Time-Resolved Optical Emission Spectroscopy Diagnosis of CO 2 Laser-Produced SnO 2 Plasma

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1 Plasma Science and Technology, Vol.18, No.9, Sep Time-Resolved Optical Emission Spectroscopy Diagnosis of CO 2 Laser-Produced SnO 2 Plasma LAN Hui ( ) 1,3, WANG Xinbing ( ) 2, ZUO Duluo ( ) 2 1 School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan , China 2 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan , China 3 School of Physics and Information Engineering, Jianghan University, Wuhan , China Abstract The spectral emission and plasma parameters of SnO 2 plasmas have been investigated. A planar ceramic SnO 2 target was irradiated by a CO 2 laser with a full width at half maximum of 80 ns. The temporal behavior of the specific emission lines from the SnO 2 plasma was characterized. The intensities of Sn I and Sn II lines first increased, and then decreased with the delay time. The results also showed a faster decay of Sn I atoms than that of Sn II ionic species. The temporal evolutions of the SnO 2 plasma parameters (electron temperature and density) were deduced. The measured temperature and density of SnO 2 plasma are 4.38 ev to 0.5 ev and cm 3 to cm 3, for delay times between 0.1 µs and 2.2 µs. We also investigated the effect of the laser pulse energy on SnO 2 plasma. Keywords: optical emission spectroscopy, laser produced plasma, CO 2 laser, electron temperature, electron density PACS: Mf, Jm, Kz DOI: / /18/9/05 (Some figures may appear in colour only in the online journal) 1 Introduction Laser-produced plasma is considered as one of the most promising technologies for next-generation extreme ultraviolet (EUV) lithography [1 3] since it has several advantages such as the debris control, high EUV conversion efficiency (CE), and so on. Cai et al. [4] studied the EUV radiation from a pure Sn target using Nd:YAG laser, and they found the EUV CE was over 1.6%. The two most challenging issues in the development of EUV sources are the in-band CE and debris emitted from the plasma. The debris limits the lifetime of the Mo/Si multilayer collector. In regard to this, to increase the EUV CE and decrease the debris, the Sn-based target gets more focus as the EUV source [5,6]. Pan et al. [7] investigated the EUV emission from SnO 2 nanofibers mat (i.e. the SnCl 4 concentration is 5%) using a Nd:YAG laser and found that the EUV emission intensity was 15% higher than that of the Sn plate at the laser power density of W/cm 2. Higashiguchi et al. [8] used (6%) SnO 2 as a target and a Nd:YAG laser as a main laser pulse. They found that the observed maximum EUV CE was 1.2% and the ion debris was significantly reduced. The optical spectrum characterization of laser-produced plasma, which can provide information on the plasma ionization balance, rate processes and the densities and temperatures, also had been investigated by many authors [9,10]. Gao et al. [11] investigated the spectroscopic emission of Fe plasma generated by Nd:YAG laser. Shen et al. [12] investigated the plasma parameters of the double-pulse-laser-produced Ni plasma. Besides, laser-produced Sn plasma had been investigated using the µm Nd:YAG [13,14] and 10.6 µm CO 2 laser [15,16], respectively. However, few works have been devoted to the analysis of the plasma characteristics of the Sn-based target. Also the research on the optical emission from CO 2 laserproduced SnO 2 plasma was not studied in detail. In this paper, the optical emission features from a ceramic SnO 2 target irradiated by a CO 2 laser with a pulse width of 80 ns (FWHM) at the wavelength of 10.6 µm have been investigated. Firstly, timeresolved measurements of the emission spectra of SnO 2 plasma in visible spectral regions have been observed. Meanwhile, the temporal evolution of line intensities has been investigated. Electron temperature is calculated using the Boltzmann plot method with the line intensities, while electron density is calculated using the method of Stark broadening. Furthermore, the electron temperature and density as a function of laser pulse energy have been analyzed. supported by National Natural Science Foundation of China (No ) and the Director Fund of WNLO 902

