Sample cooling device for scanning electron microscope

Abstract

To provide a sample cooling device for a scanning electron microscope, capable of preventing a fine oscillation of a sample as well as suppressing a temperature increase of the sample.SOLUTION: A sample cooling device 1 comprises: a chamber 3 defining a sealed space 2 in an internal part; a vacuum pump 4 that makes the sealed space 2 in this chamber 3 into a vacuum state; a sample stage 7 that can be arranged at an irradiation place of an electronic beam 6 of a lens tube 5 of the scanning electron microscope; a cylindrical cooling block 8 onto which this sample stage 7 is mounted; a cylindrical holding part 10 that is arranged around this cooling block 8, and holds the cooling block 8 with a plurality of fixed screws 9; a driving device 11 that moves this cylindrical holding part 10 in X-axis, Y-axis, and Z-axis directions with respect to the electron beam 6 radiated from the lens tube 5; a heat transmission plate 12 that is fixed at a lower end side of the cooling block 8; and a refrigeration head 13 that is provided so as to be opposite to the heat transmission plate 12, and comprises a refrigeration machine 14 for cooling this refrigeration head 13.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a sample cooling device for an electron microscope that is capable of cooling a sample during sample observation using a scanning electron microscope. [Background technology]

[0002] Patent Document 1 (JP Patent Publication 6-215719A) discloses a sample cooling device for electron microscopes and the like that cools the vicinity of the sample to prevent contamination of the sample and prevents sample vibration caused by bumping of liquid nitrogen, enabling image observation. This cooling device is provided with an electromagnetic clutch that adsorbs and separates the cooling rod, which is cooled through a liquid nitrogen tank, from the liquid nitrogen ankle, and the operation of the electromagnetic clutch adsorbs the cooling rod and the liquid nitrogen tank during cooling and separates them during image observation.

[0003] Patent Document 2 (JP Patent Publication 6-260125A) discloses a sample cooling device that can stably maintain a sample cooling temperature of from room temperature to -30°C when cooling a sample in an electron microscope. This sample cooling device holds a Peltier element inside the sample stage and cools the sample via a copper mesh wire, and the heat generated during this process is dissipated into the atmosphere using heat dissipation fins and cooling water pipes, thereby stably cooling the sample from room temperature to -30°C.

[0004] Patent Document 3 (JP Patent Publication 2000-208083 A) discloses a sample cooling device for an electron microscope that can cool an electron microscope sample to extremely low temperatures (several K) with liquid helium and use a dedicated helium cooling stage for observation and analysis at low temperatures (several tens of K) with liquid nitrogen as another cooling temperature. This cooling device comprises a sample holding member having a sample holder in the center for holding a sample and a coolant reservoir for storing a coolant for cooling the sample provided on the outer periphery of the sample holder, two coolant tanks for storing two types of coolant, and two capillaries connecting the two coolant tanks with the coolant reservoir, and a mechanism for selectively supplying the two types of coolant to the coolant reservoir is provided. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-215719 [Patent Document 2] Japanese Patent Application Publication No. 6-260125 [Patent Document 3] JP 2000-208083 A Summary of the Invention [Problem to be solved by the invention]

[0006] The methods or devices using liquid nitrogen as disclosed in Patent Documents 1 and 3 have the disadvantages of requiring a large amount of liquid nitrogen, pre-cooling the entire device to prevent bumping, etc., which results in a large device and requires a long preparation time. Furthermore, Patent Document 1 has the disadvantage that the cooling rod, although separated from the liquid nitrogen tank, is not completely thermally insulated, and the sample temperature rises relatively quickly.

[0007] As disclosed in Patent Document 2, the cooling method using a Peltier element has the disadvantage that it can only lower the temperature to about -50°C at most, making it impossible to observe living organisms, which requires low temperatures at the level of -120°C.

[0008] As a method for cooling a sample to -120°C or below without using liquid nitrogen, it is preferable to use a freezing device such as a Stirling cooler or a freezing device using a refrigerant, but since these devices use mechanical reciprocating motion, vibrations become an issue, so measures such as stopping the device itself or indirectly cooling the sample stage are necessary during high-magnification observation. In addition, when the device itself is stopped, the cooling action stops, so there is a problem that the sample temperature rises in a relatively short time, and indirect cooling methods have issues such as reduced cooling efficiency due to the complexity of the device and thermal loss, and furthermore, there is a problem that micro-vibrations from the refrigeration cycle are transmitted through the piping.

