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Ion flux characteristics in arc vapor deposition of TiN
Surface and Coatings Technology,
36 (1988)
ION FLUX CHARACTERISTICS TiN*
243
243 - 255
IN ARC VAPOR DEPOSITION OF
CLARK BERGMAN Multi-Arc
Scientific
Coatings,
261 E. Fifth Street,
St. Paul, MN 55101
(U.S.A.)
(Received June 25,1988)
Summary In this paper the ionization and energy characteristics of the particle flux impinging TiN films during arc vapor ion deposition are described. Langmuir probe measurements of voltage and current were made using a planar shielded substrate as the probe. Plasma species were observed using emission spectrometry. Substrate mass gain was measured as a function of voltage and nitrogen pressure. Ion current to the substrate was eliminated by applying a negative voltage to a fine wire screen located in front of the substrate and, at the same time, applying a positive voltage to the substrate. The ionized fraction of the titanium vapor at the substrate was determined by comparing the deposition rate obtained while excluding ions with the deposition rate obtained under normal conditions when ions were part of the depositing flux. The Langrnuir probe measurements were used to estimate the plasma parameters and the average ion energy at the substrate. The results are as follows: (1) in the absence of a nitrogen gas discharge, the density of nitrogen ions is small; (2) most of the titanium vapor is ionized; (3) the average charge carried by titanium ions to the substrate during TiN deposition is (1.6 k 0.3)e; (4) the average energy of titanium ions at the substrate is approximately 1.6e(lO + IV’l) eV, where V. is the negative substrate bias.
1. Introduction The nature of the vapor produced by metal vapor arcs and descriptions of deposition using arc sources have been given by numerous researchers. Metal vapor (or vacuum) arcs produce a vapor that is ionized to a high degree [l] and that is very energetic [2,3]. Many materials have been successfully deposited using arc sources [4]. In this process, substrates are immersed in a dense plasma that is sustained by metal ions and electrons supplied by *Paper presented at the 15th International San Diego, CA, U.S.A., April 11 - 15,1988. 0257-8972/881$3.50
Conference
on Metallurgical Coatings,
0 Elsevier Sequoia/Printed
in The Netherlands
244
cathodic arcs. Even at relatively high pressure, TiN film growth is dominated by metal ion bombardment [ 51. In this paper the ionization and energy characteristics of the particle flux impinging TiN films during arc vapor ion deposition are described. Langmuir probe measurements of voltage and current were made using a planar shielded substrate as the probe. Plasma species were observed using emission spectrometry. Substrate mass gain was measured as a function of voltage and nitrogen pressure. Ion current to the substrate was eliminated by applying a negative voltage to a fine wire screen located in front of the substrate and, at the same time, applying a positive voltage to the substrate. The ionized fraction of the titanium vapor at the substrate was determined by comparing the deposition rate obtained while ions were excluded with the deposition rate obtained under normal conditions when ions were part of the depositing flux. The Langmuir probe measurements were used to estimate the plasma parameters and the average ion energy at the substrate.
2. Apparatus Figure 1 illustrates the experimental deposition system. Titanium is evaporated from the cathode by an arc discharge. The grounded chamber is the anode. The substrate, which is 225 mm in front of the cathode, is located behind a rotatable shutter that is sandwiched between fixed apertures. The cathode, substrate and fixed apertures are perpendicular to their common symmetry axis. The cathode evaporation surface has a diameter of 64 mm. The substrate, which is 0.4 mm thick and 70 mm in diameter, is clamped on its periphery, exposing a surface of 66 mm in diameter. The fixed apertures A, and As are located 30 mm and 14 mm from the substrate and have diameters of 44.5 mm and 50.8 mm respec-
TO
PYROMETER
A TO
SPECTROMETER
Fig. 1. Experimental deposition system.
7
245
tively. Both are at ground potential. The rotatable shutter contains a blocking section, an aperture (AZ) 50.8 mm in diameter and a screened aperture (SAz) 50.8 mm in diameter. The screen in SA2, which is held in a removable clamp, is a rectangular mesh made of type 304 stainless steel wires (0.0254 mm) woven on centers (0.508 mm) and having a transparency of 90.3%. The shutter may be electrically biased. A radiant heater, located directly behind the substrate plane, was used to preheat substrates. The temperature of the front substrate surface was monitored by an IB pyrometer. In some cases, substrate temperature was also measured by an iron-constantan thermocouple spot welded to the back surface. The cylindrical chamber (22 1) was pumped by a turbomolecular pump with an effective speed of 10 1 s-l. Pressure was measured by an ionization gauge and a capacitance manometer. Nitrogen was introduced using an electronic flow controller. During deposition rate measurements, the shutter was opened only after attaining a base pressure of 5 X 10U6Ton or less, and after stabilizing nitrogen flow and pressure with the arc source operating. Plasma emission was monitored through either of two observation tubes, one directed across the cathode surface (port A), and the other aimed to intersect the symmetry axis approximately 60 mm in front of the substrate surface (port B). Emission intensity as a function of distance from the source was observed in a larger chamber where the observation tube was rotatable. The tubes were sealed with quartz windows. Light from the windows was transmitted through a quartz fiber optic cable to the input slit of a 0.5 m Ebert-type Jarrel-Ash scanning monochromator. The monochromator was sensitive in the range 230 - 900 nm with a resolution of less than 0.1 nm. The acceptance angle for all observation tubes was 29 All deposition rate measurements were made at a source current of 75 A. The substrate and shutter plate were biased using voltage-regulated d.c. power supplies. Substrate and shutter current densities were recorded continuously with a sensitivity of 1 mA mm-l. The deposition rates on the substrate and the screen were determined in terms of the mass gain per coulomb of collected charge. A microbalance, with a repeatability of a20 E.cg, was used to measure the initial and final masses. A Macbeth calorimeter was used to characterize the optical reflectance properties of deposited films.
