We demonstrated the vertical etching of Si by CO2/SF6-based deep reactive ion etching (deep-RIE) and investigated the dependence of the etching profile on the passivation process time, CO2 flow rate, inductively coupled plasma (ICP) power, and process pressure. In addition, we demonstrated the microloading effect and investigated the fluorine resistance of the passivation film by XeF2 vapor-phase etching. Furthermore, to investigate the composition of the sidewall passivation film, we performed the optical emission spectroscopy of the plasma during the deep-RIE and energy-dispersive X-ray and X-ray photoelectron spectroscopy analyses of the chemical composition of the passivation film on the etched Si sidewall. We found that the main components of the sidewall passivation film in CO2/SF6-based deep-RIE are C and O derived from the CO2 plasma and Si oxides.