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Effect of pulse width and fluence of femtosecond laser on the size of nanobump array
Applied Surface Science 253 (2007) 6555–6557 www.elsevier.com/locate/apsusc
Effect of pulse width and fluence of femtosecond laser on the size of nanobump array Yoshiki Nakata a,*, Noriaki Miyanaga a, Tatsuo Okada b b
a Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita-shi, Osaka 565-0871, Japan Graduate school of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Available online 30 January 2007
Abstract Conical nanobump arrays were generated on gold thin film processed by interfering femtosecond laser. The transition of the height and diameter as functions of fluence and pulse width was investigated. When the fluence was 87 mJ/cm2, the height and diameter were not so different at 350 fs or shorter pulse width. They decreased at longer pulse width, and no bump could be generated over 1.6 ps. The results suggest the decrease of size is due to the diffusion of electron to not-excited region, and due to heat conduction to not heated region or substrate, or change of absorbance of laser. At long pulse width of 2.4 ps and relatively higher fluence of 190 mJ/cm2, nanobump had liquid-like structure as a stop motion of a water drop. # 2007 Elsevier B.V. All rights reserved. PACS : 81.16. c; 42.25.Hz; 81.07. b; 68.55. a; 61.46.+w Keywords: Femtosecond laser; Interference; Laser modification; Laser ablation; Thin film; Nanomaterial; Nanobump; Array
1. Introduction Bottom-up technologies are mainstream in nanotechnology. Nano-sized and functional materials have been generated by bottom-up technologies, such as chemical vapor deposition, discharge, laser ablation, etc. [1,2]. They can produce quite fine structures, but have difficulties in tasks such as size, structure, density and alignment controls. For example, it is difficult to place a nanomaterial at a right position, moreover it is difficult to fabricate a precisely aligned structure by bottom-up technologies. On the other hand, structures with precise period in the order of wavelength, such as Bragg grating and distributed-feedback laser, have been fabricated by using interference of laser [3,4]. Recently, interference pattern of femtosecond laser was directly processed on a bulk surface by using femtosecond laser processing [5–10]. Among them, quite interesting structure of hollow nanobump array was found to be generated from metallic thin film processed by interfering femtosecond laser [8]. Such nano-sized, peaky and arrayed
* Corresponding author. Tel.: +81 6 6879 8729; fax: +81 6 6879 8729. E-mail address: [email protected] (Y. Nakata). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.080
structure has many applications in nanotechnology, and this is a top-down technology. It is interesting to know that the effect of pulse width on the generation of nano-bump, because it is related to photon cost, which is important in practical point of view. In this paper, we report the transition of the size and the structure of nanobump array as a function of pulse width and fluence. The effect of energy loss on the size and structure change is discussed. 2. Experimental In this experiment, a demagnification system was used to split and correlate femtosecond laser beams, as shown in Fig. 1. A femtosecond laser beam is split by a transmission beam splitter, and correlated on the surface of a material through two convex lenses configuring a demagnification system. Some beams which are not used are dumped between the lenses. The interval of the interference pattern L can be calculated by L = ML/2, where M = f 2/f 1 < 1 is the demagnification factor of the system, and L is the period of the transmission beam splitter. In this experiment, L 1.7 mm was used. This system has advantages in easy alignment and wide interfering region
6556
Y. Nakata et al. / Applied Surface Science 253 (2007) 6555–6557
Fig. 1. Experimental setup. A femtosecond laser beam is split by a transmission beam splitter, and correlated on a surface of a material by a demagnification system.
compared to the beam correlation system using mirror beam splitter [7,10]. A ultra-short laser system, of which the center wavelength was 780 nm, was used. Negative chirped pulse was used at longer pulse width. On the other hand, a gold thin film with the thickness of about 50 nm evaporated on transparent quartz glass was used as a target. All the structures shown in this paper were generated in a single shot, and the processes were carried out at room temperature and without any evacuation. The surface morphology was examined ex situ by scanning electron microscope (SEM). 3. Results Images of nanobump arrays generated at 87 mJ/cm2 and at different pulse width are shown in Fig. 2. Interfering four beams
Fig. 2. Bird’s-eye views of the transition of the nanobump array generated on gold thin film by four interfering femtosecond laser beams, observed using SEM. The femtosecond laser fluence was 87 mJ/cm2. Top left inset illustrates the beam incidence on the film, and top right inset illustrates the formation process of a bump: (a) nanobump array generated at 120 fs, (b) nanobump array generated at 355 fs, (c) nanobump array generated at 741 fs and (d) nanobump array generated at 1220 fs.
