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Coupling effects of the number of pulses, pulse repetition rate and fluence during laser PMMA ablation
Applied Surface Science 165 Ž2000. 303–308 www.elsevier.nlrlocaterapsusc
Coupling effects of the number of pulses, pulse repetition rate and fluence during laser PMMA ablation Z.Q. Liu a,) , Y. Feng a , X.-S. Yi b a b
Department of Polymer Engineering, UniÕersity of Minho, Azurem ´ 4800, Guimaraes, ˜ Portugal National Key Laboratory of AdÕanced Composites, 100095, Beijing, People’s Republic of China Received 13 April 2000; accepted 25 May 2000
Abstract PolyŽmethyl methacrylate. ŽPMMA. was ablated using a 248-nm long-pulsed KrF excimer laser operating at a pulse repetition rate ŽPRR. of 2 and 10 Hz, and fluence varying from 0.4 to 2 Jrcm2. The coupling effects of multiple shots, PRR, and fluence are found and discussed on the etching depth data and topography of PMMA. An increase in either PRR, or fluence or the number of pulses can accelerate the etching efficiency in terms of ablation rate, as a result of strengthened thermal effects. Quality of the craters such as roughness, porosity and contamination is sensitively dependent on the specific laser operating conditions. Basically, increasing the PRR and the number of pulses gives rise to a crater with smoother and less porous bottom. q 2000 Elsevier Science B.V. All rights reserved. PACS: 79.20.D; 61.80.B Keywords: PolyŽmethyl methacrylate.; Ablation; Repetition rate; Multiple shots; Fluence
1. Introduction During the past two decades, laser–polymer reactions have attracted extensive attentions both for understanding the inherent mechanisms and extending laser applications w1–5x. Diverse lasers involved there may have a wavelength from the ultraviolet through the visible into the infrared, and pulse duration from the femtosecond to millisecond w6,7x. Among a variety of laser sources, the excimer laser has been the most used for polymer applications,
) Corresponding author. Tel.: q351-253-511670-5111; fax: q351-253-510249. E-mail address: [email protected] ŽZ.Q. Liu..
such as etching, drilling, direct writing w8,9x, micromachining w10x and thin film deposition w11,12x. It has been found that the laser ablation of polymers is presumably affected by the laser wavelength, pulse temporal distribution, and absorption coefficient of the materials w7,13–16x. Moreover, the effect of laser pulse repetition rate ŽPRR. has been recently claimed, although it has not been extensively studied. Christensen w17x etched via holes in a commercially available Kaptone polyimide ŽPI. using equal numbers of pulses at repetition rates of 1, 2 and 3 kHz, and then found the etching depth increased with the increment of PRR. However, Burns and Cain w18x, who ablated PI using an excimer laser operating at 308 nm, found little effect of PRR in the range of 10–300 Hz. It, therefore, seems that PI, with an
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 9 9 - 2
304
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
absorption coefficient of 10 5 cmy1 , exhibits the PRR effect just at a high repetition rate regime. Nevertheless, a polyŽmethyl methacrylate. ŽPMMA.-based photoresist, on the other hand, showed the effect at a much lower repetition rate, due to its relatively small effective absorption coefficient. An interesting finding was that the threshold fluence of the photoresist exhibited a monotonic decrease as PRR was increased. The presence of the PRR effect would support the view that the ablation is a thermal process that, as the unique ablation mechanisms, is still open to debate w18x. However, depending on the etching conditions and composition of polymers, either a thermal decomposition w19–21x or a substantial photochemical reaction w7,22,23x or both of them w15,16,24x can, de facto, be operative during the laser irradiation process of polymers. This paper presents the ablation data and morphology evolution of PMMA subjected to various shots of a low-frequency long-pulsed 248-nm KrF excimer laser with various fluences. The coupling effects of PRR, fluence and the shot number are discussed.
The ablated surfaces were gold-coated for 5–10 min with a JFC-1100 ion sputter operating at 1.2 kV and 5 mA and then examined by a Hitachi S2400 scanning electronic microscope ŽSEM..
