Presentation high voltage electron microscope

1. High Voltage Electron Microscope

2. Contents Introduction Components Working Image formation Effect of voltage on resolution Advantages & disadvantages

3. 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!

4. 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

5. 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

6. High Voltage Source High Voltage Source is same in principle as in TEM (Heated filament) A Cockcroft Walton Rectifier-condenser stack for voltage multiplication F eedback system for regulating the output Double feed-back system is used. Fast loop for dealing with ripple slow loop for drift

7. Accelerator Multi stage accelerator are used Maximum voltage per stage depends on Design & finish of the electrodes On that of containing tube 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

8. Injector Gun The injector gun is supplied by a transistorized power unit house L arge 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

9. 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

10. Microscope Column 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.

11. Illuminating System Consists of two condenser lens 1 st is stronger & 2 nd is weaker Minimum spot size is being less than 1mm An electromagnetic double deflection system is used mounted b/w 2 nd condenser and the objective Allows a controlled tilt & shift of illuminating for alignment and for dark field imaging

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

13. 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

14. Image quality is affected by: Accelerating voltage Working distance Spot size Wavelength

15. 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.

16. 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

17. Advantages of High Voltage EM Increased Specimen Penetration: As accelerating voltage increases the ability of the beam to penetrate also increases .

18. 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 m m thick section

19. Advantages of High Voltage EM Reduced Beam Damage: The increased speed of the electrons actually decreases the likelihood of an inelastic collision.