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Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen
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Microchemical Journal 87 (2007) 175 – 179 www.elsevier.com/locate/microc
Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen Machiko Tsukiji, Kazuaki Wagatsuma ⁎ Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Received 18 June 2007; received in revised form 22 August 2007; accepted 22 August 2007 Available online 30 August 2007
Abstract Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma. © 2007 Elsevier B.V. All rights reserved. Keywords: Glow discharge optical emission spectrometry; He–N2 mixed gas; Sputtering rate; Penning ionization; Silver; Copper
1. Introduction Glow discharge optical emission spectrometry (GD-OES) is an analytical method for the direct analysis of solid samples because sample atoms are directly introduced into the plasma through cathode sputtering [1,2]. This technique is also employed as a rapid method for surface analysis because the sampling process gives a depth profile of the sample composition. The conventional GD-OES has been applied to the determination of relatively thick films such as dipped metallic coatings, electroplated films and oxide scales [3–5], whose thicknesses are more than 1 μm. The main reason for this is that the sputtering rate which can be set in the conventional apparatus is generally large: the typical value is several μm/min, which strongly depends on the kind of the cathode material [6]. However, it is recently required that various surface films having nm-order thicknesses can be analyzed by using GD-
⁎ Corresponding author. E-mail address: [email protected] (K. Wagatsuma). 0026-265X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2007.08.003
OES. A measuring method with a high-speed detection system was suggested to observe such thin films in a conventional argon GD-OES [7]. We consider in the thin-film analysis that it is important to control the sputtering rate strictly and especially to reduce the sputtering rate, because the chemical compositions of the thin layers can be investigated in more detail. The sputtering rate in GD-OES can be suppressed by lowering the discharge power applied; however, this method generally degrades the analytical performance because the faint discharge condition gives smaller intensities as well as worse signal-to-noise ratios of the emission lines. Alternatively, changing of the plasma gas could be a possible option. Helium gas produces a glow discharge plasma having smaller sputtering rate, whereas excitations of the helium species occur actively [8,9]. Therefore, mixed gas plasmas containing helium can be employed for control of the sputtering rate in GD-OES. In fact, compared to a pure argon plasma, argon–helium mixed gas plasmas could reduce the sputtering rate without degrading the analytical performance as well as the discharge stability [10]. In addition, because the excited species of helium have larger internal energies, a better detection sensitivity could be obtained
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Fig. 1. Variation in the sputtering rate of pure copper in helium–nitrogen mixed gas plasmas. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
in the determination of some elements requiring large excitation energies by use of an argon–helium mixed gas glow discharge plasma. Furthermore, the addition of different gas species to a helium plasma would provide the characteristics effective for the practical application in GD-OES. This paper describes the sputtering and the emission characteristics of a helium mixed gas plasma when small amounts of nitrogen are added to the helium plasma. The sputtering rate can be drastically raised by the nitrogen addition, and the emission intensities of sputtered species also increase when copper or silver is employed as the cathode sample. However, the intensity dependence on the amount of nitrogen is different between atomic and ionic emission lines of silver. This effect can be explained from the difference in their excitation mechanisms: the excited levels of silver ion are populated mainly through the collisions with excited states of helium such as the 1s2s metastable. 