Abstract:
A surface polishing method for enhancing crystal orientation on the surface of a tapelike metal substrate in order to enhance the critical current of a superconducting thin film. In an oxide superconductor comprising a tapelike substrate, an intermediate layer formed on the tapelike substrate, and an oxide superconducting thin film layer formed on the intermediate layer, the method for polishing a surface to be polished of the tapelike substrate comprises a step for polishing the surface to be polished while traveling the tapelike substrate continuously, wherein an initial polishing step and a finish polishing step are included. Ultimately, the average surface roughness Ra of the polished surface is 2 nanometer or less and the in-plane orientation Δφ is 5° or less.

Description:
[0001]    This application is a continuation of International Application No. PCT/JP2007/066424, filed Aug. 24, 2007 which claims priority on Japanese Patent Application 2006-229710 filed Aug. 25, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a method for polishing a target surface for forming an oxide superconductor thin film on the surface of a tape-shaped substrate, as well as to a substrate for an oxide superconductor. In particular, this invention relates to a substrate for an oxide superconductor obtained by polishing the target surface of a tape-shaped metallic substrate with a level of smoothness on the order of nanometers and forming an intermediate layer thereon. 
         [0003]    Oxide superconductors are superior superconductors having critical temperatures beyond the liquid nitrogen temperatures. 
         [0004]    Typical examples of tape-shaped superconductor wire material include tape-shaped wire materials obtained by forming a polycrystalline oriented film of yttria-stabilized zirconia (YSZ) crystalline direction-controlled for example by the IBAD (Ion Beam Assisted Deposition) method on the surface of a Hastelloy® alloy tape comprising a Ni alloy and further forming an oxide superconductor film of YBCO type (such as YBa 2 Cu 3 O 7-y ) as disclosed, for example, in Japanese Patent Publication Tokkai 9-120719. 
         [0005]    There are currently many kinds of problems to be solved in order to make the oxide superconductor materials of this kind practically usable. 
         [0006]    One of these problems is that oxide superconductors have low critical current densities. A main cause of this is that there exists electrical anisotropy in the crystal structures of oxide superconductors. It is known in particular that oxide superconductors can easily allow an electric current to flow in the directions of the a-axis and the b-axis of their crystalline axes but a current cannot easily be made to flow in the direction of the c-axis. In order to form an oxide superconductor on a substrate and to use it as a superconductor, therefore, it is necessary to form on the substrate a superconductor with good crystalline directionality, to orient the a-axis or the b-axis of the crystal of the oxide superconductor in the direction intended for the flow of the electric current, and to orient the c-axis of the oxide superconductor in another direction. 
         [0007]    As a method for achieving the above, Japanese Patent Publications Tokkai 6-145977 and 2003-36742, which are herein incorporated by reference, disclose a method of providing the surface of an elongated tape-shaped metallic substrate with an intermediate layer with controlled crystalline directionality and to form a film of oxide superconductor thereabove. 
         [0008]    If the directionality of the intermediate layer is improved, the directionality of the superconductor film formed thereabove is also improved. For obtaining a superconductor film with high critical current (Ic) and critical current density (Jc), it is particularly essential to obtain a high level of biaxial directionality. 
         [0009]    Since the directionality of the intermediate layer to be formed for this purpose depends upon the crystalline characteristic of the surface of the underlying tape-shaped substrate, the crystalline direction and the in-plane directionality of the tape-shaped substrate become important for obtaining an intermediate layer with good directionality. 
         [0010]    For crystallizing the intermediate layer film such that good directionality is obtained, it is necessary to finish the surface of the tape-shaped substrate so as to have smoothness and flatness on the level of several nanometers. 
         [0011]    U.S. Pat. No. 6,908,362 disclosed another method according to which the surface of a nickel or nickel alloy tape accurately and an oxide superconductor thin film thereafter. 
         [0012]    Another problem that is preventing superconductivity from becoming practically usable relates to the behavior of magnetic flux quanta. The Lorentz force generated by the flow through a superconductor tends to move magnetic flux quanta but the motion of magnetic flux quanta generates heat and the superconducting condition becomes destroyed. Thus, it becomes possible to increase the critical current (Ic) that can be passed through a superconductor if magnetic flux quanta can be pinned so as not to move. Japanese Patent Publications Tokkai 6-145977 and 2003-36742 are prior art documents that teach the importance of polishing the surface of the base material so as to be smooth and flat. 
         [0013]    It is technologically difficult, however, to polish the surface of a tape-shaped substrate to the level of several nanometers by a mechanical polishing process, and there has not been any example of success. 
       SUMMARY OF THE INVENTION 
