Abstract:
A thermionic or thermotunneling gap diode device consisting of two silicon electrodes maintained at a desired distance from one another by means of spacers. These spacers are formed by oxidizing one electrode, protecting certain oxidized areas and removing the remainder of the oxidized layer. The protected oxidized areas remain as spacers. These spacers have the effect of maintaining the electrodes at a desired distance without the need for active elements, thus greatly reducing costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional App. No. 60/662,058, filed Mar. 14, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This present invention is directed towards devices for cooling. In particular it is directed toward devices for cooling Integrated Circuits, such as high-performance Micro-Processors, VLSI chipsets, Infra-Red imaging sensors, and the like.  
         [0003]     In U.S. Pat. No. 6,720,704 diode devices are disclosed in which the separation of the electrodes is set and controlled using piezo-electric, electrostrictive or magnetostrictive actuators. This avoids problems associated with electrode spacing changing or distorting as a result of heat stress. In addition it allows the operation of these devices at electrode separations which permit quantum electron tunneling between them. Pairs of electrodes whose surfaces replicate each other are also disclosed. These may be used in constructing devices with very close electrode spacings.  
         [0004]     In U.S. Pat. No. 6,417,060 a method for manufacturing a pair of electrodes comprises fabricating a first electrode with a substantially flat surface and placing a sacrificial layer over a surface of the first electrode, wherein the sacrificial layer comprises a first material. A second material is placed over the sacrificial layer, wherein the second material comprises a material that is suitable for use as a second electrode. The sacrificial layer is removed with an etchant, wherein the etchant chemically reacts with the first material, and further wherein a region between the first electrode and the second electrode comprises a gap that is a distance of 50 nanometers or less, preferably 5 nanometers or less. Alternatively, the sacrificial layer is removed by cooling the sandwich with liquid nitrogen, or alternatively still, the sacrificial layer is removed by heating the sacrificial layer, thereby evaporating the sacrificial layer.  
         [0005]     In U.S. Pat. No. 6,774,003 a method for manufacturing a pair of electrodes comprises fabricating a first electrode with a substantially flat surface and placing a sacrificial layer over a surface of the first electrode, wherein the sacrificial layer comprises a first material. A second material is placed over the sacrificial layer, wherein the second material comprises a material that is suitable for use as a second electrode. The sacrificial layer is removed with an etchant, wherein the etchant chemically reacts with the first material, and further wherein a region between the first electrode and the second electrode comprises a gap that is a distance of 50 nanometers or less, preferably 5 nanometers or less. Alternatively, the sacrificial layer is removed by cooling the sandwich with liquid nitrogen, or alternatively still, the sacrificial layer is removed by heating the sacrificial layer, thereby evaporating the sacrificial layer.  
         [0006]     In U.S. Patent App. No. 2003/0068431 materials bonded together are separated using electrical current, thermal stresses, mechanical force, any combination of the above methods, or any other application or removal of energy until the bonds disappear and the materials are separated. In one embodiment the original bonding was composed of two layers of material. In another embodiment, the sandwich was composed of three layers. In a further embodiment, the parts of the sandwich are firmly maintained in their respective positions during the application of current so as to be able to subsequently align the materials relative to one another.  
         [0007]     In WO03090245 a Gap Diode is disclosed in which a tubular actuating element serves as both a housing for a pair of electrodes and as a means for controlling the separation between the electrode pair. In a preferred embodiment, the tubular actuating element is a quartz piezo-electric tube. In accordance with another embodiment of the present invention, a Gap Diode is disclosed which is fabricated by micromachining techniques in which the separation of the electrodes is controlled by piezo-electric, electrostrictive or magnetostrictive actuators. Preferred embodiments of Gap Diodes include Cool Chips, Power Chips, and photoelectric converters.  
         [0008]     In U.S. Pat. No. 3,169,200, a multilayer converter is described which comprises two electrodes, intermediate elements and oxide spacers disposed between each adjacent element. A thermal gradient is maintained across the device and opposite faces on each of the elements serve as emitter and collector. Electrons tunnel through each oxide barrier to a cooler collector, thereby generating a current flow through a load connected to the two electrodes. One drawback is that the device must contain some 106 elements in order to provide reasonable efficiency, and this is difficult to manufacture. A further drawback results from the losses due to thermal conduction: although the oxide spacers have a small contact coefficient with the emitter and collector elements, which minimizes thermal conduction, the number of elements required for the operation of the device means that thermal conduction is not insignificant.  
