Have you ever wondered how scientists and
engineers design transistors that are around the width of a strand of DNA? How do we even take pictures of such nanoscopic
transistors? Well, thatâs the role of the electron microscope
which has literally changed the way humanity sees the micro and nanoscopic world. Donât believe us? Take this European Peacock Butterfly for example. When we zoom in on its wing using a light
microscope, we see that itâs composed of tiny overlapping scales. But, when we zoom in using an electron microscope,
we can clearly see the shape of each scale, and zooming in further, we see how the scales
have a truly incredible texture entirely foreign to anything that humans manufacture. Although this wing may not be directly related
to the technology youâre familiar with, scientists and engineers have been using electron
microscopes for the past 60 years to develop smaller and smaller transistors, and with
todayâs technology this microscope can zoom in millions of times to where itâs able
to capture images of individual atoms. There are two main types of electron microscopes. The Scanning Electron Microscope or SEM is
used to see surface images like this butterfly wing, or the bristles of a used toothbrush. See, here are cells from your body, and all
around here in yellow is bacteria. Itâs gross, but letâs move on. Scanning Electron Microscopes have a maximum
resolution of around 1 nanometer. Meaning the spacing between two adjacent features
or dots of resolvable data in an image is 1 nanometer. The other type is the Transmission Electron
Microscope or TEM which is used to take images of structures that are inside materials, much
like an x-ray machine takes pictures of the bones inside our bodies. For example, TEMs are used to take the pictures
of these sections of a transistor. However, in other domains of science TEMs
can be used to take images of proteins inside mitochondria, the powerhouse of the cell,
or of nanoparticles of pure gold. Transmission Electron Microscopes are typically
more complex than SEMs and have a resolution up to 50 picometers, which is roughly the
size of a hydrogen atom. One quick note is that this videoâs sponsor,
Thermo Fisher Scientific, provided us with a basic 3D model of one of their transmission
electron microscopes and assisted in our understanding of the complex technology involved. Letâs first focus on the TEMs as they are
more commonly used in developing cutting-edge technology, and later weâll provide an overview
of the scanning electron microscope. And note that thereâs considerable overlap
between the engineering inside them both. The basic idea behind a TEM is that it generates
electrons and accelerates them to around 70% the speed of light, thus creating a beam of
electrons. Next a series of magnetic lenses focuses the
electrons down to a small area and shoots or transmits these electrons through the specimen
that weâre looking at. Depending on the different densities and materials
inside the specimen, the electrons are scattered as they pass through it, thereby imprinting
an image of whatâs inside the specimen onto the beam of electrons. The imprinted beam of electrons is then magnified
40 times using an objective lens and further magnified 50,000 times using a set of projector
lenses. At this point, the imprinted image is 5 or
so centimeters wide and large enough to be captured by a high-resolution camera sensor
at the bottom of the microscope. Weâll explore the detailed engineering in
a little bit, but for now, you might be wondering why do we have to go through the hassle of
manipulating electrons, and why canât we just use light? Well, visible light is physically limited
to magnifications up to around 2000 times, and, if you try to zoom in further the image
remains blurred without revealing any more details. On the other hand, electrons can reach meaningful
magnifications up to 2 million times. Why then is light physically limited? Well, letâs return to this image of the
European peacock butterfly and the scales on its wings. This image was captured with a camera, this
image was taken with a light microscope, and these images were captured with an electron
microscope. Letâs consider two features from the specimen
that are only 100 nanometers apart. Visible light has an average wavelength of
540 nanometers, which is larger than the distance between these two points. Due to the physics of waves, as light hits
these two features itâs bent around, thus creating a pair of propagating waves with
a diffraction pattern resulting from the interference of the two waves. If the features are substantially closer than
the wavelength of visible light, then the diffraction pattern will make the two features
appear like a single blurred feature. In short, visible light canât really resolve
features that are less than 300 nanometers apart. However, in this electron microscope, electrons
are accelerated to 70% of the speed of light and have a wavelength of 2.5 picometers which
is around 200,000 times smaller than visible lightâs wavelength. In principle, such an electron microscope
could resolve features spaced just 1 picometer apart, but, due to the magnetic lensesâ
physical limitations the real resolution is around 50 picometers, which is enough to see
individual atoms in a material. Also, if youâre wondering about the scale
of micrometers, nanometers, and picometers, hereâs a comparison of the size of each
unit. Note that there are many more details and
facts that were cut from this videoâs script and thrown into the creatorâs comments which
