L6-Scanning Electron Microscopy

8/18/2019 L6-Scanning Electron Microscopy

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SCANNING ELECTRONMICROSCOPE

CHARACTERISATIONS OFMATERIALS


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CONTENTS

Electron Microscopy

1.0 Introduction and History

• 1.1 Characteristic Information

2.0 Basic Principles

•2.1 Electron-Solid Interactions

• 2.2 Electromagnetic Lenses

• 2.3 Breakdown of an Electron Microscope

• 2.4 Signal Detection and Display

• 2.5 Operating Parameters

3.0 Instrumentation

• 3.1 Sample Prep

4.0 Artifacts and Examples

5.0 Summary


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INTRODUCTION AND HISTORY

• Electron microscopes are scientific instruments that use a beam

of energetic electrons to examine objects on a very fine scale.

•

Electron microscopes were developed due to the limitations of

Light Microscopes which are limited by the physics of light.

• In the early 1930 s this theoretical limit had been reached and

there was a scientific desire to see the fine details of the interior

structures of organic cells (nucleus, mitochondria...etc.).

• This required 10,000x plus magnification which was not possible

using current optical microscopes


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Comparison of OM,TEM and SEM

Principal features of an optical microscope, a transmission electron

microscope and a scanning electron microscope, drawn to emphasize the

similarities of overall design.


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Scanning Electron Microscopy

• SEM the most widely used type of electron microscope.

• Examine microscopic structure by scanning the surface of

materials.

• Similar to scanning confocal microscope but with much higher

resolution and much greater depth of field.

•Image is formed by a focused electron beam that scan over the

surface area of a specimen.

• The most important features of an SEM is the three dimensional

appearance of its images because of its large depth of field.


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Scanning Electron Microscopy

• SEM systems enables us to obtain chemical information from a

specimen by using various techniques, including equipping the X-

ray energy dispersive spectrometer (EDS).

•

Electron beam emitted from an electron gun is condensed to a

fine probe for surface scanning.

• Electron gun that generate electron beam;

Thermionic gun

Field emission gun (advanced SEM systems)- used FE gun because of its high beam brightness.

• Acceleration voltage for generating an electron beam is in the

range 1-40kV


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Characteristic Information: SEM

Topography

The surface features of an object or how it looks , its texture; direct relation

between these features and materials properties

Morphology

The shape and size of the particles making up the object; direct relation

between these structures and materials properties

Composition

The elements and compounds that the object is composed of and the relative

amounts of them; direct relationship between composition and materials

properties

Crystallographic Information

How the atoms are arranged in the object; direct relation between these

arrangements and material properties


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e.g. Identification of Fracture Mode

SEM micrographs of fractured surface of two BaTiO3 samples.


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OM vs. SEM


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How Fine can You See?

• Can you see a sugar cube? The thickness of a sewing needle?

The thickness of a piece of paper? …

• The resolution of human eyes is of the order of 0.1 mm, 100μm

≈ 4 mils.

• However, something vital to human beings are of sizes smaller

than 0.1mm, e.g. our cells, bacteria, microstructural details of

materials, etc.


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Scale and Microscopy Techniques


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Advantages of Using SEM over OM

Mag Depth of Field Resolution

OM: 4x – 1400x 0.5mm ~ 0.2mm

SEM: 10x – 500Kx 30mm 1.5nm

The SEM has a large depth of field, which allows a largeamount of the sample to be in focus at one time andproduces an image that is a good representation of the three-dimensional sample.

The combination of higher magnification, larger depth offield, greater resolution, compositional and crystallographicinformation makes the SEM one of the most heavily usedinstruments in academic/national lab research areas andindustry.


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Electron-Solid Interactions

When an electron beam strikes a sample, a large number of signals are

generated.

We can divide the signals into two broad catego r ies:

a) electron signals, b) photon signals


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Specimen Interaction Volume

The volume inside the specimen in which interactions occur whileinteracting with an electron beam. This volume depends on thefollowing factors:

• Atomic number of the material being examined; higher atomic

number materials absorb or stop more electrons , smallerinteraction volume.

• Accelerating voltage: higher voltages penetrate farther into thesample and generate a larger interaction volume

• Angle of incidence for the electron beam; the greater the angle(further from normal) the smaller the interaction volume.


