What are monitor wafers and dummy wafers, and how does a fab tool prove it's still OK?

Control wafers, dummy wafers, monitor wafers: they are all wafers that never ship. They aren't customer product; the fab uses them to look after its own tools. A monitor wafer (also called a control wafer) is there to be measured: a tool runs one on a schedule, and someone measures the particles on it, the thickness of a film or how well a layer conducts, to see whether the tool is behaving as usual. A dummy wafer is there to take up space: dummies fill both ends and any empty slots of a furnace boat, so customer wafers sit where temperature and gas flow are steady.

How are monitor wafers, dummy wafers and product wafers different?

Three kinds of wafer, side by side:

	Product wafer	Monitor wafer (control wafer)	Dummy wafer
Whose is it?	The customer's	The fab's own	The fab's own
Pattern on it?	Yes, layer after layer of circuits	Usually not: blank silicon	No
What is it for?	Becoming chips	Being measured, to check the tool	Filling slots, warming up a tool
Does it ship?	Yes	No	No

These are common names, and they vary from fab to fab and place to place. Dummy wafers are sometimes called filler wafers, and the two together are often called C/D (control/dummy) wafers. In the game, wafers run just to try out a tool are sometimes called test wafers.

Why not just measure the product?

Product gets measured too, but that alone isn't enough, for three reasons:

1. Product has patterns, which makes it hard to measure. The surface is covered in circuits, like looking for one speck of dust on a patterned rug. A blank monitor wafer is flat: sweep a laser across it and one extra particle shows up. On product, film thickness is often measured in small open areas left on purpose between the chips.
2. Some measurements touch or even ruin the wafer. Measuring how well a layer conducts means pressing fine needles right onto the surface, and seeing between the layers means cutting the wafer open (a cross-section), after which it can't be used. You can't do that to a customer's wafers.
3. By the time product is measured, it's late. A monitor wafer runs before customer wafers go into the tool. If you only find out from finished product, the tool may already have run several lots while it was off.

A day in the life of a monitor wafer, step by step

Step 1: Before production, or on a schedule, run one first

What: before customer wafers go into the tool, or at a set interval, put in one blank monitor wafer and run it with the usual settings. Why: what a tool produces slowly changes, and a wafer that won't ship is the cheapest thing to test on. If you skip it: the tool drifts and nobody knows, the next customer lot goes straight in, and the whole lot comes out wrong.

Step 2: Measure it

What: what gets measured depends on the tool.

• Tools that grow films (thin film, furnace): how thick the film grew, and how even it is across the wafer.
• Tools that remove material (etch, CMP, wet clean): measure the thickness before and again after; the difference tells you how much was etched, polished or cleaned off.
• Ion implant: after implanting, heat the wafer first, then measure its sheet resistance to see whether enough atoms went in.
• Many tools also count particles: count once before the run and once after; the extra ones came from the tool itself.

Why: each kind of tool tends to change in its own way. If you skip it: measure the wrong thing, and nobody is watching the part of the tool that is actually changing, so a drift goes unnoticed.

Step 3: Plot it on the control chart

What: the number goes onto this tool's own control chart, next to its numbers from every day before. Why: one number alone can't tell you whether it's the usual wobble or the start of a drift to one side; only a row of them can. If you skip it: the tool drifts a little each day, every day is "still within spec", and by the time it's really out of spec, many lots have gone through.

For how to read a control chart, see "What is an SPC control chart?".

Step 4: Inside the limits, run product; outside, stop

What: if the number falls inside the limits, the tool goes back to production and customer wafers start going in. If it falls outside, or a run of points keeps heading the same way, follow OCAP: stop the tool, keep the next lots out, check whether it was a measuring mistake, and call the engineer. Why: stop first and check after, and the lots behind it don't go in and come out wrong too. If you skip it: you think "it's probably a bad reading" and let product in, another lot goes in while you check, and that lot has to be put on hold too.

For what goes into an OCAP, see "What does OCAP mean in a chip fab?".

Step 5: After a repair or maintenance, run a monitor wafer too

What: after a tool is fixed or has had its maintenance, it first runs a few non-product wafers to settle the chamber (season), then a monitor wafer to check the result; this check is the qual run. Only when it passes does the tool go back to production (tool release). Why: a tool that has been opened up isn't necessarily the same once it's put back together; a part may not be seated right, or debris may be left behind. If you skip it: the debris lands on customer wafers, and the first lot after the repair is the one that goes wrong.

For each check after maintenance, see "What is PM (preventive maintenance)?".

What do dummy wafers do?

Dummy wafers are used a lot in a furnace. A furnace lines up more than a hundred wafers on an upright rack (the boat) and slides them into a hot tube. At the very top and bottom of the tube, temperature and gas flow aren't the same as in the middle, a bit like a tray of cookies where the ones at the edge don't bake quite like the ones in the middle.

So:

• Dummies at both ends: the dummy wafers take the hardest spots to control, and customer wafers all sit in the middle. That is a way of protecting the product.
• Dummies in the empty slots: if a run doesn't have enough wafers to fill the boat, the empty slots get dummy wafers, so gas flow and temperature in the furnace stay close to a full load.
• Monitor wafers in between: for example, one each at the top, middle and bottom; measuring them after the run tells you how evenly the whole load grew.

For how a furnace heats in zones, see "What does a furnace do in a chip fab?".

The warm-up wafers from step 5 belong to this group too.

Are used monitor and dummy wafers thrown away?

Usually not. A used wafer has a film grown on it, or something stuck to it. Chemicals strip the film, a thin layer is polished off, the wafer is cleaned, and it becomes a flat, shiny blank again, ready to serve as a monitor or dummy wafer. A wafer reused this way is called a reclaimed wafer (or reclaim wafer).

Reclaiming has limits: each polish makes the wafer a little thinner, and after a certain number of uses it can only be scrapped. Reclaimed wafers are also sorted by grade: ones whose surface isn't good enough are downgraded to jobs that ask less of them.

Don't monitor wafers cost money?

They do, and not just for the wafers:

• The wafers: monitor and dummy wafers are still wafers, and fabs use a lot of them, which is why they're reclaimed. When they run short, tools can even sit idle waiting for them.
• Tool time: while a tool runs monitor wafers on their own, it can't run customer product.

So how often to run them is a trade-off: too seldom, and a drifting tool goes unnoticed for a long time; too often, and money and time go into wafers that never ship.

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 "Implant" under the name: you're the implant process engineer. At the morning meeting you bring two weeks of sheet resistance from the ion implanter IMP-01's monitor wafers, one a day. At ten, after the beam is retuned, the first monitor wafer comes out a little above the center line, still within spec, yet the tool says the dose was just right. Two lots are waiting: put them on hold and get Equipment to check, or run them? It's your call.

Tap "Furnace" under the name instead and you're the furnace process engineer: on the morning shift you measure the three monitor wafers from the top, middle and bottom of a boat, while the slots at the very top and bottom hold dummy wafers that don't need measuring; on the evening shift you load the boat and fill the ends and the empty slots with dummy wafers. In the main story, The life of a chip, chapter 2, "Station by Station", you also load a boat in the R&D line's furnace and choose whether to put in three monitor wafers or just one in the middle. As the etch process engineer, a daytime shift can bring a random event: today's monitor wafer from the deposition tool CVD-02 shows 85 particles, limit 10. Hold per OCAP, or measure one more first?

The companies, people, tool numbers, lot numbers and figures in the game are fictional; what monitor and dummy wafers are, how they're used and how they're reclaimed follow public, general sources. How often they're run, what's measured and where the limits are drawn differ by tool and by fab. For other jargon, see Chip fab jargon in plain English.