Optical and spectral characterization of the water droplet laser plasma extreme ultraviolet source

The demanding illumination source requirements for EUV (extreme ultraviolet) lithography are now challenging physicists and engineers with the urgent task of creating new light sources in the 13 nm region having sufficient power, stability, longevity, and cleanliness to be deployed on the latest high through-put stepper machines used to fabricate next generation computer chips. The water droplet laser plasma source has been advanced as a candidate EUV source technology for EUV lithography. This study aims to obtain an accurate description of the laser plasma droplet expansion and emission dynamics.; Optical diagnostics in the form of schlieren imaging, shadowgraphic imaging, and interferometry have been employed to gain both qualitative and quantitative information about the electron density as a function of time and space and as means of studying the degree of ionization of the water droplet as a whole. Spectroscopy was employed to ensure that the appropriate laser intensity was used to optimize the EUV radiation in the spectral band of interest and also to estimate the plasma temperature. The hydrodynamic laser plasma code MEDUSA and the atomic physics code Spectra were used to model the laser plasma expansion dynamics and radiation emission, respectively. Comparisons between the experimental data and the one-dimension models show the effects of non-symmetric illumination and expansion, and expose details of the laser plasma interaction. Finally a new Sn compound doped droplet target has undergone preliminary investigations that show it to be a very promising EUV source for lithography.