High Efficiency Electroplating (220)-Orientation Nano-Twinned Copper in Methanesulfonic Acid System and Mechanism Analysis
Authors: Wei Hsiang-Sheng , Hu Chi-Chang 
Published: December 2024

Nano-twinned copper, distinguished by intricately arranged twin boundaries within each grain, manifests superior mechanical properties compared to commercial copper foil. The cohesive twin structure effectively impedes dislocation motion through intricate interactions between twin boundaries and slip bands. Consequently, copper endowed with a twinned structure achieves an ultrahigh tensile strength of approximately 1 GPa, surpassing the performance of coarse-grained polycrystalline copper, which attains only around 200 MPa under equivalent testing conditions. Furthermore, the resistivity of coherent twin boundaries is approximately half that of stacking faults and grain boundaries. This characteristic allows twinned copper to attain heightened tensile strength without significant compromises in conductivity. In the context of resistance to electromigration, the twinned structure significantly hampers copper's atomic diffusion by an order of magnitude at the triple points where twin and grain boundaries intersect, compared to commercial copper. Moreover, in certain instances, nano-twinned copper demonstrates exceptional thermal stability, maintaining elevated hardness levels even subsequent to annealing at 800 °C, in contrast to nanocrystalline copper. In a distinct scenario, annealing at 250 °C for a duration of 3 hours results in only a marginal reduction in tensile strength, stabilizing at approximately 700 MPa.

Given the exemplary performance demonstrated by nano-twinned structures, the imperative is to devise efficient methodologies for the production of high-density nano-twinned copper films. High-speed electroplating emerges as a viable approach to achieve this objective, attributed to its one-step process and facile control.

Mainly because the high current density induces the higher copper nucleation rate, making interfacial energy larger, so that copper films would easier to proceed twin formation reaction to lower the residual stress. Numerous scholarly works underscore the application of high-speed electroplating in producing nano-twinned copper films, particularly through the pulse electrodeposition mode. This mode, characterized by a clear nucleation mechanism and well-defined performance metrics, facilitates the systematic creation of nano-twinned structures. However, the incorporation of a pulse-off period resulted in a reduction of the average current density, which was suboptimal from an efficiency standpoint. Consequently, specific scholarly works explore the electroplating of a twin-crystal structure through DC electroplating, aiming for enhanced efficiency with higher average current density. Presently, the literature reports instances of achieving elevated current densities, reaching up to 40 ASD, to produce nano-twinned copper with a (111) orientation and an average twin spacing of approximately 100 nm. Nevertheless, the solubility of cupric ions serves as a limiting factor for the overall electroplating reaction rate efficiency. Consequently, an inherent upper limit to efficiency exists within the copper sulfate system, posing a challenging constraint to ameliorate through adjustments to temperature or additives.

However, existing literature indicates that the cupric ions' solubility in copper sulfate is 1.35 M, while copper methanesulfonate demonstrates an impressive solubility of up to 2 M, as confirmed through vacuum filtration. The heightened ion solubility provides the opportunity to increase current density without inducing deleterious powdery deposition issues, thereby maintaining a uniformly conductive surface. Also, the higher current density would conduct better twin formation condition. Thus, the metal methanesulfonate solution, constituting an organic acid system designed to enhance the solubility of metal ions, involves the substitution of metal sulfate with metal methanesulfonate and the replacement of sulfuric acid with methanesulfonic acid, can be considered as a feasible optimization for efficiency issue and twin formation.

In the copper sulfonate system, the induction of a nano-twinned structure involves adding organic additives to form an ionic complex with Cu+ in remediation and chloride ions, subsequently altering the deposition route and nucleating copper crystals through a stress relaxation process. Conversely, in the copper methanesulfonate system, chloride ions exhibit an antagonistic effect with organic additives. Moreover, the use of organic additives is associated with potential challenges related to aging, chain scission and cracking issues, potentially transforming the system into a batch manufacturing paradigm. Therefore, investigating the possibility of directly improving the electrochemical mechanism on the negative surface through chloride ions in this organic system to plate nano-twin crystals is a question worth exploring in order to create a sustainable electroplating process for commercial nano-twinned copper manufacture.

Here presents a novel approach to the production of (220) orientation nano-twinned copper films, achieved through the incorporation of chloride ions as additives in a high-concentration methanesulfonate copper solution. The primary objective is to leverage chloride ions for the induction of numerous twin boundaries on the copper surface, enabling meticulous control of the average twin spacing at the nanoscale.

The investigation herein provides a comprehensive analysis of the nuanced role played by chloride ion distribution in the formation of nano-twinned copper.

Figure 1