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Muhammad Zakky Nurrachman posted an update 7 years, 6 months ago
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Introduction
The recent intense and focused search for alternatives
to
3
He-based detectors has shown that detectors
based on the boron isotope
10
B are feasible alterna-
tives [
1
,
2
] both in their performance and economi-
cally [
3
–
10
].
10
B has a relatively high neutron
absorption cross section compared to
3
He, and by
optimizing the design of the detector, a detection
efficiency comparable to
3
He has been presented
[
3
,
11
]. When a neutron is absorbed by a
10
B atom,
there is a 94 % probability that the nuclear reaction
10
B
?
n
?
7
Li (0.84 MeV)
?
4
He (1.47 MeV)
?
c
(0.48 MeV) takes place, otherwise
10
B
?
n
?
7
Li
(1.02 MeV)
?
4
He (1.78 MeV).
For many
10
B-based detector applications,
10
B has
to be deposited as ‘‘high-quality’’ thin films onto
various substrate types. In this case, ‘‘high quality’’ is
defined by neutron-hard, well-adhering films of
thicknesses
[
1
l
m featuring low residual stresses,
maximum amounts of the neutron absorbing element
10
B, and thus a minimum of unfavorable impurities
like H, O, C, and N. The process must also be scalable
to several hundred square meters of two-side coated
substrates at affordable production prices. Natural
boron contains 20 %
10
B, but the isotope separation is
relatively easy and
[
95 %
10
B-enriched material is
commercially available.
In previous publications [
3
,
12
,
13
], direct current
magnetron sputtering (DCMS) was shown to provide
suitable deposition processes for the production of
10
B
4
C large area neutron detectors.
10
B
4
C is the pre-
ferred material, instead of
10
B,
10
BN, or other
10
B-
containing compounds, due to its relatively high
boron content in combination with excellent wear
resistance and thermal and chemical stability [
14
–
17
].
Additionally, the radiation hardness has recently
been shown to be appropriate for these neutron
detector applications [
18
].
Reference [
12
] addresses adhesion issues that often
arise for micrometer-range-thick B
4
C films due to
high residual film stresses in combination with low
adhesive forces between the B
4
C film and the sub-
strate. Here, the film adhesion on Al substrates was
reported to improve significantly as elevated sub-
strate temperatures between 300 and 400
°
C are used.
However, there is still a need for a well-working
process at substrate temperatures below 200
°
C,
allowing the deposition of adhering, high-quality
10
B
4
C coatings on temperature sensitive substrates.
Additionally, the inherently poor step coverage of
coatings deposited by DCMS on macrostructured
(commonly grooved) Al blades [
4
] needs further
investigation. The poor step coverage arises due to
the line-of-sight deposition nature of this technique
[
19
]. As was pointed out by Stefanescu et al.,
10
B
4
C
coating thickness non-uniformity on such
macrostructured blades may lead to detector effi-
ciency losses of up to 10 % [
20
].
A possible solution to the above mentioned con-
cerns, yet still using an industrial-scale magnetron
sputtering process, may be high-power impulse
magnetron sputtering (HiPIMS). In HiPIMS pro-
cesses, the flux of ionized target material usually
exceeds the flux of ionized working gas [
21
–
25
]. This
implies not only benefits with regard to the film
morphology and density, but also for the residual
stress [
26
,
27
] and the step coverage [
21
]. Although
ion bombardment of the growing film has frequently
been reported to yield high film stresses [
28
], the
comparison of films deposited by DCMS and HiPIMS
showed significantly reduced stresses without sacri-
ficing film hardness or density in case HiPIMS was
used [
29
]. Here, mainly the sputter gas and target
material properties, i.e., mass and ionization poten-
tials, together with appropriate bias voltage setting
were found decisive.
Therefore, this study explores DCMS and HiPIMS
process parameters for the growth of B
4
C coatings on
temperature-sensitive or macrostructured substrates.
In order to put the quality of coatings deposited
using HiPIMS or DCMS at low substrate temperature
into perspective, their properties are compared to
high-grade coatings deposited by DCMS at elevated
substrate temperature. The aim is the deposition of
uniform, high-quality B
4
C films onto Si and various
Al substrates with a thickness of
[
1
l
m at low sub-
strate temperatures without adhesion-enhancing
interlayers in order to meet the requirements of dif-
ferent
10
B-based neutron detector technologies.
Experimental details
B
4
C films were deposited in an industrial coating unit
(CC800/9, CemeCon AG, Germany). A base pressure
of less than 0.5 mPa was achieved prior to deposition.
