A photomask is a patterned optical component used to define the illumination reaching a photosensitive layer in photolithography. Its pattern controls transmission, reflection or optical phase, depending on the imaging system. In projection lithography, optics form a reduced image of the mask on the substrate.[ref]
Glossary of Lithography Terms by Chris A. Mack. Mask, Photolithography and Projection Printing
The printed boundary depends on diffraction, illumination and photoresist response as well as the drawn mask edge. Optical proximity correction deliberately changes mask geometry to compensate for those effects. A mask feature is therefore part of an imaging-and-material process, rather than a guarantee that an identical outline will appear on the wafer.
