Spatially resolved ground state atomic oxygen density during the mode transition of inductively coupled oxygen plasmas

Vacuum 164 (2019) 98–104

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Spatially resolved ground state atomic oxygen density during the mode transition of inductively coupled oxygen plasmas

T

Qingxuan Zeng, Hua Jin∗, Songhe Meng, Wei Liang, Hao Shu National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, 150080, People's Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: E/H-mode transition Ground state atomic oxygen density Spatial distribution TALIF

The ground state atomic oxygen density (nOg) and its distribution were systematically studied by two-photon absorption laser induced fluorescence (TALIF) during the discharge mode transition in inductively coupled oxygen plasmas (ICP). After distinguishing discharge mode by experiments, absolute value of nOg under different heating powers/gas pressures were measured. Increasing the heating power in discharges with a total gas pressure of 10, 60 and 80 Pa leads to E→H transition at 0.5, 1.1 and 1.3 kW, respectively. When pressure increased in 10–80 Pa, 0.1 and 2 kW plasmas maintained in E and H mode respectively, and 1 kW plasmas experienced H→E transition at 60 Pa nOg in H-modes were one order of magnitude larger than E-mode. The powerpressure composed mode transition boundaries were obtained through measuring nOg. Spatial distribution of nOg showed that spatial difference of H-mode was more obvious than E-mode. By 0.1 mm spatial resolution TALIF measurement, density gradients of rear 5 mm region in H-mode are one order of magnitude larger than E-mode. These results provide references for processing parameter selection. High spatial resolution TALIF measurement will contribute key technology for acquiring important parameters of surface reactions.

1. Introduction Among all kinds of atomic oxygen (O) species, the ground state O (3P) occupies the great majority [1]. Atomic oxygen in the low temperature plasma has been widely used in research areas such as surface modification [2], etching [3–5], material sputtering [6] and film growth [7,8]. During plasma processing, it is desirable to obtain a high density of ground state O, which could provide advantages in increasing the surface reaction and deposition rates, and thus improving the material performance after plasma processing. Among different types of plasma processing devices, the inductively coupled plasma (ICP) has the advantages of low discharge pressure, high plasma density and uniformity, therefore the behavior of ground state O has been intensively investigated in ICP facilities [9–11]. There are two ways to increase nOg in ICPs. One is to increase the heating power to obtain higher dissociation degree of O2 [12] resulting massive number of dissociated O, but also will increase atomic oxygen temperature and cause unwished extra heating effect on surface. The other is increasing the gas pressure by filling more O2. However, this method has risk that discharge mode will transit from inductive (H) to capacitive (E) mode [13–17]. Under low pressure and high power, ICP is mainly maintained by the induced electric field produced by coil ∗

current. The plasma shows H-mode feature with high plasma density and luminous intensity. With the pressure increase or power decrease to a critical threshold, the electric field which sustaining plasma discharge changes to the electrostatic field generated by capacitive coupling between coils, and the plasma changes to E-mode [18–21], in which the density and luminous intensity become much lower. Changes in plasma behavior during discharge mode transition will inevitably affect nOg. Thus in order to obtain high density of O, the ICPs should stay in Hmode and avoid entering E-mode when adjusting the processing parameters. Therefore, during the processing parameter adjustment and selection, an in-situ, direct non-invasive diagnosis method should be applied to investigate the ICP discharge mode, and especially the variation of nOg. Spectroscopy-based methods have been widely used in non-invasive plasma diagnosis. Among them, the optical emission spectroscopy (OES) and two-photon laser induced fluorescence (TALIF) methods have been applied into ICP discharge mode transition researches. The OES technique was applied in the 1.3 Pa radio frequency (RF) ICP facility by Fuller et al. [22], which shows that nOg increased by two orders of magnitude when the ICP shift from E to H mode. However, the mixed rare gases had serious affected the discharge state. Besides, the applicability hypothesis conditions of the OES-based actinometry method

Corresponding author. E-mail address: [email protected] (H. Jin).

https://doi.org/10.1016/j.vacuum.2019.03.009 Received 21 December 2018; Received in revised form 27 February 2019; Accepted 5 March 2019 Available online 09 March 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.

