Identification of Physical and Chemical Interaction Mechanisms for the

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Identification of Physical and Chemical Interaction Mechanisms for the Metals Gold, Silver, Copper, Palladium, Chromium, and Potassium with Polyimide Surfaces T. Strunskus,†,‡ M. Grunze,*,§ G. Kochendoerfer,†,| and Ch. Wo¨ll§ Laboratory for Surface Science & Technology, University of Maine, Orono, Maine 04469, and Lehrstuhl fu¨ r Angewandte Physikalische Chemie, Institut fu¨ r Physikalische Chemie, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received February 21, 1995. In Final Form: February 14, 1996X In this article we briefly review the literature for the evaporated metals gold, silver, copper, palladium, chromium, and potassium on polyimide surfaces and compare these previous results to newer experiments using Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The polyimide films were prepared by vapor phase deposition. Metal coverages range from submonolayer to several monolayers, but special emphasis in this work is put on the interaction of metals with a polyimide surface at very low metal coverages. Before our new results are presented, we discuss the various chemical and nonchemical effects, which can contribute to the change in IR absorption of polymers. For all of the metals except potassium only attenuation of polymer IR bands is observed. For the metal deposits of Au, Cu, Ag, and Pd the attenuation of the IR bands can be explained by a purely physical interaction mechanism, i.e., dynamical dipole screening and changes in the intermolecular dipole-dipole coupling between the polymer macromolecules. Deposition of potassium leads to very different and characteristic changes of the polymer IR bands and of the NEXAFS spectra which can conclusively be explained with an electron transfer from the potassium onto the polyimide. In the case of chromium the IR and NEXAFS data indicate that the chemical and physical interaction of chromium with polyimide is very complex already at the initial stage of the metal/polymer interface formation and cannot be explained conclusively with any of the interaction models suggested in the literature.

I. Introduction The study of metal/polymer interfaces has received considerable interest in recent years.1,2 Industrial applications of metallized plastics range from large area reflective coatings to densely packed structures in very large scale integrated (VLSI) microelectronic devices. Due to the high thermal and chemical stability and low dielectric constant of polyimides, these polymers are frequently used in microelectronic applications.3 Reliable adhesion between the metallization and the insulating polyimide layer will depend on the nature of interaction between the metal and the polymer; e.g., a strong chemical reaction involving metal/polymer bond formation is expected to result in strong adhesion, whereas a purely physical interaction via van der Waals or electrostatic forces is supposed to result in weak adhesion.4 Chou and Tang5 suggested a model where the bonding strength of the metal/polyimide joint was determined by the thermodynamic ability of the metal to form metal oxides. Hence, a thermodynamical classification for the chemical * To whom correspondence should be addressed. † University of Maine. ‡ Present address: Lehrstuhl fu ¨ r Materialverbunde, Technische Fakulta¨t der Universita¨t Kiel, Kaiserstrasse 2, 24143 Kiel, Germany. § Institut fu ¨ r Physikalische Chemie. | Present address: Department of Chemistry, University of California, Berkeley, CA 94720. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Metallized Plastics 1,2,3: Fundamental and Applied Aspects; Mittal, K. L., Susko, J. R., Eds.; Plenum Press: New York, 1989, 1991, 1992. (2) Metallization of Polymers; Sacher, E., Pireaux, J. J., Kowalczyk, S. P., Eds.; ACS Symposium Series 440; American Chemical Society: Washington, DC, 1990. (3) Polyimides: Synthesis, Characterization and Application; Mittal, K. L., Ed.; Plenum Press: New York, 1982, 1984; Vols. 1 and 2. (4) Mittal, K. L. J. Vac. Sci. Technol. 1976. (5) Chou, N. J; Tang, C. H. J. Vac. Sci. Technol. A 1984, 2, 751.

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reactivity of various metals with polyimides as proposed by Chou and Tang5 should be adequate, if no kinetic barriers or competing reactions, such as metal diffusion into the polymer or cluster formation on the polymer surface, prevent the system from reaching the thermodynamically most favorable state. Whereas for high metal coverages (continuous metal layers) a general agreement exists in the literature that the more reactive transition metals from strong covalent bonds and noble metals only interact via physical forces with the polyimide substrate, the situation for lowest metal coverages (i.e., individual atoms or small clusters) is not clear at all. As summarized in the following paragraphs, the results for small metal coverages on polyimide reported by different groups (in most cases using different spectroscopic techniques) are even for supposedly identical or similar experimental conditions at variance, and the interpretations offered for the same metal/polyimide system cover a wide range of plausible interaction mechanisms. This is not surprising, since, at the lowest coverages, the diffusivity of metal atoms and hence the accessibility of the (thermodynamically) most favorable reaction sites will depend on the chemical composition of the polymer surface (orientation of the polymer segments), surface morphology (crystalline or amorphous), and the balance between the cohesive energy of the metal and the adsorption energy to the polymer, which will favor metal cluster formation or chemisorption, respectively. It is quite obvious that minute differences in the experimental conditions of polymer surface preparation, metal deposition, surface cleanliness, and ambient conditions (in particular temperature) can influence the experimental results and hence lead to different conclusions about the interaction of metal atoms and clusters with polyimide surfaces. Small kinetic barriers for surface diffusion or chemisorption, which cause the system to remain in a metastable state, can hence lead to grossly wrong conclu© 1996 American Chemical Society

Interactions of Metals with Polyimide Surfaces

Figure 1. Structure formula of the PMDA/ODA polyimide repeat unit. The repeat unit consists of a pyromellitic diimide (PMDI) part and a diphenyl ether part (DPE).

sions when the interpretation of spectroscopic data is aided by calculations assuming the lowest possible energy state. To visualize the chemical heterogeneity of the polyimide surface, the polymer repeat unit for poly(N,N-diphenyl ether pyromellitimide) (PMDA/ODA polyimide) used as a substrate in this study is displayed in Figure 1. The pyromellitic diimide (PMDI) part of the repeat unit consists of two imide rings connected via one benzene ring. The planar PMDI unit allows conjugation of the carbonyl π-electrons with the π-electron system of the central benzene ring.6 The diphenyl ether (DPE) part consists of two phenylene rings bridged by an ether oxygen. According to the results of structure calculations for model monomers, the phenylene rings in the diphenyl ether part are tilted with respect to each other and are also tilted with respect to the PMDI units.6,7 Hence, no π-conjugation across the ether oxygen or across the imide nitrogen atoms is possible. As possible bonding sites for metals the carbonyl groups, the imide rings, the central benzene ring of the PMDI part, the phenylene rings of the diphenyl ether part, and the ether oxygen have to be considered. This paper presents experimental results using Fourier transform infrared reflection absorption spectroscopy (FTIRAS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) for a variety of metals (emphasizing low coverages) onto polyimide surfaces prepared by vapor deposition of PMDA and ODA and subsequent imidization in vacuum. X-ray photoelectron spectroscopy results were recorded for all metal/polyimide systems discussed here and are in good agreement with the published results obtained for vapor deposited or spincast polyimide films. We will refer to our and the published XPS results in the discussion but present here only the new FT-IRAS and NEXAFS results. We communicate the problems encountered when mechanistic interpretations are derived from results obtained by a single technique and want to present our arguments that the models presently discussed in the literature do not describe the details of metal interaction with a polyimide surface at low metal coverages. There is no disagreement between our interpretations with the conclusions in the literature for the nature of interaction between metal and polymer when the technological, relevant, high-coverage regime, i.e., continuous metal layer or large cluster formation, is considered. This paper is organized in the following way: First, we summarize the published experimental results and models for the metal/polyimide reaction, with emphasis on the metals representative for those studied in this work: Au, Ag, Cu, Pd, Cr, and K. This will be followed by the Experimental Section, where we also discuss the ambi(6) Takahashi, N.; Yoon, D. Y.; Parrish, W. Macromolecules 1984, 17, 2583. (7) Poon, T. W.; Silverman, B. D.; Saraf, R. F.; Rossi, A. R.; Ho, P. S. Phys. Rev. B 1992, 46, 11456.

