US2241432A - Electron scanning microscope - Google Patents

Description

' May 13, 1941.

M. VON ARDENNE ETAL 2,241,432 ELECTRON SCANNING MICROSCOPE Filed Sept. 7, 19158 Patented May 13, 1941 ELECTRON SCANNING LUCROSCOPE Manfred von Ardenne, Berlin-Lichterfelde, and Bodo von Berries, Berlin-Spandau, Germany Application September '7, 1938, Serial No. 228,776

6 Claims.

The invention relates to electron microscopes and more particularly to such microscope in which the object to be tested is scanned by an electron beam of small diameter and the electrons, passing through the object, or the electrons diffracted, dispersed or reflected on by the object, or the secondarily released electrons are employed to compose the image of the object from individual points.

According to the invention the electron energy modulated in the above-mentioned manner by the structure of the object is directly recorded with the aid of a photographic layer which, during the scanning movement of the electron beam, efiects a proportional movement of greater amplitude. In this manner the advantage is attained that not only the stream of the electrons but also the electron power comes into action which in the case of a scanning microscope may assume relatively high values owing to the application of anode voltages of several thousand volts. The invention presents the further advantage that the recording is highly sensitive and permits an integration for considerable periods.

According to another feature of the invention, the electron beam is sharply concentrated onto the object by means of an electron-optical reduction system whose pole pieces are arranged but a few millimeters away from the object.

The electrons diverge directly behind the object particularly as a result of the electron dispersion in the layers of the object. However, since great electron density is desirable in order to attain an intense blackening of the photographic layer, the invention further provides that the movable photographic layer is arranged directly behind the object to be tested, or that a suitably screened auxiliary lens is placed between the object and the photographic layer in order to reconcentrate the electrons. The latter method leads to great chromatic aberrations, since the speed of the electrons after they have passed through the object is non-homogeneous. However, these aberrations need not be critical, for even in the case of a considerable dispersion of the electron speeds it will be possible, when suitably dimensioning the aperture, the distance of the object and that of the image from the objective, to maintain the resulting circle of dispersion smaller than 1 mm. By the use of a diaphragm of an aperture of 1 mm. the lastmentioned requirement may be fulfilled. It should be noted in this connection that it is even desirable to operate with a recording image point of a certain fuzziness to cause the adjacent Germany September 11, 1937 recording lines to coalesce into a gapless reproduction continuously covering the entire reproduction area. This also reduces the disturbing influence of slight deviations of the scanning beam from its ideal path. The insertion of an auxiliary lens between the object and the electron indicator has not only the advantage of rendering the construction simpler but also oifers the important advantage that an additional after-acceleration is brought about in order to increase the kinetic energy of the electrons, so that the electron speed is again homogenized. In this connection it may be pointed out that also when not using an auxiliary lens, an afteracceleration of the modulated electron beam may be advantageous for the last-mentioned reason.

Even without the use of the auxiliary lens and when employing the usual pole pieces and object holders it is possible to arrange the photosensitive layer sufliciently close to the backside of the object if a narrow recording tape is employed having, for instance, a width of a few millimeters.

To obtain a complete image of the object according to the above-described method it is necessary to put together in the proper position the individual portions of the recording tape in the same manner as is the case with a telegraphic picture transmission. However, an image of the object may also be directly obtained if as in the case of picture transmission the position of the photographic layer to be recorded is so changed with respect to the electron spot or also the position of the electron spot to be recorded is so changed by means of auxiliary deviating fields with respect to the photosensitive layer that an undistorted image of the structure of the object under observation is directly produced on the photosensitive layer.

This is, for instance, accomplished by employing the means which control the change of position of the electron spot with respect to the photosensitive layer simultaneously for controlling the deflecting system in the scanning microscope. It may also be preferable to proceed in the reverse manner by causing the deflecting generators for the control of the scanning spot on the object to change at the same time the position of the recording spot with respect to the photosensitive layer. The amplitudes of deflection for the X and Y direction must have a proper ratio with respect to each other and must also maintain certain absolute magnitudes in order that an undistorted proper image be recorded.

