Apparatus, method and system for coating a substrate, in particular a superconducting tape conductor and coated superconducting tape conductor

Abstract

The present invention relates to a method for coating a substrate, in particular a superconducting tape conductor, in a vacuum environment, comprising: generating a metallic material in the gas phase, feeding the gaseous metallic material into an expansion chamber, wherein the expansion chamber is adapted to cause the gaseous metallic material to expand and be directed towards the substrate, and depositing the metallic material on at least part of the surface of the substrate. Further, the present invention relates to a coated superconducting tape conductor comprising: at least one superconducting layer and at least one metallic coating deposited on the tape conductor, wherein the thickness of the metallic coating is at least 1 m and varies over the width of the coated tape conductor by no more than 10%, preferably no more than 5%.

Claims

1. An apparatus for coating a substrate with a metallic coating, in a vacuum environment, wherein a background gas pressure in the vacuum environment is at most 1.Math.10.sup.1 Pascal, comprising: a gas source for generating a metallic material in the gas phase, wherein the vapor pressure of the metallic material in the gas source is at least 1.Math.10.sup.1 Pascal; wherein the gas source comprises an opening from which the gaseous metallic material flows into an expansion chamber installed between the gas source and a coating zone for the substrate; wherein the expansion chamber is formed as a divergent part of a Laval nozzle having an inlet opening directed to the gas source and an outlet opening directed towards the coating zone such that a flow of the gaseous metallic material is parallelized with respect to the substrate; wherein the expansion chamber is coated with an anti-adhesion coating and is actively cooled, and wherein a lamellar diaphragm is arranged between the gas source and the inlet opening of the expansion chamber.

2. The apparatus of claim 1, wherein the expansion chamber is adapted to convert at least a lateral momentum component of the particles of the gaseous metallic material into a longitudinal momentum component in the direction of the substrate; and/or wherein the expansion chamber is adapted to generate a supersonic flow of the gaseous metallic material in the direction of the substrate; and/or wherein the expansion chamber is arranged to direct the particles of the gaseous metallic material onto the substrate so that they impinge at an angle of no more than 15, preferably of no more than 10 and most preferably of no more than 5 to the surface normal of the substrate.

3. The apparatus of claim 1, wherein at least a part of the peripheral surface is treated such that absorption of thermal radiation is increased.

4. The apparatus of claim 1, wherein a ratio of a distance between the inlet opening and the substrate and a distance between the inlet opening and the outlet opening is at least 1.0, and at most 1.4, and/or wherein a ratio between a distance between the outlet and the inlet opening and a diameter of the outlet opening is at least 1.5.

5. The apparatus of claim 1, wherein the expansion chamber widens from the gas source towards the substrate in a conical or bell-shaped manner.

6. The apparatus of claim 1, wherein the expansion chamber is actively cooled by a liquid.

7. The apparatus of claim 1, wherein: the opening of the gas source comprises an opening diaphragm, the opening diaphragm having at least one fin; and/or the gas source and/or the opening diaphragm is made of tungsten, tantalum, molybdenum, carbon and/or heat-resistant ceramic.

8. The apparatus of claim 1, wherein the substrate is a flexible metal tape passing through the coating zone over the expansion chamber.

9. The apparatus of claim 8, wherein the substrate passes at least twice through the coating zone over the expansion chamber and comprises: an intermediate cooling device configured to cool the substrate after a first and before a second pass of the substrate through the coating zone; and/or at least one gas reflector arranged in or around the coating zone, the coating zone being configured to reflect particles of the metallic material in the direction of the substrate.

10. The apparatus of claim 9, wherein: at least part of said at least one gas reflector comprises an anti-adhesion coating; and/or at least part of said at least one gas reflector is configured to increase absorption of heat radiation; and/or at least part of the gas reflector is actively cooled.

11. The apparatus of claim 9, wherein the intermediate cooling device is located outside the coating zone and configured to keep a temperature of the substrate below 180 C., throughout an entire coating period.

12. The apparatus of claim 1, wherein the lamellar diaphragm is configured to reduce splashing of liquid metal droplets from the gas source.

Description

4. DESCRIPTION OF THE DRAWINGS

(1) Selected aspects of the present invention are described below with reference to the attached drawings. These drawings show:

(2) FIG. 1 a schematic structure of an arrangement for the PVD metallization of substrates according to an embodiment of the present invention;

(3) FIG. 2 a schematic structure of an arrangement for PVD metallization of tape-shaped substrates with a winder arrangement according to an embodiment of the present invention;

(4) FIG. 3A a cross-sectional preparation of a conventional, galvanically copper-plated HTS tape conductor;

(5) FIG. 3B a cross-sectional preparation of the edge of an HTS tape conductor with surrounding copper coating according to an embodiment of the present invention;

(6) FIG. 4A a cross-sectional preparation of a 12 m thick silver layer produced by a conventional high rate PVD process;

(7) FIG. 4B a cross-sectional preparation of a 12 m thick copper layer according to an embodiment of the present invention.

