Electron and Probe Microscopy Part 1 SEM and
Electron and Probe Microscopy Part 1: SEM and TEM Designer: S. H. Kazemi IASBS University, Spring 2017
Resolution Here are some of the techniques we will examine and a comparison of their lateral resolution capabilities. AFM 1Å STM 2 1 nm SAM OM 1 µm 1 mm 1 cm SEM 2/23/2021
Optical Microscopy • a diffraction experiment • basic lens components • coarse/fine focus • Mon/Bin/Tri ocular schemes • working distance • adjust interpupillary distance • quantitation with reticle • image recording 3 2/23/2021
imaging with a simple lens 4 2/23/2021
The meaning of focal length 5 2/23/2021
Aberrations of electromagnetic lenses The most important ones to consider are: • Spherical aberration • Chromatic aberration • Astigmatism 6 2/23/2021
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 7 2/23/2021
Coping with spherical aberration • Disc of minimum confusion has diameter given by: d = C α 3 C = constant} • Hence reducing α gives a large reduction in d • But for optimal resolution we need large α ! • Best compromise is with α = 10 -3 radians (= f/500) • Gives resolution = 0. 1 nm - can not be bettered 8 2/23/2021
Chromatic aberration • Light of different λ brought to different focal positions. • λ for electrons can be controlled by fixed KV and lens currents. • But λ of electrons can change by interaction with specimen ! • Rule of thumb: resolution >= (specimen thickness)/10 9 2/23/2021
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) 10 2/23/2021
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 11 2/23/2021
Astigmatism - correction • With glass optics (as in spectacles) astigmatism is corrected using an additional lens of strength and asymmetry opposed to the asymmetry of the basic (eye) lens • With electron optics, same principle employed: 1 - Electrostatic Stigmator lens apposed to main lens 2 - Strength & direction of its asymmetry user-variable • Only the OBJECTIVE lens needs accurate correction • Correction usually good for 1 -2 hours for routine work 12 2/23/2021
When we image a simple point 13 2/23/2021
Optical Microscopy Resolution • Rayleigh equation d = 0. 61 ( / N. A. ) d is distance between objects that can still be distinguished, is wavelength of light, N. A. is numerical aperture of lens = n sin(Qvertex) Q 14 2/23/2021
Definition of resolution 15 2/23/2021
SEM - scanning electron microscopy 16 2/23/2021
Scanning Electron Microscopy Electron gun Electron emitter 17 2/23/2021
The electron gun 18 2/23/2021
The electron gun A Wehnelt acts as a control grid and it also serves as a convergent electrostatic lens. The anode is biased to a high positive voltage (typically +1 to +30 k. V) relative to the emitter so as to accelerate electrons from the emitter towards the anode, thus creating an electron beam that passes through the Wehnelt aperture. The Wehnelt is biased to a negative voltage (typically − 200 V to − 300 V) relative to the emitter, which is usually a tungsten filament or Lanthanum hexaboride (La. B 6) hot cathode. The Wehnelt bias voltage creates a repulsive electrostatic field that condenses the cloud of primary electrons produced by the filament. 19 2/23/2021
wavelength & voltage l )is determined by the accelerating Wavelength of electrons voltage (V) on the filament from which they are emitted λ= 0. 1*(150/V)0. 5 (de Broglie, 1924) Therefore very high voltages (up to 100 k. V) are used to produce small values of And (λ <0. 005 nm) 20 2/23/2021
Why high vacuum ? • Mean free path of electrons v short in air - at least 10 -5 mbar usually aimed for • Also - Tungsten filaments burn out in air - Columns must be kept dust free • Achieved by 2 -fold pumping: Rotary (mechanical) pump + diffusion pump or using turbo pump 21 2/23/2021
How does a diffusion pump work? microscope column throat with valve to isolate cooling coils to mechanical pump 22 chimney OIL electrical heater 2 annular vents with jets of oil vapour emitted at high velocity 2/23/2021
The electromagnetic lens • Works at fixed focal distance & variable focal length - Like the human eye lens, but unlike light optics • Simple rheostat can vary power of lens (varies current) • Electrons also spiral thro’ lens(effect easily observed) 23 2/23/2021
Scanning Electron Microscopy Electron Gun Secondary Electron Detector Vacuum Chamber 24 2/23/2021
