US2897366A - Externally adjustable electron microscope focusing aid - Google Patents

Description

July 28, '1959 l.. 4D. cool-IRAN 2,897,366 EXTERNALLY ADJUSTABLE ELECTRON MICROSCOPE FOCUSING AID lFiled. April 1. 195s L' g losj 2 L/M BY l /foq/ '44g ATTORNEY United States Patent i EXTERNALLY ADJUSTABLE ELECTRONv MICROSCOPE FOCUSING AID Lee D. Cochran, Kensington, Md., assignor'to Canal Industrial Corporation, Bethesda, Md., a corporation of Maryland Application April 1, 1958, Serial No. 725,615

Claims. (Cl.Y Z50-49.5)

The present invention relates to electron' microscopy and more specifically to means for improving the focusing ability and yphotograph image of the` electron microscope. t

Broadly defined, the invention maybe referred to as an externally adjustable objective aperture. The invention includes a replaceable core andv insert assembly, which is adapted to the interior of the `electron microscope, and an intermediate housinglocated between objective and intermediate lenses, the housing mountingrmeans suitable for transmitting motion to the `insert assembly. In the present instance, it is the insert which carries a core for the aperture per se. In electron microscopy, the word aperture is generally recognized to include disc or cap means defining one or more apertures through'whichy the electron beam may be selectively directed to limit thecross section area of the beam. Therefore, objective apertures referred to hereinafter will linclude the means defining the aperture per se. ln the present instance, the aperture is of the back focal plane type, suitably located inside the bottom constriction of the lower pole piecel of the objective lens in an electron microscope. This aperture is generally adapted to vertical positioning at the point of minimum cross section of the'electron beams, however it is the horizontal positioning that presents the major problem, hence the present invention.

It is an object of invention to augment the ability of the electron microscope to create fine resolution by screening out background fog which may be'caused by deflected, stray electrons.

It is a further objective of invention in` electron microscopy to provide an externally adjustable objective aperture which may be readily removed for cleaning, and which by virtue of its selective disposition within the objective lens gives superior image contrast, reduces contamination of the objective aperture and permits employment of the entire viewing'field at low magnification.

The reduction of astigmatism is a further object of invention, the device being adapted to proper axial centering and to the facile removal of contaminated apertures.

Yet another object of invention includes providing means for externally adjustably mounting an objective aperture adjacent the objective lens through virtually no `modification to the existing construction of the electron microscope excepting a substitution for the intermediate bellows thereof.

Another object of invention relates to providing mechanical means in electron microscopy for selected area diffraction which said means may be'used selectively with the externally adjustable objective aperture.

The essence of invention resides-in' providing external adjustment to an internally mounted electron microscope aperture without breaking vacuum.

The details of invention are shown in preferred form in the following description and drawings wherein:

Fig. l is a vertical elevation View of'invention illustrating the relative positioning of the intermediate housing and related aperture mounting core and insert assem- ICC , bly with respect to standard focusing elements of the area diffraction as may be mounted upon the tubular housing of Figs. l and 2;

Figs. 6 and `7 are section views of aperture caps.

The basic elements of invention` include: a tubular spring-loaded telescoping housing and core and insert assembly 209' in operable association with element 1Gb.

Referring now to Fig. l it will be noted that assembly;

200 -is adapted `to engagement with the elements which support'the lower element of the pole piece of an objective lens. Whereas assembly 200 is self-supporting with respect to the pole piece and related elements, it is designed for engagement by positioning'and motion elements of the intermediate housing 100 as will be expressed hereinafter.v Tubular-housing 160 is in effect a substitution for the intermediate bellows or sylphon of' the electron microscope, housing 100 resting in vacuum seal contact with the top of the intermediate lens and with the'bottom'of the Vlower portions of the objective lens. Tubular housing 100' is an improved magnetic shield which prevents' stray electron fields from affecting the quality 'of the image formed by beam. lt is so designed' that it introduces no astigmatism into the optical system, this beingan undesirable characteristic of more usual magneticl shields. `Housing ltltlis thus located in the space between the bottom' of the microscope objective lens and the top of the intermediate lens.

