The preparation and composition design of boron-rich lanthanum hexaboride target for sputtering

Article abstract of DOI:10.1016/j.jallcom.2015.03.038

Lanthanum Hexaboride (LaB₆) nano-film has been proved to be promising transparent thermal insulation material, while its properties are limited on purity and composition. High-purity LaB₆ polycrystalline powder was prepared through boron carbide reduction method in this work. A series of techniques such as scanning electron microscopy, X-ray diffraction, laser particle analyzer and inductively coupled plasma emission spectrometer were employed to characterize LaB₆ powder. As raising the content of La₂O₃ in reactants, more uniform, finer (2.686 μm) and purer (99.5139 wt%) LaB₆ powder is prepared, with only 0.4434 wt% residual B₄C. The density of targets increases with the rise of sintering temperature and the extension of sintering time, while crystallite size increases simultaneously with the extension of sintering time. The introduction of B powder in target is conductive to sintering process, increasing hardness and flexural strength of targets. X-ray photoelectron spectrometer was used to characterize the composition and microstructure of LaB₆ nano-film which is tentatively considered to be composed of LaB₆ nanocrystalline and amorphous microstructure of La and B atoms. The film LaB_6.0627±0.02 was obtained when the ratio of B and La of sputtering target reached 12.5. The thickness and deposition rate decrease with the increase of B content in targets.

Full text of DOI:10.1016/j.jallcom.2015.03.038

Journal of Alloys and Compounds 638 (2015) 380–386
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
The preparation and composition design of boron-rich lanthanum
hexaboride target for sputtering
⇑
Defang Chen, Guanghui Min, Yan Wu, Huashun Yu, Lin Zhang
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Lanthanum Hexaboride (LaB6) nano-film has been proved to be promising transparent thermal insulation
material, while its properties are limited on purity and composition. High-purity LaB6 polycrystalline
powder was prepared through boron carbide reduction method in this work. A series of techniques such
as scanning electron microscopy, X-ray diffraction, laser particle analyzer and inductively coupled plasma
emission spectrometer were employed to characterize LaB6 powder. As raising the content of La2O3 in
Received 11 January 2015
Received in revised form 3 March 2015
Accepted 4 March 2015
Available online 16 March 2015
reactants, more uniform, finer (2.686 lm) and purer (99.5139 wt%) LaB6 powder is prepared, with only
Keywords:
Boron-rich lanthanum hexaboride
Plasma cathode
Boron carbide reduction method
Magnetron sputtering
Optical nano-film
0.4434 wt% residual B4C. The density of targets increases with the rise of sintering temperature and
the extension of sintering time, while crystallite size increases simultaneously with the extension of sin-
tering time. The introduction of B powder in target is conductive to sintering process, increasing hardness
and flexural strength of targets. X-ray photoelectron spectrometer was used to characterize the composi-
tion and microstructure of LaB6 nano-film which is tentatively considered to be composed of LaB6
nanocrystalline and amorphous microstructure of La and B atoms. The film LaB6.0627
was obtained
0.02
when the ratio of B and La of sputtering target reached 12.5. The thickness and deposition rate decrease
with the increase of B content in targets.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
target as cathode is significant for obtaining high-performance
LaB6 nano-film.
Lanthanum Hexaboride (LaB6) has been widely used in applica-
tions such as cathode emission materials [1–3], decorative coatings
[4,5] and sensors of high-resolution detector [6]. In recent years,
LaB6 cathode emission materials have been applied in plasma cath-
ode. As a pioneering work, Schelm and Smith [7,8] added LaB6 into
polyvinyl butyral (PVB) and made them into polymer film. The
results showed that LaB6 is effective in near-infrared absorption
(750–1200 nm) and visible transmittance (380–750 nm), raising a
research upsurge on the optical properties of LaB6 nano-film for
the application on building glass and automotive glass.
