Direct Formation of Patterned Nickel Film on Polymer Using ... · Direct Formation of Patterned Nickel Film on Polymer Using Selective Electroless Deposition and Electroless Anisotropic

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Baba et al.: Direct Formation of Patterned Nickel Film (1/8)

[Technical Paper]

Direct Formation of Patterned Nickel Film on Polymer Using

Selective Electroless Deposition and Electroless Anisotropic

Growth PlatingKunihito Baba*, Sayaka Arashiro*, Christopher E. J. Cordonier**, and Hideo Honma*,**

*Graduate school, Kanto Gakuin University, 1-50-1, Mutsuura-Higashi, Kanazawa-ku, Yokohama-shi, Kangawa 236-8501, Japan

**Material Engineering Surface Research Center, Kanto Gakuin University, 1-1-1, Fukuura, Kanazawa-ku, Yokohama-shi, Kanagawa 236-0004, Japan

(Received July 13, 2011; accepted November 11, 2011)

Abstract

Plating on plastic technology is widely applied in electronics-related fields. Conventionally, good adhesion strength

between metal and resin has been obtained from the anchoring effect of a roughened surface. However, as the frequency

increases, detrimental influences arise from the skin effect in the high-speed transmission. Therefore, the interface of the

conductor and substrate should be as smooth as possible. A metal film with high adhesion strength and surface smooth-

ness can be formed by surface modification with UV irradiation. This led us to the examination of selective deposition

with the UV irradiation technique. In addition, anisotropic growth has been obtained by the addition of inhibitors to the

electroless plating bath where pattern formation was possible without using resist by controlling the horizontal growth

during plating. Anisotropic growth on selectively UV-irradiated regions has been obtained by the addition of inhibitors to

the electroless nickel-plating solution and in this study, direct nickel-pattern formation was accomplished without using

resist by controlling the plating growth in the horizontal direction in a technique combining selective deposition and

anisotropic growth.

Keywords: Electroless Plating, Anisotropic Growth, UV Irradiation, Pattern Formation

1. IntroductionPlating on plastic technology is widely applied in elec-

tronics-related fields, where adhesion is conventionally

obtained from the anchoring effect of a roughened poly-

mer surface. However, as the frequency increases, the

skin effect influences high-speed transmissions, so the

conductor-substrate interface should be as smooth as pos-

sible. In addition, if the subtractive method is used for cir-

cuit formation, the risk of a short-circuit between isolated

regions of the patterned metal increases due to a high ten-

dency for bridging of the residual metal left after etching.

Therefore, the metallization of smooth polymer surfaces

for use as electronic substrates is expected to be more sat-

isfactory if adhesion can be maintained.[1–3] We have

obtained excellent adhesion with minimal influence on

substrate smoothness in experiments employing surface

activation using UV irradiation, where a selective deposi-

tion method was also examined. However, even with selec-

tive deposition and high adhesion, lateral film growth still

presents the risk of pattern crossing. Anisotropic growth

on selectively irradiated regions has been obtained by the

addition of an inhibitor to the electroless nickel-plating

solution, offering a solution for the risk of bridging. Here,

direct nickel-pattern formation was made possible without

using resist by control of the plating growth in the horizon-

tal direction in a technique combining selective deposition

with anisotropic growth.[4]

2. ExperimentalGenerally, direct pattern formation using selective irra-

diation and anisotropic growth can be used on substrates

for which deposition by UV activation is possible. Because

of comparatively easy processing and superior electrical

characteristics, cyclo-olefin polymer (COP) plastic and

ABS resin were selected as the plating substrate. The

selective deposition conditions employed were those eluci-

dated in a previous examination of plating on UV treated

plastic substrates.[1, 2]

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Transactions of The Japan Institute of Electronics Packaging Vol. 4, No. 1, 2011

