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Morphological and microstructural characterization of nanostructured pure α-phase W coatings on a wide thickness range

15 Oct 2014-Applied Surface Science (North-Holland)-Vol. 316, pp 1-8

Abstract: Nanostructured tungsten (nanoW) coatings have been deposited by DC magnetron sputtering. First, the influence of the sputtering power on the adhesion of the coatings to the substrate was investigated by depositing coatings at powers varying from 30 up to 220 W. Non-delaminated coatings were achieved at powers ≤50 W. Second, the influence of coating thickness on the morphological, microstructural and mechanical properties was investigated for films deposited at 50 W with thicknesses varying from 30 nm up to ∼4.0 μm. SEM images reveal that all the films are highly compact, consisting of nanometer sized columns that grow perpendicular to the substrate. XRD data evidence that films are monophasic, being made of pure α-phase. All coatings show compressive stress and low micro-strain. Nanoindentation tests show that coatings have a hardness higher than that reported for coarse grained W. No significant dependence of the previous properties on coating thickness was observed. Finally, the influence of the substrate on coatings properties was studied, by depositing a W coating at a power of 50 W on a commercial steel substrate: no significant dependence was found.
Topics: Coating (59%), Sputter deposition (55%), Nanoindentation (53%), Sputtering (52%)

Summary (2 min read)

1. Introduction

  • Based on Thornton works, the authors aim to optimize the sputtered W coatings by tuning the sputtering parameters.
  • The authors report on the DC sputtering parameters which lead to nondelaminated, pure ␣-W phase coatings.
  • The dependence of sample morphology, microstructure, stress/strain state and mechanical properties as a function of thickness will be discussed.
  • In addition, a preliminary study about W deposition on commercial steel substrate was performed due to the interest for possible applications in the industry.
  • In particular, coatings were deposited on steel substrate since they could be very attractive for the nuclear industry [5] [6] [7] [8] [9] , mainly because of two reasons: (i) the fact that thick, non-delaminated coatings can be deposited at room temperature on steel without any special surface treatment and/or sacrificial layer, (ii) the self-healing behavior, which is supposed for nanostructured coatings when operating under radiation environment.

2. Experimental procedure

  • In order to evaluate the mechanical response of the coatings, nanoindentation tests were carried out by using a MTS Nanoindenter XP and the continuous stiffness measurement technique together with a diamond Berkovich indenter.
  • Hardness and elastic modulus were obtained by an indentation depth of 10-15% of the coating thickness to avoid the influence of the substrate.
  • These values were determined from the unloading part of the force-depth (F-d) curve according to the procedure reported in the literature [27] .

3.1. Optimization of the sputtering parameters to achieve non-delaminated coatings

  • According to these data, depending on the sputtering power, two regions can be identified: Region I (sputtering power ≤50 W), samples are adhered to the substrate, and Region II (sputtering power >50 W), samples exhibit delamination.

3.2. Thickness dependence of the morphology, microstructure and stress state of the W coatings

  • Once deposition parameters were optimized to achieve adhered coatings, the influence of coating thickness on the morphological and microstructural properties as well as, on residual stress, microstrain and mechanical properties of the films was studied.

3.2.1. Morphology

  • Additional information about the compactness of the coatings can be obtained from density measurements.
  • The average coating density, estimated by combining RBS and SEM measurements, was calculated to be 19.50 ± 0.30 g/cm 3 .
  • This density value is similar to that tabulated for pure coarse grained W (19.25 g/cm 3 ) [31] , which indicates that the authors are dealing with highly compacted coatings.

3.2.2. Microstructure

  • A deeper analysis of the XRD patterns allows us to obtain more information about the film total stress and micro-strain.
  • The film total stress was evaluated by analysing the centroid ␣-W{1 1 0} peak profile in comparison to that reported for the conventional W, Fig. 6(a) .
  • The Hardness, Young's modulus and loaddisplacement curves for the sample deposited on steel are depicted in Fig. 9 (a) and (b), respectively, as a function of indentation depth.
  • The hardness and Young's modulus values are quite similar to those for the coatings deposited on (1 0 0) Si being 14.9 ± 0.3 GPa and 338 ± 16 GPa, respectively.
  • On these bases, the authors can conclude that the morphological, microstructural and mechanical properties for coatings deposited on commercial steel are comparable to those for coatings sputtered on (1 0 0) Si.

4. Conclusions

  • All presented results and the fact that coatings can be manufactured on steel substrates, suggest that the nanoW could be a good candidate for industrial applications.

