- Email: [email protected]
Deposition of highly oriented Teflon thin films by synchrotron radiation etching
Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247–253 www.elsevier.nl / locate / elspec
Deposition of highly oriented Teflon thin films by synchrotron radiation etching Y. Zhang*, T. Katoh, A. Endo Sumitomo Heavy Industries Ltd, 2 -1 -1 Yatocho, Nishitokyo, Tokyo 185 -8585, Japan Received 15 December 2000; received in revised form 5 March 2001; accepted 25 March 2001
Abstract Synchrotron radiation was used for deposition of highly oriented PTFE (poly tetrafluoroethylene; Teflon) thin films. In contrast with films deposited by the laser ablation, where polymer chains are parallel to the substrate surface, the films deposited by the synchrotron radiation etching have a different orientation where the polymer chains are oriented perpendicular to the substrate surface. By the mass analyses, we found that perfluoro-n-alkanes are generated as additional photo-fragments in the synchrotron radiation etching, which may play an important role for its specific orientation. 2001 Elsevier Science B.V. All rights reserved. Keywords: Teflon film; Synchrotron radiation etching; Oriented film
1. Introduction Synchrotron radiation (SR) has offered a powerful tool for studying electronic states of materials not only on surface but also in bulk [1]. On the application point of view, it has been considered as a promising source for proximity lithography in fabrication of the future electronic devices [2]. The SR induced chemical etching may also find its application in semiconductor processing [3]. We have found that, like laser ablation, the SR beam can be used to etch Teflon (polytetrafluoroethylene, PTFE) directly without using any chemicals, being a tool for direct micromachining [4] (see Fig. 1, for example). Meanwhile, the photo-fragments generated upon the SR *Corresponding author. Tel.: 181-424-68-4474; fax: 181-424-684477. E-mail address: ypz [email protected] (Y. Zhang). ]
etching in vacuum can be used to form thin films, offering a simple and versatile method for fabrication of thin films [5]. In this paper, we review our study of the PTFE thin films deposited by the SR etching.
2. Experimental The main components of our experimental setup consist of a synchrotron light source and an ultrahigh vacuum chamber with a base pressure of 10 28 mbar. The synchrotron radiation source was ‘AUROLA-II’, built by Sumitomo Heavy Industries, Ltd., which is one of the home-made compact electron storage rings (see Fig. 2). The ring was operated with electron energy of 700 MeV, stored current of 500 mA and emittance of 528 p nm rad., providing the radiation with the critical wavelength of 1.4–1.5 nm. The SR beam irradiated the target surface with the
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 01 )00300-0
248
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 1. SEM photograph of the Teflon microstructures made by the SR etching in vacuum.
photon flux of 2310 16 –1310 18 photons s 21 cm 22 . To compare with other techniques like pyrolysis and laser ablation, PTFE films were also deposited either by heating a Teflon target directly to 5508C or by ablating the target with a home-made ultrafast laser (266 nm, 10 ps, 100 Hz, more than 1 J cm 22 ). A commercial Teflon sheet of 0.5–1.5 mm in thickness was set on a target holder, while a Si(100) substrate was set on a sample holder, facing against the Teflon target. Both the target and substrate were cleaned with organic solvent before installing in the chamber. Both of them could be electrically heated by hot plates attached backsides and their temperatures (T t for the target and T s for the substrate) were measured in situ with thermocouples. Thickness of the deposited films was monitored with a quartz crystal microbalance and its distribution was measured with ellipsometry at 675 nm.
After deposition, chemical composition and crystalline features of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and 2u X-ray diffraction (XRD) (Cu Ka radiation of 8040 eV), respectively, and the surface morphology was observed with a scanning electron microscope (SEM). To obtain information of molecular weight of the deposited and starting materials, melting temperatures T m were measured with the differential scanning calorimetry (DSC) (PERKIN-ELMER, DSC-7). Molecular orientation of the deposited thin films was monitored by the Fourier transfer infrared (FTIR) spectroscopy in the range of 400–3000 cm 21 . To diagnose gaseous photo-fragments evolved upon deposition, the quadrupole mass spectrometry (QMS, INFICON-100, electronic beam energy of 110 eV) and time of flight (TOF, home-made) were in situ carried out for neutral and ionic species, respectively.
