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Ni direct electroless metallization of polymers by a new palladium-free process
Surface & Coatings Technology 200 (2006) 5028 – 5036 www.elsevier.com/locate/surfcoat
Ni direct electroless metallization of polymers by a new palladium-free process Marle`ne Charbonnier a,*, Maurice Romanda, Yves Goepfert b a
Laboratoire de Sciences et Inge´nierie des Surfaces, Universite´ Claude Bernard-Lyon 1, 69622 Villeurbanne Cedex, France b UMR CNRS # 5180 des Sciences Analytiques, Universite´ Claude Bernard- Lyon 1, 69622 Villeurbanne Cedex, France Received 16 December 2004; accepted in revised form 13 May 2005 Available online 17 June 2005
Abstract Ni films were electrolessly deposited on the surface of different polymer substrates without having to use the conventional palladium catalyst to initiate the redox reaction leading to metallization. With this in view, the surface to be coated was seeded with Ni(+2) species from an organo-nickel precursor. After reduction of the Ni(+2) ions to the Ni(0) oxidation state, it was possible to use the remarkable autocatalytic property of nickel to deposit a metallic film on the polymer by merely immersing in a Ni plating bath. The reduction of Ni(+2) species, less easy than that of Pd(+2) species commonly used in the authors’ laboratory in the case of the tin-free process, was performed in the present work either using a chemical path or by plasma treatment in a non-oxidizing atmosphere (H2 or Ar), then checked by XPS. This method was experimented on various polymer substrates using two different industrial Ni plating baths. Adhesion of Ni films evaluated by the Scotch\ tape test mainly depends on the phosphorus content of the plating bath used, i.e. on the amount of stresses generated in the film which depends on the quantity of phosphorus present, the latter originating from the plating bath reducer. Both the nature of the polymer surface treatment and especially the route used to perform the Ni(+2) reduction play an important part in the interfacial stresses generated and therefore on film adhesion. D 2005 Elsevier B.V. All rights reserved. Keywords: [B] Photoelectron spectroscopy; [B] Adhesion; [C] Electroless deposition; [C] Radio frequency (RF) plasma treatments; [D] Nickel; [X] Direct Ni electroless plating
1. Introduction In its most common applications, electroless metallization consists of a redox reaction between Ni(+2) or Cu(+2) ions and a strong reducer – sodium hypophosphite NaH 2 PO 2 for Ni(+2) and formaldehyde HCHO for Cu(+2) – both contained in the same solution. Electroless metallization of polymers is generally considered possible if a catalyst allowing the reaction initiation (palladium in the Pd(0) oxidation state) is present on the surface to be coated. The main difficulty of this process consists of chemisorbing the catalyst on the polymer surface and several routes have been proposed in the literature for this purpose. In recent work on this topic [1 – 9], we have * Corresponding author. Tel.: +33 4 72 44 83 06; fax: +33 4 72 43 12 06. E-mail address: [email protected] (M. Charbonnier). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.006
developed a tin-free method (Fig. 1) to attach the Pd(+2) species on a polymer surface and to reduce them to Pd(0) using the wet route. Under these conditions, the initiation of metal deposition is immediate when the Pd(0)-seeded surface is immersed in the plating bath. Metallization then continues thanks to the remarkable property of Ni and Cu in their metallic state to catalyze the reduction of their own ions, hence the term autocatalytic is also used to describe the plating process. Performing the direct electroless metallization of insulating substrates (polymers, glasses, ceramics) without seeding their surface with Pd(0) catalytic clusters is an interesting challenge which aims at reducing the cost of the metallization process. In this work, we consider only the electroless metallization with one of the two most commonly useable metals, viz. Ni. For this it is necessary to seed the surface to be metallized with Ni(0) clusters which
M. Charbonnier et al. / Surface & Coatings Technology 200 (2006) 5028 – 5036
Immersion in PdCl2 Immersion in H2PO2NH3 plasma (0.1 g L-1, 25°, 2 min) (10 g L-1, 85°, 3 min)
Grafting of nitro- Pd(+2) chemisorption Partial reduction genated groups on nitrogen groups of Pd(+2) toPd(0)
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Nickel film
Electroles plating
Fig. 1. Surface preparation of polymer substrates for electroless metallization by the tin-free process developed in the authors’ laboratory.
should be able to initiate the electroless plating in the same way that Pd(0) clusters do in the usual simplified electroless process [1 –9]. Over the last few years, many studies have been devoted to VUV light-induced decomposition of organo-palladium films so as to obtain the Pd seeds which act as the catalyst for the initiation of the electroless metallization. Various organo-palladium compounds were deposited by spinning-on, spraying or dip-coating then decomposed by a variety of radiation sources [10 –23] viz. laser, VUV excimer lamp, and even IR sources. Pd(0) clusters were also obtained in a single step by focusing an excimer laser beam on the substrate across a solution of the organo-palladium precursor [24 – 27]. All these methods can lead to obtaining patterned surfaces for microelectronics applications, either by moving the sample under the laser beam (laser writing), or by irradiating the organic precursor via a mask. In other works [28] the photodecomposition, (using a k = 254 nm radiation emitted by a low pressure Hg lamp), of allylcyclopentadienyl palladium in the gas phase was used to obtain a Pd layer on the substrate. Similar methods were used to deposit other noble metals such as Au or Pt [29 –33] and Cu [30,33 –40] without the intervening help of Pd to initiate the electroless reaction. In the last case, Cu deposits were achieved in one step by irradiating the substrate via a copper electroless plating solution with a variety of laser radiations [33,39,40]. Under these conditions, after the formation of Cu(0) seeds which are able to catalyze the initiation of the electroless deposition, the laser beam accelerates the plating process by heating the solution. Other authors [34 –38] use spincoating, spraying or evaporation to deposit thin films of copper salts from a solution or from the solid salt itself on various substrates. In a second step, they perform direct laser writing of copper by a local pyrolytic or photolitic decomposition of the film with a Nd:YAG, Ar+ or excimer
NH 3 or O 2 plasma
Surface functionalization
NiAc/ethanol film
Spincoating
laser beam. Another route to achieve direct copper metallization developed by Padiyath et al. [41] consists of depositing, as in the previous case, a thin copper formate film by spin-coating then reducing it by an RF H2 plasma for 15 min. Finally, a purely chemical process developed by Seita et al. [42] involves the sulfonation of the surface to be metallized by fuming sulfuric acid, adsorption of the Cu(+2) species from a cupric aqueous solution and their reduction to Cu(0) by immersion in a sodium borohydride (NaBH4) solution. To our knowledge, direct Ni metallization has not been mentioned in the literature but through a paper from Brocherieux et al. [43] who deposited Ni-based films on ABS substrates by remote MW plasma of Ni(CO)4. These films had to be treated by N2 plasma for 15 min to eliminate the remaining carbonyl species before being coated with Ni by electrodeposition. Taking into account the high price and difficulty of ready supply of nickel tetracarbonyl, we have ruled out this possibility of Ni surface seeding and turned our research to a method similar to those used for direct Cu metallization. This work takes advantage of the autocatalytic property of Ni. The first step deals with the deposition of a thin organo-nickel film and with its chemisorption on the substrate surface, which had previously been plasma treated. The second step, which is the main difficulty encountered in this work, consists of performing the reduction of Ni(+2) ions from the organo-Ni film to Ni(0) while maintaining enough Ni species strongly adsorbed on the surface to initiate the autocatalytic reaction. Different ways of reduction (chemical, photochemical, plasma) are explored and Ni metallization is performed for varying durations of time. Fig. 2 summarizes the succession of the different steps of the process studied here. Adhesion of the Ni films is evaluated and compared to that obtained for similar films using Pd to initiate the metallization.
Ni(0) seeds
Electroless Ni film
Ni(+2) reduction
Autocatalytic plating
Fig. 2. Surface preparation of polymer substrates for direct Ni electroless metallization by the new palladium-free process described in the present work.
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2. Experimental The new process described in this paper was, at first, developed on a poly(etherimide) substrate (Kapton\ HN) indexed in the following as PI and supplied by Goodefellow (UK). It was then extended to various other substrates including poly(carbonate) (PC), poly(propylene) (PP), poly(tetrafluoroethylene) (PTFE), poly(butyleneterephthalate) (PBT) or liquid crystal polymers (LCP). The surface treatment of polymers (surface cleaning, surface functionalization, reduction of Ni(+2) ions) was performed by plasma in ammonia, oxygen, argon or hydrogen atmosphere in a 13.56 MHz RF reactor (RIE 80 model from Plasma Technology, England). This reactor is equipped with two water cooled parallel and dissymmetrical electrodes and with four gas lines. The samples to be treated are placed on the RF powered electrode (17 cm in diameter) which is capacitively coupled to the power supply, through an impedance matching network. In this experimental device, the powered electrode is submitted to a bias voltage so that samples are, among others, bombarded by ions whose energy is dependent on the power density. All experiments were carried out at a power density of 0.52 W cm 2. As a consequence, the energy of the ions bombarding the sample is approximately 300 eV. The other experimental conditions were as follows: gas flow: 100 sccm, working pressure: 100 mTorr, treatment time: 30 s to 1 min. The organo-metallic precursor used was nickel acetate in ethanol solution (5 g L 1) referred to in the following as ‘‘NiAc/eth’’, the dissolution being done at about 80 -C. It was generally deposited on the substrate by spin-coating (rotation speed: 2000 rpm), but spraying or dipping were sometimes used. Nickel ions adsorbed on the substrate surface were reduced either by plasma treatment in a H2 or Ar atmosphere or chemically by dipping in a 0.5 M NaBH4 solution. BH4 is a powerful reducing agent whose decomposition in basic solution yields 8 electrons according to the following equation [44]: BH 4 þ 8OH Y BðOHÞ4 þ 4H2 O þ 8e
The redox potential E 0 (B(OH)4 / BH4) is equal to 1.24 V at pH = 14. Under these conditions, the overall redox reaction is: 0 4Ni2þ þ BH 4 þ 8OH Y BðOHÞ4 þ 4Ni þ 4H2 O:
Nickel plating baths were proprietary baths of complex and unknown composition. They work with sodium hypophosphite as a reducing agent. Two baths were used, viz. a high-phosphorus one (Europlate\ Ni 520 supplied by Mac Dermid –Frappaz, Neyron, France) and a low-phosphorus one (Enplate\ Ni 426 supplied by Enthone-OMI, France), both leading to the incorporation, into the final Ni coating, of 7 –9 and 1– 3 wt.% of phosphorus, respectively.
