A photomask displays the pattern of an integrated circuit and works as a template or stencil in the photolithographic production of the resulting device. Photomasks are used in the fabrication of high-end electronic components, semiconductor devices, displays, and many other applications. Even though transistors are becoming smaller and smaller, photomask-based UV lithography is still the industry-standard technique for micro-fabrication.
A photomask usually consists of a metal covered soda-lime or quartz plate with transparent openings. The metal absorbs the light at different wavelengths: Standard masks use chrome as absorption material for i-, g-, and h-line. The transparent image on the photomask represents the master template, where the pattern is transferred by a mask aligner or stepper into a photosensitive layer via photolithography. For semiconductor or display applications a whole set of photomasks is required to produce a 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 lithography depending on the resolution requirements. As a master template for photolithographic manufacturing, the photomask must fulfill stringent requirements. These include key specifications such as linewidth uniformity, pattern position accuracy, edge roughness, and minimum feature size. Additionally, to enable a large process window for the final process, the photomask specifications must be considerably tighter than the target application.