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
249
Fig. 2. As the synchrotron light source, a home-made compact warm ring AURORA-2S (E5700 MeV, c51.4 nm and I5500 mA) installed with self radiation-shielding at Sumitomo Heavy Industries.
3. Results SR etching, laser ablation and direct heating of the Teflon target in vacuum yielded gaseous fragments, which deposited on the Si(100) substrate as thick as 0.1–1.5 mm. The depositing rate was 7.5 nm s 21 at T t 5RT (i.e. room temperature) and 20 nm s 21 at T t 52008C, respectively, in the SR case, while it was much lower in the laser ablation case [6] where the target temperature appears to have no effect. Fig. 3 shows the SEM photos of typical surface morphology of the PTFE films deposited on the Si substrates at T s 5RT by the SR-etching (left top), by the laserablation (left bottom) and by the pyrolysis (right top). An increase of the substrate temperature causes
a change in the surface morphology in the pyrolysis (see Fig. 3, right bottom, for T s 52008C) and laser ablation case [6], but not in the SR case. A smooth film surface was obtained at any substrate temperature in the SR case, while the film surface given in the laser case was smoothed out by increasing the substrate temperature in an Ar atmosphere [6]. The surface in the pyrolytic case always has some structures [7], depending on the substrate temperature. For the deposition by the SR etching, the XRD and DSC analyses indicated that the thin films are crystalline but generally have small-size grains, and the XPS and FTIR analyses showed that the films contain mainly CF 2 and a small amount of CF 3 [5].
250
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 3. SEM photographs of typical surface morphology of the Teflon thin films deposited on Si(100) substrates at T s 5RT (i.e. room temperature) by SR etching (left top), laser ablation (left bottom) and pyrolysis (right top). The increase in the substrate temperature shows the effect in the pyrolysis (right bottom, for T s 52008C) or laser case [6], but not in the SR case. The surfaces were directly observed without coating.
All the analyses have confirmed that the deposited films are indeed the crystalline PTFE films. It is difficult to directly evaluate the molecular weight of the insoluble Teflon, but the DSC analysis may give
indirect evidence. The melting temperature measured with DSC was T53206108C for the SR case, similar to T53238C given in the laser case [8]. It is close to T53278C for the Teflon powder with a nominal
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
weight of 8500, but higher than T52628C for the powder with a nominal weight of 1500 and far higher than T51928C for C 24 F 50 [9]. It is known that the PTFE thin films deposited by rubbing its bulk material against a smooth substrate are well oriented, having its molecular axes aligned along the rubbing direction [10]. In the laser ablation case, the molecular axes have been found to be parallel to the substrate surface too, but not in one direction [8]. The molecular orientation of the deposited films can be well determined by the FTIR spectroscopy [11]. Fig. 4 shows the FTIR spectra of the PTFE films deposited by the laser ablation (top trace) and SR etching (bottom trace), respectively. It
251
is known that the E 1 (4) and E 1 (3) bands at 556 and 1156 cm 21 , respectively, have the transition moment perpendicular to the molecular axis (called perpendicular bands), while the A 2 (3) and A 2 (2) bands at 513 and 640 cm 21 , respectively, have the transition moment parallel to the molecular axis (called parallel bands) [11]. The A 2 (3) and A 2 (2) bands are clearly observed in the upper spectrum, but they almost disappear in the lower spectrum, indicating that the molecular axes for the SR case (the lower spectrum) are almost perpendicular to the substrate surface so that the normally incident IR light can not be absorbed at the A 2 (3) and A 2 (2) parallel bands. Thus, the PTFE thin films deposited by the SR etching have polymer chains highly oriented with molecular axes aligned perpendicular to the substrate surface. Fig. 5 shows typical QMS patterns (normalizing by the maximum peak) for different circumstances: under the SR etching (left top), under the laser ablation (left bottom) and with the target temperature of T t 55508C (right bottom) and with a pure perfluoro-n-alkane gas (right top) [12], respectively. It can be seen that the QMS patterns from the laser ablation case and the pyrolysis case are similar to each other, while the QMS pattern from the SR etching case appears to be close to that of perfluoron-alkane, where the CF 3 component is significant. This CF 3 component was also observed in the TOF spectra during the SR etching and in the XPS spectra of the Teflon target irradiated by the SR beam. It should be mentioned that, though the relative amount of each ionic peak may be modified by different conditions (e.g. light fluence, energy of the electronic beam used in the mass spectrometer), the fragmentation pattern given for each circumstance is typical.
