Cite this Article
Han Dao-Man, Liu Yong-Xin, Gao Fei, Liu Wen-Yao, Xu Jun, Wang You-Nian. Measurements of argon metastable density using the tunable diode laser absorption spectroscopy in Ar and Ar/O2. Chinese Physics B, 2018, 27(6): 065202  
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Measurements of argon metastable density using the tunable diode laser absorption spectroscopy in Ar and Ar/O2
Han Dao-Man1, Liu Yong-Xin1, 2, †, Gao Fei1, Liu Wen-Yao3, Xu Jun1, Wang You-Nian1
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Physics, Dalian University of Technology, Dalian 116024, China
National Demonstration Center for Experimental Physics Education, Dalian University of Technology, Dalian 116024, China
North University of China, Taiyuan 030051, China

 

† Corresponding author. E-mail: yxliu129@dlut.edu.cn

Abstract

Densities of Ar metastable states 1s5 and 1s3 are measured by using the tunable diode laser absorption spectroscopy (TDLAS) in Ar and Ar/O2 mixture dual-frequency capacitively coupled plasma (DF-CCP). We investigate the effects of high-frequency (HF, 60 MHz) power, low-frequency (LF, 2 MHz) power, and working pressure on the density of Ar metastable states for three different gas components (0%, 5%, and 10% oxygen mixed in argon). The dependence of Ar metastable state density on the oxygen content is also studied at different working pressures. It is found that densities of Ar metastable states in discharges with different gas components exhibit different behaviors as HF power increases. With the increase of HF power, the metastable density increases rapidly at the initial stage, and then tends to be saturated at a higher HF power. With a small fraction (5% or 10%) of oxygen added in argon plasma, a similar change of the Ar metastable density with HF power can be observed, but the metastable density is saturated at a higher HF power than in the pure argon discharge. In the DF-CCP, the metastable density is found to be higher than in a single frequency discharge, and has weak dependence on LF power. As working pressure increases, the metastable state density first increases and then decreases, and the pressure value, at which the density maximum occurs, decreases with oxygen content increasing. Besides, adding a small fraction of oxygen into argon plasma will significantly dwindle the metastable state density as a result of quenching loss by oxygen molecules.

PACS: 52.70.-m;52.70.Kz;52.80.Vp;
Keyword:argon metastable states;tunable diode laser absorption spectroscopy;capacitively coupled plasmas;
1. Introduction

Radio-frequency (RF) capacitively coupled plasma (CCP) has been widely studied by many researchers for its extensive applications in semiconductor manufacturing, flat panel displays, and solar cell industries.[1,2] Argon plays an important role in the industrial process when mixed with other reactive gases, and one important reason is that the existence of argon metastable atoms can lead to the dissociation of other gas molecules increasing, thus significantly affecting the plasma properties.[3,4] Argon 1s5 and 1s3 states are the two main metastable states in argon-containing plasmas, and they have many favorable physicochemical properties. For example, the energy level of the argon metastable atom is above 10 eV with respect to the ground-state atom, and its lifetime is relatively long, which may reach one second.[5] On account of the forbidden spontaneous transition to lower levels, the metastable atom generally has a relatively high density. Moreover, it has a higher collision cross section with electrons, which exceeds that of the ground-state atom by three or four orders of magnitude. Apparently, these properties of argon metastable atoms will definitely affect the discharge process.[6,7] Thus the study of the dependence of the argon metastable atom density on various external parameters and the underlying physics behind it are of great importance.

