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1. WO2005069390 - DISPOSITIFS THERMOELECTRIQUES

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THERMOELECTRIC DEVICES

TECHNICAL FIELD

The present invention generally relates to the field of thermoelectric devices. In particular, the invention relates to a thermoelectric device structure with an improved thermoelectric figure-of-merit.

BACKGROUND ART
Electronic devices such as microprocessors, laser diodes, etc. generate a lot of heat during operation. If the generated heat is not dissipated properly from such small devices, temperature buildup may occur in these devices. The buildup of temperature can adversely affect the performance of these devices. Thus, it is important to remove the generated heat in order to reduce occurrences of thermally induced failure and maintain normal operating temperatures of these devices.

Modern semiconductor manufacturing processes allow for very high circuit densities, leading to more dissipation of heat, and usage of rigorous cooling methods. Accordingly, conventional cooling techniques may not be suitable.

Conventional cooling systems for small devices are typically based on passive cooling methods and active cooling methods. The passive cooling methods include heat sinks and heat pipes. Such passive cooling methods provide limited cooling capacity due to spatial limitations. Active cooling methods include use of devices such as mechanical vapor compression refrigerators and thermoelectric coolers. The vapor compression based cooling systems generally use significant hardware such as a compressor, a condenser and an evaporator. Because of the high volume, moving mechanical parts, poor reliability and associated cost of this hardware, the use of such vapor compression based systems might not be suitable for cooling small electronic devices.

Thermoelectric cooling, for example using a Peltier device, provides a suitable cooling approach for cooling of small electronic devices. Thermoelectric cooling devices are based on the Peltier effect. Typically, a thermoelectric cooling device is a semiconductor with two metal electrodes. When a voltage is applied across these electrodes, heat is absorbed at one electrode producing a cooling effect, while heat is generated at the other electrode producing a heating effect. The cooling effect of these thermoelectric Peltier devices can be utilized for providing solid state cooling of small electronic devices.

Some typical applications of the thermoelectric cooling devices are in the field of small-scale refrigeration. Small-scale refrigeration may be used in mainframe computers, thermal management of hot chips, RF communication circuits, magnetic read/write heads, optical and laser devices, and automobile refrigeration systems.

Thermoelectric devices may provide advantages over the conventional vapor compression based cooling systems. Firstly, the thermoelectric devices have no moving parts. The lack of moving parts may make these devices more reliable and easier to maintain than the conventional cooling systems. Secondly, thermoelectric devices may be manufactured in small sizes making them attractive for small-scale applications. Thirdly, the absence of refrigerants in thermoelectric devices may provide environmental and safety benefits. Fourth, the thermoelectric coolers may be operated in vacuum and/or weightless environments and can be oriented in different directions without affecting performance.

However, typical thermoelectric devices have low efficiency as compared to the conventional cooling systems. The efficiency of a thermoelectric device is known to depend on material properties through a figure-of-merit (ZT):

ZT = S2 Tσ /λ

where, S is the Seebeck coefficient (which is a property of a material),

T is the average temperature of the thermoelectric material,

σ is the electrical conductivity of the thermoelectric material and

λ is the thermal conductivity of the thermoelectric material.

Most present day thermoelectric devices have a typical thermoelectric figure-of-merit less than 1. In order to make the thermoelectric devices as efficient as the conventional vapor compression refrigerators, the figure of merit for thermoelectric devices should be around 3.

As is evident from the above equation, a material having high electrical conductivity and low thermal conductivity will have a high figure-of-merit. This may be achieved by a reduction in thermal conductivity without a significant reduction in electrical conductivity. Various approaches have been proposed to increase the figure-of-merit of the thermoelectric devices that decrease the thermal conductivity of the material while retaining high electrical conductivity.

In one of the approaches, superlattices having reduced thermal conductivity are grown on lattice-matched substrates. (A superlattice is a periodic structure generally consisting of several to hundreds of alternating thin film layers of semiconductor material where each layer is typically between 10 and 500 Angstroms thick.) Superlattices of materials such as Bi2Te3 and Sb2Te3 are grown on GaAs and BaF2 wafers in such a way as to disrupt the thermal transport while enhancing the electronic transport in a direction perpendicular to the superlattice interfaces.

However, typical superlattices are grown on a semiconductor wafers and then may be transferred to a metal surface. This may be difficult to achieve and may make the process complex. Moreover, typical measurements on superlattice-based structures do not demonstrate larger temperature differentials or better efficiencies.

In another approach, thermal conductivity may be reduced using quantum dots and nanowires. A quantum dot is a structure where charge carriers are confined in all three spatial dimensions. Similarly, a nanowire is an ultrafine tube of a semiconductor material. Quantum confinement of carriers in reduced dimensional structures may result in larger Seebeck coefficients and hence a better thermoelectric figure of merit.

Yet another approach uses structured cold points for increasing the figure-of-merit of the thermoelectric devices. A cold point is a sharp point contact between the hot electrode and the cold electrode of a thermoelectric device. The cold points have a high ratio of electrical conductivity to thermal conductivity at the contact. This feature of the cold points may be used to improve the figure-of-merit of the thermoelectric device. Figures-of-merit in the range of 1.3 to 1.6 can be achieved using these thermoelectric devices. One such device is disclosed in U.S. Patent No. 6,467,275 Titled "Cold Point Design For Efficient Thermoelectric Coolers". The patent discloses a thermoelectric device with a cold electrode plate and a hot electrode plate. The contact between the electrodes is achieved by using a plurality of tips of the cold points on the cold electrode and the planar surface of the hot electrode.

