PROCESS FOR PREPARING A BISMUTH-TYPE SUPERCONDUCTING THIN FILM

Abstract: In a process for depositing superconducting thin film of bismuth-­ containing compound oxide on a substrate by physical vapour deposition, improvement in that the substrate is heated at a temperature between 670 and 750 °C during the deposition.

Description: Background of the Invention Field of the invention The present invention relates to a process for preparing a superconducting thin film of bismuth-containing compound oxide. More particularly, it relates to an improved process for depositing the bismuth (Bi) type superconducting thin film possessing an improved high critical current density (Jc) as well as a high critical temperature (Tc) on a substrate. Description of the related art The superconductivity is a phenomenon which is explained to be a kind of phase change of electrons under which the electric resistance become zero and the perfect diamagnetism is observed. Several superconducting devices have been proposed and developed in electronics which is a typical field to which the superconducting phenomenon is applicable. A typical application of superconductor is Josephson device in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconductors. The tunnel junction type Josephson device is expected to be a high-speed and low-power consuming switching device owing to a smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high-sensitive sensor or detector for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a precise quantum phenomenon. Development of the superconducting devices such as high-speed logic units or of no power-loss wiring materials is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption. The critical temperature "Tc" of superconductivity, however, could not exceed 23.2 K of Nb₃Ge which was the the highest Tc for the past ten years. The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Müller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p189). It had been known that certain ceramic materials of compound oxides exhibit the property of superconductivity. For example, U.S. patent No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10 K and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen is indispensable to realize superconductivity. The new type compound oxide superconductor discovered by Bednorz and Müller is represented by [La, Sr]₂CuO₄ which is called the K₂NiF₄-type oxide having a crystal structure which is similar to known perovskite type oxides. The K₂NiF₄-type compound oxides show such higher Tc as 30 K which are extremely higher than known superconducting materials. It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa₂Cu₃O 7-x having the critical temperature of about 90 K in February 1987 (Physical Review letters, Vol. 58, No. 9, p908). The other type new superconducting material is a compound oxide of Bi-Sr-Ca-Cu-O system reported by Maeda et al (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210). The bismuth-type compound oxides including this Bi-Sr-Ca-Cu-O system have such advantages that they are chemically much stable and can be prepared with reduced cost because they contain no rear earth elements. The above-mentioned new types superconducting materials can be obtained by solid reaction in a bulk form of sintered block which was obtained by sintering a powder mixture of oxides or carbonates of constituent metal elements. They can also be deposited on a substrate in a form of a thin film by physical vapour deposition (PVD) technique such as RF sputtering, vacuum deposition, ion-plating or MBE or chemical vapor deposition (CVD) technique such as thermal CVD, plasma CVD, photo-CVD or MOCVD. It is a usual practice to subject the resulting thin films to heat-treatment in an oxygen-containing atmosphere to adjust the oxygen deficiency in the crystal. The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor on a substrate in the following patent applications: European patent applications No. 88 400 268.4, No. 88 400 598.4, No. 88 401 202.2, No. 88 401 208.9, No. 88 401 320.2, No. 88 403 254.1, 88 403 328.3 or the like. The present invention completed on the same line as these patent applications. The present invention concerns the superconducting thin films of bismuth (Bi) type compound oxide. This type superconducting thin film has been prepared by deposition techniques such as RF sputtering, vacuum deposition of MO-CVD technique. Japanese Journal of Applied Physics, Vol-27, N°4, 1988 pp. L632-L633, describes sputtering in the presence of oxygen of BiSrCaCuo thin films on a MgO(100) substrate. They possess, however, very low critical current density (Jc) although they showed very high critical temperature (Tc), so that they were difficult to be utilized in practical uses as electronics devices or wiring materials. An object of the present invention is to overcome the conventional bismuth (Bi) type superconducting thin films and to provide an improved process for increasing the critical current density (Jc) thereof. Summary of the Invention The present invention provides a process for depositing a superconducting thin film of a compound oxide containing at least bismuth and copper on a substrate of Mg0 in an oxygen-containing atmosphere by a physical vapour deposition technique, characterized in that