Fundamentals and limits for the EUV emission of pinch plasma sources for EUV lithography

Future extreme ultraviolet (EUV) lithography will require high radiation intensities at a wavelength around 13.5 nm. The limits of emission in this spectral range from discharge based plasmas are discussed theoretically. The discussion is based on a simple MHD approach for a xenon plasma discharge and atomic data from the ADAS software package for radiative transitions, excitation and ionization of different ionization levels. Discharge parameters are chosen for the Philips' hollow cathode triggered pinch plasma. The calculations show that the 13.5 nm emission originates only from of Xe10+ ions and is optically thin. Ideally, the conversion efficiency is expected to scale linearly with the electron density in this case. The MHD calculations, however, show a lower increase with density. The loss channels leading to this behaviour, like leakage currents, will be discussed in detail. The identification of these losses allow, on the other hand, for a systematic improvement of the electrode system and the electrical circuit. In addition, theoretical emission spectra of xenon and tin as the most promising emitters around 13.5 nm will be compared with respect to the possible optimization potential of spectral emission characteristics.