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guities in the interpretation of the FT-IRAS data with respect to surface chemical reactions. We then present the spectroscopic results and specific discussions for the metals Au, Cu, Pd, Cr, and K evaporated onto vapor deposited polyimide surfaces. A general summary concludes this paper. Previous Studies on the Metallization of Polyimide. Most of the work on metal/polyimide interfaces has been summarized and discussed in overview articles by DiNardo,8 Ho et al.,9 Grunze et al.,10 Pireaux,11 and by Matienzo and Unertl.12 The latter excellent article not only reviews the results on surface chemical reactions but also discusses polyimide surface preparation and modifications, and mechanical tests and properties of metallized polyimide. We briefly review only some of the results which we believe are necessary for the following discussion of the FT-IRAS and NEXAFS data. Gold, Silver, Copper, and Palladium. The changes in the C 1s, N 1s, and O 1s X-ray photoelectron spectra (XPS) upon metal deposition allow experimenters to classify the metals by their chemical reactivity toward polyimide and to obtain general information about the preferential interaction sites. According to the thermodynamic model suggested by Chou and Tang the metals gold, silver, copper, and palladium should show no or only a weak chemical interaction with polyimide.5 For gold this was confirmed in a XPS study by Meyer et al.13 Silver is chemically more reactive than gold, but less reactive than copper. Interestingly an XPS study of the Ag/PET (poly(ethylene-terephthalate)) interface by Gerenser et al.14 gave qualitatively the same results as a Cu/PI interface study by Pertsin and Pashunin.15 In both studies a chemical reaction of the metal with the carbonyl group was suggested at very low coverages. But there is also experimental evidence that copper does not react chemically with polyimide, e.g., copper atoms diffuse in polyimide at elevated temperatures and only react with other copper atoms to form metallic copper clusters and particles inside the polymer matrix.16,17 Recently, Davis et al. concluded that copper does not react with polyimide at low coverages but that there is a change in bond order of the polyimide carbonyl groups induced by the copper at higher coverages.18 They found, however, no evidence for copper in an oxidized state. According to DiNardo et al. the chemical reactivity of palladium is similar to copper.19 In a high-resolution electron energy loss spectroscopy (HREELS) and ultraviolet photoemission spectroscopy (UPS) study they found (8) DiNardo, N. J. Surface Spectroscopic Techniques Applied to Metallized Plastics. In Metallized Plastics 1: Fundamental and Applied Aspects; Mittal, K. L., Susko, J. R., Eds.; Plenum Press: New York, 1989. (9) Ho, P. S.; Haight, R.; White, R. C.; Silverman, E. D.; Faupel, F. In Fundamentals of Adhesion; Lee, H. Y., Ed.; Plenum Press: New York, 1991, p 14. (10) Grunze, M.; Killinger, A.; Mainka, C.; Hahn, C.; Strunskus, T. In Metallized Plastics 2: Fundamental and Applied Aspects; Mittal, K. L., Susko, J. R., Eds.; Plenum Press: New York, 1991; p 165. (11) Pireaux, J. J. Synth. Met. 1994, 67, 39. (12) Matinzo, L. J.; Unertl, W. N. In Polyimides: Fundamental Aspects and Technological Applications; Mittal, K. L., Ghosh, M., Eds.; Dekker: New York, in press. (13) Meyer, H. M., III; Anderson, S. G.; Atanasoska, Lj.; Weaver, J. H. J. Vac. Sci. Technol. A 1988, 6 (1), 30. (14) Gerenser, L. J. J. Vac. Sci. Technol. A 1990, 8, 3682. (15) Pertsin, A. J.; Pashunin, Yu. M. Appl. Surf. Sci. 1991, 47, 115. (16) Tromp, R. M.; Legoues, E.; Ho, P. S. J. Vac. Sci. Technol. A 1985, 3, 782. (17) Faupel, F.; Gupta, B. D.; Silverman, B. D.; Ho, P. S. Appl. Phys. Lett. 1989, 55, 357. (18) Davis, G. D.; Rees, B. J.; Whisnant, P. L. J. Vac. Sci. Technol. A 1994, 12, 2378. (19) (a) DiNardo, N. J.; Demuth, J. E.; Clarke, T. C. J. Chem. Phys. 1986, 85, 6739. (b) DiNardo, N. J.; Demuth, J. E.; Clarke, T. C. J. Vac. Sci. Technol. A 1986, 4, 1060.

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no evidence for a chemical reaction of palladium with the polyimide.19 Chromium. The chemical interaction between thermally evaporated chromium (which is used as an adhesion promoter and diffusion barrier for copper/polyimide layers20) and polyimide has been a subject of numerous studies. Chromium/polyimide interfaces have been studied mainly by XPS5,21-27. The application of other techniques like HREELS,19,28 UPS,19,21 FT-IRAS,29 and NEXAFS,30 together with the use of model monomer and polymer compounds, allowed more specific information on the interaction between chromium and polyimide to be gained. In some cases the interpretation of the spectra was aided by model calculations.22,31,32 Today, most researchers agree that chromium interacts initially with the PMDI part of the polyimide and disrupts the polyimide at higher coverages to form oxidic, nitridic, and carbidic compounds. There is, however, still an ongoing controversy over the details of the interaction at very low chromium coverages. Mainly four different interaction models have been proposed in the literature. (1) π-Arene Complex Formation. On the basis of XPS results and quantum chemical calculations Haight et al. suggested that chromium forms a half-sandwich π-arene complex with the central benzene ring of the PMDI unit.21 Calculations of vibrational spectra for the proposed π-arene complex by Silverman predicted shifts of the imide related bands between 32 and 71 cm-1, an increase of the band intensities, and the appearance of a new band at 1415 cm-1.32 However, an XPS study of the chromium/BPDA-PDA polyimide interface formation revealed, that the good agreement between calculated and experimentally observed core level spectra for the π-complexation model was perhaps coincidental for ODA-PMDA polyimide.33 Clabes et al. and Andrews et al. did not detect the formation of π-complexes, even under thermodynamically favorable conditions, and presented experimental and theoretical evidence against the π-complexation model.26,27,34 (2) Electron Transfer. Clabes et al. suggest that electron transfer from chromium onto the PMDI units is the initial interaction step.26 Ionic bonds are formed between Cr2+ ions and polyimide anions. Andrews deposited chromium in an argon atmosphere into a 2-methyltetrahydrofuran solution of a polyimide model compound (N,N′-di-n(20) Kim, J.; Kowalczyk, S. P.; Kim, Y. H.; Chou, N. J.; Oh, T. S. Mater. Res. Soc. Symp. Proc. 1990, 167, 137. (21) White, R. C.; Haight, R.; Silverman, B. D.; Ho, P. S. Appl. Phys. Lett. 1987, 51, 481. (22) Haight, R.; White, R. C.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol. A 1988, 6, 2188. (23) Sanda, P. N.; Bartha, J. W.; Clabes, J. G.; Jordan, J. L.; Feger, C.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol. A 1986, 4, 1035. (24) Jordan, J. L.; Sanda, P. N.; Morar, J. F.; Kovac, C. A.; Himpsel, F. J.; Pollack, R. A. J. Vac. Sci. Technol. A 1986, 4, 1046. (25) Jordan, J. L.; Kovac, C. A.; Morar, J. F.; Pollak, R. A. Phys. Rev. B 1987, 36, 1369. (26) Clabes, J. G.; Goldberg, M. J.; Viehbeck, A.; Kovac, C. A. J. Vac. Sci. Technol. A 1988, 6, 985. (27) Goldberg, M. J.; Clabes, J. G.; Kovac, C. A. J. Vac. Sci. Technol. A 1988, 6, 991. (28) Pireaux, J. J.; Gregoire, Ch.; Vermeersch, M.; Thiry, P. A.; Vilar, M. R.; Caudano, R. In Metallization of Polymers; Sacher, E., Pireaux, J. J., Kowalczyk, S. P., Eds.; ACS Symposium Series 440; American Chemical Society: Washington, DC, 1990; p 47. (29) Dunn, D. S.; Grant, J. L. J. Vac. Sci. Technol. A 1989, 7, 253. (30) Jordan-Sweet, J. L.; Kovac, C. A.; Goldberg, M. J.; Morar, J. F. J. Chem. Phys. 1988, 89, 2482. (31) Rossi, A. R.; Sanda, P. N.; Silverman, B. D.; Ho, P. S. Organometallics 1987, 6, 580. (32) Silverman, B. D. Macromolecules 1989, 22, 3768. (33) Anderson, S. G.; Leu, J.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol. A 1993, 11, 368. (34) Andrews, M. P. In Metallization of Polymers; Sacher, E., Pireaux, J. J., Kowalczyk, S. P., Eds.; ACS Symposium Series 440; American Chemical Society: Washington, DC, 1990; p 242.

