Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
0013-4651/2014/161(5)/D235/8/$31.00 © The Electrochemical Society
D235
Tin Sensitization for Electroless Plating Review
Xingfei Weia and Donald K. Ropera,b,z
a Ralph E. Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA
b Microelectronics Photonics, 3202 Bell Engineering Center, University of Arkansas, Fayetteville, Arkansas 72701, USA
Tin sensitization prior to electroless plating improves deposition of metals on a variety of substrates. This review summarizes
relevant characteristics of the tin sensitized substrates, pre-treatment of surfaces before tin sensitization, adhesion, one- and twostep tin sensitization processes, photo-selective metal deposition (PSMD), and mechanisms and applications of tin sensitization.
Mechanistic improvements to tin sensitization such as addition of aged tin(IV) chloride, and application of accelerators to remove the
stannous shell and expose the catalytic core are evaluated. The effect of ultra-violet (UV) light on oxidation of tin(II) sensitized surface
are analyzed for PSMD. The application section concludes with examples of tin sensitization in recent use and their commercial
relevance.
© 2014 The Electrochemical Society. [DOI: 10.1149/2.047405jes] All rights reserved.
Manuscript submitted January 29, 2014; revised manuscript received March 10, 2014. Published March 25, 2014.
Recent interest in tin sensitization is growing due to its economic
and tunable capacity for precise, controllable nanoscale metallization
of surfaces on various substrates used in microcircuits, electronics,
solar cells, and catalysts. Zabetakis and Dressick recently discussed
selective electroless metallization of patterned polymer films for fabrication of electrical interconnects, plasma-etch-resistant masks, and
diffusion barriers in electronics.1 Refining the tin sensitization solutions allowed fabrication of ∼50 nm width features in metal for
industrial use and enabled fabrication of sub 10 nm metal features.
John et al. used stannous chloride (SnCl2 ) as both reducing and stabilizing agent to synthesize platinum (Pt) catalytic clusters with less than
20 atoms.2 Tin/silver (Sn/Ag) catalyst has been used as an economic
replacement for tin/palladium (Sn/Pd) catalyst in electroless plating of
silver and copper on epoxy-based polyhedral oligomeric silsesquioxane (POSS) films.3 Sensitization using SnCl2 -HCl and activation with
Pt colloid solution have been employed to coat tunable shell thickness (35–198 nm) of Ag film on polystyrene (PS) microspheres.4
Sensors to measure solution dielectric constant were made via tin sensitization with trifluoroacetic acid (TFA), ammoniacal silver nitrate
activation, and galvanic gold (Au) replacement on sodium.5 Notwithstanding this growing interest in tin sensitization in electroless plating,
its mechanism and relevant methodological characteristics have not
been systemically reviewed.
This work reviews development of the electroless plating approaches and examines critical features of common methods. Physicochemical characteristics of tin sensitized substrates, optimal treatment
of surfaces prior to electroless plating, and proposed mechanisms of
electroless plating are evaluated. Conventional two-step sensitization
is compared with a single-step catalyzing system. The review concludes with an overview of optimized methods, their current application, and future prospects in the field.
Background
The method of depositing nickel on a steel or nickel surface without
using an electric current was first reported by Brenner and Riddell.6
In 1947, they detailed use of this chemical reduction method to deposit nickel and cobalt on different surfaces and named it “electroless plating”.7 Several modifications for electroless plating on noncatalytic substrate surfaces were proposed, such as use of alkaline
solutions, making contact with a more electronegative metal, and depositing a catalytic metal layer on the surface of the non-catalytic
metal. They found different surfaces exhibited various adhesion properties for the deposited metal. For example, the adhesion of the electroless deposits to the steel was found to be enhanced by an anodic
treatment of the steel in concentrated sulfuric acid.
Use of stannous chloride (SnCl2 ) and palladous chloride (PdCl2 )
solutions as an electroless plating catalyst was first reported in 1950
z
E-mail: dkroper@uark.edu
by Bergström.8 Stannous and palladous chloride catalytic sensitization
supported electroless metallization of many kinds of surfaces that did
not reduce metal auto catalytically, such as glass. Many studies of
tin sensitization and palladium activation followed the initial report.
Marton and Schlesinger studied the nucleation and initial growth of the
nickel-phosphorous (Ni-P) film by the catalytic action of SnCl2 -PdCl2
on dielectric substrates.9 It was found that the SnCl2 -PdCl2 catalyzing
treatment of the substrates created small catalytic sites whose average
diameter was estimated at 10 Å. The number of the catalytic sites
per unit surface area was inversely related to the hydrophobicity of
the substrates: more hydrophilic substrates exhibited more catalytic
sites. Based on the SnCl2 -PdCl2 activation process, Feldstein et al.
compared two different electroless nickel plating baths, discussed the
inhibition effects of various anions in the electroless plating baths, and
investigated the metal-to-phosphorus ratio and different accelerators
affecting the electroless plating baths.10–12
Several advantages of electroless plating relative to conventional
electroplating have emerged. It allows uniform deposition over irregular and isolated surfaces. Direct deposition on nonconductors is
possible. Electroless plating results in desirable characteristics including less porosity and more corrosion resistance. It is applicable for
bulk plating.13 Due to these important benefits, electroless plating
supported by tin sensitization continues to play an important role in
industrial and academic research and manufacturing.
Critical Issues in the Field
This section examines relevant physicochemical characteristics
reported for tin sensitized solutions and substrate surfaces. First, the
composition of stannous, tin(II), and stannic, tin(IV), ions on the
substrate surface are evaluated. Contact angle, deposited catalytic
sites, and morphology of the deposited metal are then reviewed in
subsequent subsections.
Chemical composition of tin deposited electrolessly on
substrates.— Mössbauer spectroscopy study of the colloids centrifuged from the sensitizing solution and tin deposited on substrates
showed identical line positions.14 Mössbauer spectroscopy, based on
Mössbauer effect first demonstrated by Rudolf Mössbauer in 1957,
exhibits a dip in the spectrum at the frequency of resonance absorption.
