Basic Electron Microscopy Arthur Rowe The Knowledge Base

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 limits to ultimate resolution of the TEM Interactions of electrons with matter

aberrations of electromagnetic lenses the most important ones to consider are: • spherical aberration • chromatic aberration • astigmatism

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 • 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 • 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 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: • 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: • 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 _ _ _ Gun emits electrons Electric field accelerate Magnetic (and electric) field control path of electrons Electron wavelength @ 200 Ke. V 2 x 10 -12 m Resolution normally achievable @ 200 Ke. V 2 x 10 -10 m 2Å

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. .

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 matter. .

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

when electrons hit matter. .

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 matter. .

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 50 x more efficient than photon capture !

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