To provide an etching method for broadening a trench opening between
patterns of an etching mask in a dry process. The etching method of a
semiconductor substrate in which silicon and a silicon nitride film are
exposed at least on a surface of the semiconductor substrate, comprises an
oxidation step of oxidizing the silicon and the silicon nitride film from
an exposed surface to a given film thickness by spraying the semiconductor
substrate with substances excited by plasma discharge of O.sub.2 gas as a
reaction gas, and an etching step of etching the semiconductor substrate
oxidized in the oxidation step by plasma using a reaction gas comprising
at least O.sub.2 gas and CH.sub.2 F.sub.2 gas.
