Abstract
Measurements of extreme ultraviolet radiation from gadolinium, dysprosium and tungsten ions with an open n = 4 shell were performed at the National Institute of Standards and Technology. The ions were produced and confined in an electron beam ion trap, and the spectra were recorded with a flat-field grazing-incidence spectrometer in the wavelength range 3.5–17.5 nm. These data are useful for the development of future lithography sources and for diagnostics of hot plasmas in fusion devices.
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1. Introduction
Recent research in lithography has concentrated on investigating next-generation sources at shorter wavelengths—beyond extreme ultraviolet (EUV) lithography. Laser produced plasmas of gadolinium and terbium have become the industry choice as they emit strongly at 6.775 and 6.515 nm respectively due to 4d–4f and 4p–4d transitions [1–5]. Dysprosium may also be useful as a short wavelength light source as it exhibits strong emission around 6.6 nm [6–8]. Tungsten is being considered as a possible wall material in the divertor of the next generation fusion reactor, ITER, and spectroscopic data of highly charged heavy ions are crucial for ITER diagnostics [9–13]. To enhance these areas of research, EUV spectra from highly charged ions of Gd, Dy and W were measured in the National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) by varying the beam energy from 0.59 to 1.320 keV.
2. Experiment
A detailed description of the EBIT at the NIST can be found in [14]. Typically the EBIT operates at beam energies from about 1 to 30 keV. A flat field grazing incidence spectrometer [15] equipped with a gold-coated variable-spaced grating having about 1200 lines mm−1 was used to record EUV spectra between 3.5 and 17.5 nm. Metallic elements are introduced into the trap by a metal vapor vacuum arc ion source [16].
3. Results and discussion
3.1. Gadolinium spectra
In a previous work, EUV spectra of Rb-like to Cu-like gadolinium ions at (space-charge corrected) electron beam energies between 0.97 and 1.7 keV were investigated [17]. Strong intrashell n = 4 − n = 4 transitions were identified by performing detailed collisional–radiative modeling of the EBIT plasma with NOMAD [18] which utilizes atomic radiative and collisional data calculated with the flexible atomic code (FAC) [19]. A total of 73 spectral features were recorded including 59 new identifications. An example of the agreement between theory and experiment is shown in figure 1.
Figure 1. Comparison of the experimental gadolinium spectrum at a nominal beam energy of 1.320 keV and the calculated gadolinium spectrum at 1.28 keV. The second order spectrum is highlighted in red (gray) and shifted upwards to provide a clearer reading. The isoelectronic sequences for the strongest lines are shown.
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Standard image High-resolution imageGadolinium spectra recorded at electron beam energies (currents) between 0.61 keV (5 mA) and 0.91 keV (20 mA) are presented in figure 2. The beam energies reported here are obtained from the nominal beam energies by applying space charge corrections [20]. Identification of transition lines in these open 4d subshell spectra is extremely difficult. Detailed collisional radiative modeling of the EBIT plasma is required to unambiguously identify spectral features. Each open 4d subshell ion consists of thousands of energy levels due to single and double excitations, and each EBIT spectrum consists of contributions from several ion stages. As a result the total number of energy levels needed to generate a theoretical spectrum is prohibitively large.
Figure 2. Experimental spectra of gadolinium ions. Space charge corrected beam energies (in keV) are shown in the upper right corner.
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Standard image High-resolution imageInstead for comparison, single excitation 4d–4f and 4p–4d theoretical spectra of Pd-like to Rb-like gadolinium ions, calculated with FAC, are presented in figure 3. These spectra are dominated by the presence of narrow unresolved transition arrays (UTAs) [21]. The UTAs arise from transitions between the ground 4p64dN and excited 4p64dN−14f and 4p54dN+1 configurations and overlap in adjacent ion stages. This further complicates the identification of spectral features and contributing ion stages in the measured spectra presented in figure 2.
Figure 3. Pd-like through Rb-like gadolinium spectra calculated with FAC [19]. Black denotes 4d–4f transitions and red (gray) denotes 4p–4d transitions.
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Standard image High-resolution imageIn all spectra recorded at the lowest energies of about 0.61 keV there was a notable absence of the strong Pd-like resonant 4d10 1S0–4d9 4f 1P1 transition, previously recorded at 6.7636 nm by Sugar et al [6]. Very low electron beam energies are needed to record Pd-like gadolinium ion spectra (ionization potential IP = 565 eV [22]). In this regime the EBIT current was 3 mA and the reflected beam current was less than 20 μA.
3.2. Dysprosium spectra
Dysprosium ion spectra recorded at electron beam energies between 0.59 and 0.65 keV are shown in figure 4. Again the large number of energy levels and overlapping UTAs of open 4d subshell ions makes identification of spectral features and contributing ion stages a lengthy process and is not attempted here. However we do note that similar to the Gd spectra, the strong Pd-like resonant transition, previously recorded at 6.2778 nm by Sugar et al [6], is absent.
Figure 4. Experimental spectra of dysprosium ions. Space charge corrected beam energies (in keV) are shown in the upper right corner.
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Standard image High-resolution image3.3. Tungsten spectra
To understand the absence of these strong resonant transitions in Pd-like ions, spectra of tungsten ions similar to those observed in the Berlin EBIT [9, 23], were recorded at electron beam energies between 0.85 and 1.1 keV and are presented in figure 5. The ionization potential energies of Ag-like W27+, Pd-like W28+ and Rh-like W29+ ions are 0.8814 ± 0.0016, 1.1322 ± 0.0014 and 1.1799 ± 0.0014 keV respectively [24]. Also indicated in figure 5 are the positions of previously identified transitions arising from these ion stages [6, 7, 25].
Figure 5. Experimental spectra of tungsten ions. Space charge corrected beam energies (in keV) are shown in the upper right corner. The vertical lines above the spectra indicate transitions arising from different ion stages: Ag-like are shown in red (gray), Pd-like in cyan (light gray) and Rh-like in black.
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The strong resonant Pd-like transition previously recorded at 4.8948 nm is clearly seen at 0.849 keV and, with a factor of 3 smaller intensity, even at 0.797 keV. Thus, we observe Pd-like tungsten at energies lower than the IP of the Ag-like ion which is likely due to ladder-like ionization. A similar effect was observed by Sakoda et al using a compact EBIT [26]. They produced Rh-like Ba11+ with an electron beam energy below the ionization energy of Ba10+ through indirect ionization from metastable states. In tungsten the Δn = 0 excitations from the ground configuration 4d104f of the Ag-like ion result in a large number of metastable states which, once populated collisionally, enhance ionization and effectively reduce the beam energy needed to create Pd-like ions in the EBIT. Future experimental and theoretical studies will expand these observations.
4. Conclusion
EUV spectra from highly charged ions of gadolinium, dysprosium and tungsten were recorded at the NIST EBIT facility. These spectra are dominated by UTAs which overlap in open 4d subshell ions and complicate identification of spectral features. An effective lowering of the ionization potential of open 4f subshell ions was observed. This results from collisional population of numerous metastable states and will be further investigated in future studies. These results are useful in the development of next-generation sources for lithography and for diagnostics of hot plasmas in fusion devices.
Acknowledgments
This work was supported by Science Foundation Ireland under Principal Investigator research grant number 07/IN.1/I1771 and in part by the Office of Fusion Energy Sciences of the U S Department of Energy.