Patent ID: 7771903

Claim:
A method for forming an illumination pattern on a photosensitive material to print the illumination pattern in the photosensitive material when the photosensitive material is later developed, the illumination pattern being formed by light comprising a wavelength λ to which the photosensitive material is sensitive, the illumination pattern being a pattern of one or more light areas and one or more dark areas, each light area being illuminated with greater intensity of said light than each dark area, the method comprising: obtaining an optical mask comprising a pattern of one or more first features and a plurality of second features, each first feature being less transparent to said light than each second feature, wherein each first feature surrounds one or more of said second features and borders on, and is surrounded by, one of said second features; and forming said illumination pattern by illuminating the photosensitive material with said light through the optical mask to project the optical mask onto the photosensitive material; wherein for each said first feature, the optical mask has the same optical pathlength outside the first feature adjacent to an entire outer boundary of the first feature as at the one or more second features surrounded by the first feature, the optical pathlength being measured along the optical mask's thickness; for each said first feature surrounding one or more second features, the first feature and the one or more second features are projected onto one of said one or more dark areas; and wherein at least one of the following conditions (A) and (B) is true: (A) for at least one said first feature, the first feature is spaced by at least one third of the wavelength λ from any other feature less transparent than the second features and located outside of the first feature; (B) for at least one said second feature surrounded by a first feature, the second feature has a length and a width each of which is at least 0.25λ.