What is a recipe in semiconductor manufacturing?

In a chip fab, a recipe is the set of settings a tool follows to process a wafer: numbers such as temperature, pressure, gas flow, time and power, written out step by step and stored on the tool or in the fab's systems. The word comes from the kitchen: the same oven, following different recipes, turns out different dishes.

What's in a recipe?

Think of a washing machine at home. "Normal", "Quick" and "Wool" are each a preset group of numbers: how much water, how many minutes of washing, how fast the spin. You pick one, and the machine does the rest.

A fab tool works the same way, only with more numbers and more precision. A recipe usually covers:

• Gases: which ones go in, and how much of each flows (often measured in sccm).
• Pressure: what pressure to hold in the sealed space where the wafer is actually processed (the chamber).
• Power: for tools that use plasma, how strong the plasma is.
• Temperature: how hot the wafer or the furnace gets, and how fast it heats up and cools down.
• Time: how many seconds or minutes each step lasts.

And it goes step by step. Take a furnace: the wafers go in, the temperature rises slowly, the reacting gas goes in once the set temperature is reached, it holds there for a while, and finally it cools down slowly. The temperature, gas and time for every step are in the same recipe. University cleanrooms that put their etch tools' details online often show a recipe as a small table: which gases and how much of each, what pressure, how many watts, with a note on roughly how much it etches in a minute.

Why does one tool have so many recipes?

Because one tool does more than one job.

• Different things to make: one tool may handle several films and several layers. A different material often needs different gases; a different thickness needs a different time.
• Different purposes: one recipe for normal production; another for finishing what wasn't finished, such as a rework etch; and after the chamber is opened and cleaned, a few dummy wafers are run to settle the chamber walls (season), which also follows a recipe.
• Different versions: every time something is changed, there's a new version, and the old one has to be retired.

Metrology tools have recipes too, saying how and where to measure. So a tool holding several, even many, recipes is perfectly normal.

Who can change a recipe, and what comes first?

A recipe usually belongs to the process engineer responsible for that step, who decides whether and what to change (read What's the difference between a process engineer and an equipment engineer?). Technicians on the floor run lots from the work order, using the recipe the system brings up, and don't edit it themselves (read What's the difference between a fab technician and an engineer?).

Production recipes usually aren't kept on the tool for anyone to edit. They live in a central system, which sends the right one to the tool when a job starts. A change usually goes like this:

1. Test first: not on customer wafers from the start. Use test wafers, or a split lot where only a few wafers get the new settings, and compare them with the ones that didn't.
2. Review: write down which number changed, why, and what the tests showed, and have the right people look it over and sign off. The international standard for quality management asks for this too: changes to production are reviewed, and the results and who approved the change are kept on record.
3. Record: save the new version, with who changed it and when, so it can be traced if something goes wrong. Retire the old version and take it off the list people can pick from.
4. Watch afterwards: once the new settings are in use, the measurements go on a control chart to see whether they drift to one side (read What is an SPC control chart?).

Why all the fuss? Because a slightly different number can mean a slightly different result, and that small difference sometimes only shows up several steps later.

How is a recipe related to the POR?

The POR, or process of record, is the standard process production follows: which kind of tool and which conditions every step uses, all locked down. If the POR is a whole cookbook, a recipe is one page in it: the numbers a tool actually runs for that step.

So changing a production recipe means changing numbers in the POR, and that takes a formal change process. This exact set is what the customer qualified; change it and tests may need to be redone and data shown to the customer again. There are even public industry standards on how suppliers must notify customers of process changes.

One kind of "change" is agreed in advance. When litho measures the overlay as off, the system adjusts the scanner's position and angle a little so the next lot lines up. This is called a correction. Which numbers move and how they're calculated were designed beforehand; nobody is editing the recipe on the spot.

Also, the same recipe doesn't guarantee the same result forever. The chamber walls slowly change and the etch drifts with them, which is why tools also need maintenance, seasoning and qual runs (read What is etching?).

What happens if the wrong recipe is used?

The tool doesn't know you picked the wrong one. It carries out the wrong recipe very precisely: a film too thick or too thin, an etch too deep or too shallow, too many or too few atoms implanted.

Many of these mistakes can't be undone. Etched material can't be put back, and atoms shot into silicon can't be taken out. Mild cases can be redone; serious ones end in scrap.

So for normal lots, the system brings up the recipe from the work order and nobody has to pick. The riskiest moment is a manual run, where an engineer goes to the tool and picks the wafers and the recipe by hand. Two recipes whose names differ by a few characters, a screen that defaults to the one used last, an old version that should have been taken down but wasn't: all of these are traps. The better answer is mistake-proofing: make the wrong recipe impossible to pick, rather than just reminding people to be careful.

In a fab, picking the wrong recipe is one of the most typical examples of an MO (mis-operation); read What does MO mean in a chip fab?.

In fab24hr

Tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to the process engineer and tap "Etch" under the name: you're the etch process engineer. On the morning shift you open the rework-etch recipe list: two retired versions can still be picked, with names only a few characters off the current one, and this list is yours to look after. On the afternoon shift you do a manual run at ETCH-03, wafer 7 only, recipe "Rework 15 s". When the screen opens, all wafers are selected and the recipe is the one used last, out of five on the list. Chapter 8 of the main story, The life of a chip, "In a Hurry", is that afternoon. As Manufacturing, each tool can run only a few recipes, and a lot can only go to a tool that has its recipe. On the R&D line, as the implant R&D process engineer, a call comes in the middle of the night: the recipe next to the right one was picked, and the two names differ by a single letter. The companies, people, tools and recipes in the game are fictional; what a recipe is and how it's managed come from public, general sources, and real practice differs from company to company. For other jargon, see Chip fab jargon in plain English.