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Characteristics of fast electrons from Grimm glow discharge He plasmas as an electron source
SPECTROCHIMICA ACTA PART B
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 1587-1595
Characteristics of fast electrons from Grimm glow discharge He plasmas as an electron source K. Tsuji*, K. Wagatsuma, H. Matsuta Institute for Materials Research, Tohoku University, 2-I-I Katahira, Aoba, Sendai, 980-77, Japan
Received 3 March 1997: accepted 23 April 1997
Abstract
In a study of d.c. helium Grimm glow discharges applied with a maximum high voltage of 5 kV, it was found that fast electrons emitted from the cathode surface passed right through the negative glow and the hollow anode. This fast electron current corresponds to approximately 50% of the discharge current at a high voltage of 5 kV and a low helium pressure of 0.43 Torr for an aluminium cathode. The characteristics of the fast electrons under various discharge conditions, i.e. for different cathode materials, discharge voltages and helium pressures, were investigated by measuring the electron-induced X-rays, in order to consider whether the glow discharge plasma could have an application as an electron source. It was found that fast electrons were generated more efficiently under a high voltage while using a low gas pressure in the aluminium cathode. The fast electrons were then used as an electron source for the electron-induced X-ray emission analysis of Fe-Mo binary alloys. This experiment indicated that fast electrons from a Grimm glow discharge plasma are useful as a simple and inexpensive electron source in electron-induced X-ray emission analysis. © 1997 Elsevier Science B.V. Keywords: Grimm glow discharge; Fast electrons; Runaway electrons; X-ray emission; Electron source
I. I n t r o d u c t i o n
The glow discharge plasma has been utilized as an excitation source for atomic emissions. Glow discharge atomic emission spectroscopy (GD-AES) is a powerful tool for the direct analysis of solid samples [1-5]. In direct current GD-AES, a high voltage is applied between an anode and a sample cathode where cathode sputtering occurs due to ion bombardment [6]. The sputtered atoms are introduced into the negative glow region where they are excited by electron and metastable-atom impacts, leading to atomic emission. G D - A E S analysis can be performed at a high cathode sputtering rate without a high-vacuum * Corresponding author. E-mail: [email protected]
system; therefore, rapid depth analysis is possible using simple and inexpensive apparatus. This provides a major advantage over other surface analytical methods such as secondary ion mass spectrometry (SIMS). Glow discharge plasmas have also been used as an ion source. The sputtered atoms are ionized in the glow plasma, then analyzed by a mass spectrometer. This is known as glow discharge-mass spectrometry (GD-MS) [7-9]. In addition to the advantages of GDAES, G D - M S has a high detection power comparable to SIMS; therefore G D - M S is well suited for the ultratrace analysis of solid samples. Usually, a discharge voltage of less than 1 kV is applied using Ar as the discharge gas in both GDAES and GD-MS. These discharge conditions are
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0584-8547(97)00061 -X
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K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) I587 1595
necessary to obtain a stable discharge and an appropriate cathode sputtering rate. Matsuta and Wagatsuma have attempted to operate a helium glow discharge at discharge voltages of 2 kV and above [10]. It was found that a stable discharge is possible at a maximum discharge voltage of 5 kV using a normal Grimm discharge lamp. Electrons emitted from the cathode surface would have high energies corresponding to the cathode sheath voltage; thus X-ray radiation could be expected due to collisional energy transfer from the fast electrons. In fact, soft X-rays from the glow discharge lamp were measured through an X-ray transparent window [11 ]. In addition, X-ray radiation was found to originate from the X-ray window, thus indicating that highenergy electrons actually bombard the X-ray window
[ll]. In this paper, we present a study of d.c. high-
[Linearamp.] 1 [MCA]
voltage He glow discharges in order to determine optimum discharge conditions for the generation of fast electrons, and discuss the possibility of using the glow discharge plasma as an electron source. Currently, a heated filament and a tip to which a high voltage is applied are employed as electron sources in surface analytical tools such as an Auger electron spectroscopy apparatus or a scanning electron microscope. These electron sources have excellent stability and strong intensity; however, they must be operated under ultra-high-vacuum conditions. Maintenance of this ultra-high vacuum is both difficult and costly. Size constraint is also one of the most important problems to overcome in the development of an analytical apparatus. A glow discharge lamp can easily be operated under low-vacuum conditions, suggesting the possibility of a simple and inexpensive electron source. So far, there have
He gas
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Fig. 1. Experimentalset-up for the measurementof electron-induced X-rayemissionsusing a Grimmglow discharge lamp.
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
been many studies on the application of a glow discharge plasma as an optical excitation source and an ion source for analytical atomic spectroscopy. For example, fast electrons from a d.c. glow discharge plasma have been energy-analyzed in order to determine the influence of electron impact on thin film growth [12,13]. However, they have rarely been considered as an electron source for an analytical technique. We believe that fundamental spectroscopic studies of X-ray emission from a glow discharge lamp are necessary and useful when utilizing it as an electron source.
