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Determination of condensation coefficients of metals on polymer surfaces
Surface Science 454–456 (2000) 412–416 www.elsevier.nl/locate/susc
Determination of condensation coefficients of metals on polymer surfaces V. Zaporojtchenko *, K. Behnke, T. Strunskus, F. Faupel Lehrstuhl fu¨r Materialverbunde, Technische Fakulta¨t der Universita¨t Kiel, Kaiserstr. 2, 24143 Kiel, Germany
Abstract We used X-ray photoelectron spectroscopy ( XPS ) in combination with transmission electron microscopy to characterize the nucleation and growth of noble metals deposited onto polymers and to determine the condensation coefficients of the metals. The surface concentration of the adatoms was determined using a mathematical correction of the XPS intensity to take into account the cluster formation of noble metals on polymer surfaces. Condensation coefficients for Cu and Ag depend strongly on the chemical composition of the polymer and on temperature and can vary by more than three orders of magnitude already at room temperature. The influence of the deposition parameters and the metal/polymer morphology on the condensation coefficient will be discussed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Copper; Physical adsorption; Sticking; Surface diffusion; Surface structure, morphology, roughness, and topography; X-ray photoelectron spectroscopy
1. Introduction Recently, our group reported the first quantitative results on condensation coefficients of noble metals on different polymer surfaces [1,2]. The condensation coefficient C is defined as the ratio of the number of adsorbed atoms to the total number of atoms arriving at the surface [1]. At room temperature the condensation coefficient varied by more than three orders of magnitude depending on the metal/polymer combination and in particular on the polymer used. The fluoropolymer Teflon AF 1601 showed very low condensation coefficients in the range between 0.002 (Ag) and 0.02 (Cu), whereas the condensation coefficients for noble metals deposited onto polyimide * Corresponding author. Fax: +49 431 77572 603. E-mail address: [email protected] ( V. Zaporojtchenko)
at room temperature were always close to unity [1–3]. One set of measurements with silver was performed using a very sensitive and accurate radiotracer technique [1]. In the radiotracer method a radioactive metal isotope is evaporated onto a polymer sample through an aperture and the reemitted atoms are collected on a catcher foil. Using a well calibrated system this method allows to measure the fractions of the condensed and reemitted atoms with a high accuracy of ±2%. Since it is not always possible to work with radioactive isotopes we developed a method to determine C by using a combination of X-ray photoelectron spectroscopy ( XPS) and transmission electron microscopy ( TEM ) [3]. In this method intensities are calibrated by depositing metals onto substrates where the condensation coefficient is known to be very close to unity, i.e.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 3 4 -X
V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
clean metal substrates or polyimide at room temperature. To account for the effect of inelastic scattering of photoelectrons on the XPS intensity at higher coverages, spherical metal particles on a solid substrate, as observed by TEM, are assumed in order to calculate the amount of the deposited metal. Cluster sizes are determined from the intensity ratios of two metal X-ray photoelectron lines, which are well separated in their kinetic energy. The validity of this method was checked using diameters of metal particles obtained directly from TEM micrographs. A good agreement between XPS and TEM determined cluster sizes was found in all cases [3]. Determination of C for Ag on different polymers by the radiotracer and by the XPS method showed a good agreement between the two methods, supporting the validity of the XPS results and confirmed that metal diffusion into the polymers can be neglected in the determination of C [3]. Whereas in radiotracer measurements the determination of C is independent of the morphology at the metal–polymer interface, the opposite situation is encountered when XPS is used to determine C. One needs a very accurate picture of the metal distribution at the metal–polymer interface [3]. In contrast, accurate knowledge of the interfacial morphology allows one to study the influence of morphological changes on C. In this paper we focus on the close connection between metal– polymer morphology and condensation coefficient. For a discussion of the influence of the metal coverage and the deposition rate on the condensation coefficient the reader is referred to Ref. [2].
