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Evaluation of secondary ion yield enhancement from polymer material by using TOF-SIMS equipped with a gold cluster ion source
Applied Surface Science 252 (2006) 6547–6549 www.elsevier.com/locate/apsusc
Evaluation of secondary ion yield enhancement from polymer material by using TOF-SIMS equipped with a gold cluster ion source K. Aimoto a,*, S. Aoyagi b, N. Kato a, N. Iida c, A. Yamamoto c, M. Kudo a a
Department of Applied Physics, Faculty of Engineering, Seikei University, 3-3-1 Kichijioji-Kitamachi, Musashino-shi, Tokyo 180-8633, Japan b Department of Regional Development, Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu-cho, Matsue-shi, Shimane 690-8504, Japan c ULVAC-PHI, Inc., 370 Enzo, Chigasaki, Kanagawa 253-0084, Japan Available online 27 April 2006
Abstract We investigated the enhancement of the secondary ion intensity in the TOF-SIMS spectra obtained by Au+ and Au3+ bombardment in comparison with Ga+ excitation using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, the advantages of the gold cluster primary ion source over monoatomic ion could accurately be evaluated. It was observed that the degree of fragmentation decreased by the usage of cluster primary ion beam compared with monoatomic ion beam, which was observed as a shift of the intensity distribution in the spectra. It was also found out that the mass effect of Au+ and Ga+ as monoatomic primary ion, resulted in about 10–60 times of enhancement for both samples with different molecular distributions. On the other hand, the Au3+ bombardment caused intensity enhancement about 100–2600 compared with Ga+ bombardment, depending on the mass range of the detected secondary ion species. The cluster primary ion effect of Au3+, compared with Au+, therefore, was estimated to be about 10–45. # 2006 Published by Elsevier B.V. Keywords: TOF-SIMS; Gold cluster ion; Intensity enhancement; Polyethylene glycol; Fragmentation; Molecular weight distribution
1. Introduction TOF-SIMS instruments, equipped with monoatomic primary ion sources (such as Ga+ and Cs+), have been applied to the analysis of the uppermost monolayer of the surface with high sensitivity and high lateral resolution. However, secondary ions emitted from violently damaged solid surfaces by highenergy primary ion bombardment sometimes bring about difficulties in the analysis of the chemical information at the surface. Recently, as an effective approach for the solution of this problem, monoatomic primary ion guns of the TOF-SIMS instruments have been successfully replaced by such cluster primary ion guns as with SF5+, Au3+, Bi3+ and C60+ [1–3]. Due to the decrease in the impact energy per atom which composes the primary ion, decrease in damage on the surface of the sample can be expected by using cluster ion beams. It is also expected that cluster ion bombardment may cause the increase
* Corresponding author. Tel.: +81 422 37 3784; fax: +81 422 37 3871. E-mail address: [email protected] (K. Aimoto). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2006.02.098
in the surface energy density, which leads to the increase in the secondary ion yields. In this study, we investigated the secondary ion intensity enhancement obtained by Au+ and Au3+ compared with Ga+ bombardment using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, it is expected that the evaluation of intensity enhancement can be possible with a high precision over a wide mass range. We especially paid attention to the analysis of (1) the cluster ion effect on the fragmentation which appears in the mass spectra, (2) the mass dependence of the monoatomic primary ion on the secondary ion intensity, i.e., Au+ versus Ga+ and (3) the cluster ion effect on the secondary ion intensity and its mass dependence of the detected secondary ions. 2. Experimental As polymer samples, we used polyethylene glycol (Koso Chemical Co., Ltd., Tokyo, Japan), PEG, with different average molecular weight (400 and 1000, denoted as ‘‘PEG400’’ and
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K. Aimoto et al. / Applied Surface Science 252 (2006) 6547–6549
‘‘PEG1000’’ in this text). PEG samples were diluted (1/1000) with toluene and cast by spin coating method onto a Si substrate (cleaned by ultrasonication in acetone). Thus prepared samples were thick enough and no signals from the substrate could be observed. Therefore, the samples could be regarded as bulk polymer samples. The apparatuses used are a TFS2100 for Ga+ ion bombardment and a TRIFT-III for Au+ and Au3+ ion bombardment. The primary ion beam conditions (energy, ion doze) were as follows; Ga+, operated at 17 keV and Au+ and Au3+, operated at 19 keV, and the same ion doze of 3.3 1011 ions/cm2 onto the sample area of 180 mm2 was chosen both for Ga+ and Au+ bombardment, and 3.3 1010 ions/cm2 for Au3+ bombardment. 3. Results and discussion Fig. 1a–c shows the mass spectra of PEG400, at a mass range from 250 to 750, by using each primary ion source. The spectra by Au3+ bombardment were obtained with a primary ion doze of one-tenth compared with other ion sources. Quasi-molecular ion peaks (such as [M + H]+, [M + Na]+, [M + K]+; M being the entire molecule) are observed in these spectra. The peaks at 437, 481, 525, 569 and 613 are identified as sodium cationized oligomer peaks with n numbers from 9 to 13. It can be presumed that these peak intensity distributions reflected the total mass distribution of the PEG400 sample. As can be seen clearly from the intensity scale of the ordinate, the considerable intensity enhancement by Au+ and Au3+ was observed compared with Ga+ bombardment. In order to compare the intensity enhancement, all the intensities were normalized with primary ion doze.
