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Comparing study of subpicosecond and nanosecond wet etching of fused silica
Applied Surface Science 252 (2006) 4768–4772 www.elsevier.com/locate/apsusc
Comparing study of subpicosecond and nanosecond wet etching of fused silica Cs. Vass a,*, D. Sebo˝k a, B. Hopp b a
Department of Optics and Quantum Electronics, University of Szeged, H-6720 Szeged, Do´m te´r 9, Hungary b Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Do´m te´r 9, Hungary Received 3 May 2005; accepted 18 July 2005 Available online 27 October 2005
Abstract The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene–methyl-methacrylate (c = 0.85, 1.71 M) and pyrene–acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360–450 mJ cm2 depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm2 fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene–acetone solution, while using naphthalene–methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene–acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure. # 2005 Elsevier B.V. All rights reserved. Keywords: Wet etching; Fused silica; Transparent materials; Subpicosecond krF laser system
1. Introduction The laser induced backside wet etching (LIBWE) technique is an effective and high accuracy method for the laser micromachining of transparent materials, for example fused silica, pyrex, CaF2, BaF2, sapphire and different types of glasses [1–4]. This procedure makes possible among others the production of micro-optical elements: Fresnel-lenses, microlens arrays and diffractive gratings [4–6]. The most important advantages of the LIBWE procedure in contrast with other methods of transparent material micromachining are the relatively low etching threshold fluence, submicrometer depth
* Corresponding author. E-mail address: [email protected] (C. Vass). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.118
control, quasi smooth etched hole bottoms, and micropatterns free of microcracks and debris. LIBWE is an indirect method. The laser pulses do not etch directly the irradiated target surface of the transparent target. In this technique, a liquid (solution) with high absorptivity at the applied laser wavelength contacting with the backside of the transparent material is irradiated by laser pulses through the solid sample. A thin liquid layer contacting the transparent target absorbs the pulse energy warming up drastically. This hot liquid film heats the surface layer of the solid sample by heat diffusion, therefore this heated volume can soften or melt depending on the developed temperature. Meantime bubbles form and expand in the irradiated interfacial liquid reagent layer and hit the softened or molten surface of the target causing material damage and ejection. The most frequently lasers applied in the LIBWE procedure are the nanosecond excimer lasers (KrF, ArF, XeF;
Cs. Vass et al. / Applied Surface Science 252 (2006) 4768–4772
FWHM = 20–30 ns) and the most general studied transparent material is the fused silica. The femto- and subpicosecond lasers have lot of advantages in the high accuracy micromachining [7]. These lasers make possible debris free ablation without detectable thermal damages around the well-defined ablated holes. In case of the most material the quality of the ablated hole fabricated by femtosecond laser is better than that of those produced by nanosecond laser [7]. According to these facts, it is important to know whether the subpicosecond lasers are also applicable in LIBWE and whether the mentioned benefits of short pulse laser material processing also appear in the subpicosecond backside wet etching procedure, too. By analogy of the earlier results of subpicosecond laser ablation, we expected that the quality of the etched holes generated by short pulse LIBWE is also better than that of those produced by the nanosecond LIBWE. In this paper, we investigated the subpicosecond backside wet etching and compare it to the nanosecond LIBWE. In the interests of the detailed comparison, we studied one of the most important basic effects of the procedure, which is the development of the bubbles in the liquid absorber. Moreover, we calculated the contacting pressure at the quartz surface induced by the above mentioned fast bubble expansion. 2. Experimental Naphthalene–methyl-methacrylate (with a concentrations c = 0.85 and 1.71 mol dm3) and pyrene–acetone (c = 0.4 mol dm3) solutions were used as absorbing liquids in our experiments. As transparent targets, fused silica plates with a thickness of 1 mm (Suprasil II, Heraeus) were applied. Sixhundred femtoseconds pulses were produced by a high brightness KrF excimer laser (l = 248 nm, Emax = 15 mJ; the seed pulse generated by hybrid dye/excimer laser was amplified by a three-pass off-axis KrF amplifier [8]). The applied nanosecond laser was a conventional KrF excimer laser (FWHM = 30 ns). The absorption coefficients of the above mentioned liquids were measured by a plano-concave microcuvette for both nanosecond and subpicosecond lasers [9,10]. The basis of these measurements and calculations can be found in the referred studies. The presented experimental setup was used in this study. The UV beams were focused into the liquid through a quartz plate by a quartz lens ( f = 8 cm) (Fig. 1.). A blade was placed onto the outside surface of the targets to shut the half of the laser beam for the accurate
Fig. 1. Schematic diagram of the etching experimental setup.
