Anal. Chem. 1995, 67, 3167-3171
Spatial Distribution of Atoms in a dc Glow Discharge K. Hoppstockt and W. W. Harrison* Department of Chemistty, University of Florida, P.0. Box 1 17200, Gainesville, Florida 3261 1-7200
An argon glow discharge acts as a reactive chemical cell involving sputtered analyte and argon atoms and ions. The spatial distribution of atoms from several samples has been monitored by an atomic absorption and emission arrangement simultaneously with mass spectrometric monitoring of the discharge. We have examined the effect of varying the sample to exit orifice distance on resultant atom prof3es. Copper was studied by atomic absorption and iron by atomic emission, revealing complementary population shifts near the cathode surface. Argon metastable species were also monitored and correlated with the analyte response. Movement of the sample insertion probe by 90"allowed a vertical perspectiveto be obtained for the discharge atom population. 'he effect of Merent discharge voltages was also examined to show sputter variations. Atom populations are defined by many factors, includingsputter rate, diffusion rates, chemical reactivity, and discharge pressure. In recent years the importance of rapid and reliable trace analytical characterization of environmental samples, high-purity materials, and advanced ceramics has steadily increased. Glow discharge (GD) techniques, particularly glow discharge mass spectrometry (GDMS), have gained popularity especially for the direct trace elemental analysis of solid materials. To improve further the capabilities of GDMS and related GD atomic spectrometric techniques, it is beneficial to enhance the understanding of glow discharge fundamental processes that can significantly affect analytical results. The GD is usually operated at a pressure of a few millibars, and for analytical applications, the most common gas used is argon. At a discharge pressure of about 1Torr, the argon mean free path is in the order of 50 pm, and therefore, the sputtered analyte originating from the cathode will experience many collisions (e.g., with discharge gas species, contaminants, or analyte species) before reaching the exit orifice 1cm or more away from the cathode. In this study, the diffusion characteristics of the sputtered atoms were evaluated by the examination of plasma properties in different parts of the plasma. Mass spectrometry is a sensitive tool for the study of gas discharges, but it is not conducive to spatially resolved studies, as it samples only a plasma volume adjacent to the exit orifice. Optical methods such as emission, absorption, or fluorescence spectrometry are proven techniques for probing various parts of the plasma without disturbing its shape and composition. By use of both optical and mass spectrometry, complementary measurements can be made
' Present address: Research Center Jiilich GmbH, Zentralabteilung fiir Chemische Analysen, P.O. Box 1916, D-52425 Jiilich, Germany. 0003-2700/95/0367-3167$9.00/0 0 1995 American Chemical Society
to explore signal relationships among the many prominent species in the GD. Early works by H. Hormann in 1935l and J. Friedrich in 195g2 investigated high-pressure arc discharges, focusing on temperature distributions, electron densities, and luminescence density distributions rather than atom or ion distributions. Stirling and Westwood3used atomic absorption techniques to examine atom profiles in a glow discharge, in particular the sputtering processes of aluminum in Ar. Using dye laser fluorescence spectroscopy, Elbem4measured axial atom density profiles of a dc GD with Fe disk electrodes. Fereira, e t d 5determined by AAS the distribution of metastable argon atoms in a modified Grimm-type discharge, and Loving6 employed AAS and MS to study an Ar GD with a Fe cathode. Recently, Djurovic et al. reported a study of spatially and temporally resolved OES measurements of a rf-GDe7 To study the glow discharge under conditions optimized for mass spectrometry, we designed around the discharge chamber of our GDMS unit an optical system for atomic absorption and emission spectrometry, permitting careful and detailed profiling of the discharge, simultaneously with mass spectrometric measurements. EXPERIMENTALSECTION The glow discharge quadrupole mass spectrometer has been described previously.