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Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine
Journal Pre-proofs Full Length Article Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine Piotr Rytlewski, Bartłomiej Jagodziński, Rafał Malinowski, Bogusław Budner, Krzysztof Moraczewski, Agnieszka Wojciechowska, Piotr Augustyn PII: DOI: Reference:
S0169-4332(19)33245-3 https://doi.org/10.1016/j.apsusc.2019.144429 APSUSC 144429
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 June 2019 16 September 2019 16 October 2019
Please cite this article as: P. Rytlewski, B. Jagodziński, R. Malinowski, B. Budner, K. Moraczewski, A. Wojciechowska, P. Augustyn, Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144429
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine Piotr Rytlewski 1*, Bartłomiej Jagodziński1, Rafał Malinowski2, Bogusław Budner3, Krzysztof Moraczewski1 Agnieszka Wojciechowska4, Piotr Augustyn1 1) Department of Materials Engineering, Kazimierz Wielki University, Bydgoszcz, Poland 2) The Łukasiewicz Research Network - Institute for Engineering of Polymer Materials and Dyes, Toruń, Poland 3) Institute of Optoelectronics, Military University of Technology, Warsaw, Poland 4) Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370, Wroclaw, Poland Abstract In this work copper(II) L-tyrosine was firstly reported as effective precursors for laserinduced selective activation and electroless metallization of polyurethane coating. The compound in the form of powder was mixed at amount of 20 wt% with polyurethane resin to form the coating on the polycarbonate substrate. The coating was irradiated with excimer ArF laser which generated UV radiation of about 193 nm wavelength. Various laser fluences and number of laser pulses were applied to determine physicochemical changes of the coating surface layer. After irradiation coatings were electrolessly metallized. It was found that laser irradiation resulted in formation of cone-like surface geometrical structure. The high of the cones was dependent on the dose of irradiation, whereas their tops were covered with copper. The external surface layer of precipitated copper was oxidized. This, however, did not affect the possibility of the surface to be electrolessly metalized. The irradiated coatings was effectively metallized and thus deposited copper layer characterized with high adhesion strength. Keywords: copper(II) L-tyrosine, electroless metallization, lasers, precursors, surface activation * Piotr Rytlewski, [email protected]
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1. Introduction Laser irradiation and selective electroless metallization of polymer materials has a high prospect for performing metallized patterns on the 3D surfaces, which cannot be easily attained by standard chemical surface treatments. Some example of this technique has been involved in rapidly developing molded interconnect devices (3D-MID) which, in general, are an injection-molded thermoplastic parts with structured metal traces. In these devices mechanical and electrical functions are integrated [1-3]. Metallization of polymers with prior laser surface treatment can be subdivided into various methods, mainly due to the indirect or direct laser surface activation. In indirect methods surface is only preactivated and requires further chemical activation process before metallization. Preactivation can proceed in liquid or atmospheric medium and results in altering the polymer surface to enable attraction of catalyst species, like palladium to finally activate the surface [4-6]. In direct laser activation material surface is able to be directly metalized, however metal-organic/inorganic compounds (precursors) are needed to be priory incorporated into the polymer structure [7,8]. It can be realized by melt-compounding of thermoplastics or by mixing with liquid resin to form the coating. This method is often referred to as laser direct structuring (LDS). In this method laser irradiation cause ablation of organic fraction of material while precipitated metal species cover the surface, thus constituting active centers for reduction of metal ions from metallization bath. There are several commercially available metal-organic precursors for this process but most of them are expensive, usually based on palladium compounds [9]. While chemical structure of commercial additives are generally unknown, some metal-organic compounds like palladium, copper or nickel acetates or acetylacetonates were previously reported as effective metallization precursors [10-15]. Also inorganic metal compounds like copper chromium oxides, hyroxides or hydroxide phosphate were revealed as additives for polymers metallized by LDS method [8,16-18]. Besides, copper-free additives like antimony-doped tin oxide and multi-walled carbon nanotube were also reported [19,20]. Due to the significance of this technique, especially for 3D-MID new metal-organic compounds able to be effective precursors are highly desirable [21,22]. In this work the effects of laser irradiation conditions on the surface changes of polyurethane coating containing copper(II) L-tyrosine as a new effective precursor are firstly reported. It was proved that this compound can be effective precursor in LDS technique. In conventional LDS technique Nd:YAG fiber lasers generating near IR radiation (1064 nm) are applied, mainly due to the ease of process control. However, infrared radiation 2
induces temperature rise and causes thermal ablation of polymer material which is associated with surface melting. The additives present in polymer matrix are uncovered as a result of laser ablation but on the other hand are partly reembedded by accompanying surface melting. The surface becomes roughen, however, its geometrical structure is corrugated having locally rather oval than sharp profile [16]. It was proved in this work that UV laser irradiation induced highly rough surface with locally sharp cone-like structure which contributed to the excessively high adhesion strength of deposited copper layer. It is considered that in some special implementations very high adhesion can be required, thus applications of more expensive and difficult to control excimer lasers can be justified. The scientific objective of this work was to elucidate physical and chemical changes induced by ArF excimer laser irradiation in the surface layer of polymer coating with copper(II) L-tyrosine. The focus was paid to the mechanism of laser-induced cone formation and their chemical structure. Special approach of XPS fitting method was applied to prove that precipitated copper was oxidized and its forms were dependent on irradiation conditions.