The depositions were carried out in DCMS and
HiPIMS modes. All coating processes utilized two
rectangular B
4
C compound targets with an area of
J Mater Sci (2016) 51:10418–10428
10419
440 cm
2
. The B
4
C targets were mounted on two, each
other facing cathodes and sputtered in Ar
atmosphere.
Prior to deposition, the sputter system was evacuated
at full pumping speed for 2 h and the substrates were
degassed at the intended deposition temperature. The
depositionof the B
4
C filmswasconducted at75 % of the
full pumping speed. The influences of the substrate
temperature and deposition pressure on the B
4
Ccoat-
ing properties grown in DCMS and HiPIMS modes
were investigated. For our experiments, the deposition
temperatures of 100 and 400
°
C were chosen. The
working gas pressures were adjusted to 300, 450, 600,
and 800 mPa by the Ar flow and kept constant
throughout the deposition. In DCMS mode, a power of
3500 W was applied to each cathode. In HiPIMS mode,
the same average target power of 3500 W together with
a pulse frequency of 700 Hz and a pulse width of 200
l
s
was used. The pulse parameters yielded an energy per
pulse (E
pP
) of 5 Ws. No additional bias voltage was
supplied tothe substratetable in bothdeposition modes
in order to reduce residual coating stresses and to pro-
vide well-adhering coatings. The floating potential was
measured to be approximately
–
40 V.
Films with thicknesses between 1.4 and 1.9
l
m
were grown onto Si(001) wafer pieces, on flat Al
blades (alloy EN AW-5754) [
30
], and on macrostruc-
tured Al blades [
20
]. The chosen substrates allow for
various material analysis techniques and correspond
to frequently used substrates in
10
B-based neutron
detectors. All Al blades were mounted on a sample
carousel with a 2-axis planetary rotation for 2-sided
deposition. The Si wafer pieces were attached with
stainless steel wires to the flat Al blades and mounted
in a similar position as the flat Al substrates without
Si. The macrostructured Al blades were mounted
inside the deposition chamber so that the grooves
were vertical and rotated around their primary axis
using twofold rotation.
Cross-sectional scanning electron microscopy
(SEM, LEO 1550 Gemini, Zeiss, Germany) was car-
ried out in order to determine the B
4
C thickness and
hence the deposition rates of the sputter processes.
The instrument, equipped with an in-lens detector,
was operated at an acceleration voltage of 5 kV at a
working distance of
*
3 mm.
In order to study the thickness uniformity of B
4
C
coatings on grooved Al blades, cross-sectional SEM
was conducted. For sample preparation, the grooved
Al blades were cut perpendicular to the grooves,
embedded into Bakelite resin (Polyfast, Struers), and
subsequently mirror polished. The above mentioned
instrument settings were applied for SEM imaging.
The composition and bonding states of the B
4
C
films were examined by X-ray photoelectron spec-
troscopy (Axis UltraDLD, Kratos Analytical, Manch-
ester, UK) using monochromatic Al(K
a
) X-ray
radiation (h
m
=
1486.6 eV). The base pressure in the
analysis chamber during acquisition was less than
1
9
10
–
7
Pa. The XPS survey spectrum and core-level
spectra of the B 1s, Ar 2p, C 1s, and O 1s regions were
recorded on the as-received samples and after Ar
?
etching with a 4 keV Ar
?
ion beam. In order to
remove the surface oxide layer that is generated upon
exposure to air, the Ar
?
beam was rastered over an
area of 3
9
3mm
2
at an incidence angle of 20
°
.
Automatic charge compensation was applied
throughout the acquisition. After subtraction of a
Shirley-type background, the compositions were
extracted from the core-level spectra obtained from
sputter cleaned samples applying elemental cross
sections provided by Kratos Analytical.
Isotope-specific compositional analysis was per-
formed with time-of-flight elastic recoil detection
analysis (ToF-ERDA), using a 36 MeV
127
I
9
?
beam at
66
°
incidence and 45
°
recoil scattering angle. The
recoil energy of each element was converted to rela-
tive elemental depth profiles using the CONTES code
[
31
].
The residual stresses in the films were determined
by the wafer curvature method assessed by X-ray
diffraction (XRD, PANalytical Empyrean) [
32
]. The
diffractometer, equipped with a Cu K
a
1 source, was
operated at 45 kV and 40 mA. The Stoney formula for
anisotropic single crystal Si(001) was used to extract
residual coating stress from the measured substrate
curvature. Here, uniform plane stress in the film was
assumed [
33
]. The same instrument was chosen to
study the film density by X-ray reflectivity (XRR).
The density was evaluated using the PANalytical
X’Pert reflectivity software. Here, a 3-layer model,
resembling the substrate, the B
4
C films, and a surface
oxide layer, was applied.