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XPS analysis of thermal and plasma treated polyparaphenylene-vinylene thin films and their interface formed with aluminum layer
Synthetic Metals 69 (1995) 495496
XPS analysis
of thermal and plasma treated polyparaphenylene-vinylene and their interface formed with aluminum layer.
thin films
T.P. Nguyena, K. Amgaadb, M. Caillerb, V. H. TranC and S. Lefranta. Institut des Matkiaux de Names, 2 rue de la Houssinikre, 44072 Nantes Cedex 03, France. b) Laboratoire des Sciences des Surfaces et Interfaces en Mtkanique, ISITEM, 2, rue de la Houssinike, 44072 Nantes Cedex 03, France. c) Laboratoires des Mat&iaux Organiques aux PropriCt& Spkifiques, CNRS, Vemaison, BP 24,69 390 Vemaison, France. a) Laboratoire
de Physique
Cristalline,
Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the interface formed between thermally and plasma treated polyparaphenylene-vinylene thin films and an aluminium layer deposited on them. Enhancement in adhesion of the metal on thermally treated polymer films is explained by the structural change of aluminum layer. In contrast, plasma treatments induce morphological modifications of the polymer surface and favor the formation of compounds between the polymer and the metal which in turn improve the adhesive strength. 1. INTRODUCTION Thermal and plasma treatments of the polymer surface are currently used to enhance the adhesive strength of the metal layer deposited onto the polymer substrates (1). Depending on the nature of the polymer films and also on the nature of the explanations can be yielded for the used gas, several modification of the mechanical property of the polymer-metal interface. We have investigated the formation of the layer formed on treated surface of polyparaphenylene-vinylene (PPV) substrates before and after deposition of an aluminum film by XPS analysis. The results are discussed in relation to the adhesion property of the metal layer on the polymer films. 2. EXPERIMENTAL Thin films of PPV were prepared on glass substrates from a precursor solution following the method described in our previous works (2). Thermal treatment of the samples was performed under high vacuum condition (10m6 Torr) at 300°C for 3 hours. The samples were then allowed to reach room temperature before aluminum evaporation. For plasma treatments, argon, argon-oxygen or pure nitrogen were used with a pressure of 5 Pa and the applied RF power was 1OOW. The aluminum films were deposited by RF planar sputtering of a pure Al target with an average rate of 4 nm/min. The thickness of the PPV films (1 pm) as well as that of Al films (0.16 l.trn) were kept constant in all samples. The adhesive strength of Al layer on PPV substrates was determined by measuring the mean critical load LC from scratch test. This load is defined as the load applied to the stylus at which the loss of adhesion occurs. XPS measurements were performed on ESCA anlyser with Mg Kcr radiation (hv = 1253.6 eV). Spectra of carbon, oxygen (or nitrogen) and aluminum were recorded
0379-6779/95/$09.50 Q 1995 Elsevier Science S.A. All rights resewed SSDI 0379-6779(94)02541-6
throughout the interfacial region of the samples and analyzed by computer program. This region is defined as one where C Is, 01s (or N 1s) and Al 2p lines are simultaneously present. It can be reached by successive removals of the layer using argon bombardment at low voltage (1 kV) to avoid possible damaging of the analyzed surface. Binding energy data were referenced to the Au 4412 furnished by a gold plate fixed on the sample holder. 3. RESULTS
AND
DISCUSSION
The results of scratch test measurements performed on different samples are qpmmarized in table 1. We can see that the thermal treatment improves significantly the adhesion of the aluminum layer on PPV films. In contrast, at a same temperature. there is no change of k measured on Ar treated and untreated samples. The enhancement of the adhesive strength is obvious with Ar-02 and N2 treatments, the critical loads are 25 and 50g respectively as compared to 2g measured on untreated samples. We also notice that the treated samples have a brilliant aspect denoting a high reflectivity of the metal layer. Observation of the uncovered polymer film by scanning electron microscopy (SEM) reveals a smooth and uniform surface for untreated, thermally and Ar treated samples. Subsequent changes occur however in Ar-02 and N2 treated films with formation of vacancies and microcavities in the substrates. The XPS spectra recorded in the interface PPV-Al of untreated, thermally and Ar treated samples are very similar to those already reported in our previous study (2). We briefly recall that on pristine PPV film suiface, the C 1s line is located at 284.4 eV with a full width at half maximum (FWHM) of 1.6 eV which is characteristic of carbon-hydrogen bondings. Upon aluminum deposition, this line presents a small shoulder at 283.5 eV . On the other hand, 0 1s and Al 2p are also seen at
4%
T.P. Nguyen et al. / Synthetic MetaLr 69 (1995) 495496
Table 1 Mean critical load Lc measured -__
in PPV-Al system with and without treatment of the polymer
--_____--~~_____-----_______________-~_____ Untreated film
Ar treated film
surface.
