Introduction: to scan the surface and resolve the image

Introduction:

The Scanning Electron Microscope was
invented by Charles Oatley and co-workers at Cambridge University in the late
1940s. The SEM is typically a type of microscope which uses beam of electrons
to scan the surface of the specimen unlike the regular optical microscope which
uses beam of light to scan the surface and resolve the image in Millie scale.

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The typical electron microscope
consists of a pair of lenses which are magnetic in nature and is helpful in
resolving the surface of the specimen by narrowing down the electron beam to
focus the specimen. The resolution of the electron microscope is limited by the
wavelength of the electrons and the image quality of the lenses.

 

Construction
of the SEM

The Emission Gun

The Scanning Electron Microscope
consists of the regular tungsten fiber which is the source of the electrons
which is the main constituent of the emission gun. Diverse types of emission guns
are used to produce the beam of electrons. The two common types are thermionic
emission and field emission electron gun.

The thermionic emission takes place
when the tungsten filament in the emission gun which is the cathode is heated
to a temperature of about 2800K. Due to the increase in the temperature the
electrons from the valence shell is emitted. The electrons thus generated is
gathered as an electron beam, flowing into the metal plate (anode) by applying
a positive voltage to the anode. If a hole is drilled in the anode the electron
beam flow through this hole. This produces a beam of electron which is required
to produce image of the specimen surface.

The field emission electron gun works
on the principle that under the influence of strong electromagnetic field,
electrons are emitted from the surface of the conductor. Thin tungsten wire is
used as the conductor and the tip of the tungsten wire is welded with a tungsten
single crystal and is shaped to be a curvature radius of about 100nm. The only
draw back in this type of emission gun is that it requires a vacuum of 10-8
Pa.

Wehnelt Electrode

The Wehnelt electrode is placed
between the cathode (tungsten filament) and the anode and applying a negative
voltage to it can be used to adjust the current of the electron beam produced.

Lens system

The SEM consists of a pair of lens
system which is the condenser lens, and objective lens. The lens system used
here is a magnetic lens system. When electric current is passed through a coil
wound electric wire, a magnetic field is formed which behaves like a lens pushing
the electrons inwards which is like an optical lens. The optical lens converges
the light beam whereas the magnetic lens bends the electron beam inwards
producing a narrower beam.

Condenser lens

The condenser lens is right below the
electron emission gun to produce the fine electron beam to scan the surface of
the specimen. By increasing the strength of the magnetic condenser lens, the
electron probe becomes narrower whereas if the strength of the electron beam is
weakened, it might produce a broader electron probe due to the magnetic effect
of the lens. A thin metal plate called aperture is placed between the condenser
lens and objective lens that allows a part of the electron beam to be focused on
the objective lens.

Objective lens

The key role of the objective lens is
to determine the final diameter of the electron. If the performance of the objective
lens is not good, an optimally-fine electron probe cannot be produced despite all
the efforts before the action of the objective lens. Thus, it is crucial to
make the objective lens with the best performance.

Specimen stage and specimen surface

The specimen stage of the electron
microscope should can hold the specimen without drifting during the whole
microscopy process. For this purpose, a eucentric specimen stage is used. By
using this type of stage, the observation area does not shift even when the
specimen is tilted and the focus on the specimen does not change after shifting
the field view while the specimen is tilted.

The specimen surfaces

The surface of the specimen must be conductive.
If the specimen is conductive, the electrons flow through the specimen stage.
But, when the specimen is nonconductive, the electron stops in the specimen and
there is no outflow. The number of electrons thus flowing into the specimen is
not equal to the exit.

If this condition continues then negative
charge is accumulated resulting in a large negative potential. At a threshold limit,
the electrons start discharging and comes back to original potential and this
is called charging. So, the specimen should be conducting. If the specimen is non-conducting
a thin layer of conducting material is coated on the specimen to avoid
charging.
Working of a SEM

The SEM is used to produce magnified
images of the specimen which is in the micro to nanometer scale. The image of
the specimen is captured by the different signals produced when the electron
beam interacts with the specimen surface.

To produce the signals the electron
beam needs to be finely focused on the specimen. The diameter of the electron beam
can be calculated with the following diagram.

