An experimental study of asymmetry in an argon inductively coupled plasma torch using a relaxation method

S~err~himic& Acte, Vol. 438. No. 12, pp. 1431-1442.1988. Printed in Great Britain.

An experimental study of asymmetry in an argon inductively coupled plasma torch using a relaxation method E. L. BYDDER and G. P. MILLER* Physics Department, University of Waikato, Hamilton, New Zealand (Received 16 September

1987; in revised

form

10

February 1988)

Abstract-A relaxation method is used to undertake an experimental study of the degree of cytindrical symmetry present in an argon inductively coupled plasma torch, ICP. This technique, in effect a direct comparison between two different parameters, the electron (T,) and gas (rs) temperatures in the torch, is shown to be a very sensitive indicator of axisymmetry. Measurements using this technique delineate areas of axisymmetry in the ICP, and show that axisymmetry is signifi~ntIy dependent on the observation height and cootant gas flow-rate.

1. INTRODUCTION MOST studies of inductively coupled plasmas where emission spectroscopy is used as the diagnostic method for investigating the plasma require a determination of the radial intensity profiles to characterise the behaviour of the plasma parameters (7’,, Text, n, etc.). This can be obtained by taking a lateral scan across the plasma and then, with the use of Abel’s transform, converting the lateral results into radial values. However in using Abel’s transform to convert lateral intensities accurately to radial values certain pre-conditions are necessary. In particular, the plasma should be optically-thin and either axisymmetrical [l-8] or asymmetric only normal to the direction of observation [9]. Measurements have shown El@-231 that the ICP plasma is optically-thin over most of the spectral range and this requirement is easily confirmed for any particular spectral line [ 10,24]. Overall, therefore, it is apparent that the axisymmetry requirement is most critical to the accuracy of the final result. In most studies (e.g. [I1l-23,25-29]) the test for axisymmetry has been to take a lateral scan across the plasma for a ‘typical’ spectral tine or lines and if this is found to be symmetrical about the axis, it is assumed the ICP is axisymmetric for all plasma parameters. It has been noted [30,31] that the plasma is not always exactly axisymmetric, and BLADES [32] has applied an asymmetric method of solving Abel’s transform developed by YASUTOMO et al. [9] to obtain a radial distribution, where it is assumed that the asymmetry is due to load coil design and assembly. This paper describes an experimental study of the asymmetry present in an inductively coupled plasma by applying a relaxation technique deveioped by the authors [lo] which gives in effect a direct comparison between two different parameters, the electron (T,) and gas (Ts) temperatures. This allows a study to be made of the symmetry of Tc/Tg as well as of the line intensity, and shows that symmetry in the line intensity is not an adequate indication of plasma axisymmetry. Radial plasma parameters deduced from lateral results by means of an axisymmetric Abel’s transform technique, where the assumption of axisymmetry arises from the study of line intensity symmetry, must therefore be regarded with suspicion.

2. EXPERIMENTALTECHNIQUE The time-dependent behaviour of the excited argon I spectral lines have been studied in several recent papers [ 10,281. In particular in Ref. [lo] it was shown that the ICP plasma is close to equilibrium and that the excited Ar I states are coupled to the electron continuum. Thus Saha’s equation can be written in the form [lo, 33-353

*Author to whom correspondence should be addressed. 1431

1432

E. L. BYDDERand G. P.

MILLER

where n, is the electron density, np is the density of an excited state p above the resonance level, gp, giO,,the statistical weights of level p and the ionisation level respectively, m, is the mass of the electron, T, the electron temperature, h is Plan&% constant, k is Boltzmann’s constant, and E,, Eion the energy of the pth level and the ionisation