Similar cold point thermoelectric devices are disclosed in U.S. Patent No. 20020092557 entitled "Enhanced Interface Thennoelectric Coolers With Ail-Metal Tips" and U.S. Patent No. 6,384,312 enitled "Thermoelectric Coolers With Advanced Structured Coolers". These patents describe structured cold point thermoelectric devices with an enhanced figure-of-merit.

The approach of using structured cold points typically requires precise lithographic and mechanical alignments. The tolerances of the manufacturing process for these alignments may result in degraded performance. It may be difficult to maintain uniformity in radii and heights of the cold points. These factors may make it practically difficult to achieve nanometer level planarity, resulting in point intrusions or absence of contact. Current crowding effects may increase the current flowmg through point intrusions and decrease the current in points making poor contact.

Structured cold point devices may achieve only localized cooling in a small area near each cold point. Hence, the actual area of cooling (i.e. the area around the cold points between the cold electrode and the hot electrode) may be small compared to the total area to be cooled in the device. The small cooling areas may result in large thermal parasitics and poor efficiency.

Hence, there is a need for a system that achieves high figure-of-merit for thermoelectric cooling devices. There is also a need for a thermoelectric cooler that achieves lower cooling temperatures than the current thermoelectric devices.

DISCLOSURE OF INVENTION
The invention provides a thermoelectric device comprising a first electrode, a first thermoelement thermally coupled to the first electrode and a first phonon conduction impeding medium coupled with the first thermoelement. The first phonon conduction impeding medium is also thermally insulated from the first electrode. The phonon conduction impeding medium may be a liquid metal. The first thermoelement may have a thickness less than a thermalization length associated with the first theroelement. In at least one embodiment of the present invention, a second electrode is coupled to the first phonon conduction impeding medium. The thermoelectric device may include a dielectric material for maintaining spacing between the first electrode and the second electrode. The invention is also contemplated to provide methods for manufacturing and utilizing such structures.

The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail. Consequently, tliose skilled in the art will appreciate that the foregoing summary is illustrative only and that it is not intended to be in any way limiting of the invention. The inventive concepts described herein are contemplated to be used alone or in various combinations. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, may be apparent from the detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention, wherein like designations denote like elements, and in which:

FIG.. 1 shows a cross-section of a basic non-equilibrium asymmetric thermoelectric (NEAT) device structure in accordance with at least one embodiment of the present invention;

FIG.. 2 shows the variation of electron and phonon temperatures within the basic NEAT device structure in accordance with at least one embodiment of the present invention;

FIG. 3 shows variation of electron temperature and phonon temperature in a thermoelement in accordance with at least one embodiment of the present invention;

FIG. 4 shows various phonon conduction impeding mediums in accordance with at least one embodiment of the present invention;

FIG. 5 shows a NEAT device having two metal plates in accordance with at least one embodiment of the present invention;

FIG. 6a shows a nonequilibrium symmetric thermoelectric (NEST) device in accordance with at least one embodiment of the present invention;

FIG. 6b shows a nonequilibrium symmetric thermoelectric (NEST) device in accordance with at least one embodiment of the present invention;

FIG. 6c shows multiple NEST devices cascaded in series in accordance with at least one embodiment of the present invention;

FIG. 7a illustrates a cascaded NEAT device formed by merging NEAT devices together in series with alternate n-type and p-type thermoelements arranged on opposite side of liquid metal electrodes in accordance with at least one embodiment of the present invention;

FIG. 7b shows an enlarged cross section view of a single NEAT device from the cascaded NEAT device described in conjunction with Fig. 7a in accordance with at least one embodiment of the present invention;

FIG. 8 shows a cascaded NEAT device formed by merging NEAT devices together electrically in series with alternate n-type and p-type thermoelements arranged on the same side of liquid metal electrodes in accordance with at least one embodiment of the present invention; and

FIGS. 9a-9n show the process for fabricating thermoelectric devices in accordance with at least one embodiment of the present invention.

The use of the same reference symbols in different drawings indicates similar or identical items.

MODES FOR CARRYING OUT THE INVENTION
Before describing the present invention in greater detail, it is helpful to provide definitions of terms as utilized herein.

Figure of merit: The efficiency of a thermoelectric device is known to depend on material properties through a figure-of-merit ZT = S2 Tσ/λ, where S is the Seebeck coefficient, σand λ are the electrical and thermal conductivities respectively, and T is the ambient temperature. Thus a good thermoelectric material should have a high power factor (S2σ) and a low thermal conductivity.

Phonon: A phonon is a vibrational wave in a solid, and it can be viewed as a particle having energy and a wavelength. Acoustic phonons carry heat and sound tlirough a solid. They move at the speed of sound in the solid.

Phonon Glass Electron Crystal (PGEC): According to the Phonon Glass Electron Crystal (PGEC) concept, an ideal thermoelectric material should possess the electronic transport properties of a crystal and resist the passage of heat as well as glass does. The PGEC concept defines limiting characteristics of a superior thermoelectric material.