         [0009]     In U.S. Pat. No. 6,876,123 a thermotunneling converter is disclosed comprising a pair of electrodes having inner surfaces substantially facing one another, and a spacer or plurality of spacers positioned between the two electrodes, having a height substantially equal to the distance between the electrodes, and having a total cross-sectional area that is less than the cross-sectional area of either of the electrodes. In a preferred embodiment, a vacuum is introduced, and in a particularly preferred embodiment, gold that has been exposed to cesium vapor is used as one or both of the electrodes. In a further embodiment, the spacer is made of small particles disposed between the electrodes. In a yet further embodiment, a sandwich is made containing the electrodes with an unoxidized spacer. The sandwich is separated and the spacer is oxidized, which makes it grow to a required height whilst giving it insulatory properties, to allow for tunneling between the electrodes.  
         [0010]     There is a need in the art for devices having the simplicity of a layered structure to provide electrode separation without the use of active elements, in which problems of thermal conduction between the layers is reduced or eliminated.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed towards a thermionic or thermotunneling gap diode device comprising electrodes having surfaces substantially facing one another, and which are separated by spacers disposed between the electrodes so that there is a gap between the electrodes; the surface area of the spacers in contact with the electrodes is less than the surface area of the electrodes not in contact with the spacers. In a further aspect, the present invention is directed toward an electrode for use in such a device that comprises an electrode surface having protrusions.  
         [0012]     In a further aspect, the invention is directed toward a method for making a gap diode device comprising the steps of: oxidizing the surface of a substrate material; protecting selected areas of the oxidized layer in such a way that the protected areas are much less than the unprotected areas; removing areas of said oxidized layer which have not been protected, so that the protected areas remain as protrusions; and contacting an electrode so formed to another electrode, whereby the electrodes are positioned substantially facing each other, at a distance determined by the height of the protrusions.  
         [0013]     In a further aspect, the process is extended to produce multiple thermionic or thermotunneling gap diodes, and this embodiment the substrate material is a silicon sheet carrying multiple electrode regions, which is oxidized and protected as above prior to removing areas of the oxidized layer which have not been protected, so that the protected areas remain as protrusions; contacting the silicon sheet to another silicon sheet to form a composite wafer, whereby the electrodes are positioned substantially facing each other, at a distance determined by the height of the protrusions; and dicing the composite wafer to the size of the electrodes, to obtain individual thermionic or thermotunneling gap diode devices.  
         [0014]     In a further aspect the process is extended to build individual integrated circuits with integrated active cooling devices in which the step of contacting the electrode formed as described above with another electrode comprises connecting the electrode with protrusions to a sheet of integrated circuits and dicing the composite wafer to form individual integrated circuits with integrated active cooling.  
         [0015]     The approaches disclosed above utilize widely-applied silicon on insulator wafer bonding techniques to bond two silicon wafers with thin oxide layers in between. By not bonding the entire surface, for instance by trapping a small particle between two silicon wafers, due to the mechanical properties of silicon, a non-bonded area (void) of 5000 times the size (height) of the particle is created. Consequently, if two particles are spaced less than 2×5000 times the size of the particles, an even larger void is created. Using this idea, it is possible to form small “spacers” that keep the wafers at the desired distance of 5-10 nm without causing electrical or thermal “shorts”.  
         [0016]     This invention provides the simplicity of a layered structure to provide electrode separation without the use of active elements, and therefore problems of thermal conduction between the layers is reduced or eliminated.  
         [0017]     Furthermore, this method of creating gap diode devices is inexpensive as it does not require active elements such as piezoelectric actuators to build and maintain the gap.  
         [0018]     Furthermore, this invention discloses methods for manufacturing thermotunneling converters on a large scale, thus reducing costs and increasing the scope of potential applications. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0019]     Embodiments of the invention will now be described with reference to appropriate figures, which are given by way of example only and are not intended to limit the present invention.  