2. Experimental The polymer films were prepared as described previously [2]. Copper and silver were thermally evaporated from a molybdenum crucible [2]. XPS measurements were performed using an electron spectrometer (Omicron) equipped with a nonmonochromated Al/Mg Ka source operated at 240 W and a hemispherical electron analyser set to a pass energy of 30 eV. The condensation coefficients were determined according to the procedure described in Ref. [3]. The base pressure in
413
the analysis chamber during metal evaporation and XPS analysis was always better than 1×10−10 mbar. Micrographs were taken with a TEM (PHILIPS CM 30) at an accelerating voltage of 200 kV. Bright-field pictures obtained with a CCD camera were edited subsequently with standard graphic software. Using phase contrast particles larger than about 1 nm can be resolved.
3. Results and discussion 3.1. Room temperature deposition The large differences in the condensation coefficient mentioned in Section 1 may be due to differences in the chemical interaction, i.e. a favorable interaction of the metals with the polyimide carbonyl groups, and an unfavorable one with the fluoro groups of the Teflon AF. To gain further insight on the influence of these opposing interactions we studied a polyimide (PI ) and a polybenzoxazole (PBO) which contained a high fluorine content (more than 10 at.%) in their repeat units. In both cases the condensation coefficient was close to unity at room temperature, indicating that the favorable interaction with the polymer functional groups dominates the condensation of the metals on the polymers and the introduction of fluorine is not detrimental to the condensation behavior of the metals. 3.2. Temperature dependence Previously we reported that the condensation coefficient decreases with increasing temperature [1]. In case of polyimide the drop was especially steep when the temperature approached the bulk glass transition temperature [1]. It was speculated that a depression of the glass transition at the polymer surface leads to the observed decrease starting already well below the glass transition temperature. To check the influence of the glass transition temperature we investigated the temperature dependence of the copper condensation coefficient on two different polymers exhibiting a C value of unity at room temperature.
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V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
Fig. 1 shows the temperature dependence of C for copper deposited onto bisphenol A polycarbonate (BPA) and onto pyromellitic dianhydride/ oxydianiline polyimide (PMDA/ODA PI ), respectively. The condensation coefficient at room temperature is close to unity for both polymers owing to the strong interaction of copper with the carbonyl groups. Upon heating the condensation coefficient decreases for both polymers, but at a lower temperature and more steeply for BPA polycarbonate. The drop starts to occur well below the bulk glass transition temperature of the polycarbonate at T =150°C. Copper clusters formed on g BPA polycarbonate also get embedded into the polymer well below the bulk glass transition temperature in a similar way to that observed by us for gold clusters formed on TMC polycarbonate [4,5]. This indicates that the glass transition temperatures of these polymers are substantially lowered at the surface. The embedding starts at about 140°C, i.e. at a temperature where the condensation coefficient is observed to drop. One reason for a high condensation coefficient could be a high cluster density on the surface that causes all adsorbed metal atoms to be trapped by the clusters before they desorb again [5]. However, if the
clusters get embedded into the polymer the stationary concentration of clusters on the surface will be lowered, enhancing the chance for desorption of the metal atoms. It seems that exactly this happens for copper deposition on BPA polycarbonate at very low deposition rates. At high rates the embedding kinetics may be to slow to affect metal condensation. Interestingly, a similar embedding of clusters up to 350°C has not been observed by us in the case of polyimide [6 ]. We therefore looked also for other reasons to explain the steep decrease of C with increasing temperature on the polyimide surface. In TEM investigations one notes a steep drop of the cluster density at elevated temperatures. In the two TEM micrographs shown in Fig. 2 a marked difference in cluster density between copper deposition at 200 and at 270°C is observed. One notes from Fig. 1 that this is also the temperature range at which the decrease of the condensation coefficient occurs. To gain further insight into the correlation between cluster density and the condensation coefficient we investigated the temperature dependence of the cluster density. The result is shown in Fig. 3. The 1/T plot exhibits two linear ranges with
Fig. 1. Temperature dependence of the copper condensation coefficient on BPA polycarbonate (circles) and PMDA/ODA polyimide (triangles). Nominal copper coverage was 0.1 nm and the deposition rate was 0.1 nm min−1. The bulk glass transition temperature for both polymers is indicated at the temperature axis.
