Ž .Surface and Coatings Technology 133]134 2000 247]252

Deposition of Si-DLC films with high hardness, low stress andhigh deposition rates

J.C. Damascenoa, S.S. Camargo Jr a,U, F.L. Freire Jr b, R. Cariusc

aEngenharia Metalurgica e de Materiais-Uni ersidade Federal do Rio de Janeiro, Cx. Postal 68505, Rio de Janeiro, RJ, CEP 21945-970,´Brazil

b ´Departamento de Fısica-Pontificia Uni ersidade Catolica do Rio de Janeiro, Cx. Postal 38071, Rio de Janeiro, RJ, CEP 22454-970,´ ´Brazil

cISI-IPV, Forschungszentrum Julich GmbH, Postfach 1913, Julich, D-52425, Germany¨ ¨

Abstract

Ž .In this work silicon-incorporated diamond-like carbon Si-DLC films were produced by plasma enhanced chemical vaporŽ .deposition PECVD from gaseous mixtures of CH and SiH . A study of the influence of self-bias and gas composition on the4 4

mechanical and structural properties of the films was carried out. Results show that films deposited at high self-bias present highdeposition rates, low stress and surprisingly high hardness. Increasing silane concentration in the gas phase leads to anenhancement of the observed effects. Compositional and structural characterization show that deposition at high bias leads toincreased sp2 character and rather low silicon contents. Increasing the silane content in the plasma leads to an increase in the sp3

fraction of the films, and yields a further reduction of stress with almost no effect upon hardness. In this way, the possibility ofŽ . Ž . Ž .producing films with high hardness )20 GPa , low stress ;0.5 GPa and high deposition rates )40 nmrmin has been

demonstrated. This result is thought to be very important from the point of view of technological applications. Q 2000 ElsevierScience S.A. All rights reserved.

Ž .Keywords: Diamond-like carbon; Silicon; Plasma chemical vapor deposition CVD

1. Introduction

ŽSilicon-incorporated diamond-like carbon films Si-.DLC have been attracting increasing interest of re-

searchers since they have a great potential for solvingsome of the major drawbacks of pure DLC films. Forthis reason, a considerable amount of work on thesefilms has been carried out in the last 3]4 years. Indeed,Si-DLC films present: reduced residual internal stressw x w x1,2 ; high deposition rates 3 ; good adhesion to mostsubstrates, including various metal alloys, steels and

w x w xglasses 4]6 ; very high hardness 7,8,5 ; improved ther-w xmal stability 9]11 ; reduced hydrogen loss and graphi-

w xtization 12,13 ; low friction coefficients independent ofw xrelative humidity 14]16 ; resistance to oxidation, mois-

U Corresponding author.

w x w xture and corrosion 2,17,18 ; and low wettability 19 .This remarkable collection of properties makes thismaterial a promising candidate for a large number oftechnological applications as metallurgical and protec-tive coatings. In fact, Si-DLC films have already beentested as wear-resistant tribological coatings for high-stress power applications, which included car engines,transmission gears and manufacturing tools, with good

w xresults 17 .Among the different deposition techniques, the most

widely used has been plasma enhanced chemical vaporŽ .deposition PECVD . Various source gases have been

Ž . Ž . w xused, such as: methane CH and silane SiH 1,15 ;4 4w x w xCH , SiH and Ar 4 ; CH , SiH and He 3 ; benzene4 4 4 4

Ž . w xC H and H -diluted SiH 16 ; tetramethylsilane6 6 2 4Ž . w x w x Ž .TMS 18 ; CH and TMS 20 ; acetylene C H and4 2 2

w x w xTMS 5,19 ; CH , TMS, Ar and H 21 ; and CH ,4 2 4Ž . w xsilicon tetrachloride SiCl , Ar and H 14 . Despite4 2

0257-8972r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 9 3 2 - 4

( )J.C. Damasceno et al. r Surface and Coatings Technology 133]134 2000 247]252248

the various deposition conditions used by differentlaboratories, a general finding is that silicon con-tributes to the stabilization of the carbon sp3 phase,with beneficial effects on the properties of the materialw x1,7,14,15,20 . However, in spite of all the work carriedout there has been no report up to now of a systematicstudy of the influence of deposition conditions on thestructure and properties of these films.

