Surface Characterization of High-Purity Copper Electropolishing and Electrodeposition Treatments in an Ionic Liquid
Authors: Abdel-Fattah Tarek M , Loftis Derek 
Published: December 2020

High purity (>99.9% composition) copper metal specimens were used in electropolishing treatments with an acid-free ionic liquid electrolyte prepared from quaternary ammonium salts as a green polishing solution. A prominent ionic liquid composed of ethylene glycol (HOCH2CH2OH) and choline chloride (HOC2H4N(CH3)3
+Cl−), sometimes abbreviated as 2EG:1VB4, has been used in other studies to successfully electropolish various metal alloys, but has only been used in limited capacity to polish pure rare earth metals (1-5), like copper, the focus of this study. Voltammetry and chronoamperometry tests were conducted to determine the optimum conditions for electrochemical polishing, while Atomic Force Microscopy (AFM) revealed nanoscale effectiveness of the ionic liquid relative to an industry standard acid polishing treatment of 1 Mol Phosphoric Acid (H3PO4). Surface morphology comparisons summarized electrochemical polishing efficiency of these relative treatments by providing root mean square roughness prior to and post-electrochemical polishing.
Electropolishing using the ionic liquid revealed a mirror finish >10 times smoother than the same copper metal surface prior to electropolishing. This transition manifested in a marked change in root mean squared roughness from 167.15 nm to 14.744 nm and resulted in a mass loss of 0.0535 g for an electropolishing rate of 59.444 µg/s when subjected to a 900 second treatment with an average voltage of 1.5 V. By comparison, the phosphoric acid electropolishing treatment yielded an improvement in root mean squared roughness of 85.019 nm, resulted in a mass loss of 0.067 g for a higher electropolishing rate of 74.444 µg/s when subjected to the same treatment conditions. However, the it can be noted that while the elctropolishing rate via the acid solutionis greater, there are extreme peaks and troughs etched on the smoothed surface due to pitting at low current densities (Figure 1). Hydrogen contamination can be observed to affect the overall root mean square average of the region, which may be remedied via a relatively costly degassing treatment at temperatures up to 800 deg C to achieve more comparable results to the ionic liquid solution.
After these electropolishing pretreatments, the resulting copper surfaces were subsequently electroplated during a 1 hour treatment at 2 V with an 2EG:1VB4 ionic liquid solution that had previously been used to electropolish high-purity niobium metal. A thin film of niobium metal coated the polished copper during electrodeposition resulting in an average mass gain of 0.002 g, and a root mean squared roughness of increase of -2.101 nm for the ionic liquid pretreated sample, compared to the -2.417 nm resulting roughness from the phosphoric acid pretreatment.
As a result, pretreatments revealed a cheaper, recyclable, and environmentally-friendly approach to electopolishing pure copper was demonstrated to be more effective than an acidic industry standard electrolyte. Additionally, electrodeposition of niobium metals on each pretreatment sample provides a reasonably cost-effective method to combine the electroconductivity benefits of copper and ferroelasticity attributes of niobium metals.

References

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Figure 1