Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTROFORMING ELE~IENTS
Back~round
The present inventlon relates generall~r to the art of electro-
forming9 and more particularly to the art oF electroforming a heating
grid.
Electroforming of precision patterns, such as those used in -
optical systems, has been accomplished by several methods. For example,
precision mesh patterns have been produced by electroplating onto a mas-
ter pattern of lines formed by etching or ruling lines into a glass sub-
strate and depositing a conductive material into the etched or ruled
lines to form a conductive master pattern for electroplating. A major
disadvantage of this method is the limitation on the fineness and preci-
sion of etching glass.
Photolithographic techniques have also been used to produce
patterned electroforming mandrels. For example, a conductive substrate,
such as a polished stainless steel plate, is coated with a layer of
photoresist. A patterned photomask is placed over the photoresist, which
is then exposed to actinic radiation through the mask, thereby creating a
pattern of exposed and unexposed photoresist which is further developed.
Either the exposed or the unexposed portions of the photoresist are
removed, depending on whether a positive or negative pattern is desired,
resulting in a conductive pattern on the substrate. An electroplatin~
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process i5 then carried out to orm a replica of the conductive pattern
which can therea~ter be removed from the substrate. This method is also
restrlcted in the uniformity and precision of lines which can be formed,
as well as requiring reprocessing of the master pattern after limited
usage.
Il.S. Patent ~o. 3,703,450 to Bakewell discloses a method of
fabricating precision conductive ~esh patterns on a repetitively reusable
master plate comprising a conductive pattern formed on a nonconductive
substrate and a non-conductive pattern formed in the interstices of the
conductive pattern. A reproduction of the master pattern is formed by
plating of a conductive pattern onto the master pattern within a matrix
defined by the non-conductive pattern. The conductive metal master pat-
tern is typically deposi~ed onto a glass plate by evaporation of a metal
such as chromium through a ruled pattern formed on a stencil materlal.
The nonconductive pattern is formed by depositing a layer of photoresist
over the conductive pattern coated side of the glass plate. By e~posing
the photoresist to actinic radiation through the conductive pattern
coated substrate, exact registration of the conductive and nonconductive
patterns is achieved. The photoresist layer is developed and the exposed
portions are removed, leaving a pattern of photoresist over the
conductive pattern. A silicon monoxide layer is then deposlted over the
entire surface of the glass plate, covering both the photoresist/conduc-
tive pattern coated portions and the exposed glass portions. Finally,
the photoresist overlying the conductive pattern and the silicon monoxide
overlying the residual photoresist material are removed, leaving the
glass plate coated with a conductive metal pattern and an array of
silicon monoxide deposits in the interstitial spaces in the ~onductive
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pattern. Replicas of~the cond~lctive pattern are ~hen formed by electro~
plating.
Summary of the Invention
The present invention provides an alternative process for pro- -
ducing a heater element grid. A substr~te transparent to actinic radia-
tion is provided with a desired pattern for the heater element grid to
form a photomask. A substrnte to be used as the electroEorming mandrel
is coa~ed ~ith:a contin~ous~conductive;~ilm. ~ continùou~ layer of photo-
resist is deposited o~er the conduceive film. The photoresist is e~posed
to actinic radiation through the photomask, the pattern acting to mask
portions of the photoresist from exposure. The photoresist is then
developed, and the unexposed portions removed to yield a conductive
pattern of the underlying conductive film corresponding to the pattern of
the photomask. Alternatively, the exposed portions of the photoresist
may be removed to yield a conductive pattern which is a negative image of
the pattern of the photomask. The resultant article is employed as a
mandrel for the-electroforming of a metallic heater element grid. The
mandrel is in~nersed in an electroforming solution, and current is appliæd
to effect the electrodeposition of metal onto the conductive pattern area
- 20 on the mandrel. -~en~a sufficiently thick deposit is obtained, the
remaining photoresist is removed, and the electroformed heating grid is
separated from the mandrel.
Detailed Descrlptîon of the Preferred Embodiments
.
