June 1948 Radio News
[Table of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Technology builds on its own successes
in order to evolve. This article from a 1948 issue of Radio News magazine
reporting on the relatively newly perfected electron microscope. As electronics
moved from the macro scale in the form of vacuum tubes and large, high voltage-
and power-handling leaded components (resistors, inductors, capacitors) to semiconductors
and smaller, lower voltage and power components, using a standard optical type microscope
was not good due to small features on the IC die. As more powerful microscopes were
developed, engineers and scientists were able to develop semiconductor circuits
with smaller features. That enabled more compact, higher performance electronic
microscopes to be built ... and the cycle continued to where we are today. It is
sort of another way of looking at Moore's law. Every time someone states that Moore's
law has about reached its end, another breakthrough pushes the limit out even farther.
Humorous note: The collodion
mentioned here sounds like something from a
Tom Swift book.
Exploring the Infinitesimal
Fig. 1 - Latest model of the RCA-developed Electron Microscope
provides greater useful magnification than ever before possible. Magnifications
from 100 to 20,000 diameters are viewed directly on a fluorescent screen. Photographs
are usually made at magnifications of 5000 to 10,000 diameters. Because of high
resolution (clarity of detail), these photographs may be enlarged as much as twenty
times without loss of detail.
By Tom Gootée
A practical explanation of the Electron Microscope, and the use of electrons
to obtain magnification.
Man has always searched for knowledge of the infinitely small. Until a few years
ago, however, he was unable to penetrate deeply into the unknown and unexplored
universe that he knew existed but could not see.
Optical or light microscopes-in use since the Seventeenth Century-provide only
a partial solution. They have their rightful place in science, but their power is
limited, because their operation depends on light waves. They cannot see particles
much smaller than the wavelength of light, thus, their greatest magnification -
about 2000 diameters, or 2000 times - is only a teasing, long-distance glimpse of
It remained for scientists and engineers, working in the field of electron optics,
to develop a new and revolutionary magnifying device - the Electron Microscope -
the most powerful microscope in existence.
Today, this improved electronic instrument provides much higher resolving power
(better clarity) and much greater useful magnifications than have heretofore been
possible. Any degree of magnification, from less than 100 times to more than 20,000
times, can be obtained directly and viewed on a small fluorescent screen.
Magnified images can also be recorded on a photographic plate. These photographs,
known as electron micrographs, are usually made at magnifications of 500 to 10,000
diameters. Then, because of their high degree of resolution (or clarity), these
photographs can be enlarged as much as 20 times, thus providing, when necessary,
total useful magnifications as great as 100,000 diameters!
Fig. 2 - Simplified action of the electron beam striking the
specimen. The specimen is held by means of a thin supporting .film on a wire screen
in the magnetic field of the objective lens so that the specimen is "illuminated"
by the electron beam. Electrons are influenced or deflected according to the varying
densities of the specimen. Denser areas result in greater "scattering" of electrons:
less dense areas produce less "scattering." In this way. the high velocity beam
is "modulated" according to the shape and density of the specimen, and an image
of varying but sharp contrasts is obtained. Widely "scattered" electrons are unwanted.
and are removed from the electronic axis as shown. Only the main, usable portion
of the electron beam - bearing the image of the specimen - is allowed to pass through
the limiting aperture. The beam is then focused and magnified by the objective lens.
Although a massive and seemingly complex instrument (Fig. 1), the principle of
the Electron Microscope is relatively simple and in many ways similar to the ordinary
optical or light microscope. This similarity is shown (Fig. 3) by a comparison of
the basic components of the two types of magnifying systems.
Essentially, the light microscope consists of a source of illumination, a condenser
lens to concentrate the light in a beam on the object or specimen being examined,
an objective lens controlling the focus and quality of a magnified image, and a
projector lens for further magnification to produce a final image.
In the Electron Microscope, these optical elements are replaced by electronic
elements. (See Fig. 3.) A high-speed electron beam is used instead of a light beam,
and the electrons are controlled and focused by means of magnetic coils or magnetic-field
lenses - replacing the solid, glass, optical lenses.
The source of illumination is an electron gun, consisting of a filament and cathode,
and a specially shaped anode or plate. Electrons emitted by the heated filament
are attracted at a furious rate toward the anode, due to the high positive potential
of the anode, 50,000 volts, with respect to the cathode. There is a small hole in
the center of the anode, however, and most of the electrons shoot through this opening
at a tremendous velocity.
