[Coco] 6809 wafer on eBay

[Mark J. Blair]

On Aug 21, 2013, at 23:10 , Arthur Flexser <flexser at fiu.edu> wrote:
> As one who is not at all knowledgeable about the manufacture of CPU chips,
> could somebody please enlighten me as to what exactly this 4"/100 wafer
> thing is?  It's 4 inches in diameter and contains the makings of 100
> 6809s?

Correct.

>   Is each of the little squares something that gets mounted in the
> center of the actual 6809 chip in such a way as to make contact with its 40
> pins?

That is exactly correct.

>  Why would the shape be a disc with many of the squares cut off in
> the middle, rather than rectangular?

That disc-shaped wafer is a thin slice of a long, round, billet of silicon which is called a "boule":

  http://en.wikipedia.org/wiki/Boule_(crystal)

The wafer is very pure elemental silicon, and it's a single, uniform crystal with very few defects in its crystal lattice. That's important to how semiconductors ("electronic stuff") works. In order to make that gigantic, pure crystal, they start with a pool of molten silicon, and dip a small bit of solid silicon (called a "seed") into the pool. The molten silicon begins to crystallize around that seed, and they then slowly draw the seed out of the pool, with the crystal growing out of the pool as they lift the seed up. Eventually, they end up with a great big sausage-shaped boule of pure silicon, which can then be sliced up into wafers. So, that's why the wafers end up round.

A whole bunch of very complicated processing steps are then done to form many copies of the desired device (in this case, an MC6809) on each wafer. It's very expensive to make each wafer, but that cost is divided up among all of the devices on the wafer, which is why it's even economically possible to make things like computer chips.

Next, those individual devices are split up by sawing the wafer into little rectangular bits. Each one is called a "die", and the plural form is "dice". That's because the wafer is diced up much like an onion slice.

Now that they've cut that wafer up into hundreds of dice, each one of them gets mounted in a package with pins on the sides (or these days, more often balls on the bottom). The contacts on the die need to get connected to the pins of the package, and on an MC6809 that would be done with a process called wire bonding:

  http://en.wikipedia.org/wiki/Wire_bond

That was very simplified, but I hope it helps make some sense.

-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/