Patent Publication Number: US-6712258-B2

Title: Integrated quantum cold point coolers

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to devices for cooling substances such as, for example, integrated circuit chips, and more particularly, the present invention relates to thermoelectric coolers. 
     2. Description of Related Art 
     As the speed of computers continues to increase, the amount of heat generated by the circuits within the computers continues to increase. For many circuits and applications, increased heat degrades the performance of the computer. These circuits need to be cooled in order to perform most efficiently. In many low end computers, such as personal computers, the computer may be cooled merely by using a fan and fins for convective cooling. However, for larger computers, such as main frames, that perform at faster speeds and generate much more heat, these solutions are not viable. 
     Currently, many main-frames utilize vapor compression coolers to cool the computer. These vapor compression coolers perform essentially the same as the central air conditioning units used in many homes. However, vapor compression coolers are quite mechanically complicated requiring insulation and hoses that must run to various parts of the main frame in order to cool the particular areas that are most susceptible to decreased performance due to overheating. 
     A much simpler and cheaper type of cooler are thermoelectric coolers. Thermoelectric coolers utilize a physical principle known as the Peltier Effect, by which DC current from a power source is applied across two dissimilar materials causing heat to be absorbed at the junction of the two dissimilar materials. Thus, the heat is removed from a hot substance and may be transported to a heat sink to be dissipated, thereby cooling the hot substance. Thermoelectric coolers may be fabricated within an integrated circuit chip and may cool specific hot spots directly without the need for complicated mechanical systems as is required by vapor compression coolers. 
     However, current thermoelectric coolers are not as efficient as vapor compression coolers requiring more power to be expended to achieve the same amount of cooling. Furthermore, current thermoelectric coolers are not capable of cooling substances as greatly as vapor compression coolers. Therefore, a thermoelectric cooler with improved efficiency and cooling capacity would be desirable so that complicated vapor compression coolers could be eliminated from small refrigeration applications, such as, for example, main frame computers, thermal management of hot chips, RF communication circuits, magnetic read/write heads, optical and laser devices, and automobile refrigeration systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming a thermoelement for a thermoelectric cooler. In one embodiment a first substrate having a plurality of pointed tips separated by valleys wherein the substrate is covered by a metallic layer, portions of the metallic layer is covered by an insulator, and other portions of the metallic layer are exposed is formed. The other portions of the metallic layer that are exposed are covered with a thermoelectric material overcoat. A second substrate of thermoelectric material is then fused to the pointed tip side of the first substrate by, for example, heating the back of the first substrate to melt the thermoelectric material overcoat or by passing current through the pointed tips to induce Joule heating and thereby melt the thermoelectric material overcoat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a high-level block diagram of a Thermoelectric Cooling (TEC) device in accordance with the prior art; 
     FIG. 2 depicts a cross sectional view of a thermoelectric cooler with enhanced structured interfaces in accordance with the present invention; 
     FIG. 3 depicts a planer view of thermoelectric cooler  200  in FIG. 2 in accordance with the present invention; 
     FIGS. 4A and 4B depicts cross sectional views of tips that may be implemented as one of tips  250  in FIG. 2 in accordance with the present invention; 
     FIG. 5 depicts a cross sectional view illustrating the temperature field of a tip near to a superlattice in accordance with the present invention; 
     FIG. 6 depicts a cross sectional view of a thermoelectric cooler with enhanced structured interfaces with all metal tips in accordance with the present invention; 
     FIG. 7 depicts a cross-sectional view of a sacrificial silicon template for forming all metal tips in accordance with the present invention; 
     FIG. 8 depicts a flowchart illustrating an exemplary method of producing all metal cones using a silicon sacrificial template in accordance with the present invention; 
     FIG. 9 depicts a cross sectional view of all metal cones formed using patterned photoresist in accordance with the present invention; 
     FIG. 10 depicts a flowchart illustrating an exemplary method of forming all metal cones using photoresist in accordance with the present invention; 
     FIG. 11 depicts a cross-sectional view of a thermoelectric cooler with enhanced structural interfaces in which the thermoelectric material rather than the metal conducting layer is formed into tips at the interface in accordance with the present invention; 
     FIG. 12 depicts a flowchart illustrating an exemplary method of fabricating a thermoelectric cooler in accordance with the present invention; 
     FIG. 13 depicts a cross-sectional diagram illustrating the positioning of photoresist necessary to produce tips in a thermoelectric material; 
     FIG. 14 depicts a diagram showing a cold point tip above a surface for use in a thermoelectric cooler illustrating the positioning of the tip relative to the surface in accordance with the present invention; 
     FIG. 15 depicts a schematic diagram of a thermoelectric power generator; and 
     FIGS. 16A-16J depict cross sectional diagrams illustrating a process for fabricating thermoelements with pointed tip interfaces in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures and, in particular, with reference to FIG. 1, a high-level block diagram of a Thermoelectric Cooling (TEC) device is depicted in accordance with the prior art. Thermoelectric cooling, a well known principle, is based on the Peltier Effect, by which DC current from power source  102  is applied across two dissimilar materials causing heat to be absorbed at the junction of the two dissimilar materials. A typical thermoelectric cooling device utilizes p-type semiconductor  104  and n-type semiconductor  106  sandwiched between poor electrical conductors  108  that have good heat conducting properties. N-type semiconductor  106  has an excess of electrons, while p-type semiconductor  104  has a deficit of electrons. 
