The effective range of secondary electrons generated by Al Kα radiation is measured in polymethylmethacrylate (PMMA) as 400 Å. Lines with 0.1-μm linewidth with high aspect ratios are successfully replicated with x rays. Problems connected with the replication of very low contrast masks are discussed. Carbon Kα radiation is proposed as the radiation to be used for the replication of very fine geometries around or below 0.1 μm.