Introduction tem 2017 09

Why Electron Microscope?• Light Microscopes are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers.. What is electron microscopy The elec

Basics of Transmission Electron

Microscopy (TEM)

1 History

2 What is microcopy

3 Why we use electrons as a probe

4 Electron-matter interactions in a thin sample

5 Components of the TEM

6 TEM imaging modes

An extremely brief history

(A) JEM 1.25 MeV HVEM Note the size of the instrument; often the high-voltage tank is in another room above the column (B) Zeiss HRTEM with a Cs corrector and an in-column energy filter.

(C) Hitachi 200 keV dedicated STEM; note the absence of a viewing chamber Such instruments are often

designed to aid failure analysis for the semiconductor device manufacturers Specimens thinned from wafers

on the production line can be easily transferred and examined (D) JEOL 200 keV TEM/STEM; note also the absence of a viewing chamber

E) Nion 200 keV ultrahigh vacuumSuperSTEM; the only US-manufactured (S)TEM and current holder of the world record

image resolution (F) FEI Titan Comparison with Ruska’s instrument (Figure 1.1), which is 70–80 years older than these

instruments, is instructive.

What is Microscopy ? _

Wikipedia defines it: Microscopy is the technical field of using

microscopes to view objects and areas of objects that cannot be

seen with the naked eye (objects that are not within the

resolution range of the normal eye).

THERE ARE FOUR BASIC TYPES OF MICROSCOPIES:

1 Optical microscopes: Bright field microscope, Dark

Fluorescent microscope…

2 Electron microscopes: SEM, TEM…

3 Scanning probe microscopes: AFM, NSOM, STM

4 Ion microscopes: FIB…

What is Microscopy

Why Electron Microscope?

• Light Microscopes are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers.

• In the early 1930's 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 just not

possible using Light Microscopes.

What is electron microscopy

 The electron microscopy is the

use of specialized microscopes

that interact a high energy

electron beam with samples as a

means to probe a material’s

structure

 Electron microscopes have a

greater resolving power than

light microscopes, allowing it to

see much smaller objects in finer

detail (sub nanometer resolution)

of equipment, generally in a

small, specially designed room

and requiring trained personnel

to operate them.

Why we use Electrons as a probe:

1 Easy to produce high brightness electron beams

2 Easily manipulated

3 High energy electrons have a short wavelength

4 Interact strongly with matter

 Electrons are accelerated to high

energies (which gives high spatial

resolution!)

 Electron beams are

monochromatic ( having the same

energy means less chromatic

aberrations!)

 Electrons beams are shaped (e.g.,

focused, collimated etc.) and

directed onto the samples using

electron static and magnetic lens

and deflector coils

 The interaction of the high

energy electrons produces

secondary signals that have

intrinsic information, specific to

the sample’s properties.

Why we use electrons as a probe:

Why we use electrons as a probe: Resolution

Why we use electrons as a probe: Resolution

TEM at 60,000 volts has a resolving power

of about 0.0025 nm Maximum useful magnification

of about 100 million times!!!

What are electron microscopes?

Electron Microscopes are scientific instruments that use a

beam of highly energetic electrons to examine objects on a very fine scale which yield the following information:

The elements and compounds that the object is composed of

and the relative amounts of them.

4 Crystallographic Information:

How the atoms are arranged in the object.

Magnification?

Magnification is how large the image is compared to real life, whereas Resolution is the amount of information that

can be seen in the image - defined as the smallest distance below which two discrete objects will be seen as one.

► Actual Size, Image Size and Magnification are related by the

BUT you must remember to

convert values to the same unit FIRST

M A

I

Eg: Calculating Magnification

It is essential that the same unit is used for the size of the image and the size of the object:

Eg 1: If an image measures 50mm (as printed on paper)

and the object actually measures 5µm;

The size of the image should be converted to µm:

I = Size of image = 50mm = 50 000µm

A = 5µm

Therefore, magnification = 50 000/5 = 10,000

Eg2: An image of a liver cell has a real scale bar next to it

recording 10um, but you measure the scale bar and find it is

20mm What is the magnification used?

Eg3 If a red blood cell has a diameter of 8 µm and a

student shows it with a diameter of 40 mm in a drawing, what is the magnification of the drawing?

