WO2004005904A1

WO2004005904A1 - Conical specimen holder engaging mechanically a heating element comprising a conical bore

Info

Publication number: WO2004005904A1
Application number: PCT/GB2003/002947
Authority: WO
Inventors: Peter Rockett
Original assignee: Isis Innovation Limited
Priority date: 2002-07-05
Filing date: 2003-07-07
Publication date: 2004-01-15
Also published as: GB0215623D0; AU2003281293A1

Abstract

A specimen stage for providing accurate positional control of the specimen and accurate thermal control. Positional control is provided by mounting the specimen mount on a spindle, rotatable about its axis and moveable laterally and axially. The specimen spindle has a conical surface which engages a thermal control block which floats with a small downward pressure on the specimen spindle. The thermal control block is counter balanced and mounted on an unrestrained 3-axis slide. The thermal control block includes heaters and coolers to control the temperature of the specimen. The specimen spindle includes a thin-walled tube as a thermal isolator, and is in turn connected to a rotary servo and encoder and linear servo motors. Thus, thermal control is provided by the thermal block, which is independent of the motion control provided by moving the specimen spindle.

Description

CONICAL SPECIMEN HOLDER ENGAGING MECHANICALLY A HEATING ELEMENT COMPRISING A CONICAL BORE

The present invention relates to a specimen stage for supporting a specimen in a controllable position and in a controllable thermal state. In many techniques of examination and analysis it is necessary to hold a specimen for examination or analysis in a controllable position. Furthermore, it is often useful to be able to control the temperature or thermal environment of the specimen, so that it can be analysed or inspected at a desired temperature, or perhaps at different temperatures, e.g. while being heated or cooled. Thermally controllable specimen stages for fulfilling this function may typically be found in apparatus using electron beams, x- rays, u.v. or other forms of electromagnetic radiation.

As an example, Figure 1 of the accompanying drawings illustrates typical apparatus used for computed x-ray tomography which consists of an x-ray source 1 producing a divergent beam of x-rays 3 which passes through a specimen held by a specimen mount 7 which is part of a specimen stage 5. The x-rays which have passed through the specimen are detected by a large area detector 9. In the use of such apparatus the specimen is rotated about its axis, and possibly also displaced vertically (i.e. in the z-direction) to produce a series of two-dimensional images which are projections at different angles through the specimen. These two-dimensional images are then used mathematically to reconstruct a model of the internal structure of the object. In order that such a reconstruction process is accurate, it is necessary that the specimen is moved in a precise, controllable and repeatable way. For example, typically a rotational accuracy of 0.001 ° and a linear accuracy of about 0.1 μm are required. Further, typically the temperature of the specimen must be controlled to within at least 0.1 ° C during the acquisition of the projection data and these conditions should persist over a period of about 30 minutes for a simple rotational scan cycle, during which of the order of 720 image frames may be recorded. These requirements form a particularly stringent set of conditions for the design of the specimen stage.

It may also be appreciated from Figure 1 that the image magnification achieved by the use of the divergent beam is dependent upon the distance between the specimen and the x-ray source. Minimising this distance gives the possibility of a high magnification, with a corresponding increase in resolution. Therefore the specimen stage needs to be designed so that the specimen can be positioned close to the x-ray source.

Similar considerations regarding the compactness of the design, the precise and repeatable nature of the specimen movement and strict temperature control also arise in other imaging modalities.

Accordingly, the present invention provides a specimen stage for supporting a specimen in a controllable position and in a controllable thermal state, in which a thermal control stage is uncoupled from the motion stage so that these two functions can act independently. This is achieved by allowing the motion stage to support the specimen and engage mechanically with the thermal stage, but to allow the thermal system to follow passively the movements of the motion control system.

The thermal system applies only the weakest of constraints to linear displacement of the motion control system and minimum resistance to rotation of the motion system. The thermal control stage may be arranged to follow passively linear movement of the motion stage, and the motion stage is allowed to rotate relative to the thermal control stage. Preferably the thermal control stage floats free to move linearly in three orthogonal directions in response to linear movement of the motion stage, for example by being supported on an unrestrained three axis linear slide. The thermal control stage may be counterbalanced so as to float with a small downwards force in order to engage positively with the motion system. Further, alignment of the thermal control stage and the motion stage may be achieved by mounting the thermal control stage via a pitch/roll coupling.

The mechanical engagement between the motion stage and the thermal control stages preferably via mutually contacting, rotationally symmetric convex and concave elements such as a cone shaped projection, e.g. on the motion stage, and a cone shaped bore, e.g. in the thermal control stage. This cone-on-cone contact, for example at a cone angle of about 10°, provides solid location and engagement of the two parts, while keeping mechanical contact stiction and friction within acceptable bounds. It also provides for good thermal contact. The projection on the motion stage may include a specimen mount, which again may be a component releasably received in a conical cavity at the end of the projection.

The motion stage may include a thermal isolation element, such as a section of thin- walled tube, to isolate thermally the specimen mount from the actuators in the motion stage.