2. Experimental 2.1. A p p a r a t u s
In the conventional glow discharge lamp designed by Grimm [14,15], a flat sample is used both as the cathode and to vacuum-seal the discharge lamp. This lamp is suitable for the direct analysis of common metals, because the sample needs only to be sufficiently flat and large to seal the vacuum of the lamp. We custom-built the Grimm-type glow discharge lamp shown in Fig. 1. The flat cathode was attached to the discharge lamp with an O-ring. An aluminium plate was used as a typical cathode. The anode body and the hollow anode tube (8 mm inner diameter) were made of brass. Although a quartz glass plate was attached opposite the cathode in the glow discharge atomic emission spectrometer, an X-ray transparent window was necessary to detect the X-rays. In our previous experiments, we used thin films of Be, A1 and Zr, which were stuck onto a brass plate to seal a hole 3 mm in diameter, and X-rays which penetrated this thin film were monitored with a solid-state detector (SSD) [11]. The X-ray window material itself was found to emit X-rays when bombarded by fast electrons. To detect X-rays reflected from the bulk metals, we designed a target (sample) holder as shown in Fig. 1. This holder was cut at an angle of 45 ° from a brass column (30 mm in diameter), and it had a tunnel 8 mm in diameter for the electrons to pass through. The target was attached to this holder with an O-ring 18 mm in diameter; therefore, the fast electrons would bombard the target surface at an incident angle of
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45 °. X-rays were detected 25 ° from the target's surface through a Be window 25 /~m thick and 4 mm in diameter. The distance between the Be window and the Si(Li) X-ray detector (EMAX, Horiba Co.; energy resolution, 158.9 eV at Mn Ka) was 20 mm. This distance is critical as soft X-rays are easily absorbed by air molecules. Since the X-ray intensity was too strong, an X-ray slit of width 0.05 mm was attached to the front of the SSD. The detected X-rays were amplified with a linear amplifier (TC 243; Oxford Instruments Inc., UK) and were analyzed with a multichannel analyzer (MCA PC98-B; Laboratory Equipment Co.) using a personal computer. The Grimm lamp was evacuated from two ports by using two rotary pumps. The ultimate vacuum pressure was 0.02 Torr. Helium gas (purity more than 99.9995%) was introduced into the discharge lamp, monitoring the pressure with a Pirani gauge. The He pressure was adjusted by using a micrometering valve to change the flow rate of the He gas. In our previous work, a pulsed glow discharge was employed to suppress the total electric power supplied to the lamp [11]. Since a stable glow discharge plasma could only be created by a direct current discharge, the latter was employed in this work. A positive high voltage was applied to the anode body using a high-voltage power supply (EW5R 120-100; Glassman High Voltage Inc.; voltage capacity, less than 5 kV) with a grounded cathode. Both the anode body and the cathode were water-cooled. The temperature of the target was also monitored from the back by an infrared radiation thermometer (IR-BT1-GB5; Chino Co.) as shown in Fig. 1. 2.2. Samples
Although the cathode material was used as the sample in GD-AES, in this study the target was considered to be the sample, as shown in Fig. 1. Various pure A1, Si, Zr, Nb, Mo, Pd and Ag plates (20 × 20 mm2; thickness, 0.5 mm) were used as targets after their surfaces were polished with emery paper. In order to investigate the relationship between the X-ray intensity and the concentration of the element, standard F e - M o binary alloys (disk diameter, 35 mm; thickness, 5 ram; certified Mo concentration, 0.20-7.07%) were prepared.
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K. Tsuji et aL/Spectrochimica Acta Part B 52 (1997) 1587-1595
3. Results and discussion
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It is well known that secondary electrons are emitted from the cathode surface by the bombardment of ions produced by the glow discharge plasma [ 16,17]. This secondary electron emission is essential for maintaining the discharge. The emitted electrons are accelerated across the cathode sheath. The cathode sheath voltage is the sum of the applied voltage and the plasma potential, which is considerably smaller than the applied voltage [17]. The fast electrons, whose high energies correspond to the cathode sheath potential, enter the negative glow region. Slow electrons lose their energy by collisions with particles in the negative glow, resulting in ionization and excitation, and, subsequently, optical emission. It should be emphasized that the electron collision cross-section decreases with an increase in electron energy [17]. Therefore, some of the high-energy electrons pass through the negative glow without any collisions. These high-energy electrons are known as "runaway" electrons [17]. This is a significant phenomenon when discharge voltages above 2 kV are applied. Since a voltage of less than 1 kV is used in argon GDAES analysis, it is unnecessary to consider the effect of fast electrons. Moreover, in cases where a flat plate is used as the anode, fast electrons just bombard the anode plate and the only result is a rise in temperature. In the case of a Grimm lamp with a hollow anode, the
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fast electrons pass right through the negative glow and the hollow anode. Thus, we can utilize the fast "runaway" electrons from the plasma as an electron source. To observe the area bombarded by the electrons, an acrylic plate (thickness, 3 mm) was exposed to a 3 kV discharge operated for 20 s. After the target holder was removed, the acrylic plate attached opposite the cathode was used as a target. As shown in Fig. 2, the area damaged by electron impact is a circle about 10 mm in diameter, which almost corresponds to the diameter of the hollow anode. A unique feature of this electron source compared with others is the irradiation of the target by the fast electrons over a relatively large area. 3.2. Target current
Fig. 2. Photograph of an acrylic plate damaged by electron impact. The acrylic plate was placed on section paper. The damaged area is a circle about 10 mm in diameter. The discharge was operated at a discharge voltage of 3 kV for 20 s.
To evaluate the current density of the fast electrons, the target current was measured using a copper target (40 x 50 mm2; thickness, 1 mm) under various discharge conditions. The copper plate was attached opposite the cathode after the target holder was removed. The target current between the copper plate and the anode body was measured by an ammeter. Fig. 3 shows the voltage dependence of both the discharge current between the anode and the cathode, and the target current between the anode and the copper target, at an He pressure of 0.43 Torr with an aluminium cathode. The ratio of
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595 0.55
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He pressure led to more frequent collisions between the electrons and the particles in the plasma, resulting in a decrease in the number of fast electrons passing through the hollow anode. Judging from the results of Figs 3 and 4, it can be seen that fast electrons can be generated efficiently under high voltages and at low gas pressures.
