Random crystal orientation and tensile strength of nanocrystalline dumbbell-shaped copper thick films electrodeposited from acidic aqueous solutions containing polyethylene glycol

Nanocrystalline thick copper films with the thickness of ∼250 μm were electrochemically synthesized from an acidic aqueous solution containing polyethylene glycol (PEG) with the average molecular weight of 3,000 to investigate the preferential crystal orientation and mechanical properties such as microhardness and tensile strength. By addition of PEG to the electrolytic bath, the cathode potential was shifted to a less noble direction during the electrodeposition and the average crystallite size of electrodeposited copper thick films was decreased. The copper thick films electrodeposited from the solution without PEG exhibited a preferentially orientation in (220) texture while that obtained from the solution containing PEG was composed of nanocrystals with random crystal orientation that containing (111) and (200) textures. The micro-Vickers hardness, tensile strength, and elongation of the electrodeposited copper thick films reached up to 133 HV, 234 MPa, and 13.1%, respectively. These improvements in mechanical properties can be explained by the grain refinement effect and the random crystal orientation effect.


Introduction
Electrochemically synthesized metallic copper or copper oxide layers are widely used as functional materials, such as for through-hole deposition technology in printed circuit boards (PCBs) [1,2], 3D printing [3], and sensor devices [4,5], due to their excellent physical properties and cost performance.Among them, electrodeposited metallic copper layers or multilayers have been extensively studied with respect to their additives [6,7], multilayered structure [8][9][10] and mechanical properties [11,12].Above all, surface morphology and mechanical strength are important parameters affecting the quality of electrodeposited copper films.Roughness harms their mechanical properties because it induces stress concentration on the surface of the film.Further, precise control of the surface morphology will improve the performance of a PCB in a high-frequency range because a coarse morphology causes a reduction of the signal-to-noise ratio in the system.Since conventional copper films are exposed to an external force in various practical applications, they should be produced with excellent mechanical properties to provide stable performance and reliable quality.
Various methods have been used to fabricate copper films, including optimizing current density, altering the composition and temperature of electrolytic baths, and determining additives to electrolytic baths.Among them, applying an additive to the electrolytic bath is the most effective approach to realizing a copper film with an ideal surface morphology.By using an additive, it is possible to refine the crystal grains of copper films [13,14] and improve their mechanical properties [15].To electrodeposit copper films with a smooth surface, several kinds of additives, such as polyethylene glycol (PEG), gelatin, thiourea, bis(3-sulfopropyl) disulfide (SPS), and chlorides, can be applied to the bath [16,17].It is well known that the cathodic potential shifts to the less noble direction by adding additives to plating baths [18].This cathodic polarization results in refining the electrodeposited crystal grains, smoothing the surface, improving the throwing power [19], and increasing hardness.
In general, the fouling mechanism of additives during the electrodeposition of metals can be categorized as follows: (A) anti-adhesion, (B) interfacial complex formation (adhesion promotion), and (C) film formation.The additive suppresses metal deposition by adsorbing on the electrode surface in all mechanisms.In the antiadhesion mechanism (A), molecules such as saccharin, benzothiazole, thiourea, polyamide, and benzalacetone are adsorbed or deposited independently to suppress the plating reaction.By contrast, in the interfacial complex formation mechanism (B), a small number of complex ions such as chloride ion, cyanide ion, thiocyanide ion, thiourea, boric acid, oxalic acid, or malonic acid are adsorbed on the surface and coordinate with metal ions to form ion or electron bridges.In mechanism (B), the complex ions promote the deposition reaction by inducing metal ions.Finally, in the film formation mechanism (C), surfactants or polymers such as polyethylene glycol, polyvinyl alcohol, or gelatin mildly adhere to the plating surface to form a layer and suppress the plating reaction.
Researchers have investigated the effects and adsorption mechanisms of these additives.In 2007, Hakamada et al reported the Vickers hardness of copper films electrodeposited from a copper sulfate bath containing thiourea and gelatin as additives [20].They revealed that the use of thiourea suppressed the electrodeposition of copper and refined the crystal grains, thereby smoothing the surface and improving hardness.They found that the Vickers hardness of films electrodeposited from a copper sulfate bath containing thiourea increased to 250 HV as the thiourea concentration increased to 0.020 g L −1 and that the hardness gradually decreased as the thiourea concentration increased beyond 0.020 g L −1 .In 2021, Liu et al reported on a trace amount of bis-3-sulfopropyl-disulfide (SPS) being used as an additive to deposit a high-strength copper film from a copper sulfate bath [21].In their study, the effect of SPS concentration on average grain size and tensile strength was investigated.They found that the tensile strength increased to 338 MPa with increasing SPS concentrations up to 1.5 mg/L.They also reported that the maximum elongation exceeded 8%.
It is well known that the plastic deformation of a crystal is shear deformation in which dislocations move in a specific slip direction on a specific slip plane.In case of metallic copper (fcc crystal), the slip plane and slip direction are {111} and 〈 1 10〉, respectively.Usually, the copper films which are electrodeposited at a high current density exhibit a preferential crystal orientation of (220) plane.However, there is only one slip direction of [ 1 10] on (220) plane.Hence, the elongation of copper films which are electrodeposited at a high current density is typically less than 10% and smaller than that of a casted and rolled copper sheet.It is also well known that an additive in a electrolytic bath induce the transformation from a preferential crystal orientation of (220) to a random crystal orientation that containing (111) and (200) textures.Some additives, such as thiourea and SPS, contain sulfur atoms in the molecules [22].These sulfur atoms in a boundary of polycrystalline metallic materials will cause a brittle fracture under stress [23].Hence, additives not containing sulfur atoms are desirable for making an electrodeposited metallic film with ductility.For example, PEG is a water-soluble additive not containing sulfur atoms in the molecules.Therefore, in the present study, the effect of PEG addition on the crystal texture and mechanical properties of electrodeposited copper thick films was investigated.

