GB2076989A - Specimen holder - Google Patents

Abstract

A specimen holder of cylindrical configuration defines axially spaced end faces (12, 14) and an axially aligned aperture (18) extending between these faces. In one aspect, the holder is comprised of two discrete part-annular electrically conductive components (10a, 10b) separated by circumferentially spaced gaps (11) which contain electrically insulating adhesive binding the components (10a, 10b) together. In a secondary aspect, the holder is provided on its outer cylindrical surface (16) with a pair of diametrically opposite, mutually co- axial dimples (20, 21) and, at a location arcuately spaced from both dimples, with a groove (30) extending ing normally to an axial plane of the holder. <IMAGE>

Description

SPECIFICATION Specimen holder This invention relates to specimen holders for use in electron instruments such as, for example, electron microscopes and electron diffraction cameras.

To view or photograph specimens in eiectron microscopes or electron diffraction cameras it is frequently desirable to tilt the specimen by use of very small manipulating apparatus located within the microscope or camera. It is also desirable for some purposes to vary the temperature of the specimen. With this in mind, it has in the past been proposed to thermally couple a holder for the specimen to a bath of liquid nitrogen to reduce the specimen holder's temperature and to locate the specimen on a grid which forms part of the specimen holder and which can be heated by passing an electrical current through it.

In conventional electron microscopes or diffraction cameras, the objective lens pole piece is of cylindrical symmetry with a central bore of very small diameter normally of the order of 6mm. It is essential to incorporate the tilting mechanism within the focal plane of this pole piece so that the specimen can be tilted without sacrificing the resolution of the microscope. Accordingly space within the pole piece is at a premium and the tilting mechanism must be of conforming dimensions.

In Australian Patent Specification No.

291,412 there is disclosed a mechanism for supporting and tilting a specimen in an electron microscope or diffraction camera. The mechanism comprises a specimen holder supported at its periphery by three arms, two of which are positioned at opposite sides of the holder. The specimen holder is supported above a hollow conical stem with the arms extending down the outer periphery of the stem. Actuation means is provided to effect vertical movement of one or more of the arms to cause the specimen holder to pivot about an axis extending through the two opposite arms at their point of contact the periphery of the specimen holder and/or to cause tilting of such axis. The specimen holder can thereby be tilted to any inclination.

The specimen holder disclosed in Australian Patent 291,412 comprises a milled block in two body parts constructed of thermally and electrically conductive material such as brass and secured so that they are insulated from one another. A fine mesh grid is arranged to extend across the space defined by the arms of the 'U' of the base part, being wedged in a dovetail formed in the body parts of the holder. The specimen is positioned on this grid. At three positions on the periphery of the holder are provided outwardly extending conical lugs which are arranged to be located in apertures provided at the ends of the manipulator arms.

A number of disadvantages have been encountered with the specimen holder disclosed in the abovementioned patent specification.

Of primary concern is the difficulty and con-.- plication of manufacture. Milling of the two minute parts from a block of brass is relatively complex, especially when forming the dovetail to receive the grid. Positioning of the projecting conical lugs on the periphery of the body portions requires very accurate assembly on a mandrel, since two of the conical lugs determine the axis about which the specimen holder pivots. Location of the grid within a dovetail by wedging, clamping or the use of screws tends to introduce strains in the grid which, on heating of the grid, can result in an undesirable image drift or, in certain circumstances, can cause the grid to fail.The allowable tilt angle of the specimen may be undesirably decreased due to obstruction and the electrical resistance of contact with the grid may be increased.The specimen holder must be frequently cleaned, normally by ultrasonic techniques.

A specimen holder designed to resolve at least some of these difficulties is disclosed in applicant's co-pending Australian patent application 49157/79. This holder comprises a pair of sheet metal components bonded in superposition by an electrically insulating achesive. One component is divided to provide electrically isoiated sub-components across which the grid is spot-welded when the holder is in use. The conical lugs of the earlier holder are replaced by two circular and one slotted aperture provided in flanged edges of one oother of the sheet metal components.

