Role of Metallic NiCr Dots on the Adhesion, Electrical, Optical and Mechanical Properties of Diamond-like Carbon Thin Films

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Role of Metallic Ni�Cr Dots on the Adhesion,Electrical, Optical and Mechanical Properties ofDiamond-like Carbon Thin Films

Neeraj Dwivedi, Sushil Kumar,* Chandra Mohan Singh Rauthan,Omvir Singh Panwar

The effect of thermally evaporated metallic Ni�Cr dots on the adhesion, electrical, optical andthe mechanical properties of RF-PECVD grown diamond-like carbon (DLC) thin films wasinvestigated. The DLC films deposited over Ni�Cr dots containing substrates exhibited a strongadhesion to the substrate. The observed drastic increase in magnitude of the current and thedecrease of optical band gap due to presence of Ni�Cr dots revealed a considerable improve-ment in their electrical and optical properties, respect-ively. Further, load versus displacement curves wereemployed to estimate the hardness (H), elastic modulus(E), plastic resistance parameter (H/E) and elastic recov-ery (ER). The values of H and E in Ni�Cr dots containingDLC films were found to be comparatively less than thatof DLC films. Overall, Ni�Cr metal dots improvedadhesion, optical and electrical properties of DLC filmswith a little deterioration in their mechanical properties(e.g. H, E, ER and H/E).

Introduction

Plasma produced diamond-like carbon (DLC) thin films

were studied extensively due to their outstanding tribolo-

gical, mechanical properties, excellent biocompatibility

and chemical inertness.[1–6] Recently, DLC films are

introduced in a new fashion to fabricate electronic and

electromechanical devices, such as thin film transistors

(TFT’s) and microelectromechanical systems (MEMS).[7]

Ideal MEMS requires high wear resistance, good hardness

and low surface roughness etc. DLC films possess all these

properties with large wear resistance capability (10 000

times greater) than that of silicon basedMEMS.[7] Band gap

N. Dwivedi, S. Kumar, C. M. S. Rauthan, O. S. PanwarPlasma Processed Materials Group, National Physical Laboratory(CSIR), Dr. K.S. Krishnan Road, New Delhi – 110012, IndiaFax: 91þ11-45609310; E-mail: [email protected]. DwivediDepartment of Physics, Indian Institute of Technology Delhi,New Delhi – 110016, India

Plasma Process. Polym. 2011, 8, 100–107

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonline

engineering, more specifically the tuning of band gap in

large range (from 1 to 4 eV) and higher transmission in IR

region made DLC a material of great utility and led to its

many optoelectronic applications.[8,9] However, existence

of high level of residual stress in DLC structures restricted

their potential industrial applications. Many dopants (N2,

Si, Ti, Cu etc), various multilayer structures, variety of

processing techniques and post deposition annealing have

already been employed to minimize residual stress, but

most of themwere found to be limited due to highmaterial

cost, process, complexity and some of its own limitations

(e.g. post deposition annealing may damage the electronic

circuitry).[10] Materials designing was found to be a great

alternative to improve the properties ofDLCfilms. Also, due

to shrinkage in size and cost of electronic devices (single

semiconductor chip contain millions of transistor) it

became necessary to study the properties of material even

at very small dimension, which can be made possible by

exploring the materials designing. S. Paul also deposited

DLC films on metal strip to improve their adhesion and

electronic properties.[11]

library.com DOI: 10.1002/ppap.201000087

Role of Metallic Ni�Cr Dots on the Adhesion

In the present study, we explored the effect of Ni�Cr

metal dots on the adhesion, electrical, optical and

mechanical properties of DLC thin films. To visualize the

effect of Ni�Cr dots clearly, pure DLC films were also

prepared and their results then compared with Ni�Cr dots

containing DLC (Me/DLC) thin films.