(8) The following reference signs are used in the drawings: 1 vacuum chamber/environment 2 evaporation source/gas source 3 water-cooled expansion chamber/expansion nozzle 4 moving substrate tape (foil) 5 coating zone 6 tape winder/winder arrangement 7a, 7b water-cooled rear reflector/gas reflector 8 intermediate cooling device; 10 diaphragm D distance between evaporation source and substrate tape L length of the expansion chamber/expansion nozzle O.sub.i inlet diameter of the expansion chamber O.sub.a outlet diameter of the expansion chamber

5. DETAILED DESCRIPTION OF SEVERAL EXEMPLARY EMBODIMENTS

(9) A first embodiment of the present invention is illustrated in FIG. 1. A vacuum system/chamber 1 is pumped down to a residual gas pressure of less than 10.sup.2 Pascal, preferably less than 10.sup.3 Pascal, so that reactive metals (e.g. Al, Mg etc.) do not oxidize during evaporation. Chamber 1 contains an evaporation source/gas source 2 for splash-free evaporation of metals. This may be an effusion cell or a crucible which can be heated by an electron beam, a resistance heater or an induction coil until the evaporation material melts and reaches a high vapor pressure in the source in the order of more than 102 Pascal. Preferably, this vaporizer 2 has an covering diaphragm 10 with fins made of a high melting point material such as W, Ta, Mo, C or a ceramic to prevent splashing.

(10) In the upper area of the vacuum chamber 1, the substrate 4 to be coated is continuously moved through the coating zone 5 in the form of a flexible, thin tape or foil above the evaporation source 2. The tape can either come from an unwinding and rewinding device in the vacuum chamber or be continuously fed into the vacuum chamber, as described e.g. in DE 10 2009 052 873.

(11) In order to allow the metal vapor 9 to strike the substrate 4 in the area of coating zone 5 as vertically as possiblewithin an angular distribution of 15 to the substrate normaland thus avoid the column growth and high layer porosity described above, an expansion chamber 3 is installed between evaporation source 2 and coating zone 5.

(12) In the embodiment illustrated here, this expansion chamber 3 corresponds in its function to the divergent expansion part of a Laval nozzle, which is mainly used in the aerospace industry or in turbines to generate and bundle supersonic flows. The expansion chamber or Laval nozzle 3 has an inwardly concave bell shape and a round to slightly elliptical diameter. The lower inlet opening (diameter O.sub.i) is slightly wider than the outlet of the evaporation source 2 in order to thermally decouple it. The upper outlet opening (diameter O.sub.a) essentially determines the lateral dimension of coating zone 5.

(13) In some embodiments, the expansion chamber 3 with length L reaches close to the coating zone 5, so that as little metal vapor 9 as possible is laterally lost in the gap between expansion chamber 3 and substrate 4. The distance D-L is dimensioned in such a way that the substrate tape 4 does not come into contact with expansion chamber 3 even if it is slightly sagging and is also not contaminated, for example, by the anti-adhesion coating. Typically, the ratio D:L=1 to 1.4

(14) The geometric ratios of the illustrated expansion chamber or Laval nozzle 3 are as follows: The ratio between the outlet and inlet opening of the nozzle O.sub.a:O.sub.i is preferably greater than 1.5, particularly preferably greater than 2, and the ratio of the nozzle length to the outlet opening diameter is greater than L:O.sub.a=1.5. The effect of the Laval nozzle thus consists in a high separation rate from a parallel gas flow 9 and a very high material yield.

(15) However, the nozzle shapes (e.g. Laval nozzle) of the expansion chamber described here are only one possible way to achieve the directivity provided by the expansion chamber. Other forms and/or types of expansion chambers are also conceivable and are part of the present invention.

(16) In order to prevent the metal vapor 9 from adhering and condensing on the peripheral surface of expansion chamber 3, the latter is provided with an anti-adhesion coating. Suitable coatings preferably consist of long-chain PFPE compounds (trade name e.g. Fomblin). In order to keep the vapor pressure of the PFPE coating low and to dissipate the heat radiation from the gas source 2, the peripheral surface of expansion chamber 3 is actively cooled, e.g. by well heat-coupled pipes through which water flows. To prevent the peripheral surface of the expansion chamber 3 from reflecting the heat radiation from the gas source 2 onto the temperature-sensitive substrate 4, it is advisable to blacken its surface before applying the anti-adhesion coating so that it absorbs the heat radiation and dissipates it into the cooling water.

