DE102022124665A1 - Method for electroplating copper on niobium or niobium alloys and workpiece made of niobium or niobium alloy with copper coating - Google Patents

Abstract

The invention relates to a method for the galvanic deposition of copper on niobium or niobium alloys, which comprises at least the steps of first creating a workpiece made of niobium or a niobium alloy and a bath with a solution of copper tartrate hydrate with a mass concentration in the range of 2. 0 g/L to 5.0 g/L, sodium gluconate with a mass concentration ranging from 2 g/L to 5 g/L and sodium hydroxide with a mass concentration ranging from 2 g/L to 4 g/L in deionized water and with a temperature in the range of 30 °C to 90 °C. Furthermore, an anode made of copper or other conductive materials that are inert to the bath solution is provided. Subsequently, at least the surfaces of the workpiece and the anode intended for electroplating are immersed in the bath solution at a distance in the range of 20 mm to 50 mm from one another. The workpiece is then electroplated with a current density in the range of 20 mA/dm2 to 60 mA/dm2 for a duration in the range of 1 min to 60 min. In addition, a workpiece made of niobium or a niobium alloy is electroplated with a copper layer, which is a Thickness of 10 nm to 500 nm and the adhesion of which is so great that the adhesion of the layer is classified as 5A, according to the pressure sensitive tape test, which incidento the adhesive tape test carried out according to ASTM D3359, test method A (X-cut tape test). is claimed.

Description

The present invention relates to a method for electroplating copper onto niobium or niobium alloys. Furthermore, workpieces made of niobium or niobium alloys are claimed, which are galvanically coated with copper using the method according to the invention. Electroplating copper coatings on niobium is used, for example, in the production of superconducting resonators for particle accelerators.

Throughout the description, wherever niobium is mentioned, this also applies to niobium alloys with a high (≥ 50%) niobium content, unless otherwise stated.

Throughout the description, the terms “layer” and “film” are used synonymously for a thin (in the micro- or nanometer range), uniform application of solid substances to a substrate.

Superconducting devices and components such as conductors made of niobium are particularly provided with copper layers in order to improve the cooling options through better heat conduction using copper plating. In addition, the copper plating of niobium components to better connect these components is reported. The reverse process of niobium coating on a copper component is also common practice in the production of superconducting resonators, but is not relevant here.

For all applications it is crucial that the contact between the niobium and the copper is of a size that is sufficient for the application and that the adhesion is sufficiently strong even at very low temperatures (< 5 K) and the cooling time and heat-up cycles. For example, impacts on the quality of superconducting devices caused by poor contacts and low adhesion are caused by V. Palmieri and R. Vaglio discussed (Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities, Superconductor Science and Technolgy, 2016, Vol. 29, 015004 - 2-12) . The importance of good contact between niobium and copper, particularly with regard to low-temperature applications, is the topic of G. Ciovati et al. (Multi-metallic conduction cooled superconducting radio-frequency cavity with high thermal stability, Supercondtor Science and Technology, 2020, Vol. 33, 07LT01 - 1-7) . G. Ciovati et al. propose to first create a thin copper layer on the niobium surface by cold gas spraying of copper before electroplating in order to overcome the problems described. Last but not least is the work of V. Palmieri and R. Vaglio (Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities, Superconductor Science and Technology, 2016, Vol. 29, 015004 - 1-12 ) about the influence of good contact between copper and niobium on the quality of HF superconducting cavities.

Various approaches to electroplating methods for copper plating niobium have been proposed in the prior art.

In the GB 1,152,091 A discloses a method comprising the following steps. First, degrease the niobium workpiece with a suitable, preferably chlorinated, solvent; followed by deoxidation in a mixture of hydrofluoric acid and sulfuric acid; then pickling in a mixture of hydrofluoric acid, sulfuric acid and nitric acid; as well as chemical action on the surface of the metal alloy with a dilute aqueous solution of hydrofluoric acid and ammonium fluoride as the main step of the process, which gives the alloy the surface condition required for successful copper plating, and finally electrolytic copper plating in a bath of copper (II) tetrafluoroborate and Fluoroboric acid. However, nothing is said here about what the surface condition mentioned is all about.

