“Increasing vision is increasingly expensive.”

SCANNING ELECTRON MICROSCOPY

MUBASHSHIR ARIFRAHUL RANJANP HARISH

Introduction

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 optical microscopes.

The first scanning electron microscope (SEM) debuted in 1938 ( Von Ardenne) with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample.

An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes (EM) have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than visible light (photons),  and magnifications of up to about 10,000,000x, whereas ordinary, light microscopes are limited by diffraction to about 200 nm resolution and useful magnifications below 2000x.

TEMThe original form of electron microscope, the transmission electron

microscope (TEM) uses a high voltage electron beam to create an image. The electrons are emitted by an electron gun and transmitted through the specimen that is in part transparent to electrons and in part scatters them out of the beam.

When it emerges from the specimen, the electron beam carries information about the structure of the specimen that is magnified by the objective lens system of the microscope.

The spatial variation in this information (the "image") may be viewed by projecting the magnified electron image onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide.

Image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution phosphor may be coupled by means of a lens optical system or a fibre optic light-guide to the sensor of a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed on a monitor or computer.

Characteristic Information: SEM

Topography:

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

direct relation between these features and materials propertiesMorphology:

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

relation between these structures and materials propertiesComposition:

The elements and compounds that the object is composed of

and the relative amounts of them; direct relationship between

composition and materials propertiesCrystallographic Information:

How the atoms are arranged in the object; direct relation between these arrangements and material properties.

FIG: SCANNING ELECTRON MICROSCOPE

Diagram courtsey : Lowa State university

Anode [Hitachi S2300]

ELECTRON SOURCES

A scanning electron microscope (SEM) is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in araster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surfacetopography, composition, and other properties such as electrical conductivity.

Specimen and Electron Detector Geometries:-position of detectors is a function of relativeenergies of the electrons

SEM Sample Chamber [AMRAY 1830] showing positionsof SE and BSE Detectors and the Final Lens

Secondary Detector

Backscatter Detector

Final Lens

Backscattered Electron Generation-SEM-BSE-primary beam electrons -high energy -composition and topography[specimen atomic number]

Secondary Electron Generation-SEM-SE-sample electrons ejected by the primary beam [green line]-low energy-surface detail & topography

SEM Imaging Modes

X ray is produced when outer shell electron falls in to replace inner shell electron

WORKING OF SPUTTER COATER

Switch power on with main switch.Flush working chamber several time with argon gas.Set sputter time with timer digit switch.Press start button to activate sputter process.Adjust appropriate gas pressure with argon valve.Set sputter current with current potentiometer.Process stops when selected sputter time elapses.To interrupt running sputter process press stop button.Switch power off/working chamber will be vented.

FIG: SPUTTER COATER

What happens when the Electron Beam hits the sample

When the electron is bombarded by the electron beam on the specimen , electrons are ejected from the atoms of the specimen surface.

Inelastic scattering, place the atom in the excited state. The atom “wants ” to return to a ground or unexcited state. Hence the atoms will relax giving off the excess energy.

X-rays, Cathodoluminescence and Auger electrons are the three ways of relaxation.

A resulting electron vacancy is filled by an electron from a higher shell, and an X-ray is emitted to balance the energy difference between the two electrons.

Limitations of Scanning Electron Microscopy (SEM)

Samples must be solid and they must fit into the microscope chamber. Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions should not exceed 40 mm.

For most instruments samples must be stable in vacuum . Samples likely to outgas at low pressures (rocks saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clays, and samples likely to depreciate at low pressure) are unsuitable for examination in conventional SEM's.

SEM's cannot detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11.

An electrically conductive coating must be applied to electrically

insulating samples for study in conventional SEM's, unless the instrument is capable of operation in a low vacuum mode.

Advantages of Using SEM

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

The combination of higher magnification, larger depth of field, greater resolution, compositional and crystallographic information makes the SEM one of the most heavily used instruments in academic/national lab research areas and industry.

SEM at IIC, IIT ROORKEE

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