area efficient mom cap structure types

mom cap value per sq um

Hi All,

can any one tell me what could be the best value of cap can be obtained by mom cap per sq um.



thanks in advance

Re: mom cap value per sq um

sgunupuri01 said:

can any one tell me what could be the best value of cap can be obtained by mom cap per sq um.

Click to expand...

C = (ε0 εr * 1µm²) / t_iso

For a 0.18µm process, e.g. εr ≈ 7 ; t_iso ≈ 300nm ; ==> C/Area ≈ 0.2 fF/µm²

Gate oxide accumulation caps achieve ≈ 3fF/µm² in this process tech.

Re: mom cap value per sq um

sgunupuri01 said:

Hi All,

can any one tell me what could be the best value of cap can be obtained by mom cap per sq um.



thanks in advance

Click to expand...

For 90nm technology and below, capacitance density for MOM capacitors can be as high as 2 fF/um2. (IBM reported value of 1.73-1.76 fF/um2 in numerous papers, and it's probably higher in modern technologies).

According to UMC, MOM capacitor density exceeds that of MIM capacitors ("A 65-nm High-Frequency Low-Noise CMOS-Based RF SoC Technology", Yang, D.; Ding, Y.; Huang, S.; IEEE Transactions on Electron Devices, Volume 57, Issue 1, Jan. 2010 Page(s):328 - 335).

Capacitance is much higher than the value derived by Eric since many metal layers are used for MOM capacitor, the distances between metals in advanced nodes are much smaller (~0.1um), both vertical and lateral electric field fluxes may be used (even though dielectric constant is lower than 7.0 - it's ~3.9 for SiO2, and 2.5-2.9 in advanced low-K BEOL technologies).

Re: mom cap value per sq um

erikl said:

C = (ε0 εr * 1µm²) / t_iso

For a 0.18µm process, e.g. εr ≈ 7 ; t_iso ≈ 300nm ; ==> C/Area ≈ 0.2 fF/µm²

Gate oxide accumulation caps achieve ≈ 3fF/µm² in this process tech.

Click to expand...

i dourbt,does it takes into account lateral (fringing capacitance) also ???..

Is there any method to estimate capacitance of woven or comb like(interdigital) structure..???:?::?:

Re: mom cap value per sq um

deepak242003 said:

... i doubt, does it takes into account lateral (fringing capacitance) also ???..

Click to expand...

You're right, this comprises only the area dependent capacitance.

deepak242003 said:

Is there any method to estimate capacitance of woven or comb like(interdigital) structure..???:?::?:

Click to expand...

PDK docu usually states the fringing data, too, s. e.g. the TSMC 0.18µ electrical parameters below (published by MOSIS).

mom cap value per sq um

do we need to add this fringe capacitance to overlap capacitance to get total cap..??

nyways thx a lot erik....

Re: mom cap value per sq um

deepak242003 said:

do we need to add this fringe capacitance to overlap?

Click to expand...

Sure, if you need a very accurate extraction. And this depends also on the contribution of fringing cap to total cap: cap array or fingered structures contribute proportionally more fringing cap to total cap than (single) block caps. If the fringing cap contribution is more than - say - 0.1%, you must consider its contribution in case of a 10bit-converter, because otherwise the cap matching inaccuracy wouldn't show up and you would probably not recognize the ENOB reduction caused by this.

Re: mom cap value per sq um

Please note that separating the total capacitance into area and fringe (and whatever else) components is not a rigorous way to calculate capacitance. This is just a convenient approximation, allowing to estimate capacitance. It does not take into account the environment of the capacitor, other second-order effects (i.e. presence of corners in the layout, or complicated shapes), and so its accuracy is probably not better than several percent (or worse).

Accurate capacitance calculation (to an accuracy better than a few percent) requires solving Laplace equation and integrating electric field or charge, and in general case it's impossible to meaningfully separate the total capacitance into partial components.