2. Experimental The structure of the glow discharge lamp [11] and the measuring system [12] comprising a spectrometer and a directcurrent power supply have been described elsewhere. The discharge lamp was made according to the original model published by Grimm [13]. The inner diameter of the hollow anode was 8.0 mm and the distance between the anode and cathode was adjusted to be 0.2–0.3 mm. The lamp was evacuated down to ca. 2.6 Pa and then the plasma gas was introduced and was flowing during the measurement. Highpurity helium (N 99.999995%) and nitrogen (N99.99995%) were employed as the plasma gas. The partial pressures of each gas were regulated with mass flow controllers (Model 3200, KOFLOC Corp., Japan) and read on a Pirani gauge at the gas inlet of the lamp. The pressure of helium gas was fixed to be 2000 Pa and that of nitrogen gas was varied from 0 to 400 Pa. Pure copper plates (99.99% purity) were prepared to measure the sputtering rate and a silver rod (99.99% purity) was also employed as the sample. They were polished with waterproof emery papers and then finished to mirror faces. A depth
Fig. 2. Variation in the emission intensities of N2 337.13 nm (circle) and N+2 391.44 nm (square) as a function of the partial pressure of nitrogen. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
profiling meter (Surfcom 1500DX, Tokyo Seimitsu Co. Ltd., Japan) was employed for estimating the average depth of resulting craters after 60-min sputtering. 3. Results and discussion 3.1. Variation in sputtering rate in He–N2 plasmas Fig. 1 shows a variation in the sputtering rate of pure copper as a function of nitrogen partial pressure introduced into a helium plasma in the case where the pressure of helium as well as the discharge voltage is kept constant. Whereas erosion of the copper target hardly occurs in the pure helium plasma, the sputtering rate drastically increases at nitrogen pressures up to 150 Pa and then gradually increases at nitrogen partial pressures increasing from 150 to 400 Pa. Each sputtering rate was estimated for triplicate measurements and the average relative standard deviation was 5.8%. The sputtering rate was widely varied from 0.01 to 0.7 μm/min along with the nitrogen partial pressured added, and the rate of 0.7 μm/min in the plasma of 2000-Pa helium and 400-Pa nitrogen is still smaller than that
Fig. 3. Variation in the discharge current in the measurement of Fig. 1.
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whereas the rate is very small in the pure helium plasma. This observation implies that nitrogen molecular ions produced in the mixed gas plasmas are major primary ions impinging on the copper target. Fig. 4 illustrates a simplified energy level diagram where only the related levels of nitrogen and helium are presented. It is clear from this diagram that an excited state of nitrogen molecular ion, the B 2Σu state including the vibrational/ rotational excited levels, is slightly lower than the helium metastables: 1s2s 3S1 (19.82 eV) and 1s2s 1S0 (20.62 eV). It is expected under such conditions that nitrogen molecular ions are easily produced through a Penning-type ionization collision [15] as follows: g − N2 þ Hem →Nþ⁎ 2 þ He þ e ;
ð1Þ
where the superscripts m, ⁎, or g means a metastable, an excited, or a ground state. The resulting nitrogen molecular species may occupy various vibrational/rotational excited levels of the B 2 Σu state. This reaction can explain the sputtering characteristics in the helium–nitrogen mixed gas plasma, where small amounts of nitrogen added can increase the sputtering rate effectively. As shown in Fig. 2, slow-down of the ionization rate of nitrogen molecule is observed when the partial pressure of nitrogen exceeds about 200 Pa, which is derived from quenching of the helium metastables according to Eq. (1). 3.2. Spectral lines of silver emitted from He mixed gas plasmas
Fig. 4. Schematic energy level diagram representing only the related energy levels of helium and nitrogen molecules.
