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Ion beam analysis of ion-implanted polymer thin films
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 1027±1032
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Ion beam analysis of ion-implanted polymer thin ®lms Y.Q. Wang IT Characterization Facility, University of Minnesota, Minneapolis, MN 55455, USA
Abstract Ion implantation has been shown to signi®cantly alter the surface properties of polymers. The enhancement of these surface properties is attributed to the microstructural alteration of the polymers as a result of the ion beam treatment. To a great extent, the microstructure depends on the compositional constituents in the modi®ed polymer layers. This article uses ion beam analysis techniques to study the atomic composition and their depth pro®les for a set of carefully prepared polymer ®lms implanted with 50 keV nitrogen ions. The results are discussed along with the electronic properties as well as the ion energy deposition mechanisms in polymer ®lms. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61:72.Ww; 61:18.Bn; 73:61.Ph; 78:66.Qn Keywords: Polymer thin ®lms; Ion implantation; RBS; FReS; Electronic properties
1. Introduction In the past decade, ion beam technology has been extensively used to study and modify the surface properties of polymeric materials [1±6]. While such desired bulk properties of polymers as light weight, ¯exibility, easy processing, and good insulators do not change after the implantation, the beam treated polymer surfaces become electrically conductive, mechanically harder, and more resistive to wear and chemicals. It has been commonly observed that upon ion implantation, the covalent bonds in polymers break down, gaseous species (mainly H2 ) are formed and released. This outgassing process causes an irreversible change in E-mail address: [email protected] (Y.Q. Wang).
polymer structure and as a result, a hydrogen-depleted carbon network forms within the ion penetration range. While the full characterization of the H-depleted carbon network involves spectroscopic and microscopic evaluations, this paper attempts to study the elemental constituents and their distribution within the implanted layer using conventional ion beam analysis techniques (RBS and FReS). The results are discussed along with the electrical and optical properties and the ion beam energy loss mechanisms in polymers.
2. Experimental methods Poly(styrene-co-acrylonitrile) (PSA) was synthesized by uniformly mixing 80% styrene and 20%
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 8 9 - 1
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Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
acrylonitrile through the polymerization process, and the dissolved PSA solution was then spincoated onto glass substrates to form uniform thin ®lms. On implantation, the ®lms become conductive and patternable by conventional photolithographic processes, and are good sensing materials. Thin ®lm PSA samples used in this study are from two categories: varying-thickness samples (500± and etched samples. The varying-thick2500 A) ness samples were implanted with 50 keV 14 N 2 ions to a dose of 5 1016 ions=cm at a ®xed doserate of 0.5 lA/cm2 . The etched samples were prepared by etching down various regions on a single ®lm target from the surface using oxygen plasma thick sputtering. The ®lm was originally 5000 A and implanted with the same conditions above. All the varying-thickness samples were chosen to be thin enough that the ion beam can penetrate the after the implanentire polymer layer (<1300 A tation). The etched samples were designed to scan across the critical depth (ion implantation range in the polymer with a complimentary 1300 A) approach to the varying-thickness samples. The ®lm thickness was carefully measured by a Tencor P-10 Pro®ler before and after the implantation or etching. A signi®cant thickness loss (up to 50%) due to the implantation has been observed for ®lms with the initial thickness of 500±2500 A. Conventional Rutherford backscattering spectrometry (RBS) and forward recoil spectrometry (FReS) were used to study the compositional alteration of the ®lms as a result of the implantation. Two MeV He ion beam (2 mm in diameter) was impinged on the ®lm surface with a tilt angle of 73.5° in reference to its normal. RBS and FReS detectors were located at 170° and 33°, respectively, with a 12 lm thick Mylar placed in front of the FReS detector as a range foil. The solid angles subtended by RBS and FReS detectors were 5.0 and 1.2 msr, respectively. The energy spread of the RBS system is 20 keV, which corresponds to a on the PSA surface depth resolution of 570 A when the normal beam incident geometry is used. The energy spread of the FReS setup is about 40 keV, which corresponds to a depth resolution of on the PSA surface. 790 A Both H and C contents in the implanted PSA ®lms were determined using a known H and C