3. Results and discussion 3.1. Langmuirprobe Substrate current was measured as a function of applied voltage in vacuum and in nitrogen at 1, 5, 15 and 30 mTorr. Typical results are shown in Fig. 2. Plasma parameters estimated from these data are discussed in Section 3.5. In Fig. 2, it can be seen that the ion current saturates abruptly near -10 V. (Positive ions collected by a negative voltage are shown as a positive
246
f!5 !E:: mTorr 1 mTorr Vacuum
Fig. 2. Substrate current vs. voltage.
current, and electrons collected by a positive voltage are shown as a negative current.) The saturated ion current decreases with increasing pressure at a rate that is slightly greater than l/P. In vacuum, the ion current is absolutely constant from -25 to -260 V. As the nitrogen pressure is increased the ion current increases gradually between -25 and -260 V. This increase, which is proportional to voltage, is 8% at -260 V and 30 mTorr. It is probably due to electron impact ionization of nitrogen in the plasma adjacent to the substrate. The saturated current collected at a negative bias is primarily due to the flux of titanium and nitrogen ions entering the sheath separating the plasma from the substrate [6] and, to a much smaller degree, to secondary electron emission and Nz+ ions created within the sheath itself. Using Hagstrom’s data [7], the contribution from secondary electron emission was estimated to be less than 8% in vacuum and less than 4% during TiN deposition. The contribution from Nz+ produced by electron impact near the substrate is, at most, a few per cent at low pressure and bias. Results presented in the next section indicate that the production of nitrogen ions in the plasma is also a relatively small effect. 3.2. Emission spectrometry Emission spectrometry was used to detect and identify species present in titanium arc plasmas over the wavelength range 230 - 900 nm. Measure-
24’7
ments were made as a function of distance from an arc source and as a function of nitrogen pressure. The results aid in the understanding of operative mechanisms that produce and annihilate ions, and also indicate that the ratio of nitrogen ions to titanium ions is small throughout the plasma region. The emission amplitudes of Ti, Ti+, Ti’+, N2 and Nz+ are proportional to the radiative decay rates of these species within the observation region. Because the decay lifetimes are very short, such that even the most energetic titanium ions radiate before traveling 1 mm from the point of excitation, emission amplitudes are also a measure of the production rate of particular excited species within the observation region. The total production rate of a given species is obtained by integrating the emission amplitude over all lines of that species, using appropriate factors to account for variations in detector sensitivity with wavelength. This assumes the number produced in the ground state is of the same order (or less) than the number produced in detectable excited states, since species produced in their ground state are not detectable by emission spectrometry. For the purposes of this discussion, the relative production rate of an excited species will be approximated by a summation of emission amplitudes without considering sensitivity variations. Emission from decaying Ti, Ti+, Ti’+, N2 and N2+ atoms and molecules was observed, whereas emission from Ti3+, N, N+, TiN and TiN+ was not identified. Numerous intense Ti and Ti+ emission lines were observed, whereas only three relatively weak N2+ bands were identified. Ti2+ emission was observed only when the detector was aimed to collect radiation coming from locations within 50 mm of the source. Failure to detect Ti3+, which Lunev et al. [2] detected using a mass spectrometer, is an indication that its strongest lines may occur at wavelengths less than 230 nm. The absence of N and N+ lines is a reflection of the high binding energy of the N2 molecule. The absence of TiN and TiN+ lines is an indication that gas phase chemical reactions are not significant. At constant pressure, and outside the view of the brilliant cathode spots, the emission intensity varied inversely with distance from the source. In vacuum, Ti+ emission dominates that of Ti, as found by Martin et al. [8]. In N2, the radiation intensity from Ti+ and Ti species is substantially greater than from N 2+. Broad-band N2 emission gives the plasma a characteristic pink color in the source region. Figure 3 shows the variation in emission intensity (at port B) with nitrogen pressure for two of the more intense Ti and Ti+ lines together with the strongest N2+ and N2 band heads. The variation in intensity with pressure was similar for all lines of a given species. The intensities of the 521 nm Ti and 368.5 nm Ti+ lines increase linearly to approximately 50 mTorr and remain strong to more than 350 ml‘orr. The intensity of the N2+ band head at 391.4 nm increases rapidly from zero to a maximum at 1 mTorr, and then decreases just as quickly to a value that remains nearly constant to 350 mTorr. The intensity of the N2 band head at 357.7 nm reaches a maximum near 15 mTorr and then decreases at a slow exponential rate.
248
20
0
PRESSURE
(mTorr)
Fig. 3. Variation in emission intensity (at port B) with nitrogen pressure for prominent Ti+ and Ti lines and the strongest Nz+ and Nz band heads.
Since the titanium vapor enters the plasma in a highly ionized state, the reactions of greatest interest are those that neutralize titanium ions and those that ionize nitrogen molecules. Direct recombination of electrons with titanium ions is very unlikely owing to the need to conserve both energy and momentum. Furthermore, electron impact ionization of nitrogen is not likely because few electrons within the plasma will possess the required threshold energy of 15.58 eV. This leaves three-body electron recombination and charge exchange collisions as the most probable mechanisms for titanium neutralization and nitrogen ionization respectively. Electron recombination is more likely in the presence of a third particle. On the basis of the conservation laws and proximity considerations, the recombination cross-section is expected to be greatest for low energy electrons. The large increase in Ti and Ti+ emission intensity with pressure, shown in Fig. 3, suggests that three-body electron recombination is indeed a significant mechanism for neutralizing titanium ions. Clearly, the probability of Tin+ + e + N2 + Tic”-‘I+ + N2 reactions increases with the nitrogen concentration. Furthermore, the pressure at which the Ti and Ti+ emissions reach a maximum amplitude is not only a function of the local nitrogen concentration; maxima occur at decreasing pressure with increasing distance from the source. This is consistent with an increase in the three-body reaction rate in the presence of lower energy electrons. Results presented in Section 3.5 show that electron energy does decrease with increasing pressure. Between 0 and 10 mTorr, the present results are similar to those of Demidenko et al. [ 91 and Martin et al. [lo], who attribute increases in the Nz+, Ti+ and Ti emission to charge exchange reactions of the type Tin+ +
249
N, + Ticn-1)+ + N,+ proceeding through .the B *EU’ excited state of N2+. These reactions have Q values of -11.91, -5.15 and +11.91 eV, less the excitation energy of the final state of the titanium atom or ion, for n = 1,2 and 3 respectively. Assuming that nitrogen molecules are at thermal energy, the threshold for the two reactions with negative Q values is (48 + 28)/28 = 2.71 multiplied by the Q value. The threshold energies for Ti+ and Ti*+ charge exchange reactions producing the B ‘C,’ excited state of N2 are at least 32.3 eV and 14.0 eV respectively. Thus, only the Ti’+ to Ti*+ reaction is possible once titanium ions have lost their initially high energy in collisions with nitrogen molecules. The rapid decrease in the intensity of the N2’ emission shortly after reaching a maximum near 1 mTorr is an indication of this effect. These results and observations indicate that titanium ions produced at the arc source retain a high degree of charge and that nitrogen is only weakly ionized within the plasma. Nitrogen ionization occurs most efficiently at low pressure where titanium ions retain most of their initial energy. Titanium ion neutralization occurs most efficiently at high pressure and at distances where electron energy is low. Primary titanium ions radiate at the source and then travel undetected until, after electron recombination, their presence is revealed by the decay of an excited daughter possessing one less charge. Finally, the relative concentration of N2+ and titanium ions was estimated by comparing the emission intensity of the three N2+bands with 20 of the most intense Ti and Ti+ lines. The ratio of the N2+intensity to the Ti and Ti+ intensities is approximately 0.05 at 15 mTorr. Furthermore, Fig. 3 shows that this ratio decreases substantially at higher pressure, supporting the conclusion that the number density of N,+ ions to titanium ions is very small. 3.3. Charge per titanium ion The deposition rate (/.fg C-i cm-*) was measured as a function of substrate bias and nitrogen pressure with aperture A2 open. The deposition temperature was typically in the region of 425 f 50 “C; however, it exceeded 500 “C for a number of the films deposited at low pressure and high bias. Most of the films deposited in nitrogen exhibited a characteristic golden TiN color, with the CIELAB color parameters L*, a* and b* clustered in the regions 77 +_2, 2 f 1 and 36 * 2 respectively, which will be referred to as an equilibrium zone. The deposition rate was calculated from the substrate mass gain, the accumulated substrate charge and the deposition area. The collected ion charge was measured directly when the substrate bias was 25 V or more. For runs made with a bias of less than 25 V, the equivalent ion charge was determined by averaging the saturated currents observed during the preceding and following runs. The effective substrate area was 21.9 cm* in vacuum and at 1 mTorr. Owing to gas scattering, it increased to approximately 24.1 cm* at 5,15 and 30 mTorr. Deposition rate results are summarized in Fig. 4. In all cases, the net deposition rate decreases with increasing voltage. This is attributed to
250
200
100
0
-vs
(V)
Fig. 4. TiN deposition rate us. substrate bias and nitrogen pressure.
