Actinic imaging and evaluation of phase structures on extreme ultraviolet lithography masks

The authors describe the implementation of a phase-retrieval algorithm to reconstruct the phase and complex amplitude of structures on extreme ultraviolet (EUV) lithography masks. Many native defects commonly found on EUV reticles are difficult to detect and review accurately because they have a strong phase component. Understanding the complex amplitude of mask features is essential for predictive modeling of defect printability and defect repair. Besides printing in a stepper, the most accurate way to characterize such defects is with actinic inspection, performed at the design, EUV wavelength. Phase defect and phase structures show a distinct through-focus behavior that enables qualitative evaluation of the object phase from two or more high-resolution intensity measurements. For the first time, the phase of structures and defects on EUV masks were quantitatively reconstructed based on aerial image measurements, using a modified version of a phase-retrieval algorithm developed to test optical phase shifting reticles.