US2754443A - Astigmatically corrected electronic lenses - Google Patents

Description

y 10, 1956 A. ASMUS 2,754,443

ASTIGMATICALLY CORRECTED ELECTRONIC LENSES Filed Jan. 7, 1955 2 Sheets-Sheet l ff .3 1a 19 y Int entan- July 10, 1956 A. ASMUS ASTIGMATICALLY CORRECTED ELECTRONIC LENSES 2 Sheets-Sheet 2 Filed 195s Inventor".-

United States fatent ASTIGMATICALLY CORRECTED ELECTRONIC LENSES Alexander Asmus, Berlin-Spandau, Germany, assignor to Siemens & Halske Aktiengesellschaft, Berlin-Siemensstadt, Germany, a German corporation Application January 7, 1955, Serial No. 480,583

Claims priority, application Germany January 22, 1954 Claims. (Cl. 313-84) My invention relates to electronic lenses, for instance as used in electron microscopes, and is directed particularly to a compensating device for correcting axial astigmatism in such lenses.

Even with highest-precision methods of manufacture, electronic lenses cannot be produced with such accuracy as to avoid the occurrence of electron-optical lens errors due to departure of the magnetic or electric lens field from perfect rotational symmetry. These errors, especialiy in objective lenses, have the result that the resolving power of the electron optical apparatus does not attain the best expected value. The lens errors are generally caused by unevenness in the surfaces of the lensfield terminal structures such as the pole shoes or electrodes of the lens and also by inhomogenities within the material from which the lens is made. In such imperfect lenses, the intersections of the equipotential surfaces of the lens field with the planes perpendicular to the elec tron beam axis are not precisely circular but are approximately of elliptic shape. An error-free lens, however, must necessarily present precise circles as lines of intersection of the above-mentioned equipotential surfaces and perpendicular planes.

It is accordingly the principal object of my invention to devise an electron lens that readily permits correcting any residual error as manifested by axial astigmatism.

Another object is to provide an electron lens with astigmatic correction means that can readily be adjusted from the outside of an electron microscope or other electron-optical apparatus of which the lens forms a component.

To achieve these as well as the more particular objects mentioned below, and in accordance with a feature of my invention, 1 dispose around the electron-optical axis and around the field of the lens an annular corrective body for producing an elliptic field distortion adjustable as to magnitude and direction. According to another feature of the invention, the corrective annular body is disposed so as to surround the field-terminal system, i. e. the pole shoe or electrode systems, of the lens and is displaceable in the axial direction and also rotationally adjustable about the lens axis. This makes it possible to correct astigmatism with the aid of only two continuously variable adjustments, namely axial displacement and rotation of the corrective member. Since the corrective member surrounds the major lens parts, namely the pole shoe system or the lens electrodes, depending upon whether the lens is magnetic or electric, the desired lens correction is effected without necessity of having any structural part enter into the lens field proper.

There are several ways, according to my invention, of designing the corrective member. One way is to make the ring-shaped member of metal and give its inner periphery an asymmetrical configuration at one place or at two diametrically opposite places. This can be done, for example, by recessing the ring-shaped member at these places.v Another. way 'of providing for the required "ice asymmetry, applying to magnetic lenses, is to make the corrective member of brass or other non-ferromagnetic material and to provide the member, for instance at two diametrically opposite places, with iron or other ferromagnetic parts. In electrostatic lenses, the corrective member can be made of non-conducting material and be provided at suitable places with conducting metallic parts producing the desired field distortion.

According to another feature. of the. invention, the compensating device preferably has its ring-shaped corrective member provided with two control shafts operable from the outside of the evacuated envelope. of the electron-optical apparatus, one shaft serving for rotational adjustment and the other for adjusting axial displacement of the member. An especially simple construction is obtained by passing the two shafts coaxially through a common bore in the vacuum wall of the lens assembly.

