The impact of molecular emission in compositional depth profiling using Glow Discharge-Optical Emission Spectroscopy

Spectrochimica Acta Part B 63 (2008) 917–928

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Review

The impact of molecular emission in compositional depth profiling using Glow Discharge-Optical Emission Spectroscopy Arne Bengtson Corrosion and Metals Research Institute, Dr. Kristinas väg 48, Stockholm, Sweden

A R T I C L E

I N F O

Article history: Received 4 October 2007 Accepted 16 May 2008 Available online 29 May 2008 Keywords: Glow discharge Molecular emission Depth profiling

A B S T R A C T The scope of this paper is to investigate and discuss how molecular emission can affect elemental analysis in glow discharge optical emission (GD-OES), particularly in compositional depth profiling (CDP) applications. Older work on molecular emission in glow discharges is briefly reviewed, and the nature of molecular emission spectra described. Work on the influence of hydrogen in the plasma, in particular elevated background due to a continuum spectrum, is discussed. More recent work from sputtering of polymers and other materials with a large content of light elements in a Grimm type source is reviewed, where substantial emission has been observed from several light diatomic molecules (CO, CH, OH, NH, C2). It is discussed how the elevated backgrounds from such molecular emission can lead to significant analytical errors in the form of “false” depth profile signals of several atomic analytical lines. Results from a recent investigation of molecular emission spectra from mixed gases in a Grimm type glow discharge are presented. An important observation is that dissociation and subsequent recombination processes occur, leading to formation of molecular species not present in the original plasma gas. Experimental work on depth profiling of a polymer coating and a thin silicate film, using a spectrometer equipped with channels for molecular emission lines, is presented. The results confirm that molecular emission gives rise to apparent depth profiles of elements not present in the sample. The possibilities to make adequate corrections for such molecular emission in CDP of organic coatings and very thin films are discussed. © 2008 Published by Elsevier B.V.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Characteristics of molecular optical emission . . . . . . . . . . . . . . . . . 1.2. Effects of molecular emission on atomic emission spectroscopy . . . . . . . . 1.3. Previous experimental observations of molecular spectra in depth profiling using 1.4. Molecular emission intensity as a function of discharge parameters. . . . . . . 2. Experimental study of molecular emission in mixed gases . . . . . . . . . . . . . . 2.1. Addition of N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Addition of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Addition of H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Addition of N2 plus H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Effects of molecular emission on compositional depth profiling (CDP) . . . . . . . . . 3.1. Polymer coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thin films. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

E-mail address: [email protected]. 0584-8547/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.sab.2008.05.005

It goes without saying that molecular optical emission can be observed in glow discharges and other plasma sources, provided that

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at least a fraction of the discharge gas is molecular. The purpose of this paper is to investigate and discuss how such molecular emission can affect elemental analysis in glow discharge optical emission (GD-OES), particularly in compositional depth profiling (CDP) applications. The first part is an overview of work found in the literature, including a brief discussion of the characteristics of molecular emission spectra. The second part is devoted to results of new experimental work, highlighting problems related to CDP applications. There are numerous examples in the literature of observations of molecular emission in various types of plasmas, including glow discharge devices [1–7]. In fact, some of these devices were designed to use signals from molecular emission to detect organic compounds [5,6]. However, the frequently used Grimm type glow discharge source [8] is generally considered to be an “atomic source” where molecular emission is of minor concern, since the source is normally operated with argon or some other noble gas. While this does hold true for most applications to metallic samples, observations of molecular emission from both OH and N2 was detected in early types of a radio frequency (RF) powered glow discharge source, similar to a Grimm type [9,10]. In these cases, the molecular emission was attributed to small air leaks, but this was not investigated in any detail. Another early work mentions the danger of interference from OH on frequently used atomic lines for Bi and Al [11]. Smid et al. [7] has carried out an extensive investigation of the effect of nitrogen on analytical glow discharges, including high resolution studies of molecular bands. One important observation in their work was that the proportion of nitrogen added to the discharge gas does not effect the rotational intensity distribution. More recently, substantial emission in Grimm sources from several light diatomic molecules (CO, CH, OH, and NH) has been observed when sputtering polymers and other organic materials [12,13]. These observations showed that it is necessary to further investigate the role of molecular emission in glow discharge optical emission (GD-OES), in order to avoid analytical errors in such applications. 1.1. Characteristics of molecular optical emission A molecular band system, corresponding to a particular electronic transition, consists of a number of bands, each covering a broad spectral region, and containing very many lines corresponding to the transitions between individual rotation levels [14,15]. With low resolution spectrometers, such structure is often not resolved see Fig. 1 displaying part of the emission spectrum of CO. Depending on the masses of the atoms and the strength of the molecular bond, each band usually has a sharp maximum or minimum wavelength termed “band head”, a feature clearly seen in Fig. 1. Table 1 shows an example of how these band heads are classified and organised in a “Deslandres

