Patent application title: METHOD FOR FORMING THERMOELECTRIC DEVICE FROM PARTICULATE RAW MATERIALS
Andrew C. Miner (San Francisco, CA, US)
Romny Scientific, Inc.
IPC8 Class: AB22F302FI
Class name: Forming articles by uniting randomly associated metal particles consolidation of powders isostatic/hydrostatic pressing
Publication date: 2008-11-06
Patent application number: 20080274004
Patent application title: METHOD FOR FORMING THERMOELECTRIC DEVICE FROM PARTICULATE RAW MATERIALS
Andrew C. Miner
JOSHUA D. ISENBERG;JDI PATENT
Romny Scientific, Inc.
Origin: FREMONT, CA US
IPC8 Class: AB22F302FI
This invention relates to the formation manufacturing method for
constructing a thermoelectric device, by dispensing a slurry composed of
thermoelectric solids in a carrier fluid across a substrate. The process
uses a mold to confine the slurry, and heat and pressure to cure the
thermoelectric slurry into a solid. The specific method of curing the
thermoelectric material is outlined, employing a new method of condensing
the particulate solids into dense thermoelectric elements.
1. A method for fabricating a thermoelectric device, comprising: forming a
plurality of thermoelectric elements by deposition of thermoelectric
precursor material into a plurality of cavities in a mold for said
2. The method of claim 1 wherein the thermoelectric precursor material is thermoelectric solid particles dispersed in a carrier fluid.
3. The method of claim 1 wherein the thermoelectric precursor material is thermoelectric solid particles.
4. The method of claim 1 wherein the thermoelectric precursor material is treated with elevated temperature to form thermoelectric elements.
5. The method of claim 1 wherein the thermoelectric precursor material is treated with elevated pressure to form thermoelectric elements.
6. The method of claim 5 wherein where the elevated pressure is applied by submerging the device in a liquid or gas, and subjecting the device to elevated isostatic pressure.
7. The method of claim 1 wherein the thermoelectric precursor material is treated with elevated temperature and pressure to form thermoelectric elements.
8. The method of claim 7 wherein the elevated pressure is applied by submerging the device in a liquid or gas, and subjecting the device to elevated isostatic pressure.
9. The method of claim 1 wherein deposition of thermoelectric precursor material into a plurality of cavities includes using a dispensing head to deposit the precursor material into the cavities.
10. The method of claim 1 wherein deposition of thermoelectric precursor material into a plurality of cavities includes using a electrophoresis or dielectrophoresis to deposit the precursor material into the cavities.
11. The method of claim 1 wherein deposition of thermoelectric precursor material into a plurality of cavities includes forming a first mold layer having a first plurality of cavities using a electrophoresis or dielectrophoresis to deposit a precursor material of a first polarity into the cavities, removing the first mold layer, forming a second mold layer having a second plurality of cavities and locations different from locations of the first plurality of cavities, using a electrophoresis or dielectrophoresis to deposit a precursor material of a second polarity into the cavities, wherein the second polarity is opposite to the first polarity.
12. The method of claim 1 wherein the thermoelectric precursor material includes particles of thermoelectric material between 1 nanometer and 100 microns in size.
13. A method for manufacturing thermoelectric devices, comprising:a) forming a bottom substrate;b) forming bottom electrical interconnections on the bottom substrate;c) forming one or more mold layers on the bottom electrical interconnections and the bottom substrate, the mold layer including an array of open cavities;d) filling a first portion of the open cavities in the array with a p-type thermoelectric slurry;e) filing a second portion of the open cavities in the array with an n-type thermoelectric slurry;f) forming a capping layer on top of the mold layer and the filled cavities;g) densifying and curing the n-type and p-type thermoelectric slurries in the filled cavities to form thermoelectric elements;h) removing the capping layer; andi) forming top electrical interconnections on top of the thermoelectric elements.
14. The method of claim 13 further comprising: forming a top substrate on top of the top electrical interconnection.
15. The method of claim 13, wherein b) comprises forming and patterning electrically conducting materials.
16. The method of claim 13, wherein the open cavities are formed by mechanical impressions from a negative mold or by lithography.
17. The method of claim 13, wherein the thermoelectric slurry is formed by mixing particles of solid thermoelectric material with a carrier fluid.