Thermalization length: When a material is heated, electrons start moving to conduct the thermal energy. In the process, electrons collide with phonons and share energy with the phonons. As a result, the temperature of phonons starts increasing until a thermal equilibrium is attained between the electrons and the phonons. A distance traveled by electrons after which thermal equilibrium takes place is called a thermalization length.

Phonon conduction impeding medium: A material having a low acoustic velocity, e.g., materials lacking ionic order and crystal structure resulting in negligible phonon conduction. A phonon conduction impeding medium may include, without limitation, most liquids, liquid metals, liquid metal alloys (e.g., gallium and gallium alloys), some metallic solids such as indium, lead, lead-indium, thallium, interfaces created by cesium doping, liquid metal-solid interfaces, polymers, or other materials having poor phonon conductivity.

The present invention provides a thermoelectric device with an improved figure-of-merit. These results may achieved by lowering the thermal conductivity of the thermoelectric device without a significant reduction in electrical conductivity.

Referring to Fig. 1, a cross-section of an exemplary non-equilibrium asymmetric thermoelectric (NEAT) device structure in accordance with at least one embodiment of the present invention is shown.

An exemplary NEAT device structure includes a thermoelement 102 thermally coupled with an electrode (e.g., solid metal electrode 104), which is hereinafter referred to as a solid metal electrode, without limitation. Thermoelement 102 may be an ultra-thin thermoelectric semiconductor film. Solid metal electrode 104 may provide structural and mechanical stability to the thermoelement 102. An electrode, e.g., liquid metal electrode 106, is electrically as well as thermally coupled with thermoelement 102. In at least one embodiment of the present invention, liquid metal electrode 106 is a micron-sized liquid metal droplet. The liquid metal droplet may be deposited over thermoelement 102 such that it does not wet thermoelement 102. The liquid metal droplet is an example of a phonon conduction impeding medium and is used in accordance with at least one embodiment of the present invention. Any other phonon conduction impeding medium may also be used to practice the invention, but the phonon conduction impeding medium is hereinafter referred to as a liquid metal electrode, without limitation.

An electrical connection between liquid metal electrode 106 and thermoelement 102 may be established mainly by electron tunneling across a sub-nanometer tunneling gap at the interface between liquid metal electrode 106 and thermoelement 102. This tunneling gap may be formed due to non-adherence of molecules of liquid metal electrode 106 with the molecules of thermoelement 102. The electrical conduction properties of the tunneling gap are dependent on the atomic gaps, which in turn are dependent on the wetting and surface tension properties of the liquid metal. A small tunneling gap may result in an almost ideal electrical conduction.

An exemplary thermoelectric semiconductor utilized in the construction of thermoelement 102 has a high power factor (S2σ) and a thickness less than its characteristic thermalization length. For ambient applications, exemplary thermoelectric semiconductor materials include p-type Bio.5Sb1.5Te3, n-type Bi2Te2.8Seo.2, n-type Bi2Te3, and superlattices of constituent compounds such as Bi2Te3/Sb2Te3 superlattices. At higher temperatures, lead chalcogenides such as PbTe or skutteridites such as CoSb3 and traditional alloy semiconductors SiGe may be used. At low temperatures, BiSb alloys may be used. Solid metal electrode 104 may include nickel-plated copper or aluminum and may include TiW and/or Pt barriers. Thermoelement 102 may be formed on solid metal electrode 104 using techniques such as physical vapor deposition (PVD), sputtering, electrodeposition, molecular beam epitaxy (MBE), metallo-organic chemical vapor deposition (MOCVD), or other suitable technique.

Materials that may be used to form the phonon conduction impeding medium include gallium (Ga), indium (In), lead (Pb), lead-indium, lead-indium-tin, gallium-indium, gallium-indium-tin, gallium-indium with cesium doping at the surface. An exemplary composition comprises 65 to 75 % by mass gallium and 20 to 25% indium. Materials such as tin, copper, zinc and bismuth may also be present in small percentages. An exemplary composition comprises 66% gallium, 20% indium, 11% tin, 1% copper, 1% zinc and 1% bismuth. Materials like mercury, bismuth tin alloy (e.g., 58% bismuth, 42% tin by mass), bismuth lead alloy (e.g., 55% bismuth, 45% lead) may be included in the phonon conduction impeding medim.

The solid metal electrode may be replaced by any highly-doped semiconductor such as antimony or phosporus doped silicon or germanium with carrier concentrations greater than 1020 cm"3.

Hereinafter, the principle behind working of the NEAT device structure is explained in detail.

In accordance with the present invention, the figure of merit of the NEAT device structure may be increased by decreasing its thermal conductivity without causing a significant reduction in electrical conductivity.

The thermal conductivity of a thermoelectric device includes the thermal conductivity due to electrons (referred to as electron thermal conductivity λe hereinafter) and thermal conductivity due to phonons (referred to as phonon thermal conductivity λp hereinafter):

λ = λe + λp.

Thus, the value of λ can be reduced by a reduction in value of either λe or λp. However, any reduction in λe typically reduces electrical conductivity σ, thereby leading to an overall reduction in the value of figure of merit, ZT (as can be seen from the mathematical expression for ZT). Therefore, one way to reduce the value of λ without significantly affecting the value of σ includes a reduction in value of λp without significantly affecting λe.

The use of liquid metal droplet and ultra-thin thermoelectric film in the NEAT device structure results in a reduced value of λp, thereby reducing the value of λ.

