Organic electronic package and method of applying palladium-tin seed layer thereto

A method of fabricating an electronic package having an organic substrate. The substrate is formed of fiberglass and epoxy. In order to additively circuitize the electronic package substrate, an organic polyelectrolyte is deposited onto the organic substrate. A colloidal palladium-tin seed layer is deposited atop the organic polyelectrolyte. This is followed by depositing a photoimagable polymer atop the seed layer, and photolithographically patterning the photoimagable polymer to uncover portions of the seed layer. The uncovered portions of the seed layer are catalytic to the electroless deposition of copper. In this way a conductive layer of copper is deposited atop the uncovered seed layer. The organic polyelectrolyte is deposited from an aqueous solution at the pH appropriate for the desired seed catalyst coating, depending on the ionizable character of the particular polyelectrolyte employed.

FIELD OF THE INVENTION 
The invention relates to organic electronic packages, referred to herein as 
electronic packages, such as chip carriers, printed circuit boards, 
printed circuit cards, accessory cards, and the like, and more 
particularly to such electronic packages where a conductive layer, as a 
copper conductive layer, is deposited atop a seed layer, as a 
palladium-tin seed layer. This seed layer should be thick enough to be 
surface catalytic for seeding or catalyzing the deposition and growth of 
the copper, i.e., a seed coverage (expressed in micrograms per square 
centimeter or ug/cm.sup.2, but thin enough to both (1) be removable at the 
end of the fabrication process, and (2) be resistant to separation during 
electronic package fabrication. According to the invention, appropriate 
seed coverage is provided by depositing the seed layer onto a layer, film, 
or coating of an organic polymer having ionizable groups, that is, a 
polyelectrolyte, at a suitable pH. The polyelectrolyte may be a 
polyampholyte, that is, a polymer containing both positive and negative 
charges, but must contain at least one ionizable group of opposite charge 
to that of the seed particles in aqueous solution. Although strong 
ionizable groups, with a charge virtually independent of pH, may be used, 
the presence of weakly ionizable groups of either positive or negative 
charge permits greater control of seed deposition as a function of the pH 
of the polyelectrolyte solution. 
BACKGROUND OF THE INVENTION 
The seed layer is a very thin layer of metal, e.g., Pd, on a polymeric 
substrate, capable of acting as a surface catalyst for the plating of Cu 
thereon. Thus, a very critical aspect of additive plating is the seed 
layer. If the seed layer is too thin, it will not be catalytic for the 
electroless deposition of copper, while if it is too thick, it will be 
resistant to removal at the end of the fabrication process and may lead to 
resist adhesive failure during the fabrication process. The adsorption of 
polyelectrolytes, especially cationic polyelectrolytes, onto electronic 
package substrate surfaces, enhances the adsorption of colloidal Pd/Sn 
particles thereto, which, in turn catalyzes the electroless deposition of 
copper circuitization onto electronic package surfaces. In a typical 
process sequence an organic polymer having pendant ionizable groups, i.e., 
a polyelectrolyte, is adsorbed from solution onto an electronic package 
substrate, followed by adsorption of colloidal Pd/Sn as a seed layer. A 
photoresist is applied atop the seed layer and photolithographically 
imaged to uncover the regions of the seed layer to be circuitized. Copper 
is electrolessly applied to the photolithographically uncovered seed 
layer. The uncovered Pd reduces Cu in the electroless plating bath to form 
the circuitization. The remaining photoresist is removed, i.e., stripped, 
and the remaining uncovered seed and polyelectrolyte are removed, leaving 
only residual seed and polyelectrolyte underlying the Cu circuitization. 
A key requirement of the process is to achieve the appropriate Pd/Sn seed 
catalyst loading. Insufficient Pd catalyst will result in voids in the 
copper deposit, creating open circuits. Too much catalyst can cause both 
adhesive failure and lateral conduction. Adhesive failure permits the 
electroless plating solution to leak beneath the photoresist and deposit 
copper between circuit elements, causing short circuits. 
