Core level photoemission spectroscopy using synchrotron radiation was performed to study the activity of sodium on the nitridation of the (100) face of silicon. At room temperature, the exposition to molecular nitrogen of a Si (100) surface modified by a sodium monolayer induced the formation of a SiNxcompound. The sodium catalyst is removed from the surface by thermal desorption at moderate temperature. Subsequently, a clean (sodium free) Si3N4‐Si interface was formed at a much lower temperature than without the alkali metal.