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Reduction in surface resistivity of polymers by plasma source ion implantation
Surface and Coatings Technology 160 (2002) 158–164
Reduction in surface resistivity of polymers by plasma source ion implantation Hyuneui Lima, Yeonhee Leeb,*, Seunghee Hanb, Youngwoo Kimb, Jeonghee Chob, Kang-jin Kima a
Division of Chemistry and Molecular Engineering, Korea University, Seoul 136-701, South Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
b
Received 18 December 2001; accepted in revised form 22 May 2002
Abstract The surface resistivity of several polymers such as poly(styreneybutadiene copolymer), modified poly(phenyleneoxide), poly(ethylene terephthalate), and polyimide was improved by the argon gas plasma source ion implantation (Ar-PSII) technique equipped with a mesh-type conducting grid. With the grid, the surface resistivities of the modified polymers decreased up to 11 orders of magnitudes at a high ion dose, and remained nearly at the same values after 3 months. The PSII treated polymer sample with the grid provided more uniformly modified surface and lower surface resistivity than that treated without the grid. The extent of the decrease in surface resistivity depended on the polymer structures and physical properties. However, the surface resistivity was independent of the sample thickness, the grid size, and the grid height. Surface analyses using scanning electron microscopy, time-of-flight secondary ion mass spectrometry, and Raman spectroscopy provided the useful information on modified surfaces. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma source ion implantation; Grid; Polymer; Surface resistivity
1. Introduction Polymers are used in various technologies for a wide range of applications because of their excellent physical and chemical properties, coupled with their low cost. However, industrial application is sometimes limited by undesirable properties of the polymer surface, for example, poor adhesion, hardness, and electrical conductivity, etc. Printability, biocompatibility and wettability of polymer surface may also be poor, which are caused mainly by the low surface free energy. These properties need to be improved by the modification of polymer surfaces without affecting the favorable bulk properties of the polymers. Surface modification techniques of polymers generally include plasma treatment, radiation treatment, and ion beam treatment w1–3x. Great interest has been devoted to the possibility of obtaining electrically conducting structures in insulating *Corresponding author. Tel.: q82-2-958-5971; fax: q82-2-9585969. E-mail address: [email protected] (Y. Lee).
films ever since conducting polymers were discovered w4x. Electrical conductivity of polymers can be achieved by incorporating dispersible conducting fillers, the deposition of conducting materials, and use of conducting polymers w5x. In addition, surface treatment by ion or electron beam has become an increasingly popular method to enhance the surface electrical conductivity of polymers over the last two decades w6–8x. However, ion and electron beam applications for industrial purposes are restricted due to their relatively high cost and their inability to treat three-dimensional samples. Recently, plasma source ion implantation (PSII) has been introduced as an alternative ion implantation method w9x. In the PSII process, a target is immersed in a plasma and pulse-biased to a high negative voltage. Ions are accelerated to the target surface and implanted into the target across the plasma sheath. PSII technique includes both plasma treatment when the pulse is off, and ion implantation treatment when the pulse is on. The general advantages of the PSII process are: (1) its uniform treatment over non-planar and large targets without
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 6 5 - 1
H. Lim et al. / Surface and Coatings Technology 160 (2002) 158–164
target manipulation; (2) it is a simple and cost-effective method; (3) it involves high dose rates; and (4) it provides intrinsic charge compensation on the surface, especially for insulators. However, a limitation for the use of PSII technique for polymers is the difficulty of applying high bias voltage to the polymer sample and attenuation of the applied voltage by the insulating sample. Thus, Matossian et al. have treated the workpiece with a grid placed several millimeters above the surface of large epoxy specimens w10x. The reduction in sheath voltage from the dielectric potential drop was avoided because the workpiece was placed on a conducting stage and covered with a transparent and metallic grid that was electrically connected to the stage. In the present study, we improved the surface electrical conductivity of polymers such as poly(styreneybutadiene copolymer) (PSyBD), poly(ethylene terephthalate) (PET), modified poly(phenyleneoxide) (MPPO), and polyimide (PI) for packaging material of semiconductor devices, using argon implantation by the PSII technique equipped with a conducting grid. The study for optimum experimental conditions using the grid and surface structures of the polymers was also performed. 2. Experimental 2.1. Materials PSyBD films of 300 mm thickness and MPPO plates of 3 mm thickness were purchased from Yulchon Chemical (Seoul, Korea). PET films of 300 mm thickness and PI films of 125 mm thickness were purchased from SKC (Seoul, Korea) and DuPont (Wilmington, DE), respectively. Each polymer sample was cut in a round shape, having a 7 cm diameter. 2.2. Instrumentation The experiment was performed using an in-house built plasma source ion implanter. A detailed description of this apparatus and the PSII modulator characteristics are presented elsewhere w11x. The polymer film was loaded on the sample stage, which was cooled by oil flowing and covered by a metallic grid (Fig. 1). The grid was woven from the wire of 30 mm–1.5 mm in diameter, in the window screen pattern of a net size of 9.7–132.3 mm2, with an open mesh preferably having at least 70% open space. And the grid having a diameter of 21 cm was electrically connected to the sample stage (⭋s14 cm) with the post. The data for polymer samples treated by various grids having diverse open space were not different. The sample stage and the grid were applied to a high negative potential up to y50 kV with 20-ms pulse width and 200-Hz frequency by the pulse modulator. The Ar plasma was generated with a pressure of 1=10y3 Torr and a r.f. power of 100 W.
