Electrochemical and Surface Analysis of the Corrosion Protection of Copper by Nanometer-Thick Alumina Coatings Prepared by Atomic Layer Deposition

Abstract

Corrosion protection by ultra-thin (≤ 50 nm) alumina films deposited by atomic layer deposition (ALD) on copper at 250 • C was studied in 0.5 M NaCl aqueous solution by combining electrochemical and surface analytical methods. Time-of-Flight Secondary Ion Mass Spectrometry revealed a homogenous in-depth stoichiometry of the alumina coatings and their residual bulk contamination by organic and water precursor fragments. Trace contamination by a spurious Cu oxide was evidenced at the interface with the Cu substrate despite a reducing pre-treatment. Atomic force microscopy images revealed conformal substrate coverage and a granularlike coating morphology at the nanometer scale. Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS) showed no electrochemical activity of the coating and a corrosion decrease reaching 4 orders of magnitude for coated samples in comparison with bare copper. Uncoated surface fraction was quantified by application of three different methods using the LSV and EIS data which led to results in excellent agreement. The uncoated surface fraction for 50 nm Al 2 O 3 layers was ∼0.01%, confirming ALD alumina as an excellent coating for corrosion protection with spectacularly low residual corrosion activity restricted to the bottom of channel defects connecting the substrate to environment through the coating.Copper is an important engineering metal and finds widespread application in heat exchangers, electrical power lines, water utilities, oil refineries, marine environments and microelectronics owing to its high electrical and thermal conductivity and mechanical workability. 1-3 Many studies have been dedicated to the copper corrosion mechanisms, especially in presence of aggressive ions like chloride, 4-8 and various corrosion protection methods have been explored including coating techniques such as Chemical Vapor Deposition (CVD), 9,10 plasma deposition 11 or electrochemical polymerization, 12 and the use of inhibitors 13-16 or self-assembled monolayers. 17,18 In the present study, Atomic Layer Deposition (ALD) of ultra-thin (≤ 50 nm) alumina coatings has been investigated for the corrosion protection of copper. ALD, as a CVD derived method, is an ideal technique for growing high quality nanometer-thick films. It takes advantage of sequential precursor pulsing on the substrate with selflimiting saturative steps to produce conformal films with accurate thickness control. As a result of excellent step-coverage on even difficult substrate surfaces with high aspect ratios, the grown films tend to be conformal and pinhole-free. [19][20][21][22] Alumina (Al 2 O 3 ) was chosen as a well-behaved and thoroughly-studied ALD material by virtue of its high dielectric properties, good adhesion to many surfaces, amorphous structure, thermal stability and mechanical properties. 21,23-25 ALD alumina for corrosion protection has already been studied alone and/or in combination with titania on stainless steel, 23,26-28 Mg alloy, 29 Al alloy, 30 silver 31,32 and copper. 33-36 Nanolamina...

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis

“…They are indicative of silicon oxide contamination at the substrate surface. Similar residual contamination of the substrate surface by oxides has been also reported in the case of alumina, chromia or tantala protection layers grown by atomic layer deposition or filtered cathodic arc deposition on low-alloy steel, [53][54][55] stainless steel 56 and copper 57,58 substrates. One notices that, like for the Agions, the relatively high intensity of the SiO2ions in the substrate region is consistent with surface oxidation of the substrate surface pores.…”

Investigation of the Corrosion Behavior of Atomic Layer Deposited Al2O3/TiO2 Nanolaminate Thin Films on Copper in 0.1 M NaCl

“…Passivation in this manner can greatly increase initial corrosion resistance. This is evident in the work of Mirhashemihaghighi et al [ 80 , 93 ], in which polarization resistances on the order of 100 MΩ cm 2 are measured without an electroreduction step during immersion in 0.5 M NaCl. Also, this is apparent in Figure 12 , where the lack of a reduction step leads to a two order of magnitude increase in impedance of Al 2 O 3 -coated copper.…”

“…TiO 2 and ATx1 exhibit an anodic shift in corrosion potential from the uncoated copper. This has been attributed to modification of the substrate exposed through coating defects [ 55 ], as well as native oxide formation during ALD growth and aging of the oxide during storage [ 80 ]. The remaining corrosion potentials are shifted cathodically, a trend that has also been reported for ALD coatings on stainless steel [ 75 , 84 ], copper [ 79 ], and a Mg-Al alloy [ 83 ].…”

“…This suggests either incomplete removal of dissolved oxygen, coupled with a very low threshold for oxygen reduction to occur, or the generation of hydroxide ions through other means, possibly water reduction (see Equation (2)). Both scenarios likely contribute: incomplete removal of dissolved oxygen has been proposed in the past [ 80 ] and some water reduction should occur at the high cathodic overpotentials to which the samples are exposed. However, even in the absence of the reduction step (at −1.5 V) the ALD alumina film degrades similarly according to EIS spectra, displayed in Figure 12 .…”

“…Previous studies of ALD films for corrosion protection of copper have mostly focused on aluminum oxide. Mirhashemihaghighi et al [ 80 ] reported a 7x increase in polarization resistance over uncoated, polished copper with a 10 nm ALD Al 2 O 3 film and a three order of magnitude increase with a 50 nm Al 2 O 3 film in deoxygenated 0.5 M NaCl. Chai et al [ 81 , 82 ] found better corrosion protection with increasing alumina film thickness in aerated 0.1 M NaCl, as did Daubert et al [ 79 ].…”