US2424790A

US2424790A - Electron microscope

Info

Publication number: US2424790A
Application number: US519773A
Authority: US
Inventors: Charles H Bachman; Ramo Simon
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-12-01
Filing date: 1944-01-26
Publication date: 1947-07-29
Anticipated expiration: 1964-07-29
Also published as: US2424791A; GB647703A; US2424789A; GB576003A; BE576003A; BE480179A; BE483238A; US2424788A; GB669381A; FR938893A; FR58024E; FR964837A; BE474126A; US2400331A

Description

July 29, 1947. c. H. BAcHMAN ETAL.

ELECTRON MICROSCOPE Original Filed Deo. 1, 1942 T-`| .2, 126 M u; g -127 TIL/2f /26 ,2* )29 125 Fig. 4.

Inventors; Chasfles Hachmam,

lmon zaurno.` by www.

Thei` Attorljeg.

Patented July 29, 1947 ELECTRON MICROSCOPE Charles H. Bachman, Scotia, and Simon Ramo,

Niskayuna, N. Y., assignors to General Electric Company, a corporation of New York Original application December 1, 1942, Serial No. 467,530. Divided and this application January 26, 1944, Serial No. 519,773

(Cl. Z50-49.5)

19 Claims. 1

The present invention, which is a division of our copending `application S. N. 467,530, filed December 1, 1942, relates to electron microscopes, and more particularly t new and improved structure for electron microscopes of the electrostatic type.

In electron microscopes of ythe 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 adapt able or susceptible to widespread application due to the fact that the evacuated chambers have been of relatively large size because of the ltype of the electron lens systems employed, thereby involving relatively large time intervalsbetween examinations of different specimens.

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. Wherey 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 vto 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 invention described hereinafter, we provide a new and improved electron microscope which obviates the disadvantages of the prior art arrangements and which ailords simplicity of construction and operation, and `adaptability to a Wide field ofy application. The electron microscope which we provide is adapted and arranged to permit frequent opening of the evacuated chamber for the insertion and extraction of specimens to be investigated and which entails only the lapse of a relatively short interval of time between eX- aminations due to the fact that the chamber is relatively small. In addition, the specimen insertion chamber and -sealing structure thereof are arranged to permit ease of insertion and extraction 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 assembly is 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.

It is an object of our invention to provide a new land improved elec-tron microscope.

It is still 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.

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

It is a still fur-ther 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 to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Fig. 1 is a cross sectional view of the electron microscope showing the evacuated chamber, electron lens system and cathode supporting structure; Fig. 2 is a cross sectional view of the cathode supporting structure; Fig. 3 is an alternative form of the iiexible connection which may be employed therein; and Fig, 4 is an enlarged view of a portion of the sealing arrangement of the electron microscope shown in Fig. 1.

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 irradiate a specimen to be investigated and an imaging system which magnies the image produced by the impingement on the specimen into a larger image on a viewing screen such as a fluorescent screen, or upon the surface of a photographic 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 eld focusing action of .lust 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 regulated source or" 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. Although not limited to a particular resolving power or magnification, an electron microscope built in accordance with our invention operates satisfactorily having a 200 Angstrom units resolution and a useful magnification of about 10,000 As pointed out hereinafter, this magniiication may be obtained either exclusively by the design of the electron lens system itself, or by the combination of a suitable electron electrostatic lens and an associated light microscope or lens which is placed. in the vicinity of the fluorescent screen at one end of the electron microscope.

In the electron microscope described hereinafter, certain features are incorporated 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 desired focusing action of the electrostatic lens. As concerns the electron gun, this element is constructed to produce suicient 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 suilcient 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 axis is to be imaged. Furthermore, there is no reason to direct electrons at the specimen if their incident angle is such that in 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 cf 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 uncharged; consequently, the limiting angle oi the electron bundle leaving the objective lens can be established principally by the electron gun apparatus rather' than by the apertures of the objective lens system. This type of limiting is 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 an effective stop for the system, or Whether that limit is determined by the rst imaging or objective lens. It has been found that highly successful results are obtained with a small gun angle, and the gun angle produces a more readily discernible eiect upon the resolving power than does the lens aperture.

