U.S. patent application number 10/728636 was filed with the patent office on 2004-06-17 for coated and magnetic particles and applications thereof.
This patent application is currently assigned to Surfect Technologies, Inc.. Invention is credited to Griego, Thomas P..
Application Number | 20040115340 10/728636 |
Document ID | / |
Family ID | 32512327 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040115340 |
Kind Code |
A1 |
Griego, Thomas P. |
June 17, 2004 |
Coated and magnetic particles and applications thereof
Abstract
A method of using coated and/or magnetic particles to deposit
structures including solder joints, bumps, vias, bond rings, and
the like. The particles may be coated with a solderable material.
For solder joints, after reflow the solder material may comprise
unmelted particles in a matrix, thereby increasing the strength of
the joint and decreasing the pitch of an array of joints. The
particle and coating may form a higher melting point alloy,
permitting multiple subsequent reflow steps. The particles and/or
the coating may be magnetic. External magnetic fields may be
applied during deposition to precisely control the particle loading
and deposition location. Elements with incompatible
electropotentials may thereby be electrodeposited in a single step.
Using such fields permits the fill of high aspect ratio structures
such as vias without requiring complete seed metallization of the
structure. Also, a catalyst consisting of a magnetic particle
coated with a catalytic material, optionally including an
intermediate layer.
Inventors: |
Griego, Thomas P.;
(Corrales, NM) |
Correspondence
Address: |
PEACOCK MYERS AND ADAMS P C
P O BOX 26927
ALBUQUERQUE
NM
871256927
|
Assignee: |
Surfect Technologies, Inc.
Albuquerque
NM
|
Family ID: |
32512327 |
Appl. No.: |
10/728636 |
Filed: |
December 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10728636 |
Dec 5, 2003 |
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09872214 |
May 31, 2001 |
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60431315 |
Dec 5, 2002 |
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60447175 |
Feb 12, 2003 |
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60519813 |
Nov 12, 2003 |
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Current U.S.
Class: |
174/126.2 ;
204/471; 427/128; 427/97.2; 428/553; 430/315 |
Current CPC
Class: |
C25D 7/123 20130101;
H01L 24/11 20130101; H01L 2224/13099 20130101; H01L 2924/01079
20130101; H05K 2201/083 20130101; H01L 2924/01327 20130101; H01L
2224/13009 20130101; H01F 41/26 20130101; H01L 2924/01027 20130101;
H01L 2924/014 20130101; H01L 2924/19043 20130101; C25D 15/00
20130101; H01L 2924/0106 20130101; H01L 2924/15787 20130101; H01L
21/76877 20130101; H01L 2224/16 20130101; H01L 2924/1433 20130101;
H01L 2924/09701 20130101; H01L 21/76898 20130101; H01L 2224/05571
20130101; H01L 2924/01056 20130101; H01L 2924/01005 20130101; H05K
3/3473 20130101; H01L 2224/1147 20130101; H01L 2924/01033 20130101;
H05K 2203/0425 20130101; B22F 2998/00 20130101; B22F 2999/00
20130101; H01L 2224/1132 20130101; Y10T 428/12063 20150115; H05K
2201/0218 20130101; H01L 2924/01044 20130101; H01L 2924/19041
20130101; H01L 2924/01013 20130101; H05K 3/3485 20200801; H01L
2924/01029 20130101; H05K 2203/104 20130101; H01L 2924/00014
20130101; H01L 2924/01012 20130101; B23K 35/0244 20130101; H01L
2924/30107 20130101; H01L 21/2885 20130101; H01L 2224/05573
20130101; H01L 2224/11334 20130101; H01L 2924/01046 20130101; H01L
2924/01047 20130101; H01L 2924/19042 20130101; H05K 2203/043
20130101; H01L 2924/01322 20130101; H01L 24/13 20130101; H01F 41/20
20130101; H01L 24/12 20130101; H01L 2924/01006 20130101; H01L
2924/01078 20130101; C25D 15/02 20130101; H01F 41/16 20130101; H01L
2924/01082 20130101; H05K 2201/09572 20130101; H05K 2201/0215
20130101; H01L 2224/05568 20130101; B22F 2998/00 20130101; B22F
1/17 20220101; B22F 2999/00 20130101; B22F 1/17 20220101; B22F
1/145 20220101; C25D 15/00 20130101; H01L 2924/15787 20130101; H01L
2924/00 20130101; H01L 2924/00014 20130101; H01L 2224/05599
20130101; B22F 2998/00 20130101; B22F 1/17 20220101; B22F 2999/00
20130101; B22F 1/17 20220101; C25D 15/00 20130101; B22F 1/145
20220101 |
Class at
Publication: |
427/098 ;
204/471; 427/128 |
International
Class: |
C25D 013/00; C25D
001/12 |
Claims
What is claimed is:
1. A method of making a solder joint, the method comprising the
steps of: depositing particles comprising at least one coating on a
substrate; and reflowing the particles to at least partially melt
the coating, thereby forming a substantially continuous solidified
solder material.
2. The method of claim 1 wherein the depositing step comprises
depositing via a process selected from the group consisting of
electrodeposition, electrophoresis, electroplating, evaporation,
screen printing, and photostencil bumping.
3. The method of claim 1 wherein the depositing step comprises
blending the particles into a paste or ink.
4. The method of claim 1 wherein the depositing step comprises
electrodepositing in one deposition step at least two materials
with incompatible electropotentials.
5. The method of claim 1 wherein the solder material comprises
unmelted particles in a solidified matrix.
6. The method of claim 5 wherein the unmelted particles increase at
least one strength of the solder material.
7. The method of claim 6 wherein the strength is selected from the
group consisting of shear strength and compressive strength.
8. The method of claim 6 wherein the solder material is reinforced
by the unmelted particles.
9. The method of claim 1 wherein the reflowing step comprises
forming an alloy.
10. The method of claim 9 wherein the reflowing step comprises
forming an alloy which comprises substantially all of the coating
of the particles.
