Why the shape of the wafer is oval or circular?

[pandit_vlsi]

hi alll.
i want to know why the shape of the wafer is oval or circular...why not rectangular or square.
why iam asking is if the shape of the wafer is oval or circular the no.of dies will be less since usually die will be in aquare or rectangle...so if we cut oval shaped wafer into square shaped die no of die will be less...(om the periphery u will loose lot of wafer space)
so is there any specific reason to shape of the wafer....
pandit

 

[aravind]

wafer shape

it is basically circular.

but depending upon die / fab /mask it can be a oval

 

[leeenghan]

Re: wafer shape

Hi,

My guess is :

1. Uniformity. If during fabrication process (litho, etch) the machine can achieve the same uniformity across a limited area, then the best way to capture that area is a circule

2. Wafer is made in cylinder shape in the first place (to keep it simple, it is "pull" out from a poll of melted silicon in a long cylinder shape). So to make it rectanlge you have to remove alot of wafer...

Regards,
Eng Han

 

[haria]

wafer shape

Hi..
1)Even I guess that it is circular because its initial shape is cylindrical..(as mentioned by Mr.Eng Han)
2)one more reason....
As the thickness of wafer << radius of wafer,we can consider it as 2D.
If you ask a question ....
which curve has maximum area with a constant perimeter??...The answer is circle!!(using calculus of variation)
So,I guess these may be the reasons for making a wafer circular..


Regards,
Hari

 

[g3cube]

wafer shape

Hi,

A circular shape of the wafer will give a uniform gas flow inside the chamber during the fabrication. Any non-uniform gas flow will result in non-uniform process in terms of oxide thickness and variation of effectiveness of chemical reaction. Process variation can kill the devices on wafer or result in large variation of the gain of analog devices.

Regards,
G3Cube

Added after 2 minutes:

Hi,

By the way, flat edges on the wafers or cuts on the wafers are sometimes presented. They are the identification of the wafer type in terms of physical lattice arrangement.

Regards,
G3Cube