US20090019862A1

US20090019862A1 - Cryostat for studying samples in a vacuum

Info

Publication number: US20090019862A1
Application number: US11/576,982
Authority: US
Inventors: Pierre Noe; Emmanuel Picard
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2004-10-22
Filing date: 2005-10-20
Publication date: 2009-01-22
Also published as: JP2008517293A; ATE477451T1; WO2006043010A1; CN101044352A; DE602005022902D1; FR2877090A1; EP1802911B1; EP1802911A1; FR2877090B1

Abstract

Cryostat (1) for studying samples in a vacuum, which cryostat comprises a cold finger (2) equipped with a finger portion as well as a base portion (10) rigidly connected to the finger portion, which cryostat also includes a sample support (32) mounted on a free end for cooling of the finger portion, wherein this finger portion is placed in a vacuum chamber (4). The vacuum chamber is partially defined by a one-piece hollow part (6) defining an open cavity (46) with a single opening (44) through which the finger portion passes, and the sample support is located inside said open cavity (46). The chamber is also defined by a cryostat body (8) of which an external cylindrical surface with a circular cross-section (36) cooperates with a rotation device (62).

Description

TECHNICAL FIELD

This invention relates in general to a cryostat for studying samples in a vacuum using electromagnetic radiation, and more specifically to cryostats for microscope intended to maintain the sample at a temperature between 80° K and 300° K in a secondary vacuum, i.e. between approximately 10⁻⁴and 10⁻⁵mbar.
This type of cryostat is used in particular to carry out optical spectroscopy on samples, or to perform micro-photoluminescence measurements.

PRIOR ART

From the prior art, we know various types of cryostats capable of maintaining the sample in a vacuum enclosure at a temperature of around 80° K.
First, liquid helium or nitrogen continuous flow cryostats are known, such as those sold under the names “MicrostatHiRes” and “MicrostatN” (registered trademarks) of the Oxford Instruments company.
These cryostats have certain non-negligible disadvantages, including having an excessively large minimum clearance between a window provided on the external wall of the cryostat and the sample, in order to allow the use of achromatic objectives for significant magnification at a short focal distance.
In addition, the admittance angle is limited to 160° because of the constraints due to the type of cooling system used, and the use of nitrogen or helium results in substantially high operation costs.
Moreover, Joule-Thomson-type cold finger cryostats are known, which implement a principle of adiabatic expansion of pressurised gas (for example 120 bars) through a glass coil, in order to cause cooling of the sample located at the end of the finger.
Generally, the cold finger is placed in a chamber enabling it to be kept in a vacuum by pumping, and to thus limit condensation/gel problems as well as problems heat convection problems.
With such an arrangement, the sample, arranged on a sample support itself mounted on a free end of a finger portion of the cold finger, is observed through a window with a thickness of 1.5 mm, and separated by a distance greater than 2 mm from said sample. Consequently, in the same way as for liquid helium or nitrogen continuous flow cryostats, of which the minimum working distance is also on the order of 2 mm, the actual design of these cold finger cryostats of the prior art prohibits the use of certain magnifications at a short focal distance.
Furthermore, the windows on the external wall defining the vacuum chamber strongly limit the admittance angle of the cryostat, and the rotation of the sample about its own axis is impossible due to the random positioning of the cold finger and said sample inside the chamber, which moreover has a substantially square shape.

