What do the red, yellow and green lights on a tool mean?

Walk into a chip fab (or almost any automated factory) and you'll see a small light tower on top of every tool: green at the bottom, then yellow, then red. It's a tower light, also called a stack light or signal tower. One glance from across the room tells you how that tool is doing.

Three colors, roughly what they mean

Every fab and every kind of tool defines them a little differently, but the general idea is close to the common color convention for industrial indicator lights:

Light	Roughly means
Green	Running normally, processing
Yellow	Needs attention: waiting for someone, idle or running a qual, or showing a warning
Red	Fault: the tool has stopped and someone has to deal with it

Some tools add a blue or white segment for states like "operator action needed", and steady versus flashing can mean different things too.

What's the difference between yellow and red?

A tool watches itself constantly: temperature, pressure, gas flow, vacuum. When a value goes where it shouldn't, the tool raises a message. A mild one is a warning; the tool usually keeps running and shows yellow. A serious one is an alarm; the tool usually stops itself and shows red. More on the difference under "Alarm and Warning".

Red is easy: the tool has stopped, wafers can't run, everyone hurries over. The hard one is yellow.

Why is the yellow light the one people ignore?

Because when it's yellow, the tool is still running.

A tool might raise several warnings a day, and most really are nothing: a value touches a limit and comes back. Over time, people get used to clearing a yellow light and carrying on. Most of the time that's right; otherwise the whole day would go into warnings that don't matter.

But now and then a yellow light is the first sign of something bigger: a part slowly wearing out, a fitting starting to loosen, a value creeping closer to its limit every day. If it's cleared every time and nobody writes it down, by the time it turns red a batch of wafers may already be ruined and the tool down for a long repair.

So many fabs ask that a yellow light may be cleared, but it must be logged, and a warning that keeps coming back must be followed up. That's simple to say. In the middle of the night, chasing output, five minutes before you can go home, it's the hardest thing to do.

In fab24hr

fab24hr's process engineer is woken by a phone call in the middle of the night because the etch tool ETCH-03 has gone down. On the evening shift of the same day, you'll stand in front of a tool showing a yellow light and decide whether to clear it and go home. The game's two titles, "Dismissed the Yellow Light" and "Saw the Yellow Light", are about that moment. The companies, people, tools and numbers in the game are fictional; how tower lights work comes from public, general sources.