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this case is described as: N
Fig. 2. TEM micrographs showing the variation of cluster density with deposition temperature for copper deposited onto polyimide: substrate temperature during deposition was 200°C (a) and 270°C (b). Nominal coverage is 0.3 nm, and the deposition rate was 0.1 nm min−1 for both samples. The size of the pictures is 120 nm×120 nm.
different slopes. Up to 200°C one observes a relatively small decrease of the cluster density with temperature. In this temperature range the condensation coefficient is unity and the decrease in cluster density is explained by the faster surface diffusion only. Previously, we have found evidence that for noble metals on polyimide clusters are formed by random nucleation with a critical cluster size of i=1, i.e. minimal stable clusters are dimers. According to nucleation theory [7] the change of the maximum cluster density with temperature in
max
3
AB
R 1/3.5 n
exp
A
B
E /kT d , 3.5
(1)
where n is an atomic vibration frequency, R the deposition rate, and E is the activation energy of d surface diffusion. This equation can be used to determine the activation energy for surface diffusion E [2], which amounts to 0.2±0.1 eV in the d case of copper and to 0.08±0.05 eV in the case of silver. At temperatures above 200°C one observes a steeper decrease of the cluster density, but again an Arrhenius type of behavior. In this temperature range metal atoms are also reemitted into the vacuum and the change in cluster density has to be described by an equation taking desorption of metal atoms into account [7]: N
max
3
AB
R 2/3 n
exp
A
B
2 2E −E a d , 3 kT
(2)
With E known from Eq. (1) it is thus possible to d determine E from this equation, which amounts a to 0.7±0.2 eV in the case of copper on polyimide. It should be noted that the value of E for copper a on polyimide is not too different from the value of 1.02 eV calculated for the interaction of copper
Fig. 3. Change of cluster density of Cu determined by TEM plotted versus reciprocal temperature. The slopes of straight lines fitted to the two different regions can be used to determine the activation energies for surface diffusion E and for desorption E . d a
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with the carbonyl group of acetone used as a model for the copper–polyimide interaction [8].
coefficient is accompanied by a drastic decrease of the metal cluster density on the surface.
References 4. Conclusions In this paper we have reported the determination of condensation coefficients of silver and copper on polymers. The condensation coefficient at room temperature is dominated by the most favorable interaction between the metal atoms and the polymer functional groups. The decrease of the condensation coefficient with temperature was related to morphological changes occurring at the metal–polymer interface. For BPA polycarbonate embedding of metal clusters into the polymer seems to play a significant role, whereas in the case of polyimide the decrease of the condensation
[1] A. Thran, M. Kiene, V. Zaporojtchenko, F. Faupel, Phys. Rev. Lett. 82 (1999) 1903. [2] V. Zaporojtchenko, K. Behnke, A. Thran, T. Strunskus, F. Faupel, Appl. Surf. Sci. 144–145 (1999) 355. [3] V. Zaporojtchenko, K. Behnke, T. Strunskus, F. Faupel, Surf. Interf. Anal. (2000) in press. [4] C.v. Bechtolsheim, V. Zaporojtchenko, F. Faupel, Appl. Surf. Sci. 270 (1999) 119. [5] V. Zaporojtchenko, T. Strunskus, K. Behnke, C.v. Bechtolsheim, M. Kiene, F. Faupel, J. Adhes. Technol. 14 (2000) 467. [6 ] M. Kiene, T. Strunskus, R. Peter, F. Faupel, Adv. Mater. 10 (1998) 1357. [7] J.A. Venables, Surf. Sci. 299–300 (1994) 798. [8] A. Ouhlal, A. Selmani, A. Yelon, M. Andrews, Chem. Phys. Lett. 202 (1993) 51.
