- Email: [email protected]
Single and multiple ultrashort laser pulse ablation threshold of Al2O3 (corundum) at different etch phases
Applied Surface Science 154–155 Ž2000. 40–46 www.elsevier.nlrlocaterapsusc
Single and multiple ultrashort laser pulse ablation threshold of Al 2 O 3 žcorundum/ at different etch phases D. Ashkenasi ) , R. Stoian, A. Rosenfeld Max-Born-Institut fur ¨ Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, D-12489 Berlin, Germany Received 1 June 1999; accepted 21 July 1999
Abstract We present surface ablation threshold investigations on Al 2 O 3 Žcorundum. after single- and multiple-laser pulse irradiation at 800 nm in the picosecond and subpicosecond duration range. Scattered light monitoring was utilized for in situ damage detection and irradiated spots were inspected by optical and electron microscopy. We obtained fluence thresholds for two distinctively different etch phases. The threshold for ablation drops sharply for multiple-laser shot irradiation, due to material-dependent incubation effects. The threshold reduction for increased number of laser shots is related to defect accumulation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ultrashort laser pulses; Surface damage threshold; Incubation; Corundum
1. Introduction Investigations of the surface ablation and damage threshold under laser irradiation have been the subject of numerous studies w1,2x. Recently, the pulse duration dependence of laser-induced damage on dielectrics for infrared laser pulses in the nanosecond, the picosecond and the femtosecond range have been outlined in several publications w3–7x. Several vaguely defined aspects have to be reconsidered when comparing the experimental threshold levels obtained in different works, for example, the spatial definition of the damage and how many laser shots are used to determine the damage threshold. In a previous study, we demonstrated that the single-shot damage thresholds for a-SiO 2 and CaF2 with a pulse
)
Corresponding author. E-mail address: [email protected] ŽD. Ashkenasi..
duration between 0.2 and 5 ps are at least by a factor of two higher compared to multiple-shots damage threshold w5x. In addition, the type of spatial modification at threshold may differ significantly with laser shots number. This is especially true for corundum where two distinctly different etch or ablation phases were identified w8,9x. Tam et al. were the first to identify and assign the two phases as being ‘‘strong’’ and ‘‘gentle’’ w9x. The strong and gentle etch phases yield significant differences in the observed ablation rate and surface morphology w8,9x. There is experimental evidence based on ion time-of-flight spectroscopic investigations that completely different ablation mechanisms — probably thermal emission following Coulomb explosion Žgentle. and phase explosion Žstrong. — are responsible for the observed differences w10,11x. We discuss here the shot number dependency in the surface damage threshold of corundum for picosecond and subpicosecond nearinfrared laser pulses. Based on our experimental
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 9 9 . 0 0 4 3 3 - X
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
results, we can identify the threshold for each etch phase, greatly extending the results discussed in Ref. w10x.
2. Experiment The laser used in our experiments is a Ti:sapphire oscillator–amplifier system based on chirped pulse amplification. The linearly polarized laser pulses of a wavelength of 800 nm were focused by a quartz lens Ž75 mm focal length, Halle Nachf., Berlin. onto various sample surfaces. The surface focal spot size Ž1re 2 . on the target surface was measured as 700 mm2 . Target and lens are located in a vacuum chamber. A membrane pump ensured a pressure of 10y3 mbar inside the chamber. Comparative studies were also performed at ambient pressures. The repetition rate of irradiation on the sample was typically 1–2 Hz with shot numbers N s 1–100 and 20 Hz for N ) 100. Polished 2-mm-thick samples of c-Al 2 O 3 Ž0001. ŽCrystal, Berlin. were cleaned in an ultrasonic acetone bath and then mounted on a metal target holder such that the laser-processed area was not backed. The mean roughness of the samples was ca. 10 nm. For the ablation threshold investigations, we generated a dot matrix of laser-induced modifications on the sample surface by varying the number of shots and laser energy for a given pulse duration, repeating the measurements under identical conditions at least five times. The pulse duration was varied between 0.2 and 5 ps by reducing the chirp compensation in the compressor. This ensured the stability of the other laser parameters. Pulse duration, laser spot size, wavelength and laser energy were monitored regularly. The laser pulse energy was varied between 0.05 and 100 mJ by using attenuation plates. The pulse energy stability was better than 3%. To determine the surface ablation threshold at a given pulse duration and the number of laser shots, we devised the following three-step method: Surface damage is detected in situ by light scattering detection, illuminating the sample surface with a standard HeNe laser source and monitoring the image with a combination of far-field microscope, CCD-camera and video screen w8x. Using this method, slight variations in surface morphology originating from the increase in
