A method is provided for in-situ monitoring of etch uniformity during plasma etching, on the basis of the detection of interferometry patterns. The method is applicable to a reactor wherein a plasma is created in the area between the surface to be etched and a counter-surface arranged essentially parallel to the surface to be etched. The occurrence of interference patterns is detected at a location that is placed laterally with respect to the area between the surface to be etched and the counter-surface. The presence of an interference pattern at a particular wavelength is observed through the detection of oscillations of the light intensity measured by an optical detector, preferably by the standard Optical Emission Spectrometry tool of the reactor. When these oscillations are no longer detectable, non-uniformity exceeds a pre-defined limit. The counter surface is arranged such that the oscillations are detected.