Through-pellicle inspection of EUV masks

摘要

RESCAN is a metrology platform, currently under development at Paul Scherrer Institut to provide actinic inspection capability for EUV reticles. It is a lensless microscope and its defect detection protocol is based on coherent diffraction imaging. One of the key features of an actinic pattern inspection tool is the ability to operate on reticles protected by an EUV pellicle. Thanks to the absence of imaging optics in close proximity of the sample, there are no geometrical constraints preventing the inspection of a pellicle-protected reticle in RESCAN. Nevertheless, the defect detection sensitivity depends on the quality of the reconstructed images and it is therefore important to assess if and how these are affected by the presence of an EUV pellicle. We report here the results of an evaluation of the effects of different types of EUV pellicles on the reconstructed images. We observed that high-absorption silicon nitride pellicles significantly reduce the imaging quality whereas in the case of the CNT-based pellicles the imaging performance was not affected. We also observed no damage of the CNT-based pellicle. To our knowledge, this work is the first successful attempt to perform mask inspection through EUV pellicles.