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Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production
Surface & Coatings Technology 200 (2005) 649 – 654 www.elsevier.com/locate/surfcoat
Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production T. Godfroid*, J.P. Dauchot, M. Hecq Universite´ de Mons-Hainaut, laboratoire de Chimie Inorganique et Analytique, Baˆtiment Materia-Nova, Parc Initialis, Av. Copernic, B-7000 Mons, Belgium Available online 17 May 2005
Abstract An electromagnetic surface wave launched by a waveguide (type surfaguide [M. Moisan, Z. Zakrzewski, J. Phys., D., Appl. Phys. 24 (1991) 1025]) powered by a microwave generator (2.45 GHz) generates a plasma which produces atomic nitrogen by dissociation of molecular nitrogen. This plasma takes place in a quartz tube where an argon/nitrogen mixture is flowing. The dissociation rate depends directly on the power density injected into the discharge. The microwave power is therefore pulsated in order to be able to work with high instantaneous power while preserving a low mean power. The frequency of pulsation has a great influence on the effectiveness of the dissociation process: an optimal frequency has been be determined which depends on various parameters of the discharge such as power and tube diameter. The optimal conditions of pulsation seems to be due to a thermal effect and more precisely can be correlated with the radial profile of the gas temperature in the discharge. The dissociation fraction of the molecular nitrogen in atomic nitrogen is determined by following the decrease of the molecular nitrogen signal when the plasma is ignited. The latter is measured by mass spectrometry and by optical emission in the post-discharge. In order to understand the thermal effect inducing an optimum pulse frequency, the discharge is also characterized by optical spectroscopy: the change in concentration of species like argon atoms, neutral and ionized molecular nitrogen has been followed as a function of the pulse frequency and the gas discharge temperature measured by means of the rotational bands of N2+. D 2005 Elsevier B.V. All rights reserved. Keywords: Spectroscopy; Atomic nitrogen; Temperature; Microwave plasma
1. Introduction The surface wave plasma used as source of reactive species, as such atomic oxygen or nitrogen, finds utility more and more nowadays. This craze can be explained by the advantageous properties of this type of plasma source in plasma processing. For example functionalization of polymer materials surface [1], the nitriding of metallic parts [2], and also more original processes like the sterilization of medical instruments [3] or the coal purification [4] are achieved using oxygen or atomic nitrogen source. For our part we develop a source of atomic nitrogen the most effective as possible to use it thereafter in combination with a magnetron sputtering system.
* Corresponding author. Tel.: +32 65373851; fax: +32 65373841. E-mail address: [email protected] (T. Godfroid). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.190
Several parameters influence the atomic nitrogen production. For example one promising way to enhance the dissociation of atomic nitrogen is to introduce argon in the discharge. Indeed several publications [5 –7] show that the molecular nitrogen dilution in argon is a determining factor on the dissociation rate and that is preferable to work under high dilution condition. Another way to increase the dissociation rate is to pulsate the microwave power [7], which makes it possible to work with high instantaneous power in the pulse and a low mean power avoiding the destruction of the plasma engine. Despite the number of works and applications on the subject, the effect of the pulse frequency on molecular nitrogen dissociation has not been still considered. From this point of view the goal of this paper is the study of this effect on the atomic nitrogen production, by means of the emission spectroscopy which shows that the molecular nitrogen dissociation is strongly influenced by a suitable choice of the pulse parameters like pulse frequency and microwave power.
650
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
(0.01 nm) is used to determine the discharge temperature from rotational bands distribution of the N2+.
