Abstract:
An ion source impinging on the surface of the substrate to be coated is used to enhance a MOCVD, PVD or other process for the preparation of superconducting materials.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the manufacture of thick film high-temperature superconductor (HTS) coated wire having increased current capability. 
     BACKGROUND OF THE INVENTION 
     In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions. 
     Wire forms the basic building block of the world&#39;s electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary HTS compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire technology in more than a century. 
     HTS-coated wire offers best-in-class performance, carrying over one hundred times more current than conventional copper and aluminum conductors of the same physical dimension do. The superior power density of HTS-coated wire will enable a new generation of power industry technologies. It offers major size, weight, and efficiency benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways. For example, HTS-coated wire is capable of transmitting two to five times more power through existing rights of way. This new cable will offer a powerful tool to improve the performance of power grids while reducing their environmental footprint. However, to date only short samples of the HTS tape used in the manufacture of next-generation HTS-coated wires have been fabricated at high performance levels. In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS tape. 
     Vapor deposition is a process for manufacturing HTS tape where vapors of superconducting materials are deposited on a tape substrate, thereby forming an HTS coating on the tape substrate. Well-known vapor deposition processes that show promise for the high-throughput cost-effective production of HTS tapes include metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). With the use of MOCVD or PLD processes, HTS film, such as yttrium-barium-copper-oxide (YBa 2 Cu 3 O 7  or “YBCO”) film, may be deposited onto a heated buffered metal substrate to form an HTS-coated conductor. However, to date only short lengths of coated conductor wire samples have been fabricated at high performance levels with any of the above processes. Several challenges must be overcome in order to enable the cost-effective production of long lengths (i.e., several kilometers) of HTS-coated conductor. 
     One way to characterize coated conductors is by their cost per meter. Furthermore, cost and performance can be characterized as the cost per kiloamp-meter. More specifically, by increasing the current for a given cost per meter of coated conductor the cost per kiloamp-meter is reduced. This is stated as the critical current (Jc) of the deposited HTS material multiplied by the cross-sectional area of the film. 
     For a given critical current and width of coated conductor, one way to increase the cross-sectional area is by increasing the HTS film thickness. However, it has been demonstrated that with critical current as a function of thickness, the critical current may drop off and reach saturation as the thickness of a single layer of HTS film increases beyond approximately 1.5 microns. This is because beyond a film thickness of approximately 1.5 microns the HTS material becomes very porous, develops voids, and develops increased surface roughness, all of which contribute to inhibiting the flow of current. Since simply increasing the HTS film thickness does not result in a corresponding increase in critical current, a technical challenge exists in increasing the film thickness beyond 1.5 microns while also achieving a corresponding increase in critical current of an HTS-coated conductor in a cost-effective manner. 
     One approach to achieving high-quality YBCO thick films is to improve the morphology of the film, such as by increasing material density and smoothness, as the thickness exceeds 1.5 microns, thereby resulting in increased current capacity. Tatekawa, et al., U.S. Pat. No. 6,143,697, dated Nov. 7, 2000 and entitled “Method for Producing Superconducting Thick Film,” describes a method of producing a superconducting thick film that involves the steps of forming a thick layer comprising a superconducting material on a substrate; firing the thick layer formed on the substrate; subjecting the fired thick layer to cold isostatic pressing; and re-firing the thick layer subjected to cold isostatic pressing. 
     A drawback of Tatekawa, et al., is that while it is a suitable method for forming superconducting oxide thick films, it does not provide a cost-effective way to improve the morphology of the film and thus minimize the film defects, such as high porosity, voids, and surface roughness, and thereby provide thick HTS films having increased critical current. Tatekawa, et al., is therefore not suited for the cost-effective production of high-current HTS-coated conductors. 
     It is therefore an object of the invention to produce YBCO films with a thickness in excess of 1.5 microns with increased current capacity for use in the manufacture of high-current HTS-coated tape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an ion-assisted MOCVD system in accordance with the invention for producing high-current HTS-coated tapes by depositing HTS thick film with increased current capability. 
         FIG. 2  illustrates an ion-assisted PLD system in accordance with the invention for producing high-current HTS-coated tapes by depositing HTS thick film with increased current capability. 
     
    
    
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is an ion-assisted HTS thick film continuous deposition process for producing YBCO films with a thickness in excess of 1.5 microns with increased current capacity for use in the manufacture of high-current HTS-coated tape. The ion-assisted HTS thick film deposition process of the present invention includes an ion source that bombards the deposition zone within any well-known deposition process, such as an MOCVD, PLD or sputtering process. 
