JP7734404B2 - stage - Google Patents

Description

The present invention relates to a stage, and in particular to a stage to which an attachment can be attached.

In recent years, advances have been made in high-resolution analysis using electron microscopes such as transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM). For example, there is a demand for high-resolution analysis, from nanometer-order to picometer-order resolution. Recently, "in-situ observation," in which a sample is cooled (or heated, electric or magnetic field applied, or rotated) while being observed inside the electron microscope, has been attracting attention. Sample cooling is particularly considered effective in reducing damage to the sample caused by the electron beam, and sample cooling from this perspective is also being attempted. For example, a known example of a device with a cooling means is a sample processing device using a scanning electron microscope. This device is equipped with a cooling stage in the sample chamber of the scanning electron microscope on which a sample with sublimated water is placed, and a manipulator that extends above the cooling stage and extracts required components of the sample under observation by the scanning electron microscope (Patent Document 1).

Japanese Unexamined Patent Publication No. 62-85840

Despite this need for cooling, existing techniques for fixing samples to cooling stages, including the aforementioned Patent Document 1, only involve attaching the sample to the top surface of the sample pedestal, limiting cooling to the sample from below. This poses challenges for specialized applications, such as FIB (Focused Ion Beam) processing on cooling stages. For example, when performing FIB processing while cooling, the stubs holding the FIB grid must be fixed with double-sided tape or adhesive. However, these methods often use materials with low thermal conductivity. Furthermore, gaps and other inconsistencies in thermal contact occur with each fixation, resulting in heat loss and inefficiency in terms of thermal conduction. Furthermore, double-sided tape fixation is unstable and insufficient for precise FIB processing, posing problems such as the risk of peeling. While adhesives provide strong adhesion, they require removal for normal observation, which often requires significant effort. Furthermore, residual adhesive on the sample pedestal can interfere with subsequent observations, making this method undesirable. Similar difficulties are expected not only when fixing the pedestal that holds the FIB grid, but also when fixing a pre-tilted sample stage.

The present invention aims to solve the above problems by providing a stage to which attachments can be easily attached.

To achieve the above objective, the inventor conducted extensive research into the stage mechanism, and as a result, came up with the present invention.

That is, the stage of the present invention is a stage having a sample pedestal for mounting a sample and a cooling section for cooling the sample, wherein the sample pedestal is provided with an attachment and an attachment mounting device for mounting the attachment , and further has a thermoelectric element installed in close proximity to the cooling section, and the sample pedestal and the thermoelectric element are in contact with each other .

Furthermore, in a preferred embodiment of the stage of the present invention, the cooling section is characterized by having an extensible heat-conducting section.

Furthermore, in a preferred embodiment of the stage of the present invention, the attachment mounting device is a screw hole or a clamp.

Furthermore, in a preferred embodiment of the stage of the present invention, the attachment is at least one selected from an FIB grid holding attachment, a sample fixing attachment, and an electrical current attachment.

Furthermore, in a preferred embodiment of the stage of the present invention, the thermoelectric element is a thermoelectric element that utilizes at least one of the Peltier effect and the Thomson effect.

Furthermore, in a preferred embodiment of the stage of the present invention, the heat dissipation side of the thermoelectric element is in contact with the cooling section or the heat conduction section.

Furthermore, in a preferred embodiment of the stage of the present invention, the cooling section is characterized by comprising at least one of a solid refrigerant, a liquid refrigerant, and a gas refrigerant.

In a preferred embodiment of the stage of the present invention, the sample pedestal and the thermoelectric element are in contact with each other via a heat dissipation member .

Furthermore, in a preferred embodiment of the stage of the present invention, the attachment mounting device is made of a thermally conductive material.

The stage of the present invention has the advantageous effect of being able to mount an attachment. Furthermore, the stage of the present invention has the advantageous effect of being able to suppress heat loss even when an attachment is mounted.

Figure 1 shows a conceptual diagram of an example stage in one embodiment of the present invention, where Figure 1(a) shows a top view of the stage in one embodiment of the present invention, Figure 1(b) shows a side view of the stage, and Figure 1(c) shows a perspective view of the stage. Figure 2 shows an embodiment of a thermoelectric element applicable to the present invention, where Figure 2(a) shows a cross-sectional view of a Peltier element, and Figure 2(b) shows a schematic diagram of the principle of the Peltier element. FIG. 3 is a conceptual diagram of a stage showing an example in which an FIB grid is installed as an attachment in one embodiment of the present invention. FIG. 4 is a conceptual diagram of a stage showing an example of a case in which a sample is fixed from above as an attachment in one embodiment of the present invention.