4. Discussion Fig. 4. Typical changes in the FTIR spectra of the Teflon films deposited on the Si(100) substrate at T s 5RT by the SR etching (bottom trace) and by the laser ablation (top trace): In contrast with the E 1 (4) and E 1 (3) perpendicular bands detected at 556 and 1156 cm 21 , the A 2 (3) and A 2 (2) parallel bands detected at 513 and 640 cm 21 in the top trace appear to be very small in the bottom trace. Since the intense band at ca. 1210 cm 21 with a shoulder at ca. 1250 cm 21 , cannot be uniquely assigned, it is not used in the present discussion.
In the PTFE thin films deposited by the SR etching, the polymer chains are found to be highly oriented with molecular axes aligned perpendicular to the substrate surface, which is different from the orientation found in the laser ablation case. The detection of the CF 3 component in the synchrotron irradiated Teflon-target in the photo-fragments on the SR etching and in the deposited film may give us a
252
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 5. Typical QMS patterns (normalized by the maximum peak) under the SR etching (left top), laser ablation (left bottom), pyrolysis (right bottom) and with a pure perfluoro-n-alkane gas (right top). Although the relative amount of each ionic peak may be modified by different conditions (e.g. light fluence, different perfluoro-n-alkanes), the fragmentation pattern given for each circumstance is typical. The pattern given in the SR etching appears to be close to that of perfluoro-n-alkane, whereas the pattern in the laser ablation looks more similar to that of pyrolysis.
hint to understand why different orientations have been found in the PTFE-films deposited by different photo-processing. For the laser ablation case, the QMS pattern of the fragments (Fig. 5, left bottom) looks similar to that of the pyrolysis case (Fig. 5, right bottom). This can be understood based on the previous studies [8]. The laser ablation undergoes a thermally driven unzipping process similar to pyrolysis and results in mostly monomers as gaseous fragments. The radicals undergo polymerization on the substrate surface [6,8], and polymer chains are likely to form along the surface since simultaneous crystallization has to follow the chain-growth reaction kinetics [13]. In the laser ablation case, thus, the deposited PTFE films have molecular axes aligned parallel to the substrate surface. For the SR etching case, on the other hand, the QMS pattern of the photo-fragments (Fig. 5, left top) looks close to that of perfluoro-n-alkane (Fig. 5, right top), which indicates that, besides monomers, perfluoro-n-alkanes may also be produced by the SR etching as additional photo-fragments. It should be noticed that the photo-fragments produced by the SR or laser beam can hardly be detected directly with QMS due to a large degree of fragmentation from electron impact ionization but their cracking patterns in the mass spectra can indicate whether they are from
perfluoro-n-alkanes or from unsaturated monomers. The generation of perfluoro-n-alkanes can explain why the CF 3 component has been detected in the irradiated target, in the background gas and in the deposited film, though SR chemistry for Teflon remains unclear. During the film formation, since the melting temperature of the deposited macromolecules was close to that of the target and since the monomers could not be excluded, repolymerization must have occurred. On the other hand, the fact that perfluoro-n-alkane oligomers upon their crystallization result in the lamellar structure with molecular axes oriented normal to the basal plane may play the important role in the specific orientation found in the SR case [14]. For perfluoro-n-alkanes which may be electronically excited, further chain-growth may still occur on the substrate since the growing polymer chains may add to already existing ‘dead’ crystalline particles [13].
5. Conclusions The highly oriented PTFE films have been deposited by the SR etching in vacuum. The molecular orientation, which has the polymer chains perpendicular to substrate surface, has been found to be
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
different from that for the laser ablation. The generation of perfluoro-n-alkanes as additional photofragments in the SR etching has been concluded and may help understand why the different molecular orientation has been found.