[0009] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a sample cooling device for a scanning electron microscope that can suppress an increase in the temperature of the sample and prevent micro-vibrations of the sample. [Means for solving the problem]

[0010] The present invention relates to a sample cooling device for a scanning microscope in which a sample for a scanning electron microscope is placed and which cools the sample, the device comprising: a chamber defining an enclosed space therein; a vacuum pump for evacuating the enclosed space within the chamber; a sample stage which can be placed at a location irradiated by an electron beam from the column of the scanning electron microscope; a cylindrical cooling block on which the sample stage is placed; a cylindrical holding part which is placed around the cooling block and which holds the cooling block by point contact with a plurality of fixing screws; a drive unit which moves the cylindrical holding part in X-axis, Y-axis and Z-axis directions relative to the electron beam irradiated from the column; a heat transfer plate fixed to the lower end side of the cooling block; and a refrigerator which has a freezing head provided opposite the heat transfer plate and which cools the freezing head.

[0011] With the above configuration, in the stage of freezing the sample placed on the sample stage of the cooling block, the cooling block for cooling the sample stage on which the sample is placed is lowered in the Z direction by the driving device, the cooling block is brought into contact with the freezing head of the refrigerator via a heat transfer plate fixed to the lower end side of the cooling block, the refrigerator is driven to cool the cooling block, and the sample is cooled via the cooling block. Note that when a refrigerator such as a Stirling cooler is used as the refrigerator, when a refrigerator using a normal refrigeration cycle is used, the cooling temperature of the sample can reach the level of -120°C, but since it has a compressor that compresses the refrigerant, etc., there is a problem that mechanical micro-vibrations are transmitted from piping, etc.

[0012] When the sample reaches the cooling temperature and is ready to be observed, the refrigerator is stopped and at the same time, the cooling block is raised in the Z direction by the driving device to prevent the heat from the freezing head caused by the refrigerator being stopped from being transferred to the cooling block (to prevent a decrease in freezing power). This creates a gap between the heat transfer plate of the cooling block and the freezing head, preventing heat transfer, and therefore suppressing the rise in the temperature of the cooling block and, ultimately, the temperature of the sample.

[0013] The cooling block is required to have good thermal conductivity from the viewpoint of cooling speed, and also to have a large volumetric specific heat from the viewpoint of cold storage, so it is desirable to use a copper material.

[0014] Furthermore, the heat transfer plate is preferably made of a material having a high heat transfer ability, such as silver, copper, gold, aluminum, and a thermally conductive carbon material.

[0015] Furthermore, it is preferable that the fixing screw is a plastic screw and supports the cooling block by point contact, thereby suppressing the transfer of heat between the cooling block and the cylindrical support part, and thus suppressing the rise in temperature of the cooling block.

[0016] It is also preferable that the cylindrical holding portion has a flange portion formed at its upper end extending in the radial direction, the drive device has an annular drive base capable of contacting the flange portion of the cylindrical holding portion from below in the Z direction, and a biasing means is provided between the flange portion and the drive base for biasing the cylindrical holding portion downward via the flange portion. The biasing means is preferably a pressure spring.

[0017] As a result, during cooling, the driving device lowers the cylindrical holding portion (cooling block) in the Z direction so that it comes into contact with the freezing head, and the cooling block and freezing head come into contact via the heat transfer plate.When it is further lowered, the biasing means can press the freezing block and heat transfer plate firmly against the freezing head, thereby improving heat transfer.

[0018] During observation, the cooling block rises in the Z direction and is separated from the freezing head, but the cylindrical holder is strongly pressed against the drive base by the action of the pressing spring acting as a biasing means, so vibration problems do not occur during high-magnification observation. Effect of the Invention

[0019] As described above, according to the sample cooling device of the present invention, in the stage of freezing the sample, the refrigerator with high refrigeration capacity can store cold in the cooling block that cools the sample, and in the stage of observing the sample, the refrigerator is stopped and a gap is provided between the cooling block and the freezing head to block the cold, thereby maintaining the cold storage capacity of the block and maintaining the frozen state of the sample. In addition, since the refrigerator can be stopped when observing the sample, it is possible to eliminate micro-vibrations transmitted to the sample.

[0020] In addition, in conventional sample stages capable of cooling samples to liquid nitrogen levels, the movement of the sample stage was mostly limited to the XY plane because the sample stage was connected to a large-scale cooling device. However, in the present invention, the sample section is completely separated from the refrigeration device, so there are no limitations to the drive function, and it is possible to move the sample not only in the X, Y, and Z directions, but also in rotation and tilt. Furthermore, energy can be saved by turning off the power to the refrigerator during sample observation.

[0021] It is preferable that the cylindrical holding portion has a flange portion extending radially from its upper end, the drive device has an annular drive base that can contact the flange portion of the cylindrical holding portion from below in the Z direction, and a biasing means is provided between the flange portion and the drive base for biasing the cylindrical holding portion downward via the flange portion.