11. The method of claim 10 wherein the alloy has a substantially
higher melting temperature than the coating.
12. The method of claim 9 wherein the solder material contains a
substantially uniform distribution of stoichiometries.
13. The method of claim 12 wherein the solder material is
substantially uniform in composition.
14. The method of claim 1 wherein the depositing step comprises
controlling a concentration of the particles, thereby reducing a
size of the solder joint in directions parallel to the substrate
surface.
15. The method of claim 14 further comprising the step of
decreasing the pitch of solder joints on the substrate.
16. The method of claim 1 wherein the particles are magnetic.
17. The method of claim 16 wherein the depositing step comprises
controlling a particle loading with at least one external magnetic
field.
18. The method of claim 16 wherein the depositing step comprises
controlling a deposition location with at least one external
magnetic field.
19. The method of claim 16 wherein the reflowing step comprises
controlling a particle distribution in the solder joint with at
least one external magnetic field.
20. A solder material comprising particles that were coated before
being deposited on a substrate.
21. A method of co-depositing particles comprising the steps of:
suspending the particles in a suspension; applying at least one
magnetic field to the particles; co-depositing the particles along
with at least one component of the suspension; and forming a
desired structure.
22. The method of claim 21 wherein the applying step comprises
controlling at least one deposition location of the particles.
23. The method of claim 21 wherein the applying step comprises
controlling a particle loading.
24. The method of claim 21 wherein the particles are magnetic.
25. The method of claim 22 wherein the particles have been coated
with at least one coating.
26. The method of claim 25 wherein the coating is magnetic.
27. The method of claim 21 wherein the suspending step comprises
suspending the particles in an electrolytic solution.
28. The method of claim 27 wherein the co-depositing step comprises
co-depositing in one deposition step at least two materials with
incompatible electropotentials.
29. The method of claim 21 wherein the suspending step comprises
suspending the particles in an ink or paste.
30. The method of claim 22 wherein the forming step comprises
filling a via.
31. The method of claim 30 wherein the forming step comprises
accelerating a fill rate by controlling particle loading.
32. The method of claim 30 wherein the forming step further
comprises controlling the particle location with at least one
external magnetic field, thereby permitting fill electrodeposition
within the via without the presence of prior seed metallization of
an entire surface of the via.
33. A method of making a via comprising the steps of: providing
seed metallization to only a portion of a surface of the via;
filling the via with a material comprising conducting
particles.
34. A via comprising a seed metallization layer only partially
coating a surface of the via.
35. A catalyst comprising a magnetic particle coated with at least
one catalytic material.
36. An electrode comprising at least one surface layer deposited
using the catalyst of claim 35.
37. The catalyst of claim 35 further comprising a first coating of
the particle, wherein said first coating comprises a stable barrier
to prevent diffusion of elements comprising the particle into the
catalytic material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of the filing of U.S.
Provisional Patent Application Serial No. 60/431,315, entitled
"Solid core solder particles for printable solder paste", filed on
Dec. 5, 2002, U.S. Provisional Patent Application Serial No.
60/447,175, entitled "Electrochemical Devices and Processes", filed
on Feb. 12, 2003, and U.S. Provisional Patent Application Serial
No. 60/519,813, entitled "Particle Coelectrodeposition", filed on
Nov. 12, 2003. This application is a continuation-in-part of U.S.
patent application Ser. No. 09/872,214, entitled "Submicron and
Nanosize Particle Encapsulation by Electrochemical Process and
Apparatus", filed May 31, 2001. The specifications of each
application listed are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention (Technical Field)
[0003] The present invention relates to magnetic or nonmagnetic
particles which are coated with a desired material before the
particles are deposited on a substrate or surface. The coating is
thus in intimate contact with the core particle, resulting in
enhanced stoichiometric control and minimization of oxidation. The
coating may be any desired material, including but not limited to a
solder material or a catalytic material. If the core particles
and/or the coating are magnetic, external magnetic fields may be
used to enhance the deposition rate or to direct the particles to
specific locations, minimizing deposition on unwanted areas of the
substrate. Multiple types of deposition processes may be used, such
as electrodeposition, screen printing, and photostencil bumping.
The present invention also relates to the use of uncoated magnetic
or nonmagnetic particles to modify the properties of other
structures, such as semiconductor vias or bumps.
[0004] 2. Background Art
[0005] Note that the following discussion refers to a number of
publications by author(s) and year of publication, and that due to
recent publication dates certain publications are not to be
considered as prior art vis-a-vis the present invention. Discussion
of such publications herein is given for more complete background
of the scientific principles and is not to be construed as an
admission that such publications are prior art for patentability
determination purposes.
[0006] Wafer bumping technology recently has attracted considerable
attention in the high-end computing and networking markets,
primarily because this technology has enabled high performance for
high-density MPU, ASIC and memory device structures. Flip chip ball
grid array (FCBGA) is a package type that uses solder bumping
interconnection while simultaneously allowing for an area-array
configuration. This ensures signal and power/ground integrity far
superior to conventional peripheral wire bonding
interconnection.
[0007] For commodity or consumer products, such as cellular phones,
the package size is of vital importance. Chip scale packages (CSP)
already are well accepted in the industry. However, the search is
still on for even smaller solutions, such as wafer-level CSPs
(WL-CSP), which are a true chip size package. Another example,
bumped die for flip chip on board (FCOB) assembly, also can reduce
a product's final size.
[0008] There are three primary wafer bumping processes:
evaporation, electroplating and screen printing. Evaporation
methods require substantial investment in capital equipment and
typically entail high cost of ownership. Electroplating methods are
known to drive the trend for finer bump pitch, but some solder
materials are not suitable because of electroplating bath
constraints. Screen printing methods typically are the most cost
efficient; but there can be severe limitations on bump height when
the bump pitch is less than 200 microns. Deposition processes that
are useful according to the present invention also include
electrodeposition, electrophoresis, photostencil bumping, and the
like.