41
surface roughness are detected w12x. Viewing the surface using optical Nomarski and scanning electron microscopy ŽSEM. made it possible to determine the fluence interval where the surface modification becomes observable. Based on this method, the fluctuations in the determined threshold fluence values are greatest for low shot numbers, since in these cases, the modifications are faint and difficult to determine unambiguously. The visible modifications originate from that part of the illuminated region which corresponds to the peak fluence of the Gaussian laser spot. For several laser fluence levels above the obvious damage threshold, we determined the ablated area. A semi-logarithmic plot of the ablated area versus the fluence leads to an expected linear dependence w13x, from which the ablation threshold can be determined. This analysis for each individual region and pulse duration yields a ‘‘zero modification’’ threshold defined as the intersection of the linear fit with the horizontal fluence axis. This last step reduced the threshold uncertainty for single and few laser shots considerably. 3. Results and discussion The SEM picture of c-Al 2 O 3 after N s 10 laser shots at a fluence of 12.8 Jrcm2 in Fig. 1 illustrates the differences in morphology due to the two differ-
Fig. 1. SEM-picture of corundum after 10 laser shots conducted at a wavelength of 800 nm, a pulse duration of 2.3 ps and a fluence of 12.8 Jrcm2 . The depth Žprofile. has been determined by using atomic force microscopy Žrim. and optical microscopy Žcentre.. Notation dsdepth.
42
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
ent ablation phases — gentle and strong etch phase — that are easily distinguishable using optical microscopy or SEM. The gentle etch phase is characterized by an ablation rate of typically 50 nmrshot w8,9x. The pocket walls and bottom surface remain very smooth, comparable or even better than the original mean roughness of the polished sample. With increasing laser fluence Že.g., in the center of the focused Gaussian laser beam as seen in the example of Fig. 1. and with increasing shot numbers, the strong ablation phase begins to dominate. Here, the ablation rate increases considerably to values above 500 nmrshot. The mean roughness inside the processed pocket increases above 200 nm and the ablated region appears darkened in the optical microscope, facilitating the optical identification. Recently performed time-of-flight investigations on the lasersputtered Al q and O q ions, where the angular and
velocity distribution was correlated with the surface morphology, indicate that the differences in the etch phases rely on basic differences in the ablation mechanism w11x. Coulomb explosion followed by additional material removal due to shock wave expansion and thermal processes is proposed as being responsible for the particle emission in the gentle etch phase. Effects caused by the temperature increase inside the sample becomes significant at a certain degree of incubation and, hence, one obtains a transition to a strong etch phase where the ion contribution from the original electrostatic process is less dominant compared with the violent explosive thermal effects. Fig. 2a–d illustrates the surface ablation threshold for the different etch phases in c-Al 2 O 3 after N s 1–1000 shots obtained at a laser pulse duration of 0.2, 1.3, 2.3 and 4.5 ps. For N s 1–10 and laser
Fig. 2. Semi-logarithmic plot of the surface ablation threshold of the gentle Žhollow squares. and the strong etch phase Žsolid squares. versus the number of laser shots for crystalline corundum Ž c-Al 2 O 3 -0001. at a pulse duration of Ža. 0.2, Žb. 1.3, Žc. 2.3 and Žd. 4.5 ps and a laser wavelength 800 nm. Dashed line: result of fit using Eq. 1 for the gentle for N - 25 and for the strong etch phase N G 25 laser shots. Solid line: result of fit using Eq. Ž1. for the strong etch phase only. Note the increasing scale in the fluence axis.
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
43
Table 1 Single shot surface ablation threshold for the gentle and strong etch phases, Fthg Ž1. and Fths Ž1., in corundum obtained experimentally and based on the fit using Eq. 1. Also compiled are the multi-shot threshold Fth Ž`. and fit parameter k c-Al 2 O 3
Experiment
Fit using Eq. 1
tp pulse duration wpsx
Fthg Ž1.,
gentle, strong Etch phase wJrcm2 x
Fthg Ž1., Fths Ž1. gentle, strong Etch phase wJrcm2 x
0.2 1.3 2.3 4.5
3.1 " 0.3 4.2 " 0.3 4.6 " 0.5 5.0 " 0.3
Fths Ž1.
5.0 " 0.5 8.0 " 0.5 ? ?
