US2424789A

US2424789A - Electron microscope

Info

Publication number: US2424789A
Application number: US519772A
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: FR938893A; BE576003A; FR58024E; US2424788A; US2424790A; GB647703A; BE483238A; US2424791A; US2400331A; BE480179A; GB669381A; GB576003A; FR964837A; BE474126A

Description

y 1947. c. H. BACHM'AN ETAL 2,424,789

' ELECTRON MICROSCOPE I Original Filed Dec. 1, 1942 6 Sheets-$heet l 7 Inventors:

1g Charles H. Bachmam,

Simon Ramo,

Their Attorney July 29, 1947. c. H. BACHMAN EI'AL ELECTRON MICROSCOPE Original Filed Dec. 1, 1942 6 SheetsSheet 2 July 29, 1947- c. H. BAcHMAN ETAL ELECTRON MICROSCOPE Original Filed Dec. 1. 1942 6 Sheets-Sheet 5 Inventors: Charles H. Bachman k Simon Pamo, b flw flwllw y TheirA'btorney.

y 1947- c. H. VBACHMAN EII'AL 2, 24,789

ELECTRON MICROSCOPE ThZYA'btorney By 29,1947. 0. H. BACHMAN ETAL 2, 4,789

ELECTRON MICROSCOPE Original Fil ed Dec. 1, 1942 s Sheets-Sheet 5 Inventors: Charles H. Bachman, Simon Rama,

Their Attorney.

0. H. BACHMAN'ETAL ELECTRON MICROSCOPE Jul 29, 1947.

6 Sheets-Sheet 6 Original Filed Dec. 1, "1942 lnventors m W m m o H so w m M 8 YM #T C Patented July 29, 1947 2,424,789 ELECTRON MICROSCOPE Charles H. Bachman, Scotia, and Simon Ramo, Schenectady,- 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. 519,772

6 Claims. (Cl; 250-.-49.5)

The present application, which is a divisionof I our copending application, Serial No. 467,530, filed December 1, 1942. relates to electron microscopes, and more particularly to new and improved structure for electron microscopes of the electrostatic type. 1 T

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 examinations of difierent specimens.

The prior art arrangements have also been's'ubject to the disadvantage of requiring the services":

of skilled operators to efieot 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 sup plied 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. i 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 aifords simplicity of construction and operation, and adaptability to a wide field of 'ap- Y plication. The electron microscope which we provide is adapted and arranged to permit .-fre quent opening of the evacuated chamber for the-insertion and extraction of specimens to -be investigated and which entails only the-lapse of arelatively short interval of time between examinations 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, theessential elements of the electron microscope, such asthe cathode construction and the electron lenssystem, are readily demountable .for inspection and replacement. Lastly, a unitary electronilensas-- sembly 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 and improved electron microscope.

' It is 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.

' For a better understanding of our: invention, referencem-ay 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 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 across 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. 4

. a perspective view of .a manipulator which may be employed in our electron microscope, and Fig. 9 represents the cartridge-type specimen carrier. Fig. 10 represents the structure for holding and positioning an electron beam restricting'means such as a positionable apertured disk. Fig. 11

. is a view of the specimen insertionmember; Fig. 12 is a detailed view of the locket which is supported at one end of the holder shown in Fig. 11; Fig. 13i-s a view of the sectionalizedhorizontal conductor which connects corresponding electrodes of thevarious 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 an-d,20 are 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 may-be used-to photograph the image produced uponthe-fluorescent screen of the electron microscopaandsF-ig. 24 is a simplified circuit diagram showing the controllable source of voltage for the "electron microscope and the cathode filament supply.

Prior to a detailed description of the embodi ment of our invention illustrated, it is believed that it may be helpful to present generally certain fundamental aspects of the electronmicroscope disclosed herein. Generally speaking, the elec- ..tron microscope which'we provide is one employing an electrostatic type lens. The microscope comprises essentially an electron gun-whic'h produces an electron beam-toillumina-te or irradiate a specimen to be investigated and an imaging system which-magnifies the imageproduced by the impingement on the specimen-into a larger image on a viewing screen such-asa fluorescent screen, orupon 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 sizeandsconfiguration since an increase or decrease in'the electron velocity is always accompanied by a change-in the electric field focusing action of justrsuch magnitude '01 strength as'to yield precisely the same electron paths. 'By the use of *such a system'it-i's then notnecessary'to'employ a closely regulated source ofisupply voltagefor'the microscope.