Experimental Part

Two sets, first of DLC and then Me/DLC thin films were deposited

using an asymmetric capacitive coupled radio frequency plasma

enhanced chemical vapor deposition (RF-PECVD) and a thermal

evaporation techniques, on well cleaned n-type Si h100i and

corning 7059 glass substrates at different negative self biases of

�100V (DLC-1 and Me/DLC-1), �130V (DLC-2 and Me/DLC-2) and

�185V (DLC-3andMe/DLC-3). In thefirst set,DLCfilmsweregrown

over the bare substrate, whereas in the second set metal dots of

Ni�Cr having thickness of 90nm and the area of 7.85� 10�3 cm2,

respectively,werefirstgrownbythermalevaporationusingamask

at a base pressure better than 10�5 Torr. Subsequently, DLC layers

were deposited at a base pressure better than 10�5 Torr on

substrates containing theNi�Cr dots. Theworking gas pressure for

the deposition of all DLC and Me/DLC films was maintained at

50mTorr; thiswas achieved by first feeding Ar gas into the process

chamber to 1mTorr and then C2H2 gas up to 50mTorr. Thicknesses

of these films were measured by Taylor-Hobson Talystep instru-

ment. Typicalmorphological analysis ofMe/DLCfilm grownat self

bias of – 130V was carried out by scanning electron microscopy

(SEM) using JEOL, JSM-35 instrument. Transmission and reflection

spectrawereobtainedbyShimadzuUV-VIS1601spectrometer. The

obtainedresultsof transmissionandreflectionwere furtherused to

Figure 1. a) Optical image of Ni�Cr dots grown on Si substrate, b) Tysectional view of Me/DLC film grown at �130 V.


� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

estimate the optical bandgap.Mechanical properties of these films

were measured by IBIS nano indentation (M/s Fisher-Cripps

laboratories Pvt. Limited, Australia) having a Berkovich indenter

of radiusof150– 200nm. I-V characteristicswereevaluatedusinga

Keithley 6487 Pico ammeter instrument. Fourier transform

infrared (FTIR) spectroscopic analysis was carried out using a

Perkin Elmer Spectrum Bx instrument.

Results and Discussions

Scanning Electron Microscopy

Figure 1a shows the optical image of Ni�Cr dot grown on

the Si substrate. The image clearly reveals the equally

spaced and uniform patterning of the dots. Typical

morphological analysis of the Me/DLC film, grown at – 130V,

carried out by SEM, is shown in Figure 1b. The SEM

micrograph clearly reflects the homogenous and well

patterned morphology with crest and valleys visible

through out the surface of the film. Further, the cross

section view of the same film is depicted in Figure 1c. SEM

image again substantiate the crest and valley formation

having thickness of 100nmand 300 – 350nm, respectively.

Adhesion

AdhesionofDLC thinfilms to the substrateswas found tobe

a big issue to discuss. In the present work, the effort was

made to improve the adhesion of DLC through prior

pical SEM image of Me/DLC film grown at �130 V and c) SEM cross

www.plasma-polymers.org 101

Figure 2. a) I-E characteristics of DLC films deposited at �100,�130 and �185 V, b) I-E characteristics of Me/DLC films deposited

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N. Dwivedi, S. Kumar, C. M. S. Rauthan, O. S. Panwar

depositions of metallic Ni�Cr dots. Up to a self bias of

�130V, the adhesion of Me/DLC films was found to be

better than that of pure DLC films. However, beyond

�130V, the adhesion ofDLCandMe/DLCfilmswas affected

considerably, and Me/DLC films started to show poorer

adhesion than that of DLC films. At�185V, the adhesion of

Me/DLC films was found to be significant poor due to an

increase of ionic bombardment of the surface. It is to be

noted that, beyond�185V (at�275V), both types of these

films were peeled off, as soon as they were taken out from

the process chamber, due to the presence of high level of

residual stress in their structure, and therefore could not be

studied further. Thus, in the present study, the value of

�130Vwas considered as a critical self bias because at this

bias the quality and theproperties ofDLC andMe/DLCfilms

were found to be excellent. Beyond �130V, the poor

adhesion of Me/DLC films may attributed to an increase of

electric field on the metallic dots.[11] Ion energy, which

possessed linear relationship with the DC self bias and

develops at the sheath spacewas found to be an important

parameter to explain the adhesion of thin films. The

adhesion ofMe/DLCfilmswas found to be poor at the edges

ofNi�Crdotsbecause ofhigher ion impinging energyat the

edge, which resulted into higher stress, and thus causing

poor adhesion. S. Paul[11] also observed a reduction in the

adhesion of a-C:H films at the edges of Cr pad. Residual

stress of DLC films also varied proportional to the negative

self bias[11,12] because of the increasing the bias leads to an

increase of the ionic bombardment, which in turn starts

breaking away the loosely bonded hydrogen from carbon.