It is performed with an emitting (gamma source) and the absorbing
(sample) nuclei in an identical environment. Mössbauer spectroscopy
results indicated that tin colloids centrifuged from sensitizing solution
had the same form as tin present on sensitized substrates. This suggested tin colloid adhered to the substrates when they were immersed
in the solution.14 Radiochemical tracer adsorption measured a ratio
of tin(II) to tin(IV) on a Teflon substrate to be 3 to 1 after two-step
tin sensitization. Step one exposed substrate to a solution with tin(IV)
concentration of 25 mM for 1.0 min. Step two exposed substrate to a
solution with tin(II) concentration of 0.13 to 0.26 M for 1 to 15 min.15
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Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
Table I. Minimum contact angles for different substrates and different solutions.
Ref
Substrates
Contact angle (◦ )
SnCl2 (mM)
HCl (mM)
SnCl4 (mM)
Use Aged SnCl4 (mM)/(h)
Immersion time(min)
16
16
16
17
18
AZ-1350 resist
Teflon
KTFR resist
TiN
AZ-1350 resist
54
20
17
15
37
260
130
260
55
130
940
470
940
1233
470
7.5
7.5
13
0
7.5
0
0
0
0
100/60
1
1
1
n/a
n/a
AZ-1350 is positive photo resist from Shipley company. KTFR is negative photo resist from Eastman Kodak company. Teflon is fluorocarbon film from
DuPont. TiN film was made by physical vapor deposition.
Conversely, a ratio of stannous to stannic forms of 1:2 was reported for
Kepton by Mössbauer spectroscopy after substrate exposed to a solution with only tin(II) concentration at 46 mM.14 In addition to initial
exposure by tin(IV), different substrates, tin(II) content and oxidation
state, and different solutions or different measurement methods could
account for these contrasting ratios.
Contact angle on substrates.— Tin(II) concentration, exposure
time, and addition of tin(IV) to sensitization solution have been identified as important parameters that affect the contact angle of the tin
sensitization solution and the substrates. The composition of the tin
sensitization solution was optimized to improve the hydrophilicity
to the substrates. Feldstein and Weiner reported that by measuring
the contact angle of the sensitization solutions the effectiveness of
the tin sensitization solution could be quantitatively evaluated.16 By
varying the concentrations of tin(II) chloride (from 0 to 0.39 M) and
tin(IV) chloride (from 0 to 0.013 M), it was found that a certain ratio
of tin(II) to tin(IV) minimized the contact angle. A minimum contact
angle (most hydrophilic) benefited electroless plating, as hydrophobic
materials were more likely to yield a non-uniform metal thin film.
The minimum contact angle and the corresponding tin(II) to tin(IV)
ratio were found to vary on different substrates and with different
tin(IV) concentrations.16 For example, a minimum contact angle of
54◦ was reported on the AZ-1350 resist (positive photo resist from
Shipley Company) using a sensitization solution of 0.013 M tin(IV)
and ca. 0.18 M tin(II).16 High acidity (H+ concentration) of the tin
sensitization solution tended to increase the contact angle. The degree
of excess of tin(II) ions relative to tin(IV) ions decreased the rate of
tin(IV) ions adhering to AZ-1350, KTFR and Teflon surfaces.16 Minimum contact angles for different substrates and different solutions
are summarized in Table I.
Several parameters were evaluated to determine their effect on
contact angle. Contact angle measurement of tin sensitization on TiN
substrates at different SnCl2 and HCl concentrations found that decreasing the SnCl2 concentration (from 14.0 to 7.0 g/L) decreased the
contact angle (from 75◦ to 28◦ ). Increasing HCl concentration (from
30 to 90 mL/L) did not affect the contact angle as drastically (the
contact angle changed less than 10◦ ).17 A minimum contact angle of
20◦ was measured for 45 mL/L HCl at 7.0 g/L SnCl2 . During the
exposure time for sensitization, the contact angle decreased in the
initial 3–5 min, then stabilized after 5 min.17 Increasing the tin(IV)
chloride concentration in the tin(II) and tin(IV) sensitization solution
was reported to decrease the minimum contact angle, but not until the
contact angle reached a certain value, such as 53◦ for the AZ-1350
resist.16
Addition of aged tin(IV) chloride solution into conventional tin
sensitization solution (SnCl2 -HCl) was studied in detail by contact
angle, UV spectral absorption, conductance, and nephelometric measurements of the solutions.18 UV spectral absorption measurement
found that a tin(II) and tin(IV) mixture had a higher absorption value
(between wavelengths of 305 and 380 nm) than if absorption values of
pure tin(II) and pure tin(IV) solutions were summed.18 This observed
non-additive light absorption by the same element with two different
oxidation states was attributed to vibration of electrons between the
atoms at the two different oxidation states (due to intervalence charge
transfer).18
Catalytic sites on sensitized substrates.— The size and density of
catalytic sites after tin sensitization and PdCl2 activation depended on
addition of aged SnCl4 solution and HCl concentration in the SnCl2
solution. The higher the HCl concentration (from 0 to 60 mL/L), the
smaller the catalytic sites and the higher the density (Table II). It
was postulated that increasing HCl concentration depressed hydrolysis of tin(II) chloride to tin(II) hydroxide which formed tin colloids
and were purported precursors of catalytic particles. Formation of
catalytic particles was reported to be due to redox reaction between
tin(II) and palladium(II). The efficiency of the redox reaction between
tin(II) and palladium(II) during the activation step, in which stannous is oxidized and palladium reduced, was reported to be 25% at
most.15 Electron microscope studies showed incorporation of an aged
(1 week at 25◦ C) stannic component into the conventional tin sensitization solution formed finer and denser copper crystallites during
the initial stage of nucleation.19 The stannic chloride solution was
found able to improve the apparent hydrophilicity of the substrate
and enhance the electroless plating. The Pd catalytic particle density
on Formvar (polyvinylformal) coated glass substrates increased from
Table II. Catalytic particle size and density for different tin sensitization methods.
