Radio-Frequency Spectrometry

ANALYTICAL SCIENCES JUNE 1996, VOL. 12 459 Effects Argon of Helium Addition to Glow-Discharge Mass Radio-Frequency Spectrometry Jin-Chun WOOL*, Dong-Min MooN*, Tomokazu TANAKA**, Motoya MATSUNO** and Hiroshi KAWAGUCHI** *KRISS (Korea Research Institute of Standards and Science), P. 0. Box 102, Yusong, Taejon 305-606, Korea **Department of Materials Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan The addition of helium to argon up to 5% significantly decreased the sputtering rate for a brass sample, while about 30% increases of copper and zinc-ion signals were observed upon the 2% helium addition. The normalized ionization efficiencies were calculated from the ratio of the ion intensity and the atom density in the plasma measured by atomic absorption spectrometry. The ionization efficiency at 4% addition of helium was 25-times higher than that for pure argon. No change in the DC self-bias voltage was observed for various additions of helium. These characteristics were compared with the oxygen addition as well as with a DC glow discharge. The differences in the ion-signal behavior between the radio frequency and DC glow discharges are discussed. Keywords Glow-discharge mass spectrometry, argon/ helium discharge gas, radio-frequency discharge, Grimm-type ion source, absorption spectrometry, DC self-bias potential. Radio-frequency glow discharge (rf-gd) has been increasingly studied in analytical atomic spectrometry as an emission or ion source, because it has a unique capability of direct sputtering of both metal and nonconducting sample surfaces.1 6 In recent years, some successful applications of rf-gd to analytical emission and mass spectrometry have been reported with good precision and stability.'-9 Rf-GD, however, still has a limitation for the direct analysis of solid samples, because of the relatively low sensitivity and sputtering rate of samples, especially for non-conducting samples.' On the other hand, the property of an argon-glow discharge is known to be greatly influenced by the presence of a small amount of additional gas, such as helium, hydrogen, water and oxygen. Some of the gases provide positive effects, while others have serious negative effects on the analytical purpose. The effects of the mixture gas on a direct-current (DC) glow discharge have already been extensively investigated by many authors.10 13 Wagatsuma and Hirokawa14,15 reported on the effects of helium addition on the spectral emission intensities with a DC argon glow discharge. They observed an increase in the intensities of many atomic and ionic lines with the addition of helium. In the present work, we studied the effect of the controlled addition of helium to argon in rf-gd mass spectrometry. For a comparison, the effects of helium addition in DC-GD mass spectrometry were also investigated. The variation in the analyte atom densities in the rf-gd plasma was measured by atomic absorption spectrometry. This paper also includes a comparison of the addition of helium and oxygen to an rf argon glow discharge. Experimental Glow-discharge ion source A Grimm-type ion source attached to a commercial inductively coupled plasma mass spectrometer (Model SPQ 6100A, Seiko Instr. Inc.) has been described elsewhere.5,16 The source was slightly modified for this experiment; a schematic diagram is presented in Fig. 1. This source can be used in both the rf and DC modes. An aperture (G) of 1.0 mm was insulated from the anode body with Teflon sheet, and was located at a distance of 11 mm from the surface of a flat sample (C). An anode sleeve (E) was inserted in the present experiments (Fig. 1) in order to prevent an erratic discharge between the anode and the sample which was previously often necessary for a conducting sample. The clearance between the anode sleeve with an internal diameter of 7 mm and the cathode (D) was 0.2 mm. The anode body was connected electrically to the ground potential, and a positive voltage of about X65 V was applied to the aperture (G) with a DC power supply (Model PAB-160,

460 ANALYTICAL SCIENCES JUNE 1996, VOL. 12 Fig. 1 Schematic diagram of the Grimm-type glow-discharge source for a mass spectrometer (A, rf power line; B, cooling block; C, sample; D, cathode block; E, anode sleeve and block; F, gas inlet; G, aperture block; H, skimmer cone; I, direction to mass spectrometer; J, pumping line; K, Teflon; L, 0 ring; M, N, water cooling). Fig. 2 Schematic diagram of the Grimm-type glow discharge lamp for atomic absorption spectrometry (A, rf power leadplate; B, sample; C, cathode; D, anode sleeve; E, anode body; F, two optical side windows for HCL; G, window; H, gas inlet; I, pumping outlet; J, K, L, Teflon; H, 0 ring). Kikusui Electronics Co.). An rf power supply (Model RFS-002A, ULVAC, 13.56 MHz and 250 W) was used for the rf discharge along with a matching box (Model MB-002M, ULVAC). A DC power supply (Model V-703, Kawaguchi Electric Works, 2000 V, 50 ma) was used for the DC glow discharge for a comparison study. In order to introduce additional gas and argon, two pairs of a mass-flow