Positioning and cooling system

Abstract

A positioning stage for positioning and cooling an electronic device, such as a detector in an electron microscope, is disclosed. The positioning stage includes one or more thermally conductive slidin

Description

The present invention relates to positioning and cooling an electronic device, such as a detector of an electron microscope. In transmission electron microscopy, an electronic detector can be used to detect electrons of an electron beam transmitted through a sample, with the electron beam, sample and detector being arranged in vacuo. An electronic detector will typically produce heat as a by-product, and so require cooling. However, it can be challenging to provide adequate cooling in a vacuum where cooling by convection or forced air is not possible. The space constraints of an electron microscope can present further challenges, as it is usually desirable to be able to move the detector between an “inserted” position in the electron beam path and a “retracted” position out of the electron beam path, e.g. so as to allow different detectors to be positioned in the electron beam path. Known approaches to cooling a retractable detector arranged in a vacuum chamber of a transmission electron microscope include liquid cooling using flexible pipes, and the use of copper heat straps. However, these approaches can be complex and susceptible to damage, e.g. by repetitive detector motion. This can lead to damage to the detector and to the electron microscope. Furthermore, these approaches can be difficult to implement in a restricted space, and can introduce a resistance to motion which can necessitate the use of relatively large actuators. The inventors believe that there remains scope for improvements to positioning and cooling an electronic device, such as a detector of an electron microscope. According to an aspect of the present invention, there is provided a positioning stage for positioning and cooling an electronic device; the positioning stage comprising: a base assembly comprising one or more heat transfer devices; and a platform assembly that is moveable with respect to the base assembly between at least a first position and a second position, wherein the platform assembly comprises a thermally conductive platform configured to support the electronic device, and one or more thermally conductive sliding contacts in thermal communication with the thermally conductive platform; wherein the positioning stage is configured such that the or each thermally conductive sliding contact slides along a heat transfer device of the one or more heat transfer devices as the platform assembly moves between the first position and the second position such that the or each thermally conductive sliding contact is in thermal communication with the thermally conductive platform and in thermal communication with a heat transfer device of the one or more heat transfer devices when the platform assembly is in the first position and when the platform assembly is in the second position. The present invention provides a positioning stage that is configured to both position and cool an electronic device, such as a detector of an electron microscope. To facilitate positioning of the electronic device, the positioning stage includes a platform assembly that is moveable with respect to a base assembly between at least a first position and a second position. The platform assembly includes a platform for the electronic device, to allow the electronic device to be positioned in at least first and second positions. For example, and in embodiments, the first position may correspond to a retracted position in which a detector supported by the platform is positioned out of an electron beam path of an electron microscope, and the second position may correspond to an inserted position in which the detector is positioned in the electron beam path of the electron microscope. The positioning stage of the present invention is further configured to cool an electronic device that the platform is supporting. To facilitate this, the platform is thermally conductive, e.g. and in embodiments, such that the platform can thermally communicate with (e.g. transfer heat from) the electronic device it is supporting. Furthermore, the base assembly is provided with one or more heat transfer devices that are, e.g. and in embodiments, configured to transfer heat away from the platform assembly. Then, to enable heat to be transferred from the thermally conductive platform to the one or more heat transfer devices (and thus away from the platform assembly), the positioning stage is provided with one or more thermally conductive sliding contacts. A (each) thermally conductive sliding contact is configured to slide along a (respective) heat transfer device of the base assembly as the platform assembly moves between the first and second positions. In embodiments, a (each) thermally conductive sliding contact thereby remains in thermal communication with the thermally conductive platform and with a (respective) heat transfer device of the base assembly as the platform assembly moves between the first and second positions. The one or more thermally conductive sliding contacts can thereby enable heat to be transferred from the thermally conductive platform to the one or more heat transfer devices (and thus away from the platform assembly), while permitting movement of the platform assembly relative to the base assembly. The present invention thus provides a dynamic thermal interface between a platform assembly and base assembly of a positioning stage. This can allow an electronic device to be positioned and cooled using a relatively simple, robust and compact device. Moreover, the present invention permits passive cooling e.g. by conduction. This has been found to be particularly suitable for positioning and cooling a retractable detector of an electron microscope. It will be appreciated, therefore, that the present invention provides an improved positioning and cooling system for an electronic device, such as a detector of an electron microscope. The positioning stage may be configured to position a device in two or three dimensions. In embodiments, the positioning stage is a linear positioning stage that can position a device in one dimension. Thus, in embodiments, the platform assembly is moveable with respect to the base assembly along an (single) axis of motion between (at least) the first position and the second position. In order to