OBJECT OF THE INVENTION

The invention is therefore intended to propose a cryostat for studying samples in a vacuum using electromagnetic radiation, which at least partially overcomes the aforementioned disadvantages relating to the devices of the prior art.
To do this, the invention relates to a cryostat for studying samples in a vacuum using electromagnetic radiation, comprising a cold finger equipped with a finger portion as well as a base portion rigidly connected to the finger portion and enabling in particular the arrival of pressurised gas, which cryostat also includes a sample support mounted on a free end for cooling of the finger portion, wherein this same finger portion of the cold finger is placed in a vacuum chamber. The vacuum chamber is partially defined by a one-piece hollow part defining an open cavity with a single opening through which the finger portion passes, and the sample support is located inside the aforementioned open cavity. In addition, the vacuum chamber is also defined by a cryostat body comprising a through-hole through which the finger portion passes, wherein said cryostat body is connected in a sealed manner to the hollow part and to the base portion of the cold finger. Finally, the cryostat is also equipped with a rotation device cooperating with an external cylindrical surface with a circular cross-section of the cryostat body.
With such a configuration, in which the sample to be analysed is therefore intended to be located entirely inside the open cavity, and consequently to be surrounded over 360° by the hollow part defining said cavity, it should therefore be understood that this hollow part naturally forms one piece with a material transparent to the electromagnetic radiation used, so as to enable the desired study to be carried out. Thus, the positioning of the sample in the aforementioned hollow part having such a transparency in order to be capable of being passed through by said electromagnetic radiation when studying the sample, advantageously means that the possibilities of analysis and measurement of this sample are significantly increased with respect to those previously seen with the planar window solution.
In this regard, it is noted that if the material used for the hollow part is dependent on the nature of the electromagnetic radiation used to study the sample, it is also preferably chosen so as to be transparent to visible light. As an example, the material used can be quartz, insofar as it is transparent to visible light, and also to the infrared and ultraviolet radiation envisaged in various sample studies.
In addition, with the cryostat according to the invention, it is possible to use a microscope objective with very strong magnification, in particular in order to produce spectral mappings, due to the fact that the sample can be placed on the sample support at a very short distance from the hollow part produced in one piece and passed through by the electromagnetic radiation, which distance can indeed be less than or equal to 1 mm.
This potential is of course not seen in the solutions of the prior art, in which the radiation is intended to pass through a window maintained by an added wall thickness, ensuring the mechanical strength and the vacuum strength of said window. In fact, the presence of this added thickness necessarily prevents the sample from being located very close to the window, and therefore prevents the use of a microscope objective with very strong magnification.
Moreover, with such an arrangement, it is easy to envisage rotating the sample using the rotation device cooperating with the external cylindrical surface with a circular cross-section of the cryostat body, in order to produce 180° emission lobes through the quartz hollow part, and even 360° owing to a holed sample support mounted on the cold finger. In this latter case, the sample support then has a through-hole intended to be covered by the sample.
Furthermore, the cryostat according to the invention, preferably designed so as to maintain a sample located in the chamber at a temperature between 80° K and 300° K, and preferably at a temperature of around 80° K, in a secondary vacuum, makes it possible to excite and collect on the sample slice. By way of indication, it is noted that for the samples concerned, which can, for example, have a thickness of around 700 μm for a square or rectangular surface of which the sides have a dimension smaller than or equal to 10 mm, the slice of the sample is to be understood as defining all of the sides of the sample that are arranged according to thickness, perpendicularly to the surface of the sample on which the layer to be analysed is deposited.
Finally, as mentioned above, it is noted that the preferential use of quartz, and therefore silica, in order to produce the hollow part defining the chamber at the level of the sample to be analysed, makes it possible in particular to transmit by ultraviolet radiation.
As indicated above, the vacuum chamber is also defined by a cryostat body comprising a through-hole through which the finger portion passes, said wherein said cryostat body is connected in sealed manner to the hollow part and to the base portion of the cold finger. In such a case, the hollow part can then be securely assembled to an attachment flange mounted on the cryostat body, making sure for example that the hollow part is securely assembled to the attachment flange by gluing, and that the latter is mounted on the cryostat body by screwing.
In this regard, it is specified that the presence of this attachment flange makes it possible to facilitate the placement of the sample on the sample support of the cryostat, in the sense that it is simply necessary to disassemble the flange and remove the hollow part glued to the latter in order to be capable of placing the sample on the support provided for this purpose and attached to the cold finger.
Also preferably, the cryostat body is mounted on the base portion of the cold finger by screwing.
In addition, this cryostat body preferably comprises a hole allowing for the vacuum drawing of the chamber.
The quartz hollow portion preferably has a tube shape closed at one of its end, which tube can, for example, have a circular or substantially square cross-section. Thus, by using such a tube-shaped part, it is clear that analyses/measurements can be performed at a very short distance from the sample located inside the pressurised chamber.
Moreover, it is noted that, when a circular cross-section is used, there is an advantage in the fact that the emission lobes can be produced by collecting light with a constant distance and angle of incidence through the hollow part, which was obviously not the case with the planar windows used in the prior art. Furthermore, as mentioned above, it is easy to excite and collect over 180° or more, always by rotating the sample.
Finally, it is preferable for the finger portion to have the form of a straight glass slide in which a coil is etched, enabling the expansion of the pressurised gas through a series of diaphragms.
In addition, the invention also relates to an assembly for studying samples in a vacuum, said assembly including a source of electromagnetic radiation as well as a cryostat such as that defined above. In such a case, the hollow part forming one piece with the cryostat is naturally constituted by a material transparent to electromagnetic radiation, so as to enable the desired studies to be performed on the sample.
Other advantages and features of the invention will appear in the following non-limiting detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

