Substrate for epitaxial growth, manufacturing method therefor, and substrate for superconducting wire

Abstract

An objective of the present invention is to provide a copper substrate for epitaxial growth, which has higher biaxial crystal orientation, and a method for manufacturing the same. The substrate for epitaxial growth of the present invention contains a biaxially crystal-oriented copper layer, wherein the full width at half maximum of a peak based on the pole figure of the copper layer is within 5 and the tail width of the peak based on the pole figure is within 15 Such a substrate for epitaxial growth is manufactured by a 1.sup.st step of performing heat treatment of a copper layer so that is within 6 and the tail width is within 25, and after the 1.sup.st step, a 2.sup.nd step of performing heat treatment of the copper layer at a temperature higher than the temperature for heat treatment in the 1.sup.st step, so that is within 5 and the tail width is within 15.

Claims

1. A substrate for epitaxial growth, containing a biaxially crystal-oriented copper layer, wherein the full width at half maximum of a peak based on the pole figure of the copper layer is within 5, and the tail width of the peak based on the pole figure is within 15.

2. The substrate for epitaxial growth according to claim 1, further comprising a protective layer containing nickel or a nickel alloy on the copper layer, wherein the protective layer has a thickness of 1 m or more and 5 m or less, the full width at half maximum of a peak based on the pole figure of the protective layer is within 6, and the surface roughness Ra is 20 nm or less.

3. The substrate for epitaxial growth according to claim 1, further comprising a protective layer containing nickel or a nickel alloy on the copper layer, wherein the protective layer has the full width at half maximum of a peak based on the pole figure of the protective layer is within 6, and the surface roughness Ra is 20 nm or less.

4. A method for manufacturing the substrate for epitaxial growth according to claim 1, comprising a 1st step of performing heat treatment of a copper layer so that is within 6 and the tail width is within 25, and a 2nd step of performing heat treatment of the copper layer after the 1st step at a temperature higher than the temperature for heat treatment in the 1st step, so that is within 5 and the tail width is within 15.

5. A substrate for a superconducting wire, comprising the substrate for epitaxial growth according to claim 1 laminated on a nonmagnetic metal plate.

6. A substrate for a superconducting wire, comprising the substrate for epitaxial growth according to claim 2 laminated on a nonmagnetic metal plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a developed view of angle and X-ray intensity at =35 of a Cu (111) pole figure in example 1.

(2) FIG. 2 shows a developed view of 3 angle and X-ray intensity at =350 of a Cu (111) pole figure in comparative example 1.

MODES FOR CARRYING OUT THE INVENTION

(3) Hereinafter, the present invention will be described in more detail.

(4) The substrate for epitaxial growth of the present invention contains a biaxially crystal-oriented copper layer, which is characterized in that the full width at half maximum of a peak based on the pole figure of the copper layer is within 5, and preferably within 4.5, and the tail width is within 15. Here, the term full width at half maximum in the Cu (111) pole figure obtained by X-ray diffraction refers to, as shown in FIG. 1, the peak width () at a position corresponding to a half (H/2) of the peak height H that is an average value of 4 peaks that appear at =35, and the term tail width refers to the peak width () at a position corresponding to 1/50 of the peak height.

(5) As such a copper layer, a copper foil is preferably used. The thickness of the copper layer is generally preferably 7 m or more and 70 m or less in order to ensure the copper layer's own strength and achieve better processability upon the later processing of a superconducting wire.

(6) As a method for controlling and obtained from the (111) pole figure of a copper layer within predetermined ranges, so as to obtain high biaxial crystal orientation, for example, a method that involves performing cold rolling at a high reduction of at least 90%, providing uniform distortion for the entire copper layer, and then performing heat treatment for recrystallization can be employed. When a reduction is less than 90%, sufficient biaxial crystal orientation may not be obtained by heat treatment to be performed later. Such a high-reduction rolled copper foil can be generally obtained. Examples thereof include high-reduction rolled copper foil (HA foil (trade name)) manufactured by JX Nippon Mining & Metals Corp. and high-reduction rolled copper foil (HX foil (trade name)) manufactured by Hitachi Cable, Ltd.