 

Figure
1

  (http://coen.boisestate.edu/faculty-staff/files/2012/01/SEM.pdf)

In this diagram the filament diameter
is d0,
 the distance between the filament
and the condenser lens is u1 and
v1 is given to be the distance between the demagnified electron beam diameter
and the condenser lens.
So, the demagnified electron beam diameter d1 can be found as

d1= d0 x v­1/u1

From this equation it is apparent that
the stronger the condenser lens, v1 becomes shorter, and the overall
demagnified electron beam diameter will be smaller. The final electron probe
diameter d on the specimen is (from the image)

d = d1 x v­2 /
u2      = d1 x WD / u2

Where,
u2 is the distance between the demagnified source image and the objective lens
v2 is the distance between the objective lens and the specimen which is also
called working distance (WD) of the SEM
(v1 + u2) is a constant for a calibrated instrument so smaller
working distance results in smaller diameter. The probe size in an SEM is
decreased by either increasing the strength of the condenser lens or decreasing
the working distance.

Interaction of electrons with Specimens

Now that electron beam is produced, the
beam is made to enter the specimen surface. When some electron interacts with
the specimen surface, it gradually loses its energy and is absorbed. Some other
electrons on interacting with the specimen produce a variety of signals
depending on the type of interaction with the specimen. Some of the signals
thus produced are Backscattered electrons, Secondary electrons, Auger
electrons, X-rays. Each of the signal produces different type of information
about the specimen. These electrons produce the topographic, morphologic,
compositional and crystallographic information of the specimen.

Secondary electrons

When the incident electron beam
enters the specimen, secondary electrons are produced from the emission of the valence
electrons of the constituent atoms in the specimen.

 

 

Detecting the Secondary electron

The Secondary electrons are detected by
the secondary electron detector which has a scintillator (fluorescent substance)
coated on the tip of detector and a high voltage of +10kV is applied to
generate light when the secondary electrons hit the scintillator. This light is
used to produce the image of the specimen.

Backscattered electrons

Backscattered electrons are those
scattered back when the electrons interact with the nucleus of the atoms in the
specimen. Since the backscattered electrons possess higher energy than
secondary electrons, information from a relatively deep region is contained in
the backscattered electrons. The detection of backscattered electrons is done
by a specific detector.

Resolution

Resolution is defined as the “the
minimum distance that can be separated as two distinguishable points in the
image”. The resolution can be affected by factor such as structure of the specimen,
wear and tear of the instrument and so on.

In the scanning electron microscope,
resolution is limited by the source brightness.

db = (?/ 2?) (Ic /
?) ½

where,

 ? is the source brightness (controlled by the
electron gun),

Ic is the minimum
beam current required

So, to improve the
resolution of an SEM it is necessary to have a large aperture, small probe
size, and a bright electron gun; hence, for high resolution work, an SEM with a
field emission electron gun (FEG SEM) is typically used.

Limitations of SEM

Some of the
limitations of SEM is that the sample must be solid and conducting, and the
emission guns should be kept at high vacuum to avoid contamination of the
electron probe.

 

 

Electron
and Ion Beam Lithography

To structure
patterns in the nanometer scale, we require an instrument that is little enough
to fit in. Since we have mastered in producing the electron beam through
different emission guns, the same can be used to pattern structure exactly like
in an optical lithography replacing the light with electron beam.

Since electrons
are charged particle, they can be controlled by magnetic waves and can focused
to write on a resist. This makes electron beam lithography easier and
patterning structures in the sub 100nm domain to 0.1nm resolution can be
achieved.

Like EBL, ion beam
lithography can also be used. Instead of the electron beam, an ion beam is used.
The charged molecule utilized by IBL is bigger and heavier than electrons,
which actuates less diffusion when being focused on the photoresist. Both
molecules beam lithographic procedures have been utilized for designing.

Limitations of EBL and
IBL

The electrons are particles which have very less
mass and hence they are easily scattered by other molecules and hence can only
be carried out under vacuum condition as gas molecules can easily scatter an
electron. Both lithographic techniques lack efficiency. Since they require
great control over the magnetic field, it is not possible to write a lot of patterns
at the same time

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