A highly sensitive extreme ultraviolet (EUV) resist is strongly needed to reduce the development costs of high power EUV sources. Although highly sensitive chemically amplified resists based on acid-catalyzed reactions have been used in mass production lines, the chain length of acid catalytic reactions must be suppressed within several nanometers to meet the resolution requirement below the 32 nm technology node. Under such circumstances, the initial acid yield produced by EUV exposure is critical to the formation of ultrafine patterns. Using an acid sensitive dye, the authors evaluated the number of acid molecules generated by an EUV photon. The observed acid yield was well explained by the ionization model for acid generation originally proposed for chemically amplified electron beam resists. (c) 2006 American Vacuum Society.