         [0020]     For a more complete explanation of the present invention and the technical advantages thereof, reference is now made to the following description and the accompanying drawings, in which:  
         [0021]      FIG. 1  is a diagrammatic representation of a process for building thermotunneling converters;  
         [0022]      FIG. 2  is a diagrammatic representation of a process for building thermotunneling converters using a sheet of ICs as one of the electrodes;  
         [0023]      FIG. 3  shows a plurality of thermotunneling converters created on a single wafer;  
         [0024]      FIG. 4  shows a wafer on which a plurality of electrodes has been arranged, further showing the spaces in between the electrodes;  
         [0025]      FIG. 5  shows the wafer of  FIG. 4 , further comprising an oxide layer;  
         [0026]      FIG. 6  shows the wafer of  FIG. 5 , after dots have been formed on the wafer and the remaining oxide removed;  
         [0027]      FIG. 7  shows one electrode of a thermotunneling converter comprising spacer dots and a bond pad;  
         [0028]      FIG. 8  shows the electrode of  FIG. 7  further comprising an oxide seal;  
         [0029]      FIG. 9  shows the electrode of  FIG. 7 , further showing the point at which the electrode is joined to a device that is to be cooled;  
         [0030]      FIG. 10  shows a thermotunneling converter comprising two electrodes with spacers in between, showing the direction of heat flow through the converter; and  
         [0031]      FIG. 11  shows a sandwich comprising two silicon wafers with a layer of silicon oxide in between. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     Referring now to  FIG. 1 , which shows a diagrammatic representation of a process for building thermotunneling gap diode devices, in a step  201 , the surface of a silicon wafer  10  is oxidized to create a thin oxide film  12 . Preferably, film  12  has a thickness of the order of 10 nm. In a step  202 , an array of small dots  14  is created on the surface. This step may be accomplished for example and without limitation by standard photolithographic processes. In a step  203 , the oxide material  12  between spacers  14  is removed, for example, by an etching process. In a step  204 , a second silicon wafer  16 , of identical dimensions to wafer  10 , is bonded to the top of spacer array  14  formed in step three. Typically the distance between the spacers is between 10 and 10,000 times the height of the spacers.  
         [0033]     The mechanical properties of silicon are such that if a small particle is trapped in between two silicon wafers, a non-bonded area (void) of 5000 times the size (height) of the particle is created. Therefore the spacers consisting of a dot of silicon oxide topped by a protective layer will have the effect of keeping the two silicon wafers at a desired distance without the use of active elements.  
         [0034]     This forms a structure in which the thermal flux across the assembly is reduced by the ratio of surface area of these spacers to the remaining surface area. A spacer of about 1 micrometer height leads to a gap with a diameter of approximately 5000 times that size, namely, 0.5 cm. These approximations are effective for typical 4 inch diameter silicon wafers, with a thickness of about 525 micrometers. It is understood that the invention is by no means limited to these measurements or approximations, and they are mentioned merely by way of example. A detailed example is given below.  
         [0035]     Typically, the heat flux across such an assembly will be of the order of 5 W/cm 2  when the temperature across the assembly is 50K. In reality, the leakage will be even smaller, because the thermal conductivity of small dots is smaller than the published data for large quartz volumes. Furthermore, the spacers can be distributed even further out than the 2×5000×dot height (100A) used in this embodiment.  
         [0036]     Preferably the second silicon wafer supports a number of circuits to be cooled, such as a microprocessor, graphics processor, memory chip and the like.  
         [0037]     In this design, active piezo control is not required to maintain the gap, making the cooling device very inexpensive.  
         [0038]     For optimal performance it may be necessary to achieve a work function of 1 eV or less on the active wafer to increase cooling power by increasing tunneling and thermionic emission of electrons.  
         [0039]     These layers on the active wafer can be introduced using approaches commonly used in the art. For example, an active layer can be introduced on to the electrode by vacuum deposition, using materials such as zinc, lead, cadmium, thallium, bismuth, polonium, tin, selenium, lithium, indium, sodium, potassium, gallium diamond-like carbon, cesium or a combination thereof. Another possible method is sputtering, using materials such as titanium and silver. In a further example, an active layer such as copper is grown electrochemically onto the silicon layer. In another example, an electrically conducting paste, preferably silver, may be applied onto the electrode, or a thin film may be introduced using MEMS techniques. It is to be understood that the invention is in no way limited to these specific methods and they are mentioned only by way of example. Accordingly, any other suitable method may be used.  
         [0040]     To increase the tunneling current, an electrostatic field has to be applied across the Gap by applying a Voltage potential to each wafer. Based upon the electric breakdown properties of SiO 2 , a 5 Volt potential could be applied between the two wafers for an electric field of 5 MeV, well below the 8-11 MeV limit for quartz. According to published data, under these conditions the device has a cooling capacity of &gt;&gt;100 W/cm 2 , making it extremely suitable for next generation IC&#39;s.  
