Help to understand-Antenna effect

[jarillak]

Hi...
In "The art of analog layout"(Alan Hastings),
In antenna effect,(Chapter-4),
I can't understand these below lines.

"The topmost layer of metal is almost immune to antenna effects because every geometry on this layer connects to a diffusion somewhere on the die but lower metal layers do not necessarily connect to a diffusion until the top metal layer is in place."

Can anyone explain this?

 

[erikl]

As soon as there is a (any) pn-junction (diffusion) connected to a gate connection, there's no worry any more about an antenna problem: junction leakage (even in reverse) is sufficient to prevent gate break-through.

Often the lower metal layer connections to a gate don't provide a junction connection, whereas a top layer in most cases connects to an input (and/or output-) protection via MOSFET channel and/or bipolar protection diodes.

This doesn't prevent antenna violations for the lower metal layers of course (if the connection area is large compared to the gate area), because during the fabrication process the top layer is the last (metal) layer, i.e. doesn't exist before.