Fig. 1d–f shows the mass spectra of PEG1000, at a mass range from 300 to 1500, using by each primary ion source. Similar quasi-molecule ion peaks were detected from PEG1000 as were observed from PEG400. As is seen clearly in Fig. 1f, the intensity distribution which reflects the molecular weight distribution could be seen by using the Au3+ cluster primary ion beam. When Au+ primary ion beam was used, however, the peak of the molecular weight distribution was barely observed and in the case of Ga+ primary ion beam, the intensity distribution could hardly be recognized (Fig. 1d and e). These results can be attributed to the intensity enhancement and suppression of fragmentation caused by Au+ and Au3+. Since the Na cationized oligomer peaks were more intense than protonated peaks, we evaluated each secondary ion intensity of the [M + Na]+ peak and could observe the intensity enhancement by Au+ and Au3+ primary ion beams. In order to evaluate the intensity enhancement caused by the increase of the mass of the atomic primary ion (Au+ versus Ga+), comparison of the intensity of the series of the Na cationized oligomer peaks was carried out. As the lower curve in Fig. 2a the ratios of each secondary ion intensity by using Au+ primary ion beam is shown compared with Ga+ primary ion excitation for PEG400 sample. The abscissa in this figure means the number of the repeat unit n of the cationized oligomer peaks. It is evident from this figure that Au+ enhances the secondary ion intensities by ca. 8–9, compared with Ga+ bombardment. This result agrees well with the well-known fact that the higher mass peaks are formed in significantly higher yields when higher mass ion beam was used for excitation [4]. In Fig. 2a, the intensity enhancement from 100 to 200 by using Au3+ primary ion beams are also shown, comparing with
Fig. 1. TOF-SIMS spectra from PEG400 (a–c) and PEG1000 (b–f). Quasi-molecular ion peaks were detected (such as [M + H]+, [M + Na]+, [M + K]+; M being the entire molecule. m/z = 437, 481, 525, 569, 613, 657, 701, 745, 789, 833, 877, 922, 966, 1010, 1054, 1098, 1141 and 1186 are identified as [M + Na]+ peaks with n numbers from 9 to 26).
K. Aimoto et al. / Applied Surface Science 252 (2006) 6547–6549
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Fig. 2. Intensity ratio of each Na cationized secondary ion peak obtained by Au+, Au3+ and Ga+ bombardment for PEG400 (a and c) and PEG1000 (b and d). The number of the abscissa represents the number of the repeat units of the PEG samples. In order to evaluate the intensity enhancement caused by the increase of the mass of the atomic primary ion (Au+ vs. Ga+) and by the cluster ion effect (Au3+ vs. Au+), comparison of the intensity of the series of the [M + Na]+ peaks was carried out.
