Superconducting wiring lines and process for fabricating the same

Abstract: The patterned superconducting wiring lines (21,22) each consist of a portion of a thin film of an oxide superconductor deposited on a flat substrate (1), the portion having a predetermined crystal orientation (a-axis or c-axis orientation) with respect to a flat surface of the substrate. Remaining portions of the thin film of the oxide superconductor have a different crystal orientation (c-axis or a-axis orientation) from the portion and/or consist of insulation zones (23). Both of the portion and the remaining portions have a substantially identical thickness so that the thin film has a substantially flat planar surface.

Description: Background of the Invention Field of the Invention The present invention relates to a superconducting circuit, more particularly to novel patterned superconducting wiring lines each made of a thin film of oxide superconductor deposited on a substrate and a process for fabricating the same. Description of the related arts The conventional superconductors exhibit the superconductive phenomenon only at extremely low temperature so that it has not been expected to be used in actual applications. In 1986, however, new oxide type superconductors of [La, Ba] 2 CuO 4 and [La, Sr] 2 CuO 4 were discovered and after then, discovery of the other superconducting compound oxides such as Y-Ba-Cu-O system or Bi-Ca-Sr-Cu-O system was continued. In these newly discovered oxide superconductors, the superconductive phenomenon can be realized with relatively cheaper liquid nitrogen and hence the possibility of an actual utilization of the high Tc superconductors have burst onto the scene. Although these oxide superconductors were obtained initially in a bulk form of sintered block by powder sintering technique, now it becomes possible to prepare their thin films of high quality by physical vapour deposition or chemical vapour deposition techniques, to prepare a laminated structure with such super conductors (see EP-A-0 302 354) or to deposit them on a silicon base (see EP-A-0 327 493). The thin films of oxide superconductor are expected to be used in a variety of applications such as sensors, SQUID, superconducting devices including superconducting transistors and Josephson devices. In this specification, the term "oxide superconductor" means any high-Tc compound oxide which show the critical temperature of above 30 K. In superconducting circuits having superconducting wiring lines, it is desirable to integrate patterned superconducting wiring lines in the same thin film of oxide superconductor as is used to fabricate the superconducting elements or devices. However, how to fabricate the patterned superconducting wiring lines in an integrated circuit have not been established. It is known that the critical current density of oxide superconductor depend on the orientation of crystals of which a thin film of oxide superconductor is made and that the crystal possess anisotropy. This anisotropy limits the structure of a superconducting circuit including superconducting wiring lines made of a thin film of oxide superconductor. An object of the present invention is to solve the problem and provides a novel structure of patterned superconducting wiring lines each made of a thin film of oxide superconductor deposited on a substrate and a process for fabricating the same, in order to increase the freedom of circuit designing and to make the best use of high Tc oxide superconductor. Summary of the Invention The present invention provides patterned superconducting wiring lines, characterized in that each of the superconducting wiring lines consists of a portion of a thin film of an oxide superconductor deposited on a flat substrate, the portion having a predetermined crystal orientation with respect to a flat surface of the substrate, remaining portions of the thin film of the oxide superconductor having a different crystal orientation from the portion, and both of the portion and the remaining portions having a substantially identical thickness so that the film has a substantially flat planar surface. The terms "a-axis", "b-axis" and "c-axis" are crystal axes of a crystal, as are usually used in the crystallography and they are known and determined in crystalline oxide superconductors to which the present invention is applicable. Orientation of these axes is usually defined by the direction of the crystal axis with respect to a surface of the substrate. For example, "a-axis oriented thin film" is a thin film whose a-axis is perpendicular to the surface of the substrate. The superconducting circuit having superconducting wiring lines according to the present invention can be fabricated by using any oxide superconductor including compound oxide superconductors such as Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu-O system. The substrate on which the thin film of oxide superconductor is deposited is preferably a single crystalline substrate of oxide such as MgO, SrTiO 3 , CdNdAlO 4 or the like. The substrate may be a semiconductor substrate such as silicon substrate having or not having an insulating layer or buffer layer. The superconducting wiring lines can have an upper insulation layer thereon. In a special application, the upper insulation layer has preferably a thickness of less than 10 nm. In another variation, another thin film of oxide superconductor is deposited on the upper insulation layer. The present invention provides a superconducting circuit having superconducting wiring lines each made of a thin film of oxide superconductor deposited on a substrate, comprising first superconducting wiring lines each consisting of a thin film of an oxide superconductor whose c-axis is oriented perpendicularly to a surface of the substrate so that superconducting current flows in parallel with the surface of the substrate, and shield zones each consisting of a thin film of the same oxide superconductor whose a-axis is oriented perpendicularly to the surface of the substrate and whose b-axis is oriented in parallel with the direction