Scanning Electron Microscopy

Scanning Electron Microscopy  Instrument  Imaging  Chemical Analysis (EDX)  Structural and Chemical Analysis of Materials J.P. Eberhart John Wile...
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Scanning Electron Microscopy  Instrument  Imaging  Chemical Analysis (EDX)

 Structural and Chemical Analysis of Materials J.P. Eberhart John Wiley & Sons, Chichester, England, 1991.

 Scanning Electron Microscopy and X-Ray Microanalysis J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael

Kluwer Academic/Plenum Publishers, New York, 2003.

1. A column which generates a beam of electrons. 2. A specimen chamber where the electron beam interacts with the sample. 3. Detectors to monitor the different signals that result from the electron beam/sample interaction. 4. A viewing system that builds an image from the detector signal.

1 3

4 2

Reduced image of crossover

Project the crossover image onto the specimen

X-Y translation + rotation

SEM  Image not formed by focusing of lenses  X-ray maps can be displayed.  Resolution not limited by lens aberrations (in the usual sense of image forming lenses  is limited by the objective lens aberrations which determines the minimum probe size).

 Imaging involves digital processing  online image enhancement and offline image processing.  Resolution limited by probe size and beam spreading on interaction with specimen.  Hence, resolution depends on the signal being used for the formation of the image.

 A fine electron probe is scanned over the specimen.  Various detectors (Secondary Electron (SE), Back Scattered Electron (BSE), XRay, Auger Electron (AE) etc.) pick up the signals.  The amplified output of a detector controls the intensity  of the electron beam of a CRT (synchronized scanning)  of the pixel of display

Scanning Electron Beam

Various Detectors (SE, BSE, EDX, AE)

Display on CRT Note that the resolution depends on the type of signal being used

Parameter

Values

Resolution

~ 40 Å (SE); ~ (100-500) Å (BSE)

Magnification

10 – 105 

Depth of field

High (~ m)

Size of specimen

1 – 5 cm (usual range)

Importance of SEM

Many signals are generated by the interaction of the electron beam with the specimen. Each of these signals is sensitive to a different aspect of the specimen and give a variety of information about the specimen.

Backscattered Electrons (BSE)

Incident High-kV Beam

Secondary Electrons (SE) Characteristic X-rays

Auger Electrons Bremsstrahlung X- rays Visible Light Absorbed Electrons

SPECIMEN

Electron-Hole Pairs

In a SEM these signals are absent Elastically Scattered Electrons

Direct Beam

Inelastically Scattered Electrons

Signals An important point to note is the fact that the different signals are generated ‘essentially*’ from different regions in the specimen. This determines:  as to what the signal is sensitive to  the intensity of the signal.

Not to scale

• The X-rays generated by the electrons are the “Primary X-rays” • The primary X-rays can further lead to electronic transitions which give rise to the “Secondary X-rays” (Fluorescent X-rays)

 Interaction volume  volume which the electrons interact with  Sampling volume  volume from which a particular signal (e.g. X-rays) originates * Monte Carlo simulations are used to find the trajectory of electrons in the specimen and determine the probability of various processes

X-ray fluorescence and Auger electrons 1 3-10 keV e−

Electron from beam knocks out a core electron

Photoelectrons 2

Transition from higher energy level to fill core level

3 Generation of x-rays accompanying the transition

4

Further the x-ray could knock out an electron from an outer level → this electron is called the Auger electron

Photoluminescence  Photon induced light emission

Electron-hole pairs and cathodoluminescence e Incident electron excites an electron from the valence band to the conduction band → creating an electron hole pair

Cathodoluminescence  Electron induced light emission

e

Conduction band

Semiconductors

Band gap hole

Valence band

Cathodoluminescence (CL) Spectroscopy h

OR

Bias

Electron beam induced current ( EBIC)  Charge collection microscopy

Secondary Electrons (SE)  Produced by inelastic interactions of high energy electrons with valence electrons of atoms in the specimen which cause the ejection of the electrons from the atoms.  After undergoing additional scattering events while traveling through the specimen, some of these ejected electrons emerge from the surface of the specimen.  Arbitrarily, such emergent electrons with energies less than 50 eV are called secondary electrons; 90% of secondary electrons have energies less than 10 eV; most, from 2 to 5 eV.  Being low in energy they can be bent by the bias from the detector and hence even those secondary electrons which are not in the ‘line of sight’ of the detector can be captured.

Secondary Electrons

Some Z contrast! SE are generated by 3 different mechanisms:  SE(I) are produced by interactions of electrons from the incident beam with specimen atoms  SE(II) are produced by interactions of high energy BSE with specimen atoms  SE(III) are produced by high energy BSE which strike pole pieces and other solid objects near the specimen. http://www.emal.engin.umich.edu/courses/semlectures/se1.html

Back Scattered Electrons (BSE)    

Produced by elastic interactions of beam electrons with nuclei of atoms in the specimen Energy loss less than 1 eV Scattering angles range up to 180°, but average about 5° Many incident electrons undergo a series of such elastic event that cause them to be scattered back out of the specimen  The fraction of beam electrons backscattered in this way varies strongly with the atomic number Z of the scattering atoms, but does not change much with changes in E0. 

nBSE nIE

 Dependence on atomic number  BSE images show atomic number contrast (features of high average Z appear brighter than those of low average Z) http://www.emal.engin.umich.edu/courses/semlectures/se1.html

Detectors

Note that SE not in traveling in the line of sight can also be captured by the detector

Secondary Electrons

Backscattered Electrons

Magnification  The magnification in an SEM is of ‘Geometrical origin’ (this is unlike a TEM or a optical microscope)  Probe scans a small region of the sample, which is projected to a large area (giving rise to the magnification).

Area scanned on specimen Area projected onto display

Depth of field     

Dependent on the angle of convergence of the beam Depth of field is the same order of magnitude as the scan length Magnification  10,000  Scan length  10 m Depth of Field  8 m

What determines the resolution in an SEM?  Probe size (probe size is dependent on many factors)  Signal being used for imaging This is because the actual interaction volume/cross section is different from the probe diameter. Additionally, each signal is sensitive to a different aspect of the specimen.  In terms of parameters:  Accelerating voltage  Beam current  Beam diameter  Convergence angle of beam

Topographic Contrast in SEM Inclination Effect Shadowing Contrast Edge/Spike Contrast

Line of sight with the detector

Operating parameters affecting signal quality Accelerating Voltage Probe Current Working Distance Specimen Tilt Aperture Size Edge effect Contamination Charging

Operating Parameter

Values

Gun voltage

~20 keV

Working distance

~26 mm

Probe size

W filament

~30 Å

LaB6 Field Emission Vacuum

W filament

10−5 Torr

LaB6

10−8 Torr

Field Emission

10−10 Torr

 Probe current   Probe diameter   Resolution   This leads to decrease in image intensity  we have to use a brighter source (W filament < LaB6 < Field Emission gun)

Comparison of Electron Sources at 20kV Units

Tungsten

LaB6

FEG (cold)

FEG (thermal)

FEG (Schottky)

Work Function

eV

4.5

2.4

4.5

-

-

Operating Temperature

K

2700

1700

300

-

1750

Current Density

A/m2

5*104

106

1010

-

-

Crossover Size

μm

50

10

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