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The inductance of the coil was about 0.8 μH. Two ends of the coil were respectively connected to the ground and capacitor in the matching box. The matching box consisted of two vacuum capacitors, with capacitance of 100–1000 pF and 10–400 pF, respectively. The matching box was directly connected with the RF power source through a coaxial cable. The frequency of the RF power source was 13.56 MHz. During the E/H-mode transition measurement, for each set of processing parameters, the capacitors in the matching network were adjusted to reach an optimum matching condition. The reflected power were smaller than 5% of the heating power. The heating power could be continuously adjusted from 0 to 2 kW with the resolution of 0.1 kW. For the TALIF measurement, two JGS-1 quartz (extreme UV optical quartz glass with transmittance of 90.1% @ 225.7 nm) windows (70 mm in diameter) were installed symmetrically on the chamber for laser incident and energy measurement, respectively. Another JGS-2 quartz (UV optical quartz glass with transmittance of 99.9% @ 844.6 nm) window (70 × 200 mm2, the same height of top edge with two JGS-1 windows) was installed perpendicularly to the laser direction for fluorescence imaging and acquisition. High purity O2 (purity > 99.999%) was leaked in from top of the chamber with a fixed rate of 0.0338 Pa m3 s−1 (or 20 standard cubic centimeter per minute (sccm)). The gas pressure in the chamber was monitored by a vacuum gauge and a vacuum pump system installed on the bottom of the chamber, the base pressure in the vacuum chamber was 1 × 10−4 Pa. The TALIF diagnosis system mainly consists of a dye laser and a fluorescence detection system. The 355 nm fundamental laser was pumped by a Quanta-Ray YAG laser with a repetition rate of 10 Hz, and then adjusted to 451.4 nm through the Sirah Cobra dye laser (2.26 p.m. bandwidth) using Coumarin 2 dye as the working medium. Finally, the 451.4 nm laser was frequency-doubled to 225.7 nm by a beta barium borate crystal. The 225.7 nm pulse laser was introduced into the plasma chamber by a motorized positioning system (MPS) which consist of three mirrors (reflectance > 95% @ 225.7 nm) and a lens (500 mm focal length with transmittance > 90% @ 225.7 nm). The lens was adjusted to focus at the center position in the chamber. The laser beam diameter at the focus was about 0.15 mm. The laser energy out of the plasma chamber was measured by a Newport 1918-R power meter. After passing the 850 nm filter (17 nm bandwidth), the 844.6 nm fluorescence of ground state O was imaged by a 200 mm focal length lens into the Andor iStar-334T ICCD camera. The fluorescence signal was accumulated over 100 laser pulses to exclude errors from fluctuations among different laser pluses. The dye laser and ICCD camera were synchronized by a digital delay generator (DDG, Stanford Research Systems DG645). Two MPSs controlled by a synchronous control unit (SCU) were used to adjust the vertical height of optical path of incident laser and fluorescence imaging simultaneously. The minimum