In recent years, some progress has been made in theoretical and experimental studies about the production and loss process of argon metastable atoms in CCP.[3,4,8–12] In 1994, Sansonnens et al.[3] investigated the important role of argon metastable atoms in argon-diluted silane plasma by combining absorption spectroscopy (AS) and time-resolved optical emission spectroscopy (OES). The results showed that the density of metastable atom is substantially reduced with the addition of a small fraction of silane due to molecular quenching, which also greatly increases the silane dissociation rate. McMillin et al.[8,9] used planar laser-induced fluorescence (LIF) to measure the two-dimensional distributions of argon metastable density in different gas mixtures. They found that adding some Cl2 or CF4 could significantly reduce the metastable density, and cause the metastable spatial profiles to change. This was confirmed by the later simulation results obtained by Rauf et al.[10] through using a hybrid plasma equipment model. With a self-consistent fluid model, Zhang et al.[4] investigated the effect of the argon metastable atoms on plasma properties in pure argon discharge. The plasma density in the case without metastable atoms was observed to be much higher than that with metastable atoms, especially at a higher voltage, a higher pressure, and/or a higher frequency. In 2010, Ohba et al.[11] measured the spatial distribution of argon metastable density in dual-frequency (DF) CCP by using a pair of optical emission lines. Also in DF-CCP, the tunable diode laser absorption spectroscopy (TDLAS) was used by Liu et al.[12] to investigate the dependence of argon metastable state density on rf power, pressure, and CF4 content, and they found that the argon metastable state is mainly generated via the electron-impact excitation with the ground state and lost through diffusion and collision quenching processes with electrons and neutral molecules.

In addition, there are many investigations about argon metastable states in other types of plasma sources, such as inductively coupled plasmas,[13–16] surface-wave plasmas,[17,18] hollow cathode discharges,[19] and micro-discharge plasmas.[20,21] Generally speaking, almost all of the methods of measuring the metastable atom density are based on spectroscopy diagnostics, of which the AS is the most suitable way to obtain their absolute densities due to the non-radiative nature of the metastable state. However, AS was more used in other types of plasmas[13–15,17–21] than in CCP.[3,12]

Oxygen-containing plasma plays an important role in the ashing of the photoresist mask, removing polymer films, oxidation or deposition of thin film oxides and other processes.[1,2] Recently, many researches have focused on the Ar/O2 mixture discharge because of their specific features both experimentally and theoretically.[7,22–28] For example, Takechi and Lieberman[23] found that argon addition into oxygen inductive discharges can greatly improve the plasma density and the etch rate due to the increased dissociation of O2 by metastable argon atoms. In a capacitively coupled discharge, Worsley et al.[24] also observed that the O radical density is raised upon argon dilution as a result of an increasing contribution of Penning dissociation, which agreed well with the data simulated by Lee et al.[26] through using a PIC–MCC model. However, there are few researches about the density measurement of the argon metastable state in Ar/O2 mixture capacitive discharges by using AS.

In this paper, densities of argon metastable states (1s5 and 1s3) as a function of high-frequency (HF, 60 MHz) power, low-frequency (LF, 2 MHz) power, and working pressure in DF-CCP with three different gas components (0%, 5%, and 10% oxygen mixed in argon) are measured by the TDLAS. The effects of oxygen content on argon metastable state density are also studied at different pressures. Moreover, electron density is obtained by using a home-made hairpin probe to better understand the main generation and loss processes of the metastable states. The experimental details are briefly described in Section 2. Then in Section 3, experimental results are discussed. Finally, some conclusions are drawn from the present study in Section 4.

2. Experimental and data treatment details

The measurements were implemented in a 280 mm diameter cylindrical vacuum vessel as shown in Fig. 1. The vacuum vessel has been described in more detail elsewhere,[12,22,29] so only a brief description is given here. The plasma is produced between the two circular plate electrodes with 210 mm in diameter and 30 mm in spacing. The upper electrode is driven by the DF power sources (60/2 MHz, 1 kW, Thamway), while the bottom electrode and the vessel wall are grounded.

Fig. 1. (color online) Schematic of the DF capacitively coupled plasma reactor equipped with the tunable diode laser absorption spectroscopy (TDLAS) diagnostic system.

