[Characteristics of extreme ultraviolet emission from tin plasma using CO2 laser for lithography]

The extreme ultraviolet (EUV) emission characteristics from Sn plasma for lithography produced by a pulse discharge CO2 laser was investigated under different conditions. Extreme ultraviolet spectral measurements were made throughout the wavelength region of 6.5 nm to 16.8 nm using a grazing incidence flat-field spectrograph coupled with an X-ray charge-coupled device camera for detection of time-integrated spectra. The dependence of spectral properties of the EUV emission on pulse duration, incidence pulse energy, and buffer gas pressure was investigated. The results show that the peak of EUV spectra was located at 13.5 nm. The intensity of EUV emission increased with increasing laser energy ranging from 30 mJ to 600 mJ in a nonlinear manner with saturation effect. The critical energy of incident pulse laser for the generation of EUV emission is near 30 mJ in our experiment. The highest conversion efficiency of 1.2% in producing 13.5 nm EUV light with 0.27 nm bandwidth was achieved at pump energy of 425 mJ. The EUV spectra from a plate target produced by laser pulse with full width at half maximum range from 50 ns to 120 ns were recorded and negligible differences in their spectral features noticed even though higher spectral intensity was observed by shorter pulse duration. The 2% in-band EUV intensity with 52 ns pulse duration was 1.6 times higher than that with 120 ns pulse duration due to the increase in laser intensity. It was also found that the detected EUV spectral intensity rapidly decreased with increasing buffer air pressure, and the EUV emission could be totally absorbed at the pressure of 200 Pa, while weak EUV emission could be still detected at the buffer He gas pressure of 7 x 10(4) Pa. The experimental results showed that the absorption coefficient of 13.5 nm light at air buffer gas pressure of 100 Pa was 3.0 m(-1), while the absorption coefficient was 0.96 m(-1) at the same He buffer gas pressure.