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Laser-assisted metallization of composite coatings containing copper(II) acetylacetonate and copper(II) oxide or copper(II) hydroxide
Surface & Coatings Technology 259 (2014) 660–666
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Laser-assisted metallization of composite coatings containing copper(II) acetylacetonate and copper(II) oxide or copper(II) hydroxide Piotr Rytlewski ⁎ Department of Materials Engineering, Kazimierz Wielki University, ul. Chodkiewicza 30, 85-064 Bydgoszcz, Poland
a r t i c l e
i n f o
Article history: Received 2 July 2014 Accepted in revised form 4 October 2014 Available online 14 October 2014 Keywords: Polymer Coating Composite Laser Metallization
a b s t r a c t This article presents the results of laser-assisted metallization of polyurethane coatings containing 5 wt.% of Cu(acac)2 and 15 wt.% of copper(II) oxide (CuO) or copper(II) hydroxide (Cu(OH)2). Thus, two different coating compositions were compared with regard to becoming active after laser irradiation for direct electroless metallization. Coatings were irradiated with ArF excimer laser using various numbers of laser pulses at a constant fluence of 100 mJ/cm2, and then electroless metallized with copper. Surface properties resulting from laser irradiation were evaluated mainly based on surface morphology, chemical alterations and general optical assessment of deposited copper layers. It was evaluated using scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and optical imaging. It was found that surface layer of the coatings containing Cu(OH)2 could be activated, and thus copper plated at lower energy dose of laser radiation as compared to the coatings containing CuO. It resulted from more efficient laser ablation and formation of metallic copper on the coating surface. Empirical model for the change of copper content resulting from laser radiation and practical conclusions on the applicability of the coatings is presented in this work. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electroless metallization of polymer materials is a crucial process in manufacturing of electric, electronic and mechatronic devices. Metallic circuit patterns are extremely important in production of printed circuit boards (PCBs), flexible PCBs, magnetic data storage, multichip modules (MCMs) packaging, ultra large scale integrated (ULSI) circuits, and molded interconnect devices (MIDs) [1–3]. However, electroless metallization of polymer materials cannot be realized directly and surface pretreatment is required. In mass production scale polymer surface is chemically modified by means of various bath treatments to become clean, rough, oxidized and seeded with catalyst species. Many surface preparation procedures are widely reported which differ mainly in compositions, numbers and sequences of bath treatments. Due to ecological and occupational risk associated with chemical treatments physical surface modification techniques are needed and thus intensively developed [4–6]. Plasma modification is one of the most applied physical surface treatments in preparation of the polymer surface for electroless metallization. The main aim of this technique is to implement nitrogen species on the polymer surface which adsorb palladium ions from solution. Palladium atoms are highly electronegative and thus are effectively catalyzing reduction of metal ions from solution. Although expensive, palladium compounds are commonly applied in ⁎ Tel.: +48 52 3419372.
http://dx.doi.org/10.1016/j.surfcoat.2014.10.015 0257-8972/© 2014 Elsevier B.V. All rights reserved.
industry as catalysts for electroless metallization. The significant disadvantage of both chemical and plasma surface treatments is the lack of selectivity for the surface area to be treated. Lasers can be useful tools for the preparation of polymer surface to be coated with metal layer. In the PVD, CVD and electroless metallization techniques lasers can be used to clean, roughen, and/or induce chemical reactions on polymer surface prior to or along with metallization process. Conventional methods of laser surface pretreatments consist in surface oxidation along with etching [7–9]. It was found that irradiation with laser was a suitable pretreatment for metallization of polymer surfaces in these cases, where the original surface roughness was significantly increased. In the successive studies, adhesion improvement was also associated with formation of covalent bonds between aluminum and oxidized PET surface (Al–O–C) [10–12]. This type of treatment was suitable mainly for evaporated metallization of polymers improving quality of the polymer/metal interfaces. In the case of autocatalytic metallization, polymer surface has to become activated, thus other laser pretreatment procedures were proposed. Polymers were irradiated with ArF excimer laser in hydrazine atmosphere [13–15]. The water contact angle of the modified surfaces decreased significantly (from 130° to 30 °C) due to the reactions with hydrazine leading to grafting of amino groups onto the surface. These amino groups were grafted selectively, thus enabled selective electroless seeding with palladium as a catalyst for electroless metallization. Similar approach was also proposed by Romand and coworkers [16–18].
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for the change of copper content resulting from laser radiation and practical conclusions on the applicability of the coatings is presented in this work. 2. Materials and methods 2.1. Materials
Fig. 1. Cross-section of PET/coating/copper layer from bottom to top, respectively.
