In integrated circuits, all the components are implemented by various physico-chemical techniques inside a common substrate that achieves a mechanical and electrical function.
All the components have then a parasitic capacitance to the substrate. This does not really mean a parasitic capacitance to ground as it is the case on a PCB with a ground plane. The reason for that is that substrate resistivity is much higher than copper resistivity. So, all the parasitic capacitances in fact connect to a resistive network that is connected to ground at some places. This resistive network causes coupling between cells that are supposed to be independent.
There is no general approach to the substrate coupling issue. However, in any case, coupling is a three steps process:
Addressing substrate issues is usually dealing with the three steps:
Quotes are used since any cell can be both a generators and a receiver.
Two kinds of parasitic signals are injected in the substrate.
These are substrate current from devices like isolation diodes, parasitic bipolar transistors and MOS transistors. Isolation diodes should be kept reverse biased, bipolar transistors should be kept away from saturation to avoid DC currents to be injected in the substrate. This is not always possible. For MOS transistors, ionization current cannot be avoided in any case but it is usually low.
These are substrate currents from reverse biased isolation junctions and other parasitic capacitors. The injected currents amplitude is:
There is not much to do with these parameters but the best must be done. Probably the best solution is to choose a differential structure. When two signals of opposite phase inject in the substrate, the sum is theoretically zero. This is not true practically as the two signals cannot be located exactly at the same place and parasitic capacitors are not perfectly matched. However, injected signal is significantly reduced, often by an order of magnitude.
For routing parasitic capacitance, differential signals inject less current. In addition, a shielding by the first metal layer or by poly can improve the situation.
Any cell in an IC should be characterized in terms of what it injects in the substrate.
Basically, substrate attenuation results from a “pi” structure:
This attenuator has one coupling branch and two decoupling legs.
Let's run some experiments using a 3D simulator to estimate coupling resistance values:
If a current is injected punctually at the surface of a large, homogeneous and isotropic block of material with a given resistivity, constant voltage surfaces are half spheres. The resistance between two sphere halves with radius R1 and R2 ( R2>R1 )
There are basically two techniques to reduce the “receiver” sensitivity: