US2424788A - Electron microscope - Google Patents

Description

July 29, 1947.

c. H. BACHMAN ETAL 2,424,788

ELECTRON MICROSCOPE Filed Dec. 1, 1942 6 Sheets-Sheet l Invevflsors:

I v Charles H. Bachman,

8 Simon Ram o,

- Their Attorney .July 29, 1947.

c. H. BACHMAN, ET AL ELECTRON MICROSCOPE Filed Dec. 1, 1942 6 Sheets-Sheet? am 0 ML,

Inventors Charles H. Bachman Simon R by #04 (Y 1] Their Attorney,

July 29, 1947. c. H. B ACHMAN EIAL ,7

ELECTRON MICROSCOPE Filed Dec. 1, 1942 6 Sheets-Sheet 3 Inventors: Charles HBachman; Simon Rama- Their Attorney.

y- 1947. c. BACHMAN ETAL 2,424,783

ELECTRON MICROSCOPE Filed Dec. 1, 1942 s Sheets-Sheet 4 Inventors: Charles H. Bachman,

Simon Ramo,

Their/Attorney July 29, 1947. c. H. BACHMAN sf AL I 2,424,738,

ELECTRON MICROSCOPE Filed Dec. 1, 1942 e She ets-Sheet 5 Y Inventors: Charles H. Bachman, Simon Pamo,

Their At/torney.

Patented July 29, 1 947 ELECTRON MICROSCOPE Charles H. Bachman, Scotia, and Simon Ramo,

Schenectady, N. Y., assignorsto General Electric Company, a corporation of New York Application December 1, 1942, Serial No. 467,530

30 Claims. (Cl. 25o-. 49.5

and more particularly to new and improved structure for electron microscopes of the electrostatic type.

In electron microscopes of the'prior art, and particularly those which have been constructed in accordance with the prior art arrangements and teachings, the structures thereof, particularly the evacuated chamber within which the elements are placed, have not been readily adaptable or susceptible to widespread application due to the fact that the evacuated chambers have been of relatively large size because of the type of the electron lens systems employed, thereby involving relatively large time intervals between examina--'' tions of diiferent specimens. I

The prior art arrangements have also been subject to the disadvantage of requiring the services of skilled operators to effect frequent re-alinement of the electrodes or focusing apparatus.

Where, in the prior art, the magnetic type of electron microscope has been employed, in order to obtain accuracy of the image produced'and to retain a desired degree of precision in the focusing effect, it has been necessary to use a large number of regulating or controlling elements for controlling the voltage or current supplied to the electrical elements of the microscope. Due to the above factors the size of the prior art microscopes has been inordinately large, thereby restricting the degree of portability and thereby limiting the field of application.

In accordance with the teachings of our invensertion and extraction of specimens to be inves-' tigated and which entails only the lapse of a relatively hort interval of time between examina-' tions due to the fact that the chamberis relatively small. In addition, the specimen insertion chamber and sealing structure thereof are arrangned to permit ease of insertion and .ex-

traction of the specimen. Furthermore, the essential elements of the electron microscope, such as the cathode construction and the electron lens system, are readily demountable for-inspection and replacement. Lastly, a unitary electron, lens assemblyis provided so that the alinement of the electrodes thereof may be initially made at the factory, thereby obviating the necessity for frequent and time consuming re-alinement during use after manufacture.

Itis an object of our invention to provide a new and improved electron microscope.

It is another object of our invention to provide a new and improved unitary electron lens assembly of the electrostatic type for an electric discharge device.

It 'isstill another object of our invention to provide new and improved structure for an electron microscope wherein the essential elements are readily demountable and available for inspection and replacement. I

It is a further object of our invention to provide a new and improved electron microscope wherein specimens may be inserted and extracted without involving the lapse of undesirable periods of time between examinations ofdiiferent specimens.

It is a still further object of our invention to provide a new and improved demountable cath-' ode supporting and sealing structure for an electric discharge device.

It is a still further object of our invention to provide a new and improved demountable cathode supporting and positioning structure for an electron microscope.

For a better understanding of our invention, reference may be had tothe following description taken in connection with the accompanying drawings, andits scope will be pointed out in the appended claims. Fig. 1 diagrammatically illustrates an embodiment of our invention wherein the evacuated chamber containing the lens system is horizontally disposed and wherein the actuating means for moving the specimen are located within the vicinity of the viewing aperture. Fig. 2 is a cross sectional view of the cabinet supporting the microscope and shows the disposition of associated or auxiliary apparatus. Fig. 3 is a cross sectional view of the electron microscope showing the evacuated chamber, electron lens system and cathode supporting structure;

Fig. v4v is a cross sectional view of the cathode supporting structure; and Fig. 5 is an alternative form of the flexible connection which may be employed therein. Fig. 6 is a view of the actuating structure for the sealing valve adapted to open and close'the specimen insertion chamber.

1 13.19 represents the cartridge-type specimen zontal conductor which connects corresponding electrodes of the various horizontally disposed electrostatic lens system. Fig. 14 is an exploded view of the manipulator; Figs. 15 and 16 are front and side views, respectively, of the manipulator showing the horizontal motion produceable thereby; Figs. 17 and 18 are front and side views, respectively, showing the vertical transverse motion produceable thereby, and Figs. 19 and 20 are front and side views, respectively, of the manipulator showing the longitudinal or focusing action produceable by the manipulator. Fig; 21 is a detailed view of the externally accessible actuating means or wheels which control the specimen manipulator. This figure also shows bellows-type sealing structure and the member for supporting a viewing screen or fluorescent screen. Fig. 22 is a detailed view of the insertable optical or glass lens which may be used in conjunction with our electron microscope. Fig. 23 illustrates the manner in which a camera maybe used to photograph the image produced upon the fluorescent screen of the electron microscope, and Fig. 24 is a simplified circuit diagram showing the controllable source of voltage for the electron microscope and the cathode filament pp y.

Prior to a detailed description of the embodiment of our invention illustrated, it is believed that it may be helpful to present generally certain fundamental aspects of the electron microscope disclosed herein. Generally speaking, the electron microscope which we provide is one employing an electrostatic type lens. The microscope comprises essentially an electron gun which produces an electron beam to illuminate or ir radiate a specimen to be investigated and an imaging system which magnifies the image produced by the impingement on the specimen'into a larger image on a viewing screen such as a'fiuorescent screen, or :upon the surface of aphotographic plate.

In an electron microscope of the electrostatic type which we provide, the lens focal length is a function of its physical size and configuration since an increase or decrease in the electron velocity is always accompanied by a change in the electric field focusing action of just such magnitude or strength as to yield precisely the same electron paths. By the use of such a system it is then not necessary to employ a closely r'egulated source of supply voltage for the microscope.

Of course, as is well known, the desirability of employing an electron microscope in place of a light microscope for many applications is the greater resolving power incident to the use of electron irradiation as contrasted with the smaller resolving power of light microscopes. Al'- though not limited to a particular resolving power or magnification, an electron microscope built in accordance with our invention operates satis factorily having a 200 Angstrom units resolution and a useful magnification of about 10,000 As pointed out hereinafter, this magnification may be obtained either exclusively by the design of the electron lens system itself, or by the com-'- bination of a suitable electron electrostatic lens and an associated light microscope or le'ns'which is placed in the vicinity of the fluorescent screen at one end of the electron microscope.

