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Water spray assisted ultrashort laser pulse ablation
Applied Surface Science 265 (2013) 865–869
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Water spray assisted ultrashort laser pulse ablation M. Silvennoinen ∗ , J.J.J. Kaakkunen, K. Paivasaari, P. Vahimaa Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 111, Joensuu 80101, Finland
a r t i c l e
i n f o
Article history: Received 9 August 2012 Accepted 24 November 2012 Available online 5 December 2012 Keywords: Laser ablation Ultra short pulses Diffractive optical element
a b s t r a c t We have studied femtosecond ablation under sprayed thin water film and its influence and benefits compared with ablation in the air atmosphere. These have been studied in case of the hole and the groove ablation using IR femtosecond laser. Water enhances the ablation rate and in some situations it makes possible to ablate the holes with a higher aspect ratio. While ablating the grooves, the water spray allows using the high fluences without the generation of the self-organized structures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ultrashort laser pulse ablation is a powerful tool for processing solid materials with high precision [1]. Laser parameters like pulse length and fluence are important when selecting a laser for a certain processing job. When ablating with long optical pulses the surface is experiencing heating which affects the material and the details around the processing zone [2]. The shortening of the pulse reduces the heat affected zone which makes possible to produce smaller features. In theory, ablation with ultrashort pulses minimizes the amount of generated heat. Therefore femtosecond laser ablation is a versatile tool for making of small features virtually in any solid material. With femtosecond laser high fluence is desirable because it enables faster processing in form of less pulses. However, this can be problematic because femtosecond pulses are known to generate self-organized micro- and nanostructures when using high fluences [3]. Although these structures have various applications, like changing surfaces wettability properties [4,5] or increasing of light absorption on the surface [6], in most applications they are not wanted. In addition to self-organized structures, a lot of debris is formed during laser ablation and removal of it is beneficial [7,8]. With long optical pulses like nanosecond pulses, various liquids and especially water have been used to get rid of the debris. Water has been used in form of various cleaning methods like water jet, under water or backside ablation when other side of the sample is in liquid and using water steam or spray [1,9–17]. There are several studies where ablations with liquids have been used for cleaning and cooling. One of these is the ablation of glass sheet from the backside. In this setup, front side of the glass is in the air and back side is in
∗ Corresponding author. Tel.: +358 50 4423545; fax: +358 13 2513290. E-mail address: martti.silvennoinen@uef.fi (M. Silvennoinen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.135
an etchant like acetone. There have also been studies where ablation of glass is done from the front side under the water [16,18,19]. Added to this ablation of other materials like silicon and plastics have been done under relatively thick stationary water layer [10,11] and silicon carbide in alcohol [20]. Also the comparison of ablation in various gas atmosphere has been done [21,22]. Depris removal is also done with water-soluble coating which is afterwards removed [23]. All of these above mentioned methods have their disadvantages like scattering from water droplets, requirement of the additional process steps and additional coating or post cleaning. Ablation in water has additional problems when using high intensity ultrashort laser pulses because of the optical breakdown and related effect [24,25]. In this case thick layer of the water is problematic, because with high fluence laser pulses are generating gas bubbles and white-light continuum, that both absorb and remove energy from the ablation process. Thick layer is also affecting the light beam, causing it to bend and with high fluence to self-focus [24,25]. These problems can be avoided by using a very thin layer of the water, which thickness is in the range of a few microns. The thin water layer can be generated with various methods, like water jet, steam cleaner or water spray. The problem of water jet is that it does not produce a layer with even thickness on the surface, and secondly it consumes a lot of the water. The steam cleaner is also problematic, because heat deforms thin layers and materials like plastics. Steam cleaner heats sample up to 250 ◦ C and produces fog around the ablation point, which scatters the light. In this work, we demonstrate with water-assisted IR femtosecond laser ablation that hole drilling can be made more efficient than in air atmosphere and generation of the self-organized structures can be avoided. This is done by comparing the hole and the groove ablation in the air atmosphere and under the water spray using identical laser parameters. Using very thin layer of room
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M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
Distilled water Pressurized air
Sample DOE
Aperture
Waterspray
Lens
Fig. 1. Schematics of the setup used in the ablation experiments. Setup is same in ablation of holes and ablation of grooves except the diffractive optical element (DOE), which is not used in scanning of grooves. Metallic injection needle is placed in plastic pipette. Pressurized air is led into pipette and water into needle, which forms fine water spray.