Ref
Size (nm)
Density (particles/μm2 )
Substrates
Method
17
48.9
350
TiN
17
28.2
640
TiN
17
16.1
1910
TiN
19
2–3
1000
Glass coated with Formvar
19
2–3
10000
Glass coated with Formvar
20
5–10
na
TiN
Only PdCl2 (0.56 mM) with HCl (3.0 mL/L)
and HF (5.0 mL/L)
SnCl2 (36.8 mM) HCl (30 mL/L) followed by
the formal only PdCl2 process
SnCl2 (36.8 mM) HCl (60 mL/L) followed by
the formal only PdCl2 process
SnCl2 (0.53 mM) HCl (0.1cc/l) sensitization
followed by PdCl2 (0.56 mM) activation
Aged SnCl4 (5 mM) presensitizing followed the
same process as the first case (SnCl2 +PdCl2 )
SnCl2 (53 mM) with HCl (40 mL/L) then
activation PdCl2 (1.4 mM) with HCl 2.5 mL/L)
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Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
Table III. Nucleate size of different sensitizers (Ni plated for 1
min).30
1
2
3
4
5
6
7
8
9
10
11
Sensitizer
Aging (h)
Nucleate size (nm)
0.1M SnCl2 and 0.1M HCl
#1
#1
#1
#1
#1 plus 10 mg/L hydroquinone
#1 plus 10 mg/L hydroquinone
#1 plus 10 mg/L thiourea
#1 plus 10 mg/L thiourea
#1 plus 0.05wt % Triton X-100
#1 plus 0.05wt % Triton X-100
0
48
96
144
288
3
48
24
72
0
240
140.6
29.7
11.2
4.5
8.1
80.6
39.7
7.6
17.2
95.3
4.9
1000 to 10,000 particles/μm2 when stannic chloride solution aged
for 1 wk was applied.19 Field emission scanning electron microscope
(FE-SEM) study of the two-step Sn-sensitization and Pd-activation
catalyzing system on TiN surface found that Pd catalytic particle density was 350 particles/μm2 and catalytic particle diameter was 48.9
± 0.3 nm if the tin sensitization step was skipped.17 When tin sensitization with 30 mL/L HCl was applied before the palladium activation
step, the catalyzing particle density increased to 640 particles/μm2
and the particle diameter decreased to 28.2 ± 0.3 nm. Increasing HCl
concentration to 60 mL/L increased the density of catalytic particles to 1910 particles/μm2 and decreased average particle diameter to
16.1 ± 0.1 nm.17 Incorporating aged stannic chloride solution into the
sensitization solution further improved catalytic particle density. Recent research using high-resolution transmission electron microscopy
(HR-TEM) showed the conventional two-step activation (SnCl2 sensitization followed by PdCl2 activation) on tantalum nitride (TaN)
surface generated Pd catalytic sites of 5–10 nm.15
Island morphology of deposited metal.— Composition and deployment of tin sensitizing solution was found to significantly impact the
nucleate site size and the roughness of metal films in the final electroless plating step. Schlesinger and Kisel studied 11 different sensitizers
for electroless plating on glass surface (see Table III).21 Using an aged
stannous chloride sensitizer solution decreased the nucleate size from
140 nm to 4.5 nm (with increased aging time). Different surfactants
also decreased nucleate site size. For example, adding Triton X-100
reduced the size from 140 nm to 95.3 nm.
For a given sensitizer, higher density of nucleation sites corresponded to smaller size of nucleated islands. The number of metal
islands (particles/50 nm2 ) increased at a superlinear rate relative to the
sensitizer adsorption (mg/m2 ).21 An atomic force microscopy (AFM)
study of Ag electroless plated film found that the tin sensitization
process had affected the roughness of the deposited silver films.17 For
a TiN surface activated only with PdCl2 , the root mean square (RMS)
roughness was 7.71 nm. When SnCl2 sensitization (with 30 mL/L
HCl) was performed before PdCl2 activation, the RMS roughness decreased to 7.31 nm. When HCl concentration in the SnCl2 solution
was increased to 60 mL/L, the RMS roughness dropped to 4.66 nm.17
Adhesive strength between the metals and the substrates.— To improve mechanical adhesion between the metal film and the substrate,
most electroless plating processes include an etching step. As examples, soda lime glass is commonly etched using nitric acid (HNO3 )
solution;28,39 quartz substrates are etched in hydrofluoric acid (HF) or
Piranha solution, a concentrated mix of sulfuric acid (H2 SO4 ) and hydrogen peroxide (H2 O2 );36,38 and polyimide substrates are typically
pretreated.42 Etching the substrate before sensitization is hypothesized to increase the surface roughness and concomitant surface area.
This would provide more interlocking on the surface, thereby improving adhesion.62 However, micro-scale roughness created by chemical etching may not be useful for enhancing adhesion in nano-scale
fabrication.
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Pretreatments that chemically modify surfaces in advance of sensitization have also been implemented to increase adhesion between
deposited metal and the substrate. For example, KOH pretreatment of
polyimide substrates was used to generate a surface carboxylic acid
group. This was reported to increase adhesion strength.62,42 In general, oxygen-containing functional groups with associated hydrogen
ions are reported to improve adhesion. Early in the development of
elecroless plating, it was found that a β-stannic shell on Pd-Sn colloid
catalyst did not provide strong adherence to clean, hydroxylated inorganic oxide surfaces, but did adhere well to polymer photoresists.1 In
contrast, tin(II) was found to chemisorb directly to various hydroxylated surfaces and provide strong adherence.
Pre-treatment of tin sensitized surfaces.— Treatment of substrates
prior to exposure to tin sensitization solution is important achieve
particular objectives in specific applications of electroless plating. A
proposed mechanism of tin deposition on different substrates consists
of two successive actions: colloid formation in solution resulting from
air oxidation and hydrolysis of tin(II) chloride, and precipitation of
soluble tin from the liquid layer adhering to the substrate during the
water washing (due to pH increase).22 Based on this mechanism, it was
suggested that tin sensitization could be affected by the solution pH, tin
concentration, and aging. Meanwhile, tin sensitization was observed
to be independent of sensitization time (after 5 min), concentration of
the aged tin(IV) chloride (between 5 and 50 mM), and concentrations
of the tin(II) chloride (between 0.13 and 0.26 M).15
Aging stannic chloride.— Applying aged SnCl4 solution was
found able to enhance electroless plating by contact angle measurement, catalytic site formation, and mechanism evaluation. Feldstein
et al. in 1972 showed aged tin(IV) chloride solution used in the tin sensitization process improved sensitization of hydrophobic substrates.18
It was also found that adding aged SnCl4 to the conventional SnCl2
sensitization solution improved the sensitizer by increasing island
density and uniformity of the metal film for electroless Ni plating.23
Table I shows the addition of aged SnCl4 led to decreasing contact
angle from 54◦ to 37◦ . Table II shows applying an aged SnCl4 solution
increased the density of catalytic sites by 10-fold.