Argon metastable density is obtained by using the TDLAS. A tunable diode laser (LD-0773-0075-DFB-1, Toptica) is used to launch a laser beam, which travels through the mid-gap between the two electrodes after being collimated via some apertures. After passing through the chamber, the laser beam is detected by a photodiode and displayed on a digital oscilloscope (LeCroy Waverunner). A neutral density filter is placed in front of the laser entrance of the chamber, aiming to avoid light saturation in the detection procedure by the photodiode. The shutter can manage the laser beam to enter into or be blocked out of the plasma chamber. It should be noted that the tunable diode laser is tuned by a current controller unit (LDC205, Thorlabs). The current can be controlled by a 10-Hz triangle voltage signal generated by a function generator in order to scan the profile of absorption peaks of argon metastable state over the wavelength range (772.3 nm–772.5 nm). The diode temperature is set to be at 31 °C by the temperature controller unit (TED 200 C, Thorlabs).

After the adjustment of the wavelength range, the following four spectral intensity profiles should be measured to determine the absolute concentration of the metastable atoms:[12,21,30,31]

(i) IP+L when both laser and plasma are on;

(ii) IP when plasma is on, and the laser beam is shut out by the shutter;

(iii) IB when plasma is off and the laser beam is shut out by the shutter;

(iv) IL when plasma is off, and the laser on.

Here, subscripts P, L, and B refer to plasma, laser, and background, respectively. At each of the discharge parameters, the four spectral intensity profiles IP+L, IP, IB, and IL need to be simultaneously measured within a short time to ensure the accuracy. Figure 2 shows a typical example of the four spectral lines obtained in the argon plasma driven at 60 MHz, 100 W, and 12 Pa.

Fig. 2. (color online) Spectral profiles of IP+L, IP, IB, and IL in pure argon discharge driven at 60 MHz, 100 W, and 12 Pa.
Fig. 3. (color online) Measured absorption rate profile (black dash curve) and Gaussian fitting curve (red solid curve) calculated from four spectral lines shown in Fig. 2: both peak at 772.376 nm and 772.421 nm.

Following steps described above, the laser intensity I0 and its transmission beam intensity It can be written as follows: I0 = IL − IB and It = IL + p − Ip.

Then, argon metastable density can be determined with the law of Beer–Lambert

where N is the absolute density of the metastable states, l is the absorption length (approximately equal to the chamber diameter in our experiment), and the absorption cross section σ (v) can be expressed as
where f(v) is the normalized Gaussian function, A21 is Einstein’s coefficient, and g1 and g2 are the statistical weights of the lower and higher levels, respectively. Then, combining Eqs. (1) and (2), we can obtain the metastable density from
The integral in Eq. (3) is solved by a Matlab routine after a nonlinear Gaussian curve fitting.

In addition, a home-made hairpin probe is utilized to measure the electron density at the discharge center. In order to reduce the rf disturbance, the hairpin probe used in our experiment is fully floating and its detailed description can be found elsewhere.[22] The hairpin probe is based on the microwave resonance theory, which was developed by Piejak et al.[32,33] and Curley.[34] The electron density can be obtained from the following expression

where ne is the electron density, fp and f0 (with unit of GHz) are the resonant frequencies of the hairpin probe in plasma and vacuum, respectively.

3. Results and discussion

In this section, the effects of HF and LF powers, working pressure, and oxygen content on densities of argon metastable states (1s5 and 1s3) are studied. The main production and loss processes of metastable states are analyzed in Subsection 3.1. In Subsections 3.2–3.5, experimental results and a discussion are presented.