18. The method of claim 17, wherein d) comprises dispensing the thermoelectric slurry with a dispensing head composed of multiple heads in parallel.
19. The method of claim 17, wherein g) comprises: removing the carrier fluid by applying heat or reducing the ambient pressure; submerging the device in a liquid or gas, and subjecting the device to elevated isostatic pressure; and annealing the device at high temperature.
20. The method of claim 13 wherein d) comprises dispensing the p-type slurry into the portion of the open cavities with a first dispensing head.
21. The method of claim 20 wherein e) comprises dispensing the n-type slurry into the remaining open cavities with a second dispensing head.
22. The method of claim 13 wherein d) comprises depositing the p-type slurry into a plurality of cavities includes using a electrophoresis or dielectrophoresis.
23. The method of claim 13 wherein e) comprises depositing the n-type slurry into a plurality of cavities includes using a electrophoresis or dielectrophoresis.
24. The method of claim 13 wherein d) through f) comprise: forming a first mold layer having a first plurality of cavities, depositing a precursor material of a first polarity into the cavities using a electrophoresis or dielectrophoresis, removing the first mold layer, forming a second mold layer having a second plurality of cavities and locations different from locations of the first plurality of cavities, depositing a precursor material of a second polarity into the cavities using electrophoresis or dielectrophoresis, wherein the second polarity is opposite to the first polarity.
25. The method of claim 24 wherein the slurry includes particles of thermoelectric material between 1 nanometer and 100 microns in size.
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to commonly-assigned, co-pending U.S. Provisional Patent application Ser. No. 60/915,211 entitled "METHOD FOR FORMING THERMOELECTRIC DEVICES FROM PARTICULAR RAW MATERIALS" (Attorney Docket Number 050107), filed May 1, 2007, the entire disclosures of which are incorporated herein by reference.
FIELD OF THE INVENTION
This invention relates to the formation of thermoelectric materials, specifically a method to form materials from a raw material in the form of a slurry into a solid. Embodiments of this include method of forming thermoelectric materials across a substrate, as part of a process of forming a thermoelectric device.
BACKGROUND OF THE INVENTION
Applications such as thermoelectric cooling and generation require electrically conducting solids of high quality. Thermoelectrics require materials with a high z, or thermoelectric figure of merit. The thermoelectric figure of merit z is defined as S2/ρλ where S is the Seebeck coefficient, ρ is the electrical resistivity and λ is the thermal conductivity. In order to achieve high z, thermoelectric (TE) materials should have a degree of crystallinity and low resistance between crystal grains. Room temperature TEs are often made using alloys of Bismuth and Tellurium, e.g., the Bi2Te3 family. Bi2Te3 is highly anisotropic, putting a premium on proper crystal orientation in order to achieve high z, and thereby a high performance cooler or generator.
U.S. Pat. No. 6,637,210 to Bell (2003) describes a thermoelectric cooling technique based on transient effects that in one of its embodiments includes a slurry. This slurry is thermoelectric in nature, participating actively in the operation of the device in a slurry form. This invention does not describe the use of slurry materials as a raw material or as a component in an intermediate step in the formation and organization of conducting materials, such as thermoelectric materials.
U.S. Pat. No. 6,670,539 to Heremans (2003) describes the formation of thermoelectric materials by the creation of nanowires. These are aligned wires but are formed by the use of a template material that is filled by vapor deposition and other means. This patent covers a variety of material formulations in this template type method of formation, without claim to the construction and organization of solids or composite systems by slurries.
U.S. Pat. No. 6,692,652 to Takao (2004) describes the creation of perovskite-type alkali-pentavalent metal oxide compounds with a polycrystalline nature. These are organized with crystal planes in parallel from particle constituents. These may be composed of K, Na, L, Nb, Sb, or Ta, with no claim to method of organization. Slurry use is described, but the methods for dispersing across a substrate and annealing in place are not described. U.S. Pat. No. 6,806,218 to Itahara (2004) similarly claims slurry based creation methods for thermoelectric materials, but claims materials based on elements including Co Sn Mn. Organization techniques are not presented.