It was proposed that during the aging process, α-stannic acid (hydrolyzed tin(IV) chloride form) transformed to β-stannic acid, a polymeric form with proposed formula of (HO)3 -Sn-[O-Sn-(OH)2 ]n -OH or
(HO)2 -Sn-[O2 Sn]n -(OH)2 .18 Increasing the β-stannic acid concentration was reported to improve the tin sensitization process by converting
the hydrophobic surface to a more hydrophilic surface. Increasing the
temperature and decreasing the stannic chloride concentration accelerated β-stannic acid formation in the aging process. This occurred
because diluting the solution increased the stannic chloride hydrolysis
rate and higher temperatures increased the reaction rates ofhydrolysis and polymerization, respectively. On the other hand, a high acid
(HCl) content or sodium chloride concentration would inhibit the α
to β-stannic acid transformation.18 Applying aged stannic component
improved electroless metal deposition onto organic surfaces, as the
stannous ions interacted with the stannic component and reacted with
PdCl2 to form catalytic sites for electroless plating.18,24
Important aspects of sensitization, tin(II) oxidation, complex formation, tin(II) and tin(IV) hydrolysis, and aging of the sensitization
solution were discussed by Przyłuski et al.25 During initial aging (0–
50 hr), oxidation and complex formation processes predominated.
After 500 hr, hydrolysis became the predominant process. Tin(II) and
tin(IV) complexes were reported to have a maximum absorption between a wavelength 345 and 360 nm. The formula for the complex
was asserted to be [SnII 3SnIV ClI n ]14-n . Tin(II) oxidation in air was proposed to follow a zero-order reaction mechanism. The rate of tin(II)
oxidation was controlled by diffusion of oxygen from the air-solution
interface.
Different substrates.— Contact angle measurement, Mössbauer
spectroscopy, Radiochemical tracer analysis, X-ray photoelectron
spectroscopy (XPS), and X-ray fluorescence spectroscopy (XRF) have
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Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
been used to examine the properties of different substrates after tin
sensitization and activation. Contact angle measurements on AZ-1350
resist (positive photoresist, Shipley Company), KTFR resist (negative
photoresist, Eastman Kodak Company) and Teflon (fluorocarbon film,
trade mark of E. I. du Pont de Nemours & Co.) substrates after tin
sensitization found that the minimum contact angle was affected by
different tin(II) to tin(IV) ratios and different substrates (see Table I).16
Experimental results showed that the contact angle on Teflon substrate
was most sensitive to the tin(II) to tin(IV) ratio when compared with
the other two substrates). Both the Teflon and KTFR resist substrates
had a minimum contact angle around 17◦ that was much lower than
the AZ-1350 resist (at about 53◦ ).
A Mössbauer spectroscopy study of the tin sensitization on Kapton
(du Pont polyimide) showed that the tin(II) to tin(IV) ratio after tin(II)
chloride sensitization was 1:2, and after the palladium catalyzing step
the tin to palladium ratio was 3.8:1.14 Single-step tin sensitization on
graphite surface was studied by Meek and Cohen with both Rutherford
scattering technique and Mössbauer spectroscopy.26 The formation
of Pd-Sn alloy at catalytic centers and absorbtion on substrate were
confirmed.26 Similarly, a Rutherford backscattering analysis found the
Sn:Pd ratio on the Si3 N4 substrate was between 3.3:1 and 2:1 after a
single-step Pd-Sn sensitization.27 Using radiochemical tracer analysis and proton backscattering analysis, the palladium concentrations
on the soda lime glass substrate were determined to be 3.4 × 1014
atoms/cm2 and 3.25 × 1014 atoms/cm2 , respectively, after the sensitization and activation processes.28,29 Meanwhile, XPS study of a tin
sensitized and silver activated soda lime glass showed that the Sn:Ag
ratio was at about 1:2, as one Sn2+ ion could reduce two Ag+ ions.30
The total tin concentration on the alumina substrate was determined
by X-ray fluorescence spectroscopy (XRF) at 0.71 × 1015 atoms/cm2
after tin sensitization process (pH<1, SnCl2 solution).31
Recently, the single-step Pd-Sn catalyst in electroless copper plating on epoxy substrates was studied with transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX).32 It
was found that applying sodium hydroxide (NaOH) and ethylenediaminetetraacetic acid (EDTA) accelerators generated the most uniform Cu films.32 The proposed reason was that the accelerator removed
the tin outer layer (shell) on the catalytic particle and exposed the
catalytic sites to the solution. Further detail is available in the mechanism section. A high-resolution transmission electron microscopy
(HR-TEM) study of the conventional two-step activation (SnCl2 sensitization followed by a PdCl2 activation) on tantalum nitride (TaN)
surface found that the palladium catalytic sites were 5–10 nm in
size.20 A field emission scanning electron microscope (FE-SEM)
study of the two-step (Sn-sensitization and Pd-activation) catalyzing system on TiN surface found that the Pd catalytic particle size was
16.1 ± 0.1 nm (7 g/L SnCl2 , 60 mL/L HCl and Pd activation).17
These two similar tin sensitization and activation processes resulted
in slightly different catalytic site sizes (5–10 nm vs. 16 nm), possibly
due to different substrates.
Trifluoroacetic acid (TFA) in tin sensitization solution.— Trifluoroacetic acid (TFA) was identified as an acceptable replacement for
conventional hydrochloric acid (HCl) in tin sensitization solutions.
The application of trifluoroacetic acid (TFA) in tin sensitization was
first reported by Martin.33 Conventional tin-HCl solution was used by
Martin as a sensitizer for electroless silver and gold plating processes
in 1994. But in in 1995 a tin-TFA solution with 0.07 M TFA and
0.026 M SnCl2 was applied by Menon and Martin for electroless metallization of membranes.33,34 Later the tin-TFA solution sensitization
process was applied on silica spheres to achieve homogeneous silver
nanoparticle deposition.35
Later ot was reported by Roper et al. that ultrathin and uniform
gold films with enhanced features relative to sputtered gold films could
be fabricated by using tin-TFA sensitization and controlling the gold
deposition time.36 Using tin-TFA sensitization followed by silver activation and gold plating steps, regular arrays of gold nanoparticles were
fabricated by depositing metal onto an electron resist patterned by topdown electron (e)-beam lithography37 or onto a surface supporting an
array of hexagonally closed packed, self-assembledmicrospheres.38
Using continuous flow to maintain a constant thermodynamic driving
force and enhance the mass transfer rate of gold deposition onto the
surface was reported to improve the gold film quality and enhanced its
optical features.39 The dynamics of silver catalytic site formation and
gold thin film growth were studied by real-time transmission UV-vis
(T-UV) spectroscopy, after Ag0 reduced onto the Sn2+ sensitized surface in the continuous flow system.40 Formation of Ag nanoparticles
as catalytic site for galvanic displacement by Au was dynamically
recorded using T-UV spectroscopy.40 However, tin-TFA sensitization
may have some limitation, such as less concentrated tin(II) on the
substrates, larger grain size, and higher pKa than HCl (pKa of TFA is
0.23, pKa of HCl was −7.0).