The selective electroless nickel-plating bath suppresses

deposition such that metal is deposited only on the UV

irradiated areas and the anisotropic growth bath further

controls deposition such that nickel growth other than in

the horizontal direction is suppressed. Table 1 shows the

plating bath composition and operating conditions and

Table 2 shows the experiment steps. A low-pressure Hg

lamp (KOGLQ-500ULS, KOTO Co. LTD), which emits

184.9 and 253.7 nm light, was used as the UV irradiation

source in the surface modification experiments. The photo

mask shown in Fig. 1 was produced by evaporation deposi-

tion of chrome on glass. Irradiation through the photo

mask was performed to give area selectivity. The white

area in Fig. 2 is the chromium-coated area (UV impenetra-

ble region) and the black area is the bare synthetic quart

glass (UV penetrating region) of the photo mask. The syn-

thetic quartz glass used is transparent to both 184.9 and

257.3 nm light. In this experiment, the anisotropic growth

was evaluated at the 100 µm pattern region (Inside the cir-

cled area of Fig. 2).

The electroless nickel-plating film was selectively depos-

ited on a smooth UV exposed substrate employing the pre-

viously described procedure for selective deposition. The

pretreating process was carried out as follows: Exposure

Table 1 Basic bath composition and operating conditions.

E. L. Ni Selective Plating Bath E. L. Ni Anisotropic Plating Bath

Composition Concentration(mol/dm3)

Composition Concentration(mol/dm3)

NiSO4∙6H2O 0.05 NiSO4∙6H2O 0.05

NaH2PO2∙H2O 0.20 Ni(CH3COO)2∙4H2O 0.20

C2H5NO2 0.30 NaH2PO2∙H2O 0.30

(NH4)2SO4 0.20 C4H6O4 0.20

CH4N2S 0.50

Bi(NO3)3 or Pb(NO3)2 0.1~0.5 ppm Bi(NO3)3 or Pb(NO3)2 0.1~1.5 ppm

pH adjustor NaOH-H2SO4 pH adjustor NaOH-H2SO4

Bath temp. 45°C Bath temp. 80°C

Bath pH 8.00 ± 0.05 Bath pH 4.50 ± 0.05

Agitation None Agitation None

Table 2 Basic plating procedure.

UV irradiation 1 min.

▽

Degreasing NaOH 50 g/dm3 1 min. 60°C

▽

Conditioning10 vol% c/c231Rohm and Hass

1 min. 45°C

▽

Activation PdCl2 0.3 g/dm3 1 min. 45°C

▽

AccelerationNaH2PO2

0.25 mol/dm31 min. 60°C

▽

E. L. Ni p lating

Fig. 1 Schematic model of anisotropic growth.

1) UV irradiation through a photo mask2) Selective deposition3-1) Isotropic deposition (overgrowth occurs at edges)3-2)→3-3) Inhibitor concentrated at the edges promoting anisotropic film growth

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of the COP substrate to UV irradiation for 1 min., then

cleaning in an alkaline solution for 1 min., then condition-

ing for 1 min., followed by catalyst adsorption by submers-

ing in a palladium chloride solution for 1 min., and finally,

catalyst reduction by treatment in a hypophosphite solu-

tion for 1 min. These conditions were examined and found

to be optimal. An electroless nickel film of about 0.2 µm

was deposited by adjusting the immersion time during the

selective plating process. Subsequently, a film of about 10

µm was deposited by the anisotropic growth plating proce-

dure where the nickel-deposition rate was calculated from

the weight difference before and after plating. Pb and Bi

ions were used as inhibitors in the plating bath to induce

selective anisotropic growth which was evaluated from the

surface appearance observed using microscopy, laser

microscopy, and SEM.[5–7] Bismuth nitrate (Bi(NO3)3)

and lead nitrate (Pb(NO3)2) standard solutions for atomic

absorption were used as the Bi and Pb ion sources. The

cross-section of a 25-µm anisotropic-growth nickel film was

also observed by microscopy. Elemental analysis of the

deposited film was performed by EPMA. To confirm the

effect of the inhibitor, the potential at the Cu electrode

immersed in the electroless plating solution was measured

using the saturated calomel electrode (SCE). Immersion

potential analysis comparing plate and multi-dot electrodes

with respectively small and large peripheral area ratios

was also performed for which the probe with greater

peripheral area showed prominent suppression. The plate

electrode was copper-plated platinum with an effective

area of 0.8 cm2, and the multi-dot electrode was made of

copper wires bound in acrylic acid resin with an effective

area also of about 0.8 cm2. These electrodes were pro-

cessed using the steps shown in Table 2 and the electrical

potential was measured in the anisotropic plating bath.

