SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 201–207 (2000)

Investigation of the uniformity and coverage ofCxNy : H overcoatings during PECVD in aninductively coupled plasma

D. A. Zeze,* D. R. North, N. M. D. Brown and C. A. AndersonSurface Science Laboratory, University of Ulster, Coleraine BT52 1SA, Northern Ireland

The films were grown using an inductively coupled plasma with two different precursor gas mixtures: CH4/N2

and C10H16/N2. The lateral compositional uniformity of the specimens and the resultant substrate coverageare investigated here using XPS, AES, Raman spectroscopy and SEM.

X-ray photoelectron spectroscopy, because of the size of the x-ray beam used (~5 mm), gives a grossmeasure of the atomic concentration of the surface. In turn, Auger analysis, using a 1µm spot E-beam,allowed the compositional uniformity to be checked at a substantially higher lateral resolution. Additionally,Raman spectra collected from different regions of the surface confirm the outcome of the XPS and AESresults obtained. In addition to XPS, AES and Raman spectroscopy, some SEM data were generated toillustrate the compositional and structural uniformity of the coating across the substrate surface. Such SEMimages apart, no contribution of the substrates used to the XPS or AES spectra obtained confirmed that thefilms examined (150–3000A thick) were pinhole free.

These investigations clearly show that the uniformity and the substrate coverage are a function of thedeposition regime, i.e. power level, base pressure, etc., and are strongly affected by the abundance of oxygenfound in the films. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: AES; XPS; SEM; Raman; CN

INTRODUCTION

Amorphous hydrogenated carbon nitride and diamond-likecarbon (DLC) films have been the subject of intensiveresearch addressing their tribological behaviour and alsotheir electronic properties in respect of their numerouspotential applications. Many attempts have been madeto characterize, optimize and control their structure andproperties under a wide range of deposition regimes.1,2

Diamond-like carbon is commonly used, for example, asan overcoating material in many technological areas. Ina magnetic storage disc context, it is essential to deposita very thin coating film that is uniform, homogeneousand conformal with the substrate surface texture, so asto facilitate increased disc performance (i.e. areal density,reduced friction, lower fly height, faster data transfer, etc.).

Because carbon nitride films may offer better tribolog-ical properties than DLC, i.e.-C3N4 is predicted to havehardness comparable or even greater than diamond,3 it hasalso become a focus for research. In coating technology,the key requirements are related to the wear, hardness,chemical inertness, optical transparency and other salientproperties of the films used. So far, CH4 is used mainlyin nitrogen/argon mixtures for DLC/CxNy : H deposition.

* Correspondence to: D. A. Zeze, Surface Science Laboratory, Uni-versity of Ulster, Coleraine Co. Londonderry, BT52 1SA, NorthernIreland.E-mail: DA.Zeze@ulst.ac.uk

Contract/grant sponsor: Northern Ireland Industrial Research andTechnology Unit.

Contract/grant sponsor: Seagate Technology.

As stated in our previous work,4 higher molecular weightpolycyclic hydrocarbons such as C10H16 enable the synthe-sis of DLC/CxNy : H films, possibly via cluster deposition.Here, we investigate the compositional uniformity, lat-eral and uniform growth across the substrate, the so-called‘coverage’. In the present work, CxNy : H/DLC films weregrown using plasma-enhanced chemical vapour deposi-tion (PECVD) in an inductively coupled plasma (ICP)rig, with C10H16/N2 and CH4/N2 gas mixtures. Prelim-inary examination of the CxNy : H/DLC films depositedshowed compositional uniformity across the surface. Theuniformity and coverage are here examined in more detailusing XPS, AES, Raman spectroscopy and SEM.

EXPERIMENTAL

For deposition, the films were grown in an ICP processingrig using two different hydrocarbon precursors—methane(CH4) and adamantane (C10H16) vapour in a nitrogenflow—to give CH4/N2 and C10H16/N2 mixtures. Theadamantane powder used was introduced via a reservoir(inserted in the nitrogen line) heated slightly above roomtemperature to provide admantane vapour. The ICP r.f.plasma source is itself cooled by a flow of cold nitrogengas. Prior to a deposition run, the chamber was evacuateddown to a base pressure of¾10�9 Torr.