     As electrons move from electrical conductor  110  to n-type semiconductor  106 , the energy state of the electrons is raised due to heat energy absorbed from heat source  112 . This process has the effect of transferring heat energy from heat source  112  via electron flow through n-type semiconductor  106  and electrical conductor  114  to heat sink  116 . The electrons drop to a lower energy state and release the heat energy in electrical conductor  114 . 
     The coefficient of performance, η, of a cooling refrigerator, such as thermoelectric cooler  100 , is the ratio of the cooling capacity of the refrigerator divided by the total power consumption of the refrigerator. Thus the coefficient of performance is given by the equation:        η   =         α                   IT   c       -           1           2              I   2        R     -     K                 Δ                 T             I   2        R     +     α                 I                 Δ                 T                         
     where the term αIT c  is due to the thermoelectric cooling, the term ½I 2 R is due to Joule heating backflow, the term KΔT is due to thermal conduction, the term I 2 R is due to Joule loss, the term αIΔT is due to work done against the Peltier voltage, α is the Seebeck coefficient for the material, T c  is the temperature of the heat source, and ΔT is the difference in the temperature of the heat source from the temperature of the heat sink. 
     The maximum coefficient of performance is derived by optimizing the current, I, and is given by the following relation:          η   max     =       (       T   c       Δ                 T       )          [       γ        -     T   Is            T   c         γ   +   1       ]             where         γ   =       1   +           α   2        σ     λ          (         T   h     +     T   c       2     )                 and         ɛ   =       γ        -     T   h            T   c         γ   +   1                       
     where ε is the efficiency factor of the refrigerator. The figure of merit, ZT, is given by the equation:        ZT   =         α   2        σ                 T     λ                     
     where λ is composed of two components: λ e , the component due to electrons, and λ L , the component due to the lattice. Therefore, the maximum efficiency, ε, is achieved as the figure of merit, ZT, approaches infinity. The efficiency of vapor compressor refrigerators is approximately 0.3. The efficiency of conventional thermoelectric coolers, such as thermoelectric cooler  100  in FIG. 1, is typically less than 0.1. Therefore, to increase the efficiency of thermoelectric coolers to such a range as to compete with vapor compression refrigerators, the figure of merit, ZT, must be increased to greater than 2. If a value for the figure of merit, ZT, of greater than 2 can be achieved, then the thermoelectric coolers may be staged to achieve the same efficiency and cooling capacity as vapor compression refrigerators. 
     With reference to FIG. 2, a cross sectional view of a thermoelectric cooler with enhanced structured interfaces is depicted in accordance with the present invention. Thermoelectric cooler  200  includes a heat source  226  from which, with current I flowing as indicated, heat is extracted and delivered to heat sink  202 . Heat source  226  may be thermally coupled to a substance that is desired to be cooled. Heat sink  202  may be thermally coupled to devices such as, for example, a heat pipe, fins, and/or a condensation unit to dissipate the heat removed from heat source  226  and/or further cool heat source  226 . 
     Heat source  226  is comprised of p− type doped silicon. Heat source  226  is thermally coupled to n+ type doped silicon regions  224  and  222  of tips  250 . N+ type regions  224  and  222  are electrical conducting as well as being good thermal conductors. Each of N+ type regions  224  and  222  forms a reverse diode with heat source  226  such that no current flows between heat source  226  and n+ regions  224  and  222 , thus providing the electrical isolation of heat source  226  from electrical conductors  218  and  220 . 
