Initial stages in the growth of carbon films produced in an Ar-CH4-H2 microwave discharge: Composition and surface layers morphology

Article abstract of DOI:10.1063/1.115675

X-ray photoelectron spectroscopy and atomic force microscopy are employed in the characterization of the first stages in the growth of carbon layers on a (001) Si substrate which is not scratched with diamond powder before placing it in an Ar-2%CH4-H2 plasma discharge. Results show that the first layers could be formed in SiC grains where the carbon diamond particles nucleate. The high nucleation density of 1.109-5.109 nuclei. cm-2 and the low aggregates density lead to a smooth surface.

Full text of DOI:10.1063/1.115675

Initial stages in the growth of carbon films produced in an Ar–CH4–H2 microwave
discharge: Composition and surface layers morphology
L. Thomas, I. Jauberteau, J. L. Jauberteau, M. J. Cinelli, J. Aubreton, and A. Catherinot
Citation: Applied Physics Letters 68, 1634 (1996); doi: 10.1063/1.115675
View online: http://dx.doi.org/10.1063/1.115675
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/12?ver=pdfcov
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Initial stages in the growth of carbon films produced in an Ar–CH –H
4
2
microwave discharge: Composition and surface layers morphology
L. Thomas, I. Jauberteau, J. L. Jauberteau, M. J. Cinelli, J. Aubreton, and A. Catherinot
URA 320 CNRS, Facult e´ des Sciences, 123 av. A. Thomas, F-87060 Limoges, France
͑
Received 21 March 1995; accepted for publication 18 January 1996͒
X-ray photoelectron spectroscopy and atomic force microscopy are employed in the characterization
of the first stages in the growth of carbon layers on a ͑001͒ Si substrate which is not scratched with
diamond powder before placing it in an Ar–2%CH –H plasma discharge. Results show that the
4
2
first layers could be formed in SiC grains where the carbon diamond particles nucleate. The high
9
9
Ϫ2
nucleation density of 1.10 –5.10 nuclei. cm and the low aggregates density lead to a smooth
surface. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03812-8͔
Carbon or diamond layers deposited using reactive low
pressure plasma are now extensively used in a wide range of
technological areas because of their attractive physical and
chemical properties. Diamond has a high thermal conductiv-
ity, the best hardness known so far and a wide band optical
transparency. Moreover, it is quite a good electrical insulator.
However, a great number of industrial applications re-
quires a better control of the interfacial structure which is
strongly correlated to the nucleation, epitaxial growth, defect
formation, and adhesion on the substrate.1
X-ray photoelectron spectroscopy ͑XPS͒ is useful in the
characterization of the surface composition and the hybrid-
ization of surface elements. Atomic force microscopy ͑AFM͒
is a good tool for atomic scale measurements of surface mor-
phology because of its high lateral ͑0.01 nm͒ and its high
vertical ͑1 nm͒ resolution. Moreover, samples do not need
any preparation before analysis.
along the axis of the reactor from positions close to the cen-
ter of the discharge ͑6 cm͒ to positions far in the postdis-
charge ͑20 cm͒. As the layers consist almost entirely of
3
C sp bonding when the substrate is close to the center of
3
the discharge, the depositions were performed at 6 cm of the
discharge center. Under these conditions, the density of
CHxϽ3 remains low resulting in a low growth rate of the
carbon films which allows us to investigate the first stages of
carbon films growth. The microwave power is equal to 900
W. The substrate holder is heated at 800 °C. The experiments
are performed during 1, 2, and 3 h. Before each deposition,
the Si substrate is cleaned in acetone but is not scratched
with diamond powder.
,2
XPS studies are performed with a Leybold Heraeus LHS
1
2 ͑CNRS, Universit e´ of Nantes͒ using a nonmonochromatic
Mg K␣ radiation h␷ϭ1253.6 eV͒ and a spectrometer pass
energy of 50 eV.
AFM measurements are conducted with a Digital Instru-
ments Nanoscope II operating in constant force mode. A very
small tip attached to a cantilever spring ͑1ϭ200 ␮m͒ with a
The aim of this letter is to investigate the initial stages of
carbon layers growth on ͑001͒ Si wafers in an Ar–CH –H2
4
microwave plasma.
Ϫ1
force constant of 0.12 N m touches the sample surface and
The experimental setup is shown in Fig. 1. A microwave
discharge is produced within a fused silica tube ͑external
diameter ϭ19 mm and internal diameter ϭ16 mm͒. The
power supply is a SAIREM GMp 12 kE operating between 0
and 1200 W. Active species are generated in the plasma and
are carried out into the stainless-steel reactor containing the
the attractive and/or repulsive forces bend the cantilever
spring.4 The deflection of the cantilever is optically
5
detected: a laser beam is specularly reflected from the can-
tilever surface and the direction of the reflected light beam is
sensed with 4
a
position sensitive ͑two elements͒
,5
photodetectors.
sample holder which is located far from the discharge center.
Three piezoelectric ceramics x, y, and z move the tip in
Ϫ5
Before each experiment, a pressure of 10
Pa is main-
tained by means of a turbomolecular pump ͑Balzer TPU
80H͒. During deposition, the total pressure in the vessel is
kept constant using an Alcatel roots blower pump ͑70–700
1
3
Ϫ1
m h ͒ which maintains a gas drift velocity of about 5–100
Ϫ1
m s . The substrate temperature is controlled using a ther-
mocouple calibrated with a two color pyrometer ͑IRCON
mirage͒.
The carbon films are deposited with a 2% CH4 in
Ar–H gas mixture at a P͑H ͒/P͑Ar͒ partial pressure ratio of
2
2
2
in order to maximize CHxϽ3 concentrations and H atom
density, respectively. The choice of these parameters is the
result of a study described in detail in Ref. 3. At a suitable
pressure of 0.13 kPa, achieved using a total flow rate of 2000
sccm, the plasma is expanded out of the discharge center.
This reactor geometry allows us to select the impinging car-
bon species from CHxϽ3 to C H by moving the substrate
FIG. 1. Schematic of the experimental setup.
2
y
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30.63.180.147 On: Sat, 22 Nov 2014 23:21:31
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1

