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Diagnostics of a d.c. pulsed-plasma-assisted nitriding process
Surface and Coatings Technology, 59 (1993) 82—85
82
Diagnostics of a d.c. pulsed-plasma-assisted nitriding process R. Hugon, G. Henrion and M. Fabry Laboratoire de Physique des Milieux Jonisés, CNRS URA 835, Faculté des Sciences, Université de Nancy I, BP 239, 54506 Vandoeuvre Cedex (France)
Abstract Plasma diagnostics is an in situ tool well adapted to study the processes involved in surface modification. In this paper, we report some experiments carried out using optical emission spectroscopy and electrostatic probes, and performed in an iron nitriding d.c. pulsed plasma. We show that some excitation processes during the temporal postdischarge are the same as those which occur in the discharge. Moreover, results about the effect ofthe discharge power on the excitation of particular nitrogen levels are discussed. Some first results about the experimental determination of the electron energy distribution function are also presented.
1. Introduction It has been apparent for many years that plasma diagnostics is a powerful tool which can give some specific information necessary to allow process monitoring and optimization. In particular, plasma characterization by means of electrostatic Langmuir probes and optical emission spectroscopy (OES) leads to a better knowledge of the different species present in the plasma, and especially of those involved in the surface treatment process. The purpose of this paper is to study a d.c. pulsed discharge used for iron nitriding. Here, we show that, even during the postdischarge, electrons with sufficient energy remain in the plasma and are able to excite some particular levels of nitrogen, such as N2(A3) or N 2(X,v), which are of great interest in the nitriding process. The experiment we have developed allows us to make time-resolved measurements of the plasma parameters with a resolution better than I ~.ts.Special attention is paid to the experimental determination of the electron energy distribution function which gives new insights into the electron processes needed to create active particles, even when the discharge power is off.
2. Experimental apparatus
supply whose frequency and duty cycle can be adjusted and whose rise and fall times are equal to 0.2 ~is.The two other main parts of that experiment are as follows: the diagnostics apparatus, which consists of OES and Langmuir probe devices; a homemade electronic unit (described in detail in ref. 2) which synchronizes the discharge voltage, the diagnostic measurements and the data acquisition on a microcomputer.
3. The choice of working with d.c. pulsed discharge Nitriding by means of diode discharges is a wellknown process which has been used in industry for several years [3]. In such processes, the main problems concern the following: the degradation of the workpieces, owing to the
Monochro. + PM
0257—8972/93/56.00
L~
_____
-
P
Probe polarisation
Pulsed power supplyj~
S h ~inc ron.
I
L
Boxcar averager
C
L
Recorder ______________
The entire experimental system as well as the plasma reactor, have been described previously [1, 2] and we are only going to recall here their main elements (Fig. 1). Briefly, the plasma reactor is a cylindrical stainless steel chamber. The N2 discharge is generated between two electrodes, one of which is the cathode to be nitrided. The plasma is powered by means of a d.c. pulsed power
A
__________
Clock Amplifier L.1 Transient rec.
I
IH
1~LE~~ROY __________
[~cornpu~J~ Fig. 1. Schematic diagram of the arrangement of the experimental device: A, anode; C, cathode; P, probe.
©
1993
—
Elsevier Sequoia. All rights reserved
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
inopportune switching from an abnormal glow discharge to an arc regime; the sputtering of the substrate by positive ions, which implies obtaining an equilibrium between the growth and the sputtering of the nitrided layer. Nevertheless, such an ion bombardment plays a significant role in the substrate cleaning and substrate heating. Because of this ion heating, an auxiliary oven is often not needed to achieve the correct sample temperature. To solve—at least partially—these difficulties, two new ways have appeared in the last few years. The first method is the flowing afterglow, in which the substrates are placed outside the discharge. Therefore, they do not meet during ion bombardment and discharge arcing does not have any effect on the pieces themselves. However, they have to be heated independently. Such experiments have shown that active species for iron nitriding were atomic and vibrationally excited states rather than ions [4, 5]. The use of a d.c. pulsed discharge is the second method of avoiding the problems mentioned above. Such a plasma allows heating of the substrate and the sputtering is limited to the discharge duration (i.e. when the power is on), while most of the active species remain in the reactor throughout the voltage period. For such work, different treatments have been performed by varying the frequency from 50 to 1000 Hz. The metallurgical results obtained by Bougdira [6] and Petat [7] have shown that the properties of the nitride layers—analysed by means of X-ray diffraction and hardness measurements—were enhanced by using a frequency of about 100 Hz (Fig. 2). Moreover, such a method could be easily adapted to current industrial equipment. For these reasons, we have chosen the second pro-
83
cedure for our studies on plasma nitriding and we are now working with a d.c. discharge pulsed at 100 Hz, so that both the discharge time and the postdischarge duration are of the order of 5 ms. However, some diagnostic results have been performed under other experimental conditions.
