Dielectric film having high refractive index and method for preparation thereof

A method for producing a high refractive dielectric film in which a pair of cathodes for magnetron sputtering, a pair of targets placed on the pair of cathodes and a substrate, which is a film forming object, placed at a location opposite to said pair of targets are prepared in a vacuum chamber capable of producing a reduced atmosphere, and a film is formed on the surface of said substrate by a reactive neutralization sputtering method using oxygen as a reactive gas while applying alternating voltage so as to change the polarities of said pair of cathodes alternately, wherein two steps of

• a step in which a first pair of cathodes made of an alloy of titanium (Ti) and another metal (M) or a mixture of titanium (Ti) and said other metal (M) are placed at mutually adjacent positions in parallel, and alternating voltage is applied to said pair of cathodes using as a process gas an oxygen-containing gas of a first given concentration, to form on the surface of the substrate a relatively high refractive dielectric film which is an amorphous material having no columnar crystal, made of titanium oxide (TiOx:
• 1≦x≦2) and another metal oxide (MOw:
• M represents a metal, 1≦w) and
• a step in which a second pair of targets made of silicon (Si) metal are placed at mutually adjacent positions in parallel, apart from said first pair of targets, the substrate surface on which said high refractive dielectric film is formed is shifted to a location opposite to said second pair of targets and alternating voltage is applied using as a process gas an oxygen-containing gas of a second given concentration, to form a relatively low refractive dielectric film on the substrate surface carrying thereon said high refractive dielectric film formed
• are repeated a given number of times in the order as described above or in the reverse order, to form on the surface of the substrate an optical multi-layer film.