Why are wafers round?

Chips are square: every one you could pull out of a phone or a laptop is a neat little rectangle. Yet in a fab, chips are made on a round wafer and only cut apart at the end. Why not use square plates? The answer starts with how the silicon is grown.

In one line: the silicon grows as a cylinder

Chips need single-crystal silicon, where the atoms are lined up perfectly. The most common way to make it is to melt very pure silicon in a crucible, touch the surface with a small piece of single-crystal silicon as a "seed", and then pull it upward very slowly while rotating it. The molten silicon freezes onto the seed, copying its atomic order layer by layer, and what comes out is a long single-crystal rod called an ingot.

Because the whole process keeps turning, the ingot's cross-section comes out round. The ingot is then ground to an exact diameter, sliced like a salami into thin discs, and lapped and polished until it's mirror-flat. That's a wafer.

Why not grind the ingot square first?

You could, but it would be wasteful:

• Grinding a cylinder into a square bar throws away all the single-crystal silicon outside the square, and that silicon is expensive.
• Many process steps suit a circle: photoresist is coated by spinning the wafer fast so the liquid spreads into an even film, and heating, measuring and handling tools are all designed around round wafers. A circle has no sharp corners to chip or crack, and it heats more evenly.

So the whole industry, from materials to tools to the boxes wafers travel in, is built around round wafers.

Chips are square, so what about the edge?

Lay square chips out on a round wafer and the edge will always have spots where a full chip doesn't fit. Those partial chips at the edge are wasted, which is one reason wafers have grown over the decades to today's most common 300 mm (12-inch) size: a bigger wafer fits more chips, and the edge loss becomes a smaller share.

The edge is also the hardest part of the wafer to process evenly: coating, etching and deposition always differ a little between the middle and the edge, so edge chips fail more often anyway. On a wafer map you'll often see a ring of extra failures around the outside.

What's the little notch on a wafer?

Look closely and you'll see a tiny notch on the edge (smaller wafers use a short flat edge instead). It marks the direction of the crystal, and tools use it to turn every wafer to the same angle so exposure and dicing line up. A circle has many advantages, but once it spins you can't tell which way is which; the notch fixes that.

In fab24hr

The wafers in fab24hr are 30 cm (12-inch) discs, 25 to a box (a FOUP). You'll decide whether a wafer needs rework at an etch tool, read a wafer map at the metrology station, and maybe trip in the cleanroom and shatter one. The companies, people and numbers in the game are fictional; how wafers are made and why they're round come from public, general sources.