An investigation into dissociative mechanisms in nitrogenous glow discharges by optical emission spectroscopy

Vacuum/volume 46/number 7Ipages 695 to 700/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/95 $9.50+.00

Pergamon 0042-207x(94)00146-4

An investigation into dissociative mechanisms in nitrogenous glow discharges by optical emission spectroscopy K S Fancey, Research Centre in Surface Engineering, University of Hull, Hull HiJ6 7RX, UK received 9

September 7994; accepted 25 November

Department

of Engineering

Design and Manufacture,

1994

Nitrogen molecular dissociation is studied in DC diode and thermionic triode glow discharges of pure nitrogen and nitrogen-argon mixtures. Optical emission spectroscopy is used to determine the relative proportions of atomic and molecular nitrogen in the cathode sheath and plasma regions of these discharges operated at - 2 kV cathode bias in the range 0. I-10 Pa. The results show that for nitrogen-argon diode discharges at 6.67 Pa total pressure, (i) dissociation rates within the sheath compare with those of pure nitrogen discharges operated at s$nilar nitrogen pressure and the same mechanism (N;-N$ dissociative charge exchange) is believed to be primarily responsible; (ii) without argon, the proportion of atomic to molecular nitrogen in the plasma is very low compared with the sheath but increases when argon is introduced, the suggested cause being reduced quenching of electron energy by nitrogen gas, leading to increased electron impact dissociation. For nitrogen-argon thermionic triode discharges at 1.33 Pa total pressure, the proportion of atomic to molecular nitrogen in the sheath does not significantly vary with nitrogen concentration; this is thought to arise from increased incident ion energies in the sheath, compared with the higher pressure diode case, making Ar-N, dissociative charge exchange collisions more prominent. The implications of these findings are discussed in relation to plasma nitriding techniques.

1. Introduction For many years, there has been considerable interest in using techniques such as Optical Emission Spectroscopy (OES) for investigating plasma-based surface treatments and coatings processes. Although there is a tendency for much of the published work to concentrate on the plasma region of such discharges, the cathode sheath, which usually surrounds the substrate, can profoundly influence the nature of the species bombarding the substrate surface. A well known phenomenon is symmetrical charge exchange between ions accelerating across the cathode sheath and their neutral counterparts; this process, which results in a spread of bombarding energies incident at the substrate surface, was first modelled by Davis and Vanderslice’. The large collision cross section associated with symmetrical charge exchange means that the phenomenon can be expected to have some influence in all but the most collision-free cathode sheaths*. For some discharges used in plasma processing however, dissociative charge exchange events occurring in the cathode sheath can also have a significant effect on the characteristics of the bombarding species, even though the collision cross sections for these mechanisms may be comparatively small. An example is provided by the discharges studied for plasma nitriding applications: previous work, based on cathode sheath thickness

measurements and OES, has indicated that the dominant ionic species arriving at the cathode of a pure nitrogen DC diode discharge is N+, even though N: is the main ion populating the plasma region. The observation is attributed to dissociation in the cathode sheath caused by this phenomenotP5. In this paper, the study of nitrogen molecular dissociation is extended to elucidate the effects of adding argon gas to the discharge. The work is developed from a previous study’ in which OES was used to determine the relative proportions of nitrogen atomic and molecular species in the sheath and plasma regions of DC glow discharges. Diode and thermionic triode layouts are studied, the latter providing a means of enhancing the discharge so that high cathode current densities can be obtained at low gas pressures. 2. Background 2.1. Information from OES. Evidence of nitrogen dissociation and its dependence on discharge conditions can be provided by monitoring the ratio of optical emission from particular radiative levels of nitrogen atomic and molecular species. Furthermore, if the excitation threshold of the selected level for each species is similar, then the following relationship, described by Coburn and Chen6, may be applied : 695

