Electron Microscope Image Characteristics
Date: Summer 2011
When using an electron microscope, are optical qualities
(color, transparency, refraction index) shown that would appear to
optical microscopes? What prevents an electron microscope resolution
from being equal to the electrons themselves?
Electron microscope properties are very different from optical
microscopes for several reasons.
One of the first is the scale. An optical microscope is on a large
enough scale to call the light a wave rather than individual photons
(i.e. particles of light). Properties such as refraction are based on
the average effect of many photons acting as a wave. Electron
microscopes work with scattering of individual electrons.
Wavelength is part of what allows the extreme precision of the electron
microscope. Every particle has a wavelength and frequency. For
visible light, our eyes see this as color. At the level of quantum
physics, this relates to the particle's
energy: increase the energy of a particle to decrease its wavelength.
When the wavelength is smaller, precision is greater. Electron
wavelengths are much smaller than standard visible light. Very small
wavelength light (ultraviolet and beyond) is more difficult to control
and measure. It is also more destructive. Still, the precision is
limited by the electron's wavelength rather than its size. Higher
energy electrons would have even smaller wavelengths, but they would
destroy the surface rather than bouncing off of it.
Dr. Ken Mellendorf
Illinois Central College
"are optical qualities(color, transparency, refraction index) shown
that would appear to optical microscopes? "
When looking at electron micrographs you need to learn a new set of physical traits to assign to appearances.
Color and optical refraction index do not matter to electrons.
If an object has an appearance of transparency,
then that is because electrons tend to go through it
without being scattered. Films and feathers 1-to-10nm thick
do occasionally exist that have substantial transparency for SEM micrographs.
Usually in addition to being very thin, the atoms also have low Z (atomic number).
Hi-Z elements have much stronger scattering and energy-loss effect on fast electrons.
SEM micrographs tend to show a lot of "shape-contrast":
small things sticking out allow more secondary electrons to escape and reach the detector,
then in the picture pixel corresponding to that e-beam direction
you see a lighter spot.
Conversely a hole or pocket usually looks dark inside
even if the e-beam is shooting right in to the bottom of the pit.
When fast electrons of the beam penetrate the material,
they kick up slow secondary electrons going in all directions randomly,
and in a pit most of those directions are blocked.
For similar reasons the edges of a shape such as a cube often look lighter and indistinct,
almost as if they were transparent.
"What prevents an electron microscope resolution
from being equal to the electrons themselves?"
I think in some electron microscopes they are!
An electron is an irreducibly simple particle, unlike protons and neutrons.
So it is true size may be tens of orders of magnitude smaller than an atom,
but its practical size is the wave-like quantum uncertainty of its location in the given situation.
In an atom the energy well around the nucleus confines the electron's location to within about 0.1 nanometer.
But in an electron beam, the beam's angular-width and the objective lens aperture are the only confinement,
and by Heisenberg uncertainty the minimum focus spot width is inversely proportional to
the narrowness of the beam direction.
So an electron is blurred because the location is uncertain,
or because the direction is uncertain, or both.
The electron has no defined size other than this blurring effect.
It is almost more wavelike than particle-like.
When people want even more resolution, they use heavier particles such as ions,
or maybe use an objective lens with higher numeric aperture.
Other electron microscopes do have imperfect beams wider than
the quantum uncertainty determined by mass, energy, and objective lens width.
Usually the extra beam-width arises from a crude electrons source such as a filament, or from space-charge effects
(enough electrons close together in the beam to push themselves apart by repulsion.)
If one voluntarily lowers the emission current from the filament,
then there are fewer electrons in the beam and less spreading,
and more resolution is obtained,
but then it takes longer to gather enough secondary electrons to make a clear low-noise picture.
It is occasionally a tough trade-off.
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Update: June 2012