A process of producing a superconductor junction material

Abstract: A superconductor junction material is disclosed which comprises a substrate of a single crystal, and at least flux flow element, and optionally at least one Josephson junction element, provided on the surface, each of the flux flow and Josephson junction elements being formed of a superconducting oxide layer having a weak link. The flux flow and Josephson junction elements are prepared by vacuum deposition at different oxygen partial pressures.

Description: This invention relates to a process of producing a superconductor junction device having an oxide substrate provided with one or more flux flow elements and one or more Josephson junction elements. 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, the recent trend in the fabrication of Josephson junctions is toward the formation of 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 utilize 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 and difficulty in forming the link at a desired position. Newmann, C. et al propose a YBaCuO Josephson junction produced by a method in which a YBaCuO film is grown on a MgO substrate which has been damaged with a focused Ga-ion beam to form a weak link (Jpn. J. Appl. Phys., 32 , No. 2, L727-L729 1993) and Physica C, 210, 138-146 (1993)). This method permits the formation of a Josephson junction on a desired area on a substrate with good reproducibility and with a high degree of precision. The present invention provides a process of producing a superconducting junction device, comprising the steps of: (a) irradiating a surface of a single crystal substrate with a focused ion beam to form at least two damaged portions; (b) then forming a first superconducting oxide layer by vacuum deposition on said surface; (c) then selectively removing a predetermined part of said first superconducting layer to expose part of said surface together with at least one of said damaged portions and to leave at least one first pattern of said first superconducting layer intersecting at least one of the other damaged portions; (d) then forming a second superconducting oxide layer by vacuum deposition on said exposed surface including said exposed damaged portion; and (e) then selectively removing a predetermined part of said second superconducting layer to leave at least one second pattern of said second superconducting layer intersecting at least one exposed damaged portion, steps (b) and (d) being carried out at different oxygen partial pressures so that either one of said first and second patterns of said superconducting layers forms a Josephson junction element having a weak link just above the corresponding damaged portion with the other said pattern of said superconducting layers forming a flux flow junction element having a weak link just above the corresponding damaged portion (claim 1). Preferred embodiments of the process of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a perspective view schematically showing a junction material obtained according to the present invention; Figs. 2(a)-2(g) are sectional views explanatory of process steps for the fabrication of the junction material of Fig. 1; Fig. 3 is a plan view diagrammatically showing a Josephson junction prepared in Experiment 1; Fig. 4 is a graph showing current/voltage characteristics of the Josephson junction in Experiment 1 without microwave irradiation; Fig. 5 is a graph showing current/voltage characteristics of the Josephson junction in Experiment 1 with microwave irradiation; Fig. 6 is a graph showing current/voltage characteristics of the Josephson junction in Experiment 1 in accordance with the RSJ model; Fig. 7 is a graph showing current/voltage characteristics of the flux flow element obtained in Experiment 2; Fig. 8 is a graph showing a relationship between the critical temperature of an element and the oxygen partial pressure at which the YBaCuO film of the element has been formed; and Fig. 9 is a graph showing a relationship between the c-axis length of a YBaCuO film and the oxygen partial -pressure at which the film has been formed. Fig. 1 schematically depicts an example of a junction material which can be produced according to the present invention. Designated as 11 is a substrate of a single crystal such as MgO, SrTiO 3 , NdGaO 3 , LaAlO 3 or LaGaO 3 . If desired, a mixed crystal of one or more of these single crystals may be used. It is important that the substrate 11 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 permissible for the purpose of the present invention as long as the crystal orientation of a superconductor layer provided on that surface is substantially unvaried over the surface. Provided on the surface of the substrate 11 are a Josephson junction element 100 and a flux flow element 200. The Josephson