Nano materials

1. Scanning Electron Microscopy
 Instrument
 Imaging
 Chemical Analysis (EDX)
 Structural and Chemical Analysis of Materials
J.P. Eberhart
John Wiley & Sons, Chichester, England, 1991.
 Scanning Electron Microscopy and X-Ray Microanalysis
J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael
Kluwer Academic/Plenum Publishers, New York, 2003.

2. 1. A column which generates a beam of
electrons.
2. A specimen chamber where the electron
beam interacts with the sample.
3. Detectors to monitor the different signals
that result from the electron beam/sample
interaction.
4. A viewing system that builds an image
from the detector signal.
3
2
1
4

3. Reduced image
of crossover
Project the
crossover image
onto the
specimen
X-Y translation +
rotation

5.  Image not formed by focusing of lenses  X-ray maps can be displayed.
 Resolution not limited by lens aberrations
(in the usual sense of image forming lenses is limited by the objective lens aberrations
which determines the minimum probe size).
 Imaging involves digital processing
 online image enhancement and offline image processing.
SEM
 Resolution limited by probe size and beam spreading on interaction with
specimen.
 Hence, resolution depends on the signal being used for the formation of the
image.

6.  A fine electron probe is scanned over the specimen.
 Various detectors (Secondary Electron (SE), Back Scattered Electron (BSE), X-
Ray, Auger Electron (AE) etc.) pick up the signals.
 The amplified output of a detector controls the intensity
 of the electron beam of a CRT (synchronized scanning)
of the pixel of display
Scanning Electron Beam
Various Detectors
(SE, BSE, EDX,AE)
Display on CRT
Parameter Values
Resolution ~ 40 Å (SE); ~ (100-500) Å (BSE)
Magnification 10 – 105 
Depth of field High (~ m)
Size of specimen 1 – 5 cm (usual range)
Note that the resolution
depends on the type of
signal being used
Importance of SEM

7. Elastically Inelastically
Scattered Electrons Direct Scattered Electrons
Beam
Auger Electrons
Backscattered
Electrons (BSE)
Secondary
Electrons (SE)
Characteristic
X-rays
Incident
High-kV Beam
Bremsstrahlung X- rays
Visible Light
Absorbed Electrons SPECIMEN Electron-Hole Pairs
In a SEM these signals are absent
Many signals are generated by the interaction of the electron beam with the specimen. Each of
these signals is sensitive to a different aspect of the specimen and give a variety of information
about the specimen.

8. Signals
 Interaction volume  volume which the electrons interact with
 Sampling volume  volume from which a particular signal (e.g. X-rays) originates
• The X-rays generated by the electrons are the “Primary X-rays”
• The primary X-rays can further lead to electronic transitions which give rise to the “Secondary X-rays” (Fluorescent
X-rays)
An important point to note is the fact that the different signals are generated ‘essentially*’from
different regions in the specimen. This determines:  as to what the signal is sensitive to  the
intensity of the signal.
* Monte Carlo simulations are used to find the trajectory of electrons in the specimen and determine the probability of various processes
Not to scale

10. 1
2
3-10 keV
e−
Photoelectrons
X-ray fluorescence and Auger electrons
Electron from beam knocks out a core electron
Transition from higher energy level to
fill core level

11. 3
4
Generation of x-rays accompanying the transition
Further the x-ray could knock out an
electron from an outer level → this
electron is called the Auger electron

12. Semiconductors
Conduction band
Valence band
Band gap
e
e
hole
Electron beam ind
 Charge colle
uced current ( EBIC)
ction microscopy
OR
Electron-hole pairs and cathodoluminescence
h
ias
B
C y
athodoluminescence (CL) Spectroscop
Photoluminescence
Photon induced light emission
Cathodoluminescence
Electron induced light emission
Incident electron excites an electron from the valence band
to the conduction band → creating an electron hole pair