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measurement of the etch depths. The repetition rate was 2 Hz for both lasers. A Topometrix 2000 atomic force microscope (AFM) was applied in the measurement of etched depths. The high pressure induced by the very fast expansion of bubbles, which are developed in the solid–liquid interfacial layer is indispensable parameter for the understanding of LIBWE procedure. Therefore a fast photographic setup was used for the time resolved observation of the bubble formation and expansion [11]. The temporal dependence of the evolved pressure at the effected quartz surface could be calculated on the basis of the recorded images. The delay between the excimer and probe laser pulses was changed in the range of 1– 70 ms. 3. Results and discussion 3.1. Absorption coefficient measurements Our experimental work can be divided into three parts. In our earlier studies we demonstrated that the absorption coefficient of the applied liquid absorbers is a very determinative parameter in the LIBWE process [10,11]. So, in the interests of the all-embracing planning of the machining experiments and the correct interpretation of our results and observations first we measured the absorption coefficient of the used solutions for both lasers. A plano-concave microcuvette described in our former study was used for this measurement. This method is useful in the case of liquids having very high absorption coefficient. During these experiments the applied laser fluence was 0.02 mJ cm2, to avoid the damage of the applied beam profiler. This is less than the etching fluences, therefore the absorptivities of absorbers at higher fluences applied during our LIBWE experiments may differ from the measured values, but we believe that these provide good estimate. The results of these experiments are summarised in Table 1. We could not found significant differences between the absorption coefficient values for the different pulse durations in the investigated fluence range. 3.2. Nanosecond LIBWE The ablation threshold fluence of the fused silica for nanosecond KrF excimer laser pulses is more than 10 J cm2 [12,13]. It was found that the nanosecond backside wet etching Table 1 The measured absorption coefficients of the used solutions for both nanosecond and subpicosecond KrF excimer lasers Solution
c (mol dm3)
Pulse duration (FWHM)
a (1/cm)
Pyrene–acetone
0.4
30 ns 600 fs
2100 2220
Naphthalene–methylmethacrylate
0.85
30 ns 600 fs
2830 2870
Naphthalene–methylmethacrylate
1.71
30 ns 600 fs
3380 3710
4770
Cs. Vass et al. / Applied Surface Science 252 (2006) 4768–4772
threshold of quartz is more than one magnitude less. According to our measurements the threshold values for nanosecond KrF laser are 450 mJ cm2 for pyrene–acetone solution (c = 0.4 mol dm3), 440 and 360 mJ cm2 for naphthalene– methyl-methacrylate solutions with concentrations c = 0.85 and 1.71 mol dm3. These results correlate with the measured absorption coefficients of the liquid absorbers, since lower threshold fluence belongs to higher absorption coefficient. The connection between these two parameters is easily understandable. The penetration depth of the laser beam in the liquid reagent decreases with increasing its absorption coefficient. Therefore the pulse energy is absorbed in thinner liquid layer resulting in higher liquid temperature. This layer heats up the fused silica target by heat diffusion to higher temperature, which is high enough to soften or melt the quartz backside surface layer. This molten material can be removed from the target surface during the bubble expansion at lower laser fluence, too. The etch rate (the thickness of the layer removed by single laser pulse) versus laser fluence graphs (Fig. 2a–c) show that two lines having different slopes can be fitted to the measured data points in each cases. These slopes refer to the different etching mechanisms [11]. These various slopes were also observed in other studies [3,4,12,14]. In these graphs, it can be seen that at given fluence values the etch rate at the naphthalene–methyl-methacrylate solution with a concentrations 1.71 mol dm3 (highest absorptivity) is the highest and the etch rate at the pyrene–acetone solution (lowest absorptivity) is the lowest. The reason of this fact can be attributed to the
