ELECTRON MICROSCOPE

N. N. Malov, Moscow

I. Introduction

The rapid development of radio engineering, the use of ultrahigh-frequency installations in high-current technology, and also a whole series of research problems from various areas of physics have urgently required the creation of an apparatus capable of registering very short-lived (millionths of a second and less) electromagnetic processes. No mechanical construction, because of its inertia, is capable of satisfying such a requirement, but the solution of this problem can be achieved with the aid of an electron beam produced in one or another discharge tube, since this beam, first, has practically no inertia and, second, can be controlled by means of an electric or magnetic field. In a relatively short time the Braun tube, first used for recording electromagnetic processes as early as 1897, was considerably improved, and at the present time cathode oscillographs, excellently developed from the constructional point of view,^1 have become fairly widespread and make it possible to record processes having durations of the order of \(10^{-8}\) sec.

The electrons emerging from the cathode of the oscillograph have, generally speaking, differently directed velocities, mutually repel one another, and are scattered upon colliding with the gas molecules present in the tube. As a result, the cross-section of the electron beam gradually broadens as it recedes from the cathode, and when the electrons fall on the fluorescent screen used for visual observation, or on a photographic plate, they produce on it a trace that is a more or less blurred spot.

Consequently, first, the recorded curve proves to be very blurred; second, the brightness of the spot is greatly weakened, which entails a reduction in the possible recording speed, i.e., worsens the quality of the oscillograph.

In numerous attempts to concentrate the electron beam in order to obtain a bright spot of the smallest possible area,

It became clear that, under suitable conditions, there is an extraordinarily far-reaching analogy between the propagation of a bundle of light rays in some optical system and the course of an electron beam in electric or magnetic fields, which may be chosen in such a way that they will collect or scatter the electron beam analogously to the way lenses collect or scatter light rays. Furthermore, it proved possible to develop the technique for controlling an electron beam to such an extent that “electronic” images could be obtained of a source of electrons or of an object standing in the path of the electrons (“illuminated” by electrons), and the magnification of the image could be made extremely large. Thus it was possible, in the end, to construct an “electron microscope,” already now providing “electronograms” (Elektronenabbildungen) which are not inferior in quality to ordinary microphotographs and which possess, as we shall see below, a significantly greater resolving power than an ordinary microscope.

A change in the direction of a light ray in an ordinary optical system is obtained either when it is reflected, in which case the velocity of propagation remains unchanged, or when it is refracted, when the velocity of the ray changes. “Electronic reflection” can be obtained by means of a negatively charged plate or of a suitably chosen magnetic field; in the former case the magnitude of the velocity of the electrons changes continuously. Refraction of an electron ray is obtained when it passes through a corresponding condenser, or through a magnetic field. The first case corresponds to optical refraction in a medium with a continuously varying refractive index; the second, where the velocity of the electrons does not change its magnitude, has no analogy in ordinary optics. Thus in electron optics no sharp boundary can be drawn between reflection and refraction. Moreover, in ordinary optics as well this distinction sometimes disappears: for example, reflection of an electromagnetic ray from a layer of Heaviside is, in essence, the result of its multiple refraction.

The analogy between geometrical and electron optics can be continued further. A parabolic mirror corresponds to the Wehnelt cylinder of a Braun tube, successfully used to create a parallel beam of electrons. To the phenomenon of total internal reflection of light rays, which accounts, in particular, for the propagation of a ray along a narrow glass tube, one may oppose the van der Bijl phenomenon (observed for certain relations between the gas pressure and the velocity of electrons in discharge tubes, and consisting in the spontaneous concentration of the electron beam owing to the action of positive ions

...head, concentrating along the beam axis, and the formation around the beam of a sheath of negative charges, as well as, apparently, the ability of the electron beam to propagate along a narrow metal tube, discovered by Broglie’s recent experiments². Figures 1 and 2 show electron beams produced in the left glass sphere and penetrating into the right sphere, whose walls are coated with a fluorescent layer, through a copper tube 30 cm long and with an internal diameter of 2.5 mm. The spheres are filled with hydrogen at a pressure of \(10^{-2}—10^{-1}\) mm of mercury.

