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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 o

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.

rwmj 6y ago
Because of the process used to make the initial cylindrical crystal from which the wafers are sliced: https://en.wikipedia.org/wiki/Boule_(crystal) 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).
- quakeguy 6y ago
  Just looked on EBay, there are indeed many different wafers for sale. https://www.ebay.com/sch/i.html?_nkw=wafer+chip&_trksid=p2380057.m4084.l1313 https://www.ebay.com/sch/i.html?_nkw=wafer+chip&_trksid=p238... Poured in Acryl some would make cool plates.
- tromp 6y ago
  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_450_mm_transition https://en.wikipedia.org/wiki/Wafer_(electronics)#Proposed_4...
ohazi 6y ago
They're grown as a giant cylinder, which is then sliced into round wafers. https://en.wikipedia.org/wiki/Boule_(crystal) https://en.wikipedia.org/wiki/Boule_(crystal)
sharpneli 6y ago
Because the crystals as grown are circular (just first hit about the process https://youtu.be/XbBc4ByimY8 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.
- DonHopkins 6y ago
  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 https://www.youtube.com/watch?v=8QKzS_w_Ko0 Silicon Ribbons begin at 10:10. https://en.wikipedia.org/wiki/String_ribbon 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_industry https://en.wikipedia.org/wiki/Crystalline_silicon#PV_industr... >ribbon silicon (ribbon-Si), has currently no market
racingmars 6y ago
> 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!
jankeymeulen 6y ago
Because that's the way you grow them as a single crystal.
tormeh 6y ago
Also dumb question: Why do dies need to be square? Wouldn't you get less waste at the edges with a hex shape?
- fanf2 6y ago
  I expect that would be too difficult to cut up
- jl6 6y ago
  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.
  - TheOtherHobbes 6y ago
    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 https://patents.google.com/patent/US6030885A/en
  - sbierwagen 6y ago
    Could do triangular chips instead, they would be easier to dice.
- deleted 6y ago
  [deleted]

rwmj 6y ago

Because of the process used to make the initial cylindrical crystal from which the wafers are sliced: https://en.wikipedia.org/wiki/Boule_(crystal) 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).

quakeguy 6y ago
Just looked on EBay, there are indeed many different wafers for sale. https://www.ebay.com/sch/i.html?_nkw=wafer+chip&_trksid=p2380057.m4084.l1313 https://www.ebay.com/sch/i.html?_nkw=wafer+chip&_trksid=p238... Poured in Acryl some would make cool plates.
tromp 6y ago
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_450_mm_transition https://en.wikipedia.org/wiki/Wafer_(electronics)#Proposed_4...

ohazi 6y ago

They're grown as a giant cylinder, which is then sliced into round wafers. https://en.wikipedia.org/wiki/Boule_(crystal) https://en.wikipedia.org/wiki/Boule_(crystal)

sharpneli 6y ago

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

DonHopkins 6y ago
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 https://www.youtube.com/watch?v=8QKzS_w_Ko0 Silicon Ribbons begin at 10:10. https://en.wikipedia.org/wiki/String_ribbon 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_industry https://en.wikipedia.org/wiki/Crystalline_silicon#PV_industr... >ribbon silicon (ribbon-Si), has currently no market

racingmars 6y ago

> 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!

jankeymeulen 6y ago

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

tormeh 6y ago

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

fanf2 6y ago
I expect that would be too difficult to cut up
jl6 6y ago
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.
- TheOtherHobbes 6y ago
  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 https://patents.google.com/patent/US6030885A/en
- sbierwagen 6y ago
  Could do triangular chips instead, they would be easier to dice.
deleted 6y ago
[deleted]