The document summarizes electron microscopy. It describes how electron microscopy works by using a beam of electrons instead of light to image specimens. It discusses the main components of a scanning electron microscope, including the electron gun, electron lenses, and detectors. It also explains how scanning electron microscopes function by scanning a focused electron beam over a sample to produce signals containing information about the sample's surface topography and composition.

1. Electron Microscopy
Sagar Sen
Junior research fellow(JRF)
School of Instrumentation
Devi Ahilya University Indore(M.P.)

2. Content
 Introduction.
 Types of electron microscopy.
 Scanning electron microscopy(SEM).
 Main Part of SEM
* Electron Gun
* Electron Lenses
* Detector
 Summary

3. Introduction
The electron microscopy is a type of microscopy that uses a beam
of electrons to create an image of the specimen ,similar to optical
microscopy except with electrons rather than photons.
It is capable of much higher magnifications and has a greater
resolving power than a light microscope, allowing it to see much
smaller objects in finer detail.
Resolving Power
Resolution power ᶑ Numerical Aperture(NA)
Resolution power ᶑ
1

Resolving power =
𝟐𝑵𝑨
𝝀
Shorter wavelength and higher NA will provide a better
resolution
Shorter Wavelength ?

4. v≪ 𝑐 λ 𝑒 =
ℎ
2𝑚 𝑒 𝑒𝑉
v c e = h/(2meeV+ e2V2/c2)1/2
 Since electrons exhibit wave and particle behavior, the de Broglie relationship
applies:
ℎ = λ 𝑒p
 Since the electron is charged, when introduced to an electric potential difference, it
accelerates to its equilibrium momentum:
e𝑉 =
1
2
𝑝2
𝑚
→ 𝑝 = 2𝑚 𝑒 𝑒𝑉
 So particle momentum is only dependent on the electric potential difference
 We can relate wavelength to accelerating voltage:
Electron Properties
Wavelength used in microscopy
Electron microscopy
Wavelength depend on potential
difference 𝑉 λ 𝑒
10KV--->0.012nm
50KV--->.00055nm
Optical Microscopy
Wavelength used
400-700nm(Vis)Resolution power ᶑ
𝟏

Electron microscopy has better resolution power

5. Electron Microscopy
Scanning Electron Microscope(SEM) Transmission Electron Microscope(TEM)
 Electron beam scan over the
surface.
 Comparatively low resolution
and magnification.
 In SEM, the sample is placed
on the bottom of the
chamber of the instrument.
 Preparation technique- Easy
 Electron beam pass thorough the
sample.
 Higher resolution comparatively
SEM.
 TEM requires the sample to be
placed in the middle of the
chamber of the microscope.
 Skilled, Very thin sample required

6. Scanning electron microscopy
 In scanning electron microscopy (SEM) an
electron beam is focused into a small probe
scan the sample in a raster scan pattern.
 Several interactions with the sample that
result in the emission of electrons or
photons occur as the electrons penetrate the
surface.
 These emitted particles can be collected
with the appropriate detector to yield
valuable information about the material
Main Part of SEM
 Electron Gun
 Electron Lenses
 Specimen
 Detector

7. How an Electron Beam is Produced?
• Electron guns are used to produce a fine, controlled beam of
electrons which are then focused at the specimen surface.
• The electron guns may either be
(1) Thermionic gun or
(2) field-emission gun.
Cathode current density (emission current density)
For Thermionic emission
Richardson Law:
Jc = AcT2exp(-Ew/kT) in A/cm2
For Tungsten: T = 2700K Work function Ew = 4.5ev
Jc = 3.4 A/cm2
Improve current density?
Use cathode material of lower Ew
For LaB6 single crystal T=1700K
Work function Ew=2.7ev
Jc = 25 A/cm2
Brightness
[Β = 4ip / (π dp αp)2]
W filament
LaB6 Single
Crystal αp Probe convergence
angle.
dp Probe diameter or
spot size.
ip Probe current
The resolution is determined by beam
diameter.

