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Optical emission spectroscopy of Ar–N2 mixture plasma
ARTICLE IN PRESS
Journal of Quantitative Spectroscopy & Radiative Transfer 107 (2007) 361–371 www.elsevier.com/locate/jqsrt
Optical emission spectroscopy of Ar–N2 mixture plasma A. Qayyuma,, Shaista Zebb, M.A. Naveedb, N.U. Rehmanb, S.A. Ghaurib, M. Zakaullahb a Department of Physics, G.C. University, 54000 Lahore, Pakistan Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan
b
Received 30 November 2006; received in revised form 3 February 2007; accepted 5 February 2007
Abstract Optical emission spectroscopy measurements are presented to characterize the different excitation and ionization processes of both atomic and molecular species in Ar–N2 mixture plasma under different discharge conditions. Particularly, the emission intensities of nitrogen (0–0) band of second positive system at 337.1 nm and (0–0) band of first 2 þ negative systems at 391.4 nm are used to determine the dependence of their radiative states ½N2 ðC3 Pu Þ; Nþ 2 ðB Su Þ on argon fraction in the mixture. It is observed that the addition of argon gas influences the radiative states differently due to their different populating mechanisms. The results demonstrate that the addition of argon to nitrogen plasma remarkably enhance the population of N2 ðC3 Pu Þ radiative state through Penning excitation involving argon metastable states. The electron temperature is determined from Ar-I spectral line intensities, using Boltzmann’s plot method and is found to depend on argon fraction in the mixture. r 2007 Elsevier Ltd. All rights reserved. PACS: 52.80.vp; 52.70.kz; 81.65. Lp Keywords: Glow discharge plasma; Optical diagnostics; Penning effect
1. Introduction Glow discharge plasma nitriding is one of the effective methods applied to improve the surface properties such as surface hardness, wear and corrosion resistance and fatigue strength of numerous materials especially iron-based alloys [1,2]. The large number of process parameters that can be arbitrarily selected, make it attractive for the synthesis of specific structures and properties not manageable in conventionally nitriding processes [3,4]. During the nitriding process, the active species of nitrogen are generated by an electric discharge facilitating the surface interactions by transporting momentum and energy. Therefore, for better control of the surface interactions during film growth and to produce quality films it is essential to optimize the production of active species in the discharge. The generation of these active species relies on the ability of Corresponding author. Tel.: +92 51 2273999.
E-mail addresses: [email protected] (A. Qayyum), [email protected], [email protected] (M. Zakaullah). 0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.02.008
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the discharge to produce high concentration of excited states of the plasma species that carry several electron volts of energy above their ground states and can affect the surface and consequently deposition chemistry [5]. The metastable states of atoms and molecules play a significant role in the production of active species owing to their ability to accumulate a great amount of energy, which can be effective in various chemical and physical processes [6]. Therefore, addition of inert gases such as argon, neon and helium in nitrogen plasma enhances the concentration of active species through Penning excitation and ionization processes [7]. The electron temperature is an important parameter of the plasma influencing the production of active species by inelastic collisions. The addition of inert gases provides a better control on the electron temperature [8]. Therefore measurement of electron temperature at different percentages of argon in nitrogen plasma under different discharge conditions will help to know about particle collision processes, plasma reactions and concentration of active species in the plasma. The most widely used optical method for sensing atoms and molecules in plasma is optical emission spectroscopy (OES) due to its non-perturbative nature [9,10]. The basic premise of this technique is that the emission intensity of particular wavelength from an excited state is proportional to the concentration of species in that excited state [11,12]. In this paper Ar–N2 mixture plasma is characterized by means of OES to investigate the production of active species in terms of argon fraction in the mixture and operating parameters. The electron temperature, which also affects the plasma reactivity by generating the active species by inelastic collisions, is evaluated from Ar-I spectral line intensities, using Boltzmann’s plot method. The main aim of this work is to obtain insight into the species that affect the plasma reactivity and to gain better understanding of the mechanisms that govern the production of these species. In particular, the influence of argon metstables on the production 2 þ of N2 ðC3 Pu Þ and Nþ 2 ðB Su Þ radiative states is studied in this article. 2. Experimental details The experiment is carried out using nitrogen–argon mixture as working gas, and plasma is generated in an abnormal glow regime, usually used for materials processing. The discharge is sustained with 50 Hz pulsing-dc power in a parallel plate configuration of electrodes with a diameter of 75 mm and a spacing of 60 mm housed in a cylindrical stainless steel vacuum chamber of 40-cm diameter and height. The side and back of the electrodes are covered with a ceramic casing to prevent additional discharge. The power is applied to the top electrode through the inductive load, which limits the current during the discharge whereas the bottom electrode is grounded. The experimental setup is illustrated in Fig. 1. Prior to admitting the nitrogen and argon gases, the chamber is evacuated down to 105 mbar using a rotary vane pump and oil diffusion pump. The flow of nitrogen and argon gases is monitored with gas flow meters whereas pressure in the chamber is recorded by using capsule type pressure gauge. Plasma induced OES is carried out using a computer controlled system comprising a McPherson–2061 1 m monochromator having a diffraction grating with 1200 grooves/ mm coupled with a side window photo-multiplier tube (928R) and auto ranging Pico-ammeter (Keithley-485). The spectral resolution of the system is 0.01 nm; however, the spectra are recorded with a spectral resolution of 0.1 nm in order to reduce the spectra acquisitions time, which assure the temporal stability of plasma along with other operating parameters. The wavelength calibration of the monochromator is performed by using a mercury lamp. Argon gas (20–50%) is admixed with nitrogen, and emission spectra (300–800 nm) are recorded as functions of argon fractions, input powers (175–225 watts) and filling pressures (7–9 mbar), by varying one and keeping the other corresponding parameters constant. A spectrum recorded at a filling pressure of 7-mbar, input power of 200 watts and gas composition of 20% Ar is shown in Fig. 2. 3. Optical emission spectroscopy In glow discharge plasmas, electron impact excites some of the plasma species to higher electronic states that decays and emits photons of characteristic wavelength, which can be detected and analyzed by recording the emission spectrum [11]. The basic premise of this technique is that the emission intensity of a particular wavelength from an excited state is proportional to the concentration of species in that excited state [12,13]. Detection of these emission intensities provides a qualitative indicator of species concentration, which can be
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AC
Diode Chain
Nitrogen Gas
Argon Gas Capsule Type Gauge
Optical Window View Window Plasma Valve
PMT
Main Chamber Autoranging Pico-ammeter
Computer To vacuum pump
40
+
20
Ar-I (727.29 nm) Ar-I (738.39 nm)
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Ar-I (696.54 nm) Ar-I (706.72 nm)
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+
Ar-I (476.86 nm) Ar-II (488.0 nm)
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Ar-I (751.46 nm)
N2 (357.7 nm)
120 Fe- I (344.06 nm)
Emission intensity (a.u.)