of a current which propagates in the superconducting wiring line. The superconducting circuit can have second superconducting wiring lines each extending perpendicularly to the first superconducting wiring lines, and insulation zones being arranged between the second superconducting wiring lines and the shield zone. The present invention provides a process for fabricating this superconducting circuit having the superconducting wiring lines, characterized by the steps of depositing a first thin film of an oxide superconductor whose a-axis or c-axis is oriented perpendicularly to a surface of the substrate, removing predetermined areas of the first thin film to leave patterned first superconducting lines, depositing a second thin film of the oxide superconductor in such a condition that the second thin film becomes an a-axis oriented or c-axis oriented thin film which is oriented differently from the first thin film on the surface of the substrate at remaining areas where the first thin films are removed, and forming insulation zones at predetermined portions of the thin film, each of the portions corresponding to an interface between the second superconducting wiring line and the shield zone. The insulation zone can be produced by deoxygenation treatment which can be effected in a vacuum chamber or by a laser beam or a focussed ion beam. An essence of the superconducting circuit according to the present invention resides in that each of superconducting wiring lines is made of a thin film of an oxide superconductor which is juxtaposed in a single layer and can have a specific crystal orientation. In other words, in the present invention, the superconducting circuit including all superconducting wiring lines has a substantially flat planar surface. As stated above, the thin film of oxide superconductor shows anisotropy in the current property due to the anisotropy in crystal. In fact, a great value of current flows along a direction which is perpendicular to the c-axis but only extremely limited critical current flows along a direction of the c-axis. In the superconducting circuit according to the present invention, the superconducting wiring lines made of a-axis oriented thin film are used to propagate an electric current along a direction which is perpendicular to the surface of the substrate, in other words, they function as connection lines between two adjacent layers deposited one over another, while the superconducting lines made of c-axis oriented thin film are used to pass an electric current along a direction which is in parallel with the surface of the substrate. The superconducting circuit having superconducting wiring lines according to the present invention can be made up in a form of a multilayered superconducting circuit. In this case also, each layer can have the superconducting wiring lines as well as viaholes which are juxtaposed on a common plane in the mostly compacted layout. In the superconducting circuit having the superconducting wiring lines according to the present invention, all of the superconducting wiring lines are made of c-axis oriented thin film of oxide superconductor and are separated by shield zones each made of an a-axis oriented thin film of the same oxide superconductor. In this case, both of the a-axis oriented thin film and the c-axis oriented thin film co-exist in one layer and substantially no current flows between these two superconducting thin films even if these two thin films are juxtaposed because no substantial current is propagated between these two thin films. Therefore, if the superconducting wiring lines each made of the c-axis oriented thin film are produced in predetermined portions of the thin film of oxide superconductor while the shield zones each made of the a-axis oriented thin film are produced at the remaining portions of the same thin film of oxide superconductor, crosstalk between adjacent superconducting wiring lines can be minimized and hence performance of superconducting wiring is improved. Still more, the density of wiring lines also can be increased. It must be noted that, although substantially no propagation of current occurs when the propagation direction of a current passing through a superconducting wiring line is perpendicular to the c-axis of the shield zone, a current propagates from a superconducting wiring line to the shield zone when the current propagation direction becomes in parallel with the c-axis of the shield zone. Therefore, in the latter case, it is necessary to arrange an insulation zone between the superconducting wiring line and the shield zone. The insulation zone can be produced easily by deoxygenation treatment or the other means. The superconducting circuit having the superconducting wiring lines according to the present invention can be fabricated as follows: At first, a first thin film of an oxide superconductor whose c-axis is oriented perpendicularly to a surface of the substrate is formed. Then, predetermined areas of the fist thin film of c-axis oriented oxide superconductor are removed to leave pattern of first superconducting wiring lines. After this, a second thin film of the same oxide superconductor is deposited in such a condition that the second thin film becomes an a-axis oriented thin film. The processes according to the present invention have the following merits: (1) The properties of the thin film of oxide superconductor are not deteriorated during heat-treatment because both of the a-axis oriented thin film and the c-axis oriented thin film are made of an identical material. (2) Fabrication of the other elements and/or wiring lines which will be effected on the superconducting circuit of the present invention is easy, because a surface of the final circuit having the superconducting wiring lines according to the present invention has a flat planar surface. In embodiments of the present invention, superconducting wiring lines for passing superconducting current in parallel with a surface of a substrate are separated by shield zones so that crosstalk between the superconducting wiring lines can be minimized. In other words, the wiring density can be increased. Still more, since both of the superconducting wiring lines and of the shield zones are produced by an identical material, deterioration of superconducting properties of thin films of oxide superconductor caused by heat-treatment can be minimized. Now, the present invention will be described with reference to Examples but the scope of the invention should not be limited thereto. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustrative perspective view of an embodiment of a superconducting circuit obtained by the present invention. Figures 2A to 2F illustrate successive steps for fabricating a superconducting wiring pattern shown in Figure 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Figure 1 is an illustrative perspective view of an embodiment of superconducting circuit according to the present invention. The superconducting circuit of Figure 1 has a thin film layer 2 of oxide superconductor deposited on a substrate 1. The thin film layer 2 of oxide superconductor comprises c-axis oriented thin film zones 2c, a-axis oriented thin film zones 2a and insulation zones 2b. The c-axis oriented thin film zones 2c provide superconducting wiring lines in which a superconducting current propagates in parallel with a surface of the substrate 1. In these superconducting wiring lines 2c, superconducting current can flow along two directions "Y" and "X". Lines along the direction "Y" are called the first superconducting wiring lines 2c', while lines along the direction "X" are called the second superconducting wiring lines 2c". The a-axis oriented thin film zones 2a provide shield zones and each functions as a shield to separate respective superconducting wiring lines 2c. In fact, the shield zones 2a prevent crosstalk between adjacent superconducting wiring lines 2c in which superconducting current can flow along any one of two directions "Y" and "X". The insulation zones 2b are arranged at opposite sides of each second superconducting wiring line. The insulation zones 2b can be produced by deoxygenation treatment which is effected on selected portions of the a-axis oriented thin film 2a. Example 2 Figures 2A to 2F are illustrative drawings showing successive steps for fabricating the superconducting circuit shown in Figure 1. In this Example, a compound oxide superconductor of Y-Ba-Cu-O system is used as the oxide superconductor. At first, a substrate 1 having a smooth surface is prepared. In this example, the substrate 1 (Figure 2A) consists of a silicon substrate la having a buffer layer 1b which is composed of an under-layer of MgAl 2 O 3 having a thickness of 350 nm deposited by chemical vapour deposition (CVD) technique and an upper-layer of BaTiO 3 having a thickness of 75 nm deposited by sputtering technique. MgO (100) substrate, CdNdAlO 4 substrate or the like can be preferably used in place of the silicon substrate. On the buffer layer of the substrate 1, a thin film 2 of oxide superconductor having a thickness of more than 200 nm as is shown in Figure 2B is deposited. This thin film 2 can be prepared by an off-axis sputtering technique, a reactive evaporation technique or the like. In this example, the thin film 2 of oxide superconductor is prepared by the off-axis sputtering technique under the presence of a mixed sputtering gas of Ar + O 2 (volume ratio of 9 : 1) of 10 Pa. The substrate is heated at a temperature which is higher than 700°C so that the crystals deposited produce a c-axis orientated thin film of oxide superconductor in which c-axis is oriented perpendicularly to the surface of the substrate. Then, as is shown in Figure 2C, a photoresist layer 3 having a predetermined pattern is coated on the thin film 2 of oxide superconductor obtained. After the photoresist layer 3 is cured, exposed areas which are not covered with the photoresist 3 are removed by a dry-etching technique such as reactive ion etching and argon milling or by a wet etching technique such as treatment with phosphoric acid or hydrochloric acid so as to produce patterned superconducting wiring lines 2c consisting of the c-axis oriented thin film as is shown in Figure 2D. Then, as is shown in Figure 2E, an a-axis oriented thin film 2a of the same oxide superconductor is deposited on the surface of the substrate 1 on which the patterned superconducting wiring lines 2c consisting of the c-axis oriented thin film have been deposited. This thin film 2a of oxide superconductor also is prepared by the off-axis sputtering technique under the presence of a mixed sputtering gas of Ar + O 2 (volume ratio of 9 : 1) of 10 Pa but the substrate is heated at a temperature which is not higher than 650°C so that the thin film of oxide superconductor 2a deposited becomes a-axis oriented thin film. In this a-axis oriented thin film, it is preferable that its c-axis is also well-ordered. After this, a surface of the a-axis oriented thin film 2a is smoothed by a dry etching technique such as reactive ion etching or argon milling to obtain a superconducting circuit according to the present invention as is shown in Figure 2F. Namely, both surfaces of the superconducting wiring lines 2c made of c-axis oriented thin film and of the shield zones 2a made of a-axis oriented thin film make a flat planar surface or a common plane. In actual applications, an interlayer insulator layer is deposited on the resulting superconducting circuit shown in Figure 1 according to the present invention before the latter is passed to next stages for completing the superconducting circuit into a desired superconducting device. The insulation zones 2b shown in Figure 1 can be prepared by deoxygenation treatment of the thin film 2 of oxide superconductor. Namely, after predetermined areas of the thin film 2 of oxide superconductor are protected with patterned photoresist or insulator, remaining exposed areas are subjected to deoxygenation treatment so that the exposed areas are changed into non-superconductor. The deoxygenation treatment can be effected by local heating by means of a laser beam or a focussed ion beam.

