Ece 209: Chapter 6 - Dielectric Materials

Front 1

2 things that dielectrics do for capacitance:

Back 1

Why do we need to study dielectric materials?Electronic devices become smaller! Therefore, in (1), A is always decreasing as the devices shrink; d cannot decrease indefinitely as below 5-10 nm,tunneling occurs. Therefore, from (2), you have less charge stored. You cannot increase V too much as the breakdown will occur. The only way to increase it is to increase εr, which depends on the dielectric!

Front 2

why are dielectric materials efficient in storing electrostatic charge?

Back 2

• Dielectric materials are insulators. Therefore, they do not conduct electric current at room temperature.

• Electric charge deposited onto the dielectric surface cannot move and stays on the surface (e.g., being attached to surface defects).

Front 3

what is surface charging?

Back 3

Electronic interactions at surfaces are complex and are still poorly understood.

All materials have “surface states” caused by the incomplete bonds.

These strongly affect the behavior of many electronic devices.

For example:

MOSFETs : charge trapped in the oxide acts like an extra fixed voltage.

Charge-coupled-device (CCD) cameras : smudges out image

All ICs are covered with a protective layer (‘passivation”) to prevent contamination by water vapor etc. Silicon nitride orpolyimide are the most commonly used passivation layers.

Front 4

leakage current

in dielectrics, there are ideally no electrons available in.....

Back 4

In dielectrics, there are ideally no electrons available in the conduction band.

♦In reality, some electrons are present in the conduction band, caused by, for example, UV rays,cosmic rays etc.

So there will be a small leakage current

Front 5

fringing fields and the capacitance

Back 5

- caused by "edge effect". the electric field at the edges of a capacitor are non uniform. effects are more noticeable in small capacitors

Front 6

Why does capacitance increase if a dielectric is inserted into a capacitor?

polarization

Back 6

When a dielectric is inserted into parallel plate capacitor as in Figure (b),additional charge is being stored on the plates. The charge stored increases from Q0 to Q (Figure (c)).

The increase in the stored charge is due to polarization of the dielectric in the electric field.

Atoms and molecules become polarized when they are subjected to an electric field, and form electric dipoles

Front 7

what happens when we put a dielectric between the plates of a capacitor?

Back 7

This charge causes the dielectric to polarize, and therefore does not contribute to building up the potential difference across the capacitor (the net voltage across the capacitor is zero).

The same thing happens to the “next” electron’s worth of charge. → → another dipole is formed.... And so on until all atoms in the dielectric are polarized.

♦Only then do the charges induced on the capacitor plate start to contribute to build up the potential difference across the capacitor to oppose the battery voltage.

Therefore, when a dielectric is inserted between the plates of a capacitor, more charge has to flow in before the capacitor is charged up to the supply voltage. (same supply voltage) i.e. the charge storage capacity is increased.

In other words, the dielectric seems to have increased the capacitance

Front 8

define polarization

define dipole moment

Back 8

Each atom and each molecule consist of positive and negative charges. In the presence of electric field, the centers of each charge can be slightly misplaced and the particles become polarized – they become electric dipoles.

Polarization is equal to the bound charge per unit area of the dielectric surface and is measured in coulombs per square meter.

A dipole:

Consider an electric dipole with positive and negative electrical charges +Q and –Q separated by the distance d.

Front 9

Electrical polarizability

a_e

Back 9

Also called “optical polarizability” because the polarization can keep up with even optical frequencies

♦With no electric field, the electron clouds are symmetric around the nucleus, as determined by the quantum states of the atom.

♦ When a field is applied, the electron cloud is distorted.

The centers of the negative and positive charges are now offset, leading to an electric dipole, µe (= αe.Eint)

Front 10

Molecular Polarizability (1)

a_a, a_d

Back 10

Front 11

Molecular Polarizability (2)

a_a, a_d

Back 11

Front 12

interfacial polarizability

a_i

Back 12

caused by IMPERFECTIONS in the lattice

These trapped charges affect threshold voltage of MOSFET by acting like a fixed extra voltage.

♦“Semiconductor grade” chemicals have to be very pure⇒ expensive, maybe 5 x cost of regular lab chemicals

♦As these charges migrate through the dielectric, they can build up at the interface or surface. Together with the mirror charge induced in the other material, the interface charges act like dipoles, giving αi

Front 13

plotting permittivity as a function of frequency

Back 13

For electronic polarizability, only the electrons in thecloud have to be moved, so we would expectrelaxation at high frequencies because electrons are light.

♦Atomic polarizability relies on ions moving their positions so the relaxation frequency should be at frequencies of about the same as thermal oscillations of the atoms i.e. infrared

Orientational polarizability requires re-organization of groups of dipoles, so the inertia is higher than for a single dipole ⇒ frequency lower than atomic.