This leads to the development of dangling bonds and, in

consequence, carbon atoms get attached to other carbon

atoms thereby developing a 3-D cross linking network. This

induces an increase in the residual stress, and therefore

causing poor adhesion of DLC and Me/DLC films beyond

�130V.
at �100, �130 and �185 V.
Electrical Properties

The influence of Ni�Cr dots on the electrical properties of

Me/DLC films was investigated using their I-E character-

istics. The I-E characteristics of sandwich structures

comprises Al/Si/DLC/Al and Al/Si/(Me/DLC)/Al structures,

grown on n-type Si with different DLC and Me/DLC layer

thicknesses are given in Figure 2a and 2b. Amorphous DLC

filmswere characterized by high density of states that vary

in the range from 1020 to 1021 eV�1 � cm�3 and act as trap

centers for charge carriers, thus resulting in effective I-E

characteristics.[13] DLC films can be considered as p-type

material, therefore their deposition on n-type Simay result

in the formation of p-n junction hetrostructure; this

resulted into their diode-like behaviour. Recently, we

observed diode-like behaviour from n-type a-C:H/p-type

Si hetrostructure.[14] DLC materials have been explored as



p-typewindow layer forp-i-n solar cells andp-type layer for

p-C/n-Si hetrojunction solar cells.[15,16] It is to be noted that

usually DLC films show low current values. In the present

study, we also observed a low magnitude of current of

�10�10 A for the DLC films. In contrast, I-E characteristics of

Al/Si/(Me/DLC)/Al structures exhibited improved electrical

transportwithamaximumcurrent of�10�6A. Thus, due to

their higher magnitude of current, Me/DLC films can be

used as new efficient p-type layer for (Me/DLC)/Si based

hetrojunction solar cells. It is also to be noted that the

current in theMe/DLC films was found to vary inversely in

respect to the thickness of DLC layers. The influence of

Ni�Cr dots on the I-E characteristics of Me/DLC films was

found to decrease with increasing the thickness of the DLC

layers.

DOI: 10.1002/ppap.201000087


Mechanical Properties

Due to their high resolution, characteristic nanoindenta-

tion was found to be an excellent technique to study the

mechanical properties of thin films. The load versus

displacement curves of DLC and Me/DLC films are shown

in Figure 3a and 3b, respectively. From this figure, it was

observed that the maximum penetration depth (at max-

imum indentation load of 10mN) in DLC andMe/DLC films

was found to vary in range from �131.9 to�167.5 nm and

�134.2 to �154.4 nm, respectively. It is to be noted that, if

the indenter penetrates more than 10 to 25% of the total

Figure 3. a) Load-displacement curves of DLC films deposited at�100, �130 and �185 V, b) Load-displacement curves of Me/DLCfilms deposited at �100, �130 and �185 V.