table” according to the vibronic states of the electronic energy levels of the transition. The symbols ύ and ϋ refer to the vibronic levels of the upper and lower electronic levels respectively. A few of the band heads in Table 1 are identified in Fig. 1. The spectrum includes several partially overlapping sequences (transitions of the same Δν; diagonals in the Deslandres table). For further details about the structure and nature of molecular emission spectra, see [15]. 1.2. Effects of molecular emission on atomic emission spectroscopy From the broad shape of the molecular emission bands, it is readily understood that there will be spectral overlap with several atomic analytical lines. In e.g. the relatively narrow spectral segment of Fig. 1, there are frequently used analytical lines from C, N, P, S and B. Therefore, we observe elevated backgrounds from molecular emission (“line interference”) at atomic emission lines in several GD-OES applications involving e.g. polymer coatings. Since most GD-OES instruments do not record spectra but are polychromators with fixed wavelength channels, such interference is easily mistaken as real analytical signals. Therefore, in order to avoid analytical errors we must be able to identify such artefacts, and to make effective corrections to the background signals. In addition to spectral overlap, there are other effects of molecular species in the glow discharge plasma that can be detrimental to analytical accuracy. Some of the effects of hydrogen in particular were first described in 1998–2000 [16–18]. First of all, hydrogen causes a very broad “continuum” spectrum over the entire spectral range 200– 500 nm approximately. This emission is due to the excitation of the H2 2sσ3Σg state, which subsequently decays to the repulsive 2pσ3Σu state, for which the energy levels are not quantised. This leads to dissociation of the molecules, consequently the existence of this continuum hydrogen spectrum is proof that dissociation of hydrogen molecules take place in the glow discharge. The background from the hydrogen continuum is a somewhat special type of “line interference”; and in fact a majority of all analytical lines used in OES are affected. It has also been shown that when sputtering a hydride sample in pure argon, the hydrogen continuum also appears [18]. It is known from the sputtering theory [19] that atoms, not clusters or molecules, are normally predominant in sputtering of metal alloys. Therefore, the observation of the hydrogen continuum is further experimental evidence that recombination of hydrogen atoms into molecules take place in the glow discharge plasma, in agreement with modelling work of Bogaerts et al. [20–22]. Consequently, a generally elevated background is expected in any material with a high concentration of hydrogen, e.g. polymers.

Fig. 1. Part of the A1Π–X1Σ+ system of CO, recorded with an epoxy-coated steel sheet; from ref. [13]. A LECO GDS500A spectrometer was used; 4 mm DC source; 15 mA 1000 V.

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Table 1 Deslandres table of the A1Π–X1Σ+ system of CO [14] ύ\ϋ

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7 8

(154.55) 150.99 147.77 144.74 141.91 139.26

159.74 156.03 150.40 149.38 146.36 143.54 138.41 (136.19)

165.33

171.25 167.00 163.07

177.53 172.96

184.19 172.26 173.07 170.54 (166.69) 162.98 159.58

(191.33) 185.98 181.12

157.69 154.25 (151.06) (148.04) 145.23 142.59 140.11

155.97 152.77 (149.80) 144.38

164.81 161.15 (154.53) (151.58) (148.81)

172.41 (168.51)

7

8

193.12 187.87 183.02 (178.52) 174.34 170.45

200.67 195.05 189.82 (185.04)

(161.53) 153.44

The upper levels ύ are in the columns, the lower levels ϋ in the rows. The wavelengths are given in nm.