Deposition of the Pd/Sn seed or catalyst layer depends critically on the 
adsorbed polyelectrolyte. The Pd/Sn colloidal particles do not appreciably 
adsorb on the electronic package substrate surface, that is, on the 
untreated electronic package surface. Thus, a need exists to controllably 
enhance the adsorption of colloidal Pd/Sn particles onto the electronic 
package substrate. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a polyelectrolyte treatment for 
electronic package substrates that is catalytic for forming a film of 
Pd/Sn seed that is catalytic for copper deposition. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by the method and structure of our 
invention. One aspect of the invention is a method of fabricating an 
electronic package having an organic substrate. Typically the substrate is 
formed of fiberglass and epoxy. In order to additively circuitize the 
electronic package substrate, an organic polymer having ionizable groups, 
i.e., an organic polyelectrolyte, is deposited onto the organic substrate. 
A colloidal palladium-tin seed layer is deposited atop the organic 
polyelectrolyte. This is followed by depositing a photoimagable polymer 
atop the seed layer, and photolithographically patterning the 
photoimagable polymer to uncover portions of the seed layer to be 
circuitized. The uncovered portions of the seed layer are catalytic to the 
electroless deposition of copper. In this way a conductive layer of copper 
is deposited atop the uncovered seed layer. After circuitization the 
remaining photoresist is removed, i.e., stripped, and the uncovered seed 
layer and polyelectrolyte removed, leaving residual Pd/Sn seed and 
polyelectrolyte under the Cu circuitization. According to our invention 
the organic polyelectrolyte is deposited from an aqueous solution having 
the pH adjusted according to the chemical structure of the polyelectrolyte 
(when weakly acidic or basic pendant groups are present) such as to 
achieve optimum catalyst loading. 
A further embodiment of the invention is an organic electronic package 
characterized by at least one circuitization carrying layer formed of a 
photoimagable dielectric. In this embodiment the electronic package has an 
circuitization encapsulated in a photoimagable dielectric. The 
photoimagable dielectric is coated with a thin film of an organic polymer 
having ionizable groups, i.e., an organic polyelectrolyte thereon. This 
organic polyelectrolyte underlies the palladium-tin seed layer. The seed 
layer is characterized by a seed coverage of (i) less than 8 micrograms 
per square centimeter of palladium and (ii) less than 2 micrograms per 
square centimeter of tin, but thick enough to catalyze copper 
circuitization deposition. The seed layer catalyzes deposition of the 
copper wiring. The resulting copper circuitization is atop the 
photoimagable dielectric.

DETAILED DESCRIPTION OF THE INVENTION 
One aspect of our invention is a method of fabricating an electronic 
package having an organic substrate. Typically the substrate is formed of 
fiberglass and epoxy. The epoxy is typically bisphenol A based, containing 
epoxidized cresol novolac and initiated with an imidazole catalyst. 
In order to additively circuitize the electronic package substrate, an 
organic polymer having ionizable groups, i.e., an organic polyelectrolyte, 
is deposited onto the organic substrate. In one exemplification of our 
invention this is a copolymer of (1) acrylamide and (2) 
beta-methacryloxyethyltrimethyl ammonium methyl sulfate, referred to as 
AM/MTMMS, and having the structure 
##STR1## 
where the ratio n/m is about 0.1, the molecular weight by gel permeation 
is about 2 Mdaltons. 
Associated with this chemical structure may be a degree of hydrolyzed amide 
groups (typically 10 percent of the amide groups are hydrolyzed) present 
as carboxylic acid or the salt of the carboxylate anion. This 
polyelectrolyte is deposited onto the organic substrate in an aqueous 
solution containing sulfuric acid at a pH below 4. The acidic solution 
assures that the hydrolyzed pendant groups do not ionize and that only 
cations are present on the polyelectrolyte backbone. The polyelectrolyte 
is typically present at a concentration of between 0.2 and 1.2 grams per 
liter. 
