The effect of small quantities of hydrogen on a glow discharge in neon. Comparison with the argon case

Abstract

In order to gain an insight into the numerous effects caused by the addition of hydrogen to an argon glow discharge, comparison experiments have been made with neon as carrier gas. In some cases the effects are diminished or even disappear. Using various bulk samples, such as copper, steel and titanium, sputtered in a neon glow discharge, the intensities of the analytical lines are affected by the presence of hydrogen in different ways from argon; the dependent parameter (the discharge current in this work) and the sputtering rate vary less than in argon. The crater shape and roughness are also affected and these effects are discussed qualitatively. Probably the most important spectral feature caused by hydrogen in the case of a discharge in argon is the emission of a continuous background. This does not appear in neon under similar discharge conditions and only weakly at high hydrogen concentrations. This supports the suggestion, made in previous work, that an effective quenching process of the argon metastables (11.55 and 11.72 eV) is Penning excitation of the hydrogen molecules, and subsequent decay to a repulsive state with emission of the continuum; in neon the energy match does not occur. It was found with neon, as with argon, that similar features occur as when hydrogen is introduced in different ways into the glow discharge: as a molecular gas contamination or as a constituent of the sample. Glow discharge mass spectrometry (GD-MS) experiments carried out with both argon and neon support the results obtained by optical emission spectrometry (OES) and provide further relevant information.

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1. Introduction

The most important contributions in the GD-OES and GD-MS literature about the effects of impurity gases on the analytical results and on the understanding of the basic elementary processes in the glow discharge have been summarised in our previous papers.^1,2 Extensive work regarding the effect of water vapours or methane in a glow discharge source (GDS), which have similar effects to those caused by hydrogen, has been reported by various authors.^3–8 The first evidence regarding the implications of the hydrogen effect on the GD-OES analysis of steels was reported by Bengtson and Hänström in 1998,⁹ and since then further experimental work has been carried out in IFW Dresden, BAM Berlin and the University of North London, followed more and more by explanations. A theoretical review regarding the possible mechanisms taking place in an argon glow discharge containing hydrogen has been recently published by Bogaerts and Gijbels.¹⁰

Traces of hydrogen are always detected in a GDS to a greater or a lesser extent, especially at the beginning of the glow discharge analysis. These arise from residual moisture in the GDS and on the sample surface, gaseous hydrocarbons coming from the pre-vacuum oil-pumps, leakage of water vapour through the samples, etc. It has been demonstrated^2,9 that similar effects to those produced by gaseous hydrogen occur when hydrogen compounds are present in the sample. Therefore, corrections of such effects must be included in the quantification algorithms. Also disturbances in the sputtering phenomena must be taken into account when hydrogen is present in a GDS. As has been reported,^1,2,9 the sputtering rates of different samples decrease when hydrogen is added to an argon GDS, mainly due to a marked drop in the current under constant voltage and pressure conditions. However, certain sample matrices, such as titanium or silicon, are mentioned in the literature^11,12 as being predisposed to chemically reduced physical sputtering by the forming of new hydride compounds at the surface of the sample. Tabares and Tafalla¹³ suggest that the implantation of hydrogen in the metal surface is the main process responsible for the sputtering suppression.

2. Experimental

The experiments were performed with a commercial surface depth profile GD-OES instrument (LECO SDP-750, Kirchheim, near Munich, Germany) with a multi-channel polychromator and a Grimm-type source of 2.5 mm diameter to which was attached a Czerny–Turner monochromator Digi-krom 480 (CVI Laser Corporation, Albuquerque, NM, USA) with a spectral resolution of about 0.2 nm. Owing to the better possibilities for studying the emission continuum of hydrogen over a wide spectral range, a plane grating spectrograph PGS 2 (Zeiss-Jena, Germany) has been used to record the spectra emitted by a separate glow discharge source having an 8 mm anode diameter. Details of this arrangement can be found elsewhere.¹⁴ The spectral resolution of the latter spectrograph is 0.02 nm and the spectral range is about 200–770 nm. The optical signals can be integrated photographically over long periods and a microdensitometer MD100 (Zeiss-Jena,Germany) linked to appropriate software has been used to measure the photographic density of the spectra acquired on photographic plates.