different absorption coefficients of the applied liquid reagents. The explanation in this case can be also derived from the higher temperature of the smaller liquid volume that absorbs energy. Fig. 3 shows AFM images of an etched hole made by nanosecond pulses. 3.3. Subpicosecond LIBWE The ablation threshold fluence of the fused silica for 600 fs KrF excimer laser was measured to be 0.7 J cm2. This is one magnitude less than the value for the ordinary nanosecond KrF pulses, but it is approximately two times higher than the determined threshold fluence of the nanosecond LIBWE for the used solutions. In the subpicosecond LIBWE experiments, the fluences were varied up to this threshold (700 mJ cm2). During our investigations, any etched holes was not ˚ ) in this detectable using AFM (vertical resolution is A fluence range. To find the reason of this result, in our next experiments we studied the bubble origination and expansion process, which are important basic effects of the LIBWE procedure. 3.4. Time resolved investigations Several papers presented that the high pressure at the target surface caused by fast bubble expansion has important role in the backside wet etching process [11,14–16]. Therefore we completed our etching setup with a fast photographic
Fig. 2. (a–c) The etch rate as a function of the nanosecond laser fluences for each solution.
Cs. Vass et al. / Applied Surface Science 252 (2006) 4768–4772
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Fig. 4. The schema of the chemical structures of pyrene (a) and naphthalene (b) molecules.
Fig. 3. AFM images of an etched hole: edge (a); bottom (b) (nanosecond KrF, fluence: 500 mJ cm2, pulse number: 100, pyrene–acetone solution, c = 0.4 mol dm3).
arrangement for the time-resolved investigation of the bubble development and expansion in both of the subpicosecond and the nanosecond experiments. The applied laser fluence was constant, F = 460 mJ cm2. The pressure inside the bubbles can be determined on the basis of the snapshots about the bubbles measuring their radii using this equation: 3k dR 2s Rn 2s 4m dR ; (1) p R; ¼ pstat þ dt Rn R R dt R where R is the radius and Rn is the equilibrium radius of the bubble, pstat is the static ambient air pressure ( pstat = 105 Pa), s is the surface tension (smethyl-methacrylate = 0.028 N/m sacetone = 0.02346 N/m), m is the dynamic shear viscosity (mmethylmethacrylate = 0.000632 Pa s macetone = 0.000306 Pa s) and k is the polytropic exponent of the solutions (k = 4/3 for both solvents). The details of the arithmetic procedure can be found in our earlier paper and the referred study [11,17]. The results of these measurements and calculations, which are derived from the analysis of more than 270 snapshots, are summarised in Table 2. The calculated pressure values are in good agreement with the values measured by Y. Kawaguchi and co-workers, although they used other solutions [16]. The pressures are