140
Fe- I (372.0 nm) N (375.54 nm) 2 Fe-I (387.85 nm) N2 (380.49 nm) N2 (394.30 nm) N2 (391.4 nm) N2 (405.5 nm) N2 (405.94 nm) Ar-I (427.21 nm) N2 (427.8 nm)
160
N2 (337.1 nm)
Fig. 1. Schematic diagram of the experimental setup.
0 350
400
450
500 550 600 Wavelength (nm)
650
700
750
Fig. 2. Emission spectrum recorded from a mixture (20% Ar) at a filling pressure of 7-mbar, input power of 200 watts and total flow rate of 100 SCCM. The band spectra appearing at around 600 nm belongs to vibrational transition (12–8), (11–7), (9–5), (8–4), (7–3), (5–1) whereas at around 650 nm belongs to vibrational transitions (9–6), (8–5), (7–4), (6–3), (5–2), (4–1) of first negative system of N2 (B3PgP A3 + g ).
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converted into a quantitative relative or absolute species number density by knowing the EEDF and energy dependent cross sections for the electron impact excitation [13–16]. 3.1. Population of radiative states and their emission intensity In nitrogen plasma, the population of the radiative state N2 ðC3 Pu Þ is caused by many excitation and quenching processes such as electron impact excitation from the molecular ground state (X 1S+ g ) and first metastable state (A3S+ u ), associative excitation, pooling reactions, transfer of energy between collisional partners and Penning excitation. The detailed reactions are given below [6,17,18]: 3 N2 ðX1 Sþ g Þ þ e ! N2 ðC Pu Þ þ e; 3 þ N2 ðX1 Sþ g Þ þ e ! N2 ðA Su Þ þ e; 3 N2 ðA3 Sþ u Þ þ e ! N2 ðC Pu Þ þ e; 3 þ 3 1 þ N2 ðX1 Sþ g Þ þ N2 ðA Su Þ ! N2 ðC Pu Þ þ N2 ðX Sg Þ, 3 þ 3 1 þ N2 ðA3 Sþ u Þ þ N2 ðA Su Þ ! N2 ðC Pu Þ þ N2 ðX Sg Þ, 3 3 3 þ N2 ðA3 Sþ u Þ þ N2 ðC Pu Þ ! N2 ðC Pu Þ þ N2 ðA Su Þ.
In argon nitrogen mixture plasma the population of the N2 ðC3 Pu Þ excited state may also result from the transfer of the internal energy from a metastable state of argon atoms to the ground state of the nitrogen molecules [19]. The metastable states of argon Ar*m(3P2, 3P0) where subscript ‘‘m’’ stands for metastable have the higher energies 11.55 and 11.72 eV than the threshold excitation energy (11.1 eV) of nitrogen molecule. Therefore, by adding argon in nitrogen plasma a significant increase in the emission intensities and consequently the concentration of the active species can be expected by Penning effect [7] 3 3 3 N2 ðX1 Sþ g Þ þ Arm ð P2 ; P0 Þ ! N2 ðC Pu Þ þ Ar
ð411:1 eV from N2 ðX ; n ¼ 0ÞÞ.
The subsequent radiative decays emit characteristic photons of (0–0) band of second positive system at 337.1 nm: N2 ðC3 Pu ; n0 ¼ 0Þ ! N2 ðB3 Pg ; n00 ¼ 0Þ þ hn. As a result the emission intensity of (0–0) band of second positive system is proportional to the population density of the N2 ðC3 Pu Þ state [20,21]. 2 þ The Nþ 2 ðB Su Þ excited state can be populated either by direct electron impact excitation from the ground state of the molecule N2 ðX1 Sþ g Þ via [20,21] þ 2 þ N2 ð1 Sþ g Þ þ e ! N2 ðB Su Þ þ 2e ð418:5 eV from N2 ðX; n ¼ 0ÞÞ
or stepwise via electron impact ionization of the N2 molecule and then subsequent electron impact excitation of the molecular ion. þ 2 þ N2 ðX1 Sþ g Þ þ e ! N2 ðX Sg Þ þ 2e ð415:6 eV from N2 ðX; n ¼ 0ÞÞ, 2 þ þ 2 þ Nþ 2 ðX Sg Þ þ e ! N2 ðB Su Þ þ e
ð43:3 eV from Nþ 2 ðX; n ¼ 0ÞÞ at 15:6 evÞ:
The ground state molecular ions can also be produced by the metastable argon atoms (3P2, 3P0) by Penning ionization [19] 3 3 þ 2 þ N2 ðX1 Sþ g Þ þ Arm ð P2 ; P0 Þ ! N2 ðX Sg Þ þ Ar þ e: þ 2 þ 2 þ The Nþ 2 ðX Sg Þ state can be further excited to N2 ðB Su Þ radiative state either by electron impact or by impact of metastable argon atoms.