The electron microscope described hereinafter is constructed to afford proper relationship between the cathode lament point location, the ilament shield aperture diameter, anode-lament spacing and the spacing and size of the other apertures. The spacing and size oi the other apertures of the system depend upon a number of factors 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 the field due to the lament current.

As concerns the electrostatic electron lenses which magnify the electron image produced by the impingement of the electron beam on the specimen, the electrostatic lens system of the electron microscope which we provide possesses the following general characteristics. The focal length of the system is positive and relatively short without involving the use of excessively small parts. he in-focus position of the specimen is external to the high iield region of the lens and the specimen is in a region substantially entirely free from electric iields. The electrodes of the lens system are iinely machined or ground and highly polished to minimize the insulation problem and iield emission at the operating voltage. The lenses are of symmetrical configuration with a single negative potential to ground, namely the cathode potential, serving to energize the lens. rThe axial voltage drop in thc central region of the lens is maintained at an optimum value, thereby minimizing strong mag'- netic fields and chromatic aberration eiects.

In Fig. l there is illustrated a cross sectional view of an electron microscope which comprises an evacuated chamber, an electron gun, an electrostatic lens system, and a specimen carrier manipulator and actuating means therefor. The evacuated chamber' within which the microscope elements described above are positioned is defined in part by a metallic cylinder, such as a brass cylinder 24, the interior surface of which may be adapted to closely engage an insertable unitary electron lens assembly. As a means for shielding cylinder 2e, we provide a laminated metallic shielding structure which includes alu ternate 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 28. The sheath which is adjacent cylinder' 2e 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.

A unitary electrostatic electron lens assembly is defined externally by means of a metallic cylinder 29, preferably constructed of brass, the inner surface of which is iinely ground and highly polished to closely engage cylinder 24 and to support therein the elements of the electrostatic lens system. rEhe inner surface of cylinder 24 and the outer surface of cylinder 29 need not be as precisely machined as the inner surface of cylinder 2Q. Within the cylinder 29 we provide an electrostatic lens system which includes a plurality of generally similar lenses 33, 3| and rThis lens assembly may be arranged to have a total magnification of about 1500. Lenses 353 and 3i each comprises a pair of longitudinally spaced wafer-like electrodes 33 and S4 maintained at a common or anode potential and Which are provided with apertures 35 and 36, respectively. We provide an intermediate electrode 3l also of wafer or disk-like configuration in spaced relation between outer electrodes 33 and 3i'. and which is provide with an aperture 33 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 yfrom cylinder 29 and is maintained at cathode potential by means of a horizontal conductor (not shown) connected thereto. 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 tendency 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 realinement in the iield. For example, the electrodes of the lenses 36-32 may be maintained and locked in the desired illustrated position by means of a plurality of prin-cipal longitudinal and annular spacers 40-42, inclusive. In addition, the intermediate electrode 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, there is provided also a unitary structure comprising the aforementioned electrons lens system, a specimen carrier 45, and a specimen manipulator` 46. The exact structure of the elements 45 and 46 form no part of the invention of the present application and are explained in detail in our above mentioned copending application. The specimen carrier 45 is supported by manipulator 46 which in turn is supported from one end of the lens assembly.

rlhe objective electron lens 32 is essentially of the same electrical characteristics as electron lenses 30 and 3l and comprises a pair of spaced outer electrodes 41 and 43 and an intermediate electrode 49. Outer electrode 4B is `provided With an aperture 4S and a radial recess 5I adapted to receive a specimen holder, and is also provided with an enlarged aperture adapted to receive one part of a cartridge type specimen carrier.

As a part of the unitary con-struction comprising the electron lens assembly and the carrier manipulator, we also may add an electron beam defining means, such as an apertured disk '50 and supporting structure therefor. 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 cf l0-5 radians.

