Surface-treated copper foil for high-frequency circuit and method for producing the same

Abstract

The invention proposes a method for producing a surface treated copper foil (28), comprising the steps of: a) providing an untreated electrolytic copper foil (18) with a first surface (18.1) and a second surface (18.2) opposite to the first surface, preferably having a surface roughness Rz ISO inferior or equal to 0.8 µm on a first surface (18.1); b) performing a roughening treatment on at least one surface (18.1) of the copper foil (18) in an electro etching cell (20) with the copper foil (18) at anodic potential, to achieve a surface roughness Rz ISO of no more than 0.8 µm, a surface roughness Sa of 30 nm or less and a negative skewness Ssk, wherein step b) comprises applying a current density of 1 to 30 A / dm2 and wherein during the roughening treatment the current density is applied with a duration between 1 and 10 s. The invention also relates to such a treated copper foil for use in a printed circuit board.

Description

[0001] P-CIRCUI-025 / WO 1

[0002] Surface-treated copper foil for high-frequency circuit and method for producing the same

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to a surface-treated copper foil for a high-frequency circuit and more particularly relates to a surface-treated copper foil, which is excellent in adhesiveness with an insulating substrate for a high-frequency circuit and also excellent in transmission characteristics in a high-frequency region.

[0005] BACKGROUND OF THE INVENTION

[0006] Data is growing at an exponential rate and is not going to slow down, due to the popularization of information terminals like smartphones and laptops as well as social networking services and video-sharing platforms. This leads to increasing demands for transmitting massive data, which requires ever increasing signal transmission speeds between components on circuit boards. To achieve these speeds, frequency ranges are necessarily increasing from the MHz range to 1 GHz, 10 GHz or even higher. In these higher ranges, the electrical currents flow mostly near the surface of the conductors due to the well-known “skin effect”, which is the tendency of high frequency current density to be highest at the surface of a conductor and to decay exponentially towards the center.

[0007] The skin depth, where approximately 67% of the signal is carried, is inversely proportional to the square root of the frequency. Accordingly, at 1 MHz the skin depth is 65 pm, at 1 GHz it is 2.1 pm, while at 10 GHz the skin depth is only 0.65 pm. At the higher frequencies, the surface topography or roughness of the conductor becomes ever more important since a roughness in the order of, or greater than, the skin depth will impact the signal transmission through scattering. A surface roughness, Rz, on the roughened surface in the order of several pm is typical and will impact any transmission in the GHz range. The conventional design of copper foil for printed circuit boards (PCBs) is therefore constrained by the conflicting need for a roughness high enough to ensure a sufficient adhesion to a substrate, and low enough to minimize transmission loss. P-CIRCUI-025 / WO 2

[0008] In other words, in order to produce copper foil suitable for low-loss applications, the roughness of treated copper foil has to be reduced. However, reducing roughness limits the adhesion of the copper foil to the resin substrate. Producing a microroughened copper foil allows for a compromise between adhesion and signal integrity of the copper-clad laminate.

[0009] In this connection, it may be noted that in conventional printed circuit boards (PCBs), the surface of the conductor tracks is intentionally roughened to enhance adhesion characteristics to the resin layer used in the laminated PCB structures. US 10,772,199 B discloses a copper foil with microscale nodular treatment to ensure high bondability with the substrate, and US 2021 / 321514 A1 suggests increasing the microroughness of the foils by depositing micro-nodules thereon to ensure high bondability. Conventional roughening treatments comprise the deposition of nodules (nodular treatment) on the copper foil surface.

[0010] However, when producing micro-roughened foils by electrodeposition, homogeneity of the treatment at the microscopic scale is hard to achieve due to the fact that high current densities must be used, resulting in the release of hydrogen bubbles locally affecting locally nodules deposition. Moreover, when small nodules are produced, they are often very fragile and can be mechanically or chemically detached from the copper foil during lamination, which causes residues of copper in the resin that may induce short-circuits in the PCB.

[0011] Despite various approaches proposed in the prior art, there still remains a need for copper foils with controlled properties for high-frequency circuits, in particular showing good adhesion as well as the desired low transmission loss.

[0012] OBJECT OF THE INVENTION

[0013] It is an object of the present invention to provide an improved surface treated copper foil to be used in a high-frequency circuit without the afore mentioned problems. P-CIRCUI-025 / WO 3

[0014] SUMMARY OF THE INVENTION

[0015] In order to achieve the above-mentioned object, the present invention provides a method of treating a copper foil as claimed in claim 1 and a surface treated copper foil as claimed in claim 10.

[0016] According to the present invention, a method for producing a surface treated copper foil, comprises the steps of: a) providing an untreated electrolytic copper foil with a first surface and a second surface opposite to the first surface, preferably having a surface roughness Rz inferior or equal to 0.8 pm on the first surface; b) performing a roughening treatment on at least one surface of the copper foil in an electro etching cell with the copper foil at anodic potential, to achieve / produce a surface roughness Rz ISO of no more than 0.8 pm, preferably inferior or equal to 0.7 pm, more preferably inferior or equal to 0.6 pm, wherein step b) comprises applying a current density of 1 to 30 A / dm2for a duration of 1 to 10 seconds.

[0017] According to the invention, the roughening treatment of step b) is performed to produce a surface roughness (on at least one surface of the electrolytic copper foil) having a Sa of 30 nm or less and a negative skewness Ssk.

[0018] The present invention proposes a surface roughening treatment that removes, i.e. etches, material from the surface of the untreated copper foil, in contrast to conventional roughening treatment that involve electrodeposition of material, typically in the form of fine copper particles or copper nodules.

[0019] This approach goes against common practice in the art, since the inventive method relies on producing an increase of surface roughness of the foil by removing (by electrochemical etching or simply electro etching) the copper which has been obtained by electroplating, i.e. in copper foil production.

[0020] Surprisingly, the inventors found that the resulting treated copper foil - according to first results - meets the requirements for application in high frequency circuits, particularly in terms of adhesion and low transmission loss.

