Structure, and josephson junction production method

Abstract

Provided is a structure having a Josephson junction, wherein the material of a superconductor is changed to a non-superconductor by implanting an ion beam into a superconductor layer through a protective layer from above without removing the protective layer. A structure 10 includes a substrate 11, a superconductor layer 12, and a protective layer 13 laminated in sequence, wherein the substrate 11 is composed of an insulator, the superconductor layer 12 includes a non-superconductor 122 in which the material of a superconductor 121 has been changed, the superconductor layer 12 is provided with a Josephson junction 120 having a superconductor / non-superconductor / superconductor configuration, and the protective layer 13 covers all or some of the superconductor layer 12.

Description

Structure and method of manufacturing Josephson junctions

[0001] The present invention relates to a structure and a method for fabricating a Josephson junction.

[0002] A method for fabricating Josephson junctions has been known in the past, in which a focused ion beam device is used to irradiate a superconductor disposed on a substrate with an ion beam to change the superconductor material into an insulator or normal conductor. When fabricating Josephson junctions using this method, a structure is used in which a superconductor having a single-crystal structure is epitaxially grown on a substrate and a gold (Au) film approximately several hundred nanometers thick is in-situ deposited as a protective layer. This structure is irradiated with argon (Ar) ions or the like to completely remove the protective layer by milling, and then irradiated with a focused ion beam to obtain an ultra-small Josephson junction. Alternatively, a Josephson junction can be obtained by suppressing superconductivity and narrowing the superconducting region by irradiating with gallium (Ga) ions. Furthermore, a superconductor device comprising multiple structures with Josephson junctions and other electronic elements can be obtained. Therefore, this method was expected to enable the fabrication of complex Josephson junction devices.

[0003] Patent Literature 1 discloses a method for manufacturing a device using a thin film of a disorder-sensitive material such as a superconductor, a ferromagnetic material, or a semiconductor by irradiating it with a focused high-energy ion beam. Patent Literature 2 discloses a charged particle beam system including a charged particle beam column having a charged particle source, an objective lens, and a first deflection system for changing the incident position of the charged particle beam on a sample plane, and a sample chamber equipped with a sample stage for holding a sample to be processed. Patent Literature 3 discloses a method for manufacturing a microbridge superconducting quantum interference device by epitaxially growing a c-axis oriented YBCO thin film on a <100> oriented single-crystal STO substrate by pulsed laser deposition (PLD), then depositing a gold thin film, and patterning it using a Ga ion beam in an FIB system.

[0004] Non-Patent Document 1 discloses that a Josephson superconducting tunnel junction can be created by directly writing a tunnel barrier into a YBCO thin film using a focused helium ion beam with a diameter of 500 pm, and that the barrier properties can be continuously controlled from conductive to insulating by changing the irradiation dose of the focused helium ion beam. Non-Patent Document 2 discloses that a high-temperature superconducting Josephson junction (HTS-JJ) based on an artificial grain boundary is characterized in that a YBCO microchannel is protected with an Au protective layer and irradiated with a Ga focused ion beam, exhibiting normal conductivity except in a limited region. Non-Patent Document 3 discloses that a high-Tc Josephson nanojunction fabricated using a helium ion beam focused on a sub-nanometer scale on a YBCO thin film has the lowest possible noise level and is applicable to microwave and terahertz regions.

[0005] US Patent No. 10224475 Patent No. 6689602 Chinese Patent Application Publication No. 115050887

[0006] Shane A. Cybart et.al., “Nano Josephson superconducting tunnel junctions in YBa2Cu3O7-δ directly patterned with a focused helium ion beam”, [on line], JULY 2015, NATURE NANOTECHNOLOGY, <URL:http: / / www.nature.com / naturenanotechnology> K Hayashi et.al., “A Study of the HTS Josephson Junction Formed by a Ga Focused Ion Beam", [on line], 2020, Journal of Physics: Conference Series, <URL:E-mail address: tanakas@tut.jp> Francois couedo et. al., "Dynamic properties of high-Tc superconducting nano-junctions made with a focused helium ion beam", [on line], 24 June 2020, Scientific Reports, <URL:www.nature.com / scientificreports>

[0007] Previously, when fabricating Josephson junctions using a light element ion beam with a small atomic weight, such as helium, it was necessary to completely remove the Au protective layer by etching prior to ion beam irradiation. This posed a problem of degradation of the superconductor's characteristics during the etching process. Furthermore, when using element ions with a large atomic weight, such as Ga metal ions, the ion beam has a large diameter, making it difficult to control the fine shape, making it difficult to precisely control the characteristics of the Josephson junction. In the latter case, the contamination of the superconductor with Ga ions or other ions causes degradation of the superconducting properties, resulting in the degradation of the characteristics of the fabricated device over time.

[0008] Therefore, the present invention has been made in consideration of the above problems, and aims to provide a structure in which the material is changed by controlling the physical properties in an ultrafine nano-region of a superconductor by irradiating a fine beam of focused ions of a light element with a small atomic weight, and also aims to provide a method for manufacturing a Josephson junction in which the material is changed by controlling the physical properties in an ultrafine nano-region of a superconductor by irradiating a fine beam of focused ions of a light element with a small atomic weight.

[0009] The present inventors have conducted extensive research and discovered that by irradiating a laminated structure consisting of a protective layer and a superconductor with a fine beam of focused ions of a light element with a small atomic weight, it is possible to control the physical properties of the superconductor in the ultra-fine nano region and change the material, which led to the completion of the present invention. The features of the present invention are listed below.

[0010] (1) A structure in which a substrate, a superconductor layer, and a protective layer are sequentially stacked, wherein the substrate is made of an insulator, the superconductor layer has a non-superconductor obtained by varying the material of the superconductor, and a Josephson junction of superconductor-non-superconductor-superconductor is disposed, and the protective layer covers the whole or a part of the superconductor layer. (2) The protective layer has a recess in the bottom where a part of the protective layer is present, and the Josephson junction is disposed in the superconductor layer in contact with the protective layer where the recess is located. (3) The structure in (1) further comprises a cerium oxide (CeO ２ ) or silicon nitride (Si ３ N ４ The structure according to (1) or (2), further comprising a buffer layer of

[0011] (4) The oxide of the substrate is magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), strontium titanate (SrTiO ３ ), LSAT((LaAl ３ ) ０．３ (Sr ２ TaAlO ６ ) ０．７)), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ (5) The structure according to any one of (1) to (3), wherein the superconductor is any one of the group consisting of YBa ２ Cu ３ O ｘ (x is 6.3 to 7), rare earth high temperature superconductor (REBa ２ Cu ３ O ｙ (where RE represents a rare earth element, and y represents 6 to 7), bismuth-based high-temperature superconductors (Bi ２ Sr ２ Ca （ｎ－１） Cu ｎ O （２ｎ＋４） (n=1, 2 or 3), lanthanum-based high-temperature superconductors (La ２ CuO ４ , (La （１－ｘ） Sr ｘ ) ２ CuO ４ (x is 0.05 to 0.30), iron-based superconductor (REFeAsO （１－ｘ） F ｘ , AFe ２ As ２ (A is an alkali metal or alkaline earth metal), AFeAs, FeSe, FeSe （１－ｘ） Te ｘ (x is 0 to 1), ruthenium-based superconductor Sr ２ RuO ４ and MgB ２(6) The structure according to (1) or (2), wherein the protective layer is composed of a metal, an insulator, or a laminate of a metal and an insulator. (7) The structure according to (6), wherein the metal of the protective layer is any one of a group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo), and ruthenium (Ru). (8) The structure according to (7), wherein the thickness of the metal of the protective layer is in the range of 1 nm to 100 nm. (9) The structure according to (7), wherein the surface resistivity of the protective layer is in the range of 0.15 Ω / sq. to 150.00 Ω / sq., calculated per unit length in the length and width of the end faces of the superconductor layer and the protective layer. (10) The structure according to (1) or (2), wherein the insulator of the protective layer is magnesium oxide (MgO), sapphire (Al), or the like. ２ O ３ ), silicon oxide (SiO ２ ), silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ C), boron nitride (BN), silicon nitride (Si ３ N ４ (11) The structure according to (10), wherein the insulating material of the protective layer is any one of the group consisting of titanium nitride (TiN), titanium nitride (TiN), titanium carbide (TiO), titanium nitride (TiN), titanium carbide (TiO), titanium nitride (TiO ...

