Molecular Weight Evaluation of Poly(dimethylsiloxane) on Solid Surfaces Using Silver Deposition/TOF-SIMS

Transcription

1 ANALYTICAL SCIENCES DECEMBER 2004, VOL The Japan Society for Analytical Chemistry 1623 The Best Paper in Bunseki Kagaku, 2003 Molecular Weight Evaluation of Poly(dimethylsiloxane) on Solid Surfaces Using Silver Deposition/TOF-SIMS Masae INOUE, Atsushi MURASE, and Motoyasu SUGIURA Toyota Central R & D Labs., Inc., Nagakute, Aichi , Japan Molecular ions include information about end groups, functional groups and molecular weight. A method for the direct detection of these in the high mass range (m/z > 1000) from poly(dimethylsiloxane) (PDMS) on a solid surface was investigated. It was found that a TOF-SIMS analysis of silver-deposited surfaces (silver deposition/tof-sims) is useful for this purpose. Using the silver-deposition/tof-sims method, silver-cationized quasi-molecular ions were clearly detected from PDMS on solid surfaces, and their structure and molecular weight were evaluated. In addition, their images were observed without the interference of deposited silver. By applying to the analysis of paint defects etc., it was confirmed that this technique is useful to analyze actual industrial materials. Silver-cationized ions were detected not only from PDMS, but also from other organic materials, such as lubricant additives and oils on solid surfaces. Therefore, the silver deposition/tof-sims method was proved to be useful for the analysis of ultrathin substances on solid surfaces. Introduction Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the most powerful techniques for the analysis of ultrathin substances on material surfaces, owing to its surface sensitivity and selectivity. Thus, it has been widely used for solving problems in actual industrial materials, such as surface contamination on semiconductor wafers, a very small amount of foreign substance on electronic components, etc. 1,2 Poly- (dimethylsiloxane) (PDMS) often causes these problems, and analytical methods capable of determining the molecular weights and structures of PDMS are required to solve these problems. Unfortunately, in the case of conventional TOF- SIMS analysis of PDMS on material surfaces, molecular ions in the high-mass range (m/z > 1000), which include information about end groups, functional groups and molecular weights, cannot be detected, though fragment ions can be easily detected. Therefore, while conventional TOF-SIMS analysis is capable of detecting the PDMS causing these problems, it remains incapable of providing a more specific determination of the causes. For high-molecular-weight organic materials, whose molecular ions cannot be detected, it showed that silvercationized quasi-molecular ions corresponding to (M+Ag) + can be detected from thin polymer films deposited on a silver substrate in 1980s. 3 After that, many studies concerning the structure of polymers have been conducted with this technique However, it is difficult to apply it to the analysis of ultrathin substances and substances on uneven substrates, because of the need to transfer the substances to a silver To whom correspondence should be addressed. masae@mosk.tytlabs.co.jp This is an English edition of the paper which won the Best Paper Award in Bunseki Kagaku, 2003 [Bunseki Kagaku, 2003, 52(11), 979]. substrate. It is also difficult to observe the ion images of the substances on the sample. In an attempt to directly detect silver-cationized ions on polymer surfaces, a method using a TEM grid to apply silver patterning to a sample surface was proposed. 24,25 However, material in certain areas was undetectable, and no ion images of the material could be observed, because no silver was present on the marking area of the TEM grid, and a thick silver layer (about 150 nm) covered the substances in other areas. Recently, Delcorte et al. proposed a method capable of detecting gold-cationized ions in the high mass range from high-molecular-weight polymers, such as polyethylene and polypropylene via gold metallization of the surfaces. 26 They showed that using their technique, gold-cationized quasimolecular ions could be directly detected from a thin polymer film, and that their ion images could be observed. The authors have attempted to directly detect silver-cationized ions from PDMS on a solid surface by using silver metallization, which can be expected to have the greatest cationization effect. 27 It was found that TOF-SIMS analysis of silver-deposited sample surfaces (silver deposition/tof-sims) is useful for this purpose. This paper describes the utility of this method and its applications. Experimental Reagents Six types of silicone oil with different end groups, functional groups and molecular weights were used: Silicone 1: PDMS (number average molecular weight: 3800), silicone 2: PDMS (number average molecular weight: 28000), silicone 3: protonterminated PDMS, silicone 4: vinyl-terminated PDMS, silicone 5: polyhydromethylsiloxane, silicone 6: polymethylphenylsiloxane. They were dissolved in toluene at concentrations ranging from 0.02 to 0.3 mg/ml. One microliter of the solutions was deposited on a silicon substrate of approximately 50 mm 2, and the solvent was allowed to evaporate.

