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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to processes and compositions for electroless
metallization and related articles of manufacture and, more particularly,
the invention relates to the use of substrate chemical groups capable of
ligating with a variety of electroless metallization catalysts, including
tin-free catalysts, and selective electroless plating through the use of
such ligating groups.
2. Background Art
Electroless metallization procedures typically require multiple and complex
processing steps. See, for example, reviews of electroless plating in C.
R. Shipley, Jr., Plating and Surface Finishing, vol. 71, pp. 92-99 (1984);
and Metal Finishing Guidebook and Directory, vol. 86, published by Metals
and Plastics Publications, Inc. (1988), both incorporated herein by
reference. One typical procedure for metallization of polymeric substrates
employs a colloidal palladium-tin catalyst in the following sequence: (1)
pre-cleaning the substrate surface; (2) microetching, for example with a
chromic-based solution; (3) conditioning the etched substrate surface; (4)
adsorption of the palladium-tin catalyst onto the conditioned surface; (5)
treatment with an accelerator to modify and activate the absorbed
catalyst; and (6) treatment with an electroless plating solution. See, for
example, U.S. Pat. Nos. 4,061,588 and 3,011,920. A number of fundamental
studies have been performed on this and related electroless procedures.
See, for example, J. Horkans, J. Electrochem. Soc., 130, 311 (1983); T.
Osaka, et al., J. Electrochem. Soc., 127, 1021 (1980); R. Cohen, et al.,
J. Electrochem. Soc., 120, 502 (1973); and N. Feldstein, et al., J.
Electrochem. Soc., 119, 668 and 1486 (1972).
While the exact composition and structure of such a Pd/Sn catalyst have not
been confirmed, and the detailed mechanism by which a Pd/Sn colloid
adheres to a substrate is not fully understood, the following is known
and/or currently postulated. A palladium-tin electroless catalyst
typically is generated by mixing multi-molar stannous chloride and a
palladium chloride in an acidic aqueous solution containing excess
chloride ion. Sn(II) reduces the Pd(II) species, likely via an
inner-sphere redox reaction in a Pd/Sn complex, resulting in a colloidal
suspension with a dense metallic core within a less dense tin polymer
layer. The central portion of the colloid is composed of an intermetallic
compound of stoichiometry reported to be Pd.sub.3 Sn. This inner core is
believed to be a cluster containing up to 20 atoms with palladium
principally in the zero and +1 oxidation states. This inner core is the
actual catalyst in the initial metal reduction that leads to electroless
metal deposition.
Surrounding this core is a layer of hydrolyzed stannous and stannic species
that forms an outer shell of oxy- and/or hydroxy-bridged oligomers and
polymers together with associated chloride ions. This layer is known as
beta-stannic acid. The composition of the colloidal suspension contains a
high concentration (multi-molar excess) of stannous ions relative to Pd
which continue to hydrolyze and form higher oligomers on the outer surface
of the initially formed colloidal particles. Consequently the thickness
and degree of polymerization of the outer tin shell changes over time. The
resultant colloidal particle has a net negative charge.
Adhesive properties of the outerpolymeric outer shell attach the catalyst
to the substrate to be plated, known in the art as the activation process.
