Study of real defects on EUV blanks and a strategy for EUV mask inspection

The availability of defect-free masks remains one of the key challenges for inserting extreme ultraviolet lithography (EUVL) into high volume manufacturing, yet little data is available for understanding native defects on real masks. In this paper, two EUV mask blanks with known native buried phase defects were characterized with a Lasertec M7360 (266 nm wavelength), atomic force microscope (AFM), and SEMATECH's actinic inspection tool (AIT), which is an EUV-wavelength microscope. The results show that there are various kinds of native defects on the mask blank. Not surprisingly, the surface height and measured EUV intensity profile of real blank defects can differ significantly from Gaussian-shaped defects. All defects found by the M7360 were observable in the AIT, yet many do not perturb the intensity enough to be printable in isolation. This paper shows that defects come in various sizes and types and clarifies what must be done to learn more about real defect printability to achieve defect-free mask blanks.