A radial specimen insertion chamber 54 is provided in alinement with slot l'5l 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 l55 comprises a removable cover 56 which is engaged by a cross rod 51 which when pressure is exerted thereagainst maintains the cover lirmly against the upper surface of a cylinder 58, consttutingthe side wall defining means of the valve. If desired, a suitable gasket 59, such as a rubber gasket, may be employed to seal the juncture of the cover 56 and cylinder 58.

Means are provided for supplying to the evacuated chamber of the microscope air at atmospheric pressure prior to the opening of cover 56. This means may comprise a passage 60 fed by a conduit 6l 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 controlling the flow of the heated air, we provide a valve 62 seated in passage 60 and actuated by a member 63. A bellows-type sealing member 64 is sealed around the adjacent area of cylinder 58 and arm 63, a suitable actuating mechanism for cover 56 and valve 62 being provided and including the cross rod 51.

At one end of the cylinder 24 there is located a sealing and supporting structure for a filamentary cathode comprising one of the electrodes of an electron gun, which comprises a filamentary pointed cathode 65 of the hairpin type. There is also provided magnetic shielding structure comprising apertured wafer-like metallic members or disks 66 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 66 and 61 intermediate the electron beam apertures of the latter members and the position of ra 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 l68 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 61. Key 68 may also extend through annular spacers 69 and 10 which maintain disks 61 and 66 and the unitary assembly in the desired spaced relation.

Disks 56 and 61 are provided with apertures 1I and 12 through which a horizontal conductor |84 extends to impress on the intermediate electrodes of electron lenses 30, 3l 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, there may be employed an accelerating anode cup structure 13 supported by a transverse disk 13 and extending an appreciable distance longitudinally around the flamentary cathode 65 so that a suitable accelerating electric field is provided for the electrons emitted by the cathode. I

Exhaust apparatus for the evacuated chamber of the electron microscope may be connected to a tubular conduit 15 iny communication with the chamber and which is also connected to suitable evacuating pumps. y l

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 andfcomprises a compressible washer, such as a rubber washer 18, a metallic compression ring 19, a threaded retaining ring 89 adapted to engage threads provided at the end of cylinder 24, and a plurality of circumferential-Hy displaced means, such as screws 8|, for exerting longitudinal or axial pressure against the retaining ring 19, thereby compressing washer 18.

A vitreous part 82 is sealed to the supporting ring 15 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. 2.

At the other end of the evacuated chamber, particularly at the left-hand end of cylinder 24, there is provided an end wall for sealing that end of the chamber and for supporting a viewlng 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 A 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 8S. 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 S0, 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 specimen manipulator 4S are also positioned at this end of the evacuation chamber within the vicinity of the viewing screen. The manipulator 46 may be of the type designed to produce three degrees of motion mutually at right angles to each other, and may comprise three separate actuating means such as longitudinal extending rods 93, 94 and 95, only tWo of which are shown in Fig. 1 and which are controlled by means of a plurality of screw-thread devices 9S, 91 and 98, only two of which are shown in Fig. 1. The screw-thread device 91 is shown in cross-sectional detail and is also representative 0f the other two devices. For example, screw-thread device 91 comprises a head 99 which engages the associated longitudinal actuating rod 94 and is controlled in its longitudinal position by means of a screw |09 which is provided with an actuating pin |0| and which h'as one end thereof extending through disk S5 to effect an externally accessible connection adapted to receive a flexible cable (not shown). Each screw-thread device may be supported by a cylindrical member A|02 formed integral with or attached to disk 86 and which is sealedv to its associated head 99 by a deformable member such as a bellows |03 sometimes dened as a Sylphon bellows.

Reference is now made to Fig. 2 in which the cathode supporting structure is shown in greater detail. Means are provided for positioning, that is for centering the point or hairpin-type lamentary cathode 65 with respect to the alined apertures of the electron optical system. This means is of demountable character permitting rapid replacement of the filament and also permits the use of different types of filaments in connection with the other elements of 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.

