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The Synthesis of a-C:H Films From Hydrocarbon Plasma of Non-Self-Maintained Discharge

Aleksandr I. Timoshenko, Valeriy S. Taran and Vladimir I. Tereshin
Institute of Plasma Physics, NSC Kharkov Institute of Physics and Technology, Akademicheskaya St.1, 61108
Kharkov, Ukraine
Abstract. Hydrogenated amorphous carbon (a-C:H) films were synthesized from hydrocarbon plasma of non-selfmaintained
discharge, where the vacuum-arc plasma gun was used as a source of supplementary charges. The films
obtained are characterized by high transparence and good adhesion. Such parameters as temperature and density of
electrons, ionization coefficient and plasma potential distributions were determined by the single probe method in nonself-
maintained discharge plasma of both gases: nitrogen and propane-butane mixture.
Keywords: Non-self-maintained discharge, a-C:H films, nitrogen and hydrocarbon plasma.
PACS: 52.77. -j, 52.70.Ds
INTRODUCTION
The usage of glow and vacuum arc discharges for the coating deposition is prevalent now. There is also the nonself-
maintained gaseous discharge which is characterized by high ionization coefficient and high currents (dozens
and hundreds of Amperes) due to the fact that the vacuum-arc plasma gun is used in it as a source of supplementary
charges. This kind of non-self-maintained discharge first was observed by Sablev L. P. and was called by him as
two-step vacuum-arc discharge [1], featuring a metal-gaseous stage of plasma and a gaseous stage of plasma.
Although this discharge is used for the chemical heat treatment (in particular, for nitriding) of products nearly during
20 years [2], nevertheless many of its parameters still not have been investigated and what is more, there is absent
information about application of this discharge to coatings deposition processes. The last fact is concerned evidently
with a low energy of directional motion of ions in such discharge that results in a low speed of coatings deposition.
In this paper we present some results of plasma measurements, obtained by a single probe method, and show the
possibility of the a-C:H films synthesis from hydrocarbon plasma of the two-step vacuum-arc discharge.
EXPERIMENTAL DETAILS
The measurements of plasma parameters was carried out at the vacuum-arc plasma installation of “Bulat”- type
(Fig. 1), re-equipped for creating the dense, directed flow of gaseous ions. The vacuum-arc plasma gun, included
cathode 1 and anode 2, was used here as a source of supplementary charges. Anode 2 is electrically connected with
vacuum chamber 3 and is surrounded by the focusing electromagnetic coil. Vacuum-arc discharge was excited
between cathode 1 and anode 2, supplying with electrons the discharge gap between the chamber 3 walls and anode
5. To prevent the penetration of metal ions to this discharge gap, the aperture of anode 2 was overlapped by the
screen 4, that is impermeable to metal ions and permeable to electrons. Anode 5 was placed into the plasma
concentrator 6, which is 20 cm diameter cylinder, surrounded by electromagnetic coil. Vacuum arc discharge and
non-self-maintained gaseous discharge were excited with the aid of power sources PS 1 and PS 2 respectively. The
parameters of gas plasma were measured by probe 7, which could move along the axis of concentrator 6 and revolve
around it. The probe isolator could be prolonged, so the plasma parameters could be measured over the whole
chamber 3. The volt-ampere characteristic of the single probe 7 was obtained with the aid of the power unit 9. Thin
arrows show the directions of electrons drift motion and thick arrow show the motion direction of gaseous ions.
After the system was pumped to 5 ⋅10−5 Torr the power
supply sources PS 1 and PS 2 were switched on and the arc
discharge between cathode 1 and anode 2 was excited. The
arc discharge current was established on 100 A, and
discharge voltage was near 27 V. The power supply unit PS
2 provides the voltage of 110 V between anode 5 and
chamber 3 in absence of current. The gas is letting in
through the admission valve (not shown) that enable to
maintain a pressure inside chamber 3 on a specified level.
The plasma parameters were measured for both gases:
nitrogen and propane-butane mixture.
RESULTS AND DISCUSSION
The gaseous discharge between anode 5 and chamber
walls ignites starting with a pressure of (1.4 −1.6) ⋅10−4
Torr. For distinctness, the voltage on the gaseous discharge
gap was maintained at a level of 60 V both: under the
measuring of discharge characteristics and during the
determining of the plasma parameters.
Electron and Ion Currents
For experimental setup described above the gaseous
discharge current in nitrogen (Fig. 2) grows with a pressure up to values of (4 − 6) ⋅10−3 Torr. In this pressure region
when the magnetic field strength in concentrator 6 (Fig. 1) is 50 G, the discharge current is 1.5 times as much of it in
absence of magnetic field. The ion current has a similar dependence on the pressure and reach up to 3 A at a distance
of 10 cm from the outlet of concentrator. Under the same voltage the ion and electron currents in propane-butane
mixture are near of 70 % of those in nitrogen.
The ion current density has a maximum at the axis of the concentrator 6 at outlet section and reaches up to 35
mA/cm2 in a presence of magnetic field and is two times less in absence of field. Ion current quickly decreases on
two sides of the outlet section, for example, it is two times less at a distance of 10 cm from the outlet of
concentrator.
Electron Temperature and Density
Temperature and density of electrons was calculated from
probe volt-ampere characteristics that were obtained at
numerous of points inside of vacuum chamber 3 (see Fig. 1).
Maximum density was observed near the output of
concentrator 6, whereas the temperature of electrons was
practically identical over entire volume of the chamber.
The noticeable feature of two-step vacuum-arc discharge is
that electron temperature (Fig. 3) can reach up to (22 – 25) eV
that is several times higher than it is observed in vacuum arc
plasma (1.5 – 4.5 eV) [3]. Obviously, so high temperature can
have the primary electrons, which are accelerated in plasma
gun at voltage drop between cathode 1 and anode 2 (Fig. 1)
and being scattered elastically on the gas molecules in
chamber. An increase in the pressure leads to the growing of
the frequency of the inelastic collisions, which lead to the
excitation, dissociations and ionizations of the gas molecules.
As a result the temperature of electrons is reduced.
FIGURE 1. Scheme of the Experimental Setup.
1– Cathode; 2 – Anode; 3 – Vacuum chamber; 4 –
Screen; 5 – Anode; 6 – Plasma concentrator; 7 – Probe;
8 – Fixture; 9 – Power supply unit. Power sources PS 1
and PS 2 supply arc and gaseous discharges.

