Scanning Electron Microscopy

Scanning Electron Microscopy
Field
emitting tip
Grid
≤2kV
≤100kV
Anode
ZEISS SUPRA
Variable Pressure FESEM
Analytical
Workshop 2012
Dr Heath Bagshaw – CMA
bagshawh@tcd.ie

Why use an SEM?
Analytical
Workshop 2012
Fig 1. Examples of features resolvable using different imaging techniques

Improving Resolution
• Firstly, the wavelength of the imaging source is important.
In an optical microscope white light is used (λ – 380-700-nm)
• In an Electron Microscope the imaging source is a beam of electrons which has a
shorter wavelength (λ ~0.0025nm at 200kV) .
• This is approximately five times smaller than visible light and twice as small as a
typical atom – this is why electrons can ‘see’ atoms but white light can’t :-
Analytical
Workshop 2012
‘the analysis probe must be smaller than the feature being analysed’
• The wavelength of electrons is dependent on the accelerating voltage, i.e.:-
kV
Wavelength λ (pm)
20 8.588
100 3.702
200 2.508
300 1.968
• The higher the accelerating voltage the shorter the wavelength.

The Parts of an EM
• Electron Microscopes (EMs) are similar in operation to optical microscopes except
that they use a focused beam of electrons instead of light to "image" the specimen and
gain information about its structure and composition.
• There are four major regions in an Electron Microscope:-
Analytical
Workshop 2012
(1) A stream of electrons is formed (by the electron source/gun) and
accelerated toward the specimen using a positive electrical potential
(2) This stream is confined and focused using metal apertures and magnetic
lenses into a thin, focused, monochromatic beam.
(3) This beam is focused onto the sample using a magnetic lens. In an SEM
the beam is then also scanned across the surface of the sample.
(4) Interactions occur inside the irradiated sample, affecting the electron
beam which are detected and transformed into an image or signal.
• The above happens in all EMs regardless of type.

Layout of a Generic SEM
Gun
1
Aperture
Holder
2
Deflection
coils
3
Objective Lens
4
Analytical
Workshop 2012

Electron Gun
• There are two main types of gun – Thermionic and Field Emission (FEG).
• Thermionic gun :- Simplistically, a material (often a piece of twisted tungsten) is heated
to a high temperature so that it will emit electrons.
• Can also use LaB 6 crystal grown to a tip – gives a brighter beam than W for same kV.
Analytical
Workshop 2012
Tungsten filament Tungsten filament assembly LaB 6 filament tip

Thermionic Gun
Filament
Wehnelt
cylinder
-
10-10000kV
+
Anode
earth
•Filament is heated and begins to produce electrons.
•Electrons leave the filament tip with a negative potential so accelerate towards the
earthed anode and into the microscope column.
•A small negative bias on the Wehnelt then focuses the beam to a crossover which acts as
the electron source.
Analytical
Workshop 2012

Field Emission Gun (FEG)
FEG source – W tip
• A very strong magnetic field (~10 9 Vm -1 ) draws electrons from a very fine metallic tip
(usually W).
•An extraction voltage of around 2kV is applied to the first anode to create an intense
electric field to allow electrons to escape from the tip.
•The second anode is then used to accelerate the electrons into the microscope at the
required energy.
•Combination of the two anodes focuses the beam into a crossover creating a fine
beam source.

Comparison of Sources
•W filaments are very simple and inexpensive.
•LaB 6 filaments give greater brightness than W (approximately X10), but cost more.
•FEG’s give much more brightness than thermionic systems.
•FEG’s give a more monochromatic electron source and finer probe (i.e. better resolution).
•Temperatures used are much lower than for thermionic sources (particularly cold cathode
FEG’s).
•FEG’s require better vacuum systems and are more expensive.
Analytical
Workshop 2012
Comparison of the three types of source
operating at 100kV

Focusing the Beam
• After the beam is formed it is focused by a condenser lens system to form a ‘probe’.
• The lenses are electromagnetic – the focal length changes as current in the coil changes.
• After focusing, the beam is passed through an aperture which excludes electrons which are
not on the optical axis – improving resolution.
• Inconsistencies in the beam are corrected by stigmators and the beam focused onto the
sample.
A typical Electro
Magnetic Lens
Analytical
Workshop 2012

Scanning the Beam\Beam Interactions
• Deflector coils move the beam back and forth over the sample and the signal
generated from each area is collected simultaneously, building up the final image
shown on the monitor.
• Many signals are generated at the surface of the
sample and many different forms of analysis may be
performed.
• The ‘interaction volume’ is the area of the sample
excited by the electron beam to produce a signal.
Incident Electrons
(Electron Probe)
Auger Electrons
Secondary Electrons
Backscattered Electrons
Continuum X-Rays
Fluorescence X-Rays
Characteristic X-Rays
Signals generated in the interaction volume
Analytical
Workshop 2012

