Deposition of hydrogen-free diamond-like carbon film by plasma enhanced chemical vapor deposition

Article abstract of DOI:10.1063/1.116648

Hydrogen-free diamond-like carbon (DLC) films were deposited by the layer-by-layer technique using plasma enhanced chemical vapor deposition (PECVD), i.e., the alternative deposition of thin DLC layer and subsequent CF4 plasma exposure on its surface. The hydrogen-free DLC could be grown on the Si wafer by repeated deposition of the 5 nm DLC layer and subsequent 200 s CF4 plasma exposure on its surface. On the other hand, the conventional DLC deposited by PECVD contains 25 at.% hydrogen inside. The CF4 plasma exposure on the thin DLC layer appears to etch weak C-C bonds and break hydrogen bonds, resulting in a widening optical band gap and increasing conductivity activation energy.

Full text of DOI:10.1063/1.116648

Deposition of hydrogenfree diamondlike carbon film by plasma enhanced chemical
vapor deposition
Kyu Chang Park, Jong Hyun Moon, Jin Jang, and Myung Hwan Oh
Citation: Applied Physics Letters 68, 3594 (1996); doi: 10.1063/1.116648
View online: http://dx.doi.org/10.1063/1.116648
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/25?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Pulsed laser deposition of hydrogenated amorphous diamondlike carbon films from a polymer target
J. Appl. Phys. 80, 5398 (1996); 10.1063/1.362726
Textured diamond growth by low pressure flat flame chemical vapor deposition
Appl. Phys. Lett. 69, 2258 (1996); 10.1063/1.117146
Effect of gas composition and bias voltage on the structure and properties of aC:H/SiO2 nanocomposite thin
films prepared by plasmaenhanced chemicalvapor deposition
J. Vac. Sci. Technol. A 14, 2702 (1996); 10.1116/1.580191
Hydrogenfree amorphous carbon films approaching diamond prepared by magnetron sputtering
Appl. Phys. Lett. 69, 158 (1996); 10.1063/1.116906
Diamondlike carbon films grown by very high frequency (100 MHz) plasma enhanced chemical vapor deposition
technique
Appl. Phys. Lett. 69, 49 (1996); 10.1063/1.118114
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.203.227.63 On: Thu, 10 Jul 2014 12:40:03

