1. National Institute of Lasers, Plasma and Radiation Physics Romania
Metal Surface Treatments using a Nitrogen - Methane Plasma L.C. Ciobotaru and D.S. Popa
National Institute of Lasers, Plasma and Radiation
Physics
Romania

2. INTRODUCTION
In the previous papers strong changes (up to 6 times), in the N2(C) population were reported to appear in nitrogen D.C. flowing discharges when small percentage of hydrogen varying between ( ) were added. This behaviour could be explained by that the presence of a small hydrogen quantity which reduced the ionization process in nitrogen. The present work is divided in 3 parts:
The identification of active species
The identification of the main reaction products
The influence of wall material over the active species concentration

3. A schematic view of the experimental set up is presented in Fig. 1.
The discharge was produced in a Pyrex tube with 22 mm inside diameter , between two identical Ni/Cr electrodes spaced at 400 mm distance. The first electrode, in the sense of flowing gas, was the anode. Gas flows were regulated and measured by mass flow-meters with a measurement range up to 1500 sccm (1 sccm=1 cm3/ min in standard conditions), and the pressure was measured by a thermal gauge.
The after-glow discharge zone had two distinct parts: one part with the same diameter as the discharge tube and the total length of 600 mm and the other part with the inside diameter of 80 mm and the total length of 450 mm.
The second part was necessary to study the impact of the transport conditions and the walls nature on the active species. Spectral data were obtained within the after-glow discharge zone, and they were recorded by means of an optical integrated system consisting of an optical quartz fiber (OF), Optical Multi-Channel Analyzer, (OMA), and a computer.
The spectral range of the system varied between (200 – 900) nm, with the resolution of 0.05nm.

4. FIG. 1
D1, D2 – flow-meters: V1, V2, V3 – valves;
S- spectrometer; PM – photomultiplier; OMA (optical multi-channel analyzer, OP – optical fiber; G-thermal gauge

5. Total pressure of the gas mixture: 1 – 3 Torr
The experimental conditions for the D.C. nitrogen-methane flowing discharge were the following:
Total pressure of the gas mixture: 1 – 3 Torr
Nitrogen flow rate : 600 – 1000 sccm
Methane ratio to nitrogen: up to 0.2 (%)
Discharge current intensity : 50 – 100 mA
The experimental conditions for the D.C. nitrogen-methane flowing discharge were the following:
3. Methane ratio to nitrogen: up to 0.2 (%)( PER CENT)
2. Nitrogen flow rate : 600 – 1000 sccm
1. Total pressure of the gas mixture: 1 – 3 Torr
Discharge current intensity : 50 – 100 mA

6. FIG. 2 The N2 – CH4 flowing discharge
An over-view photo-image of the discharge zone is presented in Fig. 2.
FIG. 2 The N2 – CH4 flowing discharge
zone photo-image

7. The identification of active species
There were performed researches for the identification of the active species, reaction products and reaction mechanisms in nitrogen-methane plasma D.C. discharge. The main radiative species identified in this discharge were N2 ( B ) and N2 ( C ), which are emitting the spectral systems 1+ and, respectively 2+.
Generally, the nitrogen active species are the radiative species N2 ( B ), N2 ( C ) and N2+ ( B ) which belong to the N2 neutral molecule respectively to the N2+ molecular ion as well as the vibrational excited molecular nitrogen, N2 ( A ) metastable and the atoms of nitrogen.
There were performed researches for the identification (aidentificheişăn) of the active species, reaction products and reaction mechanisms in nitrogen-methane plasma discharge.
However, the main radiative species identified (aidentifaid) in this discharge were N2 ( B ) and N2 ( C ), which are emitting the spectral systems 1+ si 2+.