KS Fancey: Optical emission spectroscopy

where n,,,. and I,, are the ground state density and spectral line intensity respectively, for species x and y ; K is a constant, independent of discharge parameters. Therefore, by selecting suitable spectral lines, the resulting N/N2 spectral line intensity ratio (SLIR) can be used to provide information on changes in the proportion of atomic species. For example, a doubling in the No/ Nt SLIR would indicate that the fraction of nitrogen neutrals in atomic form (relative to the number of molecular neutrals) had doubled. 2.2. Mechanisms of nitrogen dissociation. For discharges comprising only nitrogen, the dissociative charge exchange mechanism occurring in the cathode sheath is thought to be’-’ : NT (fast) + Ni -+ NT* (slow) + N(2’

at -2 kV for all runs, creating a cathode sheath of sufficient thickness (a few cm) to enable the extraction of OES data directly from the sheath. This also facilitated the measurement of sheath thickness by direct observation, so that the OES probe could be correctly positioned. Although sheath thickness measurements have provided useful information on ion charge to mass ratios (in accordance with the Child-Langmuir relationship) in previous work3m5,this aspect was not considered in the present study, since deconvolution of the effects of at least three ionic species (Ar+, NT and N+) would be required and changes in sheath thickness were found to be small compared with measurement errors over the range of discharge conditions used. The OES work was limited to the study of spectral lines from nitrogen neutral species, since output from the corresponding ions was found either to have a poor signal to noise ratio or to be unsuitable in terms of applying equation (1). 4. Results

and Rl

NT* + N+ +N” where * denotes vibrational excitation energy. Dissociation nitrogen molecules can also occur by electron impact :

of

e+Ni

-+ 2N” fe

R2

e+Ni

+N”+Nf+2e

R3

The presence of argon in discharges containing cause the following dissociative reactions : Ar+ +Nq Ar2+ +Ni

-+ N’

+N”

+ArO

-+ N+ +N”+Ar’

nitrogen may also

4.1. Preliminary investigations with nitrogen discharges. Figure 1 shows two plots of the No/N; SLIR as a function of gas pressure in pure nitrogen discharges. Data points at the highest pressure (9.3 Pa) are from a diode discharge and the cathode current density was maintained constant with reducing pressure by adjusting the heater current supplied to the thermionic emitter. The plots, using data taken from the middle of the cathode sheath, differ from each other only in terms of the SLIR wavelengths used. Results from ref 5 are represented by the broken curve ; spectral peaks at 746.8 nm (NO) and 337.1 nm (Nt) were found to be the most suitable in terms of signal to noise ratio.

R4 R5 1

For reaction R5, NT* is formed however, this aspect is not reported R3* and R4’.

as an intermediate step’; in information on reactions

t

NO/NO, SLIR

3. Experimental A diagram of the experimental arrangement used for this study is given in ref 5. DC diode and thermionic triode discharges were investigated in a 0.1 m3 stainless steel chamber, pumped down to a base pressure of less than 5 x 10m4 Pa. The chamber contained a cathode in the form of a 24.6 cm diameter flat steel disc, horizontally mounted 30 cm above the chamber base and shielded so that bombardment by the discharge was restricted to the lower face. For the triode discharges, a thermionic emitter was used, comprising a negatively biased (- 100 V) flat loop of heated tungsten wire mounted 5 cm above the chamber base. The earthed chamber walls were anodic for all discharges studied. The OES system comprised a Bentham Instruments M300 monochromator with 30 cm focal length and a 1200 lines mm-’ grating. Spatial resolution along the vertical axis of the discharge was achieved by means of an earthed quartz fibre optic cable fitted with a 12 cm long, 0.55 cm diameter collimating tube, pointing towards the chamber central axis. Gas flow into the chamber was monitored by means of a capacitance-manometer control unit. For each run, spectral scans were performed after the gas and electrical parameters had become stable ; i.e. the working temperature had stabilised and sputter etching had minimised the influence of any contaminants present in the system. The cathode bias voltage was maintained 696

1 NITROQEN PRESSURE (Pa)

Figure 1. Measured NO/N! spectral line intensity

ratios from the middle of the cathode sheath as a function of gas pressure for pure nitrogen discharges. Thermionic emission was used to maintain the cathode current density at 0.18 mA cm-’ for a cathode voltage of -2 kV, sheath thicknesses were 2.5-3.0 cm. Solid curve shows results from the present study, using wavelengths at 493.5 nm (No) and 380.4 nm (NY) ; broken curve shows data from ref 5. Error bars represent uncertainties in spectral line intensity measurement arising from signal noise.