The reduction of phonon thermal conductivity λp may be accomplished by decoupling and separating phonon conduction from the electron conduction, e.g., by the use of an ultra-thin film semiconductor thermoelement. The phonon conduction may be selectively attenuated by the use of a phonon-blocking structure without significantly affecting the electron conduction.

Consider a thermoelectric device structure in accordance with the embodiment illustrated in Fig.. 1 wherein the thickness of the thermoelement is t. An electrical potential is applied across the thermoelement such that the electric current flows from solid metal electrode 104 to liquid metal electrode 106. Hence, the electrons will flow in the opposite direction. Once injected into the thermoelement 102 from the liquid metal electrode, electrons are not in a thermal equilibrium with phonons in the thermoelement for a finite distance A from the surface of contact of the cold electrode and the thermoelement. This finite distance A is known as thermalization length. An exemplary thickness t of the thermoelement used in the present invention is smaller than the distance A. Hence the electrons and phonons are not in a thermal equilibrium in the thermoelement and do not significantly affect each other in the energy transport.

Once the phonon transport process and the electron transport process are separated, the difference in the thermal conduction mechanisms in liquid metals and solid metals may be exploited to create the phonon-blocking or phonon-attenuating structure in the NEAT device structure, as explained below.

Thermal conduction in metals (liquid as well as solid) is due to the transport of electrons and phonons. A unique characteristic of liquid metals (and liquid metal alloys) vis-a-vis solid metals is a substantial lack of ionic order and crystal structure. This results in low acoustic velocities and negligible phonon thermal conductivity λp in the liquid metals as compared to phonon thermal conductivity of solid metals. (The phonon thermal conductivity of the liquid metals may be less than the phonon conductivity of typical solid-phase glasses or polymers with thermal conductivity values less than 0.1 W/m.K). As a result, the thermal conductivity in liquid metals is predominantly due to electrons. Therefore, when a liquid metal or other phonon conduction impeding medium is used as one of the electrodes, the electron phonon coupling is reduced in the liquid metal electrode of the NEAT device structure, as compared to electron phonon coupling of other electrodes.

There are interface thermal resistances such as Kapitza thermal boundary resistances between the liquid metal and the thermoelement that may arise due to mismatch of the acoustic velocities in the two mediums.

The liquid metal structure may be replaced by other phonon conduction impeding mediums, as described above. The net effect is that phonon thermal conductivity between the electrodes of the thermoelectric cooler may be significantly reduced.

The electronic conduction is separated from the phonon-conduction and is not substantially impeded because the liquid metals have high electronic conductivities and the electrons can tunnel through the interface barriers with reduced resistance.

Due to the reduction of phonon thermal conductivity λp to negligible amounts (because of use of liquid metal and thin thermoelectric thermoelement), the thermal conductivity in the NEAT device structure may be predominantly due to electron thermal conductivity λe Thus λ → λe. Hence there is a reduction in value of thermal conductivity λ, which in turn leads to an improved figure of merit ZT.

Fig. 2 shows the variation of electron and phonon temperatures within the NEAT structure. The temperature of liquid metal electrode 106 is T while the temperature of solid metal electrode 104 is TH. As the thermal conduction in metals is predominantly because of the electrons, the temperature of electrons in liquid metal electrode 106 is Tc, while the temperature of electrons in solid metal electrode 104 is TH. The variation of temperature 202 of electrons in thermoelement 102 is nonlinear and is governed by heat conduction equations described later.

The temperature of phonons in solid metal electrode 104 is equal to TH (because of the electron-phonon coupling within the solid). However, in the liquid metal electrode, there is no significant phonon structure due to lack of significant ionic order. The temperature of the ion cores in the liquid metal electrode is approximately the same as that of the electrons (Tc). The temperature of phonons in the thermoelectric layer at the liquid metal interface is not equal to the liquid metal temperature because of the large thermal impedance of the phonons at the interface. The temperature of the phonons in thermoelement 102 varies between the temperature of solid metal plate TH and the temperature of phonons in liquid metal electrode 106. This variation of temperature 204 is shown in Fig. 2. As is evident from the figure, the electron and the phonon temperatures in thermoelement 102 are not in equilibrium.

One-dimensional coupled equations that describe the heat transfer for the electron-phonon system within the thermoelement may be derived using the Kelvin relationship, the charge conservation equation and the energy conservation equation. The coupled equations for heat transfer may be represented as:

- V * (λ VT ) - J /σ + P(T -T ) = 0
e e' e p'
-v«(λ p vr p ) -P(T e -T p )' = O

where,

Te is the temperature of the electrons,

Tp is the temperature of the phonons,

λe is the electrical conductivity of the thermoelement,

Jis the local current density,

σis the electrical conductivity of the thermoelement,

λp is the lattice thermal conductivity of the thermoelement, and

P is a parameter that represents the intensity of the electron-phonon interaction.

More information on the parameter P representing the intensity of the electron-phonon interaction may be obtained from "Semiconductors" (31, 265 (1997)) by V. Zakordonets and G. Loginov. Additional information may be obtained from a publication entitled "Boundary Effects in Thin film Thermoelectrics" of M.
Bartkowiak and G. Mahan from Materials Research Society Symposium Proceedings, Vol. 545, 265 (1999). The parameter P may be given for three-dimensional isotropic conduction as:
P = (3Ξ2m* kBnkF)/(πph3) where,

Ξ is the deformation interaction,

m is the effective electron mass,

kB is the Boltzmann's constant

n is the electron density,

kF is the Fermi wavevector,

p is the density of the thermoelement, and

h is the reduced Planck's constant.