     Heat sink  202  is comprised of p− type doped silicon. Heat sink  202  is thermally coupled to n+ type doped silicon region  204 . N+ type region  204  is electrically conducting and is a good thermal conductor. N+ type region  204  and heat sink  202  forms a reverse diode so that no current flows between the N+ type region  204  and heat sink  202 , thus providing the electrical isolation of heat sink  202  from electrical conductor  208 . More information about electrical isolation of thermoelectric coolers may be found in commonly U.S. Pat. No. 6,222,113, the contents of which are hereby incorporated herein for all purposes. 
     The need for forming reverse diodes with n+ and p− regions to electrically isolate conductor  208  from heat sink  202  and conductors  218  and  220  from heat source  226  is not needed if the heat sink  202  and heat source  226  are constructed entirely from undoped non-electrically conducting silicon. However, it is very difficult to ensure that the silicon is entirely undoped. Therefore, the presence of the reverse diodes provided by the n+ and p− regions ensures that heat sink  202  and heat source  226  are electrically isolated from conductors  208 ,  218 , and  220 . Also, it should be noted that the same electrical isolation using reverse diodes may be created other ways, for example, by using p+ type doped silicon and n− type doped silicon rather than the p− and n+ types depicted. The terms n+ and p+, as used herein, refer to highly n doped and highly p doped semiconducting material respectively. The terms n− and p−, as used herein, mean lightly n doped and lightly p doped semiconducting material respectively. 
     Thermoelectric cooler  200  is similar in construction to thermoelectric cooler  100  in FIG.  1 . However, N-type  106  and P-type  104  semiconductor structural interfaces have been replaced with superlattice thermoelement structures  210  and  212  that are electrically coupled by doped region  204  and electrical conductor  208 . Electrical conductor  208  may be formed from platinum (Pt) or, alternatively, from other conducting materials, such as, for example, tungsten (W), nickel (Ni), or titanium copper nickel (Ti/Cu/Ni) metal films. 
     A superlattice is a structure consisting of alternating layers of two different semiconductor materials, each several nanometers thick. Thermoelement  210  is constructed from alternating layers of N-type semiconducting materials and the superlattice of thermoelement  212  is constructed from alternating layers of P-type semiconducting materials. Each of the layers of alternating materials in each of thermoelements  210  and  212  is approximately 10 nanometers (nm) thick. A superlattice of two semiconducting materials has lower thermal conductivity, λ, and the same electrical conductivity, σ, as an alloy comprising the same two semiconducting materials. 
     In one embodiment, superlattice thermoelement  212  comprises alternating layers of p-type bismuth chalcogenide materials such as, for example, alternating layers of Bi 2 Te 3 /Sb 2 Te 3  with layers of Bi 0.5 Sb 1.5 Te 3 , and the superlattice of thermoelement  210  comprises alternating layers of n-type bismuth chalcogenide materials, such as, for example, alternating layers of Bi 2 Te 3  with layers of Bi 2 Se 3 . Other types of semiconducting materials may be used for superlattices for thermoelements  210  and  212  as well. For example, rather than bismuth chalcogenide materials, the superlattices of thermoelements  210  and  212  may be constructed from cobalt antimony skutteridite materials. 
     Thermoelectric cooler  200  also includes tips  250  through which electrical current I passes into thermoelement  212  and then from thermoelement  210  into conductor  218 . Tips  250  includes n+ type semiconductor  222  and  224  formed into pointed conical structures with a thin overcoat layer  218  and  220  of conducting material, such as, for example, platinum (Pt). Other conducting materials that may be used in place of platinum include, for example, tungsten (W), nickel (Ni), and titanium copper nickel (Ti/Cu/Ni) metal films. The areas between and around the tips  250  and thermoelectric materials  210  and  212  should be evacuated or hermetically sealed with a gas such as, for example, dry nitrogen. 
     On the ends of tips  250  covering the conducting layers  218  and  220  is a thin layer of semiconducting material  214  and  216 . Layer  214  is formed from a P-type material having the same Seebeck coefficient, α, as the nearest layer of the superlattice of thermoelement  212  to tips  250 . Layer  216  is formed from an N-type material having the same Seebeck coefficient, α, as the nearest layer of thermoelement  210  to tips  250 . The P-type thermoelectric overcoat layer  214  is necessary for thermoelectric cooler  200  to function since cooling occurs in the region near the metal where the electrons and holes are generated. The n-type thermoelectric overcoat layer  216  is beneficial, because maximum cooling occurs where the gradient (change) of the Seebeck coefficient is maximum. The thermoelectric overcoats  214  and  216  are preferably in the range of 2-5 nanometers thick based upon present investigation. 
