Metallization of Polymers - American Chemical Society

Chapter 32

X-ray Photoelectron Spectroscopy of Modified Polymer Surfaces and Metal-Polymer Interfaces Correlations with Adhesion

Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch032

L. J. Gerenser Research Laboratories, Eastman Kodak Company, Rochester, NY 14650

X-ray photoelectron spectroscopy (XPS) has been used to study the interfacial chemistry between evaporated Ag and polymer surfaces with and without plasma modification. The initial stages of metallization in the sub-atomic layer regime were monitored. For an oxygen-free polymer such as polyethylene (PE), which exhibits poor adhesion with evaporated Ag, the XPS results suggest no chemical interaction with the formation of very large Ag clusters. For an oxygen-containing polymer such as poly(ethylene terephthalate) (PET), where the adhesion of evaporated Ag is greater compared to PE, the XPS results suggest charge transfer between Ag and the carbonyl oxygen in PET. Plasma treatment with oxygen introduces specific reactive sites resulting in additional chemical bond formation between the evaporated Ag and the polymer surface. These results are consistent with the significant improvement in adhesion observed for plasma treatment with a reactive gas.

Surface modification of a polymer prior to metallization is widely used to improve adhesion. The most common surface modifications employed are electric discharge (corona and plasma) and, more recently, ion-beam treatments (I). Several mechanisms have been proposed for the improved adhesion after such surface modifications (2). These include mechanical interlocking, the elimination of weak boundary layers, electrostatic attractions, and chemical bonding. All of these can play a role in adhesion depending on the surface modification used, metal/polymer system, type of metal deposition, and the extent of polymer preparation employed. However, for low power, short exposure modifications, the formation of new chemical species which can provide nucleation and chemical bonding sites for subsequent overlayers is considered to be of prime importance (3-5). Although considerable photoemission studies can be found in the literature for metallization on unmodified polymer surfaces, very few studies are reported on modified polymer surfaces. XPS has been used to investigate interactions between evaporated metals and oxygen-plasma-treated polystyrene (PS) (6.7). The XPS results suggest the formation of metal-oxygen-polymer complexes at the interface similar to that observed on oxygen-containing polymers (8). The effect of argon-ion-

0097-6156/90/044O-O433$06.00/0 © 1990 American Chemical Society Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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bombarded polyethylene (PE) on evaporated titanium films has also been studied with XPS (9.10). The XPS results indicate the formation of Ti-C bonds at the interface of both treated and untreated films; however, the treated surface produced a higher number of nucleation sites. Previously, the chemical effects of plasma treatment of PE were examined in detail with XPS (5). Argon-plasma treatments were found to produce no detectable chemical effects on the PE surface, whereas oxygen- and nitrogen-plasma treatments introduced various C - 0 and C-N functionalities into the P E surface. The effects of these plasma treatments of P E on evaporated A g were also investigated with XPS (11). It was found that argon-plasma treatment resulted in a smaller average cluster size in the early stages of metallization but did not induce any chemical interactions between the A g and PE. However, both oxygen- and nitrogen-plasma-treated P E surfaces exhibited definite chemical interactions with A g through the plasma-induced oxygen and nitrogen respectively. These results are consistent with the difference in adhesion found for evaporated A g on P E surfaces with and without plasma treatments. This paper describes the application of XPS to investigate the interfacial chemistry of evaporated A g on an oxygen-free polymer (PE) and an oxygencontaining polymer (PET), both before and after plasma treatment. The plasma treatment levels used in this study are well under the saturation level, unlike the previous study on PE (11). Whereas PE does not contain potential interaction sites for Ag, PET presents three possible interaction sites; the π electrons in the phenyl ring, the carbonyl oxygen, and the ester oxygen. Also, comparisons can be made on the extent of reactivity of the inherent oxygen in PET Versus plasma-induced oxygen. All plasma treatments and subsequent metal depositions were done in situ to prevent surface contamination on exposure to atmosphere. Metal-polymer interactions were investigated by monitoring the initial stages of metallization in the sub-atomic layer regime. Experimental The XPS spectra were obtained on a Hewlett Packard 5950A photoelectron spectrometer with a monochromatic A l Κ α x-ray source (1486.6 eV). The use of the monochromatic source minimizes sample radiation damage, which can be especially important in polymers. All materials used in this study were analyzed at ambient temperature and exhibited no evidence of x-ray damage during measurements. A l l measurements were made at an electron-takeoff angle of 3 8 ° which corresponds to an analysis depth of -50 Â Q2). The pressure in the spectrometer during analysis was typically 2 χ 10" Torr. The full width at half maximum (FWHM) for the individual components of the C ls peak in a clean polymer sample varied from 0.9 to 1.0 eV. All spectra were referenced to the C l s peak for neutral carbon in the polymer, which was assigned a value of 284.6 eV. Where necessary, a line-shape analysis routine utilizing a 90% Gaussian/10% Lorentzian line shape for the individual components was used. The PET was commercial (0.18 mm) Kodak E S T A R film base. The PE was commercial (0.10 mm) high density (HD) PE. To eliminate surface contamination, the polymer samples were ultrasonically cleaned in a series of solvents (heptane, dichloromethane, ethanol and ethyl acetate) and dried in a dry nitrogen atmosphere in a glove bag attached directly around the insertion probe of the spectrometer. Immediately after drying, the polymer samples were inserted into the preparation chamber of the spectrometer where they were evacuated to ~5 x 10' Torr. The polymer samples were than annealed (PET~90° C, PE~ 6 0 ° C) for several hours to drive off any residual solvent, water, or adsorbed gases. The plasma treatments were done in the preparation chamber of the 9