A gas mixing system using two gas cylinders (one with pure neon and one with pure hydrogen) and two mass flow controllers, MKS Instruments, Andover, MA, USA, Type 1179A (for the neon line with a maximum flow of 2000 sccm and for the hydrogen line 100 sccm), was used to obtain defined partial pressures in the GDS. The total flow rate was adjusted so that at a fixed voltage of 1000 V, the currents were comparable with those recorded for discharges in argon. As in the previous work, the total pressure in the GDS was kept constant with variation of the partial pressure of hydrogen achieved by adjusting both mass flows. The contamination of the GDS was consistently monitored and reduced to the minimum, long pre-sputtering times being selected to get accurate stable data, a prerequisite for the experiments carried out in this work. The other possibility of introducing hydrogen into the GDS has been realised by sputtering a titanium hydride layer (approximately 5 µm thick) onto pure titanium sheet metal. The crystalline phase of the TiH₂ layer has been confirmed by X-ray diffraction measurements performed in IFW Dresden.

Investigations on the sputtering rate, crater shape and roughness of some common bulk samples have been qualitatively performed by using a Mahr-Perthen (Göttingen, Germany) mechanical stylus profilometer to profile the sputtered craters.

3. Results and discussion

As this paper discusses the use of neon as a carrier gas in a glow discharge containing small quantities of hydrogen, it should be read in conjunction with a previous paper,² referring to similar experiments with argon. However, it must be noted that for neon the hydrogen content was increased until the observed quantities reached roughly stable values, so that the maximum hydrogen partial pressure was considerably higher than for argon. It should also be kept in mind that glow discharge sources were used, with different pumping systems, having different anode diameters, or even somewhat different structures, e.g., the GD-MS source.

Fig. 1 shows the intensities of the lines selected for the analysis of some common elements with the commercial GD-OES spectrometer, LECO SDP-750, using a 2.5 mm diameter anode tube, as functions of the hydrogen relative partial pressure when neon is used as a carrier gas. This figure is directly comparable with the data in Fig. 1 of reference 2 for changes in argon, except for the higher partial pressures of hydrogen. Common bulk samples, such as copper, stainless steel, titanium, aluminium and silicon wafers, have been investigated. Because no photomultiplier is set for recording a neon line, the attached monochromator was used as a supplementary acquisition channel for monitoring the intensity of an arbitrarily selected neon line, Ne I 594.48 nm.

Fig. 1 Intensity of emission lines of analytes from some common matrices depending on the content of hydrogen: (a) copper matrix, Cu I 327.3 nm, Cu II 219.2 nm; (b) steel matrix, Fe I 371.9 nm, Fe II 249.3 nm, Cr I 425.4 nm, Ni I 349.2 nm, W I 429.4 nm, Mn I 403.4 nm, Mo I 386.4 nm; (c) titanium matrix, Ti I 365.3 nm; (d) aluminium matrix, Al I 396.1 nm; and (e) silicon matrix, Si I 288.1 nm, together with the Ne I 594.48 nm and H I 121.5 nm recorded with a commercial LECO SDP-750 spectrometer and the discharge current I. GDS diameter: 2.5 mm; V_dc = 1000 V, pressure: ≈23.1 mbar (corresponding to 1200 sccm total mass flow).

The dependent parameter, the discharge current, also shown as I_dc in Fig. 1, suffers only a slight decrease or even remains unaffected by the addition of hydrogen compared with the rapid fall in argon. Sputtering rate measurements reported in ref. 1 for copper in argon suggested a direct proportionality of the sputtering rate to the discharge current, and hence a good agreement with the Boumans equation,¹⁵ and an almost constant reduced sputtering rate (sputtering rate divided by discharge current). Similar measurements performed for copper sputtered in neon mixed with different amounts of hydrogen also show a decrease of the sputtering rate, but to a lesser extent, for example at 1% v/v hydrogen in the GDS a decrease of about a fifth in neon (Fig. 2) in comparison with about a third in argon (see Fig. 1, reference 1). The variation of the current is small in the case of neon and so the reduced sputtering rate of copper falls somewhat (see Fig. 2).