The details of the deposition system are presentedelsewhere.4,5 Basically, the inductively coupled sourcewas designed and fabricated in-house and consists of acooled tubular copper (with gaseous nitrogen) r.f. coilsurrounding a quartz discharge tube covered at the exit

Copyright 2000 John Wiley & Sons, Ltd. Received 1 July 1999Revised 3 September 1999; Accepted 8 September 1999

202 D. A. ZEZEET AL.

end by a perforated ceramic plate.5 The system offers themajor advantage of eliminating the possibility of a plasmabeing generated external to the in-vacuum discharge tubeand offers precise and repeatable control of the processparameters associated with the generation of the plasmar.f. power, gas flow and gas pressure. The Al2O3TiC.10ð10 mm/ substrates used for deposition were cleanedultrasonically for¾15 min, using redistilled isopropanol.Prior to deposition, they were also cleanedin situ for5–10 min, in a vacuum with nitrogen/argon plasma.Unheated substrates were used throughout. Deposition ofthe CxNy : H films was carried out for 1.5–30 min, givingspecimens 150–3000A thick (measured by profilometry).

Data resulting from deposition experiments for morethan 30 specimens are analysed here. After deposition,characterization was carried outex situusing x-ray photo-electron spectroscopy (XPS), Auger electron spectroscopy(AES), Raman spectroscopy and scanning electron micro-scopy (SEM). The XPS analysis was carried out using anXSAM800 (Kratos) system with a hemispherical energyanalyser (hri D 127 mm, in the FAT mode with a passenergy of 38 eV) and Mg Kexcitation (13 kV and 18 mAat the anode;h� D 1253.6 eV). Owing to the x-ray beamsize at the specimen (¾5 mm), XPS gives the ‘overall’atomic concentration of the surface (analysis depth¾40A).

Auger analysis, using a PHI 660 scanning Auger micro-scope with a 1µm spot E-beam at 3 kV, allowed thecompositional uniformity to be checked over a greaternumber of much smaller separate areas (chosen randomlyacross the 10ð 10 mm specimen surface). The elec-tron beam current was brought down from 95 nA to40 nA so as to reduce beam damage.4,6 Carbon KLL,N KLL and O KLL transitions were used for quantifi-cation. The electron microscopic images (to check thelateral coverage) were also generated using the Augerinstrument, by secondary electron detection. The Ramandata referred to were collected using a 2µm spot inci-dent beam and offer further complementary lateral andstructural information. The Raman system (ISA Labram)was equipped with a microscope, coupled confocally to aspectrograph with a BX40 liquid-nitrogen-cooled charge-coupled device. Specimen excitation was achieved byan external 2 mW argon ion laser with a wavelengthof 514.53 nm. However, the laser intensity at the sur-face of films depends on the filter employed (50–10%transmission) and was kept below that leading to spec-imen damage. The operating Raman shift was in therange 0–3000 cm�1. Deconvolution and quantification ofthe Raman signature of interest (1000–1800 cm�1) wasaccomplished using the software provided with the instru-ment. A mixed Gaussian–Lorentzian lineshape functionwas used for curve fitting. The associated software pro-vides for the semi-automated iterative variation of lineposition (two lines only were assumed here) of the fullwidth at half-maximum (FWHM) of each component lineand the Gaussian/Lorentzian ratio was computed until anacceptable fit is achieved.

RESULTS AND DISCUSSION

Uniformity

Table 1 shows the XPS data, i.e. the atomic abun-dances (C : N : O : Al), of some typical specimens referred

Table 1. X-ray photoelectron spectroscopy chemical com-position of the specimens grown using an induc-tively coupled plasma in a mixture of CH4/N2 orC10H16/N2

Deposition Atomic concentration (at.%)Sample Plasma Time (min) C N O Al Ti

Al2O3TiC Blank substrate 15 39 41 5A01 C10H16/N2 10 39 14 20 25 2A02 C10H16/N2 30 70 15 10 5A03 C10H16/N2 15 54 31 15A04 C10H16/N2 20 66 28 6A05 CH4/N2 15 51 12 24 13A06 CH4/N2 10 39 19 26 16A07 CH4/N2 25 30 11 36 23A08 C10H16/N2 5 59 32 9A09 CH4/N2 18 58 34 8A10 Commercial DLCa 89 11A11 C10H16/N2 3.5 97 1 2A12 C10H16/N2 1.5 97 1 2

a DLC, diamond-like carbon.

to above. Note that samples A02, A03 and A04 weredeposited using adamantane vapour in a nitrogen plasma(C10H16/N2), whereas samples A05, A06 and A07 wereobtained from a CH4/N2 plasma. Comparison betweenthe films deposited using these two hydrocarbon sourcesis discussed elsewhere.8 However, although the XPS dataacquired give the ‘overall’ concentration of the speciespresent in the films, they do not provide any significantinformation about the local uniformity or lack of it. Thesedata can be used, of course, to investigate the lateralgrowth (coverage) of the film by monitoring the decreaseand then the loss of the aluminium signal associated withthe substrate used.

