Patent Application
20240209537
The present disclosure generally relates to the field of electrodeposited copper foils and more specifically to electrodeposited copper foils with low surface roughness.
The process and production of electrodeposited copper foil for use in printed circuit boards is basically a plating technique, as it involves arranging two electrodes (a cathode and an anode) in an electrolyte containing a copper salt, passing current between the electrodes and depositing copper on the cathode with a desired thickness. The electrodeposited copper foil is then peeled off from the surface of the cathode, and coiled onto a storage reel. The cathode is generally a rotating drum-shaped cathode and is arranged in the electrolyte to face a stationary anode.
In general, when an aqueous solution containing only copper ions and sulfate ions is employed as electrolyte, pinholes and/or micro porosities are produced in the copper foil due to an unavoidable admixture of dust and/or oil from the equipment, resulting in serious defects in practical use. Moreover, the face of the electrodeposited foil that contacts the drum, the so-called “shiny side”, is comparatively smooth, but the other (electrolyte) side, called the “matte side”, has an uneven surface.
In other words, the profile (mountain/valley) shape of the surface of the copper foil that is in contact with the electrolyte is deformed, which results in an increased surface roughness. Matte sides of electrodeposited copper foils generally present a surface roughness (as of Rz ISO) well above 0.80 μm, and commonly above 3.0 μm.
With regard to the performances required for electrodeposited copper foils, improvements in profile lowering (decreased roughness) of the matte side has been sought for decades, and became critical with the development of high frequency applications. Traditional copper foils having a surface roughness above 3.0 μm are too rough to meet the requirements enabling the fabrication of effective transmission lines for applications at frequencies of 77 GHz and above, such as for the Fifth-Generation of mobile communication (5G).
Indeed, a higher surface roughness of a copper foil induces a higher signal loss in high speed/high frequency applications. This is due to the fact that at high frequency the signal is propagated only at the surface of the conductor (due to a so-called “skin effect”). On a smoother conductor, the propagation route of the signal is therefore shorter, inducing lower loss.
Various attempts have been made in the past decades to make the matte side smoother, and as a result, electrolytes often contain additives.
In order to prevent the appearance of defects such as pinholes, chloride ions may, e.g., be added to the electrolyte and the dust and/or oil may be removed by passing the electrolyte through a filter containing active carbon or the like. Also, in order to adjust the surface roughness of the matte side and prevent micro porosities, it has long been the practice to add glue to the electrolyte and various organic and inorganic additives, apart from glue, have also been proposed.
Lowering of the profile of the matte side can also be achieved by adding to the electrolyte large quantities of a so-called brightener, such as glue and/or thiourea, for example, but as the amount of these additives is increased, there is an abrupt lowering of the room-temperature elongation rate and high-temperature elongation rate of the electrodeposited copper foil, thus resulting in a degradation of its mechanical properties.
WO 97/11210 A1 discloses an electrolyte comprising 3-mercapto-1-propane sulfonate, chloride ions, a high molecular weight polysaccharide and a low molecular weight glue. Produced copper foils exhibit good tensile strength and elongation, both at room-temperature and at high temperature. However, copper foils electrodeposited using the electrolyte of WO 97/11210 A1 hardly present a surface roughness (Rz) in the order of 1.3 μm, and the electrodeposition process may be difficult to control due to variations in naturally-sourced glue and polysaccharide used as additives.
To overcome the problem associated with the sourcing of the glue, it has been proposed to replace it by other amino compounds. EP 1 574 599 A1 and EP 1 568 802 A1 disclose the use of electrolytes comprising an organosulfur compound and a quaternary amine salt to obtain low-profile electrolytic copper foils, exhibiting a low surface roughness on their matte side. However, only surface roughness (Rz) in the order of 1-1.3 μm was achieved.
