Gd plasma source modeling at 6.7 nm for future lithography

Abstract

Plasmas containing gadolinium have been proposed as sources for next generation lithography at 6.x nm. To determine the optimum plasma conditions, atomic structure calculations have been performed for Gd11+ to Gd27+ ions which showed that n = 4 − n = 4 resonance transitions overlap in the 6.5–7.0 nm region. Plasma modeling calculations, assuming collisional-radiative equilibrium, predict that the optimum temperature for an optically thin plasma is close to 110 eV and that maximum intensity occurs at 6.76 nm under these conditions. The close agreement between simulated and experimental spectra from laser and discharge produced plasmas indicates the validity of our approach.

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.…”

Optimizing conversion efficiency and reducing ion energy in a laser-produced Gd plasma

“…The optimum electron temperature of a Gd plasma optimized for 6.7 nm emission is 3 times higher than that required to optimize a Sn plasma for 13.5 nm emission. The plasma expansion velocity is also larger as a result of this higher electron temperature, which, from theoretical modeling, has been calculated to be above 100 eV [2,21]. To limit the etendue, the plasma source size should be controlled by producing a small focal spot and shorter pulse duration in the LPP process.…”

Investigation of the ionization balance of bismuth-to-tin plasmas for the extreme ultraviolet light source based on a computer-generated collisional radiative model

“…This suggests that the calculation of the ionization balance as a function of the density and temperature of the plasma using a collisional radiative (CR) model is indispensable for the development and application of short-wavelength EUV sources. However, in previous works, 7,8 only a simple model was used, in which only the ground state of each ion was taken into account. 9 It was shown that the CR model of W for fusion plasmas may become very complex, with more than 10 3 excited states of each ion, 10 including the excited states that have a significant population and contribute to EUV emission as well as those that contribute to dielectronic recombination in the calculation of the ionization balance.…”

“…5 The emission at λ = 6.x nm from Gd and Tb 6 is considered a candidate source for future microlithography. 7 Light sources at the waterwindow wavelengths, λ = 2.4 -4.5 nm, have attracted attention for biological imaging. 8 However, the temperature of the plasma, and thus the required pumping power, is expected to increase as the wavelength of the UTA decreases because a much greater energy is needed to ionize the target atom to the Pd-like ion.…”