8. The 1+ spectral system is emitted in the nitrogen post-discharge because of two main reactions:
N+ N+ N2→N2(B,v)+N (1)
(re-association of atomic nitrogen reaction)
N2 ( B, v’=11 ) → N2 ( A,v”=7 ) + hν (1+) (2)
(de-excitation reaction of the vibrational state B of the neutral nitrogen molecule)
The most intense band from the 1+ spectral system is (11-7) with the band-head nm

9. Fig. 1 The spectral system 1+ in a pure molecular nitrogen discharge.
Modification of the 1+ spectrum intensity at the addition of 0.1% methane in pure nitrogen plasma
Fig. 1 The spectral system 1+ in a pure molecular nitrogen discharge.
Fig.2 Spectrum of a nitrogen - 0.1% methane gas mixture discharge. .
In this last case, there is a significant decrease of the 1+ system intensity, namely of the active species population N2 (B).
The figures 1 and 2 present the 1 + spectrum of the D.C. flowing plasma discharge in pure nitrogen comparative with the spectrum in nitrogen mixed with 0.1% (per cent) methane .

10. N2 ( A ) + N2 ( A ) → N2 ( B ) + N2 ( X ) (5)
There is a significant decrease of the 1+ spectral system intensity, namely of the active species population N2 (B).
This behavior was due to the extinction of the metastable N2 ( A) by the excitation transfer reaction with the methane molecule
N2 ( A ) + CH4 → N2 ( X ) + CH (4)
Consequently, the main generation mechanism of the N2 (B) species becomes inefficient:
N2 ( A ) + N2 ( A ) → N2 ( B ) + N2 ( X ) (5)
The quenching reaction of the metastable N2 ( A) with the methane molecule that leads to the N2 (A) metastable extinction by the excitation transfer reaction:
The explanation consists in the existence of the two following reactions:
N2 ( A ) + CH4 → N2 ( X ) + CH4
N2 ( A ) + N2 ( A ) → N2 ( B ) + N2 ( X )
This reaction had as consequence the fact that the N2 (B) species mechanism of generation became inefficient:

11. Fig. 3 Concentration dependence of N2(B) vs. CH4
Therefore, the N2 ( B ) concentration is directly proportional to the 1+ spectral system intensity, according to the radiative transition by which this system was emitted.
Therefore the N2 ( B ) concentration is directly proportional to the 1+ spectral system intensity, according to the radiative transition by which this system was emitted as it can see in the figure 3.

12. The second positive nitrogen system is emitted during the radiative decay of the N2(C) species :
N2(C) → N2(B) + hν (2+) (5)
which are generated by the electronic excitation of the neutral nitrogen molecule:
e + N2(X) → e + N2(C) (6) 
The intensity of the second positive system is proportional to the N2(C) population density being a convenient indicator of any modification in the density of this population occurring in the discharge.

13. Explanation:
Resulted hydrogen from methane dissociation in the electrical discharge → changes in the N2 population density by a reduction of ionization . Consequence:
-The electrical field in the discharge must increase, in order to maintain a constant discharge current ( ) mA.
-A larger electric field produces more N2(C) species (by the electronic excitation ).
- The intensity of the second system, especially the 0-0 transition (λ= nm) had a sensitive increase when the methane concentration raised, as shown in Fig. 4.

14. Fig. 4. Intensity of the 2-th positive system versus methane concentration

15. The identification of the main reaction products
In the N2- CH4 gas mixture discharge, the de-composition of the methane molecule takes place due to
the breaking of C – H chemical bonds. The main plasmo-chemical reactions from the discharge are the following:
Dissociation by electronic collisions
Dissociation by excited molecular nitrogen collisions
Reactions of radicals with nitrogen atoms
The principal reaction products obtained in these reactions are the CHx radicals and the NH/CN species.

16. CN ( B ) → CN ( X ) + hν (388 nm) ; Δv = 0 (8)
The emission band of the NH species, placed near by
λ = 336 nm, was not observed due to the great intensity of the nitrogen spectral line λ = 337 nm which belongs to the 2+ spectral system, situated in its very proximity.
I t was recorded the violet emission of the CN radical with the most intense band CN(B,7-X,7), λ = 388nm, as a result of the recombination process of the carbon atoms with atomic/molecular nitrogen:
C +N +N2 → CN (B,7) +N (7)
CN ( B ) → CN ( X ) + hν (388 nm) ; Δv = 0 (8)
It was recorded the violet emission of the CN radical with the most intense band CN(B,7-X,7) which is the result of the recombination process between the carbon atoms, atomic and molecular nitrogen, followed by its radiative de-excitation process on the fundamental state as shown in the figure 5.

