Opt 307/407 Practical Scanning Electron Microscopy

1. Opt 307/407 Practical Scanning Electron Microscopy Considerations in any microscopy: Resolution Magnification Depth of field Secondary information

2. Limits of Resolution (resolving power) Unaided eye: 0.1mm Light microscope: 0.2um SEM: 1nm TEM: 0.2nm

3. Evolution of Resolution

5. Depth of Field

6. Light Microscope vs Electron Microscope

7. General Diagram of the SEM System

8. Light Microscopy vs Electron Microscopy Advantages of EM: Resolution Magnification Depth of field Disadvantages of EM: Pricey Better if conductive (SEM) Maintenance Vacuum

10. Why do we need a vacuum anyway? Electrons are scattered by gas (or any other) molecules MFP at 1atm ~ 10cm MFP at 10-5T ~ 4m Some samples react with gases (O2) Helps keep things clean! Opt 307/407Vacuum Systems

11. Terminology Pressure Units: atm, bar, mbar Torr (mm of Hg) Pa (N/m2) 1atm=1Bar=1000mBar=760Torr=105Pa Pumping speed l/min, l/sec Opt 307/407Vacuum Systems

12. Opt 307/407Vacuum Systems

13. Quality of Vacuum Low: 760-10-2 Torr Medium: 10-2-10-5 Torr High: 10-5-10-8 Torr Ultrahigh: <~10-8 Torr Opt 307/407Vacuum Systems

14. Measuring Vacuum in EM Systems Thermocouple Gauge Pirani Gauge Cold cathode Gauge Penning Gauge Ion pump current Opt 307/407Vacuum Systems

15. Very Broad Range of Vacuum to Measure

16. Grouped Ranges for Vacuum Gauges

17. Vacuum Gauge Choices and Working Ranges

18. Thermocouple/Pirani Gauges

19. Ionization Gauges

20. Ion Gauge Collection

21. Hot Cathode Ion Gauge

22. Penning gauge

23. Penning gauge

24. Opt 307/407Vacuum Systems

25. Types of Vacuum Pumps 1- Rotary (Fore, Rough, Aux, Mechanical) 2- Turbomolecular (Turbo) 3- Diffusion (Diff) 4- Ion (Sputter-ion) Opt 307/407Vacuum Systems

26. Rotary Pump Basics Always in the Foreline of the system Exhausts pumped gases to atmosphere Pumping rate decreases as vacuum increases Usually has a low VP oil as a sealant to facilitate pumping Opt 307/407Vacuum Systems

27. Opt 307/407Vacuum Systems

28. Opt 307/407Vacuum Systems

29. Rotary Pump Problems Cannot pump <10-2 Torr Noisy Backstreams Vibration Maintenance Opt 307/407Vacuum Systems

30. Turbo Pump Basics Direct drive electric motor-gas turbine Rotor/stator assembly Moves gas molecules through the assembly by sweeping them from one to another High rotational speed (>10,000 RPM) Very clean final vacuum Opt 307/407Vacuum Systems

31. Turbo Pump Problems Needs a Foreline pump Costly Can fail abruptly Whine Needs to be protected from solid material Opt 307/407Vacuum Systems

32. Diffusion Pump Basics No moving parts Heated oil bath and condensing chamber Jet assembly to redirect condensing gas Recycle of oil Pressure gradient in condensing chamber/Foreline pump removes from high pressure side Opt 307/407Vacuum Systems

33. Opt 307/407Vacuum Systems

34. Diffusion pump problems Heat up/cool down time Needs foreline pump Can make a mess in vacuum failures/overheating Needs cooling water (usually) Opt 307/407Vacuum Systems

35. Ion Pump Basics High voltage creates electron flux Ionizes gas molecules Ions swept to titanium pole by magnetic field Titanium erodes (sputters) as ions become embedded Getters collect Ti atoms and more gas ions Current flow indicates gas pressure (vacuum) Opt 307/407Vacuum Systems

36. Opt 307/407Vacuum Systems

37. Ion Pump Problems Cannot work until pressure is <10-5 Torr Low capacity storage-type pump Needs periodic bake-out Hard to startup (sometimes) Opt 307/407Vacuum Systems

38. Summary All electron microscopes require a vacuum system. Usually consists of rotary-(turbo, diff)-(ion) pumps. System should provide clean oil-free vacuum at least 10-5 Torr or so. Vacuum is usually measured with a combination of TC and ion gauges. Vacuum problems are some of the most challenging to find and fix, and may even be caused by samples outgassing Opt 307/407Vacuum Systems

39. Opt 307/407Vacuum Systems

40. Opt 307/407Vacuum Systems Typical TEM Vacuum System

42. Electron Sources and Lenses Opt307/407

43. Types of Electron Sources Thermionic Sources Tungsten filament Lanthanum Hexaboride (LaB6) filament CeB6 Field Emission sources Cold Schottky

44. Ideal Electron Source Characteristics Low “work function” material so that it is easy to remove electrons from the material High melting point Chemically and physically stable at high temps Low vapor pressure Rugged Cheap

45. Thermionic Emission of Electrons Filament material is heated with an electrical current so that the “work function” of the material is exceeded and the electrons are allowed to leave the outermost orbital. Generates a fairly broad source of electrons (cloud)

46. Tungsten Hairpin Filaments Most common of all filaments in electron guns Low cost (~$20) Lots of beam current Not very intense illumination Emission temperature ~2700K Work function= 4.5ev Can last about 100 hours

47. Tungsten Hairpin Filament Saturation

48. Tungsten Hairpin Filament

49. LaB6 (and CeB6) Filaments Lower work function thermionic source (2.4ev) Lots brighter (~50x) than W-hairpin Relatively costly (~$700) Can be direct replacement for W-hairpin Heated to about 1700K Can last hundreds of hours

50. LaB6 Emitter Problems Need higher vacuum to reduce reactivity More difficult to make Heating/cooling must be slow (brittle material) Heating is indirect through a graphite well