Resist level critical dimension SEM metrology: challenges and developments for EUV photomasks

CD-SEM equipment remains the workhorse for advanced EUV photomask critical dimension metrology. Leading-edge EUV photomasks require ever-improving SEM capabilities, as feature size and control requirements push accuracy budgets aggressively. High-NA photomask patterning is especially difficult, as sub-resolution assist features require <25 nm structures (mask level) with tight efficacy / print margins. Furthermore, inverse lithography approaches and curvilinear data formats have made pattern types substantially more complicated than the 1-D "Manhattan" geometries of the past. Develop-Check (DC) metrology is an integral capability within the mask house to segment lithography from etch. DC Overlay (DCOL) metrology and In-die CD metrology are critical for feedback to electron beam exposure writers, ensuring mask processes remain within specifications and enabling process drift detection before masks complete their patterning steps. High-NA photomask patterning has presented a unique set of challenges, particularly concerning thinner resists, lower SEM image contrast, increased stochastics, and resist shrinking risks during e-beam exposure. Edge Placement Error (EPE) estimation by contour fitting is very sensitive to SEM measurement recipe and measurement stochastics, and DC metrology fidelity is crucial for mask process control and line yield. In this work, we demonstrate five major highlights and developments: 1) CD-SEM tool artifacts in metrology and methods to mitigate them, 2) Roughness Power spectral density as a key metrology metric for process and tool improvement, 3) overlay CDSEM metrology enhancements for thin resist 4) approaches to overcome EUV thin resist shrinkage with novel SEM measurement algorithms and 5) defining CD accurately with contour fitting improvements for EUV masks with Monte Carlo simulation. By leveraging stochastic-aware metrology, algorithmic innovations, and reducing SEM measurement dose, we address the critical needs of the mask ecosystem for CD metrology.