Another process that focuses on special preparation of the niobium surface before the actual electroplating process is in the DE 1 521 010 C3 described. Accordingly, tin is applied in small quantities to the niobium surface to be copper-plated. The tin is incorporated into the niobium grid by heating. The niobium surface is then treated with a pickling agent containing hydrofluoric acid and nitric acid and then electrolytically copper-plated.

In the EP 1 892 322 B, the surface of the niobium raw material is coated with nickel by electroforming before coating with copper.

A similar process for electrochemical copper plating of niobium, in which the niobium surface is first coated with nickel, is described in US$4,632,734 B. The method comprises the following steps: (a) blasting a niobium workpiece with aluminum oxide, (b) treating the blasted workpiece from step (a) with an alkaline cyanide bath, (c) subjecting the treated workpiece from step (b) to a preliminary electrolytic nickel deposition treatment using an acidic nickel chloride bath, (d) subjecting the treated workpiece from step (c) to a further treatment with an alkaline cyanide bath, and then (e) carrying out an electrochemical plating treatment with copper.

K. DeBry and G. Lafyatis (Electroplating process for connectorizing superconducting NbTi cables; Review of Scientific Instruments, 2018, Vo. 89, 076108 - 1-3) describe a process for electroplating NbTi cables for a connection, in which the surface of the NbTi cables is first oxidized in a basic, oxidizing electrolytic bath.

A scientific explanation for the underlying problem in forming a copper film with good adhesion to niobium is the galvanic exchange reaction between copper and niobium (niobium has a more negative electrochemical potential than copper), so precoating with a less active metal such as nickel will reduce the displacement reaction of the Copper with niobium is avoided. On the other hand, coating with nickel or another metal also presents difficulties, such as: B. the formation of a layer with a certain porosity, the formation of oxides and a certain thickness to obtain a layer. In addition, nickel is ferromagnetic, which is a limitation for certain applications.

Based on the outlined problems in the prior art, the subject of the present invention is to provide a method for the galvanic deposition of copper on niobium or niobium alloys, with which a copper layer can be obtained with improved adhesion compared to the prior art and which at the same time is simplified compared to the prior art. Workpieces made of niobium or niobium alloys are also claimed, which are coated with copper using the method according to the invention.

The problem is solved by a method with the features of claim 1 and a workpiece according to claim 2.

Unexpectedly, a process for the galvanic deposition of copper on niobium or niobium alloys has been found at the Helmholtz Center in Berlin, with which the task is solved in a given specific parameter space, with specific chemicals and according to a specific sequence of process steps.

1. a. Bereitstellen eines Werkstücks aus Niob oder einer Nioblegierung;
2. b. Bereitstellen eines Bades mit einer Lösung von Kupfertartrat-Hydrat mit einer Massenkonzentration im Bereich von 2,0 g/L bis 5,0 g/L, Natriumgluconat mit einer Massenkonzentration im Bereich von 2 g/L bis 5 g/L und Natriumhydroxid mit einer Massenkonzentration im Bereich von 2 g/L bis 4 g/L in entionisiertem Wasser und mit einer Temperatur im Bereich von 30 °C bis 90 °C;
3. c. Bereitstellen einer Anode aus Kupfer oder anderen leitenden Materialien, die gegenüber der Badlösung inert sind;
4. d. Eintauchen der Anode und zumindest der für die Galvanisierung vorgesehenen Oberflächen des Werkstücks in einem Abstand von 20 mm bis 50 mm zueinander in die Badlösung;
5. e. Elektroplattieren des Werkstücks mit einer Stromdichte im Bereich von 20 mA/dm² bis 80 mA/dm² für eine Dauer im Bereich von 1 min bis 60 min.