Fig. 1. Passage of an electron beam through a straight copper tube (\(l = 30\) cm, \(d = 2.5\) mm).

It is interesting to note that the phenomenon of reflection of electrons from charged layers created by the space charge inside the tube is so sharply expressed that an external magnetic field of the order of several gauss has no noticeable effect on the passage of electrons along the tube.

If one takes into account that it is possible easily to change the “refractive power of electromagnetic lenses” by changing the sign and magnitude of the electric and magnetic fields acting on the electron beam, and that the velocity of the electrons, which affects the refractive index, is likewise conveniently adjustable, then it becomes clear that the application of “electron optics” opens up extremely broad prospects for the scientific investigator, the full significance of which can scarcely be assessed at the present time. For example, Broglie³ admits the possibility of creating an “electron telescope,” which will collect a beam of electrons flying from the surface of the sun (of course, for this it will be necessary to rise with the telescope beyond the Heaviside layer) and will make it possible to study details of the structure of sunspots.

Fig. 2. Passage of an electron beam through a curved copper tube.

2. Obtaining an Electron-Optical Beam

In attempts to obtain an undistorted image of an electron source, or of an object illuminated by electrons, by means of electron lenses, it is necessary to take into account a number of possible disturbances that are absent in ordinary optics of light rays.

First of all, measures must be taken to eliminate the influence on the electron beam of extraneous electric and magnetic fields, analogous to the measures taken when working with an ordinary cathode oscilloscope (elimination of the magnetic field of the earth and of supply leads, prevention of the formation of static charges on the walls of the tube and on the photographic plate or fluorescent screen, etc.). Further, the field accelerating the electrons, situated between the cathode and the anode, should, as far as possible, be homogeneous and possess symmetry with respect to the axis of the tube.[^4] It is expedient to make the surface of the emitting cathode in the form of a plane, whose dimensions somewhat exceed the dimensions of the object being photographed with the electron microscope. The anode may be made in the form of a tube enclosing the cathode and provided with a diaphragm. In this case the field near the cathode proves to be almost homogeneous, but gradually weakens with distance from the cathode; as a result the lines of force are somewhat curved, producing a slight scattering of the electron beam; this scattering occurs before the beam enters the “lens” and, as Busch showed,[^5] does not distort the image obtained, but only somewhat changes the scale of the image.

The next cause that may produce distortions of the image obtained is the gradual expansion of the electron beam under the action of the forces of electrostatic repulsion of the electrons, which exceed (since the velocity of the electrons is less than the velocity of light) the forces of mutual attraction of the current lines created by the electrons. Calculation shows[^6] that even at relatively low accelerating electron voltages the scattering of the beam does not play any appreciable role if the current density is not too great; the experiments of Knoll and Ruska[^7] showed that, at the current densities required for obtaining sufficiently bright images, no expansion of the beam is observed.

Finally, the last possible cause of distortions is the partial scattering of the electron beam in collisions of the electrons with gas molecules present in the discharge tube. Such distortions will appear only when the collisions of electrons with gas molecules are relatively numerous. By reducing the gas pressure and increasing the velocity of the electrons, the mean free path of an electron can readily be made comparable with the length of the tube, owing to which

scattering of the beam becomes imperceptible (for a tube length of about 1 m and an accelerating potential difference \(V=100\) V, the pressure in the tube, in the case of air, must not exceed \(5 \times 10^{-4}\) mm of mercury; at \(V=100\) kV the pressure may be increased to \(5 \times 10^{-1}\) mm).

TABLE 1

Survey of electron-optical systems

If all the above-mentioned conditions are observed, no distortion of the rectilinear paths of the electrons will arise, and the electron beam, produced at some point of the tube, will propagate along the tube analogously to a beam of light rays.

Let us now turn to the consideration of electromagnetic systems used for obtaining images by means of such an electron-optical beam.

3. Survey of Electron-Optical Systems

Table 1 compares various methods for obtaining electronograms by means of electromagnetic “optical” systems. Fig. 3 gives a schematic representation of these same systems. Let us examine their properties.