This description will be provided with regard to the appended drawings, in which:

FIG. 1 shows a perspective view of a cryostat for studying samples in a vacuum, according to a preferred embodiment of this invention;

FIG. 2 shows a perspective view of the cold finger used in the cryostat shown in FIG. 1;

FIG. 3 shows a perspective view of the cryostat body used in the cryostat shown in FIG. 1, in association with the cold finger of FIG. 2;

FIG. 4 shows a perspective view of a quartz hollow part used in the cryostat shown in FIG. 1, as well as an attachment flange used to assemble said hollow part with the cryostat body; and

FIG. 5 shows a perspective view of an attachment between the assembly shown in FIG. 3 and the quartz hollow part of FIG. 4.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows the cryostat 1 for studying samples in a vacuum by means of electromagnetic radiation (not shown), said cryostat 1 being intended to cooperate with a microscope objective (not shown) in order to perform optical spectroscopy analyses of samples, micro-photoluminescence measurements, or any other study of samples in transmission, absorption and/or emission, known to a person skilled in the art.
In addition, this cryostat 1 is preferably designed so as to maintain the sample (not shown) at a temperature between 80° K and 300° K, and preferably at a temperature on the order of 80° K, in a secondary vacuum.
In general, the cryostat 1 comprises a cold finger 2, as well as a vacuum chamber 4 formed in particular by means of a one-piece hollow part 6, and a cryostat body 8 through which the cold finger 2 passes.
First, it can be seen in FIG. 2 that the cold finger 2 used is of the type known to a person skilled in the art, since it operates according to the same principle as that used in Joule-Thomson-type cold finger cryostats of the prior art.
Indeed, this cold finger 2 constitutes a system for cooling by expansion of a pressurised gas capable of reaching 120 bars or more, through a glass coil, wherein this adiabatic expansion is based on the known thermodynamic principle of the heat pump.
More specifically, it can be seen that the cold finger 2 comprises two distinct portions rigidly connected to one another, namely a base portion 10 comprising in particular a hole 12 for injecting pressurised gas into the finger, and from which a finger portion 14 projects forward, which finger portion has the form of a straight glass slide in which a coil 17 is etched (shown only with a dotted line in FIG. 2), enabling the expansion of the pressurised gas through a series of diaphragms.
This finger portion 14 therefore has a first end 16 rigidly connected to the base portion 10 of the cold finger 2, as well as a free cooling end 18, intended to cool the sample to be analysed with the cryostat 1 and the microscope.
By way of example, the cold finger 2 used can be identical or similar to the one sold by the MMR Technologies, Inc. company, in the product referenced “Model U377-0”.
FIG. 3 shows an exploded diagram of the cryostat body 8 assembled on the cold finger 2 by means of screws 20. Indeed, these screws 20 pass through blades 21 of the base portion 10 of the finger 2 at the level of passages 22, and are inserted into threaded holes (not visible) at the level of a rear lateral planar surface 24 of the body 8, which can then be pressed against a planar surface 26 of the base portion 10. It is noted that, as shown clearly in FIG. 3, the aforementioned planar surfaces 24 and 26 are preferably perpendicular to a main direction (not shown) of the finger portion 14 of the cold finger 2. In addition, in order to provide a sealed connection between elements 2 and 8, the planar surface 26 is provided with a flat surface on which a flat seal is positioned, or is provided with a system including a groove associated with an O-ring intended to contact the planar surface 24.
