Nucleation delay in atomic layer deposition on a thin organic layer and the role of reaction thermochemistry

Abstract

The atomic layer deposition (ALD) of a series of inorganic thin films on an interfacial organic layer has been examined, focusing on the existence of a delay in thin film nucleation and its dependence on the chemistry of each ALD reaction. The inorganic thin films examined were Al2O3, HfO2, Ta2O5, and TaNx, while the organic layer was a thin (∼5 Å thick) film of poly(ethylene imine), or PEI, on SiO2. The early stages (<50 ALD cycles) of inorganic thin film growth in each case have been characterized using a combination of surface-sensitive techniques including ellipsometry, x-ray photoelectron spectroscopy, and atomic force microscopy. PEI has the effect of attenuating growth to different degrees depending on the subsequently grown ALD thin film, leading to no attenuation of growth for Al2O3, but significant incubation periods, in increasing order, for Ta2O5, HfO2 and TaNx. Angle-resolved x-ray photoelectron spectroscopy of an Al2O3 thin film deposited on PEI|SiO2 demonstrates that, in this case, the PEI thin film is buried between the Al2O3 thin film and the SiO2 substrate, with no significant decrease in the density of nitrogen from the as deposited PEI film. Results from the HfO2 and the Ta-containing thin films indicate that the processes that lead to the attenuation of growth are associated mostly with the second half-cycle of ALD (H2O or NH3 exposure). The length of the incubation time is found to correlate with both the total enthalpy change of the overall ALD reaction, as well as the net internal energy change of a single ligand exchange reaction representative of the second half of the ALD reaction. These results suggest that the reaction thermochemistry, and activation barriers for the reactions involved the ALD process play a key role in determining the length of the incubation period caused by PEI.