Method and apparatus for forming an optical multilayer filter

A film forming apparatus for an optical multilayer filter having a predetermined number of layers in which the optical thickness of each of said layers of a λ/4-oriented structure is determined by its optimization in the neighborhood of λ/4, said apparatus comprising:

• a film forming apparatus main unit provided with a chamber in which layers of a multilayer film are formed on a substrate in a sequential order;
• a variable wavelength light source for emitting variable wavelength monitor light;
• wavelength setting means for setting the wavelength of said variable wavelength light source, for the optical thickness of said each layer to be formed, to a wavelength at which the transmittance or reflectivity of an interim multilayer film with said each layer formed as the outermost layer reaches an extreme value;
• an optical thickness monitor for detecting transmitted or reflected light of said monitor light incident on said interim multilayer film during the formation of said each layer and deciding whether said transmittance or reflectivity of said interim multilayer film has reached said extreme value;
• and
• a control device for effecting control to stop the formation of said outermost layer when it is decided by said optical thickness monitor that said transmittance or reflectivity of said interim multilayer film has reached an extreme value.