[0009] A fourth, recently developed advanced printing (photostencil
bumping) bump method uses a photosensitive resist film and provides
a solution that can address the entire range of applications, from
consumer to very high end. This partly is due to the advantage of
an advanced screen printing bumping process, which enables both a
bump height comparable to electroplating methods and a cost
structure that is competitive with standard screen printing. This
method is ideal for wafer "shuttle" services, i.e., fabrication of
different devices on a single wafer for one or many users who share
the initial tooling costs. Because a "shuttle" wafer must be
singulated prior to individual user or customer shipment, single
chip solder bumping is an effective method to apply bumps
separately on each device. Photostencil bumping has achieved a bump
pitch as low as 100-micrometers and bumping on wafers as thin as
100 microns. These advances allow for FCOB to become a very viable
solution for system miniaturization. Naturally, an optimal solution
also would have to consider the total cost for bumping, substrates,
packaging, assembly, testing and board-level assembly. Photostencil
bumping expands current capabilities for more uniform fine-pitch
bumps. The height of a bump fabricated using photostencil bumping
is similar to that enabled using electroplating and at a cost
competitive to typical screen-printing methods. For example,
photostencil bumping can produce bumps with a height of 105.about.
at a 200 micron pitch, while electroplating produces bumps 100
micron in height, and screen printing yields bumps only 75 micron
tall. One important aspect of this technology is its use of a
unique photosensitive resist film selected for its outstanding
properties in patterning, as well as the fact that it can withstand
the high temperatures required for bump formation while still
responding well to stripping by alkaline solvents. Furthermore, due
to the use of dry film openings at patterning, the height
uniformity of the bumps is improved vastly.
[0010] The present invention may be used with any method of
deposition, including all of the foregoing.
[0011] Key material, design and process considerations in solder
bumping are as follows:
[0012] 1) The bump material should ideally be high-temperature,
eutectic-forming, and lead-free
[0013] 2) The bump pitch should be as small as possible, taking
into account substrate compatibility (Bismaleimide Triazine [BT],
build-up, high-thermal expansion glass ceramic, etc.).
[0014] 3) The bump height should be sufficient to ensure
first-level reliability.
[0015] 4) The bump configuration may be area-array (MPU/ASIC) or
peripheral (memory/analog).
[0016] 5) The bump process may be wafer-level (evaporation,
electroplating, and screen printing) or single die (dimple
plate).
[0017] 6) Tailoring of melting point is desirable to accommodate
multiple reflow processing steps.
[0018] 7) Electrical contact testing must be performed both before
and after assembly.
[0019] 8) The bumps should have mechanical properties, such as
strength, sufficient to withstand possible mechanical shock,
vibration, creep, and fatigue occurring in some applications, thus
ensuring long term reliability.
[0020] 9) The material should be void-free before and after
reflow.
[0021] 10) Cost must be minimized.
[0022] The demand for lead-free bumping materials has increased
because of the Waste Electric and Electronic Equipment (WEEE) &
Restriction of Hazardous Substances (ROHS) Directive proposals in
Europe. Furthermore, lead-free bumps minimize alpha-particle
effects on memory macros in system-on-chip (SoC) devices. It has
been demonstrated that as many as 11,000 bumps can be fabricated on
a single die at 153 micron bump pitch using lead-free bumps on a
copper wiring.
[0023] There is a need for new solder materials that have the
characteristics described above. Elemental particles have been
added to existing solder compositions in an attempt to improve
these characteristics (see S. Jadhav et al., J. Electronic
Materials 30 (9), 1197 (2002), F. Guo et al., J. Electronic
Materials 30 (9) 1073 (2001), S. Hwang et al., J. Electronic
Materials 31 (11) 1304 (2002), and S. Choi et al., J. Electronic
Materials 28 (11) 1209 (1999), all of which are incorporated herein
by reference). Uncoated particles have been electrodeposited along
with a matrix or filler material. However, these approaches require
multiple processing steps, increasing the complexity and cost.
[0024] In addition, solder pastes have been blended from elemental
powders, but these have the disadvantages of poor shelf life,
stratification in the paste (which greatly reduces uniformity and
thus reliability), and the use of organic binders which are
incompatible with some applications.
[0025] The present invention also relates to semiconductor
fabrication techniques requiring the fill of blind vias with
metallic features and the fabrication of termination devices in
column or spherical shape require accelerated deposition of metals.
The existing process utilizes electrodeposition, electroless
deposition, plasma vapor deposition, and in some cases metallized
screen printing inks and pastes.
[0026] A common technique is the electroplating fill of features
defined by photoresist or photolithography. The electrodeposition
occurs by metallizing the substrates and then under conventional
electrodeposition steps the process of electrochemical deposition
builds a metallic deposit in the defined feature until the amp
minute requirement that controls the volume of fill is met. The
real time process for these types of techniques varies from two
hours to as much as ten or twelve hours to avoid occlusions or
pinches in the feature that would sacrifice the full density
structure.
[0027] The time involved in this process is not conducive to
chemical or cost-effective processing. The features that result
require a very complicated seed metallization to provide the
current buss flow to carry out the electrodeposition. This process
requires a complicated plasma vapor deposition of a seed metal
layer. This seed metal layer becomes very complicated to accomplish
when the aspect ratio of the via feature exceeds 10 to 1. The
current practice is to use more complicated methods of
cross-sputtering and still the resulting result is not sufficient
to assure a high-quality and cost-effective process.
[0028] The present invention also relates to the use of magnetic
materials for catalysis. Membrane-electrode assembly (MEA)
fabrication involves a great deal of often proprietary art, much of
which has been developed by trial and error, to achieve the right
combination of soluble Nafion, heat, and pressure for the proper
interpenetration of PEM and catalyst that gives highly active
catalyst layers. Typically inks of suspended precious metal blacks
or carbon-supported precious metals are either brushed onto carbon
felt electrodes or formed into catalyst decals by evaporation of
catalyst inks on Teflon surfaces prior to pressure-transfer onto
the PEM layer.