The Ni(2+) reduction reaction by hypophosphite is very complex and may be written, in a schematic way, as follows [44]: 0 þ Ni2þ þ 2H2 PO 2 þ 2H2 O Y Ni þ 2H2 PO3 þ H2 þ 2H
Phosphorus incorporation in the Ni deposit results from a secondary reaction due to hypophosphite disproportionation in acidic medium: 3H2 PO2 þ 2Hþ Y 2P þ H2 PO 3 þ 3H2 O: Surface composition modifications were studied by Xray photoelectron spectroscopy (XPS) with a RIBER SIA 200 instrument, using a non-monochromatic Al source. All spectra were referenced either to the C 1s peak (C\C and C\H bonds) whose binding energy was fixed at 285.0 eV or in the case of PTFE, to the C 1s peak (CF2 groups) at 292.5 eV according to the value given by Beamson and Briggs [45]. In some particularly ambiguous cases, charging effect correction was done with respect to the Au 4f7/2 peak at 84.0 eV. All samples for XPS analyses were introduced in the spectrometer, i.e. under UHV, within 5 min after the corresponding surface modifications. Surface concentrations determined from the XPS peak areas are considered as accurate to within T 10%. Microscopic observations of the metal/interface were carried out using a high resolution FEG S800 HITACHI instrument operating at 15 kV. Samples devoted to observation were surfaced by cutting at 75 -C with a diamond knife and then gold-coated. Evaluation of adhesion of nickel films on their substrate was performed using the standard ASTM D 3359 Scotch\ tape test (cross-cut tape test performed with a 3 M 250 3710 Scotch\ tape). The adhesion scale ranges from 5 (no film square removed by the tape) to 0 (more than 65% film squares removed). Although this test does not give quantitative results, it allows in some cases classification of the effects of different surface treatments and comparison of the adhesion levels of films obtained under various conditions. Film thicknesses were determined by X-ray fluorescence spectroscopy (XRFS) via the measurement of the intensity of the Ni Ka radiation with a calibration curve obtained by gravimetry used as reference. Photochemical decomposition of the metal-organic precursor was performed under primary vacuum using a Xe2* excimer lamp (Excivac Laboratory System, Heraeus Noblelight, Germany). The experimental set up consists of two contiguous chambers which can be evacuated separately (primary vacuum). The upper chamber which contains the excimer lamp is separated from the reactor by a 5 mm thick calcium fluoride window. The discharge in the lamp emits an incoherent radiation which peaks at a wavelength of 172 nm with a full width at half maximum of about 15 nm. The maximal photon flux irradiating the sample surface is estimated at 3 1016 photons cm 2 s 1 [9].
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3. Results
3.1. Reduction of Ni(+2) thin films
The development of this new Pd-free process was first performed on PI substrates [46].The choice of the organonickel precursor is governed by its solubility in various solvents. In fact, organic nickel salts are, for the most part, highly insoluble or at best only slightly soluble in common solvents. Among the most common of them, nickel formate (NiFo), nickel acetate (NiAc) and nickel acetylacetonate (NiAcAc) were investigated and of these NiAc (soluble in H2O, NH3,H2O and slightly soluble in ethanol) was retained. In the present work, the aim was to deposit an ultra-thin film of precursor on the substrate surface in order to obtain, after reduction of Ni(+2), a surface with enough Ni(0) seeds to start the autocatalytic reaction (Fig. 2). Indeed, it would be detrimental for the adhesion of the electroless nickel film to initiate the autocatalytic reaction from too ‘‘thick’’ a Ni film obtained by reduction of the organic precursor. Consequently the solvent of this precursor must be chosen with care since it plays an important part on the solution spreading out and therefore on the precursor film thickness whatever the deposition method used (spin-coating, spraying, dipping). Therefore a solution of NiAc in ethanol within the concentration range 5 to 20 g L 1 was retained. The latter, obtained by heating (¨80 -C) is probably a nickel ethanoate. A perfect spreading out of this solution was obtained by spin-coating using 1 or 2 drops. As a result, 3 to 6 Al of solution were deposited on a 9 cm2 area. Before depositing the precursor on the PI substrate, it was necessary to activate its surface in order to make it wettable and reactive towards the precursor solution. This was done by plasma surface treatment in oxygen or better still in ammonia for 1 min, which allows both the grafting of polar functional groups as well as the establishment of strong chemical bonds between precursor and substrate [1 –9].
3.1.1. Chemical reduction Taking the Pd(+2) reduction by H2PO2 in the simplified process developed in the laboratory (Fig. 1) as a model, Ni(+2) reduction with different H2PO2 solutions (0.1, 1 and 5 M) at 85 -C for different times between 15 s and 3 min was attempted. Unfortunately, H2PO2 is not sufficiently strong to cause the reduction of Ni(+2) ions before their lixiviation in the hot solution. XPS analyses show that, after such treatments, Ni virtually disappears from the PI surface. Such a behavior proves that Ni(+2) ions are less strongly adsorbed on the polymer substrate than Pd(+2) ones and that their reduction is more difficult. Another chemical reducer viz. sodium borohydride (NaBH4) which is sometimes used as a reducer in Ni plating baths [46] was also tested. Seita et al. [42] have shown that it is quite efficient in reducing Cu(+2) ions. A mere immersion of the sample coated with the NiAc precursor in a 0.5 M NaBH4 solution at room temperature for 5 to 15 s leads to a partial Ni(+2) ion reduction. Furthermore, this operation maintains enough Ni(0) species on the substrate surface to allow immediate Ni deposition when the sample is immersed in either of the two Ni industrial plating baths. Fig. 3 represents Ni 2p XPS spectra of the NiAc precursor before (a) and after (b) the NaBH4 treatment, and of Ni metal (c) for comparison. The insert provides the Ni surface concentration for the (a) and (b) samples. This figure shows the appearance of a shoulder on the low binding energy side of the Ni 2p peak (spectrum (b)) which characterizes the presence of Ni(0) species together with Ni(+2) ones (main Ni 2p peak). The very efficient reduction performed by NaBH4 is unfortunately handicapped by the fact that the solution is very unstable and cannot remain active for more than 1 or 2 h.
Fig. 3. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) NaBH4 reduction, and of Ni metal (c).
3.1.2. Plasma reduction Another way of ensuring Ni(+2) reduction is to use a hydrogen plasma as done by Padiyath et al. [41] to reduce Cu(+2) ions to Cu(0). Indeed, this method is able to reduce Ni(+2) ions to Ni(0), but, in contrast to the treatment time of 15 min recommended by Padiyath et al, we have shown by XPS that the shortest treatment times (15 –30 s) are the most efficient concerning both reduction rate and nickel surface concentration. Fig. 4 shows the energy shift of the Ni 2p peak towards the low binding energy side for various H2 plasma treatment times and in the insert, the corresponding Ni surface concentration variations. It appears that after 2 min of treatment a slight re-oxidation of Ni atoms is observed together with a decreasing of the Ni surface concentration. This Ni re-oxidation seems rather surprising in a reducing atmosphere. However in plasma, the reaction mechanisms are quite different from those occurring in the usual gas phase or in solution. In the present case, the different active species of the plasma (HI, H+, e and photons) break the bonds of the organic precursor and form
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after the shortest treatments (less than 30 s to 1 min).This is due to the rapid decomposition of the precursor and to the escape of volatile fragments. Under these conditions, Ni species which are no more drown in the organic part of the precursor are therefore present in larger concentration.