Of course, as is 'well known, the desirability of employing .an electron microscope in place of a light microscopefor many applications .is' the greater resolving power incident to .the use :of electron irradiation Pas contrasted with: thessmaller resolving power of light microscopes. Although not limited to a particular resolving power 101 magnification, :anelectron microscope built in accordance-with ourinvention operates satisfactorily having a 200-1Angstrom units resolution and a useful magnification of about 10,000 .'As pointed out hereinafter, this magnification-may be obtained eitherexclusively by thed'esign'of the electron lens systemitsel'f, or .by'thecombination of a suitable electron electrostatic lens and an associated light microscope orlens which is placed in the vicinity-of .the fluorescent screenat one end of the electron microscope.

In the electron microscope described 'hereinafter, certain features. are incorporated by virtue of the configuration and position of theeler'nents to obtain the desired operation of the electron gun, the irradiation of the specimen, 'andthe desired focusing action of the electrostatic lens. As concerns the electron gun, this element is constructed to produce-sufficient current density at the specimen without involving anappreciable 70 heating of the specimen by theelectron beam. On the other hand, the image is illuminated'to a sufficient degree of brightness for visual observation with a minimum ofcurrent striking the-specimen. It will be apparent that there isnoadvantage to irradiatea large partof-a'specimen if ,je'o'tive'l'ens system. This type of limiting is genonly 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 viewing2 screen or fluorescent screen is the result of distribution of the electrons arriving there. We

'I have found that for very thin specimens the majority of the electrons which pass through the specimen do so with their slope substantially unchanged; consequently, the limiting angle of the electron 'bund-le leaving the objective lens can be -:established principally by the electron gun ap- ...paratusrather than by the apertures of the ob- Y era'llyeasierto 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 de- 'termined by the first 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 effect uponthe resolving power than does the lens aperture.

The electron microscope described hereinafter deconstructed .to afford "proper relationship between the cathode 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 of "factors such as the location of the imaging lenssystem and the spe'cimen,'the .fila- :ment1life, the accelerating voltage, the electron beam-angle -at the "specimen and the extent of magnetic shielding of the field due to the fila- ;ment.current.

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. The in-focus position of the speci- "men is external to the high field region of the lens and the specimen is in a region substantiallyentirely free from electric fields. The electrodes o'f the lens system are finely machined or ground-and highly polished to minimize the insulation problem and field emission at the oper- 'ating voltage. "The lenses are of symmetrical. configuration with'a single negative potential to ground, namely the cathode potential, serving to energize thelen's. 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 isof a portable nature being mounted on castors 2, and which "houses associated "auxiliary apparatus for an elecftronmicroscope, and on the front of which the electron "microscopetis horizontally positioned a desk or table part 4 integral with the cabinet I 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. Therevis also positioned within the vicinity of the eye-piece or viewing aperture a plurality of externally accessible actuating means, such as knurled wheels "I, 8 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 I0 and having an actuating member I l 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 12, including a rectifier and a transformer, is employed in an upper compartment together with voltage controlling means such as a rheostatic regulator I3 and a vacuum valve l4. 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 i i and Il, both of which are supported by vibration absorption means such as springs l8 and I9. Pump It is preferably a high vacuum twostage 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 l3 of the voltage supply for the electron gun of the microscope may be provided with a handle or knob .28 which extends through the wall of the cabinet to make it readily accessible to the operator. In like manner, referring to Fig. 1, the filament supply for the cathode of the electron gun may be controlled by a knob or dial 2| also positioned on the front of the cabinet. Likewise. a main power switch 22 and a handle 23 connected to'the vacuum valve I 4 through gearin (not shown) may be made easily accessible to the operator by mounting as indicated.

In Fig. 3 there is illustrated a cross sectional view of the evacuated chamber, electron gun, the electrostatic lens system, and the 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 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 2B and alternate layers of high permeability iron 2'! and 28. 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.

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 finely ground and high- 1y 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 cylinder 29. Within the cylinder 29 We provide an electrostatic lens system which includes a.

-plurality of generally similar lenses 36, 31 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 3 5 main.- tained at a common or anode potential and which. are provided with apertures 35 and 36; respectively. We provide an intermediate electrode 32 i also of wafer or disk-like configuration in spaced? relation between outer electrodes 33 and 3E and. which is provided with an aperture 38 of sii;bstantially larger diameter than the apertures 35 and 36.

