Abstract:
A cooling device having a high cooling efficiency in a miniature size that is neither influenced by gravity nor restricted in its position of installation. The device exchanges, transports and dissipates heat generated by a heat source. The device includes a coolant storing part for storing liquid coolant, a heat absorbing part including at least one micro channel, the heat absorbing part is closely positioned to the heat generating source and connected to the coolant storing part. The liquid coolant is partly filled in the micro channel by surface tension and vaporized to become a gaseous coolant in the micro channel when heat is absorbed from the heat generating source. The device includes a heat insulating part positioned adjacent to the heat absorbing part to prevent the heat absorbed by the heat absorbing part from transferring to other zones. The device includes a condensing part for condensing the gaseous coolant. The condensing part is positioned apart from the heat absorbing part. The device includes a gas moving part positioned near the heat absorbing part and the condensing part, and the gas moving part includes a passage through which the gaseous coolant moves from the heat absorbing part to the condensing part. The device further includes a housing in which at least the heat absorbing part is housed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a micro cooling device for discharging out unnecessary heat, and more particularly to a micro cooling device for an electronic product that generates unnecessary heat in spite of a small size of product, such as an integrated circuit device. 
     2. Brief Description of the Prior Art 
     Due to a large quantity of heat generated by an integrated circuit device like a recently developed central processing unit (CPU), the device itself and a system containing it tend to be degenerated in product reliability. Especially, in a semiconductor device, a variety of parameters are affected by the operation temperature so that their values are changed, thereby causing a problem in the device to deteriorate performances of the integrated circuit. 
     One representative conventional method to solve the aforementioned problem is using a fan to forcibly cool down the device. However, the aforementioned method has its own problems like a low cooling efficiency, introduction of additional heat generating source, such as a power source for the fan, and additional heat generated by the fan itself. 
     Another method having a higher cooling efficiency is discharging heat by changing phases of a liquid material(“coolant”). In other words, a liquid material used as a coolant is passing over a heat generating source and turning into gas to discharge heat with its vaporizing energy, which has been widely used in refrigerators and air conditioners. There is also a problem in the aforementioned method in that a variety of equipment should be additionally installed for condensing the vaporized(or gaseous) coolant, so that the bulkiness of the total system and power consumption are increased. 
     Recently, a very small-sized cooling device, so called a heat pipe, has been developed by applying phase changes of a liquid material and natural convection phenomenon. Even if there are a various types of heat pipes, a double-pipe type of heat pipe having internal and external pipes is introduced as an efficient cooling device. In the double-pipe type, the coolant is filled in the external pipe, and the wall of the internal pipe has a plurality of fine holes to form a passage to the external pipe from the inside of the internal pipe. When heat from a heat source is transferred to the external pipe, the coolant in the external pipe turns into gas by absorbing the heat, and the vaporized coolant is infused into the inside of internal pipe through the holes on the internal pipe. The gas in the internal pipe further moves to the opposite end of the internal pipe through differences in buoyancy and air pressure. At the opposite end of the internal pipe, the coolant is condensed into liquid. The liquid moves through the holes of the internal pipes to the external pipe and finally returns to the original place of the coolant. 
     The heat pipe made in the aforementioned principle is small and has a favorable cooling efficiency. However, as the movement of the gas coolant inside the pipe is dependent on differences in buoyancy and air pressure and the movement of the liquid coolant is dependent on gravity, there is a limitation on the installation position or place of the heat pipe. Furthermore, since the heat pipe should be constructed in a structure in which the coolant is condensed at the opposite end of the heat generating source, as the size of the heat pipe gets smaller, its cooling efficiency gets lower and its performance gets poorer. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to solve the aforementioned problems of the conventional cooling devices and to provide a micro cooling device which can be made very small, but with high cooling performances. 
     In addition, it is another object of the present invention to provide a cooling device of high efficiency that has no restriction in installation position or place which is not influenced by gravity. 