Furthermore, the presence of e.g. H2 may well cause significant changes in the plasma impedance, affecting the electrical plasma parameters and thereby the intensity of analytical lines. These effects can occur for very low H2 concentrations e.g. ~ 0.05%v/v [18]. In most commercially available systems today, an active pressure regulation system is utilised to maintain constant electrical plasma parameters (e.g. constant current–constant voltage or constant power–constant voltage). However, in a constant pressure mode of operation, the effect of impedance variations and thereby the electrical parameters, can be severe [17,18]. With an active pressure regulation system that eliminates such variations, the pressure will be the only parameter that varies. Extensive investigations have shown that these variations have significantly smaller effects on line intensities than the variations of the electrical parameters in a constant pressure mode of operation [25–27]. Another effect of molecular gases in the plasma is a reduction in the sputtering rate [16–18], which causes a corresponding reduction in analytical line intensities. By dividing intensity with sputtering rate, a quantity known as emission yield (EY) is obtained [25]. This quantity, which is a measure of the emitted light/sputtered mass of a particular emission line of the sputtered element, is commonly used as a basis for quantification of compositional depth profiles (CDP) [25,26]. The EY is mainly a measure of the excitation probability of the upper level of the corresponding transition, but other processes related to the sample atom distribution, ionisation probability etc. also has an impact. Calculating emission intensity/sputtering rate with and without hydrogen in the plasma, it becomes obvious that hydrogen also has a significant impact on the EY of several analytical lines of other elements [16–18,23,24]. Assuming that a substantial fraction of primarily light elements (C, O, N and H) will combine to form molecules in the plasma, one

can also speculate that the degree of atomisation of these elements in the plasma will be reduced. Consequently, the atomic emission signals of these elements will also be reduced and the concept of “matrix-independent emission yield” [25–27], crucial for quantification of depth profiles, may be at least partially invalidated. In ref. [23], even spectra from metal hydrides (AlH, SiH) were observed for hydrogen content as low as 0.1 v/v. This could mean that also the atomisation of metals may be reduced in the presence of hydrogen in the plasma. 1.3. Previous experimental observations of molecular spectra in depth profiling using a Grimm type glow discharge source In previously published work [12], a slightly greasy (a fingerprint!) copper sample was run on a small CCD spectrometer (Spectruma GDA 150) with a readout rate of 10 Hz. The source was a standard DC Grimm type with a 4 mm anode, run at 20 mA 700 V. Emission from the OH molecule around 310 nm was identified during the first few seconds of the discharge, see Fig. 2. Even on a thoroughly degreased spot of the same sample, traces of the OH spectrum could be seen. This was the first confirmation that molecular emission may be responsible for some observed artefacts in near-surface depth profile analysis. The data from these measurements also revealed the existence of “Swan” bands from the carbon dimer C2 [15], see Figs. 3 and 4 (not previously published). These bands have been studied extensively by astrophysicists for decades, since they are prominent features in the spectra of several celestial objects, e.g. carbon stars [28]. Incidentally, these bands are also responsible for a major part of the visible light from hydrocarbon flames. Using a CCD spectrometer with higher resolution (LECO GDS500A), a more thorough investigation of molecular emission was carried out

Fig. 2. Part of the A2Σ+–X2Π system of OH, recorded in the top surface of a slightly greasy copper sample, from ref. [12]. A Spectruma GDA 150 spectrometer was used, 4 mm DC source; 20 mA 700 V.

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Fig. 3. Part of the A3Πg–X3Πu system of C2, recorded in the top surface of a slightly greasy copper sample. The most prominent bandhead in this spectral region is the 1–0, indicated in the figure. The identification of the indicated Ar II lines is uncertain. A Spectruma GDA 750 spectrometer was used, 4 mm RF source; 50 W (applied) power, 700 V RF.

Fig. 4. Part of the A3Πg–X3Πu system of C2, recorded in the top surface of a slightly greasy copper sample. The most prominent bandhead in this spectral region is the 0–0, indicated in the figure. The identification of the indicated Ar II line is uncertain. A Spectruma GDA 750 spectrometer was used, 4 mm RF source; 50 W (applied) power 700 V RF.

Fig. 5. Part of the A2Σ+–X2Π system of OH, recorded with an epoxy-coated steel sheet. A LECO GDS500A spectrometer was used; 4 mm DC source; 14 mA 1000 V. Note that there is an imperfect overlap of two CCD detectors in the range 312–314 nm.

by sputtering an organo-metallic paint coating (Bonazinc) [13]. A standard Grimm type source with a 4 mm anode was used, run at 14 mA 1000 V. In the spectral range 160–460 nm, emission spectra from the diatomic molecules CO, OH, NH and CH were identified [14,15] see Figs. 1, 5, 6 and 7.

