Abstract:
An integrated circuit to be cooled may be abutted in face-to-face abutment with a cooling integrated circuit. The cooling integrated circuit may include electroosmotic pumps to pump cooling fluid through the cooling integrated circuits via microchannels to thereby cool the heat generating integrated circuit. The electroosmotic pumps may be fluidically coupled to external radiators which extend upwardly away from a package including the integrated circuits. In particular, the external radiators may be mounted on tubes which extend the radiators away from the package.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/698,749, filed on Oct. 31, 2003. 
     
    
     BACKGROUND  
       [0002]     This invention relates generally to cooling integrated circuits.  
         [0003]     Electroosmotic pumps use electric fields to pump a fluid. In one application, they may be fabricated using semiconductor fabrication techniques. They then may be applied to the cooling of integrated circuits, such as microprocessors.  
         [0004]     For example, an integrated circuit electroosmotic pump may be operated as a separate unit to cool an integrated circuit. Alternatively, the electroosmotic pump may be formed integrally with the integrated circuit to be cooled. Because the electroosmotic pumps, fabricated in silicon, have an extremely small form factor, they may be effective at cooling relatively small devices, such as semiconductor integrated circuits.  
         [0005]     Thus, there is a need for better ways of cooling integrated circuits.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic depiction of the operation of the embodiment in accordance with one embodiment of the present invention;  
         [0007]      FIG. 2  is an enlarged cross-sectional view of one embodiment of the present invention at an early stage of manufacture;  
         [0008]      FIG. 3  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0009]      FIG. 4  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0010]      FIG. 5  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0011]      FIG. 6  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0012]      FIG. 7  is an enlarged cross-sectional view taken along the lines  7 - 7  in  FIG. 8  at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0013]      FIG. 8  is a top plan view of the embodiment shown in  FIG. 8  in accordance with one embodiment of the present invention;  
         [0014]      FIG. 9  is an enlarged cross-sectional view of a completed structure in accordance with one embodiment of the present invention;  
         [0015]      FIG. 10  is a depiction of a recombiner at an early stage of manufacture;  
         [0016]      FIG. 11  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0017]      FIG. 12  is an enlarged top plan view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0018]      FIG. 13  is a cross-sectional view taken general along the line  13 - 13  in  FIG. 12  in accordance with one embodiment of the present invention;  
         [0019]      FIG. 14  is an enlarged cross-sectional view at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0020]      FIG. 15  is a top plan view of the embodiment shown in  FIG. 14  at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0021]      FIG. 16  is a cross-sectional view taken generally along the line  16 - 16  in  FIG. 15  in accordance with one embodiment of the present invention;  
         [0022]      FIG. 17  is a cross-sectional view corresponding to  FIG. 16  at a subsequent stage of manufacture in accordance with one embodiment of the present invention;  
         [0023]      FIG. 17A  is a side-elevational view of a re-combiner in accordance with one embodiment of the present invention;  
         [0024]      FIG. 18  is a schematic view of a packaged system in accordance with one embodiment of the present invention;  
         [0025]      FIG. 19  is a cross-sectional view of a packaged system in accordance with another embodiment of the present invention;  
         [0026]      FIG. 20  is a cross-sectional view of a packaged system in accordance with another embodiment of the present invention;  
         [0027]      FIG. 21  is a schematic view of a cooling system in accordance with another embodiment of the present invention;  
         [0028]      FIG. 22  is a schematic view of still another embodiment of the present invention;  
         [0029]      FIG. 23  is a schematic view of still another embodiment of the present invention;  
         [0030]      FIG. 24  is an enlarged, cross-sectional view through one embodiment of the present invention taken generally along the line  24 - 24  in  FIG. 25 ; and  
         [0031]      FIG. 25  is a cross-sectional view taken generally along the line  25 - 25  in  FIG. 24  in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0032]     Referring to  FIG. 1 , an electroosmotic pump  28  fabricated in silicon is capable of pumping a fluid, such as a cooling fluid, through a frit  18 . The frit  18  may be coupled on opposed ends to electrodes  29  that generate an electric field that results in the transport of a liquid through the frit  18 . This process is known as the Electroosmotic effect. The liquid may be, for example, water and the frit may be composed of silicon dioxide in one embodiment. In this case hydrogen from hydroxyl groups on the wall of the frit deprotonate resulting in an excess of protons moving transversely to the wall or transversely to the direction of fluid movement, indicated by the arrows A. The hydrogen ions move in response to the electric field applied by the electrodes  29  in the direction of the arrows A. The non-charged water atoms also move in response to the applied electric field because of drag forces that exist between the ions and the water atoms.  
