- Email: [email protected]
Influence of the plasma current to Ti-melt on the plasma parameters and microstructure of TiN coatings in the triode ion plating system
Surface and Coatings Technology 97 (1997) 114–121
Influence of the plasma current to Ti-melt on the plasma parameters and microstructure of TiN coatings in the triode ion plating system S. Wouters *, S. Kadlec 1, C. Quaeyhaegens, L.M. Stals Limburgs Universitair Centrum, Institute for Materials Research, Universitaire Campus, Wetenschapspark 1, B-3590 Diepenbeek, Belgium
Abstract TiN coatings have been deposited by triode ion plating in a mixture of Ar and N with a varying total current through the 2 melt. The observed energy spectra of ions and neutrals are discussed with respect to the plasma parameters measured with Langmuir probes. The current I from the plasma to the Ti-melt in the crucible was modified from 0 A up to 100 A. As I is pm pm increased, major effects on the plasma and on the film microstructure (preferred orientation, stress-free lattice parameter, residual stress, thickness and microhardness) were observed. The positive voltage of the melt U with respect to the ground increased from R 0.1 to 29 V. The plasma potential (positive voltage with respect to ground) of the main plasma body increased from 4 to 16 V, while the electron temperature ranged from 2.5 to 4 eV. Plasma density and floating potential is also discussed. A high energy tail appeared in the energy distribution function of some neutral and ionic species, especially for neutral and ionized evaporated Ti. At high values of I the energy distribution of both highly energetic Ti+ and neutral Ti goes up to 25 eV and 30 eV. At low pm values of I the majority of the Ti+ ions are thermalized. pm The microstructure of the deposited TiN films develops from porous structures, with low compressive or even tensile stresses at I =0 A, up to dense structures, with high compressive stresses of more than 15 GPa at the highest value of I . Possible pm pm explanations of the observed effects are discussed, based on plasma ionization processes close to the melt and in the plasma body. © 1997 Elsevier Science S.A. Keywords: Ion plating; Plasma diagnostics; Mass spectrometry; Microstructure
1. Introduction Plasma diagnostics have been proven to offer valuable information for the development of PVD processes. Diagnostic methods used, range from optical emission spectroscopy [1–5] to Langmuir probes [1,6–12] and mass-spectrometry [4,9,10,12–15]. The production of high energy particles and ionization of the vaporized species is a physical problem important for various PVD methods, including the triode ion plating and the cathodic and anodic arc evaporation. In fact, little detail is known on the exact ways of production and acceleration of the ions and neutrals in these PVD systems, especially the energy and angular distribution of the ions and neutrals and on the effect of bombarding the growing films by these particles. * Corresponding author. Also at Katholieke Hogeschool voor Limburg (IWT ), Universitaire Campus, B-3590 Diepenbeek, Belgium. 1On leave from Institute of Physics, Academy of Sciences, Na Slovance 2, 180 40 Prague 8, Czech Republic. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 38 5 - X
Recently, measurements were published of plasma parameters during the formation of TiN coatings [16,17] and TiCN coatings [18,19], with Langmuir probes and an energy-resolved mass analyser, in triode ion plating. Typical for the triode ion plating system seems to be the occurrence of highly energetic ions. When the process settings are altered, using a grounded Ti-evaporation crucible instead of a floating one, the deposited films appear to be brownish and the coating adhesion fails due to modified film microstructure [17]. This microstructure modification seem to be related to the population of the highly energetic ions [17]. To study these effects, we have concentrated in this paper on the effects of the plasma current to the Ti melt during deposition of TiN films by triode ion plating and on the properties of TiN coatings deposited. In particular, the paper focuses on the microstructure (stress-free lattice parameter, residual stress, preferred orientation and microhardness) of the TiN films as function of the I current. The microstructure is related pm
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
to the plasma parameters measured by Langmuir probes and to the energy spectra of ions and neutrals measured by an energy-resolved mass analyser.
2. Experimental details In Fig. 1 a schematic drawing of the BAI 640 Balzers triode ion plating system in its TiN deposition mode is shown. The system consists of a deposition chamber (0.64 m3) in which the substrates are mounted, rotated and are biased (I , U ), an ionization chamber sub sub through which argon is fed to the chamber and an evaporation crucible with electron gun. Before starting the deposition, standard heating and etching processes were carried out [16 ]. The parameters used for the deposition of TiN were: total pressure P =0.20 Pa, Ar partial pressure P =0.15 Pa and tot Argon
Fig. 1. Scheme of the triode ion plating system BAI 640 with the variable crucible resistor (R ,I ,U ), the plasma-to-melt current (I ), the c R R pm LV ionization chamber (I ,U ), the rotating substrate table arc arc (I ,U ), the Ti-evaporation crucible (I ,U ), the energy-resolved sub sub E E mass analyser (PPM 421), the normal (N ) and parallel (P) Langmuir probes and the normal (N ) and parallel (P) mounted samples.
