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Materials Chemistry and Physics 134 (2012) 7– 12

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

aterials science communication

ole of ex-situ oxygen plasma treatments on the mechanical and opticalroperties of diamond-like carbon thin films

eeraj Dwivedia,b, Sushil Kumara,∗, Hitendra K. Malikb

Physics of Energy Harvesting Division, National Physical Laboratory (CSIR), K.S. Krishnan Road, New Delhi 110012, IndiaDepartment of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

r t i c l e i n f o

rticle history:eceived 26 May 2011eceived in revised form 7 December 2011ccepted 19 February 2012

a b s t r a c t

Role of ex-situ oxygen plasma (OP) treatment on mechanical properties of diamond-like carbon (DLC) thinfilms is explored. The DLC film after this treatment shows superhardness behaviour, maximum hardnessof 46.3 GPa and elastic modulus of 423.2 GPa are obtained when film grown at self bias of 100 V followedby 10 min OP treatment. Moreover, the colour of the coating is fed after OP treatment, leading it to a

eywords:hin filmslasma assisted CVDTIRechanical properties

colourless coating with significantly enhanced transmittance and improved antiglare property. Such OPtreated DLC films may find their applications as protective coatings on cutting tools, automobile parts,magnetic storage media and colourless coating in packaging, as the DLC films posses brown colour.

© 2012 Elsevier B.V. All rights reserved.

ptical properties

. Introduction

The unique tribological, mechanical and biocompatible prop-rties of carbon based thin films offer a wide range of industrialpplications including automotive, biocompatible implants andard disk [1–3]. The structure of hydrogenated amorphous carbona-C:H) consists of diamond-like sp3, graphite-like sp2 and someimes sp1 hybridized state of carbon [4]. The a-C:H films with higherp3 fraction are known as diamond like carbon (DLC) films. The dia-ond like characteristic of a-C:H films are controlled by deposition

arameters, specially ion energy and hydrogen content. However,LC films possess very high residual stresses in its structure that

ead to its poor adhesion with substrate. Kumar et al. [5] have sug-ested possible solutions to the high built-up stresses in DLC films.n view of theoretical prediction of �-C3N4 phase formation, Liu andohen [6] have made appreciable effort for the enhancement in theardness. However, its use is limited by stoichiometry maintainingroblem. Similarly, the adhesion is improved by the metal incorpo-ation in DLC films but at the cost of hardness [7]. Moreover, such aetal incorporation process in the films is quite costly. On the other

and Jiang et al. [8] have suggested that oxygen plasma preferentialtches the graphite-like sp2 clusters and enhances the diamond-

ike character of a-C:H and carbon nitride coatings. Cateledge et al.9] have suggested that oxygen in DLC helps to reduce non diamondhase such as graphite-like sp2 sites and enhance diamond-like sp3

∗ Corresponding author. Tel.: +91 11 45608650; fax: +91+11 45609310.E-mail address: skumar@nplindia.org (S. Kumar).

254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2012.02.050

sites. Recently, Santos et al. [10] have also found that oxygen plasmatreatment enhance the surface energy of DLC films that may fur-ther improve the surface hardness of DLC films. In addition, someof other authors have also devoted their keen attention towardsoxygen plasma of DLC films for improving its tribological proper-ties [11]. In the view of all these, oxygen plasma (OP) treatments ofthe DLC films may provide a new efficient approach to improve themechanical properties of the films without a need of any complexhybrid system geometry. So there is great scope to work on thistopic further.

Hence, in the present letter, we emphasize on the role of OPtreatment on mechanical properties of the DLC films and suggestthat this treatment may also provide superhard coatings with theirgood optical properties.