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2 þ The subsequent radiative decays of Nþ 2 ðB Su Þ excited state emit characteristic photons of (0–0) band of first negative system at 391.4 nm. 2 þ 0 þ 2 þ Nþ 2 ðB Su ; n ¼ 0Þ ! N2 ðX Sg ; n ¼ 0Þ þ hn.
The emission intensity of the (0–0) band of first negative system is proportional to the population of the 2 þ Nþ 2 ðB Su Þ state [20,21] The Ar+* excited state can be populated by either one-step process [13]: Ar þ eðE435 eVÞ ! Arþ or the two-step process, Ar þ eðE415:5 eVÞ ! Arþ , Arþ þ eðE419:5 eVÞ ! Arþ . For low temperature plasma (below 4 eV) there are very few electrons with the energy 35 eV required for the simultaneous excitation and ionization process. So there is a large contribution from the excitation of the ground state ions by two-step process and the emission intensity of Ar+* radiative state provides information on the ground state ion density Ar+ [22]. 3.2. Plasma electron temperature Most plasma diagnostic techniques are either electrical or optical in nature. The widely used electrical technique to measure electron temperature and electron density is the Langmuir probe due to its easy implementation. But probe technique has numerous potential problems. Being intrusive, it is difficult to use in reactive plasmas, because contamination of the probe tip and complexities in the theories used to interpret the measurement often lead to error. Thus, non-intrusive optical measurements of the plasma parameters are often desired [22]. The electron temperature can be determined by a spectroscopic technique in a variety of ways but the most commonly used spectroscopic diagnostic method to determine the electron temperature of laboratory plasma is Boltzmann’s plot method, which provides better accuracy. In this method the intensities of several spectral lines having different threshold excitation energies are employed to determine the electron temperature by assuming that the population of the emitting levels follows the Boltzmann’s distribution. By using a number of Ar-I spectral lines having common lower level, the electron temperature is obtained from the slope of the Boltzmann’s plot [23]: I ki lki Ek ln þ C, ¼ gk Aki kB T where lki is the wavelength, I ki is the measured intensity, Aki is the transition probability , gk is the statistical weight of the upper level and C is a constant for a given atomic species. The data for the observed Ar-I spectral lines are given in Table 1 [24]. In Fig. 3, the value of ln(Il/gA) is plotted versus the value of the energy of the upper level for each considered transition, and the electron temperature is obtained from the slope by using the above equation. Table 1 Ar-I transitions used for the measurements of electron temperature [24] Transition 1S4 1S4 1S4 1S4 1S4
! 3 P7 ! 3 P1 ! 3 P2 ! 3 P3 ! 3 P5
l (nm)
Aki ð106 s1 Þ
gk
E k ðcm1 Þ
427.21 667.72 727.29 738.39 751.46
0.797 0.236 1.830 8.470 40.00
3 1 3 5 1
117 151.32 108 722.61 107 496.41 107 289.70 107 054.27
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-8
ln (Iλ /gA)
-10
-12
-14
-16 105600 107800 110000 112200 114400 116600 118800
Ek (cm-1) Fig. 3. Boltzmann’s plot for a mixture of 20% Ar at a filling pressure of 7 mbar, input power of 200 watts and total flow rate of 100 SCCM.
4. Results and discussion Optical emission derived from plasma species is used to characterize the Ar–N2 mixture plasma by taking into account their collisions with electrons and with other plasma species. These excitation collisions and the subsequent radiative decays, emit characteristic photons of the plasma species involved in optical emission [13,25] 4.1. Concentrations of the active species Fig. 4 shows that the spectral intensities of the (0–0) bands of first negative and second positive systems of nitrogen are influenced by the argon mixing differently owing to the different populating mechanisms of their 2 þ 3 respective radiative states [Nþ 2 ðB Su Þ, N2 ðC Pu Þ]. The emission intensity of the first negative band head (l ¼ 391.4 nm; 0–0) decreases, whereas the emission intensity of second positive band head (l ¼ 337.1 nm; 0–0) increases with increasing argon fraction in the mixture. This fact suggests the increased population of 2 þ N2 ðC3 Pu Þ radiative state in comparison to Nþ 2 ðB Su Þ radiative state with argon addition and may be explained by the increase in number of argon metastable atoms (Ar*) in the discharge. Since the threshold excitation energy of N2 ðC3 Pu Þ radiative state, 11.1 eV, is slightly lower than those of the argon metastable atoms: 3P2 (11.55 eV) and 3P0 (11.72 eV). Therefore, the N2 ðC3 Pu Þ radiative state can be populated by the inelastic collisions of argon metastable atoms through Penning excitation. The Penning excitation of the 2 þ Nþ 2 ðB Su Þ radiative state, which is responsible for the l ¼ 391.4 nm transition, is rather difficult by argon metastable atoms due to its higher threshold excitation energy (18.7 eV). These metastable argon atoms may * + + also destroy the N+ 2 ions and generate N2 molecules by charge transfer collisions (N2 +Ar -N2+Ar ). 3 3 Furthermore, energy of the argon metastable atoms having metastable states P2 (11.55 eV) and P0 (11.72 eV) is lower than the threshold ionization energy of the N2 molecule (15.57 eV), so the ionization mechanism of N2 is not influenced considerably by argon addition. Interactions between metastable argon atoms and N2 molecules are also possible resulting in the Penning dissociation and ionization of N2 molecules, but these reactions are not expected to play an important role in low temperature plasmas due to their energetic nature [26]. Though the energy of electrons increases with argon addition, and the ionization of N2 is more sensitive to high-energy electrons than that of the excitation of (C3Pu) state, this fact is overshadowed by the enhanced
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N2 (337.1 nm)
Input power = 175 watts
N2+ (391.4 nm) Ar - II (488.0 nm) Fe (372.0 nm)
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N2 (337.1 nm) N2+ (391.4 nm) Ar- II (488.0 nm) Fe (372.0 nm)
Input power = 225 watts
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125 100
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75
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50 0.6
25 0
Emission intensity of Fe (a.u.)
