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Laser ablation efficiency of metal samples with UV laser nanosecond pulses
Applied Surface Science 138–139 Ž1999. 302–305
Laser ablation efficiency of metal samples with UV laser nanosecond pulses B. Salle, V. Detalle, J.L. Lacour, P. Mauchien, C. Nouvellon, ´ C. Chaleard, ´ A. Semerok ) CEA Saclay, DCC r DPE r SPCPr LSLA, 91191 Gif sur YÕette Cedex, France
Abstract Laser ablation of pure metals by nanosecond UV pulses is studied experimentally. The influence of laser pulse number, matter properties and pulse energy on the crater sizes was investigated. The experimental results obtained in the range 0.5 to 50 GWrcm2 , in air at atmospheric pressure are presented and compared with an existing model. A sufficiently good correlation was observed between melting temperature and ablation efficiency. q 1999 Elsevier Science B.V. All rights reserved. PACS: 79.20.D Keywords: UV nanosecond laser ablation; Ablation efficiency; Metal samples
1. Introduction The interaction of a high intensity laser pulse with a solid sample results in the crater formation. As the laser–target interaction depends on physical properties of solid, environmental conditions and laser parameters w1–4x Žwavelength, pulse duration, energy, beam diameter., the characterisation of crater by its depth, diameter and volume is an interesting method to understand the physics of the ablation process in air.
)
Corresponding author. Tel.: q33-1-69-08-65-57; Fax: q331-69-08-77-38; E-mail: [email protected]
2. Experiments The experiments were performed in air at atmospheric pressure with KrF excimer ŽLambda Physic EMG 201 MSC, 248 nm, 28 ns, size: 25 = 6 mm, divergence: 4 = 2 mrad. and Nd-YAG ŽContinuum Minilite, 266 nm, 4 ns, diameter: 1.5 mm, divergence: 2.5 mrad. lasers. A spatial filtering was used to obtain a more homogeneous intensity distribution in the zone of laser beam–surface target interaction: a diaphragm D located in the laser beam was imaged on the target surface by an optical objective F. For the KrF laser beam with the diaphragm aperture D s 0.7 mm and the lens of focal length F s 250 mm, the intensity distribution ŽFig. 1. in the interaction zone has a diameter of 150 mm ŽFWHM.. For
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 4 9 5 - 4
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
303
Fig. 1. Central section of the intensity distribution of KrF laser beam in the interaction zone.
Nd-YAG laser with D s 0.15 mm and F s 24 mm, the intensity distribution was close to the Gaussian one with a diameter of 8 mm ŽFWHM.. The laser pulse intensities were varied from 0.5 to 50 GWrcm2 . The ablation was realised for different pulse numbers with a large set of metal samples ŽMo, Fe, Mn, Cu, Al, Zn, Pb and Sn. chosen for their various matter parameters to investigate the influence of matter properties on the laser ablation process. The craters were characterised by an Optical Microscope Profilometer ŽMicroXam-Phase Shift Technology. with a lateral resolution ( 0.5 mm and a depth resolution ( 0.01 mm.
3. Experimental results and discussion Figs. 2 and 3 give the typical crater profiles obtained respectively after ten KrF laser pulses at 0.8 and 2 GWrcm2 and after one Nd-YAG laser shot at 44 GWrcm2 . On the target surface around the crater we can observe the presence of matter. The height of this matter around the crater depends on the target, the energy and the pulse number. As the energy or the number of shots was increased, the ratio of this matter height to the crater depth decreased. After one KrF laser pulse, the matter volume around the crater was approximately the same as the crater volume. For most of the targets, the crater profile did not correspond to the spatial laser intensity distribution, particularly in the case of KrF laser. We think that it is probably due to the recondensation process. Particularly at atmospheric pressure, the particles of ambient air prevent the ablated matter from escaping far away from the target surface. The more plasma is close to the surface, the more ablated matter recon-
Fig. 2. Typical central crater profiles for different targets after ten KrF laser pulses with energies Es14 mJ Žleft. and Es 5 mJ Žright.. Reference scales in mm.
densates on the target. The same effect may explain the increase of the matter height around the crater with the pulse number increase. The crater transversal dimensions were characterised by its diameter D 0.5 at FWHM and its outer diameter Dm measured on the target surface level. For KrF laser, D 0.5 was independent from the pulse number. There was a good coincidence between D 0.5 of Al and the laser beam. This was not observed for other targets. Table 1 gives the diameters Dm obtained with various metals in different irradiation conditions. As the laser pulse energy increased, Dm increased.