* A schematic diagram is shown in Figure la of the GD ion source, constructed from a six-way cross featuring 2.75in. flanges with quartz glass windows for the optical path at right angles to the ion beam extraction. The source features a cryogenic cooling coil9 coaxial about the direct insertion probe in-line with the mass spectrometer axis, which will be referred to as the x-axis in this paper. Figure l b shows that a second port on the six-way cross allows the insertion perpendicular to the x-axis, where the cryogenic cooling coil with the bellow assembly and ball valve was replaced by a blank flange. Figure 2 shows the schematic diagram of the optical arrangement. The movable arm is mounted to a translation stage, permitting precise positioning along the x-axis with a millimeter screw. A Spex 340E monochromator (Spex Industries, Inc., Edison, NY) with a focal length of 0.34 m, equipped with a (1) Harmann, H. Z. Phys. 1935,97,539-560. (2) Friedrich, J. Ann. Phys. 1959,3,327-333. (3) Stirling, A J.; Westwood, W. D. J Appl. Phys. 1970,41, 742-748. (4) Elbem, A. 1.Vac. Sci. Technol. 1979,16, 1564-1568. (5) Ferreira, N. P.; Strauss, J. A; Human H. G. C. Spectrochim. Acta 1982, 37B,273-279. (6) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983,55, 1523-1526. (7) Djurovic, S.; Roberts, J. R; Sobolewski, M. A; Olthoff, J. K. J Res. Natl. Inst. Stand, Technol. 1993,98, 159-180. (8) Bmhn, C. G.; Bentz, B. C.; Harrison, W. W. Anal. Chem. 1978,50, 373375. (9) Ohorodnik, S. IC; Harrison, W. W. Anal. Chem. 1993,65, 2542-2544.
Analytical Chemistry, Vol. 67, No. 18, September 15, 1995 3167
High Voltage
High Voltage
-
Negafive
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Pin
Sample
-f
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,
i
t
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Figure 1. Glow discharge source and direct insertion probe lor GD mass spectrometry and GD optical spectrometty: (a, let) axial insertion of sample; (b, right) 90" variation to permit alternative profile measurements. X
7- I
performed with constant dc discharge current of 4 mA. if not otherwise stated. Before the measurements were taken, the cathode was presputtered for 20 min to get a clean surface and to remove as many impurities from the system as possible. The discharge chamber was also periodically baked for several hours, after which the plasma was monitored for contaminants by mass spechometry. Cryogenic cooling was used for all measurements.
I
RESULTS AND DI!3CUSSION
Figure 2. Schematic arrangement for spatially resolved optical measurements in the glow discharge source.
Hamatsu R1547 PMT tube and a Pacilic Instruments photometer Model 110, was used for all the optical measurements. For emission studies, the photometer signal was registered on a strip chart recorder (Houston Instruments, Austin, TX). Atomic absorption measurementswith hollow cathode source lamps were performed using a mechanical chopper (Scitec Instruments) with a Model 5207 lock-in amplifier (EG&G Princeton Applied Instruments, Princeton, NYJto discriminateagainst emission. In order to obtain a high spatial resolution, an entrance slit width of 0.2 nun and a slit height of 0.167 mm were used. The exit slit of the monochromator was set to 0.2 mm. Ultrahigh-purity-grade argon @quid Air Corp.. San Francisco, CA) was used with an operating pressure of 1Torr (-1.3 mbar). The pin cathodes used in these experimentswere electrolytic iron [National Institute of Standards and Technology (Gaitherburg, MD) SRM 1265al,99.999% puratronic grade Ag and Cu wire, 2-mm diameter Uohnson Matthey Chemicals, Ward Hill, MA) and 99.97%Ti wire, 2-mm diameter (Aldrich Chemical Co., Inc., Milwaukee, W. The iron samples were machined into 2 nun diameter pins from bulk disk samples. A 4-5nun segment of the pins was exposed to the discharge. The experiments were 3168 Analytical