120
+ (391.4
Emission intensity of Fe (a.u.)
Input power = 200 watts
N2 (337.1 nm)
0.4 20
30
40
50
Ar [ % ]
Fig. 4. Variation of emission intensity of various species as a function of Ar [%] in the mixture. The data are recorded at a filling pressure of 7-mbar, total gas flow rate of 100 SCCM for different input powers.
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120 Emission intensity (a.u.)
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1.6
Ar - II (488.0 nm) Fe (372.0 nm)
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Emission intensity of Fe (a.u.)
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Filling pressure = 8-mbar
Fe (372.0 nm)
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20 0
0.4 20
N2 (337.1 nm) N2+ (391.4 nm)
1.2x102
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Ar - II (488.0 nm) Fe (372.0 nm)
1.0x102 8.0x10
30
1.0
1.4x102
Emission intensity (a.u.)
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Ar - II (488.0 nm)
0.8
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0.7 6.0x101 0.6
4.0x101 2.0x101
0.5
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Emission intensity of Fe (a.u.)
Emission intensity (a.u.)
120
Emission intensity of Fe (a.u.)
N2 (337.1 nm)
0.4 20
30
40
50
Ar [%]
Fig. 5. Variation of emission intensity of various species as a function of Ar [%] in the mixture. The data are recorded at input electrical power of 200 watts, total gas flow rate of 100 SCCM for different filling pressures.
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Penning excitation of N2 molecules by argon metastable atoms in the discharge. Furthermore, at a higher fraction of argon in the discharge there is lesser number of nitrogen molecules available to be ionized by electron collisions. The results also suggest that addition of argon increases the sputtering of the cathode, which is characterized by the increased emission intensity of the Fe spectral line. The atoms of the cathode material arrive in the glow region and are subjected to collisions with electrons and other plasma species. The excitation collisions and subsequent radiative decay emit characteristic photons of the sputtered material. The emission intensity of Ar-II line shows increasing trend with argon addition suggesting the increased concentration of argon ions. Fig. 4 also depicts an increase in the emission intensity of (0–0) band of the first negative system at 391.4 nm and (0–0) band and second positive system at 337.1 nm with input electrical power, deducing an increase in 2 þ 3 population of Nþ 2 ðB Su Þ and N2 ðC Pu Þ radiative states. This fact may be expected due to increase in electron density with increase of input power, suggesting an increase of the higher energy tail of the electron energy distribution function (EEDF). 0.45 Filling pressure = 7- mbar
175 watts 200 watts 225 watts
Electron temperature (kTe )
0.40
0.35
0.30
0.25
0.20
0.15 20
30
40
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0.45 7 -mbar 8 -mbar 9 -mbar
Electron temperature (kTe )
0.40
Input power = 200 watts
0.35
0.30
0.25
0.20
0.15 20
30
40
50
Ar [ % ] Fig. 6. Dependence of plasma electron temperature on Ar [%] for different: (a) input electrical powers, (b) filling pressures.
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Fig. 5 presents the emission intensity of radiative states as a function of argon concentration in the mixture for various filling pressures. It depicts a similar behavior of the emission intensity with argon addition as observed earlier. But a decrease in the emission intensity with filling pressure is observed, which suggests the decrease in population of these radiative states. This is due to the fact that the excitation efficiency of the discharge is generally decreased due to the increase in collisional loss of electrons kinetic energy. 4.2. Plasma electron temperature Fig. 6 shows that the electron temperature increases with argon addition. This fact may be explained as follows: The ionization cross sections and ionization potentials are nearly the same for Ar and N2 [2.5 1020 m2 and 15.7 eV, respectively], so these parameters are not likely to explain the increase in electron temperature with argon addition. The possible explanation is a difference in the EEDF especially in the high energy tail. This is probably due to a higher secondary electron yield at the target in the presence of argon in the discharge [27]. The other possible reason may be the Penning ionization of Fe atoms, which liberate energetic electrons carrying away the excess energy of argon metastables. Therefore, each penning ionization event will result in the argon neutral ground state atom, an iron ion, and an energetic electron carries away the excess energy. This mechanism and possibly other charge-exchange reactions can provide high-energy electrons, which may account for the increase in average electron temperature upon the addition of iron atoms by sputtering. Fig. 5 depicts an increase in the electron temperature with rise in input power. This fact may be explained by the increase in electron density owing to increase in power suggesting an increase of the higher energy tail of the EEDF, and the Penning ionization of Fe atoms involving argon metstables. The decrease in electron temperature with rise in filling pressure may be explained by the collisional loss of the electron energy. When pressure in the chamber increases, it causes an increase in the number of collisions between the electrons and the other plasma species. As a result the energy transferred from the electrons to the plasma species increases causing an increase in the plasma temperature by lowering the electron temperature. 