Almost immediately, the electrons are concentrated in a beam by the magnetic-field
action of the condenser lens. This doughnut-shaped coil bends the paths of the electrons,
and directs the resultant high-intensity beam on the object to be examined.
The usual microscope slide is much too thick to be penetrated by the electron
beam. For this reason, the object or specimen is placed on a very thin supporting
film or membrane, which is only a few millionths of an inch in thickness and is
effectively transparent to the electron beam. The film is made of collodion and
is strong enough to support most types of specimens, but for greater rigidity the
film is placed over a 200-mesh screen about 1/8-inch in diameter.
When the specimen is bombarded by the high-velocity beam, the path of each electron
is affected in varying degree (Fig. 2) according to the density or composition of
the specimen at the point of contact. Portions of a specimen having great density,
such as metallic oxides, cause wide "scattering" or abrupt dispersal of all electrons
striking such areas. Portions of less density cause less "scattering" of electrons,
in proportion to the density of the area where electrons strike.
In this way, a specimen is "Illuminated" by the electron beam. Electrons which
are widely "scattered" are of no importance; and are eliminated by means of a limiting
aperture. The electrons which pass through the aperture, however, bear an image
corresponding to the varying density of the specimen under observation.
The small unit (film on screen) containing the specimen is held in proper place
by means of a T-shaped specimen holder (Fig. 6) resembling a small, brass cartridge.
The specimen holder is mounted in the magnetic field of the objective lens. The
electron beam is brought to a focus by the magnetic-field action of this coil, to
form an enlarged image.
A part of the area of this magnified image is selected for further, final magnification.
Electrons forming this part of the image are allowed to enter the magnetic field
of a third coil, known as the projector lens. The electrons are brought to a focus,
producing a greatly magnified final image.
Fig. 3 - Electron Microscope is similar in basic principle to
the ordinary optical or light microscope. Essentially. the light microscope (left)
consists of a source of illumination. a condenser lens concentrating the light in
a beam on the specimen or subject under observation, an objective lens controlling
the focus of the image, a projector lens for further magnification for microphotography,
and an observation screen. In the Electron Microscope (right), optical elements
are replaced by electronic elements; the beam of light is replaced by a beam of
high speed electrons, and the solid optical lenses are replaced by magnetic field
lenses created by energizing magnetic coils. Electronic action takes place in a
vacuum within a high. cylindrical metal column. Current for controlling each lens
and the electron accelerating voltage is obtained from separate, regulated power
Since the electron beam is invisible to the human eye, a fluorescent observation
screen is placed in such a way that the beam falling upon it produces a visible
image. The screen is enclosed by the viewing chamber of the microscope, but three
glass windows in the chamber permit simultaneous observation of the final image
by several persons. The front window of the viewing chamber is also equipped with
a 2-power glass-lens magnifier to aid the operator in focusing images.
Photographs of the final image can be made by allowing the electron beam to fall
directly upon a conventional photographic plate.
Control of Operation
Efficiency of this electronic action depends to a great extent on precision control
of all factors which influence, or which might influence, the electron beam. This
is accomplished in a number of ways.
The physical appearance of the essential electronic components (Fig. 6) illustrates
their relative size and (simplified) arrangement inside the main column of the microscope.
The electron gun has an adjustment for raising or lowering the filament heater
with respect to the cathode, controlling, to a certain extent, the intensity of
the electron beam. Two thumb screws near the base provide angular or "tilt" adjustment
of the assembly and two other screws move the gun laterally at right angles to the
axis of the column, thus directing the electron beam so that it coincides exactly
with the tiny aperture of the condenser lens (Fig. 6).
The magnetic field of the condenser lens regulates the intensity of the beam
striking the specimen, and this action is controlled by the amount of current flowing
through the coil.
Focusing of the electron beam is accomplished by the magnetic field of the objective
lens, producing an enlarged image. This focusing action is controlled by the amount
of current flowing through the coil with no physical change of lens or coil position
Similarly, no change of lens or coil is required for final magnification, because
the electron beam is focused by the magnetic field of the projector lens and, again,
this action is controlled by the amount of current flowing through the coil.
