We show that the use of in situ multi-wavelength ellipsometry allows endpoint detection during the plasma etching of submicron devices in a high-density plasma reactor. In addition, a quantitative model is presented to understand the ellipsometry traces obtained while etching patterned wafers. It allows one to determine the thickness of a film in real-time as it is etched. Knowing the thickness in real-time allows greater process control, as it enables one to stop or change the process at a specified remaining film thickness. This is extremely useful in the context of device fabrication, since processing conditions can be adjusted in real-time.