Ultra-thin aluminium oxide films deposited by plasma-
enhanced atomic layer deposition for corrosion protection
Citation for published version (APA):
Potts, S. E., Schmalz, L., Fenker, M., Díaz, B., Swiatowska, J., Maurice, V., Seyeux, A., Marcus, P., Radnóczi,
G., Tóth, L., & Kessels, W. M. M. (2011). Ultra-thin aluminium oxide films deposited by plasma-enhanced atomic
layer deposition for corrosion protection.
Journal of the Electrochemical Society
,
158
(5), C132-C138.
https://doi.org/10.1149/1.3560197
DOI:
10.1149/1.3560197
Document status and date:
Published: 01/01/2011
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Ultra-Thin Aluminium Oxide Films Deposited by
Plasma-Enhanced Atomic Layer Deposition for
Corrosion Protection
S. E. Potts,
a,
*
,z
L. Schmalz,
b
M. Fenker,
b
B. Dı´az,
c
J.
Swiatowska,
c
V. Maurice,
c
A. Seyeux,
c
P. Marcus,
c
G. Radno´czi,
d
L. To´th,
d
and W. M. M. Kessels
a,
*
,z
a
Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
b
FEM Research Institute, Precious Metals & Metals Chemistry, 72525 Schwa¨bisch Gmu¨nd, Germany
c
Laboratoire de Physico-Chimie des Surfaces, CNRS (UMR 7045), E
´
cole Nationale Supe´rieure de Chimie de Paris
(Chimie ParisTech), 75005 Paris, France
d
Research Institute for Technical Physics and Materials Science, H-1525 Budapest, Hungary
We have employed plasma-enhanced and thermal atomic layer deposition (ALD) within the temperature range of 50–150
C for
the deposition of ultra-thin (10–50 nm) Al
2
O
3
films on 100Cr6 steel and aluminium Al2024-T3 alloys. [Al(CH
3
)
3
] was used as the
precursor with either an O
2
plasma or water as co-reactants. Neutral salt spray tests showed that the thicker films offered the best
corrosion-resistance. Using cyclic voltametry, the 50 nm films were found to be the least porous (<0.5%). For 10 nm thick films,
plasma-enhanced ALD afforded a lower porosity and higher film density than thermal ALD. ToF-SIMS measurements on 100Cr6
showed that the main ‘bulk’ of the films contained very few impurities, but OH and C were observed at the interfaces. TEM
confirmed that the films were conformal on all substrates and the adhesion was excellent for the films deposited by plasma-enhanced
ALD but not for thermal ALD, as delamination was observed. On the basis of these and other results, the prospects of the application
of ALD films for corrosion protection, and the use of plasma-enhanced ALD to promote their nucleation, is discussed.
V
C
2011 The Electrochemical Society. [DOI: 10.1149/1.3560197] All rights reserved.
Manuscript submitted November 19, 2010; revised manuscript received January 27, 2011. Published March 23, 2011.
Corrosion is a persistent problem for many modern high-preci-
sion applications, such as automotive
1
and aerospace
2–4
or speciality
gas handling.
5–8
Components for these applications require many
high-purity precision parts and instruments, which are commonly
manufactured from steel or aluminium alloys. As such, these parts
need to be protected by dense and defect-free coatings to prevent
corrosion. Ideally, the coatings should be ultra-thin and form a com-
plete sealing barrier to prevent contact between the alloy substrates
and the surrounding electrolytes or corrosive gases in the environ-
ment. Deposition methods used for such films have included physi-
cal vapour deposition (PVD)
9,10
and chemical vapour deposition
(CVD).
11,12
In particular, plasma-assisted CVD has been of inter-
est,
13–15
as this allows for deposition at temperatures lower than are
typical for CVD, which is beneficial when the substrates are temper-
ature-sensitive, such as heat-treated industrial alloys.