[0022] As a result, when cooling a sample, the cooling block can be pressed against the freezing head by the biasing means, thereby improving cooling capacity, and further, when observing a sample, the cooling block can be firmly fixed to the flange portion, thereby suppressing vibration. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is an explanatory diagram showing a state of a sample cooling device according to a first embodiment of the present invention during sample observation. [Diagram 2] FIG. 2 is an explanatory diagram showing a state of the sample cooling device according to the first embodiment of the present invention when freezing a sample. [Diagram 3] FIG. 3 is an explanatory diagram showing a state of the sample cooling device according to the second embodiment of the present invention during sample observation. [Figure 4] FIG. 4 is an explanatory diagram showing a state in which the sample cooling device according to the second embodiment of the present invention freezes a sample. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. EXAMPLES

[0025] 1, a sample cooling device 1 for a scanning microscope according to an embodiment of the present invention includes a chamber 3 defining an enclosed space 2 therein, a vacuum pump 4 for evacuating the enclosed space 2 in the chamber 3, a sample stage 7 that can be placed at a location irradiated with an electron beam 6 from a lens barrel 5 of the scanning electron microscope, a cylindrical cooling block 8 on which the sample stage 7 is placed, a cylindrical holder 10 that is placed around the cooling block 8 and holds the cooling block 8 with a number of fixing screws 9, a drive unit 11 that moves the cylindrical holder 10 in the X-axis, Y-axis and Z-axis directions relative to the electron beam 6 irradiated from the lens barrel 5, a heat transfer plate 12 fixed to the lower end side of the cooling block 8, and a refrigerator 14 that has a refrigerator head 13 provided opposite the heat transfer plate 12 and cools the refrigerator head 13. Reference numeral 15 denotes a detector for detecting an image signal of the electron microscope.

[0026] The refrigerator 14 is a Stirling cooler that uses a Stirling engine system, or a refrigeration device that uses a refrigeration cycle that is at least constituted by a compressor, an expander, an expansion valve, and a condenser.

[0027] Moreover, the cooling block 8 is required to have good thermal conductivity from the viewpoint of cooling speed, and also to have a large volumetric specific heat from the viewpoint of cold storage, so it is desirable to use a copper material.

[0028] Furthermore, the heat transfer plate is preferably made of a material having a high heat transfer ability, such as silver, copper, gold, aluminum, and a thermally conductive carbon material.

[0029] The fixing screws 9 are preferably made of a material with excellent heat insulation properties, particularly plastic, and support the cooling block by point contact.

[0030] With the above configuration, at the stage of freezing the sample placed on the sample stage 7 of the cooling block 8, the cooling block 8 for cooling the sample stage 7 on which the sample is placed is lowered in the Z direction by the driving device 11, and the cooling block 8 is brought into contact with the freezing head 13 of the refrigerator 14 via the heat transfer plate 12 fixed to the lower end side of the cooling block 8, and the refrigerator 14 is driven to cool the cooling block 8, and the sample can be further cooled via the cooling block 8.

[0031] When a refrigerator such as a Stirling cooler or a refrigerator using a normal refrigeration cycle is used as refrigerator 14, it is possible to achieve a cooling temperature of the sample of -120°C, but since it has a compressor that compresses the refrigerant, there is a problem that mechanical vibrations are transmitted from piping, etc. For this reason, the operation of refrigerator 14 is stopped at the stage of observing the sample to solve this problem.

[0032] When the sample reaches the cooling temperature and is to be observed, the refrigerator 14 is stopped and at the same time, the cooling block 8 is raised in the Z direction by the drive unit 11 so that the heat from the freezing head 13 caused by the stopping of the refrigerator 14 is not transferred to the cooling block 8 (so that the freezing capacity is not reduced). This creates a gap between the heat transfer plate 12 of the cooling block 8 and the freezing head 13 (thermally insulating them), preventing heat transfer, and therefore suppressing the rise in the temperature of the cooling block 8 and, ultimately, the temperature of the sample.

[0033] According to the present invention, the operation of the refrigerator is stopped to prevent the sample from vibrating slightly, but since the cooling block 8 for cooling the sample stage is made of a material with high cold storage capacity, the temperature rise of the sample can be suppressed. Also, in order to promote the cooling of the cooling block 8, a transfer plate 12 made of a material with high heat conductivity is placed at the bottom end of the cooling block 8, so that the cooling from the freezing head 13 to the cooling block 8 can be promoted.

