A robust direct method for real time control, by multiwavelength phase modulated ellipsometry (PME), of the growth of plasma deposited structures is presented here. Transparent multilayers consisting of SiO2 and SiNx alloys are investigated. This feedback control method is based on the comparison between the real time PME measurements and precomputed target trajectories. It can provide the high precision required to deposit high performance optical coatings. In particular an overall accuracy better than 1% is obtained on a fifteen layer quarter‐wave filter, designed at 670 nm. This real time procedure, which is not limited to transparent materials nor plasma processes, appears to be a useful tool for process control. © 1996 American Institute of Physics.