IR laser-induced thermolysis and UV laser-induced photolysis of 1,3-diethyldisiloxane: Chemical vapour deposition of nanotextured hydridoalkylsilicones

Article abstract of DOI:10.1039/b008437i

IR laser thermolysis and UV laser photolysis of gaseous 1,3-diethyldisiloxane proceed via different mechanisms: the former involves 1,1-H₂ and ethene elimination, whereas the latter is dominated by 1,1,H₂ and ethane elimination. The

Full text of DOI:10.1039/b008437i

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IR laser-induced thermolysis and UV laser-induced photolysis of
1,3-diethyldisiloxane: chemical vapour deposition of nanotextured
hydridoalkylsilicones{
a
Marke´ta Urbanova´, Zdenek Bastl, Jan Subrt and Josef Pola*
b
c
a
ˇ
ˇ
aInstitute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic,
165 02 Prague, Czech Republic
bJ. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
18223 Prague, Czech Republic
c
ˇ
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 25068 Rezˇ near
Prague, Czech Republic
Received 19th October 2000, Accepted 2nd April 2001
First published as an Advance Article on the web 1st May 2001
IR laser thermolysis and UV laser photolysis of gaseous 1,3-diethyldisiloxane proceed via different mechanisms:
the former involves 1,1-H2 and ethene elimination, whereas the latter is dominated by 1,1-H2 and ethane
elimination. The difference plays an important role in determining the composition of the solid nanotextured
films deposited from the gas phase.
on the structure and the decomposition mechanism of the
precursor.
Introduction
Thermal decomposition of peralkylsiloxanes under conven-
tional hot-tube conditions was examined some decades ago
(e.g. ref. 1–6) and continued to attract further attention (e.g.
ref. 7–9). It has been established that the major degradation
mechanism is the rearrangement of the Si–O bonds. No
cleavage of the weaker Si–C bond having been observed
indicated that the more stable Si–O bond is broken via a
kinetically favored path possibly involving ionic impurities or
apparatus surfaces.3
Conversely to the hot-surface heterogeneous conditions,
truly homogeneous, gas-phase IR laser-induced decomposition
of hexamethyldisiloxane takes place exclusively via cleavage of
the Si–C bonds10,11 and some Si–O bond splitting can be only
achieved under conditions close to the dielectric breakdown.12
The IR laser thermolysis13–15 of gaseous methyldisiloxanes
[(CH3)nH32nSi]2O (n~0–2) differs from UV laser photoly-
tic16,17 decomposition of these compounds: the cleavage of the
Si–O bond occurs under thermolysis, but it does not take place
under photolysis. Thus, the IR laser thermolysis of disiloxane
(n~0) is dominated by molecular elimination of silane14 and
that of 1,3-dimethyldisiloxane (n~1)15 and 1,1,3,3-tetra-
methyldisiloxane (n~2)13 proceeds by the cleavage of the Si–
C, Si–H and Si–O bonds. Conversely, the UV photolysis of
disiloxane (n~0) takes place via cleavage of the Si–H bonds16
and that of 1,3-dimethyldisiloxane (n~1) occurs via cleavage of
the Si–C and Si–H bonds.17
In this paper we examine IR and UV laser-induced
photolysis of gaseous 1,3-diethyldisiloxane (DEDSO) and
reveal that these decompositions differ in mechanism and
that they yield different solid nanotextured Si/C/H/O phases
deposited from the gas phase.
Experimental
IR laser thermolytic experiments were conducted in a Pyrex
tube-like (10 cm long, 3.5 cm in diameter) reactor (100 ml in
volume) fitted with two KBr windows. UV laser photolytic
experiments were carried out in a Pyrex reactor (140 ml in
volume) consisting of two orthogonally positioned tubes (both
3.5 cm in diameter), one (10 cm long) fitted with two quartz
plates and the other (13 cm long) furnished with two KBr
windows. Both reactors were equipped with a port for
connecting it to a vacuum manifold, a PTFE valve and a
sleeve with rubber septum. A TEA CO2 laser (Plovdiv
University) operating with a frequency 1 Hz on the R(12)
line of the 0001A1000 transition (970.55 cm21) was used
for the thermolysis experiments; the laser beam was
rectangular (0.561.7 cm) and was focused by a NaCl lens.
