Cooling apparatus and SQUID microscope using same

Abstract

There is disclosed cooling apparatus for cooling a sample or sensor to the neighborhood of the boiling temperature of a liquefied gas by transporting the gas. The cooling apparatus can be run stably such that piping or needle valve acting as a medium for transporting the liquefied gas is not clogged up. The apparatus is so constructed that a gas collection port is installed in a transfer tube and that gas produced by evaporation of the liquefied gas is collected and used as a coolant for a cooling head.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to apparatus for cooling samples made of various devices and materials (such as various semiconductor devices, semiconductor materials, magnetic materials, superconducting materials, other metal materials or inorganic materials) and maintaining the samples at low temperatures when measurements, observations, or operations are performed regarding such samples at low temperatures reaching the boiling points of liquefied gases.

[0003] 2. Description of the Related Art

[0004] Recently, high-sensitivity magnetometers having spatial resolutions of the order of micrometers and known as SQUIDs (Superconducting QUantum Interference Devices) have been put into practical use, and measurements using SQUID microscopes are increasingly performed regarding various devices and materials. Since SQUIDs use superconductivity, it is necessary to cool them at low temperatures lower than the liquid nitrogen temperature (from several K to 77 K). Furthermore, the opposite sample needs to be retained at low temperatures in many cases. In addition, where samples are observed by tunneling microscopes or atomic force microscopes as well as by SQUIDs, samples are maintained at low temperatures in some cases.

[0005] FIG. 2 is a schematic view showing one example of cooling apparatus of related art for cooling a sensor side. A three-axis scanning stage 20, a cooling head 30, a coolant introduction port 42, a sensor 50, a sample 60, etc. are installed inside a vacuum chamber 10. A vacuum pump 70, a liquefied gas storage tank 40, and a transfer tube 90 are installed outside the vacuum chamber 10.

[0006] The vacuum chamber 10 is made of stainless and maintained in a vacuum to make thermal isolation from the outside.

[0007] The three-axis scanning stage 20 is used to place the sample 60 and to control the relative position between the sensor 50 and the sample 60.

[0008] The cooling head 30 is a hermetically closed container made from oxygen-free copper to improve the thermal conduction. A first pipe 31 and a second pipe 32 forming inlet and outlet for the coolant are connected with the cooling head 30. The flow rate of the coolant flowing into the cooling head 30 is adjusted by a needle valve 33.

[0009] The liquefied gas storage tank 40 is a vacuum isolation container for storing a liquefied gas. Liquid helium is used as the liquefied gas.

[0010] The coolant introduction port 42 is used to introduce the liquid helium into the cooling head 30 installed inside a vacuum chamber. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90, and the coolant stored in the liquefied gas storage tank 40 is introduced into the cooling head 30.

[0011] A SQUID having a detection coil about 10 μm in diameter is used as the sensor 50. Niobium operating near the boiling point of liquid helium is used as a superconducting material for fabricating the SQUID. The sensor 50 is made stationary while placed in thermal contact with the cooling head 30.

[0012] The vacuum pump 70 is used to lower the pressure inside the second pipe 32, cooling head 30, first pipe 31, and transfer tube 90 and to transfer the liquid helium in the liquefied gas storage tank 40.

[0013] The procedure for cooling the cooling head 30 is as follows. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90. The vacuum pump 70 is operated and thus the liquid helium stored in the liquefied gas storage tank 40 is passed through the cooling head 30. In this way, the temperature of the cooling head 30 is cooled close to the boiling point of liquid helium.

[0014] After cooling of the cooling head 30, the sensor 50 is operated, and the relative position between the sensor 50 and the sample 60 is controlled using the three-axis scanning stage 20. A signal owing to the sensor 50 is recorded. Thus, the magnetic distribution of the sample 60 is measured.

