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Surface Modification of a Photo-Definable Epoxy Resin withPolydopamine to ImproveAdhesion with ElectrolessDeposited CopperDavid Schaubroeck a , Emilie Van Den Eeckhout a , JohanDe Baets a , Peter Dubruel b , Luc Van Vaeck c & AndréVan Calster aa Centre for Microsystems Technology (CMST)/ELIS,IMEC, Ghent University, Technologiepark 914A, B-9052,Ghent–Zwijnaarde, Belgiumb Polymer Chemistry and Biomaterials Research Group,Ghent University, Krijgslaan 281, Building S4 is, B-9000,Ghent, Belgiumc Department of Chemistry, University of Antwerp (UIA),Universiteitsplein 1, B-2610, Wilrijk, Belgium

Version of record first published: 17 May 2012

To cite this article: David Schaubroeck, Emilie Van Den Eeckhout, Johan De Baets, PeterDubruel, Luc Van Vaeck & André Van Calster (2012): Surface Modification of a Photo-Definable Epoxy Resin with Polydopamine to Improve Adhesion with Electroless DepositedCopper, Journal of Adhesion Science and Technology, 26:18-19, 2301-2314

To link to this article: http://dx.doi.org/10.1163/156856111X623104

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Surface Modification of a Photo-Definable Epoxy Resin withPolydopamine to Improve Adhesion with Electroless

Deposited Copper

David Schaubroeck a,∗, Emilie Van Den Eeckhout a, Johan De Baets a, Peter Dubruel b,

Luc Van Vaeck c and André Van Calster a

a Centre for Microsystems Technology (CMST)/ELIS, IMEC, Ghent University,Technologiepark 914A, B-9052 Ghent–Zwijnaarde, Belgium

b Polymer Chemistry and Biomaterials Research Group, Ghent University, Krijgslaan 281,Building S4 bis, B-9000 Ghent, Belgium

c Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1,B-2610 Wilrijk, Belgium

Received in final form 24 November 2011

AbstractThis paper describes the influence of polydopamine surface modifications on the adhesion strength of elec-troless deposited copper on roughened epoxy resin substrates. The surfaces are characterized with XPS andToF-S-SIMS. Next, a thorough investigation of the copper–epoxy interface is performed using SEM. Boththe polydopamine modification and the variation of the electroless plating bath temperature lead to newinsights into the different contributions of chemical and physical adhesion to the overall adhesion strength.© Koninklijke Brill NV, Leiden, 2012

KeywordsPolydopamine, electroless copper, epoxy resin, adhesion, surface modification

1. Introduction

Today, most of the printed circuit boards (PCBs) are composed of multiple alter-nating layers of patterned copper and dielectrics. The production of high densityinterconnects for integrated circuits (ICs) requires additional sequential build-up(SBU) layers, including microvias and very fine copper lines, on both sides of thePCB core [1]. New technologies and processes have been developed to support themanufacturing of these SBU layers. A key issue for the reliability of the intercon-

* To whom correspondence should be addressed. Tel.: 0032 9264 5514; Fax: 0032 9264 5374; e-mail:David.Schaubroeck@elis.ugent.be

© Koninklijke Brill NV, Leiden, 2012 DOI:10.1163/156856111X623104

Journal of Adhesion Science and Technology 26 (2012) 2301–2314

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2 D. Schaubroeck et al. /

nections is to obtain good adhesion between the deposited copper and the dielectriclayers.

Surface properties of dielectric materials, used as build-up layers in electroniccircuits, have a large impact on the adhesion strength of electrochemically depositedmetals. In most cases, these dielectric materials are composed of epoxy resins. Al-tering the chemical and physical characteristics of epoxy surfaces can result in anincreased adhesion with deposited metals [2, 3]. Wet-chemical treatments are pre-ferred for altering the characteristics of polymer surfaces due to their low cost incomparison to plasma or vacuum processes and their straightforward integration ina printed circuit board production line. These treatments include swelling, etchingby chemical oxidation, and chemical surface modification reactions [4–8].

Generally, the introduction of amine groups on the surface of polymers, siliconand other substrates leads to an increased adhesion of electroless plated copper [6,7, 9–13]. This increased adhesion can be ascribed to a better chemical and/or elec-trostatic bonding between (protonated) amine groups and Pd/Sn colloids [10, 12].These colloids are used to activate the dielectric surface and subsequently initiatethe copper electroless deposition [12].