[0012] (12) A method for manufacturing a Josephson junction, comprising: an irradiation step of irradiating a structure having a substrate, a superconductor layer, and a protective layer stacked in this order with an ion beam using a focused ion beam device, wherein in the irradiation step, the superconductor material of the superconductor layer is changed to a non-superconductor, thereby forming a superconductor-non-superconductor-superconductor Josephson junction. (13) A method for manufacturing a Josephson junction according to (12), further comprising, before the irradiation step, an etching step of etching the protective layer of the structure from the opposite side to the superconductor layer to form a bottomed recess in the protective layer while leaving a part of the protective layer, wherein after the etching step, in the irradiation step, the superconductor material of the superconductor layer in contact with the protective layer where the recess is located is changed to a non-superconductor, thereby forming the Josephson junction. (14) The method for manufacturing a Josephson junction according to (12), further comprising: a removal step of etching and removing the protective layer of the structure before the irradiation step; and a protective layer formation step of forming a new protective layer after the removal step and the irradiation step.

[0013] (15) The method for manufacturing a Josephson junction according to any one of (12) to (14), wherein the focused ion beam device generates one ion from the group consisting of helium (He) ions, lithium (Li) ions, beryllium (Be) ions, nitrogen (N) ions, and neon (Ne) ions. (16) The method for manufacturing a Josephson junction according to (15), wherein the dose per unit length of the helium ions is 50 (ions / nm) or more and less than 2000 (ions / nm). (17) The structure further comprises a cerium oxide (CeO ２ ), or silicon nitride (Si ３ N ４ 16. The method for producing a Josephson junction according to claim 15, further comprising a buffer layer of the formula (14).

[0014] (18) The oxide of the substrate is magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), strontium titanate (SrTiO ３ ), LSAT((LaAl３ ) ０．３ (Sr ２ TaAlO ６ ) ０．７ ), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ (19) The method for manufacturing a Josephson junction according to (15), wherein the superconductor is any one of the group consisting of YBa ２ Cu ３ O ｘ (x is 6.3 to 7), rare earth high temperature superconductor (REBa ２ Cu ３ O ｙ (where RE represents a rare earth element, and y represents 6 to 7), bismuth-based high-temperature superconductors (Bi ２ Sr ２ Ca （ｎ－１） Cu ｎ O （２ｎ＋４） (n=1, 2 or 3), lanthanum-based high-temperature superconductors (La ２ CuO ４ , (La （１－ｘ） Sr ｘ ) ２ CuO ４ (x is 0.05 to 0.30), iron-based superconductor (REFeAsO （１－ｘ） F ｘ , AFe ２ As ２ (A is an alkali metal or alkaline earth metal), AFeAs, FeSe, FeSe （１－ｘ） Te ｘ (x is 0 to 1), ruthenium-based superconductor Sr ２ RuO ４ and MgB ２(15) A method for manufacturing a Josephson junction according to (15), wherein the protective layer or the new protective layer is any one of a group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo), and ruthenium (Ru). (21) A method for manufacturing a Josephson junction according to (20), wherein the metal of the protective layer or the new protective layer is any one of a group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo), and ruthenium (Ru). (22) A method for manufacturing a Josephson junction according to (21), wherein the thickness of the metal of the protective layer is in the range of 1 nm to 100 nm. (23) A method for manufacturing a Josephson junction according to (21), wherein the sheet resistivity of the protective layer is in the range of 0.15 Ω / sq. to 150.00 Ω / sq., calculated per unit length in the length and width of the superconductor layer and the protective layer as end faces. (24) The insulator of the protective layer is magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ C), boron nitride (BN), silicon nitride (Si ３ N ４ (20) The method for manufacturing a Josephson junction according to (20), wherein the insulating material of the protective layer is any one of the group consisting of titanium nitride (TiN), titanium nitride (TiN), titanium carbide (TiO), titanium nitride (TiN), titanium carbide (TiO), titanium nitride (TiO ...

[0015] According to the present invention, a structure having a Josephson junction can be provided by implanting an ion beam into a superconductor layer from above through the protective layer without removing the protective layer, thereby changing the superconductor material to a non-superconductor. Also, a method for fabricating a Josephson junction can be provided by implanting an ion beam into a superconductor layer from above through the protective layer without removing the protective layer, thereby changing the superconductor material to a non-superconductor. Also, a method for fabricating a Josephson junction can be provided by implanting an ion beam into a superconductor layer from above through the protective layer without removing the protective layer, thereby changing the superconductor material to a non-superconductor in an ion beam irradiating device, even if the protective layer is removed.

[0016] 1 is a schematic diagram showing the configuration of the structure of the present invention. FIG. 2 is a schematic diagram showing another configuration of the structure of the present invention. ２ Cu ３ O ７1 is a diagram showing the crystal structure of (x=7). FIG. 2 is a diagram showing the configuration of a structure having a superconductor layer in which a Josephson junction is arranged. FIG. 3 is a diagram showing the configuration of a structure having a protective layer that covers part of the superconductor layer. FIG. 4 is a scanning electron microscope (SEM) photograph showing the cross section of a structure of the present invention. FIG. 5 is a schematic diagram showing a Josephson junction to be measured. FIG. 6 is a diagram showing the relationship between milling time and electrical resistance with respect to the thickness of an Au protective layer. FIG. 7 is a diagram showing the configuration of a structure having a bottomed recess with a part of the protective layer at the bottom. FIG. 8 is a diagram showing the configuration of a focused ion beam apparatus. FIG. 9 is a diagram showing a schematic diagram of helium ions being irradiated onto a YBCO superconductor layer in a structure. FIG. 10 is a diagram showing the characteristics of YBCO with respect to vacancy carrier concentration and temperature. FIG. 11 is a diagram showing a conventional method for manufacturing a Josephson junction using a focused ion beam apparatus. FIG. 12 is a diagram showing a method for manufacturing a Josephson junction of the present invention using a focused ion beam apparatus. FIG. 13 is a diagram showing a simulation of the behavior of helium ions when irradiated with helium ions. FIG. 14 is a schematic diagram showing the effect of helium ion irradiation on a superconductor layer with or without a protective layer. FIG. 15 is a diagram showing the beam diameter of helium ions when 1000 helium ions reach the superconductor layer. 1 is a diagram showing the relationship between temperature and electrical resistance, with the thickness of the protective layer being a variable. This diagram shows current-voltage characteristics (IV characteristics) when the helium ion dose is fixed at 536 (ions / nm) and the thickness of the protective layer is changed to (a) 40 nm, (b) 60 nm, and (c) 100 nm. This diagram shows (a) the relationship between dose and critical current, and (b) the relationship between dose and normal resistance, with the temperature fixed at 70 K and the layer thickness ranging from 0 nm without the protective layer to 16 nm as a variable. This diagram shows the results of Example 1, with (a) the structure of the structure in Example 1 and (b) the current-voltage characteristics (IV characteristics) of the Josephson junction. This diagram shows the results of Example 2, with (a) the structure of the structure in Example 2 and (b) the current-voltage characteristics (IV characteristics) of the Josephson junction. This diagram shows the results of Comparative Example 1, with (a) the structure of the structure in Comparative Example 1 and (b) the current-voltage characteristics (IV characteristics) of the Josephson junction.

[0017] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. Note that those skilled in the art can easily change or modify the present invention within the scope of the claims to create other embodiments, and these changed or modified embodiments are included within the scope of the claims. The following description is also an example of the present invention and does not limit the scope of the claims. Furthermore, in the description, the same or equivalent parts in each drawing will be assigned the same reference numerals, and redundant explanations will be omitted.

[0018] (Structure) Fig. 1 is a schematic diagram showing the configuration of a structure of the present invention. A structure 10 of the present invention is formed by sequentially stacking at least a substrate 11, a superconductor layer 12, and a protective layer 13. The substrate 11 is made of an insulator. The superconductor layer 12 has a superconductor-non-superconductor-superconductor Josephson junction 120 formed by changing the material of the superconductor 121 to a non-superconductor. The protective layer 13 is made of a metal or an insulator that covers all or part of the superconductor layer 12. These will be described in detail below.

