Cryogenic device

Abstract: A closed cycle system cryogenic device which enables a heat exchanger pipe of relatively narrow diameter to be present within a cryostat contains, as a cooling medium, one or more liquid fluorocarbons of the formula: C m F 2m+2 , wherein m is an integer of 1 to 4, an article to be cooled being immersed in the cooling medium, and a heat exchanger pipe which is disposed in an upper portion of the cryostat and is communicated with a separate liquifier, so that the cooling medium boiled and vaporised as a result of a heat generation in the cryostat is liquified within the same cryostat as a result of a heat exchange between vapour of the cooling medium and a liquid gas circulated as a heat exchange medium in the heat exchanger pipe.

Description: The present invention relates to a cryogenic device having high cooling capability. There is a particular requirement, currently, for coolants which are suitable, in particular, as cooling medium or a heat exchange medium for cooling semiconductor devices such as those which can exhibit a high speed switching at a cryogenic temperature of about 123 K or less such as 77.3 K, i.e., the boiling point of liquid nitrogen (LN₂), or other devices such as those using a superconducting material which are cooled to a working temperature thereof. Note, any cryogenic temperature around the boiling point of liquid nitrogen is referred to herein as a liquid nitrogen temperature. As is well known in the art, the mobility of a carrier in a semiconductor can be increased by reduction of the temperature to which the semiconductor is subjected, and based on this temperature characteristic, high speed switching devices which are operable at the liquid nitrogen temperature, such as high electron mobility transistors (HEMTs) or complementary metal oxide semiconductor (CMOS) devices, have been developed. For example, M. Abe, T. Mimura and M. Kobayashi, "Ultra-High-Speed HEMT LSI Technology", FUJITSU Sci. Tech. J., 24, 4, PP. 271-283 (December 1988) teach that HEMTs developed by Fujitsu Limited have achieved a 5.8 ns switching speed, and T. Vacca, D. Resnick, D. Frankel, R. Bach, J. Kreilich and D. Charlson, "A Cryogenically Cooled VLSI Super-computer", VLSI SYSTEMS DESIGN, June 1987, pp/ 80-84 teach that the first cryogenic computer using a 77K CMOS was shipped in the previous year. To operate these devices at the liquid nitrogen temperature with satisfactory results, it is necessary to use a high power coolant or cooling medium which can constantly maintain a desired cryogenic temperature such as the liquid nitrogen temperature, and also enables an effective dissipation of the heat generated in the devices during operation thereof to be obtained. Hitherto, to cool the semiconductor devices and other devices to a cryogenic temperature of about 123K or less and to maintain such a temperature, it has been proposed to arrange a heat generating part of the device in contact with a cold head of a refrigerator to dissipate the heat generated from the device through a heat conduction, or to immerse the device in a cryogenic fluid such as a liquefied gas, for example, liquid nitrogen, to dissipate the heat from the device through a boiling heat transfer. Particularly, the latter cooling method based on the boiling heat transfer can be advantageously used when the devices to be cooled have a large heat generation density and a complex structure or configuration. For example, this cooling method can be applied to the cooling of central processing units (CPUs) of a computer. Further, although a variety of liquefied gases is available, the liquefied gas usable as the cryogenic fluid in this cooling method is limited in view of the toxicity and reactivity of such gases. Suitable liquefied gases having a stable and simple composition are liquefied nitrogen, described above, and liquefied helium. Liquefied nitrogen is obtained by liquefying air and separating the resulting fluid. Use of liquefied nitrogen as the cryogenic fluid is described in, for example, F.H. Gaensslen, V.L. Rideout, E. J. Walker and J.J. Walker, "Very Small MOSFET's for Low-Temperature Operation", IEEE TRANSACTIONS ON ELECTRON DEVICES, MARCH 1977, in which FET's are cooled by direct immersion in an open pool of the liquefied nitrogen. Further, the use of liquefied fluorocarbons as the cryogenic fluid is described in, for example, "Cooling a Superfast Computer", ELECTRONIC PACKAGING & PRODUCTION, JULY 1986, in which the entire computer is immersed in a non-conductive circulating fluorocarbon liquid. Furthermore, the use of a mixture of liquefied nitrogen and liquefied fluorocarbon (CF₄) as the cryogenic fluid is described in, for example, T. Amano and M. Nagao, "Boiling Heat transfer Characteristics of Mixed Coolant", 