In this report, Boron carbide reduction method was applied to
prepare LaB6 powder under vacuum condition. The effects of
holding time, La2O3 content and purification treatment on the
properties of LaB6 powder were studied. And then vacuum
pressureless sintering was employed to prepare boron-rich LaB6
sputtering target. The influences of sintering temperature and sin-
tering time on the density and structure of target were investi-
gated. The correlation of component between LaB6 films and its
sputtering source boron-rich targets was established.
Magnetron sputtering [9] is one of the best methods to prepare
LaB6 nano-film with uniform thickness, stable component and high
purity. The preparation process of magnetron sputtering to prepare
LaB6 nano-film was studied by Xu, Zhao and Wang [10–13]. The
component of LaB6 film tends to deviate from the stoichiometric
ratio because of different deposition rates between La atom and
B atom, and there is no special LaB6 target yet. Hence, a suitable
2. Experimental methods
LaB6 micron powder was prepared by solid phase reaction using La2O3 powder
(99.999% purity, particle size <1
lm) and B4C powder (99.9% purity, particle size
5–10 m) as raw materials. The LaB6 powder synthesized above and B powder
l
(99.9% purity, particle size <1
for sputtering.
lm) were used to prepare boron-rich LaB6 target
It is very difficult to remove residual B4C from the synthetic products due to
ultrahigh melting point and insolubility in acid and alkali, by contrast it is much
easier for La2O3. Therefore the content of La2O3 in reactants was increased to pro-
mote the reaction of B4C and its influences on the morphology and crystallinity
of LaB6 powder were investigated. The molar ratio of La2O3 and B4C were 1:3 and
1:2, respectively (will be called group 1:3 and group 1:2 in the following). The
⇑
Corresponding author. Tel./fax: +86 531 88395639.
E-mail address: zhanglin2007@sdu.edu.cn (L. Zhang).
http://dx.doi.org/10.1016/j.jallcom.2015.03.038
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
mixed powder after ball-milling process was pressed into cylindrical samples
381
[14,15], which were used to synthesize LaB6 powder at 1600 °C for 2 h according
to the following total reaction equation:
La2O3 þ 3B4C ¼ 2LaB6 þ 3CO "
ð1Þ
The surface of as-prepared samples was ground off to a depth of 1 mm to remove the
impurity layer. Then the samples were pickled by 5% hydrochloric acid solution
under vacuum condition at 70 °C for 3 h. In order to study the growth process of
LaB6 particles, a resintering process for the samples of group 1:2 at 1650 °C for 2–
4 h was designed.
The mixed powder of LaB6 and B with the ratio from 1:0 to 1:7.5 (B/La: 6–13.5)
were used to prepared boron-rich LaB6 targets. The green bodies were sintered at
different temperature 1500–1650 °C for different time 3–7.5 h. LaBx nano-films
were deposited on SiO2 substrates by DC magnetron sputtering at room tempera-
ture, using the as-prepared boron-rich LaB6 targets as sputtering source. The cham-
ber was evacuated to a pressure below 4.5 ꢀ 10ꢁ4 Pa before deposition, and then
the sputtering pressure was controlled at 1.5 Pa by introducing Argon (99.9999%
purity) at the flow rate of 40 sccm. The distance between target and substrate
was kept at 55 mm when sputtering process was carried out at the power of
50 W (1.77 W cmꢁ2) for 30 min, with bias Us ꢁ100 V and ion current density
5 mA/cm2 without any intentional heating during and after the whole process.
The typical thickness and deposition rate of LaBx film were in the range of tens to
thousands nanometers and a few to tens nm/min respectively.