         [0041]     This method of building thermotunneling converters has many advantages over prior art methods, a principle advantage being that it is cheap to construct and maintain. Accordingly, many applications are possible.  
         [0042]     One primary application relates to the construction of thermotunneling converters using a sheet of ICs as one of the electrodes. ICs are usually manufactured in bulk on a large wafer sheet. At the end of the IC manufacturing process, the majority of the substrate is removed by grinding the backside to achieve a thickness of 100-200 μm to improve the heat transfer from the active device surface to a heat sink. This embodiment uses ICs at this stage of the manufacturing process, as described below.  
         [0043]      FIG. 2  shows a diagrammatic representation of a process of building thermotunneling converters using an IC sheet as one of the electrodes. Referring now to  FIG. 2 , in a step  101 , the surface of a large silicon wafer  24  is oxidized to create a thin oxide film  26 , preferably with a thickness of 10 nanometers. In a step  102 , an array of dots  28  are formed on oxidized surface  26 , using photolithographic or other methods. In a step  103 , the oxide in between dots  28  is removed by etching or other methods, to produce spacers.  
         [0044]     In a step  110 , IC silicon sheet  22 , which can be any semiconductor material, of the same dimensions as sheet  24  is polished to achieve a thickness of 100-200 μm. In a step  104 , sheet  24  with protruding spacers is then bonded to polished IC sheet  22 . This creates a sheet of thermotunneling converters using the IC sheet as one of the electrodes.  
         [0045]     Referring now to  FIG. 3 , shown is a wafer  30  with a plurality of thermotunneling converters  32  formed on the surface. In the embodiment described in  FIG. 2 , wafer  30  would be covered with an IC sheet of identical dimensions to wafer  30 . Thermotunneling converters  32  would then be diced or laser cut to size, creating a plurality of individual ICs with in-built cooling devices.  
         [0046]     Using this method, the building process can be performed very economically on the entire wafer scale, yielding hundreds of systems from a 300 mm diameter wafer.  
         [0047]     For purposes of illustration, below is a detailed example of the specifications of a thermotunneling converter built using the methods described above.  
       EXAMPLE  
       [0048]     Referring now to  FIG. 4 , shown is a circular silicon wafer  40  of 200 mm diameter. Arranged on the surface of wafer  40  are a plurality of rectangular electrodes  42 , 25×20 mm2 in size. Rectangular electrodes  42  fill as much surface space of wafer  40  as is possible, while leaving an aisle  43  of 2 mm between each device. Referring now to  FIG. 5 , wafer  40  is oxidized to create an oxide layer  41  on the entire surface of wafer  40 .  FIG. 6  shows wafer  40  after dots have been formed on rectangles  42  and the oxide in between the dots removed. Oxide is also removed from aisles  43  in between rectangles  42 .  
         [0049]     It is understood that a second wafer of identical dimensions to wafer  40  is attached to wafer  40  in order to create a sheet of thermotunneling converters, which can then be cut into individual converters.  
         [0050]      FIG. 7  shows one rectangular electrode  42  of wafer  40 . Rectangular electrode  42  comprises a silicon wafer covered with a layer of oxidized silicon. A bond pad  46  of 5×20 mm is positioned at one side of electrode  42  to hold the two electrodes in place. A plurality of spacer dots  44  with a diameter of 350 nanometers and 15 nanometers in height is arranged on rectangular electrode  42  at a distance of 100 micrometers. The oxide in between spacer dots  44  is removed.  
         [0051]     Referring now to  FIG. 8 , shown is one embodiment of electrode  42  with the addition of an oxide seal  48  positioned around the perimeter of electrode  42  to maintain the vacuum inside seal  48 .  
         [0052]     Referring now to  FIG. 9 , a rectangular area  50  on electrode  42  is in thermal contact with a device that is to be cooled. Area  50  measures approximately 14×14 mm and is positioned in the center of the area remaining after the bond pad has been positioned at one side of electrode  42 .  
         [0053]      FIG. 10  shows electrode  42  attached to a second electrode  59 , of identical dimensions as electrode  42 , thereby forming a thermotunneling converter  57 . Area  50  on first electrode  42  represents the device to be cooled, whereas area  51  on second wafer  59  is in thermal contact with a heat sink. Arrows  53  indicate the direction in which heat flows through thermotunneling converter  57 .  
