US2257774A - Electronic-optical device - Google Patents

Description

Oct. 7, 1941. M. VON ARDENNE ELECTRONIC-OPTICAL DEVICE Filed Feb. is, 1938 2 Sheets-Sheet 1 Patented Oct. 7, 1941 UNITED STATES PATENT OFFICE ELECTRONIC-OPTICAL DEVICE Manfred von Ardenne, Berlin, Germany February 18, 1987 Application February 15, 19:8. Serial a, 1903::

In Germany 'IOlaims.

sible to reproduce a primary or also a secondarysource of electrons, by the use of electric lenses or of magnetic lenses," in the form of electron images, in the same manner as it is possible to reproduce a source of light or an illuminated or light-penetrated object with the aid of optical lens systems.

Everymicroscopical observation of very small objects is absolutely limited, as is known, by diffraction phenomena, insofar as only such objects or details can be reproduced which have a sufficient size compared with the wave length of the reproducing beam. The limit of the resolving capacity for the optical microscope lies, therefore, in the order of magnitude of the wave lengths of light. The operation of the electron microscope, however, is predicated on a wave length which is related to the electron velocity by the known de- Broglie relation have a certain thickness, so that the electrons are subjected within those objects to a noticeable retardation which is different at different points of the respective object.

It is clear, therefore, that a beam of rays hava comparatively broad speed spectrum ema- This formula furnishes, for instance, for an electron beam with a speed of 100 kilovolts, a wave length which is shorter by about 10000 than the mean wave length of the visible light. 'It should therefore be possible to build an electron microscope, the resolving capacity of which is by about four ten-powers stronger than the resolving capacity of the light-microscope with the same aperture.

It was possible until now, in practice, under the most favorable conditions, merely to approach nates always from the object to be reproduced, and since it has not been possible until now to manufacture sufficiently achromatized electron objectives, it is impossible to attain even approximately the theoretically established limit'of the resolving capacity with a non-homogeneous pencil or beam of rays.

The present invention overcomes the above mentioned difflculties by point-for-point scanning of the object with the aid of an electronbeam focal spot, the diameter of which is less than one-thousandth of a millimeter. carrying out this method, the electrons coming from the electronically-optically illuminated" image elementare registered, a basis is established for the building-up of an electronic-optical image of the object, and, in fact, with a resolving factor which depends solely on the pointsharpness of the scanning focal spot in the object plane subjected to investigation.

Any desired indicator method may be used for registering the electrons coming from the object point on which the electronic beam focal spot impinges at any instant. The electrons may be counted either singly, e. g., by means of an electrometric arrangement, or the use of a counting chamber of the Geiger type, respectively, or the mean value of the resulting electron current may be measured or traced with the aid of an integrating device. Several methods to attain the object in view will be described hereinafter; however, to those experienced in the art, it will of course be clear that many other methods and means can likewise be used for ascertaining or registering that amount of electrons which starts from a given point of the object in a time unit.

The point sharpness of the electronic focal spot with which the object is scanned and which determines the attainable resolving capacity can be increased to extraordinarily high values, as will be disclosed in the following detailed explanation, and inasmuch as only this point sharpness is decisive, the invention renders it possible to extend the resolving capacity-con- I siderably beyond that of light-microscopes even with objects, the thickness of which corresponds to the thickness of a microtome. It is therefore, possible to observe and investigate microscopically objects which could not be rendered If, on'

visible until now in any way. Other advantages of the invention reside in the fact that the object to be investigated is subjected to a far smaller load, due to impinging electrons, than is the case with the normal electron microscope, and that it is not absolutely required to introduce the object into a vacuum space for carrying out the observation. These advantages are so considerable and of such importance that the invention recommends itself for use even if it is not intended to extend the resolving capacity of the electron microscope beyond the resolving capacity attainable with previously known means.