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2. Experimental 2.1.
Materials
The copper(II) L-tyrosine compound of the formula [Cu(Ltyr)2]n , further also referred to as additive, was evaluated as metallization precursors for LDS method. The scheme for this compound is presented in Fig. 1.
Fig. 1. The schemes of [Cu(L-tyr)2]n The compound presents the polymeric nature due to the one L-tyrosine ligand bridging with carboxylate groups of another linking neighbouring copper(II) centres (CuOCOCun polymer chain based on the coordination bonds. Its structure has a left-handed helical arrangement. The compound is not commercially available and was synthetized in the form of crystals powder due to the procedure described elsewhere [23]. There also detailed analysis on its physicochemical structure and properties can be found. The additive at amount of 20 wt% was mixed with polyurethane resin and then casted on the polycarbonate plates. Polyurethane resin type B4060 (Haering, Germany) was applied as a polymer matrix of the coatings. The plates being coated were injection moulded from polycarbonate of the type Xantar 19 UR (DSM Engineering Plastics, Holand) at standard processing conditions for this thermoplastic. 2.2.
Processing
The coatings were irradiated with ArF excimer laser which emits radiation at wavelength of 193 nm. The coatings were irradiated at doses varied with fluences and numbers of laser 4
pulses. The coatings were designated with regard to the irradiation conditions as presented in tab. 1. Tab. 1. Designations of coatings with regard to the conditions of laser irradiation Fluence
Number of laser pulses:
(mJ/cm2)
350
400
450
500
30
A11
A12
A13
A14
50
A21
A22
A23
A24
100
A31
A32
A33
A34
After laser irradiation samples were immersed in metallization bath which was commercially acquired as MCopper 85 (MacDermid, USA). It is a six components bath with formaldehyde as reducing agent. The samples was immersed in the bath for 60 min, while the bath was aerated, had 48°C and pH 12.8. 2.3. Examination Surface topography was examined by means of scanning electron microscopy (SEM) apparatus SU8010 (Hitachi, Japan) which was additionally equipped with energy-dispersive X-ray (EDX) detector. For recording high resolution surface topography, the samples were coated with thin conductive layer of gold deposited by evaporation in vacuum chamber. However, for EDX elemental analysis samples surface was uncoated. Submicrometer structure of surface layer was examined using photoelectron spectroscopy (XPS). The applied spectrophotometer R3000 (VG Scienta, Sweden) was equipped with Al anode, which emitted X-ray photons of energy 1486,6 eV. The main focus of this examination was to determine copper forms in thin surface layer of laser irradiated coatings. A relatively new approach for distinguishing copper forms was applied in this study [24,25]. It consists in fitting of the spectra for each of the copper form: C(0), CuO, Cu(OH)2 and Cu2O altogether to the recorded photoelectron spectra. This method enables to discern Cu(0) from Cu2O which by standard fitting was not possible due to the overlapping of photoelectron emission bands from these two forms [26]. Adhesion strength measurements were performed using tensile testing machine Instron 3367 (Instron, USA). The metal stamp was bonded to the copper layer with adhesive Araldite 2011 (Huntsman, Switzerland). After 24h the stamp was mounded in special clamps (Fig. 2) 5
and detached under constant strain (2 mm/min). Adhesion strength was calculated as the maximum force per bonded surface are of the stamp (6,5x20 mm).
Fig. 2. Set-up for adhesion strength testing, where: 1- bottom clamp, 2- top clamp, 3- screws fastening the clamps with the sample in-between, 4- stamp. 3. Results and discussion In our preliminary examination it was found that some of copper complexes are ineffective precursors for electroless metallization of laser-activated polymer coatings due to their physicochemical properties. However, it is presented in this work that copper(II) L-tyrosine can act as effective precursor. Coatings with this additive started to be metalized when irradiated with at least 350 laser pulses at fluence of at least 30 mJ/cm2 (Fig. 3).
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Fig. 3. Laser irradiated and electroless metallized coatings containing copper(II) L-tyrosine However, the deposited copper layers were not continuously plated on the surface of coatings irradiated with 350 laser pulses. The changes in physical and chemical surface structure induced by laser irradiation were investigated by SEM, EDX and XPS analysis to elucidate the main mechanism for surface activation. SEM images of irradiated coatings registered conventionally in the normal direction to the surface were not sharply focused because of the significant changes in geometrical surface structure. Therefore, cross-sectional view at a small angle was applied to gain better insight into the changes induced by laser irradiation (Fig. 4).
