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Surface and Coatings Technology 142᎐144 Ž2001. 943᎐949 Effects of plasma non-homogeneity on the physical properties of sputtered thin films V. Rigato a,U , G. Maggioni a , A. Patelli b, V. Antoni c , G. Serianni c , M. Spolaorec , L. Tramontin c , L. Depero d, E. Bontempi d b a I.N.F.N. Laboratori Nazionali di Legnaro, Via Romea 4, I-35020 Legnaro (Pd), Italy Dipartimento di Elettronica e Informatica, Uni¨ ersita ` di Pado¨ a, INFM, Unita’ di Pado¨ a, Via Gradenigo 6 r A, I-35131 Pado¨ a, Italy c Consorzio RFX, Corso Stati Uniti 4, I-35127 Pado¨ a, Italy d INFM Unita’ di Brescia and Laboratorio di Strutturistica Chimica, Uni¨ ersita ` di Brescia, ¨ ia Branze 38, 25123, Brescia, Italy Abstract The plasma generated in a two-target closed field unbalanced magnetron sputtering system for thin film deposition is characterized by means of Hall probes and cylindrical Langmuir probes as a function of the position inside the vacuum system. The plasma potential, electron density and temperature profiles in different locations are measured by two diagnostic systems equipped with cylindrical Langmuir probes. The plasma non-homogeneity due to the presence of magnetic field gradients is evaluated. In order to test the effects of measured plasma non-homogeneity on the physical properties of sputter-deposited coatings, several substrates are put inside the chamber in regions characterized by different plasma density and plasma potential. The composition, microstructure and morphology of TiN x films grown onto these substrates are then studied by means of nuclear techniques ŽRBS, n-RBS, NRA, ERDA., X-ray diffraction ŽXRD. and secondary electron microscopy ŽSEM.. The mechanical properties are determined by micro-scratch test and nanoindentation and correlated to the local plasma parameters. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma diagnostics; Unbalanced magnetron; Titanium nitride 1. Introduction Balanced and unbalanced magnetron sputtering deposition techniques are widely applied both in industrial processes and in advanced materials development and treatment. The ion density of the glow discharge is often enhanced by means of permanent magnets in order to produce a region over the target where the electron loss rate is reduced and the ionization probability enhanced due to the magnetic confinement of electrons. The knowledge of the ion flux and energy U Corresponding author. Tel.: q39-049-8068-476; fax: q39-049641925. E-mail address: [email protected] ŽV. Rigato.. of ions bombarding the growing film is of fundamental importance in ion plating processes mostly for ceramic coating production where the final coating properties like micro-structure, stoichiometry, hardness, wear resistance and adhesion are strongly dependent on plasma potential, electron temperature and density. Therefore it is of primary importance to study the plasma homogeneity and its effects on the final coating properties. The measurement of magnetic field and plasma parameters can be performed by using Hall and Langmuir probes w1x. Rohde et al. w2x studied a dual cathode, reactive sputtering system by means of a flat-discshaped Langmuir probe in order to better model the conditions for an actual substrate. However, this kind of probe is not the most suitable one for measuring the plasma potential and carrier densities as also recog- 0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 2 5 8 - 0 944 V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 nized by the authors. The present work aims at the characterization of a DC closed field unbalanced magnetron sputtering device by means of Hall probes and small cylindrical Langmuir probes best suited for precise plasma characterization. In the present work TiN x coatings were grown by reactive sputtering in ArrN2 plasma on static substrates located in regions characterized by quite different plasma density and potential due to the different magnetic fields. The composition and density of TiN x films have been studied by means of Rutherford and non-Rutherford backscattering analysis ŽRBS, non-RBS and NRA. and elastic recoil detection analysis ŽERDA.. The coating microstructure has been investigated by X-ray diffraction ŽXRD. and grazing incidence XRD ŽGIXRD.. The mechanical properties were determined by micro-scratch test and nanoindentation. The physical properties of these coatings have been related to the local plasma parameters. 