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decoupling mos capacitor

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angyp

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comparing MIM cap and MOSCAP(mos transistor), which is more suitable to be used as on-chip decoupling capacitor?why?
 

uladz55

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Hi,

MIM (like P1-P2) capacitors are more stable for different voltages then MOSCAP.
But MOSCAP capacitors have much higher (almost 10 times) specific capacitance.

Ur choice.
 

koka

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Hi~
gate leakage current is antoher concern in choosing MOSCAP as decoupling capacitor. IO MOSCAP can provide smaller leakage current , but you will pay more area to complete the specific capacitance, CORE MOSCAP has opposite relationship. It is due to the gate oxide thickness. MiM has very small leakage current (~pA), but extra mask is necessary.
 

uladz55

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why MIM capacitor need extra mask?

U can use Me-Me fringe capacitor. It's resonable for process higher then 0.35um.
 

maxwellequ

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A few notes:

1. The MOSCAP has a larger capacitance for the same area.

2. While it is true that the capacitance of the MOSCAP varies with applied voltage, if it is connected between supplies there is no such problem (variation is small because VDD is sufficiently larger than Vt).

3. The (gate) leakage current of a MOSCAP is usually negligible compared to your circuit's current consumption.

4. The channel resistance is very useful to damp the oscillations (the decoupling capacitance and the inductance of the bondwires make a resonator circuit). You don't have this in MiMs.

So, the MOSCAP is preferable for decoupling. Finally:

5. You do need an extra masking to make the MiM Capacitors. MiMs are preferable to Me-Me capacitors because their capacitance per unit of area is larger (about 4x-10x depending on the structure of Me-Me caps). Moreover the capacitance value of MiMs is much better controlled than the capacitance of Me-Me capacitors (corner capacitance variations are smaller). Most times there is even no characterization of Me-Me capacitors. Finally the matching of MiM caps is usually better. This is why, in some designs you end up paying for the additional mask.

Regards
 
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terryssw

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How about the comparison between poly-poly cap and MiM cap in terms of linerity and capacitance per unit area?
 

jordan76

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First of all,double poly is not usually availabe in generic CMOS process.

Secondly,poly-poly capacitance per unit area is larger than MIM due to its much smaller oxide thickness. In terms of linearity, both are good candidates.

Regards,
jordan76
 

CTT

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Hi,

don't you also have an ESD issue here ? MIM capacitor will handel much more than MOSCAP in a ESD zap. You may get better ESD performances
if you're doing an industrial design.

CTT
 

jordan76

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Hi CTT,
My answer above is targeted only for terryssw's question
"How about the comparison between poly-poly cap and MiM cap in terms of linerity and capacitance per unit area?".

I agree with you that MIM capacitor will handle higher voltage for ESD.

regards,
jordan76
 

take_care00

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Hi all,

poly-poly capacitance is mainly used in analog design to compensate multiple stage amplifiers for example.

Take care.
 

xinsu

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in general, mos cap and poly cap are used in analog circuit for getting higher cap value;mim used in digital circuit.
 

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