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Basic Electron Microscopy






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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
Basic Electron Microscopy

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Basic electron microscopy l.jpgSlide 1

Basic Electron Microscopy

Arthur Rowe

The Knowledge Base at a Simple Level

Introduction l.jpgSlide 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

Aberrations of electromagnetic lenses l.jpgSlide 3

aberrations of electromagnetic lenses

the most important ones to consider are:

• spherical aberration

• chromatic aberration

• astigmatism

Spherical aberration l.jpgSlide 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

Coping with spherical aberration l.jpgSlide 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

Chromatic aberration l.jpgSlide 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

Astigmatism l.jpgSlide 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)

Astigmatism arises from l.jpgSlide 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

Astigmatism correction l.jpgSlide 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

The tem column l.jpgSlide 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

Depth of focus depth of field l.jpgSlide 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

When electrons hit matter l.jpgSlide 12

when electrons hit matter ..

Slide13 l.jpgSlide 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

When electrons hit matter14 l.jpgSlide 14

when electrons hit matter ..

Slide15 l.jpgSlide 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’ l

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

When electrons hit matter16 l.jpgSlide 16

when electrons hit matter ..

Slide17 l.jpgSlide 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

When electrons hit matter18 l.jpgSlide 18

when electrons hit matter ..

Slide19 l.jpgSlide 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 !

Slide20 l.jpgSlide 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


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