A metallic base |04 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 |04 which not only transmits power to the filament 65 but also serves to impress through the longitudinal conductor |04 the negative potential of the lament on the intermediate electrodes of electron lenses 30-32. Base |04 is constructed to afford a fiexible mechanical connection between the inner` conductor 83 of the concentric line and the base. A conductive connecting member |05 is mechanically supported by base |04 but is electrically insulated therefrom by means of a glass bead or seal |05 which rigidly supports member |05. At one end of member |05 we provide a ilexible connection such as a coil spring |01 attached to the inner conductor 83.

The base |04 is also provided with a. threaded flange |08 which supports a metallic thirnble |99 provided with a shoulder ||0 which longitudinally positions the cathode positioning or centering means to be described immediately. This cathode centering means is flexibly connected to member |05 by means of a flexible connection such as a coil spring and comprises an annular' centering ring ||2 having near the periphery thereof a restricted region ||3 to obtain a substantial line engagement with shoulder llc. Ring ||2 may be positioned by a suitable mechanical expedient such as a plurality of circumferentially displaced screws ||4. Furthermore, the centering ring l2 is maintained in the desired longitudinal or axial position in iirm engagement against shoulder ||0 by means of a compression spring ||5 which is supported at its right-hand extremity by means of a threaded collar ||6 which engages threads on the inner surface of thimble |09. As a mea-ns for insulating the shell of thimble |09 and the centering means from the center conductor to the cathode, we employ a cylindrical insulator ||.1'

placed between retaining ring I2 and a cylindrical cathode base I8 adapted to receive a rigid cathode stem |9, One terminal |20 of the point cathode 65 is connected to stem ||9 through a face plate 2| which is attached to stem ||9, the latter constituting an electrical path for the inner conductor 83 through coil spring connecting member |05 and coil spring |01. The other terminal of the cathode |22 is connected to a metallic pin |23 supported by and electrically insulated from the face plate |2| by means of a glass bead |24.

As a means for facilitating ease of replacement of the cathode element E5 and to permit mechanical movement of the stem ||9 and cathode 55 without disturbing the electrical connections to the cathode, we employ a exible conductor such as a ribbon conductor |25 which extends into an opening |26 in the body of thimble |09. A screw |21 or any suitable mechanical expedient may be employed for retaining and removing the ribbon conductor'l25 in opening |26. The electrical path from terminal |22 of cathode 55 comprises pin |23, conductor |25, thimble H39 and tubular conductor 84 of the concentric line. At the end of thimble |09 we provide a cap |28 which is removable or demountable and which is adapted to be supported by thimble iti by a screwnthreaded engagement at |29. This cap |28 is pro-vided with an aperture |30 through which the electrons emerge upon acceleration due to the field produced by anode cup 13.

An alternative flexible connection for the cathode supporting structure isshown in 3 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 ll, we may employ folded strips |3| and |32 of resilient metal, respectively, connected to members HB, and |05,\83.

Fig. 4 is an enlarged cross-sectional view of the structure employed in sealing the vitreous part 32 and supporting ring l5 to cylinder 2li and spacer 11.

In operation of the electron microscope described above it has been found that the limiting angle of the electron bundle, or bundles, leaving the objective lenses 30-32 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 rlens 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 partricular 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 first objective lens, we have found that by the optimum placing of the electron gunlaperture at an appreciable longitudinal or axial distance from the object, a highly satisfactory structure is provided for producing this limiting effect. In other words, we position the electron gun so that it establishes a relatively small gun angle.

Although we do not show in the modifications of our invention illustrated a separate condenser lens for focusing electrons on the specimen, 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 ycondensing effect of the electron gun.

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 fare obtained by the proper spacing of the electrodes of each lens with respect to the longitudinal conductor which impresses the cathode potential on the intermeditae electrodes.