are often difficult to satisfy, which brought uncertainties to the final results. Using similar method, Kregar et al. [23] measured the spatial distribution of nOg in a 15 Pa oxygen ICP and shows that nOg were 1.3 × 1021 m−3 and 4 × 1018 m−3 in H and E-mode, respectively. Whereas the unknown electronic temperature brought certain discrepancy, especially in the area far away from coil. Besides, the results show that nOg varied around 50% over 1 cm spatial range, it would be better if the research could give results with higher spatial resolution than 1 cm. With better selectivity and resolution than OES, the TALIF method has been implemented to direct measure nOg in oxygen ICP by Corr et al. [12]. The 3.3 Pa ICP changed from E to H-mode when the power changed from 27 to 220 W. As a result, nOg increased from 1018 to 1020 m−3. Unfortunately, the author only studied in low pressures of 0.5–6.7 Pa, and there was no results of changing process of nOg during mode transition. In fact, research on the latter has been relatively rare, not to mention by direct measurements. Many of the existing nOg relevant researches only concerned pressures lower than 15 Pa, while plasmas with pressure of dozens of Pascal are more widely used in surface processing [24,25]. Moreover, as far as the authors know, there has been very little research on the effect of E/H-mode transition on spatial distribution of nOg. The objective of this paper is to explore how the E/H-mode transition influence nOg and its spatial distribution in the pure oxygen ICP. Through changing the heating power and pressure, the E/H-mode transition under different parameters are preliminary distinguished. The absolute value of nOg during mode transitions are direct measured by TALIF, and based on nOg the critical pressure/power thresholds at which E/H-mode transition happen are presented. Besides, the spatial distribution of nOg in both E and H-mode are obtained and the 0.1 mm high spatial resolution TALIF measurement is carried out. These results offer references for plasma processing parameter selection, extend the understanding of ground state atomic oxygen density behaviors in different discharge mode in ICP and provide key technology for acquiring important parameters of surface reactions.

2. Experimental method Fig. 1 shows the schematic diagram of the experimental setup of TALIF measurement in the pure oxygen RF-ICP and the relevant energy level transition diagram [26]. The ICP chamber was a water-cooled stainless-steel cylinder with inner diameter of 300 mm and height of 463 mm. A 20 mm thick and 270 mm diameter quartz plate was mounted on the top to separate the induction coil and plasma chamber. The two-turn water-cooled planar induction coil was made of a 6 mm diameter hollow copper tube and placed 14 mm above the quartz plate.

Fig. 1. Diagram of the experimental setup of atomic oxygen TALIF measurement and the relevant energy level transition. 99

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Fig. 2. Atomic oxygen measured by TALIF: (a) original 844.6 nm fluorescence signal captured by ICCD camera; (b) integral curve by laser wavelength scanning.

end of titration is [30],

displacement was 0.1 mm. Throughout the measurement, the dye laser wavelength scanning and ICCD signal capturing were automatically controlled by a self-programmed LabVIEW-based Virtual Instrument (VI) System. The characterization of nOg by TALIF has been described in detail in many articles [26–28], so here only a brief introduction is presented. As shown in the inserted energy level diagram shown in Fig. 1, ground state O of level J” = 2 is excited by two 225.7 nm photons via 2p4 3P2→ 3p3P1,2,0 transition, and the 844.6 nm fluorescence signal emitted through 3p3P1,2,0 → 3s3S transition is detected. The laser pulse energy is fixed to about 0.25 mJ to satisfy the quadratic dependence between laser pulse energy and fluorescence signal [29]. The qualitative value of nOg is proportional to the spectra integrals of the TALIF signal [28],

nO ∝

∫

SO (λ ) dλ Ep2

nOg =

ϕtot

ntot ≅

(T ) ϕNO 2

p ϕtot kB Tg

(3)

the NO2 flux at the titration endpoint, ϕtot the total gas flux, with ntot the total atomic density in gas, p the gas pressure, kB = 1.3806 × 1023J·K−1 the Boltzmann constant, and Tg the gas temperature. In low temperature ICP, the gas temperature Tg was approximately equal to the translational temperature of the ground state O, TOg, which can be measured by TALIF and calculated through [31], (T ) ϕNO 2

TOg =

MO c 2 × [(ΔλICP )2 − 2(ΔλL )2] 8ln2kB nA λ 02 −3

(4)