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Sacher et al.; Metallization of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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9

spectrometer (base pressure 1 χ 10~ Torr) by applying a potential to a 6 mm diameter A l rod mounted on a high-voltage feedthrough using a 5 kV, 60 Hz power supply while floating the sample rod with the chamber walls at ground potential. The sample-to-electrode distance is about 50 mm with the sample residing in the positive column of the plasma during treatment. The typical operating conditions were 10 W of primary power at 50 mTorr pressure for 15 s. Silver evaporations were done in the same preparation chamber using an inhouse-constructed evaporation assembly containing a resistively heated tungsten basket. A g coverages were controlled by a quartz crystal thickness monitor. The deposition rates were typically 0.01 to 0.1 Â / s depending on the desired coverage. Coverages were subsequendy calibrated by XPS analysis of the A g 3d and polymer core level (C ls and Ο ls) intensities as compared to known standards, and by neutron activation analysis (11). Samples for adhesion measurements were prepared in a separate chamber with a base pressure of 1 χ 10"° Torr under identical conditions. Typically, -1000 Â silver coverages were used for adhesion measurements. Adhesion strength of the deposited A g films was determined using a polyester adhesive tape and standard peel test procedures on an Instron tester. Results and Discussion General. The relative adhesion strength for evaporated A g on P E and PET with and without plasma treatments is shown in Figure 1. Several observations are immediately apparent. First, the adhesion strength of Ag on untreated PET is greater compared to A g on untreated PE. This result suggests that the inherent oxygen in PET may play a role in adhesion. Second, argon-plasma treatment provides little to no improvement in adhesion for both polymers. This result, together with the cleaning and annealing of the polymers prior to plasma treatment or metallization, suggests that boundary layers do not play a major role in the differences in adhesion observed in Figure 1. If a weak boundary layer still exists after cleaning and annealing, then plasma treatments may improve adhesion either by crosslinking or ablation of the boundary layer. However, both argon- and oxygen-plasma treatments were shown to induce a similar extent of crosslinking into PE (£). Also both argonand oxygen-plasma treatments were found to ablate polymers at a similar rate of -1 Â/s for the typical treatment times used in this study. Third, oxygen-plasma treatment provides a significant improvement in adhesion for both polymers. Again, suggesting the importance of a specific reactive species for improvement in adhesion. Scanning electron microscopy (SEM) analysis of polymer surfaces treated for 60 s, which is significantly greater than the 15 s treatment times used in this study, could detect no observable morphological differences between untreated and plasmatreated polymer surfaces for magnifications up to x20,000 (5). Based on these results, boundary layers and mechanical interlocking do not appear to play a major role in the differences in adhesion shown in Figure 1. Therefore, these differences in adhesion will be addressed from a chemical basis using XPS. Since only slight improvements were found for argon-plasma treatment and previous studies on P E (5.11) suggest no chemical effects for argon-plasma treatments, only the untreated and oxygen-plasma-treated polymers will be discussed in detail. For all depositions, high resolution spectra of the A g 3d and polymer ( C l s and/or O l s ) core levels and valence levels were accumulated. However, analysis of the A g 3d core level revealed no differences for all depositions. The Ag 3 d / peak centroid was always 368.0 ±0.1 eV. Unfortunately, chemical bonding information is difficult to obtain from the A g core levels since the +1 oxidation state exhibits little or 5