More information on this may be obtained from "Electrons and Phonons in Semiconductor Multi-layers", (Cambridge University Press, 1997, Chapter 11.7) by B. K. Ridley.

These one-dimensional coupled equations are solved subject the boundary conditions as illustrated in conjunction with Fig. 3. The figure shows variation of electron temperature 302 and phonon temperature 304 in thermoelement 102. The injected electrons in the thermoelement at the boundary x=0 have temperature approximately equal to the temperature of the liquid metal electrode. Hence,

Te(0) = Tc.

Similarly, the temperature of electrons at the other boundary of the thermoelement is approximately equal to the temperature of the solid metal electrode 104. The phonons are also at the same temperature as that of the solid metal electrode. This may be represented as:

Te(t) = Tp(t) = TH.

Also, a zero gradient for the phonon temperature across the boundary of the liquid metal electrode and the thermoelement is assumed. This boundary condition represents a desired zero phonon conduction in the liquid metal electrode. This may be represented as:


The above boundary conditions are illustrated in Fig. 3.

The one-dimensional coupled equations are solved to determine heat flux q0 as a function of the temperatures at the surfaces of the thermoelement.


where,

ζ is the factor for reduction in Joule heat backflow, and

λ rr is the effective electrical conductivity of the thermoelement.
eff

The net cooling flux Jq at the cold liquid metal electrode including the Seebeck cooling effect is given by:

Jq = STc|J| + q0.

The effective thermal conductivity for the thermoelement 102 is represented by:


It may be seen from the above equation that as 11 Λ -> 0, λ- λe, the tliermal conductivity is essentially reduced to approximately the electronic thermal conductivity. A characteristic thermalization length, A, is about 500 nanometers for exemplary materials Bi0.5Sbι.5Te3 and Bi2Te2.sSe0.2 chalcogenides. Exemplary NEAT devices including thermoelements of those exemplary materials and having film thickness of t ~ 100 nanometers thus have t/Λ of around 0.2. Hence, the thermal conductivity for the thermoelement is approximately equal to the electronic thermal conductivity.

Hence, the figure-of-merit for the NEAT structure is:

ZT = S2 Tσ /λe.

The electronic thermal conductivity is related to the electrical conductivity by the Wiedeman-Franz law by the relation λe = L0σT. Thus

ZT= S2/L0

Where L0 is the Lorenz number for the thermoelement. For pure metals, L0 = (π2/3)(k/e)2.

LQ ~ 125 microvolt/Kelvin for an exemplary thermoelement including Bi0.5Sbι.5Te3.

Hence, the thermoelement operates in the classical phonon-glass-electron-crystal (PGEC) limit at the limiting value for the figure-of-merit.

J '
The first term in the formula for q0 depicts the backflow of Joule heat to the cold electrode. In

conventional devices, half of the Joule heat developed in the thermoelement flows back to the cold electrode. But, in an exemplary device in accordance with the present invention, this backflow is reduced by a factor of ξ . The factor for reduction in Joule heat backflow ξ is given by:



The reduction of backflow of Joule heat allows for higher efficiency operation at larger temperature differentials. Also, the minimum cold end temperature for a NEAT device may be derived to be:



The maximum coefficient of performance (COP), η, (i.e. the ratio of the cooling power at the cold electrode to the total electrical power consumed by the cooler) may be given by the relation:



The thermodynamic efficiency ε is the ratio of the COP of the NEAT device to that of an ideal Carnot refrigerator operating between the same temperatures (TH and Tc),



In the case of NEAT devices based on Bi0 5Sbι.5Te3 or Bi2Te3 materials, S ~ 220 micro Volt/Kelvin and hence ε ~ 0.3. It may be seen that the thermodynamic efficiency of a NEAT device in accordance with the present invention may be competitive with mechanical vapor compression refrigerators.

Fig. 4 shows exemplary phonon conduction impeding mediums that may be used in various embodiments of the invention. Fig. 4a shows the use of liquid metal as a phonon conduction impeding medium in accordance with at least one embodiment of the invention. As shown, liquid metal 402 is disposed on the thermoelement interface 404. In at least one embodiment of the invention, a combination of liquid metal and cesium vapor doping may be used to further reduce the value of phonon thermal conductivity. As shown in Fig. 4b cesium vapor doping 406 is disposed at the interface of liquid metal 408 and thermoelement 410.

In addition to liquid metals, certain metallic solids such as indium, lead, and thallium also have poor phonon conductivity and can be used for phonon blocking. Fig. 4c shows the use of solid indium as the phonon conduction impeding medium in accordance with at least one embodiment of the invention. As shown, solid indium 412 may be sputtered on thermoelement 414, however the phonon conduction impeding medium may be formed by any suitable technique.

Dielectric dams 416, 418, 420, 422, 424, and 426 may contain the various phonon conduction impeding mediums and may be utilized to support metal links that couple electrodes 402, 408, and 412.