Fig. 4 - Complete Electron Microscope, with metal covers removed
to show location of all important electronic elements, power supplies, vacuum equipment,
control panel, and other major components. Compare this with the simplified arrangement
of essential electronic elements as shown in the diagram, Fig. 6.
Each of the three magnetic lenses contains a pole piece designed for the particular
electronic function. Pole pieces for the condenser and objective lenses are fixed.
In the case of the projector. lens, however, four distinct adjustments of the pole
piece provide four broad ranges of magnification. A gap of 3/64-inch. between the
two parts of the pole piece offers a magnification range from 7500 to about 22,000
diameters as the projector lens current is raised from minimum to maximum. When
the gap is 10/64-inch, the magnification range is from 4000 to 11,000 diameters.
If only the top half of the pole piece is used, the range is from 700 to 4000 diameters.
If no pole piece is used with the projector lens, the magnification range is from
about 75 to 750 diameters, with distortion usually apparent in the lower part of
this range. Within any of these four broad ranges of magnification, there are ten
distinct values or steps of magnification, according to the amount of current applied
to the coil of the projector lens. Any specified amount or degree of final magnification
is obtained by selecting a suitable, fixed, and stable value of energizing current
for the magnetic coil.
Only the essential components (Fig. 6) can be allowed to influence the high-velocity
electrons passing from the electron gun to the photographic plate. Stray magnetic
fields, vibration noises, and all disturbing factors and extraneous effects must
be eliminated. For this reason, all of the essential components associated with
electronic magnification are enclosed in a single, rigid, well-shielded, cylindrical
column (Fig. 4) rising above the control panel.
Since electrons are influenced and scattered by air, the path of the electron
beam must be essentially void of air. For this reason, the large, cylindrical, metal
column is evacuated by means of suitable pumping apparatus. The state of the vacuum
within the column is controlled by a single hand crank, which actuates certain valves
operating an oil-diffusion pump (Fig. 4) and a mechanical force-pump. A high vacuum
is obtained in a relatively short time. After insertion of a specimen, within 60
seconds the column can be evacuated sufficiently for normal use of the microscope.
An interlock prevents application of the filament voltage before the required vacuum
In operation, the ability of the magnetic lens system to produce an enlarged
image depends upon two important factors; the value of current energizing each of
the three coils, and the velocity of the electron beam. To obtain clear and detailed
photographs of highly magnified images, these two factors must be constant during
the time of exposure. For this reason, separate power supplies are used to provide
extremely well-regulated current for each of the three coils. These power supplies
are located in the middle and lower cubicles of the cabinet behind the microscope
column and operating panel.
Fig. 5 - Power supply furnishing an extremely stable, well-regulated
accelerating voltage for the electron beam of the microscope. Output of 50,000 volts,
up to 1 milliampere is constant to within less than one volt. Styrene condensers
play an important role in this unit. All connecting elements, shields, and tube
sockets are of metal and are gold plated.
Since the velocity of the electron stream is determined by the potential difference
between cathode and anode of the electron gun, this important accelerating potential
(50,000 volts) must be maintained with an exceptionally high degree of constancy.
This is accomplished by a novel high-voltage power supply (Fig. 5) contained on
a single, separate chassis located in the upper cubicle of the power cabinet. Input
r.f. oscillations, stabilized at 75 kc., are rectified by three type 8013-A diodes
arranged in parallel, providing a stabilized and filtered output. So well-regulated
is the output, 50,000 volts up to 1 milliampere, that it is constant to within less
than one volt. Styrene condensers are used; and connecting elements, shields, and
tube sockets are metal, and gold plated!
A total of 25 vacuum tubes, power oscillators, rectifiers, and regulators, are
used in the three power units contained in the cabinet of the microscope.
The complete Electron Microscope (Fig. 4) is considerably more complicated than
the ordinary light microscope. Yet in some ways the operation of the Electron Microscope
is much simpler and far more convenient Both types of magnifying devices have certain
limitations with respect to one another, but the chief difference between the two
types is the higher resolving power and much greater useful magnification of the
Ordinary optical or light microscopes are quite adequate for taking photographs
at magnifications of 1000 or 2000 diameters. However, an attempt at further enlargement
by photography would reveal no new information; the resultant pictures would merely
be larger and considerably less clear. This condition exists because the original
photographs do not have sufficient resolution (clarity of detail) to permit magnification
greater than 1000 to 2000 diameters. In other words, the resolving power of a light
microscope is limited.