3.1. Main production and loss processes of metastable states

As the argon 1s3 state exhibits similar production and loss processes to the 1s5 state, thus only the 1s5 was analyzed here. The 1s5 density is governed by the following one-dimensional continuity equation[3]

where nm is the metastable density, Fm(t) is the production rate, Lm(t) is the loss rate due to the collision process with other species, and Gm(t) is the diffusion loss rate, denoted by the subscript m. For a steady plasma, equation (5) can be written as
The production of argon metastable atoms is principally from the process of direct electron-impact excitation with the argon ground state: Ar(g) + e → Arm + e. Other processes such as ion-impact excitation and radiative transition from higher excited states can be ignored due to lower densities than that at the argon ground state, and smaller reaction coefficients.[3,4,12] Thus, Fm can be given by
where nAr(g) is the density of argon atoms at ground state, ne is the electron density, and kge is the rate coefficient of electron impact excitation from the argon ground state to the metastable state.

In pure argon discharges, the primary loss processes of the argon metastable state are from electron impact, i.e., the metastable argon atoms might be excited to 2p-levels, resonant and ionized states via these reactions like (i) Arm + e → Ar(2pn) + e, (ii) Arm + e → Arr + e, (iii) Arm + e → Ar+ + e. Other impact loss processes by electrons and ground argon atoms could be ignored due to lower rate coefficients.[3,4] While in the argon–oxygen mixed gas discharge, the main loss channel could be the quenching collision process by O2 molecules: (iv) Arm + O2 → O2 + Ar. So, we can give the following expression of the loss rate by these collision processes:

where kqe = kexc + kres + kion is the total collision quenching rate of the metastable state by electrons as listed in reactions (i)–(iii), and kq O2 is the collision quenching rate by O2 molecules, nm is the metastable density, and nO2 is the oxygen molecule density. As for the axial diffusion loss, the loss rate can be written as
where Dm is the metastable diffusion coefficient, Λ ≈ L/π is the characteristic length with L being the inter-electrode distance, p is the working pressure, and kD is the effective diffusive loss rate of the metastable state. Therefore, a new equation that is derived from Eqs. (5)–(9) can be obtained as follows:
In this equation, the rate coefficients of the main production and loss process for metastable atoms are listed in Table 1, where Ej is the energy threshold in each reaction.

Table 1.

Rate coefficients of the main production and loss processes for the metastable atoms.[1,25,35,36]

.

It can be clearly seen that the electron density ne and oxygen molecule density nO2 have great influences on the metastable state density. The electron temperature Te also has an effect on the density of metastable atoms by affecting rate coefficients. More discussion will be presented based on our experimental results in the following subsections.

3.2. HF power effect

In this subsection, densities of argon 1s5 and 1s3 states and electron as a function of HF power PH in single-frequency (SF) and DF capacitive discharges with different gas components (0%, 5%, and 10% oxygen mixed in argon) are investigated. Note that the working pressure is fixed at 13 Pa, and in the DF discharge, LF power PL is fixed at 50 W. The experimental results in SF and DF cases are presented in Figs. 4 and 5, respectively. One can see from Figs. 4 and 5 that the two metastable state densities exhibit similar dependence on PH. To simplify the analysis, more attention is paid to the analysis of the 1s5 state.

Fig. 4. (color online) Variations of (a) 1s5, (b) 1s3, and (c) electron densities with HF power PH in SF driven discharge with different gas components (0%, 5%, and 10% oxygen mixed in argon). Pressure is fixed at 13 Pa. Note that arrows in panels (a) and (b) roughly indicate the saturation points of the metastable densities.
Fig. 5. (color online) Variations of (a) 1s5, (b) 1s3, and (c) electron densities with PH in DF driven discharge with different gas components (0%, 5%, 10% oxygen mixed in argon). LF power PL and working pressure are fixed at 50 W and 13 Pa, respectively. Note that arrows in panels (a) and (b) roughly indicate the saturation points of the metastable state densities.

In the SF case, taking 1s5 for example (see Fig. 4(a)), with the increase of PH, the metastable density in the pure argon discharge (the black curve with squares) first rises rapidly and then becomes saturated at a value about 1.4 × 1011 cm−3 at PH > 90 W. By contrast, electron density increases almost linearly with PH increasing as shown in Fig. 4(c). The different loss processes can explain the different dependence of 1s5 and electron densities on PH, because electron and 1s5 state have similar generation processes.