2


436


10 Ε ο Ζ

c

s

1.0

II

c o Φ

Ό < 0.1

-PET-

-PEB

Untreated

I

Argon Plasma

ESI Oxygen Plasma Figure 1. Relative adhesion strength for evaporated A g on polymers.


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no shift and typically the Auger Paramater is used to distinguish between the metal and +1 oxidation state of Ag. However, the A g M W Auger peak is relatively weak and broad and due to the fact that the coverages are less than a monolayer with the possibility of multiple oxidation states present, an analysis using the Auger peak proved to be uninformative. Therefore, the following discussion will focus on analysis of the polymer core levels and the Ag valence levels. 4


Ag/untreated PE. Based on the C ls level and valence level spectra, no evidence of A g interaction with untreated PE was found. Several depositions were done, and in all cases no changes in the shape or peak positions were found. This is consistent with the poor adhesion for A g on untreated PE. The results for Ag depositions on argon-plasma-treated PE were similar, except for slight changes in the valence level spectra which will be discussed below. Ag/oxvgen-plasma-treated P E . Oxygen-plasma treatment of P E introduces a significant amount of oxygen into the PE surface (5). Depending on the treatment level and time between 1 to -20 atomic percent oxygen can be incorporated. Approximately 12 atomic percent oxygen is incorporated for the 15 s treatment time used in this work. The C ls and Ο ls spectra for oxygen-plasma-treated P E are shown in Figure 2. A distribution of various C - 0 functionalities is observed similar to those reported previously (5). An analysis of the C ls and O l s regions after deposition of - 0.6 Â A g on the oxygen-plasma-treated P E reveals significant changes as shown in Figure 2. The C ls spectrum indicates a depletion in the high binding energy features after deposition of A g and some line broadening. The regions due to C - 0 species at 286.1 eV, and OC=0 species at 288.6 eV exhibit the largest change. This effect can be due to two sources; bond breaking induced by the thermally hot A g atoms impinging on the polymer surface resulting in desorption of short-chain species or rearrangement of the modified surface and/or the formation of Ag-O-C species at the interface resulting in charge transfer to the carbon atoms through oxygen. Desorption of short-chain species is probably occuring to some extent, since an overall decrease in oxygen is observed after A g deposition. The net result of the formation of Ag-O-C species is a shift to lower binding energy for the carbon atoms involved in this interaction. A g interaction with the C - 0 species would result in a peak that falls within the envelope of the C - C species and might not be easily detected. However, Ag interaction with the 0-C=0 species should result in a peak lower in energy than the 0-C=0 peak but at a higher energy than expected for C=0 (287.6 eV). In fact, a definite increase in intensity at 288.0 eV is observed. This peak position is identical to those observed for A g salts of carboxylic acids (Table I). Examination of the Ο ls spectrum, Figure 2, reveals a shift in the peak centroid to lower binding energy, consistent with A g - 0 bond formation (13). A lineshape analysis of the Ο ls peak after A g deposition is shown in Figure 3. The lineshape analysis was done by constraining the peak positions to values previously determined in this laboratory for model compounds. The best fit provides two peaks at binding energies of 531.3 eV, and 532.7 eV. The peak at 532.7 eV is identical to that observed before A g deposition. The peak at 531.3 eV occurs at the same energy as previously determined for compounds of A g bonded to oxygen (13) and is consistent with the formation of a Ag-O-C species (Table I). The valence level spectra are shown in Figure 4. Even at coverages of less than a monolayer, the valence band spectrum is dominated by the Ag 4d band between 3-8 eV. This band directly overlaps the region of the C - H band of the untreated P E and the region where the Ο 2p lone pair orbitals occur for oxygen-plasma-modified PE (5). The two peaks comprising the C-C band (predominately C 2s character) of the PE substrate are evident in all spectra at 13.2 eV and 18.8 eV. The Ag 4d band for



438


I

I

I

I

I

I

290.5

288.5

286.5

284.5

282.5

280.5

Binding Energy (eV)

A / \

Ols

/

\

I

I

I

I

I

I

537.5

535.5

533.5

531.5

529.5

527.5

Binding Energy (eV) Figure 2. Comparison of C ls and O l s levels before (dashed line) and after (solid line) deposition of 0.6 Â A g on oxygen-plasma-treated PE.