2. Experimental 2.1. Coating deposition setup The coating deposition setup consists of a vacuum chamber Žlength 1194 mm, height 755 mm, depth 734 mm. equipped with two 250 = 140 mm2 rectangular unbalanced magnetron sources located in opposite positions. The distance between the cathode surfaces is 460 mm Žsee Fig. 1.. Two DC current-controlled power supplies ŽAdvanced Energy MDX. are used to negatively bias the cathodes. The target material is titanium. The ultimate pressure of the vacuum chamber was - 2 = 10y4 Pa. The discharge gas was high purity argon at the working pressure of 0.1 Pa. High purity nitrogen was used as reactive gas. For the TiN x coatings deposition the current on each magnetron source, Imagn , was set at 3.5 A corresponding to a total discharge current Itot s 7 A Žtotal power Pmagn s 3.7 kW.. A closed-loop optical emission monitor ŽOEM. was Fig. 1. Experimental setup used for the deposition of TiN x coatings. The picture shows the Langmuir probersample-holder system Žtop view.. Dimensions in mm. used to control the nitrogen gas flow to the deposition chamber. An optical fiber transmits the light emitted by the plasma from a region just in front of one Ti cathode to a CCD spectrometer. The intensity of a spectral line depends on the number of excited species per unit volume, so the control of the line intensity allows control of the deposited film stoichiometry in reactive sputtering by selecting the desired concentration of metal atoms in the plasma. This technique also allows us to record the entire visible spectrum emitted by the plasma and to choose the more suitable metal line to control the process. For the TiN x coatings deposition the 500-nm Ti line has been chosen and the OEM setting Ži.e. the ratio between the intensity of this line after and before nitrogen introduction into the chamber. was 65%. TiN x coatings were deposited on silicon substrates Žfor analytical purposes. and on M42 tool steel Žfor mechanical characterization .. The substrates were put in four different Ž x, y, z . locations Žsee Fig. 1.: A s s Ž0,220,0.; Bs s Žy50,220,0.; Cs s Žy100,220,0. and Ds s Žy150,220,0.. Before the TiN x coating deposition a 100᎐200-nmthick Ti underlayer was deposited in pure argon plasma. Both Ti and TiN x films were grown by DC biasing the substrate at Vs s y40 V. 2.2. Magnetic field measurements The magnetic field characterization was performed using Hall-effect sensors suitable for static field measurements. They were mounted on a mechanical supporting structure that permitted us to record simultaneously all three components. Due to the position of the permanent magnets of the two opposing magnetron sources the magnetic field configuration is characterized by symmetries with respect to the Ž x,y ., Ž y,z . and Ž x,z . planes Žsee Fig. 1., hence the magnetic field mapping was limited to the following two regions: 0 F x F 80 mm and 20 F y F 230 mm; 20 F y F 230 mm and 0 F z F 90 mm. 2.3. Langmuir probes measurements Two Langmuir probe systems were used: the first one consisted of three tungsten tips having length of 3 mm and diameter of 0.2 mm. The distance between the tips was 8 mm. Each electrode emerged from a stainless steel cylindrical pipe Ž150 mm length, 4 mm diameter. electrically isolated from it. A 25-mm-diameter stainless steel cylinder supported all three pipes and was mounted on the shaft of a manipulator allowing translation and rotation of the whole system. This first Langmuir probe system was used only in the pure argon plasma. The second system, used during TiN deposition, con- V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 sisted of four probes. Each probe was located close to each sample in order to measure the plasma parameters during the process. The probes position Žwith reference to Fig. 1. were: A p s Ž0,240,0.; Bp s Ž y 50,240,0 . ; C p s Ž y 100,240,0 . and D p s Žy150,240,0.. The length and diameter of the tungsten wire was 4 mm and 0.2 mm, respectively. A 350-mmhigh, 6-mm-diameter stainless steel vertical tube supported each probe in each sample holder. The analysis of Langmuir probe data has been performed according to the method described by Serianni et al. w3x based on the following hypotheses: Ža. the ion current portion of the Langmuir probe I᎐V characteristic curve depends on the applied voltage according to the Orbital Motion Limited theory and for voltages low enough with respect to the floating potential a linear trend was assumed; Žb. the electron current can be affected by the presence of two Maxwellian electron populations in the plasma; and Žc. the magnetic field due to magnetron sources reduces the portion of I᎐V Langmuir curve useful to evaluate the plasma parameters. 