1.4. Molecular emission intensity as a function of discharge parameters Since a primary concern in analytical spectroscopy is to minimise interference and maximise signal/background, the intensity variations of molecular emission as a function of discharge parameters was

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Fig. 6. Part of the A3Π–X3Σ− system of NH, recorded with an epoxy-coated steel sheet. A LECO GDS500A spectrometer was used; 4 mm DC source; 14 mA 1000 V.

studied [13]. Examples of resulting intensities of the CH emission band at 431 nm are shown in Fig. 8. The highest intensity is obtained at the highest voltage–lowest current setting, i.e. the highest plasma impedance. This is the opposite trend observed for all atomic analytical lines studied to date [25–27]. It is not known at present why this is the case. One clue to the explanation can be found in old data from Finkelnburg [2], who found spectra of neutral molecules in the positive column of a glow discharge at low current density, but at some critical higher current density dissociation takes place and the molecular spectra disappear. This observation would explain why we see mostly atomic emission in a Grimm type source, which normally is operated at rather high current density. It is also in agreement with the present observations in [13] that molecular emission increase at low current–high voltage conditions. It deserves mentioning here that observation of the emission in a Grimm type source is end on, making it impossible to observe how different species are distributed along the axis of the plasma. From the analytical point of view, the behaviour of the molecular emission as a function of discharge parameters is fortunate, since it provides a means to minimise the interference by lowering the impedance (increasing the pressure and current). However, too low voltage leads to a reduction in both the depth resolution and sputtering rate. The selection of suitable operating parameters for a particular application is always a matter of compromise. It should also be pointed out here, that it is not meaningful to directly compare the emission yield (EY) of molecular and atomic species. The reason for this is that in the case of molecules, both the

probabilities of molecular formation and excitation are involved. It is e.g. possible that a low current density increases the probability of molecular formation, but decreases the excitation probability to such an extent that the higher molecular fraction in the plasma is not observed as a higher relative intensity in the emission spectrum. It is very difficult to devise an experimental procedure that can separate the molecular formation process from the excitation, and the data from the experiments described here are not sufficient to do so. 2. Experimental study of molecular emission in mixed gases In order to study some more general features of molecular emission in GD-OES, a LECO GDS500A system was equipped with a system to bleed small amounts of molecular gases into the argon discharge gas flow (N2, O2, H2, and CO2), pure and as binary mixtures. The LECO GDS500A has a spectrometer of 50 cm focal length, and four CCD detector covering the range 160–460 nm with a spectral resolution of 0.06 nm. The fraction of molecular gas in the flow was approximately 1–2%, estimated from the settings of the mass flow controllers used to mix the gas flows. The total gas flow was in each case adjusted to obtain 20 mA discharge current at 1000 V. The molar fraction of the molecular gas was not possible to define with better accuracy in this experimental setup. However, as was mentioned in the introduction, the purpose of the work was not to make quantitative measurements of e.g. relative intensities at different concentrations of the molecular gases, but to identify molecular emission that may cause spectral interference of atomic emission

Fig. 7. Part of the A2Δ–X2Π system of CH, recorded with an epoxy-coated steel sheet. A LECO GDS500A spectrometer was used; 4 mm DC source; 14 mA 1000 V.

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Fig. 8. Intensities of the A2Δ–X2Π system of CH, recorded at two different current–voltage combinations at constant power 14 W: 14 mA 1000 V (top) and 28 mA 500 V (bottom). A LECO GDS500A spectrometer was used; 4 mm DC source. From ref. [13].

lines. The samples run were nearly pure Fe, Al and Cu. In this work, results from the measurements on the Cu sample are presented, since Cu has a “clean” emission spectrum with very few atomic emission lines compared with e.g. Fe. The same is true for Al, but the source was generally more stable when run with the Cu sample due to the higher operating pressure, therefore spectra obtained with the Cu sample have been selected for presentation in the following Sections 2.1–2.4.

2.1. Addition of N2 Very strong emission from several bands of N2 [15] was observed, see Fig. 9. Most of the observed bands are from the second positive system C3Πu–B3Π [15]. It is readily appreciated from Fig. 9 that N2 emission can interfere with a large number of analytical atomic lines, showing that even minor vacuum leaks can lead to artefacts in the form of false elemental signals. For instance, the most commonly

Fig. 9. Wide range spectrum of N2 recorded with nitrogen added to the argon gas flow, mainly from the C3Πu–B3Πg system, recorded with a pure Cu sample. A LECO GDS500A spectrometer was used; 4 mm DC source; 20 mA 1000 V.