       [0014]    As described above, it is necessary for obtaining a high critical current to form a tape-shaped metallic substrate with a surface that is sufficiently flat and where the crystalline directionality is easily controllable. Accordingly, the surface of the tape-shaped metallic substrate for forming the thin film thereon must be finished to a level of smoothness on the order of nanometers and also be formed such that its crystal directionality becomes good. 
         [0015]    The present inventors have succeeded in overcoming the difficulties described above as a result of their diligent studies. The present invention has been accomplished in view of the problems described above and its object is to provide a mechanical surface polishing method for finishing the surface of a tape-shaped metallic substrate to a level of smoothness on the order of several nanometers and improving the crystal directionality of the intermediate layer formed thereon such that the critical current of the superconductor film can be improved. 
         [0016]    In one aspect, the present invention relates to an oxide superconductor member comprising a tape-shaped substrate, an intermediate layer that is formed on this tape-shaped substrate and an oxide superconductor thin film layer that is formed on this intermediate layer, and more particularly to a method of polishing a target surface of this tape-shaped substrate, this method being characterized as comprising a polishing step of polishing its target surface while running the tape-shaped substrate, and including an initial polishing process and a finishing process, wherein the average surface roughness Ra of the target surface becomes 2 nanometer or less and the in-plane directionality Δφ of the intermediate layer becomes 5° or less after the polishing step. 
         [0017]    According to an embodiment of the invention, the initial polishing process comprises a first polishing process of carrying out random polishing on the target surface in one stage, and the finishing process comprises a second polishing process of carrying out random polishing on the target surface in one stage. 
         [0018]    According to another embodiment of the invention, the finishing process comprises a second polishing process of carrying out polishing surface in a running direction of the tape-shaped substrate in one stage on the target surface. In this case, polishing marks are formed in this running direction on the target surface by the second polishing process. 
         [0019]    Explained in detail, the tape-shaped substrate may be fabricated by rolling a material selected from the group consisting of nickel, nickel alloys and stainless steel. 
         [0020]    In the above, the polishing step may be carried out by employing a pad or a tape of a foamed material comprising synthetic resin and fibers and while slurry is being supplied. 
         [0021]    Explained in detail, the slurry comprises abrading particles, water and a mixture of an additive with water, wherein said abrading particles are of at least one material selected from the group consisting of monocrystalline diamond, polycrystalline diamond, fumed silica, colloidal silica, alumina, cBN and SiC. 
         [0022]    In the above, the average diameter of the abrading particles is preferably 0.1 μm-3 μm for the first polishing process and 0.02 μm-0.5 μm for said second polishing process. 
         [0023]    Explained in detail, the abrading particles used in the second polishing process are diamond particles used for polishing longitudinally along the substrate. 
         [0024]    According to an embodiment of the invention, the intermediate layer is 1 μm or less in thickness. 
         [0025]    In another aspect, the present invention relates to an oxide superconductor member comprising a tape-shaped substrate having a target surface, an intermediate layer formed on the target surface of the tape-shaped substrate, and an oxide superconductor thin film layer formed on the intermediate layer, wherein the target surface of the tape-shaped substrate has been polished by one of the methods of this invention described above such that the average surface roughness Ra of said target surface becomes 2 nanometer or less and the in-plane directionality Δφ of said intermediate layer becomes 5° or less after said polishing step. 
         [0026]    In still another aspect, the present invention relates to a base material for an oxide superconductor member comprising a tape-shaped substrate having a target surface, and an intermediate layer formed on the target surface of the tape-shaped substrate, wherein the target surface of the tape-shaped substrate has been polished by one of the methods of this invention described above such that the average surface roughness Ra of said target surface becomes 2 nanometer or less and the in-plane directionality Δφ of said intermediate layer becomes 5° or less after said polishing step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows schematically an example of polishing system for carrying out a method of this invention for polishing a tape-shaped substrate. 
           [0028]      FIG. 2  includes  FIGS. 2A ,  2 B,  2 C,  2 D and  2 E,  FIGS. 2A ,  2 B and  2 C being respectively a front view, a plan view and a side view of an example of polishing head,  FIG. 2D  showing a polishing head, and  FIG. 2E  showing another example of polishing head. 
           [0029]      FIG. 3  includes  FIGS. 3A and 3B  which are respectively a front view and a side view of a compressing mechanism in a polishing system for using a polishing method of this invention. 
           [0030]      FIG. 4  includes  FIGS. 4A and 4B  which are respectively a front view and a side view of a polishing head for polishing in the running direction in a polishing system for using a polishing method of this invention. 
