed, and it is possible to substantially maintain excellent superconductive properties, which have been attained at a film forming rate of less than 0,2 µm/min (2000 Å/min). Thus, the present invention, which can effectively improve the quality of an oxide superconducting film formed at a high speed, is particularly effective when the same is applied to preparation of an oxide superconducting wire, which is obtained by employing a long tape-type substrate and continuously forming an oxide superconducting film thereon. According to the present invention gaseous oxygen is sprayed onto a portion around a position, which is irradiated with the laser beam, on the target, to produce large quantities of oxygen excited species, while discharge plasma is generated to further increase the density of such oxygen excited species. Since such high density of excited species facilitate crystallization of the oxide superconducting film, it is possible to obtain a smooth and dense oxide superconducting film with no distinct grain boundaries even if the substrate temperature is not so much increased. Thus, it is possible to obtain an oxide superconducting film, which has excellent superconductive properties such as a high critical temperature and high critical current density. In the aforementioned case, a high-quality oxide superconducting film can be obtained even if the substrate temperature is less than 600°C. Thus, it is possible to use a substrate of a metal, such as stainless steel or nickel, which is easily oxidized at a temperature exceeding 600°C and considerably reactive with the film components, with no problem. A substrate of such a metal is advantageous in the point that the same can be easily elongated, and is flexible. Thus, according to the present invention, it is possible to obtain a high-quality oxide superconducting film at a high film forming rate through a low-temperature process. Therefore, the present invention is particularly effective when a long flexible tape-type base material is employed as a substrate and an oxide superconducting film must be continuously formed on the tape-type base material along the longitudinal direction by laser ablation. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front elevational view illustrating a state of carrying out an embodiment of the present invention; Fig. 2 illustrates surface states of oxide superconducting films which are prepared by a conventional laser ablation method and the embodiment shown in Fig. 1 at (a) and (b) respectively in an enlarged manner; and Fig. 3 is a front elevational view illustrating a state of carrying out a further embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a first embodiment of the present invention. A laser beam 32 is applied to a target 31, which contains components of an oxide superconductive material. A substrate 33 is arranged to be opposite to the target 31. When the laser beam 32 is applied to the target 31, particles are scattered in states of atoms and/or molecules from a position 34, which is irradiated with the laser beam 32, on the target 31. On the basis of such scattering of the particles, laser plasma 35 is generated from the irradiated position 34 toward the substrate 33. A gaseous oxygen conduit 36 is provided in the vicinity of the target 31 to spray gaseous oxygen around the position 34, irradiated with the laser beam 32, of the target 31, and a gaseous oxygen supply port 37, which is provided on a forward end of the gaseous oxygen conduit 36, is located in the vicinity of the irradiated position 34. Experimental Examples carried out on the basis of the embodiment shown in Fig. 1 are now described. Experimental Example 1 A Y₁Ba₂Cu₃O 7-δ sintered body was employed as a target 31. A KrF excimer laser beam 32 (wavelength: 248 nm) was converged to form a spot of 2 mm x 5 mm on the target 31. Energy density of the laser beam 32 on an irradiated position 34 of the target 31 was 2.5 J/cm². An MgO single-crystalline substrate 3 was employed and its temperature was set at 700°C. First, a film was formed on the substrate 33 at a repetition rate of the laser beam 32 of 200 Hz, with no introduction of gaseous oxygen from the gaseous oxygen supply port 37. The film was formed in an atmosphere with an oxygen pressure of 27 Pa (200 mTorr). The as-formed oxide superconducting film, which was obtained at a film forming rate of 4 µm/min., exhibited a critical temperature of 25 K. No critical current density was measured since it was impossible to measure the same at 77.3 K. Fig. 2(a) shows the surface of this oxide superconducting film in an enlarged manner. As