In this work we report on the influence of variationin the cathode self-bias and gas mixture composition onthe properties of Si-DLC films deposited by PECVDfrom gaseous mixtures of SiH and CH , and show4 4

Ž .that is possible to produce hard films )20 GPa withŽ .low internal stress ;0.5 GPa at high deposition rates

Ž .)40 nmrmin at high values of self-bias.

2. Experimental

Silicon-incorporated hydrogenated amorphous car-Ž .bon films Si-DLC were deposited from gaseous mix-

tures of methane and silane onto crystalline siliconsubstrates placed on the cathode of a conventional

Ž .radio frequency 13.56 MHz parallel-plate glow dis-charge reactor. A gas flow of approximately 10 sccm,with silane contents in the range 0]20 vol.% were fedinto the reactor through mass flow controllers. Duringall deposition runs the electrode distance and chamber

Ž .pressure monitored by a capacitance manometer werekept fixed at 2.5 cm and 2.0 Pa, respectively. Films witha thickness between 1 and 2 mm were produced at

Ž .different self-bias voltages V from y50 up to y1100BV.

It must be noted that no temperature control of thesubstrate was carried out during deposition. Therefore,substantial heating of the substrate surface may occuras a result of its interaction with the plasma, mainly incase of the depositions with high power densities, incontrast to our previous work, where the substratetemperature was found to be limited to approximately1008C at V sy200 V.B

The film composition was determined by ion beamanalysis. A 4-MeV van de Graaff accelerator providedHeq beams. For Rutherford backscattering spectrome-

Ž .try RBS measurements, we used a 2-MeV beam, withthe detector positioned at 1658 with respect to theincident ions. Hydrogen content of the samples was

Ž .determined by elastic recoil detection analysis ERDA ,with a probe of 2.2 MeV Heq incident on the sampleat an angle of 758 with respect to the surface normal.Detection of recoil protons was performed at an angleof 308. A 10-mm Mylar foil covered the detector inorder to prevent the detection of forward-scatteredalpha particles. Raman spectra were recorded in abackscattering geometry using the 488-nm line of aCoherent Innova 100 argon laser for excitation, a Spex

Fig. 1. Deposition rate as a function of negative self-bias voltage ofSi-DLC films prepared with silane concentrations in the gas of 0.2

Ž . Ž .vol.% squares and 2.0 vol.% circles .

1404 double monochromator for dispersion, and a liq-uid nitrogen-cooled CCD camera for measuring thesingle-shot spectra. Infrared spectra were measured ina vacuum Fourier transform spectrometer Bruker IFS66V.

Residual internal stress was obtained by the subs-trate bending method using a Dektak IIA stylus pro-filometer, which was also used for the thickness mea-surements. All films presented compressive stresses.Indentation of the samples was carried out with aVickers diamond micro-indenter with a 0.25-N load forapproximately 20 s, keeping the indentation depthsmaller than 20% of the sample thickness. Hardnesswas obtained from the measurement of the indentationdiagonals on an optical microscope using the differen-

Ž .tial interference contrast DIC technique. In all caseshardness was calculated from the average of a series of20 different indentations.

3. Results and discussion

The deposition rate of Si-DLC films is strongly in-creased when the RF power density dissipated in theplasma is increased in a similar manner to that observedfor pure a-C:H films. As shown in Fig. 1, the depositionrate as a function of the resulting cathode self-biaspotential presents an increase of one order of magni-tude when the self-bias voltage is increased from y50to y1100 V. In addition, comparison of the resultsobtained for films produced with two different silane

( )J.C. Damasceno et al. r Surface and Coatings Technology 133]134 2000 247]252 249

concentrations in the plasma, e.g. 0.2 and 2.0 vol.%,shows that the deposition rate can be further increasedby silane addition to the discharge. For instance, filmsdeposited with 0.2 vol.% SiH present a maximum4deposition rate of approximately 28 nmrmin, whereaswith 2.0 vol.% SiH , a deposition rate of 35 nmrmin4was achieved at V sy1100 V and under the specificBdeposition conditions employed.