(17) With this arrangement, extremely high coating rates can be achieved on the substrate 4. For economic reasons, coating rates of more than 20 nm/s, preferably more than 50 nm/s and especially preferably more than 80 nm/s are targeted. The latter can be easily achieved with the present invention even with copper as coating material. Another important economic aspect is the material yield, i.e. the ratio of the amount of material deposited on the substrate 4 to the amount of material evaporated. With the normally undirected vacuum evaporation and with the usual distances D from source 2 to substrate 4 between 30 cm and 50 cm, the material yield is usually only in the low double-digit percentage range. With the present invention, material yields of more than 50%, preferably more than 70%, and especially preferably more than 80% can be achieved without any problems.

(18) A high coating rate in normal vacuum or high vacuum on thin films or tapes inevitably leads to a high energy input due to the released condensation heat, so that the substrate 4 can heat up very quickly. Many substrate materials, such as plastics or HTS tape conductors are temperature-sensitive and can be irreversibly damaged if a temperature threshold is exceeded. The maximum permissible temperature on the one hand and the heat capacity of the substrate 4 on the other therefore determine how much material can be deposited during a run through the coating zone 5.

(19) The coating rate is thus used to calculate the transport speed for the tape substrate 4. If thicker layers are required, the substrate 4 must be coated several times. This can be done by installing several evaporation units in succession, winding the tape several times through the system along its entire length, or by feeding the tape several times through the same coating zone 5 by means of a winding device 6. The latter is particularly suitable for tape that are narrower than the width of coating zone 5. Of course, all three approaches can also be combined with each other. In addition, for all multiple coatings, after passing through coating zone 5, an intermediate cooling device 8 can ensure that heat is removed from the strip substrate and the temperature is lowered to such an extent that a new coating can take place.

(20) FIG. 2. shows a design of a system for coating substrates and especially tape conductors. For the following investigations HTS tape conductors with a width of 12 mm were coated all around with silver and copper. HTS tape conductors consist of a thin metal strip (e.g. Hastelloy C 276) with a thickness of 30-100 m, on which metal oxide buffer layers (e.g. MgO) and an HTS functional layer, e.g. cuprates from the class of RBa.sub.2Cu.sub.3O.sub.7 (R=yttrium or rare earth elements) such as GdBa.sub.2Cu.sub.3O.sub.7, were deposited. The metallization is used for contacting, protection and electrical stabilization of the tape conductor so that it does not burn through in case of overload.

(21) A metallization that is as dense, as smooth and as fully enveloping as possible with very good layer thickness homogeneity is desired. However, the superconducting cuprates tend to lose oxygen by diffusion when heated in a vacuum, which degrades their most important function, the critical current carrying capacity. The coating device in FIG. 2 guides the HTS tape conductor 4 several times through the coating zone 5 via a winder arrangement 6 comprising, for example, 15 tracks (only indicated schematically in FIG. 2).

(22) Outside the coating zone 5, each track runs through an intermediate cooling device 8. In this way, the substrate temperature can be reliably kept below a temperature of 180 C., preferably even below 150 C., throughout the entire coating period and with a layer thickness of more than 30 m of copper.

(23) A rear reflector 7a, 7b is also installed in the winder 6, which scatters the material flowing backwards through the gaps between the tracks back onto the substrate rear side at 7a or back onto the front side at 7b. The rear reflector is water-cooled in the same way as the peripheral surface of the Laval nozzle 3 and has a anti-adhesion coating. As a result, the material yield could be increased by approximately an additional 10%.

(24) In this case, the HTS tape conductor 4 was coated all around with copper. For this purpose, the tape was twisted as it was running back through the system, i.e. the back side was brought forward. The layer thicknesses on the two main surfaces and the edges of the tape 4 can be adjusted as required by the number of passages, the position of the upper reflector 7a, 7b and the width of the gaps between different winding tracks.

(25) In the manufacture of HTS tape conductors, metal layers of silver, copper, gold and tin, their alloys or a sequence of these metals are preferably used. Even though the process described here is not limited to these metals, they are the focus of the application. HTS tape conductors have therefore been coated with silver and copper with and without the process used in the invention and have been examined in detail. They have characteristic features that enable them to be directly distinguished from tape conductors coated by other metallization processes.

(26) For example, electroplated metal layers on tape conductors show a characteristic increase in layer thickness at the edges, which is inevitable due to the electrical field enhancement at the edge and the larger solid angle from which metal ions can accumulate.

(27) FIG. 3A shows an example of the cross-sectional preparation of the edge of a 12 mm wide and approx. 100 m thick HTS tape conductor which was conventionally, i.e. galvanically, coated with a copper layer nominally 20 m thick. All shown cross-section preparations (FIG. 3A, 3B, 4A, 4B) were produced by ion beam etching with Ar ions to ensure a smooth cut edge without mechanical damage to the layer structure.