Laser surface modification can also involve prior compositional modification of polymer material by incorporation of photoactive components, resulting in more intensive surface alteration. In effect, surface becomes highly activated, and thus susceptible for direct selective electroless metallization. However, minor reports on this type of surface activation can be found in scientific and technical literatures [19,20]. Recently, several articles were published on application of copper(II) oxide with copper(II) acatylacetonate in polyamide thermoplastic polymers which were subjected to laser irradiation and electroless metallization [21,22]. In this approach copper compounds were introduced into the polymer matrix at the stage of polymer extrusion, thus were uniformly dispersed in all material volume. This way is not cost-effective because laser-induced chemical alterations occur only in thin polymer surface layer. Another disadvantage can be the deterioration of mechanical properties of extruded or injection molded polymers containing copper compounds. For that reason, polymers containing amide groups are preferred being more resistant to the degradation affected by copper and copper compounds as compared e.g. to polyesters. In this work copper compounds were introduced to polyurethane coatings which are intended to coat poly(ethylene terephthalate) (PET) and after laser irradiation be electrolessly metallized. This technique has some advantages over previously proposed, in which copper compounds and polymer were mixed in all material volume as an effect of extrusion and injection molding [23,24]. Firstly, no heat treatment during production procedure is exerted on coatings, thus no heat activated reaction occurs and the coatings can be chemically stable. Moreover, less copper compounds are used because they are located only in thin polymer coating instead of all material volume. In this work application of copper hydroxide combined with copper(II) acetylacetonate is firstly reported. Cu(OH)2 is not thermally stable in the processing temperature of most thermoplastics. Therefore, its application in laser-assisted metallization of coatings is particularly justified and can result in more intensive laser-induced reactions. A comparative analysis between CuO and Cu(OH)2 each compounded with Cu(acac)2 and introduced into polyurethane matrix is presented, empirical model
The following materials were used in the investigations: (i) polyurethane coating type B4060 (Haering, Germany), poly(ethylene terephthalate) (Boryszew, Poland) as a substrate for polyurethane coating, (iii) copper(II) acetylacetonate (Cu(acac)2), (iv) copper(II) oxide (CuO), (v) copper(II) hydroxide (Cu(OH)2), and (vi) metallization bath type M-Copper 85 (MacDermid, USA). Poly(ethylene terephthalate) sheets of the size 60 × 60 × 1 mm were manufactured using an injection molding machine type 80 Eco TRX 60 (Tederic Inc., Taiwan). Polyurethane resin was mechanically mixed with 5 wt.% of Cu(acac)2 and either 15 wt.% of CuO (herein referred to as coating A) or 15 wt.% Cu(OH)2 (herein referred to as coating B). The coatings were deposited on poly(ethylene terephthalate) sheets by immersion method. The thickness of the coatings was from about 200 to about 300 μm. Exemplary image for cross-section of PET/coating/copper layer is presented in Fig. 1. 2.2. Surface treatment Deposited coatings were irradiated with ArF excimer laser type LPX 300 (Coherent, USA) using raw laser radiation. The size of laser beam spot was about 10 × 23 mm. The following irradiation parameters were applied: (i) laser wavelength: 193 nm (corresponding to a photon energy equal to 6.4 eV), (ii) laser fluence (pulse energy per unit area): 100 mJ/cm2; (iii) frequency: 5 Hz; and (iv) laser pulse duration: about 20 ns. The aim of laser irradiation was to induce formation of copper clusters on the coating surface, thus making the surface active for electroless metallization. Coatings were irradiated with different number of laser pulses at fluence (100 mJ/cm2) above the ablation threshold of the polymer coatings (typically less than 30 mJ/cm2). The exact value
Table 1 Designations of samples relating to the number of laser pulses. Number of laser pulses
Coating A (Cu(acac)2/CuO)
Coating B (Cu(acac)2/Cu(OH)2)
0 10 50 100 500
A0 A1 A2 A3 A4
B0 B1 B2 B3 B4
Fig. 2. Photos of laser irradiated and electrolessly metallized samples.
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Fig. 3. Mass loss and its temperature derivative for powders Cu(OH)2 and Cu(acac)2.
of the laser fluence was determined using an energy meter type II TOP FieldMax (Coherent Inc., USA). For the clarity of presented results coatings A and B were designated due to the number of laser pulses they were exposed to, as listed in Table 1. Laser irradiated coatings were electrolessly metallized using a sixcomponent copper plating bath type M-85 Copper (MacDermid, USA) with formaldehyde as a reducing agent. Coatings were metallized for 60 min at temperature of 46 °C while metallization bath was continuously aerated. 2.3. Examination procedures Coating ability to be electrolessly metallized was evaluated optically based on the surface area plated with copper. These observations were documented by photographs. Powders of Cu(acac)2, CuO and Cu(OH)2 as well as coatings A and B were studied by means of thermogravimetric analysis (TG) using apparatus Q500 (TA Instruments, USA). The tests were performed in nitrogen atmosphere at a flow of 60 ml/min with heating rate of 10 °C/min and temperature upper limit of 600 °C. Scanning electron microscopy (SEM) was performed using a microscope SU8010 (Hitachi, Japan). Images were recorded at an accelerating voltage of 15 kV and beam electron current of about 60 μA. Thin gold layer of about 2 nm thickness was deposited on non-metallized coatings for better imagining of the surface morphology
Fig. 5. Percentage content of copper atoms determined by EDX analysis.
using sputter coater with thickness monitor (Cressington, United Kingdom). Energy-dispersive X-ray (EDX) spectroscopy was also performed in order to analyze elemental composition of laser-activated coatings. Surface area scanned by electron beam was 1.2 × 0.9 mm and signals were recorded for each studied coating. In the case of EDX analysis coating surface was not gold-coated. X-ray photoelectron spectroscopy (XPS) was performed using a spectrometer Scient type R3000 (VG Scienta, Sweden). This spectrometer was equipped with an Al anode emitting X-rays of photon energy 1486.6 eV. To limit destruction of tested coatings, measurements were performed at low power of radiation beam (200 W). During the measurements pressure in the chamber increased from 8°10−7 to 6°10−6 Pa, which was caused by the desorption of gaseous substances from the tested coatings. Measurements were made at an angle of photoelectron emission of 90°, which corresponds to the average 4 nm examined thickness. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy of the coatings was also performed. Final spectra have an average of 16 spectra recorded for the wavenumber ranged from 4000 to 650 cm−1. Wettability tests have been carried out using an automatic DSA 100 goniometer (Kruss, Germany) after approximately 24 h after
Fig. 4. SEM images of laser irradiated coatings.
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Table 2 Percentage content of selected atoms in surface layer of coatings A and B determined based on EDX analysis. Coating
Atomic content [wt.%]
A0 A1 A2 A3 A4 B0 B1 B2 B3 B4
C
O
Cu
49.8 46.1 43.2 38.6 35.0 49.19 29.06 26.35 26.21 25.81
47.69 38.96 30.61 26 13.52 44.76 22.8 16.35 14.65 10.1
2.49 14.07 25.7 35.19 51.01 6.05 47.36 55.91 57.79 63.29
laser irradiation. Double-distilled water and diiodomethane (Sigma Aldrich, USA) were used for contact angle measurements. The surface energy was calculated based on the average values of contact angles using Owens–Wendt method. Fig. 7. ATR-FTIR spectra for coatings A0, A1, A2, A3 and A4.
