Proximity Effects of the Selective Atomic Layer Deposition of Cobalt on the Nanoscale: Implications for Interconnects

The continued scaling of transistor sizes has motivated the need to replace Cu with alternate metals to minimize resistivity, with cobalt being of interest for both interconnect via metallization as well as emerging die-bonding processes. The atomic layer deposition of cobalt using Co(tBu2DAD)2 and tertiary-butyl amine has nearly infinite selectivity (>1000 cycles) on metallic vs insulating (SiO2 or low-k SiCOH dielectric) planar samples. However, on patterned samples, selectivity under identical atomic layer deposition (ALD) conditions is limited, due to the diffusion of molecularly adsorbed metal precursors from reactive to non-reactive surfaces. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to investigate the effects of process parameters on surface precursor diffusion to determine the mechanism of selectivity loss on the nanoscale. Top-down SEM and XPS spectra of a striped test pattern of Cu and SiO2 indicated that selective vapor-phase passivation of SiO2 improved the selectivity for deposition on Cu versus SiO2 by reducing the number of insulator defects that facilitated trapping of precursor molecules and subsequent Co nucleus growth. The remaining nuclei were present due to incomplete defect passivation. Conversely, near-perfect selectivity during Co ALD was obtained with the periodic annealing of the substrate, consistent with a low temperature reflow process, allowing for Co nuclei on SiO2 defects to merge with the metallic growth surface.