temperature water there is neither heating nor the deformation of sample. In addition, white-light continuum or gas bubble generation is minimized when using thin water layer. Also debris is removed meaning that the propagation of the beam is not affected by it. Ablation made by this way is more efficient, generation of selforganized structures are suppressed and post cleaning processes are not needed. 2. Setup and experiments The femtosecond laser used in experiments has 120 fs pulse length with 790 nm center wavelength and maximum pulse energy 3.5 mJ at 1 kHz repetition rate. Schematic of the ablation set-up is shown in Fig. 1. Ablations of the hole arrays were performed with diffractive optical element (DOE). DOE produces the intensity distribution of a 5 × 5 dot array shown in Fig. 2(a) theoretical and in Fig. 2(b) experimental [26]. DOE was designed, so that with the 100 mm focal length lens it produced the dot matrix with the period of 50 m. This system is flexible, because by changing the lens with a different focal length lens, the period and spot size of the intensity distribution can be tuned. The purpose of using DOE was to test parallel processing and to speed up the ablation of hole arrays. In this case instead of ablating a hole by hole it is possible to ablate 5 × 5 holes simultaneously. In this experiment, the pulse numbers were
varied from 100 to 1000 and pulse energies on DOE aperture were varied from 0.11 to 0.56 mJ, resulting energies from 4.5 to 22.4 J per hole. These hole ablation experiments with the DOE were made into the silicon. Groove ablation were performed using the identical set-up as hole ablation, except for the DOE. In this experiment the scanning speeds were varied from 0.25 to 1.00 mm/s. The sample distances from the lens were varied so that widths of the ablated grooves were from 70 to 280 m. Average pulse numbers in the groove scans depend on width of processing area, repetition rate and scanning speed. Used average pulse numbers varied from 155 to 620. Used pulse energy was 1.0 mJ in all groove ablations, which means that fluences were varying from 1.6 to 26.0 J/cm2 . Groove ablations were studied both with and without water spray. The groove scan experiments were done into polished stainless steel. The water spray was constructed from a plastic pipette and a metallic injection needle (Fig. 1). The needle was placed inside the pipette. The needle was connected to distilled water container and the pipette to pressurized air source. The diameters of exit holes were 0.3 mm and 1.0 mm, respectively. When pressurized air is led into the pipette and water is led into the needle, fine water spray is formed. This water spray is pointed at a surface from distance of 5 cm, with 40 l/min of air and 50 ml/h of water, it produces about 1 cm2 area of water film that has constant thickness of few micrometers. Hole arrays and ablated grooves were imaged with the scanning electron microscope (SEM), without any consecutive cleaning except on grooves ablated without water spray. Large area of hole arrays were made in order to define depth and quality of the ablated holes in silicon. Samples with hole arrays were cut and crosssections were imaged using SEM. Groove topographies in steel were imaged with SEM and the cross-section were also measured with the profilometer. 3. Results Fig. 3 is shown SEM images of hole arrays ablated without (a) and with (b) water spray. As we can see in these figures, the surface ablated with water spray is clean from debris unlike the one made without water spray. Also ablation has not been interfered by debris which can be seen as slightly larger entrance aperture of the holes. Fig. 4 is shown example set of the SEM images from crosssections of the ablated holes without (a, c) and with (b, d) water spray. Depth of the holes were measured from the cross-sections made with various laser parameters and plotted into Fig. 5. These
Fig. 2. Theoretical (a) and experimental (b) far field intensity distribution of the designed diffractive optical element (DOE). In (a) lower figure is line intensity distribution from the upper one (red line). Distance between two dots is 50 m (with 100 mm focal length lens). (For interpretation of the references to color in this sentence, the reader is referred to the web version of the article.)