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butylpyromellitimide) and observed reduction of the PMDI to the anion.34 When exposed to air, the solution reoxidizes to the neutral form. Reduction of pyromellitimides leads to very characteristic shifts of the imide related vibrational bands.26 (3) Local Bonding to a Carbonyl Group. Alternatively, the chromium atoms could bond to the carbonyl group with the formation of covalent bonds to the oxygen atom or to the oxygen and carbon atoms of the carbonyl group.35,36 Model calculations considering the interaction of a chromium atom with the carbonyl group of acetone predict a shift of the carbonyl stretching band from 1760 to 1250 cm-1.36 In an early HREELS study DiNardo et al. observed a shift of the carbonyl stretching band from 1705 to 1490 cm-1.19 The shift could indicate that in polyimide the bond order of the carbon-oxygen bonds is lowered by the interaction with chromium but that it remains larger than one. (4) Oxygen Abstraction. On the basis of new calculations and experiments for the Cr/BPDA-PDA-polyimide interface Anderson et al. favored an oxygen abstraction model, where chromium would deoxygenate the carbonyl groups.33 Such a reaction has been observed for the interaction of chromium with ketones.37 Potassium. With respect to the models proposed for chromium adsorption, the question of charge transfer between metal and polymer is central. Hence, the reduction of polyimide by electrochemical means or by alkali metals can provide spectroscopic data for comparison with chromium adsorption experiments. Polyimides can be reduced electrochemically or chemically. Redox potentials for monoanion (1E° ) -0.79 V) and dianion (2E° ) -1.33 V) formation of the polyimide PMDI units have been measured with cyclovoltametry against a saturated calomel electrode.26 IR and UV absorption spectra of dissolved N,N′-di-n-alkylpyromellitic diimides, and their reduced forms have been reported in the literature.26,34 Clabes et al. demonstrated that polyimide can be reduced chemically with cesium (E° ) -3.02 V).26 Deposited onto polyimide under ultrahigh vacuum conditions cesium intercalates in the polyimide film and transfers an electron to the PMDI units of the polymer. Most of the additional charge is located on the carbonyl oxygen atoms. Potassium (E° ) -2.92 V) should reduce the PMDI units of a polyimide film similar to cesium. Clabes et al. pointed out that core level spectra of reduced polyimide obtained from electrochemical or chemical reduction are similar to spectra obtained after chromium deposition.26 Studies of Metal/Polyimide Interfaces Using Vibrational Spectroscopies. Vibrational spectroscopies can in principle provide very specific information about the nature of the chemical bonds at metal/polymer interfaces.11 High-resolution electron energy loss spectroscopy (HREELS) has been used to study the palladium/ polyimide,19 the chromium/polyimide,19,28 and aluminum/ polyimide interfaces.11,19,38 In an early HREELS study Dinardo et al. observed a shift of the carbonyl band to lower wavenumbers for chromium but no shift for palladium.19 They concluded that chromium reacts with the carbonyl group of polyimide and that palladium shows no chemical interaction with polyimide.19 Pireaux et al. (35) Chenite, A.; Selmani, A.; Yelon, A. J. Vac. Sci. Technol. A 1994, 12, 513. (36) Ouhlal, A.; Selmani, A.; Yelon, A.; Andrews, M. P. Chem. Phys. Lett. 1993, 202, 51. (37) Gladysz, J. A.; Fulcher, J. G.; Togashi, S. J. Org. Chem. 1976, 41, 3647. (38) Pireaux, J. J.; Vermeersch, M.; Gregoire, Ch.; Thiry, P. A.; Caudano, R.; Clarke, T. C. J. Chem. Phys. 1988, 88, 3353.


observed intensity changes but no additional or shifted bands in their HREELS study of the Al/PI interface.11,38 They excluded the breaking of polymer bonds and proposed the formation of a weak organometallic Al-O-C complex.11 Dunn and Grant applied Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) to study the metallization of polyimide and poly(ethyleneterephthalate) (PET) with copper and chromium.29 For 1 nm metal deposits they observed for both metals the same selective attenuation of certain polymer IR bands without the appearance of additional or shifted bands in the FT-IRA spectra. Preferably the bands characteristic for carbonyl or related groups were attenuated. Dunn and Grant concluded that both metals show a strong chemical interaction with the polymers, i.e., abstraction of oxygen at the carbonyl sites.29 In a previous study we used a combination of XPS and FT-IRAS to study the copper/polyimide interface.39 Our measurements essentially reproduced the FT-IRAS results of Dunn and Grant and the XPS results of other groups. However, we concluded that the FT-IRAS results for copper must be interpreted differently than that suggested by Dunn and Grant to be consistent with the XPS results. We suggested that in the case of copper the attenuation of the polyimide bands is not caused by chemical reactions but through screening of the dynamic dipole moments of the polymer functional groups by the highly polarizible metal atoms and clusters.39 Summarizing the results, an almost general consensus exists that gold, silver, copper, and palladium interact only weakly with polyimide, but the details of the interaction are still controversial. Chromium shows a much stronger interaction and disrupts the polymer at higher coverages to form oxidic, nitridic, and carbidic compounds. Yet, the model proposed for the adsorption mechanism of chromium atoms at lowest coverage have something to offer for every chemical subdiscipline, ranging from inorganic chemistry (oxide formation) over redox chemistry to organometallic chemistry and to coordination chemistry. II. Experimental Section Experiments were performed in two different ultrahigh vacuum systems. The one at the Laboratory for Surface Science and Technology at the University of Maine is equipped with XPS and FT-IRAS. The second, at the synchrotron storage ring BESSY in Berlin, has facilities for NEXAFS and XPS. XPS was used as a cross-reference for comparison of the FT-IRAS and NEXAFS experiments. The IR measurements were taken in situ with a commercial FT-IR spectrometer (Cygnus 100, Mattson Instruments) connected via CaF2 windows to the ultrahigh vacuum chamber.40 The measurements were done at grazing incidence (82.5° from the surface normal) in a single reflection mode with an instrumental resolution of 4 cm-1. Note that in this mode only vibrations with a component of the dynamic dipole moment perpendicular to the platinum surface can be excited (“IR surface selection rule”).41 The difference spectra in this work were obtained by subtracting the spectrum obtained after metal deposition from the spectrum prior to metal deposition; i.e., the difference spectra show only the deviations from the original polyimide spectrum. Features above the base line indicate a decrease, and features below, an increase, in absorbance intensity. For clarity of the presentation all spectra have been base line corrected and offset by adding or subtracting a constant. Some (39) Strunskus, T.; Hahn, C.; Frankel, D.; Grunze, M. J. Vac. Sci. Technol. A 1991, 9, 1272. (40) Strunskus, T. Metallisierung von aufgedampften Polyimidfilmen (in German). Ph.D. Thesis, University of Heidelberg, 1993. (41) Bradshaw, A. M.; Schweizer, E. Infrared Reflection-Absorption Spectroscopy of Adsorbed Molecules. In Advances in Spectroscopy: Spectroscopy of Surfaces; Hester, R. E., Ed.; Wiley: New York, 1988.

