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Depth profile analysis of the structure and composition of a metal-implanted polyimide surface
Surface & Coatings Technology 196 (2005) 211 – 215 www.elsevier.com/locate/surfcoat
Depth profile analysis of the structure and composition of a metalimplanted polyimide surface Tomohiro Kobayashi*, Masaya Iwaki The Institute of Physical and Chemical Research (RIKEN), 2-1, Wako, Saitama 351-0198, Japan Available online 12 October 2004
Abstract High-dose metallic ion implantation was performed into polyimide (C22H10N2O5) films, and the structure of nanoparticles and matrices were examined with high-resolution transmission electron microscopy (HRTEM). The depth-resolved profiles of electron mean free paths were analyzed with electron energy loss spectroscopy (EELS). The implantations were performed with acceleration voltages of 100 kV for Cu and 190 kV for W using electrostatic accelerators. Nitrogen implantation at 40 kV was also performed to make a reference specimen. The doses of the implantations ranged from 51016 to 11017 cm2. In this study, we fixed the thin specimens on a sample grid by using a focused ion beam (FIB) processing system equipped with a tungsten deposition system and a micromanipulator. Owing to the method of sample preparation, high-resolution observations were possible because the contraction of the specimens was suppressed during transmission electron spectroscopy (TEM) observation. The cross-sectional HRTEM and EELS observations were performed at 300 kV using a field emission TEM. The TEM observations showed migration of Cu atoms toward the surface, and the lattice images of Cu nanoparticles indicated that each particle consisted of a single crystal. We also found amorphous W nanoparticles containing carbon. The electron mean free path observations suggest that alteration reached about four times further than the mean ion range. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Polyimide; Carbonization; Nanoparticle; TEM
1. Introduction Polyimide is one of the favored candidates for forming metal-doped amorphous carbon by implantation because of its durability to radiation damage. Ion implantation into polyimide produces a carbonized hard surface showing desirable properties such as wear resistance and a barrier to gas permeation [1,2]. The structure of implanted layers, such as precipitate formation and a density gradient, is quite important for functional designs. In this study, highdosage metallic ion implantation was carried out on polyimide films, and nanoparticle formation and composition changes were observed using transmission electron spectroscopy (TEM) and electron energy loss spectro-
* Corresponding author. Tel.: +81 48 467 9358; fax: +81 484 62 4623. E-mail address: [email protected] (T. Kobayashi). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.198
scopy (EELS). There are two popular methods of analyzing the composition of an implanted surface: Rutherford backscattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS). It is difficult to determine the density of an implanted layer by these methods. Previously, we performed cross-sectional TEM observations of metallic ion-implanted polymer surfaces [3] where, for sample preparation, we used an ultramicrotome with a diamond knife to make about 100-nmthick cross-sectional specimens. The implanted layer became so hard and brittle that it was impossible to make a homogeneous specimen. Moreover, microtomed specimens held on a carbon grid tended to move during TEM observations owing to thermal deformation. In the present study, we used a focused ion beam (FIB) process to make homogeneous and well-fixed specimens without using a carbon grid. Before processing, we precoated tungsten at 1-Am thickness to protect the surface. This
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T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215
precoated layer supported the cross-sectional specimen and thermal drift was suppressed. High-resolution TEM (HRTEM) observations and EELS analyses were performed without hindrance.
