SUPERPOSITION OF TWO PLASMA BEAMS PRODUCED IN A VACUUM ARC DEPOSITION APPARATUS

V.N. Zhitomirsky', R.L. Boxman', S. Goldsmith', I. Grimberg2 and B.Z. Weiss'

'Electrical Discharge and Plasma Laboratory, Tel-Aviv University, POB 39040, Tel-Aviv 69978, Israel 'Department of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel

Abstract - Two plasma beams of different materials were produced from Ti, Zr, or Nb catholdes in a triple-cathode vacuum arc deposition apparatus. The cathodes were arranged in a circle centered on the system axis. The plasma produced was transported through a straight plasma duct with an axial magnetic field, into a deposition chamber, in which a single Langmuir probe, an array of probes, or a substrate could be mounted 560" from the cathode plane. The saturation ion current produced by one cathode, or by the simultaneous operation of two different cathodes, was measured in vacuum and in a 0.13-2.7 Pa nitrogen background. The spatial distribution of the composition of coatings deposited during simultaneous operation of different cathodes in nitrogen was also studied.

It was shown that the ion current produced by each single cathode, and by the simultaneous operation of two cathodes, decreased by a factor of 20-100 when the nitrogen pressure was increased from vacuum to P=2.7 Pa. During simultaneous operation of two cathodes, the ion saturation current was less than the sum of the ion currents produced by each cathode individually when P<0.4 Pa, while the simultaneous ion current was less than the sum of the individual ion currents when P20.4 Pa.

1. INTRODUCTION

Thin hard wear resistant coatings based on ternary nitride systems, such as Ti-Zr-N, Ti-Nb-N, etc. [ 1-31, in many cases show better performance in cutting tool and friction pair applications than TiN and other binary nitride coatings. Ternary coatings may be fabricated by vacuum arc deposition using a multi-cathode plasma source, such as the triple-cathode apparatus described in our previous studies [4, 51. In this apparatus, the plasma jet produced by the cathode spots is transported to the substrate through a magnetized plasma duct. If the cathodes, fabricated from different metallic materials, operate simultaneously, the jets o f different metal ions enter into the same plasma duct, and in the background of nitrogen, multi-component nitride coatings are deposited on the substrate. However, the distribution of elements in the coating may be non-uniform Sakamoto et al. [3] studied the element distribution along the

substrate length in reactively sputtered Ti,.,Zr,N coatings. Iin their apparatus a 100 rrim diam. target was fabricated from two single Ti and Zr segments. They showed that in their films x changed from 0.3 to 0.8 on a 10 cm long substrate. In the present apparatus [4, 51 where the cathode axes do not coilncide with the duct axis and the cathodes are positioned at some distance from each other, non-uniformity is also expected.

It was shown that plasma jet and ion current density produced by single cathode have a spatial distribution [6-81. For example, Cohen et al. [6, 71 showed thi3t the distribution profile of the plasma beam may be described by a cosine law. However, the distributioin of two or more beams produced by the different vacuum arc cathodes, and composition distribution of coatings on substrate exposed to two beams, have not been measured.

The objective of the present work was to measure tlhe ion current and composition distribution produced by the simultaneous operation of two cathodes.

2. EXPERIMENTAL SETUP AND PROCEDURE

The triple cathode vacuum arc deposition apparatus [4, 51 is shown in Fig. 1.

~ < T K I G G E K M E C H A N I S M G A T E V A L V E

SUBSTRAIE

C A T H O D E S ' C O I L I \ C O I L 2 COIL 3

Fig. 1. Vacuum arc deposition apparatus

Three cathodes of 54 mm diam. were equally distributed on a 100 mm diam circumference on the system end flange. The cathode materials were pure Ti, Nb and Zr. Two types of anodes were used: (1) a 180 mm i.d. annular anode, and (2) a 3-hole anode, in which each 54 mm diam. hole was coaxial with one of the cathode. The plasma beams were transported

0-7803-3953-3/98/$10.00 0 1998 IEEE 18th Int. Symp. on Discharges and Electrical Insulation in Vacuum-Eindhoven-1998 609

through a 160 mm diam. duct to a deposition chamber. Three Helmholtz coils produced magnetic fields of up to 10 mT at the coil centers to guide the plasma. Measurements were carried out in vacuum (1.3 mPa) and in a nitrogen background of 0.13-1.33 Pa.

The saturation ion current was measured by a 43 mm diam disk probe positioned in the deposition chamber at a distance of 560 mm from the cathode plain and centered on the duct axis. The ion current density distribution was measured along the diameter of deposition chamber which was parallel to the line connecting the centers of the two active cathodes (x axis). The relative lateral position of the cathodes the probes are shown in Fig. 3 (the cathode and probes are separated by 560 mm in the axial direction). It may be noted that lines parallel to the system axis from the center of the Nb and Ti cathodes intersect the probe plane at x=-43 mm and +43 mm respectively, and at y=-25 mm (Fig. 3a). The distribution was measured using an array of 5 probes each of 10 mm diam positioned in the center and +25 and +50 mm from the center. In all cases a bias voltage of -40 V was applied with respect to the grounded anode.

Fig. 2. Lateral positions of the cathodes and the ion current probes

Coatings were deposited onto cemented carbide substrates located at the same plane at which the probes were located, and centered on the system axis (Fig. 3). The element distribution in the coatings was studied by Auger electron spectroscopy (AES).

