Si was etched using an O2molecular beam according to the chemical reaction 2Si+O2→2SiO↑. The minimum etching temperature was decreased by 25 °C when a Ge layer had been deposited on a clean Si surface before etching. At 800 °C, the Ge‐coated Si surface was etched while the clean Si surface was not. The O2partial pressure during etching was 2×10−5Torr; the etching rate was about 80 A˚/min at 800 °C. Auger electron spectroscopy showed that the number of Ge atoms slightly decreased during Si etching. Ge atoms on the surface are thought to weaken Si back bonds by forming a thin Ge‐Si alloy layer on the surface. Undercutting at the SiO2mask edge was suppressed by this Ge predeposition technique at 800 °C because the sidewall without Ge was not etched at this temperature.