[2] [3] [4] [5] [6] [7]
Acknowledgements We thank T. Urisu of Institute for Molecular Science for supporting the FTIR measurement, S. Ikeda of Raytech Corporation for the DSC measurement, F. Kannari of Keio University for supporting the QMS measurement upon laser ablation as well as M. Yorozu and Y. Okada for technical assistance in the laser experiment.
References [1] A. Sekiyama, T. Iwasaki, K. Mastuda, Y. Saitoh, Y. Onuki, S. Suga, Nature 403 (2000) 396.
[8] [9] [10] [11] [12] [13] [14]
253
E. Spears, H.I. Smith, Electr. Lett. 8 (1972) 102. T. Urisu, H. Kyuragi, J. Vac. Sci. Technol. B5 (1987) 1436. Y. Zhang, T. Katoh, Jpn. J. Appl. Phys. 35 (1996) L186. T. Katoh, Y. Zhang, Appl. Phys. Lett. 68 (1996) 865. Y. Ueno, T. Fujii, F. Kannari, Appl. Phys. Lett. 65 (1994) 1370. C. Chang, Y. Kim, A.G. Schrott, J. Vac. Sci. Technol. A8 (1990) 3304. G.B. Blanchet, C.R. Fincher Jr., C.L. Jackson, S.I. Shah, K.H. Gardner, Science 262 (1993) 719. H. Usui, H. Koshikawa, K. Tanaka, J. Vac. Sci. Technol. A13 (1995) 2318. J.C. Wittmann, P. Smith, Nature 352 (1991) 414. M. Kobayashi, M. Sakashita, T. Adachi, M. Kobayashi, Macromolecules 28 (1993) 316. D.R. Wheeler, S.V. Pepper, J. Vac. Sci. Technol. 20 (1982) 226. B. Wunderlich, Adv. Polym. Sci. 5 (1968) 568. Chapter 4 G. Strobl, The Physics of Polymers, SpringerVerlag, Berlin, 1996.
Deposition of highly oriented Teflon thin films by synchrotron radiation etching Y. Zhang*, T. Katoh, A. Endo Sumitomo Heavy Industries Ltd, 2 -1 -1 Yatocho, Nishitokyo, Tokyo 185 -8585, Japan Received 15 December 2000; received in revised form 5 March 2001; accepted 25 March 2001
Abstract Synchrotron radiation was used for deposition of highly oriented PTFE (poly tetrafluoroethylene; Teflon) thin films. In contrast with films deposited by the laser ablation, where polymer chains are parallel to the substrate surface, the films deposited by the synchrotron radiation etching have a different orientation where the polymer chains are oriented perpendicular to the substrate surface. By the mass analyses, we found that perfluoro-n-alkanes are generated as additional photo-fragments in the synchrotron radiation etching, which may play an important role for its specific orientation. 2001 Elsevier Science B.V. All rights reserved. Keywords: Teflon film; Synchrotron radiation etching; Oriented film
1. Introduction Synchrotron radiation (SR) has offered a powerful tool for studying electronic states of materials not only on surface but also in bulk [1]. On the application point of view, it has been considered as a promising source for proximity lithography in fabrication of the future electronic devices [2]. The SR induced chemical etching may also find its application in semiconductor processing [3]. We have found that, like laser ablation, the SR beam can be used to etch Teflon (polytetrafluoroethylene, PTFE) directly without using any chemicals, being a tool for direct micromachining [4] (see Fig. 1, for example). Meanwhile, the photo-fragments generated upon the SR *Corresponding author. Tel.: 181-424-68-4474; fax: 181-424-684477. E-mail address: ypz [email protected] (Y. Zhang). ]
etching in vacuum can be used to form thin films, offering a simple and versatile method for fabrication of thin films [5]. In this paper, we review our study of the PTFE thin films deposited by the SR etching.
