Enhancing the reliability of actinic mask evaluation through high-frequency signal improvement based on EUV ptychography

초록

Extreme ultraviolet (EUV) ptychography is a promising actinic mask metrology technique capable of subwavelength and aberration-free imaging. However, its performance is limited by EUV absorption and insufficient acquisition of highfrequency (HF) diffraction signals. This study demonstrates resolution enhancement via probe refinement using a small-etendue high harmonic generation-EUV source and an illumination aperture. The source was upgraded with a narrower-bandwidth infrared driving laser, improving coherence and boosting EUV power by over 80×. Under an optical system with a 0.155 numerical aperture and 6° oblique mask illumination, Fourier transform-based simulations and experimentally obtained diffraction patterns of a Siemens star revealed a 2.69× improvement in the signal-to-noise ratio of HF diffraction signals and a 120% extension in maximum detected spatial frequency. Eventually, these enhancements enabled a reconstructed image resolution corresponding to 11.5 nm at the wafer plane. The reduced Fourier error further validated the improved phase retrieval, confirming EUV ptychography as a reliable and high-resolution actinic mask metrology tool for next-generation EUV masks.

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