Energy and mass spectra of ions in triode ion plating of Ti(C,N) coatings

SURFACE

&COI11'IiS

HC6KOLOGY

ELSEVIER

Surface and Coatings Technology 76-77 (1995) 135-141

Energy and mass spectra of ions in triode ion plating of Ti(C,N) coatings S. Wouters

1,

S. Kadlec 2, M. Nesladek, C. Quaeyhaegens, L.M. Stals

Limburgs Universitair Centrum, Institutefor Materials Research, Universitaire Campus, Wetenschapspark, B-3590 Diepenbeek, Belgium

Abstract Ti(C,N) coatings were deposited by triode ion plating in a mixture of argon, acetylene and nitrogen. The acetylene inlet was positioned at two different locations relative to the dense plasma. The energy distribution of selected ions during Ti(CxN1 - x) deposition was measured by an energy resolved mass spectrometer. The mass spectra showed wide energy distributions of Ti ions, and energy distributions of other ionized molecules, clusters and radicals. A higher probability of acetylene dissociation with the acetylene inlet directed into the plasma was observed. Preferred orientation, lattice parameters and stress for the Ti(C,N) coatings were measured by X-ray diffraction. The measured compressive stress ranged from 8 to 13 GPa, the microhardness from 0.24 to 0.27 GPa. The scratch-test revealed an adhesion comparable to that of TiN coatings. Keywords: Titanium carbonitride; Plasma diagnostics; Hard coatings; Thin film stress; Energy-resolved mass spectroscopy

1. Introduction

2. Experimental details

Ti(C,N) coatings are an interesting alternative to both TiN and TiC hard coatings owing to their different tribological properties, hardness and color [1-4]. In contact with steel, Ti(C,N) has superior friction behavior as compared with TiN. In machining applications, Ti(C,N) coatings offer improvement of cutting performance compared to both TiN and TiC, especially in interrupted cutting [4]. The film properties are related to the chemical composition and to the microstructure and depend on the preparation conditions. Triode ion plating is a suitable deposition method for TiN and Ti(C,N) coatings [5,6]. Mass spectrometry, especially combined with energy resolution, has been proven to be a powerful tool for investigation of plasma processes in deposition and etching [7-11]. The ion energy distribution during triode ion plating of TiN has previously been investigated and discussed [6]. In this study, an extra inlet of acetylene (C 2 H 2 ) was used during the deposition of Ti(C,N). The ion energy distributions and mass spectra at selected ion energies were measured.

For the deposition of the Ti(C,N) coatings, a BAI 640 Balzers triode ion plating equipment was used. The deposition process was similar to that used for TiN coatings [6] with the plasma heating and plasma etching of the substrates preceding the ion plating-evaporation stage. A mixture of argon, nitrogen and acetylene was used during the deposition of Ti(C,N) coatings. Argon is introduced through the filamentary ionization source, nitrogen through an orifice in the chamber wall. Acetylene was introduced using a gas inlet that could be positioned at two different positions, called 'up' and 'down'. In the up position the inlet was situated outside the dense plasma and in the down position it was directed into the plasma beam above the crucible, see Fig. 1. ' The Balzers PPM 421 combined energy and mass analyzer was used for in-situ monitoring of the energy distribution of selected positive ions generated in the plasma. The ions are processed with ion 'optics, selected with energy analyzer and then led to the quadrupole mass analyzer. The mass resolution of the quadrupole mass analyzer is about m]LIm = 100 (m is the mass and LIm is the peak width at 10% of the maximum intensity). The energy resolution of the energy analyzer of the PPM 421 is about OJ eV. The axis of the ion optics was put into line of sight of the crucible as in previous experiments [6J, see Fig. 1. The mass spectrometer

1 Also Katholicke Industriele Hogeschool Limburg, departement Industriele Wetenschappen en Technologie, Universitaire Campus, B-3590 Diepenbeek, Belgium. 2 On leave from Institute of Physics, Academy of Sciences, Na Slovance 2, 180 40 Prague 8, Czech Republic.