−1

is the molar mass of O, where MO = 15.999 × 10 kg mol c = 2.998 × 108 m s−1 is the speed of light, nA = 6.0221 × 1023 mol−1 is the Avogadro's constant, λ 0 is the center wavelength of the Gaussian wavelength scanning curve (nm), ΔλICP is the full width at half maximum (FWHM) of the Gaussian wavelength scanning curve (pm). The laser linewidth ΔλL = 2.26 p.m. was determined by fitting a scanning curve at a given temperature of 824 K [31]. The experiments in this study were conducted five times to obtain credible results. The uncertainties in nOg were mainly from the 2400line grating of the Sirah dye laser oscillator with the bandwidth of 2.26 p.m., which resulted in an uncertainly about 15%–20% of the final absolute values of nOg.

(1)

where SO is the intensity of the TALF signal at a certain laser wavelength λ , Ep is the laser pulse energy. The fluorescence signal captured by ICCD camera and the integral curve obtained by laser wavelength scanning are shown in Fig. 2a and b, respectively. In order to convert the integrated TALIF signal to the absolute density, the proportional relation between the TALIF signal and nOg was obtained by NO2 titration [30] through the chemical reaction of: O + NO2 → NO + O2

(T ) ϕNO 2

(2)

Small flow of NO2 gas was introduced from the bottom of chamber. The positions of NO2 outlet and TALIF detection position were 300 and 350 mm below the top quartz window, respectively. The 50 mm between them was for fully mixing and reaction of O and NO2. According to Dalton's law, the relationship between nOg and the NO2 flux at the

3. Results and discussion The different discharge modes in the pure oxygen ICPs were preliminary distinguished by plasma discharge experiment. Fig. 3a shows the plasma photos of four environmental parameter combinations

Fig. 3. Oxygen plasmas in different discharge modes: (a) Plasma photos of 0.1kW/10Pa, 1kW/10Pa, 1kW/80Pa and 2kW/80Pa; (b) Axial profiles of 844.6 nm optical emission spectroscopy (OES) intensities. The inserted figure shows enlarged curves of 0.1kW/10Pa and 1kW/80Pa. All data have errors within 5%. 100

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(heating power/gas pressure): 0.1 kW/10 Pa, 1 kW/10 Pa, 1 kW/80 Pa and 2 kW/80 Pa, and the 844.6 nm OES intensity spatial distribution of excited-state oxygen atoms are shown in Fig. 3b. The photos were taken from the outside of the laser output window (the right JGS I window shown in Fig. 1). Two circles in each photo are the two opposite JGS I windows in Fig. 1, and the white band at the top of 1kW/10Pa (or 2kW/ 80Pa) plasma photo is the quartz plate. The coordinate axis corresponds to the central axis of the chamber, which is marked in Fig. 1. The starting point of the axis (z = 60 mm), which at the same height with the top edge of the window, corresponds to 60 mm below the bottom surface of quartz plate. The measured values at z = 60 mm were somewhat blocked by the top edge of window and therefore cannot offer true information. As shown in the photos, the brightness of the right two plasmas are obviously greater than left, and this visual difference is clearly reflected by the OES intensities. The inserted figure in Fig. 3b shows the enlarged curves of 0.1kW/10Pa and 1kW/80Pa. The OES intensity errors are smaller than 5%. The OES intensities of the four plasmas decrease steadily with distance. For plasmas at pressure of 10 Pa, the largest intensity increased from 1.26 × 102 at 0.1 kW to about 3.7 × 103 at 1 kW. This increment is even greater for 80Pa plasmas, from 1.44 × 102 at 1 kW to about 5.04 × 103 at 2 kW. Although the OES intensities drop much lower at z = 120 mm, the values of 1kW/10Pa and 2kW/80Pa are still reach more than two times than 0.1kW/10Pa and 1kW/80Pa, respectively. It should be mentioned that, the NO2 titration experiments were conducted in the 1kW/10Pa plasma. From the results shown in Fig. 3a, the OES intensity at z = 120 mm is about 10 times smaller than the maximum value. Therefore, the introduced NO2 gas at z = 300 mm had little effect on the initial oxygen plasma. The effect of E/H-mode transition on nOg was directly measured by TALIF diagnosis. The measured position is 120 mm below the quartz plate on the central axis. Fig. 4 shows the variation of nOg with power under pressures of 10 Pa (black square), 60 Pa (red circle) and 80 Pa (blue triangle). The measured nOg value in this work are comparable to those simulated results in literatures. For 0.5kW/10Pa plasma, the measured value of 1.13 × 1020 m−3 here is slight less than ∼1.97 × 1020 m−3 given by Toneli et al. [32], but close to ∼1.17 × 1020 m−3 given by Gudnumdsson et al. [33], both of which were calculated in the plasma of 0.5kW/13Pa. For 1kW/10Pa plasma, the TALIF measured value of 2.89 × 1020 m−3 is in the same order of magnitude as the value of ∼9.55 × 1020 m−3 calculated by Gudnumdsson et al. [34] in a 1kW/8Pa plasma, and the difference may be

