An Introduction to the Environmental SEM and the Variable Pressure SEM
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Transcript An Introduction to the Environmental SEM and the Variable Pressure SEM
Lehigh Microscopy School 2006
An Introduction to the Environmental SEM and the
Variable Pressure SEM
John Mansfield
North Campus Electron Microbeam Analysis Laboratory
University of Michigan
417 SRB, 2455 Hayward
Ann Arbor MI 48109-2143
Phone: (313)936-3352 FAX (313)763-2282
[email protected]
http://emalwww.engin.umich.edu/people/jfmjfm/
Lehigh Microscopy School 2006
An Introduction to the Environmental SEM and the
Variable Pressure SEM
These instruments rely on relatively
Raven’s feather
new technology but have now come
to account for over 50% of the SEM
market.
Although the instruments offer
several significant advantages for
the analysis of classically “difficult”
samples, because of their unique
modes of operation, there are a
number of problems that arise from
them that must be taken into
account.
This presentation will introduce environmental SEMs and variable
pressure SEMs, discuss their differences, illustrate a number of
applications of the instruments, identify the problems that arise from
their use and the methods of overcoming these problems.
Lehigh Microscopy School 2006
What is an Environmental SEM?
A Environmental Scanning Electron Microscope is an instrument that
allows one to perform scanning electron microscopy in the
controllable presence of a significant residual gas pressure.
Any manufacturer can market an environmental scanning electron
microscope or however, the acronym ESEM, is a trademark owned by FEI,
Inc.
Hence other manufacturers have been quite inventive with alternate names:
Variable Pressure SEM, Controlled Pressure SEM, Wet SEM, Nature SEM,
EVO eXtended Variable Pressure SEM, etc.
A number of users have suggested a number of possible generic names
have been suggested: Leaky Vacuum SEM, Poor Vacuum SEM, Crappy
Vacuum SEM and Lousy Vacuum SEM, unfortunately most of them resolve
into acronyms that are already in common use.
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Comparisons of various instruments
QuickTime™ and a
TIFF (Unc ompressed) decompres sor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompress ed) dec ompres sor
are needed to s ee this pic ture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
FEI
Zeiss
JEOL
Hitachi
Quanta 200 W ESEM
10-6-20Torr
GSED LFSED, BSE
3.5nm @ 30kV 5 Torr
(FEG 2.0nm)
Evo ®XVP®SEM
10-6-22.5Torr
VPSED,BSE
4.5nm @ 30kV 5Torr
6460LV SEM
10-6-2Torr
BSE
4.0nm @ 30kV 2Torr
S3500-N SEM
10-6-2Torr
BSE
4.5nm @ 25kV 2Torr
Compiled from manufacturers website (with a little guesswork) on 9-June-2004
Lehigh Microscopy School 2006
History of “Wet” SEMs
Environmental Scanning Electron Microscopy is not that new!
1960 Thin film wet cell - R.F.M. Thornley (during initial SEM
development).
1970 Substage with controlled leakage of water vapor. Gave first
SEM images of liquid water droplets. Gas amplification was also
observed - W. C. Lane.
1973 Modified JSM2 with differential aperture allowing pressures
above one Torr - V.N.E Robinson.
1978 First commercial instruments. The Wet SEM from ISI and the
GEOSEM from JEOL.
1983 Gas amplification secondary electron detector - G. Danilatos.
1987 ESEM introduced by Electroscan.
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Gun
Simple Schematic CrossSection of a Conventional
SEM
Condenser
lens
Condenser
lens
Beam defining aperture
(in the principal plane of
the lens)
Objective lens
Everhart-Thornley Detector
Pressure
gauge
control
valve
closed
Sample
Chamber
Chamber at high vacuum
(~10-6 Torr)
High Vacuum Pump
(turbomolecular or diffusion)
Rotary pump
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Gun
Cross-Section of a
VP-SEM
Condenser
lens
Condenser
lens
Beam defining aperture
(in the principal plane
of the lens)
Everhart-Thornley Detector
(now inoperative)
Objective lens
Backscatter
Detector
Pressure
gauge
control
valve
gas flow
Sample
Chamber
Chamber at low pressure
(0.1 to 2 Torr)
Isolation Valve
High Vacuum Pump
(turbomolecular or diffusion)
Rotary pump
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Gun
Chamber
Ion Pump
G1
DIF
V2
Cross-Section
of an ESEM
Valve
Gauge
G2
1
RP1
G4
V12
VENT V3
DIF
G3
V4
V5
2
V1
G5
V6
EC1
EC2
G7
V7 V13
Older model, E3, but
principle remains the
same in the newer
instruments
RP2
V8
RP3
regulator V10
valve
Specimen Chamber
V9
Auxilary Gas
Water Vapor
V11
Vent
(Drawing courtesy FEI)
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Cross-Section
of an ESEM
FEI Quanta ESEM
(Drawing courtesy FEI)
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A Note About Vacuum Units
There are a wide variety of units for pressure measurement.
