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Controlled process for polymer micromachining using designed pulse trains of a UV solid state laser
Applied Surface Science 254 (2007) 845–849 www.elsevier.com/locate/apsusc
Controlled process for polymer micromachining using designed pulse trains of a UV solid state laser Diana Ilie *, Claire Mullan, Gerard M. O’Connor, Tony Flaherty, Thomas J. Glynn National University of Ireland, Experimental Physics Department, National Centre for Laser Applications, University Road, Galway, Ireland Received 9 June 2007; received in revised form 24 July 2007; accepted 8 August 2007 Available online 14 August 2007
Abstract A flexible workstation equipped with a solid state laser operating at 266 nm wavelength was used to machine holes in polyethylene terephthalate, polyimide and polycarbonate. An optical pulse picker was employed to reduce the high repetition rates of the laser, while a breakthrough sensor was used to avoid over-drilling of through holes. For each material, different repetition rates and designed pulse trains were tested to improve feature quality and process efficiency. Although the three polymers had very different reactions at this wavelength they all showed an improvement in feature quality with decreasing repetition rate due to a reduction in thermal effects. Up to 10 kHz the average depth per pulse remained unchanged and afterwards a slight increase was observed but this was accompanied by large uncertainties. Bursts of pulses at 40 kHz inserted inside the low repetition rate pulse train reduced the drilling time and the amount of debris redeposited without affecting the feature quality. It was found that a number of cleaning pulses after perforation eliminates the heat affected zone around exits. Holes with entrance diameters below 20 mm and exit diameters as small as 2 mm were obtained with high repeatability. # 2007 Elsevier B.V. All rights reserved. Keywords: Polymer; Micromachining; UV solid state laser; Pulse picker; Breakthrough sensor
1. Introduction In industry there is a tendency towards miniaturization of products. In the medical device industry, for instance, pulmonary inhalers have membranes with 1–2 mm holes for deep lung drug delivery [1]. The correct medication dosage depends on the size and shape of the holes. This led to development of technologies capable of producing small and repetitive features for mass production. Until recently, excimer lasers in lithography-based methods combined with chemical etching were used for micromachining of small shaped features at an industrial scale. The results in terms of feature quality were good but the associated costs for maintenance and mask fabrication were high. One less expensive alternative to excimer laser sources are solid state lasers operating at different wavelengths. These lasers are compact systems with high power efficiency, very good beam quality and have the capability to achieve very high pulse energies. Still, there are a
* Corresponding author. Tel.: +353 91 49 2811; fax: +353 91 49 4594. E-mail address: [email protected] (D. Ilie). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.021
few problems associated with micromachining of polymers using solid state lasers. Pulse energy variation in the first pulses can give unpredictable results for a small number of pulses. Repetition rates available are very high, which can be an advantage for laser cutting and surface machining but in some cases like polymer drilling thermal loading affects feature quality. A few publications in the past studied the effect of repetition rate on ablation rates for polymer machining with pulsed lasers. For example it was suggested that such an effect would be visible in polyimide machined at 308 nm wavelength only at tens or hundreds of kilohertz while for VacrelTM 8230 photoresist the onset is visible between 10 and 300 Hz [2]. Illy et al. [3] used a copper vapour laser system operating at 255 nm wavelength to machine PETG and PI at pulse frequencies between 0.75 and 15 kHz and a fluence of 0.59 J cm 2. They observed a slight increase in mean etch rate with repetition rate for PETG but no change for PI. One publication by Li et al. [4] reports higher ablation rates for polyimide with long times between pulses compared to those obtained with repetition rates between 1 and 20 kHz from a DPSS laser working at 355 nm.