facilitate positioning of an electronic device supported by the platform, the positioning stage should, and in embodiments does, comprise an (e.g. linear) actuator system configured to cause the platform assembly to move with respect to the base assembly, and a (e.g. linear) guide system configured to guide the platform assembly to move between (at least) the first position and the second position. The actuator system can be any suitable system, e.g. suitable for in vacuo operation. The (e.g. linear) actuator system may comprise a motor and lead screw that engages a lead nut, such that rotation of the lead screw by the motor causes the platform assembly to move with respect to the base assembly. Alternatively, the (e.g. linear) actuator system may comprise a piezoelectric actuator system. Similarly, any suitable (e.g. linear) guide system can be used, e.g. comprising one or more (e.g. linear) bearings or rails. In embodiments, the base assembly comprises one or more (e.g. two) (e.g. linear) guides (e.g. rails) configured to guide the platform assembly between (at least) the first position and the second position, the platform assembly is configured to be guided by the one or more guides (e.g. rails) between (at least) the first position and the second position, and the positioning stage comprises an (e.g. linear) actuator (e.g. fixed to the base assembly) that is configured to cause the platform assembly to move along the one or more guides (e.g. rails) between (at least) the first position and the second position. In order to facilitate cooling of an electronic device supported by the platform, the positioning stage should, and in embodiments does, comprise a thermal system that is configured to remove heat from (cool) an electronic device supported by the platform. The thermal system includes (at least) the one or more heat transfer devices, the thermally conductive platform, and the one or more thermally conductive sliding contacts. In embodiments, elements of the thermal system are provided separately to elements of the actuator system and the guide system. Thus, in embodiments, the one or more heat transfer devices, and / or the thermally conductive platform, and / or the one or more thermally conductive sliding contacts are provided separately / in addition to the one or more guides (e.g. rails) and / or actuator. Providing separate thermal components can allow for greater tolerances on the thermal components, which can simplify construction, for example. The thermally conductive platform is configured to support / position an electronic device while being in (direct) thermal communication with the electronic device. To facilitate this, the thermally conductive platform may comprise an upper surface configured (e.g. shaped) to be in (direct) contact with an electronic device the platform is supporting, e.g. such that heat may be transferred from the electronic device to the thermally conductive platform (by conduction). The thermally conductive platform may further comprise one or more (e.g. threaded) connectors for fixing the electronic device to the thermally conductive platform. The thermally conductive platform may have a thermal conductivity (e.g. at normal pressure and temperature) of >20 W / (m-K), such as >25 W / (m-K), such as >50 W / (m-K), such as >70 W / (m-K), such as >80 W / (m-K), such as >100 W / (m-K), such as >150 W / (m-K), such as >200 W / (m-K), such as >250 W / (m-K), such as >300 W / (m-K), such as >350 W / (m-K), such as >400 W / (m-K). The thermally conductive platform may be made from metal (i.e. including elements, alloys, compounds, etc.), such as aluminium or copper, or an alloy thereof. The thermally conductive platform could (directly) engage (and be guided by) the one or more guides (e.g. rails) of the base assembly. In embodiments, the thermally conductive platform forms (part of) an upper body of the platform assembly, and the platform assembly further comprises a lower body that may be fixed to an underside of the thermally conductive platform / upper body. The lower body may thus not make direct contact with the electronic device. The lower body of the platform assembly may form part of the guide / actuator system. For example, the lower body may be configured to engage (and be guided by) the one or more guides (e.g. rails) of the base assembly. (The thermally conductive platform / upper body may thus not directly engage the one or more guides (e.g. rails) of the base assembly.) The actuator system may be configured to apply a force (directly) to the lower body (i.e. not directly to the thermally conductive platform / upper body) in order to cause the platform assembly to move with respect to the base assembly. For example, the lower body may comprise a lead nut that a lead screw of the actuator system engages with. The lower body may have a thermal conductivity less than or equal to the thermal conductivity of the thermally conductive platform / upper body. The lower body may be made from metal, such as steel or aluminium. The lower body and the thermally conductive platform / upper body may optionally be substantially thermally insulated from each other (i.e. not in (direct) thermal communication with each other). This can help reduce or prevent heat being transferred outside of the thermal system, e.g. to the guide / actuator system. Similarly, the one or more guides (e.g. rails) of the base assembly may have a thermal conductivity less than or equal to the thermal conductivity of the thermally conductive platform / upper body and / or of the lower body. The one or more guides (e.g. rails) may be made from metal, such as steel, e.g. stainless steel. The thermally conductive platform is in thermal communication with the one or more thermally conductive sliding contacts. To facilitate this, the (upper body of the) platform assembly may comprise one or more heat transfer members via which heat can be transferred from the thermally conductive platform to the one or more thermally conductive sliding contacts (by conduction). A (each) heat transfer member may be a thermally conductive member having a first end in thermal communication with the thermally conductive platform and a second end in thermal communication with one or more thermally conductive sliding contacts. A (each) heat transfer member may extend from a side of the thermally conductive platform, e.g. and referred to as a wing. A (each) heat transfer member may have a thermal conductivity (e.g. at normal pressure and temperature) of >20 W / (m-K), such as >25 W / (m-K), such as >50 W / (m-K), such as >70 W / (m-K), such as >80 W / (m-K), such as >100 W / (m-K), such as >150 W / (m-K), such as >200 W / (m-K), such as >250 W / (m-K), such as >300 W / (m-K), such as >350 W / (m-K), such as >400 W / (m-K). A (each) heat transfer member may be made from metal, such as aluminium or copper, or an alloy thereof. A (each) heat transfer member may be made from the same material / metal as the thermally conductive platform (and have the same thermal conductivity). The platform assembly may comprise any suitable number of heat transfer members (e.g. wings), such as one, two, or more. In embodiments, the platform assembly has the same number of heat transfer members as the base assembly has heat transfer devices, and each heat transfer member of the platform assembly transfers heat from the thermally conductive platform to a respective heat transfer device of the base assembly, in embodiments via a (respective) set of one or more thermally conductive sliding contacts. For example, and in embodiments, the platform assembly comprises first and second heat transfer members (e.g. wings) extending from opposite sides of the thermally conductive platform, the base assembly comprises first and second heat transfer devices, and the first heat transfer member of the platform assembly is in thermal communication with the first heat transfer device of the base assembly (via a first set of one or more thermally conductive sliding contacts), and the second heat transfer member of the platform assembly is in thermal communication with the second heat transfer device of the base assembly (via a second set of one or more thermally conductive sliding contacts). The one or more heat transfer members may be formed separately to the thermally conductive platform, and e.g. fixed thereto, or formed integrally with the thermally conductive platform. Similarly, one or more thermally conductive sliding contacts may be formed integrally with a heat transfer member / upper body. In embodiments, the one or more thermally conductive sliding contacts are formed separately to the one or more heat transfer members / upper body, and the (one or more heat transfer members of the) platform assembly comprises one or more (e.g. two) (integrally or separately formed) thermally conductive housings configured to hold and thermally communicate with a (respective) set of one or more thermally conductive sliding contacts. In embodiments, the second end of a (each) heat transfer member (e.g. wing) forms a first side of a thermally conductive housing, and the other side of the thermally conductive housing is formed by a thermally conductive clamp fixed to the second end of the heat transfer member (e.g. wing). A (each) thermally conductive housing may hold a (respective) set of one or more thermally conductive sliding contacts by capturing the (respective) set of one or more thermally conductive sliding contacts against a (respective) heat transfer device of the base assembly. A (each) thermally conductive housing may be configured to cause a (respective) set of one or more thermally conductive sliding contacts it is holding to slide along a (respective) heat transfer device as the platform assembly moves between the first position and the second position. A (each) thermally conductive housing may partially or fully surround a (respective) heat transfer device of the base assembly. A (each) thermally conductive housing may comprise one or more internal grooves, wherein each groove is configured to capture and thermally communicate with a (respective) thermally conductive sliding contact, e.g. and in embodiments, such that walls of the groove thermally communicate with the thermally conductive sliding contact, and push the thermally conductive sliding contact to cause it to slide along a (respective) heat transfer device as the platform assembly moves between the first position and the second position. A (each) thermally conductive housing may hold any suitable number of thermally conductive sliding contacts, such as one, two, three, four, five, or more. A (each) thermally conductive housing may have a thermal conductivity (e.g. at normal pressure and temperature) of >20 W / (m-K), such as >25 W / (m-K), such as >50 W / (m-K), such as >70 W / (m-K), such as >80 W / (m-K), such as >100 W / (m-K), such as >150 W / (m-K), such as >200 W / (m-K), such as >250 W / (m-K), such as >300 W / (m-K), such as >350 W / (m-K), such as >400 W / (m-K). A (each) thermally conductive housing may be made from metal, such as aluminium or copper, or an alloy thereof. A (each) thermally conductive housing may be made from the same material / metal as the thermally conductive platform / heat transfer member (and have the same thermal conductivity). The one or more thermally conductive sliding contacts are in sliding thermal communication with the one or more heat transfer devices of the base assembly, e.g. such that heat may be transferred from a thermally conductive sliding contact to a heat transfer device of the base assembly (by conduction), while permitting sliding motion of the thermally conductive sliding contact with respect to the heat transfer device. A (each) thermally conductive sliding contact may be an elastically deformable member, e.g. biased to maintain sliding contact with a (respective) heat transfer device. A (each) thermally conductive sliding contact may partially or fully surround a (respective) heat transfer device of the base assembly. Surrounding a heat transfer device can increase contact area, and thus heat transfer. In embodiments, a (each) thermally conductive sliding contact is a spring contact, e.g. a coil spring. In embodiments, a (each) thermally conductive sliding contact is a coil spring arranged to form a loop, e.g. around a (respective) heat transfer device. In embodiments, a (each) thermally conductive sliding contact is a canted coil spring. Using a canted coil can increase contact area, and thus heat transfer. A (each) thermally conductive sliding contact may have a thermal conductivity (e.g. at normal pressure and temperature) of >20 W / (m-K), such as >25 W / (m-K), such as >50 W / (m-K), such as >70 W / (m-K), such as >80 W / (m-K), such as >100 W / (m-K), such as >150 W / (m-K), such as >200 W / (m-K), such as >250 W / (m-K), such as >300 W / (m-K), such as >350 W / (m-K), such as >400 W / (m-K). A (each) thermally conductive sliding contact may be made from metal, such as aluminium or