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Shadowgraphy of pulsed CO2 laser ablation of polymers
Applied Surface Science 248 (2005) 281–285 www.elsevier.com/locate/apsusc
Shadowgraphy of pulsed CO2 laser ablation of polymers W.O. Siew a, T.Y. Tou a,*, K.H. Wong b b
a Faculty of Engineering, Multimedia University, Cyberjaya, 63100 Selangor, Malaysia Institute for Postgraduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia
Available online 9 April 2005
Abstract Heavy plume expansions in air, helium and argon produced by pulsed CO2 laser ablations of poly(methyl methacrylate) (PMMA) and polyimide (PI) were imaged by shadowgraphy. The PMMA was melted at 1–2 mm depth beneath the surface followed by an outward expulsion of heavy plumes. A shock front was formed which detached from the first plume after several microseconds. A second, mushroom-shaped plume and a narrow ejection were also observed. In contrast, the mushroom-shaped plume structure was absent in PI ablation, but it was in the form of a single blob, which collapsed back to the target. # 2005 Elsevier B.V. All rights reserved. PACS: 42.30.Va; 47.40.Dc; 07.68+m Keywords: Polymer ablation; Molten front; Shock front; Plume collapse
1. Introduction The majority of the photo-ablation processes of polymers [1–2] used UV lasers, while very few cases were reported for pulsed CO2 lasers [3–5]. A recent review documented the chemical and spectroscopic aspects of the plume content from polymer ablation [6], while another [7] reviewed phase explosion during laser ablation of molecular structures via computer simulations. Plume expulsion was shown to occur in two delayed components [8]. The first component consists mainly of light and fast particles that generate * Corresponding author. Tel.: +60 3 83125278; fax: +60 3 83183029. E-mail address: [email protected] (T.Y. Tou).
a shock wave. The second component consists of heavy particles, and no shock wave is observed. Streak photography [9] also displayed two luminous sheaths, suggesting two time-delayed plume components. The velocity of the second plume was slower by an order of magnitude with respect to the first one that lifted off the target at 106 cm/s. The prediction of the plume trajectory usually employs either a blast-wave, drag or snow-plow models [9–12]. Shadowgraphy of UV laser ablation of poly(methyl methacrylate) (PMMA) [13] showed a narrow ejection, whose formation was attributed to plume condensation into liquid droplets. Such ejections were not observed when a pulsed CO2 laser was used [14]. Moreover, shadowgraphy of polyimide (PI) ablated by pulsed CO2 laser has never been reported before to our best knowledge.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.052
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W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
Fig. 1. Schematics of the experimental setup for shadowgraphy of the plume expulsion in pulsed CO2 laser ablation of PMMA and PI.
2. Experimental details As shown in Fig. 1, the experimental setup consists of a transversely excited atmospheric pressure CO2 laser for polymer ablation and a nitrogen-laser pumped dye laser for shadowgraphy. A gas mixture of CO2:N2:He (10:5:20) was used for the pulsed CO2 laser, which produced a 65-ns laser spike followed by a 500-ns low-power pulse. The spike contained 70– 80% of the pulsed CO2 laser output energy. The spatial laser-beam uniformity was improved by inserting a 6-mm aperture inside the resonator, adjacent to the output coupler. This tight aperture induced high diffraction losses in the transverse oscillation modes and reduced the laser output energy but produced a near-Gaussian profile. The laser beam was focused to a 0.3 mm spot on the polymer without inducing optical breakdown in the background gas. The PMMA ablation was performed in air and He gas with pressures between 700 and 1000 mbar and a laser fluence between 12 and 14 J/cm2. The PI ablation was performed with laser fluence of 20–50 J/cm2 in Argon gas at 300 mbar to obtain good shadowgraphic
images. Every laser shot was used to ablate a fresh spot on the polymer target, which was mounted on a vacuum XYZ manipulator. A nitrogen-laser pumped dye laser with 1-ns pulse duration was used for probing instead of the nitrogenlaser, because of its better beam uniformity. The dye laser output, from Rhodamine 6G (perchlorate) dissolved in enthanol, was expanded and collimated in the plume trajectory. Shadowgrams were recorded on type-667 polaroid films using a lab-made camera with a narrow band-pass window centered at 632.8 nm. A lens, used for image expansion, was focused on the ablated site. Without a spatial filter at the image plane, the arrangement resembles focused shadowgraphy [14], which allows both shadow and luminosity of the object to be imaged on the same film.
