A superconducting thin film and a method for preparing the same

Abstract: An outer surface of a superconducting thin film of compound oxide such as YBa₂Cu₃O 7-δ is protected with a protective layer which is composed of any one of (i) oxide of metal such as SiO₂, TiO₂ (ii) carbide such as SiC, (iii) nitride such as BN or (iv) the same material as the substrate such as SrTiO₃.

Description: Background of the Invention Field of the invention The present invention relates to a superconducting thin film and a method for preparing the same. More particularly, it relates to a superconducting thin film of compound oxide having a high critical temperature and also possessing lasting stability for a long period and a method for preparing the same. Description of the related art The superconductivity is a phenomenon which is explained to be a phenomenon of a kind of phase change of electrons under which the electrical resistance becomes zero and the perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power. Therefore, if the superconducting power cable is realized, the power loss of the order of 7 % which is inevitable in the conventional power cables can be reduced greatly. Realization of superconducting coils for generating a very high magnetic field is expected to accelerate development in the field of fusion power generation in which the electric power is consumed beyond its output under the present technology, as well as in the field of MHD power generation or motor-generators. The development of superconductivity is demanded also in the other industrial fields such as in the field or electric power reservation; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; or in the field of science such as NMR or high-energy physics. In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconducting bodies. Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as high sensitive sensors or detectors 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 quantum phenomenon precisely. Development of the superconducting devices 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. However, the critical temperature could not exceed 23.2 K of Nb₃Ge which was the highest Tc for all studies for the past ten years. Possibility of existence of a new type 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) 189] It had been known that certain ceramics material 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 as the K₂NiF₄-type oxide having a crystal structure which is similar to known perovskite type oxides. The K₂NiF₄-type oxides show such higher Tc as 30 K which extremely higher than those of known superconducting materials. It was also reported in the news paper 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 in the order of 90 K in February 1987. Still another type new superconducting material is a compound oxide of Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system which exhibit such high Tc as more than 100 K and which are chemically much more stable than the abovementioned YBCO type compound oxide or the like. And hence, possibility of existence of high-temperature superconductors have burst on the scene. A thin film of superconducting compound oxide is prepared by sputtering technique. For example, in Japanese patent laid-open No. 56-109824 which discloses a method for preparing a thin film of superconductor represented by BaPb 1-x Bi x O₃ (in which 0.05 ≤ x ≤ 0.35), high-frequency sputtering is carried out in an oxygen containing atmosphere and then the resulting film is further heated at 500 to 550 °C. We also described sputtering conditions for preparing superconducting thin films of compound oxide by the sputtering technique in a co-pending United States of America patent serial No. 152,714 filed on May 2, 1988. The abovementioned conventional sputtering technique itself is satisfactory but is helpless against the change in properties during storage. In fact, the superconductivity or superconducting property of the abovementioned new type superconducting materials of compound oxide is influenced by the oxygen contents in the crystalline structure since the oxygen in the crystal is not stable and is apt to escape out of the crystal, resulting in that the superconductivity is lost or deteriorated with the passing of time. Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a superconducting thin film improved in stability during storage or use and a method for preparing the same. Summary of the Invention A superconducting thin film composed of compound oxide and deposited on a substrate according to the present invention is characterized in that the outer surface of the superconducting thin film is covered with a protective layer made of one of the following materials: (i) oxides of metals selected from a group comprising Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, and Pb, (ii) carbide of SiC or TiC, (iii) nitride of BN, AlN, SiN, or TiN, and (iv) the same material as said substrate, with the provision that the protective layer is not SiO₂, Si₃N₄, AlN or Yttrium oxide (or calcium oxide) stabilised zirconia when the compound oxide superconductor is of the YBaCuO type or MLaCuO type with M being Ba or Sr. The oxide of metal of (i) may be any oxide of an element selected from a group comprising a metal element of IIa group, a metal element of IIb group, a semi-metal element of IIIb group and a semi-metal element of IVb group in the periodic table. More particularly, it can be one of oxides of following elements: Mg, Ca, Sr, Ba (IIa group); Zn, Cd, Hg (IIb group); Al, Ga, In, Tl (IIIb group) and Si, Ge, Sn, Pb (IVb group). According to the present invention, the protective layer of the present invention may be composed of oxide of at least one of elements selected from a group comprising elements of IIa group, IIb group, IIIb group and IVb group, particularly, Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn and Pb. Since the oxides of these elements are homologue of or possess very similar properties to components of the superconductor, no undesirable influence is produced when they are deposited on the surface of the superconductor. Still more, the oxides of these elements contain stable oxygen which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film, so that the superconducting property of the thin film can be maintained. The carbide of (ii) may be SiC or TiC each of which may contain other elements such as hydrogen. The carbide is chemically stable and hard and a protective layer is produced having a high density which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film to maintain the superconducting property of the thin film. The nitride of (iii) may be BN, AlN, SiN or TiN. Since the nitride is chemically stable or inactive to the superconductor there is produced a protective layer having a high density which may advantageously suppress liberation of oxygen from the crystalline superconducting thin film, the superconducting property of the thin film can be maintained for a long time. In case of (iv), the protective layer may be composed of the same material as the substrate which is made of a single crystal of MgO or SrTiO₃. The protective layer is not SiO₂, Si₃N₄, AlN or Yttrium oxide (or calcium oxide) stabilized zirconia when the compound oxide superconductor is of the YBaCuO type or MLaCuO type with M being Ba or Sr. When this material is deposited on the superconducting thin film, the deposited film grows epitaxially so that the crystalline axis of the resulting protective layer can be oriented to a predetermined direction. This means that the resulting protective film has a very high density and can prevent liberation of oxygen out of the superconducting thin film. Of course, this protective layer is inactive to the superconductor. Selection of the abovementioned protective layers depend on type or system of the compound oxide used. The superconducting thin film can be composed of compound oxide of an element α selected from IIa group of the Periodic Table, an element β selected from IIIa group of the Periodic Table and at least one element γ selected from a group comprising Ib, IIb, IIIb, IVb and VIIIa group of the Periodic Table. The element γ is copper in general. Particularly, the superconducting thin film is preferably a compound oxide represented by the general formula: (α 1-x β x )Cu y O z in which α and β means the same definition as above, x is an atomic ratio of β with respect to (α+β) and has a range of 0.1≤x≤0.9, and y and z are respective atomic ratios with respect to (α 1-x β x ) which is considered to be equal to 1 and each satisfy ranges of 0.4 ≤y≤3.0 and 1≤z≤5 respectively. According to a preferred embodiment of the present invention, the element α is Ba or Sr and the element β is at least one element selected from a group comprising Y, La, Gd, Dy, Ho, Er, Tm, Yb, Nd, Sm, Eu and Lu. From 10 to 80 % of the element α may be substituted by one or more than one of elements selected from IIa group of the Periodic Table. The element β may be a combination of more than two elements selected from IIIa group of the Periodic. A portion of these components can be substituted by at least one