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Effect of limiting orifice (anode) geometry in radio frequency glow discharge emission spectrometry
Specnochrmica Acta. Vol Prmted III Great Bntam.
47B. No
11, pp. 13W1324.
0564-8547/92 $5 Ml + .OO Q 1992 Pergamon Press Ltd
1992
Effect of limiting orifice (anode) geometry in radio frequency glow discharge emission spectrometry
Department
CHRIS LAZIK and R. KENNETH MARCUS* of Chemistry, Howard L. Hunter Chemical Laboratories, Clemson University, Clemson, SC 29634-1905, U.S.A. (Recerved
10 September 1991; accepted 1 May 1992)
Abstract-Studies are described that arc focused on the evaluation of the role that limiting orifice (anode) geometry plays in the characteristics of an rf powered glow discharge (GD) atomic emission (AES) source. In particular, the role of orifice diameter on the parametric response of resonant and non-resonant state emission is investigated with regard to increasing the sputter efficiency and sensitivity of the device. Use of an extended, cylindrical anode is investigated with regard to reducing previously observed self-absorption characteristics. The roles of orifice diameter and discharge parameters on d.c. bias potential are studied and found to behave similarly to d.c. powered GD devices. Finally, the role of orifice diameter on plasma stabilization times is investigated. In all of the above investigations, use of a small (e.g. 2 mm) limiting orifice diameter is suggested for further development and analytical applications of the rf-GD-AES technique.
1. INTRODUCTION GLOW discharge (GD) devices have a long history as excitation sources for atomic emission spectroscopy (AES) [l-4]. Most early applications of these systems centered on the study of the atomic structure of gaseous species [1,2] and of other species introduced into the discharge by cathodic sputtering [3,4]. Strictly analytical applications of GD sources did not receive great attention, however, until the introduction of the GRIMM geometry in 1968 [5]. This design and modifications thereof have since been widely used for the direct bulk analysis of metals and alloys [6-lo]. In this area, the devices are employed as alternative excitation sources to traditional spark and arc sources. Subsequent fields of application were opened with the development of compaction techniques for the analysis of non-conducting powder samples [ll-131. More recently, the Grimm design has been utilized in depth resolved analyses of thin film systems [14]. The Grimm-type source has been employed in various other spectroscopic modes such as atomic absorption (AAS), fluorescence (AFS), and mass spectrometry (MS) [X-20]. For each of these analytical modes, the Grimm-type source configuration has been modified in order to enhance its overall performance for the respective techniques. Variations in Grimm-type AES sources including anode geometry, cathodic sputtering area, gas inlet assemblies, and optical window placement have also been discussed in the literature [21-271. A comprehensive characterization of the effects of these variations, however, has not been performed. Comparison of published material is difficult owing to the vast array of materials, instrumentation, and discharge parameters used in the evaluation of these sources. In this laboratory, we have focused our recent efforts on the development of GD devices for the direct analysis of conducting and non-conducting solids. In particular, a family of radio frequency (rf) devices has been developed for MS and areas of optical spectroscopy [28-301. The original source designs [28,29] employed assemblies in which samples were mounted and inserted within the source (vacuum) chamber. As such, sample size and shape were limited to the available holder, narrowing the scope of applicability. One of the principal advantages of the Grimm design is that it accepts * Author to whom correspondence
should be addressed. 1309
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C. LAZIK and R. K. MARCUS Argon Inlet
Fused s111co
Onflce dlometer
Ifice vzkness Glass lnsuiator Copper conductor
Mole coax
RG-213/U coax cable / (to matching network)
Fig. 1. Diagrammatic representation of the rf glow discharge atomization/excitation an enlargement of the anode orifice disk.
source and
all nominally flat samples larger than the diameter of the vacuum O-ring between the sample and the cathode block. Additionally, sample holders can be used for smaller, irregular shaped samples as well as for powdered materials. Therefore, to address the limitations of the “internal” rf-GD source design, an “external” sample mount geometry akin to the Grimm source was developed for AES [30]. This source design has met the goals of allowing great flexibility in the range of sample materials amenable to the technique, a simple and easy to clean construction, and a large analyte photon flux. Having developed a prototype source and evaluated the relative emission response to discharge conditions (rf power and source pressure), we concern ourselves here with an evaluation of the role that electrode geometry has on the operating characteristics of the source. Considerations such as the geometric influence on both resonant and non-resonant emission, sputter rates, and discharge stability are of particular interest. Through the studies described here, an optimal anode geometry has been suggested that will be utilized in future studies with this system.
2. EXPERIMENTAL The rf-GD-AES source presented in Fig. 1 has been described in detail previously [30]. In that report, end-on optical viewing (“glow+cathode”), as opposed to side-on viewing (“glow only”), was determined to be best suited for AES measurements. Therefore, for this work, emission signals were monitored through the optical window located opposite the sample surface. The focus of these studies was to evaluate the influence of anode geometry on the emission characteristics of the source. To facilitate this, a removable orifice disk was employed as highlighted in Fig. 1. Eleven interchangeable orifice disks were used, six of uniform thickness (2 mm) with orifice diameters ranging from 2 to 12 mm, and five disks of uniform diameter
Effect of limiting orifice geometry
1311
(6 mm) with thicknesses from 4 to 12 mm. The disks were designed such that the sample is placed in the same position relative to the source and focusing optics for each anode geometry used. Research grade argon (99.999% purity) was employed as the discharge gas. The spectrometric equipment employed included a 0.24-m Czerny-Turner mount monochromator (Digikrom 240, CVI Laser Corp., Albuquerque, NM) with a 2400 mm holographic grating and an optical speed of f/3.9. Entrance and exit slit widths were 50 and 25 p,m, respectively. Emission from the anode orifice was focused by a quartz, plano-convex lens with a focal length of 935 mm onto the entrance slit of the monochromator with a magnification of 1:l. An RCA lP28 photomultiplier tube (PMT) was used for radiation measurement, and a Keithley 280 picoammeter was used for signal monitoring. The PMT operating voltages were held constant at 650 V for all measurements, allowing direct comparisons of photocurrent for each respective transition across the range of experiments. A Houston Instruments Model 200 x-y recorder was used to plot photocurrent vs time for the temporal stability profiles. with a A 1.0-m Jobin-Yvon (Division of Instruments, S.A., Edison, NJ) monochromator 3600 mm holographic grating cf17.5) was employed in half-width and intensity measurements used for the verification of analyte self-absorption. Emission from the anode orifice was focused onto the entrance slit by a series of two plano-convex lenses (focal length: 111 mm) at a magnification of 1:l. A Hamamatsu R955HA PMT was used for photon detection and Jobin-Yvon Emission Spectrometry Software (J-YESS program version 3.37) was employed for data processing. Clean, polished brass disks were employed as samples for the optical measurements taken in these studies, and thin (3 mm), polished copper disks were used for the weight loss determinations. Emission intensity readings were taken at constant pressure while stepping the rf power from 5 to 60 W, allowing the discharge suitable time (-1 min) to stabilize at each power setting. Temporal responses were recorded commencing with the initiation of the discharge and monitored until the emission signal was constant. Weight loss measurements were taken with pre-weighed samples that were sputtered at constant power and pressure for 30 min. The weight loss was then determined upon reweighing the cooled, sputtered samples.
3. RESULTS AND DISCUSSION The influence of anode geometry on the emission characteristics of the Grimm-type discharge lamps has not been fully investigated. For instance, many variations in the anode orifice diameter and thickness have been reported [21-271; however, a single
orifice geometry that optimizes the emission characteristics of the source has not been generally accepted. Previous studies with d.c. powered GD systems in this laboratory have shown that sample sputtering rates are directly related to the power density of the discharge at the cathode surface [31]. In this case, sputter rates should be enhanced by either increasing the discharge power or by reducing the surface area of the cathode. An increase in analyte emission signals might also be expected at higher power densities. However, spectroscopic interferences believed to be self-absorption and self-reversal became severe with increasing discharge powers [30]. Thus, by investigating the role that orifice geometry has on the emission characteristics of the source, an optimum source configuration that yields higher sputtering rates and emission intensities while minimizing absorbing interferences can hopefully be achieved. 3.1. Dependence of emission characteristics on orifice diameter In the first part of these studies, the dependence of analyte emission on cathode surface area, as dictated by the limiting anode orifice diameters, and discharge power was investigated. The experiments were performed with stock brass samples under discharge conditions of constant power and pressure. Two Cu I transitions were monitored. The resonant Cu I 324.7 nm line was chosen because of its high intensity. The non-resonant Cu I 402.3 nm line was chosen because self-absorption interferences were expected to be less severe for this high lying transition. Preliminary studies with this system have demonstrated that the parametric responses of these transitions are different, suggesting a higher degree of analyte self-absorption and possibly self-reversal for resonant transitions [30].
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C. LAZEKand R. K. M~acvs
(4 60
0
2mm
A
4mm
9
6mm
b
Smm
l
IOmm
0 12mm
Power (WI
Power ( W 1
Fig. 2. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power for various orifice diameters (source pressure = 6 torr).