The above intimated objects, and other objects and features not yet mentioned, will appear from the following detailed description which is rendered with reference to the drawings. In these drawings,

Fig. l is a representation of an electronicoptical system for the production .of a minute electron spot with which the object to be investigated is to be scanned;

Fig. 2 shows a tracing device for the registra tion of the image produced;

Fig. 3 illustrates a scanning field;

Fig. 4 represents a device for counting the electrons coming from the object;

Fig. 5 is an illustration of another embodiment of the tracing device which renders it possible to make the electronical-optically obtained image directly visible in a corresponding magnification;

Fig. 6 shows an arrangement for the formation of a particularly small electron focal spot for scanning an object, in connection with an arrangement for the observation of objects in the open air;

Fig. 7 illustrates a particularly designed electron-multiplier; and

Figs. 8-10 show several different electrodes intended to serve for the trapping of the electrons coming from the scanned object.

The following description refers to details only to an extent required for an understanding of the invention; matters that may safely be assumed to be well known to those experienced in the art, to whom the disclosure is addressed, are noted only to such extent as is necessary to support the understanding of what is new.

The highly evacuated glass tube I (Fig. 1) contains an electron emissive cathode 2 standing in a cylinder 3 of the Wehnelt type. From this cathode proceeds an electron beam transformed into a narrow pencil by the coaction of the cylinder 4 which has a positive initial voltage and 01' the anode cylinder 5, said pencil being directed towards the diaphragm opening 6. The passage opening of this diaphragm may have, for instance, a diameter of one-hundredth of a millimeter; it constitutes the electron delivery point which then is reproduced on a reduced scale on the object to be observed. In order to reproduce this electron delivery point on the object on a reduced scale, an electric or a magnetic electron object-lens may be used. Tests have demonstrated that the most favorable .reproduction is obtained with the aid of magnetic object-lenses consisting of an encased annular coil having an inner slot free of iron. With such an object-lens it is possible to obtain a focal distance down to a few millimeters. It is thus possible, with such object-lenses, to go to very great reducing scales, paying attention to direct the electron beam as accurately as possible into the corresponding object-lens diaphragm or shutter.

According to Fig. 1, the electron beams arriving from the diaphragm opening I are, therefore, trained as accurately as possible upon the passage opening of the diaphragm l of the magnetic electron object-lens III, which is done by means of a magnetic adjusting device comprising the coils l and 8 and the associated batteries and adjusting resistances, the coils-being arranged perpendicularly with respect to each other. The electron objective ID has a very short focal distance and therefore reproduces the passage opening of the diaphragm or shutter 6 on the surface of the object II to be investigated in about a fifty-fold reduction. A scanning focal spot is therefore caused at that point which has a diameter of only 5.10- mm., resulting in a resolving capacity of 5.10" for the scanning.

The electrons coming from the object II are trapped by an electrode I2 arranged preferably in the interior of a grounded cage ll of the Faraday type. This cage has only a comparatively small window and acts consequently at the same time as a shutter or diaphragm. The number of the electrons arriving at the trapping electrode I2 depends obviously at any moment on the electron stray and on the permeability to electrons of the object II at the point where the electron focal spot is located at any instant. When the object I I is moved in the plane in which the electron focal spot arises, and if this movement is carried out in such a manner that the small field of the object II to be investigated is scanned by said spot, then the number of the electrons reaching the trapping electrode I2 varies according to the permeability to electrons of the object II from point to point. It is advisable to carbonize the surface of the electrode I2 in order to obviate errors that may be caused I at this trapping electrode through reflection and by a secondary emission. Faults of the kind just mentioned may also be diminished by connecting a voltage between the object-carrier II and the trapping electrode I2. In order to register the electrons taken by the trapping electrode, this electrode may be connected with the input circuit of an amplifier I4, as illustrated in Fig. 1.

