K. MIURA et al.: DEFORMATION AND IMPROVEMENT OF THE IR TRANSMISSION OF SINGLE-CRYSTAL SILICON ...
493–497
DEFORMATION AND IMPROVEMENT OF THE IR
TRANSMISSION OF SINGLE-CRYSTAL SILICON BY DIRECT
CURRENT HEATING
DEFORMACIJA IN IZBOLJ[ANJE IR PRENOSA
MONOKRISTALNEGA SILICIJA Z ENOSMERNIM TOKOM
Kiyotaka Miura1, Yasuhiko Shimotsuma1, Masaaki Sakakura2, Shunsuke Gunji1,
Taiki Sakamoto1, Kohei Morishita3, Satoru Hachinohe4
1Kyoto University, Graduate School of Engineering, Department of Material Chemistry, Kyoto 615-8510, Japan
2Kyoto University, Hitachi Zosen Collaborative Research Division, Kyoto 615-8520, Japan
3Kyoto University, Graduate School of Engineering, Materials Science and Engineering, Kyoto 606-8501, Japan
4Proud INC., 6-9 Fudanotsuji, 2-chome, Higashi-Omi-shi, Shiga 527-0024, Japan
miura2@func.mc.kyoto-u.ac.jp
Prejem rokopisa – received: 2016-07-14; sprejem za objavo – accepted for publication: 2016-08-02
doi:10.17222/mit.2016.158
We confirmed that the deformation occurred at about 800 °C when CZ-Si was pressure and heat treated by a pulse-heating-
method, spark plasma sintering (SPS), while at the same time, the absorption peak of silicon single crystal produced using the
Czochralski process (CZ-Si), which was a major issue for infrared transparent material in the vicinity of 9 μm, was also
confirmed to have been reduced within a short time. The absorption coefficient in the vicinity of 9 μm, which was derived from
the interstitial oxygen, decreased the most at 800 °C, and the absorption derived from the stretching mode of Si–O observed in
the vicinity of 9.7 μm reached its maximum at 800 °C. This is considered to have been due to the migration of interstitial oxygen
via clusters to change the material into amorphous SiO2. It was confirmed that the impact of the applied pressure direction was
relative to crystal orientation on the peak of 9 μm. It was also found that the deformation was the maximum from the (110)
plane, that the change in absorption coefficients before and after deformation was the largest, and that the relationship turned
out to be (110)> (100)> (111). The dislocation lines in the sample after the deformation of the (100) plane were observed using
EBSD, and the polarization dependencies of transmittance in the infrared region were measured for the planes parallel and
perpendicular to the applied pressure.
Keywords: silicon, spark plasma sintering, infrared transparent material, interstitial oxygen, molding
Potrdili smo, da se je deformacija zgodila pri okoli 800 °C, ko je bila CZ-Si tla~no in toplotno obdelana s pulzno toplotno
metodo sintranja z iskre~o plazmo (angl. SPS), medtem ko je bil hkrati dose`en absorpcijski vrh silicijevega monokristala z
uporabo Czochralskijevega postopka (CZ-Si), pri katerem je glavno te`avo predstavljal infrarde~i prosojni material pri pribli`no
9 μm, saj je bil zmanj{an v zelo kratkem ~asu. Absorpcijski koeficient pri pribli`no 9 μm, ki je bil pridobljen iz intersticijskega
kisika, se je najbolj zni`al pri 800 °C in opazili smo absorpcijo, ki je izhajala iz raztezka Si-O, in je v bli`ini 9,7 μm pri 800 °C
dosegla svoj maksimum. Iz tega izhaja, da je bil zaradi migracije intersticijskega kisika preko skupin material spremenjen v
amorfni SiO2. Potrdili smo, da je bil vpliv povezan z uporabo smeri tlaka, v primerjavi s kristalno orientacijo na podro~ju 9 μm.
Ugotovljeno je bilo tudi, da je bila deformacija najve~ja na (110) ravnini in da je sprememba koeficientov absorpcije, pred in po
deformaciji, najve~ja ter da se je pokazalo razmerje (110)> (100)> (111). Opazili smo premikanje linije v vzorcu po deformaciji
(100), ravnine so bile opazovane z uporabo EBSD ter z uporabo polarizacije odvisnosti od prepustnosti v infrarde~em obmo~ju,
ki smo jo izmerili v ravnini vzporedno in pravokotno glede na tlak.
