A vertical x-ray stepper for SOR lithography has been developed. The stepper exposes wafers in a normal atmosphere to vertically-scanned SOR with continuous alignment control during exposure. The key devices of the stepper are a vertical X-Y stage and an opticalheterodyne alignment system. The X-Y stage uses airlubricated components, including air bearing lead screws, linear sliders and radial/thrust bearings. The optical-heterodyne alignment system detects the displacement between the mask and wafer gratings with precision of 8.3nm. Several exposure experiments proved that the alignment capability is better than ±0.1μm for a 3σ-value.