The invention will be more readily understood from the following description of the embodiment diagrammatically illustrated in the drawing, in which- Fig. 1 represents a sectional elevation of the electron-optical magnifying and recording system of an electron scanning microscope accord ing to the invention, while Fig. 2 shows a diagram of an entire arrangement according tothe invention, including the electric circuits necessary for producing the scanning movement of the beam and for simultaneously and proportionally operating the movable recording means.

Referring at first to Fig, 2, the vacuum vessel of the microscope is designated by 30. It contains a source 3| of a thin electron beam 32. The object to be examined is located at 5. The beam, after having passed through the object so as to be modified thereby, leaves the vacuum vessel through a Lenard window 6 and impinges upon a recording drum I I carrying a photosensitive layer. An optical lens system 33 serves to concentrate the beam so as to produce a sharp scanning spot in the plane of incidence of the object, this spot being considerably much smaller than the entire object surface so as to cover only a small fraction of the whole surface. Scanning electrodes and I9 serve to move the beam and the scanning spot over the object surface so as to compose a point-for-point and line-for-line image. While this scanning movement proceeds, the drum H is caused to elTect a synchronous movement, however, with an amplitude much greater than that of the scanning beam or spot. As a result, each short line covered by the scanning spot corresponds to a considerably longer distance simultaneously covered by the drum surface, so that a correspondingly magnified image is recorded.

Returning now to Fig. 1, a suitable design of the electron-optical lens system 33 will now be described.

By designing the pole piece I of the lens system 33 in a novel manner it is possible to arrange the object 2 in the immediate neighborhood of the center of gravity of the optical system and at the same time to dispense with constructional elements laterally of the object which would prevent the arrangement of large photographic plates or of drums provided with photosensitive material.

A metallic plate 3 sealing the vacuum chamber above it, bears against the pole piece I and is provided with a bore exactly in alinement with the aperture diaphragm 4 of the optical reduction system. If the photosensitive layer should be arranged outside the vacuum chamber, which is not absolutely necessary for carrying out the invention, it is preferable to choose said bore as well as the diameter of the object very small; for instance, T 5 mm. in diameter in order that the Lenard window necessary in this case presents a thickness as small as possible. Practical tests have shown that it is preferable to employ a Lenard window 5 consisting of a collodion film of l0 to 10* mm. in thickness which is cemented to the structural element 3 so as to form a vacuum-tight seal and which at the same time carries the object to be tested. Immediately behind the collodion film follows the diaphragm 6, which may under given circumstances be omitted, and then the photosensitive layer 1 arranged on a drum as shown in Fig, 1. If the photosensitive layer is not arranged in a vacuum chamber but is exposed to the open air it is important that this layer be guided with very great mechanical precision so that it remains as close as possible to the Lenard window in order to attain high electron densities as well as to avoid absorption losses. If this layer is arranged in the vacuum chamber the foil 5 then serves only as an object holder and may therefore be thinner than the above-mentioned him by l to 2 orders of magnitude.

A very important problem in the photographic recording is the control of the focussing of the electron spot on the plane of the object under examination. This control which would otherwise only be possible after the development of the photographic film, is facilitated by the use of an auxiliary fluorescent screen 8, an optical mirror 9 and a microscope it as shown in Fig. 1 in dotted lines.

This focussing unit is employed before placing the photosensitive layer in the path of the electron beam to recognise the variations of the amounts of electrons when slowly scanning the object from the fluctuations of the fluorescence. The more marked the fluctuations appear with a constant scanning speed and a constant fine structure of the object, the sharper the electron spot is adjusted on the object. In order to have an idea of the focussing before recording an image a Wollaston wire of, for instance, A; microns in thickness is arranged in the plane of the section through the object, 1. e., on the supporting foil of the object across the field of view and the degree of the darkening must be measured when the electron spot passes over the wire. In this case it is preferable to proceed in such a manner that by slowly changing the deflecting voltage of the pair of deflecting plates of the microscope perpendicular to the direction of the wire the electron spot is guided over the fine wire and at the same time the change in deflecting voltage is measured which is necessary to cause the fluorescence to disappear. The greatest sharpness prevails if a particularly small change in deflecting voltage causes the passage from bright to dark.

