Lecture Notes -Electron Microscopy

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ELECTRON MICROSCOPY

Transmission Electron Microscope (TEM) versus Transmission Optical Microscope (TOM)

TEM TOM

Source 100-400 kV electron gunhigh current densities:

5 x 104

Am-2

for tungsten filament1 x 106 Am-2 for field-emission source

Light source

Condenser lens Electromagnetic lens, focus adjusted bycontrolling the lens current

Glass lens, focus adjusted by lensposition

Specimen stage Allows for specimen tilt as well as some

z-adjustment

Allows for specimen tilt as well as

some z-adjustment

Objective lens Fine focusing of the image by adjustingthe lens current

Fine focusing by adjusting theposition of the specimen and theobjective lens

Final imagingsystem

Employs electromagnetic lenses toproduce image on a fluorescent screen

Eye piece forming image fordirect viewing

Recordingsystem

Computer monitor or TV Normal viewing or photographicfilms

Experimentalset-up in

Vacuum, better than 10-

torr Air, at atmospheric pressure


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Scanning Electron Microscopy (SEM)


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Major components are the electron column consisting of an electron gun and the electron lenses, and the control consoleconsisting of a cathode ray tube viewing screen and the scanning and control electronics for the electron beam.

The three dimensional appearance of the image is due to the large depth of field. A scanning electron microscope (SEM) employs a probe lens to focus the electron beam into a fine probe and scanning coils

are used to scan the probe over the sample.


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Resolution of the SEM is controlled by the probe lens. It is the inelastically scattered electrons that provide information.Usually < 30 keV electrons are used in SEM.

In optical microscopy and TEM, information is collected continuously over the full field of view (from all image pointssimultaneously) and focused by suitable lenses to form a magnified image. In SEM, information is collected sequentially.

Electron Sources:

The filament is resistively heated to 2000

2700K by applying a high voltage and a small

amount of current to a point that valence

electrons are released from its tip in what is

called a space charge cloud. The amount of

energy needed to cause electrons to leave the

filament is called the work function.

The electrons are released in all directions. The Wehnelt cap/grid has a slight negative

potential (charge) - or excess of electrons - to create negative lines of force that focus the emitted

electrons and control their emission.


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The filament orcathode is supplied with a high negative voltage, e.g. -20,000 volts. The anode, a metal plate with a hole in it, is

at ground potential (0 volts) and is greatly positive with respect to the cathode. This potential difference accelerates the electrons

toward the anode.

Along the route from the cathode to the anode, the paths of the individual electrons cross each other. This is called the point of

crossover with diameter d0. It is considered the real source in the electron gun, and its image is projected onto the specimen surface.

After the crossover, the electron beam diverges with a divergence angle 0. The condenser and objective lenses then produce a

demagnified image of the crossover on the specimen with an image diameter dp.


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The anode has a hole in it. This hole allows only a fraction of the electrons to continue down the column toward the lenses. The

remaining electrons are collected on the anode and returned via the ground to the voltage supply.

Hot Cathode Guns: Tungsten filaments and Lab-sixLaB6 crystals, producing thermionic emission of electrons.

Thermionic emission formula:

Current density I = AT2

exp(-E/kT)

A = Richardsons constant depending on the source material,

T = emission temperature K (C+273),

E = work function or the energy required to escape from the filament into the vacuum, k = Boltzmans constant.

Tungsten: LaB6:

stable beam current stable beam current

short life 10 times longer life

large tip smaller tip

large emitting area (probe diameter) smaller emitting area (probe diameter)

low brightness higher brightness (10 times higher current)

high work function lower work function

high evaporation rate medium evaporation rate

lower vacuum (10-6

Torr) higher vacuum (10-7

Torr)

low resolution higher resolution (due to thinner beam)

2700 K 1700 K


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Field Emission:

This is based on the fact that electrons can be drawn off from a material by applying a high voltage in ultra high vacuum conditions.

stable beam current if heated, unstable if cold

very long life

more monochromatic

small tip

very small emitting area (small probe diameter)

very high brightness (1000 times compared to hot guns)

concentrated electric field can tunnel through energy barrier

low evaporation rate

very high vacuum (10-10 Torr)

very high resolution (electrons are emitted from a very fine tip)

three typescold, thermal, Schottky

Electron Beam

The electrons that go through the hole in the anode and continue down the column to the specimen make up the electron beam. The flow of these electrons in called the beam current. The electron beam is manipulated by lenses and apertures.

