A thin film of oxide superconductor possessing locally different crystal orientations and a processes for preparing the same

Abstract: A thin film of oxide superconductor consisting of more than two portions (10; 11, 12) each possessing a predetermined crystal orientation and deposited on a common surface of a substrate (2). At least one selected portion (10) of the thin film of oxide superconductor is deposited on a thin under-layer (4, 100) which facilitates crystal growth of the selected portion and which is deposited previously on the substrate. The selected portions (10) may consist of a-axis oriented thin film portions while non-selected portions (ll, 12) may consists of c-axis oriented thin film portions. The thin under-layer can be a buffer layer (4) or a very thin film (100) of oxide superconductor.

Description: SPECIFICATION Background of the Invention Field of the Invention The present invention relates to a thin film of oxide superconductor possessing locally different crystal orientations and more particularly to a thin film of oxide superconductor having a-axis (or b-axis) oriented thin film portions and c-axis oriented thin film portions on a common substrate and a process for preparing the same. Description of the related arts Oxide superconductors are expected to be used in a variety of applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y-Ba-Cu-O oxide superconductors possess a critical temperature above 80 K and Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O oxide superconductors possess a critical temperature above 100 K. However, the oxide superconductors possess a crystal anisotropy in their superconducting properties. In fact, the highest critical current density is observed in a direction which is perpendicular to the c-axis of their crystal. From this fact, the direction of crystal orientation must be considered in actual utilization of oxide superconductors. In the oxide superconductors to which the present invention is applicable, the a-axis, b-axis and c-axis of their crystals are determined and known. It is also known that electric current flows in a plane defined by the a-axis and the b-axis. In the specification, only a-axis oriented thin film is referred because the difference between an a-axis oriented thin film and a b-axis oriented thin film is not important from the view point of superconducting current flow. The orientation of these axes is usually defined by the direction of the crystal axis with respect to a surface of the substrate. For example, "c-axis oriented thin film" means a thin film whose c-axis is perpendicular to the surface of substrate. When the oxide superconductors are used in superconductor electronics for producing superconducting devices or integrated superconducting circuits, it is indispensable to prepare a thin film of the oxide superconductors. However, the problem of crystal anisotropy becomes very critical in the thin films of oxide superconductors. In addition to this, in order to realize high-performance superconducting devices or integrated superconducting circuits of high density, it is requested to prepare, on a common surface of substrate, two kinds of superconducting wiring lines: one in which electric current flows in parallel with a surface of substrate and another in which electric current flows perpendicular to the surface of substrate. For example, in superconducting electrodes. current flows usually in parallel with the surface of substrate while in interlayer superconducting wiring lines connecting different layers stratified on the substrate, current must flow perpendicularly to the surface of substrate. Therefore when an oxide superconductor is used in high-performance superconducting devices or integrated superconducting circuits of high density, it is requested to deposit, on a common surface of a substrate, both of c-axis oriented thin film of oxide superconductor in which the critical current density along the direction which is in parallel with the surface of substrate is higher than along the other directions and of a-axis (or b-axis) oriented thin film of oxide superconductor in which the critical current density along the direction which is perpendicular to the surface of substrate is higher than in the direction of c-axis. Crystal orientation of the thin film of oxide superconductor can be controlled by selecting film-forming temperature which is determined usually by substrate temperature. In fact, the a-axis oriented thin film can be realized at a substrate temperature which is lower by about 50 to 100°C than the substrate temperature which is preferable to prepare the c-axis oriented thin film. From this fact, in a known process for preparing a thin film having a-axis oriented thin