In the semiconductor manufacturing of below 100 nm devices, overlay accuracy is a critical issue for high resolution on proximity x-ray lithography (PXL). We applied PXL for five layers (mark, isolation, gate, contact, and wiring) to fabricate a 100 nm n-type metal-oxide-semiconductor field effect transistor and evaluated the accuracy. Alignment exposure was done with optical heterodyne alignment and magnification correction. The overall overlay accuracy (mean +/- 3 sigma) was 25-45 nm, and the best was about 25 nm for the contact layer. The overlay error was classified into several components to estimate the error budget. The analysis revealed that the alignment error of the x-ray stepper is less than 20 nm under various conditions in device fabrication. Individual x and y scaling errors caused by mask image placement and the difference of wafer-to-mask gap was from 18-35 nm: the error of common in-plane deformation was about 20 nm. The present accuracy may make it possible to fabricate devices with a 100 nm ground rule.