Conformal Coating on Ultrahigh-Aspect-Ratio Nanopores of Anodic Alumina by Atomic Layer Deposition

Anodic alumina (AA) membranes are composed of highly uniform, nanometer-scale pores arranged in a hexagonal close-packed array. Depositing conformal films inside the nanopores is extremely difficult because the nanopores have an ultrahigh aspect ratio of L/d ≈ 103. Atomic layer deposition (ALD) is a thin film growth technique that can deposit highly uniform films on high-aspect-ratio substrates with monolayer thickness control. In this study, AA membranes were coated with Al2O3 and ZnO ALD films and subsequently analyzed using cross-sectional scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). SEM analysis of individual nanopores revealed that the AA membranes with nanopore diameters of d = 65 nm and lengths of L = 50 μm could be coated conformally by Al2O3 ALD using sufficient reactant exposure times. Zn concentration profiles measured by EPMA following ZnO ALD showed the progressive infiltration of the ZnO ALD into the nanopores with increasing exposure times for aspect ratios as high as L/d ∼5000. Monte Carlo simulation of the experimental results assuming Knudsen diffusion accurately reproduced the experimental Zn concentration profiles and predicted the minimum ALD reactant exposures necessary to achieve conformal films. The Monte Carlo simulation also predicted that the diffusion-limited deposition will become reaction-limited given a sufficiently low ALD reaction probability. To test this idea, Fourier transform infrared absorption measurements were performed during the coating of the AA membranes by Al2O3 and SiO2 ALD. The surface reactions during Al2O3 ALD have a relatively high reaction probability of ∼10-3. In contrast, the surface reactions during SiO2 ALD have a very low reaction probability of ∼10-8. In agreement with the predictions, diffusion-limited behavior with a t1/2 time dependence was observed during Al2O3 ALD. Reaction-limited behavior with a t1 time dependence was observed during SiO2 ALD.