US2819403A - Electron microscopy - Google Patents

Description

Jan. 7, 1958 .1. H. Rl-:lsNx-:R 2,819,403

ELEcTRoN MIcRoscoPY Filed July` 1, 1954 2 Sheets-Sheet 1 I'NvrN'TuR ATTORNEY .Im-IN I-l. RHSNBH Jan. 7 1958 J. H. RElsNr-:R 2,819,403

ELECTRON MICROSCOPY Filed July 1, 1954 2 Sheets-Sheet 2 INI/ENTOR.

.lm-1N H. RI-lsNn-R ,WPORNEY United States Patent ELECTRON MICROSCOPY John H. Reisner, Hadtloniield, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 1, 1954, Serial No. 440,67 7

6 Claims. (Cl. Z50-49.5)

This invention relates to electron microscopy, and more particularly to means for compensating magnetic lenses as used in electron optical apparatus, such as electron microscopes.

In electron optical apparatus such as an electron microscope, a beam of electrons is formed at an electron gun and is projected to a target positioned some distance from the gun. The electron beam is controlled and focussed in its path by means of magnetic electron lenses. Each of these lenses comprises a pair of pole pieces and an energizing coil. The pole pieces define a magnetic field through which the electrons of the beam must pass in order to reach the target. As is well known in the art, when the electrons pass through a magnetic iield their path is altered by an amount which is a function of the velocity of the electrons and the strength of the magnetic iield defined by the lens. If a beam of electrons is passed through a lens which is symmetrical with respect to the central axis of the beam and of the lens, the effect on the beam will be symmetrical. A lack oi uniformity in the effect on the beam will cause the image of the beam which impinges on the target to be distorted in accordance with the lack of uniformity. Asymmetries in the lens tield are caused largely by the anisotropy of the magnetic material from which the pole pieces are made and by mechanical maladjustments. The asymmetry of such pole pieces results in an image aberration which is termed anisotropic astigmatism. The astigmatism of such a lens field is a quantity which is characterized by both direction and magnitude.

A means for correcting field asymmetries in a magnetic lens is described in U. S. Patent No. 2,455,676, issued to Hillier on December 7, i948. ln accordance with the teachings of that patent, a plurality of ferromagnetic members is arranged about the axis of the lens and these members are adjustable to increase or decrease the quantity or" iron adjacent the lens field. By careful adjustment of these ferromagnetic members, lield asymmetries could be corrected. However, the compensation introduced by such means results in a lens field that, although compensated, is less smooth than the original asymmetric field. The ends of the adjusting screws shown in that patent produce localized areas of compensation. rihe sum of the influence of these localized areas results in an over-all compensating effect. ln order to gain a measure of resultant smoothness and versatility, a relatively large number of the adjusting screws are used, eight being the usual number. Such an arrangement, of course, requires that the several adjusting screws be adjusted separately. With eight such adjustments to make, the process would be somewhat lengthy.

lt is an object of the present invention to provide an improved and novel means for compensating for asymmetries of magnetic electron lenses.

lt is another object of the present invention to provide lens compensating means as set forth wherein a smooth compensating field is produced with fewer adjustments than has heretofore been provided.

asienta Mice The asymmetry of such magnetic electron lenses is substantially always elliptical. However, the direction and magnitude of the ield distortion may vary between lenses.

lt is a further object of the present invention to pro vide lens compensating means as set forth in which the compensation is adjustable in magnitude and direction without the necessity of providing means for rotating the compensator about the axis of the microscope.

ln accomplishing these and other objects, there has been provided, in accordance with the present invention, an improved and novel lens compensation means. The compensating means includes a pair of annuli of relatively high magnetic permeability. The two annuli are positioned in and parallel to the gap between the pole pieces of the lens structure. Each annulus is separately adjustable by a set of two adjusting screws and a spring biased plunger.

An understanding of this invention may be had from the following detailed decription when read in connection with the accompanying drawings in which:

Pig. l is a schematic representation of an electron microscope embodying the present invention,

Fig. 2 is an enlarged cross-sectional View showing the details of the structure constructed in accordance with the present invention; and,

Fig. 3 is a cross-sectional View taken along the line 3--3 of Fig. 2 viewed in the direction of the arrows.