Figure 2 depicts the dependence of copper emission intensity [(a) 324.7 nm and (b) 402.3 nm] on rf generator power over the range of orifice diameters at a pressure of 6 torr. In Fig. 2(a), the emission intensity responses for the larger orifice diameters (4-12 mm diameter) exhibit a maximum with a subsequent roll-over between 10 and 20 W discharge power. At powers above 25 W, emission intensity seems to increase slightly with increases in power density, but at much Iower rates than before the rollaver. This first type of response seems to indicate severe self-absorption or even selfreversal and will be addressed in Section 3.2. For the 2 mm diameter orifice, the resonant emission intensity is shown to increase roughly in proportion to discharge power. This response differs considerably from those of the larger orifice sizes in that it does not roll-over at the higher discharge powers. At powers above 35 W, the emission intensity does begin to level off, probably due to self-absorption or to other interferences resulting from sample heating. While power densities and corresponding sputtering rates are greater in this arrangement, the intensities at the lower power levels (before the others roll-over) are not proportionally greater than for the large orifices. A secondary process involving sample transport or the like might also be operative allowing for the higher emission intensities to be achieved relative to the other orifice sizes. These aspects will be discussed further in the next Section. The emission intensity of the Cu I 402.3 nm transition, shown in Fig. 2(b), is enhanced with increases in discharge power for each of the orifice diameters. This direct relation between emission intensity and power is expected in cases where self-
Effect
of limiting
orifice
geometry
1313
(4
(b)
Fig. 3. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power and discharge pressure with a 4 mm diameter orifice disk.
absorption interferences are small. Interestingly, the Cu I 402.3 nm emission from the larger orifices is more intense than from the small orifice sizes. A first-principles explanation for this observation is difficult to imagine. These data seem to indicate that the relationship of emission intensity to discharge power might be separate from its relation to cathodic sputtering rates. Alternatively, conditions may be such that self-absorption for non-resonant states may be greater for the small orifice cases as the plasma is restricted to a smaller region. Plasma diagnostics, such as Langmuir probe studies of the influence of power density or cathode area on the electron energy distribution function (EEDF) of the discharge, are warranted in the near future [32]. For a complete characterization of the performance of any GD device, the integral relationship between analyte response, discharge power, and source pressure must be investigated. Contour plots of Cu I emission intensity vs power and pressure for the 4 and 2 mm diameter orifices are shown in Figs 3 and 4, respectively. Resonant emission in the 4 mm orifice plot displays the characteristic maxima between 10 and 20 W with a subsequent roll-over at higher discharge powers. With the exception of the lowest source pressures, where analyte emission is low, this roll-over type character is evident for each of the larger orifice diameters as well, across all discharge parameters. The response for the Cu I 402.3 nm non-resonant transition (Fig. 3(b)) yields a smoother contour than that of the 324.7 nm line. The plots reveal that higher source pressures and powers lead to greater emission intensities. However, under extreme
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LAZIK
and R. K.
MARCUS
50
Fig. 4. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power and discharge pressure with a 2 mm diameter orifice disk.
conditions of power (>60 W) and pressure (>12 torr), excessive cathode heating occurs leading to discharge instability problems. Figure 4 shows the parametric response for the 2 mm diameter orifice case. The contour for the Cu I 324.7 nm resonant transition differs dramatically from those acquired with the larger orifices. Emission from this orifice increases roughly in proportion with discharge power, especially at source pressures above 6 torr. Possible seIf-absorption interferences, which become severe with larger orifice sizes, seem to be significantly reduced here. In addition, resonant emission from the 2 mm orifice is about an order of magnitude more intense at discharge powers above 25 W, than for the larger orifice sizes. The contour for the non-resonant transition also differs from those for the larger orifice sizes. In this case, emission seems to be less influenced by source pressure, as compared to the 4 mm diameter orifice contour plot. These results are interesting. However, Langmuir probe analyses of the electron characteristics of these rf-GD devices are probably needed in order to provide a viable explanation
L34. The parametric response of analyte emission to output power for the various orifice sizes has implications for analytical applications of the GD device. In many cases where large dynamic ranges or high precision for major alloy (ceramic) components are required, operation and monitoring of transitions within a linear or near-linear powerintensity region is most likely beneficial. This near-linear relationship is achieved with the non-resonant Cu I 402.3 nm line for each of the orifice sizes employed in this study, This transition, however, is about two orders of magnitude less intense than the resonant line used, presenting a large compromise in analytical sensitivity. With the
Effect of limiting orifice geometry
1315
2 mm diameter orifice, where probable self-absorption interferences seem to be reduced significantly, the situation arises where intense resonant transitions could be utilized in more sensitive and precise analytical applications. To this end, the smaller orifice diameters (i.e. 2 mm) seem promising for this technique in that they yield the highest intensity for “normal” GD analytical lines. Instances where self-absorption does occur for matrix species can almost always be compensated for by choice of less susceptible transitions. 3.2. Evidence for self-absorption interferences In Figs 2 and 3, the emission signal for the Cu I 324.7 nm transition is shown to exhibit an intensity maximum at -20 W discharge power, followed by a roll-over in intensity at higher powers. This effect is most dramatic with the larger anode orifice diameters. Although this roll-over character appears to result from self-absorption interferences, this has not yet been confirmed spectrometrically. In GD systems of Grimm-type geometry, self-absorption interferences result when emission from the cathode surface and the negative glow is absorbed by other analyte species in the optical path between this region and the spectrometer. Self-reversal may result in cases where large populations of absorbing analyte species are able to diffuse into the “cooler” regions of the chamber (i.e. beyond the negative glow) near the optical sampling window. Information concerning the electronic energies and distributions of analyte species within a glow discharge can be discerned by investigating the emission intensities and profiles of several different analyte transitions. Alternatively, the extent of self-absorption can be determined by comparing the response of certain transitions where self-absorption is improbable to those where it is more likely to occur. Linewidth measurements of Cu I and II transitions (with the 1.0 m monochromator) as a function of discharge conditions were limited by the resolving power of the instrument. Therefore, comparison of emission characteristics of a number of transitions was performed. The relative emission responses (log intensity vs log power) of four copper transitions are shown in Fig. 5 for the 4 and 2 mm diameter orifices. Because sample sputtering rates are enhanced with increasing discharge power, these data can be viewed as emission responses to changing concentrations of analyte species within the plasma. Emission intensities for each transition are plotted relative to the intensity for that transition at 10 W discharge power. The transitions chosen were the resonant Cu I 324.7 nm transition, the Cu I 402.3 nm line that decays to an excited electronic state (30 535 cm-i), the Cu I 510.5 nm line that originates from the same excited state as the Cu I 324.7 nm line but decays to an excited triplet state (11 203 cm-‘), and the Cu II 224.7 ionic line. Self-absorption interferences would be expected to be minimal for the last transition since the ionic populations are small compared to those of atomic species in the extended regions of the chamber. For the 4 mm orifice case (Fig. 5(a)), the relative intensity of the ionic line is strongly enhanced with increases in discharge power. This indicates that atomization, excitation, and ionization conditions within the plasma improve at higher powers. In the absence of absorbing interferences, this type of response might also be expected for the atomic transitions as well. Instead, the relative intensities of the atomic lines are only slightly affected as compared to the ionic line. In fact, the Cu I 324.7 and 510.5 nm actually exhibit a roll-over in intensity at higher powers. The relative responses for the atomic transitions indicate that they are severely repressed by self-absorption (and reversal) interferences at discharge powers greater than 20 W. This problem becomes even more pronounced with the larger orifice diameters. Apparently, gains in excitation and emission from the cathode and the negative glow brought about by increased discharge power are met with a corresponding increase in the number densities of absorbing species in the optical path. In the case of the Cu I 324.7 and 510.7 nm lines, relative self-absorption actually rises with increases in power, eventually resulting in selfreversal. In Fig. 5(b), the emission responses of the copper transitions are shown for the 2 mm diameter orifice. As was the case for the 4 mm orifice, the values for the atomic
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C. LAZIK and R. K. MARCUS (a)
Log
(Discharge power)
I
10
100
Log(Discharge power 1 Fig. 5. Relative emission intensities of four copper transitions for the (a) 4 mm and (b) 2 mm orifice diameters (224.7 nm transition is Cu II. all others Cu I).
