Specimen 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 can cool a sample when observing the sample with a scanning electron microscope. [Background technology]

[0002] Patent Document 1 (JP Patent Publication No. 6-215719) discloses a sample cooling device for electron microscopes, etc., that cools the area around the sample to prevent contamination of the sample and prevents sample vibration due to bumping of liquid nitrogen, allowing image observation. This cooling device is equipped with an electromagnetic clutch that attracts and separates the cooling rod, which is cooled through a liquid nitrogen tank, from the liquid nitrogen tank, and by operating the electromagnetic clutch, the cooling rod and liquid nitrogen tank are attracted to each other during cooling and separated during image observation.

[0003] Patent Document 2 (JP Patent Publication No. 6-260125) 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 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) using liquid helium, and use a dedicated helium cooling stage for observation and analysis at low temperatures (several tens of K) using liquid nitrogen as another cooling temperature. This cooling device has a sample holder in the center that holds the sample, a sample holder member with a refrigerant reservoir around the outer periphery of the sample holder that contains a refrigerant for cooling the sample, two refrigerant tanks for storing two types of refrigerant, and two capillaries that connect the two refrigerant tanks to the refrigerant reservoirs, and a mechanism for selectively supplying the two types of refrigerant to the refrigerant reservoirs. [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] Japanese Patent Application Laid-Open No. 2000-208083 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 drawbacks, such as the need for a large amount of liquid nitrogen and the need to pre-cool the entire device to prevent bumping, which results in a large device and requires a long preparation time. Also, Patent Document 1 has the drawback that the cooling rod, although separated from the liquid nitrogen tank, is not completely thermally insulated, causing the sample temperature to rise relatively quickly.

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

[0008] A preferred method for cooling samples to below -120°C without using liquid nitrogen is to use a freezing device such as a Stirling cooler or a freezing device that uses a refrigerant. However, these devices use mechanical reciprocating motion, which can cause vibration problems. Therefore, measures such as shutting down the device itself or indirectly cooling the sample stage are necessary during high-magnification observations. Furthermore, shutting down the device itself halts the cooling action, resulting in a relatively short rise in sample temperature. Indirect cooling methods have the drawback of reduced cooling efficiency due to the complexity of the device and thermal loss. Furthermore, there is the problem of micro-vibrations from the refrigeration cycle being transmitted through the piping.

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

[0010] The present invention provides a sample cooling device for a scanning electron 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 holds the cooling block in point contact with a plurality of fixing screws; a drive unit which moves the cylindrical holding part in the 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 a 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 a drive unit, the cooling block for cooling the sample stage is brought into contact with the freezing head of the refrigerator via a heat transfer plate fixed to the lower end 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, or a refrigerator using a normal refrigeration cycle, it is possible to achieve a sample cooling temperature of -120°C, but since it has a compressor for compressing the refrigerant, there is a problem that mechanical micro-vibrations are transmitted from the 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 a drive unit to prevent the heat from the freezing head caused by the refrigerator stopping from being transferred to the cooling block (to prevent a decrease in freezing power), creating a gap between the heat transfer plate of the cooling block and the freezing head, preventing heat transfer and thereby 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, or a thermally conductive carbon material.

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

[0016] Preferably, 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 that can come into contact with 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 to bias the cylindrical holding portion downward via the flange portion. Note that the biasing means is preferably a compression 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 the cooling block 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 pressed firmly 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. [Effects of the Invention]

[0019] As described above, the sample cooling device of the present invention has the following advantages: when freezing a sample, a refrigerator with high refrigeration capacity can store cold in the cooling block that cools the sample, and when observing the sample, the refrigerator can be stopped and a gap can be created between the cooling block and the freezing head to maintain the cold storage capacity of the block and keep the sample frozen. Furthermore, because the refrigerator can be stopped during sample observation, it is possible to eliminate micro-vibrations transmitted to the sample.