U.S. Pat. No. 6,813,931 by Yadav (2004) describes the creation of devices such as sensors form laminate or multilayer materials. These materials are formed from powders of nanoparticles, and exhibit quantum confinement effects. This does not claim the construction from slurry while narrowly referring those devices relying on quantum confinement effects.
U.S. Pat. No. 6,916,872 by Yadav (2005) describes the formation of nonspherical nanoparticle based composites. The claims include reference to materials formed for desirable characteristics such as thermal conductivity and voltage coefficient of an electrical property. Thermoelectric effects are based on voltage coefficient of a thermal property, Seebeck. This patent does not describe the formation of spherical particles into solids from a slurry.
It is within this context that embodiments of the present invention arise.
Embodiments of the present invention are related to forming solids from a slurry containing solid particles, including techniques to densify these materials and form them in ordered arrays of elements, as part of a process of forming a thermoelectric device.
Embodiments of the present invention include a method and process for creating solid materials from raw materials that are composed of particles. According to a first aspect, thermoelectric materials may be manufactured in ordered arrays of individual solid elements. These elements may be formed in ordered arrays across a substrate, as part of a process for creating a working thermoelectric device. According to a second aspect, these materials may be densified in place, on a substrate. Densification of thermoelectric materials is a key step in the creation of high performance materials for use in thermoelectric devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention may be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which
FIG. 1 shows a schematic cross section view of a thermoelectric device.
FIG. 2 shows a lower substrate on which a thermoelectric device is to be formed.
FIG. 3 shows a lower substrate with electrical interconnections formed and patterned.
FIG. 4 shows a patterned mold layer formed across the substrate and interconnections.
FIG. 5 shows a p-type thermoelectric slurry being dispensed into mold cavities by a dispensing head.
FIG. 6 shows a n-type thermoelectric slurry being dispensed into mold cavities by a dispensing head.
FIG. 7 shows low density thermoelectric materials on a substrate, formed in a mold layer, and covered with a capping layer.
FIG. 8 shows low density thermoelectric materials that are encapsulated by mechanically compliant materials on a mechanically rigid substrate, placed in a pressure vessel.
FIG. 9 shows the densified and cured thermoelectric materials on a substrate.
FIG. 10 shows condensed an annealed n and p-type thermoelectric material in mold cavities on a substrate.
FIG. 11 shows top electrical interconnections used to create an electrical circuit through the thermoelectric elements.
FIG. 12 shows an optional top substrate that may be attached using solder or thermally conductive adhesives.
FIG. 13 illustrates the principle of electrophoretic deposition.
FIG. 14 illustrates an example of depositing thermoelectric particles in cavities in a mold using electrophoretic or dielectrophoretic deposition.
FIG. 15 through FIG. 20 illustrate an example of a fabrication sequence involving depositing p-type and n-type thermoelectric particles in cavities in different mold layers using electrophoretic or dielectrophoretic deposition.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
In the drawings listed above and the discussion below, the following reference numerals refer to the following described features. 1: lower substrate 2: lower electrical interconnections 3: n-type thermoelectric elements 4: p-type thermoelectric elements 5: upper electrical interconnections 6: upper substrate 7: high potential electrical lead 8: low potential electrical lead 10: mold layer 11: open cavity 20: dispensing head for p-type thermoelectric slurry 21: p-type thermoelectric slurry being dispensed 22: p type thermoelectric slurry filling a cavity 30: dispensing head for n-type thermoelectric slurry 31: n-type thermoelectric slurry being dispensed 32: n-type thermoelectric slurry filling a cavity 40: condensed and annealed n-type thermoelectric material 41: condensed and annealed p-type thermoelectric material 50: solder or thermally conductive adhesive 60: capping layer 70: pressure vessel 71: a liquid or gas 72: a gas or mechanical ram for applying pressure 73: pressure applied to the pressure vessel using a high pressure gas or mechanical ram 74: isostatic, or spatially uniform pressure applied on the outside surface of the material placed in the pressure vessel 75: low potential electrostatic plate 76: high potential electrostatic plate 77: positively charged particles 78: negatively charged particles 79: Thermoelectric particles 80: Curved electrode 81: first mold layer 82: first group of open cavities in first mold layer 83: first polarity thermoelectric material in first group of open cavities 84: second mold layer 85: second group of open cavities in first mold layer 86: second polarity thermoelectric material in second group of open cavities
FIG. 1 shows a schematic cross section view of a thermoelectric device according to an embodiment of the present invention. A substrate 1 is used on which to build the structure. Electrically conducting materials that serve as electrical interconnections 2 are formed on the substrate 1. N-type thermoelectric elements 3 are formed in physical and electrical contact with the lower electrical interconnections 2. P-type thermoelectric elements 4 are formed in physical and electrical contact with the lower electrical interconnections 2. Electrically conducting materials form upper electrical interconnections 5, in physical and electrical contact with the n-type and p-type thermoelectric elements 3, 4. An upper substrate 6 is in physical contact with the upper electrical interconnections 5. This is the typical structure of a thermoelectric device, whether it is a thermoelectric cooler or a thermoelectric generator. When operating as a thermoelectric cooler, an electrical interconnection is held at high electrical potential 7 while another is held at low electrical potential 8, allowing electrical current to flow in series up the n-type elements 3 and down the p-type elements 4, passing through the upper and lower electrical interconnections 2, 5.
