What are WIP and moves? The two numbers a chip fab checks every day

WIP stands for work in process (also written work in progress): the wafers in a chip fab that have been started but aren't finished yet. A move counts how far they went: one wafer finishing one step is one move (some fabs count by lot). One number tells you how much is sitting on the line, the other how much moved forward today, and the board tracks both every day (moves and WIP).

What does WIP mean?

Picture a laundromat: the clothes customers have dropped off that haven't been washed and handed back yet are its work in process, whether they're spinning in a machine or waiting in a basket.

A fab is the same. From the day a wafer goes on the line at wafer start until it finishes the last step and leaves, it counts as WIP: wafers being processed in a tool, wafers waiting in a sealed wafer box, the FOUP, wafers riding the OHT on the ceiling ("How do wafers move around a fab?"), and wafers on hold, waiting for an engineer's decision. WIP can be counted in wafers and is often talked about in lots; a lot is usually the wafers in one box.

What is a move, and how is it different from output?

When a 25-wafer lot finishes the etch step, that's 25 moves if you count wafers, or 1 if you count lots. Every step counts, so a wafer racks up hundreds of moves between its first step and its last.

Output only counts wafers that finish the last step and leave the fab. Think of a marathon: moves are the total kilometers all the runners covered today; output is how many crossed the finish line today.

You need both, because a wafer takes months to make ("How long does a wafer take to make?"). The wafers leaving today were started months ago. Whether the line is flowing today shows in how many steps got done today: the moves.

How are WIP, moves and cycle time connected?

The three are tied together by a simple relationship called Little's law, named after the scholar who proved it: WIP = output per day × the average number of days each wafer stays. It describes averages over a long period and holds when the amount on the line stays roughly steady. The basic example is in "What does the manufacturing department do in a chip fab?"; here we add moves.

A made-up example (not any real fab): a line finishes 400 wafers a day, each wafer goes through 450 steps, and there are 36,000 wafers on the line.

1. Moves needed per day: 400 wafers × 450 steps = 180,000 moves.
2. Steps per wafer per day, on average: 180,000 ÷ 36,000 = 5 steps. This is often called the turn rate.
3. Days each wafer stays, on average: 450 steps ÷ 5 steps a day = 90 days. Little's law gives the same answer: 36,000 ÷ 400 = 90 days.

Now suppose the busiest step is already full, so daily output can't go up, but WIP grows to 40,000 wafers. Each wafer now stays 40,000 ÷ 400 = 100 days, 10 days longer. The extra 4,000 wafers didn't become output. They're just waiting in line.

What happens with too much or too little WIP?

Too much:

• Longer queues: with more lots in front of every step, every lot waits longer, and cycle time stretches.
• Q-time runs out more easily: after some steps, the next one has to start within a time limit, and time in the queue counts against it. The longer the queue, the more lots expire and need rework ("What is Q-time?").
• Hard to change course: squeeze in a hot lot and every step ahead of it is crowded; find a mistake at one step, and more lots have already passed it and are still on their way.
• Money stuck on the line: in accounting, WIP is inventory. The money has been spent, but it isn't a chip anyone can sell yet.

Too little: tools with nothing in front of them can only sit idle. The slowest, fullest step on the line is called the bottleneck, and an hour it sits idle is an hour the whole line loses ("What is equipment uptime?").

So there's a "just right" amount of WIP. Starting from very little, adding WIP raises output; once the bottleneck always has work waiting, output tops out, and anything more only adds waiting time. Research on wafer fabs aims at the same point: enough in front of the bottleneck to keep it busy, but not so much that cycle time stretches.

Why look at moves every day, and what does watching only moves miss?

