Study on EUV mask blank inspection with multi-wavelength high harmonic generation EUV source

The rapid advancement of extreme ultraviolet lithography (EUVL) technology has significantly increased the difficulty of achieving defect-free mask manufacturing, thereby necessitating the development of high-sensitivity inspection tools capable of detecting minor defects on mask blanks. In response to this challenge, we developed an advanced optical inspection tool based on scattering dark-field imaging (MS-DFI) system, utilizing an EUV source generated through high-order harmonic generation (HHG). The photon flux of the produced high-order harmonics, centered around the wavelength of 13.5 nm, is approximately 5.9×1010 photons per second. Our experiments demonstrated its capability to detect and provide clear inspection signals from defects as small as 2 μm in diameter on the mask sample surface. However, further analysis of the experimental results reveals that the inspection efficiency diminishes sharply with decreasing defect size, proving to be insufficient for defects smaller than the hundred-nanometer scale. To address this limitation, we employed a finite-difference time-domain (FDTD) simulation method to further investigate the potential of the HHG source in defect inspection. This method can accurately model light propagation and scattering phenomena, allowing for an in-depth analysis of the defect inspection signals. The simulation results reveal a pattern consistent with Mie scattering theory. Additionally, we conduct a study of various orders of HHG sources with different wavelengths to evaluate their effectiveness. Our findings indicate that longer wavelengths, such as 38nm, exhibit a higher potential for detecting smaller surface defects, providing a promising direction for enhancing mask defect inspection systems.