3. Results and discussion The plume and its expulsion dynamics could not be recorded in vacuum, but a small hot molten column, which penetrated into the PMMA during the first 1 ms,
W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
Fig. 2. Shadowgraphy images of plume for PMMA ablation at 14 J/ cm2 and in vacuum: a hot and luminous column penetrating into PMMA bulk is observed at 500 ns.
was seen as shown in Fig. 2. This suggests that the heavy plume was produced by thermal decomposition of the polymer by the CO2 laser pulse. This could not be seen in PI ablation because it is not a transparent material. At atmospheric pressure, the plume struc-
283
tures and the shock front were easily observed by shadowgraphy as a result of the increased particle density gradient, which in turn increased the bending of light. For PMMA, Fig. 3a–d displays the plume structures and shock front positions in air at 700, 10, 100 and 500 ms, respectively, with respect to the CO2 laser pulse. Interestingly, plume structures after more than 50 ms have not been reported elsewhere to the best of our knowledge. At a time <10 ms, the initial plume, which is bright and opaque, seems to agree with an early report [15] that attributed the plume content mainly to the monomer MMA [15]. After 10 ms, the plume continues to expand, but it appears darker due to an assumed expulsion of unknown solid materials [13,15]. The two plumes did not join together as a single blob, but cascaded in blob and mushroom-shapes. The first plume blob propagated away at 104 cm/s, but impeded the vertical movement of the second plume, which was thus forced to expand laterally, see Fig. 3b, eventually collapsing back to the
Fig. 3. Shadowgraphy images of plume for PMMA ablation at 14 J/cm2 and in air: (a) 700 ns; (b) the detachment of the shock front from the plume at 10 ms; (c) double plume structure at 100 ms and (d) partial plume collapsing at 500 ms.
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Fig. 4. Shadowgraphy image of a narrow ejection at 100 ms for PMMA ablation at 14 J/cm2 and in 700 mbar He.
target surface, see Fig. 3c. Shadowgraph images of the plume structure and the shock front were less distinct at 700–1000 mbar He. After a time >10 ms, the shock front was not captured. An image for the narrow ejection was captured at 100 ms in He gas, see Fig. 4, which was previously reported for UV, but not CO2 laser ablation of PMMA [13]. In comparison, shadowgraphy of pulsed CO2 laser ablation of PI was successful only in Ar ambient with laser fluences above 20 J/cm2. Fig. 5a shows a hemispherical expansion of plume after 5 ms for a fluence of 20 J/cm2, where the shock front was detached. When the laser fluence was raised to 50 J/ cm2, the plume and shock structures at a time <500 ns deviated from the hemispherical shape; becoming elongated at the top, see Fig. 5b. This elongated structure reminds the plume shielding of laser beam, or laser-induced absorption waves [10,15]. At a time >50 ms, the plume stopped from further expansion and collapsed back to the PI surface, see Fig. 5c. The collapsed plume expanded laterally, like a collapsing building. However, this did not mark the end of the plume expulsion from the ablated site, since the
Fig. 5. Shadowgraphy images of plume for PI ablation at laser fluences of: (a) 20 J/cm2: hemispherical plume appears and a shock front at 5 ms; (b) 50 J/cm2: elongated plume structure due to laser absorption at 500 ns; (c) 50 J/cm2: plume collapse at 60 ms and (d) 50 J/cm2: re-emission of a plume and the appearance of ejection at 150 ms.
ejection shown in Fig. 5d suggests that the plume continues to be released and attempts to push through the collapsed plume. This indicates that the ablated site of PI target was strongly melted. Hence, the second plume in CO2 laser ablation of PI as well as
W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
PMMA consists of thermally decomposed materials via melting, which are usually heavy particles.
