Method for the electrolytic deposition of metals

Abstract

The present invention relates to a method as well as and electrolyte for the electrolytic deposition of mat or half-glossy copper layers from an acid electrolyte in a pull-through device. The method according to the invention is operated with current densities comprises between 10 and 100 A/dm2 and is suitable for depositing sufficiently thick copper layers in high-speed pull-through devices. The electrolyte according to the invention comprises apart from copper, alkyl sulfonic acid.

Description

FIELD OF THE INVENTION

The present invention relates to a method for the electrolytic deposition of a mat or half-glossy copper layer from an acid electrolyte.

BACKGROUND OF THE INVENTION

Acid copper electrolytes are used for the surface coating of substrates in a manifold manner, in order to form a functional or decorative coating on the substrate surfaces.

The metallization method to be used as well as the electrolyte to be used depends on the type and nature of the substrate to be metallized. Thus, both metallic and non conductive substrates can be provided with corresponding surface layers.

In particular, in the metallization of substrates, such as wire, band, ribbon, or tube, the electrolyte and method to be used have to meet special requirements with respect to the stability and the deposition velocity. Thus, the above mentioned products are for example often metallized in high-speed pull-through devices. They are commonly referred to as “stretch products” because they are stretched or bended during service and therefore the coating must be ductile and well adhered. In order to obtain a sufficient metallization of the substrate surfaces in spite of a short contact time (high throughput rate), one has to plate up with high current densities.

Usually, acid sulphate bearing copper electrolytes are used for depositing copper on the described substrates. However, due to their insufficient stability in the presence of high current densities, such electrolytes are not suitable for the use in high-speed pull-through devices. Although apparatus and circuitry available for pull through plating of wire or tubular substrates are capable of operating at increased current densities and consequently higher pull through velocities, the quality of the coating begins to suffer from roughness and burning at operating rates well short of equipment capability. Thus, there is a need in the art for improved plating baths which can be operated at high current densities to produce high quality coatings, especially in pull through plating operations.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a method for the electrolytic deposition of a mat or half-glossy copper layer from an acid electrolyte on a substrate surface, which is suitable for being used in high-speed pull-through devices. Furthermore, it is the object of the invention to provide a suitable electrolyte for carrying out the method.

Briefly, therefore, the invention is directed to a process for the electrolytic deposition of a mat or half-glossy copper layer from an acidic electrolytic comprising contacting an elongate substrate selected from the group consisting of wire, tube, band, and ribbon with an aqueous acidic electrolytic solution comprising copper ions and an alkyl sulfonic acid by pulling the substrate through the electrolytic solution wherein a first segment of the substrate is in contact with the aqueous acidic electrolytic solution while a second segment of the substrate is not in contact with the aqueous acidic electrolytic solution; and passing a current at a density of at least about 10 A/dm² between an anode in contact with said electrolytic solution and a cathode comprising said substrate.

In another aspect the invention is directed to an electrolyte or combination of electrolytes for use in preparation of electrolytic solutions that function as plating baths for electrodeposition of copper, said electrolyte or combination of electrolytes comprising, on an anhydrous basis, between about 18 wt. % and about 40 wt. % copper and between about 22 wt. % and about 58 wt. % methane sulfonic acid.

The invention is also directed to a process for the electrolytic deposition of a mat or half-glossy copper layer on a substrate comprising reducing copper ions from an acid electrolytic solution onto said substrate at a current efficiency of at least about 75% and a current density between approximately 40 and 100 A/dm² as determined with reference to the area of the substrate being plated.

Other objects and features will be in part apparent and in part pointed out hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application claims priority from German patent application number 10 2004 041 701.6, the entire disclosure of which is explicitly incorporated by reference.

In carrying out the process of the invention, the substrate is contacted with an aqueous acidic electrolytic solution comprising copper ions and an alkylsulfonic acid. The solution is also in contact with an anode, and a current is passed through the solution at a density of at least about 10 A/dm² between the anode and a cathode comprising the substrate to be plated.

The invention has special applicability to a process in which the substrate is moved relative to the electrolytic solution as current is passed between the anode and the cathode, and more particularly to the plating of high-speed pull-through products such as wires, tubes, tape, connector material and metallized plastic wires. The term “pull-through” refers to a process in which the substrate is a relatively elongate continuous substrate which is continuously or intermittently pulled in its axial or tangential direction through the electrolytic solution as current is passed between the moving substrate and the anode in contact with the electrolytic solution, i.e., the plating bath. At any one moment, a segment of the substrate is immersed in the electrolyte and other segments of the substrate are not immersed in the electrolyte. Current is applied to the segment that is submerged. An example of such a process is the deposition of copper onto metal wire which is fed from one spool through the electrolyte to another spool. Another example of such a process is the deposition of copper onto tubing.