the Ga+ primary ion excitation for PEG400. It is also seen in this figure that the intensity enhancement by Au+ shows almost no dependence on the mass of the detected secondary ion species, although the enhancement by Au3+ showed a slight increase with the mass of the secondary ions. Fig. 2b shows the ratios of each secondary ion intensity by using Au+ and Au3+ primary ion beams compared with Ga+ primary ion excitation for the PEG1000 sample. Similar results are seen with enhancement by ca. 10–60 and 100–2600, respectively, however, the intensity enhancement showed a clear mass dependence, which is different from the PEG400 sample. In order to illustrate the distinct cluster ion beam effect and its mass dependence of the secondary ion species, the ratio of each secondary ion intensity measured by Au+ and Au3+ primary ion beams are shown in Fig. 2c and d. It is evident from these figures that the cluster effect of Au3+ gives rise to the intensity enhancement about 20 over Au+ monoatomic primary ion beam, in the case of PEG400 sample. No clear tendency of mass dependence was observed for the case of PEG400. Similar results on the intensity enhancement by cluster primary ion beam were seen for the case of PEG1000, with an enhancement value from 13 to 45, with a distinct mass dependence of the secondary ion species. The reason the mass dependence of the secondary ion intensity enhancement of PEG400 differs from that of PEG1000 is unclear at this moment. However, if the intensity of the observed peaks from each oligomer is assumed to have some contribution from several molecules with higher mass
via removal of some numbers of repeat units, it is reasonable to regard that the TOF-SIMS spectra from polymer samples do not correspond to the exact molecular distributions of the samples but are influenced by the scission process of the polymers. This means that the observed spectra only reflect the molecular distribution of the sample, and are influenced by such primary ion beam conditions as ion species, acceleration energy and so on. This fact leads to the different degrees of intensity enhancement for different polymer samples with different molecular distributions, as is the case of data processing in the present study. 4. Conclusion It was found out that Au primary ion source for TOF-SIMS instrument has advantages in such terms of (1) the mass effect of the monoatomic primary ion, i.e., Au+ versus Ga+; (2) the cluster primary ion effect of Au3+ compared with Au+ and Ga+; and (3) decrease in the degree of fragmentation by the usage of cluster primary ion beam compared with monoatomic ion beam. References [1] F. Kollmer, Appl. Surf. Sci. 231/232 (2004) 153–158. [2] G. Gillen, A. Fahey, Appl. Surf. Sci. 203/204 (2003) 209–213. [3] N. Davies, D.E. Weidel, P. Blenkinsopp, N. Lockyer, R. Hill, J.C. Vickerman, Appl. Surf. Sci. 203/204 (2003) 223–227. [4] D. Briggs, E.J. Hearn, Int. J. Mass Spectrom. Ion Proc. 67 (1985) 47.
Evaluation of secondary ion yield enhancement from polymer material by using TOF-SIMS equipped with a gold cluster ion source K. Aimoto a,*, S. Aoyagi b, N. Kato a, N. Iida c, A. Yamamoto c, M. Kudo a a
Department of Applied Physics, Faculty of Engineering, Seikei University, 3-3-1 Kichijioji-Kitamachi, Musashino-shi, Tokyo 180-8633, Japan b Department of Regional Development, Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu-cho, Matsue-shi, Shimane 690-8504, Japan c ULVAC-PHI, Inc., 370 Enzo, Chigasaki, Kanagawa 253-0084, Japan Available online 27 April 2006
Abstract We investigated the enhancement of the secondary ion intensity in the TOF-SIMS spectra obtained by Au+ and Au3+ bombardment in comparison with Ga+ excitation using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, the advantages of the gold cluster primary ion source over monoatomic ion could accurately be evaluated. It was observed that the degree of fragmentation decreased by the usage of cluster primary ion beam compared with monoatomic ion beam, which was observed as a shift of the intensity distribution in the spectra. It was also found out that the mass effect of Au+ and Ga+ as monoatomic primary ion, resulted in about 10–60 times of enhancement for both samples with different molecular distributions. On the other hand, the Au3+ bombardment caused intensity enhancement about 100–2600 compared with Ga+ bombardment, depending on the mass range of the detected secondary ion species. The cluster primary ion effect of Au3+, compared with Au+, therefore, was estimated to be about 10–45. # 2006 Published by Elsevier B.V. Keywords: TOF-SIMS; Gold cluster ion; Intensity enhancement; Polyethylene glycol; Fragmentation; Molecular weight distribution
1. Introduction TOF-SIMS instruments, equipped with monoatomic primary ion sources (such as Ga+ and Cs+), have been applied to the analysis of the uppermost monolayer of the surface with high sensitivity and high lateral resolution. However, secondary ions emitted from violently damaged solid surfaces by highenergy primary ion bombardment sometimes bring about difficulties in the analysis of the chemical information at the surface. Recently, as an effective approach for the solution of this problem, monoatomic primary ion guns of the TOF-SIMS instruments have been successfully replaced by such cluster primary ion guns as with SF5+, Au3+, Bi3+ and C60+ [1–3]. Due to the decrease in the impact energy per atom which composes the primary ion, decrease in damage on the surface of the sample can be expected by using cluster ion beams. It is also expected that cluster ion bombardment may cause the increase
