What is the highest
possible magnification?
What is the resolution?
How is an electron microscope
better than a light microscope?
How can electrons have
wave-like properties?
How are the
secondary electrons detected?
Why is it necessarry to
have a vacuum?
What can and can't be a
sample?
Didn't find your question. . . Send your
question to TTSEM Fellow Ben
Lopez.
What is the highest possible magnification?
20,000 times magnification.
What is the resolution?
10 nanometers.
How is an electron microscope better than a light
microscope?
A traditional light microscope uses visible light to resolve
an image. The wavelength of visible light ranges from
about 400-700 nanometers. These wavelengths give the light
microscope physical limitations of 500-1000 times magnification
and 200 nanometer resolution. When accelerated through
a voltage, electrons can have a wavelength on the order
of 0.1 nanometers. This allows much higher resolution
and magnification. The focal depth is also much larger.
How can electrons have wave-like properties?
Light and matter both have particle and wave-like properties!
The wavelength of a material object is given by planck's
constant (6.6*10^(-34) J*s) divided by its momentum. For
the macroscopic everyday object this wavelength is so
small as to be inconsequential. However, for something
as tiny as an electron, the wavelength is much larger,
and its wave-like properties can be utilized.
How are the secondary electrons detected?
Electrons from the electron beam impact and unbind outer-shell
electrons from atoms in the sample. These outer-shell
electrons are called the secondary electrons. These electrons
hit a detector called a scintillator-multiplier device.
the scintillator portion of the device absorbs the secondary
electron and fluoresces photons. The photomultiplier portion
of the device absorbs the photon and converts it into
a largely amplified electrical signal. The intensity of
this signal is proportional to the brightness seen on
the screen. The detector is mounted at an angle to the
sample and electron beam, this creates the shadowing effect
seen in the images.
Why is it necessarry to have a vacuum?
If the vacuum chamber was left at regular atmospheric
pressure, the electrons would interact with gas molecules
in the air and be deflected away from the electron beam
before they reached the sample. Under vacuum there are
very few gas moleculesto impede the electrons and a larger
number of electrons reach the sample.
What can and can't be a sample?
There are two main qualities necessary to have a good
image with an electron microscope. First, the sample must
be able to withstand the vacuum that it will be under
while in the vacuum chamber. Second, the sample must be
conductive. If it is not conducted the sample will absorb
charge, eject few secondary electrons, and reduce the
quality of the incident electron beam. Any metallic objects
will be quality samples to start with. Most objects that
are not metallic can be sputter coated to make them conductive.
Sputter-coating is a process of depositing a metallic
vapor onto a sample. We have the means to do sputter-coating
at the University.
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