copper, or an alloy thereof, such as beryllium copper. For example, and in embodiments, a (each) thermally conductive sliding contact is a beryllium copper canted coil spring. Beryllium copper has been found to have suitable spring and thermal properties. A (each) heat transfer device of the base assembly may be an elongate member that extends parallel to the axis of motion, e.g. such that a thermally conductive sliding contact can slide along the heat transfer device as the thermally conductive sliding contact moves with the platform assembly along the axis of motion. A (each) heat transfer device may be a solid or tubular (elongate) (e.g. metal) member. A (each) heat transfer device may be substantially cylindrical, or flattened (e.g. closer in shape to a rectangular cuboid with rounded edges), or another shape. In embodiments, a (each) heat transfer device is a heat pipe, e.g. a sealed metal pipe containing a working fluid, such as water. A (each) heat transfer device may have a (effective) thermal conductivity (e.g. at normal pressure and temperature) of >20 W / (m-K), such as >25 W / (m-K), such as >50 W / (m-K), such as >70 W / (m-K), such as >80 W / (m-K), such as >100 W / (m-K), such as >150 W / (m-K), such as >200 W / (m-K), such as >250 W / (m-K), such as >300 W / (m-K), such as >350 W / (m-K), such as >400 W / (m-K), such as >500 W / (m-K), such as >1000 W / (m-K), such as >10,000 W / (m-K). A (each) heat transfer device may be made from metal, such as aluminium or copper, or an alloy thereof. In embodiments, a (each) heat transfer is a copper heat pipe containing water. In embodiments, the base assembly comprises a thermal base that may operate as a heat sink, and the one or more heat transfer devices (e.g. heat pipes) are in thermal communication with the thermal base. The thermal base may comprise one or more (e.g. two or more) regions configured (e.g. shaped) to be in (direct) contact with a heat transfer device (e.g. heat pipe), e.g. such that heat may be transferred from the heat transfer device (e.g. heat pipe) to the thermal base (by conduction). The thermal base may comprise one or more (e.g. two or more) thermally conductive fixings (e.g. clamps) configured to hold a heat transfer device (e.g. heat pipe) in thermal communication with the thermal base, e.g. made from metal. In embodiments, both ends of an (each) elongate heat transfer device (e.g. heat pipe) are in thermal communication with (held by) the thermal base. Having both ends of a heat transfer device (e.g. heat pipe) held by the thermal base can improve heat transfer by allowing heat transfer to occur in both directions along the elongate heat transfer device (e.g. heat pipe). The thermal base may optionally be in thermal communication with an enclosure (e.g. vacuum housing), e.g. such that heat may be transferred from the thermal base to the enclosure (by conduction), and then from the enclosure (e.g. vacuum housing) to e.g. the ambient environment and / or to an external heat sink or cooling system (e.g. by convection). In embodiments, the base assembly further comprises a guide base, that may be fixed to the thermal base. The thermal base may be shaped to receive the guide base. The one or more heat transfer devices (e.g. heat pipes) may not make direct contact with the guide base. The guide base of the base assembly may form part of the guide / actuator system. For example, the one or more guides (e.g. rails) and / or the actuator system may be fixed to the guide base. In embodiments, the guide base has a thermal conductivity less than or equal to the thermal conductivity of the thermal base. The guide base may be made from metal, such as steel or aluminium. The guide base and the thermal base may optionally be substantially thermally insulated from each other (i.e. not in (direct) thermal communication with each other). This can help reduce or prevent heat being transferred outside of the thermal system, e.g. to the guide / actuator system. Embodiments of the present invention thus provide a heat transfer path from an electronic device supported by the thermally conductive platform, through the thermally conductive platform to the one or more thermally conductive sliding contacts, through the one or more thermally conductive sliding contacts to the one or more heat transfer devices, and through the one or more heat transfer devices to the thermal base. Another aspect provides an apparatus comprising a positioning stage as described above and an electronic device supported by, and in thermal communication with, the thermally conductive platform. The electronic device may be an electronic detector. The apparatus may comprise a vacuum chamber, and the positioning stage and the electronic device (e.g. detector) may be arranged in the vacuum chamber. The thermal base may be in thermal communication with a housing of the vacuum chamber. The apparatus may be an electron microscope, e.g. a transmission electron microscope, and the electronic device may be an electron detector, such as a charge coupled device (CCD), complementary metal oxide semiconductor (CMOS) camera, or direct electron detector. Thus, another aspect provides an (e.g. transmission) electron microscope comprising a positioning stage as described above and an electron detector supported by, and in thermal communication with, the thermally conductive platform. The positioning stage and the electron detector may be arranged in a vacuum chamber of the (e.g. transmission) electron microscope. The first position may correspond to a retracted position in which the detector is positioned out of an electron beam path of the electron microscope, and the second position may correspond to an inserted position in which the detector is positioned in the electron beam path of the electron microscope. Another aspect provides a method of positioning and cooling an electronic device; the method comprising: providing a positioning stage as described above; using the positioning stage to position the electronic device; and using the positioning stage to cool the electronic device. Another aspect provides a method of (e.g. transmission) electron microscopy comprising: providing a positioning stage as described above; using the positioning stage to position an electron detector; and using the positioning stage to cool the electron detector. Each aspect described herein can, and in embodiments does, include one or more, and in an embodiment all, of the features of other aspects described herein, as appropriate. Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 schematically illustrates an exemplary electron microscope; Figure 2 shows a positioning and cooling system according to an embodiment; Figure 3 is an exploded view of the system of Figure 2; Figure 4A is an exploded view of thermal components of the system of Figure 2; and Figure 4B is a cross-sectional view of thermal components of the system of Figure 2; Figure 5A, Figure 5B and Figure 5C schematically illustrate heat paths formed by thermal components of the system of Figure 2; Figure 6 shows thermal components of a positioning and cooling system according to another embodiment; Figure 7A shows a set of thermally conductive sliding contacts arranged in a housing, according to an embodiment; and Figure 7B shows assembled thermal components of a positioning and cooling system according to an embodiment; and Figure 8A shows an assembled positioning and cooling system according to an embodiment; and Figure 8B shows test results for the system of Figure 8A. As discussed above, embodiments of the present invention provide a positioning stage configured to position and passively cool an electronic device, such as an electron detector of a transmission electron microscope (TEM). The positioning stage includes one or more thermally conductive sliding contacts in thermal communication with a thermally conductive platform, that slide along a heat transfer device (e.g. heat pipe) fixed to a base as the platform moves with respect to the base. In transmission electron microscopy, a detector system is used to detect electrons of an electron beam transmitted through a sample, with the electron beam, sample and detector being arranged in vacuo. Figure 1 shows schematically a transmission electron microscope (TEM) 100. As shown in Figure 1, electrons produced by electron source (e.g. gun) 101 are accelerated by acceleration tube 102, and focussed by condenser optics 103 onto a sample held by sample holding system 104. The electron beam transmitted through the sample is focussed by objective optics 105 and may be expanded by projector optics 106 for imaging by detector 107. The detector 107 may typically comprise an electronic detector, such as a charge coupled device (CCD), complementary metal oxide semiconductor (CMOS) camera, or direct electron detector. The detector can be moved by a positioning system 109 between an “inserted” position 107 in the electron beam path 110 and a “retracted” position 108 out of the electron beam path 110, e.g. so as to allow different detectors to be positioned in the electron beam path 110. An electronic detector 107 will typically produce heat as a by-product, and so require cooling. However, it can be challenging to provide adequate cooling in the vacuum of a TEM where cooling by convection or forced air is not possible, while permitting movement of the detector. Figure 2 shows a detector positioning and cooling system according to embodiments of the present invention. The system is in the form of a linear positioning stage 200 that has a platform assembly that is moveable with respect to a base assembly along an axis of motion (that may be arranged perpendicular to the electron beam path 110). As shown in Figure 2, the base assembly includes linear guide rails 211, 212 that extend parallel to the axis of motion and are fixed to a guide base 213. The guide base 213 is fixed to a thermal base 220 which holds heat pipes 221, 222, which will be described below. The platform assembly includes an upper body 230 which forms a platform 231, and a lower body 240 that engages and runs on the guide rails 211, 212. A linear actuator system includes a stepper motor 251 and a lead screw 252 that extends parallel to the axis of motion in between the guide rails 211, 212. The screw 252 is connected at one end to the motor 251 and supported at the other end by the guide base 213. The screw 252 passes through and engages a lead nut (not shown) in the lower body 240 of the platform assembly, such that rotation of the screw 252 by the motor 251 in one direction causes linear motion of the platform assembly along the guide rails 211, 212 in a direction parallel to the axis of motion away from the motor 251, and rotation of the screw 252 by the motor 251 in the other direction causes linear motion of the platform assembly along the guide rails 211, 212 in a direction parallel to the axis of motion towards the motor 251. In the present embodiment, the linear positioning stage 200 is configured to position a detector (not shown) supported by the platform 231 in a first retracted position 108 in which the detector is positioned out of an electron beam path 110 of a TEM 100 (corresponding to the platform assembly being positioned near to the motor 251), and a second inserted position 107 in which the detector is positioned in the electron beam path 110 of the TEM 100 (corresponding to the platform assembly being positioned far from the motor 251). A suitable travel distance between the first and second positions may be about 8-10cm, such as about 9cm. The platform 231 may include one or more (e.g. threaded) connectors for connecting a detector to the platform 231. Figure 3 is an exploded view of the linear positioning stage 200. The linear positioning stage 200 includes a number of positioning components that are arranged to move / position a detector supported by the platform assembly. In particular, the base assembly includes guide rails 211, 212 that are fixed to the guide base 213. The lower body 240 of the platform assembly is configured to engage the guide rails 211,212 and be moved along the guide rails 211,212 parallel to the axis of motion by the linear actuator system 251,252 (as described above). The rails 211, 212 may be made from stainless steel, and the guide base 213 and lower body 240 may be made from aluminium. Other metals / materials are possible. Other arrangements for moving / positioning the platform assembly are possible. For example, less than or more than two rails may be used, different actuator systems may be used, etc.. The guide base 213 and the thermal base 220 may be combined into a single component. The linear positioning stage 200 further includes a number of thermal components that are arranged to remove heat from (cool) a detector supported by the platform assembly. In particular, the platform assembly further comprises a thermally conductive upper body 230 that is fixed to the lower body 240 by fixings (e.g. screws) 234, such that the upper