Using a Langmuir probe and an energy-resolved ion mass spectrometer, gas-phase kinetics of fluorocarbon ions has been investigated as a function of the Ar percentage in a mixed CF4/Ar plasma. Spatially resolved electron energy distribution function, plasma potential, and ion density are measured in an inductively coupled plasma. As the Ar percentage increases, the average electron energy decreases while the electron density remains flat. The ion density also stays constant at low Ar percentages but increases over the Ar percentage larger than 63% mainly due to the increase of the Ar+ density. The plasma potential decreases as a result of the increase of Ar partial pressure, and this is confirmed by measuring the ion energy distributions of argon and fluorocarbon ions using the ion mass spectrometry. With the mass spectrometry, it is found that the most prominent ions, CF3+ and CF+, are formed predominantly by a process of dissociative ionization or radical ionization while CF2+ ions are formed dominantly by a process of charge transfer. As a practical application of this study for SiO2 etching, the densities of the fluorocarbon ions and radicals are correlated with the SiO2 etch rate and its selectivity to photoresist. Microtrench profile is also investigated as a function of Ar percentage and it is observed that the microtrench tends to be suppressed with the increase of the Ar percentage. This tendency is correlated with changes in the plasma chemistry as the Ar percentage increases.