film thickness, then the substrate also influences the

mechanical properties and the result shows composite

substrate/film effect. Zhang et al. also studied the effect of

load on the mechanical properties of amorphous carbon

films.[17–19] They suggested that with increasing the load,

the plasticity in the structure increases due to the substrate

effect. Ultimately, the increase of plasticity means the

decrease of elastic recovery and hardness values. Although,

due carewas taken here to avoid the effect of substrate, we

were unable to completely avoid it as the thickness of these

films was not very high. However, because of this, the

hardness of Si wafer was measured before the depositions

and then hardness of substrate/film was estimated by

composite hardness model.[20] Hardness (H) and elastic

modulus (E) of DLC and Me/DLC films estimated by

unloading curve in load-unload profile are depicted in

Figure 4a and 4b. From these figures, it was confirmed that

thatH and E of DLC films initially increasedwith increasing

the self bias from�100 to�130V, but beyond�130V, their

values were found to decrease. H and E of DLC films

deposited at �100V, �130V and �185V were found to be

17.2 and 258.8GPa, 27.5 and 342.4GPa and 20 and 199GPa,

respectively.HandE inMe/DLCfilmsalso followedasimilar

trend and their values at �100V,�130V and�185V were

found to be 15.8 and 224.9GPa, 22.5 and 339.5GPa and 14.3

and 155.8GPa, respectively. DLC films consisted of mainly

sp3C�C, sp2C�Cand sp3C�Hbondings. sp3C�Chas strong

bondings and improve themechanical properties such asH

and E of theDLCfilms. sp3 C�Hhas also strong bonding, but

do not influence H.[3] On the other hand, sp2 C�C is a weak

and soft type of bonding and reduces H and E. This is to be

noted that metals were found to support the formation of

softer sp2C�Cbonding.[21] Thus, in comparison topureDLC,

the observed decrease in the values of H and E in Me/DLC

films ascribe the generation of a softer sp2 C�C structures.

The ion energy, which is a function of the self bias, is an

important parameter that can tune various hybridized

states of carbon in DLC films. Erdemir and Donnet[22]

reported that the three levels of ion energies to categorized

hydrogenated amorphous carbon are: (i) intermediate ion

energy for diamond-like, (ii) lower for polymer-like and (iii)

higher for graphite-like carbon structures. The negative self

bias below �40V may be used to obtain polymer-like

carbon due to the insufficient dissociation of hydrocarbon

precursor, �100 to �150V to obtain diamond-like carbon

due to sufficient dissociation of hydrocarbonprecursor, and

above �300V, to obtain graphite-like carbon due to

increase in sp2 induced disorder.[22] Fallon et al.[23] reported

maximumsp3 C�Cbonding infiltered cathodic vacuumarc

grown tetrahedral amorphous carbon films at �100 eV,

beyond which they observed the initiation of graphite-like

bonding. Singh et al.[1] observed themaximumsp3 bonding

withmaximumH at�150V. In present case,maximumsp3

C�C bondingwithmaximumH and E seems to be observed


Figure 4. a) Variation of negative self bias versus (i) H, (ii) E and(iii) H/E for DLC films, b) Variation of negative self bias versus(i) H, (ii) E and (iii) H/E for Me/DLC films.

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at �130V. Recently, some interesting work regarding the

effect of ion energy on various hybridization states of

amorphous carbon was carried out by Zhang et al.[17] They

explained the sp3, sp2 and sp1 bondings in amorphous

carbon on the basis of sub-implantation and stressmodels.

Further, their work also reveals that the maximum sp3

bonding occurs near�100V, and that, at higher self biases,

the sp1 bonding increased considerably. The process time

was also found to be a key parameter which tunes the



varioushybridizationsof amorphouscarbonandaffects the

hardness values.[17]

Plastic resistance parameter (H/E) was found to be an

important parameter to explain elastic-plastic and wear

resistance properties of thin films.H/E ratio for various DLC

and Me/DLC films are depicted in Figure 4a and 4b. It is

evident from these figures that H/E ratio for DLC films

increases continuously with increasing the negative self

bias from �100V to �185V. The H/E values in these DLC

films were found to vary from 0.066 to 0.1. In contrast, the

H/E values in Me/DLC films were found to first slightly

decrease and then increase with increasing the negative

self bias. Thevalues ofH/E inMe/DLCfilmsvaried from0.66

to0.09. TheH/E is basicallya ratioofhardness (related to the

density) and elastic modulus (measure of interatomic

bonding forces). Thus, H/E ratio shows the physical

response of an atomic lattice to an external force and

relates to the bulk fracture strength. The domain of validity

of H/E varies in the range between 0 and 0.1. The higher

limit shows their higher elastic behaviour, whereas lower

limit shows their elastic-plastic behaviour. For high wear

resistance coatings, H/E must be very high. In the present

case, verynominaldifferenceon themaximumvalueofH/E

between DLC (0.1) and Me/DLC (0.09) were observed.