FIG. 2. XPS spectra of Si 2p and C 1s core levels of as-deposited C layers
in an Ar–CH –H plasma at various times; ͑a͒ before C deposition, ͑b͒ 1 h,
4
2
͑c͒ 2 h, and ͑d͒ 3 h.
the scanning area ͑x, y) of up to 12 ␮mϫ12 ␮m. In the
constant force mode, the net force between the tip and the
sample is used as the feedback parameter in the z feedback
circuit which controls the displacement of the sample in the
z direction normal to the surface. The height profile is ob-
tained after each ͑x, y͒ scan.
The atomic force images are not filtered nor processed.
They are just fit by subtracting a ‘‘polynomial plane’’ which
consists of a surface whose cross section is a second order
polynomial in one axis and an horizontal line in the other
axis.
XPS spectra of Si 2p and C 1s recorded in function of
deposition time are reported in Fig. 2. Before deposition, the
Si wafer is oxidized: the Si 2p core level signal correspond-
ing to Si and SiO are found at 99.2 and 103.6 eV, respec-
2
tively. These values are in good agreement with the binding
energies in Ref. 6. A chemical shift of 1 eV towards lower
binding energy for SiO is reported in Ref. 7.
2
An Ar–H –CH plasma exposure leads to a strong de-
2
4
crease of the peak intensity at 99.2 eV. Furthermore, the peak
corresponding to the silicon oxide disappear. The SiC signal
appears at 101 eV after 1 h of deposition. The SiC binding
energy is consistent with Ref. 8 but is sightly higher by about
0.8 eV than that of SiC indicated in Refs. 6 and 7. This
binding energy shift towards higher energy could be caused
by electrostatic charging effects. These results indicate that a
thin SiC layer of about 10 nm has formed at the carbon
diamond/Si interface.
FIG. 3. AFM images of an as-deposited
C film exposed to an
The C 1s core level spectra consist of two peaks after 1
h of deposition. The signal at 283 eV, typical of carbidic
Ar–CH –H2 plasma during 1 h. ͑a͒ 5 ␮mϫ5 ␮m scan, ͑b͒ 1 ␮mϫ1 ␮m
4
scan, and ͑c͒ 250 nmϫ250 nm scan.
9
carbon is also slightly shifted towards higher binding ener-
carbon diamond layer growth.6,7,10 Its intensity increases
with increasing time deposition.
gies and corresponds to the formation of a SiC compound.
The peak at 285 eV is probably due to a spectra convolution
of the C 1s XPS signal at 285.5 eV indicated on the spectrum
The morphology of the carbon film deposited in 1 h is
shown in Fig. 3. The nucleation density is very large because
͑
a͒ and another one at lower binding energy. The peak at
85.5 eV is assigned to remaining adsorbed hydrocarbons.
The peak at lower binding energy is correlated to subsequent
9
9
Ϫ2
2
it ranges between 1.10 and 5.10 nuclei cm . The carbon
film consists of small and rather round grains very close to
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Appl. Phys. Lett., Vol. 68, No. 12, 18 March 1996 Thomas et al. 1635
30.63.180.147 On: Sat, 22 Nov 2014 23:21:31
1

each other. The largest grains are ϳ200 nm wide and 40 nm
high. According to Figs. 3͑b͒ and 3͑c͒, one grain located
above the first layers seems to be slightly faceted.
The surface structure is consistent with previous AFM
studies of MPCVD diamond films containing a large amount
We thank the IMN of Universit e´ of Nantes for XPS mea-
surements. This study has been partly supported by DRET
Contract No. 90-341 and EDF Contract No. A 7081/
AEE1333.
of carbon phase which are deposited with 2% of CH in
1
2
4
J. C. Angus and C. C. Hayman, Science 241, 913 ͑1988͒.
W. Zhu, B. R. Stoner, B. E. Williams, and J. T. Glass, Proc. IEEE 79, 621
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H2
and with an AFM structure of the early stage nucle-
13
͑
1991͒.
ation of diamond. AFM images of MPCVD carbon films
show many aggregates of particles of 30–40 nm in size.14
According to Refs. 11, 12, 14, and 15, there is no sig-
nificant difference between the layer morphology of
MPCVD carbon diamond and silicon carbide. Since XPS
data indicate a large amount of SiC, we suggest that the
layers consist of mainly SiC grains where the carbon dia-
mond particles nucleate. This is corroborated by EDS studies
that investigate the interface between silicon substrate and
3
4
5
6
L. Thomas, Ph.D. thesis, University of Limoges, 1993.
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1
1
1
4
5
6
during carbon deposition in an Ar–CH –H2 microwave
4
plasma. The high nucleation density achieved without any
surface pretreatment and the quite small amount of aggre-
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636 Appl. Phys. Lett., Vol. 68, No. 12, 18 March 1996 Thomas et al.
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