standard. This comparative method is more reliable for our thin ®lm samples than energy spectrum simulation method since the depth resolutions of both RBS (for C) and FReS (for H) are relatively poor compared with the thickness of the implanted layer in the PSA ®lms. A pristine was PSA ®lm with a known thickness of 2500 A used as our standard. Based on the molecular formula of PSA C8 H8 0:8 ± C3 H3 N0:2 n , atomic content of C and H in the pristine PSA ®lm is approximately 49.3% each, while the third element (N) is only 1.4%. The density of the ®lm was determined to be 1.12 g/cm3 by ®tting its RBS spectrum with RUMP software [7], which implies the H- and C-atomic density of 5:2 1022 3 atoms=cm each. Because of the hydrogen depletion, shrinkage and densi®cation of the polymer surface during the implantation, the density of the pristine ®lm has increased to nearly 2 g/cm3 after the implantation with 50 keV 14 N ions to a dose of 2 5 1016 ions=cm . This means that the thickness based on TRIM of the implanted layer is 1300 A Monte Carlo calculation [8]. RBS spectra were taken under two dierent beam incident geometries: normal incidence and 73.5° tilted incidence (FReS geometry). The PSA ®lms are so thin that the heavier elements in the glass substrates (mainly O, Si, Na, and Ca) are also clearly detected, which provide several plateaus for the carbon spectrum from the polymer. Under the 73.5° tilted FReS geometry, the O-signal from the substrate almost completely smears o the C-signal from the ®lm. However, if the normal beam incident geometry is used, the O-signal from the substrate contributes only a smooth plateau on which the C-signal from the ®lm is clearly isolated. By doing quadratic background subtraction, the C-peak gross counts can be found. In our measurements, the typical charge collected on each RBS spectrum (normal geometry) was 30 lC at 10 nA and the overall uncertainty in C-determination is estimated to be <5% for most of our samples. It is relevant to mention that RBS at the normal geometry and FReS at the 73.5° tilted geometry were done on dierent spots on the samples to avoid any unnecessary hydrogen loss. RBS spectra taken simultaneously with FReS at the tilted
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
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geometry are used to do charge correction in hydrogen analysis, since an accurate charge collection at a largely tilted target geometry is usually dicult. The charge collection for the FReS on each sample is typically <10 lC at 5 nA and the hydrogen loss (mobility) during the data acquisition is negligible. Although the statistical uncertainty and the beam charge measurement introduce no more than 5% error, the correction to surface H-contamination may introduce a sizable error especially for low H-containing samples (very thin ®lms). The overall uncertainty in H-determination is expected to be no more than 8% for most of our samples. 3. Results and discussions Fig. 1 shows the carbon and hydrogen spectra collected from the varying-thickness ®lms, where the carbon spectra are vertically shifted among dierent samples for clarity. The spectra from the pristine standard and a blank glass substrate are also shown in Fig. 1 for comparison. While Ccontamination on the blank glass substrate was virtually not found, the H-adsorption on the glass 2 surface was nearly 6:4 1016 atoms=cm . This amount of the surface H-contamination was assumed to exist on all our ®lms and was subsequently corrected in hydrogen calculation for the ®lms. The thickness of the pristine ®lm is noticeably reduced by the implantation, and the implanted ®lm becomes C-rich but H-depleted. Also, a slight energy shift in both C and H spectra between the pristine (1015 X cm) and the implanted (102 X cm) ®lms is attributed to the charge buildup on the insulating pristine ®lm. No doublelayer hydrogen structure is observed for this set of samples since all of them are thin enough so that after the ion implantation no pristine polymer region is left in the ®lms. Table 1 indicates that unlike the average H-concentration, the average Cconcentration remains approximately a constant with the increase of the ®lm thickness. This implies that the majority of the depleted gaseous species are hydrogen instead of hydrocarbon molecules. The increase of the average H-concentration while increasing the ®lm thickness suggests that the hy-
Fig. 1. C-RBS spectra (top) and H-FReS spectra (bottom) from the PSA pristine standard, a blank glass substrate, and varyingthickness ®lms implanted with 50 keV 14 N to a dose of 5 1016 ions=cm2 .