The increasing electron density with PH increasing is attributed to the enhancement of the electron-impact ionization reaction with argon ground-state atoms. Similarly, the generation of the 1s5 state is dominated by the collision excitation process of high-energy electrons (above 10 eV) with ground-state argon atoms. However, besides the diffusion loss process when compared with electrons, the additional quenching loss process of 1s5 by collisions with lower-energy electrons (only several eV or even lower) plays an important role. As PH increases, the quenching loss of the 1s5 state by electron collision is enhanced due to the rising of the electron and 1s5 state density. As a result, the 1s5 density tends to be saturated at a higher value of PH.

Figure 4(b) illustrates the evolution of 1s3 state density with PH. We can see that the variation of 1s3 state density with PH shows the same manners as that of 1s5 state density because they are at similar energy levels. It should be noted that the 1s3 state density is about one order of magnitude lower than the 1s5 state due to their larger difference in degeneracy.[12,37] Besides, it can be seen from Figs. 4(a) and 4(b) that the 1s5 or 1s3 state densities in the Ar/O2 mixture discharges (5% and 10% oxygen in argon) rise more slowly with increasing PH than that in the pure argon. This is because in Ar/O2 mixture discharges the metastable-state density is largely depleted due to the quenching loss process by oxygen molecules. At PH = 90 W, for instance, an addition of 5% oxygen can reduce the 1s5 state density by 75%, i.e., from 13.63 × 1010 cm−3 to 3.38 × 1010 cm−3. Therefore, the metastable density in gas mixture discharges does not come to the saturation at higher PHs under the present experimental condition. The quenching loss rate of the metastable state via collisions with oxygen molecules remains almost unchanged with increasing PH when oxygen content and working pressure are fixed. So we can expect that the density of metastable atoms in discharges of gas mixture will also come to a saturation value at a much higher value of PH

The dependence of 1s5, 1s3, and electron densities on PH is also investigated in DF CCP, and the results under otherwise identical conditions as in Fig. 4 are shown in Fig. 5. It can be seen that the metastable state densities change with increasing PH in a similar fashion to that in the SF case (in Fig. 4). Also taking the 1s5 state in pure argon for example (the black curve with squares in Fig. 5(a)), we can see that as PH increases, its density in the DF case first rises rapidly and then reaches a saturation value at PH = 50 W, which is smaller than that (90 W) in the SF case. Moreover, for the DF discharge operated in gas mixtures (5% and 10% oxygen in argon), it is observed that the metastable state density with increasing PH reaches a saturation value at PH > 110 W. The metastable state density in the DF case is higher than that in the SF case, although the electron density behavior is different. These will be discussed in detail in Subsection 3.3.

3.3. LF power effect

In Subsection 3.2, it has been seen that the variation of metastable state density in the DF case with PH is somewhat different from that in the SF case. In order to analyze their difference, we compare the dependence of the metastable state and electron densities on PH in the pure argon discharge driven by SF source with that by DF sources in Fig. 6. Note that the data shown in Figs. 6(a)–6(c) are exactly the same as in Figs. 4 and 5. The electron temperature shown in Fig. 6(d) is measured by a floating double probe. It can be observed from Figs. 6(a) and 6(b) that the metastable state densities in the DF case rise much faster with PH increasing and come to their corresponding saturation values at a lower PH than in the SF case. The density values of metastable states in the DF case are higher than in the SF case at the same PH. This is mainly because higher electron temperature in the DF case (about 2 eV higher than in the SF case, see Fig. 6(d)) will lead the rate constant kge to increase and, consequently, the more metastable atoms to be produced via electron-impact excitation with ground-state atoms. However, it can be seen from Fig. 6(c) that the electron density in the SF case is higher than that in the DF case (see Fig. 6(c)). This could be due to the fact that in the DF case the fraction of high energy electrons (with energy higher than the excitation threshold of metastable states) is much higher than in the SF discharge because of enhanced electron heating by the LF source, so that the production of metastable atoms could still be improved.

