The EUV optical system of the Reticle Imaging Microscope (RIM) for EUV mask inspection consists of a pinched Xeplasma source, a pupil-relayed Koehler-type illumination system and an equal-radii Cassegrain-type microscope with a 10x magnification1.
The 3D surface topologies were characterized over spatial wavelengths ranging from the clear apertures down to a few nanometers by using a portfolio of instruments including contacting profilometry, phase-shifting interferometry at 633 nm at various magnifications and Atomic Force Microscopy. Measured 3D topography maps were Fourier analyzed and Power Spectral Densities (PSDs) are computed over spatial periods ranging from the critical aperture down to a few nm. Integrated RMS surface errors over typically reported spatial period ranges were computed. For a different optical system we improved our polishing process to reduce surface errors for spatial periods below 10 mm. PSDs and integrated RMS surface errors will be shown in comparison with typical RIM surfaces.
All surfaces of the RIM optical system were coated with high-reflectivity coatings to maximize optical throughput. A description of the coatings and their performance had been published recently by Michael Kriese et al.2 The transmitted wavefront error (TWF) of the imager module was measured in a double pass configuration using a Fizeau-type Interferometer at 633 nm wavelength and a convex retrosphere. The measured TWF will be shown over the entire Numerical Aperture (NA = 0.0625) of the microscope. The integrated RMS of the TWF measured 0.79 nm.