SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 28, 226–230 (1999)

Tribological and Optical Properties ofHydrogen-free Amorphous Carbon Films withVarying sp3/sp2 Composition

B. K. Tay,* X. Shi, E. Liu, H. S. Tan, L. K. Cheah, J. Shi, E. C. Lim and H. Y. LeeSchool of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798

Unhydrogenated amorphous carbon films were deposited at different temperatures on a single silicon waferby a process known as Filtered Cathodic Vacuum Arc. A transition from diamond-like properties to graphiticproperties was observed at the substrate temperature of 200°C. This change of properties upon transitionincludes a change in compressive stress, frictional coefficient and density. The variation of results is consistentwith the subplantation model, which emphasizes the role of carbon atoms trapped in the subsurface layersin the evolution of a dense sp3-rich phase at room temperature. At higher temperature (>200°C) diffusionof carbon atoms to the surface of the evolving film releases internal stress and leads to the formation of agraphitic sp2 phase. Based on x-ray reflectivity measurements, it was found that a thin interlayer formedbetween the tetrahedral amorphous carbon film and the silicon substrate contributes greatly to the good fittingof the spectra. The thickness of this interlayer is independent of the temperature. The optical properties werefound to undergo a more gradual transition with the deposition temperature. Copyright 1999 John Wiley& Sons, Ltd.

KEYWORDS: cathodic arc; amorphous; carbon; friction; bandgap; x-ray reflectivity

INTRODUCTION

For over a decade, diamond-like amorphous carbon(DLC) has stimulated great interest from both scientificand industrial perspectives. Hydrogen-free DLC hasinteresting and useful properties:1 the films are smooth,transparent and exhibit a high hardness, low frictioncoefficient and chemical inertness. Therefore, this materialis important for coating technology and electronic deviceapplications. Typically, it is produced by vacuum arc2,3

or pulsed laser deposition4 methods. In contrast toconventional amorphous carbon prepared by evaporationor sputtering, which consists mostly of threefold or sp2-bonded atoms, this DLC contains a significant fraction(up to 87%) of fourfold or sp3-bonded carbon atoms,hence the name tetrahedral amorphous carbon (ta-C). Theta-C films described here were prepared using filteredbeams of carbon ions but, unlike the conventional 90°

curved filter, the Filtered Cathodic Vacuum Arc (FCVA)deposition system incorporates a novel off-plane double-bend (OPDB) filter5,6 (shown in Fig. 1) to remove allmacroparticles and neutral atoms that are produced atthe cathode, along with the plasma beam. This newfiltering technique thus makes arc deposition processespossible on a production scale. The temperature stabilityof vacuum-annealed ta-C films has been studied in anearlier experiment.7 It was found that ta-C films exhibita high resistance to degradation during annealing in highand low vacuum. They sustain their structure, thickness,

* Correspondence to: B. K. Tay, School of Electrical and ElectronicEngineering, Nanyang Technological University, Nanyang Avenue, Sin-gapore 639798.E-mail: ebktay@ntu.edu.sg

stress and hardness up to 400°C. This paper extends thework by describing the tribological and optical propertiesof ta-C films as a function of deposition temperature.

EXPERIMENTAL DETAILS

The ta-C films were deposited using an FCVA systemwhere the carbon plasma is obtained from a graphite cath-ode of 99.99% purity. The arc is initiated by contactingthe cathode with a retractable graphite rod. During deposi-tion the carbon plasma leaves the self-sustaining arc spotand the CC ions are accelerated with a d.c. bias to theout-of-sight highly doped silicon substrate clamped ontoa copper substrate holder. A toroidal magnetic field of¾40 mT was employed to produce the axial and curvilin-ear fields to steer the plasma. The substrate temperaturewas varied between 25 and 400°C by a heater attachedto the back of the copper holder. The temperature wasmeasured by a thermocouple attached behind the sub-strate holder. For depositions above room temperature, allsamples were heated to the required deposited tempera-ture. Deposition would commence once the temperaturereaches a steady value. The substrate temperature waskept withinš5 °C. The base pressure of the system was<2.0ð 10�4 Pa but rose to 1.5ð 10�3 Pa during depo-sition owing to outgasing of the cathode. Thin ta-C films(typically 40 nm) were deposited at various temperatureson h100i Si substrates.