The stage of the present invention comprises a sample pedestal for mounting a sample and a cooling unit, and the sample pedestal is characterized by being equipped with an attachment mounting device. In the present invention, the sample stage for mounting a sample is not particularly limited, including its shape and structure, as long as it is capable of mounting a sample to be observed in an electron microscope. Furthermore, in the present invention, the cooling unit is not particularly limited, including its shape and structure, as long as it is capable of cooling a sample to be observed in an electron microscope. Furthermore, in the present invention, the sample pedestal is equipped with an attachment mounting device. In other words, the only method of fixing a sample to an existing cooling stage is to attach the sample to the top surface of the sample pedestal with double-sided tape or adhesive, which results in unstable fixation. However, in the present invention, the sample pedestal is equipped with an attachment mounting device, which allows for easy attachment and removal of attachments, making it possible to interchange various attachments. The sample pedestal can be cooled by a cooling unit or Peltier element, but areas other than the cooling unit are hotter. Therefore, if an attachment is connected and fixed to an area other than the cooling unit (outside the cooling system), the attachment will receive heat from outside the cooling system and warm up. In this case, when the attachment warms up, heat is transferred to the sample, warming the cooled sample and reducing the cooling effect. In the present invention, the sample pedestal is equipped with an attachment mounting device, so the attachment is present within the cooling system via the attachment mounting device, making it possible to cool the sample with minimal heat loss.

In a preferred embodiment of the stage of the present invention, the attachment mounting device is a screw hole or a clamp, from the viewpoint of enabling attachments to be fixed firmly and with good reproducibility. In the present invention, the attachment mounting device, e.g., a screw portion, is provided, making it easy to change attachments depending on the purpose. Double-sided tape and adhesives are difficult to remove once the attachment is fixed, and are not intended for reuse. Furthermore, each time the attachment is replaced, the thermal contact between the attachment and the sample pedestal changes, and the attachment's posture is difficult to control. In contrast, in the present invention, the attachment mounting device allows attachments to be changed with good reproducibility, thereby offering the advantage of being able to flexibly accommodate a variety of applications.

Furthermore, the attachment is not particularly limited as long as it can be attached, but in a preferred embodiment of the stage of the present invention, the attachment is at least one selected from an FIB grid holder attachment, a sample fixing attachment, and an electrical current attachment. In addition to the FIB grid holder attachment and top surface fixing attachment shown in the drawings described below, when a sample is observed with the clamping orientation horizontal, a temperature gradient can be created by cooling the clamped portion toward the free end. In this invention, attachments that can impart such a temperature gradient can also be installed. Attachments that have a cold trap function to prevent contamination by placing a cooling member near the top surface of the sample are also conceivable.

When the term "cold trap" is used here, it can be understood as a device that cools and captures trace amounts of suspended matter (gases, such as hydrocarbons) in a vacuum. In other words, if there is gas in the vacuum, it is expected that it will be bombarded by the sample along with the electron beam and accumulate on the sample. When a cooling element is placed near the top surface, as in the present invention, a mechanism can be realized that surrounds the vicinity of the sample with a cooled metal element (cooling element) within a range that does not block the electron beam irradiation, creating a localized region with a high degree of vacuum. This allows for the creation of a mechanism that cools and condenses the gas that escapes in order to collect it, and ultimately prevents the gas present in the vacuum inside the electron microscope from coming into contact with the sample.

Furthermore, when considering applications that require not only cooling but also passing a current through the sample, the effects of external temperatures can be reduced by fixing the electrodes and sample clamps using (via) an attachment mounting device such as a screw section, or by fixing and passing a current through the sample on an extended table fixed by the attachment mounting device. Such attachments can also be installed in the present invention.