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Fig. 10. Enlarged detail the 0, 2 sequence of the C3Πu–B3Πg system of N2 from Fig. 9. The spectrum was recorded with a pure Cu sample, A LECO GDS500A spectrometer was used; 4 mm DC source; 20 mA 1000 V.

used lines for Zn (330.26 nm and 334.50 nm) are in the midst of the very intense 0–0 band sequence. It is also worth noting that in this case, the spectral resolution of 0.06 nm is sufficient to resolve partially some of the rotational structure in the “tails” of the vibronic bands, see Fig. 10. 2.2. Addition of CO2 A spectrum of partially overlapping sequences of CO bands in the range 160–240 nm is observed [15], see Fig. 11. The spectrum is very similar to that of Fig. 1, recorded when sputtering an organic coating. The observation of strong CO emission is one clear indication that the glow discharge plasma functions as a “chemical reactor”, since the CO molecules have most likely been formed by dissociation and possibly recombination in the plasma. Some weaker bands most likely originating from both CO and CO+ in the range 270–320 nm were also observed, but not clearly identified.

2.4. Addition of N2 plus H2 When adding two molecular gases to the argon flow, more interesting phenomena are observed. In Fig. 12, the spectral region 320–340 nm is shown, with N2 and a mixture of N2 and H2 added to the argon. With the two gases mixed, the NH band centred at 336 nm is clearly seen, superposed on the N2 band with the head at 337 nm. This is even more clear proof that the glow discharge is a “chemical reactor”, where molecules can be formed by complex processes of dissociation of molecular species and subsequent recombination of the free atoms and ions into other molecular species. While this is no surprise in itself, the strong intensity of the NH band in this case could indicate that a substantial fraction of molecules and atoms entering the glow discharge plasma can be involved in such reactions. However, it must be stressed that these data are not sufficient to substantiate this assumption, since the relative emission intensities of these molecules depend both on the relative fraction of the molecules and the transition probabilities of the emission bands.

2.3. Addition of H2 Addition of H2 produces no distinct band emission in the investigated spectral range. However, the very broad “continuum” spectrum described in Section 1.2 was clearly observed, throughout the spectral region 200–460 nm.

3. Effects of molecular emission on compositional depth profiling (CDP) As was mentioned in the Introduction, it is inevitable to have spectral overlap of several atomic emission lines when molecular

Fig. 11. Part of the A 1Π–X1Σ+ system of CO, recorded with carbon dioxide added to the argon gas flow. The sample was pure Cu; a LECO GDS500A spectrometer was used; 4 mm DC source; 20 mA 1000 V.

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Fig. 12. Spectra of N2 and NH recorded by adding nitrogen and hydrogen to the argon gas flow. The sample was pure Cu; a LECO GDS500A spectrometer was used; 4 mm DC source; 20 mA 1000 V.

emission occurs in a glow discharge. From the analytical point of view, it is important to investigate to what extent this may affect the data in CDP analysis and what can be done to make adequate corrections. From the observations so far, it appears that molecular emission is predominantly observed from molecules made up of the light elements H, C, N and O. This means that we expect spectral interference from molecular emission primarily in applications where the sample material includes one or several of these elements as majors (polymers, oxides/hydroxides etc), and in very thin film applications where adsorbed gases are always released from the lamp interior upon ignition of the plasma. In order to study these phenomena, CDP measurements were carried out on selected samples using a LECO GDS-750A spectrometer, equipped with a radio frequency (RF) powered glow discharge source. The optical system is a polychromator, equipped with dual gratings and fixed photomultiplier detectors for up to 60 selected spectral positions. The spectral resolution is approximately 0.018 nm for the 3600 grooves/mm grating and 0.035 nm for the 1800 grooves/mm grating. The RF source can be controlled to run at a constant “true power” (TP), meaning the applied (forward) power minus the reflected power and the “blind” power