The properties of the films were characterized in thefollowing manner. The stress in the films was derived bymeasuring the radius of curvature of the substrate beforeand after deposition and applying Stoney’s equation.8 Thefrictional characteristics of the ta-C films produced were

CCC 0142–2421/99/130226–05 $17.50 Received 30 November 1998Copyright 1999 John Wiley & Sons, Ltd. Accepted 8 March 1999

HYDROGEN-FREE AMORPHOUS CARBON FILMS 227

Figure 1. Schematic diagram of the FCVA system.

evaluated using a CSEM pin-on-disc tribometer. The filmswere mounted on an aluminium holder that was attachedon a turntable rotating at 400 rpm. The experiments wereperformed in a clean-air atmosphere with 60% humidityat room temperature with a fixed load of 2 N. A siliconnitride pin of 6 mm in diameter was used in the test. Filmadhesion on different substrates was investigated by usinga CSEM micro-scratch tester. The scratch tester was fittedwith a Rockwell diamond stylus (120° cone with 200µmradius hemispherical tip). A scratch length of 5 mm isdrawn over the coating by applying a progressive load ata loading rate of 7.57 N min�1 up to 15 N. The sty-lus is scratching at a table speed of 2.52 mm min�1.The tester is fitted with acoustic emission monitoringequipment that was used as an on-line failure monitor.X-ray reflectivity under grazing incidence conditions is

a powerful technique to study the film density and sur-face roughness without destroying the samples.9 – 12 X-rayreflectivity was performed using a vertical Siemens D5005x-ray diffractometer with a 401 mm measurement cir-cle that provides an accuracy of 0.007° in �–2� move-ment. The reflectivity data were collected with Cu K˛radiation (wavelength of 1.54A) at 40 kV and 40 mA.A Gobel mirror was used to achieve an intensive andparallel-beam x-ray output with divergence of¾0.05°. Theoptical bandgaps of the films were determined using aUVISEL spectroscopic phase-modulated ellipsometer inthe spectral range of 250–900 nm. The optical bandgapof the films was determined by fitting the ellipsomet-ric measurements to a Forouchi Bloomer model13 thatwas shown to be appropriate for amorphous diamond-likecarbon films.14 In this model the mechanism for optical

Copyright 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 28, 226–230 (1999)

228 B. K. TAY ET AL.

absorption is interband excitations between the bondingand antibonding� bands.15

EXPERIMENTAL RESULTS AND DISCUSSION

Figure 2 shows the relative change of compressive stresswith temperature for ta-C films. The compressive stressdecreases sharply at a transition temperature of¾200°C.A similar trend was also observed by Chhowalla usingthe FCVA technique,16 where there was a sudden fall inthe density for films deposited above a transition tempera-ture of 200°C. For the sake of comparison, the percentagedrop in stress values (35%) in our set of films is not assignificant as compared to that of Chhowalla (55%). Thiscould be due to the higher sp3 contents of the ta-C films,which can be seen from the respective film densities. Theta-C film film used in this study has a maximum densityof 3.37ð 103 kg m�3 whereas the film of Chhowalla hasa density of 3ð 103 kg m�3.16 Figure 3 shows the vari-ation of friction coefficient with deposition temperature.Again, a transition change in sp3 to sp2 content exists,with the friction coefficient increasing sharply from 0.08to 0.12. The friction coefficient increases at around thesame transition temperature of>150°C as the compres-sive stress. The above results are similar to those reportedby Lifshitz,17 where there is a sudden fall in the den-sity deposited above a certain temperature, indicative ofa transition from diamond-like to graphitic properties tak-ing place.

The role of substrate temperature in the evolution ofcarbon films from hyperthermal atoms can be described bythe subplantation model proposed by Lifshitz.17 Accord-ing to this model, ions with sufficient energy penetratethe surface layer and produce dense ta-C growth by sub-surface growth. The evolution of a dense sp3-rich phaseis facilitated by a combination of factors, including: thepreferential displacement of sp2 bonded atoms while thesp3 atoms remain intact; and the incorporation of car-bon interstitials that form internal compressive stresses

Figure 2. Relative compressive stress as a function of depositiontemperature.

Figure 3. Coefficient of friction as a function of depositiontemperature.

Figure 4. Tangential force as a function of normal force for ta-Cfilms deposited at different temperatures.

and transform spontaneously to a dense phase. At roomtemperature the carbon atoms are immobile, leading tothe above processes. At elevated temperatures, when thecarbon mobility increases, carbon atoms diffuse to thesurface, releasing the internal stresses and forming thethermodynamically stable, less dense, sp2 phase. Hence,the loss of densification at higher deposition temperatureis attributed to diffusion of the subplanted ions back tothe surface.

Figure 4 shows the tangential force as a function ofnormal force for the ta-C films deposited at different tem-perature. As seen, the critical load decreases from 6.4 Nto 1.6 N with an increase in deposition temperature from25 to 350°C. This result correlates with the detection ofacoustic emission plots, turning nearly vertical as shownin Fig. 5. Both the tangential and acoustic emission resultssupport the earlier analysis that the decrease in sp3 con-tent at higher deposition temperature causes the films tobecome less diamond-like and thus more susceptible towear and tear.