In many pull-through processes for plating a mat or half-glossy copper layer from an acid electrolyte onto a substrate it is critical to be able to plate at high speeds. In accordance with this invention, it has been discovered that incorporation of an alkyl sulfonic acid permits plating at relatively high current densities which are required for plating at high speeds. In various preferred embodiments, incorporation of a surfactant may further contribute to achievement of high current density without sacrifice of coating quality.

In accordance with this invention, the specific bath components facilitate current densities of at least about 10 A/dm2 (ASD), such as between approximately 10 and 100 ASD. The plating baths and processes of the invention are capable of current densities of greater than 20 ASD, such as between about 20 and about 100 ASD. Notably, high productivity is achievable at current densities in the range above 40 A/dm², and more particularly at current densities above about 50 A/dm², above about 60 A/dm², above about 70 A/dm2, or above about 80 A/dm². For some applications, the process of the invention may provide current densities as much as two to three times higher than those conventionally practiced on comparable substrates. Such increased current densities are realized with minimal or no sacrifice in current efficiency, and produce a mat or half-glossy copper plating that is smooth, even and unburned. Current efficiencies obtained in practice of the process of the invention are typically in the range of at least about 75%, most often greater than 80%, as measured by the weight of copper deposited vs. theoretical as computed from Faraday's law.

The invention involves plating speeds suitable for substrate travel rates of at least about 0.5 meter/second. For wire substrates, the pull rates are more typically at least about 0.8 meter/sec., preferably at least about 5 meters/second, and most typically between about 5 and about 20 meters/second. For other substrates the pull rate is most typically between about 2 and about 50 meters/minute. Notably, high productivity is achieved at travel velocities in the range above about 10 meters/second, more particularly above about 12 meters/second, 14 meters per second, 16 meters/second, or 18 meters/second. In most applications, the elongate substrate is caused to travel at the aforesaid rates relative to the electrolytic plating bath. However, productivity is ultimately determined by the dwell time in the bath rather than the relative velocity of substrate to bath. Conceivably, there may be applications wherein the bath itself may be subject to some movement relative to the cell in which the operation is conducted, e.g., in a direction partially or entirely parallel to the travel of the substrate. To encompass such alternatives, the velocity of the moving substrate can be expressed with reference to a plating vessel wherein the wire is submerged within the plating bath and current is applied, or more specifically relative to a a fixed locus of points within the vessel that is parallel or tangential to the path on which the substrate passes through the vessel. The aforesaid minimum velocity and range of velocity are applicable by this measurement as well. At these velocities, the typical submerged length of substrate subjected to electrolytic deposition conditions at the aforesaid current densities within the bath is typically between about 60 and about 300 meters. At such current densities, deposition velocities are typically in the range of about 4 to about 22 microns per minute for wire, and between about 1 and about 7 microns per minute for other pull through products.

Residence time is typically in the range between about 5 and about 25 seconds in the region of the bath wherein current is passed between the anode and cathode. For a given plating thickness at a given current density, the productivity of the process is a direct function of the residence time and thus of the length of substrate within the region of the bath wherein current is passed from anode to substrate. Where a moving substrate such as a wire is suspended from two points within the bath, the maximum submerged length of the substrate may be limited by the current density distribution within the plating bath, by the electrical resistance of the substrate, or by the elongation of the substrate as produced by the tensile stress that is produced by the substrate's own weight (less buoyancy). By enabling an increased current density at a given length of submerged substrate, the process of the invention facilitates increased deposition velocities, permits shorter residence times, increased substrate velocity, and consequently increased productivity.

According to the invention, the method is preferably carried out in a temperature range comprised between 22 and 60° C; for example, between 45 and 55° C. If the temperature is too high or too low, the plating may not be uniform.

These process conditions are suitable for the deposition of copper layers having a sufficient thickness and solidity in high-speed pull-through devices such as plastic tubing which had previously received a thin Cu coating by an immersion/electroless Cu process, and the present invention is applied to increase the Cu thickness. The thickness of copper plated in accordance with this invention may range up to about 50 microns.

