Fabrication method of semiconductor device

A method of fabricating a semiconductor device, which includes a separation step and has a high yield, is provided. A metal layer is formed over a substrate, fluorine is supplied to the metal layer, and the metal layer is then oxidized, whereby a metal compound layer is formed. A functional layer is formed over the metal compound layer, heat treatment is performed on the metal compound layer, and the functional layer is separated from the substrate with use of the metal compound layer. By performing first plasma treatment using a gas containing fluorine, fluorine can be supplied to the metal layer. By performing second plasma treatment using a gas containing oxygen, the metal layer supplied with fluorine can be oxidized.