= Photomask
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.<Glossary of Lithography Terms by Chris A. Mack>[Mask, Photolithography and Projection Printing]{ref}
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.
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