It is found in practice that, even with judicious handling, the mentioned sub-components may in some circumstances break free of the insulating adhesive.

Another known specimen holder which is suited to apparatus when heating of the specimen is not required, is composed of an elongate sample mounting tube having an external screw thread at one end for mounting in the instrument. The sample is retained on a grid at the other end by a complementary cap which firmly fits over the end of the tube. This arrangement is, of course, not suitable for use with the versatile manipulating mechanism disclosed in the aforementioned patent speclfi- cation 291,412, and indeed the orientation of the sample can only be adjusted to a minor extent, by movement of the threaded end of the mounting tube.

It is accordingly an object of the invention to provide an improved specimen holder which is easy to manufacture, reliable in assembly, and yet preserves versatility of manipulation by being co-operable with mechanism of the kind disclosed in patent specification 291,412.

The inventipn provides a specimen holder of cylindrical configuration, defining axially spaced end faces and an axially aligned aperture extending between said faces, which holder is provided on its outer cylindrical surface with a pair of diametrically opposite, mutually coaxial dimples, and, at' a location arcuately spaced from both dimples, with a groove extending to an axial plane of the holder.

The holder is thereby arranged in use to be supported on three sides in an electron instrument by the arms Df a manipulator mecha .nism. The two dimples are arranged to act as receptacles for protrusions formed on repective arms of the manipulator mechanism, and are mutually coaxial to define a pivot axis about which the specimen holder may be tilted in use.

The invention also provides-a specimen holder of cylindrical configuration, defining axially spaced end faces and an axially aligned aperture extending between said faces, the holder being comprised of two discrete partannular electrically conductiive components separated by circumferentially speced-gaps which contain electrically insulating adhesive binding the components together.

A protrusion formed on a third arm of the manipulator mechanism is arranged to be located in the slot.

According to one embodiment, the specimen holder is comprised of two discrete partannular electrically conductive components separated by circumferentially spaced gaps which contain electrically insulating adhesive binding the components together. The gaps are preferably substantially filled with electri cally insulating adhesive and are advantageously disposed at a finite angle, say 30 to 60 to an axial plane and to said end faces of the holder. For enhanced strength and longer useful life, the holder may include a separate dr contiguous filling of adhesive in a narrow slit formed in the cylindrical surface of the holder extending parallel to said end-faces.

Such filling may also assist in reducing rigidity and allowing absorbence of bending moments.

In an alternative embodiment, the specimen holder is comprised of two discrete parts, being an annular body part, and an apertured cap therefor, which cap includes an arcuate skirt dimensioned to fit firmly about the annular body part and an annular portion arranged to axially overly the annular body part. The dimples may be provided as apertures in the skirt of the cap, and the groove may be provided in the annular body part, the skirt being, cut-away to expose the groove.

For use in electron instruments, the holder may be provided with a sample mounting grid. Such a grid may be spotwelded to the holder across said aperture, either to an internal annular shoulder so as to contain said pivot axis, or to one of said end faces. As employed herein and as generally understood in the art, the term "specimen holder" does not require the existence of a grid across the holder.

The present invention also embraces a manipulator device for use in a scanning electron microscope or electron diffraction camera, incorporating the specimen holder described above.

The invention still further extends to the manufacture of the inventive specimen holder from a length of metal rod, wherein the holder1 or in the case of the alternative embodiment, the annular part, is formed as an integral end portion of the rod and then severed therefrom.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a greatly magnified perspective view from one position of a first embodiment of specimen holder in accordance with the present invention, suited to applications where heating of the sample is desired; Figure 2 is a perspective view of the holder from another position; Figure 3A is a view similar to that of Fig 2.

but showing the sample mounting grid in one of its possible positions; Figure 3B is an underside view showing the sample mounting grid in an alternative position on the holder; Figure 4 is a plan view of the bolder as seen in Figs. 1 and 2; Figures 5 and 6 are respective cross-sections on the lines 5-5 and 6-6 in Fig.4; Figure 7 depicts, in side elevation, an intermediate stage in the manufacture-of the holder shown in Figs. 1 to 6; Figure 8 is a partially cut-away perspective view of a second embodiment of specimen holder iri accordance with the invention, which does not make allowance for heating of the sample; Figures 9A and 9B are different views of the cap comprising one part of the holder shown in Fig. 8;and Figure 10 is a view of the annular part of the holder shown in Fig.8.