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D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
In this work we explore how these limitations associated with polymer drilling by DPSS lasers can be overcome by using a flexible optical setup. 2. Experimental The workstation used in this study was equipped with the High Peak Power Oscillator (HIPPO) from Spectra-Physics, a DPSS Q-switched Nd:YVO4 laser operating at 266 nm wavelength. The repetition rate ranged between 30 and 300 kHz with a maximum average output power of 2 W and a pulse duration of around 10 ns. A simplified scheme of the system setup is shown in Fig. 1. An acousto-optic modulator (AOM) was inserted in the setup to pick pulses from the pulse train without turning off the beam. When the AOM receives a driving signal it acts like a diffraction grating [5], the first order beam is directed to the sample with steering mirrors and focused by a UV achromatic objective lens, the zero order being blocked by a beam dump. As soon as the beam breaks through at the back of the sample a reflective surface placed underneath the sample directs it to the photodiode of the breakthrough sensor, which sends a signal to the pulse picker to deflect the beam from the workpiece and another signal to the CNC-driven motion stages. Then the stages move to the next machining place and the machining cycle is executed again as many times as required. For pulse energy control a combination of half waveplate and beam splitting cube was placed in the beam path. The dumped portion of the beam from the beam splitting cube was directed to a photodiode connected to a pulse counter to register the number of pulses used for machining. The thermoplastic 125 mm thick polymers used were polycarbonate (PC, linear absorption coefficient at 266 nm a266 104 cm 1), polyethylene terephthalate (PET, a266 105 cm 1) and KaptonTM HN (PI, a266 2 105 cm 1) all from Goodfellow. The linear absorption coefficients at 266 nm wavelength are estimated from [6]. Small pieces of material were mounted on a silicon frame in order to keep the sample flat for machining.
Fig. 2. Depth per pulse obtained for Kapton, PC and PET at 266 nm wavelength and 14.3 J cm 2 fluence. The values were calculated from the number of pulses required for breakthrough.
3. Results The effect of repetition rate was investigated by drilling arrays of holes in all three polymers using a fluence of 14.3 J cm 2. Ablation rates were determined from the number of pulses necessary to break through samples. For each repetition rate an array of 20 holes was machined and the results in Fig. 2 are an average of these. The results in Fig. 2 suggest that there is no dependency for ablation rates on pulse frequency up to approximately 10 kHz. A change appears at frequencies higher than 10 kHz in all three cases. At low repetition rates the error spread in depth per pulse is small, almost negligible but beyond 10 kHz they become more significant. Feature size and quality were determined with a scanning electron microscope. In all cases the diameters, both entrance and exit, increase slightly with repetition rate. A heat affected zone (HAZ) is formed around all entrances. Profiles taken with a white light interferometer revealed that the affected areas are raised above the initial surface of the sample. The thickness and
Fig. 1. Experimental setup.
D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
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Fig. 3. Entrance and corresponding exit for holes machined in PET with 14.3 J cm 2 at 10 Hz (a, entrance and d, exit), 20 kHz (b, entrance and e, exit) and 33 kHz (c, entrance and f, exit). Three cleaning pulses were used post-break through for the exits.
radius of these rings around the holes at the entrance depend on the time between successive pulses. The pictures in Fig. 3 show this evolution on PET from 10 Hz (Fig. 3a and d) to 15 kHz (Fig. 3b and e) and finally at 33 kHz (Fig. 3c and f). Similar features were formed on PC and Kapton HN samples but to a lesser extent. The evolution of the exit holes in Kapton HN machined at 13.1 J cm 2 is shown in Fig. 4. It is suggested that the pressure inside the holes builds with successive pulses and pushes the underlying thinned layer of material outwards (Fig. 4a) making an explosive perforation with the first pulse (Fig. 4b). The shape of the perforation after the first breakthrough pulse is irregular and the surrounding HAZ is
large. With our system we have the possibility to adjust the number of pulses that break through the exit aperture of the hole. For example, if we allow 2 pulses to pass, the shape improves and part of the raised layer is removed (Fig. 4c). A very clean hole with virtually no rim around it can be obtained with a small number of additional pulses. In Fig. 4d, approximately 10 pulses were used to clean the exit hole. The size of the entrance holes is around 20 mm for PET and around 16–17 mm for the other two polymers, while the exits are around 5 mm in diameter for all. These values were obtained at a fluence of 14.3 J cm 2. With lower fluences smaller diameters can be achieved. This will be addressed later in the discussion section.
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Fig. 4. Evolution of exit holes in Kapton machined at 13.1 J cm 2, just before perforation (a), after the first pulse has gone through (b), after two pulses (c) and after 10 pulses (d).
The deposition of debris around the machined features depends on the repetition rate used and the machining strategy. In Fig. 5a a large amount of debris particles, approximately 20 nm in size was observed around the ablation site at low pulse frequencies (Fig. 5a, left). A reduction in debris deposits takes place (Fig. 5a, right) with an increasing pulse repetition rate. This indicates that the interval between subsequent pulses can alter the vaporization and deposition of ablation products. Using the flexibility of our system we were able to introduce bursts of high frequency within the low frequency pulse train.
