Method of forming weak-link Josephson junction, and superconducting device employing the junction

Abstract: To form a weak-link Josephson junction of the type employing a thin film of an oxide superconductor, a crystal grain boundary produced reflecting an artificial crystal defect is utilized as the weak-link junction. The crystal grain boundary (45) is formed by a method in which atoms of different species are deposited on a predetermined part (42) of the surface of a substrate (40) the predetermined part of the surface of a substrate (40) is disturbed, or parts of different crystal face orientations are formed at the surface of a substrate (40). Then a superconducting thin film (44) is epitaxially grown on the substrate (43). Alternatively the predetermined part of the superconducting thin film (44) epitaxially grown on a substrate (43) is diffused with atoms of different species hampering a superconducting or the predetermined part of the superconductivity thin film (44) is disturbed, the superconducting thin film (44) is then annealed, Also. a superconducting device employing the weak-link junction is disclosed.

Description: The present invention relates to a method of forming a Josephson junction particularly a weak-link junction, of oxide superconductor, and also to a superconducting device employing the junction. Owing to the advent of oxide superconductors the critical temperatures (Tc) of which exceed the boiling point of liquid nitrogen (77 K), new superconducting devices are expected. However, when a device, especially an electronics device, is to be formed using the oxide superconductor, several problems are left unsolved. One of them is the formation of a Josephson junction. In a conventional Josephson junction device employing a metal superconductor, there has been formed a tunnel junction in which a very thin insulating layer is interposed between thin superconductor films. The main reasons for the realization of this junction are that, in the case of the metal superconductor, an insulating film (of, for example, SiO₂) which does not react with the thin superconductor films and which exhibits an excellent insulation even with a very small thickness can be formed, and that the metal superconductor has as great a coherent length as several nm (tens of Å). Superconducting switching devices, etc. have been developed by employing such tunnel junctions. With the high-temperature oxide superconductor, however, (1) since the coherent length thereof is as small as several tenths of nm (Å), the insulating layer must be made thinner than in the conventional case, and (2) in order to obtain thin superconductor films of high Tc, the films must be formed under the condition of a high substrate temperature or must be annealed at high temperatures. Besides, unlike the metal, the oxide superconductor is prone to react with insulating materials (in general, oxides) and is therefore very difficult of forming the tunneling type Josephson junction. For these reasons, Josephson junctions have heretofore been obtained only when a bar 10 of superconductor ceramics is pressed against the superconductor 11 as shown in Fig. 14, and a Josephson junction in the form of layered films has not been fabricated. As a Josephson junction fabricated of a thin film, there has been known a weak-link junction in which a superconducting link is weakened by thinning the film partly or by narrowing a portion for the flow of current. A practicable method of forming the weak link is one wherein, as shown in Fig. 15, a substrate 20 is provided with a step 21, and the thickness of the thin superconducting film 22 formed thereon is changed. Alternatively, there are carried out a method wherein hydrogen ions are implanted, thereby causing the superconducting current to flow through only the narrow region, a method wherein grain boundaries are utilized, and so forth. Examples of the method which utilizes the grain boundaries are described in Phys. Rev. Lett. 60 (1988) 1653 and the official gazette of Japanese Patent Application Laid-open No. 273782/1987. With these examples, however, it is expected that the grain boundaries of the polycrystalline superconducting thin-film will happen to come to a place where the junction is to be formed. Besides an inferior reproducibility, the examples have had the problem that the shape and position of the junction cannot be selected at will. Further, they have had the problem that, since the plurality of grain boundaries exist, a large number of junctions are formed in series in the direction of the current, resulting in an inferior I - V characteristic. In the technique of Japanese Patent Application Laid-open No. 273782/1987, epitaxial growth is applied for the purpose of reproducibly forming weak-link Josephson junctions of uniform characteristics at a predetermined position. According to the prior-art technique, a slit-like pattern is formed on a single-crystal substrate, and a thin superconducting film is epitaxially grown thereon. The thin superconducting film on the slit-like pattern is hindered in the epitaxial growth, to become polycrystalline at the beginning. The thickness of the film increases later, with the result that a single grain boundary is formed on the slit-like pattern. With the prior-art method, however, a photoresist process is required for forming the pattern, so that the contamination of the surface of the substrate, which is a factor for hampering the epitaxial growth, is liable to occur and degrades the reproducibility. Another problem has been that, since the pattern has its minimum width limited by microfabrication technology, the superconducting film is difficult of being thinned. The prior-art method of forming a weak-link Josephson junction by the use of a thin film of oxide superconductor is difficult of forming