Fig. 3. Typical central crater profiles for different targets after single Ž266 nm, 4 ns, 130 mJ. Nd-YAG laser pulsersurface interaction. Reference scales in mm.
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
304
Table 1 Crater diameter in mm and crater depth in mmrpulse after 10 KrF laser pulses and single Nd-YAG laser pulse Diameter Dm Žmm. Laser:
KrF
Depth Žmmrpulse. Nd-YAG
KrF
Nd-YAG
Target
E s 14 mJ
E s 5 mJ
E s 130 mJ
E s 65 mJ
E s 14 mJ
E s 5 mJ
E s 130 mJ
E s 65 mJ
Al Zn Sn Fe Pb Mn Mo Cu
230 200 280 190 270 250 170 180
190 134 220 120
23
20
16
15.5 27
1.5 1.0 3.0 0.46
20.2
12.4 34
5.5 16.9
2.5 12.5
15.4
14.5
3.0 1.5 3.9 0.55 3.1 0.75 0.66 1.0
120 120
For all matters ablated by the KrF laser, we observed that the crater depth increased linearly with the pulse number increase. In this case, the crater depth was smaller than the crater diameter. For the craters formed by the Nd-YAG laser, the depth was comparable with the diameter just after a single laser shot and the depth did not increase linearly with the pulse number. Table 1 presents the mean depth obtained with various targets and different irradiation conditions. In all cases, the depth increased with the increase of the laser energy. We tried to establish correlation between matter properties and the crater characteristics. For the same laser beam parameters, the lower the melting temperature was, the larger the crater diameter was ŽTable 1.. A sufficiently good relation was found between the crater depth and the melting temperature ŽFig. 4.. The ablation efficiency was defined as the ratio of the crater depth Žmmrpulse. to the laser fluence ŽJrcm2 , determined by dividing the pulse energy by
0.5 0.65
9.7
6.45
the intensity distribution area at FWHM.. Table 2 gives the ablation efficiency for various targets and different irradiation conditions. The ablation efficiency was found to depend on the beam spot diameter, laser fluence and target matter. Experimentally, for KrF and Nd-YAG lasers it was observed that the reducing of laser spot size resulted in the increase of the crater depth for the same laser energy. But the reducing of laser spot size led to significantly less changes in the ablation efficiency. The influence of the spot size on the ablation efficiency may be explained as follows: the lateral expansion of the near-surface plasma is faster in case of a smaller spot diameter w4–8x. This leads to a three-dimensional plasma expansion during the pulse instead of an expansion in one dimension, normal to the target surface for larger spot sizes w7x. The incident laser is attenuated by the ablated species w8x. For larger spot diameter where the plasma expansion is one-dimen-
Table 2 Ablation efficiency in wŽmmrpulse.rŽJrcm2 .x
Fig. 4. Crater depth h vs. melting temperature. Ablation parameters are the same as in Table 1 for 14 mJ KrF laser pulse.
Laser:
KrF
Target
14 mJ
5 mJ
Nd-YAG 130 mJ
65 mJ
Al Zn Sn Fe Pb Mn Mo Cu
0.038 0.019 0.049 0.007 0.039 0.009 0.008 0.013
0.054 0.036 0.107 0.016
0.078
0.123
0.021 0.065
0.019 0.096
0.037
0.05
0.018 0.023
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
305
assumption of one-dimensional plasma expansion was not valid in this case. This smaller spot size allows more spherically symmetric expansion. Thereby, other models are needed to describe the ablation process with small interaction surfaces.
4. Conclusion Fig. 5. Crater depth vs. atomic number of the target particles. Ablation parameters are the same as in Table 1 for 14 mJ KrF laser pulse. '—Experimental data, v —Evaluated data.
sional, a higher density of the ablated species is within the laser beam path. Thereby, the attenuation of the laser in plasma is greater with larger spot sizes and the effective fluence on the target surface is lower leading to a decrease of the crater depth. The ablation depths were compared with theoretical results estimated by the following expression based on the assumption of a one-dimensional plasma expansion w9x:
C 9r8 h Ž mm . ( 2.66
r Ž mgrcm3 . A1r4 Ž a.m.u. .
=I 1r2 Ž Wrcm2 . ly1 r2 Ž cm . t 3r4 Ž s .