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Fig. 1. Schematic diagram of PSII system used in the experiment: (1) vacuum chamber; (2) turbo molecular pump; (3) gas inlet system; (4) internal antenna; (5) r.f. matching network; (6) r.f. power supply; (7) sample stage; (8) high voltage pulse generator; (9) plasma; (10) permanent magnet; (11) single Langmuir probe; (12) ion gauge; (13) chamber ground; (14) Quadrupole Mass Analyzer; (15) lead shield; and (16) grid.
The surface resistivity was measured by a Keithly model 6517A high resistance meter. The surface resistivities of all untreated polymer samples were approximately 3=1015 Vysq. For the time-of-flight secondary ion mass spectrometry (TOF-SIMS) study, a Physical Electronics model PHI 7200 TOF-SIMSySALI instrument was used with Csq ion gun operated at 8 keV, at an ion current of 10 nA, and the operating spot size was 50 mm in diameter. The charge neutralization was used when modified polymer plates were analyzed. Raman spectra were measured in a backscattering geometry by using 514 nm light from an argon-ion laser and a JobinYvon T64000 triple Raman spectroscope. The spectral resolution was 1 cmy1 and the spectra were obtained in the range of 550–2500 cmy1. The surface morphologies were collected using a Hitachi model S-4100 microscope. 3. Results and discussion 3.1. Effect of grid on surface resistivity We modified polymer surfaces by the PSII treatment employing relatively low energy (-10 keV) to alter their surface wettabilities. There were no significant problems of arcing, charging, or voltage drop when low keV energies were applied to the thin polymer films. The depth profile results with respect to the implanted
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Fig. 2. Surface resistivities of Ar implanted PSyBD at a center area and a side area as a function of the post height (ion energy, y20 keV; ion dose, 1016 ionsycm2 and 5=1016 ionsycm2).
ions showed a good agreement with the data theoretically calculated by TRIM w12x. However, a relatively high energy ()10 keV) and large ion doses are required to improve the electrical conductivity of polymer surfaces. The treatment of polymers with a high energy revealed the arcing phenomena. And the arcing and local breakdown were more severe with increasing voltage and polymer film thickness. The large potential difference between the polymer surface and the stage is believed to make the pathway for the ion, which is implanted by applied voltage. Moreover, the conductivity of the polymer is low enough that the accumulated charge being implanted on the thick polymer surface cannot dissipate during the pulse. As shown in Fig. 1, we used a grid that was electrically connected to the stage with the conducting post in order to treat the thick polymer with high energy. Fig. 2 represents the surface resistivity of a treated PSyBD film as a function of the grid height above the polymer surface. At 20 keV ion energy, the surface resistivity of PSyBD decreased to approximately 6=106 Vysq. with a color change to dark brown. A considerable difference in the surface resistivity was observed by employing a grid regardless of ion dose. The surface resistivities of the treated samples in the presence of the grid decreased by 6–8 orders of magnitude uniformly independent of the post height. For the sample treated without the grid, the surface resistivity of the center area did not decrease after treatment at a
dose of 1=1016 ionsycm2, but decreased by 5 orders of magnitude at a dose of 5=1016 ionsycm2. And the surface resistivities of the side area were measured lower values compared with those of the center area. These results can be explained by the voltage drop and charging on the polymer surface with respect to the stage. The implanted ions follow a bending path to face the stage, especially for high-energy implantation, although the film is thin. However, the use of the grid enabled to prevent these phenomena. Because a conductive grid makes ions accelerate perpendicularly to the sample, the polymer sample yields more uniform and lower resistivity than that treated without the grid. The lower resistivity of the sample treated with the grid can be elucidated by the larger sheath size and the implantation with fully applied potential. Influence of sample thickness on the surface resistivity of MPPO plates was reported in Fig. 3. After an ion energy of 50 keV was used for 2 and 10 min, the surface resistivity was reduced to approximately 5=108 and 8=106 Vysq., respectively. The result shows that the surface resistivity of MPPO is virtually independent of the polymer thickness of 3–12 mm. Since the distance between sample surface and grid is found not to affect the surface resistivity, and the ions are accelerated by the applied potential to the grid, PSII technique equipped with a grid can be effectively used in modifying insulators regardless of their thickness.