almost same for pyrene–acetone solutions for both pulse durations, and the pressures in the cases of naphthalene–methylmethacrylate solutions generated during nanosecond laser irradiation are 3.5–5 times higher than that of those produced by subpicosecond pulses. This difference can be due to the different chemical and thermal effects induced by the nanosecond and subpicosecond laser irradiations. The spatial distributions of the absorbed laser energy are the same in both cases, due to the same absorption coefficients of each solution for both lasers. Presumable, the heating of the liquid absorber is the dominating effect in the nanosecond case, in contrast to the subpicosecond irradiation, where probably the chemical reactions are the significant effects [7]. Therefore, during the nanosecond LIBWE the bubble pressure does not depend significantly on the composition of the applied solutions (Table 2, 2nd column) because its value is determined by the quantity of the evaporated fragments basically depending on the absorbed energy, which was constant. In the cases of naphthalene–methyl-methacrylate solutions using subpicosecond irradiation one part of the pulse energy probably causes the polymerisation of the naphthalene–methylmethacrylate molecules producing a thin film, which can cover the contacting surface of the quartz plate [18]. The remainder pulse energy can induce the decomposition of the solution molecules. Since the pyrene molecule consists of four aromatic rings while a naphthalene molecule contains only two (Fig. 4.), therefore the ultrashort UV pulse produces more fragments breaking the pyrene molecule during the photodecomposition. More fragments induce higher pressure in the developed bubbles. These facts can explain the calculated pressure differences. 4. Conclusions
Table 2 The determined maximal pressures (MPa) of the developed bubbles Maximal bubble pressures in MPa
3
Pyrene–acetone 0.4 mol dm Naphthalene–methyl-methacrylate 0.85 mol dm3 Naphthalene–methyl-methacrylate 1.71 mol dm3
Pulse duration 30 ns
600 fs
48.1 75.0 53.8
61.0 14.3 15.7
On the basis of our results, it can be established that the subpicosecond LIBWE is unrealizable under the applied experimental conditions, using the above mentioned solutions and subpicosecond excimer laser. According to our theory, two main conditions have to be realised to accomplish the LIBWE: (i) the contacting surface of the quartz plate has to be modified (softening, melting) during the laser irradiation to decrease the
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resistance of the surface layer to the mechanical effects and (ii) on the other hand LIBWE needs significant mechanical effect (high pressure induced by bubble expansion), which is able to remove the modified quartz layer from the surface of the target. Our investigations and calculations demonstrated that the origin of bubbles is realised using pyrene–acetone solution and subpicosecond laser pulses, similarly to the nanosecond irradiation, respectively with different lower pressure values at the naphthalene–methyl-methacrylate solutions. According to these it is presumable that the first condition does not realise during the subpicosecond processing, the surface of the quartz contacting with the liquid reagent does not reach the necessary temperature, to induce the significant changes in the mechanical resistance of the target. This means that the major part of the laser pulse energy causes the fast photodecomposition of the solution molecules originating the gas-mixture composing the bubbles. For this reason, the pulse energy does not increase significantly the temperature of the liquid and indirectly the solid target, as a consequence, subpicosecond LIBWE does not occur. Acknowledgements The authors wish to thank to Prof. S. Szatma´ri for giving access to the subpicosecond laser system. The authors gratefully acknowledge the financial support of the OTKA foundation (TS049872) and the Hungarian Ministry for Culture and Education (NKFP 3A/071/2004). B. Hopp is indebted for his MTA Bolyai scholarship.