Roughly, moves get used like this every day:

1. Set the targets: from production control's start and shipment plans and the tools' capacity, work out how many moves each area needs per day. That way every section pushes its share; without targets, some areas pile up and others run dry, and nobody notices.
2. Let the system count: every time a lot finishes a step and moves out, the manufacturing execution system, MES, logs it ("What is MES?"). With so many steps and so many wafers, nobody could count by hand, and a number that shows up half a day late comes too late to act on.
3. Compare with the target every day: actual moves divided by the target is the "Moves vs. plan" figure on the board. When it drops, something is usually stuck, and you need to say at which step. Find out only when output falls short at the end of the month, and it's too late to catch up.

Looking only at total moves misses a few things:

• Easy steps are the easiest to push: every step counts as one move, hard or easy, urgent or not. Push more lots through fast steps and the number looks good, while the hard lots never move.
• Hot lots and the total pull against each other: holding a tool empty to wait for a hot lot means fewer moves during that time, yet getting the hot lot out on time often matters more than a few extra moves ("What is a hot lot?").
• Everything piles up at the bottleneck: however fast the other steps run, the lots just crowd in front of the bottleneck, and the line's output is still only what the bottleneck can do.

That's why the board shows WIP, Q-time and hot lots next to moves.

What do you do when WIP piles up at one step?

Look at which step, and which tool, the WIP is piling up in front of, not just the total. In research on fabs it's common to set a target WIP for each step, and someone looks when it runs too high or too low. A step that's always especially high is usually the bottleneck. A common one is the litho scanner: it's very expensive, and a wafer comes back to it for every layer. Bottlenecks move, too: when a tool suddenly goes down, its step can become the most congested place.

When one step piles up, it's handled roughly step by step:

1. See which tool it's piling up at: not every tool in an area can run every lot. Without checking, the lots you send over can't be run.
2. Find the cause: is a tool down, a lot on hold, or too much coming from the step before? Different causes need different people, and if you ask the wrong one, the problem stays.
3. Spread out what you can: hand lots to the tools still running; that's dispatch ("What does the manufacturing department do in a chip fab?"). If you don't, tools that could be working sit idle.
4. Ease off upstream: if the next step is blocked, finishing the step before it just means joining the queue, and lots with a Q-time may expire there. So lots headed for the blocked step wait first; lots headed elsewhere carry on, or the tool sits idle.
5. Check with production control: if the pile stays, the ship date may move, and it's worth thinking about starting fewer wafers next ("What does production control do in a chip fab?"). Research also describes capping the line's total WIP and letting new wafers in only when it falls below the cap ("What is a wafer start?").

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. A moves reading stays in the top-right corner of the screen: a light and a percentage (on a computer it's labeled MOVE; a phone leaves the label off, and a very narrow screen shows only the light). It goes up and down with how well you line up the dispatch board and how many lots get pushed past their Q-time.

On the morning shift, Chamber A of ETCH-03 has been down since 02:47, with 9 lots waiting behind it. The shift may open with Chief Chen calling at 07:33: "What percent of target can etch move this morning? Give me a number now." You can say 90% right away, or call back after you've seen the dispatch board. Then you split the 9 lots across ETCH-03 Chamber B, 05 and 07. At the 08:30 morning meeting, Yun reports "Last night we hit only 82% of our moves", and then you report the whole area's moves as a percentage of target. Tap the meeting room, and the production board on the wall shows "Moves vs. plan" at 82%, and next to it "Etch step WIP": 9 lots, limit 6 lots.

As the litho process engineer (production fab › process › litho), you may get a call on the late-night shift and have to decide which of the five lots still to expose should be held before exposure. As production control, the late-night shift's schedule sheet has an item, "Recount the total WIP for the whole fab", and it reads: "The total is not the point today. Watching which step H-2417 is at and how long it has been stuck is more useful."

The company, people, tool numbers, lot numbers and board figures in the game are fictional; WIP, moves and Little's law are written up from public, general sources and research papers, and how they're actually counted (by wafer or by lot, which steps count) differs from fab to fab. For other jargon, see Chip fab jargon in plain English.