Post on 15-Dec-2015
Low-Voltage Microscopy
When electron beams impinge on non-conducting samples a charge can build up which can make SEM imaging difficult or impossible
By operating at low beam energies this problem can often be minimized or eliminated
Charge balance
I bI b
I b
sc
Electrons cannot becreated or destroyedso currents at a pointmust sum to zero. Thecurrent flow to earth Isc
is the difference betweenthe in and out currents
If the sample is a conductor Isc can take any value (+ve or -ve) to achieve charge balance
Non-conductors
Sample can accumulate negative charges or positive charges
There can be a dynamic charge balance
For a non-conductor Isc is zero so charge accumulates
+ -
Complex materials
In the case of complex materials (e.g. layered) then the charge balance must be considered separately for each component
If a beam penetrates a layer then it will charge positively, a net electron emitter. substrate
SE BS
Imaging non-conductors
On a new SEM this will be the lowest available energy
On older machines you must decide how low to go before the performance becomes too poor to be useful for the purpose intended
Set the SEM to the lowest operating energy
Negative charging
-ve charging
E > E20
If the scan square is brighter than the background then the sample is charging negative
Positive Charging
+ve charging
E < E20
If the scan square is dark compared to the background then the sample is charging positive
Is that all there is to it?
No - charging is a complex phenomena and simply running the SEM at a low energy does not guarantee an image that is free from charge artifacts
To understand why we must look in more detail at what happens when a poorly conducting specimen is hit with an electron beam
Mechanisms for Charging
STATIC CHARGEStatic charging depends
on the net charge balance in the sample
DYNAMIC CHARGINGDynamic charging comes
from charge generated in the sample itself from electron-hole pairs
There is no global condition where this term is zero
Combining these two contributions we can synthesize a detailed model of the charging process
Charge Distribution The net amount of
negative charge injected = 1-. This is deep in the sample
The net charge that is emitted = and gives a positive region at the surface
Induced charge occurs throughout the interaction volume and could be of either sign
+ve
-ve
Incident beam Ib
Even at charge balance there is still stored charge and fields in the sample
Conductivity and Charging
The +/- charge separation produces a field which moves the induced carriers producing conductivity (EBIC)
Traps reduce the number of electrons. If the escape time from the traps is >> than the time between electron arrivals so the charge builds-up. A charged region is therefore like a leaky capacitor
EBIC is the key to dynamic charging effects
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dQ
Ramo s Theorem
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e k
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Area A
Surface Potential and Electric FieldsSurface Potential and Electric Fields
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-2.1E6
-1.6E6
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-9E5-8E5
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-5E5
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X-electric field distribution with EBIC
Cr
PMMA
Th
ickn
ess
(m
)Position (m)
The fields produced by even small amounts of charging are very high.
It is these fields which deflect the incident beam, push the secondary electrons around, move the electron-hole pairs and may even change the yield of electrons
This is seen as a drifting image
Monte Carlo calculation of fields in and above a resist sample
Minimizing dynamic charging
Reduce the beam current as the charging varies directly with IB
Change to Ultra-High resolution operating mode and lower the emission current
Reduces S/N
Dynamic Charging
Reduce the magnification
Dynamic charging depends on dose and on the magnification
Limits resolution by limiting magnification
Time dependent charging
Dynamic charging is time dependent because of the leaky capacitor effect (EBIC)
Scanning at a high speed extracts a signal before charging occurs
The whole scanned area now floats to a uniform potential allowing stable focussing and stigmation
Coating specimens
Coating should be as thin as possible, a good conductor, and a good emitter of SE
Au/Pd, Cr are good Carbon is bad (the
filler contaminates ) and the evaporator heats the sample
Coating is effective but may hide real surface detail
May be only route if high beam energy is required e.g for EDS
How coatings work
Coatings do not make the specimen conductive
They form a ground plane - eliminate fields due to charge
Increase SE yield - reduce charging
-----------------
-----------------
Charge in sample
Field deflectsincident and exit electrons
++++++++
coating
'image charge'+++++
metal is equipotential ground plane
NO EXTERNAL FIELDS
Charge in sample
Field deflects electrons
ground plane
Result of coating
Both Au-Pd and Cr effectively eliminate charging up to about 8keV
Even at higher beam energies charge-up is minimal
Thin coats do not affect EDS analysis
Other options
Heating the sample - effective for ceramics, oxides etc
Use a low pressure of a gas (VP-SEM mode or from a gas jet)
Low energy electron or ion flood beam to neutralize the charging
Use BSE detector for imaging- much less sensitive to charging
Try different SE detector, mixed or upper
Try high energy if sample is thin or on a substrate-depends on what you want to examine
Choice of detector
The choice of the detector that is used can be very significant in determining how seriously charging will appear to be
Try biasing the sample stage
Try mixing the detector signals, or switching to the lower detector if possible
S4700 TTL detector
This detector is very efficient and gives a symmetric view
These electrons are very sensitive to chemistry and to charging effects
High energy SE, BSE, and SE3 are excluded from the signal - this improves contrast
S4700 detectors
Lower (ET) and upper (TTL) detectors on S4700 have different characteristics
Lower (ET) detector accepts SE1,II and III as well as some BSE
Upper (TTL) detector accepts only low energy SE1 and 2
SE spectra
The upper detector accepts SE with energies around the peak of the SE spectrum. Peak position depends on amount of charge, chemistry, electronic structure, so these effects cause image contrast on TTL detector
Lower (ET) detector accepts everything below 50eV. Much less sensitive to charging
SE spectrum from Aluminum
Nonconducting samples
Latex paint at 1keV in Hitachi S4500
Uncoated, slow scan image at E2 energy
30kx original magnification
Lower (ET) detector for topography, reduces visibility of charging
Lower detector
Individual polymer macro- molecules on a silicon substrate imaged at 1.5keV
The lower detector shows little or no contrast
Upper detector
The upper detector easily reveals the macro- molecules
This is because they are charging negative and the TTL detector is highly sensitive to charging effects
Charging can be a useful form of contrast
Doping contrast
Chemical contrast in the SE mode
Sensitive to both P- and N-type dopants
Only visible on upper (TTL) detector
Boron doping in Si 1.5keV
Upper detector
Birds-beak dopant contrast in a device
S4500 at 1keVThis is a unique
imaging capability - 2 dimensional dopant profiling at high resolution and sensitivity (1ppm)
Damage at low energies
It is often stated that operation at low beam energies minimizes or eliminates beam induced damage
From casual observation this may appear to be true, but measurements show that the truth is just the opposite
Damage and beam energy
At high energies the damage rate is low
Damage rate rises as the energy is reduced, reaching a peak at about 100eV
At still lower energies the stopping power falls again
Experimental Stopping Power Data for Copper
Damage while scanning
If the beam is scanning then the rise in damage rate is less drastic but still considerable
however damage is confined to the near surface and not spread through a volume