2. Experimental details

The schematic of asymmetric capacitively coupled radio fre-quency (13.56 MHz)-plasma enhanced chemical vapour deposition(RF-PECVD) system used for the growth of present DLC films andex-situ OP treatments is shown in Fig. 1. RF-PECVD deposition unitconsist of gas line having gases Argon (Ar), oxygen (O2) and acety-lene (C2H2). The electrode geometry of the deposition unit wasmade quite simple where substrates were placed at powered elec-trode having small area. The other electrode which is connected to

ground was kept whole body of chamber and hence, it was muchbigger than powered electrode. At base pressure 2 × 10−5 Torr, DLCthin films were deposited on well cleaned Si wafers and corning7059 glasses under varied negative self biases of 100, 150, 200

8 N. Dwivedi et al. / Materials Chemistr

Ffi

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ig. 1. Schematic representation of RF-PECVD system used for the growth of DLClms followed by oxygen plasma treatment at different timing.

nd 250 V (samples OD-1, OD-2, OD-3 and OD-4) by maintaining aonstant working pressure of 4 × 10−2 Torr. The working pressureas achieved by incorporating Ar gas first that changed pressure

rom 2 × 10−5 Torr to 1 × 10−3 Torr and then C2H2 gas that changedressure from 1 × 10−3 Torr to 4 × 10−2 Torr.

After depositions, ex-situ OP treatments on the DLC films wereerformed at negative self bias of 100 V for 10, 20, and 30 min. Thehicknesses of pure DLC films were measured by Taylor-Hobssonalystep instrument. For thickness measurement, before deposi-ion of film onto the substrate we have masked the some portion ofubstrates and then deposited the films onto the mask containedubstrates. After deposition we removed the mask resulting in cre-tion of step between the films and the substrates. By moving stylusither from films to substrates or substrates to films the thicknessf films was measured. Fourier transform infrared (FTIR) spectro-copic measurements were carried out, for investigating bondingnvironment, using PerkinElmer spectrum Bx instrument. Ramaneasurements were performed on these samples using Renishaw

n via reflex micro Raman spectrometer with excitation wavelength14.5 nm. The mechanical properties of these films were stud-

ed by IBIS nanoindentation (M S−1 Fisher-Cripps laboratories Pvt.imited, Australia) having diamond Berkovich indenter at maxi-um indentation load of 10 mN. Finally, the optical properties were

etermined by Schimadzu UV–vis 1601 spectrometer.

. Results and discussion

The thicknesses of pure DLC thin films deposited at negativeelf biases of 100, 150, 200 and 250 V were found to be 220, 270,10 and 380 nm, respectively. Here, it is to be noted that the filmeposited at negative self bias of 250 V was peeled off within aay after deposition and could not be used for further studies. Thisay be attributed to the presence of high level residual stress in

uch a film. Based on the changes in mechanical and optical prop-rties of DLC films before and after OP treatment of 10 min, we haveuggested a chemical model given in Fig. 2(a and b), respectively.he model reveals that the oxygen atoms after OP treatment mayccupy and bond with carbon atoms. Such initiative model thateflects the bonding states of oxygen, carbon and hydrogen is also

upported by FTIR analysis shown in Figs. 2(c and d). These fig-res show the FTIR spectra of DLC films grown at 100 V before OPreatment (Fig. 2(c)) and after OP treatment of 10 min (Fig. 1(d))n the region of 2000–800 cm−1. The peaks in Fig. 2(c) at 1600

y and Physics 134 (2012) 7– 12

and 1668 cm−1 infer the C C stretching vibrations and the peaksat 1446 and 1489 cm−1 indicate sp3 CH2 and CH3 deformations,respectively. This can be noted that after OP treatment of 10 min,the C C stretching vibrations are decreased (Fig. 2(d)), which couldbe attributed for the enhancement of the number of sp3 hybridizedcarbon atoms in the carbon network. The increase in sp3 fractionafter OP treatment on the DLC films is further confirmed by opticalstudies (discussed later). Actually, oxygen bombardment results inthe breaking of C C moieties in DLC films and it may lead to the for-mation of carbonyl ( C O) and carboxylic ( COOH) groups, whichis evident from generation of peaks at 1743 cm−1 (C O stretching),1680 cm−1 (C O stretching in carboxylic group), 1241 cm−1 ( C Ostretching) and 895 cm−1 (C O H out of plane bending).