[0019] The structure 10 of the present invention can be laminated by either a wet method or a dry method. Wet methods include, for example, electrolytic plating and electroless plating. Dry methods include, for example, chemical vapor deposition (CVD) and physical vapor deposition (PVD). Specifically, a high-temperature superconductor oxide having superconducting properties, such as a copper oxide-based high-temperature superconductor YBa2O3, is deposited to a predetermined thickness by sputtering on an oxide substrate having a (100) plane as the main surface, such as an MgO substrate. ２ Cu ３ O ｘ After the single crystal is fabricated by depositing the above, a protective layer of a predetermined thickness, for example, a gold (Au) protective layer, is deposited.

[0020] (Substrate) The substrate 11 in the structure 10 of the present invention is made of a single crystal of an insulator. Insulators suitable for the substrate 11 are selected from oxides, carbides, and nitrides. The crystal structure of the substrate 11 is preferably the same single crystal structure as the superconductor layer 12 described below, and is preferably at least a single crystal with few grain boundaries and a structure that does not disrupt the uniformity of the surface morphology.

[0021] The oxides include magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), strontium titanate (SrTiO ３ ), LSAT (LaSrAlTaO), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ Examples of carbides include silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ Examples of nitrides include boron nitride (BN), silicon nitride (Si ３ N ４ ), titanium nitride (TiN), and other nitrides. In particular, oxides are preferred as the insulator in the substrate 11. Among oxides, magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ) is preferred because the metal elements Mg, Al, and Si have a strong bond with oxygen (O), resulting in high electrical insulation and a low dielectric constant. ３ ), LSAT((LaAl ３ ) ０．３ (Sr ２ TaAlO ６ ) ０．７ ), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ ) and the like contain rare earth metals, have an affinity with oxide superconductors containing the same rare earth metals, and can be laminated with a smooth surface.

[0022] Although there are no particular limitations on the thickness of the substrate 11, it is preferable that the thickness of the substrate 11 is at least 100 nm. By making the thickness of the substrate 11 100 nm or more, it is possible to reduce the influence on other elements when the superconducting element to which the structure 10 of the present invention is applied is incorporated into an electronic circuit.

[0023] (Buffer Layer) FIG. 2 is a schematic diagram showing another configuration of the structure of the present invention. In the structure 10 of the present invention, a buffer layer 111 can be provided between the substrate 11 and the superconductor layer 12. The buffer layer 111 is preferably an insulator, like the substrate 11. Furthermore, the buffer layer 111 is preferably made of a material that adjusts the incompatibility of the crystal structure between the substrate 11 and the superconductor layer 12, thereby improving the crystallinity of the superconductor layer 12. Like the substrate 11, the buffer layer 111 is selected from oxides, carbides, and nitrides. Furthermore, since high crystallinity is required, a single crystal structure is used. This makes it possible to more reliably form a single crystal structure of the oxide superconductor layer, and to form an oxide superconductor layer with better crystallinity.

[0024] The buffer layer 111 is made of cerium oxide (CeO ２ ) or silicon nitride (Si ３ N ４ In particular, cerium oxide (CeO ２ Cerium oxide has high electrical insulation properties and is a high-temperature superconductor, YBa ２ Cu ３ O ｘ It has good lattice matching with YBa ２ Cu ３ O ｘ By epitaxially growing YBa ２ Cu ３ O ｘ A single crystal can be obtained. The thickness of the buffer layer 111 is not particularly limited, but it is preferable that the thickness is such that the crystal structure between the substrate 11 and the superconductor layer 12 is matched. For example, the thickness of the cerium oxide is preferably 10 nm or more.

[0025] (Superconductor Layer) The superconductor layer 12 in the structure 10 of the present invention is a single crystal structure selected from oxide and boride superconductors. The superconductor 121 is a copper oxide-based high-temperature superconductor YBa ２ Cu ３ O ｘ (x is 6.3 to 7), rare earth high temperature superconductor (REBa ２ Cu ３ O ｙ (where RE represents a rare earth element, and y represents 6 to 7), bismuth-based high-temperature superconductors (Bi ２ Sr ２ Ca （ｎ－１） Cu ｎ O （２ｎ＋４） (n=1, 2 or 3), lanthanum-based high-temperature superconductors (La ２ CuO ４ , (La （１－ｘ） Sr ｘ ) ２ CuO ４ (x is 0.05 to 0.30), iron-based superconductor (REFeAsO （１－ｘ） F ｘ , AFe ２ As ２ (A is an alkali metal or alkaline earth metal), AFeAs, FeSe, FeSe （１－ｘ） Te ｘ (x is 0 to 1), ruthenium-based superconductor Sr ２ RuO ４ and MgB ２ In addition, since the superconductor layer 12 is formed as a single crystal, it can be formed in a vacuum using pulsed laser deposition (PLD), evaporation, sputtering, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), or metal organic deposition (MOD).

[0026] The superconductor 121 is, in particular, a copper oxide high-temperature superconductor YBa ２ Cu ３ O ｘ (x is 6.3 to 7) (hereinafter simply referred to as "YBCO"). ２ Cu ３ O ７The superconductor 121 is formed by depositing the YBCO shown in FIG. 3 in the c-axis <001> direction, and by depositing CuO atoms above and below the Y atoms. ２ In Figure 3, white circles represent each element, with the element symbols yttrium (Y), barium (Ba), copper (Cu), and oxygen (O), respectively. This allows current to flow not along the c-axis, which has a short coherence length, but along the a- or b-axis, which has a long coherence length, and is approximately perpendicular to the c-axis. Furthermore, when helium ions are irradiated from the top surface, they can be irradiated deep into the YBCO by irradiating them parallel to the <001> direction.

[0027] YBCO has a higher superconducting transition temperature Tc than other copper oxide-based high-temperature superconductors, and even a thin film with a thickness of 25 nm has a superconducting critical temperature Tc of 80 K or higher. Furthermore, the introduction of oxygen can control the superconducting transition temperature and superconducting critical current density. Reducing the amount of oxygen can reduce the superconducting critical current density. Furthermore, it is known that YBCO requires the formation of a tunnel barrier layer with a thickness comparable to the coherence length when forming a Josephson junction 120, making it suitable for manufacturing Josephson junctions 120.

[0028] Furthermore, the thickness of the superconductor layer 12 in the structure 10 of the present invention is preferably in the range of 10 nm to 100 nm, and more preferably in the range of 20 nm to 45 nm. If the thickness of the superconductor layer 12 is less than 10 nm, the superconducting transition temperature becomes low and it becomes difficult to transition to superconductivity. If the thickness of the superconductor layer 12 exceeds 100 nm, the ion beam that forms the Josephson junction 120 diffuses, making it difficult to manufacture the Josephson junction 120.

[0029] (Josephson Junction) The superconductor layer 12 in the structure 10 of the present invention has a non-superconductor 122 which is a variation of the material of the superconductor 121, and has a Josephson junction 120 of superconductor 121-non-superconductor 122-superconductor 121. Fig. 4 is a diagram showing the configuration of a structure having a superconductor layer in which a Josephson junction is arranged. As shown in Fig. 4, the superconductor layer 12 has a Josephson junction 120 of superconductor 121-non-superconductor 122-superconductor 121 arranged.

[0030] The non-superconductor 122 is formed by irradiating the superconductor 121 with a focused ion beam, for example, a helium ion beam, thereby changing the material of the superconductor 121 into the non-superconductor 122. The non-superconductor 122 can be controlled between electrically conductive and insulating by adjusting the acceleration voltage and beam amount of the irradiated helium ions. The transformed non-superconductor 122 serves as the tunnel barrier layer of the Josephson junction 120. By controlling the helium ions, two types of Josephson junctions 120 can be formed: a superconductor-normal metal-superconductor SNS junction, and a superconductor-insulator-superconductor SIS junction. The width of the non-superconductor 122 is the thickness of the tunnel barrier layer of the Josephson junction 120, and corresponds to a length approximately equal to the coherence length of the material of the non-superconductor 122. The width of the YBCO non-superconductor 122 is, for example, 2 nm, and is preferably 0.35 nm to 5.0 nm. If the width of the non-superconductor 122 is less than 0.35 nm or exceeds 5.0 nm, the Josephson effect may not be exhibited.

