[Problem to be solved] Provided is a method for processing a substrate having high etching selectivity of silicon with respect to silicon-germanium and further having a high selection ratio of silicon with respect to a silicon oxide film and/or a silicon nitride film in surface processing when manufacturing various types of silicon devices, particularly various types of silicon composite semiconductor devices containing silicon-germanium.
[Solution] A method for processing a substrate includes: bringing an etching solution into contact with a substrate including a silicon film and a silicon-germanium film to perform etching; and selectively removing the silicon film, in which the etching solution contains an organic alkali and water and has a dissolved oxygen concentration of 0.20 ppm or less.