We have developed a novel at-wavelength inspection system for extreme ultraviolet lithography (EUVL) masks that employs an EUV interferometric microscope. The main advantage of this system is the ability to observe a mask directly and detect defects as they are, thus allowing phase defects to be detected even in a finished mask with the absorber. Moreover, this system provides the high spatial resolution of an EUV microscope, the high phase resolution of interferometric measurements, and a high throughput. We investigated the defect detection capability using several types of EUVL masks with programmed defects, and found that programmed phase defects with steps as low as 5 nm can be detected. These results demonstrate the capabilities of a Mirau interferometric microscope for at-wavelength EUVL mask inspection. Considering the diffraction-limited resolution of Schwarzschild optics (similar to 50 nm), this system should be applicable to the subhundred nanometer technology node.