(7) In the present invention, heat treatment is preferably performed under different conditions in stages in order to finally obtain a copper layer having a full width at half maximum of within 5, and a tail width of within 15. Accordingly, a copper layer having high biaxial crystal orientation can be obtained. Moreover, when a protective layer made of nickel or the like is provided on a copper layer, the biaxial crystal orientation and the surface smoothness of the protective layer can be enhanced. Specifically, the method of the present invention preferably comprises a 1.sup.st step of performing heat treatment so that a copper layer has of within 6 and a tail width of within 25, and a 2.sup.nd step of performing heat treatment after the 1.sup.st step, at a temperature higher than that for heat treatment in the 1.sup.st step, so that is within 5 and the tail width is within 15. In addition, when heat treatment of a copper layer is performed by the 1.sup.st and the 2.sup.nd steps, the 2.sup.nd step may be successively performed without cooling after the 1.sup.st step. As long as a copper layer having a full width at half maximum of within 6 and a tail width of within 25 is obtained after the 1.sup.st step, cooling to the room temperature may be performed once after the 1.sup.st step, for example, and then the 2.sup.nd step of heat treatment may be performed.

(8) Specific conditions for heat treatment in the 1.sup.st and the 2.sup.nd steps are not particularly limited, as long as and can be controlled within predetermined ranges. As an example, in the 1.sup.st step, heat treatment is preferably performed at relatively low temperatures (200 C.-400 C.), and the time for heat treatment may range from about 5 to 240 minutes. Furthermore, in the 2.sup.nd step, the temperature for heat treatment is higher than that of the 1.sup.st step. However, if the temperature for heat treatment is too high, a copper layer is more likely to undergo secondary recrystallization, and crystal orientation is deteriorated. Hence, the temperature for heat treatment is preferably 1000 C. or lower and more preferably ranges from 800 C. to 900 C. Furthermore, the time for heat treatment in the 2.sup.nd step differs depending on other conditions, and is preferably a relatively short period of time. Specifically, the time for annealing is less than 10 minutes, and particularly preferably ranges from 1 to 5 minutes. The term time for annealing as used herein refers to a period of time during which a copper layer is retained within a furnace wherein the temperature reaches a predetermined temperature. When the time for annealing is 10 minutes or more, a copper layer may be deteriorated in surface roughness, or secondary recrystallization or excessive rearrangement may take place. As a result, a protective layer to be provided on the copper layer may be deteriorated in surface roughness. In addition, the above heat treatment is preferably actually performed in a process of manufacturing a substrate for a superconducting wire, after bonding a copper layer and a nonmagnetic metal plate by a surface activation bonding method or the like. The above heat treatment improves adhesion between a copper layer and a nonmagnetic metal plate, and specifically, adhesion of 10 N/cm or more, and preferably of 20 N/cm or more can be obtained.

(9) Further, a copper layer may contain a slight amount (about 1% or less) of an element in order to further improve biaxial crystal orientation by heat treatment. Examples of such an additional element that can be used herein include one or more types of element selected from among Ag, Sn, Zn, Zr, O, N, and the like. These additional elements and copper form solid solutions. If the amount thereof to be added exceeds 1%, impurities such as oxides other than a solid solution increase. This may adversely affect crystal orientation.

(10) Furthermore, a protective layer containing nickel or a nickel alloy can be formed on a biaxially crystal-oriented copper layer by plating. A nickel-containing protective layer has better oxidation resistance than that of a copper layer. Moreover, because of the presence of a protective layer, when an intermediate layer such as a CeO.sub.2 layer is formed thereon, the generation of a copper oxide film and the deterioration of crystal orientation are prevented. An element to be contained in a nickel alloy preferably leads to decreased magnetism, and examples thereof include Cu, Sn, W, and Cr. In addition, impurities may be contained, as long as they do not adversely affect crystal orientation.

