A Photomask is a metal covered soda-lime or quartz plate with transparent openings. The metal works as an absorption layer for light at different wavelengths. The standard Photomasks use chrome as absorption material for i-, g- and h-line. The transparent image on the photomask is a master template, which is transferred by a mask aligner or stepper into a photosensitive layer by photolithography. For semiconductor or display applications a set of photomasks is required to produce the complete device. For semiconductor manufacturing the photomask is protected by a foil (pellicle) to avoid any contamination.
The image on the photomask is patterned by laser lithography or e-beam depending on the requirements. Since a photomask is a master template for photolithographic manufacturing it has to fulfill highest requirements. Typical photomask specifications include line width uniformity, pattern position accuracy, edge roughness and minimum feature size. To enable a large process window for the final process the photomask specifications have to be considerably better than the target application.
Photomasks are used for prototyping in universities in all kind of fields of research but also for high end production of electronic components, semiconductor devices or displays. Photomask based UV lithography is still the workhorse for micro-fabrication.