Note: Descriptions are shown in the official language in which they were submitted.
X ~ 42458 CAN 6A
NO~LE METAL-P~LYMER COMPOSITES AND
FLEXIBLE THIN-FILM CONDUCTORS PREPARED T~EREFROM
TECHNICAL FIELD
The present invention relates to composite
articles made of noble metals deposited on thin, flexible
polymeric supports, and the use of such articles for
diverse, high performance applications, such as the
fabrication of electrodes for use in biological
environments. In another aspect, the present invention
relates to composite articles useful for the fabrication
of biocompatible, flexible, thin-film microelectrodes of
special utility as stimulating multiconductor,
multichannel bioelectrodes, e.g., cochlear implant
electrodes; to methods of preparing such articles and of
fabricating such electrodes; and to the articles and
electrodes themselves.
~ACKGROUND ART
The ~abrication of electrodes for biological
applications such as neural stimulation has gained
increased attention in recent years. Biocompatible (e.g.,
implantable), flexible, thin-film, multiconductor
microelectrode stimulation arrays have been investigated
for use as, inter alia, complex neural prostheses, such as
cochlear prostheses. See, e.g., White, et.al., Ann. N.Y.
Acad. Sci., 405:183-190 (1983). This article describes
some of the technological difficulties inherent in
fabricating thin-film neurostimulating arrays. Such
difficulties arise not because of an inability to control,
by photolithographic technology, the dimensional
resolution of the arrays, but rather because of the
inability of the art to fabricate the stable, flexible and
durable composite (e.g., metal-polymer) materials
necessary for preparing such arrays.
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For example, multiple conductor miniature
electrodes that are used for stimulating the residual
nerve fibers of an impaired human cochlea or inner ear are
subjected to demanding clinical conditions. They require
long-term reliability and stability of the metal-polymer
interface, as well as the ability to provide a suitable
charge transfer from stimulating electrodes having
geometrically small surface areas.
It is desirable that such electrodes would
ideally be made up of both a noble metal conductor, such
as platinum, and a polymeric support, such as polyimide.
Both of these materials are presumed to be biocompatible,
and are known to be bioinert. This very inertness however
makes it extremely difficult to attach the two materials
directly to each other in a manner that will enable the
resulting composite to undergo conventional photo-
lithography, as well as to then withstand the rigors of a
biological environment. The authors of the above White
et.al. article recognized this problem, and attempted to
solve it by the use of a thin intermediate layer of
tantalum or titanium between the surface of the polymer
substrate and the platinum layer. Delamination of platinum
from the polyimide substrate continued to be a vexing
problem however, see e.g., White, "System Design of a
Cochlear Implant", IEEE Engineering in Medicine and
Biology Magazine, Yol. 6, No. 2, pp. 42-46 (1987).
Other authors have similarly expressed the
fru~tration and difficulty of this problem. For instance,
Roberts et.al., 2nd Quarterly Progress Report, January 1,
1984 through March 31, 1984, NIH Contract NOl-NS-3-2352,
states that "Im]etal to polyimide adhesion after
electrical stimulation continues to be a difficult and
elusive property to achieve." This report comments on how
a composite can appear to exhibit good adhesion under a
saline soak test, but fail as soon as electrical
stimulation is applied. Since such conditions are used to
simulate those encountered in biological applications,
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this report illustrates the need to fabricate flexible
thin-film composite articles that are suita~le for
preparing electrodes for use in such biological
environments and other demanding applications.
Researchers at the solid state Electronics
Laboratory of the Bioelectrical Sciences Laboratory,
University of Michigan, have explored the development of
polyimide-tantalum thin film conductor cables. Adhesion of
metals to polyimide, and saline durability, are described
as major problems. "Multichannel Multiplexed Intracortical
Recording Arrays", Quarterly Report #~, (Contract
NIH-NINCDS-NOl-NS-7-2397) ( February 1988).
SUMMARY OF THE INVENTION
The present invention provides a composite
article useful for diverse high performance applications,
including the preparation of electrodes, such as
biocompatible, flexible, thin-film microelectrodes. The
article of the present invention comprises a polymeric
support having a coating of a noble metal deposited on at
least one surface thereof, the coating exhibiting suitable
peel force for use in high performance applications.
According to test method(s) explained more fully
below, the noble metal coating of preferred articles of
the invention exhibits a suitable peel force for its
intended purposes, e.g., à peel force of at least about
0.05 kg per millimeter width after 24 hour boiling saline
treatment.
Preferably the coating is a continuous coating
and further exhibits
(1) sheet resistance of about 3 ohms per square
or less before boiling saline treatment, and
(2) sheet resistance of about 9 ohms per square
or less after boiling saline treatment.
The invention further provides a method for
making such an article, which method comprises the steps
of (a) pretreating at least one surface of a polymeric
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support by sputter-etching, and ~b) sputter-depositing a
noble metal on the pretreated surface in order to form a
coating of the noble metal. Preferred articles of this
invention can be subjected to conventional microelectronic
photolithographic techniques in order to provide
electrodes that exhibit suitable quality, electrical
perfor~ance, and durability under demanding conditions.
8RIEF DESCRIPTION OF THE DRAWING
A more complete understanding of the invention
and its advantages will be apparent from the Detailed
Description taken in conjunction with the accompanying
Drawing, in which:
FIG. 1 illustrates a schematic view of a typical
sputter etching device.
FIG. 2 illustrates a schematic view of a typical
sputter deposition device~
FIG. 3 illustrates a test surface bearing a noble
metal coating in two strips on a pretreated polymeric
support according to the present invention.
FIG. 4 illustrates a cross section of the test
surface of FIG. 3.
FIG. 5 illustrates a perspective view of a stage
in the construction o a single sided electrode array.
FIG. 6 is an enlarged sectional view taken along
line 6-6 of FIG. 5.
FIG. 7A is an enlarged fragmentary view of
FIG. 5, and points out the fine structure of the right end
of the array in a variation of the invention.
FIG. 7B is a view similar to 7A illustrating an
alternative variation of the invention.
FIG. 8A illustrates a cross section view cut
along section lines 8A-8A in FIG. 7A.
FIG. 8B illustrates a cross section view cut
along section lines 8B-8B in FIG. 7B.
FIG. Y illustrates a perspective view of an
intermediate step in the fabrication of a two sided
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electrode array demonstrating the use of viewing ports for
aligning the photolithographic masks on the far side.
FI~. lOA is an enlarged cross section view taken
along lines 10A-10A in FIG. 9.
FIG. lOB is a view similar to FIG. 1~A showing a
variation of the invention.
DETAILED DESCRIPTI~N
The present invention provides composite articles
comprising a noble metal coating on a polymeric support,
as well a method of preparing such composite articles, and
finished articles, such as electrodes, fabricated from
such composite articles.
A composite article of the present invention can
be prepared as a polymeric support, at least one surface
of the support having been pretreated by sputter-etching,
the support thereafter having a coating of a noble metal
sputter-deposited on the pretreated surface. By
pretreating the support in the manner described in the
present invention, it is possible to deposit a noble metal
in a manner that provides suitable continuity and
adherence of the metal to the support. The resultant
composite article is well-suited for the fabrication of
finished articles, e.g., electrodes for use under
demanding conditions.