     By making the electrical conductors, such as, conductors  110  in FIG. 1, into pointed tips  250  rather than a planar interface, an increase in cooling efficiency is achieved. Lattice thermal conductivity, λ, at the point of tips  250  is very small because of lattice mismatch. For example, the thermal conductivity, λ, of bismuth chalcogenides is normally approximately 1 Watt/meter*Kelvin. However, in pointed tip structures, such as tips  250 , the thermal conductivity is reduced, due to lattice mismatch at the point, to approximately 0.1 Watts/meter*Kelvin. However, the electrical conductivity of the thermoelectric materials remains relatively unchanged. Therefore, the figure of merit, ZT, may increased to greater than 2.5 for this kind of material. Another type of material that is possible for the superlattices of thermoelements  210  and  212  is cobalt antimony skutteridites. These type of materials typically have a very high thermal conductivity, λ, making them normally undesirable. However, by using the pointed tips  250 , the thermal conductivity can be reduced to a minimum and produce a figure of merit, ZT, for these materials of greater than 4, thus making these materials very attractive for use in thermoelements  210  and  212 . 
     Another advantage of the cold point structure is that the electrons are confined to dimensions smaller than the wavelength (corresponding to their kinetic energy). This type of confinement increases the local density of states available for transport and effectively increases the Seebeck coefficient. Thus, by increasing α and decreasing λ, the figure of merit ZT is increased. 
     Conventional thermoelectric coolers, such as illustrated in FIG. 1, are capable of producing a cooling temperature differential, ΔT, between the heat source and the heat sink of around 60 Kelvin. However, thermoelectric cooler  200  is capable of producing a temperature differential greater than 100 Kelvin. Thus, with two thermoelectric coolers coupled to each other, cooling to temperatures in the range of liquid Nitrogen (less than 100 Kelvin) is possible. However, different materials may need to be used for thermoelements  210  and  212 . For example, bismuth telluride has a very low α at low temperature (i.e. less than −100 degrees Celsius). However, bismuth antimony alloys perform well at low temperature. 
     Those of ordinary skill in the art will appreciate that the construction of the thermoelectric cooler in FIG. 2 may vary depending on the implementation taking into account the desired cooling, heat transfer capacity, current and voltage supplies. For example, more or fewer rows of tips  250  may be included than depicted in FIG.  1 . The depicted example is not meant to imply architectural limitations with respect to the present invention. 
     With reference now to FIG. 3, a planer view of thermoelectric cooler  200  in FIG. 2 is depicted in accordance with the present invention. Thermoelectric cooler  300  includes an n-type thermoelectric material section  302  and a p-type thermoelectric material section  304 . Both n-type section  302  and p-type section  304  include a thin layer of conductive material  306  that covers a silicon body. 
     Section  302  includes an array of conical tips  310  each covered with a thin layer of n-type material  308  of the same type as the nearest layer of the superlattice for thermoelement  210 . Section  304  includes an array of conical tips  312  each covered with a thin layer of p-type material  314  of the same type as the nearest layer of the superlattice for thermoelement  212 . 
     With reference now to FIGS. 4A and 4B, a cross sectional views of tips that may be implemented as one of tips  250  in FIG. 2 is depicted in accordance with the present invention. Tip  400  includes a silicon cone  402  that has been formed with a cone angle of approximately 35 degrees. A thin layer  404  of conducting material, such as platinum (Pt), overcoats the silicon  402 . A thin layer of thermoelectric material  406  covers the very end of the tip  400 . The cone angle after all layers have been deposited is approximately 45 degrees. The effective point radius of tip  400  is approximately 50 nanometers. 
     Tip  408  is an alternative embodiment of a tip, such as one of tips  250 . Tip  408  includes a silicon cone  414  with a conductive layer  412  and thermoelectric material layer  410  over the point. However, tip  408  has a much sharper cone angle than tip  400 . The effective point radius of tip  408  is approximately 10 nanometers. It is not known at this time whether a broader or narrower cone angle for the tip is preferable. In the present embodiment, conical angles of 45 degrees for the tip, as depicted in FIG. 4A, have been chosen, since such angle is in the middle of possible ranges of cone angle and because such formation is easily fabricated from silicon with a platinum overcoat. This is because a KOH etch along the &lt;100&gt; plane of silicon naturally forms a cone angle of 54 degrees. Thus, after the conductive and thermoelectric overcoats have been added, the cone angle is approximately 45 degrees. 
