Ž .Thin Solid Films 377]378 2000 430]435

Optical emission spectroscopy of a PCVD process used for thedeposition of TiN on cemented carbides

S. Peter a,U, F. Richter a, R. Tabersky b, U. Konigb¨aTechnische Uni ersitat Chemnitz, 09107 Chemnitz, Germany¨

bWidia-Valenite, 45021 Essen, Germany

Abstract

In CVD processes enhanced by plasma excitation of the gaseous species the analysis of the optical emission spectra enables thedirect observation of reactive species in the close vicinity of the growing film. In this paper we report on a new OES study of thePCVD process for the deposition of titanium nitride from pulsed DC TiCl ]H ]N ]Ar discharges. From the results we expect a4 2 2better understanding of the plasma processes used in the coating production. Using optimized conditions of spectra recording, we

Ž . Ž q q q q q.selected reliable signals linesrbands of 12 species Ti, Ti , Cl, Cl , H , H, N , N , N, N , Ar and Ar involved in the TiN2 2 2deposition process. The OES measurements were made with successively modified process parameters. An analysis of thewaveforms of both the discharge current and the OES signals gave insights into the dynamics of the pulsed discharge. Thecharacteristic temporal evaluation of the discharge was attributed to the slow movement of the ionization front from the cathodeto the anode. Q 2000 Elsevier Science B.V. All rights reserved.

Keywords: CVD processes; Plasma excitation; Optical emission spectra

1. Introduction

Ž .Plasma-enhanced chemical vapor deposition PCVDcombines the advantages of CVD and low temperaturePVD deposition and is of increasing interest for thehard coating industry. PCVD TiN-coated cementedcarbides were launched to the market first by WIDIAin 1989 and they are used in many metal cuttingapplications, especially in milling, drilling, threading,

w xbut also in turning operations 1 . New coating systemsin combination with new hardmetal grades and tool

w xgeometry allow the number of applications to rise 2 .ŽIn the case of conducting film deposition among

.others TiN pulsed DC discharges offer several advan-tages, such as arc suppression, wide variation ofparameters and good control of plasma power.

U Corresponding author. Tel.: q49-371-531-8258; fax: q49-371-531-3042.

Ž .E-mail address: speter@physik.tu-chemnitz.de S. Peter .

In order to understand, develop or optimize PCVDprocesses, in situ diagnostics of the plasmardischargeare very valuable.

Ž .Optical emission spectroscopy OES is a well-estab-lished, non-invasive technique for the characterizationof gas phase species in plasmas and discharges. Theidentities of species are provided by spectral positions

Ž .and shapes of spectral features linesrbands , while theintensity of spectral features is determined by both theconcentration of the species and the excitation effi-ciency. The emission spectra contain a maximum ofinformation and they are relatively easy obtained.However, the extraction of relevant information fromthe spectra is complicated. In particular there is nouniversal model available today which allows to calcu-late ground state densities of interesting species in theglow discharge from the OES spectra.

In the present paper with optical emission spectros-copy as the main in situ method, we present our studiesto apply and optimize this technique for pulsed DC

0040-6090r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 2 7 0 - 0

( )S. Peter et al. r Thin Solid Films 377]378 2000 430]435 431

TiCl ]H ]N ]Ar discharges. Diagnostics by time-4 2 2resolved OES were complemented by discharge currentwaveform analysis.

2. Experimental

w xThe PCVD apparatus 3 used in this study was aŽ .200-l cold wall chamber stainless steel with a

Žparallel-plate reactor installed in it electrode diameter.175 mm, distance 40 mm . The power for the discharge

Žis supplied by a MAGPULS power supply voltage up.to 1000 V, current up to 10 A . The power supply

current readout in dependence on the output voltagewas precisely calibrated and improved to achieve high

Žaccuracy at low discharge currents now "2 mA in the.range of 50]200 mA .

Ž .The flows of the gases H , N , and Ar each 99.999%2 2were adjusted by mass flow controllers. Titanium tetra-

Ž .chloride 99.9% was evaporated from a vaporizer andcarried into the reactor via Ar as carrier gas. The TiCl4flow was controlled by adjusting the total pressure ofthe vaporizer by use of a precision needle valve. Thegas mixture was distributed within the parallel-platereactor by a gas distribution ring, which was on floatingpotential.