Elsevier Science S.A. SSDI 0257 -8972 (95) 02569-3

S.Wouterset al./Surface and Coatings Technology 76-77 (1995) 135-141

136

\-_-lI~L:r SUBSTRATE TABLE

IONISATION CHAMBER

AUXILIARY ANODE ---+-..... C2H2do~

1----.-......,.,.,.,

(a) Heating (b) Etching

CRUCIBLE

(c) Evaporation

changed. The total pressure was kept constant at 0.35 Pa. For this purpose, the current of the evaporation electron gun was used. When the partial pressures of the reactive gases and hence the total pressure was too high, the e-gun current was increased. Consequently, Ti evaporation and the reactive gas consumption increased, which in turn decreased the partial pressures of the reactive gases. X-ray diffraction was performed on the coatings with a Philips X-ray diffractometer equipped with Co-K« and Cu-KG( sources to determine texture coefficients, lattice parameters and the residual macroscopic stress in the films. Vickers microhardness and thickness of the Ti(C,N) coatings were measured by standard methods. Adhesion was evaluated with a CSEM scratch-adhesion tester.

Fig.!. Schematic description of the triode ion plating equipment with the mass spectrometer and the positions of the acetylene inlet.

3. Results and discussion orifice was on the floating potential, about + 2 V with respect to the chamber walls (about -7 V with respect to plasma potential). Mass spectra of ions at selected ion energy and energy distributions of multiple selected ions were collected during the deposition of Ti(C,N) coatings. Ti(C,N) coatings were deposited on polished hot work tool steel substrates (AISI H13) and on high speed steel (AISI M12). Some deposition parameters were modified to influence the ionization degree of the carbon containing species in the plasma, the C/N concentration in the films, film microstructure and film properties. The effects of different flow rates of N 2 and C2H2 and the location of the acetylene inlet are studied in this paper. The deposition parameters are summarized in Table 1. During deposition, the argon pressure (0.28 Pa) and total mass flow of acetylene and nitrogen were kept constant (90 seem) but the ratio of N 2 and C2H2 was Table 1 Deposition parameters and results of thin film characterization of Ti(CxN 1 - x) coatings. tPC2H2 and tPr;2 denote the flow rates of C2H2 and N 2 , respectively, mTi evaporation rate of Ti from crucible, and h the coating thickness Process no.

18 17 22 21 20

19

C 2H2 inlet up up up down down down

tPC2H2

tP N2

seem

mTi g min "

h

sccm

10

80 70 60 80 70 60

0.56 0.64 0.59 0.64 0.58 0.72

3.09 3.41 2.59 3.06 2.73 3.24

20 30

10 20 30

J..lI1l

Constant parameters not given in the table: substrate temperature, 330°C, deposition time, 45 min, total pressure, 0.35 Pa, Ar pressure, 0.28 Pa, substrate bias, -110 V, arc current, 130 A, e-gun voltage, lOkV.

3.1. Mass spectra Typical mass spectra Fig. 2. shows an example of the mass spectra of positive ions at three selected energies (10,12, and 17 eV) plotted up to the mass/charge ratio mlq= 75 amu. No species were observed for m/q > 80 above the detection limit (20 counts S-l). The isotopes of Ti + (46-50) and their double ionized species at m/q=23-25 are well observable, as well as clusters of the main isotope 48Ti with 0, C and N (at m!« = 60, 62 and 64). Also the TiC2 cluster is observed at mlq= 72. (It should be noted, however, that most of the labeled peaks can also be attributed to other species, e.g., mass/charge ratio mjq = 16 can be 0+, CH 4+, NH 2 +, O 2 ++, etc.) The intensity of 4°Ar+ and its isotopes 36Ar+ and 38Ar+ is usually smaller than the intensity of the ArH+ radicals (m/q=41, 37, 39) at the same energy. The hydrogen attachment seems to be a very efficient process also for other gas molecules, especially for nitrogen (peaks at m/q=28: N 2 +,29: N 2H+), acetylene (peaks at mlq = 26: C2H2 +, 27: C2H3+ and for the residual water vapor (peaks at 18: H 20 +, 19: H 30 "). For hydrogen itself, the peak at m/q=3 (H 3 +)is higher than peaks of (H 2 +) and (H +) at low energies (up to about 14 eV). At higher energies the H 3+ signal decreases more rapidly than the intensity of H 2 + and especially than that of H + . This is owing to wide energy distributions of these species, see also Fig. 3. The source of hydrogen in the plasma is apparently the dissociation of acetylene in the plasma itself and/or heterogeneous chemical reactions of hydrocarbon molecules and radicals with condensing Ti during the formation of Ti(C,N) coatings. Then, the excess hydrogen is

137

S. Wouters et al.fSurfaceand Coatings Technology 76-77 (1995) 135-141

'OArH'