Determination of condensation coefficients of metals on polymer surfaces V. Zaporojtchenko *, K. Behnke, T. Strunskus, F. Faupel Lehrstuhl fu¨r Materialverbunde, Technische Fakulta¨t der Universita¨t Kiel, Kaiserstr. 2, 24143 Kiel, Germany
Abstract We used X-ray photoelectron spectroscopy ( XPS ) in combination with transmission electron microscopy to characterize the nucleation and growth of noble metals deposited onto polymers and to determine the condensation coefficients of the metals. The surface concentration of the adatoms was determined using a mathematical correction of the XPS intensity to take into account the cluster formation of noble metals on polymer surfaces. Condensation coefficients for Cu and Ag depend strongly on the chemical composition of the polymer and on temperature and can vary by more than three orders of magnitude already at room temperature. The influence of the deposition parameters and the metal/polymer morphology on the condensation coefficient will be discussed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Copper; Physical adsorption; Sticking; Surface diffusion; Surface structure, morphology, roughness, and topography; X-ray photoelectron spectroscopy
1. Introduction Recently, our group reported the first quantitative results on condensation coefficients of noble metals on different polymer surfaces [1,2]. The condensation coefficient C is defined as the ratio of the number of adsorbed atoms to the total number of atoms arriving at the surface [1]. At room temperature the condensation coefficient varied by more than three orders of magnitude depending on the metal/polymer combination and in particular on the polymer used. The fluoropolymer Teflon AF 1601 showed very low condensation coefficients in the range between 0.002 (Ag) and 0.02 (Cu), whereas the condensation coefficients for noble metals deposited onto polyimide * Corresponding author. Fax: +49 431 77572 603. E-mail address: [email protected] ( V. Zaporojtchenko)
at room temperature were always close to unity [1–3]. One set of measurements with silver was performed using a very sensitive and accurate radiotracer technique [1]. In the radiotracer method a radioactive metal isotope is evaporated onto a polymer sample through an aperture and the reemitted atoms are collected on a catcher foil. Using a well calibrated system this method allows to measure the fractions of the condensed and reemitted atoms with a high accuracy of ±2%. Since it is not always possible to work with radioactive isotopes we developed a method to determine C by using a combination of X-ray photoelectron spectroscopy ( XPS) and transmission electron microscopy ( TEM ) [3]. In this method intensities are calibrated by depositing metals onto substrates where the condensation coefficient is known to be very close to unity, i.e.
0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 3 4 -X
V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
clean metal substrates or polyimide at room temperature. To account for the effect of inelastic scattering of photoelectrons on the XPS intensity at higher coverages, spherical metal particles on a solid substrate, as observed by TEM, are assumed in order to calculate the amount of the deposited metal. Cluster sizes are determined from the intensity ratios of two metal X-ray photoelectron lines, which are well separated in their kinetic energy. The validity of this method was checked using diameters of metal particles obtained directly from TEM micrographs. A good agreement between XPS and TEM determined cluster sizes was found in all cases [3]. Determination of C for Ag on different polymers by the radiotracer and by the XPS method showed a good agreement between the two methods, supporting the validity of the XPS results and confirmed that metal diffusion into the polymers can be neglected in the determination of C [3]. Whereas in radiotracer measurements the determination of C is independent of the morphology at the metal–polymer interface, the opposite situation is encountered when XPS is used to determine C. One needs a very accurate picture of the metal distribution at the metal–polymer interface [3]. In contrast, accurate knowledge of the interfacial morphology allows one to study the influence of morphological changes on C. In this paper we focus on the close connection between metal– polymer morphology and condensation coefficient. For a discussion of the influence of the metal coverage and the deposition rate on the condensation coefficient the reader is referred to Ref. [2].
