why are wafers round

[beyoku]

why are cpu wafers round, it seems as if they waste the dies on the outer edge?

 

[Kntx]

because the wafers are cut from these long cylindrical slabs of silicom

 

[MrThermistor]

Why not extrude?

 

[PrincessGuard]

Because the current method of growing silicon crystals (involving a seed crystal rotating in molten silicon) inherently leads to round ingots. A square process would be hard because squares do not have a constant radius so cooling won't be uniform.

You could grind the ingot to be square but this makes polishing the surface and edges difficult because of the corners, not to mention you aren't actually gaining any additional area.

Solar cells are actually made from pseudo-square wafers where the corners of the square are cut off.

 

[Eskimo]

Edge die also don't yield very well in general due to process difficulties so the waste is not as bad as it might seem. As we move to larger substrate sizes (i.e. 200->300mm) impact of silicon waste is reduced.

 

[Shalmanese]

Also, the corners of a square would pose disproportionate amounts fo stress on the wafer, giving a heterogenous composition.

 

[Dinominant]

So why not make the CPU cores cone shaped so they use the entire wafer? Uneven heating??--maybe AMD/INTEL could make a deal with some thermal compound company and literally pour it underneath there heat spreader instead of that glue.

I say we keep this tread going in hopes some AMD/INTEL engineer sees it and we spark a new idea.

 

[Shalmanese]

Cone shaped? What would be the point of that? you just wast a further 2/3 of the wafer.

 

[Mday]

Originally posted by: Dinominant
So why not make the CPU cores cone shaped so they use the entire wafer? Uneven heating??--maybe AMD/INTEL could make a deal with some thermal compound company and literally pour it underneath there heat spreader instead of that glue.

I say we keep this tread going in hopes some AMD/INTEL engineer sees it and we spark a new idea.

i think you mean wedge shaped... anyway, that usually means some unusual geometric arrangement. transistors are laid out in patterns for design and lithography. having a wedge shape would just complicate things. and the cores have to be separated. a rectangle is easier to cut out than a wedge.

 

[Mday]

server ... ffft

 

[RossGr]

Originally posted by: Dinominant
So why not make the CPU cores cone shaped so they use the entire wafer? Uneven heating??--maybe AMD/INTEL could make a deal with some thermal compound company and literally pour it underneath there heat spreader instead of that glue.

I say we keep this tread going in hopes some AMD/INTEL engineer sees it and we spark a new idea.

LOL. sorry, In general I feel that before you start patting yourself on the back for a great "idea" you should have some minimal knowledge of the field. You are displaying what is called tunnel vision, there is much,much more to a wafer based process then the die layout on the wafer.

What have you got against squares? Once the die are cut up the original shape of the wafer is not important, you would be changing a lot of things downstream simply cause you want to use the wafer real estate a bit differently. I see no benefit. You do realize that wafers must be handled so the circiutry cannot extent clear to the edge, there is an exclusion zone which cannot be used, also most wafers have a scribe, which has a unique number used to track the wafer through the process, this is ususally located in what you would call wasted space.

 

[RossGr]

Originally posted by: MrThermistor
Why not extrude?

Because wafers are made from silcone not Aluminium! Have you ever tried to bend a wafer? I did just the other night, guess what? It broke. Si Wafers are VERY brittle, and not even a little bit malable. Malability is required for extrusion. Did we mention that the unflawed, uniform crystal structure of a wafer is essential to many processes? How would you maintain that structure in an extrusion process?

 

[Particle Man]

Look under the Czochralski (CZ) pulling method. This is the most accepted way of making Si wafers for almost all of the semiconductor industry.

Yes, there is a waste of Si with the wafer, however it can be recycled. Also, this is why companies try using as much of the wafer that they can.

 

[Dinominant]

Originally posted by: Mday

Originally posted by: Dinominant
So why not make the CPU cores cone shaped so they use the entire wafer? Uneven heating??--maybe AMD/INTEL could make a deal with some thermal compound company and literally pour it underneath there heat spreader instead of that glue.

I say we keep this tread going in hopes some AMD/INTEL engineer sees it and we spark a new idea.

i think you mean wedge shaped... anyway, that usually means some unusual geometric arrangement. transistors are laid out in patterns for design and lithography. having a wedge shape would just complicate things. and the cores have to be separated. a rectangle is easier to cut out than a wedge.

Oh well, It was worth a shot, and at least I learned something

 

[RossGr]

I was considering the difficulting of dicing non square die, but aren't a lot of fabs using lasers for dicing? If so it seem that irregular shapes should not be a problem.

The problem with irreguar die would be layout of the circiuts.

 

[Wingznut]

Originally posted by: RossGr
I was considering the difficulting of dicing non square die, but aren't a lot of fabs using lasers for dicing? If so it seem that irregular shapes should not be a problem.

The problem with irreguar die would be layout of the circiuts.

No, most fabs use diamond saws to cut wafers.

Another reason that hasn't been mentioned is manufacturing. Many of the steps require spinning the wafer (such as applying/developing photoresist), and there is no way you could get consistant coverage with a square wafer. There are also plenty of other steps in the manufacturing process which would be more difficult if the wafer were not round.

But really what it comes down to is cost. The difficulties in making/manufacturing a square wafer would significantly outweigh any cost by getting a few extra die.

 

[RossGr]

Originally posted by: Wingznut

Originally posted by: RossGr
I was considering the difficulting of dicing non square die, but aren't a lot of fabs using lasers for dicing? If so it seem that irregular shapes should not be a problem.

The problem with irreguar die would be layout of the circiuts.

No, most fabs use diamond saws to cut wafers.

Another reason that hasn't been mentioned is manufacturing. Many of the steps require spinning the wafer (such as applying/developing photoresist), and there is no way you could get consistant coverage with a square wafer. There are also plenty of other steps in the manufacturing process which would be more difficult if the wafer were not round.

But really what it comes down to is cost. The difficulties in making/manufacturing a square wafer would significantly outweigh any cost by getting a few extra die.

Ok, we also use saws, just thought our little fab here was a generation behind, I guess I heard rumors that Micron was dicing with lasers. Nor sure if it is true or not.

I believe that current laser tech would be slower then saws and it also produces a hard to deal with Si vapor.