Preferably the thermal control stage comprises a thermal block and thermal control elements, such as heaters and coolers, preferably with a temperature sensor, for controlling and changing the temperature of the thermal block, and thus the specimen.

The invention will be further described by way of example with reference to the accompanying drawings in which: -

Figure 1 is a schematic view of x-ray-tomography apparatus including a specimen mount in accordance with one embodiment of the invention;

Figure 2 is a schematic diagram of a specimen stage in accordance with an embodiment of the invention;

Figures 3 a and b are schematic side and rear views of the thermal control stage in the embodiment of Figure 2; Figure 4 is a schematic cross-sectional view of the spindle assembly of the embodiment of Figure 2;

Figure 5 is a schematic cross-sectional view of the specimen spindle of the embodiment of Figure 2;

Figure 6 schematically illustrates specimen mounts usable with the embodiment of Figure 2; and

Figure 7 schematically illustrates the control system for the servo motors in the motion stage of Figure 2.

Figure 2 illustrates schematically a side view of a specimen stage in accordance with one embodiment of the invention. The stage is configured to expose a sample S held in a specimen mount 7 to an x-ray beam 3. The specimen mount 7 is releasably held in a conical aperture 21 in a projection 23 forming the end part of a specimen spindle 25 (illustrated in Figure 5). The projection 23 is, in this embodiment, made from copper in order to achieve good thermal contact with the thermal control stage (to be described later). The specimen spindle 25 includes a thermal isolation element 27 formed by a cryogenic thin-walled stainless steel tube. This, in turn, is connected to a base portion 29 which includes a flange 29a allowing it to be clamped directly to the spindle 31 of a rotary encoder 30 by means of a clamping ring 29b (see Figure 4).

Connecting the specimen spindle directly to the rotary encoder shaft in this way avoids the need for an additional drive shaft and coupling which would introduce inaccuracies and drive perturbations. As illustrated in Figure 4 rotational drive is supplied from a motor 33 A to the encoder shaft 31 via a servo drive pulley 33. A variety of specimen mounts 7 may be used depending on the type of specimen.

Two examples are shown in Figure 6, the mount 7a comprising a thin- walled beryllium tube into which unstable specimens can be inserted, and mount 7b comprising a copper platform to which stable specimens can be lightly bonded. The specimen mounts 7 have a conical base to ensure good thermal contact and alignment in the conical cavity 21. Vertical (z-axis) and lateral (x-axis) movement of the specimen is achieved by respective vertical and lateral servo-drives moving a platform 35 to which the rotary encoder 30 is attached. Preferably the lateral and vertical servo drives have an accuracy of 0.1 μm or better and the rotational servo-drive, which drives the drive pulley 33 has an accuracy of 0.001 °. A vertical (z) movement of about 100 mm is preferable in this embodiment to facilitate ease of specimen mounting and spiral/helical scanning, and a lateral (x) movement of about 40 mm to allow good specimen alignment in the x-ray beam. As shown in Figure 7 the servo motor controllers may comprise a servo motor 70 which receives its drives signals from a programmable servo controller 74 via a servo amplifier 72. The position is monitored by an incremental encoder 76 whose output is passed to the servo controller 74. The programmable servo controller 74 may, in turn, be controlled via a computer using, for example, an RS 232 link. The programmable servo controller is preferably a 3 -axis servo controller which uses the incremental encoder feedback to monitor position, speed and direction. 3 -term (PID) positional feedback control can be set for optimum response and accuracy. With this arrangement motion commands in ASCII format can be executed in real time or loaded for programmed operation. Thermal control of the temperature of this specimen and of its environment is provided by a thermal control stage 40 which includes a thermal block 42, in this embodiment made from copper, which, in operation, floats with a small downwards force on the conical surface of the projection 23 on the specimen spindle of the motion stage. The thermal block 42 includes a conical bore 44 which receives the projection 23 of the specimen spindle and controls the temperature of the specimen mount through thermal conduction across the conical contact surface. Clearly other rotationally symmetric configurations could be used, such as a curved, e.g. hemispherical, contact surface.

The contact bore 44 is offset in the thermal block 42 towards the direction of the x-ray source in order that the specimen can be placed as close as possible to the x-ray source. The use of a narrow cone angle of about 10° for the contact surface of projection 23 and bore 44 also assists this location of the specimen by reducing the diameter of the cone base. However too small cone angles would tend to lead to larger vertical errors through machining tolerances, and also increased stiction and jamming. The thermal block 42 includes a tapering x-ray passage 46 which, in this embodiment, allows a maximum cone-beam of 10° to emerge. In this embodiment the thermal block 42 is 40 mm by 40 mm in side-view, 40 mm by 30 mm in rear-view and can accommodate a maximum specimen volume of approximately a 7 mm cube.

