FROM CURRENT LITERATURE

UNIVERSAL ELECTRON MICROSCOPE FOR BRIGHT-FIELD, DARK-FIELD, AND STEREO IMAGING¹

In the last two years, descriptions have appeared in the literature of electron supermicroscopes with both magnetic² and electrostatic³ focusing.

The universal electron microscope described in M. Ardenne’s article (Fig. 1) makes it possible, as desired, to use either magnetic or electrostatic lenses (chiefly the objective). Further, this microscope makes it possible to effect a rapid transition from bright-field images to dark-field ones, and also makes it possible to obtain stereoscopic images of the observed object.

The universality of the instrument is ensured by the fact that almost all its main component parts can be removed and replaced individually without dismantling the entire microscope. Thus, for example, the pole shoes of a magnetic lens can be replaced by the electrodes of an electrostatic lens. In exactly the same way, objectives or projection lenses with different specifications and different auxiliary mechanisms can be installed at will in the path of the electron beams.

All diaphragms can be centered and replaced under vacuum. The possibility of centering the diaphragms ensures especially great sharpness of the image by reducing astigmatism and other aberrations. The possibility of replacing diaphragms under vacuum permits a rapid transition from a bright-field image to a dark-field one.

A special device makes it possible to establish the conditions corresponding to the best sharpness and to maintain them for several hours, or to reproduce them with an accuracy of up to \(10^{-4}\), which is especially important when photographing with a long exposure time. The rigid connection between the objective cartridge and the pole shoes of the objective makes the instrument insensitive to shaking. To eliminate the effects of external magnetic fields on the path of the electron beams, the instrument is shielded with permalloy. The source of electron beams is a system consisting of a tungsten wire (cathode) situated inside a cylindrical control electrode (Wehnelt cylinder), to which a small negative potential is applied, and a coaxial anode with a hole bored through its center, maintained at the potential of the accelerating voltage. With a negative voltage of 100–200 V on the control electrode and an accelerating voltage of 60 to 70 kV, this system gives a weakly diverging electron beam with a total beam current of \(2\text{–}5 \cdot 10^{-4}\) A. The distance between the beam cross section formed by the condenser optics and the center of the condenser lens is approximately twice the condenser–object distance, so that the condenser optics reduces the beam cross section by a factor of two. The irradiated surface of the object is then only a part of the field of view, which reduces the total thermal load on the object and its holder, making possible the study of objects sensitive to temperature.

The object is placed in a special cartridge, which can move in a plane perpendicular to the optical axis. The focal distan-

Labels in the diagram: Condenser; Objective; Projection camera; photo-camera.

Fig. 1.

Section of a universal electron microscope. 1—electron source; 2—magnetic shielding (permalloy); 3—device for adjusting the cathode system; 4—centering diaphragm; 5—device for adjusting the condenser coil; 6—insert with diaphragms for bright-field and dark-field illumination; 7—insert with aperture diaphragms, specimen holder, and replaceable diaphragm; 8—vacuum valves for isolating the specimen space from the main vacuum; 9—magnetic shielding (permalloy); 10—device for moving the specimen perpendicular to the optical axis; 11—device for adjusting the objective lens; 12—window for observing the intermediate image; 13—auxiliary light microscope with a single-crystal screen for observing the intermediate image; 14—insert with aperture diaphragms and replaceable diaphragm; 15—auxiliary light microscope with a single-crystal screen for observing the projection image; 16—water cooling; 17—coil housing; 18—magnetic shielding (permalloy); 19—connection to the pump; 20—window for observing the final image; 21—device for isolating the camera from the main vacuum, admitting air, and moving the cassette; 22—shutter with a fluoresci-

Fig. 2. Bright-field image of minute crystals of zinc oxide. Magnification 50,000:1

Fig. 3. Dark-field image of a thin filament of β-polyoxymethylene. Magnification 50,000:1

Fig. 4. Stereoscopic image of magnesium oxide. Magnification 25,000:1. Angle of inclination of the object plane 4°

The objective distance is 2.5 mm when an electrostatic lens is used (at a voltage of 60 kV) and 1.6–0.9 mm in the case of a magnetic lens. Owing to the presence of valves that isolate the specimen and objective space from the main vacuum, the process of replacing the specimen or objective takes only 2–3 min.

In front of the projection lens there is a fluorescent screen with a small aperture, onto which the intermediate image produced by the objective is projected. The focal length of the projection lens can be brought down to 1 mm. The distance from the lens to the image, for both the objective and the projection lens, is 650 mm, so that the maximum magnification given by the microscope is \(5 \cdot 10^5\).

The final image is projected onto a photographic chamber located in the lower part of the instrument, the shutter of which is coated with a fluorescent composition; this makes it possible both to take photographs and to observe the object under investigation visually. Owing to the presence of a hermetic lock, the cassettes are changed without admitting air into the other parts of the instrument.

To check image sharpness during adjustment and photography, the instrument is equipped with two auxiliary light microscopes with single-crystal fluorescent screens placed in the path of the electron beams: the first in the plane of the intermediate image, the second behind the projection lens at a distance equal to 10% of the distance between the projection lens and the final image.

To obtain stereoscopic images, the plane of the object can be inclined within ±10–15° by means of mechanical devices. By tilting the plane of the object to one side and the other, one can successively obtain two photographs of one and the same region of the object under investigation, differing only in the direction of the electron beams passing through the object. These photographs can then be examined with an ordinary stereoscope. Obtaining stereoscopic images is of special interest in this case, since the apertures used in the electron microscope are 2–3 orders of magnitude smaller than in the light microscope, as a result of which the depth of focus is correspondingly many times greater. The region of greatest sharpness in the direction of the axis of the electron microscope is of the order of the field-of-view diameter, which greatly improves the quality of stereoscopic images. In a number of cases, only a stereoscopic image gives an idea of the true structure of the object under investigation.

The resolving power of the universal electron microscope described, when magnetic optics are used, may in the case of bright-field imaging be confidently taken as 30 Å. However, in the photographs of Brownian particles presented in the paper, particles with a diameter even of the order of 10 Å are distinguishable. In dark-field imaging, the practically attainable resolving power is at least 50 Å.

Comparisons of magnetic and electrostatic objectives have shown that the former possess greater resolving power than the latter.

The photographs shown in Figs. 2, 3, and 4 may serve as examples of the bright-field, dark-field, and stereoscopic images obtained.

D. Zernov, Moscow

REFERENCES

1. M. Ardenne, Z. Physik, 115, 339, 1940.
2. B. Borries u. E. Ruska, Z. techn. Physik, 19, 402, 1938.
3. H. Mahl, Z. techn. Physik, 20, 316, 1939.