Characterization of nickel films deposited by cold remote nitrogen plasma on acrylonitrile-butadiene-styrene copolymer

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apptao surface science ELSEVIER

Applied Surface Science 90 (1995) 47-58

Characterization of nickel films deposited by cold remote nitrogen plasma on acrylonitrile-butadiene-styrene copolymer A. Brocherieux a, O. Dessaux a,*, p. Goudmand a, L. Gengembre b, j. Grimblot b, M. Brunel c, R. Lazzaroni d a Laboratoire de Physiochimie de l'Energ~tique et des Plasmas, EA MRES 1761, Universit~ des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France b Laboratoire de Catalyse H~t~rog~ne et Homog~ne, URA CNRS D04020, Universit~ des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France ¢ Laboratoire de Cristallographie, Centre National de Recherche Scientifique, BP 166, 38042 Grenoble cedex, France d Service de Chimie des Matdriaux Nouveaux, Universit~ de Mons-Hainaut, 7000 Mons, Belgique

Received 16 November 1994; accepted for publication 25 April 1995

Abstract Cold remote nitrogen plasma (CRNP) is used to deposit nickel films from Ni(CO)4 on an acrylonitrile-butadiene-styrene (ABS) copolymer at room temperature in a primary vacuum system. An XPS study has been realized ex-situ and in three steps: (1) the analysis of the ABS pre-treatment by CRNP, (2) the analysis of the pre-treatment followed by the metallization and the plasma post-treatment and (3) the influence of the post-treatment on the deposited films. During the ABS pre-treatment, in order to enhance the nickel adhesion, nitrogen from plasma creates functionalities on the polymer surface and the oxygen presence is strengthened on the surface. During the metallization, the plasma decomposes Ni(CO)4 vapor, leading to the nickel deposition on the ABS. The plasma post-treatment cleans the surface of the Ni film. The contaminants rate induced by the process itself is low. The conductive and adhesive properties of this nickel film coated on ABS allow a copper overlayer to be deposited by electrolysis in an acid bath. A structural and morphological observation by SEM, X-ray diffraction in glancing incidence, X-ray reflectometry and atomic force microscopy completes the film description.

1. Introduction The deposition of thin conductive films on polymers is currently investigated for numerous and varied applications, e.g. packaging or decorative coatings, recording tapes, microelectronics . . . . Today

* Corresponding author. Fax: +33-20434158.

thin films are often prepared by processes based on the vapor phase as physical vapor deposition (PVD) and chemical vapor deposition (CVD). In the PVD processes, the precursor materials are solid. In the CVD ones, the precursors are volatile and their chemical compositions are modified during the deposition process by decomposition in the vicinity of the substrate. OrganometaUic compounds with their well-known volatility properties are suitable precur-

0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0169-4332(95)00066-6

48

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

sors for the CVD processes [1-3]. With respect to its volatility and thermal stability, Ni(CO) 4 is an ideal precursor for Ni deposition and has been chosen for this present work. According to the deposition mechanisms, the CVD techniques can be classified into three groups: thermal CVD (TCVD), laser CVD (LCVD) and plasmaenhanced CVD (PECVD). TCVD and LCVD need high temperatures which can be incompatible with the substrates. Besides, area of coated surfaces by these ways are generally very small. Lots of PECVD equipments work in direct discharge or decaying plasma which are characterized by an important concentration of ions, electrons, short-lived particles and emitted photons [4-6]. When the substrates are settled in these plasmas, they are subject to energetic ion bombardment and UV light which are harmful for polymers [7]. Remote PECVD is a technique in which the depositions are performed on a substrate located far from the discharge. It is a non-ionized reactive area, rich in atoms, free radicals and molecules in different excited states with long radiative or collisional relaxation time. Besides, if the plasma gas is nitrogen, the long lifetime of nitrogen atoms, which are the main species in a cold remote nitrogen plasma (CRNP), allows reaction chambers of several cubic meters to be designed and characterized by a homogeneous concentration of reactive species [8]. The CRNP reactivity also allows films to be deposited at ambient temperature and does not require high vacuum conditions: they are the economical advantages of the process. Thin films of zinc have been coated on aluminum and polypropylene substrates by the decomposition of diethyl zinc in the CRNP [9]. Another application of the CRNP to metal deposition is the decomposition of nickel carbonyl Ni(CO) 4 [10]. The objective of this work is to obtain conductive and adhesive films on flat and complex surfaces of acrylonitrile-butadiene-styrene (ABS). This aim is well performed by the wide-spread electroplating process but the first steps and baths of preparation preceding the metallic deposit (like the palladium activation, for example) are expensive, require rigorous chemical control and present effluent treatment problems. The reactivity of the CRNP technique seems well adapted to perform this objective. The used process intrinsically includes a step of substrate

pre-treatment by CRNP before the metallization and a step of film post-treatment after the metallization. This paper presents the characterization of the nickel films deposited by the CRNP technique. The XPS study has been realized ex-situ and in three steps: (1) the analysis of the ABS pre-treatment, (2) the analysis of the pre-treatment followed by the metallization and the plasma post-treatment and (3) the influence of the post-treatment on the deposited films. Bulk nickel foils have been analyzed by XPS in the same working conditions to be used as reference. Then, an Ar + etching procedure has been carried out on nickel films deposited on two different substrates (ABS copolymer and silicon wafer) in order to compare the variation of the film composition inside the deposit and near the substrate interface. The conductive and adhesive properties of the nickel films deposited on ABS are also described and this description is completed by a structural and morphological observation with SEM, X-ray diffraction in glancing incidence, X-ray reflectometry and atomic force microscopy.