body 230 moves together with the lower body 240. Fixings (e.g. screws) 234 may be thermally conductive, e.g. metal. The upper body 230 forms a platform 231 for supporting a detector (not shown) and may have the same or greater thermal conductivity than the lower body 240. The upper body 230 may be made from aluminium or copper. Other metals / materials, such as ceramics, are possible. The upper body 230 is thus configured to remove heat from a detector supported by the platform 231. The thermally conductive upper body 230 includes a central platform region 231 and wings (heat transfer members) 232, 233 that extend from either side of the platform region 231 (perpendicular to the axis of motion), such that the upper body 230 has a greater overall width (perpendicular to the axis of motion) than the lower body 240 (and the guide base 213). The end of a (and each) wing 232, 233 forms a (respective) half of a thermally conductive housing that houses and captures a (respective) set of thermally conductive sliding contacts 237 that surround a (respective) heat pipe 221, 222. The other half of a (and each) housing is formed by a thermally conductive clamp 235 that is fixed to the end of the (respective) wing 232, 233 by fixings (e.g. screws) 236. Fixings (e.g. screws) 236 may be thermally conductive, e.g. metal. Thermally conductive clamp 235 may be made from the same material as the upper body 230, e.g. aluminium or copper. The housing formed by a wing and clamp surrounds and captures a set of thermally conductive sliding contacts 237, such that the thermally conductive sliding contacts 237 move and remain in thermal communication with the upper body 230. To facilitate this, the housing defines a set of inner grooves 239, each of which captures a (respective) thermally conductive sliding contact 237 against a heat pipe. In the present embodiment, there are five thermally conductive sliding contacts 237 in a (and each) set, and five corresponding inner grooves 239 formed in a (and each) housing, but other numbers of thermally conductive sliding contacts / grooves are possible. The upper body 230 is thus configured to transfer heat from a detector supported by the platform 231 to the thermally conductive sliding contacts 237, and also to push the thermally conductive sliding contacts 237 along the heat pipes 221, 222 as the platform assembly moves with respect to the base assembly along the axis of motion. Correspondingly, a (and each) thermally conductive sliding contact 237 is configured such that when the platform assembly moves with respect to the base assembly along the axis of motion, the thermally conductive sliding contact 237 can slide along a (respective) heat pipe 221, 222 while maintaining thermal communication between the upper body 230 and the (respective) heat pipe 221, 222. The interface between a heat pipe 221, 222 and thermally conductive sliding contacts 237 may be lubricated by grease, which may also assist with thermal transfer. Additionally or alternatively, a coating may be applied to a heat pipe and / or sliding contact to reduce friction, improve wear resistance or increase thermal conductivity. The thermally conductive sliding contacts 237 are thus configured to transfer heat from the upper body 230 to the heat pipes 221, 222 while permitting motion of the upper body 230 with respect to the heat pipes 221, 222. A (and each) thermally conductive sliding contact 237 may, for example, be a deformable metal (e.g. copper) member surrounding a heat pipe, e.g. having an inner diameter approximately equal to an outer diameter of the heat pipe it surrounds. The inventors have found using (e.g. beryllium copper alloy) canted coil springs as the sliding contacts to be particularly advantageous, as such coils can readily deform to accommodate irregularities and misalignments, while maintaining a relatively large surface area in sliding contact and thermal communication with a heat pipe. Other arrangements are possible. The heat pipes 221, 222 extend parallel to the axis of motion to permit thermally conductive sliding contacts (e.g. canted coil springs) 237 to slide along the heat pipes as the platform assembly moves with respect to the base assembly along the axis of motion. The heat pipes may be sealed aluminium or copper pipes containing a working fluid, such as water. Other arrangements are possible. The heat pipes 221,222 are fixed at both ends to the thermal base 220 by thermally conductive fixings 223, comprising thermally conductive (metal, e.g. copper or aluminium) clamp and screws. The heat pipes 221, 222 are thus configured to transfer heat from thermally conductive sliding contacts 237 to the thermal base 220 (e.g. at least partially via thermally conductive fixings 223). The thermal base 220 is shaped to receive and thermally communicate with the heat pipes 221, 222, and operates as a heat sink. The thermal base 220 may also be in thermal communication with a vacuum housing (not shown) such that the thermal base 220 transfers heat from the heat pipes 221,222 to the vacuum housing, and the vacuum housing transfers heat to the ambient environment and / or to an external heat sink or cooling system. The thermal base 220 is furthermore shaped to receive the guide base 213, and is fixed thereto by fixings (e.g. screws) 224. The thermal base 220 may have the same or greater thermal conductivity than the guide base 213. The thermal base 220 may be made from copper or aluminium (e.g. alloy). Other metals / materials are possible. As well as receiving heat generated by a detector via heat pipes 221, 222 and fixings 223, the thermal base 220 may receive heat generated by the motor 251 via motor cover 254 and fixings 253. The motor cover 254 and / or fixings 253 may be thermally conductive (e.g. metal, e.g. aluminium or copper) and configured to transfer heat from the motor 251 to the thermal base 220. This can facilitate in vacuo operation of the motor 251 without overheating (e.g. at a suitable duty cycle). Furthermore, the motor 251 may be selected to have appropriately low outgassing properties for in vacuo operation. Figures 4A and 4B more clearly show thermal components of the linear positioning stage 200. As shown in Figure 4A, the platform assembly comprises a thermally conductive (metal) upper body 230 that, in cooperation with thermally conductive (metal) clamps 235 and fixings 236, captures thermally conductive (metal) sliding contacts 237 that surround heat pipes 221, 222 in grooves 239. The base assembly