Copper Nanofilm Formation by Electrochemical ALD

Abstract

This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surface-limited redox replacement, also referred to as monolayer-restricted galvanic displacement. An automated flow-cell electrodeposition system was employed to make Cu nanofilms using 100, 200, and 500 ALD cycles. The cycle was composed of a sequence of steps: Pb underpotential deposition (UPD), rinsing with blank, introduction of at open circuit, and exchange of the Pb atoms for Cu, rinsing with a blank. The open-circuit potential was used to follow the replacement, exchange, of Pb for Cu, which shifted from that used to deposit Pb UPD (−0.44 V) up to the equilibrium potential for or −0.013 V upon a complete exchange. The resulting Cu films appeared homogeneous from inspection, optical microscopy, and scanning electron microscopy. Electron probe microanalysis showed no Pb in deposits formed using −0.44 V for Pb UPD. However, for deposits formed with Pb deposition at potentials more negative than −0.44 V, Pb was evident in the deposit. A prominent Cu(111) peak was displayed in the X-ray diffraction pattern for the Cu nanofilms. Morphology studies of the Cu films were performed using ex situ scanning tunneling microscopy and attested to the layer-by-layer growth of the Cu film. The 250 nm flat terraces suggested that a surface may have become smoother during growth rather than roughened as normally experienced during the electrodeposition or growth of thin films in general. A decrease in coulometry for Pb UPD during the first 30 cycles could also be interpreted as a decrease in surface roughness, or surface repair during ALD.