The first aspect of the present invention relates to a method for the electrodeposition of copper on niobium or niobium alloys, which comprises at least the steps:

1. a. Providing a workpiece made of niobium or a niobium alloy;
2. b. Provide a bath containing a solution of copper tartrate hydrate with a mass concentration ranging from 2.0 g/L to 5.0 g/L, sodium gluconate with a mass concentration ranging from 2 g/L to 5 g/L and sodium hydroxide with a Mass concentration ranging from 2 g/L to 4 g/L in deionized water and with a temperature ranging from 30 °C to 90 °C;
3. c. providing an anode made of copper or other conductive materials that are inert to the bath solution;
4. d. Immersing the anode and at least the surfaces of the workpiece intended for electroplating at a distance of 20 mm to 50 mm from each other in the bath solution;
5. e. Electroplating the workpiece with a current density in the range of 20 mA/dm ² to 80 mA/dm ² for a duration in the range of 1 min to 60 min.

The result of treating a workpiece made of niobium or a niobium alloy with the described method according to the invention is a homogeneous thin copper film with a thickness that depends mainly on the time of electroplating and is in the range from 10 nm to 500 nm. The copper layer formed using the method according to the invention has an adhesion that can withstand a so-called “adhesion tape test” (more on this below) and a thermal shock test with liquid nitrogen. If a thicker copper layer is required, the copper film obtained with the process according to the invention can be used as a starting point for a standard copper plating process with a copper sulfate bath, as is generally known for the galvanic copper plating of steel.

For the method according to the invention, the following remarks should advantageously be taken into account.

Go to step a. It should be mentioned that it is advisable to provide a degreased surface free of foreign particles. Any method known to those skilled in the art for producing such a surface is suitable. A useful procedure is e.g. Am GB 1,152,091 A described, in which the niobium workpiece is degreased with a chlorinated solvent and freed from oxides on the surface in a mixture of hydrofluoric acid and sulfuric acid. The standard etching bath for niobium (called “buffered chemical polishing”, BCP, HF (48%): HNO ₃ (60%): H ₃ PO ₄ (95%) = 1:1:2) is also suitable for removing coarser deposits of niobium oxides, if present, and mechanical damage after mechanical processing. After etching, the niobium must be washed thoroughly in deionized water. It should be mentioned that the niobium surface prepared in this way is covered with a thin native Nb ₂ O ₅ oxide layer 3 - 7 nm thick. It is also advantageous if the formation of grooves on the surface due to scratching or similar is avoided. Polishing the surface could therefore also be beneficial.

With regard to step b. The following should be noted when using the method according to the invention.

The most important parameters for a layer deposition using the method according to the invention are the type and concentration of the chemicals (copper tartrate hydrate “CuTH”, sodium gluconate “SG” and sodium hydroxide “NaOH”) as well as a relatively low current density.

The temperature of the solvent affects the morphology of the deposited film. As the temperature increases, the density of the Cu particles forming in the film increases. When a higher concentration of CuTH is used, the temperature becomes crucial to obtain a film with good adhesion. The higher the CuTh concentration within the given range, the higher the temperature should be within the given range. As a guideline, with a mass concentration of CuTh of 4.2 g/l, the bath temperature should not be lower than 60 °C but within the limit less than 90 °C. The order of mixing the components in the bath is preferably CuTH+NaOH+H ₂ O and heating at 40°-60°C for 10-30 min until the mixture is homogeneous (ie no undissolved residue left), after which SG is added becomes. Alternatively, the mixture of CuTH+NaOH+SG+H ₂ O is heated to 40 °C-60 °C for 10-30 min. The preparation of the solution as described offers good storage conditions (ie no decomposition of the solution).

The mechanisms and chemistry of the process according to the invention can be briefly described as follows. NaOH is added to regulate the pH of the solution and to form soluble complexes with CuTH, which is otherwise insoluble in water. This directly leads to better homogeneity of the film. SG is a chelating agent and has been experimentally proven to suppress the formation of Cu ₂ O during electroplating and also prevents the decomposition of any copper anode present. Nevertheless, a very small amount of Cu ₂ O was detected in the formed films by X-ray diffraction (XRD) measured in grazing incidence geometry at incident X-ray angles ω=0.5°-5° ( ). Further increasing the SG content is believed to further suppress the formation of _Cu2O in the deposited copper layers.

A CuTH solution used without any additives results in copper layers that are inhomogeneous over the entire cathode area and have a black hue (due to the presence of Cu ₂ O and possibly elemental copper associated with the degradation of the copper anode).

If the amount of both additives, SG and NaOH, is less than specified, the resulting deposit is similar to that obtained without any additives.