In the electron microscope described hereinafter, we have incorporated certain features by virtue of the configuration and position of the elements to obtain the desired operation of the electron gun, the irradiation of the specimen, and the 'desird'focus'ing action of the electrostatic lens. As concerns the electron gun, this element is constructed to produce sufficient current density at the specimen without involving an appreciable heating of the specimen by the electron beam. On the other hand, the image is illuminated'to a sufficient degree of brightness for "visual observation with a minimum of current striking the specimen. It will be apparent that there is no advantage to irradiate a large part of a specimen if only a small portion on the axisis to be imaged. Furthermore, there is no reason to direct electrons at the specimen if their incident angle is such that the absence of deflection they will consequently be rejected by the objective lens aperture stop. The contrast obtained at the viewing screen or fluorescent screen is the result of. distribution of the electrons arriving there. We have found that for very thin specimens the majority of the electrons which pass through the-specimen do so with their slope substantially'unchange'd; consequently, the limiting angle of the electron bundle leaving the objective lenscan be established principally by the electron gun apparatus rather than by the apertures of the objective lens system. This type of limiting s'generally easier to accomplish mechanically by'the necessary aperture in the electron gun,

and this necessary aperture may be more favorably located than the objective lens stop so as to obviate the need of punching exceedingly small diameter holes in the-lens electrodes. Of course, the lens aberrations as well as the particular thickness and material of the specimen that is under examination, play an important part in determining whether the gun angle can serve as anefi'ectiv'e stop for the system, or whether that limitis determined by the first imaging or objective': lens- We have found that highly suc- .cessful results are obtained with a small gun angle; and the gun angle produces a more readily discernible'efiect upon the resolving power than does, the lens aperture.

The electron microscope described hereinafter is constructed to afford proper relationship between thecathode filament point location, the filament shield aperture diameter, anode-filament spacing and the spacing and size of the other apertures. The spacing and size of the other apertures of the system depend upon a number offactors such as the location of the image lens-system and the specimen, the filament life, the'accelerating voltage, the electron beam angle at the specimen and the extent of magnetic shielding. of th'e'field due to the filament current.

concerns the electrostatic electron lenses which magnify the electron image produced by the impingement of the electron beam on the and highly polished to minimize the insulation problem and field emission at the operating voltage. The lenses are of symmetrical configuration with a single negative potential to ground, 3

namely the cathode potential, serving to energize the lens. The axial voltage drop in the central region of the lens is maintained at an optimum value, thereby minimizing strong magnetic fields and chromatic aberration effects.

Fig, 1 of the drawings represents a perspective view of a cabinet I, which is of a portable nature being mounted on castors 2, and which houses associated auxiliary apparatus for an electron microscope, and on the front of which the electron microscope is horizontally positioned within a cylindrical cover 3. There is provided a desk or table part 4 integral with the cabinet -l-to facilitate use of the microscope. An end cover 5 encloses the end of cylindrical cover 3 and .is provided with a viewing aperture through which an eye-piece 6 extends. This eye-piece is shown in detail in Fig, 22. There is also positioned within the vicinity of the eye-piece or viewing aperture a plurality of externally accessible actuating means, such as knurled wheels 1, B and 9 which serve to operate a manipulator (described hereinafter) to position a specimen or object to be examined. A specimen insertion chamber terminated in a sealing valve [0 and having an actuating member H is conveniently located within easy reach of the operator at the top of cover 3 and is shown in detail in Fig. 6.

Fig. 2 is a side view showing the disposition of the associated auxiliary or control apparatus within the cabinet, I. A suitable power supply means I2, including a rectifier and a transformer, is employed in an upper compartment together with voltage controlling means such as a rheostatic regulator l3 and a vacuum valve I4. Cooling means for the electron gun, such as a centrifugal blower I5, is also positioned in this compartment of the cabinet. In the lower compartment of the cabinet, we employ an evacuation means such as a pair of serially connected pumps l6 and I1, both of which are supported by vibra tion absorption means such as springs l8 and I9.

Pump I6 is preferably a high vacuum two-stage oil diffusion pump which operates in series with a mechanical pump, the latter of which furnishes the rough vacuum. Control means for the electrical apparatus and for the pumping apparatus may be positioned within the front of,

the cabinet I. For example, the rheostatic regulator I3 of the voltage supply for the electron gun of the microscope may be provided with a handle or knob 20 which extends through the,

wall of the cabinet to make it readily accessible 1 the electrostatic lens system, and thespecimen carrier manipulator and actuating means there- The evacuated chamber within which the for. microscope elements described above are positioned is defined in part by a metallic cylinder,-

such as a brass cylinder 24, the interior surface M of which may be adapted to closely engage an 6 insertable unitary electron lens assembly to be described more in detail hereinafter and which is shown in Fig. 7. As a means for shielding cylinder 24, we provide a laminated metallic shielding structure which includes alternate layers of copper and a high permeability iron. For example, we may employ cylindrical sheaths of copper 25 and 26 and alternate layers of high permeability iron 21 and 2B. The sheath which is adjacent cylinder 24 may be either iron or copper and may be turned over at its ends in order to secure the other layers in the position illustrated.

We provide a unitary electrostatic electron lens assembly which is defined externally by means of a metallic cylinder 29, preferably constructed of brass, the inner surface of which is finely ground and highly polished to closely engage cylinder 24 and to support therein the elements of the electrostatic lens system. The inner surface of cylinder 24 and the outer surface of cylinder 29 need not be as precisely machined as the inner surface of cyinder 29. Within the cylinder 29 we provide an electrostatic lens system which includes a plurality of generally similar lenses 3!), 3i and 32. This lens assembly may be arranged to have a total magnification of about 1500. Lenses 30 and 3! each comprise a pair of longitudinally spaced wafer-like electrodes 33 and 34 maintained at a common or anode potential and which are provided with apertures 35 and 36, respectively. We provide an intermediate electrode 31 also of wafer or disk-like configuration in spaced relation between outer electrodes 33 and 34 and which is provided with an aperture 38 of substantially larger diameter than the apertures 35 and 36. Electrodes 33 and 34 are constructed to have their peripheries finely ground and polished to closely engage the inner surface of cylinder 29, thereby establishing satisfactory mechanical and electrical connection to the system. Intermediate electrode 31 is electrically insulated from cylinder 29 and is maintained at cathode potential by means of a horizontal conductor connected thereto and shown in Figs. 7 and13. Electrode 31 is maintained in the position illustrated by means of a split insulating spacer or washer 39 which is supported by and in engagement with the inner surface of cylinder 29. The insulating washer or spacer 39 may be split, constituting two segmental parts to facilitate assembly thereof with the intermediate electrode 31.

It will be noted that the peripheries of apertures 35, 36 and 38 are convex, thereby reducing to a minimum the tendenc to establish undesired voltage gradients within the vicinity of the apertures or along the electron lens system,

The Various elements of the unitary electron lens assembly are susceptible of precise alinement and locking, thereby permitting an initial alinement of the electrodes at the time of manufacture and obviating the necessity of realine-' ment in the field. For example, the electrodes of the lenses 30-32 may be maintained and locked in the desired illustrated position by means of a plurality of principal longitudinal and annular spacers 40-42, inclusive. In addition, the intermediate electrod and the outer electrodes may be maintained in the desired spaced relation by means of smaller annular spacers 43 and 44 preferably of metal.

In accordance with our invention, We also provide a unitary structure comprisin the aforementioned electron lens system and a specimen 7 carrier 45, shown in Fig. 9, and a specimen manipulator 48, shown in Fig. 8. The elements 45 and 48 will be explained hereinafter in detail. The specimen carrier 45 is supported by manipulator 46 which in turn is supported from one end of the lens assembly.

The objective electron lens 32 is essentially of the same electrical characteristics as electron lenses 3t and 3i and comprises a pair of spaced outer electrodes 41 and 48 and an intermediate electrode 49. Outer electrode 48 is provided with an aperture 48 and a radial recess 5i adapted to receive a specimen holder shown in Fig. 11, and is also provided with an enlarged aperture adapted to receive one part of the cartridge-type specimen carrier shown in Fig. 9.