observed in conventional argon plasmas (typically several μm/ min). Fig. 2 shows variations in the emission intensities of nitrogen molecular band heads, the N2 337.13 nm and the N2+ 391.44 nm, as a function of the nitrogen partial pressure, and a change in the discharge current measured in the helium– nitrogen mixed gas plasma is shown in Fig. 3. The N2 337.13nm band head is assigned to the transition from C 3Πu (υ = 0) to B 3Πg (υ = 0) of nitrogen molecule, and the N2+ 391.44 nm from B 2Σu (υ = 0) to X 2Σg (υ = 0) of nitrogen molecular ion [14]. While the emission intensity of the N2 337.13 nm monotonically increases along with the amounts of nitrogen gas introduced, which well corresponds to the partial pressure, that of the N2+ 391.44 nm is predominantly elevated when the nitrogen pressure ranges from 0 to 150 Pa. The intensity variation of the N2+ 391.44 nm is very similar to the pressure dependence of the discharge current as illustrated in Fig. 3, indicating that the resultant electrons are derived predominantly from the ionization of nitrogen molecules. The dominant ionization of nitrogen molecular species can occur by any interaction with helium gas species at the lower nitrogen pressures. Furthermore, the sputtering rate well corresponds to the variation of the N2+ 391.44 nm when nitrogen gas is added,
Our previous paper has reported that observed emission lines of silver ion, which are emitted from a Grimm-style glow discharge lamp, are identified to the optical transitions from the 4d95p (10–11.3 eV) to the 4d95s (4.8–5.7 eV) excited levels or the transitions from the 4d95d (4d96s) (15–16.7 eV) to the 4d95p excited levels [16]. The Ag II 241.32 nm and the Ag II 243.78 nm belong to the former transition and the Ag II 224.62 nm and the Ag II 271.19 nm to the latter transition. These emission lines of silver ion appear particularly in an argon–helium mixed gas plasma, whereas they are hardly
Fig. 5. Variation in the discharge current when pure silver is employed as the sample. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
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Fig. 6. Dependence of the emission intensity of the Ag I 338.29 nm on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
Fig. 8. Dependence of the emission intensities of several Ag II lines derived from the 4d95d–4d95p transition on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
observed in a pure Ar plasma [16]. This phenomenon can be explained from collisions of the second kind between silver atom and helium excited species as follows:
result implies that the nitrogen addition causes similar effects in the silver sample as well as the copper sample: dominant ionization for nitrogen molecular ions and simultaneous quenching of helium excited species. The discharge current is considered to be determined principally by the amounts of nitrogen molecular ion in the plasma. Fig. 6 shows a change in the emission intensities of an atomic resonance line, Ag I 338.29 nm, as a function of the nitrogen partial pressure added. Monotonic increase in the emission intensity is observed, which would be attributed to an increase in the sputtering rate caused by nitrogen molecular ions produced through helium–nitrogen collisions. Fig. 7 shows variations in the emission intensities of silver ionic lines which are identified to the 4d95p–4d95s transition of silver ion. Their emission intensities have each sharp maximum peak at a nitrogen pressure of 30 Pa, which is much different from the intensity variation of the Ag I 338.29 nm as illustrated in Fig. 6. Also, the intensity changes of these silver ionic lines do not follow the discharge current caused by the addition of nitrogen gas. Their ionization/ excitation energies are 17.6–18.9 eV (the first ionization potential of 7.57 eV + the corresponding excitation energies). Because the total excitation energies are slightly lower than the internal energy of the helium metastables, a Penning-type ionization process (see Eq. (2)) could populate the 4d95p excited levels so that the silver ionic lines can be emitted. However, the number density of the helium metastables decreases through the quenching reaction of Eq. (1), which makes the excitations to the 4d95p levels less actively at larger nitrogen partial pressures even though larger amounts of silver atoms would be introduced into the plasma. Fig. 8 shows variations in the emission intensities of silver ionic lines which are identified to the 4d95d (4d96s) (15–16.7 eV)–4d95p transitions. This graph yields the similar pressure dependence to the result of Fig. 7 (the 4d95p–4d95s transition). Their ionization/excitation energies are 23.6–24.3 eV which is close to the ionization potential of helium (24.58 eV). Therefore, the 4d95d (4d96s) levels can be populated through a charge-transfer collision, as shown in Eq. (3), between silver atom and helium
