US Patent 7512926 Phase-shifting masks with sub-wavelength diffractive optical elements

The present invention discloses a method of designing a set of two tiled masks, as well as, a mask including: a first tile, the first tile being transparent to a light, the first tile having a first characteristic linear dimension that is 15% or less of a wavelength of the light; a second tile, the second tile being transparent to the light, the second tile having a second characteristic linear dimension that is 15% or less of the wavelength of the light; and a third tile, the third tile being opaque to the light, the third tile having a third characteristic linear dimension that is 15% or less of the wavelength of the light.