         [0054]     Because area  50  is not in close proximity to bond pad  46 , there will be very little thermal leakage through bond pad  46 . Arrows  52  indicate the path along which the heat will travel. Considering that the length of the edge zone  46  is 4*20 mm, the active wafer is thinned down to about 100 μm, and the distance between bonded edge zone  46  and the active area  50  is 5 mm, as indicated by arrow  55 , the thermal leakage through edge zone  46  will be about 5 Watt. This is illustrated in Table A:  
                                                   Heat flux through wafer and bonded area           Path in wafer 5 mm × 0.100 mm × 80 mm           ∂T = 25 C. Therm. conductivity Silicon125 W/m × K           W = 5 W                      
 
         [0055]     Table B illustrates the specifications of thermotunneling converter  57 , and the resulting heat flux that will be obtained.  
                                   TABLE B                           Area of spacer dots   diameter: L [nm]   350   9.62E−14   M 2             Height of spacer   H [nm]   15   1.50E−08   M       Distance between spacer   Multiplier   3333   1.00E−04   M   D max  = 2 × 5000 × h       Area between spacers           1.00E−08   M 2         Ratio spacer/unbonded Areas           9.62E−06       Thermal conductivity SiO 2     Max, thick films       1.1   W/m/K       Length of conductor   H [nm]   15   1.50E−08   m       Heat Flux through SiO 2  at ∂T       50   3.53E+05   W/cm 2         [K]       Heat Flux through spacers           3.4   W/cm 2     (at D = 150 μm)       Electric Field at applied       5   3.3   Mv/cm       Voltage [V]                  
 
         [0056]     As is illustrated by Table B, using the above configurations, the heat flux through the spacers will be reduced to only 3.4 W/cm 2 . This is a significant reduction and will allow for the converters to achieve efficient cooling.  
         [0057]     Referring now to  FIG. 11 , shown is a sandwich  58  comprised of two silicon wafers  60 , of the same dimensions as electrodes  42  and  59 , with a layer of oxidized silicon  62  positioned in between them. Oxidized silicon layer  62  has the same height of 15 nanometers as do spacer dots  44 . Thus, in effect, sandwich  58  shows a device of the above method with the exclusion of the stages of forming protective dots on the oxide layer and removing the remaining oxide. Using the configurations described in Table B above, the heat flux through sandwich  58  would be 353,333 W. It is thus clearly illustrated that the spacer dots provide an advantage of considerable magnitude.  
         [0058]     There are many other possible embodiments to this invention. For example, one embodiment involves oxidizing both electrodes of a converter, creating spacers protruding from both sides. In one embodiment the spacers are formed in exactly the same place on both electrodes, so when the electrodes are bonded the spacers meet and create a gap of double the height.  
         [0059]     In another embodiment, the spacers are formed on each electrode at double the distance needed in the above mentioned method. However, they are positioned in different places on each electrode so that when the two electrodes are bonded to form a converter, the resulting gaps between the spacers are of the size necessary for the converter to function efficiently.  
         [0060]     In another embodiment, a multitude of CoolChips is formed from two wafers that are prepared according to this invention. The composite wafer is then diced and the individual CoolChip dies are then bonded to individual IC dies.  
         [0061]     In another embodiment, a coolchip die is mounted into a conventional heatsink which is then mounted to a conventional IC chip package, providing cooling below ambient temperature to the IC.  
         [0062]     In other embodiments, alternative methods are used to create a layer on an electrode surface.  
         [0063]     In another embodiment, the width of the bonding pad is reduced to the same width as the oxide seal to further reduce the parasitic heat flux through the bond pad.  
         [0064]     In another embodiment, bonding is performed in a vacuum chamber to reduce the parasitic heat flux through otherwise trapped gas between the electrodes.  
         [0065]     In another embodiment, bonding is performed in a controlled atmosphere of a gas that reacts with the electrode material during the bonding anneal and forms a protective layer on either or both electrodes and creates a vacuum between the electrodes. It is to be understood that the invention is not limited to any of the above mentioned specifications:  
         [0066]     The present invention is thus directed to a method for building thermotunneling gap diode devices. One particular application of this method, as disclosed above, relates to building thermotunneling converters on a large scale for the purpose of cooling ICs.