The structure of prepared LaB6 powder was analyzed by X-ray diffraction (XRD,
DAMX-2500TC) in the range of 2h 10–80° with scan rate of 0.1°/s using the K
a
radiation of Cu (k = 0.154056 nm). Field emission scanning electron microscopy
(FESEM, JSM-6700F) with EDS (SUPRA™ 55) was used to observe the surface mor-
phology and measure the components of LaB6 powder and its boron-rich targets at
an accelerating voltage of 3 kV and current of 23 lA. The size and distribution of
LaB6 powder was measured by laser particle analyzer (Beckman Coulter
LS13320). Rockwell hardness tester (HD-1875) was employed to test hardness of
boron-rich LaB6 targets, with load at 30 kg. The samples with size
3 mm ꢀ 4 mm ꢀ 35 mm were prepared for flexural strength test by three point
bending beam method at room temperature, with span 25 mm and loading rate
0.5 mm/min. Step profiler (DEKTAK 150, Veeco) was used to measure the thickness
of films and then the deposition rates were calculated. Composition analysis of the
synthesized powder was conducted by inductively coupled plasma emission spec-
trometer (ICP, Optima 2100 DV) and carbon–sulfur Analyzer (CS-8800). The relative
atomic ratio of La and B in LaB6 nano-film was determined by X-ray photoelectron
Fig. 1. XRD patterns for LaB6 powder before (1,3) and after hydrochloric acid
pickling (2,4) synthesized using different raw materials ratio: (1,2) La2O3:B4C = 1:3
(3,4) La2O3:B4C = 1:2.
spectrometer (XPS, SCALAB250) using the K
was 90 s.
a radiation of Al and the scanning time
as the time extends the particles grow bigger and there are many
small fragments on the surface of bigger particles (Fig. 3c). These
small fragments which turned up during the grinding process
due to the high brittleness of LaB6 were removed mostly by settling
separation (Fig. 3c0).
The reduction of crystallite size would bring positive effects on
the strength of target and the deposition rate of LaB6 nano-film due
to the increase of the proportion of highly reactive grain boundary.
The effects of resintering time of LaB6 powder and sintering time of
targets on the crystallite sizes were investigated. The crystallite
size D was calculated from the full width at half maximum
(FWHM) of LaB6 (110) peak obtained by XRD according to the fol-
lowing formula [16]:
3. Results and discussion
3.1. The preparation of high-purity LaB6 micron powder
The formation of LaB6 powder is detected by XRD and FESEM
(Figs. 1 and 2). The high and sharp characteristic diffraction peaks
of LaB6 indicate that LaB6 polycrystalline powder has been pre-
pared. The synthesized LaB6 powder has slight (100) preferential
growth. The calculated lattice parameter of purified LaB6 powder
of group 1:3 is 4.16688 Å, and it is 4.16676 Å for the sample of
group 1:2. The calculated lattice parameters are both higher than
the theoretical value (4.1566 Å), which is attributed to the exis-
tence of micro-strains. Most of the impurities had been removed
after pickling and there is not any observable peaks of impurities
for the samples of group 1:2, while there is one indelible impurity
peak (2h 33°) for group 1:3 as shown in the partial amplifying
views (Fig. 1b and c) of the box part in Fig. 1a. The SEM images
show that after pickling LaB6 particles have a smoother and cleaner
surface and the particles that were bound together with impurities
disperse from each other. The LaB6 powder of group 1:2 contains
fewer impurities after pickling (Fig. 2b0) compared with the pow-
der sample of group 1:3. Meanwhile, the morphology of the pow-
der particles of group 1:2 after pickling is closer to cube and the
size of particles is more uniform than the sample of group 1:3.
The existence of agglomerates (indicated by the arrows in
Fig. 3a) would be harmful to the structural uniformity and density
of target. A repressing and resintering process at 1650 °C for the
samples of group 1:2 was performed to weaken the agglomeration.
There is not an obvious effect of the resintering process on the
phase structure. SEM images show that LaB6 powder after resinter-
ing for 2 h has better dispersion due to the complete growth, while
D ¼ ðKkÞ=ðb cos hÞ
ð2Þ
where b is FWHM corrected by instrument width correction curve.
K is Scherrer constant (0.943 for cubic particles) and k is the wave-
length of incident X-ray (1.54056 Å). The results show that the raw
materials ratio has only a small impact on the crystallite size of syn-
thesized LaB6 powder. The crystallite size of LaB6 powder increases
and becomes faster with the extension of resintering time (Fig. 4a).