The holder 10 illustrated in Figs. 1 to 6 is of cylindrical configuration, defining an external cylindrical surface 16, axially spaced endfaces 12, 14, and an axially aligned aperture 18 extending between these faces. The cylin-' drical surface 16, typically of diameter between 2.5 and 3.5mm is provided with dia metricaily opposite, substantially identical, circular dimples 20, 21 which have concave bottoms 22, 23 and are oppositely located symmetrically with respect to a diameter 24 (Fig. 4) midway between end-faces 12, 14, The concave bottoms of dimples 20, 21 ensure that matching protrusions on manipulator arms make optimum physical, and thereby electrical, contact by resting on the rims of the dimples without touching their bottoms.

At one end of a diameter 25, which is coplanar with but normal to diameter 24, surface 1 6 is flattened at 28 and traversed by a groove 30 which is of greater extent than flat 28 circumferentially of the holder. Groove 30 is arcuately spaced from, and indeed midway between, dimples 20, 21, and extends normal to an axial plane of holder 10.

Holder 10 is comprised of two discrete partannular metal components 1 0a, 1 0b separated by circumferentially spaced gaps 11 which extend between end-faces 12, 14 and contain electrically insulating adhesive 32 binding'the components together. The groove 30 is wholly in one component, 1 0a, while the dimples are arranged one in each. Each gap 11, which may, for example, be 0.1 mm wide, is filled with adhesive and is disposed at finite agles to an axial plane and to end-faces 12, 1 4. These angles, which may be between 30 and 60 , are effective to increase the total interface area between the adhesive and the metal and to confer a degree of strength to the assembly.

Extending between circumferentially spaced positions 38, 39 on the exterior of the holder is a slot which extends parallel to and midway between end-faces 12, 1 4. Slot 40 permits some axial give in the holder without risk that It will fracture under stress. It is typically filled with adhesive contiguous with adhesive 32 as is a lateral hole 41 formed in the opposite side of the holder. Adhesive in gaps 11, slot 40 and hole 41 co-operates to strongly bind the components together and to key them against relative slipping, either parallel to or normal to the axis of the assembly.

A counterbore 1 9 in end face 1 2 determines an annular shoulder 1 7 within aperture -l 8. Shoulder 1 7 is co-planar with diameters 24, 25 and thus with the centres of dimples 20, 21 and with the centre bottom of groove 30. As is evident from Figs. 1, 2 and 4 in particular, it also bisects slot 40 and hole 41 and is therefore partly defined by surfaces 40a and 41a, respectively, of the adhesive therein.

Within reference to Fig. 7 the illustrated holder may be readily and simply manufactured from a length of solid rod 7 of the chosen metal, typically of diameter 2.5 to 3.5mm. One end of the rod is axially drilled to form a blind bore 7a which subsequently gives rise to aperture 1 8. The axial extent of the holder is defined by forming a circumferential groove 43 in the rod. Counterbore 1 9 and dimples 20, 21 are provided in the bored end portion of the rod, followed by formation of flat 28 and groove 30. This end portion is then divided by forming slot 11; circumferential slot 40 and hole 41 are then curt to arrive at the stage of manufacture depicted in Fig.

7. Uncured adhesive is introduced into slots 11 and 40, and into hole 41, and allowed to set, thus coupling the spaced portions of metal. Slots 11, 40 may typically be 0.1 mm wide.