[0031] (Protective Layer) The structure 10 of the present invention has a protective layer 13 composed of a metal, an insulator, or a laminate of a metal and an insulator to cover the superconductor layer 12 and the Josephson junction 120. The superconductivity of the superconductor 121 in the superconductor layer 12 decreases when contaminated by oxygen, moisture, etc. in the atmosphere. Therefore, the protective layer 13 is provided to protect the superconductor layer 12. The structure 10 of the present invention is provided with a protective layer 13 that covers all or part of the superconductor layer 12. Figure 5 is a diagram showing the configuration of a structure provided with a protective layer that covers part of the superconductor layer. Therefore, the superconductor layer 12 is entirely covered, or at least a portion of the Josephson junction 120 formed in the superconductor layer 12 is covered and protected.

[0032] The protective layer 13 is any one of a polycrystalline, single crystalline, and amorphous body composed of a metal, an insulator, or a laminate of a metal and an insulator. A metal that is non-magnetic and stable in the atmosphere is preferred as the metal, and in particular any one of the group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo), and ruthenium (Ru). Gold (Au) is particularly preferred, as it can protect the superconductor layer 12 against oxygen and moisture in the atmosphere. The insulator is selected from oxides, carbides, and nitrides. Examples of oxides include magnesium oxide (MgO), sapphire (Al), and the like. ２ O ３ ), silicon oxide (SiO ２ Examples of carbides include silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ Examples of nitrides include boron nitride (BN), silicon nitride (Si ３ N ４ ), nitrides such as titanium nitride (TiN), etc. Oxides are preferred for the protective layer 13. A laminate of a metal and an insulator can also be used for the protective layer 13. The above-mentioned materials can be used for the metal and the insulator.

[0033] As described above, the protective layer 13 prevents the superconductor layer 12 such as YBCO from being affected by moisture and CO in the atmosphere. ２ The protective layer 13 is unstable against metals or chemicals, and suppresses deterioration of the superconducting properties due to oxidation, reduction, hydrolysis, etc. Furthermore, since YBCO is vulnerable to mechanical shock and prone to fine scratches, the protective layer 13 also protects it from physical damage. Furthermore, the protective layer 13 can be used as a metal electrode with excellent electrical conductivity, bonding suitability, and solder wettability to electrically connect the Josephson junction 120 to an external circuit. The protective layer 13 suppresses electric field noise and prevents electric field fluctuations, contributing to the stability of the Josephson junction 120. Furthermore, after the formation of the superconductor layer 12 such as YBCO, fine processing such as etching may be performed, and the protective layer 13 suppresses deterioration due to heating or plasma treatment.

[0034] Preferred metals for the protective layer 13 are Au, Ag, Cu, Al, W, Mo, and Ru. Au, Ag, and Cu are preferred because of their low electrical resistance. Au, Cu, and Al are preferred because of their excellent film-forming properties. W, Mo, and Ru are preferred because of their excellent oxidation resistance. Because the Josephson junction 120 utilizes a minute voltage or current between itself and the external electrode, Au, which has low electrical resistance and surface resistance, is the most preferred metal for the protective layer 13. Furthermore, Au is resistant to oxidation, exhibits minimal change in electrical resistance over time, and is durable. Furthermore, Au has low hardness and excellent formability. Furthermore, Au's low surface resistance can suppress attenuation and reflection of high-frequency signals due to the skin effect, thereby reducing transmission loss. Furthermore, because Au partially bypasses the current flowing through the Josephson junction 120, this effect can be used to control the characteristics of the Josephson junction 120.

[0035] The thickness of the metal of the protective layer 13, particularly Au, is preferably in the range of 1 nm to 100 nm in order to protect the superconductor layer 12 and the Josephson junction 120 from oxygen, moisture, and the like in the atmosphere. In particular, when helium ions are irradiated from above, a thickness of 1 nm to 20 nm allows the helium ions to pass through without expanding their beam diameter. Furthermore, a thickness of 20 nm to 100 nm can further reduce the resistance of the Josephson junction 120.

[0036] Figure 6 is a scanning electron microscope (SEM) photograph showing a cross section of the structure of the present invention. The structure 10, consisting of the Au protective layer 13, the YBCO superconductor layer 12, and the MgO substrate 11, was cleaved and divided using a diamond pen scribe and break process. The cleaved cross section was then observed using a scanning electron microscope (SEM) or the like. Figure 6 shows that the thickness of the Au protective layer 13 is 38 nm, and that of the YBCO superconductor layer 12 is 31 nm. This allows the thickness of the Au protective layer 13 to be directly measured.

[0037] The thickness of the Au protective layer 13 can also be measured by measuring its electrical resistance. FIG. 7 is a schematic diagram showing a Josephson junction to be measured. When the resistance of the superconductor layer 12 in the normal conducting state is sufficiently larger than that of the protective layer 13, the thickness of the protective layer 13 can be calculated from the electrical resistance, ignoring the contribution of the superconductor layer 12 to electrical conduction. As an example, thicknesses were compared using an Au protective layer 13 and a YBCO superconductor layer 12. A laminated sample of the Au protective layer 13 and the YBCO superconductor layer 12 was processed into a thin-line pattern with a known width, and the resistance was measured. The electrical resistivity of the Au protective layer 13 was found to be 30 nΩ-m. Using this electrical resistivity, the thickness of the Au protective layer 13 can be calculated from the electrical resistance value for a sample with an Au protective layer 13 as follows:

[0038] FIG. 8 shows the relationship between milling time and electrical resistance depending on the thickness of the Au protective layer. The thickness of the protective layer 13 is expressed by the following formula (1): t = (ρ × L) / (R × W): Formula (1) where t is the thickness of the protective layer (m), ρ is the surface resistivity of the protective layer material (Ω / sq.), L is the length between the measured samples (m), R is the measured electrical resistance (volume resistance: Ω·m), and W is the width of the sample (m). It can be seen that the thickness of the Au protective layer 13 directly observed by SEM and the thickness of the Au protective layer 13 measured using formula (1) after a milling time of 4 minutes are approximately equal, at 38 nm and 39 nm, respectively. Therefore, the thickness of the protective layer 13 of the structure 10 of the present invention may be measured by either method.

[0039] Furthermore, the sheet resistivity of the metal protective layer 13 is preferably in the range of 0.15 Ω / sq. to 150.00 Ω / sq., calculated per unit length for the length and width of the superconductor layer 12 and protective layer 13 as end faces. By setting the sheet resistivity of the protective layer 13 in the Josephson junction 120 in the range of 0.15 Ω / sq. to 150.00 Ω / sq., the Josephson junction 120 is excellent in current-voltage characteristics (IV characteristics) that allow the Josephson junction 120 to operate efficiently with respect to electrical signals, noise characteristics from the Josephson junction 120 such as thermal noise, high-frequency responsiveness to input signals, and signal integrity that allows input signals to be transmitted correctly without waveform distortion.

[0040] If the surface resistivity of the protective layer 13 is less than 0.15 Ω / sq., signal loss is small, but film formation control and fine processing are difficult. If it exceeds 150.00 Ω / sq., contact resistance increases and high-frequency response decreases. In particular, the surface resistivity of Au is preferably in the range of 0.30 Ω / sq. to 30.00 Ω / sq. A low surface resistivity has the advantages of small signal loss, small contact resistance, and excellent high-frequency response.

[0041] The protective layer 13 is made of an insulating material selected from oxides, carbides, and nitrides. The oxides include MgO, Al ２ O ３ , SiO ２ Examples of oxides include SiC, VC, WC, and B. ４ Examples of nitrides include carbides such as C, BN, Si, etc. ３ N ４ The insulator protects the superconductor layer 12 from oxidation by the atmosphere, moisture, CO ２ It also prevents unwanted leakage currents between electrodes when incorporated into an electric circuit, reduces noise, and prevents malfunctions. ２ O ３ has high insulation and excellent moisture resistance. MgO has high insulation and excellent crystallinity. Si ３ N ４ has excellent adhesion and is highly effective in preventing moisture. ２The insulating material of the protective layer 13 is particularly SiO. ２ The thickness of the Josephson junction 120 is preferably 1 nm or more and 200 nm or less. In particular, when helium ions are irradiated from the top surface, by making the thickness 1 nm or more and 20 nm or less, the beam diameter of the helium ions can be transmitted without being expanded. Furthermore, by making the thickness 20 nm or more and 100 nm or less, oxidation of the Josephson junction 120, moisture, CO ２ This can further enhance the resistance to contamination.

