A method of forming a process film includes the following operations. A substrate is transferred into a process chamber having an interior surface. A process film is formed over the substrate, and the process film is also formed on the interior surface of the process chamber. The substrate is transferred out of the process chamber. A non-process film is formed on the interior surface of the process chamber. In some embodiments, porosity of the process film is greater than a porosity of the non-process film.