EUV spectra of Gd and Tb ions excited in laser-produced and vacuum spark plasmas

Abstract

Extreme UV spectra of the gadolinium and terbium ions excited in the laser-produced plasma and vacuum spark sources were recorded in the 40-120 Å region and investigated on the basis of the Hartree-Fock calculations using Cowan code. The intense peaks in the 65-75 Å region of the vacuum spark spectra were interpreted as a manifold of the 4d 10 4f m -4d 9 4f m+1 transitions in the ions with a partially filled 4f shell. The drastic narrowing of these peaks was observed in the spectra of the laser-produced plasma. It was explained by a change of the 4d 10 4f m -4d 9 4f m+1 (m > 2) transition arrays mostly contributing to the intensity of the peaks in the vacuum spark spectra for the 4-4 transitions in the simplest spectra of the 4p 6 4d k (k = 8-10) and 4d 10 4f m (m = 1-2) ground configuration ions predominantly excited in hotter laser-produced plasma. The most intense lines of the 4d 10 4f 2 -4d 10 4f5d transitions in the Gd XVII and Tb XVIII spectra were classified for the first time.

The effect of viewing angle on the spectral behavior of a Gd plasma source near 6.7 nm

“…The precise value of x is yet to be determined but will be decided by the source and reflectivity combination that provides the brightest in-band EUV yield and conversion efficiency. Recent theoretical work has proposed 6.76 nm as the optimum choice for the location of the reflectivity peak of EUV optics in a future lithography system based on the fact that the strongest lines observed in this region originate from Ag-and Pd-like ions [1][2][3]. Previous experimental work [4][5][6] has shown the spectral and in-band intensity dependence of Gd plasmas on laser wavelength, intensity and target composition and concentration.…”

Sharpening of the 6.8 nm peak in an Nd:YAG laser produced Gd plasma by using a pre-formed plasma

“…The narrowest peak of 0.1 nm width was reported in ref. 10 in which an Nd:glass laser with the pulse width of 20 ns was employed. The pulse energy was 2-3 J and the laser flux was 5-8x10 11 W/cm 2 .…”

“…In the spectrum in Ref. 10, many sharp week lines are noticed, and lines at 6 nm, 6.1 nm, 7.1 nm, 7.15 nm, 11.2 nm, and 11.3 nm are assigned to emissions from 17+ Gd ions. A broad shoulder at 7.1 nm in the CO 2 LPP looks to correspond to the sharp peak at 7.1 nm in the 240 eV LHD plasma, which looks to correspond to lines at 7.1 nm and 7.15 nm in the plasma generated by the glass laser.…”

“…This behavior agrees quite well with Churilov et al's calculation. 10 Churilov et al calculated the emission wavelength for various ionization stage Gd ions. Their calculation is reproduced in Fig.…”

“…Our observation that the spectrum broadens only to a long wavelength side using both lower and higher laser irradiation intensities agrees with Churilov et al's calculation. 10 In the present paper, the experimental setup including the method of overlapping two beams is described in Section II, the observation of spectral narrowing by using a pre-formed plasma is reported in Section III, and, in Section IV, the observed spectra are compared with spectra previously reported by others.…”