Interaction Volume
•The ‘interaction volume’ is the area of the sample excited by the electron beam to
produce a signal.
•The penetration of the electron beam into the sample is affected by the accelerating
voltage used, the higher the kV the greater the penetration.
•The effective interaction volume can be calculated using the electron range, R:-
1.67
0.0276
A E0
R =
( µ m)
0.89
ρ Z
Where A is the atomic weight (g/mole), Z is the atomic number, ρ is the density (in g/cm 3 ) and E o is the energy of
the primary electron beam (in kV).
Take the example of iron:
A=55.85, Z=26, r=7.87 g/cm 3
Accelerating voltage (kV) Primary Electron Range (µm)
30 3.1
15 0.99
5 0.16
1 0.01 (10nm!)
Signals generated in the interaction volume
Analytical
Workshop 2012

Signal Detection
• The Everhard – Thornley Detector (ETD) is the most common detector used to detect
secondary electrons to image surface topography.
• Electrons are attracted to a +ve charge on a grid in front of the detector. The captured
electrons are then amplified by a photo-multiplier before being digitised and sent to a screen.
• The signal detected is transferred to a viewing screen as the beam is scanned building up the
image.
Everhard Thornley Detector
Scanning to produce an image
Analytical
Workshop 2012

Biological Samples
•Biological samples are not conductive and are particularly vulnerable to beam damage and
other heating effects when examined in an Electron Microscope.
•The level of exposure is referred to as ‘Electron Dose’ and is a measure of the number of
electrons per unit area (e/nm 2 ).
•Samples are either stained with conductive materials (e.g. OsO 4 ) or coated with Au or C.
•Samples are viewed under vacuum, so they are dried to remove all water.
a) b)
•a) SEM image of Pneumonia, and b) SEM image of Diatom (Pictures from University of Iowa)
•Preparing the samples ‘fixes’, and alters them – need a way to look at samples whilst they are
still ‘wet’.
Analytical
Workshop 2012

Variable Pressure SEM
•Localised charging is removed by the presence of gas in the sample chamber, effectively allowing the
examination of non conductive samples
•In Low vacuum mode the chamber is isolated from the high vacuum system(A) and is instead pumped
by the additional rotary pump system(B).
Analytical
Workshop 2012

Variable Pressure SEM (2)
• This allows analysis of ‘non conducting’ samples as charge is compensated by gas in the
chamber.
• Use of an SEM in VP mode does lead to some limitations in it’s operation:-
• Because the vacuum is lower in a VPSEM chamber, some resolution of the instrument is
lost due to scattering of the electron beam by the gas particles in the chamber.
• In situ heating and or cooling (with the appropriate sample stage) is possible in VPSEM
to allow direct observation of sample changes.
• Compositional analysis is still possible.
Analytical
Workshop 2012

Compositional Analysis – Back Scattered Imaging
• As mentioned previously, when the electron beam hits the sample a number of signals
are generated. Secondary electrons are used for looking at surface detail (topography).
• EM is also a very powerful technique for analysing composition and compositional
distribution in a material\sample.
B
Analytical
Workshop 2012
Signals generated in the interaction volume
• Back Scattered electrons are produced just below the surface of the sample (B) and are
scattered more by heavier elements than by lighter elements.
• The backscattered coefficient, η = (Z-1.5)/6 So, as Z increases, so does the degree of
backscatter.

Back Scattered Electrons
0.5 Backscattered
electrons
0.4
Electron yield
0.3
0.2
0.1
Secondary electrons
0 20 40 60 80
Atomic number (Z)
Electron yield (i.e. intensity) as a function of atomic number for backscattered and Secondary
electrons.
• Back Scattered electron have approximately the same energy as the primary electron
beam and are therefore easy to detect - simply by a semiconductor placed above the
sample :-
Analytical
Workshop 2012
Schematic of a backscattered electron detector.

BackScattered Imaging
• Back scattered electrons are deflected more by heavier atoms leading to a brighter
contrast in BEI images – the lighter the region the heavier the element present.
a)
b)
White
region
Dark
region
Grey
region
• a)Secondary image of a cement showing surface morphology
• b)Backscattered image of same area showing compositional inhomogeneity
• Three distinct regions in b), EDS analysis can then be used to find the different
compositions of the these regions.
Analytical
Workshop 2012

Example EDS of a Cement
• There were 3 distinct regions in the Backscattered Image
‘Light’ region is made up predominantly of
Fe. (i.e. the heaviest element)
‘Grey’ region is made up predominantly of
Ca.
‘Dark’ region is made up predominantly of Si
and Al. (i.e. the lightest elements)
Analytical
Workshop 2012

Examples Images
Imaged using ET Detector
Imaged using InLens Detector
Analytical
Workshop 2012
Low kV image of platelets

Examples Images
SE image of nano tubes
BE image of nano tubes
Analytical
Workshop 2012
SE Image of nano structure

Conclusions
•Scanning Electron Microscopes (SEM’s) are very useful tools for looking at a range of
samples\materials.
•Surface detail, homogeneity and elemental composition can be determined in one
experiment on the same sample.
•Newer Variable Pressure SEM’s allow the imaging of non conducting samples.
•ESEM’s, with cold stages and other peripherals allow imaging at 100% relative humidity
allowing imaging of ‘wet’ samples
•Electron Microscopy based analysis when used with other analysis techniques can
assist in complete characterisation\identification of materials.
•Electron Microscopes provide a very powerful analysis tool in both Materials and
biological fields.
Analytical
Workshop 2012