Ar+O2 treated film
____-_________--_________------__--------_ Substrate temperature
(Co)
Critical load (g)
N2 treated film
---__-___
25
300
25
25
25
2
8
2
15
50
______--_______----_-___-----_________-~~________-~~~______--~____------_-----____ 531.6 and 74 eV respectively. These spectra suggest that a complex of Al-O-C is formed in the interfacial region between the polymer and the metal layers. The fact that the XPS spectra recorded in PPV-Al interface are not modified in both thermally and Ar treated samples as compared to untreated one suggests that the surface of these samples has not been changed upon treatment. The increase in adhesion of Al layer can be explained threfore by its structural change (3) with possible refinement of the metallic grains. As for Ar treated sample, the effect of the bombardment would result in removing of the polymer layer without further reaction. This could explain both XPS and SEM results in this case. Figure 1 shows the XPS spectra recorded on the surface of Ar-02 treated films. Both C 1s and 0 1s lines indicate that the oxidation has occurred with formation of C-O and C=O bonds. The C 1s line is composed of i) carbon bound to other carbon or hydrogen (284.5 eV), ii) carbon singly bound to oxygen (286.1 eV), iii) carbonyl carbon (287.5 eV) and iv) ester carbon (289.1 eV). The 0 1s line has two components at 532.5 and 534.5 which correspond to C=O and C-O respectively.
compensated by a new one appeared at 531.5 eV. ii) disappearance of the O-C=0 component (289.1 eV).and diminution of the carbonyl component (287.5 eV). A new one is formed at 283.3 eV. As for the Al 2p line, the deconvoluted curve shows two peaks at 75.6 (AlxOy) and 74.2 eV. The new features observed in both carbon and oxygen spectra together with the 74.2 eV component of aluminum line strongly suggest that a complex metaloxygen-carbon is formed in the interface on the treated film. The bondings between the different atoms are on the whole similar to those found in the untreated PPV-Al interface. However, the thickness of the interfacial layer in the former case is about twofold that of the latter case indicating a deeper penetration of Al atoms in the polymer film. This corroborates the modifications of the morphology of the film surface induced by ion bombardment. In addition, we verify that the metallic atoms react preferentially with ester and carbonyl sites to form the compound as already suggested some earlier studies (4, 5). For N2 treated PPV films, both nitrogen and oxygen are found on the surface of uncovered samples. The N 1s and 0 1s lines are located at 399.5 and 531 eV respectively. Upon deposition of the Al layer, both lines are broadened showing shoulders on the high binding energies (401 and 534.5 eV). At the same time, the Al 2p line presents a peak at 74.6 eV in the interfacial region and the C 1s line shows components at 286 arts 287.1 eV (C-O and C=O bondings). Based upon data obtained for AlN (6). we suggest that a new complex has been formed between nitrogen, oxygen, carbon and aluminum. It should be noticed that the penetration of the nitrogen atoms is much more important than that of oxygen since traces of N2 is still found after complete removal of the metal layer. In conclusion, XPS analysis performed on PPV-AI systems brings out some explanations for the improvement in adhesion of aluminum layer on the PPV substrates. Both the morphology change of the polymer surface and the formation of complex in the interface seem to be responsible for the adhesion enhancement in 02 and N2 treated sample. As for the thermal treatment, it is possible that the structural change of the Al film could be at the origin of the observed increase of the adhesive strength. REFERENCES
I
80
I
I
76
I
I
72
III
II.II
286
282
I I
535
I)
525
BINDING ENERGY (eV) Fig.1 : Part A- Cls and 0 1s lines recorded on the Ar-02 treated sample ; Part B : C Is, 0 1s and Al 2p lines recorded in the PPV-Al interface of Ar-02 treated sample. Upon Al deposition (part B), these lines are modified by i) diminution of the O=C component (532.5 eV) which is
1. S. Wu, Polymer Interface and Adhesion, , Marcel Dekker. New York, 1982. 2. T.P. Nguyen , V.H. Tran, V. Massardier and A. Guyot, Synthetic Metals, 55-57 (1993) 235. 3. P. Phuku,P. Bertrand and Y De Puydt, Thin Solid Films, 200 (1991) 263. 4. B. M. DeKoven and P. L. Hagans, Appl.Surf. Sci., 27 (1986) 199. 5. M. Bou, J. M. Martin, T. Le Mogne and L. Vovelle, Appl. Surf. Sci., 47 (1991) 149. 6. H. Takaoka, J. Ishikawa and T. Tagaki, Thin Solid Films, 157 (1988) 143.