Raman analysis was also performed on pure DLC and 10 minOP treated DLC films grown at 100 V and obtained spectra areshown in Fig. 3(a–c). General survey of Raman spectra of thesefilms show square shaped peak near ∼980 cm−1, sharp G peakin the band 1500–1600 cm−1 and shoulder D peak in the band1300–1400 cm−1, where G stand for graphite and D for disorder.It is to be noted that OP treatment for 10 min on DLC films madesignificant changes, which was confirmed by shifting of G peaktowards lower wavenumber along with the increase in the intensityof square shaped peak. The square shaped peak ∼980 cm−1 corre-sponds to second order silicon and arise when film posses moretransparent diamond-like sp3 bonding [12]. It means that our asgrown DLC film (grown at 100 V) posses more sp3 bonding thatis enhanced further when OP treatment was performed for 10 min.Observed increase in the intensity of this peak after 10 min OP treat-ment may also have contribution from etching as oxygen plasmamay etch some of DLC layer. Further, OP treatment induced struc-tural changes in these films was investigated by fitting the Ramanspectra using two Gaussian components. Untreated DLC film grownat 100 V show D and G peaks at 1385 cm−1 and 1559 cm−1, respec-tively which changed to 1379 cm−1 and 1550 cm−1, respectivelywhen OP treatment was performed for 10 min on the same sam-ple. Observed shifting of G peak towards lower wavenumber dueto OP treatment of 10 min suggests the increase in sp3 bonding,which is also confirmed by FTIR analysis. In addition, we have alsoinvestigated ID/IG ratio and it was found to be 0.66–0.46 for pureDLC and 10 min OP treated DLC films, respectively grown at 100 V.Finally, by Raman analysis we observed that 10 min OP treatmenton DLC film enhances the sp3 bonding and therefore may improvethe mechanical properties. Earlier, Ferrari et al. [13,14] have alsoobserved same behaviour of G peak position and ID/IG ratio withvarying amount of sp3 fraction.

The mechanical properties of ex-situ OP treated DLC films werestudied using high resolution nanoindentation at the maximumindentation load of 10 mN. Beside OP treated DLC films, themechanical properties of substrate silicon and the reference sam-ple fused silica were also studied. The load versus displacementcurves of pure DLC, 10 min OP treated DLC, 20 min OP treatedDLC and 30 min OP treated DLC films are shown in Fig. 4(a–d),respectively. In addition, the load versus displacement curves ofsilicon substrate and fused silica are also given in Fig. 4(a). Basedon these graphs, we estimate the hardness (H), elastic modulus(E), plastic resistance parameter (H/E) and elastic recovery (ER)of as deposited and plasma treated DLC films. Since mechanicalproperties of thin films strongly depend on penetration depth, Hof Si wafer (substrate) is measured before the deposition and thenH of Si/DLC (substrate/film) is estimated by composite hardnessmodel [15]. The measurements of H, E, H/E and ER at differentex-situ OP treatments of 0, 10, 20 and 30 min are given in Table 1.The maximum value of H, E, H/E and ER as 32 GPa, 320 GPa, 0.094

and 85.6%, respectively in pure DLC films were obtained at 100 V.The mechanical properties of the DLC films depend on sp3 bond-ing, which is controlled by ion energy or self bias. Erdemir and

N. Dwivedi et al. / Materials Chemistry and Physics 134 (2012) 7– 12 9

F C filmO

Disihttgdp1optefi

TM

ig. 2. Model of (a) before ex-situ OP treated DLC and (b) after ex situ OP treated DLP treatment of 10 min.

onnet [16] have suggested that the hydrocarbon precursor at lowon energy is not sufficiently decomposed and open structures withignificant amount of unbound hydrogen are observed that resultsn soft polymer-like carbon films. At intermediate ion energy,ydrocarbon precursor is sufficiently decomposed, hydrogen con-ent is reduced and deposited films possess high density, revealinghe diamond-like structure. However, at very high ion energy,raphite like bonding dominates due to the increase in sp2 inducedisorder. Recently, we have also observed interesting mechanicalroperties of DLC and modified DLC films at self bias ranging from00 to 150 V [17–21]. In addition to self bias, ex situ OP treatmentn DLC films may further modify the mechanical properties. Suchlasma treatment for 10 min leads to drastic increase in H from 32

o 46.3 GPa and in E from 340 to 423.2 GPa, respectively. Since bondnergy of C C and C C are 610 and 345 KJ mol−1, respectively [22],rst C C bonds are broken under 10 min OP treatment. This leads

able 1easurements of H, E, H/E and % ER of pure DLC and OPT-DLC (OPT for 10, 20 and 30 min

