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PREPARATION AND CHARACTERIZATION OF DLC:N FILMS
Mihai G. MURE�ANa, Lenka Zajíčkováa,Vilma BURŠIKOVÁa,
Daniel FRANTAa, David NEČASa
a Department of Physical Electronics, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
Abstract
Diamond-like carbon (DLC) exhibits useful properties such as wide band gaps, high thermal conductivities,
high hardness and low friction coefficient. Technical difficulties due to inherent internal stress in preparation
of DLC films on metallic substrates are the cause of it's poor adhesion. Nitrogen doped diamond-like carbon
(DLC:N) films were deposited by plasma enhanced chemical vapuor deposition (PECVD) using capacitively
coupled rf discharge at 13.56 MHz in the mixture of methane, hydrogen and nitrogen on silicon, glass and
metallic substrates (stainless steel). Gas mixture, bias and discharge frequency were changed to modify the
parameters of the deposited films. The optical properties of the deposited films were investigated by:
reflectometry, ellipsometry and FTIR transmittance measurement. Depth sensing indentation technique was
used to determine hardness and elastic modulus. To improve adhesion on metallic substrates and create a
hard film, they were optimised in order to improve adhesion, by the preparation of multilayer-structures with
different deposition conditions.
1. INTRODUCTION
The DLC (Diamond-Like Carbon) also named - amorphous hydrogenated carbon (a-C:H) - thin films are a
very promising type of coating, with interesting range of properties, especially if we consider the coatings
with different impurities (N,Ti) [1] offering the possibility to choose the best performing coating for concrete
use. The nitrogen was preferred in the PECVD deposition technique because of the difficulty in obtaining
metallic precursors [2]. These primes help improve the functional properties of carbon layers in terms of
reducing the internal stress, increasing the temperature resistance and improving the adhesion. DLC
coatings can be created as smooth and compact thin films, reaching a hardness of up to 30 GPa, a low
coefficient of friction and good abrasion coefficient. The films are chemically quite inert and can be used as a
passivation layer. The development of binding layer is necessary for this kind of applications. Multilayered
thin films can be especially used for a better adhesion and lubrication.
Amorphous hydrogenated carbon films can be used, besides tribological applications, in photonics [3]. It can
include such applications like antireflective coatings, photosensitive materials or thickness monitoring. The
optical properties are important parameters for these applications, especially possible tuning of refractive
index and decrease of absorption in visible range.
Plasma-enhanced chemical vapor deposition (PECVD) is a process used to deposit thin films from a gas
state to a solid state on a substrate. The plasma is generally created by RF frequency between two
electrodes, the space between which is filled with the reacting gases. This method can be used successfully
for deposition of DLC on different substrates with low melting point, where the other traditional hard coatings
can not be deposit because of the high temperatures required.The advantage of the PECVD method is the
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lower deposition temperature combined with the precision of the coating. Another advantage is the relative
low cost of the deposition (use of hydrocarbons like methane, ethylene or acetylene).
2. EXPERIMENTAL
Diamond-Like Carbon thin films were deposited in radio frequency (rf) capacitively coupled plasma (CCP)
discharge driven at the frequency of 13.56 MHz. The reactor [4] has two parallel plate electrodes, the upper
– showerhead type is used for precursors distribution and is grounded. The bottom electrode is connected to
a CAESAR 133 13.56 MHz rf generator trough a matching unit. The distance between the electrodes is
55mm. In figure 1 is presented the reactor scheme. The pumping unit is composed from a rotary pump and a
turbomolecular one. The gas flow was controlled by electronic flow controllers. The pressure in the reactor in
controlled by a throttle valve. The base pressure in the reactor is below 10-3 Pa and the leak rate was under
0.03 sccm. Deposition conditions are showed in Table 1.
Fig. 1 – Reactor scheme
For substrates, double-side polished crystalline silicon (Si), glass, stainless steel (SS) and stainless steel
with titanium nitride (TiN) layer prepared by PECVD were used and placed on the bottom electrode. This
electrode is DC self-biased. The bias Ub determined the energy of the ions bombarding the film.