Ag þ Hem →Agþ⁎ þ Heg þ e− ;
ð2Þ
Ag þ Heþ →Agþ⁎ þ Heg ;
ð3Þ
where the superscripts , ⁎, or means a metastable, an excited or a ground state. It should be noted that the ionization/ excitation of the silver ionic lines requires larger excitation energies which can be provided only by the helium excited species. This is a typical example where the helium plasma should be employed to excite ionic emission lines effectively. Fig. 5 shows a variation in the discharge current as a function of nitrogen partial pressure introduced into a helium plasma when a silver sample is employed, yielding a similar dependence to the copper target as illustrated in Fig. 3. This m
g
Fig. 7. Dependence of the emission intensities of several Ag II lines derived from the 4d95p–4d95s transition on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
M. Tsukiji, K. Wagatsuma / Microchemical Journal 87 (2007) 175–179
ion. It can be deduced from Fig. 8 that the number density of helium ion would be also reduced by adding nitrogen gas to the helium plasma. The behaviour of such silver ionic lines is highly interesting when small amounts of nitrogen are added to a helium glow discharge plasma. As illustrated in Figs. 7 and 8, we can observe the sufficient intensities of silver ionic lines having large excitation energies, as a smaller sputtering rate maintained by employing a helium–nitrogen mixed gas. This technique is expected to be applied to in-depth analysis of thin films because the sputtering rate of the order of 0.1 μm/min can be easily controlled. 4. Conclusions The addition of small amounts of nitrogen to a helium plasma generally enhances the emission intensities of silver lines, because the sputtering rate becomes larger by bombarding with nitrogen molecular ions which are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. In such plasmas, nitrogen molecular ions work as projectiles for the cathode sputtering. The intensity of silver ionic lines is elevated at lower nitrogen pressures; however, their intensities have maximum values at a nitrogen partial pressure of 30 Pa. This variation due to the number density of the helium metastables, which are considered to work as energy donors for the excitation of the silver ionic lines, would decrease through collisions with nitrogen molecules. The helium–nitrogen mixed gas plasma has the characteristics suitable for depth analysis of thin films because smaller sputtering rates can be controlled by the nitrogen partial pressure added. Acknowledgments The authors are grateful to Noboru Yamashita, Rigaku Corp., Japan, for measurement of the surface profile meter. The authors gratefully acknowledge financial support by grants from The
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Iron and Steel Institute of Japan and by a grant from the Steel Industry Foundation for the Advancement of Environmental Protection Technology, Japan. A part of this research is supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture of Japan (No. 18360016). References [1] T. Nelis, R. Payling, N.W. Barnett (Eds.), Glow Discharge Optical Emission Spectrometry, RSC Analytical Spectroscopy Monographs, RSC, Cambridge, 2003. [2] K. Marcus, J.A.C. Broekaert (Eds.), Glow Discharge Plasmas in Analytical Spectroscopy, John Wiley & Sons, Chichester, 2003. [3] J. Pons-Corbeau, J.P. Cazet, J.P. Moreau, R. Berneron, J.C. Charbonnier, Surf. Interface Anal. 9 (1986) 21. [4] J. Angeli, T. Kaltenbrunner, F.M. Androsch, Fresenius' J. Anal. Chem. 341 (1991) 140. [5] S. Suzuki, K. Suzuki, Surf. Interface Anal. 17 (1991) 551. [6] K. Takimoto, K. Suzuki, K. Nishizaka, T. Ohtsubo, Nippon Steel Tech. Rep., Overs. 33 (1987) 28. [7] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, Surf. Interface Anal. 35 (2003) 564. [8] K. Wagatsuma, K. Hirokawa, Spectrochim. Acta 42B (1987) 523. [9] K. Wagatsuma, K. Hirokawa, Anal. Chem. 60 (1988) 702. [10] K. Wagatsuma, ISIJ Int. 42 (2003) 325. [11] K. Wagatsuma, K. Hirokawa, Surf. Interface Anal. 8 (1986) 37. [12] K. Wagatsuma, H. Morita, K. Hirokawa, Surf. Interface Anal. 17 (1991) 116. [13] W. Grimm, Spectrochim. Acta 23B (1968) 443. [14] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, Chapman & Hall Ltd., London, 1965. [15] A. von Engel, Ionized Gases, Clarendon Press, Oxford, 1965. [16] K. Wagatsuma, Z. Phys., D. 37 (1996) 231.