energy respectively. In the ICP the relaxation time for the electrons is smaller by two orders of magnitude than the time taken for any significant changes to occur in the composition of the plasma [lo]. Thus if the rf field is suddenly switched off the electrons relax to the argon gas temperature before any significant change can occur in the electron density or gas temperature. For the near-equilibrium conditions in the electron collision dominated ICP plasma it can be shown [lo] that this relaxation produces a sharp increase in the intensity of the optically thin Ar I spectral lines. The intensity change can be related to the electron temperature and the temperature difference between the electrons and gas by the equation Y-l k~

ln~=~lny+(E,,-E,) (

01

where I and I’ are the relative intensities of the Ar I excited pth state before and immediately after the removal of the rf field and y = (T,/Ts) where T, and rS are the electron and gas temperatures. As the intensity ratio (i’/i) is dependent on the temperature difference y, a lateral scan of the in~~sity ratio of a single spectral line across the plasma will indicate whether, and under what conditions, the temperature difference is axisymmetric. This provides both a test of the standard method used to check for axisymmetry and, by varying the operating conditions, an indication of the cause of any asymmetry. Since this method effectively provides a measure of the symmetry of the distribution function across the plasma, it characterises the symmetry or otherwise of the entire plasma system.

3.

INSTRUMENTATION

The rf power was supplied by a modified 2 kW communications transmitter operating at a fixed frequency of 27.12 MHz. A switching system was deveioped so that the rf power output could be reduced to zero in % 3 ps, and switched on again after a controllable delay. The repetition rate of the switch-off switch-on cycle was also adjustable, a period of about 2 s being convenient. The switching was applied simultaneously to the low rf level in the crystal oscillator buffer and at the screen of the push-pull 813 power output valves. The plasma torch assembly [29] consists of three clear fused quartz tubes, each tube mounted into an individual aluminium cylinder which facilitates the alignment of the three tubes. The torch was operated on argon (Industrial Welding grade) introduced tangentialfy into the outer tube, to provide vortex stabilisation. It was found unnecessary to provide support gas flow through the second tube. Sample introduction was provided by the centrai tube through a 1.5 mm orifice, with argon as the carrier gas. The light emitted by the plasma was focused by a 10 cm focal length quartz lens to form a x 1 image on the entrance slit of a GCA/McPherson EU700 series scanning monochromator. This uses a 1180 tine/mm grating blazed at 250.0 nm having a reciprocal dispersion of approximately 2 nm/mm. A Hamamatsu R955 photomultiplier tube (spectral range 160-900 nm) was positioned at the exit slit. The photomultiplier output was recorded on a digital storage oscilloscope. The response time of the optical detection system was measured to be x 2 ~_ls.

4.

RESULTS

The plasma axisymm~try was studied by making lateral scans across the plasma at various fixed heights above and below the top of the work-coil. Each scan was divided into approximately 30 steps. Using the Ar I 696.5nm spectral line, the intensity, I, and the intensity ratio, I’/Z, were measured, where I’/1 is the ratio of the spectral line intensity, I, just before switch-off to the peak spectral line intensity, I’, after switch-off. (This is illustrated in