The negative charge of the outer tin shell prevents aggregation of the
colloids permitting individual attachment to the substrate. The reducing
power of the Sn(II) acts as an anti-oxidant and protective layer that
maintains the catalytic core in the low valent Pd state necessary to
initiate plating. Activation is followed by an acceleration step whereby
the catalyst core is exposed. Acceleration can be achieved by a variety of
means, for instance by "subtractive" type means of dissolving the stannous
protective layer at high chloride ion concentrations to form soluble
SnCl.sub.4.sup.2-, or by oxidizing the shell to the more soluble Sn(IV) by
exposure to oxygen from the ambient. "Additive" type acceleration
sequences are also known. For example, European Patent Application
90105228.2 discloses the application of an acidic solution of PdCl.sub.2
to the intact adsorbed colloid. The stannous polymer layer of the particle
reduces the palladium ion in situ to form a metallic Pd deposit on which
plating can occur. After activation, the substrate is immersed in an
electroless plating solution. A typical electroless metal plating solution
comprises a soluble ion of the metal to be deposited, a reducing agent and
such other ligands, salts and additives that are required to obtain a
stable bath having the desired plating rate, deposit morphology and other
characteristics. Common reductants include hypophosphite ion (H.sub.2
PO.sub.2.sup.-), formaldehyde, hydrazine or dimethylamine-borane. The
reductant reacts irreversibly at the catalyst core to produce an active
hydrogen species, presumably a palladium hydride. The surface hydrogen is
also a potent reductant which transfers electrons to the soluble metal
complex in the bath and produces a metal deposit on top of the catalyst,
which eventually covers the core sufficiently to block access to the
external solution. For certain deposits, such as copper, nickel and
cobalt, the nascent layer can itself become "charged" with hydrogen and
continue to reduce metal ion to metal, leading to "autocatalytic" build-up
of an electroless deposit onto the activated surface. In a competitive
reaction, surface hydrogen atoms combine to evolve H.sub.2 gas. This
latter reaction has never been completely suppressed. Therefore, not all
available reducing equivalents in the electroless bath can be used for
metal deposition. For a properly catalyzed surface, the choice of
electroless metal plating solution is determined by the desired properties
of the deposit, such as conductivity, magnetic properties, ductility,
grain size and structure, and corrosion resistance.
Such a palladium-tin catalyst system presents a number of limitations. At a
minimum three steps are required--activation, acceleration and plating.
Often substrate pre-treatment and other additional steps are necessary to
provide uniform plating. The colloidal catalyst also is readily oxidized
and stannous ions must be replenished by regular addition of Sn(II) salts.
Further, the colloid size may fix packing density thereby making difficult
uniform plating of ultra-small objects, e.g. objects less than about 1,000
angstroms in size. Subtractive-type acceleration requires a precise and
often difficult balance of exposing the palladium core without dissolving
the portion of the stannous shell that provides adherence to the substrate
surface. Further, substrate adhesion of a Pd/Sn catalyst has been found to
be a relatively non-specific phenomenon. For example, the catalyst will
only weakly adhere to a smooth photoresist coating, requiring a pre-etch
step to provide a more textured surface and thereby increasing processing
time and costs. For many situations, such as high resolution lithography,
such pre-etching is not feasible. Further, a number of materials are
"colloidophobic", i.e. materials to which a Pd/Sn catalyst does not
adsorb. These materials include silica, certain metals and some plastics.
Recently, several electroless plating procedures have been reported, the
procedures generally employing a palladium catalyst and a polyacrylic acid
or polyacrylamide substrate coating. See, U.S. Pat. Nos. 4,981,715 and
4,701,351; and Jackson, J. Electrochem. Soc., 135, 3172-3173 (1988), all
incorporated herein by reference.
A common method for producing a patterned metallized image includes use of
a photoresist coating. In an additive metallization approach, photoresist
is applied to a substrate surface; the resist is exposed to provide
selectively soluble portions of the photoresist coating; a developer is
applied to bare selected portions of the substrate surface; those selected
portions are metallized; and the remaining resist stripped from the
substrate surface. See, generally, Coombs, Printed Circuits Handbook, ch.
11 (McGraw Hill 1988), incorporated herein by reference. A print and etch
procedure is a subtractive approach where in the case of circuit line
fabrication, a copper layer is selectively chemically etched through use
of a photoresist to define the circuit traces. For higher performance
applications, it is crucial that circuit sidewalls be uniform and
essentially vertical. Resolution limits exist with a print and etch
sequence, however, which are inherent in the subtractive nature of this
approach.
