We have evaluated and implemented a multiple-pass writing scheme that significantly improves the image-placement performance of masks for proximity x-ray lithography. Masks were fabricated using a 75 kV EL-3+ electron-beam lithography system that separates the data into fields and subfields, and exposes the images by using a variable-shaped beam. Multiple-pass writing allows averaging of system noise between multiple exposure passes written at fractional doses [Jpn. J. Appl. Phys. 32, L1707 (1993)]; stitching errors can also be averaged by offsetting the locations of the tool field and subfield boundaries for each pass [Jpn. J. Appl. Phys. 32, 5933 (1993)]. Multiple-pass writing was evaluated both with and without boundary offsets. Our experiments indicated that the offset method resulted in better image placement but negatively affected image size and defect performance because of the EL-3+system limitations. The no-offset method was optimized and implemented and achieved sub-50 nm (3 sigma) image placement. The method was then transferred to the EL-4 electron-beam Lithography system, resulting in image placement of sub-30 nm (3 sigma).