The total pressure in the chamber was measuredŽ .with a capacitance manometer range 1000 Pa and

controlled by an automatically throttle valve. Thepumping system consisted of a rotary pump and a roots

Ž 3 .blower pumping speed 300 m rh . The temperature ofŽ .the cathode substrate holder was stabilized by an

additional resistance heater.The PCVD process was investigated using spatially

and time-resolved OES. A quartz fiber and focussingquartz lenses were used to transmit the light from thedischarge to the spectrometer. The spectrographŽ .JOBIN YVON HR 460S, grating: 1200rmm was

Žequipped with a LN -cooled CCD array 1024=2562.pixels and a Peltier-cooled photomultiplier R928S as

detectors. OES signals were measured in the spectralregion from 250 to 1000 nm.

Time-resolved emission intensities as well as currentŽ .waveforms AC current probe PHILIPS PM9335 were

Žrecorded by a digital oscilloscope Tectronix TDS620B,.LeCroy LT344 .

To find out correlations between process parametersand in situ measurements a systematic variation ofindividual parameters with the others kept constant at

Ž .standard conditions see Table 1 was performed. Therelation between the TiN deposition characteristics andthe OES results will be discussed in detail in a fol-lowing paper.

3. Results and discussion

From an analysis of publications on OES studies of

Table 1Standard process parameters for the experiments

Substrate temperature 5508CPressure 300 PaTotal gas flow 400 sccmTitanium tetrachloride flow 0.9%Hydrogen flow 72.5%Nitrogen flow 12.5%Argon flow 15.0%Discharge current 120 mA

2Ž .cathode area 240 cmPulse duration time 50 msPause duration time 80 ms

w xthe PCVD of TiN and related processes 4]11 it isobvious, that the choice of the optical system and itsoperating parameters are of great importance for theinformative value of the in situ diagnostics. Therefore,we optimized our OES system with respect to the TiNPCVD process.

v Sampling of the optical emission. A collimator di-rected parallel to the cathode surface was used forstandard measurements. Optical emission was sam-pled from the negative glow and from the cathodefall region.

v Spectra recording-CCD-detector. The optimum spec-tral resolution for standard measurements was de-termined as 0.1 nm. Optical emission spectra wererecorded with two accumulation times of the CCDarray: 0.1 s and 10 s. Thereby approximately sixorders of magnitude of the intensities } fromintensive linesrbands like some emissions of N ,2Nq, Ar or the H down to weak emissions as from2 a

Nq or H } could be measured. All intensitiesd

were corrected for background radiation.v Spectra recording-photomultiplier. The photomulti-

plier was used for time-resolved measurements. Theslit widths at the spectrometer entrance and exitwere adjusted to 0.5 mm. This is accompanied witha spectral resolution of approximately 1 nmŽ .FWHM . Signal averaging over 10 000 waveformsby the digital oscilloscope was carried out for betterstatistics. When analyzing very weak emissions fivemeasured signals were added up to give additionalnoise reduction.

v Classification of linesrbands. Great care was takenŽ .to extract reliable signals linesrbands of species

involved into the TiN film formation process fromthe optical emission spectra of the discharges. Morethan 1000 lines and bands in the emission spectrumof the TiCl ]H ]N ]Ar discharges were well de-4 2 2tectable and sufficiently intensive. The classificationof the OES signals to emitting species from thedischarge was not possible on the basis of spectra

w xtables 12]17 only. The intensities listed in these

( )S. Peter et al. r Thin Solid Films 377]378 2000 430]435432

Fig. 1. Classification of emission lines from a TiCl ]H ]N ]Ar glow4 2 2discharge in the spectral range of 500]523 nm to the transitions of

Ž w x.atomic titanium standard conditions, designations according to 12 .

tables were measured under very different condi-tions. Therefore, OES investigations of pure

Žgases and special mixtures like TiCl ]H or4 2.NH ]H ]Ar under pulsed DC excitation were3 2

made. The identity of emissions and species wasaccepted only if quite complete linerband systemswere detected. As an example Fig. 1 shows theclassification of emission lines in the spectral rangeof 500]523 nm to the transitions a3F]z3D0 and

3 3 0 Ž .a F]z F in atomic titanium Ti I . For Ti I addi-3 3 0 Ž .tionally the systems a F]z G 460]470 nm ,

5 5 0 Ž . 5 5 0 Ža F]z D 837]844 nm and a F]z F 963]975. Ž w x.nm were detected designations from 12 . All

these systems have in common very low excitationenergies of 2.1]2.7 eV.

