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J. Phys. Chem. B 2000, 104, 6212-6217
Changing Molecular Orientation in Fluorocarbon Thin Films Deposited by Different Photo-Processing: Synchrotron Radiation Etching vs Laser Ablation Y. Zhang,* T. Katoh, and A. Endo Sumitomo HeaVy Industries, Ltd., 2-1-1 Yatocho, Tanashi, Tokyo 188-8585, Japan ReceiVed: September 29, 1999; In Final Form: March 3, 2000
Flexible macromolecule poly(tetrafluoroethylene) (PTFE) has shown its unique capacity for crystallization and molecular orientation: it forms long extended-chain crystalline whiskers and well orients in its thin films. Formation of the highly oriented structure is essential for oriented growth of materials and for study of the crystal, having a great impact on materials science and engineering. Like laser ablation, synchrotron radiation (SR) can be used to directly etch materials without using any chemicals, offering a simple and versatile method for forming thin films. Using Fourier transfer infrared spectroscopy, the thin PTFE films deposited by the SR etching were found to have polymer chains highly oriented with molecular axes aligned to be perpendicular to substrate surface, whereas the molecular axes align to be parallel to the substrate surface in the films deposited by the laser ablation. Studying with mass spectrometry and X-ray photoelectron spectroscopy, we come to understand this change in molecular orientation by finding molecular oligomers of perfluoro-n-alkanes as photofragments involved in the SR case.
1. Introduction As an unusual consequence of the plastically crystalline nature of polytetrafluoroethylene (PTFE), its thin films deposited by rubbing its bulk material against a smooth substrate or by hot rolling at 125 °C are well oriented, having its molecular axes aligned along the rubbing (or rolling) direction.1,2 Also, this flexible linear macromolecule is able to form long extendedchain whiskers.3 The well-oriented film of PTFE has been found to be useful as a substrate for oriented growth of materials,1 as well as for study of polymer physics4 and crystal morphology.5 Laser ablation developed as a simple and versatile method for forming thin films has been applied to depositing the PTFE thin films,6,7 where the molecular axes have been found to be parallel to the substrate surface too but not in one direction.6 With synchrotron radiation (SR) to replace laser, we can also deposit the thin films.8 Using Fourier transfer infrared (FTIR) spectroscopy, we found our PTFE films having the polymer chains aligned perpendicular to the substrate surface, different from all the previous reports.1,2,6 To understand why different molecular orientations have been formed, we have analyzed gaseous fragments evolved upon the irradiation with quadrupole mass spectrometry (QMS) and diagnosed the deposited films with FTIR and X-ray photoelectron spectroscopy (XPS). Here we report our study. 2. Experimental Section 2.1. Setup. The main components of our experimental setup consisted of a light source (i.e., either a laser or a beam-line from a synchrotron radiation ring) and an ultrahigh vacuum chamber with a base pressure of 10-8 mbar. For the laser ablation, the fourth harmonic beam (266 nm, 10 ps, 100 Hz) of a YAG laser (PULRISE) was focused to get a fluence of more than 1 J/cm2. For the SR etching, a beam (170 ps, 191 MHz) with full spectrum (its critical wavelength of 1.5 nm) from a * To whom correspondence should be addressed. E-mail: ypz_zhang@ shi.co.jp. Fax: +81-424-68-4477.