1 Measure the size of the image

… 2cm (20mm)

2 Find the actual size of the sample

… 100nm Or 1.0 x 10 2

3 Convert measured size into unit on scale bar

(in this case, nm)

Magnification of Light Microscope

M microscope = M ocular/eyepiece X M objective

M microscope = M ocular/eyepiece X M objective

Magnification

Light Microscope

Magnification?

Reading an objective

Light Microscope

Objective lens Ocular/eyepiece lens

Body Tube

Nose Piece Objective Lenses

Stage Clips

Diaphragm Light Source

► Maximum magnification: usually ×1500,

Electron Microscope

Electrons have a much lower wavelength than light (100,000 times

shorter in fact, at 0.004nm) which means that they can be used to

produce an image with resolution as great as 0.1nm Electron Microscopes can have magnifications of ×500000.

Magnification?

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.

M ob x M int x M proj = Total Mag

Electron Microscope

Magnification?

Electron Microscope

Magnification?

Electron Microscope

Magnification?

Magnification

Magnification

• Resolution is defined as the ability to

distinguish two very small and

closely-spaced objects as separate entities

• Resolution is best when the distance

separating the two tiny objects is small

• Degree to which detail in specimen is

retained in magnified image

• Resolving power:

- Unaided eye: 0.1 mm apart

- Light Microscope: 0.2 µm apart

- Electron Microscope: 0.1 nm apart

Resolution/Resolving powder?

• Limit of resolution:

Minimum distance at which two objects appears as two

distinct objects

Even if we magnify an image of two objects, we can not distinguish

them unless we have adequate resolution

Rayleigh Criterion

The Rayleigh criterion is the generally accepted criterion for the minimum resolvable detail - the imaging process is said to be diffraction-limited when the first diffraction minimum of the image of one source point coincides with the maximum of another.

Limit of resolution:

r

Airy diffraction disks

0.61/nsin

In expression for the resolution (Rayleigh’s Criterion)

r = 0.61/nsinnsin

• : wavelength of illumination

source

• n: refractive index of the medium

between the object and the objective lens

• : half angle of the cone formed

by object at objective lens.

• Numerical aperture (N.A) = nsin

Resolution!

Limit of resolution:

r

NA of an objective is a measure of its ability to gather light and resolve fine

specimen detail at a fixed object distance

n: refractive index of the imaging medium between the front lens of objective and

specimen cover glass.

NA = n(sin )

Numerical Aperture (NA)



Angular aperture One half of A-A

NA = 1 - theoretical

maximum numerical

aperture of a lens operating

with air as the imaging

Faculty of Materials Technology, Ho Chi Minh City University of Technology

268 Ly Thuong Kiet street, Ward 14, District 10, Ho Chi Minh City, Viet NamTell: 38661320; Fax: 38661843 Copyright(c) 2000 All rights reserved.

HCMUT

Factors Affecting Resolution

 Resolution = d min = 0.61/nsin(N.A.)

 Resolution improves (smaller d min ) if   or n or  

 Assuming that sin = 0.95 ( = 71.8°)

OIL IMMERSION

High energy = short wavelengths = high spatial resolution

-wavelength,wavelength,  =[1.5/(V+10V+10 -6 V 2 )] 1/2 nm

V-wavelength,accelerating voltage, n-wavelength,refractive index

-wavelength,aperture of objective lens, very small in TEM

 sin   and so r=0.61/nsin;  ~ 0.1 radians

200kV Electrons   ~ 0.0025nmnm n~1 (vacuum)

r ~ 0.02nm (0.2Å) 1/nsin10 th size of an atom!

 point is imaged

Chromatic aberration is

caused by the variation of

the electron energy and

thus electrons are not

monochromatic.

r min  0.91(C s  3 )1/nsin4

Practical resolution of microscope

Cs–coefficient of spherical aberration of lens (~mm)

as a disk.

fluorescent (TV) screen, photographic film

Human eye (retina), photographic film

Tungsten or quartz halogen lamp

Whole cells visible

Why do we use electrons as probe :

Electrons interact strongly with matter

 Electron are used as a source of illumination

particles

sufficient energy in the form of heat, the electron leave their orbit, fly off from space & are lost in atoms

source of electron

PROPERTIES OF ELECTRON

 The electron are readily absorbed &

scattered by different form of matter

sustained only in high vacuum

image formation

biological specimens to form the image

PROPERTIES OF ELECTRON

 Electron-matter interaction in a thin sample

1 Transmitted electrons (A) of the beam passes straight

through the specimen on to the screen

2 Some electron (B) of the beam lose a bit of their

energy while passing through the specimen & get

inelastically scattered electrons

3 Some electron (C) interact with atoms of specimen &

Electron deviate widely

 Electron-matter interaction in a thin sample

4 Some electron (D) get backscattered instead of getting

transmitted through the specimen

5 In some cases the electrons get absorbed by the

atoms of the specimen & instead low energy electron

(E) are emitted These electron are termed secondary electron These are very useful for forming the image

in the SEM

6 Some atom emit X-ray & light energy

 Electron-matter interaction in a thin sample

 Electron-matter interaction in a thin sample

 Electron-matter interactions in a thin sample:

– charge collective oscillation excitation (plasmons)

– excitation of surface electronic level (transition

Summary

Why we use electrons as probe

1 Easy to produce high brightness electron beams

High coherence beams allow us to generate diffraction

patterns and high spatial resolution images

2 Easily manipulated

Electron lenses and deflectors can used to easily change focal lengths and beam directions which is a necessary operating condition for flexible imaging devices

3 High energy electrons have a short wavelength

Shorter wavelengths means higher spatial resolution (Raleigh Criterion)

4 Electrons interact strongly with matter

Secondary signals have information specific to the material

Bragg diffracted electrons –structure, orientation, phase

distribution, defect content and structures, etc.

COMPONENTS OF THE TEM

1 Electron source

2 Electro-magnetic lenses

to direct and focus the electron beam inside the column

3 Vacuum pumps system

4 Opening to insert a grid with samples into the

high-vacuum chamber for observation .

5 Operation panels

6 Screen for menu and image display

7 Water supply to cool the instrument

PARTS OF TEM

Electron Gun

EDS Detector Condenser

Lens

Specimen Holder Objective Lens

Magnifying Lenses

• The electron source consists of a cathode and an anode.

• Cathode - tungsten filament which emits electrons when being heated.

• A negative cap confines the electrons into a loosely focused beam

• The beam is then accelerated towards the specimen by the positive anode

• Electron beam is tightly focused using electromagnetic lens and metal apertures.

• A platform equipped with a mechanical arm for holding the specimen and controlling its position

Electromagnetic lens system

Phosphorescent Screen Objective lens Projector lens

Components of the TEM

Electron gun: generates and accelerates the electrons to the desired energy (velocity)

them to high energies > 60keV

Components of the TEM

Electron Source

Anode [Hitachi S2300]

Electron Source

Electron Source

Electron Source

Electron Guns

We want many electrons per time unit per area (high current density) and small electron spot as possible Electron beam is generated in the

electron gun Two basic types of guns are used:

Electron Guns

when a solid is heated) Based on two types of filaments:

Tungsten (W-wire) and Lanthanum-Hexaboride (LaB6-crystal).

Employs either a thermally assisted cold field emitter or

Schottky emitter (cold guns, a strong electric field is used to extract electrons).

Single crystal of W, etched to a thin tip.

With field emission guns we get a smaller spot and higher current densities compared to thermionic guns.

Vacuum requirements are tougher for a field emission guns

Electron Guns

LaB6-crystal

Traditional guns Modern gun

Electron Beam Source

 Thermionic gun

Three elements:

- The filament (cathode)

- The Welhnelt and

- The anode

FIGURE 5.1 Schematic

diagram of a thermionic electron gun A high voltage is placed between the cathode and the anode, modified by a potential on the Wehnelt which acts as the grid in a triode system The electric field from the Wehnelt focuses the electrons into

a crossover, diameter d0

and convergence/divergence angle a0 which is the true source (object) for the lenses in the TEM illumination system.

FIGURE 5.2 The three major parts of a thermionic gun,

from top to bottom: the cathode, the Wehnelt cylinder

and the anode shown separated The Wehnelt screws into

the cathode support and both are attached to the

highvoltagecable which also contains power supplies for

heating the cathode and biasing the Wehnelt The anode

sits just below the Wehnelt and the whole assembly sits on

the top of the column of lenses thatmake up the rest of

theTEM.

(Grid Cap)

 Thermionic gun

 Functioning of the Thermionic Gun

 An positive electrical potential is applied to the anode.

 The filament (cathode) is heated until a stream of electrons is produced.

 A negative electrical potential (~500 V) is applied to the Wehnelt Cap.

 A collection of electrons occurs in the space between the filament tip and Wehnelt Cap This collection is called a space charge.

 Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in the Wehnelt Cap.

 These electrons then move down the column to be later used in imaging.