Field-Emission SEM (FESEM)

Supapan Seraphin

Department of Materials Science and Engineering

University of Arizona, Tucson, AZ85721, U.S.A.

Email: seraphin@email.arizona.edu

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)