KS Fancey: Optical emission spectroscopy

Furthermore, the excitation thresholds for these radiative levels are similar (12.0 eV at 746.8 nm and 11.1 eV at 337.1 nm), which enables equation (1) to be considered. If it is assumed that equation (1) can be applied to these results, then the proportion of N” to Ni in the fniddle of the sheath at 0.13 Pa is only about 6% of the value at 9.3 Pa. Clearly, this reduction can be attributed to reduced collision probability in the cathode sheath as gas number density is decreased but sheath thickness remains comparatively constant. The dissociation mechanism, which is thought to be R 1, has a collision cross section that increases with N; energy’“. The plateau region in Figure 1 is thought to be a consequence of thi?: as the pressure decreases, there will be fewer collisions in the sheath hence the average energy gained by accelerating ions will increase. Therefore, the collision cross section will be larger at “A” than at “B” in Figure 1, which tends to offset the pressure effect and produces the plateau. It should be noted that using a thermionic emitter can heat the discharge gas, causing some degree of rarefaction at a fixed pressure. In this work, the effect would be to reduce the probability of dissociation in the cathode sheath. The influence of this effect on the results in Figure 1, however, should be minimal, since the change in heater current supplied to the emitter was less than 3% over the range of pressures investigated. Furthermore, localised gas rarefaction in the sheath arising from heat radiated by the cathode would be predominant; hence possible discrepancies between the diode results at 9.3 Pa and the other data in Figure 1, arising from filament heating, can be expected to be negligible. To investigate discharges of nitrogen-argon mixtures, the spectral peaks at 746.8 and 337.1 nm could not be used since they became obscured by argon emission lines. The most suitable alternatives were found to be 493.5 nm (No) and 380.4 nm (NI), their excitation thresholds being 13.2 eV and 11.1 eV respectively. “.‘* The solid curve in Figure I indicates that this data is comparable with the results from ref 5, although errors arising from the effects of signal noise are larger.

4.2. Nitrogen-argon discharges. Figure 2 shows plots of the No/ N! SLIR as a function of nitrogen partial pressure in diode discharges of nitrogen-argon mixtures ; the total pressure is held constant at 6.67 Pa. Results from the middle of the cathode sheath indicate that the SLIR depends on the availability of nitrogen in the discharge. The trend is approximately linear, implying a power law relationship and, as the line of linear regression in Figure 2 has a gradient close to unity (1.27), there is almost direct proportionality. Therefore, if equation (1) is assumed to be applicable, we can deduce that the fraction of nitrogen in the middle of the sheath which is dissociated is almost directly proportional to the percentage of nitrogen in the discharge. From this, we may infer that the primary dissociation mechanism occurring in the sheath must require collisions between nitrogen species as opposed to reactions with argon; hence reaction RI can be expected to be the dominant process, as in Figure I. Also, the SLIR covers a similar range of values to the data of Figure 1, implying that dissociation events occur with comparable frequency. A more direct comparison between Figures 1 and 2 is restricted by the differences in cathode current density and sheath thickness. Thus for example, the SLIR for pure nitrogen in Figure 2 is observed to be almost three times the value indicated by Figure 1: the higher level of dissociation in Figure 2 can be attributed to the lower cathode current density

NITROGEN PARTIAL PRESSURE (%)

)

N?N; SLIR

OF CATHODE

0.1

1 NITROGEN PARTIAL PRESSURE (Pa)

10

Figure 2. No/N: spectral line intensity ratios from the middle of the cathode sheath (solid line represents least squares fit) and from the plasma, 10 cm from the cathode (broken line), as a function of nitrogen partial pressure for diode discharges of nitrogen-argon mixtures at a total pressure of 6.67 Pa. The cathode voltage and current densities were -2 kV, 0.12~0.01 mA cm-‘; sheath thicknesses were 3.2-3.9 cm.