Photo selective metal deposition (PSMD).—The photo selective metal
deposition (PSMD) method, based on redox reaction of tin(II) and
tin(IV) during tin sensitization process, was developed as a photolithography approach. It was reported that the selectivity of positive
or negative PSMD could be controlled by simply adjusting the pH of
the plating bath in electroless Ni plating.41 A recent study showed that
selective oxidization of tin(II) on the substrates with UV light irradiation could form micro-scale metal patterns on polyimide film.42
Idea development.— The concept of PSMD which combined conventional electroless plating method (SnCl2 -PdCl2 activated) with an
ultraviolet radiation treatment under a photo mask was first carried out
by Sharp.22 The proposed mechanism is shown in Figure 1. First, the
substrate was covered with a layer of divalent tin after sensitization
in SnCl2 . Second, the divalent was oxidized to tetravalent tin upon
exposure to ultraviolet radiation. Portions of the surface covered by a
photo mask remained covered by divalent tin. Third, palladium(II) in
the activation solution would be selectively reduced on the divalent tin
surface and form the Pd catalytic sites. Finally, metal was selectively
deposited by electroless plating on the substrates where Pd catalyst
was deposited. The mechanism of the divalent tin oxidation by ultraviolet radiation was first studied by Cohen et al. using Mössbauer
spectroscopy.14 They found that divalent tin on the surface was oxidized to the tetravalent form when SnCl2 sensitized substrates were
either exposed to UV light or immersed in a PdCl2 -HCl solution.14
Figure 1. Photo-selective Metal Deposition process.22
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Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
Mechanism discussion.— The mechanism of PSMD was later improved by Chow et al. who did research on applying UV light after
sensitization and after activation for electroless plating of Cu, Ni and
Co.43 It was found that for each of the three metals (Cu, Ni and
Co), plating would be inhibited as expected because of the tin(II) to
tin(IV) oxidation reaction when the UV light was applied after the tin
sensitization step. However, when the UV light was applied after the
activation step, the electroless Ni and Co plating were still inhibited.43
The reason was proposed that phosphorus was involved in Ni and Co
deposition solution, which would increase the degree of crystallization (lost activity) of the Pd catalyst in UV irradiation and inhibit
the electroless Ni and Co plating.43 On the other hand, hydrogen was
incorporated in the electroless Cu film deposition (not affect the catalyst), which would not inhibit the deposition.43 Later, it was found
that Cu deposition could be inhibited by UV light irradiation after
activation, if the water rinse times between sensitization, activation,
and metallization were shorted to 15 sec.44 The idea of using UV light
to inhibit electroless plating was also tested on plastic substrates. The
most effective inhibition was obtained from applying UV light just
after the catalyzing step.45
Baylis et al. reported a novel way of inhibiting electroless plating
via UV light by using tin(IV) chloride solution as a sensitizer.46 They
found that using tin(IV) sensitization and Pd activation, the electroless
Ni plating would deposit nickel on the substrate, but the plating would
be inhibited if UV light was applied after the Pd activation process.46
However, different result came from electroless Cu plating: the Cu was
only deposited when the UV light was applied after the Pd activating
step.46 It was proposed that the Pd catalyst was generated from the
light induced reduction of palladium(II) to palladium(0) on the tin(IV)
sensitized substrate:46
Substrate-Sn4+ -Pd2+ + UV light (hv) → Substrate-Sn4+ -Pd0
[1]
It was proposed that UV light irradiation could increase the degree
of crystallization of Pd catalyst in the presence of phosphorus and
inhibit electroless Ni plating. (This UV light inhibition assumption
was proposed by Chow et al. earlier).43 Electroless Co and Ni plating
in alkaline baths were also studied by Baylis et al. for PSMD with a
tin(IV) sensitizer.47 It was found that the reduction of Sn4+ -Pd2+ to
Sn4+ -Pd0 catalyst (under UV light) could only happen in an acidic
condition, because in a basic solution the Pd2+ was converted to
Pd(OH)2 which had a lower reduction potential than PdCl2 in an
acidic solution.
The mechanism by which UV irradiation inhibitselectroless plating remained a subject of continued exploration. Five different ways
that UV light could possibly affect the Pd catalyst havebeen proposed:
(a) UV light reduces Pd2+ to Pd0 ; (b) UV light and air oxidizes Pd2+ to
Pd4+ ; (c) UV light renders Pd0 site non-catalytic; (d) UV light breaks
bonds that connect the Pd catalyst to the substrate; or (e) UV light
dehydrates Pd2+ and forms insoluble palladium oxide.48 To clarify the
mechanism, Pd2+ on substrates and in rinse water from different tin
sensitization processes and UV light irradiation methods were quantified by a spectrophotometric method.48 More Pd2+ was recovered
in the rinse water after UV irradiation on the dried substrates comparing with the non-UV irradiation one. These experimental results
suggested UV light reduction of Pd2+ to Pd0 did not occur if Pd0
was the catalyst, because the concentration of Pd2+ in the rinse water
after UV irradiation should have been lower. The possibility that UV
light broke the bond between the Pd catalyst and the substrate was not
substantiated, because in that case the Pd catalyst in the rinse water
should remain effective for electroless plating, butexperiment showed
it was not. Althoughthe mechanism of tin(II) and tin(IV) sensitization during PSMD has not been completely elucidated,41 methods
for selective Ni and Co electroless plating47 have been duplicated by
different research groups.