compact SR source (AURORA) was selected by two pinholes to get a homogeneous beam whose flux on the target surface was between 2 × 1016 and 1 × 1018 photons s-1 cm-2. A commercial sheet of the PTFE (0.5-1.5 mm thick, called target in the following) was set on the target holder while a Si(100) substrate was set on the sample holder to face the PTFE target. Both the target and substrate were cleaned with organic solvent before being set parallel to each other in the chamber. Both of them could be electrically heated by hot plates attached to their rear. Temperatures of the target Tt and substrate Ts were measured via thermocouples. Film thickness was monitored with quartz crystal microbalance. Thickness distribution was measured with ellipsometry at 675 nm. Chemical compositions of the deposited films were analyzed by X-ray photoelectron spectroscopy (XPS), and their crystalline features were characterized by 2θ X-ray diffraction (XRD) (Cu ΚR radiation of 8040 eV), with comparison to those of the target material. Surface morphology was observed by scanning electron microscopy (SEM). To get information of molecular weight of the deposited PTFE, its melting temperature Tm was measured with differential scanning calorimetory (DSC) (Perkin-Elmer, DSC-7), compared with that of the PTFE target. To study molecular orientation in the deposited films, FTIR spectroscopy was carried out with normal and oblique transmission, where an incident angle was 0° and 80° for the normal and oblique transmission, respectively. Two FTIR spectrometers (Perkin-Elmer and JASCO) were used to measure spectra in a range of 400-3000 cm-1. For a cross-check, the film was also deposited on a metallic surface, and infrared reflection absorption spectroscopy (IRAS9) was carried out to ensure us of the right measurement given by the oblique transmission. The quadrupole mass spectrometry (QMS) was carried out with a mass spectrometer (INFICON) in a mass range between 1 and 100 to analyze gaseous photofragments evolved upon the processing. 2.2. Results. Thin PTFE films with thickness between 0.1 and 1.5 µm have been deposited on the Si(100) substrates in a
10.1021/jp993481l CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000
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Figure 1. SEM photographs of typical surface morphology of the PTFE thin films deposited on Si(100) substrates at Ts ) RT (i.e., room temperature) by different processing (from left to right): SR etching, laser ablation and pyrolysis at Tt ) 550 °C. The surfaces were directly observed without any metallic coating.
vacuum from gaseous fragments, which were yielded during either laser or SR irradiation of the PTFE target. For comparison, they were also made by vacuum pyrolysis with Tt ) 550 °C. Three SEM pictures in Figure 1 show typical surface morphology of the PTFE films deposited on the Si substrates at Ts ) RT (i.e., room temperature) by the SR beam, laser beam, and pyrolysis (from left to right): The film surface is smooth in the SR case, but by contrast it shows numerous fibrous structures in the pyrolysis case as reported.10,11 Some rough structures were also observed in the laser case, but might be smoothed out by increasing the substrate temperature in an Ar atmosphere (about 200 mTorr).7 That the deposited PTFE films are crystalline has been confirmed by XRD and DSC.6-8,10 Typical changes in the FTIR spectra are shown in Figure 2 a,b, where the bottom trace is a spectrum for the normal incidence and the top trace is one for the oblique incidence. By changing the incident angle, typical spectra of the films in the laser case were found to change in a way similar to those deposited in the pyrolysis case (shown in Figure 2b), but different from those in the SR case (shown in Figure 2a). Figure 3 shows a small band due to the CF3 stretching detected at 983 cm-1 in the SR case, but it was not detected in either the laser or the pyrolysis case. This band diminished when the target temperature was increased to Tt ) 200 °C. The CF3 component was also found in the XPS C1s level spectra (Figure 4) where, besides the CF2 peak centered at 292 eV, a small shoulder due to CF3 was detected at 295 eV for the films deposited by the SR etching (top and middle traces) but not found for the target materials (bottom trace). Again, it diminished at the higher target temperature Tt ) 200 °C. For the deposited films, F/C ) 2.01 was measured to be slightly larger than 1.95 of the PTFE target. Furthermore, the chemical compositions of the target were also checked (Figure 5): The significant CF3 component was also detected after irradiation with a low photon flux (2.5 × 1016 photons s-1 cm-2, top middle trace) and diminished at a high flux (7.5 × 1017 photons s-1 cm-2, top trace), whereas there was none before the irradiation. For the insoluble PTFE, it is hard to directly evaluate its molecular weight. The melting temperature Tm may give some information,12 however. Our DSC measurement gave Tm ) 320 ( 10 °C for the PTFE deposited by the SR, similar to Tm ) 323 °C given in the laser ablation6 but slightly lower than Tm