(leading to reduced effects from localised gas rarefaction) and a thicker cathode sheath. In contrast with the nitrogen-only results of Figure 1, there is no plateau in the sheath data of Figure 2. This is because the mean free path representing the total N: collision probability would not have changed significantly, since the total pressure (hence gas number density) is constant. Therefore, the average energy gained by N: ions accelerating between each inelastic collision in the sheath will not have been heavily influenced by the proportion of nitrogen gas in the discharge, making the energy dependence of the Rl collision cross section less relevant. Results taken from the plasma region are also plotted in Figure 2. Since the energy threshold for reaction Rl is approximately 24 eV”,14, this mechanism is not expected to occur within the plasma as ion temperatures would be too 10~‘~. Therefore, the appearance of nitrogen atomic species within the plasma must arise from other effects. When the discharge comprises 100% nitrogen, the proportion of No is very small compared with that found in the sheath and its presence in the plasma region can be attributed to two causes : (i) No diffusing into the plasma after being formed in the sheath and (ii) electron impact dissociation (R2 and R3). The effects of (i) have been used to explain the No/N: SLIR increasing in the plasma as gas pressure is reduced in pure nitrogen discharges’; however, the contribution from (ii) may also be significant. The combined effect of reactions R2 and R3 gives a collision cross section which peaks at 100 eV with a value of about 2 x IO-l6 cm* ; reaction R3 (dissociative ionisation) contributes to approximately one third of the electron impact dissociation mechanism within much of the 10-1000 eV energy range”.lh. The 697

KS Fancey: Optical emission spectroscopy

magnitude and energy dependence of the (R2+ R3) cross section is comparable to that of NT production by electron impact, the latter having a maximum value of about 2.6 x lO-‘6 cm* at 100 eV”. Hence the occurrence of electron impact dissociation events in the plasma region must almost be as frequent as the generation of NT ions. As the NT ion fraction in the plasma can be deduced to be typically 10e5 Ix. this provides some indication, in magnitude, of the fraction of nitrogen which dissociates in the plasma. There is evidence in Figure 2 that the N”/Nq SLIR from the plasma increases when argon gas is admitted into the discharge. Clearly, an effect in addition to those discussed above must be occurring, which depends on the presence of argon ; i.e. reactions R4 and R5. Reaction R4, like R 1. would not be expected to occur in the plasma region since a minimum collision energy of about 20 eV is required’. Moreover, although information on R5 seems to be limited. it can be inferred from other work7.19 that it is the only significant argon-based dissociative reaction which might occur in a plasma. As the nitrogen partial pressure is decreased, there is a slight reduction in the SLIR, but at 4%, the ratio in the plasma is greater than that found in the middle of the cathode sheath. Although R5 may be a contributory factor, this latter observation suggests that another mechanism, which would not be expected to have a significant role in the sheath, is highly significant in the plasma. It is probable that (R2+R3) is making this contribution ; the scarcity of electrons in the cathode sheath and their low cross section values associated with the high energies attained through acceleration would tend to make electron impact dissociation events in the sheath comparatively rare. In the plasma, however, the occurrence of (R2 + R3) may be greater in a mixture of argon and nitrogen in contrast with nitrogen only, because the presence of argon can be expected to reduce the quenching effect that nitrogen has on electron energy. This effect on quenching has been reported elsewhere”‘. A plot of the No/N! SLIR for thermionic triode discharges of argon-nitrogen mixtures is given in Figure 3 ; the total pressure was maintained constant at 1.33 Pa. The data, from the middle of the cathode sheath, shows little variation with nitrogen partial pressure, in contrast with the equivalent nitrogen-only thermionic triode results plotted from Figure 1. This clearly indicates that the presence of argon can make a significant contribution to the dissociation of nitrogen, particularly at low nitrogen partial pressures. As the energy threshold of reaction R4 cited earlier would not be a limiting factor in the sheath region, both R4 and R5 are probable mechanisms. This conclusion appears to be inconsistent with the inferences made from the sheath data of Figure 2, particularly since the cathode voltage and current density values are similar. However. the results of Figure 3 are from discharges operated with a five-fold reduction in total gas pressure compared with the diode data of Figure 2. Hence the mean free path between collisions in the sheath can be expected to be considerably longer for the triode discharges, resulting in ions undergoing charge exchange collisions with substantially greater incident energies. Therefore, the inconsistency might be explained by suggesting that reactions R4 or RS become more prominent than Rl as incident ion energies are increased ; i.e. the collision cross section for R4 or R5 must increase more rapidly with incident ion energy than for Rl. With the limited availability of appropriate collision cross section data, however, this inference can only be speculative. 5. Discussion 5.1. Problems associated with OES. The results have demonstrated that OES can be used to probe the cathode sheath as well 698