Atomic layer deposition (ALD) is potentially an ideal candidate
for this thin (<100 nm) film application, as it gives high-quality
films with excellent conformality, and provides precise thickness
control,
16
which arises from the sequential alternate dosing of metal
halide or organometallic and non-metal precursors. For metal
oxides, water and ozone are popular co-reactants, but recently oxy-
gen plasmas have seen greater potential because of the presence of
reactive ionic and radical species therein. The radicals in a plasma
provide higher surface reactivity than is possible with thermal
energy alone,
17–19
which allows for depositions at lower substrate
temperatures, even down to room temperature.
20
This is crucial for
temperature-sensitive substrates, such as heat-treated mild steel, for
example. Also, the presence of ions can lead to mild bombardment
of the film (depending on the plasma configuration and condi-
tions),
18
which contributes to increased density
19
and can thereby
potentially improve the sealing performance of the coatings. An
additional benefit of plasma-enhanced ALD is that a higher density
of reactive surface groups are obtained compared with thermal
ALD.
21
This, in turn, can lead to the higher growths per cycle; for
example, van Hemmen et al. reported a growth per cycle at 150
C
of 0.12 nm=cycle for Al
2
O
3
from [Al(CH
3
)
3
] and an O
2
plasma,
22
whereas where water was the oxygen source, the growth per cycle
was 0.09 nm=cycle in the same reactor.
Thermal ALD has been employed as a method for depositing
corrosion-resistant metal oxide layers onto stainless steel
23
and alu-
minium,
24
which improved the substrates’ resistance to acidic and
alkaline solutions, respectively. Additionally, the feasibility of
layers deposited by ALD as barrier materials has been demonstrated
on organic polymer substrates, which also require low deposition
temperatures, for use in organic LEDs.
25–28
It was shown that Al
2
O
3
deposited by ALD using water,
25,26
ozone
27
and an O
2
plasma
28
as
the co-reactant could be used to deposit thin films which sealed the
substrate. Langereis et al. reported that plasma-enhanced ALD gave
Al
2
O
3
with excellent sealing properties, where the films deposited at
room temperature afforded the lowest water vapour transmission
rate.
28
Despite this, to our knowledge, plasma-enhanced ALD has
not been employed for the coating of metal alloys for corrosion pro-
tection. As such, the primary focus of this article aims to assess the
suitability of Al
2
O
3
deposited by plasma-enhanced ALD, from
[Al(CH
3
)
3
] with an O
2
plasma, as a protective layer for 100Cr6 mild
steel and aluminium Al2024-T3 alloys, in comparison with thermal
ALD (using water as the oxygen source).
Experimental
Atomic layer deposition.— The experiments were carried out on
a commercially-built Oxford Instruments OpAL reactor located in a
clean-room. It is an open-load system, which operates with a rotary
pump. The base pressure was 1 mTorr and typical operating pres-
sures were 100–1000 mTorr. Trimethylaluminium, [Al(CH
3
)
3
]
(AkzoNobel, purity >99.9%,) was employed as the aluminium pre-
cursor and was held in a stainless steel bubbler at 24
C. Oxygen and
argon (both purity >99.999%) were flowed into the reaction cham-
ber at 50 and 20 sccm, respectively, throughout the plasma-
enhanced ALD process. Argon alone was used to purge the precur-
sor lines. The substrate temperature was 50, 100 or 150
C. For the
thermal ALD process, water (VWR, GBR Rectapur grade,
>99.999%) was also held in a water-cooled stainless steel bubbler
at 19
C to reduce the vapour pressure and prevent overdosing. Only
argon (200 sccm) was used as a process gas for the thermal ALD
experiments and 150
C was the only substrate temperature
employed. The cycle times ([Al(CH
3
)
3
] – purge – co-reactant –
purge) are as follows: plasma-enhanced ALD, 0.05–5–4–2 s; ther-
mal ALD, 0.05–5–0.02–10 s. For all ALD processes, the parameters
are outlined in Table I. The number of cycles chosen to obtain the
*
Electrochemical Society Active Member.
z
E-mail: s.e.potts@tue.nl; w.m.m.kessels@tue.nl
Journal of The Electrochemical Society, 158 (5) C132-C138 (2011)
0013-4651/2011/158(5)/C132/7/$28.00
V
C
The Electrochemical Society
C132
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desired film thicknesses was based on the growths per cycle reported
previously in our earlier work (plasma-enhanced ALD: 0.16, 0.14,
and 0.13 nm=cycle at 50, 100, and 150
C, respectively; thermal
ALD with water: 0.09 nm=cycle at 150
C).