The word "pretreat", and inflections thereof, as
used herein refers to the treatment of a polymeric
surface, as by sputter-etching, in order to substantially
improve the interfacial adhesion of the surface to a
sputter-deposited noble metal. The word "substantially" as
used in this sense means a degree of improvement that
renders a previously unusable surface usable in the method
of this invention for preparing composite articles having
the properties described herein.
The preferred pretreatment of the method of the
invention involves sputter~etching of a surface of the
support. Conventional sputter-etching techniques can be
--6--
applied to the method of the present invention. For
example, suitable techniques are described in V.S. Patent
Nos. 4,155,826, 4,454,186, 4,481,234, and 4,568,598, the
disclosures of each of which are hereby incorporated by
reference. The term "sputter-etching" as used herein
refers to the one-step or multi-step bombardment of a
surface with neutral species and/or ions, e.g., oxygen,
argon, carbon dioxide, nitrogen, or helium ions; this term
includes ion beam milling as well as radio-frequency (RF)
sputtering, and includes the deposition of trace
quantities of elements, e.g., chromium as the oxide, on
the inert polymer surface, as described, e.g., in U.S.
Pat. No. 4,481,234.
In the sputter-etching pretreatment of the
invention, the polymeric support preferably is specially
prepared prior to being subjected to sputter-etching, in
order to increase the effectiveness of the pretreatment
and the resultant integrity of the coating of ~etal. Such
preparation includes, for instance, the steps of ~1)
washing the support surface with appropriate cleaning
agents in order to remove oils and particulates, and (2)
thermally conditioning the washed support surface in order
to reduce volatile contaminants and adsorbed water.
Cleaning agents suitable for use in preparing
such supports include those that provide suitable
compatibility with the support itself as well as effective
removal of oils and particulates. Examples of suitable
cleaning agents are known to those skilled in the art, and
include organic solvents, such as heptane, isopropanol,
and toluene and/or fluorocarbon solvents.
Drying of the support can be accomplished in a
variety of ways that are compatible with supports of this
sort. Preferably the support is dried by heating it in a
manner that retains the dimensional and compositional
integrity of the support, yet substantially ensures the
absence of surface-absorbed water and volatile low
molecular weight contaminants to an extent that the
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support can be effectively sputter-deposited. For
instance, heating for about one to about three hours at
temperatures approaching but sufficiently below the
softening point to avoid softening of the support is
generally sufficient. For example, for "KaptonTM"
polyimide film, direct or programmed (i.e., incremental)
heating to approximately 180~C, and holding at this
temperature for a period typically on the order of about
two hours, is generally sufficient.
Supports that need to be stored after preparation
and before sputter-etching, can be stored in a manner that
avoids recontamination of the prepared surface, e.g., in
sealed containers. Prior to sputter-etching, prepared
surfaces are preferably passed under a stream of ionized
gas (e.g., nitrogen) to remove any particles that may have
become bound to the surface by electrostatic forces.
After preparation, the support can be subjected
to sputter-etching using a conventional sputtering
apparatus. Referring to FIG. 1 a schematic view of a
typical sputter etching device is illustrated. The
sputter etching device 2 includes an enclosure 4 which is
gas tight. The atmosphere within enclosure 4 can be
withdrawn through a vacuum pump attached to exhaust pipe
6, or supplemented through the gas inlet pipe 8,
controlled by the gas inlet valve 10. The workpiece 12
which is to be etched is placed on a cathode 14, which is
partially surrounded by a shield electrode 16. Suspended
above the cathode 14 is an anode 18. The chamber walls of
enclosure 4 are anodic as well. Alternating current is
applied to these electrodes by an RF power supply 20,
conditioned by RF impedance matching circuitry 22.
The surface of the support is sputter-etched to
an extent that will provide the desired adherence to a
sputter-deposited noble metal. The word "surface" as used
herein refers to the outer molecular layers of the
support, and in particular, to that depth of a polymeric
sheet surface that, when pretreated, yields a structure, .
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e.g., peak-to-valley ratio, that substantially improves
adhesion to a sputter-deposited noble metal.
In a preferred embodiment of the present
invention, e.g., in which the polymeric support is
polyimide, sputter etching is generally carried out using
a discharge power of between about 0.05 to about 0.25
watts/square centimeter, in a Perkin-Elmer discharge
chamber having a cathode area of 1940 square centimeters.
Typically, the gas pressure inside the evacuated chamber
is lowered to between about 8 x 10-5 torr to 4 x 10-~
torr. Oxygen gas is then admitted at 20 standard cubic
centimeters per minute ("SCCM") in order to maintain the
chamber pressure at about 6 x 10-3 torr during
sputter-etching. The voltage drop across the electrodes is
about 0.05 to about 1 ~V, at a standard frequency of about
13 to about 15 MHz. Under such conditions, the polyimide
is sputter-etched for between about one-half and about 10
minutes, and preferably between about 4 and about 6
minutes in order to achieve a suitable pretreatment.
As a result of sputter-etching, the pretreated
surface surprisingly exhibits improved adherence to a
sputter-deposited noble metal, and in turn the resulting
composite exhibits suitable adherence between the surface
and the coating of noble metal.
While noble metal can be deposited on a
pretreated surface by a vàriety of metal deposition
techniques, sputter-deposition of noble metal is
preferred.
The term "sputter-deposit", as used herein,
refers to a process or technique of depositing metal by
means of a sputter plasma onto an object to be coated with
the metal. The process generally involves the one-step
bombardment of a noble metal target with energetic neutral
species and ions which results in the transfer of kinetic
energy to atoms of the metal, which in turn are ejected
from the target and collide with the surface that is to
receive the metal deposit. Suitable metal deposition
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techniques include those described, e.g., in U.S. Pat.
Nos. 4,454,186, 4,568,598 and 4,481,234. In order to
ensure optimum results, sputter-deposition of the noble
metal is preferably carried out without any intervening
exposure of the pre-treated surface to ambient conditions,
e.g., exposure to air.
Preferably the metal is deposited on the
pretreated support in a manner analogous to that described
in Thin Film Processes, "Glow Discharge Sputter
Depositionn, Chapter 2, pp. 12-62, J.L. Vossen et.al.,
eds. Academic Press, New York, N.Y. (1978), the disclosure
of which is hereby incorporated by reference. In
particular such methods involve sputter-deposition using
neutral atom bombardment of a noble metal cathode.
The word "coating", as used herein, refers to the
layer of noble metal obtained on a pretreated surface,
e.g., by sputter-deposition. The coating preferably is
continuous, i.e., it has (at least within an acea of a
size suitable for its intended use) substantially no
visible pin holes or cracks therein, and exhibits suitable
continuity for its intended purpose, e.g., it exhibits a
sheet resistance suitable to allow the use of the area as
a path for electrical current.
