Polymer metallization: Low energy ion beam surface modification to improve adhesion

Beam Interactions with Materials 8 Atoms

ELSEVIER

Nuclear Instruments

and Methods in Physics Research B 131 (1997) 71-78

Polymer metallization: Low energy ion beam surface modification to improve adhesion P. Bertrand

*, P. Lambert, Y. Travaly

UnitP PCPM. Universitk Catholique de Louuain, I Place Croix du Sud, B-1348 Louuain-La-Neuue, Belgium

Abstract The interface formation between copper and poly(ethylene terephthalate) (PET) and poly(methy1 methacrylate) (PMMA) films is studied in situ by Ion Scattering Spectrometry (ISS). Very low metal fluxes (- lOi atoms/cm’ s) and hence low deposition rates are obtained by using a Knudsen’s effusion cell. This allows to reach very low metal coverages down to the sub-monolayer regime. The results indicate that without surface activation, Cu atoms interact only very weakly with both polymer surfaces. Indeed, the oxygen/carbon KS intensity ratio remains nearly unaffected by the metal deposition, showing no preferential shadowing effect. Moreover, the ISS polymer signals are still detected after exposure to Cu atom fluences corresponding to several monolayers coverage. Cu diffusion below the polymer surface is evidenced by the presence of an inelastic multiple collision contribution in the ISS spectra. It is observed that 2 keV 3He+ ion beam irradiation prior to metallization induces a drastic modification in the interface formation. Ion beam irradiation prevents the metal diffusion into the polymer bulk and leads to an increase of the metal concentration at the surface. In order to explain these results, the surface modifications produced by the ion beam on pristine polymers are studied by ISS and ToF-SIMS. Dehydrogenation and preferential loss of 0 containing fragments are found. These modifications are associated with the production of radicals leading to the creation of new adsorption sites for the Cu atoms. It is proposed that the reaction between radicals of different macrochains induces a surface crosslinking, that can prevent the diffusion for the deposited metal atoms into the polymer bulk. Keywords: Ion Scattering Spectrometry; Time-of-Flight methacrylate); Copper; Polymer metallization

Secondary

1. Introduction Metallized polymers and especially poly(ethylene terephthalate) (PET) are widely used for different applications: packaging, decorative coatings, capaci-

Corresponding [email protected] l

author.

Fax:

32-10-473452.

Email:

Ion Mass Spectrometry;

Poly (ethylene

terephthalate);

Poly (methyl

tors, magnetic tapes, etc. In such applications, a good adhesion between the polymer and the metal is required. In packaging, for example, the polyester film provides the mechanical strength whereas the metal layer, generally aluminum, acts as the gas diffusion barrier. The understanding of the physico-chemical interactions occurring at interface between the metal and the polymer is needed to overpass empirical approach.

0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-583X(97)00149-3

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It is well known that the interfacial interaction controls the quality and growth mechanisms of the metal layer. For example, in the Al/PET system, the interaction between Al and the carbonyl oxygen, which has been observed by XPS [ 1,2] and theoretically calculated recently in the frame of ab initio density-functional formalism [3], leads to an initial uniform metal covering of the substrate as seen by static SIMS and ISS [4,5]. This work concerns the study of the metal/polymer interface formation in the case where the metal reactivity towards the polymer functional groups is very low. This is undertaken in order to investigate the influence of the surface modifications produced by an ion beam bombardment. Indeed, previous results have shown that polymer surface treatments by low energy ion beam can modify deeply the surface reactivity. We reported that a pre-irradiation of polypropylene (PP) surface with a 2 keV-3Hef ion beam is able to increase drastically the Al sticking coefficient during metallization [6]. Novack et al. showed a similar effect for the Mg metallization of PP after 700 eV-Arf bombardment [7]. Moreover, it has been shown that ion beam bombardment of PMMA, used as substrate for cell culture, has a positive effect on the cell adhesion [8]. A low reactivity has been observed for the Cu/PET system [ 1,2,4], and, as a consequence, metal clustering and diffusion into the polymer bulk have been reported [9]. For this reason, Cu/PMMA and Cu/PET interface formation is investigated. Both polymers own a ester group, either as part of the backbone (PET) or in pendant group (PMMA). In situ Ion Scattering Spectroscometry (ISS) is used for the interface characterization owing to its extreme surface sensitivity [IO] so as ToF-SIMS to look at the polymer surface modifications produced by the ion irradiation.