115
N reactive gas flow of 90 sccm. The electron gun, which 2 evaporates the titanium melt, operates at 7 kV (U ) with E an emission current I of 0.4 A at the start of the E deposition process which decreases steadily to 0.3 A [18]. In the ionization chamber the hot filament low voltage (LV ) arc source U produces an argon plasma arc beam with a constant current I of 100 A. The substrate arc table is negatively biased (U =−110 V ) to attract the sub positive ions from the plasma. Two stainless steel (AISI 304) samples with a diameter of 25 mm were rotating 34 cm above the Ti-melt. One sample was directed parallel to the Ti-melt, the other was directed normal to the Ti-melt, facing the rotation axis of the substrate table. Two identical flat Langmuir probes, with a diameter of 2 mm, were positioned in the main plasma beam. One was directed normal to the Ti-melt, facing the LV ion source, and the other was directed parallel to the crucible (see Fig. 1). The plasma parameters, namely electron temperature kT , plasma density n , plasma e e potential V and floating potential V , were calculated p f from the I–V characteristics measured by both probes [20]. The differentially pumped energy-resolved mass spectrometer PPM 421 (Balzers) was mounted on top of the ion plating system (see Fig. 1) similar to previous measurements [16,18–20]. The texture coefficients T [21] were determined hkl from h–2h coupled (Bragg–Brentano) geometry measurements, performed with a Siemens X-ray diffractometer equipped with a Cu Ka source. The residual macroscopic stresses s and the stress-free lattice parameter a of the films were determined by using the glancing 0 angle diffraction technique [22], performed with a Philips X-ray diffractometer equipped with a Co Ka source. The Vickers microhardness (HV ) ranges from 0.01 0.030 GPa to 0.180 GPa. To understand the influence of the titanium melt on the discharge conditions and its effects on coating properties, TiN films were deposited on stainless steel samples with I currents (current from plasma to Ti-melt) pm between 0% and 100% of I =I +I , where I is the arc pm R R total current from the plasma to the grounded chamber walls or for the resistor current. A specific value for the current I could be selected by varying this resistor pm R (see Fig. 1). c Table 1 gives the deposition parameters for the different processes. For process A6 the total current I from the LV arc cathode is directed mainly to the arc crucible, seen as the anode (see also Fig. 1). The crucible resistor (R =6 V) is high and the total current I must c arc flow through the Ti-melt (I ). Keeping the LV arc pm current I constant, one can decrease the current I arc pm by decreasing the crucible resistor R . Consequently, the c total current through the crucible resistor I or from R plasma to ground increases, while the positive voltage
116
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Table 1 Deposition conditions for TiN coatings as a function of the cruicible resistor R , the current through the crucible resistor I , the current from the c R plasma to the melt I , the substrate current I , the power (P=I ×(U −U )) to the substrate table, the arc voltage U , the resistor U , the pm sub sub p sub arc R substrate temperature T and the mass evaporation rate of Ti from the crucible m . The following parameters were kept constant: I =100 A, Ti arc U =−110 V, P =0.20 Pa, P =0.15 Pa, N flow of 90 sccm, U =7 kV and I =0.35 A sub tot Ar 2 E E Process
I pm (A)
R C (V)
I R (A)
I sub (A)
P (W)
U arc (V)
U R (V)
T (K)
m Ti (g min−1)
A1 A2 A3 A4 A5 A6
0 2 12 32 51 96
0 0.13 0.24 0.3 0.44 6
100 98 88 68 49 4
0.5 0.6 0.7 0.8 1 1.6
57 68 80 94 120 202
27 41 43 44 44.5 45.2
0.1 12.9 20 20.5 21.7 28
493 518 523 538 558 623
0.28 0.28 0.3 0.27 0.3 0.35
on the Ti-melt U and in the main plasma body V R p decreases. While in the normal case (R =6 V) an intense c glow can be seen above the crucible, in the grounded case (R =0 V) the glow intensity is low and homogec neously distributed. While decreasing I , the substrate pm current I and, therefore, the substrate temperature T sub decreases.
