U.S. patent application number 14/455181 was filed with the patent office on 2016-02-11 for method of controlling oxygen levels for electroless plating of catalytic fine lines or features.
The applicant listed for this patent is Uni-Pixel Displays, Inc.. Invention is credited to Yieu Chyan, Danliang Jin, John-Paul O'Neil.
Application Number | 20160040294 14/455181 |
Document ID | / |
Family ID | 55266976 |
Filed Date | 2016-02-11 |
United States Patent
Application |
20160040294 |
Kind Code |
A1 |
Jin; Danliang ; et
al. |
February 11, 2016 |
METHOD OF CONTROLLING OXYGEN LEVELS FOR ELECTROLESS PLATING OF
CATALYTIC FINE LINES OR FEATURES
Abstract
A method of controlling oxygen levels for electroless plating of
catalytic fine lines or features includes selecting a substrate
that includes a plurality of catalytic lines or features that are
part of or are disposed on the substrate. The plurality of
catalytic lines or features include at least one catalytic fine
line or feature and at least one catalytic standard line or
feature. A dissolved oxygen concentration of an electroless plating
solution is regulated to a candidate controlled oxygen level. The
candidate controlled oxygen level is set to a smallest value in a
regulated range in a first pass of the method. The substrate is
submerged in the solution for a period of time sufficient to
initiate plating of the at least one catalytic standard line or
feature. The substrate is evaluated and candidate controlled oxygen
level is incremented or the previous value is selected as the
regulated oxygen level.
Inventors: |
Jin; Danliang; (The
Woodlands, TX) ; O'Neil; John-Paul; (The Woodlands,
TX) ; Chyan; Yieu; (Conroe, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Uni-Pixel Displays, Inc. |
The Woodlands |
TX |
US |
|
|
Family ID: |
55266976 |
Appl. No.: |
14/455181 |
Filed: |
August 8, 2014 |
Current U.S.
Class: |
427/8 |
Current CPC
Class: |
C23C 18/1608 20130101;
C23C 18/1617 20130101; C23C 18/1675 20130101; C23C 18/1683
20130101 |
International
Class: |
C23C 18/16 20060101
C23C018/16 |
Claims
1. A method of controlling oxygen levels for electroless plating of
catalytic fine lines or features comprising: selecting a substrate
that includes a plurality of catalytic lines or features that are
part of or are disposed on the substrate, wherein the plurality of
catalytic lines or features include at least one catalytic fine
line or feature and at least one catalytic standard line or
feature; regulating a dissolved oxygen concentration of an
electroless plating solution to a candidate controlled oxygen
level, wherein the candidate controlled oxygen level is set to a
smallest value in a regulated range in a first pass of the method;
submerging the substrate in the electroless plating solution for a
period of time sufficient to initiate plating of the at least one
catalytic standard line or feature; removing the substrate from the
electroless plating solution; evaluating the substrate to determine
whether the at least one catalytic fine line or feature initiated
at approximately the same time as the at least one catalytic
standard line or feature; evaluating the substrate to determine
whether the at least one catalytic fine line or feature exhibits
one or more failure modes; if the at least one catalytic fine line
or feature initiated at approximately the same time as the at least
one catalytic standard line or feature and did not exhibit one or
more failure modes, incrementing the candidate controlled oxygen
level by an increment and iterate the method; and if the at least
one catalytic fine line or feature exhibits one or more failure
modes, selecting the previous value of the candidate controlled
oxygen level as a regulated oxygen level.
2. The method of claim 1, wherein the at least one catalytic fine
line or feature has the smallest width among the plurality of
catalytic lines or features that are part of or are disposed on the
substrate; and wherein the at least one catalytic standard line or
feature includes the largest width among the plurality of catalytic
lines or features that are part of or are disposed on the
substrate.
3. The method of claim 1, wherein the at least one catalytic fine
line or feature has a width less than approximately 5
micrometers.
4. The method of claim 1, wherein the at least one catalytic fine
line or feature has a width in a range between approximately 5
micrometers and approximately 10 micrometers.
5. The method of claim 1, wherein the at least one catalytic fine
line or feature has a width in a range between approximately 10
micrometers and approximately 20 micrometers.
6. The method of claim 1, wherein the at least one catalytic
standard line or feature has a width on the order of magnitude of
at least 20 micrometers.
7. The method of claim 1, wherein the at least one catalytic
standard line or feature has a width on the order of magnitude of
at least a millimeter.
8. The method of claim 1, wherein the at least one catalytic
standard line or feature has a width on the order of magnitude of
at least a centimeter.
9. The method of claim 1, wherein the at least one catalytic
standard line or feature has a width on the order of magnitude of
at least a decimeter.
10. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises controlling a surface area of the
electroless plating solution exposed to air.
11. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises controlling a flow rate of inert gas that
is introduced into the electroless plating solution.
12. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises controlling a flow rate of oxygen or gas
containing oxygen that is introduced into the electroless plating
solution.
13. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises controlling a recirculation speed of the
electroless plating solution.
14. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises controlling an agitation level of the
electroless plating solution.
15. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises increasing the dissolved oxygen
concentration.
16. The method of claim 15, wherein the dissolved oxygen
concentration may be increased by: introducing oxygen or a gas
containing oxygen into the electroless plating solution and
diffusing the introduced oxygen or gas containing oxygen in the
electroless plating solution.
17. The method of claim 15, wherein the dissolved oxygen
concentration may be increased by increasing a surface area of the
electroless plating solution exposed to air.
18. The method of claim 1, wherein regulating the dissolved oxygen
concentration comprises reducing the dissolved oxygen
concentration.
19. The method of claim 18, wherein the dissolved oxygen
concentration may be reduced by introducing an inert gas into the
electroless plating solution.
20. The method of claim 18, wherein the dissolved oxygen
concentration may be reduced by pulling a vacuum.
21. The method of claim 1, wherein the regulated range is in a
range between approximately 0.6 PPM and approximately 1.6 PPM.
22. The method of claim 1, wherein the regulated range is in a
range between approximately 0.7 PPM and approximately 1.5 PPM.
23. The method of claim 1, wherein the regulated range is in a
range between approximately 0.8 PPM and approximately 1.4 PPM.
24. The method of claim 1, wherein the regulated range is in a
range between approximately 0.9 PPM and approximately 1.3 PPM.
25. The method of claim 1, wherein the regulated range is in a
range between approximately 1.0 PPM and approximately 1.2 PPM.
26. The method of claim 1, wherein the determination of whether the
at least one catalytic fine line or feature initiated at
approximately the same time as the at least one catalytic standard
line or feature comprises: examining a surface of the at least one
catalytic standard line or feature for evidence of initiation;
examining a surface of the at least one catalytic fine line or
feature for evidence, or lack thereof, of initiation; and comparing
the evidence of initiation for the at least one catalytic standard
line or feature to the evidence, or lack thereof, of initiation for
the at least one catalytic fine line or feature; wherein the at
least one catalytic fine line or feature is determined to have
initiated at approximately the same time as the least one catalytic
standard line or feature if the at least one catalytic fine line or
feature exhibits approximately the same evidence of initiation as
that of the at least one catalytic standard line or feature.
27. The method of claim 1, wherein the determination of whether the
at least one catalytic fine line or feature etched prior to
initiation comprises: examining a shape of the at least one
catalytic fine line or feature; and comparing the shape of the at
least one catalytic fine line or feature to a shape of a
corresponding line or feature in a source design; wherein the at
least one catalytic fine line or feature is determined to have
etched prior to initiation if the at least one catalytic line or
feature exhibits narrow portions, breaks, or discontinuities that
do not exist in the corresponding line or feature in the source
design.
28. The method of claim 1, wherein the dissolved oxygen
concentration is monitored by dipping a dissolved oxygen meter into
the electroless plating solution.