3. Results and discussion Laser irradiated and electrolessly metallized coatings A (containing CuO) and B (containing Cu(OH)2) are presented in Fig. 2. The aim of laser irradiation was to alter surface properties so that the coatings become activated (able to be directly electrolessly metallized). The coatings containing Cu(OH)2 were successfully metallized just after 10 laser pulses, while those containing CuO required more than 100 laser pulses at the same laser fluence. Copper plated area characterized with small holes, especially for sample B. It resulted from the fact that laser-induced photochemical and photothermal reactions were accompanied with eruption of gaseous by-products. Thus, based on the number of created holes (Fig. 2), one can conclude that these reactions were much more intensive in the case of coatings containing Cu(OH)2 than CuO. Copper(II) oxide is more thermally stable than copper(II) hydroxide which decomposes according to the reaction scheme [25]: ΔT
CuðOHÞ2 → CuO þ H2 O:
ð1Þ
It can be calculated based on atomic mass that Cu(OH)2 decomposes into 78 wt.% of CuO and 22 wt.% of water which is consistent with the results obtained from thermogravimetric analysis presenting the mass loss as a function of temperature (Fig. 3).
Fig. 6. Mass losses and its temperature derivatives for coatings A and B.
Thermal decomposition of Cu(acac)2 was previously studied and described as to follow the reaction scheme [26]:
ð2Þ Laser irradiation increases temperature up to some critical value (ablation threshold temperature) above which the excess of thermal energy is transformed into kinetic energy of material fragments being ejected (ablated) from the coating. The temperature resulting from laser ablation is high enough to dissociate Cu(OH)2 and Cu(acac) 2 , contrary to CuO which is thermally stable up to about 1000 °C [27]. For that reasons coating B presented more holes as compared to coating A which contained 15 wt.% of thermally stable CuO and 5 wt.% of Cu(acac)2. Laser ablation results in chemical and physical alteration of material surface layer. As a result of laser irradiation polymer matrix is ablated while copper compounds present in polymer coatings can be photochemically and/or photothermally reduced to metallic copper. Metallic copper was not ablated because ablation of copper requires the laser fluence of at least 2 J/cm2 [28] whereas the coatings were irradiated with laser fluence of 100 mJ/cm2. Laser-induced changes in surface morphologies of the coatings A and B are presented in Fig. 4. Many densely located bright areas representing copper agglomerates can be perceived on coating B1 and only few large on coating A1. With increasing irradiation dose minor changes in surface morphology were observed for coating B, whereas coating A was significantly altered up to 100 laser pulses (coating A3). These results are in accordance with those presenting the ability of coatings to be directly electroless metallized (Fig. 2). Coating A irradiated with 10 and 50 laser pulses could not be metallized contrary to coating B irradiated at the same energy doses. The rapid change in surface activation along with increasing number of laser pulses was associated with an increase in copper content in coating surface layer as determined by EDX analysis (Fig. 5). The most suitable fitting curve for the increase in copper content was found and expressed by the equation: −
CuN ¼ Cu∞ e
Nb NþNa
ð3Þ
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Fig. 8. ATR-FTIR spectra for coatings B0, B1, B2, B3 and B4.
where: N is a number of laser pulses, CuN is the percentage of copper content at N number of laser pulses, Cu∞ can be interpreted as percentage of copper content at infinite number of laser pulses, Nb defines the number of laser pulses required to attain 63 wt.% of Cu∞ value, and Na is an offset for laser pulses. The Na parameter can be interpreted as derived from the initial copper contents in unirradiated coatings A and B. The parameter Cu∞ equals to 53 and 61 for coatings A and B, respectively. The values of parameter Nb for coatings A and B were about 55 and 3, respectively. It can be expected that the presented empirical model will be valid for other coating compositions and/or irradiation conditions differing only in values of the equation parameters. This, however, should be experimentally verified based on the larger series of measurements. The increasing copper content was associated with decreasing carbon and oxygen contents as number of laser pulses increased (Table 2). This decrease was significantly more intensive in the case of coating B. It indicates that coating A was probably more resistant to laserinduced ablation as compared with coating B. This conclusion can be indirectly confirmed by the mass loss of coatings A and B as a result of temperature rise (Fig. 6).
Fig. 10. Deconvoluted Cu 2p3/2 core emission bands for coatings A0, A1 and A4.
This difference is observed in the temperature range from about 100 to about 400 °C, where coating B has about 10 wt.% less than coating A. Moreover, in coating B evident degradation subprocess occurred at about 330 °C represented by local maximum of mass derivative with respect to temperature. These results are in accordance with those presented in Fig. 2. Changes in chemical structure of the coatings were also studied using ATR-FTIR spectroscopy (Figs. 7 and 8). Characteristic change accompanied by increasing number of laser pulses was a decrease in CH2/CH3 absorption bands (at 3000–2800 cm−1) affected from ablation of polymer matrix. This decrease was relatively higher for coating B than A, however, initial intensity of these absorption
Fig. 9. FTIR spectra of CuO and Cu(OH)2.
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Table 4 Contact angles of water (ΘW) and diiodomethane (θD), surface energy (γs) with its polar (γp) and dispersive (γd) parts for the studied coatings. Coating
A A1 A2 A3 A4 B0 B1 B2 B3 B4
Fig. 11. Deconvoluted Cu 2p3/2 core emission bands for coatings B0, B1 and B4.
