Extreme ultraviolet mask roughness effects in high numerical aperture lithography

Area of Science:

• Optics
• Semiconductor Manufacturing
• Nanotechnology

Background:

• Extreme ultraviolet lithography (EUVL) relies on reflective optics, making surface roughness a critical factor.
• Mask multilayer roughness induces image plane speckle, leading to patterned line-edge and line-width variability.
• Future high numerical aperture (NA) systems in EUVL are expected to utilize anamorphic magnification projection optics.

Purpose of the Study:

• To investigate the impact of mask multilayer roughness on patterned line variability in high NA EUVL systems.
• To analyze the anisotropic behavior introduced by high NA and anamorphic optics.
• To quantify the increase in line variability, particularly in shadowed directions, compared to lower NA systems.

Main Methods:

• Simulations were performed to model the effects of mask roughness on image formation in high NA EUVL systems.
• Anamorphic magnification projection optics were considered for future high NA designs.
• Patterned line variability was analyzed for different NA values (0.33 NA and 0.55 NA) and directions (shadowed vs. unshadowed).

Main Results:

• Significant anisotropic behavior was observed in high NA systems.
• A substantial increase in relative patterned line variability was found in the shadowed direction for 0.55 NA compared to 0.33 NA.
• The shadowed-direction patterned line variability was twice as large as for unshadowed lines, with most increase in the low-frequency regime.

Conclusions:

• Mask roughness poses a significant challenge for high NA EUVL, leading to anisotropic line variability.
• The transition to higher NA systems exacerbates line variability issues, particularly in shadowed regions.
• Mitigating low-frequency roughness in mask multilayers is crucial for improving pattern fidelity in future EUVL applications.