Thermal transformation of solvent-processable organosilicon compounds into inorganic silicone-based thin films

Article abstract of DOI:10.1039/c0jm02093a

In this study, a solution-processable precursor, decaphenycyclopentasilane (DPCPS) was synthesized and successfully converted into an inorganic thin film through the heat treatment process. When the DPCPS coating was heat-treated at various temperatures u

Full text of DOI:10.1039/c0jm02093a

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PAPER
Thermal transformation of solvent-processable organosilicon compounds into
inorganic silicone-based thin films
a
a
b
b
a
ac
Sieun Kim, Tai-Hoon Han, Hyun-chul Jung, Yong-Soo Oh, Lyongsun Pu, Pyoung-Chan Lee*
and Jae-Do Nam*a
Received 2nd July 2010, Accepted 16th November 2010
DOI: 10.1039/c0jm02093a
In this study, a solution-processable precursor, decaphenycyclopentasilane (DPCPS) was synthesized
and successfully converted into an inorganic thin film through the heat treatment process. When the
ꢀ
DPCPS coating was heat-treated at various temperatures up to 900 C, the organic DPCPS was
converted into an inorganic compound composed of Si, C and O atoms in the form of SiC O . The
y
x
relative compositions of Si, C, and O changed with the heat-treatment temperatures changing the
carbon content, for example, from 88.9% for the pristine DPCPS to 3.0% for the specimen heat-treated
ꢀ
over 700 C, consequently becoming SiC0.05
O
1.7. The dielectric constant of the heat-treated DPCPS
could be tailored with the heat treatment temperatures from high- to low-dielectric materials, i.e., 10.8
ꢀ
ꢀ
at 300 C and 2.3 at 900 C, providing tunable dielectric properties for various optoelectronic
applications. The resulting inorganic thin films had excellent coating characteristics with low RMS
roughness (<1.0 nm).
1
. Introduction
Among
various
phenylated
cyclosilanes,
decap-
henylcyclopentasilane (DPCPS) is particularly interesting
because it is soluble, transparent, and stable in ambient condi-
tions, which makes it possible to be used as a precursor for the
Compared to conventional vacuum processes or vapour-phase
depositions, a solution-based process has attracted a great deal of
attention for the fabrication of large-area optoelectronic devices
at low costs, which ensures such large-volume production
3,8
synthesis of CPS through chemical transformation. It has been
reported that the phenyl groups in DPCPS can be easily replaced
by halogens followed by hydrogenation for further polymeriza-
1–3
processes as spin-coating or ink-jet printing. Recently, a silane-
based liquid precursor has been developed to fabricate a high
purity silicon film for specific applications in silicon thin-film
transistors (TFTs), where a hydrogenated polysilane was
synthesized in the liquid phase by the ring-opening polymeriza-
4
tion of silane compounds such as CPS.
Rather than following those complicated routes, it may be
reasonable to consider that the thermal treatment of DPCPS at
high temperatures may well give an inorganic material composed
of Si and C atoms because DPCPS contains only Si, C and H
atoms. Upon thermal treatment, the aromatic carbon groups
may be transformed into inorganic carbons via the hybridization
n
tion of cyclic silane precursors (Si H2n) via UV and heat treat-
ment and subsequently converted to a three-dimensional a-Si
3
network.
Among various cyclic silane precursors, cyclopentasilane (CPS,
3
2
11,12
ꢀ
of sp to sp carbon orbitals.
compounds containing aromatic rings generally decompose and
emit gases such as CO and CH leaving carbon and hydrogen
Around 400–600 C, organic
Si
precursor, because the molecular weight of CPS with n > 4 in
Si 2n makes it relatively stable for solution processing and it
undergoes polymerization upon irradiation with ultraviolet (UV)
5
H10) has attracted much attention as a solution-processable
4
n
H
13
atoms. After further heating, hydrogen atoms are eliminated
2
through the dehydration reaction resulting in sp -carbon
14,15
4–8
light. A successful synthesis of CPS could be achieved by
atoms.