Experimental
As summarizing in table 1, thick copper films (thickness: ∼250 μm) were electrodeposited on a dumbbellshaped metallic titanium electrode using a sulfuric acid bath (bath composition: CuSO 4 •5H 2 O (1 M), H 3 BO 3 (0.4 M), PEG (Mw: 3,000) (0, 0.01, 0.03, 0.05 g L −1 ) and pH1) with a temperature of 80 °C.In the present work, the dumbbell-shaped samples for an uniaxial tensile test were prepared refering to ASTM E08/E8M-11 standard [24,25].Here, the sample size was reduced from the ASTM standard to immerse it completely in a small amount (300 mL) of electrolytic solution [26].For comparison, pure copper film was electrodeposited from an aqueous solution without PEG.A pure copper plate (2 cm × 10 cm) and a silver/silver chloride electrode were utilized as an anode and a reference electrode, respectively.Copper films were electrodeposited by a galvanostatic mode at a current density of 500 A/m 2 .The amount of charge was fixed to 2,500 C.During the electrodeposition, the aqueous solutions were stirred at a speed of 300 rpm.After electrodeposition, the copper thick films were exfoliated from the metallic titanium electrode.The morphology of the copper thick films was examined using a scanning electron microscope (SEM, JCM-5700, JEOL Ltd, Tokyo, Japan).The roughness of the samples was investigated using SURFTEST SJ-210 (Mitutoyo, Kanagawa, Japan).The crystal structure of the electrodeposited copper thick films was determined using an X-ray diffractometer (XRD, Miniflex600-DX, Rigaku Corp., Tokyo, Japan).Furthermore, their hardness was examined using a micro-Vickers hardness testing machine (HM-211, Mitutoyo, Kanagawa, Japan).The tensile strength of the electrodeposited copper thick films was determined from stress-strain curves, which were obtained using a tensile testing machine (AG-IS 50 kN, SHIMADZU, Kyoto, Japan).