dissipated in the system without the plasma ignited. Furthermore, the source is equipped with an active pressure regulation system, capable of maintaining constant TP and constant RF voltage as several layers of varying composition are sputtered. 3.1. Polymer coatings Various polymer coatings on metal sheet are an expanding and industrially important field of application for GD-OES. The effects of molecular emission are most easily observed in polymer coatings of at least several microns thickness. One example is shown in Figs. 13–14, galvanised steel with a thin primer paint and a topcoat of polyester (PE). The RF source was run at 14 W TP and 1000 V RF. The signals at 386 nm and 339 nm, nominally, respectively, Mo and Zr (which are not present in the sample), show apparent intensity depth profiles that correlate rather well to the signals at 431 nm (CH) and 336 nm (NH). The regularly spaced low-intensity peaks in the range ~328–345 nm seen in Fig. 6 are in fact part of the A3Π–X3Σ− system of NH. Consequently, the appearance of weak NH emission at 339 nm is expected. CH has a band system with a strong bandhead at 386 nm overlapping the Mo 386.4 nm line [14], and

Fig. 13. Intensity depth profiles of spectral channels for Mo, Zr, CH and NH, recorded through a PE topcoat and a primer on a galvanised steel sheet. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power, 1000 V RF.

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Fig. 14. Intensity depth profiles of spectral channels for Mo, Zr, CH and NH in the top layer of a PE paint coat: expanded version of the first 30 s of Fig. 13. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power, 1000 V RF.

it is likely to have emission from this system simultaneously with the system at 431 nm. However, it is also very likely that part of the signal in all four spectral channels discussed here is due to the hydrogen continuum. Obviously, it is not possible to distinguish between these and other possible sources of background emission using a polychromator. However, a more close inspection of the spectrum around 339 nm in Fig. 6 indicates that most of the signal in this case is due to a NH molecular line. The differences in the intensity profiles in the beginning of the discharge from the same measurement (Fig. 14) also indicate that the dynamics of molecular formation and emission varies between molecular species, since the signal at 431 nm (CH) has a pronounced peak in the first 10 s, while the signal at 336 nm (NH) does not. Please note that the scaling on the y-axis in Fig. 14 is different from Fig. 13, since the initial peaks are of considerably higher intensity than the steadystate signals deeper into the coating. The differences between the spectral channels in the beginning of the discharge clearly show that other type of emission than the hydrogen continuum must be involved, otherwise the time–intensity profiles would be of the same general shape in all channels.

In Fig. 15, quantified depth profiles at 330 nm, (nominally Zn) and at 386 nm (nominally Mo) in a PE coating on an anodised aluminium material are shown, together with profiles of CH (431 nm) and NH (336 nm). The elements Zn and Mo are not present in this coating system, and the approximate correlation of their depth profiles to the molecular emission is readily seen also in this case. NOTE that the Zn and Mo profiles were quantified using a standard CDP calibration method [25], and calibration constants were introduced for CH and NH to give small “fictitious” concentrations. The term fictitious is used since it is not meaningful to describe the content of a polymer in terms of small dimers. The analysis was in this case done at the “standard” source conditions 14 W, 700 V RF; where the molecular emission is considerably weaker than at 14 W, 1000 V RF. As can be seen, the amounts apparently determined of both Mo and Zn are nevertheless substantial, with Zn at several percent. The reason for this is that the Zn 330 nm line is fairly insensitive; hence the rather weak background from molecular emission and/or the hydrogen continuum still translates into a substantial amount of Zn as quantified. This example clearly shows that in quantitative analysis of polymers, the influence of molecular

Fig. 15. Quantitative (false) depth profiles of Zn and Mo in a PE paint coat on anodised Al, probably resulting from molecular emission. These elements are not present in the paint coat. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power, 1000 V RF. Note that the concentration profiles of the molecular species CH and NH are fictitious, theses depth profiles are included to illustrate that they are similar to the corresponding atomic depth profiles.

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Fig. 16. Intensity depth profiles of Si 288 nm through a CVD deposited silicate layer, at constant power 14 W and three pressure settings. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power.

