Course: Physics 30
Topic: SEM electron gun
Purpose
Name:
Unit C: Cycling of Matter …
Date:
Grade: 12
The scientific purpose of this lesson is to use physics concepts to explain the operation of an SEM.
1. Background
The acceleration of electrons within a vacuum chamber has led to many technological advances and many scientific
discoveries. Some of the latest technological tools using an electron gun are nanotechnology tools, like the scanning
electron microscope (SEM). The SEM has helped to open up the world at the nano-level (10-9 m or nanometre (nm)
level). An SEM was created/invented in the mid-to-latter part of the 20th century. The SEM has a resolution down to
10 nm (10 x 10-9 m) and has a much greater (focused) depth of field than optical microscopes (providing a 3D-like
image). This is all done with accelerated and focused electrons—in a beam. To understand (describe and explain) the
SEM from a physics perspective you need to use concepts such as accelerating electrons in an electric field and
focusing electron beams in a magnetic field.
a. Gather library and Internet literature related to acceleration of electrons in general and electron guns in particular.
b. Display any cathode ray tube; e.g., from an old TV or oscilloscope. Display any old cathode ray tube used to display the
bending of electron beams with a magnet.
2. Clarifying
There are now many different types and sub-types of microscopes—or should we now say nanoscopes? Scientists now
have, for example, scanning electron microscopes (SEMs), scanning probe microscopes (SPMs), atomic force
microscopes (AFMs), and scanning tunneling microscopes (STMs). The new worlds of evidence that can be obtained
from these microscopes have revolutionized scientific work at the nano-level and normalized work at the micro-level.
We now have nano-technology which focuses on inventing technologies and making them work, and we have nanoscience which focuses on creating descriptions, explanations and predictions related to the nano-world. Technology
often focuses on how, while science focuses on why. Until the 20th century there were no examples of science leading
technology; technologies were invented and then along came science to explain the technology (e.g., the battery was
invented in the early 1800s and explained by science (and the creation of the electron) in the early 1900s). The modern
collaborative and interdisciplinary work between scientists and engineers/technologists has produced rapid advances
in both fields.
Whether science led technology or technology led science in the invention of the SEM may be debated, but the
collaborative nature of the invention is hard to debate. In this lesson we will use science to explain rather than predict
the technology. Following is a general description of the SEM.
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www.CMASTE.ca
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2. Clarifying
A scanning electron microscope (SEM) uses a high-energy beam of electrons to
scan a specimen rather than using optical (visible) light (electromagnetic
radiation). An electron gun accelerates electrons towards an anode which
collimates the electrons into a beam of parallel “rays” of electrons. A circular
magnet then focuses the electron beam on the specimen. Electrons backscattered
from the specimen are captured in detectors and electronically assimilated into an
image by a computer program. All of this must happen in a vacuum, otherwise the
electrons are scattered by air molecules and produce random “noise”. Since a
vacuum is used, wet samples cannot be used (the water is vapourized) and live
samples cannot be used (they die in a vacuum and due to the electron
bombardment). If samples are to be preserved, they are plated with a very thin
layer of gold. The gold plating also produces a better resolution and depth of
field, but this process is expensive.
[The image downloaded from www.Purdue.edu from a Google search for SEM
images. The Purdue site credits the image to Iowa State University. This webpage
also includes the following figure that illustrates the scattering of primary,
secondary and Auger electrons plus X-rays. ]
The Hitachi TM3000 pictured here is a portable
scanning electron microscope (SEM) with a
vacuum pump and computer attached. The
internal SEM is evacuated by pressing the lowerright white button. The specimen stage is in the
bottom drawer and is moved left-right and backforward as shown by the arrows by the knobs. The
magnification, focus, contrast and scanning speed
are operated by mouse clicks from the computer
screen.
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3. Labwork
Exercise
The Hitachi SM-3000 portable scanning electron microscope (SEM) uses an electron gun with an applied voltage of 5 kV or 15 kV
(for fast scanning and for slow scanning for higher resolution images).
1. What is the velocity of an electron accelerated through voltage of 5.0 kV?
2.
What is the kinetic energy of an electron accelerated by a voltage of 5.0 kV?
3.
What is the kinetic energy of an electron accelerated through an electrical potential difference of 15 kV?
4.
What is the electrical potential energy of an electron just before exiting from the cathode of the electron gun when a
voltage of 15 kV is applied?
5.
Qualitatively, what is the electric charge on the anode of the electron gun?
6.
What happens to the electrons that strike the anode?
7.
What happens to the electrons that pass through the hole in the anode?
8.
The hole in the anode acts as a collimator. What is the purpose of a collimator in conjunction with an electron gun?
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9.
Explain how the electromagnet focuses the electron beam on the specimen? First state the concept that you are using for
the explanation and then provide the explanation.
10. Explain why the SEM chamber must be evacuated of all air. What happens if the chamber is not evacuated?
11. Is the electron acting as a wave or a particle in this technological application?
12. Explain why electrons are scattered from the specimen.
13. Explain why secondary electrons may be scattered from the specimen.
14. Explain why X-rays may be emitted from the target area.
4. Summary
There are always perspectives other than scientific and technological to consider.
a. What other technology might be more economic? more time-efficient? less technologically dependent? more
environmentally friendly? more safe? more fun?
5. Homework
a. What is the general relationship between science and technology?
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b. What are the specific roles of nanoscience and nanotechnology?
c. What kind of knowledge is gathered from the SEM—empirical or theoretical?
d. Complete a literature search about the different types and sub-types of, for example, scanning electron
microscopes (SEMs), scanning probe microscopes (SPMs), atomic force microscopes (AFMs), and
scanning tunneling microscopes (STMs).
6. Review
a. Why is it necessary to place the sample on the graphite paper?
b. Why can a wet sample not be used in this SEM?
c. Why does the SEM sample-chamber have to be evacuated with a vacuum pump?
d. Why can an SEM provide a more clear, more 3D focused, and more magnified image than a light microscope?
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