bands was higher for coating A than B. It might be concluded that polymer matrix of coating B was more efficiently ablated than that of coating A which is consistent with thermogravimetric results presented in Fig. 6. With increasing frequencies of infrared radiation (IR) a linear increase in absorbance spectra was observed. The slope of the baseline was significantly higher for coating B than A. Absorbance at 4000 cm− 1 for the coating A4 was less than 0.25 whereas for coating B more than 0.41. It probably resulted from the fact that laser irradiation caused ablation of polymer matrix and uncovering fillers of copper compounds. As presented in Fig. 9 (at FTIR transmission mode) absorbance was increasing with increasing wavenumber, particularly for CuO. As previously discussed, Cu(OH)2 can be laser-decomposed into CuO with H2O by-product, thus this absorbance trend would be also justified in the case of coating B, especially taking into consideration higher ablation rate for that coating. However, this characteristic change following the number of laser pulses was reversed for coatings A4 and B4. This can result from laser-induced reduction of CuO into metallic copper along with intensive changes in surface roughness, thus affecting diffraction and reflection contributing to higher IR absorbance. In the case of coating B, additional absorption band at 3569 cm− 1
Surface energy (mJ/m2)
Contact angle ΘW
θD
γp
γd
γs
99.5 96.7 108.9 127.1 137.5 92.5 104.0 107.0 122.5 144.5
69.0 55.4 39.3 18.0 13.3 72.8 44.7 24.2 21.1 19.2
1.4 0.8 0.8 9.4 15.5 4.4 0.1 1.2 6.9 18.6
22.0 30.6 44.6 60.8 65.1 18.2 39.7 52.2 58.3 64.7
23.4 31.4 45.4 70.2 80.6 22.6 39.8 53.4 65.2 83.3
attributed to OH oscillations of Cu(OH)2 was detected. Initially, it significantly increased (coating B1) because of laser-induced ablation of polymer matrix and uncovering of embedded Cu(OH)2 particles, then with successive laser pulses it decreased and finally disappeared for coating B4. Measurement range (4000–650 cm−1) of ATR-FTIR was beyond that attributed to main peaks of CuO at 535 cm−1 and Cu(OH)2 at 416 cm−1 [29], thus investigation of laser-induced reduction reactions to metallic copper cannot be comprehensively performed by this technique. Changes in chemical composition of coatings were characterized using XPS technique. XPS measurements provide compositional information at the nanometer level of the outermost surface layer of the coatings. Deconvolutions of the Cu 2p3/2 bands for the coatings A0, A1, A4 and B0, B1, B4 are presented in Figs. 10 and 11, respectively. The component peaks at around 932.4, 932.7, 933.9, and 934.7 eV were assigned to Cu(acac)2, Cu(0), CuO, and Cu(OH)2, respectively. Detailed quantitative results of XPS analysis are listed in Table 3. Intensity of photoelectrons from copper atoms was recorded for coating A0 derived from two forms of copper compounds CuO (933.9 eV) and Cu(acac) (932.4 eV). Then, after 10 laser pulses photoelectron intensity increased and photoelectrons attributed to Cu(0) were detected. Under these irradiation conditions an increase in CuO emission band was also observed, which was caused by laser ablation of polymer matrix, and thus CuO particles were uncovered. Higher energy dose of laser radiation transformed copper compounds into the pure form of Cu(0) as presented for coating A4. In the case of coating B an initial level of Cu photoelectrons (sample B0) was higher as compared with that for coating A0. The photoelectrons were derived mainly from Cu(OH)2 (934.7 eV). Based on these observations it can be concluded that polar Cu(OH)2 has tendency to migrate toward the surface of the coating. A small component peak derived from Cu(acac)2 was also noticed in coating B0. With increasing number of laser pulses (coatings B1 and B4) single peak at about 932.7 eV was fitted and attributed to Cu(0). This increase was higher as compared with that determined for coatings A1 and A4. In many applications, it is essential to provide strong adhesion between a polymeric substrate and copper conductive tracks. The basic method to evaluate this property is to conduct pull-off adhesion
Table 3 Fitting results of peak deconvolutions of XPS Cu 2p3/2 spectra. Sample
A0 A1 A4 B0 B1 B4
CuO
Cu(acac)2
Cu(OH)2
Cu(0)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
933.9 933.9 – – – –
69.5 52.4 – – – –
1.5 2.3 – – – –
– – – 934.7 – –
– – – 93.7 – –
– – – 2.1 – –
932.4 – – 932.4 – –
30.4 – – 6.3 – –
1.5 – – 1.8 – –
– 932.8 932.6 – 932.7 932.7
– 47.6 100.0 – 100 100
– 2.3 2.2 – 1.9 2.3
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tests. However, in the case of this study, adhesion of electrolessly deposited copper layer to polymer coating was higher than the polymer coating to the coated plastic sheet. The pull-off adhesion tests resulted only in determination of adhesion strength between coating and polymer sheet which was about 1 MPa. For that reason, contact angle measurements and calculation of surface energy as an indirect measure of adhesion properties were performed. Unirradiated coatings A and B characterized with low surface energy of about 23 mJ/m2 but with successive number of laser pulses it increased to 80.6 and 83.3 mJ/m2, respectively (Table 4). The main factors affecting surface energy of a material are its chemical composition and surface geometrical structure [30]. Surface chemistry of coatings A and B differed upon laser radiation whereas surface energy change followed similar trend for each of the coatings. Therefore, one can conclude that surface geometrical structure could be the main factor affecting adhesion properties of coatings. However, caution has to be paid predicting adhesion properties based on surface energy calculations. 4. Conclusions Application of CuO or Cu(OH)2 affected the overall quality and laser activation process of coating surface. It was found that the application of Cu(OH)2 accelerated laser activation and only 10 laser pulses at fluence of 100 mJ/cm2 were sufficient for direct electroless metallization of coating. On the other hand, coating containing CuO required higher energy dose of about 100 laser pulses at the same fluence. However, more coating defects represented by numerous holes appeared in coating containing Cu(OH) 2 than CuO. The main reason for the observed differences was thermal decomposition of Cu(OH)2 associated with formation of water. It probably increased laser ablation of polymer coating leading to uncovering of copper compounds and in the next step their reduction to metallic copper. The empirical model for the change of copper content in laser irradiated surface layer was also proposed. It was noticed that Cu(OH) had tendency to migrate onto the polymer surface, thus improving rate of surface activation. Generally, interactions of copper compounds, their by-products, polymer coating, and laser radiation are very complex and can have various photothermal and/or photochemical nature. This work presents some effects of these interactions which lead to formation of activated surface prepared for direct electroless metallization. Results of this work are important in developing new approaches
