Elsevier

Surface and Coatings Technology

Volume 304, 25 October 2016, Pages 1-8
Surface and Coatings Technology

Atomic layer deposition of aluminum oxide on modified steel substrates

https://doi.org/10.1016/j.surfcoat.2016.06.064Get rights and content

Highlights

  • Aluminum oxide films were atomic layer deposited on steel foils.

  • Aluminum oxide films increased the hardness and elasticity of the surface.

  • Aluminum oxide films as insulating coatings covered rough metal surface uniformly.

Abstract

Al2O3 thin films were grown by atomic layer deposition to thicknesses ranging from 10 to 90 nm on flexible steel substrates at 300 °C using Al(CH3)3 and H2O as precursors. The films grown to thicknesses 9–90 nm covered the rough steel surfaces uniformly, allowing reliable evaluation of their dielectric permittivity and electrical current densities with appreciable contact yield. Mechanical behavior of the coatings was evaluated by nanoindentation. The maximum hardness values of the Al2O3 films on steel reached 12 GPa and the elastic modulus exceeded 280 GPa.

Introduction

Inorganic thin films are often applied on surfaces of various substrate materials in order to isolate the surface from the environmental influence or to create buffer layers between substrate and other functional coatings. Herewith, steel is a recognized hard substrate material relevant to many technological applications ranging from machinery and tool production to display electronics and solar cells. Insulating coatings on steel can be made of a variety of materials [1]. For instance, when used as substrates for the next generation flexible display devices, electrically conductive and rough stainless steel foils is to be coated with insulating silicate [2]. Aiming at increasing corrosion resistance or mechanical hardening of steel substrates, SiO2:ZrO2 nanoparticle composites [3], TiC particles [4], ZrO2 and TiO2 films [5], SiO2 [6], sputtered Cr2O3 [7], Ta2O5 films [8], TiO2 films [8], [9], [10], TiO2-Al2O3 nanolaminates [8], [11] and Ta2O5-Al2O3 mixture films [12] have been examined.

Al2O3 is an attractive material recognized as a barrier or protective coating layer, provided that appropriate deposition routes to Al2O3 films were found and accommodated. For example, electrical properties and adhesion of silica and alumina sol-gel coatings on steel have been evaluated seeking the possibility to accommodate sol–gel derived materials [13]. Al2O3 as a protective layer has been grown on naturally somewhat rough steel substrates by chemical vapor deposition [14], [15], [16], sol-gel process [13], [17], [18] and spray pyrolysis [5], [19]. Furthermore, Al2O3 [20] and aluminum silicate [21] layers have been applied as diffusion barriers between steel substrate and copper-indium-gallium sulphide absorber layer in flexible solar cells.

Among other methods suited to the synthesis of materials layers modifying the surface properties, the atomic layer deposition (ALD) [20], [22], in particular, enables growth of dense insulating and/or protective layers on large area substrates with nanometer precision. Al2O3 films can be grown by ALD uniformly over rough and even three-dimensional complex-shaped substrates, such as those of microelectromechanical devices [23], metal fibers [24], carbon fibers [25], alumina fibers [26], polymer membranes [27], and silver jewelry [28]. ALD has been applied on steel substrates in several cases to grow Al2O3 as a protective layer [8], [29], [30], [31]. In most cases, the aluminum precursor used in ALD has been trimethylaluminum, Al(CH3)3 (TMA), applied together with H2O [8], [12], [23], [24], [25], [27], [29], [30], [32], [33].

