A novel superconducting circuit and a process for fabricating the same

Abstract: A superconducting circuit having patterned superconducting wiring lines. Each wiring line consists of at least one portion (2′) of the thin film (2) of an oxide superconductor deposited on a substrate (1). The portion (2′) has a predetermined crystal orientation and the remaining portions (2″) have a different crystal orientation or changed to non-superconductor. The superconducting circuit has a planar surface. In variations, two different wiring lines (21, 22) each having a different crystal orientation are produced at different portions of a thin film of oxide superconductor, so that superconducting current flow separately through two different portions in a common thin film.

Description: Background of the Invention Field of the Invention The present invention relates to a novel superconducting circuit, more particularly, to 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. Description of the related arts Conventional superconductors became superconductive only at extremely low temperatures, and their use was excluded from many 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 has 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. The thin films of oxide superconductor are expected to be used in a variety of applications including sensors, SQUID, superconducting devices such as superconducting transistors or the like. In this specification, the term "oxide superconductor" means any high-Tc compound oxide which shows the critical temperature of above 30 K. In superconducting circuits having superconducting wiring lines, it is desirable to integrate the superconducting patterned wiring lines in the same thin film of oxide superconductor that is used to fabricate the superconducting elements or devices. However, the fabrication of patterned superconducting wiring lines in an integrated circuit has not been established. EP-A-0 410 374 discloses conventional superconducting wirings in an integrated circuit. It is known that the value of superconducting current depends on the orientation of crystals of which a thin film of oxide superconductor is made and that the crystal possesses 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 provide a novel structure of a superconducting circuit having patterned superconducting wiring lines each made of a thin film of oxide superconductor deposited on a substrate and a process for fabricating the same, so as to increase the freedom of circuit designing and to make the best use of oxide superconductor. Summary of the Invention The present invention provides a superconducting circuit according to claim 1 comprising: a flat substrate, and a circuit layer formed on said flat substrate, said circuit layer having a planar surface; wherein there is provided in said circuit layer a first wiring pattern made of thin film oxide superconductor material having the c-axis thereof oriented in a first direction; characterised in that there is further provided in the circuit layer a second wiring pattern made of said thin film oxide superconductor material and having the c-axis thereof oriented in a second direction different from said first direction, whereby the first and second wiring patterns lie on the substrate in a common layer having a continuous planar surface. The term "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 substrate. The superconducting circuit having superconducting wiring lines according to the present invention can be fabricated from 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 insulator layer or buffer layer. The first wiring pattern can be made of a thin film of the oxide superconductor whose a-axis is oriented perpendicular to the surface of the substrate. When the superconducting circuit is used in a multilayered structure, another upper thin film of oxide superconductor is further deposited on a surface of the planar surface through an insulation layer which can have a thickness of less than 10 nm. The thin film of oxide superconductor can have viaholes connecting vertically adjacent superconducting layers. It should be noted that there is no difference from the view point of flowability of superconducting current between the a-axis oriented thin film and the b-axis oriented thin film in the oxide superconductors to which the present invention is applicable, because the oxide superconductors have layered structures in which the superconducting current flows dominantly in the direction which is perpendicular to the c-axis. In a preferred embodiment of the present invention, the superconducting circuit is characterized in that each of the first superconducting wiring lines is made of a thin film of oxide superconductor whose c-axis is perpendicular to a surface of the substrate so that a superconducting current flows in parallel with the surface of the substrate, each of second superconducting wiring lines is made of a thin film of oxide superconductor whose a-axis is perpendicular to the surface of the substrate so that a superconducting current flows vertically with respect to surface of the substrate, and the superconducting circuit has a planar surface. In this case also, the superconducting circuit can have insulation zones, viaholes, another upper thin film of oxide superconductor deposited on the planar surface through an insulation layer which can have a thickness of less than 10 nm. The present invention further provides a process according to claim 9 for fabricating a superconducting circuit, comprising the steps of: depositing a first thin film of an oxide superconductor on a surface of a substrate, said deposited first thin film having the c-axis thereof oriented in a first direction relative to the substrate surface; and removing predetermined areas of said first thin film to leave a first wiring pattern; characterised in that there is further provided the steps of: depositing a second thin film of the same oxide superconductor on said first wiring pattern and on exposed areas of the substrate corresponding to the predetermined removed areas of said first thin film, said second thin film having the c-axis thereof oriented in a second direction different from said first direction; and removing an upper part of said second thin film so as to re-expose said first wiring pattern and produce a planar surface, whereby the first wiring pattern and the remaining second thin film superconductor material lie on the substrate in a common layer having a planar surface. In this case, the first thin film of oxide superconductor is preferably a c-axis oriented thin film and the second thin film of oxide superconductor is preferably an a-axis oriented thin film. In this type of superconducting circuit, all superconducting wiring lines made of an identical oxide superconductor make a planar surface in a single layer but each superconducting wiring line has a different crystal orientation. As stated above, the thin film of oxide superconductor shows anisotropy in the current property due to the anisotropy in crystal. In fact, the superconducting current flows dominantly along a direction which is perpendicular to the c-axis but almost zero current flows along a direction of the c-axis. The superconducting circuit according to the