U.S. patent application number 12/502994 was filed with the patent office on 2009-11-05 for molded dielectric layer in print-patterned electronic circuits.
This patent application is currently assigned to PALO ALTO RESEARCH CENTER INCORPORATED. Invention is credited to ANA C. ARIAS, JURGEN H. DANIEL.
Application Number | 20090275192 12/502994 |
Document ID | / |
Family ID | 39541698 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090275192 |
Kind Code |
A1 |
DANIEL; JURGEN H. ; et
al. |
November 5, 2009 |
MOLDED DIELECTRIC LAYER IN PRINT-PATTERNED ELECTRONIC CIRCUITS
Abstract
A method forms a first active electronic layer, prints an array
of pillars on the first active electronic layer, dispenses a
curable polymer over the array of pillars, molds the curable
polymer by contacting the curable polymer with a mold structure to
displace the curable polymer from upper surfaces of the pillars,
cures the curable polymer to produce a hardened polymer, and
removes the array of pillars to leave an array of holes in the
hardened polymer. Another method provides a substrate having
selected areas, prints an array of pillars on the substrate,
dispenses a curable polymer over the array of pillars, molds the
curable polymer by contacting the array of pillars with a mold
structure to displace the curable polymer from upper surfaces of
the pillars, cures the curable polymer to produce a hardened
polymer, and removes the array of pillars to leave an array of
holes in the hardened polymer corresponding to the selected areas.
Another method forms a first active electronic layer on a
substrate, prints an array of conductive pillars on the active
electronic layer on a substrate, dispenses a curable polymer on the
array of conductive pillars, molds the curable polymer by
contacting the array of pillars with a mold structure to displace
the curable polymer from the upper surfaces of the conductive
pillars, curing the curable polymer to produce a hardened polymer,
and forms a second active electronic layer on the hardened polymer
such that the second active electronic layer is in electrical
connection with the first active electronic layer through the
conductive pillars.
Inventors: |
DANIEL; JURGEN H.; (SAN
FRANCISCO, CA) ; ARIAS; ANA C.; (SAN CARLOS,
CA) |
Correspondence
Address: |
MARGER JOHNSON & MCCOLLOM/PARC
210 MORRISON STREET, SUITE 400
PORTLAND
OR
97204
US
|
Assignee: |
PALO ALTO RESEARCH CENTER
INCORPORATED
Palo Alto
CA
|
Family ID: |
39541698 |
Appl. No.: |
12/502994 |
Filed: |
July 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11615229 |
Dec 22, 2006 |
7576000 |
|
|
12502994 |
|
|
|
|
Current U.S.
Class: |
438/613 ;
257/E21.476 |
Current CPC
Class: |
H01L 2224/2784 20130101;
H01L 2224/2783 20130101; H05K 3/107 20130101; H01L 27/14683
20130101; H01L 2224/27001 20130101; H01L 29/0676 20130101; H01L
29/0673 20130101; H01L 2224/73104 20130101; H05K 2203/0582
20130101; H01L 2224/2783 20130101; H01L 2924/00012 20130101; B82Y
10/00 20130101; H05K 3/0023 20130101; H05K 2203/0278 20130101; H01L
29/0665 20130101; H01L 2224/27001 20130101; G03F 7/0017 20130101;
H01L 2924/00012 20130101; H05K 3/0017 20130101 |
Class at
Publication: |
438/613 ;
257/E21.476 |
International
Class: |
H01L 21/44 20060101
H01L021/44 |
Goverment Interests
GOVERNMENT FUNDING
[0002] This invention was made with Government support under
Cooperative Agreement No. 70NANB3H3029 awarded by the National
Institute of Standards and Technology. The Government has certain
rights in this invention.