Auger analysis using a small electron beam spot size(cf. Experimental section) allowed the compositional uni-formity to be checked over a significant number of 1µmdiameter areas, as shown in Fig. 1. Obviously, the typicaldata reported in Fig. 1 and Table 2 show clear composi-tional homogeneity across the surface. Calibration of theXPS and AES spectrometer using aluminium foil gave acompositional precision of¾š2 at.%, which is also appli-cable to the specimens analysed here. Sample A03 (Fig. 1)

Figure 1. Auger electron spectroscopy investigation of thelateral growth of CxNy : H film (sample A03) using a 1 µm spotE-beam (3 kV, 40 nA): C : N : O atomic abundances acquired over14 different areas (chosen randomly) across the surface.

Surf. Interface Anal. 29, 201–207 (2000) Copyright 2000 John Wiley & Sons, Ltd.

CxNy : H OVERCOATINGS DURING PECVD 203

Table 2. Auger electron spectroscopy elemental distributionof C : N : Al of samples A02 and A06 across thesurface average

Sample A02 Sample A06

Area no. C (at.%) N (at.%) O (at.%) AI (at.%) C (at.%) N (at.%) O (at.%)

1 70 15 9 62 67 16 11 63 68 16 11 54 68 18 9 55 69 15 10 6 54 14 326 67 16 12 5 50 15 357 70 16 9 5 53 15 328 70 16 8 6 50 15 359 68 16 11 5 52 15 32

10 71 16 9 4 52 16 3211 69 15 11 5 54 14 3512 69 15 9 6 50 16 34

Mean 69 16 10 5 52 15 33SD 1.4 0.8 1.3 0.7 1.7 0.6 1.6

exhibits a pinhole-free coating because no aluminium ortitanium signal was detected in the course of the analy-sis. Additionally, the oxygen present is<10 at.% and isattributed to superficial contamination.7 In turn, sampleA02 indicates for C : N : O : Al (Table 2) the presence ofa small amount of both oxygen and aluminium, averag-ing respectively 69 : 16 : 10 : 5 at.%. Here, the 5 at.% foraluminium is believed not to be quantitatively realisticfor the aluminium concentration owing to the noise levelcontributing to the signal at the A1 binding energies dur-ing the acquisition process, which is especially significantwith these low intensity signals, but it is sufficient to beindicative of the substrate not being entirely covered.

The compositions of sample A06 (Table 2) are partic-ularly interesting in the sense that the coating of thesesamples is not total (partial coverage), therefore the con-tribution of each element (C : N : O : A1) to the analysed.1 µm/ areas can be evaluated with confidence. The abun-dance of oxygen and the presence of aluminium at thesurface (Tables 1 and 2) are indicative of contributionsfrom the substrate Al2O3TiC. Although clearly this sub-strate was not entirely covered, AES analysis was carriedout to establish the regularity of deposition, including thepinhole distribution. Note that in the AES quantification ofthis particular sample (A06) the Al signal was not consid-ered. However, for the areas analysed Table 2 indicates, asabove, the regularity of coverage, including the effects ofpinholes. As shown in Fig.1, the uniformity of the film interms of composition is confirmed over the area examined.Finally, this uniformity (in terms of signals detected) ofthe composition across the substrate has been confirmedon the basis of the data collected on more than 30 spec-imens, all showing uniform lateral surface compositionsbut differing from one specimen to the next, reflecting thedetails of deposition.

Raman spectroscopy and microcrystallites

The actual investigation of structural uniformity was alsocarried out using Raman spectroscopy. Raman spectra oftwo regions, of the sample A03 are shown in Fig. 2 afterbackground subtraction. Both regions exhibit the sameband profile, suggesting a uniform Raman activity (see

Figure 2. Raman spectra of CxNy : H film (sample A03) showingsimilar over-all profiles in region (a) and region (b).

later) in each.Note that somesamplesdid not showanypeakin this region.However,theRamanspectrathatwereobtainedfrom differentareasof thesamesamplesshowedthe sameuniform overall profiles.

