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A study of neon–nitrogen interactions in d.c. glow discharges by optical emission spectroscopy
Thin Solid Films 398 – 399 (2001) 507–512
A study of neon–nitrogen interactions in d.c. glow discharges by optical emission spectroscopy J.C. Avelar-Batista, A.D. Wilson, A. Davison, A. Leyland, A. Matthews, K.S. Fancey* Research Centre in Surface Engineering, Department of Engineering, University of Hull, Hull HU6 7RX, UK
Abstract Neon–nitrogen d.c. diode glow discharges have been investigated through the use of spatially resolved optical emission spectroscopy. This technique has enabled the discharges to be sampled from inside the cathode sheath and plasma regions. All discharges were operated at –2 kV cathode bias and 6 Pa total pressure. By evaluating spectral line intensity ratios (SLIRs) that incorporate ion species, the principal findings are as follows: first, the cathode current density at low nitrogen partial pressures is boosted by Penning ionisation of nitrogen by neon; evidence of this mechanism is provided by maxima in nitrogen ion-based SLIRs and minima in neon ion-based SLIRs occurring at low nitrogen concentrations. Second, although the optimum nitrogen 0 partial pressure for Penning ionisation appears to be approximately 5%, the maxima in cathode current density and Nq 2 yN2 SLIRs occur at 10–15%; we speculate that a dissociative Penning ionisation mechanism is predominant at 5% but conventional (nondissociative) Penning ionisation, with possible contributions from other mechanisms (such as electron impact ionisation of nitrogen) become significant at 10–15% nitrogen concentration. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen; Optical emission spectroscopy; Penning ionisation
1. Introduction Although plasma-based surface treatment and coating deposition systems commonly use inert gas-based discharges, considerations of cost combined with useable characteristics tend to limit selection of the inert gas to argon. The need for such a gas to perform as little more than a buffer agent appears to be rarely considered. Nevertheless, the presence of argon in a discharge can influence charge exchange and dissociation behaviour with commonly used process gases such as nitrogen w1x. It has been known for many years that metal vapour, present as a minority species in an argon-based discharge, can be significantly ionised by argon metastables (Penning ionisation) w2x; the effect also occurs in neonbased discharges w3x. Of particular interest however, is that neon can also Penning ionise nitrogen: preliminary investigations through the deposition of TiN coatings by * Corresponding author. Tel.: q44-1482-465071; fax: q44-1482466664. E-mail address: [email protected] (K.S. Fancey).
reactive ion plating have suggested that neon can enhance nitrogen reactivity w4x. In a previous paper w5x, d.c. diode discharges of neon–nitrogen and argon–nitrogen mixtures were investigated by monitoring the process parameters in conjunction with spatially-resolved optical emission spectroscopy (OES). This previous work was focused on the study of mechanisms resulting in the dissociation of nitrogen molecules by evaluating the N0 yN02 spectral line intensity ratios (SLIRs) from OES data. In the present work, we utilise SLIRs that incorporate ion species from our OES data in order to understand further the ionisation and dissociation mechanisms occurring in neon–nitrogen discharges. 2. Background Neon has metastable states (Ne*) at 16.62 and 16.67 eV. Thus, as the ionisation potential of nitrogen is 14.53 eV, nitrogen molecules in a neon plasma can be Penning ionised:
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 4 3 8 - 9
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y NeUqN02™Ne0qNq 2 qe
(1)
If the nitrogen molecules are a minority species compared with the neon, a Penning mixture will be formed, in which substantial ionisation of the nitrogen can be expected. Previous work w5x revealed indirect evidence of Reaction 1 being a significant mechanism at low nitrogen partial pressures through measurement of the cathode current density, cathode sheath thickness and, from OES data, nitrogen dissociation in the cathode sheath. The principal mechanism for nitrogen molecules to dissociate in the cathode sheath is considered to be w1,6x: 0 qn 0 Nq 2 Žfast. qN2™N2 Žslow. qN2
and q 0 Nqn 2 ™N qN
(2)
where n denotes vibrational excitation energy. Since the energy threshold is ;24 eV w1x, this dissociative charge exchange process would not be expected to occur in the plasma region. Clearly, the presence of Nq ions is 2 required for Reaction 2, hence the magnitude of N0 y N02 SLIRs from the sheath provides some indication of the availability of these ions. Determination of these SLIRs gave values that increased linearly with nitrogen partial pressure in d.c. diode discharges of argon– nitrogen w1,5x but, because of Penning ionisation in corresponding discharges of neon–nitrogen, the values were found to be highest at 5–20% nitrogen partial pressure w5x. 3. Experimental A diagram of the experimental arrangement used in this study and full details of the procedures are given in Wilson et al. w5x. Briefly, d.c. diode discharge studies were performed in a chamber, constructed principally of stainless steel, approximately 60=60=60 cm. The grounded chamber (anode) housed a titanium-coated copper cathode in the form of a 25.6-cm-diameter plate, mounted horizontally, 52 cm above the chamber base. A ground shield restricted bombardment by the discharge to the lower face of the cathode. The ultimate pressure was -10y3 Pa and neon–nitrogen gas flows into the chamber were set to a total pressure of 6 Pa, with the discharge not running, for all mixture ratios. The cathode bias was maintained at –2 kV which, under neon–nitrogen conditions, created cathode sheath thicknesses of 3.5–4.5 cm. The OES system comprised a Bentham Instruments M300 monochromator with 30-cm focal length and a 1200 linesymm grating. Spatial resolution was achieved by using a grounded quartz fibre optic cable fitted with a 13.5-cm-long metal collimating tube of 0.44-cm internal diameter. This probe enabled the discharge to be
Fig. 1. Nitrogen molecular ionymolecular neutral spectral line intensity ratios, compared with the cathode current density, as a function of nitrogen partial pressure for neon–nitrogen diode discharges. All discharges were operated at –2 kV cathode bias and 6 Pa total pressure.