Concerning the electrolyte, the aims of the invention are achieved by use of a copper bearing electrolytic solution, which contains an alkyl sulfonic acid. The alkyl sulfonic acid is preferably methane sulfonic acid. The alkyl sulfonic acid is preferably included in a concentration of at least about 50 ml/L, typically 50 to about 130 ml/L. For some applications, the concentration is at least about 70 ml/L, at least about 80 ml/L or at least about 90 ml/L. For certain applications, the alkyl sulfonic acid concentration is preferably less than about 110 ml/L, such as about 90 ml/L. The purpose of the alkyl sulfonic acid, i.e., the manner in which it is believed to function to permit substantially increased current densities, is that the alkyl sulfonic acid is compatible with a much higher Cu ion concentration than is sulfuric and other typical acids. If the alkyl sulfonic acid content is too low, there is a conductivity deficit. If the alkyl sulfonic acid content is too high, there can be burning with high current densities. Also, if too high, evolution of hydrogen can increase above acceptable levels. With respect to the ranges of proportions as expressed in ml/L, it will be understood the alkyl sulfonic acid can alternatively be provided in solid form (methane sulfonic acid, mp=20° C.) and then dissolved in the aqueous medium which comprises the electrolytic solution used as the plating bath. In such case, the weight of solid alkyl sulfonic acid to be introduced can be determined from the volumes specified above, applying the appropriate density, which in the case of methane sulfonic acid is approximately 1.48 at 18° C. Densities of other alkyl sulfonic acids are available from the literature, and the above expressed volumetric ratios can readily be converted to weight ratios by anyone skilled in the art. As compared to prior art electrolytes, in one preferred embodiment the acid component of the electrolyte consists essentially of methane sulfonic acid, in that sulfuric acid and other acid components are substantially excluded.

The electrolytic solution comprises a source of copper ions which is copper in form of sulphates, nitrates, halogenides, or carboxylates thereof. A preferred copper compound source of copper ions is Cu(HSO₃CH₃)₂. The copper source is preferably included in a concentration which will provide copper ions in a concentration of at least about 40 g/L, typically between about 40 and about 90 g/L. For certain applications, the copper ion concentration is preferably at least about 75 g/L. In certain embodiments, this concentration is less than about 90 g/L, such as about 75 g/L.

The electrolytic solution also comprises a sufficient quantity of a nonionic surfactant, for example a surfactant derived from an alkylene oxide such as ethylene oxide. Preferably, the nonionic surfactant comprises an alkoxylated aromatic such as, for example, 2-naptholethoxylate[2-(2-naphthyloxy)-ethanol], such as is available from BASF under the trade name Lugalvan. The ethoxylate is normally included in a concentration of, for example, at least about 10 g/L. For some applications, the ethoxylate is preferably in a concentration which is less than about 30 g/L, to assure deposition of a satisfactorily uniform copper plating. For certain applications, it may be preferable for the nonionic surfactant concentration not to exceed, e.g., about 10 g/L. However, if the ethoxylate content is too low, the substrate may be insufficiently wetted for satisfactory plating. In selecting a suitable ethoxylate, the primary criteria are bath compatibility and whether it functions as a basic inhibitor of Cu deposition.

Furthermore, the electrolytic solution comprises a sufficient quantity of an aromatic condensation polymer comprising sulfonated aromatic repeating units such as naphthalene sulfonic acid for the purpose of further facilitating the achievement of high current densities. One suitable such naphthalene condensation product has the general formula I:

Wherein n is an integer number greater than 1. The asterisks in the formula indicate appropriate linking groups well known to the art. For example, the polymer may be prepared by condensation of naphthol with formaldehyde which generates methylene linking groups, or by condensation of α-napthol or 1-naphtol-4-sulfonic acid with a dicarboxylic acid such as maleic anhydride. One suitable such condensation product is available from BASF.

The naphthalene condensation product is included in a concentration of at least about 0.001 g/L. For most applications, the naphthalene condensation product is in a concentration which is less than about 1 g/L, for example, such as about 0.1 g/L. If the naphthalene condensation content is too high, the deposit can be uneven. In selecting a suitable naphthalene condensation product, the primary criterion to be considered is stability in high current density baths.

The electrolytic solution also contains halogenide ions, such as chloride, bromide, and iodide. The purpose of the halogenide ions is to enhance the appearance of the coating. In various exemplary embodiments, the halide ion concentration is at least about 40 mg/L. The halogenide ions are preferably in a concentration which is, for example, less than about 100 mg/L, such as about 50 mg/L. If the halogenide content is too low or too high, the deposition may not be uniform.