(11) When the thickness of a protective layer containing nickel or a nickel alloy is too thin, Cu may diffuse onto the surface of the protective layer, so as to oxidize the surface, when an intermediate layer and a superconducting compound layer are laminated thereon. When the thickness of the same is too thick, the protective layer is deteriorated in crystal orientation, and the plating distortion increases. The thickness of a protective layer is appropriately determined in view of these possibilities, and specifically is preferably 1 m or more and 5 m or less.

(12) Also, in the present invention, a copper layer is controlled to have a full width at half maximum of within 5, and a tail width of within 15, so that the crystal orientation of a protective layer to be provide on the copper layer can be well maintained, and the smoothness of the surface of the protective layer can be more improved. Specifically, a protective layer having a full width at half maximum of within 6 of a peak based on the pole figure of the protective layer, and a surface roughness Ra of 20 nm or less can be formed.

(13) Plating can be performed under appropriately employed conditions that minimize the plating distortion of a protective layer. Here, the term plating distortion refers to the degree of distortion that takes place within a plated film when plating is performed for a substrate such as a metal plate. For example, when a layer made of nickel is formed as a protective layer, this can be performed using a Watts bath or a sulfamate bath conventionally known as a plating bath. In particular, a sulfamate bath tends to minimize the plating distortion of a protective layer and thus is preferably used. The preferable range of the composition of a plating bath is as follows, but the range is not limited thereto.

(14) (Watts Bath)

(15) TABLE-US-00001 Nickel sulfate 200-300 g/l Nickel chloride 30-60 g/l Boric acid 30-40 g/l pH 4-5 Bath temperature 40 C.-60 C.
(Sulfamate Bath)

(16) TABLE-US-00002 Nickel sulfamate 200-600 g/l Nickel chloride 0-15 g/l Boric acid 30-40 g/l Additive Appropriate quantity pH 3.5-4.5 Bath temperature 40 C.-70 C.

(17) Current density when plating is performed is not particularly limited and is appropriately determined in view of a balance thereof with the time required for plating. Specifically, for example, when a plated film of 2 m or more is formed as a protective layer, low current density may result in a longer period of time required for plating, decreased line speed to secure the time therefor, lower productivity, and more difficult control of plating. Hence, current density is generally preferably 10 A/dm.sup.2 or more. Moreover, the upper limit of current density differs depending on the type of plating bath, and is not particularly limited. For example, current density in the case of Watts bath is preferably 25 A/dm.sup.2 or less, and the same in the case of sulfamate bath is preferably 35 A/dm.sup.2 or less. In general, when current density exceeds 35 A/dm.sup.2, good crystal orientation may not be obtained due to namely the burning of plating.

(18) The thus formed protective layer may undergo the formation of micropits on the surface depending on plating conditions and the like. In such a case, if necessary, the surface can be smoothed by leveling via further heat treatment after plating. The temperatures of heat treatment at this time preferably range from 700 C. to 1000 C., for example.

(19) The above substrate for epitaxial growth having a copper layer and a protective layer and a nonmagnetic metal plate are further laminated, so that a substrate for a superconducting wire can be obtained. When the substrate for a superconducting wire is manufactured, actually preferably, first, a copper layer rolled at a high reduction and a nonmagnetic metal plate are bonded, subsequently predetermined heat treatment is performed, a full width at half maximum (of a peak based on the pole figure of a copper layer) is controlled to be within 5, and a tail width is controlled to be within 15, so as to improve biaxial crystal orientation, and a protective layer is formed on the copper layer by plating to manufacture the substrate.

(20) The term nonmagnetic refers to a state, in which the relevant metal plate is not ferromagnetic at a temperature of 77K or higher, that is, a Curie point or a Neel point thereof exists at a temperature of 77K or below and the same is paramagnetic or antiferromagnetic at a temperature of 77K or higher. As such a nonmagnetic metal plate, a nickel alloy or austenitic stainless steel is preferably used since it has excellent strength and plays a role as a reinforcing material.

(21) In general, although austenitic stainless steel is in a nonmagnetic state at ordinary temperature, that is, in a state where the metal texture is 100% austenite () phase. However, when an () phase transformation point (Ms point) of a martensite which is ferromagnetic is positioned at a temperature of 77K or higher, a phase which is ferromagnetic at a liquid nitrogen temperature may appear. Accordingly, an austenitic stainless steel plate whose Ms point is designed at a temperature of 77K or below is preferably used as a substrate (for a superconducting wire) to be used at a temperature below a liquid nitrogen temperature (77K).