32. GERENSER

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I

I

I

I

I

I

537.5

535.5

533.5

531.5

529.5

527.5

Binding Energy (eV) Figure 3. O l s line-shape analysis after deposition of 0.6 Â A g on oxygenplasma-treated PE.


440



JL 20

1 15

ι 10 Binding Energy (eV)

I 5

L 0

Figure 4. Valence level region after deposition of 0.6 Â A g on (a) oxygenplasma-treated, (b) argon-plasma-treated, and (c) untreated PE.



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the untreated PE contains two distinct features reminiscent of nearly bulk-like Ag. However, for the A r plasma treated PE, the 4d band is somewhat narrower with a loss of the characteristic bulk-like A g features. For the oxygen-plasma-treated PE, the 4d band is considerably narrower with a definite feature extending to low binding energy. These results can be explained due to two effects; a difference in A g cluster size, and the formation of a Ag-O-C species at the interface. The differences between the A g 4d band on the untreated and argon-plasma-treated PE can be attributed solely to a difference in cluster size. However, on oxygen-plasma-treated P E , the differences can be attributed to both a difference in cluster size and the formation of a Ag-O-C species at the interface. Based only on clustering effects, the valence band spectra suggest a very large average cluster size for the untreated PE and near the single atom limit for the oxygenplasma-treated P E with the argon-plasma-treated P E somewhere in between. For single atom centers, the A g 4d free-atom state is split 0.55 eV due to spin-orbit interactions (14). giving rise to one relatively narrow line. As the clusters grow, the 4d band broadens and splits due to a combination of spin-orbit and crystal field effects (15). The progression from large to small clusters does predict a narrowing of the d band and has been reported on amorphous graphite (16). However, based on clustering effects alone, the shape of the d band should be symmetrical. The asymmetric shape of the d band on the oxygen-plasma-treated P E suggests the formation of a A g - 0 species. The reason for the asymmetric shape of the d band will be discussed in more detail in the section on model compounds.

Table I. Summary of XPS data for low coverage (-0.6 Â ) of A g on oxygen-plasmatreated PE and PET compared to model compounds. All data are listed in eV and are within ±0.1 eV of the stated value

Ag3dc

/2

Ag/PE Ag/PET Agbenzoate Ag acetate Ag 0 AgO 2

368.0 368.1 368.1 368.0 368.0 367.9

Cls

288.0 288.0

Ols (C^C-O) 531.2 531.2 531.3 531.3 531.4 530.5

Ag4d 1

FWHM

ionization threshold

2.6 2.7 2.4 2.7 2.1 2.9

2.0 2.1 1.9 1.8 0.7 0.6

Ag/untreated PET. The C l s and O l s spectra for A g deposition on untreated PET are shown in Figure 5. The C ls spectrum for the clean PET surface before A g deposition consists of four distinct peaks: the carbon atoms in the phenyl ring (284.6 eV), the methylene carbon atoms singly bonded to oxygen (286.1 eV), the ester carbon atoms (288.6 eV), and the peak due to the π - π * shakeup transition (290.6 eV). The three main peaks are present in a 3/1/1 ratio as expected based on P E T stoichiometry. The π - π * shakeup peak is -4.5% of the total integrated area of the main peaks. After A g deposition, slight line broadening occurs and the ester carbon peak decreases in intensity by -5-10%. The attenuation of the ester carbon peak can be attributed to A g interaction with the carbonyl oxygen. This possibility will be discussed later after examination of the O l s spectra. Another subtle effect that occurs after A g deposition is loss of intensity of the π - π * shakeup peak. The π - π * shakeup intensity is plotted as a function of A g coverage in Figure 6. There are several possible reasons for this effect. First, the



442


I 294.0

I

I

289.0

284.0

I 279.0

Binding Energy (eV)

542.0

537.0

532.0

527.0

Binding Energy (eV) Figure 5. Comparison of C ls and O l s levels before (dashed line) and after (solid line) deposition of 0.6 Â A g on untreated PET.