Fig. 6. (color online) Comparison among variations of (a) 1s5 and (b) 1s3, (c) electron densities, and (d) electron temperature with PH in SF and DF discharges operated in pure argon. Note the data in panels (a)–(c) are exactly the same as those in Figs. 4 and 5.

Figure 7 displays the variations of 1s5, 1s3, and electron densities with PL increasing in the DF driven discharges with different gas components (0%, 5%, and 10% oxygen in argon). HF power and working pressure are fixed at 50 W and 13 Pa, respectively. It can be clearly seen from Figs. 7(a) and 7(b) that the densities of 1s5 and 1s3 states each exhibit an evident increase when the LF power of 20 W is applied. However, the metastable densities are almost independent of PL at higher values of PL. This can be explained by a combined effect of increased electron temperature and shrinking plasma bulk length. In the case of the LF power, on the one hand, the electron temperature increases obviously, and thus raises the production rate of the metastable state atoms, which is similar to the results in Fig. 6. On the other hand, the higher PL enlarges the plasma sheath width, and thus the plasma bulk volume shrinks, leading to the decrease of the volume, in which metastable states can be effectively produced via electron impact. Because of the balance between the two factors, the metastable state density remains unchanged as the value of PLvaries.

Fig. 7. (color online) Variations of (a) 1s5, (b) 1s3, and (c) electron densities with LF power PL in DF discharges with different gas components (0%, 5%, and 10% oxygen mixed in argon). Value of PH is fixed at 50 W and working pressure is fixed at 13 Pa.
3.4. Pressure effect

Figure 8 exhibits the dependence of 1s5 and 1s3 states and electron densities on working pressure (from 2 Pa to 28 Pa) in the DF driven discharges with different gas components (0%, 5%, and 10% oxygen mixed in argon). The values of PH and PL are fixed at 100 W and 50 W, respectively. In the pure argon discharge, one can see from the curve with black squares in Fig. 8(a) that 1s5 state density increases rapidly when the pressure is raised from 2 Pa to 16 Pa, and then it decreases slightly with the further increase of pressure. By contrast, when the argon discharge is diluted with 5% oxygen (the curve with red circles in Fig 8(a)), the 1s5 state density increases with pressure at the low-pressure end and then decreases almost linearly after its maximum at p = 6 Pa. When the oxygen content increases to 10%, the 1s5 state density actually shows a linear decrease with pressure, indicating that the maximum density shifts to a lower pressure. Compared with the variation of the 1s5 density with pressure, the variation of electron density shows almost the same dependence on pressure in discharges with different oxygen content. Taking the pure argon discharge for example, the electron density increases rapidly in a pressure range of 2 Pa–6 Pa and then decreases with pressure further increasing.

Fig. 8. (color online) Variations of (a) 1s5, (b) 1s3, and (c) electron densities with working pressure in the DF discharges with different gas components (0%, 5%, and 10% oxygen mixed in argon). Values of PH and PL are fixed at 100 W and 50 W, respectively.

The dependence of the metastable density on pressure could be understood as follows. In the pure argon discharge operated at lower pressure, the groundstate atom density rising with pressure results in the enhancement of the production rate of the argon metastable state due to the increasing electronic excitation rate. Therefore, the metastable density increases rapidly at p < 16 Pa. However, at higher pressures, the electron-neutral collision is intensified, leading to the decrease of the electron temperature and, consequently, the decrease of the electron density. This is the reason why the 1s5 state density comes to a saturation value and even shows a slight decay at higher pressures.