The body 8 has a general ring shape with its axis parallel to the main direction of the ring portion 14, and therefore has a central through-hole 30 through which said finger portion 14 of the finger 2 passes through completely. As shown in FIG. 3, the free cooling end 18, projecting with respect to the cryostat body 8 and therefore not located inside the hole 30, is equipped with a sample support 32. This sample support 32 is typically made of copper owing to its heat conduction properties, in order to evacuate the heat, and consequently enable the cooling of the sample glued by a thermal glue to the sample support 32.
The sample support 32 is of the type with holes so as to allow 360° emission lobes to be produced. It has a front support portion 32 a on which the sample to be analysed can be placed. In addition, the sample support 32 is installed so that a main axis of a given sample resting on this support 32 merges with the axis of the cryostat body 8.
Near the contact between the planar surfaces 24 and 26, the body 8 has a hole 34 enabling the cryostat to pump, this hole 34 being intended to cooperate with suitable pumping means (not shown) arranged so as to lead to the inside of the hole 30 intended to constitute a portion of the vacuum chamber 4. The hole 34 is preferably located on an upper portion of the cryostat body 8, and leads outwardly to a bevelled interface surface 35 between the planar surface 24 and an external cylindrical surface with a circular cross-section 36 centred on the axis of the ring-shaped body 8.
Also in reference to FIG. 3, the cryostat body 8 therefore has an external cylindrical surface with a circular cross-section 36, as well as a front lateral planar surface 38 parallel to the rear lateral planar surface 24. This planar surface 38 comprises threaded holes 40 for assembly of the hollow part 6, as well as a groove 42 for receiving an O-ring intended to provide a sealed connection between elements 8 and 6.
Now in reference to FIG. 4, it can be seen that the hollow part 6, preferably forming one piece made of quartz, and also called the quartz optical nose, is preferably in the shape of a tube with a circular cross-section closed off at one of its two ends, and having an opening 44 at the other of said two ends. Of course, the tube could have any other shape, for example with a substantially square cross-section, without going beyond the scope of the invention.
This hollow part 6 therefore defines a cavity 46 open only at the level of the opening 44, said cavity 46 constituting a portion of the vacuum chamber 4. It is thus specified that it is inside said open cavity 46 that the sample to be analysed is intended to be arranged, just as the sample support 32 and at least one portion of the finger portion 14 of the cold finger 2 are also intended to be located in said cavity 46.
For its assembly on the cryostat body 8, the quartz hollow part 6 is assembled securely at the level of the opening 44 using an ultra-high vacuum glue, to an attachment flange 50, which is intended to be screwed and pressed against the front lateral planar surface 38 of the body 8, with the O-ring 52 previously placed inside the groove 42 provided for this purpose. Naturally, when the cryostat 1 is in a final assembly state as shown in FIG. 1, the attachment flange 50 is substantially passed through by the finger portion 14 of the cold finger 2, which extends forward beyond said flange 50.
The attachment flange 50 preferably has an external cylindrical surface with a circular cross-section 54 of the same diameter as the external cylindrical surface having a circular cross-section 36 of the body 8, while an internal cylindrical surface having a circular cross-section 56 of this same flange 50 has a diameter greater than or equal to the largest diameter of the hole 30 of said body 8.
In reference to FIG. 5, it can therefore be seen that the hollow tube-shaped part 6, the attachment flange 50 and the cryostat body 8 are arranged coaxially along an axis parallel to the main direction of the finger portion 14 of the cold finger 2.
In addition, it can be seen that the attachment flange 50 is assembled by a screw 58 to the body 8, wherein said screw 58 passes through passages 60 (FIG. 4) provided on this same flange 50, and cooperating with the threaded holes 40 mentioned above.