CN 111394754 A discloses an electrolyte and a process using the electrolyte for the production of copper foils suitable for application in the field of the Fifth-Generation of mobile communication. The electrolyte of CN 111394754 A proposes the use of hexylbenzylamine salt as so-called leveler in order to improve the smoothness of the matte side. While good mechanical properties, such as satisfying bonding forces, have been achieved for the copper foils, their respective surface roughness (Rz) was not decreased below 1.15 μm.
Despite constant ameliorations over the past decade, it has still not been possible to achieve production of electrodeposited coper foils exhibiting the low surface roughness that has been demanded for years, and is key to the development of the Fifth-Generation of mobile communication.
The disclosure provides a method for producing an electrodeposited copper foil with a surface roughness as low as possible without the afore mentioned problems.
More particularly, the present disclosure proposes a method for producing an electrodeposited copper foil, the electrodeposited copper foil being continuously formed in an electroforming cell comprising a rotating drum-shaped cathode, a stationary anode and an electrolyte. The present method allows forming in the electroforming cell an electrodeposited copper foil with a matte side having a roughness Rz ISO of 0.8 μm and below. The term Rz ISO refers to the roughness Rz determined according to ISO standard, as compared e.g. to Rz JIS determined according to Japanese standard.
According to the disclosure, the electrolyte comprises:
In the present text, the expression average molecular weight Mw (or simply Mw) refers to the weight average molecular weight of a polymer, by opposition to its number average molecular weight Mn or its viscosity average molecular weight Mv. The weight average molecular weight of a polymer depends not only on the number of molecules present, but also on the weight of each molecule, so a larger molecule will have a larger contribution than a smaller molecule. The average molar weight Mw is conventionally calculated as follows:
where Ni is the number of molecules of molecular weight Mi, i being an integer.
All mentioned concentrations correspond to the concentrations of the respective various components of the electrolyte being provided to the electroforming cell. The electrolyte is continuously supplied with the various components during operation of the electroforming cell to ensure that the concentrations of the various components are always in the prescribed respective ranges. Copper ions, halogen ions, 3-mercapto-1-propane sulfonate, nitrogen-containing polymer leveler and polyether suppressor can be added to the electrolyte as such, or they can be added to the electrolyte as any suitable derivative compounds resulting in the obtention of the desired respective component in the electrolyte. This applies for copper ions and halogen ions, which are commonly added to the electrolyte as a copper salt or halogen salt respectively, but also for any other component comprised in the electrolyte according to the disclosure.
In the present text, 3-mercapto-1-propane sulfonate may be referred to as MPS or as brightener. A brightener is more generally known in the field of copper deposition as an accelerant, and increases the rate of copper deposition during production of an electrodeposited copper foil.
The electrolyte further comprises a nitrogen-containing polymer leveler. A leveler exerts a strong suppression effect on copper deposition reactions in the presence of halogen ions.
The polyether suppressor may be simply referred to as suppressor, or surfactant. In the present text, a suppressor (surfactant), also exerts a strong suppression effect on copper deposition reactions in the presence of halogen ions, however with respect to a leveler, the suppressor acts in a relatively wider copper deposition current region, and can be deactivated by the use of the brightener (or accelerant).
Suppressors are inhibitors which are weakly adsorbed on the surface of electrodeposited copper foils in combination with halogen ions and are not consumed or chemically transformed on the metal surface. On the contrary, levelers are inhibitors that are strongly adsorbed and are consumed on the metal surface. The disclosure is based on the findings by the inventors that a specific combination of brightener, leveler and suppressor of different kinds at prescribed concentrations make it possible to obtain electrodeposited copper foils exhibiting the low surface roughness which has been sought for a long time, while being free of visual/surface defects, such as e.g. overgrowth, grooves, holes, craters and loss of brightness. In other words, the method according to the disclosure uses prescribed brightener, leveler and suppressor in specific concentrations to produce electrodeposited copper foils, which are free from surface defects and have a surface roughness Rz ISO of 0.8 μm and below (≤0.8 μm) on the matte side.