Single-step tin sensitization.—The single-step tin sensitization process
was developed for its convenience and greater efficiency in industrial
application. It was expected to provide more reproducible results if
the catalyzing solution could be stabilized. The catalyst particle size
D239
in the single step sensitization process was generally smaller (3 nm)
than the conventional two-step process that varied from 2–3 nm to
50 nm (see Table II). However, adisadvantage was that accelerators
(HCl, NaOH or others) had to be applied to remove the tin shell and
improve the efficiency of the catalyst.
Method development and comparison with two-step system.— In
place of the conventional two-step surface activation method (tin sensitization followed by palladium activation), a single-step catalyzing method with Sn-Pd complex was developed to increase convenience. Scanning tunneling microscopy (STM) was used to characterize the Pd catalytic clusters on graphite substrates which were formed
during the activation step.49 A pyramidal cluster with 40 Pd atoms
(4 × 5 atoms basis) was proposed. Mössbauer spectroscopy study
of the single-stage Sn-Pd complex treatment solution proposed that
the stoichiometry of the complex could be Pd2+ -3Sn2+ , that the complex was unstable and auto-reduced to a Sn-Pd alloy, and that excess
tin(II) ion in solution formed a monolayer of stannous shell on the
particle which stabilized and limited the catalyst particle size.50 On
the other hand, Meek found that the two-step sensitization process
(tin(II) sensitization and followed by palladium(II) activation) was basically different from the single-step Sn-Pd complex solution system
by a Rutherford scattering study.51 The former two-step sensitization
showed a lower Pd/Sn ratio, more Pd and Sn species lost into the
electroless plating solution, and a longer time required to initiate the
electroless plating reaction.51
Combining Mössbauer spectroscopy with electron microscopy and
using a Rutherford backscattering Cohen and Meek showed the catalytic sites for electroless plating were suspension of colloidal particles of Sn-Pd alloy with an upper limit particle size of about 30 Å in
diameter by studying the Sn-Pd complex solution sensitizing graphite
substrates (see Figure 2).26 Adding tin(IV) in single-step sensitization
solution was also studied using PSMD. The effective lifetime of the
sensitization solution for surface sensitization was improved from 45
hours to 3 months by adding HCl, which was proposed to slow down
tin(IV) chloride hydrolysis.46,52 A HR-TEM study of the single-step
Pd-Sn catalyzed surface showed that the tin solution temperature, colloid growth time, and solution aging time were the critical aspects
that determined the final catalytic particle sizes.27 The catalytic core
prepared in concentrated acidic media at room temperature was found
to have a lower tin concentration, while aging the catalyst improved
the nucleus crystalline organization.27 Heating or increasing the pH
of the catalytic solution during the colloid formation was found to
produce highly crystallized tin-rich catalytic sites.27
Accelerators.— Applying an accelerator washing step after the
single-step tin sensitization process was found to improve the catalytic
efficiency.50 An electron microscope study of the single-step sensitization showed that either applying the accelerator solution or adding the
aged tin(IV) solution to the catalyzing solution had improved the surface catalyzing process.53 An energy dispersive spectroscopy (EDS)
analysis of different accelerators applied to a graphite-epoxy composite substrate catalyzed by single-step Sn-Pd found that acceleration
Figure 2. Structure of Pd-Sn alloy catalytic site.26
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D240
Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
with 0.5 M NaOH removed an equivalent amount of tin compared
with the electrochemical method using a potential greater than 1.0 V
vs reversible hydrogen electrode (RHE). This result was better than
the HCl (8%) accelerator.54 Five different accelerators (NaOH, HCl,
H2 SO4 , NH4 OH and NH4 BF4 ) were compared in detail by TEM and
electron diffraction analysis.55 It was also found that the NaOH accelerator produced the finest remaining particles, while the accelerators
with ammoniate group (NH4 OH and NH4 BF4 ) coagulated small particles to produce high-density particles.55
Electrochemical method evaluation.— Cyclic voltammetry was
found to be useful in characterizing tin sensitization solutions. It was
reported that the concentration of tin(II), tin(IV), and Pd(II) in solutions could be measured within ±10% accuracy.56 The tin(II) ion
in solution inhibited the dissolution of the 0 valence palladium(0).
Tin(II) ion on the electrode surface could be leached by rinsing with
high chloride concentration solution. Another study using cyclic and
linear voltammetry evaluated the Pd-Sn catalyzing solution with Pd
concentration varied from 0.1 to 0.4 mM and Sn concentration ranging from 1 to 4 mM.57 It was found that electrochemical reduction
of palladium(II) was a process with two one-electron transfer steps.
Electrochemical reduction of tin(II) in 0.1 M HCl was shown to be
diffusion limited. The diffusion coefficient was measured by rotation
disk electrode to be 2.5 × 10−10 m2 /s, compared with a reported Pd(II)
diffusion coefficient of 1.17 × 10−9 m2 /s. Electrochemical reduction
of tin(IV) only happened in solutions with high HCl concentration.
Hydrolysis dominated in 0.1 M HCl solution, while in 1 M HCl solution the hydrolysis was inhibited).
Mechanism development.—The mechanism of tin sensitization and
palladium/silver activation has been discussed in literature for many
years. A widely accepted mechanism is shown in Figure 3 using
silver activation on glass substrate. The mechanism of single step SnPd activation process is different from the two-step process. It will
also be discussed here.
