photomask

A photomask, or mask, is a plate used in photolithography to transfer circuit patterns onto a substrate. It consists of a transparent plate, typically fused silica or glass, with a patterned layer of opaque material (usually chromium) that blocks light in the regions that should not be exposed. In modern semiconductor manufacturing, masks are used in projection or contact printing to imprint the mask pattern onto photoresist on silicon wafers. The mask pattern is the inverse of the desired circuit layout.

Operation: In a lithography step, ultraviolet or deep ultraviolet light passes through the transparent regions of the mask and exposes the photoresist; after development, the exposed regions are removed, revealing the underlying layer for subsequent processing. Masks can be used in contact printing, proximity printing, or projection systems called steppers or scanners. Each layer of a device requires its own mask; one mask may be used to produce many die across a wafer.

Types and variants: Transmission masks are the most common; reflective masks are used in certain DUV systems; phase-shift masks use phase-differential patterns to enhance resolution and contrast; there are also designed anti-reflective coatings and hard masks. Reticles are the masks used in projection systems; a mask can be a full wafer mask or a reticle that copies onto many wafers.

Manufacture and quality: Mask blanks are high-purity fused silica or quartz; chromium is deposited and patterned by lithography and etching, or by laser writing in advanced processes; masks undergo defect inspection, repair, and metrology. Defects cost yield, so high reliability and cleanliness are essential.