2. Experimental The polymer films were prepared as described previously [2]. Copper and silver were thermally evaporated from a molybdenum crucible [2]. XPS measurements were performed using an electron spectrometer (Omicron) equipped with a nonmonochromated Al/Mg Ka source operated at 240 W and a hemispherical electron analyser set to a pass energy of 30 eV. The condensation coefficients were determined according to the procedure described in Ref. [3]. The base pressure in
413
the analysis chamber during metal evaporation and XPS analysis was always better than 1×10−10 mbar. Micrographs were taken with a TEM (PHILIPS CM 30) at an accelerating voltage of 200 kV. Bright-field pictures obtained with a CCD camera were edited subsequently with standard graphic software. Using phase contrast particles larger than about 1 nm can be resolved.
3. Results and discussion 3.1. Room temperature deposition The large differences in the condensation coefficient mentioned in Section 1 may be due to differences in the chemical interaction, i.e. a favorable interaction of the metals with the polyimide carbonyl groups, and an unfavorable one with the fluoro groups of the Teflon AF. To gain further insight on the influence of these opposing interactions we studied a polyimide (PI ) and a polybenzoxazole (PBO) which contained a high fluorine content (more than 10 at.%) in their repeat units. In both cases the condensation coefficient was close to unity at room temperature, indicating that the favorable interaction with the polymer functional groups dominates the condensation of the metals on the polymers and the introduction of fluorine is not detrimental to the condensation behavior of the metals. 3.2. Temperature dependence Previously we reported that the condensation coefficient decreases with increasing temperature [1]. In case of polyimide the drop was especially steep when the temperature approached the bulk glass transition temperature [1]. It was speculated that a depression of the glass transition at the polymer surface leads to the observed decrease starting already well below the glass transition temperature. To check the influence of the glass transition temperature we investigated the temperature dependence of the copper condensation coefficient on two different polymers exhibiting a C value of unity at room temperature.
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V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
Fig. 1 shows the temperature dependence of C for copper deposited onto bisphenol A polycarbonate (BPA) and onto pyromellitic dianhydride/ oxydianiline polyimide (PMDA/ODA PI ), respectively. The condensation coefficient at room temperature is close to unity for both polymers owing to the strong interaction of copper with the carbonyl groups. Upon heating the condensation coefficient decreases for both polymers, but at a lower temperature and more steeply for BPA polycarbonate. The drop starts to occur well below the bulk glass transition temperature of the polycarbonate at T =150°C. Copper clusters formed on g BPA polycarbonate also get embedded into the polymer well below the bulk glass transition temperature in a similar way to that observed by us for gold clusters formed on TMC polycarbonate [4,5]. This indicates that the glass transition temperatures of these polymers are substantially lowered at the surface. The embedding starts at about 140°C, i.e. at a temperature where the condensation coefficient is observed to drop. One reason for a high condensation coefficient could be a high cluster density on the surface that causes all adsorbed metal atoms to be trapped by the clusters before they desorb again [5]. However, if the
clusters get embedded into the polymer the stationary concentration of clusters on the surface will be lowered, enhancing the chance for desorption of the metal atoms. It seems that exactly this happens for copper deposition on BPA polycarbonate at very low deposition rates. At high rates the embedding kinetics may be to slow to affect metal condensation. Interestingly, a similar embedding of clusters up to 350°C has not been observed by us in the case of polyimide [6 ]. We therefore looked also for other reasons to explain the steep decrease of C with increasing temperature on the polyimide surface. In TEM investigations one notes a steep drop of the cluster density at elevated temperatures. In the two TEM micrographs shown in Fig. 2 a marked difference in cluster density between copper deposition at 200 and at 270°C is observed. One notes from Fig. 1 that this is also the temperature range at which the decrease of the condensation coefficient occurs. To gain further insight into the correlation between cluster density and the condensation coefficient we investigated the temperature dependence of the cluster density. The result is shown in Fig. 3. The 1/T plot exhibits two linear ranges with
Fig. 1. Temperature dependence of the copper condensation coefficient on BPA polycarbonate (circles) and PMDA/ODA polyimide (triangles). Nominal copper coverage was 0.1 nm and the deposition rate was 0.1 nm min−1. The bulk glass transition temperature for both polymers is indicated at the temperature axis.