Fig. 4. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) to (g) reduction by H2 plasma for 5 s, 15 s, 30 s, 1 min, 2 min and 5 min, respectively. The Ni surface concentration variation versus the H2 plasma treatment time is represented in the insert.
small fragments or even atoms which may recombine with each other to give volatile new compounds that are evacuated by the pumping system. Simultaneously, the electrons present in the discharge reduce the Ni(+2) ions to Ni(0). Furthermore the H+ ions which bombard the sample lying on the powered electrode lead to the etching of the organic part of the precursor in a first step. When the volatile compounds corresponding to the precursor decomposition have disappeared (after a short plasma treatment time), O2 and/or H2O molecules present in the residual atmosphere (the base pressure in the plasma reactor is about 1 to 2 mTorr) may be responsible for the Ni re-oxidation, while the H+ ion bombardment provokes the progressive loss of Ni(0) species. Both for H2 plasma treatment times less than 1 min as well as for longer ones, Ni metallization is immediate with the Mac Dermid-Frappaz bath whereas it does not occur with the Enthone bath, in the composition of which there are probably very strong stabilizers. If the interpretation of the reaction mechanisms given for H2 plasma is correct, any plasma of a non reactive gas (rare gas) should give similar results. An Ar plasma which contains as active species Ar+ ions, electrons and photons allowed us to verify our hypothesis. Indeed, as shown in Fig. 5 which gives the binding energy shift of the Ni 2p peaks and, in the insert, the Ni concentration variations as a function of Ar plasma duration, the same effects as those obtained for H2 plasma were observed. The optimal reduction time is in the range 15 – 30 s. Beyond 2 min of Ar plasma, there is, as in the previous case and for the same reasons, a re-oxidation of Ni atoms and a decreasing of the Ni surface concentration. Here too, Ni metallization is immediate with the Mac Dermid bath after 30 s or 1 min of plasma treatment but not possible with the Enthone one. With both H2 and Ar plasma treatments, an increase in the Ni concentration is observed (see inserts in Figs. 4 and 5)
3.1.3. VUV-induced reduction Taking the experiments performed using UV or VUV radiations emitted by a laser or an excimer lamp to decompose various organo-Pd, Au, Pt or Cu precursors [6 – 21] as an example, we have also tried out this route to reduce organo-nickel precursors. In order to do this, ultrathin ‘‘NiAc/eth’’ films were deposited on PI and submitted to VUV radiation (k = 172 nm, E = 7.2 eV) emitted by the Xe2* excimer lamp, for 5, 10 and 20 min under primary vacuum. Contrary to what happens with Pd, Au, Pt or Cu organic salts under similar conditions, no decomposition or reduction of NiAc was observed after irradiation. Indeed, XPS analysis of these samples showed the presence of only C, O and Ni (the latter in the Ni(+2) form) characteristic of NiAc and these, in the same concentrations whatever the irradiation time. It can be concluded that NiAc is too stable to be decomposed by the radiation used. If, according to literature data [47] PdAc and CuAc are decomposed at 195 and 240 -C respectively, no indication concerning NiAc thermal stability is found. To break Ni\O bonds in NiAc in order to make free Ni, more energetic radiations e.g. the 126 nm (9.84 eV) radiation emitted by the Ar2* excimer lamp are probably necessary. However, as indicated by Macauley et al. [22], if the thermal stability of the compound plays an important part in its decomposition, another factor viz. its capability to absorb the said radiations is a key factor of the photolysis. Indeed, the latter cannot take place if the organo-metal precursor does not absorb the radiation. As an example, cobalt acetate (CoAc) which is known to decompose at
Fig. 5. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) to (g) reduction by Ar plasma for 5 s, 15 s, 30 s, 1 min, 2 min and 5 min, respectively. The Ni surface concentration variation versus the Ar plasma treatment time is represented in the insert.
M. Charbonnier et al. / Surface & Coatings Technology 200 (2006) 5028 – 5036
100 -C [47], was deposited by spin-coating on a substrate and submitted to the 172 nm UV radiation emitted by the Xe2* excimer lamp. After 10 min of irradiation, XPS analysis did not show the least reduction, which proves that the decomposition temperature is not the only factor to be considered in order to obtain the photo-reduction of such compounds.
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3.3. Discussion Table 1 clearly shows that Ni metallization is always possible for all the substrates studied after a chemical reduction of the Ni(+2) species by NaBH4 and with the two industrial plating baths used in the present study. As shown, the adhesion performances are dependent on the electroless Ni film thickness and substrate nature. However, no metallization is obtained with the Enthone bath when the Ni(+2) reduction is performed by Ar or H2 plasma (30 s– 1 min). Under the same conditions, PI, PC and PP can be metallized with the Mac Dermid plating bath, whereas PTFE cannot, even though, after Ar or H2 plasma, as after NaBH4 reduction the sample surface is greyish. This attests to the presence of Ni(0) species on the surface as do the Ni 2p XPS spectra (Figs. 5 and 6). However, after a short duration (10 to 20 s) in the plating bath, PTFE samples recover their initial white color which means that the Ni(0) species are weakly adsorbed on the surface and lixiviate in the hot Ni solution (85 -C) before the reaction initiation takes place. With PI, PPS, PBT and LCP, it is possible to obtain thick and well-adhering Ni films after Ar or H2 plasma. Though Ni metallization takes place normally with the Mac Dermid plating bath on PC and PP, problems arise when the film thickness increases (beyond ¨0.5 Am). In some cases, the film obtained is bright and apparently ‘‘perfect’’ as long as it is maintained in the plating bath but cracks as soon as it is transferred to the rinsing bath even though the temperature of this bath is raised to 85 -C in the hope of releasing the stresses. In other cases, if the film is
3.2. Extension of the method to other substrates — adhesion studies Different polymers known as difficult or even impossible to metallize using the conventional electroless process were studied with respect to direct Ni metallization. Among them, poly(butyleneterephthalate) (PBT), poly(phenylene sulfur) (PPS), liquid crystal polymers (LCP), polypropylene (PP), polycarbonate (PC) and polytetrafluoroethylene (PTFE), were surface treated by NH3 plasma (1 min), spin-coated using 1 or 2 drops of the ‘‘NiAc/eth’’ solution. Subsequently, the NiAc thin film was reduced either by NaBH4 or by Ar or H2 plasma and the surface-modified substrate was immersed in each of the industrial plating baths. The success of metallization depends on the substrate and plating bath nature as well as on the Ni(+2) reduction route. Results given in Table 1 concern Ni metallization of PI and of those polymers known to be difficult to metallize, viz. PC, PP, PTFE. The other polymers (PBT, PPS, LCP) studied in this work are metallized as easily as PI and give rise to welladherent Ni films. The corresponding results are regrouped in Table 1 with those of PI.
Table 1 Results (film thickness and adhesion) of the direct Ni metallization performed in the high and low-phosphorus baths on different polymer substrates Substrate
Mac Dermid Ni plating bath
Enthone Ni plating bath
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch test* results
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test results
10
1.8
**
5
20
2.5
**
5
0.6 1.3 0.4 1.8 2.4
**
5 0 4 0 5
10
1.2
**
5
10
1.3
**
5
PTFE
4 10 4 10 15
15
1.9
**
5
Ni(+2) reduction by Ar or H2 plasma(1 min)
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test* results
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test* results
20
2.9
**
5
5
no deposit
0.8 0.7 no deposit
**
0 0
5 5 5
no deposit no deposit no deposit
Ni(+2) reduction by NaBH4 PI-PPS PBT LCP PC PP
PI-PPS PBT LCP PC PP PTFE
5 5 5
#
** #
**
#
\
*Scotch\ test results (5) no film square removed by the tape; (4) ¨5% of film squares removed by the tape; (0) more than 65% of film squares removed by the tape. **Bright and homogeneous Ni film. # Crackled or blistered Ni film.
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Fig. 6. Ni 2p XPS spectra of the reference ‘‘NiAc/eth’’ precursor deposited on PI and not submitted to reduction (a), of this same precursor deposited on PI, PC, PP, PTFE (b) to (e), respectively and reduced by NaBH4 for 15 s at room temperature. The corresponding Ni surface concentrations are given in the insert.
too thick, it presents some blisters and spontaneously breaks up in the solution. In comparison, all the polymers that have been rendered catalytic by Pd grafting can be metallized with either of the two industrial baths, whatever the route followed for the Pd(+2) reduction (NaBH4 or NaH2PO2 solutions, Ar or H2 plasmas) (results to be published). As these films adhere well, it can be concluded that Pd(0) is a more active catalyst than Ni(0) to initiate the electroless reaction. Indeed, Pd(0) is able to lower the activation energy of this reaction, even though the plating baths contain powerful stabilizers as is probably the case for the Enthone bath. These stabilizers are used to maintain the redox solution in a metastable state and therefore to prevent its spontaneous decomposition. In the case of the catalysis using the Ni(0) species, the stabilizers may delay or even hinder the initiation of the autocatalytic reaction through a kind of poisoning of the Ni catalytic sites, hence resulting in an increase in the activation energy of the reaction. Data in Table 1 suggest that the plating bath also plays a key role on the kinetics of the autocatalytic reaction initiation. Before its use for direct metallization, a Ni bath must have functioned sufficiently long so as to operate under optimal conditions i.e. to allow rapid initiation of the electroless reaction. It is in fact well known that a new plating bath does not run spontaneously but must be activated, e.g. by immersing a piece of pure metal (here Ni) cleared of its surface natural oxide. The problems encountered with the industrial baths used in a research laboratory explain why for similar deposition times film thickness is not always constant. Indeed, for laboratory experiments, a plating bath works neither all day long nor every day. It needs to be reactivated at the time of deposition and as a result is rarely in a reproducible state. If the plating bath is not active enough (e.g. if it contains too much
stabilizer which is probably the case of the Enthone bath) and/or if the kinetics of the reaction initiation is too low – because of a too low reducer concentration in the plating bath as in the case of the low-phosphorus Enthone bath – the active species – here Ni(0) – chemisorbed on the substrate surface lixiviate in the hot plating bath (85 -C) before the deposition initiation. The success of a Pd-free metallization depends not only on the Ni plating bath composition, but also on the selected Ni(+2) reduction route and substrate nature, the two last governing the physical and chemical characteristics of the interface. Therefore besides the instantaneous characteristics of the Ni plating bath, the distribution and density of the Ni(0) species chemisorbed on the substrate surface play an important part in reaction initiation. Results given in Table 1 show that the Ni(+2) reduction by NaBH4 allows obtaining an efficient Ni deposit on all the substrates studied with both industrial baths, while the reduction by Ar plasma leads to Ni deposition only with the Mac Dermid bath and only for certain substrates. This is rather surprising if we consider the Ni 2p XPS spectra (Figs. 6 and 7) which show that Ni(+2) species that are spread out on the different substrates reach approximately the same reduction state. Furthermore, it is shown that the Ni surface concentration is about the same whatever the substrate, and that it is clearly higher after 1 min of Ar plasma treatment (¨15 at.%) than after 15 s of NaBH4 treatment (¨6 at.%) (see inserts in Figs 6 and 7). This suggests that in spite of appearances, the surface reactivity for the autocatalytic reaction which is due to the Ni(0) species is not the same in both cases and therefore that the Ni(0) surface seeding depends on the reduction route. For the present, we have no convincing explanation about this surprising behavior except for some hypotheses. The chemical reduction could give a rather homogeneous and dense distribution of the Ni(0) species. It could be conjectured that each Ni(0) atom is strongly chemisorbed
Fig. 7. Ni 2p XPS spectra of the reference ‘‘NiAc/eth’’ precursor deposited on PI and not submitted to reduction (a), of this same precursor deposited on PI, PC, PP, PTFE (b) to (e), respectively and reduced by Ar plasma for 1 min at room temperature. The corresponding Ni surface concentrations are given in the insert.