[0021] Indeed, the inventors have unexpectedly discovered that a structuration treatment (namely a roughening treatment) using electrochemical etching is more P-CIRCUI-025 / WO 4 homogeneous (i.e. results in a more homogeneous surface roughness profile of the treated side) than electrodepositing fine copper particles or nodules. Moreover, as copper is removed (i.e. etched) from the surface of the copper foil, there is no need to provide additional copper, hence reducing raw material costs / operational costs. In step b), the copper foil is thus connected to the anode in an electro-etching cell, that also comprises a cathode; the copper foil and cathode being immersed in an electrolyte. In particular, the copper foil may move in front of a cathode, e.g. a platelike cathode. The prescribed current density conventionally refers to the electric current flowing from the anode (copper foil) to the cathode; it refers to the magnitude of the current.

[0022] A treated copper foil obtained by the method according to the present invention exhibits - when compared to a copper foil subjected to roughening by electrodeposition of copper nodules - better mechanical resistance and less risk of copper particles (chemically or mechanically) detaching from the surface of the foil during lamination, thereby decreasing the risk of having copper residues in the resin (which may result in short-circuits in resulting PCB and are to be avoided).

[0023] In that context, it may be noted that the proposed roughening treatment by electro etching will produce surface-distributed nano- to micro-cavities / recesses into the surface of the electrolytic copper foil. In other words, the roughening is obtained by removing material, not by adding small copper particles that may detached from the foil.

[0024] It has been observed that the etching does not result in a simple uniform thickness reduction of the untreated copper while preserving the same roughness, but provides a different attack of the surface that modifies the roughness profile.

[0025] As understood by the skilled person, the roughening treatment in step b) is a step of electrochemical etching (also sometimes simply referred to as etching or electro etching in the present text) so that the duration during which the current density is applied, which is the duration of the roughening treatment of step b), might be referred to as “etching duration”.

[0026] Continuous direct current is advantageously used during the roughening treatment so that the applied current density is maintained throughout the prescribed duration, P-CIRCUI-025 / WO 5 which ranges from 1 to 10 seconds, without any interruption or pulses during this time interval. In contrast to pulsed electrochemical treatments, the uninterrupted application of current ensures that the electro-etching process proceeds consistently, promoting a more uniform development of surface features on the copper foil. The inventors have found that maintaining a continuous direct current avoids fluctuations in etching rate and helps forming homogeneous nano- to microscale recesses, thereby further enhancing the uniformity of the resulting roughness profile within the intended treatment interval.

[0027] Another merit of the invention is to have observed that such roughening by electro etching provides higher homogeneity, at the microscopic scale, of the structuration (etching I roughening) treatment over the whole surface of the copper foil with respect to what is conventionally obtained. In the present text “higher homogeneity” means that there is no apparent visual defects I differences in the spatial arrangement of the roughening treatment. “Homogeneity” does not mean a continuous I uniform thickness removal.

[0028] Indeed, when structuring (roughening) a surface of a copper foil by electrodeposition of copper nodules or fine particles, the copper foil is arranged at the cathode of an electrolytic cell and hydrogen reaction at this electrode results in the release of hydrogen bubbles locally affecting nodules deposition. On the contrary, in the present inventive method, the copper foil to be treated is arranged at the anode of the electrolytic cell, whereby hydrogen bubbles are formed at the counter-electrode (cathode) and do not affect homogeneity - at the microscale - of the etching process.

[0029] In the present text, it is to be understood that the electrolytic copper foil has two opposite sides (namely a drum or shiny side and an electrolyte or matte side), each side having a respective surface. However, for the sake of clarity and conciseness of explanation, when discussing the electrolytic untreated (base) copper foil, it will be referred to its surface(s) while the term “side” will be used when referring to the treated copper foil. P-CIRCUI-025 / WO 6

[0030] In embodiments, step a) of providing an untreated electrolytic copper foil may comprise a step of producing, by electroplating as conventionally known in the art, an electrolytic copper foil.

[0031] The roughening treatment b) may be carried at any time, after the copper foil electroplating, i.e. directly after or a few days after. Indeed, due to plant logistics, the untreated copper foil is conventionally stored before being subjected to further treatments. This is however not mandatory and the roughening treatment may be carried out directly after the electroplating step.

[0032] In the context of the invention, the term ‘untreated’ copper foil means the copper foil as produced at the exit of the electroplating cell (i.e. the cell in which the copper foil is formed by electrodeposition on a cathode drum), i.e. without roughening treatment or surface treatment by deposition of other species (on both sides).

[0033] As mentioned above, the roughening treatment of step b) is performed to produce a surface roughness (on at least one surface of the electrolytic copper foil) having a Sa of 30 nm or less and a negative skewness Ssk.

[0034] The invention is based on the findings by the inventors that a specific surface roughness profile, or specific surface condition, of a treated copper foil is desirable for a treated copper foil to be used in a printed circuit board, and on the identification of the corresponding surface roughness characteristics.

[0035] Surprisingly, when submitting an electrolytic copper foil to a roughening (etching) treatment as per the present invention, the surface roughness as per Rz ISO is not substantially modified, while adhesion properties I bondability of the foil is enhanced without negatively affecting signal transmission.

[0036] Another merit of the present invention is therefore to have identified that a specific surface condition, represented by 3D surface roughness parameters ranges, and more particularly by roughness parameter Sa in combination with Ssk, are suitable for accurately characterizing the surface roughness of treated copper foils, in particular but not limited to copper foils in printed circuit boards for application at high frequencies. The finding of the relevant 3D surface roughness characteristics for this application is of particular relevance. Indeed, the 3D surface roughness characteristics, also known in the art as "contactless” because they are determined P-CIRCUI-025 / WO 7 with a microscope, bring refined information on the surface roughness compared to contact measurements such as Rz.

[0037] In other words, one of the merits of the invention is to have identified that a specific surface roughness characterized by the herein prescribed values of surface roughness Sa and Ssk is suitable for a treated copper foil to be used in a printed circuit board, in particular to ensure low signal insertion loss and high, reliable bondability with a substrate (resin, pre-preg).