of laser-assisted metallization and designing new polymer coatings for that purpose. Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education as a research project No. IP2011 047371 (Iuventus Plus). References [1] [2] [3] [4]
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Laser-assisted metallization of composite coatings containing copper(II) acetylacetonate and copper(II) oxide or copper(II) hydroxide Piotr Rytlewski ⁎ Department of Materials Engineering, Kazimierz Wielki University, ul. Chodkiewicza 30, 85-064 Bydgoszcz, Poland
a r t i c l e
i n f o
Article history: Received 2 July 2014 Accepted in revised form 4 October 2014 Available online 14 October 2014 Keywords: Polymer Coating Composite Laser Metallization
a b s t r a c t This article presents the results of laser-assisted metallization of polyurethane coatings containing 5 wt.% of Cu(acac)2 and 15 wt.% of copper(II) oxide (CuO) or copper(II) hydroxide (Cu(OH)2). Thus, two different coating compositions were compared with regard to becoming active after laser irradiation for direct electroless metallization. Coatings were irradiated with ArF excimer laser using various numbers of laser pulses at a constant fluence of 100 mJ/cm2, and then electroless metallized with copper. Surface properties resulting from laser irradiation were evaluated mainly based on surface morphology, chemical alterations and general optical assessment of deposited copper layers. It was evaluated using scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and optical imaging. It was found that surface layer of the coatings containing Cu(OH)2 could be activated, and thus copper plated at lower energy dose of laser radiation as compared to the coatings containing CuO. It resulted from more efficient laser ablation and formation of metallic copper on the coating surface. Empirical model for the change of copper content resulting from laser radiation and practical conclusions on the applicability of the coatings is presented in this work. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electroless metallization of polymer materials is a crucial process in manufacturing of electric, electronic and mechatronic devices. Metallic circuit patterns are extremely important in production of printed circuit boards (PCBs), flexible PCBs, magnetic data storage, multichip modules (MCMs) packaging, ultra large scale integrated (ULSI) circuits, and molded interconnect devices (MIDs) [1–3]. However, electroless metallization of polymer materials cannot be realized directly and surface pretreatment is required. In mass production scale polymer surface is chemically modified by means of various bath treatments to become clean, rough, oxidized and seeded with catalyst species. Many surface preparation procedures are widely reported which differ mainly in compositions, numbers and sequences of bath treatments. Due to ecological and occupational risk associated with chemical treatments physical surface modification techniques are needed and thus intensively developed [4–6]. Plasma modification is one of the most applied physical surface treatments in preparation of the polymer surface for electroless metallization. The main aim of this technique is to implement nitrogen species on the polymer surface which adsorb palladium ions from solution. Palladium atoms are highly electronegative and thus are effectively catalyzing reduction of metal ions from solution. Although expensive, palladium compounds are commonly applied in ⁎ Tel.: +48 52 3419372.
http://dx.doi.org/10.1016/j.surfcoat.2014.10.015 0257-8972/© 2014 Elsevier B.V. All rights reserved.
industry as catalysts for electroless metallization. The significant disadvantage of both chemical and plasma surface treatments is the lack of selectivity for the surface area to be treated. Lasers can be useful tools for the preparation of polymer surface to be coated with metal layer. In the PVD, CVD and electroless metallization techniques lasers can be used to clean, roughen, and/or induce chemical reactions on polymer surface prior to or along with metallization process. Conventional methods of laser surface pretreatments consist in surface oxidation along with etching [7–9]. It was found that irradiation with laser was a suitable pretreatment for metallization of polymer surfaces in these cases, where the original surface roughness was significantly increased. In the successive studies, adhesion improvement was also associated with formation of covalent bonds between aluminum and oxidized PET surface (Al–O–C) [10–12]. This type of treatment was suitable mainly for evaporated metallization of polymers improving quality of the polymer/metal interfaces. In the case of autocatalytic metallization, polymer surface has to become activated, thus other laser pretreatment procedures were proposed. Polymers were irradiated with ArF excimer laser in hydrazine atmosphere [13–15]. The water contact angle of the modified surfaces decreased significantly (from 130° to 30 °C) due to the reactions with hydrazine leading to grafting of amino groups onto the surface. These amino groups were grafted selectively, thus enabled selective electroless seeding with palladium as a catalyst for electroless metallization. Similar approach was also proposed by Romand and coworkers [16–18].
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for the change of copper content resulting from laser radiation and practical conclusions on the applicability of the coatings is presented in this work. 2. Materials and methods 2.1. Materials
Fig. 1. Cross-section of PET/coating/copper layer from bottom to top, respectively.
