Lines

The 6561 has three types of conductive material: Metal, Polysilicon, and Diffusion.

Metal, diffusion and polysilicon can all be considered electrical "wires". Although polysilicon and diffusion play the important part of forming a transistor, they are also used all over the 6561 for simply carrying a signal along a path.

Metal lines or paths tend to be used for propagating a signal over longer distances due to it having much lower resistance than the polysilicon and diffusion material (metal has less than 1/500th of the resistance of diffusion and poly). For this reason signals such as the phase 1 and phase 2 clocks are taken all over the chip via metal lines. The same is true of the data and address lines. When these signals are required by a particular component then a shorter poly or diffusion line is often used to bring that signal to where it is needed.

There are a few places on the 6561 die shot where relatively long poly and diffusion lines are used though. This appears to be mainly so that such long lines can pass under many metal lines on their way to where they need to go. There's a good example of this slightly left of the centre on the 6561 die shot, where there are lots of horizontal metal lines carrying the data and address lines, and alternating poly and diffusion vertical lines carrying the counter value up to the address computation block. Having such long poly and diffusion lines can have a cost though due to the resistance of the material, where the timing of such signals can be dramatically affected, particularly in the case of polysilicon. This is discussed on page 52 of the "Introduction to VLSI Systems" book by Mead/Conway. Presumably though in the case of the 6561, the resistance of the polysilicon fabricated is low enough to be comparable to the diffusion.

The image below shows the example of alternating poly and diffusion lines mentioned above:

When metal lines are predominantly used to carry a particular signal (e.g. the phase 1 or 2 clock signal), it often needs to pass under another unrelated metal line. This can't be done in the technology used by the 6561 using metal itself (since it only has one metal layer), so a break in that metal line needs to exist, and usually a short diffusion line or poly line is used to link the two parts of that metal line so that metal line it is crossing can pass over the poly or diffusion line without affecting it. There are many examples of this in the 6561 die shot. The image below is one example, where four metal lines come in from the lower left and then must pass under several vertical metal lines. To do this it uses diffusion and poly lines to go under those metal lines and then connects to metal lines on the other side. Once again you will notice that the diffusion lines (with a slight green tinge) are wider than the polysilicon lines (with a pink tinge). It is quite common for the diffusion to be wider than the polysilicon:

The three types of conductive line will be covered in more detail in the sub pages below, but by way of an introduction, we can see the three types labelled below:

The speckled white-ish areas are all metal lines. They are semi-transparent, and so the polysilicon and diffusion lines can be seen underneath the metal lines.

Polysilicon lines are usually thinner, and have a pink tinge in the 6561 die shot. Diffusion lines are usually a bit wider and have a greenish tinge in the 6561 die shot. This is more true when lines of this type are under metal. When diffusion is not under metal, it no longer has that green tinge, but is instead grey. Polysilicon when not under metal is brownish.