[0029] The use of magnetic materials to enhance catalysis is known.
The electrode fabrication approach taken by Leddy et al. relied
upon blending carbon-supported and polymer-shrouded magnetic
particles together with soluble Nafion to form ink, which results
in considerable agglomeration of the magnetic particles and reduced
contact with the separate electrocatalyst material surface (such as
Pt). This method produces a catalyst layer containing a wide
distribution of distances between magnetic and catalytic surfaces,
yet test results were very attractive, demonstrating a three-fold
improvement in the power levels compared to controls. Quantitative
analysis of these results is complicated by lack of adequate
knowledge of the microstructure of the magnetically modified
catalyst layer, but it is estimated that only about 25% of the
catalyst is active when CO is present.
[0030] This approach has been limited to the use of relatively weak
magnetic particles, because attempts to form catalyst layers from
particles of higher magnetic strength failed to yield smooth,
physically stable layers. High field-strength particles pose
undesirable force affecting the electrodes integrity by fracturing
and deforming bed-layer compact. Introduction of magnetic particles
in the ink introduces new complications, such as how to apply and
stabilize a thin layer while magnetic forces attract the particles
together, during application and after drying of the applied ink.
See for example Leddy, et al., U.S. Pat. Nos. 5,817,221, 5,928,804,
6,001,248, 6,303,242, 6,322,676, and 6,479,176, the specifications
of which are incorporated by reference. Leddy et al.'s method has
other disadvantages, including segregation of each constituent
resulting in non-uniformity of the final product and complex
manufacturing process.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)
[0031] The present invention is both a method of depositing
structures using coated and/or magnetic particles and the resulting
structures. Deposition methods include but are not limited to
electrodeposition, electrophoresis, electroplating, evaporation,
screen printing, and photostencil bumping.
[0032] A primary advantage of the invention is that the
stoichiometry of the resulting structure is more uniform than that
of structures deposited using other methods due to the intimate
contact of the coating and particle.
[0033] A primary advantage of the invention is that by depositing
the particle and coating materials simultaneously, oxide
contamination is minimized.
[0034] A primary object of the invention is to provide a method for
making a solder joint by depositing coated particles on a substrate
and reflowing the particles. The coating is preferably of a
solderable material. The resulting solder may comprise unmelted
particles in a solidified matrix. The presence of such particles
reinforces the solder, making it more resistant to compressive and
shear stresses. The particles also change the surface tension of a
solder bump or similar structure, reducing the attainable bump
pitch and enabling a higher density of bumps. The particles and
coating may partially or completely react during reflow to form an
alloy. The alloy preferably has a higher melting point than the
coating, which permits subsequent multiple reflow steps.
[0035] A primary object of the invention is to deposit particles
that are magnetic or that are coated with a magnetic material. The
particles may be suspended in an ink or paste. Alternatively, the
particles may be co-deposited in an electrolytic solution. The
magnetic field is used to control the particle loading as well as
precisely control the depositon location of the particles. In
addition, materials with incompatible electropotentials may be
deposited in one step.
[0036] A primary object of the invention is to permit the deposit
of high aspect ratio structures, for example filling a via, without
requiring complete seed metallization of the structure. A magnetic
field may be employed to direct conducting particles into the
structure past previously metallized surfaces, thereby forming an
electrical contact and permitting the deposition to continue until
completion.
[0037] The invention is further of a catalyst comprising a magnetic
particle coated with a catalytic material. The presence of the
magnetic field is known to improve the catalyst performance. The
controlled geometry of a coating on a particle means the magnetic
field at the surface is more easily controlled. The particle may
optionally have at least one intermediate layer between the
particle and outer coating, which acts as a diffusion barrier to
prevent the magnetic particle from poisoning the catalyst.
[0038] Other objects, advantages and novel features, and further
scope of applicability of the present invention will be set forth
in part in the detailed description to follow, taken in conjunction
with the accompanying drawings, and in part will become apparent to
those skilled in the art upon examination of the following, or may
be learned by practice of the invention. The objects and advantages
of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The accompanying drawings, which are incorporated into and
form a part of the specification, illustrate several embodiments of
the present invention and, together with the description, serve to
explain the principles of the invention. The drawings are only for
the purpose of illustrating a preferred embodiment of the invention
and are not to be construed as limiting the invention. In the
drawings:
[0040] FIGS. 1A-D depict two FCBGA package configurations before
and after reflow;
[0041] FIGS. 2A-B depict the effect of particle loading on bump
shape;
[0042] FIG. 3A depicts PVD seed metallization of a via;
[0043] FIGS. 3B-D depict partial PVD seed metallization and
subsequent fill and etching of a via produced according to the
present invention;
[0044] FIG. 4 is a schematic cross section of a coated catalytic
powder which comprises an inner protective layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING
OUT THE INVENTION)
[0045] The present invention is of a solder material manufactured
using coated particles. As used throughout the specification and
claims, "solder material" also means filler metal, particle joining
material, structural joining material, brazing material, welding
material, and the like.
[0046] The coated powder preferably comprises a preferably metallic
elemental core, for example nickel or copper, coated, optionally by
electroplating, with a solderable metal or alloy such as tin,
tin/lead tin/silver or other compositions suitable for electronic
component joining. The core powder may be any size, including a few
microns or even submicron, and thus can be made compatible with any
manufacturing process. The core material may comprise nickel,
copper or other conductive powder with a melting point higher than
the melting point of the encapsulating deposit. The solder material
may form a eutectic. The coating step may be accomplished without
agglomeration. The coated powder may be electrodeposited onto the
substrate as desired. Alternatively, the coated powder may be
blended into a paste or ink that can be printed on to wafers
through conventional photostencils, i.e., screen printed, and
reflowed to create reinforced spherical bumps for, for example,
flip chip bonding.