The three most common are probably Torr, Pascal and bar.
1 Bar = 1 Atmosphere = 760mm Hg = 760 Torr = 101,080 Pascal
1Torr = 1/760th (1.3 x 10-3) of an Atmosphere/Bar = 133 Pascal
1 Pascal = 7.5 x 10-3 Torr = 9.89 x 10-6 Atmosphere/Bar
1mBar = 0.76 Torr = 101 Pascal
Pascal (Pa.) is the official System Internationale unit and is becoming more
common, although many of us find the Torr habit hard to break!
Lehigh Microscopy School 2006
Pressure above the
pressure limiting
aperture ~ 10-4 Pa
(7.5 x 10-7Torr) or
better
Differential Pumping - VPSEM
Primary
beam
Pressure limiting
aperture
Objective lens
BSED Light guide
Pressure below the pressure
limiting aperture ~ 1-270 Pa
(7.5 x 10-3-2 Torr)
10mm
Sample
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Differential Pumping - ESEM
Primary
beam
Pressure
Limiting
Aperture
Pressure above the
pressure limiting
aperture
1mm ~ 10-4 Pa
(7.5 x 10-7Torr) or
better
Secondary electron
detector ring
Pressure below the pressure
limiting aperture ~ 1-3000 Pa
(7.5 x 10-3-22.5 Torr)
Sample
(Drawing Courtesy of FEI).
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Gas in Low Vacuum SEMs
1. Scatters and broadens the probe.
2. Reduces the current available for imaging and analysis.
3. Modifies the effect of the beam-sample interaction by adding secondary
charge.
4. Produces a mechanism for secondary electron signal collection.
Are these features a GOOD thing or a BAD thing?
Consider 1 & 2 and then 3 & 4.
The variable pressure / environmental SEM
FEI
Zeiss
Hitachi
JEOL
Beam scattering (from Philips Electron Optics (1996))
Total scattering cross section (sT ) of
monotonic (argon (Ar)), diatomic (nitrogen
(N2)) and polyatomic (water vapour (H2O))
gases versus primary electron beam energy
("PE) (Danilatos =1988, Jost & Kessler 1963).
(gaseous) secondary electrons (GSEI)
the imaged SE signal is a function of:
• gas pressure
• gas type
• gas path length (?working distance)
• detector design
• detector position
• detector bias voltage
• widget (positive ion sink)
• accelerating voltage
• THE SAMPLE
- new designs/approaches under development
Gas Gain Curves dependent of Vgsed, L, gas type , P & Eo
FEI - XL30 ESEM
20kV 10mm
120
275V
330V
415V
440V
100
80
60
I = 72pA
40
10mm
15mm
20mm
30mm
80
Specimen Current (nA)
Specimen Current (nA)
140
20
60
40
20
FEI - XL30 ESEM 20kV
I = 72pA
Vgsed = 385V
0
0
0
1
2
3
4
5
6
7
8
0
1
Water Vapour Pressure (torr)
2
3
4
5
6
7
8
Water Vapour Pressure (torr)
90
50
20kV
40
10 mm
H2 O
80
Argon
70
Specimen Current (nA)
Specimen Current (nA)
60
IB= 300pA
195V
30
20
10
10kV
20kV
30kV
60
10kV 38pA
20kV 72pA
50
40
30kV 125pA
30
20
10
0
0
1
2
3
4
5
6
7
8
Pressure (torr)
-1
0
1
2
3
4
5
6
Water Vapour Pressure (torr)
- courtesy of Matthew Phillips
7
8
‘Secondary electron’ detection strategies
FEI - GSED
SIGNAL
500v
‘Danilatos’
biased
detector
‘ground’
ZEISS VPSE
SIGNAL
Luminescence
detector
500v
‘Farley-Shah’
specimen
current detector
‘ground’
SIGNAL
JEOL HITACHI
OVERVIEW - SE imaging
• secondary electron imaging in VPSEM and ESEM is
an indirect, multi-stage process
• a bias is used to enhance secondary electron chamber gas interaction for signal generation and
charge neutralisation
• ‘tertiary (gaseous)’ electrons, fields and gas
luminescence contribute to the image-forming signal
• positive ions interfere with the signal-forming
processes - above and ?on the sample
• the ‘secondary’ electron signal is a complex mix
affected by instrument and sample attributes
SE imaging - uncoated & low kV
an unlucky dragonfly
(wingspan = 50 mm)
- gas luminescence VPSE detector
SE imaging - uncoated & lowish kV
a nanolithography template
(100x100x8 mm, 10 nm Cr on SiO2)
LW = 30 +/- 6 nm
- gas luminescence VPSE detector
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Mean Free Paths in Gases
Consider a 25keV beam propagating in oxygen and look
at scattering events that deviate the electrons 1 degree.