NITROGEN PARTIAL PRESSURE (%) 10

20

50

100

l-

NO/N;_ SLIR

NITROGEN-ARGON

///

0.1 -

,I TI’

\

I

0.01 ’ 0.1

PURE NITROGEN

I

/

I

I

III,,

1 NITROGEN PARTIAL PRESSURE (Pa)

Figure 3. NO/N! spectral line intensity ratios from the middle of the cathode sheath as a function of nitrogen partial pressure for thermionic triode discharges of nitrogen-argon mixtures at a total pressure of I .33 Pa (solid curve). The cathode voltage and current densities were - 2 kV and 0.18 mA cm-* ; sheath thicknesses were 2.2-2.5 cm. For comparison, the broken curve represents data from Figure 1 over the same range of pressures.

as the plasma to provide an insight into the dissociation of gas molecules in glow discharges. Nevertheless, there are some aspects of the work which need additional comment. For example, the volume of discharge being sampled by the OES probe used in this work was ill-defined ; the depth of the sampling volume “seen” by the simple collimating tube will have depended on the optical transparency of the region. However, this probably had minimal influence on the results because spectral output was studied in relative rather than absolute terms, thereby negating any changes in signal level from this effect. Moreover, the optical transparency, either in the sheath or the plasma, would not be expected to change significantly within the restricted ranges of sampling position and discharge conditions used. The applicability of equation (1) to data extracted from the sheath region requires further consideration. The excitation threshold of the selected radiative level of each species must be similar ; then it can be assumed that the excitation efficiencies of these levels depend only on the electron energy distribution, but only if cross section effects are smal16. The use of equation (1) has been demonstrated by others with data from the plasma region6,12, where average electron energies are low, so that collisions causing excitation involve electrons close to the excitation thresholds. However, when the method is used with data from the cathode sheath, it must be more sensitive to possible differences in the excitation cross section characteristics between the selected levels. This is because electrons causing excitation in the sheath can be expected to have energies much higher than the excitation thresholds. Since there appears to be no relevant cross section

KS Fancey: Optical emission data, the influence (1) is uncertain.

spectroscopy

of this effect on the applicability

of equation

5.2. Energy transportation to the cathode. The types of discharge studied can be considered in terms of energy transportation to the cathode by the various atomic and molecular species. For nitrogen-only discharges (Figure l), the situation is relatively simple. In a nitrogen diode discharge, earlier work3-’ has provided strong evidence to suggest that N+ rather than N: ions are the main ionic species arriving at the cathode because of dissociative charge exchange collisions in the sheath (reaction Rl). However, the resulting molecular neutrals (Ni) formed by Rl will carry most of the kinetic energy from the collision process. Furthermore, symmetrical charge transfer collisions in the sheath, i.e. : N: (fast) + N!j (slow)

-+ NT (slow)

+ Nt (fast)

