Abstract:
An apparatus for providing two-phase heat transfer for semiconductor devices includes a vapor chamber configured to carry a cooling liquid, the vapor chamber having a base section, and a plurality of three-dimensional (3D) shaped members. The plurality of 3D-shaped members have interior and exterior sidewalls, the 3D-shaped members being connected to the base section so that vapor carrying latent heat can reach the respective interior sidewalls and get transferred to the respective exterior sidewalls configured to be in contact with an external coolant. The vapor chamber is configured to be in contact with a semiconductor device in order to remove heat therefrom.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/416,762 filed on May 3, 2006, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Aspects of the invention generally relate to heat transfer and, more specifically, to vapor chamber structures for efficient dissipation of heat from semiconductor devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    Removal of excessive heat from electronic packages using heat sinks is known in the semiconductor industry. To meet the increasing demand of heat flux density for high-power electronic products, various heat sink designs have been used. For example, tall, thin, flat fins can provide more cooling surface area for convective heat removal, and lower pressure drop for greater airflow. Further, the heat transfer coefficient (measured in watts per surface area per degree Celsius), and the temperature gradient, can be increased by crosscutting flat fins into multiple short sections if the direction of airflow is random. The leading and trailing edges of a fin can be augmented with a curvature on the surface to scrub dead air when air velocity is high. To overcome the extrusion ratio limit, i.e. the aspect ratio of fin height to spacing, during the extrusion process, fins can be assembled and bonded to a separate base to significantly increase the cooling surface area. Corrugated metal sheet can also be used as lightweight folded fins to increase the cooling surface area. 
         [0004]    In addition to improved fin efficiency, an ideal heat sink is desired to be made of materials that have high thermal conductivity, high machinability, low cost, low weight, and low toxicity. However, most materials that offer better properties than aluminum often are expensive to manufacture. For example, copper (390 W/m-K) offers higher thermal conductivity than aluminum (230 W/m-K), but it weighs 3 times heavier and is more difficult to machine. Graphite composite, on the other hand, is lighter but has anisotropic thermal conductivity. 
         [0005]    Since air is a not a good agent for heat transfer due to its low specific density, low specific heat, and low thermal conductivity, alternative technologies that offer higher heat removal capacities have been proposed to replace the traditional low-cost and low-maintenance air-cooling technologies. For example, the use of liquid, typically a water and glycol mixture, instead of air not only reduces the size of the heat sink while eliminating fan noise, but also easily removes heat from its source, thereby increasing system reliability. 
         [0006]    Heat pipes and vapor chambers are other promising technologies based on the principle of two-phase heat exchange. In a typical tubular heat pipe, where a vacuum-tight pipe having a wick structure and working fluid is used to connect an evaporator unit and a condenser unit, heat generated in the evaporator unit vaporizes the liquid in the wick. The vapor then carries the latent heat of vaporization and flows into the cooler condenser unit, where it condenses and releases the heat. The condensed liquid is returned to the evaporator unit through the capillary action of the wick structure. The phase change, caused by vaporization and condensation, and the two-phase flow circulation continue until the temperature gradient between the evaporator and the condenser no longer exists. 
         [0007]    Compared to a solid material such as aluminum that removes heat through thermal conduction, an average heat pipe that transports latent heat through vapor flow has an equivalent thermal conductivity more than 1000 W/m-K. A pump with micro-channels may be used if the condenser unit is far from the evaporator unit. Although water can be used as the working fluid for a wide range of temperatures, other materials such as methanol, ammonia, propylene, ethane, nitrogen, oxygen, and hydrogen are more suitable for low-temperature operation. For high-temperature operations, alkali metals such as cesium, potassium, sodium, and lithium appear to be more suitable. 
         [0008]    The passive nature and nearly isothermal heat transfer of the heat pipe technology make it attractive in many cooling and thermal control applications. More recent heat pipes have sintered wicks that could return liquid against gravity by capillary flow and provide a higher heat flux handling capability. 