The simplest method for obtaining electronograms is the method of a diaphragm with an aperture, the dimensions of which must be considerably smaller than the dimensions of the object being studied (Fig. 3, a). In this case, strictly speaking, there is no electromagnetic optical system. Magnification of the image obtained is

\[ M=\frac{b}{a}, \tag{1} \]

where \(b\) and \(a\) are the distances of the image and the object from the diaphragm.

Fig. 3. Various methods of obtaining electronograms.

If the dimensions of the aperture of the diaphragm are too large relative to the dimensions of the electron source, then the image of the source will no longer be obtained on the screen; instead, the aperture of the diaphragm will be projected. The photographs obtained by this method prove not entirely satisfactory, since the edges of the images are not especially sharp (see Fig. 6, a).

Another simple method, long known, for obtaining images is the concentration of the electron beam by a coaxial homogeneous magnetic field, produced by a coil that surrounds the discharge tube along the entire length of the path of the electron beam\(^{9}\) (Fig. 3, b). Since the magnification of the image in this case is equal to unity, this method is not of great interest.

The method shown in Fig. 3, g, in which the electron beam is self-focused with the aid of a column of space charge formed, at a suitable pressure, along the entire length of the electron beam\(^{10}\) (the van der Bijl effect), likewise does not make it possible to obtain a magnified image; this method and the preceding one are suitable for studying the structure

emitting surface, projected on the screen at its natural size, since the deflection of each electron ray in the field of the space charge (or in the magnetic field) proves to be proportional to the distance of this ray from the axis of the beam.

If a column of space charge is created not along the entire length of the ray path, but only over a small segment of it, then, owing to the change in direction of individual electron rays only over this small portion of the path, one can obtain on the screen an enlarged image of an object emitting electrons or illuminated by electrons, since such a limited space charge will act as a lens whose focal length can be varied by varying the velocity of the electrons and the magnitude of the field of the space charge. A short column of space charge can be obtained by two methods¹¹. In the first, the tube is partitioned by three electrodes having apertures arranged on the axis of the tube; an auxiliary negative potential is applied to the middle electrode, while the outer ones are grounded or connected to the anode.

With a suitable evacuation, a discharge occurs between the electrodes, and its distribution has the form shown in Fig. 3, h. Depending on the magnitude of the auxiliary potential, the refraction of the beam of electrons flying from the cathode will be more or less strong, and an image of some cross-section of the electron beam will be obtained on the screen. In the second method, two diaphragms with apertures are placed in the discharge tube. These diaphragms isolate in the tube a small volume into which air is admitted by means of a special valve. On the sides of the resulting chamber, two openings are made in the walls of the discharge tube, connected to a powerful evacuation pump. In this way it is possible to maintain, throughout the entire volume of the tube (except for the chamber), a sufficiently high vacuum preventing the formation of space charge, while inside the chamber the pressure is increased¹² (owing to the throttling action of the diaphragm apertures) and a space charge arises, causing refraction of the electron rays.

Both of these methods make it possible to obtain an arbitrary magnification of the object. Experimental investigation of such an electron lens has shown that the image obtained is distorted; this is apparently explained by the difficulty of obtaining the necessary distribution of the auxiliary discharge and by the influence of the slow electrons of this discharge on the passing electron beam. However, experiments of this kind are still very few in number, so that at present it is difficult to make a final conclusion about the suitability of this method.

Significantly better results are obtained when controlling the electron beam by means of an electrostatic field. The principal methods of changing the direction of the rays of the electron

Fig. 4. Electrostatic systems of electron optics.

of the beam by means of an electric field are shown in Fig. 4. Their action is clear without further explanation. It is necessary to note the possibility of changing the refractive power of electron-optical systems by regulating the voltage between the plates of the capacitors; when the direction of the electric field is changed, the converging system becomes a diverging one, and vice versa.

When an electron beam having a velocity of \(V\) volts passes through an infinitely thin double electric layer possessing a potential difference of \(V_0\) volts, the refractive index of the layer is

\[ n=\sqrt{1+\frac{V_0}{V}}=\frac{\sin \alpha}{\sin \beta}, \tag{2} \]

where \(\alpha\) and \(\beta\) are the angles of incidence and refraction\(^{18}\).