2. Experimental set-up The plasma is generated in a quartz tube (inner diameter 14 mm). A surface wave (2.45 GHz) travels at the interface plasma/quartz giving energy to sustain the discharge. The surface wave is launched by a surfaguide [8] powered by a microwave generator (Muegge Electronic GmbH). The maximum power is 2 kW, and can be pulsed at a frequency varying from 500 Hz to 20 kHz. The discharge is cooled by silicone oil maintained at low temperature (13 -C), flowing between the discharge tube and a surrounding Plexiglas tube. Mass flow controllers control the gas flow and the total flow can be varied from 500 to 2000 sccm. All the system is pumped by a rotary pump, which maintains the pressure between 3 and 36 torr in the discharge (depending on the gas flow) and 1.5 torr in the post discharge. A more detailed description of the experimental set-up can be found in a preceding article [7]. Concerning the spectroscopic measurements the light emitted by the post-discharge is collected by an optical fibre, at a distance of 27 cm below the end of the discharge, and is analyzed by a spectrometer (Avantes S2, grating 1800 Line Holographic LIVE, 440 to 600 N m). Spectroscopic analysis can also take place in the discharge by means of an optical fibre placed at 7 cm above the field applicator and connected either to an optical spectrometer (Avantes S2, grating 1800 line Holographic Vis, 440 to 600 N m or Avantes S4, grating 1200 line Holographic Vis, 700 N m with 1200 N m or Avantes S1, grating 1800 lines Holographic Vis, 300 N m with 460 N m). Another optical spectrometer Jobin-Yvon HR 1000 with a better resolution
3. Results and discussion 3.1. Optimum frequency To study the effect of the pulsation frequency on the atomic nitrogen production, the atomic nitrogen concentration in the post-discharge has been estimated from the intensity of the band at 580.4 nm [9]. The emission intensity at 580.4 nm is in fact linked to the atomic nitrogen concentration in the post-discharge by relationship (1). I580:4
nm
¼ u580:4
2 nm I½N
ð1Þ
where I 580.4 nm is the intensity at 580.4 nm, u 580.4 nm, a factor depending on the spectral response of the spectrometer at the corresponding wavelength and [N] the atomic nitrogen concentration [9]. (I 580.4 nm)1/2 is directly proportional to the atomic nitrogen concentration in the post-discharge. For each experiment the power in the pulse is 1600 W and the mean power is maintained at 340 W by means of the ratio between the pulse duration and the pulse repetition period. Fig. 1 shows the effect of the pulsation frequency on the atomic nitrogen production for a molecular nitrogen flow rate of 50 and 100 sccm and two different total flow rates of 500 and 1750 sccm. The atomic nitrogen concentration grows when the pulse frequency decreases and reaches a maximum value
56 54 52
50 sccm N2/500 sccm total flowrate
50
50 sccm N2/1750 sccm total flowrate
48
100 sccm N2/500 sccm total flowrate
46
2
(IN (580.4 nm))
1/2
44 42 40 38 36 34 32 30 28 26 24 22 20 0
2000
4000
6000
8000
10000
12000
Pulse frequency (Hz) Fig. 1. Influence of the nitrogen partial flow and total flow rate on the optimum frequency position.
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
E Ar+ 3s2 3p5
15,75
1
3p5 3p5 3p5
1
3p5
3p5
1
1
750.38
1
3p
P1 (1s2) P0 (1s3) 3 P1 (1s4) 3 P2 (1s5)
5
3
1
760.3 nm
Ar 3s2 3p6
0
651
A first rough information is obtained by measuring the argon line intensity at 750.3 nm and 760.3 nm as a function of the pulsation frequency. The final state of the transition corresponding these lines are radiative state and metastable state respectively as shown in Fig. 2. As shown in Fig. 3, the argon signals follow exactly the opposite tendency of that obtained for the atomic nitrogen production in the post-discharge. Consequently several assumptions can be formulated to explain the reduction in the argon signals at the optimal frequency. Firstly argon metastable levels 3p5 4p are emptied because metastables are destroyed by excitation of molecular nitrogen in the fundamental state N2(X, v=0) (X corresponding to the electronic state and v to the vibrational state of excitation) according to the reaction: Ar3p5 4s þ N2 ðX; v ¼ 0ÞYAr 3s2 3P6 þ N2 C3 Cu
ð2Þ
Fig. 2. Transition diagram of the argon transition line.