3. Results and Discussion3.1 Examination of selective plating

Figure 3 shows the COP substrate after the selective

plating process. When an inhibitor-free bath was

employed, the deposition occurred over the entire sub-

strate. However, selective deposition was achieved by the

addition of ionic Pb and Bi (Fig. 3a). Moreover, anisotropic

deposition was obtained with extended plating time (Fig.

3b). In this process, equivalent results were also observed

on the ABS resin; therefore, it was used for the following

examinations.

3.2 Examination of anisotropic growth plating: Inhibitors for anisotropic growth plating

Similar to the selective plating bath, the plating reaction

stopped upon addition of thiourea in concentrations in

excess of 10 ppm. Therefore the influence of the inhibitor

in the anisotropic plating bath was examined for ionic Pb

and Bi. Figure 4 shows topographic and cross-sectional

electron probe microscopic analyses (EPMA) of a 70 × 400

µm bump grown from a Pb (0.1 ppm) and Bi (0.4 ppm)

inclusive plating bath. The inhibitor tends to concentrate

at the edges and narrow pattern regions as in Fig. 4

according to the nonlinear diffusion model which should

result in selective growth in wide areas inducing a struc-

ture to grow vertically Pb is typically more resistant to co-

Fig. 2 Photo mask image.

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Fig. 3 Image of resolution pattern before deposition (a) and after selective deposition for 1 hr (b).

Fig. 4 EPMA analysis of deposited pattern and pattern cross-section.

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deposition under these conditions and considering the

minute and significantly lower concentration compared to

that of the Bi, detection by EPMA was not expected.

Figure 5 shows the deposition form obtained from aniso-

tropic plating, where width (W) and height (H) were mea-

sured by laser microscopy, and the line in the chart shows

the cross-sectional shape profile. When the Pb concentra-

tion was lower than 0.5 mg/dm3, nickel was deposited over

the whole substrate. Thus, selective plating did not occur

and furthermore, the inhibitor-free baths were found to

decompose. On the opposite side of the spectrum, when

the Pb concentration was 1.0 mg/dm3, pyramidal deposi-

tion morphology was observed by laser microscopy, where

the film thickness on the 100 µm square bump pattern did

not proceed beyond 2.7 µm despite the fact that the plating

time used was estimated for depositing a 10 µm film. In

large areas the target film thickness was obtained. The 100

µm square bump was chosen as the evaluation site in con-

sideration of shape control and also due to plating reaction

limitation in finer pattern regions when Pb was in excess

of 0.5 mg/dm3. Based on these results, Pb 0.5 mg/dm3

was identified as a suitable concentration. Similarly Bi in

the concentration range of 0.5–1.5 mg/dm3 was investi-

gated as an inhibitor for the promotion of anisotropic

growth (Fig. 6). Where Pb tended to lead to pyramidal

structures, the addition of Bi resulted in dome-shaped film

growth as observed by laser microscopy (Fig. 6) for which

selective deposition was attained but with some horizontal

protrusion at 0.5 and 1.0 mg/dm3. The dependence of

deposition morphology on inhibitor species is exemplified

in this comparison.

Figure 7 shows the results of measuring the immersion

potential of the two types of copper electrode in the aniso-

tropic plating bath (including Bi and Pb). The process

shown in Table 2 was used for the pretreatment of the elec-

trode. A plate type electrode and a dot type electrode were

used to measure the change in the potential while

immersed according to the difference of the shape of the

plated surface. It was confirmed that the inhibiting effect

was more significant at the larger peripheral area dot-type

electrode rather than at the plate-type electrode. This sug-

gests that metal deposition at the edges was suppressed by

this phenomenon.

3.3 Influence of plating time on anisotropic growthNext, chronological effects on film growth were exam-

ined for a plating bath containing Pb 0.5 mg/dm3 or Bi 1.0

mg/dm3. Figure 8 shows the relation of the deposition

geometry on the bump pattern and plating time. It was

reconfirmed that films only advanced vertically when Pb

was used, preventing horizontal growth as opposed to

some horizontal growth increasing with treatment time

when Bi was used.