Fig. 3. Surface resistivities of different thickness of MPPOs implanted with Ar-PSII with an energy of y50 keV.
H. Lim et al. / Surface and Coatings Technology 160 (2002) 158–164
Fig. 4. Change in surface resistivities of PET as a function of time after Ar implantation with different doses and energies.
3.2. Relation between electrical stability and ion dose and energy
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resistivity was different among polymer species at the same ion dose and energy. The surface resistivity of PET slowly decreased until an ion dose of 1=1017 ionsycm2 was reached following by rapid decrease. The surface resistivities of the other polymers were reduced almost linearly with increasing ion dose. MPPO and PSyBD shrunk and deformed after the treatment with a high ion dose and high ion energy, explaining a slightly decreased surface resistivity above 5=1016 ionsycm2 and no data at 5=1017 ionsycm2. Deformation of film invoked the saturation behavior of surface resistivity. On the other hand, the surface resistivity of PI decreased linearly according to ion dose without severe deformation and the surface resistivity of PI treated at 50 keV is lower than those of other polymers treated at 30 keV under identical ion dose. We obtained the microscopic surface images of PET, PSyBD, and PI samples to investigate the influence of surface structure on surface resistivity. Scanning electron microscopy (SEM) images of PET untreated, treated with 1=1017 ionsycm2, and treated with 5=1017 ionsy cm2 are shown in Fig. 6a–c, respectively. The PET treated with 1=1017 ionsycm2 looks like the untreated PET, whereas the PET treated with 5=1017 ionsycm2 shows severe surface damage. In case of PSyBD, surface damage was observed more severe even at a lower dose treatment than that of PET (Fig. 6d and e). The PSII treated PI showed the untreated material looking like relatively flat surface structure as shown in Fig. 6f and
Measurement for a long-term stability of the surface resistivity of implanted PET has been performed and the results are shown in Fig. 4. The surface resistivity changed rapidly within 1 day followed by a gradual increase up to 7 days, and then on the whole, the surface resistivities essentially leveled off till approximately 100 days, whereas other properties such as wettability and adhesion showed continuously large changes w13x. The beginning recovery of the reduced surface resistivity is possibly related to the extinction of long-lived free radicals and unstable dangling bonds w14x. More significant increase in the surface resistivity was observed for the samples implanted with low ion dose and low energy than those with high ion dose and high energy. It is noted that the layers with low electrical resistivity are more extensively modified and has a better long-term stability. 3.3. Surface resistivities for various polymer species The relation between surface resistivity and polymer structure was investigated. The surface resistivity decreased noticeably with increasing ion dose for the polymers studied, in agreement with other results w15,16x. In Fig. 5, the extent of decrease in the surface
Fig. 5. Surface resistivities of Ar implanted PSyBD, MPPO, PET and PI as a function of ion dose at an energy of y30, and y50 keV for PI.
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Fig. 6. SEM images of (a) PET untreated; (b) PET treated with 1=1017 ionsycm2 ; (c) PET treated with 5=1017 ionsycm2 ; (d) PSyBD untreated; (e) PSyBD treated with 1=1017 ionsycm2; (e) PI untreated; and (f) PI treated with 1=1017 ionsycm2 by Ar-PSII with an energy of y30 keV.
g. There was no relation between morphology and surface resistivity, even though surface resistivity for the samples with severe deformation could not be measured.
The electrical conductivity change in polymers irradiated by ions is still not fully understood. Ion implantation of polymers induces breaking of chemical bonds,
H. Lim et al. / Surface and Coatings Technology 160 (2002) 158–164
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Table 1 TOF-SIMS peak intensities and surface resistivities of PSyBD and PET Relative intensity (counts) CH
y 2
(myzs14)
C2H
(C2Hy yCH2y) y
(myzs25)
Surface resistivity (Vysq.)