2. Experimental A polyimide film produced by Toray-DuPont (Kapton, 30-Am-thick) was implanted with 100 keV Cu, 190 keV W, and 40 keV N ions of the relevant dosages. The mean ion ranges for these implantations were about 150 nm, as calculated by TRIM code [4]. The fluence was 51016 cm2 for Cu and N implantation and 11017 cm2 for W implantation, and the current density was kept to 0.1–3.0 AA cm2 at a pressure below 1104 Pa. During irradiation, the temperature of the surface is not observed; however, the temperature on the rear side of the specimens was below the glass transition temperature of polyimide (360 8C). The nitrogen implantation was performed to make a damaged specimen without metal ions for reference. Cross-sectional TEM observations were performed using a 300 kV Hitachi field emission electron microscope equipped with an electron scanning unit and an EELS unit to observe any growth of metallic particles. The surface layer of the implanted specimens was cut using an FIB system (Hitachi FB-2000A) and a wedgeshaped piece was extracted using a microsampling system employing a tungsten needle manipulator. Next, the piece was transferred on an edge of a 30-Am-thick copper sheet and fixed using a beam-assisted tungsten deposition system. The sample piece was planed down on both sides to leave a 100-nm-thick part for cross-sectional observations, as shown in Fig. 1. A tungsten coating on the surface and both edges prevented thin samples from deformation. The EELS observations were performed for
Fig. 1. A cross-sectional SEM image of FIB-processed ion-implanted polyimide film.
Fig. 2. A cross-sectional TEM image of copper-implanted polyimide surface.
the cross-sectional specimens every 20-nm point from the surface to the depth about 700–1200 nm. The observation time at one point was 20 ms.
3. Results and discussion 3.1. TEM observation Fig. 2 shows a cross-sectional TEM image of the Cuimplanted layer including the surface. The central depth of the nanoparticles distribution was 100 nm and it was shallower than the mean ion range calculated by TRIM code. Diffusion of implanted Cu atoms would be active at depths where the energy deposition is maximal and which causes both migration of Cu atoms toward the surface and particle formation. The bright parts (holes) shown in the image were produced by electron damage during the high-magnification observation. Fig. 3 shows a HRTEM
Fig. 3. A HRTEM image of a copper nanoparticle.
T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215
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Fig. 4. A cross-sectional TEM image of tungsten-implanted polyimide surface.
image of nanoparticles. Each particle was almost perfectly spherical and consisted of crystalline copper. Fig. 4 shows a TEM image of the W-implanted surface. The central depth of implanted tungsten is about 150 nm. This means that implanted tungsten did not migrate macroscopically. Fig. 5 shows a HRTEM image around the depth of the mean ion range. The image shows the presence of particles of about 2-nm diameter that were not observed in our previous study [3]. Crystalline particles were not observed in the image and a diffraction pattern showed no distinct ring. These particles would have been tungsten with carbon and a small amount of oxygen, judging from our previous results of X-ray photoelectron spectroscopy (XPS) [5]. In this case, diffusion of implanted tungsten atoms was localized, in contrast to the case for copper.
Fig. 5. A HRTEM image of tungsten-implanted polyimide at a depth near the mean ion range.
Fig. 6. Depth profiles of normalized electron mean free path and calculated mean ion range of (a) Cu-implanted, (b) W-implanted, and (c) N-implanted specimens.
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mean value of k between the depths of 600 and 800 nm for Cu- and W-implanted specimens and 400 and 600 nm for the N-implanted specimens. The decrease in k/k 0 was observed at a depth of around 400 nm for the Cu- and Wimplanted specimens. This means that alteration in the polyimide film extends about four times further than the mean ion range. The reasons for the alteration are still not clear; one reason may be a compaction in volume because of the heating effect during implantation. The k/k 0 values are greater than 1 at the surface of the N- and Cuimplanted specimens, indicating that the density at the surface was decreased by gas elimination. On the other hand, in the W-implanted specimen, the low-density surface did not remain, because of the sputtering during implantation. In the Cu-implanted specimen, k/k 0 is minimized at a level shallower than the mean ion range. This result supports the TEM observation that the Cu atoms migrate toward the surface. 3.3. Carbonization behavior Fig. 7 shows the depth profiles of the ratio of nitrogen and oxygen to carbon, (a) Cu-implanted and (b) Wimplanted specimens delivered from the heights of K-edges in EELS spectra. Carbonization began at the depth about 200 nm in the Cu-implanted specimen, and 170 nm in the W-implanted one. In the Cu-implanted specimen, nitrogen concentrated at the depth where nanoparticles formed that was not seen in W-implanted one. In both specimens, the ratio of oxygen decrease is larger than that of nitrogen, and nitrogen-rich amorphous carbon will be formed at the surface layer.