M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
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Fig. 3. SEM-images of the ablation made with DOE in silicon without (a) and with water spray (b). Both are made using 1000 pulses with 2.2 J energy per hole.
Fig. 4. SEM-images of the ablations made in silicon without (a), (c) and with (b), (d) water spray. Holes are made using 100 pulses and 4.5 J energy per hole. Reason why holes appear to reach different depths is because of silicon dicing is not following center of each hole in the array.
0.3
Ablation depth (mm)
100 pulses water 100 pulses 0.2
500 pulses water 500 pulses 1000 pulses water 1000 pulses
0.1
0.0
5
10
15 Pulse energy (µJ)
20
25
Fig. 5. Ablation depth as a function of the pulse energy with various pulse numbers. In graph dashed lines are made without and solid lines are made with water spray. Lines are logarithmical fittings.
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Fig. 6. SEM-images of the ablated grooves in stainless steel without (a) and with water spray (b). Both are made using average pulse number 50 and fluence 5.1 J/cm2 . Both grooves are imaged without any additional cleaning.
tests show that with water spray holes reach deeper and the quality is more consistent. From Fig. 5 one can see that two of the measured sets, without water spray using 1000 pulses and with water using 500 pulses, almost overlap each other. This means that to obtain the same ablation depth with water spray one needs only half of the pulses, which means that process speed is doubled. Reason for this is that debris is removed from the holes between the consecutive pulses. Holes ablated in air show that debris is cumulating on sides of the hole and this makes ablation harder. Holes become curved because ablation debris is blocking and randomly guiding the beam (Fig. 4(c)). With the water spray, this is avoided (Fig. 4(d)).
With DOE, pulse energy of 25 J per hole, 1000 pulses and water spray, the hole reaches the depth of 300 m resulting the aspect ratio of 15. With 3 mJ pulse energy and proper design of DOE, this means the possibility of ablating ca. 120 holes (a matrix of 11 × 11) in one second. Fig. 6 is shown SEM images of the ablated groove made without (a) and with (b) water spray. These images show that without water spray the bottom of the groove is covered with self-organized structures, debris is covering top surface and ablated material have resolidified to edges of the groove. Ablations made with water spray show that bottom of the groove is smooth and there
Fig. 7. SEM-images of the grooves made by scanning the pulses, without (left column) and with (right column) water spray. In (a) and (b) used fluence is 5.3 J/cm2 and average pulse number 155, (c) and (d) fluence is 5.3 J/cm2 and average pulse number 620, (e) and (f) fluence is 26.0 J/cm2 and average pulse number 280. In every figure is shown profilometer scan across ablated groove. Left column grooves are cleaned with a ultrasonic bath and right column grooves are imaged without any additional cleaning.
M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
are no self-organized structures. Also the surface is clean and no resolidified material is seen on edges of the groove. Fig. 7 is shown SEM images from the ablated grooves with different laser parameters. In this series of figures left column images are made without and right column with the water spray. Fig. 7(a)–(d) is made with 5.3 J/cm and (e, f) with 26 J/cm fluence. Average pulse numbers are: in (a, b) 155 pulses, in (c, d) 620 pulses and in (e, f) 280 pulses. Each of these images includes the profilometer scan made across the grooves. These figures show that without water self-organized structures are forming and feature size of the structure increases as a function of the fluence and pulse number. When grooves are ablated under water spray, self-organized structures are not formed. Grooves made with water have the bottom covered with light induced periodic surface structure (LIPSS) [27]. When ablation takes place at focal point in the air atmosphere and fluence is high, ablation debris is forming a burr around the ablated groove (Fig. 7(e)). By variation of fluence it is possible