8. Electron Generation
Thermionic Electron Gun
 Heated filament produces electrons
 Typically made of Tungsten or
Lanthanum hexaboride
 Electrons drawn towards an anode
 An aperture in the anode creates a
beam
 Electrons are accelerated by an
acceleration voltage of 1-50kV
Field Emission Gun
o A very strong electric field is used to
extract electrons from a metal filament.
o Filament typically a single tungsten
crystal.
o Requires a high vacuum.
o Similar anode setup.
o The tip of a tungsten needle is made
very sharp (radius < 0.1 mm).
o Electron probe diameter < 1 nm is
possible

9. Electron gun properties
from Reimer

10. Role of Lenses
 Condenser Lens The diameter of the first
cross-over point ~20-50µm. If we want to focus
the beam to a size < 10 nm on the specimen
surface, which is not easily attained with one
lens (say, the objective lens) only. Therefore,
condenser lenses are added to demagnify the
cross-over points.
 Objective Lens: The objective lens controls the
final focus of the electron beam by changing the
magnetic field strength .The cross-over image is
finally demagnified to a ~ 10nm beam spot
which carries a beam current of approximately
10-9 -10-13 A.
 When the beam strikes the sample interactions
occur inside the sample and are detected with
various instruments.
electrons focused by Lorentz force from electromagnetic field
F = qv x B
effectively same as optical lenses
The purpose of the electron lenses is to produce a convergent electron beam with
desired crossover diameter.

11. Electron Matter Interaction
Secondary Electrons (SE)
Produced by inelastic interactions of high energy
electrons with valence (or conduction) electrons
of atoms in the specimen, causing the ejection of
the electrons from the atoms. These ejected
electrons with energy less than 50eV are termed
"secondary electrons".
Backscattered Electrons (BSE)
BSE are produced by elastic
interactions of beam electrons
with nuclei of atoms in the
specimen and they have high
energy and large escape depth.

12. Detectors: secondary electrons
The detector for secondary
electrons is the Everhart Thornley
Detector (ETD).
This consists of a scintillator that
emits photons when hit by high-
energy electrons. The emitted
photons are collected by a light
guide and transported to a
photomultiplier for detection.
A metal grid known as a Faraday
cage surrounds the scintillator, and
is usually held at a positive
potential to attract the secondary
electrons.

13. Summary
• SEM is an analytical technique that can provide a
“quick look” of a material.
• Resolution between less than 1 nm and 20 nm can be
achieved.
• Magnification from about 10 to 500,000 times.
• Versatile platform that supports many other tools.
• Useful for the determination of the composition.

14. Research Publication
• “Tailoring optical properties of TiO2-Cr co-sputtered films
using swift heavy ions” Ratnesh Gupta, Sagar Sen,
D.M.Phase, D.K. Avasthi, Ajay Gupta, Appl. Surf. Sci. 2017.
• “Evolution of magnetic anisotropy by O ion implantation in
Fe/Co/Fe trilaye” Sagar Sen , B.K. Panigrahi , R.J. Choudhary
,Ajay Gupta Communicated Metall. Trans. A, 2017. E-
SYMP-17-1875-A.
• “Influence of preparation methods on the optical and
electronic properties of Cr-doped TiO2 thin films“Sagar Sen,
Ajay Gupta, D. M. Phase, and Ratnesh Gupta, AIP
Conference Proceedings 1832, 080074 (2017).
• “Influence of substrate temperature on the electronic and
optical properties of Cr doped TiO2” Sagar Sen, M. Gupta,
and Ratnesh Gupta AIP Conference Proceedings 1731,
080086 (2016).

15. Achievements and awards
• Received Madhya Pradesh Young Scientist award organized by
Madhya Pradesh Council Science & Technology(MPCST) Bhopal
2017.
• Received Rajiv Gandhi national fellowship (RGNF-OBC) 2015.
• Received best oral presentation in Research Scholar
workshop(RSW) at UGC IUC DAE CSR ,Indore 2014.
• Oral presentation in an International Conference on Nano
Structuring by Ion Beam (ICNIB-2017) at Devi Ahilya University
Indore 2017.
• Oral presentation in an Madam Curie international school on
Domain wall and spintronics at Spetses, Greece 2016.
• Performed experiments at India’s leading research centre IUAC
New Delhi, RRCAT Indore and UGC DAE CSR Indore.

16. Thank You