(22) From such a viewpoint, as a -stainless steel plate to be used, it is preferable to use a plate material such as SUS316 or SUS316L, SUS310, SUS305 or the like, which has a stable phase with an Ms point designed to be sufficiently lower than 77K, is widely available at a relatively low cost. Such a metal plate generally having a thickness of 20 m or more is applicable herein. When a thinner superconducting wire and the strength of the same are taken into consideration, it is preferable to set the thickness to 50 m or more and 100 m or less. However, the thicknesses are not limited to this range.

(23) The above nonmagnetic metal plate and copper layer can be laminated appropriately using a conventionally known technique such as a surface activation bonding method. In the surface activation bonding method, the surfaces of the nonmagnetic metal plate and the copper layer are each subjected to sputter etching treatment in an extremely-low-pressure inert gas atmosphere of about 10 to 110.sup.2 Pa, for example, so as to remove the surface adsorption layer and the surface oxide film for activation, and then the thus activated two surfaces are bonded by cold bonding at a reduction of 0.1%-5%. Cold bonding is preferably performed under high vacuum of 110.sup.2 Pa or less so as not to adversely affect the adhesion due to re-oxidation of the activated surfaces.

(24) Further, to well maintain the crystal orientation of an intermediate layer and a superconducting compound layer to be further laminated by epitaxial growth onto a protective layer, after bonding of a nonmagnetic metal plate and a copper layer, treatment may be performed to reduce the surface roughness Ra of the copper layer, as necessary. Specifically, methods such as rolling under pressure, buffing, electropolishing, electropolishing using electrolytic abrasive grains, and the like can be employed. With the use of these methods, surface roughness Ra is desired to be 20 nm or less and preferably 10 nm or less, for example.

(25) In addition, a biaxially crystal-oriented copper layer and a protective layer may be laminated onto only one surface of a nonmagnetic metal plate, or laminated onto both surfaces of a metal plate.

(26) An intermediate layer and a superconducting compound layer are laminated in sequence onto a protective layer of the above substrate for a superconducting wire according to a conventional method, so that a superconducting wire can be manufactured. Specifically, an intermediate layer of CeO.sub.2, YSZ, SrTiO.sub.3, MgO, Y.sub.2O.sub.3, or the like is epitaxially deposited using a means such as a sputtering method onto a protective layer formed by plating, and then a Y123-based superconducting compound layer or the like is further deposited thereon by a laser ablation method or the like, and thus a superconducting wire can be obtained. When necessary, a protective film made of Ag, Cu, or the like may further be provided on the superconducting compound layer.

EXAMPLES

(27) The present invention is explained in detail as follows based on examples and comparative examples, but is not limited to these examples.

Examples 1 and 2 and Comparative Examples 1 to 5

(28) First, a copper foil (thickness: 18 m) that had been cold-rolled at a high reduction (reduction: 96%-99%) was subjected to sputter etching treatment under an Ar gas atmosphere of 0.05 Pa-1 Pa by a surface activation bonding method and then cold-bonded at a reduction of 0.1%-1% to SUS316L (thickness: 100 m) as a nonmagnetic metal plate, thereby producing a laminated material. The copper layer surface of the laminated material was polished to have a surface roughness Ra of 20 nm or less, and then the laminated material was subjected to heat treatment under the following conditions for each step.

(29) As a 1.sup.st step, heat treatment (temperature: 250 C.-300 C., time for annealing: 5 minutes) was performed, and then as a 2.sup.nd step, heat treatment (temperature: 850 C.-875 C., time for annealing: 5 minutes) was performed (examples 1 and 2). In addition, heat treatment was performed using continuous heat treating furnace in both steps.

