1. Vacuum Technology zThe FEGSEM is only possible because some complex problems of vacuum engineering have been solved zSome 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 FEGSEM contains regions of each type laminar molecular

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

4. Ion Pumps zIonized molecules spiral in magnetic field and get buried in Ti wall coating zDiode pumps only handle some gases zTriodes pumps will handle most gases

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

6. Vacuum Hygiene zAlways keep vacuum systems running zUse LN 2 and fore-line traps if fitted zDon’t rough pump for too long zKeep fingers away from chamber zWear gloves when handling anything that will go into the sample chamber!

7. Contamination and Cleaning samples zTry not to use solvents as these are always contaminated, even when fresh from a glass container zNever use squeeze or spray bottles zCarbon Dioxide ‘snow’ cleaning may be worth investigating - no residue and good solvent action zUse a plasma cleaner or an Active Oxygen system Options available

8. Storing Samples zAs soon as a specimen is prepared for observation it begins to get dirty again zEven 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 zRemedial action is therefore required As prepared After one week

9. Plasma cleaning zPlasma 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 Intrinsic to electron beam irradiation Results from vacuum problems Both are usually important

11. Ionization Damage zOccurs 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. zMay also cause visible effects such as the formation of color centers zIn metals and semiconductors the conduction band electrons delocalize the excitation and prevent damage

12. Radiolysis zIonization damage is most important threat to organic, and some inorganic, materials. zElectrons 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 zDirect effect - destroys the crystalline structure of polymers, and other organic crystals, leaving them amorphous zProbability of radiolysis is 10x to 100x bigger than the chance of generating an X-ray zDamage competes with signal generation - damage usually wins

14. Heating zIs not usually a serious problem as the energy deposited is quite small. zFor a typical material of medium density and thermal diffusivity the temperature rise varies with energy, and beam dose Magnification 5keV15keV30keV 400x 0.1C/nA 0.24C/nA 0.56C/nA 4000x 0.15C/nA 0.34C/nA 0.79C/nA

15. Contamination - Etching zContamination 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. zEtching is removal of surface layer by impact of ions (C + OH - --> CO + H 2 ) zBoth effects are affected by surface charging and often go together zBoth 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 zAt low magnification the hydrocarbon film is polymerized into a thin sheet. zThis will charge positive (and look dark) but is not a serious problem

18. High magnification zAt high magnification the contamination grows a cone which prevents the beam reaching the surface zAvoid spot mode ! zTry and pre-pump samples before use zKeep your hands off the sample

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

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

21. Radiation doses zSEM dose is about 100 el/Å 2 zTypically at 1 - 10el/Å 2 loss of crystallinity at 10-100 el/Å 2 mass loss and above100 el/Å 2 limiting mass loss Dose for a single photo scan

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

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

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

25. Advantages of a Cold Finger zOrganic molecules tend to collect on the colder surface zReduced contamination zBetter light-element quantitative analysis

26. Vacuum and Contamination Summary zIzInsure proper vacuum zUzUse LN 2 and fore-line traps if fitted zRzReduce contamination of samples zPzProper sample preparation zUzUse cold finger when necessary