We were able to select linesrbands of 12 speciesŽ .involved in the TiN PCVD process see Table 2 . The

emission linesrbands in Table 2 are intensive and donot overlap with others.

Until now we have had no success in the detection ofTiCl. Additionally we did not find any suitable NH- andNHq-emissions from the TiCl ]H ]N ]Ar discharges.4 2 2These two radicals are prospective precursors for nitro-gen incorporation into the TiN-films. All the bands of

w xthese two species listed by Pearse and Gaydon 14

Fig. 2. Spectral region 334]339 nm measured during TiN depositionŽ .with five different TiCl flows a and recorded from a NH ]H ]Ar4 3 2

Ž .discharge b .

were too weak to be detectable in the presence of theŽ qvast amount of linesrbands of other species N , N ,2 2

q . Ž w x.Ar, Ar and H . Many authors e.g. 4]6 used the2emission intensity at 336 nm as a signal for the NHradical. However, the intensity at this wavelength is asum of the broad NH emission, the strong336.0 nm

Ž .SPS 0]0 -band of molecular nitrogen and the remark-q Ž 4 4 0able Ti emission at 336.1 nm system a F]z G of Ti

.II . Fig. 2 gives a comparison of the spectral region334]339 nm measured during TiN deposition with

Ž . Ž0.6]1.2% TiCl a and the NH bands 336.0 nm, 337.04. Ž .nm recorded from a NH ]H ]Ar discharge b . We3 2

tried to evaluate the NH signal from the emission inthe 336-nm region using the dependence on process

Žparameters of the emission of Ti II from non-over-.lapped lines . However, the NH emissions seem to be

too weak in comparison to the Ti II line mentionedabove to give reliable results.

3.1. Dependence of linerbands intensities on processparameters

We investigated the influence of the process parame-ters which are listed in Table 1 with their standardvalues. One experimental series is presented below as

Table 2Ž .Wavelength positions in nm of atomic lines and molecular bands used for in situ diagnostics of pulsed TiCl ]H ]N ]Ar discharges by OES4 2 2

Ž .Ti Ti I : 517,37 519,30 521,00 843,57q Ž .Ti Ti II : 323,45 323,66 323,90 338,38 338,78 375.93 376.13 368.52Ž .Cl Cl I : 833,33 837,60 857,53

q Ž .Cl Cl II : 489,68H : 456,81 457,27 492,86 493,422H: 410,17 434,05 486,13 656,28N : 371,05 375,54 380,492

qN : 391,44 427,81 470,92 522,702Ž .N N I : 648,27 821,63 859,40 862,92 868,03

q Ž .N N II : 566,66 567,96Ž .Ar Ar I : 750,39 751,47 763,51 772,38 794,82 801,48 810,37 811,53

q Ž .Ar Ar II : 454,50 460,96 476,49

( )S. Peter et al. r Thin Solid Films 377]378 2000 430]435 433

Ž .an example. The intensities of several except Cl IIlinesrbands listed in Table 2 for individual speciescover several orders of magnitude but were in goodapproximation proportional to each other. Therefore,for normalization and for better statistics some averag-ing was possible.

v First we normalized the intensity of each linerbandŽ .for example Ti I 519.3 nm to its mean value forthe typical five parameter points investigated withinan experimental series.

v After normalization averaging was done for eachparameter point including the intensities of several

Žlinesrbands that belong to one species i.e. all lines.of Ti I .

Varying the TiCl input into the pulsed DC dis-4Žcharge within 0.6]1.2% of the total flow all other

.parameters kept constant considerably changes onlyq q Ž .the emission intensities of Ti, Ti , Cl and Cl Fig. 3 .

The Ti II signals increased monotonously whereas theTi I emission passes through a maximum at 0.9%TiCl . The emission intensities of H and H are just as4 2little influenced by the variation of the TiCl input as4the emissions of nitrogen and argon species shown in

ŽFig. 3. That confirms that the discharge current con-

Fig. 3. Normalized emission intensities of both neutral and ionizedŽ . Ž .Ar, N and N a as well as Ti and Cl b as a function of the TiCl2 4

flow.

Fig. 4. Influence of the mean discharge current on the temporalŽ . qbehavior of the discharge current smooth curves and the N2

Ž .optical emission at 391 nm FNS 0]0 .

.stant in this series is the appropriate electricalparameter to control plasma excitation.