Since \(V_0\) may take all possible values (from \(-\infty\) to \(+\infty\)), the following electron-optical phenomena are possible (Table 2), schematically represented in Fig. 5.

In the practical realization of electrostatic electron lenses, openings have to be made in the electrodes for the passage of the electron beam (Fig. 3, \(d\)). In order to reduce field distortions, the openings are covered with a fine-mesh grid. In this case, partial deposition of electrodes on the grid and the not quite uniform distribution of the field around the grid wires somewhat distort the image.

Fig. 5. Paths of electrons in a double electric layer.

If the electron lens is made in the form of a spherical capacitor, in the plates of which small openings covered with a grid have been made, then, as calculation\(^{4}\) shows, the focal length of such a lens is

\[ f \simeq r\frac{V}{V_0}, \tag{3} \]

where \(r\) is the radius of the lens. The image of the object is, of course, inverted and may be magnified arbitrarily; by changing the focal length (by changing \(V_0\)) it is possible to obtain images of various sections of the electron beam. Knoll and Ruska\(^{8}\) (Fig. 3, \(d\)) placed between the anode and the lens a diaphragm with a T-shaped aperture, the dimensions of which exceeded the dimensions of the cathode, and, changing \(V_0\) from zero to a certain value, obtained the photographs shown in Fig. 6. Photograph 6, \(a\) was obtained at

\(V_0 = 0\) and corresponds to the projection image of the diaphragm. When \(V_0\) is increased, the action of the lens begins to be felt and an image of the cathode appears (photograph 6, \(b\)), and then also of the diaphragm (Fig. 6, \(c\)), rotated, as was to be expected, by \(180^\circ\) (Fig. 6 \(a\), \(b\), and \(c\); see inset). Owing to the distortions of the electronograms mentioned above, they are not especially sharp.

Brüche and Johannson\(^3\) used as a lens a condenser consisting of a rod electrode arranged along the axis of the electron beam and two hemispherical electrodes placed on both sides of the central electrode and electrically connected to one another. By varying \(V_0\), they obtained the photographs shown in Fig. 7, in which the refraction and focusing of the electron beam are clearly visible. An essential drawback of this lens is the falling out of the central part of the image and the impossibility of getting rid of the spherical aberration of the lens, since it is not possible fully and strictly to satisfy the condition that the retardation of the ray direction be proportional to its deviation from the axis.

TABLE 2

Refracting power of a double electric layer

To eliminate the distortions introduced by the mesh that provides the spherical form of the equipotential surfaces in a spherical condenser, one may use the combination of plane electrodes with openings for the passage of the electron beam shown in Fig. 3, \(e\). Since at the surface of the electrodes the equipotential surfaces have

To N. Malov’s article.

a  b  c

Fig. 6. Electronograms of the cathode and T-diaphragm, obtained with the aid of a spherical electrostatic lens.

Fig. 7. Demonstration Brüche-Johannson electrostatic lens.

Fig. 9. Electronogram (left) and microphotograph (right) of the surface of an oxide cathode (150-fold magnification).

ELECTRON MICROSCOPE

almost spherical shape \(^{14}\) (Fig. 8), one obtains a combination of two plano-convex lenses with focal length

\[ f \approx 2r \frac{V}{V_0}, \tag{4} \]

where \(r\) is the mean radius of the equipotential surfaces.

Fig. 9 shows an electronogram of an oxide cathode obtained in this way, at a relatively small accelerating potential (650 V), by Brüche and Johannson \(^{3}\). For comparison, alongside it is shown a microphotograph of the same object at the same magnification. The intersecting straight lines are images of a grid scratched on the surface of the cathode, which served for determining the magnification.

Combining both of the above-mentioned methods, we obtain a system (Fig. 3, f) in which the equipotential surfaces have a more regular spherical form, owing to which the distortions produced when working with the system 3, e are eliminated; at the same time, the deposition of electrons on the grids is less than in system 3, d, so that the sharpness of the image is increased. In Fig. 3, f, instead of a biconvex lens, a biconcave one is shown, but with the electric field directed oppositely.

Fig. 8. Distribution of the electric field near plane electrodes with apertures.

