What is a hot lot, and why doesn't it always go first?

A hot lot is a chip fab's rush lot: a batch of wafers marked as high priority, which usually goes first at every step. "Hot" means urgent, and a lot is one batch of wafers that travels through the fab together. In research papers on semiconductor manufacturing, the ordinary lots are sometimes called cold lots. And there is a level above hot: papers also split the most urgent ones out as super hot lots.

How does a hot lot get through faster?

A wafer goes through hundreds of steps, and it spends most of that time waiting in line in front of tools. So the best way to speed up a lot isn't to make the tools faster. It's to make the lot wait less.

Normally, which lot goes on which tool first is decided by a dispatch system following rules (how that works: "What does the manufacturing department do in a chip fab?"). Mark a lot as a hot lot, and it moves up in those rules. There are a few common levels:

1. Priority at every step: at each tool, it goes ahead of ordinary lots. Like an ambulance on the road: the cars in front pull over.
2. Priority in transport: in a 300 mm fab, wafer pods ride the OHT on rails in the ceiling, and researchers have also studied how to get hot lots onto a vehicle first ("How do wafers move around a fab?").
3. A tool kept waiting: for the most urgent lots, a tool at the next step is cleared ahead of time, with nothing else loaded, so the lot can start the moment it arrives. Research on pilot lots describes this as "freezing" tools in advance. Like clearing the intersections an ambulance is about to drive through.

The further down this list, the faster the lot goes, and the higher the cost.

Where do hot lots come from, and who decides?

Generally, hot lots come from a few places:

• A customer in a hurry: their product is about to launch, or their own production line is waiting for these chips.
• Samples and pilot runs of a new product: the first lots are out, and both the customer and the engineers are waiting for results.
• Engineers waiting on an experiment: if a split lot comes back a day sooner, the next decision can be made a day sooner. Why R&D wafers often get pushed aside: "How is an R&D line different from a production fab?".

Marking a lot as hot is often handled by Production control, the people who plan the line: first they work out who will be late if this lot cuts in, and by how much, and then they decide (the other options they have: "What does production control do in a chip fab, and how are due dates set?"). Manufacturing then pushes it through the line, step by step. If a hot lot runs into trouble on the way and is put on hold, whether it can go on is often decided by the process integration engineer for that product together with the process engineer ("What does a process integration engineer (PIE) do?"). The real procedures and names differ from company to company.

What does a hot lot cost?

The waiting a hot lot skips doesn't disappear. The lots behind it wait longer. It's like a checkout line: when someone cuts in, everyone behind them waits a bit more; the more people cut in, the longer the wait at the back, and the harder it is to tell when your turn will come.

Papers on wafer fabs started doing this math decades ago. Here, cycle time means how long a lot takes from wafer start to finish ("What is a wafer start, and why does a fab count from that day?"):

• A computer simulation of a wafer fab found that the higher the share of hot lots on the line, the longer the average cycle time of every other lot, and the more those cycle times spread out.
• Another study, using a mathematical model plus simulation, found that because hot lots get priority at every step, the cycle time and output of ordinary lots both suffer noticeably.
• A more recent paper tries to put a price tag on hot lots: the hot lot itself speeds up, but the fab as a whole can do worse, because every other lot slows down.

Keeping a tool waiting adds one more cost: while it sits empty, it could have been running other lots. Tools that take several lots at once, like a furnace, usually wait until they can load more before starting. If one starts without a full load because a hot lot can't wait, the empty spaces can't be used for anything else.

The shares and the percentages in these studies differ from fab to fab and model to model, so here is only the direction: more hot lots is not better. When everything is a hot lot, hot lots wait behind each other, and nothing is really a hot lot anymore.

Why doesn't a hot lot always go first?

Because priority doesn't mean "first, no matter what." Along with who is urgent, the person lining up lots also looks at who can't wait.

Think of a hospital emergency room: patients aren't seen in the order they arrive, or in order of importance, but by who can least afford to wait. In a fab, the lots that can least afford to wait are often the ones close to their Q-time.

After some steps, the next step must start within a set time, or the wafer surface quietly changes and the lot needs rework, or even scrap (why: "What is Q-time, and why can't wafers wait?"). A made-up example: two lots are waiting at a wet clean tool, and one clean takes 35 minutes.

	Q-time left	Clean the hot lot first	Clean the ordinary lot first
Ordinary lot	30 minutes	Waits 35 minutes, expires, rework	Cleaned right away, in time
Hot lot	2.5 hours	Cleaned right away	35 minutes later, still in time

Clean the hot lot first, and it gains 35 minutes while the other lot has to be redone. Clean the ordinary lot first, and both are fine. What a hot lot usually needs is to finish by its own deadline, not to be first at every step. As long as it will still make it, letting the other lot go ahead saves it from rework. Research on Q-time points the same way: when dispatching takes into account how much time each lot has left, fewer lots expire, without the line slowing down much.

Apart from Q-time, there are other times a hot lot has to wait:

• A lot already in the tool: a step usually isn't stopped halfway through, so the hot lot waits for it to finish.
• Only certain tools can run it: if those are down, the hot lot waits, while other lots keep going on other tools.
• Its next step isn't ready: a hot lot has Q-time too. If the next tool is down, pushing the hot lot through this step first only leaves it stuck in the middle with its clock running.

So when lots are actually lined up, the question usually isn't "where does the hot lot go in line?" It's: how much time does each lot have left, how long does each one take, and when must the hot lot leave this step? If both can be covered, cover both. Only when they really can't does someone decide who will be late, and tell Production control and the engineers early.

In fab24hr

In fab24hr, the role that runs into hot lots most is Manufacturing. Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to Manufacturing. At one in the afternoon on the afternoon shift, Production control calls: S-9X01 is a super hot lot, the customer's line is waiting for it, and it has to leave the etch area by 18:00. But two lots behind it are close to their Q-time, and it will squeeze them. You line them up yourself on the dispatch board: put it on a tool first, or fit in the lots about to expire. A technician nearby says: "A super hot lot? Third this month. Every one of them is supposedly the last." Then the OHT stops too, and you have to take it over on a cart.

The late-night shift has one too. The hot lot H-2417 is stuck in an etch tool, its first 6 wafers have been split off to go to wet clean, and other lots are already waiting there. As the etch process engineer (production fab › process › etch), you get a call from Hao on the night shift: "Your 6 wafers are a hot lot. Can I jump the queue?" First you ask him to read out how much time each lot has left; line them up right, and you get to say: "A hot lot doesn't always have to go first. Whichever lot is closest to expiring gets cleaned first." As Manufacturing, you line up those same lots at the wet clean tool yourself, on the dispatch board, that night. In chapter 6 of the main story "The life of a chip", "The Call", the lot carrying Glowmere is H-2417.

The company, people, lot numbers, tools and times in the game are fictional; what a hot lot is and why it slows other lots down come from public, general sources and research papers. For other jargon, see Chip fab jargon in plain English.