Accordingly, although the high-temperature degra-
choosing appropriate phenylated cyclosilanes, for example,
8–10
dation mechanism of DPCPS is not known, it is deemed that
the organic phenyl groups in DPCPS may well be converted to
2
sp -carbon atoms under the high-temperature heat treatment,
5
Si Ph10ꢁnX
n
with X ¼ F, C1, Br, I or H, where n ¼ 1 or 2.
a
resulting in an inorganic compound most possibly composed of
silicon and carbon. In the heat treatment process of carbosilanes,
however, it should be noted that oxygen atoms may well be
Department of Polymer Science and Engineering, Department of Energy,
Sungkyunkwan University, Suwon, South Korea. E-mail: pclee@katech.re.
kr; jdnam@skku.edu; Fax: +82 31 292 8790; Tel: +82 31 290 7285
b
Central R&D Institute, Samsung Electro-Mechanics Co. Ltd., 314
Maetan-dong, Yeongtong-gu, Suwon, South Korea
x y
inevitably included to become SiC O containing silicon dioxide
c
and carbon. In the manufacturing of silicon carbide fibres, for
example, it has been experienced that the pyrolysis of
Materials and Components R&D Division, Korea Automotive Technology
Institute, Pungse-Myun, Chonan-Si, Chungnam, South Korea
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polycarbosilane does not result in pure SiC but, in reality,
Microanalysis, EPMA1600, USA), there were no other impuri-
ties detected in the purified DPCPS.
16
SiC O containing oxygen at a nominal composition of 10–14%.
x
y
The SiC Oy compound has been used for the super low-k
x
A spin-coater was employed to coat the DPCPS solution in
benzene on a Au-coated gallium arsenide substrate (GaAs) or
conducting wafer. Then, the thermal treatment was carried out in
material because its dielectric constant is substantially lower than
SiO
bonding in the presence of carbon atoms. The low dielectric
constant and excellent thermal stability of SiC materials
make them considered appropriate for ultra large scale integra-
2
and, furthermore, it is thermally stable due to a strong Si–O
ꢁ1
a continuously-flowing argon gas (99.9999%, 50 ml min ) at
ꢀ
ꢁ1
ꢀ
x
O
y
a heating rate of 10 C min up to temperatures of 300 C, 500
ꢀ
ꢀ
ꢀ
C, 700 C and 900 C, where the specimens were held for 1 h
using tube furnace (Lindberg/Blue M). The surface
17
tion (ULSI) circuits. Currently, the plasma-enhanced chemical
a
vapour depostion (PECVD) process is usually used for such low-
morphology of the silicon based thin film was investigated using
a high resolution-atomic force microscope (NR-AFM, THERO-
MICROSCOPES, CP Research, USA). The thickness of the thin
films were measured using an alpha-step instrument (ALPHA
STEP IQ, KLA-Tencor, USA) and field emission scanning
electron microscope (JEOL, JSM7500F, Japan). Silicon-based
thin films were investigated using X-ray photoelectron spec-
troscopy (XPS, ESCA2000, UK). The capacitance-voltage
measurement of Au/thin films/Si metal-insulator-semiconductor
structure was carried out using a Capcitance-Voltage analyzer
(MST4000A, MS-TECH, Korea). The diameter of the Au dot as
a top electrode was 1.4 mm, which was prepared by evaporation
18
k materials as SiOC:H, SiC:H and a-SiC:H. Compared with
those vacuum processes, a liquid-phase solution process is
a desirable alternative due to its low-cost and large-volume
advantages.
In this study, a stable solution of DPCPS was prepared and
x y
subsequently converted into inorganic SiC O compounds at
ꢀ
various heat treatment temperatures up to 900 C. The solution-
processed inorganic thin film was achieved and characterised
using thermal gravimetric analysis, element analysis, X-ray
photoelectron spectroscopy, and dielectric measurements.
(
evaporator GVC-2000, SNT, Korea). Unless otherwise stated,
all the synthesis and analysis was performed in an inert envi-
ronment.
2
. Experimental
Tetrahydrofuran (THF) anhydride, lithium ribbon, benzene
anhydride and diphenyldichlorosilane were purchased from
Aldrich. 1 L of dried THF and 8.4 g of lithium were then charged
3
. Results and discussion
4
into the flask while being bubbled with an ultra-pure argon gas.
Analyzing the synthesized DPCPS (about 50% yield), Fig. 1
29
shows that the peak position of liquid-state Si-NMR matches
While this suspension was cooled with ice under agitation, 0.6
mol of diphenyldichlorosilane was added for 3 h. The reaction
continued until the lithium completely disappeared. After the
reaction of the mixture, THF was evaporated using an evapo-
rator. Then, 1 L of benzene was added and stirred for two days.
