Please help on RFIC spiral inductor simulation results

I simulated some on-chip spiral inductors and the resulting inductance alternates twice between positive and negative values. Could anybody tell me what's going on? Is it because something wrong with simulation settings or need some special handling? Thanks.

It's probably a resonance with your ground ring. Increase the width of the ring and see if it moves. You ground ring looks to narrow anyway; almost as much inductance as your inductor.

madengr,

Thanks for suggestion, but it seems not change much at all. Here are the new structure and simulation results. Do you have any other suggestion? Thanks.

Hi,

Look on smith chart - to see if it is capacitance resonance.
Then it may be the capaciatnce to box.
Try to increase the box - if your boundaries are all PECs try to modify at least
the top ones to Radiation boundary.

Hope it helps,

Gosha

I've looked at lots of EM results for spiral inductors from many different tools and I have never seen a result like that. I will suggest that this might be a numerical artifact, possibly because you are using a volume mesher (I am guessing this is the case). You should try with a another EM tool, preferably several of them, and compare results. Be sure to use a planar tool (I recommend Sonnet for shielded and Momentum for unshielded, I work for Sonnet). Using a volume mesher (if that is what you are doing) for a planar circuit is not a good idea. Do not analyze with a ground ring unless you are going to use the same ground ring when you include it in the final circuit (probably a good idea).