weak-link Josephson junctions of uniform characteristics at a predetermined position, and has been problematic in point of reliability. EP-A-325765, which is a document forming part of the state of the art only under Article 54(3) EPC, discloses a Josephson device in which a distorted or broken-lattice crystal region is formed in a crystal substrate, and an oxide superconductor film formed over the substrate. The superconductor film has a crystal boundary on the distorted or broken-lattice region. This forms a Josephson junction. The distorted or broken-lattice region is formed by implanting Au or Pb ions into the crystal substrate. According to a first aspect of the invention there is provided a method of forming a weak-link Josephson junction comprising the step of irradiating a substrate with Ga or Ti ions thereby to process said substrate so as either to deposit said Ga or Ti in the form of a line on a surface of said substrate or to damage the surface of the substrate in the form of a line by virtue of said Ga or Ti and the step of epitaxial growing a think film of an oxide superconductor on the processed substrate, thereby to form a crystal grain boundary on said line, said crystal grain boundary being used as a weak-link Josephson junction. According to a second aspect of the invention there is provided a method of forming a weak-link Josephson junction comprising the step of irradiating a think film of an oxide superconductor epitaxial gown on a substrate with Ga or Ti ions, for lowering a critical current density, thereby to process said think film so as either to diffuse said Ga or Ti in the form of a line within the thin oxide-superconductor film or to damage the oxide-superconductor thin film in the form of a line by virtue of said Ga or Ti and the step of annealing the processed oxide superconductor film, thereby to form a crystal grain boundary on said line, said crystal grain boundary being utilized as a weak-link Josephson junction. The concrete operation of this expedient will be described in conjunction with Figs.1(a) - 1(c). When the surface of a substrate 30 is irradiated with a focused ion beam of Ga or the like 31, the metal is deposited or a damaged layer 32 is formed down to a depth of about 10 - 20 nm (100 - 200 Å) below the surface of the substrate 30, in accordance with the magnitude of the acceleration voltage of the ion beam 31 (Fig. 1(a)). The width of the deposited region or damaged region can be changed by the way of focusing the ion beam 31, and can be made 1 µm or less. In addition, the thickness of the deposited matter is determined by an ion dose and beam width. When the beam 31 having a diameter of 1 µm is projected at a dose of 10¹⁶/cm², the deposited matter comes to have a thickness of 3.9 nm (39 Å). The substrate 30 thus processed is formed with a superconducting thin film 33 under the condition of epitaxial growth. Then, on regions not subjected to the deposition or damages, the superconducting thin film 33 is epitaxially grown, whereas on the region 34 processed by the ion beam 31, the epitaxial growth is hindered, and microcrystals 35 are produced (Fig. 1(b)). When the film 33 thickens, crystals which have grown from both the sides of the region 34 processed by the ion beam 31 collide against each other. Although the crystals on both the sides have the same crystal orientation, they involve a slight shift. Therefore, a crystal grain boundary 36 having almost no misfit is produced between the two crystals (Fig. 1(c)). Owing to the crystal grain boundary 36, the phase between both the sides of the grain boundary is held, a superconducting current across both the sides becomes low, and a weak link permitting the intrusion of a magnetic flux is formed. Another expedient is as follows: A superconducting thin film epitaxially grown on a substrate is diffused with atoms of different species which hinders a superconducting current from flowing, or it has the vicinity of its surface disturbed, and the resultant superconducting thin film is annealed, thereby to form a crystal grain boundary. An ion beam which is accelerated and focused is used as means for the diffusion or damaging. The concrete operation of this expedient will be described in conjunction with Figs. 2(a) and 2(b). When a superconducting thin film 131 epitaxially grown on a substrate 30 is irradiated with a focused ion beam of Ga or the like 132, the metal is deposited or a diffused region 133 is formed down to a depth of about 10 - 20 nm (100 - 200 Å) below the front surface of the superconducting thin film 131, in accordance with the magnitude of the acceleration voltage of the ion beam 132 (Fig. 2(a)). When the superconducting thin film 134 thus processed is annealed, recrystallization occurs to produce a crystal grain boundary 135 in the region subjected to the deposition or damaging (Fig. 2(b)). Since crystals on both the sides of the crystal grain boundary 135 have the same crystal orientation, almost no misfit is involved between the crystal grains of both the sides. Still another expedient is as follows: A part of different crystal plane orientation is artificially formed on a substrate, and superconducting thin films of different growth orientations are epitaxially grown on the substrate, whereby a crystal grain boundary which hinders a superconducting current from flowing is formed on the part of the substrate at which the crystal face orientations thereof have been changed. Oxide superconductor materials formulated by Ln₁Ba₂Cu₃O y (where Ln denotes a rare-earth metal such as Y, Er or Ho) have an orthorhombic perovskite structure, the a-axis, b-axis and c-axis of which have lengths of 0.383 nm, 0.389 nm and 1.17nm (3.83 Å, 3.89 Å and 11.7 Å), respectively. For enhancing the characteristics of the critical temperature (Tc), the critical current