Electrodes

33 and 3 8 are constructed. to have their peripheries finely ground and 1901- ished to closely engage the inner surface of cyl-- inder 29, thereby establishing satisfactory mechanical and electrical connection to the system. Intermediate electrode 3'! 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 and 13. 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 innner surface of cylinder 28. 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, 3% and 38 are convex, thereby reducing to a minimum the tendency to establish uncle sired voltag 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 re-alinement in the field. For example, the electrodes of the lenses 3fl32 may be maintained and locked in the desired illustrated position by means of a plurality of principal longitudinal and annular spacers it-l2, inclusive. In addition, the in-- termediate 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 electron lens system and a specimen carrier 65, shown in Fig. 9, and a specimen manuipulator 46, shown in Fig. 8,

Th elements

45 and 46 will be explained hereinafter in detail. The specimen carrier i5 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 3i) and Si and comprises a pair of spaced outer electrodes d"! and 48 and an intermediate electrode 49. Outer electrode 48 is provided with an aperture 58 and a radial recess 5! 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 59 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 10- radians.

A radial specimen insertion chamber 54 is provided 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 56 and also for sealing the main evacuated chamber of the microscope. Valve 55 comprises a removable cover 56 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 the juncture of the cover 58 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 50 fed by a conduit 6! 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 69 and actuated by a member 53. A bellows-type sealing member 64 is sealed around the adjacent area of cylinder 58 and arm 53. The actuating mechanism for cover 56 and valve 62 is shown in Fig. 6.

At on 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 B6 and 61 which are firmly positioned against the inner surface of cylinder 24. Electrostatic shielding means, such as a transverse metallic planar member Bl is positioned between members 66 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 61. Key 68 may also extend through annular spacers 69 and '10 which maintain disks 61 and 68 and the unitary assembly in the desired spaced relation.

Disks (SB-and 61 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 39, 3! and 32 a potential corresponding to the cathode potential. That is, the cathod 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 filamentary cathode 65 so that a suitable accelerating electric field is provided for the electrons emitted by the cathode.

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 I6 and il 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 H. 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 adapted to engage threads provided at the end of cylinder 24, and a. plurality of oircumferentially displaced means, such as screws 8!, for exerting longitudinal or axial pressure against the retaining ring 19, thereby compressing washer 19.

A vitreous part 82 is sealed to the supporting ring 18 and serves to support and seal the oathode structure. The vitreous part 82 may be of the re-e'ntrant 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 structime 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, there is provided 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 whichseats 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 99, a retaining ring 9 I and pressure exerting means such as screw 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 45 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. 3 and which are controlled by means of a plurality of screw-thread devices 96, 91 and 98, only two of which are shown in Fig. 3. The screw- 7 thread devices! is shown in cross-sectional detail and is also representative of the other'two de-. vices. 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 atscrew I which is provided with an actuating pin IOI and which has one end thereof extending through disk 86 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 which the cathode supporting structure is shown in greater detail. 7 Means are provided for positioning, that is for centering the point "or hairpin-type filamentary 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 diiferent 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. j l

A metallic base I04 is sealed at one end to the vitreous part 82 and is provided with threads to engage threads at one endof 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 filament and the intermediate-electrodes of electron lenses 3032. Base I04 is constructed to afford a flexibleniechnical connection between the inner conductor 83 of the concentric line and the base.

A conductive connecting member I05 is me-' chanically 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 providea flexible connection such as a coil spring I01 attached to the inner conductor 83. I

The base I04 is also provided with a threaded flange I08 which supports a metallic thimble I09 provided with a shoulder I I0 which'longitudinally positions the cathode positioning or centering means to be described immediately. This oath ode centering means is flexibly connected to member I05 by means of a flexible connection such as a coil spring III and comprises an annular centering ring II2 having near the periphery thereof a restricted region II3 to'obtain a substantial line engagement with shoulder I I0. Ring II2 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 firm engagement against shoulder IIO by means of a compression spring II5 which is supported atits right-hand extremity by means of a threaded collar II6 which engages threads on the inner surface of thimbl I09. As a means for insulating the shell of thimble I09 and the centering means from the center conductor to the cathode, we employ a cylindrical insulator III placed between retaining ring I12 and a cylindrical cathode base-H8 adapted to receive a rigid cath'ode stem II9. One terminal I20 of the point cathode 65 is connected to stem I I9 through a face plate I2I which is attached to stem II 9, 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 II 9 and cathode 65 without. disturbing the electrical connections to the cathode, we employ a flexible conductorsuch 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 may be 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 and tubular conductor 84 of the concentric line. At the end of thimble I09 we provide a cap I28 which is removable or demo-untable and which is adapted to be supported by thimble I09 by a screw-threaded engagement at I29." This cap I 28 is provided with an aperture I30 through which the electrons emerge upon acceleration due to the field produced by anode cup I3.

