TEM‐Based Metrology for HfO₂ Layers and Nanotubes Formed in Anodic Aluminum Oxide Nanopore Structures

Abstract

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.

A Ballistic Transport and Surface Reaction Model for Simulating Atomic Layer Deposition Processes in High‐Aspect‐Ratio Nanopores

“…ALD film properties, surface reaction conditions, and pore geometries for the four studies to which our modeling results are compared. Values in parentheses () are estimated from known quantities; for example the pressure and exposure times corresponding to the Gordon et al study were computed to match the known exposure of 9000 L (Langmuir); likewise, the pore dimensions were fitted to the known AR ¼ 43. study film Gordon [2] HfO 2 Granneman [1] HfO 2 Rubloff [19] HfO 2 George [20] …”

“…This leads to practically perfect GPC as a function of pore depth, a result consistent with the conclusions of Granneman et al [1] The non-unity sticking coefficient f i ¼ 0.1 used in this simulation further broadens the front, an observation consistent with the work of Dendooven et al [3] Finally, we note the small deviations from uniformity seen as a function of radial position, r, over the pore bottom during both exposures, resulting in a slightly convex GPC profile on the pore bottom. Column 3 of Table 1 corresponds to the high-aspect-ratio (AR ¼ 67) pore simulations of Perez et al [19] While this is the third HfO 2 ALD case we consider, there are two factors that make this study valuable for validating our simulation: 1) the system is intentionally under-dosed, resulting in partial pore filling; and 2) transmission electron microscope (TEM) images were used to directly measure the ALD film thickness profile in the region where precursor depletion presumably begins to take place. [19] Our simulation results are presented in Figure 7, together with the experimentally measured film thickness data extracted from the literature.…”

“…Column 3 of Table 1 corresponds to the high-aspect-ratio (AR ¼ 67) pore simulations of Perez et al [19] While this is the third HfO 2 ALD case we consider, there are two factors that make this study valuable for validating our simulation: 1) the system is intentionally under-dosed, resulting in partial pore filling; and 2) transmission electron microscope (TEM) images were used to directly measure the ALD film thickness profile in the region where precursor depletion presumably begins to take place. [19] Our simulation results are presented in Figure 7, together with the experimentally measured film thickness data extracted from the literature. [19] The full pore length of 4 mm is depicted in these plots; we clearly see that during the TEMAH exposure, the film surface saturation front travels only to about one-third of the full pore length, and the subsequent water dose returns the pore to its original fully oxidized state.…”

Protecting silver cultural heritage objects with atomic layer deposited corrosion barriers

“…The RMS roughness determined from the hazy area is 50.7 ± 4.9 nm, or roughly twice that from the clear area, where we measured a value of 24.0 ± 3.9 nm. Typically, minimal changes to the RMS roughness are observed after applying ALD Al 2 O 3 films [33]; indeed this type of coating produces extreme conformality [23,24]. As an uncoated sterling silver had a RMS roughness of 19.5 ± 2.4 nm and after a 100 nm thick Al 2 O 3 ALD film was applied, the RMS roughness was not significantly changed at 18.7 ± 0.8 nm.…”

“…The self-limiting nature of the chemistries implemented in ALD process produce high quality, dense and nearly pinhole-free films with thickness control at the atomic level, excellent thickness uniformity and unequalled conformality. ALD has been demonstrated to produce films that are nearly perfectly conformal even for nanoporous substrates whose pore depth to diameter ratio far exceeds ten to one [22][23][24].…”