Low-Voltage Microscopy
When electron beams impinge on nonconducting 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
Electrons cannot be
created or destroyed
so currents at a point
must sum to zero. The
current flow to earth Isc
is the difference between
the in and out currents
Ib
Ib
Ib
If the sample is a conductor Isc can take any value
(+ve or -ve) to achieve charge balance
 sc
Non-conductors
For a non-conductor Isc is zero so charge accumulates
Sample can accumulate negative charges or positive
charges
There can be a dynamic charge balance
+
-
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.
SE
BS
substrate
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 0> E2
If the scan square is
brighter than the
background then the
sample is charging
negative
Positive Charging
+ve charging
E 0< E2
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 CHARGE
Static charging depends on
the net charge balance in
the sample
DYNAMIC CHARGING
Dynamic 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
Incident beam Ib
+ve
-ve
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 buildsup. A charged region is
therefore like a leaky
capacitor
 EBIC is the key to dynamic
charging effects
dQ
L
Charge e
Vbias
Area A
Ramo' s Theorem
V
V
so work done  e dx  VdQ
L
L
dQ
V
Thus I 
 e . velocity
dt
L
V
V
If velocity  k . field  k
then by Ohm' s law R  and
L
I
L. A
resistivity  
e. k
force  e.
Surface Potential and Electric Fields
The fields produced by
even small amounts of
charging are very high.
This is seen as a drifting
image
-1.0
-0.8
2E5
-0.6
0
-1E5
3E5
4E5
-0.4
Thickness (m)
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
X-electric field distribution with EBIC
-3E5
6E5
-0.2
0.0
8.4E5
9E5
9.6E5
1E6
-1E6
-1.5E6
1.5E6
-2.5E6
1.6E6
-1.6E6 -1.1E6
9.2E5 1.2E6
-4E6
2.4E6
1.8E6
-2.2E6
-8E5
1E6
-3.2E6
-1.2E6
1.3E6 1.5E6
-9E5
1.3E68.8E5 -1.3E6
1.1E6
0.2
PMMA
0.4
0.6
-5E5
8E5
1.4E6
1.7E6 -3.9E6
1.4E6
-2.1E6
1.4E6
8E5
7E5
Cr
-7E5
-6E5
5E5
-4E5
0.8
-2E5
9.7E4
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Position (m)
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
Field deflects electrons
Field deflects
incident and
exit electrons
----- ----------
'image charge'
NO EXTERNAL FIELDS
++++
+++++
++++
coating
----- ----------
ground plane
Charge in
Charge in
sample sample
metal is equipotentia
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 substratedepends 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 Pand 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 1keV
This 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