Description
BACKGROUND OF THE INVENTION
[0001] Electroless plating is an autocatalytic reaction that may be
used to deposit a metal on the surface of catalytic portions that
are part of or are disposed on a substrate. Electroless plating may
be described as a redox reaction with both partial reactions,
anodic and cathodic, occurring at the same electrode. The anodic
partial reaction includes the oxidation of a reducing agent
contained within the electroless plating solution to yield one or
more electrons that are transferred to the metal and/or
by-products. The cathodic partial reaction includes the reduction
of free metal ions or metal complexes to a metal lattice. The
overall reaction results in metal plating onto the surface of the
catalytic portions that are part of or are disposed on the
substrate and then onto the deposited metal itself in a continuous
process.
BRIEF SUMMARY OF THE INVENTION
[0002] According to one aspect of one or more embodiments of the
present invention, a method of controlling oxygen levels for
electroless plating of catalytic fine lines or features includes
selecting a substrate that includes a plurality of catalytic lines
or features that are part of or are disposed on the substrate. The
plurality of catalytic lines or features include at least one
catalytic fine line or feature and at least one catalytic standard
line or feature. A dissolved oxygen concentration of an electroless
plating solution is regulated to a candidate controlled oxygen
level. The candidate controlled oxygen level is set to a smallest
value in a regulated range in a first pass of the method. The
substrate is submerged in the electroless plating solution for a
period of time sufficient to initiate plating of the at least one
catalytic standard line or feature and then removed from the
electroless plating solution. The substrate is evaluated to
determine whether the at least one catalytic fine line or feature
initiated at approximately the same time as the at least one
catalytic standard line or feature and is evaluated to determine
whether the at least one catalytic fine line or feature exhibits
one or more failure modes. If the at least one catalytic fine line
or feature initiated at approximately the same time as the at least
one catalytic standard line or feature and did not exhibit one or
more failure modes, the candidate controlled oxygen level is
incremented by an increment. If the at least one catalytic fine
line or feature exhibits one or more failure modes, the previous
value of the candidate controlled oxygen level is selected as a
regulated oxygen level.
[0003] Other aspects of the present invention will be apparent from
the following description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 shows an electroless plating system in accordance
with one or more embodiments of the present invention.
[0005] FIG. 2 shows catalytic lines or features having different
feature sizes in accordance with one or more embodiments of the
present invention.
[0006] FIG. 3 shows electroless plated fine lines on substrate that
exhibit issues because of prolonged exposure to electroless plating
solution prior to initiation of electroless plating.
[0007] FIG. 4 shows a catalytic fine line or feature disposed on
substrate that exhibits etching because of prolonged exposure to
electroless plating solution prior to initiation of electroless
plating.
[0008] FIG. 5 shows a method of controlling oxygen levels for
electroless plating of catalytic fine lines or features in
accordance with one or more embodiments of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0009] One or more embodiments of the present invention are
described in detail with reference to the accompanying figures. For
consistency, like elements in the various figures are denoted by
like reference numerals. In the following detailed description of
the present invention, specific details are set forth in order to
provide a thorough understanding of the present invention. In other
instances, well-known features to one of ordinary skill in the art
are not described to avoid obscuring the description of the present
invention.
[0010] FIG. 1 shows an electroless plating system 100 in accordance
with one or more embodiments of the present invention. Electroless
plating system 100 may include an electroless plating bath 110 and
a liquid electroless plating solution 120 disposed in the bath 110.
One or more control systems 130 may optionally be used to control
one or more operational characteristics of the system 100. The
electroless plating system 100 may include, for example, a
temperature control system (not independently illustrated), a pump
control system (not independently illustrated), a turbulence
control system (not independently illustrated), a dosing control
system (not independently illustrated), a sparging control system
(not independently illustrated), and/or other control system (not
independently illustrated). The one or more control systems 130 may
be discrete or one or more control systems 130, or the function or
functions that they implement, may be integrated together. One of
ordinary skill in the art will recognize that electroless plating
system 100 may include other control systems 130 in accordance with
one or more embodiments of the present invention.
[0011] As previously discussed, the one or more control systems 130
may optionally be used to control one or more operational
characteristics of electroless plating system 100. For example, a
temperature control system may be used to control a temperature of
electroless plating solution 120. A pump control system may be used
to control a recirculation speed of electroless plating solution
120 within electroless plating bath 110. A turbulence control
system may be used to control the agitation and/or the mixing of
electroless plating solution 120 within electroless plating bath
110. A dosing control system may be used to control the pH or a
component concentration level of electroless plating solution 120
by timed, manual, or sensor-activated dosing of the appropriate
chemicals at the appropriate time. A sparging control system may be
used to control a flow rate of air, oxygen, or inert gas that may
be bubbled into electroless plating solution 120. Other aspects of
electroless plating system 100 may be controlled by the combination
of one or more of the above-noted control systems 130. For example,
in one or more embodiments of the present invention, a dissolved
oxygen concentration of electroless plating solution 120 may be
controlled by a sparging control system and one or both of a pump
control system and a turbulence control system. One of ordinary
skill in the art will recognize that one or more control systems
130 may be used to control other operational characteristics of
electroless plating system 100 in accordance with one or more
embodiments of the present invention.
[0012] One or more maintenance systems 140 may optionally be used
to maintain operation of electroless plating system 100.
Electroless plating system 100 may include, for example, a cleaning
maintenance system (not independently illustrated), a fluid
transfer maintenance system (not independently illustrated), a
filtration maintenance system (not independently illustrated),
and/or other maintenance system (not independently illustrated).
The one or more maintenance systems 140 may be discrete or one or
more maintenance systems 140, or the function or functions that
they implement, may be integrated together. One of ordinary skill
in the art will recognize that electroless plating system 100 may
include other maintenance systems 140 in accordance with one or
more embodiments of the present invention.
[0013] As previously discussed, the one or more maintenance systems
140 may optionally be used to maintain operation of electroless
plating system 100. For example, a cleaning maintenance system may
be used to clean some aspect of electroless plating system 100. A
fluid transfer maintenance system may be used to remove one or more
fluids (not shown) from, add one or more fluids to, and/or transfer
one or more fluids within electroless plating system 100. A
filtration maintenance system may be used to maintain electroless
plating system 100 by filtering or removing extraneous plating
and/or particulate matter from electroless plating solution 120.
One of ordinary skill in the art will recognize that one or more
maintenance systems 140 may be used to maintain other operational
aspects of electroless plating system 100 in accordance with one or
more embodiments of the present invention.
[0014] One or more conveyor systems 150 may optionally be used to
move one or more substrates 160 in and out of the electroless
plating bath 110 as part of a production line. One of ordinary
skill in the art will recognize that the types of control,
maintenance, and/or conveyor systems used in electroless plating
system 100 may vary based on an application or design in accordance
with one or more embodiments of the present invention. One of
ordinary skill in the art will also recognize that the
configuration of electroless plating system 100 may vary based on
an application or design in accordance with one or more embodiments
of the present invention.
[0015] When a substrate 160 is submerged in the electroless plating
solution 120, an autocatalytic reaction occurs that results in the
deposition of metal (not shown) on the catalytic portions (not
shown) that are part of or are disposed on the substrate 160 and
then on the deposited metal itself in a continuous process.
Substrate 160 may be composed of one or more of a semiconductor,
glass, films, thermoplastic resins, thermosetting resins, other
polymers, ceramics, composites, fabric, paper, and/or other
material suitable for use as a substrate. The catalytic portions
comprise a material or substance that increases the rate of
reaction without being consumed by the reaction. The deposition
process continues until the catalytic portions are no longer in
contact with electroless plating solution 120, any one or more of
the reactants of the electroless plating solution 120 are depleted,
there is excessive buildup of by-products (not shown) within the
electroless plating solution 120, or the electroless plating bath
110 crashes or plates out. One of ordinary skill in the art will
recognize that electroless plating system 100 may be used to
electroless plate metals including, for example, copper, nickel,
palladium, other platinum group metals, bismuth, gold, silver,
cobalt, chromium, some composites, or alloys thereof in accordance
with one or more embodiments of the present invention.
