The invention relates to a cooling apparatus for a rotating sample holder and particularly, though not exclusively, relates to a cooling apparatus for a rotating UHV sample holder for use at temperatures of 20 Kelvin and below for studying thin film deposition.

Background

Previous efforts to optimize RF magnetron sputtering have focused on achieving the highest possible homogeneity and minimizing grain boundaries in deposited films. To this end, deposition has typically taken place at room temperature or above, which effectively permits an in situ annealing and hence promotes both structural and compositional uniformity. However, in recent years, scientists have begun to realize that in certain cases, defects and disorder in a film may actually generate interesting and desirable physical properties. Two excellent examples of this are nanocrystalline superconductors (which may display significant increases in both their critical temperatures and upper critical fields) and transition metal oxides or nitrides, in which ferromagnetism may develop in the presence of a high anionic vacancy density.

Deposition at very low temperatures is an excellent method for growing highly- disordered films without recourse to neutron bombardment or any chemically-invasive techniques. The level of disorder increases dramatically as the deposition temperature falls due to the characteristic phonon energy distribution of a material. With the exception of "rattling" modes for extremely heavy cations (which have a rather limited effect on their surrounding crystal structure), very few materials exhibit phonons with energies below - 3meV, corresponding to a temperature of ~ 35K. This implies that above this temperature, thermally-activated phonons will facilitate post-deposition annealing, reducing disorder and increasing the grain size.

Liquid nitrogen-cooled deposition stages are limited to a base temperature of 77K: any novel disorder-mediated properties (such as the enhanced superconductivity and emergent ferromagnetism mentioned above) will be significantly suppressed by deposition at such relatively high temperatures. Liquid helium-cooled deposition is thus the only method to eliminate post-annealing and grow highly-disordered or amorphous materials for use in future electronic devices.

Techniques to cool down a sample holder in UHV already exist. These usually use a cooling liquid injected inside a rotating platform of the sample holder through a rotating axle, bringing the cooling liquid into direct thermal contact with the sample holder. The cooling liquid is usually water, although liquid nitrogen has also been used. Although injecting liquid helium in a liquid nitrogen UHV device could be considered, its efficiency and its temperature stability would be extremely low. Achieving sub kelvin temperature stability at 20K and below is impossible by directly flowing cold helium gas or liquid helium on a sample holder, as its cooling power is too unstable.

Summary of the Invention

To improve temperature stability, the present invention combines cooling power to heating power from a heater directly fixed on the sample holder and controlled by a PID loop. Since connecting directly the cooling liquid, i.e. liquid helium, to the sample holder implies transmitting heating power from the heater directly to the liquid helium, which would result in high liquid helium consumption, the present invention overcomes the cooling power instability by adding an intermediate cooling block between the liquid helium and the sample holder. It improves the temperature stability and reduces the heating power needed to get sub kelvin stability, which drastically lowers the liquid helium consumption. According to a first exemplary aspect, there is provided a cooling apparatus for a rotating sample holder, the cooling apparatus comprising an intermediate cooling block; a cooling pipe in thermal contact with the intermediate cooling block and configured for passing a coolant therethrough to cool the intermediate cooling block; and at least one conducting strip providing thermal conductivity between the rotating sample holder and the intermediate cooling block to cool the rotating sample holder.

One end of the at least one conducting strip may be secured to one of the cooling block and the rotating sample holder, and another end of the at least one conducting strip may be in sliding contact with the other of the cooling block and the rotating sample holder.

The cooling pipe may be a helical pipe wound around the intermediate cooling block.

The cooling pipe may be single-walled where in contact with the intermediate cooling block and double-walled where not in contact with the intermediate cooling block. The cooling apparatus may further comprise a disc having at least one slot for passage of the at least one conducting strip therethrough, the disc configured to secure one end of the at least one conducting strip to the intermediate cooling block. The cooling apparatus may further comprise at least one support pillar for supporting the intermediate cooling block to reduce stress on the cooling pipe.

The intermediate cooling block may comprise a hole configured for a sliding fit of an axle therethrough for rotating the rotating sample holder.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.

In the drawings:

FIG. 1 is a perspective view of an exemplary cooling apparatus;

FIG. 2 is an exploded assembly view of another exemplary cooling apparatus;

FIG. 3 is a close up perspective view of cooling strips of the cooling apparatus of FIG. 2 in contact with the sample holder; and

FIG. 4 is side view of a manipulator for the sample holder comprising the cooling apparatus of FIG. 2. Detailed Description of the Exemplary Embodiments

Exemplary embodiments of a cooling apparatus 10 for a rotating sample holder 100 and a manipulator 80 for the sample holder 100 will be described with reference to FIGS. 1 to 4 below.