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Morphological and microstructural characterization of nanostructured pure α-phase W coatings on a
wide thickness range
N. Gordillo
a,b,
, M. Panizo-Laiz
a
, E. Tejado
c,d
, I. Fernandez-Martinez
e,f
, A. Rivera
a
,
J.Y. Pastor
c
, C. Gómez de Castro
g
, J. del Rio
h
, J.M. Perlado
a
, R. Gonzalez-Arrabal
a
a
Instituto de Fusión Nuclear, ETSI de Industriales, Universidad Politécnica de Madrid, C/José Gutierrez Abascal, 2, E-28006 Madrid, Spain
b
CEI Campus Moncloa, UCM-UPM, Madrid, Spain
c
Department of Materials Science, Research Centre on Safety and Durability of Structures and Materials (CISDEM), UPM-CSIC, C/Profesor Aranguren s/n,
E-28040 Madrid, Spain
d
Centro Nacional de Investigaciones Metalúrgicas, CENIM-CSIC, Madrid, Spain
e
Instituto de Energía Solar (IES), Universidad Politécnica de Madrid, Avenida Complutense s/n, E-28040 Madrid, Spain
f
Instituto de Microelectrónica de Madrid, IMM-CNM-CSIC, Isaac Newton 8 PTM, Tres Cantos, E-28760 Madrid, Spain
g
Departamento de Física de Materiales, Facultad de CC. Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain
h
Departamento de Física de Materiales, Facultad de CC. Físicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, E-28040 Madrid, Spain
Keywords:
Sputter deposition
Nanostructured tungsten
Micro-/macro-stresses
Nanoindentation
a b s t r a c t
Nanostructured tungsten (nanoW) coatings have been deposited by DC magnetron sputtering. First, the influence of the sputtering power on the
adhesion of the coatings to the substrate was investigated by depositing coatings at powers varying from 30 up to 220 W. Non-delaminated coatings
were achieved at powers 50 W. Second, the influence of coating thickness on the morphological, microstructural and mechanical properties was
investigated for films deposited at 50 W with thicknesses varying from 30 nm up to 4.0 m. SEM images reveal that all the films are highly compact,
consisting of nanometer sized columns that grow perpendicular to the substrate. XRD data evidence that films are monophasic, being made of pure -
phase. All coatings show compressive stress and low micro-strain. Nanoindentation tests show that coatings have a hardness higher than that reported
for coarse grained W. No significant depend-ence of the previous properties on coating thickness was observed. Finally, the influence of the substrate on
coatings properties was studied, by depositing a W coating at a power of 50 W on a commercial steel substrate: no significant dependence was found.
1. Introduction
Due to its properties (high melting point, low vapor pres-
sure, low physical and chemical sputtering yields, low thermal
expansion, electrical conductive properties and relative chemi-
cal inertness) tungsten- and tungsten alloy-based thin films and
coatings are attractive for applications such as diffusion barriers in
integrated circuits [1,2], radiation shielding [3], and kinetic energy
penetrators [4]. Moreover, nowadays, W-based materials is consid-
ered the best candidate as plasma facing material in both magnetic
[5–8] and laser fusion reactors [9].
W thin films have been deposited by different methods, such as
electron beam, arc deposition, pulsed laser deposition, molecular
Corresponding author at: Universidad Politécnica de Madrid, Institute of Nuclear
Fusion, Jose Gutierrez Abascal 2, 28006 Madrid, Spain. Tel.: +34913363110.
E-mail address: nuri.gordillo@gmail.com (N. Gordillo).
beam epitaxy, high-pressure torsion [10–14], being magnetron
sputtering one of the most suitable for industrial purposes. Sput-
tered W films usually present large internal stresses [15,16] which
can hamper their industrial applications because of film crack-
ing and loss of adhesion to the substrate. Therefore, the control
of stress is of high technological interest for further implementa-
tion of W coatings. It is worthwhile to mention that stress becomes
a critical point when the film thickness increases since typically
larger stresses are observed for thicker films [17]. Indeed, Maier
et al. [18] reported that without specific substrate surface treat-
ment, sufficient adhesion to the substrate was observed only for W
coatings thinner than 1 m. Different approaches have been used
to reduce stress in W thin films by using different methods: RF-
substrate biasing [19], plasma etching of the substrate [18], using
a Cr sticking layer before the deposition of W [20]. However, most
of these methods require double deposition procedure and plasma
treatments which make somehow difficult the film deposition pro-
cedure. Recently, Karabacak et al. [21,22] have shown that in-situ