     With reference now to FIG. 5, a cross sectional view illustrating the temperature field of a tip near to a superlattice is depicted in accordance with the present invention. Tip  504  may be implemented as one of tips  250  in FIG.  2 . Tip  504  has a effective tip radius at its sharpest point, a, of 10-50 nanometers. Thus, the temperature field is localized to a very small distance, r, approximately equal to 2 a or around 20-100 nanometers. Superlattice  502  need to be only a few layers thick to limit heat flow. Therefore, using pointed tips, a thermoelectric cooler with only 5-10 layers for the superlattice is sufficient. 
     Thus, fabricating a thermoelectric cooler, such as, for example, thermoelectric cooler  200 , is simplified, since only a few layers of the superlattice must be formed. Also, thermoelectric cooler  200  can be fabricated very thin (on the order of 100 nanometers thick) as contrasted to conventional thermoelectric coolers which are on the order of 3 millimeters or greater in thickness. 
     Other advantages of a thermoelectric cooler with pointed tip interfaces in accordance with the present invention include minimization of the thermal conductivity through the thermoelements, such as thermoelements  210  and  212  in FIG. 2, because of the tip interfaces. Also, the temperature/potential drops are localized to an area near the tips, effectively allowing scaling to sub-100-nanometer lengths. Furthermore, using pointed tips minimizes the number layers needed for superlattice thermoelements  210  and  212 . The present invention also permits electrodeposition of thin film structures. The smaller dimensions also allow for monolithic integration of n-type and p-type thermoelements. 
     The thermoelectric cooler of the present invention may be utilized to cool items, such as, for example, specific spots within a main frame computer, lasers, optic electronics, photodetectors, and PCR in genetics. 
     With reference now to FIG. 6, a cross sectional view of a thermoelectric cooler with enhanced structured interfaces with all metal tips is depicted in accordance with the present invention. Although the present invention has been described above as having tips  250  constructed from silicon cones constructed from the n+ semiconducting regions  224  and  222 , tips  250  in FIG. 2 may be replaced by tips  650  as depicted in FIG.  6 . Tips  650  have all metal cones  618  and  620 . In the depicted embodiment, cones  618  and  620  are constructed from copper and have a nickel overcoat layer  660  and  662 . Thermoelectric cooler  600  is identical to thermoelectric cooler  200  in all other respects, including having a thermoelectric overcoat  216  and  214  over the tips  650 . Thermoelectric cooler  600  also provides the same benefits as thermoelectric cooler  200 . However, by using all metal cones rather than silicon cones covered with conducting material, the parasitic resistances within the cones become very low, thus further increasing the efficiency of thermoelectric cooler  600  over the already increased efficiency of thermoelectric cooler  200 . The areas surrounding the contact areas of tips  650  to thermoelectric materials  210  and  212  should be vacuum or hermetically sealed with a low-thermal conductivity gas, such as, for example, argon. 
     Also, as in FIG. 2, heat source  226  is comprised of p− type doped silicon. In contrast to FIG. 2, however, silicon heat source  226  is thermally coupled to n+ type doped silicon regions  624  and  622  but does not form part of the tipped structure  650  as did silicon regions  224  and  222  do in FIG.  2 . N+ type doped silicon regions  624  and  622  do still perform the electrical isolation function performed by regions  224  and  222  in FIG.  2 . 
     Several methods may be utilized to form the all metal cones as depicted in FIG.  6 . For example, with reference now to FIG. 7, a cross-sectional view of a sacrificial silicon template that may be used for forming all metal tips is depicted in accordance with the present invention. After the sacrificial silicon template  702  has been constructed having conical pits, a layer of metal may be deposited over the template  702  to produce all metal cones  704 . All metal cones  704  may then be used in thermoelectric cooler  600 . 
     With reference now to FIG. 8, a flowchart illustrating an exemplary method of producing all metal cones using a silicon sacrificial template is depicted in accordance with the present invention. To begin, conical pits are fabricated by anisotropic etching of silicon to create a mold (step  802 ). This may be done by a combination of KOH etching, oxidation, and/or focused ion-beam etching. Such techniques of fabricating silicon pits are well known in the art. 
     The silicon sacrificial template is then coated with a thin sputtered layer of seed metal, such as, for example, titanium (step  804 ). Next, copper is electrochemically deposited to fill the valleys (conical pits) in the sacrificial silicon template. (step  806 ). The top surface of the copper is then planarized (step  808 ). Methods of planarizing a layer of metal are well known in the art. The silicon substrate is then removed by selective etching methods well known in the art (step  810 ). The all metal cones produced in this manner may then be covered with a coat of another metal, such as, for example, nickel, and then with an ultra-thin layer of thermoelectric material. The nickel overcoat aids in electrodeposition of the thermoelectric material overcoat. 
