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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.

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