Fig. 6 Plated films after immersion in anisotropic growth bath with Bi ion inhibitor.

Fig. 5 Plated films after immersion in anisotropic growth bath with Pb ion inhibitor.

1) 0.1 mg/dm3

W: –, H: –2) 0.5 mg/dm3

W: 96 µm, H: 5.3 µm3) 1.0 mg/dm3

W: 70 µm, H: 0.6 µm

1) 0.5 mg/dm3

W: 110 µm, H: 8.5 µm2) 1.0 mg/dm3

W: 105 µm, H: 9.2 µm3) 1.5 mg/dm3

W: 97 µm, H: 9.1 µm

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3.4 Examination of combined inhibitor useNext, the combined effect of Pb and Bi was examined.

Trapezoidal deposition geometry was obtained with Pb 0.1

mg/dm3 + Bi 0.4 mg/dm3, a lower total inhibitor concen-

tration than with a single species. Figure 9 shows the SEM

image of the resulting film from the two-species-inhibitor

bath, along with images of comparative films grown in

baths containing Pb or Bi only. The SEM image of the

comparative Pb-inhibited sample reconfirmed the trapezoi-

dal plating with a smooth top inclining from the corners,

where the gradual inclination is not a straight line but

rather is built up in steps. From this stair like structure,

anisotropic growth appears to occur by alternating deposi-

tion and termination reactions at the edges.

3.5 Cross-sectional observationThick-film electroless plating was performed using the

anisotropic growth baths of optimal inhibitor content

(Pb 0.5 mg/dm3, Bi 1.0 mg/dm3, Pb 0.1 mg/dm3 + Bi 0.4

mg/dm3) to prepare samples for cross-sectional examina-

tion. Figure 10 shows the resulting microscope images of

the sample cross-sections and the pattern profile. To exam-

ine the film growth history, the surface was etched by a

1:1 nitric acid + acetic acid solution. The magnified image

of the pattern edges again showed the pyramid and dome

shape growth from the effects of Pb and Bi, respectively.

Fig. 7 Immersion potential analysis correlating suppression and peripheral area ratio.

Fig. 8 Anisotropic nickel film growth shape with plating time.

1) 45 min.W: 89 µm, H: 2.8 µm

2) 90 min.W: 94 µm, H: 6.9 µm

3) 180 min.W: 94 µm, H: 12.5 µm

4) 45 min.W: 92 µm, H: 3.8 µm

5) 90 min.W: 102 µm, H: 7.9 µm

6) 180 min.W: 122 µm, H: 16.9 µm

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3.6 Process simplification by preliminary inhibitor treatment.

In the procedure described above, initial selective depo-

sition using the thiourea-inclusive selective-deposition

bath prior to anisotropic thick-film deposition was

employed, where only exposed regions were plated.

Therefore, even if the selective bath is not used, selective

plating should be possible as long as the inhibitor is

adsorbed onto the substrate surface. Table 3 shows the

steps in the revised process in which preliminary submer-

sion in an inhibitor bath containing thiourea 0.5 mg/dm3,

Pb 0.5 mg/dm3, and Bi 1.0 mg/dm3 replaces the selective-

plating bath. In effect, the processing time could be short-

ened. The inhibitor concentration used was the one dis-

cussed in the “Examination of selective plating” section,

above.

The sample prepared by thick-film plating after prelimi-

nary inhibitor treatment was similar in appearance to the

initial selective-deposition sample, as both were pyramidal

Fig. 9 Plated films after immersion in Pb and Bi ion inclusive anisotropic growth bath.

1) Pb 0.5 mg/dm3 2) Bi 1.0 mg/dm3 Pb 0.1 mg/dm3 + Bi 0.4 mg/dm3

Fig. 10 Cross sectional images of anisotropic films.

1) Pb 0.5 mg/dm3

2) Bi 1.0 mg/dm3

3) Pb 0.1 mg/dm3 + Bi 0.4 mg/dm3

Table 3 New experimental procedure (Inhibitor pretreatment).

UV irradiation 1 min.