4. OES diagnostics of the plasma Though OES is often limited to relative measurements, it remains a powerful diagnostic tool to identify atomic and molecular species and to follow their time and space variations in plasma-assisted processes. In our pulsed nitriding process, we have observed especially the radiative transitions of the second positive and the first negative bands of nitrogen: N2 (C311U+B3Hg) N~(B22~ —*X22~) respectively. Indeed, these transitions can be related to some metastable states (N 3E.~~); N 2(Ain the creation 2(X,v)) ofofnitrogen that play an important role atomic nitrogen that is the main active species for nitriding [4, 5]. By such an observation, we have shown that the emission line intensities increase with increasing power and rise to a saturation value for a power greater than 150 W (Fig. 3). Thus, the active species also reach a plateau and the power need not increase, because it would not enhance the plasma reactivity. Moreover, increasing the power would enhance the sample sputtering, as can line (Fig. 4). be seen by the observation of an iron Another interesting feature in OES is the possibility of obtaining some information about the excitation processes that occur in the plasma and in the postdis-
50
800
700
S
HVi (Vickers
100
~
40
.—
,~/__•_
80 o7 600
.3Ø~ -.
20 ~ 400
10
300
200
0
0
300 400 500 600 Frequency (Hz) Fig. 2. Effect of plasma voltage frequency on hardness and X-ray diffraction peak of nitrided layers (from ref. 7). iOO
200
/I.
•
p ~
40 0
I
______
0~ 100
/
200
300
400
___________
20
•D
0
DO ~
II,-
o~
_____________
.—
__.— i
0—
power(W) 2E.~—÷X2E~)line intensity (band head at 337.1 nm) against power Fig. 3. N~ (B for different times in the postdischarge, where t=0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz. H, 0 ~is;•, 20 Jis; H, 40 his; ~, 60 ~is; •, 80 ps; LI, 100 I.Ls.
84
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
/
faster electrons power was on. that were created when the discharge
80
5. Electron energy distribution function ~ ~
60 40
As mentioned it appears a good knowledge probe apparatusabove, and some data that analysis software that of the electron energy is necessary to explain the excita-
,
tion processes which occur in the plasma. To obtain such a parameter, we have developed some Langmuir 20 0 0
100
200 300 400 power (W) 5F—~a°D)line intensity (372.0 nm) against power for Fig. 4. Fe (z different times in the postdischarge, where t =0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz. Symbols as in Fig. 3.
charge. Indeed, we have measured emission line intensities far into the postdischarge (up to 2.5 ms after the discharge cut-off). The special case of the iron line (z5F a5D) is of particular interest if we compare its evolution with the electron density decay during the postdischarge. In Fig. 5 it can be seen that both the iron line intensity and the electron density have the same behaviour. This means that: (i) the excitation process of Fe(z5F) is the same in the post discharge as that in the discharge, namely governed by electron collisions; (ii) the secondary electron population remains important in the post discharge, though most of the slow electrons are lost by electron—ion recombinations as soon as the voltage drops to zero. In fact, the remaining slow electrons are issued owing to the thermalization of —~
~Fe
(from which the plasma potential is deduced with an accuracy of about 0.1 V) and its second derivative, which is proportional to the EEDF [9, 10]. The first results we present here deal with the evolution in the postdischarge of the EEDF shape for a plasma running at 1 kHz. The curve reported in Fig. 6 shows that the mean electron energy, calculated from JEJ(E) dE/Jf(E) dE is about 1 eV when the plasma is on and is slightly decreased to 0.5 eV at 490 j.ts into the post discharge, i.e. just before the plasma is switched on again. However, the electron density, calculated from the probe current at the plasma potential, decreases as l/t from its value of 5 x 1011 cm3 in the plasma. This decrease is specific to electron—ion recombination losses [11]. With regard to the evolution of the EEDF shape, Fig. 7 shows a shift of the right-hand part of the EEDF towards the left as one goes along in the postdischarge, while the bulk of the EEDF remains quite constant (in
Ne
~
allow us to obtain at the same time the electron density and temperature (secondary electrons), as well as the shape of the electron energy distribution function (EEDF) [8]. The probe characteristic is time resolved— with a resolution better than 1 j.ts—and is recorded over 256 points. Then, with only one data manipulation, the software leads to a smoothed curve, its first derivative
0.8 0.7
line
.4 —‘—
3 I-
\~
~0.6
L.