film portions and c-axis oriented thin film portions on a common substrate, one of the thin film portions is deposited firstly on a surface of the common substrate, then selected areas of the firstly deposited thin film portions are removed by etching technique to produce patterned first thin film portions and after that, other thin film portions are deposited on the selected areas. This known technique, however, is not effective in actual production because it is necessary to effect two deposition steps of two different thin films under different conditions. Still more, the dimension of superconducting wiring lines obtained is limited due to tolerance or precision of etching operation, so that the cross-section of a superconducting wiring line obtained becomes unnecessarily large, resulting in that it is difficult to increase the density of superconducting circuits. There is also a danger that the thin film of oxide superconductor obtained is contaminated at etched margins or non-superconductor deposit on the etched margins. Still more, when etching operation is not effected satisfactorily, for example, when a firstly deposited c-axis oriented thin film is not removed completely but is left, the next thin film deposited on the firstly deposited c-axis oriented thin film will not become an a-axis oriented thin film but becomes a c-axis oriented thin film. Even if the firstly deposited thin film can be removed completely, the resulting surface is often roughened due to delicate operational conditions of etching. In such case also, it is difficult to realize a well-ordered a-axis oriented thin film of high quality. The known process has another problems. Namely, when a c-axis oriented thin film is deposited firstly and then an a-axis oriented thin film is deposited secondly, even if the etching operation is effected ideally, oxygen atoms escape out of the margin of the c-axis oriented thin film which are exposed by etching operation, so that the stoichiometry of the thin film is spoiled at the margins of the c-axis oriented thin film, resulting in deterioration of superconducting properties at the margins. In this case, after deposition of an a-axis oriented thin film is effected, it is impossible to re-supply oxygen atoms to the marginal area of the c-axis oriented thin film by conventional oxygen supply techniques such as heat-treatment, because the marginal area is filled or closed with the a-axis oriented thin film and hence oxygen cannot enter the c-axis oriented thin film due to such a fact that oxygen can enter only through the plane defined by a-axis and b-axis. Therefore, the superconducting device obtained sometimes does not work. Still more, in the known process, it is very difficult to realize a perfect or satisfactory interface between a c-axis oriented thin film deposited firstly and an a-axis oriented thin film deposited secondly, resulting in that the capacity of electric current at the interface is limited. And, a weak junction caused by the contamination at the etching operation is often produced at the interface between the a-axis oriented thin film and the c-axis oriented thin film, resulting in that the obtained superconducting device shows non-linear property due to a tunnel current. An object of the present invention is to solve the problems of the known process and to provide an improved thin film of oxide superconductor possessing locally different crystal orientations and a process for preparing the thin film. Summary of the Invention The present invention provides a thin film of oxide superconductor consisting of more than two portions each possessing a predetermined different crystal orientation, that are a-axis oriented portions and c-axis oriented portions and deposited on a common surface of a substrate. Each portion of the thin film is deposited on a different under-layer portion which facilitates crystal growth of the thin film portion and which is deposited previously on the substrate, the differently oriented thin film portions being juxtaposed on the different under-layer portions. In a preferred embodiment of the present invention, the under-layer can be buffer layer(s) Hereafter, this preferred embodiment is described. Description of a Preferred Embodiment In this embodiment in which the thin under-layer is made of buffer layers, the thin film of oxide superconductor according to the present invention consists of first portions which are deposited on first buffer layer portions made of a material having such a crystal structure that a thin film of oxide superconductor whose c-axis is oriented along a direction which is perpendicular to the substrate is easily grown and of second portions which are deposited on second buffer layer portions made of