[0038] Whereas Rz has always been a primary parameter to characterize the roughness of copper foils in the industry, the inventors have found that it is not accurate enough to describe the surface of the foil (also because it does not vary significantly upon roughening treatment I electro etching) and can be advantageously complemented by other surface roughness parameters such as Sa and Ssk, which could hence characterize in a suitable and reproducible manner the surface of the inventive treated copper foil, where the prescribed values of Sa and Ssk provide enhanced signal transmission behaviour and reliable bondability not heretofore achieved with conventionally treated copper foils for printed circuit boards.

[0039] The surface roughness Ssk - or simply Ssk - also referred to as the skewness of the surface, is a measure of the asymmetry of the surface height distribution. It describes whether the surface has a predominance of peaks (positive Ssk) or valleys (negative Ssk) relative to a symmetrical profile. The inventors found that copper foils conventionally treated by electrodeposition of nodules present a positive Ssk while copper foils treated as per the present invention by electrochemical etching to increase their surface roughness (structuration treatment) present a negative Ssk.

[0040] As Ssk is affected by the texture (number and size of peaks and valleys) and the spatial disposition and spacing of the peaks and valleys, this parameter may advantageously further differentiate surfaces of similar roughness as expressed using 2D parameters, such as Rz, or even as expressed using 3D parameters such as Sa. Indeed, Ssk advantageously allows to differentiate foils based on differences between the spatial distribution of features or the balance of peaks and valleys. P-CIRCUI-025 / WO 8

[0041] Typically, Ssk will change signs depending on the predominance of peaks or valleys, whether or not Sa changes.

[0042] According to another aspect, the present invention also concerns a treated copper foil, preferably for use in a printed circuit board, with a first side and a second side opposite to the first side, the treated copper foil comprising an electrolytic copper foil with two opposite surfaces, a side of the treated copper foil corresponding to a surface of the electrolytic copper foil, preferably with at least one functional layer.

[0043] According to the invention, at least one of the first side and the second side has a surface roughness Rz ISO of no more than 0.8 pm, preferably inferior or equal to 0.7 pm, more preferably inferior or equal to 0.6 pm, a surface roughness Sa of 30 nm or less and a negative skewness Ssk.

[0044] In embodiments, the surface of the copper foil submitted to the roughening treatment of step b), that is subjected to the etching treatment, is free of any treatment comprising deposition of copper particles, such as e.g. nodular treatment. In other words, in embodiments there is no deposition of copper nodules prior to the etching treatment nor following the etching treatment.

[0045] What was said above regarding the surface roughness obtained by the structuration treatment according to the first aspect applies mutatis mutandis to the treated copper foil. More specifically, the inventors found that having as Sa in the prescribed range of 30 nm or less is necessary but not sufficient to ensure a satisfying behaviour when manufacturing a printed circuit board, and only treated copper foil presenting surface roughness as of Sa and Ssk in the respective prescribed ranges allow to achieve the desired low signal insertion loss (great signal transmission) and maintained bondability upon manipulation.

[0046] In some preferred embodiments, the surface roughness of the side (of the treated copper foil according to the second aspect) having a negative skewness is obtained by electro-etching a corresponding surface of the electrolytic copper foil, more preferably according to the method according to the first aspect.

[0047] In other words, the method of the first aspect is adapted to obtain the treated copper foil according to the second aspect. P-CIRCUI-025 / WO 9

[0048] In embodiments, Ssk is between -2 and 0, preferably between -1 and 0, more preferably between -0.5 and 0. Additionally or alternatively, Sa may lie between 1 and 30 nm, preferably between 5 and 30 nm, more preferably between 10 and 28 nm.

[0049] Indeed, the inventors have identified that lower Sa values may results in treated copper foils too smooth to properly adhere to another layer of material (such as e.g. a pre-preg or resin when manufacturing a printed circuit board), while higher Sa, corresponding to highly rough foils, may increase signal insertion loss and hence negatively affect the signal transmission.

[0050] Preferably, the treated copper foil presents a microstructure defined by an average grain size of at least 0.8 pm, preferably between 0.8 and 1.2 pm, more preferably between 0.9 and 1.1 pm, even more preferably of about 1 pm. Such grain size advantageously ensures satisfying electrical conductivity by minimizing electron scattering at grain boundaries, thereby reducing resistivity, while achieving desired mechanical properties. Consequently, the treated copper foil allows for high-quality signal transmission, making it particularly advantageous for high-frequency electronic applications.

[0051] In the present text, the average grain size relates to the arithmetic mean of the geodesic diameters of grains, measured from a FIB (focalized ion beam) exposed microstructure on a SEM (scanning electron microscopy) picture.

[0052] In embodiments, the electrolytic cell for the electrochemical etching comprises an acidic bath, such as e.g. a bath comprising 0 to 100 g / L of copper ions Cu2+, 40 to 150 g / L, preferably 50 to 120 g / L, more preferably 60 to 100 g / L, such as e.g. about 80 g / L, of sulfuric acid, 0 to 100 ppm, preferably 10 to 50 ppm, more preferably 20 to 30 ppm, of chloride, 0 to 10 g / L of ferrous sulfate, 0 to 20 g / L of phosphoric acid H3PO4, and 0-10 wt.-% of H2O2. More preferably, no copper is actively added to the bath upon its preparation, that is to say there is no intentional addition of copper to the bath. It is however to be understood that upon utilization of the bath (during the electrochemical etching - structuration treatment) copper concentration in the bath will increase as copper will be etched from the surface of the copper foil. Ferrous sulfate may prevent copper accumulation near the electrodes, hence improving P-CIRCUI-025 / WO 10 etching efficiency. Hydrogen peroxide H2O2 may help promoting I accelerating etching the surface of the electrodeposited (base, untreated) copper foil. Phosphoric acid may improve the etching process.

[0053] The applied current density may lie between 10 and 25 A / dm2, preferably between 20 and 25 A / dm2. As apparent to the skilled person, the structuration treatment being an electrochemical etching, the electrolytic copper foil to be treated is arranged at the anode of the electrolytic cell, while a copper plate may be arranged at the cathode. The foil to be treated being arranged at the anode, the current density is perceived here as being negative.

