The role of atomic physics in collisional-radiative modeling of tin plasmas for lithography*

We report on our continuing efforts to understand the extreme ultra-violet (EUV) emission from tin plasmas of interest to nanolithography. The intense and narrow emission of EUV radiation centered at 13.5 nm is of interest to the micro-electronics industry where it has much potential for etching smaller features on micro-processors.

The tin plasma of relevance (at a temperature around 30 eV and electron density around $10^{21}$ cm$^{-3}$) is interesting from an atomic physics perspective, since it requires highly accurate atomic structure and transition information from complex open 4p and 4d subshells of tin ions from around Sn$^{8+}$ to Sn$^{15+}$. The opacity spectra of Sn plasma in this regime are calculated, under local thermodynamic equilibrium conditions, using the Los Alamos suite of atomic codes and the opacity and plasma modeling code ATOMIC. The detailed atomic structure calculations of the complex Sn ions show an unexpectedly large contribution to EUV emission from transitions between highly-excited states, up to around 90% of the total opacity, with the more well-known EUV transitions to the ground manifold contributing only 10%. The transitions between doubly- and triply-excited states are shown to be serendipitously aligned around 13.5 nm, the wavelength of relevance in EUV light sources. Our calculations are in excellent agreement with the emissivity measured from a Nd:YAG laser-produced plasma. We also explore the properties of CO2 laser-produced tin plasmas, where non-LTE effects are more important.