Source of electrons Thermionic un E: >10MV/cm T:~1500°c Field Emission Gun W Wehnelt Cylinder(grid) field emission tip first anode Filament electron traectone (5-50um) second anode Anode first cross-over(5nm) first, or spray, apertur W and Lab6 Cold- and thermal feg Electron Gun Properties Source Brightness Stability (%o) Size Energy spread Vacuum W 3X105 LaB6 B3×106 C-FEG 109 1251 50μm3.0(eV)105(τ) 5um 10-6 5nm 0.3 10-10 T-FEG 109 < 20nm 0.7 109 Brightness- beam current density per unit solid angle
Source of Electrons T: ~1500oC Thermionic Gun W and LaB6 Cold- and thermal FEG Electron Gun Properties Source Brightness Stability(%) Size Energy spread Vacuum W 3X105 ~1 50mm 3.0(eV) 10-5 (t ) LaB6 3x106 ~2 5mm 1.5 10-6 C-FEG 109 ~5 5nm 0.3 10-10 T-FEG 109 <1 20nm 0.7 10-9 (5-50mm) E: >10MV/cm (5nm) Filament W Brightness – beam current density per unit solid angle
Why Need a vacuum? When a sem is used the electron-optical column and sample chamber must always be at a vacuum 1. If the column is in a gas filled environment, electrons will be scattered by gas molecules which would lead to reduction of the beam intensity and stability. 2. Other gas molecules, which could come from the sample or the microscope itself could form compounds and condense on the sample. This would lower the contrast and obscure detail in the Image. http://virtual.itguiucedu/training/em_tutorialexterna http://www.youtube.com/watch?v=c7evtnvhn-satN4:50-5:25 http://www.mse.iastate.edu/research/laboratories/sem/microscopy/how-does-the sem-work/high-school/the-sem-vacuum/ SEM Vacuum
Why Need a Vacuum? When a SEM is used, the electron-optical column and sample chamber must always be at a vacuum. 1. If the column is in a gas filled environment, electrons will be scattered by gas molecules which would lead to reduction of the beam intensity and stability. 2. Other gas molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image. http://www.youtube.com/watch?v=c7EVTnVHN-s at ~4:50-5:25 http://www.mse.iastate.edu/research/laboratories/sem/microscopy/how-does-thesem-work/high-school/the-sem-vacuum/ SEM Vacuum http://virtual.itg.uiuc.edu/training/EM_tutorial external
Major components and their functions Magnetic Lenses Condenser lens - focusing controls the spot size and convergence (a) of the electron beam which impinges on the sample o Objective lens final probe forming determines the final spot size of the electron beam, i.e, the resolution of a SEM http://www.youtube.com/watch?v=irxmighanbgatw1:30-1:53 http://www.youtubecom/watch?v=vwxyszpttsiatn0:32-1:02 http://www.matter.org.uk_/tem/lenses/simulation_of_condensersystemhtm
Magnetic Lenses • Condenser lens – focusing controls the spot size and convergence () of the electron beam which impinges on the sample. • Objective lens – final probe forming determines the final spot size of the electron beam, i.e., the resolution of a SEM. http://www.youtube.com/watch?v=lrXMIghANbg at~1:30-1:53 http://www.youtube.com/watch?v=VWxYsZPtTsI at~0:32-1:02 http://www.matter.org.uk/tem/lenses/simulation_of_condenser_system.htm Major components and their functions
http://www.youtubecom/watch?v=g9glw3butaqMagneticfieldinasolenoid http://www.youtubecom/watch?v=a2wudbl-g8e-inamagneticfield https://www.youtube.com/watch?v=fwikris145etom1:15andatn3:08-4:15F=-evxb) How Is Electron beam Focused? http://www.matterorg.uk/tem/lenses/electromagnetic_lenses.htm A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution. Magnetic lens P t Crossover(Beam diameter) (solenoid) o Of A Single F=-e(v x B) Path Of Electrons In The Bea Spray diaphragm Fe Lens formula: 1/f=1/p+ 1/q B Q D Demagnification: M=q/p https://www.youtube.com/watch?v=scy X_ XQgnSA&feature=related 6: 13-6: 23 f o Bo 2 Magnetic lens f can be adjusted by changing Bor i e changing the current through coil. Bo-magnetic field
How Is Electron Beam Focused? A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution. p q Magnetic lens (solenoid) Lens formula: 1/f = 1/p + 1/q Demagnification: M = q/p (Beam diameter) F = -e(v x B) f Bo 2 f can be adjusted by changing Bo, i.e., changing the current through coil. Bo - magnetic field http://www.matter.org.uk/tem/lenses/electromagnetic_lenses.htm http://www.youtube.com/watch?v=3McFA40nP0A Magnetic deflection of e- beam at~0:20-1:50 http://www.youtube.com/watch?v=G9Glw3BUTAQ Magnetic field in a solenoid http://www.youtube.com/watch?v=a2_wUDBl-g8 e- in a magnetic field https://www.youtube.com/watch?v=fwiKRis145E to~1:15 and at~3:08-4:15 F=-e(vxB) https://www.youtube.com/watch?v=sCY X_XQgnSA&feature=related 6:13-6:23 Magnetic lens
Lens formula and magnification Objective lens Optical Focal center(o) Focal Object Point Point Focal O Distance Real Focal Focal mage Plane Bi-comvex lens Plane I1-Inverted Image 11 Lens formula f-focal length(distance) f 0i o-distance of object from lens Magnification m distance of image from y obJective lens http://www.youtubecom/watch?v=-k1nniozjfo&feature=relatedat3:00-3:40
1 1 1 _ = _ + _ f O i Lens Formula f-focal length (distance) O-distance of object from lens i-distance of image from lens I1 O i i O mo= = Magnification by objective Lens formula and magnification Objective lens -Inverted image f f ho hi hi ho http://micro.magnet.fsu.edu/primer/java/lenses/converginglenses/index.html http://www.youtube.com/watch?v=-k1NNIOzjFo&feature=related at~3:00-3:40