There are two critical issues related to the EUV blanks. The first issue is high flatness for the glass substrates. In the case of optical mask blanks, flatness on the mask is mainly affected by depth of focus only in optical lithography process. On the other hand, non flat surface on the EUV mask affects undesirable image placement error (IPE) on the wafer caused by nontelecentric illumination with incident angle of 6-deg in EUV exposure. Relationship between IPE and flatness error () can be expressed by , where is reduction ratio of optics and is incident angle. Flatness error of 50 nm peak-to-valley () on the mask surface causes IPE of 1.3 nm on the wafer in EUV lithography with optics (). It means that a flatness error on the mask surface causes degradation in overlay performance in device manufacturing. Requirement of substrate flatness should be decided to meet a certain budget of overlay required in EUV exposure process. As the surface on the mask stage is a reference flat, back side on the mask must have high flatness as well as the flatness on the front side. The second issue is defects on the ML blanks. Defect size required for the ML blanks should be same as that of the optical mask blanks in the same technology node. However, defects with a few nanometer in height cause unacceptable critical dimension (CD) variation on the wafer in EUV exposure process as phase defects. This defect height is more than 20 times tighter than that for optical masks for 193 nm lithography with argon fluoride (ArF) laser due to main difference in wavelength. Moreover, as it is very difficult to repair the defects on the ML blanks, zero defects have to be achieved on raw ML blanks. According to simulation results,5 a defect with 60 nm in full width half maximum (FWHM) and 1.3 nm in height, corresponding to 22 nm sphere-equivalent-volume-diameter (SEVD), would cause CD variation of as printable defects at the 22 nm hp. Defect inspection tool with higher sensitivity is very important to manage and to improve small defects like a 22 nm SEVD. Traditionally, sensitivity of defect inspection is defined using sphere particle like poly-styrene latex (PSL) and as standard. However, in case of phase defects with low height on an EUV blank, it is impossible to discuss from sensitivity of particle. Recently, definition of SEVD was newly made as common standard for actual phase defects. A Lasertec M1350 and M7360 have sensitivity of 60 and 45 nm , respectively, which are insufficient for the target of 22 nm SEVD.6 Two kinds of inspection tools have been reported as next generation tools at the 22 nm hp and beyond. One of them is Teron blank inspection with 193 nm light developed by KLA-Tencor, known as Phasur, which is commercially available.7 The other one is actinic inspection tool with an EUV light that MIRAI-Selete have developed.8 It is facing to develop prototype tool in a program at EUVL Infrastructure Development Center, Inc. (EIDEC). Improvement of EUV blanks have been reported under inspection of M1350.2 In this paper, further improvements of flatness and defects on EUV blanks will be described.