The crystallite size of LaB6 targets increases with the extension of
sintering time and grows rapidly when the time was beyond 4.5 h
(Fig. 4b).
The size and its uniformity of LaB6 particles have a significant
impact on the density of target. And the gas in big sealed pores
in the target would bring harmful effects to the growth of film dur-
ing sputtering process. The size distribution of LaB6 powder (Fig. 5)
reveals that the mean particle size for the sample of group 1:2 is
smaller than that of group 1:3. The larger size part and the tail in
the small size side of the distribution curve are eliminated through
pickling treatment due to the removing of impurity phases, which
is well consistent with the previous analysis. The distribution

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D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
Fig. 2. SEM images of LaB6 powder before and after pickling (hydrochloric acid) synthesized using different raw materials ratio.
Fig. 3. Effects of resintering time on the morphology of LaB6 powder: (a) 0 h, (b) 2 h, (c) 4 h and (c0) 4 h, settling separation.
Fig. 4. Effects of resintering time of LaB6 powder (a) and sintering time of targets (b) on the crystallite sizes of LaB6 determined from the FWHM of LaB6 (110) peak.

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
383
certain errors for the calculation based on the premise that the
residual C atoms of purified samples were all from B4C and the
excess La atoms were all from La2O3. The calculated content of
B4C for the sample of group 1:3 after pickling is 1.2422 wt%, and
it is 0.4434 wt% for the sample of group 1:2, which demonstrates
the effect of excess La2O3 on promoting the complete reaction of
B4C. The LaB6 powder that was synthesized using excess La2O3 in
reactants achieves 99.5139 wt% purity after purification.
3.2. Design and preparation process of boron-rich LaB6 sputtering
target
The effects of sintering temperature and sintering time on the
densities of targets were shown in Fig. 6. The density of targets
increases with the increase of sintering temperature (Fig. 6a).
The density of targets increases with the extension of sintering
time and the upward trend slows down toward a stable value of
about 2.48 g cmꢁ3 (Fig. 6b). Reasons for the increase of density
lie in the rise of kinetic energy and the removing of pores.
During the process of sintering, the pores would transform from
connected pores into sealed pores, then balling, narrowing and
disappearance due to the movement of particle interface. The den-
sity is improved by the increase of kinetic energy of atoms when
the temperature increased, which is in favor of the movement of
particle interface and the removing of the pores. However, to
further increase temperature at such a high temperature would
largely increase the cost and the burden of apparatus. Therefore,
comprehensive weighting the effects of sintering temperature
and sintering time on density and crystallite size, the optimal sin-
tering parameters are 1600 °C and 4.5 h.
The added B promotes the sintering of target and the target
B/La = 12.5 has better mechanical property than the target B/La = 6.
Fig. 7 shows that the hardness and flexural strength of boron-rich
LaB6 targets both increase with the rise of B content. The cause is
that the introduction of B powder which has lower melt point com-
pared with LaB6 promotes the slip of interface and the diffusion of
atoms, transforming connected pores into sealed pores and
forming more chemical combination (Fig. 8b). It could be found
from the SEM micrographs of cross-section of LaB6 targets that,
eliminating the effects of pits, the distributions of B and La
elements for the target B/La = 6 are well consistent (Fig. 8a). The
Fig. 5. Size distribution of LaB6 powder.
curve became closer to normal distribution and the standard
deviation (
The LaB6 powder that was resintered for 2 h has the smallest mean
size 2.686 m. As resintering time further increases, the particles
r) decreased with the extension of resintering time.
l
grow up obviously, which is well consistent with the SEM results
(Fig. 3).
