Quantrainx50 Module 3.1 Electron Optics

1. place photo here Quantrainx50 Module 3.1 Electron Optics 1-2011 Confidential

2. SEM Main Components Wehnelt cylinder or FEG unit Electron Gun Demagnificationsystem Condenser lenses Scan generator Scan Unit Scan generator Focus Unit Objective and Stigmation lenses Detecting Unit Electron detector

3. SEM Main Components Electron Gun Wehneltcylinder FEG Electron Gun

4. Electron Gun Emitters • Tungsten filament (W) • Lanthanum Hexaboride filament (LaB6)(obsolete) • Cerium Hexaboride (CeB6) • Field emission filament (FEG)

5. Electron Gun Animation * * Video courtesy of Oxford Instruments

6. ą ip do specimen Electron Source Properties • Current density (brightness) • Emission current • Stability of source • Lifetime of filament • Design of electron source assembly • Ease of operation • Costs involved

7. Emission Area For Tungsten (W) Filament heating supply Filament Wehnelt cap Cross-over plane 70 A Anode High voltage supply (200 v- 30 kV)

8. Bias on Wehnelt Cap Optimum bias voltage Low bias voltage High bias voltage Equipotential lines of the Voltage Field 0 0 0 + + + Noemission High emission large spot Sufficient emission small spot

9. Emission : Autobias control Bias 255 ……………………………….. Bias 1 110µA 90µA 1 kV30 kV Autobias keeps emission between 90-110 µA for all kV 9

10. W Filament Saturation Saturation point emission current False peak / Misalignment filament current

11. Tungsten Filament

12. High Resolution, High Brightness FEG source…

13. XL Schottky FEG Theory o o o o o o o o o o • The Boersch Effect • A) Perfect beam: no interactions • B) Random beam: one dimension • C) Random beam: two dimensions • It is actually three dimensional o o o o o o o o o o o o o o o o o o A B C

14. XL Schottky FEG Theory • The Lateral Effect • lateral trajectory displacement • This effect results in a larger final spot • The diameter of the circle of confusion due to this effect. o o o o o o o o oo o o

15. Lens Defects optical axis Aperture image plane Spherical aberration Chromatic aberration Diffraction

16. Spherical Aberrations • Electrons entering into a lens at different points get focused at different points Disc of Least Confusion

17. Chromatic Aberrations • Electrons of differing energies will be focused at different places Disc of Least Confusion

18. Diffraction • The wave nature of electrons cause diffraction limitations

19. XL Schottky FEG Theory • Design Limitations • Longer electron-electron interaction times and smaller electron-electron distances lead to higher statistical aberrations at low KV • Chromatic aberration is more dominant at low voltages.

20. XL Schottky FEG Theory • Innovative solutions to reduce design limitations • A Coulomb tube designed into the column to reduce aberrations and interactions by keeping a high beam energy in the tube • Effective aperturing of the beam to remove those electrons not contributing to the probe

21. FEG Column Principle Diagram C1 10KV Drift Space (Coulomb Tube) Gun Alignment Coils C2 Objective Aperture Scan Coils Objective Lens

22. FEG gun (electron source)

23. Emission Area for FEG Filament heating supply Extractor system C1 static lens 150 A Anode High voltage supply (200 v- 30 KV)

24. Schottky Gun Design • Fil = Filament current input (2.4 Ampere) • S = Suppressor (-500V) • E = Extractor (+5000V) • C1 = Electrostatic Condenser lens Fil S E E C1

25. Schottky Tip design • M = Tip module • W = Welded tungsten Tip • Fil = Tungsten wire filament • T = Sharpened Tip • Zr = Zirconium reservoir M T W Fil Zr

26. FEG Startup Steps • Warmstart / Coldstart • Gun conditioning

27. Beam Menu Final operation status

28. FEG Column Double condenser lens • Extraction voltage changes not necessary, beam current is set by condenser lenses • C1 is electrostatic • C2 is electromagnetic • Variable lens strengths: A = high beam current mode B = low beam current mode • Final beam energy 30keV down to 200eV C1 C2 A B

29. FEG Column Double Condenser Lens • Extraction voltage changes not necessary, beam current is set by condenser lenses • C1 is electrostatic • C2 is electromagnetic • Variable lens strengths: A = high beam current mode B = low beam current mode • Final beam energy 30keV down to 200eV C1 Internal Spray Aperture C2 A B

30. FEG Column • Different paths for low and high beam current conditions through the coulomb tube, but common path to objective C1 C2 Deceleration Lens Aperture Small Spot Large Spot

31. Comparison of Columns(20KV)

32. Probe Current for FEG

33. FEG Spot Size (nM)

34. SEM Main Components Electron Gun Wehnelt cylinder Demagnification system Condenser lenses Demagnificationsystem Scan Unit Scan generator

35. L L Magnification M=L/l l ***-important

36. Scan Size Vs. Magnification • Low Mag. • Med Mag. • Hi Mag.

37. Magnifying Your Sample on Quantax50 l L L _ M= l

38. Low Magnification Scan Here Display Here

39. Intermediate Magnification Scan Here Display Here

40. Higher Magnification Scan Here Display Here

41. The viewed area (L) is fixed The smaller the area scanned on the sample results in higher viewed magnification Scan Size Vs. Magnification

42. A Focused Vs. An Unfocused Beam

43. The Crossover point on the Beam is of a Finite Size I = BeamCurrent D=Spot Size ą = Measurement of the ‘cone’

44. Current Density • Current Density remains constant through the optical path of the electron beam 4 X IAmps β = π 2 a ( ) Cm Steradians X d o X

45. Current Density (remove constants) • Current and Spot size are directly proportional IAmps β = 2 ( ) Cm d o

46. Resolution The resolution of the microscope is a measure of the smallest separation that can be distinguished in the image unresolved resolved

47. The Diameter of the Electron Beam Must Be Smaller Than the Feature to Be Resolved

48. The Electron Beam Scans From Left to Right • There can be from 512 to 4096 scan lines, at all magnifications

49. The Electron Beam Spot Size Must Be Smaller Than the Features Being Resolved • The ideal spot size

50. Too Large of Spot Size Looks Out of Focus • Too big of spot size creates an out of focus image