Patent ID: 7129028

Claim:
A method of forming a holographic grating, comprising: providing a photoresist layer on an optical substrate, forming a resist pattern in the photoresist layer to have a depth of grooves deeper than a predetermined depth of diffraction grating grooves to be formed, preparing a mixed gas containing a fluorine based gas and oxygen, said mixed gas having a mixing ratio of the fluorine based gas and oxygen such that an etching speed to the photoresist layer is greater than that to the optical substrate, and etching the photoresist layer with the resist pattern by an ion beam generated by the mixed gas containing the fluorine based gas and oxygen until the resist pattern substantially completely disappears so that the diffraction grating grooves having the predetermined depth are directly engraved on the optical substrate, wherein said ion beam, in etching the photoresist, is irradiated from a direction perpendicular to a direction of the grooves of the resist pattern and inclined with respect to a direction of a normal line of the substrate.