The effects of oxygen addition on the synchrotron radiation‐excited etching of crystal Si, poly‐Si, and SiO2using SF6reaction gas are investigated. Without oxygen, two phenomenologically classified reaction mechanisms are observed: gas phase and surface excitation mechanisms. With oxygen addition, the gas phase excitation mechanism disappears and only the surface excitation mechanism becomes dominant. Oxygen addition is extremely effective in the microfabrication of SiO2line and space patterns using poly‐Si as an etching mask. Its addition creates high selectivity between SiO2and crystal Si. Furthermore, it is considered that the added oxygen removes some etching suppressing species generated on the poly‐Si surface.