R6

have a collision cross section value which is typically 10 times greater than that of RI I”. Under DC diode conditions, symmetrical charge transfer events can be expected to occur many times for each ion entering the sheath2.‘. Thus most of the kinetic energy will be transported to the cathode by Nt, produced by reactions R6 (frequently) and Rl (less frequently). In accordance with the Child-Langmuir relationship, the high current densities achievable at low cathode bias voltages with enhanced (thermionic triode) discharges enable cathode sheath thicknesses to be significantly reduced ; furthermore, they can be operated at low pressure2,3. Therefore, the resulting cathode sheath can be made virtually collisionless, so that for the nitrogenonly case, most of the energy is transported by NT ions. For nitrogen-argon discharges, the situation is less straightforward. In addition to reactions Rl-R6, the following must also be considered : Ar+ (fast) + Nt (slow)

+ N: (slow) + Are (fast)

NC (fast) +ArO(slow)

--f Ar+ (slow) +N:(fast)

Ar+ (fast) + ArO(slow)

+ Ar+ (slow) +ArO(fast)

RI3 R9

Here, the simplest case would be a discharge in which the cathode sheath is virtually collision-free, hence most of the energy is carried by ions and the relative proportions of ionic species bombarding the cathode are primarily governed by events occurring in the plasma region. According to published data2’, the collision cross sections of reactions R7 and R8 differ markedly, but they seem to approach zero for incident ion energies of 30 eV or less ; hence their role in the plasma may be insignificant. Moreover, since the cross sections for electron impact ionisation of nitrogen and argon have similar characteristics”, the ratio of nitrogen to argon ions transporting energy to the cathode will be comparable to the partial pressures used in the discharge. However, even the low pressure nitrogen-argon data of Figure 3 is not close to this collisionless sheath situation: based on the measured cathode sheath thickness (typically 2.4 cm) and the largest charge exchange collision cross section (4 x IO-” cm2 for R9), it can be estimated 2~3that less than 40% of the energy would be transported to the cathode by ions. Thus for situations where the cathode sheath does not approach collisionless conditions, as in this work, most of the energy will be transported to the cathode by energetic neutrals. Primarily, these will be NY and Ar’ species, based on consideration of the relatively large cross sections for reactions R6 and R9. It should be noted that OES, used as set

up in this study, would be unsuitable for assessing the energy transportation characteristics of such discharges, because discrimination between the spectral output from thermal and energetic neutrals in the cathode sheath would not be possible. Finally, a comment is needed on the reliability of published cross section data. In the case of reaction R7, a recent collection of experimental data from various sources shows considerable scatter at low energies22. This, at least in part, can be attributed to variations in the states of the participating species between different sets of apparatus used for cross section measurement. For example, it has been found that rotational excitation of N, may increase the rate constant for R723. Therefore, it seems that some caution should be exercised when interpreting such data to explain discharge characteristics and this exacerbates the problems arising from the limited availability of information on collision cross sections in general. 5.3. Implications for plasma nitriding. Currently, most commercial plasma nitriding is performed with DC diode layouts, which are usually operated at 100-1000 Pa. Nitrogen-hydrogen mixtures are commonly used ; the presence of hydrogen has been found to provide benefits in terms of surface oxide depassivation and hardness properties2”28. Although the discharges considered in the present study are unsuitable for direct comparison, effects such as the dissociation of nitrogen in the cathode sheath have been observed by OES under these commercially used conditions2’. The conditions investigated in this work have greater relevance to possible future developments in commercial nitriding. In recent years, there has been a growing interest in performing plasma nitriding at very low pressures (0.5-20 Pa), using thermionically enhanced discharges. This can minimise white layer formation and the process may be performed in commercial ion plating equipment so that duplex nitriding/coating treatments are possible4,” 35. In some cases, discharges of nitrogen-argon mixtures have been used33m35and it is noteworthy that an Ar/N, ratio of l/4 was found to provide more effective nitriding than using pure nitrogen”. It is not clear though, as to whether the role of nitrogen dissociation in the cathode sheath makes a significant contribution to nitriding performance. Although the exact mechanisms of plasma nitriding seem to remain a matter of debate, the availability of nitrogen atomic species at the substrate surface must beneficially influence diffusion into the metal lattice. As discussed in Section 5.2, thermionically enhanced discharges can enable the attainment of a cathode sheath in which few collisions occur, hence most of the energetic nitrogen species bombarding the cathode are N: and the production of nitrogen atomic ions and neutrals by dissociative charge exchange events is negligible. However, under the action of energetic bombardment, molecular nitrogen can readily be expected to dissociate at the cathode surface, since the energy requirement is only 9.7 eV”. Compared with a diode layout, the energy spread of the non-thermal bombarding species in a thermionic triode can be significantly reduced, so that the average energy is closer to the maximum obtainable under the applied cathode voltage’,‘. Thus under thermionic triode nitriding conditions, the requirement for molecular dissociation at the cathode surface (9.7 eV) may be exceeded by virtually all the arriving species which are nonthermal, making the role of dissociation in the sheath less important. Despite the benefits offered by the use of thermionic triode discharges for nitriding, there are two effects which could have