[0016] For the purposes of illustration only, the chemical
mechanism of a copper-based electroless plating process using
formaldehyde as the reductant is discussed in more detail below.
One of ordinary skill in the art will recognize that the chemical
mechanism of a copper-based electroless plating process may vary
based on the chemistry, such as, for example, the composition of
the electroless plating solution used and the application, such as,
for example, the operational characteristics of the bath. One of
ordinary skill in the art will also recognize that other chemical
mechanisms may occur in other metal-based electroless plating
processes, such as, for example, a nickel-based electroless plating
process or a palladium-based electroless plating process. While the
chemical mechanisms may vary, the method of controlling oxygen
levels for electroless plating of catalytic fine lines or features
may be used to control dissolved oxygen concentration, or another
aspect of the bath that is applicable in a similar manner, for
other metal-based electroless plating processes in accordance with
one or more embodiments of the present invention.
[0017] Returning to the copper-based electroless plating example,
the chemical mechanism of an electroless plating bath can be
described as a redox reaction with both partial reactions, anodic
and cathodic, occurring at the same electrode. In certain
embodiments, the anodic partial reaction may include the hydrolysis
of HCHO to methylene glycol as set out in equation (1). The
methylene glycol may dissociate as set out in equation (2). The
intermediate may adsorb on the metal surface as set out in equation
(3). The intermediate may dissociate and desorb as set out in
equation (4). The adsorbed hydrogen may desorb as set out in
equation (5).
HCHO+H.sub.2O.fwdarw.H.sub.2C(OH).sub.2 (1)
H.sub.2C(OH).sub.2+OH.sup.-.fwdarw.H.sub.2C(OH)O.sup.-+H.sub.2O
(2)
H.sub.2C(OH)O.sup.-.fwdarw.[HC(OH)O.sup.-].sub.ads+H.sub.ads
(3)
[HC(OH)O.sup.-].sub.ads+OH.sup.-.fwdarw.HCOO.sup.-+H.sub.2O+e.sup.-
(4)
H.sub.ads.fwdarw.0.5H.sub.2 or H.sub.ads.fwdarw.H.sup.++e.sup.-
(5)
[0018] There are two cathodic partial reactions that may take
place. The first possible cathodic partial reaction occurs when
free copper ions in the electroless plating solution react directly
with the electron from the anodic half reaction as set out in
equations (6) and (7).
##STR00001##
The second possible cathodic partial reaction occurs when complexed
copper adsorbs onto the metal surface (the catalytic portions that
are part of or are disposed on the substrate) and charge is
transferred to ligands which then dissociate as set out in
equations (8) and (9), where the remaining Cu.sup.2+ may be further
reduced.
[CuL.sub.x].sup.2+xp.fwdarw.Cu.sup.2++xL.sup.p (8)
Cu.sup.2++2e.sup.-.fwdarw.Cu.sub.lattice (9)
[0019] The chemistry of the electroless plating bath is in a
constant state of change. As part of the autocatalytic reaction, a
portion of the reducing agent and a portion of the metal source in
the electroless plating solution are consumed such that the
concentration of the reducing agent and the concentration of the
metal source in the solution decrease over time. In certain
circumstances, the reduction of reactants may lead to undesirable
plating characteristics prior to depletion of any given reactant.
Undesirable plating characteristics may include, for example, poor
adhesion of the deposited metal to the catalytic portions that are
part of or are disposed on the substrate, non-uniform morphology,
high amounts of undesired extraneous plating on non-target
surfaces, undesired plating rate changes, skip plating, changes in
reflectivity, changes in brittleness, and/or any other
characteristic that may render an end product unusable for its
intended purpose. If the reducing agent is depleted, the anodic
partial reaction ceases and there is no electron source for the
cathodic reaction. If the metal source is depleted, adsorption of
the metal ceases. During operation, a dosing control system may
replenish one or more consumed reactants in the electroless plating
solution when their concentrations fall outside specified ranges to
avoid depletion. Alternatively, when the buildup of undesired
by-products has reached a threshold that produces undesirable
plating characteristics for a given process, the electroless
plating bath may be taken offline to purge and replace the
electroless plating solution.
[0020] The autocatalytic reaction produces by-products in the
electroless plating solution that may accumulate and affect the
plating characteristics of the bath. The concentration of
by-products in the solution typically increases over time and may
negatively impact or inhibit the deposition of metal. Because they
are not consumed by the reaction, concentrations of some ionic
species and complexants may also increase as bath components are
dosed to maintain metal source concentrations, reducing agent
concentration, and/or the pH of the bath. If a dosing control
system is used to maintain reactants at near constant
concentrations, the specific gravity of the bath may be used as an
approximate indicator of the buildup of by-products in the bath. To
minimize accumulation of solid by-products suspended within the
bath, a maintenance system, such as, for example, a filtration
maintenance system may be used to filter portions of electroless
plating solution 120. Otherwise, if the concentration of
by-products exceeds a certain threshold, the electroless plating
bath may be taken offline and a maintenance control system, such
as, for example, a cleaning maintenance system and/or a fluid
transfer maintenance system, may be used to purge and replace some
or all of the electroless plating solution in the bath. The
autocatalytic reaction may also produce insoluble by-products such
as, for example, metal particles, in the electroless plating
solution. These particles may increase in size as they plate in the
bath. At a certain threshold, the bath may crash, or plate out,
resulting in the uncontrolled plating of these metal particles or
other areas of extraneous metal plating causing most or all of the
remaining metal source in the electroless plating solution to
quickly plate out of the bath. If the bath crashes, the crashed
electroless plating solution may be purged, the bath may be
cleaned, and new solution may be disposed in the bath prior to
bringing the bath back online.
[0021] An electroless plating bath may be characterized by the
electroless plating solution used and the operating conditions of
the electroless plating bath. The operating conditions of the
electroless plating bath may be dictated, in part, by the
composition of the electroless plating solution used and the
commercial vendor's recommended operating conditions for their
electroless plating solution. The composition of the electroless
plating solution may vary based on the composition of the catalyst
being used to dispose, for example, the catalytic portions on
substrate. The composition of the electroless plating solution may
vary based on the type of metal source, such as, for example,
copper, nickel, or palladium, to be deposited. The composition of
the electroless plating solution may also vary in accordance with a
commercial vendor's proprietary formulation of solution.
[0022] Electroless plating solutions are inherently unstable and
the electroless plating bath chemistry tends to deteriorate over
time. While commercial vendors of electroless plating solutions do
not disclose the composition of their solutions, they typically
specify normal operating conditions for an electroless plating bath
that uses their solution. One or more control systems and/or one or
more maintenance systems may be used to sense and/or regulate the
operating conditions of the electroless plating bath to operate
within the normal operating conditions. The normal operating
conditions may include, for example, an acceptable pH range and an
acceptable temperature range for the electroless plating solution.
If the electroless plating bath is operated outside of the normal
operating conditions, the bath may become unstable and ultimately
crash. If the electroless plating bath crashes, or plates out, the
bath may be taken offline, the crashed electroless plating solution
may be purged, the bath may be cleaned, and new solution may be
disposed in the bath prior to bringing the bath back online.
[0023] While it is desirable to operate the electroless plating
bath within its normal operating conditions to avoid crashing the
bath, the bath chemistry tends to deteriorate over time from normal
usage. One measure of efficiency of an electroless plating bath is
the up-time of the bath, sometimes referred to as the bath life.
The bath life is the amount of time that a bath is online and
capable of effectively plating without undesirable plating
characteristics. The bath life may be negatively impacted by
control events that take the bath outside normal operating
conditions, maintenance events that are required because of
depletion of reactants or by-product accumulation, or other failure
modes including changes in bath chemistry such as, for example,
crashes, or plate outs.