transitions are less dependent on discharge power than the ionic line. The three atomic lines respond quite similarly, however, suggesting that the extent of any self-absorption for each of the atomic lines is negligible. In addition, self-reversal seems to have been liminated, since the Cu I 324.7 and 510.5 nm lines do not exhibit the roll-over character that is present for the 4 mm orifice case. The role of orifice diameter on the extent of self-absorption would seem to be explained in a straightforward manner. For the 2 mm orifice, the discharge is constricted by the smaller anode orifice causing the discharge to emit more like a point-source. Because of this, only the sputtered species that are able to diffuse directly out from the cathode face will be able to interfere with the analytical signal. In fact, the majority of other sputtered material will simply diffuse radially out of the direct optical path. For the larger orifice sizes, emission from a larger cathode and negative glow effectively broadens the viewing area of the spectrometer. This enables larger populations of absorbing species to remain in the optical path, giving rise to self-absorption interferences. The fact that the roll-over character present with the larger orifice diameters disappears at low source pressures (Fig. 3(b)) might also suggest a radial diffusion mechanism for reducing self-absorption interferences as high diffusion rates would lower the probability of atoms remaining within the line of sight. In large part, owing to the absence of self-absorption effects, the emission intensities for both the Cu I 324.7 and 510.5 nm lines are about an order of magnitude higher for the 2 mm orifice than for the larger orifice diameters. This gives rise to the
Effect of limiting orifice geometry
1317
possibility of employing more intense, resonant transitions, which would not otherwise be used because of self-absorption interferences, in analytical applications where high sensitivity is required. 3.3. Effects of orifice thickness on emission intensity As is the case with anode orifice diameter, or cathodic sputtering area, the influence of anode thickness on the emission character of Grimm-type sources has not been fully investigated. The use of a “thick” anode assembly essentially confines the plasma to within a cylindrical volume rising above the cathode surface. This constriction of the discharge could yield higher emission signals by coupling energy to electrons and analyte species at distances removed from the cathode surface. Because these analyte atoms would otherwise comprise an ‘absorbing’ population, self-absorption interferences could possibly be reduced with this type of assembly. In order to determine the influence of orifice thickness on the emission characteristics of the source, a series of 6 mm diameter orifices of varying thickness (2-12 mm) were employed in these experiments. The 6 mm orifice was chosen as representative of those that exhibit the Cu I 324.7 nm roll-over response. Many GD systems have been developed that utilize the “thick” anode assembly. FERREIRAet al. employed a modified hollow anode assembly in order to contain the plasma directly above the cathode in a Grimm-type source [22]. They were able to perform AES studies from the “glow only” region of the discharge by means of a small slot in the side of the cylinder. Various other “thick” anode assemblies have been born primarily out of necessity, owing to the crowding effect of additional features, such as floating anodes [27] and gas flow systems at or near the sample surface [21,24,33]. The inherent costs or benefits incurred by these modifications in anode thickness have not been of great consequence for bulk solids analysis. The influence of anode thickness on emission intensity is demonstrated in Fig. 6. For both the Cu I 324.7 (a) and Cu I 402.3 nm (b) lines, the response of emission intensity to increases in discharge power appears to be less influenced by these changes in anode geometry than in orifice diameter. In the case of the resonant line, some increase in signal intensity is realized for the moderate thickness anodes; and for the non-resonant line, the smaller orifice thickness gives rise to enhanced emission intensity. In each case, the effects of orifice thickness on the emission character of the system seem to be minimal as compared to the effects of orifice diameter and thus, these factors are probably not of great consequence in the future development of this source. 3.4. Dependence of bias potential on orifice geometry The discharge potential resident on the cathode sample is an important parameter in determining many of the properties of GD systems. This bias potential affects both cathodic sputtering rates and the EEDFs in the negative glow of the discharge [31, 321. In rf powered GDs, the d.c. self-bias potential developed at the cathode serves to drive the discharge processes. Because power losses are common in rf powered discharge systems, the d.c. bias potential can sometimes be a more useful parameter than the output power of an rf generator in determining the actual power delivered to a plasma. As was reported previously [30], this may be the case with this system as well. In Fig. 7(a), the dependence of d.c. bias potential on rf generator output power for the various orifice diameters is depicted for a 6 torr source pressure. This dependence is approximately linear for each orifice size. However, the slopes of the lines are greater for the smaller orifice diameters. Interestingly, the bias voltage for the 2 mm orifice levels-off above 30 W. As will be discussed subsequently, this may reflect the spreading of the discharge in the recess region of the discharge. The near-linear relationship between d.c. bias potential and applied rf power is evident over the range of source pressures investigated (2-10 torr) and is consistent with d.c. powered discharge operating characteristics. An important factor to be noted is that the d.c. potential is not dependent on power density [31]. For example, the d.c. bias of the 8 mm orifice
1318
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LAZIK
and R. K.
MARCUS
0
2mm
*
4mm
l
6mm
A
0mm
l
IOmm
0 12mm
I IO
0
I 20
30
I
40
I 50
i 60
Power (WI
(b)
’2
r D
2mm
k
4mm
.
6mm
A
8mm
l
IOmm
0 12mm 02
Power ( W 1 Fig. 6. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intenstty on rf generator output power for various orifice thicknesses (source pressure = 6 torr).
(40 W, 332 V) is greater than for the 4 mm orifice (10 W, 237V), although the power density at these two settings is nearly equivalent. As will be discussed in the next Section, uncertainties about the true power density of the discharge at the cathode A contour plot of d.c. bias surface might provide an explanation for this discrepancy. potential vs rf power and discharge pressure is shown for the 6 mm diameter orifice in Fig. 7(b). An inverse relationship between pressure and bias potential is evident for each discharge power. This trend is again consistent with that found in studies with d.c. powered systems and seems to support the model describing the rf discharge as a d.c. plasma with a superimposed ac. potential [30]. As was the case for analyte emission responses, the effects of orifice thickness on d.c. bias potential were found to be small relative to the effects of cathode surface area. Data for each of the orifice thicknesses are not included in this report, although the contours for each orifice thickness could simply be superimposed onto the plot shown for the 6 mm (2 mm thick) orifice in Fig. 7(b). 3.5. Weight loss characteristics The influence of orifice geometry on cathode sputtering rates can be inferred from weight loss studies. In principle, anode geometries that produce greater atomization rates should ultimately yield higher emission signals, provided that self-absorption interferences can be kept to a minimum. In the case of anode orifice diameter, the optimal diameter should provide the best compromise between cathode surface area and power delivered (current, voltage), in order to produce the highest sputter rates. Copper disks were employed as targets for the weight loss studies, with each disk sputtered for 30 min at 25 W rf power (nominal for the emission studies). Figure 8 illustrates the average weight loss determined for five replicate disks for each orifice
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Effect of limiting orifice geometry
0 2mm b 4mm n
6mm
A 8mm l
IOmm
0 12mm
100
0
I
I
IO
20
I 30
Power
I 40
I 50
I
I
60
70
(WI
Fig. 7. Dependence of d.c. self-bias potential on (a) rf generator output power for various orifice sizes (source pressure = 6 torr) and (b) rf generator power and discharge pressure with the 6 mm diameter orifice disk.
size, with the relative sample-to-sample precisions listed on the right. Power density was used as the abscissa in order to show a more direct dependence of weight loss on cathodic sputter area and power density. As expected, sample loss rates increase with power density, with sputtering rates ranging from -100 pg/min to -250 kg/min for the 12-2 mm diameter orifices, respectively. If the weight loss for the 12 mm orifice is excluded, the data show an approximately linear dependence to power density (wt loss = 3.91 +1.70x, R2= 0.981). Previously, FANG and MARCUShave also shown a direct relation between weight loss and power density for a d.c. powered system over wide ranging discharge conditions of voltage and pressure [31]. The dependency of sputter weight loss on d.c. bias found in these data is also linearly related (wt loss = 2.04 + 0.98x, R2 = 0.974). Of course the dependence of sputter weight losses on power density and d.c. bias will also be a function of source pressure. Detailed parametric studies of cathodic sputtering in rf GD sources will be initiated in the very near future. These findings seem to further indicate that the smaller orifice diameters would be optimal for most analytical applications of GD sources. At a given discharge output power, sample sputtering rates are greater while self-absorption interferences are reduced with respect to the larger orifice sizes. In certain analytical applications where
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C. LAZIK and R. K. MARCUS 84mm / RSD of measurements 8.4% 4mm orifice 8.1 % 6mm 8mm 7 5%
p’ P
8mm
IOmm
IOmm
21 0
8 2%
12mm
13.6%
I
I
I
2
Power density
(W/mm2)
Fig. 8. Weight loss dependence of copper samples on power density for 30 min sputtering period (source pressure = 6 torr).
high sample turnover rates are required or when weakly emitting species must be analyzed, the smaller orifice diameters seem to be optimal. Excluded from the data in Fig. 8 is the weight loss for the 2 mm diameter orifice. This measurement is very difficult to perform due to limited (Cl0 min) stable operating times. Under these discharge conditions plasma instabilities such as arcing, shortcircuiting, and excess cathode heating, may be more severe for the smaller orifice diameters. These instabilities are thought to originate in the recess region between the sample and the anode orifice disk. As shown in Fig. 9, this recess region is contained within the discharge chamber though it is not exposed to direct cathodic sputtering. This area is larger for the small orifice diameters because the actual sputtering area is a smaller fraction of the area inscribed by the vacuum O-ring seal (which is 14 mm in diameter). Sputtered material can diffuse into this region and redeposit onto the sample surface or onto the O-ring between the sample and source chamber, leading to the formation of a conducting film. Short circuiting through this film results in net power losses from the plasma. Some systems have been designed to prevent the diffusion of sputtered material into the recess region of Grimm-type assemblies by decreasing the spacing between the sample and anode assembly to less than 0.2 mm [15]. This approach was attempted with the rf source. However, arcing across the electrodes began to occur at interelectrode distances less than about 0.4 mm. The interelectrode distance used for these experiments was kept at 0.5 mm.
Orlflce
disk
I
Sample
Fig. 9. Diagrammatic representation
Vacuum 0-nnq
of the orifice disk assembly depicting the recess region.