In a preferred embodiment of the stage of the present invention, the attachment mounting device is made of a thermally conductive material. Examples of materials with high thermal conductivity include copper and copper alloys, aluminum and aluminum alloys, silver, and gold. In the present invention, for example, an attachment mounting device, such as a screw hole, is integrally formed on the sample base of a stage that cools a sample inside a scanning electron microscope (SEM), making it possible to mount attachments of various shapes and configurations according to the purpose. This allows the attachment to be incorporated into the cooling system, minimizing the impact of external heat and enabling a variety of cooling methods.

In a preferred embodiment of the stage of the present invention, the cooling section is characterized by comprising at least one of a solid refrigerant, a liquid refrigerant, and a gas refrigerant. The refrigerant used in the cooling section can be selected appropriately depending on the application and is not particularly limited. From the perspective of versatility, a preferred medium is a liquid. While a liquid (such as water) can be used to adjust the temperature using a general-purpose device (a cooling chiller), being a fluid can be a source of vibration. Liquid nitrogen or liquid helium can also be used as the liquid.

Furthermore, in the present invention, the cooling unit may be a solid refrigerant, as this can reduce the impact of vibrations caused by water flow or pulsating flow to nearly zero compared to water cooling. That is, in the present invention, the heat dissipation surface of a Peltier element or the like can be cooled with a solid refrigerant such as dry ice. This makes it possible to reduce the impact of vibrations caused by water flow or pulsating flow to nearly zero compared to water cooling. Furthermore, in the case of a solid, it is difficult to adjust the temperature of the cooling unit, but this can be controlled using a thermoelectric element, as described below.

On the other hand, flowing cooling gas at a minute flow rate is also less affected by vibration, and can therefore be used in the present invention. Examples of cooling gases include gasified liquid nitrogen. This allows for effective cooling of the sample. In the present invention, the cooling gas is not limited to liquid nitrogen. It is believed that simply passing weak gas through a heat dissipation surface is practically unaffected by vibration. Therefore, as described above, even when a solid refrigerant is actually used as the cooling unit, it is sufficient to pass cold air through a gap rather than pressing it against the heat dissipation surface. Similarly, when liquid nitrogen gas is used, passing the cooling gas through the heat dissipation surface allows for observation without being affected by vibration. The difficulty of adjusting the temperature of the cooling unit can be addressed by using a thermoelectric element, as described below.

In a preferred embodiment of the stage of the present invention, the cooling unit includes an extendable heat-conducting portion. That is, a sample can be cooled by providing an extendable heat-conducting portion extending laterally from the cooling surface and placing the sample on the heat-conducting portion. In the figures described below, the sample is stacked on the stage to save space, but it is also possible to fix a sample separately on the heat-conducting portion at a location separate from the stage and extend the heat-conducting path to that location. In the embodiment using a thermoelectric element described below, the thermoelectric element can be placed on the extendable heat-conducting portion. The extendable heat-conducting portion may have any shape, such as an arm or clamp. That is, in the present invention, the extendable heat-conducting portion can be made of a thermally conductive material, and the heat-conducting portion is not particularly limited, including its shape and structure, as long as it can contact the cooling unit and the sample stage and transmit the cold source from the cooling unit to the sample stage.

In a preferred embodiment of the stage of the present invention, the stage further comprises a thermoelectric element installed adjacent to the cooling unit. As described above, in an embodiment in which an extensible heat conduction unit is provided, the thermoelectric element can be installed on the heat conduction unit. In the present invention, the placement position of the thermoelectric element is not particularly limited as long as it is installed adjacent to the cooling unit. The thermoelectric element makes it possible to efficiently set the required temperature for the sample, i.e., to control the temperature. In an embodiment using a thermoelectric element, the cooling and heating response is good, minimizing the effects of thermal drift. Furthermore, the good cooling and heating response allows for precise temperature control. In an embodiment using a thermoelectric element, cooling and heating can be achieved with a single element by simply reversing the direction of current flow. At the same time, the fast cooling and heating response makes it easy to change to a desired temperature. Furthermore, precise temperature control is possible by adjusting the input power output, allowing for precise temperature control and minimizing the effects of thermal drift.