Asymmetry in argon ICPT

1433

Fig. 1). These measurements were made for a variety of operating conditions which were similar to those in which the ICP has been studied or is used. The results are summarized in the following sections. 4.1. Coolant only: minimum gas flow To isolate possible conditions for an asymmetry developing in the ICP plasma, the initial measurements were made under the simplest possible operating conditions of the torch, using an input power of 1200 W and a minimum usable coolant flow-rate of 10 l/min, with no aerosol flow (and no support gas as mentioned above). Lateral scans of the intensity of the 696.5-nm Ar I spectral line were taken at a distance of 15-mm below the top of the work-coil, and at heights of 2.5, 5 and 10-mm above the top of the work-coil. Due to the shape of the work-coil, it was impossible to obtain a full scan across the plasma in the middle of the workcoil. Figure 2 is a plot of the lateral intensity I vs the radius and Fig. 3 gives the corresponding lateral intensity ratio I'/I associated with the removal of the rf field, at various heights in the plasma. These results clearly demonstrate that the plasma is axisymmetric above the work coil in terms of both the intensity and the intensity ratio, and hence in terms of the temperature distributions. Below the coil, however, neither of these parameters can be considered to be axisymmetric. The shape of the curves in Fig. 3 shows that the largest intensity ratios are in the outer 3-4 mm of the plasma corresponding, of course, to the maximum temperature difference between the electrons and the heavy particles constituting the plasma. The fact that in the central region the intensity ratio is small is to be expected as the central region is shielded from direct rf heating of the electrons by the outer region plasma at a thickness corresponding to the skin-depth at 27 MHz ~ 3 mm. 4.2. Effect of increased coolant flow A series of measurements were made to study the effect of increasing the coolant flow-rate on the plasma axisymmetry in the ICP. To investigate the asymmetry below the coil, the coolant flow was increased in steps from 10 to 301/min. As in the previous case, lateral scans of the 696.5-nm argon I spectral line were made using the same parameters apart from flow. Increasing the flow-rate increased the length of the plasma visibly both above and below the coil. Figures 4 and 5 give comparisons of the lateral spectral intensity I and intensity ratio I'/I

Fig. 1. Photograph o f spectral line response on removal o f rf field spectral line, Ar 1696.5 nm. Timescale 0.1 ms/div. The rf field switched off at 0.08 ms, on at 0.64 ms.

E. L. BYDDER and G. P. MILLER

1434 IO -

Height II.21 --c

9-

;I,” 5

6-

7-

3-

2-

-9-6-7-6-5-4-3-2-l

0 I 2 3 4 5 6 7 8 9

Lateral distance (mm) Fig. 2. Lateral intensity scan of Ar I 696.5 nm for various

flow; input power:

5-

o-

,o ,q

-+ ___*_._ ___c--*-

heights. Coolant

flow: 10 I/min; no aerosol

1200 W.

Height -15 2.5 5 IO

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o-

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I I I 2 3 4 5 6 7 6 9

Lateral distance (mm)

Fig. 3. Lateral scan of the intensity ratio I’/1 of Ar 1696.5 nm for various heights in the plasma. Same parameters as Fig. 2.

Asymmetry in argon ICPT 8-

FLowrate ~x-I0 ----0---- 2 0 --t-30

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t t I t

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1435

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L o t e r o L distance (ram)

Fig. 4. Variation in relative intensity with increasing coolant flow-rate below the work-coil. Spectral line: Ar I 696.5 nm; height: 15 mm below top of coil; input power: 1200 W.

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x

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1.0

Fig. 5. Variation i.n intensity ratio with increasing coolant flow-rate below the work-coil Spectral line Ar I 696.5 rim; height: 15 mm below top of coil; input power: 1200 W.

1436

E. L.

BYDDER

and G. P.

