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

Abstract

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 ≈ 10 3 . 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 Al 2 O 3 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 Al 2 O 3 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 Al 2 O 3 and SiO 2 ALD. The surface reactions during Al 2 O 3 ALD have a relatively high reaction probability of ∼10 -3 . In contrast, the surface reactions during SiO 2 ALD have a very low reaction probability of ∼10 -8 . In agreement with the predictions, diffusion-limited behavior with a t 1/2 time dependence was observed during Al 2 O 3 ALD. Reaction-limited behavior with a t 1 time dependence was observed during SiO 2 ALD.

Monte Carlo simulations of atomic layer deposition on 3D large surface area structures: Required precursor exposure for pillar- versus hole-type structures

“…For arrays of holes, the exposure is independent of s 0 if H/w is large (above 50) and increases with decreasing s 0 if H/w is small. This result is in agreement with the earlier work by Elam et al 13 and Knoops et al, 22 where the reaction-limited (small H/w) and diffusion-limited (large H/w) regimes were distinguished. For structures with a small H/w ratio, the limiting factor during the deposition will be the sticking probability.…”

“…[12][13][14][15][16][17][18][19][20][21][22][23] At the start of the simulation, a MC particle is generated at a random position in a horizontal "source plane" positioned above the top surface of the 3D structures. The MC particle is emitted with a cosinedistributed random direction and its trajectory is calculated.…”

“…10,11 To have a better insight in the deposition process and to optimize the process parameters, several analytical and simulation models have been developed to describe the conformal ALD deposition in high aspect ratio structures. Among those, 2D models have been proposed to simulate thermal [12][13][14][15][16][17][18][19][20] and plasma-enhanced ALD in trenches. [21][22][23] This paper introduces a full 3D MC model, enabling simulation of thermal ALD processes in 3D structures.…”

A nanostructured cathode architecture for low charge overpotential in lithium-oxygen batteries

“…The synthesis of the new cathode architecture was done using ALD, a technique for preparing thin films that employs self-limiting chemical reactions between gaseous precursors and a solid surface allowing atomic scale control over the film thickness and composition 27 . One of the distinguishing attributes of ALD is the capability to deposit coatings on surfaces with complex topographies and to infiltrate mesoporous materials, important for the Al 2 O 3 coatings used for the cathode in this study [28][29][30] . The ALD technique can also be used to synthesize supported noble metal catalysts, such as Pd used in this work, in the size range from subnanometres to nanometres [31][32][33][34] .…”

Development of electrostatic supercapacitors by atomic layer deposition on nanoporous anodic aluminum oxides for energy harvesting applications

“…The AZO and alumina thin layer depositions were performed in an ALD reactor (Savanah 100) at 200 • C, using N 2 as the carrier and purging gas, and operating in exposure mode. This operating mode provides slower deposition processes as compared to the flow mode, but it is particularly suitable for conformal depositions on surfaces with high aspect ratio, as in the case of NAAM templates [36]. Trimethyl aluminum (TMA) and H 2 O were used as precursors to deposit the Al 2 O 3 layer at a deposition rate of 0.127 ± 0.001 nm per cycle to grow a thickness of 10.17 ± 0.07 nm dielectric layer.…”