Et07 co~~rate [cps] Et06 Et05

,oAr·~

~

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Et04 Et03

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Et05 Et04 Et03 Et02

EtOl --r-.'-.J,-JI,-...,..........T'"T""1,.......,J,-lI,..y,.~w'r-r-J\-,--r'rJ...J,..1,-v-....,..,.......,WW~1llL.y.,.1,J.,.J,..y~.-J:-l.r-lr'-IfJlL.r...J,.1rIll,Jl,.\I,J\J~..+4J).I~

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Et04 Et03 Et02 Et 01 --r-.'-.II-'\-'.,...,................,,.......,-'r"r""r"l''h-...-JIl.'r-r-J~r'rJ..1I,JI,.-h...,..,.....,..+JI-,....,....~r-T'"'T..,.a.,.J,...I,-J,..yI,.l4-,.J,-,J,-I.i~Yi.\l!l,J.,lJ..Lr~.,...,...r-T"",I\4!Ih 10 5 15 20 25 30 35 40 45 50 55 60 65 70

75

mlq Fig. 2. Typical mass spectra of positive ions, taken by the mass spectrometer PPM 421, at three selected fixed energies: 10, 12, and 17 eV. Conditions: sample no. 19, inlet position down, CzH z flow 30 seem, N, flow, 60 seem.

Energy distributions of selected ions 106

mlq~i

~, ~

1Q5~ 104

J ~

1031 10z

! -l>--v-

40 41

- - - 26

- - 28

101

-t-r-h-,..LH~=-~,--,---,~...;;..::;:.A~~-r-r-;",.~-r-r-~--rl

106 --- 12 --14 --6- 24 -v- 48

105

103

o

5

10

15

20

25

30

35

40

Energy (eV)

Fig. 3. Typical energy spectra of selected mass numbers. Conditions: sample no. 19,inlet position down, C 2H 2 flow 30 seem, N, flow 60 seem.

released from the surface of the growing film into the plasma chamber. The mass spectrum also shows presence of some hydrocarbon species and radicals, especially those corresponding to the peaks 13 (CH +), 26 (CzH z "). and 27 (CZH 3 +), as well as to the carbon peak 12 (12C+).

Typical ion energy distributions Fig. 3. shows typical energy distributions of the main selected ions, measured under the same conditions as in Fig. 2. Energy spectra of other species were also measured but for readability reasons they are not plotted in this figure. The intensity of the ion energy distributions of all ion species increase sharply at energy of about 7 eV. For most of the species this increase is just followed by the maximum of the energy distribution at about 8 to 10 V. This maximum corresponds to the plasma potential nearby the mass analyzer. Some ions accelerated across the sheath between the entrance of the PPM and the plasma can undergo collisions, for example charge exchange, and their energy can be reduced [12,13]. However, with the plasma configuration used this is not a highly probable process. For example, at the Ar pressure 0.28 Pa and cross section for charge exchange of 40 Az [14] the mean free path A is 37 mm. The sheath thickness L calculated according to the Child-Langmuir law [15] is less than 1 mm at voltages less than 20 V and at ion current densities higher than 0.1 rnA cm- z. From this a L/A ratio is obtained, of less than 0.02.

138

S. Wouters et al.jSurfaceand Coatings Technology 76-77 (1995)

Such low energy ions are also observed experimentally in the low energy tails of the energy distributions. For example they are measured in Fig. 3 in the Ar distributions with energies lower than the plasma potential (8 eV) and higher than the floating potential of the orifice (2 V). The intensity of this low energy tail is at least 2 orders of magnitude lower than the maximum, observed for thermalised ions accelerated to the full plasma potential. Therefore, the measured energy distributions are not much affected due to the collisions in the sheath. On the other hand, the mean free paths for the most probable collision processes are small compared to the dimensions of the process chamber. Effects of such collisions on the ions, generated and transported in the bulk plasma, cannot be neglected. There are differences in the ion energy distributions between different species in the high energy part observed in Fig. 3. The ions with m/q=1 (H+), 2 (Hz +),14 (CH z +,N+), 23-25 (T+'), and 46-50 (Ti ") have always a wide distribution. Other ions, except 12 (C "), 13 (CH +, not plotted) and 26 (CzHz +) have always a narrower energy distribution. Especially the energy distributions of the main Ti peaks at m]q = 24 and 48 are quite wide and relatively flat in the logarithmic plot. Sometimes a second maximum appears at about 20 to 25 eV for rr' and at 14 to 18 eV for Ti + +. A similar wide energy distribution in the Ti ion has also been observed for TiN deposition during triode ion plating [6] and during magnetron sputtering at low pressures [11]. In our case, the high energies of the Ti ions are unlikely to be owing to a high initial energy of the evaporated Ti atoms. The Ti is probably ionized in the plasma above the evaporation crucible, where the plasma potential is more positive than in places with lower electron temperature and plasma density. This has been shown using Langmuir probe measurements in TiN deposition [6]. A similar explanation can be given for the wider energy distribution of the acetylene peak at »t« = 26 with the inlet in the down position as compared with the up position, see Fig. 4. For some of the radicals, especially at m/q=1 (H+), 12 (C+), 13 (CH+) and 14 (CHz +,N+), the kinetic energy released in dissociation probably results in the observed wider spectrum than for the ionized gas molecules, e.g., for m]q = 40 (Ar +) or 28 (N z +).