density (Jc), etc. of any of these oxide-superconductor materials of thin films, it is important to select the composition of the thin films and an appropriate substrate material. In particular, since the oxide-superconductor thin-film material undergoes a high temperature process of 600 °C - 950 °C in a film forming process, a material having a low reactivity with the thin films needs to be selected as the substrate material. Further, for forming thin film crystals of favorable superconductivity properties, a substrate having few crystal grain boundaries and facilitating epitaxial growth needs to be selected. The substrate materials which are desirable from these viewpoints are strontium titanate (SrTiO₃), magnesium oxide (MgO), zirconium dioxide (ZrO₂), etc. It has been revealed that, in cases where the thin films of the oxide superconductors are formed by vapor deposition or sputtering, the growth directions of thein film crystals differ depending upon the kinds of substrate materials and the crystal plane orientations of substrates. By way of example, in a case where the thin film of the oxide superconductor is formed on the substrate of MgO (100) or ZrO₂ (100), the lattice constants of the substrate and the a- and b-axes of the thin film mismatch greatly, and hence, the film whose c-plane, most stable in term of energy, lies in contact with the plane of the substrate, is formed. In contrast, in a case where the thin film of the oxide superconductor is formed on a substrate of SrTiO₃ (100), the lattice constant (0.390 nm (3.90 Å)) of SrTiO₃ is substantially equal to the lattice constant of the b-axis or the length of 1/3 of c-axis of the oxide-superconductor thin film, and hence, the oxide-superconductor thin film is epitaxially grown on the substrate into an a-axis oriented one. Likewise, on a substrate of SrTiO₃ (110), an oxide-superconductor thin film oriented in the direction (110) is formed. Further, it has been revealed that, when a crystal plane of substrate, for example, SrTiO₃ (100), SrTiO₃ 110) or SrTiO₃ (111) is set a slight angle  off as shown in Fig. 3, the crystal orientation of the thin film of the oxide superconductor to be formed on the substrate can be controlled. By way of example, it has been found out that, when an oxide-superconductor thin film is formed under the same conditions on a region 231 in which the surface of the SrTiO₃ (100) substrate 230 is exactly conformed to the (100) plane and a region 232 in which the front surface is set or shifted off the angle  from the (100) plane, it is produced into an a-axis oriented one on the region 231 exactly conformed to the (100) plane, whereas it is produced into a c-axis oriented one on the region 232 set off the angle . In this case, the crystal orientations of the thin films differ at the boundary part of the substrate 230 at which the crystal plane orientation thereof inclines slightly, so that a crystal grain boundary 233 is formed. Herein, the off angle  should desirably be 0.5 degree or more. Besides, similar effects are attained even when SrTiO₃ (110), SrTiO₃ (111), MgO and ZrO₂ substrates are employed. In the drawings: Figs. 1(a) - 1(c), Figs. 2(a) - 2(b), and Fig. 3 are views for explaining methods of forming a crystal grain boundary at a predetermined position, respectively, Figs. 4(a) - 4(c) are views showing the manufacturing process of each of Examples 1 - 9 in the present invention, Fig. 5 is a graph showing the relationship between a dose and a film thickness for fabricating a weak-link Josephson junction as has been obtained from Examples 1 - 5, Fig. 6 is a view showing the structure of a SQUID (superconducting quantum interference device) fabricated by Example 10 in the present invention, Figs.7(a) - 7(c) are views showing the manufacturing process of each of Examples 11 - 19 in the present invention, Fig. 8 is a graph showing the relationship between a dose and a film thickness for fabricating a weak-link Josephson junction as has been obtained from Examples 11 - 15, Fig. 9 is a view showing the structure of a SQUID fabricated by Example 20 in the present invention, Figs. 10(a) - 10(b) are views showing the manufacturing process of each of Examples 21 - 22 in the present invention, Fig. 11 is a view showing the structur of a SQUID fabricated by Example 23 in the present invention, Fig. 12 is a graph showing the I - V characteristic of a grain boundary produced in the present invention, Fig. 13 is a graph showing the variation of a voltage corresponding to a quantum flux as measured by the SQUID, Fig. 14 is a sectional view of a turnneling type Josephson junction, and Fig. 15 is a sectional view of a prior-art example of a weak-link Josephson junction formed by a thin film. Now, the present invention will be described in detail in conjunction with examples. Example 1: Example 1 of the present invention will be described with reference to Figs. 4(a) - 4(c). On a single-crystal substrate 40 of strontium titanate (SrTiO₃) of plane orientation (110) which had no distortion and which was mirror-polished, gallium (Ga) 41 was irradiated in the form of lines in directions [011] and [001] by a focused ion beam (FIB) method. The conditions of the projection were an acceleration voltage of 30 kV, a line width of 1 µm (measured by SIM: scanning ion microscopy), and a line length of 0.5 mm. The doses of the Ga were 5 x 10¹¹ - 5 x 10¹⁸ /cm², and were changed one order in succession. The eight lines 42 of the respective doses were formed at intervals of 3 mm. On the resultant substrate 43 subjected to the above processing, a thin film of ErBa₂Cu₃O 7-δ 44 was formed by sputtering. In order to achieve epitaxial growth, the temperature of the substrate 43 was held at 700 °C during the formation of the film 44. The thickness of the film 44 was 0.7 µm. In the film 44 formed by