U.S. patent application number 12/724319 was filed with the patent office on 2011-07-14 for system and method for manufacturing embedded conformal electronics.
Invention is credited to Richard Gambino, Jon Longtin, Sanjay Sampath.
Application Number | 20110171392 12/724319 |
Document ID | / |
Family ID | 31495931 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110171392 |
Kind Code |
A1 |
Gambino; Richard ; et
al. |
July 14, 2011 |
System and Method for Manufacturing Embedded Conformal
Electronics
Abstract
A method for fabricating an electronic device comprises
providing a substrate, direct writing a functional material by a
thermal spray on the substrate and removing a portion of the
function material to form the electronic or sensory device.
Inventors: |
Gambino; Richard; (Stony
Brook, NY) ; Longtin; Jon; (Port Jefferson, NY)
; Sampath; Sanjay; (Setauket, NY) |
Family ID: |
31495931 |
Appl. No.: |
12/724319 |
Filed: |
March 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10491609 |
Apr 2, 2004 |
7709766 |
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PCT/US2003/024584 |
Aug 5, 2003 |
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12724319 |
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60401150 |
Aug 5, 2002 |
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Current U.S.
Class: |
427/447 ;
118/300; 118/620; 118/639; 118/696; 118/75; 427/446; 427/455 |
Current CPC
Class: |
H01L 35/32 20130101;
H01L 21/76838 20130101; H01L 35/34 20130101; H05K 2203/1344
20130101; C23C 4/185 20130101; G03F 7/70383 20130101; H05K 3/043
20130101; H01L 21/76894 20130101; B23K 26/0093 20130101; B23K
26/361 20151001; H05K 3/14 20130101; B23K 2101/36 20180801; H05K
3/04 20130101; H05K 3/027 20130101; C23C 4/18 20130101 |
Class at
Publication: |
427/447 ;
427/446; 427/455; 118/300; 118/696; 118/639; 118/620; 118/75 |
International
Class: |
B05D 5/12 20060101
B05D005/12; B05D 1/02 20060101 B05D001/02; B05D 3/02 20060101
B05D003/02; B05D 3/06 20060101 B05D003/06; B05D 5/00 20060101
B05D005/00; B05B 1/24 20060101 B05B001/24; B05C 9/12 20060101
B05C009/12; B05C 11/00 20060101 B05C011/00 |
Goverment Interests
GOVERNMENT LICENSE RIGHTS
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of grant no. N000140010654, awarded by the Department of Defense,
DARPA.
Claims
1. A method for fabricating an electronic device, comprising:
providing a substrate; depositing a functional material by a
thermal spray on the substrate; and removing a portion of the
functional material to form the electronic or sensory device.
2. The method of claim 1, wherein the substrate is flexible.
3. The method of claim 1, wherein depositing is a direct
writing.
4. The method of claim 1, wherein depositing a functional material
further comprises heat treating the functional material.
5. The method of claim 1, wherein the heat treating is preformed
one of before or after removing a portion of the functional
material.
6. The method of claim 1, wherein depositing further comprises
forming a conformal layer on the substrate.
7. The method of claim 1, wherein depositing the functional
material further comprises providing one of a metal, a
semiconductor, a ceramic, and a polymer in the thermal spray.
8. The method of claim 1, wherein depositing the functional
material further comprises providing one of a dielectric material
and an insulating material.
9. The method of claim 1, wherein removing the portion of the
functional material further comprises providing a focused laser
beam to the functional material.
10. The method of claim 1, wherein the electronic device is
fabricated in-situ.
11. The method of claim 1, further comprising coating a portion of
the electronic device.
12. The method of claim 1, further comprising: depositing an
insulating layer over the functional material after removing the
portion, wherein the functional material is a bottom metal
comprising at least two parallel strips, wherein a portion of each
of the two parallel strips is exposed on each of at least two sides
of the insulating layer; depositing a top metal of functional
material by the thermal spray over the insulating layer and exposed
portions of the two parallel strips; and removing a portion of the
top metal of functional material, forming at least one strip, the
at least one strip connecting a portion of one of the two parallel
strips exposed on a first side of the insulating layer and a
portion of a second strip of the two parallel strips exposed on a
second side of the insulting layer.
13. A system for fabricating an electronic device comprising: a
thermal spray device for depositing a conformal layer of a
functional material; and a material removal device for fabricating
an electronic device from the conformal layer of the functional
material.
14. The system of claim 13, further comprising a fixture for
retaining a substrate upon which the conformal layer of the
functional material is deposited.
15. The system of claim 13, wherein the material removal device
comprises a programmable motion device.
16. The system of claim 15, wherein the programmable motion device
comprises: a processor for receiving instructions; and an
articulated arm supporting the material removal device proximate to
the conformal layer of the functional material, the articulated arm
following the instructions received by the processor.