           [0031]      FIG. 5  includes  FIGS. 5A and 5B  which are respectively a front view and a side view of another example of polishing head for polishing in the running direction in a polishing system for using a polishing method of this invention. 
           [0032]      FIG. 6  is a computer-created image (by AFM) of the surface of a tape-shaped substrate before polishing. 
           [0033]      FIG. 7  is a computer-created image (by AFM) of the surface of Comparison Example 1 after polishing. 
           [0034]      FIG. 8  is a computer-created image (by AFM) of the surface of Comparison Example 2 after polishing. 
           [0035]      FIG. 9  is a computer-created image (by AFM) of the surface of Test Example 1 after polishing. 
           [0036]      FIG. 10  is a computer-created image (by AFM) of the surface of Test Example 2 after polishing. 
           [0037]      FIG. 11  is a computer-created image (by AFM) of the surface of Test Example 3 after polishing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The invention is described next with reference to the drawings but the examples described herein are not intended to limit the scope of the invention. 
         [0039]    Regarding an oxide superconductor of this invention comprising a tape-shaped substrate, an intermediate layer formed on this tape-shaped substrate and an oxide superconductor thin film layer formed on this intermediate layer, a method of polishing a target surface of this tape-shaped substrate is characterized as comprising the steps of carrying out initial polishing and final polishing (or finishing) while continuously running the tape-shaped substrate, such that the average surface roughness Ra of the target surface finally becomes 2 nanometers or less and the in-plane directionality Δφ of the intermediate layer becomes 5° or less. The initial polishing comprises a first polishing process for carrying out random polishing of the target surface of the tape-shaped substrate in at least one stage, and the finishing comprises a second polishing process for carrying out random polishing of the target surface of the tape-shaped substrate in at least one stage. 
         [0040]    As the tape-shaped metallic substrate in the above, pure Ni, Ni-based alloys such as Ni—Cr and Ni—W, pure Cu, Cu-based alloys such as Cu—Ni and Fe-based alloys such as Fe—Si and stainless steel which are superior in resistance against high temperatures and corrosion may be used. More specifically, Hastelloy® (of Haynes International, Inc.) and Inconel® (of Special Metals Corporation) and Ni alloys such as Ni-5% W, which are superior in resistance against corrosion and heat are usable. Such materials are fabricated by the technology of rolling into a thickness of 0.05 mm-0.5 mm, width of 2 mm-100 mm and length of several hundred meters. Metallic materials for rolling are polycrystalline and have crystalline structure aligned in the direction of the rolling. 
         [0041]    This tape-shaped substrate has linear scratches in the direction of rolling or crystalline defects. According to this invention, a random rotary polishing process is firstly carried out to remove these surface scratches formed by the rolling and the crystalline defects, and the target surface is thereafter finished by another random polishing process to become flat. An intermediate layer film and a superconductor film are sequentially formed on the target surface such that an oxide superconductor with a high critical current can be formed. 
         [0042]    The principal purpose of the first polishing process is to remove the marks and defects generated on the surface of the tape-shaped metallic substrate by the rolling process. The average surface roughness (Ra) of the tape-shaped substrate after the first polishing process is preferably 10 nm or less and more preferably 5 nm or less. 
         [0043]    The second polishing process is a process for the final finishing and its purpose is to form a surface on the tape-shaped substrate with superior crystal directionality such that an intermediate layer film with superior in-plane directionality can be formed thereon. The average surface roughness (Ra) of the tape-shaped substrate after the second polishing process is preferably 5 nm or less, more preferably 2 nm or less, and even more preferably 1 nm or less. 
         [0044]      FIG. 1  shows schematically an example of polishing system that may be employed for using a method of this invention for polishing a tape-shaped substrate for an oxide superconductor. The polishing system  100  comprises a feeder part  101   a , a back tension part  102 , a first polishing processing part  103 , a second polishing processing part  104 , a washing part  105 , an inspection part  160 , a work piece driver part  106  and a wind-up part  101   b.    
         [0045]    A tape-shaped metallic substrate  110  wound around a feeder reel of the feeder part  101   a  passes through the back tension part  102  and enters the first polishing processing part  103 . The tape-shaped substrate  110  is firstly subjected in the first polishing processing part  103  to a first polishing process to be explained in detail below. Next, the tape-shaped substrate  110  is advanced to the second polishing processing part  104  where it is subjected to a second polishing process to be explained in detail below. Thereafter, the tape-shaped substrate  110  is brought to the washing part  105  where a final cleaning process is carried out. The surface roughness Ra and the polishing marks on the tape-shaped substrate  110  thus processed are observed at the inspection part  160  to be explained in detail below. Thereafter, the tape-shaped substrate  110  passes through the work piece driver part  106  and is finally wound up around a wind-up reel of the wind-up part  101   b.    
