This article overviews NTT＇s recent progress in membrane and absorber materials for x-ray masks and the fabrication of highly accurate masks for synchrotron radiation lithography. As membrane materials, SiC and diamond films are being studied. Experimental results indicate that diamond films are better than SiC from a mask accuracy point of view because of their higher rigidity. As absorber films, Ta films deposited by electron cyclotron resonance (ECR) sputtering are studied. The Ta films have the rr structure and large grains, and are chemically very stable. All masks are produced by a membrane process in which the patterning of absorber films is done after backetching. The absorbers are patterned by a new electron beam (EB) writer with an acceleration voltage of 100 kV and ECR ion stream etching with He gas cooling. Ta patterns with widths below 100 nm are etched accurately. Placement accuracy of absorber patterns produced by this process is confirmed to be about 30 nm, using relatively simple patterns. A technique for inspecting defects on SiC membrane masks using a scanning electron beam and repair using a focused ion beam have also been developed. Ta defects on SiC membrane can be easily detected with high contrast by the system.