MILLER

with respect to coolant flow-rate at 15 mm below the top of the coil. The asymmetry in the spectral intensity below the work-coil, as shown in Fig. 4, appears to arise from the geometrical configuration of the work-coil rather than the gas flow as the asymmetry did not change with variations in flow-rate. Figure 5 illustrates that increasing the flow-rate decreases the intensity ratio below the work-coil and there is also a decrease in the asymmetry. The decrease in intensity ratio together with the increase in the intensity with increasing flow-rate is probably due to a decrease in pressure generated in the central region of the plasma by the higher gas velocities at the circumference constraining the plasma (vortex stabilization). The plasma is seen visually to increase in length as the coolant flow is increased. The effects of flow variation on the spectral intensity and intensity ratio above the workcoil can be seen by comparing Figs 6 and 7 with Figs 2 and 3. A comparison of Figs 2 and 6 shows that doubling the coolant flow-rate to 20 l/min has little effect on the intensity of the 696.5 Ar I spectral line apart from the broadening of the discharge at a height of 10 mm. However, comparing Fig. 3 with Fig. 7 shows two major changes on doubling the flow-rate; the plasma is no longer axisymmetric with respect to the intensity ratio at heights of 2.5 and 5 mm above the work-coil, and the intensity ratio has increased markedly at a height of 10 mm, especially at the periphery. To investigate this loss of axisymmetry a series of scans of the plasma were made at a fixed height (5 mm) above the work-coil with the coolant flow-rate varying from 10 to 40 l/min. The results are shown in Figs 8 and 9. Figure 8 confirms that the intensity of the spectral line emission is relatively independent of the coolant flow-rate above the work-coil. The intensity ratio, however, as shown by Fig. 9, is very dependent at the periphery of the plasma on the coolant flow-rate. The degree of asymmetry at first increased when the flow-rate was increased from 10 to 20 l/min and then decreased again as the coolant flow was increased up to 40 l/min. When these results are compared with those obtained below the work-coil (Fig. 5), peak values for Z’/Z occur on opposing sides of the plasma, with opposite effects in these two positions on the intensity ratio developing with increasing flow. It appears that these asymmetries arise from changes in

sr

Height

-9-8-7-6-5-4-3-2-l

0 Lateral

I 2

distance

3

4

5

6

7 8

9

(mm)

Fig. 6. Lateral scan of the relative intensity of Ar I 696.5 nm with coolant work-coil. Input power: 1200 W.

flow of 20 l/min above the

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Loterot distance (ram)

Fig. 7. Scan of the intensity ratio for a coolant flow of 20 l/rain at various heights above the work-coil. Spectral line: Ar I 696.5 nm; input power: 1200 W.

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Fig. 8. Variation in relative intensity with increasing coolant flow-rate. Input power: 1200 W; height: 5 mm above work-coil: spectral line: Ar I 696.5 nm. SAB 43= 1 2 - C

1437

E. L.

1438

BYDDER

15-

and G. P.

-x-

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MILLER

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Lateral distance (mm) Fig. 9. Variation in intensity ratio with increasing coolant flow-rate. Spectral line: Ar 1696.5 nm; input power: 1200 W; height: 5 mm above coil.

the Aow pattern of the vortex flow used to stabilise the plasma, as well as from the effect of the increasing volume of cold argon passing through the torch. It is also evident from the results that altering the coolant flow-rate has little effect on the central region of the plasma. Below the work-coil increasing the flow-rate initially decreased the intensity ratio after which it remained relatively steady. Above the work-coil the intensity increased slightly with an increase in coolant flow but then also remained relatively stable for flows above 20 l/min. 4.3. introduction of the ueroso~frow To use the ICP as a light source in optical emission spectroscopy it is necessary to introduce the sample to be analysed into the plasma. This is generally done in the form of an aerosol carried by a laminar flow of argon through the center of the plasma. The high frequency used to heat the plasma has a thin skin depth which assists in a channel being formed by the gas flow through the center of the plasma. This flow, with or without a sample, is called the aerosol flow. For the experiments described here the aerosol flow consisted of argon only. At flows below 1 l/min the aerosol flow lifts the plasma slightly but tends to flows around the outside of the plasma. For flow-rates of 1 l/min and higher the aerosol flow forces a “hole” through the center of the plasma. While aerosol flow-rates of 8 I/min are readily sustained by the plasma, only flow-rates of 1 and 2 l/min were investigated, as these are the range used for intr~ucing samples to the plasma. The coolant flow was maintained at a constant rate of 10 l/min for experiments involving aerosol flow. The visual effects of introducing the aerosol flow were a decrease in the brightness of the plasma, with the aerosol channel being easily visible. The change in load characteristics of the plasma necessitated a minor re-tune of the matching unit. Lateral scans of the Ar I 696.5~nm spectral line were repeated at the same heights as used in the previous section under the same conditions with the exception of the addition of an aerosol flow consisting of argon. Figures 10 and 11 illustrate the behaviour of the spectral line intensity with the introduction of 1 and 2 l/min aerosol flow. When compared with Fig. 2 the

1439

Asymmetry in argon ICPT 5.0

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Fig. 10. Variation in relative intensity with the addition of 1 l/rain aerosol flow. Gas: argon; coolant flow-rate: 10 l/rain; input power: 1200 W; spectral line: Ar 1 696.5 rim.