According to still another feature of the invention, the corrective member, to be axially and rotationally adjustable, is mounted with the aid of two rings. One of these mounting rings has gear teeth meshing with a pinion for rotational adjustment. The other ring has a screw thread which engages a threaded rotatable part and performs an axial displacement when the part is turned by another pinion. In a magnetic lens this can be carried out by mounting a corrective iron member of suitable asymmetry between two brass rings, one having an outer, annular row of gear teeth in mesh with a pinion for rotational adjustment of the member, while the other ring is externally threaded and engaged by a rotatable, internally-threaded sleeve which in turn is rotatable by means of another pinion for thus imparting axial displacement to the corrective member. According to a more specific feature, also relating to magnetic lenses, one of the pole shoes of the lens to be corrected serves as a gliding surface for the corrective member.

These and other objects, features and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 illustrates a vertical, partially cross-sectional view of a magnetic electron lens according to the invention;

Fig. 2 is a perspective view of the ring-shaped corrective member;

Fig. 3 shows the lower ring for turning the corrective member to control the radial angle at which distortion is introduced; and

Fig. 4 illustrates an alternative embodiment of the invention.

In Fig. 1, an upper pole shoe 1 and a lower pole shoe 2 form the field-terminal structures of the lens. The magnetic circuit of the lens comprises two annular cover plates 3 and 4, an outer cylindrical jacket 5, and an inner cylindrical tube 6. An excitation winding 7 is disposed in the interior of the magnetic shield or capsule thus formed and is upwardly confined by an annular disc 8 of brass. A corrective member 9 serves to correct the elliptical astigmatism of the lens. At two diametrically opposite places, the member 9 is given suitable asymmetries capable of introducing a greater or lesser elliptical distortion of the field, depending upon the positioning of the member. To this end, in the embodiment of the invention illustrated, the ring-shaped corrective member 9 has two diametrically opposed iron lugs 10 and 11 serving as the zones of asymmetry. The ring-shaped corrective member 9 itself may also consist of magnetic material but may also be made of brass. Member 9 is mounted between two brass rings 12 and '13; and the entire subassembly of parts 9,. 12 and 13 is so shaped and dimensioned that the outer surface of the lower pole shoe 2 'serves .as 'a gliding surface for that subassembly.

Two control shafts 14 and 15 serve for adjustment of the ring-shaped corrective member 9. The shafts are coaxial and extend, one within the other, through the wall of the vacuum jacket 5. The central control shaft 14 carries on its inner end a driving pinion meshing with an annular toothed portion 17 of ring 13. This drive mecha nism serves for adjusting the ring-shaped corrective member. 9 to rotate the corrective field distortion.- The two' iron lugs and 11, fixed with respect to the corrective member 9, project into complementary upper slots 18 and 19 in an upwardly extending collar 20 forming partjof ring 13, whereby the corrective member is rotated the sleeve 23, the ring 12 is prevented from turning by V a pin 24 which is securely fixed at one end to the upper Wall. plate 3 of the lens and has its other end slidingly fitted into a corresponding bore of ring 12. 'When turning the outer control shaft 15, the sleeve 23 is rotated by pinion 21, and the ring 12 is moved upwardly or downwardly and entrains the corrective member to move in the same axial direction; Turning the sleeve 23 in one direction this has the effect of moving the corrective member 9 upwardly toward the effective field gap of the lens so that the strength of corrective field distortion is increased. When turning the sleeve 23 in the other direction, a return spring 26 pushes the corrective member 9 downwardly, thus lessening the stength of field distortion.

If desired, the two control shafts 14 and 15 may extend .to the outside through releasable and non-interchangeable clutch means. In any event, the'control knobs 27. and 28 of the two shafts should have fixed angular positions relative to the respective shafts so that the knobs may serve as position-indicating means. 7 a

