Actinic imaging and evaluation of phase structures on EUV lithography masks

Actinic imaging and evaluation of phase structures on EUV lithography masks Iacopo Mochi 1 , Kenneth A. Goldberg 1 , Sungmin Huh 2 Lawrence Berkeley National Laboratory, Samsung Electronics d We describe the implementation of a phase-retrieval algorithm to reconstruct phase and complex amplitude of structures on 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, 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. I. Introduction Mask defects are one of the main issues of concern for extreme ultraviolet EUV lithography. The development of accurate defect inspection, imaging, and characterization capabilities is essential to understand mask and pattern quality and to evaluate repair efforts. Existing and emerging non- actinic defect review techniques can fail to assess the extent of defects on EUV blank and patterned masks [1] because of the strong wavelength dependence of the mask and defect optical properties in the EUV region. For this reason, with the exception of printing, actinic mask imaging appears to be the most reliable way to predict the effect that defects in the pattern or on the mask blank will have on the printed wafer. Email: IMochi@lbl.gov