The precipitate was filtered and the filtrate was purified using
petroleum ether: After standing for a few days, the insoluble
material in the mixture was removed by filtration. The product
was further recrystallized using ethyl acetate (also see Scheme 1).
with the homogeneous Si-ring structure of DPCPS in the refer-
19
ence as a single resonance. It demonstrates that the Si-ring
structure in DPCPS (Scheme 1) is successfully synthesized. In
addition, the atomic compositions of DPCPS analyzed by the
elemental analyzer and ICP-AES agree well with the theoretical
values of DPCPS (Table 1). Although it is not included here, the
synthesized DPCPS was confirmed by HPLC to be highly pure,
over 90% within the error tolerance of 2–3%.
ꢀ
DPCPS was dried at 50 C under vacuum for 24 h to remove the
As seen in Fig. 2, the major weight loss of the synthesized
ꢀ
solvent.
ꢀ
ꢀ
DPCPS occurs between 300 C and 500 C. Between 500 C and
ꢀ
29
The DPCPS structure was examined by Si-NMR spectros-
copy (Bruker, AVANCE 400 WB, Germany), elemental analyzer
7
00 C only a slight weight loss can be observed, leaving ca. 30
wt.% of SiC O residue at the end. Comparing the binding
y
x
(
Thermo Electron corporation, Flash EA 1112, USA), induc-
energies of DPCPS Si–H, Si–Si, and Si–C, at 90, 54, and 88 kcal
ꢁ1
tively coupled plasma-atomic emission spectrometry (ICP-AES,
OPTIMA 4300DV, Perkin-Elmer, USA). Thermogravimetric
analysis (TGA) was carried out in a nitrogen environment using
a Du Pont thermal analysis system (TGA, DuPont, Ltd., TGA
20
mol , respectively, it is reasonable to address that the bond
scission of Si–Si bonds begins earlier than either Si–C or Si–H
21
bonds. In these thermal treatment conditions, it is considered
that the organic covalent bonds in DPCPS would also disappear
2
910, USA). The temperature was increased linearly from room
ꢀ
temperature to various temperatures up to 900 C at a heating
ꢁ1
ꢀ
rate of 10 C min . The purity of DPCPS was estimated using
HP-LC (Agilent 1100 LC, USA) and the purity was higher than
9
0%. In the elemental analysis using EPMA (Electron Probe
2
9
Scheme 1 Synthesis of decaphenylcyclopentasilane.
026 | J. Mater. Chem., 2011, 21, 3025–3029
Fig. 1 Liquid state Si-NMR of synthesized DPCPS.
3
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ꢀ
Table 1 Elemental compositions of synthesized DPCPS compared with
the theoretical values
the heat-treatment temperature is further increased up to 700 C
ꢀ
and 900 C, the oxidative reaction seems to take place to give
a dominant Si–O peak at 103.4 eV. As already discussed, in
reality, the oxygen atoms can hardly be avoided in the heat-
Element
Ideal value
Experimental value
C
H
Si
79.05
5.56
15.41
76.90
5.58
12.88
x y
treatment conditions resulting in SiC O compounds.
Fig. 3(b) shows the carbon binding energies of the DPCPS
specimens at various thermal treatment temperatures. The
intensity of the carbon binding energy decreases with heat
treatment temperatures. In particular, the binding energy of
a
a
Si content was mesured by ICP-AES.
ꢀ
carbon above 700 C sharply decreases and eventually disap-
pears. The C–C or C–H bonding energies at the peak centred at
ꢀ
via various chain scission reactions, and consequently an inor-
ganic compound is formed via the orbital hybridization of
carbon and silicon.
2
85 eV maintain high intensity up to 500 C, indicating that the
organic phenyl groups still exist. However, the carbon organic
bonding is substantially decreased with the heat-treatment
The heat-treated DPCPS specimens at various temperatures
were analyzed by the elemental binding energies of the XPS
narrow scan measurements in Fig. 3. Fig. 3(A)–(C) shows the
XPS narrow scans of Si 2p, C 1s, and O 1s electron orbitals of the
DPCPS films heat-treated at different temperatures. As seen in
Fig. 3(a), the peak intensity of the silicon binding energies of the
DPCPS specimen increases with the heat treatment tempera-
tures. The pristine DPCPS shows a dominant peak centred at
ꢀ
temperature over 700 C leaving a broad and small peak at the
same bonding energy, seemingly demonstrating the formation of
x y
inorganic C–Si bonding of SiC O compounds. Fig. 3(c) shows
that the 1s electron peak of oxygen changes similarly to that of Si.