13.  Produced by inelastic interactions of high energy electrons with valence electrons of
atoms in the specimen which cause the ejection of the electrons from the atoms.
 After undergoing additional scattering events while traveling through the specimen, some
of these ejected electrons emerge from the surface of the specimen.
 Arbitrarily, such emergent electrons with energies less than 50 eV are called secondary
electrons; 90% of secondary electrons have energies less than 10 eV; most, from 2 to 5
eV.
 Being low in energy they can be bent by the bias from the detector and hence even those
secondary electrons which are not in the ‘line of sight’ of the detector can be captured.
Secondary Electrons (SE)

14. http://www.emal.engin.umich.edu/courses/semlectures/se1.html
SE are generated by 3 different mechanisms:
 SE(I) are produced by interactions of electrons from the incident beam with specimen
atoms
 SE(II) are produced by interactions of high energy BSE with specimen atoms
 SE(III) are produced by high energy BSE which strike pole pieces and other solid objects
near the specimen.
Some Z contrast!
Secondary Electrons

15.  Produced by elastic interactions of beam electrons with nuclei of atoms in the specimen
 Energy loss less than 1 eV
 Scattering angles range up to 180°, but average about 5°
 Many incident electrons undergo a series of such elastic event that cause them to be
scattered back out of the specimen
 The fraction of beam electrons backscattered in this way varies strongly with the atomic
number Z of the scattering atoms, but does not change much with changes in E0.
http://www.emal.engin.umich.edu/courses/semlectures/se1.html
Back Scattered Electrons (BSE)
 Dependence on atomic number  BSE images show atomic number contrast
(features of high average Z appear brighter than those of low average Z)
 
nBSE
nIE

16. Magnification
 The magnification in an SEM is of ‘Geometrical origin’
(this is unlike a TEM or a optical microscope)
 Probe scans a small region of the sample, which is projected to a large area
(giving rise to the magnification).
Area scanned on
specimen
Area projected
onto display

17.  Probe size (probe size is dependent on many factors)
 Signal being used for imaging
This is because the actual interaction volume/cross section is different from the
probe diameter. Additionally, each signal is sensitive to a different aspect of the
specimen.
What determines the resolution in an SEM?
 In terms of parameters:
 Accelerating voltage
 Beam current
 Beam diameter
 Convergence angle of beam

18. Inclination Effect
Shadowing Contrast
Edge/Spike Contrast
Topographic Contrast in SEM
Line of sight with the detector

19. Accelerating Voltage
Probe Current
Working Distance
Specimen Tilt
Aperture Size
Operating parameters affecting signal quality
Edge effect
Contamination
Charging

20. Operating Parameter Values
Gun voltage ~20 keV
Working distance ~26 mm
Probe size W filament ~30 Å
LaB6
Field Emission
Vacuum W filament 10−5 Torr
LaB6 10−8 Torr
Field Emission 10−10 Torr
 Probe current   Probe diameter   Resolution 
 This leads to decrease in image intensity  we have to use a brighter source
(W filament < LaB6 < Field Emission gun)

21. Units Tungsten LaB6 FEG (cold) FEG
(thermal)
FEG
(Schottky)
Work
Function
eV 4.5 2.4 4.5 - -
Operating
Temperature
K 2700 1700 300 - 1750
Current
Density
A/m2 5*104 106 1010 - -
Crossover
Size
μ m 50 10 <0.005 <0.005 0.015-0.030
Brightness A/cm2 sr 105 5 × 106 108 108 108
Energy Speed eV 3 1.5 0.3 1 0.3-1.0
Stability %/hr <1 <1 5 5 ~1
Vacuum PA 10-2 10-4 10-8 10-8 10-8
Lifetime hr 100 500 >1000 >1000 >1000
Comparison of Electron Sources at 20kV

22.  Probe size
Probe Current
Working Distance
Specimen Tilt
Aperture Size
Increasing Resolution
Edge effect
Contamination
Charging
 Working Distance
 strength of condenser lens
Leads to  Beam convergence angle  sphericalaberration

23. Any signal picked up by a detector can be converted to an electrical signal
and be used of imaging
 Contrast processing  +ve to –ve contrast, gamma control etc.
 Contrast quantification  contour mapping, colour mapping
 Image integration  signal integration over a number of scans ( SNR)
 Usual image analysis  phase fractions etc.
Image Processing