Untreated PSyBD PSII treated PSyBD (5=1015 ionsycm2)
1.29=103 3.44=103
1.48=103 6.12=103
1.15 1.78
3=1015 2=1012
Untreated PET PSII treated PET (5=1015 ionsycm2) PSII treated PET (1=1017 ionsycm2)
6.20=102 2.85=103 1.37=104
6.00=102 4.24=103 5.48=104
0.97 1.49 3.99
3=1015 4=1011 9=109
generation of free radicals or fragments, and physical ablation. The ablated fragments and free radicals can be incorporated in the polymer surface as new chemical functional groups or as a cross-linked structure. The conducting phase has been proposed as a graphite-like material or a three-dimensional amorphous hydrogenated carbon that is composed of randomly cross-linked networks of sp, sp2 and sp3 bonds, in sometimes in a hydrogenated state w17–19x. We performed the surface characterization of the modified polymer using TOFSIMS and Raman spectroscopy. Table 1 shows the ion intensities of PSyBD and PET from the negative ion TOF-SIMS spectra of the treatment with different ion doses. Treated polymer surfaces had a tendency of increasing the ion intensity corresponding to Cy 2 (my zs24) and C2Hy (myzs25), compared with CH2y (my zs14). C2Hy ion intensity for treated polymers increased according to the ion dose for the other polymers. Fig. 7 shows the Raman spectra obtained for PSy BD untreated and treated with an ion dose of 1=1017 ionsycm2. According to literature the D band (at 1380– 1400 cmy1) roughly corresponds to amorphous structure or ‘disorder’ in material, while the G band (at 1560– 1580 cmy1) is attributed to the ordered graphite-like
structure. The position, widths and relative intensities of these two features are found to vary with the properties and structures of the carbonaceous phases w16,19x. Treated PSyBD shows the formation of broad G peak with a small intensity. For PET, we observed no new peaks and the decreasing of specific Raman peak intensities with reducing the surface resistivity. These results suggested that the pristine polymer structure was destroyed and unsaturated bonds were increased. The different resistivities of polymer irradiated with the same ion species, energy, and dose were mainly caused by the different polymer structures. Moreover, the extent of cross-linking, heat stability, mechanical properties, and molecular weight in the same polymer structure causes the difference in modification of the polymer. 4. Conclusion PSyBD, PET, MPPO and PI were modified to improve the surface electrical conductivity by Ar-PSII equipped with a grid at an energy range of 10–50 keV. With the grid, the surface resistivity at 50 keV with a dose of 5=1017 ionsycm2 was reduced by 11 orders of magnitude and remained the same level for more than 100 days. In addition, the grid resulted in more uniformly conductive surfaces. These results can be explained by the prevention of potential drop on polymer surface in the presence of a conducting grid that was at an equipotential to the sample stage. The physical properties and chemical structures of the polymers influenced the degree of conductivity. However, the surface resistivity was independent of sample thickness, grid size and grid height above the polymer film. Surface characterization using TOF-SIMS and Raman spectroscopy provided the useful information about the carbon species on the modified polymer surfaces. Acknowledgments
Fig. 7. Raman spectra of (a) PSyBD untreated and (b) PSyBD treated with 1=1017 ionsycm2.
This work was supported by the Ministry of Science and Technology (MOST) and the Korea Science and Engineering Foundation (No.: R04-2000-00016).
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L.J. Gerenser, J. Adhes. Sci. Technol. 1 (1987) 303. ¨ A. Hollander, J. Behnisch, Surf. Coat. Technol. 98 (1988) 855. H. Dong, T. Bell, Surf. Coat. Technol. 111 (1999) 29. J. Mort, Science 208 (1980) 819. M. Narkis, A. Ram, Z. Stein, J. Appl. Poly. Sci. 25 (1980) 1515. T. Venkatesan, S.R. Forrest, M.L. Kaplan, C.A. Murray, P.H. Schmidt, B.J. Wilkens, J. Appl. Phys. 54 (1983) 3150. ¨ K.-H. Bather, U. Herrmann, Surf. Coat. Technol. 74–75 (1995) 670. K. Dworecki, T. Hasegawa, K. Sudlitz, S. Wasik, Nucl. Instr. Meth. B 166–167 (2000) 646. J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. J.N. Matossian, R.W. Schumacher, D.M. Pepper, US patent, US 5,374,456, Hughes, Aircraft Company, Los Angeles, CA, 1994.