sputtering. The deposition rate also increases with nitrogen pressure. This is consistent with a gradual decrease in the degree of titanium ionization as a result of neutralization mechanisms discussed in Section 3.2. The steeper slope of the deposition rate in vacuum is consistent with a higher sputtering yield for titanium than for TiN. The slope of the 1 mTorr curve behaves similarly between zero and 100 V, indicating that these films are undernitrided. Colorimetry measurements support this conclusion. The negative slope of deposition rate suggests that the ion arrival energy may already be above the sputtering threshold at zero bias. This is certainly true in vacuum and at 1 mTorr, since the impinging ions carry a large share of their initial 80 eV [2]. Results presented in Section 3.5 show that, even at zero bias, acceleration across the Debye sheath provides a significant source of ion energy. The curves obtained in the presence of nitrogen are virtually parallel, suggesting that the energy of sputtering ions is nearly independent of nitrogen pressure. The slight decrease in slope with increasing pressure is an indication of increased redeposition of sputtered atoms as well as a decrease in the average charge carried by titanium ions. The average charge delivered to the substrate per deposited titanium atom can be calculated directly from the data in Fig. 4 and the film composition. It is a weak function of the atomic ratio because the nitrogen atomic mass is only 25% that of titanium. Results presented by Schiller et al. [ll] indicate that the atomic ratio of nitrogen to titanium for films in the equilibrium zone is approximately 0.95 f 0.05.
251
The average charge per arriving titanium atom is the product of the titanium sticking coefficient q and the charge per deposited titanium atom. Carter and Armour [12] suggest that q = 0.9 f 0.1 for low energy metal atoms impinging surfaces to which they have a high affinity. The increase in the deposition rate (per coulomb) with pressure is caused by a decrease in the average charge of arriving titanium ions. Table 1 gives the average charge per arriving titanium ion calculated from results shown in Fig. 4. These results were obtained assuming ‘1)= 0.9 + 0.1 at zero bias and an N/Ti ratio of 0.95 f 0.05, and neglecting the contribution of secondary electron emission and N,+ ions. The combined effects of secondary electron emission and N2+ ion current would reduce the charge calculated per titanium ion by 10% or less. The charge per titanium ion in vacuum is comparable with previously published values [2,8]. The high charge per arriving titanium ion and the gradual decrease with increasing pressure are consistent with the optical emission results of Section 3.2. TABLE 1 Calculated average charge carried per titanium ion impinging on a substrate 225 mm from an arc source operating at 75 A Nitrogen pressure (mTorr) 0
1 5 15 30
Charge (e) 1.91 1.79 1.70 1.60 1.47
+ + * f f
0.21 0.22 0.21 0.20 0.18
The errors reflect uncertainties in the sticking coefficient and in stoichiometry.
3.4. Ionized titanium fraction The ionized fraction of titanium impinging on the substrate was determined in vacuum and at nitrogen pressures of 1, 5, 15 and 30 mTorr by comparing the minimum deposition rate (obtained while excluding metal ion deposition) with the maximum rate (obtained while allowing ion deposition). The ratio of the minimum rate divided by the maximum rate is the neutral vapor ratio, including condensed microdroplets. Then, the ionized vapor fraction is equal to unity minus the neutral vapor ratio. The minimum rate was determined with the screen (SA,) in front of the substrate. The screen was biased negatively at 90 V or greater to block electrons and to collect ions. At the same time, the substrate was biased at +lOO V to repel ions that penetrated the screen. The measured deposition rate was divided by the screen transparency (0.903) to compensate for screen blocking. The maximum rate was determined with the movable aperture Az open. AZ was grounded and the substrate was biased at -25 V.
252
Four effects tend to increase the observed minimum deposition rate. They are sputtering and neutralized ion reflection from the biased screen, deposition of energetic ions that pass through the screen and overnitriding. The most effective way to minimize sputtering and reflection from the screen is to use a minimum bias. Unfortunately, this allows transmission of more plasma leakage through the screen. This limits the magnitude of the positive substrate bias and may allow deposition of energetic positive ions. Plasma leakage is more. prevalent at low pressure. Unfortunately, sputtered and reflected atoms from the screen are more likely to reach the substrate at low pressure. Finally, the minimum rate can be increased slightly by overnitriding, which occurs at high pressure in the absence of energetic ion bombardment. Minimum deposition rates were determined using screen and substrate voltages of -90 V or greater and +lOO V respectively. With these conditions, no ions were able to reach the substrate. No correction was made for deposition of sputtered and reflected neutrals from the screen. Substrate sputtering reduces the observed maximum deposition rate. At a negative bias of 25 V, this effect was relatively small and could be corrected for using the data in Fig. 4. Undernitriding was also a factor that probably reduced the maximum rate determined at 1 mTorr. No correction was made for this effect. Estimated values of the titanium ionization fraction at the substrate are given in Table 2. These values are probably understated owing to the effects discussed above. The ionization fraction in vacuum is somewhat higher than values previously determined using a similar method [8,13]. In both previous cases, the screen remained in front of the substrate while measuring both the minimum and maximum deposition rates. However, owing to the Debye sheath surrounding the wires, the screen’s effective stopping power for ions is much greater than geometry would suggest. The Debye lengths shown in Table 3 indicate that the effective wire diameter could be several times the physical diameter even for energetic ions. This was not taken into account and therefore the maximum deposition rate was underestimated. TABLE 2 Estimated values of the titanium ionization fraction for an arc source operating at 75 A Nitrogen (mTorr) 0
1 5 15 30
pressure
ionization (%) 68 83 85 84 83
fraction
Distancea (N) 0 4 20 55 110
aFor reference, the source-to-substrate distance is given in terms of the number N of mean free paths for elastic collisions between titanium atoms and nitrogen molecules.
