Process for producing a superconducting thin film

Abstract: Improvement in a superconducting thin film of compound oxide represented by the formula : LnBa₂Cu₃O 7-δ (Ln is lanthanide) or (La 1-x α x )₂CUO₄ (α is Ba or Sr) deposited on a substrate or core made of MgO, SrTiO₃ or ZrO₂ by physical vapour deposition technique. The present invention is characterized by the surface roughness R max (datum length = 1,000µm) of the superconducting thin film being is less than 0.2 µm

Description: Background of the Invention Field of the invention The present invention relates to a process for producing a superconducting thin film. More particularly, it relates to a process for producing a superconducting thin film having improved high current density of superconductivity. The superconducting thin film produced according to the present invention possesses improved surface flatness as well as high critical temperature and high current density and is advantageously applicable in a variety of electronics devices including the integrated circuits. Description of the related art The superconductivity is a phenomenon which is explained to be a kind of phase changes of electrons under which the electric resistance become zero and the perfect diamagnetism is observed. In the field of electronics, a variety of superconducting devices are known. A typical application of the superconducting device 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 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 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 superconducting wire is demanded in the field of electromagnetic coils or a variety of electric parts as well as in the field of electric power transportation. For example, in the high-speed computers, the superconducting wires is demanded as a wiring material for IC packaging or the like for delivering current without any loss of power. However, the critical temperature "Tc" of superconductivity 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 a new type of superconducting material having much higher Tc was revealed by Bednorz and Müller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64 (1986) 189). 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 30K 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 90K in February 1987. And hence, the possibility of an existence of high-temperature superconductors have burst onto the scene. The "Physical Review" B, vol. 35, n°16 1st June 1987, pages 8821-8823 discloses epitaxial growth of Y₁Ba₂Cu₃O₄ and La 1.8 Sr 0.2 CuO₄ film on MgO substrate. It is said that the superconducting properties of the above-mentioned new type compound oxide superconductors are influenced by the oxygen deficiency in the crystal. In fact, if the oxygen deficiency do not exist in the crystal, Tc can not be higher and a discrepancy between the on-set temperature and a temperature where perfect zero resistance is observed become big. In Appl. Phys. Lett. 51(11), 14 September 1987, pages 858-860, the article "Preparation of superconducting YBa₂Cu₃O x thin films by oxygen annealing of multilayer metal layer" of B.Y. Tsaur et al. mentions a thin film (350nm) superconductor with a surface roughness of 20-30nm. Generally, the thin films composed of the above-mentioned new type superconducting materials are prepared by physical vapour deposition technique. However, the critical current density "Jc" observed in the superconducting thin film obtained is very low even if the thin film possesses a high critical temperature "Tc". Such low critical current density is a problem when the thin films are utilized as superconductors in practical uses. The superconducting wires composed of the compound oxides are generally produced by a wire-drawing technique in which sintered powder material of the compound oxide is compacted in a metallic sheath which is then drawn. In the superconducting wires produced by the conventional technique, however, it is difficult to improve the critical current density even if the superconducting wire shows a high critical temperature Tc. In fact, since the compound oxide is an anisotropic crystal in which electric current flow mainly along its c-axis, the maximum current density of the conventional superconducting wire which is produced by the conventional technique in which powders of such compound oxide is compacted at random is limited to a value which is insufficient to use the wire in practical uses. In the conventional sputtering technique for preparing a thin film composed of the ceramics type oxide superconductors disclosed in FR-A- 2 469 005, sputtering is effected in oxygen-containing atmosphere while the substrate is heated in order to improve ordering or orientation of the crystal and the resulting film deposited on the substrate by the sputtering stage is further heat-treated in order to increase the oxygen-content in the thin film. This patent teaches that the superconducting thin film which is represented by the formula BaPb 1-x Bi x O₃ (in which 0.05 ≦ x ≦ 0.35) and which is prepared in an oxygen-containing atmosphere by high-frequency sputtering technique should be further heat-treated at 500 to 550 °C but mentions nothing about the conditions