2.4. Coating characterization Compositional analysis has been carried out by ion beam analysis using the 7-MeV CN and the HVEC 2.5-MeV Van de Graaff accelerators at the Laboratori Nazionali di Legnaro of INFN. Contaminants like carbon and oxygen have been studied by non-Rutherford BS, using a 3.0-MeV and 2.0-MeV proton beam and by Žd,p. nuclear reaction analysis. The hydrogen content analysis has been performed by ERDA with a 2.2-MeV ␣-particle beam. The titanium depth concentration profiles have been determined using a 5.85-MeV ␣-particle beam. Coating thickness has been measured by cross-section SEM. The grazing incidence X-ray diffraction spectra and the ᎐2 patterns have been collected by a Bruker ‘D8 Advance’ diffractometer equipped with a Gobel ¨ mirror. The angular accuracy was 0.001⬚ and the angular resolution was better than 0.01⬚. The Cu K ␣ line of a conventional X-ray source Ž40 kV, 40 mA. was used. The XRD experimental data were fitted using the Topas P program w4x. The lattice parameters were calculated by the Unit Cell program w5x. The mechanical properties have been characterized by a CSEM Nano Hardness Tester ŽNHT. and Micro Scratch Tester ŽMST.. The nano-indentations have been performed with a maximum load of 10 mN using a Berkovitch diamond tip. The maximum depth of the indentation was approximately 140 nm, less than 1r10 of the total thickness of the TiN x films. The scratch tests have been performed using a 200-m radius Rockwell C diamond tip with a progressive load from 0 945 Fig. 2. Magnetic field map on the plane z s 0. to 30 N. The scratch tracks have been observed by an optical microscope and by SEM. 3. Results and discussion 3.1. Magnetic field measurements The results of the magnetic measurements obtained in the horizontal plane Ž z s 0. are presented in Fig. 2; the figure is split into two parts where different scales are used: the magnetic field ranged from approximately 2 to 20 mTorr in a closed field arrangement. 3.2. Langmuir probe measurements 3.2.1. Pure argon plasma The effect of the probe assembly insertion on the magnetron voltage was negligible and comparable to the bias variations usually observed during target lifetime. A larger perturbation effect was observed in the plasma measurements at x ) 0 ŽFig. 3.: in this case, the whole probe system was lying between the cathodes. In Fig. 3 the profiles of electron temperature and density and plasma potential are shown for y s 230 mm and z s 0 mm at 800 W. Two electron populations with different temperature and density Ž‘hot’ and ‘cold’ electrons. have been found, according to Serianni et al. w6x. At the center Ž xs 0 mm. the temperatures and densities of cold and hot electrons tend to converge. The probe I᎐V characteristics show a gradual change between an outer region where a double slope in the logarithmic plot of electron current is clearly detected and an inner region where the double slope is less marked due to higher values of temperature of cold electrons and density of hot electrons. The hot electron density increases, more rapidly than an exponential curve, by approximately two orders of magnitude going from xs y300 mm to x s 0 mm; the cold electron density increases almost exponentially by approxi- 946 V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 As already mentioned the region where x) 0 mm is affected by the perturbation of the plasma due to the probe insertion so that a non-symmetric profile of temperature, density and potential around xs 0 mm are obtained. The density of both cold and hot electrons is reduced in the perturbed portion of the plasma as can be seen from Fig. 3. Fig. 3. Electron temperature Ža., density Žb. and plasma potential Žc. profiles in pure Ar plasma: open and solid circles refer to hot and cold electrons, respectively Ž y s 230, z s 0; Pma gn s 800 W; Imagn s 1 A; Ar; p s 0.1 Pa.. mately an order of magnitude in the same direction. The density ratio of cold and hot electrons ranges from ; 300 to ; 10 going towards the center Ž xs 0 mm. and the temperature ratio Žhot over cold electron temperature. changes from ; 10 to ; 2. The floating potential decreases monotonically towards the magnetic axis from y5 to y17 V. The plasma potential, resulting from the fitting procedure, displays a maximum at xs 0 mm; an electric field of approximately 100 Vrm can be estimated between ᎐100 and 0 mm, yielding, with B ( 4 mTorr, an E= B drift velocity of ; 25 kmrs directed along the z axis. It is worth noting that a plot of Vp y Vf vs. the weighed temperature ŽTw s Tc q Ž n h rn c . ⭈ T h . is well fitted by a straight line with a slope of 5.2᎐5.6 VreV. This value is in very good agreement with that computed by assuming comparable electron and Ar ion collecting areas in the framework of a single Maxwellian theory; this last value has been used in the linearization of the Orbital Motion Limited ŽOML. theory w6x. 3.2.2. Argonr N2 plasma A set of local plasma parameter measurements has been performed during the growth of TiN x films using a reactive plasma. In this case the second diagnostic system with four Langmuir probes, described above, has been used. The reactive gas flow rate was set by the optical emission gas feeding device to a value such that the optical emission signal was 65% of the original value in pure argon plasma. This value has been chosen in order to carry out the deposition in the metallic regime just before the transition to the poisoned point w7x. The measured plasma parameters