sampled along the vertical axis; it was positioned in either the cathode sheath or the plasma (negative glow region), the centre of the collimating tube being at 1 and 10 cm below the cathode face, respectively. For the present work, the following spectral lines were evaluated (wavelength, energy threshold): Neq (371.3 nm, 31.11 eV), Ne0 (660.1 nm, 18.72 eV), Nq 2 (391.4 nm, 18.70 eV), N02 (337.1 nm, 11.10 eV) and Nq (399.5 nm, 21.60 eV). To assess repeatability, two spectral scans were performed for each set of discharge conditions. 4. Results and discussion 4.1. Initial observations from the data Fig. 1 compares the cathode current density, J, with 0 the Nq 2 yN2 SLIR as a function of nitrogen concentration in the discharges. Adding nitrogen to neon boosts J, so that at 10–15% nitrogen, J reaches a maximum value that is greater than that achieved by either neon or nitrogen alone. This synergistic effect results from Penning ionisation of nitrogen by neon, so that the majority ionic species at low nitrogen concentrations is expected to be those of nitrogen. Evidence of this mechanism is 0 provided by the maxima in Nq 2 yN2 SLIR data from both the sheath and plasma, as these correspond closely with the peak in J.
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Fig. 2. Neon ionyneon neutral spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
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Fig. 3. Nitrogen atomic ionymolecular neutral spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
Fig. 2 shows that the plasma and sheath Neq yNe0 SLIR data have minima at low nitrogen concentrations, which suggests a depletion in Neq ions. Although the minima occur at slightly lower levels of nitrogen concentration than the maxima in Fig. 1, the cause may be attributed to Penning ionisation. Neon metastables can be ionised by electron impact: NeUqey™Neqq2ey
(3)
Alternatively, they may return to ground state as a result of Penning ionisation collisions. The minimum, at least in the plasma data, can be considered to represent conditions where the collision frequency between neon metastables and electrons (with energies G5 eV) to produce ions through Reaction 3, is minimised as a result of conditions favouring the Penning process. Figs. 3 and 4 provide some indication of the level of nitrogen dissociation in the discharges. The maxima for sheath and plasma data in both figures occur at ;5% nitrogen concentration, with the Nq yNq maxima in 2 Fig. 4 being more sharply defined. The Nq yN02 peak in the sheath data from Fig. 3 may explain the discontinuity in J between 2 and 5% in Fig. 1. The rising trend beyond 20% nitrogen in the SLIR sheath data of Figs. 3 and 4 can be attributed mainly to Reaction 2, since 0 the Nq 2 –N2 collision frequency in the sheath is expected generally to increase with increasing nitrogen concentration. As Reaction 2 depends on the presence of Nq 2
Fig. 4. Nitrogen atomic ionymolecular ion spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
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however, we are less confident that this reaction is primarily responsible for the dissociation maxima at 5%; this is because Fig. 1 indicates that the availability of Nq 2 ions from the plasma reaches a maximum at a nitrogen concentration of 10%. Clearly, other observations can be made with the results of Figs. 1–4, but attempts to explain the underlying mechanisms in the following sections can only be speculative and comment is needed here on the interpretation of SLIR data. When the excitation thresholds of the radiative levels used in an SLIR are similar, the relationship described by Coburn and Chen w7x can be applied to SLIR values, providing direct information on changes in the proportion of the species concerned. This has been used for N0 yN02 SLIRs to determine changes in the proportion of dissociated nitrogen w1,5x. Since the difference in energy thresholds between the ion and neutral species used in the present study is of the order of 10 eV, it is only the Nq yNq 2 SLIR data of Fig. 4 (threshold differences2.9 eV) that may provide (with some confidence) a quantitative indication of species proportionality. 