2 LAN Hui et al.: Time-Resolved Optical Emission Spectroscopy Diagnosis of CO 2 Laser-Produced SnO 2 Plasma 2 Experimental setup The schematic of the experimental setup is given in Fig. 1. The vacuum degree of the chamber can be achieved to 10 3 Pa. The laser used in our experiments was a transversely excited atmospheric CO 2 laser. The CO 2 laser pulse duration was 80 ns (FWHM) with the output energy of 600 mj by adjusting the pressure ratio of the working gas to CO 2 : N 2 : He = 1 : 1 : 4. The laser beam was focused onto the target with 45 o to the target surface normal through a ZnSe lens. The estimated spot size at the target surface was 300 µm and the maximum power density at the target surface is W/cm 2. The 5 mm thick and 100 mm diameter planar slabs of pure ceramic SnO 2 were used as target material. High purity of SnO 2 in the sample was confirmed by XRD pattern of the target, which was more than 99.5%. The target was mounted on the mechanically rotated XY translational stage, which provided fresh surface exposure for each measurement. Fig.1 Schematic of the experimental setup The emission spectra emitted from the plasma were collected by an optical fiber normal to the direction of plasma expansion. The optical fiber was connected to the entrance slit of a Princeton SP2750i spectrograph with a focal length of 750 mm, equipped with 300 grooves/mm grating. The maximum resolution of the spectrograph was nm with a slit width of 20 µm. The ICCD system was a PI-MAX-1300 (Princeton Instruments), which was installed at the exit of the spectrograph. It was used to track the spectrum Table 1. evolution of the plasma by controlling the delay time of the image acquisition. 3 Results and discussion 3.1 Optical emission spectra All the lines visible in the nm spectrum are collected and the time evolution spectra from laserproduced SnO 2 plasma are observed in two spectral regions ( nm and nm), at the fixed gate width time of 30 ns. The delay time ranged from 0.1 µs to 2.2 µs. The experiment has been investigated at 10 3 Pa and the laser pulse energy of 600 mj. The strongest lines of O are observed at nm, nm, nm, nm and nm. The other strong lines of Sn are at nm, nm, nm, nm and nm, respectively. All the atomic/ionic line positions and relative intensities of O I, O II, Sn I, and Sn II are listed according to NIST atomic spectral database [17] and given in Table 1. It obviously shows that the emission spectrum of SnO 2 plasma in the range of nm, in which the central wavelength is set to 380 nm, mainly consists of O II lines (see in Fig. 2(a)). The emission spectrum of the plasma reveals that Sn I and Sn II lines are mainly observed between 500 nm and 600 nm (see in Fig. 2(b)). When the laser irradiates the target surface, the strong continuum emission of plasma is observed at the initial state of the plasma (0.1 µs), which results from the processes of the bremsstrahlung and electron ion recombination. The continuum rapidly decreases with the delay time and the line spectrum dominates the whole spectrum gradually with increasing time delay. At a time of 0.3 µs, the intensity of the continuous emission is weakened and the line emission dominates the whole spectrum. The line radiation is a consequence of fluorescence emission of excited atoms and ions. The line spectrum experiences a significant increase and then decreases through the process as the time delay varied from 0.1 µs to 1 µs due to the recombination. Spectroscopic parameters of the emission lines of SnO 2 plasma (NIST data) Wavelength Transition Statistical weight Transition probability Energy of the upper level λ (nm) g i g k A (10 7 s 1 ) E k (kv) O s 3 P 2 3p 3 D p 2 P 1/2 4s 2 P 1/ p 4 P 1/2 3d 4 P 1/ p 4 P 5/2 3d 4 P 5/ p 3 P 2 3d 3 P Sn p6s 3 p 1 5p 1 D d 2 D 3/2 6p 2 P 1/ d 2 D 5/2 6p 2 P 3/ f 2 F 2/5 5d 2 D 3/ f 2 F 7/2 5d 2 D 5/