         [0009]    Similar to heat pipe, which is a closed-loop, phase-change heat transfer system, a vapor chamber provides flat plate on the surface of heat source and allows direct attachment to a heat sink. In U.S. Pat. No. 6,085,831, entitled “Direct chip-cooling through liquid vaporization heat exchange” discloses a mechanism for dissipating heat from a chip. One of the drawbacks of this proposal is the limited surface area available for cooling the vapor as the interior fins inside the heat sink enclosure are not in direct contact with the cooler ambient that is outside the heat sink. To enhance heat exchange efficiency, the enclosure must be significantly larger than the conventional air-cooled heat sink to provide the necessary heat removal capacity. Since the internal fins generally provide lower cooling efficiency than external fins, it is therefore desirable to overcome the drawbacks associated with earlier designs. 
       SUMMARY OF THE INVENTION 
       [0010]    Aspects of the invention relate to a fin-shaped vapor chamber for efficient two-phase heat transfer. More specifically, aspects of the invention disclose a wick-assisted three-dimensional (3D) shaped (e.g., fin-shaped) vapor chamber structure that can be combined with other cooling mechanisms to provide efficient two-phase heat transfer for semiconductor devices. Based on the chip orientation and package configuration, a plurality of hollow fin designs are proposed to increase the surface area of vapor chamber and improve the two-phase cooling efficiency relative to conventional heat pipes and vapor chambers. Each enclosed vapor chamber is to be partially filled with a liquid-phase material whose low boiling temperature allows it to be evaporated by absorbing the heat from the chip and condensed by air cooling or liquid cooling of the fin chambers. Specific 3D-shaped vapor chamber designs with wick structures are provided for chips that are mounted horizontally, vertically, stacked vertically, or mounted at an angle. It will be appreciated that fin-shaped vapor chamber design is an example of a 3D-shaped vapor chamber design. As such, other 3D-shapes can be used. 
         [0011]    In one aspect, an apparatus for providing two-phase heat transfer for semiconductor devices includes a vapor chamber configured to carry a cooling liquid, the vapor chamber having a base section, and a plurality of 3D-shaped members. The plurality of 3D-shaped members have interior and exterior sidewalls, the 3D-shaped members being connected to the base section so that vapor carrying latent heat can reach the respective interior sidewalls and get transferred to the respective exterior sidewalls configured to be in contact with an external coolant. The vapor chamber is configured to be in contact with a semiconductor device in order to remove heat therefrom. 
         [0012]    In another aspect, an apparatus for removing excessive heat from semiconductor devices includes a vapor chamber configured to carry a cooling liquid, the vapor chamber having a base section, and a plurality of 3D-shaped members. The 3D-shaped members have interior and exterior sidewalls, the 3D-shaped members being connected to the base section so that vapor carrying latent heat can reach the respective interior sidewalls and transfer to the respective exterior sidewalls. A conduit having first and second ends and carrying a coolant is disposed to be in contact with the 3D-shaped members and configured to transfer heat from the exterior sidewalls of the 3D-shaped members to the coolant flowing through the conduit. The vapor chamber is configured to be in contact with a semiconductor device in order to remove heat from the semiconductor device. 
         [0013]    In a yet another aspect, an apparatus for removing heat from a semiconductor structure includes a top vapor chamber having a plurality of 3D-shaped members and disposed on top of the semiconductor structure; a plurality of side vapor chambers each having another set of a plurality of 3D-shaped members. The side vapor chambers are disposed to surround the semiconductor structure. The top vapor chamber and the side vapor chambers are configured to be fluidly coupled to enable transfer of fluid between the top and side vapor chambers. The side vapor chambers are mounted at an angle to enable backflow of condensed fluid by gravity towards a base portion of the semiconductor structure, the base portion being in contact with a chip carrier. The top vapor chamber and the side vapor chambers are filled with a coolant so as to completely cover sidewalls and top exterior surface of the structure in order to provide maximum surface contact and cooling capacity. 