Fig. 3. Transition of the height and diameter of nanobump as a function of pulse width at (a) 87 mJ/cm2 and (b) 114 mJ/cm2.
were used, and the top left inset illustrates beam incidence on the film. When the pulse width was shorter value of 120 fs and 355 fs, the shape was conical nanobump and they had almost same size. On the other hand, when the pulse width was elongated to 741 fs, the size was smaller. At 1220 fs, the size was much smaller. The transition of the height and diameter of nanobump as a function of pulse width at 87 mJ/cm2 and at 114 mJ/cm2 is summarized in Fig. 3. When the fluence was 87 mJ/cm2 as in Fig. 2, there are small change under about 350 fs, but they decreased over this pulse width, and no nanobump was generated over 1.6 ps. On the other hand, when the fluence was 114 mJ/cm2, the film was ablated and nanohole array was generated under about 700 fs. The size decreased with the pulse width, as well as the case at 87 mJ/cm2. The nanobump array generated at long pulse width and high fluence had liquid-like structure, as shown in Fig. 4. The pulse width was 2.4 ps, and the fluence was 190 mJ/cm2. The shape is similar to the stop motion of a water drop.
Fig. 4. Nanobump array generated at the pulse width of 2.4 ps, and at the fluence of 190 mJ/cm2.
Y. Nakata et al. / Applied Surface Science 253 (2007) 6555–6557
4. Discussion The top right inset in Fig. 2 illustrates the three-dimensional structure of a nanobump, which has been already known by exfoliating the film and observing it from the backside by using atomic force microscope (AFM) [8]. The structure is hollow, which is thought to be generated by inflation with periodical melting and evaporation caused by the interference pattern of femtosecond laser beams. The arrows represent the exhalation of vapor, and the pressure inside the bump is higher because it is closed. It is recognized that the melting structure is quite little in the case of femtosecond laser processing, compared to nanosecond laser processing. But by using lower fluence and smaller size, thermal process such as melting and evaporation can be dominant process, and characteristic structure such as hollow nanobump can be formed. In the case of femtosecond laser, transfer of energy from laser to material occurs in a quite short time. Electrons are firstly excited by femtosecond laser irradiation and thermalized in about 100 fs, then they have temperature. Then they diffuse with raising the temperature of the material in some ps. When the fluence was 87 mJ/cm2, the size of nanobump decreased at the pulse width longer than 350 fs, which is due to the loss of energy. The loss of energy can be due to the change of the absorbance of laser [11] or heat radiation, or due to the diffusion of high energy electrons from excited region to not-excited region, or heat conduction from excited region to not-heated region or to transparent substrate. There is quite few data of these phenomena between gold film and quartz glass substrate, so it is difficult to speculate about energy loss paths in detail. The loss of energy can be compensated by using higher fluence. For example, the size of nanobump at 87 mJ/cm2 and at 750 fs is almost same at 114 mJ/cm2 and at 2.9 ps. On the other hand, the basic structure is different, which is discussed as follows. At long pulse width of 2.4 ps and high fluence of 190 mJ/ cm2, they had liquid-like structure, as shown in Fig. 4. The mechanism might be similar to that of water drop, in which the viscosity and surface tension play important roles [8]. The motion is frozen as temperature down. Korte et al. supposed that the mechanism is Marangoni convection, but it should not the major mechanism because the effect is just for convection, and moreover the film thickness is too thin to form temperature distribution across the film and to be convected, which is just 50 nm [12]. The different shape should be induced by the different transition of the distribution of temperature. In the case of longer pulse width, the distribution should be relatively uniform in the lateral direction due to heat conduction. If relatively uniform temperature distribution induces such structure, less correlation of femtosecond laser beams can cause the same process. This might be another and easier way to control the liquefaction of the process. In our system shown in Fig. 1, less correlation can be obtained by just inserting
6557