SUMMARY OF THE INVENTION
The present invention comprises an electroless metal plating-catalyst
system that overcomes many of the limitations of prior systems. In one
aspect of the invention, a process is provided comprising steps of
providing a substrate comprising one or more chemical groups capable of
ligating to an electroless plating catalyst, at least a portion of the
chemical groups being chemically bonded to the substrate; contacting the
substrate with the electroless metal plating catalyst; and contacting the
substrate with an electroless metal plating solution to form a metal
deposit on the substrate. The chemical groups can be, for example,
covalently bonded to the substrate.
In another preferred aspect, the invention provides a process for selective
electroless metallization, comprising steps of selectively modifying the
reactivity of a substrate to an electroless metallization catalyst;
contacting the substrate with the electroless metallization catalyst; and
contacting the substrate with an electroless metallization solution to
form a selective electroless deposit on the substrate. The substrate
reactivity can be modified by selective treatment of catalyst ligating
groups or precursors thereof on the substrate, for example by
isomerization, photocleavage or other transformation of the ligating or
precursor groups. Such-direct modification enables selective plating in a
much more direct and convenient manner than prior selective plating
techniques. Specifically, the present invention provides selective
electroless plating without the use of a photoresist or an adsorption type
tin-containing plating catalyst.
The one or more chemical groups capable of binding to the electroless
catalyst may be provided by a variety of means. The material of
construction of the substrate may comprise the catalyst ligating groups,
for example a polyvinylpyridine substrate or an alumina substrate.
Substrates that do not inherently comprise such ligating groups may be
treated to provide the groups. For example, a source of the ligating
groups may be formulated as a material of construction of the substrate.
Alternatively, a substrate may comprise suitable precursor groups which
upon appropriate treatment provide the necessary catalyst ligating groups.
Such treatment will vary with the particular ligating group and includes,
for example, thermolysis, treatment with chemical reagents, photochemical
modification such as isomerization or photocleavage of a precursor group,
and plasma etching. Further, such treatment methods can provide precursor
groups, such as hydroxyl, carboxyl, amino and others to which a ligating
group can be bonded. Still further, the ligating chemical groups or
precursors thereof may be provided by contacting at least portions of the
substrate surface with a compound or composition comprising the ligating
groups, the ligating groups preferably adhering well to the substrate
surface, for instance by chemical and/or physical interaction. If chemical
bond formation is employed as the substrate adherence means, substrate
adhesion and catalyst ligation functions may be performed by application
of a single molecule or, alternatively, by application of multiple
molecules with subsequent linkage therebetween.
A variety of metallization catalysts may be employed, including tin-free
catalysts, with palladium (II) compounds and compositions preferred for
the generally superior catalytic activity those catalysts provide. A
substrate is preferably treated with a solution of the metallization
catalyst, for example an aqueous solution or a solution of an organic
solvent. The catalyst solution preferably comprises other materials such
as ancillary ligands, salts and buffers to enhance the stability of the
catalyst solution and thereby to provide suitable catalyst activity as
well as convenient use and storage of the solution.
The substrate to be electrolessly plated according to the present invention
may be a variety of materials such as a conductive material, a
semiconductor material, an electrically nonconductive material, and more
specifically, electronic packaging substrates such as a printed circuit
board or a precursor thereof. In a preferred aspect, the invention is
employed to metallize lipid tubule microstructures. It is believed that a
wide variety of metals may be electrolessly plated in accordance with the
present invention, for example cobalt, nickel, copper, gold, palladium and
various alloys.
As is apparent to those skilled in the art, notable advantages of the
invention include an electroless catalyst system that requires fewer and
simpler processing steps in comparison to current Pd/Sn colloid catalyst
adsorption based systems; use of more stable and convenient catalysts,
including tin-free catalysts; and improved catalyst adhesion to a
substrate allowing plating of more dense initiation and of greater
uniformity and selectivity. The invention also provides selective
patterning of substrate ligating groups, thereby enabling a selective
metal deposit without the use of a conventional photoresist patterning
sequence.