The final step of the operation is to sever the bored end-portion from the rod, resulting in a holder similar to that depicted in Figs. 1 to 6. It will be appreciated that the holder only now appears as a discrete entity: performance of the earlier steps has been greatly facilitated by its continued integrity with the rod as a whole. In particular, application of the adhesive has not required alignment and clamping of wholly separate, very small metal components. It has been found, moreover, that the holder is more mechanically stable and robust than known units, with significantly improved resistance to bond failure.

For use in electron instruments, holder 10 is provided with a fine mesh grid 80 (Fig.3A) of, for example, palladium or platinum, which in most cases is spot-welded to the annular shoulder 1 7 to bridge aperture 1 8 and electrically connect components 1 Oa, 1 0b otherwise electrically isolated by adhesive 32. Grid 80 is typically welded to the components 1 0a, 1 Ob by means of a very fine carbon electrode and in use supports the specimen under inspection.

In situ, the specimen holder 10 is fitted to a three arm manipulating mechanism which functions in a similar manner to that disclosed in Australian patent 291,412. However, the arms of the mechanism are instead provided with outwardly projecting phosphor-bronze hemi-spherical protrusions which respectively locate in the opposed dimples 20, 21 and in the groove 30 of the specimen holder. The holder can be variously tilted by selective actuation of the arms. In particular, dimples 20, 21 define a tilt or pivot axis co-incident with diameter 24. This axis, and the protrusion centerline at groove 30, can be located substantially in the plane of the shoulder 1 7 and thus substantially in the plane of the grid (80 in Fig.3A). In this way, translational movement during tilt of the sample is minimised.However, in some cases, greater clearance from the objective aperture is required.

To achieve this additional clearance, the grid may be secured to the under end face 14 of an inverted holder, rather than to shoulder 17, as shown at 82 in Fig. 3B. The latter arrangement is not suited to a case where it is desired to heat the sample but greatly facilitates the application of carbon film for specimen support, where such is required.

As previously foreshadowed, the arms of the manipulator mechanism are arranged for transmission are arranged for transmission of an electrical current through the phosphorbronze protrusions to the components 1 0a, 10b, the circuit being completed by the grid 80 which is selected to act as a resistance heater to enable the specimen positioned thereon to be heated. Furthermore, as dis closed in Australian patent 291,412, the arms df the manipulator may be thermally coupled to a source of liquid nitrogen via the base of the manipulator so that the surrounding body of the specimen holder can be cooled. An anti-contamination cap (not shown) is typically' positioned over the specimen holder when in situ.This cap is screwed to the top of a conical stem ofthe manipulating mechanism and is provided with a small aperture to allow passage of an electron beam In cases where it is not required to heat the sample, the electrically divided construction already described is unnecessarily complex and it is possible to employ the modified holder 10' depicted in Figs. 8 to 10, in which like reference numerals refer to like elements.

Holder 10' is formed in two discrete parts, an annular body part 50 and an apertured cap 52 (which is distinct from the anti-contamination cap mentioned above). Body part 50 defines an external cylindrical surface 16', axially spaced end-faces (one only illustrated at 12'), and an axially aligned aperture 18' extending between these faces. Cap 52 includes an annular portion 54 arranged to axially overly the end face 12' of the body part 50, and an arcuate skirt 56 dimensioned to fit firmly about cylindrical surface 16'. Skirt 56 extends for substantially greater than 180 of arc, and annular portion 54 is truncated at 55 on a chord bridging the ends of skirt 56.

In this case, dimples 20' 21' are provided as apertures in skirt 56, while groove 30' is formed in a flat 28'on annular body part 50, in a manner similar to the first-described em embodiment In proper operative position, which may be determined by keying means (not shown) between the cap and body part, groove 30' is exposed between the separated ends of skirt 56 and is, arcuately midway between dimples 20', 21', and in any event extends normally to an axial plane of holder 10'.

With this second embodiment, the grid 80' (Fig. 8), which has a solid rim 12a, is mounted to end face 12' so as to extend across aperture 18', and is held in place by overlying cap 52. Face 12' is dished as shown to enhance its engagement with cap 52 at the resultant pronounced rim 58 on the body part. The annular portion 54 of cap may be correspondingly dished.

