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EM simulator substrate modeling (Read 7869 times)
aaron_do
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EM simulator substrate modeling
Jul 06th, 2011, 8:23pm
 
Hi all,


I wish to add ports (formed by P diffusion) at specific points on the substrate, and extract the s-parameters of the resulting network. My problem is, I'm not sure if its possible to add a layer into the substrate which will form an electrical connection to the substrate. I'm using EMX which I think works a lot like ADS momentum. Does anybody have any ideas?


thanks,
Aaron
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RFICDUDE
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Re: EM simulator substrate modeling
Reply #1 - Jul 7th, 2011, 5:13am
 
I think that you can just define a conductive P+ substrate layer on top of the conductive substrate. You may need to define a fake metal layer to make a contact through the P+ layer down to the top of the substrate. You might be able to use a real thin perfect conductor for the fake metal, but the via through the P+ should have the same property as the P+ layer. The simulator should be able to solve for the changes in conductivity between the P+ and substrate layers around the fake metal contacts. Other folks may have better ideas than this.

I assume you are interested in the effective impedance between different ports for local contacts?

The bigger question is regarding how the bulk substrate is tied off to ground? If the substrate will be sitting on a grounded piece of metal then you might be able to assume the backside of the substrate is effectively tied to an ideal ground. If this is not the case (like in flip chip) then you may need to come up with a more realistic approach like putting real wide P+ substrate contacts fairly far away from the local ports to simulate substrate ties to ground through pad connections.

If the block you are putting these contacts in has a substrate guard ring then you need to include the ring too.


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aaron_do
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Re: EM simulator substrate modeling
Reply #2 - Jul 7th, 2011, 6:44pm
 
Hi RFICDUDE,


thanks for the help. At the moment I've defined a (rather poor) metal conductor which sits inside the substrate and is supposed to approximate p-diffusion. I also defined vias to contact the conductor.

Ultimately what I'm actually trying to do is to get a good model for an n-well diode. The n-well diode will have p-diffusion nearby, but now I'm not sure how I can define the n-well. Seems like I need to define some sort of insulator layer for a conductor to sit inside. Its giving me a massive headache. As for the nonlinear part of the diode, I'm just going to use an ideal diode circuit model.

Regarding the grounding, it is just a traditional chip with a ground plane underneath, so I guess the default simulation method is ok.

Sorry if I didn't define my original question properly, but I didn't want to add unnecessary information, and I initially overlooked the n-well part of the problem.


thanks,
Aaron
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aaron_do
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Re: EM simulator substrate modeling
Reply #3 - Jul 7th, 2011, 8:43pm
 
Hi,


actually now the problem is the "fake" metal layer is not making electrical contact with the substrate. I tried placing two such layers with about 10 um spacing, and ports on each one. The results came back as approximately a series RC instead of a parallel RC. I guess EMX cannot have metals making contact with "layers".

Does anybody know of any fix, or if this can be done with any other EM simulator?


thanks,
Aaron
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RFICDUDE
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Re: EM simulator substrate modeling
Reply #4 - Jul 9th, 2011, 7:01am
 
It is a little bit difficult to debug without a little more information about the set up.


I would start with a simple test case that finds the resistance and capacitance between a perfect conductor to ground with a conductive substrate sandwiched between them.

Then you can add complexity like two conductive layers and have some confidence the simulator is yielding reasonable results.

With each case you should be able to roughly calculate the expected resistance and capacitance from the port on the perfect conductor to the reference ground.
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