What does an AMHS engineer do in a chip fab?

An AMHS engineer (AMHS stands for automated material handling system) looks after a chip fab's whole system for moving wafer pods on its own: the OHT vehicles and their track in the ceiling, the stockers that hold pods, the lifters between floors, and the computer that decides which vehicle picks up a pod and which route it takes. The job is to make every pod of wafers get to the door of its next tool on time and be set down there.

What a wafer pod (FOUP) is, and how an OHT sets one on a tool, is covered in How do wafers move around a fab?, so it isn't repeated here.

What is there to look after?

Part	What it is	What can go wrong
OHT vehicles and track	Small vehicles hanging from rails; a large fab may have hundreds	One stops, and everything behind it is stuck
Stockers	Automated storage where pods wait for a tool, with a crane inside that shelves and fetches them	When one is full, new pods have to go to another stocker
Buffers and lifters	Some fabs keep small buffers near the tools; fabs with several floors use lifters to move pods up and down	Pods queue at the lifters too
The dispatch computer	Often called the material control system (MCS): which vehicle goes, by which route	Too many vehicles jam the track, just like cars on a road
The handshake	The signals an OHT and a tool exchange before a pod is lowered	One missing signal, and the pod can't be set down

What do they watch on a normal day?

They watch a few numbers: how long each pod takes from the moment a vehicle is requested until the pod is set down (the delivery time), how many trips are made each hour, and how busy the vehicles and the stocker cranes are. A lot spends most of its time waiting (see How long does a wafer take to make?), so when transport slows down, the waiting spreads down the line. If nobody watches, by the time Manufacturing calls to say lots aren't moving, a queue may already have formed.

The OHT vehicles need regular PM too: their wheels wear and crack with use, and a vehicle whose wheels go unchanged may stop halfway along its route.

What happens when an OHT vehicle stops on the track?

1. Find the vehicle that stopped them all: check every vehicle's position and log in the system. When the one in front doesn't move, the ones behind can only wait, so most of them aren't broken.
2. Send the vehicles not yet in around it: block off that stretch in the system so no more vehicles are sent into it, and route them another way if there is one. That keeps the jam from growing. An area with only one route just has to wait for the repair.
3. Restart it from the system, or send someone up: sometimes a restart from the system gets it moving. But if the cause isn't fixed, it may stop again soon, for the same reason as pressing reset on a tool.
4. Before going up, make sure it can't move: the OHT runs at ceiling height, so you go up on a lift platform. First block off the stretch and cut the power that needs cutting, and each person puts on their own lock and tag: that is lockout/tagout. Skip it, and the vehicle may move suddenly right next to someone's hands.
5. Take the pod off, send the vehicle for repair: a pod on the vehicle is taken off by procedure and moved on a cart instead (25 wafers, about 8 kg). The vehicle then goes to a section of track set aside for repairs, where it can be taken down without blocking anyone.
6. Before reopening, tell people, run a test, then fix the records: after a long stop, all the waiting lots come out at once, so tell Manufacturing and dispatch first and let the urgent ones go first. Test the repaired spot before reopening it. The trips people made by hand aren't in the transport records, so add them, or the next person can't find those pods.

What do they do when a stocker is almost full, or a crane needs maintenance?

When a stocker is almost full, move pods that won't be needed today, such as empty pods, to a stocker that still has room, and keep the ones close to their Q-time limit nearby. Otherwise, once it fills up, new pods have to go to another stocker and make the long trip back when they're needed.

Some stockers have two cranes in one aisle. While one is stopped for maintenance, that stocker may be a little slower, so the maintenance goes at a quiet time; afterwards the crane is tested before it touches a pod.

How do they find out why an OHT can't set a pod down?

Before an OHT lowers a pod, it and the tool exchange a series of signals called a handshake: the OHT asks "Can I lower it?", the tool answers "Go ahead", only then does the OHT lower it, and once it's down they signal each other again. The industry shares a common handshake specification, which is why different OHTs and tools can connect.

The handshake is a conversation between the two sides, and the dispatch computer often knows only that "this trip didn't complete". So someone checks on site:

1. Read both logs together: AMHS reads the vehicle side, an equipment engineer reads the tool side, and they go through it one signal at a time.
2. Find the signal where it stopped: did the vehicle not ask, or the tool not answer?
3. Check how the port is set: the platform at the tool's door (the load port) has two modes, automatic and "Manual". If someone set a pod down by hand and didn't switch it back, the OHT may never hear "Go ahead".

Work out which side it is first, and you don't have to take both sides apart.

How is it different from equipment, facilities and Manufacturing?

Job	Looks after	Where it meets AMHS
Equipment engineer	Each production tool itself	Up to the tool's door is AMHS; in a stuck handshake, each side checks its own half
Facilities engineer	The power, water, gases and air the whole fab shares	Facilities looks after what the tools consume; AMHS looks after the equipment that moves pods
Manufacturing	Which lot goes on which tool first	"Where to" is mostly dispatch and Manufacturing; "how to get there" is AMHS

Who pushes the hot lots on carts when the OHT stops differs from fab to fab. This work often sits in an automation group (see What departments does a chip fab have?), and the job titles differ too.

What background do you need, and what skills?

The work is half machines and half computers:

• Mechanical: how vehicles, cranes, wheels and track move, and where they wear.
• Electrical and controls: motors, sensors, and the signals the two sides exchange.
• Software and data: reading logs and delivery times to find where things will jam next.
• Industrial engineering: how many vehicles to run, how to lay out the track, where it will jam. At universities, much of the research on this comes from industrial engineering departments.

So people from all of these fields have a way in (see Which degrees work in a chip fab?). On top of that: safety habits that follow procedure, getting the facts straight when an on-call phone rings in the middle of the night, and explaining to many departments "where things stand and how long it will take".

In fab24hr

Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to the AMHS engineer (marked Beta). This role has no area to pick: the whole transport system belongs to your group. Which of the four shifts you get follows the time you open the game:

• Late-night shift, "The Queue": at 03:50 you're at home, and Chuan from the AMHS night shift calls: on the main track, the stretch from the front end to lithography, an OHT vehicle has stopped, and a queue has formed behind it. First you tap the vehicle that stopped them all on the system's map, then you decide whether to restart it from the system, or have Chuan block off that stretch while you go in and the two of you go up.
• Morning shift, "Stocker Almost Full": ETCH-03 has been down since the middle of the night, and the etch area's stocker is almost full, so you pick a few pods that aren't urgent to move to the front-end stocker. At 10:00, P-6A07 is due on ETCH-05; the OHT arrives, the handshake doesn't complete, and you have to find which side didn't answer.
• Afternoon shift, "That Stretch Stopped": on the etch area loop, an OHT vehicle has stopped above WET-02, and the vehicles behind it are queued back to ETCH-03. Manufacturing's afternoon shift, "The OHT stopped", is the other side of the same event: you block off the stretch and go up to fix it (or first try a restart from the system), and they push the hot lot on a cart.
• Evening shift, "Tallying up": the transport records are missing two trips, the two pods people moved by hand in the afternoon. ETCH-07's arm is stuck, so its load ports have to stop accepting pods; once it's fixed, do you wait for equipment to say it's tested before reopening, or open one port first? At 20:00, you look at today's delivery time.

The companies, people, lot numbers and tool numbers in the game are fictional; the number of vehicles and the track are simplified, and delivery time is shown only as faster or slower than usual. OHT congestion, the handshake and lockout/tagout follow public, general sources, and the real job titles and split of work differ from company to company. For other jargon, see Chip fab jargon in plain English.