[0042] Furthermore, in the structure 10 of the present invention, the protective layer 13 has a bottomed recess 131 at the bottom of which a portion of the protective layer 13 is present, and a Josephson junction 120 is disposed in the superconductor layer 12 in contact with the protective layer 13 where the recess 131 is located. FIG. 9 is a diagram showing the configuration of a structure having a bottomed recess with a portion of the protective layer at the bottom. By leaving a portion of the protective layer 13 at the bottom of the recess 131, the superconductor layer 12 and the Josephson junction 120 can be protected from contamination by ions, oxygen, and moisture in the atmosphere and from deterioration of the superconductor layer material. Furthermore, by protecting the superconductor layer 12 with the protective layer 13, deterioration due to exposure to the atmosphere, chemicals, and the like in the environment can be suppressed even when used as a superconducting element or during storage. Furthermore, by leaving the protective layer 13 at the bottom, irradiating the superconductor layer 12 with an ion beam that is appropriately diffused can prevent, for example, migration of elements other than oxygen (O) in YBCO, which could damage the crystal structure. Furthermore, when a metal is used as the protective layer 13, by shunting (electrically bypassing) the superconductors (two superconductors 121 are shown in FIG. 4), the electrical resistance in the superconductor element characteristics can be reduced, thereby making it possible to control the high-frequency characteristics.

[0043] The thickness of the protective layer 13 with a bottom left at the bottom of the recess 131 is preferably less than 100 nm to protect the superconductor layer 12 and the Josephson junction 120 from oxygen and the like in the atmosphere. In particular, by making the thickness of the protective layer 13 between 1 nm and 20 nm, helium ions can be diffused when irradiated from above, allowing the ion beam diameter to pass through without expanding. Furthermore, by making the thickness of the protective layer 13 between 20 nm and 100 nm, the oxygen concentration in the non-superconductor 122 can be easily controlled.

[0044] It is expected that the structure 10 employing the Josephson junction 120 will be applied to a medical superconducting quantum interference device (SQUID), a digital device, or a high frequency device.

[0045] (Method for manufacturing a Josephson junction) The method for manufacturing a Josephson junction of the present invention includes an irradiation step of irradiating the structure 10 with an ion beam 22 using a focused ion beam device, in which the material of the superconductor 121 of the superconductor layer 12 is changed to a non-superconductor 122 to form a superconductor-non-superconductor-superconductor Josephson junction 120.

[0046] (Focused Ion Beam Apparatus) FIG. 10 is a diagram showing the configuration of a focused ion beam apparatus. The focused ion beam apparatus 20 may use lithium (Li) or beryllium (Be) metal as the ion source 21, or may field-ionize and emit gases in the ambient atmosphere containing helium (He), nitrogen (N), or neon (Ne) at the tip of a high-melting-point metal single crystal. Therefore, the ion source 21 can generate helium (He) ions, lithium (Li) ions, beryllium (Be) ions, nitrogen (N), or neon (Ne) ions as the ion beam 22. Ga atoms, in particular, are unsuitable because they diffuse into the crystal and significantly degrade the superconducting properties. An extraction electrode 23 is provided to accelerate the emitted ions and selectively extract them. The ionized and accelerated ion beam 22 is focused by a focusing lens 24. The focused ion beam 22 is then irradiated by applying a voltage to a scanning coil 25, thereby controlling the irradiation position. Thereafter, the ion beam diameter is narrowed by the objective lens 26 so that it is irradiated onto the sample surface of the structure 10 placed on the sample stage 27 .

[0047] FIG. 11 is a schematic diagram showing the irradiation of helium ions onto the YBCO superconductor layer in the structure. The structure 10 can be processed by irradiating the surface of the structure with a fine ion beam. In particular, a helium ion beam (hereinafter simply referred to as "helium ions") 22 has small diffraction aberrations and a small convergence angle of the focused beam, resulting in a parallel beam. Furthermore, compared to Ga ions, it causes less sample contamination and less damage to the structure 10, and its small beam diameter allows for ultrafine processing. In particular, helium ions 22 can be refined to a diameter of 0.35 to 0.50 nm. The focused ion beam device 20 can control not only the beam diameter but also the acceleration voltage (kV) for the helium ions and the dose per unit length of irradiation (hereinafter simply referred to as "dose") (ions / nm). The irradiated helium ions 22 are irradiated perpendicular to the superconductor layer 12, scanning the width direction (length l). Furthermore, by repeating this operation, it is possible to irradiate a constant thickness t.

[0048] As shown in FIG. 3 , the structure 10 is formed by stacking YBCO on a substrate 11 with its (001) plane aligned in the c-axis direction of the <001> orientation. By irradiating helium ions parallel to the <001> orientation, the ions penetrate deeply into the superconductor layer 12 in the thickness direction, colliding with oxygen elements and expelling them from the structure 10, while minimizing collisions with other constituent elements other than oxygen (O). By controlling these irradiation conditions, the amount of oxygen in the superconductor layer 12, e.g., the YBCO layer, can be controlled. The same applies to lithium (Li), beryllium (Be), nitrogen (N), and neon (Ne), which have atomic weights smaller than gallium (Ga).

[0049] FIG. 12 is a diagram showing the characteristics of YBCO with respect to vacancy carrier concentration and temperature. FIG. 12 shows that YBCO exhibits superconductivity, antiferromagnetism, insulation, and metallic properties. As indicated by the arrows in FIG. 12, by controlling the irradiation conditions using the focused ion beam device 20, the vacancy carrier concentration can be controlled, and the YBCO can be controlled from a superconductor 121 (superconductivity) to a normal conductor (normal metal) due to its metallic properties, or an insulator-type antiferromagnet (anti-ferromagnet) due to its insulating properties. According to the present invention, two types of Josephson junctions 120 can be fabricated: an SNS junction having a superconductivity-normal metal-superconductor structure, and an SIS junction having a superconductivity-insulator-superconductor structure.

[0050] (Irradiation Step) In the method for manufacturing a Josephson junction of the present invention, the material of the superconductor 121 in the superconductor layer 12 is changed to a non-superconductor 122 in the irradiation step, thereby forming a Josephson junction 120 of superconductor 121-non-superconductor 122-superconductor 121. FIG. 13 illustrates a conventional method for manufacturing a Josephson junction using a focused ion beam device. Before irradiating a structure 10 having a substrate 11, a superconductor layer 12, and a protective layer 13 with focused helium ions 22 (see FIG. 13(a)), as shown in FIG. 13(b), the protective layer 13 at the irradiated portion is completely removed by milling with an inert gas with a large atomic weight, such as argon (Ar) ions, or by wet etching with an iodine solution, thereby forming an opening 132. Here, the protective layer 13 is removed without leaving any residue at the bottom of the opening 132. Thereafter, the superconductor layer 12 exposed in the opening 132 is irradiated with focused helium ions 22 to change a part of the superconductor layer 12 into a non-superconductor 122 , thereby forming a Josephson junction 120 .

[0051] However, this opening 132 cannot be formed with the focused ion beam device 20 that uses helium ions 22 with a small atomic weight. Therefore, the structure 10 in which the recess has been formed using another vacuum device must be temporarily removed from the device and then placed in the focused ion beam device 20 for forming the Josephson junction 120 in the structure 10. During the manufacturing process, the surface of the superconductor layer 12, particularly the YBCO superconductor 121, may be contaminated with oxygen or moisture in the air or gases in the atmosphere inside the device, or the superconducting properties of the superconductor layer 12 may be deteriorated by excessive etching or the like.