References [1] J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68 (1999) 111–113. [2] K. Zimmer, A. Braun, R. Bo¨hme, Appl. Surf. Sci. 208–209 (2003) 199– 204. [3] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Microelectron. Eng. 67–68 (2003) 438–444. [4] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Thin Solid Films 453–454 (2004) 31–35. [5] G. Kopitkovas, T. Lippert, C. David, S. Canulescu, A. Wokaun, J. Gobrecht, J. Photochem. Photobiol. A 166 (2004) 135–140. [6] K. Zimmer, R. Bo¨hme, A. Braun, B. Rauschenbach, F. Bigl, Appl. Phys. A 74 (2002) 453–456. [7] D. Ba¨uerle, Laser Processing and Chemistry, Springer, Berlin, 2000. [8] S. Szatma´ri, Appl. Phys. B 58 (1994) 211–223. [9] M. Csete, Zs. Bor, Appl. Opt. 36 (10) (1997) 2133–2138. [10] Cs. Vass, B. Hopp, T. Smausz, F. Igna´cz, Thin Solid Films 453–454 (2004) 121–126. [11] Cs. Vass, T. Smausz, B. Hopp, J. Phys. D: Appl. Phys. 37 (2004) 2449– 2454. [12] R. Bo¨hme, A. Braun, K. Zimmer, Appl. Surf. Sci. 186 (2002) 276– 281. [13] J. Wang, H. Niino, A. Yabe, Appl. Surf. Sci. 154–155 (2000) 571–576. [14] X. Ding, Y. Kawaguchi, H. Niino, A. Yabe, Appl. Phys. A 75 (6) (2002) 641–645. [15] H. Niino, Y. Yasui, X. Ding, A. Narazaki, T. Sato, Y. Kawaguchi, A. Yabe, J. Photochem. Photobiol. A 158 (2004) 179–182. [16] Y. Kawaguchi, X. Ding, A. Narazaki, T. Sato, H. Niino, Appl. Phys. A 79 (2004) 883–885. [17] A. Vogel, R. Engelhardt, U. Behnle, U. Parlitz, Appl. Phys. B 62 (1996) 173–182. [18] B. Hopp, Z. Toth, K. Gal, A. Mechler, Zs. Bor, S.D. Moustaizis, S. Georgiou, C. Fotakis, Appl. Phys. A 69 (Suppl) (1999) 191–194.
Comparing study of subpicosecond and nanosecond wet etching of fused silica Cs. Vass a,*, D. Sebo˝k a, B. Hopp b a
Department of Optics and Quantum Electronics, University of Szeged, H-6720 Szeged, Do´m te´r 9, Hungary b Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Do´m te´r 9, Hungary Received 3 May 2005; accepted 18 July 2005 Available online 27 October 2005
Abstract The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene–methyl-methacrylate (c = 0.85, 1.71 M) and pyrene–acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360–450 mJ cm2 depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm2 fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene–acetone solution, while using naphthalene–methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene–acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure. # 2005 Elsevier B.V. All rights reserved. Keywords: Wet etching; Fused silica; Transparent materials; Subpicosecond krF laser system
1. Introduction The laser induced backside wet etching (LIBWE) technique is an effective and high accuracy method for the laser micromachining of transparent materials, for example fused silica, pyrex, CaF2, BaF2, sapphire and different types of glasses [1–4]. This procedure makes possible among others the production of micro-optical elements: Fresnel-lenses, microlens arrays and diffractive gratings [4–6]. The most important advantages of the LIBWE procedure in contrast with other methods of transparent material micromachining are the relatively low etching threshold fluence, submicrometer depth
* Corresponding author. E-mail address: [email protected] (C. Vass). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.118
control, quasi smooth etched hole bottoms, and micropatterns free of microcracks and debris. LIBWE is an indirect method. The laser pulses do not etch directly the irradiated target surface of the transparent target. In this technique, a liquid (solution) with high absorptivity at the applied laser wavelength contacting with the backside of the transparent material is irradiated by laser pulses through the solid sample. A thin liquid layer contacting the transparent target absorbs the pulse energy warming up drastically. This hot liquid film heats the surface layer of the solid sample by heat diffusion, therefore this heated volume can soften or melt depending on the developed temperature. Meantime bubbles form and expand in the irradiated interfacial liquid reagent layer and hit the softened or molten surface of the target causing material damage and ejection. The most frequently lasers applied in the LIBWE procedure are the nanosecond excimer lasers (KrF, ArF, XeF;
