Simulation of Nitrogen and Oxygen Spectra Emitted from High Density Hot Plasma

Transcription

1 J Fusion Energ (2014) 33: DOI /s ORIGINAL RESEARCH Simulation of Nitrogen and Oxygen Spectra Emitted from High Density Hot Plasma S. Alsheikh Salo M. Akel C. S. Wong Published online: 19 June 2014 Ó Springer Science+Business Media New York 2014 Abstract The expected emission spectra of nitrogen and oxygen high density plasma have been studied for different conditions. Expected nitrogen and oxygen plasma spectra at certain electron temperature range have been plotted. Suitable electron temperature ranges for nitrogen and oxygen plasma soft X-ray emission and extreme ultraviolet emission have been investigated. Numerical experiments confirm the possibility of developing nitrogen and oxygen plasma focus as a powerful X-ray radiation source for water-window X-ray microscopy, by selecting the working gas pressure, choosing corresponding design and operating parameters of the device. We have illustrated that the results obtained from XRAYFIL simulation could be used to provide spectroscopic information of the plasma focus simulated by Lee model. Keywords Nitrogen and oxygen plasma Soft X-ray EUV emission X-rays ratio method Introduction High density plasma focus (10 20 cm -3 ) of nitrogen [1 5] and oxygen [6] gases has been used as an emitter of soft X-rays suitable for water-window X-ray microscopy [7 9]. The emission spectra of the plasma depend on the operating parameters [10, 11], therefore the plasma focus could S. Alsheikh Salo (&) M. Akel Department of Physics, Atomic Energy Commission, 6091 Damascus, Syria pscientific2@aec.org.sy C. S. Wong Physics Department, Plasma Technology Research Center, University of Malaya, Kuala Lumpur, Malaysia also be considered as a possible light source for extreme ultraviolet lithography (EUVL) at certain operating conditions [11]. This is of interest to the semiconductor manufacturing industry due to the expectation that the next generation lithography (NGL) will be using the wavelength of 135 Å [12, 13]. Various types of EUV radiation sources, including the laser produced plasma and pulsed discharge sources, such as the capillary discharge [14, 15], vacuum spark [16, 17] and plasma focus [18, 19], are being considered. These radiation sources, especially the pulsed discharge sources are favorable as X-ray and EUV radiation sources because of their lower cost and simplicity in operation, when compared to other radiation sources. Generally, it is hardly possible to get the detailed accurate knowledge of the states of the plasma. Approximate estimations, by calculations based on simplified plasma models, may be carried out. The methods of fuzzy scaling and genetic algorithms for obtaining the possibility of description of X-ray emission, scattering and applications on fusion energy have been used [20 22]. On the other hand, the most tractable plasma models are the local thermodynamic equilibrium (LTE), the non-local thermodynamic equilibrium (NLTE) or the corona equilibrium (CE), and the collisional-radiative equilibrium (CRE) [23 26]. One of the best known of these is the suite of three codes (POPULATE, SPECTRA, and RATIO) called RATION [27 30]. The POPULATE code uses the principle of detailed balance to calculate the rate of inverse processes. The SPECTRA code computes the expected emission spectrum of the plasma, while the RATIO code allows the user to view the results of POPULATE in graphical forms. The graphical outputs include the population, ratio of the intensities of selected transitions and the optical depths of transitions, as a function of temperature or density. Since the code provides details of the populations