         [0014]    In a further aspect, an apparatus for removing heat from a semiconductor device includes a solid heat sink; a vapor chamber in thermal contact with the heat sink on one side and the semiconductor device on an opposite side, each of the heat sink and the vapor chamber having a plurality of 3D-shaped members. The vapor chamber is filled with a coolant to a level so as to cover an entire surface of the semiconductor device. 
         [0015]    In an additional aspect, an apparatus for removing heat from a semiconductor device includes a vapor chamber in thermal contact with a semiconductor device, the vapor chamber having a plurality of 3D-shaped members, the vapor chamber configured to extend beyond a length of the semiconductor device; a thermally-conductive material coated as a porous film on a side surface of the vapor chamber, the side surface being in thermal contact with the semiconductor device. The vapor chamber if filled with a coolant to a level so as to cover an entire surface of the semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing and other aspects of embodiments of this invention are made more evident in the following Detailed Description of Exemplary Embodiments, when read in conjunction with the attached drawings, wherein: 
           [0017]      FIG. 1  shows a perspective view of an air-cooled fin-shaped hollow heat sink that is partially filled with working fluid in accordance with an exemplary embodiment of the invention. 
           [0018]      FIG. 2  shows a perspective view of an air-cooled fin shaped hollow heat sink with an embedded wick structure in accordance with another embodiment of the invention. 
           [0019]      FIG. 3  shows a perspective view of a liquid-cooled fin-shaped hollow heat sink partially filled with fluid in accordance with another embodiment of the invention. 
           [0020]      FIG. 4  shows a perspective view of a liquid-cooled fin-shaped hollow heat sink with embedded wick structure in accordance with another embodiment of the invention. 
           [0021]      FIG. 5  is a perspective view of a fin-shaped vapor chamber design for a three-dimensional (3D) stacked integrated circuit (e.g., chip) package in accordance with an embodiment of the invention. 
           [0022]      FIG. 6  shows a perspective view of a hybrid vapor chamber and heat sink structure for a vertically mounted chip in accordance with another embodiment of the invention. 
           [0023]      FIG. 7  shows a perspective view of an extended fin-shaped vapor chamber for vertically mounted chips in accordance with an embodiment of the invention. 
           [0024]      FIG. 8  shows side view and top view of fin structures of a vapor chamber in accordance with various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    Referring to  FIG. 1 , there is shown a perspective view of a heat dissipation module  100 . The module  100  includes an air-cooled fin-shaped hollow heat sink  102  that is partially filled with working fluid in accordance with an exemplary embodiment of the invention. The heat sink  102  has a base section in close proximity to a chip, a plurality of three-dimensional (3D) shaped members  115 . In one exemplary embodiment, the 3D-shaped members  115  can be in the form of fin-shaped members. Other 3D-shapes can be used. The heat sink  102  includes a large surface area but with an enclosed cavity inside. The heat sink  102  is also referred herein as a vapor chamber that can be manufactured by using, for example, molding, welding, or other low-cost means to form an extrusion-type heat sink container with a large surface area. However, instead of using a solid fin structure, the fins  115  are constructed with a hollow chamber connected to a base chamber so that vapor carrying latent heat can reach an interior sidewall of the fin  115  where heat can be directly transferred to the exterior sidewall that is in direct contact with a coolant disposed outside of the fins  115 . The heat sink  102  is partially filled with a liquid phase material  110  whose boiling point (e.g., low boiling point) fails within target range under vacuum or other designated pressure conditions. The heat sink  102  can be placed on top of a heat source such as chip  150 . A thermal interface material  160  can be used to join the heat sink  102  to one side of the chip  150 . The other side of the chip  150  is electrically connected to chip carrier  180  using solder balls  170 . 