Another compound that shows increased adhesion towards metals is poly-dopamine. In 2007, Messersmith and coworkers developed a mussel adhesive pro-tein (MAP) inspired polydopamine coating which can be deposited on a wide rangeof materials (polymers, ceramics, metals, etc.) [14]. This synthetic coating is ap-plied via a straightforward dopamine oligomerisation in aqueous marine conditions.The exact molecular composition of the final coating is not clear yet. However,amine, catechol and hydroquinone functions are believed to be present on the sur-face of this coating [15]. Usually, the thickness of this coating is 5–50 nm and canbe increased by multiple dopamine additions [16]. The polydopamine coating canbe used to covalently attach a variety of bio(macro)molecules to different substratesurfaces [15, 17]. More important in this work, the catechol groups are known tohave a high affinity for transition metals [18–20]. Therefore, increased adhesion todeposited copper can be anticipated.

In this study, the polydopamine surface modification is used to improve adhesionof epoxy resins to electroless deposited copper. Especially the epoxy surfaces withlower roughness are targeted due to the need for higher density substrates withsmaller features [8, 21]. In order to obtain smaller features, one has to minimize theroughness treatment of dielectric materials for two reasons. First, smaller featuresrequire the use of thin conductors. Second, low roughness is needed to avoid highfrequency losses caused by the skin effect [22, 23]. Since surface roughness is oneof the key treatments for the improvement of (metal) adhesion, chemical surfacemodification can compensate for this lower adhesion.

This work describes the surface modification of roughened epoxy resins usingpolydopamine. The polydopamine modified surfaces and their stability towards theacidic environment during electroless copper plating are investigated with XPS andToF-S-SIMS. Both the influence of this modification and the influence of the elec-

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troless plating bath temperature on the adhesion to electrolytically deposited copperare examined via peel tests and interface analyses using SEM.

2. Experimental

2.1. Preparation of a Roughened Photo-Imageable Polymer Layer on FR4

Probelec XB 7081™, provided by Huntsman, is a liquid photo-imageable dielec-tric (PID). It contains an epoxy cresol novolac (ECN) prepolymer dissolved in1-methoxy-2-propylacetate, a cationic photoinitiator and different types of fillers.Probelec™ is dip-coated on an FR4 substrate (Rogers, Belgium) (10 cm × 10 cm;1.2 mm thick). The substrate is submitted to different drying, illumination and cur-ing steps as described previously [24]. The final layer has a uniform thickness ofapproximately 40 µm. Roughness is introduced by swelling for 10 min followed bychemical etching for 1–4 min at 80°C in Shipley Circuposit Hole Prep 4425 andKMnO4/NaOH (Shipley Circuposit 3310) solutions, respectively [4].

2.2. Polydopamine Modification of Roughened ECN Surfaces

Polydopamine modification was carried out according to the method described byMessersmith et al. [14]. First, a buffer solution of 10 mM TRIS-HCl (pH 8.5) wasprepared in deionized water. Next, the substrates were introduced into the buffersolution. Subsequently, 2 mg/ml of dopamine. HCl (Sigma Aldrich, Belgium) wasadded. The substrates were left for 24 h in this solution under ambient air with slowstirring. Finally, the substrates were rinsed thoroughly with deioinized water anddried via nitrogen blowing. This procedure was taken from [14].

2.3. Electroless and Galvanic Copper Deposition

Electroless copper was deposited on the substrates via the 1-step Shipley process(Fig. 1) using Pre-dip 3340, Catalyst 3344 and Circuposit 3350-1 (electroless plat-ing bath) purchased from Rohm & Haas. The galvanic deposition was performedin Electroposit 1300 provided by Rohm & Haas. All solutions were prepared withdeionized water.

Both the electroless and the galvanic copper deposition steps were followed by adrying step in a convection oven for 1 h at 150°C. The total copper thickness of thesamples was between 17 and 25 µm.

