What is rework, and why can litho be redone when etch can't?

Rework means redoing a step that came out wrong, or finishing what it missed. The vocabulary of the international quality-management standard puts it as action that brings something nonconforming back in line with its requirements. When a fab finds a lot that isn't right, it first puts it on hold, then decides whether to release it, rework it or scrap it; who decides is in "What is a lot hold?". This article is only about rework: what can be reworked, how it's done, and what it costs.

Why is litho the step that gets reworked most?

First, what litho does. Lithography prints the circuit pattern onto the wafer: coat a layer of photoresist, expose it, develop it, and the resist that's left is the pattern. Then etch follows the pattern and cuts away the layer underneath, and finally the resist is removed.

Think of masking tape before you paint a wall: if a strip goes on crooked, you peel it off and stick it again; once the paint is sprayed, the crooked edge stays on the wall. Photoresist is that tape, a temporary stencil. After developing, the pattern exists only in the resist, and the material below hasn't been touched. If the overlay is off, the line width is wrong or there are defects, the whole resist layer is removed, the wafer is back where it was before printing, and it can be printed again.

Once it has been etched, or atoms have been shot in by ion implantation, what changed is the wafer itself, and that can't be taken back.

How is litho rework done, step by step?

Step 1: Measuring after develop finds a problem

• What: After developing, measure overlay and line width and look for defects; this check is often called after-develop inspection (ADI). How it's measured: "How does a chip fab know each step went right?".
• Why: This is the last moment the pattern can still be changed before it's etched in.
• Otherwise: The wrong pattern gets etched in, and all that's left is judging whether the wafers are still usable, and scrapping them if not.

Step 2: Decide whether to rework, and find out why it went wrong

• What: Put the lot on hold first, and an engineer decides between rework and running as is; if only a few wafers need rework, they're split off into a lot of their own (a lot split). At the same time, look for the cause: if the scanner was off, the error goes back to it as a correction (see "What is overlay?"); if a hot plate is faulty, that plate is taken out of use first.
• Why: When something is only slightly off, rework isn't always worth it; and if the cause isn't fixed, printing again comes out just as wrong.
• Otherwise: A wasted pass, and the wafers go through one more round of processing.

Step 3: Strip the resist and clean

• What: Burn the resist off with an oxygen plasma, or dissolve it in chemicals, often one after the other, then clean the wafer (see the last step of "What is photoresist?").
• Why: Not a trace of the old resist can stay, or the new coat won't go on flat and stick well.
• Otherwise: Leftovers stay on the wafer and become new defects.

Step 4: Go through litho again from the start

• What: Dry, prime, coat with resist, bake, expose, post-exposure bake, develop: the same as the first time, with no step left out.
• Why: Every step affects the printed pattern, so the conditions have to match a normal lot.
• Otherwise: Skip the primer, for example, and the resist doesn't grip, so fine lines lift off during developing.

Step 5: Measure again, and release only if it passes

• What: Measure overlay, line width and defects just like the first time; only wafers within spec go on to etch.
• Why: Rework can go wrong too, so you confirm it's right this time.
• Otherwise: Everyone thinks it's fixed, and the wrong pattern gets etched in anyway.

What else gets reworked, and what can't be?

Rework also covers finishing what was missed. If etch stopped halfway, the remaining thickness is measured and the part that wasn't etched is added; that's a rework etch. If wafers wait too long after cleaning and go past their Q-time, some can be cleaned again before moving on (see "What is Q-time?").

But many steps have no way back:

What went wrong	Can it be reworked?	Why
Litho, before etch	Usually	The pattern is only in the resist; remove it and the wafer is as it was
Etch, not deep enough	Sometimes it can be finished	Measure what's left and etch that part
Etch, too far	No	Material that's etched away can't be put back
Wrong ion implantation	No	Atoms shot into the silicon can't be picked out
Growing an oxide	Not back to the original	The oxide uses up the wafer's own silicon, and nearly half its thickness has eaten its way into the wafer; even if it's removed and regrown, that silicon doesn't come back

When rework isn't possible, all that's left is to stop, measure and assess: wafers that are still usable run on with a deviation, and the rest are scrapped. Why implantation can't be undone: "What is ion implantation?".

What does rework cost?

• Time: another pass, and hours more waiting. Studies have also found that rework adds to the lots waiting in the fab and stretches the time the reworked lots take to finish (see "How long does a wafer take to make?").
• The scanner: reworked wafers have to queue for the scanner again. Litho is often the most crowded part of a fab, so every extra print makes the lots behind wait a little longer.
• The wafer itself: another dip in chemicals, another trip in a robot arm. Stripping and cleaning aren't completely harmless to the layer below either: the effect of one pass is tiny, but it adds up over many; studies have also found that the more times wafers go through cleaning, the more the silicon's quality suffers.

So rework is chosen carefully: only the wafers that really need it. A step that keeps needing rework means something earlier isn't being looked after, such as a hot plate, a bottle of resist or a scanner, and it's time to look for the real cause; see "What is root cause analysis?".

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 "Litho" under the name: you're the litho process engineer, and the four shifts follow the time you open the site. The afternoon shift is called "Rework": the coater/developer TRK-01 came back from PM this morning, and in the two lots out of it this afternoon, the line width is wide on one side and narrow on the other. With the 12 line-width maps in front of you, you mark the wafers to rework and send the rework order; the reworked ones are measured before they go on. On the morning shift, lot P-3A08's overlay is out of spec and its next step is etch, and Ya-Ting in Integration needs an answer: rework, or run as is. On the late-night shift, an etch tool may be down, and a developed lot is queuing in front of etch with its time almost up: wake the process on-call who has just gone back to sleep, or let it go back to litho for rework in the morning. That's your call.

In the same fab, the etch process engineer's afternoon shift "In a Hurry" (also chapter 8 of the main story, The life of a chip) has you run a rework etch by hand at the tool: wafer 7 only, 15 seconds only. On the mask engineer's late-night shift, if you let a hazy photomask go on the tool first, 5 of the 25 wafers will need rework.

The company, people, lot numbers, tools and figures in the game are fictional; what rework is, how it's done and why litho can be redone follow public, general sources, and the actual practice and judgment differ from fab to fab. For other jargon, see Chip fab jargon in plain English.