Fig. 7. Depth profiles of the ratio of nitrogen and oxygen to carbon, (a) Cuimplanted and (b) W-implanted specimens.
3.2. Electron mean free path of implanted layers In this study, we observed the depth profiles of mean free paths for inelastic scattering using EELS. The total mean free path for inelastic scattering, k, is given by t=k ¼ lnðIt =I0 Þ; where t is the sample thickness, I 0 is the area of the zeroloss peak, and I t is the total area of whole spectrum [6]. The thickness of FIB-processed specimens increases at a constant rate in the direction perpendicular to the surface plane because of the spread of the processing beam. We calibrated this increase in thickness by using an unimplanted specimen. Fig. 6 shows the depth profile of the normalized mean free path of the Cu-implanted specimen (a), the W-implanted specimen (b), and the N-implanted specimen (c), with the distribution of ions being calculated by TRIM code. The normalization is carried out using the
4. Conclusion High-dosage Cu and W ion implantations into polyimide film were performed and the following results were obtained. 1. The migration of Cu atoms toward the surface was shown by TEM and EELS observations. 2. The lattice images of Cu nanoparticles indicated that each one consisted of a single crystal. 3. W nanoparticles were discovered; they contained carbon and were not crystalline. 4. The substrate alteration during implantation reached about four times further than the mean ion range. 5. Oxygen decreased faster than nitrogen and nitrogen-rich amorphous carbon layer was formed.
References [1] W. Liu, S. Yang, C. Li, Y. Sun, Wear 194 (1996) 103. [2] H. Dong, T. Bell, Surface and Coatings Technology 111 (1999) 29.
T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215 [3] T. Kobayashi, T. Iwata, Y. Doi, M. Iwaki, Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175–177 (2001) 548. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping Range of Ions in Solids, Pergamon, New York, 1977.
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[5] T. Kobayashi, A. Nakao, M. Iwaki, Surface and Coatings Technology 158–159 (2002) 399. [6] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum, New York, 1996, p. 302.
Depth profile analysis of the structure and composition of a metalimplanted polyimide surface Tomohiro Kobayashi*, Masaya Iwaki The Institute of Physical and Chemical Research (RIKEN), 2-1, Wako, Saitama 351-0198, Japan Available online 12 October 2004
Abstract High-dose metallic ion implantation was performed into polyimide (C22H10N2O5) films, and the structure of nanoparticles and matrices were examined with high-resolution transmission electron microscopy (HRTEM). The depth-resolved profiles of electron mean free paths were analyzed with electron energy loss spectroscopy (EELS). The implantations were performed with acceleration voltages of 100 kV for Cu and 190 kV for W using electrostatic accelerators. Nitrogen implantation at 40 kV was also performed to make a reference specimen. The doses of the implantations ranged from 51016 to 11017 cm2. In this study, we fixed the thin specimens on a sample grid by using a focused ion beam (FIB) processing system equipped with a tungsten deposition system and a micromanipulator. Owing to the method of sample preparation, high-resolution observations were possible because the contraction of the specimens was suppressed during transmission electron spectroscopy (TEM) observation. The cross-sectional HRTEM and EELS observations were performed at 300 kV using a field emission TEM. The TEM observations showed migration of Cu atoms toward the surface, and the lattice images of Cu nanoparticles indicated that each particle consisted of a single crystal. We also found amorphous W nanoparticles containing carbon. The electron mean free path observations suggest that alteration reached about four times further than the mean ion range. D 2004 Elsevier B.V. All rights reserved. Keywords: Ion implantation; Polyimide; Carbonization; Nanoparticle; TEM
1. Introduction Polyimide is one of the favored candidates for forming metal-doped amorphous carbon by implantation because of its durability to radiation damage. Ion implantation into polyimide produces a carbonized hard surface showing desirable properties such as wear resistance and a barrier to gas permeation [1,2]. The structure of implanted layers, such as precipitate formation and a density gradient, is quite important for functional designs. In this study, highdosage metallic ion implantation was carried out on polyimide films, and nanoparticle formation and composition changes were observed using transmission electron spectroscopy (TEM) and electron energy loss spectro-
* Corresponding author. Tel.: +81 48 467 9358; fax: +81 484 62 4623. E-mail address: [email protected] (T. Kobayashi). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.198
scopy (EELS). There are two popular methods of analyzing the composition of an implanted surface: Rutherford backscattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS). It is difficult to determine the density of an implanted layer by these methods. Previously, we performed cross-sectional TEM observations of metallic ion-implanted polymer surfaces [3] where, for sample preparation, we used an ultramicrotome with a diamond knife to make about 100-nmthick cross-sectional specimens. The implanted layer became so hard and brittle that it was impossible to make a homogeneous specimen. Moreover, microtomed specimens held on a carbon grid tended to move during TEM observations owing to thermal deformation. In the present study, we used a focused ion beam (FIB) process to make homogeneous and well-fixed specimens without using a carbon grid. Before processing, we precoated tungsten at 1-Am thickness to protect the surface. This
212
T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215
precoated layer supported the cross-sectional specimen and thermal drift was suppressed. High-resolution TEM (HRTEM) observations and EELS analyses were performed without hindrance.
