Poly(phenylsisesquioxane) (PSQ) is known for its high RIE resistance, particularly in oxygen RIE condition, because of its ladder-type polymer structure with a high silicon content. Except chlorinated and vinyl substituted ones, the lithographic sensitivity is low. Low resist sensitivities of PSQ in both photo and electron beam lithography have been improved as reported here by a chemical amplification of crosslinking of hexamethoxymethylmelamine (HMMM) to the ladder-type silicon polymer, catalyzed by a photo-acid generator like triphenylsulfonium hexafluroantimonate (TPHA) onium salt. Spectroscopic and GPC data indicates HMMM acting as a crosslinking agent to give higher molecular weight resin despite no electrophilic sites available in the resin. © 1991.