As a part of the unitary construction comprising the electron lens assembly and the carrier and manipulator, we also may add an electron beam defining means, such as an apertured disk '55 and supporting structure therefor shown in Fig. 10. This supporting structure includes means for centering and positioning the disk 50 which may be constructed of molybdenum and which serves to produce a well defined beam for the desired illumination of the specimen or object which is held by carrier 45. The apertured disk 50 is preferably of such dimension to subtend at the specimen a solid angle in the neighborhood of radians.

A radial specimen insertion chamber 54 is pro vided in alinement with slot 5| to permit ready insertion and extraction of a specimen in carrier 45. The insertion chamber may be defined by a radial tubular member 52 preferably constructed of nonmagnetic material and supporting at its upper end a valve 55 for sealing the insertion chamber 54 and also for sealing the main evacuated chamber of the microscope. Valve '55 comprises a removable cover 58 which is engaged by a cross rod which When pressure is exerted thereagainst maintains the cover firmly against the upper surface of a cylinder 58, constituting the side wall defining means of the valve. If desired, a suitable gasket 59, such as a rubber gasket, may be employed to seal th juncture of the cover 56 cylinder 58.

We also provide means for supplying to the evacuated chamber of the microscope air at atmospheric pressure prior to the opening of cover 1 56. This means may comprise a passage 60 fed by a conduit "5| through which pre-conditioned or heated air may flow to prevent the condensation of appreciable moisture on the internal parts of the microscope when it is desired to extract or insert a specimen. As a means for selectively controllin the flow of the heated air, we provide a valve 52 seated in passage 60 and actuated by a member 63. A bellows-type sealing memher 6 is sealed around the adjacent area of cylinder 58 and arm 63. The actuating mechanism for cover 56 and valve 62 is shown in Fig. 6.

At one end of the cylinder 24 we provide a sealing and supporting structure for a filamentary cathode comprising one of the electrodes of an electron gun, which comprises a filamentary pointed cathode B5 of the hairpin type. There is also provided magnetic shielding structure comprising apertured wafer-like metallic members or disks E5 and 61 which are firmly positioned against the inner surface of cylinder 24. Electrostatic shielding means, such as a transverse metallic planar member 61 is positioned between members 65 and 61 intermediate the electron beam apertures of the latter members and the position of a longitudinal conductor to be described presently, thereby defining a substantially field-free region through which the electron beam passes. Locking means are provided for maintaining all the elements of the system in a fixed longitudinal and angular position with respect to the unitary assembly described hereinafter. This locking means may comprise a longitudinal key 68 which engages an abutment of cylinder 29 or extends into a recess provided thereby, and also extends axially through openings at the peripheries of disks 66 and -6l. Key 68 may also extend through annu lar spacers 69 and 10 which maintain disks 6! and B8 and the unitary assembly in the desired spaced relation.

Disks 66 and 67 are provided with apertures H and 12 through which a horizontal conductor shown in Fig. 13 extends to impress on the intermediate electrodes of electron lenses 30, 3| and 32 a potential corresponding to the cathode potential. That is, the cathode structure to be described hereinafter and the intermediate electrodes are all maintained at the same potential.

If desired, we may employ an accelerating anode cup structure 13 supported by a transverse disk 13' and extending an appreciable distance longitudinally around the filamentary cathode 55 so that a suitable accelerating electric field is provided for th electrons emitted by the oathode.

Exhaust apparatus for the evacuated chamber of the electron microscope may be connected to a tubular conduit 15 in communication with the chamber and which is also connected to the pumps l6 and [1 shown in Fig. 2.

In order to support the cathode structure and to seal one end of the evacuated chamber defined by cylinder 24, we provide a supporting ring 16 which is seated upon a shoulder provided by cylinder 24 and which also engages an annular spacer 11. A sealing structure is employed and comprises a compressible Washer, such as a rubber Washer 18, a metallic compression ring 19, a threaded retaining ring 89 adapted to engage thread provided at the end of cylinder 24, and a plurality of circumferentially displaced means, such as screws 8|, for exerting longitudinal or axial pressure against the retaining ring 19, thereby compressing washer 12.

A vitreous part 82 is sealed to the supporting ring 18 and serves to support and seal the cathode structure. The vitreous part 82 may be of the re-entrant type through which a concentric transmission line comprising an inner conductor 83 and a tubular outer conductor 84 extends. The details of the cathode and supporting structure will be discussed with respect to Fig. 4.

At the other end of the evacuated chamber, particularly at the left-hand end of cylinder 24, we provide an end wall for sealing that end of the chamber and for supporting a viewing screen, such as a fluorescent screen 85 coated on the inner surface with a suitable fluorescent material and upon which the magnified image is produced by the impingement of the electron beam. This end wall comprises an apertured disk 86 which seats the viewing screen or fluorescent screen 85. A suitable seal for the screen 85 may be obtained by means of a rubber Washer 81 which is compressed by means of a flanged annular ring 88, the latter being attached to disk 86. The outer periphery of disk 86 is also sealed in a similar manner by means of a compressible rubber washer 89, a metallic compression ring '90, a retaining ring 9|, and pressure exerting means such as screws 92 which exert an actual pressure on compression ring 90, thereby compressing the washer 89 and sealing the juncture.

Means for actuating or controlling the speci- :1-

screw-threaded devices 96, 91 and 98, only two;

of which are shown in Fig. 3. The screw-thread device 91 is shown in cross-sectional detail and is also representative of the other two devices. For example, screw-thread device 91 comprises a head 99 which engages the associated longitudh,

nal actuating rod 94 and is controlled in its longitudinal position by means of a screw I which is provided with an actuating pin IOI and which has one end thereof extending through disk 56 to effect an externally accessible connection adapted to receive a flexible cable shown in Fig. 21. Each screw-thread device may be supported by a cylindrical member I02 formed integral with or attached to disk 86 and which is sealed to its associated head 99 by a deformable member such as a bellows I03 sometimes defined as a Sylphon bellows.

Reference is now made to Fig. 4 in whichflthe cathode supporting structure is shown in greater detail. We provide means for positioning, that s rapid replacement of the filament and also per-.

mits the use of different types of filaments in connection with the other elements 01' the electron microscope. A further advantage of the cathode supporting structure is the unique arrangement by virtue of which certain portions .of the structure not only serve as a mechanical support for the cathode but also serve as electrical conducting paths for the conduction of cathode heating current. Y

A metallic base I04 is sealed at one end'to'the vitreous part 82 and is provided with threads to engage threads at one end of the outer tubular conductor 84 to the base I04 which not only transmits power to the filament 65 but also serves to impress through the longitudinal conductor shown in Fig. 13 a negative potential on the fllament and the intermediate electrodes of electron lenses 30-32. Base I04 is constructed to afford a flexible mechanical connection between the inner conductor 83 of the concentric line and the base. Aconductive connecting member I05 is mechanically supported by base I04 but is electrically insulated therefrom by means of'a glass bead or seal I06 which rigidly supports member I05. At one end of member I05 we provide a flexible connection such as a coil spring I01 attached to the inner conductor 03. i

The base I04 is also provided with a'threaded flange I08 which supports a metallic thimble I09 provided with a shoulder IIO which longitudinally positions the cathode positioning or centering means to be described immediately. This cathode centering means is flexibly connected'to member I05 by means of a flexible connection such as a coil spring I I I and comprises anannula'r' centering ring II2 having near the periphery thereof a restricted region 3 to obtain a substantial line engagement with shoulder I I0. Ring 2' may be positioned by a suitable mechanical expedient such as a plurality of circumferentially displaced screws II4. Furthermore, the centering ring H2 is maintained in the desired longitudinal or axial position in flrm engagement againstshoulder IIO by means of a compression spring II5 which is'supported at its right-hand extremity by means of a threaded collar II6 which engages threads on the inner surface of thimble I09. As a means for insulating the shell of thimble I09 andthe centering means from the center conductor to the cathode, we employ a cylindrical insulator II1 placed between retaining ring H2 and a cylindrical cathode base -I I8 adapted to receive a rigid cathode stem II9. One terminal I20 of the point cathode is connected to stem I I 9 through a face plate I2I which is attached to stem II9, the latter constituting an electrical path for the inner conductor 83 through coil spring III, connecting member I05 and coil spring I01. The other terminal of the cathode I22 is connected to a metallic pin I23 supported by and electrically insulated from the face plate I2I by means of a glass bead I24.