you can find in the English Canadian Subtitles. That said, letâs now dive into the complex
science and engineering behind each part of this Transmission Electron Microscope. Weâll begin at the top with a device called
a field emission source which generates free electrons. The basic principle is that negatively charged
electrons are attracted to positive electric fields. Here we have a tungsten crystal needle, and
below is a ring called the extractor. This extraction ring is connected to positive
5 thousand volts, and as a result the negatively charged electrons in the tungsten are pulled
towards the extractor. The electric fieldâs effect on the electrons
is amplified by the sharply pointed tungsten crystal, which is only a few nanometers wide,
and as a result the electrons are freed from the tungsten. The next step is to accelerate them to 70%
the speed of light. To do this we use a series of metal rings
which are graduated to be tens of thousands of volts apart from one another. And, just like before, these positively charged
rings use electrostatics to attract the negatively charged electrons which are accelerated through
the center of the rings. There are two key reasons for the incredible
speed of the electron. First is so they can travel through the specimen,
whether it be a transistor, protein or a crystal lattice or something else that has been sliced
to typically only 100 nanometers in thickness; and second, as mentioned earlier, electrons
exhibit wavelike properties, and the faster they are, the shorter the wavelength and the
higher the resolution achievable. One important detail is that when the microscope
is running and electrons are being accelerated to relativistic speeds, vacuum pumps are used
to remove all the atmospheric molecules, thus creating a vacuum, similar to the vacuum of
outer space. This is because incredibly fast-moving electrons
will scatter in random directions as they collide with air molecules and thus ruin the
images of the specimen. Now that we have a beam of electrons, weâll
explore the magnetic lenses of which there are essentially three sets: the condenser,
the objective, and the projector. The role of the condenser magnetic lenses
is to focus the electrons from the source and project them onto the sample so that they
illuminate an area the size of a micrometer to several nanometers depending on the desired
magnification. Additionally, the microscope uses apertures,
or holes placed in the path of the beam to filter out any electrons that are fanning
too far from the center of the column, or optical axis, resulting in electrons more
parallel to one another before they hit the specimen. The specimen is placed on a holder which is
inserted through an airlock into the vacuum chamber. To see different aspects of the specimen such
as the crystal lattices, the holder can move, or translate the specimen in all three directions,
X,Y, and Z, and rotate the specimen along the X-axis, and with some holders, also the
Y-axis. With this we can get images exactly perpendicular
to the features such as these transistors inside. The incredibly small beam then hits the specimen
composed of different elements and densities of materials, thus scattering the electrons
in different ways thereby imprinting an image on the transmitted electron beam. The next lenses, the objective and a series
of four projector lenses, are used to resolve and magnify the miniscule image imprinted
into the electron beam up to a width of a few centimeters. This process is separated into two parts. First the objective lens â often considered
the heart of the microscope â magnifies the image by 40 times and its optical aberrations
define the final resolution. Then the projector lenses magnify the image
the rest of the way by 50,000 times. What are optical aberrations and why is 2
million times the typical maximum magnification? Well, letâs look at this image of 962 blurry
atoms of gold. With todayâs technology, the TEMâs ability
to resolve the smallest features is not limited by the electrons in the beam, but rather by
the lenses and the aberrations and distortions that they add to the image-imprinted electron
beam after it has been magnified. There are a few main types of aberrations
such as spherical and chromatic, which we wonât explore further, but the main idea
is that perfectly controlling a beam of electrons is far from trivial and the aberrations add
blurriness and impede resolution after the magnification. The projector lenses magnify what has already
been magnified by the objective lens, including the added aberrations, and this second magnification
adds its own aberrations afterwards. Therefore, a considerable amount of science
and engineering is dedicated to reducing the aberrations introduced by the objective lens,
as that is what ultimately limits the sub-nanometer scale resolution of the microscope. One thing youâre probably wondering is why
these magnetic lenses look nothing like microscope or camera lenses and how do magnetic lenses
operate on fast moving electrons? Well, inside the lens is a coil of copper
wire surrounded by an iron housing. When a current is run through these coils,
a magnetic field is produced. This magnetic field is then routed through
the iron to the pole pieces where itâs channeled into an optical column. These magnetic fields are then used to change
the trajectory of the electron by bending the electrons towards the center, or optical
axis, in a shrinking helical direction. The physics at play is the Lorentz Law. To summarize, the force on the electron is
equal to its charge, Q, times V or the electronâs velocity vector crossed with B, the magnetic