2.9 " 0.2 3.5 " 0.4 4.4 " 0.3 5.0 " 0.3
pulse duration 0.2 and 1.3 ps, the surface ablation thresholds for the gentle and strong etch phases, Fthg Ž N . and Fths Ž N ., respectively, could be determined independently ŽFig. 2a and b.. For 2.3 ps, we were unable to discriminate both ablation phases except for N s 10 laser shots Žsee the example in Fig. 1.. For 4.5 ps, we obtained either the gentle or the strong etch phase. For N G 25 shots, the surface structures on c-Al 2 O 3 show signs of the strong etch phase only. A discrimination between both phases based on surface examination was impossible in this shot number range. The decline in surface damage threshold with increasing ultrashort near-infrared laser shot numbers matches the results obtained for other dielectrics, for example, fused silica and fluorides w12,14x. The threshold decreases until N reaches about 50 laser shots. At higher laser shot numbers, the fluence threshold stabilizes at an almost constant level. This result confirms the assumption made for wide band-gap materials that above a certain fluence level, some kind of predamage incubation process is activated, that is, a precursor for ‘‘noticeable’’ changes in surface morphology after additional irradiation w15,16x. Hence, irradiation at a laser fluence F below that level, F - Fth Ž`., would require an infinite number of pulses to initiate macroscopic damage. The lines in Fig. 2a–d follow the calculations based on an exponential fit for the surface damage threshold as a function of laser shot numbers Fth Ž N . at constant pulse duration w14x: Fth Ž N . s Fth Ž ` . q Fth Ž 1 . y Fth Ž ` . eyk Ž Ny1. Ž 1 .
4.6 " 0.4 7.5 " 0.7 110 " 10 ?
FthŽ`. gentle q strong wJrcm2 x
k gentle q strong w1rN x
1.3 " 0.1 1.9 " 0.1 2.6 " 0.1 3.6 " 0.2
20 " 10 8"4 3.5 " 1.5 0.5 " 0.2
Eq. 1 uses three parameter values for each pulse duration: the single shot threshold Fth Ž1., the multiple-shot threshold for N `, Fth Ž`., and an empirical parameter k in the exponent which characterizes the strength of incubation leading to an early reduction of the threshold. We performed a fit based on the data of the strong ablation phase only Žsolid line in Fig. 2. and, wherever possible, a fit based on the combined data of the gentle etch phase for N - 25 and of the strong etch phase for N G 25 Ždashed line.. Table 1 compiles the results. The threshold values for the gentle and the strong etch phases FthŽ1. obtained from the fit using Eq. 1 compare very well with those observed experimentally. We observe a strong increase in the single-shot threshold level for the strong ablation phase, where our fit yields Fths s 100 " 10 Jrcm2 at 2.3 ps. This fluence was unattainable with the optics used in the experiment. The multiple-shot threshold Fth Ž`. also increases with longer laser pulses and the drop in the threshold Fthg Ž1.rFthŽ`. is stronger for 0.2 ps Žover 55%. than for 4.5 ps Žbelow 30%.. Short near-infrared laser pulses may generate bulk damage at a fluence level below the surface damage threshold Žlaser fluence of the focused beam on the surface.. Beam narrowing of the laser pulse for laser powers above the critical power of self-focusing can lead to irreversible changes in the bulk, for example, microcracks, controllable in size and depth w17x. Fig. 3 depicts some typical examples of bulk modifications generated with laser pulses, wavelength 800 nm and pulse duration 2.3 ps, at a fluence level, where the front surface remained unchanged, even at N s 500 laser shots. The bulk modifications appear to be
™
44
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
Fig. 3. Side view microscope picture of subsurface damage inside corundum at fluence levels below surface damage threshold after N s 500, 200, 100, 50, 25 and 10 laser shots at 2.3 ps and 800 nm: Ža. 1.2, Žb. 1.8 and Žc. 2.4 Jrcm2 . The tracks of 20 mm in diameter consist of micron-sized inclusions or cracks. The length of the tracks increases with the number of laser shots; the starting point of the tracks increases in depth with decreasing laser power Žor decreasing laser energy at constant pulse duration and laser spot size on the surface..
