EP1756881A2 - Thin film thermoelectric devices for power conversion and cooling - Google Patents

Thin film thermoelectric devices for power conversion and cooling

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Publication number
EP1756881A2
EP1756881A2 EP04821305A EP04821305A EP1756881A2 EP 1756881 A2 EP1756881 A2 EP 1756881A2 EP 04821305 A EP04821305 A EP 04821305A EP 04821305 A EP04821305 A EP 04821305A EP 1756881 A2 EP1756881 A2 EP 1756881A2
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EP
European Patent Office
Prior art keywords
thermoelectric
header
pair
heat
coupled
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04821305A
Other languages
German (de)
French (fr)
Other versions
EP1756881A4 (en
Inventor
Rama Venkatasubramanian
Brooks O'quinn
Edward Siivola
Kip Coonley
Pratima Addepalli
Randy Alley
John Posthill
Thomas Colpitts
Anil Reddy
Chris Caylor
Peter Thomas
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Laird Technologies Inc
Original Assignee
Nextreme Thermal Solutions Inc
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Publication date
Application filed by Nextreme Thermal Solutions Inc filed Critical Nextreme Thermal Solutions Inc
Publication of EP1756881A2 publication Critical patent/EP1756881A2/en
Publication of EP1756881A4 publication Critical patent/EP1756881A4/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/80—Constructional details
    • H10N10/81—Structural details of the junction
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/10—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects
    • H10N10/13—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects characterised by the heat-exchanging means at the junction
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/10—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects
    • H10N10/17—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects characterised by the structure or configuration of the cell or thermocouple forming the device
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N19/00—Integrated devices, or assemblies of multiple devices, comprising at least one thermoelectric or thermomagnetic element covered by groups H10N10/00 - H10N15/00
    • H10N19/101—Multiple thermocouples connected in a cascade arrangement
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
    • H01L23/38—Cooling arrangements using the Peltier effect
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Definitions

  • thermoelectric devices for power conversion and cooling which utilize thin film thermoelectric materials.
  • thermoelectric device can produce electrical energy when a heat flux flows through opposite conductivity types of a thermoelectric material. Furthermore, a thermoelectric device can cool an attached object when a current is flown in an appropriate direction through the thermoelectric material.
  • a thermoelectric generator for example, a Seebeck voltage generated across a thermoelectric device can be used to drive a current in a connected load circuit.
  • thermoelectric generators can generate power from a variety of heat sources.
  • a thermoelectric power device can be connected to a wall of a combustion chamber and therefore generate power from the heat flux flowing from the combustion wall.
  • thermoelectric generators typically include external electronic circuitry such as a DC-DC conversion circuits or DC-AC converters for receiving power from the thermoelectric elements at a low voltage and for delivering power at higher voltages.
  • DC-DC conversion circuits DC-AC converters
  • thermoelectric conversion efficiency is low as compared with the other direct energy conversion systems such as for example a fuel cell device.
  • thermoelectric devices had higher conversion efficiencies, then thermoelectric power conversion devices could be effectively employed even in automobiles to increase the total fuel efficiency of the automobile by converting waste heat spent from the combustion process into electrical power. Low efficiencies have also limited the application of thermoelectric devices in cooling and refrigeration applications.
  • thermoelectric coolers may be any suitable thermoelectric coolers such as those supplied by Melcor under catalog number CP 2-127-06L, the efficiency of these coolers, like those of the afore-mentioned power conversion thermoelectric devices, are limited.
  • the dimensionless thermoelectric figure of merit (ZT) is a measure of the effectiveness of the material for both cooling and power conversion applications.
  • the Seebeck coefficient (S) is a measure of how readily electrons (or holes) convert thermal to electrical energy as the electrons move across a temperature gradient. At a given temperature, the thermoelectric figure of merit ZT for a given material is maximized at an optimum doping level. In most materials, the thermoelectric figure of merit ZT is maximized at doping levels of approximately 10 cm ' .
  • thermoelectric materials have a maximum ZT of approximately 1.
  • bulk thermoelectric materials based on p-Bi x Sb 2 . x Te 3 and n-Bi 2 Te . x Se x do not have a sufficient figure-of-merit (ZT) to allow economical application in many power conversion situations.
  • ZT figure-of-merit
  • the thermal properties of superlattice structures can be improved over that of the thermal properties of bulk materials.
  • Superlattice structures in thermoelectric materials have been investigated as structures whose engineered properties can lead to better semiconductor and thermoelectric properties. The fabrication of a superlattice by molecular beam epitaxy (MBE), or other known epitaxial growth techniques, is generally known.
  • MBE molecular beam epitaxy
  • thermoelectric material For use as a thermoelectric material, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit (ZT) and Seebeck coefficient (S) are maximized.
  • ZT thermoelectric figure of merit
  • S Seebeck coefficient
  • superlattice materials are expected to have higher ZT values than bulk-materials.
  • ZT thermoelectric figure of merit
  • S Seebeck coefficient
  • thin film thermoelectric devices are presently limited by thermal mismatch and temperature gradient issues and also are practically limited by the high cost of thin-film superlattice materials.
  • Another object of the present invention is to utilize a thermoelectric device structure in which the thermoelectric properties of the device are not contravened by inadvertent heat flux around the active thermoelectric elements in the device.
  • a further object of the present invention is to provide a thermoelectric device structure for cooling power-dissipating devices.
  • Still another object of the present invention is to provide a thermoelectric device which can convert power from a wide variety of moderate to high temperature heat sources, including but not limited to fuel sources. Accordingly, one object of the present invention is to provide a multi-stage thermoelectric device for power conversion from heat sources at temperatures of 600 to 850K.
  • Still another object of the present invention is to provide a thermoelectric device which can be converted from a power conversion device to a heat pump device should coolant to a heat sink of the thermoelectric device be interrupted.
  • Yet another object of the present invention is to provide a thermoelectric device which utilizes both bulk thermoelectrics and superlattice thermoelectric films to exploit for each respective thermoelectric material those thermoelectric properties in operational temperature ranges most suitable for each. Accordingly, one object of the present invention is to provide a thermoelectric device in which there is a high internal heat-flux within an individual thermoelectric stage and a low external heat-flux across the entirety of the thermoelectric device.
  • thermoelectric device having at least one thermoelectric unit including a thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, and a thermal impedance increasing device disposed between the thermoelectric pair and one of the first and second headers.
  • FIG. 1A is a schematic of one embodiment of a thermoelectric device according to the present invention
  • Figure IB is a schematic of a thermoelectric device according to the present invention operating in a High Active Flux- Low Input-Output Flux mode
  • Figure 2 is a schematic of a multi-stage thermoelectric device according to the present invention
  • Figure 3 is a graph depicting variations of ZT with temperature for various material systems useful in the present invention
  • Figure 4 is a graph depicting state-of-the-art ZT vs carrier concentration for p and n-type Bi 2 Te 3 -based superlattice elements
  • Figure 5 is a table of estimated design efficiencies for a three-stage thermoelectric device of the present invention
  • Figure 6A is a schematic illustration depicting a thermoelectric device module of the present invention integrating a series of
  • FIG 8 is a schematic of a multi-stage thermoelectric device of the present invention employing resonant thermal energy transfer using Purcell-cavity enhancement effects and proximity-coupling of IR modes between various stages;
  • Figure 9 is a plot of apparent emission intensity as a function of the surface temperature of different engineered structures according to the present invention;
  • Figure 10 is a schematic arrangement of a three stage of thermoelectric device of the present invention with a split-header to provide thermal expansion relief;
  • Figure 11 is an electrical diagram showing a configuration for operating a thermoelectric device of the present invention in either a cooling mode or a heat pump mode;
  • Figure 12A is a schematic depicting a coupling structure for coupling a thermoelectric device to a semiconductor device chip;
  • Figure 12B is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip;
  • Figure 13 is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip;
  • Figure 14 is a schematic depicting a
  • thermoelectric device 1 of the present invention includes a thermoelectric pair of n and p-type thermoelements 2a and 2b, respectively.
  • the thermoelectric pair is connected thermally in parallel for heat conduction and electrically in series for electrical conduction.
  • thermoelectric pair of n and p-type thermoelements 2a and 2b are electrically adjoined together on the side coupled to an upper side header 3 by electrical connection 6, and are electrically connected separately on the side coupled to a lower side header 4 by electrical connection 7.
  • the thermoelectric pair of n and p-type thermoelements 2a and 2b can be coupled to the upper side header 3 by a heat pipe 5 made of high thermal conductivity materials such as Si, SiC, A1N, etc.
  • heat flowing in the thermoelectric device 1 is channeled in the heat pipe 5 into the thermoelectric pair of n and p-type thermoelements 2a and 2b.
  • thermoelectric properties of the n and p-type materials of the thermoelements 2a and 2b due to the different thermoelectric properties of the n and p-type materials of the thermoelements 2a and 2b, electrons and holes in these materials diffuse at different rates across the respective n and p-type thermoelements, thereby creating a voltage difference across the pair of n and p-type thermoelements.
  • a cooling device (not shown) is coupled to the lower side header 4 to permit dissipation of heat. Otherwise, the upper side header 3, the pair of n and p-type thermoelements 2a and 2b, and the lower side header 4 would all come to nearly equivalent temperatures.
  • electron and hole current can driven through the n and p-type thermoelements 2a and 2b by an applied voltage to thereby cool the upper side header 3. Owing to electrical barriers existing between electrical connection 6 and the pair of n and p-type thermoelements 2a and 2b, the electron and hole current will preferentially transport those electrons and holes having a higher thermal energy, thereby cooling the upper side header 3.
  • a heat dissipation device (not shown) coupled to the lower side header 4 permits dissipation of heat from the device, else the lower side header 4 would come to a temperature such that heat, via diffusion of phonons, would flow back to the upper-side header 3 at a rate equal to the heat being carried from the upper-side header 3 by the electron and hole current, thereby eliminating the cooling.
  • the optional heat pipe 5 represents a mechanism to increase the thermal impedance (through the gaps) between the upper side header 3 and the lower side header 4 to a value significantly larger (by a factor of 100 or more) than the thermal impedance across the thermoelectric pair 2a and 2b.
  • the heat pipe 5 is elongated in a direction transverse to the thermoelectric device 1 (i.e., normal to the upper side header 3).
  • the heat pipe 5 has a cross-sectional area smaller than that of an area of the upper side header 3.
  • the heat pipe 5 of the present invention provides an offset, spacing apart the upper side header 3 from the lower side header 4.
  • the offset permits for example thin-film thermoelements to be used without thermal conduction from the upper side header 3 to the lower side header 4 shunting across the gap between therebetween, thereby forcing heat flux to flow through the thermoelements 2a and 2b.
  • the offset in one embodiment of the present invention is at least 25 ⁇ m and more preferably in the range of 250 to 500 ⁇ m.
  • the offset produces a thermal impedance across a gap, where thermoelectric elements do not exist, between the upper side header 3 and the lower side header 4 whose value is at much larger than the thermal impedance across the pair of thermoelements 2a and 2b.
  • a thermal impedance across an air gap is greater than the thermal impedances for the thermoelements 2a and 2b, typically by a factor of 500 to 1000, for similar areas of air gap and thermoelectric materials.
  • the offset allows this ratio to increase, using the high thermal conductivity of the offset relative to the air-gap.
  • the thickness of the offset can be reduced as the pressure level in the gap is reduced i.e, with a high vacuum level, the thermal conductivity of the gap is reduced and so we need less of the offset thickness.
  • the offset will be at least 10 ⁇ m, and for other applications in a range of 100-500 ⁇ m.
  • the thermoelectric devices of the present invention conduct a high active heat flux, while maintaining a much lower flux through the surfaces 1 and 4, through the pair of thermoelements which permits the present invention to realize ultra-high specific power in the pair of n and p-type thermoelements 2a and 2b.
  • the present invention operates with a high active flux through each of the thermoelements while having a low input/output flux across the entirety of the thermoelectric device.
  • this aspect of the present invention is referred as High Active Flux- Low Input-Output Flux (HAF-LIOF).
  • HAF-LIOF High Active Flux- Low Input-Output Flux
  • Figure IB the heat flux through the heat gatherer (e.g. an upper side header 3) and the heat spreader (e.g. an lower side header 4) is smaller than the heat flux through the n and p-type thermoelements 2a and 2b due to the reduced packing fraction of the thermoelements.
  • thermoelements i.e., the fraction of area occupied by the pair of n and p-type thermoelements 2a and 2b relative to a unit area of for example the upper-side header 3 in one embodiment of the present invention is less than 50%, less than 20%, and can be significantly lower, 0.5-1% for example.
  • a unit area for the heat spreader is defined as that fraction of the total area of the heat spreader which principally conducts heat into one of the associated pairs of n and p-type thermoelements attached to the heat spreader.
  • the unit area would be the entire area of the upper side header 3.
  • the total heat flux into upper-side header 3 and out the lower-side header 4 can be on the order of 10 - 30 W/cm 2
  • the heat flux through each thermoelectric pair 2a and 2b can be as much as a factor of 100 times higher.
  • the preferred packing fraction is a function of the heat flux available at the heat- source, the required ⁇ T needed across the thermoelement to achieve the maximum efficiency, and the heat-flux that can be dissipated at the heat-sink. For example, if the required heat-flux through the device element is 2500 W/cm to generate a requisite ⁇ T, and the heat-flux that can be efficiently dissipated at the heat-sink is 25 W/cm 2 , then a packing fraction of 1% will be used (i.e. 25/2500 x 100%).
  • thermoelectric devices of the present invention besides using a heat pipe, other thermal impedance increasing devices can be employed.
  • a partial vacuum i.e. ⁇ 1 Torr
  • a low thermal impedance medium can be used in a module (e.g.
  • the module 10 encapsulating the thermoelectric devices shown in Figure 6 A) containing the thermoelectric devices.
  • the module 10 can be filled with a low conductivity gas such as for example Ar.
  • the headers and heat pipes of the present invention have thermally conducting properties.
  • suitable materials for the headers and heat pipes include for example A1N, !SiC, and diamond. These materials have relatively high thermal conductivities (e.g. 5-20 W/cm-K) and offer the advantage of being electrically insulating.
  • metallic or other semiconductor materials can be used for the headers and heat pipes of the present invention provided a suitable electrical insulating layer 8 is added to the surface of those materials.
  • the headers of the present invention can be made from a Si substrate of a thermal conductivity of ⁇ 1.2 to 1.6 W/cm-K having a thin (i.e. ⁇ 10 nm to 1000 nm) SiO or Si x N y layer of a thermal conductivity of- 0.015 W/cm-K deposited thereon.
  • the headers of the present invention can also be made from a Cu substrate of a thermal conductivity of ⁇ 4 W/cm-K having a thin (i.e.
  • the present invention includes a thermoelectric device having a first header (e.g.
  • thermoelements e.g. the pair of n and p-type thermoelements 2a and 2b
  • a second header e.g. the lower side header 4
  • a heat pipe coupled to the pair of n-type and p-type thermoelements (e.g. the heat pipe 5 connected to the thermoelectric pair 2a and 2b) and elongated in a transverse direction across the thermoelectric device to separate the first header from the second header.
  • the thermoelectric devices cool the first header (and thus can cool heat dissipating devices coupled thereto) by the flow of an electrical current through electrical connections 6 and 7 through the thermoelements 2a and 2b, thus transporting heat in the electrical carriers (i.e., the electrons and holes) and thereby cooling the first header.
  • the thermoelectric devices generate electrical power by the development of a voltage on the electrical connections 6 and 7 as heat flows from the first header (coupled to a heat source) through the thermoelements 2a and 2b to the second header (coupled to a heat sink).
  • Figure 2 depicts a multi-stage embodiment of the present invention.
  • Figure 2 is an illustrative schematic of a multi-stage or multi-unit thermoelectric device of the present invention including three thermoelectric stages or units.
  • the device depicted in Figure 2 is shown by way of example in a configuration for thermoelectric power conversion and, for simplicity, is discussed below only in a context of power conversion.
  • the features of the present invention depicted in Figure 2 and utilizing multiple stages and/or multiple pairs of thermoelements in a stage are, according to the present invention, applicable to thermoelectric cooling as well.
  • an upper header 12 is coupled to a heat source 14 and includes at least one upper thermoelectric stage 16 coupled to the upper header 12.
  • at least one of the depicted thermoelectric stages is dimensioned such that area for thermal conduction in the thermoelectric stage through a pair of thermoelements 20a and 20b, herein referred to as the active thermal conduction channel area, is smaller than above-noted unit area of header 22 to which the pair of thermoelements 20a and 20b are coupled to.
  • thermoelectric stage 16 is coupled to at least one lower thermoelectric stage, e.g. stage 17.
  • the lower thermoelectric stage 17 like the upper thermoelectric stage 16 has an active thermal conduction channel area through the pair of thermoelements 20a and 20b that is smaller in area than the unit area of header 22.
  • heat is dissipated from the lower-most thermoelectric stage 18 by a heat sink 24 coupled thereto. Accordingly, heat pipes in the various stages also have at least one thermal conduction channel area that is smaller than the unit areas of the associated headers.
