Wafers

[Tahir2]

Why are wafers round and not rectangular? Surely being retangular is more of an optimal layout.

 

[Bobbler]

I've often wondered that myself...

 

[the maddman]

They are grown in a furnace from liquid silicon. They start with a seed crystal which is slowly pulled away. The raw silicon comes out looking like a bell, which is sliced into round wafers.

 

[Gubbi]

Why are wafers round and not rectangular? Surely being retangular is more of an optimal layout.

It has to do with the way the silicon is refined and wafers produced. Silicon for electronics use is produced by pulling big ingots using a single crystal from a pot of melted silicon. Not only are the ingots of high purity, they are also a single crystal. Since the ingots are grown from a single small crystal they are round (same average distance to center).

The ingots are then grinded and sawed into thin slices (wafers).

You could cut the wafers to be square, but then you'd just waste wafer area earlier in the proces.

Cheers
Gubbi

 

[AlNom]

What do they do with all the unusable chips?

 

[arjan de lumens]

There are some processing techniques (in particular "spin coating") used in chip manufacturing that require the entire wafer to be rotating at very high speeds. It's presumably much easier to rotate a circular wafer fast than a rectangular one.

 

[Mintmaster]

What do they do with all the unusable chips?

The raw silicon itself is not a very large part of the cost. All the other steps work well with a round wafer, especially spinning photoresist. You can use round equipment that will likely have better uniformity characteristics especially for airflow.

In the end, though, it's likely that the cylindrical symmetry of crystal growth is the main reason, as Gubbi mentioned.

 

[nelg]

I would like to know why chips aren't octagonal.

 

[RussSchultz]

I would like to know why chips aren't octagonal.

I'm sure they are in the Battlestar Galactica universe.

 

[Geo]

Do you get to toss the excess back into the pot and melt it into the next pulled goop?

 

[AlNom]

The raw silicon itself is not a very large part of the cost....

But, what do they actually do with the unused stuff? Just dump it or reuse it?

 

[Guden Oden]

Do you get to toss the excess back into the pot and melt it into the next pulled goop?

No, of course not, as all kinds of impurities have been added (intentionally) during chip manufacturing. That would contaminate the silicon raw material.

Besides, silicon oxide is one of the more easily obtainable substances on this planet, most every lump of rock is made of it.

 

[_xxx_]

Here is a nice short summary about the process.

EDIT:
This is where it happens

 

[Simon F]

I would like to know why chips aren't octagonal.

Did you mean hexagonal?

 

[nelg]

Did you mean hexagonal?

 

[Simon F]

Then to answer your question, if they were hexagonal then imagine trying to cut apart the separate dice. With rectangular dice, you can cut (snap?) in nice straight lines right across the wafer.

 

[Sxotty]

Actually though, there would be other shapes that could be lined up so that you could cut them up in not too complicated of a manner, but isn't there other reasons than cutting for what they do?

 

[Cartoon Corpse]

the leftovers become communion wafers.

 

[nelg]

Then to answer your question, if they were hexagonal then imagine trying to cut apart the separate dice. With rectangular dice, you can cut (snap?) in nice straight lines right across the wafer.

What if you used this.

 

[Frank]

While there are multiple ways to cut up wafers holding other shapes, it would require changing much of the manufacturing process. I don't think they feel it's worth it.

For example, the wafer steppers (the machines that projects the masks onto the wafer) would need to get a new positioning system (or a serious software update). And that probably goes for most of the (extremely expensive) machinery. And you have to buy the new cutters, etc.

Further, they need to design new packaging for the chips, make new standards, change the CAD/CAM software for making printed circuits, etc.