Transcript 1Introductiona

MRC SEM Workshop
Introduce the FESEM microscope
Familiarize and Train experienced persons
to use the Hitachi S4700 FESEM
Obtain a better understanding on how to
utilize the FESEM
Workshop Outline
Introduction to the FESEM
Sample Contamination
Low Voltage SEM
High Resolution Imaging
X-ray Microanalysis
Sample Preparation
MRC Lab Procedures
Operation of the S4700 FESEM
Hands-on labs and testing for users
Introduction to the FEGSEM
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 (130keV)
The FEG SEM 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 ....
Electron Sources
W hairpin - 50µm
diameter
LaB6 - 5µm
Thermal FEG - 250Å
Cold FEG - 50Å
Nano-FEG - 5Å
Nano tips - atom sized FEG
 Nano-tips are field emitters in
which the effective size of the
tip has shrunk to a single
atom.
 They can be made by
processing normal tungsten
FE tips or from Pt-Ir, or from
carbon nanotubes
 They have exciting properties
and may be part of upcoming
SEMS but now they are still
only a laboratory curiosity
Etched
tungsten
tip
Cut PtIr tip
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
vacuum required, lifetime, cost, 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
The physical size of the tip is
not necessarily the same as the
source size
Energy Spread
 Electrons leave guns
with an energy
spread that depends
on the cathode type
 Lens focus varies
with energy
(chromatic
aberration) so
energy spread spoils
high resolution, and
low energy, images
W hairpin 2.5eV
LaB6
1.0eV
Schottky
0.75eV
Cold FEG 0.35eV
Summary of Electron Guns
 The cold FEG offers the best performance parameters in all three
categories for most purposes
 FEGs are best for high resolution, and low voltage operation
 Thermionic emitters have advantages when very high beam
currents and large spot sizes are required.
Brightness
 At a typical imaging
current FEG SEM spot
size is set only by
lens quality
 Lower brightness
guns must use bigger
spots to give same
beam - this is
brightness limited
imaging
S4700 Resolution
Performance
 The best resolution
is always obtained at
the smallest working
distance (WD)
 ..but the minimum
WD value varies with
beam energy
 At the eucentric/EDS
WD of 12mm high
quality imaging +
analysis is possible
on the 4700
Sharpness, Contrast, Depth of
Field
These are dependent upon the three major
electron-beam parameters:
Electron Probe Size dp
Electron Probe Current
ip
Electron Probe convergence angle
ip
ap
dp
ap
Beam Performance
For the highest resolution beam diameter , dp , must
be as small as possible
For the best image quality and x-ray analysis,
emission current, ip, must be as large as possible
For the best depth of field convergence angle, ap,
must be as small as possible
When dp and ap are made small, ip is also reduced
Imaging modes
On the S4700 the convergence
angle a is set by the operating
mode of the microscope
No manual adjustment to the
condenser aperture strip is
required
Don’t change the aperture!
Gun behavior
The tip must be atomically clean to perform
properly as a field emitter
Even at 10-7 torr a monolayer of gas forms in
just 1 sec so the tip must be cleaned
periodically
It is cleaned by ‘flashing’ - heating the tip to
white heat for a few seconds. This burns off
(desorbs) the gas
Flashing
The flash condition is set up at the factory
Each tip should show a consistent emission
current when it is flashed
Compare the tip current with its own usual
value not with that from other tips
Excessive flashing may blunt the tip
Gas production
 The tip gets dirty...
 Gas molecules are
desorbed from 1st
anode by electrons
 Some of these stick
on the tip making it
less sharp
 This causes the
emission current to
fall over time
The life cycle of an FEG tip
Typical characteristics
The tip is usually covered with a mono- layer of gas
after 5-10 minutes of use
The emission then stabilizes for a period of from 2
hours (new machine) to 8 hours (mature machine).
On this S4700 the tip must be re-flashed after 92
hours of operation (the software gives a warning)
On the plateau, or stable, region the total noise + drift
is only a few percent over any period of a few
minutes
The secret of successful
Field Emission Microscopy
Run the tip for at least a few hours every day
even when the microscope is not otherwise
in use
This keeps the first anode - which is the main source
of gas - clean, reducing noise and drift.
Other care
Bake the system often - at least
every six months - and on public
holidays, long weekends
If tip noise is increasing and a
bake is not possible raise the
emission current to 20 or 30µA
with the beam in the ‘freeze’
position for a couple of hours or
more for Field Emission bombard
Drift in microanalysis
For normal EDS analysis drift is not a problem
For quantitative analysis using standards, and
for line scans and X-ray maps which take
significant time to record
Drift can be an issue, this is a reason to sue a
thermonic SEM for quantitative analysis
Hidden benefits of FEGSEM
Reliability and reproducibility - no need to
change tips or break vacuum. Control by
computer ensures reset table values
Ease of use - one button operation, memory
alignment settings
Longevity - with reasonable care time between
tip changes 3-5 years (even with students
operating)
We are on the NINTH year
What isn’t good?
 The range of beam
currents available is
limited when
compared to that
from a thermionic
emitter so an FEG is
not ideally suited to
such tasks as WDS
where high currents
are needed