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Fig. 4. SEM images for laser irradiated coatings (designation of coatings in Tab. 1)
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As seen in Fig. 4, laser activated coatings characterized with formation of multiple cones, which concentration and height increased with increasing dose of laser irradiation. It was found based on EDX linear analysis that copper content was higher at the top than at the bottom of the cones (Fig. 5).
Fig. 5. SEM image of coating A34 with a cone on which EDX signal attributed to the emission band for copper was embedded In their top part copper content was about 34 at% while in the bottom about 10 at%. Based on the results, one can propose the mechanism for the formation of cones under laser irradiation. It is known from previous studies, that the ablation threshold for copper irradiated with ArF excimer laser is about 2 J/cm2 whereas for most polymers about 20 mJ/cm2 [27,28]. In this work applied laser fluences were high enough to cause ablation (ejection) of organic part of the coating, however heavier copper elements could be precipitated and agglomerated on the surface. Therefore, locally agglomerated copper constituted the mask against laser-induced ablation of organic part of the coating. The coating fragments from-between the agglomerates were ablated with successive laser pulses, thus the height of the cones increased with increasing laser pulses. Changes in copper content on a relatively large surface area (1.2 x 0.9 mm) as dependent on irradiation conditions are presented in Fig. 6.
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8
Laser fluence: 2 30 mJ/cm 2 50 mJ/cm 2 100 mJ/cm
7
Cu (at%)
6 5 4 3 2 1 0 0
100
200
300
400
500
Number of laser pulses Fig. 6. Copper content in coatings surface layer as dependent on laser irradiation conditions The increase of copper was similar for fluence of 50 and 100 mJ/cm2 while for fluence of 30 mJ/cm2 it was clearly lower. The relatively low copper content (maximally reaching about 7 at%) with respect to the content of carbon and oxygen resulted probably from precipitation of copper locally in the form of agglomerates. Photoelectron spectroscopy (XPS) was conducted to determine changes in the chemical structure of coatings depending on the parameters of laser radiation. It was confirmed that copper content increased with increasing laser pulses and their fluence (Tab. 2), however the percentage values were significantly lower as compared to those obtained from EDX analysis. Tab. 2. Content of copper, oxygen and carbon elements with binding energy (EB) of their photoelectrons for selected coatings Coating A11 A14 A21 A24 A31 A34
(at%) 0,5 0,7 0,8 1,4 0,9 1,8
Cu EB (eV) 932,63 932,88 932,68 932,93 932,88 932,51
(at%) 23,2 18,9 18,3 17,0 19,7 19,0
O EB (eV) 532,43 532,18 532,52 532,03 532,38 532,41
at% 76,2 80,4 80,9 81,6 79,4 79,2
C EB (eV) 285,18 285,08 285,23 285,13 285,18 285,41 10
One should keep in mind that in EDX technique the recorded X-ray signal came from the thickness of about 100 μm, whereas in XPS method photoelectrons are emitted from only a few nanometers thick surface layer [29]. Therefore, quantitative results from these techniques cannot be directly compared. Additional responsible factor which can be considered is high X-ray diffraction on the cone-like surface which could contribute to distortion of quantitative analysis. On the other hand XPS was helpful to define the form of copper which was precipitated from copper(II) L-tyrosine upon laser irradiation. In this work special fitting method was applied to distinguish metallic Cu(0) from oxide Cu2O form, which are practically indiscernible because their photoelectron emission bands are overlaid [26]. Using that method, it was found that copper in coatings irradiated with a smaller number of laser pulses was mainly in the form of Cu2O, CuO and Cu(OH)2, while with the increase in the number of pulses, the share of metallic copper increased (Tab 3. and Fig. 8). Tab. 3. Atomic content of individual copper forms in irradiated coatings Coating A11 A14 A21 A24 A31 A34
Cu (at%) (EB=932.7 eV) 16.4 16.7 8.8 24.4 1.6 16.8
CuO (at%) (EB=933.9 eV) 34.9 39.3 46.3 33.4 47.4 26.8
Cu2O (at%) (EB=932.4 eV) 43.6 37.3 43.7 38.2 51.1 46.8
Cu(OH)2 (at%) (EB=935.0 eV) 5.1 6.7 1.1 4.1 0.0 9.6
11
17
Cu CuO Cu2O
a)
17
Cu(OH)2
Cu CuO Cu2O
16
CPS x 10
5
16
b)
15
15
14
14
13 960 17
955
950
945
940
935
930
13 960 21
955
950
945
d)
c)
5
19
15
940
935
930
935
930
Cu CuO Cu2O
20
Cu CuO Cu2O
16
CPS x 10
Cu(OH)2
Cu(OH)2
18 17
14 16 13 960
955
950
945
940
Binding Energy (eV)
935
930
15 960
955
950
945
940
Binding Energy (eV)
Fig. 8. Cu 2p1/2 and 2p3/2 spectra for coatings: a) A11, b) A14, c) A31 and d) A34, with model spectra for Cu, CuO, Cu2O and Cu(OH)2 fitted to the recorded signal On the other hand, after irradiation with 350 laser pulses, the higher the fluence, the lower was the ratio of metallic copper, which was 16.4; 8.8 and 1.6 at%, respectively for coatings A11, A21 and A31. The decrease in metallic form of copper corresponded with the increase of Cu2O and CuO forms. As known from literature copper heated in air will oxidize first to the Cu2O and then to CuO forms [30]. Therefore, higher laser fluence favored formation of copper oxides. In the case of 500 laser pulses, when the surface already had a structure of numerous and high cones, it was difficult to determine the effect of laser fluence on the changes in structural forms of copper. The oxidized thin surface layer for copper is common, often called as native oxide layer. The oxide nanoscopic thin layer of copper did not affect the possibility of coatings to be electroless metalized (Fig. 8).