Advantageously, the inventors have found that the process reliably can be improved by replacing conventional low-molecular weight glue by a polyether. The production process is easier to control since the behavior of polyether is more consistent than the one of a low molecular weight glue.
Surprisingly, the inventors also discovered that the problem of too high roughness of known electrodeposited copper foils is solved by the use of a nitrogen-containing polymer leveler in the electrolyte. The copper foils produced with the inventive method have a significantly reduced surface roughness compared to what was obtained in the prior art. Indeed, copper foils produced with the inventive method have a surface roughness Rz ISO of no more than 0.8 μm, corresponding to a surface roughness Rz JIS of no more than 0.6 μm. The surface developed ratio (SDR) of an electrodeposited copper foil produced by the present inventive method is also reduced from 0.4% for prior art to 0.15% or even below, such as 0.1%.
The surface developed ratio (SDR) corresponds to the ratio between the area of the real developed surface and the area of the projected surface. The real surface is the surface of the produced electrolytic copper foil considering its surface roughness while the projected surface is the surface of a corresponding flat, completely smooth foil. The SDR can be calculated as follows:
The obtained electrodeposited copper foils may generally be subjected to further subsequent treatment steps. Commonly, a surface bond enhancing treatment and a passivation are deposited on the matte side of the electrodeposited copper foil. The reduction of surface roughness allows a more homogenous deposition of the treatment and passivation, inducing improved properties for the final product (such as an increased peel strength with insulating resin substrate, higher shelf life of copper foil).
It shall further be appreciated that the reduced roughness of copper foils produced according to the inventive method induces lower signal loss in high speed/high frequency applications. This is due to the fact that at high frequency the signal is propagated only at the surface of the conductor (skin effect). On a smooth conductor the propagation route of the signal is therefore shorter, inducing lower loss. This enable the fabrication of effective transmission lines for applications at frequencies of 77 GHz and above (5G, etc.)
Furthermore, first tests have shown that the ultra-low roughness electrodeposited copper foils produced according to the disclosure have mechanical properties similar to conventional copper foils. In particular, the obtained electrodeposited copper foils may have an elongation of between 10 and 25% at 20° C. for foils having a thickness of 18 μm, and between 15 and 35% at 20° C. for foils having a thickness of 35 μm. The tensile strength may be between 28 and 37 kgf/mm2 at 20° C. regardless of the thickness.
Electrodeposited copper foils produced by the method according to the disclosure or using an electrolyte according to the disclosure are however not limited to these two thicknesses, and copper foils of various thickness may be obtained. According to some embodiments, copper foils having a thickness of 9 to 70 μm may be produced.
The drum side of electrodeposited copper foil has a roughness that depends on the drum itself. In the context of the disclosure the electrodeposited copper foil may have a conventional roughness, e.g. in the order of 0.9 to 1.8 μm.
In large scale (tens or hundreds of m3 of electrolyte) continuous process, accumulation of additives degradation products can be detrimental for the quality of the copper foil deposited over long term (several days), resulting in loss of brightness and ultimately in an increase in surface roughness. Surprisingly, the inventors found out that present inventive method offers excellent long-term stability of a large volume of electrolyte in continuous use without significant drag-out.
More specifically, the inventors discovered that using polyether suppressor having an average molecular weight Mw comprised between 500 and 12000 g/mol reduces the formation of insoluble degradation products responsible for the degradation of the electrodeposited copper foil after several days of operating the production method, while ensuring a sufficient suppression effect to reduce the surface roughness of the copper foil. Moreover, only a specific range of polyether suppressor concentration in the electrolyte, from 15 to 30 ppm, allows to achieve the target surface roughness of electrodeposited copper foils while ensuring the long-term stability of the electrolyte, also by avoiding the accumulation of degradation products which lead to a degradation of copper foil aspect within several days.