4.2. Non-Penning interactions Although the data in Fig. 1 seem to indicate that the maximum effect from Penning ionisation occurs at 10– 15% nitrogen concentration, this is inconsistent with the locations of the maxima and minima in Figs. 2–4. Probably the most reliable indicator of Penning ionisation is provided by the location of the Neq yNe0 minimum in the plasma SLIR data of Fig. 2. Since this occurs at ;5%, we suggest that 5% is the (approx.) optimum nitrogen concentration for Penning ionisation. If this hypothesis is correct, there must be mechanisms in addition to Reaction 1 which contribute to the production of Nq 2 ions, causing the maxima in Fig. 1 to be located at 10–15%. To consider such mechanisms, Fig. 2 needs further examination. For 100% neon, ionisation by electron impact of Ne* (i.e. Reaction 3) and Ne0 species are the primary ionising mechanisms in the plasma; these processes would be comparatively rare in the sheath, due to the scarcity of electrons and low cross-section values associated with the high electron energies. Therefore, for 100% neon, no net change in ion population can be expected in the sheath, and it is encouraging to note that in Fig. 2, the Neq yNe0 SLIRs from the sheath and plasma are similar in the pure neon discharge. We may infer from this that the excitation mechanisms which ultimately lead to the SLIRs plotted in Fig. 2 are not heavily influenced by the different environments of the sheath and plasma regions. It follows that the SLIRs in Fig. 2 can provide some relative indication, between the sheath and plasma regions, regarding the influence of nitrogen on Neq
ions. Thus it is interesting to note that the sheath SLIRs are significantly lower than those from the plasma when nitrogen is added. In discharges of argon–nitrogen mixtures, charge exchange reactions in the sheath between ions and neutrals of the component gases have been cited w1x, and we suggest a reaction of the form: NeqqN20™Ne0qN2q
(4)
As a non-symmetrical charge exchange process, the energy threshold for Reaction 4 would preclude its occurrence in the plasma region (it can be compared with the Arq–N02 charge exchange reaction in w1x). In the sheath, however, reactions such as 4 can be expected because of the higher ion energies and this may explain the relative depletion in Neq ions within the sheath, compared with the plasma. Also, since there will be an optimum proportion of nitrogen at which the collision frequency for Reaction 4 is maximised, the shift in Fig. 2 of the sheath SLIR minimum to ;7% (compared with the plasma value at 5%) might be attributed to the effects of Reaction 4. Referring to Fig. 1, the additional source of Nq 2 ions 0 from Reaction 4 may be responsible for the Nq 2 yN2 SLIRs from the sheath being higher than those from the plasma; however, this would not explain the higher sheath SLIR at 100% nitrogen concentration in Fig. 1. Therefore, we suggest that the differences between sheath and plasma SLIRs in Fig. 1 are probably (at least) partially explained by the effect of the sheath and plasma environments on species excitation. Also, Reaction 4 could be suggested as a possible mechanism to 0 explain the location in Fig. 1 of the Nq 2 yN2 sheath SLIR maximum at 10%, especially since the minimum in the Neq yNe0 sheath SLIR data of Fig. 2 is shifted to ;7%. Nevertheless, Reaction 4, as a sheath-based reaction, does not provide an explanation for the plasma maximum in Fig. 1, which also occurs at 10%. If the optimum for Penning ionisation is close to 5% nitrogen concentration, an alternative mechanism to Reaction 4 needs to be considered, which occurs in the plasma. One such mechanism is direct electron impact ionisation of nitrogen, i.e: y N02qey™Nq 2 q2e
(5)
As more nitrogen is added to the discharges, the production rate of Nq 2 ions in the plasma by Reaction 5 will be increased because of the greater availability of N02, but decreased to some extent by the effects of electron quenching; moreover, Reaction 5 may be influenced by the effects of the electrons (;2.1 eV) from Reaction 1 on the electron energy distribution in the plasma. Thus it is possible that there is an optimum neon–nitrogen mixture for Reaction 5 which occurs at a nitrogen concentration greater than 5%, and we suggest that this could be making a contribution to the observed shift in Fig. 1.