In the visible range, light waves have lengths between 0.0004 and 0.0008 of a
millimeter. These wavelengths are relatively long when compared with the size of
the particle or object to be viewed; and it is impossible to detect objects or details
very much smaller than the wavelengths of the type of "illumination" used. For this
reason, the resolving power of a light microscope is limited by the wavelength of
its source of illumination, i.e., light.
High-velocity electrons used in the Electron Microscope, however, have a wavelength
about 1/100,000 that of visible light. Thus, image resolution with the Electron
Microscope is far greater than that obtainable with the light microscope.
Resolution is defined as the ability to distinguish between separate parts or
sections of an image.
As a practical example, the average human eye cannot resolve twodots on a piece
of paper unless they are at least 0.2 millimeter apart. One millimeter equals 1000
microns; but even the micron is too big to measure the infinitesimal field of vision
of the Electron Microscope. The Angstrom unit is used; and 1 micron is equal to
10,000 Angstrom units.
Stated another way, the average human eye cannot resolve or distinguish detail
that is smaller than about 2,000,000 Angstrom units. The Electron Microscope, however,
has a maximum resolution of better than 100 Angstrom units!
Fig. 6. Essential components of the Electron Microscope, which
originate and control the high-speed stream of electrons. The electron gun consists
of a heated cathode and "open" plate, and is the source of the electron beam. Velocity
of electrons is maintained constant by the potential difference (50,000 v.) between
cathode and anode. Downward direction of electron emission is controlled by tilting
and lateral adjustments of the electron gun assembly. Density of the electron beam,
just prior to its application to the specimen under observation, is controlled by
the magnetic field of the condenser lens. Passing through the specimen holder, the
electron beam "illuminates" the prepared specimen. Electrons are affected or influenced
according to the varying density of the specimen area being illuminated. Some electrons
are scattered out of the field completely; those that continue their journey bear
an image of varying density. Focusing of the electron beam is accomplished by adjusting
the amount of current flowing in the coil composing the objective lens, with all
distances between electronic elements remaining fixed. The amount of final magnification
of the image is determined by the amount of current flowing in the coil of the projector
lens; and again no physical movement or change of lenses is necessary to vary the
amount of magnification. Final image is obtained when the electron beam strikes
an observation screen or photographic plate.
In everyday terms, this means that particles smaller than a millionth of an inch
can be seen clearly and distinctly by the Electron Microscope.
In practical operation the microscope normally is used at magnifications between
5000 and 10,000 diameters. Obtained at such values is the best combination of image
brightness, size of the photographic field, and time of photographic exposure. Any
desired additional magnification is obtained by photographic enlargement, making
possible a much greater total useful magnification of the specimen.
In general, absorption of electrons (from the beam) does not occur in the very
small and extremely thin specimens that are usually placed under observation. Occasionally,
however, certain types of specimens absorb electrons from the beam and the released
energy appears as heat. The temperature of the specimen, itself, may often be raised
(in this manner) to such a high degree that decomposition and structural changes
take place. As an additional limitation, the electron beam generates sufficient
heat to kill all life cells, and then dehydrate the remaining inert mass.
During periods of observation, molecular or chemical changes in the specimen
often take place, due to the artificial conditions existing in the vacuum chamber
of the microscope.
Such limitations, however, are but obstacles eventually to be overcome by the
scientist and engineer.
It is important to note that it is not always advantageous to use the Electron
Microscope, when similar information supplied by the optical or light microscope
is sufficiently accurate and adequate. Contrary to sweeping statements circulated
by over-zealous press agents, the Electron Microscope has not replaced the light
A fair consideration of both types of magnifying instruments reveals that they
actually supplement each other. For low-order magnification, the light microscope
is more versatile, and entirely adequate for many forms of scientific research.
When powerful magnification is required, the Electron Microscope can be depended
upon to supply finely focused images of high resolution. The amount of magnification
plus resolution required are the determining factors as to which of the two types
of microscopes to use. Thus, there is a place in science for both instruments.
The precise ability to probe the sub-microscopic - at high magnifications with
high resolution - identifies the scientific importance of the Electron Microscope.
It makes visible many objects which have never before been seen, bringing a new
depth of vision to science and industry alike, helping to improve the way of life
for all mankind.
Posted December 29, 2021