Reticle with reduced transmission regions for detecting a defocus condition in a lithography process

A method for patterning, comprising:

• forming a first mark on a surface of a wafer using a pattern in a reticle, the reticle comprising a glass plate and an opaque material on a bottom surface of the glass plate, the opaque material comprising a first opening of the pattern, the forming the first mark comprises illuminating a top surface of the glass plate using a light source and an illumination lens, wherein light from the light source passes through the glass plate, exits the glass plate at the first opening and illuminates a projection lens before reaching the surface of the wafer, the light exiting the glass plate via the first opening has an asymmetric intensity due to attenuation of the light from the light source at a region of the glass plate on one side of the first opening;
• and
• based on a position of the first mark on the surface of the wafer, determining whether a focal plane of light reaching the surface of the wafer via the projection lens is above or below the surface of the wafer.