3. Results and discussion The effect of the number of pulses, namely the multiple-shot effect referred to in Ref. w25x, has been found also present during laser processing of PMMA in our investigation. Fig. 1 shows the etching curves at various pairs of the shot number and PRR. Although Chang and Molian w26x thought Furzikov’s model was more precise to describe their etching data of PMMA, SSB model w15x generally fits our experimental data fairly well at PRR of 10 Hz. The model-predicted data for 20, 50, 100, 200 shots are presented as the solid lines in Fig. 1b. In contrast to
2. Experimental PMMA sheets Ž2 mm in thickness. were prepared by compression moulding at a temperature of 1808C. The sheets were irradiated in air using a 248-nm KrF excimer laser with pulse duration of 30 ns. The laser beam was tuned to obtain fluence ranging from 0.4 to 2 Jrcm2 , and PRR of 2 and 10 Hz. The laser beam, after passing through a metal mask, was focused by a quartz lens with a focal length of 100 mm. The target surface perpendicular to the incident beam was located at the focus plane of the beam. For each pair of fluence and PRR, 20–500 pulses were used to shoot the target, and four ablated craters were produced for each pulse level. A Rodenstock profilometer was used to measure the depth of the ablated craters, which, divided by the number of pulses, represents the ablation rate. The ablation rates for each set of laser operating conditions Žfluence, PRR and the number of pulses. were averaged, and the standard deviation ŽSD. was worked out. In general, SD for all sets of operating conditions was below 11%, which is reasonably acceptable as a result of experimental errors.
Fig. 1. The multiple-shot effect on the ablation rate vs. fluence plots: Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
305
little multiple-shot effect at 2-Hz PRR ŽFig. 1a., the ablation curve at 10 Hz shifts upward as the shot number is increased up to 100, and afterwards, stagnates ŽFig. 1b.. The stagnation should be indicative of the saturation effect that is observed at a high fluence such as 10 Jrcm2 for PMMA w26x. Normally, the PRR effect means that a higher PRR leads to a greater ablation rate. However, it should be noted that the effect is significantly dependent on the shot number and fluence. The least repetition rate, n , to activate the PRR effect can be theoretically predicted by the following expression w18x:
n f 0.1 Da 2rN
Ž 1.
where D, a and N denote the thermal diffusivity, the effective absorption coefficient and the number of pulses, respectively. According to the method presented in Ref. w18x, a can be worked out to be 1495 cmy1 with a deviation range of 21%, that is, however, still within the reported range of 500– 10,000 cmy1 w26x. Provided D s 1.09 = 10y3 cm2rs for PMMA irradiated by the 248-nm laser beam, n is around 12 Hz for N s 20, and around 2 Hz for N s 100. This calculation agrees well with our observations, namely the PRR effect is minor at 20 shots ŽFig. 2a. but significant at 100 shots for PRR of 2 and 10 Hz ŽFig. 2b.. Although the shot number and the PRR actually involved are 20 and 2 and 10 Hz, respectively, values below the theoretical requirements, the PRR effect is, however, still noticeable at a fluence of 2 Jrcm2 ŽFig. 2a.. In fact, the PRR effect that significantly emerges at 100 shots is positively dependent on fluence. For instance, the improvement in the ablation rate upon the PRR increment jumps from 64% to 83% as fluence is increased from 0.7 to 0.9 Jrcm2 ŽFig. 2b.. It thus can be summarized that the etching efficiency Žin terms of ablation rate. can be improved identically by increasing either fluence or the shot number or PRR. For instance, an ablation rate of ca. 2 mmrpulse can be approached by using one of the following sets of laser operating conditions Žfluence, PRR, the shot number.: Ž0.7 Jrcm2 , 10 Hz, 500.; Ž0.9 Jrcm2 , 10 Hz, 50.; Ž2 Jrcm2 , 2 Hz, 20.. It is likely that the ablation of PMMA occurs with low-intensity nanosecond pulsed lasers, owning to the incubation effects during the first few pulses that
Fig. 2. The PRR effect on the ablation rate vs. fluence plots: Ža. at 20 shots; Žb. at 100 shots.