at 750 Hz. Below this optimal frequency the atomic nitrogen concentration decreases. As shown in Fig. 1 the optimum position does not depend on the total flow rate nor on the partial nitrogen flow rate. 3.2. Origin of the optimum In order determine the origin of this optimum, there were made various spectroscopic measurements on the emission lines of the species met in the discharge: neutral argon, ionized molecular nitrogen and neutral molecular nitrogen were particularly interesting from this point of view.
That should be accompanied by an increase in the molecular nitrogen signal, in the second positive system of N2, corresponding to this excited state (N2(C3Cu)). However, the evolution of this molecular nitrogen excited state, for three vibrational states, gives the same results as those obtained for argon (see Fig. 4). A second assumption is that metastable argon does not take part any more in the excitation reactions at the optimal frequency and so the levels 3p5 4s are full. However, this assumption is not satisfactory since it does not explain the reduction in the signal corresponding to a transition towards radiative levels. Consequently the only one explanation for the reduction in the argon signals is that the argon atoms are not any more in the axis of the
3800 3600
I Ar (760.3 nm)
3400
I Ar (750.3 nm)
3200 3000
I (Ar) u.a.
2800 2600 2400 2200 2000 1800 1600 1400 1200
500 sccm Total flowrate 50 sccm N2
1000 800 0
2000
4000
6000
8000
10000
12000
pulse frequency (Hz) Fig. 3. Evolution of the argon signals at 750.3 nm and 760.3 nm in discharge function of the pulse frequency.
652
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
3000
v=3 v=2 v=1 v=0
500 sccm total flowrate 50 sccm N2
2500
1500
IN
2
(C,v)
(a.u)
2000
1000
500
0 0
2000
4000
6000
8000
10000
12000
Pulse frequency (Hz) Fig. 4. Effect of the pulse frequency on the intensity of N2(C, v¶ ) lines.
discharge at the optimal frequency. A possible explanation of such an effect is a radial decrease in temperature from the discharge center to the wall. Indeed, the increase in the temperature of gas in the center of the discharge can give place to the following chain of consequences: yTg jYyN ,Yyð E=N ÞjYyTe jYyne jYyð jE ÞjYyTg The local increase in gas temperature (yT g) involves a reduction in the local density (yN) of neutral species in the center of the discharge and in turn leads to an increase in the E/N ratio (E being the electric field and N the density of neutral species). The increase of this ratio, or in other words the increase of the local electric field, involves a local increase of the electron temperature (yTe) and density (yn e) due to a higher ionization of particle in this region. This leads to an increase in current which then closes the loop since that involves an increase in the temperature of gas. There will be thus a diffusion of the neutral species from the center of the discharge towards the tube wall whereas the charged particles stay in the center of the discharge. To confirm this model temperature measurements, in the center of the discharge, were carried out by spectroscopy using the rotational bands of the N2+ [10]. Fig. 5 shows the change of the temperature in the center of the discharge as a function of the pulsation frequency. These measurements are compared with the change of the argon signal: this one varies in the opposite direction of the temperature. A strong temperature gradient between the discharge center and wall seems to explain the neutral species lowering of density observed.