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Below is an example of a typical Interaction volume for:

• Specimen with atomic number 28, 20 kV

• 0° degrees tilt, incident beam is normal to specimen surface noting the

approximate maximum sampling depths for the


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Beam Interaction Simulations


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Electron Detectors and Sample Stage


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Electromagnetic Lenses


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Electromagnetic Lenses - The Objective Lens


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Electromagnetic Lenses ;The Objective Lens – Stigmator

The objective lens is machined to very high precision andthe magnetic field pattern is very carefully designed.

However, the precision attainable by machining cannotmatch that required for controlling a beam with a 10 nmdiameter.

The stigmator, which consist of two pairs of pole-pieces arranged in the X and Y directions, is added tocorrect the minor imperfections in the objectivelens.


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The Objective Lens ; Aperture Electromagnetic Lenses

Since the electrons coming from the electron gun have spreadin kinetic energies and directions of movement, they may not be focused to the same plane to form a sharp spot.

By inserting an aperture, the stray electrons are blocked andthe remaining narrow beam will come to a narrow “Disc of

Least Confusion” Electron


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Scanning Electron Microscope

1. The "Virtual Source" at thetop represents the electrongun, producing a stream ofmonochromatic electrons.

2. The stream is condensed bythe first condenser lens(usually controlled by the"coarse probe current knob").This lens is used to both form

the beam and limit theamount of current in the beam. It works in conjunction with the condenser apertureto eliminate the high-angleelectrons from the beam.


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3. The beam is then constricted bythe condenser aperture (usuallynot user selectable), eliminatingsome high-angle electrons.

4. The second condenser lensforms the electrons into a thin,tight, coherent beam and isusually controlled by the "fineprobe current knob".

5. A user selectable objectiveaperture further eliminates high-angle electrons from the beam.


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6. A set of coils then "scan" or "sweep"the beam in a grid fashion (like atelevision), dwelling on points for aperiod of time determined by thescan speed (usually in themicrosecond range).

7. The final lens, the objective, focusesthe scanning beam onto the part ofthe specimen desired.

8. When the beam strikes the sample(and dwells for a few microseconds)interactions occur inside the sampleand are detected with variousinstruments.


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9. Before the beam moves to itsnext dwell point these

instruments count the number

of e- interactions and display a

pixel on a CRT whose intensity

is determined by this number

(the more reactions the brighter

the pixel).

10.This process is repeated until

the grid scan is finished and

then repeated, the entire

pattern can be scanned 30

times/sec.


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A Look Inside the Column

S f El t Mi


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Summary of Electron MicroscopeComponents

1. Electron optical column consists of:

– electron source to produce electrons

– magnetic lenses to de-magnify the beam

– magnetic coils to control and modify the beam

– apertures to define the beam, prevent electron spray, etc.

2. Vacuum systems consists of:

– chamber which “holds” vacuum, pumps to produce vacuum

– valves to control vacuum, gauges to monitor vacuum

3. Signal Detection & Display consists of:

– detectors which collect the signal

– electronics which produce an image from the signal


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Electron Beam Source


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Field Emission Gun

The tip of a tungsten needle is made very sharp (radius 107 V/cm)due to the sharp point effect

• Electrons are pulled out from the tip by the strong

electric field• Ultra-high vacuum (better than 7.5 x 10-9 Torr - 6 x 10-6

Pa) is needed to avoid ion bombardment to the tipfrom the residual gas.

• Electron probe diameter < 1 nm is possible


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Signal Detection and Display

If you change the target material, the high and low energypeaks remain (although their intensity may change) while thelow intensity peaks change position and are characteristic ofthe sample.

The reason we produce this type of profile is because theincident electrons we send into the sample are scattered indifferent ways.

There are two broad categories to describe electronscattering:

– elastic: Backscattered electrons

– inelastic: Secondary electrons


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Electron Detectors


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Secondary Electrons


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Detection


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Detection Sequence


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Detection Sequence


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Backscattered Electrons


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Detection


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Detection Sequence

Operating Parameters


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Operating ParametersMagnification


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Resolution


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Resolution


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Depth of Field (I)


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Depth of Field (II)


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Depth of Field (III)


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Depth of Field vs. Resolution


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Instrumentation


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Sample Preparation


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L6-Scanning Electron Microscopy

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