Process of producing a Josephson junction

Abstract: A Josephson junction is disclosed which includes a substrate of a single crystal having a substantially flat surface, a wiring pattern of an oxide superconductor formed on the flat surface of the substrate, and an altered region formed having a width of 300 nm or less and formed in the wiring pattern to intersect the wiring pattern, the crystal orientations of the wiring pattern on both sides of the altered region being equal to each other. The Josephson junction may be prepared by a process including the steps of: (a) coating a surface of a substrate of a single crystal with a normal metal to form a protecting layer over the surface of the substrate; (b) irradiating a predetermined portion of the protecting layer with a focused ion beam so that an irradiated portion is formed in the substrate; (c) removing the protecting layer from the substrate; and (d) forming a wiring pattern of an oxide superconductor on the surface of the substrate from which the protecting layer has been removed such that the wiring pattern crosses the irradiated portion of the substrate, thereby forming an altered portion in the wiring pattern at a position above the irradiated portion.

Description: This invention relates to a process for the production of a Josephson junction. In application of superconductors to electronic devices, it is very important to produce Josephson junctions with a large IcRn product. Because high Tc oxide superconductors have layered crystal structures and short coherent lengths, it is very difficult to produce Josephson junctions of a laminate type. Thus, a recent trend in the fabrication of Josephson junctions is toward the formation of a grain boundary on a substrate. Gross, R. et al propose the use of a substrate which is a laminate of two single crystal layers (Phys. Rev. Lett., 64 , 228 (1990)). Dary, K. et al propose the use of a substrate having a stepped portion (Appl. Phys. Lett., 58 , 543 (1991)). These methods, which utlize the difference in crystal orientations of the superconductors on both sides of the link, are not satisfactory because of the poor reproducibility of the Josephson junction characteristics, a small IcRn product (below 0.3 mV) and difficulty in forming the link at a desired position. Ghyselen, B. et al propose a YBaCuO/normal metal/YBaCuO junction produced with the use of a focused ion beam micromachining technique in which a 20 KeV Ga + focused ion beam with a diameter of 50 nm is used to form a trench in YBaCuO line provided on a SrTiO 3 substrate. The resulting two superconducting electrodes separated from each other by the trench are then connected by deposition of the normal metal bridge (Physica C, 198 , 215 (1992)). A method is also known which comprises the steps of: irradiating a predetermined portion of a surface of a substrate of a MgO single crystal with a Ga + focused ion beam to form a steep slope trench in the substrate; depositing YBaCuO superconductor on the surface of the substrate so that a tunnel barrier is formed in the YBaCuO layer; and patterning the YBaCuO superconductor layer to form a wiring pattern such that the tunnel barrier intersects the wiring pattern. In this method, a superconductor/normal metal/superconductor (SNS) structure is considered to be formed because the crystal orientation of the superconductor in the trench differs from that of the superconductor outside of the trench or because a grain boundary is formed on the trench. With this method, however, the reproducibility of the Josephson junction characteristics is not good and the IcRn product is as small as 0.3 mV. EP-A-0325765 discloses a process for the production of a Josephson junction in which, in a first step, chemically stable ions such as Au or Pt ions are implanted into an unprotected main surface of a crystal substrate to distort or break the crystal lattice of the substrate, and thereafter, in a second step, a superconductor film is grown on the main surface of the substrate to form a Josephson junction in the superconductor film at a portion of the distorted or broken-lattice region of the substrate. EP-A-0364101 discloses a method of forming a weak-link Josephson junction in which unprotected substrate is first irradiated with a focused Ga + ion beam so that, depending on the magnitude of the acceleration voltage of the ion beam, the metal is deposited on the substrate or a "damaged" region if formed below the surface of the substrate. Thereafter, a thin film of an oxide superconductor