Resolution limit and photon flux dependency in EUV ptychography

Abstract

With the transition of EUV lithography to high-volume manufacturing, EUV mask metrology has become a critical requirement, and for future technology nodes, the challenges will increase. RESCAN, a lensless actinic microscope dedicated to EUV mask inspection under development, is based on coherent diffraction imaging (CDI) where the complexamplitude image of the sample is obtained through its diffraction spectra measured with a pixel detector. While this approach can overcome the disadvantages and limitations of conventional optical imaging systems, in CDI, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-tonoise ratio of the recorded diffraction data depends on several parameters, such as the reflectance of the sample, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination. This paper investigates the optimal photon flux for RESCAN and analyzes the relation between the image quality and the EUV illumination intensity for a CDI-based imaging tool dedicated to EUV mask inspection and review.