The preparation of the microscopically small objects on the supporting foil as well as the focussing, and under given circumstances the arrangement of the photosensitive material in the vacuum are more or less complicated opera-- tions so that every aid facilitating said endeavor will be appreciated. Such aid consists in changing as far as possible during the manufacturing process the material photo-sensitive to the electron beam by adding coloring agents or by employing other light screening methods in such a manner that it becomes completely insensitive to ordinary light or that its sensitiveness to light is so considerably reduced that the sensitized material may be placed on the drum in a room sufiiciently lighted to handle the apparatus. Of course, the coloring agent or the protection against light must be so chosen that a subsequent removal thereof is rendered possible so that the blackening brought about by the electron efiects becomes perceptible. The electrons have even in the case of the aforem ntioned light screening still approximately the full penetrating power at the prevailing voltages of the scanning microscope, so that the sensitivity of the actinic layer to the electron effects is not materially impaired by the screening.

The application of the light-sensitive material to a drum has the advantage over the plane arrangement that the recording direction of the image is always the same and that in the case of a spiral advancement of the drum both image point coordinates are simultaneously controlled by a single drive. This rigid coupling is also the reason why the recording on a drum has come into extensive use in image telegraphy.

Fig. 2 shows by way of example the electrical connection for photographic drum recording in connection with an electron scanning microscope. The drum ll provided with the photographic layer is driven by means of a motor I2 in such a manner that upon each rotation the drum advances by the width of a line in front of the electron focal point of the scanning microscope. On the driving shaft of the drum is arranged a contact wheel l3 which during each rotation short-circuits the capacitor l4 only once and always at exactly the same phase angle. The capacitor l4 and the beam deflecting plates l5 of the scanning microscope are arranged in parallel relation to and connected with a direct current source 2!. An adjustable resistor I6 is par allel-connected with the capacitor. When a contact is made at disk l3, capacitor I4 is shortcircuited and discharged. Then after the contact is opened as disk l3 continues to revolve, the capacitor is supplied with a constant current through resistor l6 and gradually charged thereby. The voltage between the deflecting plates l5 increases accordingly so that the electron beam is gradually deflected until the next discharge of capacitor l4 occurs. The contact of the potentiometer resistance I8 is moved by the drum drive rigidly coupled therewith through the reduction gear H the movement occurring in accordance with the number of lines. Upon each rotation of the drum the potentiometer contact advances by a further step and consequently controls the voltage for the slow ordinate deflection plates I9 of the microscope deflecting system. By controlling the resistance IS, the ratio of the XY deflections with respect to each other is so adjusted that no distortion is brought about. By displacing the contact 20 along the common power source 2| the deflecting amplitude of both coordinates of the scanning microscope and therefore the magnitude of the field of view on the object under examination are controlled.

Figs. 1 and 2 show only one of the several possible forms of the invention. As already abovementioned the use of an auxiliary lens in connection with after-acceleration and the use of auxiliary deflecting plates in front of the photographic layer ln connection with the recording on a plane photographic layer represent advantageous forms of the invention. In the last case the advantage is obtained that no mechanically moved parts are employed and that a completely rigid coupling of the image point coordinates is attained by a simple electric parallel connection of the deflecting voltages or deflecting currents for either deflecting system XY. In order to carry out the invention in a proper manner it is naturally necessary that a critical spot spreading, owing to the dispersion of the electron speeds in the case of a considerable deflecting amplitude, be avoided by a suitable dimensioning and particularly by the above-mentioned afteracceleration.

Another way of recording consists in projecting the luminous spot of a fluorescent screen arranged directly behind the object by means of an optical system having a great luminous intensity on a light-sensitive layer and in then changing the position for the image synthesis. This method is, however, limited to the use of considerable streams of electrons and limited due to the fact that a correspondingly smaller resolving power is involved owing to the losses due to the fluorescence and to the optical projection.

It may finally be mentioned that also the examination of the objects in the open air may be easily efiected when employing the abovedescribed arrangement. To this end, it is only necessary to arrange, for instance, the object 2 of Fig. 1 not on the side of the object holder 5 facing the pole pieces but on the outer side facing the photographic layer.