5. Conclusions Spectroscopic measurements are presented to characterize the excitation and ionization processes of the active species in terms of their dependence on the discharge parameters. Particularly, effect of the argon mixing on the occurrence of the excited and ionized species is studied to fix its role in the generation of these 2 þ active species. An increased occurrence of the N2 ðC3 Pu Þ radiative state in comparison with Nþ 2 ðB Su Þ radiative state is observed with argon addition, whereas the occurrence of both states increases with power and decreases with filling pressure. Acknowledgments The work was partially supported by the Ministry of Science & Technology Grant, Pakistan Science Foundation Project No. PSF/R&D/C-QU/Phys.(199) and Higher Education Commission Research Project for Plasma Physics. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Journal of Quantitative Spectroscopy & Radiative Transfer 107 (2007) 361–371 www.elsevier.com/locate/jqsrt
Optical emission spectroscopy of Ar–N2 mixture plasma A. Qayyuma,, Shaista Zebb, M.A. Naveedb, N.U. Rehmanb, S.A. Ghaurib, M. Zakaullahb a Department of Physics, G.C. University, 54000 Lahore, Pakistan Department of Physics, Quaid-i-Azam University, 45320 Islamabad, Pakistan
b
Received 30 November 2006; received in revised form 3 February 2007; accepted 5 February 2007
Abstract Optical emission spectroscopy measurements are presented to characterize the different excitation and ionization processes of both atomic and molecular species in Ar–N2 mixture plasma under different discharge conditions. Particularly, the emission intensities of nitrogen (0–0) band of second positive system at 337.1 nm and (0–0) band of first 2 þ negative systems at 391.4 nm are used to determine the dependence of their radiative states ½N2 ðC3 Pu Þ; Nþ 2 ðB Su Þ on argon fraction in the mixture. It is observed that the addition of argon gas influences the radiative states differently due to their different populating mechanisms. The results demonstrate that the addition of argon to nitrogen plasma remarkably enhance the population of N2 ðC3 Pu Þ radiative state through Penning excitation involving argon metastable states. The electron temperature is determined from Ar-I spectral line intensities, using Boltzmann’s plot method and is found to depend on argon fraction in the mixture. r 2007 Elsevier Ltd. All rights reserved. PACS: 52.80.vp; 52.70.kz; 81.65. Lp Keywords: Glow discharge plasma; Optical diagnostics; Penning effect
1. Introduction Glow discharge plasma nitriding is one of the effective methods applied to improve the surface properties such as surface hardness, wear and corrosion resistance and fatigue strength of numerous materials especially iron-based alloys [1,2]. The large number of process parameters that can be arbitrarily selected, make it attractive for the synthesis of specific structures and properties not manageable in conventionally nitriding processes [3,4]. During the nitriding process, the active species of nitrogen are generated by an electric discharge facilitating the surface interactions by transporting momentum and energy. Therefore, for better control of the surface interactions during film growth and to produce quality films it is essential to optimize the production of active species in the discharge. The generation of these active species relies on the ability of Corresponding author. Tel.: +92 51 2273999.
E-mail addresses: [email protected] (A. Qayyum), [email protected], [email protected] (M. Zakaullah). 0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.02.008
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the discharge to produce high concentration of excited states of the plasma species that carry several electron volts of energy above their ground states and can affect the surface and consequently deposition chemistry [5]. The metastable states of atoms and molecules play a significant role in the production of active species owing to their ability to accumulate a great amount of energy, which can be effective in various chemical and physical processes [6]. Therefore, addition of inert gases such as argon, neon and helium in nitrogen plasma enhances the concentration of active species through Penning excitation and ionization processes [7]. The electron temperature is an important parameter of the plasma influencing the production of active species by inelastic collisions. The addition of inert gases provides a better control on the electron temperature [8]. Therefore measurement of electron temperature at different percentages of argon in nitrogen plasma under different discharge conditions will help to know about particle collision processes, plasma reactions and concentration of active species in the plasma. The most widely used optical method for sensing atoms and molecules in plasma is optical emission spectroscopy (OES) due to its non-perturbative nature [9,10]. The basic premise of this technique is that the emission intensity of particular wavelength from an excited state is proportional to the concentration of species in that excited state [11,12]. In this paper Ar–N2 mixture plasma is characterized by means of OES to investigate the production of active species in terms of argon fraction in the mixture and operating parameters. The electron temperature, which also affects the plasma reactivity by generating the active species by inelastic collisions, is evaluated from Ar-I spectral line intensities, using Boltzmann’s plot method. The main aim of this work is to obtain insight into the species that affect the plasma reactivity and to gain better understanding of the mechanisms that govern the production of these species. In particular, the influence of argon metstables on the production 2 þ of N2 ðC3 Pu Þ and Nþ 2 ðB Su Þ radiative states is studied in this article. 