Langmuir, Vol. 12, No. 11, 1996 2715 of the spectra have been smoothed slightly using an Fourier transform smoothing algorithm. The NEXAFS spectra were recorded at the electron synchrotron BESSY (Berlin) at beam line HE-TGM 2 by partial electron yield detection with an retarding voltage of -150 V. A carbon contaminated gold grid was used as a photon flux monitor; the grid photocurrent was recorded simultaneously with each NEXAFS spectrum. Normalization to the photon flux was obtained by division through a spectrum recorded for a freshly sputtered, clean gold sample. For energy calibration the spectra were referenced to characteristic peaks (due to contaminations) in the photocurrent spectrum of the monitor grid, which in turn were calibrated by measurements of reference materials. The XPS control measurements in Berlin were done with a monochromatized Al KR X-ray source. The polyimide films used in this study were prepared by vapor co-deposition of pyromellitic dianhydride (PMDA) and 4,4′diaminodiphenyl ether (ODA) (both Aldrich Gold Label, zone refined) onto a clean Pt(111) single crystal or a Si(100) wafer held at or below room temperature. The polyimide films were imidized directly after deposition by subsequent thermal curing at 575 K for at least 15 min. The polyimide films prepared on the Pt(111) crystal were used immediately for the in situ FTIRAS and XPS metallization experiments. The polyimide films on the silicon wafers were stored in air sealed containers and transported to the BESSY facility in Berlin. Prior to the NEXAFS and XPS experiments they were heated again in vacuum to 575 K for at least 15 min in order to remove adsorbed water. The chemistry of polyimide formation by vapor deposition of PMDA and ODA has been described in the literature.42-44 Vapor deposited polyimide films show qualitatively the same chemical and physical properties as polyimide films prepared by spin coating.44 The film thickness of thin polyimide films (d < 15 nm) on the platinum crystal was determined from the attenuation of the XPS Pt 4f substrate signal using an electron mean free path of λ ) 4.6 nm at 1415 eV kinetic energy (Ek). The 4.6 nm value was obtained by extrapolation from an experimentally determined value 4.2 nm for Si 2p electrons in Langmuir-Blodgett polyimide films at Ek ) 1153 eV,45 using a Ek0.5 dependence for the extrapolation.46 To determine the thickness of thicker polyimide films (d > 15 nm) on the Pt(111) substrate, the linear dependence of the FT-IRAS absorbance intensity of the polyimide carbonyl band (at 1740 cm-1) with film thickness was utilized.47 The thickness of the polyimide films prepared on the silicon substrate used for the NEXAFS experiments was determined by monitoring the film growth during vapor co-deposition with a calibrated quartz crystal oscillator.48 The metals were evaporated from a tungsten wire basket mounted in the main chamber and heated by direct current. The deposition rate was adjusted by the electrical current through the metal filled tungsten basket. The amount of metal deposited was estimated from the additional attenuation of the underlying substrate Pt 4f signal or from the intensity ratio of the most intense metal photoemission peak to the polyimide C 1s core level. In both calculations the formation of an uniform overlayer was assumed. The results are, however, only rough estimates, since significant deviations from continuous overlayer growth are expected, in particular for the noble metals where clustering and island growth have been observed.13-17 We report the approximate amount of adsorbed metal as the number of metal atoms per polyimide repeat unit, (considering only the topmost polyimide monolayer for the metal/polyimide interaction. Using (42) Salem, J. R.; Sequeda, F. O.; Duran, J.; Lee, W. Y.; Yang, R. M. J. Vac. Sci. Technol. A 1986, 4, 369. (43) Lamb, R. N.; Baxter, J.; Grunze, M.; Kong, C. W.; Unertl, W. N. Langmuir 1988, 4, 249. (44) Strunskus, T.; Grunze, M. Vapor Phase Deposition of Polyimides. In Polyimides: Fundamental Aspects and Technological Applications; Ghosh, M. K.; Mittal, K. L., Eds.; Dekker, New York, in press. (45) Killinger, A.; Thu¨mmler, C.; Grunze, M.; Schrepp, W. J. Adhes. 1992, 36, 229. (46) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (47) Lee, K. W.; Kowalczyk, S. P. In Metallization of Polymers; Sacher, E., Pireaux, J. J.; Kowalczyk, S. P., Eds.; ACS Symposium Series 440; American Chemical Society: Washington, DC, 1990; p 179. (48) Hutchings, C. W.; Grunze, M. Rev. Sci. Instrum. 1995, 66, 3943.

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Figure 2. FT-IRA spectrum of 38 nm vapor deposited polyimide film on Pt(111). Table 1. Assignment of the Polyimide IR Bands wavenumber (1/cm) (a) 1780 m (b) 1740 s (c) 1731 sh (d) 1513 sh (e) 1503 s (f) 1379 s (g) 1245 s (h) 1169 w (i) 1120 s (j) 1092 m

} }

assignment symmetric carbonyl stretch

Strunskus et al.

Figure 3. Effect of sputtering with 1 keV argon ions on the FT-IRA spectra of a 65 nm polyimide film on a Pt(111) substrate: (a) PI film for reference scaled down by a factor of 3 and difference spectra obtained after (b) sputtering for 5 min at 10-7 mbar of argon pressure, (c) additional sputtering for 10 min at 10-7 mbar of argon pressure, (d) additional sputtering for 5 min at 10-6 mbar of argon pressure, and (e) additional sputtering for 10 min at 10-6 mbar of argon pressure.

antisymmetric carbonyl stretch ν(CdO) B1u mode of 1,4-disubstituted phenylene ring, ν(C6H4) symmetric imide >NC- stretch, ν(CN) antisymmetric ether stretch, ν(COC) CH bending mode of DPE phenylene ring antisymmetric in plane imide stretch, imide mode CH bending mode of PMDI benzene ring

a bulk density of 1.42 g/cm3 49 for polyimide and a layer thickness of 0.6 nm,45 we obtain a surface density of 1.3 × 1014 (polyimide repeat units)/cm2. One monolayer of metal atoms (calculated from the bulk density) corresponds to 11-14 metal atoms per polyimide repeat unit for the metals Cr and Au, respectively. Different values for the surface density of polyimide repeat units (e.g., 9.9 × 1013/cm2) as well as for the number of metal atoms in a monolayer have been proposed in the literature.12 We feel, however, that calculating these values from bulk densities makes the least assumptions about largely unknown parameters like morphology and orientation of the polyimide units at the surface at one hand and about the packing density of the metal atoms at low coverages on the other hand. Since the values reported here are only rough estimates, they indicate a coverage regime rather than exact stoichiometric ratios of metal atoms to polyimide units. Potassium was deposited from a heated zeolite source, and the amount of adsorbed potassium was calculated from the C 1s and K 2p core level intensities using the cross sections published by Scofield.50 Different from the other metals potassium diffuses very fast in polyimide and is almost homogeneously distributed within the polyimide film after deposition. Therefore the amount of potassium is reported as the number of potassium atoms per bulk polyimide repeat unit. The polyimide films were kept at room temperature (about 300 K) for all of the metal depositions.