of elements selected from a group comprising Al, Fe, Ni, Zn, Ag and Ti. The atomic ratio of α to β depends on the type or system of the compound oxide or on the combination of α and β. Following atomic ratios are preferably used in case of Ba-Y system, Ba-La system and Sr-La system respectively: Y/(Y+Ba) 0.06 to 0.94, more preferably 0.1 to 0.4 Ba/(La+Ba) 0.04 to 0.96, more preferably 0.08 to 0.45 Sr/(La+Sr) 0.03 to 0.95, more preferably 0.05 to 0.1 Thus, the preferable compound oxides are Y-Ba-Cu-O system, La-Ba-Cu-O system and La-Sr-Cu-O system including the following special cases: Y₁Ba₂Cu₃O 7-x , Ho₁Ba₂CU₃O 7-x , Lu₁Ba₂Cu₃O 7-x , Sm₁Ba₂Cu₃O 7-x , Nd₁Ba₂Cu₃O 7-x , Gd₁Ba₂Cu₃O 7-x , Eu₁Ba₂Cu₃O 7-x , Er₁Ba₂Cu₃O 7-x , Dy₁Ba₂Cu₃O 7-x , Tm₁Ba₂Cu₃O 7-x , Yb₁Ba₂Cu₃O 7-x , La₁Ba₂Cu₃O 7-x , (La, Sr)₂CuO 4-x in which x is a number which satisfies a range of 0 < x < 1. The abovementioned oxides possess preferably perovskite type or quasi-perovskite type crystal structure. The term of quasi-perovskite type means a structure which can be considered to have such a crystal structure that is similar to Perovskite-type oxides and includes an orthorhombically distorted perovskite or a distorted oxygen-deficient perovskite or the like. The superconducting thin film may be also another type of superconductor consisting mainly of a compound oxide represented by the formula: Θ₄(Φ 1-q ,Ca q ) m Cu n O p+r in which Θ stands for Bi or Tl, Φ stands for Sr when Θ is Bi and stands for Ba when Θ is Tl, m and n are numbers each satisfying ranges of 6 ≤ m ≤ 10 and 4 ≤ n ≤ 8 respectively, p = (6+m+n), q is a number which satisfies a range of 0 < q < 1, and r is a number which satisfies a range of -2 ≤ r ≤ +2. This system is considered to be a single phase of the following compound oxide or a mixed phase consisting mainly thereof: Bi₄Sr₄Ca₄Cu₆O 20-r , Bi₂Sr₂Ca₂Cu₃O 10-r , Tl₄Ba₄Ca₄Cu₆O 20-r , Tl₂Ba₂Ca₂Cu₃O 10-r , in which r is a number which satisfies a range of -2 ≤ r ≤ +2. The substrate may be made of glass, quartz, silicon, sapphire, stainless steel or other ceramics. Particularly, the substrate consists preferably of a single crystal of MgO or SrTiO₃. Desirably, the superconducting thin film is deposited on a (001) plane or (110) plane of a single crystal of MgO or SrTiO₃. The present invention relates also to a method for preparing a superconducting thin film on a substrate. The method is characterized in that the outer surface of the superconducting thin film is covered with a protective layer made of the materials disclosed herein above. The thin film of superconductor can be prepared by the conventional physical vapour deposition technique, such as sputtering, vacuum deposition, ion plating, molecular beam epitaxial growth or the like. It is also possible to use chemical deposition technique (CVD) such as plasma CVD or photo CVD. Generally, the thin film of compound oxide is prepared by sputtering technique. According to the present invention, more than two protective layers can be formed successively on the superconducting thin film layer. Each protective layer may be different in type and composition from adjacent layers. According to a preferred embodiment of the present invention, the superconducting thin film is deposited on a substrate by sputtering technique and then a protective layer is deposited on a surface of the superconducting thin film in a common sputtering unit. Namely, the sputtering is preferably carried out by such a manner that, at first, both targets for the superconducting thin film and for the protective layer are set on respective target holders in a chamber, and then the target for the superconducting thin film is sputtered with argon gas or the like while the target for the protective layer is not sputtered. After a superconducting thin film is deposited on a substrate, the target for the protective layer is sputtered to deposit a protective layer on the superconducting thin film. Such superconducting thin film can be prepared by sputtering technique described in our co-pending United States of America patent serial No. 152,714 filed on May 2, 1988. The superconducting property can be improved by heat-treatment which is effected after the thin film of compound oxide is deposited on the substrate in oxygen containing atmosphere. The heat-treatment is preferably effected under a partial pressure of oxygen ranging from 0.01 to 15.2 MPa (0.1 to 150 atm) at a temperature between 300 and 1,500 °C. After this temperature is maintained for more than one hour, the resulting thin film is cooled slowly at a rate of less than 