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the target current to the discharge current is also shown by filled circles. The current ratio suddenly increased at voltages above 1.5 kV. This experimental result agrees with the explanation of the fast electrons described in Section 3.1, i.e. the electron collision cross-section decreases on increasing the electron energy, which is almost equivalent to the applied voltage. The target current corresponds to approximately 50% of the discharge current at voltages above 4 kV. The target current was 1.85 mA at an applied voltage of 5 kV, and the area of the electron irradiation was a circle 10 mm in diameter, as shown in Fig. 2. Therefore, the electron current density at the target was determined to be 23.6 A m -2 in this experiment. The He pressure dependence of the current ratio is shown in Fig. 4. The discharge was operated at a voltage of 5 kV with an aluminium cathode. The ratio decreased as the He pressure increased. A higher Table 1 The effects of different cathodes on current and Si Kot intensity, measured for an Si target at a voltage of 4 kV and an He pressure of 0.43 Torr Cathode
Current/mA
Si K a / ( c o u n t s s -t)
Mo W Ti Nb Cu Zr AI Si
< 0.01 < 0.01 < 0.01 0.01 0.13 0.62 0.63 1.05
15 38 63 77 755 7917 9607 10785
The effects of different cathode materials (Mo, W, Ti, Nb, Cu, Zr, A1, and Si) on both the discharge current and the X-ray intensity from the target were investigated. A silicon wafer was used as the target. The results obtained are shown in Table 1. The secondary electron coefficient is dependent on the type of cathode [18]. As shown in Table 1, the discharge current changes drastically depending on the cathode material. Since the X-ray intensity excited by the electron impact is proportional to the electron current density, we can measure the target current from the Si K a intensity. Table 1 implies that the discharge current is responsible for the generation of the electrons impinging on the target. Si, A1 and Zr are superior cathode materials for obtaining a strong target current. When selecting cathode material, we must consider its sputtering rate and stability, because the sputtering rate from ion impact also depends on the cathode material. When a Grimm lamp is used as an electron source, a smaller sputtering rate is desirable. This is because the sputtered atoms are deposited on the anode tube, leading to unstable discharges. The sputtering rates of A1 and Si were low under our experimental conditions. The stability of the discharge for the Si cathode is slightly less because of an irregular spark-like discharge. Thus, the aluminium plate was selected as the best cathode for our experiment.
3.4. Time dependence of the discharge The time dependence of the d.c. current and the target temperature was investigated. The target temperature was monitored from the back of the target by using an infrared radiation thermometer. The results for Cu and Si targets are shown in Fig. 5(a) and 5(b), respectively. The largest change in the discharge current was during the initial 200 s of discharge time;
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after that time, it showed an almost constant value. Therefore, measurements were performed after 3 min of the discharge. The Cu target temperature rose rapidly during the initial 1 min of the discharge, then decreased slightly as the discharge current decreased. The target temperature increase is caused by electron impact, which depends on the discharge current. In the case of the Si target, a surprising rise in temperature was observed, as shown in Fig. 5(b). It was difficult for us to believe that the Si temperature actually increased to almost 60°C. The temperature was monitored using an infrared radiation thermometer. Considering that Si is an infrared-transparent material, the infrared radiation observed, which was transmitted through 0.5 mm
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thick Si, might have been produced on the Si surface by electron impact. Consequently, cooling the target is necessary in order to keep its temperature steady.
3.5. Discharge voltage dependence Fig. 6(a)-6(c) show typical X-ray spectra for the AI target at the different discharge voltages 2.5 kV, 3.5 kV, and 4.5 kV, respectively. The X-ray spectra consist of a characteristic AI Kot (1.49 keV) X-ray peak and a broad peak. Since the shape and the energy of the broad peak changed depending on the applied voltage, the broad peak can be considered to be bremsstrahlung (continuous X-rays). The highest energy of the bremsstrahlung is near the applied voltage. This indicates that the energy of the fast electrons can be controlled by adjusting the applied voltage. The lower energy side of the bremsstrahlung drastically decreased below 2 keV because of soft X-ray absorption both by molecules in the air and the Be window. Therefore, the actual intensity of the AI Kot peak must be larger than the observed one shown in Fig. 6(a)-6(c). The voltage dependence of the discharge current and the AI Ko~ intensity is shown in Fig. 7. Both the current and the AI Ko~ intensity increased as the discharge voltage increased, and a similar dependence on the discharge voltage was observed. In general, the applied voltage dominates the electron energy, and the discharge current reflects the electron density in
Fig. 8. The relationship between V - Ve~ (where Vis the voltage and Vex is the excitation voltage for AI Ks) and the AI K s intensity normalized with the current, which is analyzed for the data shown in Fig. 7.
the plasma. Thus, the empirical relationship known from electron probe microanalysis (EPMA) is applied to our result, i.e. the characteristic X-ray intensity (/) is proportional to i(V- Vex)x, where i is the current, V is the voltage and Vex is the excitation voltage of the characteristic X-ray. Fig. 8 shows the relationship between log(normalized A1 Ks intensity: Hi) and l o g ( V - Vex), suggesting a linear relationship with a slope (x) of 2.3 for plots at low voltages. At higher voltages, the simple relationship described above is not applicable, probably because the X-rays are generated in a more complicated manner.
3.6. Helium pressure dependence The X-ray spectra from the AI target were measured at different He pressures at a constant voltage of 4.0 kV. As shown in Fig. 9, the discharge current increased linearly as the He pressure increased; however, the AI Ko~ intensity did not correspond to the discharge current. This is because the target current, which essentially effects the X-ray intensity, depends on the He pressure as previously described. As shown in Fig. 9, the AI Kot intensity is almost constant at He pressures above 0.35 Torr. This indicates that the excess He gas does not contribute to an increase in the X-ray intensity, and would prevent the electrons from arriving at the target. When the He pressure was less than 0.35 Torr, the A1 K s intensity decreased. This was probably caused by the decreased
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
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discharge current at such low He pressures. As described in Section 3.1, secondary electrons are produced at the cathode surface by He + ion impact; therefore, He atoms of an adequate density are necessary to obtain a high discharge current, i.e. a high target current. Therefore, the optimum He pressure is from 0.4 to 0.5 Torr under our experimental conditions.
