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MIM capacitor (Read 7399 times)
aaron_do
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MIM capacitor
Jun 06th, 2008, 7:11pm
 
hi all,

not sure if this is in the right section but anyway, i'm designing an LNA, and post-layout simulation shows that the output resonant circuit is resonating at a slightly lower frequency than in schematic simulation. I have a pretty big coupling capacitor at the output (10 pF) and i was wondering if I can compensate the frequency shift by trying to reduce the parasitic capacitance between the bottom plate and the substrate. So i was thinking i could put the MIM (M6 - M5) on top of an N-well, and use a large biasing resistor to bias the nwell at VDD. The idea is that the bottom plate capacitance would be in series with a large resistance and be suppressed.

Does anybody know if this would work?

thanks,
Aaron
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aaron_do
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Re: MIM capacitor
Reply #1 - Jun 7th, 2008, 9:01am
 
ok after thinking about it a bit longer, i doubt it would work since the cap across the nwell -psub depletion region would be in parallel with the high resistance of the nwell......
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vivkr
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Re: MIM capacitor
Reply #2 - Jun 8th, 2008, 11:15pm
 
Hi Aaron,

Not sure if I understand your problem correctly. You are saying that the bottom-plate parasitic of this 10 pF cap is causing problems for you.
But you have a MIM cap available. Can't you use that MIM to realize this 10 pF cap? That would reduce the bottom-plate considerably.
Or am I missing something?

Regards
Vivek
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aaron_do
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Re: MIM capacitor
Reply #3 - Jun 9th, 2008, 12:12am
 
Hi Vivek,

I calculated that a 10 pF MIM cap has about 150 fF of bottom plate capacitance (metal 5 to substrate). Does this number seem reasonable? Anyway if i could suppress that 150 fF, my resonant frequency would be closer to what I want.

The problem is that the inductor models that I have aren't scalable, and the circuit resonates slightly below the desired frequency, so rather than make a huge change in the inductance, i wanted to see if i could get rid of some of the parasitic capacitance.

In the end i just reduced the coupling cap size   Tongue


thanks,
Aaron
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ACWWong
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Re: MIM capacitor
Reply #4 - Jun 9th, 2008, 12:17am
 
aaron_do wrote on Jun 9th, 2008, 12:12am:
I calculated that a 10 pF MIM cap has about 150 fF of bottom plate capacitance (metal 5 to substrate). Does this number seem reasonable?


this seems reasonable... normally its better than 1~2% depending on the mim density, metal stack etc.
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