♦Finally, interfacial polarizability is caused by charge that percolates slowly through the entire thickness of the material and therefore the relaxation occurs at very low frequencies

Front 14

we can classify dielectrics broadly into three categories.

Back 14

Non-polar materials (only has electronic polarizability. ex. silicon)

Those who show variations of permittivity in the optical range of frequencies only. This includes all those dielectrics having a single type of atom, whether they be solids, liquids, or gases.

♦Polar materials (have atomic and electronic polarizability)

Like NaCl which bond ionically, display both atomic and electric polarizability (solids)

♦Dipolar materials (atomic, electronic and orientational)

Those with O-H or C=O chemical groups (e.g. water) also show orientational polarizability.

Front 15

what is the "loss angle"

Back 15

due to real capacitors not being ideal,

real capacitors have a small leakage current through the dielectric

their impedance actually consists of both real and imaginary components of 0 and 90 decree phase. the total phase difference is slightly less than 90 degrees, by an about of the loss angle.

good dielectrics must has as small loss angle as possible (small leakage current)

the frequencies (relaxation peaks) are frequencies at which the capacitive components of the capacitance cease to respond. these frequencies should be avoided

Front 16

dielectric breakdown

(graph)

Back 16

We cannot apply an infinitely large voltage across our capacitor without it breaking down. Dielectric breakdown is usually evidenced by a sudden increase in current to a very large value. Normally this damage is irreversible. The voltage that causes a dielectric breakdown is called breakdown voltage, VBD.

More commonly, electric field is used to characterize the breakdown. The maximum electric field that can be applied to a dielectric without causing a dielectric breakdown is called the dielectric strength, EBR.

In real insulators, the breakdown voltage is hard to predict since it depends on the surface conditions (dirt, damage, etc.) and on the ambient(e.g. moisture in air).

Front 17

dielectric breakdown mechanisms

Back 17

Avalanche breakdown occurs if the electric field across the insulator is high enough. In that case,the few electrons always present in the dielectric due to cosmic rays can achieve enough energy to ionize other atoms (impact ionization). The secondary electrons are also accelerated, causing further ionization and finally, avalanche develops.

Thermal breakdown occurs if the leakage current (loss angle) is large enough to cause significant heating →more leakage → more heating ...

Discharge breakdown occurs if small gas bubbles are present in the material.

Front 18

Main applications of dielectrics in electronics – capacitors and memory cells.

Back 18

Capacitors can be made as discrete elements or be integrated with other elements on the same wafer. Discrete capacitors are made from alternating layers of metal and dielectric, wrapped up in a package. The idea is to squeeze as much area as possible into a small package.

The dielectric material is chosen to get the right range of values with the minimum loss angle.

The capacitors are named after the dielectric material:e.g. ceramic caps, polysterene caps, etc...

Memory cells

The same problem of fitting a capacitor into a small space occurs in random access memory (RAM) cells.

In a RAM, data is stored as the charge in a capacitor cell with a MOSFET as a switch.

As the devices shrink, the challenge is how to maximize the charge storage and minimize the cell area.

Reducing the cell area is a vital goal for increased storage densities.The minimum dielectric thickness is also specified by the reliability of the film.

♦So, chip manufacturers make the capacitors three-dimensional, which requires complex fabrication and may be more susceptible to faults

An alternative way to increase the charge storage capacity is to increase the εr by using another material such as, for example, tantalum pentoxide Ta2O5 (εr = 20) or PZT

The challenge is to persuade manufacturers to change their fabrication process - industry people choose to make minor changes, e.g. film thickness etc., rather than to introduce a whole new material!

Front 19

Piezoelectric materials

Back 19

In these materials, a mechanical stress (tension or compression force/area) causes a dielectric polarization.

or an applied electric field will cause a mechanical strain.

Piezoelectric materials are used for electromechanical sensors and actuators (oscillators, microphones,gramophone stylus, ultrasound transmitters &receivers, cigarette lighters, …)

The mechanism of piezoelectricity involves an asymmetry in the arrangement of positive and negative ions in the material. These materials have hexagonal unit cell:

With no pressure applied, the centers of positive and negative charges coincide.When mechanical pressure is applied in vertical direction, the centers split – a dipole is formed.

In contrast, a symmetrical material would not change its dipole moment when stressed and would therefore not become strained in an electric field

Note that the pressure applied in horizontal direction does not induce polarization – piezoresistivity is anizotropic.

In a piezoelectric crystal, polarization, P, is related to mechanical stress, T,

♦And the electric stress, E, is related to the mechanical strain, S

Ferroelectric materials also rely on the built-in polarization, so they are also piezoelectric (although not necessarily vice-versa)

Quartz (crystalline SiO2) is historically the first piezoelectric material, and is still used for quartz oscillators for computer clocks, watches, radio transmitters & receivers etc