Dark-field MicroscopyRWTH Aachen | Lukas Bahrenberg
A lot of bulk materials do not exhibit high reflectance in the EUV spectral region (1nm-50nm) under near-normal incidence. Despite this fact, there are special metrology applications where EUV dark-field reflection microscopy is used as a tool for spatially-resolved stray light analysis of certain samples.
The characterization of multilayer mirror structures that are designed for industrial EUV applications requires light of the same wavelength used in metrology as will be used in the final application.
Multilayer mirrors are stacks of bilayers that consist of two different materials of different indices of refraction. Stacking a certain number of these bilayers can create structures that show EUV reflectance under near-normal incidence.
In EUV lithography, the mask consists of such a multilayer mirror structure, the mask blank, on which patterns are written. The requirements for these mask blanks include the absence of defects.
If a mask blank is put into an EUV dark-field reflection microscope, possible defects will show up as bright spot in the final microscope image. The dark-field of a microscope is the angular domain into which no light is directly reflected from the illumination. It can give important information on the scattering behavior of mask blank defects. Once a defect on a mask blank is detected and localized, steps in the mask production process can be taken to repair these defects. Thus, mask blank defect detection is an important keystone in EUV lithography.
 Juschkin, L., Freiberger, R. and Bergmann, K., "EUV microscopy for defect inspection by dark-field mapping and zone plate zooming," Journal of Physics: Conference Series, 012030 (2009).
 Maryasov, A., Herbert, S., Juschkin, L., Lebert, R., Bergmann, K. and Behringer, U. F., "EUV actinic mask blank defect inspection: results and status of concept realization," Proc. SPIE 7985, 79850C (2011).