(111) Orientation Nanotwinned Copper with Well-Controlled Morphology by System-Optimized Electropolishing
Authors: Hu Chi-Chang , Tsou Hsu 
Published: December 2024

Copper has attained indispensable status across diverse modern electronic and semiconductor applications, owing to its exceptional physical and electrical attributes. Notably, in response to the emergence of electric vehicles, copper assumes a critical role as a current collector for the negative electrode in lithium-metal batteries (LMBs). LMBs, distinguished by their high energy density in the absence of carbon-based active materials on the negative electrode side, hold immense promise for future energy storage endeavors. However, the commercialization of LMBs faces formidable challenges, including the instability of the solid electrolyte interphase, pronounced volume fluctuations, and dendrite growth of lithium. In light of the imperative to enhance the cycle life of LMBs through direct modifications to the current collector, we propose a novel approach involving the high electroplating of (111)-orientation nanotwinned copper with meticulously controlled morphology. This novel methodology is corroborated by a system-optimized electrochemical polishing system employing full factorial design principles.
Nanotwinned Cu (nt-Cu) represents a distinct crystalline structure characterized by twin-containing columnar grains. Its exceptional mechanical strength arises from twin boundaries (TBs) effectively impeding dislocation motion, thereby establishing the Hall-Petch strengthening relationship. For instance, nt-Cu demonstrates ultrahigh tensile strength, reaching approximately 1 GPa, far surpassing the 200 MPa achieved by coarse-grained Cu. Moreover, TBs contribute to enhanced thermal stability, preventing self-annealing phenomena that could alter Cu's mechanical properties. Given these advantages, incorporating high-density twin boundaries within Cu grains emerges as a promising strategy to enhance Cu interconnects' physical properties and reliability. Fortunately, a novel direct-current method, boasting a current density of up to 40 ASD, has been successfully developed for plating copper foils with densely packed (111) twin boundaries. This approach mitigates the time-consuming nature of pulse electroplating. The resulting foils exhibit the thinnest solid electrolyte interphase (SEI) formation compared to those plated with Cu (101) and Cu (001), attributed to the lower surface energy of Cu (111). This innovation complements traditional methods such as pulsed electro-deposition for manufacturing Cu deposits with dense twin boundaries.
However, a persistent issue arises in the form of uneven copper growth on the titanium plating substrate edges due to the Edge Effect. This uneven growth results in excessive copper accumulation at the edge, creating operational inefficiencies. To tackle this challenge, our approach involves utilizing electropolishing techniques (EP) to flatten the copper foil current collector while preserving its crystal orientation and defect structure integrity. By doing so, we aim to achieve a more uniform copper distribution across the substrate, thereby enhancing the performance and efficiency of the electrical system.
The electropolishing technique stands out as a remarkably efficient method for cleaning and enhancing the appearance of metals and alloys. Its efficacy has garnered significant attention both in practical applications and academic research. Numerous efforts have been made by researchers and practitioners to achieve bright and smooth surfaces on metals such as copper through the application of electropolishing methods. Several factors must be carefully considered in electropolishing, including mass transformation, choice of electrolyte, and additives. During the process, the stripping rate of copper ions often outpaces their diffusion rate, resulting in a notable accumulation of copper ions near the surface of the copper foil. This accumulation can trigger uncontrollable concentration polarization, presenting challenges in maintaining uniformity. Consequently, this phenomenon gives rise to inconsistencies in the limit current of the copper foil, resulting in uneven electropolishing rates across the surface. These variations significantly impair planarization efficiency. Additionally, the electropolishing rate of copper metals predominantly hinges on two key factors: the acidity of the solution and the resistance of the viscous layer, confirmed by alpha-step and AFM. By manipulating the electrolyte's acidity and viscosity, we can fine-tune the proportion of the electrolyte, whereby controlling the limit current without succumbing to the effects of concentration polarization. This adjustment enables us to optimize the surface morphology of the copper foil, achieving desired results effectively.
In this study, we employed design of experiment (DOE) to systematically investigate the electropolishing effect under varied conditions, encompassing factors such as distance between two electrodes, mass transformation, and different electrolyte proportions. Through statistical analysis, we systematically summarized the correlation between the roughness of the copper foil and the factors of electropolishing and develop a regression model for the composite factors, and also conduct the steepest ascent test to validate the accuracy and reliability of our regression model. As well, we correlated our findings with the electrochemical behavior observed during copper foil electropolishing. This comprehensive analysis provides valuable insights into the mechanisms and properties underlying the formation of surface profiles, enhancing our understanding of the process.

Figure 1