The cooling apparatus 10 uses liquid helium as a coolant. Liquid helium has a boiling point of minus 269 degrees Celsius, while liquid nitrogen boils at merely minus 196 degrees Celsius. As such, liquid helium is much more unstable as a cryogenic liquid compared to liquid nitrogen. Designing a device which operates down to liquid helium temperatures involves much more care and concern regarding geometry of the device, as well as stringent choices for the materials used.

In a first embodiment of the cooling apparatus 10, as shown in FIG. 1, liquid helium is disconnected from the rotating sample holder 100 by using an intermediate cooling block 12. This greatly reduces liquid helium consumption while allowing temperatures down to 4.2 Kelvin to be reached at the sample holder 100.

Heat leakage towards a sample (not shown) on the sample holder 100 is reduced by using materials with very low thermal conductivity for a rotating axle 90 of the sample holder 100. In a preferred embodiment, the rotating axle 90 is made of a non-metallic material. In combination with the intermediate cooling block 12, this allows liquid helium temperatures to be reached at the sample holder 100. The intermediate cooling block 12 as shown in FIG. 1 comprises a thermal conductive block 12 with cooling pipes 14 in thermal communication with the block 12, that is, the cooling pipes 14 are in thermal contact creating a thermal link with the block 12. At least one conducting strip 16 having two ends 16a, 16b is provided to effect thermal conductivity between the thermal block 12 and a rotating platform 102 of the sample holder 100. This may be achieved by having one end 16a fixed to the rotating platform 102 and the other end 16b in sliding contact with the thermal conductive block 12, as shown in FIG. 1. The intermediate cooling block 12 is preferably disc-shaped or cylindrical shaped. Thermal contact with the cooling pipes 14 may be effected by providing the cooling pipes 14 as a wound helical pipe 14 around the intermediate cooling block 12 as shown in FIG. 1, such that the pipe and the intermediate cooling block are thermally sunk together. The block 12 is preferably made of copper and the helical pipe 14 is preferably made of stainless steel. It should be noted that any material with a high thermal conductivity may be used for the block 12. Liquid helium is circulated through the helical pipe 14 to cool the intermediate cooling block 12.

The helical pipe 14 preferably consists of a double walled pipe 14a for all its length except where it 14b is in contact with the intermediate cooling block 12. As the sample holder 100 and cooling apparatus 10 are used in UHV, vacuum between the two surfaces of the double wall of the helical pipe 14, as usually provided in helium transfer lines, is unnecessary. The double wall is provided to reinforce the helical pipe 14 with the use of thin tubes, instead of having a single thick wall which would increase heat leakage.

The cooling apparatus 10 can still work without using a double walled pipe 14. However, either the helium consumption would be much greater or the pipe 14 will be structurally weaker.

The intermediate cooling block 12 is preferably brazed to the helical pipe 14, or it 12 may be attached by any technique suitable for ensuring a good thermal link between the block 12 and the helical pipe 14.

As shown in FIG. 3, the intermediate cooling block 12 may be supported by support pillars 18 which are fixed to the UHV chamber, to lower the stress on the helical pipe 14. These support pillars 18 preferably comprise stainless steel pipes 18, or any other material with a very low thermal conductivity to minimize heat leakage and helium consumption.

Although the support pillars 18 are not essential, lack of them 18 results in stress being laid on the helical pipe 14 to support the intermediate cooling block 122 and may reduce the overall life time of the cooling apparatus 10.

A few possible ways of increasing the cooling power delivered to the sample holder 100 are (i) increasing the flow of liquid helium running through the pipe 14, (ii) pumping on the liquid helium through an impedance to produce low temperature vapor, (iii) using more than one helical pipe 14, (iv) employing more conducting strips 16. However, for practical purposes there is a maximum achievable cooling rate, limited by the geometry of the cooling apparatus 10. Therefore, any additional heat leakage should be metered with care as these could result in an increase of the base temperature at the sample holder 100.

A hole 13 provided at an axial center of the intermediate cooling block 12 allows the rotating axle 90 of the sample holder 100 to pass through the intermediate cooling block 12 with a sliding fit. The sample holder 100 is preferably attached to a top end of the rotating axle 90 and may thus rotate freely above the intermediate cooling block 12. Alternatively, the sample holder 100 may be configured to rotate below the intermediate cooling block 12, but this configuration reduces the visible portion of a sample on the sample holder 100 beneath the intermediate cooling block 12. In an alternative embodiment of the cooling apparatus 10 as shown in FIG. 2, a plurality of conducting strips 16 or spring plates are provided. As shown in FIG. 3, one end 16a of each of the conducting strips 16 are configured to be in sliding contact with the rotating platform 102 of the sample holder 102 while another end 16b of each of the conducting strips 16 are fixed onto a surface 12a of the intermediate cooling block 12 facing the sample holder 100, as shown in FIG. 2.