Suitable composites are those exhibiting the
qualities, e.g., continuity of the metal layer, stability,
and durability, necessary for their intended use, e.g., as
bioelectrodes. Such qualities can be evaluated by a
variety of means, including visual inspection, peel force
tests, and saline soak tests, specific examples of each of
which are described in greater detail below.
According to Test Method A explained more fully
below, the noble metal coating of a preferred article of
the invention exhibits a suitable peel force for its
intended purposes, e.g., a peel force of at least about
0.05 kg per millimeter width after 24 hour boiling saline
treatment.
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Preferred are articles that exhibit a peel force
of at least about 0.1 kg/mm, and particularly preferred
are articles that exhibit a peel force of at least about
0.15 kg/mm.
Preferably the coating is a continuous coating
and further exhibits
tl) sheet resistance of about 3 ohms per square
or less before boiling saline treatment, and
(2) sheet resistance of about 9 ohms per square
or less after boiling saline treatment.
When the article is going to be used in
electrical applications, particularly preferred is a
continuous coating that exhibits sheet resistance of about
2 ohms per square or less before boiling saline treatment,
and about 5 ohms or less per square after boiling sali~e
treatment according to Test Method B. Most preferred is a
coating that exhibits sheet resistance of about ~ ohm per
square or less before boiling saline treatment and about 3
ohms or less per square after boiling saline treatment.
Referring to FIG. 2 a schematic view of a typical
sputter deposition device is illustrated. The sputter
deposition device 24 also includes an enclosure 4 which is
gas tight. The atmosphere within enclosure 4 can be
withdrawn through a vacuum pump attached to exhaust pipe
6, or supplemented through the gas inlet pipe 8,
controlled by the gas inlet valve 10. The workpiece 26 on
which material is to be deposited is placed on an anode
28, which is partially surrounded by a shield electrode
16. Suspended above the anode 28 is a noble metal cathode
30. Alternating current is applied to these electrodes by
an RF power supply 20, conditioned by RF impedance
matching circuitry 22.
Referring to FIG. 3, a test surface bearing a
noble metal coating on a pretreated polymeric support
according to the present invention is illustrated. Strips
of noble metal 32 have been sputter deposited on the
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polymeric support 34 which has been pretreated according
to the method of the present invention.
Referring to FIG. 4, a cross section of the test
surface of FIG. 3 is illustrated. The strips of noble
metal 32 are seen deposited on the pretreated polymeric
support 34. The polymeric support 34 is shown adhered by
a layer of pressure sensitive adhesive 36 to a stiff
backing 38.
Suitable materials for the preparation of the
composite articles of this invention, i.e., suitable
polymeri~ supports and noble metals, can be identified as
described below.
Suitable polymeric supports for use in the
present invention exhibit an optimal combination of such
-15 properties as inertness, optical transmissiveness, and
stability during processing, e.g., the surface of such
; supports should not soften or melt during sputter-etching.
Such supports also possess negligible amounts of
contaminants that could migrate to the surface after
sputter-etching, thereby destroying the effectiveness of
the pretreatment. Examples of suitable polymeric supports
include polyimide films, such as KaptonTM, Kapton-HS~, and
"PyralinTM PI 2555", each available from E.I. DuPont de
Nemours and Company, Wilmington, Delaware, "Kaneka ,IsM
polyimide film available from Kanegafuchi Chemical
Industries Co., Ltd., osaka Japan, and UpilexTM polyimide
film, available from U~E Industries, Ltd., Tokyo, Japan.
Other suitable polymeric supports include, but are not
limited to, polyester films, polyethylene terephthalate,
and films prepared according to known techniques from
polyester-ether block copolymers such as polybutylene
terephthalate-polytetramethylene etherglycol terephthalate
resins, e.g., those available as the "Hytrel"SM resins
from E.I. DuPont de Nemours, Inc., Wilmington, DE.
Preferred polymeric supports for use in the
present invention are in the form of sheets, e.g., films
or tapes, and exhibit an optimal combination of such
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properties as wide temperature range stability, high
decomposition temperature, and the ability to resist
undesired chemical or physical changes during the method
of the present invention. Thermoset polyimide sheet
supports are generally preferred in view of their
widespread acceptance in the field of implantable
electrodes and their ability to provide a suitable
combination of the above properties.
Examples of preferred polymeric supports include
polyimide tape, e.g., "Kapton " tape available from 3M
Company as No. 5413 tape, and polyimide film available
from DuPont as "Kapton-HTM" film. Other preferred
polymeric sheet supports include films prepared from
"Hytrel"lM block copolymers such as "Hytrel 6098", such
resins available from DuPont.
The term "noble metal", as used herein, refers to
a metal that is not readily oxidized. (See, e.g., Grant &
Hackh's Chemical Dictionary, McGraw-Hill Book Co., New
York, NY, 4th Ed., 1987). Suitable metals for use in the
composites of the present invention are the noble metals,
gold, iridium, palladium, platinum, rhodium, ruthenium,
and their alloys. When used in biological applications,
suitable metals exhibit a desired degree of conductivity,
and biocompatibility (e.g., neurocompatibility). Preferred
noble metals are exemplified by platinum and its alloys
and iridium and its alloys. While gold has not found wide
use in bioelectrodes, particularly for "above the neck"
neurostimulation human implants, gold is suitable for use
in other applications of the composite articles of this
invention, e.g., for use in functional neuromuscular
stimulation.
Particularly preferred noble metals for
biological applications are those that are bioinert when
used as an electrical charge carrier in proximity to
biological tissues, and that exhibit an optimal
combination of such properties as adherence to the
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pretreated polymeric sheet support, conductivity, cost,
and reproducibility and ease of use in sputter-deposition.
An example of a particularly preferred noble
metal for use in implantable bioelectrodes is platinum,
for example, 99.99% pure platinum target, such as that
available from Varian Associates, Inc., Specialty Metals
Division, Grove City, OH.
Following metal deposition, the resulting
composite articles can be used for a variety of
applications, e.g., as neural stimulating electrodes,
neuromuscular stimulating electrodes, biosensors, and the
like. For use as stimulating electrodes, the articles will
generally be subjected to photolithography, e.g.,
microelectronic photolithography, in order to fabricate a
multicomponent microelectrode stimulation array. The word
"fabricate" and inflections thereof, as used herein refers
to the method(s) used to prepare a device or other useful
article, e.g., an electrode, from a composite article of
this invention. Conventional photolithographic techniques
can be used for fabricating such arrays, and will not be
repeated here. Representative techniques are described,
for instance, in Elliot, Integrated Circuit Fabrication
Technology, McGraw Hill Co., 1982, the disclosure of which
is hereby incorporated by reference.
Referring to FIG. 5, a perspective view of the
penultimate stage in the construction of a single sided
electrode array, just before the array is cut from the
film, is illustrated. The conduction paths 40 fashioned
of noble metal on a polymeric support using
photolithographic methods are seen. The exact dimensions
of the array vary depending on the physiology of the
recipient, so no general dimensions are given.
