Research Article www.acsami.org
Corrosion Resistance of Atomic Layer Deposition-Generated Amorphous Thin Films Michael D. Anderson,† Brad Aitchison,‡ and David C. Johnson*,† †
Department of Chemistry, 1253 University of Oregon, Eugene, Oregon 97403, United States MLD Technologies LLC, 1000 South Bertelsen Road #14, Eugene, Oregon 97402, United States
‡
ABSTRACT: Atomic layer deposition (ALD) was used to prepare amorphous thin films of Al2O3, Nb2O5, and Ta2O5 on both silicon substrates and aluminum blocks. Etch rates in 10 M NH4OH were determined from X-ray reflectometry data collected as a function of time. Amorphous Al2O3 thin films were found to have an etch rate of 0.5 nm min−1 and an increase in roughness of ∼0.01 nm min−1. Electron microscopy data showed etch pits, consistent with the increase in roughness. Amorphous Nb2O5 and Ta2O5 films showed no appreciable etching or roughening over the course of a ∼500 h continuous immersion. An Nb2O5-coated aluminum block showed no corrosion after immersion in 10 M NH4OH for over 200 h, suggesting that the coatings were pinhole-free. These results suggest that amorphous ALD thin films of Nb2O5 and Ta2O5 are candidates as barrier layers for aluminum in caustic environments. KEYWORDS: atomic layer deposition, corrosion protection, X-ray reflectivity, aluminum corrosion, protective coatings
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materials using chemical vapor deposition (CVD),19,20 plasma,21 electrochemical,22 atomic layer deposition (ALD),23 or wet chemical techniques. Despite these prior efforts, there remains a need to form corrosion-resistant coatings on metals, particularly on complicated shapes and structures after final assembly. Ideally, the protective coatings would prevent corrosion in water containing a variety of dissolved species, across a range of pHs and at a variety of different operating temperatures. In this paper, we prepared ALD-deposited Nb2O5 and Ta2O5 coatings on aluminum and tested their ability to protect the aluminum from corrosion when immersed in basic aqueous solutions. Aluminum is an ideal material for many applications because of its low cost, light weight, and strength. The native oxide coating of aluminum that protects the metal against oxidation,24 however, does not prevent corrosion in water, acidic and basic solutions, or high-temperature steam.25−28 This corrosion makes aluminum unsuitable for applications, such as microchannel heat-exchange devices, where it would otherwise be an ideal material. The hydroxyl-terminated native oxide layer formed by aluminum metal in air, however, provides an ideal starting layer for ALD thin film depositions. ALD is a gas-phase process involving sequential, self-limiting surface chemical reactions that can produce conformal coatings on highaspect-ratio structures with atomic level control of film thickness.29,30 The self-limiting nature of the ALD surface reactions permits complicated shapes with large aspect ratios to
INTRODUCTION The properties of metals (high electrical conductivity, high thermal conductivity, strength, and ductility) make them critically important structural materials either alone or as constituents in composites, and they are critical components in electronic devices. The Achilles heel of metals is corrosion, however, with estimates of the total global cost of corrosion exceeding 3% of global gross domestic production, with corrosion of metal constituents a large fraction of this cost. For example, copper is used extensively to distribute water in buildings and in devices such as water heat exchangers. While copper is reasonably corrosion-resistant, it still has corrosion rates of ∼0.01 mg cm−2 d−1 in pure water with 1 mL/L oxygen concentration, leading to corrosion-caused failures over time.1 The mechanism behind this corrosion is an electrochemical reaction between areas remote from one another on an atomic scale2,3 and the rate of corrosion varies considerably depending on dissolved species in the water, pH, and temperature.4−8 Electrochemical corrosion is common for most metals, with corrosion rates depending on the stability of passivizing surface oxides. Some metals, such as aluminum and titanium, form dense surface oxides a few nanometers thick that prevent further oxidation. These native surface oxides are not protective in all environments, however. For example, aluminum oxide is soluble in both acidic and basic solutions, leading to significant corrosion in aqueous environments. Adjusting alloy compositions, for example, the addition of chromium to steel, is a common strategy to enhance protective oxide coatings. Another strategy to shield metals from corrosion is to deposit protective coatings such as self-assembled monolayers,9−11 organic azoles,12−14 polymers,15−18 or a variety of inorganic © 2016 American Chemical Society
Received: September 5, 2016 Accepted: October 17, 2016 Published: October 17, 2016 30644
DOI: 10.1021/acsami.6b11231 ACS Appl. Mater. Interfaces 2016, 8, 30644−30648
Research Article
ACS Applied Materials & Interfaces
using BEDE REFS, a XRR data fitting algorithm to determine film thickness, roughness, and the associated uncertainties.43 Scanning electron microscopy (SEM) images were collected using a FEI Helios DualBeam system using a 15 kV accelerating voltage and a 1.6 nA beam current. Samples were mounted to aluminum stubs with carbon tape and gold coated with a nominal ∼5 nm layer to reduce surface charging. The aluminum blocks used for the macroscale tests were prepared from 4 cm (nominal) aluminum round stock. The blocks were wetpolished using diamond abrasive pads, finishing with a 0.5 μm diamond grit. One block was coated with 150 nm of Nb2O5 in the same manner as the silicon test wafers and the other left bare. The 150 nm of Nb2O5 was applied in two coats to prevent pinholes where the blocks were supported. The blocks were etched in 10 M NH4OH for approximately 170 h as before and imaged with a consumer grade digital camera.