2.4. Analysis Methods

Scanning electron microscope (SEM) analysis was performed on a JEOL JSM-5600 instrument. This SEM instrument is equipped with an electron microprobeJED 2300 and an EDS (Energy Dispersive Spectroscopy) detector for elementalanalysis. SEM-EDS is capable of detecting all elements from carbon to uranium,with a detection limit of circa 0.2 wt% for most elements [25]. Prior to analysis, allsamples were coated with a thin gold layer (circa 20 nm) using a plasma magnetronsputter coater.

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Figure 1. Schematic illustration of the one-step electroless and galvanic processes used in this work.

XPS (X-ray Photoelectron Spectroscopy) measurements were performed on anESCA S-probe VG monochromatic spectrometer with an Al Kα X-ray source(1486 eV). A survey scan spectrum was taken from a spot of 100 µm by 200 µmon the sample. The spectral information appears from a depth of approximately5–10 nm for most polymers. XPS is inherently quantitative. This means thatthe intensity of a peak is directly proportional to the density of the atom fromwhich it derives (under fixed experimental conditions) within the sampled volume[26]. The relative atomic concentration CA of any chosen element A is obtainedfrom

CA = IA/SA∑

n(In/Sn),

where CA is expressed in at%. In is the measured peak area and Sn is the relativesensitivity factor for the specific element n. The above equation is only valid if thesolid sample is homogeneous or at least homogeneous within the sample depth.

In all spectra, the carbon 1s peak was set at 285 eV to calibrate the spectrum.After this calibration, following peak maxima were used to assign and integrate

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peaks from the other elements observed in the spectra: 531 eV (O 1s), 400 eV (N1s) and 201 eV (Cl 2p). The Casa XPS software package (from Casa Software Ltd,UK) was used to integrate the peaks. The peak areas were defined after a Shirleybackground fitting. The relative sensitivity factors (Sn) of the analyzed elements areprovided by the software which uses the Scofield yield factors [27]: SC (1s) = 1.00,SN (1s) = 1.80, SO (1s) = 2.93 and SCl (2p) = 2.29.

The ToF-S-SIMS (Time of Flight Static Secondary Ion Mass Spectrometry) anal-yses were carried out on an IONTOF V instrument (IONTOF, Münster, Germany)equipped with a Binq+ (n = 1−5, q = 1,2) liquid metal ion gun (LMIG). Massspectra were recorded by bombarding areas typically of 500 µm × 500 µm with25 keV Bi3+ ions using the so-called ‘high current bunch mode’, featuring highmass resolution and a beam spot diameter of about 2 µm. The acquisition time oftypically 100 s results in a total ion-dose density of 2.5×1011 ions cm−2. The pulselength of 1 ns allows a mass resolution of 5000 (FWHM) to be achieved at m/z 15,while the mass accuracy is lower than 100 ppm up to m/z 850. Mass spectra wererecorded for at least three distinct spots on each sample. Positive ion mass calibra-tion was performed using the ion signals at m/z 1 (H+), 15 (CH3

+), 29 (C2H5+)

and 43 (C3H7+). For the negative ion mass calibration, the ions at m/z 1 (H−), 12

(C−), 17 (OH−) and 42 (CNO−) were used. Charge compensation was performedusing low-energy electrons from a flood gun. Static secondary ion mass spectrom-etry is widely recognized as the method of choice for molecular surface analysiswith an information depth of essentially one monolayer.

The surface roughness of the samples were measured with a Wyko NT3300 non-contact optical profiler.

2.5. Peel Tests

The 90° peel tests are performed using a Dage Series 4000 equipment with a 5 kgcartridge at a peel speed of 500 µm/s.

3. Results and Discussion

3.1. Overview of Starting Material Characteristics

The composition of cured Probelec™ consisted of two major compounds:

• A photo-cured epoxy cresol novolac resin.

• Inorganic fillers: CaCO3–MgCO3 and SiO2.

In this work, Probelec resin surface is roughened with a hot KMnO4/NaOHsolution. The etching process introduces surface roughness via pore formation(Fig. 2). A linear correlation between the etching time and the roughness is ob-served (Fig. 3).

In an earlier work, the Probelec resin surface was characterized chemically andphysically via ATR-IR and SEM-EDS before and after etching [7]. Briefly, this

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Figure 2. Images obtained from the initial and roughened ECN resin surface via SEM (secondaryelectron detector) and optical profilometry (WYKO). The scale of the surface profiles is micrometer.