[0052] FIG. 14 illustrates a method for fabricating a Josephson junction of the present invention using a focused ion beam device. As shown in FIG. 14( a), a Josephson junction 120 is formed through a protective layer. As shown in FIG. 14( b), a portion of the protective layer 13 is removed, and the Josephson junction 120 is formed through the protective layer remaining in the recess 131. As shown in FIG. 14( a), in the method for fabricating a Josephson junction 120 of the present invention, a focused ion beam device 20 irradiates helium ions 22 onto the superconductor layer 12 from above the protective layer 13. The protective layer 13 covers all or part of the superconductor layer 12. The protective layer 13 prevents the superconductor 121 from being exposed to the atmosphere or the atmosphere inside the milling device or focused ion beam device 20 during the fabrication process of the Josephson junction 120. Oxide superconductors, particularly those containing YBCO, are susceptible to contamination by moisture, oxygen, and other elements remaining in the atmosphere, which can cause their superconducting properties to become unstable. Therefore, the protective layer 13 is provided on the superconductor layer 12 to prevent contamination. This makes it possible to prevent contamination by the atmosphere during the irradiation process in the focused ion beam device 20, and also to prevent the superconductor layer 12 from being exposed to the air atmosphere even after it is removed from the focused ion beam device 20.

[0053] The thickness of the metal of the protective layer 13, particularly Au, is preferably 1 nm or more and 100 nm or less. It is difficult to form the protective layer 13 uniformly without gaps or defects at a thickness of less than 1 nm. Furthermore, if the thickness of the protective layer 13 exceeds 100 nm, lithium (Li) ions, beryllium (Be) ions, nitrogen (N) ions, or neon (Ne) ions, which have atomic weights smaller than Ga ions, cannot pass through the protective layer 13, and the protective layer 13's function is fully saturated. Furthermore, when helium ions 22 are irradiated from the top surface, a thickness of 1 nm or more and 20 nm or less allows the helium ions 22 to diffuse and penetrate without expanding the ion beam diameter. Furthermore, a thickness of 20 nm or more and 100 nm or less facilitates control of the oxygen concentration of the non-superconductor 122.

[0054] (Etching Step, Irradiation Step) The method for manufacturing a Josephson junction of the present invention further includes an etching step, prior to the irradiation step, of etching the protective layer 13 of the structure 10 from the side opposite to the superconductor layer 12, to leave a portion of the protective layer 13 and form a recess 131 in the protective layer 13. In the irradiation step following the etching step, the material of the superconductor 121 in the superconductor layer 12 that contacts the protective layer 13 where the recess 131 is located is changed to a non-superconductor 122, thereby forming the Josephson junction 120.

[0055] As shown in FIG. 14( b ), the method for manufacturing a Josephson junction of the present invention includes an etching step in which the protective layer 13 is etched from the side opposite the superconductor layer 12, leaving a portion of the protective layer 13 and forming a recess 131 in the protective layer 13. The protective layer 13 is etched using a rare gas with a large atomic weight, such as argon (Ar) gas, or metal ions, such as Ga. Furthermore, the method for manufacturing a Josephson junction 120 of the present invention includes an irradiation step following the etching step in which the superconductor layer 12 is irradiated with helium ions 22 from above the protective layer 13, leaving a portion of the superconductor layer 12 in contact with the protective layer 13 where the recess 131 is located. This converts a portion of the superconductor 121 in contact with the recess 131 of the protective layer 13 into a non-superconductor 122, thereby forming the Josephson junction 120. The recess 131 prevents the non-superconductor portion 122 of the superconductor layer 12 from being contaminated by oxygen and moisture in the atmosphere, allowing the Josephson junction 120 to be formed. Furthermore, there are no particular restrictions on the position where the Josephson junction 120 is formed, and when a large scale integrated circuit device is constructed using such a Josephson junction 120, the manufacturing process can be simplified.

[0056] The thickness of the bottomed portion of the protective layer 13 remaining in the recess 131 is preferably less than 100 nm to protect the superconductor layer 12 and the Josephson junction 120 from oxygen and other atmospheric factors. In particular, a thickness of 1 nm or more and 20 nm or less allows the helium ions 22 to pass through without expanding their beam diameter when irradiated from above. Furthermore, a thickness of 20 nm or more and less than 100 nm facilitates control of the oxygen concentration in the non-superconductor 122. Here, the argon (Ar) gas etching can be performed in a vacuum device other than the focused ion beam device 20 by ion milling using Ar ions under conditions of an extraction voltage of 150 to 400 V, preferably 300 V, and an Ar pressure of 0.1 to 1.0 Pa, preferably 0.5 Pa, to selectively remove a portion of the protective layer 13.

[0057] 15A and 15B show a simulation of the behavior of helium ions when irradiated with helium ions. FIG. 15A shows the case where the thickness of the protective layer 13 is 0 nm, and FIG. 15B shows the case where the thickness of the protective layer 13 is 50 nm. The simulation conditions are as follows: <Material: protective layer> Au <Material: superconductor layer> YBCO <Material: substrate> MgO <Software> SRIM <Ion beam> Helium ions <Acceleration voltage> 30 kV <Number of ions> 100 <Other conditions> Full cascade

[0058] FIG. 16 is a schematic diagram showing the effect of helium ion irradiation on a superconductor layer with or without a protective layer. The triangular portions at the positions irradiated by the helium ions 22 indicate the portions changed by the helium ions 22. As shown in FIG. 16( a), in the structure 10 without the protective layer 13, the irradiation with helium ions 22 results in a large change in thickness of the changed non-superconductor 122 in the superconductor layer 12 in the film thickness direction. In contrast, as shown in FIG. 16( b), in the structure 10 with the protective layer 13, the irradiation with helium ions 22 results in a small change in thickness of the changed non-superconductor 122 in the superconductor layer 12 in the film thickness direction. Thus, by using a structure 10 having a certain protective layer 13, the changed non-superconductor 122 can form a uniform tunnel barrier layer and suppress excess current.

[0059] FIG. 17 shows the beam diameter of 1,000 helium ions when they reach the superconductor layer. As shown in FIG. 17 , the standard deviation of the distance from the beam center axis is proportional to the thickness of the protective layer 13 (0, 12, 16, 30, 40, 50, and 100 nm). A thinner protective layer 13 can suppress the diffusion of helium ions 22. Conversely, a thicker protective layer 13 can disperse helium ions 22, allowing for control of the material change from superconductor 121 to non-superconductor 122, e.g., the change in oxygen content. Therefore, it can be seen that the ion beam diameter when it reaches the superconductor layer 12 can be controlled by the thickness of the protective layer 13, the beam diameter of the focused helium ions 22, the acceleration voltage, the dose, etc.

[0060] FIG. 18 is a diagram showing the relationship between temperature and electrical resistance, with the thickness of the protective layer as a variable. The relationship between temperature and electrical resistance is shown for the case where helium ions 22 are irradiated at 275 (ions / nm) without a protective layer 13, and for the case where helium ions 22 are irradiated at 306 (ions / nm) with protective layers 13 of 12 nm and 16 nm. FIG. 18(a) shows the electrical resistance on a normal scale, and FIG. 18(b) shows the electrical resistance on an exponential scale. From FIGS. 18(a) and 18(b), it can be seen that the electrical resistance changes suddenly at a specific temperature, indicating superconducting properties. It can be seen that the resistance in the normal conduction region decreases as the protective layer 13 becomes thicker. Furthermore, as the protective layer 13 becomes thicker, T ｃ、ＪＪ (Josephson junction critical temperature) is increased, which indicates that the influence of the irradiation of helium ions 22 is reduced. ｃ、ＪＪ denotes the Josephson critical temperature, which is the temperature at which the superconductor 121 transitions to a superconducting state.

[0061] FIG. 19 shows the current-voltage characteristics (IV characteristics) when the helium ion dose is fixed at 536 (ions / nm) and the thickness of the protective layer is changed to (a) 40 nm, (b) 60 nm, and (c) 100 nm. As is clear from FIG. 19 , the rapid change in the current-voltage characteristics (IV characteristics) indicates that a Josephson junction 120 is formed. It is also clear that, when irradiating with helium ions 22, the critical current Ic of the Josephson junction 120 increases as the thickness of the protective layer 13 increases. In this way, by controlling the critical current Ic, a desired critical current Ic can be obtained, and further, the current level can be controlled to be easily handled by a general electronic circuit.