17. The system of claim 15, wherein the programmable motion device
comprises: a processor for receiving instructions; and an
articulated stage supporting the conformal layer of the functional
material proximate to the material removal device, the articulated
arm following the instructions received by the processor.
18. The system of claim 13, wherein the material removal device
comprises a laser.
19. The system of claim 13, wherein the material removal device is
one of a water jet, a mechanical milling machine, and an electric
discharge machine.
20. The method of claim 13, wherein the functional material is
functional as deposited.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a Continuation Application of U.S. application Ser.
No. 10/491,609 filed on Apr. 2, 2004, which is a National Stage
Application of International Application No. PCT/US2003/24584,
filed Aug. 5, 2003, which claims the benefit of U.S. Provisional
Application No. 60/401,150, filed Aug. 8, 2002, the disclosures of
which are herein incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to conformal electronic
devices, and more particularly to a method of fabricating conformal
electronics using additive-subtractive techniques.
[0005] 2. Discussion of the Prior Art
[0006] The adoption of computer-based design, engineering, and
analysis tools over the past 10-20 years has resulted in a
tremendous acceleration in the development cycle of modern
engineering systems. Modern engineering systems are lighter,
smaller, last longer, are more efficient, and are far more reliable
than their predecessors of even a few years ago. As a consequence,
however, these very same engineering systems are becoming extremely
complex, with the result that the costs involved to repair such
systems, particularly for major component failures, are
skyrocketing. Accordingly, the ability to monitor the health of
vital engineering components in-situ and non-invasively in
real-time is a vital capability that is needed for modern
engineering system designs to be fully utilized, so that
maintenance costs can be minimized, system health monitored, and
major repairs-scheduled for the most opportune times.
[0007] The sensor system should not disturb or alter any aspect of
the system it is interrogating. However, after-market sensors, even
if attached during the manufacturing process, can be unreliable,
difficult to install, and may adversely affect component
operation.
[0008] Electronic manufacturing with feature sizes in the
meso-scale regime (e.g., about 10 to 1000 micrometers) often needs
multi-step processes that include time-consuming photolithographic
methodologies. The time needed between iterations can often be
measured in terms of weeks. In addition, thick film electronics
based on ceramic multi-chip module technology, including low
temperature co-fired ceramic modules (LTCC-M) and high temperature
co-fired ceramic modules (HTCC-M) generally need firing of screen
printed pastes to moderate .about.800 C for LTCC-M or high 1400 C
for HTCC-M. The high temperature curing process gives rise to
issues associated with mismatch in thermal expansion between
dissimilar materials and can lead to premature debonding. This
needs to be accounted for during the processing through careful
tailoring of the properties of the layered materials. Current
screen printing technology is inherently limited in its fine
feature capabilities, with the line width being limited to 100
microns or higher.
[0009] Therefore, a need exists for a system and method of
fabricating conformal electronics using additive-subtractive
techniques.
SUMMARY OF THE INVENTION
[0010] Thermal spray technology coupled with precision laser
materials processing has been developed for the fabrication of
electronics and sensor fabrication. Thermal spray is implemented
for depositing a wide variety of materials that have functional
properties as deposited. The materials generally do not need
subsequent post-firing, annealing, or other time consuming, costly
post processing steps. A variety of materials can be deposited
quickly and easily using thermal spray technology. After the
deposition, precision laser micromachining using, for example,
ultrafast or UV laser systems, can be used to fabricate complex
electronic structures. The electronic structures include, for
example, resistors, capacitors, coils, transformers, and a variety
of sensors, for example, thermistors, thermocouples, thermopiles,
strain sensors, magnetic sensors, humidity sensors, gas sensors,
flow sensors, heat flux sensors, etc. Furthermore, these sensors
can be embedded within a component during manufacture to provide an
extremely robust, long-life sensing and health monitoring system
for the component, which is superior to aftermarket, add-on sensors
that must be attached manually using adhesives or other
post-manufacturing techniques. Also, because the thermal spray
technique is self-compatible, it can be used to fabricate
three-dimension electronics and sensor systems, e.g., multi-layer
sensors on the same surface area footprint, multiple-layer
thermopiles for enhanced power production, etc.
[0011] A method for fabricating an electronic device, comprises
providing a substrate, depositing a functional material by a
thermal spray on the substrate, and removing a portion of the
functional material to form the electronic or sensory device.
[0012] The substrate is flexible. Depositing is a direct
writing.
[0013] Depositing a functional material further comprises heat
treating the functional material. The heat treating is preformed
one of before or after removing a portion of the functional
material.