According to Fig. 2, the variations of the current, after having been sufficiently amplified, are transmitted to an image-tracing device. The output circuit of the amplifier I4 is for this purpose connected with a gaseous conduction or glow lamp I5, the luminous surface of which is reproduced upon the light-sensitive surface of the image roller ll by means of the objective I6. The roller I'I rotates in the usual manner below the lamp I5 and is simultaneously shifted in the direction indicated by the arrow I8.v Connected with the roller I1 is a rotary resistance I9 and a sliding resistance 20. At every rotation of the roller II the resistance I8 passes through its entire range from zero to the maximum value, and inasmuch as this resistance is connected in the circuit of a magnetic deflecting device comprising two coils 2i and 22 which are wound in opposite directions, as shown in Fig. 1, the angle of incident of the electron beam entering into the diaphragm or shutter 9 in the manner indicated by the dotted line is periodically changed. This causes the required line deflection whereby the image field J is scanned, according to Fig. 3, in the direction indicated by the arrow A1. The image deflection in the direction of the arrow A: may be brought about by means of a suitably designed coil arrangement controlled by the resistance 20. According to another embodiment,

' counting chamber."

likewise illustrated in Fla. 1, the object-carrier Ii may be secured on a strong bi-metallic rod 23 which is heated by a bifilar-wound resistance 24 located in the circuit of the sliding resistance 20. It is possible in this way to scan the object II in successive lines in accordance with the preceding continuous registration of the electrons trapped at i2 in the image-field J (Fig. 3). An imagefield (I) covering on the object merely a square of 2.10 mm. can thus be scanned with an electron focal spot of 5.10- mm. and can be traced on the roller I'I in the size of x10 cm. The

resulting traced image then shows details magni-- fied five thousand times, which cannot possibly be produced by means of a light microscope.

It is, of course, unnecessary to trace a complete image in every case; tracing of only one scanning line to obtain an answer for a distinct question as to the properties and the nature of the object II will suffice in many instances. It

' is advisable in such cases to make the tracing the desired resolving capacity is; that is to say,

the amplification difiicultles are increased in proportion to the decrease in size of the electron focal spot which scans the object II. In case of employing very small electron focal spots (order of magnitude of from 10- to 10- itis, therefore, advisable to substitute for the electrode system l2i3 a particularly sensitive arrangement, e. g., an electron multiplier" or a Geiger The latter may comprise a vacuum-tight cylindrical casing 25, along the axis of which is located the prepared filament electrode 26, and which has a Lenard window a directed toward the object II. By filling the chamber 25 with a dilute gas and providing a sufiicient voltage difierence between the wall of the chamber 25 and the filament 26, every electron penetrating into the chamber will produce' a discharge impulse between the electrode electrode 25. The inner surface of the electrode 25 may also be providedv with a suitable coating which produces a high secondary electron emission when the electrons impinge upon it so as to facilitate the discharge puncture.

The counting chamber 25-26 is preferably connected with an integrating amplifier device,-

which may comprise, according to Fig. 4, an elec tron tube 21, the grid of which couples the filament electrode 26 of the counting chamber across the resistance 28 and the condenser 29. By

choosing the leak-resistance of the grid sufiiciently high, every current impulse flowing through the counting chamber 25-26 will cause used at present in television receivers.

33. This tube therefore conducts a current which corresponds with the time integral of the electrons entering into the counting chamber. This current may, therefore, be utilized for the tracing of an image.

Current sufllcient to control a carrier wave amplifier maybe Obtained at the trapping electrode I2, provided the resolving capacity is not driven to the limit. It is possible in this case to make use of a quickly operating image-tracing device of a type such as employed in the art of television. The deflection of the scanning electron beam or pencil is then preferably brought about in both coordinates electrically or magnetically so that the object-carrier can remain stationary.

Fig. 5 shows a cathode ray tube of the type A cathode beam." coming from the cathode 34 is ac celerated in theusual manner by the electrodes 36 and 31, and the rays are assembled in the plane of the fluorescent screen 38 where they form a focal spot. The intensity of the cathode beam is controlled by means of the Wehnelt cylinder 39; two coils 40 and 4| disposed perpendicularly with respect to each other and being controlled by means of the generators 42 and 43 for tilting oscillations are provided for its a temporary blocking of the tube 21. The mean anode current fiowing in the tube 21 is thus a function of the number .of the current impulses flowing through the counting chamber 25-26 in the unit of time; it produces at the resistance 3i, which bridges the large condenser 32, a voltage drop for controlling the grid of the tube control. Connected to the generator 42, which delivers the line frequency, are the two coils 2| and 22 (Fig. 1), and parallel to the coil 4| are located the coils 44' and 45 which must be visualized as being arranged perpendicularly with respect to the coils 2| and 22 at the head end of the tube i and which transmit the image deflection to the electron beam of the electronic,- optical arrangement of Fig. 1. By connecting the Wehnelt cylinder 39 with the output circuit of the carrier-wave amplifier in the manner shown in Fig. 5, the cathode beam 35 which scans the fluorescent screen in accordance with the movements of the electron focal spot scanning the object II, will be modulated in accordance with the permeability to electrons of the object H which changes from point to point. There is thus obtained directly on the fluorescent screen 38 an electron image of the object II. The magnification obtained corresponds in this case with the ratio between the scanning paths of the two electron beams on the surface of the object ,and on the fluorescent screen.-