         [0026]    As the temperature of the chip  150  rises, the liquid  110  disposed in the heat sink  102  starts to vaporize. When vapor  130  rises and contacts the interior cold surface of the fins  115 , the vapor condenses releasing the latent heat of vaporization. As condensation occurs, condensed liquid  140  falls back to the base of the heat sink  102 . Space occupied by the module  100  can be comparable to conventional models. Forced air such as high-speed air flow can be applied directly to the outer sidewalls of the fins  115  to effectively cool the vapor of the evaporated liquid  110 . 
         [0027]      FIG. 2  shows a heat sink module  200  including an exemplary design of a wick-assisted fin-shaped vapor chamber  202 . Except for the vapor chamber  202 , the rest of the elements shown in  FIG. 2  are similar to the elements shown in  FIG. 1 , and the description of such common elements is therefore not repeated. The design of the vapor chamber  202  can be helpful to provide for efficient cooling if a chip is mounted vertically or upside down. The embedded wick structure  204  provides a simple but reliable capillary mechanism configured to deliver the condensed liquid  140  from the cooling fins to the heated base and enables the vapor chamber  202  to operate effectively at all gravity orientations. The wick structure  204  can be made of thin sheets of metal or non-metal meshes and fiber bundles. 
         [0028]      FIG. 3  shows a perspective view of a liquid-cooled fin-shaped hollow heat sink  302  that is partially filled with fluid. The heat sink  302  is designed in conjunction with a liquid cooling technique. The inventors have discovered that use of liquid cooling instead of air cooling not only improves the heat transfer efficiency, but also reduces the size of a heat sink required for similar heat flux density. As the liquid enters the heat sink  302  via inlet “A” and exits via outlet “B” of a pipe  325 , the fins  315  of the heat sink  302  can be sufficiently cooled to achieve maximal efficiency. In an exemplary embodiment, the heat-sink  302  disposed over a top surface of at least one chip  350  is designed to have a slightly larger dimension than the chip  350  to further improve the cooling efficiency. The heat sink  302 , which is partially filled with liquid  310 , is glued to the chip  350  using a layer of thermal paste  360 . The chip  350  is then bonded to a chip carrier  380  via solder balls  370 . As the temperature of the chip  350  rises, the liquid  310  evaporates to produce vapor  330 . The vapor  330  fills the interior of the heat sink  302  and eventually condenses on interior sidewalls of fins  315  of the heat-sink  302 , the funs  315  being cooled by the liquid flowing in the pipe  325 . The condensed liquid  340  would drip back to the base of the heat-sink  302  and be used in a subsequent cooling cycle. To maintain the boiling temperature within a desired range, it is preferred to have the liquid  310  partially fill the heat sink  302  under controlled temperature and pressure to gain maximum cooling efficiency. 
         [0029]      FIG. 4  shows a perspective view of a liquid-cooled fin-shaped hollow heat sink  402  with embedded wick structure  404  in accordance with another embodiment of the invention. The inventors have observed that such design in conjunction with liquid cooling provides for enhanced cooling efficiency if a chip is mounted vertically or upside down. As noted above, the embedded wick structure  404  provides a simple but reliable capillary mechanism configured to deliver the condensed liquid  340  from the cooling fins  315  back to the base, that is at a higher temperature relative to the temperature of the fins  315 , and enables the heat sink  402  to operate effectively at all gravity orientations. 
         [0030]    The combination of phase-change and liquid cooling can handle heat flux in the order of 200 W/cm 2 . Room-temperature liquid-phase material that can be used to partially fill the vapor chamber of the heat sink  402  include fluorinated ketone such as 3M™ Novec 1230 [CF 3 , CF 2 , C(O)CF(CF 3 ) 2 ], which has a boiling temperature of about 49.2° C. Low-temperature liquid-phase material that can be used to partially fill the vapor chamber of the heat sink  402  include ECARO-25 [pentafluoroethane, CF 3 -CHF 2 ], which has a boiling temperature of about −48.1° C. Other fluids such as water, ethanol, methanol, ammonia, or butane can also be used. 