[0015] With the above-described cooling apparatus of related art where stored liquefied gas is directly used as means for cooling a sensor or sample to a low temperature, the liquefied gas is often transported into a location to be cooled while using a thin pipe as a medium or the liquefied gas is transported through a minute space such as a needle valve to adjust the flow rate of the liquefied gas. The stored liquefied gas often contains impurities such as solidified carbon dioxide, oxygen, nitrogen, and water, as well as foreign substances such as microscopic dust and metal fragments. Therefore, foreign substances and impurities sometimes clog up the pipe or needle valve that is a transportation medium for the liquefied gas. Consequently, there is a problem that the apparatus ceases to function as cooling apparatus. Furthermore, where impurities adhere to the interface portion between the vacuum thermal isolation pipe and coolant introduction port, the interface portion becomes an adhesively bonded state. The vacuum thermal isolation pipe cannot be removed unless an operation for dissolving away the impurities is performed. Hence, the ending operation for the cooling apparatus cannot be performed. Thus, there is a problem that the workability is poor.

SUMMARY OF THE INVENTION

[0016] (First Means)

[0017] The present invention installs a gas collection port in a liquefied gas storage tank of a cooling apparatus. Gas produced by evaporation of the liquefied gas is collected and used as a coolant for a cooling head.

[0018] (Second Means)

[0019] In addition to the first means, a mechanism for measuring the liquid level of the liquefied gas is provided. The gas collection port is made movable vertically.

[0020] (Third Means)

[0021] In addition to the first means, a gas-cooling mechanism is provided.

[0022] (Fourth Means)

[0023] In addition to the first means, a structure is provided in which the liquefied gas storage tank is provided with a gas introduction port.

[0024] (Fifth Means)

[0025] In addition to the first means, a structure is provided in which a refrigerator and a gas introduction port are used instead of the liquefied gas storage tank.

[0026] According to the structure of the cooling apparatus owing to the first means, gas evaporated from the liquefied gas is used as a coolant for the cooling head and so even where impurities are mixed in the liquefied gas stored in the liquefied gas storage tank, a high-purity gas can be used as a coolant. Consequently, the pipe or needle valve for transporting the coolant is not clogged up. The cooling apparatus can be run stably.

[0027] Owing to the second means, the liquid level of the liquefied gas can be known. Therefore, the gas collection port can be placed close to the liquid level. Gas of lower temperature can be collected and used as a coolant. In consequence, the cooling head can be cooled to a lower temperature.

[0028] Owing to the third means, the collected gas becoming the coolant can be cooled to a lower temperature. As a result, the cooling head can be cooled to a lower temperature.

[0029] Owing to the fourth means, the pressure inside the liquefied gas storage tank can be adjusted. Therefore, the pressure inside the liquefied gas storage tank can be prevented from becoming a negative pressure. That the gas becoming the coolant cannot be transported can be prevented. Hence, the cooling apparatus can be run stably.

[0030] Owing to the fifth means, a high-purity gas can be used as a coolant without using a liquefied gas. Therefore, intrusion of foreign substances into the cooling apparatus can be prevented. The cooling apparatus can be run stably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is a schematic view showing the structure of cooling apparatus showing Embodiment 1 of the present invention;

[0032] FIG. 2 is a schematic view showing the structure of cooling apparatus of related art;

[0033] FIG. 3A is a schematic view showing the structure of cooling apparatus showing Embodiment 2 of the invention;

[0034] FIG. 3B is a schematic view showing the structure of vicinities of the front end of a transfer tube 90 in Embodiment 2 of the invention;

[0035] FIG. 4 is a schematic view showing the structure of vicinities of a liquefied gas storage tank 40 of cooling apparatus showing Embodiment 3 of the invention;

[0036] FIG. 5 is a schematic view showing the structure of vicinities of a liquefied gas storage tank 40 of cooling apparatus showing Embodiment 4 of the invention; and

[0037] FIG. 6 is a schematic view showing the structure of cooling apparatus showing Embodiment 5 of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Embodiments of the present invention are hereinafter described with reference to the drawings.