3 Fig.2 The emission spectrum of SnO 2 plasma generated by CO 2 laser at delay time: µs; (a) In nm spectral region, (b) In nm spectral region In order to better understand the detailed aspects of their optical emission, the temporal evolution of the optical emission lines from SnO 2 plasma has been investigated. Fig. 3 displays the normalized lines intensity of SnO 2 plasma produced by CO 2 laser pulses. As shown in Fig. 3, the intensity rapidly rises at first, reaches the maximum, and then decreases with the delay time. The emission intensity of Sn I ( nm) is found to reach the maximum at 0.4 µs, while that of Sn II ( nm) reaches the maximum value at the delay time of 0.5 µs. The delay between Sn I and Sn II species is probably due to different formation mechanisms. Fig.3 Emission intensity evolution of Sn I nm and Sn II nm with delay time Plasma Science and Technology, Vol.18, No.9, Sep Plasma parameters The plasma parameters of electron temperature (T e ) and electron density (N e ) can give vital information on the characteristics of spectra. It is difficult to estimate the plasma parameters at early times, just as most of the emission is the continuum radiation, which is the result from the bremsstrahlung and photo recombination radiation. As time goes on, the temperature and density can be estimated with the lines for the entire duration of the plume. The criterion for local thermodynamic equilibrium (LTE) is that the population densities of atomic and ionic electronic states are distributed according to the Boltzmann distributions. The electron temperature of LTE plasma can be calculated by using lines of information of the same atomic or ionic species, which is expressed by [18] : ( ) Imn λ mn ln = E m + ln( N(T ) ), (1) g m A mn k B T e U(T ) where λ mn, A mn and g m are the wavelength, the transition probability and the statistical weight of the upper level, respectively. E m is the upper level energy, T e is the electron temperature, k B is the Boltzmann constant, U(T ) is the partition function and N(T ) is the total number density of neutrals. The electron temperature is estimated from the Boltzmann plot of the nm, nm, nm and nm line intensities of Sn II. The spectroscopic parameters of these lines are obtained from NIST Atomic Spectra Database [17] and also given in Table 1. The electron density are estimated using Starkbroadened profiles of Sn II line at nm and Sn I line at nm, respectively, by [18] : λ 1/2 = 2ω( n e ). (2) 1016 In Eq. (2), ω is the electron impact width parameter and can be found using reference literature [19]. N e is the electron density (in cm 3 ). We ignore the effect of self-absorption on the line profile because resonance lines are not considered. The line broadening is mainly determined by the Stark broadening, the instrumental broadening and the Doppler broadening. The instrumental line broadening is nm, which is determined by the standard lamp measurement. The Doppler broadening is due to Doppler shifts (i.e., λ D = λυ/c, where λ is the wavelength of the Sn line spectra, c is the speed of light in vacuum) experienced by the ion velocity υ in the direction of observation, which changed with the delay time. The measured line broadening is corrected by subtracting the contribution of instrumental line broadening and the Doppler broadening. The temporal evolution of electron temperature (T e ) and density (N e ) of SnO 2 plasma are shown in Figs. 4 and 5. From Fig. 4, it was observed that the electron temperature decreases from (4.2 ± 0.42) ev to 904

