Extreme ultraviolet (EUV) lithography with reflective photomasks is currently being refined for high-volume manufacturing of chips with dimensions of 7 nm or less. EUV scanners can replace the most critical layers and provide lithography capabilities complementary to ArF technology. However, the fabrication and inspection of defect-free masks still remain one of the most critical issues facing EUV technology. In this review, we describe our research on the development of the 13.5-nm coherent scatterometry microscope (CMS) with high-order harmonic generation (HHG) for the mask inspection of EUV lithography. Using the HHG-CSM system, we observed programmed pattern defects in a periodic patterned mask. In the diffraction pattern from the EUV mask, a 2-nm wide line defect in an 88-nm line-and-space pattern as well as sub-100 nm sized absorber defects in a 112 nm hole pattern were both detected. By further improving the system, we demonstrated the successful reconstructions of an-88 nm periodic L/S pattern and a cross-pattern with a quantitative phase contrast. These results signify that the standalone HHG-CSM system has tremendous potential.