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Emission spectroscopy of Ar–H2 plasma
Vacuum 61 (2001) 409}412
Emission spectroscopy of Ar}H plasma V. Monna*, A. Ricard CPAT, University Paul Sabatier, CNRS, 118 rte de Narbonne, 31062, Toulouse cedex 4, France
Abstract The relative density of H atoms in Ar}H microwave glow discharge is determined from intensity of H, H and Ar radiative species. It is shown that a maximum of H atom density is obtained for 5% H }95% Ar. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microwave discharges; Plasma kinetics; Plasma spectroscopy; Hydrogen dissociation
1. Introduction Production of excited species in H plasmas is considered for surface treatments such as diamond "lm deposition [1] and more generally for etching of contaminants on material surfaces. Detection of radiative species of H plasmas by emission spec troscopy allows one to determine the H/H dis sociation rates by comparing the line intensity of H and Ar atoms where a few percent of argon gas is introduced into H (actinometry method) [1]. Here it is described how to follow the H/H dissociation rate in Ar}H plasmas from emission spectroscopy by taking into account the main kinetic reactions.
2. Experimental set-up As reproduced in Fig. 1, the microwave discharge studied in this work is incorporated in a #owing
* Corresponding author. E-mail address: [email protected] (V. Monna).
post-discharge reactor (reactor at about 60 cm from the plasma) for metal surface cleaning [2]. The plasma is produced by a microwave (surfatron) cavity, in a 5 mm diameter quartz tube. The gas #ow rates vary between 0.1 and 3 Nl/min (normal litre per minute, NTP conditions), with gas pressure between 0.1 and 10 mbar. A spectrometer (Jobin Yvon, SPEX270 M) with a CCD detector allows us to analyse the plasma radiative species near the surfatron gap.
3. Observed emission spectra Several lines and bands in Ar}H spectra, have been chosen: 4 triplets of H (a>Qd ) between 601 and 633 nm as shown in Fig. 2, 5 lines of H (>Q>) between 462 and 464 nm, 1 line of Ar (4sQ4p) at 750.4 nm and 1 line of H, H (n"2Qn"3) at 656.3 nm. ?
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 1 3 7 - 3
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V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
Fig. 1. Experimental set-up: (1) computerised detection system (CCD) and spectrometer (optical emission), (2) optical "bre, (3) post-discharge reactor, (4) pump, (5) pressure gauge (10\}10 mbar), (6) microwave plasma, (7) surfatron, (8) microwave supply (2450 MHz, 300 W), (9) hygrometer, (10) bubbler, (11) #owmeter, and (12) gas supply.
4. Kinetic study
The following intensity ratios result:
In a low-pressure Ar}H glow discharge, the Ar, H and H radiative species are mainly produced by electron collisions as follows:
I H [H ] C & J , [Ar] C I H
(4)
(a) e#ArPe#ArH,
I H [H ] C #[H] C . & J [Ar] C I H
(5)
(b) e#H Pe#HH,
Taking into account the quantity of H in the Ar}H mixture, that is x"[H ]/M, M"[Ar]# [H ], it follows: [H ]"xM and [Ar]"(1!x)M. Then for H molecules, we obtain a simpli"ed equation:
(c) e#H Pe#H#HH, (d) e#HPe#HH. The spectral line intensities can be related to the densities of species by the following equations: n [H ] C I H J[HH]" H , & & I H J[HH]"n & I
H
[H ] C #[H] C H , &
n [Ar] C , J[ArH]" H
(1)
I H x Eq. (4)N & J . I H 1!x
(6)
For H atoms, we consider two cases: (2)
I H x (1) [H ] C '[H] CN & J , I H 1!x
(3)
I H [H] (2) [H ] C ([H] CN & J N I H 1!x
where the brackets are for the species density, n the electronic density, C the electron excitation rates and the loss frequency.
I H [H](1!x) & . I H
(7)
(8)
V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
411
Fig. 2. 50% Ar}50% H spectrum, 0.5 mbar, 0.3 Nl/min, 50 W.