be uniformly coated. ALD can produce nearly pinhole-free films23 that can be effective gas diffusion barriers.31−33 Prior work has demonstrated that TiO2 films deposited by ALD protects stainless steel,34,35 CrN,36 and aluminum precoated with ALD-grown Al2O337 from electrochemical corrosion. Nb2O5 and Ta2O5 were chosen for this study because they are known to have very low solubility (∼10−6 M) in aqueous alkaline and carbonate solutions at T = 550 °C at pressures of 500 bar with low oxygen fugacity and are insoluble in aqueous solutions at lower temperatures and pressures across a wide range of pH.38,39 Since transition-metal organometallic complexes of tantalum and niobium have been developed that are excellent precursors for ALD of the oxides, we were able to produce conformal and homogeneous films on the investigated aluminum surfaces.40 One of the challenges in assessing effective corrosion barriers is the measurement of small changes in thickness and sample roughness. We used X-ray reflectivity (XRR) to measure film thickness, density, and roughness, as it is accurate, flexible, and sensitive to changes at the subnanometer length scale and is noncontacting and nondestructive. Despite the potential for XRR to be a powerful tool for measuring etching or corrosion of thin films, it has not been utilized much in this area. In this work we measure the etch rate and surface roughness as a function of time for ALD-deposited Al2O3, Nb2O5, and Ta2O5 in aqueous ammonia solutions using XRR. We found that the ALD Nb2O5 and Ta2O5 thin films were effective corrosion barriers, with dramatic differences in appearance between coated and uncoated aluminum blocks subjected to a concentrated NH4OH bath for extended periods. Postfabrication ALD-grown Nb2O5 and Ta2O5 thin films might enable the use of aluminum microchannel devices and heat exchangers at temperatures and with fluids where aluminum would normally corrode, providing the fabrication and lightweight advantages inherent to aluminum along with corrosion resistance.
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RESULTS AND DISCUSSION XRR data collected as a function of immersion time on an amorphous Al2O3 film soaked in 10 M NH4OH solution at 25 °C are shown in Figure 1. The angular period of the Kiessig
Figure 1. X-ray reflectivity data for Al2O3 as a function of etching time in 10 M NH4OH. The Kiessig fringes move farther apart as the film becomes thinner.