) 335 ( 5 °C for the PTFE target. Our data are close to Tm ) 327 °C for the PTFE power with a nominal weight of 8500, but far higher than Tm ) 262 °C for a nominal weight of 1500 and Tm ) 192 °C for C24F50.12,13 Figure 6 shows typical fragment patterns (normalizing by the maximum peak) in the QMS for the above three typical circumstances (from top to bottom): under the SR exposure, under laser exposure and with the target temperature of Tt ) 550 °C, compared to those QMS patterns from a pure perfluoron-alkane gas and from the pure monomer gas, respectively.14 Although the relative amount of each ionic peak might be modified by different conditions (e.g., different light fluence, different target temperatures, different perfluoro-n-alkanes), the QMS pattern given for each circumstance was typical. One can tell the perfluoro-n-alkanes from the monomer by their cracking patterns in the mass spectra, though primary gaseous species may not be detected solely in QMS due to a large degree of fragmentation by electron impact ionization. It should be noticed that the QMS patterns for the laser and pyrolysis case look similar to that of the monomer, whereas the QMS pattern for the SR case appears similar to that of the perfluoro-n-alkanes. To measure the removal rate, the removed depth was measured with an optical microscope and the rate in nm/pulse was given by a ratio of the depth to the pulse number in irradiation time. The irradiation time was usually set to be 1 min to prevent any error either due to beam decay or due to a temperature rise induced by the irradiation. The pulse fluence in photons/cm2 was calculated from either the given beam current (mA) in the SR case or the given pulse energy (mJ/ pulse) in the laser case for the given irradiation spots. Figures 7 gives the removal rate versus the pulse fluence for the SR etching and laser ablation of the PTFE targets, respectively. 3. Discussion Like the laser ablation, our SR etching is a photoprocess without using any chemicals. The reason we do not term our process as “SR ablation” is as follows: As seen in Figure 7, at least several tens of thousands SR pulses are needed to etch one molecular layer (a few Å), whereas the laser ablation proceeds layer by layer (here, the layer is defined by Beer’s law and usually is more than a molecular layer).15 Furthermore, there is generally a fluence threshold for the onset of laser
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Figure 3. Oblique transmission FTIR spectrum of the PTFE thin films deposited on the Si (100) substrate by the SR etching, similar to the top trace of Figure 2a between 800 and 1600 cm-1, indicating that the CF3 stretching band has been detected at 983 cm-1. This band diminished as the target temperature increased.
Figure 2. Typical changes in the FTIR spectra of the following films deposited on the Si (100) substrate at Ts ) RT by different processing. Bottom trace is a spectrum for the normal incidence (incident angle ) 0°) and top trace is for the oblique incidence (incident angle ) 80°): (a) the PTFE film deposited by SR etching; (b) the PTFE film deposited by laser ablation or pyrolysis at Tt ) 550 °C (for the laser ablation and pyrolysis cases, though the relative amount of each band may be slightly different, the spectral change looked nearly the same). The normal incidence spectrum with an improved signal-to-noise was inserted between 450 and 750 cm-1.
ablation, but it is not found in the SR etching which does not proceed layer by layer. Our consideration is in agreement with the molecular dynamics simulations of laser ablation of organic solids in which, from the microscopic point of view, the ablation is referred to a collectiVe ejection.16 On the basis of the data known,5 the molecular orientation of PTFE can be well determined by the FTIR spectroscopy. In Figure 2a, for the SR etching case, when the infrared light changes from the oblique incidence to the normal one, the A2(3) and A2(2) bands with the transition moment parallel to the molecular axis (called parallel bands5) detected at 532, 644 cm-1 are reduced to become very small, while the E1(4), E1(3) and E1(2) bands with the transition moment perpendicular to the molecular axis (called perpendicular bands5) detected at 556, 1156, 1211 cm-1 are enhanced. These spectral changes indicate that the molecular axes are almost perpendicular to the substrate surface (called perpendicular orientation for simplicity). In Figure 2b for the laser ablation case, on the other hand, the A2(3) and A2(2) bands detected at 513, 640 cm-1 are enhanced
Figure 4. XPS C1s level spectra of the PTFE thin films deposited on the Si (100) substrate at Ts ) RT by SR etching of the PTFE target at different target temperatures: Tt ) RT (top) and Tt ) 200 °C (middle) with comparison to that of the unirradiated PTFE target (bottom).
when the oblique incidence turns to the normal, indicating that the molecular axes are parallel to the substrate surface (called parallel orientation for simplicity). This means that the molecular orientation in the deposited films has changed in the different photoprocessing as follows: The molecular axes are almost perpendicular to the substrate surface in the SR case, whereas,
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J. Phys. Chem. B, Vol. 104, No. 26, 2000 6215
Figure 5. XPS C1s level spectra of the PTFE target under exposure to the SR beam with a flux of 7.5 × 1017 photons s-1 cm-2 (top) and with a reduced flux of 2.5 × 1016 photons s-1 cm-2 (middle) and that of the unirradiated PTFE (bottom).