emission and/or the hydrogen continuum can lead to relatively large analytical errors unless properly corrected. 3.2. Thin films In recent years, the interest in GD-OES applications to thin films has also shown a substantial increase [29]. With “thin films” is here understood any surface film from about 100 nm and below. In order to illustrate some of the effects that can be observed, a silicate film approximately 100 nm thick deposited by a chemical vapour deposition (CVD) technique onto a hot dipped galvanised steel sheet was studied. The film consists primarily of Si and O with C and H as impurities. It is non-conductive and was therefore run by a radio frequency (RF) source. For very thin films that are sputtered very quickly, it is difficult to measure a complete spectrum with good signal statistics. A possible alternative way to identify the existence of molecular emission from the film is to vary the plasma impedance by changing the pressure in the source, and using a standard type polychromator to record intensity depth profiles. The plasma power (“TP” as defined in Section 4) was set to 14 W. The pressure was increased in steps between approximately 3 and 6 Torr, resulting in a RF voltage drop from 850 V to 480 V and a

corresponding increase in the (effective) plasma current. In Fig. 16, the intensity depth profiles of the Si 288 nm line at 3, 4 and 6 Torr are shown. The intensities increase with pressure, or lower impedance. As was mentioned in Section 2, this is the typical behaviour of atomic analytical lines. In previous work [25–27], it has been shown that it is primarily the changes in the electrical parameters, rather than the pressure as such, that affect the atomic emission intensity. In Fig. 17, the corresponding intensity profiles at the CO 172 nm line are shown, providing further confirmation that molecular emission in general show an opposite intensity dependence on the plasma impedance [13]. Therefore, one expects that apparent atomic depth profiles caused by molecular emission have a similar dependence on plasma impedance as those of molecular emission bands. In reality, the situation is often more complex, since there may be traces of the element emitting at the same wavelength, giving rise to a real elemental signal superposed on the molecular emission. However, if the intensity as a function of impedance deviates significantly from the normal atomic emission case, it is a strong indication that molecular emission is a major part of the recorded intensity. In Fig. 18, an example is given for 180 nm (nominally S) in the silicate film. The similarity between these and the CO emission profiles in Fig. 17 leaves little doubt that the apparent S surface profiles are to a large extent artefacts due to CO emission. However, the relative

Fig. 17. Intensity depth profiles of CO 172 nm through a CVD deposited silicate layer, at constant power 14 W and three pressure settings. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power.

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Fig. 18. Intensity depth profiles of S 180 nm through a CVD deposited silicate layer, at constant power 14 W and three pressure settings. A LECO GDS-750A spectrometer was used; 4 mm RF source, 14 W plasma power.

intensities for the three pressure levels do not match the CO profiles completely, indicating that there may be some sulphur present on the surface also. 4. Discussion This review has shown that molecular emission in GD-OES can lead to analytical artefacts, primarily in CDP of organic coatings and very thin films. In principle, this problem could be handled by a standard type of “line interference” correction routinely used in all forms of OES. An emission line (or a group of lines within a certain spectral window) of each interfering molecule is measured during analysis. Appropriate fractions of the signals from these channels are then subtracted from the atomic analyte signal. Such corrections are relatively straight-forward to implement in existing spectrometer systems, and will certainly improve the analytical accuracy greatly. However, there are some remaining problems to be solved for this to become a reliable, practical tool. The first problem is of practical nature; there is at present no easy “user-friendly” way to determine the molecular correction factors when calibrating a GD-OES instrument. Further work is necessary in order to develop both procedures and reference materials for this purpose. The second problem is of more fundamental nature. As can be seen in Figs. 13–15, there is no exact correspondence between the molecular emission depth profiles and the corresponding background depth profiles of atomic channels. To some extent, this can be understood from the fact that a superposition of emission from more than one molecular species, including the hydrogen continuum, makes up the background signal. However, there is probably even more complexity involved in this problem. The molecular channels in this case, as would be the case for any polychromator or CCD spectrometer using spectral “regions of interest”, measure the emission from a limited number of rotational–vibrational emission lines, located some distance from the actual wavelength of the affected atomic emission line. It is by no means certain that the molecular lines that actually overlap the atomic line retain a constant relative intensity to those lines measured in the corresponding molecular channel throughout the analysis. The dynamics of the glow discharge plasma, particularly in the early stage, are likely to cause shifts in the relative intensities within the different molecular bands. In measurements on nitrogen band intensities in Ar/N2 mixtures, Smid et al. [7] checked that the proportion of nitrogen did not affect the rotational intensity distribution, and so was able to use the intensities of a selected group of rotational lines to study the