2. Experimental details The experiment is carried out using nitrogen–argon mixture as working gas, and plasma is generated in an abnormal glow regime, usually used for materials processing. The discharge is sustained with 50 Hz pulsing-dc power in a parallel plate configuration of electrodes with a diameter of 75 mm and a spacing of 60 mm housed in a cylindrical stainless steel vacuum chamber of 40-cm diameter and height. The side and back of the electrodes are covered with a ceramic casing to prevent additional discharge. The power is applied to the top electrode through the inductive load, which limits the current during the discharge whereas the bottom electrode is grounded. The experimental setup is illustrated in Fig. 1. Prior to admitting the nitrogen and argon gases, the chamber is evacuated down to 105 mbar using a rotary vane pump and oil diffusion pump. The flow of nitrogen and argon gases is monitored with gas flow meters whereas pressure in the chamber is recorded by using capsule type pressure gauge. Plasma induced OES is carried out using a computer controlled system comprising a McPherson–2061 1 m monochromator having a diffraction grating with 1200 grooves/ mm coupled with a side window photo-multiplier tube (928R) and auto ranging Pico-ammeter (Keithley-485). The spectral resolution of the system is 0.01 nm; however, the spectra are recorded with a spectral resolution of 0.1 nm in order to reduce the spectra acquisitions time, which assure the temporal stability of plasma along with other operating parameters. The wavelength calibration of the monochromator is performed by using a mercury lamp. Argon gas (20–50%) is admixed with nitrogen, and emission spectra (300–800 nm) are recorded as functions of argon fractions, input powers (175–225 watts) and filling pressures (7–9 mbar), by varying one and keeping the other corresponding parameters constant. A spectrum recorded at a filling pressure of 7-mbar, input power of 200 watts and gas composition of 20% Ar is shown in Fig. 2. 3. Optical emission spectroscopy In glow discharge plasmas, electron impact excites some of the plasma species to higher electronic states that decays and emits photons of characteristic wavelength, which can be detected and analyzed by recording the emission spectrum [11]. The basic premise of this technique is that the emission intensity of a particular wavelength from an excited state is proportional to the concentration of species in that excited state [12,13]. Detection of these emission intensities provides a qualitative indicator of species concentration, which can be
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AC
Diode Chain
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Argon Gas Capsule Type Gauge
Optical Window View Window Plasma Valve
PMT
Main Chamber Autoranging Pico-ammeter
Computer To vacuum pump
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+
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Ar-I (727.29 nm) Ar-I (738.39 nm)
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Ar-I (696.54 nm) Ar-I (706.72 nm)
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+
Ar-I (476.86 nm) Ar-II (488.0 nm)
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Ar-I (751.46 nm)
N2 (357.7 nm)
120 Fe- I (344.06 nm)
Emission intensity (a.u.)
140
Fe- I (372.0 nm) N (375.54 nm) 2 Fe-I (387.85 nm) N2 (380.49 nm) N2 (394.30 nm) N2 (391.4 nm) N2 (405.5 nm) N2 (405.94 nm) Ar-I (427.21 nm) N2 (427.8 nm)
160
N2 (337.1 nm)
Fig. 1. Schematic diagram of the experimental setup.
0 350
400
450
500 550 600 Wavelength (nm)
650
700
750
Fig. 2. Emission spectrum recorded from a mixture (20% Ar) at a filling pressure of 7-mbar, input power of 200 watts and total flow rate of 100 SCCM. The band spectra appearing at around 600 nm belongs to vibrational transition (12–8), (11–7), (9–5), (8–4), (7–3), (5–1) whereas at around 650 nm belongs to vibrational transitions (9–6), (8–5), (7–4), (6–3), (5–2), (4–1) of first negative system of N2 (B3PgP A3 + g ).
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converted into a quantitative relative or absolute species number density by knowing the EEDF and energy dependent cross sections for the electron impact excitation [13–16]. 3.1. Population of radiative states and their emission intensity In nitrogen plasma, the population of the radiative state N2 ðC3 Pu Þ is caused by many excitation and quenching processes such as electron impact excitation from the molecular ground state (X 1S+ g ) and first metastable state (A3S+ u ), associative excitation, pooling reactions, transfer of energy between collisional partners and Penning excitation. The detailed reactions are given below [6,17,18]: 3 N2 ðX1 Sþ g Þ þ e ! N2 ðC Pu Þ þ e; 3 þ N2 ðX1 Sþ g Þ þ e ! N2 ðA Su Þ þ e; 3 N2 ðA3 Sþ u Þ þ e ! N2 ðC Pu Þ þ e; 3 þ 3 1 þ N2 ðX1 Sþ g Þ þ N2 ðA Su Þ ! N2 ðC Pu Þ þ N2 ðX Sg Þ, 3 þ 3 1 þ N2 ðA3 Sþ u Þ þ N2 ðA Su Þ ! N2 ðC Pu Þ þ N2 ðX Sg Þ, 3 3 3 þ N2 ðA3 Sþ u Þ þ N2 ðC Pu Þ ! N2 ðC Pu Þ þ N2 ðA Su Þ.
In argon nitrogen mixture plasma the population of the N2 ðC3 Pu Þ excited state may also result from the transfer of the internal energy from a metastable state of argon atoms to the ground state of the nitrogen molecules [19]. The metastable states of argon Ar*m(3P2, 3P0) where subscript ‘‘m’’ stands for metastable have the higher energies 11.55 and 11.72 eV than the threshold excitation energy (11.1 eV) of nitrogen molecule. Therefore, by adding argon in nitrogen plasma a significant increase in the emission intensities and consequently the concentration of the active species can be expected by Penning effect [7] 3 3 3 N2 ðX1 Sþ g Þ þ Arm ð P2 ; P0 Þ ! N2 ðC Pu Þ þ Ar
ð411:1 eV from N2 ðX ; n ¼ 0ÞÞ.