An ArF (ELI 94 model) laser operating at 193 nm with a
repetition frequency of 10 Hz and an incident energy of 70 mJ
effective on the area of 2 cm2 was used for the UV photolysis of
DEDSO.
The IR laser thermolysis and UV laser photolysis of gaseous
disiloxanes10–17 as well as pyrolytic laser-induced interaction
with disiloxane aerosols18 are convenient methods for chemical
deposition of hydrogenated silicon-rich Si–O and Si–C–O films
which find applications in microelectronics and are important
in various fields of applied research. In these processes, the
composition of the films was demonstrated to differ depending
The progress of the DEDSO decompositions was monitored
by periodically removing the reactors and placing them in the
cell compartments of the FTIR (Nicolet Impact) spectrometer.
The depletion of DEDSO was followed using its diagnostic
absorption band at 1087 cm21 and the accumulation of
gaseous products was monitored by FTIR spectroscopy and
by gas chromatography (a Shimadzu GC 14A chromatograph
coupled with a Chromatopac C-R5A computing integrator,
Porapak P column, programmed (20–150 ‡C) temperature,
helium carrier gas). The hydrocarbon and organosilicon
volatile products were identified on a Shimadzu model QP
1000 quadrupole mass spectrometer (ionizing voltage 70 eV).
{Electronic supplementary information (ESI) available: GC trace of
volatile products from photolysis of DEDSO; XPS and Auger electron
spectra of the nanotextured films. See http://www.rsc.org/suppdata/jm/
b0/b008437i/
Hydrogen was determined on
a Kratos MS 80 mass
DOI: 10.1039/b008437i
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This journal is # The Royal Society of Chemistry 2001
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Chemistry in the gas phase
IR laser thermolysis. The laser thermolysis results in a visible
luminescence after each pulse, depletion of DEDSO, the
formation of gaseous products (Fig. 1) and of a solid yellowish
material depositing onto the reactor surface. The relative
amounts of the formed gaseous products are weakly dependent
on the irradiation fluence and on the number of pulses
(Table 1). The major products are ethene, hydrogen and
disiloxane which are accompanied by methane, ethane,
propane, butane, methylsilane and silane. The amounts of
the observed hydrocarbons rule out the formation of ethyl
radical being the dominant decomposition step; this species
would have undergone disproportionation and recombination
reactions to produce equal amounts of ethene and ethane and
substantially higher amounts of butane.20 The preponderance
of ethene indicates that this product is formed by molecular
expulsion and that radical steps are of low importance.
However, the small amounts of the C1–C4 hydrocarbons are
in line with a small extent of Si–C and C–C bond cleavages,
recombination of the CH3 and C2H5 radicals and abstraction
of H from DEDSO with the CH3 radical. The molecular (four-
centre) elimination of ethene can, in principle, take place from
DEDSO and ethyldisiloxane to yield disiloxane (route A), or
from intermediate silylenes (route B) originating from 1,1-H2
elimination from DEDSO (Scheme 1). The observation that
disiloxane and molecular hydrogen are the main products is in
keeping with the occurrence of both paths. We note that these
steps were also observed in the thermolysis of structurally
similar monoorganosilanes: the infrared multiphoton decom-
position of monoalkylsilanes was proposed to occur via a four-
centre elimination of silane21,22 or 1,1-H2 elimination,23 and the
shock-induced thermolysis of these compounds was interpreted
as a blend of 1,2-H2 and 1,1-H2 elimination.24,25 The minor
extent of disiloxane decomposition via elimination and
polymerization of silanone H2SiLO is supported by the
presence of silane; this reaction is a well documented process
when disiloxane is exposed to TEA CO2 laser radiation.14,26
Ethylsilane having been not observed among the products
indicates that this mode of decomposition of DEDSO and of
ethyldisiloxane into ethylsilane and silanones (e.g. route C) is
not as feasible.
