Why Silicon Wafers are circular?.?

Question

Why are silicon wafers always circular in shape?. Is it because it has least wastage after etching the dies?. I see that we have a die, the edges of the circular wafer are always covered with partial dies. This is a lot of wastage, so why not the wafers be rectangular or squarish. I beleive there will be more wastage of die space if it is squarish/rectangle. Please confirm this.

If there is any other reason please let me know.

Answer 1

It is the way they are ultra-purified; a process called zone-melting. Cylindrical rods are the most efficient way to more evenly distribute the heat. Check out wikipedia.org

Answer 2

Why Are Silicon Wafers Round

Answer 3

Silicon wafers are cut from silicon cylinders, which are grown I believe out of crystals via a series of depositions: IE, start with a strand of silicon, dip it in more silicon and coat it, allow it to crystallize under very controlled conditions. Then dip it in solution again, and allow to crystallize under very controlled conditions. The resulting built-up silicon cylinder is then machined to be circule and then sliced into the "wafers" that are used in fabrication.

I vaguely recall this out of a class or maybe it was some after school presentation, I forget. Pretty sure that's how the process starts and proceeds. And there will always be wastage when taking square units out of a circular material, or circular pieces out of square material. There's just gaps formed either way (between the circles, or around the eges of the circular wafer).

I may have gotten wrong the exact method of creating the cylinder (it's been nearly 10 years since high school, probably 12 since I heard the exact explanation), but yes, they're cut from a cylinder, that much I know.

Could be that it's poured in cylinders then heated and spun to try to get out as many impurities as possible, or something like that too. Dont' recall the exact mechanics of it.

http://encyclopedia.thefreedictionary.com/Wafer+(electronics)
http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_5/illustr/i5_2_4.html
http://encyclopedia.thefreedictionary.com/silicon

Answer 4

edthesciengeguy is basically correct. The raw silicon ingot needs to be cylindrical so that when zone melting is done, the surface tension keeps the silicon in place. This shape does have the effect of rendering many chips at the edge of the processed wafer unuseable, but some of these (as well as others scattered about the center) are exposed to different patterns for test purposes, and a full-size chip might not be needed.

Answer 5

In microelectronics, a wafer is a thin slice of semiconducting material, such as a silicon crystal, upon which microcircuits are constructed by doping (for example, diffusion or ion implantation), etching, and deposition of various materials. Wafers are thus of key importance in the fabrication of semiconductor devices such as integrated circuits.

They are made in various sizes ranging from 1 inch (25.4 mm) to 11.8 inches (300 mm), and thicknesses of the order of 0.5 mm. Generally, they are cut from a boule of semiconductor using a diamond saw or diamond wire, then polished on one or both faces.

Wafers under 200 mm generally have flats indicating crystallographic planes of high symmetry (usually the {110} face) and, in old-fashioned wafers (those below about 100 mm diameter), the wafer's orientation and doping type (see illustration for conventions). Modern wafers use a notch to convey this information, in order to waste less material.

Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic. For instance, wafer cleavage typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("dies") so that the billions of individual circuit elements on an average wafer can be separated into many individual circuits.

Hope I could convince you.

Answer 6

My wife who is a Coating Engineer says:"I think it's several reasons. 1. historical. wafers are made by pulling a ingot out of molten silicon. this is naturally a round shape. 2. purity i think i remember hearing that the edge would contain more impurities than the center of the ingot, so you cut off the outer edge to get to the more pure silicon. 3. processing. spin-on photoresist, spin-drying, better with round substrates. most processes also more uniform with a round profile"

Answer 7

These wafers are cut from a cylinder shape. The cylinderical shape is preferred because it is the best way to get single crystal layers.

Answer 8

My best guess is because of optics. The circuit patterns are imprinted on by light. In this case the optical equipment is the expensive part.