the sputtering, linear defects were noted in correspondence with the places irradiated with the Ga ions. Exactly, the linear defects were observed in correspondence with the five lines whose doses were 5 x 10¹⁴ /cm² or above. The film 44 was annealed in oxygen at 830 °C for 2 hours. The linear defects, namely, grain boundaries 45 became clear after the annealing. In order to examine the superconducting properties of the grain boundary 45, the parts of the superconducting film 44 at both the ends of the grain boundary 45 were removed to form the neck portion of this superconducting film, and the I - V characteristic of the grain boundary 45 was measured by a conventional four-probe method. As illustrated by a typical example in Fig. 12, the I - V curve exhibited the feature of a superconductivity. When current was caused to flow in a direction [110] a critical current density at 77 K in a region having no grain boundary was 2 x 10⁴ A/cm². As listed in Table 1, critical current densities across the grain boundaries 45 in the case of the film having the thickness of 0.7 µm lowered in a range from 50 % to 3 %. When currents were caused to flow in the direction [001], the absolute values of the currents decreased to about halves, but the influences of the grain boundaries 45 tended to be identical. In spite of the irradiation with the ions, the critical current density of each place where the grain boundary 45 was not observed had almost no difference from that of the region not irradiated. When the temperature dependency of the critical current density (Jc) was investigated, a relation of Jc ∝ {1 - (T/Tc)}¹˙⁶ was found. In addition, when the variation of a critical current versus a magnetic field was investigated, the cyclic dependency of the superconducting current upon the magnetic field was observed, and the fabrication of weak-link Josephson junctions was verified. Example 2: Using a substrate which was irradiated with ions under the same conditions as in Example 1, a thin film of ErBa₂Cu₃O 7-δ was formed thereon by sputtering. Conditions for the film formation were the same as in Example 1 except for a film thickness of 350 nm (3500 Å). In the film which was annealed in oxygen at 830 °C for 2 hours, grain boundaries were observed in correspondence with seven lines whose doses were 5 x 10¹² /cm² or above. As in the case of Example 1, the I - V characteristics of the grain boundaries were investigated by the conventional four-probe method. The film of Example 1 had a critical temperature (Tc) as high as 88 K, but the temperature Tc of the film being 350 nm (3500 Å) thick lowered to 83 K. This is because the film was intensely influenced by the diffusion of the impurity from the substrate. A critical current density at 77 K in a region having no grain boundary was 6 x 10⁴ A/cm². As listed in Table 1, critical current densities in the places of the grain boundaries lowered from 50 % to 1 %. In spite of the irradiation with the ions, the critical current density of the place where the grain boundary 45 was not observed had almost no difference from that of the non-irradiated region, as in Example 1. The reason why the critical current densities (Jc) were low at the grain boundaries whose ion doses were 5 x 10¹⁶ /cm² or above, is that these grain boundaries broadened because the film was thin. When the temperature dependency was investigated, the relation of Jc ∝{1 - (T/Tc)}¹˙⁶ was noted at the grain boundaries whose ion doses were 5 x 10¹⁵ /cm² or below. In addition, when the variation of the critical current versus the magnetic field was investigated, a cyclic curve was observed, and the fabrication of weak-link Josephson junctions was verified. Example 3: As in Examples 1 and 2, a thin film of ErBa₂Cu₃O 7-δ was formed to a thickness of 100 nm (1000 Å) on a substrate irradiated with ions and was annealed in oxygen. The critical temperature (Tc) of the film was 80 K. After the annealing, grain boundaries were observed in places corresponding to all lines. A critical current density at 77 K in a region having no grain boundary was 2 x 10³ A/cm², while critical current densities across the grain boundaries lowered in a range from 75 % to 0 % as listed in Table 1. Example 4: As in Example 1, a thin film of ErBa₂Cu₃O 7-δ was formed to a thickness of 1.5 µm on a substrate irradiated with ions and was annealed in oxygen. The critical temperature (Tc) of the film was 85 K. After the annealing, grain boundaries were observed in places corresponding to lines whose doses were 5 x 10¹⁶ /cm² - 5 x 10¹⁸ /cm². A critical current density at 77 K in a region having no grain boundary was 3 x 10⁴ A/cm², while critical current densities in the places of the grain boundaries lowered in a range from 60 % to 32 % as listed in Table 1. Example 5: The thickness of a thin film of ErBa₂Cu₃O 7-δ to be formed was set at 3 µm. In this case, a grain boundary was observed in only a place corresponding to a line which was irradiated at a dose of 5 x 10¹⁸ /cm². It was found that only the corresponding part could be utilized as a weak-link Josephson junction. When the results of the above examples 1 - 5 are summed up in the relationship between the film thickness and the irradiation dose, it has been revealed that a domain indicated by black dots in Fig. 5 is appropriate. Example 6: As the ion source of an FIB to be irradiated, there was used titanium (Ti) being the constituent element of a strontium titanate (SrTiO₃) substrate. The SrTiO₃ (110) substrate was irradiated with the Ti in the form of lines under the same conditions as in Example 1. A thin film of ErBa₂Cu₃O 7-δ which was 0.7 µm thick as in Example 1 was formed on the resultant substrate. Even when the ion source was altered from the Ga to the Ti, no difference was noted as to the appearance of grain boundaries. It was also found