very similar to laser-induced bulk microstructures obtained in other dielectrics by direct focusing of ultrashort pulses inside the bulk w18x. One may observe that the damage track length increases towards the surface with increasing number of laser pulses. Therefore, damage originating in a subsurface region may lead to strong changes in the surface morphology after additional laser shots act on the sample. This development based on beam narrowing effects inside the Kerr media due to self-focusing may suggest Žincorrectly. fairly low surface damage thresholds at high laser shot numbers, for example, above 1000 w12,14x. It is important to discriminate the origin of damage, since the physical processes leading to damage, that is, excitation, defect concen-
tration, plume expansion, etc., differ strongly in surface and bulk environment. Returning the attention to our results on the surface modification threshold in corundum, at least two questions remain, that is, what effect leads to the shot number dependency in the surface damage threshold and which process is responsible for the transition from the gentle to the strong etch phase observed at about N s 20 laser shots. We suggest that the answer to both questions is related to the accumulation effect of laser-induced defects inside the dielectric; possibly the generation of stable Fcenters. Nonlinear laser-induced excitation processes, multi-photon-enhanced avalanche ionization w2x, will
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
cause the development of an electron–hole plasma. Recombination of the electron–hole pairs will eventually eliminate the plasma; however, a few pairs may escape annihilation. For example, some of the electrons may be captured by structural defects or impurities w19x. There is also a finite probability that electron–hole pairs will separate to develop stable defects ŽF and Fq centers. with energy levels located deep inside the energy gap w20x. Single, low-order or resonantly enhanced high-order multi-photon excitation from the new defects generated by earlier laser pulses will additionally contribute to the plasma concentration during the next laser pulse. This may eventually cause the laser-induced electron–hole plasma to reach a critical concentration leading to damage w2x. The expected increase in defect concentration is probably fairly weak and very difficult to measure with standard spectroscopic techniques based on linear absorption. However, minor alterations in the energy structure of the wide band gap material, here corundum, may have a major impact in the absorption cross-section for highly nonlinear excitation using ultrashort Žhigh intensity. laser pulses with a fluence near damagerablation threshold. In Al 2 O 3 Žcorundum., we observe two different etch phases along with individual thresholds. Material expulsion initiated by electrostatic repulsion during the gentle phase has a lower threshold observed for low shot numbers. Thermal processes in the strong etch phase seem to dominate only after defect accumulation in the material, unless laser fluence is high enough to activate the thermally driven ablation with a rate of ca. 500 nmrshot. The fluence threshold for the strong etch phase at N - 10 laser shots depends strongly on the pulse duration, more than for the gentle etch phase. An accumulation of defects in the near subsurface region during multiple-shot exposure will certainly enhance the absorption efficiency. The amount of electrons being accelerated, hence heated, without leaving the substrate due to photo-excitation increases and subsequently the transition towards much higher ablation rates can be observed. It has been estimated that for single shots, a multi-photon excitation of the order of 6 is necessary to induce the strong ablation phase compared to 1–2 for the gentle etch phase w8x. Incubation of defects during multiple-laser pulse irradiation seems to strongly reduce the nonlinear order of excitation,
45
allowing for efficient low-order photon absorption of the near-infrared laser pulses. In addition, the accumulation of defects leads to a wider range of channels responsible for the energy transfer into the lattice, decreasing the selectivity of recombination.
4. Conclusion We have demonstrated that for corundum, the number of laser shots with picosecond and subpicosecond pulses plays a key role in the surface ablation threshold for near-infrared ultrashort laser pulses. We were able to discriminate between the gentle and strong etch phases and were able to describe the shot number dependency on threshold with a very simple model. The most dramatic change in the threshold is observed typically during the first 25 laser shots and is related to defect accumulation, for example, F-center formation, attributed tentatively as the leading incubation process. We have shown that bulk defect incubation and subsequently the development of damage inside the transparent dielectric can be induced by near-infrared ultrashort laser pulses without damaging the surface.
References w1x D. Bauerle, Laser Processing and Chemistry, Springer Ver¨ lag, Heidelberg, 1996. w2x R.M. Wood, Laser Damage in Optical Materials, Hilger, Boston, 1986. w3x B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchik, B.W. Shore, M.D. Perry, J. Opt. Soc. Am. B 13 Ž1996. 459. w4x B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchik, .W. Shore, M.D. Perry, Phys. Rev. B 53 Ž1995. 1749. w5x H. Varel, D. Ashkenasi, A. Rosenfeld, R. Herrmann, F. Noack, E.E.B. Campbell, Appl. Phys. A 62 Ž1996. 293. w6x M. Lenzner, J. Kruger, S. Satania, Z. Cheng, C. Spielmann, ¨ G. Mourou, W. Kautek, F. Krauz, Phys. Rev. Lett. 80 Ž1998. 4076. w7x A.-C. Tien, S. Backus, H. Kapteyn, M. Murname, G. Mourou, Appl. Phys. Lett. 82 Ž19. Ž1999. 3883. w8x D. Ashkenasi, A. Rosenfeld, H. Varel, M. Wahmer, E.E.B. ¨ Campbell, Appl. Surf. Sci. 120 Ž1997. 65. w9x A.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Appl. Phys. Lett. 55 Ž20. Ž1989. 2045. w10x A. Rosenfeld, D. Ashkenasi, H. Varel, M. Wahmer, E.E.B. ¨ Cambell, Appl. Surf. Sci. 127–129 Ž1998. 76. w11x R. Stoian, D. Ashkenasi, A. Rosenfeld, E.E.B. Campbell, Phys. Rev. Lett. Ž1999. submitted.
