The EB-X3 is an electron-beam mask writer designed to pattern x-ray masks with minimum feature sizes of 100 nm and below with good beam placement accuracy. The target patterning resolution is 50 nm, and a beam acceleration voltage of 100 kV is used to achieve it. This resolution has already been demonstrated using the previous version of the writer, the EB-X2 (developed last year), which also employs an acceleration voltage of 100 kV. On the other hand, the target placement accuracy for the EB-X3 is less than 10 nm, which is more than three times smaller than that for the EB-X2. To improve the placement accuracy, the resolution of the laser measurement system was changed from lambda/128 (=5 nm) to lambda/1024 (=0.6 nm), and the address unit was set to 1 nm, which yields a reproducibility of mark detection of 4 nm (3 sigma) and a calibration accuracy for deflection distortion of 5 nm(\ mean \+3 sigma). In addition, the laser measurement system monitors five axes (X, Y, yaw, and pitch [X,Y]) and the results are fed back to position the beam. Furthermore, the signal fluctuations of the deflection, lens, and aligner circuits were reduced, so that beam fluctuations are kept under 2 nm. The EB-X3 was delivered to the ASET Super-fine SR Lithography Laboratory in March 1999, and is now being evaluated for use in x-ray mask fabrication. The prospects are good for achievement of a placement accuracy of 10 nm (3 sigma) in the near future.