Bragg reflectors with IR suppression for Extreme Ultraviolet Lithography

The most promi¬sing next generation lithography technology is extreme ultraviolet lithography (EUVL). To fulfill the demands of high productivity, EUVL requires high power EUV radiation sources, with a promising technology for such sources being based on the emission of dense plasmas produced by pulsed CO2 laser radiation at 10.6 μm. However, such plasmas also scatter a significant portion of the incident laser radiation that propagate along with the EUV and reaches the wafer. So solutions are needed that provide high reflectivity in the EUV wavelength range while simultaneously suppressing the IR radiation of the laser. We have developed two multilayer based solutions that simultaneously provide high reflectivity of the EUV radiation and suppression of the CO2 laser. The first solution is a combination of a standard high reflectance EUV Mo/Si multilayer with a reflective grating. First experimental results on the optical response of such a lamellar quarter lambda reflector have been obtained, where the filtering principle is based on suppression of the 0th diffracting order for 10.6 mm radiation, with the energy being diffracted into higher orders, thus eliminating the specular reflection. The first results show 70-times suppression of the IR light at the target wavelength in the specular direction, while the EUV peak reflectance of the multilayer grating is 61% at 13.5 nm. The second solution consists of an IR-transparent B4C/Si multilayer stack which is used both as EUV-reflective coating and as a phase shift layer of the resonant IR antireflective (AR) coating. Suppression of more than two orders of magnitude at the target wavelength is demonstrated. The currently achieved EUV peak reflectance is about 45% at 13.5 nm