~ io~
•~
2
~
~A 0.5 0.4
io~ 0
-
1
0.3 0.5
1.0
1.5 Time (ms)
2.0
2.5
3.0
Fig. 5. Relative evolution of electron density (N,) and iron line intensity (both normalized to I for t = 0) vs. postdischarge time, where t = 0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz.
0
100
200 300 Time (~is)
400
5000
Fig. 6. Electron density (Ne) and mean energy () as a function of postdischarge time, where t = 0 corresponds to the discharge cut-off. Plasma frequency, I kHz.
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
the plasma and that active species remain up to 5 ms into the postdischarge. Therefore, it may be possible to
1.0 20~ts
0.6 0.8 0.4
reactivity and to reduce the sample sputtering. Secondly, an improved knowledge of nitrogen metastable with work states different is necessary characteristic to understand times to enhance better the role and the reactivity of these excited states (especially N2(A3~);N2(X,v)). For this, we have now dye laser
200 I.ts 300 ~is
~
85
1.11
0.2
00
1.0
1.5
490ps 2.0 2.5
3.0absorption. 3.5
Energy (eV) Fig. 7. Electron energy distribution function shape at different times in the postdischarge. Plasma frequency, 1 kHz.
normalized values). This is owing to the thermalization of the high energy electrons and explains the presence of slow electrons—which may induce atomic or molecular excitation—several milliseconds after the discharge is switched off.
6. Concluding remarks As described in this paper, plasma diagnostics can easily lead to some insights being obtained into the different processes involved in plasma-assisted surface treatments. A particularly important feature is the pOssibility of following some plasma processes in real time in order to control the process itself. As an example, such control is performed in plasma etching reactors where OES is used to detect the end of the etching [12]. Nevertheless, much work is necessary in order to optimize our pulsed-plasma nitriding process. First, we have to determine the best times for both the discharge and the postdischarge. Some experiments carried out in this way have shown that the plasma stabilization is only reached 2—3 ms after switching on
equipment which will allow us to measure the indensity of Thirdly, these states we need by tomeans improve ofour photoluminescence probe device order or to obtain more information about electrons with more than 2 or 3 eV. Indeed, up to now, we could not measure the tail of the EEDF. A modified experimental set-up would allow us to record the entire probe characteristic over a wide range of probe polarization with a good resolution. References I J. Bougdira, G. Henrion, M. Fabry, M. Remy and J. R. Cussenot, Mater. Sci. Eng., A139 (1991) 15. 2 G. Henrion, M. Fabry, R. Hugon and J. Bougdira, Plasma Sources Sd. Technol., 1(1992)117. 3 H. Michel, Trait. Therm., 179 (1983) 41. P. Colhgnon, Heat Treat. Met., 3 (1982) 67. 4 G. Tibbetts, AppI. Phys., 45 (1974) 5072. 5 A. Ricard, Rev. Phys. Appl., 24 (1989) 251. 6 J. Bougdira, Thesis, University of Nancy, 1990. 7 B. Petat, Thesis, University of Nancy, 1990. 8 G. Henrion, M. Fabry and R. Hugon, in L. Tsendin (ed.), Proc. XI ESCAMPIG, St. Petersburg 25—28 August 1992, Abstracts Vol. 16F, p. 135. 9 K. Wiesemann, Internal Report 87-05-164, August 1987, Institut für Experimentalphysik AGII, Ruhr Universitat, Bochum. 10 L. Schott, in W. Lochte-Holtgreven (ed), Plasma Diagnostics, North-Holland, Amsterdam, 1968, p. 668. ~ J. Bougdira, G. Henrion and M. Fabry, J. Phys. D, 24(1991)1076. 12 I. Hussla, P. Banks, P. Baumann, G. Castrischer, H. Grunwald, G. Lorentz and H. Ramisch, Proc. 1st mt. Conf on Plasma Surface Engineering (PSE’88), Garmisch-Partenkirchen, 19—23 September 1988, DGM, Oberursel, p.495.