* Corresponding author. Tel.: +81 422 37 3784; fax: +81 422 37 3871. E-mail address: [email protected] (K. Aimoto). 0169-4332/$ – see front matter # 2006 Published by Elsevier B.V. doi:10.1016/j.apsusc.2006.02.098
in the surface energy density, which leads to the increase in the secondary ion yields. In this study, we investigated the secondary ion intensity enhancement obtained by Au+ and Au3+ compared with Ga+ bombardment using polymer samples with different molecular weight distributions. Since the polymer samples used in this experiment have a wide molecular weight distribution, it is expected that the evaluation of intensity enhancement can be possible with a high precision over a wide mass range. We especially paid attention to the analysis of (1) the cluster ion effect on the fragmentation which appears in the mass spectra, (2) the mass dependence of the monoatomic primary ion on the secondary ion intensity, i.e., Au+ versus Ga+ and (3) the cluster ion effect on the secondary ion intensity and its mass dependence of the detected secondary ions. 2. Experimental As polymer samples, we used polyethylene glycol (Koso Chemical Co., Ltd., Tokyo, Japan), PEG, with different average molecular weight (400 and 1000, denoted as ‘‘PEG400’’ and
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K. Aimoto et al. / Applied Surface Science 252 (2006) 6547–6549
‘‘PEG1000’’ in this text). PEG samples were diluted (1/1000) with toluene and cast by spin coating method onto a Si substrate (cleaned by ultrasonication in acetone). Thus prepared samples were thick enough and no signals from the substrate could be observed. Therefore, the samples could be regarded as bulk polymer samples. The apparatuses used are a TFS2100 for Ga+ ion bombardment and a TRIFT-III for Au+ and Au3+ ion bombardment. The primary ion beam conditions (energy, ion doze) were as follows; Ga+, operated at 17 keV and Au+ and Au3+, operated at 19 keV, and the same ion doze of 3.3 1011 ions/cm2 onto the sample area of 180 mm2 was chosen both for Ga+ and Au+ bombardment, and 3.3 1010 ions/cm2 for Au3+ bombardment. 3. Results and discussion Fig. 1a–c shows the mass spectra of PEG400, at a mass range from 250 to 750, by using each primary ion source. The spectra by Au3+ bombardment were obtained with a primary ion doze of one-tenth compared with other ion sources. Quasi-molecular ion peaks (such as [M + H]+, [M + Na]+, [M + K]+; M being the entire molecule) are observed in these spectra. The peaks at 437, 481, 525, 569 and 613 are identified as sodium cationized oligomer peaks with n numbers from 9 to 13. It can be presumed that these peak intensity distributions reflected the total mass distribution of the PEG400 sample. As can be seen clearly from the intensity scale of the ordinate, the considerable intensity enhancement by Au+ and Au3+ was observed compared with Ga+ bombardment. In order to compare the intensity enhancement, all the intensities were normalized with primary ion doze.
Fig. 1d–f shows the mass spectra of PEG1000, at a mass range from 300 to 1500, using by each primary ion source. Similar quasi-molecule ion peaks were detected from PEG1000 as were observed from PEG400. As is seen clearly in Fig. 1f, the intensity distribution which reflects the molecular weight distribution could be seen by using the Au3+ cluster primary ion beam. When Au+ primary ion beam was used, however, the peak of the molecular weight distribution was barely observed and in the case of Ga+ primary ion beam, the intensity distribution could hardly be recognized (Fig. 1d and e). These results can be attributed to the intensity enhancement and suppression of fragmentation caused by Au+ and Au3+. Since the Na cationized oligomer peaks were more intense than protonated peaks, we evaluated each secondary ion intensity of the [M + Na]+ peak and could observe the intensity enhancement by Au+ and Au3+ primary ion beams. In order to evaluate the intensity enhancement caused by the increase of the mass of the atomic primary ion (Au+ versus Ga+), comparison of the intensity of the series of the Na cationized oligomer peaks was carried out. As the lower curve in Fig. 2a the ratios of each secondary ion intensity by using Au+ primary ion beam is shown compared with Ga+ primary ion excitation for PEG400 sample. The abscissa in this figure means the number of the repeat unit n of the cationized oligomer peaks. It is evident from this figure that Au+ enhances the secondary ion intensities by ca. 8–9, compared with Ga+ bombardment. This result agrees well with the well-known fact that the higher mass peaks are formed in significantly higher yields when higher mass ion beam was used for excitation [4]. In Fig. 2a, the intensity enhancement from 100 to 200 by using Au3+ primary ion beams are also shown, comparing with
Fig. 1. TOF-SIMS spectra from PEG400 (a–c) and PEG1000 (b–f). Quasi-molecular ion peaks were detected (such as [M + H]+, [M + Na]+, [M + K]+; M being the entire molecule. m/z = 437, 481, 525, 569, 613, 657, 701, 745, 789, 833, 877, 922, 966, 1010, 1054, 1098, 1141 and 1186 are identified as [M + Na]+ peaks with n numbers from 9 to 26).
