This article discusses recent progress in 1 x x-ray mask technology in Japan. Proximity x-ray lithography (PXL) using synchrotron radiation light with a wavelength of 0.7-1.0 nm can in principle provide as high a throughput as optical lithography, because PXL and optical lithography both employ photon beams and masks. A high-performance electron-beam mask writer called the EB-X3, suitable for PXL, has been developed by NTT under the Association of Super-Advanced Electronics Technologies (ASET) program. It has a reproducible image placement (IP) accuracy of less than 15 nm. We used the shaped-beam EB-X3 to fabricate 100 nm rule x-ray masks for the gate and contact/hole (C/H) layers of 4 Gbit dynamic random access memory (DRAM) test patterns. The ASET-NIST (National Institute of Standards and Technology) type masks consist of a 350-nm-thick TaBN absorber, a 3-mum-thick SiC membrane (27-mm square membrane area), a 1-mm-thick Si substrate, and a 6.63-mm-thick Pyrex glass frame. We have achieved an IP accuracy for 1 x x-ray masks of less than 15 nm at both the gate and C/H levels. The mask-to-mask overlay accuracy between the gate and C/H masks is less than 20 nm; and after magnification correction, it is less than 10 nm. The critical dimension (CD) variations for 100 nm features measured in a 24-mm-square area are less than 100 +/- 6.0 nm at both levels. An overlay accuracy for PXL of better than 30 nm (\mean\ + 3sigma) was obtained by the double-exposure method using the XRA volume-production x-ray stepper (Canon, Inc.) and 18 nm overlay ASET-NIST type x-ray masks (gate to contact). An IP reproducibility of less than 7 nm has been achieved for the EB-X3 itself (best data). The resist CD accuracy on the membrane for 4 Gbit DRAM patterns is better than 7 nm (Delta\mean\ + 3sigma = 6.6 nm). These data indicate that PXL has the potential for patterning 50 nm node devices with current tools.