The melting, eutectic alloy formation, and evaporation, induced locally by a scanning tunneling microscope, was studied for 20 nm Ag films, deposited on hydrogen-terminated Si(100) surfaces. The Ag thin film can be locally rearranged or evaporated with a lateral resolution of 100-150 nm. For long interaction durations 50-70 nm wide and 30 nm deep, grooves could also be cut into the Si substrate. The modification mechanism can be explained by a model involving local melting, alloy formation, and evaporation.