Enhanced nanometer-scale linewidth metrology using dark-field through-focus scanning optical microscopy

Through-focus scanning optical microscopy (TSOM) enables nondestructive nanoscale dimensional metrology by comparing measured through-focus responses with simulated references. This study experimentally compared bright-field (BF) and dark-field (DF) TSOM for linewidth estimation of subwavelength single-line structures. Through-focus stacks were acquired by deformable-mirror focus modulation without mechanical translation. Fifteen targets spanning five nominal linewidth groups (40-120 nm) were measured in both modes. Corresponding BF and DF reference libraries spanning 20-130 nm at 1-nm intervals were generated using rigorous electromagnetic simulations. Each measured TSOM image was independently normalized and compared with its corresponding library using the mean squared difference (MSD), and the linewidth producing the minimum MSD was selected. Estimates were evaluated against independent scanning electron microscopy (SEM) measurements. DF-TSOM produced smaller absolute estimation errors than BF-TSOM for 13 of 15 targets and a lower group-specific mean absolute error for all five groups. The overall mean absolute error decreased from 5.07 nm for BF-TSOM to 2.87 nm for DF-TSOM, representing an approximately 43% reduction. This improvement is consistent with suppression of the specular-reflection background, which increases the relative contribution of linewidth-dependent scattered light. These results demonstrate that DF-TSOM can improve linewidth-estimation accuracy under the investigated conditions for subwavelength structures.