We present a quantitative model to understand the ellipsometry traces recorded while etching patterned wafers in a high-density plasma reactor. This model allows one to determine the real-time thickness of a film as it etches, and data are presented showing the real-time thickness of TiN and polysilicon layers while etching. The model is generic to the film type and can be applied to any arbitrary stack of materials. 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. The model is essentially geometric and does not include the effect of diffraction. It has been applied successfully to many samples, each with different integrated circuit layouts. The results are in good agreement with scanning electron microscopy measurements.