17. λ =388 nm
The transition Δv = 0, with the band-head λ = 388 nm, was significantly intense.
Fig. 5 CN(B -X) emission spectrum of the nitrogen-methane DC discharge.

18. The CN radical emission band intensity Δv = 0 (therefore the CN radical population) increases with the increase of the methane concentration.
This dependence was a consequence of the CN radical generation reactions -eqs 8 and 9.
CH2 + N → CN + H2 (9)
CH + N → CN + H (10)
The hydrocarbon radicals concentrations CH2 and CH were directly proportional to the methane concentration – fig.6.
The hydrocarbon radicals concentrations CH2 and CH were directly proportional to the methane concentration.

19. Fig. 6 Concentration dependence of CN radical vs. CH4
The CN radical emission band intensity Δv = 0 (therefore the CN radical population) increases with the increase of the methane concentration

20. The radicals CHx, where x=1 and 3, are reacting with the nitrogen atoms by two main processes:
N+CH3 → HCN +H2 (11)
N+CH → CN+H (12)
The radical CH2 were not interacting with the nitrogen atoms so their emission was not observed.
The radicals CN from the eq. (12) are quickly destroyed trough the reaction:
N+CN → C+N2
Finally , the only important remains the recombination reaction (eq. 6).

21. The influence of wall material over the active species concentration
In the present work was studied the de-excitation process of the atomic nitrogen at the wall, using the optical emission spectroscopy method . The spectral emission of a far off after-glow discharge was investigated by placing the samples used for plasma treatment at a certain distance (more than 600 mm) from the end of the discharge tube.
The present paper studied the de-excitation process of the atomic nitrogen at the wall, using the optical emission spectroscopy method.
The spectral emission of a far off after-glow discharge was investigated by placing the samples used for plasma treatment at a certain distance (more than 600 mm) from the end of the discharge tube. The inside of the discharge tube was entirely covered with a leaflet, having a length of 200 mm.

22. In order to compare the de-excitation function on different wall materials, is defined the ratio:
RM = I11-7 M /I11-7 Px (12)
where I11-7 M si I11-7 Px are the intensities of the bands (11-7) from the 1+ spectral system with, and respectively without the leaflet cover placed inside the discharge zone of the tube.
where these notations I11-7 M and I11-7 Px correspond to the intensities of the bands (11-7) with, and respectively without the leaflet cover placed inside the discharge tube.
RM = I11-7 M /I11-7 Px
Within the 1+ system spectrum, the most intense recorded bands were (11-7). In order to compare the de-excitation function on different wall materials, were calculated the ratio, for different materials:

23. Table 1 The ratio RM for different wall materials
Teflon
Plastics
Aluminum
Copper
Q (cm3s-1)
Δt(sec)
2.5
10-1
1.02
0.41
0.2 5
5 x 10-2 1
0.43
7.5
3.3 x 10-2
1.04
0.55
0.23 10
2.5 x 10-2
1.15
0.45
As it can see from the data of the Table 1 the interaction between the nitrogen atoms and the teflon was the same as the interaction with the Pyrex glass. On the contrary, after the interaction with plastics, aluminum (eliuminiặm) and copper, the nitrogen atoms disappeared.

24. The data of the Table 1reveal:
- the interaction between the nitrogen atoms and the teflon was the same as the interaction with the Pyrex glass - after the interaction with plastics, aluminum and copper the nitrogen atoms disappeared.
The nitrogen atoms generated in the discharge by the nitrogen molecule dissociation process disappeared in the after-glow discharge zone because of the two quenching mechanisms, namely;

25. The recombination process in gaseous homogeneous phase:
N + N + N2 → N2 ( B ) + N2 ( X )
(reaction rate: cm3s-1 [7] ) (13)
The wall recombination process:
N + wall → ½ N2 + wall (14)
2. The wall recombination process .
1. The recombination process in gaseous (gheisiặs) homogeneous (homouginiặs) phase (feiz) and
The nitrogen atoms generated in the discharge by the nitrogen molecule dissociation process disappeared in the after-glow discharge zone because of the two quenching mechanisms, namely;

26. γ - the recombination probability at the wall
Kp = the reaction coefficient of the wall recombination process
(13)
γ - the recombination probability at the wall
v - the nitrogen atom thermal speed (v=500m/s at T= 300 K- room temperature)
R - the discharge tube radius
For γ = 2 x10-5 ( for Pyrex glass) →
kp = 5 x s-1
In this case we obtained (for the Pyrex glass) the reaction coefficient Kp= 5x10-2 s-1 (five multiplied by ten minus two per second).
The most used value for the recombination probability at the wall made of Pyrex glass is two multiplied by ten minus five.
The reaction coefficient Kp of this process is done by the relation thirteen where γ is the recombination probability at the wall of the nitrogen atoms, v is the thermal speed and R is the radius (reidiặs) of the discharge tube.