3.7. Quantitative analysis by fast-electron-induced X-rays Finally, we shall discuss the quantification of the 80
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Fig. 1l. Relationship between the Mo concentration and the normalized Mo La,13 intensity. A correlation coefficient of 0.99995 was obtained for triplicate measurements.
characteristic X-ray intensity excited by fast electrons. The samples used were F e - M o binary alloys which were provided by the Iron and Steel Institute of Japan as standard materials. A typical X-ray spectral measurement for a Fe-7.07% Mo alloy is shown in Fig. 10. The Mo Lot,# peak (2.3 keV) is observed against a broad background of continuous X-rays. The net intensity of the Mo L lines was integrated for a life-time of 50 s after the prerequisite 3 min discharge. The background approximated a linear curve. Triplicate measurements were performed for the series of F e - M o alloys. Fig. 11 shows the relationship between the Mo concentration (%) and the Mo Lo~,/3 intensity normalized with the discharge current, which is a mean of the initial and final currents in each measurement. Since the intensity of the characteristic X-ray is almost proportional to the discharge current as described in Section 3.5, normalization by the current is effective. The data plots are shown by l~ve -+ s (where lave is the mean of the normalized intensities for three measurements and s is the standard deviation). An excellent correlation coefficient (r) of 0.99995 was obtained without other corrections. When samples with complicated compositions are analyzed, the corrections established in EPMA will be useful. For the sample of 0.20% Mo, the Mo Lo~,/3 peak could not be distinguished from the background signal. As shown in Fig. 10, the Mo Lot,/3 peak is on the steep background slope. The shape of the background
K. Ts'uji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
on the lower energy side is influenced by the absorption of soft X-rays by air molecules. If the X-ray measurements were performed in a He atmosphere, a more precise analysis could be possible. Moreover, a 0.05 mm wide slit could be used to avoid a pile-up pulse in the SSD measurement. By using a linear amplifier with a pile-up rejecter, trace analysis should also be possible.
4. Conclusions Measurements carried out on the He Grimm glow discharges in this work demonstrated that fast electrons emitted from the cathode surface passed right through both the negative glow and the hollow anode under high voltage and low He pressure conditions. The characteristics of the fast electron were investigated under various discharge conditions, such as the type of cathode material, the discharge voltage and the He pressure, by measuring the X-rays excited by electron impact. In conclusion, we consider that the fast electrons produced by the Grimm glow discharge plasma are useful as a simple and inexpensive electron source. Compared with conventional electron sources, the electrons from the Grimm lamp have the following advantages: the Grimm discharge can be operated under low vacuum conditions (around 10 -2 Torr) resulting in low costs, a reduction in the size of the total apparatus, easy maintenance, and simplified operation. Furthermore, another of the advantages is that the fast electrons simultaneously irradiate a large target area. Studies are now under way to apply higher voltages, above 5 kV, to the Grimm lamp, so that non-destructive elemental analysis of common metals such as Fe and Cr by the electron-induced X-ray emission method is possible. An electron source operated
1595
under low vacuum conditions will also be useful for various analytical methods utilizing electron excitation. In addition, as shown in our previous paper, the X-ray emissions from the thin films used as the target may be useful as an X-ray source [1 1].
Acknowledgements Parts of this work were supported by a Grant-in-Aid for Scientific Research (A) (no. 07555260) from the Ministry of Education, Science, Sports and Culture of Japan. The authors would like to thank Mr. S. Shucart for useful suggestions.
References [I] R. Berneron,Spectrochim. Acta Part B 33 (1978) 665. [2] A. Bengtson,M. Lundholm,J. Anal.At. Spectro~. 3 (1988) 879. [3] J.A.C. Broekaert, J. Anal. At. Spectrosc. 2 (1987) 537. [4] K. Wagatsuma, K. Hirokawa, Anal. Chem. 56 (1984) 412. [5] K. Tsuji, K. Hirokawa, Surf. Interface Anal. 17 (1991) 8t9. [6] P.W.J.M. Boumans,Anal. Chem. 44 (1972) 1219. [7] J.W. Coburn, Rev. Sci. Instrum.41 (1970) 1219. [8] C.G. Bruhn, B.L Bentz, W.W. Harrison, Anal. Chem. 51 (1979) 673. [9] W.W. Harrison, K.R. Hess. R.K. Marcus, F.L. King. Anal. Chem. 58 (1986) 341A. [10] M. Matsuta and K. Wagatsuma, in preparation. [11] K. Tsuji, M. Matsuta, K. Wagatsuma, Jpn. J. Appl. Phys. 36 (1997) L446. [12] D.J. Ball, J. Appl. Phys. 43 (1972) 3047. [ 13] B.N. Chapman, D. Downer,L.J.M. Guimaraes,J. Appl. Phys. 45 (1974) 2115. [14] W. Grimm, Naturwissenschaften54 (1967) 586. [15] W. Grimm,Spectrochim. Acta Part B 23 (1968) 443. [16] A. von Engel, IonizedGases, ClarendonPress, Oxford, 1965. [17] B. Chapman, Glow Discharge Processes, John Wiley and Sons, New York, 1980. [ 18] J. Roth, in: R. Behrisch (Ed.), Sputteringby Particle Bombardment I1, Springer, Berlin, 1983.