Laser surface modification can also involve prior compositional modification of polymer material by incorporation of photoactive components, resulting in more intensive surface alteration. In effect, surface becomes highly activated, and thus susceptible for direct selective electroless metallization. However, minor reports on this type of surface activation can be found in scientific and technical literatures [19,20]. Recently, several articles were published on application of copper(II) oxide with copper(II) acatylacetonate in polyamide thermoplastic polymers which were subjected to laser irradiation and electroless metallization [21,22]. In this approach copper compounds were introduced into the polymer matrix at the stage of polymer extrusion, thus were uniformly dispersed in all material volume. This way is not cost-effective because laser-induced chemical alterations occur only in thin polymer surface layer. Another disadvantage can be the deterioration of mechanical properties of extruded or injection molded polymers containing copper compounds. For that reason, polymers containing amide groups are preferred being more resistant to the degradation affected by copper and copper compounds as compared e.g. to polyesters. In this work copper compounds were introduced to polyurethane coatings which are intended to coat poly(ethylene terephthalate) (PET) and after laser irradiation be electrolessly metallized. This technique has some advantages over previously proposed, in which copper compounds and polymer were mixed in all material volume as an effect of extrusion and injection molding [23,24]. Firstly, no heat treatment during production procedure is exerted on coatings, thus no heat activated reaction occurs and the coatings can be chemically stable. Moreover, less copper compounds are used because they are located only in thin polymer coating instead of all material volume. In this work application of copper hydroxide combined with copper(II) acetylacetonate is firstly reported. Cu(OH)2 is not thermally stable in the processing temperature of most thermoplastics. Therefore, its application in laser-assisted metallization of coatings is particularly justified and can result in more intensive laser-induced reactions. A comparative analysis between CuO and Cu(OH)2 each compounded with Cu(acac)2 and introduced into polyurethane matrix is presented, empirical model
The following materials were used in the investigations: (i) polyurethane coating type B4060 (Haering, Germany), poly(ethylene terephthalate) (Boryszew, Poland) as a substrate for polyurethane coating, (iii) copper(II) acetylacetonate (Cu(acac)2), (iv) copper(II) oxide (CuO), (v) copper(II) hydroxide (Cu(OH)2), and (vi) metallization bath type M-Copper 85 (MacDermid, USA). Poly(ethylene terephthalate) sheets of the size 60 × 60 × 1 mm were manufactured using an injection molding machine type 80 Eco TRX 60 (Tederic Inc., Taiwan). Polyurethane resin was mechanically mixed with 5 wt.% of Cu(acac)2 and either 15 wt.% of CuO (herein referred to as coating A) or 15 wt.% Cu(OH)2 (herein referred to as coating B). The coatings were deposited on poly(ethylene terephthalate) sheets by immersion method. The thickness of the coatings was from about 200 to about 300 μm. Exemplary image for cross-section of PET/coating/copper layer is presented in Fig. 1. 2.2. Surface treatment Deposited coatings were irradiated with ArF excimer laser type LPX 300 (Coherent, USA) using raw laser radiation. The size of laser beam spot was about 10 × 23 mm. The following irradiation parameters were applied: (i) laser wavelength: 193 nm (corresponding to a photon energy equal to 6.4 eV), (ii) laser fluence (pulse energy per unit area): 100 mJ/cm2; (iii) frequency: 5 Hz; and (iv) laser pulse duration: about 20 ns. The aim of laser irradiation was to induce formation of copper clusters on the coating surface, thus making the surface active for electroless metallization. Coatings were irradiated with different number of laser pulses at fluence (100 mJ/cm2) above the ablation threshold of the polymer coatings (typically less than 30 mJ/cm2). The exact value
Table 1 Designations of samples relating to the number of laser pulses. Number of laser pulses
Coating A (Cu(acac)2/CuO)
Coating B (Cu(acac)2/Cu(OH)2)
0 10 50 100 500
A0 A1 A2 A3 A4
B0 B1 B2 B3 B4
Fig. 2. Photos of laser irradiated and electrolessly metallized samples.
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Fig. 3. Mass loss and its temperature derivative for powders Cu(OH)2 and Cu(acac)2.
of the laser fluence was determined using an energy meter type II TOP FieldMax (Coherent Inc., USA). For the clarity of presented results coatings A and B were designated due to the number of laser pulses they were exposed to, as listed in Table 1. Laser irradiated coatings were electrolessly metallized using a sixcomponent copper plating bath type M-85 Copper (MacDermid, USA) with formaldehyde as a reducing agent. Coatings were metallized for 60 min at temperature of 46 °C while metallization bath was continuously aerated. 2.3. Examination procedures Coating ability to be electrolessly metallized was evaluated optically based on the surface area plated with copper. These observations were documented by photographs. Powders of Cu(acac)2, CuO and Cu(OH)2 as well as coatings A and B were studied by means of thermogravimetric analysis (TG) using apparatus Q500 (TA Instruments, USA). The tests were performed in nitrogen atmosphere at a flow of 60 ml/min with heating rate of 10 °C/min and temperature upper limit of 600 °C. Scanning electron microscopy (SEM) was performed using a microscope SU8010 (Hitachi, Japan). Images were recorded at an accelerating voltage of 15 kV and beam electron current of about 60 μA. Thin gold layer of about 2 nm thickness was deposited on non-metallized coatings for better imagining of the surface morphology
Fig. 5. Percentage content of copper atoms determined by EDX analysis.
using sputter coater with thickness monitor (Cressington, United Kingdom). Energy-dispersive X-ray (EDX) spectroscopy was also performed in order to analyze elemental composition of laser-activated coatings. Surface area scanned by electron beam was 1.2 × 0.9 mm and signals were recorded for each studied coating. In the case of EDX analysis coating surface was not gold-coated. X-ray photoelectron spectroscopy (XPS) was performed using a spectrometer Scient type R3000 (VG Scienta, Sweden). This spectrometer was equipped with an Al anode emitting X-rays of photon energy 1486.6 eV. To limit destruction of tested coatings, measurements were performed at low power of radiation beam (200 W). During the measurements pressure in the chamber increased from 8°10−7 to 6°10−6 Pa, which was caused by the desorption of gaseous substances from the tested coatings. Measurements were made at an angle of photoelectron emission of 90°, which corresponds to the average 4 nm examined thickness. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy of the coatings was also performed. Final spectra have an average of 16 spectra recorded for the wavenumber ranged from 4000 to 650 cm−1. Wettability tests have been carried out using an automatic DSA 100 goniometer (Kruss, Germany) after approximately 24 h after
Fig. 4. SEM images of laser irradiated coatings.
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Table 2 Percentage content of selected atoms in surface layer of coatings A and B determined based on EDX analysis. Coating
Atomic content [wt.%]
A0 A1 A2 A3 A4 B0 B1 B2 B3 B4
C
O
Cu
49.8 46.1 43.2 38.6 35.0 49.19 29.06 26.35 26.21 25.81
47.69 38.96 30.61 26 13.52 44.76 22.8 16.35 14.65 10.1
2.49 14.07 25.7 35.19 51.01 6.05 47.36 55.91 57.79 63.29
laser irradiation. Double-distilled water and diiodomethane (Sigma Aldrich, USA) were used for contact angle measurements. The surface energy was calculated based on the average values of contact angles using Owens–Wendt method. Fig. 7. ATR-FTIR spectra for coatings A0, A1, A2, A3 and A4.
