5. TEM - Electron Microscopy and Diffraction

$: 5. TEM - Electron Microscopy and Diffraction$
Electron Microscopy Electron Microscopy and Diffractionand Diffraction

5. 5. Transmission Electron MicroscopeTransmission Electron Microscope

Do Minh NghiepMaterials Science Center

01.01.2009 Materials Science Center, HUT 2

Electron optics and instrument Image contrast (mass thickness

contrast, phase contrast, diffraction contrast)

Magnification and electron beam adjustment

Sample preparation Application

ContentContent

Introduction Introduction

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What’s TEM What’s TEM The transmission electron microscope (TEM) is an analytical tool that allows:detailed microstructural examination through high-resolution and high-magnification imaging: magnifications of up to 500,000x and detail resolution below 1 nm are achieved routinely, investigation of crystal structures and orientations through electron diffraction pattern,determination of chemical compositions in phases, precipitates and contaminants through X-ray and electron spectroscopy: qualitative and quantitative elemental analysis can be provided from features smaller than 30 nm.


What’s TEM imageWhat’s TEM image Basic idea of a TEM is to

project a magnified image of the specimen onto a fluorescent screen where it can be viewed by the user.

The image itself is the result in intensity difference of beam electrons that are forward scattered by the specimen vs. those that are not (transmitted unscattered) .

and aperture

Objective

Projective


Pros and consPros and cons

AdvantageAdvantage

- TEM is the only technique that will allow the determination of the Burgers vector.

- TEM can also provide atomic scale resolution (2 Å). DisadvantageDisadvantage

- TEM is an expensive and destructive technique.

- Some materials are sensitive to electron beam radiation, resulting in a loss of crystallinity and mass.

- Sample preparation is very time-consuming, sample dimension small (3 mm diameter, less than 100 nm thick)

- Artifacts cause misleading for the interpretation of transmission images.

This usually causes by the fact that the TEM presents us with 2D-images of 3D-specimens viewed in transmission.


3D-object 3D-object 2D-image 2D-image

01/01/2009 Handouts-MSE4346 -K51 KHVL 7

A single projection image is plainly insufficient to infer the structure of

an object.

Each image represents a 2D projection of the 3D object

3D-object 3D-object 2D-image 2D-image

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Watch out!A cover slide!

InstrumentInstrument


TEM components TEM components and electron pathand electron path

A simplified ray diagram of a TEM consists of - an electron source, - condenser lens with aperture,- specimen, - objective lens with aperture, - projector lens and - a fluorescent screen.


SuplementarySuplementary

In actuality a modern TEM consists of many more components including- a dual condenser system,- stigmators, - deflector coils, and -a combination of intermediate and dual projector lens.

Sîi ®èt

AnètChao

Khe cè ®ÞnhCuén chØnh nguån

Condenser 1

Condensor 2

Stigmator 1Khe cè ®Þnh

DeflectorKhe tô kÝnh

VËt kÝnh

Khe vËt kÝnh

Gi¸ gi÷ mÉu

Stigmator 2

KÝnh trung gianKhe kÝnh trung gian (nhiÔu x¹)

Dual projector

Mµn huúnh quang

Focus wobble


Conventional TEMsConventional TEMs


200 kV (left) and 100 kV (right)

TEM

Modern TEMsModern TEMs


High Resolution Transmission High Resolution Transmission Electron Microscope (HRTEM)Electron Microscope (HRTEM)


HRTEM - a comparisonHRTEM - a comparison


Image contrast Image contrast


Imaging mechanismImaging mechanism


Phase contrastPhase contrast

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Bright field (BF) Dark field (DF)

Parallel incident beam

Phase contrast/ image mode

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Diffraction contrastDiffraction contrast


Diffraction contrast/ image mode (Electron microdiffraction)

Convergent incident beam



If a monochromatic e-beam of known l strikes a crystal at the appropriate Bragg’s angle a number of the diffracted electrons will be forward scattered.

Like the transmitted electrons these diffracted electrons will have nearly their same energy but will have been significantly altered from their trajectory.



The transmitted electrons will be brought to convergence in the back focal plane of the objective lens (Y).

Likewise the diffracted electrons will also be brought to convergence in the back focal plane of the lens but at a different spot (X).

.


Normally an aperture is placed in the back focal plane of the objective lens to stop widely scattered electrons from reaching the viewing screen, but in the case of diffraction it is these same scattered electrons that contain the information about the diffraction event.

To operate the TEM in diffraction mode

the objective aperture is removed from the beam path and the microscope is adjusted to focus an image of the back focal plane of the objective lens, not the image plane.




This is most easily accomplished by adjusting the strength of the objective lens so that an image of the back focal plane is projected onto the viewing screen.

Back focal plane

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DF imageDF imageBF imageBF image

Mass-thickness contrast (fringes)Mass-thickness contrast (fringes)

Materials Science Center, HUT

Contrast is caused by:- Unscattered electrons coming into screen (bright area)- Scattered electrons not coming into screen (dark area)

Contrast imageContrast image

Scattered eTransmitted e(unscattered)


MagnificationMagnification


Total magnification in the TEM is a combination of the magnification from the objective lens times the magnification of the intermediate lens times the magnification of the projector lens.