(30) Heat treatment was performed (temperature: 250 C., time for annealing: 5 minutes and 240 minutes) (corresponding to heat treatment of only the 1.sup.st step of comparative examples 1 and 2). In addition, heat treatment (the time for annealing: 5 minutes) in comparative example 1 was performed using a continuous heat treating furnace; heat treatment (the time for annealing: 240 minutes) in comparative example 2 was performed using a box-type heat treating furnace with.

(31) Heat treatment was performed (temperature: 850 C.-950 C., time for annealing: 5-10 minutes) (corresponding to heat treatment of only the 2.sup.nd step of comparative examples 3, 4 and 5). In addition, heat treatment was performed using a continuous heat treating furnace in all cases.

(32) Next, nickel was plated onto the copper layer using the laminated material as a cathode. The composition of a plating bath is as follows. In addition, the plating thickness of nickel was set to 2.5 m, the plating bath temperature was set at 70 C., and the pH of the plating bath was set at pH 4.

(33) (Sulfamate Bath)

(34) TABLE-US-00003 Nickel sulfamate 450 g/l Nickel chloride 5 g/l Boric acid 30 g/l Additive 5 ml/l
Furthermore, at the stage of the completion of heat treatment for the copper layer and at the stage of the completion of nickel plating, the full width at half maximum and the tail width of the peaks based on the pole figures of the copper layer and the protective layer were each measured using an X-ray diffractometer (RINT2000 (Rigaku Corporation)). The developed views of angles and X-ray intensities at =35 of the pole figures measured in example 1 and comparative example 1 are shown in FIG. 1 and FIG. 2, respectively.

(35) Moreover, arithmetic mean roughness within a 10 m10 m area was measured as the surface roughness Ra of the copper layer and the protective layer using AFM (Nano ScopeIIIaD3000 (Digital Instruments)). Furthermore, adhesion and Cu diffusion length of the SUS316L/copper layer interface were measured. The results are shown in Table 1. In Table 1, each measured value is shown in the form of A/B, where A denotes the average value of the measured values of 5 samples, and B denotes the maximum value of the measured values of the above 5 samples. The number of samples subjected to measurement of adhesion was 1 and the same subjected to measurement of Cu diffusion length was 1.

(36) In addition, regarding the adhesion of the SUS316L/copper layer interface, 180 peel strength for a width of 10 mm was measured using a tensile testing machine (TENSILON universal testing machine (A&D Company, Limited)). Also, the Cu diffusion length of the SUS316L/copper layer interface was observed with a transmission electron microscope (TEM, JEM-2010F (JEOL Ltd.,) and measured based on energy dispersive X-ray spectrometry (EDS, UTWSi-Li (Nolan)). The Cu diffusion length is defined as the distance from the SUS316 L/copper layer interface to a position at which the copper concentration of 2 at % or more is detected via elementary analysis of the SUS 316L side by EDS.

(37) Furthermore, in Table 1, criteria for , , and x are as follows.

(38) (Crystal Orientation)

(39) : (average value and maximum value) of the protective layer is within 5

(40) : (average value and maximum value) of the protective layer is within 5.5

(41) x: (average value or maximum value) of the protective layer is higher than 5.5 (but within 6)

(42) (Surface Roughness)

(43) : Ra of the protective layer is within 20 nm

(44) x: Ra of the protective layer is higher than 20 nm

(45) (Adhesion)