Fabrication of a thermoelectric device of the type shown in FIG. 1 may proceed according to an inventive method as shown in FIG. 2 through FIG. 12. As seen in FIG. 2 the thermoelectric device may be fabricated on a lower substrate 1 on which. Preferably, the lower substrate is a high thermal conductivity material and is compatible with high volume microelectronic assembly, such as a silicon wafer or a high thermal conductivity PCB board.
As shown in FIG. 3 electrically conducting materials may be formed and patterned to form electrical interconnections 2 on the substrate 1. The electrically conducting materials may be formed using standard microelectronic manufacturing methods including sputtering, electrode position, and screen printing. The electrically conductive materials may be patterned, e.g., using photolithography, shadow masks, or by screen printing. The electrically conductive materials preferably have high electrical conductivity, and are compatible with the process by adhering well to the substrate 1 and have limited diffusion into thermoelectric materials. Examples of applicable conductive materials include, but are not limited to, copper, nickel, aluminum, and tungsten.
Next, as shown in FIG. 4, a mold layer 10 may be formed across the lower substrate 1 and interconnections 2. This mold layer 10 is patterned to create an array of open cavities 11. These cavities 11 are located where thermoelectric elements are to be formed at a later stage of the manufacturing process. The mold layer may 10 be formed using standard semiconductor process materials such as spin photo resist, dry-film photo resist, and polyimide. The cavities 11 may be formed by mechanical impressions from a negative mold, or by photolithography.
FIG. 5 shows a p-type thermoelectric slurry 21 being dispensed into mold cavities 11 by a dispensing head 20. The thermoelectric slurry is formed by mixing particles of solid thermoelectric material with a carrier fluid. This carrier fluid can be one of several liquids, including propylene glycol, ethylene glycol, water, alcohol, etc. A dispensing head 20 is positioned over the open cavities 11 in the mold layer 10 where a p-type element is to be formed, and dispenses a precise amount of slurry. By way of example, the dispensing head 20 may be based on technology developed for high throughput dispensing of inks and glues, such as ink jet and pneumatic injection heads. The dispensing head 20 may be located precisely and may include multiple heads operating in parallel to increase throughput of slurry deposition across the substrate 1. Open cavities where n-type elements are to be formed are left empty.
As shown in FIG. 6 a n-type thermoelectric slurry 31 may be dispensed into remaining open mold cavities 11 by a dispensing head 30. The deposition of the n-type slurry 31 fills the open cavities, creating an array of alternating n and p type slurries across the mold layer. As with the p-type slurry, the dispensing head 30 can be composed of multiple heads in parallel to increase the throughput of slurry deposition across the substrate. Also, an array of dispensing heads can be used that is composed of multiple n and p type slurry dispensing heads, allowing both materials to be dispensed across the substrate in a high throughput fashion.
