Mim Cap In Vlsi

Why MIMCAP is needed?

The geometry scaling has led to thinner interconnects and reduced metal width. Interconnect lengths were also increased along with switching at gigahertz speeds to meet complex design requirement. The device scaling has increased the density of integrated transistors on the semiconductor wafer. There may be large current spikes due to simultaneous switching within short periods of time, which can cause the current resistance drop, voltage fluctuation and noise on the power supply network. These will affect reliability, speed and signal integrity. The addition of on-chip decoupling MIMCAP compensate voltage fluctuations by supplying charges to the power network. However, the capacitance must be large enough to meet the requirement.


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It is therefore very easy to create capacitors using two metals on top of each other. When you do this, you speak about MOM capacitors (Metal Oxide Metal).

These are good capacitors, but the fact is that, for mechanical and other reasons, the oxide between the metals (called Inter Metal Dielectric, IMD) cannot be made too thin. What does it mean? Well, remember the formula giving the capacitance of a parallel plate capacitor? It reads:

C=ϵA/d

Where A is the area of the metal plates, d is the distance between them. Therefore, if we can’t make d very small, it means that we need a ‘large’ area - A - to implement a certain capacitance value. We refer to this saying that the ‘capacitance density’ of MOMs is not that great.

In the MIM technology, however, we introduce an extra dielectric in the fabrication process, and also a new metal layer. This dielectric is far thinner than a normal IMD, which gives rise to a much higher cap density for MIMs.

 


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