20,22
Substrates.— 100Cr6 steel (Scha¨ffler) and Al2024-T3 aluminium
(Messier-Bugatti) alloys were employed as substrates. For the
100Cr6, fine-ground and lapped surface preparations were investi-
gated and the Al2024-T3 samples had a fine-ground surface finish.
The fine-ground surface finish was obtained using a planar grinding
machine (Kehren, type RS) using corundum (Tyrolit, 89A461H9AV2)
as a grinding medium with lubricant (Hosmac 928). The lapped surface
finish was obtained upon further treatment of the fine-ground surfaces
using an Elap 400 machine with a water-based diamond suspension
(6 lm, LamPlan Diamant MM342) for 2 h. After preparation, the
substrates were dried in compressed nitrogen (99.999%) and coated in
silicon oil (Sigma-Aldrich GmbH, no. 85409). Prior to transportation,
the samples were packaged in membrane boxes (Inca S. A. Plastic) to
prevent excess movement. Coated samples were also sent in the mem-
brane boxes for analysis. Before deposition, the required substrates
werecleanedinanultrasonicisopropanol (Emplura grade, Merck
Chemicals International) bath for 5 min and then blow-dried in room-
temperature nitrogen. Reference substrates were used in each process,
which were
1
=
4
100 mm n-type Si(100) wafers. These wafers were cov-
ered with a thin native oxide (SiO
2
, 1.5 nm) layer and did not
undergo any additional cleaning steps.
Spectroscopic ellipsometry (SE).— A J. A. Woollam Inc. M2000
rotating compensator ellipsometer on a multi-angle Gonio stage
(angles used: 65, 70, 75, and 80
) was employed to confirm the
thickness and refractive index of the films on the reference Si sub-
strates. Modelling was carried out using CompleteEASE software.
The model layers comprised Si, native SiO
2
and Al
2
O
3
. The Al
2
O
3
layer was modelled using a Cauchy layer over the wavelength range
of 190–1000 nm (1.2–6.5 eV). Further details on the SE technique
and modelling can be found in the literature.
29
Electrochemical tests.— All electrochemical tests were per-
formed using a 3-electrode system with a Pt counter electrode and a
SCE reference electrode at ambient temperature (20
C). The elec-
trolytes were 0.5 M NaCl for the Al2024-T3 substrates and 0.2 M
NaCl for the 100Cr6 substrates, prepared with ultra-pure water (re-
sistivity >18 MX cm) and reagent grade chemicals (NaCl Analar
Normapur analytical reagent VWR BDH Prolabo). These solutions
were de-aerated by bubbling argon gas through them for 30 min prior
to the measurements. The pHwas7.26 0.2 prior to the corrosion
test. The CV tests were performed afterwards from a cathodic vertex
and in a potential range depending on the substrate and its susceptibil-
ity to pitting corrosion. All CV curves (including the reverse cathodic
scan) were obtained at a scan rate of 1 mV s
1
. The electrochemical
measurements on 100Cr6 substrates were performed on polished sam-
ples and Al2024-T3 substrates on fine-ground surfaces.
Transmission electron microscopy (TEM).— TEM analysis was
carried out using a 200 kV Philips CM20 analytical TEM equipped
with an EDS detector. Prior to the analysis, all samples were thinned
by a standard mechanical grinding=ion bombardment technique.
This involved cutting the substrates, embedding them into a custom-
made Ti-holder, mechanical grinding and polishing, followed by 10
keV Ar ion milling. The final step was carried out at lower ion
energy of 3 keV in order to decrease the surface damage of the
thinned TEM specimens.