The residual internal stress of Si-DLC films dependsstrongly on the cathode self-bias potential employed. Inthe low self-bias range, the internal stress is increasedwhen the self-bias voltage increases, attains a maxi-mum, and then decreases monotonically for high valuesof self-bias, as shown in Fig. 2a. Comparing the stressvalues obtained for V sy200 and y1100 V, an inter-Bnal stress reduction larger than three-fold was achievedfor films deposited with 0.2 vol.% SiH . Furthermore,4increasing the silane content in the reactor resulted inan overall decrease in stress. In this way, films withresidual internal stresses lower than 1.0 GPa could beobtained.

The behavior observed for film hardness as a func-tion of the cathode self-bias was rather remarkable. Asis evident in Fig. 2b, a monotonic increase of hardnesscan be observed when the self-bias is increased fromy50 up to y1100 V. Even for high values of self-biasvoltage, no reduction in the hardness values was noted,in contrast to what is generally observed for purea-C:H. Also, hardness values for films deposited with2.0 vol.% silane are systematically higher than those offilms deposited with lower silane contents.

At this point, it is important to mention that the

influence of the substrate hardness, as well as thelimited thickness of the films and elastic recovery afterindentation, on the present results are expected to besmall, since hardness values measured by nanoindenta-tion on some of our samples fell within 10]15% of thepresent values. Accordingly, other authors have alsoobtained a good agreement between hardness valuesmeasured by nano- and Vickers indentation for filmswith similar thickness and within the same range of

Ž . w xvalues 20]25 GPa 22 . In this way, one can guaranteethe reliability of these results and conclude that Si-DLCfilms with high hardness, low internal stress and highdeposition rates can be produced at high values ofself-bias.

The behavior observed for the deposition rate andinternal stress as a function self-bias are similar to thatobserved years ago for pure a-C:H films deposited from

w xCH 23 . It is important to emphasize, however, that in4the present case, the addition of silane to the dischargeenhanced the deposition rate and contributed to anoverall stress relief. These facts have been alreadyobserved in our laboratory for films deposited at V sB

w xy200 V 1 . However, in the case of pure a-C:H films,Žhardness is low for the low bias material polymer-like

.films , achieves a maximum for the so-called diamond-like films that are deposited at an intermediate value ofbias, and decreases strongly in the case of films de-

Ž .posited at high bias graphite-like films . In contrast tothis, hardness values for the present films show noevidence of reduction at high self-bias but, on thecontrary, are slightly increased. Moreover, films de-

Ž . Ž .Fig. 2. a Residual internal stress and b Vickers micro hardness as a function of negative self-bias voltage of Si-DLC films prepared with silaneŽ . Ž .concentrations in the gas of 0.2 vol.% squares and 2.0 vol.% circles .

( )J.C. Damasceno et al. r Surface and Coatings Technology 133]134 2000 247]252250

Fig. 3. Atomic composition as a function of negative self-bias voltageof Si-DLC films prepared with 2.0 vol.% silane in the plasma.

posited with a larger concentration of SiH in the4plasma showed even higher hardness values.

Recently, Lacerda et al. reported on the depositionof hard a-C:H films with low stress at high values of

w xself-bias 24 . They could obtain films with a hardnessof 14 GPa, stress of 0.5 GPa and deposition rate of 14nmrmin by methane gas decomposition in a RF sput-tering system, without the addition of any impurities tothe films. However, similar to pure a-C:H, their maxi-

Ž .mum hardness values ;25 GPa were obtained atV sy200 V. Those authors attributed their results toBthe different geometry of their deposition system. Al-though a similar effect may play an important rolehere, our results clearly show that the addition ofsilane to the discharge results in a further enhance-ment of the observed effects, improving the materialproperties.