3.3. Origin of the temperature enhancement Now that the increase in temperature was highlighted, it remains to explain the origin of this increase at the optimal frequency. The solution lies in the mechanism of production of atomic nitrogen. On one hand we showed earlier that the molecular nitrogen dissociation is maximum at the optimal frequency. On the other hand the dissociation can be achieved by two ways. The first is the dissociation by direct electronic impact which is achieved by collision between a nitrogen molecule and an electron: 4 4 e þ N2 Rþ ð3Þ g ; n ¼ 0 Ye þ N S þ N S this reaction leads to the production of atomic nitrogen in the fundamental state N(4S). The second one is called dissociative recombination and is achieved by the recombination between an electron and an ionized nitrogen molecule: 4 4 e þ Nþ ð4Þ 2 YN S þ N S This last reaction releases a significant amount of energy in the form of heat into the plasma. If this reaction is favoured at the optimal frequency it would explain the increase in temperature and the effects observed. In this case the ionized nitrogen concentration should decrease when the atomic nitrogen production is maximum. In Fig. 6 the change of the ionized molecular nitrogen signal at 391.44 nm as a function of the pulse frequency is compared with the atomic production of nitrogen. Once again the signal of N2+ changes in the opposite direction of the atomic nitrogen production. This confirms the assumption of the consump-
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
653
4000
1200
3800
Tg IAr (750.3 nm)
1150
3600
1100
Tg (K)
3200
1000
3000
950
2800 2600
900
IAr (750.3 nm)
3400 1050
2400 850
2200 1750 sccm total flowrate 50 sccm N2
800
2000 1800
750 0
2000
4000
6000
8000
10000
12000
pulse frequency (Hz) Fig. 5. Comparison between the gas temperature in the axis of the discharge and the intensity of argon lines (750.3 nm) function of the pulse frequency.
tion of ionized nitrogen to produce atomic nitrogen by dissociative recombination.
4. Conclusions We have in this work evaluated the possibility of producing atomic nitrogen by molecular dissociation of nitrogen using a surface microwave sustained plasma. If the
effect of pulsation had already been highlighted, the effect of the frequency of the pulse was still vague. Measurements by optical spectrometry, in the post-discharge enabled us to determine an optimal frequency in the production of atomic nitrogen. At the same time the argon line intensity at 760.3 nm, measured in the center of the discharge, falls down at this optimal frequency. This fall of the argon signal is not due to the destruction of these argon excited states by a transfer of excitation
IN2+ (391.44 nm)
40
[N]
3800
38 3600 36 34
3200
30
2
2800
IN + (u.a)
32
3000
28
2600
2
IN (580.4 nm) (cps)
3400
26
2400 500 sccm total flow rate 50 sccm N2
2200
24
2000
22
1800
20 0
2000
4000
6000
8000
10000
12000
pulse frequency (Hz) Fig. 6. Comparison between atomic nitrogen production in post-discharge and the intensity of ionized nitrogen line (391.44 nm) function of the frequency of pulse.
654
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
mechanism to the nitrogen molecules. The signal intensity corresponding to the excited state of the nitrogen molecule shows the same fall at the optimal frequency. So the only way to explain this fall of the argon line intensity is that the argon atoms were left at the center of the discharge by diffusion towards the wall of the discharge tube. This assumption being explained by the appearance of a significant variation of the gradient temperature, between the center of the discharge and the wall of the reactor, at the optimal frequency. This gradient temperature increase can be due to the appearance of an additional reaction of dissociation of atomic nitrogen via the dissociative recombination. Reduction in the ionized nitrogen line intensity, at this frequency, confirms a consumption of the species by dissociative recombination and so validates this last assumption.
Acknowlegdements The authors thank Professor M. Moisan and Doctor Y. Kabouzi for their collaboration in the designing of the
surface wave plasma reactor and for their comments throughout this work. This work is supported by the ‘‘Fonds pour la Recherche en Industrie et en Agriculture’’.