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The glow discharge is a ready source of sputtered atoms from the sample cathode. Atoms ablated from the sample surface are subjected to a high collision rate in the 1-Torr discharge, with many atoms being returned to the sample surface, while others diffuse into the plasma. In addition to diffusion effects, atom distribution may be influenced by discharge gas flow patterns and by electric fields in the case of ions. As a result of these processes, heterogeneous steady-stateconcentrationprofiles are established across the glow discharge. Measurements along the x-Axis. To gain a better understanding of the plasma processes, it is useful to monitor various discharge species, such as ground-state and excited-state atoms, ions, and metastable (Arm*)species in various parts of the plasma. Our first interest was directed toward the distribution of the sputtered analyte, the ground-state atom population, which was studied by atomic absorption spectrometry. Four different s e w of cathode to exit orifice distance were studied (Figure 3). For a copper cathode, the absorbance profiles for each of those distance settings are shown. reflecting the changing shape of the glow d i s c h e plasma as the cathode is moved closer to the exit orifice. The measurements are quite reproducible,with variations generaUy falling within the data symbols shown. For all cathode materials used in this study (Ag, Cn, Fe, Ti). the profile exhibits this same general shape, although the measured absorbances differed signilicantly. It should be noted that the maximum absorbance is not observed immediately adjacent to the cathode surface, but instead about 0.75-1 mm in front of the surface. This phenomenon, previously noted by other inve~tigators,3,~ is some what surprising, as all the copper atoms present in the plasma
Orifice
h
t Cathode
lop
2
0
a
6
4
Cathode Top
10
Figure 3. Absorbance profiles of copper (327.4 nm) along the x-axis (see Figure 2) in an Ar plasma with a Cu cathode (4 mA, 1 Torr, and -1000 V). Distance between sample and exit orifice: (0)3, (A) 5 , (l3)7, and (0)9 mm.
0
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8
0
2
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6
a
io
12
Distance from exit orifice, mm
Distance to exit orifice, mm
10
12
Distance to exit orifice, mm
Figure 4. Emission profiles of Fe (I) at 248.3 nm along the x-axis for different distances between cathode and exit orifice in an Ar GD at 1 Torr and 4 mA (discharge voltage -1000 V). Distance between sample and exit orifice: (0)3, (A) 5, (8)7,(0)9, and (v) 12 mm.
originate from the cathode surface. This could suggest that not all the cathode material is sputtered in the form of single groundstate atoms. Possibly some multiatom species are ejected in the form of dimers, trimers, and other cluster forms. Some signiscant amount of sputtered material may also be released as excitedstate species or as ions. None of these species could be detected by our atomic absorption measurements, so whether such factors could account for the drop of absorbance near the cathode is not clear. The production of molecules or related species by the sputtering process had been reported earlier.3J0J1Winefordner, et al. reported diatomic lead in a GD as measured by laser-excited fluorescence.12 As a comparison to the measured absorbance profiles of ground-state atoms, emission measurements of the excited-state populations were also made using the same experimental conditions. Figure 4 shows spatially resolved emission profiles of Fe 0 at 248.3 nm in an argon discharge using an iron cathode. The general shapes of these curves are similar to those seen in Figure 3; a steady increase of signal intensity is found as the measurement site is moved from the anode (exit orifice) toward the cathode. Again, a drop in signal intensity is seen near the cathode surface. The decrease near the cathode is less abrupt than in the case of (10) Stirling, A. J.; Westwood, W. D.]. Phys. D: Appl. Phys. 1971,4, 246-252. 1926-1931. (11) Gough, D. S . Anal. Chem. 1976,48, (12) Patel, B. M.; Smith, B.; Winefordner, J. D. Spectrochim. Acta 1985,408, 1195- 1204.