  • thermoelectric device depicted in Figure 2 a high internal heat-flux exists within the individual thermoelectric stages (e.g., -1800 W/cm 2 for a ⁇ T across each stage of 85K to as high as -2300 W/cm with a ⁇ T across each stage of 107K). Meanwhile, a low external heat-flux exists across the entirety of the thermoelectric device (e.g., a range from 15 to 30 W/cm ).
  • the packing fraction and hence the active flux through each stage can be different or similar to the active heat flux in other stages. More specifically, the multi-stage thermoelectric device as illustrated in Figure 2 can include a high-temperature thermoelectric conversion stage (i.e.
  • thermoelectric stage 16 for example an N-SiGe/P-TAGS thermoelement pair (e.g., with a -100 ⁇ m active region thickness).
  • TAGS refers to a Tellurium, Antimony, Germanium, and Silver (TAGS) alloy composition.
  • TAGS compositions are in general denoted by (AgSbTe ) ⁇ - x (GeTe) x .
  • the most optimal composition according to the present invention for x, the GeTe mole fraction, is -0.80 to 0.85.
  • the high-temperature thermoelectric conversion stage is coupled to a mid- temperature thermoelectric conversion stage (i.e. stage 17), for example an N-PbTe/P- TAGS thermoelement.
  • the mid-temperature thermoelectric conversion stage is coupled to a lower temperature thermoelectric conversion stage (i.e. the thermoelectric stage 18), for example an n-type and p-type Bi 2 Te 3 -superlattice-thin- film thermoelement pair having a 5 to 10 ⁇ m in thickness.
  • the lower header 23 constitutes a split-header having slits formed partially (in the top side and correspondingly at certain locations on bottom side not shown) in a body of the lower header 23 to provide thermal expansion relief.
  • the splits are preferably not located near the devices.
  • the three-stage thermoelectric device depicted in Figure 2 has a design efficiency in a range of 20% and electrical power density in excess of 5 W/cm , and is capable of power conversion in a range of tens of mWatts to as high as 1 MWatt when coupled to a heat source at temperatures approaching 875K.
  • Design efficiencies in excess of 20%> and the capability to operate with hot-sides in the range of 675 to 875K permit flexibility and adaptability to real-world systems.
  • With hot-sides in the range of 675 to 875K higher Carnot efficiencies are available then would be available for example in thermoelectric devices restricted to hot-sides of 375K to 575K.
  • higher Carnot efficiency provides for higher system efficiencies which in turn facilitates load balancing by reducing the number of modules (i.e., the number of thermoelectric pairs) required to convert a given heat flux into electrical power.
  • thermoelectric device efficiency of the multi-stage thermoelectric device of the present invention.
  • Conventional bulk Bi 2 Te 3 thermoelectric devices offered by Hi-Z Inc. (San Diego, CA) exhibits a power density of 0.34 W/cm 2 at a ⁇ T of 200K for a specific power of -0.165 W/gm.
  • thermoelectric devices of the present invention can realize a specific power greater than 1 W/gm and a power density greater than 0.5 W/cm .
  • FIG. 3 is a graph depicting variations of ZT with temperature for various material systems useful in the present invention. Knowledge of the variation of ZT with temperature in suitable temperature-stable material systems, such as shown in Figure 3, permits appropriate materials selection for the above-described multi-stage design.
  • the material selections should, according to the present invention, provide a material for each thermoelectric stage which is robust in the temperature range in which the thermoelectric stage is intended to operate.
  • devices in PbTe/PbSe and Si/Ge superlattice material systems would be useful for a temperature range of 475K- 675K and 675K-875K, respectively, in the present invention.
  • the relevant properties, the lattice-mismatch and the bandgaps, of the PbTe/PbSe superlattice system are shown in Table 1. Table 1.
  • Si/Ge superlattice materials deposited at -1000K have shown a dramatic reduction in thermal conductivity and concomitantly an enhancement in ZT.
  • Si/Ge superlattices may only offer a ZT of only -0.8 at 300K, significantly below that of a Bi 2 Te /Sb 2 Te 3 superlattice or a Bi 2 Te /Bi 2 Te 3 .
  • x Se x superlattice the Si/Ge superlattices are likely to offer a much higher ZT at the higher temperatures employed in the upper thermoelectric stages depicted in Figure 2. Further, the SiGe superlattices will have higher ZT values than associated SiGe alloys of the same Ge concentration, leading to higher efficiencies.
  • PbTe-based quantum-dot superlattices are suitable materials for the n and p-type thermoelements of the present invention.
  • ZT values for these materials are achievable in a range of 1.5 to 2, in the temperature range of450K-550K.
  • ZnSb and skutterrudites i.e., materials with a known ZT >1 over a temperature range suitable for power conversion
  • these materials are in thin substrate form (i.e. in a thickness of 100 to 200 ⁇ m) and are used in a temperature range of 500K to 900K.
  • the HAF-LIOF, high-power density, multi-stage power conversion device concept of the present invention is not limited to Bi 2 Te 3 -superlattice/PbTe/SiGe material combinations.
  • superlattices of Si/Ge, PbTe/PbSe, ZnSb/CdSb, InAs/InSb, CdTe/HgCdTe, Ga x In ⁇ . x As/Gay In i-yAs can be used in the high-temperature and mid-temperature thermoelectric conversion stages, discussed above, thus providing a cascade thermoelectric device utilizing all superlattice materials.
  • FIG. 4 is a graph depicting state-of- the-art ZT vs carrier concentration for p and n-type Bi 2 Te 3 -based superlattice elements.
  • Figure 4 shows a ZT-3.5 in p-type superlattices and -2.0 in n-type.
  • ZT figure- of-merit
  • Se 0.3 superlattice device elements is improved from previously known values of -2.4 and -1.2 to values greater than 3.5 and -2, respectively.
  • the carrier levels in the p-type materials as shown in Figure 4 are controlled by varying the organometallic Tellurium flow-rate during growth, for a fixed growth rate of the superlattices which in turn can be controlled by the flow-rate of the organometallic Bismuth for Bi 2 Te 3 and organometallic Sb for Sb 2 Te 3 .
  • the carrier levels, for a constant growth rate, can also be controlled by control of growth temperature over a limited range.
  • the doping levels as shown in Figure 4 can be controlled by the growth rates of Bi 2 Te and Bi 2 Te 2.7 Se 03 layers in the superlattice as well as the growth temperatures.
  • a value of peak ZT of -2.9 demonstrated in the present invention for a 10A/50A p-type Bi 2 Te 3 - Sb 2 Te 3 superlattice is one of the highest peak ZT values observed at 300K for the p-type Bi 2 Te 3 -Sb 2 Te 3 superlattice material system.
  • Such improvements in ZT permit improved power conversion efficiencies and cooling efficiencies for the thermoelectric devices of the present indention.
  • thermoelectric properties of the fabricated p-n thermoelement couples is possible.
  • each one of the p-n thermoelement couples potentially carries a high-current (i.e., several Amps of current) and is a low- voltage thermoelectric device. Parasitic lead resistances can therefore affect the measured device efficiencies.
  • thermoelectric devices Properties of the p-n thermoelement couples have been developed by fabrication and analysis of the p-n thermoelement couples on a split semi- infinite Cu-plate.
  • electrical contact resistances in the thermoelectric devices are minimized between the metallizations and the p-n thermoelement couples, providing adequate electrical interfacing.
  • thermal interface resistances are reduced in the metal-to-dielectric interfaces of the present invention by deep-annealing of a metal into the dielectric bulk or by utilizing an AlN-diffused Al-Cu interface.
  • high aspect-ratio thermoelectric devices are provided by appropriate patterning and etching of the p-n couples (e.g.
  • An aspect ratio of the element is defined as the ratio of thickness/area of element in units of cm "1 .
  • Typical thickness of thermoelectric elements in the low-temperature stage can be about 5 to 20 microns, while the mid and high temperature stages can have thicknesses ranging from 50 to 250 microns.
  • the aspect ratio, hence the area of each element of the various stages, are based on the thermal conductivity of the materials (noting that the thermal conductivities of the p and n-type elements within a stage can be different and so their aspect ratios to allow same ⁇ T across both p and n-type elements of each stage can be different) of the various stages and the required ⁇ T for each stage for maximizing the efficiency of the overall set of stages.
  • the ⁇ T of each stage is inversely proportional to thermal conductivity of the material and directly proportional to the aspect ratio.
  • the total heat flux through each stage is same; thus the aspect ratio (and the associated packing fraction) in each stage is adjusted with this criterion to meet the design ⁇ T for each stage.
  • Power conversion efficiencies achievable in for example the three-stage thermoelectric devices of the present invention are calculable based on known equations.
  • the maximum efficiency is obtained when the load ratio r, defined as R L /R G with R L being the load resistance and R G being the thermoelectric device internal resistance, is optimized for average ZT over the entire temperature range, ZT m . See, for example, H.J. Goldsmid, Electronic Refrigeration, Pion Limited, (1986).
  • the device efficiency ⁇ can be obtained by eqn. (2), shown below for a given ⁇ T, temperature differential, and mean operating temperature T m .
  • r (l+ZT m ) 1/2 ..(1)
  • ⁇ ⁇ T/ ⁇ [(r+l)/(r-l)]*Tm + ⁇ T/2 ⁇ ..(2)
  • Power conversion efficiency estimates are shown in Table 2 of Figure 5, for a range of ZT values for each stage.
  • the power conversion efficiency estimates are shown in a range from upper to midrange to lower projected values.
  • the projections in Table 2 indicate that an efficiency in the range of 9.6% to 11.4% for a ⁇ T of 150°C in the low-temperature Bi Te -SL-based stage can be realized for a ZT value of 2 to 2.5.
  • An efficiency in the range of 4.3 to 4.7%. for a ⁇ T of 175 to 200 °C in the n-PbTe/p- TAGS-based thin N-PbTe/P-TAGS thin-substrate mid-temperature stage can be realized for a ZT of -0.75.
  • thermoelectric efficiency in the range of 3.4 to 3.7% for a ⁇ T of 200°C N- SiGe/P-TAGS thin-substrate high-temperature stage can be realized for a constant ZT of -0.75.
  • a thermoelectric efficiency of —19.4% to 17.7%) and a power density in excess of 5 W/cm are obtainable in the three-stage thermoelectric power conversion devices of the present invention.
  • Table 2 illustrates the performance of various stages of a three-stage thermoelectric device of the present invention as a function of ZT and the efficacy of thermal management at the heat-sink. The calculations in Table 2 are based on a module size of approximately -33 cm 2 .
  • a heat flux of -30 W/cm 2 is assumed to flow from the heat-source (i.e. the upper-most header) to the heat sink (i.e. the lower-most header), yielding a heat-flux dissipation range of -24.7 to 22.7 W/cm 2 at the heat-sink.
  • the power densities are calculated assuming a 5- ⁇ m thick superlattice element in the low-temperature Bi 2 Te 3 -super lattice stage and assuming a 100- ⁇ m thin- substrate for bulk thermoelements in the mid and high temperature stages.
  • Table 2 indicates that higher ZT materials for both the mid-stage (460 -660K) and high-stage (670-890K) are advantageous.
  • the aspect ratio determines the ⁇ T.
  • the efficiency is a function of ⁇ T and ZT.
  • the power generated P, in Watts, is a product of Q times efficiency.
  • power density, Pd is a function of both area, thickness, ZT and Q.
  • the thermal interface resistance can be reduced in the metal-to-dielectric interface by deep-annealing of the metal into the dielectric bulk or by utilizing an AlN-diffused Al-Cu interface, as discussed above.
  • Calculated ZT values of the inverted couples utilized for demonstration of the power conversion concepts of the present invention were about a ZT value of 1.1 at 300K.
  • the average ZT of the p-n couple of the present invention e.g. the thermoelectric pair 2a and 2b
  • increases with temperature consistent with the general trend of ZT shown in Figure 3.
  • One p-n couple of the present invention was tested up to a ⁇ T of nearly 85K, corresponding to an active device heat flux within the thermolement of -1800 W/cm 2 .
  • the present invention in one preferred embodiment includes a system for thermoelectric power conversion.
  • the system includes a thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, a thermal impedance increasing device disposed between the thermoelectric pair and one of the first and second headers, and a heat sink coupled to the second header which dissipates heat at a rate which maintains a temperature of the second header below a temperature of the first header.
  • a coupling member can be used to couple the first header to a heat source.
  • the heat source can be a combustion source, a radioactive source, a solar-heated source, and an electrical power device.
  • the coupling member in one embodiment of the present invention can be a thermally conducting member having a thermal conductance of at least Al and extending between said first header and said heat source.
  • the coupling member can be a steam pipe or a combustion exhaust pipe transporting heat flux by a convective medium such as for example steam or combustion gases.
  • FIG. 6A is a schematic illustration depicting a thermoelectric device module of the present invention integrating a series of p-n couples on a single stage.
  • Tests on p-n couple devices for open-circuit voltage (Voc), power (P), and power density (Pd in W/cm ) are shown in Figure 6B.
  • Tests on an integrated 16 p-n couple devices show expected open-circuit voltage (Voc), power (P), and power density (Pd in W/cm 2 ).
  • the open-circuit voltage has been observed to scale in the thermoelectric module to almost sixteen times that of a single-couple, for similar ⁇ T to the individual p-n couple, further supporting the HAF-LIOF concept.
  • the power levels of 38 mW per couple, for a 600 ⁇ m x 600 ⁇ m device and for a ⁇ T of about 107K correspond to an electrical power density of as much as -10.2 W/cm 2 .
  • the power level scales to 166 mWatts per couple for the 16-couple module for a ⁇ T of 77K.
  • the device fabrication methodology and the HAF-LIOF aspect of the present invention are applicable to thin substrates from two non- superlattice material systems, GaSb and InAs, corresponding to the bandgaps of the n- SiGe/p-TAGS and n-PbTe/p-TAGS systems.
  • Open-circuit voltage as a function of ⁇ T in a 100 element GaSb and InAs thin-substrate module of the present invention hare capable of Voc values of -4.5 Volts to -3.5 Volts and are capable to generate ⁇ T in excess of 100K, corresponding to active heat fluxes of -300 W/cm 2 , needed for achieving high electrical power densities.
  • thermoelectric materials utilized as the p and n-type thermoelements of the present invention are typically overgrown films on commercial substrates, the overgrown films having a thickness of for example 5 to 20 ⁇ m.
  • Substrates for the growth of, for example the thin-film superlattices or other thin film thermoelectric structures may have the same conductivity type as the overgrown films or may permit the overgrowth of opposite conductivity type films.
  • the overgrown films can be metallized to provide a low-resistance contact, such as a low-resistance Peltier contact.
  • fabrication of the thermoelectric devices of the present invention is facilitated by inverted-couple processing.
  • Table 4 shown in Figure 6C illustrates the inverted processing approach of the present invention. In inverted-couple processing, one leg of a p thermoelement and one leg of an n thermolement are attached a common electrical member adjoining the two elements.
  • fabricating the individual n-type and p-type thermoelements in each leg of the thermoelectric power conversion stages can occur in one example of the present invention by forming n-type and p-type films on separate substrates, and then attaching the individual n-type and p-type thermoelements to a common header.
  • films of a specific type can be formed on a common substrate and then selectively converting (e.g. by selective ion implantation) different regions of the formed thermoelectric material into n-type and p-type regions.
  • thermoelements could be formed intrinsic (or specifically doped) and then type- converted to another (i.e., a p-type to n-type conversion or an n-type to p-type conversion) by for example impurity-diffusion. Type-conversion would then be performed at a convenient stage in the manufacturing process. Regardless of the specific details, a set of p and n-type materials having a pre-arranged pattern are attached to a common header like SiC, A1N , low-resistivity silicon, and silicon with a thin insulating layer.
  • substrates including the individual n-type and p-type thermoelements are separated, for example by scribing or laser dicing, into individual segments.
  • the individual segments are then bonded onto a header such that alternating n-type and p-type conductivity materials exist between each adjacent thermoelement.
  • the surfaces of the header that come in contact with the n- and p-type segments are preferably metallized prior to assembly to provide low-resistance electrical connection between adjacent n- and p-type segments where necessary to electrically interconnect adjacent thermoelements.
  • the surfaces of the individual n-type and p-type thermoelements that come in contact with the header are preferably metallized prior to assembly to provide the necessary low-resistance electrical connection to the n-type and p-type thermoelements, else a high electrical contact resistance can limit the efficiency of the thermoelectric stage and the resultant thermoelectric device.
  • the substrates from each of the p- and n-segments are selectively removed for example by using selective etchants.
  • thermoelements are then patterned using photolithographic patterning followed for example by etching, or by laser ablation, to produce the desired cross-sectional thermal conduction area for the present invention (i.e., to set the aspect ratio and ultimately determine the packing fraction).
  • Low resistivity contact metallizations are then evaporated on! an upper surface of the n- and p-type thermoelements. In this step, either the same metallization can be used for both of the n- and p-type section, or different metallizations pan be used (i.e., separate evaporations), depending on the contact resistance requirements.
  • Sheet resistances, or conductances as specified above, associated with the metallizations are designed not to restrict the performance of the thermoelectric devices.
  • a top, pre-patterned metallization header can, in one embodiment of the present invention, be attached to the metallized sections to function as the aforementioned heat pipe.
  • the header itself prior to metallization can be patterned to provide the aforementioned heat pipe.
  • the formed pair of thermoelements (i.e., the n thermoelement and the p-thermoelement) including the attached header can then be flipped and bonded to a second header.