Reactive gas flow and position ofthe acetylene inlet Figs. 4 (a)-(f) show the energy distributions of reactive gas species (at mass numbers 12, 14, 26, and 28) with acetylene inlet in the up and down positions and for various CzH z flow rates. With decreasing acetylene rate and increasing nitrogen rate, increased nitrogen peak intensities are observed at

135~141

mlq = 14 (N+ + CzHz +) and 28 (N, +,CH 4 "}, while the intensities at 12 (C +) and at 26 (CzHz +) decreased. This is just what was expected. When the acetylene inlet is moved from the 'up' to the 'down' position, it can be observed that: (i) all distributions are getting wider, especially at mfq = 12 (C+), 13 (CH+, not plotted) and 26 (CzHz +); (ii) the clusters (TiC, TiN, not plotted) and the acetylene peak 26 show a smaller count rate, and (iii) the intensity of peaks at 12, 13 increases, probably owing to the presence of high energy radicals C +, CH +. These measurements suggest that the position of the acetylene inlet relative to the concentrated plasma is very important for the dissociation of the gaseous species. With the acetylene inlet in the upper position, the spectra show intense peaks at m/q=26 (CzHz "), 27 (CzHz +, not plotted), i.e.,just the spectra of ionized gas molecules. With inlet in the down position, these peaks decrease drastically but the intensity of dissociated fragment species increases, especially that at m/q= 12 (C+) and 13 (CH+). 3.2. Ti(e,N) film properties X-ray diffraction Fig. 5 shows a comparison of some properties of Ti(C,N) coatings as a function of acetylene flow with the acetylene inlet in the upper and lower positions. Texture coefficients [16] for Ti(C,N) were determined from 8-28 coupled (Bragg-Brentano] geometry measurements using as reference intensities the tabulated values for the relative intensities of the corresponding lattice plane reflections for TiN powder. A more pronounced preferred orientation can be observed with the acetylene inlet down compared to the inlet up. With the acetylene inlet up the textures are weak. Only for the film grown at 10 seem CzHz, a more pronounced (111)-(311) preferred orientation could be observed. A similar (111)-( 311) preferred orientation could be observed with the inlet down at lower flow rates of CzH z. With higher flow rates of CzHz the (220)-(311) preferred orientation appears. The residual macroscopic stresses were determined from measurements under glancing angle conditions using a constant and small incidence angle (6°) when performing a 28-scan. From the resulting a-f('P) plots, the values for the residual macroscopic stress (0') and the macroscopic stress-free lattice parameter (ao) were derived [14]. The residual macroscopic stress was calculated using the value of 640 GPa for the Young modulus and a Poisson ratio of 0.25 [5]. The resulting compressive macroscopic stresses are relatively high, from - 8 to -13 GPa. The lattice parameters are higher than those of TiN (0.4242 nm) and lower than those of TiC (0.4327 nm). An increase of the lattice parameter with increasing

139

S. Wouters et al./Surfaceand Coatings Technology 76-77 (1995) 135-141

1

'h

1

1-- 26·1

10' ij

!-<>- 12[1 '1-<>141 __ 26 1

121

1-0- 14 11

.........

1

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15

20

25

30

35

400

5

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Energy leV)

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35

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Energy leV)

Fig. 4. Energy spectra of mass numbers 12, 14,26,28 with (a) to (c) acetylene inlet up and (d) to acetylene inlet down, and with C 2H 2 flow (a) and (d) 10 seem, (b) and (e) 20 seem and (c) and (f) 30 seem.

acetylene input flow indicates increasing carbon content in the films. A remarkable increase of ao is also observed with the acetylene inlet in the 'up' position compared to the 'down' position. This seems to indicate that a more effective plasma dissociation of CzHz, as observed by the mass spectrometer, results in the increasing carbon incorporation in the coatings.