to make these self-organized structures to grow out from the surface (Fig. 7(a)), at the level of the surface or beneath of the surface (Fig. 7(c)). Note that the grooves shown in Fig. 7 without water spray are cleaned with the ultrasonic bath to show that the structures in the grooves are solid and are not removable easily. The formation mechanism of these self-organized microstructures is not fully understood [3]. This is because it is depending on many parameters such as material, laser (fluence, pulse number, polarization, etc.) and atmosphere (pressure, gas, etc.). In this article we have shown that thin layer of sprayed water prevented self-organized microstructures from forming. This could indicates that the microstructures are formed after the plasma formation since water is absent from the ablated surface during the material evaporation. In vacuum tests, not shown here, we have noticed that these generated microstructures are deeper than in gas atmosphere using identical laser parameters. In vacuum the value of fluence, which starts to build self-organized microstructures, is lower than in normal atmosphere pressure. Together with the results obtained with water it is clear that the heat transfer from the ablation spot is important parameter in the formation of the self-organized microstructures. 4. Conclusion In this work, we have demonstrated with water spray assisted IR femtosecond laser ablation that hole ablation can be made more efficient than in air atmosphere and self-organized structures can be avoided. This was done by comparing the hole and the groove ablation in the air atmosphere and under the water spray. We have shown that by using DOE and water spray, better quality holes can be made faster. Water spray not only makes the ablated holes debris free but also the surrounding areas. Added to these, it is possible to produce holes with higher aspect ratio using water-assisted ablation. We have also demonstrated that this water spraying method enhances the groove ablation, where self-organized microstructures are easily formed. It not only prevent the formation of these structures but also enhances the edge quality of the ablated grooves. Acknowledgement This article was supported by Tekes – the Finnish Funding Agency for Technology and Innovation.
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References [1] N. Barch, A. Gatti, S. Barcikowski, Improving laser ablation of zirconia by liquid films: multiple influence of liquids on surface machining and nanoparticle generation, Journal of Laser Micro/Nanoengineering 4 (2009) 66–71. [2] B.N. Chicbkov, C. Momma, S. Nolte, F. yon Alvensleben, A. Tiinnermann, Femtosecond, picosecond and nanosecond laser ablation of solids, Applied Physics A 63 (1996) 109–115. [3] B.K. Nayak, M.C. Gupta, Ultrafast laser-induced self-organized conical micro/nano surface structures and their origin, Optics and Lasers in Engineering 48 (2010) 966–973. [4] M. Silvennoinen, J. Kaakkunen, K. Paivasaari, P. Vahimaa, T. Jaaskelainen, Controlling the hydrophobic properties of material surface using femtosecond ablation, Journal of Laser Micro/Nanoengineering 5 (2010) 97–98. [5] M. Groenendijk, Fabrication of super hydrophobic surfaces by fs laser pulses, Laser Technik Journal 5 (2008) 44–47. [6] K. Paivasaari, J.J.J. Kaakkunen, M. Kuittinen, T. Jaaskelainen, Enhanced optical absorptance of metals using interferometric femtosecond ablation, Optics Express 15 (2007) 13838–13843. [7] J.H. Yoo, S.H. Jeong, X.L. Mao, R. Greif, R.E. Russo, Evidence of phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon, Applied Physics Letters 76 (2000) 783–786. [8] N.G. Semaltianos, W. Perrie, V. Vishnyakov, R. Murray, C.J. Williams, S.P. Edwardson, G. Dearden, P. French, M. Sharp, S. Logothetidis, K.G. Watkins, Nanoparticle formation by the debris produced by femtosecond laser ablation of silicon in ambient air, Materials Letters 62 (2008) 2165–2170. [9] T.A. Mai, Debris-free laser micromachining, Industrial Laser Solutions 23 (2008) 1–8. [10] G. Daminelli, J. Kruger, W. Kautek, Femtosecond laser interaction with silicon under water confinement, Thin Solid Films 467 (2004) 334–341. [11] Y. Han, C.-H. Lin, H. Xiao, H.