EXPERIMENTAL SECTION
Amorphous Al2O3 thin films were deposited at 340 °C using alternating pulses of trimethylaluminum and deionized water precursors. The Nb2O5 and Ta2O5 films were deposited at the same temperature using alternating pulses of either niobium ethoxide or tantalum ethoxide and water. Samples were prepared in a Planar Systems (now Beneq) P400A ALD chamber equipped with multiple precursor inlets and heated sources. Thin film composition was verified using a Cameca SX-100 electron microprobe (EPMA). Each thin film was analyzed at 10 sites on each sample at 5, 10, and 15 keV accelerating voltages. Data acquisition was conducted using Probe for Windows and the data matrix was postprocessed using the Stratagem software package.41 Etching was conducted in accordance with the ASTM International Standards42 taking special care to keep the etchant concentration constant during the etching process. Sample etching was conducted in a glass reactor vessel fitted with a water jacket, high-efficiency condenser column, and a glass-shrouded thermocouple using a 10 M solution of NH4OH at 25 °C. Temperatures were maintained by means of an isothermal bath connected to the water jacket. X-ray reflectivity data on the silicon samples were collected on a Bruker-AXS D8 Discover X-ray Diffractometer using Cu Kα radiation (λ = 0.15418 nm). The incident beam was conditioned and collimated using a parabolic multilayer mirror and 0.1 mm divergence slit. The exit beam was conditioned and recollimated with a 0.6 mm antiscatter slit, a Soller slit assembly, and 0.05 mm detector slit. Each sample was carefully aligned such that the beam was centered on the sample at a consistent height relative to the incident X-ray beam. XRR data were collected from 0° to 4° (2θ) with a step size of 0.003° and a data collection time of 1 s at each point. Reflectivity data were analyzed
fringes, Δθ, can be related to the total thickness, t, of the film without knowledge of the optical constants of the film according to eq 1 using the small-angle approximation: Δθ ∼ λ /2t
(1)
With increasing immersion time, the angular period increases, indicating that the film thickness decreases. The smoothness and thickness uniformity of the film determines the angle at which the Kiessig fringes are no longer observable. The higher this angle, the more Kiessig fringes that can be observed and the more accurately the film thickness can be determined.44 For the untreated film, this angle is 2θ ∼ 4°, corresponding to a median roughness of 0.8 nm. As immersion time increases, this angle decreases, indicating that the roughness increases. The accuracy with which the thickness of the film can be determined at each time determines the accuracy of the etch rate that can be determined. The measured XRR data sets were modeled to extract the maximum information possible. An example of the modeling analysis for an amorphous ALD-grown Al2O3 film can be found in Figure 2, with the inset showing the degree to which the model is able to fit the data. The modeling of the data was done using a differential evolution (DE) algorithm to automatically 30645
DOI: 10.1021/acsami.6b11231 ACS Appl. Mater. Interfaces 2016, 8, 30644−30648
Research Article
ACS Applied Materials & Interfaces
Figure 4. Electron micrograph of etched Al2O3 after 90 min etching in 10 M NH4OH. (a) 125× magnification. (b) 2500× magnification.
electron microscopy images suggest that, in addition to a fairly uniform average etch, surface pitting occurs. In contrast to the significant changes in the XRR patterns as a function of etch time found for the Al2O3 films, both the Nb2O5 and Ta2O5 XRR patterns do not change as a function of etching time, indicating that there is no change in film thickness even after immersion for ∼500 h (Figure 5). The XRR
Figure 2. Modeled and experimental X-ray reflectivity data for a representative Al2O3 film. A black line is drawn between data points and the gray crosses are the calculated intensities based on a single layer with constant electron density.
fit measured specular X-ray reflectivity curves to a calculation based on a structural model of the sample being analyzed. The measured and calculated XRR curves are objectively compared using a goodness-of-fit function whose value decreases toward zero as the agreement between the two curves improves and the model more closely represents the sample’s structure.43 The change in the thickness of the ALD Al2O3 film as a function of etching time is shown in Figure 3. The thickness
Figure 5. Al2O3, Nb2O5, and Ta2O5 film thicknesses determined from X-ray reflectivity data plotted as a function of etch time in 10 M NH4OH. Note that the horizontal axis has a log time scale.
roughness also does not increase, suggesting that pitting is not occurring even after several hundred hours of immersion in 10 M NH4OH. The XRR data is supported by SEM images of Nb2O5 films, which contain no changes in surface features after over 500 h of immersion. This suggests that thin amorphous ALD coatings of Nb2O5 and Ta2O5 could be effective diffusion barriers preventing corrosion. Since pinholes are a common failure mechanism for protective coatings, we exposed an Nb2O5 ALD-coated and an uncoated polished block of aluminum to over 170 h of immersion in 10 M NH4OH. The Nb2O5 ALD coating was done in two coats, with the block supports shifted between coats to eliminate defects due to the supports. Figure 6 compares images of these two blocks. The uncoated aluminum block (left image) has significant pitting and discoloration from the etching process. The oxidation of aluminum is supported by the observed effervescence coming from the block’s surface during the etching process. The coated block (right image) shows no such pitting or discoloration, indicating that the block has been effectively passivized with respect to the basic environment. This suggests that relatively thin ALD coatings can protect surfaces of metals from corrosion, and that XRR is an effective tool to evaluate changes in film thickness and
Figure 3. Change in film thickness as a function of etching time for an ALD-grown Al2O3 thin film. The size of the points on the graph exceeds the error of the measurement of thickness and time. An etch rate of 0.51 ± 0.04) nm min−1 was obtained from a linear least-squares fit to the thicknesses.