When all of these elements are assembled as shown, it should therefore be understood that the vacuum chamber 4 is generally constituted by the open cavity 46 and the through-hole 30, and that this chamber 4 is therefore defined from front to back successively by the quartz hollow part 6, the attachment flange 50, the cryostat body 8, and the base portion 10 of the cold finger 2. In addition, this chamber 4 is entirely sealed, and is open only at the hole 34 for the pumping necessary for vacuum drawing.
Moreover, as the sample support 32 is located entirely in the open cavity 46, the sample itself, when it is properly positioned, is therefore also placed completely inside this cavity 46, at which level it is easily possible to approach a microscope objective in order to perform the required analyses.
Again in reference to FIG. 1, it can be seen that the cryostat 1 is equipped with a rotation device 62, which is attached around the external surface 36 of the body 8, so as to rotate the latter about its own axis. More specifically, the rotation device 62 can have a hole (not shown) with a diameter substantially identical to that of the external surface 36, so that the cryostat body 8 can be inserted into the same hole, so as to be subjected to a rotation, for example by being driven by friction. Thus, as the cryostat body 8 is rigidly connected to the cold finger 2 and the quartz hollow part 6, and the sample is attached, by gluing with a thermal glue with very good heat conductivity, to the sample support 32, itself securely mounted on the cold finger 2, it is the assembly of the above-mentioned elements that is subjected to the rotation applied to the cryostat body 8 by the device 62. In addition, as the rotation occurs around the axis of the cryostat body 8, the sample centred by means of the sample support 32 is also rotated around its own main axis.
This rotation device 62, optionally power-operated, can have any shape known to a person skilled in the art, and facilitates the production of emission lobes by self-centring of the axis of the sample on the rotation axis of the cryostat body 8. In other words, as the support 32 is adjusted in order to bring the upper surface of the sample on the axis of the body 8 of the cryostat 1 also corresponding to the axis of rotation of the latter, it is easy, by means of the manual or powered rotation device 62, to rotate the sample with respect to an axis coinciding with its upper surface, referred to below as the main axis. In this way, the presence of the rotation device 62 facilitates the production of emission lobes because it enables the use of a fixed source of electromagnetic radiation.
In this regard, as mentioned above, it is noted that the sample support 32 shown in the figures is of the type with holes. In particular, it has a through-hole 32 b at the level of the front support portion 32 a. Thus, when the sample is located so as to cover the hole 32 b, it is advantageously possible to produce 360° emission lobes through the quartz hollow part, by rotating the cryostat.
While the production of the hollow part 6 using quartz constitutes a preferred embodiment of the invention, it should be understood that the material is chosen essentially on the basis of the nature of the electromagnetic radiation used to study the sample, so that this material is transparent to said radiation as well as to visible light. Consequently, the material is determined primarily on the basis of whether the source is of the visible light, X-ray, ultraviolet infrared type and so on. By way of example, beryllium (Be) can be used in the case of X-rays.
Other materials envisaged will be presented below, in association with wavelength ranges in which these different materials ensure the necessary properties transparency. Naturally, the materials listed below are provided only as examples, and the choice of the material and the thickness for the one-piece hollow part 6 can easily be made by a person skilled in the art knowing the nature of the electromagnetic radiation (wavelength), as well as the type of study conducted on the sample.