Fig. 2 Discharge current (I_dc), sputtering rate (SR) and reduced sputtering rate (SR/I_dc) during sputtering of copper at different concentrations of hydrogen in neon. SR, —●—; I_dc, —▲—; SR/I_dc, ——. GDS diameter: 2.5 mm; V_dc = 1000 V, pressure ≈23.1 mbar (corresponding to a total mass flow of 1200 sccm).

The intensities of all the analyte lines shown in Fig. 1, except Si I 288.1 nm, decrease to a greater or lesser extent with the hydrogen partial pressure, but there are considerable variations in the magnitude of the effect and between discharges in argon and neon. Despite the almost constant current and sputtering rate, the intensity recorded for Cu I 327.3 nm ("Cu1") falls with increased hydrogen content in neon, whereas it rises in argon, when the current falls greatly. This line is, of course, subject to self-absorption and self-reversal, and additional high resolution line profile measurements have shown that whereas in argon, the addition of hydrogen reduces the self-reversal, in neon it increases it! This illustrates the complexity of the processes involved. In argon the intensity of Cu II 219.2 nm ("Cu2"), partially excited by charge transfer (CT), falls in contrast to the rise in "Cu1". In neon, both lines decrease in intensity at approximately the same rate. It should be remembered that Fig. 1 only shows the intensities recorded by the LECO SDP-750 for selected analytical lines. Various other lines from a given element may behave in differing ways, as has already been reported for copper and argon lines.¹ Measurements on a number of copper lines in neon have shown that, whereas the intensities of lines mainly excited by CT in argon decrease dramatically when hydrogen is added to argon, those Cu II lines excited by CT in neon, e.g., 248.58 or 250.63 nm, decrease much less, by approximately the same amount as Cu I lines, when hydrogen is added to the neon. Thus, it appears that hydrogen quenches the argon ions efficiently and the neon ions to a lesser extent, if at all, evidence which is confirmed by the mass spectrometry measurements discussed in the final part of this paper.

The behaviour of analytical lines recorded from stainless steel and aluminium samples is very similar in neon and in argon, intensities falling to a greater or a lesser extent. With a titanium sample in neon, there is a dramatic fall in the intensity of the Ti I 365.3 nm line, possibly due to a marked reduction in the sputtering rate. Using a silicon sample, the intensity of the Si I 288.1 nm line increases with hydrogen content in both neon and argon, but the rate of change is much greater in argon (>300% for 0.1% v/v hydrogen) than in neon (∼25% for 0.1% v/v hydrogen), although in neon a change of 300% is observed with 1.5% v/v hydrogen. Measurements of numerous other Si lines excited in argon with upper energies in the range 4.92–6.62 eV show similar behaviour to that of the Si I 288.1 nm line (excitation energy 5.08 eV). So far, no explanation has been found for this effect, given the large energy range (1.7 eV), an energy selective excitation process being excluded. Generalising, one can state that the alterations to the intensities of analyte lines by hydrogen are considerable and extremely complicated, as in the argon case, with the greatest rates of change for hydrogen content up to an amount of 0.25%v/v.

Variations of the electrical parameters, sputtering rate and crater profiles have been studied in more detail using a GDS with a bigger anode diameter (8 mm). Bulk samples of copper, stainless steel and titanium were used with this GDS and representative data for 2% v/v hydrogen in the noble gas are shown in Table 1. In order to have a comparison with the argon case, data for both argon and neon are presented. Also, in order to have an appropriate basis for comparison, the neon pressure was adjusted so that for pure argon and pure neon there are similar currents in the GDS for each sample at a constant voltage of 1000 V.