     This ion source provides additional energy to the deposition process that results in improved film morphology for film thicknesses above 1.5 microns. This improved film morphology results in, for example, increased material density, improved surface roughness, and reduced porosity. Consequently, as the YBCO film grows to thicknesses exceeding 1.5 microns during the deposition process of the present invention film defects are minimized, which results in an increase in current density of the resulting YBCO thick film. 
     Ion beam-assisted electron beam evaporation is well known in, for example, optical applications where high-energy ions are focused on the film as it grows, thus forming a very dense, smooth, uniform optical structure. However, to date this technique has not been applied to HTS deposition processes to achieve similar growth enhancements. 
     The novel aspect of this invention is the inclusion of an ion source in at least the last zone of an at least two zone coating deposition process to enhance the conventional coating process that is occurring within the system of the present invention. 
     The process of this invention can produce high current density HTS tape having a total coating thickness in excess of 1.5 microns and a critical current density in excess of 200 A per centimeter. In a preferred embodiment the process produces tapes having a total coating thickness in excess of 1.5 microns and a critical current density in excess of 300 A per centimeter and in a most preferred embodiment it produces tapes having a total coating thickness in excess of 1.5 microns and a critical current density in excess of 400 A per centimeter. 
     DESCRIPTION OF THE INVENTION 
     For the purpose of illustration, the ion-assisted HTS thick film deposition process of the present invention is disclosed, firstly, in reference to an MOCVD process that is described in  FIG. 1  and, secondly, in reference to a PLD process that is described in  FIG. 2  below. However, the ion-assisted HTS thick film deposition process of the present invention is not limited to only MOCVD and PLD processes. For example, the ion-assisted HTS thick film deposition process of the present invention may be applied to evaporation and sputtering processes. 
     As a first embodiment of the invention,  FIG. 1  illustrates an ion-assisted MOCVD system  100  in accordance with the invention for producing high-current HTS-coated tapes by depositing HTS thick film with increased current capability. The ion-assisted MOCVD system  100  of the present invention includes a conventional MOCVD reactor  110 , which is a vacuum-sealed deposition chamber in which an MOCVD process occurs, such as a cold-wall reactor that may be maintained at a pressure of, for example, 1.6 Torr. 
     The MOCVD reactor  110  houses a showerhead  112  located in close proximity to a substrate heater  114 . A substrate tape  116  is positioned and translates (during operation) between the showerhead  112  and the substrate heater  114  within a deposition zone  118  formed along the length of the showerhead  112 , i.e., the area in which the substrate tape  116  is exposed to the precursor vapors. Furthermore, multiple regions within the deposition zone  118  are established, for example, a zone A and a zone B as shown in  FIG. 1 . 
     The substrate tape  116  is a flexible length of substrate formed from a variety of materials, such as stainless steel or a nickel alloy such as Inconel, upon which buffer layers, such as yttrium-stabilized zirconia (YSZ) and/or cerium oxide (CeO 2 ) have been previously deposited. The substrate tape  116  is capable of withstanding temperatures up to 950° C. and has dimensions that may vary to meet the desired finished product and system limitations. For example, the substrate tape  116  may have a thickness of 25 microns, a width of 1 cm, and a length of 100 meters. 
     The showerhead  112  is a device for uniformly distributing precursor vapors onto the substrate tape  116 . The surface of the showerhead  112  that is oriented toward the substrate tape  116  includes multiple fine holes evenly distributed throughout its area, through which the precursor vapors exit toward the substrate tape  116 . The length of the showerhead  112  and the specific composition of the vapor precursors feeding the showerhead  112  may be user defined depending on the application. 
     During the deposition process, the temperature of the substrate tape  116  is controlled via the substrate heater  114 . The substrate heater  114  is a well-known single or multiple zone substrate heater that provides heating, typically in the range of 700 to 950° C., to the substrate tape  116  via a radiant heating element, such as a lamp. Alternatively, the substrate heater  114  is a resistance heater having a heating element, such as Kanthal or MoSi 2 . 
     The ion-assisted MOCVD system  100  further includes a system for the delivery of coating precursors. An exemplary precursor delivery system includes a pump  120  that is fed by a liquid precursor source (not shown) that contains a solution containing organometallic precursors, such as tetramethyl heptanedionate (THD) compounds of yttrium (Y), barium (Ba), and copper (Cu), along with an appropriate mixture of solvents, such as tetrahydrofuran and isopropanol. The pump  120  is a high-pressure, low flow rate pump, such as a high-pressure liquid chromatography (HPLC) pump, capable of a low flow rate between 0.1 and 10 mL/min. The pump  120  feeds a precursor vaporizer  122  via a liquid line  124  formed of tubing or piping. 
