3.2.1 Surface performance of electrolytic copper foil
When SPS works together with HEC, the surface morphology of copper foils is shown in Fig. 10. The surface of the plating consists of a tumor-like structure formed by agglomerations of nonuniform spherical particles. As an example, in Fig. 1(b), particles of different sizes are irregularly distributed on the surface, accompanied by defects such as holes and cracks, leading to inhomogeneities between grain boundaries. Comparatively, the copper foil obtained by adding 0.08 g/L SPS and 0.12 g/L HEC possesses a more uniform and dense particle distribution. As shown in Fig. 1(e), the small particles are tightly bound to achieve the minimum porosity.
Figure 11 displays the SEM images of copper foils electrodeposited from the sulfate bath in the presence of SPS and collagen composite additives. In comparison to the SH additive, the surface of the deposited layer also shows numerous uniformly spreading copper nodules after the incorporation of SC additives. From Fig. 11(a) and (b), the large “tumor” structure of copper foils surface is coarse and uneven, as a result we can evidently see the undeposited titanium substrate. Apparently, the foil produced from the bath with 0.06 g/L SPS and 0.08 g/L Collagen exhibits much smoother surface than that obtained from the other bath. The average particle size in Fig .12(a) shows that the smallest grain size value of 3-SC copper foil is 6.21 µm.
X-ray diffraction analysis the effect of 5-SH and 3-SC two groups additive on copper foil deposition layer, the results are shown in Fig. 12(b). There are two distinct diffraction peaks at 2θ, 43.16° associated with the (111) crystal plane and 50.37° corresponding to the (200) crystal plane, respectively. Nevertheless, (220) and (311) crystal faces approach the disappearance state. From Fig. 12(c-d), the weave coefficient and residual stress indicate that the two-component additive reduces the grain size and enhances the residual tensile force (60.74×106 N·m− 2), but the main peak (111) remains unchanged.
The SEM images of electrolytic copper foils deposited by adding SPS, HEC and collagen additives are presented in Fig. 13. Overall, different compositions of SHC additives have various effects on the surface profile and shape of the copper grains. In the case of 5-SHC, the copper foil exhibits the flattest and densest profile with fine copper deposits covered the substrate completely. Meanwhile, the grains in 1-SHC show a similar degree of concave and convex undulation as the 5-SHC group. Apparently, the 3-SHC foils reveals a uniform orientation of the surface particles, which is conducive to improving the brightness of the copper foil. With the variation of the addition of the three additives, spherical particles of different sizes and random distribution form the tumor-like organization with distinct orientations during the deposition process. By comparing 4-SHC, 6-SHC with 9-SHC, we can find that copper foils with pores and cracks lead to incomplete coverage of the substrate and non-dense deposition. Unlike the XRD spectra of two-component additives, four diffraction peaks with different diffraction intensities appear in the 3-SHC and 8-SHC groups from Fig. 14(a). However, the fact that (111) crystal plane is the main peak suggests that SPS, HEC and collagen additives does not create a new meritocratic orientation for the growth of grains. From Fig. 14(b) and (c), the texture coefficients and residual tensile forces of (111) crystal plane in group 1 and group 5 are greater than those in group 3 and group 8.
Figures 9(b) and (c) present possible theoretical models for the mechanism of action between the additives. The SPS additive first converts metal ions with small overpotential into complex ions with large overpotential via physical adsorption on the copper surface, thereby reducing the roughness and expanding the specific surface area. Subsequently, the SPS undergoes a desorption reaction to decompose into MPS. As the MPS accumulates, the thiol functional group adsorbs on the cathode surface and the terminal sulfonate anion captures the hydrated copper ions in the plating solution, which increases the reduction rate of copper ions [27, 19]. The collagen additive is attracted to the concave corners of the copper foil, inhibiting electrodeposition and preventing the formation of pinholes [28]. Additionally, the positively charged nitrogen cations contained in collagen and the negatively charged sulfonic acid groups possessed in SPS can interact with each other through electrostatic attraction. However, the above results may be explained by the presence of both synergistic and antagonistic effects between the additives. The competitive adsorption relationship between SPS and HEC produces an inhomogeneous adsorption in opposition to the preferential adsorption occurring at the surface protrusions, causing a decrease in the refinement effect of the additives.
3.2.2 Surface roughness of electrolytic copper foil
The effect of two-component additives on the surface roughness of electrolytic copper foil is shown in Fig. 15. When SPS works with HEC, the roughness rises at low concentrations and then drops sharply to 3.12 µm as the amount of SH added increases. The SEM images in Fig. 10(e) suggests that the copper foil achieves the smallest average grain size (8.71 µm), which facilitates the uniform and orderly growth of copper particles in the deposition process and thereby reduces the roughness. Compare Fig. 15(b) and Fig. 15(a), the roughness of the copper foil is lower when SPS and collagen additives are used in combination. The Rz value of the copper foil surface is 2.30 µm at the presence of 0.06 g/L SPS and 0.08 g/L collagen.
The results of the SHC additives orthogonal experiments are listed in Table 3. Firstly, the 5-SHC group possesses the lowest surface roughness of 1.93 µm that superior to two-component additives. The effect order of different additives on the roughness of electrolytic copper foil is HEC0.12 > collagen0.10 > SPS0.05, which indicates that surface roughness value is most sensitive to HEC additive. It is because that HEC inhibits the growth of “conical peaks” of copper deposits, thus forces Cu grains to grow laterally and eventually forming a flat and smooth surface. In conclusion, the appropriate levels of each factor in the orthogonal experiment of the three-component additive package are determined as 0.06 g/L SPS, 0.12 g/L HEC and 0.10 g/L collagen.
Table 3 Orthogonal test results of SHC additive
Factor
|
Assessment indicators
|
Groups
|
SPS (g/L)
|
HEC (g/L)
|
Collagen (g/L)
|
Thickness/μm
|
Roughness/μm
|
1-SHC
2-SHC
3-SHC
4-SHC
5-SHC
6-SHC
7-SHC
8-SHC
9-SHC
|
0.05
0.05
0.05
0.06
0.06
0.06
0.07
0.07
0.07
|
0.10
0.12
0.14
0.10
0.12
0.14
0.10
0.12
0.14
|
0.06
0.08
0.10
0.08
0.10
0.06
0.10
0.06
0.08
|
7.17
5.83
5.41
6.29
5.51
5.53
7.01
6.67
5.95
|
2.66
3.54
2.01
4.33
1.93
3.22
2.97
2.23
3.34
|
Roughness
|
K1
K2
K3
R
Optimum
|
8.21
9.48
8.53
1.27
SPS0.05
|
9.95
7.71
8.56
2.24
HEC0.12
|
8.11
9.11
6.90
2.20
Collagen0.10
|
HEC0.12>
Collagen0.10
>SPS0.05
|
3.2.3 Electrochemical properties of electrolytic copper foil
The electrochemical behavior is tested to explore the effect of the composite additive on copper ion deposition. Figure 16 presents the potentiodynamic polarization curves of electrodeposited copper after the addition of SH and SC additives, respectively. When 0.08 g/L SPS is added to the base electrolyte together with 0.12 g/L HEC, the starting potential shifts positively from − 0.402 V to -0.399 V, which implies that depolarization occurs and accelerates the electrochemical deposition reaction. On the contrary, the cathodic overpotential of copper electrodeposition is negatively shifted either by introducing SPS and collagen alone or by adding 0.06 g/L SPS and 0.08 g/L collagen after compounding.
Adsorption and complexation are widely accepted theories, but some studies have demonstrated that SPS acts as a hindrance rather than a promoter of copper deposition. For example, Lin et al [29, 11] suggested that SPS additives acting alone on the cathode surface can impede grain growth and promote the formation of nuclei. Theoretically, there is a proportional relationship between nucleation rate and overpotential. The SC additives show an inhibitory effect on the deposition of copper ions, though the intensity of inhibition is lower than that of the single additive. Hence, it can be inferred that the 3-SC works opposite to the depolarizing effect exhibited by the SH additive.
The Nyquist plot depicted in Fig. 17(a) is a typical curve composed of a straight line with an upward trend in the low frequency region and a half circle in the high frequency region. Comparing the radius magnitude of the semicircular arcs after the addition of SH and SC additives, respectively, it is found that the incorporation of 0.06 g/L SPS and 0.08 g/L collagen increases the impedance, thus hindering the charge transfer process and enhancing the electrochemical polarization. The further magnified image of the EIS curves reflects that the 5-SH group possesses the smallest radius, because the copper grains grow faster than the crystallization rate, which appears as a coarse deposit layer in the SEM image. Figure 17(b) shows the cyclic voltammetry curves. The current density of reduction peak varies with the type and concentration of additives, while in the reversible reaction, the potential of reduction peak is unaffected by the additives. Therefore, the copper electrocrystallization reaction process is irreversible. In this process, Cu2+ loses electrons to be oxidized to Cu+ and copper atoms. Since the different number of electrons involved in the electrode reaction, it can be observed that the curves for different additives present various current densities. The current density increases when the presence of SPS and HEC in the solution, indicating that the SH additive has a facilitating effect on the copper deposition process. However, the presence of SC additives reduces the current density.