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on the polymer surface, each on its own site, which would make the surface very reactive towards the autocatalytic reaction. Indeed, the wet chemical route is particularly efficient in bringing the electrons necessary for the Ni(+2) ion reduction onto the whole surface, while plasma decomposition of the organic precursor operates in a totally different manner which could lead to a heterogeneous distribution of the Ni(0) species. The active species of the plasma break the chemical bonds of the precursor and the electrons present in the discharge which are able to reduce Ni(+2) ions are probably brought in a very random way to the substrate surface despite the fact that the cathode on which the sample lies is surrounded by an ion shield responsible for the etching process. The latter could lead to the formation of clusters scattered on the surface. These large clusters are probably not numerous enough to allow the initiation of the reaction with the reducer-poor, lowphosphorus Enthone bath. In any case, the bonding and distribution of the Ni(0) species on the substrate surface are probably strongly dependent on the reduction route used as shown by high resolution SEM micrographs. Fig. 8 displays the Ni/PI interface for samples whose Ni(+2) reduction was performed using the NaBH4 chemical way (a) and the Ar plasma (b).In both cases, the interfacial zone shows a good continuity between film and substrate, in that the metal film fits the surface substrate exactly. However, this continuity appears less perfect in the case of the Ar plasma reduction which supports the aforementioned hypothesis, viz. the presence of Ni(0) clusters scattered on the polymer surface. Finally, the substrate nature is to a great extent responsible for the film adhesion. Indeed, it is at the interface between Ni film and polymer substrate that stresses due to the difference of elasticity modulus of the two partners of the system are maximal [8]. Internal stresses in the Ni film strongly depend on the film composition therefore on the Ni plating bath used. A low-phosphorus film (obtained here with the Ni Enthone plating bath) which presents a low amount of Ni\P covalent bonds is less stressed than a high-phosphorus one (obtained with the Mac Dermid bath). Consequently, as
Fig. 8. High resolution SEM micrographs of the Ni/PI interface of samples whose Ni(+2) reduction was obtained (a) via a chemical way in a NaBH4 solution and (b) an Ar plasma.
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shown in Table 1, it is possible to obtain more than 1 Am thick, well-adhering Ni films with the Enthone bath while with the Mac Dermid bath films of similar thickness may not be adhering at all (case of PC and PP). As it does not appear to be possible to start the autocatalytic reaction with the Enthone bath after reduction by Ar plasma, we could not obtain adhering thick films on PC and PP by this method. It must however be noted that, when the autocatalytic reaction is initiated on PTFE (after Ni(+2) reduction by NaBH4), it leads to perfectly adhering thick Ni films whatever the Ni plating bath used. Furthermore, it is important to note that the kinetics of Ni deposition is intimately associated with that of autocatalytic reaction initiation. Experiment shows that the lower the initiation time, the higher the deposition kinetics and the better the Ni coating adhesion. It must also be pointed out that the excellent results concerning the Ni film adhesion obtained on some of the substrates studied here are quite comparable to those obtained for films of similar thickness obtained using Pd as a catalyst. On the other hand Ni films deposited on PC and PP after seeding of their surface with palladium are well-adhering, at least when their thickness does not exceed 1 to 2 Am.
4. Conclusions This work has shown how to obtain an autocatalytic Ni film on a PI surface without using the Pd conventional catalyst. For this an ultra-thin layer of an organic nickel salt must be deposited on the surface to be metallized and the corresponding Ni(+2) ions reduced to Ni(0) via either a chemical path in a solution of NaBH4 or a short H2 or Ar plasma treatment (30 s to 1 min). To obtain Ni metallization, it is necessary to rapidly initiate the autocatalytic reaction. The success of the deposition depends on: (i) the nature of the Ni plating bath which must be ‘‘rapid’’ enough to prevent the Ni(0) species lixiviation, (ii) the nature of the polymer substrate and (iii) the Ni(0) seed density and distribution on the polymer surface, i.e. the route followed to achieve the Ni(+2) species reduction. The chemical reduction by NaBH4 allows us to obtain well-adhering deposits on all the substrates with both low- and highphosphorous Ni plating baths. On the other hand, Ar plasma, despite its very efficient reduction power which keeps on the average more Ni species on the substrate surface than NaBH4 reduction does, never leads to Ni deposition when the autocatalytic process is performed with the low-phosphorous bath (Enthone). After an Ar plasma treatment and with the high-phosphorous bath (Mac Dermid), no Ni deposit is obtained on PTFE because the Ni(0) species that are able to initiate the reaction are not strongly bonded enough on its surface and lixiviate in the hot plating bath. However on PI, PBT, PPS and LCP substrates, the Ni deposition is immediate and leads to perfectly adhering thick Ni films. Lastly, Ni deposits are
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easily obtained on PC and PP but beyond 0.2 Am in thickness, they spontaneously crack and no longer adhere on their substrate. All these experiments show the key role played by the substrate nature and by the bonding of the chemical species to the sample surface, i.e. in fine, by the interface strength.
References [1] M. Alami, M. Charbonnier, M. Romand, Proceedings of the 9th International Colloquium on Plasma Processes (CIP 93), Socie´te´ Franc¸aise du Vide, Paris, 1993, p. 392. [2] M. Alami, M. Charbonnier, M. Romand, J. Electrochem. Soc. 143 (1996) 472. [3] M. Alami, M. Charbonnier, M. Romand, Plasmas Polym. 1 (1996) 113. [4] M. Charbonnier, M. Alami, M. Romand, J. Appl. Electrochem. 28 (1998) 449. [5] M. Charbonnier, M. Romand, M. Alami, Tran Minh Duc, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, vol. 2, VSP, Utrecht, The Netherlands, 2000, p. 3. [6] M. Charbonnier, M. Romand, M. Alami, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, vol. 2, VSP, Utrecht, The Netherlands, 2000, p. 29. [7] M. Charbonnier, M. Romand, E. Harry, M. Alami, J. Appl. Electrochem. 31 (2001) 57. [8] M. Charbonnier, M. Romand, Surf. Coat. Technol. 162 (2002) 19. [9] M. Charbonnier, M. Romand, U. Kogelschatz, H. Esrom, R. Seebo¨ck, in: K.L. Mittal (Ed.), Metallized Plastics 7: Fundamental and Applied Aspects, VSP, Utrecht, The Netherlands, 2001, p. 3. [10] A. Bauer, J. Ganz, K. Hesse, E. Ko¨hler, Appl. Surf. Sci. 46 (1990) 113. [11] G. Shafeev, W. Marine, H. Dallaporta, L. Bellard, A. Cros, Thin Solid Films 241 (1994) 52. [12] H. Horn, S. Beil, D.A. Wesner, R. Weichenhain, E.W. Kreutz, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 151 (1999) 279. [13] H. Esrom, Appl. Surf. Sci. 168 (2000) 1. [14] H. Esrom, J. Demny, U. Kogelschatz, Chemtronics 4 (1989) 202. [15] U. Kogelschatz, B. Eliasson, H. Esrom, Mater. Des. 12 (1991) 251. [16] H. Esrom, U. Kogelschatz, Thin Solid Films 218 (1992) 231. [17] J.Y. Zhang, H. Esrom, I.W. Boyd, Appl. Surf. Sci. 96 – 98 (1996) 399. [18] J.Y. Zhang, I.W. Boyd, Appl. Phys., A 65 (1997) 379. [19] J.Y. Zhang, S.L. King, I.W. Boyd, Q. Fang, Appl. Surf. Sci. 109 – 110 (1997) 487. [20] J.Y. Zhang, I.W. Boyd, S. Draper, Surf. Coat. Technol. 100 – 101 (1998) 469.