Pressure (Torr) Pa
Mean Free Path
10-6
10-4
10km
1
133
1.4cm
10
1330
1.4mm
50
6650
0.28mm
What effect does this have in the microscope?
Slide courtesy David Joy
Lehigh Microscopy School 2006
Ib
Analytical Expression for the Gas Scattering
n
Variation of n and beam
current (Ib) for electrons in air
at 25keV
The average number of collisions n
suffered by an electron with a path
length in the gas of GPL when the
gas mean free path is is
GPL/.
The chance of an electron reaching
the specimen unscattered (i.e. still
being in the focused probe) is
exp(-n).
Increasing the WD or the gas
pressure will therefore reduce
the beam current
Depending on the value of n, different
imaging situations are
encountered.
Slide courtesy David Joy
Lehigh Microscopy School 2006
Schematics of Scattering as a Function of Pressure
e- paths for n<<1
Minimal Scattering
99% or more of all
electrons are
unscattered.
Situation in the
normal SEM column
where the pressure is
~ 10-4 Torr
Thanks to FEI and David Joy
e- paths for n~1
Partial Scattering
30-90% of the beam
remains unscattered.
The beam profile is then
a central probe and a
surrounding skirt.
This is the ESEMVPSEM condition
e- paths for n~3
Complete Scattering
At high pressure almost all
electrons are scattering at
least once.
If n = 3 then only 3% of
beam escapes scattering.
This results in a diffuse
beam with no useful focus.
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Schematics of Scattering as a Function of Pressure
Alternate View
Overall signal level decreases as electrons are lost in the gas
Minimal Scattering
Partial Scattering
Complete Scattering
High Vacuum
LV & ESEM
Atmosphere
Thanks to FEI
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Analytical Expression for the Scattering Angle
Use Rutherford theory to estimate
the mean scattering angle.
rs=(364z/E) (P/T)1/2 WD3/2
rs is the mean scattering radius in meters.
Z is the atomic number of the gas.
P is the pressure in Pa.
E is the electron energy in eV.
WD is the working distance in m.
T is temperature in degrees K.
Thanks to David Joy
Assume
average
Assume average electron
electron
scatters at scatters
midpoint at
mid
point of path
of path
Working
Distance
WD
Probe
Skirt
Skirt radius r
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Beam Broadening with P,T, and E
The broadening varies as P1/2.
The broadening also varies as
1/Temp because the gas density
increases at lower temperature.
The broadening varies as 1/Energy
so it doubles when the energy is
reduced from 20keV to 10keV.
Consequently, low voltage
operation is difficult!
Thanks to David Joy
Lehigh Microscopy School 2006
Beam Broadening with Working Distance & Gas Type
The beam broadening varies as the
path length in the gas (working
distance) WD3/2
This is the most rapidly varying term
so always keep the WD as small
as possible
The amount of broadening varies with
the atomic number of the gas
Helium is the best choice because it
has the lowest atomic number
Argon or other heavy inert gases are
the worst choices
Thanks to David Joy
Lehigh Microscopy School 2006
What is the Result of Beam Broadening?
Sn balls on
carbon.
50Pa image
1 Pa Image
200 Pa Image
In air at
20keV, 8mm
WD Hitachi
S3000
50 Pa Image
Increasing the pressure from 1Pa to 50Pa reduces
the S/N little due to the scattering of the
incident beam
Even at 200Pa the resolution of the image remains
about the same. The resolution will degrade at
very high pressure.
Presence of the gas produces a skirt around the
beam and this lowers the contrast and reduces
the signal to noise, but does not reduce the
resolution.
Thanks to David Joy
Lehigh Microscopy School 2006
Gas in Low Vacuum SEMs
1. Scatters and broadens the probe.
2. Reduces the current available for imaging and analysis.
3. Modifies the effect of the beam-sample interaction by adding secondary
charge.
4. Produces a mechanism for secondary electron signal collection.
Are these features a GOOD thing or a BAD thing?
As long as the pressure does not get too high or the working distance too long
then 1. & 2. are not, necessarily, bad things!
What about 3 & 4?
Lehigh Microscopy School 2006
Non-conductive Sample in the SEM
Field from internal charge affects incoming electrons and
imaging.