         [0031]      FIG. 5  is a perspective view of an angled vapor chamber design  502  for a three-dimensional (3D) stacked integrated circuit (e.g., chip) package in accordance with an embodiment of the invention. The vapor chamber  502  is designed to meet the heat flux demand of a three-dimensional stack-chip  550  as illustrated. The heat sink  500  includes a top vapor chamber  510 , and multiple angled side chambers  520 A and  520 B. Hollow fins surrounding the stacked chips  550  are mounted at a tilted angle, preferably between 45° and 70°, to ensure backflow of condensed liquid by gravity. Chips  550 A to  550 H are stacked vertically where heat can be dissipated from an exterior surface of the stack structure  550 . Liquid in each of the vapor chambers surrounding the chips  550 A to  550 H should preferably be filled to a level where it completely covers the sidewalls of the chips  550 A to  550 H in order to provide maximum surface contact and cooling capacity. In one embodiment, the top vapor chamber  510  and each fin of the side chambers  520 A and  520 B are built as separate units. Such units are then assembled and mounted on the stacked chips  550 . Thermal paste is used to fill the gaps between adjacent chips (e.g., between  550 A,  550 B, and so on and so forth) and between the stacked chips  550  and the surrounding fin-shaped hollow heat sink  502 . 
         [0032]      FIG. 6  shows a perspective view of a heat sink structure  600  and a hybrid vapor chamber  602  for a vertically mounted chip in accordance with another embodiment of the invention. The embodiment shown in  FIG. 6  combines a solid heat sink  690  and a hollow vapor chamber  604  to provide efficient cooling for chips that are mounted vertically on a package (e.g., chip carrier and a chip). The solid heat sink  690  and the hollow vapor chamber  604  are together illustrated as hybrid vapor chamber  602 . The hybrid vapor chamber  602  includes a vertical section  610  that is sandwiched between a chip  650  on one side and the heat sink  690  on the other side. Thermal paste  660  is used to join the hollow vertical vapor chamber  610  to the chip  650  and the heat sink  690 . The vertical section  610  is filled with liquid so that it covers an entire surface of the chip  650 . The liquid filled in the vertical section  610  is heated by the chip  650  on one side thereby generating tiny bubbles that either rise to a top surface of the hollow chamber  604  or condense to liquid due to the cooling effect of heat sink  690 . The bubbles that reach the top surface and become vapors  630  fill a fin-shaped top chamber  615  and condense upon contact with the cool surface of the fin structure  615 . The temperature of the chip  650  can therefore be maintained below the boiling temperature of the liquid in the vapor chamber  604 . It will be appreciated that the heat sink  690  can also be replaced by other cooling structures such as, for example, a thermoelectric module. 
         [0033]      FIG. 7  shows an alternative heat sink structure design  700  of the fin-shaped vapor chamber structure  702  for vertically mounted chips shown in  FIG. 6 . Elements similar to those shown in  FIG. 6  are represented with same reference numerals. Without the use of a solid heat sink  690  shown in  FIG. 6 , fin-shaped branch chambers  704  can be extended from a top horizontal surface to a vertical side surface of the main vapor chamber  706 . To enhance the thermal exchange between cooling liquid  710  and the chip  650 , a highly thermal-conductive porous film  735  can be coated on a side surface of the vapor chamber  706 , the side surface being in direct contact with the chip  650 . In one exemplary embodiment, the highly thermal-conductive porous film can be like a sponge. Condensation of vapor  730  in fin structure  715  can be achieved, for example, either by air cooling through high-speed air flow, or via liquid cooling. 
         [0034]    The hollow fin-shaped vapor chambers described above in  FIGS. 1-7  in accordance with various embodiments can be constructed in different shapes and forms. 
         [0035]      FIG. 8  illustrates the side view and top views of some possible designs including triangular and rectangular prisms and pyramids, cones and cylinders. 
         [0036]    In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.