I.
Background
A.
Resolution in a light microscope is limited by the wave nature of light.
0.612λ
d=
n sin α
where d is the resolution (d should be small)
λ is the wavelength of the energy source
n is the index of refraction of the medium through which the
energy source travels
α is the aperture angle.
B. Since electrons have very short wavelengths, they can obtain high resolution.
C. The scanning electron microscope utilizes a focused beam of high energy
electrons that systematically scans across the surface of the specimen.
D. The interaction of the beam with the specimen produces a large number of
signals at the specimen surface, including lower energy electrons, termed
secondary electrons. These low energy electrons are easily drawn to a
positively biased signal detector.
E. The scanning of the electron beam is synchronized to the scanning of the
CRT.
II.
The Microscope Column
A. The Electron Gun
Electrons are emitted from the filament by heating it. This is termed
“thermionic emission”. The filament is negatively charged. Thus an
electrical potential is established between the filament and the grounded
anode plate. This voltage difference between the cathode and the anode
plate is referred to as the “accelerating voltage”. The Wehnelt cap is given
a slightly greater negative charge than the filament (termed the “bias”).
The combined effect of the bias and the accelerating voltage toward the
anode results in the creation of the primary electron beam within the gun.
B. As the current flowing through the filament increases, the number of
electrons emitted increases up to a point referred to as “saturation”. An
increase in current beyond this point only slightly increases further
electron emission. It is important to operate in this region to achieve good
stability.
III.
Electron Beam-Specimen Interaction
A. Incident electrons may be deflected (w/o significant energy loss) by the
attractive force experienced in passing close to a positively charged
nucleus.
B. Backscattered Electron Emission (BEI)
1. Incident electrons may be deflected through an angle
greater than 90o, and re-emerge from the surface of the
target = this is known as “backscattering”.
2. This may also occur through multiple deflections through
smaller angles.
3. For elements of high atomic #, there is a greater probability
for electrons being backscattered by single, high-angle
deflections and retaining most of their initial energy.
4. For elements of low atomic #, multiple low-angle scattering
is predominant, and more energy is lost.
5. Therefore, elements of high atomic # appear brighter in
backscatter than elements of low atomic #.
C. Secondary Electron Emission (SEI)
1. “Secondary Electrons” = electrons originally residing in the
specimen and are ejected as a result of electron bombardment.
2. Distinguishable from backscattered electrons by their much lower
energy (all electrons leaving specimen with energies <50 KeV
described as secondaries).
3. Secondaries are produced along the whole path of incident
electrons in specimen, but only those originating within a few
nanometers of the surface can escape.
IV.
Focus (2 methods)
A. Change the focal length (strength) of the lens (this is what the eye does
when it changes focus). On the SEM this focal length is called the
“working distance”.
B. Change the distance, z, between the lens and the object on which you are
focusing, (this is what you do when you change the focus on your Nikon,
petrographic microscope).
Z
C. The SEM is capable of using both methods of focusing, both changing the
strength of the final lens (working distance), and changing the distance
between the stage and the final lens (z). The image is focused when the
focal point lies on the plane of the specimen.
V.
Geometery
A. Working Distance defined as the distance between the lower pole piece of
the objective lens and the plane at which electrons are focused.
1. This distance is changed by the strength of the objective lens. If
you change the objective lens strength, or the ‘focus’, the working
distance, as displayed in millimeters on the monitor, will change.
2. In order to obtain sharp images, the sample height should be
adjusted using the Z drive of the stage until the image comes in
focus at the desired working distance.
3. For X-ray microanalysis, the working distance must be set to
10mm, which is specific to the geometry to the detector mount in
the SEM chamber.
VI.
X-Ray Analysis
A. Atomic Structure
1. Electrons always seek the lowest possible energy level
2. Removal of an electron from a low energy inner shell will result in the
immediate replacement by an electron from a higher energy outer
shell.
3. The second electron loses energy in this transfer, and this energy is
released as an x-ray.
4. An x-ray created by the filling of a vacancy in a K shell is termed a K
x-ray; the filling of an L shell creates an L x-ray, etc.
5. A vacancy filled by an electron from an adjacent shell creates an α xray. A difference of two shells creates a β x-ray. A difference of three
shells creates a γ x-ray. Thus an electron that jumps from an L shell to
a K shell creates a Kα x-ray; an electron jump from an N shell to an L
shell creates an Lβ x-ray.
6. Variations in the number of protons between elements cause
differences in the energy levels of each electron shell. As a result, the
energy differences between shells (and therefore x-ray energies) of one
atom differ from those of another atom.
B. Detection
1. EDS detector consists of a Si(Li) crystal, Field Effect
Transistor (FET), Preamplifier, Beryllium window, and Liquid
Nitrogen reservoir
2. The Si(Li) crystal converts an x-ray signal to an electronic
signal of proportional energy. When an x-ray strikes the
crystal, electrons are promoted into the conduction band,
leaving holes in the valence band. Under an applied bias, these
electrons and holes are swept apart and collected on the
electrodes on the faces of the crystal.
3. FET takes the tiny voltage pulse from the Si(Li) crystal and
separates it from the bias voltage on crystal
4. FET functions most efficiently at very low temperature.
Reduced temperature decreases extraneous signals, especially
thermal noise, and thus increases the signal-to-noise ratio.
5. Signal from FET is amplified and sent to computer to be
plotted and analyzed.
C. The Continuous X-ray Spectrum
1. When an electron passes through the strong electric field close
to an atomic nucleus, its path may be altered, causing the
electron to give up energy in the form of photons.
2. This produces a continuous spectrum = i.e. ‘continuum’ or
‘bremsstrahlung’.
3. Significance: continuum limits detectability of characteristic
lines when they are small.