5 ms·

What do they do with the edge bits of the silicon wafer that are not squares?

Cthulhu_ 6y ago
I'm not sure if they can be 'just' recycled into new wafers. Either way, given that it's just silicon I'm sure it can be recycled or safely disposed of (silicon isn't toxic as far as I know, unless you breathe it in a powder form)
- MayeulC 6y ago
  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_dopants https://en.wikipedia.org/wiki/Doping_(semiconductor)#Silicon...
- Keyframe 6y ago
  There's stuff deposited on top. Depending on the stage, it's not silicon only.
elihu 6y ago
I'm not an expert in this stuff, but I think they just get thrown away. It's one of several reasons why smaller chips are more area-efficient to make, and one of several reasons why the major semiconductor manufacturers have been so interested lately in building chips out of smaller pieces manufactured separately rather than one big monolithic chip.
- deleted 6y ago
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ohazi 6y ago
Replacement corners in case one of the full-size dies in the middle cracks during dicing.
- MayeulC 6y ago
  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.
  - ohazi 6y ago
    Yes, I was joking. :-) I think they do sometimes put test features in the corners if there's space. The electrical properties of the die can vary in interesting ways [1], but the edges are usually worse than the center. [1] https://www.google.com/search?tbm=isch&q=wafer+defect+pattern https://www.google.com/search?tbm=isch&q=wafer+defect+patter...
- jacquesm 6y ago
  Bull. Everybody knows that is because they've been cutting corners like it was crunchtime at the circle factory.
ehsankia 6y ago
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.
MayeulC 6y ago
I've wondered for quite some time why not triangles or hexagons. I guess the yield (percentage of wafer thrown away) improvements would be minimal. Plus, the temperature is often better controlled at the center, which would make the edge parts less performant anyway. That's the other advantages of chiplet design: maximize yield (a small defect renders a much smaller chip unusable), and much more granular binning (easier to sort out good/worse chips, due to placement and random issues during fabrication). Not to mention you have a much more modular design at the end, where you only have to change the cheaper (not 7nm) silicon interposer.
- rwmj 6y ago
  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 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 https://youtu.be/HW5Fvk8FNOQ?t=978
- zokier 6y ago
  Hexagons dies would be difficult to cut from the wafer, rectangles can be cut with relatively simple circular saw.
Ecco 6y ago
Another question: why do they “engrave” the squares that are going to be discarded anyway?
- borramakot 6y ago
  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.
  - Ecco 6y ago
    Awesome explanation, thanks! Kinda like dual-cavity injection molds I guess
kevin_thibedeau 6y ago
The periphery is used for wafer level test circuitry.

I'm not sure if they can be 'just' recycled into new wafers. Either way, given that it's just silicon I'm sure it can be recycled or safely disposed of (silicon isn't toxic as far as I know, unless you breathe it in a powder form)

MayeulC 6y ago
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_dopants https://en.wikipedia.org/wiki/Doping_(semiconductor)#Silicon...
Keyframe 6y ago
There's stuff deposited on top. Depending on the stage, it's not silicon only.

elihu 6y ago

I'm not an expert in this stuff, but I think they just get thrown away. It's one of several reasons why smaller chips are more area-efficient to make, and one of several reasons why the major semiconductor manufacturers have been so interested lately in building chips out of smaller pieces manufactured separately rather than one big monolithic chip.

deleted 6y ago
[deleted]
- deleted 6y ago
  [deleted]

ohazi 6y ago

Replacement corners in case one of the full-size dies in the middle cracks during dicing.

MayeulC 6y ago
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.
- ohazi 6y ago
  Yes, I was joking. :-) I think they do sometimes put test features in the corners if there's space. The electrical properties of the die can vary in interesting ways [1], but the edges are usually worse than the center. [1] https://www.google.com/search?tbm=isch&q=wafer+defect+pattern https://www.google.com/search?tbm=isch&q=wafer+defect+patter...
jacquesm 6y ago
Bull. Everybody knows that is because they've been cutting corners like it was crunchtime at the circle factory.

ehsankia 6y ago

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.

MayeulC 6y ago

I've wondered for quite some time why not triangles or hexagons. I guess the yield (percentage of wafer thrown away) improvements would be minimal. Plus, the temperature is often better controlled at the center, which would make the edge parts less performant anyway. That's the other advantages of chiplet design: maximize yield (a small defect renders a much smaller chip unusable), and much more granular binning (easier to sort out good/worse chips, due to placement and random issues during fabrication). Not to mention you have a much more modular design at the end, where you only have to change the cheaper (not 7nm) silicon interposer.

rwmj 6y ago
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 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 https://youtu.be/HW5Fvk8FNOQ?t=978
zokier 6y ago
Hexagons dies would be difficult to cut from the wafer, rectangles can be cut with relatively simple circular saw.

Ecco 6y ago

Another question: why do they “engrave” the squares that are going to be discarded anyway?

borramakot 6y ago
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.
- Ecco 6y ago
  Awesome explanation, thanks! Kinda like dual-cavity injection molds I guess

kevin_thibedeau 6y ago

The periphery is used for wafer level test circuitry.