253
Microdroplets, which have a negligible charge-to-mass ratio, are included as part of the neutral fraction. One reason why the ion fraction is larger in a nitrogen atmosphere is that the smaller microdroplets are ejected from cathodes having a nitrided surface. 3.5. Plasma parameters and ion energy Table 3 lists plasma parameters in the vicinity of the substrate. These values were obtained as follows. First, the Langmuir probe data shown in Fig. 2 were used to determine the ion current density, the floating potential V. and the plasma potential V, [6]. Vf was measured directly. Values of V, were estimated from the saturation point of negative current curves [6]. The electron saturation current is reasonably well defined both in vacuum and at 30 mTorr. The saturation current is much less clearly defined at 5 and 15 mTorr owing to the onset of an arc discharge. The value at 1 mTorr was taken to be slightly less than the vacuum value since these curves are parallel up to the vacuum plasma potential. Next, V, - V, was used to estimate the average electron temperature kT, using the method of Emmert et al. [14]. Average ion energies of 80 eV [2], 50 eV, 15 eV, 5 eV and 2.5 eV were used for the calculations at vacuum, 1 mTorr, 5 mTorr, 15 mTorr and 30 mTorr respectively. Calculated values of kT, are quite insensitive to ion energy. The plasma density n was calculated using values of the saturated ion current, V, - V, and kT, [ 141. Finally, the Debye shielding length Xn was calculated from kT, and n [14]. The thickness of the sheath d, between the plasma and the substrate was estimated as a function of pressure and substrate voltage using the Child-Langmuir equation [SJ. The ratio of d, to the mean free path A of titanium ions in nitrogen was also calculated. The results at a substrate bias of 200 V are shown in Table 4. The values of d, are smaller than A for a pressure and voltage of less than 30 mTorr and 200 V respectively. This leads to the conclusion that ions at plasma potential lose little energy in accelerating across the substrate sheath. Thus the average energy of titanium ions impinging a substrate can be written as
TABLE 3 Floating and plasma potentials at the substrate with calculated values of kTe, n and hi as a function of nitrogen pressure
PWz)
(mTorr) 0
1 5 15 30
-vf
(V)
2.2 1.4 2.0 2.2 1.7
f f + + f
0.4 0.4 0.3 0.3 0.3
22 +5 21f5 16 +4 11 f2 6fl
8.0 7.0 5.0 3.5 2.0
+ k f * f
1.5 1.5 1.0 0.5 0.3
b
;X lOlo cm+))
(mm)
1.7 1.7 2.3 2.1 1.7
0.16 0.15 0.11 0.10 0.08
+ 0.05 f 0.15 + 0.06 f 0.07 It:0.30
f + * f f
0.02 0.02 0.01 0.01 0.01
264 TABLE 4 Ratio of the plasma sheath thickness to the mean free path of titanium ions in nitrogen at a substrate bias of 200 V PW2)
(mTorr) 0
1 5 15 30
4
(mm) 0.82 0.92 1.05 1.24 1.70
+ f f * f
0.02 0.03 0.03 0.04 0.05
0.00 0.01 0.07 0.25 0.80
where (E) is given in electronvolts, V, is the negative substrate bias, VP is the local plasma potential, (z) is the average number of elementary charges e per ion and E, is the residual of the original directed energy that the titanium ions retain after traveling a distance NA from the source. Useful’estimates can be made with this equation by substituting (.a>= 1.6 from Table 1 and VP= 10 V from Table 3, and by neglecting E, for M > 20.
4. Conclusions Titanium ions are produced with great efficiency in cathode spots on an arc source. At the same time, relatively few nitrogen molecules are ionized because their concentration is several orders of magnitude less than that of titanium within the spots. In the absence of a strong gas discharge, the ratio of nitrogen to titanium ions remains small during transport to a substrate. The titanium ions retain most of their charge in passing through nitrogen gas. The average charge carried by depositing titanium ions is still approximately 1.6e after passing through a path length equivalent to 100 mean free paths. The ionized fraction of the condensing titanium vapor during TiN deposition is approximately 84%. Finally, for Nx > 20, the average energy of depositing titanium ions is approximately 1.6e( 10 + )VJ ) eV, independent of arc source parameters. References 1 C. W. Kimblin, J. Appt. Phys., 45 (1974) 5235. 2 V. M. Lunev, V. G. Padalka and V. M. Khoroshikh, Son Phys. Tech. Phys., 22 (1977) 858. 3 W. D. Davis and H. C. Miller, J. Appl. Phys., 40 (1969) 2212. 4 P. A. Lindfors, W. M. Mularie and G. K. Wehner, Surf. Coat. Z’echnol., 29 (1986) 279. 5 C. Bergman, in R. F. Hochman (ed.), Zon Plating and Implantation, American Society for Metals, Metals Park, OH, 1986, p. 115. 6 B. Chapman, Glow Discharge Processes, Wiley, New York, 1980.
255 7 H. D. Hagstrom, Phys. Rev., 104 (1956) 1516. 8 P. J. Martin, R. P. Netterfield, D. R. McKenzie, I. S. Falconer, C. G. Pacey, P. Tomas and W. G. Sainty, J. Vat. Sci. Technol. A, 5 (1987) 22. 9 I. I. Demidenko, N. S. Lomino, V. D. Ovcharenko, V. G. Padalka and G. N. Polyakova, Son Phys. Tech. Phys., 29 (1984) 895. 10 P. J. Martin, D. R. McKenzie, R. P. Netterfield, P. Swift, S. W. Filipczuk, K. H. Muller, C. G. Pacey and B. James, Thin Solid Films, 153 (1987) 91. 11 S. Schiller, G. Beister and W. Sieber, Thin Solid Films, 111 (1984) 259. 12 G. Carter and D. G. Armour, Thin Solid Films, 80 (1981) 13. 13 C. Bergman and J. H. Dontje, Znf. Co& on Metallurgical Coatings, 1986, unpublished. 14 G. A. Emmert, R. M. Wieland, A. T. Mense and J. N. Davidson, Phys. Fluids, 23 (1980) 803.