In order to try to understand the above-describedbehavior, compositional and structural characterizationof the samples was carried out. Fig. 3 shows the atomiccomposition as a function of self-bias obtained fromRBS and ERDA measurements of samples depositedwith 2.0 vol.% SiH in the discharge. It is evident that4carbon content of the films is increased as the self-biasincreases, whereas silicon content is correspondinglydecreased. Whereas at low self-bias voltages the siliconcontent of the films is much larger than in the gasphase, at high values of bias the silicon concentrationin the samples tends to reach a value similar to the gasphase. Also, the hydrogen concentration of the films issomewhat reduced by the self-bias increase. An equiva-lent behavior was obtained for the composition of films

deposited with 0.2 vol.% silane in the gas, although inthis case the silicon contents were approximately oneorder of magnitude smaller.

The observed variation in carbon and silicon con-tents of the films as a result of increasing self-bias canbe understood based on the larger dissociation cross-sections of silane compared to methane molecules andtheir dependence on the RF power density in the

Ždischarge. Therefore, at low RF power densities low.self-bias voltages the dissociation rate of silane

molecules is much higher than that of methane, leadingto a relatively larger density of Si-related radicals in theplasma than C-related ones, thus enhancing siliconincorporation into the films. As the RF power is in-

Ž .creased increased self-bias , the dissociation rate ofmethane molecules increases substantially, leading tohigher carbon contents in the samples. Consistent withthis picture, estimates based on the results presentedabove show that the deposition rate of silicon atoms isessentially independent of the self-bias in the investi-gated range, whereas the deposition rate of carbonatoms present a ten-fold increase when self-bias in-creases from V sy50 to y1100 V. In this way, filmsBwith very low silicon concentrations are obtained at thehighest values of bias. The hydrogen content of thesamples, on the other hand, is thought to be controlledby a different mechanism, namely particle bombard-ment, which is strongly enhanced by the self-bias in-crease. However, samples deposited with self-bias volt-ages from y400 up to y1100 V present similar hydro-gen contents, in contrast to this simple picture. Adeeper investigation is still needed in order to clarifythis issue.

It must be noted that the present results show that itis possible to obtain films with high hardness, low stressand high deposition rate with very low silicon contents.As already pointed out, a substantial stress decreasecan be obtained with the incorporation of just a few

w xat.% of silicon atoms 1 . These results suggest that onthe improvement of the film properties as a result ofsilane addition to the discharge, changes in plasmaandror film growth conditions may play an importantrole.

Raman investigation of Si-DLC films has shown thatan increase in the self-bias voltage has a strong effecton the structure of the material. It is evident from thespectra shown in Fig. 4a that when the self-bias voltageis increased from y200 to y1100 V, the G-peak isshifted from approximately 1500 to 1570 cmy1. Corre-spondingly, the relative intensity of the D-peak is alsogreatly increased. This behavior has been extensivelyobserved by other authors in the case of pure a-C:Hfilms, and is indicative of an increased sp2 character.Although in the present case the compositional changesinduced by the self-bias variation may also somewhataffect the Raman spectra, the observed effect on both

( )J.C. Damasceno et al. r Surface and Coatings Technology 133]134 2000 247]252 251

Ž . Ž .Fig. 4. a Raman and b infrared spectra of Si-DLC films prepared with 2.0 vol.% silane in the plasma and different negative self-bias voltages.

peak position and intensity is much larger than ex-pected from the observed variation in the carbon con-

w xtent of the films 9,20 . Therefore, one can concludethat the Raman spectra provide strong evidence of anincreased sp2 character of the films deposited at highbias. On the other hand, the comparison between theRaman spectra obtained for films deposited with twodifferent silane concentrations showed that increasingsilane concentration in the plasma enhances the sp3

character of the films, in accordance with previousw xresults 1,15,20 .