Time-of-flight secondary ion mass spectrometry (ToF-
SIMS).— Elemental depth profiles were obtained using a ToF-SIMS
5 spectrometer (IonTof). The spectrometer was run at an operating
pressure of 10
9
mbar in HC-BUNCHED mode. A pulsed 25 keV
Bi
þ
primary ion source was employed for analysis, delivering 1.8 or
0.86 pA of current over a 175 175 lm or 100 100 lm area for
the 50 and 10 nm thick coatings, respectively. Depth profiling was
performed using a 2 keV sputter beam giving a 100 or 82 nA target
current over a 400 400 lm or 1000 1000 lm area for the 50 and
10 nm thick coatings, respectively. Data acquisition and post-proc-
essing analyses were performed using the Ion-Spec software. The
profiles were recorded with negative secondary ions, which are
more sensitive to fragments originating from oxide matrices.
Neutral salt spray (NSS) testing.— Neutral salt spray tests were
performed according to the standard DIN 50021, ISO 9227 but with
the deviation that the samples were rinsed in deionised water and
dried with a hair dryer (warm, not hot air) before photographs were
taken of the samples. The electrolyte was 50 6 5gL
1
(0.86 M)
NaCl with a pH value in the range of 6.5–7.2. The tests were carried
out at a temperature of 35 6 2
C. Photographs were taken of the
samples after 2, 4, and 24 h testing. If no, or only negligible, corro-
sion products were visible, the samples were transferred back to the
salt spray test chamber and the test was continued until significant
red rust was visible on the 100Cr6 steel samples or white rust on the
Al2024-T3 substrates, after which the samples were removed from
the test chamber.
Results and Discussion
Al
2
O
3
films were deposited on industrial alloys using plasma-
enhanced and thermal ALD. For the coatings deposited by plasma-
enhanced ALD, the corrosion characteristics were tested by con-
ducting a thickness series of films of nominally 10, 20, and 50 nm
on lapped 100Cr6 steel and fine-ground Al2024-T3 substrates at a
deposition temperature of 150
C. Additionally, a temperature series
was carried out at temperatures of 50, 100 and 150
C, focussing on
10 nm thick films. Higher temperatures could not be used due to the
heat-treatment of the steel. Coatings deposited by thermal ALD (at
150
C only) with thicknesses of 10 and 50 nm were deposited on
100Cr6 and Al2024-T3 for comparison with the plasma-enhanced
process.
The film thicknesses and refractive indices, measured by SE, are
outlined in Table I. These measurements were taken from the n-type
Si reference substrate included in the reaction chamber during the
depositions. For the samples deposited by plasma-enhanced ALD,
Table I. The experimental conditions used, and the film thicknesses and refractive indices measured using spectroscopic ellipsometry (SE) on an
n-type Si substrate.
ALD
method
Deposition
temperature
(
C)
Number of
ALD cycles
Nominal
thickness
(nm)
SE
thickness on
Si (60.05 nm)
SE
refractive index
at 630 nm (60.02)
Plasma-enhanced 50 55 10 10.3 1.56
— 100 69 10 11.0 1.57
— 150 76 10 10.9 1.58
— 150 151 20 20.6 1.58
— 150 385 50 50.9 1.62
Thermal 150 112 10 10.9 1.58
— 150 556 50 54.0 1.61
Journal of The Electrochemical Society, 158 (5) C132-C138 (2011) C133
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the thicknesses were measured to be within 1 nm of the nominal
thickness, while for the 50 nm thick film for thermal ALD, the thick-
ness was 4 nm higher than intended. The refractive indices
increased slightly with increasing film thickness and deposition tem-
perature, suggesting a better bulk quality material. This is confirmed
by previously-reported Rutherford backscattering results, which
showed that films deposited at higher temperatures had higher den-
sities,
20
which was attributed to there being a lower concentration of
OH groups in those films.
CV porosity.— The porosity, which can be thought of as the per-
centage of uncoated film surface, was measured by cyclic voltame-
try.