present invention has such a novel structure that both of the a-axis oriented thin film and the c-axis oriented thin film are juxtaposed in a layer and two superconducting wiring lines make a planar surface. However, substantially no current flows between these two superconducting lines even if these two thin films of a-axis oriented thin film and c-axis oriented thin film are juxtaposed because no substantial current is propagated between these two films. This novel structure of superconducting circuit permits to realize the mostly compacted wiring design due to such a fact that both of superconducting lines made of the a-axis oriented thin film and of the c-axis oriented thin film can be juxtaposed closely on a common surface of a substrate in the mostly compacted condition which could not be done in the case of metal conductors. The superconducting wiring lines made of a-axis oriented thin film are used to pass an electric current along a direction which is perpendicular to the surface of the substrate, in other words, they function as conductor lines connecting two adjacent layers deposited one over another, while the superconducting wiring 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 according to the present invention can be used for fabricating a multi-layered superconducting circuit. In this case also, superconducting wiring lines as well as viaholes can be arranged in a juxtaposition in each layer at the mostly compacted layout and can be contacted by vertical wiring lines filled in the viaholes. In practice, the superconducting circuit can be fabricated as follows : At first, a first thin film of an oxide superconductor whose a-axis or c-axis is oriented perpendicularly to a surface of the substrate is deposited on the substrate. Then, predetermined areas of the first thin film of oxide superconductor are removed to leave patterned first superconducting lines. After this, a second thin film of the same oxide superconductor is deposited in such a condition that the second thin film becomes a different orientation from the first thin film thereon. Namely, when the first thin film is an a-axis oriented thin film, a c-axis oriented thin film is deposited and when the first thin film is a c-axis oriented thin film an a-axis oriented thin film is deposited. It is preferable to prepare firstly the c-axis oriented thin film in consideration of such a fact that a higher substrate temperature is required to prepare the c-axis oriented thin film. Deoxygenation may be effected on such portions of the superconducting lines that will not be used as wiring lines, that is, on portions which will become insulation zones. This deoxygenation treatment is described in European patent application No. 95 108 249.4 divided out of the present application. The deoxygenation can be done to the second thin film so as to produce a finally patterned superconducting circuit. The process has following merits: (1) The thin film of oxide superconductor is not deteriorated by heat-treatment during the process 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 a surface of the superconducting circuit obtained by the present invention becomes easy because the patterned superconducting circuit has a flat surface. In summary, the superconducting patterned wiring lines prepared by the present invention have a planar surface which facilitates fabrication of superconducting devices and also possess high mechanical resistance, because all portions of thin film are connected to each other through the non-superconductor which is made of the same material as the superconducting lines. Still more, two different superconducting lines for passing superconducting current in parallel with a surface of a substrate and for passing superconducting current perpendicularly to the surface of the substrate can be arranged in juxtapositions in a layer. Still more, deterioration of superconducting properties of thin films of oxide superconductor caused by heat-treatment can be prevented, because all of superconducting lines each having a different function and the insulator zones can be produced by an identical material. Now, the present invention will be described with reference to Examples. Brief Description of the Drawings Figure 1 is an illustrative perspective view of a superconducting circuit having patterned wiring lines provided by the present invention. Figures 2A to 2F illustrate successive steps for fabricating the superconducting circuit shown in Figure 1. Example 1 Figure 1 is an illustrative perspective view of a superconducting circuit provided by 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 21, a-axis oriented thin film zones 22, insulation zones 23 in which superconductivity is lost and viaholes 24. The c-axis oriented thin film zones 21 provide superconducting lines in which superconducting current is propagated in parallel with a surface of the substrate 1. The a-axis oriented thin film zones 22 provide superconducting lines in which superconducting current flow perpendicularly to the surface of the substrate 1. No superconducting current flow in the insulation zones 23. The viaholes 24 are used to connect layers in a stacked multi-layered circuit (not shown). Example 2 Figures 2A to 2F illustrate successive steps for fabricating the superconducting circuit shown in Figure 1. The oxide superconductor used in this Example 2 is a compound oxide superconductor of Y-Ba-Cu-O system. At first, a substrate 1 having a smooth surface is prepared. In this Example 2, the substrate 1 consists of a silicon base substrate 1a 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 1b 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 and the like. In this Example 2, 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 above 700 °C so that the thin film of oxide superconductor deposited shows c-axis orientation. 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 by a well-known technique. 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 lines 2c consisting of the c-axis oriented thin film as is shown in Figure 2D. Then, as is shown in Fig. 2E, an a-axis oriented thin film 2a of the same oxide superconductor is deposited on the surface of the substrate 1 having the patterned superconducting lines 2c consisting of the c-axis oriented thin film. 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 an a-axis oriented thin film. 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 in which both of the superconducting lines 2c made of c-axis oriented thin film and the superconducting lines 2a made of a-axis oriented thin film make a planar surface on a common plane or on the surface of the substrate 1. In practice, an insulator layer may be deposited on a surface of the resulting superconducting circuit of Figure 1 before the circuit is passed to next stage for completing the superconducting circuit into a desired superconducting device. The insulation zones 23 shown in Figure 1 can be prepared by deoxygenation of the thin film 2 of oxide superconductor. Namely, after predetermined areas of the thin film 2 of oxide superconductor are protected with photoresist or insulator, remaining exposed areas are subjected to a treatment of deoxygenation so that the exposed areas are changed into non-superconductor.