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4.3. Dissociation of nitrogen Further comment on the data in Figs. 3 and 4 is required. As discussed in Section 4.1, there is doubt as to whether Reaction 2 is primarily responsible for the dissociation maxima at 5%. Closer inspection of Fig. 4 provides some further clues to a possible explanation and, as mentioned earlier, it can give some quantitative indication of species proportionality. The sheath Nq yNq 2 SLIRs in Fig. 4 are greater than those from the plasma, the difference being the largest in pure nitrogen (an equivalent effect is also seen in Fig. 3). At 100% nitrogen, the principal dissociation mechanism is Reaction 2 w1x and, being a sheath-based reaction, this explains why the sheath SLIR is larger: the very small Nq yNq 2 SLIR in the plasma may arise from electron impact dissociative ionisation w1x: eyqN02™N0qNqq2ey
(6)
As the nitrogen concentration is decreased, Fig. 4 clearly shows that the sheath and plasma SLIRs become closer, so that at the maxima (5%), the plasma SLIR is ;90% of the sheath SLIR. It therefore follows that there must be, in addition to Reaction 6, a major dissociation mechanism occurring in the plasma region, the mechanism being so significant that the proportion of Nq relative to Nq 2 at 5% is increased by approximately 20-fold when compared with the conditions using 100% nitrogen. We suggest dissociation by Penning ionisation: NeUqN02™Ne0qNqqN0qey
(7)
This must explain why the maxima in dissociation from Figs. 3 and 4 occur at 5% nitrogen concentration, as Nq 2 ions are not involved (although there may be an intermediate step where, for example, Nqv ions are 2 formed, as in Reaction 2). Thus Reaction 7 is consistent with our hypothesis that the optimum conditions for Penning ionisation occur at ;5% nitrogen partial pressure. 4.4. Summarising comments Clearly, our interpretation of behaviour from the OES data above can only be speculative, and we are unsure of the possible role of sheath-based reactions, such as Reaction 4. Also, we have suggested that Reaction 5 0 may have some influence on the Nq 2 yN2 maxima in Fig. 1 being located at 10% nitrogen concentration; however, Reaction 1 must be making significant contributions to this effect as well. Since the minimum in the Neq yNe0 plasma SLIR data of Fig. 2 occurs at ;5% nitrogen concentration, it is apparent that Reaction 7 is a much more prominent mechanism than is Reaction 1 at 5%, and its overall contribution to ion production must be highly significant
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in these discharges. This means that an inference from previous work w5x, that Reaction 2 is the principal dissociation mechanism for -20% nitrogen in neon, must be re-evaluated. Our work also raises comment of a more general nature. For example, the minima in Fig. 2 are of significant magnitude. This observation may indicate that the two-step ionisation process (electron impact excitation to produce metastables, followed by ionisation in the form of Reaction 3) is the dominant ionisation mechanism in pure inert gas discharges, as opposed to the direct electron impact ionisation of ground state atoms. The relatively large metastable population and high collision cross-section values at low electron energies for Reaction 3 w8x would appear to support this view. 5. Conclusions A further evaluation of d.c. diode neon–nitrogen glow discharges has been performed, using spatially resolved OES. This has been achieved through the study of SLIRs that incorporate ion species. Although the dissimilar excitation thresholds which exist for most of the SLIR combinations used in the present study restrict quantitative evaluation, the results provide a greater insight into the mechanisms occurring in these discharges. Our principal findings can be summarised as follows: 1. Adding nitrogen to neon boosts the cathode current density, giving a maximum value at 10–15% nitrogen concentration, which is significantly higher than that achieved by either nitrogen or neon alone. The effect arises from Penning ionisation of nitrogen by neon; evidence of this is provided by the occurrence, at low nitrogen concentrations, of maxima in SLIRs q derived from Nq and minima in Neq yNe0 2 and N SLIRs. 2. Since the minimum in the Neq yNe0 SLIR plasma data occurs at a nitrogen concentration of 5%, we suggest that 5% represents the optimum nitrogen partial pressure for Penning ionisation by neon. Although speculative, we propose that dissociative Penning ionisation is the mechanism primarily responsible for ion production at 5% nitrogen concentration, and that its role is highly significant in these discharges. Furthermore, the maxima in current 0 density and Nq 2 yN2 SLIR data occurring at 10–15% may be explained by conventional (non-dissociative) Penning ionisation, with the possibility of contributions by other reactions, such as the electron impact ionisation of nitrogen. Acknowledgements We are grateful for financial support from the UK Engineering and Physical Sciences Research Council
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under their Technological Plasmas Initiative (Grant Ref. GRyM69678) and also the contribution of Ion Coat Ltd for a CASE award to A.D. References w1x K.S. Fancey, Vacuum 46 (1995) 695. w2x J.W. Coburn, E. Kay, Appl. Phys. Lett. 18 (1971) 435.
w3x D. Serxner, R.L. Smith, K.R. Hess, Appl. Spectrosc. 45 (1991) 1656. w4x K.S. Fancey, A. Leyland, F.M. Badow, A. Matthews, Mater. Sci. Eng. A262 (1999) 227. w5x A.D. Wilson, J.C. Avelar-Batista, S.J. Dowey, J. Robson, A. Leyland, A. Matthews, K.S. Fancey, Surf Coat Technol (in press). w6x K.S. Fancey, A. Matthews, Surf. Coat. Technol. 33 (1987) 17. w7x J.W. Coburn, M. Chen, J. Appl. Phys. 51 (1980) 3134. w8x M. Johnston, K. Fujii, J. Nickel, S. Trajmar, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 531.