Klju~ne besede: silicij, sintranje z iskre~o plazmo, infrarde~i prozorni material, intersticijski kisik, modeliranje
1 INTRODUCTION
Pyroelectric infrared sensors, commonly used in
motion sensors and similar devices, are required to
efficiently detect infrared radiation with a wavelength of
approximately 9 μm. Silicon produced using the Czo-
chralski process (CZ Silicon), can be obtained at low
prices, but can only be used for IR rays in a relatively
narrow band (around 1.2–6 μm) since the oxygen eluted
from the quartz crucible is trapped in the crystal, which
results in the presence of absorption near 9 μm that arises
from interstitial oxygen.1–3 Silicon can be produced as
either a single-crystalline form by the Czochralski (CZ)
crystal-growth process or the float zone (FZ) method. FZ
silicon is grown without a quartz crucible and is thus
almost completely oxygen-free, making the absorption
of 9 μm much less pronounced, but FZ silicon is expen-
sive. If this absorption band at 9 μm is improved, the
wavelength range of infrared transparent windows or
lenses of CZ silicon crystal may possibly be expanded. It
is known that the absorption band at 9 μm due to inter-
stitial oxygen can be reduced by a long heat treatment
(for several to several dozen hours) because of the
conversion of Si into SiOx (x2).4–6 In addition, silicon,
which is naturally brittle owing to its covalent character,
easily cracks under a small load. Thus, the conventional
process used to obtain a specific lens shape is expensive.
Moreover, it is difficult to shape aspherical lenses, which
are ideal and highly efficient shapes for focusing. To
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 3, 493–497 493
UDK 67.017:622.785:621.365.46 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)493(2017)
solve these problems, a silicon lens fabricated by hot
pressing has been investigated.7 However, in hot press-
ing, single-crystal silicon is pressed at 1405 °C, just
below the silicon’s melting temperature. Therefore, the
dislocation density is large. As a result, the transmittance
is reduced to 10–20 %. When the sample is recrys-
tallized, the dislocation density is reduced, and the
transmittance is restored to about 40 %. However, a heat
treatment is required after hot pressing.7
We have recently succeeded in manufacturing a
silicon lens at around 800 °C, (roughly 600 °C lower
than the temperature for hot pressing) by direct current
heating using a spark plasma sintering (SPS) system.
Furthermore, we confirmed that the interstitial oxygen
decreased in a short time by adding pressure with direct
current heating. The aims of the present study are to 1)
provide a silicon material for an infrared transmitting
member that secures transmittance for infrared of wave-
length around 9 μm and can be used for a wide range of
wavelengths and 2) provide an infrared transmitting
member made of the silicon material.
2 EXPERIMENTAL PART
Single-crystal silicon (#6 × 4mm) grown by the
Czochralski process was used as the starting material.
The samples have 100, 110, and 111 planes perpendi-
cular to the axis of pressing. The samples were heated to
the desired temperature under a uniaxial compressive
pressure in a carbon die with an inside diameter of
15 mm by the SPS system (LABOX-125, Spark Plasma
Sintering System, Sinter Land, Japan). The SPS system
is made up of a uniaxial press, punch electrodes, vacuum
chamber, direct current (DC) pulse generator and
position, temperature, and pressure measuring units. The
temperature was increased from 700 °C to 1100 °C. The
heating rate was 100 °C/min from room temperature to
the desired temperature. The sample temperature was
controlled by adjusting the pulse duration, current and
voltage, and measured on the silicon surface directly by
a radiation thermometer. The uniaxial pressure was
changed between 0.1 kN and 8 kN. The pressure rising
rate was changed between 0.05 kN/min and 2 kN/min.
After the samples had been heated to the pre-set
temperature under a uniaxial compressive pressure of
0.4 kN, the pressure was increased to the pre-set pressure
while keeping the temperature the same. In the heat/press
deformation of the plasma sintering system (SPS) in this
study, when the temperature of silicon reached about
400 °C, there was a change in electrical resistance,
which confirmed the start of direct power supply to the
interior of silicon. After being cut into 1-mm-thick
chunks, the specimens were mirror polished and then
used for infrared transmission measurements.
Infrared transmission was measured in the range of
2–12 μm at room temperature by FT-IR to determine the
absorption coefficient. In addition, to compare the
differences between a plane perpendicular and a plane
parallel to the axis of the pressure, the polarization
dependence of the infrared transmission spectrum was
investigated using a holographic wire grid polarizer. The
crystal orientation and the distribution of dislocations for
the CZ silicon crystal after transformation were also
characterized by electron backscatter diffraction
(EBSD).