Claims
1. A method, comprising: forming a first active electronic layer on
a substrate; printing an array of conductive pillars on the active
electronic layer on a substrate; dispensing a curable polymer on
the array of conductive pillars; molding the curable polymer by
contacting the array of pillars with a mold structure to displace
the curable polymer from the upper surfaces of the conductive
pillars; curing the curable polymer to produce a hardened polymer;
and forming a second active electronic layer on the hardened
polymer such that the second active electronic layer is in
electrical connection with the first active electronic layer
through the conductive pillars.
2. The method of claim 1, wherein printing an array of conductive
pillars comprises printing one of metal pillars, polymer bumps
covered with a conductive material, conductive pillars mixed with
structural particles of another material.
3. The method of claim 1, wherein molding the curable polymer
comprises contacting the array of conductive pillars with a
conductive mold to determine if any residual polymer is on a
surface of the conductive pillars.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 11/615,229, entitled MOLDED DIELECTRIC LAYER
IN PRINT-PATTERNED ELECTRONIC CIRCUITS, filed Dec. 22, 2006, the
disclosure of which is herein incorporated by the reference in its
entirety.
BACKGROUND
[0003] It is possible to form electronic circuits using printing
technologies, typically ink-jet printing, where the `ink` is
actually liquids that can form the structures. Printed lines and
dots used in forming the circuit have relatively large features
sizes that are not conducive to some electronic devices.
[0004] In one example, assume an active matrix display backplane
with high fill-factor pixels. Fill factor is generally a ration of
the area of pixel that is actively controlling, or in the case of
active-matrix sensor arrays receiving/sensing light, to the area of
the entire pixel. Each pixel of a thin-film transistor (TFT) active
matrix backplane generally has a switching transistor and a pixel
pad for each pixel.
[0005] Each transistor typically consists of several layers: a gate
layer, a gate dielectric layer, a source/drain layer and a
semiconducting layer. In typical pixels with bottom-gate TFTs, the
data lines which apply the data signals to the pixels are on the
same level as the source/drain layer of the TFT and of the pixel
pad. If the data lines are wide, the region of the pixel pad
becomes smaller within a given area for a pixel. The area of the
pixel is limited because image quality generally comes from a
number of pixels per image, and as many pixels as possible are
squeezed onto a given backplane.
[0006] In an approach to overcome this problem an additional metal
layer is introduced which extends or `mushrooms` the pixel drain
pad layer over the transistor circuitry and partially over the data
lines. This also shields the TFT channel region from light which is
essential for low charge-leakage in the TFT off-state. Vias are
formed in a dielectric layer over the transistor to allow
connection between this `mushroom metal` pixel pad and the
underlying drain pad which is connected to the drain of the TFT.
The example of a display backplane is compelling, but forming of
vias for interconnects between layers is also important in other
electronic circuits where often multiple dielectric layers separate
conducting, semiconducting or otherwise functional layers, referred
to here as active electronic layers or active layers. Moreover,
forming simple via holes in a dielectric layer is required for
other applications such as in microfluidic circuits where the
dielectric layer may contain a fluid and sensing elements lie in a
layer underneath the dielectric.
[0007] However, forming vias in a printing technology can be a
problem. Printing technologies tend to be additive, where things
are added together to form images, such as in color printing where
colors are added together to form a final color. Circuits formed
from printing technologies are generally formed by adding layers to
other layers to form the structures. In one example, a conventional
semiconductor fabrication process deposits a continuous dielectric
layer. To form vias, the process must etch the vias into the
dielectric. In another example, a micromolding process molds a
polymeric dielectric layer with vias in a single step, if alignment
is included in the step. However, a thin surface layer typically
remains at the bottoms of the vias that has to be removed by
etching.
[0008] Generally, etching, such as wet-chemical or plasma etching,
does not occur in printing technologies. This makes forming the
vias problematic, as vias are normally formed by etching.
SUMMARY
[0009] An embodiment is a method that forms a first active
electronic layer, prints an array of pillars on the first active
electronic layer, dispenses a curable polymer over the array of
pillars, molds the curable polymer by contacting the curable
polymer with a mold structure to displace the curable polymer from
upper surfaces of the pillars, cures the curable polymer to produce
a hardened polymer, and removes the array of pillars to leave an
array of holes in the hardened polymer.