comprises a thermal (metal) base 220 with heat pipes 221,222 fixed thereto by thermally conductive (metal) clamps 223. Figure 4B shows a cross-sectional view through a heat pipe 222. As shown in Figure 4B, heat pipe 222 passes through a set of (five) canted coil springs 237 that are held in grooves 239 formed in a thermally conductive housing 233. The heat pipe 222 comprises a metal (e.g. copper) casing that forms a vapour cavity 401 containing a working fluid (e.g. water) that can be evaporated by heat from sliding contacts 237. Vapour generated in an evaporation region proximate to sliding contacts 237 travels through vapour cavity 401 away from the evaporation region and then condenses back to liquid in one or more condensation regions. Condensed liquid may return to the evaporation region using a sintered (or other) wick structure. A (and each) heat pipe may typically have an effective thermal conductivity much greater than that of the metal casing alone, e.g. at least 10 times greater. The diameter of a (and each) heat pipe may be, for example, 1-10mm, such as 6mm. The length of a (and each) heat pipe length may be, for example, 100-1000mm, such as 150mm. Figure 5A-C schematically illustrate resulting heat transfer paths provided by the thermal components of the linear positioning stage 200. As illustrated by Figures 5A-C, heat produced by a detector (not shown) supported by thermally conductive platform 231 may be transferred through the thermally conductive platform 231 to wings 232, 233, through wings 232, 233 (and grease) to thermally conductive sliding contacts 237 (not visible in Figures 5A-C), and through thermally conductive sliding contacts 237 (and grease) to heat pipes 221, 222. The heat pipes 221, 222 may then transfer heat away from the platform assembly, and through thermally conductive clamps 223 and into thermal base 220. As illustrated in Figure 5A, when the platform assembly is positioned in the first, retracted position (near to the motor 251), the heat pipes 221, 222 may transfer heat away from the platform assembly predominantly in one direction towards the cooler end (far from the motor 251). Similarly, as illustrated in Figure 5B, when the platform assembly is positioned in the second, inserted position (far from the motor 251), the heat pipes 221, 222 may transfer heat away from the platform assembly predominantly in one direction towards the cooler end (near to the motor 251). As illustrated in Figure 5C, when the platform assembly is positioned between the first and second positions, the heat pipes 221, 222 may transfer heat away from the platform assembly in both directions. Heat produced by a detector supported by the platform 231 can thus be transferred away from the detector, while permitting motion of the platform 231 and detector with respect to the base. This can provide a particularly robust and compact combined positioning and cooling arrangement. Moreover, since heat transfer is passive and predominantly by conduction and by phase transition in a sealed vapour chamber, the arrangement is particularly suited to operating in a vacuum chamber, e.g. of a TEM. Figure 6 shows an exploded view of thermal components of the platform assembly according to an embodiment. As shown in Figure 6, the platform assembly includes an upper body 230 that forms a thermally conductive platform 231, and a set of thermally conductive sliding contacts 237 in the form of canted coil springs. Thermally conductive clamps 235 are fixed to wing ends 232, 233 by fixings 236 to capture the canted coils 237 around heat pipes (not shown) in grooves 239. Vacuum grease 238 is used for lubrication purposes and to assist in thermal transfer to the heat pipes. The vacuum grease 238 may be selected to have appropriately low outgassing properties for in vacuo operation. As shown in Figure 6, in this embodiment the thermally conductive clamps 235 are located on the upper side of the platform assembly, rather than on the lower side of the platform assembly. Figure 7A shows a set of five thermally conductive sliding contacts 237 arranged in a housing half 232 formed at the end of a wing of the upper body 230. In this embodiment, each thermally conductive sliding contact 237 is a beryllium copper canted coil spring that is held by a respective inner groove 239 of the housing 232, which is made from aluminium. Figure 7B then shows the housing clamped around a copper heat pipe 221 (with the heat pipe 221 passing through the canted coil springs 237). (Other materials / metals are possible.) Figure 8A shows an assembled linear positioning stage 200 according to an embodiment. A heating element 801 was attached to the thermally conductive platform 231, and thermocouples TC1-TC4 were attached for testing purposes: TC1 was positioning to measure the temperature of the heating element 801, TC2 was positioning to measure the temperature at a wing-heat pipe interface, TC3 was positioned to measure the temperature of the thermal base 220, and TC4 was positioned to measure the temperature of a housing 802 that the thermal base 220 was in thermal communication with. A fifth thermocouple, TC5 (not shown), was used to measure the ambient room temperature. Figure 8B shows the results of a test in which the platform 231 was subjected to about 15,000 insertion and retraction cycles, and then heated to a temperature of about 40°C by the heating element 801 in a vacuum chamber. Figure 8B shows that the temperatures recorded by TC1-TC4 tended to reach a plateau, indicating stable heat transfer away from the platform 231 and into the housing 802, even after a large number of insertion and retraction cycles. Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims. For example, although heat pipes in embodiments described above are substantially cylindrical with a substantially circular cross-section, it would be possible for heat pipes to have another cross-sectional shape. For example, a heat pipe may be flattened, e.g. having a substantially (e.g. rounded) rectangular crosssection. In this case, a (and each) thermally conductive sliding contact may comprise a flat surface in sliding thermal contact with a (flat) side of a heat pipe, and there may be thermally conductive sliding contacts in sliding thermal contact with some or all (flat) sides of a heat pipe. Although the linear positioning stage has been described as operating in a vacuum, it would be possible to operate the linear positioning stage in other environments, such as ambient conditions.