2 1624 ANALYTICAL SCIENCES DECEMBER 2004, VOL. 20 Fig. 1 TOF-SIMS spectra of silicone 1. (a) Sample on a silicon substrate; (b) silver-deposited sample using the diode sputtering method; (c) silver-deposited sample using the vacuum evaporation coating method; (d) sample on a silver substrate. Fig. 2 Secondary ion images of silver ion and silver-cationized ions obtained from silicone 4 (upper). 200 µm 2 field of view. Silver deposition/tof-sims spectrum of silicone 4 (lower). Silver deposition Two methods of silver deposition, the diode sputtering method and the vacuum evaporation coating method, were tried. The former method was performed using an Eico Engineering IB-3 ion sputter device. The target current was 3 ma, the argon pressure was 10 Pa and the accelerating voltage was 1.4 kv. The distance between the silver target and sample was approximately 60 mm, and various silver coating times ranging from 0.5 to 60 s were used. The latter method was performed using a JEOL JEE-5B vacuum evaporation coating device. The pressure was Pa and the silver deposition rate ranged from approximately 27 to 210 nmol/cm 2 (corresponding to thicknesses of 3 20 nm). Measurement by TOF-SIMS TOF-SIMS measurements were performed using a Physical Electronics TFS-2100 (TRIFT II) instrument. Mass spectra were acquired using bunched 69 Ga + ion pulses with an impact energy of 15 kev, an ion current of 600 pa (DC), a pulse width of 13 ns and a total collection time of 5 min from the raster area of 100 µm 100 µm. The total ion doses in these measurements were approximately ions/cm 2 under a static limit. Ion images were acquired using a total collection time of 30 min from the raster area of 200 µm 200 µm. The total ion doses in these measurements were approximately ions/cm 2 under a static limit. Results and Discussion Silver deposition conditions With the silver deposition/tof-sims method, there is concern that the sample may suffer damage due to the direct deposition of silver on the sample surface. If damage occurs on the sample surface, it can be expected that either no silvercationized ions will be detected, or that the molecular weight distribution of the silver-cationized ions will differ from that of the sample deposited on the silver substrate. This section describes the silver deposition conditions that did not cause damage to the sample surfaces. Fig. 3 AFM image of the silver deposited surface using the vacuum evaporation coating method. Diode sputtering method This method is more convenient than the vacuum evaporation coating method, because it does not require as high a vacuum. The deposition time required is about 5 min, about one tenth that of the vacuum evaporation coating method. With deposition at room temperature (about 20 C), neither silver-cationized ions nor fragment ions were detected. Optical microscope observations of the silver-deposited sample surfaces revealed that the surfaces were covered with a thin film, which was identified as being SiO 2 by reflection infrared spectroscopy. It was considered that the SiO 2 film was generated by the thermal oxidation of PDMS; in other words, the sample surfaces suffered damage by silver deposition under the conditions used. It was impossible to control this phenomenon by changing the silver deposition conditions, such as the target current, coating time and accelerating voltage. Therefore, an attempt was made to cool the samples during silver deposition so as to prevent the occurrence of damage on the surface due to thermal oxidation. Samples deposited on silicon substrates were cooled during silver deposition by placing them on an aluminum block that had been cooled to about 5 C. As a result of cooling the samples, the silver-cationized ions were detected from a sample, and it was found that cooling the samples was an effective way to prevent damage due to thermal oxidation. However, silvercationized ions were not detected from all of the samples. It was suggested that this temperature was not sufficient to prevent damage. Therefore, it was considered to be necessary to cool the sample to a temperature lower than 5 C in order to prevent damage.