that the I - V characteristic and the value of Jc exhibited very high reproducibilities. Example 7: In the irradiation of Ga ions, the acceleration voltage thereof was lowered from 30 kV to 15 kV. Under the same conditions as in Example 1 except for the acceleration voltage, an SrTiO₃ (110) substrate was irradiated with the Ga ions in the form of lines. A thin film of ErBa₂Cu₃O 7-δ having a thickness of 0.35 µm was formed on the resultant substrate by sputtering. Grain boundaries after the annealing of the thin film were observed in places corresponding to the lines whose doses were 5 x 10¹⁴ /cm² or above. The reason why the irradiation was less effective for the formation of the grain boundaries than in the case of the acceleration voltage of 30 kV, will be that the substrate was damaged at a lower degree, to approximate the situation of the vapor deposition of Ga. Example 8: SrTiO₃ (100) was employed for a substrate, which was irradiated with Ga ions in the form of lines under the same conditions as in Example 1. A thin film of YBa₂Cu₃O 7-δ having a thickness of 0.7 µm was formed on the substrate subjected to the ion beam processing. The thin film was grown so that the a-axis [100] thereof might become perpendicular to the plane of the film. Although the c-axis and b-axis of the film mixed within the plane thereof, grain boundaries were clearly observed in correspondence with the linear parts irradiated with the Ga and formed under the same conditions of doses as in Example 1. This indicated that the grain boundaries were hardly affected by the orientation of the substrate and the material of the superconducting thin film. Example 9: Magnesium oxide (MgO) (100) was used for a substrate, which was irradiated with Ti ions at an acceleration voltage of 30 kV and at doses of 5 x 10¹¹ /cm² - 5 x 10¹⁸ /cm² as in Example 6. A thin film of YBa₂Cu₃O 7-δ having a thickness of 0.35 µm was formed on the resultant substrate. The thin film was such that the c-axis thereof became perpendicular to the plane of the substrate, and that the a-axis and b-axis thereof could not be distinguished. However, grain boundaries appeared in places corresponding to lines whose doses were 5 x 10¹² - 5 x 10¹⁸ /cm². The value of Jc in a region where no grain boundary was formed, was substantially stipulated by the film thickness and the dose irrespective of the kind of the substrate. Example 10: The crystal grain boundaries formed by the method indicated by the above examples 1 - 9 were utilized for the weak-link Josephson junctions, and a dcSQUID (direct-current superconducting quantum interference device) was fabricated. An SrTiO₃ (110) substrate 70 was irradiated with Ga ions in the form of two serial lines having a line width of 1 µm, a length of 0.5 mm and an interval of 0.8 mm, under the conditions of an acceleration voltage of 30 kV and a dose of 5 x 10¹⁵ /cm². An ErBa₂Cu₃O 7-δ thin film 71 having a thickness of 0.35 µm was formed on the resultant substrate under the same conditions as in Example 1. The thin film thus formed was processed by photolithography and chemical etching into the pattern of the SQUID which utilized two grain boundaries 72 as shown in Fig. 6. A magnetic flux was measured at 76 K by causing a bias current to flow through the SQUID. As a result, a voltage corresponding to a quantum flux was observed as shown in Fig. 13. It has accordingly been indicated that the weak-link junctions which utilize the grain boundaries formed by the present invention are applicable to superconducting devices. Example 11: Example 11 of the present invention will be described with reference to Figs. 7(a) - 7(c). On a single-crystal substrate 140 of strontium titanate (SrTiO₃) of plane orientation (110) which had no distortion and which was mirror-polished, a thin film of ErBa₂Cu₃O 7-δ 141 was formed by sputtering. In order to effect epitaxial growth, the temperature of the substrate 140 was held at 700 °C during the formation of the film 141. The thickness of the film 141 was 0.7 µm. A focused ion beam (FIB) of gallium (Ga) 142 was projected in the form of lines in directions [011] and [001] on the thin film 141. The conditions of the irradiation were an acceleration voltage of 30 kV, a line width of 1 µm (measured by SIM: scanning ion microscopy), and a line length of 0.5 mm. The doses of the Ga were 1 x 10¹² - 1 x 10¹⁹ /cm², and were changed one order in succession. The eight lines 143 of the respective doses were formed at intervals of 3 mm. The resultant thin film 144 subjected to the processing was annealed in oxygen at 830 °C for 2 hours. After the annealing, the grain boundaries of defects appeared in the places bombarded with the ions. Exactly, the linear defects were observed in the five lines whose doses were 1 x 10¹⁵ /cm² or above. The linear defects, namely, grain boundaries 145 of the film 144 became clear. In order to examine the superconducting properties of the grain boundary 145, the parts of the superconducting film 144 at both the ends of the grain boundary 145 were removed to form the neck portion of this superconducting film, and the I - V characteristic of the grain boundary 145 was measured by the conventional four-probe method. As illustrated by a typical example in Fig. 12, the I - V curve exhibited the feature of a superconductivity. A critical current density in a direction [110], at 77 K in a region having no grain boundary was 2 x 10⁵ A/cm². As listed in Table 2, critical current densities across the grain boundaries 145 in the case of the film thickness of 0.7 µm lowered in a range from 52 % to 11 %. The absolute values of the currents in the direction [001] decreased to about halves, but the influences of the grain boundaries 145 tended to be identical. In spite of the irradiation with the ions, the critical current density of each place where