Fig. 4 illustrates schematically another embodiment of a corrective device for a magnetic lens. Only the essential parts of the pole shoe system of the lens itself are shown. The pole shoe system comprises an upper pole shoe 3!, a lower pole shoe 32, and a non-magnetic intermediate member 33 rigidly securing the pole shoes to each other. A ring-shaped corrective member 34 surrounding the pole shoe system is provided for compensating axial astigmatism in the lens. The compensating member is made of brass and has a lower annular flange 35. The necessary asymmetry is provided by diametrically opposed iron lugs 36 and 37 incorporated in the ring. For varying the direction of corrective field distortion, the corrective member 34 is cooperatively connected with a control shaft 37 carrying at its inner'end a driving 7 pinion 38 meshing with an annular toothed portion of a brass ring 39. A wedged-shaped brass. lug 41 fixed with respect to the brass ring 39, or integrated therewith and projecting inwardly, is fitted within a complementary notch 42 of the ring-shaped corrective 34 and is slidable up and down therein. The brass lug 41, upon turning, carries with it the ring-shaped corrective member 34 and turns it about the pole shoe system for changing the direction of field distortion.

For varying the strength of field distortion by axial adjustment of the ringeshaped corrective member 34, the lens system comprises a control shaft 43 which carries on its inner end a driving pinion 44 meshing with an annular toothed portion 45 of a positioning ring 46. The'turning movement of this positioning ring is imparted to a ring member 49 through two downwardly-extending pins 47, 48 each fixed at one end in positioning ring 46 and fitting slidably at their other ends in complementary openings in the ring member 49; The ring member 49 has an' inner annular groove 5% within which the peripheral edge of a flange portion of the ring-shaped corrective member 34 is seated. The ring member 49 has an external screw thread 51 screwed within an internally-threaded fixed member 52, whereby, when turning the ring member 46, the ring member 49 will be screwed upwardly or downwardly in the internally-threaded member 52. Axial movement is thereby imparted to the corrective member 34 thus varying the strength of field distortion. By suitable proportioning of the frictional conditions, the corrective member 34 is prevented from being carried along by the turning motion of the ring member 4?. The outer control knobs 53 and 54 in this embodiment of the invention are fixed to the ends of the respective control shafts 37 and 43, which extend through the vacuum wall (not shown in Fig. 6) to be outside of the apparatus.

It will be apparent to those skilled in the art upon study of this disclosure thatthe invention permits of various embodiments, modifications and uses other than those specifically described herein without departing from the essential features of the invention and Within the scope of the claims annexed hereto.

I claim:

1. An electron-optical lens, comprising annular lensfield terminal structures fixed and coaxially spaced relative to each other and defining a lens field and a lens axis, a corrective body of annular shape surrounding said structure, said corrective body being asymmetrically designedat diametrically opposed locations for elliptically distorting the lens field, said annular body being axially displaceable relative to said structure and being rotatably adjustable about the lens axis for compensating .axial astigmatism.

2. An electron-optical lens, comprising two annular magnet pole shoes fixed and coaxially spaced fromeach other to form a lens-field gap defining a lens axis, a corrective body of annular shape having a larger inner diameter than said pole shoes and surrounding said gap, said corrective body being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, said annular body being axially displaceable relative to said structure and being rotatably adjustable about the lens axis for compensating axial astigmatism.

3. An electronic lens, comprising a vacuum envelope, annular lens-field terminal structures within said envelope coaxially spaced from each other and defining a lens field and a lens axis, an annular corrective memberdisposed in said envelope and surrounding said axis, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, and manually-controllable drive means'for independently varying the axial and rotary positions of said corrective 'member with respect to said axis, said drive means extending from within to the outside of said envelope.

'4. An electronic lens, comprising annular lens-field terminal structures coaxially spaced relative each other and defining a lens' field and lens axis, an annular corrective member surrounding said axis, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, a first drive mechanism for controlling the axial position of said corrective member along saidaxis, and a second drive mechanism for controlling the rotary position of said corrective member with'respect to said axis.

5. An electronic lens, comprising a vacuum envelope, annular lens-field terminal structures coaxially spaced from each other within said envelope and defining a lens field and a lens axis, an annular corrective member disposed in said envelope surrounding said axis, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, a first drive mechanism for controlling the axial position of said corrective member along said axis, and a second drive mechanism for controlling the rotaryposition of said corrective member with respect to said axis,

said first and second drive mechanisms being disposed within said envelope and comprising a pair of manually operable drive shafts, one of said shafts being tubular and having the other of said shafts coaxially disposed therein, and said two shafts extending from Within to the outside of said envelope.