Hardness and elasticity of the coatings are common physical criteria for their mechanical performance and quality. Regarding the hardness, Vickers [15], [11], [34], [35] and Berkovich [3] indentation tests can be applied to evaluate the hardness of a variety of materials. Al2O3 films sputtered on silicon [35], and on CVD-grown [14] or spray-pyrolysed [19] Al2O3 films on steel have been investigated by Berkovich indentation. Hardness of Al2O3 films has so far been evaluated on two dimensional relatively smooth Si or SiO2/Si substrates [32], [33], [36], glass plates [37], Al2O3-TiC wafers [38], cemented carbides [35] and on substrates of quite different hardness, such as Cu, glass, alumina and sapphire [34]. Nanoindentation using Berkovich tip has earlier been performed on ALD-grown Al2O3 films on silicon [37], [39] and glass substrates [37]. In certain applications functional coatings have to be grown to passivate, insulate or protect relatively rough surfaces, such as those of metals. In such cases it may occur reasonable to protect the surface first with possibly less dense but faster growing material, e.g. using physical vapor deposition, and continue with thin homogeneous layers, which may grow relatively slowly, e.g. during atomic layer deposition, thereby providing sufficiently thick and dense cap within the stacked coating offering improved mechanical and chemical protection. For instance, CrN films physical vapor deposited on low alloy steel have been sealed by 50 nm thick mixed layers of Al2O3 and Ta2O5, covering the relatively rough and pinhole-rich surface of the CrN coatings uniformly [40].

Al2O3 thin films have been grown by ALD on steel substrates to thicknesses from 10 to 100 [29], [30], [31] to 500 nm [8] with the aim to study their resistance to corrosion. It was realized that the inherent chemical instability of amorphous alumina does not provide sufficient protection against strongly corrosive environments, and, for this reason, the Al2O3 layers were further applied in the coatings on steel together with complementary materials layers, e.g. creating Al2O3-TiO2 [8] or Al2O3-Ta2O5/CrN [40] sandwiched structures. In these works, however, electrical insulating properties of the Al2O3 films were not directly addressed, neither were the films characterized in terms of their hardness and elasticity. However, if the coatings are to be developed as adhesive insulators for flexible electronics, such as displays or solar cells [2], [20] the basic dielectric performance should be evaluated together with the mechanical behavior.

This work is devoted to the atomic layer deposition of Al2O3 films on steel foil substrates. Some of the foil substrates were covered by SiO2 films by physical vapor deposition prior to the ALD process. The coatings grown to different thicknesses were evaluated electrically and by nanoindentation.

Section snippets

Experimental details

The Al2O3 films were grown in a Picosun SUNALE R-150 ALD reactor from trimethylaluminum, Al(CH3)3, and H2O. In all experiments, the substrate temperature was kept at 300 °C. The deposition cycle times were 0.1-5.0-0.1-5.0 s, corresponding to the sequence of Al(CH3)3 pulse, purge, H2O pulse and the second purge time, respectively. The number of deposition cycles was varied between 100 and 1000 (Table 1). Sandvik stainless steel foils with a thickness of 50 μm and the maximum surface area of 5 × 5 cm2

Film structure, morphology, and composition

The Al2O3 films were expectedly amorphous as determined by XRD. Fig. 1 represents optical and scanning electron microscope images taken from a 90 nm thick Al2O3 film on a steel substrate without the SiO2 intermediate layer. The images visualize the basically rough surface of the steel sample covered with 90 nm thick alumina coating. Evidently, the Al2O3 film has covered the features on the surface conformally, following the surface topology, concurrently complicating the location and

Summary

Amorphous and continuous Al2O3 thin films were grown by atomic layer deposition using Al(CH3)3 and H2O as precursors uniformly on steel surfaces, exhibiting reproducible dielectric properties. To ensure the reproducibility in terms of the laterally uniform electrical insulation, the thickness of the metal oxide had to be increased above 10 nm. The maximum nanohardness of the Al2O3 films on steel reached 12 GPa and the elastic modulus could exceed 280 GPa. The Al2O3 thin films deposited on both

Acknowledgements

The work has been supported by Finnish Centre of Excellence in Atomic Layer Deposition (Academy of Finland, project no. 284623), institutional research grant IUT24-2 (Estonian Ministry of Science and Education), Estonian Academy of Sciences, and Latvian national project on multifunctional materials and composites, photonics and nanotechnology (IMIS2).

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