can change PMMA from transparent to 248-nm light to strongly absorbing of 248-nm light w4x. Once photochemical decomposition begins with the significant absorption, the feedback between the temperature rise and increased absorption coefficient gives rise to surface melting. Thermodegradation occurs with 100% monomer products when the surface temperature approaches 4008C for PMMA w27x. For long-pulsed lasers Žpulse duration ) 20 ns. such as the laser used in our investigation, melt expulsion driven by the monomer vapour pressure, as well as
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Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
relaxation time, or when the amount of laser pulses is large enough to coerce the occurrence of heat accumulation. The arguments can be evidenced in Fig. 3, which shows topography of PMMA craters ablated by 20 shots of laser beam with a fluence of 0.7 Jrcm2 . Although the ablation rate at 10 Hz is almost the same as that at 2 Hz ŽFig. 2a., thermal stress cracks, indicative of striking thermal effects, are observed in the crater at 10-Hz PRR ŽFig. 3b., whereas the crater at 2-Hz PRR shows no cracks but particulate contamination ŽFig. 3a. that normally imply the domination of photochemical mechanisms during irradiation. Bibliographically, a large thermal damage zone with debris around the ablated crater, which shows more thermal cracks in the bottom, has been observed when using almost the same laser but with a PRR of 100 Hz w26x.
Fig. 3. SEM photographs of PMMA craters ablated by 20 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
the recoil pressure of the laser beam, facilitates the materials removal. It is generally agreed that thermal effects are negligible at low fluences, but become increasingly significant with increments of fluence and the number of pulses w15,16x. When the long-pulsed laser operates at a low repetition rate, heat that is converted from the absorbed single- or multi-photons has enough time to diffuse and get lost into the environment. Therefore, the ablation process involving the 248-nm excimer laser with a pulse width of 30 ns, a low fluence, and a low PRR is characterized prevailingly by photochemical decomposition. However, as the fluence is increased such that the energy input outranges the heat energy dissipated, thermal effects can emerge to be significant. The thermal effects is also standing out when the repetition rate is high enough to significantly shorten the pulse–pulse
Fig. 4. SEM photographs of PMMA ablated by 100 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
The PRR effect on morphology evolution is significantly distinct for 100 pulse-irradiated PMMA ŽFig. 4.. The crater bottom for 2-Hz PRR is rough, porous and debris-contaminated ŽFig. 4a., whereas that for 10 Hz presents quite an even surface with little contamination ŽFig. 4b.. With the 2-Hz pulsed laser beam, increasing the shot number from 100 to 300 leads to an observable reduction in the amount of pores and particulate contamination ŽFig. 5a., although the surface still remains uneven. Nevertheless, for a PRR of 10 Hz, the shot number increment results in a smoother surface, on which some fine debris but no pores are observed ŽFig. 5b.. Thus, it seems plausible that a PRR rise leads to smoothing the ablated surfaces, and a shot number increment results in minimizing pores on the surfaces. In fact,
307
the increases in both PRR and the shot number render the thermal effects more striking. As a result, the sharper the temperature rise is, the more rapidly the gaseous products explode away carrying some of the surrounding substrates, with little risk being entrapped in the bulk polymer during resolidification, therefore, the less porous the ablated surface are. In particular, as PRR is increased, the irradiation process involving a given number of shots is accomplished in a shorter period, followed by a rapid resolidification process in air. Consequently, this prevents the polymer melt that is induced by diffused heat from spreading or flowing, driven by surface tension and back-pressure over the crater w14x, thereby leaving neat surfaces behind. However, particulate contamination is somewhat inevitable without a purge gas employed in the irradiation process since some of the ejected fragments are likely to react with ambient oxygen in air, and form the sooty debris that may be redeposited on the ablated surfaces.
4. Conclusions The coupling effects of PRR, fluence and the number of pulses are found existent on the etching depth data and topography of PMMA, ablated using a low-frequency 248-nm long-pulsed KrF excimer laser. The ablation rate can be improved identically by increasing either PRR, or fluence or the number of pulses. The multiple-shot effect strikes at a high repetition rate, and so does the PRR effect at a large number of shots. The PRR effect becomes more pronounced with increasing fluence. Morphologically, increasing PRR and the number of pulses gives rise to a crater with smoother and less porous bottom. It is thus recommended that both etching efficiency and morphology evolution should be taken into account when optimizing the laser operating conditions.