drogen at the implanted layer is not uniformly distributed: the deeper in the implanted region, the more hydrogen exists. Fig. 2 shows the carbon and hydrogen spectra collected from the etched samples, where the carbon spectra are vertically shifted among dierent samples for clarity. Only one of the etched samples shows a single-layer (etched thickness 1489 A) hydrogen peak (from the remaining unimplanted region) since its implant layer (thickness 1300 A) has been completely etched away. The rest show a double-layer hydrogen structure as shown in Fig. 2 (bottom). The fact that the carbon spectra in these samples do not seem to show any double-layer feature is because the carbon change in the implanted layer is not as obvious as hydrogen. The lower hydrogen region in the spectra corresponds
50.00 ± 12.54 15.19 20.10 24.41 1:30 1018 0 2:22 1017 4:55 1017 1:06 1018 1:16 1018 2500 A 1 mm 230 A 500 A 1150 A 1300 A 2500 A 1 mm 500 A 1000 A 2000 A 2500 A Pristine standard Glass substrate 500 A 1000 A 2000 A 2500 A
1:30 1018 6:38 1016 3:18 1016 8:17 1016 2:67 1017 3:74 1017
5:20 1022 ± 1:38 1022 1:63 1022 2:32 1022 2:88 1022
5:20 1022 ± 9:65 1022 9:10 1022 9:22 1022 8:92 1022
Relative hydrogen ([H]/[C] + [H]) (at.%) Average carbon concentration (atoms/cm2 ) Total carbon content (atoms/cm2 ) Average hydrogen concentration (atoms/cm2 ) Total hydrogen content (atoms/cm2 ) Thickness after implant Thickness before implant Sample type
Table 1 Hydrogen and carbon contents in varying-thickness PSA ®lms
1.12 ± 1.95 1.84 1.88 1.83
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
Film density (g/cm3 )
1030
Fig. 2. C-RBS spectra (top) and H-FReS spectra (bottom) from was imthe etched samples. Before etching, the ®lm (5000 A) planted with 50 keV 14 N to a dose of 5 1016 ions=cm2 .
to the implanted layer, whereas the higher hydrogen region corresponds to the pristine ®lm beneath the implanted layer. As listed in Table 1, H-depletion and C-rearrangement in the implanted ®lms contribute to the increased ®lm densities. As compared with the densities of three known carbon structures (diamond: 3.515 g/cm3 , graphite: 2.267 g/cm3 , and aC:H: 1.8±2.1 g/cm3 ), the ion implanted PSA ®lms may be generally described as a hydrogenated amorphous carbon. If the H and C content in a thinner ®lm is subtracted out from a thicker one, we can obtain the H and C distributions at different depths. Fig. 3 (top) shows the relative hydrogen content, [H]/([H]+[C]), as a function of the depth below the ®lm surface. It clearly indicates that hydrogen increases with increasing the depth and reaches the H-level (50%) in the pristine ®lm
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
Fig. 3. (Top) deduced H-depth pro®le within the implant layer. (Center) calculated energy loss distribution of 50 keV 14 N ions in the PSA ®lm by TRIM. (Bottom) electrical resitivity and optical gap as a function of actual ®lm thickness after the implantation.
This is consistent with the at a depth of 1200 A. as estimated by implantation range of 1300 A TRIM [8]. However, a similar subtraction method did not produce the similar H-depth pro®le from the etched samples. We attribute these irregular results due to the oxygen plasma etching process, since the H and C alteration in the implanted ®lm during the plasma sputtering is not well known and could not be considered at the calculation. The non-uniform characteristic of H-distribution within the implanted layer is directly related to the dierent kinds of damages induced by the ion beam along the ion track [9]. Fig. 3 (center) shows the calculated results of the energy loss distributions for 50 keV N ions in the PSA ®lm
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by TRIM [8]. During the implantation, the ion loses its energy primarily by the electronic ionization/excitation (75%), then by creating phonons (23%) and by producing vacancies (1%). Large energy transfer from the ion through the ioniza breaks tion in the near surface region (0±900 A) large amount of covalent bonds between C and H, produces large amount of dissociated hydrogen atoms and large carbon-based free radicals. These hydrogen atoms drift through the matrix, form hydrogen gas, and diuse out from the sample. As a result, less hydrogen is left in the near surface region. Meanwhile, the remaining carbon-based free radicals crosslink each other as the matrix cools down and eventually form a large three-dimensional (3D) carbon network near the surface. When the ion penetrates to the bottom region of the implanted layer, the electronic (900±1300 A) energy loss decreases rapidly. However, the energy loss through nuclear collisions becomes important. In other words, a signi®cant amount of the ionÕs remaining energy is used to vibrate the matrix atoms (phonons) and to dislocate the atoms (vacancies). As a result, the polymer chain is likely fragmented into many but small size carbon-based radicals. Because of the existence of the large amount of the phonons and vacancies, dissociated hydrogen atoms are likely to be trapped and thus are less able to escape from the deep region. As the matrix cools down, these hydrogen atoms can join back the surrounding free radicals and form stable fragments or molecules. As a result, the crosslinking among these fragments or molecules is unlikely to occur and thus the large 3D carbon network formed near the surface region is less likely to form in the bottom region of the implanted layer. Instead, an individually fragmented 1D structure is dominant in the region. The observed electrical and optical properties of the ion-implanted PSA ®lms also re¯ect this non-uniform characteristic of the implanted region [10]. Fig. 3 (bottom) shows the room-remperature surface resistivity and the optical gap as a function of the actual ®lm thickness after the implantation. The optical energy gap (Eg ) was obtained by ®tting the absorption data with the indirect transition 1=2 formula: a hm / hm ÿ Eg , where hm is the photon energy and a is the absorption coecient.
