Extreme ultraviolet source at 6.7 nm based on a low-density plasma

Abstract

We demonstrate an efficient extreme ultraviolet (EUV) source for operation at λ = 6.7 nm by optimizing the optical thickness of gadolinium (Gd) plasmas. Using low initial density Gd targets and dual laser pulse irradiation, we observed a maximum EUV conversion efficiency (CE) of 0.54% for 0.6% bandwidth (BW) (1.8% for 2% BW), which is 1.6 times larger than the 0.33% (0.6% BW) CE produced from a solid density target. Enhancement of the EUV CE by use of a low-density plasma is attributed to the reduction of self-absorption effects.

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].…”

“…To test this hypothesis, spectra were also obtained with the 10-ns laser from a foam target with a 30% composition of Gd by mass, known to produce spectra that are optically thinner [6]. The results are presented in Fig.…”

“…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]. As the ion stages involved in the 6.x nm radiation from Gd plasmas are higher than those of the 13.5 nm emitting ions of Sn plasmas, higher electron temperatures 4 are needed and thus higher laser power densities.…”

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

“…12 However, while there are large numbers of papers reporting a Gd plasma generated by a YAG laser, all papers reported very broad spectra with a width larger than 1nm. 13,14 So, generating a 6.8 nm narrow peak in a Gd plasma by using a Nd:YAG laser is a challenging work.…”

Feasibility study of broadband efficient “water window” source

“…Efficient, high-power extreme-ultraviolet (EUV) sources for semiconductor lithography at 13.5 [6] and 6.7 nm [7][8][9] based on laser-produced plasmas have been demonstrated in high-volume manufacturing of integrated circuits (IC) having node sizes of 22 nm or less [10].…”