1988 Cryogenic Society of Japan Spring Meeting, C1-4, May 1988, in which a superconducting material having a high Tc is directly immersed in a mixed coolant consisting of the liquefied nitrogen and liquefied CF₄ in an open vessel. The cooling method taught by Amano et al is illustrated in Fig. 1. To ascertain the effectiveness of the mixed coolant when cooling the high Tc superconducting material, they used the illustrated experimental apparatus in which a Pt wire 1 is dispersed in a mixed coolant 2 in an open vacuum vessel 3. A thermocouple 4, as a temperature sensor, connected with a recorder 8 is also disposed in the mixed coolant 2. In the apparatus, 5 is a balance-scale used to determine a change of the weight of the coolant 2, 6 is a DC supply, and 7 is a resistor. Surprisingly, the experiments using this apparatus showed that the mixed coolant of the liquefied nitrogen and 8 mol % of the liquefied CF₄ can increase the cooling capability to about twice the cooling capability obtained when liquefied nitrogen is used alone. Although this paper does not teach the use of the mixed coolant (LN₂ plus liquefied CF₄) when cooling semiconductor devices and related devices, it is understood that such a use will be accompanied by many drawback, for example: (1) Since an open system is employed, atmospheric air or moisture in the atmosphere can be taken up by and frozen in the mixed coolant; (2) Since the coolant is boiled and vaporized, the gas generated can cause atmospheric pollution and an additional coolant must be supplied to the open vessel to compensate for vaporization loss of the coolant. To avoid the drawbacks (1) and (2), use of the mixed coolant in a closed system has been considered, but since the mixed coolant must contain a higher amount of the liquefied CF₄, i.e., as high as 8 to 20 mol %, the composition of the mixed coolant will be changed upon liquefaction of the vaporized coolant, thereby causing variations in the cooling capability of the coolant. Further, in the vaporized coolant, the high boiling components thereof tend to be condensed and separated in a liquefied or refrigerator used to again liquefy the vaporized coolant, because of a high concentration of the high boiling components. The separated components will cause clogging of the pipes, stoppage of the apparatus and other unavoidable drawbacks. Furthermore, not all fluorocarbons except for CF₄, which are considered to be appropriate for admixture to the liquefied nitrogen, provide a solution after they are directly admixed with the liquefied nitrogen in accordance with conventional mixing methods; in practice, they are solidified and precipitated in a mixing apparatus. It is of note, furthermore, that the present inventors also tried to use a mixed coolant consisting of liquefied nitrogen and liquified argon or liquified krypton, but could not obtain satisfactory cooling effects. On the other hand, to obtain an increased cooling capability, it has been suggested to use a combination of a cryostat or closed cryogenic container and a liquefier, particularly a refrigeration unit, a heat exchanger pipe of which liquefier is disposed in an upper inner portion of the cryostat. An article to be cooled is immersed in a coolant or cooling medium in the cryostat, and the vaporized coolant is liquefied as a result of heat exchange between a vapor of the coolant and a heat exchange medium circulated through the heat exchanger pipe. In this cryogenic cooling system, liquefied nitrogen is generally used as the coolant, and helium gas is used as the heat exchange medium. This system, however, suffers from the following drawbacks: (1) To avoid undesirable effects on the article to be cooled due to vibration of the liquefier, the liquefier must be disposed far from the cryostat; (2) Since helium gas is used as the heat exchange medium, the resulting cooling capability is not satisfactorily high, as apparently the heat exchange is based on gas-gas contact; (3) To increase the cooling capability of the gas-gas contact system, it is possible to increase the diameter of the heat exchanger pipe used. This, however, will result in an increase of the power and equipment area of the apparatus and thus increase operation costs. Therefore, it is desired to provide an improved cryogenic device using a coolant in a closes system of capability and not having the drawbacks of the prior art cryogenic devices of like type. According to the present invention, there is provided a cryogenic device having a closed cycle system, which comprises a closed cryostat containing, as a cooling medium, one or more liquid fluorocarbons of the formula: C m F 2m+2 , wherein m is an integer of 1 to 4, an article to be cooled being immersed in the cooling medium, and a heat exchanger pipe