EUV Spectroscopy of Xenon Ions Created Using an Electron Beam Ion Trap

Laser-produced plasma source development for Extreme Ultra Violet (EUV) lithography has concentrated on xenon, since XeXI emits at 13.5 nm, the wavelength at which the reflectivity of MoSi mirrors is centred. However it is not obvious that the required conversion efficiencies can be achieved using xenon, and tin has been identified as a strong emitter at this wavelength. The transitions responsible in tin are 4p 4d 4p 4d + 4p 4d4f occurring in a number of adjacent ion stages that merge to form an unresolved transition array (UTA). This UTA is similar to a feature that appears between 10 nm to 11 nm in xenon, which thus provides information directly relevant to tin. The present experimental studies on xenon were performed at the NIST Electron Beam Ion Trap (EBIT). EBITs were developed to perform spectroscopic studies of highly charged ions. The experiments involved changing EBIT parameters, such as the electron beam energy, so that the distribution of ion stages within the plasma changed systematically. Analysis of the corresponding EUV spectra yields information about the contribution of various ion stages to the evolution of the UTA between 10 nm – 11 nm. Previously reported data for the ion stages XeVII through to XeXI are used to identify features occurring in the EBIT spectrum. When the EBIT relative intensities are compared to those from vacuum spark sources they are found to give better agreement with the calculated gA values (statistically weighted Einstein A-coefficients).