Vacuum Technology

1. Vacuum Technology • The FEGSEM is only possible because some complex problems of vacuum engineering have been solved • Some basic knowledge of vacuum technology is useful in getting the best from the machine and maintaining the vacuum integrity

2. Qualitative Vacuum Ranges Low or rough vacuum 760 to 1 Torr Medium vacuum 1 to 10-3 Torr High vacuum 10-3 to 10-6 Torr Very high vacuum 10-6 to 10-9 Torr Ultra-high vacuum 10-9 and lower laminar molecular FEGSEM contains regions of each type

3. Vacuum pumps • For each of the vacuum ranges identified there is one or more type of pump that is best suited • Pumps are always used in combination with one pump used to start the next • The sequencing of the pump down is important. Now under computer control - do not try to do this by hand

4. Ion Pumps • Ionized molecules spiral in magnetic field and get buried in Ti wall coating • Diode pumps only handle some gases • Triodes pumps will handle most gases

5. Ion pump performance • “The” UHV pump • Requires no backing - works best in a closed system • Requires periodic bake-out into rough pumped system to clean the pump

6. Vacuum Hygiene • Always keep vacuum systems running • Use LN2 and fore-line traps if fitted • Don’t rough pump for too long • Keep fingers away from chamber • Wear gloves when handling anything that will go into the sample chamber!

7. Contamination and Cleaning samples • Try not to use solvents as these are always contaminated, even when fresh from a glass container • Never use squeeze or spray bottles • Carbon Dioxide ‘snow’ cleaning may be worth investigating - no residue and good solvent action • Use a plasma cleaner or an Active Oxygen system Options available

8. Storing Samples • As soon as a specimen is prepared for observation it begins to get dirty again • Even storing the sample in a vacuum dessicator will not prevent the growth of surface contaminant films because the source of the problem is carried in by the specimen itself • Remedial action is therefore required As prepared After one week

9. Plasma cleaning • Plasma cleaning provides a rapid and efficient way of removing the build-up of surface contaminants and restoring the sample to a pristine condition Same sample after plasma cleaning

10. Unwanted Beam Interactions Radiation Damage Ionization Displacement Heating Contamination Etching Results from vacuum problems Intrinsic to electron beam irradiation Both are usually important

11. Ionization Damage • Occurs when the beam generates high energy excitations lasting long enough for relaxation of ion cores to occur. This causes a bonding instability and the structure falls apart. • May also cause visible effects such as the formation of color centers • In metals and semiconductors the conduction band electrons delocalize the excitation and prevent damage

12. Radiolysis • Ionization damage is most important threat to organic, and some inorganic, materials. • Electrons are the most intense source of ionizing radiation available - the typical dose in an SEM is equivalent to standing 6 foot from a 10 megaton H-bomb Compare SEM to Sun and SPEAR* *Stanford Positron Electron Accelerating Ring

13. Effects of radiolysis • Direct effect - destroys the crystalline structure of polymers, and other organic crystals, leaving them amorphous • Probability of radiolysis is 10x to 100x bigger than the chance of generating an X-ray • Damage competes with signal generation - damage usually wins

14. Heating • Is not usually a serious problem as the energy deposited is quite small. • For a typical material of medium density and thermal diffusivity the temperature rise varies with energy, and beam dose Magnification 5keV 15keV 30keV 400x0.1C/nA 0.24C/nA 0.56C/nA 4000x0.15C/nA 0.34C/nA 0.79C/nA

15. Contamination - Etching • Contamination is beam induced polymerization of hydrocarbons on the sample surface. The organic molecules come from the oil vapors of the vacuum pumps and the outgassing of any organic material present in the instrument. • Etching is removal of surface layer by impact of ions (C + OH - --> CO + H2 ) • Both effects are affected by surface charging and often go together • Both are changed by temperature

16. Contamination and Etching Electrons break down contamination film. The residue charges +ve and the field pulls in other contaminant. If water vapor is present then OH- ions go to the + ve charge region and etch that area away

17. Low magnification • At low magnification the hydrocarbon film is polymerized into a thin sheet. • This will charge positive (and look dark) but is not a serious problem

18. High magnification • At high magnification the contamination grows a cone which prevents the beam reaching the surface • Avoid spot mode ! • Try and pre-pump samples before use • Keep your hands off the sample

19. Cones • Contamination cones can grow to a height of hundreds of angstroms and are very tough • Prevent growth by irradiating area at low magnification before going to a high magnification

20. Beam currents • The beam currents and current densities available in an FEG SEM are high even for small probe sizes • This can cause problems on radiation sensitive samples such as organic materials and biological tissue • Always try to minimize the radiation dose

21. Radiation doses • SEM dose is about 100 el/Å2 • Typically at 1 -10el/Å2loss of crystallinity at 10-100 el/Å2 mass loss and above100 el/Å2limiting mass loss Dose for a single photo scan

22. Temperature effects • Altering both the temperature of the sample and its surroundings will switch contamination to etching as the temperature falls • This is because water vapor condenses on sample.

23. Temperature Effects II • Holding the sample at RT but placing a cold surface close to it can dramatically reduce the contamination rate • Such a device is usually called a “Cold Finger”

24. The Cold Finger • The finger is held at LN2 temperatures, very close to the specimen surface • After filling the cold finger allow the sample enough time to reach thermal equilibrium before starting to image

25. Advantages of a Cold Finger • Organic molecules tend to collect on the colder surface • Reduced contamination • Better light-element quantitative analysis

26. Vacuum and Contamination Summary • Insure proper vacuum • Use LN2 and fore-line traps if fitted • Reduce contamination of samples • Proper sample preparation • Use cold finger when necessary