< Engineering Physics - I > · PDF file< Engineering Physics - I > Material prepared by: <...

< Engineering Physics - I > <Quantum Physics: Microscopes>

Material prepared by: < Physics Faculty > Topic No: < 6 > of 13

Introduction

Learning Objectives

On completion of this chapter you will be able to:

1. Learn about Electron microscope

2. Learn about Scanning Electron microscope

3. Learn about Transmission Electron microscope



Electron microscope

An electron microscope is a type of microscope that

uses electrons to illuminate a specimen and create an

enlarged image. Electron microscopes have much

greater resolving power than light microscopes and can

obtain much higher magnifications. Some electron

microscopes can magnify specimens up to 2 million

times, while the best light microscopes are limited to

magnifications of 2000 times. Both electron and light

microscopes have resolution limitations, imposed by

their wavelength. The greater resolution and

magnification of the electron microscope is due to the

wavelength of an electron, its de Broglie wavelength,

being much smaller than that of a light photon,

electromagnetic radiation.

The electron microscope uses electrostatic and electromagnetic lenses in forming the

image by controlling the electron beam to focus it at a specific plane relative to the

specimen in a manner similar to how a light microscope uses glass lenses to focus light

on or through a specimen to form an image

Disadvantages of electron microscope

Electron microscopes are expensive to build and maintain, but the capital and running

costs of confocal light microscope systems now overlaps with those of basic electron

microscopes. They are dynamic rather than static in their operation, requiring

extremely stable high-voltage supplies, extremely stable currents to each

electromagnetic coil/lens, continuously-pumped high- or ultra-high-vacuum systems,

and a cooling water supply circulation through the lenses and pumps. As they are very

sensitive to vibration and external magnetic fields, microscopes designed to achieve

high resolutions must be housed in stable buildings (sometimes underground) with



special services such as magnetic field cancelling systems. Some desktop low voltage

electron microscopes have TEM capabilities at very low voltages (around 5 kV) without

stringent voltage supply, lens coil current, cooling water or vibration isolation

requirements and as such are much less expensive to buy and far easier to install and

maintain, but do not have the same ultra-high (atomic scale) resolution capabilities as

the larger instruments.

The samples largely have to be viewed in vacuum, as the molecules that make up air

would scatter the electrons. One exception is the environmental scanning electron

microscope, which allows hydrated samples to be viewed in a low-pressure (up to

20 Torr/2.7 kPa), wet environment.

Scanning electron microscopes usually image conductive or semi-conductive materials

best. Non-conductive materials can be imaged by an environmental scanning electron

microscope. A common preparation technique is to coat the sample with a several-

nanometer layer of conductive material, such as gold, from a sputtering machine;

however, this process has the potential to disturb delicate samples.

Small, stable specimens such as carbon nanotubes, diatom frustules and small mineral

crystals (asbestos fibres, for example) require no special treatment before being

examined in the electron microscope. Samples of hydrated materials, including almost

all biological specimens have to be prepared in various ways to stabilize them, reduce

their thickness (ultrathin sectioning) and increase their electron optical contrast

(staining). There is a risk that these processes may result in artifacts, but these can

usually be identified by comparing the results obtained by using radically different

specimen preparation methods. It is generally believed by scientists working in the field

that as results from various preparation techniques have been compared and that there

is no reason that they should all produce similar artifacts, it is reasonable to believe

that electron microscopy features correspond with those of living cells. In addition,

higher-resolution work has been directly compared to results from X-ray

crystallography, providing independent confirmation of the validity of this technique



Electron microscopy application areas

Semiconductor and data

storage

• Circuit edit

• Defect analysis

• Failure analysis

Biology and life sciences

• Cryobiology

• Protein localization

• Electron tomography

• Cellular tomography

• Cryo-electron microscopy

• Toxicology

• Biological production and

viral load monitoring

• Particle analysis

• Pharmaceutical QC

• 3D tissue imaging

• Virology

• Vitrification

Research

• Electron beam induced depostion

• Materials qualification

• Materials and sample preparation

• Nanoprototyping

• Nanometrology

• Device testing and characterization

Industry

• High-resolution imaging

• 2D & 3D micro-characterization

• Macro sample to nanometer metrology

• Particle detection and characterization

• Direct beam-writing fabrication

• Dynamic materials experiments

• Sample preparation

• Forensics

• Mining (mineral liberation analysis)

• Chemical/Petrochemical



Scanning electron microscope

The scanning electron microscope (SEM) is a type of electron microscope that images

the sample surface by scanning it with a high-energy beam of electrons in a raster scan

pattern. The electrons interact with the atoms that make up the sample producing

signals that contain information about the sample's surface topography, composition

and other properties such as electrical conductivity.