The electronogram shown in Fig. 9 proves quite clearly that, by means of electrostatic electron lenses, it is possible to obtain excellent images.

The last method that remains for us to consider—the magnetic lens—gives equally good results. Whereas a magnetic field coaxial with the electron beam and extending along its whole length promotes concentration of the electron beam, the application of a magnetic field limited to a small region must cause refraction of the electron rays over a small segment of their path and may, under suitable conditions, be used as a collecting lens (Fig. 3, c). This was first pointed out by Busch \(^{15}\), who showed that a sufficiently short coil behaves with respect to an electron beam in the same way as an ordinary lens does with respect to light, and that the focal length of such a magnetic lens is determined by the relation:

\[ f=\frac{ab}{a+b}=\left(\frac{2v}{e/m}\right)^2 \frac{1}{\displaystyle \int_{-\infty}^{+\infty} H^2(z)\,dz}, \tag{5} \]

where \(v\) is the velocity of the electrons, \(e/m\) is their specific charge, \(H(z)\) is the intensity of the magnetic field of the coil in the direction of its axis; the magnification of the image is determined by the usual optical formula:

\[ M=\frac{L_{\mathrm{im}}}{L_{\mathrm{pr}}}=\frac{b}{a}, \tag{6} \]

where \(b\) and \(a\) are the distances of the image and the object from the lens.

Since the coil has finite dimensions, if one wishes to obtain a very large magnification it may happen that the distance \(a\) must be reduced so much that the object will lie in the region where the field of the coil already has an appreciable value. Then the portion of the field lying beyond the object will, of course, not take part in the refraction of the electron rays; the theoretical relation (5) loses its force, and the calculation of the lens for this case becomes greatly complicated.

Therefore, when it is necessary to obtain a very large magnification, the field of the coil must be concentrated over as short a segment as possible; for this purpose the coil is provided with an iron casing, closely adjoining the walls of the discharge tube, owing to which the field is concentrated only in a very small space devoid of iron. Busch’s theory was subjected to experimental verification by Knoll and Ruska \(^{18}\), who found a quite satisfactory agreement between the theory and the experimental data. The photographs obtained were distinguished by great sharpness and brightness. Fig. 10 represents an electronogram of a circular aperture of a diaphragm, taken with the aid of a magnetic lens (13-fold magnification). Comparison of it with the corresponding electronogram (Fig. 6, \(b\)), obtained with the aid of an electrostatic lens with spherical electrodes, clearly shows the advantages of the magnetic lens. By changing the current in the coil and, consequently, its focal distance, Knoll and Ruska were able to obtain images of various cross sections of the electron beam and, in particular, of various diaphragms placed one after another along the beam; the electronograms obtained were quite satisfactory.

Because of the unequal velocity of the individual electrons of the beam, distortions of the image are possible analogous to the chromatic aberration of ordinary optical systems. It was established experimentally that, at sufficiently high accelerating potentials, these distortions do not play an appreciable role. Fluctuations of the voltage supplying the discharge tube may cause a change in the sharpness and intensity of the whole image, since it is displaced somewhat from the focal plane; this becomes especially noticeable at high current densities produced by the electron beam. The installation of a suitable filter in the rectifier circuit feeding the discharge tube makes it possible to eliminate these distortions.

Distortions arising due to the non-proportional deflection of electrons relatively far from the axis of the electron beam (spherical aberration) proved to lie beyond the limits of observational accuracy, since the cross section of the electron beam is, generally speaking, very small, and the electrons enter the lens very close to its axis. For the same reasons, electronograms also do not show other distortions associated with the incidence of rays on the lens at very large angles (astigmatism, coma, etc.).

4. Electron microscope

Since the possibility of obtaining electronograms by means of one or another electromagnetic lens had been experimentally demonstrated, the idea naturally arose of combining in a single discharge tube two or several optical systems which would make it possible, without excessively increasing the length of the tube, to obtain strongly magnified images of a source of electrons or of an object “illuminated” by them (the electron microscope).

Theoretically, it is conceivable to combine any of the types of electron lenses considered above. Practically, up to the present time, combined electron microscopes giving large magnifications (up to 400-fold) have been constructed by Knoll and Ruska⁴ and by Knoll, Gautermann and Schulze¹⁷, who used a combination of magnetic lenses made in the form of short coils provided with an iron casing.

