Re: Metal antenna ratio is cumulative, but why?
The "input current" is the plasma charge-throw as it
etches the present layer. Dry etch plasma can't get
at the already-etched layers as they are hard-masked
by the ILD between present layer and lower ones. A
lower attached layer may "share" charge but this does
not mean it -adds- charge or current. All of that is
received at the layer under etch.
The "antenna" is the periphery of the feature, just as
it it being cut free. Before this there is a shorting sheet
with many attached vias for plasma charge to exit, and
the not-etched feature is soft masked by photoresist.
Once the layer's features separate, plasma charge on a
line has only diodes (benign) or MOS gates ("programmable")
to get out through. You don't want randomly programmed
VT and you want latent oxide defectivity or outright gate
rupture even less.
So antenna rules, if done right, would check net
connectivity for any attached diodes, and only flag
nets that have only MOS gates connected.
Thing is, CAD dudes (and the very occasional dudette)
are lazy bastages and won't do the work. So you get
conservative rules that assume no help from diodes,
in some technology cases I have seen.
Capacitance doesn't really enter it. Etch times are long.
Interconnect capacitances are small.
You will charge up until either diode photoconduction
or MOS gate tunneling takes a balancing current.
So if you look carefully you ought to see antenna rules
working against a ratio of feature periphery to attached
MOS gate area (it's current density through the gate
that leaves trapped charge and oxide damage
proportionally).