' As a means for facilitating ease of replacement of the cathode element 65 and to permit mechanical movement of the stem I I9 and cathode'65 without disturbing the electrical connections to the cathode, We employ a flexible conductor such as a ribbon conductor I25 which extends into an opening I26 in the body of thimble I09. A screw I21 or any suitable mechanical expedient maybe employed for retaining and removing the ribbon conductor I25 in opening I26. The electrical path from terminal I22 of cathode 65 comprises pin I23, conductor I25, thimble I09 1 and tubular conductor 84 of the concentric line.

At the end of thimble I09 We provide a cap I26 which is removable or demountable and which is adapted to be supported by thimble I09 by a screw-threaded engagement at I29. This cap I28 is provided with an aperture I30 through which the electrons emerge upon acceleration due to the field produced byanode cup 13.

An alternative flexible connection for the oathode supporting structure is shown in Fig. 5 and corresponding elements have been assigned like reference numerals. Instead of employing coiled springs as the flexible connection and extending one of the springs through member I I8, we may employ folded strips I3I and I32 of resilient metal,

respectively, connected to members II8, I05 and The actuating mechanism for releasing the pressure on cover 56 of the valve 55 closing the specimen insertion chamber is shown in Fig. 6. Rod 51 which exerts pressure against cap 56 may be removed therefrom by movement of handle I I in a clockwise direction, causing the clockwise rotation of a pivoted actuating arm I33 which is pivoted at some point to the right of the figure and which is connected to rod 51 through a crank I34. Member 63 which actuates valve 62 is also connected to actuating arm I33 by means of a connecting rod I35 which is positioned to actuate valve 62, thereby admitting air to the insertion chamber and the main evacuation chamber so that the cover 56 may be removed.

In' Fig. 7 we have there diagrammatically illustrated, partially in perspective and partially in cross section, the unitary electron lens assembly showing the plurality of spaced electrostatic 11 lens systems 30, 3I and 32, andcorresponding elements have been assigned like referencenumerals.

Referring to the electron lens 32, it will. be noted that the intermediate electrode. 49' thereof, which corresponds to the intermediate electrodes 31 of electron lenses 30. and 3I, is supportedby an insulator I36 which holds the. intermediate electrode 49 firmly in position and alinement. Provision ismade for the impression on the intermediate electrodes of. each lenssystem of mptential corresponding to the cathode. potential by means of the conductor shown in Fig; 13. This conductor extends longitudinally oftheelectron gun and is placed within thecylinder 29 extending through apertures I31-I4I. These apertures are also finely machined or ground to have convex peripheries to minimize. high electric field gradients.

The conductor is a sectionalized. conductorrto facilitate assembly and disassembly of. theelectron lens unit, and comprises a pluralityof; conductors I42, I43 and I44 whichare weldedv or otherwise attached to they intermediate: electrodes and are provided with recesses to receive telescoping intermediate members I45: and. I46.

We also provide within the, electron. lens assembly electrostatic shieldingmeansspaced. longitudinally between the sectionalized conductor and the electron path. Thisshie'lding means'may comprise a plurality of planarmetalllc shielding members I41 and I48 which closely engage the inner surface of cylinder 29 and abut facing-outer electrodes of the electron lenses 3D.32, inclusive.

As means for facilitating assembly of the en tire electron. lens unit and forv positioning: the actuating rods 93-95 in a position whichismechanically expedient and which will: not deleteriously affect the operation of the electron lens system, we provide longitudinal grooveslyihg. in cylinder 29 and adapted to receive and permit longitudinal movement. of rods 9.395. Only'one of these grooves, that is groove. I'49' which; re:- ceives rod 93,.is shown in Fig. 7. Each. of-yrods S395 is terminated in a foot I59; which engages heads 99 of thescrew-thread:devices.9698 shown in Fig. 3.

As a means for locking or: keying th electron lens assembly in the desired or fixed. angular position withincylinder 29, we may employsuitable means such as a tongue II constituting an integral part of or attached to-the cylinder, and which may be adapted to engage one of the annular spacing rings such as ring 69 shown1-in Fig.3.

The manipulator 46 is shown in detail in: Fig. 8 and constitutes a system of, multiplewayswhich are mutually adjustable 0r controllable to produce three independent movements at rightv angles to each other. That is, the manipulator 46 upon actuation by rods 93-95 produces independent horizontal-transverse, vertical-trans verse and longitudinal or axial motion. The transverse longitudinal and vertical. motions: are employed for. scanning purposes and. the axial motion is employed. for focusing purposes, the latter being necessary in view of the fact that the focal length of the electrostatic. lens system provided by the microscope is fixed by the: geometry of the electrodes.orlensesthereof.

The manipulator shown in. Fig. 8 is provided with three actuating levers I52,v I53 and I54, only two of which are shown in. Fig. 8. The details of construction and operation of themanipulator will be explained in connection with Figs..14-20, inclusive. A's to the general features of operationit may be saidhere that the specimen carrier 45 is supported at one end by an annular ring I55 constituting a part of the manipulators, and this ring by virtue of the aforementioned ways moves the carrier 45 for scanning. and focusing purposes. The entire manipulator. issupportedgbya face plate I56 which is maintained in the desired axial position by engagement with the inner surface of cylinder 29.

Fig. 9 is a detailed cross sectional view of the specimen carrier 45 which may be of the cartridge-type comprising-a retaining shell I51 having a-longitudinal opening I58 through which the" electron beam passes, and having a resilient holding'or-positioning'member such as a plunger I59 also apertured. The plunger I59 may be biased by means of a compression spring I 69. The carrier I45 is adapted to receive a locket shown in Fig. 12 which holds a specimen, and the distance between the inner vertical surface of shell I51 and the face of plunger I59 is such that'a slight pressure must be exerted upon the locket to obtain an insertion and in order firmly tohold the locket during adjustment of the manipulator 46.

In Fig; 10 there is illustrated the structure for supporting'the beam limiting disk 59. This structure comprises a face plate I6I which is supported from face plate I56 by means of a plurality of spaced supporting rods I62 and I63. Other rods, not shown, may also be employed to afford a firm support. Molybdenum disk 50 is held-in position by a shroud ring I64, the latter-of'which is positioned-by a plurality of thumbvided with apertures I69 and I68 which, how

ever, for the purpose of construction and arrangement are not in alinement with the corresponding apertures I31I4I inthe disks or electrodesof the electron lens assembly or aperture I10 of face plate I66 shown in Fig. 8. These latter mentioned apertures are provided for the purpose of receiving the sectionalized horizontal conductor shown in Fig. 13 and which will be described in detail hereinafter.

A specimen holder is shown in Fig, 11 and is arranged to be inserted through the specimen insertion chamber 54 shown in Fig. 3 and into slot 5I and includes a locket I1'l comprising a recessed ring I12 adapted to receive a, fine removablemesh upon which the specimen is placed and: a locking and. retaining ring I13. Rings I12 and L13 are shown in cross sectional detail in Fig. 12.