Figure 3. Plot of root mean square surface roughness (Rrms) versus etch time.

etching process includes two phenomena:

• Polymer chain breakdown via chemical oxidation.

• Decrease of the calcite filler concentration.

3.2. Polydopamine Surface Modification and Characterization

Surfaces which had been etched for 1, 2, 3 or 4 min were modified with poly-dopamine as described in the experimental section. Generally, the influence of thismodification step on the roughness is negligible.

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Table 1.XPS analyses of three substrates: non-modified (1 and 2), polydopamine modified (1-DOPA and 2-DOPA) and polydopamine modified samples after HCl stability tests (1-DOPA-HCl and 2-DOPA-HCl)

Substrate Etch time # of C (at%) O (at%) N (at%) Cl (at%) N/C (%)(min) measurements

1 1 3 78 ± 0.5 22 ± 0.5 — — —2 2 3 77 ± 0.1 23 ± 0.1 — — —1-DOPA 1 6 70 ± 0.4 24 ± 2.3 6.4 ± 1.3 — 9.1 ± 1.82-DOPA 2 6 73 ± 2.0 22 ± 0.7 5.3 ± 1.3 — 7.3 ± 2.01-DOPA-HCl 1 4 73 ± 0.7 22 ± 0.6 5.0 ± 0.9 — (0.4) 6.8 ± 1.22-DOPA-HCl 2 4 74 ± 0.4 22 ± 0.9 4.2 ± 1.2 — (0.3) 5.7 ± 1.7

XPS analysis was performed to obtain the surface atomic composition beforeand after the modification (Table 1). Before modification, only carbon and oxygenatoms are present on the surface. Nitrogen is introduced after the modification stepwith polydopamine.

Theoretically the N/C ratio of a fully covered surface is 1/8 (12.5%). Here, N/Cratios of 7.3 and 9.1% are obtained in case of 1-DOPA and 2-DOPA, respectively.This can be ascribed to incomplete polydopamine coverage, a polydopamine layerthickness smaller than the XPS penetration depth (5–10 nm) or the rough nature ofthe surface.

ToF-S-SIMS analyses of polydopamine modified substrates reveal several addi-tional peaks compared to the original surface (not shown). In the negative spectrumadditional peaks at m/z 26 (CN−) and 42 (CNO−) are observed. This means thatnitrogen containing species are present on the surface. The positive spectra revealadditional peaks at m/z 30 (CH2NH2

+), 123 (C7H7O2+) and 402. The fragments

at m/z 30 and 123 originate from dopamine-like structures. Until now, we are notable to ascribe the intense peak at m/z 402 to a specific fragment. Probably it issome kind of trimer of dopamine.

3.3. Polydopamine Stability Tests Towards Acidic Environment

During the electroless deposition process, the samples were immersed in the Pre-dip solution (1 min) followed by a colloidal Pd/Sn suspension (3 min) (Fig. 1). Bothsolutions have a pH value of roughly 1 due to the presence of hydrochloric acid. Inorder to test the stability of the polydopamine coating in the above-mentioned acidicsolutions, they were immersed in 1 M HCl for 4 min.

XPS analyses after these stability tests show a small decrease in the nitrogencontent on the surface (Table 1). This nitrogen content decrease can be ascribedto desorption of protonated (poly)dopamine. Nevertheless, polydopamine is stillpresent after the HCl treatment. Thus, bonding (electrostatic or coordinative) of thecatechol and surface amine with the negatively charged Pd/Sn colloids is possibleduring the electroless process steps.

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Table 2.Overview of all substrates and their specific electroless plating conditions

Substrate Etch time Electroless Cu plating Electroless Cu plating Peel strength(min) temperature (°C) time (min) (N/mm)

1-LOW 1 35 60 NA2-LOW 2 35 60 0.41 ± 0.023-LOW 3 35 60 0.69 ± 0.074-LOW 4 35 60 0.91 ± 0.051-HIGH 1 47 45 NA2-HIGH 2 47 45 0.37 ± 0.033-HIGH 3 47 45 0.60 ± 0.044-HIGH 4 47 45 0.57 ± 0.091-LOW-DOPA 1 35 60 0.18 ± 0.042-LOW-DOPA 2 35 60 0.36 ± 0.093-LOW-DOPA 3 35 60 0.82 ± 0.024-LOW-DOPA 4 35 60 0.96 ± 0.041-HIGH-DOPA 1 47 45 0.28 ± 0.072-HIGH-DOPA 2 47 45 0.91 ± 0.073-HIGH-DOPA 3 47 45 0.34 ± 0.034-HIGH-DOPA 4 47 45 0.38 ± 0.03

