An apparatus for deep-ultraviolet interferometric Lithography is described, in which the interfering beams illuminate the substrate through a fused silica prism and a layer of index-matching liquid. The liquid-immersion technique was found to be compatible with a commercially available, chemically amplified photoresist. The apparatus was used with a 257 nm light source to write gratings having a period of 97 nm and linewidth of approximately 40 nm.