Fig. 5. Variation of atomic oxygen density (nOg) with pressure under powers of 0.1 kW (black square), 1 kW (red circle) and 2 kW (blue triangle). The range of nOg in E and H-mode plasmas are roughly distinguished by two dotted black lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

due to the different flow rate. Increasing the heating power in these three plasmas, nOg first goes through a period of slow growth in E-mode, then suddenly surge at a specific power, following by the steady growth in H-mode. The values of nOg in H-modes are generally one order of magnitude larger than in E-modes. The E/H-mode transition happens at 0.5, 1.1 and 1.3 kW for pressures of 10, 60, and 80 Pa, respectively. The trend of transition power threshold increases with pressure is consistent with literature [14]. The results of nOg show no power interval in which the plasma in the E/H-hybrid mode, as mentioned by Wegner et al. [35]. This may be due to the much lower resolution of the heating power adjustment in this work (0.1 kW) than in the literature (1 W). Nonetheless, the variation trend of nOg during the E to H mode transition is consistent with the measured results given by Corr et al. [12] and the calculated results given by Meichsner et al. [21]. As shown in Fig. 4, nOg decreases with pressure no matter in E or Hmode. This trend has been shown more clearly in Fig. 5, in which the variation of nOg with pressure from 10 to 80 Pa under powers of 0.1 kW (black square), 1 kW (red circle) and 2 kW (blue triangle) are shown. At power of 0.1 kW, the plasmas stay in E-mode for the entire pressure range. The value of nOg decreases from 1.58 × 1019 m−3 at 10 Pa to 1.39 × 1019 m−3 at 80 Pa with no dramatic change. Similar change also appears in the H-mode plasmas with power of 2 kW. Besides the slight increase at 20 Pa, nOg decreased from 7.24 × 1020 m−3 at 10 Pa to 4.62 × 1020 m−3 at 80Pa, also with no sudden change. As far as the pressure range studied here is concerned, the plasmas with power of 0.1 and 2 kW are always maintained in E and H-mode, respectively. In general, for the same pressures, nOg at 2 kW are more than one order of magnitude larger than 0.1 kW. Different from these two powers, the unique change appears in the plasma of 1 kW. In lower pressures of 10–50 Pa, the plasmas stay in Hmode and nOg decreases with pressure from 2.89 × 1020 m−3 to 1.57 × 1020 m−3. When the pressure rises above 50 Pa, nOg decreases by almost an order of magnitude, from 1.57 × 1020 m−3 to 2.32 × 1019 m−3 at 60 Pa. The sudden decrease of nOg clearly indicates that the plasma has experienced H to E mode transition between 50 and 60 Pa. It should be noted that the variation trends of nOg with pressure are different from the literature. Corr et al. [12] and Gudmundsson et al. [33] had obtained the increasing trends of nOg with pressure, both for the E and H-mode. In this paper, however, except for the slight increment from 10 to 20 Pa at power of 2 kW, nOg decreases with pressure for both E and H-mode. The different trends can be attributed to the different pressure ranges focused in literature and this work. Corr et al.