36 (1988)
ION FLUX CHARACTERISTICS TiN*
243
243 - 255
IN ARC VAPOR DEPOSITION OF
CLARK BERGMAN Multi-Arc
Scientific
Coatings,
261 E. Fifth Street,
St. Paul, MN 55101
(U.S.A.)
(Received June 25,1988)
Summary In this paper the ionization and energy characteristics of the particle flux impinging TiN films during arc vapor ion deposition are described. Langmuir probe measurements of voltage and current were made using a planar shielded substrate as the probe. Plasma species were observed using emission spectrometry. Substrate mass gain was measured as a function of voltage and nitrogen pressure. Ion current to the substrate was eliminated by applying a negative voltage to a fine wire screen located in front of the substrate and, at the same time, applying a positive voltage to the substrate. The ionized fraction of the titanium vapor at the substrate was determined by comparing the deposition rate obtained while excluding ions with the deposition rate obtained under normal conditions when ions were part of the depositing flux. The Langrnuir probe measurements were used to estimate the plasma parameters and the average ion energy at the substrate. The results are as follows: (1) in the absence of a nitrogen gas discharge, the density of nitrogen ions is small; (2) most of the titanium vapor is ionized; (3) the average charge carried by titanium ions to the substrate during TiN deposition is (1.6 k 0.3)e; (4) the average energy of titanium ions at the substrate is approximately 1.6e(lO + IV’l) eV, where V. is the negative substrate bias.
1. Introduction The nature of the vapor produced by metal vapor arcs and descriptions of deposition using arc sources have been given by numerous researchers. Metal vapor (or vacuum) arcs produce a vapor that is ionized to a high degree [l] and that is very energetic [2,3]. Many materials have been successfully deposited using arc sources [4]. In this process, substrates are immersed in a dense plasma that is sustained by metal ions and electrons supplied by *Paper presented at the 15th International San Diego, CA, U.S.A., April 11 - 15,1988. 0257-8972/881$3.50
Conference
on Metallurgical Coatings,
0 Elsevier Sequoia/Printed
in The Netherlands
244
cathodic arcs. Even at relatively high pressure, TiN film growth is dominated by metal ion bombardment [ 51. In this paper the ionization and energy characteristics of the particle flux impinging TiN films during arc vapor ion deposition are described. Langmuir probe measurements of voltage and current were made using a planar shielded substrate as the probe. Plasma species were observed using emission spectrometry. Substrate mass gain was measured as a function of voltage and nitrogen pressure. Ion current to the substrate was eliminated by applying a negative voltage to a fine wire screen located in front of the substrate and, at the same time, applying a positive voltage to the substrate. The ionized fraction of the titanium vapor at the substrate was determined by comparing the deposition rate obtained while ions were excluded with the deposition rate obtained under normal conditions when ions were part of the depositing flux. The Langmuir probe measurements were used to estimate the plasma parameters and the average ion energy at the substrate.
2. Apparatus Figure 1 illustrates the experimental deposition system. Titanium is evaporated from the cathode by an arc discharge. The grounded chamber is the anode. The substrate, which is 225 mm in front of the cathode, is located behind a rotatable shutter that is sandwiched between fixed apertures. The cathode, substrate and fixed apertures are perpendicular to their common symmetry axis. The cathode evaporation surface has a diameter of 64 mm. The substrate, which is 0.4 mm thick and 70 mm in diameter, is clamped on its periphery, exposing a surface of 66 mm in diameter. The fixed apertures A, and As are located 30 mm and 14 mm from the substrate and have diameters of 44.5 mm and 50.8 mm respec-
TO
PYROMETER
A TO
SPECTROMETER
Fig. 1. Experimental deposition system.
7
245
tively. Both are at ground potential. The rotatable shutter contains a blocking section, an aperture (AZ) 50.8 mm in diameter and a screened aperture (SAz) 50.8 mm in diameter. The screen in SA2, which is held in a removable clamp, is a rectangular mesh made of type 304 stainless steel wires (0.0254 mm) woven on centers (0.508 mm) and having a transparency of 90.3%. The shutter may be electrically biased. A radiant heater, located directly behind the substrate plane, was used to preheat substrates. The temperature of the front substrate surface was monitored by an IB pyrometer. In some cases, substrate temperature was also measured by an iron-constantan thermocouple spot welded to the back surface. The cylindrical chamber (22 1) was pumped by a turbomolecular pump with an effective speed of 10 1 s-l. Pressure was measured by an ionization gauge and a capacitance manometer. Nitrogen was introduced using an electronic flow controller. During deposition rate measurements, the shutter was opened only after attaining a base pressure of 5 X 10U6Ton or less, and after stabilizing nitrogen flow and pressure with the arc source operating. Plasma emission was monitored through either of two observation tubes, one directed across the cathode surface (port A), and the other aimed to intersect the symmetry axis approximately 60 mm in front of the substrate surface (port B). Emission intensity as a function of distance from the source was observed in a larger chamber where the observation tube was rotatable. The tubes were sealed with quartz windows. Light from the windows was transmitted through a quartz fiber optic cable to the input slit of a 0.5 m Ebert-type Jarrel-Ash scanning monochromator. The monochromator was sensitive in the range 230 - 900 nm with a resolution of less than 0.1 nm. The acceptance angle for all observation tubes was 29 All deposition rate measurements were made at a source current of 75 A. The substrate and shutter plate were biased using voltage-regulated d.c. power supplies. Substrate and shutter current densities were recorded continuously with a sensitivity of 1 mA mm-l. The deposition rates on the substrate and the screen were determined in terms of the mass gain per coulomb of collected charge. A microbalance, with a repeatability of a20 E.cg, was used to measure the initial and final masses. A Macbeth calorimeter was used to characterize the optical reflectance properties of deposited films.