is epitaxially grown on the substrate, with the result that on regions of the substrate which were not subjected to the Ga + ion irradiation a superconductive thin film is epitaxially grown whereas on the regions subjected to the irradiation the epitaxial growth is hindered and microcrystals are produced. There is thus produced a crystal grain boundary which can be used as a weak-link Josephson junction. The present invention has been made to provide a Josephson junction having a large IcRn product and a process of defining such a Josephson junction on any desired area on a substrate with good reproducibility and with a high degree of precision. More particularly, the present invention provides a process for the production of a Josephson junction, comprising the steps of: (a) coating a surface of a substrate of a single crystal with a normal metal to form a protecting layer over the surface of said substrate; (b) irradiating a predetermined portion of said protecting layer with a focused Ga + ion beam so that an irradiated portion is formed in said substrate; (c) removing said protecting layer from said substrate; and (d) forming a wiring pattern of an oxide superconductor on the surface of said substrate from which said protecting layer has been removed such that said wiring pattern crosses said irradiated portion of said substrate, thereby forming an altered portion in said wiring pattern at a position above said irradiated portion. Step (d) preferably includes the substeps of: depositing said oxide superconductor on the surface of said substrate from which said protecting layer has been removed to form a layer of said oxide superconductor and to form said altered portion in said oxide superconductor layer at a position above said irradiated portion; and patterning said oxide superconductor layer to form said wiring pattern such that said altered portion intersects said wiring pattern. The present invention will now be described in detail below with reference to the accompanying drawings in which: Fig. 1 is an enlarged plan view diagrammatically showing a typical Josephson junction produced according to the present invention; Figs. 2(a)-2(e) are sectional views explanatory of process steps in accordance with the present invention for the fabrication of the Josephson junction of Fig. 1, Fig. 2(e) being a sectional view taken on line II-II in Fig. 1.; Fig. 3 is a graph showing current/voltage characteristics of the Josephson junction produced in Example 1; Fig. 4 is a diagram showing the Shapiro step characteristics of the Josephson junction produced in Example 1; and Fig. 5 is a graph showing temperature dependency of superconducting critical electric current density of Josephson junctions obtained in Example 1. One preferred embodiment of the fabrication of a Josephson junction according to the present invention will now be described with reference to Figs. 2(a)-2(e). The process uses a substrate 1 of a single crystal such as MgO, SrTiO 3 , NdGaO 3 , LaAlO 3 . If desired a mixed crystal of one or more of these single crystals may be used. It is important that the substrate 1 have a substantially flat surface. The term "substantially flat surface" used herein is intended to refer to such a surface that a superconductive layer provided thereon has crystal orientation substantially equal throughout that surface. Thus, the presence of small grooves or steps in a surface of the substrate 1 is permissive for the purpose of the present invention as long as the crystal orientation of a superconductor layer provided on that surface is substantially the same. As shown in Fig. 2(a), the substrate 1 is first coated with a normal metal, such as Au, Pd or Pt, to form a protecting layer 2 over the flat surface thereof. The formation of the protecting layer 2 may be suitably performed by vacuum deposition. The protecting layer 2 generally has a thickness of 0.01 to 0.4 µm, preferably 0.02 to 0.2 µm. The coated substrate 1 is then irradiated with a focused Ga + ion beam at any desired position so that the irradiated portion of the protecting layer 2 is etched and removed and an irradiated portion 3 is defined in the substrate 1 at the desired position, as shown in Fig. 2(b). The irradiated portion 3 contains the implanted Ga + ions and serves to form an altered portion 5 in the superconductor layer 4 provided on the substrate 1 and the altered portion 5 in turn provides a link or junction of superconductor wiring pattern 4a, as described hereinafter. The diameter of the focused Ga + ion beam