82
Diagnostics of a d.c. pulsed-plasma-assisted nitriding process R. Hugon, G. Henrion and M. Fabry Laboratoire de Physique des Milieux Jonisés, CNRS URA 835, Faculté des Sciences, Université de Nancy I, BP 239, 54506 Vandoeuvre Cedex (France)
Abstract Plasma diagnostics is an in situ tool well adapted to study the processes involved in surface modification. In this paper, we report some experiments carried out using optical emission spectroscopy and electrostatic probes, and performed in an iron nitriding d.c. pulsed plasma. We show that some excitation processes during the temporal postdischarge are the same as those which occur in the discharge. Moreover, results about the effect ofthe discharge power on the excitation of particular nitrogen levels are discussed. Some first results about the experimental determination of the electron energy distribution function are also presented.
1. Introduction It has been apparent for many years that plasma diagnostics is a powerful tool which can give some specific information necessary to allow process monitoring and optimization. In particular, plasma characterization by means of electrostatic Langmuir probes and optical emission spectroscopy (OES) leads to a better knowledge of the different species present in the plasma, and especially of those involved in the surface treatment process. The purpose of this paper is to study a d.c. pulsed discharge used for iron nitriding. Here, we show that, even during the postdischarge, electrons with sufficient energy remain in the plasma and are able to excite some particular levels of nitrogen, such as N2(A3) or N 2(X,v), which are of great interest in the nitriding process. The experiment we have developed allows us to make time-resolved measurements of the plasma parameters with a resolution better than I ~.ts.Special attention is paid to the experimental determination of the electron energy distribution function which gives new insights into the electron processes needed to create active particles, even when the discharge power is off.
2. Experimental apparatus
supply whose frequency and duty cycle can be adjusted and whose rise and fall times are equal to 0.2 ~is.The two other main parts of that experiment are as follows: the diagnostics apparatus, which consists of OES and Langmuir probe devices; a homemade electronic unit (described in detail in ref. 2) which synchronizes the discharge voltage, the diagnostic measurements and the data acquisition on a microcomputer.
3. The choice of working with d.c. pulsed discharge Nitriding by means of diode discharges is a wellknown process which has been used in industry for several years [3]. In such processes, the main problems concern the following: the degradation of the workpieces, owing to the
Monochro. + PM
0257—8972/93/56.00
L~
_____
-
P
Probe polarisation
Pulsed power supplyj~
S h ~inc ron.
I
L
Boxcar averager
C
L
Recorder ______________
The entire experimental system as well as the plasma reactor, have been described previously [1, 2] and we are only going to recall here their main elements (Fig. 1). Briefly, the plasma reactor is a cylindrical stainless steel chamber. The N2 discharge is generated between two electrodes, one of which is the cathode to be nitrided. The plasma is powered by means of a d.c. pulsed power
A
__________
Clock Amplifier L.1 Transient rec.
I
IH
1~LE~~ROY __________
[~cornpu~J~ Fig. 1. Schematic diagram of the arrangement of the experimental device: A, anode; C, cathode; P, probe.
©
1993
—
Elsevier Sequoia. All rights reserved
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
inopportune switching from an abnormal glow discharge to an arc regime; the sputtering of the substrate by positive ions, which implies obtaining an equilibrium between the growth and the sputtering of the nitrided layer. Nevertheless, such an ion bombardment plays a significant role in the substrate cleaning and substrate heating. Because of this ion heating, an auxiliary oven is often not needed to achieve the correct sample temperature. To solve—at least partially—these difficulties, two new ways have appeared in the last few years. The first method is the flowing afterglow, in which the substrates are placed outside the discharge. Therefore, they do not meet during ion bombardment and discharge arcing does not have any effect on the pieces themselves. However, they have to be heated independently. Such experiments have shown that active species for iron nitriding were atomic and vibrationally excited states rather than ions [4, 5]. The use of a d.c. pulsed discharge is the second method of avoiding the problems mentioned above. Such a plasma allows heating of the substrate and the sputtering is limited to the discharge duration (i.e. when the power is on), while most of the active species remain in the reactor throughout the voltage period. For such work, different treatments have been performed by varying the frequency from 50 to 1000 Hz. The metallurgical results obtained by Bougdira [6] and Petat [7] have shown that the properties of the nitride layers—analysed by means of X-ray diffraction and hardness measurements—were enhanced by using a frequency of about 100 Hz (Fig. 2). Moreover, such a method could be easily adapted to current industrial equipment. For these reasons, we have chosen the second pro-
83
cedure for our studies on plasma nitriding and we are now working with a d.c. discharge pulsed at 100 Hz, so that both the discharge time and the postdischarge duration are of the order of 5 ms. However, some diagnostic results have been performed under other experimental conditions.