One series of measurements done with the collima-tor directed more to the middle of the inter-electrodespace } and therefore closer to the gas inlet ring }resulted in different dependence. Away from the cath-ode the atomic titanium intensity increased more than

Ž .the signals from ionized titanium Ti II when increas-ing the titanium tetrachloride input.

3.2. Temporal beha¨ior of the discharge current and theoptical emissions

The shape of the current pulse is changed fromrectangular } observed for example during a H ]Ar2discharge in a clean chamber } to much more compli-cated waveforms in TiCl ]H ]N ]Ar discharges. Cor-4 2 2

w xresponding to other authors 18,19 we found that thequantity of the TiCl flow plays the major role for the4evaluation of the discharge current with time.

( )S. Peter et al. r Thin Solid Films 377]378 2000 430]435434

w xUsing a video system it was shown 20 that in aŽ 3.larger reactor approx. 100 dm the discharge reaches

Ž .some cathode-parts tolls to be coated with a delay ofup to approximately 100 ms. In contrast, the volume ofour parallel-plate reactor is only approximately 1 dm3

but very similar dependence was observed. Fig. 4 showsthe temporal behavior of the discharge current for four

Ž .different mean current values 50]200 mA . Additio-nally, time-resolved emission intensities of ionizedmolecular nitrogen are included in Fig. 4. The emissionintensities were normalized for comparability with thecurrent measurements. The oscillations in the currentwaveform after switching of the pulse at 50 ms are alsoimaged in the OES-signals.

The current waveforms in Fig. 4 show a sharp peakafter switching on the pulse. But during this shortcurrent peak no optical emission was detected. Afterthis peak the current decreases, passes a minimumvalue and increases again. This last increase in currentis accompanied with increasing optical emission. Forboth discharge current and optical emission intensity

Žthe time to reach a certain level 50% of maximum.values is nearly the same. With increasing mean dis-

charge current it decreases from approximately 25 msŽ . Žat 50 mA 510 V to approximately 6 ms at 200 mA 600

.V .ŽThis means a similar behavior of the integral over

.the cathode area measured current waveform and thelocal detected optical emission signal. We expect thatthe observed effects are caused by the movement of anionization front in the direction from the cathode tothe anode. When this ionization front reaches theanode, the conductivity of the discharge strongly in-creases. As a consequence both discharge current andoptical emission intensity will rise. The velocity of theionization front in low pressure glow discharges de-pends above all on the gas atmosphere, the pressure

Žand the applied voltage approx. 60 cmrms for 300 Paw x.and 1000 V in argon 21 . In the TiCl ]H ]N ]Ar4 2 2

discharges this velocity should be two orders of magni-tude lower.

As an interesting result, significant differences in thetemporal behavior of the optical emission intensitybetween Ti I, Ti II and the other species were observedŽ .see Fig. 5 . The emission of ionized titanium sets inbefore others. In contrast, the signal from atomic tita-nium is detected later, rises up until the end of thepulse and decreases not so suddenly as the other emis-

Ž .sions but similar to the current compare Fig. 4 . Wealso measured the waveform for Clq } it looks likethat of H and Nq. The different waveforms cannot bea 2explained by the different excitation energies from

Žground state of the species Ti I : 2.4 eV; Ti521.0 nmII : 3.9 eV; H : 12.1 eV; Nq : 3.1 eV from323.6 nm a 2 391.4 nmNq ground state or as much as 18.5 eV from N2 2

Fig. 5. Temporal behavior of the optical emission lines of Nq 3912Ž . Ž . Ž .nm a , Ti I 520.9 nm b and Ti II 323.4 nm c and H 656.3 nma

Ž . Ža]c standard conditions, the waveform of the H line is drawna

.each time for better comparability .

w x.ground state 12,15 . We expect that different spatialdistributions of Ti I and Ti II and the propagation ofthe ionization and excitation in the parallel-plate reac-tor during the current pulses are responsible for thedifferent waveforms. Further time- and spatial-resolvedOES measurements and a series of mass spectrometricinvestigations of neutrals and ions at the cathode willhave to be carried out to have more information on thekey chemical species and the plasmachemical reactionsthat link them.

4. Conclusions

Optical emission spectroscopy and current waveformanalysis were used to study the process of TiN deposi-tion from pulsed DC TiCl ]H ]N ]Ar discharges. We4 2 2optimized the spectra recording with respect to signalintensity and spectrometer resolution. Reliable signalsŽ . Ž q qlinesrbands of 12 species Ti, Ti , Cl, Cl , H , H,2

q q q.N , N , N, N , Ar and Ar were selected. These2 2intensive and non-overlapping linesrbands were usedto study the spatial and temporal evaluation of opticalemission from species involved in the TiN depositionprocess.