2. Experimental 2.1. Materials

ABS substrates are supplied by Goodfellow as 1.5 mm thick slabs of commercial quality. Before any experiment, they are cleaned in an ultrasonic bath with trichlorotrifluoroethane for 10 min and dried for 12 h at 60°C. The ABS copolymer is a hydrophilic material, it contains as received about 4% water. Bulk nickel foils, also provided by Goodfellow, are 99.98% rich and 1 mm thick. To reduce the oxides layer, the sample is polished in ethanol before any analysis or treatment. Silicon substrates are monocrystal with an orientation (100). They are ultrasonically cleaned in ethanol and dried at 60°C. 2.2. Plasma set-up

The plasma was created inside a quartz tube (27 mm inner diameter) by a resonant cavity [11] connected to a 2450 MHz microwave generator (Fig. 1). The transmitted power was typically 800 W. A

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58 O.04m N2

NI(CO) 4 i

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continuous pumping conveys the plasma to the reaction chamber of 12 / in borosilicate glass (150 mm inner diameter), the capacity of the primary pump was 33 m3/h. The initial base pressure measured on a Pirani gauge was about 10 -2 hPa. The Ni(CO) 4 gas injection was collinear to the plasma flow and the injector end was located at 3 to 4 cm upstream from the substrate. 90 ° bends separated the resonant cavity from the reaction chamber in order to eliminate any effect due to UV radiation of the discharge on the substrate. Two configurations of injection were used in this study. The main part of the work was realized in a non-symmetrical geometry characterized by an angle of about 10° between the Ni(CO) 4 injection and the reactor axis. In a second configuration, the injector was aligned with the reactor axis. This last configuration has been used for the study by ionic etching of the deposit core and of the metal/substrate interface. The justification of both these kinds of configuration is presented in Section 3.2.3. In particular experimental conditions [12], a secondary ionization zone (SI) or Pink Afterglow appears downstream from the discharge in nitrogen plasma, this zone precedes the far afterglow one. SI is characterized by important gradients in temperature profile and in concentration of ionic species and electrons. With nitrogen pressure and microwave power fixed, its spatial extension increases with the discharge time till the steady state is reached. In our experimental conditions, this steady state came after about 15 min of discharge working. So, the distance between the discharge and the polymer (denoted d) was fLxed to 230 cm in order to avoid the disruptive

49

influence of SI on the sample. But for a working time of the discharge lower than 15 min, the polymer could be set at a shorter distance from the discharge: d = 130 cm, downstream from the SI. Nitrogen used to generate the plasma was from Air Liquide (U grade - industrial quality) and was regulated by a MKS mass flow regulator. Nickel tetracarbonyl was purchased from Strem Chemicals, it was regulated by a ball flowmeter. The process was carried out in three stages (a, b, c) which occur without air exposure: (a) Pre-treatment of the polymer surface. The nitrogen remote plasma worked at 6 hPa pressure (which corresponded to a nitrogen flow of 2.65 / / m i n ) and during a constant time t a = 5 min to pre-treat the polymer settled in the reaction chamber. The influence of the plasma pre-treatment duration t~ on the metallic film adherence to the substrate has not been studied. (b) Metallic deposition. Nickel carbonyl was admitted in the reactor during this stage. The operating pressure varied between 8 and 5.8 hPa and the Ni(CO) 4 flow rate was lower than 2 sccm. The deposition duration t b varied from 1 to 27 min. The action of the CRNP decomposed Ni(CO) 4, leading to nickel deposition onto the polymeric sample. (c) Post-treatment. The existence of this third stage (c) was imposed by the necessity to destroy any Ni(CO) 4 trace remaining in gas mains and in the reactor. The nitrogen pressure was 5.8 hPa. During the post-treatment (t c = 15 min unless another value is specified), the metallic deposit stayed in the reactor and the nickel carbonyl content in the CRNP decreased.