III. Results and Discussion The FT-IRA spectrum of a clean 38 nm thick vapor deposited polyimide film is shown in Figure 2.40 The assignment of the bands is taken from the paper from Ichida et al.51 and is summarized in Table 1. Only the five strongest bands at 1740, 1503, 1379, 1245, and 1120 cm-1 were used for the analysis of the metallization experiments. We will use the following notation for the polyimide bands: ν(CdO) for the band at 1740 cm-1; ν(C6H4) for the band at 1503 cm-1; ν(CN) for the (49) Dressler, H.; Holter, S. N. In Encyclopedia of Chemical Technology; Kirk-Othmer, Ed.; Wiley: New York, 1982; p 705. (50) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129. (51) Ichida, H.; Wellinghoff, S T.; Boer, E. Koenig, J. L. Macromolecules 1980, 13, 826.

band at 1380 cm-1; ν(COC) for the band at 1245 cm-1, and imide mode for the band at 1120 cm-1. The ν(CdO), the ν(CN), and the imide mode are “pyromellitimide (PMDI) modes” located on the pyromellitimide part of the polyimide repeat unit, whereas the ν(C6H4) and the ν(COC) modes are “diphenyl ether (DPE) modes” located on the diphenyl ether part of the polyimide repeat unit. Before we present our data on metal deposition on polyimide, we want to point out the ambiguities encountered by interpreting the FT-IRAS results for metals on polymers, i.e., the problems in identifying chemical changes at the polymer surface such as chemical bond breaking and forming. Observables in the IR spectra as a result of metal deposition are the appearance of new absorption bands and changes in absorbance, where the latter can be inhomogeneous over the envelope of an absorption band. The first case, appearance of new bands, can be unambiguously related to chemical changes in the polymer. More difficult (if not impossible) is the interpretation of changes in absorbance, because these can be associated with fragmentation of the polymer, screening of the dynamical dipoles39,40 or changes in intra- and intermolecular dipole-dipole coupling of polymer chains52 through the adsorbed metal. In order to verify that disruption of the polyimide chains (as suggested by Dunn and Grant for chromium) does lead to a decrease in absorbance of the vibrational bands, but not to new features in the accessible frequency range of our experiments (1000-4000 cm-1), we sputtered a polyimide film with argon ions to erode the polymer surface.40 As shown in Figure 3, at a low argon ion dose only the imide related bands are attenuated; for higher ion doses all polyimide bands are effected. It has been suggested that argon sputtering leads to the disrupture of the imide rings;53,54 hence, the preferential attenuation of the imide related bands in our data is fully consistent with this model. However, as indicated in the FT-IRAS spectra in Figure 3c-e, at higher argon doses also other parts of the polymer are disrupted. Despite the fragmentation of the polyimide, no additional or shifted IR (52) Lin, S. B.; Koenig, J. L. J. Polym. Sci., Polym. Phys. Ed. 1983, 21, 2067. (53) Sengupta, K. S.; Birnbaum, H. K. J. Vac. Sci. Technol. A 1991, 9, 2928. (54) Jeong, H. S.; White, R. C. Mat. Res. Soc. Symp. Proc. 1992, 236, 325.


Figure 4. Temperature dependence of the FT-IRAS data of a 70 nm thick polyimide film. The spectrum of the polyimide film was taken at T ) +178 °C (a). The difference spectra were obtained after cooling to T ) +35 (b), -35 (c), -83 (d), and -123 °C (e). Features above the base line indicate an intensity increase; features below indicate a decrease.

bands are detected in the frequency range of our experiments. The latter observation is not consistent with a model proposed in the literature for the effect of Ar+ ions on polyimide, where the formation of hydroxyl or new ether groups has been proposed.55 Most important for the following discussion is that disrupture of the polyimide does not lead to the appearance of characteristic new bands in the FT-IRA difference spectra at wavenumbers above 1000 cm-1. Note that IR band intensities in the sputter difference spectra are up to an order of a magnitude larger than in the metallization experiments presented below; i.e., the sputter experiment should be much more sensitive for the detection of new or shifted bands. We conclude that the absence of new bands upon metal deposition does not exclude polymer degradation. A little but important distinction between the sputter and the metallization experiments is that in the latter case the attenuation of the vibrational bands is not homogeneous over the band envelope. Another important effect to be considered is a possible attenuation of the vibrational bands due to a change in the vibrational coupling between high-frequency intramolecular and low-frequency intermolecular vibrations.56 Changes in vibrational coupling in polymers are evident in temperature dependent measurements of the IR absorbance.57,58 Snyder et al. reported that in IR transmission experiments the absorbance of spun-on polyimide films has a larger temperature dependence than expected for a simple Boltzmann distribution of the oscillators.59 Since adsorbed metal particles, in particular if they diffuse into the polymer film, will also change the vibrational coupling, we also studied the temperature dependence of the absorbance in reflection geometry in thin vapor deposited polyimide films. The FT-IRAS spectrum of a 70 mm thick polyimide film held at 450 K (a) and the difference spectra recorded at decreasing temperature are displayed in Figure 4. The difference spectra show an increase in the high-frequency component of the ν(CdO) and of the ν(COC) and ν(CN) modes with decreasing temperature. The ν(C6H4) mode shifts to higher wavenumbers at lower temperature, as does the (55) Marletta, G.; Iacona, F.; Toth, A. Macromolecules 1992, 25, 3190. (56) Ovander, L. N. Opt. Spectrosc. 1961, 11, 68; 1962, 12, 401. (57) Hannon, M. J.; Koenig, J. C. J. Polym. Sci. A 1969, 2, 1085. (58) Ogura, K.; Kawamura, S.; Sobue, H. Macromolecules 1971, 4, 79. (59) Snyder, R. W.; Sheen, C. W.; Painter, P. C. Appl. Spectrosc. 1981, 42 (3) 503.

Langmuir, Vol. 12, No. 11, 1996 2717

Figure 5. Intensity of the antisymmetric carbonyl band at 1740 cm-1 versus temperature for the 70 nm thick polyimide film (circles). The dotted line was obtained by linear regression of the data points and the intensity at the lowest temperature (T ) 151 K) was normalized to 1. The solid line is the intensity decrease calculated from a Boltzmann distribution of the oscillators.59

ν((OC)2NC) mode. Note, that these changes are fully reversible with temperature and are also observed in a polyimide film of 9 nm thickness. The relative intensity changes (determined according to ref 60) of about 15% are linear with temperature and, within the experimental errors, the same for all bands in the temperature range studied here. As an example, the relative change in the ν(CdO) mode with temperature is plotted in Figure 5. Figure 5 also includes the very small absorbance change expected from a Boltzmann distribution of the oscillators according to

A(T2) A(T1)

)

e-hcνj/kT1 + 1 e-hcνj/kT2 + 1

(1)

where A is the absorbance at temperature Tx and νj is the wavenumber of the respective IR band.59 Clearly, the experimentally determined change with temperature is much stronger than expected from a Boltzmann Ansatz. Snyder et al. suggested that the intensity change in the IR spectra is caused by conformal changes within the polyimide chains.59 We favor an alternative model, first suggested by Lin et al. for poly(ethylene-terephthalate) (PET).52 The intensity changes were as large as 48% for a 200 K temperature difference52 and were explained with changes in the local environment of the oscillators effecting the vibrational coupling between high-frequency intramolecular and low-frequency intermolecular vibrations.56 We suggest the same model for our FT-IRAS data on thin vapor deposited polyimide films. The lateral and vertical thermal expansion coefficient for similar polyimides is 9 × 10-6 and 1.1 × 10-4 K-1, respectively.61 The lateral expansion per repeat unit is rather small, but the vertical expansion of a 70 nm thick polyimide film is 2.3 nm over the temperature range of 300 K. Assuming a parallel orientation of the polymer chains with respect to the substrate surface and taking the average thickness of 0.6 nm for a polyimide monolayer,45 this translates into an increase in the interlayer spacing by 3% (0.02 nm). This small increase in separation changes the vibrational coupling between the polymer chains and influences the intra- and intermolecular dipole-dipole coupling of the molecular subunits (which scales with 1/r3). Changes in (60) Hirschfeld, T. In FTIR Spectroscopy; Ferrara, J. R., Basille, L., Eds.; Academic Press: New York, 1979. (61) Elsner, G.; Kempf, J.; Bartha, J. W.; Wagner, H. H. Thin Solid Films 1990, 185, 189.

2718


Strunskus et al.