Electromagnetic lenses and focusing:


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Since electrons have a charge, their direction of travel can be altered by an electromagnetic field. An electron traveling in off-axis to a uniform magnetic field follows a helical path. Electrons can be brought to focus by engineering the electrostatic

and/or magnetic fields. In the region of electron gun, the beam is influenced by the electrostatic field. All the subsequent

focusing is achieved by electromagnetic lenses.

The condenser lens concentrates (or demagnifies) the beam of electrons into a small area called a spot. It is like the condenseron an optical transmission microscope which concentrates light from the light source (e.g,. a light bulb) into a small area as it

passes through the specimen slide. The size of the spot can be adjusted. It can be a large area or it can be adjusted to be a

small area. Condenser orspot size control is one of the most important controls on an SEM.

In the electromagnetic lens, the intensity of the field (the magnetic flux) causes a radial vector along the optical axis, sowhen an electron is accelerated through the pole pieces, it takes a helical path through the lens. The rotational force is the

product of the electron velocity and the density of the magnetic flux. This vector interaction also results in focusing as the

strength of the lens is changed.


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Focal length of the electromagnetic lens is controlled by varying the lens current. The focal length is approximatelyproportional to V/(NI)

2where V is the accelerating voltage, N is the number of turns in the magnet coil and I is the current.


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Electron lenses demagnify the image of the beam crossover (with d0 ~ 50m for a heated electron gun) in the electron gun tothe final spot size on the specimen of ~10 nm. This represents a demagnification by 5000 times.

These lenses are created with high precision and even a hairline scratch can distort their magnetic field and will have to bereplaced. Most electromagnetic lenses are cooled with water to prevent extra heating.

Their functions are similar to optical lenses. A condenser lens can condense electrons; an objective lens can focus electrons onthe specimen, and a projector lens can project an image onto a screen.


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1/f = 1/p + 1/q

Demagnification m = p/q = d0/d1

Like in optical microscopy, spherical aberration makes peripheral electrons to be deflected more than electrons that arecloser to the center. Electrons in the beam with slightly different velocities or wavelengths causes chromatic aberration.

Both types of aberrations results in a disk rather than a point where all the rays converge. Both types of aberrations make theimage blurred and reduce resolution.

Diameter of the disk ds in the case of spherical aberrationds = Cs

3

Where Cs is the spherical aberration coefficient (usually few millimeters) directly related to the accelerating voltage and the

focal length of the lens and is the aperture angle.

Diameter of the disk in the case of chromatic aberrationdc = E/E Cc

WhereE/E is the fractional variation in electron beam voltage, C c is the chromatic aberration coefficient related to the focal

length of the lens and is the aperture angle.


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Increasing the working distance produces a larger spot size at the specimen and a consequent degradation of the imageresolution.

Increasing the condenser lens strength increases the demagnification of each lens by reducing the probe size dp.

Neglecting chromatic aberration, the dependence of probe size dp on probe current ip can be written as:


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where constant K ~ 1. The value of dp can be obtained in nm when Cs and are in nm, ip in Amperes and brightness in A/cm2

sr.


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Depending on the type of electron source, and its inherent brightness, the focused beams can have sizes ranging fromnanometers to micrometers carrying currents ranging from picoamperes to microamperes. The thermionic guns have a

minimum probe size of ~ 10 nm. Field emission guns have the smallest probe diameters ~ 1.2 nm suitable for higher resolution

SEM measurements.

Resolution can be improved by increasing the accelerating voltage. But one has to worry about radiation damage at such highvoltages.

The lenses in a SEM are generally electromagnets, coils of copper wire around a hollow iron core through which the electronspass as they are accelerated down the column. By applying a direct current through the copper coil, we can create a magnetic

field in the hollow of the lens that will slightly change the path of the electron beam, and we can control that path by varying

the current in the coil, which in turn changes the strength of the electromagnetic field.

The electromagnetic lens is a notoriously poor lens when compared to the glass lens that focuses light. Some aberrations cannot be

corrected as they can in glass lenses.

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Lecture Notes -Electron Microscopy