Effect of limiting orifice geometry
1321
Sample vaporization caused by extreme heating at the sample surface can also lead to discharge instabilities. This problem would be expected to be most pronounced for the smaller orifice diameter cases where higher power densities lead to greater sample surface temperatures. While little evidence has been found for vaporization in the sputtering region in this system, it is most likely to affect redeposited sample species in the recess region leading to the reintroduction of sample material into the plasma. Additionally, vaporized material can redeposit onto the insulating vacuum seal, leading to short circuiting problems. Sputtering also seems to take place within the recess region of the discharge under highly energetic discharge conditions. At moderate power densities, the cathodic sputtering area is defined by the size of the limiting anode orifice. When the plasma energy is increased the discharge seems to extend into the recess region. Under these conditions, the cathodic sputtering area extends out to the inner diameter of the Oring. This situation gives rise to uncertainties in sample sputtering rates and discharge power densities at the cathode surface. These uncertainties can probably be alleviated by employing smaller diameter O-rings to reduce the recess region area. This might also effectively minimize plasma instability problems associated with the recess region. The effects of orifice thickness on sample weight loss rates were found to be minimal. Weight loss rates were of the order of 130 l.i,g/min with an RSD of -5% for each of the orifice thicknesses employed, nearly identical to the value listed in Fig. 8 for the 6 mm diameter orifice. The data are consistent with the emission intensity responses and the d.c. bias potential measurements discussed previously, indicating that the influence of anode orifice thickness (over the range of 2-12 mm) on the emission and sputtering characteristics of the source is minimal. 3.6. Temporal stability measurements The ability of a glow discharge source to establish a stable plasma quickly is critical for many analytical applications. In particular, these devices are frequently employed in metal alloy production facilities, where knowledge of melt composition is needed rapidly. Stabilization times of the order of 2-3 min or less would allow for the prompt analysis of a reasonable number of samples in a given period of time. In addition, many discharge stability problems, such as short-circuiting, can be eliminated with short sample analysis times. Temporal profiles for the various orifice diameters used in these experiments are depicted in Fig. 10. The Cu I 324.7 nm transition was monitored. For each orifice size, the emission intensity reaches a stable value within two minutes after the plasma is initiated. The sharp peak at the start of the discharge is interesting and could represent a “true” emission from the plasma without the presence of self-absorption interferences (i.e. before large ground state populations exist in the chamber). The pre-peak might also be caused by loosely-bound matrix elements present on the sample surface that are sputtered efficiently at the onset of the discharge. This seems unlikely, however, since the sample surface is polished before being sputtered in these experiments. Finally, the peak may represent the higher instantaneous discharge voltages present as discharge is initiated and begins eroding. The emission profile for the 2 mm orifice differs significantly from those of the other orifice diameters. The resonant emission signal, as shown in both Figs 3(a) and 10, is about an order of magnitude greater for the smaller orifice with the signal-tobackground ratio (SIB) being at least an order of magnitude greater for the 2 mm diameter orifice. The signal-to-noise ratio (SIN) as described by long-term stability (-20 s), however, is compromised. In large part, this is the result of a low frequency oscillation in the emission signal. The magnitude of the short term noise is in fact comparable to the other traces. (It should be mentioned that background intensities do not fluctuate in this manner.) This may result from periodic changes in the position of the negative glow relative to the cathode face and focusing optics. Based on the observations of sputtering in the recess region discussed previously, it is believed that the low frequency oscillations are the result of a plasma wander in and around the
1322
C. LAZIK and R. K. MARCUS
Fig. 10. Temporal profiles (break-in curves) of Cu I 324.7 nm emission intensity of brass samples for each anode orifice diameter (power = 25 W, source pressure = 8 torr).
recess region. Reduction of the O-ring diameter, which would eliminate the recess region, is expected to remedy this problem. If successful, it is seen in the trace that the improvements in S/B will be seen in the SIN. The application of the rf-GD-AES technique for the analysis of non-conducting samples is a major goal of this research and has been demonstrated in a previous publication [30]. An exemplary spectrum of a Macor (Accuratus Ceramic Corp., Washington, NJ) sample was included in that paper. In Fig. 11, a temporal profile of
IO-
Time (mwd
Fig. 11. Temporal profile of Si I 288.1 nm emission intensity of a Macor sample (power = 25 W, source pressure = 8 torr).
Effect of limiting orifice geometry
1323
the Si I 288.1 nm transition from a Macor sample is shown for a 6 mm diameter orifice case. The Si concentration in Macor is -15%. As with the conducting samples, the emission is stabilized within 3 min of discharge ignition. The origin of the spike present at about 4 min is unknown. However, the discharge returned to the original stable reading soon thereafter.
4. CONCLUSIONS Studies of the effects of limiting anode orifice geometry on the emission characteristics of an rf-GD-AES source have been performed. Emission intensity measurements with brass samples indicate that the 2 mm orifice gives rise to intense analytical signals for resonant Cu transitions. Self-absorption interferences with this orifice are reduced substantially as compared with the larger orifice diameters. The effects of limiting anode thickness were much less pronounced than those of orifice diameter. However, the thinner orifices do yield slightly higher emission intensities than the thicker ones. Choice of an optimal anode orifice diameter thus seems to be a more important consideration in the future development of this type of system. Direct current bias potential measurements reveal a linear relationship to rf generator power and an inverse proportionality to source pressure. Bias potentials are larger for the smaller orifice diameters at a given discharge power, owing to greater power densities at the cathode surface. Under discharge conditions of constant source pressure and power density, d.c. potentials are found to differ for each orifice diameter, indicating that the relationship of the bias potential to power density is not direct Sputter weight losses are shown to be greater for the smaller orifice diameters. This also has been attributed to higher power densities at the sample surface. The effects of discharge power density on sample sputtering rates has not been precisely elucidated, owing to uncertainties caused by such factors as sample redeposition, and vaporization within the recess region of the source for the small diameter cases. Sputtering processes within the recess region have also created uncertainties as to the true power density at the cathode face. The effects of orifice thickness on weight loss were found to be negligible. The observed dependences on power density and d.c. bias potential are found to be very similar to those of d.c. GD devices. Temporal emission profiles were obtained for each of the anode orifice geometries. At moderate discharge power and pressure, the emission signal stabilizes within 2 min upon initiation for each orifice size. This is also the case when non-conducting samples are used. The response for the 2 mm orifice yields a greater emission intensity and signal-to-background ratio than the larger orifice sizes. However, the signal-to-noise ratio is appreciably reduced. The presence of a low frequency oscillation in the emission signal seems to be primarily responsible for this, and efforts to characterize and eliminate the source of this noise are currently being made in this laboratory. The smaller limiting anode orifice sizes, primarily the 2 mm orifice, seem to yield more analytically attractive emission characteristics with the rf-GD-AES system in this laboratory. Attempts to reduce or even eliminate the apparent compromise in signal-to-noise ratio for this orifice size, such as the development of a 2 mm orifice with a smaller recess region or the use of a water cooling system to prolong temporal stability, are currently underway. Additionally, the use of discharge gas flow onto the cathode surface is also being investigated as a possible means of improving the performance of this system. This rf-GD-AES system should thus prove to be an excellent atomization/excitation source for many applications in atomic emission spectrometry. Acknowledgements-This work is based upon work supported by the National Science Foundation under grant numbers CHE-8091788 and CHE-9117152. Financial support from Jobin-Yvon, Division of Instruments SA, is also gratefully appreciated. Helpful discussions with the Editor (P. W. J. M. B.) are also acknowledged.
