Note: Descriptions are shown in the official language in which they were submitted.
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1 METHOD
FOR FORMING THIN FILM CONDUCTORS ON A SUBSTRATE
2
3 BACKGROUND OF THE INVENTION
4
1. Technical Field
6
7 The
present invention relates to thin films in general, and, in particular, to
8 a method of forming thin film conductors on a substrate.
9
2. Description of Related Art
11
12
Photonic curing is the high-temperature thermal processing of a thin film
13 using
light pulses from a flashlamp. Photonic curing allows thin films on low-
temperature
14 substrates to be processed in much shorter time periods (about 1
millisecond) than with an
oven (which takes seconds to minutes).
- -
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, a thin
film
precursor material is initially deposited onto a porous substrate. The thin
film precursor material
is then irradiated with a light pulse in order to transform the thin film
precursor material to a
thin film such that the thin film is more electrically conductive than the
thin film precursor
material. Finally, compressive stress is applied to the thin film and the
porous substrate to
further increase the thin film's electrical conductivity.
Certain exemplary embodiments can provide a method for forming a thin film
conductor
on a substrate, said method comprising: depositing a thin film precursor
material onto a porous
substrate; irradiating said thin film precursor material with a light pulse to
transform said thin
film precursor material to a thin film, wherein said thin film is more
electrically conductive than
said thin film precursor material; and applying compressive stress to said
thin film and said
porous substrate by a pair of pinch rollers to further increase said thin
film's electrical
conductivity, wherein said pinch rollers are driven at co¨v/r, where co is an
angular velocity of
said pinch rollers, r is a radius of said pinch rollers, and v is a moving
speed of said thin film.
Certain exemplary embodiments can provide a method for forming a thin film
conductor
on a substrate, said method comprising: depositing a thin film precursor
material onto a porous
substrate; irradiating said thin film precursor material with a light pulse to
transform said thin
film precursor material to a thin film, wherein said thin film is more
electrically conductive than
said thin film precursor material; and applying compressive stress to said
thin film and said
porous substrate to further increase said thin film's electrical conductivity,
wherein said
applying of compressive stress oscillates in magnitude with time.
All features and advantages of the present invention will become apparent in
the
following detailed written description.
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1 BRIEF DESCRIPTION OF THE DRAWINGS
2
3 The
invention itself, as well as a preferred mode of use, further objects, and
4
advantages thereof, will best be understood by reference to the following
detailed
description of an illustrative embodiment when read in conjunction with the
accompanying
6 drawings, wherein:
7
8 Figure
1 is a high-level process flow diagram of a method for forming thin
9 film
conductors on a substrate, in accordance with a preferred embodiment of the
present
invention;
11
12 Figure
2 is a diagram of a photonic curing apparatus, in accordance with a
13 preferred embodiment of the present invention; and
14
Figure 3 is a graph showing the height profile of a copper film on a paper
16 substrate before and after being compressed using the method depicted in
Figure 1.
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1 DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
2
3 The relatively short processing time enabled by photonic curing
can cause
4 problems. One of the artifacts of photonic curing is that the rapid
heating of a thin film
can generate gas within the thin film. If the gas generation is violent
enough, the thin film
6 will undergo a complete cohesive failure, i.e., it may explode. More
commonly, the thin
7 film develops a slight porosity. Often, the porosity is inconsequential,
but it can, under
8 certain conditions, cause the thin film to be more mechanically fragile
than its denser
9 counterparts. Furtheitnore, if the thin film has any electronic
functionality, such as
electrical conductivity, its sheet resistance will be higher as a result of
the porosity. The
11 increased porosity in the thin film can also exhibit increased surface
roughness as well.
12 This can inhibit the attachment of electrical components as well as
diminish the cosmetic
13 appearance of the thin film. In addition, the increased porosity can
cause enhanced
14 degradation of the thin film over time if the processed thin film is
sensitive to elements,
such as water or oxygen, commonly found in the environment. Thus, it would be
desirable
16 to provide an improved method for forming thin film conductors on a
substrate.
17
18 Referring now to the drawings and in particular to Figure 1,
there is depicted
19 a flow diagram of a method for founing thin film conductors on a
substrate, in accordance
with a preferred embodiment of the present invention. Starting at block 100, a
thin film
21 precursor material is initially deposited onto a substrate, as shown in
block 110. The
22 material is then thettnally processed with a photonic curing apparatus
such that the thin film
23 precursor material becomes a thin film material, as depicted in block
120. The electrical
24 conductivity of the thin film material is higher than that of the thin
film precursor material.
