What is sEM? Working principles of SEM Major components and their functions How Scanning Electron Microscopes Work B How Sturio Electron Vacuum Gun Chamber Anode Condenser Lens Electrol Condenser Beam Lens Objective Chamber Backscatter Detector Detector Secondary Sample Detector
• What is SEM? • Working principles of SEM • Major components and their functions
How an electron beam is produced? o Electron guns are used to produce a fine, controlled beam of electrons which are then focused at the specimen surface. o The electron guns may either be thermionic gun or field-emission gun
How an Electron Beam is Produced? • Electron guns are used to produce a fine, controlled beam of electrons which are then focused at the specimen surface. • The electron guns may either be thermionic gun or field-emission gun
Electron beam source Single Crystal LaB6 La b6 Heating Cup Heat ing wire(w or Re) Support posts Lanthanum Hexaboride Single Crystal Filament Electron Beam Source W W or LaB6 Filament Thermionic or Field Emission Gun http://www.youtube.com/watch?v=fxevsnzt8l8atN2:10-2:20 http://www.youtube.com/watch?v=vwxyszpttsiatw1:05-1:40thermionicgun
Electron beam Source W or LaB6 Filament Thermionic or Field Emission Gun http://www.youtube.com/watch?v=VWxYsZPtTsI at ~1:05-1:40 thermionic gun http://www.youtube.com/watch?v=fxEVsnZT8L8 at ~2:10-2:20 w LaB6
ttp//www.matter.org.uk/tem/electron_gun/electronsources.htm Tungsten http://www.matter.orguk/tem/electron_gun/electrongun_simulation.htm (Cathode) Wehnelt Thermionic Emission Gun Cylinde A tungsten filament Anode heated by dc to Filament Heating Supply approximately 2700K or Filament LaBe rod heated to around 2000K Bias A vacuum of 10-3 Pa(10-4(cylinder) Resistor Pa for Lab) is needed to prevent oxidation of the Voltage filament Equipotentials Emission Electrons" boil off" from Current the tip of the filament Anod Plate Electrons are accelerated by an acceleration voltage of 1-50kV Beam Current http://www.youtube.com/watch?v=zijljilxdhyElectrongundetail
Thermionic Emission Gun • A tungsten filament heated by DC to approximately 2700K or LaB6 rod heated to around 2000K • A vacuum of 10-3 Pa (10-4 Pa for LaB6) is needed to prevent oxidation of the filament • Electrons “boil off” from the tip of the filament • Electrons are accelerated by an acceleration voltage of 1-50kV - + http://www.matter.org.uk/tem/electron_gun/electron_gun_simulation.htm http://www.matter.org.uk/tem/electron_gun/electron_sources.htm http://www.youtube.com/watch?v=ZIJ1jI1xDhY Electron gun detail
Field Emission Gun The tip of a tungsten needle is made very sharp(radius<0. 1 um) The electric field at the tip is very strong(> 10/ V/cm)due to the sharp point effect Electrons are pulled out from the tip by the strong electric field 150kv is Ultra-high vacuum(better than 10-6 Pa)is needed to avoid ion2nd bombardment to the tip fromAnode the residual gas Field-emission gun Electron probe diameter< 1 nm is possible http://www.matterorguk/tem/electron_gun/electron_sources.htm
Field Emission Gun • The tip of a tungsten needle is made very sharp (radius < 0.1 mm) • The electric field at the tip is very strong (> 107 V/cm) due to the sharp point effect • Electrons are pulled out from the tip by the strong electric field • Ultra-high vacuum (better than 10-6 Pa) is needed to avoid ion bombardment to the tip from the residual gas. • Electron probe diameter < 1 nm is possible http://www.matter.org.uk/tem/electron_gun/electron_sources.htm