5.0

Height --x~ -15 ----o---- 2.5

4.5

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Fig. I I. Variation in relative intensity with the addition of 2 I/rain aerosol flow. Same parameters as Fig. 10.

1440

E.

L. BYCIDERand G. P. MILLER

reduction in intensity is evident as in the annular shape of the ICPT plasma characteristic of this mode of operation. The effect of the aerosol flow on the intensity ratio r/r is shown in Figs 12 and 13. Apart from slightly improving the symmetry below the work-coil, the introduction of the aerosol fIow does not appear to affect the axisymmetry provided the aerosol tube is aligned correctly when the torch is assembled. Misalignment of this tube not only destroys the axisymmetry, but also makes it difficult to push a channel through the plasma and so tends to extinguish the discharge. 5. CONCLUSIONS BLADE’S conclusion [31] that the geometric configuration of the load coil can be the cause of introducing asymmetries into the plasma is confirmed. The presence of this asymmetry is easily determined by a lateral scan across the plasma and can be handled with the use of an asymmetric Abel’s inversion (191. However, by giving a direct comparison between the electron and gas temperatures the relaxation method reveals an asymmetry not evident from the lateral scan. From the results, it isevident that to assume axisymmetry solely on the basis of a lateral scan of spectral line intensity is unsatisfactory. More importantly, the development of asymmetry with increasing coolant flow and the variation of asymmetry with height implies an asymmetrical temperature difference between the electron and gas tem~ratures arising from the flow pattern of the coolant gas. At the lowest sustainable coolant flow (10 l/minf and at a height of 5 mm above the work-coil the plasma appears to be axisymmetric for both the intensity and intensity ratio. However, at observation heights above and below the top of the work-coil with gas flows greater than 10 l/min, lateral scans across the plasma of the intensity ratio indicate a departure from axisymmetry. Therefore, with a range of Ar I spectral lines and a suitable Abel’s inversion technique, the relaxation method can provide a spatial resolution of tem~rature difference giving an insight into the energy properties of the plasma and thereby the validity of the various theoretical models. This work is proceeding.

Height -X. ..*+..-

-15 2.5

I.01 1 1 1 1 1 I 1 f 1 I I 1 1 I 1 I 1 1 -9-8-7-6-5-4-3-2-1

0 I 2 3 4 5 6 7 6 9

Lateral distance(mm)

Fig. 12. Variation in intensity ratio with the introduction ofaerosol flow, 1 I/min. Same parametersas Fig. 10.

1441

Asymmetry in argon ICPT

-x. ...+_.. _**--

Height -15 2.5 *

-*-IO

IO’1

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-9-8-7-6-5-4-3-2-l

1 III/

11 11 111

I

0 I 2 3 4 5 6 7 6 9

Lateral distance brnn)

Fig. 13. Variation in intensity ratio with the introduction ofaerosol flow, 2 l/min. Same parametersas Fig. 10.

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[1] [2] [3] [4] [S] [6] [7] [S] [P] [lo]

1442 [32] [33] [34] [35]

E. L. BYDDERand G. P. MILLER M. W. Blades, Appl. Specrrosc. 37, 71 (1983). D. R. Bates, A. E. Kingston and R. W. P. McWhirter, Proc. R. Sot. Land. A. A267, 297 (1962). D. R. Bates, Atomic and Molecular Processes. Academic Press, New York (1962). D. R. Bates and G. Dalgamo, Atomic and Molecular Processes. Academic Press, New York (1962).