Cathodic polarization behavior during copper electrodeposition
Figure 1 show the effect of PEG concentration on the cathodic polarization curves (a) and cathodic overpotential (b) for copper electrodeposition.The cathode potential was swept from +0.20 V to −1.50 V versus Ag/AgCl at a sweep rate of 100 mVs −1 .The cathodic polarization curve that was obtained from the solution containing PEG was slightly polarized to the less noble direction than that without PEG.The equilibrium potential of Cu/Cu 2+ , E , cu eq was estimated as +0.138V versus Ag/AgCl by the following Nernst's equation (1).where the values are as follows: standard electrode potentials, E cu 0 (+0.337V versus Ag/AgCl), Faraday constant, F = 96,485 C mol −1 , the gas constant, R = 8.3 JK −1 mol −1 , the bath temperature, T = 313 K, the valence number of Cu 2+ , n = 2, and molar concentration of Cu 2+ ions, [Cu 2+ ] = 1, [Cu 0 ] = 1.As shown in figure 1(a), a rising cathodic current was observed at a cathode potential that was almost identical to the above estimation value (+0.138V versus Ag/AgCl).Hence, the electrodeposition of copper from an acidic aqueous solution seems to proceed without accompanying an overpotential [27,28].With increasing current density, the cathodic polarization increased, and the slope of the curves became smaller in the current density range above 1000 A/ m 2 .Hence, in the high current density range, copper ions appear to be in a mass-transport rate-limiting process.By contrast, in the current density range below 1000 A m −2 , copper ions seem to be in a charge-transfer rate-limiting process.Therefore, considering the productivity and morphology of metallic copper, we determined that the optimum current density is 500 A m −2 , which corresponds to the chargetransfer state of Cu 2+ ions.Figure 1(b) show the effect of PEG concentration on the cathodic overpotential for copper electrodeposition at the current density of 500 A/ m 2 .the cathodic overpotential increased up to 0.182 V with increasing the PEG concentration up to 0.05 g/L.This polarization effect seems to be caused by the adhesion of PEG on the cathode.The current efficiency for the copper deposition from the solution without PEG was 97.1%, whereas that from the solution containing PEG was slightly decreased down to 93.5%.