[0024] FIG. 2 shows catalytic lines or features on substrate 160
having different feature sizes in accordance with one or more
embodiments of the present invention. In certain embodiments, the
catalytic portions 210, 230 that are part of or are disposed on
substrate 160 may have different feature sizes such as, for
example, a different width and/or a different thickness. For
example, substrate 160 may include one or more catalytic fine lines
or features 210 disposed on substrate 160 that have a feature size,
such as, for example, a width 220, which is on the order of
magnitude of nanometers or micrometers. In certain embodiments, the
catalytic fine lines or features 210 may have a width 220 less than
approximately 5 micrometers. In other embodiments, the catalytic
fine lines or features 210 may have a width 220 in a range between
approximately 5 micrometers and approximately 10 micrometers. In
still other embodiments, the catalytic fine lines or features 210
may have a width in a range between approximately 10 micrometers
and approximately 20 micrometers. In certain embodiments, substrate
160 may also include one or more catalytic standard lines or
features 230 disposed on substrate 160 that have a feature size,
such as, for example, a width 240, of at least 20 micrometers or
larger. In other embodiments, substrate 160 may also include one or
more catalytic standard lines or features 230 disposed on substrate
160 that have a feature size, such as, for example, a width 240,
which is on the order of magnitude of millimeters or larger. In
still other embodiments, substrate 160 may also include one or more
catalytic standard lines or features 230 disposed on substrate 160
that have a feature size, such as, for example, a width 240, which
is on the order of magnitude of centimeters or larger. In still
other embodiments, substrate 160 may also include one or more
catalytic standard lines or features 230 disposed on substrate 160
that have a feature size, such as, for example, a width 240, which
is on the order of magnitude of decimeters or larger.
[0025] In the electroless plating bath (e.g., electroless plating
bath 110 of FIG. 1), the plating process includes two stages: an
initiation stage (also referred to as the induction stage) and a
buildup stage (also referred to as the bulk plating stage). In the
initiation stage, the catalytic portions 210, 230 that are part of
or are disposed on the substrate 160 initiate the autocatalytic
reaction. The catalytic portions 210, 230 may be comprised of a
catalytic metal, catalytic metal alloy, catalytic metal compound,
or materials containing catalytic metals, alloys, or compounds that
may be the same as, or different from, the metal being plated by
the bath. The exposed surfaces of the catalytic portions 210, 230
may form the active surface for plating during initiation. Once the
electroless plating process is initiated, the autocatalytic
reaction continues to deposit metal (not shown) during the buildup
stage. The exposed surface of the metal plated during the
initiation stage may form the active surface for plating during
buildup. The amount of time substrate 160 is submerged in the
electroless plating bath may be controlled to achieve a desired
thickness of deposited metal. For example, if an application or
design requires a thin layer of deposited metal, the substrate may
be submerged in the electroless plating bath for a shorter amount
of time than an application or design that requires a thicker layer
of deposited metal. However, the bath is caustic and if the
substrate is submerged in solution for too long, without uniform
initiation of plating that can form a protective metal surface, the
catalytic portions 210, 230 that are part of or are disposed on the
substrate 160, as well as the substrate 160 itself, may be
damaged.
[0026] The plating rate is a measure of the plating thickness
achieved per unit of time in a particular application or design.
Generally, the plating rate is a function of the time required to
initiate the autocatalytic reaction and the application-specific
time required to achieve a buildup of a desired thickness of metal.
For catalytic standard lines or features 230, the initiation time
may be on the order of magnitude of seconds to minutes, depending
on the bath conditions. In certain embodiments, the initiation time
of catalytic standard lines or features 230 may be in a range
between approximately 10 seconds and approximately 40 seconds. As
previously noted, once initiated, bulk plating begins and
deposition continues until the substrate is removed from the
electroless plating bath or the bath chemistry fails. However, for
catalytic fine lines or features 210, the initiation time is
substantially longer than that of catalytic standard lines or
features 230 in a conventional electroless plating bath that uses
commercially available electroless plating solution operated at
normal operating conditions. The initiation time for catalytic fine
lines or features 210 may be at least twice, and potentially
substantially longer, than that of catalytic standard lines or
features 230. As such, catalytic fine lines or features 210 do not
initiate, if they initiate at all, until after their catalytic
standard line or features 230 counterpart. Consequently, in a
conventional electroless plating bath, a substrate 160 that
includes catalytic lines or features with diverse feature sizes,
but include at least one catalytic fine line or feature 210, plates
in a non-uniform manner because the catalytic fine lines or
features 210 take substantially longer to initiate, if they
initiate at all, and then buildup.
[0027] Because the catalytic fine lines or features 210 take longer
to initiate and plate to a desired thickness, the catalytic
standard lines or features 230 spend more time in the bulk plating
stage and the buildup of deposited metals on the catalytic standard
lines or features is substantially thicker than that of the
catalytic fine lines or features 210. The non-uniform deposition of
metal may affect the electrical performance of the deposited metal.
For example, thin layers of deposited metal may have a higher
electrical resistance and lower conductivity than thick layers of
deposited metal. As a consequence, the electrical performance of
the deposited metal and, by extension, the end product that
incorporates it, may be negatively impacted. Similarly, the
non-uniformity may negatively impact reliability. The thicker
deposits of metal on the catalytic standard lines or features 230
may cause adhesion issues that cause the deposited metal to peel
off of the substrate 160. As a consequence, the reliability of the
deposited metal and, by extension, the end product that
incorporates it, may be reduced.
[0028] Because the catalytic fine lines or features 210 take longer
to initiate and plate to a desired thickness, the overall plating
rate of a substrate 160 with catalytic lines or features of
different sizes may be dominated by the plating rate of the
catalytic fine lines or features 210 and it may be difficult to
achieve high-volume in a production environment. To increase the
number of substrates 160 that may be plated in a certain amount of
time, such as, for example, a production run, one or more
additional electroless plating baths may be required. Each
additional electroless plating bath requires significant physical
space, increases capital costs, increases material costs, and
increases operational costs.
[0029] Another issue with non-uniform plating is potential damage
to the catalytic portions that are part of or are disposed on
substrate, as well as the substrate itself, when the substrate is
submerged in the electroless plating solution for an extended
period of time required to initiate and plate the catalytic fine
lines or features. FIG. 3 shows electroless plated fine lines 310
on substrate 160 that exhibit issues because of prolonged exposure
to electroless plating solution prior to initiation of electroless
plating. In certain applications, such as, for example, touch
sensor applications, flexographic printing may be used to print a
catalytic ink image (not shown) of a conductive pattern on
substrate 160 for subsequent electroless plating. In this example,
the catalytic ink serves as the catalytic portions that are
disposed on the substrate 160 and are metallized by the electroless
plating process. When the catalytic ink image includes a mix of
catalytic standard lines or features (not shown) and catalytic fine
lines or features, such as, for example, micrometer-fine catalytic
lines 310, the substrate 160 may be submerged in the electroless
plating solution longer than would otherwise be required for the
catalytic standard lines or features alone. If the substrate is
submerged too long, the solution may etch away portions of the
catalytic ink image on substrate, especially the portions of the
catalytic ink image that correspond to the catalytic fine lines or
features 310, prior to initiation, if it initiates at all. As such,
portions of the catalytic ink image printed on substrate may be
etched away and, as a consequence, the electroless plating process
may plate on top of the altered catalytic ink image, if it plates
at all. Plated fine lines or features 310 may be altered such that
the deposited metal may include unintended narrow portions 320,
breaks or discontinuities 330, or wide portions 340 that negatively
impact the electrical performance, functionality, reliability,
optical properties, and/or yield of the end product (not
independently illustrated).
[0030] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features regulates the dissolved oxygen
concentration of the electroless plating solution to ensure
consistent plating of catalytic lines or features with diverse
feature sizes that include at least one catalytic fine line or
feature. The method increases the plating rate, provides
substantially uniform plating for diverse feature sizes, improves
production efficiency, reduces production costs, improves
reliability, and improves yield.