EXAMPLES OF POSSIBLE MATERIALS

germanium (Ge): 1.7-28 μm
silicon (Si): 1.1-8 μm and 22-70 μm
gallium arsenide (GaAs): 1.1-31 μm
cadmium tellurium (CdTe): 1.1-30 μm
zinc sulphide (ZnS): 0.75-12 μm
zinc selenide (ZnSe): 0.6-19 μm
silver bromide (AgBr): 0.6-35 μm
thallium bromoiodide (KRS-5): 0.6-40 μm
lithium niobate (LiNbO3): 0.-6 μm
silver chloride (AgCl): 0.5-21 μm
titanium dioxide (TIO2): 0.5-2.5 μm
thallium bromoiodide (KRS-6): 0.4-26 μm
caesium iodide (CsI): 0.30-50 μm
sodium chloride (NaCl): 0.28-18 μm
IR fused silica (SiO2): 0.28-3.3 μm
potassium bromide (KBr): 0.28-22 μm
potassium chloride (KCl): 0.27-18 μm
caesium bromide (CsBr): 0.25-38 μm
calcite (CaCO3): 0.22-2.2 μm
sapphire (Al2O3): 0.25-2.8 μm
quartz (SiO2): 0.18-2.3 μm
UV fused silica (SiO2): 0.17-2 μm
calcium fluoride (CaF2): 0.17-9 μm
barium fluoride (BaF2): 0.18-11 μm
magnesium fluoride (MgF2): 0.15-6.5 μm
lithium fluoride (LiF): 0.14-6 μm
Of course, various modifications can be made by a person skilled in the art to the cryostat 1 for studying samples in a vacuum, described above only by way of a non-limiting example.

Claims

1. Cryostat (1) for studying samples in a vacuum using electromagnetic radiation, said cryostat comprising a cold finger (2) equipped with a finger portion (14) as well as a base portion (10) rigidly connected to the finger portion (14) and enabling in particular the arrival of pressurised gas, which cryostat also includes a sample support (32) mounted on a free end for cooling (18) of the finger portion (14), wherein this same finger portion (14) of the cold finger (2) is placed in a vacuum chamber (4), wherein said vacuum chamber (4) is partially defined by a one-piece hollow part (6) defining an open cavity (46) with a single opening (44) through which the finger portion (14) passes, and the sample support (32) is located inside the aforementioned open cavity (46), characterised in the said vacuum chamber (4) is also defined by a cryostat body (8) comprising a through-hole (30) through which the finger portion (14) passes, wherein said cryostat body (8) is connected in a sealed manner to the hollow part (6) and to the base portion (10) of the cold finger (2), and in that the cryostat is also equipped with a rotation device (62) cooperating with an external cylindrical surface with a circular cross-section (36) of the cryostat body (8).

2. Cryostat (1) according to claim 1, characterised in that said hollow part (6) is securely assembled to an attachment flange (50) mounted on said cryostat body (8).

3. Cryostat (1) according to claim 2, characterised in the said hollow part (6) is assembled securely to the attachment flange (50) by gluing, and in that the latter is mounted on said cryostat body (8) by screwing.

4. Cryostat (1) according to claim 1, characterised in that said cryostat body (8) is mounted on said base portion (10) of the cold finger (2) by screwing.

5. Cryostat (1) according to claim 1, characterised in that said cryostat body (8) comprises a hole (34) allowing for the vacuum drawing of said chamber (4).

6. Cryostat (1) according to claim 1, characterised in that said hollow part (6) has a tube shape closed off at one of its two ends.

7. Cryostat (1) according to claim 6, characterised in that said hollow part (6) has a tube shape with a circular cross-section.

8. Cryostat (1) according to claim 6, characterised in that said hollow part (6) has a tube shape with a substantially square cross-section.

9. Cryostat (1) according to claim 1, characterised in that it is designed so as to maintain a sample located in said chamber (4) at a temperature of around 80° K, and in a secondary vacuum.

10. Cryostat (1) according to claim 1, characterised in that said hollow part (6) defining the open cavity (46) is made of quartz.

11. Cryostat (1) according to claim 1, characterised in that said sample support (32) has a through-hole (32 b) intended to be covered by the sample.

12. Cryostat (1) according to claim 1, characterised in that said finger portion (14) has the form of a straight glass slide in which a coil (17) is etched, enabling the expansion of the pressurised gas through a series of diaphrams.

13. Assembly for studying samples in a vacuum, characterised in that it includes a source of electromagnetic radiation, as well as a cryostat (1) according to any one of the previous claims.