2. Experimental A polyimide film produced by Toray-DuPont (Kapton, 30-Am-thick) was implanted with 100 keV Cu, 190 keV W, and 40 keV N ions of the relevant dosages. The mean ion ranges for these implantations were about 150 nm, as calculated by TRIM code [4]. The fluence was 51016 cm2 for Cu and N implantation and 11017 cm2 for W implantation, and the current density was kept to 0.1–3.0 AA cm2 at a pressure below 1104 Pa. During irradiation, the temperature of the surface is not observed; however, the temperature on the rear side of the specimens was below the glass transition temperature of polyimide (360 8C). The nitrogen implantation was performed to make a damaged specimen without metal ions for reference. Cross-sectional TEM observations were performed using a 300 kV Hitachi field emission electron microscope equipped with an electron scanning unit and an EELS unit to observe any growth of metallic particles. The surface layer of the implanted specimens was cut using an FIB system (Hitachi FB-2000A) and a wedgeshaped piece was extracted using a microsampling system employing a tungsten needle manipulator. Next, the piece was transferred on an edge of a 30-Am-thick copper sheet and fixed using a beam-assisted tungsten deposition system. The sample piece was planed down on both sides to leave a 100-nm-thick part for cross-sectional observations, as shown in Fig. 1. A tungsten coating on the surface and both edges prevented thin samples from deformation. The EELS observations were performed for
Fig. 1. A cross-sectional SEM image of FIB-processed ion-implanted polyimide film.
Fig. 2. A cross-sectional TEM image of copper-implanted polyimide surface.
the cross-sectional specimens every 20-nm point from the surface to the depth about 700–1200 nm. The observation time at one point was 20 ms.
3. Results and discussion 3.1. TEM observation Fig. 2 shows a cross-sectional TEM image of the Cuimplanted layer including the surface. The central depth of the nanoparticles distribution was 100 nm and it was shallower than the mean ion range calculated by TRIM code. Diffusion of implanted Cu atoms would be active at depths where the energy deposition is maximal and which causes both migration of Cu atoms toward the surface and particle formation. The bright parts (holes) shown in the image were produced by electron damage during the high-magnification observation. Fig. 3 shows a HRTEM
Fig. 3. A HRTEM image of a copper nanoparticle.
T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215
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Fig. 4. A cross-sectional TEM image of tungsten-implanted polyimide surface.
image of nanoparticles. Each particle was almost perfectly spherical and consisted of crystalline copper. Fig. 4 shows a TEM image of the W-implanted surface. The central depth of implanted tungsten is about 150 nm. This means that implanted tungsten did not migrate macroscopically. Fig. 5 shows a HRTEM image around the depth of the mean ion range. The image shows the presence of particles of about 2-nm diameter that were not observed in our previous study [3]. Crystalline particles were not observed in the image and a diffraction pattern showed no distinct ring. These particles would have been tungsten with carbon and a small amount of oxygen, judging from our previous results of X-ray photoelectron spectroscopy (XPS) [5]. In this case, diffusion of implanted tungsten atoms was localized, in contrast to the case for copper.
