Penetration depth variation in atomic layer deposition on multiwalled carbon nanotube forests

Abstract

Atomic layer deposition (ALD) of Al2O3 on tall multiwalled carbon nanotube forests shows concentration variation with depth in discrete steps. While ALD is capable of extremely conformal deposition in high aspect ratio structures, decreasing penetration depth has been observed over multiple thermal ALD cycles on 1.3 mm tall multiwalled carbon nanotube forests. Scanning electron microscopy imaging with energy dispersive x-ray spectroscopy elemental analysis shows steps of decreasing intensity corresponding to decreasing concentrations of Al2O3. A study of these steps suggests that they are produced by a combination of diffusion limited precursor delivery and the increase in precursor adsorption site density due to nuclei growing during the ALD process. This conceptual model has been applied to modify literature models for ALD penetration on high aspect ratio structures, allowing two parameters to be extracted from the experimental data. The Knudsen diffusion constant for trimethylaluminum (TMA) in these carbon nanotube forests has been found to be 0.3 cm2 s−1. From the profile of the Al2O3 concentration, the sticking coefficient of TMA in the TMA/water thermal ALD process was found to be 0.003.