2.3. Surface analysis At each stage of the process, XPS data were collected by a LHS 10 Spectrometer after air exposure time t d of about 30 min (unless another value is specified) for transferring the samples. It utilized non-monochromatized A1 K a X-radiation as excitation source and operated with 13 kV and 20 mA current emission, at an electron take-off angle of 90° relative to the surface plane. When the sample was isolated from the spectrometer, the binding energies BE of the different analyzed elements were refer-

50

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

enced to the C ls peak which has been assigned a value of 285.0 eV; the charging effect was so corrected [13]. But when the sample was a conductor and connected to ground (nickel films studied by ionic etching, Section 3.2.2), the reference was based on hv = BE + KE + ~s = 1486.6 eV where KE is the measured kinetic energy and ~s is the spectrometer constant calibrated against the AU4fT/2 peak at 84.0 eV. On some samples, an Ar + etching procedure was used to eliminate progressively the overlayer, The following conditions were chosen: 2000 eV for the accelerating voltage of the Penning gun and 560 V and 7 mA for the discharge, the estimation of the ion current on the sample is about 2 /zA/cm 2. The etching duration varied from sample to sample and will be indicated. From the measured intensity ratios of the photopeaks, the atomic stoichiometries were determined from the following equation:

n A / n B = IAi/IBj X K a j / K A i , where Ia,(n ) is the i(j) photopeak intensity of the element A(~) and Ka,(n,) is the resultant term between the cr°ss'secti°n o~ the i(j) core level orbital, the inelastic mean free path and the transmission factor of the analyzer, both of the latter being kinetic energy-dependent. The efficiency of the electron detector was considered to be constant. For the pre-treated and metallized ABS samples, the C ls photopeak intensity was also followed. The intensity of an element was determined by the area under the corresponding spectrum. The C ls intensity (noted I in this paper) of the pre-treated or coated ABS was normalized by that of the untreated ABS in order to follow the cleaning of the polymer during the pre-treatment or to study the polymer coverage and the carbon rate inside the deposit. A line-shape analysis has been effected on the N ls and O ls spectra. No decomposition of the C ls spectrum has been tested because of the multitude of possible bonds in a small interval of binding energies. The decomposition routine utilized a Gaussian line shape for the individual components. The full width at half maximum (FWHM) for these individual components was fixed around 2 eV for N ls and O Is peaks. Nevertheless, for the thicker deposits, FWHM of the O ls peaks could be adjusted around 1.5 eV.

The scanning electron microscope gives structural and topographical information about the deposits. Glancing angle X-ray diffraction concerns the crystalline quality of film (monocrystal, polycrystalline or amorphous) and the determination of grain size. But it did not allow, in this case, to determine the chemistry of the present phases in the film. The spectra have been performed with an INEL detector and at 45 kV, 200 nA with the Cu K a radiation; the incidence angles have been set at 0.5 ° and 1°. X-ray reflectometry (40 kV, 20 or 50 hA) enables the density and the roughness of the surface to be defined. Further information on the morphology of the surface of the deposited films has been obtained by atomic force microscopy (AFM). A Nanoscope M apparatus was used; it was operated in room conditions in the constant force mode. Several (5 to 7) 1 × 1 /zm 2 areas were imaged for a given sample in order to ensure that the collected data properly represent the surface morphology. Compared to electron microscopy, AFM has the advantage of providing structural information in the direction perpendicular to the surface, which allows for instance the surface roughness to be determined.

3. Results and discussion The industrial objective of this metallization process involves the operation conditions which are intrinsically sources of oxidation. They can be listed as follows: the residual oxygen-containing impurities relative to the plasma process (plasma reactions with the glass walls of the reaction chamber, impurities in the plasma gas and in the primary vacuum system) and relative to the commercial nature of the ABS substrate. The last source is the necessary air exposure of the substrates after their treatment. The results described hereafter will be always considered in the light of these potential sources of oxidation.

3.1. The ABS substrate 3.1.1. Untreated ABS ABS is an oxygen-free polymer as regards its chemical formulation. But XPS measurements confirm the ABS sensitivity to moisture, the initial

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58 Table 1 XPS analysis of ABS before and after the pretreatment by CRNP (t a = 5 rain, d = 130 cm)

C Is Nls

Ols

Global envelope

Untreated ABS

Pretreated ABS

I FWHM (eV) N / C ( × 10 ' 2 ) FWHM (eV) BE (eV) O / C ( X 10 +2 ) FWHM (eV) BE (eV)