Figure S. Emission profiles of argon metastable species (811.5 nm) along the x-axis. Distance between sample and exit orifice: (0)3, (A) 5, (0)7, (0)9, and (v) 12 mm.
the ground-state atoms but clearly indicates the reduced population in this area. The data in Figures 3 and 4 show that both ground-state and excited-state populations are diminished near the cathode. Metastable argon atoms have been implicated as key energy transfer species in glow discharges. Therefore, their axial distribution was determined in order to provide some additional insight into the profile shapes of sputtered species. Figure 5 shows emission profiles of argon metastables (Arm*)taken at five different sample to exit orifice distances. These plots have some general similarities to the sputtered species plots, but also significant differences. Indeed, factors affecting the respective populations should be quite different. There is an overall decrease in metastable population moving from the cathode to the exit orifice, but the changes are more abrupt. A sharp transition occurs for each distance plot, occurring each time just into the negative glow. At that location in the plasma, about 2/3 of the signal decrease occurs withii -1-2 mm of distance in the plasma. Most likely this is related to optimum electron energy distributions affecting metastable formation. A small, but distinctive drop in metastable population also occurs just inside the dark space of the glow discharge, similar to that observed for sputtered species. W i l e this could be some factor in the decreased analyte emission, it does not follow that the larger abrupt metastable reduction in the negative glow necessarily has any relationship to the corresponding decrease of the sample atoms. As seen in Figure 3, a much smoother decline in intensity occurs. Profiles for the various species show that formation and loss mechanisms do not affect all the critical species uniformly. Perpendicular Probe Insertion. Radial Measurements. The optical contigurationshown in Figure 2 allows measurements along one axis only. To study the plasma along other axes would have required major reorientation of the optical system. Instead, we took advantage of the symmetry of the six-way cross to move the direct insertion probe 90" from the conventional measurement mode. In this manner, the pin sample is perpendicular to the z-axisthrough the top port, as shown in the inset of Figure 6. With this configuration, driving the movable arm along the x-axis probes the plasma radially at varying distances from the cathode center. The distance away from the cathode tip is varied by moving the direct insertion probe up or down along the paxis (Figure 6). The data collected by this process permit construction of the absorbance profile shown in Figure 6. A copper pin was used as Analytical Chemistry, Vol. 67, No. 18, September 15, 1995
3169
__
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Flgure 6. Absorbance profile of copper (327.4 nm) in an argon glow discharge; probe inserted perpendicular to the x-axis. Distance of cathode center to exit orifice, 16 mm.
the cathode in a 1-Torr argon glow discharge. The ground-state copper atom population decreases (as expected) with increasing distance from the cathode. Of more interest, when the probing beam (0.17 mm x 0.20 mm) passes just beneath the cathode top, we observed a drop in absorbance in the cathode center region, corresponding to the results previously shown (Figure 3), where the absorbance also decreased in front of the cathode. (This center region will be discussed in more detail when Figure 7 is considered.) Comparing the radial decrease in absorbance on either side of the cathode, the slope on the exit orifice side (left side in Figure 6) becomes significantly steeper than that observed on the opposite side away from the exit orifice. The GDMS system requires an intense differential pumping system to maintain the high vacuum on the mass spectrometer side simultaneously with the much greater pressure (-1 Torr) in the discharge chamber. The resulting pressure differential creates a jet expansion of the argon/sputtered-atom plasma mixture through the exit orifice. This effect enhances the diffusion-based material transport and also dilutes the plasma mixture by introducing discharge gas from outer plasma regions. These results indicate that the plasma in this particular setup is not as symmetrical as might be expected. The asymmetry may explain why our attempts to employ Abel conversion computation^^^,^^ of the data did not create reliable results. It also indicates that atom distribution profiles obtained in this radial direction may differ slightly in configuration from those taken axially as a result of the differential pumping effect. The drop in absorbance near the cathode was detailed more completely as shown by the data in Figure 7 . Measurements were taken at 0.5" intervals and at four different discharge voltages. Increasing voltages increases the currrent and the sputter rate, and consequently, the absorbance. The contrast between the maximum absorbance (-1 mm from the pin center) and the minimum absorbance at the cathode center remains generally constant within the precision of these measurements. It seems unlikely that reduced sputtering is occurring at the center. The results do indicate, however, the presence of fewer sputtered ground-state atoms, for reasons that may involve excited states, polyatomics, clusters, or some other unknown effect. (13) Sacks, R D.; Walters, J. P.Anal. Chem. 1970, 42, 61-84. (14) Cremers, C. J.; Birkeland, R. C. Appl. Opt. 1966, 5, 1057-1063.
