Momentum EM simulation - capacitor

[big_fudge98]

Hi,

I have a basic question related to EM simulations. (I am using ADS Momentum (ADS 2021) in Cadence Virtuoso (!C618) environment). I would like to EM simulate a 30 fF MOM cap. The way I am doing this is as follows:
I attach an input pin and an output pin. Define these as the input port and the output port. Perform EM simulations, and then measure capacitance using Y parameters.

My question is, do I need to attach 4 pins overall and use differential ports at the input and output? Is return current involved here? I am quite confused at what happens to the return path in a lumped component like a capacitor.

Thanks.

 

[volker@muehlhaus]

You can use 2 port with implicit ground (infinite ground at bottom of substrate) and then evaluate the differential capacitance between port 1 and 2

OR

you can make pin 2 the ground reference for port 1. That is done in the port editor by drag & drop. In this case, you evaluate capacitance of the 1-port element.

Both methods will provide the same result for series capacitance of your MOM. If you also need the shunt capacitance of MOm to the substrate, then you need the 2-port simulation.

Good luck!
Volker

 

[big_fudge98]

You can use 2 port with implicit ground (infinite ground at bottom of substrate) and then evaluate the differential capacitance between port 1 and 2

OR

you can make pin 2 the ground reference for port 1. That is done in the port editor by drag & drop. In this case, you evaluate capacitance of the 1-port element.

Both methods will provide the same result for series capacitance of your MOM. If you also need the shunt capacitance of MOm to the substrate, then you need the 2-port simulation.

Good luck!
Volker

Hello Volker, thanks for your answer! I have a follow-up question:
Why would both cases give the same result? Is it because the implicit ground at the bottom is a PEC with zero resistance, and due to the fact that the return current undergoes no phase shift along the implicit ground since this is a small lumped component?

 

[volker@muehlhaus]

I see your point and admit that I ignored a possible return path effect in the PEC boundary. In this case, the DUT is small and I think both results will be the same.

I'm working a lot on EM anaylsis of RFIC inductors and usually evaluate the 1-port differential data that are calculated from the 2-port. Equations as shown at the end of my appnote here:

Dr. Mühlhaus Consulting & Software GmbH » Inductor EM simulation: 1-port or 2-port?

Your RF EDA solution partner since 1993

muehlhaus.com

 

[big_fudge98]

I see your point and admit that I ignored a possible return path effect in the PEC boundary. In this case, the DUT is small and I think both results will be the same.

I'm working a lot on EM anaylsis of RFIC inductors and usually evaluate the 1-port differential data that are calculated from the 2-port. Equations as shown at the end of my appnote here:

Dr. Mühlhaus Consulting & Software GmbH » Inductor EM simulation: 1-port or 2-port?

Your RF EDA solution partner since 1993

muehlhaus.com

Hi, thanks for the reply. I read your appnote and found this statement interesting "The ports have a large distance between the (+) pin on the metal layer, and the infinite ground at the bottom of the substrate. "

How about defining a ground plane on M1/M2 layer? Then, the input pin and output pin of the cap (which lies in the M7 plane) can be referenced with this ground plane and hence be defined as ports. Does this seem like a more accurate approach?

(I know this sounds like nit-picking, but I am working at 250 GHz, and hence would like as high accuracy as possible).

 

[volker@muehlhaus]

Yes, you can create a defined ground on a lower metal layer. Just make sure it doesn't create an issue with shunt capacitance of the device.

With a client (IHP) we created an rfcmim element some years ago, and that also has a defined metal ring around the MIM for controlled shunt path behaviour. https://ieeexplore.ieee.org/document/7839730

 

[big_fudge98]

Yes, you can create a defined ground on a lower metal layer. Just make sure it doesn't create an issue with shunt capacitance of the device.

With a client (IHP) we created an rfcmim element some years ago, and that also has a defined metal ring around the MIM for controlled shunt path behaviour. https://ieeexplore.ieee.org/document/7839730

Thanks. This has been useful