DETAILS ON 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 >= (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 @ 200KeV
 2x10-12 m
_ Resolution normally achievable
@ 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
> the ejected electron is a low-energy, secondary electron
- detected & used to from SEM images
> the original high-energy electron is scattered
- known as a ‘back-scattered’ electron (SEM use)
> an outer-shell electron drops into the position formerly
occupied by the ejected electron
> 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
> undergo varying degree ofdeflection (inelastic scattering)
> undergo loss of energy - again varying
> lost energy appears as X-rays of varying wavelength
> this X-ray continuum is identical to that originating from
an X-ray source/generator (medical, XRC etc)
> original electrons scattered in a forward direction will
enter the imaging system, but with ‘wrong’ 
> 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
> either removing or inserting an electron
> results in free radical formation
> this species is extremely chemically active
> reactions with neighbouring atoms induce massive change
in the specimen, especially in the light atoms
> this radiation damage severely limits possibilities of EM
> examination of cells in the live state NOT POSSIBLE
> 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
> these transmitted electrons can be used to form an image
> this is called imaging by subtractive contrast
> can be recorded by either
(a) TV-type camera (CCD) - very expensive
(b) photographic film - direct impact of electrons
Photographic film
> silver halide grains detect virtually every electron
> 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