Finally, compressive stress is applied to the thin film material located on
the substrate to
26 cause the thin film material to densify such that its electrical
conductivity of the thin film
27 material can be further increased, as shown in block 130.
28
29
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1 A. Depositing thin film precursor material
2 The
thin film precursor material can be in a particulate form. The thin film
3
precursor material can also be dispersed in a liquid. The thin film precursor
material can
4 be
deposited onto a substrate by one or combinations of printing methods such as
screen
printing, inkjet, aerosol jet, flexographic, gravure, laser, pad, dip pen,
syringe, or coating
6 methods such as airbrush, painting, roll coating, slot die coating, etc.
7
8
Alternatively, the thin film precursor material can be deposited without a
9 liquid
including vacuum deposition techniques such as chemical vapor deposition
(CVD),
PECVD, evaporation, sputtering, etc. Other dry coating techniques in which the
thin film
11 precursor material can be deposited include electrostatic deposition,
xerography, etc.
12
13 The
thin film precursor material is preferably contains a metal and/or a metal
14
compound such as an oxide, salt, or organometallic. The thin film precursor
material can
be copper, nickel, cobalt, silver, carbon, aluminum, silicon, gold, tin, iron,
zinc, titanium,
16 etc.
Examples of oxides include Cu2O, CuO, Co304, Co203, NiO, etc. Examples of
salts
17
include copper (II) nitrate, copper (II) chloride, copper (II) acetate, copper
(II) sulphate, as
18 well
as nitrates, chorides, acetates, and sulphates of cobalt, nickel, silver, etc.
If the thin
19 film
precursor material contains a metal compound, a reducing agent generally
accompanies
it as well.
21
22 The
substrate, which may be porous, preferably has a maximum working
23
temperature of less than 450 C. Examples include polymers and cellulose.
Examples of
24 porous
substrates include fiber based films that are calendered such as cellulose
(e.g., paper)
or polyethylene (e.g., Tyvek manufactured by DuPont ). Alternatively, the
porosity may
26 be induced in the substrate by foaming the substrate material.
27
28 B. Photonic curing of the thin film precursor material
29 When
the thin film precursor material is printed within a liquid, thermal
processing of the thin film precursor material evaporates the solvent. If the
thin film
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1
precursor material is the particulate form of the final thin film, the
photonic curing
2
additionally sinters the thin film precursor material. If the thin film
precursor material is
3
composed of multiple species designed to chemically react with each other
(such as a metal
4
compound and a reducing agent), then the theinial processing additionally
reacts the
precursor thin film material to form the final thin film which is generally a
metal.
6
7 Thin
film precursor material can be processed thermally using a photonic
8 curing
apparatus. With reference now to Figure 2, there is illustrated a diagram of a
9
photonic curing apparatus, in accordance with a preferred embodiment of the
present
invention. As shown, a photonic curing apparatus 200 includes a conveyor
system 210, a
11 strobe
head 220, a relay rack 230, and a reel-to-reel feeding system 240. Photonic
curing
12
apparatus 200 is capable of irradiating a thin film precursor material 202
deposited on a
13
substrate 203 situated on a web being conveyed past strobe head 220 at a
relatively high
14 speed.
16 Strobe
head 220 includes a high-intensity xenon flashlamp 221 for curing
17 thin
film precursor material 202 located on substrate 203. Xenon flashlamp 221 can
18
provide pulses of different intensity, pulse length, and pulse repetition
frequency. For
19
example, xenon flashlamp 221 can provide 10 ,as to 10 ms pulses with a 3" by
6" wide
footprint at a pulse repetition rate of up to 1 kHz. The spectral content of
the emissions
21 from
xenon flashlamp 221 ranges from 200 nm to 2,500 nm. The spectrum can be
adjusted
22 by
replacing the quartz lamp with a ceria doped quartz lamp to remove most of the
23
emission below 350 nm. The quartz lamp can also be replaced with a sapphire
lamp to
24 extend
the emission from approximately 140 nm to approximately 4,500 nm. Xenon
flashlamp 221 can also be a water wall flash lamp that is sometimes referred
to as a
26 Directed Plasma Arc (DPA) lamp.
27
28 Relay
rack 230 includes an adjustable power supply, a conveyance control
29
module, and a strobe control module. The adjustable power supply can produce
pulses with
energy of up to 4 kJ per pulse. Adjustable power supply is connected to xenon
flashlamp
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1 221, and the intensity of the emission from xenon flashlamp 221 can be
varied by
2 controlling the amount of current passing through xenon flashlamp 221.
