Accelerator Surface Phase Associated with Superconformal Cu Electrodeposition

Superconformal film growth is a key process in state-of-the-art Cu metallization of electronic devices. Superfilling of recessed surface features results from the competition between electrolyte additives that accelerate or inhibit Cu electroplating. In situ scanning tunneling microscopy is used to image the accelerating bis-(3-sodiumsulfopropyl disulfide) (SPS)-Cl- surfactant phase that is responsible for disrupting and preventing the formation of the inhibiting poly(ethylene glycol)-Cl- layer. Various aspects of competitive and coadsorption of Cl- and SPS on Cu(100) were examined for industrially relevant additive concentrations. At potentials associated with superfilling, a saturated, c(2x2) Cl- ordered adlayer forms on the surface. When as-received SPS is added, individual SPS and (3-mercaptopropyl)sulfonate (MPS) molecules are imaged as a mobile two-dimensional gas diffusing on the Cl- adlattice. The SPS-Cl- surfactant accounts for many aspects of the additive function previously observed and stipulated by the curvature enhanced accelerator model of superconformal film growth. SPS-derived species of differing mobility and tunneling contrast appear with exposure time. The lattice gas species are sensitive to the imaging conditions with tip-molecule interactions particularly evident at higher tunneling currents. At negative potentials, the c(2x2) Cl- adlayer is disrupted by an order-disorder transition, followed by desorption at more negative potentials. This allows direct access of SPS to the Cu metal whereupon irreversible sulfide formation occurs.