Scanning process and image formation

In a typical SEM, an electron beam is thermionically emitted from an electron gun

fitted with a tungsten filament cathode. Tungsten is normally used in thermionic

electron guns because it has the highest melting point and lowest vapour pressure of all

metals, thereby allowing it to be heated for electron emission, and because of its low

cost. Other types of electron emitters include lanthanum hexaboride (LaB6) cathodes,

which can be used in a standard tungsten filament SEM if the vacuum system is

upgraded and field emission guns (FEG), which may be of the cold-cathode type using

tungsten single crystal emitters or the thermally-assisted Schottky type, using emitters

of zirconium oxide.

The electron beam, which typically has an energy ranging from a few hundred eV to 40

keV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in

diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in

the electron column, typically in the final lens, which deflect the beam in the x and y

axes so that it scans in a raster fashion over a rectangular area of the sample surface.

When the primary electron beam interacts with the sample, the electrons lose energy

by repeated random scattering and absorption within a teardrop-shaped volume of the

specimen known as the interaction volume, which extends from less than 100 nm to

around 5 µm into the surface. The size of the interaction volume depends on the

electron's landing energy, the atomic number of the specimen and the specimen's

density. The energy exchange between the electron beam and the sample results in the



reflection of high-energy electrons by elastic scattering, emission of secondary

electrons by inelastic scattering and the emission of electromagnetic radiation, each of

which can be detected by specialized detectors. The beam current absorbed by the

specimen can also be detected and used to create images of the distribution of

specimen current. Electronic amplifiers of various types are used amplify the signals

which are displayed as variations in brightness on a cathode ray tube. The raster

scanning of the CRT display is synchronised with that of the beam on the specimen in

the microscope, and the resulting image is therefore a distribution map of the intensity

of the signal being emitted from the scanned area of the specimen. The image may be

captured by photography from a high resolution cathode ray tube, but in modern

machines is digitally captured and displayed on a computer monitor and saved to a

computer's hard disk.



Magnification

Magnification in a SEM can be controlled over a range of about 5 orders of magnitude

from x25 or less to x 250,000 or more. Unlike optical and transmission electron

microscopes, image magnification in the SEM is not a function of the power of the

objective lens. SEMs may have condenser and objective lenses, but their function is to

focus the beam to a spot, and not to image the specimen. Provided the electron gun

can generate a beam with sufficiently small diameter, an SEM could in principle work

entirely without condenser or objective lenses, although it might not be very versatile

or achieve very high resolution. In an SEM, as in scanning probe microscopy,

magnification results from the ratio of the dimensions of the raster on the specimen

and the raster on the display device. Assuming that the display screen has a fixed size,

higher magnification results from reducing the size of the raster on the specimen, and

vice versa. Magnification is therefore controlled by the current supplied to the x,y

scanning coils, and not by objective lens power.

Resolution of the SEM

The spatial resolution of the SEM depends on the size of the electron spot, which in

turn depends on both the wavelength of the electrons and the electron-optical system

which produces the scanning beam. The resolution is also limited by the size of the

interaction volume, or the extent to which the material interacts with the electron

beam. The spot size and the interaction volume are both large compared to the

distances between atoms, so the resolution of the SEM is not high enough to image

individual atoms, as is possible in the shorter wavelength (i.e. higher energy)

transmission electron microscope (TEM). The SEM has compensating advantages,

though, including the ability to image a comparatively large area of the specimen; the

ability to image bulk materials (not just thin films or foils); and the variety of analytical

modes available for measuring the composition and proprties of the specimen.

Depending on the instrument, the resolution can fall somewhere between less than 1

nm and 20 nm. The world's highest SEM resolution is obtained with the Hitachi S-5500.



Resolution is 0.4nm at 30kV and 1.6nm at 1kV. In general, SEM images are easier to

interpret than TEM images.

The Transmission Electron Microscope

Transmission electron microscopy (TEM) is a microscopy technique, a beam of electrons

is transmitted through an ultra thin specimen. An image is formed from the electrons

transmitted through the specimen, magnified and focused by an objective lens and

appears on an imaging screen, it may be either fluorescent screen, or on a layer of

photographic film, or to be detected by a CCD camera.

Principle

The electron microscopes are worked on the basis of , when a beam of electrons hit on

a bulk material it can be either reflected or backscattered by the bulk material or the

electrons share energy to the atomic electrons that are present in a solid and which can

then release secondary electrons. In electron microscopy the incoming electrons are

focused by electric or magnetic field and pass through the solid specimen and collect

the amplified scattered electrons and the secondary electrons by a sensor.