An output shutter or diaphragm 6 having the previously noted opening diameter of 0.01 mm. can be accurately manufactured only with very great difficulty; and it is, therefore, advisable to use instead of a real shutter a so-called potential shutter or diaphragm for a' primary electron delivery source which is then reproduced on a reduced scale. The potential shutter can in this case be produced by the coaction of the anode and the Wehnelt cylinder. With an implement actually made in this way, the flat oxidecathode was located at a distance of 0.3 mm. in back of the opening of 0.6 mm. of the Wehneltcylinder which was located opposite the anode carrying 25,000 volts at a distance of .3 mm. With this arrangement could be obtained in the interior of' the passageway of 1 mm. diameter of the anode an electron focal spot, the diameter,

of which was considerably below 0.1 mm. and

optical device as compared with the arrangement shown in Fig. 1.

In order to obtain a possibly strong reduction,

the distance between the source of the electrons and the electronic-optical object-lens must be chosen as great as possible. However, the earth magnetic field constitutes a disturbing factor, insofar as it causes a curvature of the electron beam. This curvature can be considered to a certain extent in the manufacture of the device, or incident to adjusting it, but variations of the earth magnetic field give rise to considerable disturbances. In order to avoid these disturbances,

, the electron beam may be disposed in-the direction of the earth magnetic lines of force, or the path of the beam may be screened by means of a suitably designed and arranged iron shell. Another possibility is to compensate the earth magnetic field by the provision of a corresponding magnetic counterfield produced preferably by two flat coils (so-called Helmholtz coils) arranged coaxially with respect to the discharge tube and sufiiciently spaced therefrom. The passage of the current through these coils is then readjusted according to the variations of the earth magnetic field which may be ascertained by suitable measurement as desired.

A particularly important means for increasing the sharpness of the electron focal spot caused upon the object to be investigated consists in a multi-stage reduction of the reproduction of the original electron source. Such an arrangement is shown in Fig. 6. The electrons emanating from the cathode 46 are assembled by the coaction of the Wehnelt cylinder 41 and the anode 48 in an electron focal spot P1, from which they pass through a magnetic screening tube 49 in the direction of the magnetic objective lens 50. The iron-encased lens 50' produces by means of the magnetic field arising in its iron-free slot a strongly reduced real image of the electron focal point P1 in the point P2, from which the electrons pass through a second, particularly effective, screening tube toward a second magnetic objective lens 62, in front of which are arranged deflecting coils 53 and 54. The lens 52 reproduces the additionally reduced electron focal point P: on the surface of the object to be investigated.

In the embodiment illustrated in Fig. 6, the object to be investigated is not housed within the extremely evacuated electron beam arrangement 49, 6|, but is located in the open air. In order to render this possible, the arrangement 49, Si is closed by a disk 53a. which is provided with a tube having a minute aperture closed in turn by a Lenard window 54a. This window need not consist of metal, but may be made, for instance, of collodium or the like, by reason of the slight total scanning electron current. The window may be produced by the evaporation of a solvent down to a thickness of 5.10 In accordance with this invention, it may also serve as a support for the object to be investigated. The Lenard window a is arranged as close as possible to the trapping electrode 55 which is directly connected with the grid of the tube 56.