In Fig. 11 we have also shown a suitable specimen holder which comprises a stem I14 spring biased to the position illustrated with respect to a cylinder I15. Stem I14 telescopes cylinder I15 and the-latter is attached to a curved holding plate I16 whereby spring I11, in the absence of pressureon knob I16, draws the stem I15 upward. The recessed disk I12 is flexibly connected to the; stem I14 by suitable means such ass fine-linkrchain. I19. The lower end of stem I14 is provided with a curvature to engage the periphery of disk I12 upon depression of mem ber I18, thereby firmly holdin the locket in engagement for the purpose of extending and psitioning the locket in carrier 45 shown in Fig. 9.

In Fig, 13 there is shown a portion of a longitudinal :zcctio-nalized conductor for impressing cathode potentialon the intermediate electrodes of the various electron lenses 3Ii32 and corresponding elements have been assigned reference numerals employed in Fig. 7. Due to the fact that the apertures I68 and I99 of the beam limiting structure are not in alinement with apertures I 31I4I of the lens assembly and aperture I of the manipulator face plate I56, we provide an elbow I 80 which telescopes section I 42 of the conductor .and which is provided with a highly polished and finely ground cup I8I adapted to receive a. resiliently mounted pin I82 constituting a part of section I83 and which, in turn, tele-.

scopes a second elbow section I84 which is in electrical contact with and received by thimble I09 of the cathode supporting structure shown in Fig. 4:.

Referring now to the details of the specimen manipulator shown in Fig. 8, reference is here had to the exploded view of the manipulator shown in Fig. 14. The only elements there illustrated are the face plate and supporting ways. The manipulator per se is disclosed and claimed in United States Letters Patent No. 2,380,209, granted July 10, 1945, to Charles H. Bachman and Jacob 'Ischopp.

There is provided on face plate I56 a pair of vertical ways I85 and I86 having therein races I81, preferably of V-shaped configuration to support a plurality of friction reducing bearings such as miniature ball bearings Q88. A vertical transversing member I99 is also provided with races I00 and I 9I to engage the ball bearings I88. Attached to the transversing member I89 there is provided a pair of horizontal ways I92 and I93 also comprising V-shaped races I94 and I95 adapted to receive miniature ball bearings I96. A horizontal transversing member I91 provided with suitable races E98 and I99 engages the ball bearings 955, A third set of ways namely the horizonta1 or axial ways for focusin purposes comprising ways 200 and 29L are supported by the horizontal transversing member I91 and also comprise V-shaped races 202 and 203 likewise provided with miniature ball bearings 2 04 for reducing friction. Ring I55 constitutes a part of an L-shaped member 205 provided with races to engage the ball bearings held by races 202 and 203 an is ultimately supported by ways 200 and 20 I.

Appreciation of the manner of operation of the manipulator shown in detail ma be obtained by referring to Figs. -20, inclusive. Figs. 15-20, inclusive, do not show all of the elements of the manipulator 46. The associated groups I5, I6; I1, I8 and I9, show only those elements deemed necessary in order to emphasize the various independent movements produceable by the manipulator, the complete perspective view being shown in Fig. 8. Figs. 15 and 16, which are front and side views respectively of the manipulator, when operated to produce horizontal transverse motion by actuation of lever I52, indicate that upon movement of lever I52 forward, that is away from the drawing, lever I52 actuates crank 206 (shown in Fig. 8) which in turn rotates connecting rod 201 to move cam 208, thereby exerting a pressure against the structure for transversing 14 member I91, and thereby moving the horizontal transversing member I99 to the right. There is also provided means in connection with each of the movements for substantially eliminatin or reducing back-lash which, in connection with the arrangements shown in Figs. 15 and 16, com prises a spring 209 which biases member I99 towards the left observed from the front view shown in Fig. 15,

Operation of the vertical transvers motion produced by the manipulator may be obtained by referring to Figs. 17 and 18 which are, respectively, front and side views showing additional structure for obtaining this motion. Lever I54 is connected to rod 2I0 which in turn operates a cam 2| I connected to the way I92.

Back-lash prevention means is also provided in connection with member 202 to bias this member downward and toward member 205, and may comprise a spring 2I2a. Forward motion, that is a motion to the left of lever I54 viewed from Fig. 18, raises ways I92 and I93 thereby raising ring I55 and the associated carrier 45.

The manner in which the focusing or longitudinal movement of ring I55 is obtained may be appreciated by referring to Figs. 19 and 20. Actuating lever I53 is connected to member 205 by means of a crank or connecting rod 2 I 3 and a cam 2M, the latter of which engages member 205 to move it horizontally or longitudinally. The spring 2 I21) serves to prevent back-lash.

The structure through which the actuating wheels, including illustrated wheels 1 and 8, for controlling the manipulator are joined to and 0peratively associated with the screwthread devices may be more fully appreciated by referring to Fig.

21. Certain of the elements there illustrated have been described hereinbefore and corresponding elements have been assigned like reference numerals. The end plate or disk 86 shown at the left-hand part of 3 is presented in enlarged form in Fig. 21 and shows the manner in which the contact or connecting pins IOI extend through that disk affording suitable connections for 'a plurality of flexible cables 2I5, 2I6 and 2I1 which are provided with adaptors 218 and which are connected to the knurled wheels 1, 8 and 9.

A viewing aperture mounted at the end of the horizontally disposed cover for the electron gun, and represented by the reference numeral 6 in Fig. 22, may be designed to be removable and comprises an eye-piece 2I9, a light. or glass lens v220 supported by a cylinder 22I which is attached to eye-piece 2I9 and which is insertable within a holding cylinder 222. The light lens 220 is in alinement with the fluorescent screen provided by disk shown in Fig. 21 and serves to magnify the image appearing on this screen.

We also provide an arrangement whereby in place of using a light magnifying lens a camera 223 may be supported at the end of the cover cap 5 as illustrated in Fig. 23. In such instances, the camera 223 which may also include a magnifying lens is provided with an adaptor 224 which is inpower outlets. For example, the system may be energized from an alternating current circuit 221 of commercial frequency. The main switch 2?. shown in Fig. 1 may be connected in the position illustrated to control the application of voltage and current to the microscope as well as to control the application of power to the pumps I6 and I1 and centrifugal blower I5. A unidirectional voltage is produced by means of a rectifier circuit which may include a single unidirectiona1 conducting device such as an electric discharge device 228 which is energized from a transformer 229. The voltage controlling means I3 may comprise means, such as a rheostat 230, connected in series relation with the primary winding thereof to control the magnitude of the unidirectional voltage applied to the microscope and may be actuated by knob shown in Fig. 1. One terminal of the secondary winding of transformer 229 is connected to ground. By virtue of the polarization of the discharge device 228, conductor 231 is maintained at a negative potential with respect to ground. Current limiting resistances and may be connected in the positions illustrated not only to limit the current trans mitted by discharge device 228 but also to limit the current supplied to the anode-cathode circuit of the electron microscope. In order to effeet the application of a substantially constant unidirectional voltage to the microscope, we may provide suitable filtering means such as a capacitance 234 which is connected between ground and the common juncture of resistances 232 and 233.

Transformer 235 is employed as a source of cathode heating current or filament current for the discharge device 228. Variation of the cathode heating current supplied to cathode 65 of the electron gun may be obtained by any suitable arrangement such as a transformer 236 controlled by means such as a rheostat 231, the latter of which may be actuated by switch or dial 2I shown in Fig. 1.

Conductors 238 constitute means for supplying cathode heating current to the concentric line comprising inner conductor 83 and the tubular outer conductor 84 shown in Fig. 3. An adaptor (not shown) may be employed for connecting conductors 23l and 238 to the elements of the electron gun through a shielded cable 239 comprising a grounded metallic shield 24!], conductors 238 which extend therethrough centrally, and a tubular conductor-2 the latter of which is connected to the outer tubular conductor 84 of the concentric line.