1 1.9 4 2.0 400.0 8--,13 2.7 ---, 2.8 532.8 ---,533.0

0.87 2.1 16 2.7 400.1 18 2.9 532.6

oxygen content can vary from one sample to another while the nitrogen content is stable (Table 1). Nevertheless, the C ls spectrum collected twice, at the beginning and at the end of the XPS spectra recording, shows a good stability during the analysis in the spectrometer. Other impurities, detected as traces by XPS on our substrates are Na and Ca, probable fillers of the ABS manufacturing. Oxygen detection by XPS on the surface of oxygen-free polymers has been reported frequently in the literature. Yasuda et al. observed it on polyethylene, polystyrene and polyacrylonitrile [14]. In the same way, Lub et al. found 1.5 at% of oxygen on the polystyrene surface [15] and Morra et al. 2.7 at% [16]. On the ABS surface, Burkstrand et al. measured 7.6 wt% oxygen [17]. The decomposition of the N ls and O Is spectra suggests two components for each element. The N ls spectrum reveals its main component at 400.0 eV: this position is due to C = N species present in the polymer framework. The second peak appears as a small shoulder at 402.0 eV and results probably from the oxidation of the previous nitrile function. The examination of the O Is spectrum shows two peaks at 532.4 and 533.9 eV, the position at 532.4 eV is consistent with the BE region where different species like C=O, C-NO, C - O H are found and the peak at 533.9 eV could be due to the water adsorbed i n / o n the polymer. 3.1.2. Pre-treated ABS The XPS investigation of the pre-treated ABS includes two steps: the sample pre-treatment by

51

plasma during 5 min and its air exposure for the transfer to the spectrometer. The increase of the atomic ratios N / C and O / C recorded on the pre-treated ABS copolymer relative to the untreated ABS suggests the creation of new chemical functions on its surface. The ABS pre-treatment by CRNP favors nitrogen and oxygen incorporation (Table 1). The presence of oxygen on polymeric surfaces during or after treatment with a nitrogen plasma is a common phenomenon [15,18-20]. Radicals or functional groups created on the polymer surface by the plasma treatment could be rapidly oxidized or hydroxilated. Besides, the oxygen content recorded on this pre-treated ABS has a threefold origin as it has been previously mentioned. The FWHM of the global N Is spectrum recorded after the pre-treatment increases relative to the untreated ABS. The line-shape analysis of the N ls envelope reveals the presence of a new peak at 398.9 eV. The formation of this new nitrogen species to lower binding energy suggests a reduction by a hydrogen transfer from ABS and the formation of imine C = N H from the nitrile function at 400 eV. The two peaks previously determined on the untreated ABS are always there, at the same binding energy. The O ls spectrum does not show a new component and the FWHM of the global envelope remains constant: O ls results again from the same two contributions observed on the untreated ABS. The O / C ratio growth indicates that these contributions are only strengthened on the ABS surface. To study the ageing effect on the pre-treated samples, a second recording of C ls, O ls and N ls spectra has been effected after an air exposure of 5 h. No important variation has been detected in comparison with the values obtained after only the 30 min required for transfer to XPS. The N / C and O / C ratios are not affected by any ageing phenomenon beyond 30 min to 5 h. In the same way, after a CRNP treatment, the nitrogen incorporation on a surface of polyethylene terephtalate (PET) is not modified after ageing in ambient air for 84 days, no new oxygen functionality being detected [21]. On polypropylene (PP) treated by CRNP, the N / C and O / C ratios remain constant after 15 days of air exposure "[8]. But for both these references and in this work, a rapid ageing effect is not excluded

A. Brocherieuxet al. /Applied Surface Science 90 (1995) 47-58

52

during the first minutes, required for the sample transfer to XPS.

3.2. The metallization 3.2.1. XPS study of the nickel films deposited on ABS subs[rates The influence of the deposition time t b has been studied according to the evolution of different photoelectron spectra and atomic ratios recorded on coated ABS samples (Table 2). The atomic ratios are calculated relative to nickel because their variations take better into consideration the nickel coverage rate. The results of these investigations are compared (Table 3) with the numerous data on the N i - O system available in the literature and with our own data obtained on bulk nickel foils, for reliable assignments. The increase of the post-treatment duration t c will be analyzed Section 3.2.1.2.

3.2.1.1. Ni films deposited by N 2 plasma Starting from the first minutes of the nickel deposition, the Ni 2p3/2 spectrum shows two components with a shake-up satellite structure (Fig. 2). The XPS signals of Ni 2p3/2 reveal a quite complex structure induced by multiplet splitting and shake-up satellites which complicate quantitative peak-fitting [22]. No decomposition test has been carried out on the nickel spectrum, but the contribution of each component is clearly evidenced by the evolution of the nickel global envelope. The component " a " appears at about 854.0 + 0.2 eV and progresses in intensity during the metallization. The second component " b "

Table 2 Global envelopes of Ni2p, Cls, Nls and Ols obtained on ABS metallized as a function of the deposition time t b t b (rain) 4 6 Ni2p3/2 BE a (eV) BE b (eV) C/Ni(Xl0 +z) Cls I FWHM (eV) N/Ni(X10 +2) Nls FWHM (eV) O/Ni(×10 +2) Ols FWHM (eV)

10

19

854.1 854.1 854.0 854.0 856.9 857.0 857.1 856.0 70 53 59 234 0.3 0.2 0.2 0.5 2.5 2.7 2.0 2.5 74 35 40 30 3.6 3.9 3.0 3.3 102 95 55 82 3.1 3.3 3.7 3.0

