Polysilicon

Polysilicon (or "Poly) lines are easily identifiable by their colour. Depending on whether it is under metal or not, and passing over diffusion or not, it can take on one of about three to four different colours. As a general rule though, it is always better to trust the colours that are not under metal. If a line is under metal, it is a good idea to follow that line until it is no longer under metal and see what colour it is at that point. If it is a shade of brown then it is a polysilicon line. If the whole of a line is under metal then another way to recognise it as polysilicon is by looking at the metal contacts with that line. If such contacts are a dirty grey in colour then the line must be a polysilicon line. If under the metal, it quite often has a pink-ish tint, but this is not always true, particularly when it is wider than a simple connecting line. This is the main reason for advising to follow the line until it is no longer under metal, to see if it is brown. When not under metal, polysilicon is one shade of brown if "over" diffusion and a different shade of brown when not "over" diffusion.

I have put the word "over" in double quotes to draw attention to the fact that polysilicon actually never passes over diffusion. We just say it does. This is the scenario where a transistor is formed, and rather than the polysilicon passing over the diffusion, it instead passes over some insulating oxide. The diffusion comes right up to the edge of the polysilicon and appears to pass under it, but actually it doesn't. The different colour for the polysilicon when it appears to pass over the diffusion will be due to the oxide between the polysilicon and the substrate. So when polysilicon is not under metal, the two different colours are either when it is directly over the substrate, or when it is over the oxide (i.e. for a transistor gate). In the 6561 die shot, both are shades of brown.

The following diagram labels a few poly silicon lines:

When poly passes under metal, it often has a pink tint. This is due to thin film effects. The vast majority of poly lines are the same width (unlike diffusion), and so almost always have this pink tint (when under metal).

Depletion mode transistors use a thicker poly line for their gate. In general this also takes on a pink tint when under metal, although the diffusion part that it crosses usually shows a green tint when under metal.

There are several places in the die shot where much thicker lines of poly are used. Not only for the depletion mode transistors, but also for capacitors and resistors. When such thicker lines of poly pass under a metal line, they are speckled white like the metal line itself (in much the same way that thicker diffusion lines are also speckled white). This highlights once again the danger of putting too much faith in the thin film effect colours for identifying the different lines. Thicker diffusion and thicker polysilicon, when under metal, can both appear to look the same. Once again this is why it is best to follow the line back to either a contact, or until it is no longer under metal, to determine what type of material it is.

For poly lines though, since the vast majority are very thin, they usually have the pink tint. But as noted above, when poly lines do not pass under metal, they can have one of two different shades of brown, and this colouring appears to be regardless of how thick they are (unlike when under metal). The different shades of brown are highlighted in the image above.