XPS analysis
of thermal and plasma treated polyparaphenylene-vinylene and their interface formed with aluminum layer.
thin films
T.P. Nguyena, K. Amgaadb, M. Caillerb, V. H. TranC and S. Lefranta. Institut des Matkiaux de Names, 2 rue de la Houssinikre, 44072 Nantes Cedex 03, France. b) Laboratoire des Sciences des Surfaces et Interfaces en Mtkanique, ISITEM, 2, rue de la Houssinike, 44072 Nantes Cedex 03, France. c) Laboratoires des Mat&iaux Organiques aux PropriCt& Spkifiques, CNRS, Vemaison, BP 24,69 390 Vemaison, France. a) Laboratoire
de Physique
Cristalline,
Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the interface formed between thermally and plasma treated polyparaphenylene-vinylene thin films and an aluminium layer deposited on them. Enhancement in adhesion of the metal on thermally treated polymer films is explained by the structural change of aluminum layer. In contrast, plasma treatments induce morphological modifications of the polymer surface and favor the formation of compounds between the polymer and the metal which in turn improve the adhesive strength. 1. INTRODUCTION Thermal and plasma treatments of the polymer surface are currently used to enhance the adhesive strength of the metal layer deposited onto the polymer substrates (1). Depending on the nature of the polymer films and also on the nature of the explanations can be yielded for the used gas, several modification of the mechanical property of the polymer-metal interface. We have investigated the formation of the layer formed on treated surface of polyparaphenylene-vinylene (PPV) substrates before and after deposition of an aluminum film by XPS analysis. The results are discussed in relation to the adhesion property of the metal layer on the polymer films. 2. EXPERIMENTAL Thin films of PPV were prepared on glass substrates from a precursor solution following the method described in our previous works (2). Thermal treatment of the samples was performed under high vacuum condition (10m6 Torr) at 300°C for 3 hours. The samples were then allowed to reach room temperature before aluminum evaporation. For plasma treatments, argon, argon-oxygen or pure nitrogen were used with a pressure of 5 Pa and the applied RF power was 1OOW. The aluminum films were deposited by RF planar sputtering of a pure Al target with an average rate of 4 nm/min. The thickness of the PPV films (1 pm) as well as that of Al films (0.16 l.trn) were kept constant in all samples. The adhesive strength of Al layer on PPV substrates was determined by measuring the mean critical load LC from scratch test. This load is defined as the load applied to the stylus at which the loss of adhesion occurs. XPS measurements were performed on ESCA anlyser with Mg Kcr radiation (hv = 1253.6 eV). Spectra of carbon, oxygen (or nitrogen) and aluminum were recorded
0379-6779/95/$09.50 Q 1995 Elsevier Science S.A. All rights resewed SSDI 0379-6779(94)02541-6
throughout the interfacial region of the samples and analyzed by computer program. This region is defined as one where C Is, 01s (or N 1s) and Al 2p lines are simultaneously present. It can be reached by successive removals of the layer using argon bombardment at low voltage (1 kV) to avoid possible damaging of the analyzed surface. Binding energy data were referenced to the Au 4412 furnished by a gold plate fixed on the sample holder. 3. RESULTS
AND
DISCUSSION
The results of scratch test measurements performed on different samples are qpmmarized in table 1. We can see that the thermal treatment improves significantly the adhesion of the aluminum layer on PPV films. In contrast, at a same temperature. there is no change of k measured on Ar treated and untreated samples. The enhancement of the adhesive strength is obvious with Ar-02 and N2 treatments, the critical loads are 25 and 50g respectively as compared to 2g measured on untreated samples. We also notice that the treated samples have a brilliant aspect denoting a high reflectivity of the metal layer. Observation of the uncovered polymer film by scanning electron microscopy (SEM) reveals a smooth and uniform surface for untreated, thermally and Ar treated samples. Subsequent changes occur however in Ar-02 and N2 treated films with formation of vacancies and microcavities in the substrates. The XPS spectra recorded in the interface PPV-Al of untreated, thermally and Ar treated samples are very similar to those already reported in our previous study (2). We briefly recall that on pristine PPV film suiface, the C 1s line is located at 284.4 eV with a full width at half maximum (FWHM) of 1.6 eV which is characteristic of carbon-hydrogen bondings. Upon aluminum deposition, this line presents a small shoulder at 283.5 eV . On the other hand, 0 1s and Al 2p are also seen at
4%
T.P. Nguyen et al. / Synthetic MetaLr 69 (1995) 495496
Table 1 Mean critical load Lc measured -__
in PPV-Al system with and without treatment of the polymer
--_____--~~_____-----_______________-~_____ Untreated film
Ar treated film
surface.