4. Conclusion Shadowgraphy of plume expulsion was obtained in air and argon at high pressures. Hot molten front penetrating into PMMA suggests thermal decomposition. In contrast to the cascaded, double plume structure, there was only a simple plume structure in the PI ablation. A narrow ejection was observed for both PMMA and PI, which were previously reported to be absent in the UV laser ablation of PMMA.
Acknowledgement We would like to acknowledge the Malaysian Ministry of Science, Technology and Innovation for financial support of this project.
285
References [1] Y. Kawamura, K. Toyoda, S. Namba, Appl. Phys. Lett. 40 (1982) 374. [2] R. Srinivasan, S. Mayne-Banton, Appl. Phys. Lett. 41 (1982) 576. [3] G. Koren, Appl. Phys. Lett. 45 (1984) 10. [4] R. Braun, R. Nowak, P. Hess, H. Oetzmann, C. Schmidt, Appl. Surf. Sci. 43 (1989) 352. [5] P.E. Dyer, G.A. Oldershaw, J. Sidhu, Appl. Phys. B 48 (1989) 489. [6] T. Lippert, J.T. Dickson, Chem. Rev. 103 (2003) 453. [7] L.V. Zhigilei, E. Leveugle, B.J. Garrison, Y.G. Yingling, M.I. Zeifman, Chem. Rev. 103 (2003) 321. [8] R. Kelly, A. Miotello, B. Braren, C.O. Otis, Appl. Phys. Lett. 60 (1992) 2980. [9] K.H. Wong, T.Y. Tou, K.S. Low, J. Appl. Phys. 83 (1998) 2286. [10] P.E. Dyer, J. Sidhu, J. Appl. Phys. 64 (1988) 4657. [11] P.G. Ventzek, R.M. Gilgenbach, H.C. Chi, R.A. Lindley, J. Appl. Phys. 72 (1992) 1696. [12] R. Srinivasan, B. Braren, K.G. Casey, J. Appl. Phys. 68 (1990) 1842. [13] R. Srinivasan, J. Appl. Phys. 73 (1993) 2743. [14] M.A. Levine, J.C. Hegarty, Appl. Opt. 2 (1963) 78. [15] R. Srinivasan, Appl. Phys. A 56 (1993) 417.
Shadowgraphy of pulsed CO2 laser ablation of polymers W.O. Siew a, T.Y. Tou a,*, K.H. Wong b b
a Faculty of Engineering, Multimedia University, Cyberjaya, 63100 Selangor, Malaysia Institute for Postgraduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia
Available online 9 April 2005
Abstract Heavy plume expansions in air, helium and argon produced by pulsed CO2 laser ablations of poly(methyl methacrylate) (PMMA) and polyimide (PI) were imaged by shadowgraphy. The PMMA was melted at 1–2 mm depth beneath the surface followed by an outward expulsion of heavy plumes. A shock front was formed which detached from the first plume after several microseconds. A second, mushroom-shaped plume and a narrow ejection were also observed. In contrast, the mushroom-shaped plume structure was absent in PI ablation, but it was in the form of a single blob, which collapsed back to the target. # 2005 Elsevier B.V. All rights reserved. PACS: 42.30.Va; 47.40.Dc; 07.68+m Keywords: Polymer ablation; Molten front; Shock front; Plume collapse
1. Introduction The majority of the photo-ablation processes of polymers [1–2] used UV lasers, while very few cases were reported for pulsed CO2 lasers [3–5]. A recent review documented the chemical and spectroscopic aspects of the plume content from polymer ablation [6], while another [7] reviewed phase explosion during laser ablation of molecular structures via computer simulations. Plume expulsion was shown to occur in two delayed components [8]. The first component consists mainly of light and fast particles that generate * Corresponding author. Tel.: +60 3 83125278; fax: +60 3 83183029. E-mail address: [email protected] (T.Y. Tou).
a shock wave. The second component consists of heavy particles, and no shock wave is observed. Streak photography [9] also displayed two luminous sheaths, suggesting two time-delayed plume components. The velocity of the second plume was slower by an order of magnitude with respect to the first one that lifted off the target at 106 cm/s. The prediction of the plume trajectory usually employs either a blast-wave, drag or snow-plow models [9–12]. Shadowgraphy of UV laser ablation of poly(methyl methacrylate) (PMMA) [13] showed a narrow ejection, whose formation was attributed to plume condensation into liquid droplets. Such ejections were not observed when a pulsed CO2 laser was used [14]. Moreover, shadowgraphy of polyimide (PI) ablated by pulsed CO2 laser has never been reported before to our best knowledge.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.03.052
282
W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
Fig. 1. Schematics of the experimental setup for shadowgraphy of the plume expulsion in pulsed CO2 laser ablation of PMMA and PI.