30
For the films deposited by plasma-enhanced ALD on 100Cr6,
the porosity was highest for the thinnest films, at 0.6%, with the
porosity decreasing with increasing thickness to 0.07% for 50 nm
thick films (see Fig. 1a). Where thermal ALD was used, the porosity
was an order of magnitude higher for the 10 nm films, suggesting
that nucleation and film densification was not as efficient using this
process, leaving 3.65% of the 100Cr6 surface uncoated. The case
was seemingly reversed for the 50 nm films, although the porosity
of the films deposited using thermal ALD varied greatly over the
samples, as shown by the error bar; therefore, the porosities of the
films deposited by the two techniques were considered equivalent at
a thickness of 50 nm. The thicker films have a higher probability of
covering surface features more efficiently due to the greater amount
of material being deposited. This thickness dependence could possi-
bly be resolved by using a surface pre-treatment.
On Al2024-T3 (Fig. 1b), the porosities were generally signifi-
cantly higher, suggesting that there were significant nucleation prob-
lems for these substrates and=or coating delamination occurring
during the electrochemical testing as a result of poor adhesion.
However, some of porosity values (for example, 56%) are unrealisti-
cally high to be considered as porosities, which might suggest that
dissolution of the film during the measurements was occurring for
such a large surface area to be seemingly bare. However, the trend
was similar to the 100Cr6 substrates, where there was a general
decrease in porosity with increasing film thickness. Again for the
thermal ALD, the porosity of the films was higher than the plasma-
enhanced method for the 10 nm thick films (56 and 30%, respec-
tively) but equivalent for the 50 nm thick films (10%).
The temperature series was carried out for 10 nm thick films de-
posited by plasma-enhanced ALD (Fig. 2). For 100Cr6 steel, there
was an almost linear decrease in the film porosity with increasing
deposition temperature. It is known that Al
2
O
3
films grown at lower
temperatures tend to have lower densities, primarily due to the
increased incorporation of OH groups.
20,22
Such densities leave the
film prone to having a higher concentration of defects, therefore
higher porosities, through which corroding media can travel. The
plasma-enhanced method did, however, give a much lower film po-
rosity (at 150
C) than the corresponding thermal ALD process,
which is attributed mainly to the increased film density resulting
from the higher reactivity provided by the plasma and the improved
nucleation properties of the films. It should be noted that the thermal
ALD process was not carried out at deposition temperatures lower
than 150
C due to the long purge times required when using water;
for example, Groner et al. reported necessary water purge times of
180sat33
C.
31
At 150
C, we found that only a 10 s water purge
was necessary for thermal ALD, but for plasma-enhanced ALD, 2 s
was sufficient for all temperatures.
For Al2024-T3 substrates, the opposite trend was observed,
whereby the lowest deposition temperatures afforded the lowest film
porosities (12%), which almost doubled at 150
C. However, in a
similar manner to the 100Cr6, the porosity for the thermal ALD
films was substantially higher than that for the plasma-enhanced
ALD films deposited at 150
C.
TEM imaging.— Cross-sectional bright field TEM images
showed that the Al
2
O
3
deposited by both plasma-enhanced and ther-
mal ALD were conformal on fine-ground Al2024-T3 (Fig. 3) and
lapped 100Cr6 (Fig. 4). For the films deposited by plasma-enhanced
ALD on Al2024-T3, the coating was present even in deep cracks in
the substrate (Fig. 3), which shows that the O
2
plasma radicals can
reach down trenches. Radicals formed in a plasma are known to
Figure 1. (Color online) Variation of film porosity as a function of film
thickness for Al
2
O
3
films deposited at 150
C on (a) lapped 100Cr6 and (b)
fine-ground Al2024-T3. Note the break in the vertical axis for 100Cr6.
Figure 2. (Color online) Variation of film porosity as a function of deposi-
tion temperature for 10 nm thick Al
2
O
3
films deposited by plasma-enhanced
ALD on (a) lapped 100Cr6 and (b) fine-ground Al2024-T3. The porosity for
films deposited by thermal ALD at 150
C is given for comparison.
Journal of The Electrochemical Society, 158 (5) C132-C138 (2011)C134
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recombine at surfaces, although oxygen radicals have a low recom-
bination probability on metal oxides.
19,32
Therefore, it is likely that
the native oxide on the Al2024-T3 allows the high aspect ratio
trench to be coated.