     One advantage to this method of producing all metal cones is that the silicon substrate mold is reusable if the copper is peeled from the silicon substrate as the separation process. The silicon substrate mold may be reused up to around 10 times before the mold degrades and becomes unusable. 
     Forming a template in this manner is very well controlled and produces very uniform all metal conical tips since silicon etching is very predictable and can calculate slopes of the pits and sharpness of the cones produced to a very few nanometers. 
     Other methods of forming all metal cones may be used as well. For example, with reference now to FIG. 9, a cross sectional view of all metal cones  902  formed using patterned photoresist is depicted in accordance with the present invention. In this method, a layer of metal is formed over the bottom portions of a partially fabricated thermoelectric cooler. A patterned photoresist  904 - 908  is then used to fashion all metal cones  902  with a direct electrochemical etching method. Often the tips are further sharpened by focused ion beam milling. 
     With reference now to FIG. 10, a flowchart illustrating an exemplary method of forming all metal cones using photoresist is depicted in accordance with the present invention. To begin, small sections of photoresist are patterned over a metal layer, such as copper, of a partially fabricated thermoelectric cooler (step  1002 ). The photoresist may be patterned in an array of sections having photoresist wherein each area of photoresist within the array corresponds to areas in which tips to the all metal cones are desired to be formed. The metal is then directly etched electrochemically (step  1004 ) to produce cones  902  as depicted in FIG.  9 . The photoresist is then removed and the tips of the all metal cones may then be coated with another metal, such as, for example, nickel (step  1006 ). The second metal coating over the all metal cones may then be coated with an ultra-thin layer of thermoelectric material (step  1008 ). Thus, all metal cones with a thermoelectric layer on the tips may be formed which may used in a thermoelectric device, such as, for example, thermoelectric cooler  600 . The all metal conical points produced in this manner are not as uniform as those produced using the method illustrated in FIG.  8 . However, this method currently is cheaper and therefore, if cost is an important factor, may be a more desirable method. 
     The depicted methods of fabricating all metal cones are merely examples. Other methods may be used as well to fabricate all metal cones for use with thermoelectric coolers. Furthermore, other types of metals may be used for the all metal cone other than copper. 
     With reference now to FIG. 11, a cross-sectional view of a thermoelectric cooler with enhanced structural interfaces in which the thermoelectric material rather than the metal conducting layer is formed into tips at the interface is depicted in accordance with the present invention. Thermoelectric cooler  1100  includes a cold plate  1116  and a hot plate  1102 , wherein the cold plate  1116  is in thermal contact with the substance that is to be cooled. Thermal conductors  1114  and  1118  provide a thermal couple between electrical conducting plates  1112  and  1120  respectively. Thermal conductors  1114  and  1118  are constructed of heavily n doped (n+) semiconducting material that provides electrical isolation between cold plate  1116  and conductors  1112  and  1120  by forming reverse biased diodes with the p− material of the cold plate  1116 . Thus, heat is transferred from the cold plate  1116  through conductors  1112  and  1120  and eventually to hot plate  1102  from which it can be dissipated without allowing an electrical coupling between the thermoelectric cooler  1100  and the substance that is to be cooled. Similarly, thermal conductor  1104  provides a thermal connection between electrical conducting plate  1108  and hot plate  1102 , while maintaining electrical isolation between the hot plate and electrical conducting plate  1108  by forming a reverse biased diode with the hot plate  1102  p− doped semiconducting material as discussed above. Thermal conductor  1104  is also an n+ type doped semiconducting material. Electrical conducting plates  1108 ,  1112 , and  1120  are constructed from platinum (Pt) in this embodiment. However, other materials that are both electrically conducting and thermally conducting may be utilized as well. Also, it should be mentioned that the areas surrounding tips  1130 - 1140  proximate thermoelectric materials  1122  and  1110  should be evacuated to produce a vacuum or should be hermetically sealed with a low thermal conductivity gas, such as argon. 
     In this embodiment, rather than providing contact between the thermoelements and the heat source (cold end) metal electrode (conductor) through an array of points having metal in the point electrodes as in FIGS. 2 and 6, the array of points of contact between the thermoelement and the metal electrode is provided by an array of points  1130 - 1140  composed of thermoelements  1124  and  1126 . The tips  1130 - 1140  of the present embodiment may be formed from single crystal or polycrystal thermoelectric materials by electrochemical etching. 