         [0046]    It is preferable to wash the tape-shaped substrate  110  after each polishing process is carried out (as shown at  120   a ,  120   b  and  120   c ). In this manner, remaining abrading particles, polishing debris and residual slurry can be removed. 
         [0047]    As will be explained below, the running motion of the tape-shaped substrate is controlled by the back tension part  102  and the work piece driver part  106  while it is maintained in a condition under a specified tension. There are also a plurality of width-regulating guides  140   a ,  140   b  and  140   c  provided at appropriate intervals as explained in detail below for preventing positional displacement of the tape-shaped substrate. Slackness-detecting sensors  150   a  and  150   b  are further provided on the downstream side of the feeder reel and the upstream side of the wind-up reel for detecting the slackness of the tape-shaped substrate and controlling the rotary speed of the wind-up reel. 
         [0048]    The tape-shaped metallic substrate  110  provided with the specified tension is subjected to the first polishing process at the first polishing processing part  103 . Although  FIG. 1  shows the polishing system as polishing only the lower surface  111  of the tape-shaped metallic substrate  110 , this is not intended to limit the scope of the invention. The polishing system may be adapted to polish the upper surface of the tape-shaped metallic substrate  110 . 
         [0049]    The first polishing processing part  103  comprises at least one polishing station (two polishing stations  103   a  and  103   b  being shown in  FIG. 1 ) each having a polishing head  401  (shown in  FIG. 2 ) and a compressing mechanism  440  (shown in  FIG. 3 ) and one or more washing devices  120   a  and  120   b  each provided on the downstream side of the corresponding polishing station.  FIGS. 2A ,  2 B and  2 C are respectively a front view, a plan view and a side view of an example of polishing head  401 , comprising a feeding mechanism for feeding a polishing tape  410  onto a polishing table  413  and a rotating mechanism for rotating the polishing table  413  around an axial line perpendicular to its polishing surface. 
         [0050]    The feeding mechanism comprises a feeder reel  411  with the polishing tape  410  wound around it, at least one supporting roller, a wind-up reel  412  for winding up the polishing tape after the polishing process and a driver motor (not shown) dynamically connected to both the feeder reel  411  and the wind-up reel  412 . They are all enclosed inside a housing  414 . Woven cloth, non-woven cloth and flocked cloth made of foamed urethane, suede, polyester or nylon may be used as the polishing tape  410 . Explained parenthetically, the housing  414  is wrapped inside a cover  420  for preventing slurry from scattering to the exterior during the polishing process. As the driver motor is operated, the polishing tape  410  is fed out of the feeder reel  411 , passes on the polishing table  413  through the supporting reels and is finally wound up by the wind-up reel  412 . An unused portion of the polishing tape  410  is always being supplied onto the polishing table  413  for polishing the target surface of the tape-shaped metallic substrate  110 . It is preferable to supply the aforementioned slurry during the polishing process. The slurry is preferably one comprising abrading particles, water and a mixture of an additive (such as a lubricant and a dispersant for the abrading particles) with water. Examples of the abrading particles include diamond (monocrystalline and polycrystalline), silica (colloidal silica and fumed silica), alumina, SiC and cBN. The average diameter of the abrading particles in the slurry is 0.1 μm-3 μm for the first polishing process and 0.02 μm-0.5 μm for the second polishing process. A plurality of polishing devices may be provided for each polishing process, each polishing device using abrading particles with a different average diameter. Alternatively, polishing may be carried out continuously by using abrading particles with the same average diameter according to the requirement on the target surface and for reducing the polishing time. 
         [0051]    The rotating mechanism, on the other hand, comprises a spindle  416 , which is at a lower part of the housing  414  and is connected coaxially to the rotary axis X of the polishing table  413 , a motor  417  and a belt  415  for communicating the rotary power of the motor  417  to the spindle  416 . There is also provided a supporting table  419  for supporting the motor  417  and the housing  414 . The spindle  416  is inside the supporting table  419  and is rotatably attached thereto. Explained additionally, the supporting table  419  is carried on a pair of rails  421 , and a handle  420  is provided to the supporting table  419  for moving the polishing station on the rails. As the motor  417  is operated, its rotary power is communicated through the belt  415  to the spindle  416 , and the housing  414  rotates around the axial line X. It is possible to provide a plurality of polishing stations. In such a case, their housings may be rotated in opposite directions (such that the polishing tapes rotate in opposite directions) such that the polishing efficiency can be improved. 
         [0052]      FIG. 2D  shows a variation whereby the motor  417 ′ is also contained inside the supporting table  419 . 
