Exploring the Infinitesimal
June 1948 Radio News

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 acknowledged.

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

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 the infinitesimal.

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!

Basic Principle

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.

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 necessary.

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.

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 is obtained.

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.

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.

Resolution

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 Electron Microscope.

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!

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 microscope!

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