Fig. 1 Typical GC trace of volatile products from IR laser thermolysis
of DEDSO (1, methane; 2, silane; 3, ethene; 4, ethane; 5, methylsilane;
6, propane; 7, disiloxane; 8, butane; 9, ethyldisiloxane; 10, DEDSO.
spectrometer using standard mixtures of hydrogen and
methane. The quantitative analysis of gaseous products
relied on the knowledge of the FID or TIC response factors
of the authentic samples from our stock.
In order to evaluate properties of the deposited solid material
by FTIR and X-ray photoelectron spectroscopy (XPS), as well
as by electron microscopy, the films were produced on different
substrates (KBr lumps, metal sheets) accommodated in the
reactors before irradiation.
The X-ray photoelectron and Auger electron spectra were
recorded in ESCA 310 (Gammadata Scienta) and ESCA 3
MkII electron spectrometers. The background pressure during
the spectral measurement was in the 1027 Pa range. Electron
spectra were recorded using Al Ka radiation. Detailed spectral
scans were taken over the Si 2p, C 1s, O 1s and Si KLL regions.
The samples were measured as received and after sputtering by
argon ions (E~5 keV, I~20 mA, t~5 min). The aim of the ion
sputtering was to remove the superficial layers which could be
oxidized during the sample transport from the reactor to the
spectrometer. The elemental surface concentrations were
calculated from the photoelectron peak intensities using
theoretical photoionization cross-sections.
Routes A and B of the DEDSO decomposition in Scheme 1
lead to different final products; the former affords perhydrido-
silicone, while the latter yields a silicone polymer incorporating
some Si–Si bonds and ethyl and H substituents. Route B
affords various silylenes which can be inferred24,25 to undergo
insertions into Si–H bonds of DEDSO and of the siloxanes
produced by elimination of silanes from these products of
insertion. These reactions make the total scheme of the
formation of solid macromolecules rather complex.
The SEM analyses were conducted on a Philips XL30 CP
Scanning Electron Microscope.
DEDSO was prepared according to the literature proce-
dure19 and its purity was better than 96% as checked by gas
chromatography.
Results and discussion
Routes A and B are reflected in the stoichiometry of the
deposit as discussed below.
Gaseous DEDSO (5 Torr) was decomposed by tuning the TEA
CO2 laser radiation into the absorption (dSiH2) band centered
at 969 cm21 (absorptivity 0.021 Torr21 cm21). The ArF laser-
induced photolysis of gaseous DEDSO (5 or 20 Torr) in an
excess of helium gas (atmospheric pressure) was conducted by
irradiating a gaseous sample at 193 nm hitting the tail of
the UV absorption band with the absorptivity at 193 nm,
UV laser photolysis. The laser photolysis results in the
formation of volatile hydrocarbons and hydrogen together
with a white fog observed throughout all the inside of the
reactor. The fog slowly descends onto the reactor bottom and
produces white solid films. No silicon-containing volatile
1.661024 Torr21 cm21
Table 1 Distribution of products in laser thermolysis of DEDSO
Gaseous products (relative mol%)
.
Fluence/J cm22
No. of pulses
Depletion (%)
CH4
C2H4
C2H6
C3H8
C4H10
SiH4
CH3SiH3
(H3Si)2O
H2/CH4
0.43
0.67
1.72
1.72
100
700
30
11.6
24.6
18.7
34.0
5.2
1.5
6.0
5.3
57.6
59.8
62.3
60.1
4.6
0.7
0.8
0.5
0.2
0
0.2
0.3
0
0.1
0
0.3
0.1
0.3
1.0
2.7
0.7
2.1
2.1
29.4
37.1
28.3
30.7
4.3
—
10
24
70
0
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Scheme 1
products have been detected. Typical hydrocarbon product
distribution as dependent on the photolysis progress (Fig. 2)
reveals ethane (a major product) accompanied by smaller
amounts of ethene, methane, propane and butane. The absence
of Si-containing volatile products indicates that the major
decomposition step is the cleavage of the Si–C and/or Si–H but
not that of the Si–O bond. The energy delivered by the photons
at 193 nm is equal to ca. 620 kJ mol21 and is in excess of the
energy required to split the Si–C (y370 kJ mol21), C–H
(y410 kJ mol21) and Si–H (y380 kJ mol21) and even of the
strong Si–O bond (y530 kJ mol21).27 However, the Si–O bond
cleavage is not a feasible photochemical step28,29 and the
expected primary photolytic processes are 1,1-H2 and 1,1-
[hydrocarbon] eliminations.16,30,31
indicates that ethane originates via molecular extrusion. This
route gets support from the known preference for combination
over disproportionation of small radicals in the gas phase.20
The low amounts of butane observed in our experiments are
thus in line with an insignificant occurrence of ethyl radicals
which recombine to butane and disproportionate to ethane and
ethene. The observation of propane reflects the cleavage of the
C–C bond and the recombination of ethyl and methyl radicals.