2 678 J Fusion Energ (2014) 33: a b Fig. 2 Computed EUV oxygen plasma focus spectra at different temperatures for NLTE model Fig. 1 a Calculated full, Brem. and Recomb. spectra for nitrogen plasma with T e = 100 ev. b Computed full EUV nitrogen plasma focus spectra at T e = 100 ev for NLTE model in Lithium-like through fully stripped ions, an X-ray filter analysis code, XRAYFIL, [31] has been developed to allow more accurate non-dispersive X-ray plasma diagnosis with absorption filters by using a series of trial spectra of the emitting species. The code calculates a set of emission spectra for a given plasma using RATION, and convolves it with the transmission characteristics of the filter set used, as well as the response function of the detector chosen. Comparison of the ratio of the signal through the different filters from these calculated values to that recorded in the experiment allows us to obtain a measurement of the plasma temperature and density. The code incorporates a number of options for various emission scenarios and detectors. The main purpose of XRAYFIL code is to calculate the emission spectra of nitrogen and oxygen plasma and then to work out the number of photons passing through the chosen composite filter (BPX65 PIN diode detector, in our case). So, the emission spectra (full, Bremsstrahlung, recombination, and line) are computed using XRAYFIL code (in unit of number of photons/cm 3 / Angstrom/Sec./Sterad.) [31]. In this work, the XRAYFIL code coupled with the POPULATE code is used to study the EUV and X-ray emissions of nitrogen and oxygen plasma focus. The spectra of radiation emissions (full, Bremsstrahlung, recombination, and line) from the nitrogen and oxygen plasma focus have been simulated for different plasma conditions. The calibrated X-ray ratio curves for electron temperature measurements of nitrogen and oxygen plasma focus have been deduced. Results and Discussion of Numerical Experiments Spectrum of the Nitrogen and Oxygen Plasma in the Extreme Ultraviolet Range Many numerical experiments have been carried out using POPULATE and XRAYFIL codes for calculations of nitrogen and oxygen plasma spectra in the extreme ultraviolet range at different temperatures for NLTE (for nitrogen ev and for oxygen ev). Figure 1a presents the expected radiative emissions (full, Bremsstrahlung, recombination, and line) of nitrogen plasma at T e = 100 ev for NLTE model, N e = cm -3, N i = cm -3 for a wide range of the wavelength ( Å). From this figure it can be seen that the rectangle and its sharp edge obtained in the full spectra are related to the variation of the recombination emission versus wavelength. The obtained results in the extreme

3 J Fusion Energ (2014) 33: Fig. 3 Computed EUV oxygen plasma focus spectra at T e = 125 ev for NLTE model, N e = cm -3 Fig. 5 Computed spectra of nitrogen plasma at three different temperatures for N e = cm -3 Number of photons/cm 3 /sec./sterad./a o 1E29 1E28 1E27 1E26 1E25 1E24 1E23 1E22 1E21 1E20 Soft X-ray nitrogen spectrum at Te = 100 ev λ = A o λ = 24.9 A o Wavelength, A o Fig. 4 Computed spectrum for nitrogen plasma with T e = 100 ev ultraviolet range showed that T e = 100 ev is the optimum value for the strong emission at or near wavelength of 135 Å (see Fig. 1b). While for oxygen plasma T e = 125 ev is the suitable temperature for higher EUV emission (see Figs. 2, 3). Nitrogen and Oxygen Plasma as Soft X-ray Source The ion distribution in the plasma has been investigated for various conditions assuming the NLTE model. Then the plasma radiations in the X-ray range have been simulated using POPULATE and XRAYFIL codes. The computed spectral lines emitted from a plasma are broadened by Doppler broadening [27, 32]. The nitrogen plasma spectrum has been computed at different temperatures (T e in the range of ev), electron density (N e in the range of cm -3 ) and ion density (N i in the range of cm -3 ). Figure 4 presents the expected radiative X-ray emissions of nitrogen plasma focus at T e = 100 ev for NLTE model, N e = cm -3,N i = cm -3. The effects of electron temperature and density on the plasma emissions have been studied. The electron plasma temperature influence on the radiative emission has been found to be more dominant than electron density. Figure 5 shows the variations of the expected full emission spectra of plasma focus at various temperatures. As expected from theoretical consideration of plasma emission [32], the continuum of the X-ray emission spectrum is observed to shift towards shorter wavelength (higher photon energy) with increasing electron temperature. The relative population of the ionic species present is also affected by the temperature. Based on the corona model [33 35] the prominent species present in nitrogen plasma at electron temperature of 90 ev are N?5 and N?6, while at 150 ev, N?5,N?6 are present with small fraction of N?7. Finally at electron temperature higher than 200 ev, N?7 become prominent, together with N?6. This will affect the recombination and line radiations. At electron temperature much higher than 300 kev, when the plasma becomes fully ionized, Bremsstrahlung is expected to dominate. Numerical experiments have also shown that the shapes of spectra for different electron densities ( cm -3 ) are similar, but different in amplitude, due to the N e 2 -dependence. Figure 6 shows variation of the soft X-ray intensity (Hlike ions) versus electron temperature. From Fig. 6 it can be seen that the most suitable temperatures for H-like is found to be about ev.