(28) In the electron microscope image of the cut edge in FIG. 3A at the top of the front side of the metal substrate, the buffer and HTS layers are clearly visible as bright bands. Above and around them is the electrodeposited copper layer. This is dimensionally stable over most of the tape surface with a thickness of 20 m, but becomes continuously thicker towards the edge of the tape and at 45 m reaches values more than twice as high at the edge. Although this so-called dog-bone effect can be reduced by suitable arrangement of the anodes in the electrolyte bath, it never completely disappears during depositing at economically interesting rates of more than 20 nm/s, and is therefore a characteristic distinguishing feature over PVD coating, whose layer thickness is practically constant right up to the edge.

(29) In the best case, a thickness ratio of edge to center of 1.2-1.3 is observed for electroplated tape conductors, whereas this ratio is below 1.1, preferably even below 1.05 for the PVD metal layers produced here (see FIG. 3B). This dimensional stability is desired mainly for the construction of magnet coils, since an inhomogeneous conductor thickness leads to unnecessary gaps between the winding layers.

(30) In comparison, FIG. 3B shows the cross-sectional preparation of a tape conductor 300 coated all around with copper using the high rate PVD process of the present invention. Clearly visible in the electron microscopic image of the cut edge in FIG. 3B at the top of the front side of the metal substrate are the buffer and HTS layers 310 as bright bands. In the tape conductor in FIG. 3B, the front and rear sides were coated with different thicknesses, here 14 m at the front and 6 m at the rear. The excellent homogeneity of the layer thickness 320 and edge coverage are clearly visible. The variation of the metal layer thickness 320 is less than 10%, preferably less than 5% of the average value.

(31) The present invention thus makes it possible to achieve the high quality of PVD deposited metal layers even in the range of high coating rates and large metal layer thicknesses. The process is particularly suitable for the metallization of HTS tape conductors if between 1-30 m, preferably between 1-20 m and especially preferably between 3-20 m of metal layer thickness are applied on each side.

(32) FIGS. 4A and 4B show a comparison of the results of conventional high rate evaporation (FIG. 4A) and the present invention (FIG. 4B) using two HTS tape conductors coated with 12 m thick metal layers.

(33) FIG. 4A shows a silver layer in cross section which was produced at a high rate >50 nm/s and intermediated cooling, but without the expansion chamber 3 provided by the present invention. The vapor from the source is reflected back into the chamber by the side walls of a chamber until it impinges on the substrate, where it is incorporated into the metal layer. This causes the vapor to travel from all directions in space (i.e., non-directional) up to grazing incidence on the substrate surface.

(34) In FIG. 4A, the HTS tape conductor at the bottom only shows the interface to the HTS layer and a 1.5 m thin, crystalline silver layer, which was annealed at temperatures >300 C. after an initial PVD coating. Clearly visible in the high rate silver layer in this arrangement is the column growth, which leads to large pores and gaps 440 in the layer and characteristic surface structures. The thickness variation and roughness is several m and thus more than 20% of the average layer thickness.

(35) By contrast, FIG. 4B shows the cross-section of a 12 m thick copper layer 320 deposited on a comparable HTS tape conductor surface with the present invention. On closer inspection, a layer structure can be seen within the copper layer 320, which is the result of multiple passes through the coating zone. In contrast to FIG. 4A, this layer is very dense and smooth. Pores and gaps 440 as in FIG. 4A are almost completely absent.

(36) In the cross-sectional preparation applied here by ion beam etching and electron microscopic observation (magnification 5000) perpendicular to the substrate surface, cavities or pores 440 make up less than 1% of the cross-sectional area and thus also of the volume of the metal layer 320. The thickness variation of the metal layer 320 measured on this cross-sectional preparation by ion beam etching is at least less than 10% in some embodiments even less than 5% of the average local layer thickness.

(37) The HTS tape conductors 300 produced by one of the embodiments of the present invention are also characterized by a very low area density of metal sputter 450 on the surface. During high rate evaporation, crucibles often experience turbulent processes in the molten metal and spattering due to the strong overheating. The resulting metal droplets 450 have a diameter of >10 m, can damage the substrate on impact due to local overheating or can be pressed into the underlying HTS layer 310 when the substrate is guided over rollers and break it. Splashing can be effectively avoided by using an evaporation source with a cover plate made of high-melting material such as W, Ta, Mo, C or ceramic. The metal layers 320 produced here are therefore also characterized by a very low surface density <0.1/cm.sup.2 of splashes and embedded particles 450 with an average diameter of more than 10 m.