-L. Tsai, Femtosecond laser-induced silicon surface morphology in water confinement, Microsystem Technologies 15 (2009) 1045–1049. [12] A. Kruusing, Underwater and water-assisted laser processing: part 2—etching, cutting and rarely used methods, Optics and Lasers in Engineering 41 (2004) 329–352. [13] M. Geiger, W. Becker, T. Rebhan, J. Hutfless, N. Lutz, Increase of efficiency for the XeCl excimer laser ablation of ceramics, Applied Surface Science 96-98 (1996) 309–315. [14] H. Liu, F. Chen, X. Wang, Q. Yang, H. Bian, J. Si, X. Hou, Influence of liquid environments on femtosecond laser ablation of silicon, Thin Solid Films 518 (2010) 5188–5194. [15] A. Kruusing, Underwater and water-assisted laser processing: part 1—general features, steam cleaning and shock processing, Optics and Lasers in Engineering 41 (2004) 307–327. [16] S. Lee, Femtosecond laser nanomachining and application to micro/nanofluidics for single cell analysis, Dissertation (Mechanical Engineering), University of Michigan, 2008. [17] J. Ren, M. Kelly, L. Hesselink, Laser ablation of silicon in water with nanosecond and femtosecond pulses, Optics Express 30 (13) (2005) 1740–1743. [18] D.J. Hwang, T.Y. Choi, C.P. Grigoropoulos, Liquid-assisted femtosecond laser drilling of straight and three-dimensional microchannels in glass, Applied Physics A 79 (2004) 605–612. [19] R. An, M.D. Hoffman, M.A. Donoghue, A.J. Hunt, S.C. Jacobson, Water-assisted femtosecond laser machining of electrospray nozzles on glass microfluidic devices, Optics Express 16 (2008) 15206–15211. [20] C. Li, X. Shi, J. Si, T. Chen, F. Chen, S. Liang, Z. Wu, X. Hou, Alcohol-assisted photoetching of silicon carbide with a femtosecond laser, Optics Communication 282 (2009) 78–80. [21] E.A. Waddell, L.E. Locascio, G.W. Kramer, UV laser micromachining of polymers for microfluidic applications, Journal of the Association for Laboratory Automation 7 (2002) 78–82. [22] E. Stratakis, V. Zorba, M. Barberoglou, C. Fotakis, G.A. Shafeev, Femtosecond laser writing of nanostructures on bulk Al via its ablation in air and liquids, Applied Surface Science 255 (2009) 5346–5350. [23] T. Lizotte, Removal of UV YAG or excimer laser debris after micro via drilling, Advanced Packaging 1 (2002). [24] A. Brodeur, S.L. Chin, Ultrafast white-light continuum generation and selffocusing in transparent condensed media, Journal of the Optical Society of America B 16 (1999) 637–650. [25] K. Yang, Y. Zhou, Q. Ren, J.Y. Ye, C.X. Deng, Dynamics of microbubble generation and trapping by self-focused femtosecond laser pulses, Applied Physics Letters 95 (2009), 051107-051107-3. [26] J. Turunen, F. Wyrowski, Diffractive Optics for Industrial and Commercial Applications, Akademie Verlag, Berlin, 1997. [27] M. Birmbaum, Semiconductor surface damage produced by ruby lasers, Journal of Applied Physics 36 (1965) 3688–3690.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Water spray assisted ultrashort laser pulse ablation M. Silvennoinen ∗ , J.J.J. Kaakkunen, K. Paivasaari, P. Vahimaa Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 111, Joensuu 80101, Finland
a r t i c l e
i n f o
Article history: Received 9 August 2012 Accepted 24 November 2012 Available online 5 December 2012 Keywords: Laser ablation Ultra short pulses Diffractive optical element
a b s t r a c t We have studied femtosecond ablation under sprayed thin water film and its influence and benefits compared with ablation in the air atmosphere. These have been studied in case of the hole and the groove ablation using IR femtosecond laser. Water enhances the ablation rate and in some situations it makes possible to ablate the holes with a higher aspect ratio. While ablating the grooves, the water spray allows using the high fluences without the generation of the self-organized structures. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Ultrashort laser pulse ablation is a powerful tool for processing solid materials with high precision [1]. Laser parameters like pulse length and fluence are important when selecting a laser for a certain processing job. When ablating with long optical pulses the surface is experiencing heating which affects the material and the details around the processing zone [2]. The shortening of the pulse reduces the heat affected zone which makes possible to produce smaller features. In theory, ablation with ultrashort pulses minimizes the amount of generated heat. Therefore femtosecond laser ablation is a versatile tool for making of small features virtually in any solid material. With femtosecond laser high fluence is desirable because it enables faster processing in form of less pulses. However, this can be problematic because femtosecond pulses are known to generate self-organized micro- and nanostructures when using high fluences [3]. Although these structures have various applications, like changing surfaces wettability properties [4,5] or increasing of light absorption on the surface [6], in most applications they are not wanted. In addition to self-organized structures, a lot of debris is formed during laser ablation and removal of it is beneficial [7,8]. With long optical pulses like nanosecond pulses, various liquids and especially water have been used to get rid of the debris. Water has been used in form of various cleaning methods like water jet, under water or backside ablation when other side of the sample is in liquid and using water steam or spray [1,9–17]. There are several studies where ablations with liquids have been used for cleaning and cooling. One of these is the ablation of glass sheet from the backside. In this setup, front side of the glass is in the air and back side is in
∗ Corresponding author. Tel.: +358 50 4423545; fax: +358 13 2513290. E-mail address: martti.silvennoinen@uef.fi (M. Silvennoinen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.135
an etchant like acetone. There have also been studies where ablation of glass is done from the front side under the water [16,18,19]. Added to this ablation of other materials like silicon and plastics have been done under relatively thick stationary water layer [10,11] and silicon carbide in alcohol [20]. Also the comparison of ablation in various gas atmosphere has been done [21,22]. Depris removal is also done with water-soluble coating which is afterwards removed [23]. All of these above mentioned methods have their disadvantages like scattering from water droplets, requirement of the additional process steps and additional coating or post cleaning. Ablation in water has additional problems when using high intensity ultrashort laser pulses because of the optical breakdown and related effect [24,25]. In this case thick layer of the water is problematic, because with high fluence laser pulses are generating gas bubbles and white-light continuum, that both absorb and remove energy from the ablation process. Thick layer is also affecting the light beam, causing it to bend and with high fluence to self-focus [24,25]. These problems can be avoided by using a very thin layer of the water, which thickness is in the range of a few microns. The thin water layer can be generated with various methods, like water jet, steam cleaner or water spray. The problem of water jet is that it does not produce a layer with even thickness on the surface, and secondly it consumes a lot of the water. The steam cleaner is also problematic, because heat deforms thin layers and materials like plastics. Steam cleaner heats sample up to 250 ◦ C and produces fog around the ablation point, which scatters the light. In this work, we demonstrate with water-assisted IR femtosecond laser ablation that hole drilling can be made more efficient than in air atmosphere and generation of the self-organized structures can be avoided. This is done by comparing the hole and the groove ablation in the air atmosphere and under the water spray using identical laser parameters. Using very thin layer of room
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M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
Distilled water Pressurized air
Sample DOE
Aperture
Waterspray
Lens
Fig. 1. Schematics of the setup used in the ablation experiments. Setup is same in ablation of holes and ablation of grooves except the diffractive optical element (DOE), which is not used in scanning of grooves. Metallic injection needle is placed in plastic pipette. Pressurized air is led into pipette and water into needle, which forms fine water spray.
