The wavelength of choice for the next extreme UV lithography (EUVL) step, 13.5 nm, is based on the availability of MoSi multilayer mirrors with excellent reflectivity within a reflectance bandwidth of approximately 0.5 nm at this wavelength. A wide variety of pulsed discharge sources using xenon or xenon/helium mixtures are being actively investigated. However all of the conversion efficiencies reported to date for these devices are close to 0.8%. Considerable work has also been expended on exploring the feasibility of using laser produced plasmas of xenon clusters or gas puffs from nozzles or solid xenon targets. The highest conversion efficiencies (1.2% into 2% bandwidth) have been achieved using solid xenon. In xenon, the transitions responsible are 4d - 5p in Xe XI[1]. The required conversion efficiency has now been revised upwards to better than 3% conversion into a 2% bandwidth. In the xenon spectrum an intense unresolved transition array (UTA) arising from 4p-4d and 4d-4f transitions occurs near 11 nm and is significantly brighter than the 13.5 nm emission. In tin the same UTA occurs in the 13.5 nm region and consequently tin should be potentially the brightest emitter at this wavelength [2,3]. The transitions responsible in tin arise from stages VIII through XIII that overlap in energy because of final state configuration interaction effects. If the tin concentration is reduced to <10%, the peak brightness in laser produced plasmas from slab targets actually increases due to enhanced radiation transport at lower density. Furthermore, if the remaining >90% of the constituents are low Z materials the radiation emitted is concentrated in a band 1-2-nm wide centred near 13.5 nm. EUV output is also sensitive to laser pulse profile and duration. The extreme brightness of these features is a direct result of the transfer of oscillator strength from the continuum 4d-&epsilonf resonance known to dominate the EUV absorption of the neutral and low ion stages to discrete 4d-4f transitions. In order to optimise a source, spectroscopic data on the extent of the emission from the relevant ion stages and information on excitation and recombination rates etc. are needed in order to develop a comprehensive plasma model.

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