[21] J.Y. Zhang, I.W. Boyd, Thin Solid Films 318 (1998) 234. [22] D.J. Macauley, P.V. Kelly, K.F. Mongey, G.M. Crean, Appl. Surf. Sci. 138 – 139 (1999) 622. [23] Z. Geretovszky, I.W. Boyd, Appl. Surf. Sci. 138 – 139 (1999) 401. [24] A.G. Schrott, B. Braren, E.J.M. O’Sullivan, R.F. Saraf, P. Bailey, J. Roldan, J. Electrochem. Soc. 142 (1995) 944. [25] K. Korda´s, J. Be´ke´si, K. Bali, R. Vajtai, L. Na´nai, T.F. George, S. Leppa¨vuori, J. Mater. Res. 14 (1999) 3690. [26] K. Korda´s, L. Na´nai, K. Bali, K. Ste´pa´n, R. Vajtai, T.F. George, S. Leppa¨vuori, Appl. Surf. Sci. 168 (2000) 68. [27] K. Korda´s, S. Leppa¨vuori, A. Uusima¨ki, T.F. George, L. Na´nai, R. Vajtai, K. Bali, J. Be´ke´si, Thin Solid Films 384 (2001) 185. [28] R.R. Thomas, J.M. Park, J. Electrochem. Soc. 136 (1989) 1661. [29] G.J. Fisanick, M.E. Gross, J.B. Hopkins, M.D. Fennell, K.J. Schnoes, A. Katzir, J. Appl. Phys. 57 (1985) 1139. [30] M. Wehner, F. Legewie, B. Theisen, E. Beyer, Appl. Surf. Sci. 10 (1996) 406. [31] J.Y. Zhang, I.W. Boyd, J. Mater. Sci. Lett. 16 (1997) 996. [32] G.J. Berry, J.A. Cairns, M.R. Davidson, Y.C. Fan, A.G. Fitzgerald, J. Thomson, W. Shaikh, Appl. Surf. Sci. 162 – 163 (2000) 504. [33] K. Korda´s, J. Be´ke´si, R. Vajtai, L. Na´nai, S. Leppa¨vuori, A. Uusima¨ki, K. Bali, T.F. George, G. Galba´cs, F. Igna´cz, P. Moilanen, Appl. Surf. Sci. 172 (2001) 178. [34] A. Gupta, R. Jagannathan, Appl. Phys. Lett. 51 (1987) 2254. [35] M. Bode, E.W. Kreutz, M. Kro¨sche, Appl. Surf. Sci. 46 (1990) 148. [36] H.G. Mu¨ller, Appl. Phys. Lett. 56 (1990) 904. [37] H.G. Mu¨ller, K. Buschick, S. Schuler, A. Paredes, Appl. Surf. Sci. 46 (1990) 143. [38] J.Y. Zhang, H. Esrom, Appl. Surf. Sci. 54 (1992) 465. [39] K. Korda´s, K. Bali, S. Leppa¨vuori, A. Uusima¨ki, L. Na´nai, Appl. Surf. Sci. 154 (2000) 399. [40] X.C. Wang, H.Y. Zheng, G.C. Lim, Appl. Surf. Sci. 200 (2002) 165. [41] R. Padiyath, M. David, S.V. Babu, in: K.L. Mittal (Ed.), Metallized Plastics 2: Fundamental and Applied Aspects, Plenum Press, New York, 1991, p. 113. [42] M. Seita, M. Kusaka, H. Nawafune, S. Mizumoto, Plating Surf. Finish. 83 (1996) 57. [43] A. Brocherieux, O. Dessaux, P. Goudmand, L. Gengembre, J. Grimblot, M. Brunel, R. Lazzaroni, Appl. Surf. Sci. 90 (1995) 47. [44] G.O. Mallory, in: G.O. Mallory, J.B. Hajdu (Eds.), Electroless Plating. Fundamentals and Applications, American Electroplaters and Surface Finishers Society, Inc., Orlando, FL, 1990, p. 1. [45] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers. The Scienta ESCA 300 Database, John Wiley and Sons, Chichester, UK, 1992, p. 1. [46] M. Charbonnier, M. Romand, Y. Goepfert, Trans. Mater. Heat Treat. 25 (5) (2004) 1106. [47] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 58th edition, CRC Press, Cleveland, Ohio, 1977/1978.
Ni direct electroless metallization of polymers by a new palladium-free process Marle`ne Charbonnier a,*, Maurice Romanda, Yves Goepfert b a
Laboratoire de Sciences et Inge´nierie des Surfaces, Universite´ Claude Bernard-Lyon 1, 69622 Villeurbanne Cedex, France b UMR CNRS # 5180 des Sciences Analytiques, Universite´ Claude Bernard- Lyon 1, 69622 Villeurbanne Cedex, France Received 16 December 2004; accepted in revised form 13 May 2005 Available online 17 June 2005
Abstract Ni films were electrolessly deposited on the surface of different polymer substrates without having to use the conventional palladium catalyst to initiate the redox reaction leading to metallization. With this in view, the surface to be coated was seeded with Ni(+2) species from an organo-nickel precursor. After reduction of the Ni(+2) ions to the Ni(0) oxidation state, it was possible to use the remarkable autocatalytic property of nickel to deposit a metallic film on the polymer by merely immersing in a Ni plating bath. The reduction of Ni(+2) species, less easy than that of Pd(+2) species commonly used in the authors’ laboratory in the case of the tin-free process, was performed in the present work either using a chemical path or by plasma treatment in a non-oxidizing atmosphere (H2 or Ar), then checked by XPS. This method was experimented on various polymer substrates using two different industrial Ni plating baths. Adhesion of Ni films evaluated by the Scotch\ tape test mainly depends on the phosphorus content of the plating bath used, i.e. on the amount of stresses generated in the film which depends on the quantity of phosphorus present, the latter originating from the plating bath reducer. Both the nature of the polymer surface treatment and especially the route used to perform the Ni(+2) reduction play an important part in the interfacial stresses generated and therefore on film adhesion. D 2005 Elsevier B.V. All rights reserved. Keywords: [B] Photoelectron spectroscopy; [B] Adhesion; [C] Electroless deposition; [C] Radio frequency (RF) plasma treatments; [D] Nickel; [X] Direct Ni electroless plating
1. Introduction In its most common applications, electroless metallization consists of a redox reaction between Ni(+2) or Cu(+2) ions and a strong reducer – sodium hypophosphite NaH 2 PO 2 for Ni(+2) and formaldehyde HCHO for Cu(+2) – both contained in the same solution. Electroless metallization of polymers is generally considered possible if a catalyst allowing the reaction initiation (palladium in the Pd(0) oxidation state) is present on the surface to be coated. The main difficulty of this process consists of chemisorbing the catalyst on the polymer surface and several routes have been proposed in the literature for this purpose. In recent work on this topic [1 – 9], we have * Corresponding author. Tel.: +33 4 72 44 83 06; fax: +33 4 72 43 12 06. E-mail address: [email protected] (M. Charbonnier). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.05.006
developed a tin-free method (Fig. 1) to attach the Pd(+2) species on a polymer surface and to reduce them to Pd(0) using the wet route. Under these conditions, the initiation of metal deposition is immediate when the Pd(0)-seeded surface is immersed in the plating bath. Metallization then continues thanks to the remarkable property of Ni and Cu in their metallic state to catalyze the reduction of their own ions, hence the term autocatalytic is also used to describe the plating process. Performing the direct electroless metallization of insulating substrates (polymers, glasses, ceramics) without seeding their surface with Pd(0) catalytic clusters is an interesting challenge which aims at reducing the cost of the metallization process. In this work, we consider only the electroless metallization with one of the two most commonly useable metals, viz. Ni. For this it is necessary to seed the surface to be metallized with Ni(0) clusters which
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Immersion in PdCl2 Immersion in H2PO2NH3 plasma (0.1 g L-1, 25°, 2 min) (10 g L-1, 85°, 3 min)
Grafting of nitro- Pd(+2) chemisorption Partial reduction genated groups on nitrogen groups of Pd(+2) toPd(0)
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Nickel film
Electroles plating
Fig. 1. Surface preparation of polymer substrates for electroless metallization by the tin-free process developed in the authors’ laboratory.
should be able to initiate the electroless plating in the same way that Pd(0) clusters do in the usual simplified electroless process [1 –9]. Over the last few years, many studies have been devoted to VUV light-induced decomposition of organo-palladium films so as to obtain the Pd seeds which act as the catalyst for the initiation of the electroless metallization. Various organo-palladium compounds were deposited by spinning-on, spraying or dip-coating then decomposed by a variety of radiation sources [10 –23] viz. laser, VUV excimer lamp, and even IR sources. Pd(0) clusters were also obtained in a single step by focusing an excimer laser beam on the substrate across a solution of the organo-palladium precursor [24 – 27]. All these methods can lead to obtaining patterned surfaces for microelectronics applications, either by moving the sample under the laser beam (laser writing), or by irradiating the organic precursor via a mask. In other works [28] the photodecomposition, (using a k = 254 nm radiation emitted by a low pressure Hg lamp), of allylcyclopentadienyl palladium in the gas phase was used to obtain a Pd layer on the substrate. Similar methods were used to deposit other noble metals such as Au or Pt [29 –33] and Cu [30,33 –40] without the intervening help of Pd to initiate the electroless reaction. In the last case, Cu deposits were achieved in one step by irradiating the substrate via a copper electroless plating solution with a variety of laser radiations [33,39,40]. Under these conditions, after the formation of Cu(0) seeds which are able to catalyze the initiation of the electroless deposition, the laser beam accelerates the plating process by heating the solution. Other authors [34 –38] use spincoating, spraying or evaporation to deposit thin films of copper salts from a solution or from the solid salt itself on various substrates. In a second step, they perform direct laser writing of copper by a local pyrolytic or photolitic decomposition of the film with a Nd:YAG, Ar+ or excimer
NH 3 or O 2 plasma
Surface functionalization
NiAc/ethanol film
Spincoating
laser beam. Another route to achieve direct copper metallization developed by Padiyath et al. [41] consists of depositing, as in the previous case, a thin copper formate film by spin-coating then reducing it by an RF H2 plasma for 15 min. Finally, a purely chemical process developed by Seita et al. [42] involves the sulfonation of the surface to be metallized by fuming sulfuric acid, adsorption of the Cu(+2) species from a cupric aqueous solution and their reduction to Cu(0) by immersion in a sodium borohydride (NaBH4) solution. To our knowledge, direct Ni metallization has not been mentioned in the literature but through a paper from Brocherieux et al. [43] who deposited Ni-based films on ABS substrates by remote MW plasma of Ni(CO)4. These films had to be treated by N2 plasma for 15 min to eliminate the remaining carbonyl species before being coated with Ni by electrodeposition. Taking into account the high price and difficulty of ready supply of nickel tetracarbonyl, we have ruled out this possibility of Ni surface seeding and turned our research to a method similar to those used for direct Cu metallization. This work takes advantage of the autocatalytic property of Ni. The first step deals with the deposition of a thin organo-nickel film and with its chemisorption on the substrate surface, which had previously been plasma treated. The second step, which is the main difficulty encountered in this work, consists of performing the reduction of Ni(+2) ions from the organo-Ni film to Ni(0) while maintaining enough Ni species strongly adsorbed on the surface to initiate the autocatalytic reaction. Different ways of reduction (chemical, photochemical, plasma) are explored and Ni metallization is performed for varying durations of time. Fig. 2 summarizes the succession of the different steps of the process studied here. Adhesion of the Ni films is evaluated and compared to that obtained for similar films using Pd to initiate the metallization.