2. Experimental details As shown in Fig. 1, the experimental setup consists of a transversely excited atmospheric pressure CO2 laser for polymer ablation and a nitrogen-laser pumped dye laser for shadowgraphy. A gas mixture of CO2:N2:He (10:5:20) was used for the pulsed CO2 laser, which produced a 65-ns laser spike followed by a 500-ns low-power pulse. The spike contained 70– 80% of the pulsed CO2 laser output energy. The spatial laser-beam uniformity was improved by inserting a 6-mm aperture inside the resonator, adjacent to the output coupler. This tight aperture induced high diffraction losses in the transverse oscillation modes and reduced the laser output energy but produced a near-Gaussian profile. The laser beam was focused to a 0.3 mm spot on the polymer without inducing optical breakdown in the background gas. The PMMA ablation was performed in air and He gas with pressures between 700 and 1000 mbar and a laser fluence between 12 and 14 J/cm2. The PI ablation was performed with laser fluence of 20–50 J/cm2 in Argon gas at 300 mbar to obtain good shadowgraphic
images. Every laser shot was used to ablate a fresh spot on the polymer target, which was mounted on a vacuum XYZ manipulator. A nitrogen-laser pumped dye laser with 1-ns pulse duration was used for probing instead of the nitrogenlaser, because of its better beam uniformity. The dye laser output, from Rhodamine 6G (perchlorate) dissolved in enthanol, was expanded and collimated in the plume trajectory. Shadowgrams were recorded on type-667 polaroid films using a lab-made camera with a narrow band-pass window centered at 632.8 nm. A lens, used for image expansion, was focused on the ablated site. Without a spatial filter at the image plane, the arrangement resembles focused shadowgraphy [14], which allows both shadow and luminosity of the object to be imaged on the same film.
3. Results and discussion The plume and its expulsion dynamics could not be recorded in vacuum, but a small hot molten column, which penetrated into the PMMA during the first 1 ms,
W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
Fig. 2. Shadowgraphy images of plume for PMMA ablation at 14 J/ cm2 and in vacuum: a hot and luminous column penetrating into PMMA bulk is observed at 500 ns.
was seen as shown in Fig. 2. This suggests that the heavy plume was produced by thermal decomposition of the polymer by the CO2 laser pulse. This could not be seen in PI ablation because it is not a transparent material. At atmospheric pressure, the plume struc-
283
tures and the shock front were easily observed by shadowgraphy as a result of the increased particle density gradient, which in turn increased the bending of light. For PMMA, Fig. 3a–d displays the plume structures and shock front positions in air at 700, 10, 100 and 500 ms, respectively, with respect to the CO2 laser pulse. Interestingly, plume structures after more than 50 ms have not been reported elsewhere to the best of our knowledge. At a time <10 ms, the initial plume, which is bright and opaque, seems to agree with an early report [15] that attributed the plume content mainly to the monomer MMA [15]. After 10 ms, the plume continues to expand, but it appears darker due to an assumed expulsion of unknown solid materials [13,15]. The two plumes did not join together as a single blob, but cascaded in blob and mushroom-shapes. The first plume blob propagated away at 104 cm/s, but impeded the vertical movement of the second plume, which was thus forced to expand laterally, see Fig. 3b, eventually collapsing back to the
Fig. 3. Shadowgraphy images of plume for PMMA ablation at 14 J/cm2 and in air: (a) 700 ns; (b) the detachment of the shock front from the plume at 10 ms; (c) double plume structure at 100 ms and (d) partial plume collapsing at 500 ms.