3. Results and discussion 3.1. Langmuirprobe Substrate current was measured as a function of applied voltage in vacuum and in nitrogen at 1, 5, 15 and 30 mTorr. Typical results are shown in Fig. 2. Plasma parameters estimated from these data are discussed in Section 3.5. In Fig. 2, it can be seen that the ion current saturates abruptly near -10 V. (Positive ions collected by a negative voltage are shown as a positive
246
f!5 !E:: mTorr 1 mTorr Vacuum
Fig. 2. Substrate current vs. voltage.
current, and electrons collected by a positive voltage are shown as a negative current.) The saturated ion current decreases with increasing pressure at a rate that is slightly greater than l/P. In vacuum, the ion current is absolutely constant from -25 to -260 V. As the nitrogen pressure is increased the ion current increases gradually between -25 and -260 V. This increase, which is proportional to voltage, is 8% at -260 V and 30 mTorr. It is probably due to electron impact ionization of nitrogen in the plasma adjacent to the substrate. The saturated current collected at a negative bias is primarily due to the flux of titanium and nitrogen ions entering the sheath separating the plasma from the substrate [6] and, to a much smaller degree, to secondary electron emission and Nz+ ions created within the sheath itself. Using Hagstrom’s data [7], the contribution from secondary electron emission was estimated to be less than 8% in vacuum and less than 4% during TiN deposition. The contribution from Nz+ produced by electron impact near the substrate is, at most, a few per cent at low pressure and bias. Results presented in the next section indicate that the production of nitrogen ions in the plasma is also a relatively small effect. 3.2. Emission spectrometry Emission spectrometry was used to detect and identify species present in titanium arc plasmas over the wavelength range 230 - 900 nm. Measure-
24’7
ments were made as a function of distance from an arc source and as a function of nitrogen pressure. The results aid in the understanding of operative mechanisms that produce and annihilate ions, and also indicate that the ratio of nitrogen ions to titanium ions is small throughout the plasma region. The emission amplitudes of Ti, Ti+, Ti’+, N2 and Nz+ are proportional to the radiative decay rates of these species within the observation region. Because the decay lifetimes are very short, such that even the most energetic titanium ions radiate before traveling 1 mm from the point of excitation, emission amplitudes are also a measure of the production rate of particular excited species within the observation region. The total production rate of a given species is obtained by integrating the emission amplitude over all lines of that species, using appropriate factors to account for variations in detector sensitivity with wavelength. This assumes the number produced in the ground state is of the same order (or less) than the number produced in detectable excited states, since species produced in their ground state are not detectable by emission spectrometry. For the purposes of this discussion, the relative production rate of an excited species will be approximated by a summation of emission amplitudes without considering sensitivity variations. Emission from decaying Ti, Ti+, Ti’+, N2 and N2+ atoms and molecules was observed, whereas emission from Ti3+, N, N+, TiN and TiN+ was not identified. Numerous intense Ti and Ti+ emission lines were observed, whereas only three relatively weak N2+ bands were identified. Ti2+ emission was observed only when the detector was aimed to collect radiation coming from locations within 50 mm of the source. Failure to detect Ti3+, which Lunev et al. [2] detected using a mass spectrometer, is an indication that its strongest lines may occur at wavelengths less than 230 nm. The absence of N and N+ lines is a reflection of the high binding energy of the N2 molecule. The absence of TiN and TiN+ lines is an indication that gas phase chemical reactions are not significant. At constant pressure, and outside the view of the brilliant cathode spots, the emission intensity varied inversely with distance from the source. In vacuum, Ti+ emission dominates that of Ti, as found by Martin et al. [8]. In N2, the radiation intensity from Ti+ and Ti species is substantially greater than from N 2+. Broad-band N2 emission gives the plasma a characteristic pink color in the source region. Figure 3 shows the variation in emission intensity (at port B) with nitrogen pressure for two of the more intense Ti and Ti+ lines together with the strongest N2+ and N2 band heads. The variation in intensity with pressure was similar for all lines of a given species. The intensities of the 521 nm Ti and 368.5 nm Ti+ lines increase linearly to approximately 50 mTorr and remain strong to more than 350 ml‘orr. The intensity of the N2+ band head at 391.4 nm increases rapidly from zero to a maximum at 1 mTorr, and then decreases just as quickly to a value that remains nearly constant to 350 mTorr. The intensity of the N2 band head at 357.7 nm reaches a maximum near 15 mTorr and then decreases at a slow exponential rate.
248
20
0
PRESSURE
(mTorr)
Fig. 3. Variation in emission intensity (at port B) with nitrogen pressure for prominent Ti+ and Ti lines and the strongest Nz+ and Nz band heads.
Since the titanium vapor enters the plasma in a highly ionized state, the reactions of greatest interest are those that neutralize titanium ions and those that ionize nitrogen molecules. Direct recombination of electrons with titanium ions is very unlikely owing to the need to conserve both energy and momentum. Furthermore, electron impact ionization of nitrogen is not likely because few electrons within the plasma will possess the required threshold energy of 15.58 eV. This leaves three-body electron recombination and charge exchange collisions as the most probable mechanisms for titanium neutralization and nitrogen ionization respectively. Electron recombination is more likely in the presence of a third particle. On the basis of the conservation laws and proximity considerations, the recombination cross-section is expected to be greatest for low energy electrons. The large increase in Ti and Ti+ emission intensity with pressure, shown in Fig. 3, suggests that three-body electron recombination is indeed a significant mechanism for neutralizing titanium ions. Clearly, the probability of Tin+ + e + N2 + Tic”-‘I+ + N2 reactions increases with the nitrogen concentration. Furthermore, the pressure at which the Ti and Ti+ emissions reach a maximum amplitude is not only a function of the local nitrogen concentration; maxima occur at decreasing pressure with increasing distance from the source. This is consistent with an increase in the three-body reaction rate in the presence of lower energy electrons. Results presented in Section 3.5 show that electron energy does decrease with increasing pressure. Between 0 and 10 mTorr, the present results are similar to those of Demidenko et al. [ 91 and Martin et al. [lo], who attribute increases in the Nz+, Ti+ and Ti emission to charge exchange reactions of the type Tin+ +
249
N, + Ticn-1)+ + N,+ proceeding through .the B *EU’ excited state of N2+. These reactions have Q values of -11.91, -5.15 and +11.91 eV, less the excitation energy of the final state of the titanium atom or ion, for n = 1,2 and 3 respectively. Assuming that nitrogen molecules are at thermal energy, the threshold for the two reactions with negative Q values is (48 + 28)/28 = 2.71 multiplied by the Q value. The threshold energies for Ti+ and Ti*+ charge exchange reactions producing the B ‘C,’ excited state of N2 are at least 32.3 eV and 14.0 eV respectively. Thus, only the Ti’+ to Ti*+ reaction is possible once titanium ions have lost their initially high energy in collisions with nitrogen molecules. The rapid decrease in the intensity of the N2’ emission shortly after reaching a maximum near 1 mTorr is an indication of this effect. These results and observations indicate that titanium ions produced at the arc source retain a high degree of charge and that nitrogen is only weakly ionized within the plasma. Nitrogen ionization occurs most efficiently at low pressure where titanium ions retain most of their initial energy. Titanium ion neutralization occurs most efficiently at high pressure and at distances where electron energy is low. Primary titanium ions radiate at the source and then travel undetected until, after electron recombination, their presence is revealed by the decay of an excited daughter possessing one less charge. Finally, the relative concentration of N2+ and titanium ions was estimated by comparing the emission intensity of the three N2+bands with 20 of the most intense Ti and Ti+ lines. The ratio of the N2+intensity to the Ti and Ti+ intensities is approximately 0.05 at 15 mTorr. Furthermore, Fig. 3 shows that this ratio decreases substantially at higher pressure, supporting the conclusion that the number density of N,+ ions to titanium ions is very small. 3.3. Charge per titanium ion The deposition rate (/.fg C-i cm-*) was measured as a function of substrate bias and nitrogen pressure with aperture A2 open. The deposition temperature was typically in the region of 425 f 50 “C; however, it exceeded 500 “C for a number of the films deposited at low pressure and high bias. Most of the films deposited in nitrogen exhibited a characteristic golden TiN color, with the CIELAB color parameters L*, a* and b* clustered in the regions 77 +_2, 2 f 1 and 36 * 2 respectively, which will be referred to as an equilibrium zone. The deposition rate was calculated from the substrate mass gain, the accumulated substrate charge and the deposition area. The collected ion charge was measured directly when the substrate bias was 25 V or more. For runs made with a bias of less than 25 V, the equivalent ion charge was determined by averaging the saturated currents observed during the preceding and following runs. The effective substrate area was 21.9 cm* in vacuum and at 1 mTorr. Owing to gas scattering, it increased to approximately 24.1 cm* at 5,15 and 30 mTorr. Deposition rate results are summarized in Fig. 4. In all cases, the net deposition rate decreases with increasing voltage. This is attributed to
250
200
100
0
-vs
(V)
Fig. 4. TiN deposition rate us. substrate bias and nitrogen pressure.