3 RESULTS AND DISCUSSION
Figure 1 shows a photograph and the IR absorption
spectra of the CZ silicon that was compressed by
changing the temperature at a pressure of 4 kN
perpendicular to a plane of silicon (100). The absorption
coefficient around 9 μm, derived from the interstitial
oxygen decreases to its minimum at 800 °C, and then
increased as the temperature rose from 900 °C to 1100
°C. In contrast, the absorption derived from the Si-O
stretching mode observed around 9.7 μm was at its
maximum at 800 °C. This is because the interstitial
oxygen had moved and changed to amorphous Silica
SiO2 over the cluster (SiO4). It is well known that inter-
stitial oxygen is transformed into oxygen precipitation
and into amorphous SiO2 by heat treatment over a long
K. MIURA et al.: DEFORMATION AND IMPROVEMENT OF THE IR TRANSMISSION OF SINGLE-CRYSTAL SILICON ...
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Figure 1: a) Image and b) IR absorption spectra of CZ silicon
obtained by changing the temperature reached by applying a pressure
(4 kN) perpendicular to a plane of silicon (100)
Slika 1: a) Slika in b) IR spektri absorpcije CZ silikona, pridobljenega
s spreminjanjem temperature, dose`ene z dodajanjem tlaka (4 kN)
pravokotno na povr{ino silikona (100)
time.8,9 The reason for the maximum reduction of inter-
stitial oxygen at 800 °C is controlled oxygen diffu-
sion.10,11 We also found that in the high-temperature
region beyond 800 °C, the super-saturation of the SiO2
that formed was reduced and the process of crystal
growth from nucleation did not progress further. In addi-
tion, SiO2 that had segregated once in the low-tempera-
ture region, decomposed again. Therefore, the optimum
temperature of the decreasing absorption band at 9 μm is
estimated to be 800 °C.
Similarly, when the ultimate pressure is changed, the
9 μm absorption decreased proportionately with the
pressure, and it was confirmed that the pressure at the
time of deformation, greatly contributes to the movement
of interstitial oxygen. Figure 2 shows the IR spectrum of
the samples pressed at 1, 2, 4, 6, and 8 kN by fixing the
temperature at 900 °C. The absorption band at 9 μm de-
creases in proportion to increasing loads, except for the 8
kN load. This result indicates that the precipitation of
SiO2 clusters or particles is increased by dislocations due
to high pressure. In contrast, the absorption band at 9 μm
of the sample that is pressed at 8 kN tends to increase.
The crack of the surface of this sample is observed using
an optical microscope. This result indicates that the
crack generates the light scattering.
From these results, it is clear that deformation due to
pressure is used with heat treatment for the diffusion of
interstitial oxygen. Thus, the pressing in optimum
conditions can be expected to be applied for an infrared
transmission lens because not only are the samples
molded but also the absorption property at 9 μm is im-
proved. An example succeeded in manufacturing a sili-
con single crystal lens at around 800 °C (actual sample
temperature) is shown Figure 3, which is about 600 °C
lower than the molding temperature for the hot-pressing
method,6 by using an SPS system.
In order to determine the impact of the applied press-
ure direction relative to crystal orientation on the peak of
9 μm, each sample was prepared and placed so that the
crystalline planes were perpendicular to the pressing axis
on the 100, 110 and 111 planes. Figure 4 and Table 1
show the obtained results. It was found that the defor-
mation was a maximum from the (110) plane, that the
change in absorption coefficients before and after defor-
mation was the largest, and that the relationship turned
out to be (110)> (100)> (111). During the deformation of
the (110) plane, the transformation was parallel to the
axis of the applied pressure, while, in the case of the de-
formation of the (100) plane, due to mutual intersection
of slip planes, the interaction due to slip planes became
larger and the change was smaller when compared to the
(110) plane. In the (111) plane, the change was small
because the slip plane is perpendicular to the loading
axis.