said thin film is deposited on a (110) plane of the Mg0 substrate which is heated at a temperature between 670 and 750 °C, preferably between 670 and 720 °C, more prefembly between 680 and 710 °C during the deposition. According to a preferred embodiment, the deposited thin film is further annealed at a temperature between 870 and 910 °C, preferably between 880 and 905 °C. This annealing operation is effected preferably for a time duration between 10 minutes and 10 hours. This annealing operation is carried out preferably in oxygen gas containing atmosphere or in the presence of oxygen gas. The pressure of oxygen gas is preferably about 1 atom. The present invention completed on the basis of such finding of the present inventors that the superconducting property of thin films of compound oxide containing at least bismuth and copper deposited by physical vapour deposition technique is influenced critically by the substrate temperature and hence the substrate temperature must be controlled precisely. In fact, a superconducting thin film such as a thin film made of Bi-Sr-Ca-Cu type compound oxide which was prepared by the conventional processes a very poor critical current density (Jc), although the thin film possesses a very high critical temperature (Tc) of higher than 100 K. Namely, the highest critical current density (Jc) observed in the known thin film of bismuth (Bi) type compound oxide deposited on the substrate has been about 10,000 A/cm² which is inadequate for practical applications. The present inventors tried to find the best conditions in order to improve the critical current density (Jc) by a variety of experiments and found that the quality of the thin film deposited is influenced critically by the substrate temperature during the deposition stage and finally found that the critical current density (Jc) can be improved remarkably by controlling the substrate temperature precisely. According to the experiments which conducted by the present inventors, the critical current density (Jc) drop sharply when the substrate temperature is selected outside a range between 670 and 750 °C, more precisely between 670 and 720 °C and more precisely between 680 and 710 °C during the deposition. The present inventors also found that an effective result is obtained when the annealing of the deposited thin film is effected in a predetermined condition. Namely, according to the present invention, it is very important to heat the substrate at a temperature between 870 and 910 °C, preferably between 880 and 905 °C. It was experienced that the superconducting property of the thin film became relatively worse when the annealing is effected outside the above-mentioned annealing condition. Description of the Preferred Embodiment The substrate on which the thin film is deposited is preferably a single crystal of MgO. The superconducting thin film according to the present invention is deposited on a {110} plane of these single crystals. The superconducting thin film according to the present invention can be deposited on a substrate by physical vapour deposition (PVD) technique such as sputtering, vacuum deposition or ion-plating, preferably by RF (radio-frequency) magnetron sputtering. The target is preferably a sintered mass or sintered powder of a compound oxide composed of constituent metal elements from which the thin film is made and oxygen. Such sintered mass or sintered powder can be prepared by mixing powders of oxides, carbonates, nitrate or fluorides each containing at least one of constituent metal elements of the thin film to be deposited in such a manner that a powder mixture contains the constituent metal elements in predetermined atomic ratios and then by sintering the powder mixture. The sputtering gas is preferably a mixture of argon and oxygen, the percentage of oxygen is preferably 10 to 60 %. The pressure of the sputtering gas is preferably between 1 x 10⁻² and 1 x 10⁻¹ Torr. During the sputtering, it is preferable to apply high-frequency of 0.064 to 1.27 W/cm² to the target. The sputtering is continued for a time which is required to obtain a thin film of a desired thickness. The deposition rate is preferably between 0.05 and 1 Å/min. The sintered mass or sintered powder can be used as a vapour source in the vacuum deposition technique and ion plating technique. The present invention is applicable for any superconductor composed of compound oxide containing bismuth (Bi), preferably a typical bismuth type compound oxide of Bi, Sr, Ca and Cu represented bythe general formula: Bi₄(Sr 1-x , Ca x ) m Cu n O p+y in which "m", "n", "x" and "y" are numbers each satisfying respective range of 6 ≦ m ≦ 10, 4 ≦ n ≦ 8, 0 < x < 1 and -2 ≦ y ≦ +2 and p = (6+m+n). As typical compound oxides of this type, the following systems can be mentioned: Bi₂Sr₂Ca₂Cu₃O 10-y and Bi₄Sr₄Ca₄Cu₆O 2(10-y) . As the other type bismuth-containing compound oxides to which the present invention is applicable, it can be mentioned the following systems: wherein "Ln" stands for lanthanide elements. The thin film of the bismuth type compound oxide obtained by the process according to the present invention shows remarkably improved critical current density (Jc) without spoiling its inherent high critical temperature (Tc), so that it becomes possible to develop a variety of