Method for preparing a superconducting thin film of compound oxide

Abstract: Method for preparing a superconducting thin film of compound oxide on a substrate (6) by laser evaporation technique. A surface of a target (7) onto which a laser beam, impinges is curved in such a manner that a created plume diverges.

Description: Background of the Invention Field of the invention The present invention relates to an improvement in a method for preparing a superconducting thin film of compound oxide having a large area by laser evaporation technique. Description of the related art Superconductivity have been thought, for a long time, to be a phenomenon observable at ultra-low temperatures realized with liquid helium as a cryogen. However, the possibility of an existence of a new type of superconducting material was revealed by Bednorz and Müller, who discovered (La, Sr) 2 CuO 4 which exhibit the superconductivity at 30 K in 1986. C. W. Chu et al. discovered, in the United States of America, another superconducting material of YBa 2 Cu 3 O 7-δ having the critical temperature of about 90 K in 1987. Maeda et al discovered so-called bismuth type superconducting material. And hence, the possibility of an actual utilization of high-Tc superconductors have burst onto the scene because the superconductivity can be realized with cheap cryogen of liquid nitrogen. The above-mentioned new type superconducting materials were obtained in a form of sintered mass prepared by powder metallurgy. But now, they can be prepared in a thin film form of very high quality. Laser evaporation technique is one of the most promising techniques because superconducting thin films which exhibit improved superconducting properties can be prepared by the laser evaporation technique with no after-treatment such as annealing of thin films deposited on a substrate. In the laser evaporation method, a target is irradiated with a laser beam to create a plume which is then deposited on a substrate. Therefore, an area of a thin film prepared by the laser evaporation method is limited to the dimension or an effective cross-sectional area of the plume which is limited by a diameter of a laser beam. However, since available laser is limited, the diameter of laser beam is limited and hence the dimension or area of thin film prepared is limited. In J. Appl. Phys. 68 (12), 1990, pages 6331-6335, the preparation of superconducting oxide films by laser irradiation of a cylindrical target of sintered Ba 2 YCu 3 O 7-x is described. A plume of particles which diverges is created. However, the particles diverge in only two opposite directions and the area of the film prepared is therefore limited in the other directions. GB-2-189 509 discloses a laser evaporation method using a cylindrical ceramic target comprising a tip onto which a laser beam impinges and being rotated about its axis in order to insure that the evaporation of the ceramic material is uniform. Therefore, an object of the present invention is to provide a novel process for preparing a superconducting thin film of oxide possessing a large area. Summary of the Invention The present invention provides a method of preparing a superconducting thin film of compound oxide by laser evaporation technique, characterized in that the surface of a target onto which a laser beam impinges is spherically or conically curved in such a manner that a created plume diverges radially. In the known laser evaporation method, the plume created from the target by a laser beam which impinges onto the target is emitted perpendicularly to a surface of the target. According to the present invention, the target has a convex surface so that the plume created from the target by a laser beam which impinges onto the target diverges radially. Therefore, the diameter of the plume on the surface of a substrate becomes very large and hence a thin film of compound oxide of large area is obtained. The plume can be diverged by using a target having a spherically or conically curved surface. The laser evaporation technique itself is well-known and usual operational condition can be used in the present invention. Followings are typical operational conditions: Substrate temperature 600 to 750 °C Gas pressure in chamber 13.33 N.m -2 (100mTorr) to 133.3 N.m -2 (1,000mTorr) Distance between the target and the substrate 40 to 100 mm Laser density 1.0 to 3.0 J/cm 2 Deposition rate: 1 to 100 nm/min (10 to 1,000 Å/min) Any laser can be used but an excimer laser is usually used. The substrate is preferably a single crystal substrate of oxide such as MgO, SrTiO 3 , LaAlO 3 , LaGaO 3 and yttrium stabilized zirconia (YSZ). The process according to the present invention can be used for preparing thin films of any known high-Tc superconductors such as La-Sr(Ba)-Cu-O, Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O. The process according to the present invention permits to prepare a superconducting thin film of compound oxide possessing a large surface area. Brief Description of the Drawing Fig.1 illustrates a structure of a film forming apparatus which can be used in the process according to the present invention. Fig.2 illustrates targets used in the apparatus of Fig. 1 in cross section. Fig.3 are contour maps of film thickness prepared in Example. The film forming apparatus shown in Fig. 1 comprises a vacuum chamber 1 in which a substrate holder 2 and a target holder 3 are positioned and a laser 5 which emits a laser beam to a target through a transparent window 4 secured to the vacuum chamber 1. The substrate holder 2 and the target holder 3 are constructed in such a manner that each of substrate 6 and target 7 can be positioned at desired level, orientation and angle. The laser beam emitted from the laser 5 is converged by an optical system 8. Fig. 2 illustrates cross-section of targets 7 which can be used in the apparatus shown in Fig. 1. The target 7 has a convex surface such as a spherical surface as shown in Fig. 2(a) or a conical surface shown in Fig. 2(b) each illustrated in a cross-section. According to the present invention, a plume created by a laser beam which impinges onto the target 7 diverge radially from the target 7 and deposit on the substrate 6 at a larger area. Now, the present invention will be described with reference to Example, but the scope of the invention should not be limited to the Description of the Preferred Embodiment A superconducting thin film of Y-type compound oxide by using the apparatus shown in Fig. 1. A thin film was deposited on a (100) plane of MgO single crystal substrate by laser evaporation method. As a target, a sintered body of compound oxide represented by a composition of YBa 2 Cu 3 O y was used. Sample 1 was prepared by using a target (diameter of 15 mm, height of 5 mm) having a spherical surface (curvature of 10 mm). Sample 2 was prepared by using a conical target having a diameter of 15 mm and a height of 5 mm. For comparison, sample 3 was prepared by using a pyramid target having a square base of 15 mm side and a height of 5 mm. Also for comparison, sample 4 is prepared in the same apparatus as above but using a flat target. Operational conditions used are summarized in Table 1. Substrate temperature 700°C Gas pressure 53.32N.m -2 (400mTorr) Laser energy density 1.5 to 2.0 J/cm 2 Film thickness 200 nm (2,000 Å) Distance between substrate and target 50 mm Fig. 3 illustrates contour maps of film thickness prepared. Relation between samples and the contour maps is shown in Table 2. Sample 1 Fig. 3 (a) Sample 2 Fig. 3 (b) Sample 3 Fig. 3 (c) Sample 4 Fig. 3 (d)