3
4 The adjustable power supply controls the emission intensity of
xenon
flashlamp 221. The power, pulse duration, and pulse repetition frequency of
the emission
6 from xenon flashlamp 221 are electronically adjusted in real time and
synchronized to the
7 web speed to allow optimum curing of thin film precursor material 202
without damaging
8 substrate 203, depending on the optical, thermal, and geometric
properties of thin film
9 precursor material 202 and substrate 203. Preferably, the time duration
of irradiation of
each light pulse is less than the time to thermal equilibration time of the
stack comprising
11 thin film precursor material 202 on substrate 203.
12
13 During the irradiation with light pulses, substrate 203 as well
as thin film
14 precursor material 202 is being moved by conveyor system 210. Conveyor
system 210
moves thin film precursor material 202 under strobe head 220 where thin film
precursor
16 material 202 is cured by rapid light pulses from xenon flashlamp 221.
The power,
17 duration, and repetition rate of the emissions from xenon flashlamp 221
are controlled by
18 the strobe control module, and the speed at which substrate 203 is being
moved past strobe
19 head 220 is determined by the conveyor control module.
21 When xenon flashlamp 221 is emitting light pulses, thin film
precursor
22 material 202 is momentarily heated to provide the energy for curing thin
film precursor
23 material 202. When a rapid pulse train is synchronized to moving
substrate 203, a uniform
24 cure can be attained over an arbitrarily large area as each section of
thin film precursor
material 202 may be exposed to multiple light pulses, which approximates a
continuous
26 curing system such as an oven.
27
28 C. Compressing processed material
29 After thin film precursor material 202 located on substrate 203
has been
photonically cured with flashlamp 221 to form a thin film material 202',
compressive stress
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1 is applied to thin film material 202' and substrate 203 in order to
densify thin film material
2 202 and substrate 203. Thin film material 202' on substrate 203 can be
compressed by one
3 or combinations of existing technologies such as stamping, forging,
rolling, calendering,
4 pressing, embossing, laminating, etc.
6 Rolling is preferably used in a reel-to-reel manufacturing
setting by a set of
7 pinch rollers 260. Pinch rollers 260 are loaded, in compression, such
that the peak pressure
8 applied to thin film material 202' and substrate 203 exceeds 25% of the
ultimate tensile
9 strength (UTS) of the bulk thin film material after photonic curing at
standard conditions.
For a relatively soft and ductile metal like copper, the preferred compression
pressure range
ii is between 7,500 and 30,000 psi (i.e., 25% to 100% of its ultimate
tensile strength at
12 standard conditions).
13
14 Because substrate 203 is porous, it is compressible and
responds to
compression by reducing in thickness while keeping the same width, such as a
fiber based
16 substrate like paper. This single dimensional change ensures that thin
film material 202'
17 is not damaged by lateral deformation of substrate 203. The peak
pressure capable of being
18 applied by pinch rollers 260 to polymer substrates that are non-porous,
such as PET, may
19 be limited because PET is a low-temperature polymer that tends to be
relatively soft. PET
will deform laterally at a lower pressure threshold than other substrates,
which can cause
21 damage to thin film material 202' and substrate 203.
22
23 Pinch rollers 260 are driven at angular velocity co = v/r,
where a) is the
24 angular velocity of pinch rollers 260 and r is the radius of pinch
rollers 260, adjusted, and
synchronized to the web speed, v, to allow optimum densification of thin film
material 202'
26 without damaging substrate 203, depending on the mechanical and
geometric properties of
27 thin film material 202' and substrate 203.
28
29 In certain situations, it may be advantageous to apply dynamic
compressive
stress (oscillating magnitude over time) with pinch rollers 260, driven at a
certain
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1 frequency, to thin film material 202' on substrate 203 to achieve high
peak pressures with
2 a lower average force on pinch rollers 260 to extend tool lifetime and/or
increase maximum
3 web speeds.
