After etching a Si-containing low permittivity insulating film with chlorine based gas, the etched wafer is subjected to an etching aftertreatment process comprising introducing oxygen gas to a vacuum processing chamber with a pressure as low as 0.2 Pa to 1 Pa and a flow rate as low as 5 cc to 20 cc/min, generating plasma within the chamber, heating the wafer 2 being subjected to aftertreatment between 50° C. and 200° C., applying a wafer bias power within the range of 50 W to 200 W, and exposing the wafer to the generated plasma, thereby simultaneously removing the photoresist components, the antireflection film components and the halogen components.