  • the second header thermally connects the n thermolelement to the p- thermoelement, but contains patterned electrical connections such that electrically the n thermoelement and the p-thermoelement are individually connected, as shown in Figures 1 A and IB.
  • the bottom header thus functions as an electrical member having, as shown in Figures 1 A and IB, a split electrical contact (i.e., an electrical contact only contacting individually the n-type and p-type thermoelements), while as a thermal member the bottom header functions as a continuous thermal contact. Regardless of the formation approach (i.e.
  • thermoelectric devices can then be added using similar procedures, or by attaching subsequent pre-fabricated stages, or by attaching selective members of subsequent stages.
  • inverted couple processing is given below: P-type and n-type superlattice thermoelectric films are deposited on GaAs substrates.
  • the deposited superlattice films are patterned with Cr/Au/Ni/Au metallizations.
  • the deposited superlattice films and/or the GaAs substrates are etched in preparation for dicing. Diced strips of the p-type and n-type superlattice films attached to the GaAs substrates are then bonded in an alternating conductivity type pattern to a header.
  • the header includes an AlN substrate having Ti/Au metallization (annealed) and having a subsequent Cu/No/Au topmost metallization. A Sn preform bonds the diced strips to the AlN substrate (functioning as a first header).
  • the GaAs substrate is then etch removed, and Cr/Au/Ni/Au contacts are evaporated through shadow masks or evaporated and patterned to form electrical contacts to the superlattice thin films. Thick metal pads are then formed on the evaporated contacts to define for example a heat pipe structure. Dies containing the superlattice thin films and the attached AlN header are placed and bonded to a separate split metallized header (functioning as a second header) allowing for individual electrical connection to each of the n- and p-type thin film thermoelements.
  • Table 5 depicts various bonding steps used to fabricate the three stage device shown in Figure 2. As depicted in Table 5, in stage 3 of the thermoelectric device (i.e.
  • a preferred heat spreader is AlN or Si.
  • a AgCu eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned N-SiGe/P-TAGS thermoelements.
  • Metallization layers on the N- SiGe/P-TAGS thermoelement pair include silver having a typical diffusion barrier layer of W or Mo orNi or similar refractory metal to prevent diffusion of Ag into the N-SiGe and P-TAGS materials. Table 5
  • a preferred heat spreader is AlN.
  • a AgAuGe eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned N-PbTe/P- TAGS thermoelement pair.
  • Metallization layers on the N-PbTe P-TAGS thermoelement pair include silver having a typical diffusion barrier layer of Ni or W or Mo to prevent diffusion of Ag into the N-PbTe and P-TAGS materials.
  • a preferred heat spreader is SiC.
  • a Sn eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned n- and p-type Bi 2 Te 3 - superlattice-thin-film pair.
  • Metallization layers on the n- and p-type Bi 2 Te 3 - superlattice-thin-film pair include Cu having a diffusion barrier layer of Ni or a similar metal to prevent diffusion of Cu into the N-Bi 2 Te 3 SL and/or P-type Bi 2 Te 3 SL materials.
  • the uppermost stage i.e.
  • stage 3 can be bonded to the heat-source header (e.g., a Cu plate) using a AgCuP eutectic bond.
  • Stages 2 and 3 can be eutectic bonded together using a Auln eutectic.
  • Stages 1 and 2 can be eutectic bonded together using a SnAu eutectic.
  • Stage 1 can be bonded to the heat-sink header (e.g., an Al plate) using an InSn eutectic bond.
  • successively-lower-melting-point eutectics bond the successive stages so that the previously-bonded stages are in tact.
  • thermoelectric device of the present invention can be attached to an integrated circuit die or wafer element (preferably to the backside) with or without an intervening layer of thermal grease or plastic adhesive.
  • a thin electrically insulating layer or a series of back-side p-n isolation junctions can be utilized to permit electrical isolation of the integrated circuit on the die from the currents flowing in the thermoelectric devices.
  • the "top" header joining the thermoelectric device of the present invention to the integrated circuit die is made of thick metallizations which form the electrical contact and the heat pipe to the p-n thermoelements.
  • FIG. 7A is a schematic depicting the bonding of a pre-fabricated thermoelectric device 72 to a semiconductor device chip 74.
  • bonding is used to bond the pre-fabricated thermoelectric devices to an integrated circuit die. As such, this approach separates the thermoelectric device fabrication steps from the semiconductor device chip.
  • thermoelectric device 72 a number of bonding techniques (to be discussed below) can be used to join an upper header 75 of the pre-fabricated thermoelectric device 72 to the semiconductor device chip 74. Accordingly, in the attached structure, heat from power dissipating devices 76 in the semiconductor device chip 74 will be pumped through the thermoelements 73 and dissipated at a heat sink (not shown) attached to the lower header 77.
  • Figure 7B is a schematic depicting the attachment of the present invention of a thermoelectric device without a top header onto a semiconductor device chip. Specifically, Figure 7B depicts the bonding of pair of thermoelements 73 to the semiconductor device chip 74.
  • thermoelectric device 72 As shown in Figure 7B, the bonding to the semiconductor device chip 74 is facilitated by the pre-deposition or growth of an electrically insulating layer 82 on the semiconductor device chip 74 (e.g. silicon dioxide formation on a Si device wafer). As shown in Figure 7B, the semiconductor device chip 74 itself serves as a header spreading heat from power dissipating devices 76. Thus, in this embodiment of the present invention, the thermoelements are attached directly to the semiconductor device chip 74 without the utilization of a top header.
  • Figure 7C is a schematic depicting the sequential formation of a thermoelectric device 72 on the present invention onto a semiconductor device chip 74.
  • the fabrication process of the present invention may utilize bonding at separate steps in the process to bond thermoelectric materials and/or superlattice layers to other layers of the thermoelectric device, and to provide a mechanism for direct attachment of the thermoelectric devices of the present invention to the semiconductor device chip 74.
  • an electrically insulating layer 82 is formed on the semiconductor device chip 74.
  • the layer 82 can be a thermal oxide grown on a Si device chip; the layer 82 can be a deposited oxide such as SiO 2 or Si 3 N 4 deposited for example by sputtering or vapor deposition.
  • thermoelectric devices 72 are deposited and patterned in accordance with the layout of the thermoelectric devices 72 on the semiconductor device chip 74.
  • the metallizations 83 can include for example structures to form the heat pipe 5, described in Figure 1.
  • pair of thermoelements 73 are attached (deposited or bonded) to the metallizations 83.
  • the pair of thermoelements 73 can include for example the features of thermoelectric device 1 shown in Figure 1.
  • a set of metallizations 85 are formed on thermoelements 73.
  • contact and annealing using the procedures detailed below complete the fabrication by joining a header 84 to the metallizations 85.
  • various approaches of the present invention include: (1) attaching a pre-fabricated thermoelectric module to a device wafer, (2) building a thermoelectric device from the backside of a device wafer, or (3) building a part of a thermoelectric device onto the backside of a device wafer (e.g. building heat pipes onto the backside of a device wafer), and completing fabrication by attachment of remaining pre-fabricated components.
  • attachment several attachment methods can be used including: (1) soldering, (2) brazing, (3) friction bonding, and (4) insulator-insulator bonding similar to wafer bonding.
  • a hybrid "reactive" bonding process is utilized in which insulator surfaces, having a thin reactive metal layer, are placed opposed to one another and then contacted and heated to react the metal layer with the insulator surfaces and thereby bond the opposed components together.
  • a metal include for example Ti, W, Cr, Mo, etc., or alloys thereof. These metals readily oxidize and form in a preferred embodiment suicides which melt at temperatures of 300°C or less.
  • the hybrid reactive bonding process of the present invention relies on the reactivity of the thin metal layer with the respective insulators to achieve a bond.
  • the metal layer is preferably thin (e.g., less than 500 A) such that all of the reactive layer is consumed, or reacts with, the insulating layers.
  • hybrid reactive bonding bonding is achieved when the surfaces are brought into contact and then heated such that the metal reacts with one or both insulating surfaces.
  • friction bonding according to the present invention can be used to bond thermoelectric materials and/or superlattice layers to other layers on the thermoelectric device and provide a mechanism for direct attachment of the thermoelectric devices of the present invention to an integrated circuit die or wafer element. Friction bonding can, in one embodiment of the present invention, be implemented with intervening reactive layers.
  • thermoelectric bonding In friction bonding, linear motion over a short distance using an ultrasonic transducer provides the friction necessary to produce heating at the surface that will produce the bond. Heating to a high fraction of the melting temperature and then using a friction bonding technique to produce local heating and thereby for instance melting or plastic deformation at the contact surfaces can produce a suitable bond. Heating from one direction and cooling from another side can be used to locally heat to a small area near the contact surfaces. Small amounts of material or thin sheets of material in a perform cut to die or strip size to match the thermoelectric component parts. Materials in thin sheets or thin film form are used, according to the present invention, to provide for a buffer that protects the thermoelectric superlattice materials during the friction bonding. Materials in thin sheets avoid mechanical damage to the top layers of the thermoelectric materials.
  • the thin layer is selected to form a eutectic that allows friction bonding.
  • friction bonding the surfaces of the materials to be bonded can be roughened if necessary to improve the friction and thus the efficacy of forming a friction bond.
  • diffusion bonding or thermal annealing can be used to bond the thermoelectric devices to a semiconductor device chip. Bonding can occur between a top header of a thermoelectric devices and a semiconductor device chip by any number of the processes described herein, and can utilize a bonding material to facilitate coupling of the semiconductor device chip to the semiconductor device chip. Regardless of approach, a thermally conductive and mechanically stable connection or bond is preferred between the top header and the semiconductor device chip.
  • Bonding to the semiconductor device chip to the thermoelectric devices of the present invention can be accomplished in the following non-limiting examples by: 1. utilization of thermal adhesives or thermally conductive epoxy, 2. soldering, 3. diffusion bonding using electroplated or evaporated metal contacts, 4. utilization of anisotropic thermal adhesives, 5. utilization of thermoplastic conductive polymers, and 6. utilization of silicon to silicon molecular bonding (in the case where the cooling header of the thermoelectric device is made from silicon). Heat flux from the semiconductor device chip can be dissipated from the thermoelectric devices of the present invention by a heat sink operating at a temperature above that of the semiconductor device chip.
  • thermoelectric devices of the present invention when not cooling, can sense a heat flux from the chip and thus can provide active sensing of the device chip performance.
  • a template of such thermoelectric devices can therefore, in one embodiment of the present invention, provide a mapping of device component utilization (i.e. those device components having higher utilizations will produce higher amounts of heat flux into the proximate thermoelectric devices).
  • the present invention can utilize a number of approaches using bonding to facilitate thermal transfer by thermal conduction from one thermoelectric power conversion stage to another. Such techniques and other techniques known in the art can be used to appropriately bond the various stages together. These approaches, according to the present invention, realize high-quality thermal interfacing.
  • thermoelectric conversion stages utilizes radiant thermal energy transfer using Purcell- enhancement cavity transmitter/receiver structures such as those described in the afore- mentioned U.S. Provisional Application No. 60/253,743, the entire contents of which are inco ⁇ orated herein by reference, entitled "Spontaneous emission enhanced heat transport method and structures for cooling, sensing, and power generation" for heat transfer from one thermoelectric power conversion stage to another.
  • the radiant portion if not the dominant process, plays a substantial role in managing thermal stress by providing less-rigidly-bonded interfaces.
  • Figure 9 is a schematic illustration in which radiant coupling is used between adjacent thermoelectric conversion stages.
  • thermoelectric stage 16 is coupled via radiant coupling mechanism 217 to the thermoelectric stage 17.
  • Stage 17 is in turn coupled via radiant coupling mechanism 218 to the thermoelectric stage 18.
  • Electrical connections 220 provide connections to the thermoelements on each stage.
  • the present invention can utilize Purcell enhancement from an enhanced density of radiative modes in small-scale structures (similar to enhanced electronic density of states in quantum-confined systems) for enhanced spontaneous emission using patterned / ⁇ m-size-range, appropriately-spaced, structures for specific temperatures, on the heat spreader.
  • engineered micro-fins can also potentially enhance spontaneous radiative heat transport.
  • Spontaneous emission enhanced heat transport may additionally enhance emission at infra-red wavelengths near 300K.
  • the inco ⁇ oration of micron or sub-micron size Purcell cavities will provide for the theoretical maximum radiative emission at peak wavelengths of 10 ⁇ m which will enhance heat transport by as much as a factor of 1000 at 300K, leading to a radiative dissipative flux of ⁇ SEEHT of 44 W/cm 2 .
  • Such micron size particles inco ⁇ orated by impregnation or self-assembly, followed by overgrowth, permit the scope for radiative heat transfer mechanisms to be considerably enhanced.
  • a radiative coupling mechanism includes a thermally conductive layer having dispersed therein particles including one of metal, semimetal, and semiconductor particles to thereby enhance black body radiation from the thermally conductive layer and support radiative heat transfer across the interfaces of the thermoelectric device components without the necessity of physical bonding.
  • the present invention recognizes that of enhanced emission with these Purcell cavity structures can be further enhanced/realized if there is matching of "increased density of states" in emitters with “increased density of states” with receivers/absorbers, i.e., resonant thermal energy transfer.
  • resonant thermal energy transfer by Purcell-cavity effects, other "proximity coupling of radiative infrared modes” can be exploited as well, in the present invention.
  • Utilization of radiation coupling will, according to the present invention, reduce thermal stress, by removing (strong) physical interfacial contacts between various stages. Further, the mechanical alignment of the resonant structures will not be a significant issue, given that the typical size of the inverted couple headers are -300 ⁇ m x 300 ⁇ m.
  • Thermoelectric Modules For automatic assembly of the modules described above a dicer and pick-and- place tools (standard to the IC industry) can be used.
  • a wafer dicer and robotic pick- and-place tool provide not only cost-effective but also reliable fabrication of both mini- modules and large-scale modules.
  • the tool is used to dice p-n couple dies from a processed wafer and to assemble these inverted couple dies into a range of devices, from mini-modules to large-array of mini-modules.
  • the dicer and pick-and-place tools, described above can also be used according to the present invention to assemble thin- substrate (i.e.
  • FIG. 10 is a schematic arrangement of a three stage or unit thermoelectric device of the present invention having a split-header on a lower stage 18 to provide thermal expansion relief.
  • the lower-stage 18 of the multi-stage thermoelectric device can be nearly instantly converted to a heat pump for short periods of time, thus avoiding catastrophic device damage due to for example an interruption in coolant flow, and further enhancing reliability.
  • Figure 10 represents a multi-stage thermoelectric device of the present invention in which electrical reconfiguration avoids catastrophic failures during coolant flow loss to the lower stage 18 and thus the risk of deterioration of the thermoelectric properties of the thermoelements at this stage (i.e., due to deterioration of the thermoelectric materials and/or contacts) is minimized.
  • An electrical configuration for permitting avoidance of a catastrophic failure is depicted in Figure 11.
  • Figure 11 is an electrical diagram showing an exemplary configuration for operating a thermoelectric device of the present invention in either a heat pump mode or a power conversion mode. As shown in Figure 11 , each stage of a thermoelectric device 102 is temperature monitored by temperature monitor block 104.
  • Each stage is electrically controlled/sensed by a switch block 106 and a controller 108.
  • the controller 108 supplies power out from the thermoelectric device 102.
  • the controller can include filters and dc-ac converters to output ac power or can include dc-dc power converters to output higher voltage dc power than directly available from the thermoelectric device 102.
  • Figure 12 depicts a cooling system 110 which for illustrative pu ⁇ oses depicts a heat exchanger 112 utilized to pump with pump 114 a fluid to a heat sink plate 116.
  • a flow sensor 118 monitors the flow of coolant to the heat sink plate 116.
  • a temperature of the lower stage and the output power from the lower stage is monitored by a power unit 120.
  • the controller 108 can electrically reconfigure the thermoelectric device from a power conversion mode to a heat pump mode. As a result, heat will be pumped to the heat source upon this electrical reconfiguration permitting the temperature of the lower stage devices to at least momentarily avoid overheating and catastrophic destruction.
  • the thermoelectric device can continue in the "cooling" mode until the heat source is turned off, removed, or otherwise decoupled from the thermoelectric device 102. As such, catastrophic failures can be avoided.
  • the controller 108 being connected to the separate stages can switch the stages from the above-noted power conversion mode and heat pump (i.e. a cooling mode) to a heat flux sensing mode.
  • thermoelectric devices of the present invention In a heat sensing mode, the current through the thermoelements are measured as an indicator of the heat flux through the thermoelements coming from a heat source. Such information can be used as a measure of the heat being dissipated from the heat source.
  • the controller can thus process a signal indicative of heat flux from the thermoelectric pair of n-type and p-type thermoelements.
  • the controller of the present invention connected electrically to the thermoelectric devices of the present invention can be configured to switch the thermoelectric devices between at least one of a cooling mode, a heat pump mode, a power conversion mode, and a heat flux sensing mode.
  • thermoelectric devices and thermoelectric device modules of the present invention can be accomplished through a number of mechanisms.
  • Thermoelectric devices can be coupled to such devices for the pu ⁇ oses of "hot spot" cooling those parts of the device most susceptible to exceeding operational temperature limits.
