Photomask

rdf:langString Photomask

rdf:langString A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography and the production of integrated circuits (ICs or "chips") in particular. Masks are used to produce a pattern on a substrate, normally a thin slice of silicon known as a wafer in the case of chip manufacturing. Several masks are used in turn, each one reproducing a layer of the completed design, and together they are known as a mask set. Previously, photomasks used to be produced manually by using rubylith and mylar. As complexity continued to grow, manual processing of any sort became difficult. This was solved with the introduction of the which automated the process of producing the initial large-scale pattern, and the step-and-repeat cameras that automated the copying of the pattern into a multiple-IC mask. The intermediate masks are known as reticles, and were initially copied to production masks using the same photographic process. The initial stages produced by the generators have since been replaced by electron beam lithography and laser-driven systems. In these systems there may be no reticle, the masks can be generated directly from the original computerized design. Mask materials have also changed over time. Initially, the rubylith was directly used as the mask. As feature size shrank the only way to properly focus the image was to place it in direct contact with the wafer. These contact aligners often lifted some of the photoresist off the wafer and the mask had to be discarded. This helped drive the adoption of reticles, which were used to produce thousands of masks. As the power of the lamps exposing the masks increased, film became subject to distortion due to heat, and was replaced by silver halide on soda glass. This same process led to the use of borosilicate and then quartz to control expansion, and from silver halide to chromium which has better opacity to the ultraviolet light used in the lithography process.