Primary beam
Sample
charge
interferes
with
imaging
-
-
-
-
-
-
Charge collects in surface region
Lehigh Microscopy School 2006
Charge Neutralization & Signal Amplification in an ESEM with
Gaseous Secondary Electron Detector
Detected
electron
signal
Primary beam
GSED
+ +
+ +
++ +
+ +
+ + +
Collection ring at
high voltage
Signal amplification
by gas ionisation
+ +
+
+ ++ ++
ground
Slide courtesy FEI
+
+
+
non-conductive specimen
+ ++ + +
ground
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Low Vacuum Charge Neutralization Large Field Detector Secondary Imaging
Electron
beam
Detected
electron
signal
EDX
Detector
10mm WD
Sample
Slide courtesy FEI
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Prototype VPSE - II (GSD)
gas luminescence
Image
PMT
+ve bias
External PMT and
Bias power supply
sample
Glass light guide with conductive
coating
Thanks to Brendan Griffin of the University of Western Australia for this slide
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Prototype VPSE - II (GSD)
Gas luminescence around primary beam and biased VPSED
Thanks to Brendan Griffin of the University of Western Australia for this slide
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Charge Neutralization -ESEM Example
High vacuum
5 Torr gas
Non-conductive silicon nitride
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Gas in Low Vacuum SEMs
1. Scatters and broadens the probe.
2. Reduces the current available for imaging and analysis.
3. Modifies the effect of the beam-sample interaction by adding secondary
charge.
4. Produces a mechanism for secondary electron signal collection.
Are these features a GOOD thing or a BAD thing?
3 & 4 can help suppress charging effects and provide a way of imaging with
secondary electrons!
Lehigh Microscopy School 2006
Can a VP-SEM or ESEM do “normal” SEM?
Although an ESEM or VP-SEM might not use the same detection
strategies as a “regular” SEM, they can be used for what may be
deemed “routine” SEM.
Evaluation of Ag/Cu
Roman coins.
25µm
Stainless steel in Bakelite mount
5 Torr, 30kV 7mm WD
CuK
CuL
Ag
100µm
O
X-ray maps of
a coin with
O, Cl & S
phases
Lehigh Microscopy School 2006
Ideal Samples for VP-SEM and ESEM
No clay filler
Above: Paper Wipe
20kV 5Torr 7.5mm WD.
Right: Post-It Note
15kV 5Torr 7.5mm WD.
Both uncoated.
Insulating, wet and dirty samples
are ideal candidates for VP or
ESEM.
Considerable amount of clay filler for smooth
writing surface
Lehigh Microscopy School 2006
Ideal Samples for VP-SEM and ESEM
Micro-fabricated Electrodes for Biomechanical Applications - Biosensor
Electrodes embedded in guinea pig skull 5 Torr, 20kV 7mm WD.
Guinea Pig Skull
Si Probes
Pads are coated
with poly-peptides
to encourage
tissue adhesion
coated
50µm
Contact pads
(Au, Pt, Ir, etc.)
uncoated
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New Contrast Modes
CCI (charge contrast
imaging) is a unique
mode of the ESEM or
VPSEM.
This reveals the
crystallography and
growth process of natural
and artificial minerals
with high sensitivity.
CCI of Gibbsite at 20keV.
CCI of
Gibbsite
20keV
Image
courtesy
Davidat
Joy.
Lehigh Microscopy School 2006
Environmental Scanning Electron Microscope:
An Ideal Instrument for a Large Multi-User Facility
e.g. U of M EMAL
Many users (>30 at any point in time)
Many novice users (constant turnover)
Laboratory accessible 24 hours per day
ESEM is:
Easy to Use
Easy to train users
Tolerant to misuse (gloves unnecessary for sample handling)
Minimal sample preparation
Flexible stage exchange from one experiment to
the next
Excellent teaching tool
Lehigh Microscopy School 2006
Environmental Scanning Electron Microscope:
An Ideal Instrument for a Large Multi-User Facility
The ElectroScan E3 in the UM EMAL
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The FEI Quanta 200 3D in the UM
EMAL
Lehigh Microscopy School 2006
Working with Water, ESEM Mode
Saturated vapor pressure of H2O varies from ~ 2.3kPa at 20°C to 600Pa at
0°C.
To image liquid water at room temperature a pressure of 20Torr is required.
Lower pressures require a cooled sample
Pressure - Torr
15
Solid
phase
10
Liquid phase
ESEM
Gaseous phase
5
LOW VAC
0
-10
0
10
20
Temperature - Celsius
30
Lehigh Microscopy School 2006
Really Wet Samples
Table of the partial pressure of water vapor as a function of temperature & relative
humidity.