sputtering. The deposition rate also increases with nitrogen pressure. This is consistent with a gradual decrease in the degree of titanium ionization as a result of neutralization mechanisms discussed in Section 3.2. The steeper slope of the deposition rate in vacuum is consistent with a higher sputtering yield for titanium than for TiN. The slope of the 1 mTorr curve behaves similarly between zero and 100 V, indicating that these films are undernitrided. Colorimetry measurements support this conclusion. The negative slope of deposition rate suggests that the ion arrival energy may already be above the sputtering threshold at zero bias. This is certainly true in vacuum and at 1 mTorr, since the impinging ions carry a large share of their initial 80 eV [2]. Results presented in Section 3.5 show that, even at zero bias, acceleration across the Debye sheath provides a significant source of ion energy. The curves obtained in the presence of nitrogen are virtually parallel, suggesting that the energy of sputtering ions is nearly independent of nitrogen pressure. The slight decrease in slope with increasing pressure is an indication of increased redeposition of sputtered atoms as well as a decrease in the average charge carried by titanium ions. The average charge delivered to the substrate per deposited titanium atom can be calculated directly from the data in Fig. 4 and the film composition. It is a weak function of the atomic ratio because the nitrogen atomic mass is only 25% that of titanium. Results presented by Schiller et al. [ll] indicate that the atomic ratio of nitrogen to titanium for films in the equilibrium zone is approximately 0.95 f 0.05.
251
The average charge per arriving titanium atom is the product of the titanium sticking coefficient q and the charge per deposited titanium atom. Carter and Armour [12] suggest that q = 0.9 f 0.1 for low energy metal atoms impinging surfaces to which they have a high affinity. The increase in the deposition rate (per coulomb) with pressure is caused by a decrease in the average charge of arriving titanium ions. Table 1 gives the average charge per arriving titanium ion calculated from results shown in Fig. 4. These results were obtained assuming ‘1)= 0.9 + 0.1 at zero bias and an N/Ti ratio of 0.95 f 0.05, and neglecting the contribution of secondary electron emission and N,+ ions. The combined effects of secondary electron emission and N2+ ion current would reduce the charge calculated per titanium ion by 10% or less. The charge per titanium ion in vacuum is comparable with previously published values [2,8]. The high charge per arriving titanium ion and the gradual decrease with increasing pressure are consistent with the optical emission results of Section 3.2. TABLE 1 Calculated average charge carried per titanium ion impinging on a substrate 225 mm from an arc source operating at 75 A Nitrogen pressure (mTorr) 0
1 5 15 30
Charge (e) 1.91 1.79 1.70 1.60 1.47
+ + * f f
0.21 0.22 0.21 0.20 0.18
The errors reflect uncertainties in the sticking coefficient and in stoichiometry.
3.4. Ionized titanium fraction The ionized fraction of titanium impinging on the substrate was determined in vacuum and at nitrogen pressures of 1, 5, 15 and 30 mTorr by comparing the minimum deposition rate (obtained while excluding metal ion deposition) with the maximum rate (obtained while allowing ion deposition). The ratio of the minimum rate divided by the maximum rate is the neutral vapor ratio, including condensed microdroplets. Then, the ionized vapor fraction is equal to unity minus the neutral vapor ratio. The minimum rate was determined with the screen (SA,) in front of the substrate. The screen was biased negatively at 90 V or greater to block electrons and to collect ions. At the same time, the substrate was biased at +lOO V to repel ions that penetrated the screen. The measured deposition rate was divided by the screen transparency (0.903) to compensate for screen blocking. The maximum rate was determined with the movable aperture Az open. AZ was grounded and the substrate was biased at -25 V.
252
Four effects tend to increase the observed minimum deposition rate. They are sputtering and neutralized ion reflection from the biased screen, deposition of energetic ions that pass through the screen and overnitriding. The most effective way to minimize sputtering and reflection from the screen is to use a minimum bias. Unfortunately, this allows transmission of more plasma leakage through the screen. This limits the magnitude of the positive substrate bias and may allow deposition of energetic positive ions. Plasma leakage is more. prevalent at low pressure. Unfortunately, sputtered and reflected atoms from the screen are more likely to reach the substrate at low pressure. Finally, the minimum rate can be increased slightly by overnitriding, which occurs at high pressure in the absence of energetic ion bombardment. Minimum deposition rates were determined using screen and substrate voltages of -90 V or greater and +lOO V respectively. With these conditions, no ions were able to reach the substrate. No correction was made for deposition of sputtered and reflected neutrals from the screen. Substrate sputtering reduces the observed maximum deposition rate. At a negative bias of 25 V, this effect was relatively small and could be corrected for using the data in Fig. 4. Undernitriding was also a factor that probably reduced the maximum rate determined at 1 mTorr. No correction was made for this effect. Estimated values of the titanium ionization fraction at the substrate are given in Table 2. These values are probably understated owing to the effects discussed above. The ionization fraction in vacuum is somewhat higher than values previously determined using a similar method [8,13]. In both previous cases, the screen remained in front of the substrate while measuring both the minimum and maximum deposition rates. However, owing to the Debye sheath surrounding the wires, the screen’s effective stopping power for ions is much greater than geometry would suggest. The Debye lengths shown in Table 3 indicate that the effective wire diameter could be several times the physical diameter even for energetic ions. This was not taken into account and therefore the maximum deposition rate was underestimated. TABLE 2 Estimated values of the titanium ionization fraction for an arc source operating at 75 A Nitrogen (mTorr) 0
1 5 15 30
pressure
ionization (%) 68 83 85 84 83
fraction
Distancea (N) 0 4 20 55 110
aFor reference, the source-to-substrate distance is given in terms of the number N of mean free paths for elastic collisions between titanium atoms and nitrogen molecules.
