hfss mesh outside
Eric -
I believe that the root cause of the discrepancy of your simulation results and analytical model (of parallel plate capacitor) is EDGE effects, or fringing, or periphery, capacitance. For the geometry you are using (10x10um plates with 1 um separation), the difference you are quoting is comparable with my estimates:
assuming that fringing/periphery capacitance is equivalent to adding an area to the plates by oversizing them by the spacing between the plates (1.0 um), you get an additional area of 4x10=40 um2 on top of 100 um2 - that is ~40% increase (assuming there is 3.9 epsilon everywhere).
You did not specify the thickness of the metal (thicker metals would increase fringing capacitance), and I am not sure if you really set epsilon=1 outside the capacitor - so there is no point to make more accurate estimates.
Practical suggestion - increase the size of the plates to 1000x1000 um (I hope HFSS would not choke at that), so that the edge effect becomes very small, then compare HFSS results with analytical model. Any discrepancy higher than ~1% should be then attributes either to inaccuracy in HFSS simulations (mesh effects etc.), or incorrect setting of your simulations in HFSS.
I did not have a direct experience with HFSS for MIM/MOM cap simulations, but some of my partners/customers have done so. HFSS builds a huge 3D mesh, requires very long simulation time (~1day per simulation run with frequency sweep), and provides relatively accurate values for capacitance and RF characteristics (resonant frequency, quality factor, etc.).
I am using a random walk based capacitance extraction, which is a rigorous field solver, it provides all required parameters (capacitance between the plates, capacitance to ground, capacitances to neighboring nets, quality factor, resonant frequency) with very short simulation times (~few minutes - for MIM/MOM capacitor of any size and with any metal interconnect environment). It reads in standard files (GDS layouts (or DFII or CCI/SVDB databases), technology files in any of the standard formats (itf, proc, ircx, ict - with all the advanced manufacturing effects taken into account), etc.), so that simulation setup is very simple and fast.
Max
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