the grain boundary 145 was not observed had almost no difference from that of the region not irradiated. When the temperature dependency of the critical current density (Jc) was investigated, the relation of Jc ∝ {1 - (T/Tc)}¹˙⁶ was found. In addition, when the variation of a critical current versus a magnetic field was investigated, the cyclic dependency of the superconducting current upon the magnetic field was observed, and the fabrication of weak-link Josephson junctions was verified. Example 12: A thin film of ErBa₂Cu₃O 7-δ having a thickness of 0.35 µm was formed by sputtering under the same conditions for the film formation as in Example 11, and was irradiated with ions under the same conditions as in Example 11. Thereafter, the thin film was annealed in oxygen at 830 °C for 2 hours. In the thin film, grain boundaries were observed in correspondence with seven lines whose doses were 1 x 10¹³ /cm² or above. As in the case of Example 11, the I - V characteristics of the grain boundaries were investigated by the conventional four-probe method. The film of Example 11 had as high a critical temperature (Tc) as 88 K, but the temperature Tc of the film being 0.35 µm thick lowered to 83 K. This is because the film was intensely influenced by the diffusion of the impurity from a substrate. A critical current density at 77 K in a region having no grain boundary was 6 x 10⁴ A/cm². As listed in Table 2, critical current densities in the places of the grain boundaries lowered in a range from 50 % to 1 %. In spite of the irradiation with the ions, the critical current density of the place where the grain boundary 145 was not observed had almost no difference from that of the non-irradiated region, as in Example 11. The reason why the critical current densities (Jc) were low at the grain boundaries whose ion doses were 1 x 10¹⁷ /cm² or above, is that these grain boundaries broadened because the film was thin. When the temperature dependency was investigated, the relation of Jc ∝ {1 - (T/Tc)}¹˙⁶ was noted at the grain boundaries whose ion doses were 1 x 10¹⁶ /cm² or below. In addition, when the variation of the critical current versus the magnetic field was investigated, a cyclic curve was observed, and the fabrication of weak-link Josephson junctions was verified. Example 13: As in Example 11, a thin film of ErBa₂Cu₃O 7-δ was formed to a thickness of 100 nm (1000 Å), and it was irradiated with ions and then annealed in oxygen. The critical temperature (Tc) of the film was 80 K. After the annealing, grain boundaries were observed in the places of all lines. A critical current density at 77 K in a region having no grain boundary was 2 x 10³ A/cm², while critical current densities across the grain boundaries lowered in a range from 39 % to 0 % as listed in Table 2. Example 14: A thin film of ErBa₂Cu₃O 7-δ with 1.5 µm thick was formed on a substrate, and it was irradiated with ions and then annealed in oxygen as in Example 11. The critical temperature (Tc) of the film was 85 K. After the annealing, grain boundaries were observed in places corresponding to lines whose doses were 1 x 10¹⁷ /cm² - 1 x 10¹⁸ /cm². A critical current density at 77 K in a region having no grain boundary was 3 x 10⁵ A/cm², while critical current densities across the grain boundaries lowered in a range from 64 % to 35 %. Example 15: The thickness of a thin film of ErBa₂Cu₃O 7-δ to be formed was set at 7 µm. In this case, a grain boundary was observed in only a place corresponding to a line which was irradiated at a dose of 1 x 10¹⁹ /cm². It was found that only the corresponding part could be utilized as a weak-link Josephson junction. When the results of the above examples 11 - 15 are summed up in the relationship between the film thickness and the irradiation dose, it has been revealed that a domain indicated by black dots in Fig. 8 is appropriate. Example 16: As the ion source of an FIB to be irradiated, there was used titanium (Ti) being the constituent element of a strontium titanate (SrTiO₃) substrate. A thin film of ErBa₂Cu₃O 7-δ having a thickness of 0.7 µm as in Example 11 was formed on the substrate, and was irradiated with the Ti in the form of lines under the same conditions as in Example 11. Even when the ion source was altered from the Ga to the Ti, no difference was noted as to the appearance of grain boundaries. It was also found that the I - V characteristic and the value of Jc exhibited very high reproducibilities. Example 17: In the irradiation of Ga ions, the acceleration voltage thereof was lowered from 30 kV to 15 kV. Under the same conditions as in Example 11 except for the acceleration voltage, an ErBa₂Cu₃O 7-δ thin film 0.35 µm thick formed on an SrTiO₃ (110) substrate was irradiated with the Ga ions in the form of lines. Grain boundaries after the annealing of the thin film were observed in places corresponding to the lines whose doses were 1 x 10¹⁵ /cm² or above. The reason why the irradiation was less effective for the formation of the grain boundaries than in the case of the acceleration voltage of 30 kV, will be that the thin film was damaged at a lower degree, to approximate the situation of the vapor deposition of Ga. Example 18: SrTiO₃ (100) was employed for a substrate, on which a thin film of YBa₂Cu₃O 7-δ having thickness of 0.7 µm was formed. The thin film was irradiated with Ga ions in the form of lines under the same conditions as in Example 11. The thin film subjected to the ion beam processing had been grown so that the a-axis [100] thereof might become perpendicular to the plane thereof. Although the c-axis and b-axis of the film mixed within the plane thereof, grain boundaries were clearly observed in correspondence with the linear parts irradiated with the Ga and formed under the same conditions of doses as in Example 11. This indicated that the grain boundaries were hardly affected by the orientation of the substrate and the material of the superconducting thin film. Example 19: Magnesium oxide (MgO) (100) was used for a substrate, on which a thin film of YBa₂Cu₃O 7-δ having a thickness of 0.35 µm was formed. As in Example 16, the thin film was irradiated with Ti ions at an acceleration voltage of 30 kV and at doses of 1 x 10¹² - 1 x 10¹⁹ /cm². The thin film was such that the c-axis thereof became perpendicular to the plane of the substrate, and that the a-axis and b-axis thereof could not be distinguished. However, grain boundaries appeared in places corresponding to lines whose doses were 1 x 10¹³ - 1 x 10¹⁹ /cm². The value of Jc in a region where no grain boundary was formed, was substantially stipulated by the film thickness and the dose irrespective of the kind of the substrate. Example 20: The crystal grain boundaries formed by the method indicated by the above examples 11 - 19 were utilized for the eak-link Josephson junctions, and a dcSQUID was fabricated. An SrTiO₃ (110) substrate 170 was overlaid with an ErBa₂Cu₃O 7-δ thin film 171 at a thickness of 0.35 µm under the same conditions as in Example 11. The thin film 171 was irradiated with Ga ions in the form of two serial lines having a line width of 1 µm, a length of 0.5 mm and an interval of 0.8 mm, under the conditions of an acceleration voltage of 30 kV and a dose of 5 x 10¹⁵ /cm². The thin film thus formed was processed by photolithography and chemical etching into the pattern of the SQUID which utilized two grain boundaries 172 as shown in Fig. 9. A magnetic flux was measured at 76 K by causing a bias current to flow through the SQUID. As a result, a voltage corresponding to a quantum flux was observed as shown in Fig. 13. It has accordingly been indicated that the weak-link junctions which utilize the grain boundaries formed by the present invention are applicable to superconductor devices. According to Examples 1 - 20 of the present invention, a single crystal grain boundary can be formed at a predetermined position, and it can therefore be utilized for a superconducting weak-link Josephson junction. It is also a great merit that a bias current during the fabrication of a device can be controlled by selecting the thickness of a film and the level of a dose. Incidentally, the method of forming a thin film in the present invention has been sputtering or layered deposition. Needless to say, however, the thin film of oxide superconductor can also be formed by reactive evaporation, and other methods of forming a film are applicable. Example 21: Example 21 of the present invention will be described with reference to Figs. 10(a) and 10(b). First, as shown in Fig. 10(a), the surface of a single-crystal substrate 240 of strontium titanate (SrTiO₃) having a plane orientation (100) was cut into a surface part 241 which was exactly conformed to the plane orientation (100) and a surface part 242 which was inclined an angle  relative to the surface part 241. The respective surface parts were polished into mirror surfaces having no distortion. There were prepared substrate samples whose inclination angles  were set at 0.5 degree, 1 degree, 2 degrees, 6 degrees, 10 degrees and 14 degrees. Next, while a substrate temperature during film formation was held at 700 °C, a layered film 243 was formed on the substrate by vacuum deposition. The layered film 243 consisted of layers of Er₂O₃, BaO and Cu, which were respectively 6 nm, 18.5 nm and 6 nm (60 Å, 185 Å and 60 Å) thick in terms of each single layer, and it had a total thickness of 0.7 µm. Subsequently, the thin film 243 was annealed in oxygen at 900 °C for 2 hours, thereby to form oxide-superconductor films of Er₁Ba₂Cu₃O₇ as shown in Fig. 10(b). Owing to the annealing, the oxide-superconductor film oriented in the a-axis direction of the superconductor was formed on the region 241 exactly conformed to the plane orientation (100). On the other hand, the oxide-superconductor film oriented in the c-axis direction of the superconductor was formed on the region 242 inclined the angle  relative to the plane orientation (100). In thin film, a crystal grain boundary 244 was created by the difference of the orientations fo the thin film crystals at the boundary part of the substrate 240 at which the crystal plane thereof began to incline. In order to examine the superconducting properties of the grain boundary 244, as in Example 1, the parts of the superconducting film 243 at both the ends of the grain boundary 244 were removed to form the neck portion of this superconducting film, and the I - V characteristic of the grain boundary 244 was measured by a conventional four-probe method. As illustrated by a typical example in Fig. 12, the I - V curve exhibited the feature of a superconductivity. A critical current density at 77 K in the region having no grain boundary was 2 x 10⁵ A/cm². As listed in Table 3, critical current densities in the places of the grain boundaries 244 in the samples lowered in a range from 52 % to 11 % in accordance with the values of the inclination angles . When the temperature dependency of the critical current density (Jc) was investigated, a relation of Jc ∝{1 - (T/TC)}¹˙⁵ was found. In addition, when the variation of a critical current versus a magnetic field was investigated, the cyclic dependency of the superconducting current upon the magnetic field was observed, and the fabrication of weak-link Josephson junctions was verified. Inclination angle (degree) Jc/Jco (%) Example 21 Example 22 0 100 100 0.5 52 60 1 45 55 2 37 47 6 28 32 10 10 23 14 11 12 Example 22: The surface of a single-crystal substrate of magnesium oxide (MgO) having a plane orientation (100) was worked into a shape similar to that of Example 21 as shown in Fig. 10(a), whereupon the respective surface