Abstract:
A cooling apparatus has a storage vessel for storing a liquefied gas and an evaporated gas produced by evaporation of the liquefied gas, a cooling head cooled by the evaporated gas to approximately a boiling temperature of the liquefied gas, piping for conveying the evaporated gas from the storage vessel to the cooling head, and a gas collection port provided at one end portion of the piping above a liquid level of the liquefied gas for collecting only the evaporated gas from the storage vessel. Thus, contamination of a gas line, needle valve, or the like, can be avoided. In addition, a liquid level sensor may be provided in the storage vessel for sensing the liquid level of the liquefied gas, so that the gas collection port may be vertically moved within the storage vessel to collect cooled gas close to the liquid level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Description of the Related Art 
     Recently, high-sensitivity magnetometers having spatial resolutions on the order of micrometers and known as SQUIDs (Superconducting Quantum Interference Devices) have been put into practical use, and measurements using SQUID microscopes have been increasingly performed on various devices and materials. Since SQUIDs use superconductivity, it is necessary to cool them at temperatures lower than the temperature of liquid nitrogen (from several K to 77 K). Furthermore, the sample also 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. 
     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 . 
     The vacuum chamber  10  is made of stainless steel and maintained in a vacuum state to make provide thermal isolation from the outside. 
     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 . 
     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 an inlet and an 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 . 
     The storage vessel is a liquefied gas storage tank  40  which is a vacuum isolation container for storing a liquefied gas  41 . Liquid helium is used as the liquefied gas  41 . 
     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 . 
     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 . 
     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 . 
     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. 
     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. 
     With the above-described cooling apparatus of the related art, where stored liquefied gas is directly used as means for cooling a sensor or a 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 in 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 in that the workability is poor. 
     SUMMARY OF THE INVENTION 
     (First Means) 
     In accordance with the present invention, a gas collection port is provided 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. 
     (Second Means) 
     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. 
     (Third Means) 
     In addition to the first means, a gas-cooling mechanism is provided. 
     (Fourth Means) 
     In addition to the first means, a structure is provided in which the liquefied gas storage tank is provided with a gas introduction port. 
     (Fifth Means) 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     FIG. 1 is a schematic view showing the structure of cooling apparatus showing Embodiment 1 of the present invention; 
     FIG. 2 is a schematic view showing the structure of cooling apparatus of related art; 
     FIG. 3A is a schematic view showing the structure of cooling apparatus showing Embodiment 2 of the invention; 
     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; 
     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; 
     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 
     FIG. 6 is a schematic view showing the structure of cooling apparatus showing Embodiment 5 of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are hereinafter described with reference to the drawings. 
     (Embodiment 1) 
     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. 
     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 . 
     The vacuum chamber  10  is made of stainless steel. The inside is maintained in a vacuum state to provide thermal isolation from the outside. 
     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 . 
     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 . 
     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. 
     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 (a larger tube and a smaller tube). Vacuum thermal isolation and a 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. 
     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 . 
     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 . 
     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 . 
     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. 
     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. 
     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. 
     (Embodiment 2) 
     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 . 
     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 . 
     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. 
     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 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. 
     (Embodiment 3) 
     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 . 
     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. 
     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 . 
     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. 
     (Embodiment 4) 
     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 . 
     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/cm 2 . Cooling is done while supplying helium gas into the liquefied gas storage tank  40 . 
     (Embodiment 5) 
     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. 
     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. 
     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. 
     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. 
     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. 
     After cooling the cooling head  30 , the magnetic distribution of the sample is measured in the same way as in Embodiment 1. 
     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. 
     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. 
     In addition, a gas of a lower temperature can be used. Therefore, the cooling head can be cooled to a lower temperature. 
     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. 
     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. 
     According to the present invention, the SQUID microscope can be measure and observe stably.