[0031] In a production environment, there are a number of
interrelated goals for the electroless plating process. From an end
product perspective, the electroless plating bath should not
significantly alter the catalytic portions that are part of or are
disposed on the substrate prior to the deposition of metal, provide
substantially uniform plating for diverse feature sizes on
substrate that include at least one catalytic fine line or feature,
provide appropriate connectivity, resistivity, and conductivity of
the deposited metal, and the deposited metal should adhere to the
substrate. From an electroless plating bath perspective, the
electroless plating bath should provide the highest possible
plating rate, substantially uniform plating for diverse feature
sizes, and the highest possible up-time or bath life.
[0032] In an electroless plating bath, dissolved oxygen in the
electroless plating solution serves as a stabilizer. If the
dissolved oxygen concentration in the electroless plating solution
is too low, the bath life, or time in which the bath may be
operated, is reduced. When the dissolved oxygen concentration is
too low, the bath may crash or plate out and spontaneously
precipitate metal throughout the solution instead of depositing
metal on the catalytic portions that are part of or are disposed on
the substrate. If the dissolved oxygen concentration in the
electroless plating solution is too high, exceeding the amount a
given solution was designed for, any plating in process will cease
and no further plating will take place. When the dissolved oxygen
concentration is too high, the oxygen renders the catalyst less
effective in catalyzing the reaction such that metal deposition
does not take place.
[0033] Conventional commercially available electroless plating
solutions typically include a high dissolved oxygen concentration.
Conventional commercially available electroless plating solutions
typically include at least 5.0 parts-per-million ("PPM") dissolved
oxygen concentration or more. Because the concentration of
commercially available electroless plating solutions is sufficient
to plate catalytic standard lines or features, such as, for
example, copper clad circuit board blanks, the dissolved oxygen
concentration is typically not monitored or regulated, nor do the
manufacturers of commercial electroless plating solution recommend
doing so.
[0034] To the extent some commercial vendors do consider dissolved
oxygen concentration, some manufacturers of commercially available
electroless plating solutions recommend bubbling air into the
electroless plating solution to maintain the high dissolved oxygen
concentration or potentially increase it without monitoring the
actual concentration. This high dissolved oxygen concentration is
necessary to achieve stable plating for the high bath loading
factor that is typically used in a conventional electroless plating
bath application. The bath loading factor is the ratio of the
surface area to be plated per volume of electroless plating
solution in the bath. Commercially available electroless plating
solutions for high bath loading factor baths typically require a
great deal of air to be bubbled through the solution because oxygen
is the principle stabilizer for the bath. Adding this oxygen
partially inhibits undesired extraneous plating, but still allows
plating on the desired catalytic lines or features because the
surface area of the catalyst is so high that consumption of the
reactants and the plating rate is high enough to overcome the
inhibition of plating due to dissolved oxygen. In these
conventional applications with a high bath loading factor, the rate
of consumption of reactants is so high that high dosing rates, high
agitation levels, and high levels of air bubbling are required to
prevent the bath from crashing. As the bath ages and shows signs of
instability, the bath loading factor may be reduced to prolong the
bath life. Some manufacturers of commercially available electroless
plating solutions recommend increasing the dissolved oxygen
concentration to prevent plating in one or more control systems
and/or one or more maintenance systems that come into contact with
the electroless plating solution, but are not involved in the
substantive deposition of metal.
[0035] However, when a substrate that includes one or more
catalytic fine lines or features and one or more catalytic standard
lines or features is electroless plated in a conventional
electroless plating bath using commercially available electroless
plating solution, the high dissolved oxygen content of the solution
prolongs or inhibits the initiation stage of the electroless
plating process for the one or more catalytic fine lines or
features. As a consequence, the one or more catalytic fine lines or
features are exposed to the electroless plating solution for a
longer period of time and are prone to etching prior to metal
deposition. FIG. 4 shows a scanning electron microscope image of a
catalytic ink pattern 410 printed on substrate 160 that is etched
prior to electroless plating. When a catalytic ink pattern 410 is
exposed to electroless plating solution for an extended period of
time prior to initiation (if it initiates at all), such as, for
example, in a case where a high dissolved oxygen concentration
poisons catalytic sites, portions of the catalytic ink pattern 410
may be damaged by etching. This etching phenomenon may alter the
catalytic ink pattern 410, resulting in, for example, unintended
narrow portions, breaks or discontinuities, wide portions, or
removal of one or more surface layers. In this instance, the damage
due to etching has removed one or more surface layers of the
catalytic ink pattern 410, exposing one or more metal catalyst
nanoparticles 420 of the catalytic ink pattern 410. If this
catalytic ink pattern 410 initiates and plates, poor adhesion will
result because one or more metal catalyst nanoparticles 420 are no
longer embedded in the ink 410, but sit loose on the surface of the
ink itself.
[0036] During the up-time of an electroless plating bath, the
dissolved oxygen concentration may decrease over time because of
one or more of the aging of the bath chemistry, increased side
reactions, buildup of by-products, which can reduce the solubility
of oxygen in the solution or eventually precipitate out and provide
nucleation sites for further electroless plating to occur, or other
causes. As the dissolved oxygen concentration decreases, the bath
activity, such as, for example, the plating rate, increases
resulting in further reduction of dissolved oxygen and the bath
life decreases. If the dissolved oxygen concentration decreases to
a certain threshold, the bath life may be significantly reduced.
The threshold may vary based on a particular commercial vendor's
formulation of electroless plating solution or the amount of
chemical stabilizer added to the bath.
[0037] In a conventional electroless plating bath, the amount of
time required to initiate may vary based on the feature sizes of
the catalytic lines or features. In certain embodiments, such as,
for example, embodiments where flexographic printing is used to
print a catalytic ink pattern on substrate for subsequent
electroless plating, the catalytic ink may include catalyst
nanoparticles with a loading percentage by weight that is typically
less than that of the polymer content of the ink. As such, the
catalytic ink may be considered a polymer network containing
embedded catalytic sites. Initiation is an inherently probabilistic
process in which the relevant reactants need to diffuse within
close proximity of the catalytic sites. Dissolved oxygen may act as
a catalytic poison and render some of these catalytic sites
inactive while adsorbed to the surface of the catalytic component
of the ink. While one would expect the same percentage of larger
versus fine catalytic sites to be affected by the adsorption of
oxygen, there may be a feature size dependence. For example, once
initiated, catalytic activity tends to increase such that it is no
longer impacted by oxygen adsorption. Once the initial nucleation
occurs and the size of the catalytic site increases, the impact of
oxygen becomes smaller as oxygen adsorption is less likely to
impact the entire catalytic line or feature. This may be assisted
by the actual change in catalytic effectiveness as the metal
composition changes. This effect is in competition with an etching
effect in which catalytic sites may be etched by the etchant. As
such, catalytic standard lines or features tend to initiate faster
than catalytic fine lines or features in a conventional electroless
plating bath as more catalytic sites are available for local
diffusion, even when they have the same ratio of oxygen deactivated
sites to active catalytic sites.
[0038] In a conventional electroless plating bath, the amount of
time required to initiate may vary based on the feature sizes of
the catalytic lines or features. In certain embodiments, such as,
for example, embodiments where flexographic printing is used to
print a catalytic ink pattern on substrate for subsequent
electroless plating, catalytic fine lines or features tend to have
a thickness that is less than that of catalytic standard lines or
features as an artifact of the flexographic printing process. In
certain embodiments, catalytic ink is largely comprised of a
polymeric material. The diffusion rate of oxygen, which is
contained within a polar media such as, for example, water, may
vary from the diffusion rate of the reducing agent, which is
typically a small organic molecule. Thus, the surface of catalytic
sites tend to be passivated first, leaving the interior of the
catalytic line or feature still catalytically active. As the
thickness of catalytic lines or features increases, larger
(thicker) catalytic lines or features tend to have more active
sites available. As such, catalytic standard lines or features tend
to initiate faster than catalytic fine lines or features in a
conventional electroless plating bath.
