Method of controlling sidewall profile by using intermittent, periodic introduction of cleaning species into the main plasma etching species

A method of plasma etching deeply recessed features having an aspect ratio of about 15:1 or greater to a depth of 1.5 μm or greater through a silicon-containing hard mask into a carbon-containing layer in a semiconductor substrate, said method comprising the intermittent use of a cleaning species within a continuous etching process, where at least one cleaning species is added to already present etchant species of said continuous etching process for a limited time period, wherein the length of each cleaning species addition time period ranges from about 5% to about 100% of the time length of an etching time period which either precedes or follows said cleaning species addition time period during said continuous etching process, whereby a specific nominal depth of etching is obtained essentially without bowing of etched sidewalls of said deeply recessed features.