SEM Electron gun

Question 1

The various components of the SEM can be categorized as

Answer 1

1. The Electron Column
2. The Specimen Chamber
3. The Vacuum Pumping System
4. The Electronic Control and Imaging System

Question 2

The Electron Column

Contains the:

Answer 2

a. Electron Gun
b. Magnetic Lenses
c. Scan Coil
d. Apertures

Question 3

(1) The filament (cathode) or electron source

Answer 3

) The filament (cathode) or electron source, which generates electrons and is thus held at negative potential with respect to the ground.
The most common type of electron source is a 0.25 mm (0.01 in) diameter tungsten filament heated to approximately 2500 °C. The electron essentially boil off (thermionic emission) the sharply bend tip of the filament and are attracted to the anode which is maintained at a positive voltage relative to the filament.

Question 4

(2) The shield (Wehnelt cylinder)

Answer 4

is biased negatively to collimate the electron from the filament and direct them toward the anode.

Question 5

The anode

Answer 5

is at large potential difference relative to the filament, causing acceleration of the electron. The difference in potential between the filament and the anode is accelerating voltage.
A range of voltage between 1 and 30 keV is available on most SEMs. The choice of accelerating voltage depends upon specimen type and the type of SEM application.

Question 6

Electron Beam

Answer 6

Electron gun, source of electron, regulates the intensity of the beam and direct it onto the specimen. It provides:

- large stable current (intense)

- symmetrical
- small electron beam

High Intensity:
Sufficient number of electrons passes through small area of the sample, creates higher signal to noise ratio.

Symmetrical:
For good alignment

Small electron beam: for

• Small area investigation,
• High resolution,
• Smaller power consumption

Question 7

The emission of electron from a substance can be induced by application of:

Answer 7

• Heat (Thermionic emission)
• Strong electric-field or field emission
• Electromagnetic radiation (photoelectric emission)
• Atomic particles (secondary)
Only first two are used to generate electron beam in electron microscopes

Question 8

Polarization field

Answer 8

Is due to atoms in the surface layer having other atoms exerting forces on them from underneath, but no forces from on top. This result negative electron layer at the surface and positive under it.
When a negative electron enters into this field it is attracted by the positive layer and repelled by the negative, therefore, it requires higher energy for electron to escape.

Question 9

Image field

Answer 9

This results after an electron has passed through the polarization field, it leaves a positive charge at the surface of the metal that is an electrostatic image of the electron.
This positive electrostatic image pulls the electron back toward the surface of the metal.
For electron to escape from the surface of the metal, it should have sufficient energy to overcome image filed.

Question 10

Electron Emission

Answer 10

1. Heat (Thermionic emission)

2. Strong electric-field or field emission

Question 11

Characteristics of the Thermionic Emitting Materials

Answer 11

They should possess:
1. Low Work Function
Work Function: Potential energy to give to electron to leave material, e.g.,
2. Low Oxidation (for low vacuum)
3. Low evaporation (for low operating temperature for satisfactory emission)

Question 12

1. Thermionic emitters:

Answer 12

Use an electrical current to heat up the filament which lowers the work function of the filament material. When the work function is lowered, electrons can be more readily drawn off of the filament with an electric field.
– Tungsten Thermionic
– LaB6 (lanthanum hexaboride)

Question 13

Tungsten Thermionic

Answer 13

This filament is a loop of tungsten which functions as the cathode. A voltage is applied to the loop, causing it to heat up. The anode, which is positive with respect to the filament, forms powerful attractive forces for electrons. This causes electrons to accelerate toward the anode. Some accelerate right by the anode and on down the column, to the sample.

Question 14

thermionic emitters (Work function)

Answer 14

The ability to give up electrons is related to a material’s “work function.”
The work function of a material can be given by the equation:
E = Ew + Ef
where E is the total amount of energy needed to remove an electron to infinity from the lowest free energy state, Ef is the highest free energy state of an electron in the material and Ew is the work function or work required to achieve the difference.
• Materials with a small work function are better thermionic emitters than those with a large work function, but there is a trade off.
• Although tungsten has a relatively high work function it also has the highest melting point of all metals. A large number of electrons can be obtained below its melting point giving tungsten filaments a longer working life and making them useful filaments.

Question 15

saturation of tungsten thermionic electron gun

Answer 15

• A filament is said to be “saturated” when further heating of the filament does not result in an increase in the number of electrons emitted.
• False peak caused by regions of surface irregularities of filament that reaches emission temp before tip

Question 16

Lanthanum hexaboride-LaB6

Answer 16

The LaB6 filament is also a thermal filament. However, its work function is lower than for a tungsten filament, so it is more efficient.
LaB6 (lanthanum hexaboride)
- Made from sintered powder
- Tip radius 1 -10 mm
- Height 0.5 mm
- Body diameter 100 mm
- Has 10 times brightness and longer life than tungsten
- Life time: 200-1000 hrs
- Requires high vacuum of 10-7 torr (10-5 Pa)

Question 17

Field emission electron gun

Answer 17

Cold cathode field emission sources do not heat the filament material. The electrons are drawn from field emission gun by placing the filament at a huge electrical potential gradient, so large that the work function of the material is overcome, and electrons are drawn off of the filament.
Field emission systems require extremely high, clean vacuums in which to operate

