Dielectric
Constant, k
One important
property of a dielectric material is its permittivity.
Permittivity
(ε) is
a measure of the ability of a material to be polarized by an electric
field.
It is, however, easier to
grasp the concept of permittivity by first discussing a
closely related property,
capacitance (C). Capacitance is a measure of the ability of a material
to hold charge if a voltage is applied across it, and is best modeled by
a dielectric layer that's sandwiched between two parallel conductive
plates.
If a voltage
V is applied across a capacitor of capacitance C, then the charge Q that
it can hold is directly proportional to the applied voltage V, with the
capacitance C
as the proportionality constant.
Thus,
Q = CV,
or C = Q/V. The unit of measurement for capacitance is the farad
(coulomb per volt).
The
capacitance of a capacitor depends on the permittivity
ε
of the
dielectric layer, as well as the area A of the capacitor and the
separation distance d between the two conductive plates.
Permittivity and capacitance are mathematically related as follows:
C =
ε
(A/d).
When the
dielectric used is
vacuum,
then the capacitance Co
=
εo
(A/d), where
εo
is the permittivity of vacuum
(8.85 x 10-12 F/m).
The
dielectric
constant (k)
of a material
is the ratio of its permittivity
ε
to the permittivity of vacuum
εo,
so k =
ε/εo.
The dielectric constant is
therefore also known as the relative permittivity of the material. Since
the dielectric constant is just a ratio of two similar quantities, it is
dimensionless.
Given its
definition, the dielectric constant of vacuum is 1. Any
material is able to polarize more than vacuum, so the k of a material is
always > 1. Note that the dielectric constant is also a function of
frequency in some materials, e.g., polymers, primarily because
polarization is affected by frequency.
A
low-k
dielectric
is a dielectric that has a low permittivity, or low ability to polarize
and hold charge. Low-k dielectrics are very good insulators for
isolating signal-carrying conductors from each other. Thus, low-k
dielectrics are a necessity in very dense multi-layered IC's, wherein
coupling between very close metal lines need to be suppressed to prevent
a degradation in device performance.
A
high-k
dielectric,
on the other hand, has a high permittivity. Because high-k
dielectrics are good at holding charge, they are the preferred
dielectric for capacitors. High-k dielectrics are also used in
memory cells that store digital data in the form of charge.
See also:
Dielectric
Home