Higher accelerating voltages in transmission electron microscopes (TEMs) allow for shorter electron wavelengths and improved resolution. A 1 MeV TEM can achieve resolutions around 0.5 angstroms. Maintaining a stable high voltage, such as within 0.5 volts for a 1 MeV microscope, is important. Increasing the voltage improves resolution and penetration depth but can also increase specimen charging and damage.

4. The greater the
accelerating
voltage the shorter
the
Therefore, a 50,000 volt (50 kV) electron has a
wavelength of 0.0055nm and a 1MeV electron has
a wavelength of 0.00123nm!

5. 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

6. 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

7.  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

8.  Multi stage accelerator are used
 Maximum voltage per stage depends on
1. Design & finish of the electrodes
2. On that of containing tube
3. 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

9.  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

10. Effects of increasing
voltage in electron gun:
Resolution increased
( decreased)
Penetration increases
Specimen charging
increases (insulators)
Specimen damage
increases
Image contrast
decreases
Chromatic aberration is
decreased

11. In 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.

13.  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

14.  It consists of two projectors
 1st is stronger & 2nd is weaker
 The overall magnification range in normal
operations
2000× to 140000 ×

15.  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

18.  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

19.  Increased Specimen Penetration:
As accelerating voltage increases the
ability of the beam to penetrate also
increases.

20.  Increased Depth of Information:
The great depth of information allows an
entire thick section to be viewed at once.
Dendrite in 3 m
thick section

21.  Reduced Beam Damage:
The increased speed of the electrons
actually decreases the likelihood of an
inelastic collision.