Analternative 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 I05, 83.

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 h'andie 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 ofa connecting rodI35 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 there is diagrammatically illustrated, partially in perspective and partially in cross section,fthe unitary electron lens assembly showing the plurality of spaced electrostatic lens systems 30, 3I and 32, and corresponding elements have been assigned like reference numerals.

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 lense 30 and 3|, is supported by an insulator I36 which holds the intermediate electrode 49 firmly in position and alinement. Provision is made for the impression on the intermediate electrodes of each lens system of a potential corresponding to the oathode potential by means of the conductor shown in Fig. 13. This-conductor extends longitudinally of the electron gun and is placed within the cyl- These apertures are also finely machined or ground to have convex peripheries to minimize high electric field gradients.

The conductor is a sectionalized conductor to facilitate assembly and disassembly of the elec-i tron lens unit, and comprises a plurality of conductors I42, I43 and I44 which are welded or otherwise attached to the intermediate electrodes and are provided with recesses to receive telescoping intermediate members I45 and I45.

There is also provided within the electron lens assembly electrostatic shielding means spaced lon= gitudinally between the sectionalized conductor and the electron path. This shielding means may comprise a plurality of planar metallic shielding members I4! and I48 which closely engage the inner surface of cylinder 29 and abut facing outer electrodes of the electron lenses 30-32, in-

clusive.

As means for facilitating assembly of the'entire electron lens unit and for positioning the actuating rods 93-95 in a position which is mechanically expedient and which will not deleteriously affect the operation of the electron lens system, we provide longitudinal grooves lying incylinder. 29 and adapted to receive and permit longitudinal movement of rods 9395. Only one of these grooves, that is groove I49 which receives rod 93, is shown in Fig. '7. Each of rods 9395 is terminated in a. foot I50 which engages heads 99 of the screw-thread devices 96-98 shown in Fig. 3.

As a means for looking or keying the electron lens assembly in the desired or fixed angular position within cylinder 29, there may be employed suitable means such as a tongue II constituting an integral part of or attached tothe cylinder,

and which may be adapted'to engage one of the 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 or lenses thereof.

The manipulator shown in Fig. 8 is provided with three actuating levers I52, I53 and I54, only two of which are shown in Fig. 8. The details of construction and operation of the manipulator will be explained in connection with Figs. 14-29, inclusive. As to the general featuresof operation, it may be said here 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 is supported by a face plate I55 which is maintained in the desired axial position by engagement with the inner surface of cylinder29.

Fig, 9 is a detailed cross-sectional view of the specimen carrier 45 which may be of the cartridge type comprising a retaining shell I5I having a longitudinal opening I58 through which. the electron beam passes, and having aresilient holding or positioning member such as a plunger;

I59 alsoapertured. The plunger I 59 may be biased by means of a compression spring I50. 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 aslight pressure must be exerted upon the locket toobtain an insertion and in order firmly to hold the locket during adjustment of the mani ulator.

In Fig. 10 there is illustrated the structure for supporting the beam limiting disk 50. This structure comprises a face plate IBI which is supported from face plate I55 by means of a plurality of spaced'supporting rod I62 and I63. Other rods, not shown, may also be employed to afford a firm support. Molybdenum disk 55 is held in position by a shroud ring I54, the latterof which is positioned by a plurality of thumbscrews I55 which may be adjusted to obtain the desired alinement of the aperture in disk 50 with respect to the electron gun and the apertures of tlieelectrodes constituting the electron lens system. Thumbscrews I55 are supported by an annular memberj I65 having a central aperture within which shroud ring I 64 and disk 53 are positioned and is provided with a clamp I61 to control the axial displacement of these members. Face plate I5I and annular member I65 are provided with apertures I69 and I58 which, however, for the purpose of construction and arrangement are not in alinement with the corresponding apertures I3'I-I4I in the disks or electrodes of the electron lens assembly or aperture I'IIl of face plate I55 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 III comprising a recessed ring I'IZ adapted to receive a fine removable' mesh upon which the specimen is placed and alockingand retaining ring I'I3. Rings I12 and H3 are shownin cross-sectional detail in Fig. 12.