[0014] Depositing further comprises forming a conformal layer on
the substrate. Depositing the functional material further comprises
providing one of a metal, a semiconductor, a ceramic, and a polymer
in the thermal spray. Depositing the functional material further
comprises providing one of a dielectric material and an insulating
material.
[0015] Removing the portion of the functional material further
comprises providing a focused laser beam to the functional
material.
[0016] The electronic device is fabricated in-situ.
[0017] The method comprises coating a portion of the electronic
device.
[0018] The method further comprises depositing an insulating layer
over the functional material after removing the portion, wherein
the functional material is a bottom metal comprising at least two
parallel strips, wherein a portion of each of the two parallel
strips is exposed on each of at least two sides of the insulating
layer, depositing a top metal of functional material by the thermal
spray over the insulating layer and exposed portions of the two
parallel strips, and removing a portion of the top metal of
functional material, forming at least one strip, the at least one
strip connecting a portion of one of the two parallel strips
exposed on a first side of the insulating layer and a portion of a
second strip of the two parallel strips exposed on a second side of
the insulting layer.
[0019] A system for fabricating an electronic device comprises a
thermal spray device for depositing a conformal layer of a
functional material, and a material removal device for fabricating
an electronic device from the conformal layer of the functional
material. The system comprises a fixture for retaining a substrate
upon which the conformal layer of the functional material is
deposited.
[0020] The material removal device comprises a programmable motion
device. The programmable motion device comprises a processor for
receiving instructions and an articulated arm supporting the
material removal device proximate to the conformal layer of the
functional material, the articulated arm following the instructions
received by the processor. The programmable motion device comprises
a processor for receiving instructions and an articulated stage
supporting the conformal layer of the functional material proximate
to the material removal device, the articulated arm following the
instructions received by the processor.
[0021] The material removal device comprises a laser. The material
removal device is one of a water jet, a mechanical milling machine,
and electric discharge machine.
[0022] The functional material is functional as deposited.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Preferred embodiments of the present invention will be
described below in more detail, with reference to the accompanying
drawings:
[0024] FIG. 1 is a Ni-Cu strain gauge deposited by thermal spray,
forming a K-type thermocouple, exemplifying the use of a selective
overcoat, according to an embodiment of the present invention;
[0025] FIG. 2 is a diagram of laser processing of thermal spray
deposit to fabricate a strain gauge according to an embodiment of
the present invention;
[0026] FIG. 3 is a diagram of a laser patterned Ni-Cu strain gauge
deposited by thermal spray according to an embodiment of the
present invention;
[0027] FIG. 4 is a graph of resistance versus strain for a NiCr
thermal spray strain gauge patterned using a laser system;
[0028] FIG. 5A is a flow diagram of a method for fabricating an
electronic device according to an embodiment of the present
invention;
[0029] FIG. 5B is a diagram of the stages of thermal spray/laser
patterning of a multiplayer thermopile according to an embodiment
of the present invention;
[0030] FIG. 6 is a diagram of a multilayer thermopile, showing
connectivity between bottom and top layers according to an
embodiment of the present invention;
[0031] FIG. 7 is a diagram of a 40-element thermopile fabricated
with NiCr/NuCu on alumina, with both positive and negative
connector leads on the left-hand side of device according to an
embodiment of the present invention;
[0032] FIG. 8 is a diagram of a "star" thermopile concept, where
the two thermocouple materials are represented by different shaded
lines, according to an embodiment of the present invention;
[0033] FIG. 9 is a close-up diagram of the star thermopile
interface between dissimilar materials at inner and outer radii
according to an embodiment of the present invention;
[0034] FIG. 10 is a diagram of a micro-heater laser patterned into
NiCr coating on an alumina substrate according to an embodiment of
the present invention;
[0035] FIG. 11 is a graph of heater temperature versus input power
for heater in FIG. 10;
[0036] FIG. 12 is a diagram of an ultrafast laser trimmed of
thermal spray line according to an embodiment of the present
invention;
[0037] FIG. 13 is a diagram of laser-machined vias in a multilayer
thermal spray structure according to an embodiment of the present
invention; and
[0038] FIG. 14 is a diagram of a laser trimming process according
to an embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0039] Direct write electronics technologies provide an opportunity
to integrate mesoscopic electronic devices with the physical
structure on which the electronic systems will be used, eliminating
the need for a traditional printed circuit board. The ability to
print electronic features on flexible and conformal substrates
enables unique applications for deployable electronics, such as
placing electronics in projectiles, for flexible satellite solar
arrays, usage in rolled-up forms that can be inserted into
symmetric or odd shapes, installed on military gear, as well as
various surveillance equipment. This can save space and reduce
weight through 3-D integration. It can provide a dramatic cost
savings by eliminating the majority of passive components in
automated fabrication, while minimizing procurement. It can reduce
inventories of electronic components or parts, enable the building
of specialty parts on the "fly" without mass production set-up
costs, and increase the reliability of rugged electronic components
due to the automated assembly process and the absence of solder
joints.
