Time and space resolved visible spectroscopic imaging CO2 laser produced extreme ultraviolet emitting tin plasmas

Abstract

Experiments involving laser produced tin plasma have been carried out using a CO2 laser with an energy of 800 mJ/pulse and a full width at half maximum (FWHM) of 80 ns in vacuum. Time-integrated extreme ultraviolet spectral measurement showed that the peak of the extreme ultraviolet lithography spectrum was located at 13.5 nm and the spectrum profile’s FWHM of the unresolved transition arrays was 1.1 nm. Plasma parameters of the electron temperature and density measurements in both axial and radial directions at later times had been obtained from a two-dimensional time and space resolved image spectra analysis. The axial spatial distribution of the electron density showed a 1/d2.6 decrease profile, and the radial spatial distribution of the electron density showed a 1/r1.1 profile, in which d is the axial distance from the target surface and r is the radial distance. The electron density was found to maintain symmetry across the radial distance at all delay times. Near the plasma plume center, the electron temperature Te varied slightly with increasing axial or radial distance, which was related to collisional decoupling and reheating of the ionized species in the plasma at distances longer than 3 to 4 mm. The space averaged electron temperature was measured in the range of 3.4–1.0 eV, and the space averaged electron density was measured in the range of 2.0 × 1017 to 2.2 × 1016 cm−3, as the time delay varied from 1.6 μs to 3.6 μs with respect to the pulse discharge. Time evolutions of the plasma temperature and density were found to have an apparent rise at a delay time of 2.4 μs in the corresponding time of the laser pulse tail peak. This suggests that plasma parameters and extreme ultraviolet emission intensity can be controlled by a double pulse combined laser.