What is Q-time, and why can't wafers wait?

In a chip fab you'll often hear: "This lot's Q-time is almost up." Q-time is short for queue time, and in the fab it means a limit: after one step finishes, a later step must start within a set time. Miss it, and that lot can't simply carry on down its route.

Why can't a wafer wait?

Between steps, wafers aren't kept in a vacuum. They sit in sealed boxes in the cleanroom's air. After many steps, the wafer surface is in a "just right, still fresh" state, and it changes if it's left too long:

• After cleaning: cleaning strips the thin oxide and contamination off the surface, leaving bare silicon. But bare silicon slowly grows back a very thin oxide in air, and picks up traces of organic molecules. If the next step grows a film or oxide where quality matters a lot, that little extra on the surface changes the result.
• After developing: the photoresist pattern from litho needs to go to etch soon. Left too long, the resist can change, and the etched line width drifts with it.
• After some films: the surface absorbs moisture or slowly reacts with air, which affects how well the next layer sticks.

You can't see these changes, and measurements don't always catch them right away; by the time anyone notices, several more steps may have been done. So instead of checking afterwards, fabs set a rule up front: past the limit, the lot doesn't move on.

What happens when Q-time runs out?

It depends on the step and how far over. In mild cases, the step is redone (rework), for example cleaning again before going into a furnace. Some can't be undone, and the wafer is scrapped. In between, the lot is put on hold while an engineer decides whether it can still be used. Either way it costs time and tool capacity, so everyone would rather not go over at all.

So why does it still happen?

Because the tools in a fab always have a queue. A wafer spends most of its time waiting for tools, and Q-time ties two steps together: the countdown starts the moment the first step finishes. If the next tool happens to go down, is in maintenance, or has a stack of hot lots ahead, the clock keeps running.

So the people scheduling lots often think backwards: if the next step isn't ready, don't start the first one yet. Better for a wafer to wait a little before the countdown starts than to get stuck halfway. That's why dispatching in a fab isn't just first come, first served; it also depends on how many countdowns each lot is carrying.

In fab24hr

In fab24hr, much of Manufacturing's job is Q-time: which lots on the dispatch board are close to their limit, which stopped tool will hold up whom, and phoning an engineer at night to ask "can this lot still wait?". The time limits, lot numbers and tools in the game are fictional settings; what Q-time is and why it exists come from public, general sources. For other jargon, see Chip fab jargon in plain English.