Fig. 11. General view of an electron microscope with magnetic lenses.

The finished construction of the electron microscope of Knoll, Gautermann and Schulze is shown in Fig. 11; Fig. 12 gives the diagram of this electron microscope.

A heated equipotential cathode, made of a massive nickel plate coated (on the surface facing the lenses) with a layer of an electron-emitting substance, was introduced into the glass discharge tube on a stem. On the opposite side of the cathode there was placed a heating filament. For po-

of the radiation along, as far as possible, parallel equipotential surfaces; the cathode was provided with a guard ring. The considerable thickness of the nickel plate reduced the distorting action of the magnetic field of the heating current. To determine the resulting magnification on the emitting surface of the cathode, a grid was ruled with the aid of a dividing machine. The anode tube surrounded the cathode and extended far along the length of the electron beam, protecting it from external electric fields. The lenses were made in the form of coils placed outside the tube and suspended on Cardan suspensions. At small magnifications only one coil operated (the one situated closer to the cathode). To control the focusing of the image produced by the first coil at large magnifications, the second coil was provided with a control grid, which at the same time served as an ocular micrometer.

Fig. 12. Diagram of the electron microscope.

The influence of the earth’s magnetic field was compensated by a permanent magnet placed near the tube. In visual observations the fluorescent screen was placed directly on the wall of the tube, which, as usual, had a slightly curved form ensuring greater strength of the tube. When photographing electronograms, carried out with a camera placed outside the microscope, the rear wall of the tube was made plane-parallel. The tube was constantly under evacuation. The accelerating voltage was small (up to 2 kV); the current through the tube amounted (depending on the nature of the object under study and on the magnification) to from \(10^{-3}\) to \(10^{-6}\) A; the exposure was from 0.5 to 30 sec. The whole apparatus was mounted on an optical bench, along which, if desired, the magnetic lenses could easily be moved.

Since slight mechanical vibrations of the microscope caused noticeable distortions at large magnifications and long exposures, special measures had to be taken to eliminate them.

Another specimen of an electron microscope, constructed by Knoll and Ruska,\(^4\) operated at high accelerating voltages (from 10 to 100 kV); in it there was used not a heated but a cold cathode, and, in addition to the two lens coils installed in the first design and playing the role of objective and ocular, a third coil was also used, placed between the cathode and the illuminated object and concentrating the electron beam on the object (analogously to the condenser of an ordinary microscope). In this design the tube was

To p. N. N. Malov.

Fig. 10. Electronogram of a round aperture, taken with a magnetic lens (13-fold enlargement).

Fig. 13. Electronogram (left) and microphotograph (right) of a mesh made of molybdenum wire. Electronogram taken at 70 kV accelerating voltage, using one magnetic lens (12-fold enlargement).

Fig. 14. Electronogram of a mesh made of molybdenum wire, taken at 65 kV accelerating voltage; (10- and 150-fold enlargement).

a          b

Fig. 16. Electronogram (a) and microphotograph (b) of a cathode coated with a thin oxide layer (13-fold enlargement, 2000 V).

metallic, and the fluorescent screen was made of a thin glass plate (0.1 mm), coated with a metallic layer (about 100 μμ thick), obtained by cathodic sputtering.

By changing the currents flowing through the coils, it was possible to vary the focal lengths of the magnetic lenses and to obtain images of various sections of the electron beam at the most diverse magnifications (up to 400-fold).

In Fig. 13 is shown an electronogram of a molybdenum wire mesh placed near the condenser lens, taken with the second microscope at an accelerating voltage of 70 kV, using one magnetic lens (12-fold magnification). For comparison, beside it is placed an optical microphotograph of the same object, which proves to be much less contrasty.

In Fig. 14 the same mesh was photographed with the electron microscope twice: once at 10-fold magnification (one lens), the other time (both lenses) at 150-fold magnification. The accelerating voltage in both cases was 65 kV. The blurring of the edges of the wire at high magnification is explained by vibrations of the microscope caused by the operation of the pump being evacuated.

a  b  c  d  e  f  g

Fig. 15. Electronograms of a diaphragm with a T-shaped opening, taken with the aid of two magnetic lenses while changing the distance between the objective and the screen. Magnification from 1 to 18 times.