3 ANALYTICAL SCIENCES DECEMBER 2004, VOL Fig. 4 Silver deposition/tof-sims spectra of six kinds of silicone. The number in parentheses is the number average molecular weight. (a) Silicone 1 (PDMS); (b) silicone 2 (PDMS); (c) silicone 3 (proton-terminated PDMS); (d) silicone 4 (vinyl-terminated PDMS); (e) silicone 5 (polyhydromethylsiloxane); (f) silicone 6 (polymethylphenylsiloxane). An aluminum block had been cooled to about 20 C, and then the samples were cooled during silver deposition by placing them on the aluminum block. One of the silver deposition/tof-sims spectra obtained is shown in Fig. 1(b). Silver-cationized ions were detected from all of the samples with silver deposited, and the molecular weight distributions were similar to that of the sample on the silver substrate (Fig. 1(d)). This result indicates that cooling the samples to around 20 C during silver deposition can essentially prevent thermal damage to the sample surfaces. Vacuum evaporation coating method It can be expected that sample surfaces will suffer less damage by using this method than that by using the diode sputtering method, because of the low kinetic energy applied, which is about 1/100 that of the diode sputtering method. The silver deposition/tof-sims spectrum obtained using this method is shown in Fig. 1(c). Silver-cationized ions were clearly detected from all of the samples with silver deposited at room temperature (about 20 C), and the molecular weight distributions were similar to that of the sample on the silver substrate (Fig. 1(d)). The correspondence between these spectra indicates that the damage of the sample surfaces suffered by this method was negligible. From this perspective, the vacuum evaporation coating method possesses an advantage over the diode sputtering method. In addition, by cooling the sample to around 20 C during silver deposition, it is possible to apply this method to substances that have a low boiling point, and are easily damaged by thermal oxidation. The vacuum evaporation coating method was used for subsequent experiments. Effects on ion images The secondary ion images of silver ion and silver-cationized ions obtained from the sample deposited silver at a rate of approximately 50 nmol/cm 2 are shown in Fig. 2. The image of the silver-cationized ions showed no interference from the deposited silver. The reason why there are hardly any silver or silver-cationized ions visible in the bottom portion of these images is attributable to the fact that the PDMS was greater than the monolayer thickness in the center of the droplet, and thus the deposited silver sank completely into the PDMS. An AFM (atomic force microscope) image of the silverdeposited surface is shown in Fig. 3. The deposited silver had the appearance of an island of about several dozen nanometers, a size sufficiently smaller than the lateral resolution of TOF- SIMS (about 0.2 µm). Therefore, it was considered that the image of the silver-cationized ions showed no interference from the deposited silver. Molecular weight evaluation and silicone structure The spectra of six types of silicone oil deposited silver at a rate of approximately 50 nmol/cm 2 are shown in Fig. 4. Among them, two spectra (silicones 1 and 2) in the mass range m/z are shown in Fig. 5. By using the silver deposition/tof- SIMS method, two types of silver-cationized ions, silvercationized linear fragments (a) and silver-cationized cyclic fragments (F), were detected from all of the samples. Silvercationized linear fragments have a formula identical to that of the molecular ions and silver-cationized cyclic fragments represent a series corresponding to an integral number of repeat units. With these silver-cationized fragments, we attempted to determine the structure and the molecular weight. The