y In housing 100 it will be'noted that sections lZ and are retained in telescoping relation, section 120 having its inner flange surrounding the corresponding flange of section 102. The anges of both` define axial channels, the lower and narrower being section 104 and the upper and larger being indicated as at 124. Shelves 106 and 128 respectively retain the helical spring 122. ln the lower section suitable recesses 108 and 11i) serve to retain the circular vacuum seals 114 and 116 respectively. Within the upper section of the telescoping tubular housing 100 there is-provided a core positioner projection 126, extending generally centrally of the sectionv 1253 and radially inwardly toward the axis of the housing to engage a corresponding recess of the core of assembly 200. A seal retaining recess is milled in the top of section 122 to receive circular vacuum seal 132. Converging bores 134 extend radially inwardly of section 120 from the exterior wallY into channel 124,-V these bores accommodating a suitable bearing plug for screw drives and defining a channel suitable for sealing O-ring 136. Vacuum seal rotary drives 140 are so mounted within section 122 that the stems 142 thereof may be shifted horizontally into and out of registry with corresponding control screws in the core element of assembly 200. The rotary drive 140 includes stem 142 of tubular cross section, a resilient member 148 fixed at one end to the interior of stem 122-and mounting blade 146 at the other end of said resilient member. This mounting is shiftable with respect to the semi-fixed stem 122 and its corresponding external control knob- In order that reciprocal motion may be effected in the seal frictionally connecting lower section 102 and upper section 120, and to provide reciprocal and rotary motion for the drives 140, the respective seals 116 and 136 are lubricated with a vacuum grease. All seals in the presentv construction are preferably greased to enhance positioning-of the elements carryn'ng the seals 3 and to insure against loss of vacuum in the column of the electron microscope.

Assembly 200 as shown in Fig. 3 includes core and insert portions, the core portion identified as 200 is a generally cylindrical brass element which is adapted to t snugly within the corresponding interior of the lower portion of the pole piece beneath the bottom constriction. The core member 200 defines a cylindrical channel 202 and includes horizontal cut-out 204 at its base to receive core positioning projection 126 of the housing 100. Other areas include vertical cavity 206 for the expansion screw; annular recess 208, accomodating a circular friction seal; orifice 210 which is directed toward the leaf spring assembly 216; vertical slots 212 on opposite sides of the core extending from bottom substantially half way to the top thereof; and threaded orifice 214 at the top of the core. Insert 220 is an elongated, tubular member which is adjustable vertically with respect to the core element 200 by virtue of the internal threading circumferentially beneath the conical top of the insert 220. This threading meshes rockably with the inwardly proiecting portion 214 of the core 200. Insert 220 is spring loaded by contact of spring assembly 216, opposing contact of the control screws 218. In Figure 2, the control screws are shown within the bottom of the core 200. Upper portion 224 of the insert denes a substantially conical head having a large opening` at its top. 'I'he aperture cap 226 rests atop the insert and contains or defines the aperture for the objective lens as at 228. This cap 226 is removable, replaceable, and may comprise a housing for a disc having one or more suitable apertures. As a mie such caps are fabricated of brass and the discs of a platinum, copper or other non-magnetic metal. The remaining parts of the core and insert are preferably constructed of non-magnetics such as brass.

Referring now to Fig. 4, a modification 300 of invention is shown, wherein an adjustable ball mount for the insert 320 is illustrated, a portion of the drawing being broken away to reveal the inter-connection between insert and core member. O-rings 322 act as ball seats for the ball 324. Ball bearing 324 is threaded internally at 323 to accommodate external threading 321 of insert 320. Pressure of the O-rings against the ball prevents turning and vibratory movement to the aperture caps 326. A suitable ball retaining ring 325 is seated within the channel of core 300.

In Figure 5 selected area ditfraction device 40) includes a rotary shaft 402 to which is attached an adjusting knob 404. This rotary shaft is fixed to the cube 406. The entire assembly is adapted to be mounted in the lower portion of the housing 100, shaft 402 being in vacuum sealed contact with bore 124 for rotary and reciprocal motion. See Figures 1 and 3. Cube 406 is hollow and has four square apertures in opposed relation as shown. Corresponding aperture plates are preferably fabricated of sheet platinum or copper, the entire cube mount being preferably made of brass. It will be appreciated that should the selected area diffraction device be plunged inwardly into the path of the electron beam, as is intended in selected arca diffraction work, an ever-diminishing portion of the image of the electron miscroscope may be created on the screen by slight rotation of the cube. For instance, a square shadow cast by the uppermost of two of the apertures Within the cube 400, assuming the cube to be parallel to the plane of the screen, will be irninished in size as the cube is rotated slightly, opposite portions of the rectangular apertures casting converging shadows on the screen to present a selected square area of the magnified specimen image.