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Fig. 8. SEM images of coatings A12, A14, A32, A34 after electroless metallization With increasing irradiation doses more copper was deposited as proved by EDX analysis. Detected content of Cu was 26, 40, 75 and 83 at% for coatings A11, A14, A32, A34, respectively. The electroless deposited copper layers were electrically conductive even in the case of coating A11, which seemed to be non-continuous. While Cu content increased with increasing irradiation dose, the ratio of C/O was constant (2:1). The geometrical structure of electroless deposited copper layer reflected that formed after laser irradiation (Fig. 9).
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Fig. 9.SEM images of coatings A12, A14, A32, A34 after electroless metallization (side-view) However, the tops of the cones became more rounded which resulted probably from higher deposition rate of copper on the tops, where Cu was more intensively precipitated by laser irradiation, than in other sites. It was found that the surface of cones as well as between them was completely covered with copper and no significant changes in Cu content due to the specific localization were noticed (Fig. 10).
Fig. 10. SEM image of metalized coating A34 with EDX signal attributed to the emission band of copper (85±5 at% of Cu)
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The adhesive strength of deposited copper layers was evaluated by the pull-off test. In all cases regardless of irradiation conditions, copper layer could not be separated from the polymer coating. During the tests the polymer coating was broken cohesively (Fig. 11a) or the adhesive partly remained on the stamp (Fig. 11b).
Fig. 11. Exemplary photos of coatings and stamps after pull-off tests
The destruction strength was about 3,1 0±0,3 MPa. It can be expected that such a high adhesion strength of the copper layer resulted from the cone-shaped surface structure of the coating created by laser irradiation, which enabled a very good anchoring of the copper layer and a large contact area with the adhesive through which the stamp was fixed.
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4. Conclusions In this work copper(II) L-tyrosine was firstly reported as effective precursors for selective activation and electroless metallization of polymer coating. Irradiation with ArF excimer laser resulted in formation of cone-like surface geometrical structure. The copper was precipitated from copper(II) L-tyrosine and agglomerated upon laser radiation. These agglomerates constituted the tops of the cones and thus the main centers for the reduction of copper ions from metallization bath. Formation of cones was attributed to the difference in ablation threshold of polymer matrix and metallic copper. Agglomerated copper constituted a mask against laser ablation of polyurethane coating, thus successive laser pulses and increase of their fluences contributed to the increase in height of the cones. The surface layer of precipitated copper was oxidized, however the oxidation state was dependent on the irradiation doses. At lower doses Cu2O and Cu(OH)2 forms were dominant but with increasing laser pulses these forms were transformed into the CuO. External thin oxidized layer of copper did not affect possibility of successful metallization. After electroless metallization the deposited copper layer had the geometrical structure reflected that induced by laser irradiation. Adhesion strength of copper layer was exceeding that of the polyurethane coating and polycarbonate substrate (about 3 MPa), as determined based on pull-off tests. The excessively high adhesion strength of deposited copper layer resulted probably from highly rough surface with locally sharp cone-like structure. This effect is practically not possible when IR laser are applied, which is commonly the case in laser direct structuring. The obtained copper layers, as highly conductive, can be used for further metal deposition using electroplating methods. Acknowledgement This work has been financed from the funds of the National Centre of Science granted upon decision DEC-2013/11/D/ST8/03423.
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*Highlights (for review)
Highlights:
Copper(II) L-tyrosine is effective precursors for laser-induced activation of polymer coating.
ArF excimer laser irradiation induced cone-like surface geometrical structure.
The tops of the cones was covered with copper precipitated from copper(II) Ltyrosine.
The forms of precipitated copper was determined by XPS using vectors fitting method.
Geometrical surface structure of metalized layer reflected that induced by laser irradiation.
A very high adhesion strength of deposited copper layer was obtained.