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D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
w12x A. Rosenfeld, M. Lorenz, R. Stoian, D. Ashkenasi, Appl. Phys. A Ž1999. in press. w13x J. Liu, Opt. Lett. 7 Ž1982. 196. w14x D. Ashkenasi, R. Stoian, A. Rosenfeld, Appl. Surf. Sci. 150 Ž1999. 101. w15x L.D. Merkle, N. Koumvakalis, M. Bass, J. Appl. Phys. 55 Ž1984. 772–775. w16x S. Jones, P. Braunlich, R. Casper, X. Shen, P. Kelly, Opt. Eng. 28 Ž1989. 1039.
w17x D. Ashkenasi, H. Varel, A. Rosenfeld, S. Henz, J. Herrmann, E.E.B. Campbell, Appl. Phys. Lett. 72 Ž1998. 1442–1444. w18x E.N. Glezer, E. Mazur, Appl. Phys. Lett. 71 Ž7. Ž1997. 882. w19x K.S. Song, R.T. Williams, Self Trapped Excitons, second edition, 105 Springer Series in Solid State Physics. Springer Verlag, Berlin, 1996. w20x V.A. Skuratov, Nucl. Instrum. Methods Phys. Res., Sect. B 146 Ž1998. 385–392.
Single and multiple ultrashort laser pulse ablation threshold of Al 2 O 3 žcorundum/ at different etch phases D. Ashkenasi ) , R. Stoian, A. Rosenfeld Max-Born-Institut fur ¨ Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, D-12489 Berlin, Germany Received 1 June 1999; accepted 21 July 1999
Abstract We present surface ablation threshold investigations on Al 2 O 3 Žcorundum. after single- and multiple-laser pulse irradiation at 800 nm in the picosecond and subpicosecond duration range. Scattered light monitoring was utilized for in situ damage detection and irradiated spots were inspected by optical and electron microscopy. We obtained fluence thresholds for two distinctively different etch phases. The threshold for ablation drops sharply for multiple-laser shot irradiation, due to material-dependent incubation effects. The threshold reduction for increased number of laser shots is related to defect accumulation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Ultrashort laser pulses; Surface damage threshold; Incubation; Corundum
1. Introduction Investigations of the surface ablation and damage threshold under laser irradiation have been the subject of numerous studies w1,2x. Recently, the pulse duration dependence of laser-induced damage on dielectrics for infrared laser pulses in the nanosecond, the picosecond and the femtosecond range have been outlined in several publications w3–7x. Several vaguely defined aspects have to be reconsidered when comparing the experimental threshold levels obtained in different works, for example, the spatial definition of the damage and how many laser shots are used to determine the damage threshold. In a previous study, we demonstrated that the single-shot damage thresholds for a-SiO 2 and CaF2 with a pulse
)
Corresponding author. E-mail address: [email protected] ŽD. Ashkenasi..
duration between 0.2 and 5 ps are at least by a factor of two higher compared to multiple-shots damage threshold w5x. In addition, the type of spatial modification at threshold may differ significantly with laser shots number. This is especially true for corundum where two distinctly different etch or ablation phases were identified w8,9x. Tam et al. were the first to identify and assign the two phases as being ‘‘strong’’ and ‘‘gentle’’ w9x. The strong and gentle etch phases yield significant differences in the observed ablation rate and surface morphology w8,9x. There is experimental evidence based on ion time-of-flight spectroscopic investigations that completely different ablation mechanisms — probably thermal emission following Coulomb explosion Žgentle. and phase explosion Žstrong. — are responsible for the observed differences w10,11x. We discuss here the shot number dependency in the surface damage threshold of corundum for picosecond and subpicosecond nearinfrared laser pulses. Based on our experimental
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 9 9 . 0 0 4 3 3 - X
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
results, we can identify the threshold for each etch phase, greatly extending the results discussed in Ref. w10x.