24. Backscattered Electron Images
 Emission of Backscattered electrons
= f(composition, surface topography, crystallinity, magnetism of the specimen)
 Composition  Z number
 Topography and composition information is separated using detector
 Crystallinity  channeling contrast (& EBSD)
(the BSE intensity changes drastically on or around Bragg’s condition)
 Poorer spatial resolution
 Electron Backscattered Diffraction(EBSD)

25. Backscattered Electron Signals
SignalA
Signal B
A + B
A − B
Topography (TOPO)
Composition (COMPO)
Detectors

26. 20 kV, 1100 
Specimen: Metallic
TOPO: A − B
COMPO: A + B
Ref: SEM Manual,JEOL

27. Backscattered Electron Image (BEI)
20 kV, 1100 
Specimen: Metallic
Composition via:  BEI  EDX
Secondary Electron Image (SEI)
X-ray image (Si) X-ray image (Al)
Ref: SEM Manual,JEOL

28. www.nanoed.org/courses/zheng_electron/Dravid_part2.pdf

29. Accelerating
Voltage
 High Resolution  Unclear surface structures
 More edge effect
 More charge-up
 More damage
 Clear surface structures
 Less damage
 Less charge-up
 Less edge effect
 Low resolution

30. Low atomic number High atomic number
Low
accelerating
voltage
High
accelerating
voltage

31. 30 kV
5 kV
2500 
Specimen: Toner
 Accelerating voltage
 Increased contribution of BSE
 Low surface contrast
 Charging
Ref: SEM Manual,JEOL

32. 25 kV
5 kV
7200 
Specimen: Sintered powder
 Accelerating voltage
 Better surface contrast
 Not sharp at high magnifications
 WD or  probe diameter
Ref: SEM Manual,JEOL

33. 25 kV
5 kV
36000 
Specimen: Evaporated Au particles
 Accelerating voltage
 Better image sharpness
 Improved resolution
Ref: SEM Manual,JEOL

34. 25 kV
5 kV
2500 
Specimen: Paint coat
 Accelerating voltage
 Low surface contrast
 More BSE 
contributions from within
the specimen
Ref: SEM Manual,JEOL

35. Specimen tilt
0 
5kV, 1100 
Specimen: IC chip
TILT
 Improve quality of SE images
 complete survey of topography
 Stereo images  images at 2 angles
45 
Ref: SEM Manual,JEOL

36. Probe current
 Smooth image  Deteriorated resolution
 More damage
 High-resolution obtainable  Grainy image

37. 1 nA
10 kV, 5400 
Specimen: Ceramic
 Probe current
  image sharpness
  surface smoothness
10 pA
0.1 nA
Ref: SEM Manual,JEOL

38. Working Distance
 Greater depth of field
 Low depth of field
 Low resolution
 High resolution
The working distance is the distance between the final condenser lens and the specimen
working distance 
 spherical aberration 
(spot size 
resolution improves)
working distance 
 Depth of field 
(wide cone of electrons)

39. Aperture size
(objective lens)
 Large current
 Grainy image
 Low resolution
 Smaller depth of field
 High resolution
 Greater depth of field
e.g. Better for EDX

40. Edge Effect
25 kV
5 kV
Tilt: 50, 720 
 Accelerating voltage
 Greater the edge effect
(edges become brighter)
 SE emission from protrusions and circumferences appear bright
Ref: SEM Manual,JEOL
Specimen: IC chip

41. Charging
 Due to low conductivity of sample
 Coating with a conducting material to avoid charging
 To  charging   Voltage,  probe current, tilt specimen
10 kV
4 kV
 Accelerating voltage
  Charging
Ref: SEM Manual,JEOL
Specimen: Foreleg of vinegar fly

42. Contamination
 Due to residual gas in the vicinity of the electron probe
 Leads to reduced contrast and loss in image sharpness
 Usually caused by scanning a small region for long time
Specimen: ITO
5 kV 18000 
Contamination
Ref: SEM Manual,JEOL