[0039] In one or more embodiments of the present invention, the
dissolved oxygen concentration is regulated to a regulated oxygen
level, catalytic fine lines or features initiate quickly and at
approximately the same time as larger, or standard, catalytic lines
or features. When the dissolved oxygen concentration is within a
certain regulated range, the time to initiate is independent of the
feature size such that catalytic lines or features with diverse
feature sizes initiate at approximately the same time. In certain
embodiments, the initiation time may be in a range between
approximately 10 seconds and approximately 40 seconds, regardless
of feature size. The regulated dissolved oxygen concentration range
may vary based on the shape, size, and configuration of the
catalytic lines or features on substrate. If no historical data
from using the method exists to indicate which regulated range may
be appropriate for a given application or design, the largest
regulated range may be used to focus in on the appropriate
controlled oxygen level. In certain embodiments, the regulated
dissolved oxygen concentration may be in a range between
approximately 0.6 PPM and approximately 1.6 PPM. In other
embodiments, the regulated dissolved oxygen concentration may be in
a range between approximately 1.0 PPM and approximately 1.2 PPM. In
still other embodiments, the regulated dissolved oxygen
concentration may be in a range between approximately 0.9 PPM and
approximately 1.3 PPM. In still other embodiments, the regulated
dissolved oxygen concentration may be in a range between
approximately 0.8 PPM and approximately 1.4 PPM. In still other
embodiments, the regulated dissolved oxygen concentration may be in
a range between approximately 0.7 PPM and approximately 1.5
PPM.
[0040] In certain embodiments, the dissolved oxygen concentration
may be controlled by one or more of controlling a depth at which a
substrate is submerged in electroless plating solution, controlling
the exposed surface area of the electroless plating solution to
air, controlling the bubbling of one or more of air, oxygen, a gas
that contains oxygen, or an inert gas into the electroless plating
solution, and controlling the recirculation and/or agitation of the
electroless plating solution to affect diffusion. In certain
embodiments, where the electroless plating solution has a
non-volatile reducing agent, a pressure/vacuum control system may
be used alone, or in combination with one or more of the above, to
control the dissolved oxygen concentration.
[0041] A depth at which a substrate is submerged in electroless
plating solution may be controlled by a system (not shown)
configured to submerge the substrate at a desired depth of
electroless plating solution. In certain embodiments, where the
agitation level of the electroless plating solution in the bath is
relatively low, a gradient of localized dissolved oxygen
concentrations may occur. Typically, the highest localized
dissolved oxygen concentration may be observed nearest the surface
of the electroless plating solution and decreases with depth.
Certain bath chemistries, including, for example,
formaldehyde-based electroless plating solutions, exhibit this
effect because the reaction of the electroless plating chemistry
reduces the dissolved oxygen concentration. The reduction of
dissolved oxygen concentration is more pronounced near the active
surface of the substrate where plating occurs. In the absence of
significant agitation, oxygen must diffuse through the electroless
plating solution to replace the oxygen reduced by the reaction. If
the substrate is located near the surface of the electroless
plating solution, diffusion of oxygen from the air-solution
interface may exceed the rate at which oxygen is depleted by the
reaction. However, if the substrate is submerged at a certain
depth, diffusion of oxygen from the air-solution interface may be
less than the rate at which oxygen is depleted by the reaction. As
such, an electroless plating bath with a given electroless plating
solution may be characterized such that the localized dissolved
oxygen concentration levels within the gradient are identified by
depth. Thus, a depth at which a substrate is submerged in
electroless plating solution may be controlled to ensure that the
substrate and the reaction take place at a desired localized
dissolved oxygen concentration. For example, a roller system (not
shown) may be used to submerge a substrate at a depth of, for
example, two to three inches, below the surface of the electroless
plating solution. One or more of the rollers of the roller system
configured to submerge the substrate at depth may have a diameter
suitable to depress the substrate at a desired depth. The diameter
of the roller may have a bend radius that does not exceed a maximum
bend radius allowed by the flexibility of the substrate material
being used or the plated material. In certain embodiments, such as,
for example, a production electroless plating line configured for
electroless plating of roll-to-roll substrate material, a first
depth setting roller may be disposed near an entry point of the
substrate material to the electroless plating solution and a second
depth setting roller may be disposed near an exit point of the
substrate material from the electroless plating solution.
[0042] The exposed surface area of the electroless plating solution
to air may be controlled with a cover (not shown). The cover may be
placed over the electroless plating bath (110 of FIG. 1) to control
the exposure of the electroless plating solution to air. If a
conveyor system (150 of FIG. 1) is used, the conveyor may cause an
input and/or output gap, however, the cover should be capable of
substantially covering the bath. In certain embodiments, the cover
may allow for variable amounts of coverage such that the exposed
surface area may be increased or decreased as needed. The exposed
surface area of the electroless plating solution to air may be
reduced to promote reduction in the dissolved oxygen concentration
or to regulate a dissolved oxygen concentration when a nitrogen, or
another inert gas, blanket is employed. The exposed surface area of
the electroless plating solution to air may be increased to promote
an increase in the dissolved oxygen concentration from the
solution/air interface when recirculation or agitation is employed.
Recirculation or agitation may be employed to move electroless
plating solution from the bottom to the top of the bath, thereby
exposing other portions of the electroless plating solution to
air.
[0043] The bubbling of one or more of air, oxygen, a gas that
contains oxygen, or an inert gas, such as nitrogen, may be
controlled with a sparging control system. Air, oxygen, or a gas
that contains oxygen may be bubbled into the electroless plating
solution to increase the dissolved oxygen concentration. Nitrogen,
or another inert gas, may be bubbled into the electroless plating
solution to decrease the dissolved oxygen concentration. At any
given temperature, pressure, and total solute concentration, there
is a maximum amount of gas that will be soluble in a solution.
Typically, the relative concentrations of the gasses will closely
reflect the partial pressures of the gas with which the liquid is
in contact with. By bubbling nitrogen the effective partial
pressures of nitrogen that the solution is exposed to will be
higher. The nitrogen, or inert gas, may also form a blanket over
the exposed surface area of the electroless plating solution
reducing the introduction of oxygen into the electroless plating
solution from the solution/air interface. Blanketing works in a
similar manner by reducing the partial pressure of oxygen in the
gas above the plating solution. For this reason, the higher the
surface area of the bubbles, the more effective sparging may be in
reducing the dissolved oxygen concentration.
[0044] The recirculation and/or agitation of the electroless
plating solution may be controlled with a pump control system
and/or a turbulence control system. A pump control system may
control a recirculation speed that may be used to control the
mixing and/or diffusion of oxygen within the electroless plating
solution. A turbulence control system may control agitators that
may be used to control the mixing and/or diffusion of oxygen within
the electroless plating solution. A sparging control system may
also be a source of agitation as a gas is bubbled through the
electroless plating solution.
[0045] In certain embodiments, if an operator of the electroless
plating system wishes to reduce the dissolved oxygen concentration,
nitrogen, or another inert gas, may be bubbled into the electroless
plating solution. The nitrogen, or inert gas, forms a blanket over
the electroless plating solution that reduces or eliminates the
introduction of oxygen into the electroless plating solution from
the air that would otherwise be disposed above the electroless
plating solution. In other embodiments, if the operator wishes to
reduce the dissolved oxygen concentration, a vacuum may be pulled.
In still other embodiments, the electroless plating system may
include a roller system configured to submerge the substrate at a
depth where the localized dissolved oxygen concentration is lower.
One of ordinary skill in the art will recognize that one or more of
the above-noted techniques for reducing the dissolved oxygen
concentration may be used in combination in accordance with one or
more embodiments of the present invention. One of ordinary skill in
the art will also recognize that the one or more of the above-noted
techniques used may vary based on an application or design in
accordance with one or more embodiments of the present
invention.
[0046] In certain embodiments, if the operator of the electroless
plating system wishes to increase the dissolved oxygen
concentration, air, oxygen, or a gas that contains oxygen may be
bubbled into the electroless plating solution by a sparging control
system or other control system. A pump control system may be used
to increase the recirculation of the electroless plating solution
within the bath and promote mixing and/or diffusion of oxygen
throughout the solution. A turbulence control system may be used to
agitate the electroless plating solution within the bath and
promote mixing and/or diffusion of oxygen throughout the solution.