The terms-"ligate" or "ligating" or "ligation", as used herein in reference
to the interaction between an electroless metallization catalyst and the
substrate chemical groups of the invention, refers to any attraction,
binding, complexing, chelating or sequestering, whatever the nature or
extent of such attraction, binding, complexing, chelating or sequestering,
between the catalyst and the substrate chemical group.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Many substrates or substrate surfaces capable of being plated according to
the present invention intrinsically contain chemical groups, or
appropriate precursors of chemical groups, that are able to ligate an
electroless plating catalyst. For example, a polyvinylpyridine film
intrinsically contains such chemical groups with the pendant pyridine
serving as the catalyst ligating group. As discussed herein, the pyridyl
group has been found to be a particularly preferred ligating group for a
palladium catalyst. For a discussion of the pyridyl moiety as a ligating
group, see Calvert, et al., Inorganic Chemistry, 21, 3978 (1982),
incorporated herein by reference. Similarly, a substrate comprising
aluminum oxide will bind a palladium catalyst by the AlO and AlOH groups
of the alumina. Further, the ligating material need not be the sole
component of the substrate. Thus, the ligating material may be physically
blended as one of multiple components comprising the substrate if
sufficient ligating moieties are accessible at the substrate surface to
ligate to the catalyst.
A possible shortcoming of such a blending approach is that incorporation of
large quantities of a second material may impair the film-forming or other
properties of the bulk material. A potential solution to this problem is
to incorporate a surfactant form of the ligating component into the bulk
material by proper choice of the relative solubility/polarity
characteristics of the ligating component and the surfactant. By
incorporating a small percentage of the surfactant into the bulk, a high
surface concentration of the ligating component could be produced. An
analogous approach has been employed in the photoresist arts, where a
small quantity of surfactant is formulated into the resist to enhance film
planarity by reducing the surface tension through a high surface
concentration of surfactant.
Many substrates that do not inherently comprise suitable ligating groups
may be readily modified to possess the necessary ligating groups.
Substrate modification methods include, but are not limited to,
thermolysis, reaction of the surface with one or more chemical reagents,
irradiation with photons or ions, vapor phase modification, graft
polymerization, x-ray and nuclear radiation treatment or, more generally,
any treatment that effects the desired conversion of the substrate. One
potential modification sequence provides hydrolysis of a polyimide surface
and reacting the hydrolyzed surface with a silane reagent possessing a
suitable ligating group, such as .beta.-trimethoxysilylethyl-2-pyridine.
Another method provides chemically etching a polyethylene surface with a
Cr.sub.2 O.sub.7.sup.2- solution to provide hydroxyl groups on the
substrate surface. The hydroxyl groups should then condense with a
suitable compound containing a ligating group, for example nicotinoyl
chloride with its pyridyl ligating group. Some of the surface substrate
modification methods permit convenient selective treatment of a substrate
surface. For example, if a surface can undergo a photochemical conversion
to reveal a ligating group, that surface can be exposed to masked
radiation and directly produce a pattern of catalyst binding sites. After
treatment with a suitable catalyst, the patterned catalyst surface can
then be electrolessly metallized to produce a negative tone image of the
mask employed.
Rather than directly modifying the substrate, the substrate may be imparted
with suitable ligating groups by indirect modification of the surface. For
example, a substrate may be coated with one or more film layers, each
layer comprising one or more suitable ligating agents. The film layer
preferably adheres well to the substrate, for example by containing a
functional group that will chemically and/or physically adhere to the
substrate.
The adhesive and ligation functions of such a film may be performed by
application of a single molecule or, alternatively, by application of
multiple molecules with subsequent linkage between each of the molecules.
For example, .beta.-trimethoxysilylethyl-2-pyridine provides both ligating
and substrate-adherent functionalities. The alkoxysilane group can
chemically bind the compound to a substrate. For instance, the
trimethoxysilyl group reacts with surface hydroxyl (silanol) functions of
a quartz substrate, displacing methanol to directly bond to the substrate.
The thus bound pyridyl moiety of the silylpyridyl molecule serves as a
ligand for chelating with the plating catalyst.