Increasing the TiCl flow from 0.6 to 1.2% of total4flow the signals from Tiq, Cl and Clq increasedmonotonously whereas the Ti emission passed a maxi-mum. All other emissions were nearly unchanged bythe variation of the precursor flow. A further signifi-cant difference between Ti and Tiq was observed inthe temporal behavior of their optical emission inten-sity. All other analyzed emissions showed nearly identi-cal waveforms.

In spite of the small volume of the used reactor, themeasured current waveforms were comparable to re-sults obtained from industrial reactors. The character-istics of the temporal evaluation of the discharge withinthe parallel-plate reactor were attributed to the slowmovement of the ionization front from the cathode tothe anode.

( )S. Peter et al. r Thin Solid Films 377]378 2000 430]435 435

References

w x1 U. Konig, R. Tabersky, H. van den Berg, Surface Coatings¨Ž .Technol. 50 1991 57.

w x2 R. Tabersky, H. van den Berg, U. Konig, Thin Solid films.¨w x3 S. Peter, F. Richter, in J.V. Heberlein, D.W. Ernie, J.T. Roberts

Ž .Eds , Proceedings of the 12th International Symposium onŽ .Plasma Chemistry ISPC-12 , Minneapolis, MN, USA,

1995:2137.w x Ž .4 Y. Ishii et al., Surface Coatings Technol. 49 1991 279.w x5 K.-T. Rie, A. Gebauer, J. Wohle, Plasma Chem. Plasma Proc.¨

Ž . Ž .13 1 1993 93.w x6 R. Hochreiter, J. Laimer, H. Stori, D. Heim, Surface Coatings¨

Ž .Technol. 74-75 1995 443.w x7 K. Hiramatsu, H. Ohnishi, T. Takahama, K. Yamanishi, J. Vac.

Ž . Ž .Sci. Technol. A 14 3 1996 1037.w x8 K.-T. Rie, A. Gebauer, J. Wohle, Surface Coatings Technol.¨

Ž .86-87 1996 498.w x9 M. Boumerzoug, M. Boudreau, P. Mascher, Plasma Chem.

Ž . Ž .Plasma Proc. 17 2 1997 181.w x Ž . Ž .10 Y. Ohshita, K. Watanabe, J. Electrochem. Soc. 145 7 1998

2558.w x11 K.S. Mogensen, S.S. Eskildsen, C. Mathiasen, J. Bottiger, Sur-

Ž .face Coatings Technol. 102 1998 41.

w x12 A.R. Striganov, N.S. Sventitskii, Tables of Spectral Lines ofNeutral and Ionized Atoms, IFIrPlenum, New York, Washing-ton, 1968.

w x13 N. Saidel, W.K. Prokofiew, S.M. Raiski, Spektraltabellen, Ver-lag Technik, Berlin, 1955.

w x14 R.W.B. Pearse, A.G. Gaydon, The Identification of MolecularSpectra, Fourth Edition, Chapman & Hall, London, 1976.

w x15 G. Herzberg, Spectra of Diatomic Molecules, Princeton, 1950.w x Ž .16 H.M. Crosswhite Ed. , The Hydrogen Molecule Wavelength

Tables of Gerhard Heinrich Dieke, Wiley-Interscience, NewYork, London, Sydney, Toronto, 1972.

w x17 A. Lofthus, P.H. Krupenie, J. Phys. Chem. Ref. Data 6, 1Ž .1977 113.

w x18 K.S. Mogensen, C. Mathiasen, S.S. Eskildsen, H. Stori, J. Bot-¨Ž .tiger, Surface Coatings Technol. 102 1998 35.

w x19 T.A. Beer, J. Laimer, H. Stori, Surface Coatings Technol.¨Ž .120-121 1999 331.

w x20 T.A. Beer, C. Kugler, J. Laimer, H. Stori, Proceedings of the¨14th International Symposium on Plasma Chemistry, August2]6, 1999, Prague, Czech Republic, Volume III, 1541.

w x Ž .21 J.I.F. Palop et al., in: Lukac, Kosinar, Skalny Eds , ES-CAMPIC 96, Europhysics Conference Abstracts, Vol. 20E, PartA, 157.