2. Experimental The main components of our experimental setup consist of a synchrotron light source and an ultrahigh vacuum chamber with a base pressure of 10 28 mbar. The synchrotron radiation source was ‘AUROLA-II’, built by Sumitomo Heavy Industries, Ltd., which is one of the home-made compact electron storage rings (see Fig. 2). The ring was operated with electron energy of 700 MeV, stored current of 500 mA and emittance of 528 p nm rad., providing the radiation with the critical wavelength of 1.4–1.5 nm. The SR beam irradiated the target surface with the
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 01 )00300-0
248
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 1. SEM photograph of the Teflon microstructures made by the SR etching in vacuum.
photon flux of 2310 16 –1310 18 photons s 21 cm 22 . To compare with other techniques like pyrolysis and laser ablation, PTFE films were also deposited either by heating a Teflon target directly to 5508C or by ablating the target with a home-made ultrafast laser (266 nm, 10 ps, 100 Hz, more than 1 J cm 22 ). A commercial Teflon sheet of 0.5–1.5 mm in thickness was set on a target holder, while a Si(100) substrate was set on a sample holder, facing against the Teflon target. Both the target and substrate were cleaned with organic solvent before installing in the chamber. Both of them could be electrically heated by hot plates attached backsides and their temperatures (T t for the target and T s for the substrate) were measured in situ with thermocouples. Thickness of the deposited films was monitored with a quartz crystal microbalance and its distribution was measured with ellipsometry at 675 nm.
After deposition, chemical composition and crystalline features of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and 2u X-ray diffraction (XRD) (Cu Ka radiation of 8040 eV), respectively, and the surface morphology was observed with a scanning electron microscope (SEM). To obtain information of molecular weight of the deposited and starting materials, melting temperatures T m were measured with the differential scanning calorimetry (DSC) (PERKIN-ELMER, DSC-7). Molecular orientation of the deposited thin films was monitored by the Fourier transfer infrared (FTIR) spectroscopy in the range of 400–3000 cm 21 . To diagnose gaseous photo-fragments evolved upon deposition, the quadrupole mass spectrometry (QMS, INFICON-100, electronic beam energy of 110 eV) and time of flight (TOF, home-made) were in situ carried out for neutral and ionic species, respectively.
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
249
Fig. 2. As the synchrotron light source, a home-made compact warm ring AURORA-2S (E5700 MeV, c51.4 nm and I5500 mA) installed with self radiation-shielding at Sumitomo Heavy Industries.
3. Results SR etching, laser ablation and direct heating of the Teflon target in vacuum yielded gaseous fragments, which deposited on the Si(100) substrate as thick as 0.1–1.5 mm. The depositing rate was 7.5 nm s 21 at T t 5RT (i.e. room temperature) and 20 nm s 21 at T t 52008C, respectively, in the SR case, while it was much lower in the laser ablation case [6] where the target temperature appears to have no effect. Fig. 3 shows the SEM photos of typical surface morphology of the PTFE films deposited on the Si substrates at T s 5RT by the SR-etching (left top), by the laserablation (left bottom) and by the pyrolysis (right top). An increase of the substrate temperature causes
a change in the surface morphology in the pyrolysis (see Fig. 3, right bottom, for T s 52008C) and laser ablation case [6], but not in the SR case. A smooth film surface was obtained at any substrate temperature in the SR case, while the film surface given in the laser case was smoothed out by increasing the substrate temperature in an Ar atmosphere [6]. The surface in the pyrolytic case always has some structures [7], depending on the substrate temperature. For the deposition by the SR etching, the XRD and DSC analyses indicated that the thin films are crystalline but generally have small-size grains, and the XPS and FTIR analyses showed that the films contain mainly CF 2 and a small amount of CF 3 [5].
250
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 3. SEM photographs of typical surface morphology of the Teflon thin films deposited on Si(100) substrates at T s 5RT (i.e. room temperature) by SR etching (left top), laser ablation (left bottom) and pyrolysis (right top). The increase in the substrate temperature shows the effect in the pyrolysis (right bottom, for T s 52008C) or laser case [6], but not in the SR case. The surfaces were directly observed without coating.