how to prepare the thin film of high-Tc superconductor which was discovered after the filing date of this patent. The present applicant already proposed several processes for preparing the thin films of the high-Tc superconductor in the following patent applications: EP-A- 0 277 885, EP-A- 0 284 489, EP-A- 0 292 382, EP-A- 0 292 387, EP-A- 0 294 285. Although the processes disclosed in these patent applications are themselves useful and satisfactory, it is still requested to improve further the critical temperature and particularly the critical current density. The present invention is an extension of and an improvement in these patent applications. Therefore, an object of the present invention is to provide an improved thin film which is composed of compound oxide type superconducting material and which has a high critical density of superconductivity in addition to the high critical temperature and a process for producing the same. Another object of the present invention is to solve the abovementioned problem and to provide an improved compound oxide type superconducting wire having higher current density. Summary of the Invention The subject matter of the present invention is a process for preparing a thin film of oxide superconductor on a heated substrate by RF- sputtering according to claim 1. The expression of "the substantial portion " mean that the majority, for example more than 80 % of the surface of the superconducting thin film is smooth even if voids or defects which are inevitably produced when a thin film is deposited on a large area by the physical vapour deposition technique are observed in the remaining minor portion of the surface. The smoothness of the superconducting thin film is evaluated by such a fact that the surface roughness R max (datum length = 1,000µm) of the superconducting thin film composed mainly of compound oxide is less than 0.2 µm. The value of the surface roughness R max is determined by observing the thin film obtained microscopically, particularly by SEM. Experiments which were conducted by the present inventors revealed such a fact that the critical current density "Jc" observed on the thin film obtained decrease sharply if the surface roughness R max is not less than 0.2 µm. The superconducting thin film produced according to the present invention is composed mainly of compound oxide represented by the following general formula (1) and (2): (1) Ln₁Ba₂Cu₃O 7-δ in which "Ln" stands for at least one lanthanide element selected from a group comprising La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Yb, Tm and δ is a number which satisfies a range of 0 ≦ δ < 1. The "Ln" is preferably Y, Er, Ho or Dy. The compound oxide is considered to be consisted mainly of perovskite type of quasi-perovskite type oxide. An atomic ratio of the lanthanide element "Ln" : Ba : Cu is preferably 1 : 2: 3 as is defined by the formula but the atomic ratio is not restricted strictly to this ratio. In fact, the other compositions whose atomic ratio deviate from the above-mentioned value by 50 %, more preferably by 20 % can be included in the scope of the present invention. The expression of "composed mainly of" defined in Claims means that compound oxides whose atomic ratio of Ln : Ba : Cu is not equal to 1 : 2: 3 are also included in the scope of the present invention. This definition means also that the compound oxide can contain impurities which exist inevitably at the concentration of ppm order as well as another component which is added intentionally in order to improve the other properties of the thin film obtained. As the components added intentionally, it can be mentioned Sr, Ca, Mg or Be of the IIa group of the periodic table and an element which is selected from elements of IIIa, Ib, IIb, IIIb, IVa and VIIIa groups of the periodic table such as Ti, V but which is not the elements contained in the formula. (2) (La 1-x α x )₂CuO₄ in which α stands for Ba or Sr and 0<x<1. The compound oxide is considered to be consisted mainly of perovskite type or quasi-perovskite type oxide. It is preferable that an atomic ratio of La : Ba (or Sr) : Cu satisfies the value defined by the formula but the atomic ratio is not restricted strictly to the ratio. In fact, it is often observed that the other compositions whose atomic ratio deviate from the above-mentioned ratio by 50 %, more preferably by 20 % also show effective superconducting property. Therefore, the expression of "composed mainly of" defined in Claims means that compound oxides whose atomic ratio are not equal to the value defined by the formula are also included in the scope of the present invention. This definition means also that the compound oxide can contain impurities which exist inevitably at the concentration of ppm order as well as another component which is added intentionally in order to improve the other properties of the thin film obtained. As the components which can be added intentionally, it may be mentioned Sr, Ca, Mg or Be of IIa group of the periodic table and an element selected from elements of IIIa, Ib, IIb, IIIb, IVa and VIIIa groups of the periodic table such as Ti, V which are not the elements contained in the formula. The