FIG. 6 depicts the layer structure at the edge of
the construction illustrated iQ FIG. 5. Onto a stiff
backing 38, a polymeric sùpport 34 is shown adhered by a
layer of pressure sensitive adhesive 36. The layer of
adhesive is an optional feature which can be dispensed
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with in the making of this structure by, for example,
binding the polymeric support 34 to the stiff backing 38
by an annulus of adhesive tape. An insulative layer 42
covers the upper layer of the polymeric sheet support 34
at this point in the fabrication.
FIG. 7A depicts the fine structure of the end of
an array in a variation of the invention. The conduction
paths 40 are seen through the transparent insulative layer
42. Each conduction path 40 terminates in a pad 44, which
delivers current to the external environment through a
window 45 in the insulative layer 42. Also shown is
alignment mark 48.
FIG. 7~ depicts the fine structure of the end of
an array in an alternative variation. The conduction paths
40 are seen through the transparent insulative layer 42.
Each conduction path 40 terminates in a pad 44, which
delivers current to the external environment through a
stimulating plate 46 which extends through a window 45 in
the insulative layer 42.
Referring to FIG. 8A, a cross section view cut
along section lines 8A-8A in FIG. 7A is illustrated. The
conduction paths 40 are seen end on, running along and
deposited on the polymeric support 34. The insulative
layer 42 protects the flow of current within the
conduction paths 40 except where they terminate in a pad
44. Here, the insulative layer has been removed over a
section of the pad 44, creating a window 45 through which
the pad 44 is in electrical contact with the external
environment.
Referring to FIG. 8B, a cross section view cut along
section lines 8B-8B in FIG. 7B is illustrated. In this
view, an additional layer of platinum is deposited to form
the stimulating plates 46, and additional insulative
material is deposited covering the edges of the
stimulating plates 46. This additional insulative
material merges with the previously laid down insulative
layer 42. The conduction paths 40 are seen end on,
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running along and deposited on the polymeric substrate 34.
The insulative layer 42 protects the flow of current
within the conduction paths 40 except where they terminate
in a pad 44. Here, the insulative layer has been removed,
; 5 forming a window 45. Above the window 45 additional
conductive material is deposited forming a stimulating
plate 46, which is in electrical contact with the pad 44
and with the external environment.
Referring to FIG. 9, a perspective view of an
intermediate step in the fabrication of a two-sided
electrode array is illustrated. A single sided array is
fabricated on one side of a flexible backing. On the
opposite side is deposited a layer of noble metal from
which the second array will be prepared. Viewing ports
for aligning the photolithographic masks are left on the
far side by the simple expedient of covering small regions
on the far side opposite the alignment marks on the near
side with a bit of adhesive tape during the deposition
step. Several of these alignment marks 48 can be seen in
this view.
Referring to FIG. 10A, a cross section view cut
along section lines 10A-lOA in FIG. 9 is illustrated. The
structure is shown having a flexible backing 50, which in
turn has two layers of polymeric support 34 and 54 adhered
to its two sides by two layers of pressure sensitive
adhesive 36 and 56. On one side, the elements of the
above-described single sided array, i.e., the conduction
paths 40, the insulative layer 42, and the pads 44 exposed
through windows 45, have been fabricated. On the opposite
30- side of the flexible backing 50, a second coating of noble
metal 58 has been deposited to allow the fabrication of a
second set of conduction paths. A viewing port 60 has
been left uncovered by metal so that the alignment mark 48
on the first side can be seen through the various
translucent layers, allowing coordinated placement of the
conduction paths on both sides of the flexible backing.
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Referring to FIG~ 10B, a cross section view cut
along section lines 10~-lOA in FIG. 9 of a variation of
the invention analogous to FIG. 8B iS illustrated. As in
FIG. lOA, the structure is shown having a flexible backing
50, which has two layers of polymeric support 34 and 54
adhered to its two sides by two layers of pressure
sensitive adhesive 36 and 56. On one side, the elements
of the above described single sided array, the conduction
paths 40, the insulative layer 42, the pads 44, and the
stimulating plates 46 which contact the pads 44 through
the windows 45 in the insulative layer 42 have been
fabricated. On the opposite side of the flexible backing
50, a second coating of noble metal 58 has been deposited
to allow the fabrication of a second set of conduction
lS paths. A viewing port 60 has been left uncovered by metal
so that the alignment mark 48 on the first side can be
seen through the various translucent layers, allowing
coordinated placement of the conduction paths on both
sides of the flexible backing.
Those skilled in the art will recognize the
applicability of articles of the present invention for a
variety of other applications as well. For instance, in
view of the improved adherence between the metal and the
support, such articles can be used as films, multiple
conductor flexible cables, catalytic metal supported on
high performance polymer films, sensors (such as
biosensors), passive electrical elements (such as
inductors, capacitors, resistors, connector contacts and
the like), or components of such elements, as well as
stimulators.
The invention is further illustrated by the
following EXAMPLES, but the particular materials and
amounts thereof recited in these examples, as well as
other conditions and details, should not be construed to
unduly limit this invention.
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EXAMPLES
TEST METE~OD A: PEEL FOP~CE
A 5000 ml Pyrex reaction flask having one ground
glass port connected to a borosilicate condenser was used.
~round glass joints were sealed with silicone grease (High
Vacuum Grease, Dow Corning, Midland, Michigan). A
conventional external heater or heating mantle was used to
maintain a solution temperature of 100 +/- 2C. All
internal surfaces of glassware were cleaned using
detergent solution followed by rinsing with water which
was of at least 5 Megohm-cm resistivity. A saline solution
was prepared by combining high purity water of at least 5
Megohm-cm resistivity with reagent grade NaCl compound to
obtain a 0.154 molar (0.9% NaCl by weight) NaCl solution.
A volume of the saline solution sufficient to
immerse all test materials in the glassware vessel was
used. The specimens were placed into the vessel for a
period of 24 hours at 100C. Within three minutes after
removal from the saline, specimens were rinsed with water
of at least 5 Megohm-cm resistivity at a temperature of at
least 23C but not greater than 100C. Care was taken to
ensure that the force of the rinse water was not
sufficient to itself dislodge metal from the specimen
surface. The wet specimens were brie1y rinsed with
isopropanol and allowed to dry under ambient conditions
for 15 minutes.
A testing machine having tensile testing
capabilities was used (Materials Testing System 'INo.
810.05", MTS Systems, Corp., Eden Prairie, MN), having a
movable member that could be operated at constant rates of
speed in the range of 0.5 to 5000 cm/min. (0.2-1968
in./min.) and an electronic load cell to simultaneously
measure dynamic forces in the range of 0 to 22.4 kilograms
(0 to 50 lbs). Also a high speed, single event recording
storage oscilloscope was used which was capable of
-18-
recording the peel foroe signal from the load cell, and
transmitting the recorded signal to a printing machine.
