High Voltage Electron Microscope

Contents

IntroductionComponentsWorking Image formationEffect of voltage on resolutionAdvantages & disadvantages

The greater the accelerating voltage the shorter the l

Therefore, a 50,000 volt (50 kV) electron has a wavelength of 0.0055nm and a 1MeV electron has a wavelength of 0.00123nm!

A million volt (MeV) TEM must maintain an accelerating voltage that is stable to within 0.5 volts. With this coupled with a field emission source one can achieve resolutions that are in the range of 0.5Å

The HVEM consists of the following major parts

The illumination system Electron gun Condenser

The image forming system Objective lens

The projective system Several projector lens

Aperture Affect the image and diffraction

pattern

High Voltage Source

High Voltage Source is same in principle as in TEM (Heated filament)

A Cockcroft Walton Rectifier-condenser stack for voltage multiplication

Feedback system for regulating the output

Double feed-back system is used. Fast loop for dealing with ripple slow loop for drift

Accelerator

Multi stage accelerator are used

Maximum voltage per stage depends on

1. Design & finish of the electrodes2. On that of containing tube3. Degree of vacum

Raising the voltage in steps of 25-50 kV at intervals of a few minutes

In U.S steel installation max. achievable voltage per stage is 150 Kv

In Japanese HVEM more stages (25) are used

Injector Gun The injector gun is

supplied by a transistorized power unit house

Large aluminium dome is used

It acts as corona shield over the accelerator column.

Operating conditions of the gun are controlled

by: Servo-motors driving

insulating rods Light beams

Bias (Wehnelt)Cylinder

Filament (20-100 KV)

Anode

stream of electrons originating from outer shell of filament atoms

Effects of increasing voltage in electron gun:

Resolution increased ( decreased)

Penetration increasesSpecimen charging

increases (insulators)Specimen damage

increasesImage contrast

decreasesChromatic aberration

is decreased

Effects of increasing voltage in electron gun:

Resolution increased ( decreased)

Penetration increasesSpecimen charging

increases (insulators)Specimen damage

increasesImage contrast

decreasesChromatic aberration

is decreased

Microscope ColumnIn this column Electron beam is generated under

vacuum, focused to a small diameter & scanned across the specimen by electromagnetic lens

Lower portion of the column is called specimen chamber

Japanese microscopes have more massive columns, up to 50 cm in diameter.

It provide adequate protection in all operating conditions.

Illuminating System

Consists of two condenser lens

1st is stronger & 2nd is weaker

Minimum spot size is being less than 1mm

An electromagnetic double deflection system is used mounted b/w 2nd condenser and the objective

Allows a controlled tilt & shift of illuminating for alignment and for dark field imaging

Imaging System It consists of two projectors 1st is stronger & 2nd is weaker The overall magnification range in

normal operations 2000× to 140000 ×

Viewing Screen & Camera

As the range of 750-1000 kv electrons is about 30 times that at 100 kv the viewing screen has to be modified.

Thicken the fluorescent layer leads to blurring of image details.

Best solution is to mount phosphor on sheet of plastic

Optimum thickness is a compromise b/w image brightness & resolution at high temperature

Image quality is affected by:

Accelerating voltage Working distance Spot size Wavelength

Effects of accelerating voltage

Choosing the right accelerating voltage is critical for obtaining a good clear image and it depends on the material being examined.The more conductive the material the better it will under high voltages otherwise the specimen will be damage.

Advantages of High Voltage EM

Increased Resolution: Although still a long way from a theoretical

resolution of 0.0006 nm. the best actual resolution has been achieved with a 1 MeV field emission TEM.

1 MeV 200 kV 100kV

Advantages of High Voltage EM

Increased Specimen Penetration: As accelerating voltage increases

the ability of the beam to penetrate also increases.

Advantages of High Voltage EM

Increased Depth of Information: The great depth of information

allows an entire thick section to be viewed at once.

Dendrite in 3 mm thick section

Advantages of High Voltage EM

Reduced Beam Damage: The increased speed of the

electrons actually decreases the likelihood of an inelastic collision.