temperature water there is neither heating nor the deformation of sample. In addition, white-light continuum or gas bubble generation is minimized when using thin water layer. Also debris is removed meaning that the propagation of the beam is not affected by it. Ablation made by this way is more efficient, generation of selforganized structures are suppressed and post cleaning processes are not needed. 2. Setup and experiments The femtosecond laser used in experiments has 120 fs pulse length with 790 nm center wavelength and maximum pulse energy 3.5 mJ at 1 kHz repetition rate. Schematic of the ablation set-up is shown in Fig. 1. Ablations of the hole arrays were performed with diffractive optical element (DOE). DOE produces the intensity distribution of a 5 × 5 dot array shown in Fig. 2(a) theoretical and in Fig. 2(b) experimental [26]. DOE was designed, so that with the 100 mm focal length lens it produced the dot matrix with the period of 50 m. This system is flexible, because by changing the lens with a different focal length lens, the period and spot size of the intensity distribution can be tuned. The purpose of using DOE was to test parallel processing and to speed up the ablation of hole arrays. In this case instead of ablating a hole by hole it is possible to ablate 5 × 5 holes simultaneously. In this experiment, the pulse numbers were
varied from 100 to 1000 and pulse energies on DOE aperture were varied from 0.11 to 0.56 mJ, resulting energies from 4.5 to 22.4 J per hole. These hole ablation experiments with the DOE were made into the silicon. Groove ablation were performed using the identical set-up as hole ablation, except for the DOE. In this experiment the scanning speeds were varied from 0.25 to 1.00 mm/s. The sample distances from the lens were varied so that widths of the ablated grooves were from 70 to 280 m. Average pulse numbers in the groove scans depend on width of processing area, repetition rate and scanning speed. Used average pulse numbers varied from 155 to 620. Used pulse energy was 1.0 mJ in all groove ablations, which means that fluences were varying from 1.6 to 26.0 J/cm2 . Groove ablations were studied both with and without water spray. The groove scan experiments were done into polished stainless steel. The water spray was constructed from a plastic pipette and a metallic injection needle (Fig. 1). The needle was placed inside the pipette. The needle was connected to distilled water container and the pipette to pressurized air source. The diameters of exit holes were 0.3 mm and 1.0 mm, respectively. When pressurized air is led into the pipette and water is led into the needle, fine water spray is formed. This water spray is pointed at a surface from distance of 5 cm, with 40 l/min of air and 50 ml/h of water, it produces about 1 cm2 area of water film that has constant thickness of few micrometers. Hole arrays and ablated grooves were imaged with the scanning electron microscope (SEM), without any consecutive cleaning except on grooves ablated without water spray. Large area of hole arrays were made in order to define depth and quality of the ablated holes in silicon. Samples with hole arrays were cut and crosssections were imaged using SEM. Groove topographies in steel were imaged with SEM and the cross-section were also measured with the profilometer. 3. Results Fig. 3 is shown SEM images of hole arrays ablated without (a) and with (b) water spray. As we can see in these figures, the surface ablated with water spray is clean from debris unlike the one made without water spray. Also ablation has not been interfered by debris which can be seen as slightly larger entrance aperture of the holes. Fig. 4 is shown example set of the SEM images from crosssections of the ablated holes without (a, c) and with (b, d) water spray. Depth of the holes were measured from the cross-sections made with various laser parameters and plotted into Fig. 5. These
Fig. 2. Theoretical (a) and experimental (b) far field intensity distribution of the designed diffractive optical element (DOE). In (a) lower figure is line intensity distribution from the upper one (red line). Distance between two dots is 50 m (with 100 mm focal length lens). (For interpretation of the references to color in this sentence, the reader is referred to the web version of the article.)
M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
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Fig. 3. SEM-images of the ablation made with DOE in silicon without (a) and with water spray (b). Both are made using 1000 pulses with 2.2 J energy per hole.
Fig. 4. SEM-images of the ablations made in silicon without (a), (c) and with (b), (d) water spray. Holes are made using 100 pulses and 4.5 J energy per hole. Reason why holes appear to reach different depths is because of silicon dicing is not following center of each hole in the array.
0.3
Ablation depth (mm)
100 pulses water 100 pulses 0.2
500 pulses water 500 pulses 1000 pulses water 1000 pulses
0.1
0.0
5
10
15 Pulse energy (µJ)
20
25
Fig. 5. Ablation depth as a function of the pulse energy with various pulse numbers. In graph dashed lines are made without and solid lines are made with water spray. Lines are logarithmical fittings.
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Fig. 6. SEM-images of the ablated grooves in stainless steel without (a) and with water spray (b). Both are made using average pulse number 50 and fluence 5.1 J/cm2 . Both grooves are imaged without any additional cleaning.
tests show that with water spray holes reach deeper and the quality is more consistent. From Fig. 5 one can see that two of the measured sets, without water spray using 1000 pulses and with water using 500 pulses, almost overlap each other. This means that to obtain the same ablation depth with water spray one needs only half of the pulses, which means that process speed is doubled. Reason for this is that debris is removed from the holes between the consecutive pulses. Holes ablated in air show that debris is cumulating on sides of the hole and this makes ablation harder. Holes become curved because ablation debris is blocking and randomly guiding the beam (Fig. 4(c)). With the water spray, this is avoided (Fig. 4(d)).
