A defect inspection technique on an extreme ultraviolet lithography mask is described. There are two kinds of defects, amplitude defects and phase defects due to the multilayer coating. The technique utilizes a microscope using the same 13.5 nm wavelength as the light used for exposure, and producing a magnified image of defects on a mask. Using this microscope, amplitude defects on practical masks and phase defects are observed. A phase defect was formed by a multilayer coated on a line pattern with a height of 5 nm and width of 90 nm on a glass substrate. Although the detected defect is made beforehand, it is detected by reflection of the light which penetrated inside of a multilayer. These results show that it is possible to detect the internal reflectivity distribution, without depending on surface perturbations.

Interferometric lithography is a well-known methodology to fabricate the periodic nano-scale structures such as nano-wires and nano-dots. It has also been widely studied by using extreme-ultraviolet light source because this approach might be able to characterize the light source, optics, and photo-resist to be used for the next generation projection lithography. Typically, the incident beam into the periodic grating diffracts and interferes at the downstream imaging position. These interference fringes, i.e. sinusoidal profiles, can be patterned on the photo-resist.In this paper, the basic concept of interferometric lithography is described. In addition, the Lloyd's mirror inter-