Effect of substrate roughness on Mo/Si multilayer optics for EUVL produced by UHV-e-beam evaporation and ion polishing

We have examined the growth of Mo/Si multilayers fabricated by UHV e-beam evaporation and in-situ ion-polishing on Zerodur and ULE substrates of different high-spatial frequency roughnesses (HSFR 0.1-0.6 nm). Structural information about the multilayer structure, interface roughness correlations and interlayer formation has been obtained by grazing-incidence X-ray reflectivity and scattering measurements, cross-sectional TEM micrographs and normal-incidence EUV reflectance measurements. Buffer layers (single layer and multilayer) have been applied to the substrate surface before Mo/Si multilayer deposition for a partial smoothing and consecutive improvement of the achieved EUV reflectance. Amorphous carbon films have shown a significant smoothing effect on substrates with enhanced HSFR (0.2 - 0.6 nm r.m.s.) which results in an absolute EUV reflectivity enhancement of about 1% measured at 13.4 nm wavelength. Additionally, local reflectance inhomogenities caused by nanometer-size substrate defects (holes and asperities) are effectively smoothed-out. (Authors)