' In Fig. 11 there isalso shown a suitable specimen holder which comprises a stem I'I4 spring biased to the position illustrated with respect to a cylinder I'I5. Stem I'I4 telescopes cylinder I15 and the latter is attached to a curved holding plate H6 whereby spring I'I'I', in the absence of pressure 'on knob I18; draws the stem I'I5 upward. The recesseddisk H2 is flexibly connected" to the stem I'I4 by suitable means such as a fine-link. chain I19. The lower end of stem I'lltg' is'provided with a curvature to engage the p'eripheryof disk In upon depression of member I118 thereby firmly. holding the locket in engagement for the purpose of extending and positioning theJocket-in carrier 45 shown in Fig. 9.

In Fig. 13 there is shown a portion of a longitudinal sectionalized conductor for impressing cathode. potential on the intermediate electrodes of the various electron lenses 3II--32 and corresponding elements have been assignedv reference numeralsem-ployed in Fig. 7. Due to the fact that the apertures I68 and-I69 of the beam limiting structure are not in alinement with apertures I3] I4 I ofthe, lens assembly and aperture IIll oflthe manipulator face plate" I55, we provide an elbow I89; which; telescopes section I42 of the conductor andwhich is provided with a highly pclishedand finely ground cup IBI adapted tov receive a resiliently mounted pin I92 constituting a part of section I83 and which, in turn, telescopes 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 Tschopp.

There is provided on face plate I56 a pair of vertical ways I 95 and I86 having therein races I91, preferably of V-shaped configuration to support a plurality of friction reducing bearings such as miniature ball bearings I88. A vertical transversing member I89 is also provided with races I90 and I9! to engage the ball bearings I 98. 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 I98 and 199 engages the ball bearings I96. A third set of ways, namely, the horizontal or axial ways for focusing purposes comprising ways 200 and 20I, are supported by the horizontal transversing member I91 and also comprise V-shaped

races

202 and 209 likewise provided with miniature ball bearings 204 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 and is ultimately supported by ways 200 and 29I.

Appreciation of the manner of operation of the manipulator shown in detail may be obtained by referring to Figs. 1.5-20, inclusive. Figs. 15-20, inclusive, do not show all of the elements of the 14 comprise a spring 2I2a. Forward motion, that is a motion to the left of lever I54 viewed from Fig. 18, raises ways I92 and 193 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 3205 by means of a crank or connecting rod 2I3 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 I and 0, for

manipulator 95. The associated groups I5, I6; I'I,

I8 and I9, 20 show only those elements deemed necessary in order to emphasize the various independent movements produceable by the manipby 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 297 to move cam 209, thereby exerting a pressure against the structure for traversing 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 eliminating or reducing back-lash which, in connection with the arrangements shown in Figs. 15 and 16, comprises a spring 299 which biases member I99 towards the left observed from the front view shown in Fig. 15.

Operation of the vertical transverse 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 2 I 0 which in turn operates a cam 2II connected to the way I92.

Back-lash prevention means is also provided in connection with member 202 to bias this member controlling the manipulator are joined to and operatively 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 corre-' sponding elements have been assigned like reference numerals. The end plate or disk 96 shown at the left-hand part of Fig. 3 is presented in enlarged form in Fig. 21 and shows the manner in which the contact or connecting pins IIJI extend through that disk affording suitable connections for a plurality of flexible cables 2I5, 2I6 and 2II which are provided with adaptors 2I8 and which are connected to the knurled wheels I, 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 andv comprises an eye-piece 2I9, a light or glass lensv 220 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.

An arrangement is provided 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 insertable within holding cylinder 222 and may alsobe provided with horizontal supporting members; 225 and a further supporting member 226.

Fig. 24 diagrammatically illustrates an electric: circuit diagram which may be employed for the. application of anode-cathode voltage to the elec-- tron microscope and for the control of the oathode heating current. The electron microscope which we provide is capable of being energized from any suitable source of alternating current such as that readily available at the usual commercial power outlets. For example, the system may be energized from an alternating current circuit 22'Iof commercial frequency. The main switch 22 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 I! and centrifugal blower I5. A unidirectional voltage is produced by means of a rectifier circuit which may include a single unidirectional 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 20 shown in Fig. 1. One terminal of the secondary winding of trans-- 15.. former 229 is connected toground. By virtue of the polarization of the discharge device 228, conductor 2-3I is maintained at a negative potential with respect to ground. Current limiting