These bursts are at the laser repetition rate of 40 kHz. Rows of via holes machined at 100 Hz (Fig. 5b, left) and 5 kHz (Fig. 5b, right) with varying pulses per burst were produced and then examined with the scanning electron microscope. As can be seen in Fig. 5b for up to four pulses per burst at 100 Hz and 5 kHz on PET these bursts help to produce cleaner samples without affecting the removal rate or the quality of the holes. With five pulses per burst at 5 kHz a significant HAZ is formed. It appears that there is a limitation on how many pulses per burst can be used to obtain cleaner samples. The number depends on
Fig. 5. Scanning electron micrographs showing debris redeposition on PET machined at 14.3 J cm 2. On the left (a) different repetition rates were used as indicated under each column with the values in kHz. Arrays of holes were machined with repetition rates of 100 Hz (b, left) and 5 kHz (b, right) and different pulses per burst. The number of pulses per burst is indicated under each column.
D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
the type of material used and on processing parameters such as fluence. The same experiment on PC and Kapton revealed that using up to 10 pulses per burst does not produce an increased HAZ or an increase in diameters (not shown). 4. Discussion In this paper we report ablation rates (average depth attained per pulse) obtained by counting the actual pulses used for breakthrough. This method provides more accurate ablation rates than those obtained previously by timing the laser drilling process where uncertainties in the timer underestimated the duration for breakthrough [7]. We observed that parameters such as fluence affect the results in terms of depth per pulse, amount of debris redeposited, size and quality of the feature. Lower fluences could minimize the dimensions of the hole and the HAZ but the drilling time would increase or material removal can cease before breakthrough. Exit holes obtained after the first perforation pulse were very small but the quality was poor. Applying a few cleaning pulses afterwards improved the shape and removed HAZ. We used three cleaning pulses for PET machined at 14.3 J cm 2 and up to 10 pulses for Kapton machined at 13.1 J cm 2. The number of cleaning pulses can be influenced by various factors like fluence, material type and the quality desired. The amount of debris deposited around holes can be minimized using high repetition rates or burst of high repetition in low frequency trains. Further investigations with the scanning electron microscope revealed that the debris particles around features machined at 100 Hz are grouped into clusters around 100 mm in size. When pulse frequency increases or bursts are inserted in the pulse train, the cluster size starts to decrease and they almost disappear at 40 kHz. A better
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fragmentation of the ablation products results in smaller deposits around the machined site. There are no definitive values for the best parameters, rather their combined effects on the material to be processed should be taken into account in meeting the requirements for the final feature. 5. Conclusions Reproducible arrays of holes in polymers were made using a highly flexible system equipped with a 266 nm laser and an acousto-optic modulator. A slight increase in ablation rates was observed for the frequencies higher than 10 kHz but it was accompanied by large uncertainties. Heat accumulated at the irradiation site from successive pulses affects the quality and size of the features. A long time interval between pulses allows heat to dissipate into the bulk before the next pulse arrives but also gives the ablation products enough time to settle around the hole. Different machining strategies such as changing the number of pulses per burst can further improve the quality of hole features. References [1] S. Farr, J. Schuster, N. Christa, Drug Deliv. Technol. 2 (3) (2002). [2] F.C. Burns, S.R. Cain, J. Phys. D-Appl. Phys. 29 (1996) 1349–1355. [3] E.K. Illy, D.J.W. Brown, M.J. Whitford, J.A. Piper, IEEE J. Selected Top. Quantum Electron. 5 (1999) 1543–1548. [4] Z. Li, S.Z. You, L. Lui, N.L. Yakovlev, P.M. Moran, Proc. SPIE 4637 (2002) 43–53. [5] J. Wilson, J.F.B. Hawkes, Optoelectronics: An Introduction, Prentice Hall Ltd., UK, 1989, p. 99. [6] D. Bauerle, Laser Processing and Chemistry, in: Advanced Texts in Physics, third rev. and enlarged ed., Springer, 2000,, p. 702. [7] C. Mullan, D. Ilie, G.M. O’Connor, S. Favre, T.J. Glynn, Proc. SPIE 6459 (2007) 64590G.
