Method for forming a thin film of a composite metal compound and apparatus for carrying out the method

A method for forming a thin film of a composite metal compound comprising the steps of:

• sputtering independent targets formed of at least two different metals so as to form on a substrate an ultra-thin film of a composite metal or an incompletely-reacted composite metal;
• forming a flow of a reactive gas containing electrically neutral, activated species from a plasma including enriching the electrically neutral, activated species relative to electrically charged species;
• irradiating the ultra-thin film with the flow of reactive gas so as to convert the composite metal or the incompletely-reacted composite metal to a composite metal compound through the reaction of the ultra-thin film with the electrically neutral, activated species in the reactive gas;
• and
• sequentially repeating the step of forming the ultra-thin film and the step of irradiating so as to form on the substrate a thin film of the composite metal compound having a desired thickness,
• whereby the thin film is formed to have a refractive index within the range between the optical refractive index intrinsic to a constituent metal compound of the thin film and the optical refractive index intrinsic to another constituent metal compound of the thin film.