All the analyses have confirmed that the deposited films are indeed the crystalline PTFE films. It is difficult to directly evaluate the molecular weight of the insoluble Teflon, but the DSC analysis may give
indirect evidence. The melting temperature measured with DSC was T53206108C for the SR case, similar to T53238C given in the laser case [8]. It is close to T53278C for the Teflon powder with a nominal
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
weight of 8500, but higher than T52628C for the powder with a nominal weight of 1500 and far higher than T51928C for C 24 F 50 [9]. It is known that the PTFE thin films deposited by rubbing its bulk material against a smooth substrate are well oriented, having its molecular axes aligned along the rubbing direction [10]. In the laser ablation case, the molecular axes have been found to be parallel to the substrate surface too, but not in one direction [8]. The molecular orientation of the deposited films can be well determined by the FTIR spectroscopy [11]. Fig. 4 shows the FTIR spectra of the PTFE films deposited by the laser ablation (top trace) and SR etching (bottom trace), respectively. It
251
is known that the E 1 (4) and E 1 (3) bands at 556 and 1156 cm 21 , respectively, have the transition moment perpendicular to the molecular axis (called perpendicular bands), while the A 2 (3) and A 2 (2) bands at 513 and 640 cm 21 , respectively, have the transition moment parallel to the molecular axis (called parallel bands) [11]. The A 2 (3) and A 2 (2) bands are clearly observed in the upper spectrum, but they almost disappear in the lower spectrum, indicating that the molecular axes for the SR case (the lower spectrum) are almost perpendicular to the substrate surface so that the normally incident IR light can not be absorbed at the A 2 (3) and A 2 (2) parallel bands. Thus, the PTFE thin films deposited by the SR etching have polymer chains highly oriented with molecular axes aligned perpendicular to the substrate surface. Fig. 5 shows typical QMS patterns (normalizing by the maximum peak) for different circumstances: under the SR etching (left top), under the laser ablation (left bottom) and with the target temperature of T t 55508C (right bottom) and with a pure perfluoro-n-alkane gas (right top) [12], respectively. It can be seen that the QMS patterns from the laser ablation case and the pyrolysis case are similar to each other, while the QMS pattern from the SR etching case appears to be close to that of perfluoron-alkane, where the CF 3 component is significant. This CF 3 component was also observed in the TOF spectra during the SR etching and in the XPS spectra of the Teflon target irradiated by the SR beam. It should be mentioned that, though the relative amount of each ionic peak may be modified by different conditions (e.g. light fluence, energy of the electronic beam used in the mass spectrometer), the fragmentation pattern given for each circumstance is typical.
4. Discussion Fig. 4. Typical changes in the FTIR spectra of the Teflon films deposited on the Si(100) substrate at T s 5RT by the SR etching (bottom trace) and by the laser ablation (top trace): In contrast with the E 1 (4) and E 1 (3) perpendicular bands detected at 556 and 1156 cm 21 , the A 2 (3) and A 2 (2) parallel bands detected at 513 and 640 cm 21 in the top trace appear to be very small in the bottom trace. Since the intense band at ca. 1210 cm 21 with a shoulder at ca. 1250 cm 21 , cannot be uniquely assigned, it is not used in the present discussion.
In the PTFE thin films deposited by the SR etching, the polymer chains are found to be highly oriented with molecular axes aligned perpendicular to the substrate surface, which is different from the orientation found in the laser ablation case. The detection of the CF 3 component in the synchrotron irradiated Teflon-target in the photo-fragments on the SR etching and in the deposited film may give us a
252
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
Fig. 5. Typical QMS patterns (normalized by the maximum peak) under the SR etching (left top), laser ablation (left bottom), pyrolysis (right bottom) and with a pure perfluoro-n-alkane gas (right top). Although the relative amount of each ionic peak may be modified by different conditions (e.g. light fluence, different perfluoro-n-alkanes), the fragmentation pattern given for each circumstance is typical. The pattern given in the SR etching appears to be close to that of perfluoro-n-alkane, whereas the pattern in the laser ablation looks more similar to that of pyrolysis.