4 LAN Hui et al.: Time-Resolved Optical Emission Spectroscopy Diagnosis of CO 2 Laser-Produced SnO 2 Plasma (0.5 ± 0.11) ev, for delay times between 0.1 µs and 2.2 µs. Fig. 5 shows the temporal evolution of the electron density from both Sn I ( nm) and Sn II ( nm) lines for SnO 2 plasma. The electron density of both lines has decay trends. From the Sn I line, it is observed that the maximum value of electron density is (1.32 ± 0.5) cm 3 at delay time of 0.1 µs, whereas the maximum value of electron density is (1.14 ± 0.43) cm 3 at delay time of 0.4 µs from the Sn II line. Fig.4 Temporal evolution of the electron temperature of SnO 2 plasma produced by the laser of 600 mj. The air pressure in the chamber is maintained at 10 3 Pa. The ICCD gate width had a constant value of 30 ns means the kinetic energy of the SnO 2 plasma is higher than that of Sn plasma. However, the conclusion of the plasma parameters (electron temperature and density) is slightly different from that of their results, which is probably due to the fact that we measure time-resolved spectra of the entire space. Besides, in Cummings et al. [20], Fig. 5 showed that the electron temperature range of the SnO 2 plasma was wider than those of the Sn plasma, therefore, for the entire space, the SnO 2 plasma parameters may be slightly higher than Sn plasma. Moreover, the difference of SnO 2 and Sn plasma parameters is attributed to the thermodynamic properties of the target material, which directly affects the energy absorbed by the plasma from the incident laser pulse. Generally, the absorptance of metals decreases with increasing wavelength, and the absorptance of oxides increases drastically with increasing wavelength [21]. So, with the 10.6 µm wavelength of the CO 2 laser, the absorptance of SnO 2 is much higher than that of Sn. The incident laser pulse energy has a significant influence on the electron temperature and density of laser-produced SnO 2 plasma. Fig. 6(a) and (b) show the temporal evolution of electron temperature (T e ) and density (N e ) of SnO 2 plasma generated by the different incident laser pulses. Fig.5 Temporal evolution of the electron density of SnO 2 plasma produced by the laser of 600 mj. The air pressure in the chamber is maintained at 10 3 Pa. The ICCD gate width had a constant value of 30 ns To better understand the SnO 2 plasma parameters, we have also performed the experiment with Sn plasma under the same experimental conditions. Using the time-of-flight (TOF) method, we derived the ion kinetic energy of plasmas. It is found that SnO 2 plasma has larger kinetic energy when compared with the Sn plasma, which corresponds to KEs of 3.25 kev for SnO 2 plasma and 1.84 kev for Sn plasma. Moreover, the electron temperature of Sn plasma varies as (3.8± ±0.13) ev, whereas the electron density of Sn plasma varies as (8.24±0.3) (1.07±0.1) cm 3, for the delay time from 0.1 µs to 2.2 µs. Compared with Cummings et al. [20], we both have the same characteristics of the kinetic energy, which Fig.6 Temporal variation of (a) electron temperature and (b) density of SnO 2 plasma produced by the different incident laser pulse energies (72 mj, 142 mj, 343 mj, and 600 mj). The air pressure in the chamber is maintained at 10 3 Pa. The ICCD gate width had a constant value of 30 ns 905

5 Plasma Science and Technology, Vol.18, No.9, Sep The laser energy can be changed from 72 mj to 600 mj by adjusting the GaAs crystal attenuator. During the time interval of the laser pulse, the plasma expands isothermally initially, the target surface constantly absorbs radiation, which is due to absorption of the laser radiation by means of electrons via the inverse bremsstrahlung (IB) absorption process. After termination of the laser pulse, no more energy is pumped into the plasma, the plasma expansion velocities decrease rapidly with delay time, thereby causing the electron temperature and density to drop as delay time. The electron temperature and density increase with increasing laser pulse energy due to higher laser plasma energy transfer result from the higher absorption and/or reflection of the laser photon by the plasma. With the increase of the laser pulse energy, the target releases more excited species, ions and free electrons and interacts with the incident laser photon, so as to promote the target further heating and ionizing, and absorbing more laser energy. 