Deposition of hydrogen-free diamond-like carbon film by plasma enhanced
chemical vapor deposition
Kyu Chang Park, Jong Hyun Moon, and Jin Janga)
Department of Physics, Kyung Hee University, Dongdaemoon-ku, Seoul 130-701, Korea
Myung Hwan Oh
Division of Electronics and Information Technology, Korea Institute of Science and Technology,
P.O. Box 131, Cheongrang, Seoul 130-650, Korea
͑Received 23 January 1996; accepted for publication 16 April 1996͒
Hydrogen-free diamond-like carbon ͑DLC͒ films were deposited by the layer-by-layer technique
using plasma enhanced chemical vapor deposition ͑PECVD͒, i.e., the alternative deposition of thin
DLC layer and subsequent CF4 plasma exposure on its surface. The hydrogen-free DLC could be
grown on the Si wafer by repeated deposition of the 5 nm DLC layer and subsequent 200 s CF4
plasma exposure on its surface. On the other hand, the conventional DLC deposited by PECVD
contains 25 at. % hydrogen inside. The CF4 plasma exposure on the thin DLC layer appears to etch
weak C–C bonds and break hydrogen bonds, resulting in a widening optical band gap and
increasing conductivity activation energy. © 1996 American Institute of Physics.
͓S0003-6951͑96͒03125-7͔
The diamond-like carbon ͑DLC͒ film with no incorpo-
rated hydrogen has considerable interest, because of its high
hardness and substitutional doping capability compared to its
hydrogenated counterpart.1–3 The DLC film deposited by
plasma enhanced chemical vapor deposition ͑PECVD͒ has
much hydrogen inside, typically higher than 20 at. %.4 The
incorporated hydrogen reduces film hardness. Hydrogen-free
DLC film can be obtained by a filtered vacuum arc deposi-
tion or by ion beam deposition.5,6 Mckenzie et al., deposited
hydrogen-free DLC with more than 85% sp3 fraction by a
filtered vacuum arc deposition.7 However, large area uniform
deposition is not easy when using this method.
measured using a Perkin-Elmer UV-VIS-IR spectrophotom-
eter and the optical band gap was obtained using Tauc’s
plot.9
Figure 1 shows the FTIR transmittance spectra of the
DLC films. The absorption peaks at 2870 cmϪ1, 2925
cmϪ1, and 2960 cmϪ1 corresponding, respectively, to sp3
CH3 ͑symmetrical͒, sp3 CH2 ͑asymmetrical͒ and sp3 CH3
͑asymmetrical͒ modes,10 appear in the FTIR spectrum for a
conventional DLC film. The hydrogen content obtained from
the absorption coefficient8 for the conventional DLC film
was 25 at. %. However, the hydrogen content for the DLC
film deposited with 200 s CF4 plasma exposure time was
found to be less than 1 at. %. As can be seen from the figure,
the CHn vibration intensity disappears completely when the
CF4 plasma exposure time is 200 s. Therefore, we can de-
posit a hydrogen-free DLC film by PECVD using layer-by-
layer deposition technique.
In the present work, the layer-by-layer deposition
method, i.e., the deposition of thin DLC layer and subse-
quent exposure of its surface to CF4 plasma, was applied to
deposit hydrogen-free DLC film.
We used a conventional PECVD system, in which rf
power was applied to the substrate holder. CH4/H2/He and
CF4/He were introduced for the deposition of the DLC layer
and surface treatment, respectively. A glass plate and silicon
wafer were used as the substrates for film deposition.
Table I depicts the layer-by-layer deposition conditions
for the DLC films. The flow rates of He, H2, and CH4 were
fixed at 50 sccm, 5 sccm, and 1 sccm, respectively, for DLC
deposition and the flow rate of CF4 was fixed at 30 sccm for
the plasma treatment. The self-bias voltage was found to be
Ϫ120 V at a fixed rf power of 100 W, and it depended
strongly on the gas pressure and on the rf power used. The
100 s growth under the deposition mode shown in Table I
resulted in 5 nm thick DLC layer. We have carried out 50
times repeated deposition and plasma exposure to obtain 200
nm thick DLC film which was used to measure FTIR ͑Fou-
rier transform infrared͒ absorption. The stretching mode ab-
sorptions due to CHn ͑nϭ1,2,3͒ were investigated and hydro-
gen content was calculated by using the absorption
coefficients.8 Interband optical absorption coefficients were
Figure 2 shows the optical band gap (Eogpt) of the DLC
film obtained from Tauc’s plot. The optical band gap in-
creases from 1.2 to 1.4 eV with increasing CF4 plasma ex-
posure time. The increase in the optical band gap appears to
be due to the preferential etching of graphite phase in DLC
by CF4 plasma exposure.11 The bonding and antibonding
states of C–C sp2 lie in the inner side of those for
C–C sp3 ͑Ref. 12͒. The removal of the sp2 bonds, there-
fore, results in the widening band gap of the DLC.
TABLE I. Layer-by-layer deposition conditions for the DLC films.
Condition
Deposition
CF4 plasma exposure
rf power ͑W͒
Pressure ͑mbar͒
Flow rates ͑sccm͒
He
100
400
100
450
50
5
50
0
H2
CH4
1
0
CF4
0
30
Sub. temp. ͑K͒
Time ͑s͒
300
100
300
0–200
a͒
Electronic mail: jjang@nms.kyunghee.ac.kr
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
3594 Appl. Phys. Lett. 68 (25), 17 June 1996 0003-6951/96/68(25)/3594/2/$10.00 © 1996 American Institute of Physics
132.203.227.63 On: Thu, 10 Jul 2014 12:40:03

