PURPOSE: To improve verticality of etching, and increase the patterning precision of a minute thin film pattern, by forming a modified layer on a resist film, after the resist film is silylated.

CONSTITUTION: After an Al thin film 3 is formed on an Si substrate 1 on which an insulating film 2 is formed, a positive resist film 4 is formed on the film 3; prebaking is performed; the substrate is moved in a tightly closed chamber; a silylated layer 5 is formed on the surface part of film 4 by vapor phase treatment. When exposure is performed via a light transmitting region 7 of a photo mask 6, the layer 5 in the irradiated region of UV rays 8 is extinguished by substitution for H2. When this substrate is inserted into a microwave downflow equipment, and the film 4 surface is treated by the downflow of N2 plasma, the layer 5 is turned into a modified layer 9 which has [Si-N] bonds and is highly resistant to ion shock. The film 3 is selectively eliminated by RIE treatment using O2 gas wherein the layer 9 is used as a mask, and a resist mask 10 having the layer 9 is completed. An Al wiring 103 is formed.