Fig. 4. Variation of atomic oxygen density (nOg) with power under pressures of 10 Pa (black square), 60 Pa (red circle) and 80 Pa (blue triangle). The range of nOg in E and H-mode plasmas are roughly distinguished by two dotted black lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 101

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[12] and Gudmundsson et al. [33] focused on the low pressure lower than 13.3 Pa, lower than the range of 10–80 Pa in this paper. This difference will lead to different dissociation process of O2 in plasma. According to the kinetic model proposed by Kim [36], the dissociation rate at low pressure is very high and increases with the pressure, especially for the high heating powers. This leads to an increase in atomic density with pressure, as shown by the results of nOg in Ref. [12], and the results of 10–20 Pa at heating power of 2 kW in Fig. 5. However, when the pressure is higher, the dissociation rate significantly decreases with increased pressure, and the source of O(3P) gradually changed from molecular oxygen dissociation to the energy transfer reaction [19]. As a result, the higher the gas pressure, the less dissociated O produced, as shown by the overall trend in Fig. 5. Beside, O(3P) is mainly annihilated by recombination at wall of the stainless steel chamber. The sticking coefficient (γO ) of O(3P) only decreased about 2.9% when pressure increased from 10 to 80 Pa [21]. For these reasons, the decrease trends of nOg with increased pressure was obtained. The above results clearly indicates that the discharge mode transition will change nOg more than one order of magnitude. Besides, the discharge mode transition threshold will change with the processing parameters. Therefore, in order to obtain the high density of O by selecting the appropriate processing parameters, the most basic thing that should be clear is the parameter threshold for the occurrence of the discharge mode transition. The transition threshold was obtained by directly and systematically measurement of nOg in a broader processing parameter range. Fig. 6 shows the result of E/H-mode boundaries composed of heating power and gas pressure. The areas below the dashed line and above the solid line correspond to the E and H-mode, respectively. It should be noted that these two boundaries are obtained by measuring nOg with gradually increasing the power at the specific pressures, and may be different with the situation of decreasing powers. This is because as the power decrease, the hysteresis of the H to E-mode transition may happen [37], resulting in the transition threshold different with the power increasing. It should be also explained that due to the power and pressure adjusting resolution used here (0.1 kW and 10 Pa, respectively), there is a middle area between the two boundaries, as shown in Fig. 6. Based on the current experimental results, it is difficult to determine which mode this middle area belongs to. However, it can be speculated that the precise discharge mode transition point must located in this middle area, including the two boundaries. Besides, by adjusting the power and pressure with much higher resolution, such as 1 W in literatures [17,18], the E/H-hybrid mode might be measured. Nevertheless, these boundaries and areas will provide references for plasma parameter selection of different discharge modes in ICPs.

Fig. 7. Spatial distributions of atomic oxygen density (nOg) in four plasmas: (a) Variation of nOg with distance; (b) Variation of the normalized values (ñOg) with distance, two E-mode curves are inserted in the top right corner.