K. Aimoto et al. / Applied Surface Science 252 (2006) 6547–6549
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Fig. 2. Intensity ratio of each Na cationized secondary ion peak obtained by Au+, Au3+ and Ga+ bombardment for PEG400 (a and c) and PEG1000 (b and d). The number of the abscissa represents the number of the repeat units of the PEG samples. In order to evaluate the intensity enhancement caused by the increase of the mass of the atomic primary ion (Au+ vs. Ga+) and by the cluster ion effect (Au3+ vs. Au+), comparison of the intensity of the series of the [M + Na]+ peaks was carried out.
the Ga+ primary ion excitation for PEG400. It is also seen in this figure that the intensity enhancement by Au+ shows almost no dependence on the mass of the detected secondary ion species, although the enhancement by Au3+ showed a slight increase with the mass of the secondary ions. Fig. 2b shows the ratios of each secondary ion intensity by using Au+ and Au3+ primary ion beams compared with Ga+ primary ion excitation for the PEG1000 sample. Similar results are seen with enhancement by ca. 10–60 and 100–2600, respectively, however, the intensity enhancement showed a clear mass dependence, which is different from the PEG400 sample. In order to illustrate the distinct cluster ion beam effect and its mass dependence of the secondary ion species, the ratio of each secondary ion intensity measured by Au+ and Au3+ primary ion beams are shown in Fig. 2c and d. It is evident from these figures that the cluster effect of Au3+ gives rise to the intensity enhancement about 20 over Au+ monoatomic primary ion beam, in the case of PEG400 sample. No clear tendency of mass dependence was observed for the case of PEG400. Similar results on the intensity enhancement by cluster primary ion beam were seen for the case of PEG1000, with an enhancement value from 13 to 45, with a distinct mass dependence of the secondary ion species. The reason the mass dependence of the secondary ion intensity enhancement of PEG400 differs from that of PEG1000 is unclear at this moment. However, if the intensity of the observed peaks from each oligomer is assumed to have some contribution from several molecules with higher mass
via removal of some numbers of repeat units, it is reasonable to regard that the TOF-SIMS spectra from polymer samples do not correspond to the exact molecular distributions of the samples but are influenced by the scission process of the polymers. This means that the observed spectra only reflect the molecular distribution of the sample, and are influenced by such primary ion beam conditions as ion species, acceleration energy and so on. This fact leads to the different degrees of intensity enhancement for different polymer samples with different molecular distributions, as is the case of data processing in the present study. 4. Conclusion It was found out that Au primary ion source for TOF-SIMS instrument has advantages in such terms of (1) the mass effect of the monoatomic primary ion, i.e., Au+ versus Ga+; (2) the cluster primary ion effect of Au3+ compared with Au+ and Ga+; and (3) decrease in the degree of fragmentation by the usage of cluster primary ion beam compared with monoatomic ion beam. References [1] F. Kollmer, Appl. Surf. Sci. 231/232 (2004) 153–158. [2] G. Gillen, A. Fahey, Appl. Surf. Sci. 203/204 (2003) 209–213. [3] N. Davies, D.E. Weidel, P. Blenkinsopp, N. Lockyer, R. Hill, J.C. Vickerman, Appl. Surf. Sci. 203/204 (2003) 223–227. [4] D. Briggs, E.J. Hearn, Int. J. Mass Spectrom. Ion Proc. 67 (1985) 47.