4
Heating pinch rollers 260 to a temperature between standard temperature and
6 the maximum working temperature of substrate 203 can decrease the
required pressure to
7 achieve a similar result with standard temperature pinch rollers 260 due
to the softening of
8 thin film material 202' during compression.
9
Compressive stress applied to thin film material 202' deposited on substrate
11 203 can increase the density of thin film material 202'. A particle or
solution-based
12 deposited material has a density lower than the bulk precursor material
due to a residual
13 pore structure within the deposited layer. Additionally, the photonic
curing process may
14 introduce additional porosity in thin film material 202'. The volume of
pore space relative
to layer volume (volume fraction) will vary depending on material, process,
and particle
16 size. Reducing the pore space volume fraction densifies the material
improving its
17 performance in teaus of increased electrical conductivity if it is
conductive, improved
18 mechanical stability and hardness, alters the surface properties like
reducing surface
19 roughness and improving solder-ability, and improved chemical resistance
if the material
is prone to corrosion by reducing the surface area to volume ratio.
Compressing thin film
21 material 202' increases it density, which brings deposited thin film
material 202' closer to
22 the properties of the bulk thin film material.
23
24 The following examples illustrate various methods of applying
compressive
stress to thin film materials located on a substrate. The results of
compressive stress are
26 densification of thin film material on the substrate such that
conductivity, mechanical
27 stability, and chemical resistance of the thin film material are
improved.
28
29
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1
Example 1: Compressive stress applied to thin films of mesoporous copper on
paper
2 substrates
3 A
screen printable version of a copper oxide reduction ink (part no. ICI-021
4
available from NovaCentrix in Austin, Texas) was printed on Wausau 110 lb
exact index
paper with a 230 mesh flat screen. The print was then dried in a 140 C oven
for 5 minutes
6 to
remove excess solvents. Initially, the ink had a sheet resistance that was ¨1
GQ/111.
7 That is, the resistance as measured by an ordinary multimeter was an open
circuit.
8
9 The
ink was converted to a conductive mesoporous copper thin film using
a photonic curing apparatus (such as PulseForge 3300 X2 photonic curing
system
11
manufactured by NovaCentrix in Austin, Texas). The settings on the machine
used for
12 curing
were 430 V, 1,600 ms, overlap factor of 5, and at a web speed of 16 feet per
13 minute. The sheet resistance after photonic curing was 17.2 mK2/111.
14
The mesoporous copper thin film underwent densification via the following
16
process: A pair of steel rollers (1.7" diameter x 3.0" length) applied a
compressive force
17 of
2,875 lbf to the foamed copper thin film on paper as it was drawn through the
rollers.
18 The
cross sectional area of compression was 0.074 in', yielding an average 38,850
psi
19
applied to the printed conductors. Densification, via compression, of the
mesoporous
copper reduced the sheet resistance to 9.3 mf2/ EL Thus, compressing the
mesoporous
21 copper decreased its resistivity by 46%.
22
23
Additional benefits to the overall perfoimance of the compressed copper
24 film,
besides improved electrical conductivity, became apparent during surface
mounted
device (SMD) attachment evaluation, mechanical stability testing, and
environmental
26 testing.
27
28
Compressed copper has demonstrated a significantly improved success rate
29 of
attaching SMD components over as-converted mesoporous copper. The failure rate
was
50% for thermode bonded SIVID silicon chips to the mesoporous copper.
Compressed
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1 copper
demonstrated a much higher success rate of 90% due to its low surface
roughness.
2
Additionally, the reduced surface roughness alters the optical properties of
the converted
3 copper from matte to nearly specular reflectivity at high pressures.
4
Referring now to Figure 3, there is illustrated a graph showing a height
6
profile of foamed copper on paper before and after compression. Both the total
height and
7 the
surface roughness are reduced indicating increased density and reduced surface
8
roughness of the copper film. Specifically, the surface roughness was reduced
from 25
9 micron
to 5 microns. The entire thin film stack was reduced in thickness by about 50
microns.