Oxygen plasma treatment Samples

OD-1 (100 V) OD-2 (15

H (GPa) E (GPa) H/E ER (%) H (GPa)

0 min 32 340 0.094 85.6 25.1

10 min 46.3 423.2 0.1 77.2 31

20 min 34.2 350 0.098 78.1 23.1

30 min 23 291 0.079 70 27

s and FTIR spectra of DLC film grown at 100 V (c) before OP treatment and (d) after

to the formation of more C C bonding and generation of carbonyl(C O) and carboxylic ( COOH) groups, which is sketched in andis also revealed from FTIR analysis. Although carboxylic groupcontain only some fraction of sp2 bonding, overall percentage ofsp2 bonding was found to be less in OP treated DLC film comparedto pure DLC film. Jiang et al. [8] have reported that oxygen ionspreferentially etch the soft graphite-like sp2 clusters in sp3 andsp2 carbon network. Cateledge et al. [9] have also suggested thatoxygen in DLC films help to convert sp2 bonding into sp3 bondingand hence improve the mechanical properties. On increasingthe time of OP treatment for 20 min, the magnitudes of H andE get reduced slightly, which may be due to the initiation ofbreaking of even sp3 C C bonds, damage and implantation of

some of oxygen ions. However, further increase in time of OPtreatment to 30 min leads to a drastic reduction in H and E, whichmay be due to breaking of more sp3 C C bonds and increase in

) films grown at self biases of 100, 150 and 200 V.

0 V) OD-3 (200 V)

E (GPa) H/E ER (%) H (GPa) E (GPa) H/E ER (%)

303.5 0.083 72.8 20.4 312 0.065 69.2319.3 0.097 71.1 27 288 0.094 81.2261.7 0.088 59.2 10 131 0.076 45.5296 0.091 78 14 203 0.069 70

10 N. Dwivedi et al. / Materials Chemistry and Physics 134 (2012) 7– 12

200018001600140012001000800

0

800

1600

2400

3200(a)100 V

Inte

ns

ity

(a

.u.)

Raman Shift (cm-1

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Before OP treatment

After OP treatment of 10 min.

1000 120 0 140 0 160 0 180 0

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(b)Before OP treatment

Inte

nsit

y (

a.u

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Raman Shift (cm-1)

100 V

1000 120 0 140 0 160 0 180 0

1550 cm-1

1379 cm-1

(c)After OP treatment of 10 min.

Inte

nsit

y (

a.u

.)

Raman Shift (cm-1)

100 V

Fig. 3. (a) General scan Raman spectra of pure DLC film and 10 min OP treated DLC film grown at 100 V (sample OD-1), (b) fitted Raman spectra of untreated DLC film growna .

dhwahttOacgIftc1bwtfr1aoi

t 100 V and (c) fitted Raman spectra of 10 min OP treated DLC film grown at 100 V

isorder caused by more damage and implantation. Further, weave also plotted change in hardness (�H) due to OP treatmentith respect to original hardness of non plasma treated films, which

s shown in Fig. 5. The 0 in the graph represent the base (original)ardness of non plasma treated DLC films. From figure, it is evidenthat in case of 10 min OP treatment; the �H was found to be posi-ive for all films grown at 100, 150 and 200 V. However, for 20 minP treatment although �H was found to be positive for film grownt 100 V but it was negative for films grown at 150 and 200 V. Inase of 30 min OP treatment, �H was found to be negative for filmsrown at 100 and 200 V but it was positive for film grown at 150 V.t was realized that 10 min OP treatment was found to be feasibleor improving the mechanical properties of these films. Beside OPreated DLC films, the H and E of silicon and fused silica were alsoalculated. The values of H and E in silicon were found to be 10.4 and30 GPa, respectively, whereas H and E in fused silica were found toe 9 and 78 GPa, respectively. In order to explore elastic–plastic andear resistance properties of the films, we have estimated the plas-

ic resistance parameter (H/E). The H/E ratio is related to the bulkracture strength, higher the H/E value, better the elastic and wearesistance properties. Maximum value of H/E was obtained after

0 min OP treatment. However, for larger times of OP treatment (20nd 30 min) the H/E ratio gets decreased. This infers more fractionf work is consumed in plastic deformation and large plastic strains expected when contacting a material. To elucidate the elastic

information further, % ER [17–21,23] was estimated and themaximum ER was observed for DLC film grown at 100 V. However,ER varied in the range of 85.6–70% for pure DLC grown at 100 Vand when the OP treatment was done for 30 min. These resultsof ER were found to be in good agreement with H, E and H/Emeasurements.