In a preferred embodiment of the present invention, a glass
master plate is provided with a pattern representing the configuration of
the heating grid to be produced by electroforming. While the pattern may
be formed by a coatin~, a most preferred embodiment of the present
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invention utili~es a glass photomask to provide the pattern, preferably a
glass photomask having a pattern formed by stain produclng metal infused
into the glass. Preferred techniques for producing stained glass
photomasks are described in detail in U.S. Patents 4,144,066 and
4,155,735 to Ernsberger.
A continuous conductive film is deposited on the surface of a
substrate to be used as the electroforming mandrel. The conductive film
may be a metal or an electroconductive metal ox$de such as tin oxide or
lndium oxide. The conductive film may be deposited by an conventional
coating technique such as vacuum deposition, cathode sputtering, chemical
vapor deposition or pyrolytic coating techniques. In a most preferred
embodiment of the present invention, a conductive film comprising indium
oxide is deposited by magnetron sputtering. The conductive film is
preferably deposited on a glass substrate. In a most preferred
embodiment of the present invention, a conductive film is sputtered from
a cathode comprising 80 to 90 percent indium and 10 to 20 percent tin.
A continuous layer of photoresist is applied over the conduc-
tive film. Any conventional photoresist with sufficient resolution is
acceptable. In a preferred embodiment of the present invention, photo-
resist in sheet form is laminated to the conductive film. Thephotoresist is exposed to actinic radiation through the photomask to cure
the exposed portions of the photoresist. The photomask pattern masks
portions of the photoresist from exposure, and these portions remain
uncured. Following exposure of the photoresist, and a post-curing cycle
if necessary, the photoresist is developed. Preferably, the photoresist
is contacted with a chemical solution which dissolves and removes the
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unexposed, uncured portions of the photoresist, thereby providing a
pattern of the underlying conductive film which is a positive image of
the pattern in the photomask. The remaining exposed, cured portions of
the photoresist surrounding the conductive pattern form walls within
which the electroformed heating grid is subsequently depositedO In an
alternative embodiment of the present invention a positive working pho~o-
resist may be employed to form a conductive film pattern which is a
negative image of the photomask pattern.
The resulting article is employed as a mandrel for the electro-
forming of a metallic heating grid which is a replication of the pattern
on the conductive film. As here described, the substrate bearing a
conductive film having a pattern defined by the photomask pattern is
contacted with a conventional metal-containing electrodeposition solution.
An electrical circuit is established, using the conductlve Eilm as the
cathode and an electrode of the metal to be deposited as the anode. An
electrical potential is applied, and metal is deposited on the conductive
film in the pattern defined by the nonconductive photoresist. Electro-
deposition is continued until the desired thickness is obtained for the
electroformed heating grid. The substrate bearing the conductive film,
photoresist, and electroformed heating grid is removed from the electro-
deposition solution. Separation of the electroformed article from the
mandrel may be effected by various means, such as alternately heating
and chilling. In certain applications wherein the electroformed article
is very thin and/or comprises very fine lines, the remaining photoresist
is first removed, preferably by dissolution. Then the electroformed
article is lifted off the mandrel. In other applications, the electroformed
article may be separated from the mandrel without removing the remaining
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photoresist, permitting immedia~e reuse of the mandrel. In most preferred
embodiments of the present invention wherein the electroformed article is
a heating grid and comprises very fine lines, a preferred method for
separating the electroformed heating element from the mandrel is to remove
the photoresist, contact the electroformed article with a polymeric
material to which the article adheres, and remove the heating element
attached to the polymeric material. Preferably, the polymeric material
is an interlayer sheet to be laminated to a rigld sheet to form an aircraft
transparency. In a most preferred embodiment, the polymeric material is
a sheet of poLyvinyl b~ltyral, a surface of which i5 chemically treated to
soften the surface. The tacky surface is used to pick the heatlng grid
off the mandrel. The polyvinyl butyral sheet ls then laminated to a
second polymer sheet with the heating grid between them. Various solvents
may be used to soften the polyvinyl butyral; diethylene glycol monobutyl
ether is preferred.
The present invention will be further understood from the
descriptions of specific examples which follow.