The temperature of the thermal block 42 is controlled by means of heaters 48 visible in Figure 3b, which may be 16W cartridge heaters together with two 70W thermoelectric coolers 49 which dump heat to fluid-cooled heat-exchangers 50. An environmental chamber (not shown) with thin beryllium x-ray path windows, encloses the thermal control stage and a controlled dry gas bleed prevents water droplet condensation. Platinum resistance thermometers (not shown) monitor the temperature of the block 42 in two locations, one near the specimen cavity at the top of the block and the other at the top of the cone near the specimen mount. A tunable PID controller, which is RS 232 linked to a PC, controls the temperature to the desired set point to an accuracy of 0.1 °C. The set point temperature can be programmed so that precise thermal cycling is available, or manually entered for step variation. In this embodiment the working temperature range is ± 50 °C with a thermal response of about 0.2 °C per second. The illustrated assembly of thermal block 42, thermoelectric coolers 49 and heat exchangers 50 is held together under compression by disc springs (not shown) confined with a restraining frame (not shown). This arrangement allows the free expansion and contraction of the assembly and avoids damage to the thermoelectric coolers. Clearly additional cooling or heating can be included to increase the temperature range provided. The thermal block 42 (together with heaters and coolers and heat exchangers) is attached to an unrestrained 3-axis miniature slide 52 which permits free motion within a 40 mm cubic space in response to movement of the specimen spindle 25. The thermal block assembly is also counterbalanced through a sprocket and chain mechanism 54 to a counterbalance weight 56 and the thermal block assembly and counterbalance mechanism are both attached via a pitch/roll coupling 58 to a support frame 60. The pitch/roll coupling 58 allows angular alignment of the thermal block to the specimen spindle, the specimen spindle then being driven vertically to lift the thermal block, thus providing a good contact between the projection 23 and conical thermal contact surface 44. The use of the 3-axis slide 52 and counterbalance 56 means that the thermal block provides very little restraint on the motion stage, while the conical contact between the projection 23 and surface 44 allow virtually free rotation of the specimen spindle relative to the thermal control stage.

Claims

1. A specimen stage for supporting a specimen in a controllable position and in a controllable thermal state, the specimen stage comprising a motion stage for controlling the position of the specimen and a thermal control stage for controlling the thermal state of the specimen, the thermal control stage engaging mechanically the motion stage and being adapted passively to follow movement of the motion stage.

2. A specimen stage according to claim 1 , wherein the thermal control stage is arranged passively to follow linear movement of the motion stage.

3. A specimen stage according to claim 1 or 2, wherein the motion stage is arranged to provide rotational movement of the specimen relative to the thermal control stage.

4. A specimen stage according to claim 1, 2 or 3, wherein the thermal control stage floats free to move linearly in three orthogonal directions relative to a support in response to movement of the motion stage.

5. A specimen stage according to claim 4, wherein the thermal control stage is connected to said support via a 3-axis linear slide to provide said free linear movement.

6. A specimen stage according to claim 4 or 5, further comprising a counterbalance mechanism mounted on said support for counterbalancing the weight of the thermal control stage.

7. A specimen stage according to claim 4, 5 or 6 wherein the thermal control stage is connected to said support via a pitch/roll coupling to allow relative angular alignment of the thermal control stage and the motion stage.

8. A specimen stage according to any one of the preceding claims wherein the motion stage provides vertical and lateral movement and axial rotation of the specimen.

9. A specimen stage according to any one of the preceding claims wherein the motion stage engages the thermal control stage via mutually contacting rotationally symmetric convex and concave elements.

10. A specimen stage according to claim 9 wherein the mutually contacting rotationally symmetric convex and concave elements comprise mutually contacting conical surfaces.

11. A specimen stage according to claim 9 or 10 wherein the motion stage comprises a conically tapering projection as one of said elements and the thermal control stage comprises a corresponding conically tapering bore as the other of said elements, said bore receiving said projection.

12. A specimen stage according to claim 9, 10 or 11 wherein the projection has at its end a specimen mount.

13. A specimen stage according to claim 12 wherein the specimen mount is releasably received in an aperture at the end of the projection.

14. A specimen stage according to claim 13 wherein the aperture for receiving the specimen mount is conical.

15. A specimen stage according to any one of the preceding claims wherein the motion stage comprises a thermal isolation element.

16. A specimen stage according to claim 15 wherein the thermal isolation element comprises a thin-walled tube.

17. A specimen stage according to any one of the preceding claims wherein the motion stage is arranged to lift the thermal control stage to maintain said mechanical engagement.

18. A specimen stage according to any one of the preceding claims wherein the thermal control stage comprises a thermal block and thermal control elements for changing the temperature of the thermal block.

19. A specimen stage according to claim 18 wherein the thermal control elements comprise at least one of a heater and a cooler.

20. A specimen stage according to claim 18 or 19 wherein the thermal block substantially surrounds said specimen, said specimen being positioned, in use, in a through-passage in said thermal block.

21. A specimen stage according to claim 20 wherein the motion stage engages the thermal control stage in a bore which intersects said through-passage.

22. A specimen stage constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.