[0054] The bath is preferably kept at a temperature between 20 and 50 °C, more preferably between 20 and 30 °C. The etching duration (i.e. duration of application of the current density of step b) may be preferably between 2 and 8 s.

[0055] In embodiments, the etching duration may be adjusted by varying the feeding speed of the untreated electrolytic copper foil through the electrolytic bath. Alternatively, in other embodiments the feeding speed may be set by other process steps, such as e.g. the production rate of the electrolytic copper foil, so that the etching duration may be adjusted by varying the volume of the electrolytic bath or modifying the size (length) of the copper plate used as the cathode.

[0056] Preferably, at least one surface of the (base, untreated) electrolytic copper foil, preferably the surface corresponding to the side with the (resulting, after electro etching) negative skewness Ssk, presents a surface roughness Rz between 0 and 0.8 pm, preferably between 0.2 and 0.7 pm, more preferably between 0.3 and 0.6 pm, even more preferably between 0.3 and 0.5 pm.

[0057] Low surface roughness of the base (untreated electrolytic) copper foil helps achieving low insertion loss, i.e. minimal signal degradation at high frequencies.

[0058] Additionally or alternatively, the treated copper foil may present, on at least the side with the negative Ssk, a surface roughness Rz ISO lower than 0.8 pm, preferably lower than 0.6 pm, more preferably lower than 0.5 pm.

[0059] In embodiments, the surface receiving the structuration treatment (i.e. the surface of the copper foil corresponding to the side of the treated copper foil having a P-CIRCUI-025 / WO 11 resulting skewness Ssk being negative) is the electrolytic side (or matte side, opposed to the drum - or shiny - side) of the electrolytic copper foil.

[0060] According to the same or other embodiments, the method for treating a copper foil comprises, after the structuration treatment (i.e. electrochemical etching) as further step of forming a treatment stack comprising at least one functional layer, preferably on the etched surface of the copper foil.

[0061] Correspondingly, the treated copper foil further comprises a treatment stack comprising at least one functional layer, the treatment stack being arranged on at least one surface of the electrolytic copper foil, preferably on the surface corresponding to the side with the negative skewness Ssk.

[0062] Preferably, the treatment stack comprises at least one functional layer selected from:

[0063] - a heat-resistant layer comprising mainly molybdenum, cobalt, zinc and / or nickel,

[0064] - an anti-corrosion layer comprising mainly chromium, and

[0065] - an adhesion promoting layer comprising a silane compound, preferably a silane with an epoxy and / or amine and / or phenyl and / or vinyl function, such as e.g. N- Phenyl-3-aminopropyltrimethoxysilane, and I or N-(Vinylbenzyl)-2-aminoethyl- 3-aminopropyltrimethoxysilane hydrochloride.

[0066] The term mainly here is to be understood as comprising more than 50 wt.% of the compound, preferably more than 60, 70 or 80 wt.%.

[0067] In embodiments, the heat-resistant layer may comprise molybdenum, cobalt, zinc and / or nickel, in a total amount of up to 120 mg / m2; where the respective concentration of each metal is between 0 and 100 mg / m2A minimum total amount of metal (molybdenum, cobalt, nickel and / or zinc) for this layer is however 3 mg / m2.

[0068] In embodiments, the anti-corrosion layer comprises chromium in an amount of between 2 and 50 mg / m2, preferably between 5 and 25 mg / m2.

[0069] In embodiments, the adhesion promoting layer comprises silane in an amount of between 3 and 50 mg / m2, preferably between 5 and 25 mg / m2. P-CIRCUI-025 / WO 12

[0070] Preferably the treatment stack comprises all three layers, the heat-resistant layer being the inner layer (on the electrochemically etched surface of the electrolytic copper foil), the anti-corrosion layer in the middle and the adhesion promoting layer as outer layer.

[0071] Alternatively, the functional layers may be formed as combinations of molybdenum, cobalt, zinc, nickel and / or chromium, covered by the adhesion promoting layer.

[0072] The functional layers are generally continuous layers and may be formed by any appropriate / know process.

[0073] Preferably, the treatment stack presents a thickness defined by the nature and number of treatment layers, regardless of a thickness of the (untreated, base) copper foil.

[0074] It may be noted that the functional layers do not substantially change the surface roughness of the copper foil side on which it is formed. In other words, each layer of the treatment stack tends to follow I reproduce the surface roughness of the underlying layer, and ultimately the one of the etched (structured) electrolytic copper foil. That is to say, the functional layers of the treatment stack are rather thin layers that do not sensibly modify the surface roughness of the treated copper foil, which is mainly determined by the underlying roughness of the structured (etched) copper foil.

[0075] It may thus be considered that the surface roughness of a side of the treated copper foil (with all its treatments) corresponds to the surface roughness of the underlying surface of the etched electrolytic copper foil.

[0076] Preferably, the untreated electrolytic copper foil has a copper purity of at least 99.8 wt.-%.

[0077] The treated copper foil may have an area weight of 40 to 1300 g / m2It is desirable that the treated copper foils are as homogenous as possible, with small to no thickness variations. In this context, referring to an area weight to characterize a copper foil is similar to referring to a thickness of the foil, as the foil have a substantially constant thickness. Treated copper foils with the highest area weight are the thickest and treated copper foils with the lowest area weight are the thinnest. P-CIRCUI-025 / WO 13

[0078] The thickness of the untreated copper foil may lie between 6 and 140 pm, preferably between 10 and 70 pm such as between 15 and 40 pm, and e.g. be of about 18 pm or about 35 pm.

[0079] The tensile strength may typically be in the range of 20 to 60 kgf / mm2, preferably 25 to 55 kgf / mm2at 20°C and / or the elongation may be in the range between 10 and 35 %.

[0080] In a third aspect, the present invention concerns the use in a printed circuit board of the treated copper foil of the second aspect, or of a copper foil treated according to the method of the first aspect.

[0081] In still another aspect, the present invention relates to a copper clad laminate comprising a surface treated copper foil according to the second aspect, or a copper foil treated according to the method of the first aspect. In embodiments, the treated copper foil is laminated onto a PPE substrate at 200°C for 2 h, wherein the treated copper foil has a peel strength superior or equal to 0.8 N / mm.

