Why is the lithography area in a chip fab yellow?

Photos and videos from inside a chip fab often show one whole area bathed in yellow light, windows included. That's the lithography area, and in the industry its nickname is simply "litho" (in Chinese-speaking fabs, literally "the yellow light"). The yellow isn't a design choice. The reason is in what happens there.

What happens in the litho area

The circuits on a chip are printed on, one layer at a time. The printing method is photolithography, or litho for short:

1. Coat the wafer with a thin layer of photoresist, a material that changes when light hits it.
2. Shine light through a photomask onto the wafer with an exposure tool (a scanner or stepper). The circuit pattern is "printed" into the resist.
3. Develop: a chemical washes away the exposed (or unexposed, depending on the resist) parts. What's left is the pattern.

Next, etch or another step follows that pattern into the layer below. A chip goes through this loop many times, and every layer starts with litho defining the pattern.

Why yellow: photoresist reacts to blue and ultraviolet light

Photoresist is designed to react to the light from the exposure tool. Classic exposure light is ultraviolet and the violet-blue light just next to it: textbooks mention the mercury-lamp g-line (436 nm, violet-blue) and i-line (365 nm, ultraviolet), and advanced processes use even shorter deep-ultraviolet light.

The catch is that ordinary white lighting also contains blue light and a little ultraviolet. Under normal white lamps, wafers waiting between tools would slowly get exposed by the room lights, and the resist would change before the real pattern was ever printed.

So the lamps in the litho area filter out blue light and everything shorter. What's left looks yellow or amber. The yellow film on the windows does the same job for the white light coming in from outside.

In one line: the litho area is yellow because photoresist reacts to blue and ultraviolet light, and yellow light doesn't expose it.

Is it to protect the workers' eyes?

No. The yellow is for the resist, not for people. If anything, working for hours under light of a single color isn't especially comfortable: a university study comparing workers in a yellow-lit litho area with workers in neighboring areas found poorer color discrimination and more eye fatigue in the litho group. Under yellow light, reds, oranges and whites are hard to tell apart, which people who work there will tell you about.

Why not just turn the lights off?

Because people still have to work there: watch tools, move wafers, read labels, handle problems. Total darkness would be dangerous and unworkable. Yellow is the compromise: it barely affects the resist, and human eyes can still see well.

Why not red? Red light is safe for resist too, but the human eye is much less sensitive to red, so the same lamp power looks far dimmer. Yellow and amber sit right between "safe for the resist" and "easy for people to see".

Is the whole fab yellow?

No. Only the areas that handle photoresist, the coating, exposure and developing, need yellow light. The rest of the fab (etch, deposition, cleaning, metrology) uses ordinary white light. Looking down a cleanroom corridor, there's often just one yellow block.

Yellow lamps aren't automatically safe forever, either. Filter tubes and sleeves age, and the wrong lamp can leak a little ultraviolet. Researchers have tested yellow-room lighting using photoresist itself as the detector and found lighting that slowly exposed it. That's one reason wafers sitting too long in the litho area is a problem in its own right.

A litho mistake can be undone

Litho has an advantage no other step has: you can redo it. If an exposure is off or the overlay is out of spec, as long as the wafer hasn't gone to etch, the resist can be stripped and the wafer coated and exposed again. That's called rework. Once etch has removed material, there's no going back. That's why the measurement right after litho is one of the most important checkpoints on the whole line.

In fab24hr

In fab24hr's production fab, the litho area is the yellow floor with two curtains on the left side of the cleanroom. The litho engineer's four shifts go from a stuck coater/developer track and the temperature of the post-exposure bake, to deciding whether out-of-spec overlay needs rework, to one defect on a photomask that prints the same flaw onto every wafer. The companies, people and numbers in the game are fictional; what litho does and why it's yellow come from public, general sources. For the jargon, see Chip fab jargon in plain English.