Evaluation of Finished Extreme Ultraviolet Lithography (EUVL) Masks Using a EUV Microscope

Low-defect-density mask blanks are one of the key technological requirements for successful extreme ultraviolet lithography (EUVL). We have developed an EUVL experimental exposure system and evaluated the characteristics of the mask in the exposure area of 10 mm×2 mm. These imaging patterns are replicated as a result of the total performance of imaging optics aberration, resist process, and illumination condition. Furthermore, as the feature size continues become smaller, close to 50 nm or 35 nm nodes, it is becoming more difficult to determine the quality of the mask only from the replicated results. Therefore, we have proposed an at-wavelength mask inspection system based on the EUV microscope, which is the best way to observe the mask directly. Meanwhile, an EUV Mirau interferometric microscope has been developed at NTT, which has the advantage of direct observation of a finished mask at the wavelength of 13 nm. Using this system, preliminary experiments to examine the resolution of the EUV microscope were carried out. Until now, the Cr absorber pattern of a 250 nm line width corresponding to 50 nm on a wafer and 750 nm hole patterns on a mask could be clearly observed. Furthermore, as an example of defect observation, this system could find mask defects, which are due to insufficient etching of the buffer layer in mask fabrication. We confirmed the EUV microscope is a useful tool for evaluating a finished EUVL mask.