How Wafers work?

[Xenon14]

From my understanding a Wafer is a disc that has x amount of chips on its surface. (The smaller the manufacturing process... .18, .13, the more chips can fit on the wafer "duh"). My question is this though, the edges of the wafer have incomplete chips, so obviously those will be scrapped. Why can't they make Rectangular Wafers to accomodate the whole surface area towards chip development and increase efficiency?

 

[zodder]

This sounds like a great question to pose in Highly Technical.

 

[jbond04]

Hey Xenon14, I think I can give you at least part of the answer that you're looking for. A lot of wafer processing steps in the fab involve spinning the wafer (such as depositing photoresist onto wafers before they are run through photolithography). As you might imagine, spinning square or rectangular wafers isn't a very ideal situation.

However, I think in order to best answer this question, you would have to look at how the wafers themselves were manufactured. I'm not very well versed on this topic, but perhaps a senior EE student or fab technician could answer your question completely. The way I understand it, silicon wafers are manufactured by sort of "pulling" a large silicon rod out of a circular mold, grinding it smooth, then cutting each wafer up individually, followed by polishing it smooth. Again, this process is much easier to perform with circular objects, as opposed to something square or rectangular.

Here, I even looked up a site that tells a little about how silicon wafers are manufactured:
Silicon Wafer Production Steps

I hope I answered at least part of your question, and I hope that someone with a little more knowledge than I can come in fully explain everything.

 

[microAmp]

Flat and round wafers aren't the only thing being used. A company called Ball Semiconductor is working on a sphere shape.

Front page
Sphere design.

 

[Gagabiji]

Originally posted by: zodder
This sounds like a great question to pose in Highly Technical.

lol I was thinking the EXACT same thing...

 

[UnklSnappy]

Because of the way the crystals are grown.
Silicon crystal growth and wafer production

 

[splice]

There are round because that is how the current process (Czochralski method) works.

Read Page 4 of this document: http://iisme.org/etp/Silicon_Wafer_Processing.pdf

 

[Tbirdkid]

Because of the way the crystals are grown.
Silicon crystal growth and wafer production

There ya go.....

 

[Eskimo]

In addition to the reasons given above a circular shape is superior from mechanical and thermal standpoints. A rectangular or square substrate would have corners which would heat and cool at a different rate than the rest of the edge leading to thermal induced stress which is important considering the very high temperatures we heat wafers to (800-1000C). Also I would imagine the corners would be more prone to damage by mechanical handling systems.

Due to the realities of process induced variations and defectivity issues it's rare for even whole edge die to yield (work) on any shape of wafer. Thus even if you had complete rows of 'complete' edge die it is doubtful that you would actually get functioning chips from them.

*not speaking for any company in particular