Referring to Fig. 1 which indicates in detail certain aspects of the electron lens assembly, we have found that a high degree of precision is obtained by spacing the outer electrodes 33 and 54 at distances from the intermediate electrode 3l which are at least equal to or which approach the diameter d of the electron aperture of intermediate electrode 3l. 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 3l 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 electrodes 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. u

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 ytype of electrostatic lens. It should be understood that the focal properties 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.

One of the important advantages afforded by the electron microscope which we provide and which contributes in a large measure to the dee sired precise operation thereof, by eliminating 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. 1, 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 affording the desired shielding of the specimen and the electron beam from the field due to the longitudinal conductor.

A further important advantage of an electron microscope built in accordance With our inven tion is the structure which entails the use of onlir a single conductor connected to the cathode 1 1 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 modifications 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 in the United States is:

1. In an electron microscope, the combination including an evacuated chamber, a unitary electron lens system within said chamber, an electron gun comprising a cathode and an accelerating anode, a specimen carrier intermediate the electron lens system and said accelerating anode, a beam conning means intermediate said anode and said specimen carrier, and means for centering said cathcde with respect to said electron lens system.

2. In an electron microsc-ope, an evacuated chamber defined by a metallic cylinder, a unitary lens system including a metallic cylinder adapted to be inserted within and to be in close engagement with the inner surface of the first mentioned cylinder and containing therein an electrostatic electron lens system, a viewing screen at one end of said first mentioned cylinder, a specimen carrier supported by the second mentioned cylinder, an electron gun comprising a lamentary cathode and accelerating anode structure displaced longitudinally from said specimen carrier, and sealing structure at the other end of said rst mentioned cylinder comprising an annular ring, a re-entrant vitreous part sealed to said ring for supporting said filamentary cathode and comprising externally accessible terminals for supplying current to said filament, and a metallic thimble supported by said seal for establishing one connection to said filament.

3. In an electron discharge device having an evacuated chamber defined by a metallic cylinder, means for supporting the cathode of an electron gun comprising a ring in close engagement with one end of said cylinder, a vitreous part sealed to said ring and supporting a metallic thimble which constitutes one connection to said lainent from an external source or" current, a central conducting means to complete the circuit for said iilament comprising a flexible connection from the base of said thimble to said filament thereby permitting accurate positioning of said iilament, and adjustable supporting means for said iilament supported by said thimble.

4. In an electron discharge device having an evacuated chamber defined by a metallic cylinder, an electron gun at one end of said cylinder and including a cathode and an accelerating anode structuie, means for supporting said cathode and for sealing that end of said evacuated chamber comprising an annular metallic ring abutting a shoulder of the said cylinder and a vitreous part sealed to said ring and the cathode structure, and means for adjusting the position of said cathode with respect to said anode.

5. In an electron discharge device having an evacuated chamber, an electron gun comprising a cathode including a lament and an accelerating anode, a pair of conductors for supplying current to said filament, means for supporting said filament comprising a ring at one end of said chamber, a re-entrant vitreous part sealed to said ring and supporting a. metallic thimble, means for connecting one terminal of said iilament to said thimble, said thimble being connected to one of said conductors, and means for centering and adjusting said filament comprising an insulating member positioned within said thimble, a rigid metallic member connected to the other terminal of Said filament and supported by said insulated ring, means for moving said rigid member with respect to said thimble, and flexible conductor means intermediate said last mentioned member and the other conductor.

6. In an electron discharge device having an evacuated chamber defined by a metallic cylinder, an electron gun at one end of said cylinder and including a iilamentary cathode and an accelerating anode structure, means for supporting said cathode comprising an annular ring in abutment with a shoulder provided by said cylinder, a compressible washer seated in engagement with said ring and said cylinder, an annular metallic compression ring engaging said first mentioned ring and said washer, a metallic retaining ring threaded to said cylinder, and means for exerting pressure on said compression ring.

'7. In an electron discharge device having an evacuated chamber defined by a metallic cylinder, an electron gun at one end of said cylinder and including a cathode and an accelerating anode structure, means for supporting said cathode and for sealing that end of said evacuated chamber comprising an annular metallic ring abutting a shoulder of said cylinder and a vitreous part sealed to said ring and the cathode structure, a compressible washer engaging said cylinder and said ring, a metallic compression ring engaging the first mentioned ring and said washer, a threaded retaining ring engaging said cylinder, and means for exerting pressure on said compression ring.