Microhardness The Vickers microhardness HV 15 , was measured at load 0.15 N, see also Fig. 5. The values range from 0.24 GPa to 0.27 GPa. These values are comparable with those of TiN coatings.

Adhesion The critical load was measured by the scratch-test method for the Ti(C,N) coatings deposited onto different substrate materials. On the polished tool steel substrates (AISI H13), the critical load corresponding to chipping of the coating ranged from about 30 to 60 N. First failures were observed at a load of about 15 to 40 N. On high speed steel (AISI M3: 2) substrates and also on cemented carbide inserts the critical load was much higher than on polished tool steel. In this case, first failures were observed at a load of 50 to 80 N. 4. Conclusions

The recorded mass spectra showed wide energy distributions of Ti ions and narrower energy distributions of

140

S. Wouters et al./Surface and Coatings Technology 76-77 (1995) 135-141

inlet down

inlet up

.428

.428

.....E .426

..... E.426

-

~

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C

If·424

If.424

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10

20

30

40

0

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2.0

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40

C2H2 flow [seem]

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2'1 1-~.5

10

0.5

0.5

0.0

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10

20

30

40

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20

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C2H2 flow [seem]

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Fig. 5. Comparison of properties of Ti(C,N) coatings as a function of acetylene flow with the acetylene inlet up and down position. ao is the macroscopic stress-free lattice parameter, Thkl is the texture coefficient corresponding to the lattice planes (hkl), (J is the macroscopic stress determined from X-ray diffraction and HV15 is the Vickers microhardness measured at load 0.15 N.

other ionized molecules, clusters and radicals. The energy distributions of simple light ionized radicals, especially H+, C+, and CH+, are wider than what is observed for ionized gas molecules, such as Ar +, N 2 +, and C2H2 +. Higher probability of acetylene dissociation with the acetylene inlet situated down was proved by massspectrometry. The higher values of the lattice parameter with the acetylene inlet situated down suggest that the coatings tend to contain more carbon than the coatings deposited with the acetylene inlet situated up, i.e. outside the dense plasma. Preferred orientation, lattice parameters and stress in the Ti(C,N) coatings were measured by X-ray diffraction. Compressive stress ranged from 8 to 13 GPa, microhardness from 0.24 to 0.27 GPa. The scratch-test revealed adhesion comparable to that of TiN coatings. The observations also indicate that the radicals and

ions formed from the dissociated acetylene, such as C +, and CH + are more reactive than the acetylene molecules and form the compound with Ti on expense of nitrogen.

Acknowledgments Thanks are due to the Belgian Science Supporting Institute for financial support, research Contracts IIKW-400004.91, IIKW-400002.91, IIKWAOOOO1.86. This text presents research results of the Belgian Program on Inter-University Attraction Poles, initiated by Belgian Prime Minister's Office, Science Policy Programming (Brussels). The research program is supported by the European Commission and the Flemish Government in the framework of the Future Contract for the Belgian Province of Limburg. One of the authors

S. Wouters et al./Surface and Coatings Technology 76-77 (1995) 135-141

(SK) acknowledges the scholarship from the Belgium Prime Minister's Office, Science Policy Programming, enabling his stay at the Limburg University Center.

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[7] J.W. Coburn, Thin Solid Films, 171 (1989) 65. [8] M. Hecq and A. Hecq, Thin Solidfilms, 76 (1991) 35.

[9] G. Keller, I.Barzen, W.Datter, R. Erz, S. Ulrich, K. Jung and H. Erhardt, Mater. Sci. Eng. A, 139 (1991) 137. [10] I. Petrov, A. Myers, J.E. Greene and J.R. Abelson, J. Vac. Sci. Technol. A, 12 (1994) 2846. [11] R. Roth, J. Schubert and E. Fromm, Proc. Conf. Plasma Surf. Eng., Garmish-Partenkirchen, 1994, to be published. [12] W.D. Davis and T.A. Vanderslice, Phys. Rev., 131 (1963) 219. [13] K.S. Fancey and A. Matthews, Appl. Phys. Lett.. 55 (1989) 834. [14] R.S. Robinson, J. Vac. Sci. Technol., 16 (1979) 185. [15] F.F. Chen, Introduction to Plasma Physics, Plenum, New York, 1974. [16] D.S. Rickerby, A.M. Jones and B.A. Bellamy, Surf. Coat. Technol., 37 (1988) 631. [17] AJ. Perry, V. Valvoda and D. Rafaja, Thin Solid Films, 214 (1992) 169. [18] AJ. Perry, Thin Solid Films, 193-194 (1990) 463.