Methane is likely formed by the abstraction of H from
DEDSO. These products of radical reactions indicate that a
small portion of ethane can be produced by abstraction of H by
ethyl radicals from DEDSO, or by recombination of methyl
radicals.
The observation of H2 (in amounts of 1.5–1.8 times those of
ethane) can be explained by 1,1-molecular elimination of H2.
Some of the plausible major and minor steps are shown in
Scheme 2, where path B is that given in Scheme 1. These steps
imply that, compared to DEDSO, the solid product is poorer in
H and C but has the same Si : O ratio.
The predominance of ethane among the hydrocarbons
The formation of solid agglomerates is compatible with
aggregation of silylenes taking place via sequences of the
insertion of silylenes into the Si–H bonds of DEDSO and the
siloxanes produced by elimination of silanes from these
products of insertion. Thus, the total scheme of the formation
of solid macromolecules is, similarly to that for the IR laser
decomposition, rather complex.
Mass spectra of DEDSO and ethyldisiloxane. Comment on
the fragmentation of DEDSO and ethyldisiloxane under
electron impact is pertinent. The mass spectra of DEDSO
[m/z (relative intensity, %)~134 (10), 133 (41), 132 (29), 106
(13), 105 (92), 104 (20), 103 (22), 78 (16), 77 (100), 76 (14), 75
(50), 74 (11), 73 (17), 72 (7)] and of ethyldisiloxane [m/z (relative
intensity, %)~106 (11), 105 (60), 104 (10), 78 (12), 77 (100), 76
(30), 75 (48), 74 (9), 73 (17), 72 (8)] consist of signals
corresponding to the loss of ethene and of 1–5 or 1–6 H
atoms. Both mass spectra were obtained by GC/MS analysis. It
Fig. 2 Hydrocarbon product yield at different photolysis progress.
($ ethane, % ethene, 6 methane, # propane, +butane).
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Table 2 FTIR spectra of the solid deposit from IR laser thermolysis
(f~0.67 J cm22 (a) and 1.72 J cm22 (b)) and UV laser photolysis (c) of
DEDSO
Absorptivitya
Vibration (wavenumber/cm21
)
a
b
c
n(Si–C)zd(Si–H) (850)
n(Si–C) (880)
dSiH2 (970)
n(Si–O) (1080)
n(Si–H) (2150)
n(C–H) (2980)
n(C–H) (2920)
n(C–H) (2960)
0.72
0.66
0.60
1.0
0.47
0.15
0.17
0.26
0.63
0.59
0.54
1.0
0.24
0.07
0.11
0.12
1.20
—
0.36
1.0
0.47
0.17
0.18
0.27
Scheme 2
aNormalized to that of the n(Si–O) band.
is evident that the electron impact-induced fragmentation
of the (C2H5)H2SiOSiH2X (X~H, C2H5) molecules has
similar features to the IR laser thermolysis of (C2H5)H2SiO-
SiH2(C2H5).
differ and suggests that proposed route B is a significant
pathway, since the exclusive operation of route A would have
afforded an unobserved Si–O material. The determined
stoichiometry of the topmost layers of the deposits reveals
that the material from the UV laser photolysis possesses more
carbon than the deposits from the IR laser thermolysis. This is
in accordance with the occurrence of carbon-richer polymeriz-
able transients in the UV laser-induced process. The amount of
oxygen in all the deposits being higher than 0.5 of that of silicon
indicates that the superficial layers are prone to oxidation and
that they uptake oxygen from air when the deposit is
transferred for the XPS analysis.