  • Integrated semiconductor devices requiring hot spot cooling include for example microprocessors, graphic processors and other power dissipating devices fabricated in silicon, germanium, silicon-germanium, gallium arsenide, or any such semiconductor material.
  • the integration can occur for example via recesses in the semiconductor chip and/or in the headers. In this approach, vertical and lateral vias or recesses are fabricated in the semiconductor chip and/or the heat spreader.
  • FIG. 12A is a schematic depicting a coupling structure for coupling a thermoelectric device of the present invention to a semiconductor device chip.
  • Figure 12A depicts a thermoelectric device attached to a semiconductor device chip 74.
  • the semiconductor device chip 74 includes the aforementioned power dissipating devices 76 in a region proximate to the recesses 78. Selective cooling occurs via the cooling of recesses 78 in the semiconductor device chip 74.
  • the recesses 78 permit alignment of the thermoelectric devices 72 to the power dissipating devices 76.
  • an electrically insulating interlayer 82 can be preferably inte ⁇ osed between the thermoelectric devices and the semiconductor material of the chip.
  • the properties of the electrically insulating interlayer 82 are such to permit electrical isolation without impeding heat flux between the semiconductor device chip 74 and the thermoelectric devices.
  • An electrically insulating material such as SiO 2 can be suitable for the electrically insulating interlayer 82, although other insulating materials known in the art are likewise applicable.
  • the electrically insulating interlayer 82 is at least applied to the semiconductor chip in regions where the thermoelectric devices are to be attached.
  • metallized contacts 83 can be made on the electrically insulating interlayer 82 to provide electrical connections to the thermoelectric devices.
  • Metallized contacts can be made using one or more of known metallization techniques such as for example but not limited to evaporation, electroplating, or soldering.
  • thermoelectric devices can be used to bond the thermoelectric devices to the chip. Bonding occurs between a top header 79 of the thermoelectric devices and the semiconductor device chip 74 by any number of the processes described above, and can utilize a bonding material 80 to facilitate coupling of the semiconductor device chip 74 to the semiconductor device chip 74. Regardless of approach, a thermally conductive and mechanically stable connection or bond is preferred between the top header 79 and the semiconductor device chip 74. Heat flux from the semiconductor device chip 74 is dissipated from the thermoelectric devices 72 by a heat sink operating at a temperature above that of the semiconductor device chip 74.
  • FIG 12B is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip.
  • the thermoelectric devices 72 can be directly attached to the semiconductor device chip 74 without utilization of recesses 78.
  • placement of the thermoelectric devices 72 on the semiconductor device chip 74 aligns the thermoelectric devices 72 opposite the power dissipating devices 76.
  • coupling of thermoelectric devices 72 to the semiconductor device chip 74 occurs with or without the utilization of recesses.
  • FIG 13 is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip.
  • thermoelectric devices 72 are attached in recesses 78 existing in both the semiconductor device chip 74 and a heat spreader plate 90.
  • the heat spreader plate 90 can utilize a bonding material 80 to couple the heat spreaders 84 to the heat spreader plate 90.
  • Figure ,14 is a schematic depicting another coupling structure for coupling thermoelectric devices to a semiconductor device chip.
  • the coupling structure includes vias for electrical connections to the thermoelectric devices. Power requirements for the thermoelectric devices require that electrical leads to the thermoelectric devices be electrically insulated.
  • the electrical leads 92 can be integrated directly in a header (e.g.
  • header plate 90 by vias 94, which either vertically or laterally provide electrically conductive channels.
  • the vias 94 can be either etched or prefabricated such that the vias 94 run through the header plate 90.
  • Insulating material 96 for the vias 94 can be any high temperature plastic or ceramic of sufficient dielectric quality which encapsulates metal (e.g., copper, gold or, any high electrically conductive metal).
  • Vias in the header plate 90 can be created by laser ablation and can have electroplated metal deposited therein. This structure shown in Figure 14 having insulated electrical leads facilitates independently power or sensing of individual ones of the thermoelectric devices.
  • the header plate 90 would be made of materials that match the coefficient of thermal expansion of silicon (e.g. within a 5% match in terms of linear expansion coefficient).
  • FIG 15 is a schematic depicting another coupling structure for coupling thermoelectric devices to a semiconductor device chip.
  • the coupling structure includes vias for electrical connections to the thermoelectric devices and includes high thermal conductivity materials in the header plate 90.
  • a combination of high thermal conductive materials 98 like highly oriented pyrolytic graphite, carbon foams, graphite foams can be used to enhance the rejection of heat from the hot side of the thermoelectric device. These materials can be used as a part or for the entirety of the heat spreader plate 90. Thermal conductivity of these materials can be five times that of the thermal conductivity of copper. These materials would be extremely efficient in dissipating heat from the heat sink 84 of the thermoelectric devices 72. As illustrated in Figure 15, lateral electrical connections 100 connect to the thermoelectric devices 72.
  • the lateral electrical connections 100 like the electrical leads 92 are formed with electrically conductive materials and insulated from the heat sink plate 90.

Abstract

A thermoelectric device having at least one thermoelectric unit including at least one thermoelectric pair of n-type and p-type thermoelements, a first header couplet to one side of the thermoelectric pair, and a second header coupled to a second side of the thermoelectric pair. The thermoelectric pair has a termal conduction channel area smaller than an area of the first header or the second header such that the thermal conduction area is a fraction of the area of the first header or the second header.

Description

TITLE OF THE INVENTION
THIN FILM THERMOELECTRIC DEVICES FOR POWER CONVERSION AND COOLING
Statement Regarding Federally Sponsored Research The U.S. Government, by the following contracts, may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms, as provided for by the terms in High-Performance Thin-film Thermoelectric Devices for Cooling and Power Generation, DARPA/ONR Contract No. N00014-97-C-0211, Thin-film Thermoelectric Palm Power Technologies, DAR A/ARO Contract No. DAAD19-01-C-0070, and Meta-Material Structures for Super-Radiant Structures, DARPA/AFOSR Contract No. F49620-01-C-0038.
Cross- Reference to Related Documents This application is related to U.S. Provisional Application No. 60/372, 139 entitled "Thermoelectric device technology utilizing double-sided Peltier junctions" filed on April 15, 2002, the entire contents of which is incoφorated herein by reference. This application is related to U.S. Pat No 6,300,150 entitled "Thin-film thermoelectric device and fabrication method of same" issued October 9, 2001, the entire contents of which is incoφorated herein by reference. This application is related to U.S. Pat No 6,071,351 entitled "Low temperature chemical vapor depositor and etching apparatus and method" issued June 6, 2002, the entire contents of which is incoφorated herein by reference. This application is related to U.S. Pat No 6,505,468 entitled "Cascade cryogenic thermoelectric cooler for cryogenic and room temperature applications" issued January1 14, 2003, the entire contents of which is incoφorated herein by reference. This application is also related to U.S. Provisional Application No. 60/253,743 entitled "Spontaneous emission enhanced heat transport method and structures for cooling, sensing, and power generation", filed November 29, 2000, the entire contents of which is incoφorated herein by reference, and subsequently filed as PCT Application No. PCT US01/44517 filed November 29, 2001. This application is related to U.S. Provisional Application No. 60/428,753, "Three-Thermal-Terminal (T3) Trans-Thermoelectric Device", filed November 25, 2002, the entire contents of which is incoφorated herein by reference.
DISCUSSION OF THE BACKGROUND Field of the Invention This invention relates to thermoelectric devices for power conversion and cooling which utilize thin film thermoelectric materials.
Background of the Invention A thermoelectric device can produce electrical energy when a heat flux flows through opposite conductivity types of a thermoelectric material. Furthermore, a thermoelectric device can cool an attached object when a current is flown in an appropriate direction through the thermoelectric material. In a thermoelectric generator, for example, a Seebeck voltage generated across a thermoelectric device can be used to drive a current in a connected load circuit. Indeed, thermoelectric generators can generate power from a variety of heat sources. For example, a thermoelectric power device can be connected to a wall of a combustion chamber and therefore generate power from the heat flux flowing from the combustion wall. In this example, a fuel is burnt to produce the heat flux for thermoelectric power generation; however, other heat sources applicable for thermoelectric generators include solar-heated sources, radioisotope-heated sources, and nuclear reactor waste heat sources. Thermoelectric generators typically include external electronic circuitry such as a DC-DC conversion circuits or DC-AC converters for receiving power from the thermoelectric elements at a low voltage and for delivering power at higher voltages. In conventional thermoelectric devices, the efficiency of thermoelectric conversion is at about 6 to 8%. Accordingly, there is a drawback that thermoelectric conversion efficiency is low as compared with the other direct energy conversion systems such as for example a fuel cell device. If thermoelectric devices had higher conversion efficiencies, then thermoelectric power conversion devices could be effectively employed even in automobiles to increase the total fuel efficiency of the automobile by converting waste heat spent from the combustion process into electrical power. Low efficiencies have also limited the application of thermoelectric devices in cooling and refrigeration applications. Chen et al. in U.S. Pat. No. 5,713,208, the entire contents of which are incoφorated by reference, describe a thermoelectric cooling apparatus which includes a plurality of thermoelectric coolers each having a hot side connected to a heat sink and a cold side coupled to an object to be cooled. While Chen et al. describe that thermoelectric coolers may be any suitable thermoelectric coolers such as those supplied by Melcor under catalog number CP 2-127-06L, the efficiency of these coolers, like those of the afore-mentioned power conversion thermoelectric devices, are limited. The dimensionless thermoelectric figure of merit (ZT) is a measure of the effectiveness of the material for both cooling and power conversion applications. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) convert thermal to electrical energy as the electrons move across a temperature gradient. At a given temperature, the thermoelectric figure of merit ZT for a given material is maximized at an optimum doping level. In most materials, the thermoelectric figure of merit ZT is maximized at doping levels of approximately 10 cm' . Currently, the best non- superlattice thermoelectric materials have a maximum ZT of approximately 1. Today, bulk thermoelectric materials based on p-Bix Sb2.x Te3 and n-Bi2 Te .x Sex do not have a sufficient figure-of-merit (ZT) to allow economical application in many power conversion situations. In contrast to bulk materials, the thermal properties of superlattice structures can be improved over that of the thermal properties of bulk materials. Superlattice structures in thermoelectric materials have been investigated as structures whose engineered properties can lead to better semiconductor and thermoelectric properties. The fabrication of a superlattice by molecular beam epitaxy (MBE), or other known epitaxial growth techniques, is generally known. The choice of materials and the relative amounts of the materials which make up the superlattice are factors in determining the characteristics of the superlattice. For use as a thermoelectric material, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit (ZT) and Seebeck coefficient (S) are maximized. Thus, superlattice materials are expected to have higher ZT values than bulk-materials. Despite the higher ZT of superlattice thin-film materials, thin film thermoelectric devices are presently limited by thermal mismatch and temperature gradient issues and also are practically limited by the high cost of thin-film superlattice materials. SUMMARY OF THE INVENTION One object of the present invention is to provide a thermoelectric device structure utilizing high ZT thermoelectric materials. Another object of the present invention is to utilize a thermoelectric device structure in which the thermoelectric properties of the device are not contravened by inadvertent heat flux around the active thermoelectric elements in the device. A further object of the present invention is to provide a thermoelectric device structure for cooling power-dissipating devices. Still another object of the present invention is to provide a thermoelectric device which can convert power from a wide variety of moderate to high temperature heat sources, including but not limited to fuel sources. Accordingly, one object of the present invention is to provide a multi-stage thermoelectric device for power conversion from heat sources at temperatures of 600 to 850K. Still another object of the present invention is to provide a thermoelectric device which can be converted from a power conversion device to a heat pump device should coolant to a heat sink of the thermoelectric device be interrupted. Yet another object of the present invention is to provide a thermoelectric device which utilizes both bulk thermoelectrics and superlattice thermoelectric films to exploit for each respective thermoelectric material those thermoelectric properties in operational temperature ranges most suitable for each. Accordingly, one object of the present invention is to provide a thermoelectric device in which there is a high internal heat-flux within an individual thermoelectric stage and a low external heat-flux across the entirety of the thermoelectric device. Various of these and other objects of the present invention are accomplished in several embodiments of the present invention. One exemplary embodiment includes a novel thermoelectric device having at least one thermoelectric unit including a thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, and a thermal impedance increasing device disposed between the thermoelectric pair and one of the first and second headers.
BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1A is a schematic of one embodiment of a thermoelectric device according to the present invention; Figure IB is a schematic of a thermoelectric device according to the present invention operating in a High Active Flux- Low Input-Output Flux mode; Figure 2 is a schematic of a multi-stage thermoelectric device according to the present invention; Figure 3 is a graph depicting variations of ZT with temperature for various material systems useful in the present invention; Figure 4 is a graph depicting state-of-the-art ZT vs carrier concentration for p and n-type Bi2Te3-based superlattice elements; Figure 5 is a table of estimated design efficiencies for a three-stage thermoelectric device of the present invention; Figure 6A is a schematic illustration depicting a thermoelectric device module of the present invention integrating a series of p-n couples on a single stage; Figure 6B(l)-(3) depict tests on p-n couple devices of the present invention for open-circuit voltage (Voc), power (P), and power density (Pd in W/cm ; Figure 6C is a table illustrating the inverted processing approach of the present invention; i Figure J7A is a schematic depicting the attachment of a pre-fabricated thermoelectric device of the present invention onto a semiconductor device chip; Figure 7B is a schematic depicting the attachment of a thermoelectric device of the present invention onto a semiconductor device chip; Figure 7C is a schematic depicting the sequential formation of a thermoelectric device on the present invention onto a semiconductor device chip. Figure 8 is a schematic of a multi-stage thermoelectric device of the present invention employing resonant thermal energy transfer using Purcell-cavity enhancement effects and proximity-coupling of IR modes between various stages; Figure 9 is a plot of apparent emission intensity as a function of the surface temperature of different engineered structures according to the present invention; Figure 10 is a schematic arrangement of a three stage of thermoelectric device of the present invention with a split-header to provide thermal expansion relief; Figure 11 is an electrical diagram showing a configuration for operating a thermoelectric device of the present invention in either a cooling mode or a heat pump mode; Figure 12A is a schematic depicting a coupling structure for coupling a thermoelectric device to a semiconductor device chip; Figure 12B is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip; Figure 13 is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip; Figure 14 is a schematic depicting a coupling structure including vias for electrical connections to the thermoelectric devices; and Figure 15 is a schematic depicting a coupling structure including high thermal conductivity materials in a heat sink plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical, or corresponding parts throughout the several views, and more particularly to Figure 1A thereof, Figure 1 A depicts a schematic of one embodiment of a thermoelectric device according to the present invention. As shown in Figure 1 A, a thermoelectric device 1 of the present invention includes a thermoelectric pair of n and p-type thermoelements 2a and 2b, respectively. The thermoelectric pair is connected thermally in parallel for heat conduction and electrically in series for electrical conduction. The thermoelectric pair of n and p-type thermoelements 2a and 2b are electrically adjoined together on the side coupled to an upper side header 3 by electrical connection 6, and are electrically connected separately on the side coupled to a lower side header 4 by electrical connection 7. As shown illustratively in Figure 1 A, the thermoelectric pair of n and p-type thermoelements 2a and 2b can be coupled to the upper side header 3 by a heat pipe 5 made of high thermal conductivity materials such as Si, SiC, A1N, etc. In this example of the present invention, heat flowing in the thermoelectric device 1 is channeled in the heat pipe 5 into the thermoelectric pair of n and p-type thermoelements 2a and 2b. In one embodiment of the present invention, due to the different thermoelectric properties of the n and p-type materials of the thermoelements 2a and 2b, electrons and holes in these materials diffuse at different rates across the respective n and p-type thermoelements, thereby creating a voltage difference across the pair of n and p-type thermoelements. In this embodiment, a cooling device (not shown) is coupled to the lower side header 4 to permit dissipation of heat. Otherwise, the upper side header 3, the pair of n and p-type thermoelements 2a and 2b, and the lower side header 4 would all come to nearly equivalent temperatures. As a consequence, the electrons and holes in the n and p-type thermoelements 2a and 2b would stop diffusing to the lower-side header. In another embodiment of the present invention, electron and hole current can driven through the n and p-type thermoelements 2a and 2b by an applied voltage to thereby cool the upper side header 3. Owing to electrical barriers existing between electrical connection 6 and the pair of n and p-type thermoelements 2a and 2b, the electron and hole current will preferentially transport those electrons and holes having a higher thermal energy, thereby cooling the upper side header 3. A heat dissipation device (not shown) coupled to the lower side header 4 permits dissipation of heat from the device, else the lower side header 4 would come to a temperature such that heat, via diffusion of phonons, would flow back to the upper-side header 3 at a rate equal to the heat being carried from the upper-side header 3 by the electron and hole current, thereby eliminating the cooling. The optional heat pipe 5 represents a mechanism to increase the thermal impedance (through the gaps) between the upper side header 3 and the lower side header 4 to a value significantly larger (by a factor of 100 or more) than the thermal impedance across the thermoelectric pair 2a and 2b. As shown in Figure 1 A, the heat pipe 5 is elongated in a direction transverse to the thermoelectric device 1 (i.e., normal to the upper side header 3). The heat pipe 5 has a cross-sectional area smaller than that of an area of the upper side header 3. The heat pipe 5 of the present invention provides an offset, spacing apart the upper side header 3 from the lower side header 4. The offset permits for example thin-film thermoelements to be used without thermal conduction from the upper side header 3 to the lower side header 4 shunting across the gap between therebetween, thereby forcing heat flux to flow through the thermoelements 2a and 2b. The offset in one embodiment of the present invention is at least 25 μm and more preferably in the range of 250 to 500 μm. As such, the offset produces a thermal impedance across a gap, where thermoelectric elements do not exist, between the upper side header 3 and the lower side header 4 whose value is at much larger than the thermal impedance across the pair of thermoelements 2a and 2b. For example, a thermal impedance across an air gap is greater than the thermal impedances for the thermoelements 2a and 2b, typically by a factor of 500 to 1000, for similar areas of air gap and thermoelectric materials. Thus, the offset allows this ratio to increase, using the high thermal conductivity of the offset relative to the air-gap. The thickness of the offset can be reduced as the pressure level in the gap is reduced i.e, with a high vacuum level, the thermal conductivity of the gap is reduced and so we need less of the offset thickness. For many applications, the offset will be at least 10 μm, and for other applications in a range of 100-500 μm. Regardless of the heat pipe, the thermoelectric devices of the present invention conduct a high active heat flux, while maintaining a much lower flux through the surfaces 1 and 4, through the pair of thermoelements which permits the present invention to realize ultra-high specific power in the pair of n and p-type thermoelements 2a and 2b. Thus, the present invention operates with a high active flux through each of the thermoelements while having a low input/output flux across the entirety of the thermoelectric device. As described herein, this aspect of the present invention is referred as High Active Flux- Low Input-Output Flux (HAF-LIOF). This aspect is illustrated pictorially in Figure IB. In this aspect, the heat flux through the heat gatherer (e.g. an upper side header 3) and the heat spreader (e.g. an lower side header 4) is smaller than the heat flux through the n and p-type thermoelements 2a and 2b due to the reduced packing fraction of the thermoelements. A packing fraction of the thermoelements relative to the area of the heat spreader(s) permits the utilization of thinner thermoelements, thereby reducing the fabrication costs that would be involved should for example thicker sections of high ZT materials be required to maintain the requisite ΔT across the thermoelectric device. The packing fraction of thermoelements (i.e., the fraction of area occupied by the pair of n and p-type thermoelements 2a and 2b relative to a unit area of for example the upper-side header 3) in one embodiment of the present invention is less than 50%, less than 20%, and can be significantly lower, 0.5-1% for example. A unit area for the heat spreader is defined as that fraction of the total area of the heat spreader which principally conducts heat into one of the associated pairs of n and p-type thermoelements attached to the heat spreader. For the single pair of n and p-type thermoelements shown in Figure IB, the unit area would be the entire area of the upper side header 3. For instance, while the total heat flux into upper-side header 3 and out the lower-side header 4 can be on the order of 10 - 30 W/cm2, the heat flux through each thermoelectric pair 2a and 2b can be as much as a factor of 100 times higher. The preferred packing fraction is a function of the heat flux available at the heat- source, the required ΔT needed across the thermoelement to achieve the maximum efficiency, and the heat-flux that can be dissipated at the heat-sink. For example, if the required heat-flux through the device element is 2500 W/cm to generate a requisite ΔT, and the heat-flux that can be efficiently dissipated at the heat-sink is 25 W/cm2, then a packing fraction of 1% will be used (i.e. 25/2500 x 100%). However, if the heat-flux that can efficiently be dissipated at the sink is only 2.5 W/cm , then a packing fraction needed would be 0.1% for the same heat-flux of 2500 W/cm through each thermoelectric pair. A smaller packing fraction, however, can result in increased parasitic thermal transfer losses through any medium (such as air, Nitrogen, Helium) between the hot and cold-sides of the thermoelectric module. Therefore, in thermoelectric devices of the present invention, besides using a heat pipe, other thermal impedance increasing devices can be employed. For example, a partial vacuum (i.e. ~1 Torr) as a low thermal impedance medium can be used in a module (e.g. the module 10 encapsulating the thermoelectric devices shown in Figure 6 A) containing the thermoelectric devices. Alternatively, the module 10 can be filled with a low conductivity gas such as for example Ar. The headers and heat pipes of the present invention have thermally conducting properties. Examples of suitable materials for the headers and heat pipes include for example A1N, !SiC, and diamond. These materials have relatively high thermal conductivities (e.g. 5-20 W/cm-K) and offer the advantage of being electrically insulating. Alternatively, metallic or other semiconductor materials can be used for the headers and heat pipes of the present invention provided a suitable electrical insulating layer 8 is added to the surface of those materials. The thermal conductance through the electrical insulating layer preferably presents no substantial impediment to vertical heat flow through the pair of n and p-type thermoelements 2a and 2b. For example, the headers of the present invention can be made from a Si substrate of a thermal conductivity of ~ 1.2 to 1.6 W/cm-K having a thin (i.e. ~10 nm to 1000 nm) SiO or SixNy layer of a thermal conductivity of- 0.015 W/cm-K deposited thereon. For example, the headers of the present invention can also be made from a Cu substrate of a thermal conductivity of ~ 4 W/cm-K having a thin (i.e. -100 to 1000 nm) SiO2 or SiχNy layer of a thermal conductivity of- 0.015 W/cm-K deposited thereon. If a SiC header or heat pipe is used that is electrically conducting (e.g. a doped SiC material), a similar insulating layer can be applied as well. Since the thermoelements are low voltage, high current devices, the electrical connections are preferably highly conductive having a low resistance (preferably 1/10th or less than that of the Ohmic resistance of the thermoelements). A low conductance for the electrical connections will undermine the efficiency of the thermoelectric stage by resistive losses in current flow through the thermoelectric device. Accordingly, in general, the present invention includes a thermoelectric device having a first header (e.g. the upper side header 3) coupled to a heat source, a pair of n- type and p-type thermoelements (e.g. the pair of n and p-type thermoelements 2a and 2b) coupled to the first header and configured to conduct heat from the first header, a second header (e.g. the lower side header 4) coupled to the pair of n-type and p-type thermoelements and configured to conduct heat from the thermoelectric pair, and a heat pipe coupled to the pair of n-type and p-type thermoelements (e.g. the heat pipe 5 connected to the thermoelectric pair 2a and 2b) and elongated in a transverse direction across the thermoelectric device to separate the first header from the second header. In one embodiment of the present invention, the thermoelectric devices cool the first header (and thus can cool heat dissipating devices coupled thereto) by the flow of an electrical current through electrical connections 6 and 7 through the thermoelements 2a and 2b, thus transporting heat in the electrical carriers (i.e., the electrons and holes) and thereby cooling the first header. In another embodiment of the present invention, the thermoelectric devices generate electrical power by the development of a voltage on the electrical connections 6 and 7 as heat flows from the first header (coupled to a heat source) through the thermoelements 2a and 2b to the second header (coupled to a heat sink). Further, the present invention is not limited to single stage and/or single pair thermoelement devices but can utilize multiple stages and/or multiple pairs of thermoelements in a single stage. Figure 2 depicts a multi-stage embodiment of the present invention. Specifically, Figure 2 is an illustrative schematic of a multi-stage or multi-unit thermoelectric device of the present invention including three thermoelectric stages or units. The device depicted in Figure 2 is shown by way of example in a configuration for thermoelectric power conversion and, for simplicity, is discussed below only in a context of power conversion. However, the features of the present invention depicted in Figure 2 and utilizing multiple stages and/or multiple pairs of thermoelements in a stage are, according to the present invention, applicable to thermoelectric cooling as well. Further, the number of stages and the number of thermoelectric pairs per stage in the present invention are not restricted to that illustrated in Figure 2. As shown in Figure 2, an upper header 12 is coupled to a heat source 14 and includes at least one upper thermoelectric stage 16 coupled to the upper header 12. Of the thermoelectric stages 16, 17, and 18 depicted in Figure 2, at least one of the depicted thermoelectric stages is dimensioned such that area for thermal conduction in the thermoelectric stage through a pair of thermoelements 20a and 20b, herein referred to as the active thermal conduction channel area, is smaller than above-noted unit area of header 22 to which the pair of thermoelements 20a and 20b are coupled to. Such dimensioning as discussed with regard to Figure 1 establishes the HAF-LIOF aspect for the multi-stage thermoelectric device of Figure 2. As shown in Figure 2, the upper thermoelectric stage 16 is coupled to at least one lower thermoelectric stage, e.g. stage 17. The lower thermoelectric stage 17 like the upper thermoelectric stage 16 has an active thermal conduction channel area through the pair of thermoelements 20a and 20b that is smaller in area than the unit area of header 22. In the device depicted in Figure 2, heat is dissipated from the lower-most thermoelectric stage 18 by a heat sink 24 coupled thereto. Accordingly, heat pipes in the various stages also have at least one thermal conduction channel area that is smaller than the unit areas of the associated headers. In the multi-stage thermoelectric device depicted in Figure 2, a high internal heat-flux exists within the individual thermoelectric stages (e.g., -1800 W/cm2 for a ΔT across each stage of 85K to as high as -2300 W/cm with a ΔT across each stage of 107K). Meanwhile, a low external heat-flux exists across the entirety of the thermoelectric device (e.g., a range from 15 to 30 W/cm ). In a multi-stage device, the packing fraction and hence the active flux through each stage can be different or similar to the active heat flux in other stages. More specifically, the multi-stage thermoelectric device as illustrated in Figure 2 can include a high-temperature thermoelectric conversion stage (i.e. the upper thermoelectric stage 16), for example an N-SiGe/P-TAGS thermoelement pair (e.g., with a -100 μm active region thickness). TAGS refers to a Tellurium, Antimony, Germanium, and Silver (TAGS) alloy composition. TAGS compositions are in general denoted by (AgSbTe )ι-x(GeTe)x. The most optimal composition according to the present invention for x, the GeTe mole fraction, is -0.80 to 0.85. As illustrated in Figure 2, the high-temperature thermoelectric conversion stage is coupled to a mid- temperature thermoelectric conversion stage (i.e. stage 17), for example an N-PbTe/P- TAGS thermoelement. As illustrated in Figure 2, the mid-temperature thermoelectric conversion stage is coupled to a lower temperature thermoelectric conversion stage (i.e. the thermoelectric stage 18), for example an n-type and p-type Bi2Te3-superlattice-thin- film thermoelement pair having a 5 to 10 μm in thickness. As shown in Figure 2, the lower header 23 constitutes a split-header having slits formed partially (in the top side and correspondingly at certain locations on bottom side not shown) in a body of the lower header 23 to provide thermal expansion relief. In one embodiment of the present invention, the splits are preferably not located near the devices. The three-stage thermoelectric device depicted in Figure 2 has a design efficiency in a range of 20% and electrical power density in excess of 5 W/cm , and is capable of power conversion in a range of tens of mWatts to as high as 1 MWatt when coupled to a heat source at temperatures approaching 875K. Design efficiencies in excess of 20%> and the capability to operate with hot-sides in the range of 675 to 875K permit flexibility and adaptability to real-world systems. With hot-sides in the range of 675 to 875K, higher Carnot efficiencies are available then would be available for example in thermoelectric devices restricted to hot-sides of 375K to 575K. In turn, higher Carnot efficiency provides for higher system efficiencies which in turn facilitates load balancing by reducing the number of modules (i.e., the number of thermoelectric pairs) required to convert a given heat flux into electrical power.
Materials Selection While the concepts of the present invention are not restricted to any particular family of thermoelectric materials, the use of superlattice materials improves thermoelectric device efficiency of the multi-stage thermoelectric device of the present invention. Conventional bulk Bi2Te3 thermoelectric devices offered by Hi-Z Inc. (San Diego, CA) exhibits a power density of 0.34 W/cm2 at a ΔT of 200K for a specific power of -0.165 W/gm. Yet, even unoptimized thin-film Bi2Te3-superlattice mini- module devices fabricated by the HAF-LIOF aspect of the present invention, even without the mid and high temperature thermoelectric power conversion stages discussed above, have demonstrated a power density of 0.7 W/cm2, with a ΔT of 77K, and a specific power of 16.7 W/gm. Specific power defined as the power produced per unit weight of the thermoelectric device. Both the specific power and the power density levels would increase with the addition of mid and high-temperature stages and further optimization of the low-temperature superlattice stage. Accordingly, the thermoelectric devices of the present invention can realize a specific power greater than 1 W/gm and a power density greater than 0.5 W/cm . On the other hand, specific powers in a range of less than 0.0001 W/gm, 0.001 W/gm, and 0.01 W/gm, are possible with the present invention and are applicable to applications such as for example bio-medical devices. In these applications, the HAF- LIOF devices could be used for heat gathering from low temperature sources in a range of 30-40°C.such as the human body for the powering of pacemakers or neuro- stimulators used to alleviate symptoms of Parkinson's disease or other neurological disorders. Figure 3 is a graph depicting variations of ZT with temperature for various material systems useful in the present invention. Knowledge of the variation of ZT with temperature in suitable temperature-stable material systems, such as shown in Figure 3, permits appropriate materials selection for the above-described multi-stage design. The material selections should, according to the present invention, provide a material for each thermoelectric stage which is robust in the temperature range in which the thermoelectric stage is intended to operate. For example, devices in PbTe/PbSe and Si/Ge superlattice material systems would be useful for a temperature range of 475K- 675K and 675K-875K, respectively, in the present invention. The relevant properties, the lattice-mismatch and the bandgaps, of the PbTe/PbSe superlattice system are shown in Table 1. Table 1.
Both materials PbTe and PbSe have similar thermal expansion coefficients; so that lattice mismatch at higher temperatures would be similar to that at 300K. Also, kT at the temperatures of 500 to 650K is about 0.05 eV, comparable or larger to the bandgap difference between PbTe and PbSe. Thus, no barriers or quantum confinement effects will be present within the material, but a significant thermal conductivity reduction is expected in these superlattice materials. Regarding the SiGe materials family, Si/Ge superlattice materials, according to the present invention, are attractive for high temperature applications (i.e., 650 to 850K). For example, Si/Ge superlattice materials deposited at -1000K have shown a dramatic reduction in thermal conductivity and concomitantly an enhancement in ZT. Although Si/Ge superlattices may only offer a ZT of only -0.8 at 300K, significantly below that of a Bi2Te /Sb2Te3 superlattice or a Bi2Te /Bi2Te3.xSex superlattice, the Si/Ge superlattices are likely to offer a much higher ZT at the higher temperatures employed in the upper thermoelectric stages depicted in Figure 2. Further, the SiGe superlattices will have higher ZT values than associated SiGe alloys of the same Ge concentration, leading to higher efficiencies. Indeed, development of high ZT thin films in the PbTe/PbSe and in the SiGe material systems have been described by M. Lee et al. in Appl. Phys. Lett., 70, 2957 (1997) and R. Venkatasubramanian, Phys. Rev. B 61, p. 3091 (2000) and by R. Venkatasubramanian et al. in Proc. of 17th International Conference on Thermoelectrics, 191, (1998), the entire contents of which are incoφorated herein by reference. In addition to superlattices, other quantum-confined structures and structures can be utilized according to the present invention in which a strong thermal conductivity reduction is obtained while at the same time maintaining electrical conduction across the thermoelectric stages. For example, PbTe-based quantum-dot superlattices (QDSL) are suitable materials for the n and p-type thermoelements of the present invention. ZT values for these materials are achievable in a range of 1.5 to 2, in the temperature range of450K-550K. Furthermore, even ZnSb and skutterrudites, (i.e., materials with a known ZT >1 over a temperature range suitable for power conversion) are also applicable in the present invention provided these materials are in thin substrate form (i.e. in a thickness of 100 to 200 μm) and are used in a temperature range of 500K to 900K. Thus, the HAF-LIOF, high-power density, multi-stage power conversion device concept of the present invention is not limited to Bi2Te3-superlattice/PbTe/SiGe material combinations. Indeed, superlattices of Si/Ge, PbTe/PbSe, ZnSb/CdSb, InAs/InSb, CdTe/HgCdTe, GaxInι.x As/Gay In i-yAs can be used in the high-temperature and mid-temperature thermoelectric conversion stages, discussed above, thus providing a cascade thermoelectric device utilizing all superlattice materials. For the lower temperature thermoelectric conversion stages of the present invention and for single stage cooling devices operating near room temperature, superlattice stages of Bi2Te3-Sb2Te3 can be used. Figure 4 is a graph depicting state-of- the-art ZT vs carrier concentration for p and n-type Bi2Te3-based superlattice elements. Figure 4 shows a ZT-3.5 in p-type superlattices and -2.0 in n-type. According to the present invention, by control and engineering of dopant carrier levels , the 300K figure- of-merit (ZT) achievable with p-type Bi2Te3/Sb2Te3 and n-type Bi2Te3/Bi2Te2. Se0.3 superlattice device elements is improved from previously known values of -2.4 and -1.2 to values greater than 3.5 and -2, respectively. The carrier levels in the p-type materials as shown in Figure 4 are controlled by varying the organometallic Tellurium flow-rate during growth, for a fixed growth rate of the superlattices which in turn can be controlled by the flow-rate of the organometallic Bismuth for Bi2Te3 and organometallic Sb for Sb2Te3. The carrier levels, for a constant growth rate, can also be controlled by control of growth temperature over a limited range. For the n-type materials, the doping levels as shown in Figure 4 can be controlled by the growth rates of Bi2Te and Bi2Te2.7Se03 layers in the superlattice as well as the growth temperatures. A value of peak ZT of -2.9 demonstrated in the present invention for a 10A/50A p-type Bi2Te3- Sb2Te3 superlattice is one of the highest peak ZT values observed at 300K for the p-type Bi2Te3-Sb2Te3 superlattice material system. Such improvements in ZT permit improved power conversion efficiencies and cooling efficiencies for the thermoelectric devices of the present indention.
