Photomasks

Masks in Photolithographic Production

A photomask displays the pattern of an integrated circuit and works as a template in the photolithographic production of the resulting device. Masks are used in the fabrication of high-end electronic components, semiconductor devices, or displays. Even though transistors are becoming smaller and smaller, photomask-based UV lithography is still the industry-standard workhorse 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, the pattern which is transferred by a mask aligner or stepper into a photosensitive layer by photolithography. For semiconductor or display applications a whole 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. A a master template for photolithographic manufacturing, the mask has to fulfill stringent requirements. Those include key specifications like linewidth uniformity, pattern position accuracy, edge roughness and minimum feature size. In addition, to enable a large process window for the final process, the photomask specifications have to be considerably better than the target application.

High line width uniformity and low edge roughness, considerably better than required for the final application

Precise pattern positioning and plate-to-plate accuracy to enable accurate alignment of multilayer structures

Good Mura condition to avoid disturbances in regular periodic patterns, especially for display applications

High repeatability to ensure stable photomask quality

High resolution

enables the production of photomasks for optical, UV, and DUV mask-based lithography

CD Uniformity

to create a production master with well-defined structure sizes

Edge roughness

considerably better than required for the target application

Placement accuracy

for pattern consistency throughout the complete mask set and to reduce Mura effects

Application images

Systems

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