stress reduction in W films is possible by using a nanostructured W
compliant layers sandwiched between the film and the substrate. In
this case, the compliant layer has been deposited by oblique angle
deposition. This way of reducing stress seems to be very promis-
ing but still requires rotation of the substrate during deposition.
On the other hand, as illustrated by Thornton [23], the stress state
of a sputtered film can be tuned to the desired value by changing
the deposition parameters (i.e. substrate temperature, working gas
pressure, plasma power or chamber geometry).
Based on Thornton works, we aim to optimize the sputtered
W coatings by tuning the sputtering parameters. In this paper,
we report on the DC sputtering parameters which lead to non-
delaminated, pure -W phase coatings. The dependence of sample
morphology, microstructure, stress/strain state and mechanical
properties as a function of thickness will be discussed. In addi-
tion, a preliminary study about W deposition on commercial steel
substrate was performed due to the interest for possible applica-
tions in the industry. In particular, coatings were deposited on steel
substrate since they could be very attractive for the nuclear indus-
try [5–9], mainly because of two reasons: (i) the fact that thick,
non-delaminated coatings can be deposited at room temperature
on steel without any special surface treatment and/or sacrificial
layer, (ii) the self-healing behavior, which is supposed for nano-
structured coatings when operating under radiation environment.
In this work, the substrate influence on sample morphology and
microstructure is investigated.
2. Experimental procedure
In order to study the plasma power influence on the adhe-
sion to the substrate of the W coating, a first set of samples was
deposited on (1 0 0) Si at different sputtering plasma power varying
from 30 W up to 220 W while keeping constant the Ar gas pressure
(8 ×10
3
mbar) and the target-substrate distance (8 cm).
Once the plasma power that leads to well adhered coatings was
found out, the second objective was to study the influence of coat-
ing thickness on morphology, microstructure, micro-strain/stress
state and mechanical properties. For this purpose, a second set of
samples with thicknesses ranging from 30 nm to 4.0 m was sput-
tered at a plasma power 50 W, known to produce well adhered
coatings and the maximum deposition rate, which is very desirable
for industrial purposes.
The deposition setup consists of a high vacuum chamber (main
chamber) with a base pressure in the 10
8
mbar range, a lock-in
chamber, and a 2 inch diameter magnetron designed and manu-
factured by Nano4Energy SLNE [24]. The lock-in chamber allows
loading the samples in the main chamber without destroying the
high vacuum, which is very relevant for the purity of the samples.
Coatings were deposited from a pure W commercial target (99.95%)
in the presence of a high pure Ar atmosphere (99.9999%). Deposi-
tion took place at room temperature (RT). Before each growth the
W cathode was cleaned by argon plasma etching during 5 min in
order to avoid contamination by the possible target oxidation. It is
important to note that the deposition setup was only devoted to
the fabrication of W coatings.
For transmission electron microscope (TEM) observation a spe-
cially designed nanoW film with a thickness of about 30 nm was
deposited onto BO
x
coated Si substrates. The BO
x
layer with a thick-
ness of 100 nm was evaporated in the same experimental setup by
a Joule heating filament on a (1 0 0) Si substrate. After deposition,
the sample was immersed in ultra pure water to dissolve the BO
x
layer and the remaining nanoW thin film was captured on a TEM
grid for subsequent observation.