w11x S. Han, Y. Lee, H. Kim, J. Lee, J. Yoon, G. Kim, Surf. Coat. Technol. 93 (1997) 261. w12x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, vol. 1, Pergamon Press, Oxford, 1985. w13x H. Lim, Y. Lee, S. Han, J. Cho, K.-J. Kim, J. Vac. Sci. Technol. A 19 (2001) 1490. w14x B. Wasserman, Phys. Rev. B 34 (1986) 1296. w15x B.J. Bachman, M.J. Vasile, J. Vac. Sci. Technol. A 7 (1989) 2709. w16x A. De Bonis, A. Bearzotti, G. Marletta, Nucl. Instr. Meth. B 151 (1999) 101. w17x G.R. Rao, K. Monar, E.H. Lee, J.R. Treglio, Surf. Coat. Technol. 64 (1994) 69. w18x Y.Q. Wang, R.E. Giedd, M.G. Moss, J. Kaufmann, Nucl. Instr. Meth. B 127–128 (1997) 710. w19x T. Venkatesan, S.R. Forrest, M.L. Kaplan, et al., J. Appl. Phys. 56 (1984) 2778.
Reduction in surface resistivity of polymers by plasma source ion implantation Hyuneui Lima, Yeonhee Leeb,*, Seunghee Hanb, Youngwoo Kimb, Jeonghee Chob, Kang-jin Kima a
Division of Chemistry and Molecular Engineering, Korea University, Seoul 136-701, South Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
b
Received 18 December 2001; accepted in revised form 22 May 2002
Abstract The surface resistivity of several polymers such as poly(styreneybutadiene copolymer), modified poly(phenyleneoxide), poly(ethylene terephthalate), and polyimide was improved by the argon gas plasma source ion implantation (Ar-PSII) technique equipped with a mesh-type conducting grid. With the grid, the surface resistivities of the modified polymers decreased up to 11 orders of magnitudes at a high ion dose, and remained nearly at the same values after 3 months. The PSII treated polymer sample with the grid provided more uniformly modified surface and lower surface resistivity than that treated without the grid. The extent of the decrease in surface resistivity depended on the polymer structures and physical properties. However, the surface resistivity was independent of the sample thickness, the grid size, and the grid height. Surface analyses using scanning electron microscopy, time-of-flight secondary ion mass spectrometry, and Raman spectroscopy provided the useful information on modified surfaces. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma source ion implantation; Grid; Polymer; Surface resistivity
1. Introduction Polymers are used in various technologies for a wide range of applications because of their excellent physical and chemical properties, coupled with their low cost. However, industrial application is sometimes limited by undesirable properties of the polymer surface, for example, poor adhesion, hardness, and electrical conductivity, etc. Printability, biocompatibility and wettability of polymer surface may also be poor, which are caused mainly by the low surface free energy. These properties need to be improved by the modification of polymer surfaces without affecting the favorable bulk properties of the polymers. Surface modification techniques of polymers generally include plasma treatment, radiation treatment, and ion beam treatment w1–3x. Great interest has been devoted to the possibility of obtaining electrically conducting structures in insulating *Corresponding author. Tel.: q82-2-958-5971; fax: q82-2-9585969. E-mail address: [email protected] (Y. Lee).
