Actinic patterned mask inspection for EUV lithography

Abstract

In 2019, Lasertec successfully developed ACTISTM, the world’s first actinic patterned mask inspection (APMI) system, and has since been providing it as a solution to customers for use in EUV lithography production processes. APMI, is a type of inspection that utilizes the same 13.5nm EUV light used in EUV lithography. ACTIS can perform high-resolution, high-throughput inspection of EUV photomasks and detect all types of printable defects, the mask defects that would be printed on wafers in the EUV lithography process. DUV inspection can detect some of the printable defects. However, DUV inspection cannot detect phase defects, whereas APMI can. A comparison between the inspection performance of DUV and APMI for several defects, including phase defects, is provided in this paper. High-NA lithography is expected to be used for the EUV process at the technology nodes of N3 and beyond. High-NA lithography will be achieved by utilizing anamorphic optics, where magnification scales of projection differ along the horizontal and vertical axes. ACTIS has the extendibility to meet the requirements of high-NA lithography as it can be modified to have the NA on the mask extended along a single axis. Lasertec will develop a new generation ACTIS for use in high NA EUV lithography with higher NA projection optics. Discussions on increasing the NA of projection optics give rise to questions about inspection resolution due to light being blocked by the absorber on the EUV photomask. This paper will discuss this question using the results of simulations at varying absorber heights and NA values.