27. d[N]/dt = -kN[N]2[N2]–kp[N] (15) ► 1/[N]z = 1/[N]zo+kN[N2]/kp (16)
The temporal variation of the nitrogen atoms density is described by the equation:
d[N]/dt = -kN[N]2[N2]–kp[N] (15)
After integration in the given experimental conditions of the discharge :
Total pressure 1-3 Torr;
Residence time values similar with the Table 1
The molecular nitrogen density - [ N2 ] ~1017 cm-3
► /[N]z = 1/[N]zo+kN[N2]/kp (16)
RM = expo (-2 kp z / v)
The temporal variation of the nitrogen atoms density is described by the equation forteen.
After integration in the given experimental conditions of the discharge, namely :
The molecular nitrogen density around ten at seventeen power per cubic sentimitar - [ N2 ] ~10 17 cm-3- calculated for the given total pressure
Residence time values from the Table 1
Total pressure 1-3 Torr;
By using the equation 15 and the RM values from the table one we can now calculate the recombination probability at the wall of the nitrogen atoms for different materials.
The equation (14) can be aproximated by a simplier form as (urmeaza ec. 15).

28. Calculated values for the recombination probabilities:
γplastics = (3±1)
γAl = ( 6 ± 3)
Published values:
γAl = ( 6.5 ± 2.5)
γAl2O3 = ( 1.6 ± 0.4)
Conclusion: The determined recombination probability was corresponding to the aluminum existence and not to the aluminum oxide layer. The same calculations lead to the fact that recombination probability are corresponding to the copper not copper-oxide.

29. D.C flowing discharge at low pressure in a nitrogen-methane plasma
CONCLUSION
D.C flowing discharge at low pressure in a nitrogen-methane plasma
Notable diminution of the N2(B) population due to the quenching reaction of the metastable N2(A) with the methane molecule.
Notable increase of the N2(C) population due to the associative ionization reduction.
Notable increase of the CN radical population with the methane concentration.
Studies of the nitrogen interaction with the different wall materials reveal the fact the for the Teflon the interaction is the same than in the case of Pyrex glass, the interaction with Plastics, Aluminum and Copper lead to a faster loss of the nitrogen atoms and for the Copper foil the nitrogen atoms are totally inactivated.

30. References: [1]. J. Loureiro, Chem. Phys. 157 (1991)
[2]. J. Looureiro, A. Ricard, J. Phys. B: At. Mol.Opt. Phys., 25 (1992)
Popa S.D.; Journal of Phys D – Appl Phys, Vol. 29, 1996,  
[3.] Popa S.D., Hochard L, Ricard A; Journal Phys III France, Vol. 7, 1997
[4] Popa S.D., Chiru P, Ciobotaru L; Journal Phys D – Appl Phys, Vol. 31, 1998
[5]. Sa P.A., Guerra V, Loureiro J & all. ; Journal of
Phys. D- Appl. Phys., Vol. 37(2), 2004

31. The dissociation of a nitrogen molecule in atomic nitrogen is produced, in the given experimental condition , essentially by V-V (vibration-vibration) transfer, as a transition from the last bond level on a pseudo-level situated in continuum zone , by a collision with a vibrational excited molecule as follows:
N2(X,v) +N2(X,v=45) → N2(X,v-1) +N+N
The presence of the hydrogen, even in a very small quantity produces a strong de-activation of the vibrational high levels because of the transfer V-T (vibration-translation) between molecular nitrogen and molecular hydrogen, which becomes dominant for the vibration big numbers :
H2 + N2(X,v) → H2 + N2(X,v-1)
The result is a strong diminution of the dissociation rate of the molecular nitrogen with the hydrogen concentration.