Table 1 – Deposition conditions
Sample CH4
[sccm]
H2
[sccm]
N2
[sccm]
Power
[W]
Self-bias
[V]
Pressure
[Pa]
Time
[min]
CH16 8.5 2.5 2.5 100 -130 12 60
CH17 8.5 2.5 2.5 50 -50 12 60
CH30 8.5 5 0 100 -75 12 30
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The samples were previously cleaned in ultrasonic bath for 15 min in a solution of isopropyl alcohol and
cyclohexane with a ration of 1:1. The substrates were treated in H2 and Ar mixture plasma with a flow rate of
10 and 3 sccm respectively, for 10 min before deposition. Total working pressure was 8 Pa. The power was
100 W and the self-bias -150 V.
Another set of samples (Table 2) was made to confirm the decrease of the internal stress. For this batch a
constant bias was used, to eliminate the differences between films.
Table 2 – Deposition condition for the second batch
Sample CH4
[sccm]
H2
[sccm]
N2
[sccm]
Power
[W]
Self-bias
[V]
Pressure
[Pa]
Time
[min]
CH50 8.5 5 0 45 -80 12 30
CH51 4.7 2.5 2.5 35 -65 12 30
CH53 4.7 2.5 2.5 35 -65 12 30
CH54 4.7 2.5 2.5 50 -65 12 120
CH55 4.7 2.5 2.5 50 -65 12 120
Characterization of the samples was performed by:
• Ellipsometry in UV-NIR range (Jobin Yvon UVISEL phase-modulated variable-angle spectroscopic
ellipsometer) at five angles of incidence from 55o to 75o
• Ellipsometry in VUV-UV range (BESSY II synchrotron rotating analyzer) at an angle of incidence was
67.5o
• Reflectance in UV-NIR range (PerkinElmer Lambda 45 spectrophotometer)
• Transmitance in IR range (Bruker Vertex 80v Fourier transform spectrophotometer) equipped with a
parallel beam transmittance accessory
• Hardness and elastic modulus (Fischerscope H100) using depth sensing indentation technique
• Atomic composition (RBS - Rutherford backscattering spectroscopy)
• Hydrogen content (Elastic Recoil Detection Analysis)
3. RESULTS AND DISCUSSIONS
3.1 Optical characterization
All data was fitted simultaneously by a single, consistent structural and dispersion model [5,6]. Dispersion
models used for fitting were Kramers-Kronig consistent in the entire spectral range. A schematic diagram of
the density of states (DOS) of A-C:H:N films is given in Figure 2, which is similar to the one of pure DLC but
with the addition of nitrogen in the 1s core level state.
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Fig. 2 - Schematic diagram of the density of states
The density of states is similar to a-C:H film because 2s and 2p valence electrons of carbon and nitrogen
form common valence and conduction bands. Only valence and conduction bands were used for dielectric
response. The dielectric function of transition layers was modeled by parametrized joint DOS (PJDOS). In
the case of the high relative thickness of the samples (100-200nm) the informations are provided mostly by
the a-C:H:N films. For a correct measurement, very thin films are required.
Fig. 3 - Real and imaginary parts of the dielectric function at their top and bottom interfaces
In the IR spectrum the relative transmittance was calculated by dividing the measured transmittance to the
transmittance of the substrate. This transmittance was used to find the film absorbance, proportional with
Lambert-Beer law. The IR transmittances were fitted together with the other optical data using a Kramers-
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Kronig consistent model. The joint density of states (JDOS) correspond with phonon absorption and they are
described by Gaussian peaks.
Fig. 5 - Chemical structure from IR spectrum – imaginary part of dielectric function
Transmittance below 1200 cm-1 are related to substrate absorption, and for that was neglected. From IR
spectra we were able to locate CH stretching between 3000-3080 cm-1 for sp2 and between 2855- 2960 cm-1
for sp3; CH deformation between 1375-1460 cm-1. Wide peaks were observed between 2949-3493 cm-1 and
were attributed to OH radicals, the peak at 3372 cm-1 to NH and the one at 2205 cm-1 to CN bondings [7].