3. Results Results of Langmuir probes and energy-resolved mass spectrometry experiments carried out in the Ar–Ti–N 2 plasma with varying I are described in this section. pm The effects on the film microstructure of the normaland parallel-mounted samples are also compared as function of I . pm 3.1. Langmuir probes In Fig. 2a the plasma potential V and the floating p potential V , measured with the normal and parallel f probes, are shown as function of I . Fig. 2a shows a pm linear increase of the plasma potential V and of the p floating potential V with increasing I . f pm In Fig. 2b the electron temperature kT and the e plasma density n , measured with the normal and parallel e probes, are shown as a function of I . The plasma pm density n shows, for both the normal and parallel e probes, a ( local ) maximum at I =10 A. A local maxipm mum for the electron temperature kT is only observed e for the normal probe at around I =30 A. pm 3.2. Energy spectra of ions and neutrals The energy spectra of ions as function of I are pm given for 40Ar+ in Fig. 3(a), 48Ti+ in Fig. 3b, 14N+ in 2 Fig. 4a and 14N+ in Fig. 4b. At low I values an energy pm distribution with a maximum at 1 or 2 V lower than the plasma potential V of the main plasma body (see p Fig. 2a) is observed as the sharp rising edge of the spectra. With increasing I , this energy distribution of pm
low energetic ions shifts to a higher energy value, like the plasma potential shifts to a higher potential value (see Fig. 2a), and gets broader. Starting at I =12 A, for Ti+ (m/q=48; Fig. 3b), a pm second energy distribution arises with a maximum almost at the potential of the evaporating crucible U R (see Table 1) and the count-rate for the low energetic ions decreases. By further increase of I , both energy pm distributions shift to higher energy values, like the plasma potential U and the crucible potential U do. p R The ions Ar+ (m/q=40; Fig. 3a), N+ (m/q=28; Fig. 4a) 2 and especially N+ (m/q=14; Fig. 4(b)) always show a broad spectrum. Ar+ gains the energy from the LV arc source and the energy of N+ originates from the dissociation processes in the main plasma body. The energy spectra of neutrals as function of I are pm given for 40Ar in Fig. 5a and 48Ti in Fig. 5b. The energy distribution for the thermalized neutrals at 0 eV and the small side energy distribution between 2.5 and 5 eV do not change with varying I . As Ar and N (not shown) pm 2 are both gaseous particles, they are present in the PPM 421 as an isotropic gas and the energy spectra show a high background at high energies [19]. The origin of the side energy distribution and of the higher background at high energies lies in the potential distribution in the PPM 421 ionization chamber. Neutral titanium (48Ti; m/q=48; Fig. 5b) as a nongaseous species, has a much lower isotropic background. It shows two main distributions. The intensity of the first energy distribution at 0 eV, the energy distribution of the thermalized neutrals, increases little with increasing I . The maximum of the second energy distribution, pm the energy distribution of energetic Ti-neutrals, is situated at about 5 eV lower than the maximum of the high energetic ions (see also Fig. 3b). As discussed in Ref. [19], these energetic Ti-neutrals are probably originating from charge exchange collisions with energetic titanium ions. Owing to the low count-rate of neutral Ti [19], this distribution appears above the detection limit at 12 A I current (Fig. 5b). With increasing I pm pm the intensity increases, the maximum gets broader and shifts to a higher energy value.
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
117
Fig. 2. Plasma characteristics, (a) plasma potential V and floating potential V , (b) electron temperature kT and plasma density n , of parallelp f e e and normal-mounted flat Langmuir probes as a function of I . pm
3.3. Microstructure 3.3.1. Preferred orientation In Fig. 6a the preferred orientation as function of I is described for the samples mounted normal to the pm Ti-melt and in Fig. 6b for the samples mounted parallel to the Ti-melt. For the gold–yellow-coloured TiN samples, deposited at 96 A I , the (111) preferred pm orientation is as expected. With decreasing I for the pm parallel mounted samples the (111) preferred orientation holds, except at 12 A were the (200) orientation appears,
and for the normal mounted samples the (200) preferred orientation is observed. At the lowest I value no clear pm preferential orientation can be observed. 3.3.2. Lattice parameter and stress Fig. 7 shows the stress-free lattice parameter a and 0 the residual macroscopic stress s as function of I . The pm stress-free lattice parameter does not change very much with changing I , which is an indication for the coating pm composition. On the other hand, the residual stresses of both normal- and parallel-mounted samples decrease
118
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 3. The energy spectra of the ions (a) 40Ar+ and (b) 48Ti+ as a function of I . pm
Fig. 4. The energy spectra of the ions (a) 14N+ and (b) 14N+ 2 as a function of I . pm
from high compressive stresses, at high I values, to pm zero stresses or low tensile stresses at low I values. pm
20] confirm that Ti ions are the primary species which acquire the high energy and always have a high energy maximum (or a high energy tail ) at the highest energy of all the ion species. Other ions, e.g. Ar+, and Ti neutrals also have a high energy tail, probably due to collisions with the high energy Ti ions. This result clearly indicates that the volume close to the melt contains a plasma responsible for production of the high energy tails [16,18,19,17]. The present experiments prove that this plasma is only formed when at least part of the arc current flows through the melt. The ionization by the arc at the grounded melt only produces a plasma with low plasma potential, and mostly thermalized ions almost without the high energy tails. The biggest step in the residual stress and the microhardness, for both the parallel- and normal-mounted samples, is seen when changing I from 12 to 30 A. pm SEM observations reveal that the film microstructure changes from porous to compact in this region. A similar densification effect has been observed as a function of deposition rate [23]. In our experiments, the power of the ion bombardment at the substrates also increases with increasing I (see Table 1) as both I pm sub
3.3.3. Thickness and microhardness In Fig. 8 the film thickness h and Vickers microhardness (HV ) are given as a function of I . The samples 0.01 pm mounted parallel to the Ti-melt are twice as thick as the samples mounted normal to the Ti-melt, except for the grounded-melt deposition where the thickness value is the same. With increasing I , the samples show an pm increase of the film microhardness, even when the thickness decreases. At high I , where the thickness is low, pm the film microhardness is influenced by the low substrate hardness (0.025 GPa) and, therefore, can be higher in reality than is measured.