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Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
Both the surface resistivity and the optical gap increase with the increase of the ®lm thickness, but since the level o for a thickness above 1500 A The thickness of the implanted layer is <1500 A. results suggest that the implanted layer is not uniformly conductive and does not absorb the light uniformly. Compared with the bottom region of the implanted layer, the near-surface region is more electrically conductive and has a higher light absorption coecient. The temperature dependence of the surface resistivity measurements further suggest that the electrical conduction in thinner ®lms follows 3D hopping mechanism 1=4 q T exp T3 =T , whereas the 1D hopping 1=2 q T exp T1 =T dominates in the thick ®lm [10]. This is consistent with the 3D/1D structure of the implanted layer discussed above. In summary, RBS and FReS have been successfully used to determine the carbon and hydrogen content in ion-implanted PSA thin ®lms. Hydrogen is found to play a very important role in determining the structure of the carbon network within the implanted layer. While the depth resolutions of RBS and FReS are poor for carbon and hydrogen analysis in polymers, a carefully prepared set of varying-thickness samples allow us to obtain the carbon and hydrogen distribution within the implanted layer. Compared with the bottom region of the implanted layer, the nearsurface region is more conductive, exhibits greater optical absorption, and contains less amount of hydrogen. Electronic energy loss of ions in the polymer matrix is mainly responsible for the formation of 3D carbon network structure, the observed hydrogen depletion, the ®lm thickness loss, the enhanced electrical conductivity and the in-
creased optical absorption in ion implanted polymer ®lms. Acknowledgements The author would like to thank R.E. Giedd, M.G. Moss, and J. Kaufmann for their assistance in the earlier part of this work. The PSA ®lms were provided by Brewer Science Inc. in Rolla, Missouri. The implantation was done at Southwest Missouri State University. Part of the ion beam data acquisition was carried out at The University of Michigan. References [1] T. Venkatesan, L. Calcagno, B.S. Elman, G. Foti, in: P. Mazzoldi, G.W. Arnold (Eds.), Ion Beam Modi®cations of Insulators, Elsevier, Amsterdam, 1987 (Chapter 8). [2] J. Davenas, G. Boiteaux, Adv. Mater. 2 (11) (1990) 521. [3] L. Calcagno, G. Foti, Nucl. Instr. and Meth. B 59/60 (1991) 1153. [4] C.J. So®eld, S. Sugden, J. Ing, L.B. Bridwell, Y.Q. Wang, Vacuum 44 (1993) 285. [5] E.H. Lee, G.R. Rao, L.K. Mansur, TRIP 4 (7) (1996) 229. [6] Y.Q. Wang, L.B. Bridwell, R.E. Giedd, in: R. Arshady (Ed.), Desk Reference of Functional Polymers: Synthesis and Applications, ACS Book Publisher, 1997, pp. 371±404 (Chapters 2.5 and 2.6). [7] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, Oxford, 1985. [9] Y.Q. Wang, R.E. Giedd, L.B. Bridwell, Nucl. Instr. and Meth. B 79 (1993) 659. [10] Y.Q. Wang, R.E. Giedd, M.G. Moss, J. Kaufmann, in: J.L. Duggan, I.M. Morgan (Eds.), Application of Accelerators in Research and Industry, vol. CP392, AIP Press, New York, 1997, p. 985.