FIGURE 2. Dependence of Gaseous Discharge
Current on the Pressure of Nitrogen for Two
Magnetic Field Strengths: B = 0 and B = 50

Coefficient of Ionization
This parameter of two-step vacuum-arc discharge not had been measured earlier and, in view of high discharge
current, it was surmised that it may reach up to dozens of percents. By knowing the density of electrons, one can
calculate the coefficient of ionization as ratio of electron density to the gas molecules concentration at different
pressures. As it follows from Fig. 4, the maximum of coefficient of ionization is observed at pressure region of
8 ⋅10−4 − 2 ⋅10−3 Torr and not exceeds of 0.1%. So, the coefficient of ionization in plasma of two-step vacuum-arc
discharge is considerably lower than one is in vacuum arc discharge (where it may approach to 100 % [4]) and it is
several orders higher than degree of ionization in glow-discharge column (10-6 – 10-4) % [5].
Floating Potential Distribution
It is known, that floating potential differs from the plasma potential in several Volts [6]. So, the distribution of
plasma potential may be evaluated by information about course of floating potential. The main feature of the floating
potential distribution (Fig. 5) inside the experimental setup is its steep slope at the outlet of concentrator 6 (Fig. 1).
Therefore, the largest value of ion energy may be expected just after output of concentrator, as later on the ions will
be lose in its energy due to collision with the gas molecules. It may be noted also the decrease in both: the value of
plasma potential and potential gradient when the pressure is growing. Such tendency takes place in a pressure range
from 2 ⋅10 −4 to 1⋅10 −2 Torr.
A-C:H Films Deposition
For the a-C:H films deposition, the non-self-maintained discharge was exited in propane-butane mixture under a
pressure of 4 ⋅10−3 Torr. The discharge current was 50 A and the voltage between anode 5 and chamber 3 (see Fig.
1) was 90 V. The films were deposited on glass and stainless steel substrates that were rotating about the axis of the fixture 8, periodically leaving from a zone of the densest ion flow moved from concentrator 6 (Fig. 1). Under this
conditions the coatings growth rate was 4.5 micron/hour. These films are transparent in optical band even when its
thickness reach 5μ m and has a good adhesion to the substrate surfaces. Their Vickers microhardness was 1200 –
1500 kG/mm2 that is essentially lower than vacuum arc deposited coatings have. It is quite possible that the reason
of this fact is in insufficient degree of dissociation and low energy of ions.
CONCLUSION
Temperature and density of electrons, ionization coefficient and potential distribution were determined by the
single probe method in nitrogen and propane-butane plasma of non-self-maintained discharge, where the vacuum arc
plasma gun is used as a source of supplementary charges. In contrast to glow discharge, directed and dense flux of
ions was produced at anode region. It enables synthesizing of a-C:H films with a high rates of growths without using
of high-voltage RF-oscillator that is applied usually for excitation of glow discharge.
REFERENCES
1. L. P. Sablev, A. A. Andreev, S. N. Grigoriev, and A. S. Metel,. U.S. Patent No. 5,503,725 (2 April 1996).
2. A. A. Andreev, I. V. Bubnov, A. S. Vereschaka, V. G. Padalka L. P. Sablev and R. I. Stupak, USSR Patent No. 1307886
(1987).
3. A. Anders, Phys. Rev. Lett. 55, 969, (1997).
4. A. A. Plutto, V. N. Ryzhkov and A. T. Kapin, Russ. Sov. Exp. Tech. Phys., 47, (2), 494-507 (1964).
5. J. P. Raizer, Physics of Gaseous Discharge (in Russian), Moscow, “Nauka”, 1987, p. 15.
6. R. H. Huddlestone and S. L. Leonard, Plasma Diagnostic Techniques, NY-London, Academic Press, 1965, pp. 145-146.

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