[0020] Furthermore, in conventional sample stages capable of cooling samples to liquid nitrogen levels, the sample stage is connected to a large-scale cooling device, so movement of the sample stage is mostly limited to the X and Y planes. However, in this invention, the sample section is completely separated from the refrigerator, so there are no restrictions on drive functions, 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 at 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 to bias 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 the cooling capacity, and further, when observing a sample, the cooling block can be firmly fixed to the flange portion, thereby suppressing vibration. [Brief explanation 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 when a sample is observed. [Figure 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. [Figure 3] FIG. 3 is an explanatory diagram showing a state of the sample cooling device according to the second embodiment of the present invention when a sample is observed. [Figure 4] FIG. 4 is an explanatory diagram showing a state of the sample cooling device according to the second embodiment of the present invention when freezing a sample. DETAILED DESCRIPTION OF THE INVENTION

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

[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 within the chamber 3, a sample stage 7 that can be placed at a location irradiated by 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 disposed 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-, Y-, 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 of the cooling block 8, and a refrigerator 14 that has a refrigerator head 13 disposed opposite the heat transfer plate 12 and cools the refrigerator head 13. Also, reference numeral 15 denotes a detector for detecting image signals from 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 composed of a compressor, an expander, an expansion valve, and a condenser.

[0027] Furthermore, the cooling block 8 is required to have good thermal conductivity from the viewpoint of cooling speed, and also to have a large volume 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, or 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 then be cooled via the cooling block 8.

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

[0032] When the sample reaches the cooling temperature and is ready 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 to prevent the heat from the freezing head 13 caused by the stopping of the refrigerator 14 from being transferred to the cooling block 8 (to prevent a decrease in freezing capacity), creating 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 thereby 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 the cooling block 8 for cooling the sample stage is made of a material with high cold storage capacity, so that the temperature rise of the sample can be suppressed. Also, 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 during the sample observation stage, 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 14 from affecting the cooling block 8 through the heat transfer plate 12. [Example]

[0035] 3 , a sample cooling device 1 for a scanning microscope according to a 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 within 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 disposed 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-, Y-, 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 of the cooling block 8, and a refrigerator 14 that has a refrigerator head 13 disposed opposite the heat transfer plate 12 and cools the refrigerator head 13. Also, reference numeral 15 denotes a detector for detecting an image signal from the electron microscope.

[0036] In the sample cooling device 1 of this Example 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 device 11, surrounding the periphery of 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-described configuration, when cooling cooling block 8 as shown in FIG. 4, when drive arm 11A is lowered by drive device 11, cylindrical holding portion 10 is lowered via pressing spring 16, and cooling block 8, which is point-supported on cylindrical holding portion 10 by fixing screws 9, comes into contact with freezing head 13 via heat transfer plate 12, and is pressed against freezing head 13 by the urging force of pressing spring 16, thereby making tight contact, thereby improving the cooling efficiency of freezing head 13.

[0038] Furthermore, when the sample reaches the cooling temperature and is ready for observation, the refrigerator 14 is stopped and, at the same time, the drive arm 11A is raised by the drive unit 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 (so that the freezing capacity does not decrease). As a result, the drive base 11B abuts against the flange portion 10A, so that the cylindrical holder 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 heat transfer and suppressing an increase in the temperature of the cooling block 8 and, ultimately, the sample. Furthermore, the action of the pressing spring 16 presses the cylindrical holder 10, cooling block 8, and sample stage 7 firmly against the drive base, so that vibration problems do not occur during high-magnification observation. [Explanation of symbols]

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

Claims

1. 1. A sample cooling device for a scanning electron microscope, in which a sample of a scanning electron microscope is placed and which cools the sample, a chamber defining an enclosed space therein; a vacuum pump that creates 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 electron microscope; a cylindrical cooling block on which the sample stage is placed; a cylindrical holding portion 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 holding unit in X-axis, Y-axis, and Z-axis directions relative to the electron beam emitted from the electron 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 made of a material selected from the group consisting of silver, copper, gold, aluminum, and thermally conductive carbon material.

3. 3. The sample cooling device according to claim 1, wherein the fixing screws are plastic screws and support the cooling block by point contact.

4. The sample cooling device according to claim 1, characterized in 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 to bias the cylindrical holding portion downward via the flange portion.

Images