Scanning Electron Microscopy - IITKhome.iitk.ac.in/~anandh/MME441/SEM.pdf · Scanning Electron Microscopy ... 4. A viewing system that builds an image from the detector signal. 3

Scanning Electron MicroscopyScanning Electron Microscopy

Instrument

Imaging

Chemical Analysis (EDX)

Structural and Chemical Analysis of Materials

J.P. EberhartJohn 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.

32

14

Reduced image of crossover

Project the crossover image onto the specimen

X-Y

translation + rotation

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.

SEM

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), X-

Ray, 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

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)

Note that the resolution depends on the type of

signal being used

Importance of SEM

Absorbed Electrons Electron-Hole Pairs

Direct Beam

SPECIMEN

Elastically Scattered Electrons

InelasticallyScattered Electrons

Bremsstrahlung

X-

raysAuger Electrons

Visible Light

BackscatteredElectrons (BSE)

Secondary Electrons (SE)

Characteristic

X-rays

Incident High-kV Beam

In a SEM

these signals are absent

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.

Signals

Interaction volume volume which the electrons interact with

Sampling volume volume from which a particular signal (e.g. X-rays) originates

•

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)

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.

* Monte Carlo simulations are used to find the trajectory of electrons in the specimen and determine the probability of various processes

Not to scale

1

2

e−3-10 keV

Photoelectrons

X-ray fluorescence and Auger electrons

Electron from beam knocks out a core electron

Transition from higher energy level to fill core level

3

4

Generation of x-rays accompanying the transition

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

SemiconductorsConduction band

Valence band

Band gap

e

e

hole

Electron beam induced current ( EBIC)

Charge collection microscopyOR

Electron-hole pairs and cathodoluminescence

h

Bias

Cathodoluminescence

(CL) Spectroscopy

Photoluminescence Photon induced light emission

Cathodoluminescence

Electron induced light emission

Incident electron excites an electron from the valence band to the conduction band → creating an electron hole pair

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 (SE)

http://www.emal.engin.umich.edu/courses/semlectures/se1.html

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.

Some Z contrast!

Secondary Electrons

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

.

http://www.emal.engin.umich.edu/courses/semlectures/se1.html

Back Scattered Electrons (BSE)

Dependence on atomic number

BSE images show atomic number contrast (features of high average Z appear brighter than those of low average Z)

BSE

IE

nn

Secondary Electrons

Backscattered Electrons

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

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

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.

What determines the resolution in an SEM?

In terms of parameters:

Accelerating voltage

Beam current

Beam diameter

Convergence angle of beam

Inclination Effect

Shadowing Contrast

Edge/Spike Contrast

Topographic Contrast in SEM

Line of sight with the detector

Accelerating Voltage

Probe Current

Working Distance

Specimen Tilt

Aperture Size

Operating parameters affecting signal quality

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)

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

μ

m 50 10 <0.005 <0.005 0.015-0.030

Brightness A/cm2

sr 105 5 ×

106 108 108 108

Energy Speed eV 3 1.5 0.3 1 0.3-1.0

Stability %/hr <1 <1 5 5 ~1

Vacuum PA 10-2 10-4 10-8 10-8 10-8

Lifetime hr 100 500 >1000 >1000 >1000

Comparison of Electron Sources at 20kV

Probe size

Probe Current

Working Distance

Specimen Tilt

Aperture Size

Increasing Resolution

Edge effect

Contamination

Charging

Working Distance

strength of condenser lens

Leads to Beam convergence angle spherical aberration

Any signal picked up by a detector can be converted to an electrical signal and be used of imaging

Contrast processing +ve

to –ve

contrast, gamma control etc.

Contrast quantification contour mapping, colour

mapping

Image integration signal integration over a number of scans ( SNR)

Usual image analysis phase fractions etc.

Image Processing

Backscattered Electron Images

Emission of Backscattered electrons = f(composition, surface topography, crystallinity, magnetism of the specimen)

Composition Z number

Topography and composition information is separated using detector

Crystallinity

channeling contrast (& EBSD)

(the BSE intensity changes drastically on or around Bragg’s condition)

Poorer spatial resolution

Backscattered Electron Signals

Signal A

Signal B

A + B

A −

B

Topography (TOPO)Composition (COMPO)

Detectors

20 kV, 1100 Specimen: Metallic

TOPO: A −

BCOMPO: A + B

Ref: SEM Manual, JEOL

Backscattered Electron Image (BEI)

20 kV, 1100 Specimen: Metallic

Composition via:

BEI

EDX

Secondary Electron Image (SEI)

X-ray image (Si) X-ray image (Al)


www.nanoed.org/courses/zheng_electron/Dravid_part2.pdf

AcceleratingAccelerating

VoltageVoltage

High Resolution

Unclear surface structures

More edge effect

More charge-up

More damage

Clear surface structures

Less damage

Less charge-up

Less edge effect

Low resolution

Low atomic number High atomic number

Low accelerating

voltage

High accelerating

voltage

30 kV

5 kV

2500

Specimen: Toner

Accelerating voltage Increased contribution of BSE

Low surface contrast

Charging


25 kV5 kV

7200

Specimen: Sintered powder


Better surface contrast

Not sharp at high magnifications WD or probe diameter


25 kV

5 kV

36000

Specimen: Evaporated Au particles


Better image sharpness

Improved resolution


25 kV

5 kV

2500

Specimen: Paint coat


Low surface contrast

More BSE contributions

from within the specimen


Specimen tilt

0

5kV, 1100

Specimen: IC chipTILT

Improve quality of SE images

complete survey of topography

Stereo images images at 2 angles

45


Probe currentProbe current

Smooth image

Deteriorated resolution

More damage

High-resolution obtainable

Grainy image

1 nA

10 kV, 5400

Specimen: Ceramic

Probe current

image sharpness

surface smoothness10 pA

0.1 nA


Working DistanceWorking Distance

Greater depth of field

Low depth of field

Low resolution

High resolution

The working distance is the distance between the final condenser

lens and the specimen

working distance

spherical aberration

(spot size

resolution improves) working distance

Depth of field

(wide cone of electrons)

Aperture sizeAperture size

(objective lens)(objective lens)

Large current

Grainy image

Low resolution

Smaller depth of field

High resolution

Greater depth of field

e.g. Better for EDX

Edge Effect

25 kV5 kV

Tilt: 50, 720


Greater the edge effect (edges become brighter)

SE emission from protrusions and circumferences appear bright

Ref: SEM Manual, JEOLSpecimen: IC chip

Charging

Due to low conductivity of sample

Coating with a conducting material to avoid charging

To

charging Voltage,

probe current, tilt specimen

10 kV4 kV


ChargingRef: SEM Manual, JEOL

Specimen: Foreleg of vinegar fly

Contamination

Due to residual gas in the vicinity of the electron probe

Leads to reduced contrast and loss in image sharpness

Usually caused by scanning a small region for long time

Specimen: ITO

5 kV 18000

Contamination


Backscattered Electron Diffraction

Diffraction of Backscattered electrons:

1) Channeling contrast, 2) Diffraction patterns (EBSD)

Weaker than atomic number contrast required good BSE detector

The BSE intensity changes drastically on or around Bragg’s condition

Poorer spatial resolution

EBSD

A stationary electron beam strikes a tilted crystalline sample and the diffracted electrons form a pattern on a fluorescent screen.

Pattern is characteristic of the crystal structure and orientation of the sample region from which it was generated.

Used to measure the crystal orientation, measure grain boundary misorientations, discriminate between different materials, and provide information about local crystalline perfection.

http://www.ebsd.com/basicsofebsd2.htm

A diffraction pattern from nickel collected at 20 kV accelerating voltage


The nickel crystal unit cell superimposed on the diffraction pattern in the orientation which generates this pattern. The crystal planes are labelled

which correspond to the (2-20) and (020) Kikuchi bands in the diffraction pattern.

Indexing: Kikuchi bands are labelled

with the Miller indices of the crystal planes that generated them (red). The planes project onto the screen at the centre of the bands. Kikuchi band intersections are labelled

with crystal direction that meets the screen at this point (white). This direction is the zone axis of the planes corresponding to the intersecting Kikuchi bands.

The Kikuchi band width depends on the d-spacing of the corresponding plane. The (200) plane d-spacing is wider than the (2-20) plane so the Kikuchi bands from (200) planes are narrower than those from (2-20) planes.


The symmetry of the crystal is shown in the diffraction pattern. For example, four fold symmetry is shown around the [001] direction by four symmetrically equivalent <013> zone axes.


Changes in the crystal orientation result in movement of the diffraction pattern. The simulated diffraction pattern is from a sample tilted 70°

to the horizontal and the crystal orientation is viewed along the direction perpendicular to the sample

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Scanning Electron Microscopy - IITKhome.iitk.ac.in/~anandh/MME441/SEM.pdf · Scanning Electron Microscopy ... 4. A viewing system that builds an image from the detector signal. 3