Background: With the adoption of extreme ultraviolet (EUV) lithography in the semiconductor manufacturing, actinic EUV mask metrology has become a crucial technology to ensure the required defect sensitivity and throughput for high-volume manufacturing. Reflection-mode EUV scanning microscope (RESCAN) is a lensless actinic microscope dedicated to EUV mask metrology based on coherent diffraction imaging (CDI) as an alternative approach for EUV mask metrology and inspection.

Aim and Approach: In CDI, the complex-amplitude image of the sample is obtained through its measured diffraction. Though this approach can overcome the disadvantages and limitations of conventional imaging systems, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-to-noise ratio of the recorded diffraction data depends on several parameters, such as the sample’s reflectance, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination.

Result: We investigate the optimal photon flux for RESCAN and provide a systematic study on the relation between the image resolution and the illumination intensity for CDI-based imaging at EUV wavelength.

Conclusions: The insights provided will be helpful for the optimization of CDI for EUV imaging, in particular for increasing the throughput of EUV mask inspection with low power sources.