Ni(0) seeds
Electroless Ni film
Ni(+2) reduction
Autocatalytic plating
Fig. 2. Surface preparation of polymer substrates for direct Ni electroless metallization by the new palladium-free process described in the present work.
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2. Experimental The new process described in this paper was, at first, developed on a poly(etherimide) substrate (Kapton\ HN) indexed in the following as PI and supplied by Goodefellow (UK). It was then extended to various other substrates including poly(carbonate) (PC), poly(propylene) (PP), poly(tetrafluoroethylene) (PTFE), poly(butyleneterephthalate) (PBT) or liquid crystal polymers (LCP). The surface treatment of polymers (surface cleaning, surface functionalization, reduction of Ni(+2) ions) was performed by plasma in ammonia, oxygen, argon or hydrogen atmosphere in a 13.56 MHz RF reactor (RIE 80 model from Plasma Technology, England). This reactor is equipped with two water cooled parallel and dissymmetrical electrodes and with four gas lines. The samples to be treated are placed on the RF powered electrode (17 cm in diameter) which is capacitively coupled to the power supply, through an impedance matching network. In this experimental device, the powered electrode is submitted to a bias voltage so that samples are, among others, bombarded by ions whose energy is dependent on the power density. All experiments were carried out at a power density of 0.52 W cm 2. As a consequence, the energy of the ions bombarding the sample is approximately 300 eV. The other experimental conditions were as follows: gas flow: 100 sccm, working pressure: 100 mTorr, treatment time: 30 s to 1 min. The organo-metallic precursor used was nickel acetate in ethanol solution (5 g L 1) referred to in the following as ‘‘NiAc/eth’’, the dissolution being done at about 80 -C. It was generally deposited on the substrate by spin-coating (rotation speed: 2000 rpm), but spraying or dipping were sometimes used. Nickel ions adsorbed on the substrate surface were reduced either by plasma treatment in a H2 or Ar atmosphere or chemically by dipping in a 0.5 M NaBH4 solution. BH4 is a powerful reducing agent whose decomposition in basic solution yields 8 electrons according to the following equation [44]: BH 4 þ 8OH Y BðOHÞ4 þ 4H2 O þ 8e
The redox potential E 0 (B(OH)4 / BH4) is equal to 1.24 V at pH = 14. Under these conditions, the overall redox reaction is: 0 4Ni2þ þ BH 4 þ 8OH Y BðOHÞ4 þ 4Ni þ 4H2 O:
Nickel plating baths were proprietary baths of complex and unknown composition. They work with sodium hypophosphite as a reducing agent. Two baths were used, viz. a high-phosphorus one (Europlate\ Ni 520 supplied by Mac Dermid –Frappaz, Neyron, France) and a low-phosphorus one (Enplate\ Ni 426 supplied by Enthone-OMI, France), both leading to the incorporation, into the final Ni coating, of 7 –9 and 1– 3 wt.% of phosphorus, respectively.
The Ni(2+) reduction reaction by hypophosphite is very complex and may be written, in a schematic way, as follows [44]: 0 þ Ni2þ þ 2H2 PO 2 þ 2H2 O Y Ni þ 2H2 PO3 þ H2 þ 2H
Phosphorus incorporation in the Ni deposit results from a secondary reaction due to hypophosphite disproportionation in acidic medium: 3H2 PO2 þ 2Hþ Y 2P þ H2 PO 3 þ 3H2 O: Surface composition modifications were studied by Xray photoelectron spectroscopy (XPS) with a RIBER SIA 200 instrument, using a non-monochromatic Al source. All spectra were referenced either to the C 1s peak (C\C and C\H bonds) whose binding energy was fixed at 285.0 eV or in the case of PTFE, to the C 1s peak (CF2 groups) at 292.5 eV according to the value given by Beamson and Briggs [45]. In some particularly ambiguous cases, charging effect correction was done with respect to the Au 4f7/2 peak at 84.0 eV. All samples for XPS analyses were introduced in the spectrometer, i.e. under UHV, within 5 min after the corresponding surface modifications. Surface concentrations determined from the XPS peak areas are considered as accurate to within T 10%. Microscopic observations of the metal/interface were carried out using a high resolution FEG S800 HITACHI instrument operating at 15 kV. Samples devoted to observation were surfaced by cutting at 75 -C with a diamond knife and then gold-coated. Evaluation of adhesion of nickel films on their substrate was performed using the standard ASTM D 3359 Scotch\ tape test (cross-cut tape test performed with a 3 M 250 3710 Scotch\ tape). The adhesion scale ranges from 5 (no film square removed by the tape) to 0 (more than 65% film squares removed). Although this test does not give quantitative results, it allows in some cases classification of the effects of different surface treatments and comparison of the adhesion levels of films obtained under various conditions. Film thicknesses were determined by X-ray fluorescence spectroscopy (XRFS) via the measurement of the intensity of the Ni Ka radiation with a calibration curve obtained by gravimetry used as reference. Photochemical decomposition of the metal-organic precursor was performed under primary vacuum using a Xe2* excimer lamp (Excivac Laboratory System, Heraeus Noblelight, Germany). The experimental set up consists of two contiguous chambers which can be evacuated separately (primary vacuum). The upper chamber which contains the excimer lamp is separated from the reactor by a 5 mm thick calcium fluoride window. The discharge in the lamp emits an incoherent radiation which peaks at a wavelength of 172 nm with a full width at half maximum of about 15 nm. The maximal photon flux irradiating the sample surface is estimated at 3 1016 photons cm 2 s 1 [9].
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3. Results
3.1. Reduction of Ni(+2) thin films
The development of this new Pd-free process was first performed on PI substrates [46].The choice of the organonickel precursor is governed by its solubility in various solvents. In fact, organic nickel salts are, for the most part, highly insoluble or at best only slightly soluble in common solvents. Among the most common of them, nickel formate (NiFo), nickel acetate (NiAc) and nickel acetylacetonate (NiAcAc) were investigated and of these NiAc (soluble in H2O, NH3,H2O and slightly soluble in ethanol) was retained. In the present work, the aim was to deposit an ultra-thin film of precursor on the substrate surface in order to obtain, after reduction of Ni(+2), a surface with enough Ni(0) seeds to start the autocatalytic reaction (Fig. 2). Indeed, it would be detrimental for the adhesion of the electroless nickel film to initiate the autocatalytic reaction from too ‘‘thick’’ a Ni film obtained by reduction of the organic precursor. Consequently the solvent of this precursor must be chosen with care since it plays an important part on the solution spreading out and therefore on the precursor film thickness whatever the deposition method used (spin-coating, spraying, dipping). Therefore a solution of NiAc in ethanol within the concentration range 5 to 20 g L 1 was retained. The latter, obtained by heating (¨80 -C) is probably a nickel ethanoate. A perfect spreading out of this solution was obtained by spin-coating using 1 or 2 drops. As a result, 3 to 6 Al of solution were deposited on a 9 cm2 area. Before depositing the precursor on the PI substrate, it was necessary to activate its surface in order to make it wettable and reactive towards the precursor solution. This was done by plasma surface treatment in oxygen or better still in ammonia for 1 min, which allows both the grafting of polar functional groups as well as the establishment of strong chemical bonds between precursor and substrate [1 –9].
3.1.1. Chemical reduction Taking the Pd(+2) reduction by H2PO2 in the simplified process developed in the laboratory (Fig. 1) as a model, Ni(+2) reduction with different H2PO2 solutions (0.1, 1 and 5 M) at 85 -C for different times between 15 s and 3 min was attempted. Unfortunately, H2PO2 is not sufficiently strong to cause the reduction of Ni(+2) ions before their lixiviation in the hot solution. XPS analyses show that, after such treatments, Ni virtually disappears from the PI surface. Such a behavior proves that Ni(+2) ions are less strongly adsorbed on the polymer substrate than Pd(+2) ones and that their reduction is more difficult. Another chemical reducer viz. sodium borohydride (NaBH4) which is sometimes used as a reducer in Ni plating baths [46] was also tested. Seita et al. [42] have shown that it is quite efficient in reducing Cu(+2) ions. A mere immersion of the sample coated with the NiAc precursor in a 0.5 M NaBH4 solution at room temperature for 5 to 15 s leads to a partial Ni(+2) ion reduction. Furthermore, this operation maintains enough Ni(0) species on the substrate surface to allow immediate Ni deposition when the sample is immersed in either of the two Ni industrial plating baths. Fig. 3 represents Ni 2p XPS spectra of the NiAc precursor before (a) and after (b) the NaBH4 treatment, and of Ni metal (c) for comparison. The insert provides the Ni surface concentration for the (a) and (b) samples. This figure shows the appearance of a shoulder on the low binding energy side of the Ni 2p peak (spectrum (b)) which characterizes the presence of Ni(0) species together with Ni(+2) ones (main Ni 2p peak). The very efficient reduction performed by NaBH4 is unfortunately handicapped by the fact that the solution is very unstable and cannot remain active for more than 1 or 2 h.