The electrolytic solution can additionally comprise typical process materials, such as leveling agents and surface-active agents, also combinations thereof, as they are known in literature.

The invention is further directed to an electrolyte or combination of electrolytes for use in preparation of electrolytic solutions that function as plating baths for electrodeposition of copper. On an anhydrous basis, Such electrolyte or combination of electrolyte comprises between 18 wt. % and about 40 wt. % copper and between about 22 wt. % and about 58 wt. % methane sulfonic acid. Preferably, such electrolyte or combination further comprises between about 2 wt. % and about 13 wt. % of a naphtholethoxylate anion surfactant, and more preferably further comprises a condensation polymer of naphthol bearing ring-substituted sulfonic acid groups. The electrolyte or combination may further comprise between about 0.02 wt. % and about 0.04 wt. % halide ions.

It will be understood that the electrolyte can be supplied as a pre-packaged combination which can be directly introduced into water to provide an electrolytic solution which may serve as the plating bath for use in the method of the invention. Alternatively the combination may be provided as a supply or kit comprising separately packaged components, or with some components pre-mixed but others separately packaged.

The following example describes an electrolyte and electrolytic solution according to the invention, wherein the invention is not to be limited to the exemplary realization.

EXAMPLE 1

Composition of an aqueous electrolytic solution for the deposition of copper in a pull-through device:

copper:                   40 to 90 g/l, preferably 75 g/l
methane sulfonic acid:    50 to 130 ml/l, preferably 90 ml/l
halogenide ions:          40 to 100 mg/l, preferably 50 mg/l
2-naphtolethoxylate:      5 to 30 g/l, preferably 10 g/l
Naphthalene condensation: 0.001 to 1 g/l, preferably 0.1 g/l

Preferably, the electrolyte comprises chloride as halogenide ion.

Using the electrolytic solution of this example as the plating bath, a 1 mm diameter wire is plated with copper in a pull-through continuous electrolytic coating under ambient temperature conditions at a line velocity of 600 m/min., a current density of 80 A/dm², and a residence time of 10 seconds. Submerged length of the wire is 100 meters. A uniform copper coating having a thickness of 3 μm is obtained on the wire.

Example 2 shows typical process conditions for the method according to the invention.

EXAMPLE 2

Using an apparatus similar to that of Example 1, a 1 mm diameter wire is electroplated under the process conditions described below for the deposition of mat or half-glossy copper layers on a substrate from an electrolyte according to the invention:

Substrate:                       Brass
Temperature:                     25° C.
Current density:                 10 A/dm2
Pull-through velocity            50 m/min
Layer thickness of the deposited copper layer: 5 μm

EXAMPLE 3

A 1 mm diameter wire substrate is coated in a pull through device under the conditions of Example 2 in a methane sulfonic acid (MSA) bath and a comparative sulfuric acid bath. The baths have the following compositions:

MSA Bath

copper: 75 g/L

methane sulfonic acid: 90 g/L

chloride ions: 50 mg/L

2-naphtolethoxylate: 10 g/L

Naphthalene condensation: 0.1 g/L

Sulfuric Acid Bath

copper: 50 g/L

sulfuric acid:55 g/L H₂SO₄

chloride ions: 60 mg/L

2-naphtolethoxylate: absent

Naphthalene condensation: absent

Polyethylene glycol: 1 g/L

EXAMPLE 4

Using the same bath compositions described in Example 3, laboratory plating experiments were conducted under the temperature and current density conditions generally described in Example 2. Comparisons were made of the elongation characteristics of the wire coated in the bath of the invention vs. that coated in the conventional sufuric acid bath. In this context, “elongation” means the maximum stretching of the wire that can be tolerated without damage to the copper coating.

Elongation tests were conducted according to specification ASTM DIN EN 0 002. The methane sulfonic acid based electrolytes in terms of the percentage elongation show no disadvantage in comparison to the sulphuric acid based electrolytes.