ꢀ
As the heat treatment temperature reaches over 700 C, it is
believed that the debonded Si is likely to react with oxygen to
give an increased amount of oxygen in the specimen. The atomic
compositions of the heat-treated DPCPS specimens are
summarized in Table 2 as measured by XPS experiments. The
pristine DPCPS specimen is composed of 10.4% of Si and 88.9%
of C, containing only a small amount of oxygen at 0.7%. When
the specimen is further heat-treated, the relative composition of
silicon and oxygen is increased while carbon is decreased. In
particular, it can be seen that the carbon content is substantially
1
00.5 eV, which corresponds to the Si–C bonding of the DPCPS.
ꢀ
For the specimen heat-treated at 300 C, a peak of bonding
energy begins appearing at a higher energy level, which becomes
ꢀ
clear as the heat-treatment temperature reaches 500 C. The peak
ꢀ
appearing at the heat-treatment temperature of 500 C corre-
sponds to the Si–Si bonding centred at 101.8 eV, which demon-
strates that the organic carbon is mostly eliminated between 300
ꢀ
ꢀ
ꢀ
ꢀ
decreased between 500 C and 700 C complying with the TGA
results in Fig. 2. The relative content of Si, C, and O atoms seems
C and 500 C, to form a silicon-based inorganic compound. As
ꢀ
to be stabilized over 700 C at 36.4% of Si, 2.0% of C, and 61.6%
of O resulting in SiC0.05O1.7. It should be noted that the similar
Table 2 Summary of atomic percentage of DPCPS thin films heat-
treated at different temperatures as measured by XPS
Atomic percentage (%)
Heat treatment
ꢀ
temperature ( C)
Si
C
O
No heating
10.4
12.5
14.8
36.4
37.5
88.9
79.6
71.7
2.0
0.7
7.9
14.5
61.6
59.4
3
5
7
9
00
00
00
00
3.0
ꢀ
ꢁ1
Fig. 2 TGA diagram of DPCPS heated at 10 C min in nitrogen.
Fig. 3 XPS diagram of DPCPS thin films thermally treated at different temperatures: (a) Si 2p electrons, (b) C 1s electrons and (c) O 1s electrons.
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composition and oxygen content was detected in the XPS
experiments even when the thermal treatment was performed in
air. It demonstrates that the oxidative reaction can hardly be
avoided in the heat treatment process of unbound Si atoms and,
subsequently, the formation of SiC0.05O1.7 is not significantly
influenced by the presence of oxygen in the environment.
Fig. 4 shows the cross-sectional FE-SEM images of the
DPCPS specimens coated on the gold surface after being heat-
treated at different temperatures. Fig. 4(a) exhibits the pristine
DPCPS film at a thickness of 1.2 mm, which is physically too soft
to be used as a coating material. However, when it is heat-treated
ꢀ
up to 300 C, the film turns hard and the thickness decreases to
3
50 nm, which further decreases to 54 nm at the heat treatment
ꢀ
temperature of 500 C. As seen in Fig. 4(f), the specimen thick-
ꢀ
ness stays almost constant at around 40 nm over 500 C,
complying with the consistent weight and composition as
measured in TGA (Fig. 2) and XPS (Table 2), respectively. The
x y
heat-treated SiC O films are physically hard and optically
transparent exhibiting well-developed continuous, firm, and
uniform features.
Fig. 5 AFM images of DPCPS thin films thermally treated at different
ꢀ
ꢀ
ꢀ
temperatures: (a) no heating, (b) 300 C, (c) 500 C, (d) 700 C, and (e)
ꢀ
The film surface was examined by the AFM images in Fig. 5.
The pristine DPCPS film shows a very flat surface at a RMS
roughness of 0.2 nm. The RMS roughness increases to 4.2 nm
9
00 C. The RMS roughness muasured herein are plotted in (f) as
a function of heat treatment temperature.