A test tape was used having an adhesive coating
of about 76 microns (0.0030 in.) adhered to a polyester
tape backing having a thickness of about 130 microns
(0.0050 in.) ("No. 56 tape", 3M Electrical Products
Division, St. Paul, MN). Each 2.54 cm (1.0 in.) wide test
tape was cut to a minimum length of q3.18 cm (17.0 in.)
and gently laid onto an exposed 2.54 cm (1.0 in.) x 20.32
cm (8 in.) subregion of a specimen. Four passes
~1 second/pass) were made over each test tape with the
roller previously described. A piece of test tape
~approximately 27.94 cm (11.0 in.) long) was adhered to
the portion of test tape not covering specimen, so as to
cover up the tacky adhesive portion. Each test
tape-specimen-test panel sample was heated at 130 ~/- 2C
for 2 hours in an oven, in order to enhance the bond
strength, and allowed to cool at ambient temperature for
one-half hour.
Peel force was evaluated as described below
according to ASTM Test Method D903-49, Section 8
Paragraph 1, the disclosure of which is incorporated
herein by reference. At room temperature (22 +/- 3C), two
clamping fixtures, one fixed and the other mounted to the
moving member element, were aligned with the test tape.
The test tape was peeled forward by hand a distance of
approximately 2.54 cm tl in.) and the clamping fixtures
were attached in order to provide a 180 degree peel angle.
The test tape was then peeled at a constant peel rate
until a steady force level was observed, or until it was
observed that there was adhesive failure of the noble
metal from the polymer (i.e., composite bond failure). The
peel rate, peel force, and the observation of the absence
or presence of composite bond failure were each recorded.
The highest rate of peel for which composite bond failure
was absent was identified. The peel force(s) measured at
this highest rate of peel were then determined. An Average
--19-
Peel Force ( F) value and a Standard Deviation (Sr) were
calculated in a normalized form as force divided by test
tape width, for each composite.
~ = Average Peel Force (the average of at least two
total measurements). In general, ~ F~)/n)
in kg/mm.
SF = Standard Deviation of Peel Force (calculated
from data for at least two measurements).
SF ~ ( 1/( n-1) x (~ (Fi_~) 2 ) ) 1/2
Note: F1 - Individ~al peel force of
measurement #i.
n e The number of measurements.5
To evaluate the noble metal/polymer composite
portion of an article such as an electrode, it is
necessary to remove any coatings or passivation layers of
materials covering the composite. Articles that are not in
planar form can be tested by applying adhesive film (e.g.,
Ablefilm 550 Adhesive Film, Ablestik Laboratories,
Gardena, CA) to each test panel using the method described
above. One or more such articles can then be laid onto a
test panel-adhesive film structure in a manner that
corresponds with the location of a "specimen subregion",
as described above.
When more than one article is to be bonded within
a particular "specimen subregion", the multiple articles
can be positioned in a closely packed arrangement such
that no adhesive is visible between the articles. The
test specimens can be heated for 2 hours at 150F (66C)
with a constant 34,000 pascals (5 PSI) compressive stress
applied to the specimen and allowed to cool to room
temperature. ~n oligomeric dimethylsiloxane (e.g., #360
Medical Fluid, Dow Corning, Midland, Michigan) is applied
at a thickness of about 12.7 microns (0.0005 in.), to any
surface that would correspond to the location of a
. ~ .
,
-20-
"specimen subregion" of each test panel except surfaces of
articles to be peel tested, or surfaces located within a
distance of 50 microns (0.002 in.) of articles to be peel
tested. The width of the articles (or group of articles)
to be peel tested is measured in several places. If the
width varies more than 5%, edge portions of the article
(or assemblage of articles) can be cut away so that the
resultant width variation does not exceed 5% of the
average width.
TEST METHOD B: S~EET RESISTANCE
The electrical resistance of the noble metal
coating was evaluated in the manner described below
according to ASTM Method ~ 539-80, as described in Maissel
et.al., "Handbook of Thin Film Technology", pp.' 13-5 to
13-7, McGraw-Hill ~ook Co. (1983), the disclosure of which
is hereby incorporated by reference.
Noble metal/polymeric support composite samples
were prepared as 7.62 cm x 20.32 cm (3 in. x 8 in.)
specimens. Each specimen had at least two subregions, each
of which was 2.54 cm (1.0 in.) x 20.32 cm (8.0 in.) in
size.
Test panels of borosilicate low-expansion glass,
such as Corning 7740, available from Corning Glass Works,
Corning, NY were washed using detergent solution followed
by rinsing with water of at least 1 Megohm-cm resistivity.
The panels were then rinsed with a solution containing 70
parts heptane to 30 parts isopropanol by volume and dried
for 10 minutes at 100C. Specimens were bonded to test
panels using either the adhesive portion of the specimen
itself (if provided as a tape), or an adhesive prepared as
follows: A liquid acrylic adhesive solution of the
pressure sensitive adhesive ~PSA) variety, or a solid such
as acrylic pressure sensitive transfer adhesive; te.g.,
Scotch brand "467MP Laminating Adhesive" (Converter
specialties Division, 3M, 5t. Paul, MN)) was uniformly
- : .. . . ..
-,
.
. : ,.. ..
-21-
applied to test panels in order to form a uniform solid
coating of 25-50 microns (0.001-0.002 in.) thickness.
Solvents were removed from the prepared adhesive by baking
the coated panels for 2 hours at 90C (with protection
from airborne dust contamination). In order to minimize
the amount of entrapped air between the adhesive layer and
the panel surface, a release liner wae placed aver the
adhesive surface and four rolling passes (1 second/pass)
were made over all parts of the release liner using a
2016-2240 gram (4.5-5.0 lb.) roller having a 9.53 cm
rolling surface. The release liner was then peeled away to
expose the underlaying adhesive surface.
The uncoated back portion of each specimen
prepared from a polymeric film (as opposed to tape) was
cleaned using a solvent solution containing 70 parts
heptane to 30 parts isopropanol by volume and allowed to
dry under ambient conditions for 5 minutes. Specimens were
gently laid onto the adhesive surface, and a soft cotton
tipped applicator was used to gently press the advancing
contact line of specimen onto the adhesive surface while
wiping from side to side. The resulting "sandwich" was
rolled using four complete passes ~1 second/pass) of a
2016-2240 gram (4.5-5.0 lb.) roller having a smooth rubber
rolling surface.
A probe for measuring sheet resistance was
constructed having individual, spring loaded metal
contacts having curved tip surfaces (rounded to a radius
of curvature of approximately 38.5 mm). The tips were
constructed so as not to impart a compressive stress on
the test sur$ace in excess of 0.11 Megapascals (16 psi).
This particular stress was found to be useful with all but
the softest supports (e.g., films prepared from ~ytrel~M
resin). With soft supports it is necessary to use a probe
or other technique that does not itself disrupt the
coating, e.g., by deforming the underlying support as the
probe is brought into contact with the surface. For
specimens 2.54 cm x 2.54 cm (1.0 in. x 1.0 in.) or larger,
, .: , . ,, ~ :
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2Q~.4~
-22-
a probe having 4 elements, mounted in a collinear
arrangement with a probe tip to probe tip spacing distance
of 0.423 cm (0.167 in.) in an electrically insulating
support was used. The probe elements were each
independently connected through insulated low resistance
wires to a constant DC current source and voltmeter, such
as Model 3478A, ~ewlett Packard Co., Palo Alto, CA,
capable of providing a constant DC current of 1 milliamp
and measure DC voltages in the range of 1.0 millivolt to 1
volt.