4. Discussion The measurements of the energy distributions of most ions and neutrals show that the intensity of the high energy tail increases with increasing current I . This pm raises the question of the origin of the high energy part of the ion energy spectra. All our measurements [16,18–
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 5. The energy spectra of the neutrals (a) 40Ar and (b) 48Ti as a function of I . pm
and (V −U ) increase with increasing I . p sub pm Nevertheless, at the conditions of the highest microstructure change (12
119
Fig. 6. The preferred orientations for the (a) normal- and (b) parallelmounted samples as a function of I . pm
The film microstructures give the impression that the films cannot be densified further when exceeding I =50 A, although the plasma parameters, energy pm spectra and process parameters show differences. Only the texture of the normal-mounted sample switches to (111) at the highest I value. pm 5. Conclusions The effects of the total current I between the plasma pm and the Ti-melt on the triode ion plating process of TiN depositions were studied. A high population of the evaporated material in the triode ion plating has been observed. Under normal deposition conditions there are more neutral Ti atoms with energies above 2 eV than the low energy ones (energy below 2 eV ). This high population of the high energy neutrals is quite unexpected in a PVD method based on thermal evaporation. However, we have shown that the plasma current to the Ti melt has a tremendous effect on the energy spectra of both ions and neutral species, even if the plasma density at the substrates is modified only slightly. This
120
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 7. The stress-free lattice parameter a and the residual stress s for the parallel- and normal-mounted samples as a function of I . 0 pm
Fig. 8. The thickness h and microhardness (HV
means that the plasma adjacent to the evaporated cathode is most important in the formation of both the high energy ions and the high energy neutrals. The films made with low I are thicker porous films, pm with low microhardness and stress, while the films at
) as a function of I . 0.01 pm
high I are thinner dense films, with high stress and pm microhardness. The high energy Ti neutrals are most probably formed by charge transfer collisions with the Ti+ ions. Probe measurements and energy distributions of ions
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
and neutrals measured by energy-resolved mass spectrometry show that the energy distributions of ions and neutral titanium are important for the film microstructure. These parameters show important changes as a function of I correlated with the film microstructure. pm
Acknowledgement Thanks are due to the NFWO (National Fund for Scientific Research) for granting this research under contract G.4.0004.94. This text presents research results of the Belgian Program on Inter-University Attraction Poles, initiated by the Belgian Prime Minister’s Office, Science Policy Programming (Brussels). The research is executed in the framework of the Objective-2 region program 1996–1998 for Limburg (Belgium) and financed by the EU ( EFRD-action) and the Flemish Government (Limburgfonds). Scientific responsibility is assumed by the authors. One of the authors (S. Kadlec) acknowledges the support for his postdoctoral scholarship stay at the Limburg University Center.
References [1] J.A. Thornton, J. Vac. Sci. Technol. 15 (2) (1978) 188. [2] B. Kulakowska-Pawlak, W. Zyrnicki, Thin Solid Films 230 (1993) 115.
121
[3] K. Behringer, U. Fantz, J. Phys. D: 27 (1994) 2128. [4] R. Roth, J. Schubert, E. Fromm, Surf. Coat. Technol. 74–75 (1995) 461. [5] G. Baravian, G. Sultan, E. Damond, H. Detour, C. Hayaud, P. Jacquot, Surf. Coat. Technol. 77 (1–3) (1995) 687. [6 ] R.M. Clements, J. Vac. Sci. Technol. 15 (2) (1978) 193. [7] T.E. Sheridan, M.J. Goeckner, J. Goree, J. Vac. Sci. Technol. A 9 (3) (1991) 688. [8] I. Ivanov, Sv. Statev, V. Orlinov, Vacuum 43 (1992) 837. [9] H.-M. Katsch, E. Quandt, J. Phys. D: 25 (1992) 430. [10] N. Hershkowitz, H.L. Maynard, J. Vac. Sci. Technol. A 11 (4) (1993) 1172. [11] I.D. Sudit, R.C. Woods, Rev. Sci. Instrum. 64 (9) (1993) 2440. [12] R. Kaltofen, G. Weise, Surf. Coat. Technol. 74–75 (1995) 469. [13] W.D. Davis, T.A. Vanderslice, Phys. Rev. 131 (1963) 219. [14] J.W. Coburn, E. Kay, Solid State Technol. (1971) 49. [15] P.Y. Jouan, G. Lemperiere, Vacuum 42 (14) (1991) 927. ´ [16 ] M. Nesladek, C. Quaeyhaegens, S. Wouters, L.M. Stals, E. Bergmann, G. Rettinghaus, Surf. Coat. Technol. 68–69 (1994) 339. [17] L.M.M. Stals, M. Nesladek, C. Quaeyhaegens, Surf. Coat. Technol. in press. ´ [18] S. Wouters, S. Kadlec, M. Nesladek, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 76 (1995) 135. [19] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 92 (1997) 56. [20] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Thin Solid Films in press. [21] D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1988) 631. [22] C. Quaeyhaegens, G. Knuyt, L. Stals, J. Vac. Sci. Technol. A 14 (4) (1996) 2462. [23] S. Kadlec, J. Musil, V. Valvoda, R. Cerny, in: U. Ehlemann, H.G. Lergon, K. Wiesemann (Eds.), Proc. 10th Int. Symp. on Plasma Chemistry, Bochum, Germany, IUPAC, Bochum, 1991, pp. 2.4-4, 1–6.