Fig. 3. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) NaBH4 reduction, and of Ni metal (c).
3.1.2. Plasma reduction Another way of ensuring Ni(+2) reduction is to use a hydrogen plasma as done by Padiyath et al. [41] to reduce Cu(+2) ions to Cu(0). Indeed, this method is able to reduce Ni(+2) ions to Ni(0), but, in contrast to the treatment time of 15 min recommended by Padiyath et al, we have shown by XPS that the shortest treatment times (15 –30 s) are the most efficient concerning both reduction rate and nickel surface concentration. Fig. 4 shows the energy shift of the Ni 2p peak towards the low binding energy side for various H2 plasma treatment times and in the insert, the corresponding Ni surface concentration variations. It appears that after 2 min of treatment a slight re-oxidation of Ni atoms is observed together with a decreasing of the Ni surface concentration. This Ni re-oxidation seems rather surprising in a reducing atmosphere. However in plasma, the reaction mechanisms are quite different from those occurring in the usual gas phase or in solution. In the present case, the different active species of the plasma (HI, H+, e and photons) break the bonds of the organic precursor and form
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after the shortest treatments (less than 30 s to 1 min).This is due to the rapid decomposition of the precursor and to the escape of volatile fragments. Under these conditions, Ni species which are no more drown in the organic part of the precursor are therefore present in larger concentration.
Fig. 4. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) to (g) reduction by H2 plasma for 5 s, 15 s, 30 s, 1 min, 2 min and 5 min, respectively. The Ni surface concentration variation versus the H2 plasma treatment time is represented in the insert.
small fragments or even atoms which may recombine with each other to give volatile new compounds that are evacuated by the pumping system. Simultaneously, the electrons present in the discharge reduce the Ni(+2) ions to Ni(0). Furthermore the H+ ions which bombard the sample lying on the powered electrode lead to the etching of the organic part of the precursor in a first step. When the volatile compounds corresponding to the precursor decomposition have disappeared (after a short plasma treatment time), O2 and/or H2O molecules present in the residual atmosphere (the base pressure in the plasma reactor is about 1 to 2 mTorr) may be responsible for the Ni re-oxidation, while the H+ ion bombardment provokes the progressive loss of Ni(0) species. Both for H2 plasma treatment times less than 1 min as well as for longer ones, Ni metallization is immediate with the Mac Dermid-Frappaz bath whereas it does not occur with the Enthone bath, in the composition of which there are probably very strong stabilizers. If the interpretation of the reaction mechanisms given for H2 plasma is correct, any plasma of a non reactive gas (rare gas) should give similar results. An Ar plasma which contains as active species Ar+ ions, electrons and photons allowed us to verify our hypothesis. Indeed, as shown in Fig. 5 which gives the binding energy shift of the Ni 2p peaks and, in the insert, the Ni concentration variations as a function of Ar plasma duration, the same effects as those obtained for H2 plasma were observed. The optimal reduction time is in the range 15 – 30 s. Beyond 2 min of Ar plasma, there is, as in the previous case and for the same reasons, a re-oxidation of Ni atoms and a decreasing of the Ni surface concentration. Here too, Ni metallization is immediate with the Mac Dermid bath after 30 s or 1 min of plasma treatment but not possible with the Enthone one. With both H2 and Ar plasma treatments, an increase in the Ni concentration is observed (see inserts in Figs. 4 and 5)
3.1.3. VUV-induced reduction Taking the experiments performed using UV or VUV radiations emitted by a laser or an excimer lamp to decompose various organo-Pd, Au, Pt or Cu precursors [6 – 21] as an example, we have also tried out this route to reduce organo-nickel precursors. In order to do this, ultrathin ‘‘NiAc/eth’’ films were deposited on PI and submitted to VUV radiation (k = 172 nm, E = 7.2 eV) emitted by the Xe2* excimer lamp, for 5, 10 and 20 min under primary vacuum. Contrary to what happens with Pd, Au, Pt or Cu organic salts under similar conditions, no decomposition or reduction of NiAc was observed after irradiation. Indeed, XPS analysis of these samples showed the presence of only C, O and Ni (the latter in the Ni(+2) form) characteristic of NiAc and these, in the same concentrations whatever the irradiation time. It can be concluded that NiAc is too stable to be decomposed by the radiation used. If, according to literature data [47] PdAc and CuAc are decomposed at 195 and 240 -C respectively, no indication concerning NiAc thermal stability is found. To break Ni\O bonds in NiAc in order to make free Ni, more energetic radiations e.g. the 126 nm (9.84 eV) radiation emitted by the Ar2* excimer lamp are probably necessary. However, as indicated by Macauley et al. [22], if the thermal stability of the compound plays an important part in its decomposition, another factor viz. its capability to absorb the said radiations is a key factor of the photolysis. Indeed, the latter cannot take place if the organo-metal precursor does not absorb the radiation. As an example, cobalt acetate (CoAc) which is known to decompose at
Fig. 5. Ni 2p XPS spectra of the ‘‘NiAc/eth’’ precursor deposited on PI, before (a) and after (b) to (g) reduction by Ar plasma for 5 s, 15 s, 30 s, 1 min, 2 min and 5 min, respectively. The Ni surface concentration variation versus the Ar plasma treatment time is represented in the insert.
M. Charbonnier et al. / Surface & Coatings Technology 200 (2006) 5028 – 5036
100 -C [47], was deposited by spin-coating on a substrate and submitted to the 172 nm UV radiation emitted by the Xe2* excimer lamp. After 10 min of irradiation, XPS analysis did not show the least reduction, which proves that the decomposition temperature is not the only factor to be considered in order to obtain the photo-reduction of such compounds.
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3.3. Discussion Table 1 clearly shows that Ni metallization is always possible for all the substrates studied after a chemical reduction of the Ni(+2) species by NaBH4 and with the two industrial plating baths used in the present study. As shown, the adhesion performances are dependent on the electroless Ni film thickness and substrate nature. However, no metallization is obtained with the Enthone bath when the Ni(+2) reduction is performed by Ar or H2 plasma (30 s– 1 min). Under the same conditions, PI, PC and PP can be metallized with the Mac Dermid plating bath, whereas PTFE cannot, even though, after Ar or H2 plasma, as after NaBH4 reduction the sample surface is greyish. This attests to the presence of Ni(0) species on the surface as do the Ni 2p XPS spectra (Figs. 5 and 6). However, after a short duration (10 to 20 s) in the plating bath, PTFE samples recover their initial white color which means that the Ni(0) species are weakly adsorbed on the surface and lixiviate in the hot Ni solution (85 -C) before the reaction initiation takes place. With PI, PPS, PBT and LCP, it is possible to obtain thick and well-adhering Ni films after Ar or H2 plasma. Though Ni metallization takes place normally with the Mac Dermid plating bath on PC and PP, problems arise when the film thickness increases (beyond ¨0.5 Am). In some cases, the film obtained is bright and apparently ‘‘perfect’’ as long as it is maintained in the plating bath but cracks as soon as it is transferred to the rinsing bath even though the temperature of this bath is raised to 85 -C in the hope of releasing the stresses. In other cases, if the film is
3.2. Extension of the method to other substrates — adhesion studies Different polymers known as difficult or even impossible to metallize using the conventional electroless process were studied with respect to direct Ni metallization. Among them, poly(butyleneterephthalate) (PBT), poly(phenylene sulfur) (PPS), liquid crystal polymers (LCP), polypropylene (PP), polycarbonate (PC) and polytetrafluoroethylene (PTFE), were surface treated by NH3 plasma (1 min), spin-coated using 1 or 2 drops of the ‘‘NiAc/eth’’ solution. Subsequently, the NiAc thin film was reduced either by NaBH4 or by Ar or H2 plasma and the surface-modified substrate was immersed in each of the industrial plating baths. The success of metallization depends on the substrate and plating bath nature as well as on the Ni(+2) reduction route. Results given in Table 1 concern Ni metallization of PI and of those polymers known to be difficult to metallize, viz. PC, PP, PTFE. The other polymers (PBT, PPS, LCP) studied in this work are metallized as easily as PI and give rise to welladherent Ni films. The corresponding results are regrouped in Table 1 with those of PI.
Table 1 Results (film thickness and adhesion) of the direct Ni metallization performed in the high and low-phosphorus baths on different polymer substrates Substrate
Mac Dermid Ni plating bath
Enthone Ni plating bath
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch test* results
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test results
10
1.8
**
5
20
2.5
**
5
0.6 1.3 0.4 1.8 2.4
**
5 0 4 0 5
10
1.2
**
5
10
1.3
**
5
PTFE
4 10 4 10 15
15
1.9
**
5
Ni(+2) reduction by Ar or H2 plasma(1 min)
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test* results
Deposition time (min)
Ni film thickness (Am)
Aspect
Scotch\ test* results
20
2.9
**
5
5
no deposit
0.8 0.7 no deposit
**
0 0
5 5 5
no deposit no deposit no deposit
Ni(+2) reduction by NaBH4 PI-PPS PBT LCP PC PP
PI-PPS PBT LCP PC PP PTFE
5 5 5
#
** #
**
#
\
*Scotch\ test results (5) no film square removed by the tape; (4) ¨5% of film squares removed by the tape; (0) more than 65% of film squares removed by the tape. **Bright and homogeneous Ni film. # Crackled or blistered Ni film.