In operation, the specimen or object to be investigated is placed upon a fine wire mesh which is inserted within the recess of ring I12 constituting one part of locket I1I shown in Fig. 12, and the looking or retaining ring I13 is employed to maintain the mesh firmly against the inner shoulder of ring I12. The holding plate 516 is grasped and the knob I18 is depressed, thereby extending the rigid stern I14 so that the curved lower portion thereof firmly engages the outer periphery of ring I12, thereby positively holding it. Cover 53 of valve 55 is removed and the specimen holder is extended through chamber 54 and slot SI and the locket is inserted in specimen carrier 45, particularly between the inner vertical surface of shell I51 and the face of plunger I59, the latter serving to hold the locket firmly. The knob I18 is then released which permits retraction of the stem I14 upward into cylinder I15, which action renders the locket IH free to be positioned by the carrier and manipulator 46. The holder is then permitted to rest against the side or vertical wall of insertion chamber 54, the length of the holder being such that it does not extend sufiiciently upward to prohibit the closing or placement of the cover 56.

The valve is then sealed by placing the transverse rod 51 in the groove provided in cover 55, and handle I I of the valve is moved in a clockwise direction, viewed from Fig. 6, to apply sealing pressure to the cover.

Evacuation of the chamber enclosing the electron gun, the lens system and other associated elements is then initiated by the operation of handle or knob 23 shown in Fig. 1. After the pressure within the chamber has been reduced to a suitable value, power may be applied to the electron microscope; that is, anode-cathode voltage may be applied to the electron gun by operation of knob or dial 29 and the filament current may be controlled by knob 2|, both of which are illustrated in Figs. 1 and 24. Of course, rheostat I3 permits adjustment of the voltage applied to the anode and cathode of the electron gun, and correspondingly controls the magnitude of the voltage impressed across the outer electrodes and the intermediate electrode of each of the electrostatic lenses 3032.

The electron image formed at the specimen plane is magnified by the electrostatic objective lenses 3032 to produce an enlarged image on the fluorescent screen 85. Where desired, the light optical magnifying lens shown in Fig. 22 may be employed to obtain further magnification.

Movement of the object for viewing and focusing purposes may be effected through operation of the knurled wheels l-9 within the vicinity of the eye-piece 6 which, in turn, position heads 99 of the screw-thread devices 9698, inclusive, which are operatively connected to levers I52- I54 of manipulator 46 through rods 9395.

The specimen may be removed in the following manner: Handle II shown in Fig. 6 is raised effecting release of the pressure on cover 56 so that this cover may be removed. However, prior to the removal of the cover, as stated above, heated air is admitted to the insertion chamber 54 and to the main evacuated chamber through passage 60 by means of valve 62 which is actuated by arm I33, connecting rod I35 and member 63. As soon as the differential between atmospheric pressure and the pressure within the chamber is sufiiciently reduced, cover 56 is then readily removable.

We have found that the limiting angle of the electron bundle, or bundles, leaving the objective lenses 3U32 can be established mainly by the electron gun aperture rather than by the apertures of the objective lenses. This type of limitation as to the divergence of the electron beam affords distinct advantages from a mechanical standpoint because the last aperture in the electron gun preceding the specimen determines that angle, and that last aperture may be more favorably located than the objective lens stop, thereby avoiding the necessity for providing extremely small diameter holes in the electrodes constituting the elements of the various electrostatic lenses. Although the lens aberrations as well as the particular thickness and material of the specimen under examination play an important, part in determining whether the gun angle may serve as the effective stop of the system, or whether that limit is best determined by the use of such a condenser lens is optional in the electron microscope which we provide. The electron gun, since it is a means for performing such focusing operation, includes in it the focusing properties of a condenser lens, and we place the electron gun at an optimum position to utilize to a great extent the condensing effect of the electron gun. a v

In accordance with our invention, we provide an electron microscope of the type employing electrostatic lenses wherein particular configuration of the electrodes, the spacing thereof and the means for energizing the lenses offer distinct advantages and produce results not obtained by the prior art arrangements. More specifically, in accordance with one aspect of our invention we provide optimum spacing not only between the various lenses of the electrostatic system but we also have found that there is a desired relationship between the spacing of the electrodes constituting each electrostatic'lens and the electron apertures of the electrodes. In addition, we have found that highly satisfactory results are obtained by the proper spacing of the electrodes of each lens with respect to the longitudinal conductor which impresses the cathode potential on the intermediate electrodes.

Referring to Figs. 3 and 7, which indicate in detail certain aspects of the electron lens assembly, we have found that a high degree of precision is obtained by spacing the outer electrodes33 and 34 at distances from the intermediate electrode 31 which are at least equal to or which approach the diameter d of the electron aperture of intermediate electrode 31. In addition, it has been found advantageous as concerns operation of the electron lens system as a whole, to make the distance s between center lines of each electron lens at least two times as great as the diameter d of the aperture of the intermediate electrode 31 or at least two times as great as the spacing h between the outer electrodes 33 and 34 and the intermediate electrode 31. As concerns the relative diameters of the apertures in the outer electrode 33 and 34 and the aperture of the intermediate electrode, it has been found that the aperture of the inner electrode should have a diameter at least twice the value of the aperture diameters of the outer electrodes.

As a further matter, referring to Fig. 7 in conjunction with Fig. 3, we have found that in order to obtain the desired electrostatic shielding between the longitudinal sectionalized conductor and the electron beam, thelongitudinal conductor should be spaced at a distance .c which is at least as great as the distance h between the intermediate electrode 31 and the outer electrodes 33 and 34.

Although in the illustrated embodiment of our invention we have shown the electrostatic lens of the type comprising an intermediate electrode having an aperture of larger diameter than the apertures in the outer electrodes, it will be appreciated that our invention is not limited to that particular type of electrostatic lens. It should 18 be understood that the focal propertites of a lens of. this type are greatly determined by the distribution of potential along the axis and that the distribution of potential is readily determinable by the electrode position and configuration. A large variety of electrode constructions and placements may be utilized for attainment of a desired potential distribution. The structure which we provide is, therefore, readily adaptable to a wide variety of electrostatic lens assemblies and configurations.

' the effect of extraneous fields, is the substantially continuous shielding of the electron beam provided by the structure. For example, beginning at the electron gun as shown in cross sectional view in Fig. 3, the disk-like members 66 and 61 which constitute the magnetic shielding means and the transverse electrostatic shielding member 61 define a substantially field-free region through which the electron beam passes after being accelerated by the anode cup 13. It will be noted that the planar member 61 is interposed between the electron path and the longitudinal conductor. Furthermore, the specimen carrier 46 may be constructed of a suitable shielding material such as brass, thereby afiording the desired shielding of the specimen and the electron beam from the field due to the longitudinal conductor. As pointed out above in connection with Fig. 7, the planar members 141 and I48 also provide electrostatic shielding within the unitary electron lens assembly.

A further important advantage of an electron microscope built in accordance with our invention is the structure which entails the use of only a single conductor connected to the cathode structure and the intermediate electrodes of the electrostatic lenses for impressing the desired potential on the elements of the system.

While we have shown and described our invention as applied to a particular device and as embodying various other devices, it will be obvious to those skilled in the art that changes and modification may be made without departing from our invention, and. we, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A unitary electrostatic lens system comprising a metallic cylinder and a plurality of spaced electrostatic lenses therein each including a pair of spaced apertured wafer-like disks closely fitted to the inner surface of said cylinder and an intermediate aperture wafer-like electrode electrically insulated from said cylinder and means for supporting said intermediate electrode and aligning the aperture therein with the aperture of said pair of disks comprising a disk-like insulator closely fitted to the inner surface of said cylinder.

2. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, a unitary electron lens assembly comprising a second metallic cylinder in conductive contact with said first cylinder, said second cylinder having therein a plurality of electrostatic lenses each including a pair of spaced outer electrodes having alined apertures and closely fitted to the inner surface of said cylinder game 19 and an intermediate electrode electrically insu lated therefrom and in spaced relation. .between said outer electrodes, and means for maintaining said assembly in a predetermined position within the first mentioned cylinder andcomprisingan annular spacer. v V e H 3. In an electron microscope, the combination including an evacuated chamber definedby a naetallic cylinder, a unitary electron lens assembly comprising a second metallic cylinder in conductive contact with said first cylinder, saidse'corid cylinder having therein a plurality of electrostatic lenses each including a pair of spaced outer electrodes having alined apertures and closely fitted' to the inner surface of said cylinder and an intermediate electrode electrically insulated therefrom and in spaced relation between'fs'aid outer electrodes, and spacingmeans for main taining said assembly in a' predetermined posi tion within the first mentioned cylinder and co'm'f 2 prising an annular spacer adapted to receiv e a tongue extending from said second cylinder.{

4. In an electron microscope, the combination including a unitary imaging element comprising a metallic cylinder and a plurality ofelectrostatic electron lenses positioned-within said cylinder, each of said lenses comprising a pair of spaced wafer-like electrodes having alined apertures therein and each including an intermediateelectrode electrically insulated from said cylinder one of said pair of electrodes having an additional aperture, and means for impressing a relatively negative potential on the intermediate electrodes comprising a longitudinally extending conductor positioned within said cylinder connected to said intermediate electrodes and extending through said additional apertures. I

5. In an electron microscope, the combination including a unitary electrostatic electron lens system comprising a metallic cylinder and having therein a plurality of associated spaced sets of electrodes, each set of electrodes comprising a pair of spaced wafer-like partitions having alined apertures and closely fitted to the inner surface of said cylinder and an intermedaite electrode having an aperture alined with the'first mentionde apertures and electrically insulated from said cylinder, metallic spacing means between each pair of electrodes and the intermediateelectrode and between sets of electrodes, and means for impressing a relatively negative potential on the intermediate electrode of each set comprising a longitudinally positioned rod connected to and supported by the intermediate electrodes and extending through apertures in the outer electrodes.

6. In an electron microscope, the combination including a unitary electron lens system comprising a metallic cylinder and including therein a plurality of spaced electron lenses each including a pair of spaced outer electrodes of wafer like construction having alined apertures and an intermediate electrode electrically insulated from said cylinder and supported thereby by insulating means, and means for impressing a relatively negative potential on the intermediate electrode of each lens comprising a longitudinally disposed conductor positioned within said cylinder and extending through the outer electrodes and electrically connected to and supported by the intermediate electrodes, and electrostatic shielding means positioned longitudinally and spaced between said apertures and said conductor.

'7. In an electron microscope, the combination including an evacuated chamber defined by a metallic member, a unitary electron lens-system comprising a metallic cylinder containing therein aplurality of spaced electron lenses said cylinder being slidably supported by said member, an electromgun located within said metallic member, and a specimen carrier supported by said cylinder at one end thereof intermediate said electron gun andsaid lenses.

an an electron microscope, the combination including an evacuated chamber defined by a metallic=cylinden a unitary electron lens assembly comprising a second cylinder slidably supported b y said first cylinder and having therein a plurality of spaced electron lenses, a specimen carriersupported at one end of said assembly, an electron gunat one end of said first mentioned cylinder and including a filamentary cathode and an accelerating anode structure, and locking means for preventing rotation of said lens assembIy- W ith respect to said anode structure.

"a m-an electron microscope, the combination including an evacuated chamber defined by a metallic cylinden'aunitary electron lens assembly comprising a second cylinder having therein a p'lur'alit 'of spaced electron lenses, a specimen ca'rfie uppor'ted at one end of said assembly and-"formed in part by one of said lenses, an electron gunat one end of said first mentioned cylincler and including a filamentary cathode and an accelerating anode structure, and locking means to prevent relative rotation of said anode structure and said lens assembly comprising a longitudinal key adapted to engage said lens assembly and said anode structure.

10. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, a unitary electron lens assembly including a second metallic cylinder adapted to be inserted within the first mentioned cylinder and comprising a plurality of spaced electrostatic lenses aspecimen carrier mounted on one end of said lens assembly and formed in part by one of said lenses, an electron gun at one end of the first mentioned cylinder and comprising a filamentary cathode and an accelerating anode structure, and means for confining the electron beam-produced by said gun including an apertured disk positioned intermediate said specimen carrier and said accelerating anode structure.

111 In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, an electron lens assembly including a second cylinder adapted to be inserted within the first mentioned cylinder and comprising an electrostatic electron lens, a specimen carrier supported at one end of said second cylinder and formed in part by one of said lenses, an electron gun at one end of the first mentioned cylinder and including a filamentary cathode and an accelerating anode structure, an apertured disk positioned between said specimen carrier and said anode structure, and centering means for adjusting said apertured disk.

4 12. In an electron microscope, the combination including an evacuated chamber defined by a cylindrical member, a unitary lens assembly adapted to be inserted in said member and comprising a' plurality of electrostatic electron lenses and means for supporting said lenses in spaced relation, an electron gun, a specimen carrier supported at one end of said lens assembly and intermediate said gun and said lens, and an annular ring at'one end of said member having sealed thereto a re-entrant vitreous part supporting said electron gun.

-1'3- In an electron microscope,.the combination including an-evacuated chamber defined by a metallic cylinder, shielding means for said metallic cylinder comprising alternate layers of copper and high permeability iron surrounding said cylinder, and a unitary electron lens assembly comprising a second metallic cylinder adapted to be inserted within and to engage closely the inner surface of the first mentioned cylinder and including therein a plurality of spaced electrostatic lenses.

14. In a unitary electrostatic lens system for insertion in a chamber to be evacuated, the combination comprising a cylinder, a plurality of axially displaced electrostatic lenses each comprising a pair of spaced outer electrodes and a spaced intermediate electrode electrically insulated from said cylinder, each of the intermediate electrodes having attached thereto a section of a longitudinal connecting conductor positioned within said cylinder, a plurality of sections for connecting the first mentioned sections, an electron un within said chamber, and means for applying cathode potential to the longitudinal conductor.

15. In a unitary electrostatic lens assembly for insertion in an electron microscope chamber to be evacuated, the combination including a cylinder, a plurality of axially displaced electrostatic lenses each comprising a pair of outer apertured disk electrodes and an intermediate electrode spaced therebetween and electrically insulated from said cylinder, an electron gun within said chamber, the intermediate electrodes each being provided with and supporting a longitudinal conductor section extending through apertures in the associated outer electrodes which are radially displaced from the first mentioned apertures, and a plurality of conductor sections for interconnecting the conductor sections attached to the intermediate electrodes and the cathode of said electron gun.

16. In a unitary electrostatic lens assembly for insertion in a chamber to be evacuated, the combination including a cylinder, a plurality of axially displaced electrostatic lenses each comprising a pair of spaced and apertured outer disk electrodes and an intermediate apertured disk electrode spaced therebetween and electrically insulated from said cylinder, the apertures of each lens providing a passage for an electron beam therethro-ugh, each of the intermediate electrodes being provided with and supporting an attached section of a longitudinal conductor, a plurality of intermediate sections of a longitudinal conductor for interconnecting the first mentioned sections, and longitudinal shielding means between the path of the electron beam and the sections of said conductor.

17. In an electron microscope, the combination including an electron gun comprising a cathode, an electrostatic lens system, a viewing screen, a specimen carrier intermediate said gun and said lens system, an accelerating anode in the vicinity of said cathode, and means intermediate said accelerating anode and said carrier for shielding an electron optical image formed of a specimen from magnetic fields due to said cathode.