What is claimed is:

1. Electron scanning microscope having means for producing an electron beam, means for supporting an object in the path of the beam, means for concentrating said beam so as to produce a scanning spot on the object, means for moving said beam to scan the surface of the object, a recording member for accommodating a photosensitive layer, said recording member being arranged in the path of said beam behind the object and movable transversely to said beam, and mechanical control means connected with said recording member for moving said member in proportional relation to the scanning movements of said beam, yet with an amplitude greater than that of said movements, whereby an enlarged image of the object is produced upon said layer.

2. Electron scanning microscope having means for producing an electron beam, means for supporting an object in the path of the beam, means for concentrating said beam so as to produce a scanning spot on the object, scanning means for moving said beam to scan the surface of the 0bj ect, a movable recording member for accommodating a photosensitive layer, said member being disposed directly behind the object in the direction of the beam travel, means for directing the beam, modulated by the structure of the object, onto said sensitive layer to be recorded there, and mechanical control means connected with said recording member for moving said member relatively to said object in proportional relation to the scanning movements of said beam, yet with an amplitude greater than that of said scanning movements, said control means being electrically connected with said scanning means so as to operate in synchronism therewith, whereby an enlarged image of the object is produced upon said layer.

3. Electron beam scanning microscope comprising means for holding an object to be examined, means for producing an electron beam having a diameter of the order of 10- mm. in the plane of incidence at the object to be examined, scanning means for moving said beam to scan the surface of the object, a movable recording member for accommodating a photosensitive layer, means for directing the beam, modulated by the structure of the object, onto said sensitive layer, mechanical control means connected with said recording member for moving said member relatively to said object in proportional relation to the scanning movements of said beam, yet with an amplitude greater than that of said scanning movements, said control means being electrically connected with said scanning means so as to operate in synchronism therewith, whereby an enlarged image of the object is produced upon said layer, and an electric auxiliary field arranged between the object and said sensitive layer, for accelerating the electrons.

4. Electron beam scanning microscope comprising means for holding an object to be examined, means for producing an electron beam having a diameter of the order of 10- mm. at the object to be examined, electrostatic scanning means for deflecting said beam in perpendicular directions to scan the surface of the object, means for accommodating a photosensitive layer disposed behind the object in the direction of the beam travel, means for directing said beam, modulated by the structure of the object, onto said sensitive layer to be recorded there, mechanical control means for moving said layer relative to the object and transverse to the path of the beam, and electric control means for synchronously operating said electrostatic scanning means and said control means so as to move said photosensitive layer in proportional relation to the travel of the beam on said object, yet with an amplitude greater than said travel, whereby an enlarged image of the object is produced upon said layer.

5. Electron beam scanning microscope com prising a vacuum vessel, means in said vessel for producing an electron beam, means for holding an object to be examined, means for concentrating said beam so as to produce a scanning spot on the object to be examined, means for moving said beam to scan the surface of the object, an object carrier comprising a supporting film for the object disposed across the path of the beam and forming a Lenard window closing said vacuum vessel, a movable recording member arranged outside of said vessel for accommodating a photosensitive layer, said member being disposed in the path of said beam behind said ob ject carrier, and mechanical control means connected with said recording member for moving said member relatively to said object in propor tional relation to the scanning movements of said beam, yet with an amplitude greater than that of said scanning movements, said control means being electrically connected with said scanning means so as to operate in synchronism therewith, whereby an enlarged image of the object is produced upon said layer.

6. Electron beam scanning microscope com prising means for holding an object to be examined, means for producing an electron beam having a diameter of the order of 10* mm. at the object to be examined, scanning means for moving said beam to scan the surface of the object, a recording member for accommodating a photosensitive layer, said member being arranged in the path of said beam behind the object, mechanical control means connected with said recording member for moving said member relatively to said object in proportional relation to the scanning movements of said beam yet with an amplitude greater than that of said movemerits, said control means being electrically connected with said scanning means to operate in synchronism therewith in order to produce an enlarged image of the object on said layer, and electron-optical concentrating lens means disposed between said object and said sensitive layer.

MANFRED VON ARDENNE. BODO v. BORRIES.

Images