a material having such a crystal structure that a thin film of oxide superconductor whose c-axis is oriented along a direction which is in parallel to the substrate is easily grown. The thin film of oxide superconductor according to the present embodiment is prepared by the steps of depositing a first buffer layer on said substrate, removing selected portions of said first buffer layer to expose a surface of said substrate, depositing a second buffer layer on the exposed surface, and then depositing a thin film of oxide superconductor on the first buffer layer-portions remained and on the second buffer layer. The obtained thin film of oxide superconductor consists of c-axis oriented portions in which the c-axis of crystal is oriented along a direction which is perpendicular to the substrate and a-axis oriented portions in which the c-axis of crystal is oriented along a direction which is in parallel with the substrate, so that the directions of electric current can be selected according to respective uses of the thin film of oxide superconductor of which superconducting wiring lines, superconducting electrodes or the like are fabricated, resulting in that high-performance superconducting devices or integrated superconducting circuits can be realized. The process of this embodiment is applicable to any known oxide superconductor and is advantageously applicable to Y-Ba-Cu-O oxide superconductor, Bi-Sr-Ca-Cu-O oxide superconductor and Tl-Ba-Ca-Cu-O oxide superconductor which have the most attractive properties including the critical temperature. The buffer layers to be deposited on the substrate are selected preferably in such a manner that the portion of the buffer layer on which the c-axis oriented thin film of oxide superconductor is to be deposited is made of LaAlO 3 , PrGaO 3 or the like, while the portion of the buffer layer on which the a-axis oriented thin film of oxide superconductor is to be deposited is preferably made of Al 2 O 3 , MgO or the like. This is because the crystal lattice of LaAlO 3 , PrGaO 3 or the like well-matches or is compatible with that of a-axis or b-axis oriented thin film of oxide superconductor to be deposited thereon and a c-axis oriented thin film of oxide superconductor can be grown easily on these buffer layers. The thin film of oxide superconductor is preferably prepared at a substrate temperature in which a c-axis oriented thin film of oxide superconductor grows. This is because a desired crystalline thin film can be obtained at higher substrate temperatures, and growth of the c-axis oriented thin film of oxide superconductor requires a higher substrate temperature than that for the a-axis oriented thin film of oxide superconductor. In the process according to the present invention, both of c-axis and a-axis oriented thin film portions of oxide superconductor grow spontaneously at the same substrate temperature. In the process according to the present invention, a thin film of oxide superconductor possessing locally different crystal orientations can be realized easily by a single step of deposition and the superconducting properties of the superconducting oxide thin film are not spoiled because the thin film is not subjected to undesirable operations. The thin film of oxide superconductor prepared by the process according to the present invention possesses locally different crystal orientations whose interfaces have good conditions and hence shows improved superconducting properties which are requested for realizing superconducting devices or integrated superconducting circuits. Now, the present invention will be described with reference to Example but the scope of the invention should not be limited thereto. Brief Description of the Drawings Fig. 1 to 6 illustrate successive steps of an embodiment of the present invention. Description of a Preferred Embodiment Example A thin film of Y 1 Ba 2 Cu 3 O 7-x having a-axis oriented portions and c-axis oriented portions is deposited on a common surface of MgO substrate by a process according to the embodiment of the present invention. The process according to the present invention will be described with referring to the attached drawing. At first, a buffer layer (3) of LaAlO 3 having a thickness of about 200 nm is deposited on a substrate (2) as is shown in Fig. 1 by sputtering. The substrate (2) is preferably of insulator such as MgO (100), SrTiO 3 , CdNdAlO 4 or the like or of semiconductor such as silicon. The buffer layer (3) can be also of PrGaO 3 . A photo-resist layer (50) is formed on the surface of the buffer layer (3) as is shown in Fig. 2. When LaAlO 3 is selected as the buffer layer (3), selected portion of the buffer layer (3) at which an a-axis oriented thin film of oxide superconductor is to be deposited is removed by patterning process. Fig. 3 shows a patterned photo-resist layer having remaining photoresist area (51, 52). Then, the patterned photo-resist layer is subjected to ion milling or reactive ion etching operation to remove the non-protected portion (5) of the buffer layer (3) until a surface of the substrate (2) is exposed between two adjacent buffer layer portions (31, 32) protected by the photo-resist area (51. 