Electron microscopy v SEM - scanning electron microscopy v Tiny electron beam scanned across surface of specimen v Backscattered BSE) or secondary electrons (SE) detected v signal output to synchronized display 25 2/23/2021
Scanning Electron Microscopy 26 2/23/2021
Scanning Electron Microscopy 27 2/23/2021
Scanning Electron Microscopy Electron gun Don't make x-rays - use electrons directly Wavelength: NOT = hc/E (massless photons) = h/(2 melectronq. Vo) (non-relativistic) = h/(2 melectronq. Vo + q 2 Vo 2/c 2)1/2 relativistic = h / (2 melectronq. Vo + q 2 Vo 2/c 2)1/2 = 1. 22639 / (Vo + 0. 97845 · 10 -6 Vo 2)1/2 (nm) & Vo(volts): 10 k. V ——> 0. 12 Å & 100 k. V —> 0. 037 Å 28 2/23/2021
Scanning Electron Microscopy = h/(2 melectronq. Vo + q 2 Vo 2/c 2)) ØEffects of increasing voltage in electron gun: ØResolution increased ( decreased) ØPenetration increases ØSpecimen charging increases (insulators) ØSpecimen damage increases ØImage contrast decreases 29 2/23/2021
SEM Lens Electrons focused by Lorentz force from electromagnetic field F = qv x B effectively same as optical lenses Lenses are ring-shaped Coils generate magnetic field, electrons pass thru hollow center. Lens focal length is continuously variable & apertures control limit beam 30 2/23/2021
SEM- sample preparation Conducting : Little or no preparation attach to mounting stub for insertion into instrument may need to provide conductive path with Ag paint Non-conducting : Usually coat with conductive very thin layer (Au, C, Cr) 31 2/23/2021
Scanning Electron Microscopy Interaction volume Backscattered electrons come from whole volume (high energy) Secondary electrons come from neck only (low energy) 32 2/23/2021
Scanning Electron Microscopy Interaction volume 33 2/23/2021
Scanning Electron Microscopy 34 2/23/2021
Electron Re-emission e– Elastically scattered Backscattered Inelastically scattered SEM Relative Intensity Secondary electron emission 35 Fraction of Incident Beam Energy 2/23/2021
BSE vs. 2° Detection Both can be used, different information, different detection scheme. 36 BSE 2° Electrons Specular reflection Isotropic emission Higher energy Very low energy Encode some chemical information Better structural contrast 2/23/2021
Scanning Electron Microscopy Specimen-electron interactions What comes from specimen? Backscattered electrons Secondary electrons Fluorescent X-rays composition - EDS 37 high energy compositional contrast Brightness of regions in image increases as atomic number increases (less penetration gives more backscattered electrons) low energy topographic contrast 2/23/2021
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Schematic of a backscattered electron detector (BSD) for scanning electron microscopy (SEM) a light pipe to carry the photon signal from the scintillator inside the evacuated specimen chamber of the SEM to the photomultiplier outside the chamber The detector consists primarily of a scintillator inside a Faraday cage inside the specimen chamber of the microscope. A low positive voltage is applied to the Faraday cage to attract the relatively low energy (less than 50 e. V by definition) secondary electrons. 39 Example: Everhart-Thornley Detector (E-T detector or ET detector) 2/23/2021
Scanning Electron Microscopy Backscattered electron detector - solid state detector Electron energy up to 30 -50 ke. V Annular around incident beam Repel secondary electrons with negative biased mesh Images are more sensitive to chemical composition (electron yield depends on atomic number) Line of sight necessary 40 2/23/2021
Scanning Electron Microscopy Secondary electron detector - scintillation detector • Positive bias mesh needed in front of detector to attract low energy electrons • Line of sight unnecessary 41 2/23/2021
Scanning Electron Microscopy Choose correct detector- topography example Fracture surface in iron Backscattered electrons Secondary electrons 42 2/23/2021
Scanning Electron Microscopy Composition - what elements present at a particular spot in specimen? Use solid state detector, and do energy scan for fluorescent X-rays 43 2/23/2021
Scanning Electron Microscopy Composition mapping - x-ray fluorescence Use solid state detector set for X-ray energy for a particular element in specimen 44 Image X-ray map 2/23/2021