Cs. Vass et al. / Applied Surface Science 252 (2006) 4768–4772
FWHM = 20–30 ns) and the most general studied transparent material is the fused silica. The femto- and subpicosecond lasers have lot of advantages in the high accuracy micromachining [7]. These lasers make possible debris free ablation without detectable thermal damages around the well-defined ablated holes. In case of the most material the quality of the ablated hole fabricated by femtosecond laser is better than that of those produced by nanosecond laser [7]. According to these facts, it is important to know whether the subpicosecond lasers are also applicable in LIBWE and whether the mentioned benefits of short pulse laser material processing also appear in the subpicosecond backside wet etching procedure, too. By analogy of the earlier results of subpicosecond laser ablation, we expected that the quality of the etched holes generated by short pulse LIBWE is also better than that of those produced by the nanosecond LIBWE. In this paper, we investigated the subpicosecond backside wet etching and compare it to the nanosecond LIBWE. In the interests of the detailed comparison, we studied one of the most important basic effects of the procedure, which is the development of the bubbles in the liquid absorber. Moreover, we calculated the contacting pressure at the quartz surface induced by the above mentioned fast bubble expansion. 2. Experimental Naphthalene–methyl-methacrylate (with a concentrations c = 0.85 and 1.71 mol dm3) and pyrene–acetone (c = 0.4 mol dm3) solutions were used as absorbing liquids in our experiments. As transparent targets, fused silica plates with a thickness of 1 mm (Suprasil II, Heraeus) were applied. Sixhundred femtoseconds pulses were produced by a high brightness KrF excimer laser (l = 248 nm, Emax = 15 mJ; the seed pulse generated by hybrid dye/excimer laser was amplified by a three-pass off-axis KrF amplifier [8]). The applied nanosecond laser was a conventional KrF excimer laser (FWHM = 30 ns). The absorption coefficients of the above mentioned liquids were measured by a plano-concave microcuvette for both nanosecond and subpicosecond lasers [9,10]. The basis of these measurements and calculations can be found in the referred studies. The presented experimental setup was used in this study. The UV beams were focused into the liquid through a quartz plate by a quartz lens ( f = 8 cm) (Fig. 1.). A blade was placed onto the outside surface of the targets to shut the half of the laser beam for the accurate
Fig. 1. Schematic diagram of the etching experimental setup.
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measurement of the etch depths. The repetition rate was 2 Hz for both lasers. A Topometrix 2000 atomic force microscope (AFM) was applied in the measurement of etched depths. The high pressure induced by the very fast expansion of bubbles, which are developed in the solid–liquid interfacial layer is indispensable parameter for the understanding of LIBWE procedure. Therefore a fast photographic setup was used for the time resolved observation of the bubble formation and expansion [11]. The temporal dependence of the evolved pressure at the effected quartz surface could be calculated on the basis of the recorded images. The delay between the excimer and probe laser pulses was changed in the range of 1– 70 ms. 3. Results and discussion 3.1. Absorption coefficient measurements Our experimental work can be divided into three parts. In our earlier studies we demonstrated that the absorption coefficient of the applied liquid absorbers is a very determinative parameter in the LIBWE process [10,11]. So, in the interests of the all-embracing planning of the machining experiments and the correct interpretation of our results and observations first we measured the absorption coefficient of the used solutions for both lasers. A plano-concave microcuvette described in our former study was used for this measurement. This method is useful in the case of liquids having very high absorption coefficient. During these experiments the applied laser fluence was 0.02 mJ cm2, to avoid the damage of the applied beam profiler. This is less than the etching fluences, therefore the absorptivities of absorbers at higher fluences applied during our LIBWE experiments may differ from the measured values, but we believe that these provide good estimate. The results of these experiments are summarised in Table 1. We could not found significant differences between the absorption coefficient values for the different pulse durations in the investigated fluence range. 3.2. Nanosecond LIBWE The ablation threshold fluence of the fused silica for nanosecond KrF excimer laser pulses is more than 10 J cm2 [12,13]. It was found that the nanosecond backside wet etching Table 1 The measured absorption coefficients of the used solutions for both nanosecond and subpicosecond KrF excimer lasers Solution
