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In situ Surface X-ray Diffraction Studies of the Copper-Electrolyte Interface: Atomic Structure and Homoepitaxial Growth

Copper electrodeposition is the predominantly used technique for on-chip wiring in the fabrication of ultra-large scale integrated (ULSI) microchips. In this `damascene copper 
electroplating’ process, multicomponent electrolytes containing organic additives realize void-free filling of trenches with high aspect ratio (`superconformal deposition’). Despite manifold studies, motivated by the continuous trend to shrink wiring dimensions and thus 
the demand of optimized plating baths, detailed knowledge on the growth mechanism - in presence and absence of additives - is still lacking. 
 
Using a recently developed hanging meniscus x-ray transmission cell, brilliant synchrotron x-rays and a fast, one-dimensional detector system, unique real-time in situ surface x-ray diffraction studies of copper electrodeposition were performed under realistic reaction conditions, approaching rates of technological relevance. Preparatory measurements of the electrochemical dissolution of Au(001) in chloride-containing electrolyte demonstrated the capability of this powerful technique, specifically the possibility to follow atomic-scale deposition or dissolution processes with a time resolution down to five milliseconds. 
 
The electrochemical as well as structural characterization of the Cu(001)- and Cu(111)-electrolyte interfaces provided detailed insight into the complex atomic-scale structures in 
presence of specifically adsorbed chloride on these surfaces. The interface of Cu(001) in chloride-containing electrolyte exhibits a continuous surface phase transition of a disordered 
Cl adlayer to a c(2×2) Cl adlayer with increasing potential. The latter was found to induce a small vertical corrugation of substrate atoms, which can be ascribed to lattice relaxations induced by the presence of coadsorbed water molecules and cations in the outer part of the 
electrochemical double layer. The study of the specific adsorption of chloride on Cu(111) from acidic aqueous electrolyte revealed a hexagonal, rotated adlayer structure, which was not reported before for this system. In comparison to other halide-metal(111) systems, the potential dependence of this structure suggests a strong adsorbate-adsorbate interaction. 
 
Operating under diffusion-limited conditions, i.e., at constant deposition rate, homoepitaxial growth of the Cu(001) single crystal electrode in chloride-containing solution has 
been investigated in situ for 1 and 5 mM Cu ion concentrations as a function of deposition overpotential. Detailed insight into the complex relationship between the atomic-scale structure of the solid-liquid interface, the growth behavior, and the resulting surface morphology was gained, revealing a pronounced mutual interaction of the Cu growth process and the Cl adlayer order. Depending on the latter, transitions from step-flow to layer-by-layer to 3D growth are observed, attributed to a reduction in the Cu surface mobility with increasing order. The kinetics of the c(2×2) adlayer ordering, in turn, are strongly affected during Cu deposition as compared to results obtained in Cu-free solution. Moreover, an oscillatory average strain in the surface layer is observed during layer-by-layer growth, 
indicating an expansion of the topmost layer occurring periodically for fractional coverages. 
 
Addition of polyethylene glycol (PEG), a commonly used inhibitor in the industrial damascene process, considerably changes the growth conditions. The chloride ordering 
kinetics are influenced such that the c(2×2) covered phase is stabilized in a widened potential regime. The onset of the transition to 3D growth is observed at more negative 
potentials, limiting the occurrence of layering oscillations to a narrower potential regime. Compared to the PEG-free electrolyte, the deposition rate is notably slowed down by a 
factor of approximately 3. The present study reports new direct experimental observations 
of the growth mechanisms at electrochemical interfaces on the atomic-scale.