         [0033]     As a result, a pumping effect may be achieved without any moving parts. In addition, the structure may be fabricated in silicon at extremely small sizes making such devices applicable as pumps for cooling integrated circuits.  
         [0034]     In accordance with one embodiment of the present invention, the frit  18  may be made of an open and connected cell dielectric thin film having open nanopores. By the term “nanopores,” it is intended to refer to films having pores on the order of 10 to 1000 nanometers. In one embodiment, the open cell porosity may be introduced using the sol-gel process. In this embodiment, the open cell porosity may be introduced by burning out the porogen phase. However, any process that forms a dielectric film having interconnected or open pores on the order of 10 to 1000 nanometers may be suitable in some embodiments of the present invention.  
         [0035]     For example, suitable materials may be formed of organosilicate resins, chemically induced phase separation, and sol-gels, to mention a few examples. Commercially available sources of such products are available from a large number of manufacturers who provide those films for extremely low dielectric constant dielectric film semiconductor applications.  
         [0036]     In one embodiment, an open cell xerogel can be fabricated with 20 nanometer open pore geometries that increase maximum pumping pressure by a few orders of magnitude. The xerogel may be formed with a less polar solvent such as ethanol to avoid any issues of water tension attacking the xerogel. Also, the pump may be primed with a gradual mix of hexamethyldisilazane (HMDS), ethanol and water to reduce the surface tension forces. Once the pump is in operation with water, there may be no net forces on the pump sidewalls due to surface tension.  
         [0037]     Referring to  FIGS. 2-9 , the fabrication of an integrated electroosmotic pump  28  using a nanoporous open cell dielectric frit  18  begins by patterning and etching to define an electroosmotic trench.  
         [0038]     Referring to  FIG. 2 , a thin dielectric layer  16  may be grown over the trench in one embodiment. Alternatively, a thin etch or polish-stop layer  16 , such as a silicon nitride, may be formed by chemical vapor deposition. Other techniques may also be used to form the thin dielectric layer  16 . The nanoporous dielectric layer  18  may than be formed, for example, by spin-on deposition. In one embodiment, the dielectric layer  18  may be in the form of a sol-gel. The deposited dielectric layer  18  may be allowed to cure.  
         [0039]     Then, referring to  FIG. 3 , the structure of  FIG. 2  may be polished or etched back to the stop layer  16 . As a result, a nanoporous dielectric frit  18  may be defined within the layer  16 , filling the substrate trench.  
         [0040]     Referring next to  FIG. 4 , openings  24  may be defined in a resist layer  22  in one embodiment of the present invention. The openings  24  may be effective to enable electrical connections to be formed to the ends of the frit  18 . Thus, the openings  24  may be formed down to a deposited oxide layer  20  that may encapsulate the underlying frit  18 . In some embodiments, the deposited oxide layer  20  may not be needed.  
         [0041]     The resist  22  is patterned as shown in  FIG. 4 , the exposed areas are etched and then used as a mask to form the trenches  26  alongside the nanoporous dielectric layer  18  as shown in  FIG. 5 . Once the trenches  26  have been formed, a metal  29  may be deposited on top of the wafer In one embodiment, sputtering can be used to deposit the metal. The metal  29  can be removed by etching or lift-off techniques in such a manner as to leave metal only in the trench at the bottom of the trenches  26  as shown in  FIG. 6 . The metal  29  is advantageously made as thin as possible to avoid occluding liquid access to the exposed edge regions of the frit  18 , which will ultimately act as the entrance and exit openings to the pump  28 . The metal  29  may be thick enough, however, to assure adequate current flow without damage to the electrodes. Additionally, it is advantageous if the metal  29  also is deposited along the edges of the frit to a thickness which does not block the pore openings. This assures a uniform electric field along the entire depth of the frit.  