Ar+O2 treated film
____-_________--_________------__--------_ Substrate temperature
(Co)
Critical load (g)
N2 treated film
---__-___
25
300
25
25
25
2
8
2
15
50
______--_______----_-___-----_________-~~________-~~~______--~____------_-----____ 531.6 and 74 eV respectively. These spectra suggest that a complex of Al-O-C is formed in the interfacial region between the polymer and the metal layers. The fact that the XPS spectra recorded in PPV-Al interface are not modified in both thermally and Ar treated samples as compared to untreated one suggests that the surface of these samples has not been changed upon treatment. The increase in adhesion of Al layer can be explained threfore by its structural change (3) with possible refinement of the metallic grains. As for Ar treated sample, the effect of the bombardment would result in removing of the polymer layer without further reaction. This could explain both XPS and SEM results in this case. Figure 1 shows the XPS spectra recorded on the surface of Ar-02 treated films. Both C 1s and 0 1s lines indicate that the oxidation has occurred with formation of C-O and C=O bonds. The C 1s line is composed of i) carbon bound to other carbon or hydrogen (284.5 eV), ii) carbon singly bound to oxygen (286.1 eV), iii) carbonyl carbon (287.5 eV) and iv) ester carbon (289.1 eV). The 0 1s line has two components at 532.5 and 534.5 which correspond to C=O and C-O respectively.
compensated by a new one appeared at 531.5 eV. ii) disappearance of the O-C=0 component (289.1 eV).and diminution of the carbonyl component (287.5 eV). A new one is formed at 283.3 eV. As for the Al 2p line, the deconvoluted curve shows two peaks at 75.6 (AlxOy) and 74.2 eV. The new features observed in both carbon and oxygen spectra together with the 74.2 eV component of aluminum line strongly suggest that a complex metaloxygen-carbon is formed in the interface on the treated film. The bondings between the different atoms are on the whole similar to those found in the untreated PPV-Al interface. However, the thickness of the interfacial layer in the former case is about twofold that of the latter case indicating a deeper penetration of Al atoms in the polymer film. This corroborates the modifications of the morphology of the film surface induced by ion bombardment. In addition, we verify that the metallic atoms react preferentially with ester and carbonyl sites to form the compound as already suggested some earlier studies (4, 5). For N2 treated PPV films, both nitrogen and oxygen are found on the surface of uncovered samples. The N 1s and 0 1s lines are located at 399.5 and 531 eV respectively. Upon deposition of the Al layer, both lines are broadened showing shoulders on the high binding energies (401 and 534.5 eV). At the same time, the Al 2p line presents a peak at 74.6 eV in the interfacial region and the C 1s line shows components at 286 arts 287.1 eV (C-O and C=O bondings). Based upon data obtained for AlN (6). we suggest that a new complex has been formed between nitrogen, oxygen, carbon and aluminum. It should be noticed that the penetration of the nitrogen atoms is much more important than that of oxygen since traces of N2 is still found after complete removal of the metal layer. In conclusion, XPS analysis performed on PPV-AI systems brings out some explanations for the improvement in adhesion of aluminum layer on the PPV substrates. Both the morphology change of the polymer surface and the formation of complex in the interface seem to be responsible for the adhesion enhancement in 02 and N2 treated sample. As for the thermal treatment, it is possible that the structural change of the Al film could be at the origin of the observed increase of the adhesive strength. REFERENCES
I
80
I
I
76
I
I
72
III
II.II
286
282
I I
535
I)
525
BINDING ENERGY (eV) Fig.1 : Part A- Cls and 0 1s lines recorded on the Ar-02 treated sample ; Part B : C Is, 0 1s and Al 2p lines recorded in the PPV-Al interface of Ar-02 treated sample. Upon Al deposition (part B), these lines are modified by i) diminution of the O=C component (532.5 eV) which is
1. S. Wu, Polymer Interface and Adhesion, , Marcel Dekker. New York, 1982. 2. T.P. Nguyen , V.H. Tran, V. Massardier and A. Guyot, Synthetic Metals, 55-57 (1993) 235. 3. P. Phuku,P. Bertrand and Y De Puydt, Thin Solid Films, 200 (1991) 263. 4. B. M. DeKoven and P. L. Hagans, Appl.Surf. Sci., 27 (1986) 199. 5. M. Bou, J. M. Martin, T. Le Mogne and L. Vovelle, Appl. Surf. Sci., 47 (1991) 149. 6. H. Takaoka, J. Ishikawa and T. Tagaki, Thin Solid Films, 157 (1988) 143.