Fig. 3. H /Ar intensity ratio versus H quantity in Ar}H mixture, 0.5 mbar, 0.3 Nl/min, 70 W ((}䊐}) H (462}464 nm)/Ar, (}䊏}) H (601}633 nm)/Ar, (*) x/(1!x)).
Fig. 4. H /Ar intensity ratio versus H quantity in Ar}H mixture, 0.5 mbar, 0.3 Nl/min, 70 W ((}䊏}) H /Ar, (*) x/(1!x)). ?
5. Experimental results
H (> Q> ). The experimental results of T T I /I intensity variations versus x are reproduced & in Fig. 3. It is clear that, for the observed H bands, I /I follows x/(1!x) as given by Eq. (7). & For H atoms as shown in Fig. 4, the "rst case (Eq. (7)) appears to be obtained for higher percentages of H in Ar}H (x'0.7). The excess of I /I &
By observing the emission spectra, we can determine the line intensities. We sum all the rotational and vibrational transition lines of "x H spectrum (Fig. 2) to obtain the total intensity of H (aQd) and also all the rotational lines of
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V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
6. Conclusion With these kinetic study and spectroscopy measurements, we have selected the dominant reactions in the dissociation of H in an Ar}H micro wave plasma. It is concluded that, if a majority of H radiative atoms comes from the dissociation of H molecules when H contributes more than 50% in the Ar}H mixture it is not the case for lower H percentages where the direct excitation of H atoms appears to be the most relevant reaction. Work is in progress to determine the H atom density in the post-discharge reactor by using Ni catalytic probes [3].
Fig. 5. (1!x) I /I versus H quantity in Ar}H , 0.5 mbar, & 0.3 Nl/mn, 50 W.
as observed at low H percentages (x(0.5) in Fig. 4 is the result of case 2 (Eq. (8)). Variation of (1!x)I /I versus x is reproduced in Fig. 5 indic& ating a maximum of H atom density for 5% of H .
References [1] Hassouni K, Farhat S, Scott CD, Gicquel A. J Phys III 1996;6:1229. [2] Gaillard M, Raynaud P, Ricard A. Plasmas Polym 1999;4:243. [3] Mozetic M, Drobnic M. Vacuum 1998;50:319.
Emission spectroscopy of Ar}H plasma V. Monna*, A. Ricard CPAT, University Paul Sabatier, CNRS, 118 rte de Narbonne, 31062, Toulouse cedex 4, France
Abstract The relative density of H atoms in Ar}H microwave glow discharge is determined from intensity of H, H and Ar radiative species. It is shown that a maximum of H atom density is obtained for 5% H }95% Ar. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Microwave discharges; Plasma kinetics; Plasma spectroscopy; Hydrogen dissociation
1. Introduction Production of excited species in H plasmas is considered for surface treatments such as diamond "lm deposition [1] and more generally for etching of contaminants on material surfaces. Detection of radiative species of H plasmas by emission spec troscopy allows one to determine the H/H dis sociation rates by comparing the line intensity of H and Ar atoms where a few percent of argon gas is introduced into H (actinometry method) [1]. Here it is described how to follow the H/H dissociation rate in Ar}H plasmas from emission spectroscopy by taking into account the main kinetic reactions.
2. Experimental set-up As reproduced in Fig. 1, the microwave discharge studied in this work is incorporated in a #owing
* Corresponding author. E-mail address: [email protected] (V. Monna).
post-discharge reactor (reactor at about 60 cm from the plasma) for metal surface cleaning [2]. The plasma is produced by a microwave (surfatron) cavity, in a 5 mm diameter quartz tube. The gas #ow rates vary between 0.1 and 3 Nl/min (normal litre per minute, NTP conditions), with gas pressure between 0.1 and 10 mbar. A spectrometer (Jobin Yvon, SPEX270 M) with a CCD detector allows us to analyse the plasma radiative species near the surfatron gap.
3. Observed emission spectra Several lines and bands in Ar}H spectra, have been chosen: 4 triplets of H (a>Qd ) between 601 and 633 nm as shown in Fig. 2, 5 lines of H (>Q>) between 462 and 464 nm, 1 line of Ar (4sQ4p) at 750.4 nm and 1 line of H, H (n"2Qn"3) at 656.3 nm. ?