A study of neon–nitrogen interactions in d.c. glow discharges by optical emission spectroscopy J.C. Avelar-Batista, A.D. Wilson, A. Davison, A. Leyland, A. Matthews, K.S. Fancey* Research Centre in Surface Engineering, Department of Engineering, University of Hull, Hull HU6 7RX, UK
Abstract Neon–nitrogen d.c. diode glow discharges have been investigated through the use of spatially resolved optical emission spectroscopy. This technique has enabled the discharges to be sampled from inside the cathode sheath and plasma regions. All discharges were operated at –2 kV cathode bias and 6 Pa total pressure. By evaluating spectral line intensity ratios (SLIRs) that incorporate ion species, the principal findings are as follows: first, the cathode current density at low nitrogen partial pressures is boosted by Penning ionisation of nitrogen by neon; evidence of this mechanism is provided by maxima in nitrogen ion-based SLIRs and minima in neon ion-based SLIRs occurring at low nitrogen concentrations. Second, although the optimum nitrogen 0 partial pressure for Penning ionisation appears to be approximately 5%, the maxima in cathode current density and Nq 2 yN2 SLIRs occur at 10–15%; we speculate that a dissociative Penning ionisation mechanism is predominant at 5% but conventional (nondissociative) Penning ionisation, with possible contributions from other mechanisms (such as electron impact ionisation of nitrogen) become significant at 10–15% nitrogen concentration. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen; Optical emission spectroscopy; Penning ionisation
1. Introduction Although plasma-based surface treatment and coating deposition systems commonly use inert gas-based discharges, considerations of cost combined with useable characteristics tend to limit selection of the inert gas to argon. The need for such a gas to perform as little more than a buffer agent appears to be rarely considered. Nevertheless, the presence of argon in a discharge can influence charge exchange and dissociation behaviour with commonly used process gases such as nitrogen w1x. It has been known for many years that metal vapour, present as a minority species in an argon-based discharge, can be significantly ionised by argon metastables (Penning ionisation) w2x; the effect also occurs in neonbased discharges w3x. Of particular interest however, is that neon can also Penning ionise nitrogen: preliminary investigations through the deposition of TiN coatings by * Corresponding author. Tel.: q44-1482-465071; fax: q44-1482466664. E-mail address: [email protected] (K.S. Fancey).
reactive ion plating have suggested that neon can enhance nitrogen reactivity w4x. In a previous paper w5x, d.c. diode discharges of neon–nitrogen and argon–nitrogen mixtures were investigated by monitoring the process parameters in conjunction with spatially-resolved optical emission spectroscopy (OES). This previous work was focused on the study of mechanisms resulting in the dissociation of nitrogen molecules by evaluating the N0 yN02 spectral line intensity ratios (SLIRs) from OES data. In the present work, we utilise SLIRs that incorporate ion species from our OES data in order to understand further the ionisation and dissociation mechanisms occurring in neon–nitrogen discharges. 2. Background Neon has metastable states (Ne*) at 16.62 and 16.67 eV. Thus, as the ionisation potential of nitrogen is 14.53 eV, nitrogen molecules in a neon plasma can be Penning ionised:
0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 4 3 8 - 9
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y NeUqN02™Ne0qNq 2 qe
(1)
If the nitrogen molecules are a minority species compared with the neon, a Penning mixture will be formed, in which substantial ionisation of the nitrogen can be expected. Previous work w5x revealed indirect evidence of Reaction 1 being a significant mechanism at low nitrogen partial pressures through measurement of the cathode current density, cathode sheath thickness and, from OES data, nitrogen dissociation in the cathode sheath. The principal mechanism for nitrogen molecules to dissociate in the cathode sheath is considered to be w1,6x: 0 qn 0 Nq 2 Žfast. qN2™N2 Žslow. qN2
and q 0 Nqn 2 ™N qN
(2)
where n denotes vibrational excitation energy. Since the energy threshold is ;24 eV w1x, this dissociative charge exchange process would not be expected to occur in the plasma region. Clearly, the presence of Nq ions is 2 required for Reaction 2, hence the magnitude of N0 y N02 SLIRs from the sheath provides some indication of the availability of these ions. Determination of these SLIRs gave values that increased linearly with nitrogen partial pressure in d.c. diode discharges of argon– nitrogen w1,5x but, because of Penning ionisation in corresponding discharges of neon–nitrogen, the values were found to be highest at 5–20% nitrogen partial pressure w5x. 3. Experimental A diagram of the experimental arrangement used in this study and full details of the procedures are given in Wilson et al. w5x. Briefly, d.c. diode discharge studies were performed in a chamber, constructed principally of stainless steel, approximately 60=60=60 cm. The grounded chamber (anode) housed a titanium-coated copper cathode in the form of a 25.6-cm-diameter plate, mounted horizontally, 52 cm above the chamber base. A ground shield restricted bombardment by the discharge to the lower face of the cathode. The ultimate pressure was -10y3 Pa and neon–nitrogen gas flows into the chamber were set to a total pressure of 6 Pa, with the discharge not running, for all mixture ratios. The cathode bias was maintained at –2 kV which, under neon–nitrogen conditions, created cathode sheath thicknesses of 3.5–4.5 cm. The OES system comprised a Bentham Instruments M300 monochromator with 30-cm focal length and a 1200 linesymm grating. Spatial resolution was achieved by using a grounded quartz fibre optic cable fitted with a 13.5-cm-long metal collimating tube of 0.44-cm internal diameter. This probe enabled the discharge to be
Fig. 1. Nitrogen molecular ionymolecular neutral spectral line intensity ratios, compared with the cathode current density, as a function of nitrogen partial pressure for neon–nitrogen diode discharges. All discharges were operated at –2 kV cathode bias and 6 Pa total pressure.