In Fig. 15 are reproduced electronograms of a diaphragm with a T-shaped aperture, obtained (with an unchanged distance between the diaphragm and the screen) by changing the focal lengths of two magnetic lenses. The magnification changes from one-fold to 18-fold. Comparison of these electronograms with electronograms of the same diaphragm taken with an electrostatic lens with spherical electrodes (Fig. 6, b) clearly demonstrates the advantages of magnetic lenses.

By selecting the focal lengths of the magnetic lenses in such a way that an image of the emitting cathode is obtained on the screen, one can study the processes occurring on it under different conditions of temperature and pressure of the surrounding gas.

In Fig. 16, a is shown an electronogram of a heated cathode coated with a thin oxide layer, taken at an accelerating voltage of 2000 V. It is clearly visible in it that, despite the uniform coating of the cathode, not all parts of the oxide layer emit with equal intensity. The ordinary optical microphotograph of the same cathode shown in Fig. 16b (taken in reflected light) makes it possible to see the “dark” (with respect

(emitted electrons) regions of the cathode with a mesh deposited on them. The bright spot emitting electrons in the electronogram on the microphotograph, of course, had lost considerably in brightness.

In Fig. 17 a series is given of successive electronograms of a thin oxide layer that emitted in vacuum over the course of 8 hours (the photographs were taken with the first microscope at an accelerating voltage of 2000 V). In the photographs, gradual changes are clearly visible in the intensity of emission in individual parts of the cathode, the formation of islands of emission and their gradual destruction.

The same picture is even more sharply noticeable at high (100-fold) magnification (Fig. 18). Of particular interest are the bright, strongly emitting bands, which are cracks that have formed in a thick oxide layer operating under conditions of a gas discharge.

Numerous photographs of the emitting cathode made with the aid of the electron microscope make it possible to study in detail the operating conditions of the cathode, the distribution of emission intensity, etc.

With an increase in the cathode filament current, an image that had previously been perfectly sharp becomes partially veiled (Fig. 19, a and b). If the setting of the electron microscope is slightly changed by changing the current in the magnetic lens, then a sharp image of the emitting island again appears; it is thus proved that the veiling of the photograph is produced by a cloud of space charge formed above the intensely emitting part of the cathode (Fig. 19, c).

Without dwelling on further results of the study of the emitting cathode with the aid of the electron microscope, since at present they have only the character of preliminary data, let us briefly consider the possibilities that the electron microscope opens up for scientific investigations.

The study of the structure of meshes, small apertures, the determination of the sizes of grains and of the nonuniform density of layers of substances that are completely or partially permeable to the electron beam can be carried out in “transmitted electron light,” by placing the body under investigation in the path of the beam in front of the lenses.

If the surface of a body is capable of reflecting electrons, then the study of the structure of the surface can be carried out in reflected electron light, if a homogeneous beam directed onto the surface of the body is reflected by it into the electron-optical system.

If the surface of a body emits electrons, then a direct study of the distribution of emission over the surface of the body is possible. By this method one can study the properties of cold and heated cathodes of discharge tubes, as well as photocathodes.

Reference to the article by N. N. Malov.

a  b  c  d  e

f  g  h  i  k

Fig. 17. Formation and change over time of emitting islands on a thin oxide layer operating in vacuum for 8 hours. The electronograms were taken at 2000 V.

Fig. 18. Intense emission from cracks in a thick oxide layer (100-fold magnification, 2000 V).

a  b  c

Fig. 19. Formation of a space-charge cloud near the surface of a thick oxide layer. a—space charge is absent; b—blurring of the emitting island by the space charge when the cathode incandescence is increased; c—restoration of a sharp image of the emitting island by changing the focal length of the electron-optical system.

By changing in this case the focal length of the system, one can obtain images of various sections of the electron beam located above the emitting surface and study the distribution of space charge in them.