c (mol dm3)
Pulse duration (FWHM)
a (1/cm)
Pyrene–acetone
0.4
30 ns 600 fs
2100 2220
Naphthalene–methylmethacrylate
0.85
30 ns 600 fs
2830 2870
Naphthalene–methylmethacrylate
1.71
30 ns 600 fs
3380 3710
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threshold of quartz is more than one magnitude less. According to our measurements the threshold values for nanosecond KrF laser are 450 mJ cm2 for pyrene–acetone solution (c = 0.4 mol dm3), 440 and 360 mJ cm2 for naphthalene– methyl-methacrylate solutions with concentrations c = 0.85 and 1.71 mol dm3. These results correlate with the measured absorption coefficients of the liquid absorbers, since lower threshold fluence belongs to higher absorption coefficient. The connection between these two parameters is easily understandable. The penetration depth of the laser beam in the liquid reagent decreases with increasing its absorption coefficient. Therefore the pulse energy is absorbed in thinner liquid layer resulting in higher liquid temperature. This layer heats up the fused silica target by heat diffusion to higher temperature, which is high enough to soften or melt the quartz backside surface layer. This molten material can be removed from the target surface during the bubble expansion at lower laser fluence, too. The etch rate (the thickness of the layer removed by single laser pulse) versus laser fluence graphs (Fig. 2a–c) show that two lines having different slopes can be fitted to the measured data points in each cases. These slopes refer to the different etching mechanisms [11]. These various slopes were also observed in other studies [3,4,12,14]. In these graphs, it can be seen that at given fluence values the etch rate at the naphthalene–methyl-methacrylate solution with a concentrations 1.71 mol dm3 (highest absorptivity) is the highest and the etch rate at the pyrene–acetone solution (lowest absorptivity) is the lowest. The reason of this fact can be attributed to the
different absorption coefficients of the applied liquid reagents. The explanation in this case can be also derived from the higher temperature of the smaller liquid volume that absorbs energy. Fig. 3 shows AFM images of an etched hole made by nanosecond pulses. 3.3. Subpicosecond LIBWE The ablation threshold fluence of the fused silica for 600 fs KrF excimer laser was measured to be 0.7 J cm2. This is one magnitude less than the value for the ordinary nanosecond KrF pulses, but it is approximately two times higher than the determined threshold fluence of the nanosecond LIBWE for the used solutions. In the subpicosecond LIBWE experiments, the fluences were varied up to this threshold (700 mJ cm2). During our investigations, any etched holes was not ˚ ) in this detectable using AFM (vertical resolution is A fluence range. To find the reason of this result, in our next experiments we studied the bubble origination and expansion process, which are important basic effects of the LIBWE procedure. 3.4. Time resolved investigations Several papers presented that the high pressure at the target surface caused by fast bubble expansion has important role in the backside wet etching process [11,14–16]. Therefore we completed our etching setup with a fast photographic
Fig. 2. (a–c) The etch rate as a function of the nanosecond laser fluences for each solution.
Cs. Vass et al. / Applied Surface Science 252 (2006) 4768–4772
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Fig. 4. The schema of the chemical structures of pyrene (a) and naphthalene (b) molecules.
Fig. 3. AFM images of an etched hole: edge (a); bottom (b) (nanosecond KrF, fluence: 500 mJ cm2, pulse number: 100, pyrene–acetone solution, c = 0.4 mol dm3).
