Diffusion

A diffusion line (also called "active") is one that isn't a poly line or a metal line. Metal and poly lines are fairly easy to spot, so through a process of elimination, all other lines are diffusion. The colour of the contact can also tell us if the line it connects to is diffusion or not. If the contact is pure white then it connects to diffusion. Diffusion lines are also generally wider than poly lines, sometimes much wider.

Diffusion lines are sometimes a bit more difficult to spot than the other two because they don't really have a colour as such. The wider diffusion lines when passing under metal will look the same colour as the metal. See the following image:

In the example above, f1 and f2 connect under a large area of metal using fairly wide diffusion lines. Under the large metal area, they have a grey edge but internally it looks the same colour as the metal.

The thinnest lines of diffusion will usually have a greenish tint when passing under metal. There are a few examples in the image above.

This greenish tint is due to thin film effects like those seen in the surface of a soap bubble. Note, however, that this shouldn't be relied on to identify diffusion. Some areas of metal can appear to have a green tint when there is no diffusion under them at all. This can happen, for example, when two poly lines are close together, as in the following example:

There is no diffusion between the horizontal poly lines and yet there is a green tint. This is why it is dangerous to rely on the green colour to identify diffusion, particularly when under metal.

It is usually easier to spot diffusion in the areas of the die shot that are not under metal. For areas of the chip where there is no metal, poly or diffusion, the die has a certain grey shade (slighty greenish perhaps, but predominantly grey). Areas where diffusion exists that isn't under metal has a slightly darker shade of grey (with perhaps a bluish/silvery tint, but once against predominantly grey). The difference is subtle, but if you squint your eyes, you can see that the diffusion has a slightly darker shade.

In the case of the 6561 die shot, the thinnest of diffusion lines are slightly wider than the thinnest of the polysilicon lines. The VLSI design rules of the time allowed for the minimum width of polysilicon and diffusion to be the same, but for whatever reason, the designers of the 6561 chose to make their minimum diffusion line wider than the minimum polysilicon line. This appears to have also extended to the amount of the material surrounding a metal contact, i.e. the diffusion around a metal to diffusion contact is always wider than the polysilicon around a metal to polysilicon contact. Luckily for us, we can use this fact to help us in reversing the chip.