The pump control system and the turbulence control system may be
used independently or at the same time to achieve the desired
mixing and/or diffusion. In other embodiments, if the operator
wishes to increase the dissolved oxygen concentration, a spray or
cascade system may be used to increase the exposed surface area and
promote the introduction of oxygen into the electroless plating
solution. The spray or cascade system may be used with or in place
of a sparging control system that may be used to bubble oxygen, or
a gas mixture that contains oxygen, into the electroless plating
solution. In still other embodiments, if the operator wishes to
increase the dissolved oxygen concentration, the exposed surface
area of the electroless plating solution to air may be increased
using any suitable means for increasing the exposed surface area. A
pump control system and/or a turbulence control system may be used
to promote the diffusion of oxygen throughout the solution. One of
ordinary skill in the art will recognize that one or more of the
above-noted techniques for increasing the dissolved oxygen
concentration may be used in combination in accordance with one or
more embodiments of the present invention. One of ordinary skill in
the art will also recognize that the one or more of the above-noted
techniques used may vary based on an application or design in
accordance with one or more embodiments of the present
invention.
[0047] One of ordinary skill in the art will recognize that any
other means of controlling the exposed surface area of the
electroless plating solution, controlling the bubbling of air,
oxygen, a gas that contains oxygen, or an inert gas into the
electroless plating solution, controlling the recirculation and/or
agitation of the electroless plating solution, or other means of
controlling the dissolved oxygen concentration level may be used in
accordance with one or more embodiments of the present invention.
One of ordinary skill in the art will also recognize that any
combination or permutation of any of the above methods of control
may be used in accordance with one or more embodiments of the
present invention.
[0048] FIG. 5 shows a method of controlling oxygen levels for
electroless plating of catalytic fine lines or features in
accordance with one or more embodiments of the present invention.
One of ordinary skill in the art will recognize that the method may
be performed at laboratory scale, e.g., in a beaker, or in a
production scale, e.g., production electroless plating bath in
accordance with one or more embodiments of the present invention.
If the method is performed at laboratory scale, one of ordinary
skill in the art will recognize that the results may be used in
production scale in accordance with one or more embodiments of the
present invention.
[0049] In step 510, a substrate may be selected that includes a
plurality of catalytic lines or features that are part of or are
disposed on the substrate. The plurality of catalytic lines or
features include at least one catalytic fine line or feature and at
least one catalytic standard line or feature. In certain
embodiments, the at least one catalytic fine line or feature may
have a width less than approximately 5 micrometers. In other
embodiments, the at least one catalytic line or feature may have a
width in a range between approximately 5 micrometers and
approximately 10 micrometers. In still other embodiments, the at
least one catalytic line or feature may have a width greater than
10 micrometers, but on the order of magnitude of micrometers. In
certain embodiments, the at least one catalytic standard line or
feature may have a width on the order of magnitude of at least a
millimeter or more. In other embodiments, the at least one
catalytic standard line or feature may have a width on the order of
magnitude of at least a centimeter or more. In still other
embodiments, the at least one catalytic standard line or feature
may have a width on the order of magnitude of at least a decimeter
or more.
[0050] For purposes of testing the diversity of feature sizes, the
at least one catalytic fine line or feature may represent the
smallest width among the plurality of catalytic lines or features
that are part of or are disposed on the substrate. If the substrate
is a test substrate used to determine a dissolved oxygen
concentration range that works well for a production substrate, the
at least one catalytic fine line or feature may represent the
smallest width among a plurality of catalytic lines or features
that are part of or are disposed on the production substrate.
Similarly, the at least one catalytic standard line or feature may
represent the largest width among the plurality of catalytic lines
or features that are part of or are disposed on the substrate. If
the substrate is a test substrate used to determine a dissolved
oxygen concentration for a production substrate, the at least one
catalytic standard line or feature may represent the largest width
among a plurality of catalytic lines or features that are part of
or are disposed on the production substrate.
[0051] In step 520, the dissolved oxygen concentration of the
electroless plating solution may be regulated to a candidate
controlled oxygen level. Generally, the highest dissolved oxygen
concentration level that initiates the at least one catalytic fine
line or feature as fast as the at least one catalytic standard line
or feature, but does not exhibit one or more failure modes, is
desirable from a plating performance/bath life tradeoff
perspective. However, a lower dissolved oxygen concentration level
may be used at the expense of reduced bath life. The candidate
controlled oxygen level may be determined by incrementing the
previous candidate controlled oxygen level by an increment or
setting the candidate controlled oxygen level to the smallest value
in a regulated range of dissolved oxygen concentration.
[0052] In a first pass of the method, the candidate controlled
oxygen level may be set to the smallest value in the regulated
range of dissolved oxygen concentration. The regulated range may
vary based on the shape, size, and configuration of the catalytic
lines or features on substrate. If no historical data from using
the method exists to indicate which regulated range may be
appropriate for a given application or design, the largest
regulated range disclosed herein may be used to focus in on an
appropriate controlled oxygen level. In certain embodiments, the
regulated dissolved oxygen concentration may be in a range between
approximately 0.6 PPM and approximately 1.6 PPM. In other
embodiments, the regulated dissolved oxygen concentration may be in
a range between approximately 1.0 PPM and approximately 1.2 PPM. In
still other embodiments, the regulated dissolved oxygen
concentration may be in a range between approximately 0.9 PPM and
approximately 1.3 PPM. In still other embodiments, the regulated
dissolved oxygen concentration may be in a range between
approximately 0.8 PPM and approximately 1.4 PPM. In still other
embodiments, the regulated dissolved oxygen concentration may be in
a range between approximately 0.7 PPM and approximately 1.5
PPM.
[0053] In a subsequent pass of the method, the candidate controlled
oxygen level may be increased by an increment that may vary based
on an application or design. For example, the regulated dissolved
oxygen concentration range may be divided into a number of equal
increment values that are used to increment the candidate
controlled oxygen level. The number of equal increment values may
vary based on an application or design.
[0054] The dissolved oxygen concentration of the electroless
plating solution may be regulated by one or more of controlling the
surface area of the electroless plating solution exposed to air,
controlling a flow rate of inert gas introduced into the
electroless plating solution, controlling a flow rate of oxygen or
a gas that contains oxygen into the electroless plating solution,
controlling a recirculation speed of the electroless plating
solution, and/or controlling an agitation of the electroless
plating solution. The candidate controlled oxygen level may be
achieved by increasing or reducing the dissolved oxygen
concentration of the electroless plating solution.
[0055] In certain embodiments, the dissolved oxygen concentration
of the electroless plating solution may be increased by introducing
oxygen or a gas that contains oxygen into the electroless plating
solution and diffusing the introduced oxygen or gas containing
oxygen in the electroless plating solution. In certain embodiments,
the oxygen or gas that contains oxygen may be introduced into the
electroless plating solution with a sparging control system that
bubbles oxygen or gas that contains oxygen into the electroless
plating solution. In other embodiments, a spray or cascade system
may be used in place of, or in addition to, the sparging control
system to increase the surface area of electroless plating solution
that is exposed to air. In still other embodiments, the surface
area of electroless plating solution exposed to air may be
increased to promote the introduction of oxygen into the
electroless plating solution from the solution/air interface by,
for example, removing a cover and/or removing an inert gas blanket
that may be disposed over the electroless plating solution. The
increase in exposed surface area may be used in place of or in
addition to the sparging control system and the spray or cascade
system. The electroless plating solution may be recirculated by a
pump control system and/or agitated by a turbulence control system
to promote mixing and diffusion of the introduced oxygen throughout
the electroless plating solution.