If the NaOH concentration is lower than specified, CuTH does not dissolve completely in water (copper tartrate hydrate is insoluble in water), resulting in copper layers that resemble those without additives.

If the NaOH concentration is too high compared to the specified value, you will not get a continuous copper deposit, but rather copper-containing islands mixed with Cu ₂ O phases.

If the SG content is lower than stated or is completely absent, the copper film formed is slightly darker in color (due to Cu ₂ O, which is detected by XRD) and the copper anode oxidation is pronounced.

If the concentration of CuTH is increased to 10 g/l, the copper film is peeled off. In this case, crystals of Cu ₂ O were observed under the formed copper film.

Particularly good results are achieved in a range of 3.5 g/l to 4.0 g/l CuTH at temperatures in the range of 35 °C to 65 °C. At temperatures lower than 30 °C, the Cu ₂ O content is increased in certain areas and the layer adhesion in these areas is impaired.

If no potential is applied, no precipitate will form (i.e. an electroless process using the chemicals themselves is not possible).

Regarding steps c. to e. of the method according to the invention, the disclosure is supplemented by the following.

In step c. An anode made of copper or other conductive materials that are inert to the bath solution is provided. Although a copper anode is preferred, others can also be used conductive materials inert to the bath solution such as platinum, graphite, carbon fiber, glassy carbon, etc. are used as materials for an anode.

The anode and at least the surfaces of the workpiece intended for electroplating are immersed in the bath solution at a distance of 20 mm to 50 mm from one another.

The electroplating of the workpiece with a current density in the range of 20 mA/dm ² to 80 mA/dm ² is carried out for a period in the range of 1 min to 60 min, depending on the desired layer thickness of the copper film to be formed. A specific duration of electroplating for a desired layer thickness may need to be determined experimentally, since the duration is not the only parameter on which the layer thickness depends, although it is by far the one with the greatest weight.

Another aspect of the invention is a workpiece made of niobium or a niobium alloy, which is electroplated with a copper film with a thickness in the range of 10 nm to 500 nm and has such great adhesion that the adhesion of the film after the adhesion test ASTM D3359, Test Method A (X-cut Tape Test) is rated 5A and passes a liquid nitrogen thermal shock test. The adhesion classification according to ASTM D3359 with 5A means “No removal or detachment of the layer in the test procedure” after the film has been and is cut/scored at the point to be tested down to the substrate (the base, the workpiece). the highest possible test result. This test result qualifies the copper coating as suitable for the use of such coated workpieces in superconductivity, especially in particle accelerators. What is particularly noteworthy is that the “tape test” is passed after the thermal shock test has been carried out.

The advantage of the method according to the invention is that it allows the electroplating of niobium with copper in a simple and inexpensive manner, obtaining good and even improved results in terms of the adhesion of the copper film. Workpieces made of niobium or niobium alloys that have been galvanically coated with a copper film using the method according to the invention are covered with a homogeneous copper film with strong adhesion.

Example

An exemplary galvanic coating of niobium with copper using the method according to the invention and the coated workpiece obtained thereby is described below and completed by the figures.

shows X-ray diffractograms obtained under grazing incidence at angles 2θ of ω = 0.5°, 1°, 3°, 5° and in Bragg-Brentano geometry (BB) of the copper layer on niobium obtained by the exemplary electroplating; b) shows the X-ray diffractogram at ω = 0.5° grazing angle of incidence and is plotted logarithmically on the abscissa. The three phases observed - niobium, copper, copper oxide (Cu ₂ O) - are indicated above the reflections.

and b) show SEM images of the copper film formed in the example at two different magnifications. The scales at the bottom left of the images indicate 200 nm in a) and 100 nm in b).

The galvanic coating of niobium with copper in the example was carried out with the parameters given below on a rectangular cuboid made of niobium with dimensions of 2.8 x 10 x 40 mm as a workpiece.

• - Lösung von 3,5 g/L Kupfertartrat-Hydrat (C₄H₄CuO₆·H₂O) in entionisiertem Wasser sowie
• - Natriumhydroxid 2 g/L (NaOH) und
• - Natriumgluconat 2 g/L (C₆H₁₁NaO₇).

