The electrodeposition of copper from supercritical CO₂/acetonitrile mixtures and from supercritical trifluoromethane

The electrochemistry of [Cu(hfac)₂], where hfac is hexafluoroacetylacetonate, and [Cu(MeCN)₄]⁺ were investigated in liquid acetonitrile (MeCN), supercritical CO₂/MeCN and supercritical trifluoromethane (CHF₃) at 310-311 K and 17-20 MPa using either [NBuⁿ₄][BF₄] or [NBuⁿ₄][B(3,5-(CF₃)₂C₆H₃)₄] as the supporting electrolyte. In liquid acetonitrile it is possible to deposit metallic Cu from both ([Cu(MeCN)₄][BF₄]) and [Cu(hfac)₂] but voltammetry for the [Cu(hfac)₂] system is more complex and there is evidence of stripping of the Cu by reaction with Cu(ii). Voltammetry of the two copper complexes in scCO₂/MeCN showed typical plating and stripping features but with slightly increased diffusion limited currents for copper reduction due to the decreased viscosity of the supercritical solvent. In scCO₂/MeCN the Cu(i) complex, tetrakis(acetonitrile)copper(i) tetrafluoroborate ([Cu(MeCN)₄][BF₄]), was found to produce better quality copper deposits than the Cu(ii) complex ([Cu(hfac)₂]). The Cu(i) complex has the advantages that it is stable and does not undergo comproportionation with copper(0) and that its ligands are totally compatible with the scCO₂/MeCN solvent system. The solubility of ([Cu(MeCN)₄][BF₄]) is limited in scCO₂/MeCN but can be significantly improved by changing the anion for tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([B(3,5-(CF₃)₂C₆H₃)₄]⁻). It was possible to deposit smooth copper films of high purity and low resistivity (down to 4.0 × 10^-6 Ω cm) from the Cu(i) complex. Copper was also deposited from supercritical CHF₃ using [Cu(hfac)₂] as a precursor. Although the plating and stripping features in the voltammetry are complicated by the lack of cosolvent and electroreduction of the solvent or free ligands, it was possible to produce copper films with resistivities as low as 5.8 × 10^-6 Ω cm.