decreases linearly as a function of etch time corresponding to an etch rate of (0.51 ± 0.04) nm min−1. The linear decay as a function of time suggests that the chemical etching is the ratelimiting step, not the diffusion of the species away from the surface. The film roughness also increases linearly with etching time, increasing from 0.7 to 1.6 nm over 90 min with a roughening rate of ∼0.01 nm min−1. While the presence of sample roughening is not surprising, the magnitude of the rate suggests that the etching process is relatively planar. This suggests that rates of pitting or spalling typically seen in failure tests are relatively slow. The observed etching is consistent with the enhanced solubility of aluminum oxides, oxyhydroxides, and hydroxides in range of pH investigated.45 Electron microscopy of the Al2O3 films, Figure 4, shows significant changes in the surface appearance when compared to the pre-etch specimen, which is consistent with the etching seen in the XRR data. The 30646
DOI: 10.1021/acsami.6b11231 ACS Appl. Mater. Interfaces 2016, 8, 30644−30648
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ACS Applied Materials & Interfaces Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) through grant DMR-1266217. The authors gratefully acknowledge the assistance of MLD Technologies, LLC, for sample preparation. M.D.A. acknowledges support by the National Science Foundation IGERT Fellowship Program under Grant No. DGE-0549503.
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Figure 6. Photographs of two aluminum blocks after immersion in 10 M NH4OH for 170 h. The block in Figure 6a was a polished aluminum block without a protective coating. The block in Figure 6b was coated with 150 nm of amorphous ALD-deposited Nb2O5. The scratches in the block on the right were introduced before application of the Nb2O5 coating and do not change as a function of immersion time.
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roughness as a function of different environments and exposure times.
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CONCLUSIONS ALD produced Al2O3, Nb2O5, and Ta2O5 thin films were deposited on silicon wafers and polished aluminum surfaces. Xray reflectivity was shown to be an effective tool to monitor the thickness and roughness of the films as a function of exposure time. The Al2O3 film thickness was found to change linearly with respect to time in 10 M NH4OH with an etch rate of 0.5 nm min−1. The Al2O3 film roughness was found to increase with respect to time at a rate of 0.01 nm min−1, suggesting that pitting was occurring as confirmed by SEM images. The Nb2O5, and Ta2O5 films, however, showed no appreciable etching or increase in film roughness over the course of a 500 h etching. An amorphous ALD coating of Nb2O5 was shown to effectively protect an aluminum block, indicating that pinholes were not formed during the exposure time. Amorphous ALD-grown Nb2O5 and Ta2O5 are potentially effective protective coatings for aluminum in caustic environments. Numerous ALD precursors have been developed for the deposition of oxide compounds containing various metals, including Hf, Cr, Ni, Ta, Nb, Si, Zn, Zr, V, and W. Specific oxides can be chosen as protective coatings based on their solubility in the working environment, with the properties of the bulk oxides being a useful first guide to likely candidates. XRR provides a quick means to test the etch rate of the ALD-produced oxides in different environments. The object to be coated needs a hydroxyl or hydrogen-terminated surface for the ALD process. The oxygen-containing precursor for the ALD process also should not react with the object to form a poorly adhered oxide. In addition to oxides, ALD-produced metal fluorides and/or metal nitride coatings are alternative candidates as protective coatings and could also potentially serve as an oxygen/hydroxyl diffusion barrier.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (D.C.J.). Present Address #
Michael D. Anderson, SAS Institute Inc., 100 SAS Campus Drive, Cary, NC 27513-2414. Author Contributions
The manuscript was written through contributions of all authors. 30647
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