Fig. 5. A HRTEM image of tungsten-implanted polyimide at a depth near the mean ion range.
Fig. 6. Depth profiles of normalized electron mean free path and calculated mean ion range of (a) Cu-implanted, (b) W-implanted, and (c) N-implanted specimens.
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mean value of k between the depths of 600 and 800 nm for Cu- and W-implanted specimens and 400 and 600 nm for the N-implanted specimens. The decrease in k/k 0 was observed at a depth of around 400 nm for the Cu- and Wimplanted specimens. This means that alteration in the polyimide film extends about four times further than the mean ion range. The reasons for the alteration are still not clear; one reason may be a compaction in volume because of the heating effect during implantation. The k/k 0 values are greater than 1 at the surface of the N- and Cuimplanted specimens, indicating that the density at the surface was decreased by gas elimination. On the other hand, in the W-implanted specimen, the low-density surface did not remain, because of the sputtering during implantation. In the Cu-implanted specimen, k/k 0 is minimized at a level shallower than the mean ion range. This result supports the TEM observation that the Cu atoms migrate toward the surface. 3.3. Carbonization behavior Fig. 7 shows the depth profiles of the ratio of nitrogen and oxygen to carbon, (a) Cu-implanted and (b) Wimplanted specimens delivered from the heights of K-edges in EELS spectra. Carbonization began at the depth about 200 nm in the Cu-implanted specimen, and 170 nm in the W-implanted one. In the Cu-implanted specimen, nitrogen concentrated at the depth where nanoparticles formed that was not seen in W-implanted one. In both specimens, the ratio of oxygen decrease is larger than that of nitrogen, and nitrogen-rich amorphous carbon will be formed at the surface layer.
Fig. 7. Depth profiles of the ratio of nitrogen and oxygen to carbon, (a) Cuimplanted and (b) W-implanted specimens.
3.2. Electron mean free path of implanted layers In this study, we observed the depth profiles of mean free paths for inelastic scattering using EELS. The total mean free path for inelastic scattering, k, is given by t=k ¼ lnðIt =I0 Þ; where t is the sample thickness, I 0 is the area of the zeroloss peak, and I t is the total area of whole spectrum [6]. The thickness of FIB-processed specimens increases at a constant rate in the direction perpendicular to the surface plane because of the spread of the processing beam. We calibrated this increase in thickness by using an unimplanted specimen. Fig. 6 shows the depth profile of the normalized mean free path of the Cu-implanted specimen (a), the W-implanted specimen (b), and the N-implanted specimen (c), with the distribution of ions being calculated by TRIM code. The normalization is carried out using the
4. Conclusion High-dosage Cu and W ion implantations into polyimide film were performed and the following results were obtained. 1. The migration of Cu atoms toward the surface was shown by TEM and EELS observations. 2. The lattice images of Cu nanoparticles indicated that each one consisted of a single crystal. 3. W nanoparticles were discovered; they contained carbon and were not crystalline. 4. The substrate alteration during implantation reached about four times further than the mean ion range. 5. Oxygen decreased faster than nitrogen and nitrogen-rich amorphous carbon layer was formed.
References [1] W. Liu, S. Yang, C. Li, Y. Sun, Wear 194 (1996) 103. [2] H. Dong, T. Bell, Surface and Coatings Technology 111 (1999) 29.
T. Kobayashi, M. Iwaki / Surface & Coatings Technology 196 (2005) 211–215 [3] T. Kobayashi, T. Iwata, Y. Doi, M. Iwaki, Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 175–177 (2001) 548. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping Range of Ions in Solids, Pergamon, New York, 1977.
215
[5] T. Kobayashi, A. Nakao, M. Iwaki, Surface and Coatings Technology 158–159 (2002) 399. [6] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum, New York, 1996, p. 302.