4 1626 ANALYTICAL SCIENCES DECEMBER 2004, VOL. 20 Fig. 5 Silver deposition/tof-sims spectra of silicone 1 and silicone 2 (upper). The number in parentheses is the number average molecular weight. The structures of linear fragment (lower left) and cyclic fragment (lower right). a, Linear fragment; F, cyclic fragment. Fig. 6 An example for the analysis of paint defect. a, Linear fragment of PDMS; F, cyclic fragment of PDMS. (a) TOF-SIMS spectrum of the paint defect; (b) silver deposition/tof-sims spectrum of the paint defect; (c) silver deposition/tof-sims spectrum of PDMS, which is the cause of the paint defect. functional groups could be determined from the intervals of these fragments, which corresponded to the repeat units. As shown in Fig. 5, the interval of the fragments of PDMS (silicones 1 4) was m/z = 74, which corresponded to repeat units of PDMS ( Si(CH 3) 2 O ). In the case of polyhydromethylsiloxane (silicone 5), the interval of the fragments was m/z = 60, which corresponded to Si(H)CH 3 O, and in the case of polymethylphenylsiloxane (silicone 6), the interval of the fragments was m/z = 136, which corresponded to Si(C 6H 5)CH 3 O. The end groups could be determined based on the silver-cationized linear fragments, which had an integral number of repeat units plus two end groups. Thus, the structure of six types of silicone oils could be determined. Dong et al. studied the fragmentation of various types of PDMS deposited on silver substrates, and demonstrated that the relative intensity of the silver-cationized cyclic fragments increased as the molecular weight increased. 28,29 In the case of low-molecular-weight PDMS (silicone 1), the relative intensity of the linear fragments (a) was high, whereas in the case of high-molecular-weight PDMS (silicone 2), the relative intensity of the cyclic fragments (F) was high. These results agree with the results of PDMS deposited on silver substrates. With our method, it was found that the molecular weight could be evaluated by comparing the relative intensities of these fragments as well as the conventional method of using a silver substrate. We conclude that the molecular weight and structure of ultrathin silicone oil on material surfaces can be evaluated by using the silver deposition/tof-sims method. Analysis of paint defects In the automotive painting process, contaminants, such as PDMS, fluorine oil, etc. on paint surfaces cause paint defects and the specific determination of contaminants is very important to solve these problems. With conventional TOF-SIMS method, however, it is difficult to solve these problems, because the molecular ions of PDMS, fluorine oil, etc. could not be detected and the structure of contaminant cannot be specifically determined. Therefore, we applied the silver deposition/tof- SIMS method to determine the cause of a paint defect. To make a paint defect sample, PDMS dissolved in toluene was sprayed on an aluminum plate and polyester-melamine resin was painted on the plate. Then, the paint plate was baked at 140 C for 30 min. The spectra are shown in Fig. 6. Silver-cationized ions of PDMS that caused the paint defect were detected on the paint defect surface, and the structure and the relative molecular weight were evaluated. It was confirmed that this technique is useful for the analysis of actual industrial problems. Segregation of PDMS on silicon substrate Silicone 4 deposited onto a silicon substrate was analyzed using the silver deposition/tof-sims method. Figure 7 shows the ion images of the two types of silver-cationized fragments and the mass spectra extracted from the selected area, which are indicated in the image. It was found that the ratio of silvercationized linear fragments to silver-cationized cyclic fragments varied with the measured area, and that the cyclic fragments were concentrated outside of the circle, and the linear fragments were distributed toward the center of the circle. It was considered that the cyclic fragments came from a highermolecular-weight PDMS in the sample, and the linear fragments came from a lower-molecular-weight PDMS. Therefore, the ion images indicate that PDMS with different molecular weights was segregated when the solvent evaporated from the silicon substrate. Analysis of other organic materials The silver deposition/tof-sims method was applied to detection of other organic materials, such as certain types of lubricant additives (a succinimide-type dispersant). Silvercationized quasi-molecular ions were detected on solid surfaces (Fig. 8) and their structure was determined. Silver-cationized quasi-molecular ions of oil, which was the solvent for the additive, were detected as well. Consequently, it was