[0034] Furthermore, when the operation of the refrigerator 14 is stopped at the stage of observing the sample, a gap 16 is formed between the freezing head 13 of the refrigerator 14 and the transfer plate 12 of the cooling block 8, thereby preventing the temperature rise of the freezing head 13 caused by the stopping of the refrigerator 1 from affecting the cooling block 8 through the heat transfer plate 12. EXAMPLES

[0035] 3, the sample cooling device 1 for a scanning microscope according to the second embodiment of the present invention includes a chamber 3 defining an enclosed space 2 therein, a vacuum pump 4 for evacuating the enclosed space 2 in the chamber 3, a sample stage 7 that can be placed at a location irradiated with an electron beam 6 from a lens barrel 5 of the scanning electron microscope, a cylindrical cooling block 8 on which the sample stage 7 is placed, a cylindrical holder 10 that is placed around the cooling block 8 and holds the cooling block 8 with a plurality of fixing screws 9, a drive unit 11 that moves the cylindrical holder 10 in the X-axis, Y-axis and Z-axis directions relative to the electron beam 6 irradiated from the lens barrel 5, a heat transfer plate 12 fixed to the lower end side of the cooling block 8, and a refrigerator 14 that has a refrigerator head 13 provided opposite the heat transfer plate 12 and cools the refrigerator head 13. Reference numeral 15 denotes a detector for detecting an image signal of the electron microscope.

[0036] In the sample cooling device 1 of this embodiment 2, a flange portion 10A extending radially is formed at the upper end of the cylindrical holding portion 10, and a ring-shaped drive base 11B is provided at the tip of a drive arm 11A extending from a drive unit 11 so as to surround the cylindrical holding portion 10 with a predetermined gap therebetween, and the drive arm 11A and the flange portion 10A are movably connected by a pressure spring 16.

[0037] With the above-mentioned configuration, when cooling the cooling block 8 as shown in Figure 4, when the drive arm 11A is lowered by the drive unit 11, the cylindrical holding portion 10 is lowered via the pressing spring 16, and the cooling block 8, which is point-supported on the cylindrical holding portion 10 by the fixing screw 9, comes into contact with the freezing head 13 via the heat transfer plate 12, and is pressed against the freezing head 13 by the biasing force of the pressing spring 16, thereby improving the cooling efficiency by the freezing head 13.

[0038] In addition, at the stage where the sample reaches the cooling temperature and is observed, the refrigerator 14 is stopped and at the same time, the driving arm 11A is raised by the driving device 11 so that the heat of the freezing head 13 caused by the stopping of the refrigerator 14 is not transferred to the cooling block 8 side (so that the freezing capacity does not decrease), and the driving base 11B abuts against the flange portion 10A, so that the cylindrical holding portion 10 rises and the cooling block 8 rises in the Z direction, creating a gap between the heat transfer plate 12 of the cooling block 8 and the freezing head 13 (thermally isolating them), preventing the transfer of heat, and thus suppressing the rise in the temperature of the cooling block 8 and the temperature of the sample. In addition, the cylindrical holding portion 10, the cooling block 8, and the sample stage 7 are strongly pressed against the driving base by the action of the pressing spring 16, so that vibration problems do not occur during high magnification observation. [Explanation of symbols]

[0039] 1. Sample cooling device 2 Closed space 3. Chamber 4. Vacuum pump 5. Telescope tube 6. Electron Beam 7 Sample stage 8 Cooling Block 9 Fixing screw 10 Cylinder holder 10A Flange part 11 Drive unit 11A Drive arm 11B Drive base 12 Heat transfer plate 13 Frozen Head 14 Refrigeration Machine 15 Detector 16 Compression spring

Claims

1. A sample cooling device for a scanning electron microscope, in which a sample of the scanning electron microscope is placed and which cools the sample, comprising: A chamber defining an enclosed space therein; a vacuum pump for creating a vacuum in the sealed space within the chamber; a sample stage that can be placed at an irradiation point of the electron beam of the lens barrel of the scanning electron microscope; a cylindrical cooling block on which the sample stage is placed; a cylindrical holder disposed around the cooling block and holding the cooling block by point contact with a plurality of fixing screws; a driving device that moves the cylindrical holder in X-axis, Y-axis, and Z-axis directions with respect to the electron beam irradiated from the lens barrel; A heat transfer plate fixed to a lower end side of the cooling block; a cooling head provided opposite said heat transfer plate, and a refrigerator for cooling said cooling head.

2. 2. The sample cooling device according to claim 1, wherein the heat transfer plate is selected from the group consisting of silver, copper, gold, aluminum and thermally conductive carbon materials.

3. 3. The sample cooling device according to claim 1, wherein the fixing screw is a plastic screw and supports the cooling block by point contact.

4. The sample cooling device of any one of claims 1 to 3, characterized in that the cylindrical holding portion is formed with a flange portion extending radially from its upper end, the driving device is provided with an annular driving base that can contact the flange portion of the cylindrical holding portion from below in the Z direction, and a biasing means is provided between the flange portion and the driving base for biasing the cylindrical holding portion downward via the flange portion.

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