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Spectral measurement of ultra line-narrowed F2 laser

F2 lasers are the light source of choice for microlithographic tools enabling structures below the 70 nm technology node. Accurate measurements of the spectrum of F2 lasers are therefore very important. We have succeeded in measuring the spectrum of an ultra line narrowed F2 laser using a VUV grating spectrometer calibrated with a 153 nm coherent light source (153CLS). As a first step in the development of a 157 nm coherent light source (157CLS), the less complex 153CLS has been built. Using resonant two-photon processes and four-wave mixing in Xe, this method provided a tunable laser system with high conversion efficiency and a very narrow linewidth, which can be approximated by a (delta) function. The 153CLS included a pulsed, single-mode tunable Ti:sapphire laser (768.0 nm), a third harmonic generation unit (256.0 nm) and an Xe gas cell. The 153CLS had a linewidth of 0.007pm (FWHM) and a power of 0.05mW at 1000 Hz. The VUV grating spectrometer and a Michelson interferometer for F2 lasers have also been developed. The instrument function of the spectrometer has been measured with the 153CLS. Experimental and theoretical instrument functions were in good agreement (FWHM: 0.30pm). The instrument function at 157 nm was therefore estimated to have the theoretical FWHM of 0.31 pm. The spectral linewidth of the line-selected F2 laser has been measured under various laser conditions with the spectrometer as well as with the interferometer. Results show good agreement between both measurements. The spectrum of the ultra line narrowed F2 laser was measured with the VUV grating spectrometer calibrated using the 153CLS. The laser's FWHM of the deconvolved spectrum was 0.29pm. The deconvolved spectral purity containing 95% of the total laser energy is less than 0.84pm.