The subsequent radiative decays emit characteristic photons of (0–0) band of second positive system at 337.1 nm: N2 ðC3 Pu ; n0 ¼ 0Þ ! N2 ðB3 Pg ; n00 ¼ 0Þ þ hn. As a result the emission intensity of (0–0) band of second positive system is proportional to the population density of the N2 ðC3 Pu Þ state [20,21]. 2 þ The Nþ 2 ðB Su Þ excited state can be populated either by direct electron impact excitation from the ground state of the molecule N2 ðX1 Sþ g Þ via [20,21] þ 2 þ N2 ð1 Sþ g Þ þ e ! N2 ðB Su Þ þ 2e ð418:5 eV from N2 ðX; n ¼ 0ÞÞ
or stepwise via electron impact ionization of the N2 molecule and then subsequent electron impact excitation of the molecular ion. þ 2 þ N2 ðX1 Sþ g Þ þ e ! N2 ðX Sg Þ þ 2e ð415:6 eV from N2 ðX; n ¼ 0ÞÞ, 2 þ þ 2 þ Nþ 2 ðX Sg Þ þ e ! N2 ðB Su Þ þ e
ð43:3 eV from Nþ 2 ðX; n ¼ 0ÞÞ at 15:6 evÞ:
The ground state molecular ions can also be produced by the metastable argon atoms (3P2, 3P0) by Penning ionization [19] 3 3 þ 2 þ N2 ðX1 Sþ g Þ þ Arm ð P2 ; P0 Þ ! N2 ðX Sg Þ þ Ar þ e: þ 2 þ 2 þ The Nþ 2 ðX Sg Þ state can be further excited to N2 ðB Su Þ radiative state either by electron impact or by impact of metastable argon atoms.
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2 þ The subsequent radiative decays of Nþ 2 ðB Su Þ excited state emit characteristic photons of (0–0) band of first negative system at 391.4 nm. 2 þ 0 þ 2 þ Nþ 2 ðB Su ; n ¼ 0Þ ! N2 ðX Sg ; n ¼ 0Þ þ hn.
The emission intensity of the (0–0) band of first negative system is proportional to the population of the 2 þ Nþ 2 ðB Su Þ state [20,21] The Ar+* excited state can be populated by either one-step process [13]: Ar þ eðE435 eVÞ ! Arþ or the two-step process, Ar þ eðE415:5 eVÞ ! Arþ , Arþ þ eðE419:5 eVÞ ! Arþ . For low temperature plasma (below 4 eV) there are very few electrons with the energy 35 eV required for the simultaneous excitation and ionization process. So there is a large contribution from the excitation of the ground state ions by two-step process and the emission intensity of Ar+* radiative state provides information on the ground state ion density Ar+ [22]. 3.2. Plasma electron temperature Most plasma diagnostic techniques are either electrical or optical in nature. The widely used electrical technique to measure electron temperature and electron density is the Langmuir probe due to its easy implementation. But probe technique has numerous potential problems. Being intrusive, it is difficult to use in reactive plasmas, because contamination of the probe tip and complexities in the theories used to interpret the measurement often lead to error. Thus, non-intrusive optical measurements of the plasma parameters are often desired [22]. The electron temperature can be determined by a spectroscopic technique in a variety of ways but the most commonly used spectroscopic diagnostic method to determine the electron temperature of laboratory plasma is Boltzmann’s plot method, which provides better accuracy. In this method the intensities of several spectral lines having different threshold excitation energies are employed to determine the electron temperature by assuming that the population of the emitting levels follows the Boltzmann’s distribution. By using a number of Ar-I spectral lines having common lower level, the electron temperature is obtained from the slope of the Boltzmann’s plot [23]: I ki lki Ek ln þ C, ¼ gk Aki kB T where lki is the wavelength, I ki is the measured intensity, Aki is the transition probability , gk is the statistical weight of the upper level and C is a constant for a given atomic species. The data for the observed Ar-I spectral lines are given in Table 1 [24]. In Fig. 3, the value of ln(Il/gA) is plotted versus the value of the energy of the upper level for each considered transition, and the electron temperature is obtained from the slope by using the above equation. Table 1 Ar-I transitions used for the measurements of electron temperature [24] Transition 1S4 1S4 1S4 1S4 1S4
! 3 P7 ! 3 P1 ! 3 P2 ! 3 P3 ! 3 P5
l (nm)
Aki ð106 s1 Þ
gk
E k ðcm1 Þ
427.21 667.72 727.29 738.39 751.46
0.797 0.236 1.830 8.470 40.00
3 1 3 5 1
117 151.32 108 722.61 107 496.41 107 289.70 107 054.27
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-8
ln (Iλ /gA)
-10
-12
-14
-16 105600 107800 110000 112200 114400 116600 118800
Ek (cm-1) Fig. 3. Boltzmann’s plot for a mixture of 20% Ar at a filling pressure of 7 mbar, input power of 200 watts and total flow rate of 100 SCCM.
4. Results and discussion Optical emission derived from plasma species is used to characterize the Ar–N2 mixture plasma by taking into account their collisions with electrons and with other plasma species. These excitation collisions and the subsequent radiative decays, emit characteristic photons of the plasma species involved in optical emission [13,25] 4.1. Concentrations of the active species Fig. 4 shows that the spectral intensities of the (0–0) bands of first negative and second positive systems of nitrogen are influenced by the argon mixing differently owing to the different populating mechanisms of their 2 þ 3 respective radiative states [Nþ 2 ðB Su Þ, N2 ðC Pu Þ]. The emission intensity of the first negative band head (l ¼ 391.4 nm; 0–0) decreases, whereas the emission intensity of second positive band head (l ¼ 337.1 nm; 0–0) increases with increasing argon fraction in the mixture. This fact suggests the increased population of 2 þ N2 ðC3 Pu Þ radiative state in comparison to Nþ 2 ðB Su Þ radiative state with argon addition and may be explained by the increase in number of argon metastable atoms (Ar*) in the discharge. Since the threshold excitation energy of N2 ðC3 Pu Þ radiative state, 11.1 eV, is slightly lower than those of the argon metastable atoms: 3P2 (11.55 eV) and 3P0 (11.72 eV). Therefore, the N2 ðC3 Pu Þ radiative state can be populated by the inelastic collisions of argon metastable atoms through Penning excitation. The Penning excitation of the 2 þ Nþ 2 ðB Su Þ radiative state, which is responsible for the l ¼ 391.4 nm transition, is rather difficult by argon metastable atoms due to its higher threshold excitation energy (18.7 eV). These metastable argon atoms may * + + also destroy the N+ 2 ions and generate N2 molecules by charge transfer collisions (N2 +Ar -N2+Ar ). 3 3 Furthermore, energy of the argon metastable atoms having metastable states P2 (11.55 eV) and P0 (11.72 eV) is lower than the threshold ionization energy of the N2 molecule (15.57 eV), so the ionization mechanism of N2 is not influenced considerably by argon addition. Interactions between metastable argon atoms and N2 molecules are also possible resulting in the Penning dissociation and ionization of N2 molecules, but these reactions are not expected to play an important role in low temperature plasmas due to their energetic nature [26]. Though the energy of electrons increases with argon addition, and the ionization of N2 is more sensitive to high-energy electrons than that of the excitation of (C3Pu) state, this fact is overshadowed by the enhanced
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N2 (337.1 nm)
Input power = 175 watts
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+ (391.4
Emission intensity of Fe (a.u.)
