Probing Buried Defects in Extreme Ultraviolet Multilayer Blanks Using Ultrasound Holography

Imaging high-resolution subsurface defects nondestructively in Mo-Si multilayer (ML) blanks used in extreme ultraviolet lithography is a challenge and no known metrology tools are available to identify such defects in a nondestructive way. The understanding of their growth mechanism during ML deposition necessitates the monitoring of these defects, which can then lead to fabricating defect-free ML blanks. Here, we report for the first time, a unique and novel application of scanning near-field ultrasound holography (SNFUH) in nondestructive imaging of high-resolution e-beam patterned lines and bumps buried under Mo/Si ML film stacks used for ultraviolet lithography. Our results indicate the successful identification of buried defects under ML blanks using SNFUH.