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Ion beam analysis of ion-implanted polymer thin ®lms Y.Q. Wang IT Characterization Facility, University of Minnesota, Minneapolis, MN 55455, USA
Abstract Ion implantation has been shown to signi®cantly alter the surface properties of polymers. The enhancement of these surface properties is attributed to the microstructural alteration of the polymers as a result of the ion beam treatment. To a great extent, the microstructure depends on the compositional constituents in the modi®ed polymer layers. This article uses ion beam analysis techniques to study the atomic composition and their depth pro®les for a set of carefully prepared polymer ®lms implanted with 50 keV nitrogen ions. The results are discussed along with the electronic properties as well as the ion energy deposition mechanisms in polymer ®lms. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61:72.Ww; 61:18.Bn; 73:61.Ph; 78:66.Qn Keywords: Polymer thin ®lms; Ion implantation; RBS; FReS; Electronic properties
1. Introduction In the past decade, ion beam technology has been extensively used to study and modify the surface properties of polymeric materials [1±6]. While such desired bulk properties of polymers as light weight, ¯exibility, easy processing, and good insulators do not change after the implantation, the beam treated polymer surfaces become electrically conductive, mechanically harder, and more resistive to wear and chemicals. It has been commonly observed that upon ion implantation, the covalent bonds in polymers break down, gaseous species (mainly H2 ) are formed and released. This outgassing process causes an irreversible change in E-mail address: [email protected] (Y.Q. Wang).
polymer structure and as a result, a hydrogen-depleted carbon network forms within the ion penetration range. While the full characterization of the H-depleted carbon network involves spectroscopic and microscopic evaluations, this paper attempts to study the elemental constituents and their distribution within the implanted layer using conventional ion beam analysis techniques (RBS and FReS). The results are discussed along with the electrical and optical properties and the ion beam energy loss mechanisms in polymers.
2. Experimental methods Poly(styrene-co-acrylonitrile) (PSA) was synthesized by uniformly mixing 80% styrene and 20%
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 8 9 - 1
1028
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
acrylonitrile through the polymerization process, and the dissolved PSA solution was then spincoated onto glass substrates to form uniform thin ®lms. On implantation, the ®lms become conductive and patternable by conventional photolithographic processes, and are good sensing materials. Thin ®lm PSA samples used in this study are from two categories: varying-thickness samples (500± and etched samples. The varying-thick2500 A) ness samples were implanted with 50 keV 14 N 2 ions to a dose of 5 1016 ions=cm at a ®xed doserate of 0.5 lA/cm2 . The etched samples were prepared by etching down various regions on a single ®lm target from the surface using oxygen plasma thick sputtering. The ®lm was originally 5000 A and implanted with the same conditions above. All the varying-thickness samples were chosen to be thin enough that the ion beam can penetrate the after the implanentire polymer layer (<1300 A tation). The etched samples were designed to scan across the critical depth (ion implantation range in the polymer with a complimentary 1300 A) approach to the varying-thickness samples. The ®lm thickness was carefully measured by a Tencor P-10 Pro®ler before and after the implantation or etching. A signi®cant thickness loss (up to 50%) due to the implantation has been observed for ®lms with the initial thickness of 500±2500 A. Conventional Rutherford backscattering spectrometry (RBS) and forward recoil spectrometry (FReS) were used to study the compositional alteration of the ®lms as a result of the implantation. Two MeV He ion beam (2 mm in diameter) was impinged on the ®lm surface with a tilt angle of 73.5° in reference to its normal. RBS and FReS detectors were located at 170° and 33°, respectively, with a 12 lm thick Mylar placed in front of the FReS detector as a range foil. The solid angles subtended by RBS and FReS detectors were 5.0 and 1.2 msr, respectively. The energy spread of the RBS system is 20 keV, which corresponds to a on the PSA surface depth resolution of 570 A when the normal beam incident geometry is used. The energy spread of the FReS setup is about 40 keV, which corresponds to a depth resolution of on the PSA surface. 790 A Both H and C contents in the implanted PSA ®lms were determined using a known H and C
