SCANNING ELECTRON MICROSCOPE (SEM)

CHAITRALI.V.JADHAV 07/M.SC-1

PAPER-4

AIMTo study principle and working of scanning electron microscope.

HISTORYAccount of early history - mcmullan.

Photo with 50mm object field-width showing channeling contrast - Max knoll.

True microscope with magnification - Manfred von Ardenne 1937.

First SEM developed for bulk samples - Zworykin et al. in 1942.

First commercial SEM developed - Cambridge Scientific Instrument Company as “Stereoscan” in 1965.

PRINCIPLEBasic principle : A beam of eˉ is generated by a suitable source ( tungsten filament or a field emission gun).

Electron beam is then accelerated through a high voltage (e.g. 20 kV).

Passed through a system of apertures and electromagnetic lenses to produce a thin beam of eˉ.

Then the beam scans the surface of the specimen.

Electrons are emitted from the specimen by the action of scanning beam.

Collected by a suitable positioned detector.

CONSTRUCTIONElectron Gun (Filament)

Condenser Lenses

Objective Aperture

Scan Coils

Chamber (specimen test)

Detectors

Computer Hardware & Software

INFRASTRUCTURE REQUIREMENTS Power supply

Vaccum system

Cooling system

Vibration-free floor

Room free of ambient magnetic & electric fields.

The scanning electron microscope

Electron guns are typically one of two types:

Electron guns

Thermionic guns

Field emission

guns

Thermionic guns:

Most common type.Apply thermal energy to a filament.

Usually made of tungsten (high M.P).

Field emission guns:

Create strong electric field.

Located either at very top or at very bottom of an SEM.

These eˉ are not specific in direction…

Electron Gun

CONDENSER LENSES Just like optical microscopes, SEMs use condenser lenses to produce

clear & detailed images.

The condenser lenses in these devices, however, work differently.

They aren’t made of glass.

Instead, made of magnets capable of bending the path of eˉ.

Hence, focus & control eˉ beam, ensuring that the eˉ end up precisely where they need to go.

OBJECTIVE APERTURE The objective aperture arm fits above the objective lens in the SEM.

It is a metal rod that holds a thin plate of metal containing four holes. Over this fits a much thinner rectangle of metal with holes (apertures) of different sizes. By moving the arm in and out different sized holes can be put into the beam path.

An aperture holder: this arm holds a thin metal strip with different sized holes that line up with the larger holes. The metal strip is called an Aperture strip.

The aperture stops electrons that are off-axis or off-energy from progressing down the column. It can also narrow the beam below the aperture, depending on the size of the hole selected.

SCAN COILSThe scanning coils consist of two solenoids oriented in

such a way as to create two magnetic fields perpendicular to each other.

Varying the current in one solenoid causes the electrons to move left to right.

Varying the current in the other solenoid forces these electrons to move at right angles to this direction (left to right) and downwards.

CHAMBER (specimen test) The sample chamber of an SEM is where researchers place the

specimen that they are examining.

Because the specimen must be kept extremely still for the microscope to produce clear images, the sample chamber must be very sturdy and insulated from vibration.

In fact, SEMs are so sensitive to vibrations that they're often installed on the ground floor of a building.

The sample chambers of an SEM do more than keep a specimen still.

They also manipulate the specimen, placing it at different angles and moving it so that researchers don't have to constantly remount the object to take different images.

DETECTORS SEM's various types of detectors as the eyes of the microscope.

These devices detect the various ways that the electron beam interacts with the sample object.

For instance, Everhart-Thornley detectors register secondary electrons, which are electrons dislodged from the outer surface of a specimen. These detectors are capable of producing the most detailed images of an object's surface.

Other detectors, such as backscattered electron detectors and X-ray detectors, can tell researchers about the composition of a substance.

VACCUM CHAMBERSEMs require a vacuum to operate.

Without a vacuum, the electron beam generated by the electron gun would encounter constant interference from air particles in the atmosphere.

Not only would these particles block the path of the electron beam, they would also be knocked out of the air and onto the specimen, which would distort the surface of the specimen.

APPLICATIONS Vareity of applications in a # of scientific & industry related fields

(characterisation of solid materials is beneficial).

In addition to topographical, morphological and compositional & crystallographic information, SEM can detect and analyze surface fractures, provide information in microstructures, examine surface contaminations, reveal spatial variations in chemical compositions, provide qualitative chemical analyses and identify crystalline structures.

SEMs have practical, industrial and technological applications such as semiconductor inspection, production line of miniscale products and assembly of microchips for computers.

They can be as essential research tool in fields such as life science, biology, gemology, medical and forensic science, metallurgy.

ADVANTAGES Wide array of applications.

3D & topographical imaging.

Versatile information gathered from different detectors.

Easy to operate (user friendly).

Works faster ( completing SEI, BSE & EDS).

Technological advances allow generation of data in digital form.

Most SEM samples require minimal preparation action.

DISADVANTAGES Expensive, large & must be housed

in an area free of electric, magnetic and vibration interference.

• Maintenance involves keeping a steady voltage, currents to electromagnetic coils and circulation of cool water.

• Special training is required to operate an SEM as well as prepare samples.

• SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can handle moderate vacuum pressure.

POLLEN GRAINS taken on a SEM show characteristic depth of field of SEM micrographs.