shown in Fig. 2(a), the oxide superconducting film was obtained as an aggregate of fine crystal grains of not more than 1 µm, with clear appearance of grain boundaries. Then, a gaseous oxygen conduit 36 of a metal tube having an inner diameter of 2 mm was arranged to locate its gaseous oxygen supply port 37 at a position separated by 1 mm from the irradiated position 34. Gaseous oxygen of 300 SCCM was blown out from the gaseous oxygen supply port 37, and the atmosphere pressure was set at 27 Pa (200 mTorr), identically to the aforementioned comparative example. The as-formed oxide superconducting film, which was obtained at a film forming rate of 4.2 µm/min., exhibited a critical temperature of 89 K, and critical current density of 1.7 x 10⁶ A/cm² at 77.3 K. Fig. 2(b) shows the surface of this superconducting film in an enlarged manner. As shown in Fig. 2(b), the surface was smoothed and densified, and connection between the crystal grains was increased. Although Fig. 2(b) shows apparent grain boundaries in virtual lines for the purpose of illustration, substantially no such lines were recognized in actual observation. Experimental Example 2 The pulse repetition rate (Hz) of the laser beam 32 was varied in a range of 10, 20, 30, 40 and 50, to form oxide superconducting films with and without introduction of gaseous oxygen respectively. Table 1 shows film forming rates (µm/min.) (Å/min.), critical current density values J C (A/cm²) at 77.3 K, and zero-resistance critical temperatures T C (K) of the oxide superconducting films measured in the aforementioned conditions. Other conditions were similar to those in Experimental Example 1. It is understood from Table 1 that superconductive properties attained at film forming rates of less than 0,2 µm/min. (2000 Å/min.) are still substantially maintained at film forming rates exceeding 0,2 µm/min., due to introduction of gaseous oxygen. When no gaseous oxygen is introduced, on the other hand, superconductive properties are reduced as the film forming rate is increased. Comparing the films particularly in relation to the film forming rates in excess of 0,2 µm/min., it is understood that the oxide superconducting film obtained according to the inventive method exhibits superior superconductive properties as compared with the oxide superconducting film obtained with no introduction of gaseous oxygen. Thus, the present invention has a remarkable effect particularly under a high film forming rate. Experimental Example 3 A Y₁Ba₂Cu₃O 7-δ sintered body was employed as a target 31. An ArF excimer laser beam 32 (wavelength: 193 nm) was converged to form a spot of 4 x 8 mm² on the target 31. Energy density at an irradiated position 34 of the target 31 was 1.2 J/cm². An MgO single-crystalline substrate 33 was employed, and its temperature was set at 600°C. First, oxygen was introduced from a wall of a film forming chamber with no introduction of gaseous oxygen from the gaseous oxygen supply port 37, and the overall film forming chamber was brought into an oxygen pressure state of 20 Pa (150 mTorr), to form a film on the substrate 33 at a laser repetition rate of 200 Hz. The as-formed oxide superconducting film, which was obtained at a film forming rate of 2.2 µm/min., exhibited a critical temperature of 18 K. Then, a gaseous oxygen conduit 36 of a metal tube having an inner diameter of 2 mm was arranged to locate its gaseous oxygen supply port 37 at a position separated by 0.5 mm from an irradiated position 34, and gaseous oxygen of 200 SCCM was supplied from the gaseous oxygen supply port 37, to form a film under an atmosphere pressure of 20 Pa (150 mTorr), identically to the above comparative example. The repetition rate of a laser beam 32 was set at 200 Hz, identically to the above comparative example. The as-formed oxide superconducting film, which was obtained at a film forming rate of 2.7 µm/min., exhibited a critical temperature of 90 K, and critical current density of 1.9 x 10⁶ A/cm² at 77.3 K. Fig. 3 shows a state of carrying out a further embodiment of the present invention. A laser beam 42 is applied to a target 41, which contains components of an oxide superconductive material. A substrate 43 is arranged to be opposite to the target 41. When the laser beam 42 is applied to the target 41, particles are scattered in states of atoms and/or molecules from a position 44, which is irradiated with the laser beam 42, on the target 41. On the basis of such scattering of the particles, laser plasma is generated from the irradiated position 44 toward the substrate 43. A gaseous oxygen conduit 45 is provided in the vicinity