standard. This comparative method is more reliable for our thin ®lm samples than energy spectrum simulation method since the depth resolutions of both RBS (for C) and FReS (for H) are relatively poor compared with the thickness of the implanted layer in the PSA ®lms. A pristine was PSA ®lm with a known thickness of 2500 A used as our standard. Based on the molecular formula of PSA C8 H8 0:8 ± C3 H3 N0:2 n , atomic content of C and H in the pristine PSA ®lm is approximately 49.3% each, while the third element (N) is only 1.4%. The density of the ®lm was determined to be 1.12 g/cm3 by ®tting its RBS spectrum with RUMP software [7], which implies the H- and C-atomic density of 5:2 1022 3 atoms=cm each. Because of the hydrogen depletion, shrinkage and densi®cation of the polymer surface during the implantation, the density of the pristine ®lm has increased to nearly 2 g/cm3 after the implantation with 50 keV 14 N ions to a dose of 2 5 1016 ions=cm . This means that the thickness based on TRIM of the implanted layer is 1300 A Monte Carlo calculation [8]. RBS spectra were taken under two dierent beam incident geometries: normal incidence and 73.5° tilted incidence (FReS geometry). The PSA ®lms are so thin that the heavier elements in the glass substrates (mainly O, Si, Na, and Ca) are also clearly detected, which provide several plateaus for the carbon spectrum from the polymer. Under the 73.5° tilted FReS geometry, the O-signal from the substrate almost completely smears o the C-signal from the ®lm. However, if the normal beam incident geometry is used, the O-signal from the substrate contributes only a smooth plateau on which the C-signal from the ®lm is clearly isolated. By doing quadratic background subtraction, the C-peak gross counts can be found. In our measurements, the typical charge collected on each RBS spectrum (normal geometry) was 30 lC at 10 nA and the overall uncertainty in C-determination is estimated to be <5% for most of our samples. It is relevant to mention that RBS at the normal geometry and FReS at the 73.5° tilted geometry were done on dierent spots on the samples to avoid any unnecessary hydrogen loss. RBS spectra taken simultaneously with FReS at the tilted
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
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geometry are used to do charge correction in hydrogen analysis, since an accurate charge collection at a largely tilted target geometry is usually dicult. The charge collection for the FReS on each sample is typically <10 lC at 5 nA and the hydrogen loss (mobility) during the data acquisition is negligible. Although the statistical uncertainty and the beam charge measurement introduce no more than 5% error, the correction to surface H-contamination may introduce a sizable error especially for low H-containing samples (very thin ®lms). The overall uncertainty in H-determination is expected to be no more than 8% for most of our samples. 3. Results and discussions Fig. 1 shows the carbon and hydrogen spectra collected from the varying-thickness ®lms, where the carbon spectra are vertically shifted among dierent samples for clarity. The spectra from the pristine standard and a blank glass substrate are also shown in Fig. 1 for comparison. While Ccontamination on the blank glass substrate was virtually not found, the H-adsorption on the glass 2 surface was nearly 6:4 1016 atoms=cm . This amount of the surface H-contamination was assumed to exist on all our ®lms and was subsequently corrected in hydrogen calculation for the ®lms. The thickness of the pristine ®lm is noticeably reduced by the implantation, and the implanted ®lm becomes C-rich but H-depleted. Also, a slight energy shift in both C and H spectra between the pristine (1015 X cm) and the implanted (102 X cm) ®lms is attributed to the charge buildup on the insulating pristine ®lm. No doublelayer hydrogen structure is observed for this set of samples since all of them are thin enough so that after the ion implantation no pristine polymer region is left in the ®lms. Table 1 indicates that unlike the average H-concentration, the average Cconcentration remains approximately a constant with the increase of the ®lm thickness. This implies that the majority of the depleted gaseous species are hydrogen instead of hydrocarbon molecules. The increase of the average H-concentration while increasing the ®lm thickness suggests that the hy-
Fig. 1. C-RBS spectra (top) and H-FReS spectra (bottom) from the PSA pristine standard, a blank glass substrate, and varyingthickness ®lms implanted with 50 keV 14 N to a dose of 5 1016 ions=cm2 .