Device Performance In the present invention, the advances in ZT for the p and n-type superlattice materials have been incoφorated into fabricated p-n thermoelement couples. By flipping the fabricated p-n couples onto a split semi-infinite Cu-plate as opposed to flipping the fabricated p-n couples onto a header with a limited-thickness metallization, a more accurate measure of the thermoelectric properties of the fabricated p-n thermoelement couples is possible. For instance, each one of the p-n thermoelement couples potentially carries a high-current (i.e., several Amps of current) and is a low- voltage thermoelectric device. Parasitic lead resistances can therefore affect the measured device efficiencies. Properties of the p-n thermoelement couples have been developed by fabrication and analysis of the p-n thermoelement couples on a split semi- infinite Cu-plate. In the fabricated thermoelectric devices, electrical contact resistances in the thermoelectric devices are minimized between the metallizations and the p-n thermoelement couples, providing adequate electrical interfacing. In addition to managing electrical contact resistances, thermal interface resistances are reduced in the metal-to-dielectric interfaces of the present invention by deep-annealing of a metal into the dielectric bulk or by utilizing an AlN-diffused Al-Cu interface. Further, high aspect-ratio thermoelectric devices (either with thicker superlattice films or smaller-area devices) are provided by appropriate patterning and etching of the p-n couples (e.g. the thermoelectric pair 2a and 2b). An aspect ratio of the element is defined as the ratio of thickness/area of element in units of cm"1. Typical thickness of thermoelectric elements in the low-temperature stage can be about 5 to 20 microns, while the mid and high temperature stages can have thicknesses ranging from 50 to 250 microns. The aspect ratio, hence the area of each element of the various stages, are based on the thermal conductivity of the materials (noting that the thermal conductivities of the p and n-type elements within a stage can be different and so their aspect ratios to allow same ΔT across both p and n-type elements of each stage can be different) of the various stages and the required ΔT for each stage for maximizing the efficiency of the overall set of stages. The ΔT of each stage is inversely proportional to thermal conductivity of the material and directly proportional to the aspect ratio. Hence, according to the present invention, the total heat flux through each stage is same; thus the aspect ratio (and the associated packing fraction) in each stage is adjusted with this criterion to meet the design ΔT for each stage. Power conversion efficiencies achievable in for example the three-stage thermoelectric devices of the present invention are calculable based on known equations. The maximum efficiency is obtained when the load ratio r, defined as RL/RG with RL being the load resistance and RG being the thermoelectric device internal resistance, is optimized for average ZT over the entire temperature range, ZTm. See, for example, H.J. Goldsmid, Electronic Refrigeration, Pion Limited, (1986). Once r is known as per eqn. (1) shown below, the device efficiency ψ can be obtained by eqn. (2), shown below for a given ΔT, temperature differential, and mean operating temperature Tm. r = (l+ZTm)1/2 ..(1) ψ = ΔT/{[(r+l)/(r-l)]*Tm + ΔT/2} ..(2)
Power conversion efficiency estimates are shown in Table 2 of Figure 5, for a range of ZT values for each stage. The power conversion efficiency estimates are shown in a range from upper to midrange to lower projected values. The projections in Table 2 indicate that an efficiency in the range of 9.6% to 11.4% for a ΔT of 150°C in the low-temperature Bi Te -SL-based stage can be realized for a ZT value of 2 to 2.5. An efficiency in the range of 4.3 to 4.7%. for a ΔT of 175 to 200 °C in the n-PbTe/p- TAGS-based thin N-PbTe/P-TAGS thin-substrate mid-temperature stage can be realized for a ZT of -0.75. An efficiency in the range of 3.4 to 3.7% for a ΔT of 200°C N- SiGe/P-TAGS thin-substrate high-temperature stage can be realized for a constant ZT of -0.75. Thus, according to the present invention, a thermoelectric efficiency of —19.4% to 17.7%) and a power density in excess of 5 W/cm are obtainable in the three-stage thermoelectric power conversion devices of the present invention. Indeed, Table 2 illustrates the performance of various stages of a three-stage thermoelectric device of the present invention as a function of ZT and the efficacy of thermal management at the heat-sink. The calculations in Table 2 are based on a module size of approximately -33 cm2. For these examples, a heat flux of -30 W/cm2 is assumed to flow from the heat-source (i.e. the upper-most header) to the heat sink (i.e. the lower-most header), yielding a heat-flux dissipation range of -24.7 to 22.7 W/cm2 at the heat-sink. The power densities are calculated assuming a 5-μm thick superlattice element in the low-temperature Bi2Te3-super lattice stage and assuming a 100-μm thin- substrate for bulk thermoelements in the mid and high temperature stages. Table 2 indicates that higher ZT materials for both the mid-stage (460 -660K) and high-stage (670-890K) are advantageous. For a given heat flow Q, in Watts, the aspect ratio determines the ΔT. The efficiency is a function of ΔT and ZT. The power generated P, in Watts, is a product of Q times efficiency. Thus, power density, Pd, is a function of both area, thickness, ZT and Q. Hence, according to the present invention, one approach to achieve power densities of greater than 20 W/cm2 is to use thinner sections of thermoelement materials (e.g., preferably less than 300 μm). Calculations of thermal -to-electrical conversion efficiency, corrected for lead resistances, versus internal ΔT across the flipped inverted p-n couple, and the ZT obtained from the efficiency equations indicate that about 85% of the external ΔT (and consequently the heat flux) is applied across the device. The 15% loss is primarily due to the thermal interface resistance between the metallization and the A1N header.
According to the present invention, the thermal interface resistance can be reduced in the metal-to-dielectric interface by deep-annealing of the metal into the dielectric bulk or by utilizing an AlN-diffused Al-Cu interface, as discussed above. Calculated ZT values of the inverted couples utilized for demonstration of the power conversion concepts of the present invention were about a ZT value of 1.1 at 300K. The average ZT of the p-n couple of the present invention (e.g. the thermoelectric pair 2a and 2b) increases with temperature, consistent with the general trend of ZT shown in Figure 3. One p-n couple of the present invention was tested up to a ΔT of nearly 85K, corresponding to an active device heat flux within the thermolement of -1800 W/cm2. For this tested p-n couple, the external heat flux was about 28 W/cm2 with a temperature rise of about 28K at the heat-sink. Application of improved thermal management practices to operate even this test device with a ΔT of 150K, even with the present ZT of -1.5, will permit an efficiency of ~8.6%> to be realized. Improvements in ZT values from 1.5 to -2 will realize in the present invention efficiencies of -11 % for a ΔT of 150K. Accordingly, the present invention in one preferred embodiment includes a system for thermoelectric power conversion. The system includes a thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, a thermal impedance increasing device disposed between the thermoelectric pair and one of the first and second headers, and a heat sink coupled to the second header which dissipates heat at a rate which maintains a temperature of the second header below a temperature of the first header. Further, a coupling member can be used to couple the first header to a heat source. The heat source can be a combustion source, a radioactive source, a solar-heated source, and an electrical power device. The coupling member in one embodiment of the present invention can be a thermally conducting member having a thermal conductance of at least Al and extending between said first header and said heat source. Alternatively, the coupling member can be a steam pipe or a combustion exhaust pipe transporting heat flux by a convective medium such as for example steam or combustion gases.
Figure 6A is a schematic illustration depicting a thermoelectric device module of the present invention integrating a series of p-n couples on a single stage. Tests on p-n couple devices for open-circuit voltage (Voc), power (P), and power density (Pd in W/cm ) are shown in Figure 6B. Tests on an integrated 16 p-n couple devices show expected open-circuit voltage (Voc), power (P), and power density (Pd in W/cm2). In particular, the open-circuit voltage has been observed to scale in the thermoelectric module to almost sixteen times that of a single-couple, for similar ΔT to the individual p-n couple, further supporting the HAF-LIOF concept. The power levels of 38 mW per couple, for a 600 μm x 600 μm device and for a ΔT of about 107K correspond to an electrical power density of as much as -10.2 W/cm2. The power level scales to 166 mWatts per couple for the 16-couple module for a ΔT of 77K.
Both p-type and n-type thin-film superlattice thermoelements of the present invention have been stable. Exposures to temperatures of -450K and times of up to 60 hours have showed no marked deterioration in the device ZT of these elements, as shown in Table 3, below. Table 3.
Type ZT as made t ZT after t anneaι ZT after t anneal ZT after t anneal ZT after t anneal ~| 0 UTS. ~ 15 hrs. - 30 hrs. - 45 hrs. anneal ~ 60 hrs. 1.7 ± 0.25 2.0 ± 0.25 2.25 ± 0.25 2.25 ± 0.25 1.95 ± 0.25
N 0.59 ± 0.1 0.68 ± 0.1 0.84 ± 0.15 0.71 ± 0.1 0.76 ± 0.15
Besides superlattices, the device fabrication methodology and the HAF-LIOF aspect of the present invention are applicable to thin substrates from two non- superlattice material systems, GaSb and InAs, corresponding to the bandgaps of the n- SiGe/p-TAGS and n-PbTe/p-TAGS systems. Open-circuit voltage as a function of ΔT in a 100 element GaSb and InAs thin-substrate module of the present invention hare capable of Voc values of -4.5 Volts to -3.5 Volts and are capable to generate ΔT in excess of 100K, corresponding to active heat fluxes of -300 W/cm2, needed for achieving high electrical power densities.
Manufacture of the Thermoelectric Devices and Modules Various methods are available for the manufacturing of the devices and the device components of the present invention. Given below for puφoses of illustration are steps and methods involved in fabricating the individual p and n-type thermoelectric pairs of the present invention and are steps and methods for integration of the thermoelectric pairs into modules. The steps and method described below are illustrative and are not given to imply any limitations on the present invention. The thermoelectric materials utilized as the p and n-type thermoelements of the present invention are typically overgrown films on commercial substrates, the overgrown films having a thickness of for example 5 to 20 μm. Substrates for the growth of, for example the thin-film superlattices or other thin film thermoelectric structures, may have the same conductivity type as the overgrown films or may permit the overgrowth of opposite conductivity type films. The overgrown films can be metallized to provide a low-resistance contact, such as a low-resistance Peltier contact. In one fabrication method of the present invention, fabrication of the thermoelectric devices of the present invention is facilitated by inverted-couple processing. Table 4 shown in Figure 6C illustrates the inverted processing approach of the present invention. In inverted-couple processing, one leg of a p thermoelement and one leg of an n thermolement are attached a common electrical member adjoining the two elements. Fabricating the individual n-type and p-type thermoelements in each leg of the thermoelectric power conversion stages can occur in one example of the present invention by forming n-type and p-type films on separate substrates, and then attaching the individual n-type and p-type thermoelements to a common header. Alternatively, films of a specific type can be formed on a common substrate and then selectively converting (e.g. by selective ion implantation) different regions of the formed thermoelectric material into n-type and p-type regions. For example, the thermoelements could be formed intrinsic (or specifically doped) and then type- converted to another (i.e., a p-type to n-type conversion or an n-type to p-type conversion) by for example impurity-diffusion. Type-conversion would then be performed at a convenient stage in the manufacturing process. Regardless of the specific details, a set of p and n-type materials having a pre-arranged pattern are attached to a common header like SiC, A1N , low-resistivity silicon, and silicon with a thin insulating layer. In one fabrication method of the present invention, substrates including the individual n-type and p-type thermoelements are separated, for example by scribing or laser dicing, into individual segments. The individual segments are then bonded onto a header such that alternating n-type and p-type conductivity materials exist between each adjacent thermoelement. The surfaces of the header that come in contact with the n- and p-type segments are preferably metallized prior to assembly to provide low-resistance electrical connection between adjacent n- and p-type segments where necessary to electrically interconnect adjacent thermoelements. Likewise, the surfaces of the individual n-type and p-type thermoelements that come in contact with the header are preferably metallized prior to assembly to provide the necessary low-resistance electrical connection to the n-type and p-type thermoelements, else a high electrical contact resistance can limit the efficiency of the thermoelectric stage and the resultant thermoelectric device. Following bonding of n- and p-type segments, the substrates from each of the p- and n-segments are selectively removed for example by using selective etchants. The thermoelements are then patterned using photolithographic patterning followed for example by etching, or by laser ablation, to produce the desired cross-sectional thermal conduction area for the present invention (i.e., to set the aspect ratio and ultimately determine the packing fraction). Low resistivity contact metallizations are then evaporated on! an upper surface of the n- and p-type thermoelements. In this step, either the same metallization can be used for both of the n- and p-type section, or different metallizations pan be used (i.e., separate evaporations), depending on the contact resistance requirements. Sheet resistances, or conductances as specified above, associated with the metallizations are designed not to restrict the performance of the thermoelectric devices. < A top, pre-patterned metallization header can, in one embodiment of the present invention, be attached to the metallized sections to function as the aforementioned heat pipe. Alternatively, the header itself prior to metallization can be patterned to provide the aforementioned heat pipe. The formed pair of thermoelements (i.e., the n thermoelement and the p-thermoelement) including the attached header can then be flipped and bonded to a second header. The second header, referred to for the puφose of illustration as a bottom header, thermally connects the n thermolelement to the p- thermoelement, but contains patterned electrical connections such that electrically the n thermoelement and the p-thermoelement are individually connected, as shown in Figures 1 A and IB. The bottom header thus functions as an electrical member having, as shown in Figures 1 A and IB, a split electrical contact (i.e., an electrical contact only contacting individually the n-type and p-type thermoelements), while as a thermal member the bottom header functions as a continuous thermal contact. Regardless of the formation approach (i.e. direct deposition or bonding), thick metallizations and pattering can be used, according to one embodiment of the present invention, to form the noted bottom header and to provide the afore-mentioned split electrical contact. Direct attachment by deposition or bonding would permit a large number of patterned thermoelements each possessing the requisite thermal conduction area to be fabricated and electrically connected in series to provide either an output electrical contact for power conversion or for cooling. Subsequent stages of thermoelectric devices could then be added using similar procedures, or by attaching subsequent pre-fabricated stages, or by attaching selective members of subsequent stages. One illustrative example of inverted couple processing is given below: P-type and n-type superlattice thermoelectric films are deposited on GaAs substrates. The deposited superlattice films are patterned with Cr/Au/Ni/Au metallizations. The deposited superlattice films and/or the GaAs substrates are etched in preparation for dicing. Diced strips of the p-type and n-type superlattice films attached to the GaAs substrates are then bonded in an alternating conductivity type pattern to a header. The header includes an AlN substrate having Ti/Au metallization (annealed) and having a subsequent Cu/No/Au topmost metallization. A Sn preform bonds the diced strips to the AlN substrate (functioning as a first header). The GaAs substrate is then etch removed, and Cr/Au/Ni/Au contacts are evaporated through shadow masks or evaporated and patterned to form electrical contacts to the superlattice thin films. Thick metal pads are then formed on the evaporated contacts to define for example a heat pipe structure. Dies containing the superlattice thin films and the attached AlN header are placed and bonded to a separate split metallized header (functioning as a second header) allowing for individual electrical connection to each of the n- and p-type thin film thermoelements. By way of example, Table 5 depicts various bonding steps used to fabricate the three stage device shown in Figure 2. As depicted in Table 5, in stage 3 of the thermoelectric device (i.e. the uppermost header), a preferred heat spreader is AlN or Si. In this stage, a AgCu eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned N-SiGe/P-TAGS thermoelements. Metallization layers on the N- SiGe/P-TAGS thermoelement pair include silver having a typical diffusion barrier layer of W or Mo orNi or similar refractory metal to prevent diffusion of Ag into the N-SiGe and P-TAGS materials. Table 5
As depicted in Table 5, in stage 2 of the thermoelectric device, a preferred heat spreader is AlN. In this stage, a AgAuGe eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned N-PbTe/P- TAGS thermoelement pair. Metallization layers on the N-PbTe P-TAGS thermoelement pair include silver having a typical diffusion barrier layer of Ni or W or Mo to prevent diffusion of Ag into the N-PbTe and P-TAGS materials. As depicted in Table 5, in stage 3 of the thermoelectric device, a preferred heat spreader is SiC. In this stage, a Sn eutectic is used as a solder material for bonding the heat spreader to the metallization layers on the afore-mentioned n- and p-type Bi2Te3- superlattice-thin-film pair. Metallization layers on the n- and p-type Bi2Te3- superlattice-thin-film pair include Cu having a diffusion barrier layer of Ni or a similar metal to prevent diffusion of Cu into the N-Bi2Te3 SL and/or P-type Bi2Te3 SL materials. As depicted in Table 5, the uppermost stage (i.e. stage 3) can be bonded to the heat-source header (e.g., a Cu plate) using a AgCuP eutectic bond. Stages 2 and 3 can be eutectic bonded together using a Auln eutectic. Stages 1 and 2 can be eutectic bonded together using a SnAu eutectic. Stage 1 can be bonded to the heat-sink header (e.g., an Al plate) using an InSn eutectic bond. Hence, in the present invention, successively-lower-melting-point eutectics bond the successive stages so that the previously-bonded stages are in tact. Such measures serve to match or reduce the mismatch in the coefficients of thermal expansion between the various stages as successive bonding can cause thermal stress. An example of stacking of the various stages to obtain efficient thermal interfaces between each of the adjacent stacks by using eutectic metal bonding is described by R. Venkatasubramanian et al in Appl. Phys. Lett., Vol. 60, 886 (1992), the entire contents of which are incoφorated by reference.
