September 1942 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Television and radio facsimile machine electronics technology was
credited for aiding in the development of a new type of scanning
electron microscope (SEM)
that could image the surface of opaque object to a resolution of
100 angstroms (Å). The television contribution part of the
technology was precisely controlling a raster path for the electron
beam. The facsimile part was the knowledge of how to assemble a
printed image from streaming data. It is interesting to note that
in order for an object to be imaged via SEM, its surface must be
electrically conductive. Accordingly, non-metallic objects like
bugs, plants, plastic, wood, etc., must receive a coating of metal
by a process such as sputtering. Doing so can leave a few atoms
thick later without losing too much resolution due to softening
New Scanning Electron Microscope Revealed to Radio Engineers
Dr. James Hillier (foreground). Dr. V. K. Zworykin (seated)
and Richard L. Snyder demonstrating the recently perfected
Dr. V. K. Zworykin describes perfection of new instrument
at Convention of Institute of Radio Engineers at Cleveland,
The picture from the facsimile printer of the instrument
shows etched nickel.
Perfection by a group of scientists of a scanning electron microscope
which enables the study of surfaces of opaque objects, including
metal, in far greater detail than heretofore possible, was revealed
recently by Dr. V. K. Zworykin, associate director of RCA Laboratories,
in a scientific paper read at the Institute of Radio Engineers'
convention in Cleveland, Ohio. The paper was jointly prepared by
Dr. Zworykin, Dr. James Hillier and Richard L. Snyder, all of RCA
Laboratories, who contributed to development of the instrument.
While Dr. Zworykin cautioned that, as with any new instrument,
it is not possible to judge the full range of utility of the scanning
electron microscope at the present time, it was pointed out by associates
that the new device has important, and possibly far-reaching significance
to the field of metallurgy. Investigation of grain structure in
metals on an order of minute detail never before realized becomes
The principles of television, electron microscopy and
facsimile are joined to make this new instrument.
Schematic arrangement of the scanning electron microscope.
Skeleton of the new electron microscope, showing the
placement of lenses in relation to the electron beams.
The scanning electron microscope, Dr. Zworykin said, is the result
of utilizing principles and devices taken from three outstanding
developments in radio and electronics - television, the electron
microscope and radio facsimile.
"Think how stupendous the undertaking would be," said Dr. Zworykin,
"if someone were to conceive an electron scanning microscope and
then set out to research for it without having the knowledge at
his fingertips which we have of television and electron microscopes,
as well as the art of radiophoto or facsimile. It would be an endless
"But, as often happens in science, the discovery of one good
thing leads to another. In addition to the developments mentioned,
the electron multiplier tube is another vital factor in this latest
scanning achievement. After passing through an intricate process
of magnetic scanning, the electron beam containing the picture information
is converted to light at a fluorescent screen. The light is concentrated
by a wide-aperture lens on the photo-cathode of the multiplier,
which in turn operates a printing device."
Explaining how the new instrument works, Dr. Zworykin said that
the formation of an image may result from one of two methods. One
is the simultaneous imaging of all the elements, as in the projection
by optical lenses of a picture on a screen, on a photographic plate,
or on the retina of the human eye. The second method is by the successive
recording of individual image elements, the process used in television.
"Modern high-power electron microscopes may utilize either of
these procedures," Dr. Zworykin said. "The conventional electron
microscope - also perfected by RCA Laboratories - like any ordinary
optical instrument, records the entire array of elements simultaneously
in the form of an electron-optical image. The scanning microscope,
however, adopts the methods of television, that is, records one
picture element at a time ..."
Each type of electron microscope has its own particular sphere
of usefulness, Dr. Zworykin pointed out. The conventional electron
microscope, he said, is primarily suited for the study of "transparent"
objects, specimens less than a micron in thickness; while the scanning
microscope has as its special province the investigation of the
surface of objects, which are opaque to electrons. Roughly, the
two types of electron microscopes correspond in application to the
light microscopes used in biological and metallurgical work, but
the electron instruments "see" much greater detail.
"As simple as the basic principle of the scanning electron microscope
may appear," continued Dr. Zworykin, "the realization of an instrument
with a resolving power comparable with that of the electron microscope
was confronted with very great difficulties. These obstacles arose
from the necessity of making the scanning spot no larger than the
least separation which is to be resolved, that is, no larger than
one two-millionth of an inch (100 Angstrom Units) in diameter. It
was no easy job to form a spot of this size, but we have succeeded."
In fact, so accurate and delicate is the control of the electrons
in passing through the instrument to help "see" the surface of the
metal, or object being observed and photographed, that the tiny
electrons pass through the scanning tube about one yard apart. The
mastery achieved is indicated further by the fact that, theoretically,
it would require 30 billion, billion, billion of electrons to weigh
The scanning electron microscope was brought to perfection by research
and development work which extended over a period of years, and
was participated in by a large number of scientists in RCA Laboratories.
The authors of the paper, Zworykin, Hillier and Snyder, called particular
attention to the early work of Arthur W. Vance and L. E. Flory,
and to the mathematical contributions of Dr. E. G. Ramberg, all
of RCA Laboratories.
A summary of the technical paper prepared by the authors follows:
In order to examine the surface of bulk material with the high
resolving power afforded by the use of an electron beam a new electron
microscope of the scanning type has been developed in which an extremely
fine and stationary electron probe is produced by a two-stage reducing
electrostatic-lens system. The specimen is moved mechanically in
such a way that each point of its surface is scanned in a systematic
fashion by the electron probe. The secondary electrons which are
emitted from the point of the specimen bombarded by the electrons
of the probe are accelerated and projected on a fluorescent screen.
The intensity of the light emitted by the fluorescent screen varies
in accordance with the secondary-emission properties of successive
points of the specimen. This modulated light signal is converted
into an electrical signal by means of a multiplier phototube and
then synthesized in a printed picture by an amplifier and facsimile
printer system. The use of the electronic-light-electronic transformation
of the image signal improves the signal-to-noise ratio by at least
an order of magnitude over that found in conventional methods of
collection and voltage amplification. An experimental model has
been constructed and has been successful in producing images of
etched metal surfaces at magnifications as high as 10,000 diameters
and with a resolving power considerably better than 50 millimicrons.
The blackening of the image points has been found to be a function
of the three-dimensional contour of the specimen as well as of its
Posted October 19, 2014