Field-Emission SEM (FESEM) - MTEC · PDF fileField-Emission SEM (FESEM) Sometime called •...
Transcript of Field-Emission SEM (FESEM) - MTEC · PDF fileField-Emission SEM (FESEM) Sometime called •...
Field-Emission SEM (FESEM)
Supapan Seraphin
Department of Materials Science and Engineering
University of Arizona, Tucson, AZ85721, U.S.A.
Email: [email protected]
Field-Emission SEM (FESEM)Sometime called
• Field Emission Gun SEM (FEG-SEM )
• High resolution SEM
• Low-Voltage SEM
Hitachi S4800 Hitachi S5500Hitachi S8020
Why do we need FEG-SEMs?
• The FEG SEM offers high performance not just high resolution
• This means large probe currents (up to a few nanoamps), and small diameter electron probes(from 1 to 3nm), over a wide energy range (from 0.5 -30keV)
• The FEG SEM performance package involves both the gun and the probe forming lens
The Electron Gun
• The device which provides the electron beam is the called the ‘gun’
• This is the single most important component of the SEM because it determines the level of performance that can be achieved
• Electrons can be produced in several different ways ....
Thermionic Emitters
• Boil electrons over the top of the
energy barrier. T ~ 2800K
• The current density Jc depends on
the temperature and the cathode
work function - Richardson’s
equation…..
• Jc=AT2exp(-e/kT)
• Cheap to use, modest vacuum
required (W only)
• Can also use LaB6 which has a
better performance
but requires a higher
quality vacuum
work
function
eV
conduction band
vacuum level
thermionicelectronic
Thermionic
electrons
Schematic Model of
Thermionic Emission
Cold Field Emitters
• Electrons tunnel out from the metal because of the high field
• The field is obtained by using a sharp tip (100nm) and a high voltage
• Jc=AF2/.exp(-B1.5/ F)
• Note that the emission is temperature independent
• Needs UHV but gives long life and high performance
work
function
eV
conduction band
vacuum level
po
ten
tia
l
distance
barrier
FieldF V/cm
Care and Feeding of the FEG
• The field emitter tip is installed in a Butler-Crewe gun which is a high performance electrostatic lens
• V1 and V2 give independent control of the emission current and of the beam
energy
Vacc Vtip
Tip
Real Image
Vacc = beam energy
Vtip = extraction voltage
Gun behavior
• The tip must be atomically clean to perform properly as a field emitter
• Even at 10-7 torr (“one Langmuir”) a monolayer of gas is deposited in just 1 sec so the tip must be cleaned every time before it is used
• Cleaning is performed by ‘flashing’ - heating the tip to white heat for a few seconds. This burns off (desorbs) the gas
The new ‘FE’ - Schottky FEG
• This tip is not sharp enough
for direct field emission
• In the Schottky emitter the
field F is used to reduce the
work function -
= 3.80E-4 FeV
• Cathode behaves like a
thermionic emitter with
• The cathode is also enhanced
by adding ZrO2 to lower the
value of
• A Schottky gun is a field-
assisted thermionic emitter
work
function
eV
conduction band
vacuum level
po
ten
tia
l
distance
barrier
FieldF V/cm
ZrO dispenser
Comparing emitters
• The various types of electron emitters can be compared by looking at three parameters -brightness, source size, energy spread
• Other quantities are also important - e.g the vacuum required, the lifetime between tip changes, the cost of the vacuum system and tip replacement, and the expected mode of use of SEM
Source Size
• …is the apparent size of the disc from which the electrons come
• Small is good - for high resolution SEM less demagnification
• Big is sometimes good -e.g. for large probe sizes and high beam currents
• Tungsten hairpin -
50µm diameter
• LaB6 - 5µm
• Schottky FEG - 25nm
• Cold FEG - 5nm
• Nano-FEG - 0.5nm
The physical size of the tip is
not necessarily the same as
the source size
Emitter brightness
• Brightness is the most useful measure of gun performance
• Brightness varies linearly with energy (Langmuir’s equation) so we must compare different guns at the same beam energy to be fair
• High brightness is not the same as high current
• At 20keV typical brightness values (units -A/cm2/str)
4 W hairpin 105
4 LaB6 106
4 FEGs 108
4 nano-FEG 1010
Energy Spread
• Electrons leave guns with an energy spread that depends on the cathode type
• Lens focus varies with energy (chromatic aberration) so a high energy spread spoils high resolution, and low energy, images
• The energy spread of a W thermionic emitter is about 2.5eV, and 1eV for LaB6
• For field emitters the spread varies
0.7eV
0.3eV
1.5eV
Summary
• Field Emitters offers the best performance parameters in all three categories for most purposes
• Cold FEGs are best for the highest resolution, and lowest voltage operation
• Schottky field emitters have advantages when stability, reliability, very high beam currents, and large spot sizes are required.