52). Then, another buffer layer (4) of Al 2 O 3 is deposited up to the same thickness as the adjacent buffer layer portions (31, 32) as is shown Fig. 4. The buffer layer Al 2 O 3 may be replaced by a buffer layer of MgO. Then, layers (41, 42) of Al 2 O 3 deposited on the photo-resist area (51, 52) are removed by lift-off technique of the latter so that the buffer layer portions (31, 32) are exposed as is shown in Fig. 5. In this case. if necessary, side walls can be eliminated by side-etching technique. Removal of portions of the buffer layer (3) can be effected by other known techniques. For example, a hole of non-protected portion (5) of the buffer layer (3) is filled with suitable material and then the whole surfaces of deposited layers are subjected to etchback operation. After the buffer layer portions (31, 32) are exposed as is shown in Fig. 5, the whole surface of exposed buffer layer portions (31, 32) of LaAlO 3 and of buffer layer (4) of Al 2 O 3 is subjected to a cleaning operation which can be carried out by irradiating the whole surface with slant ion shower (in which the axis of ion shower makes an acute angle with respect to the surface). Finally, as is shown in Fig. 6, a thin film of oxide superconductor is deposited on the buffer layers (31, 32) of LaAlO 3 and on a buffer layer (4) of Al 2 O 3 by a known technique such as off-axis sputtering, reaction-evaporation, molecular beam epitaxy (MBE) or chemical vapour deposition (CVD), so that c-axis oriented thin film portions (11, 12) are grown on the buffer layer portions (31, 32) of LaAlO 3 while an a-axis oriented thin film portion (10) is grown on the buffer layer (4) of Al 2 O 3 . The off-axis sputtering is carried out for example under following operational conditions: Sputtering gas Ar : 90 % O 2 : 10 % Pressure : 10 Pa Substrate temperature : 650 to 700°C

Claims: A thin film of oxide superconductor consisting first portions (11,12) consisting of c-axis oriented thin film of oxide superconductor and second portions (10) consisting of a-axis oriented thin film of oxide superconductor, each portion (11, 12 and 10) being deposited on a different buffer layer portion (31, 32 and 4) deposited on a substrate (2), and said differently oriented portions of thin film of oxide superconductor (11, 12 and 10) being juxtaposed on said different buffer layer portions (31, 32 and 4). The thin film set forth in claim 1, wherein said first buffer layer portions (31, 32) are made of LaAlO 3 or PrGaO 3 and said second buffer layer portions (4) are made of Al 2 O 3 or MgO. The thin film set forth in claim 1 or 2, wherein said buffer layer portions (31, 32 and 4) have a thickness of about 200 nm. The thin film set forth in any one of claims 1 to 3, wherein said substrate (2) is MgO (100), SrTiO 3 , CdNdAlO 4 or semiconductor substrate. A process for preparing a thin film of oxide superconductor comprising first portions (11, 12) consisting of c-axis oriented thin film of oxide superconductor and second portions (10) consisting of a-axis oriented thin film of oxide superconductor, characterized by the steps of depositing a first buffer layer (3) on said substrate (2), removing selected portions (5) of said first buffer layer (3) to expose a surface of said substrate (2), depositing a second buffer layer (4) on the exposed surface, and depositing a thin film of oxide superconductor on the first buffer layer (31, 32) remained and on the second buffer layer (4), so that differently oriented portions (11, 12 and 10) of said thin film of oxide superconductor are deposited and juxtaposed on different buffer layer portions (31, 32 and 4). The process set forth in claim 5, wherein said first buffer layer portions (31, 32) are made of LaAlO 3 or PrGaO 3 and said second buffer layer portions (4) are made of Al 2 O 3 or MgO. The process set forth in claim 5 or 6, wherein said buffer layer portions (31, 32 and 4) have a thickness of about 200 nm. The process set forth in any one of claims 5 to 7, wherein said substrate is MgO (100), SrTiO 3 , CdNdAlO 4 or semiconductor substrate.

Inventors: IIYAMA MICHITOMO, INADA HIROSHI, IIYAMA, MICHITOMO, INADA, HIROSHI, IIYAMA, MICHITOMO, C/O OSAKA WORKS, INADA, HIROSHI, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1992
Main classification: H01L39
IPC codes: H01L39