Table 2. Dipole Screening and Localization Energies (For Formulas See Text)

metal

atomic radius63 (nm)

polarizability64 (nm3)

dipole screening ∆A (%)

localization energy (kJ/mol)

Cu Ag Au Pd Cr

0.128 0.144 0.144 0.137 0.128

0.0061 0.0072 0.0058 0.0048 0.0116

53 53 44 40 84

5 4 3 3 10

interchain spacing will also occur when metals are adsorbed and diffuse into the polymer and thus change the local environment of the adjacent vibrating dipoles and the longer range intermolecular vibrational coupling. Hence, a decrease in absorbance due to metal adsorption (as observed for all of the metals studied in this work) could also be explained by the same physical model as discussed above for the temperature dependence. Another effect of physical origin which will decrease the absorbance of specific vibrational bands is depolarization (screening) of the dynamical dipoles by the metal particles as discussed by us previously for copper on polyimide.39 To estimate the dipole screening effect, we considered a situation where a metal atom (or small cluster) is adsorbed directly adjacent to the carbonyl group of the pyromellitimide unit in the polymer. The dynamical dipole of the carbonyl group induces a dynamical dipole in the metal particle.39 The ratio between the original and the induced dipole is given by

pind 1 R )porig 4π0 r3

(2)

where R is the polarizability of the metal particle and r is the distance between the dipole and induced dipole. Approximating the metal particle as a sphere of radius rcluster, the polarizability R is given by

R ) 4π0rcluster3

(3)

Using eq 3 in eq 2, we obtain

pind rcluster3 )porig (r + d)3

(4)

cluster

where d is the distance between dipole and cluster. For large clusters with rcluster f ∞ the surface selection rule follows from eq 4 with pind ) -porig. Using the van der Waals radius for the carbonyl oxygen atom (0.14 nm62), the Goldschmidt atomic radius for the metals,63 and the tabulated polarizabilities of the metal atoms,64 we can calculate the decreases in absorbance of the carbonyl vibration by dipole screening through a metal atom according to

(

pind ∆A ) 1A porig

)

2

(5)

The values are summarized in Table 2 and range from 40 to 84% for palladium and chromium atoms, respectively. We will show that this can explain the experimental results without invoking any chemical interactions. Further, the (62) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (63) Goodfellow Corp., Data table for physical properties of metals, 1995. (64) Tabulated Atomic and Molecular Polarizabilities. CRC Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1993.

Figure 6. FT-IRA difference spectra for gold deposits on polyimide films: (a) 15 nm PI reference spectrum (scaled down by a factor of 6), (b) 15 Au atoms/surface PI repeat unit on a 12 nm PI film, (c) 8 Au atoms/surface PI repeat unit on a 15 nm PI film, (d) 11 Au atoms/surface PI repeat unit on a 15 nm PI film, and (e) 17 Au atoms/surface PI repeat unit on a 15 nm PI film.

static dipole/induced dipole interaction between the carbonyl group (p ) 2.3 D 65) and a metal atom will stabilize the adjacent adsorption site by an energy E:

E)

p2R 1 6 r (4π0)2

(6)

The energies for the metal atoms are summarized in Table 2 and range from 5 kJ/mol for palladium and gold atoms to 10 kJ/mol for chromium atoms, i.e., sufficient to localize metal atoms preferentially at the sites with the highest dipole moments. Another point to consider is absorption, scattering, or reflection of IR light caused by the metal deposit. Absorption or scattering of IR light would lead to a broad band increase of the absorbance in the IR spectrum, but this was experimentally not observed. Variations in base line absorbance were always less than 0.5%, i.e., small compared to the attenuation of the polyimide bands. An increase in reflectivity of the polymer surface caused by the metal deposit should lead to an attenuation of the polymer bands without a change in base line absorbance. This agrees with the experimental observation, but a significant increase in reflectivity of the polyimide surface is only expected for higher metal coverages when the deposited metal atoms start to exhibit metallic properties, i.e., when they start to form larger clusters or a continuous layer. The above considerations and arguments demonstrate that a decrease in absorbance can indicate drastic chemical changes due to polymer fragmentation, as in the ion bombardment experiments, or could originate from subtle physical effects such as very small changes in the density or screening of the dynamical dipoles by atoms or small metal clusters. In the following we will present and discuss the data for the individual metals. (A) Gold. Gold is one of the metals which does not interact chemically with polyimide and hence a reference case for determining the effect of physical bonding on the FT-IRAS spectra. In Figure 6 we present FT-IRA difference spectra for increasing gold coverages onto cured polyimide films, which show the same general effects but also emphasize that the details of the spectral changes differ from experiment to experiment although the (65) Barrow, G. M. Physical Chemistry, 4th ed.; McGraw-Hill: New York, 1979.


procedures followed the same protocol.40 As will be discussed below, this indicates that subtle changes of some experimental parameters, e.g., the preparation of the polyimide films and changes in the metal deposition conditions, have a strong effect on the attenuation behavior of the polymer bands. As a reference for the metallization results the FT-IRA spectrum of a 15 nm polyimide film (scaled down by a factor of 6) is shown in Figure 6a. The IR difference spectrum for a deposit of 15 gold atoms/PI repeat unit on a 12 nm polyimide film is displayed in Figure 6b. The IR intensity of the ν(CdO) band at 1735 cm-1 is reduced after gold deposition selectively at the higher frequency side of the band. The imide band at 1120 cm-1, the ν(CN) band at 1380 cm-1, and the ν(COC) band at 1250 cm-1 show some attenuation; the ν(C6H4) band at 1500 cm-1 is not affected. Difference spectra obtained after sequential gold depositions onto a 15 nm polyimide film are shown in Figure 6c-e. Deposition of 8 gold atoms/unit (a smaller coverage than in Figure 6b) leads to attenuation of the ν(CdO) and the imide mode but also of the ν(C6H4) mode at 1500 cm-1 (Figure 6c). Also, in this experiment the attenuation of the ν(CdO) band is more even, and the slight decrease in the difference spectra at the lower frequency side of the bands indicate an increase in intensity (broadening) of the bands at lower frequencies with metal deposition. Only a weak attenuation is observed for the ν(COC) band, and no change is observed for the ν(CN) band. Increasing the gold coverage to 11 and 17 atoms/unit leads to further attenuation of the polyimide bands (Figure 6d,e). The ν(CN) band at 1380 cm-1 shows the weakest attenuation. The relative attenuation of the ν(CdO) and the ν(C6H4) band is similar for all coverages in this experiment and increases from 7% for the lowest gold coverage (8 atoms/unit, Figure 6c) to 15% for the highest gold coverage (17 atoms/unit, Figure 6e). In the gold experiments the ν(CdO) and the imide band showed always the strongest relative attenuation, whereas the relative attenuation of the ν(C6H4), the ν(CN), and the ν(COC) bands varied from experiment to experiment. Significant is the difference in the attenuation of the ν(CdO) band in the two experiments displayed: Attenuation of the high-frequency side of the band indicates that only specific carbonyl groups, probably those in more crystalline areas of the polymer surface, are affected, whereas the more even attenuation and broadening of the band in the second data set more closely resembles the temperature dependent measurements of the polyimide film (Figure 4) and hence changes in dipole-dipole coupling in the polymer due to incorporation of gold (clusters). We speculate that the difference in the two data sets indicates a subtle balance between the occupation of specific adsorption sites by atoms and small clusters, intermixing with the polymer, and gold cluster formation, depending on the very specific film properties and ambient temperature. After deposition of 40 gold atoms/unit on a 12 nm polyimide film we observe an attenuation of 25% of the ν(CdO) band intensity. For the higher metal coverages we do not observe a selective interaction with the carbonyl groups but a nearly proportional attenuation of all of the polyimide vibrational bands. This could indicate an increase in reflectivity of the metal covered polyimide surface compared to the clean polyimide surface. Also, a higher index of refraction in the interphase, where metal and polymer are intermixed, causes a shorter and less grazing path of the light through the intermixed layer and hence a decrease in absorbance of the polyimide IR bands. That some intermixing of gold with the polymer occurs

Langmuir, Vol. 12, No. 11, 1996 2719

Figure 7. FT-IRA difference spectra for gold deposited with different deposition rates on polyimide films: (a) 17 Au atoms/ surface PI repeat unit deposited with a rate of 0.56 atoms/unit min, (b) 41 Au atoms/surface PI repeat unit deposited with a rate of 2.7 atoms/unit min, and (c) 70 Au atoms/surface PI repeat unit deposited with a rate of 14 atoms/unit min.