The head of Antarctic krill Electron microscope image

of the compound eye

higher magnification of the

krill's eye by SEM



Working of Transmission Electron microscope

The TEM operates on the same basic principles as the light microscope except uses

electrons instead of light. Figure shows the schematic of the TEM. The virtual source at

the top represents the electron gun with the energy of about 100-400 keV, producing a

stream of monochromatic electrons. These electrons are focused to a small, thin,

coherent beam by the use of condenser lenses 1 and 2. The first lens largely determines

the pot size and the second lens changes the size of the spot on the sample, changing it

from a wide dispersed spot to a pinpoint beam. The beam is restricted by the

condenser aperture and hit the sample by high angle electrons. The beam strikes the

sample and parts of it transmit. The transmitted portion of the electron is focused by

the objective lens into an image. The objective and selected area metal apertures can

restrict the beam, the objective aperture enhancing contrast by blocking out high angle

diffracted electrons, the selected area aperture enabling the used to examine the

periodic diffraction of electrons by ordered arrangements of atoms in the sample. The

image is passed down the column through the intermediate and projector lens. The



output may be observed by fluorescent screen. The image strikes the phosphor image

screen and light is generated, allowing the user to see the image.

The darker areas of the image represent those areas of the sample that fewer electrons

were transmitted through (they are thicker or denser). The lighter areas of the image

represent those areas of the sample that more electrons were transmitted through

(they are thinner or less dense). The TEM builds an image by way of differential

contrast. Those electrons that pass through the sample go on to form the image while

those that are stopped or deflected by dense atoms in the specimen are subtracted

from the image. In this way a black and white image is formed. Some electrons pass

close to a heavy atom and are thus only slightly deflected. Thus many of these

"scattered" electrons eventually make their way down the column and contribute to the

image. In order to eliminate these scattered electrons from the image we can place an

aperture in the objective lens that will stop all those electrons that have deviated from

the optical path. The smaller the aperture we use the more of these scattered

electrons we will stop and the greater will be our image contrast.

Another important element of the TEM is the vacuum system. The main reason to use

vacuum is to avoid collisions between electrons of the beam and stray molecules.

Limitations

1. In TEM, the materials require extensive sample preparation, we need to produce

a sample thin enough to be electron transparent.

2. It is a time consuming process

3. The structure of the sample may be changed during the preparation process

4. The region of analysis is too small, the possibility that the region analysed may

not be characteristic for the whole sample

5. The sample may be damaged by the electron beam, particularly in the case of

biological samples

Applications of the TEM

1. The TEM is used in both material science, metallurgy and the biological sciences.



2. In biological applications it is used to create tomographic reconstructions of

small cells or thin sections of larger cells and 3D reconstructions of individual

molecules.

3. In material science it is useful to find the dimensions of powders or nanotubes.

4. Faults in crystals or metals can be identify by TEM, by careful selection of

defects it is useful to locate the position of the defects and also the nature of

the defect present.

5. High Resolution TEM (HRTEM) technique allows detecting the crystal structure

directly.

6. The resolution of image is too high compare with the optical magnifying systems

Bamboo vascular bundles. (Left to right): light micrograph with safranin/alcian blue stain; SEM at

similar scale; TEM of fibre cells (a few of the pink cells from the light micrograph), note the

higher magnification and resolution: lamellation of the cell wall is clear.



Check your understanding

1. Choose the right answer from the options given below:

The scanning electron microscope (SEM) is a type of optical microscope that

images the sample surface by scanning it with a high-energy beam of electrons in

a raster scan pattern

a) True

b) False

2. State if the following statement is true or false?

The TEM operates on the same basic principles as the light microscope except

uses electrons instead of light

a) True

b) False

Check the correct answers on page 14.

Summary

On completion of this chapter you have learned that:

1. Learned about Electron microscope

2. Learned about Scanning Electron microscope

3. Learned about Transmission Electron microscope

Activity



Learn the basics about optical and electron microscopes

Suggested Reading

1. Optics of high-performance electron Microscopes, Sci. Technol. Adv. Mater. 9 (2008) 014107 (30pp) 2. http://en.wikipedia.org/wiki/Transmission_electron_microscope

Answers to CYU.

Provide the right answers to the Check your understanding section here.

1. b

2. a

< Engineering Physics - I > · PDF file< Engineering Physics - I > Material prepared by: <...

Documents

Transcript of < Engineering Physics - I > · PDF file< Engineering Physics - I > Material prepared by: <...