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5. Scanning electron microscope (SEM) Dr Indranil Bhaumik
Introduction to electron microscopy: Limit of resolution under OM 0.2 micron!! (Abbe criterion).
Way out : Magnification via electron beam ! Electron microscopy takes advantage of the wave nature of rapidly moving electrons. Electrons can be accelerated under electrostatic potential difference λ can be reduced (better resolution) Associated ‘de Broglie wavelength’ ? In non-relativistic regime : velocity of electron v= √(2eV/mo ) [V applied voltage, morest mass of electron] de Broglie wavelength λ = h/p = h/mov = h /√(2mo eV) λ (in nm) ~ 1.23/√ √E (in eV)
(#ignore inconsistency in units)
For 40 keV electron λ ~0.0062 nm Note : For higher energy electron (especially used in TEM) one should introduce relativistic correction as the velocity may approach to a considerable fraction of velocity of light.
Two types of EMs : # Scanning electron microscopy (SEM): used for seeing the surface detail of the sample. # Transmission electron microscopy (TEM): used for imaging as well as diffraction pattern Limit!!! Electron microscopes, so far, are limited to magnifications of around 1,000,000 x, primarily because of spherical and chromatic aberrations of the lenses. 1
Scanning electron microscopy (SEM) The first true SEM: in 1942 by Zworykin et al.
Basic operation of SEM: generation a beam of electrons in a vacuum. collimated by electromagnetic condenser lenses, focused by an objective lens, surface of the sample scanned by EM deflection coils. secondary electrons are released by the sample secondary electrons are detected by a scintillation material that produces flashes of light from the electrons. The light flashes are then detected and amplified by a photomultiplier tube. Finally to CRT for image formation.
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# Electron beam generation (source) : To provide an intense beam of high energy electrons. There are two main types of electron gun. 1. Thermionic electron gun (by heating) Tungsten wire filament (or lanthanum hexaboride, LaB6), is heated by passing an electrical current through it. When energy is more than the ‘work function (W)’ of the element material electrons are ejected out. J = A T2 e-W/kT 2. Field emission gun (FEG) (electrons 'tunneling' past the work function of the metal tip helped by the high electrical field gradients) high voltage ~ 109 V (field ~109 V /cm) applied between a pointed cathode and a plate anode caused a current (electron) to flow out of the element.
SEM Cathode Comparison
Source Size Brightness (A/cm2 /steradian) Vacuum (Torr)
Tungsten LaB6
FEG
100 µ
5µ
<100 Ang
105
106
108
10-5
10-6
10-9
While a tungsten filament cathode can be used with a simple diffusion pump, the higher vacuum requirements can mean more exotic vacuum system pumps and seals.
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Whenelt Cap : A negative electrical potential (~ -500 V) is applied to the Whenelt Cap As the electrons move toward the anode those emitted from the filament's side are repelled by the Whenelt Cap toward the optic axis (horizontal center) Electrons ejects out of gun area through the small (<1 mm) hole in the Whenelt Cap.
The electrons are then accelerated down as positive electrical potential is applied to the anode 10 KV DC (typical value, variable),
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# Electron beam lenses Light optics: refraction (or reflection) of light. Electron optics: electrostatic / magnetic fields to influence the trajectories of beams of electrons. Electrical mutual repulsion between electrons in the beam will cause the beam to diverge.
