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Design >> Analog Design >> MiM cap shielding
https://designers-guide.org/forum/YaBB.pl?num=1287242967

Message started by newic on Oct 16^th, 2010, 8:29am

Title: MiM cap shielding
Post by newic on Oct 16^th, 2010, 8:29am

My MiM cap is formed by M6-M5. What is the effective way to shield the MiM cap?

I use dummy Mim cap to surround the core MiM cap but with smaller size.
Is there other way?

How about the shielding at the bottom side? Shall I shield it with psub tap or just leave it blank?

Title: Re: MiM cap shielding
Post by vp1953 on Oct 18^th, 2010, 5:35pm

Hi Newic,

I dont think it is possible to shield the bottom parasitic coupling cap to substrate. Putting a plain metal structure between the bottom plate and the substrate increases the coupling cap to substrate (since the effective thickness of the cap has come down due to the thickness of the additional metal layer). Psub tap may increase the coupling cap depending on what exactly your psub is.

Title: Re: MiM cap shielding
Post by vp1953 on Oct 18^th, 2010, 5:36pm

Hi Newic,

there might be some active way of shielding the cap but at substantial premiums of power that might be unacceptable. If you do learn of some way do the shielding, please do let me know.

Title: Re: MiM cap shielding
Post by ACWWong on Oct 23^rd, 2010, 4:15pm

newic wrote on Oct 16^th, 2010, 8:29am:

How about the shielding at the bottom side? Shall I shield it with psub tap or just leave it blank?

Leaving it "blank" usually means metal fill will be under and around that capacitor anyway unless you draw metal exclusion layers. If using a MiM cap pcell, often they'll include a layer to do this exclusion depending on your pdk. metal fill is needed to planarise the back-end-of-line processing.
if you can exclude fill without violating drc, then shielding the bottom plate with say metal 1 will increase the parasitic capacitance but now to a net of your choice (usually something quiet), and the Q of the parasitic capacitor can be improved, if that matters to you.

Title: Re: MiM cap shielding
Post by newic on Oct 23^rd, 2010, 6:58pm

thanks for the advice!!

Title: Re: MiM cap shielding
Post by vp1953 on Oct 24^th, 2010, 10:33am

Hi ACWWong,

Thank you much for your explanation. I was clearly mistaken about not being able to avoid substrate coupling.

More than getting a higher Q (which can be 50 or higher for MiM caps), in many instances it is important to avoid any parasitics to gnd. For an AC signal, both metal 1 tied to a voltage (as you have stated in your example) and substrate are at virtual gnd. Would this method not increase the effective parasitic to (virtual) gnd?

Clearly there are benefits of using metal 1 tied to a voltage say, to avoid a any coupling of signals via substrate.

Title: Re: MiM cap shielding
Post by ACWWong on Oct 24^th, 2010, 1:09pm

vp1953 wrote on Oct 24^th, 2010, 10:33am:

Would this method not increase the effective parasitic to (virtual) gnd?

yes, as i said in my last post the parasitic value does increases (due to less distance between "plates"), but now you have a MiM parasitic, and not a potentially lossy, albeit lower valued, parasitic where one plate is substrate. Clearly it depends on you application whether you want to just minimise parasitic cap or you want to control the parastic quality/crosstalk.

cheers
aw

Title: Re: MiM cap shielding
Post by newic on Oct 25^th, 2010, 6:27am

I was thinking that if the bottom plate M5 of the mim cap is ground. Could I still fill M1-M2-M3 with ground plate underneath the mim cap (metal density issue)?
The parasitic cap between M5 to Metal-n should not be an issue because they are both ground. Doe it matter?