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Comment by ehsankia | Hacker News Reader

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

CDs and DVDs write in a circular pattern starting from the middle going outwards, but the actual chips on these wafers seem to be their own individual squares.

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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).

And migration to the next proposed size of 450mm may never happen, as the economics are not favorable:

https://en.wikipedia.org/wiki/Wafer_(electronics)#Proposed_4...

Just looked on EBay, there are indeed many different wafers for sale.

https://www.ebay.com/sch/i.html?_nkw=wafer+chip&_trksid=p238...

Poured in Acryl some would make cool plates.

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

That’s a great question.

I would guess that dies are built from modular sections (e.g. SRAM cells), and it’s important that two identical modules perform identically - signal propagation time is relevant at this scale, so the shape and layout of each module must be identical. I would further guess that rectangular layouts are easiest to reason about, easiest to make masks for, easiest to pack efficiently at the transistor level, and easiest to test.

But I don’t know of a fundamental reason why a sufficiently advanced VHDL “compiler” couldn’t produce hex-cell or even circular layouts.

Chip dicing hardware can produce hex-cells, or any other cell with straight edges. (Not circular - that's not a good shape to expect from crystalline silicon.)

But - as you say - the modular sections are rectangular, and for most applications there's no good reason to make the dies any other shape.

There's actually a patent for hex-cell chips, but it doesn't seem to have been used for any significant projects.

https://patents.google.com/patent/US6030885A/en

Could do triangular chips instead, they would be easier to dice.

I expect that would be too difficult to cut up

> 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 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.

Here's another video that shows different ways silicon wafers are manufactured, including "silicon ribbon" which makes rectangular ribbons!

https://www.youtube.com/watch?v=8QKzS_w_Ko0

Silicon Ribbons begin at 10:10.

https://en.wikipedia.org/wiki/String_ribbon

>Ribbon solar cells are a 1970s technology most recently sold by Evergreen Solar (which is now in receivership, i.e. bankrupt and liquidated), among other manufacturers.

https://en.wikipedia.org/wiki/Crystalline_silicon#PV_industr...

>ribbon silicon (ribbon-Si), has currently no market

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

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

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

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