4. OES diagnostics of the plasma Though OES is often limited to relative measurements, it remains a powerful diagnostic tool to identify atomic and molecular species and to follow their time and space variations in plasma-assisted processes. In our pulsed nitriding process, we have observed especially the radiative transitions of the second positive and the first negative bands of nitrogen: N2 (C311U+B3Hg) N~(B22~ —*X22~) respectively. Indeed, these transitions can be related to some metastable states (N 3E.~~); N 2(Ain the creation 2(X,v)) ofofnitrogen that play an important role atomic nitrogen that is the main active species for nitriding [4, 5]. By such an observation, we have shown that the emission line intensities increase with increasing power and rise to a saturation value for a power greater than 150 W (Fig. 3). Thus, the active species also reach a plateau and the power need not increase, because it would not enhance the plasma reactivity. Moreover, increasing the power would enhance the sample sputtering, as can line (Fig. 4). be seen by the observation of an iron Another interesting feature in OES is the possibility of obtaining some information about the excitation processes that occur in the plasma and in the postdis-
50
800
700
S
HVi (Vickers
100
~
40
.—
,~/__•_
80 o7 600
.3Ø~ -.
20 ~ 400
10
300
200
0
0
300 400 500 600 Frequency (Hz) Fig. 2. Effect of plasma voltage frequency on hardness and X-ray diffraction peak of nitrided layers (from ref. 7). iOO
200
/I.
•
p ~
40 0
I
______
0~ 100
/
200
300
400
___________
20
•D
0
DO ~
II,-
o~
_____________
.—
__.— i
0—
power(W) 2E.~—÷X2E~)line intensity (band head at 337.1 nm) against power Fig. 3. N~ (B for different times in the postdischarge, where t=0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz. H, 0 ~is;•, 20 Jis; H, 40 his; ~, 60 ~is; •, 80 ps; LI, 100 I.Ls.
84
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
/
faster electrons power was on. that were created when the discharge
80
5. Electron energy distribution function ~ ~
60 40
As mentioned it appears a good knowledge probe apparatusabove, and some data that analysis software that of the electron energy is necessary to explain the excita-
,
tion processes which occur in the plasma. To obtain such a parameter, we have developed some Langmuir 20 0 0
100
200 300 400 power (W) 5F—~a°D)line intensity (372.0 nm) against power for Fig. 4. Fe (z different times in the postdischarge, where t =0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz. Symbols as in Fig. 3.
charge. Indeed, we have measured emission line intensities far into the postdischarge (up to 2.5 ms after the discharge cut-off). The special case of the iron line (z5F a5D) is of particular interest if we compare its evolution with the electron density decay during the postdischarge. In Fig. 5 it can be seen that both the iron line intensity and the electron density have the same behaviour. This means that: (i) the excitation process of Fe(z5F) is the same in the post discharge as that in the discharge, namely governed by electron collisions; (ii) the secondary electron population remains important in the post discharge, though most of the slow electrons are lost by electron—ion recombinations as soon as the voltage drops to zero. In fact, the remaining slow electrons are issued owing to the thermalization of —~
~Fe
(from which the plasma potential is deduced with an accuracy of about 0.1 V) and its second derivative, which is proportional to the EEDF [9, 10]. The first results we present here deal with the evolution in the postdischarge of the EEDF shape for a plasma running at 1 kHz. The curve reported in Fig. 6 shows that the mean electron energy, calculated from JEJ(E) dE/Jf(E) dE is about 1 eV when the plasma is on and is slightly decreased to 0.5 eV at 490 j.ts into the post discharge, i.e. just before the plasma is switched on again. However, the electron density, calculated from the probe current at the plasma potential, decreases as l/t from its value of 5 x 1011 cm3 in the plasma. This decrease is specific to electron—ion recombination losses [11]. With regard to the evolution of the EEDF shape, Fig. 7 shows a shift of the right-hand part of the EEDF towards the left as one goes along in the postdischarge, while the bulk of the EEDF remains quite constant (in
Ne
~
allow us to obtain at the same time the electron density and temperature (secondary electrons), as well as the shape of the electron energy distribution function (EEDF) [8]. The probe characteristic is time resolved— with a resolution better than 1 j.ts—and is recorded over 256 points. Then, with only one data manipulation, the software leads to a smoothed curve, its first derivative
0.8 0.7
line
.4 —‘—
3 I-
\~
~0.6
L.