drogen at the implanted layer is not uniformly distributed: the deeper in the implanted region, the more hydrogen exists. Fig. 2 shows the carbon and hydrogen spectra collected from the etched samples, where the carbon spectra are vertically shifted among dierent samples for clarity. Only one of the etched samples shows a single-layer (etched thickness 1489 A) hydrogen peak (from the remaining unimplanted region) since its implant layer (thickness 1300 A) has been completely etched away. The rest show a double-layer hydrogen structure as shown in Fig. 2 (bottom). The fact that the carbon spectra in these samples do not seem to show any double-layer feature is because the carbon change in the implanted layer is not as obvious as hydrogen. The lower hydrogen region in the spectra corresponds
50.00 ± 12.54 15.19 20.10 24.41 1:30 1018 0 2:22 1017 4:55 1017 1:06 1018 1:16 1018 2500 A 1 mm 230 A 500 A 1150 A 1300 A 2500 A 1 mm 500 A 1000 A 2000 A 2500 A Pristine standard Glass substrate 500 A 1000 A 2000 A 2500 A
1:30 1018 6:38 1016 3:18 1016 8:17 1016 2:67 1017 3:74 1017
5:20 1022 ± 1:38 1022 1:63 1022 2:32 1022 2:88 1022
5:20 1022 ± 9:65 1022 9:10 1022 9:22 1022 8:92 1022
Relative hydrogen ([H]/[C] + [H]) (at.%) Average carbon concentration (atoms/cm2 ) Total carbon content (atoms/cm2 ) Average hydrogen concentration (atoms/cm2 ) Total hydrogen content (atoms/cm2 ) Thickness after implant Thickness before implant Sample type
Table 1 Hydrogen and carbon contents in varying-thickness PSA ®lms
1.12 ± 1.95 1.84 1.88 1.83
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
Film density (g/cm3 )
1030
Fig. 2. C-RBS spectra (top) and H-FReS spectra (bottom) from was imthe etched samples. Before etching, the ®lm (5000 A) planted with 50 keV 14 N to a dose of 5 1016 ions=cm2 .
to the implanted layer, whereas the higher hydrogen region corresponds to the pristine ®lm beneath the implanted layer. As listed in Table 1, H-depletion and C-rearrangement in the implanted ®lms contribute to the increased ®lm densities. As compared with the densities of three known carbon structures (diamond: 3.515 g/cm3 , graphite: 2.267 g/cm3 , and aC:H: 1.8±2.1 g/cm3 ), the ion implanted PSA ®lms may be generally described as a hydrogenated amorphous carbon. If the H and C content in a thinner ®lm is subtracted out from a thicker one, we can obtain the H and C distributions at different depths. Fig. 3 (top) shows the relative hydrogen content, [H]/([H]+[C]), as a function of the depth below the ®lm surface. It clearly indicates that hydrogen increases with increasing the depth and reaches the H-level (50%) in the pristine ®lm
Y.Q. Wang / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 1027±1032
Fig. 3. (Top) deduced H-depth pro®le within the implant layer. (Center) calculated energy loss distribution of 50 keV 14 N ions in the PSA ®lm by TRIM. (Bottom) electrical resitivity and optical gap as a function of actual ®lm thickness after the implantation.
This is consistent with the at a depth of 1200 A. as estimated by implantation range of 1300 A TRIM [8]. However, a similar subtraction method did not produce the similar H-depth pro®le from the etched samples. We attribute these irregular results due to the oxygen plasma etching process, since the H and C alteration in the implanted ®lm during the plasma sputtering is not well known and could not be considered at the calculation. The non-uniform characteristic of H-distribution within the implanted layer is directly related to the dierent kinds of damages induced by the ion beam along the ion track [9]. Fig. 3 (center) shows the calculated results of the energy loss distributions for 50 keV N ions in the PSA ®lm
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by TRIM [8]. During the implantation, the ion loses its energy primarily by the electronic ionization/excitation (75%), then by creating phonons (23%) and by producing vacancies (1%). Large energy transfer from the ion through the ioniza breaks tion in the near surface region (0±900 A) large amount of covalent bonds between C and H, produces large amount of dissociated hydrogen atoms and large carbon-based free radicals. These hydrogen atoms drift through the matrix, form hydrogen gas, and diuse out from the sample. As a result, less hydrogen is left in the near surface region. Meanwhile, the remaining carbon-based free radicals crosslink each other as the matrix cools down and eventually form a large three-dimensional (3D) carbon network near the surface. When the ion penetrates to the bottom region of the implanted layer, the electronic (900±1300 A) energy loss decreases rapidly. However, the energy loss through nuclear collisions becomes important. In other words, a signi®cant amount of the ionÕs remaining energy is used to vibrate the matrix atoms (phonons) and to dislocate the atoms (vacancies). As a result, the polymer chain is likely fragmented into many but small size carbon-based radicals. Because of the existence of the large amount of the phonons and vacancies, dissociated hydrogen atoms are likely to be trapped and thus are less able to escape from the deep region. As the matrix cools down, these hydrogen atoms can join back the surrounding free radicals and form stable fragments or molecules. As a result, the crosslinking among these fragments or molecules is unlikely to occur and thus the large 3D carbon network formed near the surface region is less likely to form in the bottom region of the implanted layer. Instead, an individually fragmented 1D structure is dominant in the region. The observed electrical and optical properties of the ion-implanted PSA ®lms also re¯ect this non-uniform characteristic of the implanted region [10]. Fig. 3 (bottom) shows the room-remperature surface resistivity and the optical gap as a function of the actual ®lm thickness after the implantation. The optical energy gap (Eg ) was obtained by ®tting the absorption data with the indirect transition 1=2 formula: a hm / hm ÿ Eg , where hm is the photon energy and a is the absorption coecient.
