Ah yes, I was aware of this but it slipped my mind when posing the question...was thinking about the device physics!
![Grin Grin](https://designers-guide.org/forum/Templates/Forum/default/grin.gif)
While I'm here, I want to check my understanding.
So, you are talking about IR drop through the metal which causes a difference in voltage between substrate contacts. Let's say I have substrate contacts on opposite sides of the chip both connected by a metal1 line to ground. If current through metal1 is low such that there is not much of an IR drop. there should be no difference in voltage and thus no substrate current, right?
However, if I had a MOSFET in the center of the chip, it would still be injecting substrate current due to impact ionization and reverse leakage, right? This current would have to find it's way to the substrate contact via the substrate and thus encounter a lot of substrate resistance, right? Because the current flows through a large resistance, there is enough of a voltage drop to activate the parasitic BJTs and cause latch up, right?
In this situation, to fix the problem, I would have to put a substrate contact (connected to ground and the other substrate contacts via metal1) in the center of the chip next to the MOSFET. This contact would give the substrate current a path to get to ground via the metal1 line and thus avoid causing a voltage drop in the substrate, right?
In the case where I have a high IR drop on the metal1 line, does adding more substrate contacts help? There would still be a voltage drop between the contacts and thus substrate current flow would occur. It seems like making the metal1 wider would be the solution.
Also, I've heard of soft contacts before. You put in substrate contacts but they aren't connected via metal. They make their contact via the substrate. Sounds like a bad idea because you encourage the current to flow through the substrate.
It would seem like the 2 basic rules are:
1) put substrate contacts next to devices to provide a non-substrate path for substrate current
2) use wide lines to avoid IR drop, which keeps the voltage difference between substrate contacts to a minimum (thus reducing substrate current flow)
And, I believe that using a capacitor between Vdd and Gnd (bypass cap) to supply charge during large transients will prevent excessive current from being drawn from the supplies at any one time (thus avoiding IR drop on the supply lines).
Thanks!
Marc