As noted, the adhesive and ligating functions may be performed by multiple
chemical groups with bond formation or other linkage between each of the
groups. The linkage connecting the multiple functional groups may be of
variable length and chemical composition. Examples include
3-(trimethoxysilyl) propylamine and a quinoline-8-sulfonic acid chloride.
The aminosilane is applied as the substrate adsorbent. The coated surface
is then reacted with a quinoline-8-sulfonic acid chloride, the SO.sub.2 Cl
group coupling to the amine group of the coated surface to form a
sulfonamide linkage, and the quinolinic group serving as a catalyst
ligation moiety. Similarly, 3-(trimethoxysilyl) propylamine can be applied
to a substrate and then reacted with the acid chloride group of
4,4'-dicarbonyl chloride-2,2'-bipyridine to form an amide linkage. The
pyridyl moieties of this complex serve as catalyst ligating groups. Other
silyl amines can be condensed in a similar manner, for example
3-(triethoxysilyl) propylamine. Another sequence provides condensing the
hydroxyl groups of a chemically etched polyethylene substrate with a
suitable ligating precursor, for example, 3-(trimethoxysilyl)propylamine,
which after formation of the oxygen-silyl bond by methanol displacement,
the amino group can condense with a suitable ligating compound such as
nicotinoyl chloride.
A ligating chemical group comprising a radiation sensitive chromophore can
provide selective photochemical patterning and metallization where
selective photolysis or radiation ablation modifies the chemical groups on
the substrate surface to substantially reduce or eliminate ligating
ability in the selected film surface areas. Subsequent exposure to the
plating catalyst and metallization solutions provides a positive tone
image of the photomask employed. For example, the pyridyl group of
.beta.-trimethoxysilylethyl-2-pyridine serves as a chromophore for
convenient patterning and subsequent selective metallization of the
substrate surface through microlithographic techniques.
Analogously, a ligating film can be employed where selective photolysis
transforms a non-ligating group within the film into a ligating group. For
example, azoxybenzene derivatives photoisomerize from a weakly or
non-ligating azoxybenzene group to the ligating 2-hydroxyazobenzene group.
The chelating ability of 2-hydroxyazobenzene and
2-(2-pyridylazo)-1-naphthol has been described in Calabrese, et al.,
Inorq. Chem., 22, 3076 (1983), incorporated herein by reference. The
Photo-Fries reaction is another potential means to provide suitable
ligating groups. By this reaction, for example, polyacetoxystyrene can be
irradiated with ultraviolet radiation to provide the ligating
2-hydroxyacetophenone moiety.
Depending on the nature of the radiation-sensitive materials employed, such
transformations may be accomplished with a variety of exposure sources and
imaging tools. For example, ultraviolet or visible light will be suitable
for certain transformations, while other transformation may require
exposure sources such as electron beam or x-ray treatment. Such energy
sources can be provided by image tools known to those in the art, for
example ultraviolet contact printers and projection steppers, electron
beam writers and x-ray proximity printers.
For such patterning of a ligating film, the film preferably is an ultrathin
film, which is a film defined herein to mean a film of a composite
thickness of between about ten molecular layers and a single molecular
(monomolecular) layer. Such a film can be formed through dip coating or
vapor phase deposition procedures as are known in the art. A ligating film
composed of multiple layers of ligating groups may not provide highly
selective plating. Radiation exposure may fail to penetrate sufficiently
the entire thickness of a multiple layer film leaving intact ligating
groups in undesired substrate surface areas, thereby resulting in
non-selective plating. As irradiation can readily penetrate through the
thickness of an ultrathin film, such a film can be patterned with greater
precision resulting in greater image resolution.
As described above, the invention provides a substrate surface containing a
chemical functional group capable of binding metallization catalysts from
solution. One way of binding a catalyst to a surface is by a metal-ligand
complexation, or ligation reaction. Though not wishing to be bound by
theory, the ability of a substrate ligand L to bind an electroless
catalyst, for example a palladium (II) catalyst, should be readily
determined by examining the formation equilibrium constant K.sub.f for the
generalized complexation reaction (I):
Pd.sup.2+ +L<-->PdL.sup.2+ (I)
wherein, K.sub.f is equal to the ratio of concentration of products to
reactants in reaction (I), i.e.,
##EQU1##
Large values of K.sub.f would indicate strong, or essentially
irreversible, binding of the catalyst to the ligand. See, generally, A.
