The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Modeling >> Passive Devices >> MIM capacitor https://designers-guide.org/forum/YaBB.pl?num=1212804698 Message started by aaron_do on Jun 6th, 2008, 7:11pm

 Title: MIM capacitor Post by aaron_do on Jun 6th, 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

 Title: Re: MIM capacitor Post by aaron_do on 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......

 Title: Re: MIM capacitor Post by vivkr on 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? RegardsVivek

 Title: Re: MIM capacitor Post by aaron_do on 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   :Pthanks,Aaron

Title: Re: MIM capacitor
Post by ACWWong on 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.