[0082] Such a peel strength advantageously provides for sufficient bondability with a substrate (pre-preg) when producing a printed circuit board.

[0083] In yet another aspect, the present invention also concerns a printed circuit board comprising the treated copper foil of the second aspect or a copper foil treated according to the method of the first aspect.

[0084] Advantageously, such printed circuit board presents good transmission properties even at high to really high frequencies (in the order of the tens of GHz) and low signal insertion loss.

[0085] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings.

[0086] BRIEF DESCRIPTION OF THE DRAWINGS

[0087] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: P-CIRCUI-025 / WO 14

[0088] Figure 1 : are SEM (Scanning Electron Microscope) views of surface treated copper foils: a) and b) according to the present invention (examples 1 and 2), and c) and d) according to comparative embodiments (comparative examples 1 and 2);

[0089] Figure 2: are cross-sectional SEM views of surface treated copper foils: a) and b) according to the present invention (examples 1 and 2), and c) according to a comparative embodiment (comparative example 1 );

[0090] Fig. 3: are schematic representation of the surface of a copper foil for positive and negative values of parameter Ssk;

[0091] Fig. 4: are schematic views of a) an electroforming cell and b) an electro etching cell;

[0092] Fig. 5: is a schematic sectional view of a treated copper foil according to the invention; and

[0093] Fig.6: are SEM views of surface treated copper foils according to comparative examples 3 to 6.

[0094] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0095] As explained above, and with reference to Fig. 5, the present invention provides a treated copper foil with a specific surface roughness. The treated copper foil 28 has a first side 28.1 and a second side 28.2 opposite to the first side 28.1 , and comprises an electrolytic copper foil 18 with two opposite surfaces 18.1 , 18.2, a side 28.1 , 28.2 of the treated copper foil 28 corresponding to a surface 18.1 , 18.2 of the electrolytic copper foil 18, preferably with at least one functional layer 40, 42, 44.

[0096] In preferred embodiments, there are three functional layers, namely a heat-resistant layer 40 comprising molybdenum, cobalt, zinc and / or nickel, an anti-corrosion layer 42 comprising chromium and an adhesion promoting layer 44 comprising a silane compound.

[0097] Each treatment layer 40, 42, 44 is rather thin (thinner than what is represented with respect to the thickness of the copper foil - Fig. 5 is for informative purpose only) and tends to follow I reproduce the surface roughness of the underlying layer, and ultimately the one of the etched (structured) electrolytic copper foil 18, so that a P-CIRCUI-025 / WO 15 surface roughness of a side 28.1 of the treated copper foil 28 substantially corresponds to a surface roughness of the corresponding surface 18.1 of the electrolytic copper foil 18.

[0098] In the embodiment of Fig. 5, only a first surface 18.1 of the electrolytic copper foil 18 has been submitted to the electro etching treatment and then coated with the treatment layers 40, 42, 44. It is however possible and still within the scope of the present invention to also submit the opposite, second side 18.2 of the untreated copper foil 18 to a roughening (electro etching) treatment and / or to the deposition / coating of functional layer(s).

[0099] Figures 2a and 2b illustrate embodiments of such surface treated copper foils 28, 28’, having a respective first side 28.1 , 28.1’ which corresponds to a respective electro etched first surface 18.1 , of an electrolytic copper foil 18. It is to be noted that the foils imaged on Fig.2 have been glued to a substrate before being introduced into the scanning electron microscope, so that both the glue 46 and the substrate 48 are visible on the pictures of Fig. 2a-2c.

[0100] A surface treated copper foil as per the invention is produced by submitting an untreated copper foil, previously produced by electrodeposition (also called electrolytic or electrodeposited copper foil) to a structuration (i.e. roughening) process I treatment.

[0101] An embodiment of the method will be described with reference to Fig.4.

[0102] In a first step (Fig.4a), an electrodeposited copper foil 18 with a first surface 18.1 and an opposite, second surface 18.2 is produced in an electroforming cell 10, wherein an electrolyte is passed through an apparatus comprising a drum-shaped cathode 14 (the surface of which is made of stainless steel or titanium) which is rotating and a stationary anode 16 (a lead or a titanium electrode covered by a precious metal oxide) which is provided opposite the cathode 14. An electric current is passed through both electrodes 14, 16 to deposit copper on the surface of the cathode 14 with a desired thickness, thus forming an electrodeposited copper foil 18. The electrodeposited copper foil 18 is then peeled off from the surface of the cathode 14. The foil thus prepared is generally referred to as untreated copper foil 18. P-CIRCUI-025 / WO 16

[0103] An exemplary electrolyte for the electroplating (electrodeposition) process may comprise 50-100 g / L of copper Cu2+, 40-100 g / L of sulfuric acid, 20-50 ppm halogen ion such as e.g. chloride, and possibly organic additives such as brighteners, suppressors, and leveling agents to improve the quality and uniformity of the copper deposit.

[0104] In a second step (Fig.4b), at least one surface 18.1 , 18.2 of the electrolytic untreated copper foil 18 is roughened. The roughening treatment (i.e. structuration process) comprises etching - by applying a current density, i.e. electro etching or electrochemically etching - the surface of the base (untreated) copper foil.

[0105] In such process, the copper foil to treat (i.e. the foil to be surface-treated) is arranged at the anode 26 of an electrolytic cell 20 comprising a cathode 24, an anode 26 and an electrolyte 22, i.e. a current density with reverse polarities as compared to an electrodeposition process is applied. Such reverse current density, compared to an electrodeposition process, removes copper from the surface of the foil, hence enhancing its rugosity.

[0106] An exemplary electrolyte 22 for the etching process may comprise 0-100 g / L of copper Cu2+, 40-150 g / L of sulfuric acid, 0-100 ppm of chloride, , 0 to 10 g / L of ferrous sulfate, and 0-10 wt.-% of H2O2. The electrolyte may be kept at 20-50 C. The electrolysis may be performed using a current density e.g. of about 1 to 30 A / dm2 (with the copper foil to treat being at the anode - i.e. applying reverse polarities as when electrodepositing a foil), and the duration of the electrolysis may be between 2 and 8 seconds.

