Post on 05-Jan-2016
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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
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
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
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
Ion pump performance
“The” UHV pumpRequires no backing
- works best in a closed system
Requires periodic bake-out into rough pumped system to clean the pump
Vacuum Hygiene
Always keep vacuum systems runningUse LN2 and fore-line traps if fittedDon’t rough pump for too longKeep fingers away from chamberWear gloves when handling anything
that will go into the sample chamber!
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
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
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
Unwanted Beam Interactions
Radiation Damage
IonizationDisplacementHeating
ContaminationEtching
Intrinsic to electron beam irradiation
Results fromvacuum problems
Both are usually important
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
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
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
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
400x 0.1C/nA 0.24C/nA 0.56C/nA
4000x 0.15C/nA 0.34C/nA 0.79C/nA
Contamination - EtchingContamination 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
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
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
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
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
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
Radiation doses
SEM dose is about 100 el/Å2
Typically at 1 -10el/Å2 loss of crystallinity
at 10-100 el/Å2 mass loss
and above100 el/Å2 limiting mass loss
Dose for a single photo scan
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.
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”
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
Advantages of a Cold Finger
Organic molecules tend to collect on the colder surface
Reduced contamination
Better light-element quantitative analysis
Vacuum and Contamination Summary
Insure proper vacuumUse LN2 and fore-line traps if fittedReduce contamination of samplesProper sample preparationUse cold finger when necessary