2. Experiment The laser used in our experiments is a Ti:sapphire oscillator–amplifier system based on chirped pulse amplification. The linearly polarized laser pulses of a wavelength of 800 nm were focused by a quartz lens Ž75 mm focal length, Halle Nachf., Berlin. onto various sample surfaces. The surface focal spot size Ž1re 2 . on the target surface was measured as 700 mm2 . Target and lens are located in a vacuum chamber. A membrane pump ensured a pressure of 10y3 mbar inside the chamber. Comparative studies were also performed at ambient pressures. The repetition rate of irradiation on the sample was typically 1–2 Hz with shot numbers N s 1–100 and 20 Hz for N ) 100. Polished 2-mm-thick samples of c-Al 2 O 3 Ž0001. ŽCrystal, Berlin. were cleaned in an ultrasonic acetone bath and then mounted on a metal target holder such that the laser-processed area was not backed. The mean roughness of the samples was ca. 10 nm. For the ablation threshold investigations, we generated a dot matrix of laser-induced modifications on the sample surface by varying the number of shots and laser energy for a given pulse duration, repeating the measurements under identical conditions at least five times. The pulse duration was varied between 0.2 and 5 ps by reducing the chirp compensation in the compressor. This ensured the stability of the other laser parameters. Pulse duration, laser spot size, wavelength and laser energy were monitored regularly. The laser pulse energy was varied between 0.05 and 100 mJ by using attenuation plates. The pulse energy stability was better than 3%. To determine the surface ablation threshold at a given pulse duration and the number of laser shots, we devised the following three-step method: Surface damage is detected in situ by light scattering detection, illuminating the sample surface with a standard HeNe laser source and monitoring the image with a combination of far-field microscope, CCD-camera and video screen w8x. Using this method, slight variations in surface morphology originating from the increase in
41
surface roughness are detected w12x. Viewing the surface using optical Nomarski and scanning electron microscopy ŽSEM. made it possible to determine the fluence interval where the surface modification becomes observable. Based on this method, the fluctuations in the determined threshold fluence values are greatest for low shot numbers, since in these cases, the modifications are faint and difficult to determine unambiguously. The visible modifications originate from that part of the illuminated region which corresponds to the peak fluence of the Gaussian laser spot. For several laser fluence levels above the obvious damage threshold, we determined the ablated area. A semi-logarithmic plot of the ablated area versus the fluence leads to an expected linear dependence w13x, from which the ablation threshold can be determined. This analysis for each individual region and pulse duration yields a ‘‘zero modification’’ threshold defined as the intersection of the linear fit with the horizontal fluence axis. This last step reduced the threshold uncertainty for single and few laser shots considerably. 3. Results and discussion The SEM picture of c-Al 2 O 3 after N s 10 laser shots at a fluence of 12.8 Jrcm2 in Fig. 1 illustrates the differences in morphology due to the two differ-
Fig. 1. SEM-picture of corundum after 10 laser shots conducted at a wavelength of 800 nm, a pulse duration of 2.3 ps and a fluence of 12.8 Jrcm2 . The depth Žprofile. has been determined by using atomic force microscopy Žrim. and optical microscopy Žcentre.. Notation dsdepth.
42
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
ent ablation phases — gentle and strong etch phase — that are easily distinguishable using optical microscopy or SEM. The gentle etch phase is characterized by an ablation rate of typically 50 nmrshot w8,9x. The pocket walls and bottom surface remain very smooth, comparable or even better than the original mean roughness of the polished sample. With increasing laser fluence Že.g., in the center of the focused Gaussian laser beam as seen in the example of Fig. 1. and with increasing shot numbers, the strong ablation phase begins to dominate. Here, the ablation rate increases considerably to values above 500 nmrshot. The mean roughness inside the processed pocket increases above 200 nm and the ablated region appears darkened in the optical microscope, facilitating the optical identification. Recently performed time-of-flight investigations on the lasersputtered Al q and O q ions, where the angular and
velocity distribution was correlated with the surface morphology, indicate that the differences in the etch phases rely on basic differences in the ablation mechanism w11x. Coulomb explosion followed by additional material removal due to shock wave expansion and thermal processes is proposed as being responsible for the particle emission in the gentle etch phase. Effects caused by the temperature increase inside the sample becomes significant at a certain degree of incubation and, hence, one obtains a transition to a strong etch phase where the ion contribution from the original electrostatic process is less dominant compared with the violent explosive thermal effects. Fig. 2a–d illustrates the surface ablation threshold for the different etch phases in c-Al 2 O 3 after N s 1–1000 shots obtained at a laser pulse duration of 0.2, 1.3, 2.3 and 4.5 ps. For N s 1–10 and laser
Fig. 2. Semi-logarithmic plot of the surface ablation threshold of the gentle Žhollow squares. and the strong etch phase Žsolid squares. versus the number of laser shots for crystalline corundum Ž c-Al 2 O 3 -0001. at a pulse duration of Ža. 0.2, Žb. 1.3, Žc. 2.3 and Žd. 4.5 ps and a laser wavelength 800 nm. Dashed line: result of fit using Eq. 1 for the gentle for N - 25 and for the strong etch phase N G 25 laser shots. Solid line: result of fit using Eq. Ž1. for the strong etch phase only. Note the increasing scale in the fluence axis.