27 854.1 856.2 30 0.1 1.8 32 2.5 58 3.7

Table 3 BE values (eV) of Ni2p3/2 contributions in different oxidation states and of O ls observed in nickel-oxygen systems according to the literature Ni° NiO Ni3+ Ni(OH)2 Ref. (surface defect) 852.6 852.6 852.2 853.0 852.5 852.7 853.1 + 0.1

854.6-856.5 854.6 856.5 854.1 855.6 855.2 854.0 853.7 855.8 854.8+ 0.2

856.3-857.6 [22] [23] [24] 856.1 [25] 855.6 [26] [27] 856.7 + 0.3 Our work on bulk nickel foil

Water ( *) or adsorbed oxygen-containing species

OH- 0 2- associatedwith Ni 8÷ Ref. cations 8 + = +3 8 + = +2

532.8* 533.4*

531.6 531.2 531.4 531.0

533.1" 532.0 531.8 531.2 532.1 ~(c03) 2- 531.3 533.0 ~ o - c - o 532.2 + 0.2

529.8 529.4 529.6 530.0 529.5 529.7 530.1 + 0.2

[22] [24] [23] [28] [29] [27] [25] [25] Our work on bulk nickel foil

is located at 857.0 5:0.2 eV for t b ~ 10 min and at 856.1 + 0.2 eV for higher values t b (Table 2). In relation to the first component " a " , its intensity decreases during the nickel coating. Furthermore, after an Ar ÷ ionic etching on the metallic film deposited during 6 min, this component " b " disappears and the nickel lines move to a spectrum similar to metallic species: the separation between Ni2P3/2 and Ni 2Pl/2 maxima after 4 min etching is 17.2 eV, like for Ni °. Based on the above observations, the higher binding energy component " b " of the Ni 2p3/2 spectrum is a surface component, appearing essentially during the air exposure and easily eliminated by ionic etching. This component is attributed to a combination of nickel hydroxide and oxide the relative proportions of which depend on the deposition time. The 856.1 eV position would correspond to a combination richer in oxidized species and the

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

410

390 540

525

900

880

860

840

410

390 540

525

900

880

860

840

410

390 540

525

900

880

860

840

410

390 540

525

900

880

860

840

410

390 540

&A

,

,

i

525

900

t

~

880

i

i

860

p---q

840

Binding energy (oV)

Fig. 2. Evolutionof the differentcontributionsof Ni2p, Nls and O ls spectraon a nickel film depositedon ABS (non-symmetrical geometryof injection)as a functionof the depositiontime tb.

857.0 eV value to one richer in nickel hydroxides (Table 3). The component " a " grows with the film deposition time, it evolves as a bulk component. Its BE position recorded on the surface of the film (854.0 eV) corresponds to a combination of the oxidized contribution of the surface and the metallic species Ni °. Concerning the carbon analysis, the C ls normalized intensity ( I ) and C / N i ratio roughly decrease during the metallization. The FWHM of the global C is envelope increases very little till t b = 10 min and then sharpens with the important deposits. Indeed, a thick nickel coating hides more the polymeric substrate and so reduces the carbon detection possibility. But the accurate detection of contamination rate coming from the process itself has been carded out on a silicon substrate (el. Section 3.2.2). Examination of the N Is spectra provides further

53

insight into the N ls evolution during the metallization. Till t b --10 min, the global envelope widens out and for the long deposition times, the spectrum shows a sharper shape: the FWHM is 2.5 eV. The decomposition of the global envelope points out 4 components (Fig. 2). These ones are roughly stable in binding energy but their relative areas change with the deposition time and induce a FWHM modification of the global envelope. As regards their relative area, the lower BE component (397.6 + 0.2 eV) not very important at the beginning of the metallization becomes dominant for the long coating times. It should be identified as a nitride-like species bound to nickel. The second component in terms of relative area for the long metallization times is located at 399.2 ± 0.2 eV. This component is the main one for the short coatings, till t b -----6 min. It appears as an oxidized form of the previous component. The oxidation is deeper when the metallic film is thin: it comes from the water desorption of polymer and from the air exposure of the film surface. The two other components (401.0 + 0.2 eV and 403.4 ± 0.3 eV) are probably different oxidation states of the previous components. Besides, the N ls contributions identified on the untreated and pre-treated ABS are hidden by the nickel film. The evolution of the O ls spectra during the metallization enriches again the argument on the film oxidation. The FWHM of the global envelope increases with the deposition time; in this case, three components come from the decomposition of the global spectrum (Fig. 2). The main one for all deposition times t b appears at 531.9 ± 0.3 eV. This component is a combination between O H - species bound to nickel (Table 3) and to functions associating N and O. The lower BE contribution which is located at 530.0 ± 0.2 eV is, like on bulk nickel, associated to O 2- bound to Ni 2+. And finally, the higher BE component (533.9 + 0.2 eV) should come from the water adsorbed on the metal surface (Table 3). The atomic ratio O / N i decreases when the metallization time t b increases from 4 to 10 rain at d - - 130 cm and from 19 to 27 rain at d = 230 cm. The values of the O / N i ratio obtained for t b = 27 min at d -- 230 cm and for t b = 10 rain at d = 130 cm are approximately equal. This observation can be explained by the increase of the discharge distance where the only effect is the rise of the oxygen incorporation.