Table 1 GDS parameters of argon and neon at voltage = 1000 V and similar currents in each gas, and with 2% v/v hydrogen in each case (the discharge current is the dependent parameter), for three common bulk samples. GDS diameter: 8 mm

	Discharge in argon			Discharge in neon
Sample	Total pressure/mbar	Pure Ar:Current/mA	2% v/v H2/Ar:Current/mA	Total pressure/mbar	Pure Ne:Current/mA	2% v/v H2/Ne:Current/mA
Copper	4.0	76	62	10.3	78.3	83.2
Stainless steel	4.4	62.5	45	8.7	64	67.5
Titanium	4.2	86.5	54.4	10.4	84.8	80.5

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As can be clearly seen, the considerable decrease in the discharge current (the dependent GDS-parameter) in the case of argon is replaced in the case of neon with small variations (increase for copper and stainless steel and decrease for titanium). Despite the slight variation of the current in the case of neon, the reduced sputtering rate of copper (sputtering rate divided by discharge current) is, as in the case of argon, roughly constant. It should be noted that the reduced sputtering rates do not differ greatly in neon and argon (about 7 µm s⁻¹ mA⁻¹).

Other phenomena of the sputtering process, such as modification of the sputtering crater shape and even of the roughness must be taken into account when hydrogen is present in a GDS. For different samples, hydrogen causes decreases in the sputtering rates with neon though smaller than in argon, even despite the slight increase of the discharge current (copper and stainless steel). For copper and stainless steel, either in neon or in argon, the presence of hydrogen causes a more convex crater shape and seems to reduce the roughness. A comparative example is illustrated in Fig. 3, where copper and stainless steel are sputtered in pure argon and neon and in mixtures of each with 2% v/v hydrogen. Quantitative roughness measurements have not yet been performed. Due to the various changes observed for different samples, future experiments related to the changes of the crater shape and roughness are planned. In the case of argon, it is unclear whether the changed shape and roughness are the result of the presence of hydrogen, or of the lower current; with neon the current change is small (<5%), implying that the changes are clearly attributable to the presence of hydrogen!

Fig. 3 Crater profilograms for copper (Cu) and stainless steel (SS) sputtered in argon and neon under the conditions presented in Table 1.

One of the relevant findings in the previous experiments carried out in an argon GDS was the emission continuum observed in the spectral range of ≈220–440 nm. This was independent of the sample used and could be attributed to the excitation of the H₂^* (2sσ ³Σ_g) state and its subsequent decay to the repulsive H₂^* (2pσ ³Σ_u) state, leading to dissociation of the hydrogen molecule (process 1):

The excitation could be by Penning excitation (PE) of the hydrogen molecule by argon metastables, as also suggested by Prince et al.¹⁶ The [italic v]′ = 0 vibrational state of the 2sσ ³Σ_g lies 11.79 eV above the ground state, and could be excited by the argon metastables at 11.55 eV and especially at 11.72 eV, the excess energy required being supplied by thermal energy^16,17 (process 2).

An alternative explanation is that the hydrogen upper molecular state is excited by electron impact. However, with neon, where PE of the hydrogen molecule is not possible, no significant continuum was observed under similar experimental conditions (except at considerably higher partial pressures of hydrogen). Relevant spectra, which show the emission of the continuum in the argon case only, are presented in Fig. 4, with titanium as a sample. Very similar spectra, in terms of the emission continuum, were obtained for the copper and steel samples. The absence of a continuum in the neon case, although electron impact excitation of the hydrogen molecule would still be possible, strongly supports the hypothesis that Penning excitation is the relevant process in the argon case.

Fig. 4 Emission scan spectra (230–360 nm) of a titanium bulk sample sputtered in pure argon and neon and in mixtures of 2% v/v hydrogen in argon and neon, after 5 min acquisition time with a plane grating spectrograph. The GDS parameters correspond to the Ti case in Table 1: GDS anode diameter, 8 mm; V_dc = 1000 V; pressure, 4.2 mbar for argon and 10.4 mbar for neon.

