Basic Table-top SEM imaging
•
•
•
•
1
Imaging concepts
Contrast mechanisms – optical
Contrast mechanisms – SEM (BSE)
Detector geometry – compositional and
topographic contrast
Contrast and Brightness
Brightness is the intensity of a
signal. In a digital grayscale image
(such as a black and white photo)
the brightness of a pixel is
represented by a number. In an 8bit image, the number can range
from 0 to 255. This is 28 levels of
gray.
Contrast is the relative difference
in intensity between two areas.
I1  I 2
C
I2
2
Contrast Mechanisms in “real life”
A contrast mechanism is the physical process that causes an image to have
brightness variations.
There are several contrast mechanisms that allow our eyes utilize light to
observe the objects around us. The most common are:
1. Angle contrast – surfaces that directly face a source of illumination are
brighter
2. Shadow contrast – objects that are blocked from the illumination source
are dark
3. Color contrast - compounds in surfaces absorb some wavelengths of light
and reflect others. Objects with different composition will generally have
different colors.
The human brain automatically and instantly processes these contrast
mechanisms to interpret a scene. The best interpretations are possible
when our eyes can see all three types of contrast.
3
Contrast Mechanisms in “real life” – minus
color
The brightnesses of several
regions were measured with
an image processing program,
ImageJ. Can you identify all
three contrast mechanisms in
this image? (Hint: all three
mechanisms are present)
4
Contrast Mechanisms in “real life” – minus
color
Let’s start by looking at these three regions. What’s the contrast
mechanism?
5
Contrast Mechanisms in “real life” – minus
color
Let’s start by looking at these three regions. What’s the contrast
mechanism?
A reasonable guess is angle
contrast - they are simply angled
differently with respect to the
illumination source. Our brains
recognize that the object is a Rubik’s
cube – and this knowledge aids in the
interpretation
But how do we know that the three
regions aren’t different colors? What
do you think?
6
Contrast Mechanisms in “real life” – minus
color
And what’s up with these regions? What’s the contrast
mechanism?
7
Contrast Mechanisms in “real life”
When all three contrast
mechanisms can be seen in the
photograph, we can be more
certain of the interpretation
The Table-top SEM is equipped
with “eyes” that “see” in black and
white. Color information is lost –
so you must make inferences
based on black and white contrast.
Research microscopes usually
have an attachment that adds
compositional information – so
they can “see” in “color.”
8
Imaging in the Phenom – the Backscattered
Electron Detector
• The BSE detector is an annular ring
separated into 4 segments
• The primary beam passes through the
annulus. The detector then intercepts
some of the electrons backscattered from
the specimen
• Adding the signals from all 4 segments
generates compositional contrast or Z
contrast
• Subtracting one block of segments from
another generates topographical contrast
9
BSE Compo contrast (FULL mode on the
Phenom)
Primary Beam
An atom with more protons in the
nucleus (i.e. high Z) will deflect
more electrons by higher angles
and will thus direct more BSE’s at
the detector.
This is known as compositional
contrast.
Note that not all of the
backscattered electrons are
intercepted by the detector
Backscattered
electrons
High Z – emits
many BSEs
10
Low Z – emits
fewer BSEs
Example: a flat rock surface
The rock (a basalt) is
composed of many
different mineral
grains. Each type of
mineral has a different
composition and
therefore a different
average Z. Each
mineral type therefore
has a different average
brightness.
How many different
types of minerals do
you see?
How many different
grains can you count?
11
BSE Topographic contrast (topo mode)
Primary Beam
In the illustration, the illuminated
surface is angled more towards B than
A. Thus side B receives more of the
emitted (which contrast mechanism is
this?)
Side A
Side B
The two quadrants of side B are
subtracted from side A. This creates
topographical contrast, which consists
of both angle and shadow contrast.
Objects that face towards side A will be
brighter than those that face side B.
This also cancels out the Z contrast
(why? We’ll come back to that)
Specimen with topographic variation
12
Subtracting BSD segments for topo
contrast
This is the same
area on the same
rock as the one
you just saw. The
specimen’s Z
contrast has been
subtracted out,
and contrast is
therefore due to
angle and shadow
contrast.
We can see that
the surface is flat
but rough.
13
Compare Topo and FULL modes
Topo A mode
BSE compo
Note that compo (FULL) mode cancels out much of the topographical (angle
and shadow) contrast!!
14
Orienting the topographical shading
Phenom images of a chicken bone
Topo A – shadows oriented to the
bottom of the image
15
Topo B – shadows oriented to the
right
Watch out for weird subtraction artifacts in
topo mode . . . .
FULL mode – notice the deep holes
16
Topo mode – deep holes appear to
be “filled”
So how does this work?
detector
To understand how to get compositional
and topographical modes out of the
same detector, we have think about
the signal that a detector collects.
In the illustration at left, we see that
when the beam illuminates the high
Z material, the detector collects
about twice as many backscattered
electrons as when the beam
illuminates the low Z material.
We can plot the detector output vs
beam location to graphically see the
image brightness as the beam
scans across the sample
High Z
Low Z
detector
output
beam position
17
Adding BSD segments for compositional contrast
In the case illustrated at right, each
detector collects about three times
as much signal from the high Z
material as from the low Z material.
In other words, when the beam is on
the left side of the specimen, both
detectors simultaneously receive a
strong signal. When the beam is on
the right (green) side of the
specimen, both detectors
simultaneously receive a weak
signal.
Adding the detector outputs (FULL
mode) increases this compositional
contrast.
Side A
Side B
High Z Low Z
Signal A
Signal B
detector
output
A+B
(FULL)
Subtracting one output from the other
gives no contrast. Thus, Topo
mode cancels out the compositional
contrast.
18
A-B
(Topo)
beam position
Subtracting signals for topographic contrast
In the case illustrated at right, the
specimen has a raised feature.
When the beam is aimed at the left
side of this feature, detector A
receives a strong signal while
detector B receives a weak signal.
The situation is reversed when the
beam is aimed at the right side of
the feature.
Adding the detector outputs cancels out
this topographical contrast.
Subtracting one output from the other
increases the topographical contrast
A
B
specimen
Signal A
Signal B
A+B
A-B
19