junction element 100 is formed of a first superconductive oxide layer 15a having weak links 16b and 16c and is utilized, in the illustrated embodiment, as a highly sensitive sensor such as for detecting magneticity, light or a high frequency wave. The flux flow element 200 is formed of a second superconductive oxide layer 15b having a weak link 16a and is utilized as a transistor. One preferred embodiment of the fabrication of the junction material of Fig. 1 will be described with reference to Figs. 2(a)-2(e). As shown in Fig. 2(a), the substrate 11 is first coated with a normal metal, such as Au, Pd or Pt, to form a protecting layer 12 over the flat surface thereof. The formation of the protecting layer 12 may be suitably performed by vacuum deposition. The protecting layer 12 generally has a thickness of 0.01-0.4 µm, preferably 0.02-0.2 µm. The coated substrate 11 is then irradiated with a focused ion beam at any desired position so that the irradiated portion of the protecting layer 12 is etched and removed and damaged portions 14a-14c are defined in the substrate 1 at the desired position, as shown in Fig. 2(b). The damaged portions 14a-14c contain the implanted ion and serves to form weak links as described hereinafter. The focused ion beam is preferably Ga + ion beam. The beam diameter 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 12, the scattering of the ion can be minimized so that the width of the damaged portions 14a-14c can be maintained as small as 300 nm or less. The protecting layer 12 also serves to prevent the formation of a large trench in the substrate 11. Each of the damaged portions 14a-14c is of a flat type and the depth thereof is generally not greater than 100 nm. After the completion of the ion beam irradiation, the protecting layer 12 is completely removed from the substrate 11 as shown in Fig. 2(b). For this purpose, an argon ion milling method or a wet method using an aqueous solution containing KI and I is suitable for reasons of prevention of adverse affection on the substrate 11. A first layer 15a of an oxide superconductor is then formed by vacuum deposition on the surface of the substrate 11 from which the protecting layer 12 has been removed and which bears the damaged portions 14a-14c (Fig. 2(c)). Since those portions of the superconductor layer 15a that are located just above the damaged portions 14a-14c are grown under an influence of the damaged portions, the crystallinity of those portions is disturbed, so that weak links 16a-16c are defined in the superconducting layer 15 at positions just above the damaged portions 14a-14c, respectively. 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 15a is generally in the range of 0.05-0.6 µm, preferably 0.1-0.4 µm. The formation of the superconductor layer 15a may be performed by a pulse laser deposition method, a magnetron sputtering method or like conventional method at a specific, first oxygen partial pressure as described hereinafter. It is preferred that the deposition of the superconductor layer 15a be performed while maintaining the temperature of the substrate 11 at 720-800°C, more preferably 740-790°C, for reasons of obtaining a desired superconducting oxide crystal film. The superconductor layer 15a is then overlaid with a photoresist mask 18 (Fig. 2(d)) and patterned by, for example, photolithography and dry etching with argon ion or chemical etching, thereby to form a pattern 15a (Fig. 1 and Fig. 2(e)) which intersects the damaged portions 14b and 14c. Thus, a Josephson junction element 100 having weak links 16b and 16c is formed on the substrate 11. Next, a second, oxide superconductor layer 15b is formed on the substrate 11 bearing the Josephson junction element 100 in the same manner as in the formation of the first superconductor layer 15a except that a different, second oxygen partial pressure is used as described hereinafter (Fig. 2(f)). The second superconductor layer 15b is then subjected to photolithography and patterning in the same manner as described above to form a pattern 15b (Fig. 1 and Fig. 2(g)) which intersects the damaged portions 14a. Thus, a flux flow element 200 having weak links 16a is formed on the substrate 11. It has been unexpectedly found that the junction characteristics can be controlled by control of the oxygen partial pressure at which the vacuum deposition of an oxide superconductor on a damaged portion-bearing substrate is performed and that a Josephson junction element and a flux flow element can be selectively obtained at different oxygen partial pressures. Thus, for example, when the oxygen partial pressure (the first pressure in the above embodiment) is not greater than 19.95 Pa (150 mTorr), preferably between 9.975-19.95 Pa (75-150 mTorr), more preferably 13.3-19.285 Pa (100-145 mTorr), a Josephson junction is