Acknowledgements
Fig. 5. SEM photographs of PMMA ablated by 300 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
The authors acknowledge the financial support from the National Doctoral Research Foundation of the State Education Commission of China.
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Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
References w1x R. Srinivasan, V. Mayne-Banton, Appl. Phys. Lett. 41 Ž1982. 576–578. w2x H.H.G. Jellinek, R. Srinivasan, J. Phys. Chem. 88 Ž1984. 3048–3051. w3x T. Uchida, N. Shimo, H. Sugimura, H. Masuhara, J. Appl. Phys. 76 Ž1994. 4872–4878. w4x M.D. Shirk, P.A. Molian, J. Laser Appl. 10 Ž1998. 18–28. w5x G.B. Blanchet, J. Appl. Phys. 80 Ž1996. 4082–4089. w6x P.P. Pronko, S.K. Dutta, D. Du, J. Appl. Phys. 78 Ž1995. 6233–6240. w7x R. Srinivasan, J. Appl. Phys. 72 Ž1992. 1651–1653. w8x S.I. Bozhevolnyi, I.V. Potemkin, V.B. Svetovoy, J. Appl. Phys. 71 Ž1992. 2030–2032. w9x Y. Nakayama, T. Matsuda, J. App. Phys. 80 Ž1996. 505–508. w10x H. Watanabe, M. Yamamoto, J. Appl. Polym. Sci. 64 Ž1997. 1203–1209. w11x M. Gitay, B. Joglekar, S.B. Ogale, Appl. Phys. Lett. 58 Ž1991. 197–199. w12x G.B. Blanchet, S.I. Shah, Appl. Phys. Lett. 62 Ž1993. 1026– 1028. w13x G.C. D’Couto, S.V. Babu, F.D. Egitto, C.R. Davis, J. Appl. Phys. 74 Ž1993. 5972–5980.
w14x F.D. Eggito, C.R. Davis, Appl. Phys. B 55 Ž1992. 488–491. w15x R. Srinivasan, M.A. Smrtic, S.V. Babu, J. Appl. Phys. 59 Ž1986. 3861–3867. w16x Y. Feng, Z.Q. Liu, X.-S. Yi, Appl. Surf. Sci. 156 Ž2000. 177–182. w17x C.P. Christensen, Photonics Spectra Ž1991. 98–100, August. w18x F.C. Burns, S.R. Cain, J. Phys. D 29 Ž1996. 1349–1355. w19x G.B. Blanchet, C.R. Fincher, C.L. Jackson, S.I. Shah, K.H. Gardner, Science 262 Ž1993. 719–722. w20x B. Luk’yanchuk, N. Bityurin, S. Anisimov, D. Bauerle, Appl. ¨ Phys. A 57 Ž1993. 367–385. w21x S.R. Cain, J. Phys. Chem. 97 Ž1993. 7572–7576. w22x Y. Feng, Z. Liu, R. Vilar, X.-S. Yi, Appl. Surf. Sci. 150 Ž1999. 131–136. w23x R. Srinivasan, J. Appl. Phys. 73 Ž1993. 2743–2749. w24x S. Kuper, J. Brannon, Appl. Phys. A 57 Ž1993. 255–259. ¨ w25x Z. Ball, T. Feurer, D.L. Callahan, R. Sauerbrey, Appl. Phys. A 62 Ž1996. 203–211. w26x T.-C. Chang, P.A. Molian, J. Manuf. Proc. 1 Ž1999. 1–17. w27x N. Grassie, Products of thermal degradation of polymers, in: J. Brandup, E.H. Immergut ŽEds.., Polymer Handbook vol. II Wiley-Interscience, New York, 1989, pp. 367–369.
