Nanoscale topography control for the fabrication of advanced diffractive optics

Gray-scale electron beam lithography has been used to create high-efficiency (63%, output normalized) blazed gratings suitable for use at extreme ultraviolet (EUV) wavelengths (i.e., 13.4 nm). The total blaze height at these wavelengths is ≈7 nm. The surface topography was generated in a single processing step in hydrogen silsesquioxane (HSQ). This material converts to SiO2 upon exposure and forms a robust substrate for subsequent operations, unlike conventional organic resists. The HSQ is overcoated with a Mo/Si multilayer to provide reflectivity at EUV wavelengths. The grating efficiency is determined by the fidelity of the profile to the ideal and by the surface roughness of the HSQ. A region of the resist response curve was identified that enabled sufficient topography to be generated while maintaining the surface roughness of the resist below 2.5 nm root mean square. Large area (0.5×2.0 mm2) gratings were fabricated, and the resulting dose profile was adjusted during the course of the exposure to compensate for observed delay-time/reciprocity effects in HSQ.