ꢀ
and 6.3 nm at the heat treatment temperatures of 300 C and 500
ꢀ
pristine DPCPS specimen in this study. The DPCPS film heat-
ꢀ
C, respectively, seemingly due to the active decomposition
ꢀ
treated at 300 C gives a dielectric constant value of 8.0, which
reactions of organic components. Over 500 C, the RMS
gradually decreases to 4.0, 2.4 and 2.5 with the heat-treatment
ꢀ
roughness becomes below 0.9 nm. Overall, the surface roughness
of the films is good enough to be used for most optoelectronic
applications without any cracks or defects.
temperature increased to 500, 700 and 900 C, respectively.
Compared to the dielectric constant of silica at 3.9, the
ꢀ
SiC0.05
O
1.7 film, developed by heat treatment over 700 C in this
Fig. 6 shows the dielectric constant of the silicon-based thin
films after heat treatment at different temperatures. Since the
pristine DPCPS coating was too soft to be used as a robust
coating film, the dielectric constant was not measured for the
study, gives a dielectric constant of around 2.4–2.5, which is
lower than SiO , seemingly because the carbon atoms incorpo-
2
rated into this film disturb the atomic lattice structure of SiO . It
2
demonstrates that a low-k inorganic thin film can be fabricated
by the DPCPS organic solution process to give dielectric
constants of 2.4–2.5 through a facile heat treatment method. It
should be mentioned that by adjusting the heat treatment
temperature of DPCPS coating, both high-k and low-k dielectric
constants could be tailored, in the range of 2.4 and 8.0.
Fig. 4 FE-SEM images of DPCPS thin films thermally treated at
ꢀ
ꢀ
ꢀ
different temperatures: (a) no heating, (b) 300 C, (c) 500 C, (d) 700 C,
ꢀ
and (e) 900 C. The thicknesses measured herein are plotted in (f) as
Fig. 6 Dielectric constant of DPCPS thin films plotted as a function of
a function of heat treatment temperature.
heat treatment temperature.
3
028 | J. Mater. Chem., 2011, 21, 3025–3029
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4
5
6
E. Hengge and G. Bauer, Monatsh. Chem., 1975, 106, 503.
E. Hengge and G. Bauer, Angew. Chem., 1973, 85, 304.
4
. Conclusions
J. E. Kilpatrick, K. S. Pitzer and R. Spitzer, J. Am. Chem. Soc., 1947,
6
x y
The silicon-based SiC O inorganic thin films were fabricated
9, 2483.
W. Cui, F. Li and N. L. Allinger, J. Am. Chem. Soc., 1993, 115, 2943.
E. Hengge and R. Janoschek, Chem. Rev., 1995, 95, 1495.
using a solution process of DPCPS and heat treatment up to
ꢀ
7
8
9
00 C. The quality of the coated films was excellent. During the
conversion steps of the specimens from organic to inorganic
materials, detailed chemical compositions, coating thickness, and
surface roughness were investigated. Depending on the heat
9 U. Poschl, H. Siegl and K. Hassler, J. Organomet. Chem., 1996, 506,
3.
9
1
0 E. Hengge and H. Marketz, Monatsh. Chem., 1970, 101, 528.
1 J. R. Dahn, Tao Zheng, Yinghu Liu and J. S. Xue, Science, 1995, 270,
590.
1
ꢀ
treatment temperatures between 300 and 700 C, the dielectric
1
2 P. L. Walker Jr., Ed. Chemistry and Physics of Carbon, M. Dekker,
New York, vol. 1–26, pp. 1965.
constants could be tailored from 2.4 to 8.0 by changing the heat
treatment temperatures, resulting from different compositions of
13 K. Ouchi and H. Honda, Fuel, 1959, 38, 429.
14 J.-D. Nam and J. C. Seferis, Carbon, 1992, 30(5), 751.
15 J.-D. Nam and J. C. Seferis, J. Polym. Sci., Part B: Polym. Phys.,
x y
SiC O films.
1
999, 37, 907.
Acknowledgements
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This research was supported by the WCU (World Class
University) program through the Korea Science and Engineering
Foundation funded by the Ministry of Education, Science and
Technology (R31-2008-000-10029-0). We also appreciate project
and equipment support from Gyeonggi Province through the
GRRC program in Sungkyunkwan University.
1
1
8 G. Carlotti, N. Cherault, N. Casanova, C. Goldberg and G. Socino,
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