In a further series of experiments using neon, higher voltages and lower pressures, the continuum was still not observed with 2% v/v hydrogen, scarcely at 5% v/v, but became clearly visible at a hydrogen concentration of 10% v/v. The continuum was observed for the other samples (copper, steel, etc.) and its appearance was not dependent on the sputtered material. Copper was chosen in Fig. 5 as an example for this case. Hence, in argon, the dominant process of exciting the hydrogen continuum is the PE by the argon metastables, while in neon another one, probably electron impact, becomes significant at higher hydrogen concentrations (process 3):

Fig. 5 Scan spectra (230–360 nm) of a copper sample sputtered in neon containing relevant selected relative partial pressures of hydrogen: (a) 0% v/v; (b) 2% v/v; (c) 5% v/v; and (d) 10% v/v. GDS diameter: 8 mm; V_dc = 1500 V; pressure, 4.58 mbar (corresponding to a 700 sccm total flow); current, (a) 32.2 mA, (b) 42.2 mA, (c) 41.5 mA and (d) 41.6 mA.

In order to get a picture of the variation of the dependent parameter (the current) for different samples and different hydrogen concentrations in neon, some representative data are included in Table 2. It should be noted that the most significant variation of the current ranges below 1% v/v hydrogen.

Table 2 Variations of the discharge current as the dependent parameter in a neon GDS for different common samples. GDS diameter: 8 mm; V_dc = 1200 V; pressure = 4.58 mbar

	Hydrogen in neon (% v/v)
	0	1	2	5	10
Sample	Discharge current/mA
Copper	24.3	28.4	29.1	29.1	28.5
Stainless steel	27.8	33.0	33.4	34.8	34.7
Titanium	25.0	24.5	24.7	24.9	24.3

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In the investigation of changes induced by hydrogen in the GDS features, special attention was paid to the comparison of the effects caused by the use of different hydrogen sources: (i) as a molecular impurity introduced into the GDS together with the neon; and (ii) as a sample constituent. Fig. 6 illustrates the similar values of the parameters, measured by the commercial LECO GDS-750, observed when hydrogen is liberated into the neon discharge from a TiH₂ layer and then when hydrogen is added externally while the titanium substrate is sputtered. The various hydrogen concentrations could be set with the existing gas mixing system and one can see that comparable values with the sputtering of TiH₂ in pure neon can be reached when the Ti substrate is sputtered in neon containing 0.066% v/v hydrogen, in terms of I_dc, the neon line Ne I 594.48 nm, hydrogen line H I 121.5 nm and roughly the Ti I 365.3 nm line. The neon line was measured by use of the external acquisition channel provided by the attached monochromator. Also, due to the instrumentation, the order of the changes in hydrogen concentrations is towards low values, in contrast to the similar picture (Fig. 4) in ref. 2 using argon.

Fig. 6 Depth profile of a TiH₂/Ti sample in pure neon up to 300 s, followed by sputtering of the titanium substrate when different contents of hydrogen are added. GDS diameter: 2.5 mm; V_dc = 1000 V; pressure ≈ 23.1 mbar (corresponding to a total mass flow of 1200 sccm). The two hatched areas show the similar intensity patterns for the two different origins of hydrogen.

A comparison of the spectra emitted by the TiH₂ layer in pure argon and in neon is also interesting from the point of view of the hydrogen continuum. Fig. 7 shows the clear difference regarding the emission background. Similar to the case when hydrogen is introduced into the argon GDS externally as a molecular impurity (see Fig. 4), an emission continuum can also be observed when TiH₂ is sputtered in pure argon. Detailed measurements are reported in ref. 2. By contrast with the case of argon, and similar to the emission spectra of titanium in pure neon, see Fig. 4, no emission background can be observed when TiH₂ is sputtered in pure neon, despite the higher pressure (6.4 mbar in neon in comparison with 4.2 mbar in argon) and sensible lower currents. One can draw the conclusion that, independent of the hydrogen origin, and independent of the carrier gas (argon or neon), similar processes take place in the glow discharge plasma and at sputtering, because similar experimental features are measured. Bogaerts and Gijbels¹⁰ have shown that once hydrogen is present in the discharge, dissociation and recombination processes balance, and the degree of dissociation depends on the experimental parameters.