KS Fancey:

Optical emission spectroscopy

for some commercial applications: (i) bombardment intensity decreases exponentially with distance from the thermionic emitter and is also lower on surfaces remotely positioned from the emitter 2,36; (ii) contamination of substrate surfaces by tungsten sputtered from the filaments used for thermionic emission might become significant for longer processing timesi6. Discharge enhancement may be achieved by other means, although the magnitude of enhancement available would not necessarily be as high. For example, the nitrogen could be subjected to Penning ionisation, if mixed in suitable proportions with an appropriate gas such as neon or helium, to increase the cathode current density derived from nitrogen ions. A diode configuration with such a discharge might provide considerable advantages in nitriding performance over a more conventional gas mixture in a similar layout. Moreover, an OES system which can sample from the cathode sheath, as used in this study, could give invaluable information in respect of optimising the gas mixture for the Penning effect and other process parameters. implications

increased incident ion energies arising from the lower total gas pressure used. The various collisions and resulting reactions which can occur in nitrogenous discharges make them a complex case for study. The situation is exacerbated by the fact that the available published information on many of the key reactions is scarce or possibly unreliable. Nevertheless, this study has demonstrated how a simple OES system can be employed to contribute towards a further understanding of collision mechanisms and their role in plasma processing applications. Acknowledgements 1 would like to thank A Matthews, A Leyland and A S James of the Research Centre in Surface Engineering for their invaluable help and useful comments. The work reported utilises data gathered on projects originally funded with support from the Engineering and Physical Sciences Research Council. References Phys Rev, 131,219 (1963). IEEE Tram Plasma Sci, 18,869 (1990). 'K S Fancey and A Matthews, SurfCoat Tech&, 33, 17 (1987). 4A Leyland, K S Fancey, A S James and A Matthews, SurfCoat Technol, 41,295 (1990). ‘K S Fancey and A Matthews, Vacuum, 41,2196 (1990). ‘J W Coburn and M Chen. J. Appr Phys, 51, 3134 (1980). ’ F Howorka, J Chem Phys, 68,804 (1978). ’ D Rapp, P Englander-Golden and D D Briglia, J Chem Phys, 42,408 1 (1965). ‘W B Maier 11, J Chem Phys, 41, 2174 (1964). “‘W McGowan and L Kerwin, Can J Php, 42,2086 (1964). ” S V Dresvin (ed.) Physics and Technology oflow Temperature Plasmas, p.56. Iowa State University Press, Ames (1977). ” L Petitjean and A Ricard, J Phys D : Appi Phys, 17,919 (1984). “W B Maier II, J Chem Phgs, 55, 2699 (1971). “W B Maier II and R F Holland, J Chem Phys, 59, 4501 (1973). “B N Chapman, Glow Discharge Processes, p.52. Wiley, New York (1980). ” H F Winters, J Chem Phys, 44, 1472 (1966). ” D Rapp and P Englander-Golden, J Chem Phys, 43, 1464 (1965). ” K S Fancey and A Matthews, Advanced Surf&e Coatings: a Handbook et’ Surfhce Engineering, D S Rickerby and A Matthews (eds), p. 140. Blackie. New York (1991). I9J M Poitevin and G Lemperiere, Thin Solid Fibns, 120, 223 (1984). ‘“G Lemperiere and J M Poitevin, Vacuum, 37, 825 (1987). ” R C Amme and H C Hayden, J Chem Phys, 42,201l (1965). *’ G Parlant and E A Gislason, J Chem Phys, 86,6183 (1987). “A A Viggiano, J M Van Doren, R A Morris and J F Paulson, J Chem Phys, 93, 4761 (1990). ‘4 M Hudis, J Appl Phys, 44, 1489 (1973). “C K Jones, S W Martin, D J Sturges and M Hudis, Proc. Conf. On Hear Treafment, London, 1973, P.71. Metals Society, London (1975). 