Comparisonof the Ramanspectrashownhere(Figs 2and3) showsthat thesearevery similar to thosereportedby Turbanetal.9 Althoughour Ramanenvelopeis decon-volutedhereinto two peaksto giveanexcellentfit with theexperimental,as opposedto the four usedelsewhere,9,11

the samemain peakpositions,respectivelyat 1360 and1580 cm�1 (the latter derived from what would be thedoubly degenerateE2g in-plane mode in pure graphite),identifiedastheD andG bands,areapparentasexpected.By comparisonwith the Ramanspectraof purediamondand pure graphite, which exhibit, respectively,narrowpeaksat 1340and1570cm�1, thesebandsarecommonlyassociatedwith the sp3 and sp2 phasesof carbon.If it isestablishedunambiguouslythat the bandat 1580cm�1 isassociatedwith sp2 (graphite) type12,13 structuralcontri-butions,thereis in fact no clear evidenceto assignsys-tematicallyany bandappearingin the region 1400 cm�1

to sp3 carbon (diamond), especially for quantification.Indeed,the bandat 1360cm�1 arisesfrom the disorderedstructure12,14 of the specimen,anda slight frequencyshiftto higherwavenumber(¾1600cm�1) of the secondband

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 201–207 (2000)

204 D. A. ZEZEET AL.

Figure 3. Comparison of Raman spectra of a CxNy : H film(sample A04 with a clear band separation) (a) and a diamond-likecarbon film (commercial blank DLC) (b).

(the G band) appearsas a function of increasingdisor-der, particularly in the more graphitic samplesinvolvingsmallmicrocrystallitesizes.14,15 This banddoesnot appearto dependon the mutual arrangementof graphitic plainsbecauseit is found at the sameposition in turbostraticcarbons16 andothercarbons.Rather,this band(attributedto the in-plane A1g mode, which is Raman inactive ingraphite)appearsto be renderedactiveby the breakdownof thenormalRamanselectionrulesbroughtaboutby thelossof symmetryin thedisorderedstructuresconcerned.Afull studyof theRamanspectroscopyof thecarbon-relatedmaterialsis in handandwill be publishedshortly.

Nevertheless,one can typically attribute the bands(Dand G) seenin amorphouscarbon(DLC) to a diamond-like carbon ‘fingerprint’ that is reproducible,althoughsome slight frequencyshifts can appearas a result ofsurfacemodification. Therefore,the more than usuallyclear separationof D and G bandsillustrated in Figs 2and 3(a) underlinesthe probablestructuralmodificationhereof the Ramanactivity of the DLC-like films in thepresenceof nitrogen. Note that, Fig. 3(b) is a Ramanenvelopeof a hydrogenatedDLC film, whereasFigs 2and3(a)aretypical of spectraof amorphousnitrogenatedcarbonfilms. Amorphousand/ornitrogenatedcarbonfilms

commonlyexhibit peaksin theRamanpositions,asshownin Figs 2 and3. Thesearetakento indicatethe presenceof nanocrystallitesin thefilms,9–11,17–19 giving ratio ID/IG

(ratio of D/G band intensities),which is indicative ofthe size and the relative order of the microcrystallitespresent.The observedintensity of the ‘disorder’ bandhasbeenshownto be a good indicator of the crystallitesize.12,16 Indeed, some SEM images seem to confirmthis hypothesisbecausethey show shiny micro points(possibly graphitic in character)spreadover the surfaceof the specimens[Fig. 4(b)].

Coverage

Coverageis seenhere as the ability to deposit a uni-form hole-free coating layer acrossthe substrate.Thefilm viewed in plan should look homogenous.Becauseof some experimentaldifficulties at one stage,notablya poor systembasepressure(correctedsuccessfully),asystematicinvestigationof coverageusing SEM imag-ing, given its immediacy, was carried out to qualify

Figure 4. Scanning electron microprobe pictures of CxNy : Hfilms. (a) partial coverage showing obvious pinholes (sampleA01); magnification 40ð. (b,c,d) Pinhole-free specimens withmagnification 500ð (for sample A04) and, 40ð and 1400ð (forsample A12).

Surf. InterfaceAnal. 29, 201–207 (2000) Copyright 2000JohnWiley & Sons,Ltd.

CxNy : H OVERCOATINGS DURING PECVD 205

Figure 4. (continued).

the effects on deposition of poor base pressure con-ditions. Figure 4 shows typical SEM images of threespecimens. Sample A01 [Fig. 4(a), magnification 40ð]is partially coated, with several uncoated areas seenon the surface, illustrating non-uniform lateral coverageof the substrate. In turn, samples A04 [Fig. 4(b)] andA12 [Fig. 4(c,d)]—magnification 500ð, 40ð and 1400ð,respectively—indicate a pinhole free covered surface.When the base pressure is poor (¾10�6 Torr), the qual-ity of the coating is obviously affected and also correlateswith the higher abundance of oxygen in the resultant films.For instance, specimens A01 and A04 show 20 at.% and6 at.% oxygen, respectively, associated with poor andacceptable base pressures.