References [1] A. Hollander, J.E. Klemberg-sapeiha, M.R. Wertheimer, Surf. Coat. Technol. 74 – 75 (1995) 55. [2] Enrique Camps, Stephen Mulh, Sau`l Romero, Vacuum 51 (3) (1998) 385. [3] A. Ricard, M. Moisan, S. Moreau, J. Phys. 34 (2001) 1203. [4] P.M. Kanilo, V.I. Kazantsev, N.I. Rasyukc, K. Schu¨nemann, D.M. Vavriv, Fuel 82 (2003) 187. [5] J. Henriques, E. Tatarova, V. Guerra, C.M. Ferreira, Vacuum 69 (1 – 3) (2002 (December 24)) 171. [6] J. Henriques, E. Tatarova, V. Guerra, C.M. Ferreira, J. Appl. Phys. 91 (9) (2002 (May 1)). [7] T. Godfroid, J.-P. Dauchot, M. Hecq, Surf. Coat. Technol. 173 – 174 (2003) 1276. [8] M. Moisan, Z. Zakrzewski, J. Phys. D: Appl. Phys. 24 (1991) 1025. [9] A.-M. Diamy, J.-C. Legrand, A. Moritts, A. Ricard, Surf. Coat. Technol. 112 (1999) 38. [10] Se Youn Moon, W. Choe, Spectrochim. Acta, Part B 58 (2003) 249.
Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production T. Godfroid*, J.P. Dauchot, M. Hecq Universite´ de Mons-Hainaut, laboratoire de Chimie Inorganique et Analytique, Baˆtiment Materia-Nova, Parc Initialis, Av. Copernic, B-7000 Mons, Belgium Available online 17 May 2005
Abstract An electromagnetic surface wave launched by a waveguide (type surfaguide [M. Moisan, Z. Zakrzewski, J. Phys., D., Appl. Phys. 24 (1991) 1025]) powered by a microwave generator (2.45 GHz) generates a plasma which produces atomic nitrogen by dissociation of molecular nitrogen. This plasma takes place in a quartz tube where an argon/nitrogen mixture is flowing. The dissociation rate depends directly on the power density injected into the discharge. The microwave power is therefore pulsated in order to be able to work with high instantaneous power while preserving a low mean power. The frequency of pulsation has a great influence on the effectiveness of the dissociation process: an optimal frequency has been be determined which depends on various parameters of the discharge such as power and tube diameter. The optimal conditions of pulsation seems to be due to a thermal effect and more precisely can be correlated with the radial profile of the gas temperature in the discharge. The dissociation fraction of the molecular nitrogen in atomic nitrogen is determined by following the decrease of the molecular nitrogen signal when the plasma is ignited. The latter is measured by mass spectrometry and by optical emission in the post-discharge. In order to understand the thermal effect inducing an optimum pulse frequency, the discharge is also characterized by optical spectroscopy: the change in concentration of species like argon atoms, neutral and ionized molecular nitrogen has been followed as a function of the pulse frequency and the gas discharge temperature measured by means of the rotational bands of N2+. D 2005 Elsevier B.V. All rights reserved. Keywords: Spectroscopy; Atomic nitrogen; Temperature; Microwave plasma
1. Introduction The surface wave plasma used as source of reactive species, as such atomic oxygen or nitrogen, finds utility more and more nowadays. This craze can be explained by the advantageous properties of this type of plasma source in plasma processing. For example functionalization of polymer materials surface [1], the nitriding of metallic parts [2], and also more original processes like the sterilization of medical instruments [3] or the coal purification [4] are achieved using oxygen or atomic nitrogen source. For our part we develop a source of atomic nitrogen the most effective as possible to use it thereafter in combination with a magnetron sputtering system.
* Corresponding author. Tel.: +32 65373851; fax: +32 65373841. E-mail address: [email protected] (T. Godfroid). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.190
Several parameters influence the atomic nitrogen production. For example one promising way to enhance the dissociation of atomic nitrogen is to introduce argon in the discharge. Indeed several publications [5 –7] show that the molecular nitrogen dilution in argon is a determining factor on the dissociation rate and that is preferable to work under high dilution condition. Another way to increase the dissociation rate is to pulsate the microwave power [7], which makes it possible to work with high instantaneous power in the pulse and a low mean power avoiding the destruction of the plasma engine. Despite the number of works and applications on the subject, the effect of the pulse frequency on molecular nitrogen dissociation has not been still considered. From this point of view the goal of this paper is the study of this effect on the atomic nitrogen production, by means of the emission spectroscopy which shows that the molecular nitrogen dissociation is strongly influenced by a suitable choice of the pulse parameters like pulse frequency and microwave power.