Influence of the plasma current to Ti-melt on the plasma parameters and microstructure of TiN coatings in the triode ion plating system S. Wouters *, S. Kadlec 1, C. Quaeyhaegens, L.M. Stals Limburgs Universitair Centrum, Institute for Materials Research, Universitaire Campus, Wetenschapspark 1, B-3590 Diepenbeek, Belgium
Abstract TiN coatings have been deposited by triode ion plating in a mixture of Ar and N with a varying total current through the 2 melt. The observed energy spectra of ions and neutrals are discussed with respect to the plasma parameters measured with Langmuir probes. The current I from the plasma to the Ti-melt in the crucible was modified from 0 A up to 100 A. As I is pm pm increased, major effects on the plasma and on the film microstructure (preferred orientation, stress-free lattice parameter, residual stress, thickness and microhardness) were observed. The positive voltage of the melt U with respect to the ground increased from R 0.1 to 29 V. The plasma potential (positive voltage with respect to ground) of the main plasma body increased from 4 to 16 V, while the electron temperature ranged from 2.5 to 4 eV. Plasma density and floating potential is also discussed. A high energy tail appeared in the energy distribution function of some neutral and ionic species, especially for neutral and ionized evaporated Ti. At high values of I the energy distribution of both highly energetic Ti+ and neutral Ti goes up to 25 eV and 30 eV. At low pm values of I the majority of the Ti+ ions are thermalized. pm The microstructure of the deposited TiN films develops from porous structures, with low compressive or even tensile stresses at I =0 A, up to dense structures, with high compressive stresses of more than 15 GPa at the highest value of I . Possible pm pm explanations of the observed effects are discussed, based on plasma ionization processes close to the melt and in the plasma body. © 1997 Elsevier Science S.A. Keywords: Ion plating; Plasma diagnostics; Mass spectrometry; Microstructure
1. Introduction Plasma diagnostics have been proven to offer valuable information for the development of PVD processes. Diagnostic methods used, range from optical emission spectroscopy [1–5] to Langmuir probes [1,6–12] and mass-spectrometry [4,9,10,12–15]. The production of high energy particles and ionization of the vaporized species is a physical problem important for various PVD methods, including the triode ion plating and the cathodic and anodic arc evaporation. In fact, little detail is known on the exact ways of production and acceleration of the ions and neutrals in these PVD systems, especially the energy and angular distribution of the ions and neutrals and on the effect of bombarding the growing films by these particles. * Corresponding author. Also at Katholieke Hogeschool voor Limburg (IWT ), Universitaire Campus, B-3590 Diepenbeek, Belgium. 1On leave from Institute of Physics, Academy of Sciences, Na Slovance 2, 180 40 Prague 8, Czech Republic. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 38 5 - X
Recently, measurements were published of plasma parameters during the formation of TiN coatings [16,17] and TiCN coatings [18,19], with Langmuir probes and an energy-resolved mass analyser, in triode ion plating. Typical for the triode ion plating system seems to be the occurrence of highly energetic ions. When the process settings are altered, using a grounded Ti-evaporation crucible instead of a floating one, the deposited films appear to be brownish and the coating adhesion fails due to modified film microstructure [17]. This microstructure modification seem to be related to the population of the highly energetic ions [17]. To study these effects, we have concentrated in this paper on the effects of the plasma current to the Ti melt during deposition of TiN films by triode ion plating and on the properties of TiN coatings deposited. In particular, the paper focuses on the microstructure (stress-free lattice parameter, residual stress, preferred orientation and microhardness) of the TiN films as function of the I current. The microstructure is related pm
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
to the plasma parameters measured by Langmuir probes and to the energy spectra of ions and neutrals measured by an energy-resolved mass analyser.
2. Experimental details In Fig. 1 a schematic drawing of the BAI 640 Balzers triode ion plating system in its TiN deposition mode is shown. The system consists of a deposition chamber (0.64 m3) in which the substrates are mounted, rotated and are biased (I , U ), an ionization chamber sub sub through which argon is fed to the chamber and an evaporation crucible with electron gun. Before starting the deposition, standard heating and etching processes were carried out [16 ]. The parameters used for the deposition of TiN were: total pressure P =0.20 Pa, Ar partial pressure P =0.15 Pa and tot Argon
Fig. 1. Scheme of the triode ion plating system BAI 640 with the variable crucible resistor (R ,I ,U ), the plasma-to-melt current (I ), the c R R pm LV ionization chamber (I ,U ), the rotating substrate table arc arc (I ,U ), the Ti-evaporation crucible (I ,U ), the energy-resolved sub sub E E mass analyser (PPM 421), the normal (N ) and parallel (P) Langmuir probes and the normal (N ) and parallel (P) mounted samples.
