Patent Description:
An electron microscope uses a focused beam of electrons to interact with atoms on the surface of a sample and produces information about the sample's surface topography and composition. A well-known type of electron microscope is a scanning electron microscope (SEM).

It can be desirable to move the sample in the electron microscope, both linearly and rotationally. For example, rotation is important to allow the correct orientation of the specimen under the electron beam for many imaging and preparation operations. An electron microscope typically has a specimen stage onto which the sample is either mounted directly, or indirectly, via a sample holder. Modules may be provided between the specimen stage and the sample, for example a rotate module may be provided to provide rotational motion. The specimen stage may provide linear movement.

Electron microscopy takes place under vacuum conditions. Some samples (either biological or non-biological) are sensitive to electron beams and require freezing to limit damage from the electron beam. When biological samples or samples with a high water content are exposed to the vacuum conditions of an electron microscope, any liquid in the sample immediately begins to evaporate, which causes major deformation. To overcome these problems, samples are rapidly frozen prior to being placed in the electron microscope. They are then held as close as possible to the temperature of liquid nitrogen (-<NUM>) whilst analysis in the microscope takes place.

A cold stage is typically used to maintain the low temperature of the sample during scanning. The sample is placed on the cold stage, which maintains the low temperature of the sample. The cold stage is mounted directly or indirectly on the specimen stage. Current cold stage designs use either copper braids connecting the stage to a reservoir of liquid nitrogen or pass cold nitrogen gas through tubes connected to the stage to reach the required temperature. Biological samples must be kept below -<NUM> to avoid deterioration. The use of gas cooled cold stages allows cooling to about -<NUM> and the use of braid cooled cold stages allows cooling to about -<NUM>.

Cold stages have the disadvantage that they do not allow both cooling and rotation of the sample. This is due to the braids and tubes connecting the liquid nitrogen reservoir to the cold stage preventing rotation of the cold stage. As the cold stage is tethered to either the braids or gas tubes, rotation would cause shearing of the braids or gas tubes.

It is an object of the invention to overcome disadvantages associated with the prior art.

Document <CIT> discloses a rotatable stage comprising a heating/cooling element via bearings.

A first aspect of the invention provides a rotatable stage for an analytical apparatus, the rotatable stage comprising:.

A second aspect of the present invention provides an electron microscope comprising a vacuum chamber and a rotatable stage according to the first aspect of the invention, wherein the stator, rotor and bearing are located in the vacuum chamber, the electron microscope optionally comprising a specimen stage, with the stator mounted on the specimen stage via a thermally insulated support.

A third aspect of the invention provides a method for cooling the rotor of a rotatable stage, the method comprising:.

The bearing may comprise a material having a thermal conductivity of greater than about 170Wm-<NUM>K-<NUM>.

The use of a highly thermally conductive bearing allows the rotor to be efficiently cooled. This enables the low temperature of a sample mounted on the rotor to be maintained.

The bearing is configured to provide multiple points of contact between the bearing and the stator and between the bearing and the rotor respectively. The multiple points of contact enhance the thermal connection between the stator and rotor. The bearing may be configured to provide at least <NUM> points of contact between the bearing and the stator and the bearing and the rotor respectively. In one embodiment, the bearing may be configured to provide at least <NUM> points of contact between the bearing and the stator and the bearing and the rotor respectively.

The bearing may comprise one continuous component. The bearing comprises a coiled spring. The use of a coiled spring has the advantage of providing many points of contact between the stator and the bearing and between the rotor and the bearing respectively, thereby providing an efficient thermal link between them, whilst having low friction. For example, each coil of the coiled spring may have one point of contact with the stator and one point of contact with the rotor. The coiled spring is very tolerant of machining variations in the component parts. The coiled spring may comprise a canted coiled spring. The canted coiled spring may have an elliptical cross-section.

The material of the bearing may comprise at least one of copper, phosphor bronze, and CuBe<NUM>. The material of the bearing may comprise an alloy. In one embodiment the material of the bearing comprises a copper alloy.

Suitably, the bearing may comprise a material having a Rockwell hardness of between about 45Rb and about 94Rb.

The analytical apparatus may comprise a microscope, for example an electron microscope. Examples of suitable analytical apparatus include scanning electron microscopes (SEM), focussed ion beam instruments (FIB) such as a focussed ion beam scanning electron microscopes (FIB-SEM), and X-ray beamline end stations.

The heat exchanger may comprise a gas tube, wherein the gas tube is configured to receive cold gas. The cold gas may be cooled, i.e. below ambient temperature. The cold gas may have a temperature of -<NUM> or below. The gas may comprise nitrogen gas. For example, the cold gas may comprise cooled nitrogen gas, at a temperature in the range of just below ambient temperature to -<NUM>. In an alternative embodiment, the heat exchanger may comprise a cooled braid. The braid may comprise a copper braid.

The rotatable stage may comprise a flexible connection providing thermal conduction between the heat exchanger and a second heat exchanger allowing movement of the stator relative to the electron microscope. For example, the stator may be mounted on a specimen stage adapted for linear movement, for example in X and Y. The connection may comprise a portion of the gas tube or the thermally conductive braid.

The second heat exchanger may be configured to remove heat. The second heat exchanger may be in contact with liquid nitrogen.

