Abstract: In the manufacturing of 16 Mbit DRAM chips, the deep trench formation process in a silicon wafer by plasma etching is a very critical step when the etching gas includes 0.sub.2. As a result, the monitoring of the trench formation process and thus the etch end point determination is quite difficult. The disclosed monitoring method is based on zero order interferometry. The wafer is placed in a plasma etcher and a plasma is created. A large area of the wafer is illuminated through a view port by a radiation of a specified wavelength at a normal angle of incidence. The reflected light is collected then applied to a spectrometer to generate a primary signal S of the interferometric type. Next, this signal is applied in parallel to two filters. A low-pass filter produces a first secondary signal S1 that contains data related to the deposition rate and the redeposited layer thickness. A band-pass filter produces a second secondary signal S2 that contains data related to the trench etch rate and depth.