hint to understand why different orientations have been found in the PTFE-films deposited by different photo-processing. For the laser ablation case, the QMS pattern of the fragments (Fig. 5, left bottom) looks similar to that of the pyrolysis case (Fig. 5, right bottom). This can be understood based on the previous studies [8]. The laser ablation undergoes a thermally driven unzipping process similar to pyrolysis and results in mostly monomers as gaseous fragments. The radicals undergo polymerization on the substrate surface [6,8], and polymer chains are likely to form along the surface since simultaneous crystallization has to follow the chain-growth reaction kinetics [13]. In the laser ablation case, thus, the deposited PTFE films have molecular axes aligned parallel to the substrate surface. For the SR etching case, on the other hand, the QMS pattern of the photo-fragments (Fig. 5, left top) looks close to that of perfluoro-n-alkane (Fig. 5, right top), which indicates that, besides monomers, perfluoro-n-alkanes may also be produced by the SR etching as additional photo-fragments. It should be noticed that the photo-fragments produced by the SR or laser beam can hardly be detected directly with QMS due to a large degree of fragmentation from electron impact ionization but their cracking patterns in the mass spectra can indicate whether they are from
perfluoro-n-alkanes or from unsaturated monomers. The generation of perfluoro-n-alkanes can explain why the CF 3 component has been detected in the irradiated target, in the background gas and in the deposited film, though SR chemistry for Teflon remains unclear. During the film formation, since the melting temperature of the deposited macromolecules was close to that of the target and since the monomers could not be excluded, repolymerization must have occurred. On the other hand, the fact that perfluoro-n-alkane oligomers upon their crystallization result in the lamellar structure with molecular axes oriented normal to the basal plane may play the important role in the specific orientation found in the SR case [14]. For perfluoro-n-alkanes which may be electronically excited, further chain-growth may still occur on the substrate since the growing polymer chains may add to already existing ‘dead’ crystalline particles [13].
5. Conclusions The highly oriented PTFE films have been deposited by the SR etching in vacuum. The molecular orientation, which has the polymer chains perpendicular to substrate surface, has been found to be
Y. Zhang et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 247 – 253
different from that for the laser ablation. The generation of perfluoro-n-alkanes as additional photofragments in the SR etching has been concluded and may help understand why the different molecular orientation has been found.
[2] [3] [4] [5] [6] [7]
Acknowledgements We thank T. Urisu of Institute for Molecular Science for supporting the FTIR measurement, S. Ikeda of Raytech Corporation for the DSC measurement, F. Kannari of Keio University for supporting the QMS measurement upon laser ablation as well as M. Yorozu and Y. Okada for technical assistance in the laser experiment.
References [1] A. Sekiyama, T. Iwasaki, K. Mastuda, Y. Saitoh, Y. Onuki, S. Suga, Nature 403 (2000) 396.
[8] [9] [10] [11] [12] [13] [14]
253
E. Spears, H.I. Smith, Electr. Lett. 8 (1972) 102. T. Urisu, H. Kyuragi, J. Vac. Sci. Technol. B5 (1987) 1436. Y. Zhang, T. Katoh, Jpn. J. Appl. Phys. 35 (1996) L186. T. Katoh, Y. Zhang, Appl. Phys. Lett. 68 (1996) 865. Y. Ueno, T. Fujii, F. Kannari, Appl. Phys. Lett. 65 (1994) 1370. C. Chang, Y. Kim, A.G. Schrott, J. Vac. Sci. Technol. A8 (1990) 3304. G.B. Blanchet, C.R. Fincher Jr., C.L. Jackson, S.I. Shah, K.H. Gardner, Science 262 (1993) 719. H. Usui, H. Koshikawa, K. Tanaka, J. Vac. Sci. Technol. A13 (1995) 2318. J.C. Wittmann, P. Smith, Nature 352 (1991) 414. M. Kobayashi, M. Sakashita, T. Adachi, M. Kobayashi, Macromolecules 28 (1993) 316. D.R. Wheeler, S.V. Pepper, J. Vac. Sci. Technol. 20 (1982) 226. B. Wunderlich, Adv. Polym. Sci. 5 (1968) 568. Chapter 4 G. Strobl, The Physics of Polymers, SpringerVerlag, Berlin, 1996.