[0010] Another embodiment is a method that provides a substrate
having selected areas, prints an array of pillars on the substrate,
dispenses a curable polymer over the array of pillars, molds the
curable polymer by contacting the array of pillars with a mold
structure to displace the curable polymer from upper surfaces of
the pillars, cures the curable polymer to produce a hardened
polymer, and removes the array of pillars to leave an array of
holes in the hardened polymer corresponding to the selected
areas.
[0011] Another embodiment is a method that forms a first active
electronic layer on a substrate, prints an array of conductive
pillars on the active electronic layer on a substrate, dispenses a
curable polymer on the array of conductive pillars, molds the
curable polymer by contacting the array of pillars with a mold
structure to displace the curable polymer from the upper surfaces
of the conductive pillars, curing the curable polymer to produce a
hardened polymer, and forms a second active electronic layer on the
hardened polymer such that the second active electronic layer is in
electrical connection with the first active electronic layer
through the conductive pillars.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Embodiments of the invention may be best understood by
reading the disclosure with reference to the drawings, wherein:
[0013] FIG. 1 shows an example of an array of pillars.
[0014] FIG. 2 shows a more detailed view of a pillar.
[0015] FIGS. 3-8 show an example of a method of forming vias using
printing techniques.
[0016] FIGS. 9-11 show an alternative example of a method of
forming vias using printing techniques.
[0017] FIGS. 12-15 show an example of a method of forming extended
regions of a dielectric layer.
[0018] FIGS. 16-20 show examples of alternative methods of forming
vias using printing techniques.
[0019] FIGS. 21-24 show an example of a method of forming via
contacts using printing techniques.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0020] FIG. 1 shows an example of sacrificial pillars printed on a
substrate. These pillars such as 10 define a via area in which will
be formed a via contact. Wax, such as Kemamide-based wax is just
one possible material that could be used to form the pillars. Waxes
have a low viscosity above their melting point and therefore are
suitable for jet-printing. Below their melting point they solidify.
Since these materials are printed without solvents, the volume
shrinkage is relatively low which enables the printing of rather
tall structures. Other materials include solvent-based polymers
such as ProLift.TM. manufactured by Brewer Science or heat
decomposable polymers such as the family of Unity.TM. polymers
manufactured by Promerus. Except for the heat decomposable polymer,
the materials would be later removed in a solvent that would not
attack the subsequently applied dielectric, typically a polymer.
ProLift.TM. is as lift-off polymer which is rather temperature
stable and resistant to organic solvents, but it retains solubility
in aqueous developers. In the case of the heat decomposable
material, the material would be removed at a later stage of the
process by heating beyond a decomposition temperature. While much
of the discussion here may focus on individual pillars, the pillars
may also consist of walls or other geometries such as linear
structures, rectangular structures, extended circular structures,
etc. that later form reservoirs, channels or regions.
[0021] FIG. 2 shows an example of a pillar structure formed by
printing. Ink-jet printing may be one example printing method, but
other printing methods may also be chosen, such as dip-pen
deposition, flexographic printing, screen printing or other
deposition methods that are capable of selectively depositing small
quantities of a material. Examples of ink-jet printing include
piezo-ink-jet printing, thermal ink-jet printing, electrostatic
ink-jet printing, etc. To achieve a vertical extent higher than a
single drop of ink-jetted material, the pillars may be formed of a
stack of drops of ink. FIG. 2 shows an example of a pillar formed
from three drops of the ink, 2, 4 and 6. It must be noted that as
the term `ink` is used here, it includes any liquid dispensed using
printing technologies for the formation of electronic circuits and
components and microstructures.