In order to study the possible influence of substrate material on
coating morphology, a sample with a thickness of 1800 nm was
deposited at a plasma power of 50 W on commercial steel substrate.
Prior to deposition, the steel substrate was mechanically polished
by using napless synthetic cloth. For the first rough polishing step a
0.5 m colloidal alumina was used. The second polishing was done
by using a 0.03 m colloidal alumina.
Before deposition, all substrates underwent cleaning processes
which consist of the following steps: (i) washing the substrate with
ultra-pure water soap solution, (ii) bathing in acetone during 5 min,
(iii) immersion in an isopropanol ultrasound bath, and finally, (iv)
drying by blowing with nitrogen gas.
The coating morphology and topography were characterized by
optical microscope, field emission gun-scanning electron micro-
scope (FEG-SEM) and atomic force microscope (AFM). FEG-SEM
images were taken by using a JEOL JSM 6335FZEISS AURIGA micro-
scope (Carl Zeiss, Oberkochen, Germany) operated at different
acceleration voltages from 1 to 5 kV. AFM was operated in the
dynamic mode (Dulcinea control system from Nanotec Electrónica
S.L.). Topography data and root mean square (rms) roughness were
analyzed by the free software WsxM, from Nanotec Electrónica S.L.
[25].
TEM measurements were performed with a JEOL JEM 2100
microscope operated at 200 kV and equipped with an X-ray energy
dispersive spectroscopy detector.
The thickness of the samples was initially measured by pro-
filometer (Veeco Dektak Profilometer) and further verified by
cross-sectional FEG-SEM images.
Structural characterization of the samples was carried out
by X-ray diffraction (XRD) using a Philips X-PERT four cycle
diffractometer with a Cu
K
( = 0.15405 nm) radiation source. XRD
measurements were performed in Bragg–Brentano geometry.
The elemental composition and the areal density of the coatings
(at/cm
2
) were characterized by proton induced X-ray emission
(PIXE) and Rutherford backscattering spectroscopy (RBS), respec-
tively. PIXE and RBS measurements were performed at the Institute
of Ion Beam Physics and Materials Research (Helmholtz-Zentrum
Dresden-Rossendorf) [26] by using a H
+
beam at an energy of
1.2 MeV impinging perpendicular to the sample surface. The char-
acteristic X-ray emission was collected by a Li-doped Si detector
located at 120
to the beam direction. Backscattered ions were
detected by a standard Si-barrier detector located at an angle of
170
with respect to the beam direction.
In order to evaluate the mechanical response of the coatings,
nanoindentation tests were carried out by using a MTS Nanoin-
denter XP and the continuous stiffness measurement technique
together with a diamond Berkovich indenter. Hardness and elastic
modulus were obtained by an indentation depth of 10–15% of the
coating thickness to avoid the influence of the substrate. These val-
ues were determined from the unloading part of the force-depth
(Fd) curve according to the procedure reported in the literature
[27]. A minimum of fifteen indentations were done on each sample.
3. Results and discussion
3.1. Optimization of the sputtering parameters to achieve
non-delaminated coatings
The influence of sputtering power on the adhesion of the
coatings to the substrate was studied by varying the sputtering
power from 30 to 220 W while keeping fixed other sputtering
parameters (i.e. gas pressure, cathode-substrate distance and sub-
strate temperature). A brief overview of the sputtering parameters,
deposition rate, sample thickness and adhesion to the substrate is
illustrated in Table 1.
The adhesion of the coatings to the substrate is firstly inves-
tigated by optical microscopy. In Fig. 1, the images for two