as previously reported,6 the molecular axes are parallel to the substrate surface in the laser case. The significant blue-shift (∼19 cm-1) of the A2(3) band in Figure 2 (a) gives further evidence to support the change in crystal morphology of the deposited films.5 What has resulted in the change in chain orientation of the films deposited by the different photoprocessing? Obviously, it has little to do with the surface morphology (Figure 1). The difference in the QMS patterns (Figure 6) for these two photoprocesses may give us a hint to get an answer to it. For the laser case, the QMS pattern looks similar to that of the monomers, like the pyrolysis case. This is agreed with the previous studies, indicating that an unzipping process occurs as dose in the pyrolysis, yielding mostly radicals of the monomer.17,18 The radicals undergo polymerization on the substrate surface,6,7 and polymer chains are likely to form along it (i.e., the parallel orientation), since simultaneous crystallization has to follow chain-growth reaction kinetics19 and the surface may play the same role as does the surfactant in granular polymerization of TFE.3 For the SR case, however, the QMS pattern appears to be closer to that of the perfluoro-n-alkane, though there is a small component of C2F3+ due to the monomers, indicating that molecular oligomers of perfluoron-alkanes may be predominant over the monomers in the fragments. 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 help understand the perpendicular orientation found in the SR case.5,20 That the
Figure 6. Typical QMS patterns (normalized by the maximum peak) under the three processing, compared to the pattern of a pure perfluoron-alkane or monomergas. Although the relative amount of each ionic peak may be modified by different conditions (e.g., light fluence, different perfluoro-n-alkanes), the pattern given for each circumstance is typical. The pattern in the SR etching appears similar to be that of the perfluoro-n-alkane, while those patterns in both the laser ablation and pyrolysis look similar to that of the monomer.
perfluoro-n-alkanes may be involved in the film formation is consistent with the detection of a fraction of CF3 in the deposited film with FTIR (Figure 3) and XPS (Figure 4). 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 film formation. Although the perfluoro-n-alkanes (which may be highly electronically excited) are not so reactive as the monomers, further chain-growth may still occur on the substrate since the growing polymer chains may add to already existing “dead” crystalline particles.19 On the basis of the DSC data, the macromolecules deposited by the SR etching should have the mean molecular weight of more than 7500, and thus, the perfluoro-n-alkanes CnF2n+2 (ng150) may also be included in the deposited PTFE films since they can hardly be distinguished from the PTFE. However, the CF3 component in the deposited films has diminished when the target temperature was increased to Tt ) 200 °C (Figure 4).
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Figure 8. Dependence of the A2(3) band-position and the band ratio of A2(2)/E1(3) on the substrate temperature between Ts ) 50 and Ts ) 300 °C (lines are to guide the eyes): With increasing Ts, the blue-shift of the A2(3) band as well as the reduction in the band ratio of A2(2) (parallel band)/E1(3) (perpendicular band) indicate that the crystal morphology starts changing at Ts > 200 °C.
4. Conclusion
Figure 7. Removal rates versus pulse fluence for the SR etching (top) and laser ablation (bottom) of PTFE. The fluence threshold is found in the laser ablation, but not in the SR etching. Several tens of thousands SR pulses are needed to remove a molecular layer (a few Å), whereas a single laser pulse can ablate more than one.