films ever since conducting polymers were discovered w4x. Electrical conductivity of polymers can be achieved by incorporating dispersible conducting fillers, the deposition of conducting materials, and use of conducting polymers w5x. In addition, surface treatment by ion or electron beam has become an increasingly popular method to enhance the surface electrical conductivity of polymers over the last two decades w6–8x. However, ion and electron beam applications for industrial purposes are restricted due to their relatively high cost and their inability to treat three-dimensional samples. Recently, plasma source ion implantation (PSII) has been introduced as an alternative ion implantation method w9x. In the PSII process, a target is immersed in a plasma and pulse-biased to a high negative voltage. Ions are accelerated to the target surface and implanted into the target across the plasma sheath. PSII technique includes both plasma treatment when the pulse is off, and ion implantation treatment when the pulse is on. The general advantages of the PSII process are: (1) its uniform treatment over non-planar and large targets without
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 3 6 5 - 1
H. Lim et al. / Surface and Coatings Technology 160 (2002) 158–164
target manipulation; (2) it is a simple and cost-effective method; (3) it involves high dose rates; and (4) it provides intrinsic charge compensation on the surface, especially for insulators. However, a limitation for the use of PSII technique for polymers is the difficulty of applying high bias voltage to the polymer sample and attenuation of the applied voltage by the insulating sample. Thus, Matossian et al. have treated the workpiece with a grid placed several millimeters above the surface of large epoxy specimens w10x. The reduction in sheath voltage from the dielectric potential drop was avoided because the workpiece was placed on a conducting stage and covered with a transparent and metallic grid that was electrically connected to the stage. In the present study, we improved the surface electrical conductivity of polymers such as poly(styreneybutadiene copolymer) (PSyBD), poly(ethylene terephthalate) (PET), modified poly(phenyleneoxide) (MPPO), and polyimide (PI) for packaging material of semiconductor devices, using argon implantation by the PSII technique equipped with a conducting grid. The study for optimum experimental conditions using the grid and surface structures of the polymers was also performed. 2. Experimental 2.1. Materials PSyBD films of 300 mm thickness and MPPO plates of 3 mm thickness were purchased from Yulchon Chemical (Seoul, Korea). PET films of 300 mm thickness and PI films of 125 mm thickness were purchased from SKC (Seoul, Korea) and DuPont (Wilmington, DE), respectively. Each polymer sample was cut in a round shape, having a 7 cm diameter. 2.2. Instrumentation The experiment was performed using an in-house built plasma source ion implanter. A detailed description of this apparatus and the PSII modulator characteristics are presented elsewhere w11x. The polymer film was loaded on the sample stage, which was cooled by oil flowing and covered by a metallic grid (Fig. 1). The grid was woven from the wire of 30 mm–1.5 mm in diameter, in the window screen pattern of a net size of 9.7–132.3 mm2, with an open mesh preferably having at least 70% open space. And the grid having a diameter of 21 cm was electrically connected to the sample stage (⭋s14 cm) with the post. The data for polymer samples treated by various grids having diverse open space were not different. The sample stage and the grid were applied to a high negative potential up to y50 kV with 20-ms pulse width and 200-Hz frequency by the pulse modulator. The Ar plasma was generated with a pressure of 1=10y3 Torr and a r.f. power of 100 W.
159
Fig. 1. Schematic diagram of PSII system used in the experiment: (1) vacuum chamber; (2) turbo molecular pump; (3) gas inlet system; (4) internal antenna; (5) r.f. matching network; (6) r.f. power supply; (7) sample stage; (8) high voltage pulse generator; (9) plasma; (10) permanent magnet; (11) single Langmuir probe; (12) ion gauge; (13) chamber ground; (14) Quadrupole Mass Analyzer; (15) lead shield; and (16) grid.
The surface resistivity was measured by a Keithly model 6517A high resistance meter. The surface resistivities of all untreated polymer samples were approximately 3=1015 Vysq. For the time-of-flight secondary ion mass spectrometry (TOF-SIMS) study, a Physical Electronics model PHI 7200 TOF-SIMSySALI instrument was used with Csq ion gun operated at 8 keV, at an ion current of 10 nA, and the operating spot size was 50 mm in diameter. The charge neutralization was used when modified polymer plates were analyzed. Raman spectra were measured in a backscattering geometry by using 514 nm light from an argon-ion laser and a JobinYvon T64000 triple Raman spectroscope. The spectral resolution was 1 cmy1 and the spectra were obtained in the range of 550–2500 cmy1. The surface morphologies were collected using a Hitachi model S-4100 microscope. 3. Results and discussion 3.1. Effect of grid on surface resistivity We modified polymer surfaces by the PSII treatment employing relatively low energy (-10 keV) to alter their surface wettabilities. There were no significant problems of arcing, charging, or voltage drop when low keV energies were applied to the thin polymer films. The depth profile results with respect to the implanted
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Fig. 2. Surface resistivities of Ar implanted PSyBD at a center area and a side area as a function of the post height (ion energy, y20 keV; ion dose, 1016 ionsycm2 and 5=1016 ionsycm2).