A further embodiment of this invention is deposition of the above 
polyelectrolyte onto the organic substrate in an aqueous solution 
containing sodium hydroxide at a pH above 10. The alkaline solution 
ensures complete ionization of the weakly acidic hydrolyzed groups and 
produces an ampholyte containing both positive and negative groups. 
Polyelectrolyte concentration is typically again between 0.2 and 1.2 grams 
per liter. 
Another polyelectrolyte is a cationic polyamide-amine, which contains only 
cations on the polyelectrolyte backbone which do not vary with solution pH 
and has no pendant weakly acidic or weakly basic groups. A neutral aqueous 
solution is typically employed with a polyelectrolyte concentration again 
of between 0.2 and 1.2 grams per liter. 
A colloidal palladium-tin seed layer is deposited atop the organic 
polyelectrolyte. This is followed by depositing a photoimagable polymer 
atop the seed layer, and photolithographically patterning the 
photoimagable polymer to uncover portions of the seed layer. The uncovered 
portions of the seed layer are catalytic to the electroless deposition of 
copper. In this way a conductive layer of copper is deposited atop the 
uncovered seed layer. 
The seed layer is a Pd/Sn colloidal suspension prepared by mixing 100 grams 
of SnCl.sub.2, 2 grams of PdCl.sub.2, 175 grams of NaCl, 0.1 grams of 3M 
Corporation FC-95 fluorocarbon surfactant, and 200 milliliters of 37% HCl 
in 1 liter of water. 
After depositing the seed layer, a photoimagable polymer is deposited, 
i.e., a photo resist. The resist is photolithographically processed to 
uncover seed layer in the intended pattern of the circuitization, and Cu 
is electrolessly deposited on the uncovered seed layer. The photoresist is 
then stripped. The uncovered seed and polyelectrolyte are removed, leaving 
residues thereof overlaid by the Cu circuitization. 
According to a particularly preferred alternative embodiment of the 
invention, a photoimagable dielectric is deposited atop a patterned 
conductive layer, thereby encapsulating the circuitization, and 
photoimaged. Photoimaging can include photo formation of vias. This 
photoimagable dielectric is, typically, an epoxy resin system of a polyol 
resin condensation product of (i) an epihalohydrin and (ii) an epoxidized 
octafunctional bisphenol A formaldehyde novolac resin. Where flame 
retardant properties are desired, it can include an epoxidized glycidyl 
ether of tetrabromo bisphenol A. This photoimagable dielectric is 
typically deposited to a thickness of up to about 20 mils, generally from 
about 1 mil to 20 mils, and preferably from about 2 mils to about 20 mils. 
A polyelectrolyte is deposited as described hereinabove. In this preferred 
embodiment of the invention the colloidal palladium-tin seed layer is 
deposited to a seed coverage of less than 8 micrograms per square 
centimeter of Pd and less than 2 micrograms per square centimeter of Sn 
atop the organic polyelectrolyte. 
A further embodiment of the invention is an organic electronic package 
characterized by at least circuitization carrying layer formed of a 
photoimagable dielectric. In this embodiment the electronic package has a 
circuitized organic substrate. The circuitization is encapsulated in a 
photoimagable dielectric. The photoimagable dielectric is roughened and 
coated with a layer of an organic polymer having ionizable groups, i.e., 
an organic polyelectrolyte. This organic polyelectrolyte underlies the 
palladium-tin seed layer. The seed layer is characterized by a seed 
coverage of (i) less then 8 micrograms per square centimeter of palladium 
and (ii) less then 2 micrograms per square centimeter of tin. The seed 
layer is catalytic for deposition of the copper wiring, that is, the seed 
layer underlies a patterned or circuitized conductive layer after the 
processing shown in the FIGURE and described hereinabove. The 
photoimagable dielectric layer is adapted to also carry circuitization. 