1324
C. LAZIK and R. K. MARCUS REFERENCES
[l] [2] [3] [4] [5] [6] [7] [S] [9]
[lo] [ll] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26]
[27] [28]
[29] [30]
[31] [32] [33]
F. Paschen, Ann. Phys. N. Y. 50, 901 (1916). H. Z. Schuler, Physics 59, 149 (1929). P. W. J. M. Boumans, Anal. Chem. 44, 1219 (1972). P. J. Slevin and W. W. Harrison, Appl. Spectrosc. Rev. 10, 201 (1975). W. Grimm, Spectrochim. Acta. 23B, 443 (1968). H. Jager, AnaLChim. Acta 60, 303 (1972). H. W. Radmacher and M. C. De Swardt, Spectrochim. Acta 30B, 353 (1975). N. P. Ferreira and L. R. P. Butler, Analyst 103. 607 (1978). K. Wagatsuma and K. Hirokawa, Anal. Chem. 57, 2901 (1985). J. Pons-Corbeau, Surf Inter. Anal. 7, 169 (1975). M. Dogan, K. Laqua and H. Massman, Spectrochim. Acta 4OB, 65 (1972). G. S. Lombdahl and J. V. Sullivan, Spectrochim. Acta 398, 1395 (1984). M. R. Winchester and R. K. Marcus, Spectrochim. Acta 42B, 615 (1991). A. Bengtson. Spectrochim. Acta 40B, 631 (1985). D. S. Gough, Anal. Chem. 48, 1926 (1976). K. Ohls, Fresenius’ Z. Anal. Chem. 327, 118 (1987). H. Bubert, Spectrochim. Acta 39B, 1377 (1984). P. Hannaford and A. Walsh, Spectrochim. Acta 43B, 1068 (1988). H. J. Kim, E. H. Piepmeier, G. L. Beck, G. G. Brumbaugh and 0. T. Farmer III, Anal. Chem. 62, 639 (1990). N. Jakubowski,
D. Stiiwer and W. Vieth, Fresenid Z. Anal. Chem. 331, 145 (1988). A. E. Bernhard, Spectroscopy 2, 24 (1987). N. P. Ferreira, J. A. Strauss and H. G. C. Human, Spectrochim. Acta 38B, 899 (1983). M. Dogam, K. Laqua and H. Massman, Spectrochim. Acta 26B, 631 (1971). C. L. Chakrabarti, K. L. Headrick, J. C. Hutton, P. C. Bertels and M. H. Back, Spectrochim Acta 46B, 183 (1991). F. Leis, J. A. C. Broekaert and K. Laqua, Spectrochim. Acta 42B, 1169 (1987). H. J. Kim and E. H. Piepmeier, Anal. Chem. 60, 2040 (1988). J. B. Ko, Spectrochim. Acta 39B, 1405 (1984). D. C. Duckworth and R. K. Marcus, Anal. Chem. 61, 1879 (1989). M. R. Winchester and R. K. Marcus, J. Anal. At. Spectrom. 5, 575 (1990). M. R. Winchester, C. Lazik and R. K. Marcus, Spectrochim. Acta 46B, 483 (1991). D. Fang and R. K. Marcus, Spectrochim. Acta 43B, 1451 (1988). D. Fang and R. K. Marcus, Spectrochim Acta 45B, 1053 (1990). P. R. Banks and M. W. Blades, Spectrochim Acta 46B, 501 (1991).
47B. No
11, pp. 13W1324.
0564-8547/92 $5 Ml + .OO Q 1992 Pergamon Press Ltd
1992
Effect of limiting orifice (anode) geometry in radio frequency glow discharge emission spectrometry
Department
CHRIS LAZIK and R. KENNETH MARCUS* of Chemistry, Howard L. Hunter Chemical Laboratories, Clemson University, Clemson, SC 29634-1905, U.S.A. (Recerved
10 September 1991; accepted 1 May 1992)
Abstract-Studies are described that arc focused on the evaluation of the role that limiting orifice (anode) geometry plays in the characteristics of an rf powered glow discharge (GD) atomic emission (AES) source. In particular, the role of orifice diameter on the parametric response of resonant and non-resonant state emission is investigated with regard to increasing the sputter efficiency and sensitivity of the device. Use of an extended, cylindrical anode is investigated with regard to reducing previously observed self-absorption characteristics. The roles of orifice diameter and discharge parameters on d.c. bias potential are studied and found to behave similarly to d.c. powered GD devices. Finally, the role of orifice diameter on plasma stabilization times is investigated. In all of the above investigations, use of a small (e.g. 2 mm) limiting orifice diameter is suggested for further development and analytical applications of the rf-GD-AES technique.
1. INTRODUCTION GLOW discharge (GD) devices have a long history as excitation sources for atomic emission spectroscopy (AES) [l-4]. Most early applications of these systems centered on the study of the atomic structure of gaseous species [1,2] and of other species introduced into the discharge by cathodic sputtering [3,4]. Strictly analytical applications of GD sources did not receive great attention, however, until the introduction of the GRIMM geometry in 1968 [5]. This design and modifications thereof have since been widely used for the direct bulk analysis of metals and alloys [6-lo]. In this area, the devices are employed as alternative excitation sources to traditional spark and arc sources. Subsequent fields of application were opened with the development of compaction techniques for the analysis of non-conducting powder samples [ll-131. More recently, the Grimm design has been utilized in depth resolved analyses of thin film systems [14]. The Grimm-type source has been employed in various other spectroscopic modes such as atomic absorption (AAS), fluorescence (AFS), and mass spectrometry (MS) [X-20]. For each of these analytical modes, the Grimm-type source configuration has been modified in order to enhance its overall performance for the respective techniques. Variations in Grimm-type AES sources including anode geometry, cathodic sputtering area, gas inlet assemblies, and optical window placement have also been discussed in the literature [21-271. A comprehensive characterization of the effects of these variations, however, has not been performed. Comparison of published material is difficult owing to the vast array of materials, instrumentation, and discharge parameters used in the evaluation of these sources. In this laboratory, we have focused our recent efforts on the development of GD devices for the direct analysis of conducting and non-conducting solids. In particular, a family of radio frequency (rf) devices has been developed for MS and areas of optical spectroscopy [28-301. The original source designs [28,29] employed assemblies in which samples were mounted and inserted within the source (vacuum) chamber. As such, sample size and shape were limited to the available holder, narrowing the scope of applicability. One of the principal advantages of the Grimm design is that it accepts * Author to whom correspondence
should be addressed. 1309
1310
C. LAZIK and R. K. MARCUS Argon Inlet
Fused s111co
Onflce dlometer
Ifice vzkness Glass lnsuiator Copper conductor
Mole coax
RG-213/U coax cable / (to matching network)
Fig. 1. Diagrammatic representation of the rf glow discharge atomization/excitation an enlargement of the anode orifice disk.
source and
all nominally flat samples larger than the diameter of the vacuum O-ring between the sample and the cathode block. Additionally, sample holders can be used for smaller, irregular shaped samples as well as for powdered materials. Therefore, to address the limitations of the “internal” rf-GD source design, an “external” sample mount geometry akin to the Grimm source was developed for AES [30]. This source design has met the goals of allowing great flexibility in the range of sample materials amenable to the technique, a simple and easy to clean construction, and a large analyte photon flux. Having developed a prototype source and evaluated the relative emission response to discharge conditions (rf power and source pressure), we concern ourselves here with an evaluation of the role that electrode geometry has on the operating characteristics of the source. Considerations such as the geometric influence on both resonant and non-resonant emission, sputter rates, and discharge stability are of particular interest. Through the studies described here, an optimal anode geometry has been suggested that will be utilized in future studies with this system.
2. EXPERIMENTAL The rf-GD-AES source presented in Fig. 1 has been described in detail previously [30]. In that report, end-on optical viewing (“glow+cathode”), as opposed to side-on viewing (“glow only”), was determined to be best suited for AES measurements. Therefore, for this work, emission signals were monitored through the optical window located opposite the sample surface. The focus of these studies was to evaluate the influence of anode geometry on the emission characteristics of the source. To facilitate this, a removable orifice disk was employed as highlighted in Fig. 1. Eleven interchangeable orifice disks were used, six of uniform thickness (2 mm) with orifice diameters ranging from 2 to 12 mm, and five disks of uniform diameter
Effect of limiting orifice geometry
1311
(6 mm) with thicknesses from 4 to 12 mm. The disks were designed such that the sample is placed in the same position relative to the source and focusing optics for each anode geometry used. Research grade argon (99.999% purity) was employed as the discharge gas. The spectrometric equipment employed included a 0.24-m Czerny-Turner mount monochromator (Digikrom 240, CVI Laser Corp., Albuquerque, NM) with a 2400 mm holographic grating and an optical speed of f/3.9. Entrance and exit slit widths were 50 and 25 p,m, respectively. Emission from the anode orifice was focused by a quartz, plano-convex lens with a focal length of 935 mm onto the entrance slit of the monochromator with a magnification of 1:l. An RCA lP28 photomultiplier tube (PMT) was used for radiation measurement, and a Keithley 280 picoammeter was used for signal monitoring. The PMT operating voltages were held constant at 650 V for all measurements, allowing direct comparisons of photocurrent for each respective transition across the range of experiments. A Houston Instruments Model 200 x-y recorder was used to plot photocurrent vs time for the temporal stability profiles. with a A 1.0-m Jobin-Yvon (Division of Instruments, S.A., Edison, NJ) monochromator 3600 mm holographic grating cf17.5) was employed in half-width and intensity measurements used for the verification of analyte self-absorption. Emission from the anode orifice was focused onto the entrance slit by a series of two plano-convex lenses (focal length: 111 mm) at a magnification of 1:l. A Hamamatsu R955HA PMT was used for photon detection and Jobin-Yvon Emission Spectrometry Software (J-YESS program version 3.37) was employed for data processing. Clean, polished brass disks were employed as samples for the optical measurements taken in these studies, and thin (3 mm), polished copper disks were used for the weight loss determinations. Emission intensity readings were taken at constant pressure while stepping the rf power from 5 to 60 W, allowing the discharge suitable time (-1 min) to stabilize at each power setting. Temporal responses were recorded commencing with the initiation of the discharge and monitored until the emission signal was constant. Weight loss measurements were taken with pre-weighed samples that were sputtered at constant power and pressure for 30 min. The weight loss was then determined upon reweighing the cooled, sputtered samples.