Controlled process for polymer micromachining using designed pulse trains of a UV solid state laser Diana Ilie *, Claire Mullan, Gerard M. O’Connor, Tony Flaherty, Thomas J. Glynn National University of Ireland, Experimental Physics Department, National Centre for Laser Applications, University Road, Galway, Ireland Received 9 June 2007; received in revised form 24 July 2007; accepted 8 August 2007 Available online 14 August 2007
Abstract A flexible workstation equipped with a solid state laser operating at 266 nm wavelength was used to machine holes in polyethylene terephthalate, polyimide and polycarbonate. An optical pulse picker was employed to reduce the high repetition rates of the laser, while a breakthrough sensor was used to avoid over-drilling of through holes. For each material, different repetition rates and designed pulse trains were tested to improve feature quality and process efficiency. Although the three polymers had very different reactions at this wavelength they all showed an improvement in feature quality with decreasing repetition rate due to a reduction in thermal effects. Up to 10 kHz the average depth per pulse remained unchanged and afterwards a slight increase was observed but this was accompanied by large uncertainties. Bursts of pulses at 40 kHz inserted inside the low repetition rate pulse train reduced the drilling time and the amount of debris redeposited without affecting the feature quality. It was found that a number of cleaning pulses after perforation eliminates the heat affected zone around exits. Holes with entrance diameters below 20 mm and exit diameters as small as 2 mm were obtained with high repeatability. # 2007 Elsevier B.V. All rights reserved. Keywords: Polymer; Micromachining; UV solid state laser; Pulse picker; Breakthrough sensor
1. Introduction In industry there is a tendency towards miniaturization of products. In the medical device industry, for instance, pulmonary inhalers have membranes with 1–2 mm holes for deep lung drug delivery [1]. The correct medication dosage depends on the size and shape of the holes. This led to development of technologies capable of producing small and repetitive features for mass production. Until recently, excimer lasers in lithography-based methods combined with chemical etching were used for micromachining of small shaped features at an industrial scale. The results in terms of feature quality were good but the associated costs for maintenance and mask fabrication were high. One less expensive alternative to excimer laser sources are solid state lasers operating at different wavelengths. These lasers are compact systems with high power efficiency, very good beam quality and have the capability to achieve very high pulse energies. Still, there are a
* Corresponding author. Tel.: +353 91 49 2811; fax: +353 91 49 4594. E-mail address: [email protected] (D. Ilie). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.021
few problems associated with micromachining of polymers using solid state lasers. Pulse energy variation in the first pulses can give unpredictable results for a small number of pulses. Repetition rates available are very high, which can be an advantage for laser cutting and surface machining but in some cases like polymer drilling thermal loading affects feature quality. A few publications in the past studied the effect of repetition rate on ablation rates for polymer machining with pulsed lasers. For example it was suggested that such an effect would be visible in polyimide machined at 308 nm wavelength only at tens or hundreds of kilohertz while for VacrelTM 8230 photoresist the onset is visible between 10 and 300 Hz [2]. Illy et al. [3] used a copper vapour laser system operating at 255 nm wavelength to machine PETG and PI at pulse frequencies between 0.75 and 15 kHz and a fluence of 0.59 J cm 2. They observed a slight increase in mean etch rate with repetition rate for PETG but no change for PI. One publication by Li et al. [4] reports higher ablation rates for polyimide with long times between pulses compared to those obtained with repetition rates between 1 and 20 kHz from a DPSS laser working at 355 nm.