formed on the substrate. On the other hand, a flux flow element is obtained when the oxygen partial pressure (the second pressure in the above embodiment) is greater than 19.95 Pa (150 mTorr), preferably between 20.61-39.9 Pa (155-300 mTorr), more preferably 20.61-26.6 Pa (155-200 mTorr). The flux flow element is distinguished from the Josephson junction element in that a magnetic flux which has penetrated into the weak link of the former element can flow therethrough, whereas a magnetic flux which has penetrated into the weak link of the latter element is pinned thereat. In the foregoing embodiment, only one Josephson junction element 100 and only one flux flow junction are described as formed on the substrate 11 for sake of simplicity. It is without saying that any desired number of such elements can be formed on a single substrate in the same manner as above. Further, the formation of the Josephson junction element(s) may be preceded by the formation of the flux flow element. The oxide superconductor layer 15a may be formed of an oxide superconductor having a composition which is the same as or different from that of the layer 15b. The following experiments and Example will further illustrate the present invention. Experiment 1 Fabrication of Josephson Junction Element: A Josephson junction element as shown in Fig. 3 was prepared. One surface of a single crystal MgO (100) substrate 11 was coated with Au by vacuum deposition at 1.33x10 -7 Pa (10 -6 mTorr) 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, in a micro area of 0.1 µm x 20 µm, with focused Ga + ion beam at an acceleration voltage of 30 keV and a beam current of 1 pA so that there was formed a damaged portion 14 into which Ga infiltrated and which had a shallow groove with a depth of 40-80 nm. The Au layer was then removed by chemical etching with a solution obtained by dissolving 4 g of KI and 1 g of I in 150 g of distilled water. A YBa 2 Cu 3 O 7-x was then deposited on the substrate 11 by a pulse laser deposition method at an oxygen partial pressure of 13.3 Pa (100 mTorr) and a substrate temperature of 770°C to form a YBaCuO superconductor layer 15 having a thickness of 300 nm. The YBaCuO layer 15 was deposited under the influence of the damaged portion 14 of the substrate 11 so that the crystallinity of that portion of the YBaCuO layer 15 positioned just above the damaged portion 14 was disturbed. The YBaCuO layer 15 was then patterned by photolithography and dry etching with argon ion to form a pattern as shown in Fig. 3 composed of two electrode sections each having a width of 1 mm and a length of 5 mm and a linking section which had a width of 5 µ m and a length of 30 µ m and which was intersected by the damaged portion 14. The intersecting region of the YBaCuO pattern represents a weak link 16 of the thus obtained Josephson junction element. Fig. 4 shows voltage-current characteristics of the above Josephson junction element at 14 K without microwave irradiation, while Fig. 5 shows Shapiro step characteristics of the above Josephson junction element measured at 15 K with a strong microwave irradiation of 25 GHz. A Josephson junction may be explained by a resistively shunted junction (RSJ) model in which a distributed capacity, a quasiparticle conductance and a nonlinear Josephson junction element are connected in parallel. Fig. 6 shows voltage-current characteristics of the above Josephson -junction, from which it will be noted that the curve is downwardly convex and, thus, is of a RSJ type. From the results shown in Figs. 5 and 6, it will be appreciated that the Josephson junction thus formed exhibits excellent performance. Experiment 2 Fabrication of Flux Flow Element: Experiment 1 was repeated in the same manner as described except that the deposition of the YBaCuO layer was performed at an oxygen partial pressure of 26.6 Pa (200 mTorr). The thus obtained oxide superconductor junction element showed voltage-current characteristics as shown in Fig. 7. The curve is upwardly convex in the plus voltage region, suggesting that the element is of a flux flow type rather that a Josephson junction type. The above procedure was repeated in the same manner as described at various oxygen partial pressures of 6.65 Pa, 9.975 Pa and 20.61 Pa (50, 75 and 155 mTorr). At 9.975 Pa (75 mTorr), a Josephson junction element was obtained. At 20.61 Pa (155 mTorr), a flux flow element was obtained. Figs. 8 and 9 show oxygen partial pressure dependence of the critical temperature and c-axis length, respectively. With an oxygen partial pressure of 6.65 Pa (50 mTorr), the critical temperature is significantly lowered with the simultaneous increase of the c-axis length. Example 1 Fabrication of Superconductor Junction Material Having Josephson Junction Element and Flux Flow Element: A superconductor junction