Fig.
1.
Optical
images
of
the
surface
for
samples
deposited
on
(1
0
0)
Si
at
different
plasma
power.
(a)
Delamination
and
cracks
are
evident
for
the
sample
deposited
at
160
W
(sample
II).
(b)
Smooth
surface
is
seen
for
the
deposited
at
48
W
(sample
V).
(c)
Cross
sectional
SEM
images
for
a
coating
with
a
thickness
of
4000
nm
deposited
at
46
W
(sample
VII).
representative
samples
(II
and
V)
are
shown.
For
films
deposited
at
sputtering
power
larger
than
50
W,
the
surfaces
are
clearly
delami-
nated.
As
an
example,
an
optical
image
for
a
film
deposited
at
160
W
is
shown
in
Fig.
1(a).
In
this
figure
large
color
contrast
is
observed.
Grey
color
regions
correspond
to
parts
of
the
sample
where
W
is
still
present
(scarcely
sticking)
whereas
black
regions
correspond
to
naked
Si
surface
after
W
has
been
completely
peeled
off.
Sim-
ilar
delamination
behavior
for
W
has
been
previously
reported
to
be
due
to
the
high
compressive
stress
stored
in
this
kind
of
films
[21,22]
related
mainly
to
the
“atomic
peening”
process
[23].
For
films
deposited
at
sputtering
power
50
W,
no
delamination
is
observed,
here
the
optical
image,
Fig.
1(b),
reveals
homogeneous
surface,
which
is
a
first
indication
of
adhesion
to
the
substrate.
In
order
to
have
a
deeper
insight
about
the
adhesion
of
the
coating
to
the
substrate,
SEM
measurements
were
carried
out.
A
typical
cross-sectional
SEM
image
of
a
layer
deposited
at
46
W
is
shown
in
Fig.
1(c).
In
this
figure
a
sharp
but
net
interface
between
the
W
coat-
ing
and
the
(1
0
0)
Si
substrate
is
seen.
Thus,
SEM
data
corroborate
optical
microscope
images,
illustrating
that
the
interface
between
coatings
and
substrate
is
good.
Indeed,
it
is
worthwhile
to
mention
that
coatings
sputtered
at
powers
50
W
pass
the
scotch
tape
test.
According
to
these
data,
depending
on
the
sputtering
power,
two
regions
can
be
identified:
Region
I
(sputtering
power
50
W),
samples
are
adhered
to
the
substrate,
and
Region
II
(sputtering
power
>50
W),
samples
exhibit
delamination.
3.2.
Thickness
dependence
of
the
morphology,
microstructure
and
stress
state
of
the
W
coatings
Once
deposition
parameters
were
optimized
to
achieve
adhered
coatings,
the
influence
of
coating
thickness
on
the
morphological
Table
1
Sample
code,
substrate,
plasma
power,
thickness,
deposition
rate
and
adhesion
of
the
samples
studied
in
this
work.
The
growth
pressure
was
kept
constant
in
all
cases
at
8
×
10
3
mbar.
Sample
code
Substrate
Plasma
power
(W)
Thickness
(nm)
Deposition
rate
(nm/min)
Adhesion
I
Si
220
×
II
Si
160
×
III
Si
100
×
IV
Si
50
30
15
V
Si
48
2000
20
VI
Si
46
600
20
VII
Si
46
4000
17
VIII
Si
45
1200
20
IX
Si
45
250
17
X
Si
30
200
13
XI
Steel
47
1800
10
and
microstructural
properties
as
well
as,
on
residual
stress,
micro-
strain
and
mechanical
properties
of
the
films
was
studied.
For
this
purpose,
a
series
of
samples
with
thicknesses
varying
from
30
nm
to
4.0
m
was
deposited
on
Si
(samples
from
IV
to
IX)
at
fixed
sputtering
parameters
(see
Table
1.
3.2.1.
Morphology
FEG-SEM
top
view
and
cross-sectional
images
are
shown
in
Fig.
2
for
coatings
with
different
thickness.
All
coatings
exhibit
very
simi-
lar
morphology
constituted
of
columns
that
grow
perpendicular
to
the
substrate.
Columns
present
an
inverted
pyramidal
shape
which
is
compatible
with
the
zone
T
in
the
Thornton’s
morphology
dia-
gram
[28].
Because
of
the
pyramidal
shape,
the
column
average
diameter
close
to
the
sample
surface
became
larger
with
increas-
ing
sample
thickness,
being
70
nm
in
width
for
the
coating
with
a
thickness
of
250
nm
film
and
400
nm
for
that
with
a
thickness
of
4.0
m.
Such
evolution
is
illustrated
by
the
histograms
depicted
on
Fig.
2.
Fig.
3
shows
AFM
images
of
two
meaningful
samples
(X
and
VIII).
The
topography
is
similar
to
that
observed
by
SEM
images
in-plane
(left
column
on
Fig.
2).
The
coating
surface
is
regular
and
smooth.
The
root
mean
square
(rms)
roughness,
as
extracted
from
the
quantitative
height
values,
is
lower
than
3
nm
for
all
samples.
One
important
parameter
to
be
considered
when
dealing
with
nanostructured
materials
is
their
grain
boundary
architec-
ture,
since
it
highly
influences
the
nanomaterial
properties
(i.e.
radiation-resistance,
mechanical
properties)
[29].
Because
of
this
reason,
the
nature
of
grain
boundaries
in
the
sputtered
coatings
was
investigated
by
TEM.
Dark
field
TEM
image
of
a
30
nm
in
thickness
nanostructured
W
sample,
especially
designed
for
TEM
observation
(see
experimental)
are
depicted
in
Fig.
4(a)
and
(b).
In
these
figures,
grains
presenting
anisotropic
shapes,
with
sizes
ranging
from
50
to
150
nm
are
seen.
All
morphological
data
evidence
that
samples
microstructure
is
quite
dense.
Indeed,
cross-sectional
FEG-SEM
images
reveal
the
absence
of
voids
in
between
columns,
within
the
SEM
resolution
limit.
This
result
differs
from
those
reported
for
similar
materials
deposited
by
oblique
angle
deposition
in
which
voids,
associated
to
the
shadowing
effect
taking
place
during
sputtering,
are
typically
present
among
the
columns
[30].
Then,
the
lack
of
voids
may
indi-
cate
that
energetic
ions
are
the
main
responsible
for
the
columnar
growth.
Additional
information
about
the
compactness
of
the
coatings
can
be
obtained
from
density
measurements.
The
average
coating
density,
estimated
by
combining
RBS
and
SEM
measurements,
was
calculated
to
be
19.50
±
0.30
g/cm
3
.
This
density
value
is
similar
to