controller (Model 3810, KOFLOC) and a needle valve were used. Argon gas of high purity (>99.999%) was used as the discharge gas. The helium and oxygen used for the additional gases were referred to as commercially high purity. Glow-discharge lamp for atomic absorption spectrometry In order to observe the effect of additional gas on the analyte atom density in the GD plasma, a Grimm-type lamp which could be attached to an atomic absorption spectrometer (Model IL-250, Instrumental Lab. Ltd.) instead of a burner was constructed; a schematic diagram is given in Fig. 2. Two quartz windows (F) with a diameter of 20 mm were provided at the side of the body for the light path from a hollow-cathode lamp. The distance between the sample surface and the optical axis was 18 mm. The clearance between the anode sleeve (D) and the cathode (C) was 0.2 mm, and the diameter of the anode sleeve was 7 mm. An rf power supply (Model RS-5S, RF Plasma Product Inc., 13.56 MHz and 500 W) was used for this lamp along with a matching box (Model AM-5, RF Plasma Product Inc.). In this experiment, a Convectron gauge (Ganville-Philips Co.) was used for the gas-flow controller. Sample and miscellaneous Brass samples were used for most of the measurements. The sample surface was polished with alumina abrasive paper (above 300 grit) and washed in an ultrasonic bath. For the measuring the DC self-bias voltage, a digital multimeter with 110 Mfl input impedance was employed with a series choke coil of 80 µh inductance and a shunt capacitor of 1000 pf. The crater profiles were measured with a surface-roughness meter (Model Surfcom 110B, Tokyo Seimitsu). For measuring the sputtering rate, the amounts of material ablated were determined by weighing the sample before and after a discharge for 5-30 min, depending on the operating conditions. When the sample loss was too small to be weighed, it was estimated from the sputtered volume measured by the roughness meter. Results and Discussion Effect of the discharge pressure In a previous paper, it was reported that the variation in the emission intensity and sputtering rate as a function of the pressure has a maximum at a pressure of about 2 Torr in an rf-gd operated at a constant rf power.' This behavior is in contrast with the fact that they increase exponentially as the pressure increases in a DC glow discharge operated in a constant-voltage mode. In rf-gd mass spectrometry, the variation in the Cu and Zn-ion signal intensities as a function of the pressure was measured; the results are given in Fig. 3. This figure indicates that the ion signal intensities have their maxima at about 4 Torr when the discharge is operated at a constant rf power of 120 W. Since the sputtering rates

ANALYTICAL SCIENCES JUNE 1996, VOL. 12 461 Table 1 Typical helium addition conditions used for the experiment of Fig. 3 Dependence of the argon pressure on intensities. Sample, brass; rf power, 120 W. the ion signal measured at each pressure condition always have similar maxima as those of the ion signal intensities, this behavior could be explained by the same mechanism as that of the rf-gd emission spectrometry described previously.' In absorption spectrometry with the discharge tube shown in Fig. 2 operated at a constant rf power of 120 W, the absorbance of copper measured at 324.7 nm for a brass sample also showed a maximum at about 3 Torr. Considering the difference in the location of the pressure gauges relative to the discharge for mass and atomic absorption spectrometry, the difference in the pressures showing the maxima of the ion intensities and the absorbance may be assumed not to be significant. The subsequent experiments were carried out at the optimum pressure in each discharge mode, as shown in Table 1. The linearity of the atomic absorption spectrometry was ascertained with low-alloy steel samples (SS401-405 Certified Reference Materials, Bureau of Analysed Samples Ltd.) for manganese in the DC glow-discharge mode. Although the absorbance of manganese (0.112 for 1.84% of Mn) is about one forth lower than that reported in the literaturel', the sensitivity was sufficient for the measuring of the absorbance of copper in brass samples. Other experimental conditions optimized for the experiments involving helium addition are summarized in Table 1. Effect of helium on the sputtered atom density The absorbances of the Cu 324.7 nm line were measured for a brass sample in the range of 0-5% helium addition to argon in both the rf and DC glow discharge. As an example, the absorbance variation in the rf glow discharge operated at 120 W is presented in Fig. 4. A similar variation was observed in the DC glow discharge. The sputtering rates of the sample were measured at each different helium content; showing that the variation of sputtering rate was quite similar to those of the Fig. 4 Effect of the helium ratio on the atomic absorption of Cu 324.7 nm in an rf argon glow discharge. Sample, brass; rf power, 120W. Fig. 5 Correlation between the atomic absorption and sputtering rate at various ratios of helium in an rf argon glow discharge.