54

A. Brocherieux et aL /Applied Surface Science 90 (1995) 47-58

3.2.1.2. Influence of the post-treatment duration For the destruction of the residual Ni(CO) 4 the sample needs to be kept in the plasma for about 15 min after the nickel deposition. However to understand the plasma influence on the metallized sample, a simple test is effected: following a deposition time of 27 min, two samples are held in the N 2 plasma for two different times t c = 15 min and 150 min. These two samples show different atomic ratios. A long stay in the plasma implies a decrease of C ls intensity (from 13 to 10) and thus a soft fall of the C / N i ratio (from 0.33 to 0.20). The O / N i ratio slowly rises (from 0.58 to 0.65) while N / N i slightly drops (from 0.32 to 0.21). Moreover, the higher binding energy component (856.2 eV) of the Ni 2p spectrum becomes dominant over the 854.1 eV component. And finally, an intensity inversion between the two BE components appears on the O ls spectrum. The decrease of the C ls intensity in correlation with that of the C / N i ratio testifies to the reduction of the carbon contamination rate on the metallic deposit when the post-treatment duration increases. In the same way, an important decrease of the carbon content has been also observed after the plasma treatment of the bulk nickel foil. Simultaneously, a reinforcement of the 0 2- component is noticed on the sample surface by the intensity inversion on the O ls spectrum.

3.2.2. Ionic etching of nickel films Both substrates ABS and silicon have been coated during 15 min, in the symmetrical configuration of injection; the film thickness measured on the silicon sample is about 3200 ,~. The line-shape analysis of the N Is and O Is spectra carded out on these ABS and silicon substrates before the etching indicates that these fingerprints of the nickel deposit are really similar. Moreover, the nitrogen components so pointed out on both substrates are only due to the metallization process and without relation with the substrate. An Ar + etching has been realized to follow the composition variation of the nickel films depending on the substrate where they are deposited (Fig. 3). Till about 10 min of etching, the evolution of the elements intensity is really comparable on both substrates and reveals the influence of the air exposure: during this period of 10 min, the Ni 2p peak intensity

1000 T a

~

10000

o

J 0

20

I

i

40 E t ch i n g time (rain)

i lOOO~ 800

,

i

60

80

~ - - ~

600

10000 8000

~f"-~---~

6oo0

200

2000

0

I 0

30

60 90 E t ch i n g time (rain)

J~ I(NIs) I ~l(Ni2p )

0 120

150

Fig. 3. Evolution of C ls, O Is, N ls and Ni2p intensities versus

etchingtime for a nickel film depositedon silicon(a) and on ABS (b) (symmetricalgeometryof injection, tb = 15 min, d = 230 cm).

progressively rises with the etching time while the intensities of all the other elements roughly decrease. The observation of the C ls behavior is interesting at the beginning of the procedure: the C ls intensity reaches a minimum value after 2 min and gradually increases to an asymptotic value. This minimum value represents, under the layer oxidized by air, the carbon intensity obtained by the plasma post-treatment. Nevertheless, the great difference between the composition profiles of the nickel films on both substrates is the oxygen variation under the contamination layer of the air exposure. On the silicon substrate, the O ls intensity keeps a constant value while on ABS, it passes through a maximum. This phenomenon recorded on ABS is due to the proximity of the polymer surface which is reached since ~ 120 min of etching as evidenced by the decrease of the Ni 2p intensity and the rise of the C ls intensity. Besides at 150 min of Ar + sputtering, the atomic ratios N / C and O / C are similar to those found on the untreated ABS (Table 1): N / C = 0.05 and O / C = 0.10. The polymer surface is an oxidation source for the nickel which is depositing during the first early minutes of the metallization. Indeed, the ABS pre-treatment by plasma favors the water and oxygen desorption out of the polymer. As the nickel film is depositing by clusters (Section 3.2.4.4), it does not mask totally the ABS substrate and this explains the high values of the O ls intensity. Then,

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

55

the O ls intensity decreases with the complete cover of the ABS by the assembling of these nickel clusters. This phenomenon of oxidation at the film/substrate interface is characteristic of the ABS, it is not observed on the silicon substrate. Besides, the contamination rate due to the process can be approximated on the silicon substrate. Indeed, under the layer oxidized by air, the different peak intensities and the atomic ratios remain constant during the penetration inside the coating core. After 80 man of etching, C / N i and N / N i reach 0.06 and O / N i is ~ 0.04. Relative to other processes, these different ratios are low: for example, by photo-assisted decomposition from Ni(CO)4, 9% to 10% of carbon is found in the deposited film [1].