6. An electronic lens, comprising annular lens-field terminal structures coaxially spaced relative each other and defining a lens field and a lens axis, an annular corrective member surrounding said axis, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, an externally-threaded ring member coaxial With said corrective member and rotatable with respect thereto, a rotatable internally-threaded member Within which said externally-threaded member is threaded, said internally-threaded member having a first annular gear portion, a first drive gear in mesh with said first annular gear portion for turning said internally threaded member, a second ring member coaxial with said corrective member, means interconnecting said second ring member and said corrective member for simultaneous cooperative rotary motion, said second ring member having a second annular gear portion, and a second drive gear in mesh with said second annular gear portion for turning said second ring member.

7. An electronic lens, comprising two fixed magnet pole shoes coaxially spaced from each other to form a lensfield gap defining a lens axis, a corrective member of annular shape having a larger inner diameter than the outer diameter of said pole shoes and surrounding said gap, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, a first drive mechanism for controlling the axial position of said corrective member along said axis, and a second drive mechanism for controlling the rotary position of said corrective member With respect to said axis.

8. An electronic lens, comprising two fixed magnet pole shoes coaxially spaced from each other to form a lensfield gap defining a lens axis, one of said pole shoes having an annular outer slide surface, a corrective member of annular shape having a larger inner diameter than the outer diameter of said pole shoes and surrounding said gap, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, holding means seated upon said slide surface and connected with said corrective member whereby said member is rotatable and axially displaceable relative to said slide surface, a first drive mechanism engaging said holding means for controlling the axial position of said corrective member along said axis, and a second drive mechanism engaging said holding means for controlling the rotary position of said corrective member with respect to said axis.

9. An electronic lens, comprising two fixed magnet pole shoes coaxially spaced from each other to form a lensfield gap defining a lens axis, a. corrective member of annular shape having a larger inner diameter than the outer diameter of said pole shoes and surrounding said gap, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, an externally-threaded ring member coaxial with said corrective member and rotatable with respect thereto, a rotatable internally-threaded member within which said externally-threaded member is threaded, said internally-threaded member having a first annular gear portion for turning said internally theaded tember, a second ring member coaxial with said corrective member, means interconnecting said second ring member and said corrective member for simultaneous cooperative rotary motion, said second ring member having a second annular gear portion, and a second drive gear in mesh with said second annular gear portion for turning said second ring member.

l0. An electronic lens, comprising two fixed magnet pole shoes coaxially spaced from each other to form a lens-field gap defining a lens axis, a corrective member of annular shape having a larger inner diameter than the outer diameter of said pole shoes and surrounding said gap, said corrective member being asymmetrically designed at diametrically opposed locations for elliptically distorting the lens field, an externally-threaded annular member coaxial with said corrective member and rotatable with respect thereto, means interconnecting said externally-threaded member and said corrective member for simultaneous axial motion, a fixed, internally-threaded member within which said externally-threaded member is threadedly engaged, a first ring member coaxial with said corrective member and having a first annular gear portion, relative axially slidable means interconnecting said first ring member and said externallythreaded member for simultaneous rotary motion, a first manually-operable drive gear in mesh with said first annular gear portion for turning said first ring member, a second ring member coaxial with said corrective member and having a second annular gear portion, means interconnecting said second ring member and said corrective member for simultaneous rotary motion and allowing relative axial motion, and a second manually operable drive gear in mesh with said second annular gear portion for turning said second ring member and said externallythreaded annular member.

References Cited in the file of this patent UNITED STATES PATENTS 2,219,193 Mynall Oct. 22, 1940 2,416,687 Fry Mar. 4, 1947 2,5 87,942 Wessenberg et al Mar. 4, 1952 2,637,000 Page Apr. 28, 1953 2,679,018 Reisner et al May 18, 1954 2,714,678 Wolff Aug. 2, 1955

Images