% RELATIVE HUMIDITY
100
95
90
85
80
75
70
65
60
55
50
0
4.6
4.3
4.1
3.9
3.6
3.4
3.2
3.0
2.7
2.5
2.3
2.0
1.8
1.6
1.4
1.3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
4.9
5.3
5.7
6.1
6.5
7.0
7.5
8.0
8.6
9.2
9.8
10.5
11.2
12.0
12.8
13.6
14.5
15.4
16.4
17.5
18.6
19.8
21.0
22.3
23.7
25.2
26.7
28.3
30.0
31.8
33.7
35.6
37.7
39.9
42.1
44.5
47.0
4.7
5.0
5.4
5.8
6.2
6.6
7.1
7.6
8.2
8.7
9.3
10.0
10.6
11.4
12.1
12.9
13.8
14.7
15.6
16.6
17.7
18.8
20.0
21.2
22.5
23.9
25.4
26.9
28.5
30.2
32.0
33.8
35.8
37.9
40.0
42.3
44.7
4.4
4.7
5.1
5.5
5.9
6.3
6.7
7.2
7.7
8.3
8.8
9.4
10.1
10.8
11.5
12.2
13.1
13.9
14.8
15.8
16.8
17.8
18.9
20.1
21.3
22.7
24.0
25.5
27.0
28.6
30.3
32.1
33.9
35.9
37.9
40.1
42.3
4.2
4.5
4.8
5.2
5.5
5.9
6.4
6.8
7.3
7.8
8.3
8.9
9.5
10.2
10.8
11.6
12.3
13.1
14.0
14.9
15.8
16.8
17.9
19.0
20.2
21.4
22.7
24.1
25.5
27.0
28.6
30.3
32.0
33.9
35.8
37.9
40.0
3.9
4.2
4.5
4.9
5.2
5.6
6.0
6.4
6.9
7.3
7.9
8.4
9.0
9.6
10.2
10.9
11.6
12.4
13.2
14.0
14.9
15.8
16.8
17.9
19.0
20.1
21.4
22.6
24.0
25.4
26.9
28.5
30.2
31.9
33.7
35.6
37.6
3.7
4.0
4.2
4.6
4.9
5.2
5.6
6.0
6.4
6.9
7.4
7.9
8.4
9.0
9.6
10.2
10.9
11.6
12.3
13.1
14.0
14.8
15.8
16.8
17.8
18.9
20.0
21.2
22.5
23.8
25.2
26.7
28.3
29.9
31.6
33.4
35.3
3.4
3.7
4.0
4.3
4.6
4.9
5.2
5.6
6.0
6.4
6.9
7.3
7.8
8.4
8.9
9.5
10.2
10.8
11.5
12.3
13.0
13.9
14.7
15.6
16.6
17.6
18.7
19.8
21.0
22.2
23.6
24.9
26.4
27.9
29.5
31.2
32.9
3.2
3.4
3.7
3.9
4.2
4.5
4.9
5.2
5.6
6.0
6.4
6.8
7.3
7.8
8.3
8.8
9.4
10.0
10.7
11.4
12.1
12.9
13.7
14.5
15.4
16.4
17.4
18.4
19.5
20.7
21.9
23.2
24.5
25.9
27.4
28.9
30.6
2.9
3.2
3.4
3.6
3.9
4.2
4.5
4.8
5.2
5.5
5.9
6.3
6.7
7.2
7.7
8.2
8.7
9.3
9.9
10.5
11.2
11.9
12.6
13.4
14.2
15.1
16.0
17.0
18.0
19.1
20.2
21.4
22.6
23.9
25.3
26.7
28.2
2.7
2.9
3.1
3.3
3.6
3.8
4.1
4.4
4.7
5.1
5.4
5.8
6.2
6.6
7.0
7.5
8.0
8.5
9.0
9.6
10.2
10.9
11.6
12.3
13.0
13.8
14.7
15.6
16.5
17.5
18.5
19.6
20.7
21.9
23.2
24.5
25.9
2.4
2.6
2.8
3.0
3.3
3.5
3.7
4.0
4.3
4.6
4.9
5.2
5.6
6.0
6.4
6.8
7.3
7.7
8.2
8.8
9.3
9.9
10.5
11.2
11.9
12.6
13.3
14.2
15.0
15.9
16.8
17.8
18.8
19.9
21.1
22.3
23.5
2.2
2.4
2.5
2.7
2.9
3.1
3.4
3.6
3.9
4.1
4.4
4.7
5.0
5.4
5.7
6.1
6.5
7.0
7.4
7.9
8.4
8.9
9.5
10.1
10.7
11.3
12.0
12.7
13.5
14.3
15.1
16.0
17.0
17.9
19.0
20.0
21.2
2.0
2.1
2.3
2.4
2.6
2.8
3.0
3.2
3.4
3.7
3.9
4.2
4.5
4.8
5.1
5.4
5.8
6.2
6.6
7.0
7.4
7.9
8.4
8.9
9.5
10.1
10.7
11.3
12.0
12.7
13.5
14.2
15.1
15.9
16.9
17.8
18.8
1.7
1.8
2.0
2.1
2.3
2.4
2.6
2.8
3.0
3.2
3.4
3.7
3.9
4.2
4.5
4.8
5.1
5.4
5.8
6.1
6.5
6.9
7.4
7.8
8.3
8.8
9.3
9.9
10.5
11.1
11.8
12.5
13.2
14.0
14.7
15.6
16.5
1.5
1.6
1.7
1.8
2.0
2.1
2.2
2.4
2.6
2.8
2.9
3.1
3.4
3.6
3.8
4.1
4.4
4.6
4.9
5.3
5.6
5.9
6.3
6.7
7.1
7.6
8.0
8.5
9.0
9.5
10.1
10.7
11.3
12.0
12.6
13.4
14.1
1.3
1.4
1.6
1.7
1.8
1.9
2.1
2.2
2.4
2.5
2.7
2.9
3.1
3.3
3.5
3.7
4.0
4.2
4.5
4.8
5.1
5.4
5.8
6.1
6.5
6.9
7.3
7.8
8.2
8.7
9.3
9.8
10.4
11.0
11.6
12.2
12.9
°C
45
40
35
30 27.5
Vapor Pressure in Torr
25 22.5
1.1
20 17.5
15 12.5
10
7.5
5
2.5
1.0
0.9
0.8
0.7
0.6
0.5
0.3
0.2
0.1
0.