8. In a sealing and electrode supporting structure for an electric discharge device including an evacuated chamber defined by a metallic cylinder, the combination including an annular ring engaging said cylinder, a reentrant vitreous part sealed to said ring, a concentric line construction terminated in a metallic annular base sealed to said vitreous part, a metallic thimble supported by said base, and a iiexible connection between said electrode and the inner conductor of said concentric line.

9. In a sealing and electrode supporting structure for an electric discharge device including an evacuated chamber defined by a metallic cylinder, the combination including an annular ring engaging said cylinder, a. reentrant vitreous part sealed to said ring, a concentric line construction terminated in a metallic annular base sealed to. said vitreous part, a metallic thimble supported by said base, a iiexible connection between the inner conductor of said concen-tric line and a metallic connecting member supported by said base, a longitudinal member for supporting said electrode, centering means supported by said 'nimble foradjusting said member and a second flexible connection between said centering means and said metallic connecting member.

l0. In a sealing and electrode supporting struc ture for an electric discharge device including an evacuated chamber defined by a metallic cylinder, the combination including an annular ring engaging said cylinder, a reentrant vitreous part 'sealed to said ring, a concentric line construction terminated in a metallic annular base sealed to said vitreous part, a metallic thimble supported by said base, a metallic connecting member supported by said base, a ilexible connection between the inner conductor of said concentric line and said connecting member, a positionable member for supporting said electrode, centering means for supporting said positionable member, means for insulating said positionable member from said thimble, spring biasing means for maintaining said centering means in a predetermined axial position, and a second flexible connection between said centering means and said connecting member.

11. In a demountable cathode supporting structure for an elec-tric discharge device including an evacuated chamber dened by a metallic cylinder, the combination including an annular ring seated on a shoulder of said cylinder at one end thereof, a vitreous re-entrant part sealed to said ring, a concentric line including an inner conductor and a tubular outer conductor terminated in a metallic base sealed to said vitreous part, a metallic thimble supported by said base, a connecting member supported by said base, a ilexible connection between said connecting member and said inner conductor, a positionable support for said cathode, centering means for the cathode support comprising a metallic centering ring engaging an inner shoulder of said thimble and insulated from said support, spring biasing means for maintaining said centering ring in engagement with the associated shoulder thereby establishing the axial position of said cathode, and a second exiible connection between said connecting member and said centering means.

12. In a demountable cathode supporting structure for an electric discharge device including an evacuated chamber rdefined by a metallic cylinder, the combination including an annular ring seated on a shoulder of said cylinder at one end thereof, a vitreous re-entrant part sealed to said ring, a concentric line including an inner conductor and a tubular outer conductor terminated in a metallic base sealed to said vitreous part, a metallic thimble supported by said base, a connecting member supported by said base, a flexible connection between said connecting member and said inner conductor, a positionable support for said cathode, centering means for said cathode support comprising a metallic centering ring engaging an inner shoulder of said thimble and insulated from said support, spring biasing means for maintaining said centering ring in engagement with the associated shoulder thereby establishing the axial position of said cathode, a second ilexible connection between said connecting member and said centering means, and a plurality of circumferentially spaced means about said thimble for adjusting said centering ring.

13. In a demountable cathode supporting structure for an electric discharge device including an evacuated chamber, the combination including an annular ring, a vitreous part sealed to said ring, a base member sealed to said vitreous part, externally accessible terminals, a metallic thimble supported by said base member, a connecting member electrically insulated from and supported by said base member, a eXible connection between one of said terminals and said connecting member, a lamentary cathode having a pair of terminals and supported by a longitudinally extending positionable member and providing an electrical connection to one ter- 'minal of said cathode, an insulated pin carried by `said support and constituting the other terminal of said cathode, and a ilexible conductor connected to said pin and in engagement with said thimble thereby completing the circuit for said cathode through said thimble and said base member to the other externally accessible terminal.