[0040] According to an embodiment of the present invention, thermal
spray technology coupled with precision laser materials processing
have been developed for the fabrication of electronics and sensor
fabrication. Thermal spray is implemented for depositing a material
having functional properties as deposited, e.g., without the need
for subsequent post-firing, annealing, or other time consuming,
costly post processing steps in most cases, although these
processes can be performed if desired. A variety of materials can
be deposited quickly and easily using thermal spray technology.
After the deposition, precision laser micromachining using, for
example, ultrafast or UV laser systems, can be used to fabricate
complex electronic structures, for example, resistors, capacitors,
coils and transformers, and can also be used to fabricate a variety
of sensors, for example, thermistors, thermocouples, thermopiles,
strain sensors, magnetic sensors, humidity sensors, gas sensors,
flow sensors, heat flux sensors, etc. Furthermore, these sensors
can be embedded within a component during manufacture to provide an
extremely robust, long-life sensing and health monitoring system
for the component, which is superior to aftermarket add-on sensors
that need to be attached manually using adhesives or other
post-manufacturing techniques. Also, because the thermal spray
technique is self-compatible, it can be used to fabricate
three-dimension electronics and sensor systems, e.g., multi-layer
sensors on the same surface area footprint, multiple-layer
thermopiles for enhanced power production, etc.
[0041] A sensor that is directly embedded into the component in a
coordinated manner has substantial advantages in terms of
reliability, longevity, and minimal disturbance of component
function.
[0042] According to an embodiment of the present invention, a
system and method has been developed for the fabrication of sensors
and electronics for condition based maintenance and remote health
monitoring of engineering systems. Direct-writing technology can be
implemented for wide ranging functional electronics and sensor
structures including metals, semiconductors, ceramics and polymers
on virtually any surface. Direct-write line widths can be in the
range of 200 microns and larger. According to an embodiment of the
present invention, single layer and multi-layer electronic devices
can be fabricated through additive mask-free, environmentally
benign electronics processing technology. Direct writing systems
can be used for prototyping concepts in manufacturing as well as
provide new capabilities for the fabrication of novel embedded
electronics and sensor systems.
[0043] According to an embodiment of the present invention, systems
and methods can combine additive-subtractive fabrication using
direct write thermal spray for material addition, followed by an
ultrafast, UV, or other laser processing step for material removal.
This can allow a substantial reduction in line width to the
10-micron level and below, as well as the ability to use virtually
any material. This approach can enhance the flexibilities of both
processes, e.g., flexibility of thermal spray to deposit virtually
any material/create multiple layers on low temperature substrates,
and the advantage of ultrafast or UV pulsed lasers to non-thermally
remove materials with minimal thermal damage. Other material
removal systems can be used, for example, a water jet, electric
discharge machining, or milling machine.
[0044] The capabilities include demonstration of the hybridized
thermal spray/laser subtraction concept for an embedded sensor
system for remote health monitoring of harsh environment
engineering systems. An extended capability will involve
incorporating wireless concepts for passive or semi-passive
embedded sensors using R-L-C circuits for untethered monitoring of
the components.
[0045] The potential applications of such technology are unique and
far-reaching. Examples include strain gauges, thermistors,
thermocouples, thermopiles (thermocouples in series for power
generation), magnetic and piezo sensors, interdigitated capacitors
for L-C circuits, antennas, microheaters (for integration into
chemical and biological sensors), among others. It will allow novel
sensor and electronic devices to be prepared in-situ and, to do so
in environmentally friendly lean manufacturing methods.
[0046] Direct write electronics technologies provide an opportunity
to integrate mesoscopic electronic devices with the physical
structure on which the electronic systems will be used, eliminating
the need for a traditional printed circuit board. The ability to
print electronic features on flexible substrates enables unique
applications for deployable electronics, such as placing
electronics in projectiles, for flexible satellite solar arrays,
usage in rolled-up forms that can be inserted into symmetric or odd
shapes, installed on military gear, as well as various surveillance
equipment. This saves space and reduces weight through 3-D
integration. It provides a dramatic cost savings by eliminating the
majority of passive components in automated fabrication and
minimizing procurement. It reduces inventories of electronic
components or parts, enables the building of specialty parts on the
"fly" without mass production set-up costs, and increases the
reliability of rugged electronic components due to the automated
assembly process and the absence of solder joints.