Referring now to the drawings in more detail, schematically in Fig. l and in more detail in Figs. 2 and 3, the representation of an electron microscope includes an evacuable chamber 2 enclosed in the microscope column structure Ll. Ari electron gun 6 is positioned at one end of the cylindrical column 4. A beam of electrons generated at the electron gun is directed towards a target 8 placed adjacent the opposite end of the column Intermediate the gun and the target, there is positioned a magnetic lens lll which focusses the electron beam on the target. An object stage l2 is positioned adjacent the lens between the lens and the gun. As is well known in the art, the object stage l?. includes suitable means, not shown, for holding a specimen which is to be subjected to examination in the microscope. The lens comprises an upper pole piece member 14 and lower pole piece member lo separated by a non-magnetic spacer lg. Surrounding the pole pieces, in energizing relation thereto, is a field coil Zd which is supported in the column d. Supported by tno nonmagnetic spacer 1S, there is positioned a pair of annali ZZ and 2d respectively. These annuli are made of a suitable pararnagnetic material eXY hibiting high magnetic permeability. The upper annulus is laterally adjustable with respect to the microscope column by means of a pair of adjusu'ng screws 26 and a plunger 25% which is, in turn, biased by a spring 3d. Similarly, the lower annulus 245 is laterally adjustable by means of a pair of adjusting screws 32 and a plunger 3d which is also biased by a spring 36. The two annuli may be positioned to operate with the upper one resting on the lower one as shown in the drawing or, with a thin nonmagnetic spacer therebetween. The nonmagnetic spacer would facilitate the relative motion by relieving the interu facial friction. The adjusting screws 26 and 32 are provided with suitable adjusting knobs or handles dd. .Each of the knobs fill is provided with a shaft l2 which passes through the walls of the column structure fr, through the field coil windings 2l) to engage the ends of the adjusting screws. These shafts are provided with suitable vacuum seals so that the internal vacuum may be maintained. The knobs lll and shafts may be retracted from engage ment with the screws to allow the lens structure l@ to be removed.

The annuli thus positioned constitute field compensating means in accordance with the present invention. By

asiaaos proper adjustment of the adjusting screws Z6 and 32, the two annuli may be adjusted to various positions with respect to each other and with respect to the axis of the microscope. For example, the two annuli may be adjusted to such a position that they are coaxial with respect to each other but eccentrically positioned with respect to the beam axis of the microscope. On the other hand, if the two annuli are adjusted to a position such that their respective axes do not coincide, a relatively elliptical field or influence will be described by the intersecting circles. Since the two rings are circular and separately adjustable, it may readily be seen that the direction of the major axis of the elliptical field of influence may be oriented in any chosen direction perpendicular to the beam axis of the microscope. The ellipticity of the compensating field may be adjusted by increasing or decreasing the amount by which axes of the two annuli depart from coincidence.

In this manner, it may readily be seen that the compensating field resulting from the adjustable annuli may f be varied both as to magnitude and direction without the necessity of having means for rotating the compensation device about the beam axis of the microscope. Also, since the compensation is accomplished through the use of two continuous surfaces, the resulting field of influence will be considerably smoother than that achieved by the aforementioned screw type compensation means.

What is claimed is:

l. Electron optical apparatus having means for defining a magnetic lens field for affecting an electron beam passing therethrough comprising at least two annuli of magnetizable material, said annuli being disposed in said lens field and aligned for the successive passage of said beam therethrough, and said annuli being laterally displaceable with respect to each other to compensate for asymmetries of said lens field.

2. In a magnetic lens system for an electron microscope, a pair of spaced pole pieces for defining a magnetic lens field, a first and Second pararnagnetic annulus, said annuli being mounted between said spaced pole pieces with the openings therein to be threaded by said field, and means for separately adjusting said annuli to compensate for inherent asymmetries of the lens field defined by said pole pieces.

3. A magnetic lens for an electron microscope comprising an upper pole piece, a lower pole piece, means for mounting said pole pieces in spaced relation, means for magnetically energizing said pole pieces to define a magnetic lens field, a first paramagnetic annulus, a second paramagnetic annulus, means for mounting said an nuli in parallel relation in the space between said pole 4 l pieces, the axes of said aunuli being disposed in the direction of said lens field which is established in the region between said upper and said lower pole pieces, and means for laterally adjusting each of said annuli to compensate for inherent asymmetries in the lens eld defined by said pole pieces.

4. A magnetic lens for an electron microscope comprising a pair of spaced pole pieces, means for magneti cally energizing said pole pieces to define a magnetic lens field, means including a pair of paramagneti-c annuli mounted between said pole pieces for introducing a substantially elliptical compensating field between said pole pieces to compensate for inherent asymmetries of said lens field defined by said pole pieces, and means for adjusting the ellipticity and the direction of said compensating field.

5. An electron microscope comprising a microscope column defining an evacuable chamber, an electron gun for producing a beam of electrons, target means upon which said beam of electrons is focussed, a magnetic electron lens mounted between said electron gun and said target means within said evacuable chamber, said lens comprising a pair of spaced pole pieces, means for magnetically energizing said pole pieces to define a magnetic lens field, means including a pair of paramagnetie annuli mounted between said pole pieces for introducing a substantially elliptical compensating field between said pole pieces to compensate for inherent asymmetries of the lens field defined by said pole pieces, means for adjusting the ellipticity and direction of said compensating field, and means accessible externally of said evacuable chamber for effecting said adjustment.

6 A magnetic lens system for an electron microscope comprising an upper pole piece, a lower pole piece, said pole pieces defining a magnetic lens field for focusing an electron beam passing through said pole pieces, a first and a second paramagnetic annulus, said annuli being mounted in said magnetic field and aligned for the successive passage of said electron beam through the openings therein, and means for separately adjusting said annuli to compensate for inherent asymmetries of said lens field.

References @ited in the le of this patent UNITED STATES PATENTS 2,247,524 Schuchmann et al July l, 1941 2,253,542 Von Ardenne Aug. 26, 1941 2,455,676 Hillier Dec. 7, 1948 FOREIGN PATENTS 666,168 Great Britain Feb. 6, 1952

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