These data confirm the electrical conductivity of the nickel films deposited by plasma. Nevertheless, these values which are 200 to 400 times higher than the bulk nickel resistivity (6.8 /xll. cm) are the consequence of the structural and chemical features of the films. The nickel grains which form the deposits are set in a less compact set than in bulk nickel (Section 3.2.4) and the current percolation is carded out by the only joining of the nickel clusters. The electrical conductivity is also diminished by the presence of the nickel nitride species in the film composition (Section 3.2.1.1).

3.2.3. Macroscopic characteristics of the nickel deposit: adhesion, thickness and resistivity

3.2.4.1. Scanning electron microscopy

The adhesion properties have been tested on ABS metallized in the non-symmetrical geometry of injection ( t b ----35 man). Two kinds of tests have been carded out. The results of the scratch test are satisfactory, the percentage of the removed area ranges from 0% to 5%. The second test is an electrolytic deposit of copper, at room temperature and from an acid bath (200 g / f H2SO 4) on the nickel film deposited by plasma. The observation of no flakes after the copper coating demonstrates the good adhesion and conductivity of the nickel film on the ABS substrate. Concerning the thickness of the films, the nonsymmetrical geometry of injection induces an important gradient (the differences in thickness are about 80%), but allows wider areas to be coated. With an injection tube of 7 mm inner diameter, it is possible, in our experimental conditions, to cover an area of about 70 cm 2. In the symmetrical geometry, the metallized surface has diminished to 20 CIIl 2 but has a better homogeneity an thickness: it varies only by 8%. This configuration of injection allows one also to measure the deposition rate by a piezoelectric stylus: it is 200 A/man on a perfectly flat substrate of silicon. Besides, the resistivity has been measured by a four-poant probe on ABS samples metallized an the symmetrical configuration. The 3300 ,~ thick films show a resistivity of 3100 /xl-l.crn, the resistivity for thicker deposits (4000 A) is 1300 /zl)-cm.

3.2.4. Structural and morphological characterization of the nickel deposit

The examination of the nickel films by SEM has been performed in two steps (Fig. 4). The first observations have been realized on a very than deposit (t b = 1 min) in the symmetrical configuration of injection. The film seems to be constituted by small grains ( _< 0.5/zm) disposed everywhere on the ABS surface, on the flat parcels as well an the cavities of the copolymer. Their granulometry is regular.

A film coated during 15 rain an the same symmetrical configuration has been also observed. In this case, the relief of the ABS surface is strongly attenuated, most cavities of ABS are masked. A great disparity an the grain arrangement appears: either the grains are deposited in an isolated manner or they form more or less important groups. This inhomogeneity in the grain layout creates voids inside the film.

3.2. 4.2. X-ray diffraction in glancing incidence Two nickel films on ABS ( t b = 27 man) and on silicon ( t b = 50 man) have been analyzed by X-ray diffraction. On both samples, only one Bragg peak is measured at 0-- 21.12 ° on ABS and 21.78 ° on silicon (Fig. 5). The width of this Bragg peak is important, about 3°: this observation means that the grains have really a small size. The Scherrer formula: L -- A (/3 cos 0) -1 with L the size of the grain, A(Cu K a ) = 1.54 ,~ and fi the width of the Bragg peak ~n 20, gives for the grains a mean size L = 16 A. Besides, the position of this peak does not give

56

A. Brocherieuxet al. /Applied Surface Science 90 (1995) 47-58

information about the surface state of the film. The mean calculated value d = 2.12 ,~-1 is next to the main reflections noticed in the JCPDS tables of o 1 metallic nick61 (d = 2.03 A - , hkl = 111, ref. 4-850) o 1 and nickel oxide ( d = 2.09 A - , h k l = 200, ref. 4-855). The absence of other diffracted lines does not allow a conclusion about the surface structure.

469.00

35t.75

' ~ 2M.50

117.25

0.00 5.01

3.2.4.3. X-ray reflectometry

As a good planarity of the sample is necessary for this method, the natural roughness of the ABS copolymer has limited the analyses to the deposited film on silicon. The critical angle obtained on the film (0.344 °) is lower than the angle determined on a bulk nickel sample (0.398°). This critical angle is

14.00

23.00

32.00

41.00

50.00

Fig. 5. X-raydiffractionpatternof a nickelfilm on silicon.

related to the electronic density n at the surface of the analyzed material as (n) 1/2 [30]. So the method points out again the porous structure of the deposited film where the electronic density is lower than the one of bulk nickel by the ratio (0.3440/0.3980) 2 = 75%. 3.2.4.4. Atomic force microscopy

t~

Fig. 4. SEM imagesfromnickelfilm depositedon ABS: (a) tb 1 =

rain, (b) t b = 15 rain.