[0047] According to an embodiment of the present invention, a
system and method combines the thermal spray capabilities with
complementary precision laser subtraction to provide substantially
improved capabilities for manufacturing embedded, conformal
electronics and sensors. For example, according to an embodiment of
the present invention, the material versatility of thermal spray
for material deposition and course patterning is coupled with the
fast, precision material removal capabilities of ultra fast and UV
lasers, which use non-thermal material removal mechanisms that
minimize thermal damage associated with more traditional laser
processing. This combination capitalizes on the strengths of both
techniques: wide material versatility coupled with high-precision
(.about.10 .mu.m) rapid patterning ability. Also, multi-layer
structures can be built with this technology, and electrical
connections, e.g., vias, have been successfully created to make
electrical connections across both layers.
[0048] Thermal spray is a directed spray process in which material
is accelerated to high velocities and impinged upon a substrate,
where a dense and strongly adhered deposit is rapidly built.
Material is injected in the form of a powder, wire, or rod into a
high velocity combustion or thermal plasma flame, or wire arc, or a
cold-spray (non-thermal) spray process, which imparts thermal and
kinetic energy to the particles. By controlling the plume
characteristics and material state (e.g., molten, softened), it is
possible to deposit a wide range of materials (metals, ceramics,
polymers and combinations thereof) onto virtually any substrate in
various conformal shapes. The ability to melt, soften, impinge,
rapidly solidify, and consolidate introduces the possibility of the
synthesizing useful deposits at or near ambient temperature. The
deposit is built-up by successive impingement of droplets, which
yield flattened, solidified platelets, and referred to as `splats`.
The deposit microstructure and, thus, properties, aside from being
dependent on the spray material, rely on the processing parameters,
which are numerous and complex.
[0049] Thermal spray has been used for decades for large-scale
applications, including, for example, TBCs in turbine engines,
internal combustion engine pistons and cylinder bores, and
corrosion protection coatings on ships and bridges. Thermal spray
can be used for meso-scale (e.g., about 100 .mu.m-10 mm)
structures, particularly for electronic applications. Thermal spray
methods can be used to form thick (e.g., greater than about 20
.mu.m), smooth deposits of a wide range of ceramics, including
alumina, spinel, zirconia, and barium titanate. Additionally, thin
(e.g., less than about 200 .mu.m wide) metallic lines of Ag, Cu, as
well as Ni-based alloys, can be produced with square sides and that
have electrical conductivities as good as, and in some cases
superior to, conductor lines formed using thin-film methods. Spray
production technologies for coatings and direct-write lines include
for example, combustion, wire arc, thermal plasmas and even cold
spray solid-state deposition.
[0050] The advantages of direct-write thermal spray for sensor
fabrication include, for example, robust sensors integrated
directly into coatings, thus providing unparalleled coating
performance monitoring, high-throughput manufacturing and
high-speed direct-write capability, and useful materials electrical
and mechanical properties in the as-deposited state. In some cases,
the properties can be further enhanced by appropriate post-spray
thermal treatment. Further advantages include being cost effective,
efficient, and able to process in virtually any environment,
robotics-capable for difficult-to-access and severe environments,
can be applied on a wide range of substrates and conformal shapes,
and is rapidly translatable development to manufacturing.
[0051] Thermal spray methods offer means to produce blanket
deposits of films and coatings as well as the ability to produce
patches, lines, and vias. Multiple layers can be produced on
plastic, metal, and ceramic substrates, both planar and conformal.
Embedded functional electronics or sensors can be over coated with
protective coating, allowing applications in harsh environments.
Such embedded harsh environment sensors can be used for
condition-based maintenance of engineering components.
[0052] High-power ultra fast laser systems, in which the laser
pulse duration is measured in femto- or pico-seconds have
advantages over their thermal-based counterparts, including,
minimal temperature rise and thermal damage in processed material,
a wide range of applicable materials, precision machining
capabilities, sub-surface (3-D) machining, and high-aspect-ratio
processing.
[0053] Ultra fast systems can use titanium-doped sapphire
(Ti:sapphire) as the lasing medium, and chirped-pulse amplification
(CPA) to produce femtosecond laser pulses with millijoule energy
levels.
[0054] UV-wavelength, nanosecond-pulse lasers implement a pulse
duration tens of thousands of times longer than an amplified
femto-second system, and use a wavelength in the UV region
(typically about 355 nm or shorter), which results in direct
bond-breaking by the incident photons. As such, like the ultra fast
lasers, material is removed in a non-thermal mechanism (thermal
damage is minimized), though not to the same extent as ultra fast
lasers.
[0055] The use of ultra fast and UV lasers for precision materials
processing works well with a wide variety of thermal spray
materials that can be deposited for sensor and electronic
applications. The combination of these two technologies provides
for the capability to fabricate robust, embedded sensors in
functional components.