In addition to the interest in the variation of nOg during the ICP discharge mode transition, it is also very concerned how the transition affect the nOg spatial distributions. Because it is the particle density spatial distribution, more specifically the density gradient near the surface that significantly affect the plasma-surface reactions [38,39]. Fig. 7 shows the spatial distribution of nOg in 65–120 mm range of the four plasmas shown in Fig. 3 measured by TALIF with 5 mm spatial resolution. As shown in Fig. 7a, for the entire space, the nOg in H-mode are more than one order of magnitude larger than in E-mode for the same pressure of 10 or 80 Pa. The spatial difference of nOg in H-mode are more obvious than that in E-mode. To better see the spatial variation trends, the absolute values of nOg are normalized (labelled as ñOg) at the end point of z = 120 mm as shown in Fig. 7b. It can be clearly seen that the spatial variation of ñOg is so different for two discharge modes. In E-modes no obvious difference exists between 85 and 120 mm, and only slight increment of ñOg exist from 85 to 65 mm, as shown in the inserted curves in Fig. 7b. Since the overall densities are at a very low level in E-mode, the spatial difference is very small, and the maximum value of ñOg under 10 and 80 Pa only increased about 0.15 and 0.4 times, respectively, compared to the end point of z = 120 mm. In H-mode, however, the maximum values increase significantly to about 5 and 13 times under 10 and 80 Pa, respectively. In addition, more obvious spatial distribution differences have appeared. Large number of the oxygen atoms aggregate in the beginning area. Therefore, for both of the two H-mode plasmas, most oxygen atoms concentrate in the approximate area between 65 and 80 mm that closest to the discharge coil. This approximate area is marked as Discharge Zone

Fig. 6. E/H-mode boundaries composed of power and pressure. 102

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(I), as shown in Fig. 7b. The ñOg distribution of two H-mode plasmas is also quite different. The aggregation phenomenon of atomic oxygen in Zone (I) is more remarkable for high pressure plasma, but as the distance increases, the decrease rate of ñOg is also much faster. This distribution feature is attributed to the atomic oxygen motion behavior. Considering the movement of atomic oxygen in plasma, the relationship between the mean free path (λ O ) and gas temperature (Tg ) and pressure ( p ) is,

λ O ∝ ng−1 ∝ Tg p−1

(5)

in which ng is the number density of gaseous species. For low temperature plasma, the gas temperature (Tg ) can be approximated by atomic oxygen temperature (TO ). For 1kW/10Pa and 2kW/80Pa plasmas, the value of TO at z = 70 mm were determined to be 636 K and 700 K, respectively, by the method of TALIF of Eq. (4). For the 1kW/ 10Pa plasma, the gas temperature value of 636 K in this work is close to the value of 600 K in the 10Pa H-mode plasma by Wegner et al. [35], but lower than the value of 900 K in the 0.5kW/10.6Pa plasma by Foucher et al. [40]. This temperature discrepancy might be due to the lower flow rate in this work and the different measurement principles. In this way, at the position of z = 70 mm, the mean free path of O in 1kW/10Pa plasma is about 7.27 times that in 2kW/80Pa plasma. Furthermore, according to Chapman-Enskog equation [41], the atomic oxygen diffusion coefficient (DO ) has relationship of,

DO ∝

Tg3/2 p−1

Fig. 8. Spatial distribution of normalized atomic oxygen density (ñOg) by 0.1 mm resolution TALIF measurement in 115–120 mm. Table 1 Gradient and R2 of linear fittings of normalized atomic oxygen density (ñOg) in 115–120 mm.

−1

(6)