In a preferred embodiment of the stage of the present invention, the heat dissipation side of the thermoelectric element is in contact with the cooling unit or the heat conduction unit. In other words, in this invention, the thermoelectric element may be positioned adjacent to the cooling unit or the heat conduction unit. For example, the cooling unit or the heat conduction unit, such as a solid refrigerant, may be pressed against the heat dissipation side (heat dissipation surface side), or a gap may be provided to allow cold air to be applied. When applying cold air, either natural convection or forced convection using a fan or the like may be used. However, if forced convection generates vibrations, natural convection is preferable, depending on the level of forced convection. Even in the case of natural convection, the solid refrigerant has a sufficiently low temperature, creating a large temperature gradient between the heat dissipation surface side and the cold air from the cooling unit, such as a solid refrigerant. This creates sufficient heat transfer and is believed to allow the heat dissipation surface to be adequately cooled. Note that forced convection is more effective than natural convection, and water cooling is more effective than air cooling for heat dissipation.

In a preferred embodiment of the present invention, the thermoelectric element is a thermoelectric element that utilizes at least one of the Peltier effect and the Thomson effect. The Peltier effect (also called the Peltier effect) is the effect of converting electrical energy into thermal energy, and is a phenomenon in which a temperature difference occurs between the two ends of two dissimilar metals (or semiconductors) when the two ends are connected and an electric current is passed through them. This is particularly called a Peltier element and is used to cool precision instruments, wine cellars, and the like. The Thomson effect is the effect of generating heat other than Joule heat (heat is absorbed when the current is reversed) that occurs when an electric current is passed through a uniform metal (or dissimilar metals) with a temperature gradient. Both are capable of generating and absorbing heat.

In addition, a heat dissipation member may be installed between the thermoelectric element and the cooling unit, etc., to efficiently dissipate heat from the thermoelectric element.

In a preferred embodiment of the stage of the present invention, the thermoelectric element is a Peltier element, which provides good cooling and heating response and minimizes the effects of thermal drift. Peltier elements are also called Peltier elements (thermo-modules), which is a general term for elements that utilize the Peltier effect. The currently mainstream structure that is considered to have the best performance is called the "π-type," which has a structure like that shown in Figure 2. By passing a current through a PN junction using a P-type semiconductor and an N-type semiconductor, heat can be dissipated between the P-N and absorbed between the N-P.

The principle is as follows. Figure 2 shows one embodiment of a thermoelectric element applicable to the present invention. Figure 2(a) shows a cross-sectional view of a Peltier element, and Figure 2(b) shows a schematic diagram of the Peltier element's principle. In Figure 2(a), 21 indicates the hot-side metal (mainly Cu), 22 indicates the ceramic substrate (mainly alumina), 23 indicates the heat dissipation surface, 24 indicates the N-type semiconductor, 25 indicates the P-type semiconductor, 26 indicates the electric wire, 27 indicates the power source, 28 indicates the heat absorption, 29 indicates the conduction band of the N-type semiconductor, 30 indicates the heat dissipation, 31 indicates the positive side, 32 indicates the heat absorption side, 33 indicates the valence band, 34 indicates the heat dissipation side, 35 indicates the negative side, 36 indicates the cold-side metal (mainly Cu), 37 indicates the cold-side metal (mainly Cu), 38 indicates electrons, 39 indicates holes, and 40 indicates the conduction band of the P-type semiconductor.

In Figure 2(a), the negative pole is connected to metal 36 on the N-type semiconductor 24 side. Therefore, voltage pushes electrons from the conduction band of metal 36 up into the conduction band 29 of N-type semiconductor 24. At this time, because there is an energy gap between the conduction band of metal 36 and the conduction band 29 of N-type semiconductor 24, the electrons absorb thermal energy from metal 36, thereby cooling it. The electrons then flow and fall from the conduction band 29 of N-type semiconductor 24 into the conduction band of metal 21. The energy gap between the two bands causes the electrons to release thermal energy. In this way, metal 21 on the hot side is heated. Furthermore, the flowing electrons fall from the conduction band of metal 21 into holes 39 flowing through the P-type semiconductor 25, releasing thermal energy and heating metal 21 on the hot side. In the P-type semiconductor 25, voltage generates holes 39, which flow from the cold side 37 to the hot side 21. The electrons generated at that time are pushed up into the conduction band of the cold-side metal by the voltage, absorbing thermal energy according to the energy gap and cooling the cold-side metal 37. In this way, heat is carried from the cold side to the hot side of the Peltier module as a result of the current flow. In addition to the thermal energy carried by the current, there is also thermal energy carried by thermal conduction, but because the thermal energy carried by thermal conduction flows in the opposite direction, the less of it there is, the better the performance of the Peltier module. In other words, removing the thermal energy from the hot side as quickly as possible using a heat sink or similar will allow the Peltier module to perform well. Simply put, electrons carry (remove) heat.