With DOE, pulse energy of 25 J per hole, 1000 pulses and water spray, the hole reaches the depth of 300 m resulting the aspect ratio of 15. With 3 mJ pulse energy and proper design of DOE, this means the possibility of ablating ca. 120 holes (a matrix of 11 × 11) in one second. Fig. 6 is shown SEM images of the ablated groove made without (a) and with (b) water spray. These images show that without water spray the bottom of the groove is covered with self-organized structures, debris is covering top surface and ablated material have resolidified to edges of the groove. Ablations made with water spray show that bottom of the groove is smooth and there
Fig. 7. SEM-images of the grooves made by scanning the pulses, without (left column) and with (right column) water spray. In (a) and (b) used fluence is 5.3 J/cm2 and average pulse number 155, (c) and (d) fluence is 5.3 J/cm2 and average pulse number 620, (e) and (f) fluence is 26.0 J/cm2 and average pulse number 280. In every figure is shown profilometer scan across ablated groove. Left column grooves are cleaned with a ultrasonic bath and right column grooves are imaged without any additional cleaning.
M. Silvennoinen et al. / Applied Surface Science 265 (2013) 865–869
are no self-organized structures. Also the surface is clean and no resolidified material is seen on edges of the groove. Fig. 7 is shown SEM images from the ablated grooves with different laser parameters. In this series of figures left column images are made without and right column with the water spray. Fig. 7(a)–(d) is made with 5.3 J/cm and (e, f) with 26 J/cm fluence. Average pulse numbers are: in (a, b) 155 pulses, in (c, d) 620 pulses and in (e, f) 280 pulses. Each of these images includes the profilometer scan made across the grooves. These figures show that without water self-organized structures are forming and feature size of the structure increases as a function of the fluence and pulse number. When grooves are ablated under water spray, self-organized structures are not formed. Grooves made with water have the bottom covered with light induced periodic surface structure (LIPSS) [27]. When ablation takes place at focal point in the air atmosphere and fluence is high, ablation debris is forming a burr around the ablated groove (Fig. 7(e)). By variation of fluence it is possible to make these self-organized structures to grow out from the surface (Fig. 7(a)), at the level of the surface or beneath of the surface (Fig. 7(c)). Note that the grooves shown in Fig. 7 without water spray are cleaned with the ultrasonic bath to show that the structures in the grooves are solid and are not removable easily. The formation mechanism of these self-organized microstructures is not fully understood [3]. This is because it is depending on many parameters such as material, laser (fluence, pulse number, polarization, etc.) and atmosphere (pressure, gas, etc.). In this article we have shown that thin layer of sprayed water prevented self-organized microstructures from forming. This could indicates that the microstructures are formed after the plasma formation since water is absent from the ablated surface during the material evaporation. In vacuum tests, not shown here, we have noticed that these generated microstructures are deeper than in gas atmosphere using identical laser parameters. In vacuum the value of fluence, which starts to build self-organized microstructures, is lower than in normal atmosphere pressure. Together with the results obtained with water it is clear that the heat transfer from the ablation spot is important parameter in the formation of the self-organized microstructures. 4. Conclusion In this work, we have demonstrated with water spray assisted IR femtosecond laser ablation that hole ablation can be made more efficient than in air atmosphere and self-organized structures can be avoided. This was done by comparing the hole and the groove ablation in the air atmosphere and under the water spray. We have shown that by using DOE and water spray, better quality holes can be made faster. Water spray not only makes the ablated holes debris free but also the surrounding areas. Added to these, it is possible to produce holes with higher aspect ratio using water-assisted ablation. We have also demonstrated that this water spraying method enhances the groove ablation, where self-organized microstructures are easily formed. It not only prevent the formation of these structures but also enhances the edge quality of the ablated grooves. Acknowledgement This article was supported by Tekes – the Finnish Funding Agency for Technology and Innovation.
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