ions showed a good agreement with the data theoretically calculated by TRIM w12x. However, a relatively high energy ()10 keV) and large ion doses are required to improve the electrical conductivity of polymer surfaces. The treatment of polymers with a high energy revealed the arcing phenomena. And the arcing and local breakdown were more severe with increasing voltage and polymer film thickness. The large potential difference between the polymer surface and the stage is believed to make the pathway for the ion, which is implanted by applied voltage. Moreover, the conductivity of the polymer is low enough that the accumulated charge being implanted on the thick polymer surface cannot dissipate during the pulse. As shown in Fig. 1, we used a grid that was electrically connected to the stage with the conducting post in order to treat the thick polymer with high energy. Fig. 2 represents the surface resistivity of a treated PSyBD film as a function of the grid height above the polymer surface. At 20 keV ion energy, the surface resistivity of PSyBD decreased to approximately 6=106 Vysq. with a color change to dark brown. A considerable difference in the surface resistivity was observed by employing a grid regardless of ion dose. The surface resistivities of the treated samples in the presence of the grid decreased by 6–8 orders of magnitude uniformly independent of the post height. For the sample treated without the grid, the surface resistivity of the center area did not decrease after treatment at a
dose of 1=1016 ionsycm2, but decreased by 5 orders of magnitude at a dose of 5=1016 ionsycm2. And the surface resistivities of the side area were measured lower values compared with those of the center area. These results can be explained by the voltage drop and charging on the polymer surface with respect to the stage. The implanted ions follow a bending path to face the stage, especially for high-energy implantation, although the film is thin. However, the use of the grid enabled to prevent these phenomena. Because a conductive grid makes ions accelerate perpendicularly to the sample, the polymer sample yields more uniform and lower resistivity than that treated without the grid. The lower resistivity of the sample treated with the grid can be elucidated by the larger sheath size and the implantation with fully applied potential. Influence of sample thickness on the surface resistivity of MPPO plates was reported in Fig. 3. After an ion energy of 50 keV was used for 2 and 10 min, the surface resistivity was reduced to approximately 5=108 and 8=106 Vysq., respectively. The result shows that the surface resistivity of MPPO is virtually independent of the polymer thickness of 3–12 mm. Since the distance between sample surface and grid is found not to affect the surface resistivity, and the ions are accelerated by the applied potential to the grid, PSII technique equipped with a grid can be effectively used in modifying insulators regardless of their thickness.
Fig. 3. Surface resistivities of different thickness of MPPOs implanted with Ar-PSII with an energy of y50 keV.
H. Lim et al. / Surface and Coatings Technology 160 (2002) 158–164
Fig. 4. Change in surface resistivities of PET as a function of time after Ar implantation with different doses and energies.
3.2. Relation between electrical stability and ion dose and energy
161
resistivity was different among polymer species at the same ion dose and energy. The surface resistivity of PET slowly decreased until an ion dose of 1=1017 ionsycm2 was reached following by rapid decrease. The surface resistivities of the other polymers were reduced almost linearly with increasing ion dose. MPPO and PSyBD shrunk and deformed after the treatment with a high ion dose and high ion energy, explaining a slightly decreased surface resistivity above 5=1016 ionsycm2 and no data at 5=1017 ionsycm2. Deformation of film invoked the saturation behavior of surface resistivity. On the other hand, the surface resistivity of PI decreased linearly according to ion dose without severe deformation and the surface resistivity of PI treated at 50 keV is lower than those of other polymers treated at 30 keV under identical ion dose. We obtained the microscopic surface images of PET, PSyBD, and PI samples to investigate the influence of surface structure on surface resistivity. Scanning electron microscopy (SEM) images of PET untreated, treated with 1=1017 ionsycm2, and treated with 5=1017 ionsy cm2 are shown in Fig. 6a–c, respectively. The PET treated with 1=1017 ionsycm2 looks like the untreated PET, whereas the PET treated with 5=1017 ionsycm2 shows severe surface damage. In case of PSyBD, surface damage was observed more severe even at a lower dose treatment than that of PET (Fig. 6d and e). The PSII treated PI showed the untreated material looking like relatively flat surface structure as shown in Fig. 6f and
Measurement for a long-term stability of the surface resistivity of implanted PET has been performed and the results are shown in Fig. 4. The surface resistivity changed rapidly within 1 day followed by a gradual increase up to 7 days, and then on the whole, the surface resistivities essentially leveled off till approximately 100 days, whereas other properties such as wettability and adhesion showed continuously large changes w13x. The beginning recovery of the reduced surface resistivity is possibly related to the extinction of long-lived free radicals and unstable dangling bonds w14x. More significant increase in the surface resistivity was observed for the samples implanted with low ion dose and low energy than those with high ion dose and high energy. It is noted that the layers with low electrical resistivity are more extensively modified and has a better long-term stability. 3.3. Surface resistivities for various polymer species The relation between surface resistivity and polymer structure was investigated. The surface resistivity decreased noticeably with increasing ion dose for the polymers studied, in agreement with other results w15,16x. In Fig. 5, the extent of decrease in the surface
Fig. 5. Surface resistivities of Ar implanted PSyBD, MPPO, PET and PI as a function of ion dose at an energy of y30, and y50 keV for PI.