     In order to accomplish the aforementioned objects of the present invention, there is provided a cooling device which discharges out heat generated by a heat generating source, said device comprising: a coolant storing part for storing liquid coolant; a heat absorbing part comprising at least one micro channel and being positioned to be close to said heat generating source and connected to said coolant storing part, said liquid coolant being partly filled in said micro channel by surface tension and vaporized to be gaseous coolant in said micro channel when heat is absorbed from said heat generating source; a heat insulating part being adjacent to said heat absorbing part for preventing said heat absorbed by said heat absorbing part from transferring to other zones; a condensing part for condensing said gaseous coolant and being positioned apart from said heat absorbing part; a gas moving part being close to said heat absorbing part and said condensing part and being a passage through which said gaseous coolant moves from said heat absorbing part to said condensing part; and a housing in which at least said heat absorbing part is comprised. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Objects and aspects of the invention will become apparent from the following description of an embodiment with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic cross-sectional view for illustrating a cross-section of an XZ plane of a cooling device in accordance with an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view for illustrating the cooling device shown in FIG. 1 as viewed from line a-a′; 
     FIG. 3 is an enlarged schematic view for illustrating one of micro channels in a heat absorbing part of the cooling device shown in FIG. 1; and 
     FIG. 4 is a cross-sectional view for illustrating a cross-section of an XZ plane of the cooling device  100 ′ in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic cross-sectional view for illustrating a cross-section of an XZ plane of a cooling device in accordance with an embodiment of the present invention. The cooling device  100  of the present invention includes a coolant storing part  102  to store liquid coolant (indicated by wave patterns in the drawing) and a heat absorbing part  106  which is positioned close to the coolant storing part  102  and neighboring the heat generating source(not shown). The heat absorbing part  106  includes a plurality of micro channels  114  (indicated by slant lines in the drawing). The coolant stored in the coolant storing part  102  is partly filled in the micro channels  114  by surface tension of each of the micro channels according to the capillary phenomenon. 
     The cooling device  100  of the present invention further includes a gas moving part  104  positioned across from the coolant storing part  102  and separated by the heat absorbing part  106 . The cooling device  100  also includes a heat insulating part  108  (which is a heat insulating element separate from the heat absorbing part  106  and the condensing part  110 ) placed adjacent to the heat absorbing part  106  for preventing heat from transferring to the other parts. The cooling device also includes a condensing part  110  positioned across from the heat absorbing part  106  and separated in Z-axis by the heat insulating part  108 . 
     Preferably, the coolant storing part  102 , the heat absorbing part  106 , the gas moving part  104 , the heat insulating part  108  and the condensing part  110  is formed in a housing  112 , which forms an embodiment of the cooling device  100  of the present invention. 
     In order to more clearly describe the geometrical structure of an embodiment of the present invention, FIG. 2 is shown for a cross-sectional view of the cooling device  100  in FIG. 1 seen from line a-a′. The cooling device  100  includes a coolant storing part  102  formed apart in the X-axis from the gas moving part  104  with the heat absorbing part  106  being inserted therebetween. As mentioned, a plurality of micro channels  114  are formed in the heat absorbing part  106 . 
     Next, operational processes of the cooling device  100  will be described with reference to FIGS. 1 through 3. As shown in FIG. 1, a heat transferring direction is illustrated in block arrows  120  and  122 . Heat generated by the external heat generating source (not shown) is conveyed to the heat absorbing part  106  of the cooling device  100 . It is preferred that a thermal contact is kept between the external heat generating source and the external wall of the housing  112  of the cooling device  100  where the heat absorbing part  106  is neighbored. 
     The housing  112  can be made of a variety of materials including semiconductor materials like silicon Si or Gallium Ga, layering materials like self-assembled mono-layer, metals like copper Cu or aluminum Al or either alloy, ceramics or crystalline materials like diamond. Especially, if the external heat generating source is a semiconductor device, the cooling device  100  of the present invention can be made of the same semiconductor material used for the semiconductor device. The cooling device  100  of the present invention, as will be described below, can be integrally manufactured as one of the following fabrication processes. Therefore, the cooling device  100  can be made in the same size (for instance, several or tens of square centimeters of area in XY plane) as the external heat generating source, so that the thermal resistance in the cooling device  100  of the present invention can be minimized. 
     As shown in FIG. 2, the heat transferred from the external heat generating source is absorbed at the heat absorbing part  106 . As shown in FIG. 2, the heat absorbing part  106  has a plurality of micro channels  114 , and the coolant stored in the coolant storing part  102  is filled up to a predetermined portion of the channels  114  according to the capillary phenomenon. This is illustrated in detail in FIG.  3 . As shown in FIG. 3, which is an enlarged view for schematically illustrating one of the micro channels  114  of the heat absorbing part  106 , the coolant fills from the coolant storing part  102  up to a position designated as “A” of the micro channel. 