The thermo electric slurries 21, 31, may particles made from thermoelectric materials that are the same or similar to current materials used for operation near room temperature. Examples of n-type thermoelectric materials include, but are not limited to alloys (also known as solid solutions) of Bismuth (Bi), Tellurium (Te), and sometimes Selenium (Se). By way of example, and without limitation, an alloy of Bi(2)Te(2.7)Se(0.3), where the numbers in parentheses indicate the relative proportions of each element, may be used in an n-type slurry. Examples of p-type thermoelectric materials include, but are not limited to, are usually alloys of Bi, Antimony (Sb), and Te. By way of example, an alloy of Bi(0.5)Sb(1.5)Te(3) may be used in a p-type slurry. The particles in the slurries 21, 31 may be of any suitable size, e.g. from several nanometers in diameter (more than 1 nanometer) to tens of microns (less than 100 microns).
As shown in FIG. 7 low density thermoelectric materials 32, 22 formed on the substrate 1 in the mold layer 10 may be covered with a capping layer 60 prior to undergoing a densification and curing process. A carrier fluid may first be removed from the slurries that form the low density thermoelectric materials 32, 22, e.g., by the application of heat and/or reducing the ambient pressure, leaving a cluster of low density thermoelectric solids 32, 22. The capping layer 60 and the mold layer 10 may be formed using spin coated or dipped materials that are much more mechanically compliant than the rigid substrate 10. The capping layer 60 covers the low density thermoelectric materials 32, 22 all across the substrate, encapsulating them.
As shown in FIG. 8 the low density thermoelectric materials 32, 22 that have been encapsulated by mechanically compliant materials 10, 60 on the mechanically rigid substrate 1 may be placed in a pressure vessel 70. In an adaptation of a process used in powder metallurgy, known as isostatic pressing, the pressure vessel 70 may be filled with a liquid or a gas 71, and high pressure 73 is applied to the inside of the vessel using a high pressure gas or mechanical ram 72. The submerged substrate 1, thermoelectric materials 32, 22, mold layer 10, and capping layer 60, experience a uniform pressure 74 on their outside surfaces. Since the substrate 1 is much more mechanically rigid that the other materials, it deforms very little. However, the compliant mold and capping layers 10, 60 and the low density thermoelectric materials 32, 22 deform. This applied pressure compresses the thermoelectric materials 32, 22, and densifies them. This process may also include the application of heat to the system, in a process similar to hot isostatic pressing, influencing the thermoelectric properties of the thermoelectric elements.
FIG. 9 shows the densified and cured thermoelectric materials on a substrate after the removal from the pressure vessel 73. Due to the application of isostatic pressure in the pressure vessel, the thermoelectric materials 40, 41 are highly compressed, and have a smaller volume than before. This process allows these individual elements that are arrayed across the substrate to each be compressed uniformly, creating a high quality, uniform thermoelectric device manufacturing process. The mold layer 10 may be removed, and the device may be annealed at high temperature to further enhance the thermoelectric properties of the thermoelectric materials 3, 4. The mold layer 10 may be reapplied to facilitate subsequent manufacturing. Specifically, as shown in FIG. 10 the mold layer may be formed around condensed an annealed n and p-type thermoelectric material 40, 41 in mold cavities on the substrate 1.
FIG. 11 shows top electrical interconnections 5 used to create an electrical circuit through the thermo-electric elements. Electrically conducting materials may be formed using standard microelectronic manufacturing methods including sputtering, electrodeposition, and screen printing. The conducting materials that form the interconnections 5 may be patterned using photolithography, shadow masks, or by screen printing. Preferably, the conducting materials that make up the interconnections 5 are characterized by high electrical conductivity, and have limited diffusion into the thermoelectric materials. Examples of suitable materials for the top electrical interconnections 5 include copper, nickel, aluminum, and tungsten.
As shown in FIG. 12 an optional top substrate 6 may be attached using solder or thermally conductive adhesives 50. The top substrate 6 provides a stable surface on which the device to be cooled may be attached, or that may be attached to a heat source (in the case of power generation). The mold layer 10 can optionally be removed by chemical wet etching or other means.