[0022] FIG. 3 shows the beginning of a process to form vias using
printing processes, rather than conventional processes employing
etching. A via is a hole or opening in a layer that allows
connections between layers above and below the layer in which the
via exists. Vias may be left empty to expose a selected area on the
underlying substrate, they may be filled or partially filled with
metal, either before or after formation and they may be also filled
or partially filled with a semiconducting material or other
functional material. Examples of the selected areas on the
substrate may be areas in which a sensor or other sensitive
material resides, or an actuator for which it is desirable to leave
the area open to the surrounding environment. Selected areas are
defined regions on the substrate for which it is desirable that
they be accessible from higher layers. The sensor may be a heat
sensor such as a thermistor, it may be a pressure sensor or a flow
sensor or a chemical or biosensor such as an ion sensor, pH sensor
or surface acoustic wave sensor, for example. As an actuator, a
heater may be located on the substrate that heats a fluid above.
Alternatively, a membrane actuator or an ultrasonic actuator may be
located on the substrate, for example. Vias or holes filled or
partially filled or coated with metal or other conductive material
may be referred to here as via contacts or conductive paths.
[0023] In FIG. 3, a printing process deposits or otherwise forms an
array of sacrificial pillars such as 10 on a substrate 18. As
discussed above, the printing process may repeat as needed to
achieve the proper height for the pillars. In one example the
printing process is a jet-printing process and the deposited
material is a hot-melt wax.
[0024] In FIG. 4, a curable polymer 12 covers the pillars. It is
dispensed by any known dispensing methods for liquids or flowable
substances, including pipette dispensing, syringe dispensing, other
pressure driven dispensing methods, including extrusion-type
dispensing, but also spray dispensing or simple pouring from a
reservoir, etc. A curable polymer is a substance that is a liquid
or flowable substance/polymer until it undergoes a curing process
that causes it to harden. The polymer material may include organic
polymers, inorganic polymers, organic-inorganic hybrids and
composites, for example.
[0025] The curing process may involve the application of heat or
radiation, including UV light. The curing process may also be a
catalytic curing process. At the point of the process shown by FIG.
4, the curable polymer remains in its moldable, uncured state.
Example polymers are the UV curable polymers Norland optical
adhesive 68 or Norland optical adhesive 60 (Norland Products, Inc.
Cranbury, N.J.) and the two component epoxy Devcon 5 Minute Epoxy
(ITW Performance Polymers, Riviera Beach, Fla.). If the pillar
structures are relatively heat resistant, such as for the later
mentioned metal pillars, or if the pillars have a relatively high
melting/softening point, the curable polymer may also be a
thermoplastic polymer. Here `curing` does not refer to permanent
curing, but it would be used in the context of `hardening` or
solidifying by lowering the process temperature. Example polymers
would be thermoplastic polymers often used in molding applications
such as PMMA (polymethylmethacrylate), PC (polycarbonate), PSU
(polysulfone), COC (cyclo-olefine copolymer), PS (polystyrene),
etc.
[0026] In FIG. 5, the process applies a mold 14 to the uncured
polymer. The mold may consist of a silicone elastomer, such as
Sylgard 184 silicone manufactured by Dow, Gel-Film.RTM. (Gel-Pak)
materials, etc. The mold material may be an elastomeric material or
it may be a rigid material and it may be coated with a
low-surface-energy coating such as a fluorocarbon coating or it may
be treated with commonly known mold-release agents. During the
molding process, the surface of the mold contacts the top surfaces
of the pillars and displaces the curable polymer.
[0027] In the case of an elastomeric mold the contact between the
pillar top surface and the mold may be increased due to the
deformation of the elastomer. For larger arrays of pillars, this
can make the displacement of the polymer on all the pillars more
reliable. In the case of thermoplastic pillars or wax pillars, one
may also heat the substrate at this point to cause the pillars to
soften, which in combination with a slight pressure from the mold
may result in an improved via profile due to the occurring
deformation in the pillars. Softening the pillars may also assist
in displacing the curable polymer from the tops of the pillars.
[0028] FIG. 5 also shows the application of UV light, assuming that
the curable polymer is a UV curable polymer, just one example of
many different types of curable polymers suitable for this process.