462 ANALYTICAL SCIENCES JUNE 1996, VOL. 12 absorbance. The correlation between the absorbance and the sputtering rate measured in the rf glow-discharge mode is shown in Fig. 5. Even though the measurement error of the sputtering rate were relatively large under the condition of a low sputtering rate, an excellent linear relationship (r=0.96) between the sputtering rate and absorbance was observed. Effect of helium addition on the ion intensities The ion intensities of Cu, Zn+ and Ar+ were measured at various helium ratios in the rf argon glow discharge; the results are given in Fig. 6. Intensities of the Cu and Zn ions first increased along with the helium addition, and then maximized at 2% addition of helium. Although the intensity of Ar+ gradually increased, it showed no maximum in the range of 0-5% helium. The intensities of Cu+ and Zn+ at their maxima are about 30% higher than those for pure argon. The sputtering rate measured at each helium ratio in the rf glow discharge is also shown at the bottom of Fig. 6. While the ion intensities of Cu+ and Zn+ increased along with helium addition until 2%, the sputtering rate of the sample decreased from the beginning. Assuming that the ionization efficiency of an atom is defined as the ionization probability of the atom in a plasma without the consideration of a sputtering effect from solid sample, the increase in Cu+ and Zn+ intensities can be explained as an increase in the ionization efficiency. Since the meta stable state (19.8 and 20.7 ev) and ionization potential (24.58 ev) of helium are much higher than those of argon, the increase in the ionization efficiency may be a direct consequence of the presence of higher energy state species in the plasma. The sputtering, on the other hand, decreases along with an increase in the helium ratio, as shown in Figs. 4 and 6. Therefore, the maxima of the intensities of Cu+ and Zn+ may be caused by two opposite competing effects: a decrease in the atom density and an increase in the ionization efficiency along with an increase in the helium ratio. In Fig. 6, the intensity of Ar+ gradually increases along with an increase in helium. This may be reasonable, since the argon population is not related to the sputtering. As shown in Fig. 5, there is a linear relationship between the sputtering rate and the sputtered-atom density of copper in the plasma. The ionization efficiency can therefore be obtained from the ratio of the ion intensity and sputtering rate. The ionization efficiencies of copper normalized to that for pure argon were calculated and plotted against the helium content (Fig. 7). The ionization efficiency increased along with increasing the ratio of helium, and became about 25- times higher at 4% addition than that of pure argon. The ionization efficiency of argon became only about 2.5- times higher at 4% helium addition. This may have been due to the difference in the ionization potential of copper (7.68 ev) and argon (15.75 ev). Comparison with a DC glow discharge The effect of helium addition to argon on the ion Fig. 6 Variation in the ion intensity and sputtering rate as a function of the helium ratio in an rf argon glow discharge. Sample, brass; rf power, 120 W. Fig. 7 Normalized ionization efficiency for various helium ratios in an rf argon glow discharge. intensities and sputtering rate was measured in the DC glow discharge mode for a comparison with the rf glow discharge; the results are given in Fig. 8. The increase in the argon ion signal and the decrease in the sputtering rate are similar to those of an rf glow discharge, while only a slight increase in the ion signals of Cu+ and Zn+ could be seen at 1 % of helium addition. These results suggest that effect of the helium addition on the ionization efficiency is greater in the rf glow discharge than in the DC glow discharge. To clarify the reason, the variation in the DC self-bias potential was measured,