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 1587-1595
Characteristics of fast electrons from Grimm glow discharge He plasmas as an electron source K. Tsuji*, K. Wagatsuma, H. Matsuta Institute for Materials Research, Tohoku University, 2-I-I Katahira, Aoba, Sendai, 980-77, Japan
Received 3 March 1997: accepted 23 April 1997
Abstract
In a study of d.c. helium Grimm glow discharges applied with a maximum high voltage of 5 kV, it was found that fast electrons emitted from the cathode surface passed right through the negative glow and the hollow anode. This fast electron current corresponds to approximately 50% of the discharge current at a high voltage of 5 kV and a low helium pressure of 0.43 Torr for an aluminium cathode. The characteristics of the fast electrons under various discharge conditions, i.e. for different cathode materials, discharge voltages and helium pressures, were investigated by measuring the electron-induced X-rays, in order to consider whether the glow discharge plasma could have an application as an electron source. It was found that fast electrons were generated more efficiently under a high voltage while using a low gas pressure in the aluminium cathode. The fast electrons were then used as an electron source for the electron-induced X-ray emission analysis of Fe-Mo binary alloys. This experiment indicated that fast electrons from a Grimm glow discharge plasma are useful as a simple and inexpensive electron source in electron-induced X-ray emission analysis. © 1997 Elsevier Science B.V. Keywords: Grimm glow discharge; Fast electrons; Runaway electrons; X-ray emission; Electron source
I. I n t r o d u c t i o n
The glow discharge plasma has been utilized as an excitation source for atomic emissions. Glow discharge atomic emission spectroscopy (GD-AES) is a powerful tool for the direct analysis of solid samples [1-5]. In direct current GD-AES, a high voltage is applied between an anode and a sample cathode where cathode sputtering occurs due to ion bombardment [6]. The sputtered atoms are introduced into the negative glow region where they are excited by electron and metastable-atom impacts, leading to atomic emission. G D - A E S analysis can be performed at a high cathode sputtering rate without a high-vacuum * Corresponding author. E-mail: [email protected]
system; therefore, rapid depth analysis is possible using simple and inexpensive apparatus. This provides a major advantage over other surface analytical methods such as secondary ion mass spectrometry (SIMS). Glow discharge plasmas have also been used as an ion source. The sputtered atoms are ionized in the glow plasma, then analyzed by a mass spectrometer. This is known as glow discharge-mass spectrometry (GD-MS) [7-9]. In addition to the advantages of GDAES, G D - M S has a high detection power comparable to SIMS; therefore G D - M S is well suited for the ultratrace analysis of solid samples. Usually, a discharge voltage of less than 1 kV is applied using Ar as the discharge gas in both GDAES and GD-MS. These discharge conditions are
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0584-8547(97)00061 -X
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K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) I587 1595
necessary to obtain a stable discharge and an appropriate cathode sputtering rate. Matsuta and Wagatsuma have attempted to operate a helium glow discharge at discharge voltages of 2 kV and above [10]. It was found that a stable discharge is possible at a maximum discharge voltage of 5 kV using a normal Grimm discharge lamp. Electrons emitted from the cathode surface would have high energies corresponding to the cathode sheath voltage; thus X-ray radiation could be expected due to collisional energy transfer from the fast electrons. In fact, soft X-rays from the glow discharge lamp were measured through an X-ray transparent window [11 ]. In addition, X-ray radiation was found to originate from the X-ray window, thus indicating that highenergy electrons actually bombard the X-ray window
[ll]. In this paper, we present a study of d.c. high-
[Linearamp.] 1 [MCA]
voltage He glow discharges in order to determine optimum discharge conditions for the generation of fast electrons, and discuss the possibility of using the glow discharge plasma as an electron source. Currently, a heated filament and a tip to which a high voltage is applied are employed as electron sources in surface analytical tools such as an Auger electron spectroscopy apparatus or a scanning electron microscope. These electron sources have excellent stability and strong intensity; however, they must be operated under ultra-high-vacuum conditions. Maintenance of this ultra-high vacuum is both difficult and costly. Size constraint is also one of the most important problems to overcome in the development of an analytical apparatus. A glow discharge lamp can easily be operated under low-vacuum conditions, suggesting the possibility of a simple and inexpensive electron source. So far, there have
He gas
anode body
II F
insulator
silt
negative glow
(Comp° er j
anode tube
target rll[I
IR-thermometer
J/~/ vacuum
high-voltage powersupply
Fig. 1. Experimentalset-up for the measurementof electron-induced X-rayemissionsusing a Grimmglow discharge lamp.
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
been many studies on the application of a glow discharge plasma as an optical excitation source and an ion source for analytical atomic spectroscopy. For example, fast electrons from a d.c. glow discharge plasma have been energy-analyzed in order to determine the influence of electron impact on thin film growth [12,13]. However, they have rarely been considered as an electron source for an analytical technique. We believe that fundamental spectroscopic studies of X-ray emission from a glow discharge lamp are necessary and useful when utilizing it as an electron source.