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W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
Fig. 4. Shadowgraphy image of a narrow ejection at 100 ms for PMMA ablation at 14 J/cm2 and in 700 mbar He.
target surface, see Fig. 3c. Shadowgraph images of the plume structure and the shock front were less distinct at 700–1000 mbar He. After a time >10 ms, the shock front was not captured. An image for the narrow ejection was captured at 100 ms in He gas, see Fig. 4, which was previously reported for UV, but not CO2 laser ablation of PMMA [13]. In comparison, shadowgraphy of pulsed CO2 laser ablation of PI was successful only in Ar ambient with laser fluences above 20 J/cm2. Fig. 5a shows a hemispherical expansion of plume after 5 ms for a fluence of 20 J/cm2, where the shock front was detached. When the laser fluence was raised to 50 J/ cm2, the plume and shock structures at a time <500 ns deviated from the hemispherical shape; becoming elongated at the top, see Fig. 5b. This elongated structure reminds the plume shielding of laser beam, or laser-induced absorption waves [10,15]. At a time >50 ms, the plume stopped from further expansion and collapsed back to the PI surface, see Fig. 5c. The collapsed plume expanded laterally, like a collapsing building. However, this did not mark the end of the plume expulsion from the ablated site, since the
Fig. 5. Shadowgraphy images of plume for PI ablation at laser fluences of: (a) 20 J/cm2: hemispherical plume appears and a shock front at 5 ms; (b) 50 J/cm2: elongated plume structure due to laser absorption at 500 ns; (c) 50 J/cm2: plume collapse at 60 ms and (d) 50 J/cm2: re-emission of a plume and the appearance of ejection at 150 ms.
ejection shown in Fig. 5d suggests that the plume continues to be released and attempts to push through the collapsed plume. This indicates that the ablated site of PI target was strongly melted. Hence, the second plume in CO2 laser ablation of PI as well as
W.O. Siew et al. / Applied Surface Science 248 (2005) 281–285
PMMA consists of thermally decomposed materials via melting, which are usually heavy particles.
4. Conclusion Shadowgraphy of plume expulsion was obtained in air and argon at high pressures. Hot molten front penetrating into PMMA suggests thermal decomposition. In contrast to the cascaded, double plume structure, there was only a simple plume structure in the PI ablation. A narrow ejection was observed for both PMMA and PI, which were previously reported to be absent in the UV laser ablation of PMMA.
Acknowledgement We would like to acknowledge the Malaysian Ministry of Science, Technology and Innovation for financial support of this project.
285
References [1] Y. Kawamura, K. Toyoda, S. Namba, Appl. Phys. Lett. 40 (1982) 374. [2] R. Srinivasan, S. Mayne-Banton, Appl. Phys. Lett. 41 (1982) 576. [3] G. Koren, Appl. Phys. Lett. 45 (1984) 10. [4] R. Braun, R. Nowak, P. Hess, H. Oetzmann, C. Schmidt, Appl. Surf. Sci. 43 (1989) 352. [5] P.E. Dyer, G.A. Oldershaw, J. Sidhu, Appl. Phys. B 48 (1989) 489. [6] T. Lippert, J.T. Dickson, Chem. Rev. 103 (2003) 453. [7] L.V. Zhigilei, E. Leveugle, B.J. Garrison, Y.G. Yingling, M.I. Zeifman, Chem. Rev. 103 (2003) 321. [8] R. Kelly, A. Miotello, B. Braren, C.O. Otis, Appl. Phys. Lett. 60 (1992) 2980. [9] K.H. Wong, T.Y. Tou, K.S. Low, J. Appl. Phys. 83 (1998) 2286. [10] P.E. Dyer, J. Sidhu, J. Appl. Phys. 64 (1988) 4657. [11] P.G. Ventzek, R.M. Gilgenbach, H.C. Chi, R.A. Lindley, J. Appl. Phys. 72 (1992) 1696. [12] R. Srinivasan, B. Braren, K.G. Casey, J. Appl. Phys. 68 (1990) 1842. [13] R. Srinivasan, J. Appl. Phys. 73 (1993) 2743. [14] M.A. Levine, J.C. Hegarty, Appl. Opt. 2 (1963) 78. [15] R. Srinivasan, Appl. Phys. A 56 (1993) 417.