14. In a demountable cathode support for an electric discharge device, the combination including a supporting ring, a re-entrant vitreous part sealed to said ring, a concentric transmission line terminated in a metallic base sealed to the re-entrant section of said part, a connect-- ing member electrically insulated from and supported by said base member, a ilexible connec-l tion between the inner conductor of said line and. said connecting member, a metallic thimble provided with a screw thread to engage threads of' said base member, a metallic support for saidl cathode constituting an electrical connection to.-

kone terminal thereof and supported by a centering means within said thimble, said support being electrically insulated from said thimble, a flexible: connection between said centering means and said connecting member, an electrically insulated pin connected to the other terminal of said cathode, a ilexible conductor connected to said pin. and extending through an opening in said thimble retaining means for engaging said flexiblev conductor in electrical contact with said thimble, and a demountable metallic cap supported by said thimble and substantially surrounding said cathode.

15. In combination, an electron microscope including an evacuated chamber, an electron lens system within said chamber, an electron gun comprising a cathode having a filament and an associated accelerating anode, means for sealing one end of said chamber and for supporting said filament comprising a vitreous part supporting a metallic thimble which substantially surrounds said lament and which is provided with an aperture in alinement with apertures of said accelerating anode and the lens system, means for adjusting the position of said cathode with respect to said apertures, and magnetic shielding means longitudinally spaced from said lament.

16. In combination, an electron microscope including an evacuated chamber, an electron lens system within said chamber, an electron gun comprising a cathode having a iilament and an associated accelerating anode, means for sealing one end of said chamber and for supporting said lament comprising a vitreous part supporting a metallic thimble which substantially surrounds said filament and which is provided with an aperture in alinement with apertures of said accelerating anode and the lens system, means for adjusting the position of said cathode with respect to said apertures, and magnetic shielding means between said accelerating anode and said electron lens system.

17. In an electron microscope, the combination including a cathode, an accelerating electrode and an electron lens system, a specimen carrier intermediate said electron lens system and said accelerating electrode, and means including a heat conductive member having an aperture therein interposed between said accelerating electrode and said specimen carrier for limiting the size of the beam of electrons which irradiates said specimen carrier.

18. An electron microscope comprising a source of electrons, means for accelerating said electrons, an electron lens system, a viewing screen, a specimen carrier supported between said electron lens system and said accelerating means, and an adjustable heat conductive member supported between said accelerating means and said carrier, said member having an aperture therein alignable with said source and said lens for limiting the size of the beam of electrons which irradates said specimen carrier.

19. In an electron microscope, the combination comprising an electron gun, an electrostatic lens system comprising a plurality of apertured electrodes, a viewing screen at one end, a specimen carrier in proximity to the electrostatic lens system, a cathode aperture Within the vicinity of said cathode and axially displaced from said specimen carrier to subtend a relatively small angle at the plane of the specimen thereby affording a minimizing effect on the divergence of the electrons leaving the lens of said system, and beam limiting means intermediate said electron gun and said specimen carrier.

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 2,319,061 Hillier May 11, 1943 2,266,717 Von Borries et al. Dec. 16, 1941 2,305,459 Schuchmann et al. Dec. 15, 1942 2,234,281 Ruska Mar. 11, 1941 2,208,406 Benedict July 16, 1940 2,275,234 Ruedy Mar 3, 1942 2,356,633 Von Ardenne Aug. 22, 1944 2,058,914 Rudenberg Oct. 27, 1936 2,195,914 Baier Apr. 2, 1940 1,836,829 Weinhart Dec. 15, 1931 2,218,639 Diels Aug. 30, 1938 2,244,358 Ewald June 3, 1941 2,185,807 Gabor et al. Jan. 2, 1940 OTHER REFERENCES Review of Scientific Instruments, vol. 12, pages 91 to 93, February 1941, 250-495.