of the target 41, to spray gaseous oxygen around the position 44, irradiated with the laser beam 42, of the target 41, while a gaseous oxygen supply port 46, which is provided on a forward end of the gaseous oxygen conduit 45, is located in the vicinity of the irradiated position 44. A ring-shaped electrode 47, for example, is arranged at a substantially intermediate position between the target 41 and the substrate 43. DC or RF power is applied to the electrode 47, whereby discharge plasma is generated around the electrode 47, to be superimposed on the aforementioned laser plasma. Experimental Examples carried out on the basis of the embodiment shown in Fig. 3 are now described. Experimental Example 4 A Y₁Ba₂Cu₃O 7-δ sintered body was employed as a target 41. A KrF excimer laser 42 (wavelength: 248 nm) was converged to form a spot of 2 mm x 5 mm on the target 41. Energy density of the laser beam 42 at an irradiated position 44 of the target 41 was 2.5 J/cm². An MgO single-crystalline substrate 43 was employed, and its temperature was set at 450°C. A gaseous oxygen conduit 45 of a metal tube having an inner diameter of 2 mm was arranged to locate its gaseous oxygen supply port 46 in a position separated by 1 mm from the irradiated position 44. Gaseous oxygen of 300 SCCM was blown out from the gaseous oxygen supply port 46, and the atmosphere pressure was set at 27 Pa (200 mTorr). RF power of 13.56 MHz was applied to a ring-shaped electrode 47, to generate discharge plasma in the vicinity of the electrode 47. The repetition rate of the laser beam 42 was set at 200 Hz, to form an oxide superconducting film on a substrate 43 at a film forming rate of 3.5 µm/min. The as-formed oxide superconducting film exhibited a zero-resistance critical temperature of 85 K, and critical current density of 7.1 x 10⁵ A/cm² at 77.3 K. For the purpose of comparison, another film was formed with no generation of discharge plasma at a film forming rate of 3.4 µm/min. The as-formed oxide superconducting film exhibited a critical temperature of 62 K. No critical current density was measured since it was impossible to measure the same at 77.3 K. It is conceivable that no excellent superconductive properties were attained in the aforementioned comparative example particularly since its substrate 43 was at a low temperature of 450°C. For the purpose of reference, the temperature of such a substrate 43 was increased to 700°C with no generation of discharge plasma, to form an oxide superconducting film. The as-formed film exhibited a critical temperature of 89 K, and critical current density of 1.7 x 10⁶ A/cm² at 77.3 K. Experimental Example 5 The pulse repetition rate (Hz) of a laser beam 42 was varied in a range of 10, 20, 30, 40 and 50, to form films with and without generation of discharge plasma respectively. In the former case, a DC constant voltage source of +300 V was coupled to an electrode 47, to generate discharge plasma in the vicinity of the electrode 47. Further, gaseous oxygen was blown out from a gaseous oxygen supply port 46 through a gaseous oxygen conduit 48, and a pumping speed was adjusted to attain a gas pressure of 27 Pa (200 mTorr) in a vessel for laser ablation. The temperature of a substrate 43 was set at 400°C. Other experimental conditions were similar to those in Experimental Example 4. Table 2 shows film forming rates µm/min. (Å/min.), critical current density values J C (A/cm²) at 77.3 K and zero-resistance critical temperatures T C (K) of the as-formed oxide superconducting films. It is understood from Table 2 that, due to generation of the discharge plasma, excellent superconductive properties are still attained at such a low substrate temperature of 400°C, regardless of values of film forming rates. When no discharge plasma is generated, on the other hand, overall superconductive properties are relatively inferior, and gradually deteriorated particularly with increase of the film forming rate. At film forming rates exceeding 0,2 µm/min. (2000 Å/min.), the comparative film, which was prepared with no generation of discharge plasma, exhibited extremely inferior or substantially no superconductive properties. Thus, comparing the films particularly in relation to the film forming rates exceeding 0,2 µm/min. (2000 Å/min.), it is understood that the inventive oxide superconducting film obtained with generation of discharge plasma exhibits extremely superior superconductive properties as compared with the comparative film obtained with no generation of discharge plasma, and this embodiment has a remarkable effect particularly under a high film forming rate.