[0039] (Embodiment 1)

[0040] FIG. 1 is a schematic view showing the structure of cooling apparatus showing Embodiment 1 of the present invention. Cooling apparatus of the invention is used for cooling of a sensor of a SQUID microscope.

[0041] A three-axis scanning stage 20, a cooling head 30, a coolant introduction port 42, a sensor 50, a sample 60, etc. are installed inside a vacuum chamber 10. A vacuum pump 70, a liquefied gas storage tank 40, and a transfer tube 90 are installed outside the vacuum chamber 10.

[0042] The vacuum chamber 10 is made of stainless. The inside is maintained in a vacuum to make thermal isolation from the outside.

[0043] The three-axis scanning stage 20 is used to place the sample 60 and to control the relative position between the sensor 50 and the sample 60.

[0044] The cooling head 30 is a hermetically closed container made from oxygen-free copper to improve the thermal conduction. A first pipe 31 and a second pipe 32 forming inlet and outlet for the coolant are connected with the cooling head 30. The flow rate of the coolant flowing into the cooling head 30 is adjusted by a needle valve 33.

[0045] The liquefied gas storage tank 40 is a vacuum thermal isolation container for storing a liquefied gas. Liquid helium is used as the liquefied gas 41. The space over the liquid level of liquid helium inside the liquefied gas storage tank 40 is filled with low-temperature helium gas 47 produced by evaporation of liquid helium.

[0046] The transfer tube 90 is used to transfer the coolant held in the liquefied gas storage tank 40 to the cooling head 30. The tube is made up of two flexible tubes (larger tube and smaller tube). Vacuum thermal isolation and radiation thermal shield are provided between the two flexible tubes. With respect to the structure of the transfer tube 90 on the side of the liquefied gas storage tank 40, the front end is closed such that the liquefied gas is not collected. Instead, a gas collection port 91 for collecting helium gas is formed at a position higher than the height of the liquid level of the coolant.

[0047] The coolant introduction port 42 is intended to introduce the coolant into the cooling head 30 installed inside the vacuum chamber. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90, and the coolant is introduced into the cooling head 30 from the liquefied gas storage tank 40.

[0048] A SQUID having a detection coil about 10 μm in diameter is used as the sensor 50. Niobium operating near the boiling point of liquid helium is used as a superconducting material for fabricating the SQUID. The sensor 50 is made stationary while kept in thermal contact with the cooling head 30.

[0049] The vacuum pump 70 is used to lower the pressure inside the second pipe 32, cooling head 30, first pipe 31, and transfer tube 90 and to transfer the liquid helium in the liquefied gas storage tank 40.

[0050] The liquefied gas storage tank 40 is a container for transporting and storing liquefied gas that becomes a coolant. At this time, helium is used as the coolant because of the used material of the sensor 50. A container inside the liquefied gas storage tank 40 is filled with liquid helium and helium gas produced by evaporation of the liquid helium.

[0051] The procedure for cooling the cooling head 30 is as follows. The coolant introduction port 42 and liquefied gas storage tank 40 are connected by the transfer tube 90. The vacuum pump 70 is operated. Low-temperature helium gas produced by evaporation of the liquid helium inside the liquefied gas storage tank 40 is passed through the cooling head 30 to cool the cooling head 30 close to the boiling temperature of helium.

[0052] After cooling of the cooling head 30, the sensor 50 is operated, and the relative position between the sensor 50 and the sample 60 is controlled using the three-axis scanning stage 20. A signal owing to the sensor 50 is recorded. Thus, the magnetic distribution of the sample 60 is measured.

[0053] (Embodiment 2)

[0054] FIG. 3A is a schematic view showing the structure of cooling apparatus showing Embodiment 2 of the invention. This embodiment is by no means different from Embodiment 1 except that the distance between the front end of the transfer tube 90 and the gas collection port 91 is shortened and that a liquid level sensor 44 is fitted to the front end of the transfer tube 90.