Embodiments of the present invention allow for thermoelectric devices to be formed in a highly manufacturable process, as many elements, and therefore many devices may be processed across a single substrate in parallel. Electrical interconnections 2 are formed using wafer scale deposition and patterning techniques, creating uniform and precise structures across a substrate 1, as shown in FIG. 3. Similarly, microelectronic assembly techniques allow cavities 11 to be formed in a mold layer 10 that are located accurately with respect to the position of the electrical interconnections 2, as seen in FIG. 4. These cavities 11 become the receptacle for the thermoelectric slurries (21, 31) that are injected or dispensed by an automated print head (FIGS. 5 and 6). Many thousands of cavities may be filled across a wafer, allowing many hundreds of thermoelectric devices to result from this volume manufacturing method. Each cavity may be filled with n-type or p-type thermoelectric material precursor slurries (21, 31) as the design requires. The thermoelectric material precursors (21, 31) may be delivered in the form of particles dispersed in a carrier fluid.
High quality thermoelectric materials are critical for a viable product, and a key to embodiments of the invention is the ability to perform a densification and high temperature anneal (or cure) to each individual element in place across the substrate. FIGS. 7 and 8 illustrate how the carrier fluid can be removed by the application of heat and/or reducing the ambient pressure, and each individual element can be encapsulated in a capping layer 60. By applying isostatic pressure to the entire system, as shown in FIG. 8 the loosely packed thermoelectric precursors 32, 22 are compressed into highly dense solids (3,4 in FIG. 9). This is accomplished by submerging the system in a liquid or gas 71 filled chamber 70 and increasing the pressure 73. Isostatic pressure 74 compresses the system on all sides of the system, and since the mold layer 10 and capping layer 60 are mechanically compliant materials, and the substrate 1 is a rigid material, the thermoelectric elements 32, 22 are highly compacted. This pressing may be done at ambient temperature, or at elevated temperature. The thermoelectric properties of the elements 3, 4 are further improved by curing or annealing them at high temperature, as shown in FIG. 9. This process may necessitate removing the mold layer 10, depending on the nature of the mold layer 10 and the annealing conditions.
The volume production process continues, as shown in FIG. 10 with the reapplication of a mold layer 10, as needed. Upper electrical interconnections 5 are formed using standard microelectronic fabrication and patterning processes, and a top substrate 6 may be attached as the application requires (FIGS. 11 and 12). This top substrate is attached by reflowing solders 50 or by the use of high thermal conductivity adhesives 50.
This invention offers several advantages. Since the thermoelectric materials are dispersed in a slurry and deposited on the wafer just where they are needed, there is little raw material waste. This is a significant advantage over alternatives where large thermoelectric crystals are sawed into pieces, creating waste, or where thermoelectric materials are deposited over an entire wafer and then selectively etched. Both of these alternatives are very wasteful processes, and dramatically increase costs. By the application of high pressure to the system while under a liquid, isostatic pressure is experienced on all the outer surfaces, allowing each element to experience the same pressure. This allows for a high uniformity in the thermoelectric element formation process, and greatly reduces the potential for the substrate to break or crack. Should an alternative, such as the application of pressure to the elements using a mechanical press, be used, the applied pressure would not be uniform on all sides of the substrate, making it likely that the substrate would break or crack. By forming many thermoelectric devices across a substrate in a parallel manufacturing process, costs are reduced and uniformity in the product can be greatly improved over traditional hand-made thermoelectric device processes.
A number of variations on the above-described process are possible. For example, instead of using a dispensing head to deposit precursor material into the cavities, electrophoretic or dielectrophoretic methods may be used to deposit these materials.
Dielectrophoretic and Electrophoretic deposition are related approaches that are used to deposit materials such as paints in a variety of applications and phosphor particles in the manufacture of CRT televisions and other displays. These two techniques may also be applied to the deposition of patterned thin and thick film thermoelectric materials. Electrophoretic deposition describes the movement of charged particles in an applied electric field towards a surface. As the particles impact the surface they form a desired layer or film. As shown in FIG. 13 two plates 75, 76 separated by a gap of width w have an electrical potential V applied between them. The region between the plates may be approximated to have a uniform electric field with a strength of V/w. Particles 77, 78 with both positive and negative charge are shown in the region between the plates. Positively charged particles 77 experience an electrophoretic force towards the plate 75 with low electrical potential, and negatively charged particles 78 experience a similar force towards the high potential plate 78. Although a uniform electric field is shown in FIG. 13 for the sake of example, this is not a requirement for the movement of charged particles 77, 78. The movement of the particles largely follows the direction of the field lines E, altered by other forces such as gravity, viscous drag, etc. In a typical deposition application only one type of charged particle is typically used. For example, particles of thermoelectric material may be negatively charged and dispersed in a fluid between a positively charged substrate 1 part and a negatively charge plate. The thermoelectric particles are drawn to the substrate by the electric field E filling the cavities in the mold.