The term ‘LOW’ in the substrate column means that the electroless copper deposition on this sub-strate was performed at 35°C. The terms ‘HIGH’ and ‘LOW’ in the substrate column mean that theelectroless copper deposition on this substrate was performed at 47 and 35°C, respectively. The nameof the substrates modified with polydopamine contains the suffix ‘DOPA’. The last column representsthe peel strength (± estimated standard deviation) obtained after galvanic plating.

3.4. Copper Deposition on Polydopamine Modified and Non-Modified EtchedECN Substrates

Both modified and non-modified substrates were coated with copper via electrolesscopper deposition (2–5 µm) followed by galvanic copper deposition (15–20 µm).An overview of the samples plated with copper is presented in Table 2. Two elec-troless plating temperatures were applied: 35 and 47°C. The ideal temperaturedescribed by the supplier for the electroless copper bath is 47°C.

3.5. Influence of Electroless Plating Temperature and Polydopamine Modificationon the Adhesion to Plated Copper

Figure 4 is a graph of the measured adhesion (peel) strength versus the roughnessfor non-modified copper plated samples at low (35°C) and high (47°C) electrolessplating bath temperatures. Increased roughness leads to higher adhesion strengthdue to increased mechanical interlocking. Unexpectedly, the prescribed plating tem-perature (47°C) results in lower adhesion compared to lower plating temperature.The explanation for this phenomenon is given below.

The results obtained for polydopamine modified substrates are even much morecomplicated. A study of the interface is needed to explain these results. Three spe-

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Figure 4. Peel strength (adhesion strength) versus Rrms surface roughness for non-modified and poly-dopamine modified ECN resins plated with copper. Each graph contains two series of measurements,one at low (35°C) and one at high (47°C) electroless plating temperatures.

Figure 5. Three different areas for SEM investigation on the copper plated samples after peel test.

cific areas were inspected using an SEM detector in the backscattered electronmode: the backside of the copper strip, the peel-off area, and the interface (Fig. 5).The last is obtained via embedding the sample in epoxy resin followed by severalgrinding and polishing steps.

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(a) (b)

(c) (d)

Figure 6. Compositional contrast images obtained with backscattered SEM analysis from the backsideof the four different copper strips. (a) Sample 4-HIGH with a peel strength of 0.57 N/mm. (b) Sam-ple 4-HIGH-DOPA with a peel strength of 0.38 N/mm. (c) Sample 4-LOW with a peel strength of0.91 N/mm. (d) Sample 4-LOW-DOPA with a peel strength of 0.96 N/mm. SEM-EDS analysis indi-cated (not shown here) that the black areas on the images were composed of carbon and oxygen.

A closer look at the backside of the copper strips reveals large differences be-tween low and high electroless plating temperatures (Fig. 6). Higher plating tem-peratures lead to a larger amount of small craters in the copper, especially on sample4-HIGH-DOPA and to a lesser amount on sample 4-HIGH. The origin of the smallcraters can be found in the incomplete filling of the pores with electroless copperdue to higher deposition speed. This effect is more pronounced in case of poly-dopamine modified samples. This may be ascribed to a higher concentration ofinitiating Pd0 species on the surface due to the surface modification. Therefore, thefailure is mainly in the mechanically weak copper in case of high plating tempera-ture.