253
Microdroplets, which have a negligible charge-to-mass ratio, are included as part of the neutral fraction. One reason why the ion fraction is larger in a nitrogen atmosphere is that the smaller microdroplets are ejected from cathodes having a nitrided surface. 3.5. Plasma parameters and ion energy Table 3 lists plasma parameters in the vicinity of the substrate. These values were obtained as follows. First, the Langmuir probe data shown in Fig. 2 were used to determine the ion current density, the floating potential V. and the plasma potential V, [6]. Vf was measured directly. Values of V, were estimated from the saturation point of negative current curves [6]. The electron saturation current is reasonably well defined both in vacuum and at 30 mTorr. The saturation current is much less clearly defined at 5 and 15 mTorr owing to the onset of an arc discharge. The value at 1 mTorr was taken to be slightly less than the vacuum value since these curves are parallel up to the vacuum plasma potential. Next, V, - V, was used to estimate the average electron temperature kT, using the method of Emmert et al. [14]. Average ion energies of 80 eV [2], 50 eV, 15 eV, 5 eV and 2.5 eV were used for the calculations at vacuum, 1 mTorr, 5 mTorr, 15 mTorr and 30 mTorr respectively. Calculated values of kT, are quite insensitive to ion energy. The plasma density n was calculated using values of the saturated ion current, V, - V, and kT, [ 141. Finally, the Debye shielding length Xn was calculated from kT, and n [14]. The thickness of the sheath d, between the plasma and the substrate was estimated as a function of pressure and substrate voltage using the Child-Langmuir equation [SJ. The ratio of d, to the mean free path A of titanium ions in nitrogen was also calculated. The results at a substrate bias of 200 V are shown in Table 4. The values of d, are smaller than A for a pressure and voltage of less than 30 mTorr and 200 V respectively. This leads to the conclusion that ions at plasma potential lose little energy in accelerating across the substrate sheath. Thus the average energy of titanium ions impinging a substrate can be written as
TABLE 3 Floating and plasma potentials at the substrate with calculated values of kTe, n and hi as a function of nitrogen pressure
PWz)
(mTorr) 0
1 5 15 30
-vf
(V)
2.2 1.4 2.0 2.2 1.7
f f + + f
0.4 0.4 0.3 0.3 0.3
22 +5 21f5 16 +4 11 f2 6fl
8.0 7.0 5.0 3.5 2.0
+ k f * f
1.5 1.5 1.0 0.5 0.3
b
;X lOlo cm+))
(mm)
1.7 1.7 2.3 2.1 1.7
0.16 0.15 0.11 0.10 0.08
+ 0.05 f 0.15 + 0.06 f 0.07 It:0.30
f + * f f
0.02 0.02 0.01 0.01 0.01
264 TABLE 4 Ratio of the plasma sheath thickness to the mean free path of titanium ions in nitrogen at a substrate bias of 200 V PW2)
(mTorr) 0
1 5 15 30
4
(mm) 0.82 0.92 1.05 1.24 1.70
+ f f * f
0.02 0.03 0.03 0.04 0.05
0.00 0.01 0.07 0.25 0.80
where (E) is given in electronvolts, V, is the negative substrate bias, VP is the local plasma potential, (z) is the average number of elementary charges e per ion and E, is the residual of the original directed energy that the titanium ions retain after traveling a distance NA from the source. Useful’estimates can be made with this equation by substituting (.a>= 1.6 from Table 1 and VP= 10 V from Table 3, and by neglecting E, for M > 20.
4. Conclusions Titanium ions are produced with great efficiency in cathode spots on an arc source. At the same time, relatively few nitrogen molecules are ionized because their concentration is several orders of magnitude less than that of titanium within the spots. In the absence of a strong gas discharge, the ratio of nitrogen to titanium ions remains small during transport to a substrate. The titanium ions retain most of their charge in passing through nitrogen gas. The average charge carried by depositing titanium ions is still approximately 1.6e after passing through a path length equivalent to 100 mean free paths. The ionized fraction of the condensing titanium vapor during TiN deposition is approximately 84%. Finally, for Nx > 20, the average energy of depositing titanium ions is approximately 1.6e( 10 + )VJ ) eV, independent of arc source parameters. References 1 C. W. Kimblin, J. Appt. Phys., 45 (1974) 5235. 2 V. M. Lunev, V. G. Padalka and V. M. Khoroshikh, Son Phys. Tech. Phys., 22 (1977) 858. 3 W. D. Davis and H. C. Miller, J. Appl. Phys., 40 (1969) 2212. 4 P. A. Lindfors, W. M. Mularie and G. K. Wehner, Surf. Coat. Z’echnol., 29 (1986) 279. 5 C. Bergman, in R. F. Hochman (ed.), Zon Plating and Implantation, American Society for Metals, Metals Park, OH, 1986, p. 115. 6 B. Chapman, Glow Discharge Processes, Wiley, New York, 1980.
255 7 H. D. Hagstrom, Phys. Rev., 104 (1956) 1516. 8 P. J. Martin, R. P. Netterfield, D. R. McKenzie, I. S. Falconer, C. G. Pacey, P. Tomas and W. G. Sainty, J. Vat. Sci. Technol. A, 5 (1987) 22. 9 I. I. Demidenko, N. S. Lomino, V. D. Ovcharenko, V. G. Padalka and G. N. Polyakova, Son Phys. Tech. Phys., 29 (1984) 895. 10 P. J. Martin, D. R. McKenzie, R. P. Netterfield, P. Swift, S. W. Filipczuk, K. H. Muller, C. G. Pacey and B. James, Thin Solid Films, 153 (1987) 91. 11 S. Schiller, G. Beister and W. Sieber, Thin Solid Films, 111 (1984) 259. 12 G. Carter and D. G. Armour, Thin Solid Films, 80 (1981) 13. 13 C. Bergman and J. H. Dontje, Znf. Co& on Metallurgical Coatings, 1986, unpublished. 14 G. A. Emmert, R. M. Wieland, A. T. Mense and J. N. Davidson, Phys. Fluids, 23 (1980) 803.