The obtained copper layers, as highly conductive, can be used for further metal deposition using electroplating methods.
S0169-4332(19)33245-3 https://doi.org/10.1016/j.apsusc.2019.144429 APSUSC 144429
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 June 2019 16 September 2019 16 October 2019
Please cite this article as: P. Rytlewski, B. Jagodziński, R. Malinowski, B. Budner, K. Moraczewski, A. Wojciechowska, P. Augustyn, Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc. 2019.144429
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Laser-induced surface activation and electroless metallization of polyurethane coating containing copper(II) L-tyrosine Piotr Rytlewski 1*, Bartłomiej Jagodziński1, Rafał Malinowski2, Bogusław Budner3, Krzysztof Moraczewski1 Agnieszka Wojciechowska4, Piotr Augustyn1 1) Department of Materials Engineering, Kazimierz Wielki University, Bydgoszcz, Poland 2) The Łukasiewicz Research Network - Institute for Engineering of Polymer Materials and Dyes, Toruń, Poland 3) Institute of Optoelectronics, Military University of Technology, Warsaw, Poland 4) Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370, Wroclaw, Poland Abstract In this work copper(II) L-tyrosine was firstly reported as effective precursors for laserinduced selective activation and electroless metallization of polyurethane coating. The compound in the form of powder was mixed at amount of 20 wt% with polyurethane resin to form the coating on the polycarbonate substrate. The coating was irradiated with excimer ArF laser which generated UV radiation of about 193 nm wavelength. Various laser fluences and number of laser pulses were applied to determine physicochemical changes of the coating surface layer. After irradiation coatings were electrolessly metallized. It was found that laser irradiation resulted in formation of cone-like surface geometrical structure. The high of the cones was dependent on the dose of irradiation, whereas their tops were covered with copper. The external surface layer of precipitated copper was oxidized. This, however, did not affect the possibility of the surface to be electrolessly metalized. The irradiated coatings was effectively metallized and thus deposited copper layer characterized with high adhesion strength. Keywords: copper(II) L-tyrosine, electroless metallization, lasers, precursors, surface activation * Piotr Rytlewski, [email protected]
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1. Introduction Laser irradiation and selective electroless metallization of polymer materials has a high prospect for performing metallized patterns on the 3D surfaces, which cannot be easily attained by standard chemical surface treatments. Some example of this technique has been involved in rapidly developing molded interconnect devices (3D-MID) which, in general, are an injection-molded thermoplastic parts with structured metal traces. In these devices mechanical and electrical functions are integrated [1-3]. Metallization of polymers with prior laser surface treatment can be subdivided into various methods, mainly due to the indirect or direct laser surface activation. In indirect methods surface is only preactivated and requires further chemical activation process before metallization. Preactivation can proceed in liquid or atmospheric medium and results in altering the polymer surface to enable attraction of catalyst species, like palladium to finally activate the surface [4-6]. In direct laser activation material surface is able to be directly metalized, however metal-organic/inorganic compounds (precursors) are needed to be priory incorporated into the polymer structure [7,8]. It can be realized by melt-compounding of thermoplastics or by mixing with liquid resin to form the coating. This method is often referred to as laser direct structuring (LDS). In this method laser irradiation cause ablation of organic fraction of material while precipitated metal species cover the surface, thus constituting active centers for reduction of metal ions from metallization bath. There are several commercially available metal-organic precursors for this process but most of them are expensive, usually based on palladium compounds [9]. While chemical structure of commercial additives are generally unknown, some metal-organic compounds like palladium, copper or nickel acetates or acetylacetonates were previously reported as effective metallization precursors [10-15]. Also inorganic metal compounds like copper chromium oxides, hyroxides or hydroxide phosphate were revealed as additives for polymers metallized by LDS method [8,16-18]. Besides, copper-free additives like antimony-doped tin oxide and multi-walled carbon nanotube were also reported [19,20]. Due to the significance of this technique, especially for 3D-MID new metal-organic compounds able to be effective precursors are highly desirable [21,22]. In this work the effects of laser irradiation conditions on the surface changes of polyurethane coating containing copper(II) L-tyrosine as a new effective precursor are firstly reported. It was proved that this compound can be effective precursor in LDS technique. In conventional LDS technique Nd:YAG fiber lasers generating near IR radiation (1064 nm) are applied, mainly due to the ease of process control. However, infrared radiation 2
induces temperature rise and causes thermal ablation of polymer material which is associated with surface melting. The additives present in polymer matrix are uncovered as a result of laser ablation but on the other hand are partly reembedded by accompanying surface melting. The surface becomes roughen, however, its geometrical structure is corrugated having locally rather oval than sharp profile [16]. It was proved in this work that UV laser irradiation induced highly rough surface with locally sharp cone-like structure which contributed to the excessively high adhesion strength of deposited copper layer. It is considered that in some special implementations very high adhesion can be required, thus applications of more expensive and difficult to control excimer lasers can be justified. The scientific objective of this work was to elucidate physical and chemical changes induced by ArF excimer laser irradiation in the surface layer of polymer coating with copper(II) L-tyrosine. The focus was paid to the mechanism of laser-induced cone formation and their chemical structure. Special approach of XPS fitting method was applied to prove that precipitated copper was oxidized and its forms were dependent on irradiation conditions.
