How do wafers move around a fab? FOUPs and the overhead hoist transport

Wafers get around a fab with two things: a sealed box called a FOUP, and an overhead hoist transport (OHT) that runs on rails in the ceiling. The wafers ride in the box, up to 25 to a box, and the OHT carries the box from one tool to the next and sets it down at the tool's door. The whole system that moves wafers around automatically is called an AMHS, short for automated material handling system. A wafer goes through hundreds of steps and has to move after almost every one, so this system runs all day and all night.

What is a FOUP, and how many wafers does it hold?

FOUP stands for Front Opening Unified Pod: a standard pod that opens at the front. People also just call it a pod or a wafer pod. It's a plastic box with a door on the front. Inside, both sides have rows of slots; each wafer sits in its own slot, spaced apart so they never touch.

These pods are made for 12-inch wafers (30 cm across), so each side of the box is a bit more than 30 cm. A pod holds up to 25 wafers. In a fab, one pod is often one lot: one lot number, moving from step to step together.

The pod's shape and size follow a public standard the industry wrote together. So a pod from any maker fits on another maker's tool and can be carried by another maker's OHT.

Why are wafers shut in a sealed box?

Because the lines on a chip are extremely fine. One particle too small to see can land on a wafer, break a line, and ruin that chip. Cleanroom air is already very clean, but people still walk around in it, and people are the biggest source of particles in a cleanroom (see Why do chip fab workers wear full bunny suits?).

So once a wafer leaves a tool, it's best if it never meets the room's air again. With the pod closed, dust from outside can't get in. A wafer is either in its pod or inside a tool, and both of those small spaces can be cleaner than the aisles where people walk. How cleanroom classes are counted, and why the aisles don't need the top class, is covered in How clean is a cleanroom?.

Still, the pod is full of air, not a vacuum. A wafer's surface slowly changes in air, which is why there's a time limit between some steps (see What is Q-time, and why can't wafers wait?). For wafers that are especially sensitive to oxidation or moisture, some pods are also filled with nitrogen or very dry air to push the moisture out.

What is the OHT, and why is it on the ceiling?

The OHT is a small vehicle that hangs from rails in the ceiling. It doesn't carry people; mostly it carries wafer pods.

Why the ceiling? Running overhead, the OHT can drive right above a tool or a stocker and drop off or pick up pods from there, without ever crossing the floor where people walk. The rails loop above the tools in each area, and the areas are linked by more rails or other transport equipment. Most 12-inch fabs today rely on the OHT to move pods.

How does the OHT set a pod on a tool?

On the front of each tool are a few platforms for pods, called load ports. The OHT drives to the spot right above a load port, stops, lowers the pod straight down onto the platform, lets go and pulls its hoist back up. It's a bit like a claw machine, except this time it's putting something down. To take away a finished pod, it does the reverse: lower the hoist, grab the pod, lift it up.

Once the pod is on the load port:

1. The platform lines the pod up and holds it in place.
2. The tool reads the ID on the pod to check that this pod belongs at this tool.
3. If it does, the tool opens the pod's door, and a robot arm inside the tool takes the wafers in one at a time.
4. When they're done, they usually go back into the same pod one at a time, the door closes, and the pod waits for the OHT.

How the OHT and the load port hand off ("Can I put it down?" "It's down.") follows a public industry standard too, so different tools all connect the same way.

If the next tool is still busy, the pod goes to a stocker to wait. A stocker is an automated warehouse with a robot arm inside that puts pods on shelves and takes them down; the OHT drops off and picks up pods at its ports.

Who decides where a pod goes next?

Not the OHT. Which steps each lot goes through, and what happens at each one, is set from the start, like a very long itinerary. The factory's computer system, the MES (manufacturing execution system), keeps track of which step every lot is on.

When a step is done, the system records it, and now it knows what kind of tool the next step needs. There are often several tools of the same kind. Deciding which lot goes on which tool first is called dispatch. In most 12-inch fabs, a dispatch system does this automatically by rules, such as first come, first served, nearest due date first, and hot lots at the front of the line. Once that's decided, the transport system calls an empty OHT vehicle to pick up the pod.

So most of the time, nobody needs to touch a pod. Only when a tool goes down or a hot lot cuts in, and the planned order has to change, does Manufacturing step in by hand (see What does the manufacturing department do in a chip fab?). A lot actually spends most of its time waiting: for a tool, for the OHT, for the lot ahead to finish.

Why don't people just carry the pods?

1. They're heavy: a full pod of 12-inch wafers, box included, weighs about 7 to 9 kilograms (roughly 15 to 20 pounds), like a big bag of rice. Lifting one now and then is easy enough. But a fab moves pods a huge number of times a day, and bending and lifting all day wears out wrists and backs. Older 8-inch fabs had lighter boxes, and people often moved them on carts or by hand; with 12-inch wafers the pods got heavier and quality demands rose, so fabs switched to automated handling on a large scale.
2. People bring particles: the fewer people near the wafers, the better.
3. Mistakes happen: grab the wrong pod or take it to the wrong tool, and the whole pod may need rework, or even be scrapped. Trip halfway and drop the pod, and several wafers inside can shatter. Letting the system move pods, and letting tools check the ID, takes away many of these chances to go wrong.

Still, people haven't stopped carrying pods entirely. When the OHT breaks down or a hot lot can't wait, someone pushes the pod on a cart. Then the cart goes slowly and stops before each corner, because all 25 wafers in that pod are in one person's hands.

In fab24hr

In fab24hr's production fab, the ceiling of the etch area has a loop of OHT rail, with a few OHT vehicles running on it and stopping above the tools and the stocker to drop off and pick up pods. Tap the rail and its card tells you that which tool a lot goes to is decided by the system, according to dispatch. Tap any pod belonging to another product and the camera follows it, so you can see where the OHT takes it.

To push one yourself, play Manufacturing: tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to "Manufacturing". In the afternoon shift a super hot lot cuts in, and of course the OHT stops halfway. You push the 25-wafer pod on a cart from the stocker to a tool at the far end; the pod wobbles, and when it tips one way, you steady it from the other side. In the morning and evening shifts you may also get the random event "The OHT is stuck" and have to decide whether the hot lot goes by cart first. Tap "Take it apart" on a tool's card, and on some tools (a cluster tool, for example) the wafer pods and the platforms they sit on are right at the front.

The fab, people, lot numbers and tool numbers in the game are fictional, and the number of OHT vehicles and the rails are simplified. Twenty-five wafers to a pod, a full pod at around 8 kg, and the OHT setting pods on load ports follow public, general sources. For other jargon, see Chip fab jargon in plain English.