[0047] In this embodiment, during reflow the solder material melts,
wetting and spreading over the entire core particle and substrate
surface, and forming a solder bump. Upon solidification the solder
coating joins together the individual particles and bonds with the
substrate. The solidified bump contains embedded unmelted elemental
core particles. FIG. 1 depicts two FCBGA package configurations:
FIG. 1A depicts a bump for a glass ceramic substrate before reflow;
FIG. 1B shows the same bump after reflow. FIG. 1C depicts a bump as
deposited on a build-up substrate before reflow, and FIG. 1D shows
the bump after etching and reflow. FIGS. 1C and 1D show a composite
solder bump comprising unmelted particles.
[0048] The resulting structure is a composite, or aggregate,
material with a significantly higher compressive and shear
strengths than solder that doesn't contain any particles. This
reinforcement increases reliability not only because of resistance
to mechanical shock, vibration, and the like, but also because the
resulting bumps are strong enough to withstand shear stresses due
to the mismatch in the thermal expansion coefficients of the solder
and the substrate, even at higher temperatures. This eliminates the
need for underfill, saving cost and a manufacturing step.
[0049] Although the core material has not melted in this
embodiment, there may be a reaction between the core particle and
the coating. During reflow, a solid solution or intermetallic
compound may be formed at the interface between the core particle
and coating due to interdiffusion of the atoms in each. This
diffusion may occur when the solder is liquid, resulting in a
compositional change in the interfacial liquid, in which case the
reaction is known as transient liquid phase bonding. Alternatively,
if reflow is performed at a temperature less than the melting point
of the solder coating, the reaction is called solid state diffusion
bonding. In either case, the resulting interfacial compound may
have a higher melting temperature than the original solder coating,
thus enabling the material to withstand multiple reflow cycles at
higher temperatures than the melting point of the original solder
material, especially if all of the solder material has reacted with
the core material. This bonding serves to drastically increase the
strength of the solder joints. Traditional solder pastes composed
of multiple elemental powders have the problem of non-uniformity
due to the non-intimate contact of the reactants. These pastes thus
cannot withstand multiple reflow cycles because they undergo
secondary reflow due to prior incomplete reaction of their
constituents. Multiple reflow capability is an important aspect of
Level 1 soldering of an electronic device, because the device must
be soldered further, joining on to the substrates of a mother board
or electronic substrates as it is combined into a system.
[0050] In another embodiment, rather than using a core particle
which doesn't melt during reflow, a powder material may be chosen
which melts and thereby alloys with the coating during reflow. The
alloy may be eutectic. For example, silver and copper particles
have been coated with tin. The coated particles are screened onto a
substrate. During reflow at a temperature higher than the
temperature of both elemental metals, a eutectic alloy solder is
created. Although these alloys are known, the advantage of this
method is that they allow these materials to be deposited
economically. Because for many alloys each of the elemental powders
which form the alloy's components must be electrodeposited (that
is, delivered ionically) in separate steps due to incompatibility
of the electrolytic solutions, manufacturing time and cost is
increased. In addition, there is a greater risk of oxidation of the
surface of the already deposited powder because the substrate must
be moved between multiple plating cells to deposit each element.
Any oxide formation will inhibit the reaction of the elements.
Another method entails evaporation of elemental multilayers which
are then reacted; however, this process is very slow and expensive.
These methods have the drawback of non-uniform stoichiometry of the
final product due to incomplete reaction and non-intimate contact
of the reactant elements. Another prior method has been to deposit
the material already alloyed, which requires very high reflow
temperatures, greatly reducing solderability and possibly adversely
impacting other device components which cannot withstand high
temperatures. In all embodiments the coated powders of the present
invention may be electrodeposited in a single step, thereby
avoiding the aforementioned problems and reducing cost and
manufacturing time. And because the coating and core powder are
always in intimate contact, wetting is greatly enhanced and
oxidation cannot occur.
[0051] The coated powders of the present invention can be used with
any type of interconnections, for example Level 1 or chip-level
interconnections, such as flip chip solder bumps, wire bonds, or
stitch bonds, or Level 2 interconnections, which are the
traditional printed circuit board solder joints including surface
mount or through-hole configurations.
[0052] By matching the chemical and physical properties of the
coatings and core powders, other desirable properties of the final
material can be achieved, such as higher thermal conductivity to
enhance package cooling and improved electrical properties such as
current capacity or higher inductance if desired. And because
adding particles changes the surface tension of the melted solder,
at high particle loading (i.e. particle concentration or density),
a lower bump pitch (i.e., increased bump density) can be achieved.
Rather than forming a sphere during reflow, the bump may form an
elliptical shape with steeper sides, which does not extend
laterally to the same extent as a sphere, allowing bumps to be
placed closer together. This is illustrated in FIG. 2, which
schematically depicts the effect of particle loading on the
reflowed solder bumps of FIG. 1. FIG. 2A is identical to FIG. 1D;
as the particle loading is increased, the bump shape changes from a
sphere to one having a narrower profile (FIG. 2B).
[0053] In addition, the final chemical composition of the material
can be chosen to enhance the stability of properties such as
electromigration of the solder material, for example pure tin. This
will increase the reliability of the solder joint by, among other
things, preventing the formation of solid state dendrites, which
have been shown to cause gross failure in electronic components.
These properties are applicable to any structure created according
to the present invention, including but not limited to solder
bumps, bond rings, and vias.
[0054] By choosing core particles which are magnetic, external
magnetic fields can be utilized to enhance deposition of the
solder, including but not limited to using the methods of
electrodeposition, photostencil bumping, and screen printing. The
particles, or paste or ink that include them, can be more precisely
directed exactly to the desired deposition location. The particle
loading can be more precisely controlled. In addition, the unique
properties of a magnetic core particle, made from an element or
alloy such as nickel, could also have importance in novel
fabrication techniques using magnetic field enhancements and the
electrical testing of bumped die. For example, a magnetic field may
be applied during reflow to control the spatial distribution of the
powders in the solder joint, which may change the surface tension
as well as wetting or other properties of the solder material.
