The dielectric constant, k, is a parameter defining ability of material to store charge.
In Si technology the reference is a value of k of silicon dioxide, $SiO_2$, which is 3.9.
Dielectrics featuring k>3.9 are referred to as “high”-k dielectric while dielectric featuring k<3.9 are defined as “low”-k dielectrics.
In cutting edge silicon nano electronics both high- and low-k dielectrics are needed to implement fully functional very high-density integrated circuit, although, for drastically different reasons.
Need for high-k dielectric:
- Silicon dioxide (SiO2) has been used as a gate oxide material for decades.
- As transistors have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance and thereby drive current, raising device performance.
- As the thickness scales below 2 nm, leakage currents due to tunneling increase drastically, leading to high power consumption and reduced device reliability.
- Replacing the silicon dioxide gate dielectric with a high-k material allows increased gate capacitance without the associated leakage effects.
Need for low-k dielectric:
- In digital circuits, insulating dielectrics separate the conducting parts (wire interconnects and transistors) from one another.
- As components have scaled and transistors have gotten closer together, the insulating dielectrics have thinned to the point where charge build-up and crosstalk adversely affect the performance of the device.
- Replacing the silicon dioxide with a low-k dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.