2. Experimental 2.1. A p p a r a t u s
In the conventional glow discharge lamp designed by Grimm [14,15], a flat sample is used both as the cathode and to vacuum-seal the discharge lamp. This lamp is suitable for the direct analysis of common metals, because the sample needs only to be sufficiently flat and large to seal the vacuum of the lamp. We custom-built the Grimm-type glow discharge lamp shown in Fig. 1. The flat cathode was attached to the discharge lamp with an O-ring. An aluminium plate was used as a typical cathode. The anode body and the hollow anode tube (8 mm inner diameter) were made of brass. Although a quartz glass plate was attached opposite the cathode in the glow discharge atomic emission spectrometer, an X-ray transparent window was necessary to detect the X-rays. In our previous experiments, we used thin films of Be, A1 and Zr, which were stuck onto a brass plate to seal a hole 3 mm in diameter, and X-rays which penetrated this thin film were monitored with a solid-state detector (SSD) [11]. The X-ray window material itself was found to emit X-rays when bombarded by fast electrons. To detect X-rays reflected from the bulk metals, we designed a target (sample) holder as shown in Fig. 1. This holder was cut at an angle of 45 ° from a brass column (30 mm in diameter), and it had a tunnel 8 mm in diameter for the electrons to pass through. The target was attached to this holder with an O-ring 18 mm in diameter; therefore, the fast electrons would bombard the target surface at an incident angle of
1589
45 °. X-rays were detected 25 ° from the target's surface through a Be window 25 /~m thick and 4 mm in diameter. The distance between the Be window and the Si(Li) X-ray detector (EMAX, Horiba Co.; energy resolution, 158.9 eV at Mn Ka) was 20 mm. This distance is critical as soft X-rays are easily absorbed by air molecules. Since the X-ray intensity was too strong, an X-ray slit of width 0.05 mm was attached to the front of the SSD. The detected X-rays were amplified with a linear amplifier (TC 243; Oxford Instruments Inc., UK) and were analyzed with a multichannel analyzer (MCA PC98-B; Laboratory Equipment Co.) using a personal computer. The Grimm lamp was evacuated from two ports by using two rotary pumps. The ultimate vacuum pressure was 0.02 Torr. Helium gas (purity more than 99.9995%) was introduced into the discharge lamp, monitoring the pressure with a Pirani gauge. The He pressure was adjusted by using a micrometering valve to change the flow rate of the He gas. In our previous work, a pulsed glow discharge was employed to suppress the total electric power supplied to the lamp [11]. Since a stable glow discharge plasma could only be created by a direct current discharge, the latter was employed in this work. A positive high voltage was applied to the anode body using a high-voltage power supply (EW5R 120-100; Glassman High Voltage Inc.; voltage capacity, less than 5 kV) with a grounded cathode. Both the anode body and the cathode were water-cooled. The temperature of the target was also monitored from the back by an infrared radiation thermometer (IR-BT1-GB5; Chino Co.) as shown in Fig. 1. 2.2. Samples
Although the cathode material was used as the sample in GD-AES, in this study the target was considered to be the sample, as shown in Fig. 1. Various pure A1, Si, Zr, Nb, Mo, Pd and Ag plates (20 × 20 mm2; thickness, 0.5 mm) were used as targets after their surfaces were polished with emery paper. In order to investigate the relationship between the X-ray intensity and the concentration of the element, standard F e - M o binary alloys (disk diameter, 35 mm; thickness, 5 ram; certified Mo concentration, 0.20-7.07%) were prepared.
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K. Tsuji et aL/Spectrochimica Acta Part B 52 (1997) 1587-1595
3. Results and discussion
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3.1. Generation of fast electrons in a Grimm lamp ~
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It is well known that secondary electrons are emitted from the cathode surface by the bombardment of ions produced by the glow discharge plasma [ 16,17]. This secondary electron emission is essential for maintaining the discharge. The emitted electrons are accelerated across the cathode sheath. The cathode sheath voltage is the sum of the applied voltage and the plasma potential, which is considerably smaller than the applied voltage [17]. The fast electrons, whose high energies correspond to the cathode sheath potential, enter the negative glow region. Slow electrons lose their energy by collisions with particles in the negative glow, resulting in ionization and excitation, and, subsequently, optical emission. It should be emphasized that the electron collision cross-section decreases with an increase in electron energy [17]. Therefore, some of the high-energy electrons pass through the negative glow without any collisions. These high-energy electrons are known as "runaway" electrons [17]. This is a significant phenomenon when discharge voltages above 2 kV are applied. Since a voltage of less than 1 kV is used in argon GDAES analysis, it is unnecessary to consider the effect of fast electrons. Moreover, in cases where a flat plate is used as the anode, fast electrons just bombard the anode plate and the only result is a rise in temperature. In the case of a Grimm lamp with a hollow anode, the
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Discharge Voltage (kV)
Fig. 3. Voltage dependence of both ([]) the discharge current and (A) the Cu target current at an He pressure of 0.43 Torr. The ratio of the target current to the discharge current is also shown by filled circles.
fast electrons pass right through the negative glow and the hollow anode. Thus, we can utilize the fast "runaway" electrons from the plasma as an electron source. To observe the area bombarded by the electrons, an acrylic plate (thickness, 3 mm) was exposed to a 3 kV discharge operated for 20 s. After the target holder was removed, the acrylic plate attached opposite the cathode was used as a target. As shown in Fig. 2, the area damaged by electron impact is a circle about 10 mm in diameter, which almost corresponds to the diameter of the hollow anode. A unique feature of this electron source compared with others is the irradiation of the target by the fast electrons over a relatively large area. 3.2. Target current
Fig. 2. Photograph of an acrylic plate damaged by electron impact. The acrylic plate was placed on section paper. The damaged area is a circle about 10 mm in diameter. The discharge was operated at a discharge voltage of 3 kV for 20 s.
To evaluate the current density of the fast electrons, the target current was measured using a copper target (40 x 50 mm2; thickness, 1 mm) under various discharge conditions. The copper plate was attached opposite the cathode after the target holder was removed. The target current between the copper plate and the anode body was measured by an ammeter. Fig. 3 shows the voltage dependence of both the discharge current between the anode and the cathode, and the target current between the anode and the copper target, at an He pressure of 0.43 Torr with an aluminium cathode. The ratio of
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595 0.55
.
,
.
,
.
,
.
,
.
.
.
He pressure led to more frequent collisions between the electrons and the particles in the plasma, resulting in a decrease in the number of fast electrons passing through the hollow anode. Judging from the results of Figs 3 and 4, it can be seen that fast electrons can be generated efficiently under high voltages and at low gas pressures.
.
