A method for dry-etching a Si substrate or a Si layer in a processing chamber includes the step of supplying an etching gas into the processing chamber, wherein the etching gas is a mixture gas including Cl₂, O₂ and NF₃ and a residence time of the etching gas is equal to or greater than about 180 msec, the residence time being defined as:

where p represents an inner pressure of the processing chamber; V, an effective volume of etching space formed on the Si substrate or the Si layer; and Q, a flow rate of the etching gas.