100°C/min. Advantage of the heat-treatment cannot be obtained if the heat-treatment is effected outside the abovementioned conditions. For example, if the thin film is heated at a temperature which is higher than 1,500 °C, the abovementioned advantage cannot be obtained but the superconductivity will be disappeared. In case of the compound oxide type superconductor, oxygen deficiency in its crystal is a critical factor for realizing the superconductivity. Particularly, a heat-treatment under a relatively higher partial pressure of oxygen is very preferable and is considered to be indispensable to realize superior superconductivity. However, it is impossible to prevent the thin film of compound oxide from deterioration which occur during storage even if the thin film is heat-treated completely. The deterioration of superconductivity, in other words a phenomenon that the superconducting property is lost gradually in time may be caused by disappearance or liveration of oxygen in the crystal because the superconductivity of compound oxide is observed in unstable or quasi-stable phase. This is a big problem of compound oxide type superconductors in their actual use. This problem is solved by the present invention in which the outer surface of the thin film of superconductor is covered by a stable and hard protective layer which has a high density to prevent oxygen from escaping from its crystal structure. In a stage of deposition of the protective layer on the superconducting thin film, the substrate on which the thin film is deposited should not be heated at a temperature which is higher than 500 °C. In fact, if the substrate is heated at a temperature which is higher than 500 °C, the superconducting property is damaged because oxygen in its crystal is apt to be lost. The thin film of compound oxide is preferably deposited on a (001) plane or (110) plane of a single crystal of MgO or SrTiO₃. In fact, the thin film of compound oxide possesses anisotropy in its crystal structure, so that a current passing along the direction which is in parallel with a plane defined by a-axis and b-axis of the crystal show a very high critical current density but the other current passing along the other directions is relatively lower. Therefore, when the c-axis is orientated to a direction which is perpendicular to the plane defined by a-axis and b-axis of the crystal, a current which has much higher critical current density (Jc) can be delivered along a direction which is in parallel with the plane. According to a preferred embodiment of the present invention, the superconducting thin film composed of a single crystal or polycrystals of compound oxide whose c-axis is orientated to a direction which is perpendicular to the plane defined by by a-axis and b-axis of the crystal is deposited on a (100) plane of a substrate of single crystal of MgO or SrTiO₃ which has such lattice constant or lattice constants in a-axis and/or b-axis that are proximate to those of the thin film such as Ln₁Ba₂Cu₃O₇ in order to realize a high critical current density (Jc) along the direction which is in parallel with a surface of the substrate. In case of (iv) where the protective layer is composed of the same material as the substrate which consist of a single crystal of MgO or SrTiO₃, the deposited protective layer grows epitaxially so that the crystalline axis of the resulting protective layer is oriented to a predetermined direction, resulting in that protective film has a very high density and can prevent escape of oxygen out of the superconducting thin film. It is apparent from the description abovementioned that the superconducting thin film of compound oxide according to the present invention have improved stability compared with conventional superconducting thin film and hence they can be utilized advantageously in a applications of thin film devices, such as Matisoo switching elements or Josephson device, Anacker memory device or Superconducting Quantum Interference Device (SQUID) Now, the present invention will be described with reference to attached drawings which illustrate an apparatus used for carrying out the present invention. But, the present invention is not limited thereto. Brief Description of the drawing FIG. 1 is an illustrative view of a magnetron sputtering unit which is used in the present invention. Referring to FIG. 1 illustrating a magnetron sputtering unit which is used in the present invention, the sputtering unit includes, in a chamber 1, two target holders for a superconducting thin film 2 and for a protective layer 3, a pair of magnetron