Basic Electron Microscopy

1
Basic Electron Microscopy

• Arthur Rowe

The Knowledge Base at a Simple Level
2
Introduction

• These 3 presentations cover the fundamental
  theory of electron microscopy
• In presentation 2 we cover
• lens aberrations and their importance
• how we correct for lens astigmatism
• limits to ultimate resolution of the TEM
• Interactions of electrons with matter

3
aberrations of electromagnetic lenses
the most important ones to consider are
spherical aberration chromatic aberration
astigmatism
4
spherical aberration
object plane
arises because a simple lens is more powerful
at the edge than at the centre is not a
problem with glass lenses (can be ground to
shape) disc of minimum confusion results
instead of point focus is not correctable for
electromagnetic lenses
5
coping with spherical aberration
disc of minimum confusion has diameter given
by d C ?? C constant
hence reducing ? gives a large reduction in d .
. . but for optimal resolution we need large ?
! best compromise is with ? 10-3 radians (
f/500) gives resolution 0.1 nm - can not be
bettered
6
chromatic aberration
light of different???brought to different focal
positions ??for electrons can be controlled by
fixed KV and lens currents but ? of electrons
can change by interaction with specimen ! rule
of thumb resolution gt (specimen thickness)/10
7
astigmatism
minimal confusion
arises when the lens is more powerful in one
plane than in the plane normal to it causes
points to be imaged as short lines, which flip
through 90 degrees on passing through focus
(minimal confusion)
8
astigmatism - arises from

• inherent geometrical defects in circular bore
  of lens
• inherent inhomogeneities in magnetic properties
  of pole piece
• build-up of contamination on bore of pole-piece
  and on apertures gives rise to non-conducting
  deposits which become charged as electron strike
  them
• hence astigmatism is time-dependent
• and cannot be designed out
• inevitably requires continuous correction

9
astigmatism - correction

• with glass optics (as in spectacles)
  astigmatism is corrected
• using an additional lens of strength asymmetry
• opposed to the asymmetry of the basic (eye) lens
• with electron optics, same principle employed
• electrostatic stigmator lens apposed to main
  lens
• strength direction of its asymmetry
  user-variable
• only the OBJECTIVE lens needs accurate
  correction
• correction usually good for 1-2 hours for
  routine work

10
The TEM Column

• Gun emits electrons
• Electric field accelerate
• Magnetic (and electric) field control path of
  electrons
• Electron wavelength _at_ 200KeV ? 2x10-12 m
• Resolution normally achievable _at_ 200KeV ? 2 x
  10-10 m ? 2Å

11
depth of focus - depth of field

• depth of useful focus (in the specimen) is
  primarily limited by chromatic aberration effects
  
• the absolute depth of focus is larger than this
  for all practical purposes, everything is in
  focus to same level
• . . . So one cannot rack through focus (as in a
  light or even scanning electron) microscope
• depth of field (in the image plane) is - for all
  practical purposes infinite

12
when electrons hit matter ..
13
when electrons hit matter ..
(1) they may collide with an inner shell
electron, ejecting same gt the ejected electron
is a low-energy, secondary electron - detected
used to from SEM images gt the original
high-energy electron is scattered - known as a
back-scattered electron (SEM use) gt an
outer-shell electron drops into the position
formerly occupied by the ejected electron gt
this is a quantum process, so a X-ray photon of
precise wavelength is emitted - basis for X-ray
microanalysis
14
when electrons hit matter ..
15
when electrons hit matter ..
(2) they may collide or nearly collide with an
atomic nucleus gt undergo varying degree
ofdeflection (inelastic scattering) gt undergo
loss of energy - again varying gt lost energy
appears as X-rays of varying wavelength gt this
X-ray continuum is identical to that originating
from an X-ray source/generator (medical, XRC
etc) gt original electrons scattered in a forward
direction will enter the imaging system, but
with wrong l gt causes a haze and loss of
resolution in image
16
when electrons hit matter ..
17
when electrons hit matter ..
(3) they may collide with outer shell
electrons gt either removing or inserting an
electron gt results in free radical formation gt
this species is extremely chemically active gt
reactions with neighbouring atoms induce massive
change in the specimen, especially in the light
atoms gt this radiation damage severely limits
possibilities of EM gt examination of cells in
the live state NOT POSSIBLE gt all examinations
need to be as brief (low dose) as possible
18
when electrons hit matter ..
19
when electrons hit matter ..
(4) they may pass through unchanged gt these
transmitted electrons can be used to form an
image gt this is called imaging by subtractive
contrast gt can be recorded by either (a)
TV-type camera (CCD) - very expensive (b)
photographic film - direct impact of
electrons Photographic film gt silver halide
grains detect virtually every electron gt at
least 50x more efficient than photon capture !
20
when electrons hit matter ..

• beam damage occurs
• light elements (H, O) lost very rapidly
• change in valency shell means free radicals
  formed
• . . . consequent chemical reactions causing
  further damage
• beam damage is minimised by use of
• low temperatures (-160)
• high beam voltages
• minimal exposure times