Conventional two-step system.— In the mechanism of tin(II) sensitization, two processes could contribute to tin(II) deposition on the
substrate : colloid formation in solution due to tin(II) chloride oxidation in air and hydrolysis in water; and precipitation of soluble tin
species from the adhering film of solution during the water rinse step
due to a rapid pH increase.22 Therefore, tin sensitization was hypothesized to be affected by solution properties such as pH, concentration
Figure 3. Tin sensitization and silver activation on soda lime glass.30
and aging; substrate properties and pretreatment; and immersion and
water rinse processes.22 A Mössbauer spectroscopy study of tin(II)
sensitization on Kapton (du Pont polyimide) substrate showed that
the ratio of tin(II) to tin(IV) ions on the substrate was about 1:2,
and the total amount of the tin on the substrate remained at about 10
μg/cm2 before and after the activation step with Pd(II) solution.14 After activation, the atomic ratio of tin to palladium was approximately
at 3.8:1, which proved the amount of tin(II) oxidized and the amount
of the Pd(II) reduced were the same:14
Sn2+ (s) + Pd2+ (aq) → Sn4+ (s) + Pd0 (s)
[2]
A radiochemical tracer analysis study showed that the surface concentration of tin(II) and tin(IV) on the tin sensitized Teflon substrate
were 1.2 × 1016 atoms/cm2 and 0.36 × 1016 atoms/cm2 respectively,
which was the only report that quantified the tin(II) concentration.15 In
comparison, the tin(II) concentration on the tin sensitized soda-lime
glass was found between 0.53 × 1015 and 0.93 × 1015 atoms/cm2
in our lab (unpublished result: tin concentration was 29 mM, trifluoroacetic acid concentration was 72 mM, and sensitization time was
3 min).58 Many studies reported the total tin concentration at different electroless plating steps, because tin(II) was easily oxidized when
exposed in air. A Rutherford scattering study of tin sensitization on
cleaved graphite substrates showed that the total tin concentration on
the surface was at about 1.5 × 1016 atoms/cm2 , the ratio of O/Sn (the
Sn counted for the total amount of tin on the substrate) was about
1.3, and the Cl/Sn ratio was close to 0.1.51 The same study found that
after Pd catalyzing process the O/Sn ratio increase to 2 and Pd/Sn
ratio was approximately at 0.2, while the Pd/Sn ratio after activation
on Kapton found by Mössbauer spectroscopy was about 1:3.8.14,51
The slight difference could be due to different substrates or different
analytic methods applied. Another study of Rutherford backscattering
analysis of single-step Pd-Sn catalyzing the Si3 N4 substrate found that
the palladium and total tin concentration concentrations were at 1.6
× 1015 atoms/cm2 and 3.5 × 1015 atoms/cm2 , respectively.27 Dynamic study found that the Pd/Sn ratio was found to increase from 0.3
to 0.5 during the first 5 min immersion time, while after 5 min it was
stabilized.27 Introducing a step in which substrates were immersed in a
silver nitrate solution between the stannous chloride and the palladium
chloride steps was found to generate a very homogeneous nucleation
on soda lime glass substrates:28
Sn2+ (s) + 2Ag+ (aq) → Sn4+ (s) + 2Ag0 (s)
[3]
2Ag0 (s) + Pd2+ (aq) → 2Ag+ (aq) + Pd0 (aq)
[4]
Radioactive tracer analysis of Sn, Ag and Pd showed that the total
amount of tin after SnCl2 sensitization and rinsing was about 0.12
μg/cm2 (6.1 × 1014 atoms/cm2 ), the silver concentration followed by
AgNO3 activation and rinsing was about 0.16 μg/cm2 (8.9 × 1014
atoms/cm2 ), and the palladium concentration followed by PdCl2 activation and rinsing was about 0.06 μg/cm2 (3.4 × 1014 atoms/cm2 ).28
If the AgNO3 solution activation was skipped, the Pd concentration
decreased to about 0.04 μg/cm2 (2.3 × 1014 atoms/cm2 ), which was
only 2/3 the amount found with a Ag step.28 Similarly, a Pd concentration of 3.25 × 1014 atoms/cm2 was reported by proton backscattering
analysis of the Pd catalyzed soda lime glass.29 Baylis et al. suggested
a minimum Pd catalyst coverage of 5 × 1013 atoms/cm2 was required
to initiate electroless metal deposition on glass.29 X-ray fluorescence
spectrometry (XRF) was used to quantify the amount of tin, silver and
palladium on the alumina substrate after each step.31 It was reported
that tin was at 0.71 × 1015 atoms/cm2 after tin sensitization, silver
was at 1.3 × 1015 atoms/cm2 after silver nitrate intermediate step, and
palladium was at 0.5 × 1015 atoms/cm2 after PdCl2 activation.31 Because different analysis methods were performed in different regions
(depth) of the catalytic site, from the core to the shell, the Pd/Sn ratios
could be varied.13 Table IV summarizes all the different tin, silver and
palladium concentration previously reported and cited in this review
(references contain detailed information).
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Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
D241
Table IV. Surface concentration of tin, silver and palladium (atoms/cm2 ).
Ref.
Tin(II)
Tin(IV)
Tin (total)
Ag
Pd
15
27
28
29
31
51
58
1.2 × 1016
na
na
na
na
na
0.54–0.93 × 1015
0.36 × 1016
na
na
na
na
na
na
na
3.5 × 1015
6.1 × 1014
na
0.71 × 1015
1.5 × 1016
na
na
na
8.9 × 1014
na
1.3 × 1015
na
na
na
1.6 × 1015
3.4 × 1014
3.25 × 1014
0.5 × 1015
na
na
Single-step catalysis systems.— The mechanism of the single-step
catalysis system (briefly discussed in a previous section) was found
by Mössbauer spectroscopy study of single-step Sn-Pd complex sensitization on Kapton substrates.50 It showed a mixture of tin(II) and
tin(IV) lines on the spectra. After ‘accelerator’ washing, the tin(II) line
was absent and the catalytic activity of the surface was increased.50
Based on these observations, it was concluded that a tin(II) shell was
stabilizing the Sn4+ -Pd0 catalyst as in Figure 2 (except the stannic shell
was proposed to be a stannous shell).50 Applying electron microscopy
and electron diffraction analysis of the single-step catalysis followed
by acceleration process, Feldstein et al. proposed that the presence of
Pd3 Sn prior to the metallization step would produce the best plating
results.53 Another Mössbauer spectroscopy study of tin sensitizing on
graphite substrates identified the formation of a catalytic Pd-Sn alloy.26
The single-step tin sensitization processes were suggested to be: first,
colloidal particles of Pd-Sn alloy adsorb to substrates; second excess
stannic hydroxide is removed by accelerator (20 mL Shipley 19 in 100
mL water); and third, additional removal of stannic hydroxide occurs
during electroless Cu plating (in the Mac-Dermid 9070D electroless
Cu plating solution) (see Figure 2).26 A stannic shell was proposed to
occur on the Pd-Sn alloy core. Because Cohen et al. suggested both
stannic and stannous shells, and report26 was later than,50 information
in the former is regarded as more complete, which supports a stannic
(Sn4+ ) shell on the catalytic core.