2. Experimental

procedure

2. I. Samples PMMA powder was dissolved in dichloromethane and thin films deposited onto Si wafers were prepared by spin coating from the solution. PET substrates consisted in a 12 p_rn biaxially stretched semi-crystalline polyester film (Mylar from Du Pont

de Nemours-Luxembourg) and they were fixed on the sample holder by means of double sided scotch tape (Permacel). 2.2. Surface analysis Ion scattering measurements were carried out with a CMA energy analyzer coaxially mounted with the ion gun (Kratos WG-541). In this configuration, the primary ion beam is at normal incidence to the surface and the 139.5” scattered ions are energy analyzed. A 2 keV-3Hef ions beam was rastered on a 2.1 X 1.75 mm’ surface area. To minimize irradiation damage, the ion beam current was limited to 10 nA and acquisition time to 200 s. The spectra were recorded with a programmable multichannel analyzer (EG&G ORTEC ACETM MCS). Typical spectrum acquisition included 50 energy scans of 4 s each over 5 12 memory channels and corresponding to scattered ion energy ratio (E/E,) varying from 0.3 to 1.0. This gives a total incident ion fluence of 3.4 X lOI ions/cm* per spectrum. A low energy electron flood gun @SW EG2) was used to compensate the surface charge building up during the ion bombardment of the insulating polymer substrates. Uncomplete charge neutralization leads to the settling of a surface potential and consequently, this produces an energy shift of the peaks towards a higher energy. This shift is more important for the peaks at lower energy. Indeed, if V, > 0 is the surface potential, the ion energy after scattering becomes: E, = K (E, - eV,> + eV, = K E, + eV, (1 K), where K is the kinematic factor. The energy shift is then: AE = E,* - K E, = eV, (1 - K). Raw spectra are mass calibrated on the basis of the binary elastic collision (BEC) model. The energy position of the BEC peak Ej is given by the kinematic factors K, = EL/E, calculated at the 139.5” scattering angle (K, = 0.407, K, = 0.513, K,, = 0.847) [IO]. In the figures with superposition of different spectra, the energy scales have been slightly adjusted in order to display always the C peak at the same position independently of the surface charging, if present. For intensity measurements, the spectra are fitted with Gaussian curves after a non-linear background substraction and smoothing with a least square algorithm. Each peak intensity (area) is normalized by division by the incident ion current and by an relative sensitivity factor F, taking into ac-

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count the differential scattering cross section and the analyzer transmission ( F, = 1, F, = 2.1, F,, = 22.4) [lo]. The detector efficiency was supposed to be constant. ToF-SIMS spectra were obtained with a Charles Evans & Associates TFS400-MM1 spectrometer working in the following conditions [ 1I]: a pulsed 15 keV-Gaf incident ion beam (530 pA dc current, 11 kHz pulsing rate, 1.2 ns pulse width) was rastered on a 100 X 100 km2 surface area. The spectrum acquisition time was 300 s, corresponding to a total ion per spectrum. Low fluence of N lo’* ions/cm* energy (20 eV> electrons pulses (1 electron pulse for 10 ion pulses) were used for charge neutralization. 2.3. Metallization Cu was thermally deposited in a UHV chamber (residual pressure in the lo-” mbar range) connected to the ISS analysis chamber. A Knudsen’s effusion cell located - 15 centimeters away from the polymer substrate was used for the deposition. The metal flux was continuously controlled with a water-cooled quartz crystal monitor placed at the same position as the sample. With the Cu atomic density of 8.45 X lo** atoms/cm’, one Cu monolayer corresponds to 1.93 X 1Ol5 atoms/cm*. Typical metallization rates used in this study were IO” - lOI atoms/cm* s.

3. Results and discussion Fig. la shows the ISS spectrum obtained for a pristine PET sample. The spectrum exhibits two peaks due to 3He+ backscattered after one binary elastic collision (BEC) with surface carbon and oxygen atoms respectively. The energy position of these peaks Ei corresponds to their respective kinematic factors K, = EJE,. The normalized O/C intensity ratio is about 1.6 and deviates from the stoechiometric value O/C = 0.4 of PET (C,,H,O,). This is mainly due to a higher neutralization of 3He+ ions on the C atoms than on the 0 atoms [12]. The intensity increase observed at low energy in the spectra is caused by species including hydrogen sputtered by the beam during the analysis [12]. ISS spectra were recorded after different metal deposited

0.3

0.4

0.5

73

0.6

0.7

0.8

0.9

1

Energy ratio [E/ED]