The stator may comprise a temperature sensor.

The rotatable stage may comprise means for:.

The system may comprise a controller and suitably comprises a control unit or computational device having one or more electronic processors.

Controlling the temperature of the heat exchanger may comprise controlling the flow of cold gas through the gas tube.

A feedback loop to maintain a fixed temperature at the stator is thus provided.

An aspect of the invention provides an electron microscope comprising a specimen stage and wherein the stator is mounted on the specimen stage via a thermally insulated support.

The electron microscope may comprise a stage rotate module, mounted directly or indirectly on the specimen stage, wherein the rotor is mounted via thermally isolating supports to the stage rotate module.

The bearing may comprise a material having a thermal conductivity of greater than about 170Wm-<NUM>K-<NUM>. The bearing may comprise a material having a thermal conductivity of greater than about 140Wm-<NUM>K-<NUM>, greater than about <NUM> Wm-<NUM>K-<NUM>, greater than about <NUM> Wm-<NUM>K-<NUM>, or greater than about <NUM> Wm-<NUM>K-<NUM>. The bearing may comprise a material having a thermal conductivity in the range of about <NUM> Wm-<NUM>K-<NUM> to about <NUM> Wm-<NUM>K-<NUM>.

The heat exchanger may comprise a gas tube, and the method may comprise the step of passing cold gas through the gas tube.

The gas tube may be cooled to at least -<NUM>.

The method may comprise simultaneously cooling and rotating the rotor.

The rotatable stage may be used for apparatus which requires both cooling and rotation of the sample. For example, analytical instruments or apparatus for coating samples.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover, the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

<FIG> shows an embodiment of the rotatable stage in a scanning electron microscope. The scanning electron microscope <NUM> is provided with a vacuum chamber <NUM> which houses an electron microscope specimen stage <NUM>.

The rotatable stage comprises a static part of the rotatable stage (stator) <NUM>, a rotating part of the rotatable stage (rotor) <NUM> and bearing <NUM> between them. The rotor <NUM> is mounted inside the stator <NUM>, with the bearing <NUM> enabling rotation between the stator <NUM> and rotor <NUM>. The rotor <NUM> is mounted on a microscope stage rotate module mounted on the electron microscope specimen stage <NUM>, via a thermally isolating support (not shown). The stator <NUM> is mounted by thermally insulating supports (not shown) to the main electron microscope specimen stage <NUM>.

A heat exchanger in thermal connection with the stator <NUM> is provided in the form of a cooling tube <NUM> located around the outer circumference of the stator <NUM>. Nitrogen gas cooled close to -<NUM> is passed through the cooling tube <NUM>, to thereby cool the stator.

The supply of cooled nitrogen gas is provided via a second heat exchanger <NUM> placed in a liquid nitrogen Dewar flask <NUM>. A flow of warm, dry nitrogen gas <NUM> is passed through the heat exchanger submerged in liquid nitrogen where it is cooled to close to -<NUM>. The cooled nitrogen gas <NUM> is passed through a vacuum feedthrough <NUM> into the vacuum chamber <NUM> and into the cooling tube <NUM>.

The bearing <NUM> acts as a high efficiency thermal link which connects the stator to the rotor. In this embodiment, the bearing is a circular copper canted coiled spring.

<FIG> is a plan view of the rotatable stage in more detail, showing the stator <NUM> and rotor <NUM>. A sample holder <NUM> is shown on the rotor <NUM>. The stator <NUM> is connected to the electron microscope specimen stage <NUM> via a thermally isolated locator <NUM>. A cooling tube <NUM> is shown around the outer circumference of the stator <NUM>, enabling the cooled nitrogen gas to circulate around it, thereby cooling the stator.

<FIG> is a section through the rotatable stage of <FIG>. The rotor <NUM> is shown mounted on the microscope stage rotate module <NUM> via thermal insulator <NUM>. The rotor <NUM> is mounted in stator <NUM> via bearing <NUM>, which comprises a circular copper canted coiled spring. The cooling tube <NUM> is shown around the periphery of the stator <NUM>.

In an alternative embodiment, the cooling tube could be replaced by a cooling braid.

The cold parts of the system (for example the cooling tube <NUM> and sample holder <NUM>) are made of gold plated copper for high thermal conductivity and corrosion resistance. The insulating parts (for example thermally isolated locator <NUM> and thermal insulator <NUM>) are of ceramic and Torlon™ (a high vacuum compatible polymer).

A temperature sensor on the stator (not shown) is used in a feedback loop to maintain a set, fixed temperature by varying the gas flow through the cooling tube <NUM>.

The use of a canted coiled spring has the advantage of ease of assembly, as the rotor, stator and bearing to simply click together.

Claim 1:
A rotatable stage for an analytical apparatus, the rotatable stage comprising:
a stator (<NUM>);
a heat exchanger (<NUM>) in thermal connection with the stator;
a rotor (<NUM>); and
a thermally conductive bearing (<NUM>) comprising a coiled spring;
wherein the bearing is located between the stator and the rotor, and
wherein the bearing is configured to provide multiple points of contact between the bearing and the stator and multiple points of contact between the bearing and the rotor, so that the bearing provides a thermal connection between the stator and rotor.