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D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
In this work we explore how these limitations associated with polymer drilling by DPSS lasers can be overcome by using a flexible optical setup. 2. Experimental The workstation used in this study was equipped with the High Peak Power Oscillator (HIPPO) from Spectra-Physics, a DPSS Q-switched Nd:YVO4 laser operating at 266 nm wavelength. The repetition rate ranged between 30 and 300 kHz with a maximum average output power of 2 W and a pulse duration of around 10 ns. A simplified scheme of the system setup is shown in Fig. 1. An acousto-optic modulator (AOM) was inserted in the setup to pick pulses from the pulse train without turning off the beam. When the AOM receives a driving signal it acts like a diffraction grating [5], the first order beam is directed to the sample with steering mirrors and focused by a UV achromatic objective lens, the zero order being blocked by a beam dump. As soon as the beam breaks through at the back of the sample a reflective surface placed underneath the sample directs it to the photodiode of the breakthrough sensor, which sends a signal to the pulse picker to deflect the beam from the workpiece and another signal to the CNC-driven motion stages. Then the stages move to the next machining place and the machining cycle is executed again as many times as required. For pulse energy control a combination of half waveplate and beam splitting cube was placed in the beam path. The dumped portion of the beam from the beam splitting cube was directed to a photodiode connected to a pulse counter to register the number of pulses used for machining. The thermoplastic 125 mm thick polymers used were polycarbonate (PC, linear absorption coefficient at 266 nm a266 104 cm 1), polyethylene terephthalate (PET, a266 105 cm 1) and KaptonTM HN (PI, a266 2 105 cm 1) all from Goodfellow. The linear absorption coefficients at 266 nm wavelength are estimated from [6]. Small pieces of material were mounted on a silicon frame in order to keep the sample flat for machining.
Fig. 2. Depth per pulse obtained for Kapton, PC and PET at 266 nm wavelength and 14.3 J cm 2 fluence. The values were calculated from the number of pulses required for breakthrough.
3. Results The effect of repetition rate was investigated by drilling arrays of holes in all three polymers using a fluence of 14.3 J cm 2. Ablation rates were determined from the number of pulses necessary to break through samples. For each repetition rate an array of 20 holes was machined and the results in Fig. 2 are an average of these. The results in Fig. 2 suggest that there is no dependency for ablation rates on pulse frequency up to approximately 10 kHz. A change appears at frequencies higher than 10 kHz in all three cases. At low repetition rates the error spread in depth per pulse is small, almost negligible but beyond 10 kHz they become more significant. Feature size and quality were determined with a scanning electron microscope. In all cases the diameters, both entrance and exit, increase slightly with repetition rate. A heat affected zone (HAZ) is formed around all entrances. Profiles taken with a white light interferometer revealed that the affected areas are raised above the initial surface of the sample. The thickness and
Fig. 1. Experimental setup.
D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
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Fig. 3. Entrance and corresponding exit for holes machined in PET with 14.3 J cm 2 at 10 Hz (a, entrance and d, exit), 20 kHz (b, entrance and e, exit) and 33 kHz (c, entrance and f, exit). Three cleaning pulses were used post-break through for the exits.
radius of these rings around the holes at the entrance depend on the time between successive pulses. The pictures in Fig. 3 show this evolution on PET from 10 Hz (Fig. 3a and d) to 15 kHz (Fig. 3b and e) and finally at 33 kHz (Fig. 3c and f). Similar features were formed on PC and Kapton HN samples but to a lesser extent. The evolution of the exit holes in Kapton HN machined at 13.1 J cm 2 is shown in Fig. 4. It is suggested that the pressure inside the holes builds with successive pulses and pushes the underlying thinned layer of material outwards (Fig. 4a) making an explosive perforation with the first pulse (Fig. 4b). The shape of the perforation after the first breakthrough pulse is irregular and the surrounding HAZ is
large. With our system we have the possibility to adjust the number of pulses that break through the exit aperture of the hole. For example, if we allow 2 pulses to pass, the shape improves and part of the raised layer is removed (Fig. 4c). A very clean hole with virtually no rim around it can be obtained with a small number of additional pulses. In Fig. 4d, approximately 10 pulses were used to clean the exit hole. The size of the entrance holes is around 20 mm for PET and around 16–17 mm for the other two polymers, while the exits are around 5 mm in diameter for all. These values were obtained at a fluence of 14.3 J cm 2. With lower fluences smaller diameters can be achieved. This will be addressed later in the discussion section.
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Fig. 4. Evolution of exit holes in Kapton machined at 13.1 J cm 2, just before perforation (a), after the first pulse has gone through (b), after two pulses (c) and after 10 pulses (d).
The deposition of debris around the machined features depends on the repetition rate used and the machining strategy. In Fig. 5a a large amount of debris particles, approximately 20 nm in size was observed around the ablation site at low pulse frequencies (Fig. 5a, left). A reduction in debris deposits takes place (Fig. 5a, right) with an increasing pulse repetition rate. This indicates that the interval between subsequent pulses can alter the vaporization and deposition of ablation products. Using the flexibility of our system we were able to introduce bursts of high frequency within the low frequency pulse train.