did not show appreciable differences when compared with those obtained in a pure Ar plasma in the same experimental conditions. The results obtained during TiN x deposition for the hot and cold electron temperatures and the respective densities are shown in Fig. 4. The electron densities increase towards the center of the discharge Ž xs 0 mm., with more than an exponential increase for the hot electron density. The ratios n crn h and TcrT h range from 500 to 10 and from 20 to 3, starting from xs y150 mm towards xs 0 mm. We can then estimate a local ion current density js , impinging on each substrate varying from 0.5 mArcm2 at xs y150 mm to 3.5 mArcm2 at x s 0 mm. During the reactive deposition, we obtain plasma potential values in the range between q3 and q6 V moving from xs y150 mm to xs 0 mm. The energy of ions impinging on the growing films, as calculated by plasma potential and substrate bias, is in the range 40᎐50 eV. 3.3. Coating properties Cross-section SEM images Žnot reported in this paper. show that the TiN x coatings deposited in the different positions are characterized by a void-free, dense structure. Fig. 5 shows the titanium deposition rate in the argon plasma as a function of x position Ž y s 220 mm; z s 0 mm. as measured by RBS analysis. The Ti deposition rate in ArrN2 plasma in the four sample positions is also reported. The Ti deposition profile deviates from the expected Ti sputtering profile typical of magnetron sources: in the 100-mm-wide central region the deposition rate clearly decreases; the deposition profile shows a maximum value at approximately xs y75 mm. At xs y100 mm the deposition V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 Fig. 4. Temperature and density profiles measured along the x-axis for hot electrons Žopen circles. and cold electron Žsolid circles. during TiN x deposition Ž Pmagn s 3.7 kW; Imagn s 3.5 A; ArrN2 ; pAr s 0.1 Pa.. rate starts to decrease as expected. The experimental findings indicate that the Ti re-sputtering in the central region is quite pronounced as deduced from the current density data measured by the Langmuir probes: as a matter of fact the current density at xs 0 mm is approximately seven times that at xs y150 mm. A similar trend is observed for Ti deposition rate in ArrN2 plasma. From the measured deposition rate 947 and current density the ion-to-Ti-atom ratio at the substrate, JirJ Ti , has been evaluated. These data are reported in Fig. 6a: as it can be seen the JirJ Ti values range from approximately 2 Ž xs y150 mm. to 10 Ž xs 0 mm.. The stoichiometry of TiN x coatings deposited in the four x positions is characterized by a value NrTis 0.95" 0.05 for all the samples. The contaminant concentration resulted below 1 at.%. From the thickness data and the composition analysis the coatings density has been determined. As is shown in Fig. 6b the density decreases from 5.5 grcm3 to 4.3 grcm3 in passing from xs y150 to 0 mm. The color of the coatings was bright gold. From the ᎐2 X-ray scans a trend of the average crystallite size has been determined as a function of the position using the Scherrer formula: the crystallite size is higher at xs 0 mm Ž91 nm. and decreases to 18 nm at xs 100 mm Žsee Fig. 6a.. From the grazing angle X-ray spectra collected at s 0.2⬚, 0.5⬚, 1.0⬚ and 2.0⬚ Žnot shown in this paper. it was deduced that at increasing grazing angles the TiN reflections shift down to lower 2 values, indicating an increase of the lattice parameter a of this phase at the substraterfilm interface. The trend is observed in all the samples but it is much more pronounced in the samples deposited at x s y100 mm and xs y150 mm. At a fixed grazing angle, the lattice parameter increases as a function of x position: at s 2.0⬚ it ˚ at x s 0 mm to as 4.275 A ˚ increases from as 4.246 A at xs y100 mm ŽFig. 6c.. These findings show that the coatings are characterized by a compressive stress which is less pronounced for the samples deposited at x s 0 mm. Fig. 5. Titanium deposition rate in Ar plasma ŽI. and in reactive ArrN2 plasma ŽB. Ž Pmagn s 3.7 kW; Imagn s 3.5 A; pAr s 0.1 Pa. as measured by Rutherford Backscattering. 948 V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 Fig. 6. Ža. JirJ Ti ion-to-Ti-atom ratio incident at the substrate and average crystallite size of deposited TiN x coatings; Žb. deposition rate Žmrh. and density of TiN x coatings; Žc. nanohardness and lattice parameter Žat s 2.0⬚. of TiN x coatings as a function of x position. Also the texture of the coatings showed a trend with the position. As shown in Fig. 7, which reports the ᎐2 X-ray scans for the different samples, the films obtained at xs 0 mm Ž JirJ Ti ; 10. are characterized by a very strong Ž200. preferred orientation. On the contrary the samples obtained under a less intense ion bombardment Ž xs y100 mm, x s y150 mm. are characterized by a Ž111. preferred orientation. Thus