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Both the surface resistivity and the optical gap increase with the increase of the ®lm thickness, but since the level o for a thickness above 1500 A The thickness of the implanted layer is <1500 A. results suggest that the implanted layer is not uniformly conductive and does not absorb the light uniformly. Compared with the bottom region of the implanted layer, the near-surface region is more electrically conductive and has a higher light absorption coecient. The temperature dependence of the surface resistivity measurements further suggest that the electrical conduction in thinner ®lms follows 3D hopping mechanism 1=4 q T exp T3 =T , whereas the 1D hopping 1=2 q T exp T1 =T dominates in the thick ®lm [10]. This is consistent with the 3D/1D structure of the implanted layer discussed above. In summary, RBS and FReS have been successfully used to determine the carbon and hydrogen content in ion-implanted PSA thin ®lms. Hydrogen is found to play a very important role in determining the structure of the carbon network within the implanted layer. While the depth resolutions of RBS and FReS are poor for carbon and hydrogen analysis in polymers, a carefully prepared set of varying-thickness samples allow us to obtain the carbon and hydrogen distribution within the implanted layer. Compared with the bottom region of the implanted layer, the nearsurface region is more conductive, exhibits greater optical absorption, and contains less amount of hydrogen. Electronic energy loss of ions in the polymer matrix is mainly responsible for the formation of 3D carbon network structure, the observed hydrogen depletion, the ®lm thickness loss, the enhanced electrical conductivity and the in-
creased optical absorption in ion implanted polymer ®lms. Acknowledgements The author would like to thank R.E. Giedd, M.G. Moss, and J. Kaufmann for their assistance in the earlier part of this work. The PSA ®lms were provided by Brewer Science Inc. in Rolla, Missouri. The implantation was done at Southwest Missouri State University. Part of the ion beam data acquisition was carried out at The University of Michigan. References [1] T. Venkatesan, L. Calcagno, B.S. Elman, G. Foti, in: P. Mazzoldi, G.W. Arnold (Eds.), Ion Beam Modi®cations of Insulators, Elsevier, Amsterdam, 1987 (Chapter 8). [2] J. Davenas, G. Boiteaux, Adv. Mater. 2 (11) (1990) 521. [3] L. Calcagno, G. Foti, Nucl. Instr. and Meth. B 59/60 (1991) 1153. [4] C.J. So®eld, S. Sugden, J. Ing, L.B. Bridwell, Y.Q. Wang, Vacuum 44 (1993) 285. [5] E.H. Lee, G.R. Rao, L.K. Mansur, TRIP 4 (7) (1996) 229. [6] Y.Q. Wang, L.B. Bridwell, R.E. Giedd, in: R. Arshady (Ed.), Desk Reference of Functional Polymers: Synthesis and Applications, ACS Book Publisher, 1997, pp. 371±404 (Chapters 2.5 and 2.6). [7] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, Oxford, 1985. [9] Y.Q. Wang, R.E. Giedd, L.B. Bridwell, Nucl. Instr. and Meth. B 79 (1993) 659. [10] Y.Q. Wang, R.E. Giedd, M.G. Moss, J. Kaufmann, in: J.L. Duggan, I.M. Morgan (Eds.), Application of Accelerators in Research and Industry, vol. CP392, AIP Press, New York, 1997, p. 985.