The issue, to what extent the state of a surface after an etching plasma has been extinguished represents the state of the surface under actual plasma conditions, has been addressed for reactive ion etching (RIE) of silicon in CF4/O2 using ellipsometry during and after RIE, and x-ray photoemission spectroscopy (XPS) after transferring the sample in vacuum. In XPS, a, 1-2-nm-thick reaction layer consisting of silicon bound to fluorine and oxygen is observed. An increase in the oxygen content of the feed gas mixture produces a thicker SiFxO layer which contains more oxygen. Qualitatively similar reaction layer trends as a function of the process parameters were seen in the ellipsometry raw data taken during the plasma process. Upon extinguishing the plasma, the ellipsometric variables changed in a systematic fashion, which is shown to be primarily due to substrate cooling effects. In addition, a rapid change of the optical parameters of the substrate upon termination of the discharge is observed which cannot be explained by sample cooling. This change of the ellipsometric variables appears to be due to the adsorption of 1 monolayer of material upon extinguishing the plasma. The reaction layer thicknesses determined by ellipsometry before and after extinction of the plasma agree with each other and the XPS thickness estimates reasonably well (to within a few percent for the 25% 02 to 50% 02 range, where a simple model of a transparent overlayer can be used to describe the data), if substrate cooling is taken into account. This result validates postplasma surface reaction layer studies. © 1992, American Vacuum Society. All rights reserved.