substrate on which the superconducting thin film of compound oxide is deposited can be a substrate made of perovskite type crystal or oxide, or a substrate made of metal or semiconductor on which a buffer layer composed of perovskite type crystal or oxide is deposited. The substrate is preferably made of single crystal of MgO, SrTiO₃, ZrO₂, YSZ or Al₂O₃, polycrystal of Al₂O₃ or made of metal or semiconductor having a buffer layer made of these materials and having a deposition surface on which the superconducting thin film is deposited. The thin film is preferably deposited on a {001} plane or {110} plane of the substrate made of a single crystal of MgO or SrTiO₃. The processes for producing a superconducting composite according to the present invention include a physcial vapour deposition stage for depositing a superconducting thin film composed mainly of compound oxide on a substrate or on a core, and are characterized in that the physical vapour deposition is carried out under such an operational condition that the substantial portion of a surface of the superconducting thin film obtained finally has a surface roughness R max of less than 0.05 µm (datum length = 1000 µm) by controlling at least one of the parameters selected from a group comprising (1) a deposition rate, (2) atmosphere gas pressure, (3) oxygen gas contents and (4) high-frequency power. The conditions of (1) to (4) are described hereafter in more details. The superconducting thin films composed of compound oxides obtained by the processes according to the present invention are thin films composed mainly of the following compound oxides (1) and (2) : Ln₁Ba₂Cu₃O 7-δ (1) (La 1-x α x )₂CuO₄ (2) The details of these compound oxides are already described in the first and second subject matters of the present invention and hence are not repeated here. At first, we will describe the physical vapour deposition technique on which the processes according to the present invention based. According to the present invention, sputtering technique, more preferably RF (radio frequency) sputtering technique which itself is well-known is used in the physical vapour deposition. In the physical vapour deposition technique, an atomic ratio of metal elements contained in a vapour source is adjusted according to difference in the evaporation rate as well as in the deposition possibility of each metal element. The vapour source is preferably constructed of a sintered mass which is prepared by powder sintering technique of material power mixture comprising metal elements and/or their oxides or carbonates and whose atomic ratio of the metal elements is adjusted according to respective vaporization rates of the metal elements, or is constructed of a sintered powder which is obtained by pulverizing the sintered mass. The vapour source may consist of a plurality of segments each selected from powders of constituent metal elements, oxide or carbonate such as powders of Y₂O₃, CuO and BaCuO₂. For example, the vapour source can consist of two segments of elemental Cu powder and a compound oxide of Ba and Y. The substrate on which the superconducting thin film of compound oxide is deposited is preferably a single crystal whose lattice distance is nearly same as the lattice distance of the crystal of the compound oxide such as a single crystal of MgO, SrTiO₃ or ZrO₂. The deposition surface is preferably {001} plane or {110} plane of the single crystal of MgO or SrTiO₃. According to a preferred embodiment, the substrate or the core is heated during the sputtering stage. The heating temperature for the substrate or the core is 200 to 950 °C, preferably 500 to 920 °C. If the temperature of the substrate or core is not higher than 200 °C, the deposited compound oxide do not possess proper crystalline structure but become glass and hence do not show superconducting property. To the contrary, if the temperature of the substrate or core exceed 950 °C, another crystalline structure which does not show superconducting property is produced in the deposited compound oxide. Now, we will describe the above-mentioned sputtering conditions (1) to (4). These conditions (1) to (4) are determined from different points of view on the present invention and have respective ranges defined hereafter. In the present invention, each of the conditions can be selected solely or more than two conditions selected from these conditions can be combined with each other. (1) Deposition rate The deposition rate which is defined according to the first aspect of the present invention must be within a range of 0.005 to 0.1 nm (0.05 to 1Å/sec), preferably within a range of 0.01 to 0.08 nm/s (0.1 to 0.8Å/sec). Experiments which were conducted by the present inventors revealed such a fact that, if the deposition rate during the physical vapour deposition exceed 1Å/sec, the critical current density observec in the superconducting thin film obtained deteriorate sharply, so that such superconducting thin film can not be used in practical uses. To the contrary, if the deposition rate during the physical vapour deposition is not higher than 0.005nm/s (0.05Å/sec), the deposition speed become too much slow to perform the deposition in the industrial