▽

Degreasing NaOH 50 g/dm3 1 min. 60°C

▽

Conditioning10 vol% c/c231Rohm and Hass

1 min. 45°C

▽

Activation Pd Cl2 0.3 g/dm3 1 min. 45°C

▽

AccelerationNaH2PO2

0.25 mol/dm31 min. 60°C

▽

Inhibitor pretreatmentCH4N2S0.1 ppm

1 min. 45°C

▽

E. L. Ni anisotropic bath

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in shape, as shown in Fig. 11. Selective deposition only

occurred when preliminary immersion in a thiourea-inclu-

sive bath was performed. Other, thiourea-free, conditions

resulted in deposition over the whole substrate area so

even if Pb and Bi had adsorbed on the substrate before-

hand, selective deposition was not obtained. Based on the

anisotropic-growth plating examination, it seems thiourea

is a powerful deposition suppressor: deposition did not

occur when thiourea was present in the thick-film plating

bath, and it was particularly effective in suppressing initial

deposition resulting in good control of the selectivity in

electroless nickel plating. Though different in behavior

from Pb and Bi (for example, in film growth control), thio-

urea also displayed unique inhibitive effects.

4. ConclusionsBy combining UV-induced selective plating with the

anisotropic-growth technique, lateral-growth-controlled

metal bump-pattern films were formed. Similar anisotropic

growth can be expected with line patterns as well.

Anisotropic growth by electroless nickel plating was

found to depend on nonlinear diffusion of the Pb and Bi

inhibitors at the pattern edges, where the geometry of the

film growth changed depending on the species. When Pb

was employed, growth with a trapezoidal geometry

occurred which was advantageous to pattern formation.

Moreover, when Pb was used in combination with Bi, the

anisotropic growth became possible at lower concentra-

tions. Regarding the selective deposition phenomenon, the

direct pattern plating process was improved by initial

adsorption of thiourea onto the surface, achieved by

immersing the substrate in a solution that contained the

inhibitor instead of initial selective plating.

AcknowledgementThis work was supported by the environmental friendly

functional surface project of the KANAGAWA Academy of

Science and Technology. (KAST)

References[1] K. Tashiro, T. Bessho, and H. Honma, “Environmental

Benign Pretreatment Process Using Photocatalyst,”

Journal of Japan Institute of Electronics Packaging,

Vol. 8, No. 5, pp. 426–429, 2005 (in Japanese).

[2] M. Watanabe, K. Matsui, M. Sugimoto, and H. Honma,

“Smooth Circuit Formation on Cycloolefin Polymers,”

Journal of Japan Institute of Electronics Packaging,

Vol. 10, No. 3, pp. 229–233, 2007 (in Japanese).

[3] T. Sugiyama, Y. Iimori, K. Baba, M. Watanabe, and H.

Honma, “Surface Metallization on High Temperature

Liquid-Crystal-Polymer Film by UV-Irradiation Pro-

cess,” J. Electrochem. Soc., Vol. 156, No. 9, pp. 360–

363, 2009 (in Japanese).

[4] M. Sugimoto and H. Honma, “Adhesion Mechanism

of Plating on Surface Reformed Resin by UV irradia-

tion,” The Surface Finishing Soc. of Japan, Vol. 59,

No. 5, pp. 294–298, 2008 (in Japanese).

[5] A. M. T van der Putten and G. de Bakker, “Anisotropic

Deposition of Electroless Nickel. Bevel Plating,” J.

Electrochem. Soc., Vol. 140, No. 8, p. 2229, 1993.

[6] A. M. T van der Putten and G. de Bakker, “Anisotropic

Deposition of Electroless Nickel. Bevel Plating,” J.

Electrochem. Soc., Vol. 140, No. 8, p. 2231, 1993.

[7] K. Hatada, Proceedings of The Surface Finishing Soci-

ety of Japan 118th meeting, p. 312, 2008 (in Japanese).

Fig. 11 Effect of pretreatment with thiourea on selective deposition using a Pb 0.1 mg/dm3 + Bi 0.4 mg/dm3 inclusive anisotropic growth bath.

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Direct Formation of Patterned Nickel Film on Polymer Using ... · Direct Formation of Patterned Nickel Film on Polymer Using Selective Electroless Deposition and Electroless Anisotropic