e-beam metrology of thin resist for high NA EUVL

One of the many constraints of high numerical aperture extreme ultraviolet lithography is related to resist thickness. A critical consequence of moving from the current 0.33 to 0.55 NA (high NA) is depth of focus reduction. The question we seek to answer in the present study is how the resist thickness reduction, required to compensate for the drop in NA, impacts the e-beam metrology needed to characterize the process. The impact of film thickness on e-beam metrology was first investigated by critical dimension scanning electron microscopy (CD SEM) using our current best-known methods (BKMs). Optimized settings minimizing such an impact were then studied using CD SEM as well as low-voltage SEM. Atomic force microscopy was used to accurately characterize the sample thickness. Our results indicate that alternative operating conditions and BKMs are sometimes needed to meet the metrology requirements. However, even in the case of materials showing a large sensitivity to resist thinning, we were able to identify e-beam imaging conditions capable of meeting metrology specifications.