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2. Experimental 2.1.
Materials
The copper(II) L-tyrosine compound of the formula [Cu(Ltyr)2]n , further also referred to as additive, was evaluated as metallization precursors for LDS method. The scheme for this compound is presented in Fig. 1.
Fig. 1. The schemes of [Cu(L-tyr)2]n The compound presents the polymeric nature due to the one L-tyrosine ligand bridging with carboxylate groups of another linking neighbouring copper(II) centres (CuOCOCun polymer chain based on the coordination bonds. Its structure has a left-handed helical arrangement. The compound is not commercially available and was synthetized in the form of crystals powder due to the procedure described elsewhere [23]. There also detailed analysis on its physicochemical structure and properties can be found. The additive at amount of 20 wt% was mixed with polyurethane resin and then casted on the polycarbonate plates. Polyurethane resin type B4060 (Haering, Germany) was applied as a polymer matrix of the coatings. The plates being coated were injection moulded from polycarbonate of the type Xantar 19 UR (DSM Engineering Plastics, Holand) at standard processing conditions for this thermoplastic. 2.2.
Processing
The coatings were irradiated with ArF excimer laser which emits radiation at wavelength of 193 nm. The coatings were irradiated at doses varied with fluences and numbers of laser 4
pulses. The coatings were designated with regard to the irradiation conditions as presented in tab. 1. Tab. 1. Designations of coatings with regard to the conditions of laser irradiation Fluence
Number of laser pulses:
(mJ/cm2)
350
400
450
500
30
A11
A12
A13
A14
50
A21
A22
A23
A24
100
A31
A32
A33
A34
After laser irradiation samples were immersed in metallization bath which was commercially acquired as MCopper 85 (MacDermid, USA). It is a six components bath with formaldehyde as reducing agent. The samples was immersed in the bath for 60 min, while the bath was aerated, had 48°C and pH 12.8. 2.3. Examination Surface topography was examined by means of scanning electron microscopy (SEM) apparatus SU8010 (Hitachi, Japan) which was additionally equipped with energy-dispersive X-ray (EDX) detector. For recording high resolution surface topography, the samples were coated with thin conductive layer of gold deposited by evaporation in vacuum chamber. However, for EDX elemental analysis samples surface was uncoated. Submicrometer structure of surface layer was examined using photoelectron spectroscopy (XPS). The applied spectrophotometer R3000 (VG Scienta, Sweden) was equipped with Al anode, which emitted X-ray photons of energy 1486,6 eV. The main focus of this examination was to determine copper forms in thin surface layer of laser irradiated coatings. A relatively new approach for distinguishing copper forms was applied in this study [24,25]. It consists in fitting of the spectra for each of the copper form: C(0), CuO, Cu(OH)2 and Cu2O altogether to the recorded photoelectron spectra. This method enables to discern Cu(0) from Cu2O which by standard fitting was not possible due to the overlapping of photoelectron emission bands from these two forms [26]. Adhesion strength measurements were performed using tensile testing machine Instron 3367 (Instron, USA). The metal stamp was bonded to the copper layer with adhesive Araldite 2011 (Huntsman, Switzerland). After 24h the stamp was mounded in special clamps (Fig. 2) 5
and detached under constant strain (2 mm/min). Adhesion strength was calculated as the maximum force per bonded surface are of the stamp (6,5x20 mm).
Fig. 2. Set-up for adhesion strength testing, where: 1- bottom clamp, 2- top clamp, 3- screws fastening the clamps with the sample in-between, 4- stamp. 3. Results and discussion In our preliminary examination it was found that some of copper complexes are ineffective precursors for electroless metallization of laser-activated polymer coatings due to their physicochemical properties. However, it is presented in this work that copper(II) L-tyrosine can act as effective precursor. Coatings with this additive started to be metalized when irradiated with at least 350 laser pulses at fluence of at least 30 mJ/cm2 (Fig. 3).
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Fig. 3. Laser irradiated and electroless metallized coatings containing copper(II) L-tyrosine However, the deposited copper layers were not continuously plated on the surface of coatings irradiated with 350 laser pulses. The changes in physical and chemical surface structure induced by laser irradiation were investigated by SEM, EDX and XPS analysis to elucidate the main mechanism for surface activation. SEM images of irradiated coatings registered conventionally in the normal direction to the surface were not sharply focused because of the significant changes in geometrical surface structure. Therefore, cross-sectional view at a small angle was applied to gain better insight into the changes induced by laser irradiation (Fig. 4).