650
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
(0.01 nm) is used to determine the discharge temperature from rotational bands distribution of the N2+.
2. Experimental set-up The plasma is generated in a quartz tube (inner diameter 14 mm). A surface wave (2.45 GHz) travels at the interface plasma/quartz giving energy to sustain the discharge. The surface wave is launched by a surfaguide [8] powered by a microwave generator (Muegge Electronic GmbH). The maximum power is 2 kW, and can be pulsed at a frequency varying from 500 Hz to 20 kHz. The discharge is cooled by silicone oil maintained at low temperature (13 -C), flowing between the discharge tube and a surrounding Plexiglas tube. Mass flow controllers control the gas flow and the total flow can be varied from 500 to 2000 sccm. All the system is pumped by a rotary pump, which maintains the pressure between 3 and 36 torr in the discharge (depending on the gas flow) and 1.5 torr in the post discharge. A more detailed description of the experimental set-up can be found in a preceding article [7]. Concerning the spectroscopic measurements the light emitted by the post-discharge is collected by an optical fibre, at a distance of 27 cm below the end of the discharge, and is analyzed by a spectrometer (Avantes S2, grating 1800 Line Holographic LIVE, 440 to 600 N m). Spectroscopic analysis can also take place in the discharge by means of an optical fibre placed at 7 cm above the field applicator and connected either to an optical spectrometer (Avantes S2, grating 1800 line Holographic Vis, 440 to 600 N m or Avantes S4, grating 1200 line Holographic Vis, 700 N m with 1200 N m or Avantes S1, grating 1800 lines Holographic Vis, 300 N m with 460 N m). Another optical spectrometer Jobin-Yvon HR 1000 with a better resolution
3. Results and discussion 3.1. Optimum frequency To study the effect of the pulsation frequency on the atomic nitrogen production, the atomic nitrogen concentration in the post-discharge has been estimated from the intensity of the band at 580.4 nm [9]. The emission intensity at 580.4 nm is in fact linked to the atomic nitrogen concentration in the post-discharge by relationship (1). I580:4
nm
¼ u580:4
2 nm I½N
ð1Þ
where I 580.4 nm is the intensity at 580.4 nm, u 580.4 nm, a factor depending on the spectral response of the spectrometer at the corresponding wavelength and [N] the atomic nitrogen concentration [9]. (I 580.4 nm)1/2 is directly proportional to the atomic nitrogen concentration in the post-discharge. For each experiment the power in the pulse is 1600 W and the mean power is maintained at 340 W by means of the ratio between the pulse duration and the pulse repetition period. Fig. 1 shows the effect of the pulsation frequency on the atomic nitrogen production for a molecular nitrogen flow rate of 50 and 100 sccm and two different total flow rates of 500 and 1750 sccm. The atomic nitrogen concentration grows when the pulse frequency decreases and reaches a maximum value
56 54 52
50 sccm N2/500 sccm total flowrate
50
50 sccm N2/1750 sccm total flowrate
48
100 sccm N2/500 sccm total flowrate
46
2
(IN (580.4 nm))
1/2
44 42 40 38 36 34 32 30 28 26 24 22 20 0
2000
4000
6000
8000
10000
12000
Pulse frequency (Hz) Fig. 1. Influence of the nitrogen partial flow and total flow rate on the optimum frequency position.