• Nanotips may be the future ‘super source’
Resolution and Working Distance
• The best resolution is always obtained at the smallest working distance (WD)
• .. but the minimum WD value varies with beam energy
Why choose a low beam energy?
• At low energies the
electron range falls
from the micrometer
values found above
10keV to just a few
nanometers at energies
of 0.5keV
• This huge variation has
profound implications
on every aspect of
scanning microscopy
Range from modified Bethe
equation
Seeing is believing
• The sample is a 30nm film of carbon on a copper grid
• At 20keV the carbon film is transparent because it is penetrated by the beam.The SE signal comes from the carbon film but is produced by electrons backscattered from the copper
SE image of TEM grid 20keV
Electron range at low energy
• At 1keV - by comparison - the carbon appears solid and opaque because the beam does not penetrate through the film, and the copper grid is not visible at all
• The variation of beam range with energy is dramatic and has significant results on what we see in the low voltage SEM
Same area as before but 1keV beam
Some consequences of low energy operation
• The interaction volume decreases in size and
shrinks towards the surface
• Spatial resolution is improved in all image modes
• The SE yield rises significantly improving images
and as a result
Interaction volume
• The interaction volume falls
with beam energy E as about
1/E5
• The interaction volume no
longer samples the bulk of
the specimen but is now
restricted to the near-surface
regions only
• The information in the
signals produced is therefore
much more surface oriented
at low energies than at high
Monte Carlo simulations of
interactions in silicon
The best approach - try a wide range of energies and modes
SEM images of carbon nanotubes with an iron core
Spatial resolution…..
• At high energy the SE1 signal typically comes from a volume 3-5nm in diameter, but the SE2signal from a volume of 1-3µmin diameter
• High resolution contrast information is therefore diluted by the low spatial resolution SE2 background
SE2 come from the full width of
interaction volume
But at low energies…...
• ..the SE1 and SE2 electrons emerge from the same volume because of the reduction in the size of the interaction volume
• So SE1, SE2 and BSE images will all exhibit high resolution….
the interaction volume shrinks
Mode : Pure
SE Vacc. :
1.5kV
Mag. : x200k
Photo-resist (Bird’s eye view )
Low voltage BSE imaging
• BSE mode provides high resolution Z contrast, topographic detail, and provides freedom from charging artifacts
• Conventional BSE detectors are not good at low energies, and they require a long WD but the new ExB filter solves this problem
GaAs/GaAlAs quantum wells at 3keV
5nm wide
Alumina / Nickel
CompositeCourtesy of Associate Prof.. T. Sekino,
ISIR, Osaka Univ.
Pure SE
BSE-H
Composite Rich
SE+BSE-L
Mixed Signal Modes using ExB
Why the SE yield changes
• SE escape depth is ~ 3-5nm
• At high energies most SE are produced too deep to escape so the SE yield dis low
• But at lower energies the incident range is so small that most of the SE generated can escape so the SE yield rises rapidly
• At very low energies fewer SE are produced because less energy is available so the SE yield falls again
interaction volumes
low
voltagehigh
voltage
The high energy image
• The changes discussed above affect the form of the image
• At high energies we see the classic SEM ‘three dimensional’appearance
• Surface detail is revealed by topographic contrast
• Because the interaction volume is large features above the surface are highlighted
The LVSEM image
• The low voltage images appears much flatter and less three dimensional than the high voltage image
• This is because topographic contrast is reduced
• There is also no highlighting of features on the surface
• Greater visibility of surface marks and contamination
• Volume of interaction and image contrast will be most affected
• 3-D perception affected
• Surface Imperfections
(Joy & Joy, 1996, p. 251)
If all else fails…..coat the sample
• Coatings do not make the sample a conductor
• They form a ground plane - i.e. the free electrons in the metal move so as to eliminate the external field
• The charge is not eliminated but the disruptive field is removed
• Successful coating means paying attention to the details discussed earlier
Charge in
sample
Field deflects electrons
ground plane
Field lines do not leak away from the surface
Result of coating
• Thin films of either Au-Pd and Cr can effectively eliminate charging up to about 8keV
• Even at higher beam energies the charge-up potential is minimal
• Thin metal coats do not degrade EDS analysis - they improve it because they stabilize the beam landing energy
Experimental Charging Data from
Alumina (Sapphire)