2hA M Staines and T Bell, Thin SolidFihns, 86, 201 (1981). “A Grill and D Itzhak, Thin SolidFilms, 101, 219 (1983). ‘*B Xu and Y Zhang, Surf&g, 3,226 (1987). “K Rusnak and J Vlcek, J Phys D: Appl Phys, 26,585 (1993). “A S Korhonen and E H Sirvio, Thin Solid Films, 96, 103 (1982). ” A Matthews, J Vat Sci Technol A, 3,2354 (1985). “A Leyland, K S Fancey and A Matthews, SurfEng, 7,207 (1991). “M Van Stappen, B. Malliet, L. Stals, L De Schepper, J R Roos and J P Celis. Muter Sri Eng, A140, 554, (1991). “J D’Haen, C Quaeyhaegens, L M Stals and M Van Stappen, Surf Coat Technol, 61, 194 (1993). ‘“E I Meletis and S Yan, J VQCSci Technol A, 11,25 (1993). ” K S Fancey and A Matthews, Thin Solid Films, 193/194, 17 1 (1990).

’ W D Davis and T A Vanderslice,

6. Conclusions By using OES to study DC diode and thermionic triode discharges of nitrogen and nitrogen-argon mixtures, the following information has been obtained. (i) For diode discharges of nitrogen-argon mixtures at a constant total pressure of 6.67 Pa, measurements from the middle of the cathode sheath indicate that the relationship between the proportion of nitrogen in atomic form and the nitrogen number density in the sheath (at constant sheath thickness) approximates to a power law with an index close to unity. From this, it can be inferred that the dominant dissociation mechanism is the same as for nitrogen-only discharges, i.e., charge exchange by NT-N: collisions, as opposed to reactions involving argon species. Furthermore, the frequency of dissociation events in the sheath seems to be comparable to that of pure nitrogen discharges operated over the same range of nitrogen pressures. (ii) For the nitrogen-only case, the proportion of atomic nitrogen in the plasma of the discharges in (i) is very small when compared with that of the sheath region. When argon is added, this increases and, if the N,/Ar gas ratio is very low, the fraction of atomic nitrogen is higher in the plasma than in the sheath. Although dissociation by Ar2+ -Ni collisions in the plasma may be making a contribution, it is believed that dissociation by electron impact is highly significant; this latter mechanism becomes more prominent at very low N,/Ar gas ratios as the presence of argon is thought to reduce the quenching effect that nitrogen has on electron energy. (iii) For thermionic triode discharges of nitrogen-argon mixtures at a constant total pressure of 1.33 Pa, measurements made as in (i) show that the proportion of atomic nitrogen in the sheath does not significantly change with nitrogen number density. This is attributed to the increased prominence of dissociation by Ar+NY and Ar2+-N: collisions over those of NT-NY. In contrast with the higher discharge pressure used in (i) it is suggested that the argon-based collisions become more significant here because of possible changes in collision cross sections as a result of

700

’ K S Fancey and A Matthews,