POLE PIECE FOR A TRANSMISSION ELECTRON MICROSCOPE

Abstract

A pole piece for a transmission electron microscope, the pole piece comprising an upper pole piece, containing a first pathway for an electron beam, a lower pole piece which is coupled to the upper pole piece and which contains a second pathway operatively connected to the first pathway, the upper pole piece and lower pole piece being separated by a gap between the first pathway and the second pathway. The pole piece has a mechanism which can extend or reduce the distance between the upper pole piece and the lower pole piece by changing the distance between the first pathway and the second pathway.

Claims

1. A pole piece for a transmission electron microscope, the pole piece comprising: an upper pole piece, containing a first pathway for an electron beam, a lower pole piece which is coupled to the upper pole piece and which contains a second pathway operatively connected to the first pathway, the upper pole piece and lower pole piece being separated by a gap between the first pathway and the second pathway, characterised in that: the pole piece comprises a mechanism which can extend or reduce the distance between the upper pole piece and the lower pole piece by changing the distance between the first pathway and the second pathway, wherein, the mechanism comprises a spacer which couples the outer part of the upper pole piece to the lower pole piece; and an anulus rotatably mounted in a substantially circular channel between the outer part and the inner part of the upper pole piece and connected to an actuator wherein rotation of the annulus causes the actuator to move the inner part to extend or reduce the distance between the upper pole piece and the lower pole piece.

2. The pole piece as claimed in claim 1 wherein the mechanism adjusts the position of the upper pole piece.

3. The pole piece as claimed in claim 1 wherein, the mechanism moves the upper pole piece towards or away from the lower pole piece.

4. The pole piece as claimed in claim 1 wherein, the upper pole piece comprises an outer part with a concentrically mounted inner part.

5. The pole piece as claimed in claim 4 wherein, the inner part is coupled to the mechanism and is moveable to extend or reduce the distance between the upper pole piece and the lower pole piece.

6. The pole piece as claimed in claim 4 wherein, the mechanism comprises a bearing mounted between a sleeve, the sleeve formed from an inner concentric surface of the outer part and an outer concentric surface of the concentrically mounted inner part.

7. The pole piece as claimed in claim 6 wherein, the bearing is actuated by a rotatable cam with the bearing acting as the follower which experiences linear motion.

8. The pole piece as claimed in claim 6 wherein, the bearing and sleeve have cooperating threads which allow the bearing to be rotated and moved linearly.

9. The pole piece as claimed in claim 1 wherein, the actuator comprises an inclined ratchet and pawl mechanism.

10. The pole piece as claimed in claim 9 wherein, the inclined ratchet comprises a stepped surface of the outer part of the upper pole piece and the pawl comprises an engaging lower surface of the annulus.

11. The pole piece as claimed in claim 10 wherein, stepped surface the stepped surface comprises a plurality of steps, the height of the steps on the stepped surface define discrete values of the size of the distance between the upper pole piece and the lower pole piece and therefore, the gap between the first channel and the second channel.

12. The pole piece as claimed in claim 1 wherein, a drive mechanism is used to rotate the annulus.

13. The pole piece as claimed in claim 12 wherein, the drive mechanism comprises a set of gear teeth mounted on its outer circumference of the annulus which are operatively connected to one or more cog which couples the annulus to a drive shaft.

14. The pole piece as claimed in claim 13 wherein, the drive shaft is manually operable.

15. The pole piece as claimed in claim 13 wherein, the drive shaft is machine operable.

16. The pole piece as claimed in claim 1 comprising a feedback mechanism to note the position of, or number of revolutions of, the drive shaft such that the separation of the upper and lower pole pieces is known.

17. The pole piece as claimed in claim 16 wherein the distance between the upper and pole piece can be visually displayed.

18. The pole piece as claimed in claim 17 wherein the visual display can be a numerical value or colour indicator.

19. The pole piece as claimed in claim 1 wherein a switch/trigger monitors the position of the upper and/or lower pole piece.

20. The pole piece as claimed in claim 1 wherein access is provided for a camera or similar device to observe the position of the upper and/or lower pole piece.

21. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

[0056] FIG. 1 is a schematic diagram of a TEM as generally known;

[0057] FIG. 2 is a schematic cross-section of a symmetrical objective-lens of a TEM including a known pole piece;

[0058] FIG. 3 is a perspective view of a known fixed gap pole piece;

[0059] FIG. 4 is a graph which plots the aberration coefficient 43 against the focal length of the spherical and chromatic aberrations;

[0060] FIGS. 5a and 5b are schematic representations of a first embodiment of the present invention;

[0061] FIG. 6 is a cut away perspective view of a second embodiment of the present invention;

[0062] FIG. 7 is a perspective view of the second embodiment of the present invention;

[0063] FIG. 8 is a perspective view of the second embodiment of the present invention;

[0064] FIGS. 9a to 9c are side views of the second embodiment of the present invention which show differing gap heights;

[0065] FIG. 10 is a graph which plots optical axis distance against magnetic flux density for a 1.5 mm pole piece gap;

[0066] FIG. 11 is a graph which plots optical axis distance against magnetic flux density for a 4 mm pole piece gap; and

[0067] FIG. 12 is a graph which plots optical axis distance against magnetic flux density for a 6.5 mm pole piece gap.

DETAILED DESCRIPTION OF THE DRAWINGS

[0068] The present invention provides a pole piece for a transmission electron microscope (TEM) which allows the pole piece gap, that is, the gap between the upper pole piece and the lower pole piece to be adjusted without the cost in time and money of disassembling the TEM to replace the pole piece with one having a different sized pole piece gap. Moreover the pole piece of the present invention can work in a vacuum when incorporated into a transmission electron microscope. In other words the upper pole piece and the lower pole piece can be adjusted in a vacuum.

[0069] FIGS. 5a and 5b are schematic diagrams which show a pole piece 51 which has an upper pole piece (UPP) 53 and a lower pole piece (LPP) 55. Pathway inside the UPP 53 illustrates the cavity running the length of the UPP at its centre where the magnetic field is concentrated. The pathway and magnetic field focusses the electrons as they travel from the TEM's electron source to its specimen stage (not shown). A similar pathway is shown in the LPP for electrons once transmitted from the specimen towards the objective and imaging lenses.

[0070] In this example of the present invention a mechanism for linear movement of the UPP 53 comprises a carriage or bearing 71 which is positioned between an outer part 61 of the UPP 53 and an inner part 59. The inner part 59 and the outer part 61 are concentric, the centre of the pathway being aligned with the centre of the UPP 53. The bearing is mounted in a sleeve 69 which forms an inner concentric surface of the outer part 61. A spacer (not shown) fixedly can connect the outer part 61 of the UPP 53 to the LPP 55 in practice. The inner part 59 is not so connected and may move linearly as shown by arrow 58 to and from the pole end of the LPP 55.

[0071] The carriage or bearing 71 may be actuated in a number of ways, for example a small cam may be mounted on the bearing which may be resiliently mounted such that rotation of the cam will cause linear movement of the bearing which will move the inner part 59 of the UPP 53 linearly as shown by arrow 58. Alternatively, the bearing 71 and sleeve 69 may have cooperating threads which allow the bearing to be rotated and moved in direction 58.

[0072] FIGS. 6 to 9 show a second and another embodiment of the present invention. In FIGS. 6 to 9 reference the same numerals are used to describe the same features when illustrated in different drawings.

[0073] FIGS. 6 to 9 show a pole piece 81 which has an upper pole piece (UPP) 93 and a lower pole piece (LPP) 85. Pathway 84 inside the UPP illustrates the cavity running the length of the UPP at its centre where the magnetic field is concentrated. A similar pathway 86 is shown in the LPP for electrons once transmitted from the specimen towards the objective and imaging lenses. Pathways 84 and 86 are accurately aligned to allow the electrons to pass through to an objective lens (not shown).

[0074] In this example, the UPP comprises an outer part 89 and an inner part 91. The outer pole part 89 is supported by a spacer 113 which connects it to the LPP 85. The inner part 91 comprises a hollow centred generally cylindrical body having a flange 105 for engagement with a coupling ring or anulus at one end with a frustoconical second end which narrows to meet the pole end of the lower pole piece 85 at the gap 90. The cylindrical surface of the inner part 91 is slidable connected to the outer part 89 at surface 93.