arrangement for the time-resolved investigation of the bubble development and expansion in both of the subpicosecond and the nanosecond experiments. The applied laser fluence was constant, F = 460 mJ cm2. The pressure inside the bubbles can be determined on the basis of the snapshots about the bubbles measuring their radii using this equation: 3k dR 2s Rn 2s 4m dR ; (1) p R; ¼ pstat þ dt Rn R R dt R where R is the radius and Rn is the equilibrium radius of the bubble, pstat is the static ambient air pressure ( pstat = 105 Pa), s is the surface tension (smethyl-methacrylate = 0.028 N/m sacetone = 0.02346 N/m), m is the dynamic shear viscosity (mmethylmethacrylate = 0.000632 Pa s macetone = 0.000306 Pa s) and k is the polytropic exponent of the solutions (k = 4/3 for both solvents). The details of the arithmetic procedure can be found in our earlier paper and the referred study [11,17]. The results of these measurements and calculations, which are derived from the analysis of more than 270 snapshots, are summarised in Table 2. The calculated pressure values are in good agreement with the values measured by Y. Kawaguchi and co-workers, although they used other solutions [16]. The pressures are
almost same for pyrene–acetone solutions for both pulse durations, and the pressures in the cases of naphthalene–methylmethacrylate solutions generated during nanosecond laser irradiation are 3.5–5 times higher than that of those produced by subpicosecond pulses. This difference can be due to the different chemical and thermal effects induced by the nanosecond and subpicosecond laser irradiations. The spatial distributions of the absorbed laser energy are the same in both cases, due to the same absorption coefficients of each solution for both lasers. Presumable, the heating of the liquid absorber is the dominating effect in the nanosecond case, in contrast to the subpicosecond irradiation, where probably the chemical reactions are the significant effects [7]. Therefore, during the nanosecond LIBWE the bubble pressure does not depend significantly on the composition of the applied solutions (Table 2, 2nd column) because its value is determined by the quantity of the evaporated fragments basically depending on the absorbed energy, which was constant. In the cases of naphthalene–methyl-methacrylate solutions using subpicosecond irradiation one part of the pulse energy probably causes the polymerisation of the naphthalene–methylmethacrylate molecules producing a thin film, which can cover the contacting surface of the quartz plate [18]. The remainder pulse energy can induce the decomposition of the solution molecules. Since the pyrene molecule consists of four aromatic rings while a naphthalene molecule contains only two (Fig. 4.), therefore the ultrashort UV pulse produces more fragments breaking the pyrene molecule during the photodecomposition. More fragments induce higher pressure in the developed bubbles. These facts can explain the calculated pressure differences. 4. Conclusions
Table 2 The determined maximal pressures (MPa) of the developed bubbles Maximal bubble pressures in MPa
3
Pyrene–acetone 0.4 mol dm Naphthalene–methyl-methacrylate 0.85 mol dm3 Naphthalene–methyl-methacrylate 1.71 mol dm3
Pulse duration 30 ns
600 fs
48.1 75.0 53.8
61.0 14.3 15.7
On the basis of our results, it can be established that the subpicosecond LIBWE is unrealizable under the applied experimental conditions, using the above mentioned solutions and subpicosecond excimer laser. According to our theory, two main conditions have to be realised to accomplish the LIBWE: (i) the contacting surface of the quartz plate has to be modified (softening, melting) during the laser irradiation to decrease the
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resistance of the surface layer to the mechanical effects and (ii) on the other hand LIBWE needs significant mechanical effect (high pressure induced by bubble expansion), which is able to remove the modified quartz layer from the surface of the target. Our investigations and calculations demonstrated that the origin of bubbles is realised using pyrene–acetone solution and subpicosecond laser pulses, similarly to the nanosecond irradiation, respectively with different lower pressure values at the naphthalene–methyl-methacrylate solutions. According to these it is presumable that the first condition does not realise during the subpicosecond processing, the surface of the quartz contacting with the liquid reagent does not reach the necessary temperature, to induce the significant changes in the mechanical resistance of the target. This means that the major part of the laser pulse energy causes the fast photodecomposition of the solution molecules originating the gas-mixture composing the bubbles. For this reason, the pulse energy does not increase significantly the temperature of the liquid and indirectly the solid target, as a consequence, subpicosecond LIBWE does not occur. Acknowledgements The authors wish to thank to Prof. S. Szatma´ri for giving access to the subpicosecond laser system. The authors gratefully acknowledge the financial support of the OTKA foundation (TS049872) and the Hungarian Ministry for Culture and Education (NKFP 3A/071/2004). B. Hopp is indebted for his MTA Bolyai scholarship.
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