At 5% oxygen content, the variation of 1s5 state density with pressure behaves similarly to that of the electron density. When argon plasma is diluted by 10% oxygen, it seems that argon 1s5 state density reaches a maximum value at p = 2 Pa or 4 Pa, and decreases linearly with pressure increasing, as shown in Fig 7(a). In a pure argon discharge, the changes of 1s5 and 1s3 with the pressure behave quite similarly, while in argon discharge with 5% or 10% oxygen dilution, it seems that the 1s3 density exhibits weaker dependence on pressure than 1s5 density. As the working pressure increases, 1s3 density shows a slight increase at p < 10 Pa, and then a slow decline at higher pressures, which is true for both 5% and 10% oxygen dilution cases.

3.5. Oxygen content effect

Figure 9 shows 1s5, 1s3, and electron densities as a function of oxygen content at three different working pressures (6.7 Pa, 13 Pa, and 20 Pa). The values of PH and PL are fixed at 100 W and 50 W, respectively. From Figs. 9(a) and 9(b), one can clearly see that adding a small fraction of oxygen will significantly reduce the densities of 1s5 and 1s3 states. Taking 1s5 at 20 Pa for example (the curve with blue triangles in Fig. 9(a)), with an addition of 5% oxygen, its density is reduced by 77%, i.e., from 18.50 × 1010 cm−3 to 4.15 × 1010 cm−3. By contrast, the electron density (see Fig. 9 (c)) falls by 19% at 20 Pa when adding 5% oxygen in argon. The dramatic depletion of metastable states can be understood with Eq. (10) in Subsection 3.1. The electron temperature remains almost the same by adding 5% oxygen in argon,[38] and the discharge structure or electric field structure is weakly influenced by the oxygen dilution, so that the electron power absorption mode remains unchanged. Therefore, the production rate of the metastable state is believed to change slightly when only 5% oxygen is added. The sharp fall of the metastable density is mainly attributed to the quenching loss process by oxygen molecules, because the diffusion loss rate kD and quenching loss rate electrons, kqene, show much weaker dependence on the oxygen fraction than the quenching loss rate by the oxygen molecule, which has a relatively high rate coefficient kqO2.

Fig. 9. (color online) Variations of (a) 1s5, (b) 1s3, and (c) electron densities with oxygen content in DF driven discharges at three different gas pressures (6.7 Pa, 13 Pa, and 20 Pa). Values of PH and PL are fixed at 100 W and 50 W, respectively.

With the increase of pressure, it can be seen from Figs. 9(a) and 9(b) that Ar 1s5 and 1s3 densities show more remarkable decreases with oxygen content, indicating that the quenching loss by oxygen molecules is enhanced at a higher pressure. To be more specific, at 6.7 Pa and 13 Pa, with 5% oxygen dilution, the values of 1s5 density decrease by 65% and 75%, respectively. When pressure increases to 20 Pa, the 1s5 density decreases by 77%. This phenomenon is consistent with that in Subsection 3.4.

4. Conclusions

In this paper, we have measured the densities of two metastable states (1s5 and 1s3) in argon and argon/oxygen mixture DF-CCP by using the tunable diode laser absorption spectroscopy. Effects of HF power, LF power, and working pressure on the metastable state density are studied in discharges operated with different gas components (0%, 5%, and 10% oxygen mixed in argon). The dependence of the metastable state density on oxygen content is also investigated at different working pressures. It is found that the metastable state densities exhibit different dependence on HF power in discharges with different content of oxygen added in argon. As HF power increases, the metastable density in the pure argon discharge first increases rapidly and then tends to be saturated at higher HF powers. When the argon plasma is diluted with 5% or 10% oxygen, metastable state density would be saturated at a higher HF power than the case of pure argon plasma. In DF-CCP, LF power is found to weakly affect the metastable state density. With the increase of working pressure, the metastable state density first increases, and then decreases slowly after reaching its peak value. The pressure value at which the maximum metastable density occurs decreases with the increase of the oxygen content. A small fraction of oxygen will greatly deplete the metastable state atoms, and this becomes more significant at a higher pressure due to the enhanced quenching process by oxygen molecules.

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