Structure of electrodeposited copper thick films
Figure 2 show the effect of PEG concentration on the SEM images of electrodeposited copper thick films ((a) PEG: free, (b) PEG: 0.01 g L −1 , (c) PEG: 0.03 g L −1 , (d) PEG: 0.05 g L −1 ).The effect of PEG concentration (e) and cathodic overpotential (f) on the surface roughness, R a of electrodeposited copper thick films are also shown in figure 2. The morphology exhibited a nodulous structure with numerous domains of several tens micrometers, which was electrodeposited from the aqueous solution containing PEG-free, 0.01 g/L and 0.03 g/L as shown in figures 2(a), (b), (c).By contrast, in the solution containing 0.05 g/L PEG, the surface morphology was transformed to exhibit a coarse nodulous structure with numerous hexagonal domains (figure 2(d)).This alternation in the surface appearance seems to have been caused by PEG adhesion on the cathode to suppress the heterogeneous nucleation of electrochemically reduced metallic copper atoms.Figures 2(e) and (f) show the effect of PEG concentration and cathodic overpotential on the surface roughness, R a , respectively.The surface roughness of copper thick film that was electrodeposited from the solution without PEG was 2.94 μm.The addition of 0.01 g/L PEG slightly reduced the surface roughness to 2.56 μm.By contrast, as shown in figure 2(e), the surface roughness was enhanced up to 6.42 μm with increasing PEG concentration up to 0.05 g/L.As shown in figure 2(f), this enhancement in the surface roughness seems to be caused by increasing the cathodic overpotential due to the excess amount of PEG adhesion on the cathode.According to the electrochemical nucleation theory, during the electrodeposition of metals, the homogeneous nucleation rate increases with increasing the cathodic overpotential [29].On the contrary, in the present study, the excess amount of PEG adhesion seems to induce the heterogeneous nucleation that results in the enhancement of surface roughness.
Figure 3 show the effect of PEG concentration on the XRD profiles of electrodeposited copper thick films ((a) PEG: free, (b) PEG: 0.01 g L −1 , (c) PEG: 0.03 g L −1 , (d) PEG: 0.05 g L −1 ).The diffraction peaks derived from fcc-Cu (111), ( 200) and (220) were observed in all samples.In the copper thick film electrodeposited from an aqueous solution without PEG, the preferential crystal orientation in fcc-Cu (220) was observed as shown in figure 4(a).This tendency corresponds well to the results reported by Wang et al [2].By contrast, in the copper thick films electrodeposited from an aqueous solution containing PEG, the preferential crystal orientation in fcc-Cu (220) was diminished while the random crystal orientation was observed as shown in figures 4(b),(c),(d).This tendency corresponds well to the results (when gelatin was used as an additive) reported by Li et al [12].This drastic change in crystal orientation seems to be caused by the PEG adhesion effect on the cathode where the nucleation of copper crystals on the cathode is suppressed and the cathodic overpotential is enhanced as shown in figure 1(b).This increase in the cathodic overpotential induces the random crystal orientation.
Figure 4 show the effect of PEG concentration (a) and cathodic overpotential (b) on the crystallite size of electrodeposited copper thick films.The effect of PEG concentration (c) and cathodic overpotential (d) on the crystal orientation index, X (111) , X (200) , X (220) of electrodeposited copper thick films are also shown in figure 4. The average crystallite size, d, of the electrodeposited copper thick films was determined using the following equation (2) (Scherrer's formula): where K, λ, and β correspond to the Scherrer's constant (0.94), X-ray wavelength (Cu − K α = 0.15418 nm), and half width of diffraction peaks, respectively.As shown in figure 4(a), the average crystallite size of copper thick film electrodeposited from solution without PEG was approximately 48 nm.By contrast, the average crystallite size of copper thick film electrodeposited from solution containing PEG was decreased to about 32 nm.This tendency corresponds well to the results (when thiourea was used as an additive) reported by Kumar et al [14].This decrease in crystallite size also seems to be induced by the PEG adhesion effect on the cathode, which reduces the crystal growth rate of electrodeposited metallic copper.These results are consistent with the trend of surface appearance of electrodeposited copper thick films as shown in figures 2(a),(b),(c),(d).Therefore, the morphology transformation on the electrodeposited copper thick films seems to be quite sensitive even at very small amounts of PEG addition.Figure 4(c) show the effect of PEG addition on the orientation index of (220), ( 200) and (111) of electrodeposited copper thick films.Here, the orientation index, X (hkl) was defined as the following Harris Formula [30].  were 100%, 46%, and 20%, respectively.N is the number of diffraction planes considered for the determination of X (hkl) .As is shown in figure 4(c), the preferential orientation index, X (220) decreased by PEG addition, whereas X (111) and X (200) increased and the random crystal orientation was enhanced by PEG addition.Figure 4(d) show the effect of cathodic overpotential on the preferential orientation index, X (220) , X (200) and X (111) , respectively.X (220) was decreased with increasing the cathodic overpotential, whereas X (111) and X (200) was increased and the random crystal orientation was realized with an increase in cathodic overpotential.