Microchemical Journal 87 (2007) 175 – 179 www.elsevier.com/locate/microc
Emission characteristics of Grimm-style glow discharge plasmas with helium matrix plasma gas containing small amounts of nitrogen Machiko Tsukiji, Kazuaki Wagatsuma ⁎ Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan Received 18 June 2007; received in revised form 22 August 2007; accepted 22 August 2007 Available online 30 August 2007
Abstract Glow discharge plasmas with helium–(0–16%) nitrogen mixed gas were investigated as an excitation source in optical emission spectrometry. The addition increases the sputtering rate as well as the discharge current, because nitrogen molecular ions, which act as primary ions for the cathode sputtering, are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. The intensity of a silver atomic line, Ag I 338.29 nm, is monotonically elevated along with the nitrogen partial pressure added. However, the intensities of silver ionic lines, such as Ag II 243.78 nm and Ag II 224.36 nm, gave different dependence from the intensity of the atomic line: Their intensities had maximum values at a nitrogen pressure of 30 Pa when the helium pressure and the discharge voltage were kept at 2000 Pa and 1300 V. This effect is principally because the excitations of these ionic lines are caused by collisions of the second kind with helium excited species such as helium metastables and helium ion, which are quenched through collisions with nitrogen molecules added to the helium plasma. The sputtering rate could be controlled by adding small amounts of nitrogen to the helium plasma, whereas the cathode sputtering hardly occurs in the pure helium plasma. © 2007 Elsevier B.V. All rights reserved. Keywords: Glow discharge optical emission spectrometry; He–N2 mixed gas; Sputtering rate; Penning ionization; Silver; Copper
1. Introduction Glow discharge optical emission spectrometry (GD-OES) is an analytical method for the direct analysis of solid samples because sample atoms are directly introduced into the plasma through cathode sputtering [1,2]. This technique is also employed as a rapid method for surface analysis because the sampling process gives a depth profile of the sample composition. The conventional GD-OES has been applied to the determination of relatively thick films such as dipped metallic coatings, electroplated films and oxide scales [3–5], whose thicknesses are more than 1 μm. The main reason for this is that the sputtering rate which can be set in the conventional apparatus is generally large: the typical value is several μm/min, which strongly depends on the kind of the cathode material [6]. However, it is recently required that various surface films having nm-order thicknesses can be analyzed by using GD-
⁎ Corresponding author. E-mail address: [email protected] (K. Wagatsuma). 0026-265X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2007.08.003
OES. A measuring method with a high-speed detection system was suggested to observe such thin films in a conventional argon GD-OES [7]. We consider in the thin-film analysis that it is important to control the sputtering rate strictly and especially to reduce the sputtering rate, because the chemical compositions of the thin layers can be investigated in more detail. The sputtering rate in GD-OES can be suppressed by lowering the discharge power applied; however, this method generally degrades the analytical performance because the faint discharge condition gives smaller intensities as well as worse signal-to-noise ratios of the emission lines. Alternatively, changing of the plasma gas could be a possible option. Helium gas produces a glow discharge plasma having smaller sputtering rate, whereas excitations of the helium species occur actively [8,9]. Therefore, mixed gas plasmas containing helium can be employed for control of the sputtering rate in GD-OES. In fact, compared to a pure argon plasma, argon–helium mixed gas plasmas could reduce the sputtering rate without degrading the analytical performance as well as the discharge stability [10]. In addition, because the excited species of helium have larger internal energies, a better detection sensitivity could be obtained
176
M. Tsukiji, K. Wagatsuma / Microchemical Journal 87 (2007) 175–179
Fig. 1. Variation in the sputtering rate of pure copper in helium–nitrogen mixed gas plasmas. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