Infrared analysis of the films has led to consistentconclusions about their bonding properties. Fig. 4bshows the spectra obtained in the range of the C]H

Ž y1 .stretching mode 2700]3100 cm . It is evident that astrong reduction in the peaks in the region around2900 cmy1 } which are generally attributed to hydro-gen bonded to sp3 carbon } occurs as a consequenceof increasing the self-bias voltage. The observed de-crease is much larger than expected from a simplereduction in the hydrogen content. On the other hand,in the region above 3000 cmy1 } which is related tohydrogen bonded to sp2 carbon } a significant reduc-tion in the absorbance is not observed, suggesting anincreased sp2 fraction of the films deposited at highbias, in accordance with the Raman results. Again, theanalysis of infrared spectra of films deposited withdifferent silane concentrations has shown that the ef-fect of silane addition to the discharge is to increasethe sp3 character of the films.

Thus, one can conclude that films obtained at highvalues of self-bias present good mechanical properties

and an increased sp2 character. The addition of largersilane contents to the discharge, on the other hand,

3 Ž 2 .increases the sp character reduces sp and con-tributes to a further improvement in the mechanicalproperties. This puzzling situation can be understood in

Ž .two different ways: i one may admit that the materialstructure is such that sp2 carbon atoms also contributeto the mechanical properties, as the results of Lacerda

w xet al. for a-C:H films seem to indicate 22,24 ; or, moreŽ .plausibly, ii one must consider that the mechanical

properties are not only dependent on the sp2rsp3

fraction, but also on the detailed material bondingproperties, in particular on the hydrogenation of thesp3 matrix. In this way, films deposited at high bias canpresent higher hardness due to their reduced density ofŽ 3.sp C]H bonds, as the results in Fig. 4b suggest,which would imply a more rigid network. Still, a deeperanalysis of this issue will be the subject of a futurepublication.

In order to investigate to what extent the propertiesof Si-DLC could be improved by silicon addition, aseries of depositions with varying gas composition wascarried out at high values of self-bias. Fig. 5 shows theeffect of increasing silane content in the plasma on thehardness of films produced at y800 V. Although somevariation in the hardness can be observed, with some-what higher values obtained for silane contents in therange 2]5 vol.%, all films present a hardness higherthan 20 GPa. The insert in Fig. 5 shows that almoststress-free films can be obtained in this way, since theresidual stress of the films is strongly reduced to ap-proximately 0.5 GPa. The corresponding maximum de-

( )J.C. Damasceno et al. r Surface and Coatings Technology 133]134 2000 247]252252

Fig. 5. Vickers micro hardness as a function of silane content in theplasma for Si-DLC films deposited at a self-bias of y800 V. Theinsert shows the results for the residual internal stress of the samefilms.

position rate achieved with 20 vol.% silane is largerthan 40 nmrmin. This is a very important result fromthe point of view of technological applications, since itdemonstrates the possibility of producing hard Si-DLCfilms with low stress and a high deposition rate.

4. Conclusions

A study of the influence of deposition parameters,namely self-bias and gas composition, on the propertiesof Si-DLC films deposited by PECVD from gaseousmixtures of silane and methane has been carried out.Films deposited at high self-bias show high depositionrates, low stress and surprisingly high hardness. In-creasing silane concentration in the gas phase leads toan enhancement of the observed effects. Compositionaland structural characterization of the films showed thatSi-DLC films deposited at high values of bias show asilicon content similar to the silicon concentration inthe gas phase, and an increased sp2 character. Increas-ing silane contents up to 20 vol.% yielded a furtherreduction in stress, with almost no effect on hardness.

In this way, it has been demonstrated that Si-DLC filmsŽ . Ž .with high hardness )20 GPa , low stress ;0.5 GPa

Ž .and high deposition rates )40 nmrmin can be de-posited by PECVD.

Acknowledgements

This work was supported by the Finep and CNPqBrazilian agencies, the CNPqrDLR International

Ž .Cooperation Program and DLRrBMBF Germany .

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