Photomask

Notes

A leading-edge mask set can exceed twenty million dollars, which is why NRE dominates low-volume chip economics.

Overview

A photomask, or mask, is the patterned master plate whose image is projected onto the wafer during lithography, one mask for each patterned layer of a chip. It is usually a plate of high-purity quartz carrying the pattern in a thin opaque film such as chrome, or for extreme ultraviolet lithography a reflective multilayer mirror with a patterned absorber. A full set of masks for an advanced process is one of the largest fixed costs of a new chip.

How it works

After tapeout, the layout is converted into mask data. This mask data preparation includes optical proximity correction, which adjusts shapes so they print as intended, and fracturing into the simple shapes a mask writer can draw. An electron beam or laser writer exposes the pattern in resist on a coated blank, which is then developed and etched to leave the absorber pattern.

Masks are inspected for defects, measured for pattern size and placement, and repaired where needed. A thin transparent membrane called a pellicle is mounted a short distance above the pattern, so that dust lands out of focus instead of printing on every wafer. Most scanners reduce the mask image four times, so mask features are larger than the printed ones but must be exact.

Masks for extreme ultraviolet lithography work by reflection, because almost every material absorbs that light. They have a stack of alternating molybdenum and silicon layers under the absorber, and their blanks must be almost free of defects. Specialist mask shops, often owned by or tied to large fabs, make most masks, and the complete set for a product is called a mask set.

Examples

• A full mask set for a new processor, with dozens of masks for transistor and wiring layers.
• A metal-only revision that replaces just a few wiring masks to fix a bug found at bring-up.
• A multi-project wafer, where many designs share one mask set to cut prototype cost.
• An EUV mask with a reflective molybdenum and silicon multilayer and a patterned absorber.
• A pellicle mounted on a mask to keep particles out of the focal plane.

Why it matters

Masks link design to manufacturing: a mistake in the design becomes a mistake in every mask that carries it, and a design fix after tapeout usually means paying for new masks. Mask data preparation engineers, mask shop engineers and lithography teams own the masks, while programme managers watch mask cost and cycle time.

In the chip lifecycle, masks are made right after tapeout, before the first wafers start, and new masks are needed for each metal fix or new revision of a design.

It matters most to: Mask Data Preparation Engineer, Mask Inspection and Repair Engineer, Mask Shop Interface Engineer, Photomask Engineer, Photomask Process Engineer

Common confusions

Not the same as reticle

In practice reticle and photomask usually mean the same plate. Strictly, a reticle holds the pattern for one exposure field that is stepped across the wafer.

Not the same as photoresist

The photoresist is the light-sensitive film on the wafer. The mask is the master plate the light passes through, or reflects from, before reaching it. Photomask vs photoresist

Not the same as hard mask

A hard mask is a film deposited on the wafer to protect areas during etch. It has nothing to do with the photomask apart from the name.

Sources

1. Photronics (opens in a new tab), Photronics, 2026-10-22
2. Lithography principles (opens in a new tab), ASML, 2026-10-22
3. Photomask (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22
4. Extreme ultraviolet lithography (Wikipedia) (opens in a new tab), Wikimedia Foundation, 2026-10-22

Related terms

• Mask blank
• Mask shop
• OPC

Where it appears on the Hub

Used on 160 pages of the Hub. In each group, the pages that mention it most come first.