3170 Analytical Chemistry, Vol. 67, No. 18, September 15, 7995
Figure 7. Absorbance profiles directly in front of the cathode surface using an Ar glow discharge with a Cu cathode (inserted along the y-axis); operated with different discharge voltages. Applied voltages: (A)1000, (0)1250, (m) 1500, and (0)1750 V.
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15
Figure 8. Profiles of absorbance (364.3 nm) and emission (Ti (11) at 300 nm, and Arm' at 81 1.5 nm) taken directly in front of the cathode surface of an Ar glow discharge with a Ti pin (inserted along the y-axis): ( 0 )OES Arm' 81 1.5 nm, ( x ) OES Ti (11) 323.4 nm, and (+) AAS Ti 364.3 nm.
Another possible cause for the absorbance drop could involve the formation of ions in this central cathode region. By mass spectrometry, we can only sample ions that form near the ion exit oritice, but by monitoring optical emission from some suitable element, spatial measurements of the ion populations can be tletermined. Ti was selected as the test element due to its strong ion lines. In this way, sputtered titanium atoms could be measured by atomic absorption and titanium ions by atomic emission. Figure 8 shows a comparison of ground-state and ion distributions obtained in an Ar GD with a Ti pin as the cathode. In addition, argon metastable atoms were monitored by their atomic emission. The profile of Ti atoms is quite similar to that of Cu (Figure 3), with the same type of characteristic dip in front center of the cathode. The shape observed for the Arm*emission generally tracks the ground-state metal atom population, suggesting that the forces affecting the sputtered atom population may also influence metastable formation. The Ti ion emission profile presents an interesting contrast, as highest emission intensity is observed at the cathode center. In fact, the sharp increase of ions coincides with the drop in atom population. That implies that a fraction of Ti could be sputtered in ionic form or ionized within the first few micrometers (which cannot be resolved with the present system). Sputtered ions should be returned to the cathode by the dark space field adjacent to the cathode, but ions
'
~
~
formed in plasma interactions beyond that field would be in a generally field-free region. The multiple peaks observed with the ion profile are surprisingly reproducible. An ion source design that samples cathodically through a center ion orifice might be w ~ r t hreconsidering. The hollow cathode ion source15 that provided the impetus in our laboratory for initial GDMS work featured this mode of sampling. High-intensity ion beams of sputtered analytes were obtained, although the ion energy spread was too large to be suitable for a quadrupole system. We are currently considering the possibility of revisiting this type of ion source. CONCLUSIONS
Sputtered atom populations in a glow discharge are influenced by a number of factors. Diffusion of atoms away from the cathode sample causes significant concentration gradients toward the source extremities. The ion exit port of a GDMS source (15) Harrison,W. W.; Magee, C. W. Anal. Chem. 1973,46, 461-464
contributes its own effect as a result of the large differential pressure across the ion exit orifice, and this is at the very site where ions are produced for GDMS. A small, but well-defined reduction in sputtered atom density occurs near the center of the cathode face, possibly due to ionization processes. The atom distribution plots show that mass spectral sensitivity may be unduly reduced by the present source designs. A GDMS source that sampled ions nearer the cathode may be worth further consideration. ACKNOWLEDGMENT
This research was supported by the Department of Energy, Division of Basic Energy Sciences. In addition, K.H. was s u p ported by the Deutsche Forschungsgemeinschaft. Received for review January 3, 1995. Accepted June 20,
1995.B AC950009X @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
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