Martell, et al., "Critical Stability Constants", Plenum Press, New York
(1975), where examples of complexation reactions have been tabulated.
While little data has been reported for palladium (II) ligation reactions,
the general trends for complexation reactions can be obtained by examining
the formation constant values for Ni(II). Palladium is directly below
nickel in the periodic table and has similar coordination properties. The
results with nickel ion shows that through a chelate effect a multidentate
ligand (chelate) provides a greater K.sub.f than a corresponding
monodentate group, where the term monodentate group refers to a chemical
group that can provide only one ligand binding site, and the term
multidentate group refers to chemical group or groups that can provide
greater than one ligand binding site. For example, chelation of Ni(II) by
2,2'-bipyridine results in a complex that is 10,000 times more stable than
a pyridine complex, and 30 times more stable than a bis-pyridine complex.
Further, it is believed that a higher K.sub.f provides a metal
deposit-with relatively greater adhesion to a substrate upon subsequent
metallization.
Thus, a bipyridyl is preferred over a monopyridyl for the relatively
stronger bond the bipyridyl forms with an electroless metallization
catalyst and the higher quality metal deposit thereby provided. Use of
suitable multidentate ligating groups has enabled deposition of thick
adherent metal plates, including metal plates of thickness equal to and
greater than about 2500 angstroms on smooth, unetched surfaces. In
addition to bipyridyl, numerous other multidentate groups should also
serve as suitable ligating groups, for example 2,2':6,2''-terpyridine,
oxalate, ethylene diamine, 8-hydroxyquinoline and 1,10-phenanthroline.
Organophosphines, nitriles, carboxylates and thiols should also ligate
well, i.e. exhibit a significant K.sub.f, with a palladium electroless
metallization catalyst. For example, 3-mercaptopropyltriethoxysilane,
2-(diphenylphosphino)ethyltriethoxysilane, and cyanomethylphenyl
trimethoxysilane should serve as suitable catalyst ligating groups in
accordance with the invention. Also preferred are ligating groups with
antibonding (pi*) orbitals in the ligand, for example aromatic
heterocycles such as pyridine and other nitrogen containing aromatics.
Such groups give rise to dpi.fwdarw.pi* backbonding interactions that
favor complex formation. It has thus been found that a benzyl chloride
group provides poor ligating ability whereas an alkylpyridyl provides good
ligation to an electroless catalyst.
A variety of compounds may be employed as the electroless catalyst in
accordance with the invention, such as palladium, platinum, rhodium,
iridium, nickel, copper, silver and gold. Palladium or
palladium-containing compounds and compositions generally provide superior
catalytic activity and therefore are preferred. Particularly preferred
palladium species include bis-(benzonitrile)palladium dichloride,
palladium dichloride and Na.sub.2 PdCl.sub.4. Other salts of
PdCl.sub.4.sup.2- should also be suitable, such as potassium and
tetraethylammonium salts.
The electroless metallization catalysts useful in the processes of the
invention are preferably applied to the substrate as a solution, for
example as an aqueous solution or a solution of an organic solvent.
Suitable organics include dimethylformamide, toluene, tetrahydrofuran, and
other solvents in which the metallization catalyst is soluble at effective
concentrations.
Means for contacting a substrate with a catalyst solution may vary widely
and include immersion of the substrate in a solution as well as a spray
application. The catalyst solution contact time required to provide
complete metallization of the contact area can vary with catalyst solution
composition and age.
A variety of catalyst solutions have been successfully employed, with
solutions stabilized against decomposition preferred. Thus, the catalyst
solution may comprise ancillary ligands, salts, buffers and other
materials to enhance catalyst stability. Though again not wishing to be
bound by theory, it is believed many of the catalyst solutions useful in
the present invention decompose over time by oligomerization and formation
of insoluble oxo-compounds, for example as reported by L. Rasmussen and C.
