Low-n absorber EUV masks: mask manufacturing assessment through process characterization and optimization with early look at EUV Low-NA and High-NA imaging performance

Industry has made significant progress advancing EUV Lithography technology for the past several years leading to its full adoption starting at N7 node and volume production through N3 node. To extend EUVL technology well into sub-2nm node regime and beyond, a new High NA EUV platform (Hi-NA) is needed and will be introduced. These new Hi-NA scanner platforms will use anamorphic optics with 0.55NA with EUV illumination incidence angle lower from current scanners arriving light of 6° incidence. At mask level, the combination of EUV light at oblique incidence, absorber thickness, and non-uniform mirror reflectance through incidence angle, creates mask-induced imaging aberrations, known as mask 3D effects (M3D). Additionally, these Hi-NA systems introduce mask related imaging complications such as non-telecentricity, which has shown to cause H-V bias due to shadowing, pattern shift through focus, and image contrast loss due to apodization by the reflective mask coatings. All these factors suggest a need for novel EUV mask absorbers, which will reduce these effects. Of the various novel materials, the low refractive index (low-n) absorber materials, characterized as EUVPSM, have received most attention and acceptance. EUVPSM promises to address mask 3D effects while improving wafer image quality with enhanced contrast. In this work, we have embarked on characterization studies to evaluate a EUV low-n absorber material (EUVPSM), which meets EUV mask fabrication requirements for impending 2nm and below advanced semiconductor device nodes. While a number of low-n materials were considered, through simulation and material evaluation the selection was narrowed to one material, which met thickness, PSM phase angle, and EUV reflectivity targets. Characterization consisted of studying mask manufacturing modules including patterning, post-exposure processing, cleaning, defect inspection and repair. In this study, the material was evaluated for critical patterning performance looking at key CD control metrics such as resolution, uniformity, linearity, proximity and LER/LWR. We also characterized pattern fidelity looking at complex mask designs for 2nm node class and beyond using internally developed advanced mask characterization and optimization methodology emphasizing 2D pattern characterization, including sidewall angle (SWA) assessment of patterned absorber and resultant imaging. Another module studied is the EUV Black-Border patterning process which must improve to meet tightening dimensional, placement and EUV reflectivity controls needed for the Hi-NA regime, especially to enable half-field stitching requirements. With a mask manufacturing process established and characterized, EUVPSM mask lithographic performance was evaluated both to validate the simulation models, and to assess overall EUV printability at Low-NA, where the lithography process and tools are more established. Additionally, an early assessment at Hi-NA conditions is planned to examine critical imaging metrics while comparing the EUVPSM to standard Ta-based binary mask absorber, including early assessment of BB process on both mask and wafer.