11
12 For
copper on paper, there is a saturation point for what pressures improve
13 the
electrical conductivity below the UTS of pure copper. As-converted mesoporous
copper
14
measured about 30 mn/111 in sheet resistance. Mesoporous copper film
compressed at
8,300 psi (27% UTS of pure copper) measured 22 m52/111 in sheet resistance. At
a pressure
16 of
12,000 psi (40% UTS of pure copper) the sheet resistance reached a minimum
value of
17 20
mS2/ El (saturation point). Further increasing the applied pressure to 25,000
psi (83%
18 UTS of
pure copper) saw no improvement on the sheet resistance of the copper films.
19
However, when tracking the conductivity over time it was observed that copper
films
compressed at 12,000 psi gained in sheet resistance by 20% over 40 days in
air. The
21 copper
films compressed at 25,000 psi only gained in sheet resistance by 5% over 40
days
22 in
air. Therefore, pressures beyond 40% UTS of pure copper (12,000 psi) are
required for
23 corrosion resistance and stability over time for the copper thin film.
24
Even though increasing the applied stress by 2x did not improve the
26
electrical conductivity of the films, the stability was greatly improved. This
means that the
27 pore
space volume fraction was reduced with the increased pressure (25,000 psi)
and/or the
28 copper
material was completely yielded and did not "spring back" like the films
compressed
29 at
half the pressure. The spring back effect is commonly seen in traditional
sheet metal
forming. In a manufacturing environment, in order to reduce a piece of sheet
metal in
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1 gauge,
the material must be compressed or rolled through multiple stages. A single
stage
2 gauge
reduction is not useful due to the metal's tendency to expand in thickness
after being
3
reduced because of the elastic deformation component of the process. In this
case, the
4 foamed
copper compressed at 80% UTS yields completely and prevents residual elastic
stress from degrading overall performance and stability of the compressed
film.
6
7 After
photonic curing, the converted mesoporous copper has a high surface
8 area
to volume ratio contributing to its poor native corrosion resistance.
Compressing the
9
mesoporous copper greatly reduces the surface area to volume ratio of the
copper and
improves the material's corrosion resistance. Environmental testing was
perfolmed on bare
11 as-
converted and compressed copper films on paper substrate. Compressed copper
12
demonstrates a significantly improved corrosion resistance when tested in an
environment
13 at 85
C/100% relative humidity for 24 hours. Uncoated mesoporous copper on paper
does
14 not
survive such an environmental test, but compressed copper survives un-coated
and
without a detectable change in conductivity. Uncoated compressed copper films
passed an
16
industry standard (1,000 hours at 85 C/85% relative humidity) with only an
increase in
17
resistivity by 20%. Additional cost benefits pertaining to production become
apparent as
18
required volumes of materials for encapsulating the compressed copper films
are decreased
19 relative to as-converted copper.
21 When
it is desirable to only reduce the surface roughness of the thin film,
22
significantly lower pressure may be used. As-cured films of mesoporous copper
on paper
23
substrate exhibiting average surface roughness of 5 microns were compressed at
2,600 psi
24 (9%
UTS of pure copper) reducing the average surface roughness to 2 microns. At
this
pressure, the electrical conductivity of the mesoporous copper films was
unchanged.
26
27
Example 2: Compressive stress applied to porous thin films of nickel on paper
substrates
28 A
screen printable version of a nickel flake ink (part no. 79-89-16 available
29 from
NovaCentrix in Austin, Texas) was printed on Wausau 110 lb exact index paper
with
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a 230 mesh flat screen. The prints were dried in a 150 C oven for 5 minutes to
remove
2 excess solvents. After oven drying the sheet resistance measured 77
Q/171.
3
4 The
dried ink was photonically cured to form a highly conductive porous
nickel thin film using a photonic curing apparatus (such as PulseForge 3300
X2 photonic
6 curing
system manufactured by NovaCentrix in Austin, Texas). The settings on the
7
photonic curing apparatus used for curing were 540 V, 1,100 ms, overlap factor
of 4, at a
8 web
speed of 14 feet per minute. Photonic curing reduced the sheet resistance of
the nickel
9 film on the paper substrate to 550 mS2/III.
11 The
porous nickel thin film underwent densification via the following
12
process: A pair of steel rollers (1.7" diameter x 3.0" length) applied a
compressive force
13 of
2,464 lbf to the porous nickel thin films on paper as they were drawn through
the
14
rollers. The cross-sectional area of compression was 0.074 in2, yielding an
average 33,300
psi applied to the printed conductors. Densification, via compression, of the
porous nickel
16
reduced the sheet resistance to 60 m52/111. Compressing the porous nickel
decreased its
17 resistivity by 89%.