Fig. 1. ISS spectra recorded with a 2 keV ‘He scattered at 139.5” with an ion fluence of 3.44~ lOI ions/cm’ for (a) pristine PET and after in situ Cu metallization with a fluence of (b) 0.12 X 1OlL at./cm’; (c) 0.98X 10” at./cm’; (d) 2.79~ lOI at./cm*; (e) 8.67~ IO” at./cm’.

fluences. It is seen (Fig. lb) that a new peak appears at E/E, close to K,, = 0.847, which is due to 3He ions elastically scattered by Cu atoms lying on top of the PET surface. This Cu BEC signal appears only as a high energy shoulder in graphs labeled (c), (d) and (e), recorded after higher metal deposition fluences. Indeed, as the Cu deposition fluence increases, a broad distribution appears in the low energy side = 0.8) of the Cu BEC peak. This distribution (E/E, is produced by 3He ions backscattered after multiple and inelastic collisions with Cu atoms located underneath the PET uppermost surface layer [ 131. Indeed, for 3He backscattering on metallic Cu (thin layer or surface clusters), only the BEC surface peak is present in the spectra. This is due to the high neutralization of the ions backscattered below the surface. This distribution increases proportionally to the Cu deposition fluence as seen in Fig. 2 and shows that a lot of Cu atoms are buried below the PET surface. Moreover, the spectra exhibit the C and 0 BEC peaks coming from the PET (see also in Fig. 2), even after a Cu deposition of - 8 X lOi at./cm* which should correspond to four Cu monolayers in the case of an uniform coverage. This indicates that the Cu surface coverage is not uniform and that Cu clusters are formed at the surface in agreement with the persistence of the elastic shoulder in the broad distribution due to Cu. This behavior is very differ-

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those observed in the case of Al metallization of PET where an uniform surface coverage was found

0 4

2

0

Cu deposited

6

fluence

8

10

[I 0” at/cm’]

Fig. 2. Evolution of the ISS intensity with Cu fluence: (a) carbon BEC peak; (b) oxygen BEC peak and (c) copper broad distribution (reduced by a factor 2.41, for metallized PET samples.

ent from that observed in the case of Al metallization of PET where only a BEC peak was found for Al [6]. Fig. 3 presents the evolution of the O/C normalized intensity ratio as a function of the metal fluence. It is seen that the O/C ratio is nearly constant with the metal deposition fluence. This indicates that there is no preferential shadowing of the PET surface atoms by the deposited Cu atoms and confirms that there is no preferential Cu interaction with PET oxygen functionalities. The uncovered surface regions exhibit nearly the same composition as the pristine PET surface. These results showing an important diffusion of Cu underneath the PET surface, as well as Cu aggregation onto the polymer surface are in opposition to

2

0 Cu

4

deposited

6

fluence

[lCI”

8

10

al/cm’]

Fig. 3. Evolution of the normalized ISS O/C intensity ratio as a function of Cu fluence for metallized PET sample.

141. ISS measurements were also performed on pristine PMMA sample and after different Cu deposited fluences (not shown). For pristine PMMA (C,H,O,), an ISS O/C normalized intensity ratio (- 1.2) is found lower than for PET although the stoichiometric O/C ratio is the same (O/C = 0.4) for both polymers. The lower ISS O/C ratio of PMMA is understood by the difference in the surface molecular structure. In PMMA, the ester pendant groups are able to reorientate towards the bulk for minimizing the surface free energy when in contact with the apolar UHV medium of the analysis chamber. This tendency is also favored by the PMMA amorphicity which enhances the segment mobility. Moreover, a preferential 0 loss during the ISS analysis in the case of PMMA cannot be excluded [ 121. Indeed the carbonyl oxygen is more easily sputtered by the incident ions when it belongs to the pendant methacrylathe group (PMMA case> than in the case of PET, where the ester group is part of the backbone and is protected by the phenyl ring [ 171. After Cu deposition on PMMA, results similar to those observed for PET are found. Here also, no Cu binary collision peak was observed but a broad distribution due to multiple and inelastic collisions, indicating that Cu atoms diffuse underneath the PMMA surface as reported in the case of PET metallization. The evolution of the C, 0, Cu intensities and the O/C intensity ratio with the metal fluence is similar to what was found for PET. This allows to draw the same conclusions about the low interfacial reactivity and the Cu deposition mechanism. It is well established that the metal/polymer interface formation is governed by the balance between adatom-adatom and adatom-substrate interaction [ 1,2,4]. The former dominates when the reactivity of the incoming metal atom towards the polymer surface functionalities is not high enough to immobilize it in an adsorption site, allowing its surface/bulk migration until it meets an other adatom to start the formation of an island. This is the case at the Cu/PET and Cu/PMMA interfaces. A copper clustering at the polymer surface is indeed expected although the very slow deposition rate used in our