For all thermal ALD samples, there was a void present (20 nm)
between the substrate and coating (filled with glue used in the TEM
preparation), suggesting that the Al
2
O
3
film was not well adhered.
The same was the case on 100Cr6 substrates (Fig. 4), where the film
was separated from the substrate by 8 nm. The use of a plasma in
the ALD process therefore has a beneficial influence on the adhesion
of the film, which can be crucial for the coating’s resistance to
corrosion.
ToF-SIMS depth profiles.— Time-of-flight secondary ion mass
spectroscopy studies were carried out on nominally 50 and 10 nm
thick Al
2
O
3
films on lapped 100Cr6 substrates in order to compare
the plasma-enhanced and thermal ALD techniques. For the 50 nm
thick samples (Fig. 5), there was no significant difference in the
width of the interfacial region between the film and the substrate.
This was not unexpected as this width is mostly governed by the
roughness of the alloy surface, which causes shadowing effects dur-
ing sputtering. Consequently, the slight differences in film thickness
(Table I) between the plasma-enhanced and thermal ALD processes
could not be resolved. In terms of the composition of the interfacial
region, the signals of the FeO
2
–
and CrO
2
–
ions were slightly lower
for the plasma-enhanced process, suggesting less contamination by
the substrate oxide. However, the C
–
and OH
–
signals were compa-
rable for both samples. In the bulk of the coatings there was a lower
concentration of OH for the plasma-enhanced process in the later
stages of deposition, which is a result of the higher reactivity of the
plasma than water in the ALD process, causing film densification
when the influence of interfacial contamination decreased. Film
densification has been verified by Rutherford backscattering data on
Al
2
O
3
films, which has shown that films with lower OH concentra-
tions have a higher density, and that films deposited by plasma-
enhanced ALD are slightly denser than those deposited by thermal
ALD.
22
Our ToF-SIMS data show that this effect is more sensitive
away from the interfacial region. However, the presence of OH in
the films is not necessarily detrimental to the coating efficiency
since the lowest porosity was obtained with the 50 nm thick thermal
ALD sample (which contained the more OH in the bulk of the coat-
ing). A similar trend has been observed in the use of Al
2
O
3
as a pro-
tective coating for organic LEDs,
28
where the films containing more
OH (which were deposited by plasma-enhanced ALD at room tem-
perature) exhibited better moisture-barrier properties than films de-
posited at higher temperatures.
Regarding the coating surface, Fig. 6 shows that OH was present
further below the surface for the plasma-enhanced ALD samples
(up to 30 s sputtering) than for the thermal ALD sample (10 s
sputtering), although the overall concentration of OH in the former
was lower. This surface variation appears to be related to the AlO
2
–
ion profile (Fig. 5), which shows a hump in the same near-surface
region of the plasma-enhanced ALD sample, suggesting a deeper
penetration of surface hydroxylation. This could be due to environ-
ment-induced modifications occurring during ex situ transfer for
sample analysis. It is likely that this is promoted by the higher den-
sity of reactive surface sites on the plasma-enhanced ALD samples
compared with those deposited by thermal ALD. A higher density
of reactive surface groups is a characteristic of plasma-enhanced
ALD and is one of the main contributing factors to the higher
growths per cycle obtained.
21
For the 10 nm thick samples, the difference between the two depo-
sition methods is significantly more prominent (Fig. 7). For the film
deposited by plasma-enhanced ALD, a sputtering time of 400 s was
needed to reach the interface, whereas for thermal ALD, only 110 s
was required (both of these samples were analysed with the same
Figure 3. Bright-field TEM image of 20 nm Al
2
O
3
deposited on Al2024-
T3 by plasma-enhanced ALD at 150
C. Inset: close-up of the trench area.
Figure 4. Bright-field TEM image of 55 nm Al
2
O
3
deposited on 100Cr6
by thermal ALD at 150
C, showing a void filled with glue, from the TEM
sample preparation, between the coating and the substrate.
Figure 5. (Color online) ToF-SIMS depth profiles for 50 nm Al
2
O
3
depos-
ited at 150
C on 100Cr6 using (a) plasma-enhanced ALD and (b) thermal
ALD.
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