electrodes 4 each surrounding respective targets 2 and 3, high-frequency power sources 5 for actuating the magnetron electrodes 5 and a substrate holder on which a substrate is secured. The chamber 1 has an exhaust port 8 which is connected to a vacuum source (not shown) and a gas inlet port 7 for introducing an atmosphere gas. The substrate holder is provided with a heater 9 for controlling a temperature of the substrate 6. Such sputtering unit itself is known and used in a variety of applications. In operation, after a substrate 6 is secured to the substrate holder and targets 2 and 3 are set on respective target holder in the chamber 1, vacuum is created in the chamber 1 and a suitable gas such as argon and/or oxygen at a predetermined pressure is introduced into the chamber 1. After the substrate 6 is heated to a predetermined temperature by the heater 9, high-frequency power is applied to one of the magnetron electrode 4 surrounding the target 2 for the superconducting thin film to start sputtering operation. Introduction of the atmosphere gas in the chamber 1 is controlled at a predetermined pressure during the sputtering operation. The deposition of the superconducting thin film of compound oxide is carried out by the conventional sputtering technique. After the thin film of compound oxide is deposited, another the magnetron electrode 4 surrounding the target 3 for the protective layer is energized so that a thin film of the protective layer is deposited on the superconducting thin film. Now, several embodiments of the method according to the present invention will be described by Examples, but the scope of the present invention should not be limited thereto. In the following Examples, both of the superconducting thin film and the protective layer are prepared in the sputtering unit shown in Fig. 1. For comparison, two series of samples are prepared so that the first series possesses the protective layer while second series do not possess the same. Example 1 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of h-BN are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0x10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻² Torr) are introduced and the substrate is heated at 620 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec.) After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that h-BN is sputtered to deposit a protective layer of 200nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450 °C. Measurement of critical temperature Resistance of the resulting thin film is measured on such a sample that has vacuum-deposited aluminum electrodes at opposite sides of the thin film of compound oxide deposited on the substrate 6. Measurement of the critical temperature Tc and Tcf was carried out by a conventional four probe method in which the sample was immersed in liquidized helium to cool the sample down to a temperature of 8 K in a cryostat. Then, the temperature dependence of resistance of the sample was determined with rising the temperature gradually to determine a temperature of Tcf at which the perfect superconductivity start to be lost and resistance begins to appear and a temperature of Tc at which the superconductivity is lost and ordinary resistance begins to appear. Changes of Tcf and Tc are determined by comparing two values observed on same sample just after the protective layer is deposited and after one month. The result as well as main operational parameters are shown in Table 1. Example 2 The same procedure as Example 1 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. The result as well as main operational parameters are shown in Table 1. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 1 1.35 Pa (1.0×10⁻²) 620 BN 91 78 90 78 - 89 77 36 - 2 0.945 Pa (7.0×10⁻³) 680 BN 58 47 59 48 - 57 48 8 - (Note) -- : Superconductivity is not observed in liquid helium Example 3 The same procedure as Example 1 is repeated except that the target of h-BN is replaced by a target of TiN. The result as well as main operational parameters are shown in Table 2. Example 4 The same procedure as Example 2 is repeated except that the target of h-BN is replaced by a target of TiN. The result as well as main operational parameters are shown in Table 2. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 3 1.08 Pa (8.0×10⁻³) 730 TiN 96 85 95 86 - 98 88 33 - 4 0.675 Pa (5.0×10⁻³) 690 TiN 51 38 53 37 - 49 37 17 - (Note) -- : Superconductivity is not observed in liquid helium Example 5 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of SiC are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻²Torr) are introduced and the substrate is heated at 630 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 0.1 MPa (1 atm) is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that SiC is sputtered to deposit a protective layer of 200nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450 °C. Measurement of critical temperature Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 3. Example 6 The same procedure as Example 5 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. The result as well as main operational parameters are shown in Table 3. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 5 1.35Pa (1.0×10⁻²) 630 SiC 86 72 87 71 - 85 72 33 - 6 0.945Pa (7.0×10⁻³) 670 SiC 61 47 60 48 - 62 50 - - (Note) -- : Superconductivity is not observed in liquid helium Example 7 The same procedure as Example 5 is repeated except that the target of SiC is replaced by a target of TiC. The result as well as main operational parameters are shown in Table 4. Example 8 The same procedure as Example 6 is repeated except that the target of SiC is replaced by a target of TiC. The result as well as main operational parameters are shown in Table 4. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 7 1.080 Pa (8.0×10⁻³) 710 TiC 93 82 94 81 - 91 80 31 - 8 0.675 Pa (5.0×10⁻³) 685 TiC 53 39 51 38 - 52 38 - - (Note) -- : Superconductivity is not observed in liquid helium Example 9 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a powder target 3 composed of MgO powder are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The substrate of MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻² Torr) are introduced and the substrate is heated at 650 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 0.1M Pa (1 atm) is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that MgO is sputtered to deposit a protective layer of 250 nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450 °C. Measurement of critical temperature Measurement of the critical temperature Tc and Tcf was carried out by the same method as Example 1. The result as well as main operational parameters are shown in Table 5. Example 10 The same procedure as Example 9 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. The result as well as main operational parameters are shown in Table 5. No. Partial pressure of O₂ (Torr) Pa Tempature of Substrate (°C) Protective Layer Jc (A/cm²) Just after Deposition After 1 month Tc (K) Tcf (K) Tc (K) Tcf (K) 9 1.35 Pa (1.0×10⁻²) 650 MgO 3×10⁶ 79 67 77 68 - 1×10⁵ 78 68 11 - 10 0.945 Pa (7.0×10⁻³) 620 MgO 5×10⁵ 56 39 56 38 - 8×10³ 55 40 - - (Note) -- : Superconductivity is not observed in liquid helium Example 11 The same procedure as Example 9 is repeated except that the powder target of MgO is replaced by a target of SrTiO₃ block. The result as well as main operational parameters are shown in Table 6. Example 12 The same procedure as Example 10 is repeated except that the substrate of MgO is replaced by a substrate composed of a single crystal of SrTiO₃ and that the target of powder MgO is also replaced by a target of SrTiO₃. The thin film of compound oxide is deposited on (100) plane of the substrate of SrTiO₃ block. The result as well as main operational parameters are shown in Table 6. No. Partial pressure of O₂(Torr) Pa Tempature of Substrate (°C) Protective Layer Jc (A/cm²) Just after Deposition After 1 month Tc (K) Tcf (K) Tc (K) Tcf (K) 11 1.08 Pa (8.0×10⁻³) 700 SrTiO₃ 2×10⁶ 89 76 88 77 - 9×10⁴ 90 78 25 - 12 0.675 Pa (5.0×10⁻³) 640 SrTiO₃ 7×10⁵ 49 34 48 35 - 3×10⁴ 48 26 - - (Note) -- : Superconductivity is not observed in liquid helium Example 13 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of MgO are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0x10⁻² Torr) are mtroduced and the substrate is heated at 600 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 1 atom is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that MgO is sputtered to deposit a protective layer of 200 nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450 °C. Measurement of critical temperature Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 7. Example 14 The same procedure as Example 13 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. The result as well as main operational parameters are shown in Table 7. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 13 1.35 Pa (1.0×10⁻²) 600 MgO 89 71 90 72 - 77 68 39 - 14 0.945 Pa (7.0×10⁻³) 700 MgO 54 36 55 37 - 55 34 18 - (Note) -- : Superconductivity is not observed in liquid helium Example 15 The same procedure as Example 13 is repeated except that the target of MgO is replaced by a target of SiO₂. The result as well as main operational parameters are shown in Table 8. Example 16 The same procedure as Example 14 is repeated except that the target of MgO is replaced by a target of SiO₂. The result as well as main operational parameters are shown in Table 8. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 15 1.08 Pa (8.0×10⁻³) 650 SiO₂ 95 77 94 78 - 92 75 34 - 16 0.675 Pa (5.0×10⁻³) 680 SiO₂ 47 35 48 37 - 46 36 20 - (Note) -- : Superconductivity is not observed in liquid helium Example 17 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of TiO₂ are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻² Torr) are introduced and the substrate is heated at 600 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 0.1M Pa (1 atm) is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that TiO₂ is sputtered to deposit a protective layer of 200 nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 450 °C. Measurement of critical temperature Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 9. Example 18 The same procedure as Example 17 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. The result as well as main operational parameters are shown in Table 9. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 17 1.35 Pa (1.0×10⁻²) 600 TiO₂ 88 72 89 71 - 79 67 41 - 18 0.945 Pa (7.0×10⁻³) 700 TiO₂ 57 39 56 41 - 58 36 21 - (Note) -- : Superconductivity is not observed in liquid helium Example 19 The same procedure as Example 17 is repeated except that the target of TiO₂ is replaced by a target of ZrO₂. The result as well as main operational parameters are shown in Table 10. Example 20 The same procedure as Example 18 is repeated except that the target of TiO₂ is replaced by a target of ZrO₂. The result as well as main operational parameters are shown in Table 10. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 19 1.08 Pa (8.0×10⁻³) 650 ZrO₂ 93 77 95 76 - 91 76 38 - 20 0.675 Pa (5.0×10⁻³) 680 ZrO₂ 48 36 49 37 - 47 34 21 - (Note) -- : Superconductivity is not observed in liquid helium Example 21 Preparation of a target for compound oxide At first, powders of Y₂O₃ and BaCO₃ were mixed at an atomic ratio of Y : Ba = 1 : 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 950 °C to obtain a sintered block of YBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of Y₂O₃ are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻² Torr) are introduced and the substrate is heated at 680 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 0.1 M Pa (1 atm) is introduced in the chamber 1 and the temperature of the substrate is adjusted at 650 °C. The deposited thin film is left under this condition for 15 hour and then cooled slowly at a cooling rate of 7 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that Y₂O₃ is sputtered to deposit a protective layer of 200 nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 430 °C. Measurement of critical temperature Measurement of the critical temperature Tc and Tcf was carried by the same method as Example 1. The result as well as main operational parameters are shown in Table 11. Example 22 The same procedure as Example 21 is repeated except that a sintered block of LaBa₂Cu₃O₇ is used as a target for a thin film of compound oxide which is prepared by following procedure. Preparation of a target for compound oxide Powders of La₂O₃ and BaCO₃ were mixed at an atomic ratio of La : Ba = 1: 2 and then a powder of CuO was added to the resulting powder mixture at 10 % excessive proportion with respect to an atomic ratio of Y : Ba : Cu = 1 : 2 : 3. Then, the resulting powder mixture was sintered at 970 °C to obtain a sintered block of LaBa₂Cu₃O₇ which is used as a target 2 for a superconducting thin film. Deposition of thin films The resulting target 2 and a target 3 composed of La₂O₃ are set on respective target holder and a substrate 6 consisting of a single crystal of MgO is secured on the substrate holder. The MgO crystal is set in such manner that its surface on which the thin film is deposited has a (001) plane. After the chamber 1 is vacuumed, argon gas at a partial pressure of 6.75 Pa (5.0 x 10⁻² Torr) and oxygen gas at a partial pressure of 1.35 Pa (1.0 x 10⁻² Torr) are introduced and the substrate is heated at 680 °C. Then, the magnetron electrode 4 for the target 2 is energized with high frequency of 3 W/cm² to prepare a thin film of compound oxide of 1 µm on the substrate at a film forming rate of 0.05 nm/sec (0.50 Å/sec). After deposition of the thin film of compound oxide complete, oxygen gas of 0.1 M Pa (1 atm) is introduced in the chamber 1 and the temperature of the substrate is adjusted at 730 °C. The deposited thin film is left under this condition for 20 hour and then cooled slowly at a cooling rate of 8 °C/min. Then, another magnetron electrode for the protective layer 3 is energized so that La₂O₃ is sputtered to deposit a protective layer of 200 nm (2,000 Å) on the thin film of compound oxide which is prepared in the abovementioned step. During the deposition of the protective layer, the temperature of the substrate is maintained at 380 °C. The result as well as main operational parameters are shown in Table 11. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 21 1.35 Pa (1.0×10⁻²) 680 Y₂O₃ 88 72 89 71 - 79 67 41 - 22 0.945 Pa (7.0×10⁻³) 710 La₂O₃ 57 39 56 41 - 58 36 11 - (Note) -- : Superconductivity is not observed in liquid helium Example 23 The same procedure as Example 21 is repeated except that the target of Y₂O₃ is replaced by a target of BaTiO₃. The result as well as main operational parameters are shown in Table 12. Example 24 The same procedure as Example 22 is repeated except that the target of Y₂O₃ is replaced by a target of BaTiO₃. The result as well as main operational parameters are shown in Table 12. No. Partial pressure of O₂ Pa (Torr) Tempature of Substrate (°C) Protective Layer Just after deposition One month later Tc (K) Tcf (K) Tc (K) Tcf (K) 23 1.08 Pa (8.0×10⁻³) 640 BaTiO₃ 93 77 95 76 - 91 76 38 - 24 0.675 Pa (5.0×10⁻³) 670 BaTiO₃ 48 36 49 37 - 47 34 - - (Note) -- : Superconductivity is not observed in liquid helium

Claims: A thin film of compound oxide superconductor having a crystal structure of perovskite and deposited on a substrate made of glass, quartz, silicon, sapphire or other ceramics, characterized in that an outer surface of said thin film is covered with a protective layer made of one of following materials: (i) oxides of one of metals selected from a group comprising Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn and Pb. (ii) carbide of SiC or TiC. (iii) nitride of BN, AlN, SiN or TiN, and (iv) the same material as said substrate, with the provision that the protective layer is not SiO₂, Si₃N₄, AlN or Yttrium oxide (or calcium oxide) stabilised zirconia when the compound oxide superconductor is of the YBaCuO type or MLaCuO type with M being Ba or Sr. The thin film set forth in claim 1 wherein said superconductor is a compound oxide represented by the formula: (α 1-x ,β x ) Cu y O z in which α is IIa group element and β is IIIa group element, x is an atomic ratio of β with respect to (α+β) and has a range of 0.1≤x≤0.9, and y and z are respective atomic ratios with respect to (α 1-x β x ) which is considered to be equal to 1 and each satisfy ranges of 0.4 ≤y≤3.0 and 1 ≤z≤5 respectively. The thin film set forth in claim 2 wherein said said element α is Ba and said element β is at least one element selected from a group comprising Y, La, Gd, Dy, Ho, Er, Tm, Yb, Nd, Sm, Eu and Lu. The thin film set forth in claim 2 wherein said element α is Sr and said element β is La. A thin film of a compound oxide superconductor represented by the formula : Θ₄(Φ 1-q ,Ca q ) m Cu n O p+r in which Θ stands for Bi or Tl, Φ stands for Sr when Θ is Bi and stands for Ba when Θ is Tl, m and n are numbers each satisfying ranges of 6 ≤ m ≤ 10 and 4≤n≤8 respectively, p = (6+m+n), q is a number which satisfies a range of 0<q<1, and r is a number which satisfies a range of -2≤r≤+2 said film having a crystal structure of perovskite and being deposited on a substrate the outer surface of said thin film being covered with a protective layer made of one of following materials: (i) oxides of metals selected from a group comprising Mg, Ca, Sr, Ba, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn and Pb, (ii) carbide of SiC or TiC, (iii) nitride of BN, AIN, SiN or TiN, and (iv) the same material as said substrate. The thin film set forth in any one of claims 1 to 5 wherein said substrate is a single crystal of MgO or SrTiO₃. The thin film set forth in claim 6 wherein said thin film is deposited on a (001) plane or (110) plane of a single crystal of MgO or SrTiO₃.

Inventors: FUJITA NOBUHIKO, ITOZAKI HIDEO, JODAI TETSUJI, TANAKA SABURO, YAZU SHUJI, FUJITA, NOBUHIKO, ITOZAKI, HIDEO, JODAI, TETSUJI, TANAKA, SABURO, YAZU, SHUJI, FUJITA, NOBUHIKO C/O ITAMI WORKS, ITOZAKI, HIDEO C/O ITAMI WORKS, JODAI, TETSUJI C/O ITAMI WORKS, TANAKA, SABURO C/O ITAMI WORKS, YAZU, SHUJI C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: H01L39
IPC codes: C01B13, C01G1, C23C16, H01B3, C01G3, H01L39, H01B12, C04B41, H01B13, C23C14