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experiments should favor the formation of an eventual adatom-substrate interaction. In order to study the effect of the ion beam surface treatment on the interface formation, the PET and PMMA films were first bombarded with 2 keV3He+ with fluences ranging from 0 up to m 3 X lOI ions/cm’. They were then metallized in situ at constant deposition rate with a Cu metal fluence of u 2 X 10” atoms/cm’, which would correspond to one monolayer deposition in the case of uniform covering. They were then analyzed by ISS. The results show that, for PET and PMMA, the copper distribution undergoes a continuous transformation when the pre-irradiation fluence is increased: the Cu signal changes from a broad distribution to a well defined BEC peak located at the correct kinematic value. This is illustrated in Figs. 4 and 5 for PET and PMMA respectively. The presence of the Cu BEC peak indicates that more metal atoms are now lying in the first monolayer. Also, the increase of the Cu BEC peak with the pre-irradiation fluence is well correlated with the decrease of the PET signals. New Cu adsorption sites are created at surface by ion bombardment. An influence of the metallization rate has been also observed, the increase of the rate leading, for the same total Cu fluence, to an increase of the BEC Cu peak. Indeed, for a given surface atom mobility, increasing the deposition rate increases the adatom-adatom encounter probability

‘::~&z~~] 013

0.4

0.5

0.;

0:7

08

0:9

1

Energy ratio [E/Eo]

Fig. 5. ISS spectra recorded on PMMA samples pre-irradiated with 2 keV ‘He+ and an ion fluence of (a) none; (b) 0.32 X IO” ions/cm”; (c) 0.63X IO” ions/cm’; (d) 1.27X IO” ions/cm*; (e) 2.32X IO” ions/cm? and, after, metallized with 2.00X 10” Cu at./cm’ and 2.82~ lOI* at./cm’s deposition rate.

and the nucleation of metal island at the surface. This prevents metal atom diffusion into the bulk. Similar effects have been observed at the Cu-polyimide interface by le Goues et al. [2]. As seen in Figs. 4 and 5, another effect is also occurring when metallization is done after pre-irradiation: a preferential decrease of oxygen as compared to carbon is observed. In order to know if this effect is due either to 0 shadowing by the deposited Cu atoms or to 0 preferential sputtering during the ion

1.2

OJJ 0

Energy ratio [E/Eo]

0.5

1

1:s

2keV-3He’

2

2:5

Pre irradiation

3

315

4

fluence

[l 0i5 ions/cm’]

Fig. 4. ISS spectra recorded on PET samples pre-irradiated with 2 keV ‘He+ and an ion fluence of (a) none; (b) 0.40X 10” ions/cm’; (c) 0.68 x 10 ” ions/cm*; (d) 1.32X IO” ions/cm*; (e) 2.52X lOI ions/cm’ and after, metallized with 2.00X 10’i Cu at./cm’ and 2.66X IO” at./cm*s deposition rate.

Fig. 6. Evolution of the relative normalized ISS O/C intensity ratio as a function of the pre-irradiation fluence for (a) PET and (b) PMMA sample. The data are normalized to unity at their maximum value (1.6 for PET and 1.2 for PMMA).

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treatment, the surface modifications produced by the ion beam on pristine PET and PMMA samples have been investigated by ISS and ToF-SIMS (only for PET). Many different surface modifications can be produced by ion beam bombardment [14]. A dehydrogenation under the 3He+ irradiation has already been mentioned to explain the low energy intensity increase observed in the ISS spectra of the pristine polymers. Moreover, Fig. 6 shows the evolution of the ISS O/C ratios as a function of the pre-irradiation fluence. The data have been normalized to unity at their maximum value in order to compare PET and PMMA. The O/C decrease evidences the pref-

6000

erential sputtering of 0 containing fragments. One may notice that this deoxygenation is faster for PMMA than for PET, as a result of the greater fragility of the pendant methacrylate groups of PMMA as mentioned above. This deoxygenation has been confirmed by ex situ ToF SIMS analyses. The different oxygen containing fragments are seen to decrease with the 2 keV-3Hef bombardment. The intensity decrease with the ion fluence is also observed for the PET characteristic peaks (Fig. 7). These peaks are directly related to the fragmentation of the monomer repeat unit M: C,Hi (@) (76 Da), @-CO+ (104 Da) and CO-@-COOH+ (149 Da), (M k H)+ are also detected at 191 Da and 193 Da