         [0053]      FIG. 2E  shows another example of polishing head, using a polishing pad  431  instead of a polishing tape. This polishing head  430  comprises a platen  432  to which is pasted the polishing pad  431  for polishing the tape-shaped substrate  110 , a spindle  433  for supporting the platen  432 , a belt  436  and a motor  434 . The spindle  433  is rotatably attached to the supporting table  435 , and the motor  434  is contained inside this supporting table  435 . As the motor  434  is operated, its rotary power is communicated through the belt  436  to the spindle  433 , and the polishing pad  431  rotates and polishes the tape-shaped substrate  110 . At the time of the polishing process, it is preferable to supply the aforementioned slurry approximately to the center of the polishing pad  431 . 
         [0054]    The compressing mechanism  440  is explained next.  FIGS. 3A and 3B  are respectively a front view and a side view of a compressing mechanism  440  used in a polishing system of this invention. The compressing mechanism  440  comprises an air cylinder  441 , a compressing plate  443  and a holder plate  445  provided on the central line of the compressing plate  443  along the running direction of the tape-shaped substrate. The lower surface of the holder plate  445  is provided with a guide groove  446  corresponding to the width of the tape-shaped substrate  110  for preventing the generation of positional displacement of the tape-shaped substrate  110  during the polishing process. The holder plate  445  is appropriately exchangeable with another plate, depending on the size (width and thickness) of the tape-shaped metallic substrate  110 . Explained additionally, there is a position-adjusting handle  442  connected to the side surface of the compression mechanism  440 , serving to make adjustments such that the center in the direction of the width of the tape-shaped metallic substrate  110  will coincide with the center of the compression mechanism  440 . In this manner, the pressure from the air cylinder  441  can be transmitted to the tape-shaped substrate  110  through the compressing plate  443  and the holder plate  445 . There is also provided an adjustment screw  444  at an upper portion of the compressing plate  443 . Prior to the polishing process, the parallel relationship between the compressing plate  443  and the polishing table  413  is adjusted by means of this adjustment screw  444 . The descriptions given above are not intended to limit the compressing mechanism. A compressing mechanism with a different specification may be employed for the purpose of this invention. 
         [0055]      FIGS. 4A and 4B  are respectively a front view and a side view of a preferable example of polishing head  610  used in the second polishing process for the tape-shaped substrate of this invention. The polishing head  610  comprises a cylindrical drum  601  having a resin sheet  602  wound around a cylindrical drum base made of stainless steel, for example, a driving motor  603  for rotating the cylindrical drum  601  and a driving mechanism (not shown) such as having a driving wheel. Foamed urethane, a woven cloth or a non-woven cloth may be used as the resin sheet  602 . The cylindrical drum  601  is contained inside a housing  606 . A motor  605  for causing the cylindrical drum  601  to oscillate in a direction perpendicular to the running direction of the tape-shaped substrate  110  may be additionally included. This oscillatory motion serves to prevent the tape-shaped metallic substrate  110  from being polishing at a same place on the cylindrical drum  601 . At the time of the polishing, it is preferable to supply aforementioned slurry onto the resin sheet  602 . 
         [0056]      FIGS. 5A and 5B  are respectively a front view and a side view of another preferable example of polishing head  620  used in the second polishing process for the tape-shaped substrate of this invention. This polishing head  620  comprises a contact roller  622  for pressing a polishing belt  621  onto the tape-shaped substrate  110 , a polishing belt driving means  623 , support rollers  625  and a driver motor  624  which is connected too the polishing belt driving means  623 . The contact roller  622 , the support rollers  625  and the polishing belt driving means  623  are contained inside a housing  628 . A tape made of woven or non-woven cloth of synthetic fibers or a foamed material may be used for the polishing belt  621 . As the driver motor  624  is operated, the polishing belt  621  is moved by the polishing belt driving means  623  through the contact roller  622  and the support rollers  625  and the target surface of the tape-shaped substrate  110  is polished. At the time of the polishing, it is preferable to supply aforementioned slurry onto the polishing belt  621 . A motor  626  for oscillating the contact roller  622  perpendicularly to the direction of running motion of the tape-shaped substrate  110  may additionally be provided. This oscillatory motion serves to prevent the tape-shaped metallic substrate  110  from being polished by a same part of the polishing belt  621 . 
         [0057]    These polishing heads  610  and  620  are characterized in that the polishing surface of the cylindrical drum  601  or the polishing belt  621  rotates in or against the direction of motion of the tape-shaped substrate  110 . The polishing head  610  or  620  comprises a polishing station together with the compressing mechanism  440  explained with reference to  FIG. 3 . A plurality of polishing stations may be connected in series for the second polishing process. In such an arrangement, it is preferable to provide a washing machine described above on the downstream side of each polishing station. 