Claims (24)

1. A specimen holder of cylindrical confi duration, defining axially spaced end faces and an axially aligned aperture extending between said faces, which holder is provided onits outer cylindrical surface with a pair of diametrically opposite, mutually co-axial dimples, and, at a location arcuately spaced from both dimples, with a groove extending normally to an axial plane of the, holder.

2. A specimen holder according to claim 1, comprised of two discrete part-annular e'ec- trically conductive components separated b9 circumferentially spaced gaps which contain electrically insulating adhesive binding the components together.

3. A specimen holder according to claim 2, wherein each of said gaps is disposed at a finite angle to an axial plane and to said end faces of the holder.

4. A specimen holder according to claim; 3, wherein said finite angle is in the range 30 to 60 .

5. A specimen holder according to claim 2, 3 or 4, further including a separate or contiguous filling of adhesive in a narrow s it formed in the cylindrical surface of the holcler extending parallel to said end-faces.

6. A specimen holder according to any one of claims 2 to 5, further including a radial or approximately radial hole in said holder, which hole intersects one of said gaps and is filled with insulating adhesive.

7. A specimen holder according to claim 1, comprised of two discrete parts, being an annular body part, and an apertured cap therefor, which cap includes an arcuate skirt dimensioned to fit firmly about the annular body part and an annular portion arranged to axially overly the annular body part.

8. A specimen holder according to claim 7, wherein the dimples are provided as apertures in the skirt of the cap, and the groove is provided in the annular body part, said skirt being positioned to expose the groove.

9. A specimen holder according to claim 7 or 8, wherein at least one of the end faces of the annular body part is dished.

10. A specimen holder according to an' one of claims 7 to 9, in combination with E grid mounted to one of said end faces so a; to extend across said axially aligned aperture, the cap being placed so that the annular part thereof immediately overlies the grid.

11. A specimen holder of cylindrical configuration, defining axially spaced end faces and an axially aligned aperture extending between said faces, the holder being comprised of two discrete part-annular electrically con ductile components separated by circumferentially spaced gaps which contain electrically insulating adhesive binding the components together.

1 2. A specimen holder according to claim 11, wherein each of said gaps is disposed at a finite angle to an axial plane and to said end.

faces of the holder.

1 3. A specimen holder according to claim 12, wherein said finite angle is in the range 30 to 60 .

14. A specimen holder according to claim 11, 12 or 13, further including a separate or contiguous filling of adhesive in a narrow slit formed jn the cylindrical surface of the holder extending parallel to said end-faces.

1 5. A specimen holder according to any one of claims 11 to 14, further including a radial or approximately radial hole in said holder, which hole intersects one of said gaps and is filled with insulating adhesive.

1 6. A specimen holder according to any preceding claim, wherein said groove extends across a flat formed on the cylindrical surface of the holder.

1 7. A specimen holder according to any preceding claim, formed with an internal annular shoulder for mounting a grid in a diametral plane containing the co-axis of said dimples.

1 8. A specimen holder according to claim 17, in combination with a grid mounted on said shoulder to extend across said axially aligned aperture.

1 9. A specimen holder according to any preceding claim, wherein said dimples and groove extend for a substantial portion of the axial dimension of said holder.

20. A specimen holder according to any preceding claim, wherein the dimples and groove are of substantially equal width.

21. A specimen holder according to any preceding claim, where the diameter of the holder is substantially greater than its axial length.

22. A manipulator device for use in a scanning electron microscope or electron diffraction camera, incorporating a specimen holder according to any preceding claim.

23. A method of manufacturing a specimen holder according to any preceding claim, wherein the holder, or said annular part thereof, is formed as an integral end portion of the rod and then severed therefrom.

24. A specimen holder substantially as hereinbefore described with reference to Figs.

1 and 6 or 8 to 10 ot the accompanying drawings.