Feasibility of highly line-narrowed F2 laser for 157-nm microlithography

Highly line-narrowed F2 laser operation in the VUV has been achieved for the first time by means of a master oscillator/power amplifier laser design. Different concepts have ben investigated experimentally for the master oscillator (MO) in order to obtain narrowband spectra. The diffraction grating based design showed to be limited to a FWHM of approximately 0.4 pm. The spectral FWHM of the MO could be further reduced to below 0.3 pm with a double etalon-based resonator. Single pass amplification was employed to increase the beam energy density of the beam up to 50 mJ/cm2. The spectral FWHM of the amplified light is slightly larger than the FWHM of the correspondent MO radiation, indicating saturation and/or inhomogeneous broadening of the F2 amplifier medium. Experimental data obtained from broadband operation and ASE measurements suggests that the free running bandwidth of F2 lasers result form spectral gain-narrowing of the laser medium.