         [0058]    The tape-shaped metallic substrate  110  is subjected for the second polishing process at the aforementioned second polishing processing part  104 . In the case of the polishing system shown in  FIG. 1 , the second polishing process is carried out by a random polishing method in two stages. At the time of the polishing, it is preferable to use slurry comprising abrading particles, water and a mixture of an additive (such as a lubricant and a dispersant for the abrading particles) with water. SiO 2 , Al 2 O 3 , diamond, cBN, SiC and colloidal silica may be used as the abrading particles although they are not limited by these examples. The average diameter of the abrading particles to be used is 0.02-0.1 μm, and preferably 0.02-0.07 μm. 
         [0059]    Appropriate polishing programs can be incorporated in the polishing system described above by combining the devices according to the given polishing conditions. When each polishing process is carried out in a plurality of stages, for example, the polishing conditions (such as the rotational speed of the polishing head and the diameters of the abrading particles in the slurry) may be appropriately adjusted. 
         [0060]    As explained above, the first polishing process according to the polishing method of this invention comprises the step of polishing the target surface of the tape-shaped substrate such that its average surface roughness Ra will become 10 nm or less, and preferably 5 nm or less. 
         [0061]    As explained above, furthermore, the second polishing process according to the polishing method of this invention comprises the step of polishing the target surface of the tape-shaped substrate such that its average surface roughness Ra will become 5 nm or less, preferably 2 nm or less, and more preferably 1 nm or less. 
         [0062]    Preferably, the first polishing process is carried out as a normal random polishing process and the final finishing of the second polishing process is carried out such that polishing marks along the running direction of the tape-shaped substrate will be formed on the target surface that is polished. 
         [0063]    The intermediate layer is formed on the surface of the tape-shaped substrate thus obtained. The IBAD (Ion Beam Assisted Deposition) method or the ISD (Inclined Substrate Deposition) method may be used to vapor-deposit a zirconia (YSZ) film stabilized by MgO, CeO 2 , SrTiO 2  and Y 2 O 3  as an intermediate layer having in-plane biaxial directionality. An oxide superconductor film is vapor-deposited on the intermediate layer. Thus, the tape-shaped substrate and the intermediate layer become the substrate for an oxide superconductor film. The present invention makes it possible to form an intermediate layer which is superior in crystal directionality and as a result to form a superconductor thin film with a high critical current. 
       EXAMPLES 
       [0064]    Tape-shaped substrates were polished under various polishing conditions according to this invention and the directionality of the intermediate layers vapor-deposited thereon was examined. The results of these experiments will be described next. 
         [0065]    A polishing device according to the continuous polishing system shown in  FIG. 1  was used for the purpose. The polishing processes were carried out according to a polishing program either in a single stage or several stages. In the case of a polishing process in several stages, the abrading particles in the slurry were appropriately varied from fine to rough kinds and changes were appropriately made between random polishing and polishing in the running direction (in the direction of electric current). 
         [0066]    Tape-shaped metallic substrates of Hastelloy C-276 (for example, 58% Ni-17% Mo-15% Cr-5% Fe-4% W) were used. These substrates were fabricated by a rolling technology such that the thickness of 0.1 mm, width of 10 mm and length of several hundred meters were obtained.  FIG. 6  is a computer image (by AFM) of the surface of a tape-shaped substrate before polishing. The average surface roughness Ra of the substrates before the polishing was 15-30 nm. The maximum surface roughness R max  was 200-500 nm, and the speed protrusions are traces of rolling generated by the rolling process. 
         [0067]    A MgO (in-plane biaxial directional) film was formed as an oriented (directional) intermediate layer by using an IBAD (Ion Beam Assisted Deposition) device. It is to be noted here that an intermediate layer with thickness of 1 μm or greater used to be conventionally used because the value of Δφ generally tends to become smaller if the intermediate layer is thicker. In the present application, tests were conducted by intentionally keeping the thickness of the intermediate layer less than 1 μm (or specifically about 0.5 μm). 
         [0068]    An assist beam was projected at an angle of 55° from the normal line to the substrate surface. The conditions for the forming of the film included the substrate temperature of 500°-600°, and the pressure in the oxygen atmosphere was several hundred mTorr (×0.133 Pa). 
         [0069]    The in-plane directionality Δφ and the perpendicular directionality Δω were measured by evaluating the full width at half-maximum (FWHM) obtained by measuring the X-ray polar diagram. 
       Comparison Example 1 
       [0070]    According to a polishing program, only a random polishing was carried out in one stage as the first polishing process (only  103   a  of  FIG. 1 ). Slurry with polycrystalline diamond with average particle diameter D50 of 1 μm was used. A 30-weight % aqueous solution of glycol compounds, glycerol and fatty acid was used as the additive and it was adjusted to pH8. The polishing head was rotated while a tape of foamed polyurethane was supplied and the polishing process was carried out under the following conditions while a tape-shaped substrate was passed between a compression pad and slurry was being supplied. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Rotational speed of the polishing head: 
                 60 
                 rpm 
               