     The position “A” in which the coolant is filled depends on type of the coolant and dimension of the micro channels  114 . Particularly, the type of coolant may be different according to the material of the housing  112  because the coolant may go through a chemical reaction with the surface of micro channels  114  or housing  112 . In consideration of environment pollution, a non-CFC type of new coolant may be preferred. As a coolant compatible with the material of the housing  112 , for instance, in the electronic products like integrated circuit, water H 2 O, or alcohol like methanol or ethanol may be preferably selected. The large thermal capacity of the aforementioned coolant and its small surface tension angle with a semiconductor device result in a great flow rate of the coolant to transfer great quantity of heat. Furthermore, there is no problems related to environment pollution. Even when there is a defect in the housing  112  (for instance, a fine crack on the surface of the housing), it is less probable that the coolant will be discharged out of the housing  112 . 
     In general, though there is surface tension in macroscopic system, an influence of the gravity force is superior. Therefore, it is difficult to efficiently take an advantage of the surface tension in macroscopic system. In order to make the influence of the gravity force negligible, the size of the system gets smaller. Thus, preferably, the width of each micro channels  114  suitable to the cooling device  100  of the present invention is approximately within a range of 1 nm to 1000 um, and the length of the channels  114  is approximately within a range of 0.5 to 5 cm. In addition, the cross-sectional surface of the each micro channel  114  may be formed in circle, oval, rectangle, square, multi-angular shape, etc. As described below, the cross-sectional area may get larger or smaller in a predetermined direction to control the magnitude of the surface tension between the internal wall of the channels  114  and coolant. 
     As described above, if heat is applied from the external heat generating source to the micro channels  114  of the heat absorbing part  106  filled with coolant, fine bubbles are generated by evaporation of some coolant filled in the micro channels  114 , so that there forms turbulence in the coolant. Such fine bubbles and turbulence in the coolant generate more fine bubbles (not shown) in the micro channels  114 . These fine bubbles move to gas moving part  104  where no coolant is stored. Since the bubbles move only in a distance of only several millimeters, the influence of the gravity can be negligible. Therefore, even if the coolant storing part  102  and gas moving part  104  are respectively positioned at high and low places, the bubbles can move to the gas moving part  104  from the coolant storing part  102  due to the difference in pressure in the heat absorbing part  106 . The movement of the bubbles is described in detail hereinafter. 
     The aforementioned movement of the bubbles having a predetermined direction can be established by the nodes  116  formed on the internal surface of the micro channels  114  in the heat absorbing part  106 . In other words, as shown in FIG. 3, a plurality of nodes  116  are protruded on the internal surface of the micro channels  114  in a zone close to the coolant storing part  102 . As the cross-sectional area of the micro channel  114  gets smaller to the direction toward the gas moving part  104  (that is, in the increasing direction of the X axis) in which the surface tension gets larger in such a direction. The aforementioned increase in the surface tension enables the coolant to have potential energy which causes coolant to move in the direction from the coolant storing part  102  to the gas moving part  104 . In summary, according to the directional potential energy of the coolant, most of the bubbles generated in the coolant tend to move in an increasing direction of the X axis. 
     As shown in FIG. 1, the gas moving part  104  is formed as an empty space at first. The bubbles moved from the heat absorbing part  106  to the gas moving part  104  break into gases (gaseous coolant). As the gaseous coolant erupts out of the heat absorbing part  106 , the gaseous coolant moves to the condensing part  110  due to the difference in pressure at the zone adjacent to the heat absorbing part  106  and the condensing part  106 . 
     As the number of the bubbles in a unit volume is increased, the cooling efficiency of the cooling device of the present invention is increased. Therefore, it is preferable to increase the possibility of generation of such bubbles. For example, a plurality of micro grooves (not shown) may be formed on the internal surface of the channels  114  in the heat absorbing part  106 . Alternatively, a microwave generator (not shown) may be used to provide microwave energy to the cooling device  100  for finely vibrating the cooling device thereby increasing the possibility of bubble generation. 
     Then, the gas coolant loses its vaporization energy in the condensing part  110  to change into liquid state coolant. In order to more efficiently carry out condensation of the coolant, a plurality may be mounted on the external surface of the housing  112  close to the condensing part  110 . The aforementioned fins may be made in microscopic size. Further, for instance, if micro actuators are made together with the micro fins, the heat discharged out of the condensing part  110  can be recycled to circulate surrounding air. Or, if the fin is made of a thermoelectric device, the heat discharged out of the condensing part  110  can be transformed into electrical energy which can be used for other electronic devices. Furthermore, in accordance with another embodiment of the present invention, the condensing part  110  may be formed bigger than that of the heat absorbing part  106  (for instance, about 10 times), so that convection of surrounding atmosphere can also be helpful for condensation of the gaseous coolant. Further, micro fins may be formed on the inside surface of the condensing part  110 , thereby increasing the condensing efficiency of the coolant. 