In contrast, the substrates plated at low temperature show almost no craters.Moreover, larger amounts of carbon are present on these copper strips (black re-gions). This means that the failure is partially located in the ECN resin which leadsto higher peel strengths (>0.8 N/mm). The same effect can be seen in the imagesof the peel-off area (Fig. 7). A higher amount of copper is present in case of higher

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(a) (b)

(c) (d)

Figure 7. Compositional contrast images obtained with backscattered SEM analysis from the‘peel-off’ areas. (a) Sample 4-HIGH with a peel strength of 0.57 N/mm. (b) Sample 4-HIGH-DOPAwith a peel strength of 0.38 N/mm. (c) Sample 4-LOW with a peel strength of 0.91 N/mm. (d) Sample4-LOW-DOPA with a peel strength of 0.96 N/mm. SEM-EDS analysis indicated (not shown here) thatthe black areas on the images are composed of carbon and oxygen. The white areas were composedof copper.

plating temperature. Again, this effect is more pronounced in case of polydopaminemodified substrates. In general, the other samples with lower roughness (not shownhere) showed the same trends, although less pronounced.

Interface analyses confirm the conclusions stated above. Cross-sectional analysisof the plated substrates indicates that lower plating temperature leads to better fillingof the pores with electroless plated copper (Fig. 8). Therefore, increased mechanicalinterlocking is achieved; hence, higher peel strengths are obtained.

Two major conclusions can be drawn from these SEM analyses:

• Electroless copper grows faster on polydopamine modified substrates due toincreased concentration of Pd/Sn colloids on the surface.

• A lower temperature of this specific electroless plating bath (35°C) results in abetter filling of the pores and, therefore, leads to increased adhesion strength.

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(a) (b)

(c) (d)

Figure 8. Compositional contrast images obtained with backscattered SEM analysis from copper–epoxy interfaces of four samples. (a) Sample 4-HIGH with a peel strength of 0.57 N/mm. (b) Sam-ple 4-HIGH-DOPA with a peel strength of 0.38 N/mm. (c) Sample 4-LOW with a peel strength of0.91 N/mm. (d) Sample 4-LOW-DOPA with a peel strength of 0.96 N/mm. The scale bar indicated onimage (a) is also valid for the other images.

This information from the SEM analyses facilitates further interpretation ofthe obtained adhesion strength values versus roughness from copper plated poly-dopamine modified and non-modified samples (Fig. 4). Non-modified substratesreveal higher peel strengths in case of lower plating temperature due to better fill-ing of the pores. Substrates with an Rrms lower than 400 nm are not possible to platewith electroless copper. Hence, they are not included in the graph. In contrast, thepolydopamine modified samples with similar roughness can be plated with electro-less copper due to a sufficient Pd/Sn colloid surface concentration.

Except from the substrate 2-HIGH-DOPA (Table 2 and Fig. 4), high plating tem-perature applied to polydopamine modified substrates leads to low peel strengths.The peel strengths are even lower compared to non-modified samples, presumablydue to faster electroless deposition caused by higher Pd/Sn colloid surface con-centration. In case of lower plating temperature, the peel strength increases withincreasing roughness due to improved mechanical interlocking caused by betterpore filling.

Sample 2-HIGH-DOPA shows unexpected high peel strength compared to allother samples with a similar roughness. Up to now, this can only be ascribed toincreased chemical adhesion. More experiments are needed to gain deeper insightinto these observations.

4. Conclusion

Generally, higher peel strengths are obtained in case of polydopamine modificationand lower temperature of the electroless plating bath. The latter causes a more opti-mal filling of the pores which leads to increased mechanical interlocking. However,one unexpected result shows that increased adhesion due to polydopamine modi-

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fication is possible on samples with lower roughness. More research is needed toobtain a full explanation for these observations.

In future, the influence of the polydopamine modification on the surface con-centration of initiating species (Pd/Sn colloids or Pd0 containing complexes) needsto be examined. Next, the deposition speed of the electroless copper on (modified)substrates has to be monitored as function of plating temperature. Finally, the influ-ence of deposition speed, polydopamine modification and surface concentration ofinitiating species on the adhesion strength should be studied.

Acknowledgements

The FWO-Flanders is gratefully acknowledged for the financial support. The au-thors appreciate technical support and valuable discussions with A. Gielen and Dr.D. Cuypers.

References

1. C. F. Coombs, Printed Circuits Handbook. McGraw-Hill, New York (2001).2. S. Siau, A. Vervaet, S. Nalines, E. Schacht and A. Van Calster, J. Electrochem. Soc. 151, C816–

C830 (2004).3. S. Siau, A. Vervaet, S. Nalines, E. Schacht and A. Van Calster, J. Electrochem. Soc. 151, C831–

C849 (2004).4. S. Siau, A. Vervaet, L. Van Vaeck, E. Schacht, U. Demeter and A. Van Calster, J. Electrochem.