Claims: Method of preparing an oxide superconducting film, comprising the steps of: preparing a target (31; 41) containing components of an oxide superconductive material; arranging a substrate (33; 43) to be opposite to said target (31; 41) applying a laser beam (32; 42) to said target (31; 41), thereby generating laser plasma (35) and scattering particles from said target (31; 41); depositing said scattered particles on said substrate (33; 43), thereby forming an oxide superconducting film on said substrate (33; 43); and supplying gaseous oxygen toward said laser plasma (35); characterized in that oxygen is sprayed around a position (34; 44) on said target (31; 41) being irradiated with said laser beam (32; 42) by means of a conduit (36; 45) having a supply port (37; 46), said supply port (37; 46) being positioned apart from said irradiated position (34; 44) by a distance of 0.5 mm or 1 mm. The method of preparing an oxide superconducting film in accordance with claim 1, wherein a film forming rate for said oxide superconducting film is selected to be at least 0.2 µm/min. The method of preparing an oxide superconducting film in accordance with claim 1, wherein said substrate (33; 43) is set at a temperature of less than 600°C. The method of preparing an oxide superconducting film in accordance with claim 1 or 3, further comprising a step of providing an electrode (47) between said target (43) and said substrate (41), thereby generating discharge plasma around said electrode (47). Apparatus for preparing an oxide superconducting film using laser ablation, comprising: a target (31; 41) containing components of an oxide superconductive material; a laser source for applying a laser beam (32; 42) to said target (31; 41); a substrate (33; 43) arranged to be opposite to said target (31; 41); and a means (36, 37; 45, 46) for supplying gaseous oxygen towards a laser plasma (35) generated by application of said laser beam (32; 42), characterized in that said means (36, 37; 45, 46) for supplying gaseous oxygen towards said laser plasma (35) comprises a conduit (36; 45) having a supply port (37; 46), said supply port (37; 46) being positioned apart from the irradiated position (34; 44) of the target by a distance of 0.5 mm or 1 mm. Apparatus according to claim 5, further comprising an electrode (47) positioned between said target (41) and said substrate (43).

Inventors: HARA TSUKUSHI C O THE TOKYO EL, HAYASHI NORIKI C O OSAKA WORKS, OKANIWA KIYOSHI C O THE TOKYO, OKUDA SHIGERU C O OSAKA WORKS, TAKANO SATOSHI C O OSAKA WORKS, YAMAMOTO TAKAHIKO C O THE TOKY, YOSHIDA NORIYUKI C O OSAKA WOR, HARA, TSUKUSHI, C/O THE TOKYO ELECTRIC POWER, HAYASHI, NORIKI, C/O OSAKA WORKS OF SUMITOMO, OKANIWA, KIYOSHI, C/O THE TOKYO ELECTRIC POWER, OKUDA, SHIGERU, C/O OSAKA WORKS OF SUMITOMO, TAKANO, SATOSHI, C/O OSAKA WORKS OF SUMITOMO, YAMAMOTO, TAKAHIKO, C/O THE TOKYO ELECTRIC POWER, YOSHIDA, NORIYUKI, C/O OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, TOKYO ELECTRIC POWER CO, SUMITOMO ELECTRIC INDUSTRIES, LTD., THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1991
Main classification: H01L39
IPC codes: C23C14, H01L39, H01B13, C01G1, C30B29, H01B12, C01B13, C01G3, C30B23