1.2
1.1
1.3
1.2
1.4
1.3
1.5
1.4
1.6
1.5
1.7
1.6
1.9
1.7
2.0
1.8
2.1
1.9
2.3
2.1
2.5
2.2
2.7
2.4
2.8
2.5
3.0
2.7
3.2
2.9
3.4
3.1
3.6
3.3
3.9
3.5
4.1
3.7
4.4
3.9
4.7
4.2
4.9
4.5
5.3
4.7
5.6
5.0
5.9
5.3
6.3
5.7
6.7
6.0
7.1
6.4
7.5
6.7
7.9
7.2
8.4
7.6
8.9
8.0
9.4
8.5
10.0
9.0
10.5
9.5
11.1 10.0
11.8 10.6
1.0
1.1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
2.0
2.1
2.2
2.4
2.6
2.7
2.9
3.1
3.3
3.5
3.7
4.0
4.2
4.5
4.7
5.0
5.3
5.7
6.0
6.4
6.7
7.1
7.5
8.0
8.4
8.9
9.4
0.9
0.9
1.0
1.1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
2.0
2.1
2.2
2.4
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
4.2
4.4
4.7
5.0
5.2
5.6
5.9
6.2
6.6
7.0
7.4
7.8
8.2
0.7
0.8
0.8
0.9
1.0
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.2
2.3
2.5
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.5
4.8
5.0
5.3
5.7
6.0
6.3
6.7
7.1
0.6
0.7
0.7
0.8
0.8
0.9
0.9
1.0
1.1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.1
2.2
2.3
2.5
2.6
2.8
3.0
3.1
3.3
3.5
3.7
4.0
4.2
4.5
4.7
5.0
5.3
5.6
5.9
0.5
0.5
0.6
0.6
0.7
0.7
0.7
0.8
0.9
0.9
1.0
1.0
1.1
1.2
1.3
1.4
1.5
1.5
1.6
1.8
1.9
2.0
2.1
2.2
2.4
2.5
2.7
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.5
4.7
0.4
0.4
0.4
0.5
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.8
0.8
0.9
1.0
1.0
1.1
1.2
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.4
2.5
2.7
2.8
3.0
3.2
3.3
3.5
0.2
0.3
0.3
0.3
0.3
0.3
0.4
0.4
0.4
0.5
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.8
0.8
0.9
0.9
1.0
1.1
1.1
1.2
1.3
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.4
0.1
0.1
0.1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.3
0.3
0.3
0.4
0.4
0.4
0.4
0.5
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.7
0.8
0.8
0.9
0.9
1.0
1.1
1.1
1.2
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
Lehigh Microscopy School 2006
Really Wet Samples
°C
0
1
2
3
4
5
6
7
8
9
10
11
Relative Humidity %
100
95
90
4.6
4.3
4.1
4.9
4.7
4.4
5.3
5.0
4.7
5.7
5.4
5.1
6.1
5.8
5.5
6.5
6.2
5.9
7.0
6.6
6.3
7.5
7.1
6.7
8.0
7.6
7.2
8.6
8.2
7.7
9.2
8.7
8.3
9.8
9.3
8.8
Part of the table of the partial pressure of water
vapor as a function of temperature and and
relative humidity. Operation of the microscope
with the sample in a Peltier stage held at 3°C
allows the operator to vary the humidity such
that liquid water can be condensed on the
surface. Allows the studies of fluid drying.
QuickTime™ and a
Video decompressor
are needed to see this picture.