Soc. 152, C442–C455 (2005).5. S. Siau, A. Vervaet, E. Schacht, S. Degrande, K. Callewaert and A. Van Calster, J. Electrochem.

Soc. 152, D136–D150 (2005).6. D. Schaubroeck, J. De Baets, T. Desmet, P. Dubruel, E. Schacht, L. Van Vaeck and A. Van Calster,

Appl. Surface Sci. 256, 6269–6278 (2010).7. D. Schaubroeck, J. De Baets, T. Desmet, S. Vanvlierberghe, E. Schacht and A. Van Calster, Appl.

Surface Sci. 255, 8780–8787 (2009).8. J. Ge, R. Tuominen and J. K. Kivilahti, J. Adhesion Sci. Technol. 15, 1133–1143 (2001).9. W. J. Dressick, C. S. Dulcey, J. H. Georger, G. S. Calabrese and J. M. Calvert, J. Electrochem.

Soc. 141, 210–220 (1994).10. Z. J. Yu, E. T. Kang and K. G. Neoh, Polymer 43, 4137–4146 (2002).11. M. Charbonnier, M. Romand, E. Harry and M. Alami, J. Appl. Electrochem. 31, 57–63 (2001).12. D. Zabetakis and W. J. Dressick, ACS Appl. Mater. Interfaces 1, 4–25 (2009).13. Z. J. Yu, E. T. Kang and K. G. Neoh, Langmuir 18, 10221–10230 (2002).14. P. B. Messersmith, H. Lee, S. M. Dellatore and W. M. Miller, Science 318, 426–430 (2007).15. P. B. Messersmith, H. Lee and J. Rho, Adv. Mater. 21, 431–434 (2009).16. V. Ball, F. Bernsmann, A. Ponche, C. Ringwald, J. Hemmerle, J. Raya, B. Bechinger, J. C. Voegel

and P. Schaaf, J. Phys. Chem. C 113, 8234–8242 (2009).17. C. B. Park, S. H. Ku, J. Ryu, S. K. Hong and H. Lee, Biomaterials 31, 2535–2541 (2010).18. P. B. Messersmith, Science 328, 180–181 (2010).19. L. Q. Zhang, W. C. Wang, Y. Jiang, Y. Liao, M. Tian and H. Zou, J. Colloid Interface Sci. 358,

567–574 (2011).20. F. Zhou, Q. Ye and W. M. Liu, Chem. Soc. Rev. 40, 4244–4258 (2011).

2313D. Schaubroeck et al. / J. Adhesion Sci. Technol. 26 (2012) 2301–2314

Dow

nloa

ded

by [

Uni

vers

ity o

f G

ent]

, [M

r D

avid

Sch

aubr

oeck

] at

04:

37 2

0 A

ugus

t 201

2

14

21. S. Siau, A. Vervaet, A. Van Calster, I. Swennen and E. Schacht, Appl. Surface Sci. 237, 456–461(2004).

22. H. W. Johnson and M. Graham, High-Speed Signal Propagation: Advanced Black Magic. PrenticeHall, Upper Saddle River, NJ (2003).

23. A. Deutsch, C. W. Surovic, R. S. Krabbenhoft, G. V. Kopcsay and B. J. Chamberlin, IEEE Trans.Adv. Packaging 30, 279–287 (2007).

24. S. Siau, A. Vervaet, S. Degrande, E. Schacht and A. Van Calster, Appl. Surface Sci. 245, 353–368(2005).

25. J. Goldstein, Scanning Electron Microscopy and X-Ray Microanalysis. Kluwer Academic/PlenumPublishers, New York (2003).

26. D. Briggs and J. T. Grant, Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, p.345. IM Publications, Chichester (2003).

27. J. H. Scofield, J. Electron Spectr. Related Phenom. 8, 129–137 (1976).

2314 D. Schaubroeck et al. / J. Adhesion Sci. Technol. 26 (2012) 2301–2314

Dow

nloa

ded

by [

Uni

vers

ity o

f G

ent]

, [M

r D

avid

Sch

aubr

oeck

] at

04:

37 2

0 A

ugus

t 201

2