More detailed insight into the chemistry of these materials is
provided by the Si 2p core level spectra as well as the more
resolved Si KL23L23 Auger electron spectra that reveal that
these films are composed of different components. Three
chemical states of Si are observed in the electron spectra of
samples from the IR laser thermolysis, while only one state has
been identified in samples from the UV laser photolysis
(Table 4).
The films deposited by the IR laser thermolysis are composed
of small amounts of elemental silicon (designated as Si(1)), a
silicon in a polymer whose chemical environment is similar to
that in oxosilanes (designated as Si(2)), and of an Si(3) silicon
which can be interpreted as a product of interaction of the Si(2)
state with atmospheric oxygen and/or moisture. The Si(3) state
predominates in the as received samples, but upon ion
sputtering its population decreases and those of the Si(1) and
Si(2) states increase. The above assignments are based on
comparison of the measured spectral data with the published
ones.33
The films produced by the UV laser photolysis contain only
one chemical state of Si which is not identical to any of the
states found in the deposits produced by the IR laser
thermolysis. The measured binding energy of Si 2p electrons
as well as the kinetic energy of Si KLL electrons fall within the
range of values expected for Si/C/O/H polymers.33 The ion
sputtering of this material leads to broadening of the spectra
and shifting of the Auger peak by 2.4 eV towards higher kinetic
energies. This shift is much higher than the shift of the Si 2p
line, 0.3 eV and can be interpreted as being caused by increased
extraatomic relaxation due to modification of the polymer
structure by argon ions. We did not observe such large shifts of
Deposited films
The solid films deposited from IR laser thermolysis and UV
laser photolysis of DEDSO show similar IR spectral patterns
(Fig. 3), consisting of bands assigned in Table 2. The
absorptivity ratios A[n(C–H)] : A[n(Si–H)] for the deposit
from the low-fluence and high-fluence IR laser thermolysis
(0.55 and 0.50, respectively) and that from the UV laser
photolysis (0.57) reveal similar ratios of C–H and Si–H bonds
in all the three materials. These values are in line32 with the
more than double content of the H(C) centers compared to the
H(Si) centers. The absorptivity ratios A[n(Si–H)] : A[n(Si–O)]
with these solids are, in the given order, 0.47, 0.24 and 0.47;
these values are compatible with fewer H(Si) (and also H(C))
centers in the deposit from the high-fluence IR laser thermo-
lysis and show that reaction steps in which Si–H and C–H
bonds get lost are more important in this process. The deposits
from low-fluence IR laser photolysis and UV laser photolysis
possess similar contents of H(Si) and H(C) centers, but their
spectral pattern at 850 and 880 cm21 is different and in keeping
with a different Si–C–(O) framework.
XPS and Auger electron spectral analyses of the deposited
films before and after sputtering by Ar ions allow us to draw
inferences about the chemistry of these materials and about the
relative importance of the paths proposed to take place in the
gas phase.
The surface stoichiometry assessed from the intensities of the
photoemission lines of the deposited films (Table 3) is discussed
first. The values from the Table 3 can be compared to the
stoichiometry of DEDSO Si1C2O0.5 and reconciled with the
observed escape of the Si, C and O elements from the formed
solid product into the volatile products. The stoichiometry of
the topmost (ca. 5 nm in depth) layers of the deposits from the
low- and high-fluence laser thermolysis does not practically
Table 3 Stoichiometrya of the deposits determined by XPS analysis
Deposition method
Beforeb
Afterc
Laser thermolysis, f ~0.67 J cm22
Laser thermolysis f~1.72 J cm22
UV laser photolysis
Si1.0C0.8O1.2
Si1.0C1.0O1.5
Si1.0C1.8O0.9
Si1.0C0.6O0.9
Si1.0C0.8O1.1
Si1.0C1.1O0.8
b
c
a¡10%. Sample as received. Sample after sputtering by argon ions.
Fig. 3 FTIR spectra of the deposit from IR laser thermolysis (a) and
UV laser photolysis (b) of DEDSO.