Each of which is capable of approximately 100x.

Mob x Mint x Mproj = Total Mag so Mtotal can be a magnitude of 106

What’s magnificationWhat’s magnification


Under different conditions of high and low magnification the same component can have different functions:

MagnificationMagnification

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Contrast forming

Image focus

Diffraction focus

Diffraction stigmation

Image stigmation

Area selection

Low magnificationHigh magnification

Objective lens

Diffraction lens

Objective aperture

SA aperture

Object. stigmator

Diffract. stigmator

Image focus

Diffraction focus

Contrast forming

Area selection

Diffraction stigmation

Image stigmation

Components

Depth of Field (Dfi): the range of distance at the specimen parallel to the illuminating beam in which the object appears to be in focus. Depth of Focus (Dfo): the range of distance at the image plane (i.e. the eyepiece, camera, or photographic plate) in which a well focussed object appears to be in focus.

Magnification and focusing Magnification and focusing


In TEM thanks to great Dfi and Dfo the image is well focuced even at high magnification

Magnification and focusing Magnification and focusing

The depth of focus is so great for the projector lens system that an image that is in focus on the screen will also be in focus on plate film beneath the screen or even a TV camera below the plate film

Fluorescent screen

Binocular viewing scope

Viewport

Plate camera


Adjustment of Adjustment of electron beamelectron beam


Adjustment of condenserAdjustment of condenser

The role of the condenser lenses is to make the beam that is striking the specimen as nearly parallel as is possible.

Condenser lens

Specimen

Primary beam


Adjustment of condenserAdjustment of condenser

As magnification increases the condenser lens must be adjusted to properly illuminate the specimen. When the lens is brought to its smallest spot the beam is said to be at the crossover point.

Screen before focal point: large beam (under focused)

Screen at focal point: minimum beam

size/crossover, maximum brightness (focused)

Screen behind focal point: large beam

(over focused)

(-) Change in condenser lens current (+)


Holey (not Holy) Formvar is used to critically adjust the stigmation of a TEM. When the beam is under or over focused on the specimen a Fresnel fringe becomes visible due to the effects of diffraction around the edges of the whole. When this Fresnel fringe is evenly distributed then the beam is said to be stigmated.

StigmatorStigmator


Deflection coils

In older TEMs functions such as gun and beam alignment were accomplished by physically moving components in the column.

Today they are achieved by use of electromagnetic deflection coils that are positioned throughout the column.


Shift / tilt of beam by deflecton coilsShift / tilt of beam by deflecton coils

Using the deflection coils the beam can be: shifted so that the focused beam is centered in the back focal plane of the lens and tilted so that the beam is centered on the specimen. - deflection angle


Pivot pointPivot point

We call these centering spots “pivot points” and as the beam is shifted or tilted back and forth there should be no apparent movement if the microscope is properly aligned

Pivot point


Specimen stage positionSpecimen stage position

Likewise the position of the specimen relative to the beam is of critical importance. To adjust the height or “Z” position of the specimen it is physically rocked back and forth until an object in the center no longer moves.

Specimen stage

Specimen


SpecimenSpecimen stagestage

In-column motion of specimen: Z-Y-Z shift, rotation/ tilt around a common axis by coresponding mechanism.


Sample tilt in single obj. lensSample tilt in single obj. lens A major problem with TEM

lens design is the fact that there is very little space between the pole pieces of the objective lens to accommodate the specimen and objective aperture.

This also puts severe constraints on how far the specimen can be tilted within the lens.


Twin/double objective lensTwin/double objective lens

A clever solution to this problem came in the invention of the double objective or “twin” lens.

In this lens design the specimen actually lies between two separate lens fields allowing for tilt angles as large a 60o from perpendicular to the optical axis.


Focus Wobbler shifts the image back and forth, and when the movement is stable the image is focused.


Specimen Specimen preparationpreparation


Wedge sample Wedge sample -”windows” technique-”windows” technique


ElectropolishingElectropolishing


Sample sections after Sample sections after electropolishing and sample gridselectropolishing and sample grids


Cutting thin sample Cutting thin sample by microtomy by microtomy


Replica foilsReplica foils


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TEM grids

3 mm

Instrumentation Instrumentation


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Sample positions

Cooling

Standard

Heating

Sample holdersSample holders


ApplicationsApplications


Dislocation structureDislocation structure(diffraction contrast) (diffraction contrast)


Contrast image Contrast image and and

microdiffraction microdiffraction pattern of two pattern of two

screw dislocation screw dislocation groups with groups with

different Burgers different Burgers vectors vectors


BF and DF BF and DF images of images of

dispersive dispersive precipitates precipitates

in Al-Cr-Zr in Al-Cr-Zr alloyalloy


Lattice Lattice image of image of

(111) (111) planes in planes in

Ga-AsGa-As


HR image HR image of of

phase in phase in Al alloysAl alloys


HR contrast HR contrast image of image of

metal atom metal atom arrangement arrangement

in 1,97 nm in 1,97 nm thick thick

specimen specimen of Nb-W-O of Nb-W-O


5. TEM - Electron Microscopy and Diffraction

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