Ž 1. where C s 0.5 A Ža.m.u.. w Z 2 Ž Z q 1.xy1 r3, A is the atomic mass, Z the charge state of the ions Žwe supposed ions single charged with Z s 1., r the target density, I the laser beam intensity and t the pulse duration. Fig. 5 compares the experimental depth obtained with the KrF laser beam with the values calculated by Eq. Ž1.. The theoretical ablation depth was found in relatively good agreement with the experimental results. Thus, we concluded that the KrF laser experiments Žspot diameter 150 mm. can be described on the basis of the model proposed in Ref. w9x. The plasma expansion during the pulse can be considered one-dimensional. For the Nd-YAG experiments with a spot diameter of 8 mm, the theoretical ablation depth was significantly lower Ž10 times and more. than the experimental results. The
The nanosecond UV laser ablation experiments at atmospheric pressure for different metal samples allowed us to determine the dependence of the ablation efficiency with the parameters of laser–target interaction. The influence of the pulse number, the incident laser energy, the laser beam diameter and the properties of metals was studied. Experimental ablation depths obtained in excimer laser experiments were in good agreement with the theoretical values calculated with the Phipps et al. expression w9x. But further experimental and theoretical investigations are needed for a better understanding of the correlation obtained between ablation efficiencies and melting temperatures.
References w1x M. Von Allmen, Laser Beam Interactions with Materials, Springer, Berlin, 1987. w2x A.M. Prokhorov, V.I. Konov, I. Ursu, I.N. Mihailescu, Laser Heating of Metals, Hilger, 1990. w3x E.N. Sobol, Phase Transformations and Ablation in LaserTreated Solids, Wiley-Interscience Publication, NY, 1995. w4x D. Bauerle, Laser Processing and Chemistry, Springer, Berlin, ¨ 1996. w5x M. Eyett, D. Bauerle, Appl. Phys. Lett. 51 Ž1987. 2054. ¨ w6x J. Heitz, X.Z. Wang, P. Schwab, D. Bauerle, L. Schultz, J. ¨ Appl. Phys. 68 Ž1990. 2512. w7x A. Giordini Guidoni, R. Kelly, A. Mele, A. Miotello, Plasma Sources Sci. Technol. 6 Ž1997. 260. w8x B. Wolff-Rottke, J. Ihlemann, H. Schmidt, A. Scholl, Appl. Phys. A 60 Ž1995. 13. w9x C.R. Phipps, T.P. Turner, R.F. Harrison, G.W. York, W.Z. Osborne, G.K. Anderson, X.F. Corlis, L.C. Haynes, H.S. Steele, K.C. Spicochi, T.R. King, J. Appl. Phys. 64 Ž1988. 1083.
Laser ablation efficiency of metal samples with UV laser nanosecond pulses B. Salle, V. Detalle, J.L. Lacour, P. Mauchien, C. Nouvellon, ´ C. Chaleard, ´ A. Semerok ) CEA Saclay, DCC r DPE r SPCPr LSLA, 91191 Gif sur YÕette Cedex, France
Abstract Laser ablation of pure metals by nanosecond UV pulses is studied experimentally. The influence of laser pulse number, matter properties and pulse energy on the crater sizes was investigated. The experimental results obtained in the range 0.5 to 50 GWrcm2 , in air at atmospheric pressure are presented and compared with an existing model. A sufficiently good correlation was observed between melting temperature and ablation efficiency. q 1999 Elsevier Science B.V. All rights reserved. PACS: 79.20.D Keywords: UV nanosecond laser ablation; Ablation efficiency; Metal samples
1. Introduction The interaction of a high intensity laser pulse with a solid sample results in the crater formation. As the laser–target interaction depends on physical properties of solid, environmental conditions and laser parameters w1–4x Žwavelength, pulse duration, energy, beam diameter., the characterisation of crater by its depth, diameter and volume is an interesting method to understand the physics of the ablation process in air.
)
Corresponding author. Tel.: q33-1-69-08-65-57; Fax: q331-69-08-77-38; E-mail: [email protected]
2. Experiments The experiments were performed in air at atmospheric pressure with KrF excimer ŽLambda Physic EMG 201 MSC, 248 nm, 28 ns, size: 25 = 6 mm, divergence: 4 = 2 mrad. and Nd-YAG ŽContinuum Minilite, 266 nm, 4 ns, diameter: 1.5 mm, divergence: 2.5 mrad. lasers. A spatial filtering was used to obtain a more homogeneous intensity distribution in the zone of laser beam–surface target interaction: a diaphragm D located in the laser beam was imaged on the target surface by an optical objective F. For the KrF laser beam with the diaphragm aperture D s 0.7 mm and the lens of focal length F s 250 mm, the intensity distribution ŽFig. 1. in the interaction zone has a diameter of 150 mm ŽFWHM.. For
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 4 9 5 - 4
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
303
Fig. 1. Central section of the intensity distribution of KrF laser beam in the interaction zone.
