B4C/Si based EUV multilayer mirror with suppressed reflectivity for CO2 laser radiation

We have developed a multilayer mirror for extreme ultraviolet (EUV) radiation which has near-zero reflectance for CO2 laser radiation (10.6 mm). The EUV reflecting multilayer coating is based on alternating B4C and Si layers. Substantial transparency of these materials with respect to the infrared radiation allows utilizing the multilayer coating in a resonant absorbing structure for 10.6 mm. We have integrated the multilayer structure with a well known quarter-wavelength thin film absorber. Experimental samples were manufactured using magnetron sputtering deposition technique. The samples demonstrate suppression of the infrared radiation by more than two orders of magnitude. At the same time EUV peak reflectance amounts 45% at 13.5 nm, with FWHM being about 0.284 nm that matches the 2% bandwidth of a typical EUV litho tool optical system. Therefore such a mirror can replace a standard Mo/Si mirror in an EUV lithography tool to form an efficient solution for the suppression of unwanted CO2 laser radiation.