The possibility of investigating objects by means of an electronic-optical microscope without the necessity of introducing them into the high vacuum of the electron beam tube presents a particularly important advantage of the present invention. This opens a way for the electronmicroscopic investigation of substances, the texture of which is changed under vacuo (for instance, by withdrawal of water), and which, ,therefore, could not be investigated up to now by electron-optical microscopes without affecting their nature or properties.

I have already mentioned previously that the electrons starting from the scanning point of the v surface of the object concerned can be trapped in an electron multiplier". A particularly advantageousembodiment, which is distinguished by an exceedingly small initial current, is illustrated in Fig. 7. This "electron-multiplier consists of a vacuum vessel 61, in the interior of which are provided the prepared grids Il-BI as well as the closing plate 62, the diameter of the latter increasing from step to step. In front of the first grid 68 is arranged the diaphragm or shutter 63, and over its aperture is disposed the Lenard I window 64. This is substituted for the trapping electrode I! of Fig. 1, or 66 of Fig. 6. The rebounding electrodes 66-62 are connected with increasing positive voltage delivery points of the battery 65. In the supply connection to the last electrode 62 is provided a resistance 66, the voltage drop of which controls the amplifier ll. It was found that, by the use of such an electronmultiplier" for scanning the object to be investigated, about 20 electrons per image element are suflicient to control the amplifier H in a satisfactory manner.

In the examples discussed in the foregoing, the practical application of the invention has been described mainly in connection with the so-called penetration-radiation method". In contradistinction to the previously known electron-microscopes, the present improved devices not only permit a very high resolving capacity in connection with thin layers of an object, when using said method, but also render possible, while maintaining this capacity, an electronic-optical observation in plan view (exclusive observation with returning electrons), and also an electronicoptical observation after the so-called darkfield method (exclusive observation of the laterally strayed electrons). Suitable embodiments of the trapping electrodes which serve as registering surface are shown in Figs, 8-10, whereby the known modes of illumination of the lightmicroscope are imitated.

In Fig. 8 the numeral 64a denotes, drawn to an enlarged scale, the Lenard window onto which impinges the electron beam 66a under a very narrow convergence angle. The Lenard window carries the object 61 to be investigated. If, analogous to Fig. 1, a trapping electrode 66 located in agrounded cage 69 having a narrow window is arranged opposite the object 61, the electrons which penetrate the object 61 without being deflected, as well as a limited portion of the laterally straying electrons will obviously reach the electrode 69. This arrangement therefore corresponds with the bright-field transparent observation in case of the light microscope.

In the further example illustrated in Fig. 9 is arranged an annular trapping -electrode II in front of the Lenard-window, this electrode trapping exclusively returning electrons. This observation method, which is used, for instance, for the investigation of the surface texture of ground metal parts, corresponds to the observation with incident light by means of a light microcope. In order to protect the trapping electrode 10 against the impingement of electrons not returning from the Lenard window 54a, a grounded annular shutter or diaphragm II is provided, which surrounds the electron beam 66.

In Fig. is shown an electron arrangement for .an observation method which corresponds with the dark-field observation" of the optical microscope. The electrons which penetrate the object 61 to be investigated without being deflected are trapped by the grounded circular disk 12. Only the annular surface is therefore used as registering surface, which remains free between the disk 12 and the cage electrode 13. Only the straying or deflected electrons therefore reach the trapping electrode 14, which start from the object within the range of a predetermined angle. It is clear that this method yields images very rich in contrasts even with extremely thin object layers. The electrodes shown in Fig. 10 may also be given such geometrical configuration that only those electrons exert an action incident to the image reproduction which correspond with certain distinct diffraction figures. The distribution of micro-crystalline particles in the object can be made visible in this manner.

Another mode of observation is presented by the possibility of utilizing the object support itself (object-carrier I I, Lenard window 54) for a trapping electrode. A pure electron absorption image may be attained in this manner. In order to carry out this method, it is solely necessary to use a conducting object support, which is sumciently insulated with respect to the other members of the apparatus. The manufacture of conducting object supports can be effected, for instance, by means of cathode atomization. Making use of this method of manufacture is also recommended if separate trapping electrodes are employed and if it is desired to place between the trapping electrode and the object an acceleration potential (for instance, for the acceleration of secondary electrons), or a brake voltage as used, for instance, to keep oil! of the trapping electrode all electrons, the speed of which is below a certain limit.