[0056] Sensors can include, for example, thermistors and
thermocouples for temperature measurement as well as serpentine
strain gauges for strain measurement. Temperature and strain are
two of the most important parameters in engineering systems such as
internal combustion and turbine engines, power transmission
systems, fluid power components, transportation equipment, general
manufacturing systems, etc. A thermopile can be fabricated, which
is a series of thermocouples (as many as 100-200) in series to
produce useful voltage and current, for power generation in-situ
using an existing temperature difference in the system.
[0057] The flexibility of thermal spray in its material deposition
capability combined with the simplicity and reliability of the
thermocouple as a temperature sensor and the strain gauge as a
strain sensor make thermal-spray-based thermocouples and strain
gauges a natural choice. E-type and K-type thermocouples, and
serpentine strain gauges can be fabricated using variations of
thermal spray. Substrates sprayed include pure alumina and spinel
coated steel.
[0058] FIG. 1 shows a bare thermal-sprayed thermocouple (left) as
well as a thermocouple that has been coated with alumina (right) to
demonstrate the ability to embed such sensors underneath functional
coatings.
[0059] According to an embodiment of the present invention, thermal
barrier coatings can be used to introduce a temperature difference
in the presence of an otherwise uniform heat load or temperature
field. Thermal barrier coatings (TBCs) can be been traditionally
used to provide enhanced component lifetime in high temperature,
harsh environments by providing additional thermal resistance to
heat flow to the device. The TBC is thermal sprayed over the
component, though other coatings for wear and corrosion can also be
used. The TBC material is chosen to have a low thermal
conductivity, hence in service heat will experience a resistance in
moving from the top of the TBC to the component that is being
protected underneath. Temperature differences of about 100.degree.
C. are can be experienced. By using a TBC to selectively coat one
side of a thermopile, for example, a non-uniform temperature
distribution would be produced in the presence of a uniform heat
flux, for example from a flame, or a panel exposed to solar
radiation. The temperature difference, in turn, can be used with
the thermopile concept to produce useable electricity as discussed
above.
[0060] Thermal spray technology can be used to fabricate integral
strain gauges directly onto system components or surfaces.
Furthermore, the combination of a strain gauge and a temperature
sensor, which provides both material temperature and compensation
for the strain gauge, represent an extremely powerful combination.
One popular material for high-temperature metallic strain gauges is
NiCr. NiCr can be sprayed to form, for example, heaters and other
laser patterned devices. The initial strain gauge development was
based on NiCr, an inexpensive, readily obtainable material that
also has useful properties.
[0061] Strain gauge fabrication using thermal spray can be obtained
using a single material for the sensor device itself. The same
material, e.g., NiCr, may also be used for both the strain sensor
and the lead wires, provided the width and thickness of the lead
wires are increased such that the effective resistance of the lead
wire is negligible compared to the strain gauge element. In
practice this can be done by increasing the spray line width, while
also depositing the lead wires at a slower velocity--with the same
material feed rate--to increase line thickness. A five-fold
increase in line width and thickness over the strain sensor line
dimension, for example, results in a lead wire resistance of only
4% that for an equivalent length of sensor patterning.
[0062] Referring to FIG. 2, the strain gauge pattern can be
fabricated using either the ultra fast or UV lasers, and the
patterns follow conventional strain gauge design, with a serpentine
series of thermal spray traces forming the gauge. Specific
dimensions are determined by the desired gauge resistance, size,
sensitivity, and maximum expected strain.
[0063] Testing can be performed using precision multimeters or a
standard Whetstone-bridge-based system to record resistance while
the test specimen is strained a known amount. During testing, a
thermocouple can be attached directly over the strain gauge to
compensate for temperature during the measurement. A functioning
prototype strain gauge fabricated using the ultra fast laser is
shown in FIG. 3.
[0064] The results for a similar strain gauge fabricated using
thermal spray technology followed by ultrafast laser materials
processing is shown in FIG. 4. Repeatability between devices, in
this case two gauges, linearity, and lack of hystersis are
attributes of devices fabricated according to an embodiment of the
present invention. The gauge is fabricated on an alumina substrate,
which is then fixed at one end as a cantilever beam, while the free
end is displaced a known amount. A commercial strain gauge was
attached to the sample as well to provide a reference for the true
strain of the specimen.
[0065] More sophisticated patterns are also possible, including
depositing two mutually orthogonal patterns to measure strain in
the x and y-directions simultaneously. Thermal spray protective
overcoats can be applied for protection to the same gauge, which
will then be re-tested to assess how the overcoat influences gauge
operation. It is also possible to fabricate arrays of strain
sensors to determine variation in strain as a function of location
on a component. Strain gauge design can also be designed to
minimize temperature drift.
[0066] Strain gauges are ubiquitous and indispensable in devices
that range from micro-weight scales to structure health monitors in
buildings and bridges. Commercial devices are usually
pre-fabricated, packaged and bonded or otherwise attached to the
structure to be monitored. Our approach to mesoscale manufacturing
allows strain gauges to be fabricated in situ. Further, the sensor
might even be hardened with a final spray coat of a suitable
impervious material.