Coupling effects of the number of pulses, pulse repetition rate and fluence during laser PMMA ablation Z.Q. Liu a,) , Y. Feng a , X.-S. Yi b a b
Department of Polymer Engineering, UniÕersity of Minho, Azurem ´ 4800, Guimaraes, ˜ Portugal National Key Laboratory of AdÕanced Composites, 100095, Beijing, People’s Republic of China Received 13 April 2000; accepted 25 May 2000
Abstract PolyŽmethyl methacrylate. ŽPMMA. was ablated using a 248-nm long-pulsed KrF excimer laser operating at a pulse repetition rate ŽPRR. of 2 and 10 Hz, and fluence varying from 0.4 to 2 Jrcm2. The coupling effects of multiple shots, PRR, and fluence are found and discussed on the etching depth data and topography of PMMA. An increase in either PRR, or fluence or the number of pulses can accelerate the etching efficiency in terms of ablation rate, as a result of strengthened thermal effects. Quality of the craters such as roughness, porosity and contamination is sensitively dependent on the specific laser operating conditions. Basically, increasing the PRR and the number of pulses gives rise to a crater with smoother and less porous bottom. q 2000 Elsevier Science B.V. All rights reserved. PACS: 79.20.D; 61.80.B Keywords: PolyŽmethyl methacrylate.; Ablation; Repetition rate; Multiple shots; Fluence
1. Introduction During the past two decades, laser–polymer reactions have attracted extensive attentions both for understanding the inherent mechanisms and extending laser applications w1–5x. Diverse lasers involved there may have a wavelength from the ultraviolet through the visible into the infrared, and pulse duration from the femtosecond to millisecond w6,7x. Among a variety of laser sources, the excimer laser has been the most used for polymer applications,
) Corresponding author. Tel.: q351-253-511670-5111; fax: q351-253-510249. E-mail address: [email protected] ŽZ.Q. Liu..
such as etching, drilling, direct writing w8,9x, micromachining w10x and thin film deposition w11,12x. It has been found that the laser ablation of polymers is presumably affected by the laser wavelength, pulse temporal distribution, and absorption coefficient of the materials w7,13–16x. Moreover, the effect of laser pulse repetition rate ŽPRR. has been recently claimed, although it has not been extensively studied. Christensen w17x etched via holes in a commercially available Kaptone polyimide ŽPI. using equal numbers of pulses at repetition rates of 1, 2 and 3 kHz, and then found the etching depth increased with the increment of PRR. However, Burns and Cain w18x, who ablated PI using an excimer laser operating at 308 nm, found little effect of PRR in the range of 10–300 Hz. It, therefore, seems that PI, with an
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 9 9 - 2
304
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
absorption coefficient of 10 5 cmy1 , exhibits the PRR effect just at a high repetition rate regime. Nevertheless, a polyŽmethyl methacrylate. ŽPMMA.-based photoresist, on the other hand, showed the effect at a much lower repetition rate, due to its relatively small effective absorption coefficient. An interesting finding was that the threshold fluence of the photoresist exhibited a monotonic decrease as PRR was increased. The presence of the PRR effect would support the view that the ablation is a thermal process that, as the unique ablation mechanisms, is still open to debate w18x. However, depending on the etching conditions and composition of polymers, either a thermal decomposition w19–21x or a substantial photochemical reaction w7,22,23x or both of them w15,16,24x can, de facto, be operative during the laser irradiation process of polymers. This paper presents the ablation data and morphology evolution of PMMA subjected to various shots of a low-frequency long-pulsed 248-nm KrF excimer laser with various fluences. The coupling effects of PRR, fluence and the shot number are discussed.
The ablated surfaces were gold-coated for 5–10 min with a JFC-1100 ion sputter operating at 1.2 kV and 5 mA and then examined by a Hitachi S2400 scanning electronic microscope ŽSEM..
