U.S. patent application number 12/111873 was filed with the patent office on 2008-11-06 for method for forming thermoelectric device from particulate raw materials.
This patent application is currently assigned to Romny Scientific, Inc.. Invention is credited to Andrew C. Miner.
Application Number | 20080274004 12/111873 |
Document ID | / |
Family ID | 39939652 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080274004 |
Kind Code |
A1 |
Miner; Andrew C. |
November 6, 2008 |
METHOD FOR FORMING THERMOELECTRIC DEVICE FROM PARTICULATE RAW
MATERIALS
Abstract
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.
Inventors: |
Miner; Andrew C.; (San
Francisco, CA) |
Correspondence
Address: |
JOSHUA D. ISENBERG;JDI PATENT
809 CORPORATE WAY
FREMONT
CA
94539
US
|
Assignee: |
Romny Scientific, Inc.
Richmond
CA
|
Family ID: |
39939652 |
Appl. No.: |
12/111873 |
Filed: |
April 29, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60915211 |
May 1, 2007 |
|
|
|
Current U.S.
Class: |
419/68 ;
419/66 |
Current CPC
Class: |
B22F 1/0074 20130101;
B22F 10/10 20210101; C22C 1/04 20130101; Y02P 10/25 20151101 |
Class at
Publication: |
419/68 ;
419/66 |
International
Class: |
B22F 3/02 20060101
B22F003/02; B22F 7/04 20060101 B22F007/04 |
Goverment Interests
GOVERNMENT INTERESTS
[0002] This invention was made with Government support during an
effort supported by a Small Business Innovation Research (SBIR),
award number SBIR 0637734, awarded by the National Science
Foundation (NSF). The Government has certain rights in the
invention.
Claims
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 elements.
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; and i)
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.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] 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
[0003] 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
[0004] 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
S.sup.2/.rho..lamda. where S is the Seebeck coefficient, .rho. is
the electrical resistivity and .lamda. 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 Bi.sub.2Te.sub.3 family. Bi.sub.2Te.sub.3
is highly anisotropic, putting a premium on proper crystal
orientation in order to achieve high z, and thereby a high
performance cooler or generator.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] It is within this context that embodiments of the present
invention arise.
SUMMARY
[0011] 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.
[0012] 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
[0013] Embodiments of the present invention may be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which
[0014] FIG. 1 shows a schematic cross section view of a
thermoelectric device.
[0015] FIG. 2 shows a lower substrate on which a thermoelectric
device is to be formed.
[0016] FIG. 3 shows a lower substrate with electrical
interconnections formed and patterned.
[0017] FIG. 4 shows a patterned mold layer formed across the
substrate and interconnections.
[0018] FIG. 5 shows a p-type thermoelectric slurry being dispensed
into mold cavities by a dispensing head.
[0019] FIG. 6 shows a n-type thermoelectric slurry being dispensed
into mold cavities by a dispensing head.
[0020] FIG. 7 shows low density thermoelectric materials on a
substrate, formed in a mold layer, and covered with a capping
layer.
[0021] FIG. 8 shows low density thermoelectric materials that are
encapsulated by mechanically compliant materials on a mechanically
rigid substrate, placed in a pressure vessel.
[0022] FIG. 9 shows the densified and cured thermoelectric
materials on a substrate.
[0023] FIG. 10 shows condensed an annealed n and p-type
thermoelectric material in mold cavities on a substrate.
[0024] FIG. 11 shows top electrical interconnections used to create
an electrical circuit through the thermoelectric elements.
[0025] FIG. 12 shows an optional top substrate that may be attached
using solder or thermally conductive adhesives.
[0026] FIG. 13 illustrates the principle of electrophoretic
deposition.
[0027] FIG. 14 illustrates an example of depositing thermoelectric
particles in cavities in a mold using electrophoretic or
dielectrophoretic deposition.
[0028] 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
[0029] 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.
REFERENCE NUMERALS
[0030] In the drawings listed above and the discussion below, the
following reference numerals refer to the following described
features. [0031] 1: lower substrate [0032] 2: lower electrical
interconnections [0033] 3: n-type thermoelectric elements [0034] 4:
p-type thermoelectric elements [0035] 5: upper electrical
interconnections [0036] 6: upper substrate [0037] 7: high potential
electrical lead [0038] 8: low potential electrical lead [0039] 10:
mold layer [0040] 11: open cavity [0041] 20: dispensing head for
p-type thermoelectric slurry [0042] 21: p-type thermoelectric
slurry being dispensed [0043] 22: p type thermoelectric slurry
filling a cavity [0044] 30: dispensing head for n-type
thermoelectric slurry [0045] 31: n-type thermoelectric slurry being
dispensed [0046] 32: n-type thermoelectric slurry filling a cavity
[0047] 40: condensed and annealed n-type thermoelectric material
[0048] 41: condensed and annealed p-type thermoelectric material
[0049] 50: solder or thermally conductive adhesive [0050] 60:
capping layer [0051] 70: pressure vessel [0052] 71: a liquid or gas
[0053] 72: a gas or mechanical ram for applying pressure [0054] 73:
pressure applied to the pressure vessel using a high pressure gas
or mechanical ram [0055] 74: isostatic, or spatially uniform
pressure applied on the outside surface of the material placed in
the pressure vessel [0056] 75: low potential electrostatic plate
[0057] 76: high potential electrostatic plate [0058] 77: positively
charged particles [0059] 78: negatively charged particles [0060]
79: Thermoelectric particles [0061] 80: Curved electrode [0062] 81:
first mold layer [0063] 82: first group of open cavities in first
mold layer [0064] 83: first polarity thermoelectric material in
first group of open cavities [0065] 84: second mold layer [0066]
85: second group of open cavities in first mold layer [0067] 86:
second polarity thermoelectric material in second group of open
cavities
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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).
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
Operation
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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."
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