Magnetic lens (via Lorentz force): Current flowing in coil of copper wires inside the iron pole pieces Field is inhomogeneous: weak in the center of the gap /stronger close to the bore. Electrons close to the center are less strongly deflected than those passing the lens far from the axis. The overall effect is that a beam of parallel electrons is focused into a spot (called cross-over). Electrostatic vs magnetic lens: Electrostatic lenses:
require conducting surfaces very close to the
path of an electron beam in order to produce an electrical field of high intensity. These surfaces must be accurately formed, extremely smooth and are easily contaminated. Magnetic lenses:
formed by solenoid coils that are located
completely outside of the vacuum system. Therefore, they suffer none of the contamination problems inherent in electrostatic lenses. However electrostatic lenses can be made extremely small, faster response for beam deflections. In practical instruments, electrostatic lenses are used only in the electron gun (Whenelt cap), Magnetic lenses are used through the rest of an instrument. 5
# Imaging with SEM Bulk Specimen Interactions Back scattered electron Primary electron Sample
Secondary electron (SE) Characteristic X-ray Auger electron
*Backscattered Electrons: Head-on collision with atoms. Electrons scattered "backward" at 180o. KE > 50 eV *Secondary Electrons: Knocks out electron with low energy (5 eV) from the atom of the sample. *X-rays: Since a lower (usually K-shell) electron was emitted from the atom during the secondary electron process an inner (lower energy) shell now has a vacancy. A higher energy electron can "fall" into the lower energy shell, filling the vacancy. As the electron "falls" it emits energy, usually X-rays to balance the total energy of the atom. characteristic energy which is unique to the element from which it originated. (EDAX Energy Dispersive X-ray analysis This x ray can again knock out another electron from outer shell (Auger electron) Also unique characteristics of elements. 6
SE has low energy: hence SE generated in the bulk gets absorbed within the sample. Only those generated in the surface of the sample escape and detected. Only topographic feature. Collection and detection of SE: by a "collector” (Faraday cage) in conjunction with the secondary electron detector (SED). * Faraday cage: a grid or mesh with a +200V potential applied to it which is placed in front of the detector attracts the negatively charged SE pass through the grid-holes and into the detector to be counted.
High energy backscattered electrons do not get deflected by the weak potential of the Faraday cage do not contribute in the image formation ! 7
Detector surface inside faraday cage (+12 kV) accelerates electron Scintillator layer gives off photons when struck by electrons Light travels down the light tube (LG) and hits the photocathode and converted back to electrons Amplified CRT to form image
For imaging the sample is scanned point by point, line by line. Each point on the sample being scanned correspond to a point in the CRT. Full scan of the selected area on the sample generates final image on the CRT screen
Note: For non conducting sample: it has to be quoted with a thin conducting layer (gold; typical few hundred nm) for ground the electron accumulated on the sample surface otherwise repels the following electron beam 8
# Effect of Accelerating Voltage · As accelerating voltage increases, the beam penetrates deeper into specimen, i.e. the zone of interaction gets deeper. · As beam penetrates deeper into the specimen, less secondary electrons can escape. · Secondary electrons can only escape from a limited depth (E) below the surface.
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# Contrast Creation : Factors Affecting SEM Image Specimen Topography In the diagram, as the beam scans from left to right, areas marked B will be bright because they are scanned by the beam and in the line of sight of the detector. Areas marked I are intermediate in brightness because they are out of the line of sight of the detector. Regions D will be dark because they are not scanned with the beam at all. Beam angle relative to surface topography The edges of objects are often brighter than their centers due to interaction of the beam with the specimen surface. More secondary electrons escape from the edges because the zone of interaction is closer to the surface. Surface projections and the edges of depressions in the specimen surface also generate more secondary electrons than flat surfaces, thus giving them a brighter appearance than flat surfaces. This effect is also due to zone of interaction between beam and specimen. Pics from: Bozzola and Russell
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# Resolution: Smaller the beam size better the resolution. Resolve more detailed structures of the sample as compared to a beam with a bigger spot size. Decrease the beam diameter by increasing the current in the condenser lens (i.e., increasing the strength of the CL) or using more CL.
The disadvantage of obtaining a better resolution: shorter focal length will cause the beam to diverge so that many electrons are not able to enter the final lens resulting in a decreasing beam current as well a lower beam current fewer electrons interact with the sample fewer SE. lower brightness. Electronically this signal can be increased to compensate for the low beam current but this will also result in an increase in electronic noise. Because of the increased noise ('snowy' image on the viewing screen) it will be more difficult to focus the specimen and the final image will start to look grainy as well. A correct balance between spot size and beam current needs to be found. 11
Case study (SEM)
Alumina template
PZT nano-tube on alumina template Heating
Sintering (heating) of PMN-PZ ceramic (Grain growth) 12
Few additional points: Most of the times SEM is equipped with an Energy Dispersive X-ray analysis (EDX) system that enables it to perform compositional analysis on specimens identify materials and contaminants, as well as estimating their relative concentrations on the surface of the specimen.
EDX plot of Al65Cu20Fe15 alloy.
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