Formation of overgrowth defects on electrodeposited copper foils is advantageously prevented by controlling the formation of nitrogen-containing polymer leveler degradation products. In order to do so, used nitrogen-containing polymer leveler has an average molecular weight Mw comprised between 1000 and 30000 g/mol, and its concentration should not exceed 12 ppm. However, to ensure a sufficient levelling effect, the electrolyte comprises at least 5 ppm of nitrogen-containing polymer leveler.
In embodiments, the average molecular weight Mw of the nitrogen-containing polymer leveler is comprised between 1 500 and 15 000 g/mol, preferably between 2 000 and 5 000 g/mol. Additionally or alternatively, the nitrogen-containing polymer leveler is present in the electrolyte at a concentration comprised between 6 and 11 ppm, preferably between 7 and 10 ppm.
In general, the nitrogen-containing polymer leveler may comprise or consist of one or more polymers. Advantageously, the nitrogen-containing polymer leveler is selected from polyvinylpyrrolidone, polyallylamine, polyethyleneimine, and mixtures thereof. In embodiments wherein the leveler is a mixture of polymers, the concentration of nitrogen-containing polymer leveler in the electrolyte corresponds to the total concentration of all polymers forming the mixture, and each one of the polymer forming the mixture has an average molecular weight Mw comprised between 1000 and 30000 g/mol, preferably between 1500 and 15000 g/mol, more preferably between 2000 and 5000 g/mol. Alternatively, polymers forming the mixture may have higher or lower average molecular weights, but the polymer mixture has an average molecular weight Mw comprised between 1000 and 30000 g/mol, preferably between 1500 and 15000 g/mol, more preferably between 2000 and 5000 g/mol.
In embodiments, the average molecular weight Mw of the polyether suppressor is comprised between 500 and 6000 g/mol, preferably between 1000 and 3500 g/mol. Additionally or alternatively, the polyether suppressor is present at a concentration comprised between 12 and 28 ppm, preferably between 15 and 25 ppm.
In general, the polyether suppressor may comprise or consist of one or more polymers. Advantageously, the polyether suppressor is selected from polyethylene glycol, polypropylene glycol, copolymers of polyethylene glycol and polypropylene glycol, and mixtures thereof. In embodiments wherein the polyether suppressor is a mixture of polymers, the concentration of polyether suppressor in the electrolyte corresponds to the total concentration of all polymers forming the mixture, and each one of the polymer forming the mixture has an average molecular weight Mw comprised between 500 and 12000 g/mol, preferably between 500 and 6000 g/mol, more preferably between 1000 and 3500 g/mol. Alternatively, polymers forming the mixture may have higher or lower average molecular weights, but the polymer mixture has an average molecular weight Mw comprised between 500 and 12000 g/mol, preferably between 500 and 6000 g/mol, more preferably between 1000 and 3500 g/mol.
In embodiments, copper is added to the electrolyte as copper sulfate. According to the same or alternative embodiments, the copper is present in the electrolyte at a concentration comprised between 60 and 100 g/L, preferably between 70 and 90 g/L.
In embodiments, the halogen ion is a chloride and/or bromide ion, and/or the halogen ion is present in the electrolyte at a concentration comprised between 35 and 50 ppm.
In embodiments, the electrolyte may further comprise sulfuric acid at a concentration comprised between 65 and 85 g/L, preferably between 70 and 80 g/L. Using sulfuric acid at such concentrations has the advantageous effect of reducing the electric resistance between the anode and the cathode, thereby decreasing the electric power and electric consumption needed to produce the electrodeposited copper foil. Production costs may therefore be reduced.
According to the same or alternative embodiments, the electrodeposited copper foil is formed by applying a current density between the cathode and the anode, the current density being comprised between 40 and 80 A/dm2, preferably between 40 and 60 A/dm2, more preferably between 45 and 55 A/dm2. Such current densities advantageously allow for a strong levelling effect of the electrolyte used in the method according to the disclosure. Moreover, using such current densities advantageously allow for a faster copper deposition, with respect to the use of lower current densities, which enhances the productivity.