     The precursor vaporizer  122  is a well-known device in which a precursor solution is flash vaporized and mixed with an inert carrier gas, such as argon or nitrogen, for delivery to the showerhead  112 . The inert carrier gas is fed into the precursor vaporizer  122  via a gas line  126  formed of tubing or piping. The precursor vapors exit the precursor vaporizer  122  via a precursor vapor line  128  that connects to the inlet of the showerhead  112 . The vapor line  128  is a connecting tube or pipe through which the precursor vapor and its inert carrier gas pass on their way from the precursor vaporizer  122  to the showerhead  112 . 
     Just prior to the vapor line  128  entering the MOCVD reactor  110 , an oxygen line  130  opens into the vapor line  128 . The oxygen line  130  is a tube or pipe through which oxygen passes for introduction to the precursor vapor and its inert carrier gas flowing within the vapor line  128 . 
     The ion-assisted MOCVD system  100  includes an ion source  132  that emits an ion beam  134  that is directed toward the substrate tape  116  within the MOCVD reactor  110 . The ion source  132  may be an inexpensive gridless ion bombardment source that is capable of generating a collimated or non-diffused ion beam at a power level typically in the range of 0.5 to 10 KW. An example of the gridless ion source  132  is commercially available from Veeco Instruments, [2330 E Prospect Fort Collins, Colo. 80525] operates at voltages up to 100-1000 eV, and has dimensions of 6 cm by 66 cm. The size and orientation of the ion source  132  is determined based on the length of the substrate tape  116  irradiated and the design of the MOCVD reactor  110 . The ion source  132  does not have to be located in close proximity to the deposition zone  118 , as the ions of ion beam  134  can travel long distances. 
     Alternatively, the ion source  132  may be a gridded ion source. However, a gridded ion source is likely to be less desirable than a gridless ion source because, typically, gridded ion sources are more costly than gridless ion sources and function with more stringent pressure requirements than gridless ion sources, i.e., 10 −4  to 10 −6  Torr as compared with 10 −2  to 10 −3  Torr for gridless ion sources. 
     The pressure interface between the ion source  132  and the MOCVD reactor  110  is accomplished via a pressure differential  136  mounted within the outer wall of the MOCVD reactor  110 . The pressure differential  136  is a device that allows the ion source  132  to be held at a typical vacuum pressure in the range of approximately 10 −4  to 10 −2  Torr, while at the same time allowing the MOCVD reactor  110  to be held at a vacuum pressure typically in the range of 1-50 Torr. This can be accomplished by means of a turbomolecular pump or a cryopump. The pressure differential  136  also includes an opening that allows the ion beam  134  to pass into the MOCVD reactor  110 . 
     With reference to the ion-assisted MOCVD system  100  of  FIG. 1 , the basic MOCVD process is well known in the art and can be summarized as follows. Within the MOCVD reactor  110  of the ion-assisted MOCVD system  100 , HTS film, such as YBCO, is deposited by vapor-phase precursors onto the heated substrate tape  116  via chemical reactions that occur at the surface of the substrate tape  116 . More specifically, the linear translation of the substrate tape  116  through the deposition zone  118  begins in a direction progressing from zone A to zone B (the mechanisms for translating the substrate tape  116  are not shown), the pump  120  is activated, the precursor vaporizer  122  is activated, and the substrate heater  114  is activated. 
     The vapor line  128  delivers the yttrium-barium-copper vapor precursor to the showerhead  112 , which uniformly directs this vapor precursor toward the substrate tape  116  within the deposition zone  118 . The result of the oxygen reacting with the yttrium-barium-copper vapor precursors and then this reacting combination coming into contact with the heated substrate tape  116  within the deposition zone  118  causes the yttrium-barium-copper vapor precursor to decompose and form a layer of YBCO atop the substrate tape  116  as it translates through the deposition zone  118 . 
     The substrate tape  116  experiences the initial accumulation of YBCO film within zone A of the deposition zone  118  where the film thickness builds from zero microns up to 1.0 to 1.5 microns. The substrate tape  116  subsequently experiences further accumulation of YBCO film within zone B of the deposition zone  118 , where the film thickness continues to build from approximately 1.5 microns up to 5 microns. 