Another very important advantage which this invention presents appears particularly from the Pigs, 8-10. It is based onthe narrow convergence angle of the scanning electron beam resulting in a very great depth sharpness. It is known that with a microscope the depth sharpness lies in' the order of magnitude of the resolving capacity. With strong microscopes it is usual to make use of aperture values'which approach the value One". If apertures of the same value for generating a cathode ray, means for focusing the ray to a point on an object to be examined, the diameter of said point being of the order of the wavelength of visible light, a target beyond the object in the direction of the ray for intercepting electrons thereof that pass through the object, a circuit in which current flow is established by electrons intercepted on said target, reproducing means controlled responsive to variations in said current flow, said reproducing means including an element movable in two directions, and means electrically responsive to movement of said element for producing corresponding and proportionate movements of microscopic dimensions of the focus point of said ray on said object.

2. An electronic microscope comprising means for generating a cathode ray. means for focusing the ray to a point on an object to be examined, the diameter of said point being. of the order of the wavelength of visible light, a target beyond the object in the direction of the ray for intercepting electrons thereof that pass through the object, a circuit in which current flow is established by electrons intercepted on said target, reproducing means controlled responsive to variations in said current flow, said reproducing means including an element movable in two directions, and means electrically responsive to the two movements of said element, respectively, for moving the focus point of the said ray in one direction and the object in a different direction to cause said point to scan a microscopic area of said object.

3. In an electronic microscope, means for producing a cathode ray, means for focusing the ray to a point on an object'to be examined. the diameter of said point being of the order of the wouldbe employed in connection with the present electron-microscope, the resolving capacity of which extends to 10- and 10-", it would become necessary to make the plane in which the elec-- tron spot moves when the object is being scanned coincide with the object layer to be investigated with a degree of accuracy amounting to from 10 to 10-". If, however, electron object-lenses, which have an aperture of about 0.01 mm. and can be manufactured comparatively easily by magnetic means, are used, the depth sharpness rises by two tens-powers and it is sufilcient to make the plane of the object to be scanned coincide with the observation plane with an accuracy of from 10- to 10- This accuracy is still within the range of what can be attained with electric or mechanical auxiliary means.

Changes may be made within the scope and spirit of the appended claims, wherein I have defined what is believed to be new and desired to have protected by Letters Patent of the United States.

Whatis claimed is:

1. An electronic microscope comprising means wavelength of visible light, reproducing means controlled in accordance with the number of electrons of the ray which pass through the object at the point where the ray impinges. said reproducing means including a movable element, means for producing movement of said element in two directions, and means electrically responsive to the movement of said element for producing corresponding but relatively microscopic movements of the focus point of the ray on said object.

4. In an electron microscope, means for producing an electron beam, means for focusing said beam at a point on an object to be examined, the diameter of said point being-of the order of the wavelength of visible light, scanning means for causing said point to scan an area of said object whereby said impinging electron beam is modified by said object, a collecting electrode adjacent said object, and a shield interposed between said object and target, said shield having an annular aperture therein surrounding an opaque portion located in line with controllable in response to variations in said current flow, said reproducing means including a movable element, and means electrically responsive to the movements of said element for producing similar but greatly reduced movement between said object and said beam.

6. In an electronic microscope. means for producing an electron beam, means for focusing said beam on a point or an object to be examined to cause electrons to be released therefrom,

the diameter of said point being less than the ducing an electron beam, means for Iocusing said beam on a point or an object to be examined to cause electrons to be released therefrom, the diameter or said point being less than the wave length or visible light, electrode means tor collecting said electrons, an output circuit coupled to said electrode means in which a current flow is established proportional to the number of said electrons, reproducing means coupled to said output circuit having a reproducing surface and a recording element, means for producing relative movement between said surface and said element, and means electrically responsive to said relative movement for producing corresponding movement of microscopic proportions between said object and said beam whereby said beam scans a microscopic area of said object.

MANFRED von ARDENNE.

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