18. In an electron microscope, the combination including an electron gun comprising a cathode, an electrostatic lens system, a viewing screen, a specimen carrier intermediate said gun and said lens system, an accelerating anode in the vicinity of said cathode, and means for shielding an electron optical image formed of a specimen. from magnetic fields due to said cathode comprising a pair of apertured wafer-like electrodes in spaced relation intermediate said accelerating anode and said specimen carrier.

19. In an electron microscope, the combination including an electron gun comprising a cathode, an electrostatic lens system, a viewing screen, a specimen carrier intermediate said gun and said lens system, an accelerating anode in the vicinity of said cathode, and means intermediate said accelerating anode and said specimen carrier for shielding an electron optical image formed of a specimen from magnetic fields due to said cathode comprising a pair of wafer-like apertured magnetic shielding members and a longitudinal electrostatic shielding member spaced between said magnetic shielding members.

20. In an electron microscope, the combination including an electron gun, a viewing screen, an electrostatic lens system comprising a cylinder, an electron lens within said cylinder and including a pair of Wafer-like apertured electrodes and a spaced intermediate wafer-like electrode having an aperture diameter at least twice the diameter of the apertures in the outer electrodes, and a single conductor extending longitudinally within said cylinder for interconnecting said gun and said intermediate electrode.

21. In an electron microscope, the combination comprising an electron gun, a viewing screen, an electrostatic lens system comprising a cylinder, an electron lens supported by said cylinder and including a pair of outer wafer-like electrodes in spaced relation and having alined apertures and an intermediate apertured electrode spaced between the outer electrodes and having an aperture diameter of the same order of magnitude as the spacing between the intermediate electrode and the outer electrodes, and a metallic conductor extending longitudinally within said cylinder for impressing electron gun potential on said intermediate electrode.

22. In an electron microscope having means defining an evacuated region, the combination supported within said region including an elec tron gun, a viewing screen, and an electrostatic lens system comprising an electron lens including a pair of wafer-like outer electrodes having alined apertures and an intermediate spaced electrode having an aperture the diameter of which is substantially equal to the spacing between the intermediate electrode and said outer electrodes.

23. In an electron microscope having means defining an evacuated region, the combination supported within said region including an electron gun and a viewing screen, and an electrostatic lens system comprising a cylinder, a plurality of axially spaced electron lenses removably positioned in said cylinder and each comprising a pair of spaced outer electrodes having alined apertures and an intermediate spaced electrode having an aperture the diameter of which is substantially greater than the diameters of the first mentioned apertures and substantially equal to the spacing between the intermediate electrode and said outer electrodes, the distance between the consecutive lenses of said system being at least twice the aperture diameter of the intermediate electrode.

24. In an electron microscope, the combination including an electron gun, a viewing screen, an electrostatic lens comprising a pair of spaced outer electrodes having alined apertures and an intermediate electrode having an aperture alined with the first mentioned apertures, axially extending conductor means for impressing cathode potential on the intermediate electrode, said conductor means being displaced radially from said apertures by a distance at least equal to the distance between said intermediate electrode and said outer electrodes.

25. In an electron microscope, the combination including an electron gun comprising a cathode and an accelerating anode, a viewing screen, an electrostatic lens including a pair of spaced electrode having alined apertures and an intermediate electrode spaced between the outer electrodes and having an aperture in alinement with the first mentioned apertures, and a longitudinal conductor connected to and supported by said cathode and said intermediate electrode, said longitudinal conductor being spaced radially from the center line of said apertures by a distance at least equal to the spacing between said intermediate electrode and said outer electrodes.

26. In an electron microscope, the combination including an evacuated metallic chamber, an electron gun, an electrostatic lens system comprising a plurality of electron lenses each including a pair of spaced outer electrodes connected to said chamber and an intermediate electrode insulated therefrom, and a single conductor extending longitudinally within said chamber interconnecting said gun and said intermediate electrode whereby the electron gun and said lens system are energized upon impression of a single potential difference between said gun and ground.

27. In an electron microscope comprising mean defining an evacuated region, the combination including an electron gun, a viewing screen, an electrostatic lens system, a conductor located within said region and connected to said gun and said lens system, and a specimen carrier intermediate said lens system and said gun and constructed of a material to afford substantial electrostatic shielding at the plane of the specimen for fields due to said conductor.

28. In an electron microscope comprising means defining an evacuated region, the combination including an electron gun for producing a beam of electrons, a viewing screen for receiving said beam, an electrostatic lens system positioned between said gun and said screen comprising an electron lens having a pair or outer electrodes and a spaced intermediate electrode insulated therefrom, a conductor extending longitudinally within said region for connecting said gun to said intermediate electrode, longitudinally extending electrostatic shielding means positioned between said conductor and the electron beam produced by said gun, and a metallic specimen carrier intermediate said electron lens and said gun and affording substantial shielding at the plane of said specimen for electrostatic fields due to said conductor.

29. In an electron microscope, the combination including an evacuated chamber defined by a metallic member, a unitary electron lens system comprising a metallic cylinder containing therein a plurality of electron lenses, means including an electron gun located within said metallic member for projecting an electron beam longitudinally of said chamber, and means for positioning an object to be examined in the path of said beam comprising a specimen carrier located at one end of said cylinder intermediate said electron gun and said lens and formed in part by one of said lenses.

30. In an electron microscope, the combination including an evacuated chamber defined by a metallic cylinder, shielding means for said metallic cylinder comprising alternate layers of copper and high permeability iron surrounding said cylinder, and a unitary electron len assembly comprising a second metallic cylinder adapted to be inserted within and to engage closely the inner surface of the first-mentioned cylinder and including therein a plurality of spaced electron lenses.

CHARLES H. BACHMAN. SIMON RAMO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,267,082 Dailey Dec. 23, 1941 2,200,694 Gerecke et al May 14, 1940 2,303,166 Laico NOV. 24, 1942 2,301,490 Winans Nov. 10, 1942 2,227,034 Schlesinger Dec. 31, 1940 2,225,455 Klauer Dec. 17, 1940 2,215,794 Muller Sept. 24, 1940 2,200,095 Marton May 7, 1940 2,284,710 Zowrykin et a1 June 2, 1942 2,277,414 Ramo Mar. 24, 1942 2,058,914 Rudenberg Oct. 27, 1936 2,075,141 Schlesinger Mar. 30, 1937 2,227,033 Schlesinger Dec. 31, 1940 2,313,018 Krause Mar. 2, 1943 2,234,281 Ruska Mar. 11, 1941 2,227,189 Knoll Dec. 31, 1940 2,036,532 Knoll et a1. Apr. 7, 1936 2,336,774 Bracknet et al. Dec. 14, 1943 2,301,303 Marton Nov. 10, 1942 2,292,087 Ramo Aug. 4, 1942 2,344,238 Finch Mar. 14, 1944 2,362,084 Stahl Nov. 7, 1944 2,147,372 Knoll Feb. 14, 1939 FOREIGN PATENTS Number Country Date 849,698 France Aug. 21, 1939 849,648 France Aug. 21, 1939 OTHER REFERENCES Von Ardenne, Uber ein Universal-Elektronenmikroskop, etc., Zeit fur Physik, vol. 115, pp. 340 to 351, March 1940. 250-495.

Electron Microscopy by V. K. Zworykin and Hillier, published in the Scientific Monthly, Sept. 1944, vol LIX, pages -179. (Copy in Div. 54, 250-495.)

Electron Optics by V. K. Zworykin, published in the Journal of Franklin Institute, May 1933, pages 535-55. (Copy in Div. 54, 250-160.)

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