[0055] The co-deposition of particles, preferably magnetic, can
improve both the production process and final material properties
of other microfabricated structures, such as vias, as well. Many of
the advantages discussed above are applicable to these other
structures. The particles may be uncoated particles, magnetic or
nonmagnetic, or coated particles. The coated particles, if magnetic
properties are desired, may have a magnetic core such as nickel.
Alternatively they may comprise a nonmagnetic core coated with a
magnetic material. By optionally using magnetic fields the
particles can be more precisely directed to the desired deposition
location. Also, dissimilar materials, for example those with widely
differing electropotentials or incompatible electrolytic solutions,
may be co-deposited in one step, saving time, manufacturing cost,
and eliminating the potential for oxide contamination occurring
between process steps. In addition, particle loading of the
deposited material may be more precisely controlled using magnetic
assistance. The final material may consist of the particles
embedded in a matrix of the deposited material. The particles
and/or the coatings thereon may react with the matrix material
during further processing steps.
[0056] By co-depositing particles, preferably with magnetic
assistance to more precisely direct the particles to the desired
location, during fill electrodeposition, the current requirement
for a complete film to be formed during prior seed layer
metallization performed by PVD (plasma vapor deposition) could be
relaxed. Thus the base of the via could be metal free and the
particles being drawn in by the magnetic field would extend the
electrical current buss into the base of the via, once they are in
contact with a shoulder metallization that follows into the via.
FIG. 3 shows the cross section of a via in silicon or ceramic
substrate 300. FIG. 3A shows PVD seed metallization 310 that was
complete and coats the walls of the full three-dimensional geometry
of the via. FIG. 3B depicts the same via geometry with PVD
metallization 320 which is incomplete and tapers off towards the
base of the via, leaving the base and possibly one-third to
two-thirds of the length of the via without seed metallization.
[0057] Complete metallization would not be necessary, because by
subsequently co-depositing conducting, preferably magnetic
particles 330, preferably by magnetic assistance, into the via
during electrodeposition of the fill, they will extend the current
flow into the base of the un-metallized via and provide the
electrical continuity to provide a consistent reliable and
repeatable electrodeposited fill of the via.
[0058] By introducing particles into the electrolyte and preferably
directing them magnetically into the via, the rate of fill for the
via can be accelerated linearly over a large range of particle
concentrations; for example, a 60% solid concentration may increase
the deposition rate by 60%, depending on the particle size and rate
of loading. A typical volume ratio of particles to electrodeposit
is approximately three to one, although other ratios are possible.
As depicted in FIG. 3C, resulting fill 340 would be composed of
particles 350 bound in the fully densified matrix of the
electrodeposit. By co-depositing such particles, more favorable
current conditions are created which allow acceleration of both the
deposition and densification of the process.
[0059] According to FIG. 3D, wafer or substrate 360 may then be
plasma etched on back side 370, removing the substrate material and
exposing bump 380 on the back surface of the wafer substrate which
forms a through via interconnect. The rate of etch and the amount
of substrate removed would define the geometry, including aspect
ratio and the height, of the resulting bump on the back side of the
wafer. In addition to the stated improvements in the via fill
process, the presence of particles, preferably nickel particles,
will also provide appreciable improvements in the thermal
conductivity of the via and will provide a consistent solderable
surface. Note that the drawings in FIG. 3 are schematic and are not
meant to represent any particular relative size of the particles
and via, or any particular particle concentration.
[0060] Another structure that may be deposited according to the
present invention are bond rings, which are typically composed of a
tin-gold eutectic solder. Preferably, 1-2 micron nickel particles
are coated with tin, suspended in a gold electrolyte, and are
co-deposited in a single step along with the gold. Multiple layers
may be employed. Although a specific size range is disclosed, any
particle size may be employed in order to optimize the properties
of the structure. The magnitude and duration of an external
magnetic field will partially determine the fill proportion and
final composition of the deposited structure. After subsequent
reaction a tin-gold composition may be formed, preferably an 80:20
eutectic composition. The nickel particles will mechanically
reinforce the bond rings. Alternatively, pure tin particles
suspended in the gold electrolyte may be co-deposited with the
gold, again with the goal of producing a desired eutectic
composition. In the latter embodiment, magnetic fields would not be
employed to assist with the co-deposition.
[0061] By selecting various particulate material, coated or
uncoated, for its catalytic, electronic or other surface
properties, the present invention can be used to create embedded
passive component devices in a substrate during the
microfabrication process. Such devices include but are not limited
to resistors, capacitors and inductors. Choice of the particle,
optional coating, and electrolyte materials would define further
intrinsic properties that may be valuable in defining the
properties of, for example, electronic components, hydrogen storage
fields, and inductive or magnetic transducers.
EXAMPLE
[0062] Magnetic Core Catalyst Particles
[0063] An example of the use of coated magnetic core powders is in
the manufacture of electrocatalyst materials for applications
including, but not limited to, fuel cells. Not only does the use of
magnetic core powders improve manufacturability of the device, it
enhances its efficiency as well. The present invention comprises
coating a magnetic particle, preferably Ni, with a catalytic
material, preferably a metal, and preferably platinum. Optionally,
other elements such as ruthenium may be added to the surface,
either entirely encapsulating the particle or partially coating the
surface, to tailor the catalyst's mechanical, electrochemical,
electronic, and/or magnetic properties. The partial coatings may
comprise isolated islands of the additional element. The ruthenium
may optionally be oxidized.
[0064] The use of a magnetic material in catalyst electrodes
results in improved catalytic properties. The magnetic moment of
the core particles improves efficiency of device, and makes the
catalyst more resistant to contamination. By fixing the
electrocatalyst to the surface of the magnetic particle, and thus
effectively providing a single distance from the magnetic material
to the catalyst surface rather than a distribution of distances, a
reasonable certainty to estimates of the local magnetic field at or
through the catalytic surface is provided, so that a quantitative
relationship can be established for the magnetic effect on CO
tolerance. This is another advantage over the existing art.