[0067] In many remote sensor-monitoring situations, wireless
concepts are required since access is not easy. For active wireless
systems, local power is essential to drive the circuit. One way to
obtain this power, for example, in hot component monitoring, is
power harvesting through thermo-piles which is an extension to
thermocouple technology.
[0068] Thermocouples produce a voltage proportional to the
temperature difference across their junctions. As temperature
sensors, they work very well. Their output voltage, however, is on
the order of several tens of millivolts per .degree. C., making
useful voltage levels for powering electronic circuits, e.g., 1-5V
difficult without extremely large temperature variations. A
thermopile is a collection of thermocouples wired electrically in
series and thermally in parallel so that their voltages add. The
idea is to fabricate a thermopile into a component that normally
experiences some form of a temperature gradient during operation,
e.g., an exhaust manifold, heat sink, friction-heated surface, or
substrate for a chemical reaction. In the presence of a temperature
difference, the thermopile will convert some of the heat flow
directly to electric power, which can be used local activation of
circuits.
[0069] The total thermopile output voltage (assuming a very high
load resistance so that current draw does not alter the voltage) is
NS.sub.ab .DELTA. T, where N is the number of thermocouples,
S.sub.ab the Seebeck coefficient, and .DELTA. T the temperature
difference between hot and cold temperature sources. For a given
thermocouple material and temperature difference, only N can be
increased to increase the output voltage. Recent work has focused
on the design and fabrication of multi-element thermopiles for
power generation and enhanced sensor applications using thermal
spray and MICE technology. A unique feature of this design is the
multilayer capability of thermal spray.
[0070] A method for fabricating an electronic device comprises
providing a substrate (501), direct writing a functional material
by a thermal spray on the substrate (502) and removing a portion of
the function material to form the electronic or sensory device
(503) (see FIG. 5A).
[0071] In this design a substrate is coated with an optional
insulating layer (504) and then the first alloy of the thermocouple
(NiCr in this example) is deposited (505). The sample is then sent
to the ultra fast processing laboratory in which the NiCr patch is
cut into a collection of N parallel strips (506). The sample is
then sent back to the thermal spray facility for an insulating
overcoat (507), followed by the deposition of the second
thermocouple alloy (NiCr in this case) (508). Finally the top
layers are patterned using the ultra fast laser again to provide
electrical separation between layers, while providing an electrical
series connection (509). This is done by slightly staggering the
top laser pattern to connect the positive terminal of one
thermocouple to the negative of the next. Proof-of-concept designs
were successfully completed with N=4. For a K-type thermocouple
(NiCr/NuCu), each thermocouple produced approximately 5.5 mV for a
total potential of .about.22 mV with a temperature difference of
.about.125.degree. C. A figure of the device in the various stages
of fabrication is shown in FIG. 5B, and a schematic of the device
is shown in FIG. 6.
[0072] Second-generation devices have been fabricated with N
ranging from 20-250. A recent K-type thermopile device with N=40
produced a voltage of .about.0.5V for a temperature difference of
.about.300.degree. C. between hot and cold junctions, and is shown
in FIG. 7.
[0073] In addition to the linear thermopile described above, a
radial thermopile can also readily be fabricated, as shown in FIGS.
8 and 9. In this design, one junction is formed on the inside of
the ring structure, and the second junction is formed at the
outside ring. As for the linear thermopile the two thermoelectric
materials are alternately deposited side by side and connected at
their ends to form the thermoelectric junctions. A heat source can
be applied at the geometric center of the thermopile array, for
example, by using a flame, torch, or by attaching a conducting
material that is thermally connected to a heat source. The outer
edge of the circle is maintained at a lower temperature either by
natural means, for example, natural convection or by the use of
fins, or by active cooling, using flowing gas, liquid or other
means to maintain a temperature difference between the center and
periphery of the star thermopile. Note that the device can work
equally well by reversing the heat source and heat sink, e.g., by
heating the edges and cooling the center.
[0074] Microheaters are resistive elements designed to deliver heat
locally to a device. They find wide application in everything from
gas flow sensors to microfluidic lab-on-a-chip devices. A
thermistor is a device whose resistance is a sensitive (and known)
function of temperature. Together, microheaters and thermistors
allow closed-loop control of temperature, even under dynamic
conditions such as ambient temperature or varying thermal load.
