3D Surface characterisation - Atomic Force and Scanning Electron Microscopy for Imaging 3D Structures

3D Surface Characterisation
Dr Daniel J. Thomas
Atomic Force and Scanning Electron Microscopy for
Imaging 3D Structures
daniel.thomas@engineer.com

Scanning Electron Microscopy
• A scanning electron microscope produces
images of a sample by scanning it with a
focused beam of electrons.
• These electrons interact with atoms in the
sample, producing various signals that can be
detected and that contain information about
the samples surface topography and
composition.
• A Tungsten filament cathode is used in a
thermionic electron gun because it has a high
melting point and low vapour pressure,
thereby allowing it to be heated for electron
emission.

How an electron beam works

• The electron beam, which typically has an energy
ranging from 0.2 to 40keV, which is focused by one
or two condenser lenses to a spot about 0.4 to 5nm
in diameter.
• The beam passes through pairs of scanning coils or
pairs of deflector plates in the electron column,
typically in the final lens, which deflect the beam in
the x and y axes so that it scans in a over an area of
the sample surface.
• Chemical characterisation is performed using energy
dispersive X-ray analysis (EDX). The electron beam
stimulate the atoms and they send out X-rays of
specific energies for each element, the so called
characteristic X-rays.

The Principle
•When the primary electron beam interacts with the
sample, the electrons lose energy by repeated
random scattering known as the interaction volume,
which extends from less than 100nm to
approximately 5μm into the surface.
•The size of the interaction volume depends on the
electrons landing energy, the atomic number of the
specimen (density). The energy exchange between
the electron beam and the sample results in the
reflection of high-energy electrons by elastic
scattering.

Magnification
• Magnification in a SEM is controlled over a
range 10 to 500,000 times. Image
magnification is not a function of the power
of the objective lens.
• SEMs may have condenser and objective
lenses, but their function is to focus the
beam to a spot, and not to image the
specimen. Provided the electron gun can
generate a beam with sufficiently small
diameter.

•The most common mode of detection is by
secondary electrons emitted by atoms excited
by the electron beam. The number of
secondary electrons is a function of the angle
between the surface and the beam.

High Resolution High Magnification
• By carefully controlling electron beam properties and using
backscatter we can image very complex non-conductive materials,
such as these aggregate particles embedded into the surface of a
ceramic tile.

Tilt Scanning
• On a flat surface, the plume of secondary electrons is mostly
contained by the sample, but on a tilted surface, the plume is
partially exposed and more electrons are emitted. By scanning the
sample and detecting the secondary electrons, an image
displaying the tilt of the surface is created.

Imaging Silicon
• SEM image of laser scribed silicon – the scribe is 25μm
across. The SEM is used extensively for capturing detailed
images of silicon surfaces. We can see details of the laser
pulse damage on the surface.

3D Image Capture
• SEMs do not naturally provide 3D images. However 3D
data can be obtained using two main methods, such as:
• Photogrammetry taken from two or three images from a
tilted specimen.
• Vertical stacks of SEM micrographs plus image-processing
software
• Possible applications for this include roughness
measurement or measurement of internal structures for
modelling applications.

Rebuilding an Image in 3D
• 3D reconstructed SEM of bone in
comparison with CT models showing the
reticulated pore structure. Some finer-scale
porosity is also observed in the SEM.

Atomic Force Microscopy
• Atomic force microscopy (AFM) is a very high-resolution
non optical-based microscopy, with demonstrated
resolution on the order of fractions of a nanometer.
• The AFM is one of the foremost tools for imaging,
measuring, and manipulating matter at the nanoscale.
• The information is gathered by feeling the surface with a
mechanical probe. Piezoelectric elements that facilitate
tiny but accurate and precise movements on electronic
command enable very precise scanning.

Atomic Force Microscopy
• The precursor to the AFM was the Scanning
Tunnelling Microscope (STM) in which its tip
interacts with the valence bands of the atoms.
Atomic Force MicroscopeScanning Tunnelling Microscope

Types of Tip
• The main measurement instrument is the
tip
Normal Si Tip Si Supertip Si Ultralever
Diamond-coated tip Sharpened tip Gold-coated Si3N4 tip

Scanning modes
• Contact mode imaging is heavily influenced by frictional
and adhesive forces, and can damage samples and distort
image data.
• Non-contact imaging generally provides low resolution and
can also be hampered by a contaminant such as liquid
which can interfere with oscillation.
Contact Non-Contact

Scanning modes
Non Contact Scan Contact Scan

Tapping Mode
• Tapping Mode imaging takes advantages of the two
modes. It eliminates frictional forces by intermittently
contacting the surface and oscillating with sufficient
amplitude to prevent the tip from being trapped by
adhesive meniscus forces from a contaminant layer.
Contact Non-Contact Tapping

Generating a Surface 3D Profile
• By scanning multiple paths then we can generate a three
dimensional axonometric profile of the surface.
The structure can then be
rebuilt and profile
measurement, such as surface
roughness parameters
extracted.

AFM scans of Cancer Biomarkers
Thus attachment of the antibodies
has modified the surface
topography compared with linking
groups only.
The
biofunctionalised
silicon surface with
the antibody when
investigated using
AFM showing
changes in the
surface roughness.
When qualitatively
comparing them,
the sample with the
antibody attached
has more
pronounced
features.

Conclusions
• Scanning electron microscopy and Atomic Force
Microscopy are useful instruments in acquiring 3D surface
profiles.
• These techniques can also be used for many more aspects
of imaging. Allowing for the surface topography and
composition of samples to be understood.
• By combing both SEM and AFM techniques then we are
able to understand and characterise the properties of novel
materials.

Dr Daniel Thomas
3Dynamic Systems

3D Surface characterisation - Atomic Force and Scanning Electron Microscopy for Imaging 3D Structures

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