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Summary form only given. F/sub 2/ lasers are taken notice of the light source for lithographic tools enabling structures below the 70 nm technology node. The F/sub 2/ laser linewidth is essential for the design of the lithography projection system. A dioptric design system requires the F/sub 2/ laser linewidth of about 0.2 pm (FWHM), which is due to the large dispersion of the refractive index of CaF/sub 2/ at 157 nm. Therefore the spectral measurement of a line-narrowed F/sub 2/ laser is very important. We have developed a 157 nm coherent light source (157CLS) to calibrate a VUV spectrometer. The 157CLS is required to have a very narrow linewidth, which can be approximated by a /spl delta/ function to a VUV spectrometer. This light source can also be used to inspect the optical systems by interferometry.