Suitable resistor materials can be deposited on a variety of
insulating subtracted included alumina and spinel, as well as
plastic, wood, and ceramics. Similar to the strain gauge devices
discussed above, these materials are precision laser patterned
using an ultrafast or UV laser to form a heater element with the
desired geometry, resistance, surface area, and temperature
variation (if desired). Semiconductor thermistor material can also
be deposited in the vicinity of the heater to operate as a
thermistor sensing device. Such a combination will facilitate
tighter temperature control and faster response. Thermal sprayed
thermistors as well as heater elements can be fabricated. A
photograph of the device is shown in FIG. 10, and the device
temperature as a function of input power is shown in FIG. 11.
[0075] Thermal spray can be used to deposit thin lines of material
for direct-write of electronics. These lines, while achieving line
widths of 300 .mu.m or larger, are difficult to fabricate in sized
much smaller than this. Ultrafast laser processing can be used to
pattern thermal spray deposited lines for even finer feature
resolution. To trim a thermal spray line, the laser makes multiple
passes on both sides of the line, starting from the outside and
working towards the center. The thickness of the line is determined
by stopping at a prescribed distance from the centerline. The depth
of the machining into the SPL and substrate is determined by the
stage speed. The motion control system provides for positioning
accuracy of 0.5 .mu.m.
[0076] An SEM image of a trimmed line is shown in FIG. 12. The
material is Ag sprayed onto a Ti substrate. The original line width
as sprayed is roughly 500 .mu.m. The laser-trimmed region is 80-100
.mu.m in width, and 200 .mu.m in length. For this case, 10 strips
were used on each side of the line with the laser making two passes
over each strip. The stage speed was 5 mm/s, and the process
proceeds from the outside towards the centerline of the line such
that the final pass on each side is closest to the centerline,
which is done to avoid re-deposition of material on the trimmed
portion of the line. Feature quality and uniformity are good.
[0077] The laser-machined regions cut into the substrate as well as
the SPL. This happens because there is no indicator at this time to
instruct the laser to stop cutting when the SPL line has been
completely removed and the substrate is being removed. To guarantee
the entire SPL line was removed, the stage speed was run slower
than needed. To optimize the technique, parameters can be
empirically determined to provide sufficient removal of material.
Alternatively, the laser-processed region can be dynamically
monitored to determine when the substrate has been reached. For
example, the laser-processed feature can be monitored using a video
camera or the ablated material can be analyzed using a fiber-optic
spectrometer, shutting off the laser when substrate material begins
to be ablated.
[0078] Another observation made is that the trimmed lines are not
perfectly sharp. Referring to FIG. 13, it can be seen that the
spatial profile of the laser beam influences the trimmed line. The
tighter the beam is focused (for a smaller spot size), the more
sharp the "hourglass" shape of the beam becomes. If sharp,
rectangular features are mandatory, it may be possible to prescribe
a more complicated laser-material path to minimize beam profile
effects that tend to round the tops of the trimmed lines.
[0079] Vias can be fabricated into a thermal-sprayed multilayer
structure using the motion control system. Feature quality can be
improved substantially. The vias, as with the handmade case, are
done in a thermal sprayed electrical inductor comprising several
layers, for example: Ti-substrate, bonding layer, ceramic
insulator, bottom Ag conductor, ceramic insulator, ferrous inductor
material, insulator, and top Ag conductor.
[0080] Feature quality and edge definition is very good. The
perspective view on the right in FIG. 14 is slightly deeper near
the edges. This occurs because the stage cannot accelerate or
decelerate infinitely fast, and the stage velocity is slower in
this region, resulting in more pulses per site and corresponding
deeper features. This issue has been addressed and corrected
recently.
[0081] Thermal spray technology is suited for developing multilayer
sensors for enhanced performance. The thermopile concepts discussed
above, for example, can be extended by fabricated several devices
on top of one another. For example subsequent linear thermopiles
can be fabricated on top of previous devices by thermal spraying an
insulating layer between devices. In this fashion, all thermopiles
would experience approximately the same temperature difference,
however the individual devices could be electrically connected in
either parallel or series, depending on the needs of the electrical
load that the thermopile will drive.
[0082] Similarly, multiple sensors or devices could be fabricated
on the same physical area on a substrate, for example, a
thermocouple for temperature measurement, a strain gauge for strain
measurement, a magnetic multilayer device and a microheater for
periodic burn off of contaminants could be fabricated on the same
physical footprint by using a multilayer fabrication approach, and
is a natural extension of the thermal spray capabilities and
strengths.
[0083] Having described embodiments for a method of fabricating
conformal electronics using additive-subtractive techniques, it is
noted that modifications and variations can be made by persons
skilled in the art in light of the above teachings. It is therefore
to be understood that changes may be made in the particular
embodiments of the invention disclosed which are within the scope
and spirit of the invention as defined by the appended claims.
Having thus described the invention with the details and
particularity required by the patent laws, what is claimed and
desired protected by Letters Patent is set forth in the appended
claims.
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