[0055] FIG. 3B is a schematic view showing the structure of vicinities of the front end of the transfer tube 90. The front end of the transfer tube 90 is closed not to collect liquefied gas. The liquid level sensor 44 for judging whether it is inside the coolant or not is fixed. The gas collection port 91 is formed at a position that is several centimeters above the liquid level sensor 44.

[0056] The procedure for cooling the cooling head 30 is as follows. The transfer tube 90 is inserted into the liquefied gas storage tank 40. When the liquid level sensor 44 is located at the liquid level of the liquid helium, a Wilson seal port of the liquefied gas storage tank 40 is closed in. The transfer tube is fixed. The vacuum pump 70 is operated. Helium gas close to the liquid level of the liquid helium inside the liquefied gas storage tank 40 is supplied into the cooling head 30, thus cooling it.

[0057] After cooling of the cooling head 30, the sensor 50 is operated, and the relative position between the sensor 50 and the sample 60 is controlled using the three-axis scanning stage 20. A signal owing to the sensor 50 is recorded. Thus, the magnetic distribution of the sample 60 is measured.

[0058] Since the liquid level in the liquefied gas storage tank 40 drops with the cooling time, the position of the front end of the transfer tube is reset to the liquid level of the liquid helium at appropriate times.

[0059] (Embodiment 3)

[0060] FIG. 4 is a schematic view showing the structure of the vicinities of a liquefied gas storage tank 40 of cooling apparatus showing Embodiment 3 of the invention. This embodiment is by no means different from Embodiment 1 except that a gas-cooling mechanism 92 is fitted at the front end of the transfer tube 90 and that a liquid level sensor is mounted at an underpart of the gas collection port 91.

[0061] The gas-cooling mechanism 92 is a mechanism for cooling helium gas to the temperature of liquid helium by passing the helium gas through liquid helium once. The gas-cooling mechanism 92 is made up of a pipe for passing helium gas and fins for enhancing the thermal exchange efficiency by increasing the surface area. The material of the gas-cooling mechanism 92 is fabricated using oxygen-free copper at this time. It can be replaced by other material such as aluminum that has good thermal conductivity.

[0062] The procedure for cooling the cooling head 30 is as follows. The transfer tube 90 is inserted into the liquefied gas storage tank 40. When the liquid level sensor 44 is located at the liquid level of the liquid helium, the Wilson seal port in the liquefied gas storage tank 40 is closed in. The transfer tube is fixed. The vacuum pump 70 is operated. Thus, helium gas taken in from the gas collection port 91 is cooled to a temperature comparable with the liquid helium temperature by the gas-cooling mechanism 92 and transported to the cooling head 30.

[0063] After cooling of the cooling head 30, the sensor 50 is operated, and the relative position between the sensor 50 and the sample 60 is controlled using the three-axis scanning stage 20. A signal owing to the sensor 50 is recorded. Thus, the magnetic distribution of the sample 60 is measured. Since the liquid level in the liquefied gas storage tank 40 drops with the operation time, the position of the front end of the transfer tube is reset to the liquid level of the liquid helium at appropriate times.

[0064] (Embodiment 4)

[0065] FIG. 5 is a schematic view showing the structure of the vicinities of a liquefied gas storage tank 40 of cooling apparatus showing Embodiment 4 of the invention. This embodiment is by no means different from Embodiment 1 except that a gas introduction port 46 for introducing helium gas is formed in an inner-layer container of the liquefied gas storage tank 40.

[0066] The gas introduction port 46 is connected with a pipe for helium gas or a helium gas cylinder via a rubber tube and a regulator. The secondary side pressure of the regulator is set to about 0.3 kg f/cm2. Cooling is done while supplying helium gas into the liquefied gas storage tank 40.

[0067] (Embodiment 5)

[0068] FIG. 6 is a schematic view showing the structure of cooling apparatus showing Embodiment 5 of the invention. Instead of the liquefied gas storage tank and liquid helium in Embodiment 1, the apparatus is so constructed that it uses a refrigerator, helium gas, and a gas precooler.