3. Results and discussion The effect of the number of pulses, namely the multiple-shot effect referred to in Ref. w25x, has been found also present during laser processing of PMMA in our investigation. Fig. 1 shows the etching curves at various pairs of the shot number and PRR. Although Chang and Molian w26x thought Furzikov’s model was more precise to describe their etching data of PMMA, SSB model w15x generally fits our experimental data fairly well at PRR of 10 Hz. The model-predicted data for 20, 50, 100, 200 shots are presented as the solid lines in Fig. 1b. In contrast to
2. Experimental PMMA sheets Ž2 mm in thickness. were prepared by compression moulding at a temperature of 1808C. The sheets were irradiated in air using a 248-nm KrF excimer laser with pulse duration of 30 ns. The laser beam was tuned to obtain fluence ranging from 0.4 to 2 Jrcm2 , and PRR of 2 and 10 Hz. The laser beam, after passing through a metal mask, was focused by a quartz lens with a focal length of 100 mm. The target surface perpendicular to the incident beam was located at the focus plane of the beam. For each pair of fluence and PRR, 20–500 pulses were used to shoot the target, and four ablated craters were produced for each pulse level. A Rodenstock profilometer was used to measure the depth of the ablated craters, which, divided by the number of pulses, represents the ablation rate. The ablation rates for each set of laser operating conditions Žfluence, PRR and the number of pulses. were averaged, and the standard deviation ŽSD. was worked out. In general, SD for all sets of operating conditions was below 11%, which is reasonably acceptable as a result of experimental errors.
Fig. 1. The multiple-shot effect on the ablation rate vs. fluence plots: Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
305
little multiple-shot effect at 2-Hz PRR ŽFig. 1a., the ablation curve at 10 Hz shifts upward as the shot number is increased up to 100, and afterwards, stagnates ŽFig. 1b.. The stagnation should be indicative of the saturation effect that is observed at a high fluence such as 10 Jrcm2 for PMMA w26x. Normally, the PRR effect means that a higher PRR leads to a greater ablation rate. However, it should be noted that the effect is significantly dependent on the shot number and fluence. The least repetition rate, n , to activate the PRR effect can be theoretically predicted by the following expression w18x:
n f 0.1 Da 2rN
Ž 1.
where D, a and N denote the thermal diffusivity, the effective absorption coefficient and the number of pulses, respectively. According to the method presented in Ref. w18x, a can be worked out to be 1495 cmy1 with a deviation range of 21%, that is, however, still within the reported range of 500– 10,000 cmy1 w26x. Provided D s 1.09 = 10y3 cm2rs for PMMA irradiated by the 248-nm laser beam, n is around 12 Hz for N s 20, and around 2 Hz for N s 100. This calculation agrees well with our observations, namely the PRR effect is minor at 20 shots ŽFig. 2a. but significant at 100 shots for PRR of 2 and 10 Hz ŽFig. 2b.. Although the shot number and the PRR actually involved are 20 and 2 and 10 Hz, respectively, values below the theoretical requirements, the PRR effect is, however, still noticeable at a fluence of 2 Jrcm2 ŽFig. 2a.. In fact, the PRR effect that significantly emerges at 100 shots is positively dependent on fluence. For instance, the improvement in the ablation rate upon the PRR increment jumps from 64% to 83% as fluence is increased from 0.7 to 0.9 Jrcm2 ŽFig. 2b.. It thus can be summarized that the etching efficiency Žin terms of ablation rate. can be improved identically by increasing either fluence or the shot number or PRR. For instance, an ablation rate of ca. 2 mmrpulse can be approached by using one of the following sets of laser operating conditions Žfluence, PRR, the shot number.: Ž0.7 Jrcm2 , 10 Hz, 500.; Ž0.9 Jrcm2 , 10 Hz, 50.; Ž2 Jrcm2 , 2 Hz, 20.. It is likely that the ablation of PMMA occurs with low-intensity nanosecond pulsed lasers, owning to the incubation effects during the first few pulses that
Fig. 2. The PRR effect on the ablation rate vs. fluence plots: Ža. at 20 shots; Žb. at 100 shots.
can change PMMA from transparent to 248-nm light to strongly absorbing of 248-nm light w4x. Once photochemical decomposition begins with the significant absorption, the feedback between the temperature rise and increased absorption coefficient gives rise to surface melting. Thermodegradation occurs with 100% monomer products when the surface temperature approaches 4008C for PMMA w27x. For long-pulsed lasers Žpulse duration ) 20 ns. such as the laser used in our investigation, melt expulsion driven by the monomer vapour pressure, as well as
306
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
relaxation time, or when the amount of laser pulses is large enough to coerce the occurrence of heat accumulation. The arguments can be evidenced in Fig. 3, which shows topography of PMMA craters ablated by 20 shots of laser beam with a fluence of 0.7 Jrcm2 . Although the ablation rate at 10 Hz is almost the same as that at 2 Hz ŽFig. 2a., thermal stress cracks, indicative of striking thermal effects, are observed in the crater at 10-Hz PRR ŽFig. 3b., whereas the crater at 2-Hz PRR shows no cracks but particulate contamination ŽFig. 3a. that normally imply the domination of photochemical mechanisms during irradiation. Bibliographically, a large thermal damage zone with debris around the ablated crater, which shows more thermal cracks in the bottom, has been observed when using almost the same laser but with a PRR of 100 Hz w26x.