at room temperature is also evident in deposition rate dependent measurements. In Figure 7 we show IR difference spectra for gold deposits where the deposition rates were varied at 1:5:25 in spectra a-c, respectively. Note that the final coverages as determined by XPS are not the same but are 17, 41, and 70 gold atoms/unit in Figure 7a-c, respectively. The polyimide films had nearly the same thickness for all three samples. The highest attenuation of the polyimide bands is observed for a medium gold coverage deposited at a medium rate (Figure 7b). Although the highest gold coverage is about four times higher than the lowest coverage, the attenuation of the carbonyl band is comparable. The attenuation of the DPE ring mode is weaker for the sample with the higher gold coverage and the faster deposition rate. In studies of the copper/polyimide system with medium energy ion scattering (MEIS) and transmission electron microscopy (TEM) it was found that the deposition rate has a strong influence on the distribution of the metal on and within the polyimide and that copper can be localized below the polyimide surface, even for room temperature deposition.16 The same applies for gold: at the higher deposition rate the stationary coverage of gold atoms on the polymer surface is high, and larger gold clusters and islands are formed on the polyimide surface, which affects the polyimide spectra less than in the case of gold diffusion into the polymer. With decreasing deposition rate more gold atoms diffuse into the polyimide and form clusters inside the polymer; i.e., more intermixing of the gold and the polyimide occurs for the slower deposition rate, resulting in a stronger attenuation of the bands. At the lowest deposition rate, the stationary surface concentration (and final coverage) is lowest leading to a high degree of dispersion of gold in the polymer. For the copper/ polyimide system we observed a similar dependence of the FT-IRAS results on the deposition rate of the copper.39 In general, the FT-IRAS data for low gold coverages can be explained by dynamical dipole screening and changes in the intra- and intermolecular dipole coupling of polymer chains by the metal atoms. (B) Copper, Silver, and Palladium. Qualitatively the FT-IRAS results for the copper/polyimide interface reported previously by us are very similar to the results for the gold/polyimide interface presented here.39 We therefore refrain from showing the FT-IRAS data again but would like to present XAES (X-ray Auger electron spectroscopy) data supporting the conclusion that copper does not interact chemically with polyimide.

2720


Figure 8. X-ray photoelectron spectroscopy data of the Cu Auger L3M4,5M4,5 transition for a 0.2 nm copper deposit (1.3 Cu atoms/surface PI repeat unit) on a 18 nm spun-on polyimide film:66 (a) room temperature (300 K) deposit, (b) same as a after heating to 473 K for 30 min, and (c) same as b after heating to 523 and 573 K for 30 min. The vertical line at 1849.5 eV is a guide for the eye.

Our conclusion that copper forms no chemical bonds with polyimide was in disagreement with the suggestion by Pertsin and Pashunin derived from their XAES data that at low copper coverages the copper atoms are bonded to the polyimide carbonyl groups and a Cu+ species exists on the surface.15 In the Cu L3M4,5M4,5 Auger spectra for low copper coverages on polyimide they observe an additional Auger transition separated by 2 eV from the main transition, which was not observed on less polar polymers like polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and poly(dimethylphenylene oxide) (PDMPO). Their XPS results for copper on polyimide were reproduced on a vapor deposited polyimide film40 and recently on a spun-on polyimide film.66 In Figure 8 we show selected Cu L3M4,5M4,5 spectra for a deposit of 1.3 Cu atoms per PMDI unit (0.02 nm) on a spun-on polyimide film using the Auger parameter R′ ) Eb(Cu 2p3/2) + Ek(Cu L3M4,5M4,5) on the energy axis.66 The Auger parameter of copper metal is R′ ) 1851.4 eV. In the room temperature deposit (Figure 8a) the additional L3M4,5M4,5 Auger transition (marked with an arrow) is clearly visible at R′ ) 1847.5 eV separated by about 2 eV from the main transition at R′ ) 1849.5 eV. The shift of the main transition to a lower Auger parameter compared to copper metal was explained as a cluster size effect, whereas the additional 2 eV shift exhibited by the Auger transition at 1847.5 eV was seen as evidence for copper in the Cu+ state bonded covalently to the polyimide carbonyl groups.15 Heating the sample to 473 K for 30 min (Figure 8b) leads to a decrease in intensity of the Auger transition at 1847.5 eV and to an increase in intensity of the main transition appearing at 1849.6 eV. Heating the sample further to 573 K leads to cluster growth as indicated by the shift of the main transition to a larger Auger parameter (R′ ) 1850.6 eV). The changes observed in the Auger spectrum can only be explained with a high mobility of the copper atoms on the surface of the hot polyimide films, which is in contradiction to the view that copper atoms are bonded covalently to the polyimide carbonyl groups. We therefore believe that the data of Pertsin and Pashunin have to be interpreted in a different way. Using an electrostatic model Moretti and Porta calculated the change in the Auger parameter R′ (as introduced (66) Kiene, M.; Strunskus, T.; Faupel, F. Manuscript in preparation.

Strunskus et al.

Figure 9. Rate dependence for the deposition of palladium onto a 18 nm thick polyimide film: (a) FT-IRA spectrum of polyimide film, (b) FT-IRA difference spectra obtained after deposition of 60 Pd atoms/surface PI repeat unit within 1 h, and (c) FT-IRA difference spectra deposited after deposition of 60 Pd atoms/surface PI repeat unit within 90 s.

by Wagner et al.67) for small copper clusters adsorbed on dielectric substrates.68 Their calculations revealed, that the Auger parameter of copper clusters depends strongly on their polarizability.68 The largest decrease of the Auger parameter is expected for highly dispersed copper atoms and very small copper clusters, which have a much lower polarizability than larger clusters or copper metal. Therefore the additional L3M4,5M4,5 Auger transition observed for low copper coverages on polyimide is not necessarily an indication for copper in the +1 oxidation state but could be due to single copper atoms or very small copper clusters, coexisting with the larger clusters on or beneath the polyimide surface. The nonuniform distribution of cluster size can be rationalized with the localization of copper atoms near different polar polyimide groups as discussed above, initially reducing the formation of larger clusters. On nonpolar polymers like polyethylene such a localization is not possible and accordingly no additional Auger transition is observed experimentally.15 The FT-IRAS and XPS results for silver resemble very closely those of copper and give also no evidence for chemical bond formation. A representative FT-IRAS spectrum for silver on polyimide is shown in Figure 15. The FT-IRAS data of palladium adsorbed on polyimide are, with respect to the changes on the polyimide vibrations, identical to those for gold (Figure 6) and copper.39 We note, however, that the absolute attenuation at the highest palladium coverages for different polyimide film thicknesses is the same and that the attenuation is also independent on the deposition rate (see Figure 9). This indicates that palladium in contrast to copper and gold does not intermix with the polymer but remains adsorbed on the surface. Our XPS data69 show the same attenuation effects as those observed for copper or gold on polyimide, except that a broadening of the N 1s emission to lower binding energies is observed. Whether this indicates a preferential interaction with the imide nitrogen in polyimide has to be investigated in future studies. The following summarizes the results for Au, Ag, Cu, and Pd on polyimide: The experimental results for these metals are consistent with a purely physical interaction between metal and polymer and can be explained without invoking chemical reactions. The initial preferential attenuation of the carbonyl C 1s and O 1s emissions observed in XPS measurements13-15,39,40,69 and of the (67) Wagner, C. D. Anal. Chem. 1972, 44, 972. (68) Moretti, G.; Porter, P. Surf. Sci. 1993, 287/288, 1076. (69) Kochendo¨rfer, G. Diplomarbeit Heidelberg, 1992.