5 ANALYTICAL SCIENCES DECEMBER 2004, VOL Fig. 8 TOF-SIMS spectra of lubricant additive (succinimide type dispersant). (a) Sample on a silicon substrate; (b) silver deposited sample using the vacuum evaporation coating method. Fig. 7 Secondary ion images of silver-cationized linear fragment (a) and silver-cationized cyclic fragment (F) obtained from silicone 4 (upper). 200 µm 2 field of view. Silver deposition/tof-sims spectra extracted from selected area (a) and (b) indicated in the image (lower). determined that silver-cationized ions were detected not only from PDMS, but also from other organic materials, such as certain types of lubricant additives on solid surfaces. Therefore, the silver deposition/tof-sims method was proved to be useful for the analysis of ultrathin substances on solid surfaces. Conclusions A method for the direct detection of silver-cationized quasimolecular ions from high-molecular-weight substances on solid surfaces was investigated, and it was found that the silver deposition/tof-sims method was useful for this purpose. With this method, images of the substances were obtained without any interference from the deposited silver. Based on the silver-cationized ions detected by this method, it was possible to determine the structure of the PDMS and to evaluate its molecular weight. By applying this method to the analysis of paint defect, the evaluation of lateral distribution of PDMS on silicon substrate, etc., it was confirmed that this method is useful to analyze actual industrial materials. Silver-cationized quasi-molecular ions were detected not only from PDMS, but from other organic materials on solid surfaces as well. Therefore, silver deposition/tof-sims method was proved to be useful for the analysis of ultrathin substances on solid surfaces. Acknowledgements The authors would like to thank the members of the Environmental Analysis Lab. and Materials Analysis Lab. of Toyota Central R & D Labs., Inc., for their useful discussions and advice. References 1. A. Hattori, J. Non-Crystalline Solids, 1997, 218, G. G. Goodman, P. M. Lindley, and L. A. McCaig, Proc. Institute of Environmental Sciences and Technology, 1999, I. V. Bletsos, D. M. Hercules, D. Greifendorf, and A. Benninghoven, Anal. Chem., 1985, 57, I. V. Bletsos, D. M. Hercules, D. van Leyen, and A. Benninghoven, Macromolecules, 1987, 20, I. V. Bletsos, D. M. Hercules, J. H. Magill, D. van Leyen, E. Niehuis, and A. Benninghoven, Anal. Chem., 1988, 60, J. Lub, D. van Leyen, and A. Benninghoven, Polym. Commun., 1989, 30, D. van Leyen, B. Hagenhoff, E. Niehuis, A. Benninghoven, I. V. Bletsos, and D. M. Hercules, J. Vac. Sci. Technol., 1989, 7, I. V. Bletsos, D. M. Hercules, D. van Leyen, A. Benninghoven, C. G. Karakatsanis, and J. N. Rieck, Anal. Chem., 1989, 61, J. Lub and A. Benninghoven, Org. Mass Spectrom., 1989, 24, L. O Toole, R. D. Short, F. A. Bottino, A. Pollicino, and A. Recca, Polym. Degrad. Stabil., 1992, 38, M. P. Chiarelli, A. Proctor, I. V. Bletsos, D. M. Hercules, H. Feld, A. Leute, and A. Benninghoven, Macromolecules, 1992, 25, J. A. Treverton, A. J. Paul, and J. C. Vickerman, Surf. Interface Anal., 1993, 20, P. A. Zimmerman, D. M. Hercules, and A. Benninghoven, Anal. Chem., 1993, 65, H. van der Wel and J. Lub, Surf. Interface Anal., 1993, 20, P. A. Zimmerman and D. M. Hercules, Appl. Spectrosc., 1994, 48, M. A. Peters, A. M. Belu, R. W. Linton, L. Dupray, T. J. Meyer, and J. M. DeSimone, J. Am. Chem. Soc., 1995, 117, L. R. H. Cohen, D. M. Hercules, C. G. Karakatsanis, and J. N. Rieck, Macromolecules, 1995, 28, K. Xu, A. Proctor, and D. M. Hercules, Int. J. Mass Spectrom., 1995, 143, K. Endo, N. Kobayashi, M. Aida, and T. Hoshi, Polym. J.,

6 1628 ANALYTICAL SCIENCES DECEMBER 2004, VOL , 28, A. Delcorte, B. G. Segda, and P. Bertrand, Surf. Sci., 1997, 381, M. Deimel, H. Rulle, V. Liebing, and A. Benninghoven, Appl. Surf. Sci., 1998, 134, S. S. Reddy, X. Dong, R. Murgasova, A. I. Gusev, and D. M. Hercules, Macromolecules, 1999, 32, A. Delcorte, I. Wojeciechowski, X. Gonze, B. J. Garrison, and P. Bertrand, Int. J. Mass Spectrom., 2002, 214, R. W. Linton, M. P. Mawn, A. M. Belu, J. M. DeSimone, M. O. Hunt, Jr., Y. Z. Menceloglu, H. G. Cramer, and A. Benninghoven, Surf. Interface Anal., 1993, 20, A. M. Belu, M. O. Hunt, Jr., and R. W. Linton, Surf. Sci. Spectra, 1998, 4, A. Delcorte, N. Medard, and P. Bertrand, Anal. Chem., 2002, 74, B. Hagenhoff, TOF-SIMS, Surface Analysis by Mass Spectrometry, ed. J. C. Vickerman and D. Briggs, 2001, IM Publications and Surface Spectra, Manchester, X. Dong, A. Proctor, and D. M. Hercules, Macromolecules, 1997, 30, X. Dong, A. Gusev, and D. M. Hercules, J. Am. Soc. Mass Spectrom., 1998, 9, 292.