[0107] The treated copper foil 28 exiting the electrolytic cell 20 (etching cell) presents a first side 28.1 and an opposite second side 28.2, wherein the first side 28.1 corresponds to the etched first surface 18.1 of the electrolytic copper foil 18. T reated (etched I roughened I structured) copper foil 28 may be coiled onto a storage reel 30.

[0108] In a (some) subsequent step(s) (not shown), the etched (roughened) copper foil may be submitted to a chemical or (further) electrochemical surface treatment, to form at least one functional layer. The surface treatment may be e.g. a bond enhancing treatment and / or a passivation treatment. P-CIRCUI-025 / WO 17

[0109] In embodiments, the at least one functional layer is chosen from a heat-resistant layer comprising molybdenum, cobalt, zinc and / or nickel, an anti-corrosion layer comprising chromium and an adhesion promoting layer comprising a silane compound.

[0110] In preferred embodiments, there may be at least two functional layers, wherein a first functional layer is a heat-resistant layer comprising between 3 and 120 mg / m2(total amount) of molybdenum, cobalt, zinc and / or nickel, each metal not being present at a concentration of more than 100 mg / m2, and the second functional layer is an anti-corrosion layer comprising between 2 and 50 mg / m2of chromium. If present, an adhesion promoting layer may comprise 3 to 50 mg / m2of silane compound.

[0111] Examples

[0112] Electrolytic copper foils with an initial surface roughness Rz of about 0.55 pm, a Sa of about 8 nm and a Ssk of about 0.1 were produced by a method well known in the art.

[0113] As produced, untreated copper foils were then submitted to a structuration treatment according to the invention (examples 1 and 2) or to respective comparative structuration treatments (comparative examples 1 to 6, not part of the invention).

[0114] The treatment according to the invention corresponds to an electrochemical etching of the surface of the electrolytic copper foil, while the treatment according to comparative examples is either the electrodeposition of copper nodules or an electrochemical etching without the prescribed duration and / or intensity for the applied current density, or both.

[0115] In practice, all the treatments are performed in an electrolytic cell comprising a bath comprising sulfuric acid, maintained at a temperature of about 23 °C. Electrodeposition (comparative examples 1 , 2 and 6) is performed by applying a positive current density to the copper foil, while etching (examples according to the invention, and comparative examples 3 to 6) is performed by applying a negative current density to the copper foil. P-CIRCUI-025 / WO 18

[0116] Bath composition and current density for the examples and comparative examples are resumed in Table 1 below.

[0117] In Table 1 , positive current densities are shown for the comparative examples, i.e. wherein the copper foil to be treated was arranged at the cathode of the electrolytic cell, while for the examples, the foil to be treated was arranged at the anode, hence the reverse polarities and negative current densities in Table 1 .

[0118] The concentrations shown in Table 1 correspond to the concentrations of the various compounds of the electroplating bath at the start of the surface treatment. It is to be understood that copper concentration in the bath of comparative examples 1 or 2, or second step of comparative example 6 will decrease as copper is electrodeposited in the form of copper nodules. On the contrary, copper concentration in the bath of an example according to the invention, or comparative examples 3 to 5 or first step of comparative example 6 will increase as copper will be etched from the surface of the copper foil. [Table 1 ] P-CIRCUI-025 / WO 19

[0119] The thus obtained surface treated copper foils were then analysed to determine their surface characteristics, such as surface roughness (as of Rz, Sa and Ssk), and to determine some structural and physical characteristics, such as average grain size, peel strength on a substrate or insertion loss.

[0120] Surface treated copper foils are analysed as follows:

[0121] Roughness determination as of Rz

[0122] 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.

[0123] Roughness determination as of Sa

[0124] The arithmetical mean height (as of parameter Sa) is determined according to ISO 25178-2 dated 2021.

[0125] By way of example, for the purpose of explanation only, Sa may be determined by performing the following steps.

[0126] 1. AFM Imaging of the Surface

[0127] 1.1. Use an atomic force microscope (AFM) to scan the surface of the copper foil.

[0128] 1 .2. Set the scan area to a sguare of 10pmx10pm.

[0129] 1.3. Collect height data z(x,y) for the scanned surface. The height values are measured at discrete points in the grid, forming a topographic map.

[0130] 2. Data Processing

[0131] 2.1. Extract the height distribution from the AFM image. P-CIRCUI-025 / WO 20

[0132] 2.2. Represent the height data as a discrete matrix Zy, where i,j index the spatial coordinates (x,y).

[0133] 3. Calculation of Sa

[0134] 3.1. Compute the mean absolute height deviation Sa using the following formula: where:

[0135] • A is the total scanned area, A=Lx[_ (here 10 pm x 10 pm).

[0136] • |z(x,y) I represents the absolute deviation of the height values from the mean plane.

[0137] 3.2. In practice, for discrete height data obtained from the AFM scan, the integral is approximated as: where:

[0138] • N is the total number of height points in the scanned area.

[0139] • Izil represents the height at the l-th point relative to the mean plane.

[0140] Roughness determination as of Ssk

[0141] The skewness of the surface (as of parameter Ssk) is determined according to ISO 25178-2 dated 2021.

[0142] By way of example, for the purpose of explanation only, Ssk may be determined by performing the following steps.

[0143] 1. AFM Imaging of the Surface

[0144] 1.1. Use an atomic force microscope (AFM) to scan the surface of the copper foil.

[0145] 1 .2. Set the scan area to a square of 10pmxl 0pm.

[0146] 1.3. Collect height data z(x,y) for the scanned surface. The height values are measured at discrete points in the grid, forming a topographic map. P-CIRCUI-025 / WO 21

[0147] 2. Data Processing

[0148] 2.1. Extract the height distribution from the AFM image.

[0149] 2.2. Represent the height data as a discrete matrix Zy, where i,j index the spatial coordinates.

[0150] 3. Calculation of Skewness

[0151] 3.1. Compute the mean height (z) of the surface: where N is the total number of points in the scanned area, and zi are the individual height values.