A more accurate determination of the content of each compo-
nent in the powder was provided to characterize the purity of
LaB6 powder (Table 1). The results show that there are trace
amounts of impurities such as carbon which is mainly from resid-
ual B4C powder, and silicon which is mainly from the raw materials
and environmental pollution, etc. The results of XRD (Fig. 1) and
EDS, however, show that there is not C atom in LaB6 powder. The
contradiction is due to the lower sensitivity of XRD for C element
and the interference of LaB6 layer around residual B4C [17]. From
the calculated value, it could be reconfirmed that most of residual
La2O3 was removed by pickling treatment. There are several rea-
sons for the negative value turned up at the content of La2O3.
Firstly, B4C is very difficult to be combusted completely during
CS analysis owing to the high melting points, resulting in the
reduction of test value compared to actual one. And there are
Table 1
Composition analysis of purified LaB6 powder synthesized using different raw materials ratio.
Raw materials (La2O3:B4C)
La (wt%)
B (wt%)
C (wt%)
B4C (wt%)
LaB6 (wt%)
La2O3 (wt%)
Other impurities (wt%)
1:3
1:2
67.2726
67.8490
32.4160
32.0223
0.2698
0.0964
1.2411
0.4434
98.7896
99.5139
ꢁ0.0723
0.0416
0.0323
0.0104
Fig. 6. Variation of the density of boron-rich LaB6 targets with sintering temperature (a) and sintering time at 1650 °C (b).

384
D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
demonstrates that the content of B in the surface of particles is
much higher.
The composition of LaB6 nano-film was measured by XPS.
Fig. 9a and b shows an example for the XPS fitting results of the
LaB6 nano-film that was deposited using the target B/La = 12.5.
The B 1s and La 3d5/2, 3d3/2 core level spectra of LaB6 nano-films
reveal that the chemical states of B and La atoms are not single. The
peaks of La 3d5/2 core level lies in binding energy 837.3 eV and
835.7 eV, from which two components LaB6 [18,19] and La are
identified. The peaks of B 1s core level at 187.23 eV and 187.4 eV
are approximately corresponding to LaB6 [18,19] and amorphous
B [20]. The LaB6 nano-film is tentatively considered to be com-
posed of LaB6 nanocrystalline and amorphous microstructure of
La and B atoms [21]. There is not obvious diffractive peaks could
be found in the XRD spectra (Fig. 9c), which is attribute to the
stronger surface scattering effect caused by finer grains and the
interference of the metastable structure of LaB6 on XRD test. The
crystallinity and grain size of LaB6 nano-film decrease with the
increase of B content in targets [22]. For a certain substance, the
binding energy of electron in a certain atomic level is constant.
Hence, the relative atomic ratio of B and La (RB/La) were calculated
by measuring the kinetic energy of photoelectrons according to the
following formula [23,24]:
Fig. 7. Mechanical properties of LaB6 and boron-rich LaB6 targets: (a) hardness (b)
flexural strength.
content of B is higher on the edge of particles than inside for the
target B/La = 12.5 (Fig. 8b). This indicates that LaB6 particles are
surrounded by smaller B particles and big pores are filled up with
B particles. This result could be further confirmed from the SEM
image of cross-section without polishing (Fig. 8c), which
RB=La ¼ ðIB=SBÞ=ðILa=SLaÞ
ð3Þ
Fig. 8. Micrographs and composition analysis of LaB6 target cross-section. (a)(a0)(a00) are the micrograph, composition distribution of lanthanum and boron of LaB6 target with
B/La = 6 after polishing, respectively; (b)(b0)(b00) are the corresponding results of LaB6 target with B/La = 12.6 after polishing; and (c)(c0)(c00) are the results of the same target as
(b) before polishing.

D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
385
Fig. 9. Chemical and structure characteristics of LaB6 films: fitting X-ray photoemission spectra of (a) B 1s and (b) La3d core level, and (c) XRD patterns of LaB6 nano-films for
the sample deposited by the target B/La = 12.5. (d) The correlation of the composition between the LaB6 nano-films and boron-rich targets.
