Patent ID: 7261745

Claim:
An apparatus for trim etching a structure on a semiconductor substrate, the trim etching employing an etchant gas supplied to an etch chamber via a first mass flow controller, and an inert gas supplied to the etch chamber via a second mass flow controller, the apparatus comprising; a first optical device for producing a first signal representative of an etchant gas concentration; a second optical device for producing a second signal representative of an inert gas concentration; an element for relating the first and the second signals to produce a normalized etchant gas concentration signal; and a comparator having a first input terminal receiving the normalized etchant gas concentration signal and a second input terminal receiving a reference signal representing a desired etchant gas concentration, the comparator for outputting an error signal for controlling the first mass flow controller, said error signal representing a difference between said reference signal and said normalized etchant gas concentration signal.