               
                   
                 Compressive force: 
                 300 
                 g/cm 2   
               
               
                   
                 Flow rate of slurry: 
                 10 
                 ml/min 
               
               
                   
                 Running speed of the tape-shaped substrate 
                 20 
                 m/hr 
               
               
                   
                   
               
             
          
         
       
     
       Comparison Example 2 
       [0071]    According to a polishing program, two identical polishing machines were connected in series (as  103   a  and  103   b  of  FIG. 1 ) for only the first polishing process ( 103  of  FIG. 1 ), carrying out random polishing in two stages under the same conditions as in Comparison Example 1. 
       Test Example 1 
       [0072]    According to a polishing program, the first polishing process ( 103  of  FIG. 1 ) was carried out by rotating the polishing head while feeding a foamed polyurethane tape thereto, passing a tape-shaped substrate between the compression pad and supplying slurry to carry out a random polishing process in two stages ( 103   a  and  103   b  of  FIG. 1 ) under the following polishing conditions. The slurry for the first stage contained polycrystalline diamond particles with average diameter D50 of 1 μm and that for the second stage contained polycrystalline diamond particles with average diameter D50 of 0.5 μm. The additive was a 30-weight % aqueous solution of glycol compound, glycerol and fatty acid, adjusted to pH8. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Rotational speed of the polishing head: 
                 60 
                 rpm 
               
               
                   
                 Compressive force: 
                 300 
                 g/cm 2   
               
               
                   
                 Flow rate of slurry: 
                 10 
                 ml/min 
               
               
                   
                 Running speed of the tape-shaped substrate 
                 20 
                 m/hr 
               
               
                   
                   
               
             
          
         
       
     
         [0073]    The second polishing process ( 104  of  FIG. 1 ) was carried out by rotating the polishing head while feeding a tape of non-woven cloth comprising polyester fibers, passing the tape-shaped substrate between the compression pad and supplying slurry with a random polishing device of the same form as for the first polishing process under the following polishing conditions. Slurry of colloidal silica with average particle diameter D50 of 0.03 μm was used. The additive was an aqueous solution with ammonium oxalate, potassium oxalate and glycerol added to DEMOL EP (trade name) of Kao Chemical Company, adjusted to pH9. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Rotational speed of the polishing head: 
                 60 
                 rpm 
               
               
                   
                 Compressive force: 
                 200 
                 g/cm 2   
               
               
                   
                 Flow rate of slurry: 
                 10 
                 ml/min 
               
               
                   
                 Running speed of the tape-shaped substrate 
                 20 
                 m/hr 
               
               
                   
                   
               
             
          
         
       