     In one embodiment, thermoelement  1124  is comprised of a super lattice of single crystalline Bi 2 Te 3 /Sb 2 Te 3  and Bi 0.5 Sb 1.5 Te 3  and thermoelement  1126  is formed of a super lattice of single crystalline Bi 2 Te 3 /Bi 2 Se 3  and Bi 2 Te 2.0 Se 0.1 . Electrically conducting plate  1120  is coated with a thin layer  1122  of the same thermoelectric material as is the material of the tips  1130 - 1134  that are nearest thin layer  1120 . Electrically conducting plate  1112  is coated with a thin layer  1110  of the same thermoelectric material as is the material of the tips  1136 - 1140  that are nearest thin layer  1112 . 
     With reference now to FIG. 12, a flowchart illustrating an exemplary method of fabricating a thermoelectric cooler, such as, for example, thermoelectric cooler  1100  in FIG. 11, is depicted in accordance with the present invention. Optimized single crystal material is first bonded to a metal electrode  1301  by conventional means or the metal electrode is deposited onto the single crystal material to form the electrode connection pattern (step  1202 ) as depicted in FIG.  13 . The other side of the thermoelectric material  1314  is then patterned (step  1204 ) by using photoresist  1302 - 1306  as a mask and the metal electrode as an anode in an electrochemical bath to etch the surface (step  1206 ). The tips  1308 - 1312  as depicted in FIG. 13 are formed by controlling and stopping the etching process at appropriate times. 
     A second single crystal substrate is thinned by chemical-mechanical polishing and then electrochemically etching the entire substrate to nanometer films (step  1210 ). The second ultra-thin substrate forms the cold end. The two substrates (the one with the ultra-thin thermoelectric material and the other with the thermoelectric tips) are then clamped together with light pressure (step  1212 ). This structure retains high crystallinity in all regions other than the interface at the tips. Also, similar methods can be used to fabricate polycrystalline structures rather than single crystalline structures. 
     With reference now to FIG. 14, a diagram showing a cold point tip above a surface for use in a thermoelectric cooler illustrating the positioning of the tip relative to the surface is depicted in accordance with the present invention. Although the tips, whether created in as all-metal or metal coated tips or as thermoelectric tips have been described thus far as being in contact with the surface opposite the tips. However, although the tips may be in contact with the opposing surface, it is preferable that the tips be very near the opposing surface without fully touching the surface as depicted in FIG.  14 . The tip  1402  in FIG. 14 is situated near the opposing surface  1404  but is not in physical contact with the opposing surface. Preferably, the tip  1402  should be a distance d on the order of 1 nanometer or less from the opposing surface  1404 . In practice, with a thermoelectric cooler containing thousands of tips, some of the tips may be in contact with the opposing surface while others are not in contact due to the deviations from a perfect plane of the opposing surface. 
     By removing the tips from contact with the opposing surface, the thermal conductivity between the cold plate and the hot plate of a thermoelectric cooler may be reduced. Electrical conductivity is maintained, however, due to tunneling of electrons between the tips and the opposing surface. 
     The tips of the present invention have also been described and depicted primarily as perfectly pointed tips. However, as illustrated in FIG. 14, the tips in practice will typically have a slightly more rounded tip as is the case with tip  1402 . However, the closer to perfectly pointed the tip is, the fewer number of superlattices needed to achieve the temperature gradient between the cool temperature of the tip and the hot temperature of the hot plate. 
     Preferably, the radius of curvature r 0  of the curved end of the tip  1402  is on the order of a few tens of nanometers. The temperature difference between successive layers of the thermoelectric material below surface  1404  approaches zero after a distance of two (2) to three (3) times the radius of curvature r 0  of the end of tip  1402 . Therefore, only a few layers of the super lattice  1406 - 1414  are necessary. Thus, a superlattice material opposite the tips is feasible when the electrical contact between the hot and cold plates is made using the tips of the present invention. This is in contrast to the prior art in which to use a superlattice structure without tips, a superlattice of 10000 or more layers was needed to have a sufficient thickness in which to allow the temperature gradient to approach zero. Such a number of layers was impractical, but using only 5 or 6 layers as in the present invention is much more practical. 
     Although the present invention has been described primarily with reference to a thermoelectric cooling device (or Peltier device) with tipped interfaces used for cooling, it will be recognized by those skilled in the art that the present invention may be utilized for generation of electricity as well. It is well recognized by those skilled in the art that thermoelectric devices can be used either in the Peltier mode (as described above) for refrigeration or in the Seebeck mode for electrical power generation. Referring now to FIG. 15, a schematic diagram of a thermoelectric power generator is depicted. For ease of understanding and explanation of thermoelectric power generation, a thermoelectric power generator according to the prior art is depicted rather than a thermoelectric power generator utilizing cool point tips of the present invention. However, it should be noted that in one embodiment of a thermoelectric power generator according to the present invention, the thermoelements  1506  and  1504  are replaced cool point tips, as for example, any of the cool point tip embodiments as described in greater detail above. 