Integration of Thermoelectric Devices and Modules to Semiconductor Chip Packages A thermoelectric device of the present invention can be attached to an integrated circuit die or wafer element (preferably to the backside) with or without an intervening layer of thermal grease or plastic adhesive. For direct attachment without intervening layers of thermal grease or plastic adhesive, a thin electrically insulating layer or a series of back-side p-n isolation junctions can be utilized to permit electrical isolation of the integrated circuit on the die from the currents flowing in the thermoelectric devices. In this approach, the "top" header joining the thermoelectric device of the present invention to the integrated circuit die is made of thick metallizations which form the electrical contact and the heat pipe to the p-n thermoelements. Suitable thermoelectric materials deposition/attachment/formation, type conversion, and patterning form the p-n thermoelements and the heat pipe structures. Subsequent metallizations complete a "bottom" header and provide in one embodiment of the present invention a place for attachment to a heat sink device (e.g., an air-cooled plate or a heat-pipe) Figure 7A is a schematic depicting the bonding of a pre-fabricated thermoelectric device 72 to a semiconductor device chip 74. In this approach, bonding is used to bond the pre-fabricated thermoelectric devices to an integrated circuit die. As such, this approach separates the thermoelectric device fabrication steps from the semiconductor device chip. In this approach, a number of bonding techniques (to be discussed below) can be used to join an upper header 75 of the pre-fabricated thermoelectric device 72 to the semiconductor device chip 74. Accordingly, in the attached structure, heat from power dissipating devices 76 in the semiconductor device chip 74 will be pumped through the thermoelements 73 and dissipated at a heat sink (not shown) attached to the lower header 77. Figure 7B is a schematic depicting the attachment of the present invention of a thermoelectric device without a top header onto a semiconductor device chip. Specifically, Figure 7B depicts the bonding of pair of thermoelements 73 to the semiconductor device chip 74. As shown in Figure 7B, the bonding to the semiconductor device chip 74 is facilitated by the pre-deposition or growth of an electrically insulating layer 82 on the semiconductor device chip 74 (e.g. silicon dioxide formation on a Si device wafer). As shown in Figure 7B, the semiconductor device chip 74 itself serves as a header spreading heat from power dissipating devices 76. Thus, in this embodiment of the present invention, the thermoelements are attached directly to the semiconductor device chip 74 without the utilization of a top header. Figure 7C is a schematic depicting the sequential formation of a thermoelectric device 72 on the present invention onto a semiconductor device chip 74. In this approach, the fabrication process of the present invention may utilize bonding at separate steps in the process to bond thermoelectric materials and/or superlattice layers to other layers of the thermoelectric device, and to provide a mechanism for direct attachment of the thermoelectric devices of the present invention to the semiconductor device chip 74. In the simplified process depicted in Figure 7C, at step (a), an electrically insulating layer 82 is formed on the semiconductor device chip 74. The layer 82 can be a thermal oxide grown on a Si device chip; the layer 82 can be a deposited oxide such as SiO2 or Si3N4 deposited for example by sputtering or vapor deposition. At step (b), metallizations 83 (including interconnections between the thermoelements not shown for the sake of simplicity) are deposited and patterned in accordance with the layout of the thermoelectric devices 72 on the semiconductor device chip 74. The metallizations 83 can include for example structures to form the heat pipe 5, described in Figure 1. At step (c), pair of thermoelements 73 are attached (deposited or bonded) to the metallizations 83. The pair of thermoelements 73 can include for example the features of thermoelectric device 1 shown in Figure 1. At step (d), a set of metallizations 85 are formed on thermoelements 73. At step (e), contact and annealing using the procedures detailed below complete the fabrication by joining a header 84 to the metallizations 85. Accordingly, various approaches of the present invention include: (1) attaching a pre-fabricated thermoelectric module to a device wafer, (2) building a thermoelectric device from the backside of a device wafer, or (3) building a part of a thermoelectric device onto the backside of a device wafer (e.g. building heat pipes onto the backside of a device wafer), and completing fabrication by attachment of remaining pre-fabricated components. For attachment, several attachment methods can be used including: (1) soldering, (2) brazing, (3) friction bonding, and (4) insulator-insulator bonding similar to wafer bonding. Furthermore, in a preferred embodiment, a hybrid "reactive" bonding process is utilized in which insulator surfaces, having a thin reactive metal layer, are placed opposed to one another and then contacted and heated to react the metal layer with the insulator surfaces and thereby bond the opposed components together. Such a metal include for example Ti, W, Cr, Mo, etc., or alloys thereof. These metals readily oxidize and form in a preferred embodiment suicides which melt at temperatures of 300°C or less. The hybrid reactive bonding process of the present invention relies on the reactivity of the thin metal layer with the respective insulators to achieve a bond. In one embodiment of the present invention, the metal layer is preferably thin (e.g., less than 500 A) such that all of the reactive layer is consumed, or reacts with, the insulating layers. In hybrid reactive bonding, bonding is achieved when the surfaces are brought into contact and then heated such that the metal reacts with one or both insulating surfaces. Furthermore, friction bonding according to the present invention can be used to bond thermoelectric materials and/or superlattice layers to other layers on the thermoelectric device and provide a mechanism for direct attachment of the thermoelectric devices of the present invention to an integrated circuit die or wafer element. Friction bonding can, in one embodiment of the present invention, be implemented with intervening reactive layers. In friction bonding, linear motion over a short distance using an ultrasonic transducer provides the friction necessary to produce heating at the surface that will produce the bond. Heating to a high fraction of the melting temperature and then using a friction bonding technique to produce local heating and thereby for instance melting or plastic deformation at the contact surfaces can produce a suitable bond. Heating from one direction and cooling from another side can be used to locally heat to a small area near the contact surfaces. Small amounts of material or thin sheets of material in a perform cut to die or strip size to match the thermoelectric component parts. Materials in thin sheets or thin film form are used, according to the present invention, to provide for a buffer that protects the thermoelectric superlattice materials during the friction bonding. Materials in thin sheets avoid mechanical damage to the top layers of the thermoelectric materials. The thin layer is selected to form a eutectic that allows friction bonding. In friction bonding, the surfaces of the materials to be bonded can be roughened if necessary to improve the friction and thus the efficacy of forming a friction bond. Further, diffusion bonding or thermal annealing can be used to bond the thermoelectric devices to a semiconductor device chip. Bonding can occur between a top header of a thermoelectric devices and a semiconductor device chip by any number of the processes described herein, and can utilize a bonding material to facilitate coupling of the semiconductor device chip to the semiconductor device chip. Regardless of approach, a thermally conductive and mechanically stable connection or bond is preferred between the top header and the semiconductor device chip. Bonding to the semiconductor device chip to the thermoelectric devices of the present invention can be accomplished in the following non-limiting examples by: 1. utilization of thermal adhesives or thermally conductive epoxy, 2. soldering, 3. diffusion bonding using electroplated or evaporated metal contacts, 4. utilization of anisotropic thermal adhesives, 5. utilization of thermoplastic conductive polymers, and 6. utilization of silicon to silicon molecular bonding (in the case where the cooling header of the thermoelectric device is made from silicon). Heat flux from the semiconductor device chip can be dissipated from the thermoelectric devices of the present invention by a heat sink operating at a temperature above that of the semiconductor device chip. Moreover, once bonded, the attached thermoelectric devices of the present invention, when not cooling, can sense a heat flux from the chip and thus can provide active sensing of the device chip performance. A template of such thermoelectric devices can therefore, in one embodiment of the present invention, provide a mapping of device component utilization (i.e. those device components having higher utilizations will produce higher amounts of heat flux into the proximate thermoelectric devices). Thus, as illustrated above, the present invention can utilize a number of approaches using bonding to facilitate thermal transfer by thermal conduction from one thermoelectric power conversion stage to another. Such techniques and other techniques known in the art can be used to appropriately bond the various stages together. These approaches, according to the present invention, realize high-quality thermal interfacing. Another alternative approach of the present invention for integrating the various thermoelectric conversion stages utilizes radiant thermal energy transfer using Purcell- enhancement cavity transmitter/receiver structures such as those described in the afore- mentioned U.S. Provisional Application No. 60/253,743, the entire contents of which are incoφorated herein by reference, entitled "Spontaneous emission enhanced heat transport method and structures for cooling, sensing, and power generation" for heat transfer from one thermoelectric power conversion stage to another. In this approach, the radiant portion, if not the dominant process, plays a substantial role in managing thermal stress by providing less-rigidly-bonded interfaces. Figure 9 is a schematic illustration in which radiant coupling is used between adjacent thermoelectric conversion stages. As shown in Figure 8 a heat source 14 is coupled via radiant coupling mechanism 216 to a thermoelectric stage 16. Stage 16 is in turn coupled via radiant coupling mechanism 217 to the thermoelectric stage 17. Stage 17 is in turn coupled via radiant coupling mechanism 218 to the thermoelectric stage 18. Electrical connections 220 provide connections to the thermoelements on each stage. Further, the present invention can utilize Purcell enhancement from an enhanced density of radiative modes in small-scale structures (similar to enhanced electronic density of states in quantum-confined systems) for enhanced spontaneous emission using patterned / μm-size-range, appropriately-spaced, structures for specific temperatures, on the heat spreader. Thus, engineered micro-fins can also potentially enhance spontaneous radiative heat transport. These μm-size geometries are achievable with photolithography a large-area wafers for a cost-effective implementation. Spontaneous emission enhanced heat transport (SEEHT) may additionally enhance emission at infra-red wavelengths near 300K. The incoφoration of micron or sub-micron size Purcell cavities, will provide for the theoretical maximum radiative emission at peak wavelengths of 10 μm which will enhance heat transport by as much as a factor of 1000 at 300K, leading to a radiative dissipative flux of ΦSEEHT of 44 W/cm2. Such micron size particles incoφorated by impregnation or self-assembly, followed by overgrowth, permit the scope for radiative heat transfer mechanisms to be considerably enhanced. Such particles can further be incoφorated in high-thermal conductivity heat spreader such as SiC, AlN, Si, diamond, etc. Enhancement of such intensities, even compared to highly-emissive (Ti) surfaces, showing both structure dependence and wavelength dependence, is shown in Figure 9. Indeed, Figure 9 is a plot of apparent emission intensity as a function of the surface temperature of different engineered structures according to the present invention. As such, in one embodiment of the present invention, a radiative coupling mechanism includes a thermally conductive layer having dispersed therein particles including one of metal, semimetal, and semiconductor particles to thereby enhance black body radiation from the thermally conductive layer and support radiative heat transfer across the interfaces of the thermoelectric device components without the necessity of physical bonding. While not limited to the following theory, the present invention recognizes that of enhanced emission with these Purcell cavity structures can be further enhanced/realized if there is matching of "increased density of states" in emitters with "increased density of states" with receivers/absorbers, i.e., resonant thermal energy transfer. In addition to "resonant thermal energy transfer" by Purcell-cavity effects, other "proximity coupling of radiative infrared modes" can be exploited as well, in the present invention. Utilization of radiation coupling will, according to the present invention, reduce thermal stress, by removing (strong) physical interfacial contacts between various stages. Further, the mechanical alignment of the resonant structures will not be a significant issue, given that the typical size of the inverted couple headers are -300 μm x 300 μm.
Thermoelectric Modules For automatic assembly of the modules described above a dicer and pick-and- place tools (standard to the IC industry) can be used. A wafer dicer and robotic pick- and-place tool, provide not only cost-effective but also reliable fabrication of both mini- modules and large-scale modules. The tool is used to dice p-n couple dies from a processed wafer and to assemble these inverted couple dies into a range of devices, from mini-modules to large-array of mini-modules. The dicer and pick-and-place tools, described above can also be used according to the present invention to assemble thin- substrate (i.e. 100 μm to 250 μm thick) bulk modules as well as substrate die containing the afore-mentioned superlattice thin-film structures. In one aspect of the present invention, large-scale headers are used, on which a large array of mini-modules are assembled to produce large-scale modules and proportionally larger powers. Figure 10 is a schematic arrangement of a three stage or unit thermoelectric device of the present invention having a split-header on a lower stage 18 to provide thermal expansion relief. In another aspect of the present invention, the lower-stage 18 of the multi-stage thermoelectric device can be nearly instantly converted to a heat pump for short periods of time, thus avoiding catastrophic device damage due to for example an interruption in coolant flow, and further enhancing reliability. As such, Figure 10 represents a multi-stage thermoelectric device of the present invention in which electrical reconfiguration avoids catastrophic failures during coolant flow loss to the lower stage 18 and thus the risk of deterioration of the thermoelectric properties of the thermoelements at this stage (i.e., due to deterioration of the thermoelectric materials and/or contacts) is minimized. An electrical configuration for permitting avoidance of a catastrophic failure is depicted in Figure 11. Figure 11 is an electrical diagram showing an exemplary configuration for operating a thermoelectric device of the present invention in either a heat pump mode or a power conversion mode. As shown in Figure 11 , each stage of a thermoelectric device 102 is temperature monitored by temperature monitor block 104. Each stage is electrically controlled/sensed by a switch block 106 and a controller 108. In a power conversion mode, the controller 108 supplies power out from the thermoelectric device 102. The controller can include filters and dc-ac converters to output ac power or can include dc-dc power converters to output higher voltage dc power than directly available from the thermoelectric device 102. Figure 12 depicts a cooling system 110 which for illustrative puφoses depicts a heat exchanger 112 utilized to pump with pump 114 a fluid to a heat sink plate 116. A flow sensor 118 monitors the flow of coolant to the heat sink plate 116. A temperature of the lower stage and the output power from the lower stage is monitored by a power unit 120. In the event of coolant failure, the controller 108 can electrically reconfigure the thermoelectric device from a power conversion mode to a heat pump mode. As a result, heat will be pumped to the heat source upon this electrical reconfiguration permitting the temperature of the lower stage devices to at least momentarily avoid overheating and catastrophic destruction. Once the coolant failure is restored, or in the event of permanent failure of the cooling system 110, the thermoelectric device can continue in the "cooling" mode until the heat source is turned off, removed, or otherwise decoupled from the thermoelectric device 102. As such, catastrophic failures can be avoided. Further, the controller 108 being connected to the separate stages can switch the stages from the above-noted power conversion mode and heat pump (i.e. a cooling mode) to a heat flux sensing mode. In a heat sensing mode, the current through the thermoelements are measured as an indicator of the heat flux through the thermoelements coming from a heat source. Such information can be used as a measure of the heat being dissipated from the heat source. The controller can thus process a signal indicative of heat flux from the thermoelectric pair of n-type and p-type thermoelements. Thus, the controller of the present invention connected electrically to the thermoelectric devices of the present invention can be configured to switch the thermoelectric devices between at least one of a cooling mode, a heat pump mode, a power conversion mode, and a heat flux sensing mode.