Claims: A superconducting circuit, comprising: a flat substrate (1), and a circuit layer (2) formed on said flat substrate (1), said circuit layer (2) having a planar surface; wherein there is provided in said circuit layer (2) a first wiring pattern (21/22) made of thin film oxide superconductor material having the c-axis thereof oriented in a first direction; characterised in that there is further provided in the circuit layer (2) a second wiring pattern (22/21) made of said thin film oxide superconductor material and having the c-axis thereof oriented in a second direction different from said first direction, whereby the first and second wiring patterns lie on the substrate (1) in a common layer (2) having a continuous planar surface. The superconducting circuit set forth in claim 1, wherein the first wiring pattern (22) is made of a thin film of an oxide superconductor having the a-axis thereof oriented perpendicular to the substrate surface. The superconducting circuit set forth in claim 2, wherein the first wiring pattern (21) is made of thin film oxide superconductor material having the c-axis thereof perpendicular to the surface of the substrate so as to support a superconducting current flowing parallel to said substrate surface and the second wiring pattern (22) is made of thin film oxide superconductor material having the α-axis thereof oriented perpendicular to the substrate surface so as to support a superconducting current flowing perpendicularly with respect to said substrate surface. The superconducting circuit set forth in any previous claim, wherein said circuit layer comprises insulation zones (23). The superconducting circuit set forth in any previous claim, wherein an upper protective layer is formed on a surface of said superconducting circuit. The superconducting circuit set forth in any previous claim, wherein another upper thin film of oxide superconductor is deposited on said planar surface of said circuit layer (2) through an insulation layer. The superconducting circuit set forth in claim 6, wherein said insulation layer has a thickness of less than 10 nm. The superconducting circuit set forth in any previous claim, wherein the superconducting circuit further comprises via holes (24) in said circuit layer (2). A process of fabricating a superconducting circuit, comprising the steps of: depositing a first thin film of an oxide superconductor (2) on a surface of a substrate (1), said deposited first thin film (2) having the c-axis thereof oriented in a first direction relative to the substrate surface; and removing predetermined areas of said first thin film (2) to leave a first wiring pattern (2c); characterised in that there is further provided the steps of: depositing a second thin film (2a) of the same oxide superconductor on said first wiring pattern and on exposed areas of the substrate corresponding to the predetermined removed areas of said first thin film (2), said second thin film (2a) having the c-axis thereof oriented in a second direction different from said first direction; and removing an upper part of said second thin film so as to re-expose said first wiring pattern (2c) and produce a planar surface, whereby the first wiring pattern (2c) and the remaining second thin film superconductor material (2a) lie on the substrate (1) in a common layer having a planar surface. The process set forth in claim 9, wherein said first thin film of oxide superconductor is a thin film having the c-axis thereof oriented perpendicular to the substrate surface and said second thin film of oxide superconductor is a thin film having the a-axis thereof oriented perpendicular to the substrate surface. The process set forth in claim 9 or 10, and further comprising the step of forming insulation zones (24) in said superconducting circuit. The process set forth in claim 11, wherein the insulation zone formation step comprises a deoxygenation treatment of thin film oxide superconductor material in said common layer.

Inventors: IIYAMA MICHITOMO, INADA HIROSHI, NAKAMURA TAKAO, IIYAMA, MICHITOMO, INADA, HIROSHI, NAKAMURA, TAKAO, 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: H01L21
IPC codes: H01L21, H01L23