sampled along the vertical axis; it was positioned in either the cathode sheath or the plasma (negative glow region), the centre of the collimating tube being at 1 and 10 cm below the cathode face, respectively. For the present work, the following spectral lines were evaluated (wavelength, energy threshold): Neq (371.3 nm, 31.11 eV), Ne0 (660.1 nm, 18.72 eV), Nq 2 (391.4 nm, 18.70 eV), N02 (337.1 nm, 11.10 eV) and Nq (399.5 nm, 21.60 eV). To assess repeatability, two spectral scans were performed for each set of discharge conditions. 4. Results and discussion 4.1. Initial observations from the data Fig. 1 compares the cathode current density, J, with 0 the Nq 2 yN2 SLIR as a function of nitrogen concentration in the discharges. Adding nitrogen to neon boosts J, so that at 10–15% nitrogen, J reaches a maximum value that is greater than that achieved by either neon or nitrogen alone. This synergistic effect results from Penning ionisation of nitrogen by neon, so that the majority ionic species at low nitrogen concentrations is expected to be those of nitrogen. Evidence of this mechanism is 0 provided by the maxima in Nq 2 yN2 SLIR data from both the sheath and plasma, as these correspond closely with the peak in J.
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Fig. 2. Neon ionyneon neutral spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
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Fig. 3. Nitrogen atomic ionymolecular neutral spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
Fig. 2 shows that the plasma and sheath Neq yNe0 SLIR data have minima at low nitrogen concentrations, which suggests a depletion in Neq ions. Although the minima occur at slightly lower levels of nitrogen concentration than the maxima in Fig. 1, the cause may be attributed to Penning ionisation. Neon metastables can be ionised by electron impact: NeUqey™Neqq2ey
(3)
Alternatively, they may return to ground state as a result of Penning ionisation collisions. The minimum, at least in the plasma data, can be considered to represent conditions where the collision frequency between neon metastables and electrons (with energies G5 eV) to produce ions through Reaction 3, is minimised as a result of conditions favouring the Penning process. Figs. 3 and 4 provide some indication of the level of nitrogen dissociation in the discharges. The maxima for sheath and plasma data in both figures occur at ;5% nitrogen concentration, with the Nq yNq maxima in 2 Fig. 4 being more sharply defined. The Nq yN02 peak in the sheath data from Fig. 3 may explain the discontinuity in J between 2 and 5% in Fig. 1. The rising trend beyond 20% nitrogen in the SLIR sheath data of Figs. 3 and 4 can be attributed mainly to Reaction 2, since 0 the Nq 2 –N2 collision frequency in the sheath is expected generally to increase with increasing nitrogen concentration. As Reaction 2 depends on the presence of Nq 2
Fig. 4. Nitrogen atomic ionymolecular ion spectral line intensity ratios as a function of nitrogen partial pressure, from the same discharges used in Fig. 1.