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V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
this case is described as: N
Fig. 2. TEM micrographs showing the variation of cluster density with deposition temperature for copper deposited onto polyimide: substrate temperature during deposition was 200°C (a) and 270°C (b). Nominal coverage is 0.3 nm, and the deposition rate was 0.1 nm min−1 for both samples. The size of the pictures is 120 nm×120 nm.
different slopes. Up to 200°C one observes a relatively small decrease of the cluster density with temperature. In this temperature range the condensation coefficient is unity and the decrease in cluster density is explained by the faster surface diffusion only. Previously, we have found evidence that for noble metals on polyimide clusters are formed by random nucleation with a critical cluster size of i=1, i.e. minimal stable clusters are dimers. According to nucleation theory [7] the change of the maximum cluster density with temperature in
max
3
AB
R 1/3.5 n
exp
A
B
E /kT d , 3.5
(1)
where n is an atomic vibration frequency, R the deposition rate, and E is the activation energy of d surface diffusion. This equation can be used to determine the activation energy for surface diffusion E [2], which amounts to 0.2±0.1 eV in the d case of copper and to 0.08±0.05 eV in the case of silver. At temperatures above 200°C one observes a steeper decrease of the cluster density, but again an Arrhenius type of behavior. In this temperature range metal atoms are also reemitted into the vacuum and the change in cluster density has to be described by an equation taking desorption of metal atoms into account [7]: N
max
3
AB
R 2/3 n
exp
A
B
2 2E −E a d , 3 kT
(2)
With E known from Eq. (1) it is thus possible to d determine E from this equation, which amounts a to 0.7±0.2 eV in the case of copper on polyimide. It should be noted that the value of E for copper a on polyimide is not too different from the value of 1.02 eV calculated for the interaction of copper
Fig. 3. Change of cluster density of Cu determined by TEM plotted versus reciprocal temperature. The slopes of straight lines fitted to the two different regions can be used to determine the activation energies for surface diffusion E and for desorption E . d a
416
V. Zaporojtchenko et al. / Surface Science 454–456 (2000) 412–416
with the carbonyl group of acetone used as a model for the copper–polyimide interaction [8].
coefficient is accompanied by a drastic decrease of the metal cluster density on the surface.
References 4. Conclusions In this paper we have reported the determination of condensation coefficients of silver and copper on polymers. The condensation coefficient at room temperature is dominated by the most favorable interaction between the metal atoms and the polymer functional groups. The decrease of the condensation coefficient with temperature was related to morphological changes occurring at the metal–polymer interface. For BPA polycarbonate embedding of metal clusters into the polymer seems to play a significant role, whereas in the case of polyimide the decrease of the condensation
[1] A. Thran, M. Kiene, V. Zaporojtchenko, F. Faupel, Phys. Rev. Lett. 82 (1999) 1903. [2] V. Zaporojtchenko, K. Behnke, A. Thran, T. Strunskus, F. Faupel, Appl. Surf. Sci. 144–145 (1999) 355. [3] V. Zaporojtchenko, K. Behnke, T. Strunskus, F. Faupel, Surf. Interf. Anal. (2000) in press. [4] C.v. Bechtolsheim, V. Zaporojtchenko, F. Faupel, Appl. Surf. Sci. 270 (1999) 119. [5] V. Zaporojtchenko, T. Strunskus, K. Behnke, C.v. Bechtolsheim, M. Kiene, F. Faupel, J. Adhes. Technol. 14 (2000) 467. [6 ] M. Kiene, T. Strunskus, R. Peter, F. Faupel, Adv. Mater. 10 (1998) 1357. [7] J.A. Venables, Surf. Sci. 299–300 (1994) 798. [8] A. Ouhlal, A. Selmani, A. Yelon, M. Andrews, Chem. Phys. Lett. 202 (1993) 51.