it can be concluded that the films obtained at xs 0 mm Ž JirJ Ti ; 10. are characterized by a quite different microstructure than those obtained at higher distances from the center, due to the effect of the more intense ion bombardment as already reported in Sundgren w8x. This finding is also confirmed by the nano-indentation and microscratch tests performed on the same samples. The Vickers hardness of the coatings, ranging from 2000 to 2500 kgfrmm2 , is reported in Fig. 6c. It clearly increases with the lattice parameter and is a minimum for the samples deposited at x s 0 mm. This behavior can be related to the increase of film density accompanied by the increase of the compressive stress of the films. Microscratch measurements on samples deposited at xs 0 mm performed with 200-m radius tip, show lateral spallation failures, while on the bottom of the track the TiN x film is still present. It has been verified by energy dispersive X-ray analysis that the TiN x layer detaches from the Ti underlayer and not from the M42 substrate indicating that the TirTiNx interface is characterized by a weaker adhesion, most probably due to the different growth mechanism of the Ž200. preferentially oriented TiN x layer onto a crystallized Žapprox. 100-nm-thick. Ti underlayer Žalso subject to intense ion plating during deposition.. The sample deposited at xs y50 mm did not show any failure up to 30 N, while conformal cracking occurred in the samples deposited at x s y100 mm and xs y150 mm. at 11 and 20 N, respectively. This failure mode can be related to the higher internal stress of these samples. Fig. 7. ᎐2 X-ray scans of TiN x coatings grown in different positions, from x s 0 mm Žbottom. to xs y150 mm Žtop.. V. Rigato et al. r Surface and Coatings Technology 142᎐144 (2001) 943᎐949 4. Conclusions A two-cathode DC unbalanced magnetron sputtering device has been characterized by means of Hall and Langmuir probes. Langmuir probe measurements show the existence of two main plasma regions characterized by different electron density, temperature and plasma potential related to the magnetic field. In the high density region the plasma density is approximately 10 17 ionsrm3 in the reported experimental conditions. Outside this high density region a decrease of plasma density, temperature and plasma potential are measured. Different ion numbers JirJ Ti have been calculated as a function of the position in the case of sputtering of Ti and TiN x films. The JirJ Ti values ranged from 2 to 10 by going towards the high density region. The physical properties of the TiN x films statically deposited in regions characterized by different values of JirJ Ti are quite different. The films obtained with most intense ion plating in the central high density region show a lower density, higher average crystallite size, Ž200. preferred orientation, lower hardness and a less stressed micro-structure. The coatings deposited in the region characterized by JirJ Ti of approximately 2 showed a preferred Ž111. orientation and a much smaller average crystallite size. As a result they were harder and denser. The samples deposited in the region characterized by JirJ Ti of approximately 7 show the best adhesion properties. The sample composition is independent on the position and is characterized by a NrTi value of 0.95" 0.05. These findings obtained during a static deposition of 949 TiN x with a dual cathode closed field system indicate that the final properties of coatings grown on this particular system are strongly dependent on position. This effect can be averaged out if samples are moved inside the chamber provided that motion parameters are properly chosen. The present study should be extended to systems with four and more opposed polarity unbalanced magnetrons which should be characterized by a wider region of homogeneous confined plasma. Acknowledgements The authors wish to thank V. Cervaro for the technical support. References w1x P. Spatenka, J. Vlcek, J. Blazek, Vacuum 55 Ž1999. 165 and references w2᎐8x reported therein. w2x S.L. Rohde, I. Petrov, W.D. Sproul, S.A. Barnett, P.J. Rudnik, M.E. Graham, Thin Solid Films 193r194 Ž1990. 117. w3x G. Serianni, L. Tramontin, M. Spolaore, V. Antoni, M. Begatin, R. Cavazzana, D. Desideri, E. Martines, Proc. of XXIV International Conference on Phenomena in Ionised Gases, vol. II, Polish Academy of Sciences, Warsaw, Poland, 1999, p. 9. w4x P. Topas, copyright Bruker AXS Version 1.0.1, 1999. w5x T.J.B. Holland, S.A.T. Redfern, Mineral. Mag. 61 Ž1997. 65. w6x G. Serianni, V. Antoni, R. Cavazzana, G. Maggioni, E. Martines, N. Pomaro, V. Rigato, M. Spolaore, L. Tramontin, Proc. of EPS 27th Conference on Controlled Fusion and Plasma Physics, Budapest, Hungary Ž2000. 24 B, pp. 17᎐20. w7x S. Berg, T. Larsson, C. Nender, H.-O. Blom, J. Appl. Phys. 63 Ž3. Ž1988. 887. w8x J.-E. Sundgren, Thin Solid Films 128 Ž1985. 21᎐44.