115
N reactive gas flow of 90 sccm. The electron gun, which 2 evaporates the titanium melt, operates at 7 kV (U ) with E an emission current I of 0.4 A at the start of the E deposition process which decreases steadily to 0.3 A [18]. In the ionization chamber the hot filament low voltage (LV ) arc source U produces an argon plasma arc beam with a constant current I of 100 A. The substrate arc table is negatively biased (U =−110 V ) to attract the sub positive ions from the plasma. Two stainless steel (AISI 304) samples with a diameter of 25 mm were rotating 34 cm above the Ti-melt. One sample was directed parallel to the Ti-melt, the other was directed normal to the Ti-melt, facing the rotation axis of the substrate table. Two identical flat Langmuir probes, with a diameter of 2 mm, were positioned in the main plasma beam. One was directed normal to the Ti-melt, facing the LV ion source, and the other was directed parallel to the crucible (see Fig. 1). The plasma parameters, namely electron temperature kT , plasma density n , plasma e e potential V and floating potential V , were calculated p f from the I–V characteristics measured by both probes [20]. The differentially pumped energy-resolved mass spectrometer PPM 421 (Balzers) was mounted on top of the ion plating system (see Fig. 1) similar to previous measurements [16,18–20]. The texture coefficients T [21] were determined hkl from h–2h coupled (Bragg–Brentano) geometry measurements, performed with a Siemens X-ray diffractometer equipped with a Cu Ka source. The residual macroscopic stresses s and the stress-free lattice parameter a of the films were determined by using the glancing 0 angle diffraction technique [22], performed with a Philips X-ray diffractometer equipped with a Co Ka source. The Vickers microhardness (HV ) ranges from 0.01 0.030 GPa to 0.180 GPa. To understand the influence of the titanium melt on the discharge conditions and its effects on coating properties, TiN films were deposited on stainless steel samples with I currents (current from plasma to Ti-melt) pm between 0% and 100% of I =I +I , where I is the arc pm R R total current from the plasma to the grounded chamber walls or for the resistor current. A specific value for the current I could be selected by varying this resistor pm R (see Fig. 1). c Table 1 gives the deposition parameters for the different processes. For process A6 the total current I from the LV arc cathode is directed mainly to the arc crucible, seen as the anode (see also Fig. 1). The crucible resistor (R =6 V) is high and the total current I must c arc flow through the Ti-melt (I ). Keeping the LV arc pm current I constant, one can decrease the current I arc pm by decreasing the crucible resistor R . Consequently, the c total current through the crucible resistor I or from R plasma to ground increases, while the positive voltage
116
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Table 1 Deposition conditions for TiN coatings as a function of the cruicible resistor R , the current through the crucible resistor I , the current from the c R plasma to the melt I , the substrate current I , the power (P=I ×(U −U )) to the substrate table, the arc voltage U , the resistor U , the pm sub sub p sub arc R substrate temperature T and the mass evaporation rate of Ti from the crucible m . The following parameters were kept constant: I =100 A, Ti arc U =−110 V, P =0.20 Pa, P =0.15 Pa, N flow of 90 sccm, U =7 kV and I =0.35 A sub tot Ar 2 E E Process
I pm (A)
R C (V)
I R (A)
I sub (A)
P (W)
U arc (V)
U R (V)
T (K)
m Ti (g min−1)
A1 A2 A3 A4 A5 A6
0 2 12 32 51 96
0 0.13 0.24 0.3 0.44 6
100 98 88 68 49 4
0.5 0.6 0.7 0.8 1 1.6
57 68 80 94 120 202
27 41 43 44 44.5 45.2
0.1 12.9 20 20.5 21.7 28
493 518 523 538 558 623
0.28 0.28 0.3 0.27 0.3 0.35
on the Ti-melt U and in the main plasma body V R p decreases. While in the normal case (R =6 V) an intense c glow can be seen above the crucible, in the grounded case (R =0 V) the glow intensity is low and homogec neously distributed. While decreasing I , the substrate pm current I and, therefore, the substrate temperature T sub decreases.
3. Results Results of Langmuir probes and energy-resolved mass spectrometry experiments carried out in the Ar–Ti–N 2 plasma with varying I are described in this section. pm The effects on the film microstructure of the normaland parallel-mounted samples are also compared as function of I . pm 3.1. Langmuir probes In Fig. 2a the plasma potential V and the floating p potential V , measured with the normal and parallel f probes, are shown as function of I . Fig. 2a shows a pm linear increase of the plasma potential V and of the p floating potential V with increasing I . f pm In Fig. 2b the electron temperature kT and the e plasma density n , measured with the normal and parallel e probes, are shown as a function of I . The plasma pm density n shows, for both the normal and parallel e probes, a ( local ) maximum at I =10 A. A local maxipm mum for the electron temperature kT is only observed e for the normal probe at around I =30 A. pm 3.2. Energy spectra of ions and neutrals The energy spectra of ions as function of I are pm given for 40Ar+ in Fig. 3(a), 48Ti+ in Fig. 3b, 14N+ in 2 Fig. 4a and 14N+ in Fig. 4b. At low I values an energy pm distribution with a maximum at 1 or 2 V lower than the plasma potential V of the main plasma body (see p Fig. 2a) is observed as the sharp rising edge of the spectra. With increasing I , this energy distribution of pm