4 680 J Fusion Energ (2014) 33: Fig. 6 Variation of H-like ion emission line intensity of nitrogen plasma with T e for NLTE model Fig. 8 Variation of He-like ion emission line intensity of oxygen plasma with T e for NLTE model Fig. 7 Computed spectrum for oxygen plasma with T e = 200 ev Fig. 9 Variation of H-like ion emission line intensity of oxygen plasma with T e for NLTE model Numerical experiments have also been performed on the oxygen plasma to find the emission spectra at different conditions. Figure 7 presents the expected radiative X-ray emissions of oxygen plasma focus at T e = 200 ev for NLTE model, N e = cm -3, N i = cm -3. The electron temperature and density effects on the plasma emissions have also been studied. Based on the corona model [34 36] the prominent species present in oxygen plasma at electron temperature of ev are O?6 and O?7, while at electron temperature higher than 300 ev, O?8 become prominent, together with O?7. Figures 8 and 9 show variations of the soft X-ray intensity (He-like and H-like ions) versus electron temperature. From Fig. 8 it can be seen that the most suitable temperature for He-like is found to be about 125 ev. While the most suitable temperature for H-like ions is found to be about 200 ev (see Fig. 9). The plasma focus device is the simplest low-cost high density hot plasma, and so many numerical experiments simulating X-ray radiations emitted from plasma focus devices and optimizing the plasma focus devices operating with nitrogen and oxygen gases using Lee model for generating a maximum soft X-ray yield have been conducted [33 39]. Numerical experiments using Lee model on plasma focus operated with nitrogen and oxygen gases showed that the suitable temperature windows for generating soft X-ray from nitrogen and oxygen are and ev for nitrogen and oxygen, respectively. And it

5 J Fusion Energ (2014) 33: has been shown that optimized operational and geometrical conditions are required for improvement of the X-ray yield. As an example, numerical experiments for optimizing the UNU/ICTP PFF plasma focus device using Lee model showed that the peak axial speeds suitable for maximum nitrogen and oxygen soft X-ray yields are about 2.5 cm/ls [37] and 4 cm/ls [39], respectively. Practically, these needed axial speeds, with other optimized parameters to reach ranges of temperatures suitable for generating soft X-ray from low atomic number gases (nitrogen and oxygen), could be achieved in the plasma focus devices. While for plasma focus operated with high atomic number gases like krypton and xenon, the required axial speeds suitable for soft X-ray generation are higher than 11 cm/ls [40], and it is not clear whether plasma focus devices will operate in such high speed regimes for Kr and Xe. However, based on our obtained X-ray spectroscopic results using the POPULATE and XRAYFIL codes, it can be said that the suitable T e ranges for soft X-ray from nitrogen and oxygen plasma correspond to the T e windows used in the numerical experiments performed using Lee model. Electron Temperature Measurements of Nitrogen and Oxygen Plasma Based on Ratio Method The electron temperature of the plasma can be determined from the analysis of radiation in the X-ray region [41 43] using the five channels BPX65 PIN diodes with foils of different thicknesses [44 48]. The attenuated radiative emissions of plasma through different channels of BPX65 PIN diodes with varying absorption filters have been calculated using the Ratio-BPX65 code [49, 50]. Briefly, the code has been written in FORTRAN 77 for studying the effect of the response of BPX65 photodiode, with Mylar and aluminium foils. The attenuated plasma spectrum through aluminized mylar foils and detected by the BPX65 photodiode can be determined by using the following formula [31]: Z I 0 ¼ Pðk; T e ÞSðkÞexp l mylar ðkþx mylar dk ð1þ Similarly, the X-ray emission detected by diodes with additional aluminium foils of various thicknesses can be expressed as: Z I¼ Pðk;T e ÞSðkÞexp l mylar ðkþx mylar þl Al ðkþx Al dk ð2þ Finally, the ratio of the X-ray signals obtained by diodes with additional aluminium foils against that with aluminized mylar only can then be calculated as R = I/I 0 : R¼ I IR 0 ¼ Pðk;T e ÞSðkÞexp l mylar ðkþx mylar þl Al ðkþx Al dk R Pðk;T e ÞSðkÞexp l mylar ðkþx mylar dk ð3þ In Eqs. (1), (2) and (3) S(k) is the BPX65 sensitivity, l is mass absorption coefficient of material, and x is the absorption foil thickness. As an example, the radiative emission from the nitrogen and oxygen plasma focus, actually detected by the BPX65 PIN diode with 12 lm aluminized Mylar and with additional aluminum foils of varying thicknesses (10 30 lm), have been calculated. Figures 10 and 11 show the detected nitrogen and oxygen plasma emission, respectively, which show the attenuated X-ray intensities recorded after passing through different filters at T e = 100 ev for nitrogen and T e = 200 ev for oxygen. The signals recorded by the BPX65 detector provide information about the time evolution of the X-rays produced by the plasma focus and these can be used to determine the electron temperature of the plasma focus by the X-ray foil absorption technique. For this purpose, the sets of nitrogen and oxygen plasma spectra for different temperatures in the range of T e = ev for nitrogen and T e = ev for oxygen have been calculated to get the X-ray signal ratio R = I/I 0 (see Figs. 12, 13). Number of photons/cm 3 /sec./sterad./a o 1E28 1E26 1E24 1E22 1E20 1E18 1E16 1E14 1E12 1E10 1E8 1 Nitrogen specta Te = 100 ev NLTE, Ne = 1e19, Full spectra Spectra + BPX μm Mylar Spectra + BPX μm Mylar + 10 μm Al Spectra + BPX μm Mylar + 30 μm Al 10 Wavelength, A o Fig. 10 Computed spectra of nitrogen plasma at electron temperature T e = 100 ev through different sets of filters [BPX65 PIN diode with 12 lm aluminized Mylar (D.1), BPX65 PIN diode with 12 lm aluminized mylar coupled to aluminum foil thickness of 10 lm (D.2), and BPX65 PIN diode with 12 lm aluminized mylar coupled to aluminum foil thickness of 30 lm (D.3))]