4. Possible applications of OP treated DLC films

4.1. As hard, protective, encapsulated and wear resistance coating

Since OP treated DLC films have better mechanical properties,they can be used as hard and protective coatings on cutting toolsand automotive parts, protective and encapsulate layer on solarcells and high wear resistance coating on magnetic storage media.

4.2. As an antiglare coating

In order to investigate the transmittance using UV–vis spec-troscopy, we have also grown a DLC film at negative self bias of100 V on large area (15 cm × 13 cm) glass substrate followed by ex-

situ OP treatment for 10 min (Fig. 6). The transmittance spectrumof this film before and after OP treatment reveals that the plasmatreatment enhances the transmittance of the as deposited film.The film also shows antiglare properties, which was confirmed by

N. Dwivedi et al. / Materials Chemistry and Physics 134 (2012) 7– 12 11

Displacement ( μμm)

0.160.120.080.040.00

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Displacement ( m)

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OD-2

OD-3

Silicon Substrate

Fused Silica

00 V f

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Recently, DLC thin films have been used as gas barrier coating

Displacement ( μm)

Fig. 4. Load versus displacement graphs of DLC films grown at 100, 150 and 2

erforming several non scientific tests such as this coating made itossible to see clearly even in fogy season and blurt texts could beead easily. Moreover, the antiglare property in OP DLC film elimi-

ated the requirement of an additional hard and protective layer onntiglare coating because it itself is a hard coating and it can min-mize the product cost. More work in this direction is in progress.

10 20 30

-10

-5

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5

10

15

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ΔH positi ve

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ness,ΔH

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100 V

150 V

200 V

ig. 5. Variation of change in hardness (�H) with varying OP treatment time for allLC films.

Displacement ( μm)

ollowed by OP treatments for (a) 0 min, (b) 10 min, (c) 20 min and (d) 30 min.

4.3. For packaging

in industries because it enhances the oxygen gas barrier 20–30times in comparison with uncoated polyethylene terephthalate

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20

40

60

80

100

After OP treatment of 10 min.

Before OP treatment

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[22] H.Y. Sang, W.J. Zhang, T.M. Wang, Vacuum 68 (2003) 203.[23] M. Lejeune, M. Benlahsen, P. Lemoine, Solid State Commun. 135 (2005)

2 N. Dwivedi et al. / Materials Che

PET) bottles. This gas barrier property is found to be very usefulor beer and wine industries. However, brown colour of DLC thatrises due to graphite-like sp2 bonding restricts its industrial appli-ation as the government of most of the countries (like Japan) hasanned colour coatings for packaging [24]. However, the colour ofLC films is fed after OP treatment. Hence, the plasma treated filmsay be used as colourless, hard and biocompatible coating for the

ackaging purpose.

. Conclusions

Using RF-PECVD technique, we have grown DLC films under var-ed self bias and studied for mechanical and optical properties. Filmeposited at 100 V exhibited excellent mechanical properties withigh H ∼ 32 GPa. The mechanical properties were further improvedy performing ex-situ OP treatment on these films. For example Has enhanced from 32 GPa to 46.3 GPa with 10 min OP treatment.owever, OP treatment for larger duration reduces the mechan-

cal properties. On the other hand, plasma treatment for 10 minemoved the colour of the films, enhanced the transmission andmproved the antiglare property. Observed changes in mechani-al and optical properties were also fairly correlated with FTIR andaman measurements.

cknowledgements

The authors are grateful to the Director, National Physical Labo-atory, New Delhi (India) for his kind support. Authors are thankfulo Mr. Ishpal for his help in Raman measurements. We gratefully

cknowledge Dr. O.S. Panwar and Mr. C.M.S. Rauthan for their helps.ne of author (ND) acknowledges CSIR, India for providing SRF

ellowship. This research was sponsored by CSIR, Govt. of India,hrough the Network Project NWP-0027.

[

y and Physics 134 (2012) 7– 12

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