3. Results and discussion Laser irradiated and electrolessly metallized coatings A (containing CuO) and B (containing Cu(OH)2) are presented in Fig. 2. The aim of laser irradiation was to alter surface properties so that the coatings become activated (able to be directly electrolessly metallized). The coatings containing Cu(OH)2 were successfully metallized just after 10 laser pulses, while those containing CuO required more than 100 laser pulses at the same laser fluence. Copper plated area characterized with small holes, especially for sample B. It resulted from the fact that laser-induced photochemical and photothermal reactions were accompanied with eruption of gaseous by-products. Thus, based on the number of created holes (Fig. 2), one can conclude that these reactions were much more intensive in the case of coatings containing Cu(OH)2 than CuO. Copper(II) oxide is more thermally stable than copper(II) hydroxide which decomposes according to the reaction scheme [25]: ΔT
CuðOHÞ2 → CuO þ H2 O:
ð1Þ
It can be calculated based on atomic mass that Cu(OH)2 decomposes into 78 wt.% of CuO and 22 wt.% of water which is consistent with the results obtained from thermogravimetric analysis presenting the mass loss as a function of temperature (Fig. 3).
Fig. 6. Mass losses and its temperature derivatives for coatings A and B.
Thermal decomposition of Cu(acac)2 was previously studied and described as to follow the reaction scheme [26]:
ð2Þ Laser irradiation increases temperature up to some critical value (ablation threshold temperature) above which the excess of thermal energy is transformed into kinetic energy of material fragments being ejected (ablated) from the coating. The temperature resulting from laser ablation is high enough to dissociate Cu(OH)2 and Cu(acac) 2 , contrary to CuO which is thermally stable up to about 1000 °C [27]. For that reasons coating B presented more holes as compared to coating A which contained 15 wt.% of thermally stable CuO and 5 wt.% of Cu(acac)2. Laser ablation results in chemical and physical alteration of material surface layer. As a result of laser irradiation polymer matrix is ablated while copper compounds present in polymer coatings can be photochemically and/or photothermally reduced to metallic copper. Metallic copper was not ablated because ablation of copper requires the laser fluence of at least 2 J/cm2 [28] whereas the coatings were irradiated with laser fluence of 100 mJ/cm2. Laser-induced changes in surface morphologies of the coatings A and B are presented in Fig. 4. Many densely located bright areas representing copper agglomerates can be perceived on coating B1 and only few large on coating A1. With increasing irradiation dose minor changes in surface morphology were observed for coating B, whereas coating A was significantly altered up to 100 laser pulses (coating A3). These results are in accordance with those presenting the ability of coatings to be directly electroless metallized (Fig. 2). Coating A irradiated with 10 and 50 laser pulses could not be metallized contrary to coating B irradiated at the same energy doses. The rapid change in surface activation along with increasing number of laser pulses was associated with an increase in copper content in coating surface layer as determined by EDX analysis (Fig. 5). The most suitable fitting curve for the increase in copper content was found and expressed by the equation: −
CuN ¼ Cu∞ e
Nb NþNa
ð3Þ
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Fig. 8. ATR-FTIR spectra for coatings B0, B1, B2, B3 and B4.
where: N is a number of laser pulses, CuN is the percentage of copper content at N number of laser pulses, Cu∞ can be interpreted as percentage of copper content at infinite number of laser pulses, Nb defines the number of laser pulses required to attain 63 wt.% of Cu∞ value, and Na is an offset for laser pulses. The Na parameter can be interpreted as derived from the initial copper contents in unirradiated coatings A and B. The parameter Cu∞ equals to 53 and 61 for coatings A and B, respectively. The values of parameter Nb for coatings A and B were about 55 and 3, respectively. It can be expected that the presented empirical model will be valid for other coating compositions and/or irradiation conditions differing only in values of the equation parameters. This, however, should be experimentally verified based on the larger series of measurements. The increasing copper content was associated with decreasing carbon and oxygen contents as number of laser pulses increased (Table 2). This decrease was significantly more intensive in the case of coating B. It indicates that coating A was probably more resistant to laserinduced ablation as compared with coating B. This conclusion can be indirectly confirmed by the mass loss of coatings A and B as a result of temperature rise (Fig. 6).
Fig. 10. Deconvoluted Cu 2p3/2 core emission bands for coatings A0, A1 and A4.
This difference is observed in the temperature range from about 100 to about 400 °C, where coating B has about 10 wt.% less than coating A. Moreover, in coating B evident degradation subprocess occurred at about 330 °C represented by local maximum of mass derivative with respect to temperature. These results are in accordance with those presented in Fig. 2. Changes in chemical structure of the coatings were also studied using ATR-FTIR spectroscopy (Figs. 7 and 8). Characteristic change accompanied by increasing number of laser pulses was a decrease in CH2/CH3 absorption bands (at 3000–2800 cm−1) affected from ablation of polymer matrix. This decrease was relatively higher for coating B than A, however, initial intensity of these absorption
Fig. 9. FTIR spectra of CuO and Cu(OH)2.
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Table 4 Contact angles of water (ΘW) and diiodomethane (θD), surface energy (γs) with its polar (γp) and dispersive (γd) parts for the studied coatings. Coating
A A1 A2 A3 A4 B0 B1 B2 B3 B4
Fig. 11. Deconvoluted Cu 2p3/2 core emission bands for coatings B0, B1 and B4.