Another peaks were induced by nitrogen content: at 1248 cm-1 and between 1513-1553 cm-1 are sp2C; pure
sp2C bonds were observed at 1300, 1600 and 1680 cm-1.
Because of different values of self-bias from the first set of samples were not the same, a new batch was
made with a constant bias (-65 V) to better compare the nitrogen role in the films. Refractive index and
extinction coefficient were calculated for this samples and given in Figure 6.
Fig. 6 – Refractive index n and extinction coefficient k of the new sample batch
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The measurements revealed two groups – with lower and respectively higher indexes. This change can be
due to the heating of the substrates because of longer deposition time, but showed a good reproducibility
when using same deposition parameters.
3.2 Mechanical characterization
The hardness was obtained from analysis of loading and unloading curves. The load and the corresponding
indentation depth were recorded as a function of time for both loading and unloading processes. Maximum
load was 100 mN and the accuracy of depth measurement was ±1 nm. The indentations were repeated for
16 times.
Fig. 7 - Loading-unloading dependences and dependence of the differential
hardness on the indentation depth for CH30
The “pop-in” can be observed on the loading curves. This effect is related to creation of indentation induced
cracking and delamination. The critical indentation depth (i.e. critical load) interfacial crack creation
increased after annealing (440oC), also observed in the differential hardness dependence on the indentation
depth.
The differential hardness mathematical expression is
(L is the load, A is the contact area, k is geometric constant, h is indentation depth) is the ratio of the small
load change and the corresponding change in the contact area. The differential hardness dependence on the
indentation depth is used to visualize the indentation induced changes in the tested material such as creation
of cracks or delamination.
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Fig. 8 - Differential hardness on the indentation depth for CH16 and CH17
3.3 Chemical composition
Complete chemical composition (Table 3) was performed by RBS in combination with ERDA. The
measurements revealed also some trace of argon (0.3%) explained by incorporation during treatment time
and oxygen caused by aging effect in atmosphere.
Table 2 – Elemental composition
Sample C
[%]
H
[%]
N
[%]
O
[%]
CH16 56 28 13 3
CH17 46 38 10 6
CH30 65 34 0 1
4. CONCLUSIONS
DLC films were successfully prepared by capacitively coupled rf discharge. Samples were measured with a
wide range of optical means, which concluded in good quality results. Pure DLC films were not stable on SS
or TiN substrates. The problem was also their high compressive stress combined with rough surface of
metallic samples. DLC:N films could be used as intermediate layer on SS but their adhesion was even better
on PECVD-TiN layers. The hardness decreases with the nitrogen incorporation from 21.7 Gpa (CH30) to
18.5 Gpa (CH16).
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5. ACKNOWLEDGEMENT
This research has been supported by Czech Ministry of Education, under project MSM 0021622411, by the
Grant Agency of the Czech Republic GACR No. 104/09/H080 and GACR No. 202/07/1669.
LITERATURE
[1] M. Nothe, U. Breuer, F. Koch, H. J. Penkalla, W. P.Rehbach, H. Bolt, Appl. Surf. Sci. 179 (2001) 122–128.
[2] F. Rabbani, R. E. Galindo, W. M. Arnoldbik, S. van der Zwaag, A. van Veen, H. Schut, Diam. Relat. Mat. 13 (2004) 1645–1657.
[3] Y. Hayashi, K. M. Krishna, H. Ebisu, T. Soga, M. Umenob, T. Jimbo, Diam. Relat. Mat. 10 (2001) 1002–1006.
[4] L. Zajıckova, D. P. Subedi, V. Bursıkova, K. Veltruska, Acta Physica Slovaca 53 (6) (2003) 489–504.
[5] D. Franta, D. Necas, L. Zajıckova, Opt. Express 15 (2007) 16230–16244.
[6] D. Franta, D. Necas, L. Zajıckova, V. Bursıkova, C. Cobet, Diamond Relat. Mater. 19 (2010) 114–122.
[7] D. Mayo, F. A. Miller, R. W. Hannah, Course Notes on the Interpretation of Infrared and Raman Spectra, Wiley-Interscience, New
York, 2004.