Claims: Method of preparing a superconducting thin film of compound oxide on a substrate (6) disposed in a vacuum chamber (1) by laser evaporation technique, characterized in that the surface of a target (7) onto which a laser beam impinges is spherically or conically curved in such a manner that a created plume diverges radially. The method set forth in claim 1 wherein the substrate is heated at a temperature between 600 and 750°C. The method set forth in claim 1 to 2 wherein the gas pressure in chamber is adjusted between 13.33 N m -2 (100 m Torr) and 133.3 N m -2 (1,000 m Torr). The method set forth in any one of claim 1 to 3 wherein the distance between the target and the substrate is adjusted between 40 and 100 mm. The method set forth in any one of claim 1 to 4 wherein a laser beam has a energy density of between 1.0 and 3.0J/cm 2 . The method set forth in any one of claim 1 to 5 wherein the deposition rate is adjusted between 1 and 100 nm/min (10 and 1,000 Å/min). The method set forth in any one of claim 1 to 6 wherein the compound oxide is a high-Tc superconductor. The method set forth in claim 7 wherein the compound oxide is selected from a group consisting of La-Sr(Ba)-Cu-O,Y-Ba-Cu-O,Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O.

Inventors: ITOZAKI HIDEO, NAGAISHI TATSUOKI, NAKANISHI HIDENORI, TANAKA SABURO, ITOZAKI, HIDEO, NAGAISHI, TATSUOKI, NAKANISHI, HIDENORI, TANAKA, SABURO, ITOZAKI, HIDEO, C/O ITAMI WORKS OF SUMITOMO, NAGAISHI, TATSUOKI, C/O ITAMI WORKS OF SUMITOMO, NAKANISHI, HIDENORI, C/O ITAMI WORKS OF SUMITOMO, TANAKA, SABURO, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39, C23C14, C30B29, H01B12, H01B13, H01L21