bands was higher for coating A than B. It might be concluded that polymer matrix of coating B was more efficiently ablated than that of coating A which is consistent with thermogravimetric results presented in Fig. 6. With increasing frequencies of infrared radiation (IR) a linear increase in absorbance spectra was observed. The slope of the baseline was significantly higher for coating B than A. Absorbance at 4000 cm− 1 for the coating A4 was less than 0.25 whereas for coating B more than 0.41. It probably resulted from the fact that laser irradiation caused ablation of polymer matrix and uncovering fillers of copper compounds. As presented in Fig. 9 (at FTIR transmission mode) absorbance was increasing with increasing wavenumber, particularly for CuO. As previously discussed, Cu(OH)2 can be laser-decomposed into CuO with H2O by-product, thus this absorbance trend would be also justified in the case of coating B, especially taking into consideration higher ablation rate for that coating. However, this characteristic change following the number of laser pulses was reversed for coatings A4 and B4. This can result from laser-induced reduction of CuO into metallic copper along with intensive changes in surface roughness, thus affecting diffraction and reflection contributing to higher IR absorbance. In the case of coating B, additional absorption band at 3569 cm− 1
Surface energy (mJ/m2)
Contact angle ΘW
θD
γp
γd
γs
99.5 96.7 108.9 127.1 137.5 92.5 104.0 107.0 122.5 144.5
69.0 55.4 39.3 18.0 13.3 72.8 44.7 24.2 21.1 19.2
1.4 0.8 0.8 9.4 15.5 4.4 0.1 1.2 6.9 18.6
22.0 30.6 44.6 60.8 65.1 18.2 39.7 52.2 58.3 64.7
23.4 31.4 45.4 70.2 80.6 22.6 39.8 53.4 65.2 83.3
attributed to OH oscillations of Cu(OH)2 was detected. Initially, it significantly increased (coating B1) because of laser-induced ablation of polymer matrix and uncovering of embedded Cu(OH)2 particles, then with successive laser pulses it decreased and finally disappeared for coating B4. Measurement range (4000–650 cm−1) of ATR-FTIR was beyond that attributed to main peaks of CuO at 535 cm−1 and Cu(OH)2 at 416 cm−1 [29], thus investigation of laser-induced reduction reactions to metallic copper cannot be comprehensively performed by this technique. Changes in chemical composition of coatings were characterized using XPS technique. XPS measurements provide compositional information at the nanometer level of the outermost surface layer of the coatings. Deconvolutions of the Cu 2p3/2 bands for the coatings A0, A1, A4 and B0, B1, B4 are presented in Figs. 10 and 11, respectively. The component peaks at around 932.4, 932.7, 933.9, and 934.7 eV were assigned to Cu(acac)2, Cu(0), CuO, and Cu(OH)2, respectively. Detailed quantitative results of XPS analysis are listed in Table 3. Intensity of photoelectrons from copper atoms was recorded for coating A0 derived from two forms of copper compounds CuO (933.9 eV) and Cu(acac) (932.4 eV). Then, after 10 laser pulses photoelectron intensity increased and photoelectrons attributed to Cu(0) were detected. Under these irradiation conditions an increase in CuO emission band was also observed, which was caused by laser ablation of polymer matrix, and thus CuO particles were uncovered. Higher energy dose of laser radiation transformed copper compounds into the pure form of Cu(0) as presented for coating A4. In the case of coating B an initial level of Cu photoelectrons (sample B0) was higher as compared with that for coating A0. The photoelectrons were derived mainly from Cu(OH)2 (934.7 eV). Based on these observations it can be concluded that polar Cu(OH)2 has tendency to migrate toward the surface of the coating. A small component peak derived from Cu(acac)2 was also noticed in coating B0. With increasing number of laser pulses (coatings B1 and B4) single peak at about 932.7 eV was fitted and attributed to Cu(0). This increase was higher as compared with that determined for coatings A1 and A4. In many applications, it is essential to provide strong adhesion between a polymeric substrate and copper conductive tracks. The basic method to evaluate this property is to conduct pull-off adhesion
Table 3 Fitting results of peak deconvolutions of XPS Cu 2p3/2 spectra. Sample
A0 A1 A4 B0 B1 B4
CuO
Cu(acac)2
Cu(OH)2
Cu(0)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
Peak (eV)
Area (%)
FW (eV)
933.9 933.9 – – – –
69.5 52.4 – – – –
1.5 2.3 – – – –
– – – 934.7 – –
– – – 93.7 – –
– – – 2.1 – –
932.4 – – 932.4 – –
30.4 – – 6.3 – –
1.5 – – 1.8 – –
– 932.8 932.6 – 932.7 932.7
– 47.6 100.0 – 100 100
– 2.3 2.2 – 1.9 2.3
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tests. However, in the case of this study, adhesion of electrolessly deposited copper layer to polymer coating was higher than the polymer coating to the coated plastic sheet. The pull-off adhesion tests resulted only in determination of adhesion strength between coating and polymer sheet which was about 1 MPa. For that reason, contact angle measurements and calculation of surface energy as an indirect measure of adhesion properties were performed. Unirradiated coatings A and B characterized with low surface energy of about 23 mJ/m2 but with successive number of laser pulses it increased to 80.6 and 83.3 mJ/m2, respectively (Table 4). The main factors affecting surface energy of a material are its chemical composition and surface geometrical structure [30]. Surface chemistry of coatings A and B differed upon laser radiation whereas surface energy change followed similar trend for each of the coatings. Therefore, one can conclude that surface geometrical structure could be the main factor affecting adhesion properties of coatings. However, caution has to be paid predicting adhesion properties based on surface energy calculations. 4. Conclusions Application of CuO or Cu(OH)2 affected the overall quality and laser activation process of coating surface. It was found that the application of Cu(OH)2 accelerated laser activation and only 10 laser pulses at fluence of 100 mJ/cm2 were sufficient for direct electroless metallization of coating. On the other hand, coating containing CuO required higher energy dose of about 100 laser pulses at the same fluence. However, more coating defects represented by numerous holes appeared in coating containing Cu(OH) 2 than CuO. The main reason for the observed differences was thermal decomposition of Cu(OH)2 associated with formation of water. It probably increased laser ablation of polymer coating leading to uncovering of copper compounds and in the next step their reduction to metallic copper. The empirical model for the change of copper content in laser irradiated surface layer was also proposed. It was noticed that Cu(OH) had tendency to migrate onto the polymer surface, thus improving rate of surface activation. Generally, interactions of copper compounds, their by-products, polymer coating, and laser radiation are very complex and can have various photothermal and/or photochemical nature. This work presents some effects of these interactions which lead to formation of activated surface prepared for direct electroless metallization. Results of this work are important in developing new approaches
of laser-assisted metallization and designing new polymer coatings for that purpose. Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education as a research project No. IP2011 047371 (Iuventus Plus). References [1] [2] [3] [4]
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