Input power = 200 watts
N2 (337.1 nm)
0.4 20
30
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Ar [ % ]
Fig. 4. Variation of emission intensity of various species as a function of Ar [%] in the mixture. The data are recorded at a filling pressure of 7-mbar, total gas flow rate of 100 SCCM for different input powers.
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Emission intensity of Fe (a.u.)
Emission intensity (a.u.)
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Emission intensity of Fe (a.u.)
N2 (337.1 nm)
0.4 20
30
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Ar [%]
Fig. 5. Variation of emission intensity of various species as a function of Ar [%] in the mixture. The data are recorded at input electrical power of 200 watts, total gas flow rate of 100 SCCM for different filling pressures.
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Penning excitation of N2 molecules by argon metastable atoms in the discharge. Furthermore, at a higher fraction of argon in the discharge there is lesser number of nitrogen molecules available to be ionized by electron collisions. The results also suggest that addition of argon increases the sputtering of the cathode, which is characterized by the increased emission intensity of the Fe spectral line. The atoms of the cathode material arrive in the glow region and are subjected to collisions with electrons and other plasma species. The excitation collisions and subsequent radiative decay emit characteristic photons of the sputtered material. The emission intensity of Ar-II line shows increasing trend with argon addition suggesting the increased concentration of argon ions. Fig. 4 also depicts an increase in the emission intensity of (0–0) band of the first negative system at 391.4 nm and (0–0) band and second positive system at 337.1 nm with input electrical power, deducing an increase in 2 þ 3 population of Nþ 2 ðB Su Þ and N2 ðC Pu Þ radiative states. This fact may be expected due to increase in electron density with increase of input power, suggesting an increase of the higher energy tail of the electron energy distribution function (EEDF). 0.45 Filling pressure = 7- mbar
175 watts 200 watts 225 watts
Electron temperature (kTe )
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Electron temperature (kTe )
0.40
Input power = 200 watts
0.35
0.30
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Ar [ % ] Fig. 6. Dependence of plasma electron temperature on Ar [%] for different: (a) input electrical powers, (b) filling pressures.
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Fig. 5 presents the emission intensity of radiative states as a function of argon concentration in the mixture for various filling pressures. It depicts a similar behavior of the emission intensity with argon addition as observed earlier. But a decrease in the emission intensity with filling pressure is observed, which suggests the decrease in population of these radiative states. This is due to the fact that the excitation efficiency of the discharge is generally decreased due to the increase in collisional loss of electrons kinetic energy. 4.2. Plasma electron temperature Fig. 6 shows that the electron temperature increases with argon addition. This fact may be explained as follows: The ionization cross sections and ionization potentials are nearly the same for Ar and N2 [2.5 1020 m2 and 15.7 eV, respectively], so these parameters are not likely to explain the increase in electron temperature with argon addition. The possible explanation is a difference in the EEDF especially in the high energy tail. This is probably due to a higher secondary electron yield at the target in the presence of argon in the discharge [27]. The other possible reason may be the Penning ionization of Fe atoms, which liberate energetic electrons carrying away the excess energy of argon metastables. Therefore, each penning ionization event will result in the argon neutral ground state atom, an iron ion, and an energetic electron carries away the excess energy. This mechanism and possibly other charge-exchange reactions can provide high-energy electrons, which may account for the increase in average electron temperature upon the addition of iron atoms by sputtering. Fig. 5 depicts an increase in the electron temperature with rise in input power. This fact may be explained by the increase in electron density owing to increase in power suggesting an increase of the higher energy tail of the EEDF, and the Penning ionization of Fe atoms involving argon metstables. The decrease in electron temperature with rise in filling pressure may be explained by the collisional loss of the electron energy. When pressure in the chamber increases, it causes an increase in the number of collisions between the electrons and the other plasma species. As a result the energy transferred from the electrons to the plasma species increases causing an increase in the plasma temperature by lowering the electron temperature. 5. Conclusions Spectroscopic measurements are presented to characterize the excitation and ionization processes of the active species in terms of their dependence on the discharge parameters. Particularly, effect of the argon mixing on the occurrence of the excited and ionized species is studied to fix its role in the generation of these 2 þ active species. An increased occurrence of the N2 ðC3 Pu Þ radiative state in comparison with Nþ 2 ðB Su Þ radiative state is observed with argon addition, whereas the occurrence of both states increases with power and decreases with filling pressure. Acknowledgments The work was partially supported by the Ministry of Science & Technology Grant, Pakistan Science Foundation Project No. PSF/R&D/C-QU/Phys.(199) and Higher Education Commission Research Project for Plasma Physics. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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