The application of the UV light causes the curable polymer in this
example to `cure` or harden. In another example, UV exposure may
not be required and the polymer may cure by catalytic curing such
as in a two-component epoxy polymer, for example. The polymer 12
may also contain inorganic particles such as titania or barium
titanate nanoparticles if an increased dielectric constant is
desired. FIG. 6 shows the resulting structure after removal of the
mold. The hardened polymer has formed a layer having the top
surfaces of the array of pillars exposed.
[0029] It must be noted that using a rigid mold instead of an
elastomeric mold, or if the pillars soften after heating, the
pillars would not extend from the surface in FIG. 6. The extending
pillars shown in FIG. 6 are seen if one uses a soft elastomer mold
because it deforms slightly around the pillars.
[0030] FIG. 7 shows the hardened polymer after removal of the
pillars. In the example of a Kemamide-wax pillar, the pillars would
dissolve in a warm isopropanol solution. The solvent should not
attack the curable polymer, which could be one of the polymers
mentioned above. The hardened polymer has an array of holes or
openings 20, which may have any shape.
[0031] If desired, a conductive layer 22 may form electrical
contacts in the openings, between a first functional layer on the
substrate and a second functional layer on the polymer layer. FIG.
8 shows an example of a conductive layer. The conductive layer may
consist of a patterned metal, a polymeric conductor, a
semiconductor or other conducting or semiconducting material. The
material may be deposited for example from a solution by
jet-printing or by conventional vacuum deposition methods. In one
example, the conducting material is jet-printed silver deposited
from a solution of silver nanoparticles. The holes or openings in
the polymer resulting from the molding process may have different
profiles, depending upon the molding techniques. The openings may
tilt, or have an enhanced profile with a decreasing diameter from
bottom to top. These alternatives come from variations in the
printing process such as altering the diameters of the drops, or
moving a second drop slightly to one side of a first drop. The
printing process discussed up to now has not mentioned any
alignment.
[0032] FIG. 9 shows an example of a structure having a first
functional layer for which the process should include an alignment
process. This example has a transistor structure having source 26
and drain 24 contacts on the functional layer on the substrate. The
process would form the polymer layer over the source and drain
contacts, with the openings aligned to the source and drain
contacts. Functional or active layers may include contacts, as in
the source and drain contacts, active areas, such as implanted,
semiconductive regions, conductive traces, components, etc.
[0033] In FIG. 10, the process deposits a semiconductive material
28 into the openings over the source and drain. This material forms
the channel of the thin-film transistor structure. In the shown
example the semiconductor material is deposited from solution such
as by ink-jet printing. Many solution processable semiconductors
are available, including polymeric semiconductors and semiconductor
precursors. In one example, the semiconductor is solution deposited
polythiophene PQT-12
(poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene]). Here, the
function of the via is to contain the semiconductor solution and to
prevent excessive spreading. The described transistor structure may
be a bottom gate transistor in which case the gate electrode lies
on the substrate below the source-drain contacts and below a gate
dielectric. An insulating layer 29 may be deposited above the
semiconductor, which also acts as a barrier layer against moisture
and air.
[0034] The transistor structure may also be a top-gate transistor
in which case the gate is located above the semiconductor within
the via. A gate dielectric is deposited above the semiconductor.
Deposition may occur by jet-printing of the dielectric material
from solution or the semiconductor and dielectric may be deposited
from a blended solution of both materials. In the latter process,
the dielectric material phase separates on top of the semiconductor
during the drying process. In one example, the polythiophene PQT-12
and the polymer PMMA (Poly-methyl-methacrylate) form such a polymer
blend that phase separates.
[0035] In the case of a top-gate transistor, the structures shown
in FIG. 10 may be used as ISFET (ion selective field effect
transistor) structures in which a liquid fills the vias which act
as a reservoir. The metal gate of a conventional field effect
transistor is replaced by a reference electrode located somewhere
in the liquid.