Jorgenson, Acta. Chem. Scand., 22, 2313 (1986). It is believed the
presence of catalyst oligomers in the catalyst solution can affect the
ability of the catalyst to induce metallization and/or inhibit selectivity
of metallization of a patterned substrate. For example, as such catalyst
oligomers increase in molecular weight, their solubilities decrease and
precipitation of the catalyst can occur.
Suitable agents for stabilizing a catalyst solution can vary with the
particular catalyst employed, as is apparent to those skilled in the art.
For instance, a metallization catalyst of PdCl.sub.4.sup.2- can be
stabilized in aqueous solution by addition of excess chloride ion and
decreasing pH to inhibit formation of oxo-bridged oligomers of the
catalyst, of proposed structures such as Cl.sub.3 PdOPdCl.sub.2 (H.sub.2
O).sup.3-, and Cl.sub.3 PdOPdCl.sub.3.sup.4-. This is supported by the
greater stability of catalyst solutions comprising sufficient
concentrations of sodium chloride or tetraethylammonium chloride (TEACl)
relative to the stability of PdCl.sub.4.sup.2- solutions not containing
such agents. Such catalyst stabilization can be accomplished by adjustment
of chloride ion concentration during preparation of the catalyst solution,
or by adjustment of chloride ion concentration after the catalyst solution
has attained full catalytic activity. In addition to chloride, other
anions that prevent the formation of catalyst oligomers should also be
suitable agents for stabilizing a catalyst solution, for example bromide
and iodide ions.
Cation effects have also been observed. For example, suitable use of sodium
chloride with Na.sub.2 PdCl.sub.4 provides an active and stablilized
catalyst solution. Replacing sodium chloride with ammonium chloride in
such a solution, however, results in a solution with little or no activity
as a metallization catalyst. In this case, it is believed that the lack of
catalytic activity may be the result of the formation of stable cis- or
trans-(NH.sub.3).sub.2 PdCl.sub.2 species in solution. Replacing sodium
chloride with TEACl provides a solution that requires a shorter induction
period to reach full activity, and once active remains selective and
stable only for a few days. It is further noted that while a number of
cations may be suitable, cation selection may be dictated by the specific
metallization process. For example, for advanced microelectronic
applications, use of sodium ions generally is avoided if possible and,
therefore, use of TEACl as a catalyst solution stabilizer may be
preferred.
It also has been found that catalyst solutions of higher (less acidic) pH,
e.g. pH of greater than 4, can be stabilized with a suitable buffer
solution. Preferably, pH of a catalyst solution is controlled by a buffer
component which does not appreciably coordinate with the metallization
catalyst. For a Pd(II) metallization catalyst, a preferred buffering agent
is 2-(N-morpholino)ethane sulfonic acid, referred to herein as MES,
available from the Aldrich Chemical Company. This buffer has a pK.sub.a of
6.15 and has been described in Good, et al., Biochemistry, 5(2), pp.
467-477 (1966).
Additionally, it has been found that solution preparation methods can
affect the stability and metallization activity of a catalyst solution
useful in the invention. For example, the catalyst solutions disclosed in
Examples 16 and 17 herein are prepared using approximately equivalent
initial quantities of acetate buffer, sodium chloride and Na.sub.2
PdCl.sub.4 .times.3H.sub.2 O. In Example 16 herein, an aqueous catalyst
solution comprising NaCl and Na.sub.2 PdCl.sub.4 .times.3H.sub.2 O reaches
full activity as a metallization catalyst about 24 hours after preparation
at room temperature. Addition of a prescribed amount of acetate buffer to
this active solution maintains its catalytic activity. In contrast,
preparation of a catalyst solution as described in Example 17 herein by
simultaneous mixing of acetate buffer, NaCl and Na.sub.2 PdCl.sub | | |