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Table 4 Si 2p core level binding energies and populations of individual chemical states of Si in the deposits
Si 2p binding energy/eV [population (%)]
Deposit from laser thermolysis,
f~0.67 J cm22
Deposit from laser thermolysis,
f~1.72 J cm22
Deposit from UV laser photolysis
Silicon environment
Beforea
Afterb
Beforea
Afterb
Beforea
Afterb
Si(1)
Si(2)
Si(3)
Si
99.5 [9]
101.0 [12]
102.6 [79]
—
99.5 [15]
101.1 [57]
103.0 [28]
—
99.9 [8]
101.0 [11]
102.8 [81]
—
99.8 [56]
101.7 [30]
102.8 [14]
—
—
—
—
—
—
102.0 [100]
—
102.3 [100]
aSample as received. bSample after sputtering by argon ions.
Auger spectra with ion sputtering for films produced by IR
laser thermolysis.
presence of unsaturated silicon centers which are not
appreciably produced by the UV photolysis.
In an attempt to explain the presence of the Si(1), Si(2) and
Si(3) forms in the deposit from the IR laser-induced thermo-
lysis, and the absence of these forms in the deposit from the UV
laser photolysis, we assume that the elemental silicon is mostly
formed from the detected silane. Indeed, the infrared multi-
photon decomposition of silane into elemental silicon is
well documented in the literature.34–36 The population of the
elemental Si in the deposit obtained at the low-fluence
irradiation is lower than that obtained at the high-fluence
irradiation; this indicates that the higher fluences of radiation
facilitate silane decomposition. Both the Si(2) form produced by
IR laser photolysis and the Si form produced by UV laser
photolysis are silicones, but their differing Si 2p binding
energies result from different structures. The formation of the
Si(2) type is caused by reactions of silanone, H2SiLO
which is formed by the IR laser-induced decomposition of
disiloxane.26 The silanone can isomerize into hydroxysilylene
(HO)HSi:37,38 which can decompose into elemental silicon and
water, or react with molecular hydrogen to yield silanol. These
steps were shown to occur in IR laser-induced thermolysis of
disiloxane,26 but their occurrence in the UV laser photolysis is
not substantiated. The formation of the Si(3) form upon
exposure of the Si(2) form to air obviously relates to some
Scanning electron microscopy analysis reveals that all the
deposits, regardless of their formation under different irradia-
tion conditions, consist of nanotextured agglomerates which
are bonded together (Fig. 4). This feature is in keeping with the
agglomeration occurring in the gas phase and may indicate that
once deposited, the agglomerates increase their size also after
having been deposited on the substrate.
All the deposits are insoluble in organic solvents (ethanol,
diethyl ether, chloroform, hexane and toluene), which indicates
their high molecular weight and/or cross-linked structure.
Conclusion
The TEA CO2 laser thermolysis of DEDSO is initiated by
elimination of H2 and ethene, whereas ArF laser photolysis
begins as elimination of H2 and ethane. Both processes proceed
as a multitude of subsequent reactions and differ in the final
products. The IR laser-induced thermolysis of DEDSO yields
hydrogen, hydrocarbons and Si- and O-containing volatile
compounds and affords chemical vapour deposition of
solid hydridoalkylsilicone films containing elemental silicon.
The UV laser-induced photolysis of DEDSO results in the
Fig. 4 SEM of the deposit obtained by IR laser thermolysis (a) and UV laser photolysis (b) of DEDSO.
J. Mater. Chem., 2001, 11, 1557–1562
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ˇ
16 J. Pola, M. Urbanova´, Z. Bastl, J. Subrt and H. Beckers, J. Mater.
Chem., 1999, 9, 2429.
formation of hydrogen and hydrocarbons and affords chemical
vapour deposition of solid hydridoalkylsilicone films.
ˇ
17 J. Pola, A. Ouchi, Z. Bastl, J. Subrt, M. Sakuragi, A. Gal´ıkova and
A. Gal´ık, Chem. Vap. Deposition, 2001, 7, 19.
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Acknowledgements
The work was supported by GAAVCR of the Czech Republic
ˇ
(grant no. A4072806). The authors thank Mr J. Kupc´ık for MS
determination of hydrogen.
22 J. W. Thoman and J. I. Steinfeld, Chem. Phys. Lett., 1986, 124, 35.
23 D. M. Rayner, R. P. Steer, P. A. Hackett, C. L. Wilson and
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