scale. (2) Atmosphere gas pressure The atmosphere gas pressure which is defined according to the second aspect of the present invention must be within a range of 0.1 to 66 Pa (0.001 to 0.5 Torr), preferably within a range of 1 to 44 Pa (0.01 to 0.3 Torr). (3) Oxygen gas contents In the present invention, a mixed gas consisting of inert gas and oxygen is selected as the atmosphere gas in the sputtering stage. The proportion of oxygen or the oxygen contents in the mixed gas which is defined according to the third aspect of the present invention must be within a range of 5 to 95 %, preferably within a range of 10 to 80 %. The inert gas or sputtering gas which can be mixed with oxygen is preferably argon. (4) High-frequency power In the present invention, the sputtering is preferably performed by RF (radio frequency) magnetron sputtering technique. The high-frequency power applied during the sputtering stage which is defined according to the forth aspect of the present invention must be within a range of 5 to 200 W with respect to a target having a diameter of 10 cm, that is 0.064 to 2.55 W/cm². When the oxygen contents in the sputtering gas is low, the high-frequency power is also reduced relatively to a range of 5 to 100 W with respect to a target having a diameter of 10 cm, that is 0.064 to 1.27 W/cm². To the contrary, if the oxygen contents in the sputtering gas is higher than 30 to 95 % by molecule, particularly 40 to 80 % by molecule, the high-frequency power is preferably increased to a range of 1.27 to 2.55 W/cm². According to the processes of the present invention, the sputtering is carried out under such a condition that the the substantial portion of a surface of said superconducting thin film becomes smooth. The smoothness of the superconducting thin film obtained is evaluated by such a fact that the surface roughness R max (datum length = 1,000 µm) of the superconducting thin film composed mainly of compound oxide is less than 0.05 µm. The value of the surface roughness R max is determined by observing the thin film obtained microscopically, particularly by SEM. According to a preferred embodiment of the processes of the present invention, after deposition of the thin film complete, the resulting thin film is further heat-treated or annealed in an oxygen-containing atmosphere and then is cooled slowly. The heat-treatment is preferably carried out at a temperature ranging 800 to 960 °C and the cooling rate of the thin film is preferably less than 10 °C/min. The heat-treatment is preferably carried out at a partial oxygen pressure ranging 10⁴ to 10⁶Pa (0.1 to 10 atm). The purpose of the heat-treatment is to adjust the oxygen deficiency in the compound oxide. In fact, the thin film which is not subjected to the heat-treated often shows poor superconducting property and in the worst case does not exhibit superconducting property. Therefore, the heat-treatment is preferable. The superconducting thin film of the above-mentioned type compound oxides shows anisotropy in the critical current density. Namely, the current passing along a 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, while the current passing along the other directions is relatively lower. Therefore, in order to accord the crystalline orientation of the superconducting thin film with that of the substrate, it is a general practice to deposit the superconducting thin film on the specified plane of a single crystal such as MgO, SrTiO₃, YSZ, ZrO₂ or the like whose lattice distance is nearly same as the lattice distance of the compound oxide crystal. However, the maximum critical current density "Jc" of the conventional processes have been limited to a rather low value of 1 x 10⁵ A/cm² because the orientation of crystal could not ordered satisfactorily. The advantage of the processes according to the present invention reside in that the critical current density "Jc" of the superconducting thin film can be increased at one effort to the level of 1 x 10⁶ A/cm² which is one order higher than the conventional value. Such great improvement in the critical current density result from such a fact that the surface roughness of the superconducting thin film of compound oxide is improved by adjusting at least one of the conditions (1) to (4) to the optimum value. Although the theoretical reason why such important improvement in the current density "Jc" can be achieved by the improvement in the surface roughness is not clear under the present situation, we think that the improvement in the current density "Jc" may come from such a reason that the direction of crystalline orientation of c-axis in the superconducting thin film prepared by the present invention is ordered perpendicularly or nearly perpendicularly with respect to a surface of the substrate or the core. Therefore, it is preferable to deposit the superconducting thin film on {001} plane of a single crystal of MgO or SrTiO₃. It is also possible to use {110} plane. In this case, the c-axis become in parallel with a surface of a substrate, so that current is flown perpendicularly with respect to the c-axis in the practical uses. Still more, since the thermal expansion coefficient of MgO or SrTiO₃ is similar to