EUV ptychographic imaging for high fidelity actinic inspection of periodic mask patterns with EUV ptychography microscope

Abstract

High-resolution patterned mask inspection technologies are increasingly critical as extreme ultraviolet (EUV) lithography advances for sub-3 nm processes. In EUV inspection, the 13.5 nm wavelength leads to strong absorption in most materials, challenging efficient imaging optics. To address these limitations, actinic patterned mask inspection utilizing coherent diffractive imaging (CDI) has been actively investigated. However, highly periodic patterns complicate image reconstruction due to limited diffraction diversity. In this study, we employed a multi-shot CDI-based actinic inspection tool, the EUV ptychography microscope, utilizing coherent EUV light from a high-harmonic source and a 6°oblique illumination system. We enhanced the spatial coherence of the probe by applying a 30 µm illumination aperture directly above the patterned mask, achieving a 1.41 times improvement in the signal-to-noise ratio of high frequency signals. Additionally, probe structuring created a distinct probe intensity distribution with a defined boundary, mitigating inverse problems during object extraction. We integrated momentum terms into the ptychographic algorithm to improve the convergence rate and incorporated a modulus-enforced probe with the algorithm to reduce crosstalk errors between the object and the probe, thereby enhancing the reconstructed image. Reconstruction of a 200 nm half-pitch line pattern demonstrated a 0.98π phase difference, closely matching the theoretical 0.96π. Reconstruction accuracy improved by 26%, and convergence increased by 1.8 times. These results validate the EUV ptychography microscope as a next-generation actinic mask evaluation tool.