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M. Charbonnier et al. / Surface & Coatings Technology 200 (2006) 5028 – 5036
Fig. 6. Ni 2p XPS spectra of the reference ‘‘NiAc/eth’’ precursor deposited on PI and not submitted to reduction (a), of this same precursor deposited on PI, PC, PP, PTFE (b) to (e), respectively and reduced by NaBH4 for 15 s at room temperature. The corresponding Ni surface concentrations are given in the insert.
too thick, it presents some blisters and spontaneously breaks up in the solution. In comparison, all the polymers that have been rendered catalytic by Pd grafting can be metallized with either of the two industrial baths, whatever the route followed for the Pd(+2) reduction (NaBH4 or NaH2PO2 solutions, Ar or H2 plasmas) (results to be published). As these films adhere well, it can be concluded that Pd(0) is a more active catalyst than Ni(0) to initiate the electroless reaction. Indeed, Pd(0) is able to lower the activation energy of this reaction, even though the plating baths contain powerful stabilizers as is probably the case for the Enthone bath. These stabilizers are used to maintain the redox solution in a metastable state and therefore to prevent its spontaneous decomposition. In the case of the catalysis using the Ni(0) species, the stabilizers may delay or even hinder the initiation of the autocatalytic reaction through a kind of poisoning of the Ni catalytic sites, hence resulting in an increase in the activation energy of the reaction. Data in Table 1 suggest that the plating bath also plays a key role on the kinetics of the autocatalytic reaction initiation. Before its use for direct metallization, a Ni bath must have functioned sufficiently long so as to operate under optimal conditions i.e. to allow rapid initiation of the electroless reaction. It is in fact well known that a new plating bath does not run spontaneously but must be activated, e.g. by immersing a piece of pure metal (here Ni) cleared of its surface natural oxide. The problems encountered with the industrial baths used in a research laboratory explain why for similar deposition times film thickness is not always constant. Indeed, for laboratory experiments, a plating bath works neither all day long nor every day. It needs to be reactivated at the time of deposition and as a result is rarely in a reproducible state. If the plating bath is not active enough (e.g. if it contains too much
stabilizer which is probably the case of the Enthone bath) and/or if the kinetics of the reaction initiation is too low – because of a too low reducer concentration in the plating bath as in the case of the low-phosphorus Enthone bath – the active species – here Ni(0) – chemisorbed on the substrate surface lixiviate in the hot plating bath (85 -C) before the deposition initiation. The success of a Pd-free metallization depends not only on the Ni plating bath composition, but also on the selected Ni(+2) reduction route and substrate nature, the two last governing the physical and chemical characteristics of the interface. Therefore besides the instantaneous characteristics of the Ni plating bath, the distribution and density of the Ni(0) species chemisorbed on the substrate surface play an important part in reaction initiation. Results given in Table 1 show that the Ni(+2) reduction by NaBH4 allows obtaining an efficient Ni deposit on all the substrates studied with both industrial baths, while the reduction by Ar plasma leads to Ni deposition only with the Mac Dermid bath and only for certain substrates. This is rather surprising if we consider the Ni 2p XPS spectra (Figs. 6 and 7) which show that Ni(+2) species that are spread out on the different substrates reach approximately the same reduction state. Furthermore, it is shown that the Ni surface concentration is about the same whatever the substrate, and that it is clearly higher after 1 min of Ar plasma treatment (¨15 at.%) than after 15 s of NaBH4 treatment (¨6 at.%) (see inserts in Figs 6 and 7). This suggests that in spite of appearances, the surface reactivity for the autocatalytic reaction which is due to the Ni(0) species is not the same in both cases and therefore that the Ni(0) surface seeding depends on the reduction route. For the present, we have no convincing explanation about this surprising behavior except for some hypotheses. The chemical reduction could give a rather homogeneous and dense distribution of the Ni(0) species. It could be conjectured that each Ni(0) atom is strongly chemisorbed
Fig. 7. Ni 2p XPS spectra of the reference ‘‘NiAc/eth’’ precursor deposited on PI and not submitted to reduction (a), of this same precursor deposited on PI, PC, PP, PTFE (b) to (e), respectively and reduced by Ar plasma for 1 min at room temperature. The corresponding Ni surface concentrations are given in the insert.
M. Charbonnier et al. / Surface & Coatings Technology 200 (2006) 5028 – 5036
on the polymer surface, each on its own site, which would make the surface very reactive towards the autocatalytic reaction. Indeed, the wet chemical route is particularly efficient in bringing the electrons necessary for the Ni(+2) ion reduction onto the whole surface, while plasma decomposition of the organic precursor operates in a totally different manner which could lead to a heterogeneous distribution of the Ni(0) species. The active species of the plasma break the chemical bonds of the precursor and the electrons present in the discharge which are able to reduce Ni(+2) ions are probably brought in a very random way to the substrate surface despite the fact that the cathode on which the sample lies is surrounded by an ion shield responsible for the etching process. The latter could lead to the formation of clusters scattered on the surface. These large clusters are probably not numerous enough to allow the initiation of the reaction with the reducer-poor, lowphosphorus Enthone bath. In any case, the bonding and distribution of the Ni(0) species on the substrate surface are probably strongly dependent on the reduction route used as shown by high resolution SEM micrographs. Fig. 8 displays the Ni/PI interface for samples whose Ni(+2) reduction was performed using the NaBH4 chemical way (a) and the Ar plasma (b).In both cases, the interfacial zone shows a good continuity between film and substrate, in that the metal film fits the surface substrate exactly. However, this continuity appears less perfect in the case of the Ar plasma reduction which supports the aforementioned hypothesis, viz. the presence of Ni(0) clusters scattered on the polymer surface. Finally, the substrate nature is to a great extent responsible for the film adhesion. Indeed, it is at the interface between Ni film and polymer substrate that stresses due to the difference of elasticity modulus of the two partners of the system are maximal [8]. Internal stresses in the Ni film strongly depend on the film composition therefore on the Ni plating bath used. A low-phosphorus film (obtained here with the Ni Enthone plating bath) which presents a low amount of Ni\P covalent bonds is less stressed than a high-phosphorus one (obtained with the Mac Dermid bath). Consequently, as
Fig. 8. High resolution SEM micrographs of the Ni/PI interface of samples whose Ni(+2) reduction was obtained (a) via a chemical way in a NaBH4 solution and (b) an Ar plasma.
5035
shown in Table 1, it is possible to obtain more than 1 Am thick, well-adhering Ni films with the Enthone bath while with the Mac Dermid bath films of similar thickness may not be adhering at all (case of PC and PP). As it does not appear to be possible to start the autocatalytic reaction with the Enthone bath after reduction by Ar plasma, we could not obtain adhering thick films on PC and PP by this method. It must however be noted that, when the autocatalytic reaction is initiated on PTFE (after Ni(+2) reduction by NaBH4), it leads to perfectly adhering thick Ni films whatever the Ni plating bath used. Furthermore, it is important to note that the kinetics of Ni deposition is intimately associated with that of autocatalytic reaction initiation. Experiment shows that the lower the initiation time, the higher the deposition kinetics and the better the Ni coating adhesion. It must also be pointed out that the excellent results concerning the Ni film adhesion obtained on some of the substrates studied here are quite comparable to those obtained for films of similar thickness obtained using Pd as a catalyst. On the other hand Ni films deposited on PC and PP after seeding of their surface with palladium are well-adhering, at least when their thickness does not exceed 1 to 2 Am.
4. Conclusions This work has shown how to obtain an autocatalytic Ni film on a PI surface without using the Pd conventional catalyst. For this an ultra-thin layer of an organic nickel salt must be deposited on the surface to be metallized and the corresponding Ni(+2) ions reduced to Ni(0) via either a chemical path in a solution of NaBH4 or a short H2 or Ar plasma treatment (30 s to 1 min). To obtain Ni metallization, it is necessary to rapidly initiate the autocatalytic reaction. The success of the deposition depends on: (i) the nature of the Ni plating bath which must be ‘‘rapid’’ enough to prevent the Ni(0) species lixiviation, (ii) the nature of the polymer substrate and (iii) the Ni(0) seed density and distribution on the polymer surface, i.e. the route followed to achieve the Ni(+2) species reduction. The chemical reduction by NaBH4 allows us to obtain well-adhering deposits on all the substrates with both low- and highphosphorous Ni plating baths. On the other hand, Ar plasma, despite its very efficient reduction power which keeps on the average more Ni species on the substrate surface than NaBH4 reduction does, never leads to Ni deposition when the autocatalytic process is performed with the low-phosphorous bath (Enthone). After an Ar plasma treatment and with the high-phosphorous bath (Mac Dermid), no Ni deposit is obtained on PTFE because the Ni(0) species that are able to initiate the reaction are not strongly bonded enough on its surface and lixiviate in the hot plating bath. However on PI, PBT, PPS and LCP substrates, the Ni deposition is immediate and leads to perfectly adhering thick Ni films. Lastly, Ni deposits are
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easily obtained on PC and PP but beyond 0.2 Am in thickness, they spontaneously crack and no longer adhere on their substrate. All these experiments show the key role played by the substrate nature and by the bonding of the chemical species to the sample surface, i.e. in fine, by the interface strength.
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