[0062] FIG. 20 shows (a) the relationship between dose and critical current Ic and (b) the relationship between dose and normal resistance Rn, with the temperature fixed at 70 K and the layer thickness varying from 0 nm without a protective layer to 16 nm. It can be seen that the critical current Ic increases with the thickness of the protective layer 13. Furthermore, it can be seen that the normal resistance Rn decreases with the thickness of the protective layer 13. In devices employing Josephson junctions 120, logic circuits can be constructed using the pulsed voltage generated when current flows through the Josephson junctions 120. In this case, the switching speed is proportional to the IcRn product. Therefore, the larger the IcRn product, the faster the device can operate. Since the IcRn product increases in proportion to the critical temperature Tc of the superconductor 121 used in the Josephson junctions 120, it can be seen from FIGS. 20(a) and 20(b) that IcRn can be controlled by adjusting the dose.

[0063] As explained above, in the method for manufacturing the Josephson junction 120 of the present invention, the protective layer 13 is etched by the etching step, leaving a part of the protective layer 13 and forming a recess 131 in the protective layer 13, and then the superconductor layer 12 is irradiated with helium ions 22 through the remaining protective layer 13 to form the Josephson junction 120. Furthermore, by controlling the film thickness of the protective layer 13, the Josephson junction critical temperature Tc ＪＪ、 It is clear that the critical current Ic and normal resistance Rn can be controlled.

[0064] Furthermore, the method for manufacturing a Josephson junction 120 of the present invention further includes a removal step of etching and removing the protective layer 13 of the structure 10 before the irradiation step, and a protective layer formation step of forming a new protective layer after the removal step and the irradiation step. The method for manufacturing a Josephson junction 120 of the present invention includes a removal step of etching and removing the protective layer 13 of the structure 10 before the irradiation step. It also includes a removal step of etching the entire protective layer 13 of the structure 10. In particular, before the irradiation step, the protective layer 13 in the portion where the Josephson junction 120 is to be formed is completely removed. Next, helium ions 22 are directly irradiated onto the superconductor layer 12 of the structure 10 where the protective layer 13 is not present. While controlling the acceleration voltage, dose, beam diameter, etc. of the helium ions 22 irradiation, the material of the superconductor 121 of the superconductor layer 12 is changed to a non-superconductor 122, thereby forming the Josephson junction 120. Next, after the removal step and the irradiation step, a protective layer formation step is performed in which a new protective layer is formed on the formed Josephson junction 120. At this time, a new protective layer can be formed on all or part of the superconductor layer 12. In this way, a structure 10 having the substrate 11, the superconductor layer 12, and the protective layer 13 can be formed.

[0065] As described above, the method for manufacturing a Josephson junction of the present invention, unlike the conventional Josephson junctions 120, is free from variations that occur in the photolithography process, even when fabricating a large-scale integrated circuit device, and allows integration without restrictions on the position where the Josephson junction 120 is formed. Furthermore, it becomes possible to manufacture two different types of Josephson junctions 120, SNS junctions and SIS junctions, with reproducibility and uniformity.

[0066] The present invention will be described in detail based on the following examples, but the present invention is not limited to the examples shown below.

[0067] (Example 1) <Fabrication of Example 1> Figure 21 shows the results of Example 1, where (a) shows the structure of the structure in Example 1 and (b) shows the current-voltage characteristics (IV characteristics) of the Josephson junction. As shown in Figure 21(a), the structure of Example 1 is composed of a substrate 11, a superconductor layer 12, a protective layer 13, and a recess 131. This structure was fabricated as follows. First, in a thin-film fabrication device equipped with a vacuum chamber for forming a high vacuum, MgO material was evaporated using a pulsed laser or the like to epitaxially grow on an MgO seed substrate. Next, a YBCO material formed to a predetermined composition was evaporated using a pulsed laser and deposited on the MgO substrate to form a 25 nm-thick superconductor layer. Next, a 100 nm-thick protective layer was formed on the YBCO superconductor layer by in-situ deposition. Next, the 100 nm-thick protective layer was formed by argon (Ar) ion milling. At this time, the milling rate of the Au protective layer during argon (Ar) ion milling was calculated. Next, by controlling the milling time, a recess was formed, leaving 12 nm of the Au protective layer. In this way, a structure was fabricated in which the protective layer with a recess, the superconductor layer, and the substrate were sequentially stacked.

[0068] Next, the fabricated structure was placed on the sample stage of a focused ion beam apparatus, and helium in the atmosphere was ionized using a W ion source and released into space. The ionized helium ions were irradiated onto the recess of the above-mentioned structure under the fabrication conditions of beam diameter: 0.35 nm, acceleration voltage: 30 kV, and dose: 268 (ions / nm) to form a Josephson junction, thereby obtaining the structure of Example 1.

[0069] <Evaluation of Example 1> Next, FIG. 21(b) shows the measurement results of the current-voltage characteristics (IV characteristics) of the Josephson junction. The measurement shows the current-voltage characteristics (IV characteristics) at a low temperature of 41.2 K. As is clear from FIG. 21(b), the obtained structure exhibits the Josephson effect. The critical current Ic was 735 μA, and the normal resistance Rn was 0.9 Ω. In particular, the IV characteristics show that the voltage rise at the critical current Ic is steep, indicating excellent voltage response.

[0070] (Example 2) <Manufacturing of Example 2> Figure 22 is a diagram showing the results of Example 2, where (a) shows the structure of the structure in Example 2 and (b) shows the current-voltage characteristics (IV characteristics) of the Josephson junction. As shown in Figure 22(a), the structure of Example 1 is composed of a substrate 11, a buffer layer 111, a superconductor layer 12, a protective layer 13, and a recess 131. This structure was manufactured as follows. First, in a thin-film manufacturing device in which a vacuum container for forming a high vacuum is placed, cerium oxide (CeO ２ ) material is evaporated by a pulse laser or the like to form sapphire (Al ２ O ３ ) substrate, and cerium oxide (CeO ２ Next, a material formed to a predetermined composition of YBCO is evaporated by a pulse laser to form a buffer layer 111 of CeO ２ A 25 nm thick superconductor layer was formed by deposition on the buffer layer 111. Next, a 100 nm thick protective layer was formed on the YBCO superconductor layer by in-situ film formation. Next, the 100 nm thick protective layer was milled with argon (Ar) ions. At this time, the milling rate of the Au protective layer during argon (Ar) ion milling was calculated. Next, by controlling the milling time, a recess was formed, leaving 12 nm as part of the Au protective layer. In this way, a structure was fabricated in which a protective layer with a recess - a superconductor layer - a substrate were sequentially stacked.

[0071] Next, in the same manner as in Example 1, the fabricated structure was placed on the sample stage of a focused ion beam apparatus, and helium in the atmosphere was ionized using a W ion source and released into space. The ionized helium ions were irradiated onto the recess of the above-mentioned structure under the fabrication conditions of beam diameter: 0.35 nm, acceleration voltage: 30 kV, and dose: 459 (ions / nm), forming a Josephson junction and producing the structure of Example 2.

[0072] <Evaluation of Example 2> Next, FIG. 22(b) shows the measurement results of the current-voltage characteristics (IV characteristics) of the Josephson junction. The measurements show the current-voltage characteristics (IV characteristics) at low temperatures of 40 K, 45 K, 50 K, and 55 K. As is clear from FIG. 22(b), the obtained structure exhibits the Josephson effect. In particular, the IV characteristics show a steep voltage rise at the critical current Ic, indicating excellent voltage response.

[0073] (Comparative Example 1) <Manufacturing of Comparative Example 1> FIG. 23 is a diagram showing the results of Comparative Example 1, where (a) shows the structure of the structure in Comparative Example 1 and (b) shows the current-voltage characteristics (IV characteristics) of the Josephson junction. As shown in FIG. 23(a), the structure 10 of Example 1 is composed of a substrate 11, a superconductor layer 12, a protective layer 13, and an opening 132. This structure was manufactured as follows. In Comparative Example 1, the Au protective layer 13 of Example 1 was milled entirely by argon ion milling to form openings 132, thereby exposing the YBCO superconductor layer 12. Next, the superconductor layer 12 of the structure was irradiated with ion beams under the same manufacturing conditions as in Example 1 to form Josephson junctions, thereby producing the structure of Comparative Example 1.