The AFM images of the Ni films deposited on silicon show a reproducible pattern made of well-defined rectangular grains with flat surfaces and fairly sharp edges: these are small nickel crystals (Fig. 6). The majority of the grains has a size of 110 × 80 nm 2, larger grains (,-, 200 × 150 nm 2) are sometimes detected. This size distribution probably corresponds to the clustering of smaller crystallites, the value of 16 ,~ obtained by X-ray diffraction should be the mean size of these small crystallites. The nickel film morphology is very similar to that observed on the surface of bulk nickel after treatment with a mild corroding agent; that treatment leads to intercrystalline corrosion and reveals the presence of rectangular crystals of about the same size as in the deposited films. Nevertheless, the main difference relative to bulk nickel is the fact that the packing of the grains is not dense in the deposited films. Numerous voids appear around the nickel grains: in Fig. 6, they are represented by the dark areas. The figure confirms what has already l~een observed by SEM and X-ray reflectometry, but this loose structure does not stop the conductivity of these films. However, the films surface is fairly flat: the AFM-determined root-mean-square surface

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

57

Fig. 6. AFM imagesfrom nickelfilm depositedon silicon:(a) Z, (b) X - Y views.

roughness, measured on 1 X 1 /zm 2 areas, is typically 6 - 7 nm.

4. Conclusion A metallization process by decomposition of nickel tetracarbonyl Ni(CO)4 in cold remote nitrogen plasma (CRNP) has been studied to deposit, at room temperature, a metallic film on an ABS copolymer of commercial quality. The film examination by SEM, X-ray diffraction and X-ray reflectometry reveals a loose structure: the nickel grains grow by the cluster-

ing of very small crystallites but their growth does not completely fill the numerous interstitial voids observed between the clusters. The conductive and adhesive properties of this nickel film coated on ABS allow a copper overlayer to be deposited by electrolysis in an acid bath. Under a contamination layer formed of nickel species oxidized and hydroxilated by the ambient atmosphere, the deposit is essentially constituted of Ni °. The presence of this metallic phase explains the good conductivity of the film. A small contribution of a nitride-like species bound to nickel has also been identified. The content of impurities in the core

58

A. Brocherieux et al. /Applied Surface Science 90 (1995) 47-58

deposit is low: C / N i and N / N i - - 0 . 0 6 and O / N i - - 0 . 0 4 . Besides, the post-treatment o f the nickel deposit reduces the carbon contamination rate. The adhesion o f the nickel film is quite satisfactory: during the pre-treatment o f the substrate, C R N P creates n e w nitrogenated functions and reinforces the o x i d i z e d ones on the A B S surface. T h e s e functionalities f a v o r the film adhesion, particularly o x y g e n functions since nickel has a g o o d affinity for oxygen.

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[13] S. Nowak, M. Collaud, G. Dietler, P. GriSning and L. Schlapbach, J. Vac. Sci. Technol. A 11 (1993) 481. [14] H. Yasuda, H.C. Marsh, S. Brandt and C.N. Reilley, J. Polym. Sci. 15 (1977) 991. [15] J. Lub, F.C. van Vroonhoven, E. Bruninx and A. Benninghoven, Polymer 30 (1989) 40. [16] M. Morra, E. Occhiello and E. Garbassi, Angew. Makromol. Chem. 189 (1991) 125. [17] J.M. Burkstrand, J. Vac. Sci. Technol. 15 (1978) 223. [18] R. Foerch, Le Vide, les Couches Minces 246 (1989) 213. [19] R. Foerch, N.S. McIntyre, R.N. Sodhi and D.H. Hunter, J. Appl. Polym. Sci. 40 (1990) 1903. [20] F. Poncin-Epaillard, B. Chevet and J.C. Brosse, J. Adhes. Sci. Technol. 8 (1994) 455. [21] B. Mutel, O. Dessaux, P. Goudmand, L. Gengembre and J. Grimblot, Surf. Interf. Anal. 20 (1993) 283. [22] H.W. Hoppe and H.H. Strehblow, Surf. Interf. Anal. 14 (1989) 121. [23] A.R. Gonzalez-Elipe, J.P. Hoigado, R. Alvarez and G. Munuera, J. Phys. Chem. 96 (1992) 3080. [24] S. Uhlenbrock, C. Scharfschwerdt, M. Neumann, G. Illing and H.J. Freund, J. Phys.: Condens. Matter 4 (1992) 7973. [25] D.B. Mitton, J. Walton and G.E. Thompson, Surf. Interf. Anal. 20 (1993) 36. [26] N.S. Mclntyre and M.G. Cook, Anal. Chem. 47 (1975) 2208. [27] C.L. Bianchi, M.G. Cattania and P. Villa, Appl. Surf. Sci. 70/71 (1993) 211. [28] J.C. Fuggle, in: Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Ed. D. Briggs (Heyden, London, 1977) p. 273. [29] G. Deniau, P. Viel, G. Lecayon and J. Delhalle, Surf. Inteff. Anal. 18 (1992) 443. [30] J.C. Bruyere, C. Savall, B. Reynes, M. Bmnel and L. Ortega, J. Phys. D (Appl. Phys.) 26 (1993) 713.