Table 2
The measured thickness of films and the calculated deposition
rates were shown in Table 2. The thickness and deposition rate
Thickness and deposition rate of films using LaB6 + xB targets.
x
Thickness h (nm)
Deposition rate aD (nm/min)
decrease with the increase of B content in targets. This indicates
that B has lower deposition rate compared with La, so that when
the content of B increases the number of La that deposited in the
substrate is dramatically decreased while the amount of B does
not increase in a same extent. This result is well consistent with
the above analysis.
0.0
3.0
4.0
6.0
6.5
7.5
625
500
409
429
318
273
20.83
16.67
13.63
14.30
10.60
9.10
4. Conclusion
where IB and ILa are the peak areas of B and La elements respec-
tively, and SB and SLa are the sensitivity factors. The correlation of
atomic ratio of B and La between films and targets was established
(Fig. 9d). The corporation of B atom in LaB6 nano-film increases with
the increase of B content in targets. The film with composition
The main conclusions of this work are shown in the following:
(1) As raising the content of La2O3 in reactants, uniform, fine
(2.686 lm) and highly pure (99.5139 wt%) LaB6 polycrys-
LaB6.0627
was obtained when the atomic ratio of B and La of
0.02
talline powder was prepared by boron carbide reduction
method at 1600 °C for 2 h, resintering process at 1650 °C
for 2 h and pickling treatment.
sputtering target reached 12.5. The cause for the dramatically fall
of B content in films than in targets is the significant dispersion
effect of B during the sputtering and deposition process. The electric
and magnetic fields have a smaller bound effect for B ion and the
low relative atomic mass of B makes it easier to deviate from the
deposition direction. Generally, the phenomenon that sputtered B
reacts with the oxygen presented in a residual atmosphere of the
deposition chamber and B is released from the film as B2O3 gas will
also causes the decrease of B content in the film [25]. However, B
has higher chemical inertness than La which means that La is easier
to react with O. In order to get clear on this question, two more
films were prepared under pressure 2.5 ꢀ 10ꢁ4 Pa and
6.5 ꢀ 10ꢁ4 Pa, respectively. While the results of ICP test show that
the effect is negligible. Therefore, the reaction of sputtered B with
O is not considered to be the cause of the enormous difference of
B content between films and targets.
(2) The density of targets increases with the increase of sinter-
ing temperature. The density of targets increases with the
extension of sintering time and the upward trend slows
down toward a stable value of about 2.48 g cmꢁ3, while
the crystallite size increase with the extension of sintering
time simultaneously. Big pores are filled up with B particles
which promote the sintering of target as well. The hardness
and flexural strength of boron-rich LaB6 targets both
increase with the rising of B content. In this study, the
LaB6 target that was sintered at 1600 °C for 4.5 h is the opti-
mal one.
(3) The LaB6 nano-film is tentatively considered to be composed
of LaB6 nanocrystalline and amorphous microstructure of La
and B atoms. The proportion of B atom in LaB6 nano-films

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D. Chen et al. / Journal of Alloys and Compounds 638 (2015) 380–386
ˇ
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sents an enormous reduction than the B content in targets.
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The film with composition LaB6.0627
was obtained when
0.02
the atomic ratio of B and La of sputtering target reached
12.5. The thickness and deposition rate decrease with the
increase of B content in targets, which indicates that B has
lower deposition rate than La.
[13] D. Wang, L. Zhang, Effect of heat treatment on the properties of dc magnetron
sputtered LaB6/ITO films, Appl. Surf. Sci. 25 (2011) 6418–6423.
[14] S.Q. Zheng, G.H. Min, Z.D. Zou, Synthesis of LaB6 powder by reaction in La2O3–
B4C system, Acta Metall. Sin. 37 (2001) 419–422.
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[15] X.H. Xv, H.N. Xiao, W.M. Guo, Preparation and reaction mechanism of LaB6
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This research was funded by National Natural Science
Foundation of China (NSFC, No. 51102154). The authors wish to
acknowledge the financial support of this foundation.
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