~ io~
•~
2
~
~A 0.5 0.4
io~ 0
-
1
0.3 0.5
1.0
1.5 Time (ms)
2.0
2.5
3.0
Fig. 5. Relative evolution of electron density (N,) and iron line intensity (both normalized to I for t = 0) vs. postdischarge time, where t = 0 corresponds to the discharge cut-off. Plasma frequency, 100 Hz.
0
100
200 300 Time (~is)
400
5000
Fig. 6. Electron density (Ne) and mean energy (
R. Hugon et al.
/
Pulsed-plasma-assisted nitriding
the plasma and that active species remain up to 5 ms into the postdischarge. Therefore, it may be possible to
1.0 20~ts
0.6 0.8 0.4
reactivity and to reduce the sample sputtering. Secondly, an improved knowledge of nitrogen metastable with work states different is necessary characteristic to understand times to enhance better the role and the reactivity of these excited states (especially N2(A3~);N2(X,v)). For this, we have now dye laser
200 I.ts 300 ~is
~
85
1.11
0.2
00
1.0
1.5
490ps 2.0 2.5
3.0absorption. 3.5
Energy (eV) Fig. 7. Electron energy distribution function shape at different times in the postdischarge. Plasma frequency, 1 kHz.
normalized values). This is owing to the thermalization of the high energy electrons and explains the presence of slow electrons—which may induce atomic or molecular excitation—several milliseconds after the discharge is switched off.
6. Concluding remarks As described in this paper, plasma diagnostics can easily lead to some insights being obtained into the different processes involved in plasma-assisted surface treatments. A particularly important feature is the pOssibility of following some plasma processes in real time in order to control the process itself. As an example, such control is performed in plasma etching reactors where OES is used to detect the end of the etching [12]. Nevertheless, much work is necessary in order to optimize our pulsed-plasma nitriding process. First, we have to determine the best times for both the discharge and the postdischarge. Some experiments carried out in this way have shown that the plasma stabilization is only reached 2—3 ms after switching on
equipment which will allow us to measure the indensity of Thirdly, these states we need by tomeans improve ofour photoluminescence probe device order or to obtain more information about electrons with more than 2 or 3 eV. Indeed, up to now, we could not measure the tail of the EEDF. A modified experimental set-up would allow us to record the entire probe characteristic over a wide range of probe polarization with a good resolution. References I J. Bougdira, G. Henrion, M. Fabry, M. Remy and J. R. Cussenot, Mater. Sci. Eng., A139 (1991) 15. 2 G. Henrion, M. Fabry, R. Hugon and J. Bougdira, Plasma Sources Sd. Technol., 1(1992)117. 3 H. Michel, Trait. Therm., 179 (1983) 41. P. Colhgnon, Heat Treat. Met., 3 (1982) 67. 4 G. Tibbetts, AppI. Phys., 45 (1974) 5072. 5 A. Ricard, Rev. Phys. Appl., 24 (1989) 251. 6 J. Bougdira, Thesis, University of Nancy, 1990. 7 B. Petat, Thesis, University of Nancy, 1990. 8 G. Henrion, M. Fabry and R. Hugon, in L. Tsendin (ed.), Proc. XI ESCAMPIG, St. Petersburg 25—28 August 1992, Abstracts Vol. 16F, p. 135. 9 K. Wiesemann, Internal Report 87-05-164, August 1987, Institut für Experimentalphysik AGII, Ruhr Universitat, Bochum. 10 L. Schott, in W. Lochte-Holtgreven (ed), Plasma Diagnostics, North-Holland, Amsterdam, 1968, p. 668. ~ J. Bougdira, G. Henrion and M. Fabry, J. Phys. D, 24(1991)1076. 12 I. Hussla, P. Banks, P. Baumann, G. Castrischer, H. Grunwald, G. Lorentz and H. Ramisch, Proc. 1st mt. Conf on Plasma Surface Engineering (PSE’88), Garmisch-Partenkirchen, 19—23 September 1988, DGM, Oberursel, p.495.