ANALYTICAL SCIENCES JUNE 1996, VOL. 12 463 Fig. 9 Variation in the ion intensity as a oxygen ratio in an rf argon glow discharge. rf power,120 W. function Sample, of the brass; Fig. 8 Variation in the ion intensity and sputtering rate as a function of the helium ratio in a DC argon glow discharge. Sample, brass; discharge voltage, -1000 V. but did not explain the above results as described later. Comparison with oxygen addition For a comparison, the ion intensities of Cut, Zn, and Ar+ were measured at various oxygen ratios in the rf-gd. As shown in Fig. 9, the intensities of Cu+ and Zn+ decreased drastically upon increasing the oxygen ratio. The intensities then slightly increased after the minima at the 2% addition. A rapid decrease in the sputtering rate was also observed along with an increase in the oxygen ratio. These results indicate that the addition of oxygen reduces both of the ionization efficiency and the sputtering rate. Effect of helium addition on the DC self-bias potential The DC self-bias voltage generated at the sample electrode in the rf-gd has been well considered in many papers.4'' The self-bias voltage is induced to the sample electrode, which is electrically isolated in terms of DC from the rf power source by a blocking capacitor in the matching box. The variation in the DC self-bias voltage was measured as a function of the helium content, and is given in Fig. 10. For comparison, the voltage was also measured at various additions of oxygen and is plotted in the same figure. The self-bias potential did not change for either helium or oxygen addition within the experimental error. Since a DC self-bias voltage is generated by a difference in the ion and electron mobilities, the change in the average ion mobilities might be compensated for a change in the electron concentration. Fig. 10 Variation in the DC self-bias voltage as a function of the helium or oxygen ratio in an rf argon glow discharge. Sample, brass; rf power, 120 W. References 1. D. L. Donohue and W. W. Harrison, Anal. Chem., 47, 1528 (1975). 2. D. C. Duckworth and R. K. Marcus, Anal. Chem., 61, 1879 (1989). 3. D. C. Duckworth and R. K. Marcus, Appl. Spectrosc., 44, 649 (1990). 4. M. R. Winchester, C. Lazik and R. K. Marcus, Spectrochim. Acta, 46B, 483 (1991). 5. H. Kawaguchi, T. Tanaka and H. Fukaya, Anal. Sci., 7 (supplement), 537 (1991). 6. T. R. Harville and R. K. Marcus, Spectrochim. Acta, 65B, 3636 (1993). 7. J. C.Woo, K. H. Cho, T. Tanaka and H. Kawaguchi, Spectrochim. Acta, 49B, 915 (1994). 8. S. De Gendt, R. E. Van Grieken, S. K. Ohrodnik and W.

464 ANALYTICAL SCIENCES JUNE 1996, VOL. 12 W. Harrison, Anal. Chem., 67,1026 (1995). 9. R. Haville and R. K. Marcus, Anal. Chem., 67, 1271 (1995). 10. M. Hecq, A. Hecq and M. Fontignies, Anal. Chim. Acta, 155, 191 (1983). 11. P. L. Larkins, Spectrochim. Acta, 46B, 291 (1991). 12. M. Saito, Anal. Sci., 7 (supplement), 541 (1991). 13. W. Fischer, A. Naoumidis and H. Nickel, J. Anal. At. Spectrom., 9, 375 (1994). 14. K. Wagatsuma and K. Hirokawa, Spectrochim. Acta, 42B, 523 (1987). 15. K. Wagatsuma and K. Hirokawa, Anal. Chem., 60, 702 (1988). 16. T. Tanaka, T Kubota and H, Kawaguchi, Anal. Sci.,10, 895 (1994). 17. D. S. Gough, P. Hannaford and R. M. Lowe, Anal. Chem., 61, 1652 (1989). (Received November 27, 1995) (Accepted February 29, 1996)