superconducting devices including Josephson Device which are operable at high temperature above the boiling point of liquid nitrogen. Now, the present invention will be described with reference to examples, but the scope of the invention should not be limited to them. Examples In this example, several superconducting thin films of Bi-Sr-Ca-Cu type compound oxide were prepared by RF-magnetron sputtering method. A target used in the RF-magnetron sputtering was prepared by mixing intimately powders of commercially available BiCO₃, SrCO₃, CaCO₃ and CuO in such a manner that the atomic ratios of respective elements Bi, Sr, Ca and Cu in the resulting powder mixture became 1.4 : 1.0 : 1.0 : 1.5 and then sintering the powder mixture at 800 °C for 8 hours. The resulting sintered mass of Bi-Sr-Ca-Cu type compound oxide is used as the target. The substrate was a single crystal of MgO. The thin film was deposited on {110} plane of the MgO substrate. Six samples of thin films were prepared at different substrate temperatures. The RF-magnetron sputtering was effected under the following common conditions except the substrate temperature for all samples: Sputtering gas : a mixed gas of Ar and O₂ O₂/(Ar+O₂)=0.2 (vol %) Sputtering gas pressure : 2 x 10⁻² Torr High-frequency power : 50 W (0.64 W/cm²) Deposition of all thin films was continued until the thin film of 2,000 Å thick was obtained. It was confirmed that the resulting thin films became superconductors by measuring the critical temperature. After then, the critical current density (Jc) of respective thin films was measured at 77.3 K. Table 1 shows the critical current density (Jc) of respective thin films obtained as well as the substrate temperatures adopted. Then, the other five samples were prepared by the same conditions as Sample 3 which showed the highest critical current density (Jc). Then, these five samples were subjected to annealing at different temperatures shown in Table 2 in an oxygen gas stream which is supplied at a rate of 0.6 liter/cm² for 60 minutes. The critical current density (Jc) of the resulting thin films was measured by the same method as above. The results are summarized in Table 2. From the results shown above, it is apparent that the critical current density (Jc) of superconducting thin films of Bi-Sr-Ca-Cu type compound oxide are influenced strongly by the substrate temperature during deposition and by the annealing temperature. In other words, the critical current density of the superconducting thin films of this type can be improved by controlling these temperatures.

Claims: A process for depositing a superconducting thin film of a compound oxide containing at least bismuth and copper on a substrate of Mg0 in an oxygen containing atmosphere by a physical vapour deposition technique, characterized in that said thin film is deposited on a (110) plane of the Mg0 substrate which is heated at a temperature between 670 and 750 °C during the deposition. The process set forth in Claim 1 wherein the deposited thin film is further annealed at a temperature between 870 and 910 °C. The process set forth in Claim 1 or 2 wherein the annealing is effected for more than 10 minutes. The process set forth in any one of Claim 1 to 3 wherein the annealing is effected in oxygen-containing atmosphere. The process set forth in any one of Claim 1 to 4 wherein the physical vapour deposition technique is sputtering. The process set forth in Claim 5 wherein a target used in the sputtering is a sintered block or sintered powder of a compound oxide containing metal elements which are constituent elements of the compound oxide of the thin film to be deposited. The process set forth in Claim 6 wherein said sintered block or sintered powder is prepared by mixing powders of oxides, carbonates, nitrate of fluorides each containing at least one of contituent metal elements of the thin to be deposited in such a manner that a powder mixture contains the constituent metal elements in predetermined atomic ratios and then by sintering the powder mixture. The process set forth in any one of Claim 5 to 7 wherein the sputtering gas is a mixture of argon and oxygen, the percentage of oxygen being 10 to 60 %. The process set forth in any one of Claim 5 to 8 wherein the pressure of the sputtering gas is between 1 x 10⁻² and 1 x 100⁻¹ Torr. The process set forth in any one of Claim 5 to 9 wherein high-frequency power density of 0.064 to 1.27 W/cm² is applied to the target during the sputtering. The process set forth in any one of Claim 5 to 10 wherein the thin film is deposited at a rate between 0.05 and 1 Å/min. The process set forth in any one of Claim 1 to 11 wherein the thin film deposited is made of an compound oxide of Bi, Sr, Ca and Cu. The process set forth in Claim 12 wherein the compound oxide consists mainly of compound oxides represented by the general formula: Bi₄(Sr 1-x , Ca x ) m Cu n O p+y in which "m", "n", "x" and "y" are numbers each satisfying respective ranges of 6 ≦ m ≦ 10, 4 ≦ n ≦ 8, 0 < x < 1 and -2 ≦ y ≦ +2 and p = (6+m+n).

Inventors: FUJIMORI, NAOJI C/O ITAMI WORKS, HARADA, KEIZO C/O ITAMI WORKS, HIGAKI, KENJIRO C/O ITAMI WORKS, ITOZAKI, HIDEO C/O ITAMI WORKS, YAZU, SHUJI C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: H01B12
IPC codes: C23C14, C04B41, H01B13, C01G1, C01G29, H01L39