Method of etching porous film

A method of etching a porous film, wherein the method is performed in a state where a workpiece having the porous film and a mask provided on the porous film and providing an opening exposing the porous film partially is placed on a stage provided in a chamber of a plasma processing apparatus, the method comprising:

• supplying a first gas to the chamber, the first gas being gas consisting of perfluorotetraglyme;
• and
• generating plasma of a second gas for etching of the porous film, to etch the porous film at the same time as said supplying a first gas or after said supplying a first gas, and
• wherein partial pressure of the first gas in the chamber or pressure of the first gas in the chamber when only the first gas is supplied to the chamber is higher than critical pressure causing capillary condensation of the first gas in the porous film and lower than saturated vapor pressure of the first gas at a temperature of the workpiece during execution of said supplying a first gas.