3. RESULTS AND DISCUSSION The influence of anode geometry on the emission characteristics of the Grimm-type discharge lamps has not been fully investigated. For instance, many variations in the anode orifice diameter and thickness have been reported [21-271; however, a single
orifice geometry that optimizes the emission characteristics of the source has not been generally accepted. Previous studies with d.c. powered GD systems in this laboratory have shown that sample sputtering rates are directly related to the power density of the discharge at the cathode surface [31]. In this case, sputter rates should be enhanced by either increasing the discharge power or by reducing the surface area of the cathode. An increase in analyte emission signals might also be expected at higher power densities. However, spectroscopic interferences believed to be self-absorption and self-reversal became severe with increasing discharge powers [30]. Thus, by investigating the role that orifice geometry has on the emission characteristics of the source, an optimum source configuration that yields higher sputtering rates and emission intensities while minimizing absorbing interferences can hopefully be achieved. 3.1. Dependence of emission characteristics on orifice diameter In the first part of these studies, the dependence of analyte emission on cathode surface area, as dictated by the limiting anode orifice diameters, and discharge power was investigated. The experiments were performed with stock brass samples under discharge conditions of constant power and pressure. Two Cu I transitions were monitored. The resonant Cu I 324.7 nm line was chosen because of its high intensity. The non-resonant Cu I 402.3 nm line was chosen because self-absorption interferences were expected to be less severe for this high lying transition. Preliminary studies with this system have demonstrated that the parametric responses of these transitions are different, suggesting a higher degree of analyte self-absorption and possibly self-reversal for resonant transitions [30].
1312
C. LAZEKand R. K. M~acvs
(4 60
0
2mm
A
4mm
9
6mm
b
Smm
l
IOmm
0 12mm
Power (WI
Power ( W 1
Fig. 2. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power for various orifice diameters (source pressure = 6 torr).
Figure 2 depicts the dependence of copper emission intensity [(a) 324.7 nm and (b) 402.3 nm] on rf generator power over the range of orifice diameters at a pressure of 6 torr. In Fig. 2(a), the emission intensity responses for the larger orifice diameters (4-12 mm diameter) exhibit a maximum with a subsequent roll-over between 10 and 20 W discharge power. At powers above 25 W, emission intensity seems to increase slightly with increases in power density, but at much Iower rates than before the rollaver. This first type of response seems to indicate severe self-absorption or even selfreversal and will be addressed in Section 3.2. For the 2 mm diameter orifice, the resonant emission intensity is shown to increase roughly in proportion to discharge power. This response differs considerably from those of the larger orifice sizes in that it does not roll-over at the higher discharge powers. At powers above 35 W, the emission intensity does begin to level off, probably due to self-absorption or to other interferences resulting from sample heating. While power densities and corresponding sputtering rates are greater in this arrangement, the intensities at the lower power levels (before the others roll-over) are not proportionally greater than for the large orifices. A secondary process involving sample transport or the like might also be operative allowing for the higher emission intensities to be achieved relative to the other orifice sizes. These aspects will be discussed further in the next Section. The emission intensity of the Cu I 402.3 nm transition, shown in Fig. 2(b), is enhanced with increases in discharge power for each of the orifice diameters. This direct relation between emission intensity and power is expected in cases where self-
Effect
of limiting
orifice
geometry
1313
(4
(b)
Fig. 3. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power and discharge pressure with a 4 mm diameter orifice disk.
absorption interferences are small. Interestingly, the Cu I 402.3 nm emission from the larger orifices is more intense than from the small orifice sizes. A first-principles explanation for this observation is difficult to imagine. These data seem to indicate that the relationship of emission intensity to discharge power might be separate from its relation to cathodic sputtering rates. Alternatively, conditions may be such that self-absorption for non-resonant states may be greater for the small orifice cases as the plasma is restricted to a smaller region. Plasma diagnostics, such as Langmuir probe studies of the influence of power density or cathode area on the electron energy distribution function (EEDF) of the discharge, are warranted in the near future [32]. For a complete characterization of the performance of any GD device, the integral relationship between analyte response, discharge power, and source pressure must be investigated. Contour plots of Cu I emission intensity vs power and pressure for the 4 and 2 mm diameter orifices are shown in Figs 3 and 4, respectively. Resonant emission in the 4 mm orifice plot displays the characteristic maxima between 10 and 20 W with a subsequent roll-over at higher discharge powers. With the exception of the lowest source pressures, where analyte emission is low, this roll-over type character is evident for each of the larger orifice diameters as well, across all discharge parameters. The response for the Cu I 402.3 nm non-resonant transition (Fig. 3(b)) yields a smoother contour than that of the 324.7 nm line. The plots reveal that higher source pressures and powers lead to greater emission intensities. However, under extreme
1314
C.
LAZIK
and R. K.
MARCUS
50
Fig. 4. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intensity on rf generator output power and discharge pressure with a 2 mm diameter orifice disk.
conditions of power (>60 W) and pressure (>12 torr), excessive cathode heating occurs leading to discharge instability problems. Figure 4 shows the parametric response for the 2 mm diameter orifice case. The contour for the Cu I 324.7 nm resonant transition differs dramatically from those acquired with the larger orifices. Emission from this orifice increases roughly in proportion with discharge power, especially at source pressures above 6 torr. Possible seIf-absorption interferences, which become severe with larger orifice sizes, seem to be significantly reduced here. In addition, resonant emission from the 2 mm orifice is about an order of magnitude more intense at discharge powers above 25 W, than for the larger orifice sizes. The contour for the non-resonant transition also differs from those for the larger orifice sizes. In this case, emission seems to be less influenced by source pressure, as compared to the 4 mm diameter orifice contour plot. These results are interesting. However, Langmuir probe analyses of the electron characteristics of these rf-GD devices are probably needed in order to provide a viable explanation
L34. The parametric response of analyte emission to output power for the various orifice sizes has implications for analytical applications of the GD device. In many cases where large dynamic ranges or high precision for major alloy (ceramic) components are required, operation and monitoring of transitions within a linear or near-linear powerintensity region is most likely beneficial. This near-linear relationship is achieved with the non-resonant Cu I 402.3 nm line for each of the orifice sizes employed in this study, This transition, however, is about two orders of magnitude less intense than the resonant line used, presenting a large compromise in analytical sensitivity. With the
Effect of limiting orifice geometry
1315
2 mm diameter orifice, where probable self-absorption interferences seem to be reduced significantly, the situation arises where intense resonant transitions could be utilized in more sensitive and precise analytical applications. To this end, the smaller orifice diameters (i.e. 2 mm) seem promising for this technique in that they yield the highest intensity for “normal” GD analytical lines. Instances where self-absorption does occur for matrix species can almost always be compensated for by choice of less susceptible transitions. 3.2. Evidence for self-absorption interferences In Figs 2 and 3, the emission signal for the Cu I 324.7 nm transition is shown to exhibit an intensity maximum at -20 W discharge power, followed by a roll-over in intensity at higher powers. This effect is most dramatic with the larger anode orifice diameters. Although this roll-over character appears to result from self-absorption interferences, this has not yet been confirmed spectrometrically. In GD systems of Grimm-type geometry, self-absorption interferences result when emission from the cathode surface and the negative glow is absorbed by other analyte species in the optical path between this region and the spectrometer. Self-reversal may result in cases where large populations of absorbing analyte species are able to diffuse into the “cooler” regions of the chamber (i.e. beyond the negative glow) near the optical sampling window. Information concerning the electronic energies and distributions of analyte species within a glow discharge can be discerned by investigating the emission intensities and profiles of several different analyte transitions. Alternatively, the extent of self-absorption can be determined by comparing the response of certain transitions where self-absorption is improbable to those where it is more likely to occur. Linewidth measurements of Cu I and II transitions (with the 1.0 m monochromator) as a function of discharge conditions were limited by the resolving power of the instrument. Therefore, comparison of emission characteristics of a number of transitions was performed. The relative emission responses (log intensity vs log power) of four copper transitions are shown in Fig. 5 for the 4 and 2 mm diameter orifices. Because sample sputtering rates are enhanced with increasing discharge power, these data can be viewed as emission responses to changing concentrations of analyte species within the plasma. Emission intensities for each transition are plotted relative to the intensity for that transition at 10 W discharge power. The transitions chosen were the resonant Cu I 324.7 nm transition, the Cu I 402.3 nm line that decays to an excited electronic state (30 535 cm-i), the Cu I 510.5 nm line that originates from the same excited state as the Cu I 324.7 nm line but decays to an excited triplet state (11 203 cm-‘), and the Cu II 224.7 ionic line. Self-absorption interferences would be expected to be minimal for the last transition since the ionic populations are small compared to those of atomic species in the extended regions of the chamber. For the 4 mm orifice case (Fig. 5(a)), the relative intensity of the ionic line is strongly enhanced with increases in discharge power. This indicates that atomization, excitation, and ionization conditions within the plasma improve at higher powers. In the absence of absorbing interferences, this type of response might also be expected for the atomic transitions as well. Instead, the relative intensities of the atomic lines are only slightly affected as compared to the ionic line. In fact, the Cu I 324.7 and 510.5 nm actually exhibit a roll-over in intensity at higher powers. The relative responses for the atomic transitions indicate that they are severely repressed by self-absorption (and reversal) interferences at discharge powers greater than 20 W. This problem becomes even more pronounced with the larger orifice diameters. Apparently, gains in excitation and emission from the cathode and the negative glow brought about by increased discharge power are met with a corresponding increase in the number densities of absorbing species in the optical path. In the case of the Cu I 324.7 and 510.7 nm lines, relative self-absorption actually rises with increases in power, eventually resulting in selfreversal. In Fig. 5(b), the emission responses of the copper transitions are shown for the 2 mm diameter orifice. As was the case for the 4 mm orifice, the values for the atomic
1316
C. LAZIK and R. K. MARCUS (a)
Log
(Discharge power)
I
10
100
Log(Discharge power 1 Fig. 5. Relative emission intensities of four copper transitions for the (a) 4 mm and (b) 2 mm orifice diameters (224.7 nm transition is Cu II. all others Cu I).