[0065] One embodiment of the present invention is the production of
magnetic electrode materials that can be cast on or pressed into
ionomer membranes in a reliable and predictable fashion to give
stable, uniform catalyst loading and membrane-electrode assemblies
(MEAs) with highly active electrodes, even when utilizing high
field-strength materials, such as Nd--Fe--B, are used that exert
strong forces of self-attraction. The present invention provides
superior MEA performance and tolerance to CO levels present in
hydrogen from reformed hydrocarbons, as well as improved abrasion
resistance. An optional protective interfacial layer can render the
core particles inert with respect with the catalytic reaction, and
provide a robust interface ideal for addition of preferably Pt
and/or Pt/Ru catalyst layers, directly on each encapsulated
magnetic particle, where the field strength is the strongest.
Preforming the precious metal layer onto magnetic particles
completely encapsulated with a non-corroding metal bonding layer
will place the electrocatalyst as close as possible to the surface
of the magnetic material, regardless of subsequent processing steps
to form MEAs. In the case of encapsulation by a protective Ni
layer, or Ni-Pd layer if better corrosion resistance is needed, the
protective barrier metal is also magnetic and should enhance the
magnetic effect.
[0066] An example process for producing coated particles according
to a preferred embodiment of the present invention is as
follows.
[0067] 1. Use optimal particle geometries from available metal and
metal oxide powders that are suitable to process in aqueous
electroplating solutions. Criteria should include magnetic
saturation, geometry aspect ratio for dipole susceptibility,
size/distribution, and surface morphology.
[0068] 2. Determine a suitable barrier coat metal that can
withstand the corrosive environment. Verify by acid test the
required weight gain of the deposit to achieve full particle
encapsulation without agglomeration.
[0069] 3. Calculate the equivalent specific weight of platinum
based on the previous catalyst loading reported. Determine
parameters for electrodeposition of a very thin, uniform platinum
coating on to the encapsulated magnetic particles, to give a 3 to
10 percent by weight platinum.
[0070] 4. Optional deposition of a partial surface coverage of
ruthenium onto the platinum coated encapsulated metallic particles,
under conditions to yield a high degree of nucleation and formation
of small islands of ruthenium on the surface of the particle.
[0071] 5. Apply stable, uniform catalyst layers to membrane or
gas-diffusion electrodes and form MEAs using Nafion 112 and
demonstrate highly active electrodes layers.
[0072] 6. Deposition of uniform layer of magnetically supported
particles blended with Nafion polymers and mounted to carbon
felt.
[0073] To ensure chemical and physical stability of the magnetic
particles, we use inert metallic encapsulation techniques based on
electrodeposition using a rotary-flow-thru electroplater that will
provide magnetic beads encapsulated by a protective, corrosion-free
barrier. The encapsulation process is based on electrodeposition
using a patented rotary electroplater, specifically designed for
electrolytic application of coatings onto particles in the
few-micron to sub-micron range of diameters. One example of such a
process is disclosed in U.S. Patent Application Ser. No.
09/872,214, entitled "Submicron and Nanosize Particle Encapsulation
by Electrochemical Process and Apparatus", filed May 31, 2001,
incorporated herein by reference. This process is applicable to the
manufacture of coated particles according to any of the embodiments
of the present invention. A very durable platinum or
palladium/nickel alloy coating may be applied and annealed to a
nickel undercoat to keep the magnetic material from leaching into
the cell and to put the Platinum or ruthenium catalytic element
electrodeposited onto surface at the location of highest possible
field strength. This approach provides a more robust, chemically
inert layer than polystyrene, which is known to be unstable as the
ionomer-base polymer in PEM fuel cells, PEM electrolysers, or
hydrocarbon reformers and will advance the development of the
critical microstructure responsible for the beneficial effects of
magnetic particles in MEAs. This method will provide an integrated
composite of the materials and mitigate the uncertainty due to the
art of blending electrocatalyst and magnetic materials.
[0074] The Rotary Flow-thru electrodeposition on powder
encapsulation process utilizes centrifugal force to compact bulk
materials in aqueous solution against an electrolytic cathode
contact. The particle material is loaded through the top opening
and the plating cell is rotated at sufficiently high rpm to
centrifugally cast the powder against the cathode contact.
Electroplating solution is continuously introduced at the top
opening of the rotating cell through the immersed anode and flows
through the cell exiting through a sintered porous plastic ring
layered between the domed top, cathode contact ring, and base
plate. Electroplating is carried out with a cycle of periodic
stopping and/or counter rotation and sequential switching of the DC
power supply to the cell to circulate the particle position for
even coverage and prevention of agglomeration (bridging).
[0075] Optionally, the anode and cathode can be switched to operate
the apparatus in anodic rather than cathodic mode. The sequential
positioning of the nozzles, anodes (the anode can be easily removed
and switched to provide for deposition of different metals), and
drain port provides a method to expose the materials being plated
to a multiple step chemical process without intermixing the
chemistry. Furthermore, the continuous immersion of the plated work
prevents oxidation that normally occurs on the substrate when
transferred from tank to tank in the conventional barrel plating
process. The continuous immersion is preferably achieved by
performing all steps of the process in the same cell. The chemical
solutions are sequentially returned via the porous ring to the
appropriate return drain for a discrete circulation of each
chemical solution. Then by introducing the rinse water during
high-speed rotation the chemical solutions are exchanged with
minimal dilution due to the differing specific weights. Subsequent
steps are then carried out.
[0076] The preferred cell process flow for electrolytic
encapsulation of discrete particles with nickel plate (as an
example) is as follows:
[0077] 1 Load conductive powder;
[0078] 2 Rinse;
[0079] 3 Hot soak;
[0080] 4 Nickel electroplate with start/stop cycle;
[0081] 5 Rinse;
[0082] 6 Hot rinse; and
[0083] 7 Vacuum dry.
