Electronic – What capacitors are available in chip design


I've heard of MIM caps and MOM caps in chip design; what are the differences between them? What, if any, other capacitor types available to chip designers? What are the relative advantages and disadvantages between the types?

Best Answer

MIM (Metal-Insulator-Metal) and MOM (Metal-Oxide-Metal) capacitors are both metal-to-metal capacitors.

In MIM capacitors, metal plates are stacked on top of each other and separated by a (thin) layer of silicon oxide. Usually this thin oxide is made in a special processing step as the "normal" oxide between metal layers is much thicker (for robustness), which would result in much less capacitance per area. I have seen MIM caps provide around 1-2 femto Farads per square micrometer.

Most MIM capacitors use Metal 5 as the bottom plate, a thin oxide layer, and then a "Metal MIM" as the top plate which is then connected with vias to Metal 6 which will be the usable top plate connection. Direct connections to the "Metal MIM" are not allowed.

I have also seen "dual MIM" structures where there is a second thin oxide layer on top of the "Metal MIM" which connects (via Metal 6) to the Metal 5 bottom plate. This can almost double the density of a MIM cap.

Another type of capacitor is the Fringe capacitor, which uses only one metal layer. This capacitor relies on the fringe capacitance (side capacitance). A top view would look like:

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MOM Capacitors are composed of stacked fringe capacitors:

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Of these three, the MIM cap gives the highest capacitance per area in my experience .

That sums up the Metal capacitors which:

  • can have quite accurate values, for a large cap. tolerance can be 1%

  • the capacitance is independent of the voltage, in other words, these caps are very linear.

  • often you can place these caps on top of other circuits as they are metal-only.

  • they can take quite a lot of area.

  • For the MIM cap (with the thin oxide) There are often special design rules to prevent ESD and manufacturing damage.

Other capacitor types are the non-metal ones:

MOS capacitors: these are often like a PMOS where the gate is the top plate and the Drain/Source connections are the bottom plate. The MOS capacitor's value is very dependent on the applied DC voltage!

Diode based capacitors: these are basically varicaps as their capacitance changes with the DC voltage.

Both these caps are non-linear as their capacitance changes with the applied DC biasing voltage (as opposed by the metal-caps which do not suffer from this).

Their density can be higher than a MIM cap provided you DC-bias these capacitors at the right voltage. For local supply decoupling (where the DC voltage is constant anyway) especially the MOS capacitor is quite useful.