transitions are less dependent on discharge power than the ionic line. The three atomic lines respond quite similarly, however, suggesting that the extent of any self-absorption for each of the atomic lines is negligible. In addition, self-reversal seems to have been liminated, since the Cu I 324.7 and 510.5 nm lines do not exhibit the roll-over character that is present for the 4 mm orifice case. The role of orifice diameter on the extent of self-absorption would seem to be explained in a straightforward manner. For the 2 mm orifice, the discharge is constricted by the smaller anode orifice causing the discharge to emit more like a point-source. Because of this, only the sputtered species that are able to diffuse directly out from the cathode face will be able to interfere with the analytical signal. In fact, the majority of other sputtered material will simply diffuse radially out of the direct optical path. For the larger orifice sizes, emission from a larger cathode and negative glow effectively broadens the viewing area of the spectrometer. This enables larger populations of absorbing species to remain in the optical path, giving rise to self-absorption interferences. The fact that the roll-over character present with the larger orifice diameters disappears at low source pressures (Fig. 3(b)) might also suggest a radial diffusion mechanism for reducing self-absorption interferences as high diffusion rates would lower the probability of atoms remaining within the line of sight. In large part, owing to the absence of self-absorption effects, the emission intensities for both the Cu I 324.7 and 510.5 nm lines are about an order of magnitude higher for the 2 mm orifice than for the larger orifice diameters. This gives rise to the
Effect of limiting orifice geometry
1317
possibility of employing more intense, resonant transitions, which would not otherwise be used because of self-absorption interferences, in analytical applications where high sensitivity is required. 3.3. Effects of orifice thickness on emission intensity As is the case with anode orifice diameter, or cathodic sputtering area, the influence of anode thickness on the emission character of Grimm-type sources has not been fully investigated. The use of a “thick” anode assembly essentially confines the plasma to within a cylindrical volume rising above the cathode surface. This constriction of the discharge could yield higher emission signals by coupling energy to electrons and analyte species at distances removed from the cathode surface. Because these analyte atoms would otherwise comprise an ‘absorbing’ population, self-absorption interferences could possibly be reduced with this type of assembly. In order to determine the influence of orifice thickness on the emission characteristics of the source, a series of 6 mm diameter orifices of varying thickness (2-12 mm) were employed in these experiments. The 6 mm orifice was chosen as representative of those that exhibit the Cu I 324.7 nm roll-over response. Many GD systems have been developed that utilize the “thick” anode assembly. FERREIRAet al. employed a modified hollow anode assembly in order to contain the plasma directly above the cathode in a Grimm-type source [22]. They were able to perform AES studies from the “glow only” region of the discharge by means of a small slot in the side of the cylinder. Various other “thick” anode assemblies have been born primarily out of necessity, owing to the crowding effect of additional features, such as floating anodes [27] and gas flow systems at or near the sample surface [21,24,33]. The inherent costs or benefits incurred by these modifications in anode thickness have not been of great consequence for bulk solids analysis. The influence of anode thickness on emission intensity is demonstrated in Fig. 6. For both the Cu I 324.7 (a) and Cu I 402.3 nm (b) lines, the response of emission intensity to increases in discharge power appears to be less influenced by these changes in anode geometry than in orifice diameter. In the case of the resonant line, some increase in signal intensity is realized for the moderate thickness anodes; and for the non-resonant line, the smaller orifice thickness gives rise to enhanced emission intensity. In each case, the effects of orifice thickness on the emission character of the system seem to be minimal as compared to the effects of orifice diameter and thus, these factors are probably not of great consequence in the future development of this source. 3.4. Dependence of bias potential on orifice geometry The discharge potential resident on the cathode sample is an important parameter in determining many of the properties of GD systems. This bias potential affects both cathodic sputtering rates and the EEDFs in the negative glow of the discharge [31, 321. In rf powered GDs, the d.c. self-bias potential developed at the cathode serves to drive the discharge processes. Because power losses are common in rf powered discharge systems, the d.c. bias potential can sometimes be a more useful parameter than the output power of an rf generator in determining the actual power delivered to a plasma. As was reported previously [30], this may be the case with this system as well. In Fig. 7(a), the dependence of d.c. bias potential on rf generator output power for the various orifice diameters is depicted for a 6 torr source pressure. This dependence is approximately linear for each orifice size. However, the slopes of the lines are greater for the smaller orifice diameters. Interestingly, the bias voltage for the 2 mm orifice levels-off above 30 W. As will be discussed subsequently, this may reflect the spreading of the discharge in the recess region of the discharge. The near-linear relationship between d.c. bias potential and applied rf power is evident over the range of source pressures investigated (2-10 torr) and is consistent with d.c. powered discharge operating characteristics. An important factor to be noted is that the d.c. potential is not dependent on power density [31]. For example, the d.c. bias of the 8 mm orifice
1318
C.
LAZIK
and R. K.
MARCUS
0
2mm
*
4mm
l
6mm
A
0mm
l
IOmm
0 12mm
I IO
0
I 20
30
I
40
I 50
i 60
Power (WI
(b)
’2
r D
2mm
k
4mm
.
6mm
A
8mm
l
IOmm
0 12mm 02
Power ( W 1 Fig. 6. Dependence of (a) Cu I 324.7 nm and (b) Cu I 402.3 nm emission intenstty on rf generator output power for various orifice thicknesses (source pressure = 6 torr).
(40 W, 332 V) is greater than for the 4 mm orifice (10 W, 237V), although the power density at these two settings is nearly equivalent. As will be discussed in the next Section, uncertainties about the true power density of the discharge at the cathode A contour plot of d.c. bias surface might provide an explanation for this discrepancy. potential vs rf power and discharge pressure is shown for the 6 mm diameter orifice in Fig. 7(b). An inverse relationship between pressure and bias potential is evident for each discharge power. This trend is again consistent with that found in studies with d.c. powered systems and seems to support the model describing the rf discharge as a d.c. plasma with a superimposed ac. potential [30]. As was the case for analyte emission responses, the effects of orifice thickness on d.c. bias potential were found to be small relative to the effects of cathode surface area. Data for each of the orifice thicknesses are not included in this report, although the contours for each orifice thickness could simply be superimposed onto the plot shown for the 6 mm (2 mm thick) orifice in Fig. 7(b). 3.5. Weight loss characteristics The influence of orifice geometry on cathode sputtering rates can be inferred from weight loss studies. In principle, anode geometries that produce greater atomization rates should ultimately yield higher emission signals, provided that self-absorption interferences can be kept to a minimum. In the case of anode orifice diameter, the optimal diameter should provide the best compromise between cathode surface area and power delivered (current, voltage), in order to produce the highest sputter rates. Copper disks were employed as targets for the weight loss studies, with each disk sputtered for 30 min at 25 W rf power (nominal for the emission studies). Figure 8 illustrates the average weight loss determined for five replicate disks for each orifice
1319
Effect of limiting orifice geometry
0 2mm b 4mm n
6mm
A 8mm l
IOmm
0 12mm
100
0
I
I
IO
20
I 30
Power
I 40
I 50
I
I
60
70
(WI
Fig. 7. Dependence of d.c. self-bias potential on (a) rf generator output power for various orifice sizes (source pressure = 6 torr) and (b) rf generator power and discharge pressure with the 6 mm diameter orifice disk.
size, with the relative sample-to-sample precisions listed on the right. Power density was used as the abscissa in order to show a more direct dependence of weight loss on cathodic sputter area and power density. As expected, sample loss rates increase with power density, with sputtering rates ranging from -100 pg/min to -250 kg/min for the 12-2 mm diameter orifices, respectively. If the weight loss for the 12 mm orifice is excluded, the data show an approximately linear dependence to power density (wt loss = 3.91 +1.70x, R2= 0.981). Previously, FANG and MARCUShave also shown a direct relation between weight loss and power density for a d.c. powered system over wide ranging discharge conditions of voltage and pressure [31]. The dependency of sputter weight loss on d.c. bias found in these data is also linearly related (wt loss = 2.04 + 0.98x, R2 = 0.974). Of course the dependence of sputter weight losses on power density and d.c. bias will also be a function of source pressure. Detailed parametric studies of cathodic sputtering in rf GD sources will be initiated in the very near future. These findings seem to further indicate that the smaller orifice diameters would be optimal for most analytical applications of GD sources. At a given discharge output power, sample sputtering rates are greater while self-absorption interferences are reduced with respect to the larger orifice sizes. In certain analytical applications where
1320
C. LAZIK and R. K. MARCUS 84mm / RSD of measurements 8.4% 4mm orifice 8.1 % 6mm 8mm 7 5%
p’ P
8mm
IOmm
IOmm
21 0
8 2%
12mm
13.6%
I
I
I
2
Power density
(W/mm2)
Fig. 8. Weight loss dependence of copper samples on power density for 30 min sputtering period (source pressure = 6 torr).