Lehigh Microscopy School 2006
Examples - Plant Tissue (Wet Leaves)
Leaf Surface
4.9 Torr, 10kV, 11mm WD, 3°C
Lehigh Microscopy School 2006
Plant Tissue - Seedlings and Early Plant Growth
Rootlet embedded in Agar
Sample Cup
Agar Jelly
Peltier Cooling Block
Schematic of
Sample Holder
Wet Root Surface
5.0 Torr, 10kV
8mm WD 3°C
30µm
Sample kept wet by pumping to 5 Torr and then repeated flooding of the sample
chamber to 9 Torr (modern versions of the ESEM do this automatically).
See Cameron et al, J. Microscopy, 173 (1994), pp227-237.
Lehigh Microscopy School 2006
Plant Tissue - Seedling Root & Root Hairs
Hairs are easily damaged during mounting and
pump down and can’t be re-hydrated.
Wet Root Surface,
5.1 Torr, 10kV,
8mm WD, 3°C
30µm
Lehigh Microscopy School 2006
Application Examples - Plant Tissue
While it is possible to keep a cell
Leaf Surface, 5.5 Torr, 10kV,
sample (leaf or other tissue) fully
hydrated in the microscope, if you make 11mm WD, 3°C
the mistake of dehydrating it, there is no
recovery.
Leaf Surface, 4.9 Torr, 10kV,
11mm WD, 3°C
50µm
50µm
Focusing damage, too high a
magnification for too long can rupture
the cells.
Lehigh Microscopy School 2006
Low Magnification - Overcome Aperture Vignetting
The differential pumping aperture
can limit the field of view of the
sample. For very low magnification
it is sometimes necessary to stitch
together many images. This can
be done manually as seen in this
cross section of a rapeseed
(used to make Canola Oil).
Wet samples, 3.5Torr, 15kV, Room
Temperature.
Montage of ~30 individual images
Lehigh Microscopy School 2006
Low Magnification - Overcome Aperture Vignetting
200µm
Low magnification view of substantial portion of a tissue engineering scaffold,
a single sodium alginate bead. Montage of 18 separate images automatically assembled
by Quickstitch™.
Lehigh Microscopy School 2006
Wetting Experiment - Sodium Alginate Beads
Tissue Engineering Scaffolds Sodium
alginate slab in Peltier stage in
ESEM, temperature approx. 3°C.
Pressure rapidly increased from 3
Torr to 5.5 Torr. Pores of the slab
almost all close up during
hydration.
Selected from microscope video acquired to a Sony DV Camera and edited with
Final Cut™ Pro and iMovie™ on an Apple Macintosh.
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
Primary beam
Schematic Diagram of the beam skirting that
occurs as the primary electron beam travels
through the environment of an environmental
scanning electron microscope.
Beam Skirt
Sample
Diameter of Skirt up to ~2mm
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
10000
9000
Copper K Alpha Intensity (counts)
8000
7000
6 Torr
4 Torr
2 Torr
"0" Torr
Effect of Pressure on X-ray Spectrum Spatial
Resolution at 15kV Accelerating Voltage.
See: Griffin, B.J. Proc. 50th EMSA, 1992, p1324.
6000
5000
4000
3000
2000
1000
0
0
100
200
300
400
500
Diatance from edge (µm)
600
700
800
e- beam
stepped
away from
Cu block
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
10000
9000
Copper K Alpha Intensity (counts)
8000
15kV
20kV
25kV
7000
6000
5000
4000
3000
2000
1000
0
0
Effect of Accelerating Voltage. on X-ray
Spectrum Spatial Resolution at 4 Torr.
See: Griffin, B.J. Proc. 50th EMSA, 1992,
p1324.
100
200
300
400
500
600
Distance from edge (µm)
700
e- beam
stepped
away from
Cu block
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
EMAL ESEM Simulations
Produced by Electron Flight Simulator E
http://members.aol.com/members/smworld100/
Working Distance 7.5mm
Pressure 5Torr
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
EMAL ESEM Simulations
Produced by Electron Flight Simulator E
http://members.aol.com/members/smworld100/
Working Distance 6.5mm
Pressure 1Torr
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
Primary e beam
P3 > P2 > P1
P1
P2
Pressure Variation Method
Schematic diagram of beam skirting as a function of
pressure.
Method assumes that to a good approximation the
skirt shape is independent of pressure in the single
scattering regime and only the intensity of the skirt
varies with pressure.
P3
Beam Skirt
Sample
Several 100µm
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
Left: Schematic diagram of pressure
variation experiment which employs
a manufactured composite of GaP
and Ti-34Al-1.3V (by weight)
Profile of electron skirt intensity
Primary beam axis
BakeliteЄ Mount
Irregular particles are
flakes of GaP
Spherical particles are
Ti34Al1.3V (by weight)
Below: ESEM image of area of such
a sample used in pressure variation
tests.
