PURPOSE: To obtain a precise thin film pattern for manufacturing LSI, etc. in a high yield by using a prepolymer of a monomer having three or more functional groups and causing polymn. on being irradiated with light, electron beams or the like or a precopolymer contg. the monomer.

CONSTITUTION: Al vapor-deposited film 2 for the wiring of the transistor formed on substrate 1 is coated with a prepolymer of polyvalent carboxylic acid ester having three or more COOH groups in the benzene ring and represented by formula I or org. acid ester having a triazine ring and represented by formula II or a precopolymer of the org. acid ester and methyl methacrylate as a negative type resist material to form a photoresist film 3. Film 3 is scanned and irradiated with electron beams or the like to crosslink the resist of the irradiated parts, forming increased mol.wt. parts 3A, 3B, 3C, and after removing the unirradiated parts by development, Al film 2 is plasma etched by applying a high frequency electric field in CCl4 gas. Thus, predetermined patterns 8AW8C are formed.