% Elongation
	Methane sulfonic acid	Sulphuric Acid
	Exp.	1	7.54	6.43
		2	6.96	5.24
		3	6.96	7.12
		4	6.84	5.42
		5	9.74	6.04
		Ø	7.6	6.05
		(Avg)
		max.	9.74	7.12
				untempered
		1	13.37	14.3
		2	17.28	17.65
		3	13.92	17.92
		4	9.28	13.6
		5	15.66	10.65
		Ø	13.9	14.82
		(Avg)
		max.	17.28	17.92
				tempered

When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in any accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

1. A process for the electrolytic deposition of a mat or half-glossy copper layer from an acidic electrolytic comprising:

contacting an elongate substrate selected from the group consisting of wire, tube, band, and ribbon with an aqueous acidic electrolytic solution comprising copper ions and an alkyl sulfonic acid by pulling the substrate through the electrolytic solution wherein a first segment of the substrate is in contact with the aqueous acidic electrolytic solution while a second segment of the substrate is not in contact with the aqueous acidic electrolytic solution; and

passing a current at a density of at least about 10 A/dm2 between an anode in contact with said electrolytic solution and a cathode comprising said substrate.

2. The process of claim 1 wherein said pulling is continuous.

3. The process of claim 1 wherein said pulling is intermittent.

4. The process of claim 2 wherein said pulling is at a linear velocity of the substrate relative to said electrolytic solution of at least about 0.5 m/sec.

5. The process of claim 2 wherein said pulling is at a linear velocity of the substrate relative to said electrolytic solution of at least about 5 m/sec.

6. The process of claim 2 wherein said pulling is at a linear velocity of the substrate relative to said electrolytic solution of between about 5 m/sec and about 20 m/sec.

7. The process of claim 1 wherein said electrolytic solution is contained in a plating vessel and said substrate is caused to move within the electrolytic solution relative to said vessel.

8. The process of claim 1 wherein the substrate is pulled continuously or intermittently through said electrolytic solution at a linear velocity of at least about 0.8 m/sec. relative to a fixed locus of points within said vessel parallel or tangential to a path on which said substrate passes through said vessel.

9. The process as set forth in claim 8 wherein said substrate comprises a wire and said velocity is between about 5 and about 20 m/sec.

10. The process of claim 1 wherein the substrate has a residence time of between about 5 and about 25 seconds within a region of said electrolytic solution wherein current is passed between said anode and said cathode at said current density.

11. The process of claim 1 wherein said electrolytic solution further comprises a naphtholethoxylate nonionic surfactant.

12. The process of claim 1 wherein said electrolytic solution further comprises a condensation polymer of naphthol bearing ring-substituted sulfonic acid groups.

13. The process of claim 1 wherein the electrolytic solution comprises at least about 40 g/L Cu ions, a naphtholethoxylate nonionic surfactant, a condensation polymer of naphthol bearing ring-substituted sulfonic acid groups, halide ions, and at least about 50 ml/L methane sulfonic acid.

14. The process of claim 1 wherein the electrolytic solution comprises at least about 75 g/L Cu ions.

15. The process of claim 13 wherein the electrolytic solution comprises at least about 75 g/L Cu ions.

16. The process of claim 1 wherein sulfuric acid and other acid components are substantially excluded from the electrolytic solution.

17. The process of claim 1 wherein said current density is at least about 40 A/dm2 and said plating occurs at a current efficiency of at least about 75%.

18. The process of claim 1 wherein said current density is at least about 80 A/dm2 and said plating occurs at a current efficiency of at least about 75%.

19. A process for the electrolytic deposition of a mat or half-glossy copper layer on a substrate comprising reducing copper ions from an acid electrolytic solution onto said substrate at a current efficiency of at least about 75% and a current density between approximately 40 and 100 A/dm2 as determined with reference to the area of the substrate being plated.

20. An electrolyte or combination of electrolytes for use in preparation of electrolytic solutions that function as plating baths for electrodeposition of copper, said electrolyte or combination of electrolytes comprising, on an anhydrous basis, between about 18 wt. % and about 40 wt. % copper and between about 22 wt. % and about 58 wt. % methane sulfonic acid.

21. The electrolyte or combination of electrolytes of claim 20 further comprising between about 2 wt. % and about 13 wt. % of a naphtholethoxylate nonionic surfactant.

22. The electrolyte or combination of electrolytes of claim 21 further comprising a condensation polymer of naphthol bearing ring-substituted sulfonic acid groups.

23. The electrolyte or combination of electrolytes of claim 20 further comprising between about 0.02 wt. % and about 0.04 wt. % halide ions.

24. An electrolyte or combination as set forth in claim 20 that is pre-packaged for introduction into water to provide an electrolytic solution which may serve as the plating bath for electrodeposition of copper.

25. A supply or kit comprising the components of the electrolyte or combination of claim 20 wherein at least some but not all components are pre-mixed and packaged separately from one or more other components.