6 682 J Fusion Energ (2014) 33: Number of photons/cm 3 /sec./sterad./a o 1E28 1E26 1E24 1E22 1E20 1E18 1E16 1E14 1E12 1E10 Nitrogen specta Te = 100 ev NLTE, Ne = 1e19, Full spectra Spectra + BPX μm Mylar Spectra + BPX μm Mylar + 10 μm Al Spectra + BPX μm Mylar + 30 μm Al Ratio Spectra of oxygen plasma Ne = 1e19, NLTE T e = 100 ev T e = 150 ev T e = 200 ev 1E Wavelength, A o Fig. 11 Computed spectra of oxygen plasma at electron temperature T e = 200 ev through different sets of filters [BPX65 PIN diode with 12 lm aluminized Mylar (D.1), BPX65 PIN diode with 12 lm aluminized mylar coupled to aluminum foil thickness of 10 lm (D.2), and BPX65 PIN diode with 12 lm aluminized mylar coupled to aluminum foil thickness of 30 lm (D.3)] 1E Al foil thickness ( μm) Fig. 13 Calculated X-ray ratio (R = I/I 0 ) curves of BPX65 PIN diode coupled to mylar (12 lm) and sets of BPX65 PIN diode coupled to mylar (12 lm) with different aluminum foil thicknesses (10, 20, 30, 40, and 50 lm) for X-rays of oxygen plasma (NLTE, N e = cm -3 ) at various temperatures based on electron temperatures and ion densities of studied plasma focus obtained by Lee model, we can proceed to obtain spectroscopic information for EUV and X-ray emissions of the plasma focus using XRAYFIL code. Conclusions Fig. 12 Calculated X-ray ratio (R = I/I 0 ) curves of BPX65 PIN diode coupled to mylar (12 lm) and sets of BPX65 PIN diode coupled to mylar (12 lm) with different aluminum foil thicknesses (10, 20, 30, 40, and 90 lm) for X-rays of nitrogen plasma (NLTE, N e = cm -3 ) at various temperatures These ratio curves can be used as calibration curves for the measurement of electron temperatures for nitrogen and oxygen plasmas. Finally, these numerical experiments showed that for generation of EUV and soft X-ray from plasma focus operated with nitrogen and oxygen optimized operational and geometrical conditions are required. Then, The radiation emission spectra of nitrogen and oxygen plasma at various plasma parameters have been computed using the XRAYFIL code by assuming a NLTE model for the plasma. Nitrogen and oxygen plasma focus spectra have been calculated for plasma focus operation as soft X-ray and EUV sources. The suitable electron temperature ranges for soft X-ray and EUV emissions from studied hot plasma were found. The calibration X-ray ratio curves for electron temperature deduction of nitrogen and oxygen plasma have been computed. These ratio curves could be used as calibration curves for the measurement of electron temperatures for nitrogen and oxygen plasma focus. Finally, we believe that the simulation results presented here may also be useful to set the condition for any pulsed hot plasma source to be considered as EUV or soft X-ray source. Acknowledgments The authors would like to thank general director of AECS for support, guidance and encouragement. C. S. Wong s participation in this work is supported by University of Malaya research Grant RG204-11AFR.

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