The electrolyte preferably is maintained at a temperature higher than 50° C. to prevent copper sulfate crystallization in the electrolyte. More preferably the temperature of the electrolyte is comprised between 50 and 60° C., in order to simultaneously enhance copper dissolution and to both prevent copper sulfate crystallization and degradation of the surface roughness of the electrodeposited copper foil.
Advantageously, the method is a continuous process and the electrolyte has an endless life time, i.e. degradation products and by-products formed during the use of the electrolyte in the method according to the disclosure do not affect the quality of the produced electrolytic copper foil, in particular the brightness of the electrolytic copper foil is not altered. In other words, the quality of the produced electrolytic copper foil, in particular its brightness and its surface roughness, is not impaired by accumulation in the electrolyte of reaction by-products or degradation products, such as e.g. degradation products of the leveler, the suppressor or the brightener. It should be appreciated that the electrolyte has a life-time of more than three days, preferably more than seven days, more preferably more than fifteen days. Such a long life-time of the electrolyte enables the production of electrodeposited copper foils with constant properties, i.e. without any quality loss in the production over a few days of utilization of the electrolyte.
According to another aspect, the disclosure concerns an electrolyte for the production of an electrodeposited copper foil comprising:
What was said regarding advantages and embodiments of the inventive method applies mutatis mutandis to the inventive electrolyte.
In yet another aspect, the disclosure also concerns an electrodeposited copper foil, in particular as produced by the inventive method or produced by using the inventive electrolyte, the electrodeposited copper foil having a bright electrolyte side, with a surface roughness Rz of 0.8 μm and below, a surface developed ratio below 0.15%, preferably of 0.1%, and being free of structural defects.
As indicated above, the present electrodeposited copper foil exhibits suitable mechanical properties for industrial use, as well a weight deviation of less than 5%, or even 3% and less.
In the present context, any given numeric value covers a range of values form −10% to +10% of said numeric value, preferably a range of values form −5% to +5% of said numeric value, more preferably a range of values form −1% to +1% of said numeric value.
Further details and advantages of the present disclosure will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings.
The present disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:
The operative principle of an electroforming cell will first be described with reference to
As explained above, the present disclosure provides a method for producing an electrodeposited copper foil, the electrodeposited copper foil being continuously formed in an electroforming cell, as well as an electrolyte for the production of an electrodeposited copper foil, the produced copper foil having a very low surface roughness and being free of defects.
An electrodeposited copper foil is produced by using an electroforming cell 10 (referred as plating machine in the industry) as shown in
In a subsequent step, the electrodeposited copper foil 18 may be subjected to an electrochemical or chemical surface treatment, such as a bond enhancing treatment and/or a passivation treatment (not shown).
Electrodeposited copper foils were produced using either a method according to the disclosure (examples 1 to 6) or a comparative method (comparative examples 1 to 6) not forming part of the disclosure. The method according to the disclosure and the comparative method differ from each other only by the composition of the electrolyte. According to both methods, the electrolyte is maintained at a temperature of 55° C. and the current density applied between the cathode and the anode is 50 A/dm2.
Electrolyte compositions for the various examples are presented in Table 1, and electrolyte compositions for various comparative examples are presented in Table 2. MPS stands for 3-mercapto-1-propane sulfonate in Table 1 and Table 2.
The concentrations shown in Table 1 and Table 2 correspond to the concentrations of the various compounds of the electrolyte being provided to the electroforming cell. Before starting the electroforming cell (or plating machine), each electrolyte is prepared by solubilizing, in a suitable amount of water, the compounds shown in Table 1 and Table 2. Each electrolyte also includes copper, which is dissolved in the electrolyte with sulfuric acid by oxidizing metallic copper. The copper concentration is 80 g/l. During operation of the electroforming cell, each electrolyte is continuously supplied with the various components to ensure that the concentrations of the various components are always in the prescribed respective ranges.