     Concurrent with the normal deposition process occurring within the ion-assisted MOCVD system  100  as described above, the ion source  132  is activated and thus emits the ion beam  134 . The stream of positive ions forming the ion beam  134  is accelerated toward the substrate tape  116  within the deposition zone  118 . More specifically, the ion beam  134  emitting from the ion source  132  is focused upon the substrate tape  116  as it translates through zone B of the deposition zone  118 , where the YBCO film is further accumulating and approaching and/or exceeding a thickness of 1.5 microns. Although the process is shown as having two deposition zones A and B, there may be multiple deposition zones, with the requirement that those deposition zones where the substrate has a coating in excess of 1.5 microns thick, have an ion source focused on the substrate tape as it translates through that deposition zone. While it is not an absolute requirement, it may be preferable to have the ion source focused on the substrate even in the first deposition zone where the film is grown to a thickness of 1.5 microns. In this way, it could be assured that a template of a dense film is available for subsequent growth. 
     As a result, the YBCO deposition process occurring within zone B of the deposition zone  118  is influenced by the ion bombardment provided by the ion beam  134 . Due to this ion bombardment, additional energy is added to the deposition process within zone B of the deposition zone  118 , which has the effect of minimizing film defects, such as high porosity, voids, and surface roughness, thereby maintaining a high-quality growth template as the YBCO film accumulates by vapor deposition upon the substrate tape  116 . As a result, the ion-assisted MOCVD system  100  of the present invention is capable of producing a YBCO film with a thickness in excess of 1.5 microns that has increased material density and smoothness that results in increased current capacity. 
     There is no particular orientation requirement for the ion source  132  in relation to zone B. Rather, the orientation is governed by the design of the MOCVD reactor  110  because there is an optimum distance between the substrate tape  116  and the showerhead  112 . In particular, the orientation of the incident ion beam  134  is governed by the dimension of the showerhead  112  and the substrate heater  114 . 
     The ion beam  134  from the ion source  132  is not focused upon the substrate tape  116  within zone A of the deposition zone  118  where the YBCO film is forming with a thickness that is less than, for example, 1.0 to 1.5 microns. This is because, as stated above, the quality of the YBCO film morphology within the first 1.0 to 1.5 microns of growth is very high and the current capacity is not inhibited. 
     Although not required to obtain the benefits of the present invention, the ion beam may be allowed to impinge upon the substrate in zone A as well. Ion bombardment may be used within zone A of the deposition zone  118  where the thickness of the YBCO film is less than 1.0 to 1.5 microns to assure that the film is dense, thereby providing a good template for subsequent layers. 
       FIG. 2  illustrates a second embodiment of the invention, an ion-assisted PLD system  200  for producing high-current HTS-coated tapes by depositing HTS thick film with increased current capability. The ion-assisted PLD system  200  of the present invention includes a conventional deposition chamber  210 , which is a vacuum chamber designed specifically for pulsed laser deposition applications. An example of such a vacuum chamber is a 12- or 18-inch vacuum chamber commercially available by Neocera, [10000 Virginia Manor Road Beltsville, Md. 20705] although those skilled in the art will appreciate that a number of alternative vendors manufacture vacuum chambers in a variety of shapes and sizes. The deposition chamber  210  is maintained at a pressure of, for example, 200 mTorr. In this example, the deposition chamber  210  houses a first target  212  and a second target  214  that are located in close proximity to a substrate heater  216 . The substrate tape  116  as described in  FIG. 1  is positioned and translates (during operation) between the targets  212  and  214  and the substrate heater  216 . The targets  212  and  214  are composed of HTS material, such as YBCO, and are available commercially from suppliers such as Praxair Surface Technologies, Specialty Ceramics [16130 Wood-Red Rd., #7, Woodinville, Wash. 98072] and Superconductive Components, Inc. [1145 Chesapeake Ave., Columbus, Ohio 43212]. 
     During the deposition process, the temperature of the substrate tape  116  is controlled via the substrate heater  216 . Like the substrate heater  114  of  FIG. 1 , the substrate heater  216  is a well-known single or multiple zone substrate heater that provides heating, typically in the range of 750 and 830° C., to the substrate tape  116  via a radiant heating element such as a lamp. 
     Finally, the ion-assisted PLD system  200  includes an ion source  218  that emits an ion beam  220  that is directed toward the substrate tape  116  within the deposition chamber  210 . The ion source  218  is an inexpensive gridless ion bombardment source that is capable of generating a collimated or non-diffused ion beam at a power level typically in the range of 0.5 to 10 KW. An example of a gridless ion source  218  is commercially available from Veeco Instruments, [2330 E Prospect Fort Collins, Colo. 80525] operates at voltages up to 100-1000 eV, and has dimensions of 3 to 6 cm in diameter. The size of the ion source  218 , especially the length of the ion source  218 , is the similar to the length of the film deposition zone. 
