Photomask

A photomask (also simply called a mask) is an opaque plate with transparent areas that allow light to shine through in a defined pattern.

Photomasks are commonly used in photolithography for the production of integrated circuits (ICs or "chips") to produce a pattern on a thin wafer of material (usually silicon).

As feature size shrank, the only way to properly focus the image was to place it in direct contact with the wafer.

Lithographic photomasks are typically transparent fused silica plates covered with a pattern defined with a chromium (Cr) or Fe2O3 metal absorbing film.

[6] A set of photomasks, each defining a pattern layer in integrated circuit fabrication, is fed into a photolithography stepper or scanner, and individually selected for exposure.

[7] As used in steppers and scanners which use image projection,[8] the reticle commonly contains only one copy, also called one layer of the designed VLSI circuit.

A photomask with several copies of the integrated circuit design is used to reduce the number of steppings required to expose the entire wafer, thus increasing productivity.

[11] A 2005 study by IMEC found that thinner absorbers degrade image contrast and therefore contribute to line-edge roughness, using state-of-the-art photolithography tools.

The commonly used attenuated phase-shifting mask is more sensitive to the higher incidence angles applied in "hyper-NA" lithography, due to the longer optical path through the patterned film.

[16] EUV photomasks work by reflecting light,[17] which is achieved by using multiple alternating layers of molybdenum and silicon.

Beginning in the 1960s, thin film stretched on a metal frame, also known as a "pellicle", was used as a beam splitter for optical instruments.

It has been used in a number of instruments to split a beam of light without causing an optical path shift due to its small film thickness.

In 1978, Shea et al. at IBM patented a process to use the "pellicle" as a dust cover to protect a photomask or reticle.

Current pellicles are made from polysilicon, and companies are exploring other materials for high-NA EUV and future chip making processes.

A photomask. This photomask has 20 copies, also called layers, of the same circuit pattern or design.
A schematic illustration of a photomask (top) and an IC layer printed using it (bottom)
A simulated photomask. The thicker features are the integrated circuit that is desired to be printed on the wafer. The thinner features are assists that do not print themselves but help the integrated circuit print better out-of-focus. The zig-zag appearance of the photomask is because optical proximity correction was applied to it to create a better print.
Pellicle Mounting Machine MLI