There are no particular restrictions on the semiconductor material, and any can be used, but Bi-Te semiconductors are considered to have the best performance and are the mainstream.

Generally, Peltier device performance can be measured by the temperature difference ΔT that can be achieved relative to the temperature Th when the temperature on the heat-dissipating side is constant. For example, for Th = 75, 50, or 25°C, ΔT = 93, 85, or 75. If the heat-dissipating surface were simply cooled to, say, liquid nitrogen temperature (-196°C), the heat-absorbing surface would likely exceed minus 200°C. However, due to the characteristics of the material, in reality, ΔT is estimated to be 10°C at temperatures near liquid nitrogen. The lower the temperature, the less heat there is to excite electrons, reducing the Peltier cooling capacity. Additionally, the lower the temperature, the greater the electrical resistance of the semiconductor, causing it to self-heat due to current, resulting in a reduction in overall cooling capacity.

In addition, in a preferred embodiment of the present invention, the heat dissipation side of the thermoelectric element is in contact with the cooling section or the heat conduction section, from the viewpoint that it is possible to set a lower temperature by cooling the heat dissipation side of the thermoelectric element. The following examples mainly describe the case of cooling, but in the present invention, heating is also possible using the thermoelectric element. Since the bottom surface of the thermoelectric element cools during heating, it is necessary to heat the cooling section (process at a higher temperature than the bottom surface). In this case, it can function as a heating section rather than a cooling section.

While the phenomenon is simply reversed when it comes to heating compared to cooling, the practicality of Peltier elements and other thermoelectric devices (whether multi-stage or not) also influences their practicality. Basically, when aiming for the lowest possible temperatures, multi-stage thermoelectric devices, such as Peltier elements, can be used. In this case, a pyramid-like structure can be used, with the heat-absorbing surface (top layer) being smaller and the heat-dissipating surface becoming larger. This structure is used because, since the larger the surface area, the greater the heat absorption capacity, the heat absorbed by the smaller upper layer can be dissipated by the larger lower layer. When using a device for heating by reversing the polarity of the current, it is not as simple as this. Heat from the larger lower layer tends to flow into the smaller upper layer all at once. If the upper layer cannot absorb the heat, heat accumulates in the middle layer, which tends to be higher than the upper layer. For this reason, it is thought that most Peltier elements, even when used for heating, will reach around +100°C (a temperature at which the solder at the joint does not deteriorate). Peltier elements create heat absorption and heat dissipation surfaces through the movement of electrons, and by controlling the amount of current passed through the Peltier element, it is, in principle, possible to precisely control the temperature from near room temperature to the negative range. These effects enable stable, high-resolution observations.

Furthermore, in a preferred embodiment of the stage of the present invention, the sample pedestal and the thermoelectric element are in contact with each other, from the perspective of cooling the area below the sample pedestal.

The following describes the stages of one embodiment of the present invention with reference to the drawings, but the present invention is not intended to be limited to these.

Figure 1 shows a conceptual diagram of an example stage in one embodiment of the present invention. Figure 1(a) shows a top view of the stage in one embodiment of the present invention, Figure 1(b) shows a side view of the stage, and Figure 1(c) shows a perspective view of the stage. In Figure 1, 1 shows the sample base, 2 shows the attachment mounting device, 3 shows the thermoelectric element (if a thermoelectric element is required), 4 shows the heat dissipation member, and 5 shows the cooling unit.

With reference to the figures, the role of each component and the connections between them for a stage in one embodiment of the present invention will be explained below. First, there is the stage body. In Figure 1, the stage has a cooling unit 5. However, in the case of the heating described above, the underside of the thermoelectric element cools during heating, so the cooling unit must be heated (processed at a higher temperature than the underside). In this case, it can function as a heating unit rather than a cooling unit. Also, in this example, a thermoelectric element 3 is also used, but in embodiments that do not use the thermoelectric element 3, the thermoelectric element 3 can be omitted. Since this example uses a thermoelectric element 3, a heat dissipation member 4 may be provided as needed, as shown in the figure. Also, in this example, the attachment mounting device 2 is specifically a screw hole, but as mentioned above, it may also be a clamp or the like. There is no particular limitation as long as it allows for attachment and removal. The attachment mounting device 2 is preferably made of a thermally conductive material. This further reduces heat loss.