in the determination of some elements requiring large excitation energies by use of an argon–helium mixed gas glow discharge plasma. Furthermore, the addition of different gas species to a helium plasma would provide the characteristics effective for the practical application in GD-OES. This paper describes the sputtering and the emission characteristics of a helium mixed gas plasma when small amounts of nitrogen are added to the helium plasma. The sputtering rate can be drastically raised by the nitrogen addition, and the emission intensities of sputtered species also increase when copper or silver is employed as the cathode sample. However, the intensity dependence on the amount of nitrogen is different between atomic and ionic emission lines of silver. This effect can be explained from the difference in their excitation mechanisms: the excited levels of silver ion are populated mainly through the collisions with excited states of helium such as the 1s2s metastable. 2. Experimental The structure of the glow discharge lamp [11] and the measuring system [12] comprising a spectrometer and a directcurrent power supply have been described elsewhere. The discharge lamp was made according to the original model published by Grimm [13]. The inner diameter of the hollow anode was 8.0 mm and the distance between the anode and cathode was adjusted to be 0.2–0.3 mm. The lamp was evacuated down to ca. 2.6 Pa and then the plasma gas was introduced and was flowing during the measurement. Highpurity helium (N 99.999995%) and nitrogen (N99.99995%) were employed as the plasma gas. The partial pressures of each gas were regulated with mass flow controllers (Model 3200, KOFLOC Corp., Japan) and read on a Pirani gauge at the gas inlet of the lamp. The pressure of helium gas was fixed to be 2000 Pa and that of nitrogen gas was varied from 0 to 400 Pa. Pure copper plates (99.99% purity) were prepared to measure the sputtering rate and a silver rod (99.99% purity) was also employed as the sample. They were polished with waterproof emery papers and then finished to mirror faces. A depth
Fig. 2. Variation in the emission intensities of N2 337.13 nm (circle) and N+2 391.44 nm (square) as a function of the partial pressure of nitrogen. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
profiling meter (Surfcom 1500DX, Tokyo Seimitsu Co. Ltd., Japan) was employed for estimating the average depth of resulting craters after 60-min sputtering. 3. Results and discussion 3.1. Variation in sputtering rate in He–N2 plasmas Fig. 1 shows a variation in the sputtering rate of pure copper as a function of nitrogen partial pressure introduced into a helium plasma in the case where the pressure of helium as well as the discharge voltage is kept constant. Whereas erosion of the copper target hardly occurs in the pure helium plasma, the sputtering rate drastically increases at nitrogen pressures up to 150 Pa and then gradually increases at nitrogen partial pressures increasing from 150 to 400 Pa. Each sputtering rate was estimated for triplicate measurements and the average relative standard deviation was 5.8%. The sputtering rate was widely varied from 0.01 to 0.7 μm/min along with the nitrogen partial pressured added, and the rate of 0.7 μm/min in the plasma of 2000-Pa helium and 400-Pa nitrogen is still smaller than that
Fig. 3. Variation in the discharge current in the measurement of Fig. 1.
M. Tsukiji, K. Wagatsuma / Microchemical Journal 87 (2007) 175–179
177
whereas the rate is very small in the pure helium plasma. This observation implies that nitrogen molecular ions produced in the mixed gas plasmas are major primary ions impinging on the copper target. Fig. 4 illustrates a simplified energy level diagram where only the related levels of nitrogen and helium are presented. It is clear from this diagram that an excited state of nitrogen molecular ion, the B 2Σu state including the vibrational/ rotational excited levels, is slightly lower than the helium metastables: 1s2s 3S1 (19.82 eV) and 1s2s 1S0 (20.62 eV). It is expected under such conditions that nitrogen molecular ions are easily produced through a Penning-type ionization collision [15] as follows: g − N2 þ Hem →Nþ⁎ 2 þ He þ e ;
ð1Þ
where the superscripts m, ⁎, or g means a metastable, an excited, or a ground state. The resulting nitrogen molecular species may occupy various vibrational/rotational excited levels of the B 2 Σu state. This reaction can explain the sputtering characteristics in the helium–nitrogen mixed gas plasma, where small amounts of nitrogen added can increase the sputtering rate effectively. As shown in Fig. 2, slow-down of the ionization rate of nitrogen molecule is observed when the partial pressure of nitrogen exceeds about 200 Pa, which is derived from quenching of the helium metastables according to Eq. (1). 3.2. Spectral lines of silver emitted from He mixed gas plasmas
Fig. 4. Schematic energy level diagram representing only the related energy levels of helium and nitrogen molecules.