Table 1: Absorption coefficient difference at 9 μm and deformation
amount of height for samples in Figure 4
Tabela 1: Koeficient absorpcije – razlika pri 9 μm in deformacijska
koli~ina vi{ine za vzorce na Sliki 4
Crystal plane
Absorption
coefficient
difference at 9.0 μm
(cm–1)
Deformation
amount
(mm)
(100) 2.37 0.28
(110) 2.68 0.44
(111) 1.55 0.12
K. MIURA et al.: DEFORMATION AND IMPROVEMENT OF THE IR TRANSMISSION OF SINGLE-CRYSTAL SILICON ...
Materiali in tehnologije / Materials and technology 51 (2017) 3, 493–497 495
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Figure 2: IR absorption spectra of CZ silicon obtained by changing
the ultimate pressure at 900 °C
Slika 2: IR absorpcija spektrov CZ silikona, pridobljenega s spremi-
njanjem kon~nega tlaka pri 900 °C
Figure 4: Shape difference of samples obtained by pressure per-
pendicular to the pressing axis on 100, 110, and 111 planes at 1000 °C
and 4 kN
Slika 4: Razli~ne oblike vzorcev, pridobljene s tlakom pravokotno s
potisno osjo na 100, 110 in 111 pri 1000 °C in 4 kN
Figure 3: A silicon single-crystal lens obtained from a CZ silicon disk
(#22 × 4 mm) by molding at around 800 °C (actual sample tempe-
rature), which is about 600 °C lower than the melting temperature, by
using an SPS system
Slika 3: Silikonski monokristalni le~i, pridobljeni iz CZ silikonskega
diska (#22 × 4 mm) z dolivanjem na okoli 800 °C (dejanska
temperatura vzorca), ki je za 600 °C ni`ja kot talilna temperatura, pri
uporabi SPS-sistema
The dislocation lines in the sample after the defor-
mation of the (100) plane were observed using EBSD
and the results are shown in Figure 5a and 5b.
As seen in Figure 5a, where the plane is perpen-
dicular to the applied pressure, the dislocation lines enter
randomly, while in Figure 5b, where the plane is parallel
to the applied pressure, the dislocations lines are oriented
horizontally. Then, the polarization dependencies of
absorption in the infrared region were measured for each
case. The changes in IR absorption spectra are shown in
Figure 6a and 6b. In the sample in Figure 5a, no pola-
rization dependency was observed, while in the sample
of Figure 5b, where the transition lines are horizontally
oriented, polarization dependency was observed in the
absorption of 9 μm. Interstitial oxygen is known to move
through the central position of Si-Si bond along the
(110) planes12 and as a result, we can say that the
interstitial oxygen segregated along the loading axis
towards the dislocation lines. It can be predicted that in
the pressure temperature region (700–1100 °C), the
carrier density must have increased due to the flow of
direct current in the silicon, and due to the weakening of
covalent bonds, a metal-like deformation with weak
orientation was formed. This indicates that the disloca-
tion lines in Figure 5a and 5b do not originate from the
crystal structure and that the impact of the axis of the
applied pressure is reflected in a much larger way.
4 CONCLUSION
We have shown that deformation occurred at 800 °C
and 6 kN when CZ-Si was pressure and heat treated by
direct current heating using the SPS system, while at the
same time, the absorption peak of CZ-Si, which had
been a major issue for the infrared transparent material
in the vicinity of 9 μm, was also confirmed to have been
reduced within a short time. It was also found that the
deformation was the maximum from the (110) plane, that
the change in absorption coefficients before and after
molding was the largest, and that the relationship turned
K. MIURA et al.: DEFORMATION AND IMPROVEMENT OF THE IR TRANSMISSION OF SINGLE-CRYSTAL SILICON ...
496 Materiali in tehnologije / Materials and technology 51 (2017) 3, 493–497
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Dislocation lines in samples after the deformation of (100) plane observed using electron backscatter diffraction (EBSD):
a) perpendicular and b) parallel to the applied pressure
Slika 5: Dislokacijske linije v vzorcih po deformaciji (100), izmerjene z uporabo EBSD: a) pravokotno in b) paralelno glede na uporabljeni tlak
Figure 6: Polarization dependence of absorption in the infrared region
obtained for samples in Figures 5a and 5b
Slika 6: Polarizacijska odvisnost na infrarde~em podro~ju, pridob-
ljena za vzorce iz Slike 5a in 5b
out to be (110)> (100)> (111). Furthermore, we have
shown that a silicon lens could be molded at around
800 °C, which is about 600 °C lower than the melting
temperature, by using an SPS system. Although further
studies are required for elucidating the detailed mecha-
nism behind low-temperature deformation, we can ex-
pect that single-crystal silicon formation can be realized
at low temperatures of 800–900 °C and that this tech-
nique where the interstitial oxygen absorption is reduced
in a short period of time at the same time as formation,
can be applied in the making of infrared transmission
windows and lenses.
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