Langmuir, Vol. 12, No. 11, 1996 2721

Figure 11. Scheme for reduction of polyimide with potassium (after ref 26). Figure 10. FT-IRA spectra and difference spectra for potassium deposited onto a 15 nm polyimide film: (a) without potassium, (b) with trace levels of potassium, and with (c) 0.3, (d) 0.8, (e) 1.6, and (f) 2.2 potassium atoms/volume repeat unit.

carbonyl vibration in the IR data suggests that for these metals an adsorption site adjacent to the carbonyl groups is favored. (C) Potassium and Chromium. We finally want to address the central question, how reactive metals such as chromium interact with polyimide at lowest metal coverages. As discussed in the Introduction, different models have been proposed for the chemistry of chromium on polyimide. Clabes et al. found that polyimide films can be reduced chemically with cesium, and they suggested that chromium also reduces the pyromellitimide units.26 Inspired by their results we used potassium as a reducing agent for our vapor deposited polyimide films to obtain reference data for the interpretation of the chromium experiments. Reduction of the polymer, i.e., formation of the mono- or dianion of polyimide, should have drastic effects on the vibrational spectra. (1) Potassium. We first present the FT-IRAS results since they provide a easy distinction between the formation of the mono- and dianion as a function of K coverage. In Figure 10 the FT-IRA spectra and the respective difference spectra between two subsequent steps are displayed. The small numbers in the figure indicate the oxidation state of the polyimide: 0 ) neutral; 1 ) radical anion; 2 ) dianion. After the initial potassium deposition (the concentration is too low to be estimated from the XPS spectra) small but very characteristic changes are detected in the FT-IRA spectrum (Figure 10b). The carbonyl mode and the imide mode show a decrease in intensity, and three new vibrational bands appear at 1660, 1438, and 1355 cm-1. Their intensity increase after the second and the third potassium deposition step together with a decrease of the imide ring related bands (Figure 10c,d). In the difference spectra the new vibrational bands are shifted to higher wavenumbers and appear now at 1680, 1447, and 1357 cm-1 (Figure 10c,d). A small decrease of the ODA ring mode at 1507 cm-1 and the phenyl ether mode at 1250 cm-1 is observed.

Table 3. IR Band Positions (cm-1) for Neutral, Radical Anion, and Dianion of PMDA-BA Diimide Model Compound in Solution and Thin ODA/PMDA Polyimide Film, Respectively neutral radical anion dianion

PMDA-BA70

PMDA-ODA

1773 1725 1394 1656, 1647 1435 1350 1605 1567 1517 1320

1780 1740, 1731 1379 1660-1680 1438-1447 1355-1357 1622-1630 1572-1585 1524-1530 1330-1335

After deposition of 1.6 K/PI unit, not only the polyimide IR bands but also the vibrational bands which appeared after the first deposition steps show a decrease in intensity and a new set of vibrational bands appear. These bands can be located in the difference spectrum at 1622, 1572, 1524, 1492, and 1335 cm-1 (Figure 10d,e). They increase after the last potassium deposition, whereas the bands marked with “1” decrease further. Kovac et al. reported FT-IR spectra of PMDA-BA and its two reduced forms in THF solution.70 Their and our band positions are summarized in Table 3. Generally our values are higher (up to 25 cm-1) but still close to the values reported by Kovac et al. Considering the differences between the two systems, i.e., FT-IR transmission measurements of a di-n-alkyl pyromellitimide in solution and FT-IRA spectra of a solid PMDA/ODA polyimide, respectively, the agreement between the spectra is excellent. The most significant difference seems to be that we observe a band at 1495 cm-1 for the dianion, and no equivalent band exists in the spectra of Kovac et al.70 The proposed reduction scheme is shown in Figure 11 (after ref 26). After the initial potassium deposition the polyimide is reduced to the radical anion form. The radical anion form can be identified by characteristic vibrational bands at 1670, 1445, and 1355 cm-1 (marked “1” in (Figure 10). For coverages up to 0.8 K atoms/PI repeat unit only (70) Kovac, C. A.; Jordan-Sweet, J. L.; Goldberg, M. J.; Clabes, J. G.; Viehbeck, A.; Pollak, R. A. IBM J. Res. Dev. 1988, 32, 603.

2722


Figure 12. NEXAFS spectra of the carbon K-edge for potassium on polyimide: (a) without potassium and (b) with 0.5 potassium atoms/volume repeat unit. Table 4. Assignments of the NEXAFS Transitions in Figures 12 and 1473 no.

energy (eV)

assignment of final orbital

1 2 3 4 5 6 7 8 1 2 3 4 5 1 2 3 4

284.8 285.2 286.6 287.4 289.2 291.9 295.4 301.1 401.8 404.3 407.6 409.4 413.7 531.1 535.6 540.4 544.3

π*(CdC), PMDI benzene ring π*(CdC), DPE phenylene ring π*(CdC), DPE phenylene ring π*(CdO) π*(CdC), second PMDI σ*(CN), σ*(CO) σ*(CC) σ*(CO), σ*(CC), second DPE π* PMDI benzene ring, (CdO) π* PMDI benzene ring, (CdO) π*(CN), σ* σ*(CN) σ*(CC) π*(CdO) π*(CdC) PMDI σ*(C-N), σ*(CO) σ*(CO)

Strunskus et al.

Figure 13. FT-IRA difference spectra for chromium deposits on 13 nm polyimide film: (a) 13 nm PI film as reference divided by 3 and (b) 0.6, (c) 3, (d) 10, (e) 40, and (f) 112 Cr atoms/surface PI repeat unit.

formation of the radical anion is observed. This suggests that the potassium diffuses into the polyimide film and is distributed almost evenly within the film. Clabes et al. observed a homogeneous depth profile for cesium in polyimide by angular resolved photoemission spectroscopy.26 Haushalter and Krause showed that potassium ions can diffuse freely in a reduced polyimide film.71 This is the reason why we assumed a homogeneous distribution of potassium within the polyimide film in our calculations of the potassium concentration. Upon increase of the potassium concentration to 1.6 K atoms per pyromellitimide unit the vibrational bands of the dianion form are observed at 1625, 1575, 1530, 1490, and 1330 cm-1 (marked “2” in Figure 10). Correspondingly the IR bands of the radical anion form decrease. Even though the potassium amount would be sufficient to reduce the polyimide at least to the monoanion form, weak vibrational bands of unreduced pyromellitimide units at 1718 and 1114 cm-1 are still observed. This is an indication that some regions of the polyimide film were not accessible for the potassium atoms. The effect on the DPE modes is minimal for both reduction steps. The transferred electron is localized on the pyromellitimide unit. This is in agreement with previous studies which showed that the reduction potential depends only weakly on the substituents on the pyromellitimide nitrogens but are strongly influenced by changes within the pyromellitimide unit.72 The experi-

ments with potassium unambiguously demonstrate that reduction of polyimide by adsorbed alkali metals is accompanied by distinct changes in the FT-IRAS data. In Figure 12 we present NEXAFS spectra at the carbon K-edge obtained for a potassium deposit of 0.5 K atoms per volume polyimide repeat unit on a vapor deposited polyimide film. To increase the relative effect of the potassium on the polyimide NEXAFS spectrum, we subtracted 50% of the normalized polyimide spectrum from the spectrum obtained after potassium deposition and renormalized it afterward. This procedure is justified because, after deposition of such a small amount of potassium, a substantial fraction of the polyimide will remain unchanged. The transition energies are tabulated and assigned to specific K-edge transitions in the polymer in Table 4.73 After deposition of 0.5 K atoms/repeat unit the π* resonances of the PMDI part (1, 4, 5) are changed in intensity and/or shifted. No change is detected for the π* resonance (2) of the DPE phenylene rings. This is in total agreement with the changes observed in the infrared spectra and further supports the reduction model. A full set of NEXAFS and core level photoemission spectra (XPS) for potassium on spin-cast and on vapor deposited polyimide films will be presented and discussed in detail in a forthcoming publication.74 (2) Chromium. In the following we will present our FT-IRAS and NEXAFS results for chromium on polyimide and compare them to the data presented and discussed in the previous section of this paper. The corresponding core level data were in good agreement with previously reported spectra for chromium deposits on spun-on polyimide films.22,26 Chromium has been deposited onto freshly cured polyimide films, and the FT-IRA spectra were recorded immediately after metal without exposing the samples to the ambient, i.e., avoiding the reaction of the chromium deposit with air. In Figure 13 we show a sequence of FT-IRA difference spectra for chromium deposits on a 13 nm polyimide film. After the first chromium deposition (

Identification of Physical and Chemical Interaction Mechanisms for the

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