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Fig. 6. SEM images of (a) PET untreated; (b) PET treated with 1=1017 ionsycm2 ; (c) PET treated with 5=1017 ionsycm2 ; (d) PSyBD untreated; (e) PSyBD treated with 1=1017 ionsycm2; (e) PI untreated; and (f) PI treated with 1=1017 ionsycm2 by Ar-PSII with an energy of y30 keV.
g. There was no relation between morphology and surface resistivity, even though surface resistivity for the samples with severe deformation could not be measured.
The electrical conductivity change in polymers irradiated by ions is still not fully understood. Ion implantation of polymers induces breaking of chemical bonds,
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Table 1 TOF-SIMS peak intensities and surface resistivities of PSyBD and PET Relative intensity (counts) CH
y 2
(myzs14)
C2H
(C2Hy yCH2y) y
(myzs25)
Surface resistivity (Vysq.)
Untreated PSyBD PSII treated PSyBD (5=1015 ionsycm2)
1.29=103 3.44=103
1.48=103 6.12=103
1.15 1.78
3=1015 2=1012
Untreated PET PSII treated PET (5=1015 ionsycm2) PSII treated PET (1=1017 ionsycm2)
6.20=102 2.85=103 1.37=104
6.00=102 4.24=103 5.48=104
0.97 1.49 3.99
3=1015 4=1011 9=109
generation of free radicals or fragments, and physical ablation. The ablated fragments and free radicals can be incorporated in the polymer surface as new chemical functional groups or as a cross-linked structure. The conducting phase has been proposed as a graphite-like material or a three-dimensional amorphous hydrogenated carbon that is composed of randomly cross-linked networks of sp, sp2 and sp3 bonds, in sometimes in a hydrogenated state w17–19x. We performed the surface characterization of the modified polymer using TOFSIMS and Raman spectroscopy. Table 1 shows the ion intensities of PSyBD and PET from the negative ion TOF-SIMS spectra of the treatment with different ion doses. Treated polymer surfaces had a tendency of increasing the ion intensity corresponding to Cy 2 (my zs24) and C2Hy (myzs25), compared with CH2y (my zs14). C2Hy ion intensity for treated polymers increased according to the ion dose for the other polymers. Fig. 7 shows the Raman spectra obtained for PSy BD untreated and treated with an ion dose of 1=1017 ionsycm2. According to literature the D band (at 1380– 1400 cmy1) roughly corresponds to amorphous structure or ‘disorder’ in material, while the G band (at 1560– 1580 cmy1) is attributed to the ordered graphite-like
structure. The position, widths and relative intensities of these two features are found to vary with the properties and structures of the carbonaceous phases w16,19x. Treated PSyBD shows the formation of broad G peak with a small intensity. For PET, we observed no new peaks and the decreasing of specific Raman peak intensities with reducing the surface resistivity. These results suggested that the pristine polymer structure was destroyed and unsaturated bonds were increased. The different resistivities of polymer irradiated with the same ion species, energy, and dose were mainly caused by the different polymer structures. Moreover, the extent of cross-linking, heat stability, mechanical properties, and molecular weight in the same polymer structure causes the difference in modification of the polymer. 4. Conclusion PSyBD, PET, MPPO and PI were modified to improve the surface electrical conductivity by Ar-PSII equipped with a grid at an energy range of 10–50 keV. With the grid, the surface resistivity at 50 keV with a dose of 5=1017 ionsycm2 was reduced by 11 orders of magnitude and remained the same level for more than 100 days. In addition, the grid resulted in more uniformly conductive surfaces. These results can be explained by the prevention of potential drop on polymer surface in the presence of a conducting grid that was at an equipotential to the sample stage. The physical properties and chemical structures of the polymers influenced the degree of conductivity. However, the surface resistivity was independent of sample thickness, grid size and grid height above the polymer film. Surface characterization using TOF-SIMS and Raman spectroscopy provided the useful information about the carbon species on the modified polymer surfaces. Acknowledgments
Fig. 7. Raman spectra of (a) PSyBD untreated and (b) PSyBD treated with 1=1017 ionsycm2.
This work was supported by the Ministry of Science and Technology (MOST) and the Korea Science and Engineering Foundation (No.: R04-2000-00016).
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