field vector. In short, if the electron were to have a velocity
away from the optical axis, it would be forced by the magnetic field down towards the center. However, if the electron were traveling perfectly
down the center along the optical axis, it wouldnât experience any Lorentz force from
the magnetic fields and would just continue down the center. As a result, the magnetic lenses act as convex
or converging lenses, focusing all the electrons down to a focal point. As the electrons continue their trajectory
past the focal point and expand, they produce a magnified image. This magnification depends on the strength
of the magnetic fields, the position of the lenses, and the position of the detectors
and cameras. Letâs move further down the microscope and
explore how we turn electrons into images. There are two separate systems. First, we have a phosphorescent screen which
has a special coating that glows when electrons hit it and a camera is used to view the screen. This system is used to align the microscope
and provide an overview of the specimen. When youâre ready to capture a high-resolution
image, the phosphorescent screen moves out of the way, and the image is captured using
the second system with a more sensitive CMOS camera that has a higher resolution and dynamic
range. The purpose of having two systems is that
the phosphorescent screen and camera is used to ensure that the electron beam and magnetic
lenses are set up properly, as an incorrectly focused beam could damage the sensitive CMOS
camera. Weâve covered many key parts of the microscope,
but there are other pieces of equipment and modules that provide additional features. For example, there are X-Ray detectors, energy
filters, phase plates, monochromators, multipole correctors, mechanisms to hold and adjust
apertures, water cooling for the magnetic lenses, tons of circuitry to control the magnetic
lenses and the field emission source, vacuum pumps, power supplies, and much more. Additionally, the entire microscope sits on
air cushions to remove external vibrations. Undoubtedly, this microscope represents an
incredible amount of science and engineering, and weâre thankful to this videoâs sponsor,
Thermo Fisher Scientific, for allowing us to look inside. In addition to electron microscopes, Thermo
Fisher also makes a wide range of laboratory equipment such as centrifuges, incubators,
xâray and mass spectrometers, and in fact they make PCR systems that can be used to
test for Covid 19. Undeniably, Thermo Fisher products are some
of the backbones of scientific research in labs across the world. Thermo Fisher isnât sponsoring this video
because they want you to buy a multi-million-dollar electron microscope, but rather, just like
us at Branch Education, they believe that the future of humanity lies in the hands of
scientistsâ and engineersâ abilities to discover, innovate, and engineer solutions
to the problems that face humanity. If youâre pursuing a career in science or
engineering, take a look at Thermo Fisher Scientific. You too could work on creating the tools that
propel science and engineering forward. Now that we understand the transmission electron
microscope, letâs look at the Scanning Electron Microscope or SEM which Thermo Fisher Scientific
also manufactures. The main idea is that, instead of illuminating
an area of a specimen and imprinting the image all at once, with a SEM we create a focused
spot, and scan this spot across the object weâre trying to magnify. These electrons then bounce off, and, in the
process, create secondary electrons, back-scattered electrons and X-Rays, which we measure to
get details as to the surface topology and chemical composition. For example, this process was used to create
these images of the butterfly wing, or of this salt crystal. The issue with SEM is that it only takes images
of the surfaces of materials and the resolution is limited by how small we can create the
focused spot and by how deep the electrons penetrate into the sample, or the so-called
interaction volume. The practical resolution is typically around
1 nanometer. Additionally, a useful variation of the Transmission
Electron Microscope thatâs worth mentioning is called an STEM, where the S is for scanning. This microscope is similar to the TEM, but
like the SEM, we focus the beam into a spot and then use deflection coils to scan the
spot through the specimen. The benefit of STEM is that it has a different
mechanism for creating image contrast and, when paired with an x-ray detector, is capable
of elemental analysis of the sample. More expensive TEMs typically have the optical
elements and circuitry to perform both TEM and STEM, and the user can toggle between
the two modes. Weâre sure you have many questions; feel
free to put them in the comments below, and weâll try to answer them in the top pinned
comment. Also, one of the scientists from Thermo Fisher
who works on these microscopes and helped us to research and write this script, has
written the creatorâs comments with loads of additional information, so take a look
at them in the English Canadian Subtitles. We believe the future will require a strong
emphasis on engineering education and weâre thankful to all our Patreon and YouTube Membership
Sponsors for supporting this dream. If you want to support us on YouTube Memberships,
or Patreon, you can find the links in the description. This is Branch Education, and we create 3D
animations that dive deeply into the technology that drives our modern world. Watch another Branch video by clicking one
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