Finally, by combining electron lenses with ordinary optical systems, one may hope to obtain a very highly perfected microscope, since distortions that may arise in electron diffraction patterns at excessively high magnification may be compensated to one degree or another by an ordinary optical system, whose properties are at present known to us in greater detail than the properties of electron lenses.

The above summary does not, of course, cover all possible applications of the electron microscope, whose properties have as yet been studied far from completely. Its undoubted advantage over the optical microscope lies in the ease with which one can change the focal length of the electron lenses and the magnification of the image obtained; moreover, the resolving power of the electron microscope is considerably higher than that of an ordinary one. Indeed, in an ordinary optical system the resolving power is

\[ d=\frac{\lambda}{A}\ \text{cm}, \tag{7} \]

where \(\lambda\) is the wavelength and \(A\) the aperture. A normal microscope has a resolving power of about \(0.2\,\mu\); in determining the resolving power of an electron microscope, one must consider the de Broglie wave associated with the moving electron, whose length is

\[ \lambda=\frac{h}{mv}=1.22\cdot 10^{-7}\frac{1}{\sqrt{V}}\ \text{cm}, \]

where \(h\) is Planck’s constant, \(m\) and \(v\) are the mass and velocity of the electron, and \(V\) is the accelerating voltage, expressed in volts. At \(V=2000\ \mathrm{V}\) we obtain \(\lambda=2.7\cdot 10^{-9}\ \text{cm}\). With an increase in the accelerating voltage, the wave associated with the electron will be still smaller.

Since, in order to obtain undistorted electron diffraction patterns by means of magnetic lenses, one must work with electrons whose paths form small angles with the axis of the lens, the aperture of the electron microscope proves to be considerably smaller than that of the optical one. Knoll and Ruska find for their microscope an aperture \(A_1=0.02\). With these data, the resolving power of the electron microscope is found to be

\[ d_1=\frac{2.7\cdot 10^{-9}}{0.02}=1.35\cdot 10^{-7}\ \text{cm}=13.5\ \text{\AA} \]

which exceeds the resolving power of the optical microscope by approximately 150 times.

The aperture of the electron microscope, in the opinion of its designers, can still be considerably increased, so that a further increase in resolving power, bringing us close to the limits of the molecular world, appears entirely possible.

In conclusion, let us point out that, theoretically, the construction of an ion microscope is also possible, in which illumination of the object is produced not by electrons, but by ions. Since, however, the mass of an ion considerably exceeds the mass of an electron, the magnetic lens for ions, at the same focal distance and accelerating voltage, must have a considerably larger number of ampere-turns (approximately 100–200 times larger), which greatly complicates the construction of such a microscope.

More favorable in this respect are electric lenses, whose focal distance does not depend on the mass of the particle undergoing refraction. However, until recently there has been no information in the literature on the construction of an ion microscope.

The problem of constructing the electron microscope was solved only a few months ago. The existing designs, of course, are still very far from the perfected type of electron microscope which, perhaps in the near future, will allow us to see and register a whole series of phenomena that until now have been inaccessible to direct observation.

Literature

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2. E. Brüche, “Z. Physik” 78, 177, 1932.

3. E. Brüche, “Naturwiss.” 20, 353, 1932.

4. M. Knoll and E. Ruska, “Z. Physik” 78, 318, 1932.

5. H. Busch, “Arch. f. Elektrot.” 18, 588, 1927.

6. E. E. Watson, “Phil. Mag.” 3, 849, 1927.

7. M. Knoll and E. Ruska, “Z. f. techn. Phys.” 12, 3–9, 1931.

8. M. Knoll and E. Ruska, “Ann. d. Phys.” 12, 607, 641, 1932.

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12. W. Wien, “Ann. d. Phys.” 76, 117, 1925.

13. H. Bethe, “Naturwiss.” 15, 785, 1927.

14. F. Ollendorf, Potentialfelder d. Elektrotechnik, p. 297, Berlin, 1927, Verlag J. Springer.

15. H. Busch, “An. d. Phys.” 81, 974, 1926.

16. M. Knoll and E. Ruska, “Z. f. techn. Phys.” 12, 448, 1932; see also 7.

17. M. Knoll, F. G. Houtermans and W. Schulze, “Z. Physik” 78, 340, 1932.