[0075] The coupling ring couples the inner part 91 to a drive mechanism. The outer circumference of the ring has gear teeth 107 which are operatively connected to toothed cogs 109 which, in turn are connected to a gear shaft 117, worm gear 115 and drive shaft 111. It will be appreciated that only one of the three cogs 109 can be is connected to the drive shaft. The other two cogs are free rotating and are used to stabilise the toothed ring. Any combination of cogs can be used

[0076] The coupling ring has a tiered bottom surface and is located in a channel 88 which comprises a tiered upwardly pointing surface 98 of the outer UPP 89. The top tier of the upwardly pointing surface comprises an inclined stepped surface, that is, the size of the step increases around the circumference of the channel. The cooperative engagement between the tiered bottom surface of the ring and the steps acts to change the distance between the inner UPP 91 and the LPP 85 upon rotation of the ring. The stepped shape also acts to prevent contra-rotation of the ring, in other words, the inner UPP 91 is engaged on an inclined ratcheting plane so as to lift when rotated. This lifting is then released and the inner portion locks into a new raised position.

[0077] The design achieves the required rotation and lifting of the inner UPP using the gear shaft 117 to translate mechanical motion via the worm gear 115 from the external drive shaft 111. A feedback mechanism (not shown) can be provided to cooperate with the drive shaft 111 or gear shaft 117 to note the position of, or number of revolutions of, the drive shaft such that the separation of the upper and lower pole pieces is known. Suitably the distance between the upper and pole piece can be visually displayed. The visual display can be a numerical value or colour indicator. For example, a numerical counter can be used to display the position of the upper and lower pole pieces with respect to each other. A switch/trigger can be provided to monitor the position of the upper and/or lower pole piece. It will be appreciated that the pole piece can be dimensioned to allow access by a camera or other viewing device to visually inspect the pole piece or visually display the physical position of the pole piece within a microscope.

[0078] FIGS. 9a to 9c show an enlargement of the central portion of the adjustable pole-piece 81 as shown in the embodiment of FIGS. 6 to 8 with similar reference numerals.

[0079] FIG. 9a shows the pole piece 81, with an inner UPP 91, ratchet surface in the channel of the outer UPP 89. Pathways 84 and 86 are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap 119 of 1.5 mm to the LPP 85.

[0080] FIG. 9b shows the pole piece 81, with an inner UPP 91, ratchet surface in the channel of the outer UPP 89. Pathways 84 and 86 are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap 121 of 4.0 mm to the LPP 85.

[0081] FIG. 9c shows the pole piece 81, with an inner UPP 91, ratchet surface in the channel of the outer UPP 89. Pathways 84 and 86 are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap 123 of 6.5 mm to the LPP 85.

[0082] FIG. 10 is a graph 131 of optical axis distance 133 against magnetic flux density 135 for a 1.5 mm pole piece gap. The curve 137 is a gaussian curve with a single peak. The intensity distribution as a function of the distance from the optical axis 139 is also shown.

[0083] FIG. 11 is a graph 141 which plots optical axis distance against magnetic flux density for a 4 mm pole piece gap. The curve 143 is a gaussian curve with a narrow double peak. The intensity distribution as a function of the distance from the optical axis 149 is also shown.

[0084] FIG. 12 is a graph 151 which plots optical axis distance against magnetic flux density for a 6.5 mm pole piece gap. The curve 153 is a gaussian curve with a wide double peak. The intensity distribution as a function of the distance from the optical axis 159 is also shown.

[0085] In the context of the present invention the terms ‘first pathway’ and ‘second pathway’ is used to describe a bore or channel through and between the pole pieces that allows correct operation of an electron microscope and understood by a person skilled in the art of electron microscopes.

[0086] In general, a single peak with a sharp Gaussian distribution of magnetic flux density with respect to optical axis distance would be considered to be an ideal lens, whereas at the more expanded setting the optical quality is poorer but allows a spectrometer to be closer to the sample or larger sample tilts.

[0087] In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

[0088] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.