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top plate parasitic capacitance of MIM capacitor (Read 4805 times)
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top plate parasitic capacitance of MIM capacitor
Mar 21st, 2009, 8:55am
Hi all,

I am a new member in designers-guide.
About MIM cap, I am using it as a coupling capacitor, but when I extract the layout, in extracted file there is a big parasitic capacitance for top plate which degrade the NF of my LNA and also change its frequency.
My coupling capacitors are 2pF and the value of extracted top plate parasitic capacitor is 150fF.
From the topics here and also the pdf of desingkit it seems that the reasonable value for parasitic capacitance of the MIM caps are 1-2 % of the main cap. So, I would like to know that the results of the extraction are reasonable or not.
My process MIM caps have been constructed in M8 (both of the plates).

Thank you in advance,
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Reiner Franke

Posts: 165
Re: top plate parasitic capacitance of MIM capacitor
Reply #1 - Mar 29th, 2009, 4:19am
150fF at 2pF is 7.5% which not unreasonable. But DK could not forsee every case.

The top plate cap depend on which metal is above the cap top metal plate.

1. If it is a metal it has a defined cap and could be forseen in the DK device by either a separate symbol or by a switch which change the mode. But be aware that the shield generate a further pin. So the symbol should be unique.

2. If there is no metal above by metal fill block ther remains stray caps on the perimeter. For very critical analog nodes (your LNA input is the highest sensitive point) consider also stray shield in perimeter to the cap on the same layer than the top metal plate.

3. On more digital dominated DK often you have no control over the metal fill. Then use your  own metal fill which you could control and connect to quite shield.

The story is that the digital fabs earn the money by digital gate content but the delivery is often defined by the analog content.

It has interessting similarities with capital crisis  :(

PS: Please check your LNA. If it is sensitive to 7.5% par cap it will not work because the main cap have also around that spread.
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