Pro and Cons of the different Types of IC capacitors that can be introduced in a IC chip.
Integrate circuits technology allows to create a variety of devices on the silicon die. The most common single devices integrated on IC chips are: Transistors, diodes, resistors, capacitors and inductors.
This article explores the different types of IC capacitors to be integrated on-chip
Yes, Inductors can also be integrated!! You might be interested on how the inductors are integrated on chip
In microelectronics, where the area means money, often capacitors are the bulkiest device. Consequently, when choosing a certain technology, one relevant and deterministic parameter can be the capacitance density, depending on the application. This is how much fF can I integrate per squared micra. The density is measured in fF/µm² and varies from 0.2 to ~7, depending on the technology.
How are the ideal Capacitors?
let’s start with the ideal case…. IC-Capacitors in silicon are far from being ideal, but how is an ideal cap?
The properties of an ideal capacitor:
- high quality factor (Q) / low losses
- high breakdown voltages
- capacitance being voltage-independent
- high area capacitance
- low parasitic capacitance
- low leakage currents
The capacitance of a cap is determined by the formula:
Therefore to obtain high-density capacitors in integrated circuits, three factors could be changed:
- A better isolator with higher dielectric constant
- Shorter distances between layers
- Increase the area
The distance between layers depend on the technology, but they are not constant and the thick can not be determined with precission. That is why the capacitors have typically around ~20% of error from the nominal value and bring the large nominal capacitance variations.
The die-area is a very valuable factor in chip-design, and should be reduced as much as the design allows it. Remember area = money.
IC Capacitor types
There are several types of capacitors commonly used in integrated circuits (ICs). Some of the most common types include:
- Metal-Oxide-Semiconductor (MOS) Capacitors
- Metal-Insulator-Metal (MIM) Capacitors
- Mosfet Capacitors
- Metal Fringe Capacitors
- Deep Trench Capacitors
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 widely used, because of the high capacitance per area unit with the lowest parasitics. The drawback is that they require more process steps during the fabrication. Mainly, a new mask and step are added to deposit the insulator between the metal layers. Therefore, a fabrication process with MIM caps is more expensive than others using only MOM-caps.
They can be single, double, triple, etc. It depends on the number of layers of metal and insulators. The thickness of the insulator layer is in the range of 25 nm (depending on the fabrication technology).
The quality factor is very high because both plates are made out of metal and the voltage dependency is negligible.
High density (capacitance per area unit)
C is indpendent of Voltage
More process steps and layers (than MOS caps)
For example, in X-Fab XT018 technology cmm3t or cmmh4t with a density between 1 and 6 fF/µm². In TSMC18, mimcap_2p0_sin with C = 1.00 fF/µm².
They are very similar to the MIMs caps but with an oxide layer between metals is usually made by interdigiating metal layers with the process oxide. So the capacitance is created in two places: laterally capacitance with the other fingers and vertically capacitance with the other layers.
They have a lower capacitance per unit area, but they are cheaper than MIM.
Low C density
Transistors MOS Capacitor
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.
They are also known historically as MOS-Capacitors (Metal-Oxide-Semiconductor)
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 very 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. Another drawback is the parasitic resistance of the bottom plate.
High C density
huge variation with the voltage
High parasitic resistance of the button plate
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 teh devices csf3 or csft4.
Low C density
This kind of caps are still an 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 (almost any). The capacitance density is up to 700 nF/mm² by a submicrometer pores array. The pores could be up to few hundreds nanometers big.
Not mature. Almost not available
Diodes (pn-junctions), when they are reverse-biased, introduce a small capacitance, which varies a lot with the voltage applied.
MOS-caps and pn-caps are very voltage-dependent capacitances.
The density of the junction capacitances is around 1pF/um2.
Only to use as a voltage variable capacitance
Not really an advantage. marginally usefull
These are just a few types, the most popular and used in the industry. There may be other specialized capacitor types used in specific applications. The choice of capacitor type depends on factors such as capacitance requirements, voltage ratings, temperature stability, and integration feasibility within the IC fabrication process.
I hope this article has provided the information you expected and has facilitated your understanding. If it has been helpful, then you may be interested in further reading with IC inductors: how they are and pros and cons in the other article.