Because of the process used to make the initial cylindrical crystal from which the wafers are sliced: https://en.wikipedia.org/wiki/Boule_(crystal) It involves spinning a seed crystal and drawing a cylinder out of a bath of molton ultra-pure silicon. Spinning => cylinder => circular wafers. To reduce waste of the "edge bits" the industry has moved over time to larger and larger wafers. I have some wafers from the 90s which are 6" and 8" in diameter (amazing what you can buy on eBay), but modern ones are all 12" (actually 300mm).

They're grown as a giant cylinder, which is then sliced into round wafers.

https://en.wikipedia.org/wiki/Boule_(crystal)

Because the crystals as grown are circular (just first hit about the process https://youtu.be/XbBc4ByimY8)

From that one slices the wafers and then the processors get made. The ”extra” ones in the edges have pretty much zero marginal cost.

> Even dumber question, why do they need to be circular?

The wafer is round because it's cut from a cylinder of silicon. And the cylinder is a cylinder because spinning is involved in the process to make it. Hence, thanks to centripetal force, it ends up being round!

Because that's the way you grow them as a single crystal.

Also dumb question: Why do dies need to be square? Wouldn't you get less waste at the edges with a hex shape?

Not sure how easy it would be to recycle those as chips, given it will have dopants [1] inside. It will likely be unfit for computing applications unless purified, but since it's on the order of one dopant atom per 1e12 silicon atoms, it would basically be 100% pure in other industries.

Some metallic contacts (mostly aluminium), silicon oxide and other residues are likely present as well, depending on the masking process.

[1] https://en.wikipedia.org/wiki/Doping_(semiconductor)#Silicon...

There's stuff deposited on top. Depending on the stage, it's not silicon only.

Partly it's "path dependency" - everything is set up for rectangular dies, so everything would need to change for uncertain benefits. Not just the tooling, but also the design software. While I was looking at this I found that Intel has a patent for an octagonal die with a smaller square one fitted in the gaps between: https://patents.google.com/patent/US20060278956

BTW the dicing used (in the 1970s) to be done partly by hand. You can see a video of someone doing it here: https://youtu.be/HW5Fvk8FNOQ?t=978

Hexagons dies would be difficult to cut from the wafer, rectangles can be cut with relatively simple circular saw.

I know you re probably joking, but please mind Poe's law.

For the uninitiated, yeah, some dies can die during dicing. But I think you'd have trouble finding the cracks, and then it's just infeasible to precisely cut both halves where they would need to be cut, then reattach them. The issues would be the cut thickness, not damaging the circuits near it, precisely aligning the circuits, and then electrically connecting the circuits.

Alignment is probably the hardest part, we can barely do it for flip-chip wafers/silicon interposers on the order of the µm, imagine doing it at less than 7nm, which is the transistor pitch here.

Bull. Everybody knows that is because they've been cutting corners like it was crunchtime at the circle factory.

So, the main result of designing a CPU is a series of masks that essentially indicate where to put what. For example, in this layer, inject boron anywhere the mask doesn't block. The masks aren't wafer sized- they are pretty small, and a machine moves the mask from position to position across the wafer to re-use it. But, at least when I was working on this, some masks would be larger than an individual square (die)- maybe the mask could do 2x2 at a time. In that case, maybe the application of the mask would get you one complete die, and three die off the edge.