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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.