[0069] The gas introduction port 46 is intended to introduce helium gas into the gas precooler 100. The gas introduction port 46 is connected with a helium gas cylinder or helium gas pipe via a rubber tube and a regulator.

[0070] The gas precooler 100 is apparatus for cooling ordinary-temperature helium gas introduced from the gas introduction port 46 to a low temperature in the neighborhood of 100 K. At this time, apparatus is used in which piping for thermal exchange is placed in a vacuum vessel for holding liquid nitrogen. The piping for thermal exchange is made up of thin tubes and fins for increasing the surface area. The gas precooler 100 and a refrigerator head 80 are connected by thermal insulation piping.

[0071] The refrigerator head 80 is combined with a refrigerator compressor 81 and a flexible hose 82, whereby the head becomes a refrigerator capable of cooling to about 2 K at minimum. Gas piping connected with the gas precooler 100 is installed in the refrigerator head 80 while in thermal contact with it.

[0072] The procedure for cooling the cooling head 30 is as follows. The vacuum pump 70 is operated, so that helium gas is introduced into the gas precooler 100 via the gas introduction port 46. The helium gas is cooled to about 77 K to 100 K by the gas precooler 100 and then introduced into the piping in thermal contact with the refrigerator head 80. Thus, the gas is cooled to a low temperature of about 4 K and transported to the cooling head 30, thus cooling the cooling head 30 to a low temperature.

[0073] After cooling the cooling head 30, the magnetic distribution of the sample is measured in the same way as in Embodiment 1.

[0074] According to the present invention, gas evaporated from liquefied gas is used as a coolant for a cooling head. Therefore, even where impurities are mixed in liquefied gas stored in a liquefied gas storage tank, a gas of high purity can be used as a coolant. Consequently, pipes or needle valve for transporting the coolant is not clogged up. The cooling apparatus can be run stably.

[0075] Furthermore, no impurities adhere to the interface portion between vacuum thermal isolation piping and a coolant introduction port. Therefore, the cooling ending operation can be performed without problem. Hence, the workability is improved.

[0076] In addition, a gas of a lower temperature can be used. Therefore, the cooling head can be cooled to a lower temperature.

[0077] Moreover, the pressure inside the liquefied gas storage tank can be adjusted. Therefore, the pressure inside the liquefied gas storage tank can be prevented from becoming a negative pressure. That the gas becoming a coolant cannot be transported can be prevented. The cooling apparatus can be run stably.

[0078] Further, a high-purity gas can be used as a coolant without using a liquefied gas. Consequently, intrusion of foreign substances can be completely prevented. The cooling apparatus can be run quite stably.

[0079] According to the present invention, the SQUID microscope can be measure and observe stably.

Claims

1. Cooling apparatus comprising: a cooling head for cooling to approximately boiling temperature of a liquefied gas; a liquefied gas storage tank for storing the liquefied gas becoming a coolant that cools said cooling head; piping acting as a medium for transporting the coolant; a drive source for transporting the coolant; a vacuum chamber for accommodating at least the cooling head; and a gas collection port for collecting gas produced by evaporation of the liquefied gas instead of the liquefied gas, the gas collection port being formed at an end portion of said piping.

2. The cooling apparatus according to claim 1, wherein a mechanism for measuring the liquid level of the liquefied gas is mounted inside the liquefied gas storage tank, and wherein the gas collection port is made movable in a vertical direction.

3. The cooling apparatus according to claim 1, wherein there is provided a gas cooling mechanism.

4. The cooling apparatus according to claim 1, wherein the liquefied gas storage tank is provided with a gas introduction port.

5. The cooling apparatus according to claim 1, wherein a refrigerator and a gas introduction port are used instead of the liquefied gas storage tank.

6. A SQUID microscope comprising: the cooling apparatus according to claim 1;a sample stage for installing a sample inside the vacuum chamber; and a SQUID sensor; wherein said SQUID sensor is cooled by the cooling apparatus.