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5

ζ

ι

0

•

1 1

1

1

1

2

1 3

1

1

4

Coverage (A)

Figure 6. Intensity of C ls shakeup peak as a function of A g coverage.



444


aforementioned interaction of A g with the carbonyl oxygen can induce a net increase in electron density in the phenyl ring through the ester carbon atom. A second possibility is charge transfer from A g to the phenyl ring via the formation of a Ag-π complex with the phenyl ring. The formation of A g complexes with π systems has been discussed by Cotton and Wilkinson (17). Changes in the intensity of shakeup peaks for aromatic systems has been discussed by Clark and Dilks (18) where the effect of electron donating groups on the phenyl ring was found to decrease the shakeup intensity. A third possibility is breakup of the phenyl ring due to the thermally hot A g atoms impinging on the PET surface. The line broadening observed after A g deposition also suggests damage to the polymer surface possibily by bond breaking. Possibilities 2 and 3 cannot be excluded with the present information. Examination of the Ο ls spectra (Figure 5) can provide further insights into the Ag/PET interactions. The O l s spectrum for the clean PET surface contains three distinct peaks due to the carbonyl oxygen atoms at 532.2 eV, the ester oxygen atoms at 533.5 eV, and the π - π * shakeup peak at 538.5 eV. The main peaks are present in 1/1 ratio as expected for clean PET. The π - π * shakeup peak is ~ 2.5% of the total integrated area of the main peaks. After A g deposition, the Ο ls spectrum broadens slightly, similar to the C ls spectra, and exhibits a definite skew to lower binding energy. The π - π * shakeup peak also decreases in intensity as a function of A g coverage similar to that observed for the C ls shakeup peak. The skew to lower binding energy suggests the formation of a new oxygen species. A line-shape analysis of the Ο ls spectrum after A g deposition suggests the presence of a new peak at -531.2 eV. This peak occurs at the same energy as that found on the oxygen-plasma-treated PE and is identical to the value previously determined for A g carboxylate compounds where the Ag-oxygen bond has a significant degree of covalent character (13). Based on the line-shape analysis, this peak is estimated to be approximately 5-10% of the Ο ls total integrated area. These results are consistent with the formation of a Ag-O-C species, but the formation of A g 0 by abstraction of oxygen from the polymer cannot be excluded based on the XPS data. Three possible reaction schemes are shown in Figure 7 to account for these observed changes. Reaction 1 involves breaking of the ester oxygen/ester carbon bond and the formation of a Ag-O-C species with the ester oxygen. This is consistent with the observed decrease in the ester carbon intensity. This reaction scheme also produces isolated carbonyls which is consistent with the observed intensity increase in the carbonyl region. However, such a scheme would result in a net shift to lower binding energy for the C - 0 (286.1 eV) species bonded to Ag. Experimentally this is not observed. Such a small change might be difficult to detect due to overlap of the C - 0 peak (286,1 eV) with the peak due to the carbon atoms in the phenyl ring (284.6 eV). Reaction 2 involves breaking of the methylene carbon/ester oxygen bond and the formation of a Ag-carboxylate structure. This structure would result in a net negative charge on the ester carbon atom causing a shift of the ester carbon peak to lower binding energy, consistent with the observed decrease in the ester carbon intensity. However, this reaction scheme would eliminate one C - 0 species for each Ag-carboxylate formed. Experimentally this is not observed. Reaction 3 requires no bond-breaking and is simply an interaction of A g with the carbonyl oxygen. This interaction results in a net transfer of electron density to the ester carbon through the carbonyl oxygen and produces a net shift to lower binding energy for the ester carbon, consistent with all the XPS data. The valence level spectra are shown in Figure 8. The valence level spectrum for clean PET consists of five main bands of which the most important are the bands between 3-10 eV. The highest occupied valence level (3-4 eV) should have significant contribution from the π electrons in the phenyl ring and are not resolvable as a separate band. The bands between 4-10 eV have significant contributions from 2



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Reaction 1

Ο

Η

Η

Ο

- C — Ο — C — C — 0 - - A g H

.c

+

(Cj)—

H

Reaction 2

H

H

I

I

I

I

Ag

- C — O — C — O H

+

+

Metallization of Polymers - American Chemical Society

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