[0152] 3.2. Calculate the root mean square (RMS) roughness (q) as:

[0153] 3.3. Determine the skewness (Ssk) using the formula: where:

[0154] Ssk>0 indicates a surface dominated by peaks (Fig. 3a)

[0155] Ssk<0 indicates a surface dominated by valleys (Fig. 3b).

[0156] Peel test

[0157] The copper foil is laminated on a resin substrate of PPE (polyphenylene ether) for 2h at 200 °C. The peel strength is measured at 90°. The test was carried out according to IPC-TM-650 Method 2.4.8.5.

[0158] Insertion Loss

[0159] Insertion Loss measurements conducted from 10 MHz to 67 GHz made on PNA E8361 C on microstrip PCB design using following characteristics: Microstrip design P-CIRCUI-025 / WO 22 on Astra77 material (Dk = 3.0); Copper thickness: 1.8 MIL - 18pm; Track width: 0.47pm; dielectric thickness: 8 MIL; Impedance = 50 Q; No soldermask; No plating finishing; Track length: 25 cm; Connectors ELF-67-002.

[0160] Grain size

[0161] In the present text, the average grain size relates to the arithmetic mean of the geodesic diameters of grains, measured from a FIB (focalized ion beam) exposed microstructure on a SEM (scanning electron microscopy) picture.

[0162] In embodiments, the grain size is determined by performing the following steps:

[0163] 1. Preparation of the Sample and Image Acquisition

[0164] A focused ion beam (FIB) is utilized to prepare a cross-section of the copper foil, exposing its microstructure. A scanning electron microscopy (SEM) image is captured to visualize the grains in the exposed region.

[0165] 2. Image Processing in Imaged Software

[0166] The SEM image is imported into the Imaged software, and an area to be analyzed, measuring 30 pm x 10 pm and representing only the microstructure is selected. Any irrelevant areas such as background are excluded from the area to be analyzed in the Imaged software.

[0167] 3. Application of the Morphological Segmentation Algorithm

[0168] In the Imaged software, the MorpholibJ plugin is used to perform segmentation of the SEM image, in particular to perform segmentation of each grain of the microstructure in the area to be analyzed. The algorithm involves two key steps:

[0169] Gradient Enhancement: A morphological gradient filter is applied with a radius of 3 to enhance grain boundaries within the image.

[0170] Watershed Segmentation: Watershed segmentation is applied with a tolerance parameter set to 14, ensuring accurate separation of grains without oversegmentation.

[0171] 4. Grain Size Determination

[0172] Once segmentation is complete, the grains are distinctly separated. The geodesic diameter of each grain is measured to quantify its size. The geodesic diameter, P-CIRCUI-025 / WO 23 defined as the maximum internal distance within a grain, is selected as the characteristic parameter for grain size.

[0173] The average grain size is then the arithmetic mean of all grain size values.

[0174] Discussion of the examples and counter examples Roughened foils of examples (by electrochemical etching) and counter-examples (by electrodeposition of copper nodules or electrochemical etching or both) were submitted to series of test, the results of which are summarized in Table 2.

[0175] [Table 2]

[0176] All copper foils, according to the examples and comparative examples, have an average grain size in the prescribed range of 0.8-1 .2 pm, and (except for comparative example 2) similar surface roughness as of Rz. Electrochemically etching the surface of the copper foil instead of electrodepositing copper nodules P-CIRCUI-025 / WO 24 does not influence the crystalline structure (as of grain size) of the electrolytic copper foil.

[0177] Surface treated copper foils corresponding to the present invention have a sharper I rougher surface profile (Fig 1 a and Fig 2a corresponding to Example 1 ; Fig 1 b and Fig 2b corresponding to Example 2) than surface treated copper foils of the comparative examples not according to the invention (Fig 1 c and Fig 2c corresponding to Comparative example 1 ; Fig 1 d corresponding to Comparative example 2; Fig. 6a corresponding to Comparative example 3; Fig. 6b corresponding to Comparative example 4; Fig. 6c corresponding to Comparative example 5; Fig. 6d corresponding to Comparative example 6).

[0178] Indeed, as shown in Table 2, when (solely) etching the surface of the copper foil, the skewness as of Ssk of the resulting surface is negative, while it is positive for a copper foil surface-treated by electrodeposition of copper nodules (independently of whether or not the surface was previously etched - see comparative examples 1 , 2 and 6).

[0179] Such difference in the skewness of the profile does not negatively impact the bondability of the surface treated copper foil to a substrate (as of peel strength - see Table 2) while improving the signal transmission by decreasing the insertion loss at 30 GHz (Table 2).

[0180] However, it is observed that when etching is carried out with a current density above 30 A / dm2, both the surface roughness as of Sa and the insertion loss are too high (comparative example 2).

[0181] When the current density is applied for less than 1 s, bondability (as expressed by the peel strength on PPE) is too low (comparative example 4). Same is true when the current density is applied for more than 10 s (comparative example 5). Moreover, applying the current density for more than 10 s further results in a too rough copper foil as per Sa (comparative example 5).

[0182] If copper nodules are electrodeposited on the etched surface of the copper foil (comparative example 6), the resulting surface skewness is positive, and both surface roughness as per Sa and insertion loss are too high. P-CIRCUI-025 / WO 25

[0183] Only performing an electro-etching with the prescribed current density for the prescribed duration and without copper deposition in the form of copper nodules allows obtaining a treated copper foil exhibiting the desired surface roughness and performances in terms of bondability and transmission of signal. Moreover, when an operator runs a finger over the treated copper foil of examples 1 or 2, nothing comes off (no deposit onto their finger), contrary to comparative example 1 , for which copper detaches quite easily from the surface.

[0184] Furthermore, treatment using electrochemical etching is more homogeneous than electrodeposition of nodules. To sum up, the surface treated copper foil of the present invention, treated by electrochemical etching (without electrodeposition of copper nodules), exhibits better mechanical resistance with less chance of detached copper particles, while providing for a better signal transmission (lower insertion loss) and sufficient bondability (as of peel strength - similar to what is obtained for a comparative, conventional surface treatment).