These bursts are at the laser repetition rate of 40 kHz. Rows of via holes machined at 100 Hz (Fig. 5b, left) and 5 kHz (Fig. 5b, right) with varying pulses per burst were produced and then examined with the scanning electron microscope. As can be seen in Fig. 5b for up to four pulses per burst at 100 Hz and 5 kHz on PET these bursts help to produce cleaner samples without affecting the removal rate or the quality of the holes. With five pulses per burst at 5 kHz a significant HAZ is formed. It appears that there is a limitation on how many pulses per burst can be used to obtain cleaner samples. The number depends on
Fig. 5. Scanning electron micrographs showing debris redeposition on PET machined at 14.3 J cm 2. On the left (a) different repetition rates were used as indicated under each column with the values in kHz. Arrays of holes were machined with repetition rates of 100 Hz (b, left) and 5 kHz (b, right) and different pulses per burst. The number of pulses per burst is indicated under each column.
D. Ilie et al. / Applied Surface Science 254 (2007) 845–849
the type of material used and on processing parameters such as fluence. The same experiment on PC and Kapton revealed that using up to 10 pulses per burst does not produce an increased HAZ or an increase in diameters (not shown). 4. Discussion In this paper we report ablation rates (average depth attained per pulse) obtained by counting the actual pulses used for breakthrough. This method provides more accurate ablation rates than those obtained previously by timing the laser drilling process where uncertainties in the timer underestimated the duration for breakthrough [7]. We observed that parameters such as fluence affect the results in terms of depth per pulse, amount of debris redeposited, size and quality of the feature. Lower fluences could minimize the dimensions of the hole and the HAZ but the drilling time would increase or material removal can cease before breakthrough. Exit holes obtained after the first perforation pulse were very small but the quality was poor. Applying a few cleaning pulses afterwards improved the shape and removed HAZ. We used three cleaning pulses for PET machined at 14.3 J cm 2 and up to 10 pulses for Kapton machined at 13.1 J cm 2. The number of cleaning pulses can be influenced by various factors like fluence, material type and the quality desired. The amount of debris deposited around holes can be minimized using high repetition rates or burst of high repetition in low frequency trains. Further investigations with the scanning electron microscope revealed that the debris particles around features machined at 100 Hz are grouped into clusters around 100 mm in size. When pulse frequency increases or bursts are inserted in the pulse train, the cluster size starts to decrease and they almost disappear at 40 kHz. A better
849
fragmentation of the ablation products results in smaller deposits around the machined site. There are no definitive values for the best parameters, rather their combined effects on the material to be processed should be taken into account in meeting the requirements for the final feature. 5. Conclusions Reproducible arrays of holes in polymers were made using a highly flexible system equipped with a 266 nm laser and an acousto-optic modulator. A slight increase in ablation rates was observed for the frequencies higher than 10 kHz but it was accompanied by large uncertainties. Heat accumulated at the irradiation site from successive pulses affects the quality and size of the features. A long time interval between pulses allows heat to dissipate into the bulk before the next pulse arrives but also gives the ablation products enough time to settle around the hole. Different machining strategies such as changing the number of pulses per burst can further improve the quality of hole features. References [1] S. Farr, J. Schuster, N. Christa, Drug Deliv. Technol. 2 (3) (2002). [2] F.C. Burns, S.R. Cain, J. Phys. D-Appl. Phys. 29 (1996) 1349–1355. [3] E.K. Illy, D.J.W. Brown, M.J. Whitford, J.A. Piper, IEEE J. Selected Top. Quantum Electron. 5 (1999) 1543–1548. [4] Z. Li, S.Z. You, L. Lui, N.L. Yakovlev, P.M. Moran, Proc. SPIE 4637 (2002) 43–53. [5] J. Wilson, J.F.B. Hawkes, Optoelectronics: An Introduction, Prentice Hall Ltd., UK, 1989, p. 99. [6] D. Bauerle, Laser Processing and Chemistry, in: Advanced Texts in Physics, third rev. and enlarged ed., Springer, 2000,, p. 702. [7] C. Mullan, D. Ilie, G.M. O’Connor, S. Favre, T.J. Glynn, Proc. SPIE 6459 (2007) 64590G.