For the four element (equi-spaced and collinear)
probe, the wires of the outer two probe elements were
connected to the output connections of the constant DC
current source. The wires of the inner two probe elements
were connected to the voltage measurement input terminals
of the voltmeter. The measured voltage (volts) and
current (amps) values were used to determine sheet
resistance (R~). For such a probe R~ (V x 4.532)/I
(ohms/square).
The noble metal surface of each specimen mounted
onto a test panel to form a "sandwich" was cleaned using
the heptane/isopropanol solvent. The specimens and a
platinum metal reference foil were dried for five to ten
minutes at 100C in an oven. The probe tips were
simultaneously brought into contact with the metal surface
of a region of a specimen and the voltage drop ~volts) and
applied current (amps) were observed. Resistance levels on
each of the two subregions on each specimen, and on the
reference foil were calculated as follows:
Using six measurements for each composite
specimen in units of ohms per 'square':
n = The number of measurements.
;
A = Average Sheet Resistance before/after exposure to boiling saline.
A = ((~ Ai)/n) in ohms/square.
.: ,:
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-23-
SA = Standard Deviation of Sheet Resistance.
SA = (1/(n-1~ x (~ (Ai-A) )) / in ohms/square.
Note: Ai = Individual sheet resistance of
measurement #i.
EXAMPLE 1
Sputter-Etching Pretreatment of a Polyimide Surface
Polyimide film ("Kapton"~M), was prepared by
adhering #5413 Plastic Film tape (3M Company, St. Paul,
MN) to an aluminum test panel. The tape has a polyimide
film backing having a thickness of approximately 0.0038 cm
(0.0015 in.) and a silicone adhesive layer having a
thickness of approximately 0.0033 cm (0.0013 in.). The
back surface of the film was cleaned by rinsing with
heptane and isopropanol mixed in a 70 to 30 parts volume
ratio ("heptane/isopropanol") to remove oils. The
absorbed water was removed by baking in a Model 26 oven
(Sigma Systems, San Diego, CA) at 180C in air in a
covered holder for a period of two hours. The surface was
cleaned of dust with a jet of ionized nitrogen gas and
then subjected to sputter-etching.
Sputter-etching was carried out using an oxygen
sputter plasma in a Randex sputtering system model 2400
radio frequency diode sputtering apparatus (Perkin Elmer
Co., Palo Alto, CA) operated at a frequency of 13.56 MHz.
Samples were placed on a cathode of 1940 sq. cm. area. The
chamber was first evacuated to a pressure of 8 x 10-5
torr, then oxygen gas was introduced at a flow rate of 20
standard cubic centimeters per minute ("SCCM"). An
equilibrium pressure of approximately 6 millitorr was
maintained as oxygen was continuously introduced and
removed from the system. The treatment lasted 5 minutes at
an RF power level of 100 watts. -
The pretreated, i.e., sputter-etched, surface of
the polyimide was further processed by sputter deposition
of pure platinum metal. Sputter deposition was performed
-24-
using argon ion bombardment of a pure platinum target in
the same chamber as described for sputter etching, without
breaking the vacuum between the steps. The chamber was
operated with a discharge power of 250 watts, provided by
an RF generator tuned to a frequency of 13.56 MHz and
coupled to a platinum cathode. First, the chamber was
evacuated to a base pressure of one millitorr, then argon
was introduced into the system at a constant rate of 30
SCCM, in order to maintain a working pressure of 6 x 10-3
torr when balanced against the continuing operation of the
vacuum system.
Specimens were positioned on an annular anode
which rotated at a rate of 2.5 rotations per minute
("RPM") during sputter deposition, in a manner such that
each specimen was brought directly under the target during
each rotation. The noble metal target was sputtered for 5
minutes with the aperture to the specimens closed, after
which the aperture was opened. Using a cathode having
effective dimensions of 9.5 cm (3.75 in.) x 4.45 cm
(1.75 in.) x 0.32 cm (1/8 in.), a platinum coating having
a thickness of 3000 A was achievable within nine hours.
The thickness of the coating was estimated based on
previous measurements of thickness that were made using a
thickness measuring device ("Dektak II" surface profile
measuring system, Veeco Instruments, Sloan Technology Inc.
Santa Barbara, CA.) and correlated with deposition time.
The resultant composite was used to fabricate an
electrode as described below in EXAMPLE 2. Other
composites were prepared in substantially the manner
described above for the evaluation of sheet resistance and
peel force. Using the above-described Test Method B, the
sheet resistance prior to boiling saline for these
composites was determined to be 1.42 +/- 0.46 ohms per
square. The sheet resistance after boiling saline, was
determined to be 4.7 +/- 1.3 ohms per square. The peel
force of these composites after boiling saline, as
.. : ..
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2~
-25-
determined by Test Method A, was found to be greater than
O.1 kg/~m.
These results indicate that the thin film of
platinum sufficiently resisted the corrosive and
degradative effects of boiling saline.
Example 2
Fabrication of a One-Sided Electrode Array
The platinum/polyimide composite of EXAMPLE 1 was
fabricated using conventional photolithographic techniques
to create a one-sided electrode array in the following
manner. The composite was cleaned with
heptane/isopropanol and allowed to dry at room temperature
in air. The composite was baked at 85C in a natural
atmosphere in a Model CR07-256 B/C oven (Blue M Co., Blue
Island, IL) for 30 minutes. A positive photoresist
("AZ1370SF", Shipley Co., Newton, MA) was applied, at a
nominal thickness of 3 microns, to the platinum surface by
spin coating in an automated "Omnichuck" spin coating
machine (Machine Technology, Inc., Parsippany, NJ). This
coating machine was set for a fluid ejection pressure of
2.1 to 2.5 kg/cm2 (30 to 35 psi). Photoresist was applied
for 3.5 seconds at 6 RPM followed by an 8 second spin at
3600 RPM to remove excess material. The photoresist was
then baked in the same manner described in EXAMPLE 2
above, and then allowed to absorb water vapor under
ambient conditions for 10 minutes.
The composite with photoresist applied thereon
was then placed in a model 3001CHRZ Wafer Alignment system
(Eaton Semiconductor Equipment, Kaspar Instruments model
3001, Sunnyvale; CA), and covered with a designed mask
defining the conduction path geometry for the physical
dimensions of a cochlea, for use as a cochlear implant.
Besides the conduction paths and charge injection pads,
the mask also defined alignment marks in order to allow
alignment of a second masking, described below.
-26-
The composite thus held in the alignment system
with the mask in place was then exposed to UV radiation in
order to chemically modify the photoresist and make it
vulnerable to a developing step. This exposure was
carried out with a mercury arc UV lamp operated by a model
762 Intensity Control System (Optical Assoc., Santa Clara,
CA) which provided 3.4 milliwatts/cm2 at a wavelength in
the range of 365 to 436 nm with an exposure duration of 35
seconds. The energy density of UV radiation under the
recited conditions was at least 120 millijoules/cm2.