Electronics Application Accordingly, integration of the thermoelectric devices and thermoelectric device modules of the present invention to electronic devices such as semiconductor chip packages can be accomplished through a number of mechanisms. Thermoelectric devices can be coupled to such devices for the puφoses of "hot spot" cooling those parts of the device most susceptible to exceeding operational temperature limits. Integrated semiconductor devices requiring hot spot cooling include for example microprocessors, graphic processors and other power dissipating devices fabricated in silicon, germanium, silicon-germanium, gallium arsenide, or any such semiconductor material. The integration can occur for example via recesses in the semiconductor chip and/or in the headers. In this approach, vertical and lateral vias or recesses are fabricated in the semiconductor chip and/or the heat spreader. The above-noted bonding techniques join the semiconductor chip to the heat spreader. Figure 12A is a schematic depicting a coupling structure for coupling a thermoelectric device of the present invention to a semiconductor device chip. Specifically, Figure 12A depicts a thermoelectric device attached to a semiconductor device chip 74. The semiconductor device chip 74 includes the aforementioned power dissipating devices 76 in a region proximate to the recesses 78. Selective cooling occurs via the cooling of recesses 78 in the semiconductor device chip 74. The recesses 78 permit alignment of the thermoelectric devices 72 to the power dissipating devices 76. Further, an electrically insulating interlayer 82 can be preferably inteφosed between the thermoelectric devices and the semiconductor material of the chip. As noted previously, the properties of the electrically insulating interlayer 82 are such to permit electrical isolation without impeding heat flux between the semiconductor device chip 74 and the thermoelectric devices. An electrically insulating material such as SiO2 can be suitable for the electrically insulating interlayer 82, although other insulating materials known in the art are likewise applicable. The electrically insulating interlayer 82 is at least applied to the semiconductor chip in regions where the thermoelectric devices are to be attached. Following application, metallized contacts 83 can be made on the electrically insulating interlayer 82 to provide electrical connections to the thermoelectric devices. Metallized contacts can be made using one or more of known metallization techniques such as for example but not limited to evaporation, electroplating, or soldering. Following metallization, diffusion bonding, or thermal annealing can be used to bond the thermoelectric devices to the chip. Bonding occurs between a top header 79 of the thermoelectric devices and the semiconductor device chip 74 by any number of the processes described above, and can utilize a bonding material 80 to facilitate coupling of the semiconductor device chip 74 to the semiconductor device chip 74. Regardless of approach, a thermally conductive and mechanically stable connection or bond is preferred between the top header 79 and the semiconductor device chip 74. Heat flux from the semiconductor device chip 74 is dissipated from the thermoelectric devices 72 by a heat sink operating at a temperature above that of the semiconductor device chip 74. As such, heat flux from the power dissipating devices 76 is more efficiently dissipated to the outside environment due to the higher temperature differential existing between the temperature of the heat sink and the ambient than the temperature of the semiconductor device chip 74 and the ambient. Figure 12B is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip. As shown in Figure 8B, the thermoelectric devices 72 can be directly attached to the semiconductor device chip 74 without utilization of recesses 78. In this coupling structure, placement of the thermoelectric devices 72 on the semiconductor device chip 74 aligns the thermoelectric devices 72 opposite the power dissipating devices 76. Thus, coupling of thermoelectric devices 72 to the semiconductor device chip 74 occurs with or without the utilization of recesses. Figure 13 is a schematic depicting another coupling structure for coupling a thermoelectric device to a semiconductor device chip. As shown in Figure 13, thermoelectric devices 72 are attached in recesses 78 existing in both the semiconductor device chip 74 and a heat spreader plate 90. The heat spreader plate 90 can utilize a bonding material 80 to couple the heat spreaders 84 to the heat spreader plate 90. Figure ,14 is a schematic depicting another coupling structure for coupling thermoelectric devices to a semiconductor device chip. The coupling structure includes vias for electrical connections to the thermoelectric devices. Power requirements for the thermoelectric devices require that electrical leads to the thermoelectric devices be electrically insulated. As shown in Figure 14, the electrical leads 92 can be integrated directly in a header (e.g. header plate 90) by vias 94, which either vertically or laterally provide electrically conductive channels. The vias 94 can be either etched or prefabricated such that the vias 94 run through the header plate 90. Insulating material 96 for the vias 94 can be any high temperature plastic or ceramic of sufficient dielectric quality which encapsulates metal (e.g., copper, gold or, any high electrically conductive metal). Vias in the header plate 90 can be created by laser ablation and can have electroplated metal deposited therein. This structure shown in Figure 14 having insulated electrical leads facilitates independently power or sensing of individual ones of the thermoelectric devices. The header plate 90 would be made of materials that match the coefficient of thermal expansion of silicon (e.g. within a 5% match in terms of linear expansion coefficient). Figure 15 is a schematic depicting another coupling structure for coupling thermoelectric devices to a semiconductor device chip. The coupling structure includes vias for electrical connections to the thermoelectric devices and includes high thermal conductivity materials in the header plate 90. A combination of high thermal conductive materials 98 like highly oriented pyrolytic graphite, carbon foams, graphite foams can be used to enhance the rejection of heat from the hot side of the thermoelectric device. These materials can be used as a part or for the entirety of the heat spreader plate 90. Thermal conductivity of these materials can be five times that of the thermal conductivity of copper. These materials would be extremely efficient in dissipating heat from the heat sink 84 of the thermoelectric devices 72. As illustrated in Figure 15, lateral electrical connections 100 connect to the thermoelectric devices 72. The lateral electrical connections 100 like the electrical leads 92 are formed with electrically conductive materials and insulated from the heat sink plate 90. Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:
1. A thermoelectric device comprising: at least one thermoelectric unit comprising, at least one thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, and said thermoelectric pair having a thermal conduction channel area smaller than an area of at least one of the first header and the second header such that the thermal conduction area is a fraction of the area of said at least one of the first header and the second header, and said fraction is less than 50%>.
2. The device of Claim 1, wherein said fraction is less than 20%>
3. The device of Claim 1, wherein said fraction is less than 10%>.
4. The device of Claim 1, wherein said fraction is less than 1%>. 5. The device of Claim 1 , wherein said fraction is less than 0.
5%).
6. The device of Claim 1, further comprising: a thermal impedance increasing device disposed between the thermoelectric pair and one of said first and second headers and configured to set a thermal impedance across a gap between the first and second headers to be at least comparable to a thermal impedance across said thermoelectric pair.
7. The device of Claim 6, wherein the thermal impedance increasing device comprises: a heat pipe disposed between one of said first and second headers, coupled to said thermoelectric pair, and elongated in a transverse direction to space apart the first header from the second header.
8. The device of Claim 7, wherein the heat pipe comprises: a metal member disposed between said thermoelectric pair and one of said first and second headers, extending in said transverse direction, and having a width comparable to a width of the thermoelectric pair.
9. The device of Claim 7, wherein said heat pipe is configured to separate said first header from said second header by a distance such that a thermal impedance across a gap between the first and second headers is at least comparable to a thermal impedance across the thermoelectric pair.
10. The device of Claim 9, wherein said distance is at least 100 to 500 μm.
11. The device of Claim 9, wherein said distance is at least 10 μm.
12. The device of Claim 6, wherein the thermal impedance increasing device comprises: an evacuated housing enclosing the thermoelectric pair and said one of the first and second headers.
13. The device of Claim 12, wherein said housing comprises a vacuum level of no greater that 1 Torr.
14. The device of Claim 6, wherein the thermal impedance increasing device comprises: a housing enclosing the thermoelectric pair and said one of the first and second headers, said housing filled with a low thermal conductivity medium to reduce a thermal impedance across a gap between the first and second headers to be at least comparable to a thermal impedance across the thermoelectric pair.
15. The device of Claim 14, wherein said low thermal conductivity medium comprises Ar gas.
16. The device of Claim 1, wherein said thermoelectric pair, upon conduction of said heat through the thermoelements, produces an electrical potential.
17. The device of Claim 1, wherein said thermoelectric pair, upon a current flow through the thermoelements, cools said first header.
18. The device of Claim 1 , further comprising: electrical connections to said thermoelectric pair, having a resistance less than 1/10th of an Ohmic resistance of the thermoelectric pair.
19. The device of Claim 18, wherein said electrical connections comprise metal connections.
20. The device of Claim 19, wherein said metal connections comprise
Cr/Au/Ni/Au contacts to said thermoelectric pair.
21. The device of Claim 1, wherein said thermoelectric pair comprises: a thermoelectric material having a figure of merit of at least 1.
22. The device of Claim 1, wherein said thermoelectric pair comprises: a thermoelectric material having a figure of merit of at least 2.
23. The device of Claim 1, wherein said thermoelectric pair comprises: at least one of a superlattice and a quantum dot superlattice.
24. The device of Claim 23, wherein said superlattice comprises: at least one of a Bi2Te3/Sb2Te3 superlattice, a SiGe superlattice, and a PbTe/PbSe superlattice.
25. The device of Claim 1, wherein at least one of said first header and second header comprises: a thermally conducting member.
26. The device of Claim 25, wherein the thermally conducting member comprises: an electrically conducting member and an electrical insulation member formed on the electrically conducting member; and a patterned conductor formed on the electrical insulation member and connecting to said thermoelectric pair.
27. The device of Claim 26, wherein said electrically conducting member comprises at least one of Al, Cu, doped Si, and doped SiC.
28. The device of Claim 26, further comprising: plural thermoelectric pairs connected in series by said patterned conductor.
29. The device of Claim 25, wherein the thermally conducting member comprises: an electrically insulating plate; and a patterned conductor provided on the electrically insulating plate and connecting to said thermoelectric pair.
30. The device of Claim 29, wherein said electrically insulating plate comprises at least one of AlN, SiC, Si, and diamond.
31. Th 1e device of Claim 29, further comprising: plural thermoelectric pairs connected in series by the patterned conductor.
32. The device of Claim 3, wherein the heat pipe has a thermal conduction channel area that is smaller than an area of said one of the first header and the second header.
33. The device of Claim 3, wherein the heat pipe comprises a heat pipe having a temperature drop of less than 5K for heat fluxes in a range about 25 W/cm2.
34. The device of Claim 3, wherein the heat pipe comprises a heat pipe coupled to said thermoelectric pair and said one of the first and second headers by a metal bond.
35. The device of Claim 34, wherein the metal bond comprises at least one of a solder-bump-bond, a friction bond, and a reactive-metal bond.
36. The device of Claim 3, wherein the heat pipe comprises a radiative coupling mechanism coupling heat from the heat pipe to at least one of said first header, said thermoelectric pair, and said second header.
37. The device of Claim 36, wherein the radiative coupling mechanism comprises a Purcell-enhancement cavity transmitter/receiver structure.
38. The device of Claim 37, wherein the Purcell-enhancement cavity transmitter/receiver structure comprises: a thermally conductive layer including dispersed therein one of metal, semimetal, and semiconductor particles.
39. The device of Claim 37, wherein the Purcell-enhancement cavity transmitter/receiver structure comprises: at least one of μm-size and sub-micron-size radiation fins configured to enhance spontaneous radiative heat transport.
40. The device of Claim 1, further comprising: a controller configured to control a current through said thermoelectric pair such that said thermoelectric device operates in at least one of a cooling mode, a heat pump mode, a power conversion mode, and a heat flux sensing mode.
41. The device of Claim 40, wherein said controller is configured in said heat flux sensing mode to measure said current through said thermoelectric pair.
42. The device of Claim 1, wherein said second header comprises: a body having slits formed partially through the body.
43. The device of Claim 1, wherein said second header comprises a material having a thermal conductivity higher than Cu.
44. The device of Claim 1, wherein said first header comprises: an integrated circuit element; and a heat spreader disposed between said integrated circuit element and said thermoelectric pair.
45. The device of Claim 1, wherein said at least one thermoelectric unit is configured to have a specific power greater than 1 W/gm.
46. The device of Claim 1 , wherein said at least one thermoelectric unit is configured to have a specific power in a range of 0.0001 W/gm to 0.01 W/gm.
47. The device of Claim 46, wherein the first header is coupled to a heat source in a range of 30-40 °C.
48. The device of Claim 1, wherein said at least one thermoelectric unit is configured to produce a power density greater than 0.5 W/cm .
49. The device of Claim 1, wherein said at least one thermoelectric unit is configured to produce a power density in a range from 0.0005 - 0.5 W/cm .
50. A thermoelectric device comprising: plural cascaded thermoelectric units, each unit including, at least one thermoelectric pair of n-type and p-type thermoelements, a first header coupled to one side of the thermoelectric pair, a second header coupled to a second side of the thermoelectric pair, said at least one thermoelectric pair having a thermal conduction channel area smaller than an area of at least one of the first header and the second header and the thermal conduction area is a fraction of the area of said at least one of the first header and the second header, and said fraction is less than 50%.
51. The device of Claim 50, wherein one of said plural cascaded thermoelectric units comprises: at least one upper thermoelectric unit coupled to a heat source; at least one lower thermoelectric unit coupled to the at least one upper thermoelectric unit; and a heat sink thermally coupled to the at least one lower thermoelectric unit and configured to dissipate heat from the at least one lower thermoelectric unit.
52. The device of Claim 51, wherein said fraction in said at least one upper thermoelectric unit is in a range from 80% to 10%.
53. The device of Claim 51, wherein said fraction in said at least one lower thermoelectric unit is in a range from 20%) to 0.5%).
54. The device of Claim 51, wherein said fraction in said at least one lower thermoelectric unit is less than 0.5%.
55. The device of Claim 51, wherein said thermoelectric pair in at least one of the at least one upper thermoelectric unit and the at least one lower thermoelectric unit comprises: at least one of Bi2Te3, Sb2Te3, SiGe, (AgSbTe2)ι-x(GeTe)x, PbTe, PbSe, ZnSb, and skutterrudites.
56. The device of Claim 55, wherein the (AgSbTe2)ι.x(GeTe)x comprises a GeTe mole fraction of -0.80 to 0.85.
57. The device of Claim 51, wherein the at least one upper thermoelectric unit is configured to operate at temperatures from 670K to 870K.
58. The device of Claim 51, wherein the at least one lower thermoelectric unit is configured to operate at temperatures from 470K to 670K. [
59. The device of Claim 51, wherein the at least one lower thermoelectric unit is configured to operate at temperatures from 300K to 470K.
60. The device of Claim 51, wherein said plural cascaded thermoelectric units are configured to have a specific power greater than 1 W/gm.
61. The device of Claim 51, wherein said plural cascaded thermoelectric units are configured to have a specific power in a range of 0.0001 W/gm to 0.01 W/gm.
62. The device of Claim 61, wherein the first header is coupled to said heat source in a range of 30-40 °C.
63. The device of Claim 51, wherein said plural cascaded thermoelectric units are configured to produce a power density greater than 0.5 W/cm2.
64. The device of Claim 51 , wherein said plural cascaded thermoelectric units are configured to produce a power density in a range of 0.0005 W/cm to 0.5 W/cm .
65. A thermoelectric device comprising: at least one thermoelectric pair of n-type and p-type thermoelements having a doping concentration in respective of said n-type and p-type thermoelements ranging from 1 1019 to 6 x 1019/cm3; a first header coupled to one side of the thermoelectric pair; and a second header coupled to a second side of the thermoelectric pair..
66. The device of Claim 65, wherein at least one said n-type and p-type thermoelements have a ZT of greater than 1.
67. The device of Claim 66, wherein at least one said n-type and p-type thermoelements comprise a superlattice structure.
68. A system for thermoelectric power conversion, comprising: a thermoelectric pair of n-type and p-type thermoelements; a first header coupled to one side of the thermoelectric pair; a second header coupled to a second side of the thermoelectric pair; said thermoelectric pair having a thermal conduction channel area smaller than an area of at least one of the first header and the second header such that the thermal conduction area is a fraction of the area of said at least one of the first header and the second header, said fraction being less than 50%; and a heat sink coupled to the second header and configured to dissipate heat at a rate which maintains a temperature of the second header below a temperature of the first header.
69. The system of Claim 68, wherein said fraction is less than 20%)
70. The system of Claim 68, wherein said fraction is less than 10%.
71. The system of Claim 68, wherein said fraction is less than 1%>.
72. The system of Claim 68, wherein said fraction is less than 0.5%
73. The system of Claim 68, further comprising: a coupling member coupling the first header to a heat source.
74. The system of Claim 73, wherein said coupling member comprises at least one of a steam pipe, a combustion exhaust pipe, and a thermally conducting member having a thermal conductance of at least Al and extending from said first header to said heat source.
75. The system of Claim 68, further comprising: a thermal impedance increasing device disposed between the thermoelectric pair and one of said first and second headers
76. The system of Claim 68, further comprising: plural cascaded thermoelectric units, each comprising said first header, said thermoelectric pair, and said second header.
77. The system of Claim 76, wherein one of said plural cascaded thermoelectric units comprises: at least lone upper thermoelectric unit coupled to a heat source; at least one lower thermoelectric unit coupled to the at least one upper thermoelectric unit; and a heat sink thermally coupled to the at least one lower thermoelectric unit and configured to dissipate heat from the at least one lower thermoelectric unit.
78. The system of Claim 77, wherein said thermoelectric pair in at least one of the at least one upper thermoelectric unit and the at least one lower thermoelectric unit comprises: at least one of Bi2Te3, Sb Te3, SiGe, (AgSbTe2)ι-x(GeTe)x , PbTe, PbSe, ZnSb, and skutterrudites.
79. The system of Claim 78, wherein the (AgSbTe2)ι-x(GeTe)x comprises a GeTe mole fraction of -0.80 to 0.85.
80. The system of Claim 77, wherein said thermoelectric pair in at least one of the at least one upper thermoelectric unit and the at least one lower thermoelectric unit comprises: at least one of a superlattice and a quantum dot superlattice.
81. The system of Claim 80, wherein said superlattice comprises: at least one of a superlattice of Si/Ge, PbTe/PbSe, ZnSb/CdSb, InAs/InSb, CdTe/HgCdTe, GaxInι-xAs/GayInι-yAs.
82. The system of Claim 77, wherein the at least one upper thermoelectric unit is configured to operate at temperatures from 670K to 870K.
83. The system of Claim 77, wherein the at least one lower thermoelectric unit is configured to operate at temperatures from 470K to 670K.
84. The system of Claim 77, wherein the at least one lower thermoelectric unit is configured to operate at temperatures from 300K to 470K.
85. The system of Claim 76, wherein said plural cascaded thermoelectric units and said heat sink are configured to have a specific power greater than 1 W/gm.
86. The system of Claim 76, wherein said plural cascaded thermoelectric units are configured to have a specific power in a range of 0.0001 W/gm to 0.01 W/gm.
87. The system of Claim 86, wherein the first header is coupled to a heat source in a range of 30-40 °C.
88. The system of Claim 76, wherein said plural cascaded thermoelectric units and said heat sink are configured to produce a power density greater than 0.5 W/cm2.
89. The system of Claim 76, wherein said plural cascaded thermoelectric units are configured to produce a power density in a range of 0.0005 W/cm2 to 0.5 W/cm2.
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