[0036] In a conventional top-gate field effect transistor a metal
deposited into the via will form the gate. Shown in FIG. 11 is the
cross-section of a bottom gate thin-film transistor with
`mushroom-metal` that contacts the drain pad. The conductive layer
30 in FIG. 11 establishes contact to the drain of a transistor and
it extends over the transistor area thereby shielding the channel
area from ambient light. The dielectric layer above the
semiconductor in FIG. 11 is deposited thick enough so that the
`mushroom metal` does not have any gating effect on the transistor
channel.
[0037] Other layers such as for diode structures may be deposited
and patterned onto the conductive layer 30, thus turning the pixels
into light sensors. In this manner, an electrical connection forms
between the functional layer on the substrate and a subsequent
second functional layer, such as a pixel pad in a display or image
sensor back plane, through the openings formed from the sacrificial
pillars.
[0038] In addition to the openings, the polymer may have formed in
it extended regions. Referring to FIG. 12, one can see that the
mold 14 differs in its structure now, having relief regions such as
34. When the mold 14 comes into contact with the curable polymer in
its soft or liquid state, application of pressure causes the
polymer to fill the relief region. After hardening and removal of
the sacrificial pillars, the hardened layer has extended regions
such as 36, shown in FIG. 13. This structure may provide
microfluidic channels, wells or reservoirs with vias to the
substrate. The vias may expose sensing circuits, actuators or
heaters to manipulate the fluid or to sense properties of the fluid
or to detect components of the fluid. As mentioned earlier, the via
openings may also be elongated openings, for example patterned
along the channel length, so that the fluid inside the channels can
be sensed or actuated by circuitry on the substrate all the time
during its flow within the channel.
[0039] FIG. 14 shows filling of the vias with a conductive material
or otherwise functional material, the vias being filled either
fully such as 38 or partially, such as 39. Whether a via becomes
fully or partially filled depends upon the application for which
the structure is intended, as well as the materials and process
used. The fully or partially filled vias form conductive paths.
[0040] One application of the extended regions includes using the
extended regions to form at least one reservoir. As shown in FIG.
15, a reservoir 40 forms between two of the extended regions and
the lid 41. The via contacts may provide electrical signals to the
region defined by the reservoir to cause a reaction in the material
to achieve a particular effect, depending upon the application or
electrical signals may be read out through the vias.
[0041] These processes would fill the reservoirs with different
materials. For example, for sensor applications, a charge
generation material may reside in the space between the reservoirs
and the lid, charge generation materials may include lead-iodide
and mercury-iodide. Typically for display applications, materials
in the reservoirs include electrophoretic ink, liquid crystals,
electrochromic materials or electrowetting display fluids. The
reservoirs may also form liquid or gas microchannels.
[0042] Having presented a general discussion of the formation of a
molded polymer or molded dielectric layer, various alternatives and
modifications become available. For example, FIG. 16 shows several
spacer beads such as 42, on the substrate between the sacrificial
pillars. Example materials are glass spheres, polystyrene spheres
or glass fiber spacers. In one particular example, the spacers are
5 micron fiber spacers from EM Industries of Hawthorne, N.J., and
they are typically employed as spacers in the fabrication of liquid
crystal displays. The spacer material may be chosen based upon the
desired thickness of the dielectric layer. The spacer beads serve
to prevent the molding process from compressing the curable polymer
excessively and to assure a uniform thickness of the molded
dielectric layer over a large area. The spacer beads may be
deposited by spraying or by inkjet printing from a dispersion and
subsequent drying of the solvent.
[0043] In an alternative process, the curable polymer 12 covers the
pillars and the spacer beads, as shown in FIG. 17. Instead of
applying the spacer beads first, they may have been dispersed in
the polymer. In the molding process of FIG. 18, a mold having
relief regions is shown. It must be noted that the mold may or may
not have relief regions as used in this process; it is one option
to be considered. FIG. 18 shows the option of heating the substrate
to soften the thermoplastic polymer pillars prior to compression by
the mold 14. For purposes of this discussion, thermoplastic polymer
materials includes waxes. The spacer beads ensure that the mold
does not compress too much, over-flattening the pillars for any
particular application.