Comparatively lower H/E ratio observed below �185V in

DLC and Me/DLC films exhibited a larger fraction of work

being consumed in plastic deformation and large plastic

strain is to be expectedwhen contacting amaterial. Also at

�185V, comparatively less H/E (0.09) in Me/DLC than H/E

(0.1) in DLC showed some fraction ofwork being consumed

in a plastic deformation and little plastic strain are to be

expected when contacting a material. Elastic recovery (ER),

ratio of residual displacement at no load to the displace-

ment atmaximumload (dres /dmax) andplastic deformation

energy (Ur)werealso foundtobean importantparameter to

explain the elastic properties of thin films. The variation of

ER, dres /dmax and Ur versus self bias for DLC and Me/DLC

filmsare shown inFigure5aand5b. The%ERofDLCandMe/

DLC films was calculated by using the relation:[24]

%ER ¼ ½ðdmax �dresÞ=dmax� � 100 (1)

where dmax and dres are the displacement at the maximum

load and residual displacement after load removal,

respectively. The values of ER of DLC films deposited at

�100, �130 and �185V were found to be 91.5%, 70% and

95%, respectively, which changed to 71.4%, 73.8% and 73%,

respectively for Me/DLC films deposited at same negative

self biases. The presence of metallic Ni�Cr dots in Me/DLC

films may be a possible cause of comparatively lower ER

due to mainly two reasons: (i) metals are easily deformed

under load and thus show some plastic deformation and

(ii) Ni�Cr dots develops several interfacial states (at

boundary of Ni�Cr and DLC film), and therefore create

DOI: 10.1002/ppap.201000087

Figure 5. a) Variation of negative self bias versus (i) ER, (ii)dres/dmax and (iii) Ur for DLC films, b) variation of negative selfbias versus (i) ER, (ii) dres/dmax and (iii) Ur for Me/DLC films.


disturbance in loading-unloading curves. The plastic

deformation, which is responsible for lower values of ER

in Me/DLC films, may occur due to an increase of

dislocation motion at room temperature. Plastic deforma-

tion energy (Ur)[25] is also a very important parameter to

study the elastic properties of thin films. The Ur possessed

a relation with H [26] given by:

Plasma

� 2011

Ur ¼ P3=2=3H1=2 ½ðwotan2cÞ�1=2� (2)

Process. Polym. 2011, 8, 100–107

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

wherewo is the geometry constant and attains the value of

1.3 for pyramid indenter, P is the load, and c is the half

angle of Berkovich indenter and have the value 65.38. Thevariation of Ur against negative self bias for DLC and Me/

DLC films is shown in Figure 5a and 5b. The values ofUr for

DLC films deposited at �100V, �130V and �185V were

found to be 1� 10�9 J, 8.1� 10�10 J and 9.5� 10�10 J,

respectively, which changed to �1� 10�9 J, 8.9� 10�10 J

and 1.1� 10�9 J, respectively, for Me/DLC films deposited

at same negative self biases. dres /dmax is also an important

parameter to study the elastic properties of thin films and

gives information similar to ER with different domains of

validity.[27] The domain of validity for this parameter

varies between 0 and 1. Lower limit shows fully elastic

behaviour, where the upper limit corresponds to rigid-

plastic behaviour of thin films. The dres /dmax ratios for DLC

films were found to be 0.085, 0.3 and 0.048 at �100V,

�130V and �185V, respectively, which changed to 0.286,

0.26 and 0.27, respectively for Me/DLC films at same

negative self biases. It is to be noted that dres /dmax ratio in

DLC andMe/DLC filmswas closer to the lower limit (0), and

thus exhibitedmore elastic behaviour and less rigid-plastic

behaviour.

Optical Properties

DLCfilmsareknownfor theirhigher optical transmission in

IR region, which led to their potential application for IR

devices. Transmission spectra of DLC andMe/DLC films are

shown in Figure 6a and 6b. From Figure 6a, it can be seen

that the transmission of DLC films varied in the range from

80 to 95% in the near IR region. The transmission of DLC

filmsdepositedat�100Vand�130Vwas foundtosaturate

beyond �600nm. However, film deposited at �185V

showed continuous increase in transmission. Observed

higher transmission in these films also confirmed their

diamond-like character. Transmission spectra of various

Me/DLC films are given in Figure 6b. The maximum

transmission in these films varied in the range from 80

to 85%. Observed comparatively less transmission in Me/

DLC filmsmay be due to the presence of Ni�Cr dots. Optical

band gap (Eg) of DLC and Me/DLC films were estimated

using the Tauc equation. No significant variation of Egagainst self bias was observed in DLC filmswhich varied in

range from 2.4 to 2.7 eV. However, comparatively lower

values of Egwere observed inMe/DLCfilms. Thevalues of Egfor Me/DLC films were found to be in the range from 1.4 to

