Spectroscopic ellipsometry was applied to study the damage induced by chlorine plasma etching of crystalline silicon (100). Two etching modes (with different de bias voltages) were investigated; (1) reactive ion etching with radiofrequency (rf) power applied only to the wafer stage and (2) a high density helicon plasma with and without rf power on the stage. Bias voltages from 0 to -350 V were employed. A two layer model was used to interpret the ellipsometry data. The interfacial layer was modeled as a mixture of amorphous silicon (a-Si), crystalline silicon (c-Si), and chlorinated silicon (SiClx), and the top surface was modeled as a mixture of a-Si and SiClx. The evolution of the damaged surface layer was investigated as a function of plasma exposure time and de bias. Realtime measurements show that the damaged surface layer reaches a saturated thickness and composition after about 4 a of plasma exposure. The thickness of the surface layer increases with mean ion energy (i.e., negative de bias voltage) from 9 Angstrom at 0 V to 32 Angstrom at -350 V. Ion energy also increases the equivalent thicknesses of a-Si and SiClx, within the surface layer. In addition, real-time ellipsometric traces indicate that the stable surface layer present after etching critically depends on the sequence and delay between the extinction of the plasma source and the stage power. When power is turned off abruptly, the surface that was present during etching does not change with time (in the absence of air exposure) indicating that post-plasma surface diagnostic measurements such as x-ray photoelectron spectroscopy reflect the surface that is present during etching.