The exposed photoresist was then developed, in
order to remove the exposed material, for approximately 16
seconds in a solution consisting of 3.5 parts deionized
water and 1 part "AZ351 Developer"TM developing solution
(American Hoechst Co., Sommerville, NJ). The composite
article was then immediately rinsed in running deionized
water until a temperature compensated conductivity monitor
(Model 920-20M, Balsbaugh, Foxboro, MA) indicated that the
water had returned to a resistivity of at least 1
Megohm-cm. The article was then dried with a stream of
ionized nitrogen.
Sputter-etching with an argon plasma was then
carried out to remove platinum not protected by
photoresist. A radio frequency diode sputtering apparatus
(Randex Model 2400, Perkin Elmer Co., Palo Alto, CA) was
first evacuated to a pressure in the range of 8 x 10-6 to
5 x 10-5 torr, argon gas was introduced at a flow rate of
20 SCCM and the electrodes operated at a frequency of
13.56 MHz. An equilibrium pressure of approximately one
millitorr was maintained as argon was continuously
introduced and pumped through the system. 500 W power was
required for this operation, with a duration of 60 to 75
minutes.
Sputter-etching with an oxygen plasma was then
carried out in order to remove photoresist over the
platinum conductor pathways. The radio frequency diode
sputtering apparatus, operated at a frequency of
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-27-
13.56 MHz, was first evacuated to a pressure of one
millitorr, then oxygen gas was introduced at a flow rate
of 20 SCCM. An equilibrium pressure of approximately
5 x 10-3 torr was maintained as oxygen was continuously
introduced and removed from the system. The power
required for this step was 300 W, and this rate was
maintained for approximately 20 minutes.
A "Pyralin solution" was mixed consistinq of
equal amounts of Pyralin PI-2555 (E.I. DuPont de
Nemours, Inc., Wilmington, DE), "AZ Thinner" ~Shipley Co.,
~ewton, MA), and reagent grade N-methyl-2-pyrrolidone.
The composite was baked at 85C in air in a Model CR07-256
~/C oven (Blue M Co., Blue Island, IL) for 30 minutes,
then spin coated with the Pyralin solution in an automated
Omnichuck spin coating machine (Machine Technology, Inc.,
Parsippany, NJ) set for a fluid ejection pressure of 30 to
40 psi. The fluid application time was set to 3 seconds
at 6 RPM followed by an 8 second, 600 RPM spin to remove
excess material. The composite was then baked at 85C in
air in the Model CR07-256 B/C oven once again for 30
minutes, and allowed to cool in ambient conditions for at
least 10 minutes, but preferably no longer than about 8
hours before beginning the next step.
The insulating pyralin layer was then removed at
selected points, thereby creating "windows" through which
charge could be delivered into the excitable nerve fibers
of the body of a patient. To accomplish this, photoresist -.
material was applied for a second time, in the same manner
described above. Similarly, a second optical mask set to
expose only the window areas to the W irradiation step
was placed, keying on the platinum metal alignment marks
generated in the first masking. The composite article was
then exposed to UV illumination sufficient to chemically
change the unmasked areas and render them vulnerable to
the developer. It was then immersed in the developer
described earlier, in order to remove not only the exposed
photoresist, but also the pyralin immediately below these
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-28-
areas. The composite was then rinsed sequentially in
acetone and isopropanol to remove the unexposed
photoresist. It was found that best results were achieved
if the surface of the composite was kept wetted by solvent
at all times during this procedure.
The impedance of the resultant electrode pads was
determined in a circuit made up of the pads immersed in
physiological saline and attached to an AC meter/AC power
source by platinum wire. Monopolar pad impedance was
determined using a low frequency impedance analyzer ~model
4192A, Yokogawa-Hewlett Packard, Ltd., Tokyo, Japan) set
at a stimulating voltage of 0.75 to 1.0 volt-RMS at 1000
Hz. All pads were 50 micron x 100 micron in size. The
monopolar impedance was determined to be 4600 ohms ~/-
1000 ohms for 152 pads out of 154 produced on different
arrays.
Finally, the electrodes were heated in a
convection oven to render the pyralin fully imidized
(i.e., "cured"). Further curing was carried out according
to the manufacturer's suggestions by heating in an oven
~model DC-256-C, 81ue M Co., Blue Island, IL) for 35
minutes at 105C, then for one hour at 310C, after which
they were slowly allowed to cool to room temperature.
Under conditions that approximate those
encountered under physiological use, electrodes fabricated
from composites of the present invention demonstrated
desirable performance and uniformity.
EXAMPLE 3
Preparation of a Two-Sided Array
A SilasticTM brand medical grade silicone rubber
sheet (Cat. Number 500-3, Dow Corning, Midland, MI) having
a thickness of 0.025 cm (0.010 in.) was washed with a hot
-~ water rinse to remove talc. The sheet was then rinsed
with high purity water and dried for 12 hours at 60C and
under a vacuum of 38 cm (15 in.) Hg in a vacuum oven. A
circular disk was cut from the sheet having a diameter of
. .
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:
2~
-29-
6.6 cm (2.6 in.) and laid on a lint free surface. Lint
and other particles were removed from the rubber sheet by
using a mildly tacky tape such as 3M's MagicTM brand
transparent tape (3M Company, St. Paul, MN). The ruSber
disk was then laid very flat on a clean, lint- and
dust-free 7.62 cm (3 in.) silicon wafer having an indexing
flat at one part of its edge.
3M brand "Kapton" tape (#5413, 3M Company,
St. Paul, MN) from a 10.2 cm (4 in.) wide roll was then
applied to the rubber disk with an air jet from a pen
shaped probe. Using the air jet it was possible to lay
the tape down very flat using appropriate force without
stretching or scratching the polyimide surface.
This structure was then laid on a flat,
lS mirror-finish stainless steel panel and placed in a
hydraulic platen press (Model 12-10-2T, Wabash Metal
Products Company, Wabash, Indiana) set to a heat of 100C.
A light contact force of approximately 3.45 to 6.39 bars
(50 to 100 psi) was applied to improve the adhesion
between the rubber and the pressure-sensitive adhesive. A
TeflonTM liner was used to prevent contamination of the
polyimide surface during this operation. ~o assure that
symmetric pressing was achieved, the samples were pressed
four times with the wafer rotated 90 degrees after each
one minute press cycle.
Within eight hours, and preferably within one
hour, the Rapton surface was washed with
heptane/isopropanol and with soft paper towels to remove
contaminants. The rubber/polyimide structure was then
gently removed from the silicon wafer and inverted onto a
new clean silicon wafer so that Kapton tape could be
applied to the second side. It was found that a delay in
performing this step resulted in excessive adhesion of
the silicon wafer, making qentle removal impossible. The
same lint removal, tape application, pressing, and
cleaning steps described above were carried out to form a
second side of an identical nature.
.
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.
'
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-30-
The circular edge of the three-layer structure
(Rapton tape-silicone rubber-Kapton tape) was then taped
down to the wafer using an annulus of the same Kapton tape
cut from a wide roll. with the aid of a hand tool havinq
a smooth rounded end surface this tape was laid down
carefully so as to minimize any stretching of the
structure, while obtaining as airtight a seal as possible.