[0044] The flattened pillars may result in an improved via profile.
The exact via profile also may depend upon the wetting properties
of the mold by the wax, if it becomes liquid. Good wetting
properties typically result in a small contact angle and a via that
is wider towards the top. An example of the resulting via 46 is
shown in FIG. 20, after the curing process of FIG. 19. Vias with a
greater diameter near the top are advantageous for contact
metallization or contact coating.
[0045] As an alternative to the pillar removal process, a fully
additive process may simplify making via contacts between the first
and second layers. As shown in FIG. 21, rather than depositing
sacrificial pillars, the process deposits metal or conductive
pillars such as 50 onto the substrate. The dispensing of the
curable polymer 12 in FIG. 22 and the application of the mold 14 of
FIG. 23 are similar to those processes already discussed. The
resulting polymer layer in FIG. 24 still has openings, but the
openings now accommodate the metal pillars 50.
[0046] One issue that may arise in formation of the pillars
concerns the height of the pillars. Higher pillars may be desired
that are not easily formed from repeated jetting of ink drops in a
stack. FIG. 25 shows pillar 50 and alternative structures. The
metal pillars may include metal particles, such as silver
nanoparticles, resulting in a metal pillar 50. To achieve higher
pillars, the silver nanoparticles may be mixed with other
dispersible larger particles such as styrene particles, glass
spheres, etc., shown as 52 in FIG. 25. These particles may have
dimensions on the order of micrometers or tens of micrometers and
will be referred to here as `structural particles.` The silver
nanoparticles may form a conductive layer around the structural
particles. The silver nanoparticles may be first mixed with those
structural particles and then the solution is dispensed.
Alternatively, the structural particles may be first dispensed and
the conductive material is then deposited or printed on top.
[0047] Yet another alternative involves depositing a relatively
tall polymer bump and then coating the bump with a layer of
conductor, as shown by 54. Both the bump and the coating would
result from printing processes. The polymer pump may be jet-printed
UV-curable polymer and the conductor may be a jet-printed layer of
silver nanoparticles or conductive polymer such as PEDOT:PSS
(Baytron P).
[0048] Another possible concern with this process arises with
regard to the top surfaces of the pillars. Any residue of the
curable polymer on the tops of the pillars may cause problems in
the electrical connections. FIGS. 26 and 27 show alternative ways
of ensuring that no curable polymer hardens on the top surfaces of
the via contacts.
[0049] In FIG. 26, the substrate 18 would be substantially
transparent, allowing the curing with light to occur through the
substrate. Any residual polymer on the top surface 60 of the
pillars would not cross-link, and therefore not harden, because the
pillars themselves would block the light from reaching that portion
of the polymer. This would allow the residue to be removed with
solvents.
[0050] In FIG. 26, the mold 14 shown in several previous figures
could be conductive, such as carbon filled silicone. If the pillars
are conductive, one could determine the quality of the electrical
contact between the mold and the pillars before curing the polymer
by measuring the electrical current flow from the mold 14 to the
substrate 18. A capacitive measurement of the capacitance between
the mold and the substrate may also be used. Good electrical
contact would indicate that little or no residue remains on the
tops of the pillars. The resulting structure is shown in FIG.
27.
[0051] In this manner, vias allow electrical connections to be made
between a first printed circuit and a second printed circuit, or to
expose areas of the first printed circuit. These vias are formed in
an additive process, rather than involving etching or removal
processes.
[0052] It will be appreciated that several of the above-disclosed
and other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or
applications. Also that various presently unforeseen or
unanticipated alternatives, modifications, variations, or
improvements therein may be subsequently made by those skilled in
the art which are also intended to be encompassed by the following
claims.
* * * * *