2.1 eV. The decrease of Eg in Me/DLC filmmay be due to an

increase of sp2 C�C bonding as discussed in previous

section. Further, Eg was correlated with H. Eg and H vary

proportional to sp3 C�C and inversely proportional to sp2

C�C type of bonding. The DLC films show higher sp3 C�C

bonding and therefore, higher Eg and H values. In contrast,


Figure 6. a) Transmission spectra of DLC films and figure in insetshows the variation of Eg versus negative self bias for DLC films,b) Transmission spectra of Me/DLC films and figure in inset showsthe variation of Eg versus negative self bias for Me/DLC films.

Figure 7. FTIR spectra of a) DLC film deposited at negative self biasof �100 V and b) Me/DLC film deposited at �100 V.

106


Me/DLC films show comparatively lower Eg and therefore

lowerHvalues. The tuningofEg from lower tohigher values

in these films may be very useful for their optoelectronic

applications.

FTIR Analysis

Typical FTIR spectra of DLC and Me/DLC films deposited at

�185V are presented in Figure 7a and 7b. The several peaks

observed in the region from500 to1000 cm�1 correspond to

bonding state of Si with C, H and O. It is to be noted that

region 500 to 1 000 cm�1 was found to be slightly different

in DLC and Me/DLC films due to presence of several



interfacial Ni�Cr dots in Me/DLC films, which disturbs the

Si�C bonds. Thismight also be a one of the causes for lower

H values in Me/DLC films. In addition, DLC and Me/DLC

films showed C�H stretching band with sharp peak at 2

918 cm�1. The region above 2 960 cm�1 correspond to the

sp2 CHx bonding (up to 3 200 cm�1), whereas below 2

960 cm�1 showed sp3 CHx bonding (up to 2 700 cm�1). The

peak observed beyond 3 200 cm�1 may be attributed to sp1

type of bonding. Various peaks observed in the region 1 600

– 1 800 cm�1 reveal the existence of sp2 C¼C bonds. The

region from 1000 cm�1 to 1 250 cm�1 with sharp peak at

1 156 cm�1 corresponds to C�C/C�H bonds. These films

also exhibited strong CO2 peaks due to the atmospheric

contamination. Since there is no significance of this peak,

therefore the region between 2 225 and 2 407 cm�1 was

deducted fromthe FTIR spectra. The FTIR resultswere found

to be good in agreements with results reported in

literature.[28,29]

DOI: 10.1002/ppap.201000087


Conclusion

DLC and Me/DLC thin films were deposited on Si and

corning 7059 glass substrates under varied self bias from

�100 to �185V, using combined RF-PECVD and thermal

evaporation techniques. By sacrificing slightly themechan-

ical properties, the adhesion electrical and optical proper-

ties were improved considerably in the Me/DLC films.

Maximum H in DLC film was found to be 27.5GPa, which

slightly decreased to 22.5GPa in Me/DLC film. The

magnitude of current in DLC films were found be �10�10

A which significantly improved to �10�6 A by employing

the Me/DLC structure. Similarly, the values of Eg were

tuneable by using theMe/DLC structure. FTIR result reveals

that the type of bonding was almost unaffected above

1 000 cm�1.

Acknowledgements: The authors are grateful to the Dr. R. C.Budhani, director of the National Physical Laboratory, New Delhi(India) for his kind support. Authorswish to thank Dr. S. N. Sharmaand Mr. K. N. Sood for their assistance with the FTIR and SEMmeasurements, respectively. Authors also acknowledge Ministryof New and Renewable Energy Government of India for providingNRE fellowship to Mr. Neeraj Dwivedi. This research is sponsoredby CSIR, Government of India, through the Network Project NWP-0027.

Received: July 1, 2010; Revised: September 14, 2010; Accepted:September 24, 2010; DOI: 10.1002/ppap.201000087

Keywords: adhesion; diamond-like carbon (DLC); indentation;PECVD; thin films

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Role of Metallic NiCr Dots on the Adhesion, Electrical, Optical and Mechanical Properties of Diamond-like Carbon Thin Films