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
E Ar+ 3s2 3p5
15,75
1
3p5 3p5 3p5
1
3p5
3p5
1
1
750.38
1
3p
P1 (1s2) P0 (1s3) 3 P1 (1s4) 3 P2 (1s5)
5
3
1
760.3 nm
Ar 3s2 3p6
0
651
A first rough information is obtained by measuring the argon line intensity at 750.3 nm and 760.3 nm as a function of the pulsation frequency. The final state of the transition corresponding these lines are radiative state and metastable state respectively as shown in Fig. 2. As shown in Fig. 3, the argon signals follow exactly the opposite tendency of that obtained for the atomic nitrogen production in the post-discharge. Consequently several assumptions can be formulated to explain the reduction in the argon signals at the optimal frequency. Firstly argon metastable levels 3p5 4p are emptied because metastables are destroyed by excitation of molecular nitrogen in the fundamental state N2(X, v=0) (X corresponding to the electronic state and v to the vibrational state of excitation) according to the reaction: Ar3p5 4s þ N2 ðX; v ¼ 0ÞYAr 3s2 3P6 þ N2 C3 Cu
ð2Þ
Fig. 2. Transition diagram of the argon transition line.
at 750 Hz. Below this optimal frequency the atomic nitrogen concentration decreases. As shown in Fig. 1 the optimum position does not depend on the total flow rate nor on the partial nitrogen flow rate. 3.2. Origin of the optimum In order determine the origin of this optimum, there were made various spectroscopic measurements on the emission lines of the species met in the discharge: neutral argon, ionized molecular nitrogen and neutral molecular nitrogen were particularly interesting from this point of view.
That should be accompanied by an increase in the molecular nitrogen signal, in the second positive system of N2, corresponding to this excited state (N2(C3Cu)). However, the evolution of this molecular nitrogen excited state, for three vibrational states, gives the same results as those obtained for argon (see Fig. 4). A second assumption is that metastable argon does not take part any more in the excitation reactions at the optimal frequency and so the levels 3p5 4s are full. However, this assumption is not satisfactory since it does not explain the reduction in the signal corresponding to a transition towards radiative levels. Consequently the only one explanation for the reduction in the argon signals is that the argon atoms are not any more in the axis of the
3800 3600
I Ar (760.3 nm)
3400
I Ar (750.3 nm)
3200 3000
I (Ar) u.a.
2800 2600 2400 2200 2000 1800 1600 1400 1200
500 sccm Total flowrate 50 sccm N2
1000 800 0
2000
4000
6000
8000
10000
12000
pulse frequency (Hz) Fig. 3. Evolution of the argon signals at 750.3 nm and 760.3 nm in discharge function of the pulse frequency.
652
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
3000
v=3 v=2 v=1 v=0
500 sccm total flowrate 50 sccm N2
2500
1500
IN
2
(C,v)
(a.u)
2000
1000
500
0 0
2000
4000
6000
8000
10000
12000
Pulse frequency (Hz) Fig. 4. Effect of the pulse frequency on the intensity of N2(C, v¶ ) lines.
discharge at the optimal frequency. A possible explanation of such an effect is a radial decrease in temperature from the discharge center to the wall. Indeed, the increase in the temperature of gas in the center of the discharge can give place to the following chain of consequences: yTg jYyN ,Yyð E=N ÞjYyTe jYyne jYyð jE ÞjYyTg The local increase in gas temperature (yT g) involves a reduction in the local density (yN) of neutral species in the center of the discharge and in turn leads to an increase in the E/N ratio (E being the electric field and N the density of neutral species). The increase of this ratio, or in other words the increase of the local electric field, involves a local increase of the electron temperature (yTe) and density (yn e) due to a higher ionization of particle in this region. This leads to an increase in current which then closes the loop since that involves an increase in the temperature of gas. There will be thus a diffusion of the neutral species from the center of the discharge towards the tube wall whereas the charged particles stay in the center of the discharge. To confirm this model temperature measurements, in the center of the discharge, were carried out by spectroscopy using the rotational bands of the N2+ [10]. Fig. 5 shows the change of the temperature in the center of the discharge as a function of the pulsation frequency. These measurements are compared with the change of the argon signal: this one varies in the opposite direction of the temperature. A strong temperature gradient between the discharge center and wall seems to explain the neutral species lowering of density observed.