4Y

Fig. 3. Lateral position of the cathodes and the sample for concentration distribution study

3. EXPERIMENTAL RESULTS

3.1 Ion current dependence on pressure

Fig. 4 shows the pressure dependence of the ion current density when Ti and Zr cathodes were operated separately, the sum of these current and the ion current density when the two cathodes were operated simultaneously, as measured on the 43 mm diam probe. The arc currents for both cathodes were 250 A, the annular anode was used, and the magnetic fields produced in the duct by Coils 1, 2 and 3 were 1, 7.5 and 10 mT, at the center of the respective coils.

I. Sum Ti+Zr 40

. . . . . . . . . . . . . . . . . . . : . . - 0 0.5 I 1.5 2 2.5 3

p, Pa

Fig. 4. Ion current density as a function of nitrogen pressure for separate and simultaneous operation of Ti and Zr cathodes

It may be seen that with increasing nitrogen pressure, the Ti ion current at first slightly increased and then decreased, while the Zr ion current decreased monotonically. The sum of the Ti and Zr currents decreased with pressure, while the ion current during simultaneous Ti and Zr cathode operation was approximately constant for P<0.4 Pa, and then decreased with pressure for P>0.4 Pa. Generally, the ion current produced by each single cathode, and by the simultaneous operation of two cathodes, decreased by a factor of 20-100 with increasing the nitrogen pressure from vacuum to P=2.7 Pa. At P<0.4 Pa the sum of the ion currents produced during single cathode operation was larger than that produced during simultaneous operation, whereas at P20.4 Pa - the sum of ion currents became smaller than the simultaneous operation ion current..

3.2 Ion current distribution

The ion current density distribution was measured during separate and simultaneous operation of Ti and Nb cathodes with arc currents of 200 and 250 A respectively (Fig. 2). The three-hole anode was used, and axial field produced by coils 1-3 were 2.5, 7.5 and 7.5 mT respectively. The ion current density

610

distributions during separate and simultaneous operation with a nitrogen pressure of 0.67 Pa are presented in Fig. 5, and the pressure dependence of the ion current density at the central (0 mm) probe is shown in Fig. 6 .

40 -

E

E e-

Ti --- t

0 4 . I I . , . . I I , . . I , , 1 d -50 -25 0 15 50

x , m m

Fig. 5 . Ion current density distribution during separate and simultaneous operation of Nb and Ti cathodes

’” -I I

0 0.5 1 1.5 P, Pa

Fig. 6 . Ion current density as a function of nitrogen pressure during separate and simultaneous operation of Nb and Ti cathodes

It may be seen that the maximum Nb ion current at P=0.67 Pa was recorded by the probe positioned at -25 mm, whereas the largest Ti ion currents were collected by the +25 and +-50 probes (Fig. 5). The sum of the individual ion currents had a maximum at -25 mm and was almost constant from 0 to +50 mm. However, ion current during simultaneous operation of the two cathodes had a maximum at the - 25 mm probe, and a minimum at the +25 mm probe.

The central probe current during simultaneous operation was less than the sum of the individual currents in vacuum, but greater than the sum, at higher pressures (Fig. 6), simillar to the behaviour observed with the Ti and Zr electrode pair (Fig. 4).

3.3 Element distribution in a coating produced during simultaneous cathode operation

Fig. 7 shows the element distribution in a (Nb,Zr)N coating along its 40 mm length, with the sample positioned as shown in Frg. 3. The nitrogen pressure was 0.67 Pa, and the substri& temperature was 400°C. It may be seen that the concentrations of Zr and Nb had factors of 2 variations along the sample length, whereas the nitrogen concentration slowly increased with x.

50

40

C 30 m v

U” 20

10

0

0 10 20 30 40

x ,mm

Fig. 7. Element distribution along the sample in the Nb-Zr-N mating

4. DISCUSSION

Tlhe ion current during siinultaneous cathode operation differed significantly from the sum of the individual currents, indicating that the combined plasma flux is not merely a linear superposition of the individual plasma beams (Figs. 4, 6 ) . The lower ion current during simultaneous cathode operation in low pressures ,compared to the sum of the individual ion currents, may be due to the collisions of the different ions in the plasma. Gidalevich et i d . [9] analyzed the formation and shape of a double shock front, which should appear in the interaction region of two supersonic plasma jets. Their model did not include magnetic collimation, and thus their numerical calculationis are not directly applicable to the present case. The higher ion current during simultaneous cathode operation (relative to the sum of the individual ion currents) at higher pressures may be related to gas ionization by thermal electrons [lo].

The results show that multi-component nitride coatings obtained during simultaneous deposition from two plasma beams may have a strong non-uniformity in the composition (Fig. 7), as observed previously during co-sputtering [3]. Means must be employed to improve the compositional uniformity, such as rapid sample motion and/or plasma beam sweeping [S, 111.

61 1

5. CONCLUSIONS

0 The ion flux in magnetically collimated vacuum arc plasma beams generally vary across the beam cross section, limiting the uniformity in coatings obtained from these beams.

The ion flux distribution obtained from the simultaneous operation of dissimilar cathodes generally differs from the sum of the individual cathode plasma beams.

In the present experiment, the simultaneous flux was less than the sum of the individual fluxes at low pressure, and higher at high pressures.

0 The composition of coatings obtained during simultaneous cathode operation had non-uniform compositions.

Substrate motion or plasma beam sweeping is suggested for improving the compositional uniformity.

e

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