The obtained electrodeposited copper foils were then analysed to determine their surface characteristics at the matte side, such as surface roughness (as of Rz ISO) and surface developed ratio (SDR) and to detect the presence of visual defects.
Electrodeposited copper foils are analysed as follows:
The roughness of copper foils is measured with a contact profilometer consisting of a diamond needle (stylus) sliding on the surface. From this measurement a 2D profile of the surface is created, and Rz is calculated as the average distance between the highest peak and lowest valley over 8 sampling lengths. Here the surface roughness Rz refers to ISO 4287:1997.
The surface developed ratio of the matte side of each electrodeposited copper foil is measured using non-contact three-dimensional white light interferometry.
The principle is to divide a light beam in two paths, directing one to a reference mirror and the other one to the sample surface. This measurement beam travel different distances depending on the surface profile. The two waveforms are then recombined and create specific interference patterns depending on their phase difference. Those patterns are analyzed to calculate the height of the sample at each point (pixel) scanned. Roughness parameters are then calculated from this 3D profile. Here the surface roughness SDR refers to ISO 2517 and is typically measured on a 200×1000 μm sample surface.
Produced electrodeposited copper foils are controlled using an optical microscope to detect whether they present structural defects. Scanning electron microscope might be used afterwards to identify the type of defect (craters and/or overgrowth defects).
The loss of brightness is controlled through the visual aspect of the foil and is correlated with an abrupt increase of Rz (above 2.0 μm).
Surface characteristics of electrodeposited copper foils produced by a method according to the disclosure are presented in Table 1 and surface characteristics of electrodeposited copper foils produced by a comparative method are presented in Table 2.
All copper foils produced by using electrolytes according to the disclosure (examples 1 to 6) have a surface roughness comprised between 0.7 and 0.8 μm, a SDR comprised between 0.10 and 0.15%, and are free of visual and surface defects (see Table 1 and
As shown in Table 2, when using concentrations of polyether higher than 30 ppm, the electrolyte is not stable over long term due to accumulation of degradation products (comparative example 1). In other words, the electrolyte loses brightness after a few days of use. This leads to a degradation of the produced copper foil aspect within several days, such as but without being limited to an increase in the SDR. The same effect can be observed when using a polyether with an average molecular weight Mw above 12000 g/mol (comparative example 5).
Using electrolyte comprising less than 15 ppm of polyether does not allow to achieve target surface roughness (comparative example 2, Table 2), although produced electrodeposited copper foils do not present visual defect.
As shown in Table 2, when using concentrations of nitrogen-containing polymer higher than 12 ppm, target surface roughness can be achieved but degradation of the copper foil aspect is observed, with the appearance of localized overgrowth defects (comparative example 3, see also
The appearance of overgrowth defects is also observed when using nitrogen-containing polymer with an average molecular weight higher than 30000 g/mol (comparative example 4). Moreover, in this case, target surface roughness could not be achieved and produced copper foil had a surface roughness Rz ISO of 5.1 μm and a SDR of 10.8%.
When using less than 30 ppm of halogen ion (comparative example 6), the target surface roughness could not be achieved, and the produced electrodeposited copper foil lacks brightness (see Table 2).
Only electrolyte compositions corresponding to the present disclosure, i.e. comprising a halogen ion, a polyether (as suppressor agent) and a nitrogen-containing polymer (as leveller agent) with the prescribed average molecular weights Mw, and within the prescribed concentrations, allow to achieve the desired reduction in the surface roughness of electrodeposited copper foils without the appearance of visual/surface defects.
| Number | Date | Country | Kind |
|---|---|---|---|
| LU500134 | May 2021 | LU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/062331 | 5/6/2022 | WO |