Claims

1. A positioning stage for positioning and cooling an electronic device; the positioning stage comprising:a base assembly comprising one or more heat transfer devices; anda platform assembly that is moveable with respect to the base assembly between at least a first position and a second position, wherein the platform assembly comprises a thermally conductive platform configured to support the electronic device, and one or more thermally conductive sliding contacts in thermal communication with the thermally conductive platform;wherein the positioning stage is configured such that the or each thermally conductive sliding contact slides along a heat transfer device of the one or more heat transfer devices as the platform assembly moves between the first position and the second position such that the or each thermally conductive sliding contact is in thermal communication with the thermally conductive platform and in thermal communication with a heat transfer device of the one or more heat transfer devices when the platform assembly is in the first position and when the platform assembly is in the second position.

2. The positioning stage of claim 1, wherein:the platform assembly further comprises a lower body fixed to the thermally conductive platform;the base assembly comprises one or more guides configured to guide the platform assembly between the first position and the second position;the lower body is configured to engage the one or more guides of the base assembly; andthe positioning stage comprises an actuator configured to apply a force to the lower body of the platform assembly to cause the platform assembly to move along the one or more guides.

3. The positioning stage of claim 1 or 2, wherein the platform assembly comprises one or more thermally conductive housings, and wherein the or each thermally conductive housing is configured to hold a set of one or more of the one or more thermally conductive sliding contacts.

4. The positioning stage of claim 1,2, or 3, wherein the or each thermally conductive sliding contact is elastically deformable.

5. The positioning stage of any preceding claim, wherein the or each thermally conductive sliding contact surrounds a heat transfer device of the one or more heat transfer devices of the base assembly.

6. The positioning stage of any preceding claim, wherein the or each thermally conductive sliding contact is a canted coil spring.

7. The positioning stage of any preceding claim, wherein the or each heattransfer device of the base assembly is a heat pipe.

8. The positioning stage of any preceding claim, wherein the base assembly comprises a thermal base in thermal communication with the one or more heat transfer devices.

9. The positioning stage of claim 8, wherein the or each heat transfer device extends between a first end and a second end, and the thermal base holds the or each heat transfer device at or near the first end and at or near the second end.

10. The positioning stage of claim 8 or 9, wherein the base assembly further comprises a guide base fixed to the thermal base, wherein one or more guides configured to guide the platform assembly between the first position and the second position are fixed to the guide base.

11. The positioning stage of any preceding claim, wherein the one or more heat transfer devices, the thermally conductive platform, and the one or more thermally conductive sliding contacts comprise aluminium or copper.

12. An apparatus comprising an electronic device and a positioning stage as claimed in any preceding claim; wherein the electronic device is supported by and in thermal communication with the thermally conductive platform.

13. The apparatus of claim 12, further comprising a vacuum chamber; wherein the electronic device and the positioning stage are arranged within the vacuum chamber.5 14. The apparatus of claim 12 or 13, wherein the apparatus is an electronmicroscope and the electronic device is a detector.

15. A method of positioning and cooling an electronic device; the method comprising:10 providing a positioning stage as claimed in any one of claims 1 to 11;using the positioning stage to position the electronic device; and using the positioning stage to cool the electronic device.

16. A method of electron microscopy comprising positioning and cooling a15 detector using the method of claim 15.

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