4 Conclusion The optical emissions produced by the interaction of CO 2 laser (10.6 µm, 80 ns) with plate ceramic SnO 2 have been investigated using the measurement of timeresolved optical emission spectroscopy (OES). At the initial stage (< 0.1 µs) of plasma, the strong continuum spectrum is observed and then decreases, and thereafter the isolated atomic and ionic lines can be distinguished easily. In the spectral region of nm, the emission observed is rich in O II of SnO 2 plasma. In the spectral region of nm, the emissions observed are mainly in Sn I and Sn II of SnO 2 plasmas. Temporal variation of the emission spectrum shows that the peak emission intensity of Sn I ( nm) is prior to that of Sn II ( nm) and the intensity of Sn I line is also much higher. Electron temperature is determined with Sn II lines using the Boltzmann diagram method, while the electron densities are estimated using the method of the Stark broadening. The plasma parameters are directly related to the emission characteristics of the plume species, so it is important to optimize a plasma emission source. With the pressure of 10 3 Pa and the laser pulse of 600 mj, it is observed that the electron temperature of SnO 2 plasma decreases from (4.2±0.42) ev to (0.5±0.11) ev and the electron density of SnO 2 plasma decreases from (1.14±0.43) cm 3 to (5±0.1) cm 3, for delay times between 0.1 µs and 2.2 µs. Compared with the plasma parameters (electron temperature and density) of Sn plasma under the same experiment conditions, it is found that the plasma parameters of SnO 2 plasma are higher. Compared with Cummings et al. [20], the electron temperature and density with their conclusions are slightly different from ours, which is probably due to the fact that we measured timeresolved spectra of the entire space. The variation of plasma parameters (electron temperature and density) with laser pulse energy has been analyzed. The SnO 2 plasma parameters increase with increasing of laser pulse energy, because more energy is absorbed from the higher incident laser pulse. References 1 Yuspeh S, Tao Y, Burdt R A, et al. 2011, Appl. Phys. Lett., 98: Wu T, Wang X B, Wang S Y. 2013, Plasma Science and Technology, 15: Chen H, Lan H, Chen Z Q, et al. 2015, Acta Physica Sinica, 64: (in Chinese) 4 Cai Y, Wang W T, Yang M. 2008, Acta Physica Sinica, 57: 5100 (in Chinese) 5 Hayden P, Cummings A, Murphy N, et al. 2006, J. Appl. Phys., 99: Freeman J R, Harilal S S, Verhoff B, et al. 2012, Plasma Sources Sci. Technol., 21: Pan C, Gu Z Z, Nagai K. 2006, J. Appl. Phys., 100: Higashiguchi T, Dojyo N, Hamada M. 2006, Appl. Phys. Lett., 88: Baig M A, Qamar A, Fareed M A, et al. 2012, Physics of Plasmas, 19: Hanif M, Salik M, Baig M A, et al. 2011, Plasma Science and Technology, 13: Gao X, Guo K M, Song X W, et al. 2010, Chinese Journal of Lasers, 37: 877 (in Chinese) 12 Shen J, Yang Z C, Liu X L, et al. 2015, Plasma Science and Technology, 17: Harilal S S, O Shay B, Tillack M S, et al. 2005, J. Appl. Phys., 98: O Shay B, Najmabadi F, Harilal S S, et al. 2007, J. Phys.: Conf. Ser., 59: Shaikh N M, Tao Y, Burdt R A, et.al. 2010, J. Appl. Phys., 108: Wu T, Wang X B, Wang S Y, et al. 2012, J. Appl. Phys., 111: See persistent lines of singly ionized Sn for information about Boltzmann plot, Tables/tintable4.htm 18 Griem H R, 1997, Principles of Plasma Spectroscopy. Cambridge University Press, Cambridge, NewYork 19 Coons R W, Harilal S S, Polek M, et al. 2011, Anal. Bioanal. Chem., 400: Cummings A, O Sullivan G, Dunne P, et al. 2006, J. Phys. D: Appl. Phys., 39: Tolochko N K, Laoui T, Khlopkov Y V, et al. 2000, Rapid Prototyping J., 6: 155 (Manuscript received 14 September 2015) (Manuscript accepted 19 January 2016) address of LAN Hui: hlan306@hust.edu.cn 906