observed in conventional argon plasmas (typically several μm/ min). Fig. 2 shows variations in the emission intensities of nitrogen molecular band heads, the N2 337.13 nm and the N2+ 391.44 nm, as a function of the nitrogen partial pressure, and a change in the discharge current measured in the helium– nitrogen mixed gas plasma is shown in Fig. 3. The N2 337.13nm band head is assigned to the transition from C 3Πu (υ = 0) to B 3Πg (υ = 0) of nitrogen molecule, and the N2+ 391.44 nm from B 2Σu (υ = 0) to X 2Σg (υ = 0) of nitrogen molecular ion [14]. While the emission intensity of the N2 337.13 nm monotonically increases along with the amounts of nitrogen gas introduced, which well corresponds to the partial pressure, that of the N2+ 391.44 nm is predominantly elevated when the nitrogen pressure ranges from 0 to 150 Pa. The intensity variation of the N2+ 391.44 nm is very similar to the pressure dependence of the discharge current as illustrated in Fig. 3, indicating that the resultant electrons are derived predominantly from the ionization of nitrogen molecules. The dominant ionization of nitrogen molecular species can occur by any interaction with helium gas species at the lower nitrogen pressures. Furthermore, the sputtering rate well corresponds to the variation of the N2+ 391.44 nm when nitrogen gas is added,
Our previous paper has reported that observed emission lines of silver ion, which are emitted from a Grimm-style glow discharge lamp, are identified to the optical transitions from the 4d95p (10–11.3 eV) to the 4d95s (4.8–5.7 eV) excited levels or the transitions from the 4d95d (4d96s) (15–16.7 eV) to the 4d95p excited levels [16]. The Ag II 241.32 nm and the Ag II 243.78 nm belong to the former transition and the Ag II 224.62 nm and the Ag II 271.19 nm to the latter transition. These emission lines of silver ion appear particularly in an argon–helium mixed gas plasma, whereas they are hardly
Fig. 5. Variation in the discharge current when pure silver is employed as the sample. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
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Fig. 6. Dependence of the emission intensity of the Ag I 338.29 nm on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
Fig. 8. Dependence of the emission intensities of several Ag II lines derived from the 4d95d–4d95p transition on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
observed in a pure Ar plasma [16]. This phenomenon can be explained from collisions of the second kind between silver atom and helium excited species as follows:
result implies that the nitrogen addition causes similar effects in the silver sample as well as the copper sample: dominant ionization for nitrogen molecular ions and simultaneous quenching of helium excited species. The discharge current is considered to be determined principally by the amounts of nitrogen molecular ion in the plasma. Fig. 6 shows a change in the emission intensities of an atomic resonance line, Ag I 338.29 nm, as a function of the nitrogen partial pressure added. Monotonic increase in the emission intensity is observed, which would be attributed to an increase in the sputtering rate caused by nitrogen molecular ions produced through helium–nitrogen collisions. Fig. 7 shows variations in the emission intensities of silver ionic lines which are identified to the 4d95p–4d95s transition of silver ion. Their emission intensities have each sharp maximum peak at a nitrogen pressure of 30 Pa, which is much different from the intensity variation of the Ag I 338.29 nm as illustrated in Fig. 6. Also, the intensity changes of these silver ionic lines do not follow the discharge current caused by the addition of nitrogen gas. Their ionization/ excitation energies are 17.6–18.9 eV (the first ionization potential of 7.57 eV + the corresponding excitation energies). Because the total excitation energies are slightly lower than the internal energy of the helium metastables, a Penning-type ionization process (see Eq. (2)) could populate the 4d95p excited levels so that the silver ionic lines can be emitted. However, the number density of the helium metastables decreases through the quenching reaction of Eq. (1), which makes the excitations to the 4d95p levels less actively at larger nitrogen partial pressures even though larger amounts of silver atoms would be introduced into the plasma. Fig. 8 shows variations in the emission intensities of silver ionic lines which are identified to the 4d95d (4d96s) (15–16.7 eV)–4d95p transitions. This graph yields the similar pressure dependence to the result of Fig. 7 (the 4d95p–4d95s transition). Their ionization/excitation energies are 23.6–24.3 eV which is close to the ionization potential of helium (24.58 eV). Therefore, the 4d95d (4d96s) levels can be populated through a charge-transfer collision, as shown in Eq. (3), between silver atom and helium
Ag þ Hem →Agþ⁎ þ Heg þ e− ;
ð2Þ
Ag þ Heþ →Agþ⁎ þ Heg ;
ð3Þ
where the superscripts , ⁎, or means a metastable, an excited or a ground state. It should be noted that the ionization/ excitation of the silver ionic lines requires larger excitation energies which can be provided only by the helium excited species. This is a typical example where the helium plasma should be employed to excite ionic emission lines effectively. Fig. 5 shows a variation in the discharge current as a function of nitrogen partial pressure introduced into a helium plasma when a silver sample is employed, yielding a similar dependence to the copper target as illustrated in Fig. 3. This m
g
Fig. 7. Dependence of the emission intensities of several Ag II lines derived from the 4d95p–4d95s transition on the partial pressure of nitrogen added. Discharge voltage: 1300 V; pressure of helium: 2000 Pa.
