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Basic Electron Microscopy. Arthur Rowe. The Knowledge Base at a Simple Level. 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

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Basic electron microscopy l.jpg

Basic Electron Microscopy

Arthur Rowe

The Knowledge Base at a Simple Level


Introduction l.jpg
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


Aberrations of electromagnetic lenses l.jpg

aberrations of electromagnetic lenses

the most important ones to consider are:

• spherical aberration

• chromatic aberration

• astigmatism


Spherical aberration l.jpg

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


Coping with spherical aberration l.jpg

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


Chromatic aberration l.jpg

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


Astigmatism l.jpg

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)


Astigmatism arises from l.jpg

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


Astigmatism correction l.jpg

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


The tem column l.jpg
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


Depth of focus depth of field l.jpg

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



Slide13 l.jpg

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



Slide15 l.jpg

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’ l

> causes a ‘haze’ and loss of resolution in image



Slide17 l.jpg

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



Slide19 l.jpg

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 !


Slide20 l.jpg

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


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