Mechanical properties of electrodeposited copper thick films
Figure 5 show the effect of PEG concentration (a), surface roughness (b), crystallite size (c) and crystal orientation index, X (220) (d) on the microhardness of electrodeposited copper thick films.The hardness was approximately 103 HV in the sample electrodeposited from the aqueous solution without PEG.By contrast, the hardness increased up to approximately 133 HV in the samples electrodeposited from the aqueous solutions containing PEG.This hardness value is greater than that of a casted and rolled copper sheet (about 110 HV).In the present study, the applied load, F was adjusted to 0.1 kgf (0.98 N) and the dwell time was fixed to 10 s.In case the hardness, H V is 133 kgf mm −2 , the average length of the diagonal left by the indenter, d can be estimated to be around 37.3 × 10 -3 mm according to the following equation (4).( ) Furthermore, the surface area of the resulting indentation, A can be estimated to be around 752 x 10 -6 mm 2 according to the following equation (5).
Hence, the applied stress, σ can be estimated to be around 1303 MPa.Liu et al reported that the nanoindentation and scratching mechanism of a single crystalline copper [31,32].According to their experimental condition, the applied load, F and groove width, d were around 60 μN and 0.5 μm, respectively.Thus, the applied stress, σ can be estimated to be around 445 MPa.It is well known that the tensile strength of a casted and rolled copper sheet is about 200 MPa.Therefore, in the present study, the applied load seems to be enough to realize the plastic deformation of electrodeposited copper crystals.Figures 5(b), (c) and (d) show the effect of surface roughness, crystallite size and crystal orientation on the micro-Vickers hardness of electrodeposited copper thick films, respectively.
The micro-Vickers hardness increased with decreasing the crystallite size as shown in figure 5(c).The increase in the hardness can be explained by the mechanism of crystal grain refinement strengthening, which will enhance the grain boundary area and suppress the movement of dislocations by the random crystal orientation as shown in figure 5(d) [33].
Water-soluble polymers which added to an aqueous solution can be adsorbed on a metallic cathode surface.The polymer additives can inhibit the diffusion of metal atoms on the cathode surface and enhance the crystal nucleation frequency.This enhancement of crystal nucleation by the polymer additives will reduce the average crystallite size of electrodeposited metal.In a previous study, Hakamada et al reported that the micro-Vickers hardness of copper films electrodeposited from a sulfate bath was strongly affected by additives such as thiourea and gelatin [20].They revealed that the average crystallite size of copper films, which were electrodeposited from aqueous solutions containing thiourea or gelatin, became smaller (down to 31 nm) and the micro-Vickers hardness increased (up to 266 HV).In the present study, the average crystallite size of copper films decreased to approximately 32 nm (figure 3(d)) and the micro-Vickers hardness increased up to about 133 HV (figure 5(a)) by the effect of PEG addition.Hence, the fouling mechanism of PEG seems to be similar to that of gelatin.
Figure 6 shows the effect of PEG addition on the nominal stress-strain curves of electrodeposited copper thick films.The tensile tests were performed with a crosshead speed of 0.5 mm min −1 on each specimen after the mechanical exfoliation process.where σ, σ 0 , k, and d correspond to the yield stress, internal stress, sliding constant, and crystal grain size, respectively.As shown in figure 7(c), the results in the present study follows the above Hall-Petch equation.Figure 8 show the effect of PEG concentration on the sample appearance (optical microscope images) of electrodeposited copper thick films before and after the tensile tests ((a) PEG: free, (b) PEG: 0.01 g L −1 , (c) PEG: 0.03 g L −1 ).A −1 s −1 is depicted in figure 8, the dumbbell shape was precisely transferred from the titanium   cathode to the electrodeposited copper thick films after the mechanical exfoliation process.After the tensile tests, all samples were elongated by several millimeters along the narrow parallel part in the dumbbell and fractured in a ductile mode at the edge of the narrow parallel part.
Figure 9 show the effect of PEG concentration (a), surface roughness (b), crystallite size (c) and crystal orientation index, X (220) (d) on the elongation of electrodeposited copper thick films.As shown in figure 9(a), the elongation increased significantly, from 5.51% to 13.1%, with the addition of PEG (0.01 g L −1 ).A −1 s −1 is indicated in figures 9(c) and (d), this PEG addition effect may be attributed to the crystallite size and the random crystal orientation, respectively.Generally, it is difficult to make a random crystal orientation if samples are electrodeposited from an aqueous solution without adhesive additives [34].On the contrary, in the present study, the addition of PEG resulted in the random crystal orientation.Liu et al reported that a trace amount of SPS was effective to electrodeposit a high-strength copper film from a sulfate bath [21].They found that the tensile strength increased parabolically with increasing SPS concentration; the highest tensile strength was increased to 338 MPa at an SPS concentration of 1.5 m g /L .However, according to their report, the elongation was decreased to 8%, and the preferential crystal orientation of electrodeposited copper films was transformed to the (220) plane by the addition of SPS.Hence, the elongation seems to be affected by the preferential crystal orientation of electrodeposited metals.Meng et al reported that benzyl-containing quaternary ammonium salt (BZC) was an effective additive for micro-via copper electroplating [35].In their report, the preferential crystal orientation of electrodeposited copper in a micro-via was strongly affected by the BZC concentration.They determined that BZC promoted the random crystal orientation.The effect of BZC in their study corresponds well to the effect of PEG in the present study.It is well known that the slip plane and slip direction of fcc crystals are {111} and 〈 1 10〉, respectively, because there are three slip directions such as [ 1 10], [ 101] and [ 011] on (111) plane.Meanwhile, there is two slip directions of [011] and [ 011] on (200) plane.On the contrary, there is only one slip direction of [ 1 10] on (220) plane.Hence, in the present study, the enhancement of the elongation due to the addition of PEG seems to be caused by the transformation of the preferential crystal orientation from (220) to the random crystal orientation that containing (111) and (200) textures.