18
19 Example 3: Compressive stress applied to thin films of silver on paper
substrates
A screen printable version of a silver flake ink (part no. HPS-03OLV
21
available from NovaCentrix in Austin, Texas) was printed on Wausau 110 lb
exact index
22 paper
with a 230 mesh flat screen. The print was dried in a 170 C oven for 5 minutes
to
23 remove
excess solvents and cause sintering of the silver flakes. After oven drying
the sheet
24 resistance measured 16.9 mf2/111.
26 The 5
micron thick silver trace on paper substrate underwent densification
27 via
the following process: A pair of steel rollers (1.7" diameter x 3.0" length)
applied a
28
compressive force of 1,848 lbf to the silver thin Elms on paper as they were
drawn through
29 the
rollers. The cross sectional area of compression was 0.074 in2, yielding an
average
24,970 psi applied to the printed conductors. Densification, via compression,
of the silver
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1
reduced the sheet resistance to 14.2 m2'E1. Compressing the silver film
decreased the
2 resistivity by 16%.
3
Example 4: Compressive stress applied to thin films of mesoporous copper on
PET
substrates
6 A
screen printable version of a copper oxide reduction ink (part no. ICI-021
7
available from NovaCentrix in Austin, Texas) was printed on ST505 polyethylene
8
terephthalate (PET) film with a 230 mesh flat screen. The print was then dried
in a 140 C
9 oven
for 5 minutes to remove excess solvents. Initially, the ink had a sheet
resistance that
was ¨IGO/E . That is, the resistance as measured by an ordinary multimeter was
an open
11 circuit.
12
13 The
ink was converted to a conductive mesoporous copper thin film using
14 a
photonic curing apparatus (PulseForge 3300 X2 photonic curing system
manufactured
by NovaCentrix in Austin, Texas). The settings on the machine used for curing
were 360
16 V,
2,500 ms, overlap factor of 1, and at a web speed of 16 feet per minute. The
sheet
17 resistance after photonic curing was 46 inf2/0.
18
19 The
mesoporous copper thin film underwent densification via the following
process: A pair of steel rollers (1.7" diameter x 3.0" length) applied a
compressive force
21 of
1,027 lbf to the foamed copper thin film on paper as it was drawn through the
rollers.
22 The
cross sectional area of compression was 0.074 in2, yielding an average 13,873
psi
23
applied to the printed conductors. Densification, via compression, of the
mesoporous
24 copper
reduced the average sheet resistance to 34 mS2/0. Thus, compressing the
mesoporous copper decreased its resistivity by 26% and reduced its surface
roughness.
26
27 The
pressure applied to the thin films of mesoporous copper on PET was
28 nearly
half the pressure used in Example 1. This was done to preserve the copper film
due
29 to the
tendency of PET to deform laterally at pressures exceeding its yield pressure
of
15,000 psi.
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1 When compressive stress is applied only to the printed areas of
a thin film
2 (i.e., not the entire thin film and substrate), significantly higher
pressures (greater than the
3 yield pressure of the substrate such as PET) may be applied to the thin
film of mesoporous
4 copper and nonporous PET substrate to increase the density and electrical
conductivity of
the thin film. The limitation of rolling compression at pressures greater than
the yield
6 pressure of the nonporous substrate is removed as lateral deformation
local to the thin film
7 conductor does not disrupt the thin film conductor's contiguity, where
complete areal
8 compression does. This type of area specific compression of printed
circuits may be
9 accomplished through the use of a stamping tool such as an embossed
roller. The
embossed roller may have a raised pattern matching the printed circuit pattern
and would
11 contact and compress only in the printed regions on the substrate,
leaving the majority of
12 substrate uncompressed. Generally, this technique is useful for printed
depositions covering
13 less than 50% of the substrate. As the percentage of deposition area
increases to 100%, the
14 area specific compression tends to behave more like rolling compression
where the entire
web of substrate is compressed, thus forfeiting the advantage.
16
17 As has been described, the present invention provides a method
for forming
18 thin film conductors on a substrate.
19
While the invention has been particularly shown and described with reference
21 to a preferred embodiment, it will be understood by those skilled in the
art that various
22 changes in form and detail may be made therein without departing from
the spirit and scope
23 of the invention.
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