Question 18

Different Types of Field Emission

Answer 18

1. Cold Field Emission (FE)
2. Thermal Filed Emission (TF)
3. Schottky Emission (SE)

Question 19

Cold field emission (FE)

Answer 19

• Needs clean vacuum > 10-10 torr (10-8 pa).
• Foreign gas monolayer reduced and unstable ie.
• Heating tip to 2500 k (flashing) cleans the tip and gas atoms. After flashing for 10-15 min it goes reduction slows down.
• There is no fixed maximum current.
• Higher emission gradually reduces due to gas absorption and eventually become unstable.
• Filament life time depends on number of flashes, more flashes, blunter it gets (no burnout).
• Extraction voltage has to increase as tip getting blunter. The tip should be changed and sharpen when extraction voltage has to increase to maximum (after months and years).
• Tips are vulnerable to catastrophic failure due to high voltage arc discharges.
• Advantages of cold emission:
1. Virtual source is small, therefore small demagnification to form a small spot of 1-2 nm,
2. Energy spread is very low, therefore good low voltage performance.

Question 20

Thermal field emission (TF)

Answer 20

• Needs vacuum > 10-7 torr
• Heating tip to <100> single crystal of W
• Continuous heating 1300-2000 k prevent most of the gas for building up.
• Higher beam energy spread.

Question 21

Schottky Emitter Operation (SE)

Answer 21

• Operates at 1800 K
• Uses ZrO coating on <100> W facet at the tip to reduce work function from 4.5 to 2.8 eV.
• Flat emitting area results in stable emission
• Large beam size has to be demagnified to 1-2 nm, therefore will lose its current density

Question 22

Field Emission Stability and total current:

Answer 22

• High current (1 nA) into small electron probe (1-2 nm) produces excellent SEM images.
• Problem is stability which can be improved by feedback loop.
• For X-ray analysis , F.E. is not suitable and overall for probe size > 200nm W or LaB6 are better

Question 23

Resolution and Different Types of Electron Guns

Answer 23

• The two factors that determine resolution in the scanning electron microscope are:
– accelerating voltage
– initial crossover diameter
• According to Abbe’s equation as stating that the resolution of an instrument is dependent upon the wavelength of its illumination source.
• The accelerating voltage of a scanning electron microscope is variable, usually in the range 500 - 30,000 volts.
• An electron accelerated by a potential of 30Kv has a shorter wavelength than one accelerated by a 5Kv potential. Thus, the 30Kv electron should give us better point to point resolution.
• The other component of resolution, initial crossover (beam diameter)
• To resolve a feature on the surface of a specimen, the beam must still have a smaller diameter than that feature, yet still contain enough electrons (referred to as beam current density) to generate acceptable amounts of signal.
• The smaller that the initial crossover is, the less that the electromagnetic lenses have to work to demagnify the beam into a usable probe.
• A tungsten hairpin filament will have a crossover diameter of about 50 microns.
• A field emitted probe can be used for imaging without any subsequent focusing action of electromagnetic lenses. This makes the field emission system the highest resolution instrument.
• Source size for cold and thermal field emission <5 nm, and for Schottky is 15-30 nm.
• There is an inherent fluctuation of emission with the FESEM. Any contaminants that come to rest on the filament cause the emission current to fluctuate. This prevents the FESEM from being interfaced with a quantitative microanalyzer, since a prerequisite of good quantitative analysis is a steady beam current.

Question 24

different types of Electron Current

Answer 24

1. Filament Current
2. Emission Current
3. Beam Current
4. Probe Current

Question 25

1. Filament Current

Answer 25

Filament heating current, If (filament heating current) of electron gun using thermionic emitters (W and LaB6) heats the emitter by resistance heating to the point at which electron emission occurs.

Question 26

2. Emission Current

Answer 26

Emission Current, Most of the electron emitted is absorbed by the anode and return to the high voltage supply this current is measured as emission current (is circulating current, about 100 mA).

Question 27

3. Beam Current,

Answer 27

Beam Current, ib, is the portion of current passing through the anode hole.

Question 28

4. Probe Current

Answer 28

Probe Current, ip, current at the sample, at each lens and aperture along the column the beam current becomes smaller at the specimen. The beam is several order of magnitude smaller at the sample surface.

Question 29

Brightness of Illumination

Answer 29

• Important property of an electron gun is the number of electrons per unit of solid angle of beam and not the total intensity of the beam. This means concentrated electron in small beams.
• The number of electron in the beam is defined by brightness.
• Brightness = (Beta) = current density per unit of solid angle = (current/area) Amp/cm2 steradian

Question 30

Solid angle

Answer 30

• The angle that, seen from the center of a sphere, includes a given area on the surface of that sphere. The value of the solid angle is numerically equal to the size of that area divided by the square of the radius of the sphere.
• The maximum solid angle is ~12.57, corresponding to the full area of the unit sphere, which is 4.
 Solid Angle =  = A / r2
• Standard unit of a solid angle is the Steradian (sr).