The parameters for the bath composition in the example are as follows:

• - Solution of 3.5 g/L copper tartrate hydrate (C ₄ H ₄ CuO ₆ ·H ₂ O) in deionized water and
• - Sodium hydroxide 2 g/L (NaOH) and
• - Sodium gluconate 2 g/L (C ₆ H ₁₁ NaO ₇ ).

• - Badtemperatur: 35 °C;
• - Stromdichte: 40 mA/dm²;
• - Beschichtungsdauer: 30 min;
• - Abstand zwischen Kathode und Anode: 35 mm;
• - Anodenmaterial: Kupfer.

The parameters for electroplating are:

• - Bath temperature: 35 °C;
• - Current density: 40 mA/dm ² ;
• - Coating time: 30 min;
• - Distance between cathode and anode: 35 mm;
• - Anode material: copper.

The thickness of the copper film obtained on the workpiece is 200 nm. The film is characterized using grazing incidence X-ray diffraction. The result is in shown, with the intensity on the abscissa in is logarithmically scaled to highlight low intensity reflections. Note that the reflection assigned to the (111) reflection of Cu ₂ O is without the logarithmic scaling in is not visible at all, indicating the negligible proportion of copper oxide. This finding is confirmed by an electron diffraction X-ray spectroscopy analysis of the copper film on niobium in the initial state (not shown).

The texture of the copper layer is in the and b) SEM images, which demonstrate a narrow distribution of copper grain size and a general homogeneity and smoothness of the layer.

The film adhesion is so good that the film passed an adhesive tape test according to ASTM D3359, Test Method A, using the adhesion test kit from TQC Sheen® (Industrial Physics Inks & Coatings B.V.) and a thermal shock test by rapidly cooling the resulting copper film on niobium Withstands 300K to 80K with liquid nitrogen. The Scotch tape test was carried out after the cold shock and was rated 5A according to ASTM D3359, which corresponds to the highest adhesion according to the test procedure. The test kit uses a 25mm wide/150µm thick ScotchPar™ polyester film tape specifically manufactured for such purposes.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• GB 1152091 A [0007, 0019]
• DE 1521010 C3 [0008]
• EP 1892322 [0009]
• US 4632734 [0010]

Non-patent literature cited

• V. Palmieri and R. Vaglio discussed (Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities, Superconductor Science and Technolgy, 2016, Vol. 29, 015004 - 2-12) [0005]
• G. Ciovati et al. (Multi-metallic conduction cooled superconducting radio-frequency cavity with high thermal stability, Supercondtor Science and Technology, 2020, Vol. 33, 07LT01 - 1-7) [0005]
• V. Palmieri and R. Vaglio (Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities, Superconductor Science and Technology, 2016, Vol. 29, 015004 - 1-12 [0005]
• K. DeBry and G. Lafyatis (Electroplating process for connectorizing superconducting NbTi cables; Review of Scientific Instruments, 2018, Vo. 89, 076108 - 1-3) [0011]

Claims (2)

A method for electroplating copper on niobium or niobium alloys, comprising at least the following steps: a. providing a workpiece made of niobium or a niobium alloy; b. providing a bath containing a solution of copper tartrate hydrate with a mass concentration in the range of 2.0 g/L to 5.0 g/L, sodium gluconate with a mass concentration in the range of 2.0 g/L to 5.0 g/L and sodium hydroxide with a mass concentration in the range of 2.0 g/L to 4.0 g/L in deionized water and at a temperature in the range of 30 °C to 90 °C; c. providing an anode made of copper or other conductive materials which are inert towards the bath solution; d. immersing at least the surfaces of the workpiece and the anode intended for electroplating at a distance of 20 mm to 50 mm from each other in the bath solution; e. Galvanizing the workpiece with a current density in the range of 20 mA/dm ² to 60 mA/dm ² and for a duration in the range of 1 min to 60 min. Niobium or niobium alloy workpiece coated with a copper film having a thickness in the range of 10 nm to 500 nm which has an adhesion great enough to maintain the adhesion of the film in accordance with ASTM D3359, Test Method A Adhesion test is classified as 5A.

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