which is disposed in an upper portion of the cryostat and is communicated with a separate liquifier so that the cooling medium boiled and vaporised as a result of a heat generation in the cryostat is liquified within the same cryostat as a result of a heat exchange between vapour of the cooling medium and a liquid gas circulated as a heat exchange medium in the heat exchanger pipe. For a better understanding of the invention and to show how the same can be carried into effect reference will now be made, by way of example only, to the accompanying drawings, wherein: FIGURE 1 is a diagrammatic cross-sectional view of the cooling apparatus in accordance with a prior art method and already described herein; and FIGURE 2 is a diagrammatic cross-sectional view of a cooling apparatus in accordance with the present invention. The coolant used herein is a fluorocarbon C m F 2m+2 , wherein m is as defined above. The fluorocarbon used is thus CF₄, C₂F₆, C₃F₈, C₄F₁₀ or a mixture thereof. The use of C m F 2m+2 wherein n is 5 or more must be avoided, because such fluorocarbons having a higher number of carbon atoms have an increased boiling point. In the practice of the present invention, the coolant is placed as a cooling medium in a closed cryostat of the cryogenic device. An upper inner portion of the cryostat has a heat exchanger pipe disposed at a suitable position therein and communicated with a separate liquefier, so that the cooling medium boiled and vaporized as a result of heat generation in the cryostat is liquified within the same cryostat, as a result of a heat exchange between vapour of the cooling medium and a heat exchange medium in the heat exchanger pipe. The heat exchange medium circulated through the heat exchanger pipe is preferably a liquid nitrogen. Of course, other heat exchange media may be circulated, if desired. The liquefier used may have any conventional structure, but preferably the liquefier comprises a Stirling cycle refrigerator with a compressor. As described above, any heat exchange medium may be used in the practice of the present invention, but to obtain a satisfactory heat exchange effect, it is necessary to circulate a heat exchange medium having a lower boiling point than that of the cooling medium used simultaneously in the cryostat, through the heat exchanger pipe of the liquefier. If the above requirements are satisfied, a heat transfer by boiling occurs at the portion of the heat exchanger pipe in the cryostat, and thus a higher efficiency heat transfer, compared with that of the convection heat transfer obtained by using a helium gas in the heat exchanger pipe, is ensured. This is why it is preferred to use liquid nitrogen as the heat exchange medium in the heat exchanger pipe. The present invention can be applied to the cooling of any articles which are to be cooled to a cryogenic temperature of 123 K or less. Typical examples of such articles, although not limited thereto, are semiconductor devices such as high electron mobility transistors or complementary metal oxide semiconductor devices, devices using a superconducting material which exhibits a superconductivity at the cryogenic temperature, or electronic equipment which is operable at the cryogenic temperature. Surprisingly, according to the present invention, large-scale computers can be cooled, with a high efficiency, in a cryogenic device having a simple structure. For the cryogenic cooling of an article to be cooled to the cryogenic temperature, the coolant of the present invention is placed in a closed cryostat. The cooling using the cryostat will be described hereinafter with reference to Fig. 2. The cryogenic device of Fig. 2 is of the closed system type which comprises a cryostat 33 and a liquefier 13 set apart from one another. The liquefier 13 consists of a Stirling cycle refrigeration unit 14 and a high-pressure compressor 15. A heat exchanger pipe 38 from the liquefier 13 is disposed in an upper inner portion of the cryostat 33 to effect cooling and liquification of the vaporised cooling medium. A heat exchange medium 39 circulated through the heat exchanger pipe 38 is liquid nitrogen. Vaporised heat exchange medium which is returned to the liquifier through the lower limb of pipe 38 is cooled under an applied pressure of about 1.3 to 1.5 at m to form the liquid N₂ coolant. The gas is cooled by cooling a bottom portion of the container 17 with a cold head (not shown) of the refrigeration unit 14. Namely, a specific cooling gas having, for example, a temperature of about 70K, such as liquid helium of 11 to 13 kg/cm² is compressed in the compressor 15, and the compressed gas is adiabatically cooled with a piston 16, and as a result of an indirect contact of the thus cooled gas in the cold head with the returned gas