parts of the substrate were polished into mirror surfaces having no distortion. Subsequently, as shown in Fig. 10(b), an oxide-superconductor film of Er₁Ba₂Cu₃O₇ having a thickness of 0.3 µm was formed on the substrate by sputtering. The temperature of the substrate during the film formation was set at 700 °C. Thereafter, the film was annealed in oxygen at 830 °C for 2 hours. The sample obtained was such that thin films of the oxide superconductor c-axis oriented perpendicularly to the corresponding substrate surface parts were formed on both the region 241 ( = 0 degree) which was exactly conformed to the plane orientation (100) and the region 242 which was inclined the angle  relative to the former region 241, and that a grain boundary was created by the inclination of the orientations of the crystals of the thin films at the boundary part of the substrate at which the crystal plane thereof began to incline. Incidentally, in this example, the film became superconductor during the sputtering, so that the subsequent annealing process was not always necessary. In order to examine the superconducting properties of the grain boundary, as in the case of Example 21, both the sides of the grain boundary were removed, and the I - V characteristic of the grain boundary was investigated by a conventional four-probe method. The I - V characteristic exhibited the feature of the superconductivity as illustrated in Fig. 12. A critical current density at 77 K in the region having no grain boundary was 6 x 10⁴ A/cm². As listed in Table 3, critical current densities in the places of the grain boundaries in the samples lowered from 60 % to 12 %. The reason why the critical current density at the grain boundary was lower as the inclination angle  was greater, is that the inclination of the growth axis of the thin film crystal increased, so the width of the grain boundary enlarged. When the temperature dependency of the critical current density was investigated, the relation of Jc ∝{1 - (T/Tc)}¹˙⁵ was found as to the grain boundaries of the thin films formed using the substrates whose inclination angles were 0.5 degree or above. In addition, the fabrication of weak-link Josephson junctions was verified from the variation of a critical current versus a magnetic field. Example 23: A dcSQUID as shown in Fig. 11 was fabricated by utilizing the weak-link Josephson junctions in Examples 21 - 22 of the present invention. Using a substrate 260 in which a part of the surface of a single-crystal substrate of magnesium oxide (MgO) having a plane orientation (100) was cut at an inclination angle  = 2 degrees, a thin film of Er₁Ba₂Cu₃O₇ 261 having a thickness of 0.3 µm was formed under the same conditions as in Example 22. In the thin film thus formed, a grain boundary 262 was produced at the position of the substrate 260 at which the surface part began to incline. The grain boundary part of the thin film thus formed was processed by photolithography and chemical etching into the pattern of the SQUID which utilized two grain boundaries 262 as shown in Fig. 11. A magnetic flux was measured at 76 K by causing a bias current to flow through the SQUID. As a result, a voltage corresponding to a quantum flux was observed as shown in Fig. 13. It has accordingly been indicated that the weak-link junctions which utilize the grain boundaries formed by the present invention are applicable to superconductor devices. According to Examples 21 - 23 of the present invention, a single crystal grain boundary can be formed at a predetermined position, and it can therefore be utilized for a superconducting weaklink Josephson junction. Incidentally, the method of forming a thin film in the present invention has been sputtering or layered deposition. Needless to say, however, the thin film of oxide superconductor can also be formed by reactive evaporation, and other methods of forming a film are applicable.

Claims: A method of forming a weak-link Josephson junction comprising the step of irradiating a substrate with Ga or Ti ions thereby to process said substrate so as either to deposit said Ga or Ti in the form of a line on a surface of said substrate or to damage the surface of the substrate in the form of a line by virtue of said Ga or Ti and the step of epitaxially growing a thin film of an oxide superconductor on the processed substrate, thereby to form a crystal grain boundary on said line, said crystal grain boundary being used as a weak-link Josephson junction. A method of forming a weak-link Josephson junction defined in Claim 1, wherein the processing of said substrate is performed by a focused ion beam. A method of forming a weak-link Josephson junction comprising the step of irradiating a thin film of an oxide superconductor epitaxially grown on a substrate with Ga or Ti ions, for lowering a critical current density, thereby to process said thin film so as either to diffuse said Ga or Ti in the form of a line within the thin oxide-superconductor film or to damage the oxide-superconductor thin film in the form of a line by virtue of said Ga or Ti and the step of annealing the processed oxide superconductor film, thereby to form a crystal grain boundary on said line, said crystal grain boundary being utilized as a weak-link Josephson junction. A method of forming a weak-link Josephson junction as defined in Claim 3, wherein the irradiation with said substance is performed by a focused ion beam.

Inventors: AIDA TOSHIYUKI, FUKAZAWA TOKUUMI, FUTAMOTO MASAAKI, HIRATANI MASAHIKO, HONDA YUKIO, KAWANAMI YOSHIMI, MADOKORO YUUICHI, MIYAUCHI KATSUKI, TAKAGI KAZUMASA, AIDA, TOSHIYUKI, FUKAZAWA, TOKUUMI, FUTAMOTO, MASAAKI, HIRATANI, MASAHIKO, HONDA, YUKIO, KAWANAMI, YOSHIMI, MADOKORO, YUUICHI, MIYAUCHI, KATSUKI, TAKAGI, KAZUMASA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: H01L39
IPC codes: H01L39