Integrate circuits technology allows creating many types of devices on the silicon die. The most common devices are transistors, diodes, resistors or capacitors. Now, the different kinds of capacitors that could be integrated on-chip are discussed.
In microelectronics, where the area means money, the most important parameter is the capacitance density. The density is measured in fF/µm² and varies from 0.2 to ~7, depending on the technology.
MIM Capacitor (Metal-Insulator-Metal)
It is formed by two parallel metal layers and has a high-k dielectric between them. The button layer forms a small parasitic cap with the substrate.
This type is the most used, because of the high capacitance per unit area with the lowest parasitics. The drawback is that they require more process steps during the fabrication. Basically, a new mask and step are added to deposit the insulator between the metal layers.
They can be single, double, triple, etc. It depends on the number of layers of metal and insulators
For example, in X-Fab XT018 technology cmm3t or cmmh4t with a density between 1 and 6 fF/µm²
MOM Capacitors (Metal-Oxide-Metal) are very similar to the MIMs caps but with an oxide layer between metals is usually made by interdigiating metals with the process oxide. They have lower capacitance per unit area, but they are cheaper.
Transistors can be used as a cap using the thin oxide layer from the gate as an insulator. The Gate acts as the top plate and the Drain and Source as the button plate.
The drawback is that if the voltage at the gate varies, the depletion area changes at the gate varying the dielectric properties and modifying the capacitance. So, the MOS capacitor’s value is dependent on the applied DC voltage.
The capacitance per unit area is higher than the MIM caps, because the insulator of the gate (SiO2) is much thinner than the insulator between the metal layers. But the variation with the voltage is a big disadvantage.
They are useful for local supply decoupling where the DC voltage is constant.
Metal Fringe Capacitors
They use only one metal layer, relying on the side capacitance property and the matching is better than the MIMs caps.
For example, in X-FAB technology csf3 or csft4.
This kind of caps are still object of study and development. Deep narrow pores are made in the silicon substrate to gain more surface and create high-density capacitor saving die area.
These are useful to save area, but they are not available in many technologies. The capacitance density is up to 700 nF/mm² by a submicrometer pores array. The pores could be up to few hundreds nanometers big.