     In the condensing part  110 , the gaseous coolant is condensed and collected into liquid coolant. When enough liquid coolant is collected, the liquid coolant moves to the coolant storing part  102  through the micro channels formed in the condensing part  110 . The condensed coolant moves to the coolant storing part  102  under the same principle as described above. Similar to the structure of the heat absorbing part  106 , the channels in the condensing part  110  may include a plurality of nodes  118  on the inside surface in a zone adjacent to the gas moving part  104 . At this time, the nodes  118  are formed in an opposite direction to those  116  formed in the heat absorbing part  106 . The coolant condensed into liquid returns to the coolant storing part  102 , thereby completing the circulation of the coolant in the cooling device  100 . 
     As described above, the circulation of the coolant in the cooling device  100  of the present invention is carried out in itself without external driving force, especially through the capillary phenomenon by surface tension of the liquid coolant, overcome the influence of gravity. As a plurality of micro channels  114  are included in the heat absorbing part  106 , the surface tension is greater than gravity in such cases. 
     As micro dynamics are applied in the present invention, there are a variety of methods to make the cooling device  100  of the present invention. For instance, there may be used MEMS(micro electro mechanical system) or SAM(self assembled mono-layer) method, or a super-precision structure manufacturing method using laser or plasma. 
     Now, another embodiment of the present invention will be described with reference to FIG. 4, which is a cross-sectional view for illustrating the XZ plane of the cooling device  100 ′ in accordance with another embodiment of the present invention. As shown in the drawing, the cooling device  100 ′ can be formed in a multi-layered structure with expansion of the mono-layered structure of the cooling device  100 . 
     The coolant circulation of the cooling device  100 ′ is described as below. The coolant turns into gas by the heat absorbed in the heat absorbing part  100 ′ and the gaseous coolant begins to move by the same mechanism as described in the mono-layered structure of the cooling device  100 . Then, the coolant, as much as what has flowed out of the coolant storing part  102 ,′ re-fills from the condensing part  110 ′ to the coolant storing part  102 ′ according to the principle of continuity. The gaseous coolant turns into liquid coolant again at the condensing part  110 ′ through the gas moving part  104 ′, so that the amount of the coolant, which flowed into the coolant storing part  102 ′ from the condensing part  110 ′, is compensated. Thereby, the circulation of the coolant in the cooling device  100 ′ is completed. 
     As shown in the drawing, the cooling device  100 ′ is different from the cooling device  100  in the multi-layered structure of the condensing part  110 ′, but all the essential principles, such as circulation of the coolant, phase changes or heat generation, are the same in both cooling devices  100  and  100 ′. The multi-layered structure of the condensing part  110 ′ includes a plurality of micro channels (slant zones) and is divided by the insulating part  108 ′. A plurality of nodes  118 ′ are formed for inducing the directional characteristics of the coolant in the micro channels. Of course, such nodes may be formed in the internal surface of all the micro channels of the condensing part  110 ′ to strongly keep a predetermined directional characteristics. Similarly in the mono-layered structure of the cooling device  100 , nodes  116 ′ may also be formed in the heat absorbing part  106 ′ to induce the directional characteristics of the coolant. 
     As described above, a plurality of layers of the condensing parts  110 ′ are formed to improve the condensation efficiency of coolant, thereby improve cooling efficiency of the cooling device  100 ′. 
     In accordance with the present invention, there is provided a microscopic cooling device having a characteristic of a high efficient heat cooling, which also improves performances and reliability of products with the cooling device of the present invention. 
     Having described specific preferred embodiments of the invention, it is to be understood that the invention is not limited to those disclosed embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. 
     For example, the subject matter of the present invention may be embodied in a cooling device which includes a separate housing for the coolant storing part or the condensing part interconnected with the heat absorbing part through tubes as the gas moving part. In this particular embodiment, the size of the separate housing can be larger than the heat absorbing part, so that the condensing efficiency can be increased. 
     Alternatively, the above described parts of the cooling device of the present invention may be formed on a plane, so that the thickness of the cooling device can be reduced. In this particular embodiment, the heat absorbing part and the condensing part are formed on, for example, the XY plane, thermally insulated from each other by the insulation part which is also formed on the same XY plane, and connected with each other through coolant storing part and the gas moving part also formed on the same XY plane. 
     Further, the micro channels in the heat absorbing part may be formed as curved lines rather than straight lines.