A tool has been fixed. Why does it still need a qual run?

A tool that has been fixed still needs a qual run because "fixed" only proves the tool runs. It doesn't yet prove that what it makes is right. In a qual run, after maintenance or a repair, a few blank wafers that never ship (test wafers) go into the tool first, run through one fixed set of conditions, and the results are measured. Only if they are in spec, and close to this tool's usual numbers, does the tool get its tool release and go back to production. The full word is qualification; when it is done again after maintenance, it is also called requalification.

It's a bit like a repair shop that has just replaced your brakes: the mechanic drives the car around the block and tries the brakes a few times before handing back the keys. The full list of checks after maintenance is in "What is PM (preventive maintenance)?"; this article is only about the qual run.

Once it's fixed, why not just run product?

Three reasons:

1. The chamber isn't what it was. After a part swap or a chamber open, the walls, the moisture left inside and the positions of the parts all differ from before the tool stopped. Freshly cleaned walls actually make the first few wafers etch differently from usual; researchers call this the "first wafer effect."
2. The repair itself can go wrong. A part that isn't seated, a fitting that isn't fully tightened: often you can't tell on the spot.
3. Losing the bet costs far more. Test wafers are blank; if one is ruined, you take another. A customer's pod holds up to 25 wafers, and each one has already been through many steps. Try the tool out on a lot, get it wrong, and you lose far more than a few test wafers.

How does a qual run work? Step by step

Step 1: The tool stops

What: the tool goes into scheduled maintenance, or breaks and throws an alarm. Its status goes down, and the lots behind it stay out. Why: someone has to open it and work inside, so nothing can be running in it. If you skip it: lots keep going in on schedule and get ruined in a tool that is broken or half taken apart.

Step 2: Equipment fixes it and hands it back

What: the equipment engineer swaps the part, puts everything back, pumps the chamber down and does a leak check, makes sure the hardware is OK, then does the tool handback to the process engineer. Why: settle "does it run?" first, so that the qual run measures "does it make the right thing?" If you skip it: a fitting is still leaking, the qual run keeps failing, and no one can tell whether it's the hardware or the conditions.

Why the pump-down takes time: "Why do semiconductor tools need a vacuum?".

Step 3: Run the test wafers and measure

What: first a few dummy wafers go through so the freshly cleaned walls settle (season); then the test wafers run through one fixed set of test conditions and go off to be measured. Who does these two varies from fab to fab. For an etch tool, three things are commonly checked:

• Etch rate: the test wafer carries a film. Its thickness is measured before etching and again after; the difference divided by the time is how much it etches per minute.
• Uniformity: many points on the same wafer are measured, to see whether the center and the edge etched the same.
• Particles: they are counted before the run and again after. The extra particles came from the tool itself.

Why: the three answer "did it etch enough?", "is the whole wafer the same?" and "is it clean?". If you skip it: check only one, say the etch rate is right, and a shower of flakes goes unnoticed.

Tools that grow films or implant measure different things; see "What are monitor wafers and dummy wafers?".

Step 4: Check the spec, and compare with the tool's usual numbers

What: the numbers are compared with the spec first, then with this tool's own past numbers, for example its own control chart. Why: the spec says "pass or fail"; the usual numbers say "same as before or not." In spec but clearly different from usual means something inside the tool has changed, and a few more wafers are often run to confirm. If you skip it: every qual run "just passes," the tool drifts a little further each time, and by the time it is really out of spec, many lots have been made.

How to read a control chart: "What is an SPC control chart?".

Step 5: Both sides pass, then the tool is released

What: the process engineer confirms the results, puts the tool back into production and tells Manufacturing it can take lots. If it fails, it goes back to step 2: Equipment looks again, fixes it, and the qual run is done again. Why: Equipment hands over "the tool runs," Process confirms "what it makes is right," and each half has someone answering for it. If you skip it: when only one side has the say, nobody looks at the other half.

How the two kinds of engineer split the work: "What's the difference between a process engineer and an equipment engineer?".

Step 6: After the release, look a little harder at the first lots

What: after the tool release, the measurements on the first few lots get an extra look, and the sensor readings tied to the new part are watched for a few days; some fabs measure more of those lots than usual. Why: a qual run is only a few wafers and one set of conditions, and a problem with a new part doesn't always show on the first wafer. If you skip it: the kind of problem that gets worse slowly is found only many lots later.

How a tool's sensors are watched: "What is FDC?".

How is a new tool's first qualification different from a qual run after maintenance?

Both mean "prove it first, then produce," but they prove different things:

	A new tool's first qualification	Qual run after maintenance or a repair
When	Just moved in and installed	After every PM, repair or chamber open
What it proves	The tool does what was agreed and can be used for production	The tool is back to how it was before it stopped
Compared with	Conditions agreed in advance by buyer and seller	The spec, plus this tool's own past numbers
How long	Long, with many items to test	Short: a few test wafers

A new tool has no "usual" to compare with yet. After it passes acceptance under the contract, it still has to run for a while and build up its own numbers before its control limits can be drawn. Every qual run after that is compared with those numbers. How a new tool gets from move-in to a signed acceptance: "What does an equipment vendor's field service engineer (FSE) do?".

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.

• Morning shift, "Firefighting": Chamber A of ETCH-03 has been down since 02:47. At the morning meeting the chief asks you, "What time will it be back up?" Later, Kai from Equipment tells you, "All we're handing back is 'the tool runs.' Whether what it makes is right only counts once your qual run passes." After the hardware handback, you first use a vacuum wand to move three test wafers into the pod, and if your hand shakes you may drop one. In the qual run you drag two settings, how many season wafers and how long to pump down and bake, until etch rate and particles added (10 or fewer) are both in spec. Do too little and it won't pass; do too much and every lot behind it waits. Once it passes, you tell Yun in Manufacturing she can send lots, and Kai says he'll watch the helium flow on the new ESC for three days. Chapter 7 of The life of a chip, "Firefighting", is the same morning.
• Evening shift, "Almost Out the Door": if you don't leave early, Kai asks you to keep an eye on Chamber A's helium flow. Each wafer reads a little higher than the last, it touches the warning line, and the yellow light comes on. Dismiss the yellow light and go home, or open a ticket: your call.
• From the afternoon review meeting until the evening handover, you can also go back to the metrology tool for a look: three lots have been measured since the tool release. Is the line width back around the center line?

As the etch equipment engineer (swipe to the equipment engineer and tap "Etch" under the name), the morning shift ends with the tool handback after the ESC swap. You may be able to skip some steps before the handback; skip the leak check, and on the first test wafer the helium flow goes over the warning line, so the chamber has to be opened again.

The companies, people, tool numbers and figures in the game are fictional, and the specs on the qual run screen are marked as illustrative only; what a qual run is and why it's done come from public, general sources. Which items are tested, and what the specs are, differ for every kind of tool and every fab. For other jargon, see Chip fab jargon in plain English.