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 1 3 7 - 3
410
V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
Fig. 1. Experimental set-up: (1) computerised detection system (CCD) and spectrometer (optical emission), (2) optical "bre, (3) post-discharge reactor, (4) pump, (5) pressure gauge (10\}10 mbar), (6) microwave plasma, (7) surfatron, (8) microwave supply (2450 MHz, 300 W), (9) hygrometer, (10) bubbler, (11) #owmeter, and (12) gas supply.
4. Kinetic study
The following intensity ratios result:
In a low-pressure Ar}H glow discharge, the Ar, H and H radiative species are mainly produced by electron collisions as follows:
I H [H ] C & J , [Ar] C I H
(4)
(a) e#ArPe#ArH,
I H [H ] C #[H] C . & J [Ar] C I H
(5)
(b) e#H Pe#HH,
Taking into account the quantity of H in the Ar}H mixture, that is x"[H ]/M, M"[Ar]# [H ], it follows: [H ]"xM and [Ar]"(1!x)M. Then for H molecules, we obtain a simpli"ed equation:
(c) e#H Pe#H#HH, (d) e#HPe#HH. The spectral line intensities can be related to the densities of species by the following equations: n [H ] C I H J[HH]" H , & & I H J[HH]"n & I
H
[H ] C #[H] C H , &
n [Ar] C , J[ArH]" H
(1)
I H x Eq. (4)N & J . I H 1!x
(6)
For H atoms, we consider two cases: (2)
I H x (1) [H ] C '[H] CN & J , I H 1!x
(3)
I H [H] (2) [H ] C ([H] CN & J N I H 1!x
where the brackets are for the species density, n the electronic density, C the electron excitation rates and the loss frequency.
I H [H](1!x) & . I H
(7)
(8)
V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
411
Fig. 2. 50% Ar}50% H spectrum, 0.5 mbar, 0.3 Nl/min, 50 W.
Fig. 3. H /Ar intensity ratio versus H quantity in Ar}H mixture, 0.5 mbar, 0.3 Nl/min, 70 W ((}䊐}) H (462}464 nm)/Ar, (}䊏}) H (601}633 nm)/Ar, (*) x/(1!x)).
Fig. 4. H /Ar intensity ratio versus H quantity in Ar}H mixture, 0.5 mbar, 0.3 Nl/min, 70 W ((}䊏}) H /Ar, (*) x/(1!x)). ?
5. Experimental results
H (> Q> ). The experimental results of T T I /I intensity variations versus x are reproduced & in Fig. 3. It is clear that, for the observed H bands, I /I follows x/(1!x) as given by Eq. (7). & For H atoms as shown in Fig. 4, the "rst case (Eq. (7)) appears to be obtained for higher percentages of H in Ar}H (x'0.7). The excess of I /I &
By observing the emission spectra, we can determine the line intensities. We sum all the rotational and vibrational transition lines of "x H spectrum (Fig. 2) to obtain the total intensity of H (aQd) and also all the rotational lines of
412
V. Monna, A. Ricard / Vacuum 61 (2001) 409}412
6. Conclusion With these kinetic study and spectroscopy measurements, we have selected the dominant reactions in the dissociation of H in an Ar}H micro wave plasma. It is concluded that, if a majority of H radiative atoms comes from the dissociation of H molecules when H contributes more than 50% in the Ar}H mixture it is not the case for lower H percentages where the direct excitation of H atoms appears to be the most relevant reaction. Work is in progress to determine the H atom density in the post-discharge reactor by using Ni catalytic probes [3].
Fig. 5. (1!x) I /I versus H quantity in Ar}H , 0.5 mbar, & 0.3 Nl/mn, 50 W.
as observed at low H percentages (x(0.5) in Fig. 4 is the result of case 2 (Eq. (8)). Variation of (1!x)I /I versus x is reproduced in Fig. 5 indic& ating a maximum of H atom density for 5% of H .
References [1] Hassouni K, Farhat S, Scott CD, Gicquel A. J Phys III 1996;6:1229. [2] Gaillard M, Raynaud P, Ricard A. Plasmas Polym 1999;4:243. [3] Mozetic M, Drobnic M. Vacuum 1998;50:319.