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
43
Table 1 Single shot surface ablation threshold for the gentle and strong etch phases, Fthg Ž1. and Fths Ž1., in corundum obtained experimentally and based on the fit using Eq. 1. Also compiled are the multi-shot threshold Fth Ž`. and fit parameter k c-Al 2 O 3
Experiment
Fit using Eq. 1
tp pulse duration wpsx
Fthg Ž1.,
gentle, strong Etch phase wJrcm2 x
Fthg Ž1., Fths Ž1. gentle, strong Etch phase wJrcm2 x
0.2 1.3 2.3 4.5
3.1 " 0.3 4.2 " 0.3 4.6 " 0.5 5.0 " 0.3
Fths Ž1.
5.0 " 0.5 8.0 " 0.5 ? ?
2.9 " 0.2 3.5 " 0.4 4.4 " 0.3 5.0 " 0.3
pulse duration 0.2 and 1.3 ps, the surface ablation thresholds for the gentle and strong etch phases, Fthg Ž N . and Fths Ž N ., respectively, could be determined independently ŽFig. 2a and b.. For 2.3 ps, we were unable to discriminate both ablation phases except for N s 10 laser shots Žsee the example in Fig. 1.. For 4.5 ps, we obtained either the gentle or the strong etch phase. For N G 25 shots, the surface structures on c-Al 2 O 3 show signs of the strong etch phase only. A discrimination between both phases based on surface examination was impossible in this shot number range. The decline in surface damage threshold with increasing ultrashort near-infrared laser shot numbers matches the results obtained for other dielectrics, for example, fused silica and fluorides w12,14x. The threshold decreases until N reaches about 50 laser shots. At higher laser shot numbers, the fluence threshold stabilizes at an almost constant level. This result confirms the assumption made for wide band-gap materials that above a certain fluence level, some kind of predamage incubation process is activated, that is, a precursor for ‘‘noticeable’’ changes in surface morphology after additional irradiation w15,16x. Hence, irradiation at a laser fluence F below that level, F - Fth Ž`., would require an infinite number of pulses to initiate macroscopic damage. The lines in Fig. 2a–d follow the calculations based on an exponential fit for the surface damage threshold as a function of laser shot numbers Fth Ž N . at constant pulse duration w14x: Fth Ž N . s Fth Ž ` . q Fth Ž 1 . y Fth Ž ` . eyk Ž Ny1. Ž 1 .
4.6 " 0.4 7.5 " 0.7 110 " 10 ?
FthŽ`. gentle q strong wJrcm2 x
k gentle q strong w1rN x
1.3 " 0.1 1.9 " 0.1 2.6 " 0.1 3.6 " 0.2
20 " 10 8"4 3.5 " 1.5 0.5 " 0.2
Eq. 1 uses three parameter values for each pulse duration: the single shot threshold Fth Ž1., the multiple-shot threshold for N `, Fth Ž`., and an empirical parameter k in the exponent which characterizes the strength of incubation leading to an early reduction of the threshold. We performed a fit based on the data of the strong ablation phase only Žsolid line in Fig. 2. and, wherever possible, a fit based on the combined data of the gentle etch phase for N - 25 and of the strong etch phase for N G 25 Ždashed line.. Table 1 compiles the results. The threshold values for the gentle and the strong etch phases FthŽ1. obtained from the fit using Eq. 1 compare very well with those observed experimentally. We observe a strong increase in the single-shot threshold level for the strong ablation phase, where our fit yields Fths s 100 " 10 Jrcm2 at 2.3 ps. This fluence was unattainable with the optics used in the experiment. The multiple-shot threshold Fth Ž`. also increases with longer laser pulses and the drop in the threshold Fthg Ž1.rFthŽ`. is stronger for 0.2 ps Žover 55%. than for 4.5 ps Žbelow 30%.. Short near-infrared laser pulses may generate bulk damage at a fluence level below the surface damage threshold Žlaser fluence of the focused beam on the surface.. Beam narrowing of the laser pulse for laser powers above the critical power of self-focusing can lead to irreversible changes in the bulk, for example, microcracks, controllable in size and depth w17x. Fig. 3 depicts some typical examples of bulk modifications generated with laser pulses, wavelength 800 nm and pulse duration 2.3 ps, at a fluence level, where the front surface remained unchanged, even at N s 500 laser shots. The bulk modifications appear to be
™
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D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
Fig. 3. Side view microscope picture of subsurface damage inside corundum at fluence levels below surface damage threshold after N s 500, 200, 100, 50, 25 and 10 laser shots at 2.3 ps and 800 nm: Ža. 1.2, Žb. 1.8 and Žc. 2.4 Jrcm2 . The tracks of 20 mm in diameter consist of micron-sized inclusions or cracks. The length of the tracks increases with the number of laser shots; the starting point of the tracks increases in depth with decreasing laser power Žor decreasing laser energy at constant pulse duration and laser spot size on the surface..