The photoimagable dielectric is preferably an epoxy resin system of a 
polyol resin condensation product of (i) an epihalohydrin and (ii) an 
epoxidized octafunctional bisphenol A formaldehyde novolac resin. The 
system can further include an epoxidized glycidyl ether of tetrabromo 
bisphenol A. This layer typically has a thickness of from about 2 mils to 
about 20 mils. 
EXAMPLES 
The invention can be understood by reference to the following examples. 
Example 1 
Two epoxy-glass panels that had been previously roughened were dipped for 2 
minutes into aqueous solutions containing 0.05 % (grams/gram) of AM/MTMMS. 
One solution was prepared with 2% (volume/volume) sulfuric acid and the 
other with 0.3N sodium hydroxide. 
The panels were rinsed, seeded in a colloidal Pd/Sn bath, rinsed, 
accelerated in a sodium hydroxide bath, rinse, and hot air dried. Coupons 
were taken from each panel for seed analysis. The panels were laminated 
with DuPont T-168 photoresist, exposed, and developed. The seed bearing 
laminate was exposed in areas where electroless copper plating was to 
occur. Both panels were plated to about 1.7 mils with copper. The resist 
was stripped in a normal manner and both parts were tested for leakage. 
The panel processed in the acidic solution of AM/MTMMS had more than 50 
leakage current nets which exhibited leakage whereas the panel processed 
in the basic solution had only three leakage current nets. Seed analysis 
confirmed that the acid processed panel had a Pd/Sn ratio of 5.2/0.9 
ug/cm.sup.2 whereas the basic processed panel had a Pd/Sn ratio of 4.0/0.7 
ug/cm2. 
Seed retention is even more affected by this treatment if the epoxy-glass 
panel material is hole cleaned in standard permanganate chemistry prior to 
seed. Hole cleaned samples seeded with acidic AM/MTMMS had a Pd/Sn ratio 
of 6.7/2.0 (micrograms per square centimeter/micrograms per square 
centimeter) whereas those seeded with pH 12 or greater AM/MTMMS had a 
1.9/0.5 ug/cm.sup.2 ratio. 
Example 2 
A dielectric layer of 2.8 mil dry film of a photoimagable dielectric formed 
of a polyol resin of (1) an epichlorohydrin and (2) an epoxidized 
octafunctional bisphenol A formaldehyde novolac resin was laminated to an 
epoxy-glass panel laminate that had previously been stripped of copper. 
The photoimagable dielectric was exposed and cured to produce a panel 
suitable for studying seed uptake on a photoimagable dielectric surface. 
The panel was imaged and was then surface roughened to ensure good copper 
to dielectric adhesion. Coupons were taken from this core and prepared for 
electroless copper deposition as described below. 
A 0.05% solution of AM/MTMMS containing 3.0% (v/v) of sulfuric acid was 
prepared. A second solution was prepared which contained 0.05% AM/MTMMS 
and 0.1N sodium hydroxide. One coupon was dipped in the acidic solution of 
AM/MTMMS for 2 minutes followed by a 1 min deionized water rinse. The 
coupon was then immersed in a colloidal Pd/Sn seed bath for 3 minutes, 
followed by a 1 minute deionized water rinse and then treatment in a 0.5N 
sodium hydroxide acceleration bath followed by a final 1 min deionized 
water rinse. The other coupon was treated in the same manner except the 
0.05% basic AM/MTMMS solution was used. The coupons were air dried. 
The acidic AM/MTMMS solution had a Pd/Sn ratio of 17.6/13.2ug/cm2 whereas 
the basic solution had a ratio of 8.0/1.9 ug/cm2. 
While the invention has been described with respect to certain preferred 
embodiments and exemplifications, it is not intended to limit the scope of 
the invention thereby, but solely by the claims appended hereto.