from the cryostat 33, the coolant is produced in the container 17. The cooling capability of the used cooling medium is evaluated by defining it as the heat flux at the shift from nucleate boiling to layer or film boiling. The shift point is decided by measuring the relationship between the chip temperature and power dissipation. The chip temperature is measured at diodes formed on the chip surface. The results obtained were satisfactory. The cooling capabilities per unit area of the LSI chips at the point of transition to the layer boiling state was in excess of two to three times higher than when liquid nitrogen was used as a coolant in a comparative experiment. Note, for the illustrated device, since the heat exchange medium is not a gas but a liquid such as liquid nitrogen, it is possible to reduce a diameter of the heat exchanger pipe, without reducing the cooling capability. For example, when cooling of 1 KW is intended, the heat transfer can be increased to 10⁵ W/m²K (10³ W/m²K for the use of He gas), and the diameter of the heat exchanger pipe can be reduced to 25 mm (50 mm for He gas). In Fig. 2, although not shown, a heater may be attached to a bottom portion of the container 17. The heater can be advantageously used to control the temperature of the cooling medium to an approximate boiling point thereof, or to control the speed of the heat exchange reaction. The inventors have found that the similar and satisfactory results can be obtained whichever of the aforementioned fluorocarbons is used as coolant in the cryostat. Any liquified gas may be used as the heat exchange gas in the heat exchanger pipe of the liquefier, depending upon the specific properties of the selected fluorocarbon. Generally, the liquified gas in the heat exchanger pipe has a lower boiling point than that of the fluorocarbon used in the cryostat. As in the previously described example, liquid nitrogen is preferably used as the heat exchange medium. Using the cryogenic device of Fig. 2 and the fluorocarbon CF₄ as the cooling medium, the following experiment was carried out. A circuit board 32 100 mm x 100 mm in size and having 4 x 4 LSI chips 31 10 mm x 10 mm in size mounted thereon was dipped in the cooling medium (liquid CF₄) 34 in the cryostat 33. On application of an electric voltage through an electric cable (not shown) to the LSI chips 31, the boiling of the cooling medium 34 began and small bubbles 35 of the vapor of the medium 34 were generated on the surface of the heated LSI chips 31. The chips 31 were thus cooled as a result of a removal of vaporization heat from the chip surface. Further, the vaporized cooling medium was subjected to an indirect heat exchange with the heat exchange medium (liquid N₂) in the heat exchanger pipe 38, and as a result, the medium vapor was cooled to about 90K and the thus liquified medium was dropped in the not-vaporized cooling medium 34. In the heat exchanger pipe 38, the circulating liquid N₂ was first boiled with the heat from the CF₄ vapor to form a flow of the gas-liquid mixture, and returned to the liquifier 14 where it was liquified. The results of the experiment showed that the cooling capability of the device is 500 W, i.e., about five times higher than that (100 W) of the prior art device in which a He gas is used as the heat exchange medium. Note, although the liquid CF₄ was used in the above experiment, it may be replaced with other fluorocarbons, i.e., liquid C₂F₆, C₃F₈, C₄F₁₀ or a mixture thereof, and a suitable heat exchange medium may be selected for each fluorocarbon.

Claims: A cryogenic device having a closed cycle system, which comprises a closed cryostat containing, as a cooling medium, one or more liquid fluorocarbons of the formula: C m F 2m+2 , wherein m is an integer of 1 to 4, an article to be cooled being immersed in the cooling medium, and a heat exchanger pipe which is disposed in an upper portion of the cryostat and is communicated with a separate liquifier so that the cooling medium boiled and vaporised as a result of a heat generation in the cryostat is liquified within the same cryostat as a result of a heat exchange between vapour of the cooling medium and a liquid gas circulated as a heat exchange medium in the heat exchanger pipe. A device according to claim 1, in which liquid nitrogen is used as the heat exchange medium. A device according to claim 1 or 2, in which the liquefier comprises a Stirling cycle refrigerator with a compressor. A cryogenic device as claimed in any preceding claim, wherein the fluorocarbon is liquid CF₄.

Inventors: KAMEHARA NOBUO, NIWA KOICHI, YAMADA MITSUTAKA, YOKOUCHI KISHIO, KAMEHARA, NOBUO, NIWA, KOICHI, YAMADA, MITSUTAKA, YOKOUCHI, KISHIO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: C09K5
IPC codes: C09K5, H01L23, F25D3