     In a thermoelectric power generator  1500 , rather than running current through the thermoelectric device from a power source  102  as indicated in FIG. 1, a temperature differential, T H −T L , is created across the thermoelectric device  1500 . Such temperature differential, T H −T L , creates a current flow, I, as indicated in FIG. 15 through a resistive load element  1502 . This is the opposite mode of operation from the mode of operation described in FIG. 1 
     Therefore, other than replacing a power source  102  with a load resistor  1502  and maintaining heat elements  1512  and  1516  at differential temperatures T H  and T L  respectively with heat sources Q H  and Q L  respectively, thermoelectric device  1500  is identical in components to thermoelectric device  102  in FIG.  1 . Thus, thermoelectric cooling device  1500  utilizes p-type semiconductor  1504  and n-type semiconductor  1506  sandwiched between poor electrical conductors  1508  that have good heat conducting properties. More information about thermoelectric electric power generation may be found in  CRC Handbook of Thermoelectrics , edited by D. M. Rowe, Ph.D., D.Sc., CRC Press, N.Y., (1995) pp. 479-488 and in  Advanced Engineering Thermodynamics , 2nd Edition, by Adiran Bejan, John Wiley &amp; Sons, Inc., N.Y. (1997), pp. 675-682, both of which are hereby incorporated herein for all purposes. 
     With reference now to FIGS. 16A-16J, cross sectional diagrams illustrating a process for fabricating thermoelements with pointed tip interfaces is depicted in accordance with the present invention. The thermoelements fabricated with this method may be used as thermoelements for a thermoelectric cooler such as, for example, thermoelectric cooler  200 . To begin, a pointed tip substrate  1602  such as, for example, a silicon substrate or copper substrate peeled from silicon molds as described above, is formed as depicted in FIG.  16 A. Next, the pointed tip substrate  1602  is coated with a metal layer  1604 , such as, for example, titanium (Ti) or platinum (Pt), by, for example, a sputtering or an evaporation process, as depicted in FIG. 16B. A thin insulator  1606 , such as, for example, silicon dioxide, is deposited over the metal layer  1604  as depicted in FIG.  16 C. The valleys between tips  1610 - 1612  are filled with a sacrificial planarizing dielectric  1608  such that only the tips  1610 - 1612  of the metallic and insulator coated pointed tip substrate  1602  is exposed as depicted in FIG.  16 D. 
     Next, the sacrificial dielectric  1608  and thin insulator  1606  are etched together until the tips  1610 - 1612  are exposed as depicted in FIG. 16E. A thermoelectric material overcoat  1613 - 1615  is then selectively grown by electrochemical methods or chemical vapor deposition (CVD) over the tips  1610 - 1612  to a thickness of approximately five (5) nanometers as depicted in FIG.  16 F. The sacrificial dielectric  1608  is then removed as depicted in FIG.  16 G. The pointed tip substrate  1602  with pointed tips  1610 - 1612  is mechanically aligned with a substantially flat surfaced thermoelectric substrate  1617  as depicted in FIG.  16 H. The single crystal thermoelectric substrate  1617  is polished on one side  1619  and metallized by sputter deposition of Ni  1618  on the opposite side. The end of pointed tip substrate  1602  opposite pointed tips  1610 - 1612  is heated to approximately 550 degrees Celsius in order to melt the TE overcoats  1613 - 1615  and fuse the TE materials on the tips  1610 - 1612  to thermoelectric substrate  1617  as depicted in FIG.  16 I. Alternatively, a current may be passed through the tips  1610 - 1612  to the point that Joule heat melts the thermoelectric material  1613 - 1615  near the tips  1610 - 1612  in order to fuse the tips  1610 - 1612  to thermoelectric substrate  1617 . 
     The present invention has been described primarily with reference to conically shaped tips, however, other shapes of tips may be utilized as well, such as, for example, pyramidically shaped tips. In fact, the shape of the tip does not need to be symmetric or uniform as long as it provides a discrete set of substantially pointed tips through which electrical conduction between the two ends of a thermoelectric cooler may be provided. The present invention has applications to use in any small refrigeration application, such as, for example, cooling main frame computers, thermal management of hot chips and RF communication circuits, cooling magnetic heads for disk drives, automobile refrigeration, and cooling optical and laser devices. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.