PURPOSE: To form a highly accurate pattern and contribute to the improvement of yield rate and device properties by etching it with a reaction gas including hydrogen bromide gas, and posttreating the particles consisting of created silicon bromides with a fluoride gas or a solution containing hydrofluoric acid.

CONSTITUTION: A field insulating film 6 is formed on a semiconductor substrate 1, and an insulating film 5 is created in a transistor formation area, and then it is coated with a gate electrode 7 consisting of polysilicon, and then it is covered with a resist film 8. Next, the semiconductor device is put in a PIE device, and hydrogen bromide gas is let flow in to etch the gate electrode 7 with a resist film 8 as a mask by reactive ions. Next, the semiconductor substrate is put in an ashing device, and oxygen gas, to which fleon gas is added, is let flow in and it reacts with particles, whereby the particles are removed by sublimation together with the resist film, and the gate electrode is patterned accurately, and also the surface of the semiconductor substrate is cleaned.