As long as the Peltier element and sample base are in thermal contact, they can be directly above each other or offset to the side and extended. Also, while the clamp is shown standing vertically in the illustration, it can also clamp the sample horizontally. One important feature is that the attachment mounting holes are integrated with the sample base; the mounting holes can be machined as a single piece, or the screw holes can be manufactured separately and then press-fit or glued in later.

Figure 3 is a conceptual diagram of an example stage in which an FIB grid is installed as an attachment in one embodiment of the present invention. In Figure 3, 50 denotes an FIB grid holder, 51 denotes a copper screw, and 52 denotes an FIB grid holder attachment. In this example, the FIB grid holder attachment 52 is attached and detached via copper screw 51, which is a thermally conductive material. However, there are no particular limitations as long as it is attachable and detachable. This example shows an example in which a FIB sample holder is fixed to a sample holder as a cooling attachment. The FIB mesh can be clamped vertically in the center of the gap. This reduces damage caused by electron beams and ion beams due to the cooling effect. While this example shows a FIB sample holder, it is also possible to clamp and observe a regular sample (if the base portion is made a little larger). The structure may also be designed to clamp the sample horizontally.

Figure 4 shows a conceptual diagram of a stage in one embodiment of the present invention, illustrating an example of a case in which a sample is fixed from above as an attachment. In Figure 4, 60 indicates the observation sample, and 61 indicates the sample fixing attachment (for fixing the upper surface). If the sample is fixed from above as an attachment and the attachment is made of a thermally conductive material, it is possible to cool the sample from above as well.

Attachments are easy to replace, and in-situ observation is possible even under cooling conditions, making it applicable to a wide range of technical fields.

1. Sample stand
2. Attachment mounting device 3. Thermoelectric element (if thermoelectric element is required)
4 Heat dissipation member 5 Cooling part 21 Hot side metal (mainly Cu)
22 Ceramic substrate (mainly alumina)
23 Heat dissipation surface 24 N-type semiconductor 25 P-type semiconductor 26 Electric wire 27 Power source 28 Heat absorption 29 Conduction band of N-type semiconductor 30 Heat dissipation 31 Positive side 32 Heat absorption side 33 Valence band 34 Heat dissipation side 35 Negative side 36 Cold side metal (mainly Cu)
37 Cold side metals (mainly Cu)
38 Electrons 39 Holes 40 Conduction band of P-type semiconductor 50 FIB grid holding base 51 Copper screws 52 FIB grid holding attachment 60 Observation sample 61 Sample fixing attachment (for fixing the upper surface)

Claims (9)

A stage having a sample pedestal for mounting a sample and a cooling unit for cooling the sample, wherein the sample pedestal is equipped with an attachment and an attachment mounting device for mounting the attachment, and further has a thermoelectric element installed adjacent to the cooling unit, and the sample pedestal and the thermoelectric element are in contact with each other. A stage as described in claim 1, characterized in that the cooling section has an extensible heat conductive section. A stage as described in claim 1 or 2, wherein the attachment mounting device is a screw hole or a clamp. The stage described in any one of claims 1 to 3, characterized in that the attachment is at least one selected from an FIB grid holding attachment, a sample fixing attachment, and an electrical current attachment. The stage described in claim 1, characterized in that the thermoelectric element is a thermoelectric element that utilizes at least one of the Peltier effect and the Thomson effect. 3. The stage according to claim 2 , wherein the heat radiation side of the thermoelectric element is in contact with the cooling portion or the heat conduction portion. A stage as described in any one of claims 1 to 6, characterized in that the cooling section is made of at least one of a solid refrigerant, a liquid refrigerant, and a gas refrigerant. A stage according to claim 1, characterized in that the sample pedestal and the thermoelectric element are in contact with each other via a heat dissipation member. 9. The stage according to claim 1, wherein the attachment mounting device is made of a thermally conductive material.