A TEM study of the Pd-Sn catalyst system showed that the high
catalytic activity was due to large colloidal particles, and the accelerator (1 M NH4 BF4 or 1:1 HCl) caused coagulation of small particles that
increased activity.59 X-ray photoelectron spectroscopy (XPS) showed
that the Pd/Sn ratio was not constant for particles with different catalytic activities, and the Pd and Sn could exist as Pd-Sn intermediate
or alloys.59 Another XPS study of SnCl2 and SnCl4 sensitization (both
concentrations were 2.5 mM) on soda lime glass surface found that
the tin coverages were the same between 1: 5 and 1:10 (Sn:Si).30 After
the tin(II) and tin (IV) sensitized glasses were treated in ammoniac
silver nitrate solution, the Ag to Sn ratio on the glass surface were
estimated at 2.3.30 It was higher than the former reported ratio of 1.6
measured by radioactive tracer analysis, because in the former case
the SnCl2 concentration was lower (0.44 mM).28,30 The mechanisms
of tin sensitization and silver activation are summarized in Figure
3.30 Energy dispersive X-ray analysis (EDX) found the Pd/Sn ratio
was ∼0.3 before acceleration, increased to ∼1 after HCl acceleration,
and became >2 when accelerated by NaOH.32 Other discussion of
single-step activation process occurred in a previous section.
Following the steps of SnCl2 solution sensitization, rinse, PdCl2 activation, and rinse, electroless plating of Ni, Ni-Co alloy, and Co were
studied.9,60 It was proposed that the structure of deposited Co could be
either face centered cubic (FCC) or hexagonal close packing (HCP),
while the Ni and Ni-Co alloy films were in FCC structures.60 The PdSn catalyst fabricated by cyclic voltammetry and conoamperometric
binary electro-deposition was studied in nitrate reduction reaction,
and the best catalytic performance was at the ratio of Pd30 Sn70 .61
Future Perspectives
The chemical composition of deposited tin compounds on tin
sensitized substrates has been found to have different ratios of
tin(II)/tin(IV), varying from 3:1 to 1:2. Differences in the ratio arose
from different applied solution concentrations and different analytical methods. Measured contact angles have been shown to vary with
substrate composition, tin solution concentration, and solution aging
time. For example, at the same tin sensitization conditions, AZ-1350,
Teflon, and KTFR substrates showed different contact angles (54◦ ,
20◦ , and 17◦ , respectively) while aging the SnCl4 solution decreased
the contact angle from 54◦ to 37◦ on the AZ-1350 substrate.
The diameter and density of catalytic sites resulting from sensitization were improved by increasing the HCl concentration in the
SnCl2 sensitization solution, by adding aged SnCl4 solution, and by
adding surfactants. By increasing the HCl concentration from 0 to 30
to 60 mL/L, the catalytic particle size decreased from 48.9 to 28.2 to
16.1 nm, and the particle density increased from 350 to 640 to 1910
particles/μm2 . The addition of aged SnCl4 solution increased the density of the catalytic sites by a factor of 10, and applying a 144 h aged
sensitizer achieved a minimum nucleate size of 4.5 nm. The addition
of Triton X-100 as surfactant decreased the nucleate size from 140.6
nm to 95.3 nm without aging, the addition of hydroquinone at 3 h aging yielded an 80.6 nm nucleate size, and adding thiourea and aging
the solution 24 h reduced the catalytic size to 7.6 nm. Trifluoroacetic
acid (TFA) was found to to avoid tin(II) hydrolysis, especially for
membrane metallization. Photo selective metal deposition (PSMD)
method was developed based on UV light-induced tin(II) oxidation
and catalyst inhibition. Different metal patterns were fabricated using
PSMD. However the fundamental mechanism appeared to differ for
different electroless plating systems and remains a subject of further
research.
The mechanism of the sensitization and activation steps was evaluated in detail by different analytical methods (radio chemical tracer
analysis, Mössbauer spectroscopy, X-ray fluorescence). Based on the
surface concentrations of tin and palladium (or silver) on the substrates
during each step, a widely accepted mechanism has been proposed.
First, the divalent tin compound isadsorbed on the substrates. After
tin sensitization, the palladium(II) or silver(I) would be reduced to
Pd0 (Ag0 ) following a redox reaction mechanism and become a catalytic site on the surface. The electroless plating could then happen
on catalytic sites so formed.
Outlook
Commercial importance.— Literature reports indicate that the
two-step SnCl2 and PdCl2 (Ag or Pt) activation process is used to fabricate monodisperse catalyst, metal monolayers, and to control metal
deposition on glass or membranes.2–5,35,36 The single-step Sn-Pd complex activation process was developed for convenient operation and is
reported to have been used for electroless Cu plating on printed circuit boards.48,49 The photo selective metal deposition (PSMD) method
was developed in 1970s for fabricating metal patterns.43–48 Itwas reported in 2006 again by Kim et al. for selective metal deposition on
polyimide.42 A reviewer of the submitted manuscript reported that
tin sensitized surfaces are of particular value in catalyzing electroless
plating on circuit boards. The Sn/Pd system is critical and widely
used in this process. However, while it remains an interesting topic
for research, the PSDM process is reported to be of less commercial
significance.
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D242
Journal of The Electrochemical Society, 161 (5) D235-D242 (2014)
Current applications of the tin sensitization and electroless plating
methods are in fabricating metal film for micro-circuits, solar cell, and
making catalysts. The key conditions that affect the tin sensitization
process are: addition of aged SnCl4 , selection of substrate, choice and
concentration of acid stabilizer (HCl vs. trifluoroacetic acid), and UV
light exposure. Single-step sensitization affords greater convenience
and controllability in specific situations. Substrate exposure to an aged
SnCl4 solution before the conventional SnCl2 solution has been shown
to enhance the absorption of tin(II) and improve electroless plating.
Future opportunities.—Improving the tin sensitization process could
support fabrication of a monolayer of tin(II) on the substrate. Ideally, amonolayer of deposited tin(II) could be a premier condition
for producing uniform metal thin films, comparable to vapor deposition. Addition of SnCl4 solution appears to be a way to approach this
goal. For these reasons, the mechanism of tin(II) deposition on the
surface and its oxidation under conditions important for electroless
plating deserves examination in greater detail. Selective oxidation of
tin(II) that has been uniformly deposited on asubstrate could pattern
the surface for electroless metallization. Methods to protecttin(II) that
could complement UV oxidation could be considered. Alternatively,
selective silver or palladium activation subsequent to tin sensitization
could also be explored to pattern the surface for metallization. Ultimately, patterned metal structures, even 3D nanostructures, could be
fabricated by appropriate control and patterning of tin(II) sensitized
substrates.
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