The deposition rate under normal running conditionswas found to be 0.6 µm h�1, leading to an investiga-tion of the minimum time required to grow a film thatexhibits both compositional uniformity and uniform lat-eral coverage. The XPS data reported in Table 1 sum-marize the surface composition of deposited DLC andCxNy : H specimens, along with that of a substrate blankAl2O3 TiC. The minimum time required for total cover-age was found to be close to 90 s, yielding films with<2 at.% oxygen under the deposition conditions reported

previously4 (70 W, 40 mTorr, 4.2 sccm). Coverage is con-firmed by there being no detectable diagnostic substratesignals (Al, Ti) in the corresponding XPS data. SpecimenA12 [pinhole-free, Fig. 4(d)], deposited at 70 W, 40 mTorr,with 4.2 sccm of nitrogen flow, shows a very low nitro-gen content (<2 at.%). This is attributed to the presenceof excess adamantane vapour, because of the adamantanereservoir overheating, i.e. this film looks more like a nor-mal DLC film. However, because the deposition rate wasestimated to be 0.6 µm�1, the film obtained after 90 sshould be at least 150A thick. Figures 4(c) and 4(d) (sam-ple A12), magnified 100ð and 1400ð, respectively, givesome visual evidence for uniform coverage; this is con-firmed by Table 1, which gives the composition acrossa similar specimen (A09) but does not in fact show anysignal from the substrate.

Figure 5 gives the corresponding XPS spectra of somespecimens, showing to signal from the substrate (which

Figure 5. X-ray photoelectron spectra of CxNy : H (a) and dia-mond-like carbon (b) films grown onto Al2O3TiC substrates usingan inductively coupled plasma with C10H16/N2 mixtures.

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 201–207 (2000)

206 D. A. ZEZEET AL.

Figure 5. (continued).

exhibits Al 2s Al 2p and Ti 2p bandsvisible on thecorrespondingspectrumof theblanksubstrate).Here,too,C10H16 and CH4 were used for thesedepositionsand,as shown in Fig. 5, the coverageis total regardlessofthe hydrocarbonsource.Note also that the quality ofthe coatingwill dependmainly on the depositionregimeand the oxygen abundancefound in the film. The XPS

spectraof CxNy : H and DLC coatingsare presentedinFigs 5(a) and 5(b), respectively.Visual comparisonofthesespectrasuggeststhe conclusionthat the CxNy : Htype of coating tends to entrain and bond readily withoxygen,formingNO andCOin thematrix.4,5,8,9 In turn,asopposedto the nitrogenatedfilms, DLC-type films grownin the depositionregime alluded to above show muchlower oxygen abundances.Additionally, a commercialDLC sample(A10: 1500A, Fig. 5(b), top spectrum)showsahigheramountof oxygenthanthosegrownfor thisstudy.However,theXPSdataderivedfrom thespectraof Fig. 5andthe correspondingatomicconcentrationssummarizedin Tables1 and 2 again confirm the uniformity of thedepositionin termsof chemicalcompositionandcoverageacrossthe substrate.

CONCLUSION

In this report, film compositionaluniformity in CxNy : Hfilms was investigated using a combination of XPS,AES Raman spectroscopyand SEM. In terms of sig-nals detected,the elementaluniformity and structureofthe depositedfilms indicate homogeneity.The coverageacrossthe substrate(lateral growth) has also beencon-firmed on the basisof the data collectedon more than30 specimens,all showingboth uniform compositionandcompletecoverageacrossthe substrate.

Ramandatadrawnfrom the literatureandthat of someexperiments(not reportedhere) indicate the presenceofthe two bands(¾1400 and ¾1600 cm�1) identified innearly all types of carbon,suggestingthat the band at1400 cm�1 observedin DLC and analoguescannot beascribedsystematicallyandspecificallysolely to sp3 car-bon (diamond) contributions.However, Ramanspectraof the hydrogenatedcarbonnitrides and DLC examinedshowa clearseparationof the bandsat¾1400cm�1 and¾1600 cm�1, in contrast to that proposedelsewhere.17

Spectracollectedfrom different regionsof the depositedsamplesillustrated similar overall band profiles, con-firming structuraluniformity and the completecoveragedemonstratedby AES, Auger and SEM. The minimumtime required to lay down a completefilm of uniformcompositionandstructureunderthe conditionsusedwasfound to be 90 s, giving a film ¾150 A thick.

Acknowledgements

Theauthorswish to thankSeagateTechnologyandtheNorthernIrelandIndustrialResearchandTechnologyUnit for their financialsupport,andalso colleaguesin the SeagateConsortiumand othersfor their helpfuldiscussionsandcontributions.

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