US2301987A - Electron microscope - Google Patents

Description

Nov. 17, 1942. v VON R l 2,301,987

ELECTRON MICROSCOPE Filed Nov. 16, 1939 i 2 Sheets-Sheet 1 Nov. 1

B. VON BORRIES ELECTRON MICROSCOPE Filed NOV. 16, 1959 2 Sheets-Sheet 2 5000 va/v Bee/was ATTORJVE'YS.

Patented. Nov. 17, 1 942 ELECTRON MICROSCOPE Bodo von Berries, Berlin-Spandau, Germany; vested in the Alien Property Custodian Application November 16, 1939, Serial No. 304,763 In- Germany October 21, 1938 8 Claims.

The present invention relates to electron microscopes and, particularly, to means for centering the electron beam.

Electron microscopes of the known type usually contain a radiating apparatus consisting of an incandescent cathode, a Wehnelt cylinder and an anode diaphragm for producing an electron beam which is directed through a field coil serving the function of a condensing lens, the support of the object to be examined and one or fluctuations in the operation of the apparatus,

however, may not be entirely avoided, and a shifting of the image constitutes a very disturbing factor particularly if the image is to be reproduced photographically.

It is therefore the principal object of the present invention to prevent such undesired shifting of an image formed in an electron microscope. Y

Another object of the invention is to render an electron microscope insensitive to the fluctuations of the current and voltage, above.

Another object of the invention is to center the electron beam relative to the optical'axis of the portraying system.

For attaining these objects the invention pro vides the following method and means. I'he electrical values determining or controlling the electron beam arevaried arbitrarily, while adjusting means provided for varying the direction and position of the electron beam are adjusted until the shifting of the image caused by the arbitrary variations is practically eliminated. For adjusting the position and direction of the electron beam, means may be provided whereby one or more parts of the radiating. apparatus may be adjusted relative to the lens system, pref erably by shifting and tilting theseparts or by either or these methods. These adjusting means make it possible to control the electron beam produced by the radiating apparatus so as to co incide exactly with the optical axis of the por traying lens system. The use, according to the as mentioned v invention, of arbitrary variations of the electrical values producing or controlling the electron beam permits determining by direct observation and in a simple manner whether or not the adlusting movements are performed correctly.

The adjustment is continued until the electron beam has been centered. Then, a shifting of the image, due to fluctuations of the current of the condensing coil and of the voltage of the radiating apparatus, is prevented since these fluctuations are now only able to change the cross-sectional area of the electron beam but not the direction thereof.

In executing the process, the electron beam is centered preferably by-arbitrarily varying the current or voltage producing or controlling the electron beam. This may be performed, for ex ample, by arbitrarily varying the current flow:- ing through the objective coil or the high tension producing the electron beam. More particularly, the method according to the invention may be performed by varying the respective electrical value, for example the high tension for producing the electron beam, by several predetermined amounts, preferably by first arbitrarily making small changes from the normal value and thereafter one or more progressively greater changes. The adjustment of the electron beam may be thus continuously corrected so as to decrease progressively the sensitivity of the apparatus to fluctuations of the current or voltage.

These 'and other objects, features and advantages of the present invention will appear from the following detailed description and the accompanying drawings, in which Figural is explanatory and diagrammatically illustrates one method for attaining the objects of the present invention.

Figure 2 illustrates similarly a modification of the same method;

Figure 3 shows an embodiment of a circuit for producing the above-mentioned electric variations according to the method represented by Figure 2.

Figure 4 shows another electric control circuit for producing such fluctuations in accordance with the invention.

Referring at first to Figure 5, the electron microsccpe proper, here illustrated schematically, is more fully described in the copending application, Serial No. 303,180, filed November '7, i939, and is'shown only as a convenient example, no claim being made in thepresent application as to the structural features of this microscope and it being understood that any known electron shown in Figure 5 contains a radiating device having a cathode I02 and an anode I04. These elements are firmly secured to an insulator I40 so as to form an integral radiating unit. This unit is laterally displaceable and tiltable relative to the stationary portion I09 of the microscope vessel. Adjusting screws, not shown, may be provided for controlling the angular and lateral positions of the'radiating unit.

I05 designates the condensing coil of the microscope. This coil is also tiltable and transversely displaceable, for instance by means of screws (not illustrated). been adjusted, it may, as a rule, be left in its proper position. The object I06 is also displaceable transversely to the stationary portions of the microscope. The upper portions of the microscope vessel are schematically indicated by broken lines and designated by III) and III.

After the coil I05 has once These portions may contain flexible sleeves or the like resilient means for joining the radiating unit with the condenser coil and the latter with the lower portion ofthe microscope. The magnifying lens system proper consists of an objective lens I01 and a projection lens I08. The elec-' tron beam issuing from the radiating unit passes first through the condensing lens I05, then through the object I06 where it is modified in ac cordance with the structure of the object, thence through the objective lens I01 which produces an intermediate image, and finally through the projection lens which throws a magnification of the intermediate image onto a screen or photographic plate in the bottom portion of the microscope vessel. As mentioned above, the final image is apt to be disturbed by shifting if the elecated push-button switches 4 and 5.

tric currents or voltages energizing the irradiating unit and the lens system fluctuate unless the beam coincides accurately with the optical axis 01 the lens system.