Fig.
2.
Top
view
(left
column),cross-section
FEG-SEM
images
(middle
column)
and
histograms
(right
column)
representing
the
average
column
diameter
on
the
sample
surface
for
coatings
with
different
thickness:
(a)
250
nm
(black),
(b)
600
nm
(red),
(c)
1200
nm
(green),
(d)
2000
nm
(dark
blue),
(e)
4000
nm
(light
blue)
deposited
on
(1
0
0)
Si
and
(f)
1800
nm
(magenta)
deposited
on
commercial
steel
substrates.
The
scale
bar
is
800
nm
for
all
photos
and
the
thicknesses
are
indicated
on
each
image.

Fig. 3. AFM images of two nanoW samples, both deposited on (1 0 0) Si substrate with thickness of (a) 30 nm (sample IV) and rms = 1.0 nm and (b) 1200 nm (sample VIII) and
rms = 2.5 nm.
that tabulated for pure coarse grained W (19.25 g/cm
3
) [31], which
indicates that we are dealing with highly compacted coatings.
3.2.2. Microstructure
Fig. 4(c) shows a selected area electron diffraction (SAED) ring
pattern from a region of Fig. 4(a). This ring pattern indicates that
W film is polycrystalline. The ring pattern is well indexed by
the -W (space group Im-3m) structure, Fig. 4(c). Other authors
report the presence of some rings assigned to both -W and -
W structures [32,33] for similar as-deposited nanostructured W
films. They conclude that both phases of W were presented in their
films. Regarding the -phase (A15), -W{2 1 0} and -W{2 1 1}
are the most intense reflections. The -phase (space group Pm-
3n) is mainly seen for thin films. This phase is metastable but it
may be stabilized by impurities (e.g. oxygen) [33,34]. Shen et al.
[34] reported that the -phase is stabilized when O concentrations
larger than 5–15% are reached.
In our case, since the interplanar distance (d) of -W{1 1 0}
(d
-W{1 1 0}
= 0.2238 nm) and -W{2 1 0} (d
-W{2 1 0}
= 0.2256 nm)
are similar, reflections are very close and cannot be distin-
guished in the ring pattern depicted in Fig. 4(c). However,
if the -phase were present in our coatings, the -W{2 1 1}
reflection (d
-W{2 1 1}
= 0.2062 nm) with a relative intensity (I),
I
-W{2 1 1}
/I
-W{2 1 0}
= 0.96, would be observed in the SAED pattern
between -W{1 1 0} and -W{2 0 0} reflections, but this is not the
case. Other SAED spots measured at different regions were ana-
lyzed and the same ring pattern was obtained. The fact that we do
not observe any sign of the -phase, indicates that this phase could
not be stabilized, which excludes an O concentration beyond 5–15%
[34]. Apart of this indirect estimate, we could not determine the O
concentration, either with RBS or with PIXE because the sensitivity
limit of both techniques to light species is very poor.
XRD diffraction patterns for films with different thickness are
depicted in Fig. 5. All coatings exhibit four Bragg peaks correspond-
ing to the -{1 1 0}, -{2 0 0}, -{2 1 1} and -{2 2 0} reflections
of the thermodynamically stable body-centered cubic (bcc) -W
phase [17]. These results are in agreement with the SAED pattern
as previously discussed, indicating that coatings are polycrystalline
and monophasic (pure -phase). We carried out PIXE experiments
to detect the presence of Ar, a common impurity in coatings
deposited by sputtering. The results (not shown) reveal that the
Ar content in the coatings is 0.04%, notably lower than typical
reported values (2%) [35,36].
A deeper analysis of the XRD patterns allows us to obtain more
information about the film total stress and micro-strain. The film
total stress was evaluated by analysing the centroid -W{1 1 0}
peak profile in comparison to that reported for the conventional
W, Fig. 6(a). The Si peak in Fig. 5 corresponds to the substrate and
is used as an internal reference. Then, any shifts in the 2 pos-
itions of W to the tabulated ones can be attributed to stress in
Fig. 4. Dark field TEM images measured at two different scales (a), (b) and SAED pattern (c).

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