Figure 6 shows the plot of measured reflectivity (points)with simulated reflectivity (line) for thin films depositedat 25°C (curves a and b) and 315°C (curve c). It is very

Surf. Interface Anal. 28, 226–230 (1999) Copyright 1999 John Wiley & Sons, Ltd.

HYDROGEN-FREE AMORPHOUS CARBON FILMS 229

Figure 5. Acoustic emission as a function of normal force forta-C films deposited at different temperatures.

Figure 6. X-ray reflectivity for ta-C films deposited at 25 °Cand 315 °C. The solid line is the fit of the data at differenttemperatures.

interesting to note that a good fit between experiment andsimulation was obtained by using ta-C/interlayer/silicon(curve b), whereas using only a single layer of ta-C on thesilicon substrate (curve a) was found to be insufficient tomatch all interference fringes. The formation of this inter-face layer could be due to SiC, which may be explained bythe subplantation mechanism whereby the medium-energycarbon ions are capable of penetrating the surface layerof h100i-oriented silicon during the early deposition stage.However, the exact mechanism responsible for this inter-layer is not known at present and is currently under inves-tigation. The film mass density, roughness and thicknessof the layers used in the fitting are listed in Table 1. As

seen, the mass density decreases from 3.4 to 2.8 g cm�3

with increasing deposition temperature. The stress reliefphenomena and higher frictional coefficient were also ver-ified by the fitting results, which show surface rougheningat elevated temperature associated with a decrease in thecarbon density.

The existence of an interlayer is also proven by spectro-scopic phase-modulated ellipsometry. The optical propertyof the ta-C film was derived based on the Forouhi andBloomer amorphous semiconductor model. It has beenshown that the microstructure of these films deposited onsilicon wafer was simulated successfully by a four-layermodel consisting of a roughness layer, a ta-C layer, agraded layer and the silicon substrate.18 Figure 7 showsthe experimental and simulated values of the variation ofreal and imaginary pseudodielectric functions with pho-ton energy for one of the ta-C films deposited on acrystalline silicon substrate. As shown, the fit to exper-imental data is good. The chi-squared value for all thefilms was in the range 0.03–0.2. The thickness of theinterlayer (¾1 nm) obtained from both x-ray reflectiv-ity and ellipsometry analysis is comparable and seems tobe independent of the temperature. The absorption coef-ficient derived from the model simulation is shown inFig. 8. As seen, the edge of the absorption coefficientshifts towards lower energy with increasing temperature.Figure 9 shows the variation of optical bandgap with depo-sition temperature. The optical bandgap falls from valuestypical of sp3 bonding to much lower values typical ofsp2 bonding. However, the variation in optical propertieswas more gradual, with the deposition temperature in con-trast to the sharper drop in the compressive stress and

Figure 7. Simulated real (dashed line) and imaginary (solid line)parts of the pseudodielectric functions as a function of energyfor a ta-C film. Experimental data are depicted by the crosses.

Table 1. Values of the fitting parameters used for the reflectivity curves of ta-C filmsdeposited at different temperatures

Deposition temperature (°C)25 100 150 200 254 315

Surface roughness (nm) 0.05 0.05 0.05 0.08 0.1 0.3Carbon layer density .g cm�3/ 3.4 3.3 3.1 3.0 2.9 2.8Carbon layer thickness (nm) 45.0 39.2 36.5 42.5 48.7 33.0Interlayer roughness (nm) 1.0 1.0 1.0 1.0 1.0 1.0Interlayer density .g cm�3/ 2.5 2.5 2.4 2.5 2.5 2.5Interlayer thickness (nm) 1.0 0.8 1.0 0.8 0.8 1.0Si substrate roughness (nm) 0.1 0.1 0.1 0.1 0.1 0.1

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230 B. K. TAY ET AL.

Figure 8. Absorption coefficient as a function of depositiontemperature.

friction coefficient. This decreasing trend in the bandgapwith deposition temperature is in good agreement withthose reported by Chhowalla.16 It was suggested that thegradual change in optical properties is attributed to themedium-range order of sp2 sites.

CONCLUSION

The tribological and optical properties of ta-C films werefound to be dependent on the deposition temperature,

Figure 9. Optical bandgap as a function of deposition tempera-ture.

changing to sp2 above a transition temperature of 200°C.This transition is attributed to the diffusion of subplantedatoms to the surface via thermal diffusion, leading to therelaxation of stress and density.

Acknowledgement

The authors would like to acknowledge Mr P. Y. Rui for carrying outthe x-ray reflectivity measurements.

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Surf. Interface Anal. 28, 226 230 (1999) Copyright 1999 John Wiley & Sons, Ltd.