CPU die wafers

[Über~PhLuBB]

I just saw a P4 commercial, which has the P4 logo displayed over an orange CG wafer of cores (though the average consumer would obviously NEVER make the connection).

Then a question popped into my head. Why the heck are silicon wafers ROUND? The big thing to talk about when you're speaking of CPU cost is how many chips per wafer. Why round? If you had a square wafer the same size as the diameter of a circle, there would be like 40% more area! That would make CPU production cheaper, plus there would be NO wasted material (little triangular shaped areas around the circumfrance of the circle where a full square die could not be fit), and a whole lot of other things.

So, for the third time in a single post (lol), why make silicon CPU die wafers round?

[Monaco]

That's an excellent question! Now I wanna know too :D

[Über~PhLuBB]

Originally posted by Monster of Rock
That's an excellent question! Now I wanna know too :D

See, I could understand if maybe they had to be spun somewhere in the CPU making process, like maybe they have to be polished under a buffer or something, but even so, they could STILL be square, or rectangular. I can buff my computer case panel without messing up the edges with uneven pressure. So I'm sure some computer controlled machine that only takes off one molecular level of material could do the same thing with a square wafer (that is, if they spin at all).

So come on Hoot, or someone. Out with it!

[Monaco]

ok, this is bothering me now! The CPU die itself is square, so why isn't the wafer square? Like you said, that's a huge waste of space! What do they do with all the extra silicon they have to trim off that circle? Square is easier to package, store, and handle than circle. Why why why?!?

I Googled around for info on how they actually make CPUs...nothing good yet. This (http://www.hardwareguys.com/supplement/cpu-mfg.html) place describes the process- CPUs start out as an ingot, which is sliced into wafers, which get the actual CPUs built on them. Nothing about the shape. They DO mention polishing, though....

I'm stumped.

[Über~PhLuBB]

Originally posted by Monster of Rock
ok, this is bothering me now! The CPU die itself is square, so why isn't the wafer square? Like you said, that's a huge waste of space! What do they do with all the extra silicon they have to trim off that circle? Square is easier to package, store, and handle than circle. Why why why?!?

Exactly, they could just make a wafer, say, 20 by 30 dies wide (because dies are rectangular) and not waste all that extra material. Even on circular wafers, not all the CPU's work. But if they increase the number of CPUs per wafer (like they do when they shrink die sizes), they'd have a higher number, of which there would a higher yield. The PERCENTAGE of duds per wafer would increase along with the total number of dies, but the AMMOUNT of working dies would increase as well.

But of course, to compensate for the additional surface area, they could simply make MORE wafers. Well, the big thing I'm interested here is the actual loss of material around the edge.

[Gravity Man]

Maybe there's something about the way the wafers are made that makes them need to be round.

[Über~PhLuBB]

Originally posted by Gravity Man
Maybe there's something about the way the wafers are made that makes them need to be round.

That is precisely the information I'm looking for. ;)

[cack01]

I was actually told why they are round, but of course I didn't feel like taking notes in class that day.

I bet Hawk could ask someone for us.

[skou]

I don't know why, but I do know that they have been round since the beginning. My father was in the industry since the early fifties, and when he worked at Intel in the seventies, I saw the 3 inch wafers they used. Back then Intel was only making memory, the 1101, with a massive 1048 kb of storage- that's kb not kB.

If I'm not mistaken, it's the way the silicon crystal is grown.

steve

[Ottoman]

yeah i was told what it was but i forget, but it had to do with the manufacturing process... and getting prefect flawless samples...

[cack01]

Originally posted by Ottoman
and getting prefect flawless samples...

(memory slowly returning) Ottoman is right. I just can't remember why. Also I think it helps lower wasted silicon.

[Über~PhLuBB]

Well there MUST be a reason, or they would be square or rectangular.

But I thought wafers are CUT from ingots. They're not grown in circles, they're CUT into circles. Why not cut 'em into squares? =P

[ThePerfectCore]

I think I might have the answer.

*Flips through book*

By the way, screw all of you for making me stay up this late. :p

*Flips some more*

Okay, here we go.

"The silicon used to make computer chips is extracted from common rocks and sand. It is melted down into a form that is 99.9 percent oure silicon and then doped with chemicals to make it electrically positive or electrically negative."

(Well duh)

And then it has a picture of a dude holding a giant silicon cylinder.

"The molten silicon is then 'grown' into cylindrical ingots in a process similar to candle dipping."

And there's your answer, folks. Seems it's just easier to dip into a vat of silicon a few times and pull out a dozen or so cylinders than it is to worry about forming and pressing silicon into cubes.

And silicon is the sixth most common substance on the planet, so we're not going to run out of sand anytime soon. At least it's not Geranium... :rolleyes:

[CrystalMethod]

They're round for two reasons. First is because they're cut from cylinders, hence the round shape (duh!). Second has to do with handling them during the maunfacturing process. Apparently they are much stronger in the round shape rather than a square or rectangular one. This question was asked at the Intel channel conference here in Montreal, and those were the two reasons we got in reply.

[Über~PhLuBB]

Well, that explains it. Make sense too, like you said, why waste time forming it into a square when you could just "dip" another cylender up for the cost of some purified sand. =)

[deathstar13]

from what i remember TPC has the correct answer.there is/was a place in wilmington ohio that grew/made the cylinders and cut em.

back 14 years ago while i was in collage for electronics engineering we visisted the place.didnt seem important to me at the time.cant say i even remember much.as i do remember me and a buddy had a fifth of liquer in the car awaiting us when we were done.stupid me!

was really interesting from what i do remember.kinda looked like a stack of cdr's that that were solid.clean rooms all over the place.and they said once they cut a slice of the silicon its then shipped to cali.

im gonna do a few searches and see if i can find anything about the place.mabey still there.

"edit" i guess its still there, Crysteco is the name.http://www.winbmdo.com/scripts/sbir/firm.asp?firm_id=598
is a link to a little what they do.click on the SBIR link at the top :D
i didnt follow that link much.dont need mililtary def: after me :P