The photolithographic procedure for creating a single
sided array as outlined in EXAMPLE 2 was then carried out
on the exposed areas of the second side of the structure.
Sputter-etching and sputter-deposition procedures
as described in EXAMPLE 1 were carried out on the first
side of the structure, after first inverting the 3-layer
structure and remounting it to the wafer with a piece of
tape, and then placing tape strips on the first side at
positions opposite the alignment marks located on the
second side. The procedure described above and in EXAMPLE
2 was repeated on the first side of the article, except
that the tape covering the alignment areas was removed at
the time of spin coating photoresist onto side one.
Reflected light microscopy through the translucent
Kapton-silicone-Kapton silicone rubber structure allowed
this masking to be aligned with the alignment marks still
remaining on the face-down side. The electrode in its
final dimensions was cut away from the structure with a
surgical razor.
Electrodes prepared as described in this EXAMPLE
have been soaked in 37C saline and subjected to
intermittent electrical stress for months. After 600
hours, 39% of the electrodes had monopolar impedance below
1600 ohms, and 21% had impedance between 1600 and 40,000
ohms. The remaining 40~ percent were damaged by residual
DC current which dissolved the Pt metal. These results
suggest that a periodic balanced AC electrical stress does
not harm the platinum construction any more than does
saline exposure alone.
-31-
EXAMPLE 4
Utility of the Electrode Array In Vivo
An electrode prepared as described in EXAMPLE 3
was used in testing an animal subject. The electrode had
fourteen conduction paths forming seven channels, each
having two poles. It was inserted into the cochleas of a
cat in order to test its ability to deliver neural
stimulation. The thresholds were somewhat high, but the
neural responses elicited were quite normal.
EXAMPLE S
other Polymeric Supports
Various polymeric supports were pretreated by
sputter-etching and then sputter deposited with platinum
according to the method of the present invention, and
evaluated according to Test Methods A and B.
The results in TAB~E A below compare the maximum
time elapsed, after samples were placed in boiling saline,
before uniform loss of metal from the support was visible.
Comparisons made in this manner were found to serve as a
suitable pre-screening of materials (e.g., supports) and
of the effectiveness of the sputter-etching and sputter-
deposition procedures, in that higher values (e.g.,
greater than about 50 hours, and preferably greater than
about lO0 hours) tended to indicate which samples would
perform well in terms of peel force and sheet resistance
as determined by Test Methods A and B, respectively.
TABLE A also compares the samples prepared by a
sputter-etching pretreatment followed by sputter
deposition, according to the method of the present
invention, with samples similarly sputter deposited
although without such pretreatment. AS can be seen, for
each support other than the particular polyvinylidene
fluoride sample used, at least one sample pretreated by
sputter-etching exhibited a significantly longer maximum
::
'
-32-
elapsed time before visible metal loss than did any sample
not pretreated in this manner.
The particular supports used in TABLE A were as
follows:
Polyimide: #5413 Plastic Film Tape, 3M Company,
St. Paul, MN, having a backing of DuPont KaptonTM
polyimide.
Polyethylene terephthalate ("PET"): prepared
internally according to standard techniques as extruded,
biaxially oriented PET, having 0.3% AlSi slip agent.
"HytrelTM": extruded and biaxially oriented film
prepared internally according to standard technigues from
Hytrel 6098TM resin (DuPont).
Polyvinylidene fluoride (PVF2): prepared
internally according to standard techniques as extruded,
biaxially oriented PVF2.
TABLE A
Maximum elapsed time (hours)
Sputter etch, No sputter etch
SupportSputter-deposit Only sputter-deposit
Polyimide:
3M-Rapton~M Tape>300 45
Polyethylene
terephthalate 80 30
Hytrel 6098 -
oriented >160 30
Polyvinylidene
fluoride 16 14
. .
-33-
The results in TABLE B below compare the peel
force and sheet resistance for samples prepared in the
same manner as those listed in TABLE A. As can be seen in
TABLE B, samples prepared using the same polyimide, PET,
and HytrelSM 6098 films used above, each exhibited
suitable peel force properties for use as composites of
the present invention, whereas the particular
polyvinylidene fluoride sample used was unsuitable for
such use, under the preparation conditions used. These
results correlate well with the maximum elapsed time
determinations set forth in TABLE A.
The relatively high sheet resistance values for
the HytrelSM support are believed to be due to the soft,
rubbery nature of this support. The stress of the
electrical probes used accordinq to Test Method B appeared
to slightly deform the support in a manner that broke the .
continuity of the coating, thereby providing
uncharacteristically high sheet resistance readings.
.
.i . .
.
-34-
TMLe B
Peel Force
(Kg/mm) Sheet Resistance
after 100C (ohms/square)
Support Saline Aging Before After
Polyimide:
3M-KaptonTM Tape >0.1 1.42 ~/- 0.46 4.7 ~/- 1.3
Polyethylene
terephthalate 0.1 1.46 +/- 0.52 5.2 +/- 2.8
HytrelT~ 609B -
oriented >0.1 3.4 +/- 2.5 11 l/- 8
Polyvinylidene
fluoride 0.05 1.71 +~- 0.66 5.2 ~/- 1.3
EXAMPLE 6
Other Noble Metal~
Specimens were prepared by sputter depositing
iridium and gold, in the manner described above with
respect to the sputter deposition of platinum. It was
found that iridium exhibited suitable properties, while
gold appeared to readily debond. It is likely that the
adherence of all noble metals, including gold, could be
improved by specifically optimizing the pretreatment and
sputter deposition according to the teaching of this
invention.
E:XAMPLE 7
Other Support Shape6
As described previously, the application of this
invention is not limited to planar, sheet-like support
-35-
surfaces. Samples of cylindrical shapes, pretreated and
sputter-deposited on their exterior surfaces, were
prepared to demonstrate that em~odiments analogous to
wires could readily be prepared. Such structures would
find particular usefulness with modern connector
technology and cabling for both AC and DC signal
transmission.
Tube-like structures having outer diameters of
5.08 x 10-2 cm (0.02 in.) and 1.27 x 10-2 cm (0.005 in.)
were prepared according to the methods described in
EXAMPLE l. Polyimide tubes ~Micro-~ore M~ and polyimide
coated flexible fused silica tubing were both obtained -;
from Polymicro Technologies, Phoenix, AZ. Lengths of
tubing up to 28 cm (11.0 in.) were coiled and mounted to
silicon wafers covered with XaptonTM tape using two pieces
of KaptonTM tape to hold each tube. All cleaning was done
prior to mounting. After baking 2 hours at 180C the
specimens were exposed to ionized nitrogen prior to
sputter etching and sputter deposition.
Sheet resistance of the resultant tubes could not
be measured directly using the planar 4 point probe, but
could be estimated by multiplying the resistance per unit
length by the outer diameter. Sheet resistance was
estimated to be 1.4 ohms per square for the 5.08 x 10-2 cm
(0.02 in.) diameter tube, and 1.8 ohms per square for the
1.27 x 10-2 cm ~0.005 in.) tube. Both of these values
correlate well with data for similar planar composite
shapes.
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