EXAMPLE I
A glass photomask is prepared by coating a glass plate with a
photographic emulsion comprising silver halide which is exposed to
actinic radiation through a master pattern in the shape of the part to be
electroformed. Exposed areas of the photographic emulsion form a latent
image which is developed by immersion in developing solutions which con-
~ert the silver halide to colloidal silver. The coated glass plate is
subjected to an electric field which induces migration of the silver ion
into the glass. The silver ions are reduced to elemental silver which
agglomerates into colloidal, microcrystalline color centers which form a
stained pattern within the glass which corresponds with the master
pattern of the article to be electroformed. An electroforming mandrel is
prepared by coating a glass substrate surface with a continuous conduc-
tive film by magnetron sputtering of a cathode comprising 90 percent
in~um and 10 percent tin. The preferred indium oxide film has a surface
resistvity less than 20 ohms per square. A continuous layer Gf
phDtoresist is applied over the conductive film by laminflting a sheet of
photoresist to the indium oxide at a temperature of 235F. (about
113C.). A photoresist layer having a thickness of 0.001 inch (about
0.025 millimeter) is available from Thiokol/Dynachem Corp. of Tustln,
California. The photoresist is exposed to actlnic radiation (Colight
M-218) through the glass photomask for 20 seconds and cured. The photo-
resist is developed with a solvent which removes the unexposed portions
of the photoresist thereby providing a pattern of the underlying indium
oxide corresponding with the pattern in the photomask which in turn corre- -
sponds with the master pattern in the shape of the article to be electro-
formed. The resultant article is used as an electroforming mandrel in
the following process.
EXAMPLE II
A glass mandrel 3 by 7 inches ~about 7.6 by 17.8 centimeters)
is prepared as in Example I having a screen pattern comprising lines
0.0012 inch (about 0.03 millimeter) wide spaced 0.02~ inches (about 0.5
millimeters) apart. The mandrel is prep~red for electroforming by
sequent~al dipping into 2 dilute solution of hydrochloric and nitric
acids, and isopropanol, each followed by a water rinse to clean and we~ -
the electroforming surface. The glass mandrel is dipped into the
electroforming solution several times to completely wet the surface and
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remove air bubbles before the electroforming process co~ences. The
electroforming solution comprises nickel sulfamate~ and is maintained at
a temperature of 110~F. (about 43C.). A cathode contact is applied to
the indtum oxide film of the glass electroforming mandrel. An anode
contact is applied to a depolarized nickel plate. Both the mandrel and
the plate ~re immersed into the nickel sulfamate solution. At a current
denslty o~ 10 amps per square foot, electroforming proceed~s at a rate of
0.001 inch ~0.025 millimeter) per 100 minutes. When the electroformed
part reaches the desired thickness~ 0.0005 inches (about 0.013
~illime~er~3, the mandrel is removed from the solutlon. The remaining
photoresist i9 dissolved and removed with sodium hydroxide solution at
150F. (about 66C.). The electroformed heating grid is removed from the
mandrel by contacting the surface with a sheet of polyvinyl butyral, the
contacting surface of which has been treated with diethylene glycol
monobutyl ether to produce an adhesive surface. As the polyvinyl butyral
sheet is pulled away from the mandrel, the grid remains attached to the
tacky surface of the polyvinyl butyral. To form a heatable interlayer,
the polyvinyl butyral sheet bearing the heating grid is laminated to
a~other polymeric sheet with the heating grid between the sheets.
EXAU~LE III
An optical grid is produced by electroforming as in Exampla II,
~cep~ that the conductive pattern on the mandrel comprises finer lines
more ~osely spaced. An optical grid is produced comprising lines 0.001
i~b (about 0.025 millimeter) wide spaced 0.003 inch ~about 0.076
meter) apart.
The above examples are offered to illustrate the present inven-
io~. Va~ious modifications are included within the scope of the present
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invention. For e~ample, metallic sub~traees may be used for the
e}ectroforming mandrel, and other metals may be deposit~d by
electroforming, such as copper, iron, lead, tin and zinc. The
electroformed elements of the present invention need not be grid
patterns, but may be produced in any shape or configuration, limited only
by the artwork. The scope of the present in~ention is defined by the
following claims.