low energetic ions shifts to a higher energy value, like the plasma potential shifts to a higher potential value (see Fig. 2a), and gets broader. Starting at I =12 A, for Ti+ (m/q=48; Fig. 3b), a pm second energy distribution arises with a maximum almost at the potential of the evaporating crucible U R (see Table 1) and the count-rate for the low energetic ions decreases. By further increase of I , both energy pm distributions shift to higher energy values, like the plasma potential U and the crucible potential U do. p R The ions Ar+ (m/q=40; Fig. 3a), N+ (m/q=28; Fig. 4a) 2 and especially N+ (m/q=14; Fig. 4(b)) always show a broad spectrum. Ar+ gains the energy from the LV arc source and the energy of N+ originates from the dissociation processes in the main plasma body. The energy spectra of neutrals as function of I are pm given for 40Ar in Fig. 5a and 48Ti in Fig. 5b. The energy distribution for the thermalized neutrals at 0 eV and the small side energy distribution between 2.5 and 5 eV do not change with varying I . As Ar and N (not shown) pm 2 are both gaseous particles, they are present in the PPM 421 as an isotropic gas and the energy spectra show a high background at high energies [19]. The origin of the side energy distribution and of the higher background at high energies lies in the potential distribution in the PPM 421 ionization chamber. Neutral titanium (48Ti; m/q=48; Fig. 5b) as a nongaseous species, has a much lower isotropic background. It shows two main distributions. The intensity of the first energy distribution at 0 eV, the energy distribution of the thermalized neutrals, increases little with increasing I . The maximum of the second energy distribution, pm the energy distribution of energetic Ti-neutrals, is situated at about 5 eV lower than the maximum of the high energetic ions (see also Fig. 3b). As discussed in Ref. [19], these energetic Ti-neutrals are probably originating from charge exchange collisions with energetic titanium ions. Owing to the low count-rate of neutral Ti [19], this distribution appears above the detection limit at 12 A I current (Fig. 5b). With increasing I pm pm the intensity increases, the maximum gets broader and shifts to a higher energy value.
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
117
Fig. 2. Plasma characteristics, (a) plasma potential V and floating potential V , (b) electron temperature kT and plasma density n , of parallelp f e e and normal-mounted flat Langmuir probes as a function of I . pm
3.3. Microstructure 3.3.1. Preferred orientation In Fig. 6a the preferred orientation as function of I is described for the samples mounted normal to the pm Ti-melt and in Fig. 6b for the samples mounted parallel to the Ti-melt. For the gold–yellow-coloured TiN samples, deposited at 96 A I , the (111) preferred pm orientation is as expected. With decreasing I for the pm parallel mounted samples the (111) preferred orientation holds, except at 12 A were the (200) orientation appears,
and for the normal mounted samples the (200) preferred orientation is observed. At the lowest I value no clear pm preferential orientation can be observed. 3.3.2. Lattice parameter and stress Fig. 7 shows the stress-free lattice parameter a and 0 the residual macroscopic stress s as function of I . The pm stress-free lattice parameter does not change very much with changing I , which is an indication for the coating pm composition. On the other hand, the residual stresses of both normal- and parallel-mounted samples decrease
118
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 3. The energy spectra of the ions (a) 40Ar+ and (b) 48Ti+ as a function of I . pm
Fig. 4. The energy spectra of the ions (a) 14N+ and (b) 14N+ 2 as a function of I . pm
from high compressive stresses, at high I values, to pm zero stresses or low tensile stresses at low I values. pm
20] confirm that Ti ions are the primary species which acquire the high energy and always have a high energy maximum (or a high energy tail ) at the highest energy of all the ion species. Other ions, e.g. Ar+, and Ti neutrals also have a high energy tail, probably due to collisions with the high energy Ti ions. This result clearly indicates that the volume close to the melt contains a plasma responsible for production of the high energy tails [16,18,19,17]. The present experiments prove that this plasma is only formed when at least part of the arc current flows through the melt. The ionization by the arc at the grounded melt only produces a plasma with low plasma potential, and mostly thermalized ions almost without the high energy tails. The biggest step in the residual stress and the microhardness, for both the parallel- and normal-mounted samples, is seen when changing I from 12 to 30 A. pm SEM observations reveal that the film microstructure changes from porous to compact in this region. A similar densification effect has been observed as a function of deposition rate [23]. In our experiments, the power of the ion bombardment at the substrates also increases with increasing I (see Table 1) as both I pm sub
3.3.3. Thickness and microhardness In Fig. 8 the film thickness h and Vickers microhardness (HV ) are given as a function of I . The samples 0.01 pm mounted parallel to the Ti-melt are twice as thick as the samples mounted normal to the Ti-melt, except for the grounded-melt deposition where the thickness value is the same. With increasing I , the samples show an pm increase of the film microhardness, even when the thickness decreases. At high I , where the thickness is low, pm the film microhardness is influenced by the low substrate hardness (0.025 GPa) and, therefore, can be higher in reality than is measured.