     
       Test Example 2 
       [0074]    According to a polishing program, the first polishing process ( 103  of  FIG. 1 ) was carried out with the same polishing conditions as in Test Example 1 and the second polishing process ( 104  of  FIG. 1 ) was carried out by connecting two polishing machines as used in Test Example 1 ( 104   a  and  104   b  of  FIG. 1 ) to carry out random polishing in two stages under the same conditions as in Test Example 1. All the other polishing conditions were the same as in Test Example 1. 
       Test Example 3 
       [0075]    According to a polishing program, the first polishing process ( 103  of  FIG. 1 ) was carried out with the same polishing conditions as in Test Example 1 and the second polishing process ( 104  of  FIG. 1 ) was by carrying out two-stage polishing in the running direction by connecting two polishing machines of the same type in series ( 104   a  and  104   b  of  FIG. 1 ). Polycrystalline diamond with average diameter D50 of 1 μm was used as slurry. All the other polishing conditions were the same as in Test Example 1. 
       Evaluation 
       [0076]    Results of the experiments were evaluated regarding the average surface roughness Ra of the target surface after the tape-shaped substrate was polished and the root mean square (RMS, or the square root of the averaged valued of the squares of the deviations between the average line and the measured curve) as well as the in-plane directionality and the perpendicular directionality of the intermediate layer after the intermediate layer film was formed on the polished surface. The average surface roughness Ra and the RMS of the surface of the tape-shaped substrate after the polishing process were measured by using a scanning probe microscope (Nanoscope Dimension 3100 Series, produced by Digital Instruments Inc.) 
         [0077]      FIGS. 6-11  are (AFM) photographs of three-dimensional images created by a computer by scanning arbitrarily selected surface areas of 10 μm×10 μm of various tape-shaped substrates.  FIG. 6  shows the surface of a tape-shaped substrate before the polishing.  FIGS. 7 and 8  show the surface conditions of the tape-shaped substrate respectively of Comparison Examples 1 and 2 after the polishing process.  FIGS. 9 ,  10  and  11  show the surface conditions of the tape-shaped substrate respectively of Test Examples 1, 2 and 3 after the polishing process. 
         [0078]    Table 1 shows the results of the comparison experiments. 
         [0079]    As shown in Table 1, if the target surface of the tape-shaped substrate is polished such that its surface roughness Ra becomes 2 nm or less (preferably 1 nm or less) and its RMS is 2.0 nm or less (preferably 1.5 nm or less), the in-plane directionality Δφ of the intermediate layer film (MgO) becomes 5° or less and its perpendicular directionality Δω becomes 1.5° or less and hence that superior directionality characteristics can be achieved. 
         [0080]    If Test Example 2 with only random polishing and Test Example 3 which is a combination of both random polishing and polishing in the running direction, it can be understood that Test Example 3 gives a high value for Ra than Test Example 2 but that Test Example 3 gives a lower value for Δφ, making it possible to realize an extremely high level of flatness less than 4°. This indicates that the value of Δφ depends on the direction of polishing more than Ra and can be improved particularly by polishing in the running direction. 
         [0081]    It has also been made clear that in-plane directionality Δφ of less than 4° can be realized with the thickness of intermediate layer less than 1 μm (0.5 μm according to the Test Examples) if a polishing method of this invention is used. Thus, this invention makes it possible to form the whole of a superconductor thin film thinner and applications in a wide range may be expected. 
         [0082]    Although the invention has been shown above with reference to only some specific examples, they are merely intended to be demonstrative and not to limit its scope. For example, the number of stages in the random polishing device, the material of the polishing tape or the polishing pad, the rotational speed of the polishing head, the magnitude of the compressive force, the kind, the particle size and the flow rate of the slurry to be used, and the running speed of the tape-shaped substrate may be appropriately varied. Such variations are all intended to be in the spirit of and hence to be included within the scope of the invention. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Average 
                   
                 Thickness 
                   
                 Perpen- 
               
               
                   
                 surface 
                   
                 of inter- 
                 In-plane 
                 dicular 
               
               
                   
                 roughness 
                 RMS 
                 mediate 
                 directionality 
                 directionality 
               
               
                   
                 Ra (nm) 
                 (nm) 
                 layer (μm) 
                 Δφ (°) 
                 Δω (°) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Comparison 
                 3.5 
                 4.9 
                 0.5 
                 7.4 
                 2.8 
               
               
                 Example 1 
               
               
                 Comparison 
                 1.8 
                 2.4 
                 0.5 
                 7.9 
                 3.3 
               
               
                 Example 2 
               
               
                 Test 
                 0.9 
                 1.3 
                 0.5 
                 4.9 
                 1.5 
               
               
                 Example 1 
               
               
                 Test 
                 0.2 
                 0.3 
                 0.5 
                 4.5 
                 1.2 
               
               
                 Example 2 
               
               
                 Test 
                 0.5 
                 0.6 
                 0.5 
                 3.7 
                 1.3 
               
               
                 Example 3