a method of fabricating a high aspect ratio deep trench in a semiconductor substrate comprising reducing the formation of a passivation film during the etching of the trench by including a first step of contacting the substrate in which the deep trench is to be formed with a fluorine poor or low concentration of a fluorine gas in the plasma of etchant gases for etching the high aspect ratio deep trench , followed by a second step of increasing the concentration of the fluorine containing gas to create a fluorine - rich plasma while lowering the chamber pressure of the reactor and rf power . preferably , the second step is introduced periodically during the etching of a deep trench in an alternating manner .