[0074] <Evaluation of Comparative Example 1> As shown in Figure 23(b), the current-voltage characteristics (IV characteristics) were measured at low temperatures of 42 K, 57.6 K, 63.9 K, 70.6 K, and 73.3 K. As is clear from Figure 23(b), the obtained structure exhibits the Josephson effect. However, looking at the IV characteristics at 42 K, it can be seen that the voltage rise is not steep, and the voltage response is inferior to that of Example 1.

[0075] REFERENCE SIGNS LIST 10 Structure 11 Substrate (MgO substrate, sapphire substrate) 111 Buffer layer 12 Superconductor layer (YBCO superconductor layer) 120 Josephson junction 121 Superconductor 122 Non-superconductor 13 Protective layer (Au protective layer) 131 Recess 132 Opening 14 End face 20 Focused ion beam device 21 Ion source 22 Ion beam (helium ions) 23 Extraction electrode 24 Focusing lens 25 Scanning coil 26 Objective lens 27 Sample stage

Claims

1. A structure in which a substrate, a superconductor layer, and a protective layer are sequentially stacked, wherein the substrate is made of an insulator, the superconductor layer has a non-superconductor obtained by varying the material of the superconductor, a superconductor-non-superconductor-superconductor Josephson junction is disposed in the superconductor layer, and the protective layer covers all or part of the superconductor layer.

2. The structure according to claim 1, wherein the protective layer has a recess at the bottom of which a part of the protective layer is present, and the Josephson junction is disposed in the superconductor layer in contact with the protective layer where the recess is located.

3. The structure has cerium oxide (CeO ２ ), or silicon nitride (Si ３ N ４ 3. The structure of claim 1, further comprising a buffer layer of 4. The oxide of the substrate is magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), strontium titanate (SrTiO ３ ), LSAT((LaAl ３ ) ０．３ (Sr ２ TaAlO ６ ) ０．７ ), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ 3. The structure of claim 1, wherein the structure is any one of the group consisting of:

5. The superconductor is YBa ２ Cu ３ O ｘ (x is 6.3 to 7), rare earth high temperature superconductor (REBa ２ Cu ３ O ｙ (where RE represents a rare earth element, and y represents 6 to 7), bismuth-based high-temperature superconductors (Bi ２ Sr ２ Ca （ｎ－１） Cu ｎ O （２ｎ＋４） (n=1, 2 or 3), lanthanum-based high-temperature superconductors (La ２ CuO ４ , (La （１－ｘ） Sr ｘ ) ２ CuO ４ (x is 0.05 to 0.30), iron-based superconductor (REFeAsO （１－ｘ） F ｘ , AFe ２ As ２ (A is an alkali metal or alkaline earth metal), AFeAs, FeSe, FeSe （１－ｘ） Te ｘ (x is 0 to 1), ruthenium-based superconductor Sr ２ RuO ４ and MgB ２ 3. The structure of claim 1, wherein the structure is any one of the group consisting of:

6. The structure according to claim 1 or 2, wherein the protective layer is composed of either a metal, an insulator, or a laminate of a metal and an insulator.

7. The structure of claim 6, wherein said metal of said protective layer is any one of the group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo), and ruthenium (Ru).

8. The structure of claim 7, wherein the thickness of the metal in the protective layer is in the range of 1 nm to 100 nm.

9. The structure according to claim 7, wherein the surface resistivity of said protective layer is in the range of 0.15 Ω / sq. to 150.00 Ω / sq. when the length and width of said superconductor layer and said protective layer are converted per unit length.

10. The insulator of the protective layer is made of magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ C), boron nitride (BN), silicon nitride (Si ３ N ４ 7. The structure of claim 6, wherein the metal is any one of the group consisting of titanium nitride (TiN).

11. The structure of claim 10, wherein the thickness of the insulator of the protective layer is in the range of 1 nm to 200 nm.

12. A method for manufacturing a Josephson junction, comprising: an irradiation step of irradiating an ion beam onto a structure in which a substrate, a superconductor layer, and a protective layer are sequentially stacked, using a focused ion beam device; in the irradiation step, the superconductor material of the superconductor layer is changed to a non-superconductor, thereby forming a superconductor-non-superconductor-superconductor Josephson junction.

13. A method for manufacturing a Josephson junction as set forth in claim 12, further comprising an etching step, prior to said irradiation step, of etching said protective layer of said structure from the side opposite to said superconductor layer to form a recess with a bottom in said protective layer while leaving a part of said protective layer, and after said etching step, in said irradiation step, changing the superconductor material of said superconductor layer in contact with said protective layer where said recess is located into a non-superconductor, thereby forming said Josephson junction.

14. The method for manufacturing a Josephson junction according to claim 12, further comprising: a removal step of etching and removing the protective layer of the structure before the irradiation step; and a protective layer formation step of forming a new protective layer after the removal step and the irradiation step.

15. A method for manufacturing a Josephson junction according to any one of claims 12 to 14, wherein the focused ion beam device generates one ion from the group consisting of helium (He) ions, lithium (Li) ions, beryllium (Be) ions, nitrogen (N) ions, or neon (Ne) ions.

16. The method for manufacturing a Josephson junction according to claim 15, wherein the dose of the helium ions per unit length is equal to or greater than 50 (ions / nm) and less than 2000 (ions / nm).

17. The structure has cerium oxide (CeO ２ ), or silicon nitride (Si ３ N ４ 16. The method of claim 15, further comprising a buffer layer of 18. The oxide of the substrate is magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), strontium titanate (SrTiO ３ ), LSAT((LaAl ３ ) ０．３ (Sr ２ TaAlO ６ ) ０．７ ), YSZ, YAl ２ O ３ , NdGaO ３ , lanthanum aluminate (LaAlO ３ ) and dysprosium scandate (DyScO ３ 16. The method of claim 15, wherein the Josephson junction is any one of the group consisting of:

19. The superconductor is YBa ２ Cu ３ O ｘ (x is 6.3 to 7), rare earth high temperature superconductor (REBa ２ Cu ３ O ｙ (where RE represents a rare earth element, and y represents 6 to 7), bismuth-based high-temperature superconductors (Bi ２ Sr ２ Ca （ｎ－１） Cu ｎ O （２ｎ＋４） (n=1, 2 or 3), lanthanum-based high-temperature superconductors (La ２ CuO ４ , (La （１－ｘ） Sr ｘ ) ２ CuO ４ (x is 0.05 to 0.30), iron-based superconductor (REFeAsO （１－ｘ） F ｘ , AFe ２ As ２ (A is an alkali metal or alkaline earth metal), AFeAs, FeSe, FeSe （１－ｘ） Te ｘ (x is 0 to 1), ruthenium-based superconductor Sr ２ RuO ４ and MgB ２ 16. The method of claim 15, wherein the method is any one of the group consisting of:

20. The method for manufacturing a Josephson junction according to claim 15, wherein the protective layer or the new protective layer is made of either a metal, an insulator, or a laminate of a metal and an insulator.

21. A method for manufacturing a Josephson junction as claimed in claim 20, wherein the metal of the protective layer or the new protective layer is any one of the group consisting of gold (Au), silver (Ag), copper (Cu), aluminum (Al), tungsten (W), molybdenum (Mo) and ruthenium (Ru).

22. The method for manufacturing a Josephson junction according to claim 21, wherein the thickness of the metal in the protective layer is in the range of 1 nm to 100 nm.

23. A method for manufacturing a Josephson junction as set forth in claim 21, wherein the surface resistivity of the protective layer is in the range of 0.15 Ω to 150.00 Ω per unit length when the length and width of the superconductor layer and the protective layer are taken as end faces.

24. The insulator of the protective layer is made of magnesium oxide (MgO), sapphire (Al ２ O ３ ), silicon oxide (SiO ２ ), silicon carbide (SiC), vanadium carbide (VC), tungsten carbide (WC), boron carbide (B ４ C), boron nitride (BN), silicon nitride (Si ３ N ４ 21. The method of claim 20, wherein the material is any one of the group consisting of titanium nitride (TiN), titanium nitride (TiN), and titanium carbide (CuN).

25. The method for manufacturing a Josephson junction according to claim 24, wherein the thickness of the insulator in the protective layer is in the range of 1 nm to 200 nm.

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