Scanning Electron Microscopy Contrast Comes from any kind of interaction with electron beam • Topography • Composition • Elements • Phases • Grain (crystal) orientation • Static Charging affects 45 2/23/2021
An SEM example Trochodiscus longispinus in OM and SEM. Note improved depth of field and resolving capability of the SEM experiment. 46 2/23/2021
Another SEM example Changing the Y content in the Ni electrolyte bath from 1 to 5 g/L. Preferential growth directions are altered as the nucleation rates are changed by the co-depositing material. 47 2/23/2021
Transmission electron Microscope TEM 48 2/23/2021
TEM Typical Accel. volt. = 100 -400 k. V (some instruments - 1 -3 MV) Spread broad probe across specimen - form image from transmitted electrons Diffraction data can be obtained from image area Many image types possible (BF, DF, HR, . . . ) - use aperture to select signal sources Main limitation on resolution: aberrations in main imaging lens Basis for magnification - strength of post- specimen lenses 49 2/23/2021
TEM compared to SEM 50 2/23/2021
TEM v Condenser system : lenses & apertures for controlling illumination on specimen. v Specimen chamber assembly v Objective lens system : image-forming lens - limits resolution; aperture - controls imaging conditions. v Projector lens system : magnifies image or diffraction pattern onto final screen. 51 2/23/2021
Transmission electron Microscope Specimen preparation Types: Replicas Films Slices Powders, fragments Foils As is, if thin enough ultra. Microtomy Crush and/or disperse on carbon film Foils: 3 mm diam. disk very thin (<0. 1 - 1 micron - depends on material, voltage) 52 2/23/2021
TEM Sample Holder: Mesh for normal samples 53 2/23/2021
TEM Cryogenic transmission electron microscopy (Cryo-TEM) uses a TEM with a specimen holder capable of maintaining the specimen at liquid nitrogen or liquid helium temperatures. This allows imaging specimens prepared in vitreous ice, the preferred preparation technique for imaging individual molecules or macromolecular assemblies, imaging of vitrified solid-electrolye interfaces, and imaging of materials that are volatile in high vacuum at room temperature, such as sulfur. 54 2/23/2021
TEM Typical images: carbon nanotube 55 2/23/2021
TEM Typical images: Mesoporous structure 56 2/23/2021
TEM Typical images: Mesoporous structure of nanoparticles 57 2/23/2021
TEM Diffraction Use Bragg's law - = 2 d sin But much smaller: (0. 0251Å at 200 k. V) if d = 2. 5Å, = 0. 288° 58 2/23/2021
TEM: diffraction mode 59 2/23/2021
TEM Diffraction pattern in TEM Cr 23 C 6 - F cubic a = 10. 659 Å 60 Ni 2 Al. Ti - P cubic a = 2. 92 Å 2/23/2021
TEM Polycrystalline regions polycrystalline Ba. Ti. O 3 spotty Debye rings 61 2/23/2021
TEM Sources of contrast Diffraction contrast - some grains diffract more strongly than others; defects may affect diffraction Mass-thickness contrast – absorption/scattering. Thicker areas or mat'ls w/ higher Z are dark 62 2/23/2021
TEM Bright field imaging Only main beam is used. Aperture in back focal plane blocks diffracted beams Image contrast mainly due to subtraction of intensity from the main beam by diffraction 63 2/23/2021
TEM What else is in the image? Many artifacts Surface films Local contamination Differential thinning Others Also get changes in image because of annealing due to heating by beam 64 2/23/2021
TEM Instead of main beam, use a diffracted beam Move aperture to diffracted beam or tilt incident beam 65 2/23/2021
TEM Images can be combined to get the most information out of a sample. Here is a diffraction pattern on the left and a high resolution electron image on the right for the same material: Nd 13 Ca. O 7 66 2/23/2021
TEM Lattice imaging Use many diffracted beams Slightly off-focus Need very thin specimen region Need precise specimen alignment See channels through foil Channels may be light or dark in image Usually do image simulation to determine features of structure 67 2/23/2021
TEM: An example M 23 X 6 (figure at top left). L 21 type b'-Ni 2 Al. Ti (figure at top center). L 12 type twinned g'-Ni 3 Al (figure at bottom center). L 10 type twinned Ni. Al martensite (figure at bottom right). 68 2/23/2021
TEM: diffraction vs. image mode 69 2/23/2021
TEM Synthetic image of three fcc nanoparticles with different orientations on a amorphous carbon substrate 70 2/23/2021
TEM 71 2/23/2021
Thanks for patience 72 2/23/2021
- Slides: 72