[0056] In certain embodiments, the dissolved oxygen concentration
of the electroless plating solution may be reduced by bubbling
nitrogen, or another inert gas, into the electroless plating
solution that forms an inert gas blanket over the electroless
plating solution. The nitrogen, or inert gas, blanket reduces or
eliminates the introduction of oxygen into the electroless plating
solution from the solution/air interface. In other embodiments,
using electroless plating solution with non-volatile components, a
vacuum may be pulled either directly upon the plating vessel or on
a suitable porous membrane to reduce the dissolved oxygen
concentration.
[0057] In step 530, the substrate may be submerged in the
electroless plating solution for a period of time sufficient to
initiate plating of the at least one catalytic standard line or
feature. The time may vary based on the feature size of the at
least one catalytic standard line or feature. This time may be
determined from historical data, empirical data, or trial and
error. In step 540, the substrate may be removed from the
electroless plating solution for evaluation. In step 550, the
substrate may be evaluated to determine whether the at least one
catalytic fine line or feature initiated at approximately the same
time as the at least one catalytic standard line or feature. The at
least one catalytic standard line or feature and the at least one
catalytic fine line or feature may be examined with an optical
microscope, scanning electron microscope, atomic force microscope,
laser profilometry, or other characterization tool. A surface of
the least one catalytic standard line or feature may be examined
for evidence of initiation on the surface. A surface of the least
one catalytic fine line or feature may be examined for evidence of
initiation, or lack thereof, on the surface. The evidence of
initiation for the at least one catalytic standard line or feature
may be compared to the evidence of initiation, or lack thereof, of
initiation for the least one catalytic fine line or feature. The at
least one catalytic line or feature may be determined to have
initiated at approximately the same time as the least one catalytic
standard line or feature if the at least one catalytic fine line or
feature exhibits approximately the same evidence of initiation on
its surface as that of the least one catalytic standard line or
feature. If the at least one catalytic fine line or feature did not
initiate at approximately the same time as the at least one
catalytic standard line or feature, the candidate controlled oxygen
level may be incremented and the method may iterate as shown in
step 560. If the at least one catalytic fine line or feature
initiated at approximately the same time as the at least one
catalytic standard line or feature, the next determination may be
made.
[0058] In step 570, the substrate may be evaluated to determine
whether the at least one catalytic fine line or feature exhibits
one or more failure modes. The substrate may be examined with an
optical microscope, scanning electron microscope, atomic force
microscope, laser profilometer, or other characterization tool and
tested for electrical functionality and various stress tests that
simulate long term reliability. The one or more failure modes may
include etching from prolonged exposure to electroless plating
solution prior to initiation, narrow portions, wide portions,
breaks or discontinuities, brittleness, grain size issues, adhesion
issues, porosity issues, color issues, or any other indicator of
plating issues. A shape of the at least one catalytic fine line or
feature may be examined. A shape of the at least one catalytic fine
line or feature may be compared to a shape of a corresponding line
of feature in a source design, such as, for example, a design file
that was used to generate the at least one catalytic fine line or
feature. The at least one catalytic fine line or feature may be
determined to have etched prior to initiation if the at least one
catalytic line or feature exhibits narrow portions, breaks, or
discontinuities that do not exist in the corresponding line or
feature in the source design. If the at least one catalytic fine
line or feature did not exhibit one or more failure modes, the
candidate controlled oxygen level may be incremented and the method
may iterate as shown in step 560. If the at least one catalytic
fine line or feature exhibits one or more failure modes, the
previous value of the candidate controlled oxygen level may be
selected as the regulated oxygen level that may be used for the
substrate in production as shown in step 580. If there is no
previous value of the candidate controlled oxygen level, the method
may be repeated using one of the larger regulated ranges.
[0059] In this way, a regulated dissolved oxygen concentration
range, that is substantially lower than the dissolved oxygen
concentration of commercially available electroless plating
solutions may be explored to identify the highest dissolved oxygen
concentration within the regulated range that initiates the at
least one catalytic fine line or feature at approximately the same
time as the at least one catalytic standard line or feature and
does not exhibit one or more failure modes. The candidate
controlled oxygen level may be incremented and the method may
repeat using the newly incremented candidate controlled oxygen
level until one or more failure modes are exhibited. When one or
more failure modes are exhibited, the previous value of the
candidate controlled oxygen level may be selected as the regulated
oxygen level for production. The dissolved oxygen concentration may
be regulated to the regulated oxygen level for production.
[0060] If the at least one catalytic fine line or feature initiated
at approximately the same time as the at least one catalytic
standard line or feature and did not exhibit one or more failure
modes, the candidate controlled oxygen level may be incremented by
an increment and the method may be repeated using the newly
incremented candidate controlled oxygen level. In this way, the
process may explore the highest possible candidate controlled
oxygen level that may be used so as to extend the bath life while
still providing the benefit of fast initiation of catalytic fine
lines or features.
[0061] In certain embodiments, further optimization may be achieved
by repeating the method starting with the selected regulated oxygen
level, a narrower regulated range, and a smaller increment size to
focus in on the highest possible dissolved oxygen concentration
that may be used. The bath life may be extended by using the
highest dissolved oxygen concentration within the regulated range
that does not exhibit one or more failure modes.
[0062] Advantages of one or more embodiments of the present
invention may include one or more of the following:
[0063] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features monitors and regulates the
dissolved oxygen concentration of the electroless plating solution
during electroless plating operations.
[0064] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features regulates the dissolved oxygen
concentration of the electroless plating solution to a certain
range such that the time required to initiate is feature size
independent and is not sensitive to variations in feature size,
width, and/or thickness of the catalytic portions that are part of
or are disposed on the substrate.
[0065] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features regulates the dissolved oxygen
concentration of the electroless plating solution to a certain
range that ensures consistent plating of lines or features with
diverse feature sizes that include at least one fine line or
feature.
[0066] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features regulates the dissolved oxygen
concentration of the electroless plating solution to a certain
range that ensures uniform plating of lines or features with
diverse feature sizes that include at least one fine line or
feature.
[0067] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features reduces or eliminates etching of
the catalytic portions that are part of or are disposed on the
substrate that are submerged in electroless plating solution.
Because all lines or features initiate at approximately the same
time, the fine lines or features are not exposed to electroless
plating solution for a prolonged period of time prior to
initiation.
[0068] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features reduces or eliminates peeling
caused by poor adhesion between the plated metal and the catalytic
portion that is part of or is disposed on the substrate or by poor
adhesion between the catalytic portion that is part of or is
disposed on the substrate.
[0069] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features provides substantially uniform
plating for lines or features with diverse feature sizes.
[0070] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features provides substantially uniform
plating for lines or features on substrate that have diverse
feature sizes and include at least one fine line or feature.
[0071] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features provides substantially uniform
plating for lines or features with diverse feature sizes that are
disposed on one or more sides of a substrate, including opposing
sides of the substrate.
[0072] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features reduces or eliminates peeling
caused by non-uniform plating that results in some lines or
features that have a substantially thicker buildup of metal than
other lines or features on substrate.
[0073] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features increases the up-time or bath life
by tuning the oxygen content for the specific application.
[0074] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features increases the range of temperature
in which the electroless plating may be operated.
[0075] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features is not sensitive to a type of
substrate material, a type of catalytic portions that are part of
or are disposed on the substrate including precursor, or catalytic,
ink, an age of a precursor, or catalytic, ink, a feature size such
as width or height of the catalytic portions that are part of or
are disposed on the substrate.
[0076] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features provides a consistent and a
uniform plating of lines or features with diverse feature sizes and
include at least one fine line or feature.
[0077] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features improves the plating rate.
[0078] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features improves production
efficiency.
[0079] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features reduces material cost.
[0080] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features improves yield.
[0081] In one or more embodiments of the present invention, a
method of controlling oxygen levels for electroless plating of
catalytic fine lines or features improves reliability.
[0082] While the present invention has been described with respect
to the above-noted embodiments, those skilled in the art, having
the benefit of this disclosure, will recognize that other
embodiments may be devised that are within the scope of the
invention as disclosed herein. Accordingly, the scope of the
invention should be limited only by the appended claims.
* * * * *