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however, we are less confident that this reaction is primarily responsible for the dissociation maxima at 5%; this is because Fig. 1 indicates that the availability of Nq 2 ions from the plasma reaches a maximum at a nitrogen concentration of 10%. Clearly, other observations can be made with the results of Figs. 1–4, but attempts to explain the underlying mechanisms in the following sections can only be speculative and comment is needed here on the interpretation of SLIR data. When the excitation thresholds of the radiative levels used in an SLIR are similar, the relationship described by Coburn and Chen w7x can be applied to SLIR values, providing direct information on changes in the proportion of the species concerned. This has been used for N0 yN02 SLIRs to determine changes in the proportion of dissociated nitrogen w1,5x. Since the difference in energy thresholds between the ion and neutral species used in the present study is of the order of 10 eV, it is only the Nq yNq 2 SLIR data of Fig. 4 (threshold differences2.9 eV) that may provide (with some confidence) a quantitative indication of species proportionality. 4.2. Non-Penning interactions Although the data in Fig. 1 seem to indicate that the maximum effect from Penning ionisation occurs at 10– 15% nitrogen concentration, this is inconsistent with the locations of the maxima and minima in Figs. 2–4. Probably the most reliable indicator of Penning ionisation is provided by the location of the Neq yNe0 minimum in the plasma SLIR data of Fig. 2. Since this occurs at ;5%, we suggest that 5% is the (approx.) optimum nitrogen concentration for Penning ionisation. If this hypothesis is correct, there must be mechanisms in addition to Reaction 1 which contribute to the production of Nq 2 ions, causing the maxima in Fig. 1 to be located at 10–15%. To consider such mechanisms, Fig. 2 needs further examination. For 100% neon, ionisation by electron impact of Ne* (i.e. Reaction 3) and Ne0 species are the primary ionising mechanisms in the plasma; these processes would be comparatively rare in the sheath, due to the scarcity of electrons and low cross-section values associated with the high electron energies. Therefore, for 100% neon, no net change in ion population can be expected in the sheath, and it is encouraging to note that in Fig. 2, the Neq yNe0 SLIRs from the sheath and plasma are similar in the pure neon discharge. We may infer from this that the excitation mechanisms which ultimately lead to the SLIRs plotted in Fig. 2 are not heavily influenced by the different environments of the sheath and plasma regions. It follows that the SLIRs in Fig. 2 can provide some relative indication, between the sheath and plasma regions, regarding the influence of nitrogen on Neq
ions. Thus it is interesting to note that the sheath SLIRs are significantly lower than those from the plasma when nitrogen is added. In discharges of argon–nitrogen mixtures, charge exchange reactions in the sheath between ions and neutrals of the component gases have been cited w1x, and we suggest a reaction of the form: NeqqN20™Ne0qN2q
(4)
As a non-symmetrical charge exchange process, the energy threshold for Reaction 4 would preclude its occurrence in the plasma region (it can be compared with the Arq–N02 charge exchange reaction in w1x). In the sheath, however, reactions such as 4 can be expected because of the higher ion energies and this may explain the relative depletion in Neq ions within the sheath, compared with the plasma. Also, since there will be an optimum proportion of nitrogen at which the collision frequency for Reaction 4 is maximised, the shift in Fig. 2 of the sheath SLIR minimum to ;7% (compared with the plasma value at 5%) might be attributed to the effects of Reaction 4. Referring to Fig. 1, the additional source of Nq 2 ions 0 from Reaction 4 may be responsible for the Nq 2 yN2 SLIRs from the sheath being higher than those from the plasma; however, this would not explain the higher sheath SLIR at 100% nitrogen concentration in Fig. 1. Therefore, we suggest that the differences between sheath and plasma SLIRs in Fig. 1 are probably (at least) partially explained by the effect of the sheath and plasma environments on species excitation. Also, Reaction 4 could be suggested as a possible mechanism to 0 explain the location in Fig. 1 of the Nq 2 yN2 sheath SLIR maximum at 10%, especially since the minimum in the Neq yNe0 sheath SLIR data of Fig. 2 is shifted to ;7%. Nevertheless, Reaction 4, as a sheath-based reaction, does not provide an explanation for the plasma maximum in Fig. 1, which also occurs at 10%. If the optimum for Penning ionisation is close to 5% nitrogen concentration, an alternative mechanism to Reaction 4 needs to be considered, which occurs in the plasma. One such mechanism is direct electron impact ionisation of nitrogen, i.e: y N02qey™Nq 2 q2e
(5)
As more nitrogen is added to the discharges, the production rate of Nq 2 ions in the plasma by Reaction 5 will be increased because of the greater availability of N02, but decreased to some extent by the effects of electron quenching; moreover, Reaction 5 may be influenced by the effects of the electrons (;2.1 eV) from Reaction 1 on the electron energy distribution in the plasma. Thus it is possible that there is an optimum neon–nitrogen mixture for Reaction 5 which occurs at a nitrogen concentration greater than 5%, and we suggest that this could be making a contribution to the observed shift in Fig. 1.