is preferably in the range of 10-100 nm and the beam current is in the range of 1 pA - 60 nA. Because of the presence of the protecting layer 2, the scattering of the Ga + ions can be minimized so that the width of the irradiated portion 3 can be maintained as small as 300 nm or less. The protecting layer 2 also serves to prevent the formation of a large trench in the substrate 1. After the completion of the Ga + ion beam irradiation, the protecting layer 2 is completely removed from the substrate 1 as shown in Fig. 2(c). For this purpose, an argon ion milling method or a wet method using an aqueous solution containing KI and I are suitable. A layer 4 of an oxide superconductor is then formed on the surface of the substrate 1 from which the protecting layer 2 has been removed and which bears the irradiated portion 3 (Fig. 2(d)). As a result, an altered portion 5 is defined in the superconducting layer 4 at a position above the irradiated portion 4. The oxide superconductor is preferably a YBaCuO superconductor, such as YBa 2 Cu 3 O 7-x or YBa 2 Cu 4 O 8-x . The thickness of the superconductor layer 4 is generally in the range of 0.05 to 0.6 µm, preferably 0.1 to 0.4 µm. The formation of the superconductor layer 4 may be performed by such conventional methods as a pulse layer deposition method or a magnetron sputtering method. The superconductor layer 4 is thereafter patterned to define a wiring pattern 4a (Fig. 1 and Fig. 2(e)) such that the altered portion 5 intersects the wiring pattern 4a. Electrodes 6 are subsequently provided on the wiring pattern 4a by any suitable known method. The patterning may be suitably effected by conventional photolithography. In the thus prepared Josephson junction, the crystal orientations of the wiring pattern 4a on both sides of the altered region 5 are equal to each other. Preferably, the average crystal orientation of the altered region 5 is equal to the crystal orientation of the wiring pattern 4a on each side of the altered region 5. In the foregoing embodiment, only one Josephson junction is formed on the substrate. This is, however, only for simplifying explanation, and any desired number of such junctions can be formed on a single substrate in the same manner as above. The method according to the present invention is effective in producing integrated Josephson junctions utilized for the application to SQUIDS. The following example will further illustrate the present invention. Example 1 One surface of a single crystal MgO (100) substrate was coated with Au by vacuum deposition to form a protecting layer of Au having a thickness of about 100 nm. This was then disposed in a focused ion beam apparatus and was irradiated with Ga + ion beam at 30 KeV focused ion beam. The beam with a diameter of 50 nm was scanned through a length of 10 µm for 40 seconds with a beam current of 1 pA so that there was formed an irradiated portion into which Ga was infiltrated. The width of the irradiated portion was about 130 nm. The Au layer was then removed by 450 eV Ar + ion milling. A YBa 2 Cu 3 O 7-x was then deposited on the substrate by a pulse laser deposition method to form a superconductor layer having a thickness of 300 nm and having an altered portion above the irradiated portion. The YBaCuO layer was then patterned to form an H-shaped wiring pattern which had a length of 30 µm and a width of 5 µm and which was intersected by the altered portion. An electrode was applied to each of the four leg portions of the H-shaped wiring pattern to form a junction structure. When this structure was irradiated with a microwave at 65 K, Shapiro steps were observed, indicating that the structure was a Josephson junction. Fig. 3 shows voltage-current characteristics of the above Josephson junction at 4.2 K under conditions in which no microwave was irradiated (line 11) or a high power microwave was irradiated (line 12). The critical current (Ic), which is a current at a voltage of zero when no microwave power is applied, was 7 mA. It is known that the critical current is perfectly supressed and the resistivity approaches normal resistance (Rn) when a sufficiently large microwave power is applied. In the case of Fig. 3, the normal resistance is about 8 ohms. Fig. 4 shows Shapiro step characteristics of the above Josephson junction measured at 4.2 K while irradiating a strong microwave of 10.2 GHz. The Ga + ion beam was scanned twice through a length of 10 µm for 20 seconds with a beam current of 1 pA. From the results shown in Fig. 4, it will be appreciated that the