Extreme ultraviolet emission spectra of Gd and Tb ions

Abstract

Theoretical EUV emission spectra of gadolinium and terbium ions calculated with the Cowan suite of codes and the FAC relativistic code are presented. 4d−4f and 4p−4d transitions give rise to unresolved transition arrays in a range of ions. The effects of configuration interaction are investigated for transitions between singly excited configurations. Optimization of emission at 6.775 nm and 6.515 nm is achieved for Gd and Tb ions respectively by consideration of plasma effects. The resulting synthetic spectra are compared with experimental spectra recorded using the laser produced plasma technique.

Gd plasma source modeling at 6.7 nm for future lithography

“…In Fig. 5 the theoretical spectrum is compared with the LPP data of [4] which gives a good fit for an electron temperature of 144 eV and compares well with the earlier FAC code calculation which arrived at an identical value [5]. The laser flux on the target surface was estimated as (5 -8 ) x 10 11 Wcm -2 which yields an average electron temperature is around 135 -180 eV [15].…”

“…These studies have shown that plasmas containing tin, where transitions of the type 4p 6 4d n -4p 5 4d n+1 + 4d n-1 4f overlap in ion stages from Sn 9+ -Sn 13+ to form an unresolved transition array (UTA) centered near 13.5 nm are the strongest emitters at this wavelength. To keep pace with Moore's law, research on shorter wavelength EUV sources has already begun [3][4][5][6][7][8].…”

“…Theoretical spectra were subsequently calculated by Kilbane and O'Sullivan using the Flexible Atomic Code (FAC) for Gd and Tb plasmas [5] and Sasaki et al using the HULLAC code for Tb plasmas [6]. It should be noted that there are discrepancies between these two theoretical simulations [5,6].…”

“…It has been shown that configuration interaction (CI) plays an important role in the simulation of Gd spectra [4,5], so the choice of electronic configurations to be used is critical. The configurations chosen in the present case are given in Table I.…”

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.…”