Nd-YAG laser with D s 0.15 mm and F s 24 mm, the intensity distribution was close to the Gaussian one with a diameter of 8 mm ŽFWHM.. The laser pulse intensities were varied from 0.5 to 50 GWrcm2 . The ablation was realised for different pulse numbers with a large set of metal samples ŽMo, Fe, Mn, Cu, Al, Zn, Pb and Sn. chosen for their various matter parameters to investigate the influence of matter properties on the laser ablation process. The craters were characterised by an Optical Microscope Profilometer ŽMicroXam-Phase Shift Technology. with a lateral resolution ( 0.5 mm and a depth resolution ( 0.01 mm.
3. Experimental results and discussion Figs. 2 and 3 give the typical crater profiles obtained respectively after ten KrF laser pulses at 0.8 and 2 GWrcm2 and after one Nd-YAG laser shot at 44 GWrcm2 . On the target surface around the crater we can observe the presence of matter. The height of this matter around the crater depends on the target, the energy and the pulse number. As the energy or the number of shots was increased, the ratio of this matter height to the crater depth decreased. After one KrF laser pulse, the matter volume around the crater was approximately the same as the crater volume. For most of the targets, the crater profile did not correspond to the spatial laser intensity distribution, particularly in the case of KrF laser. We think that it is probably due to the recondensation process. Particularly at atmospheric pressure, the particles of ambient air prevent the ablated matter from escaping far away from the target surface. The more plasma is close to the surface, the more ablated matter recon-
Fig. 2. Typical central crater profiles for different targets after ten KrF laser pulses with energies Es14 mJ Žleft. and Es 5 mJ Žright.. Reference scales in mm.
densates on the target. The same effect may explain the increase of the matter height around the crater with the pulse number increase. The crater transversal dimensions were characterised by its diameter D 0.5 at FWHM and its outer diameter Dm measured on the target surface level. For KrF laser, D 0.5 was independent from the pulse number. There was a good coincidence between D 0.5 of Al and the laser beam. This was not observed for other targets. Table 1 gives the diameters Dm obtained with various metals in different irradiation conditions. As the laser pulse energy increased, Dm increased.
Fig. 3. Typical central crater profiles for different targets after single Ž266 nm, 4 ns, 130 mJ. Nd-YAG laser pulsersurface interaction. Reference scales in mm.
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
304
Table 1 Crater diameter in mm and crater depth in mmrpulse after 10 KrF laser pulses and single Nd-YAG laser pulse Diameter Dm Žmm. Laser:
KrF
Depth Žmmrpulse. Nd-YAG
KrF
Nd-YAG
Target
E s 14 mJ
E s 5 mJ
E s 130 mJ
E s 65 mJ
E s 14 mJ
E s 5 mJ
E s 130 mJ
E s 65 mJ
Al Zn Sn Fe Pb Mn Mo Cu
230 200 280 190 270 250 170 180
190 134 220 120
23
20
16
15.5 27
1.5 1.0 3.0 0.46
20.2
12.4 34
5.5 16.9
2.5 12.5
15.4
14.5
3.0 1.5 3.9 0.55 3.1 0.75 0.66 1.0
120 120
For all matters ablated by the KrF laser, we observed that the crater depth increased linearly with the pulse number increase. In this case, the crater depth was smaller than the crater diameter. For the craters formed by the Nd-YAG laser, the depth was comparable with the diameter just after a single laser shot and the depth did not increase linearly with the pulse number. Table 1 presents the mean depth obtained with various targets and different irradiation conditions. In all cases, the depth increased with the increase of the laser energy. We tried to establish correlation between matter properties and the crater characteristics. For the same laser beam parameters, the lower the melting temperature was, the larger the crater diameter was ŽTable 1.. A sufficiently good relation was found between the crater depth and the melting temperature ŽFig. 4.. The ablation efficiency was defined as the ratio of the crater depth Žmmrpulse. to the laser fluence ŽJrcm2 , determined by dividing the pulse energy by
0.5 0.65
9.7
6.45
the intensity distribution area at FWHM.. Table 2 gives the ablation efficiency for various targets and different irradiation conditions. The ablation efficiency was found to depend on the beam spot diameter, laser fluence and target matter. Experimentally, for KrF and Nd-YAG lasers it was observed that the reducing of laser spot size resulted in the increase of the crater depth for the same laser energy. But the reducing of laser spot size led to significantly less changes in the ablation efficiency. The influence of the spot size on the ablation efficiency may be explained as follows: the lateral expansion of the near-surface plasma is faster in case of a smaller spot diameter w4–8x. This leads to a three-dimensional plasma expansion during the pulse instead of an expansion in one dimension, normal to the target surface for larger spot sizes w7x. The incident laser is attenuated by the ablated species w8x. For larger spot diameter where the plasma expansion is one-dimen-
Table 2 Ablation efficiency in wŽmmrpulse.rŽJrcm2 .x
Fig. 4. Crater depth h vs. melting temperature. Ablation parameters are the same as in Table 1 for 14 mJ KrF laser pulse.