Fig. 3. SEM photographs of PMMA craters ablated by 20 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
the recoil pressure of the laser beam, facilitates the materials removal. It is generally agreed that thermal effects are negligible at low fluences, but become increasingly significant with increments of fluence and the number of pulses w15,16x. When the long-pulsed laser operates at a low repetition rate, heat that is converted from the absorbed single- or multi-photons has enough time to diffuse and get lost into the environment. Therefore, the ablation process involving the 248-nm excimer laser with a pulse width of 30 ns, a low fluence, and a low PRR is characterized prevailingly by photochemical decomposition. However, as the fluence is increased such that the energy input outranges the heat energy dissipated, thermal effects can emerge to be significant. The thermal effects is also standing out when the repetition rate is high enough to significantly shorten the pulse–pulse
Fig. 4. SEM photographs of PMMA ablated by 100 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
Z.Q. Liu et al.r Applied Surface Science 165 (2000) 303–308
The PRR effect on morphology evolution is significantly distinct for 100 pulse-irradiated PMMA ŽFig. 4.. The crater bottom for 2-Hz PRR is rough, porous and debris-contaminated ŽFig. 4a., whereas that for 10 Hz presents quite an even surface with little contamination ŽFig. 4b.. With the 2-Hz pulsed laser beam, increasing the shot number from 100 to 300 leads to an observable reduction in the amount of pores and particulate contamination ŽFig. 5a., although the surface still remains uneven. Nevertheless, for a PRR of 10 Hz, the shot number increment results in a smoother surface, on which some fine debris but no pores are observed ŽFig. 5b.. Thus, it seems plausible that a PRR rise leads to smoothing the ablated surfaces, and a shot number increment results in minimizing pores on the surfaces. In fact,
307
the increases in both PRR and the shot number render the thermal effects more striking. As a result, the sharper the temperature rise is, the more rapidly the gaseous products explode away carrying some of the surrounding substrates, with little risk being entrapped in the bulk polymer during resolidification, therefore, the less porous the ablated surface are. In particular, as PRR is increased, the irradiation process involving a given number of shots is accomplished in a shorter period, followed by a rapid resolidification process in air. Consequently, this prevents the polymer melt that is induced by diffused heat from spreading or flowing, driven by surface tension and back-pressure over the crater w14x, thereby leaving neat surfaces behind. However, particulate contamination is somewhat inevitable without a purge gas employed in the irradiation process since some of the ejected fragments are likely to react with ambient oxygen in air, and form the sooty debris that may be redeposited on the ablated surfaces.
4. Conclusions The coupling effects of PRR, fluence and the number of pulses are found existent on the etching depth data and topography of PMMA, ablated using a low-frequency 248-nm long-pulsed KrF excimer laser. The ablation rate can be improved identically by increasing either PRR, or fluence or the number of pulses. The multiple-shot effect strikes at a high repetition rate, and so does the PRR effect at a large number of shots. The PRR effect becomes more pronounced with increasing fluence. Morphologically, increasing PRR and the number of pulses gives rise to a crater with smoother and less porous bottom. It is thus recommended that both etching efficiency and morphology evolution should be taken into account when optimizing the laser operating conditions.
Acknowledgements
Fig. 5. SEM photographs of PMMA ablated by 300 laser pulses with a fluence of 0.7 Jrcm2 : Ža. at 2 Hz PRR; Žb. at 10 Hz PRR.
The authors acknowledge the financial support from the National Doctoral Research Foundation of the State Education Commission of China.
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