M. Tsukiji, K. Wagatsuma / Microchemical Journal 87 (2007) 175–179
ion. It can be deduced from Fig. 8 that the number density of helium ion would be also reduced by adding nitrogen gas to the helium plasma. The behaviour of such silver ionic lines is highly interesting when small amounts of nitrogen are added to a helium glow discharge plasma. As illustrated in Figs. 7 and 8, we can observe the sufficient intensities of silver ionic lines having large excitation energies, as a smaller sputtering rate maintained by employing a helium–nitrogen mixed gas. This technique is expected to be applied to in-depth analysis of thin films because the sputtering rate of the order of 0.1 μm/min can be easily controlled. 4. Conclusions The addition of small amounts of nitrogen to a helium plasma generally enhances the emission intensities of silver lines, because the sputtering rate becomes larger by bombarding with nitrogen molecular ions which are produced through Penning-type ionization collisions between helium metastables and nitrogen molecules. In such plasmas, nitrogen molecular ions work as projectiles for the cathode sputtering. The intensity of silver ionic lines is elevated at lower nitrogen pressures; however, their intensities have maximum values at a nitrogen partial pressure of 30 Pa. This variation due to the number density of the helium metastables, which are considered to work as energy donors for the excitation of the silver ionic lines, would decrease through collisions with nitrogen molecules. The helium–nitrogen mixed gas plasma has the characteristics suitable for depth analysis of thin films because smaller sputtering rates can be controlled by the nitrogen partial pressure added. Acknowledgments The authors are grateful to Noboru Yamashita, Rigaku Corp., Japan, for measurement of the surface profile meter. The authors gratefully acknowledge financial support by grants from The
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Iron and Steel Institute of Japan and by a grant from the Steel Industry Foundation for the Advancement of Environmental Protection Technology, Japan. A part of this research is supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture of Japan (No. 18360016). References [1] T. Nelis, R. Payling, N.W. Barnett (Eds.), Glow Discharge Optical Emission Spectrometry, RSC Analytical Spectroscopy Monographs, RSC, Cambridge, 2003. [2] K. Marcus, J.A.C. Broekaert (Eds.), Glow Discharge Plasmas in Analytical Spectroscopy, John Wiley & Sons, Chichester, 2003. [3] J. Pons-Corbeau, J.P. Cazet, J.P. Moreau, R. Berneron, J.C. Charbonnier, Surf. Interface Anal. 9 (1986) 21. [4] J. Angeli, T. Kaltenbrunner, F.M. Androsch, Fresenius' J. Anal. Chem. 341 (1991) 140. [5] S. Suzuki, K. Suzuki, Surf. Interface Anal. 17 (1991) 551. [6] K. Takimoto, K. Suzuki, K. Nishizaka, T. Ohtsubo, Nippon Steel Tech. Rep., Overs. 33 (1987) 28. [7] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, Surf. Interface Anal. 35 (2003) 564. [8] K. Wagatsuma, K. Hirokawa, Spectrochim. Acta 42B (1987) 523. [9] K. Wagatsuma, K. Hirokawa, Anal. Chem. 60 (1988) 702. [10] K. Wagatsuma, ISIJ Int. 42 (2003) 325. [11] K. Wagatsuma, K. Hirokawa, Surf. Interface Anal. 8 (1986) 37. [12] K. Wagatsuma, H. Morita, K. Hirokawa, Surf. Interface Anal. 17 (1991) 116. [13] W. Grimm, Spectrochim. Acta 23B (1968) 443. [14] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, Chapman & Hall Ltd., London, 1965. [15] A. von Engel, Ionized Gases, Clarendon Press, Oxford, 1965. [16] K. Wagatsuma, Z. Phys., D. 37 (1996) 231.