     There is no particular orientation of the ion source  218  relative to the film deposition zone. Rather, the orientation is governed by the design of the deposition chamber  210  because there is an optimum distance between the substrate tape  116  and the targets  212  and  214 . In particular, the orientation of the incident ion beam  218  is governed by the dimension of the targets  212  and  214  and the substrate heater  216 . The ion source  218  does not have to be located in close proximity to the substrate tape  116 , as the ions of ion beam  220  can travel long distances. Alternatively, the ion source  218  is a gridded ion source. 
     With reference to the ion-assisted PLD system  200  of  FIG. 2 , the basic PLD process is well known in the art and need only be summarized as follows. Within the deposition chamber  210  of the ion-assisted PLD system  200 , HTS film, such as YBCO, is deposited by the evaporation of HTS material and the subsequent exposure of the heated substrate tape  116  to this evaporant. More specifically, the linear translation of the substrate tape  116  through the deposition chamber  210  begins in a direction that first passes by the target  212  and then by the target  214  that are arranged along the substrate tape  116  line of travel (the mechanisms for translating the substrate tape  116  are not shown). The substrate heater  216  is activated. 
     A first laser source (not shown) is activated and generates a laser beam  222  that impinges upon the surface of the target  212 , causing the formation of a plume  224 , which emanates from that portion of the target  212  radiated by the laser beam  222  toward the substrate tape  116  in a highly forward-directed fashion. In like manner, a second laser source (not shown) is activated and generates a laser beam  226  that impinges upon the surface of the target  214 , causing the formation of a plume  228 , which emanates from that portion of the target  214  radiated by the laser beam  226  toward the substrate tape  116  in a highly forward-directed fashion. 
     The plumes  224  and  228  are plasma clouds resulting from the material of targets  212  and  214 , respectively, melting and subsequently evaporating explosively when impinged upon by the laser beams  222  and  226 , respectively. 
     The YBCO particles contained in the plume  224  are thus deposited onto the surface of the substrate tape  116  as the tape translates through the deposition chamber  210  at a predetermined speed. 
     The substrate tape  116  experiences the initial accumulation of YBCO film via exposure to the YBCO particles contained in the plume  224  as the substrate tape  116  translates through the deposition chamber  210  at a predetermined speed. Due to exposure to the particles of the plume  224 , the film thickness upon the surface of the substrate tape  116  builds from zero microns up to 1.0 to 1.5 microns. The substrate tape  116  subsequently experiences further accumulation of YBCO film via exposure to the YBCO particles contained in the plume  228  as the substrate tape  116  translates through the deposition chamber  210  at a predetermined speed. Due to exposure to the particles of the plume  228 , the film thickness upon the surface of the substrate tape  116  builds from approximately 1.5 microns up to 5 microns. 
     Concurrent with the normal deposition process occurring within the ion-assisted PLD system  200  as described above, the ion source  218  is activated and thus emits the ion beam  220 . The stream of positive ions forming the ion beam  220  is accelerated toward the substrate tape  116  and is focused upon the substrate tape  116  as it translates through the particles of the plume  228  where the YBCO film is further accumulating and approaching and/or exceeding a thickness of 1.5 microns. As a result, the YBCO deposition process occurring via exposure to the particles of the plume  228  is influenced by the ion bombardment provided by the ion beam  220 . Although the process is shown as having two plumes, there may be multiple plumes, with the requirement that those deposition zones where the substrate has a coating in excess of 1.5 microns thick, have an ion source focused on the substrate tape as it translates through the plume defining that deposition zone. 
     Due to this ion bombardment, additional energy is added to the deposition process occurring due to exposure to the particles of the plume  228 , which has the effect of minimizing film defects, such as high porosity, voids, and surface roughness, thereby maintaining a high-quality growth template as the YBCO film accumulates by vapor deposition upon the substrate tape  116 . As a result, the ion-assisted PLD system  200  of the present invention is capable of producing a YBCO film with a thickness in excess of 1.5 microns that has increased material density and smoothness that results in increased current capacity. 
     The ion beam  220  from the ion source  218  need not be focused upon the substrate tape  116  as it is exposed to the plume  224  where the YBCO film is forming with a thickness that is less than, for example, 1.0 to 1.5 microns. This is because, as stated above, the quality of the YBCO film morphology within the first 1.0 to 1.5 microns of growth is still very high and, thus, the current capacity is not inhibited. 
     Alternatively, however, ion bombardment may be used in the area where the substrate tape  116  is exposed to the plume  224  where the thickness of the YBCO film is less than 1.0 to 1.5 microns. In particular, using ion bombardment in the area where the substrate tape  116  is exposed to the plume  224  can assure that the film is dense, thereby providing a good template for subsequent layers.