4. Discussion The measurements of the energy distributions of most ions and neutrals show that the intensity of the high energy tail increases with increasing current I . This pm raises the question of the origin of the high energy part of the ion energy spectra. All our measurements [16,18–
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 5. The energy spectra of the neutrals (a) 40Ar and (b) 48Ti as a function of I . pm
and (V −U ) increase with increasing I . p sub pm Nevertheless, at the conditions of the highest microstructure change (12
119
Fig. 6. The preferred orientations for the (a) normal- and (b) parallelmounted samples as a function of I . pm
The film microstructures give the impression that the films cannot be densified further when exceeding I =50 A, although the plasma parameters, energy pm spectra and process parameters show differences. Only the texture of the normal-mounted sample switches to (111) at the highest I value. pm 5. Conclusions The effects of the total current I between the plasma pm and the Ti-melt on the triode ion plating process of TiN depositions were studied. A high population of the evaporated material in the triode ion plating has been observed. Under normal deposition conditions there are more neutral Ti atoms with energies above 2 eV than the low energy ones (energy below 2 eV ). This high population of the high energy neutrals is quite unexpected in a PVD method based on thermal evaporation. However, we have shown that the plasma current to the Ti melt has a tremendous effect on the energy spectra of both ions and neutral species, even if the plasma density at the substrates is modified only slightly. This
120
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
Fig. 7. The stress-free lattice parameter a and the residual stress s for the parallel- and normal-mounted samples as a function of I . 0 pm
Fig. 8. The thickness h and microhardness (HV
means that the plasma adjacent to the evaporated cathode is most important in the formation of both the high energy ions and the high energy neutrals. The films made with low I are thicker porous films, pm with low microhardness and stress, while the films at
) as a function of I . 0.01 pm
high I are thinner dense films, with high stress and pm microhardness. The high energy Ti neutrals are most probably formed by charge transfer collisions with the Ti+ ions. Probe measurements and energy distributions of ions
S. Wouters et al. / Surface and Coatings Technology 97 (1997) 114–121
and neutrals measured by energy-resolved mass spectrometry show that the energy distributions of ions and neutral titanium are important for the film microstructure. These parameters show important changes as a function of I correlated with the film microstructure. pm
Acknowledgement Thanks are due to the NFWO (National Fund for Scientific Research) for granting this research under contract G.4.0004.94. This text presents research results of the Belgian Program on Inter-University Attraction Poles, initiated by the Belgian Prime Minister’s Office, Science Policy Programming (Brussels). The research is executed in the framework of the Objective-2 region program 1996–1998 for Limburg (Belgium) and financed by the EU ( EFRD-action) and the Flemish Government (Limburgfonds). Scientific responsibility is assumed by the authors. One of the authors (S. Kadlec) acknowledges the support for his postdoctoral scholarship stay at the Limburg University Center.
References [1] J.A. Thornton, J. Vac. Sci. Technol. 15 (2) (1978) 188. [2] B. Kulakowska-Pawlak, W. Zyrnicki, Thin Solid Films 230 (1993) 115.
121
[3] K. Behringer, U. Fantz, J. Phys. D: 27 (1994) 2128. [4] R. Roth, J. Schubert, E. Fromm, Surf. Coat. Technol. 74–75 (1995) 461. [5] G. Baravian, G. Sultan, E. Damond, H. Detour, C. Hayaud, P. Jacquot, Surf. Coat. Technol. 77 (1–3) (1995) 687. [6 ] R.M. Clements, J. Vac. Sci. Technol. 15 (2) (1978) 193. [7] T.E. Sheridan, M.J. Goeckner, J. Goree, J. Vac. Sci. Technol. A 9 (3) (1991) 688. [8] I. Ivanov, Sv. Statev, V. Orlinov, Vacuum 43 (1992) 837. [9] H.-M. Katsch, E. Quandt, J. Phys. D: 25 (1992) 430. [10] N. Hershkowitz, H.L. Maynard, J. Vac. Sci. Technol. A 11 (4) (1993) 1172. [11] I.D. Sudit, R.C. Woods, Rev. Sci. Instrum. 64 (9) (1993) 2440. [12] R. Kaltofen, G. Weise, Surf. Coat. Technol. 74–75 (1995) 469. [13] W.D. Davis, T.A. Vanderslice, Phys. Rev. 131 (1963) 219. [14] J.W. Coburn, E. Kay, Solid State Technol. (1971) 49. [15] P.Y. Jouan, G. Lemperiere, Vacuum 42 (14) (1991) 927. ´ [16 ] M. Nesladek, C. Quaeyhaegens, S. Wouters, L.M. Stals, E. Bergmann, G. Rettinghaus, Surf. Coat. Technol. 68–69 (1994) 339. [17] L.M.M. Stals, M. Nesladek, C. Quaeyhaegens, Surf. Coat. Technol. in press. ´ [18] S. Wouters, S. Kadlec, M. Nesladek, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 76 (1995) 135. [19] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Surf. Coat. Technol. 92 (1997) 56. [20] S. Wouters, S. Kadlec, C. Quaeyhaegens, L.M. Stals, Thin Solid Films in press. [21] D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1988) 631. [22] C. Quaeyhaegens, G. Knuyt, L. Stals, J. Vac. Sci. Technol. A 14 (4) (1996) 2462. [23] S. Kadlec, J. Musil, V. Valvoda, R. Cerny, in: U. Ehlemann, H.G. Lergon, K. Wiesemann (Eds.), Proc. 10th Int. Symp. on Plasma Chemistry, Bochum, Germany, IUPAC, Bochum, 1991, pp. 2.4-4, 1–6.