element having a Josephson junction element 100 and a flux flow element 200 as shown in Fig. 1 was prepared. One surface of a single crystal MgO (100) substrate 11 was coated with Au by vacuum deposition at 1.33x10 -7 Pa (10 -6 mTorr) 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, in a micro area of 0.1 µm x 20 µm, with focused Ga + ion beam at an acceleration voltage of 30 keV and a beam current of 1 pA so that there were formed damaged portion 14a-14c into each of which Ga infiltrated and each of which had a shallow groove with a depth of 40-80 nm. The Au layer was then removed by chemical etching with a solution obtained by dissolving 4 g of KI and 1 g of I in 150 g of distilled water. A YBa 2 Cu 3 O 7-x oxide was then deposited on the substrate 11 by a pulse laser deposition method at an oxygen partial pressure of 13.3 Pa (100 mTorr) and a substrate temperature of 770°C to form a YBaCuO superconductor layer having a thickness of 300 nm. The YBaCuO layer was deposited under the influence of the damaged portions 14a-14c of the substrate 11 so that the crystallinity of those portions of the YBaCuO layer positioned just above the damaged portions 14a-14c was disturbed. The YBaCuO layer was then patterned by photolithography and dry etching with argon ion to form a pattern 15a having weak links 16b and 16c each having a width of 5 µm and a length of 30 µm and each located just above the damaged portions 14b and 14c. A YBa 2 Cu 3 O 7-x oxide was then deposited on the substrate 11 by a pulse laser deposition method at an oxygen partial pressure of 26.6 Pa (200 mTorr) and a substrate temperature of 770°C to form a YBaCuO superconductor layer having a thickness of 300 nm. The YBaCuO layer was then patterned by photolithography and dry etching with argon ion to form a pattern 15b having weak link 16a which had a width of 5 µ m and a length of 30 µ m and which was located just above the damaged portions 14a.

Claims: A process of producing a superconducting junction device, comprising the steps of: (a) irradiating a surface of a single crystal substrate (11) with a focused ion beam to form at least two damaged portions (14a-14c); (b) then forming a first superconducting oxide layer (15a) by vacuum deposition on said surface; (c) then selectively removing a predetermined part of said first superconducting layer (15a) to expose part of said surface together with at least one of said damaged portions (14a) and to leave at least one first pattern of said first superconducting layer (15a) intersecting at least one of the other damaged portions (14b, 14c); (d) then forming a second superconducting oxide layer (15b) by vacuum deposition on said exposed surface including said exposed damaged portion; and (e) then selectively removing a predetermined part of said second superconducting layer (15b) to leave at least one second pattern of said second superconducting layer (15b) intersecting at least one exposed damaged portion (14a), steps (b) and (d) being carried out at different oxygen partial pressures so that either one of said first and second patterns of said superconducting layers forms a Josephson junction element (100) having a weak link (16b, 16c) just above the corresponding damaged portion with the other said pattern of said superconducting layers forming a flux flow junction element (200) having a weak link (16a) just above the corresponding damaged portion (14a). A process as claimed in Claim 1, wherein step (b) is performed at an oxygen partial pressure of not lower than 13.3 Pa (100 mTorr) but not higher than 19.95 Pa (150 mTorr) to form said Josephson junction element (100) and step (d) is performed at an oxygen partial pressure of higher than 19.95 Pa (150 mTorr) but not higher than 26.6 Pa (200 mTorr) to form said flux flow element (200). A process as set forth in Claim 1 or Claim 2, wherein before step (a) said surface is coated with a gold layer (12), said gold layer being removed before step (b). A process as claimed in any preceding claim, wherein said focused ion beam is a focused Ga + ion beam. A process as claimed in any preceding claim, wherein steps (c) and (e) are each performed by chemical etching or plasma etching. A process as claimed in any preceding claim, wherein steps (b) and (d) are each performed by pulsed laser deposition at a substrate temperature of 720-800°C.

Inventors: ENOMOTO YOUICHI, FUJIMOTO MANABU, SUZUKI KATSUMI, TANAKA SHOJI, ENOMOTO, YOUICHI, FUJIMOTO, MANABU, SUZUKI, KATSUMI, TANAKA, SHOJI, ENOMOTO, YOUICHI, C/O SUPERCON.RES.LAB., TANAKA, SHOJI, C/O SUPERCON.RES.LAB.
Applicants: SHARP KK, SUPERCONDUCTIVITY RESEARCH LAB, SHARP CORPORATION, SUPERCONDUCTIVITY RESEARCH LABORATORY OF INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER
Application year: 1995
Main classification: H01L39
IPC codes: H01L39, H01L27