Claims: A superconducting circuit having superconducting wiring lines (2c) each made of a thin film of oxide superconductor deposited on a substrate (1), each of said superconducting wiring lines (2c) consisting of a thin film of the oxide superconductor oriented such that the c-axis of the crystal structure thereof is perpendicular to a surface of the substrate (1) whereby superconducting current flows, in said superconducting wiring lines, parallel to said surface of the substrate, and shield zones (2a) each consisting of a thin film of said oxide superconductor deposited on said substrate such that the a-axis of the crystal structure thereof is perpendicular to the surface of the substrate (1) and the b-axis of the crystal structure thereof is parallel to the direction of a current which propagates in said superconducting wiring lines, wherein the shield zones (2a) are arranged to separate respective superconducting wiring lines (2c) from each other. The superconducting circuit set forth in claim 1, wherein said superconducting wiring lines comprise first superconducting wiring lines (2c') extending in a first direction (Y) and second superconducting wiring lines (2c") extending in a direction (X) perpendicular to said first direction, and wherein there are further provided insulation zones (2b) arranged between said second superconducting wiring lines (2c") and said shield zones (2a). A process for fabricating a superconducting circuit set forth in claim 1, characterized by the steps of: depositing a first thin film of an oxide superconductor such that the a-axis or c-axis of the crystal structure thereof is oriented perpendicularly to a surface of the substrate (1), removing predetermined areas of said first thin film to leave patterned superconducting wiring lines (2c); depositing a second thin film of said oxide superconductor, on said surface of the substrate at remaining areas in which said first thin film has been removed, such that said second thin film has the c-axis or a-axis of the crystal structure thereof oriented perpendicular to the surface of the substrate, the orientation of the crystal structure of the second thin film being different from that of said first thin film. A process for fabricating a superconducting circuit set forth in claim 1, characterized by the steps of: depositing a first thin film of an oxide superconductor such that the a-axis or c-axis of the crystal structure thereof is oriented perpendicularly to a surface of the substrate (1), removing predetermined areas of said first thin film to leave patterned superconducting wiring lines (2c), said superconducting wiring lines comprising first superconducting wiring lines (2c') extending in a first direction (Y) and second superconducting wiring lines (2c") extending in a direction (X) perpendicular to said first direction; forming shield zones (2a) by depositing a second thin film of said oxide superconductor, on said surface of the substrate (1) at remaining areas in which said first thin film has been removed, such that said second thin film has the c-axis or a-axis of the crystal structure thereof oriented perpendicular to the surface of the substrate, the orientation of the crystal structure of the second thin film being different from that of said first thin film; and forming insulation zones (2b) at the interfaces between said second superconducting wiring lines (2c") and shield zones (2a). The process set forth in claim 4, wherein said insulation zones (2b) are produced by deoxygenation treatment. The process set forth in claim 5, wherein said deoxygenation treatment is effected by vacuum, a laser beam or a focussed ion beam.

Inventors: IIYAMA MICHITOMO C O OSAKA WOR, INADA HIROSHI C O OSAKA WORKS, NAKAMURA TAKAO C O OSAKA WORKS, IIYAMA, MICHITOMO, C/O OSAKA WORKS, INADA, HIROSHI, C/O OSAKA WORKS, NAKAMURA, TAKAO, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: H01P3
IPC codes: H01P3, H01L39