The radiating device is connected with a highvoltage circuit I43 containing a rectifier I25 and a capacitor, and connected to the high-voltage secondary I23 of a transformer whose low voltage primary I24 is fed from a circuit I44 con-. taming an alternating current source I21, re-

sistors I28, I29 and I30,-and 'contactors (pushbuttons) I 3! and I32. This circuit permits,varying the high voltage of the irradiating apparatus for the purpose aforementioned. The objective coil I01 is energized from a circuit I containing an C. source I31, resistors I38, I35, I36 and contactors I33 and I34. The circuit I45 permits varying the energizing current of the lens coil I01. By operating in either circuit I44and I45, one, the other, or both push-buttons, the resistance of the circuit is changed and thus the effective voltage or current varied as explained inthe following. The variations cause the magnified image to effect shifting movements. While observing these movements, the observer changes the adjustment of the radiating unit or condenser lens until the shifting/ceases.

As illustrated in Fig. 1, the ,arbitrary fluctuations of the electrical value (voltage or current) may" be performed', for example, by starting with the normal value N, the electrical value is increased-each time for a period t by an amount d1.

If by adjusting the electron microscope the apparatus has been made insensitive to these fluctuations d1, correspondingly greater fluctuations d2 maybe performed, whereupon the microscope is further-adjusted so as to be insensitive also to these fluctuations. I v

As illustrated in Fig. 2, the desired result may also be obtained by arbitrarily adjusting the variations of the electrical-value N by alternately increasing and reducing the same by the same amount 11. Progressively increasing positive and negative deviations from the normal value N may also be provided so as to center the electron I beam of the microscope as exactly as possible.

by influencing the low-tension circuit of the high-tension transformer orthe control circuit of a thermionic regulator of theexciter.

Fig. 3 illustrates separatelya circuit for vary ing arbitrarily the high tension necessary for producing the, electron beam by a definite positive and negative amount. The arrangement shown in Fig. 3 comprises the exciting winding I of the high-tension generator, two resistances 2 and 3 in the exciting circuit, and manually oper- When in normal position, as shown in the drawings, the exciting winding I is connected in series with t e resistance 2, in which case the normal value N of the exciting voltage may be adjusted. For obtaining an arbitrary increase of the voltage, the push-button 4 is operated was to bridge the resistance 2. The voltage is then correspondinglydecreased by operating the push-button 5 whereby the two resistances 2 and 3 are connected in series with the exciting winding I. The circuits I44 and I45 are similar to that of Fig. 3 and hence operate substantially in the way as just mentioned. a I

As illustrated in Fig. 4, it is also possible that the same push-button switch II be used for increasing as well as decreasing the voltage. For this purpose, the v ltage is reduced if the push increase and reduction of the high tension necessary for producing the electron beam. For this purpose, after the push-button II is depressed, further push-buttons I2 or I3 are operated for connecting greater resistances into the circuit. The changes in voltage may also be obtained by switching on or ofl portions of the transformer windings. The manually operated switches for obtaining arbitrary changes of the electrical values are preferably mounted at a place easily accessible from the place of observation of the microscope. 4

I claim:

1. Themethod of centering the electron beam of varying the electric energization of at least one of said means to cause shifting of said image so long as said different means are cute! optical alignment. and mechanically adjusting said diiiferentmeans relative to one another while said varying is continued until said shifting disappears.

2. The method of centering the image-producing electron beam in an electron microscope having an objective lens and means for mechanically adjusting the beam relative to the lens, comprising the steps of varying the energization of the objective lens to cause shifting of the image when said lens and said beam are in electron-optical misalignment, and actuating. said mechanical adjusting means while said varying is continued until said shifting is substantially liminated.

3. The method of centering the image-producing electron beam in an electron microscope having a high voltage irradiating device for producing the beam and a magnifying lens system mechanically adjustable relative to each other, comprising the steps of periodically varying the high voltage of the irradiating device to cause shifting of the image when the beam and the lens are in misalignment, and varying the relative position of said device and said lens simultaneously with said periodic voltage variation is effected until said shifting substantially ceases.

4. The methodoi centering the image-producing electron beam in an electron microscope having a high voltage irradiating device for producing the beam and an objective lens mechanically adjustable relative to each other, which comprises the steps of periodically varying the high voltage of the irradiating device andthe energization of the objective lens to cause shifting of the image when the beam and the lens are in misalignment, and simultaneously varying the relative position of said device and said lens until said shifting substantially ceases.

5. The method of centering the electron beam of electron-optical apparatus having electrically energized means for producing the beam and electrically energized means for modifying the beam to produce an image, comprising the steps of varying the electric energization of at least one of said means by a relatively small amount to cause shifting of the image when said difierent means are in electron-optical misalignment, me-

chanically adjusting said different means relative to one another until said shifting is substantially reduced, varying said energization to an extent greater than before to again cause shifting of the image, and further adjusting said difierent means mechanically relative to one another to again re- 7 "of varying the electric energization of at least one of said means by alt'rnateh increasing and decreasing it by substantially equal amounts to cause shifting of the image when said different means are in electron-optical misalignment, mechanically adjusting said different means relative to one another until said shifting is substantially eliminated.

7. With an -electron microscope having electrically energized means for producing an electron beam, lens means for modifying the beam to produce an image, and an adjustingdevice for varying the position of said different means relative to one another for centering the beam, in combination, an electric energizing circuit connected with one of said means, said circuit comprising a variable circuit element for varying the energization to causeshifting of the image unless said different means are adjusted by said device to be in correct optical alignment, and control means arranged in said circuit for putting at will said variable circuit element in and out of operation.

8. With an electron microscope having electrically energized means for producing an electron beam, lens means for modifying the beam to produce an image. and an adjusting device for varying the position of said different means relative to one another for centering the beam, in combination, an electric circuit for energizing one of said means comprising manually operated switch means for increasing and decreasing the energlzation to cause shifting of the image so long as said different means are in electron-optical misalignment.

BODO von BORRIES.

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