Dielectrophoretic deposition describes the movement of uncharged particles in an applied non-uniform electric field. Movement of these uncharged particles requires that the dielectric constant of the particles and the medium in which they reside to be different. A particle with no net charge in a nonuniform field undergoes an internal charge separation due to its presence in an electrical field, e.g., the particle has a surplus of positive charges at the side near low electrical potential, and a surplus of negative charges at the side near the high electrical potential. Since the particle is in a nonuniform electric field, either the positive or negatively charged region will be in a region of higher electric field strength. As such, there is a net electrostatic force on the particle that causes the particle to move. The net force may cause the particle to move towards the low potential electrode or the high potential electrode, based on the relative polarizability of the particle with respect to the medium. If the particle is more polarizable than the medium, it will experience net force towards the high electrical field region. This is the case, for example, of a metallic particle (high dielectric constant) in a medium such as isopropyl alcohol (moderate dielectric constant). The opposite is true for particles of low polarizability (low dielectric constant), such as plastic in a medium with higher polarizability, such as isopropyl alcohol.
Either electrophoresis or dielectrophoresis may be applied to the deposition of thermoelectric materials into massively parallel arrays. An example of deposition of suspended particles into patterned mold materials in order to create solid thermoelectric elements is shown in FIG. 14. A substrate 1 is prepared with a mold layer 10, also known as a template. The template is made of a material having a low dielectric constant, such as photo resist. Since the dielectric constant is lower than the fluid in which the thermoelectric particles 79 are dispersed, the electric field lines E from a curved electrode 80 preferentially flow into the open cavities 11, to the substrate 1. As such, charged or uncharged thermoelectric particles largely follow these electric field lines into the open cavities 11 in the template 10, by means of electrophoretic or dielectrophoretic deposition. Such techniques may be applied to deposition of the n-type and/or p-type slurries in the open cavities 11.
It is noted that the open cavities 11 may be formed and filled in two stages by electrophoretic or dielectrophoretic deposition. For example as shown in FIG. 15 in a first stage, a first mold layer 81 may be formed and a first group of open cavities 82 may be formed in the first mold layer 81 (e.g., by photolithography) over lower electrical connections 2 on a lower substrate 1. The open cavities 82 may be filled with one polarity (e.g., p-type) thermoelectric material 83 by electrophoretic or dielectrophoretic deposition, as shown in FIG. 16. Subsequently, the first mold layer 81 may be removed as shown in FIG. 17 and a second mold layer 84 may be formed and a second group of open cavities 85 may be formed in the second mold layer 84 over the lower electrical connections 2, but in different locations than the thermoelectric material 83 as shown in FIG. 18. The second group of open cavities 85 may then be filled with an opposite polarity (e.g., n-type) thermoelectric material 86 by electrophoretic or dielectrophoretic deposition as shown in FIG. 19. The second mold layer 84 may then be removed as shown in FIG. 20. Subsequent fabrication may then proceed as described above with respect to FIG. 10 through FIG. 12.
There are several advantages of forming thermoelectric elements using the techniques described above. For example, thermoelectric elements may be formed on a substrate in a massively parallel manner. Thermoelectric elements may be formed by the described method from particle raw materials dispersed in a fluid (e.g., a slurry) in a manner resulting in little wasted raw material. Where electrophoretic or dielectrophoretic techniques are used, the relative dielectric constants of particles, suspending fluid, and template material allow the flow of particles to be designed such that they deposit to a high degree only in the cavities formed by the template. The size and length of the thermoelectric elements can be controlled by the design of the template layer, composition of the thermoelectric slurry, and (for electrophoretic or dielectrophoretic deposition) applied electric field, and the time of deposition.
While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature described herein, whether preferred or not, may be combined with any other feature described herein, whether preferred or not. In the claims that follow, the indefinite article "A", or "An" refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. In the claims that follow, the expressions first and second are used to distinguish between different elements and do not imply any particular order or sequence. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase "means for."