Plasma etching has been widely used to etch the materials in the fabrication of semiconductor devices. For further development of integrated semiconductor devices, more precise control of the etching process is required. Thus, we have developed a mass-analyzed ion beam apparatus; energy controlled single species ions are irradiated onto the surface under an ultra-high vacuum condition, and thus we can study the roles of individual reactive ion irradiation without gas phase reactions or neutral radicals irradiation. We have investigated the desorption products and etching yields for silicon and silicon dioxide by CF_x⁺ (x=1, 2, 3) ion bombardments. Desorbed products could be detected with a quadrupole mass spectrometer by the pulse ion beam method, and we estimate the kinetic energy of desorbed products from the time delay of waveform of incident ion pulse. Etching yields of CF_x⁺ increased with increasing ion energy and with increasing number of fluorine atoms in the ions. Above 1000 eV, etching yields is gradually saturated. Below 500 eV, etching yields abruptly dropped with decreasing ion energy, and fluorocarbon films grew on the surfaces. These results suggest that the etching reactions are affected by chemical reactivity of the incident ions.