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Fig. 4. SEM images for laser irradiated coatings (designation of coatings in Tab. 1)
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As seen in Fig. 4, laser activated coatings characterized with formation of multiple cones, which concentration and height increased with increasing dose of laser irradiation. It was found based on EDX linear analysis that copper content was higher at the top than at the bottom of the cones (Fig. 5).
Fig. 5. SEM image of coating A34 with a cone on which EDX signal attributed to the emission band for copper was embedded In their top part copper content was about 34 at% while in the bottom about 10 at%. Based on the results, one can propose the mechanism for the formation of cones under laser irradiation. It is known from previous studies, that the ablation threshold for copper irradiated with ArF excimer laser is about 2 J/cm2 whereas for most polymers about 20 mJ/cm2 [27,28]. In this work applied laser fluences were high enough to cause ablation (ejection) of organic part of the coating, however heavier copper elements could be precipitated and agglomerated on the surface. Therefore, locally agglomerated copper constituted the mask against laser-induced ablation of organic part of the coating. The coating fragments from-between the agglomerates were ablated with successive laser pulses, thus the height of the cones increased with increasing laser pulses. Changes in copper content on a relatively large surface area (1.2 x 0.9 mm) as dependent on irradiation conditions are presented in Fig. 6.
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8
Laser fluence: 2 30 mJ/cm 2 50 mJ/cm 2 100 mJ/cm
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Cu (at%)
6 5 4 3 2 1 0 0
100
200
300
400
500
Number of laser pulses Fig. 6. Copper content in coatings surface layer as dependent on laser irradiation conditions The increase of copper was similar for fluence of 50 and 100 mJ/cm2 while for fluence of 30 mJ/cm2 it was clearly lower. The relatively low copper content (maximally reaching about 7 at%) with respect to the content of carbon and oxygen resulted probably from precipitation of copper locally in the form of agglomerates. Photoelectron spectroscopy (XPS) was conducted to determine changes in the chemical structure of coatings depending on the parameters of laser radiation. It was confirmed that copper content increased with increasing laser pulses and their fluence (Tab. 2), however the percentage values were significantly lower as compared to those obtained from EDX analysis. Tab. 2. Content of copper, oxygen and carbon elements with binding energy (EB) of their photoelectrons for selected coatings Coating A11 A14 A21 A24 A31 A34
(at%) 0,5 0,7 0,8 1,4 0,9 1,8
Cu EB (eV) 932,63 932,88 932,68 932,93 932,88 932,51
(at%) 23,2 18,9 18,3 17,0 19,7 19,0
O EB (eV) 532,43 532,18 532,52 532,03 532,38 532,41
at% 76,2 80,4 80,9 81,6 79,4 79,2
C EB (eV) 285,18 285,08 285,23 285,13 285,18 285,41 10
One should keep in mind that in EDX technique the recorded X-ray signal came from the thickness of about 100 μm, whereas in XPS method photoelectrons are emitted from only a few nanometers thick surface layer [29]. Therefore, quantitative results from these techniques cannot be directly compared. Additional responsible factor which can be considered is high X-ray diffraction on the cone-like surface which could contribute to distortion of quantitative analysis. On the other hand XPS was helpful to define the form of copper which was precipitated from copper(II) L-tyrosine upon laser irradiation. In this work special fitting method was applied to distinguish metallic Cu(0) from oxide Cu2O form, which are practically indiscernible because their photoelectron emission bands are overlaid [26]. Using that method, it was found that copper in coatings irradiated with a smaller number of laser pulses was mainly in the form of Cu2O, CuO and Cu(OH)2, while with the increase in the number of pulses, the share of metallic copper increased (Tab 3. and Fig. 8). Tab. 3. Atomic content of individual copper forms in irradiated coatings Coating A11 A14 A21 A24 A31 A34
Cu (at%) (EB=932.7 eV) 16.4 16.7 8.8 24.4 1.6 16.8
CuO (at%) (EB=933.9 eV) 34.9 39.3 46.3 33.4 47.4 26.8
Cu2O (at%) (EB=932.4 eV) 43.6 37.3 43.7 38.2 51.1 46.8
Cu(OH)2 (at%) (EB=935.0 eV) 5.1 6.7 1.1 4.1 0.0 9.6
11
17
Cu CuO Cu2O
a)
17
Cu(OH)2
Cu CuO Cu2O
16
CPS x 10
5
16
b)
15
15
14
14
13 960 17
955
950
945
940
935
930
13 960 21
955
950
945
d)
c)
5
19
15
940
935
930
935
930
Cu CuO Cu2O
20
Cu CuO Cu2O
16
CPS x 10
Cu(OH)2
Cu(OH)2
18 17
14 16 13 960
955
950
945
940
Binding Energy (eV)
935
930
15 960
955
950
945
940
Binding Energy (eV)
Fig. 8. Cu 2p1/2 and 2p3/2 spectra for coatings: a) A11, b) A14, c) A31 and d) A34, with model spectra for Cu, CuO, Cu2O and Cu(OH)2 fitted to the recorded signal On the other hand, after irradiation with 350 laser pulses, the higher the fluence, the lower was the ratio of metallic copper, which was 16.4; 8.8 and 1.6 at%, respectively for coatings A11, A21 and A31. The decrease in metallic form of copper corresponded with the increase of Cu2O and CuO forms. As known from literature copper heated in air will oxidize first to the Cu2O and then to CuO forms [30]. Therefore, higher laser fluence favored formation of copper oxides. In the case of 500 laser pulses, when the surface already had a structure of numerous and high cones, it was difficult to determine the effect of laser fluence on the changes in structural forms of copper. The oxidized thin surface layer for copper is common, often called as native oxide layer. The oxide nanoscopic thin layer of copper did not affect the possibility of coatings to be electroless metalized (Fig. 8).
