Etching method and apparatus, computer program and computer readable storage medium

An etching method for etching a silicon nitride film in a laminated body wherein the silicon nitride film is laminated on a silicon oxide film and is covered with a hard mask whose principal component is a silicon oxide, and a patterned resist film is formed on the hard mask, the method comprising:

• (a) an etching step of etching the hard mask by using the resist film as a mask to form a mask pattern in the hard mask;
• (b) an ashing step of ashing the resist film;
• (c) an oxidizing step of oxidizing the surface of the hard mask under a different condition from that in the ashing;
• (d) a main etching step of etching the silicon nitride film by using the patterned hard mask as a mask;
• and
• (e) an overetching step of overetching the silicon nitride film under a selectivity condition of the silicon nitride film to the silicon oxide film that an etching of the silicon oxide film exposed under the silicon nitride film is suppressed,
• wherein the main etching step (d) is performed after the step (a) and before the overetching step (e) at a selectivity of the silicon nitride film to the silicon oxide film smaller than the selectivity in the overetching step (e).