Referring primarily to Fig. 5, a macroscopic NEAT thermoelectric device is illustrated in accordance with at least one embodiment of the present invention. Fig. 5 shows an exemplary NEAT device having two metal plates. A first metal plate 502 is coupled to a thermoelement 504. An exemplary thermoelement includes a thin layer (10-100 nm) of a semiconductor material like Bi0.5Sbι.5Te3 or Bi2Te3, but may be any of the materials described above. Thermoelement 504 is coupled with a liquid metal electrode 506. In at least one embodiment, liquid metal electrode 506 may be a micron-sized droplet of liquid metal. However, other materials, for example, those materials described above in conjunction with the NEAT device structure explained of Fig. 1 may be included at least one embodiment of the invention. Liquid metal electrode 506 is thermally and electrically coupled to a second conductive plate, e.g., second metal plate 508, which is hereinafter referred to as a second metal plate, without limitation. Second metal plate 508 acts as a contact surface with an object to be cooled. Second metal plate 508 is thermally insulated from a first conductive plate, e.g., first metal plate 502, which is hereinafter referred to as a first metal plate, without limitation. In at least one embodiment, the lateral dimension of the metal plates is in the range of approximately 10-100 micrometers while the vertical dimension is in the range of approximately 10-100 micrometers.

In at least one embodiment of the invention, dielectric material spacers 510 may be placed between the metal plates for maintaining and controlling the spacing between the metal plates. The dielectric material spacers may include a thermally insulating dielectric material. The spacers can be made in different forms, and may include thin film low-K dielectrics such as SiLK (SiLK resin is a solution of a low-molecular-weight aromatic thermosetting polymer) or aerogels, insulating epoxies and polystyrene beads, or other suitable materials. The surface tension of liquid metal allows for the use of various compatible forms of spacers and results in thermal stress-free NEAT devices. In at least one embodiment, the solid metal electrodes may be preplated with gold or indium based solders and integrated into NEAT device structures in cooler configurations. Gold and indium solder plating may be used for low temperature soldering of the NEAT devices in the conventional electrically-series and thermally-parallel cooler configurations as described in conjunction with Figs. 7 and 8.

Referring primarily to Fig. 6a, at least one embodiment of the thermoelectric device in accordance with the present invention is described. This is a nonequilibrium symmetric thermoelectric (NEST) device.

A first electrode, e.g., solid metal electrode 602, which is hereinafter referred to as a solid metal electrode, without limitation, is thermally coupled to a thermoelectric material, e.g., first thermoelectric thin film 604. Thermoelectric thin film 604 is coupled to another electrode, e.g., liquid metal electrode 606. Liquid metal electrode 606 is coupled to a second thermoelectric material, e.g., second thermoelectric thin film 608, which is in turn electrically and thermally coupled to a second electrode, e.g., solid metal electrode 610. Spacing between the two solid metal electrodes 602 and 610 is maintained using a dielectric material 612 in a similar manner as the embodiment described in conjunction with Fig. 5.

At least one embodiment of the thermoelectric device in accordance with the present invention is described in Fig. 6b. A thennoelectric material, e.g., thermoelectric thin film 614, is coupled to two electrodes, e.g., liquid metal electrodes 616 and 618. Thermoelectric thin film 614 may be supported at the ends using adhesives, e.g., epoxy resin. Liquid metal electrode 616 is electrically and thermally coupled to a first solid metal electrode 620 while the second liquid metal electrode 618 is electrically and thermally coupled to a second solid metal electrode 622. Spacing between the two solid metal electrodes 620 and 622 may be maintained using a dielectric material 624 in a similar manner as the embodiments described in conjunction with Fig. 5 and Fig. 6a.

The embodiment described in Fig. 6b may be more complex to fabricate than other embodiments. The structural robustness of such an embodiment may be improved by replacing at least one of the liquid electrodes with an alternate phonon conduction impeding medium, e.g., solid indium, lead, or indium-lead.

The NEAT or NEST devices as described in conjunction with Figs. 5, 6a, and 6b can also be cascaded or coupled in series to increase the temperature differentials across a unit element. Fig. 6c shows a cascaded

NEST device comprising a stack of the devices of Fig 6a. The temperature differentials achieved by individual units get added linearly to obtain the final temperature differential of the cascaded system. These macroscopic elements can then be assembled in electrically-series and thermally-parallel cooler configurations by processes well established in the conventional thermoelectric technology. More information about the electrically series and thermally parallel cooler structures and their fabrication can be found in "Thermoelectrics: Basic

Principles and New Materials Development" by G. Nolas, J. Sharp, and H. Goldsmid, Springer-Verlag, Berlin Heidelberg, 2001. In at least one embodiment of the invention, the abovementioned NEAT and NEST devices may be integrated into a thermoelectric cooler using a thin film process.

Referring to Fig. 7a, a cascaded NEAT device formed by merging two substrates of single (n-type or p-type) polarity thermoelements deposited on solid metal electrodes is illustrated, consistent with at least one embodiment of the present invention.