Claims

P-CIRCUI-025 / WO 26Claims1 . A method for producing a surface treated copper foil (28), comprising the steps of: a) providing an untreated electrolytic copper foil (18) with a first surface (18.1 ) and a second surface (18.2) opposite to the first surface, preferably having a surface roughness Rz ISO inferior or equal to 0.8 pm on the first surface (18.1 ); b) performing a roughening treatment on at least one surface (18.1 ) of the copper foil (18) in an electro etching cell (20) with the copper foil (18) at anodic potential, to achieve a surface roughness Rz ISO of no more than 0.8 pm, a surface roughness Sa of 30 nm or less and a negative skewness Ssk, wherein step b) comprises applying a current density of 1 to 30 A / dm2for a duration of 1 to 10 seconds.

2. The method as claimed in claim 1 , wherein step b) comprises applying a current density of 10 to 25 A / dm2, more preferably 20 to 25 A / dm2and / or wherein the current density is applied for a duration of 2 to 8 s.

3. The method as claimed in claim 1 or 2, wherein step b) is carried out using a continuous direct current.

4. The method as claimed in claim 1 , 2 or 3, wherein in step b) the electro etching cell (20) comprises an acidic bath (22), the bath (22) preferably comprising 0 to 100 g / L of copper ions Cu2+, 40 to 150 g / L, preferably 50 to 120 g / L, more preferably 60 to 100 g / L, of sulfuric acid, 0 to 100 ppm, preferably 10 to 50 ppm, more preferably 20 to 30 ppm, of chloride, 0 to 10 g / L of ferrous sulfate, 0 to 20 g / L of phosphoric acid, and 0-10 wt.-% of H2O2.

5. The method as claimed in the preceding claim, wherein no copper is actively added to the bath.

6. The method as claimed in any one of the preceding claims, wherein in step b) the bath (22) is maintained at a temperature between 20 and 50 °C, preferably between 20 and 30 °C.P-CIRCUI-025 / WO 277. The method as claimed any one of the preceding claims, wherein Ssk is between -2 and 0, preferably between -1 and 0, more preferably between -0.5 and 0.

8. The method as claimed in any one of the preceding claims, wherein Sa is between 1 and 30 nm, preferably between 5 and 30 nm, more preferably between 10 and 28 nm.

9. The method as claimed in any one of the preceding claims, comprising a further step c) of forming at least one functional layer (40, 42, 44) on the roughened copper foil (28).

10. The method as claimed in the preceding claim, wherein the at least one functional layer (40, 42, 44) is chosen from a heat-resistant layer comprising molybdenum, cobalt, zinc and / or nickel, an anti-corrosion layer comprising chromium and an adhesion promoting layer comprising a silane compound.11 .A treated copper foil (28), preferably for use in a printed circuit board, with a first side (28.1 ) and a second side (28.2) opposite to the first side, the treated copper foil (28) comprising an electrolytic copper foil (18) with two opposite surfaces (18.1 , 18.2), a side of the treated copper foil corresponding to a surface of the electrolytic copper foil, preferably with at least one functional layer (40, 42, 44), wherein at least one of the first side (28.1 ) and the second side (28.2) has a surface roughness Rz ISO of no more than 0.8 pm, a surface roughness Sa of 30 nm or less and a negative skewness Ssk.

12. The treated copper foil (28) as claimed in claim 11 , wherein Ssk is between -2 and 0, preferably between -1 and 0, more preferably between -0.5 and 0.

13. The treated copper foil (28) as claimed in claim 11 or 12, wherein Sa is between 1 and 30 nm, preferably between 5 and 30 nm, more preferably between 10 and 28 nm.

14. The treated copper foil (28) as claimed in any one of claims 11 to 13, wherein the treated copper foil (28) presents a microstructure defined by an average grain size, wherein the average grain size is at least 0.8 pm, preferably between 0.8 and 1.2 pm, more preferably between 0.9 and 1.1 pm, even more preferably of about 1 pm.P-CIRCUI-025 / WO 2815. The treated copper foil (28) as claimed in any one of claims 11 to 14, wherein the surface roughness of the side (28.1 ) having a negative skewness is obtained by electro-etching a corresponding surface (18.1 ) of the electrolytic copper foil (18), preferably according to a method as claimed in any one of claims 1 to 10.

16. The treated copper foil (28) as claimed in any one of claims 11 to 15, wherein at least one surface (18.1 ) of the electrolytic copper foil (18), preferably the surface (18.1 ) corresponding to the side (28.1 ) with the negative skewness Ssk, presents a surface roughness Rz ISO between 0 and 0.8 pm, preferably between 0.2 and 0.7 pm, more preferably between 0.3 and 0.6 pm.

17. The treated copper foil (28) as claimed in any one of claims 11 to 16, further comprising a treatment stack comprising at least one functional layer (40, 42, 44), the treatment stack being arranged on at least one surface (18.1 , 18.2) of the electrolytic copper foil (18), preferably on the surface (18.1 ) corresponding to the side (28.1 ) with the negative skewness Ssk.

18. The treated copper foil (28) as claimed in the preceding claim, wherein the treatment stack comprises as functional layer (40, 42, 44) at least one of a heat- resistant layer comprising molybdenum, cobalt, zinc and / or nickel, an anticorrosion layer comprising chromium and an adhesion promoting layer comprising a silane compound.

19. Use in a printed circuit board of a treated copper foil (28) as claimed in any one of claims 11 to 18 or of a copper foil (18) treated according to a method as claimed in any one of claims 1 to 9.

20. A copper clad laminate comprising a treated copper foil (28) according to any one of claims 11 to 18, or a copper foil (18) treated according to a method as claimed in any one of claims 1 to 10.

21. The copper clad laminate of the previous claim, wherein the copper foil is laminated onto a PPE substrate at 200°C for 2h, and wherein the treated copper foil has a peel strength superior or equal to 0.8 N / mm.

22. Printed circuit board comprising a treated copper foil (28) as claimed in any one of claims 11 to 18 or a copper foil (18) treated according to a method as claimed in any one of claims 1 to 10.

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