Gradient (-mm R2

Thus it can be calculated that DO in 1kW/10Pa plasma is about 6.93 times that in 2kW/80Pa plasma. Therefore, the mean free path and diffusion coefficient of 2kW/80Pa plasma are much smaller than 1kW/ 10Pa, which causes the free movement or diffusion of O in 2kW/80Pa are very limited. Thus it appears in the spatial distribution that 2kW/ 80Pa plasma shows a much higher decline rate of ñOg, resulting the lower value of ñOg in the range of 80–105 mm. This approximate range is marked as Diffusion Zone (II) in Fig. 7b. The results of spatial distribution of ñOg are consistent with the statement that plasma tends to shrink toward the high electric field when the pressure increases, especially for the plasma composed of high electron loss gases such as oxygen [39]. Finally, in the range of 105–120 mm, ñOg of the two Hmode plasmas decrease to a very low level with similar tendencies. The research of Fletcher et al. [38] on surface catalytic recombination has shown that, it is nOg and its spatial gradient in a few millimeters near the surface that determines the reaction rate of O on material surface. The results shown by Fig. 7 have indicated that nOg will change dramatically within a few millimeters of space, especially in the Discharge and Diffusion Zones. Therefore, obtaining the high spatial resolution information of nOg is the key to characterize the reaction rate of O on the material surface in the future. It should be mentioned that, since the TALIF signal is dominated by the central portion of the Gaussian laser intensity profile [38], the slightly larger laser beam (∼0.15 mm) will not significantly degrade the spatial resolution of 0.1 mm. Therefore, the 0.1 mm spatial resolution TALIF measurement is implemented in the range of 115–120 mm (marked as Gas-Solid Interaction Zone (III) in Fig. 7b). This spatial range is selected because during the gas-surface interaction experiments, the specimen surface are often fixed at z = 120 mm in order to exclude the effect of electromagnetic field as much as possible. The absolute values of nOg are normalized at z = 120 mm and shown in Fig. 8. It can be clearly seen that the spatial variation of H-mode is more pronounced than E-mode. Fig. 8 also shows the linear fittings of the results. The gradient and coefficient of determination (or R2) of the fitting lines are listed in Table 1. In general, the gradient of ñOg in H-mode are one order of magnitude larger than E-mode. For the H-mode plasmas, the higher pressure results in a greater density gradient. Although nOg in Zone (III) is much lower than others, there is still an obvious density gradient in the H-mode plasma, which may have a significant effect on the material

)

0.1 kW/10 Pa

1 kW/10 Pa

1 kW/80 Pa

2 kW/80 Pa

0.00259 0.79

0.01783 0.89

0.00253 0.73

0.02711 0.95

surface located here. In the future study of O-surface interactions, the high spatial resolution measurement will provide the valuable atomic information in the near surface space above different materials. Based on this advantage, the key parameters such as reaction rate can be determined.

4. Summary In this work, the ground state atomic oxygen density (nOg) and its spatial distribution during E/H-mode transition in the inductively coupled oxygen plasmas were systematically studied. The plasma discharge mode was preliminary distinguished by plasma discharge experiment and the optical emission spectrum. Results have shown that the 0.1 kW/10 Pa and 1 kW/80 Pa plasmas were in E-mode with low level of brightness and the 844.6 nm OES intensities (IOES). While the 1 kW/10 Pa and 2 kW/80 Pa plasmas were in H-mode with high levels of brightness and IOES. The absolute value of nOg measured by TALIF has shown that with power increased in 0.1–2 kW, the 10, 60 and 80 Pa plasma experienced the E→H transition at power of 0.5, 1.1 and 1.3 kW, respectively. When pressure increased in 10–80 Pa, the 0.1 and 2 kW plasmas stayed in E and H discharge mode, respectively, while 1 kW plasma underwent the H→E transition happened at 60 Pa. Generally, nOg in H-mode was more than one order of magnitude higher than E-mode. Based on the variation of nOg, the E/H-mode boundaries composed of power and pressure were obtained. Compared to E-mode plasmas, the spatial differences of nOg in H-mode were much more obvious. Atomic oxygen aggregation near the discharge coil, and the spatial variation determined by skin effect, mean free path and diffusion coefficient could be clearly seen. Results of the 0.1 mm spatial resolution TALIF measurement in the rear 5 mm region indicated that the density gradients in H-mode were one order of magnitude larger than E-mode. The results in this paper provide references for plasma parameter selection, and the high spatial resolution TALIF measurement will contribute key technology for acquiring important parameters of surface reactions.

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Acknowledgements This work was supported by the Heilongjiang Postdoctoral Research Foundation [grant numbers LBH-Q16095], the National Natural Science Foundation of China [grant numbers 11502058], the Fundamental Research Funds for the Central Universities [grant No HIT.NSRIF.201823] and the National Key Laboratory Fund.

[20]

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