4

7 ;

5000

m N

-:

c s 3

4000 C,H,-CO 3000

d E

2000

1000

i

::

sll,

50

CO-C,H,xOOH

1% ,ll,

._I. ,; ,,, ll,,.

lbll

IljO

?I-HM+H ------Y

7000

6000 F z

CiiO 5000

C-H_

$ w 4000 ;. z J e d E

3000

2000

1000

Fig. 7. ToF-SIMS spectra of pristine (a) PET and (b) after a 2 keV ‘He+ Irradiation with an ion fluence of 1.22

X

10” ions/cm2

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[ 151. Some saturated hydrocarbon peaks like C, H 5 (29 Da), C,HT (43 Da), C,H9f (57 Da) which cannot be explained by the PET structure, are seen to increase with the irradiation fluence. It is believed that they are due to an hydrocarbon contamination of the surface during the sample transfer from the metallization chamber to the ToF-SIMS spectrometer. Similar effects of ion beam bombardment have already been reported for PMMA and PET [16,17]. Formation of macroradicals are associated with the sputtering of atoms from the macromolecular chains. These radicals can react with other radicals into the same chain to produce unsaturation and branching or between chains to produce cross-linking [ 181. The ability of plasma treatment with noble gas (also containing ions) to crosslink polymer surface has been recognized since long time and used to improve surface cohesion of polymer [ 191. If the radicals are long living, they could also react later on with the deposited metal. It is to be noted that the increase of Cu atoms immobilized at the surface while decreasing the concentration of oxygen functional groups underlines the non reactivity of Cu towards these groups. The decrease of the PET ToF-SIMS characteristic peaks with the pre-irradiation fluence could be the signature of the surface crosslinking. The ion beam induced surface crosslinking can lead to the formation of a very cohesive and dense surface skin which could act as a Cu diffusion barrier preventing its migration into the bulk. Nevertheless, due to the low Cu-polymer interaction, this is only a kinetic barrier to diffusion and interface relaxation effects may occur if the surface treatment is not strong enough. Indeed, a relaxation effect has been observed. Fig. 8 presents KS spectra recorded at different times (5, 20 and 30 minutes) after the metal deposition (2 X 10” Cu atoms/cm2) on a pre-bombarded PET sample (2 keV- 3He’, 2.3 X 1015 ions/cm2). A rapid decrease down to a saturation is observed for the Cu BEC peak intensity, and this is associated with the development of the broad Cu scattering distribution. A similar effect has been observed for the Cu/PMMA interface. This effect can be explained either by Cu clustering at the surface, either by Cu diffusion into the polymer bulk. Since no intensity increase is seen for the C and 0 polymer signals as it should be expected if only clustering at the surface was occurring, relaxation

*Ooo /

0.3

0.4

0.5

0.6

07

oa

0.9

1

Energy ratio [E/Eo]

Fig. 8. ISS spectra recorded on PET sample pre-irradiated with 2 keV ‘He+ fluence of 2.33X 10f’5 ions/cm’ and metallized with a Cu deposited fluence of 2.00X lOI at./cm’. The spectra are recorded (a) 5 min; (b) 20 min and (c) 30 min after the metal deposition.

with Cu clusters burying under the surface is proposed. Such a mechanism was also reported in the literature in order to explain the decrease in the Cu2p component in XPS study of the Cu/PET interface

[201.

4. Conclusion This ISS study of the Cu/PET and Cu/PMMA interface formation shows that the copper atoms do not interact with PET and PMMA surface ester functionalities. Therefore, their mobility is high and they diffuse along the surface as well as into the polymer bulk. The interface formation and growth are seen to be extremely dependent on the exact experimental conditions (primary ion beam preirradiation fluence, relaxation time, metal deposition rate and fluence). An 3He pre-irradiation of the polymer induces surface modifications leading to preferential oxygen loss and creating new adsorption sites for Cu (radicals, unsaturation, etc.). Moreover a surface crosslinking is suspected, hindering the Cu diffusion into the polymer bulk. The interface created by this way is nevertheless not at equilibrium and further diffusion can accompany the relaxation process. II. ION IRRADIATION

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Acknowledgements This work is supported by the Belgian Interuniversity Attraction Poles research program on the Sciences of Interfacial and Mesoscopic Structures.

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