Rare-earth plasma extreme ultraviolet sources at 6.5–6.7 nm

Abstract

We have demonstrated a laser-produced plasma extreme ultraviolet source operating in the 6.5–6.7 nm region based on rare-earth targets of Gd and Tb coupled with a Mo/B4C multilayer mirror. Multiply charged ions produce strong resonance emission lines, which combine to yield an intense unresolved transition array. The spectra of these resonant lines around 6.7 nm (in-band: 6.7 nm ±1%) suggest that the in-band emission increases with increased plasma volume by suppressing the plasma hydrodynamic expansion loss at an electron temperature of about 50 eV, resulting in maximized emission.

Gd plasma source modeling at 6.7 nm for future lithography

“…However while the agreement with the earlier calculation validates both approaches, the experimental spectrum is integrated over the full duration of a laser pulse and thus a range of plasma conditions. Moreover, the plasma from pure Gd is known to be optically thick since the emission profile is very sensitive to the Gd concentration in the target [7,8,19]. Self-absorption due to lower ionic charge states and neutral vapor in the expanding Gd plasmas will reduce the intensity at wavelengths greater than 6.8 nm.…”

“…Based on the results of [15] this electron temperature implies a laser flux close to 2.4 × 10 11 W/cm 2 for a CO 2 LPP. From initial experimental studies [7,8] it is clear that LPPs produced by Nd:YAG lasers are optically thick. Further work is needed to find both the optimum temperature and source conditions using calculations that allow for full radiation transport and plasma hydrodynamics.…”

“…Following the earliest studies on extreme ultraviolet (EUV) emission from laser produced plasmas of elements with 62≤Z≤74, it is evident that both Gd and Tb ions have 4p 6 4d n -4p 5 4d n+1 + 4d n-1 4f transitions near this wavelength and emit an intense unresolved transition array (UTA) [11]. Recently, the suitability of Nd:yttrium-aluminum-garnet (Nd:YAG) laser-produced plasma (LPP) EUV sources based on Gd and Tb has been demonstrated for high power operation [3,7]. In order to achieve higher conversion efficiency (CE) from laser energy to EUV emission energy and spectral purity, the effects of optical thickness were evaluated by changing the laser wavelengths to change the plasma density and opacity [8].…”

“…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].…”

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

“…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. Spectra from pure Gd targets in the 6.7 nm region were shown to be optically thick, however, the opacity could be reduced by decreasing the Gd concentration or increasing the laser wavelength [5,6].…”