Spheres of Ti-34Al-1.3V
GaP
flake
Bakelite
™ mount
Microscopy
School 2006
Mansfield Version of PressureLehigh
Variation
Method
Can we do XEDS in the Environmental SEM?
Working Distance of 7.2mm
Working Distance of 6.7mm
4 10
4
3.5 10
4
6 10 4
5 10 4
3 10 4
2.5 10
P (Coun ts)
P (Coun ts)
4.5 10 4
P (Counts)
Ga (Counts)
4
4 10 4
3 10 4
P (Counts)
Ga (Counts)
2 10 4
1.5 10
2 10 4
4
1 10 4
1 10 4
0
0.5
1
1.5
2
2.5
3
0
0.5
Press ure (Torr)
P (Coun ts)
3.5 10
4
3 10
4
Working Distance
2.5 10 4
P (Counts)
Ga (Counts)
4
1 10
4
0.5
1
1.5
2
P Intensity/Ga Intensity
6.2mm
0.33 ±0.05
6.7mm
0.38 ±0.04
7.2mm
0.36 ± 0.02
Working Distance
0
2.5
Ratios for Zero Pressure
2 10 4
1.5 10
2
Press ure (Torr)
Working Distance of 6.2mm
4 10
1.5
Intercept Ratios from Pressure Series
4.5 10 4
4
1
2.5
3
P Intensity/Ga Intensity
6.2mm
0.33 ± 0.05
6.7mm
0.34 ±0.05
7.2mm
0.35 ±0.05
Press ure (Torr)
3
Lehigh Microscopy School 2006
Can we do XEDS in the Environmental SEM?
In a word, yes, if you are very careful.
Working Ranges for Pressure Variation Method
Results indicate that:
1. Working distances of 6.2 to 7.2mm are acceptable.
2. Pressures should be in the range of 0.5 to 2.5 Torr (67 Pa to 334 Pa).
3. Higher accelerating voltages are better.
Other methods can be found here:
E. Doehne, "A new correction method for energy-dispersive spectroscopy analysis under
humid conditions", Scanning, Vol. 18, 3 (1996), 164.
E. Doehne, "A new correction method for high-resolution energy-dispersive x-ray analyses in
the environmental scanning electron microscope", Scanning, Vol. 19, 2 (1997), 75.
J.B. Bilde-Sørensen & C.C. Appel, “Energy-Dispersive X-ray Spectrometry in the
Environmental Scanning Electron Microscope”, Extended abstracts of the 48th Annual Meeting
of Scandinavian Society for Electron Microscopy, 1996, Ed. A.B. Maunsbach, Printed by SVF’s
Fællestrykkeri, Aarhus, Denmark, pp4-5.
J.B. Bilde-Sørensen & C.C. Appel, “X-ray Spectrometry in the ESEM & LVSEM: Corrections
for Beam Skirt Effects”, Extended abstracts of the 49th Annual Meeting of Scandinavian
Society for Electron Microscopy, 1997, Ed. A.B. Tholen, Printed by SVF’s Fællestrykkeri,
Aarhus, Denmark, pp12-15.
Lehigh Microscopy School 2006
A Hamburger, the ultimate ESEM sample
Lehigh Microscopy School 2006
A Hamburger, the ultimate ESEM sample
Sample mounting is no problem.
Lehigh Microscopy School 2006
A Hamburger, the ultimate ESEM sample
Bun
Hamburger viewed in the ESEM in
Peltier cooling stage. Pressure 4.6 Torr,
Temperature 3°C Accelerating Voltage
30kV.
Pickle
Cheese
Meat
Lettuce
Bun
It is, of course, necessary to
disassemble the burger to image it!
ESEM of a Hamburger
Lehigh Microscopy School 2006
Summary Comments
VPSEMs and ESEMs are excellent general purpose SEMs.
Many samples require little or no sample preparation.
The pressure is an extra parameter to be considered in imaging.
Wet samples can be imaged, but many times all that may be visible is
the water surface.
The VPSEM and ESEM isn’t just for wet and sloppy samples!
XEDS can be done in the ESEM with care.
Lehigh Microscopy School 2006
An Introduction to the Environmental SEM and the
Variable Pressure SEM
John Mansfield
North Campus Electron Microbeam Analysis Laboratory
University of Michigan
417 SRB, 2455 Hayward
Ann Arbor MI 48109-2143
Phone: (313)936-3352 FAX (313)763-2282
[email protected]
http://emalwww.engin.umich.edu/people/jfmjfm/
My daughter, Betsy,
thought of the trick that
allows me to bring you
the next slide.
QuickTime™ and a
Microsoft Video 1 decompressor
are needed to see this picture.
Betsy’s Fruit Fly