Whereas the patent to Dornfeld 2,464,382 described means for selectively inserting or removing an aperture in the electron beam path, the present invention is adapted to eect accurate positioning of an aperture within the back focal plane of the objective lens externally, and

without breaking vacuum in the electron microscope. This invention is particularly suited to externally centering an aperture in the back focal plane of the microscope. ln addition suicient excursion is provided for the aperture disc to accommodate two or more actual apertures. In this manner one aperture can be used to obtain image contrast. A second aperture in the same disc can then be selected by rotation of the adjusting screws 218. This second aperture is of suflcient diameter (.015 inch) to permit passage and study of diffraction rings or for other purposes as indicated in Dornfelds patent. It is noteworthy that this second aperture can be selected and centered at will without breaking vacuum or otherwise disturbing column elements. In other words, after use, the larger aperture can be moved out of the beam and a smaller aperture selected for greater image contrast work without breaking vacuum or disturbing the column elements. Optionally, image contrast apertures of different sizes may be located in the same disc so that apertures can be chosen to give desired image contrast as above recited.

By way of illustration, installation of the components may be effected substantially as follows. After the microscope has been aligned and the specimen removed, the intermediate bellows and intermediate lens are Iboth removed from the microscope and insert 220 is adjustably positioned within core 230 so that the lower portion of the insert is slightly below the horizontal plane of adjusting sc-rews 218. Now the insert is centered by eye within the core, manipulating the adjusting screws by hand and at this time the aperture cap 226 or the like may be placed at the tip ofthe insert. The assembled core consisting of elements 200 and 220 is placed up into the lower portion of the pole piece until the assembly stops, whereupon the core assembly is tightened in place through tightening of the expansion screw 206 effecting a lirm seal through the compression of the circular vacuum seal 209 against the Spanner nut which normally retains the lower portion of the pole piece in position, and also through expansion of the core into the bore of the pole piece. At this time, the intermediate lens of the microscope is replaced, and thereafter the telescoping housing is compressed through the use of suitable tongs engaging the studs 112. Telescoping housing 100 is placed into position in lieu of the common sylphon or intermediate bellows. Alignment projection 126 of housing 100 is carefully positioned within corresponding slot 212 of the core 200, thus the housing is centered with respect to the bottom of the objective lens. The tongs are removed from studs 112 and vacuum seal is made both to objective lens above and intermediate lens below by expansion of the elements 102 and 120 of the housing.

The thumb knobs of the assemblies at this time are plunged slowly inward with a twisting motion until blades 144 engage the slots of screws 218 on the core. The resilient construction of the inner portion of assemblies 140 permits alignment of the blades 144 with corresponding slots of core screws 218 irrespective of slight misalignment between housing 100 and core-insert assembly 200. Through rotation of the knobs careful centering of the objective aperture at the opposite end of the core-insert assembly may be effected wholly externally during operation of the electron microscope. The excursion thus given the aperture is by rotary motion, sufiieient to semi-fix a selected aperture in the path or" the electron beam.

In the modification of Figure 4 suspension of the aperture holding insert 320 in side core 300 is eifected by means of a full-threaded ball and socket connection permitting smooth operation and full excursion in all directions. Insert 220 may be adjusted all the way down for use with a wide angle pole piece, or pole piece with longer focal length.

Figures 6 and 7 illustrate suitable aperture caps for the insert. Figure 6, for instance, shows a unitary cap withv aperture and Figure 7 a cap with disc insert, the disc having at least two apertures of varying size. The excursion permitted by the invention makes a selected aperture available for different image contrast values or for diffraction studies without removal of the core and insert assembly from the microscope. Aperture 332 is enlarged (.012" to .015) over the apertures 334.

I claim:

l. In combination with an electron microscope having electro-magnetic lenses in column, a leus aperture cap positioning assembly, externally adjustable including: au insert seated axially within one lens, an externally centerable core held by the insert and in axial relation thereto, said core retaining the aperture within the back focal plane of the lens and a housing removably sealed against the bottom of the objective lens, said housing mountingY externally manipulable core positioning means, the positioning means being sealed in the housing in engageable relation to the core.

2. The device according to claim 1 in which core positioning means includes at least two externally manipulable drives.

3. The device according to claim l in which the insert engages the core pivotally in external concentric relation.

4. The device according to claim 3 in which the insert contains a ball-joint pivot said pivot being engageable with the core.

5. In combination with an electron microscope having electro-magnetic lenses in column, at least one of which includes an objective lens with pole piece, an externally adjustable aperture positioning assembly, comprising: a rigid insert fixed to the base of the pole piece and extending upwardly thereotf and an aperture positioning core impinging upon said insert and having its top extending into the back focal plane of the objective lens, aperture defining means on the top of the core, an eXpansible housing removably seated between the base of microscope objective lens and top of intermediate lens and externally manipulable core positioning means sealed in the housing in engageable relation to the core whereby motion to the bottom of the core moves the aperture delning means transversely with respect to the objective lens.

References Cited in the le of this patent UNITED STATES PATENTS

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