very similar to laser-induced bulk microstructures obtained in other dielectrics by direct focusing of ultrashort pulses inside the bulk w18x. One may observe that the damage track length increases towards the surface with increasing number of laser pulses. Therefore, damage originating in a subsurface region may lead to strong changes in the surface morphology after additional laser shots act on the sample. This development based on beam narrowing effects inside the Kerr media due to self-focusing may suggest Žincorrectly. fairly low surface damage thresholds at high laser shot numbers, for example, above 1000 w12,14x. It is important to discriminate the origin of damage, since the physical processes leading to damage, that is, excitation, defect concen-
tration, plume expansion, etc., differ strongly in surface and bulk environment. Returning the attention to our results on the surface modification threshold in corundum, at least two questions remain, that is, what effect leads to the shot number dependency in the surface damage threshold and which process is responsible for the transition from the gentle to the strong etch phase observed at about N s 20 laser shots. We suggest that the answer to both questions is related to the accumulation effect of laser-induced defects inside the dielectric; possibly the generation of stable Fcenters. Nonlinear laser-induced excitation processes, multi-photon-enhanced avalanche ionization w2x, will
D. Ashkenasi et al.r Applied Surface Science 154–155 (2000) 40–46
cause the development of an electron–hole plasma. Recombination of the electron–hole pairs will eventually eliminate the plasma; however, a few pairs may escape annihilation. For example, some of the electrons may be captured by structural defects or impurities w19x. There is also a finite probability that electron–hole pairs will separate to develop stable defects ŽF and Fq centers. with energy levels located deep inside the energy gap w20x. Single, low-order or resonantly enhanced high-order multi-photon excitation from the new defects generated by earlier laser pulses will additionally contribute to the plasma concentration during the next laser pulse. This may eventually cause the laser-induced electron–hole plasma to reach a critical concentration leading to damage w2x. The expected increase in defect concentration is probably fairly weak and very difficult to measure with standard spectroscopic techniques based on linear absorption. However, minor alterations in the energy structure of the wide band gap material, here corundum, may have a major impact in the absorption cross-section for highly nonlinear excitation using ultrashort Žhigh intensity. laser pulses with a fluence near damagerablation threshold. In Al 2 O 3 Žcorundum., we observe two different etch phases along with individual thresholds. Material expulsion initiated by electrostatic repulsion during the gentle phase has a lower threshold observed for low shot numbers. Thermal processes in the strong etch phase seem to dominate only after defect accumulation in the material, unless laser fluence is high enough to activate the thermally driven ablation with a rate of ca. 500 nmrshot. The fluence threshold for the strong etch phase at N - 10 laser shots depends strongly on the pulse duration, more than for the gentle etch phase. An accumulation of defects in the near subsurface region during multiple-shot exposure will certainly enhance the absorption efficiency. The amount of electrons being accelerated, hence heated, without leaving the substrate due to photo-excitation increases and subsequently the transition towards much higher ablation rates can be observed. It has been estimated that for single shots, a multi-photon excitation of the order of 6 is necessary to induce the strong ablation phase compared to 1–2 for the gentle etch phase w8x. Incubation of defects during multiple-laser pulse irradiation seems to strongly reduce the nonlinear order of excitation,
45
allowing for efficient low-order photon absorption of the near-infrared laser pulses. In addition, the accumulation of defects leads to a wider range of channels responsible for the energy transfer into the lattice, decreasing the selectivity of recombination.
4. Conclusion We have demonstrated that for corundum, the number of laser shots with picosecond and subpicosecond pulses plays a key role in the surface ablation threshold for near-infrared ultrashort laser pulses. We were able to discriminate between the gentle and strong etch phases and were able to describe the shot number dependency on threshold with a very simple model. The most dramatic change in the threshold is observed typically during the first 25 laser shots and is related to defect accumulation, for example, F-center formation, attributed tentatively as the leading incubation process. We have shown that bulk defect incubation and subsequently the development of damage inside the transparent dielectric can be induced by near-infrared ultrashort laser pulses without damaging the surface.
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