Conclusion
The surface roughness and crystallite size of electrodeposited copper thick films were significantly reduced following PEG addition.These effects are caused by PEG adsorption on the cathode surface to suppress the diffusion of copper atoms on the cathode surface.Structure of electrodeposited copper thick films was transformed to a random crystal orientation by the effect of PEG addition.Furthermore, the micro-Vickers hardness of the films increased up to 133 HV was achieved by the effect of PEG addition, and it was greater than that of a cast rolled copper (about 110 HV).This enhancement in the micro-Vickers hardness can be explained by the grain refinement strengthening effect.The tensile strength of electrodeposited copper thick films increased up to 234 MPa via a grain refinement strengthening effect.Elongation also increased significantly, from 5.51% to 13.1%.This improvement in ductility may be realized by the formation of a random crystal orientation.

Figure 1 .
Figure 1.Effect of PEG concentration on the cathodic polarization curves (a) and cathodic overpotential (b) for copper electrodeposition.

Equation ( 3 )
describes the analysis of the relative peak intensities dependent on I hkl , i.e., the intensities observed from (h k l) lattice planes of the sample, and I hkl 0 denotes the intensities of standard Cu powders.In the present study, the intensity ratios of I

Figure 4 .
Figure 4. Effect of PEG concentration (a) and cathodic overpotential (b) on the crystallite size of electrodeposited copper thick films.Effect of PEG concentration (c) and cathodic overpotential (d) on the crystal orientation index, X (111) , X (200) , X (220) of electrodeposited copper thick films.

Figure 7
show the effect of PEG concentration (a), surface roughness (b), crystallite size (c) and crystal orientation index, X (220) (d) on the tensile strength of electrodeposited copper thick films.As shown in figure 7(a), the tensile strength increased from 151 MPa to 234 MPa by addition of PEG (0.01 g/L).This result is presumably due to a strengthening mechanism by the crystal grain refinement as shown in figure 7(c) and the random crystal orientation (figure 7(d)).It is well known that the relationship between yield stress and grain size can be expressed by the following Hall-Petch equation (equation (6)):

Figure 5 .
Figure 5.Effect of PEG concentration (a), surface roughness (b), crystallite size (c) and crystal orientation index, X (220) (d) on the microhardness of electrodeposited copper thick films.

Figure 7 .
Figure 7. Effect of PEG concentration (a), surface roughness (b), crystallite size (c) and crystal orientation index, X (220) (d) on the tensile strength of electrodeposited copper thick films.

Figure 6 .
Figure 6.Effect of PEG addition on the nominal stress-strain curves of electrodeposited copper thick films.

Figure 8 .
Figure 8.Effect of PEG concentration on the sample appearance (optical microscope images) of electrodeposited copper thick films before and after the tensile tests ((a) PEG: free, (b) PEG: 0.01 g L −1 , (c) PEG: 0.03 g L −1 ).