high sample turnover rates are required or when weakly emitting species must be analyzed, the smaller orifice diameters seem to be optimal. Excluded from the data in Fig. 8 is the weight loss for the 2 mm diameter orifice. This measurement is very difficult to perform due to limited (Cl0 min) stable operating times. Under these discharge conditions plasma instabilities such as arcing, shortcircuiting, and excess cathode heating, may be more severe for the smaller orifice diameters. These instabilities are thought to originate in the recess region between the sample and the anode orifice disk. As shown in Fig. 9, this recess region is contained within the discharge chamber though it is not exposed to direct cathodic sputtering. This area is larger for the small orifice diameters because the actual sputtering area is a smaller fraction of the area inscribed by the vacuum O-ring seal (which is 14 mm in diameter). Sputtered material can diffuse into this region and redeposit onto the sample surface or onto the O-ring between the sample and source chamber, leading to the formation of a conducting film. Short circuiting through this film results in net power losses from the plasma. Some systems have been designed to prevent the diffusion of sputtered material into the recess region of Grimm-type assemblies by decreasing the spacing between the sample and anode assembly to less than 0.2 mm [15]. This approach was attempted with the rf source. However, arcing across the electrodes began to occur at interelectrode distances less than about 0.4 mm. The interelectrode distance used for these experiments was kept at 0.5 mm.
Orlflce
disk
I
Sample
Fig. 9. Diagrammatic representation
Vacuum 0-nnq
of the orifice disk assembly depicting the recess region.
Effect of limiting orifice geometry
1321
Sample vaporization caused by extreme heating at the sample surface can also lead to discharge instabilities. This problem would be expected to be most pronounced for the smaller orifice diameter cases where higher power densities lead to greater sample surface temperatures. While little evidence has been found for vaporization in the sputtering region in this system, it is most likely to affect redeposited sample species in the recess region leading to the reintroduction of sample material into the plasma. Additionally, vaporized material can redeposit onto the insulating vacuum seal, leading to short circuiting problems. Sputtering also seems to take place within the recess region of the discharge under highly energetic discharge conditions. At moderate power densities, the cathodic sputtering area is defined by the size of the limiting anode orifice. When the plasma energy is increased the discharge seems to extend into the recess region. Under these conditions, the cathodic sputtering area extends out to the inner diameter of the Oring. This situation gives rise to uncertainties in sample sputtering rates and discharge power densities at the cathode surface. These uncertainties can probably be alleviated by employing smaller diameter O-rings to reduce the recess region area. This might also effectively minimize plasma instability problems associated with the recess region. The effects of orifice thickness on sample weight loss rates were found to be minimal. Weight loss rates were of the order of 130 l.i,g/min with an RSD of -5% for each of the orifice thicknesses employed, nearly identical to the value listed in Fig. 8 for the 6 mm diameter orifice. The data are consistent with the emission intensity responses and the d.c. bias potential measurements discussed previously, indicating that the influence of anode orifice thickness (over the range of 2-12 mm) on the emission and sputtering characteristics of the source is minimal. 3.6. Temporal stability measurements The ability of a glow discharge source to establish a stable plasma quickly is critical for many analytical applications. In particular, these devices are frequently employed in metal alloy production facilities, where knowledge of melt composition is needed rapidly. Stabilization times of the order of 2-3 min or less would allow for the prompt analysis of a reasonable number of samples in a given period of time. In addition, many discharge stability problems, such as short-circuiting, can be eliminated with short sample analysis times. Temporal profiles for the various orifice diameters used in these experiments are depicted in Fig. 10. The Cu I 324.7 nm transition was monitored. For each orifice size, the emission intensity reaches a stable value within two minutes after the plasma is initiated. The sharp peak at the start of the discharge is interesting and could represent a “true” emission from the plasma without the presence of self-absorption interferences (i.e. before large ground state populations exist in the chamber). The pre-peak might also be caused by loosely-bound matrix elements present on the sample surface that are sputtered efficiently at the onset of the discharge. This seems unlikely, however, since the sample surface is polished before being sputtered in these experiments. Finally, the peak may represent the higher instantaneous discharge voltages present as discharge is initiated and begins eroding. The emission profile for the 2 mm orifice differs significantly from those of the other orifice diameters. The resonant emission signal, as shown in both Figs 3(a) and 10, is about an order of magnitude greater for the smaller orifice with the signal-tobackground ratio (SIB) being at least an order of magnitude greater for the 2 mm diameter orifice. The signal-to-noise ratio (SIN) as described by long-term stability (-20 s), however, is compromised. In large part, this is the result of a low frequency oscillation in the emission signal. The magnitude of the short term noise is in fact comparable to the other traces. (It should be mentioned that background intensities do not fluctuate in this manner.) This may result from periodic changes in the position of the negative glow relative to the cathode face and focusing optics. Based on the observations of sputtering in the recess region discussed previously, it is believed that the low frequency oscillations are the result of a plasma wander in and around the
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C. LAZIK and R. K. MARCUS
Fig. 10. Temporal profiles (break-in curves) of Cu I 324.7 nm emission intensity of brass samples for each anode orifice diameter (power = 25 W, source pressure = 8 torr).
recess region. Reduction of the O-ring diameter, which would eliminate the recess region, is expected to remedy this problem. If successful, it is seen in the trace that the improvements in S/B will be seen in the SIN. The application of the rf-GD-AES technique for the analysis of non-conducting samples is a major goal of this research and has been demonstrated in a previous publication [30]. An exemplary spectrum of a Macor (Accuratus Ceramic Corp., Washington, NJ) sample was included in that paper. In Fig. 11, a temporal profile of
IO-
Time (mwd
Fig. 11. Temporal profile of Si I 288.1 nm emission intensity of a Macor sample (power = 25 W, source pressure = 8 torr).
Effect of limiting orifice geometry
1323
the Si I 288.1 nm transition from a Macor sample is shown for a 6 mm diameter orifice case. The Si concentration in Macor is -15%. As with the conducting samples, the emission is stabilized within 3 min of discharge ignition. The origin of the spike present at about 4 min is unknown. However, the discharge returned to the original stable reading soon thereafter.
4. CONCLUSIONS Studies of the effects of limiting anode orifice geometry on the emission characteristics of an rf-GD-AES source have been performed. Emission intensity measurements with brass samples indicate that the 2 mm orifice gives rise to intense analytical signals for resonant Cu transitions. Self-absorption interferences with this orifice are reduced substantially as compared with the larger orifice diameters. The effects of limiting anode thickness were much less pronounced than those of orifice diameter. However, the thinner orifices do yield slightly higher emission intensities than the thicker ones. Choice of an optimal anode orifice diameter thus seems to be a more important consideration in the future development of this type of system. Direct current bias potential measurements reveal a linear relationship to rf generator power and an inverse proportionality to source pressure. Bias potentials are larger for the smaller orifice diameters at a given discharge power, owing to greater power densities at the cathode surface. Under discharge conditions of constant source pressure and power density, d.c. potentials are found to differ for each orifice diameter, indicating that the relationship of the bias potential to power density is not direct Sputter weight losses are shown to be greater for the smaller orifice diameters. This also has been attributed to higher power densities at the sample surface. The effects of discharge power density on sample sputtering rates has not been precisely elucidated, owing to uncertainties caused by such factors as sample redeposition, and vaporization within the recess region of the source for the small diameter cases. Sputtering processes within the recess region have also created uncertainties as to the true power density at the cathode face. The effects of orifice thickness on weight loss were found to be negligible. The observed dependences on power density and d.c. bias potential are found to be very similar to those of d.c. GD devices. Temporal emission profiles were obtained for each of the anode orifice geometries. At moderate discharge power and pressure, the emission signal stabilizes within 2 min upon initiation for each orifice size. This is also the case when non-conducting samples are used. The response for the 2 mm orifice yields a greater emission intensity and signal-to-background ratio than the larger orifice sizes. However, the signal-to-noise ratio is appreciably reduced. The presence of a low frequency oscillation in the emission signal seems to be primarily responsible for this, and efforts to characterize and eliminate the source of this noise are currently being made in this laboratory. The smaller limiting anode orifice sizes, primarily the 2 mm orifice, seem to yield more analytically attractive emission characteristics with the rf-GD-AES system in this laboratory. Attempts to reduce or even eliminate the apparent compromise in signal-to-noise ratio for this orifice size, such as the development of a 2 mm orifice with a smaller recess region or the use of a water cooling system to prolong temporal stability, are currently underway. Additionally, the use of discharge gas flow onto the cathode surface is also being investigated as a possible means of improving the performance of this system. This rf-GD-AES system should thus prove to be an excellent atomization/excitation source for many applications in atomic emission spectrometry. Acknowledgements-This work is based upon work supported by the National Science Foundation under grant numbers CHE-8091788 and CHE-9117152. Financial support from Jobin-Yvon, Division of Instruments SA, is also gratefully appreciated. Helpful discussions with the Editor (P. W. J. M. B.) are also acknowledged.
1324
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