D92
Journal of The Electrochemical Society, 156 ͑3͒ D92-D97 ͑2009͒
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013-4651/2009/156͑3͒/D92/6/$23.00 © The Electrochemical Society
Patterned Electroless Nickel Plating on a Tacky Photopolymer
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Andrei S. Petrov and Jan B. Talbot*
Chemical Engineering Program, University of California, San Diego, La Jolla, California 92093, USA
The feasibility of using a photopolymer as a substrate for electroless metal plating was demonstrated. This photopolymer was first
patterned by polymerization on exposure to UV radiation. Then chemical surface modification with ethylenediamine allowed for
fixation of palladium seed catalyst, without the implementation of a sensitization step, onto which nickel was deposited. A set of
conditions was developed to successfully demonstrate the patterning of electroless nickel plating.
©
2009 The Electrochemical Society. ͓DOI: 10.1149/1.3058584͔ All rights reserved.
Manuscript submitted July 22, 2008; revised manuscript received November 26, 2008. Published January 9, 2009.
Cromalin is a tacky photopolymer developed in the 1970s by
Experimental
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DuPont as a means to pattern toner particles for color proofing.
Cromalin was also used for particle mounting for microscopy and a
The polymerization of positive TackyDot was carried out using a
5 W UV lamp with a UV power output of 2.6 W, a radiation spec-
trum of 315–400 nm, and max of 352 nm. TackyDot sheets, 2
ϫ 5 cm, were exposed to UV irradiation for 60 s at a distance of
cm from the UV lamp. A negative USAF 1951 resolution test
target ͑from Edmund Optics͒ was used as a mask to pattern the
positive photopolymer. The block length and width of the USAF test
chart can be determined from the following equations
1
2
,3
technology called TackyDot was developed for solder bumping.
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Hurt and Talbot successfully extended the TackyDot technology to
a dry screening process for emissive displays by immobilizing phos-
phor particles on a patterned Cromalin surface, as well as for attach-
7
5
ment of carbon, silver, and magnetite nanoparticles and a TiO2
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photocatalyst for use in a photocatalytic reactor for removal of or-
6
ganic compounds from aqueous streams. The objective of this re-
2−
͑−1͒/6
2−͑−1͒/6
search was to determine the feasibility of using Cromalin as a sub-
strate for electroless metal plating.
ᐉ = 2.5 mm ϫ
;
w = 0.5 mm ϫ
␥
␥
2
2
Cromalin is manufactured for use as positive and negative pho-
topolymers. UV radiation in the range of 360–380 nm is used to
activate the TackyDot photopolymer. As shown in Fig. 1, exposure
where w and ᐉ are bar width and length. The length and width are
functions of ␥ and , which are, respectively, the group and element
numbers where the set of bars are located. For example, a bar lo-
cated in group 3 and element 4 has a length of 221 m and a width
of 44 m.
As the positive TackyDot undergoes polymerization, the color of
the Cromalin layer changes from nearly colorless to light blue,
which is useful in determining not only the location of the polymer-
ized areas, but also the extent of the polymerization reaction. Fol-
lowing the polymerization step, the protective Mylar cover film was
removed using Scotch or another adhesive tape. This exposed the
patterned TackyDot surface for further chemical modification, as
illustrated in Fig. 1c. To obtain the best possible image resolution
the mask was positioned as close as possible to the TackyDot sur-
face in order to reduce light scattering and should create distinct
pattern boundaries.
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of positive TackyDot to UV radiation through a mask mimics the
pattern of the mask, leaving unexposed areas “tacky” ͑i.e., the un-
polymerized material͒ which may be used for particle attachment or
chemical modification. Exposure of negative TackyDot to UV radia-
tion gives the negative image of the mask, making the exposed areas
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polymerized and tacky. As demonstrated by Hurt, negative Tacky-
Dot is soluble in aqueous solutions and therefore was not considered
as a viable substrate for electroless nickel plating. However, positive
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TackyDot is considerably less soluble in water and we have used it
in an aqueous system for an extended period of time, indicating it
could be a possible substrate for electroless nickel plating.
The positive TackyDot is comprised of three layers: a 4 m
tacky photoimageable Cromalin photopolymer layer sandwiched be-
tween a top 12 m Mylar cover film and an 18 m polypropylene
support sheet, as shown in Fig. 1. Note that the photopolymer may
be adhered to a different substrate upon removal of the polypro-
pylene support sheet. The positive Cromalin photopolymer consists
of acrylic polymer binders ͓28.6 wt % Elvacite 2008 polymethyl-
methacrylate ͑PMMA͒ and Elvacite 2051 PMMA 350C, and
The UV-exposed patterned TackyDot surface was chemically
modified by exposure to an atmosphere saturated with 1,2–
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diaminoethane vapor ͑v.p. = 10.4 mmHg at 20°C ͒ for 30–40 min.
The chemical vapor deposition time was experimentally optimized.
The polymer was then gently washed with deionized ͑DI͒ water for
5
–10 s in order to remove excess condensed ethylenediamine and
reaction products. The photopolymer on the polypropylene support
was then taped to a glass microscope slide to prevent the deforma-
tion which occurs during the drying process. In order to prevent
undesired polymerization of the TackyDot photopolymer, the proce-
dure was carried out in a darkroom in the absence of other UV
sources.
21.9 wt % Ebecryl 3903͔, acrylic monomers ͓25.4 wt % pentaeryth-
ritol triacrylate ͑PETA͒ and 15.78 wt % triethylene glycol dimethy-
acrylate ͑TDMA͔͒, a photoinitiator, a chain transfer agent, and
2
sensitizer. The various ester groups, R –C͑vO͒–O–R , present in
1
2
TDMA, PMMA, PETA, and Ebecryl are possible sites for the sur-
face functionalization with amines. The nitrogen-containing amine
groups allow for fixation of the palladium seed catalyst without the
Deposition of the palladium catalytic sites necessary for subse-
quent nickel growth onto the amine-modified TackyDot surface was
performed by dipping the polymer into a 0.1 wt % palladium chlo-
ride ͑PdCl2͒ and 1.0 wt % hydrogen chloride ͑HCl͒ solution for
10 min. The TackyDot sheet was then rinsed with DI water to re-
move any excess PdCl2. The palladium͑II͒ catalyst seed on the
implementation of a sensitization step with SnCl , which is due to
2
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the strong affinity between palladium and nitrogen. The methods
for surface modification, catalyst mobilization, and nickel reduction
on positive TackyDot photopolymer are discussed in this paper. Fur-
thermore, a set of plating conditions has been determined that suc-
cessfully demonstrated patterned electroless nickel plating onto the
photopolymer.
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TackyDot surface was then reduced to Pd with a 0.5–1.0 wt %
aqueous sodium hypophosphite solution ͑NaPO2H2͒ for approxi-
mately 3–5 min, followed by a DI water rinse.
An electroless plating bath was prepared by dissolving nickel
chloride hexahydrate, NiCl ·6H O ͑46 g/L͒, sodium citrate dihy-
2
2
drate, C H O Na ·2H O ͑74 g/L͒, and ammonium chloride, NH Cl
*
Electrochemical Society Fellow.
E-mail: jtalbot@ucsd.edu
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3
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͑40 g/L͒ in purified DI water which was heated to 70 Ϯ 0.5°C. The