A positive chemical amplification resist system has been developed for 50 kV electron-beam direct-writing lithography. This resist consists of a meta-cresol/para-cresol novolak resin, tetrahydropyranyl-protected polyvinylphenol (THP-M) as a polymeric dissolution inhibitor, and tri(methanesulfonyloxy)benzene (MeSB) as an acid generator. Radiation-induced hydrolysis of MeSB in the novolak resin matrix was confirmed to yield methanesulfonic acid and phenol derivatives by using gel-permeation and high-performance liquid chromatography. The catalytic chain length of THP-M deprotection was estimated to be approximately 300 in the matrix. It was found that the compatibility of a polymeric dissolution inhibitor with a novolak matrix relates to their polarity characteristics, and that the increase of para-cresol ratio in the novolak matrix can enhance the compatibility with THP-M and dissolution-inhibition capability of the composition. The optimized composition and process of the resist system achieved high aspect ratio patterns (0.3-mum contact holes/1.6-mum thickness) with high sensitivity (3.5 muC/cm2 at 50 kV).