Josephson junction exhibits excellent performance. The IcRn of this junction was found to be more than 50 mV. This indicates that a good tunnel barrier is formed. Similar procedures were repeated a number of times. Josephson junctions having large RnIc products were obtained with excellent reproducibility. Fig. 5 shows temperature dependency of the superconducting critical current per unit area (critical current density (jc)) of the Josephson junctions. The curves 13 and 14 are for the samples with irradiation widths of 130 nm and 60 nm, respectively. The curve 15 is for the non-irradiated sample. From the results shown in Fig. 5, it will be appreciated that the current density (jc) decreases with the increase in irradiation width. The reason for this is considered to be because that portion of the YBaCuO layer above the irradiated region is altered and converted into a normal conductor so that there is formed an SNS (superconductor/normal metal/ superconductor) structure. Namely, it is believed that the crystalinity of the portion of the MgO substrate irradiated with Ga ion beam is disturbed so that a stress or crystal disturbance is also caused in that portion of the YBaCuO layer above the irradiated portion of the MgO substrate. The Ga which has implanted into the irradiated portion of the MgO substrate is diffused into the YBaCuO layer during the formation of the YBaCuO layer which is performed at a high temperature. The crystal disturbance and the Ga diffusion synergetically cause oxygen defficiency in that portion of the YBaCuO layer above the irradiated portion of the MgO substrate. Since the YBaCuO layer is C-axis oriented, the vertical direction of the junction plane of the plannar structure becomes a-b axis having a long coherent length. Further, the crystal structure and chemical composition of the tunnel layer are similar to those of the superconductor layer. For these reasons, a superconductive tunnel current is considered to flow.

Claims: A process for the production of a Josephson junction, comprising the steps of: (a) coating a surface of a substrate (1) of a single crystal with a normal metal to form a protecting layer (2) over the surface of said substrate; (b) irradiating a predetermined portion of said protecting layer with a focused Ga + ion beam so that an irradiated portion (3) is formed in said substrate; (c) removing said protecting layer (2) from said substrate; and (d) forming a wiring pattern (4a) of an oxide superconductor (4) on the surface of said substrate from which said protecting layer has been removed such that said wiring pattern crosses said irradiated portion of said substrate, thereby forming an altered portion (5) in said wiring pattern at a position above said irradiated portion. A process according to Claim 1, wherein step (d) includes the substeps of: depositing said oxide superconductor (4) on the surface of said substrate (1) from which said protecting layer (2) has been removed to form a layer of said oxide superconductor and to form said altered portion (5) in said oxide superconductor layer at a position above said irradiated portion; and patterning said oxide superconductor layer to form said wiring pattern (4a) such that said portion intersects said wiring pattern. A process according to Claim 1 or Claim 2, wherein said altered region (5) formed in step (d) has an average crystal orientation which is equal to the crystal orientations of said wiring pattern (4a) on both sides of said altered region. A process according to any preceding claim, wherein said normal metal is gold. A process according to any preceding claim, wherein the protecting layer formed in step (a) has a thickness of 0.02 to 0.2 µm. A process according to any preceding claim, wherein said substrate (1) is formed of a single crystal of an oxide selected from MgO, SrTiO 3 , NdGaO 3 , LaAlO 3 , LaGaO 3 and mixed crystals thereof. A process according to any preceding claim, wherein said oxide superconductor is a YBaCuO superconductor. A process according to any preceding claim, wherein the thickness of the oxide superconductor forming said wiring pattern (4a) is from 0.1 to 0.4 µm.

Inventors: ENOMOTO YOUICHI, NEUMANN CHRISTIAN, SUZUKI KATSUMI, TANAKA SHOJI, ENOMOTO, YOUICHI, NEUMANN, CHRISTIAN, SUZUKI, KATSUMI, TANAKA, SHOJI, TANAKA, SHOJI, CITY CORP. KASHIWAGI 103
Applicants: INT SUPERCONDUCTIVITY TECH, NIPPON ELECTRIC CO, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, NEC CORPORATION
Application year: 1993
Main classification: H01L39
IPC codes: H01L39