Copper is one of the key materials of the twenty-first century and has attracted tremendous attention over the last two decades due to its nowadays realized application as silicon chip interconnect material. A particular scientific goal of this contribution is to achieve a more sophisticated understanding of the interaction of electrosorbed halide ad-layers with copper electrodes and of the resulting geometric structures of its surface and interface on a microscopic scale. Our efforts are motivated by experiments demonstrating that already trace amounts of halides in combination with particular organic additives are sufficient in order to improve the copper plating processes. A full understanding of the atomic scale processes is still lacking due to missing structural data.
The surface structure of Cu(100) surfaces modified by chloride and iodide have been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface sensitive x-ray scattering with a particular focus on adsorbate and potential dependent surface relaxation effects. An extraordinary large Cu-Cl bond length of 2.63 Å is found for the (2×2)-Cl phase on the basis of diffraction data analysis at positive potentials close to the on-set of the copper dissolution reaction. This finding points to a largely ionic character of the Cu-Cl interaction on Cu(100) with chloride particles likely retaining their full charge upon adsorption. The ionic Cu-Cl bond will be discussed as the origin of the observed 2.2% outward relaxation between the first two copper layers. This result indicates that the bond between the first and the second copper layer is significantly weakened which seems to be the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 Å the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.