Fig. 7 Scan spectra (230–360 nm) of TiH₂ layer (after 5 min exposure time with a plane grating spectrograph) sputtered in pure argon and pure neon. GDS diameter: 8 mm; V_dc = 1000 V; argon GDS pressure = 4.2 mbar and neon GDS pressure = 6.4 mbar.

In order to obtain supplementary information, a glow discharge source based on the plane cathode Grimm source, but specially designed for mass spectrometry,¹⁸ was used. Copper and titanium were chosen as samples for comparative experiments in argon and in neon. Again in neon the pressure needed to be higher than in argon, i.e., 8.4 mbar (corresponding to 200 sccm) compared with 3.92 mbar (corresponding to 50 sccm). Only two situations were tested: (i) pure argon and neon; and (ii) their mixtures with 2% v/v hydrogen. The detected ionic signals are shown for each case in Fig. 8.

Fig. 8 Mass spectrometry signals of: (a) Cu sputtered in pure Ar and in 2% v/v H₂/Ar; (b) Cu sputtered in pure Ne and in 2% v/v H₂/Ne; (c) Ti sputtered in pure Ar and 2% v/v H₂/Ar; (d) Ti sputtered in pure Ne and in 2% v/v H₂/Ne; and the corresponding currents. GDS diameter: 8 mm; V_dc = 800 V; pressure: 3.92 mbar, corresponding to 50 sccm for argon and 8.4 mbar, corresponding to 200 sccm for neon.

Argon case

A clear quenching of the argon ion signal, independent of the sample, can be observed as a result of the hydrogen presence in the glow discharge. This may be partly due to a drop in the argon metastable population as suggested in ref. 1, but is at least partly due to the formation of ArH⁺, which is very prominent in the mass spectrum, the reaction occurring with a high rate coefficient (in the order of 10⁻¹⁰–10⁻⁹ cm³ s⁻¹), as pointed out in ref. 10. Similar to the results of Saito,¹⁹ the copper signal increases considerably and the Ti one varies slightly. The fact that the signals recorded for masses 46, due only to ⁴⁶Ti (natural abundance 8.0) and 47, which could be due to ⁴⁷Ti (natural abundance 7.5), or ⁴⁶TiH, vary in the same way when hydrogen is introduced into the GDS shows that no significant quantity of titanium hydride is formed in the plasma. The decrease of the current is more or less similar to the OES-sources, as expected. The great reduction in the argon ion intensity explains the marked decrease in the intensity of Cu II spectral lines excited by charge transfer,¹ despite the fact that the intensity of Ar II spectral lines increase;¹ it was previously suggested that these levels were excited directly from the ground state of the atom and were not affected by changes in the populations of atomic metastables or ground state ions.

Neon case

By contrast with the argon case, for a copper sample the Ne⁺ signal increases considerably when hydrogen is introduced (by a factor of about 3 for a 50% increase in current) and for a titanium sample both current and Ne⁺ signal remain almost constant. Both the Cu⁺ signal and the Ti⁺ one increase considerably on addition of hydrogen. Again, the fact that the Ne⁺ signal is not quenched explains the OES observation that Cu II lines excited by CT in neon are only moderately affected by the presence of hydrogen, although Ne II spectral lines fall in intensity. This stresses the need for both OES and MS experiments to get a fuller picture of processes in the discharge. Again no titanium hydride is formed. Similar to the OES sources, the current increases for copper and slightly decreases for titanium.

Concluding, considering only the influence of hydrogen on the glow discharge, the use of neon as a carrier gas in the place of argon eliminates the analytical problems caused by the hydrogen continuum, but the intensity of analytical lines can still be seriously affected. It also provides some further information on the elementary processes taking place in a GDS.

ack

The authors are indebted to R. Dorka, G. Pietzsch, M. Kunstár (IFW Dresden), R.-D. Schulze, and A. Lippitz (BAM Berlin) for the technical assistance. V.-D. H. thanks the EU-GDS Network for funding visits to IFW-Dresden.

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Footnote

† Presented at the Tenth Biennial National Atomic Spectrometry Symposium (BNASS), Sheffield, UK, July 17–20, 2000.

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