The relation between SiO2 etch rates and incident fluxes of reactive species in a dual-frequency (27 MHz and 800 kHz) parallel-plate system was evaluated by using various in situ measurements tools. C4F8/Ar/O-2 was used for etching gases. The steady-state thickness TC-F of a fluorocarbon polymer layer on the etched SiO2 surface was also measured. The SiO2 etch rate could be related to total F atom flux Gamma(F-total), which depends on both the incident fluxes of C-F reactive species and the surface reaction probability s. The s is a function of the net energy on the reactive layer (V-net). This energy is determined by the incident ion energy and the energy loss at the C-F polymer on the etched surface. A change in V-net from 500 to 1450 V was estimated to correspond to a change in s from 0.01 to 0.1. The steady-state thickness of the C-F polymer TC-F increased when excess C-F species were supplied to the etched surface. A thick polymer (TC-F>1 nm) decreases the ion energy and slows or stops the etching in fine holes. A polymer 5 nm thick can decrease the ion energy by about 750 V. The TC-F must therefore be controlled when high-aspect contact holes are etched.