In one configuration, the ends 16b of the conducting strips 16 may be fixed onto the surface 12a using a disc 20 with a plurality of slots 22 therethrough, the plurality of slots being at least equal to the plurality of conducting strips 16 provided. This may be effected by passing the end 16a of each conducting strip 16 that is in sliding contact with the rotating platform 102 through one of the slots 22 in the disc 20 while the other end 16b is bent to engage the disc 20, such that securing of the disc 20 onto the surface 12a of the intermediate cooling block 12 results in the bent end 16b of the conducting strips 16 being clamped between the disc 20 and the intermediate cooling block 12. The disc 20 may be secured onto the surface 12a of the cooling block 12 by means of screws (not shown) or other suitable securing means. In other embodiments, other means may be used to secure the conducting strips 16 to the surface 12a of the intermediate cooling block 12, such as by individually screwing one end 16b of each of the conducting strips 16 to the surface 12a. The disc 20 and screws are preferably made of copper, but any other material with a high thermal conductivity may be used.

The conducting strips 16 or metallic spring plates are preferably distributed all around the rotating platform 102 of the sample holder 100 and the surface 12a of the intermediate cooling block 12. The cooling apparatus 10 still works even if only one spring plate or conducting strip 16 is used, as long as the spring plate 16 provides sufficient thermal link between the intermediate cooling block 12 and the sample holder 100. More spring plates or conducting strips 16 fixed on or in contact with the sample holder 100 would enhance the effective cooling rate and allow the temperature of the sample holder 100 to approach to 4.2 Kelvin.

All spring plates or conducting strips 16 must be in good thermal contact with the intermediate cooling block 12 and the sample holder 100 to ensure a high cooling power for the sample holder 100. The spring plates or conducting strips 16 are preferably made of copper-beryllium, but any other resilient material with a high thermal conductivity may be used. The coefficient of friction between the material used for the spring plates or conducting strips 16 and the intermediate cooling block 12 (in the case of the embodiment of FIG. 1) or the rotating platform 102 of the sample holder 100 (in the case of the embodiment of FIG. 2) is also very important, as a low coefficient of friction will reduce the amount of debris that will stick to the intermediate cooling block 12 (in the case of the embodiment of FIG. 1) or the rotating platform 102 of the sample holder 100 (in the case of the embodiment of FIG. 2) and increase the cooling power. The sample holder 100 must be rigid, preferably with a mass as low as possible to reduce its specific heat and hence increase the cooling power of the invention. In any case, its 100 mass must be carefully chosen to balance a suitably fast thermal response time with the requirement of thermal stability. The rotation of the axle 90 sets the sample holder 100 in rotation against the intermediate cooling block 12. Thermal energy is therefore removed from the sample holder 100 via the spring plates or conducting strips 16.

The load on the spring plates or conducting strips 16 must be optimised so as to maintain sufficient thermal contact between the intermediate cooling block 12 and the sample holder 100, while keeping heating due to friction to a minimum. The spring plates or conducting strips 16 and the intermediate cooling block 12 must be highly polished. The intermediate cooling block 12 may be considered a thermal bath since it has a cold and very stable temperature. The geometry of the spring plates or conducting strips 16, i.e., their width x thickness x length must be optimized for an ideal elastic coefficient.

The spring plates or conducting strips 16 should be as light as possible (and therefore as thin as possible), but it is important that they 16 keep enough spring effect or maintain sufficient elasticity to effect good thermal contact with the intermediate cooling block 12 (in the case of the embodiment of FIG. 1) or the rotating platform 102 of the sample holder 100 (in the case of the embodiment of FIG. 2) when the sample holder 100 is rotating.

The load of the spring plates or the conducting strips 16 on the intermediate cooling block 12 (in the case of the embodiment of FIG. 1) or the rotating platform 102 of the sample holder 100 (in the case of the embodiment of FIG. 2) can be simply adjusted by varying the distance between the rotating platform 102 of the sample holder 100 and the intermediate cooling block 12. In order to do so, as shown in FIG. 4, a UHV rotary feed through 40 which rotates the axle 90 is mounted on a UHV bellow 42, allowing the rotating axle 90 to move linearly along its 90 longitudinal axis. Other linear UHV translation devises and UHV rotary devices may also be used instead of a bellow 42 and a rotary feed through 40.

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations in details of design, construction and/or operation may be made without departing from the present invention.