An exemplary substrates for forming the thermoelectric devices, e.g., silicon wafers 702, includes an insulating material, e.g., thin films of silicon dioxide 704. Substrate materials may include gallium arsenide, indium phosphide, glass, thermally-conducting polished ceramics, polished metal, or other suitable materials. Solid metal electrodes 706 are formed over silicon dioxide film 704. Single polarity thermoelements (typically approximately 10-100 nm thick) are alternately arranged on solid metal electrodes 706 so that they form an electrical series circuit. The alternate thermoelements are of opposite polarity. For example, a p-type thermoelement 708 and an n-type thermoelement 710 are arranged alternately to form an electrical series circuit. Electrodes, e.g., electrodes of liquid metal 712, are coupled to the thermoelements. This embodiment can be seen as a number of NEAT devices (incorporating thermoelements of opposite polarity arranged alternately) combined together in series. The process of fabrication of such thermoelectric devices is explained in detail in conjunction with Fig. 9. The n and p NEAT devices form an electrically series and thermally parallel circuit, similar to thermoelectric modules using conventional thermoelements. The two substrates may be spaced apart by dielectric standoffs 714 at the edges. Similar to the other embodiments, the compressibility of the liquid metal dots allows stress-free assembly.

Fig. 7b shows an enlarged cross section of a single NEAT device from the thermoelectric device described in conjunction with Fig. 7a. Multiple patterned metal electrodes 716 are formed on an insulating material, e.g., ultra-thin (10 -100 nm) silicon dioxide, silicon nitride, thermal oxide, CVD tetra-ethyl-ortho-silicate (TEOS) oxide, PECVD oxide, spin-on-glass, or other suitable material. Exemplary insulating materials provide electrical isolation of thermoelements in the series circuit, while reducing the thermal resistance between each metal electrode 716 and each silicon substrate 702. Exemplary electrodes may include nickel-plated copper, aluminum, or other suitable materials. A platinum layer may be included at the thermoelectric boundary for reducing electromigration at high current densities and improving metal-semiconductor contacts. In addition, ultra-thin (10-30 nm) layers of titanium/tungsten may be included added for better adhesion, e.g., improving adhesion of platinum to aluminum or of copper to silicon dioxide.

Referring to Fig. 8, a thermoelectric device in accordance with at least one embodiment of the present invention includes substrates, e.g., silicon wafers 802, with dielectric materials, e.g., silicon dioxide film 804, formed on them. Electrodes, e.g., solid metal electrodes 806, are formed on silicon dioxide film 804. Single polarity thermoelements are alternately ananged on solid metal electrodes 806 so that they form an electrically series circuit. The alternate thermoelements are of opposite polarity. For example, a p-type thermoelement 808 and an n-type thermoelement 810 are arranged alternately to form an electrically series circuit. Electrodes, e.g., liquid metal electrodes 812, are disposed between the thermoelements. Alternate n-type and p-type thermoelements are arranged monolithically on the same side of liquid metal electrodes 812. This is in contrast with Fig. 7a where alternate n-type and p-type thermoelements are arranged on opposite sides of liquid metal electrodes 712.

The fabrication process for forming abovementioned embodiments of the invention is hereinafter explained in detail. The diagrams illustrate exemplary process sequences of fabricating one pair of cascaded NEAT devices. However, the batch process described herewith can be generalized to fabrication of multiple pairs of cascaded NEAT devices (typical of practical thermoelectric coolers). Fig. 9a shows a base structure 900 including a substrate, which may be any of the substrates described above. An exemplary substrate includes silicon wafer 902 having a thickness of 100-500 microns. A blanket layer of a dielectric material, e.g., silicon dioxide 904 having a thickness of 0.5 microns, is formed on the surface of wafer 902 by chemical vapor deposition (CVD), plasma-enhanced CVD processes using tetra-ethyl-ortho-silicate (TEOS), direct thermal oxidation of silicon, or other suitable technique. Silicon dioxide 904 may then patterned by conventional optical lithography, electron beam, focused ion beam, direct writing , or other suitable technique, and etched by plasma etching techniques or reactive ion etching to form pits in the oxide. A conductive material, e.g., a copper seed, which may be TaN/ Ta/Cu, is formed in the pits, e.g., by physical vapor deposition (PVD) or other suitable techniques. Additional conductive material, e.g., copper, is then added, e.g., by electrochemically plating onto the seed layers, to substantially cover up the pits. The conductive material may also be aluminum, or other suitable material. The surface is then polished by chemical and mechanical polishing (CMP), or other suitable technique. Thin blanket layers (e.g., layers less than 20 nm thick) of TiW and Pt may be formed by PVD and patterned over the copper links by plasma etching techniques. These metallization techniques result in a composite metal structure 906. A thermoelectric material, e.g., approximately 10-100 nm film of thermoelectric material 908 is then formed by physical vapor deposition (PVD), electro-deposition, metallo-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other suitable technique on base structure 900. Fig. 9b shows resulting structure 910 after forming thermoelectric film 908.

Structure 910 is then patterned by typical semiconductor patterning techniques (e.g., forming a layer of photoresist 912 on the substrate, selectively exposing the photoresist to define areas to be etched, selectively etching areas of photoresist based upon those areas selectively exposed, and then etching the underlying and now exposed material layer). The coating of photoresist layer 912 may be done in such a manner that the lateral dimensions of the photoresist layer is same as the desired lateral dimensions of the thermoelement. Fig. 9c shows a resulting structure 914 after a layer of photoresist has been formed and patterned.

This is followed by etching of thermoelectric layer, e.g., by plasma etching techniques, wet-etching using a combination of dilute hydrochloric acid and nitric acid, reactive ion etching, or other suitable technique. Next the photoresist is removed, e.g., by dissolution in organic solvents that do not affect the thermoelectric layer 908. Resulting structure 916, shown in Fig. 9d, may be formed after removal of an exposed photoresist layer.