050 0.45 .fl t~ c
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3.3. Cathode materials 0.30
0.25
o0
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Fig. 4. He pressure d e p e n d e n c e o f the ratio o f the Cu target current to the discharge current at a constant discharge voltage o f 5.0 kV.
the target current to the discharge current is also shown by filled circles. The current ratio suddenly increased at voltages above 1.5 kV. This experimental result agrees with the explanation of the fast electrons described in Section 3.1, i.e. the electron collision cross-section decreases on increasing the electron energy, which is almost equivalent to the applied voltage. The target current corresponds to approximately 50% of the discharge current at voltages above 4 kV. The target current was 1.85 mA at an applied voltage of 5 kV, and the area of the electron irradiation was a circle 10 mm in diameter, as shown in Fig. 2. Therefore, the electron current density at the target was determined to be 23.6 A m -2 in this experiment. The He pressure dependence of the current ratio is shown in Fig. 4. The discharge was operated at a voltage of 5 kV with an aluminium cathode. The ratio decreased as the He pressure increased. A higher Table 1 The effects of different cathodes on current and Si Kot intensity, measured for an Si target at a voltage of 4 kV and an He pressure of 0.43 Torr Cathode
Current/mA
Si K a / ( c o u n t s s -t)
Mo W Ti Nb Cu Zr AI Si
< 0.01 < 0.01 < 0.01 0.01 0.13 0.62 0.63 1.05
15 38 63 77 755 7917 9607 10785
The effects of different cathode materials (Mo, W, Ti, Nb, Cu, Zr, A1, and Si) on both the discharge current and the X-ray intensity from the target were investigated. A silicon wafer was used as the target. The results obtained are shown in Table 1. The secondary electron coefficient is dependent on the type of cathode [18]. As shown in Table 1, the discharge current changes drastically depending on the cathode material. Since the X-ray intensity excited by the electron impact is proportional to the electron current density, we can measure the target current from the Si K a intensity. Table 1 implies that the discharge current is responsible for the generation of the electrons impinging on the target. Si, A1 and Zr are superior cathode materials for obtaining a strong target current. When selecting cathode material, we must consider its sputtering rate and stability, because the sputtering rate from ion impact also depends on the cathode material. When a Grimm lamp is used as an electron source, a smaller sputtering rate is desirable. This is because the sputtered atoms are deposited on the anode tube, leading to unstable discharges. The sputtering rates of A1 and Si were low under our experimental conditions. The stability of the discharge for the Si cathode is slightly less because of an irregular spark-like discharge. Thus, the aluminium plate was selected as the best cathode for our experiment.
3.4. Time dependence of the discharge The time dependence of the d.c. current and the target temperature was investigated. The target temperature was monitored from the back of the target by using an infrared radiation thermometer. The results for Cu and Si targets are shown in Fig. 5(a) and 5(b), respectively. The largest change in the discharge current was during the initial 200 s of discharge time;
1592
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595 i
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Fig. 5. Time dependence of both (D) the discharge current and (Q) the temperature of (a) Cu and (b) Si targets at a discharge voltage of 4.0 kV and an He pressure of 0.43 Ton.
80
after that time, it showed an almost constant value. Therefore, measurements were performed after 3 min of the discharge. The Cu target temperature rose rapidly during the initial 1 min of the discharge, then decreased slightly as the discharge current decreased. The target temperature increase is caused by electron impact, which depends on the discharge current. In the case of the Si target, a surprising rise in temperature was observed, as shown in Fig. 5(b). It was difficult for us to believe that the Si temperature actually increased to almost 60°C. The temperature was monitored using an infrared radiation thermometer. Considering that Si is an infrared-transparent material, the infrared radiation observed, which was transmitted through 0.5 mm
60
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Energy (keY) Fig. 6. X-ray spectra emitted from the AI target at discharge voltages of (a) 2.5 kV, (b) 3.5 kV, and (c) 4.5 kV.
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595 4.0
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thick Si, might have been produced on the Si surface by electron impact. Consequently, cooling the target is necessary in order to keep its temperature steady.
3.5. Discharge voltage dependence Fig. 6(a)-6(c) show typical X-ray spectra for the AI target at the different discharge voltages 2.5 kV, 3.5 kV, and 4.5 kV, respectively. The X-ray spectra consist of a characteristic AI Kot (1.49 keV) X-ray peak and a broad peak. Since the shape and the energy of the broad peak changed depending on the applied voltage, the broad peak can be considered to be bremsstrahlung (continuous X-rays). The highest energy of the bremsstrahlung is near the applied voltage. This indicates that the energy of the fast electrons can be controlled by adjusting the applied voltage. The lower energy side of the bremsstrahlung drastically decreased below 2 keV because of soft X-ray absorption both by molecules in the air and the Be window. Therefore, the actual intensity of the AI Kot peak must be larger than the observed one shown in Fig. 6(a)-6(c). The voltage dependence of the discharge current and the AI Ko~ intensity is shown in Fig. 7. Both the current and the AI Ko~ intensity increased as the discharge voltage increased, and a similar dependence on the discharge voltage was observed. In general, the applied voltage dominates the electron energy, and the discharge current reflects the electron density in
Fig. 8. The relationship between V - Ve~ (where Vis the voltage and Vex is the excitation voltage for AI Ks) and the AI K s intensity normalized with the current, which is analyzed for the data shown in Fig. 7.
the plasma. Thus, the empirical relationship known from electron probe microanalysis (EPMA) is applied to our result, i.e. the characteristic X-ray intensity (/) is proportional to i(V- Vex)x, where i is the current, V is the voltage and Vex is the excitation voltage of the characteristic X-ray. Fig. 8 shows the relationship between log(normalized A1 Ks intensity: Hi) and l o g ( V - Vex), suggesting a linear relationship with a slope (x) of 2.3 for plots at low voltages. At higher voltages, the simple relationship described above is not applicable, probably because the X-rays are generated in a more complicated manner.
3.6. Helium pressure dependence The X-ray spectra from the AI target were measured at different He pressures at a constant voltage of 4.0 kV. As shown in Fig. 9, the discharge current increased linearly as the He pressure increased; however, the AI Ko~ intensity did not correspond to the discharge current. This is because the target current, which essentially effects the X-ray intensity, depends on the He pressure as previously described. As shown in Fig. 9, the AI Kot intensity is almost constant at He pressures above 0.35 Torr. This indicates that the excess He gas does not contribute to an increase in the X-ray intensity, and would prevent the electrons from arriving at the target. When the He pressure was less than 0.35 Torr, the A1 K s intensity decreased. This was probably caused by the decreased
K. Tsuji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
1594 2.0
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He Pressure (Ton') Fig. 9. The He pressure dependence of both (R) the discharge current and (D) the AI Ko~ intensity at a discharge voltage of 4.0 kV.
discharge current at such low He pressures. As described in Section 3.1, secondary electrons are produced at the cathode surface by He + ion impact; therefore, He atoms of an adequate density are necessary to obtain a high discharge current, i.e. a high target current. Therefore, the optimum He pressure is from 0.4 to 0.5 Torr under our experimental conditions.