         [0042]     Referring to  FIG. 7 , a chemical vapor deposition material  34  may be formed over the frit  18  and may be patterned with photoresist and etched, as indicated at  32 , to provide for the formation of channels  38  shown in  FIG. 8 . The channels  38  are etched through the deposited material  34  over the substrate  40 . The channels  38  act as conduits to convey liquid to and from the rest of the pump  41 . Also, electrical interconnections  36  may be fabricated by depositing metal (for example by sputtering), and removing the metal in selected areas (for example by lithographic patterning and etching across the wafer to enable electrical current to be supplied to the electrodes  29 . This current sets up an electric field that is used to draw the fluid through the pump  28 .  
         [0043]     Referring to  FIG. 9 , the fluid may pass through the microchannels  38  and enter the frit  18  by passing over the first electrode  29 . The fluid is drawn through the frit  18  by the electric field and the disassociation process described previously. As a result, the fluid, which may be water, is pumped through the pump  28 .  
         [0044]     Referring now to  FIGS. 10 through 17 , one embodiment of a fabrication technique for making an integrated re-combiner is illustrated. Initially, a semiconductor substrate  60 , such as a silicon wafer, may have a trench  62  formed therein by patterning and etching techniques, for example. Thereafter, a catalyst material  64 , such as platinum, is sputter deposited as shown in  FIG. 10 . The catalyst material  64  is polished off the top of the wafer substrate  60  so only the portion  66  remains as shown in  FIG. 11 . A resist may be spun-on and patterned to form microchannels  68   a  and  68   b , shown in  FIGS. 12 and 13 .  
         [0045]     The microchannels  68   a  and  68   b  may be etched to the depth of the top of the catalyst material  66  and the resist used to do the etching may be cleaned. Then a resist  70  may be spun-on and ashed to clear the top of the wafer substrate  60 , as shown in  FIG. 14 . A barrier, such as TiTiN, and copper  72  may be sputtered on top of the wafer substrate  60 . A resist lift off may be used to remove the copper from the top of the catalyst material  66  and the microchannels  68   a  and  68   b  as shown in  FIG. 17 .  
         [0046]     A porous Teflon layer (not shown) may be deposited over the wafer surface and either etched back or polished so that the Teflon covers the catalyst material  66  while having the copper  72  exposed. The Teflon layer protects the catalyst material  66  from getting wet when re-combined gas turns into water.  
         [0047]     A pair of identical substrates  60 , processed as described above, may then be combined in face-to-face abutment to form a re-combiner  30  as shown in  FIG. 17A . The substrates  60  may be joined by copper-to-copper bonding where there is no trench  16  or channel  68 . Other bonding techniques, such as eutectic or direct bonding, may also be used to join the two wafers together. The trenches  16  and channels  68  may be aligned to form a passage for cooling fluid circulation over the catalyst material  66 .  
         [0048]     The re-combiner  30  may be used to reduce the buildup of gas in the cooling fluid pumped by the pump  28 . Exposure of the gases to catalytic material  66  results in gas recombination. The re-combiner  30  may be made deep enough to avoid being covered with water formed from recombined gas and the cooling fluid itself.  
         [0049]     Electroosmotic pumps  28  may be provided in a system  100  coupled by fluid passageways as indicated in  FIG. 18 . The passageways couple a radiator  132 , a re-combiner  30 , and a set of microchannels  116  in a circuit or pathway for fluid. Thus, the fluid pumped by the pump  28  passes through the channel  116  and the re-combiner  30  to the radiator  132  where heat is removed to the surrounding environment. Thus, the microchannels  116 , associated with an integrated circuit not shown in  FIG. 18 , provide cooling for that integrated circuit.  
         [0050]     Referring to  FIG. 19 , a surface mount or flip-chip package  129  may support an integrated circuit  124  having bump connections  126  to the package  129 . The top side of the integrated circuit  124  faces towards the package  129 .  
         [0051]     The die  114  active semiconductor  124  is underneath the bulk silicon  122 . The die  114  may be coupled to another die  112  by a copper-to-copper connection  120 . That is, copper metal  120  on each die  112  and  114  may be fused to connect the dice  112  and  114 . The die  112  may be bonded by glass, polymers, or dielectric bonding to the die  140 .  
         [0052]     The die  112  may include a dielectric layer  118  and a plurality of microchannels  116 , which circulate cooling fluid. On the opposite side of the die  112  are a plurality of electroosmotic pumps  28  formed as described previously. A dielectric layer  136  couples the die  112  to a die  140 , which forms the re-combiner  30 . The re-combiner/condenser  30  may be coupled to an external radiator  132  such as a finned heat exchanger.  