variation of the band intensity. However, particularly for species such as CO, changes in intensity distributions between band systems may easily be caused by changing discharge conditions. These phenomena are, to the best of the knowledge of the author, largely unexplored in glow discharges. In terms of a practical and reliable method to subtract the line interference, it will probably be necessary to measure the molecular emission very close to each atomic line. One obvious method to accomplish this is to use CCD spectrometers that can record a sufficient part of the spectrum surrounding the analytical line in real time. However, to date CCD detectors are not quite fast enough for demanding thin film applications. They are also more limited than PM tubes in terms of dynamic range. We will undoubtedly see further technical development of OES instruments and detectors improving the possibilities to address these issues more effectively. It should also be stressed that the problems of background emission from molecular species will be less in an instrument with high resolving power. However, the LECO GDS-750A instrument used in the work presented here on polymer coatings and thin films has a very good resolution of approximately 0.018 nm, and substantially higher resolving power cannot be obtained in any commercially available instruments. An interesting characteristic of the molecular emission is the dependence on the discharge parameters; at a given power the highest intensities were observed at high voltage–low current conditions [13]. As was pointed out in Section 1.4, the opposite is true for all atomic lines studied to date. It was also found that these characteristics of molecular emission are in general agreement with older work on glow discharges, showing that molecular formation and emission is favoured by “soft” conditions, i.e. a low current density [2]. However, it is not known if the high voltage also increases the molecular formation and/or excitation, as the work in [13] would suggest. More work is needed to understand these phenomena better. One more aspect of molecular formation in glow discharges was briefly mentioned in the introduction. If a substantial fraction of the light atoms sputtered from the sample recombine to molecules, it is possible that the degree of atomisation is significantly reduced. This would happen when more than one of the light atoms is present in abundance in the plasma. One consequence would be that the concept of “matrix-independent emission yield” [25], crucial for the present models of quantification in CDP, would be invalidated. There are indications that this is the case for e.g. the determination of carbon in polymers. One interesting investigation related to the molecular formation was carried out by Smid et al. [21], who found that the intensity/H2 plots for the H lines at 656 and 486 nm show very

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pronounced curvature. These lines, unlike the H 121.5 nm line, are not subject to self absorption, so that these curves can only be explained by a change in the atom/molecule balance, or a marked change in excitation conditions. While these observations are very interesting, it can still be concluded that the processes of molecular formation and excitation in glow discharges remain largely unexplored to date. Finally, there is the obvious question if molecular emission in GDOES contains useful analytical information about the sample. As was mentioned earlier, describing the chemical content of e.g. a polymer in terms of dimers present in the plasma is not meaningful. However, the relative intensities of the emission from different molecular species may very well contain other analytical information of interest. Here is yet another highly interesting aspect of analytical glow discharge spectrometry to be explored. Acknowledgements The author is greatly indebted to Kevin Brushwyler, Kimberly Miller and Kim Marshall of Leco Corporation for performing experimental work on a LECO GDS500A, as well as valuable discussions. M. Analytis at Spectruma Analytik GmbH is likewise thanked for the experimental work on SPECTRUMA GDA 150 and GDA 750 spectrometers, and valuable discussions. The financial support of the European Union and the Swedish Ironmasters' association (Jernkontoret) are gratefully acknowledged. References [1] H. Kayser, Handbuch der spectroscopie, vol. 5, Verlag von S. Hirzel, Leipzig, 1910. [2] W. Finkelnburg, Auflage, Einführung in die atomphysik, vol. 4, Springer Verlag, Berlin, 1956. [3] W.B. Hurt, W.W. Robertson, Atomic and molecular emission in the negative glow of a helium discharge, J. Chem. Phys. 42 (1964) 556–560. [4] J.A.C. Broekaert, The development of microplasmas for spectrochemical analysis, Anal. Bioanal. Chem. 374 (2002) 182–187. [5] J.C.T. Eijkel, H. Stoeri, A. Manz, An atmospheric pressure dc glow discharge on a microchip, J. Anal. At. Spectrom. 15 (2000) 297–300. [6] I.G. Koo, W.M. Lee, Molecular emission spectrometric detection of low level sulfur using hollow cathode glow discharge, Bull. Korean Chem. Soc. 25 (2004) 73–78. [7] P. Smid, E.B.M. Steers, Z. Weiss, J. Vlcek, The effect of nitrogen on analytical glow discharges studied by high resolution Fourier transform spectroscopy, J. Anal. At. Spectrom. 18 (2003) 549–556. [8] W. Grimm, Eine neue Glimentladungslampe für die optische Emissionsspektralanalyse, Spechrochim. Acta Part B 23 (1968) 443–454.

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