Laser:
KrF
Target
14 mJ
5 mJ
Nd-YAG 130 mJ
65 mJ
Al Zn Sn Fe Pb Mn Mo Cu
0.038 0.019 0.049 0.007 0.039 0.009 0.008 0.013
0.054 0.036 0.107 0.016
0.078
0.123
0.021 0.065
0.019 0.096
0.037
0.05
0.018 0.023
B. Salle´ et al.r Applied Surface Science 138–139 (1999) 302–305
305
assumption of one-dimensional plasma expansion was not valid in this case. This smaller spot size allows more spherically symmetric expansion. Thereby, other models are needed to describe the ablation process with small interaction surfaces.
4. Conclusion Fig. 5. Crater depth vs. atomic number of the target particles. Ablation parameters are the same as in Table 1 for 14 mJ KrF laser pulse. '—Experimental data, v —Evaluated data.
sional, a higher density of the ablated species is within the laser beam path. Thereby, the attenuation of the laser in plasma is greater with larger spot sizes and the effective fluence on the target surface is lower leading to a decrease of the crater depth. The ablation depths were compared with theoretical results estimated by the following expression based on the assumption of a one-dimensional plasma expansion w9x:
C 9r8 h Ž mm . ( 2.66
r Ž mgrcm3 . A1r4 Ž a.m.u. .
=I 1r2 Ž Wrcm2 . ly1 r2 Ž cm . t 3r4 Ž s .
Ž 1. where C s 0.5 A Ža.m.u.. w Z 2 Ž Z q 1.xy1 r3, A is the atomic mass, Z the charge state of the ions Žwe supposed ions single charged with Z s 1., r the target density, I the laser beam intensity and t the pulse duration. Fig. 5 compares the experimental depth obtained with the KrF laser beam with the values calculated by Eq. Ž1.. The theoretical ablation depth was found in relatively good agreement with the experimental results. Thus, we concluded that the KrF laser experiments Žspot diameter 150 mm. can be described on the basis of the model proposed in Ref. w9x. The plasma expansion during the pulse can be considered one-dimensional. For the Nd-YAG experiments with a spot diameter of 8 mm, the theoretical ablation depth was significantly lower Ž10 times and more. than the experimental results. The
The nanosecond UV laser ablation experiments at atmospheric pressure for different metal samples allowed us to determine the dependence of the ablation efficiency with the parameters of laser–target interaction. The influence of the pulse number, the incident laser energy, the laser beam diameter and the properties of metals was studied. Experimental ablation depths obtained in excimer laser experiments were in good agreement with the theoretical values calculated with the Phipps et al. expression w9x. But further experimental and theoretical investigations are needed for a better understanding of the correlation obtained between ablation efficiencies and melting temperatures.
References w1x M. Von Allmen, Laser Beam Interactions with Materials, Springer, Berlin, 1987. w2x A.M. Prokhorov, V.I. Konov, I. Ursu, I.N. Mihailescu, Laser Heating of Metals, Hilger, 1990. w3x E.N. Sobol, Phase Transformations and Ablation in LaserTreated Solids, Wiley-Interscience Publication, NY, 1995. w4x D. Bauerle, Laser Processing and Chemistry, Springer, Berlin, ¨ 1996. w5x M. Eyett, D. Bauerle, Appl. Phys. Lett. 51 Ž1987. 2054. ¨ w6x J. Heitz, X.Z. Wang, P. Schwab, D. Bauerle, L. Schultz, J. ¨ Appl. Phys. 68 Ž1990. 2512. w7x A. Giordini Guidoni, R. Kelly, A. Mele, A. Miotello, Plasma Sources Sci. Technol. 6 Ž1997. 260. w8x B. Wolff-Rottke, J. Ihlemann, H. Schmidt, A. Scholl, Appl. Phys. A 60 Ž1995. 13. w9x C.R. Phipps, T.P. Turner, R.F. Harrison, G.W. York, W.Z. Osborne, G.K. Anderson, X.F. Corlis, L.C. Haynes, H.S. Steele, K.C. Spicochi, T.R. King, J. Appl. Phys. 64 Ž1988. 1083.