that of the superconductor of compound oxide, the thin film is not subjected to undesirable stress caused by difference in the thermal expansion coefficient which will be exerted during the heating and cooling stages. In conclusion, the superconducting thin film obtained by the process according to the present invention exhibit very higher critical current density than those that are prepared by the conventional processes. Now, several embodiments of the process for producing the superconducting composites according to the present invention will be described by Examples, but the scope of the present invention should not be limited thereto. The experiments shown in the following Examples were conducted for the purpose of determining the optimum ranges for each of the conditions (1) to (4) with respect to Ln₁Ba₂Cu₃O 7-δ type superconductors and (La 1-x α x )₂CuO₄ type superconductors. In the Examples, the critical temperature of superconductivity (Tc) is determined by the conventional four probe method. The critical current density (Jc) is determined at 77.0 K and is expressed by A/cm². Namely, electric resistance is measured with increasing the amount of current and a value of the current intensity where resistance detected at first is divided by a cross sectional area through which current pass. The value of the surface roughness R max is determined by taking a photo of the thin film deposited by SEM (Scanning Electron Microscopy). Ln₁Ba₂Cu₃O 7-δ type superconducting thin films Example 1 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are compound oxides of Ln-Ba-Cu-O ceramics each having an atomic ratio of Ln : Ba : Cu = 1 : 2.24 : 4.35. "Ln" stands for lanthanide elements shown in Table 1. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under same operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Atmosphere gas: O₂/(O₂+Ar) = 20 Atmosphere gas pressure: (0.1 Torr) 13.3 Pa Substrate temperature: 700 °C High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate = (0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated at 900 °C for three hours and then cooled to ambient temperature at a cooling rate of 5 °C/min. The critical temperature and the critical current density observed on the resulting thin films are summarized in Table 1. As comparative examples, sputtering is carried out under the same conditions as above except that the high-frequency power is increase to 150 W (1.9 W/cm²). The results are also shown in Table 1. It is apparent from the results shown in Table 1, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. From such a fact that we can not find any unevenness on the surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications, while there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples, we can estimate that the structure of the superconducting thin films of compound oxides obtained by the process according to the present invention is uniform and compacted Example 2 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are ceramics of compound oxides of Ln-Ba-Cu-O which are prepared by usual sintering technique in which material powders whose atomic ratio of Ln : Ba : Cu is adjusted to 1: 2.24 : 4.35 is sintered. "Ln" stands for lanthanide elements shown in Table 2. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 700 °C Atmosphere gas: O₂/(O₂+Ar) =20 Atmosphere gas pressure: (0.1 Torr) 13.3 Pa High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate =(0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated in O₂ of atmospheric pressure at 900 °C for one hour and then cooled to ambient temperature at a cooling rate of 5 °C/min. The critical temperature and the critical current density observed on the resulting thin films are summarized in Table 2. As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increase to 0.15 nm/s (1.5 Å/sec). The results are also shown in Table 2. It is apparent from the results shown in Table 2, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. From such a fact that we can not find any unevenness on the surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications, while there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples, we can estimate that the structure of the superconducting thin films of compound oxides obtained by the process according to the present invention is uniform and compacted Example 3 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are ceramics of compound oxides of Ln-Ba-Cu-O which are prepared by usual sintering technique in which material powders whose atomic ratio of Ln : Ba : Cu is adjusted to 1: 2.24 : 4.35 is sintered. "Ln" stands for lanthanide elements shown in Table 3. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 700 °C Atmosphere gas pressure: (0.1 Torr) 13.3 Pa Sputtering gas: O₂(20 %)/Ar(80%) High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm Deposition rate: ( 0.35 Å/sec) 0.035 nm/s Annealing: 900 °C/3 hours (cooled at a cooling rate of 5 °C/min) The critical temperature, the critical current density and the surface roughness observed on the resulting thin films are summarized in Table 3. As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increase to 1.5 Å/sec (0.15 nm/s ). The results are also shown in Table 3. It is apparent from the results shown in Table 3, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. In the case of the present invention, although voids of several microns are observed in a very small portion (about 1 % of the whole surface) of surfaces of the thin films prepared by the process according to the present invention, we can not find unevenness on the majority surface of the thin films when the surfaces are observed by SEM at 10,000 magnifications. To the contrary, in the case of the thin films of compound oxide obtained by the comparative examples, we find innumerable grains of several microns on the surfaces. Example 4 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are ceramics of compound oxides of Ln-Ba-Cu-O which are prepared by usual sintering technique in which material powders whose atomic ratio of Ln : Ba : Cu is adjusted to 1 : 2.24 : 4.35 is sintered. "Ln" stands for lanthanide elements shown in Table 4. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 700 °C Atmosphere gas: O₂/(O₂+Ar) = 50 Atmosphere gas pressure: (0.1 Torr) 13.3 Pa High-frequency power: 150 W (1.9 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate =(0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated in O₂ of atmospheric pressure at 900 °C for one hour and then cooled to ambient temperature at a cooling rate of 5 °C/min. The critical temperature and the critical current density observed on the resulting thin films are summarized in Table 4. As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increase to 0.15 nm/s (1.5 Å/sec). The results are also shown in Table 4. It is apparent from the results shown in Table 4, the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. From such a fact that we can not find any unevenness on the surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications, while there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples, we can estimate that the structure of the superconducting thin films of compound oxides obtained by the process according to the present invention is uniform and compacted Example 5 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are ceramics of compound oxides of Ln-Ba-Cu-O which are prepared by usual sintering technique in which material powders whose atomic ratio of Ln : Ba : Cu is adjusted to 1 : 2.24 : 4.35 is sintered. "Ln" stands for lanthanide elements shown in Table 5. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 690 °C High-frequency power: 100 W (1.27 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm Deposition rate (0.35 Å/sec) 0.035 nm/s Atmosphere gas pressure: (0.15 Torr) 20 Pa Atmosphere gas: O₂/Ar (20/80) After the deposition complete, the resulting thin film deposited on the substrate is heated in O₂ of atmospheric pressure at 910 °C for three hours and then cooled to ambient temperature at a cooling rate of 5 °C/min. The critical temperature and the critical current density observed on the resulting thin films are summarized in Table 5. As comparative examples, sputtering is carried out under the same conditions as above except that the atmosphere gas pressure is changed to 0.1 Pa to 93 Pa (0.0008 Torr and 0.7 Torr) and the high-frequency power is increased to 150 W (1.0 W/cm²). The results are also shown in Table 5. It is apparent from the results shown in Table 5, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. We can not find unevenness on about 98 % of the whole surfaces of the thin fims of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications. To the contrary, there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples. Example 6 Superconducting wires are produced by RF magnetron sputtering technique. A core used is a rod of MgO having a length of 150 mm and a diameter of 1 mm which is manufactured by molten drawing technique. The rod is rotated about its own axis in front of a substrate holder in a chamber of a RF magnetron sputtering machine during the sputtering operation. Targets used in the Examples are ceramics of compound oxides of Ln-Ba-Cu-O which are prepared by usual sintering technique in which material powders whose atomic ratio of Ln : Ba : Cu is adjusted to 1 : 2.24 : 4.35 is sintered. "Ln" stands for lanthanide elements shown in Table 6. Each targets has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Heating temperature of the core: 700 °C Sputtering pressure: (0.1 Torr) 13.3 Pa Oxygen contents: O₂(20 %)/Ar(80%) High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Deposition rate: (0.30 Å/sec) 0.030 nm/s Annealing: 900 °C/3 hours (cooled at a cooling rate of 5 °C/min) The critical temperature, the critical current density and the surface roughness observed on the resulting thin films are summarized in Table 6. In the case of the present invention, although voids of several microns are observed in a very small portion (about 1 % of the whole surface) on surfaces of the thin films prepared, we can not find unevenness on the majority surface of the thin films when the surfaces are observed by SEM at 10,000 magnifications. To the contrary, in the case of the thin films of compound oxide obtained by the comparative examples, we find innumerable grains of several microns on the surfaces. (La 1-x α x )₂CuO₄ type superconductors Example 7 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are sintered blocks of compound oxides whose atomic ratio of La : Ba (or Sr) : Cu is adjusted to 1.8 : 0.2 : 1. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the following operational conditions: Substrate: MgO {001} plane Atmosphere gas: O₂/(O₂+Ar) = 20 Atmosphere gas pressure: (0.1 Torr) 13.3 Pa Substrate temperature: 700 °C High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate ( 0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated in air at 900 °C for three hours and then cooled to ambient temperature at a cooling rate of 5 °C/min. As comparative examples, sputtering is carried out under the same conditions as above except that the high-frequency power is increased to 150 W (1.9 W/cm²). The results are also shown in Table 7. Example 8 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are sintered blocks of compound oxides whose atomic ratios of La : Ba (or Sr) : Cu are adjusted to 1.8 : 0.2 : 1. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the following operational conditions: Substrate: MgO {001} plane Atmosphere gas: O2/(O₂+Ar) = 20 Atmosphere gas pressure: (0.1 Torr) 13.3 Pa Substrate temperature: 700 °C High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate (0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated in air at 900 °C for one hour and then cooled to ambient temperature at a cooling rate of 5 °C/min. As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increased to (1.5 Å/sec) 0.15 nm/s. The results are also shown in Table 8. Example 9 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are sintered blocks of compound oxides of an element α (Ba or Sr), La and Cu which are prepared by usual sintering technique in which material powders whose atomic ratio of La : α : Cu is adjusted to 1.8 : 0.2 : 1 is sintered. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 700 °C Atmosphere gas pressure: (0.1 Torr) 13.3 Pa Sputtering gas: O₂ (20 %)/Ar (80 %) High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm Deposition rate (0.35 Å/sec) 0.035 nm/s Annealing 900 °C/3 hours (Cooled at a cooling rate of 5 °C/min) As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increased to (1.5 Å/sec) 0.15 nm/s. The results are also shown in Table 9. It is apparent from the results shown in Table 9, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. Although voids of several microns are observed at a very small portion (about 1 % of the whole surface) of surfaces of the thin films prepared by the present invention, we can not find unevenness on the majority surface of the thin films when the surfaces are observed by SEM at 10,000 magnifications. To the contrary, in the case of the thin films of compound oxide obtained by the comparative examples, we find innumerable grains of several microns on the surfaces. Example 10 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are sintered blocks of compound oxides whose atomic ratios of La : Ba (or Sr) : Cu are adjusted to 1.8 : 0.2 : 1. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the following operational conditions: Substrate: MgO {001} plane Atmosphere gas: O₂/(O₂+Ar) = 50 Substrate temperature: 700 °C Atmosphere gas pressure: (0.1 Torr) 13.3 Pa High-frequency power: 150 W (1.9 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm (Deposition rate (0.35 Å/sec) 0.035 nm/s) After the deposition complete, the resulting thin film deposited on the substrate is heated in air at 900 °C for one hour and then cooled to ambient temperature at a cooling rate of 5 °C/min. As comparative examples, sputtering is carried out under the same conditions as above except that the deposition rate is increased to (1.5 Å/sec) 0.15 nm/s. The results are also shown in Table 10. From such a fact that we can not find any unevenness on the surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications, while there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples, we can estimate that the structure of the superconducting thin films of compound oxides obtained by the process according to the present invention is uniform and compacted. Example 11 Superconducting thin films are prepared by RF magnetron sputtering technique. Targets used in the Examples are sintered blocks of compound oxides of an element α (Ba or Sr), La and Cu which are prepared by usual sintering technique in which material powders whose atomic ratio of La : α : Cu is adjusted to 1.8 : 0.2 : 1 is sintered. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under the identical operational conditions for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 690 °C High-frequency power: 100 W (1.27 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.88 µm Deposition rate (0.35 Å/sec) 0.035 nm/s Atmosphere gas pressure: (0.15 Torr) 20 Pa Atmosphere gas: O₂/Ar (20.: 80) After the deposition complete, the resulting thin film deposited on the substrate is heated in air at 910 °C for three hours and then cooled to ambient temperature at a cooling rate of 5 °C/min. The results are also shown in Table 11. As comparative examples, sputtering is carried out under the same conditions as above except that the atmosphere gas pressure is changed to 0.1 Pa to 93 Pa (0.0008 Torr and 0.7 Torr). The results are also shown in Table 11. We can not find unevenness on about 98 % of the whole surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications. To the contrary, there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples. Ln₁Ba₂Cu₃O 7-δ type superconducting thin films Example 12 Superconducting thin films are prepared by RF sputtering technique. Targets used in the Examples are compound oxides of Ln-Ba-Cu-O ceramics each having an atomic ratio of Ln : Ba : Cu = 1 : 2.24 : 4.35. "Ln" stands for lanthanide elements shown in Table 12. Each target has a disk-shape having a diameter of 100 mm. The sputtering is carried out under an identical operational condition for all samples. The operational conditions are as following: Substrate: MgO {001} plane Substrate temperature: 700 °C Atmosphere gas pressure: (0.01 to 0.1 Torr) 1.3 Pa to 13.3 Pa High-frequency power: 40 W (0.51 W/cm²) Sputtering time: 6 hours Thickness of the thin film: 0.8 µm After the deposition complete, the resulting thin film deposited on the substrate is heated at 900 °C for one hour and then cooled to ambient temperature at a cooling rate of 5 °C/min. As comparative examples, a superconducting thin film of compound oxide containing Ho is prepared under the same conditions as above except that the high-frequency power is increase to 150 W (1.9 W/cm²). The results are also shown in Table 12. It is apparent from the results shown in Table 12, that the superconducting thin films prepared in the condition defined by the present invention exhibit much higher critical current density than the comparative examples. From such a fact that we can not find any unevenness on the surfaces of the thin films of compound oxides obtained by the present invention when the surfaces are observed by SEM at 10,000 magnifications, while there exist grains of several microns on the surfaces of the thin films obtained by the comparative examples, we can estimate that the structure of the superconducting thin films of compound oxides obtained by the process according to the present invention is uniform and compacted.

Claims: A process for preparing a thin film of oxide superconductor on a heated substrate by RF-sputtering, characterized in that in order to obtain a thin film of oxide superconductor having a Jc greater than 1.10⁵ A/cm², said sputtering is carried out at a deposition rate of 0.05 to 1 Å/sec with controlling gas pressure, oxygen gas content and high-frequency power in such a manner that the substantial portion of a surface of said thin film obtained finally has a surface roughness R max (datum lenght=1,000µm) of less than 0.05µm. The process set forth in claim 1 wherein said gas pressure is in a range from 0.1 to 66 Pa (0.001 to 0.5 Torr), preferably from 1 to 44 Pa (0.01 to 0.3 Torr). The process set forth in any one of claim 1 or 2 wherein said sputtering is carried out in a mixed gas of inert gas and oxygen, a proportion of oxygen in said mixed gas being within a range of 5 to 95 vol %, preferably 10 to 80 vol %. The process set forth in any one of claim 1 to 3 wherein said high-frequency power is in a range from 0.064 to 2.55 W/cm², preferably from 0.064 to 1.27 W/cm². The process set forth in any one of claim 1 to 4 wherein said substrate is heated to a temperature ranging from 200°C to 950°C, preferably from 500 to 920°C. The process set forth in any one of claim 1 to 5 wherein, after the thin film is deposited, the resulting thin film is further heat-treated in an oxygen-containing atmosphere. The process set forth in claim 5 wherein said heat-treatment is effected at a temperature ranging 800 to 960°C. The process set forth in claim 6 or 7 wherein said heat-treatment is carried out at a partial oxygen pressure ranging (0.1 to 10 atm) 10⁴ to 10⁶ Pa. The process set forth in any one of claim 6 to 8 wherein the heat-treated thin film is cooled slowly at a cooling rate of less than 10°C/min. The process set forth in any one of claim 1 to 9 wherein said compound oxide is represented by the general formula : Ln₁Ba₂Cu₃O 7-δ in which Ln stands for at least one lanthanide element selected from a group comprising La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Yb, Tm and Lu and δ is a number which satisfies a range of 0≦δ<1. The process set forth in any one of claim 1 to 9 wherein said compound oxide is represented by the general formula : (La 1-x α x )₂CUO₄ in which α stands for Ba or Sr. The process set forth in any one of claim 1 to 11 wherein said substrate is made of a single crystal of oxide whose lattice constants are nearly same as those of said compound oxide. The process set forth in claim 12 wherein said substrate is made of MgO, SrTiO₃ or ZrO₂. The process set forth in any one of claim 1 to 13 wherein said superconducting thin film is deposited on (001) plane or (110) plane of a substrate made of a single crystal of MgO or SrTiO₃.

Inventors: HIGAKI KENJIRO 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, HIGAKI, KENJIRO 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: H01L39