(46) : adhesion is 10 N/cm or more

(47) x: adhesion is less than 10 N/cm

(48) TABLE-US-00004 TABLE 1 Comparative Comparative Example 1 Example 2 example 1 example 2 Step 1.sup.st step.fwdarw.2.sup.nd step 1.sup.st step.fwdarw.2.sup.nd step 1.sup.st step 1.sup.st step Temperature Low temperature.fwdarw. Low temperature.fwdarw. Low temperature Low temperature High temperature High temperature ( C.) (250 + 850) (300 + 875) (250) (250) Time for annealing Short time Short time Short time Long time (min) (5 min each) (5 min each) (5) (240) Crystal orientation (XRD) (Cu/) (4.36/4.48) (4.60/4.82) (4.39/4.42) (4.33/4.45) 1.sup.st step (Cu Tail width /) (19.14/19.88) (18.32/19.15) (19.14/19.88) (18.32/19.45) 2.sup.nd step (Cu Tail width /) (14.67/15.30) (14.32/14.86) (Ni/) (4.79/4.91) (4.75/4.96) (4.17/4.42) (4.22/4.39) Surface roughness (AFM) x x (Cu Ra/nm) (5.97/6.60) (9.85/13.87) (5.51/6.36) (5.33/6.77) (Ni Ra/nm) (12.22/14.20) (11.76/12.46) (52.2/55.5) (48.9/52.2) Cu/SUS Adhesion x x Cu diffusion length (nm) 100 >100 <10 <10 Adhesion (N/cm) Breaking of material Breaking of material <3 <3 Unmeasurable Unmeasurable Comparative Comparative Comparative example 3 example 4 example 5 Step 2.sup.nd step 2.sup.nd step 2.sup.nd step Temperature High temperature High temperature + High temperature + ( C.) (850) (950) (950) Time for annealing Short time Short time Short time + (min) (5) (5) (10) Crystal orientation (XRD) x (Cu/) (4.90/5.11) (4.82/5.00) (4.78/4.91) 1.sup.st step (Cu Tail width /) 2.sup.nd step (Cu Tail width /) (16.11/16.74) (15.92/16.63) (16.08/16.82) (Ni/) (5.43/5.70) (5.23/5.41) (5.11/5.35) Surface roughness (AFM) x x (Cu Ra/nm) (5.42/6.47) (8.30/9.55) (29.2/34.5) (Ni Ra/nm) (13.36/15.55) (27.8/31.2) (53.8/61.2) Cu/SUS Adhesion Cu diffusion length (nm) 100 >120 >120 Adhesion (N/cm) Breaking of material Breaking of material Breaking of material Unmeasurable Unmeasurable Unmeasurable Supplementary notes) Crystal orientation, Surface roughness . . . N5ave./N5max.

(49) In comparative examples 1 and 2, low-temperature heat treatment (corresponding to heat treatment in the 1.sup.st step) alone was performed. The of the thus obtained copper layer was low and the surface was smooth, however, the orientation within the copper layer was insufficient, such that fine and random crystals were present. It was thus revealed that the formation of a protective layer made of nickel by plating results in the rough surface of the protective layer. Moreover, adhesion between the SUS plate and the copper layer was insufficient.

(50) In comparative example 3, high-temperature treatment alone was performed using a continuous heat treating furnace. In the process of recrystallization (Cu (100) orientation) from rolling texture, crystals deviating from Cu (100) grew, and was not decreased. A sharp increase in Cu diffusion length at a heat treatment temperature of around 800 C. is also assumed to be a cause of inhibiting crystal growth.

(51) In comparative examples 4 and 5, the temperature for heat treatment was set at a higher temperature. In comparative example 5, the time for annealing was increased. Therefore, excess rearrangement of crystal took place in the copper layer. It was revealed that this made the surface of the protective layer rough, upon the formation of the protective layer made of nickel by plating. These results revealed that, regarding conditions for the 2.sup.nd step when heat treatment is performed in stages, the heat treatment temperatures should be set to range from 800 C. to 900 C., and the time for annealing should be set to be less than 10 minutes.

(52) In contrast to these comparative examples, in examples 1 and 2, where heat treatment was performed in stages in the 1.sup.st and the 2.sup.nd steps, crystals of the copper layer were preferentially Cu (100)-oriented in the 1.sup.st step, and then crystal rearrangement was caused to take place in the copper layer in the 2.sup.nd step. Accordingly, the surface of the protective layer made of nickel was smooth. At this time, within the copper layer, nuclei of Cu (100)-oriented crystal grains were generated in the 1.sup.st step, and thus rearrangement was thought to take place in the 2.sup.nd step to grow the nuclei generated in the 1.sup.st step. Through high crystal orientation of the copper layer in such a manner, the nickel protective layer provided on the upper layer thereof also had high crystal orientation, and the surface became smooth. Furthermore, adhesion between the SUS plate and the copper layer was also excellent.

(53) All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety.