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
laminar
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
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 pump
Requires 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 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!
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
Same sample after plasma cleaning
Plasma cleaning
provides a rapid and
efficient way of
removing the buildup of surface
contaminants and
restoring the sample
to a pristine
condition
Unwanted Beam Interactions
Radiation Damage
Ionization
Displacement
Heating
Intrinsic to electron
beam irradiation
Contamination
Etching
Results from
vacuum 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 Hbomb
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 - 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
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 vacuum
Use LN2 and fore-line traps if fitted
Reduce contamination of samples
Proper sample preparation
Use cold finger when necessary