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Fig. 8. SEM images of coatings A12, A14, A32, A34 after electroless metallization With increasing irradiation doses more copper was deposited as proved by EDX analysis. Detected content of Cu was 26, 40, 75 and 83 at% for coatings A11, A14, A32, A34, respectively. The electroless deposited copper layers were electrically conductive even in the case of coating A11, which seemed to be non-continuous. While Cu content increased with increasing irradiation dose, the ratio of C/O was constant (2:1). The geometrical structure of electroless deposited copper layer reflected that formed after laser irradiation (Fig. 9).
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Fig. 9.SEM images of coatings A12, A14, A32, A34 after electroless metallization (side-view) However, the tops of the cones became more rounded which resulted probably from higher deposition rate of copper on the tops, where Cu was more intensively precipitated by laser irradiation, than in other sites. It was found that the surface of cones as well as between them was completely covered with copper and no significant changes in Cu content due to the specific localization were noticed (Fig. 10).
Fig. 10. SEM image of metalized coating A34 with EDX signal attributed to the emission band of copper (85±5 at% of Cu)
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The adhesive strength of deposited copper layers was evaluated by the pull-off test. In all cases regardless of irradiation conditions, copper layer could not be separated from the polymer coating. During the tests the polymer coating was broken cohesively (Fig. 11a) or the adhesive partly remained on the stamp (Fig. 11b).
Fig. 11. Exemplary photos of coatings and stamps after pull-off tests
The destruction strength was about 3,1 0±0,3 MPa. It can be expected that such a high adhesion strength of the copper layer resulted from the cone-shaped surface structure of the coating created by laser irradiation, which enabled a very good anchoring of the copper layer and a large contact area with the adhesive through which the stamp was fixed.
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4. Conclusions In this work copper(II) L-tyrosine was firstly reported as effective precursors for selective activation and electroless metallization of polymer coating. Irradiation with ArF excimer laser resulted in formation of cone-like surface geometrical structure. The copper was precipitated from copper(II) L-tyrosine and agglomerated upon laser radiation. These agglomerates constituted the tops of the cones and thus the main centers for the reduction of copper ions from metallization bath. Formation of cones was attributed to the difference in ablation threshold of polymer matrix and metallic copper. Agglomerated copper constituted a mask against laser ablation of polyurethane coating, thus successive laser pulses and increase of their fluences contributed to the increase in height of the cones. The surface layer of precipitated copper was oxidized, however the oxidation state was dependent on the irradiation doses. At lower doses Cu2O and Cu(OH)2 forms were dominant but with increasing laser pulses these forms were transformed into the CuO. External thin oxidized layer of copper did not affect possibility of successful metallization. After electroless metallization the deposited copper layer had the geometrical structure reflected that induced by laser irradiation. Adhesion strength of copper layer was exceeding that of the polyurethane coating and polycarbonate substrate (about 3 MPa), as determined based on pull-off tests. The excessively high adhesion strength of deposited copper layer resulted probably from highly rough surface with locally sharp cone-like structure. This effect is practically not possible when IR laser are applied, which is commonly the case in laser direct structuring. The obtained copper layers, as highly conductive, can be used for further metal deposition using electroplating methods. Acknowledgement This work has been financed from the funds of the National Centre of Science granted upon decision DEC-2013/11/D/ST8/03423.
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*Highlights (for review)
Highlights:
Copper(II) L-tyrosine is effective precursors for laser-induced activation of polymer coating.
ArF excimer laser irradiation induced cone-like surface geometrical structure.
The tops of the cones was covered with copper precipitated from copper(II) Ltyrosine.
The forms of precipitated copper was determined by XPS using vectors fitting method.
Geometrical surface structure of metalized layer reflected that induced by laser irradiation.
A very high adhesion strength of deposited copper layer was obtained.
The obtained copper layers, as highly conductive, can be used for further metal deposition using electroplating methods.