3.3. Origin of the temperature enhancement Now that the increase in temperature was highlighted, it remains to explain the origin of this increase at the optimal frequency. The solution lies in the mechanism of production of atomic nitrogen. On one hand we showed earlier that the molecular nitrogen dissociation is maximum at the optimal frequency. On the other hand the dissociation can be achieved by two ways. The first is the dissociation by direct electronic impact which is achieved by collision between a nitrogen molecule and an electron: 4 4 e þ N2 Rþ ð3Þ g ; n ¼ 0 Ye þ N S þ N S this reaction leads to the production of atomic nitrogen in the fundamental state N(4S). The second one is called dissociative recombination and is achieved by the recombination between an electron and an ionized nitrogen molecule: 4 4 e þ Nþ ð4Þ 2 YN S þ N S This last reaction releases a significant amount of energy in the form of heat into the plasma. If this reaction is favoured at the optimal frequency it would explain the increase in temperature and the effects observed. In this case the ionized nitrogen concentration should decrease when the atomic nitrogen production is maximum. In Fig. 6 the change of the ionized molecular nitrogen signal at 391.44 nm as a function of the pulse frequency is compared with the atomic production of nitrogen. Once again the signal of N2+ changes in the opposite direction of the atomic nitrogen production. This confirms the assumption of the consump-
T. Godfroid et al. / Surface & Coatings Technology 200 (2005) 649 – 654
653
4000
1200
3800
Tg IAr (750.3 nm)
1150
3600
1100
Tg (K)
3200
1000
3000
950
2800 2600
900
IAr (750.3 nm)
3400 1050
2400 850
2200 1750 sccm total flowrate 50 sccm N2
800
2000 1800
750 0
2000
4000
6000
8000
10000
12000
pulse frequency (Hz) Fig. 5. Comparison between the gas temperature in the axis of the discharge and the intensity of argon lines (750.3 nm) function of the pulse frequency.
tion of ionized nitrogen to produce atomic nitrogen by dissociative recombination.
4. Conclusions We have in this work evaluated the possibility of producing atomic nitrogen by molecular dissociation of nitrogen using a surface microwave sustained plasma. If the
effect of pulsation had already been highlighted, the effect of the frequency of the pulse was still vague. Measurements by optical spectrometry, in the post-discharge enabled us to determine an optimal frequency in the production of atomic nitrogen. At the same time the argon line intensity at 760.3 nm, measured in the center of the discharge, falls down at this optimal frequency. This fall of the argon signal is not due to the destruction of these argon excited states by a transfer of excitation
IN2+ (391.44 nm)
40
[N]
3800
38 3600 36 34
3200
30
2
2800
IN + (u.a)
32
3000
28
2600
2
IN (580.4 nm) (cps)
3400
26
2400 500 sccm total flow rate 50 sccm N2
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mechanism to the nitrogen molecules. The signal intensity corresponding to the excited state of the nitrogen molecule shows the same fall at the optimal frequency. So the only way to explain this fall of the argon line intensity is that the argon atoms were left at the center of the discharge by diffusion towards the wall of the discharge tube. This assumption being explained by the appearance of a significant variation of the gradient temperature, between the center of the discharge and the wall of the reactor, at the optimal frequency. This gradient temperature increase can be due to the appearance of an additional reaction of dissociation of atomic nitrogen via the dissociative recombination. Reduction in the ionized nitrogen line intensity, at this frequency, confirms a consumption of the species by dissociative recombination and so validates this last assumption.
Acknowlegdements The authors thank Professor M. Moisan and Doctor Y. Kabouzi for their collaboration in the designing of the
surface wave plasma reactor and for their comments throughout this work. This work is supported by the ‘‘Fonds pour la Recherche en Industrie et en Agriculture’’.
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