What does the manufacturing department do in a chip fab, and how are lots scheduled?

In one sentence: a fab's manufacturing department is the team that keeps wafers going from step to step on time. A wafer goes through hundreds of steps, and it spends most of that time waiting in line in front of tools. Which lot goes on which tool first, how to route around a stopped tool, where a hot lot cuts in: that is Manufacturing's day. The core of it has its own name in the fab: dispatching.

How does Manufacturing split the work with process and equipment engineers?

The same tool, three people looking at different things:

Who	Looks after	Typical question
Process engineer	Whether what the tool makes is right	Is this lot within spec? Can it go on?
Equipment engineer	Whether the tool itself runs	Which part failed? When will it be fixed?
Manufacturing	Whether the lots keep going on time	Where is this lot? Which tool next? Will it make it?

For the first two, see "What's the difference between a process engineer and an equipment engineer?". Manufacturing doesn't decide the settings a tool runs with (the recipe), and it doesn't fix tools; it watches the flow of the whole line. When a tool's light turns red, the people on shift watching the line are often the first to see it. They follow the OCAP, the out-of-control action plan: stop what needs stopping, keep lots away from the tool, then call the engineer. Most of the hands-on tool work is done by technicians; for how they differ from engineers, see "What's the difference between a fab technician and an engineer?".

One more that people mix up: production control. Production control plans when each lot must be finished and when it ships; Manufacturing is who actually gets the lot there inside the fab. One plans the trip, the other drives.

What is dispatching?

Picture a busy restaurant kitchen: lots of tables waiting, only a few burners. Which table you cook for first changes how long every table waits. Dispatching is the fab's version of that: deciding which lot goes on which tool first.

A fab is harder to schedule than a kitchen, for three reasons:

• The same wafer keeps coming back. A chip is built layer by layer, and for each layer the wafer goes back through litho, etch and the other areas. So new lots just put on the line and old lots nearly done often compete for the same tool.
• Not every tool can run every lot. Some lots can only go on certain tools.
• Tools stop. While a tool is down, in maintenance or running test wafers, it can't take lots.

Most fabs don't plan every step in advance. A dispatch system decides on the spot which lot is next, by rules: first come, first served; earliest due date first; or comparing the time left until the due date with the work still left to do. Each rule has trade-offs, and none is best on every number. So the system handles ordinary hours, and people step in when something happens.

How do you decide which lot runs first?

People usually step in for these:

1. Hot lots. A hot lot goes to the front at every step. But a hot lot is fast because other lots give way: studies have found that as hot lots increase, the cycle time of regular lots grows clearly longer. So more hot lots is not better.
2. Q-time running out. After some steps, the next step has to start within a time limit, or the step is redone, or the wafers are even scrapped. Lots near the limit go first; better still, think backwards: if the next tool isn't free, don't start the earlier step yet. Why the limit exists: "What is Q-time, and why can't wafers wait?".
3. Tool status. When a tool goes down, the lots queued for it are spread over the tools that can still run them; a lot that only that tool can run has to wait until it's fixed. The day a tool's maintenance is scheduled also changes where tomorrow's lots should go first.
4. Will the next step jam? Push every lot out at once and the next step can't keep up either. Some tools take several lots at a time, such as the high-temperature furnace, so you also decide whether to wait for a full load or start now.

All of this comes together in the MES, the manufacturing execution system: where each lot is, when it finished a step, where it goes next.

What do moves and WIP tell you?

Manufacturing follows two numbers on the board every day: moves and WIP.

• Move: one wafer finishing one step counts as one move (some fabs count by lot). Many moves today means lots are going forward.
• WIP: work in process, the lots still on the line and not yet finished.

Read them together. There is a simple relationship between WIP and how long each wafer stays: WIP = output per day × the average number of days each wafer stays. Say a line finishes 1,000 wafers a day and has 30,000 on the line: on average, a wafer stays 30 days. With the same daily output, the more WIP piles up, the longer each wafer waits. So "put more wafers in" doesn't necessarily make them come out faster.

Moves have a blind spot too: every step counts as one, urgent or not. Push extra lots through easy steps and the number looks great, while the hot lot stuck in front of a stopped tool hasn't gone anywhere. So the board shows more than moves: which steps lots are piling up at, and which lots are close to their Q-time.

What gets handed over between shifts?

Fabs run around the clock; for why, see "Why do chip fabs run 24 hours a day?". The people watching the line often work 12-hour shifts with several crews rotating, so the line changes hands at least twice a day.

A handover passes on what isn't finished: which tools are down and who they're waiting for; which lots are on hold, why, and who decides; which lots are close to their Q-time; where the hot lot is right now. Public workplace-safety guidance recommends handovers that are face to face and two-way, both spoken and written, so the person taking over can ask questions on the spot; when a repair carries on across the shift change, that matters even more. The longer the shift, the more you rely on the lines you wrote down.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to Manufacturing. You run the etch area and open the dispatch board every shift: tap a lot, then a tool, watch the Q-time count down, and follow the two lines for planned and actual wafers finished. On the late-night shift, the etch tool ETCH-03 turns red at 02:47, and it's your call whether to follow the OCAP and phone the on-call engineer or press reset first; then you line up four lots at the wet clean tool and write the handover sheet. On the morning shift, Chamber A of ETCH-03 is still down, the 9 lots waiting for it have to be spread over the chamber and tools still running, the morning meeting wants a moves number, and you have to talk Zhou, a senior engineer, into giving up the tool he was about to use for an experiment, for a lot close to its Q-time. On the afternoon shift, Production control calls with a super hot lot, and the OHT in the ceiling happens to stop. On the evening shift, all five lines on the handover sheet are right, but a lot is missing. Sometimes a technician calls in sick, or the dispatch system goes offline. The same card also has a Production control role. The company, people, tool numbers, lot numbers and figures in the game are fictional; what Manufacturing does and how dispatching works follow public, general sources, and the real division of work differs from company to company.