FIG. 3. Temperature dependence of conductivity for the deposited DLC
films.
FIG. 1. Typical FTIR spectra for the deposited DLC films.
Figure 3 shows the temperature dependence of dark con-
ductivity of the DLC films. The conductivity of the DLC
shows an activated form and the activation energy increases
from 0.25 to 0.55 eV with increasing CF4 plasma exposure
time up to 200 s. This means that the Fermi level moves
toward the midgap after CF4 plasma exposure because the
optical band gap of hydrogen-free DLC is 1.4 eV. The con-
ventional DLC shows a p-type behavior and the activation
energy of conductivity is around 0.2 eV ͑Ref. 2͒. The newly
developed DLC indicates a relatively high value ͑0.55 eV͒ of
activation energy.
We deposited 200 nm thick DLC on the Si wafer to
measure FTIR transmittance, which was nearly the maxi-
mum thickness we could grow in case of hydrogen-free
films. The maximum thickness depended strongly on the
substrate used because of the delamination of the film due to
the stress build-up.
The CF4 plasma exposure to thin DLC layer appears to
etch some bonds and to relax the remaining network. The
weak C–C and hydrogen atoms bonded to C atoms can be
etched by F radicals. During this process the carbon atoms
can be relaxed, resulting in hydrogen-free DLC. The Fermi
level appears to move toward the midgap, because the mate-
rial composition changes remarkably.
In summary, we have developed hydrogen-free DLC
film by a PECVD using the layer-by-layer deposition tech-
nique. The conductivity activation energy increases from
0.25 to 0.55 eV and the optical band gap increases from 1.2
to 1.4 eV with increasing CF4 plasma exposure time up to
200 s.
This work was supported by University Basic Research
Program by Ministry of Information and Communication and
by Ministry of Education through the Basic Research Sci-
ence Institute Project No. BSRI 95-2443.
1 J. C. Angus and F. Jansen, J. Vac. Sci. Technol. A 6, 1778 ͑1988͒.
2 V. S. Veersamy G. A. J. Amaratunga, C. A. Davis, A. E. Timbs, W. I.
Milne, and D. R. Mckenzie, J. Phys. Condens. Matter 5, L169 ͑1993͒.
3
¨
W. Jacob and W. Moller, Appl. Phys. Lett. 63, 1771 ͑1993͒.
4 L. H. Chou, J. Appl. Phys. 72, 2027 ͑1992͒.
5 S. Aisenberg and R. Chabot, J. Appl. Phys. 42, 2953 ͑1971͒.
6 R. Lossy, D. Pappas, R. A. Roy, and J. J. Cuomo, Appl. Phys. Lett. 61, 17
͑1992͒.
7 D. R. Mckenzie, D. Muller, and B. A. Pailthorpe, Phys. Rev. Lett. 67, 773
͑1991͒.
8 K. Nakazawa, S. Ueda, M. Kumeda, A. Morimoto, and T. Shimizu, Jpn. J.
Appl. Phys. 21, L617 ͑1992͒.
9 J. Tauc, Amorphous and Liquid Semiconductors, edited by J. Tauc
͑Plenum, London, 1974͒, p. 15.
10 N. Mutsukura, S. Inoue, and Y. Machi, J. Appl. Phys. 71, 43 ͑1992͒.
11 Y. Shimada, N. Mutsukura, and Y. Mach, J. Appl. Phys. 71, 4019 ͑1992͒.
12 J. Robertson and E. P. O’Reilly, Phys. Rev. B 35, 2946 ͑1987͒.
FIG. 2. Optical band gap of the deposited DLC films.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Appl. Phys. Lett., Vol. 68, No. 25, 17 June 1996 Park et al. 3595
132.203.227.63 On: Thu, 10 Jul 2014 12:40:03