Droplets of liquid metal 918 or other phonon conduction impeding medium may then be deposited on the thermoelectric layer 908 by e.g., micropipette dispensing techniques, pressure fill techniques, jet printing, sputtering methods, or other suitable techique. Fig. 9e shows a NEAT thermoelectric device structure 920 as described in conjunction with Fig. 1.

Hereinafter, a method for fabricating NEAT thermoelectric devices in accordance with embodiments of Fig. 7 and 8 is described.

As described earlier, the embodiment of Fig. 7 combines two substrates of single (n-type or p-type) polarity thermoelements and arranges them alternately to form an electrically series and thermally parallel circuit.

To manufacture a NEAT device in accordance with Fig. 7, structure 920 may be used and a second liquid metal droplet 922 or other phonon conduction impeding medium may be disposed on composite metal layer 906, resulting in structure 924 as depicted in Fig. 9f.

Thereafter, a structure 926 (as shown in Fig. 9g) may be formed using the method as described in conjunction with Figures 9a through 9d. Structure 926 is similar to structure 916 (of Fig. 9d) except that structure 926 comprises an additional composite metal layer 928. Structure 926 includes a semiconductor thermoelement 929 that has a polarity opposite to that of thermoelement 908 in structure 924. For example, in case the thermoelement of structure 924 is n-type, structure 926 will have a p-type thermoelement and vice-versa. The two structures 924 and 926 may be combined to form a structure 930, as illustrated in Fig. 9h. The structures may be combined, e.g., by flip-chip backside-to-front aligners, and held in place, e.g., by polymer resin, epoxy seals, or other suitable materials on tire periphery of the structure. The structures 924 and 926 may be separated using e.g., dielectric standoffs 931 at the edges. Thus, structure 930 combines complementary polarity thermoelements 908 and 929 electrically series and thermally parallel.

As described earlier, embodiments consistent with Fig. 8 combines two substrates, one with thermoelectric elements (both n-type and p-type) and the other with conductive links and arranges them to form an electrically series and thermally parallel circuit.

To manufacture a NEAT device in accordance with Fig. 8, a layer of photoresist may be formed on structure 916 and patterned except in at least one region where a thermoelement of opposite polarity is to be formed. Resulting structure 932 is shown in Fig. 9i. Thereafter, a thermoelectric film of opposite polarity may be formed on the surface of structure 932, by techniques as described above, resulting in structure 934, as illustrated in Fig. 9j.

The photoresist film may be removed to leave behind structure 936 as illustrated in Fig. 9k. As shown, Fig. 9k has opposite polarity thermoelements 908 and 933 deposited on it. Liquid metal drops 938 may be dispensed on the thermoelements 908 and 933 by techniques described above, resulting in structure 940 as illustrated in Fig. 91.

Thereafter, a structure 942 (as shown in Fig. 9m) is formed using a method as described in conjunction with Fig. 9a. Structure 942 is similar to basic structure 900 (of Fig. 9a) except that structure 942 comprises an additional metal electrode 944. The two structures 940 and 942 may then be combined, e.g., by flip-chip backside-to-front aligners, and held in place, e.g., by polymer resin, epoxy seals, or other suitable material on the periphery of the structure, to form a structure 946, which includes thermoelements electrically in series and thermally in parallel. Structure 946 is illustrated in Fig. 9n. The structures 940 and 942 may be separated, e.g., using dielectric standoffs 945 at the edges. As shown, structure 946 combines two substrates, one with thermoelectric elements (both n-type and p-type) and the other with simple metal links and arranges them to form an electrically series and thermally parallel circuit. In structure 946 alternate n-type and p-type thermoelements 908 and 933 are arranged monolithically on the same side of liquid metal droplets 938.

The thermal and electrical operation of the embodiments shown in Fig.7a and Fig. 8 are identical. An embodiment consistent with Fig. 7 may have fabrication and processing conditions of a p-type thermoelement substrate that are substantially different from the conditions for an n-type thermoelement substrate. This flexibility allows substantially different types of n-type and p-type thermoelectric materials to be integrated in a device. An embodiment consistent with Fig. 8 includes only one substrate undergoing processing techniques of deposition of thermoelectric materials. The other substrate, without the thermoelements, includes metal links, and may be implemented on the backside of an external device. The external device could be a silicon-based microprocessor, a gallium arsenide optoelectronic chip, the cold plate of a refrigerator, or other suitable device.

Cascaded NEST structures may be fabricated by a method similar to that used to manufacture cascaded NEAT structure shown in Fig. 8 (where alternate n-type and p-type thermoelements are arranged on the same side of liquid metal electrodes).

Although the present invention has been described primarily with reference to a thermoelectric cooling device, the invention may be used as a power generator for generation of electricity. When used in the Peltier mode (as described above) the thermoelectric cooling device may be used for refrigeration while in the Seebeck mode the device may be used for electrical power generation. More information about electrical power generation maybe found in "CRC Handbook of Thermoelectrics", edited by D.M. Rowe, Ph.D., D.Sc, CRC Press, New York, (1995) pp. 479-488 and in "Advanced Engineering Thermodynamics", 2nd Edition by Adiran Bejan, John Wiley & Sons, Inc., New York (1997) pp. 675-682.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.