This might be due to desorption of larger oligomers from the target surface at a higher temperature. Obviously, the perfluoron-alkanes appeared to have been yielded as the photofragments on the target surface (Figure 5), though SR chemistry for PTFE remains unknown. Since the PTFE macromolecules can undergo chain reorientation by mobile twin-helix reversal defects and the perpendicular orientation possesses lower formation energy than the parallel orientation,4,20 the following experiments were further carried out to confirm the change in chain orientation feasible. (1) The film deposited on the substrate at Ts ) RT by the pyrolysis was heated to 350 °C for 1 min and then cooled back to room temperature. Its FTIR spectra showed that the parallel orientation had changed to the perpendicular orientation, consistent with a previous study on recrystallization of PTFE from the molten state.21 (2) The PTFE films deposited by the pyrolysis on the substrates with different temperatures Ts showed their chains changed from the parallel orientation (at Ts ) RT) to the perpendicular orientation (at Ts ) 265 °C). Figure 8 gives the blue-shift of the A2(3) band as well as the abundance ratio of the A2(2) band to the E1(3) band (in the normal transmission) versus Ts to show this change starting at Ts > 200 °C, similar to a previous study.12 A rise in the substrate temperature may increase mobility of the forming chains so as to orient the helical axes parallel to each other and then to pack themselves laterally in a regular manner into the lamellar structure with the perpendicular orientation during the crystallization.20 This change may occur without a significant change in crystallinity of the formed film12 and the temperature for this change should be dependent on the size of fragments, thus on the molecular weight of the target12 and the decomposition process as well.22
The PTFE thin films have been deposited by different photoprocessing, namely SR etching and laser ablation, and found to have different molecular orientations, that is, the chains are aligned to be perpendicular to substrate surface in the SR case whereas they are parallel to the substrate surface in the laser case. The photofragments detected upon the two photoprocessing may account for the change in the chain orientation, that is, the laser ablation yields mainly the radical fragments of the monomer resulting in the parallel orientation by their polymerization and simultaneous crystallization on the substrate, while the SR etching yields perfluoro-n-alkanes as additional photofragments resulting in the perpendicular orientation upon their crystallization on the substrate. Acknowledgment. We thank T. Urisu of the Institute for Molecular Science for supporting the FTIR measurement, S. Ikeda of Raytech Corp. 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 and Notes (1) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (2) Robinson, T. S.; Price, W. C. Proc. Phys. Soc. 1953, LXVI, II-B, 969. (3) Folda, T.; Hoffmann, H.; Chanzy, H.; Smith, P. Nature 1988, 333, 565. (4) Kimming, M.; Strobl, G.; Stu¨hn, B. Macromolecules 1994, 27, 2481. (5) Kobayashi, M.; Sakashita, M.; Adachi, T.; Kobayashi, M. Macromolecules 1993, 28, 316. (6) Blanchet, G. B.; Fincher, C. R., Jr.; Jackson, C. L.; Shah, S. I.; Gardner, K. H. Science 1993, 262, 719. (7) Ueno, Y.; Fujii, T.; Kannari, F. Appl. Phys. Lett. 1994, 65, 1370. (8) Katoh, T.; Zhang, Y. Appl. Phys. Lett. 1996, 68, 865. (9) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107. (10) Nason, T. C.; Moore, J. A.; Lu, T. M. Appl. Phys. Lett. 1992, 60, 1866. (11) Chang, C.; Kim, Y.; Schrott, A. G. J. Vac. Sci. Technol. 1990, A8, 3304. (12) Usui, H.; Koshikawa, H.; Tanaka, K. J. Vac. Sci. Technol. A 1995, 13, 2318. (13) Starkweatherm, H. W., Jr. Macromolecules 1986, 19, 1131. (14) Wheeler, D. R.; Pepper, S. V. J. Vac. Sci. Technol. 1982, 20, 226. (15) Srinivasan, R.; Braren, B. Chem. ReV. 1989, 89, 1303.
Fluorocarbon Thin Films Deposited by Photo-Processing (16) Zhigilei, L. V.; Kodali, P. B. S.; Garrison, B. J. J. Phys. Chem. B 1998, 102, 2845. (17) Goodwin, P. M.; Otis, C. E. J. Appl. Phys. 1991, 69, 2584. (18) Siegle, J. G.; Muus, L. T.; Lin, T.; Larsen, H. A. J. Polym. Sci. A 1964, 2, 391. (19) Wunderlich, B. AdV. Polymer Sci. 1968, 5, 568.
J. Phys. Chem. B, Vol. 104, No. 26, 2000 6217 (20) Strobl, G. The Physics of Polymers; Springer-Verlag: Berlin, 1996; p 143. (21) Symons, N. K. J. J. Polym. Sci. A 1963, 1, 2843. (22) The surface temperature seemed to have little effect in the laser ablation case (see ref 6) since very large fragments even droplets are usually involved.