resistances

232 and 233 may be connected in the positions illustrated not only to limit the current transmitted by discharge device 228 but also to limit the current supplied to the anode-cathode circuit. of the electron microscope. In order to effect the application of a substantially constant unidirectional voltage to the microscope, there may be provided 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 21 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 240, conductors 238 which extend therethrough centrally, and a tubular conductor 24L 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 I'll shown in Fig. 12, and the looking or retaining ring I13 is employed to maintain the mesh firmly against the inner shoulder of ring H2. The holding plate H6 is grasped and the knob H8 is depressed, thereby extending the rigid stem I'M so that the curved lower portion thereof firmly engages the outer periphery of ring H2, thereby positively holding it. Cover 550i Valve 55 is removed and the specimen holder is extended through chamber 54 and slot and the locket is inserted in specimen carrier 45, particularly between the inner vertical surface of shell l5! and the face 1'6 ode'voltage may be applied to the electron gun by operation of knob or dial 20 and the filament current may be controlled by knob 2!, both of which are illustrated in Figs. 1 and 24. Of course, rheostat l3 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 30-32.

The electron image formed at the specimen plane is-magnified by the electrostatic objective the eye-piece 6 which, in turn, position heads 99 of plunger I59, the latter serving to hold the locket firmly. The knob H8 is then released which permits retraction of the stem I14 upward into cylinder I15, which action renders the locket Ill free to be positioned by the carrier 45 and manipulator 4B. 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 sufliciently upward to prohibit the closing or placement of the cover 56.

The valve is then sealed by placing the transverse rod 57 in the groove provided in cover 56, and handle ll of the valve i 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 Figs. 1 and 24. After the pressure within the chamber has been reduced to a suitable value, power may be applied to the electronmicroscope; that is, anode-cathof the screw-thread devices 96-98, inclusive, which are operatively connected to levers 152- I54 of manipulator 46 through rods 53-95.

The specimen may be removed in the following manner: Handle H shown in Fig, 6 is raised effectihg release of the pressure on cover 56 so that this cdver 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 65 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 suifi'ciently reduced, cover 56 is then readily removable.

It has been found that the limiting angle of the electron bundle, or bundles, leaving the objective lenses 38-32 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 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 thespecimen under examination play an important part in determinin 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 optimumplacing of the electron gun aperture at an appreciable longitudinal or axial distance from theobject, ,a highly satisfactory structure is provided for producing this limiting efiect. In other words, we position the electron gun so that it establishesa-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 ajgreat extent the condensing effect of the :electron chamber, an electron lens system within said chamber, means for supporting said electron gun and chamber in the horizontal position, aviewing aperture at one end of said chamber, means within said chamber for positioning :a specimen to be investigated, actuating means within the vicinity of said aperture for controlling the last mentioned means, and a substantially vertical specimen insertion chamber in communication with .the first mentioned chamber.

5. In an electron microscope, the combination including a horizontally disposed electron gun chamber, an electron lens system within said chamber, means for supporting said electron gun and chamber in a horizontal position, a viewing aperture at one end of said .chamber, means within said chamber for positioning a specimen to be investigated, a specimen insertion chamber in communication with the first mentioned chamher, and externally accessible means supported in the vicinity of said aperture for adjusting said specimen positioning means in three mutually perpendicular directions whereby the positioning of said specimen in accordance with the image produced at said aperture is facilitated.

6. In an electron microscope, the combination including a horizontally disposed electron gun chamber, a cabinet for supporting said chamber and containing therein a power supply for the electron gun, an evacuation means for reducing the pressure within said chamber, a light optical viewing aperture at one end of said chamber, a horizontal table part attached to said cabinet below said chamber, means within said chamber for positioning a specimen to be investigated, actuating means Within the vicinity of said apertureforcontrolling the last-mentioned means, a

specimen insertion chamber in communication with said first mentioned chamber, and means supported by said cabinet above said table part for controlling the power supply to the electron gun. f

CHARLES H. BACHMAN. SIMONRAMO.

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

UNITED STATES PATENTS Number Name Date 2,360,872 Hillier Oct. 24, 1944 2,052,914v Rudenberg Oct. 27, 1936 2,132,076 Kotraschek et a1. Oct. 4, 1938 2,215,979 Schuchmann Sept. 24, 1940 1,419,241- Edwards June 13, 1922 OTHER REFERENCES