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4.3. Dissociation of nitrogen Further comment on the data in Figs. 3 and 4 is required. As discussed in Section 4.1, there is doubt as to whether Reaction 2 is primarily responsible for the dissociation maxima at 5%. Closer inspection of Fig. 4 provides some further clues to a possible explanation and, as mentioned earlier, it can give some quantitative indication of species proportionality. The sheath Nq yNq 2 SLIRs in Fig. 4 are greater than those from the plasma, the difference being the largest in pure nitrogen (an equivalent effect is also seen in Fig. 3). At 100% nitrogen, the principal dissociation mechanism is Reaction 2 w1x and, being a sheath-based reaction, this explains why the sheath SLIR is larger: the very small Nq yNq 2 SLIR in the plasma may arise from electron impact dissociative ionisation w1x: eyqN02™N0qNqq2ey
(6)
As the nitrogen concentration is decreased, Fig. 4 clearly shows that the sheath and plasma SLIRs become closer, so that at the maxima (5%), the plasma SLIR is ;90% of the sheath SLIR. It therefore follows that there must be, in addition to Reaction 6, a major dissociation mechanism occurring in the plasma region, the mechanism being so significant that the proportion of Nq relative to Nq 2 at 5% is increased by approximately 20-fold when compared with the conditions using 100% nitrogen. We suggest dissociation by Penning ionisation: NeUqN02™Ne0qNqqN0qey
(7)
This must explain why the maxima in dissociation from Figs. 3 and 4 occur at 5% nitrogen concentration, as Nq 2 ions are not involved (although there may be an intermediate step where, for example, Nqv ions are 2 formed, as in Reaction 2). Thus Reaction 7 is consistent with our hypothesis that the optimum conditions for Penning ionisation occur at ;5% nitrogen partial pressure. 4.4. Summarising comments Clearly, our interpretation of behaviour from the OES data above can only be speculative, and we are unsure of the possible role of sheath-based reactions, such as Reaction 4. Also, we have suggested that Reaction 5 0 may have some influence on the Nq 2 yN2 maxima in Fig. 1 being located at 10% nitrogen concentration; however, Reaction 1 must be making significant contributions to this effect as well. Since the minimum in the Neq yNe0 plasma SLIR data of Fig. 2 occurs at ;5% nitrogen concentration, it is apparent that Reaction 7 is a much more prominent mechanism than is Reaction 1 at 5%, and its overall contribution to ion production must be highly significant
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in these discharges. This means that an inference from previous work w5x, that Reaction 2 is the principal dissociation mechanism for -20% nitrogen in neon, must be re-evaluated. Our work also raises comment of a more general nature. For example, the minima in Fig. 2 are of significant magnitude. This observation may indicate that the two-step ionisation process (electron impact excitation to produce metastables, followed by ionisation in the form of Reaction 3) is the dominant ionisation mechanism in pure inert gas discharges, as opposed to the direct electron impact ionisation of ground state atoms. The relatively large metastable population and high collision cross-section values at low electron energies for Reaction 3 w8x would appear to support this view. 5. Conclusions A further evaluation of d.c. diode neon–nitrogen glow discharges has been performed, using spatially resolved OES. This has been achieved through the study of SLIRs that incorporate ion species. Although the dissimilar excitation thresholds which exist for most of the SLIR combinations used in the present study restrict quantitative evaluation, the results provide a greater insight into the mechanisms occurring in these discharges. Our principal findings can be summarised as follows: 1. Adding nitrogen to neon boosts the cathode current density, giving a maximum value at 10–15% nitrogen concentration, which is significantly higher than that achieved by either nitrogen or neon alone. The effect arises from Penning ionisation of nitrogen by neon; evidence of this is provided by the occurrence, at low nitrogen concentrations, of maxima in SLIRs q derived from Nq and minima in Neq yNe0 2 and N SLIRs. 2. Since the minimum in the Neq yNe0 SLIR plasma data occurs at a nitrogen concentration of 5%, we suggest that 5% represents the optimum nitrogen partial pressure for Penning ionisation by neon. Although speculative, we propose that dissociative Penning ionisation is the mechanism primarily responsible for ion production at 5% nitrogen concentration, and that its role is highly significant in these discharges. Furthermore, the maxima in current 0 density and Nq 2 yN2 SLIR data occurring at 10–15% may be explained by conventional (non-dissociative) Penning ionisation, with the possibility of contributions by other reactions, such as the electron impact ionisation of nitrogen. Acknowledgements We are grateful for financial support from the UK Engineering and Physical Sciences Research Council
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under their Technological Plasmas Initiative (Grant Ref. GRyM69678) and also the contribution of Ion Coat Ltd for a CASE award to A.D. References w1x K.S. Fancey, Vacuum 46 (1995) 695. w2x J.W. Coburn, E. Kay, Appl. Phys. Lett. 18 (1971) 435.
w3x D. Serxner, R.L. Smith, K.R. Hess, Appl. Spectrosc. 45 (1991) 1656. w4x K.S. Fancey, A. Leyland, F.M. Badow, A. Matthews, Mater. Sci. Eng. A262 (1999) 227. w5x A.D. Wilson, J.C. Avelar-Batista, S.J. Dowey, J. Robson, A. Leyland, A. Matthews, K.S. Fancey, Surf Coat Technol (in press). w6x K.S. Fancey, A. Matthews, Surf. Coat. Technol. 33 (1987) 17. w7x J.W. Coburn, M. Chen, J. Appl. Phys. 51 (1980) 3134. w8x M. Johnston, K. Fujii, J. Nickel, S. Trajmar, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 531.