[0084] According to another embodiment, the Rotary Flow-Thru
electrodeposition technique is used to encapsulate iron oxide
(ferrite) powder to create a chemically inert magnet core, which
will subsequently be rendered to inert permanent magnet beads with
a platinum layer deposited on the nickel barrier. The process steps
of this embodiment are as follows.
[0085] Use metal alloy powder with a particle size range of 3-5
.mu.m-diameters, which is electroplated in a nickel sulfamate
solution at an amperage density of <0.2 amps/dm.sup.2.
[0086] This material is then rinsed and dried in a vacuum oven for
further processing.
[0087] The total amp hour requirement is controlled by weight gain
percentage using the physical constants established to deposit
nickel: 0.91308-ampere hours to deposit 1 gram of 2-valency nickel
metal.
[0088] After determining the weight gain percentage that assures
chemical resistance and inertness. The platinum weight gain is
determined by calculating specific weight of the active catalyst in
an amount less than approximately 0.4 mg/cm.sup.2. As a rule of
thumb, the specific surface area goes down by about a factor of 3
for a 10-fold increase in particle radius, so somewhat lower Pt and
Ru loadings may be required to keep the thickness of the catalyst
layer to less than 20 microns for 1 to 5 micron diameter magnetic
supports.
[0089] This applied equivalent weight is controlled by the physical
constant for electrodepositing platinum metal: 0.54957-ampere hours
to deposit 1 gram of 4-valency platinum metal.
[0090] The resulting electroplated particles are examined using a
scanning electron microscope and EPMA mapping of the electroplated
platinum deposit, which measures Pt surface coverage, to verify
complete and uniform Pt deposition. Should the Ni-encapsulated
ferrite-based magnetic materials prove less stable than desired,
either a different alloy may be used as the barrier layer, or
higher loading of precious metal may be applied, or an alternative
magnetic material, such as Ni--Fe or Al--Ni--Co, may be used.
[0091] After determining the coated particles meet the design
specifications the particles can be permanently magnetized as
powder with a medium energy (440 Joules), low voltage, capacitor
discharge type magnetizer capable of saturating Alnico and Barium
Ferrite magnetic materials.
[0092] A typical 3-5 .mu.m diameter coated particle produced
according to this embodiment is schematically depicted by the cross
section in FIG. 4. The particles are magnetically charged and ready
for blending into ink for deployment as the catalyst electrode at
either the cathode or anode. Note that the particles may be of any
diameter, from submicron to over a hundred microns, depending on
the requirements of the application.
[0093] The particle-size distribution, geometry, and degree of
porosity may be determined by combined BET and Scanning electron
Microscopy (SEM), coupled with energy dispersive X-ray analysis may
be used to ascertain the depth of catalyst deposition and the
purity of the applied platinum catalyst layer.
[0094] The quality of coatings is assessed by placing the particles
in acid such as nitric acid, to determine if iron leaches from the
core. If particles remain intact and no significant yellowing of
the solution is observed, the particles are incorporated into a
Nafion film on a glassy carbon electrode at .about.15% loading.
[0095] The catalyst must now be applied to the electrodes. The
direct application of catalyst layers to electrodes, which is the
standard approach, is attractive from a commercial point of view,
because of its compatibility with the demands of high volume
manufacturing processes. Methods devised in the laboratory, such as
decal transfer of ink layers cast first onto non-stick vellum, can
be cost prohibitive in the real world. On the other hand, simple
application methods, such as brushing of catalyst solutions
directly onto porous gas-diffusion electrodes can be ill-defined
and difficult to perform reproducibly to obtain the best degree of
loading, penetration and uniformity. A simple, direct application
method could yield spontaneous self-assembly of particles into the
pores, driven by matching of relative particle sizes, when narrow
size distributions are involved. Catalyst layers may be deposited
as inks on membranes or carbon paper electrodes obtained from
commercial sources as described in the literature, such as in a
fashion according to the methods of Leddy et al., supra, to form a
dense layer of catalyst particle in contact with the PEM layer.
Tight tolerance on the thickness and uniformity of the catalyst
layer may be achieved by controlling the viscosity of the ink
solution, which is controlled by concentration of Nafion.
[0096] A slurry of catalyst layer may be prepared by dispersing the
catalyst coated magnetic particles with carbon black Vulcan XC72R
for the anode into the solvent substituted Nafion solution this was
coated on the micro-porous layer formed electrode by tape casting.
The particle distribution is assured by strong magnetic field on
the backside of the casting surface.
[0097] One advantage of the present invention is the ability to use
magnetic fields to assist in depositing a monolayer of particles on
the MEA. This minimizes or eliminates the platinum which is not at
the surface, and thus is not in direct contact with the flow
stream, drastically reducing the cost of the device. In addition,
magnetic fields may be used during screen printing to direct the
deposition of catalytic particles, limiting them only to the
pattern the flow stream will follow. Thus, the particles are not
deposited where they will not be used, again dramatically reducing
costs. For the present embodiment, the deposition method is
preferably screen printing.
[0098] In addition to fuel cells, the present invention is
applicable for batteries, including rechargeable batteries,
hydrogen-based energy developers, electronics, and MEMS, delivering
faster charge cycles, longer life, higher power, and smaller
size.
[0099] A further embodiment of the present invention is to create
multilayer or stratified compositions where on one layer a target
material could be co-deposited followed by a second layer
co-deposited with a reactive material, providing the capability to
create solid state battery fields deposited on a substrate. By
having the ability to deposit in stratified layers, many
electrochemical devices, including fuel cell membranes, can be
fabricated in the layered composition with the chemistry of the
composition selected to perform the counter electrode properties of
a normal electrochemical cell in this area.
[0100] Although the invention has been described in detail with
particular reference to these preferred embodiments, other
embodiments can achieve the same results. Variations and
modifications of the present invention will be obvious to those
skilled in the art and it is intended to cover all such
modifications and equivalents. The entire disclosures of all
patents and publications cited above are hereby incorporated by
reference.
* * * * *