3.7. Quantitative analysis by fast-electron-induced X-rays Finally, we shall discuss the quantification of the 80
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Fig. 1l. Relationship between the Mo concentration and the normalized Mo La,13 intensity. A correlation coefficient of 0.99995 was obtained for triplicate measurements.
characteristic X-ray intensity excited by fast electrons. The samples used were F e - M o binary alloys which were provided by the Iron and Steel Institute of Japan as standard materials. A typical X-ray spectral measurement for a Fe-7.07% Mo alloy is shown in Fig. 10. The Mo Lot,# peak (2.3 keV) is observed against a broad background of continuous X-rays. The net intensity of the Mo L lines was integrated for a life-time of 50 s after the prerequisite 3 min discharge. The background approximated a linear curve. Triplicate measurements were performed for the series of F e - M o alloys. Fig. 11 shows the relationship between the Mo concentration (%) and the Mo Lo~,/3 intensity normalized with the discharge current, which is a mean of the initial and final currents in each measurement. Since the intensity of the characteristic X-ray is almost proportional to the discharge current as described in Section 3.5, normalization by the current is effective. The data plots are shown by l~ve -+ s (where lave is the mean of the normalized intensities for three measurements and s is the standard deviation). An excellent correlation coefficient (r) of 0.99995 was obtained without other corrections. When samples with complicated compositions are analyzed, the corrections established in EPMA will be useful. For the sample of 0.20% Mo, the Mo Lo~,/3 peak could not be distinguished from the background signal. As shown in Fig. 10, the Mo Lot,/3 peak is on the steep background slope. The shape of the background
K. Ts'uji et al./Spectrochimica Acta Part B 52 (1997) 1587-1595
on the lower energy side is influenced by the absorption of soft X-rays by air molecules. If the X-ray measurements were performed in a He atmosphere, a more precise analysis could be possible. Moreover, a 0.05 mm wide slit could be used to avoid a pile-up pulse in the SSD measurement. By using a linear amplifier with a pile-up rejecter, trace analysis should also be possible.
4. Conclusions Measurements carried out on the He Grimm glow discharges in this work demonstrated that fast electrons emitted from the cathode surface passed right through both the negative glow and the hollow anode under high voltage and low He pressure conditions. The characteristics of the fast electron were investigated under various discharge conditions, such as the type of cathode material, the discharge voltage and the He pressure, by measuring the X-rays excited by electron impact. In conclusion, we consider that the fast electrons produced by the Grimm glow discharge plasma are useful as a simple and inexpensive electron source. Compared with conventional electron sources, the electrons from the Grimm lamp have the following advantages: the Grimm discharge can be operated under low vacuum conditions (around 10 -2 Torr) resulting in low costs, a reduction in the size of the total apparatus, easy maintenance, and simplified operation. Furthermore, another of the advantages is that the fast electrons simultaneously irradiate a large target area. Studies are now under way to apply higher voltages, above 5 kV, to the Grimm lamp, so that non-destructive elemental analysis of common metals such as Fe and Cr by the electron-induced X-ray emission method is possible. An electron source operated
1595
under low vacuum conditions will also be useful for various analytical methods utilizing electron excitation. In addition, as shown in our previous paper, the X-ray emissions from the thin films used as the target may be useful as an X-ray source [1 1].
Acknowledgements Parts of this work were supported by a Grant-in-Aid for Scientific Research (A) (no. 07555260) from the Ministry of Education, Science, Sports and Culture of Japan. The authors would like to thank Mr. S. Shucart for useful suggestions.
References [I] R. Berneron,Spectrochim. Acta Part B 33 (1978) 665. [2] A. Bengtson,M. Lundholm,J. Anal.At. Spectro~. 3 (1988) 879. [3] J.A.C. Broekaert, J. Anal. At. Spectrosc. 2 (1987) 537. [4] K. Wagatsuma, K. Hirokawa, Anal. Chem. 56 (1984) 412. [5] K. Tsuji, K. Hirokawa, Surf. Interface Anal. 17 (1991) 8t9. [6] P.W.J.M. Boumans,Anal. Chem. 44 (1972) 1219. [7] J.W. Coburn, Rev. Sci. Instrum.41 (1970) 1219. [8] C.G. Bruhn, B.L Bentz, W.W. Harrison, Anal. Chem. 51 (1979) 673. [9] W.W. Harrison, K.R. Hess. R.K. Marcus, F.L. King. Anal. Chem. 58 (1986) 341A. [10] M. Matsuta and K. Wagatsuma, in preparation. [11] K. Tsuji, M. Matsuta, K. Wagatsuma, Jpn. J. Appl. Phys. 36 (1997) L446. [12] D.J. Ball, J. Appl. Phys. 43 (1972) 3047. [ 13] B.N. Chapman, D. Downer,L.J.M. Guimaraes,J. Appl. Phys. 45 (1974) 2115. [14] W. Grimm, Naturwissenschaften54 (1967) 586. [15] W. Grimm,Spectrochim. Acta Part B 23 (1968) 443. [16] A. von Engel, IonizedGases, ClarendonPress, Oxford, 1965. [17] B. Chapman, Glow Discharge Processes, John Wiley and Sons, New York, 1980. [ 18] J. Roth, in: R. Behrisch (Ed.), Sputteringby Particle Bombardment I1, Springer, Berlin, 1983.