         [0053]     The external radiator  132  may be spaced from the rest of the system atop tubes  133  that enable fluid to be circulated through the body of the radiator  132 . The use of an external radiator  132  enables the removal of more heat.  
         [0054]     Exterior edges of the stack  110  may be sealed except for edge areas needed to provide fluid inflow and egress of the microchannels  116 .  
         [0055]     Thus, fluid may be circulated by the pumps  28  through the microchannels  116  to cool the die  114  active semiconductor  124 . That fluid may be passed upwardly through appropriate passageways in the die  112  to the electroosmotic pumps  28 . A pump liquid may then be communicated by appropriate passageways to the re-combiner/condenser  30 .  
         [0056]     In some embodiments, by providing a vertical stack  110  of three dice, a compact footprint may be achieved in a conventional package  129 . The re-combiner  30  may be thermally insulated by the dielectric layer  136  from the lower, heat producing components.  
         [0057]     Referring to  FIG. 20 , the structure shown therein corresponds in most respects to the structure shown in  FIG. 19 . The only difference is that the copper-to-copper bonding is eliminated. In this case, a glass, polymer, or dielectric bond process may be utilized to connect the dice  112  and  114 , as well as the dice  112  and  140 .  
         [0058]     Referring to  FIG. 21 , a bumpless build-up layer (BBUL) package  142  is illustrated. The package  142  has build-up layers because the package is “grown” (built up) around the silicon die, rather than being manufactured separately and bonded to it. Bumpless build-up layer packaging is similar to flip-chip packaging except that no bumps are utilized and the device or core is embedded with the package. The build-up layers  144  provide multiple metal interconnection layers that enable electrical connections between the package pins and contacts on the dice  112  and  114  without the need for bumps.  
         [0059]     The dice  112  and  114  are separately fabricated and, in this case, are bonded by a copper/copper bond as illustrated. The re-combiner  30  is inserted in the BBUL package  142  separately from the stack of the dice  112  and  114 . Build-up layers  144  may be provided between the BBUL package  142  and the radiator  132  and on the bottom of the package  142 . The build-up layers  144  serve to couple the re-combiner  30  to the stack including the dice  112  and  114 . Channels may be built unto the layers  144  to couple fluid from pumps  28  and microchannels  116  to the recombiner  30  and from the recombiner  30  to the radiator  132 .  
         [0060]     Referring to  FIG. 22 , the structure therein corresponds to the structure shown in  FIG. 21  but, again, the copper-to-copper bonding between the dice  112  and  114  is replaced with either polymer, dielectric, or glass bonding processes.  
         [0061]     Referring next to  FIG. 23 , a BBUL package  142  corresponds to the embodiment shown in  FIG. 21 , except that the dice  112  and  114  are not stacked. A build-up layer  144  couples the die  112  to the die  116  and the re-combiner  30 .  
         [0062]     Via channels may be used to couple the dice  112 ,  114 , and  140 . Alternatively, channels or tubes may be utilized for this purpose. The channels or tubes may be formed in the same structure or may be separate structures physically joined to the dies  112 ,  114 , and  140  for this purpose.  
         [0063]     As another example, referring to  FIG. 24 , a package  156  may have a first trench  154  and a second trench  160  which are isolated from one another. Interior edges of the trenches  154 ,  160  are defined by the die  114  which is inserted into the package  156 . The trenches  154  and  160  may communicate with ports  158  and  162  which allow fluid to be added or exhausted from the package exterior. The edges of the die  114  are in communication with the fluid filled trench  154 . Fluid from the fluid filled trench  154  may enter the stack  110  and may leave through the fluid filled trench  160 . Fluid may be recirculated by tubing  168  which connects the ports  162  and  158 . A radiator  132  may be coupled by the tubing  168 .  
         [0064]     Referring to  FIG. 25 , the fluid filled trench  154  may fluidically communicate with one or more microchannels  122 , that in turn communicate with one or more electroosmotic pumps  28  and re-combiners  30 . In this way, fluid may be pumped by the electroosmotic pump  28  for selective cooling of hot areas of the die  114 . Upper and lower covers  164  and  166  may be included on the package in one embodiment of the present invention.  
         [0065]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.