Can Use Top Al layer to transmit High Speed data?

[NovelPanda]

Dear All:

I have an small circuit that has intensive interconnections which may suffer from noticeable crosstalk. To alleviate this I may resort to using the top aluminium (M11) to transmit high speed data (>30Gb/s). However I did not use this metal layer before except for PAD. In addition to Antenna effect (when the line is long enough), what other potential risk I need to consider?

Thanks in Advance!

 

[dick_freebird]

Often this layer is thick`(3u or so) with larger minimum
width and space rules. Presumably you won't be routing
a whole lot of 30GBPS signals in close proximity so this
may not be a concern.

But since it's likely to have the highest current capacity
and lowest DC resistance, you may find you're competing
with, or must yield to, top level power bussing. Cell level
designers may be directed to not use top metal in order
to let the chip integrators / physical designers have a
free hand with top level power routing.

And then you'd be forced down to (say) M10 with no
idea of what's slabbed overhead until after your cell
design is done (bummer).

Depending on approach you might want to make top
metal the ground plane and top-1 be your signals,
in a controlled-impedance transmission line structure.
I don't think you want to treat those signals as
"wires". You can then put side guards on top-1 and
"close the coax" with top-2 for fully shielded lines or
pairs, perhaps. Copious vias can make a pretty decent
coaxial structure. Presuming you like the natural
impedance you can get from such a thing (not always
ideal for larger signals, power-draw-wise, to have a
low-Z on chip tline).