transparent glasses to change the optical paths of suitable beams. As mentioned above, the size is not so different with the pulse width shorter than about 350 fs, as shown Fig. 2 (a and b). Within the pulse width, energy loss processes are not prominent and not effective to size down. Availability of longer pulse results in longer lifetime of optics used in femtosecond laser and correlation system. For example, power density at 300 fs is one third of that at 100 fs. This point of view has infrequently discussed and investigated, but is very important in practical point of view. Longer lifetime results in low photon cost, which is essential to practical use of femtosecond laser, because it is thought to have relatively higher photon cost. In summary, we investigated the transition of the height and diameter of nanobump as a function of fluence and pulse width. The size decrease according to the elongation of pulse width in some ps suggest that the energy loss is thought to be due to diffusion of electron and heat conduction or change of absorbance of laser. A pulse width of 350 fs is enough short to generate nanobump array without loss of energy. A liquidlike structure is thought to be generated with relatively uniform temperature distribution. Acknowledgments The experiments were performed using femtosecond laser system at the Institute for Ionized Gas and Laser research, Kyushu University. The measurement of surface morphology was made using SEM at the Center of Advanced Instrumental Analysis, Kyushu University. This research was partly supported by Iketani science and technology foundation, Amada foundation for metal work technology, Murata science foundation and CASIO science promotion foundation. References [1] S. Iijima, Nature 354 (1991) 56. [2] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [3] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. [4] C.V. Shank, J.E. Bjorkholm, H. Kogelnik, Appl. Phys. Lett. 18 (1971) 395. [5] Y. Nakata, T. Okada, M. Maeda, Appl. Phys. Lett. 81 (2002) 4239. [6] Y. Nakata, T. Okada, M. Maeda, Jpn. J. Appl. Phys. 42 (2003) L379. [7] Y. Nakata, T. Okada, M. Maeda, Appl. Phys. A 77 (2003) 399. [8] Y. Nakata, T. Okada, M. Maeda, Jpn. J. Appl. Phys. 42 (2003) L1452. [9] K.K. Seet, V. Mizeikis, S. Juodkazis, H. Misawa, J. Non-cryst. Sol. 352 (2006) 2390. [10] K. Kawamura, M. Hirano, T. Kamiya, H. Hosono, J. Non-cryst. Sol. 352 (2006) 2347. [11] D.R. Bach, D.E. Casperson, D.W. Forslund, S.J. Gitomer, P.D. Goldstone, A. Hauer, J.F. Kephart, J.M. Kindel, R. Kristal, G.A. Kyrala, K.B. Mitchell, D.B. van Hulsteyn, A.H. Williams, Phys. Rev. Lett. 50 (1983) 2082. [12] F. Korte, J. Koch, B.N. Chichkov, Appl. Phys. A 79 (2004) 879.
Effect of pulse width and fluence of femtosecond laser on the size of nanobump array Yoshiki Nakata a,*, Noriaki Miyanaga a, Tatsuo Okada b b
a Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita-shi, Osaka 565-0871, Japan Graduate school of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Available online 30 January 2007
Abstract Conical nanobump arrays were generated on gold thin film processed by interfering femtosecond laser. The transition of the height and diameter as functions of fluence and pulse width was investigated. When the fluence was 87 mJ/cm2, the height and diameter were not so different at 350 fs or shorter pulse width. They decreased at longer pulse width, and no bump could be generated over 1.6 ps. The results suggest the decrease of size is due to the diffusion of electron to not-excited region, and due to heat conduction to not heated region or substrate, or change of absorbance of laser. At long pulse width of 2.4 ps and relatively higher fluence of 190 mJ/cm2, nanobump had liquid-like structure as a stop motion of a water drop. # 2007 Elsevier B.V. All rights reserved. PACS : 81.16. c; 42.25.Hz; 81.07. b; 68.55. a; 61.46.+w Keywords: Femtosecond laser; Interference; Laser modification; Laser ablation; Thin film; Nanomaterial; Nanobump; Array
1. Introduction Bottom-up technologies are mainstream in nanotechnology. Nano-sized and functional materials have been generated by bottom-up technologies, such as chemical vapor deposition, discharge, laser ablation, etc. [1,2]. They can produce quite fine structures, but have difficulties in tasks such as size, structure, density and alignment controls. For example, it is difficult to place a nanomaterial at a right position, moreover it is difficult to fabricate a precisely aligned structure by bottom-up technologies. On the other hand, structures with precise period in the order of wavelength, such as Bragg grating and distributed-feedback laser, have been fabricated by using interference of laser [3,4]. Recently, interference pattern of femtosecond laser was directly processed on a bulk surface by using femtosecond laser processing [5–10]. Among them, quite interesting structure of hollow nanobump array was found to be generated from metallic thin film processed by interfering femtosecond laser [8]. Such nano-sized, peaky and arrayed
* Corresponding author. Tel.: +81 6 6879 8729; fax: +81 6 6879 8729. E-mail address: [email protected] (Y. Nakata). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.080
structure has many applications in nanotechnology, and this is a top-down technology. It is interesting to know that the effect of pulse width on the generation of nano-bump, because it is related to photon cost, which is important in practical point of view. In this paper, we report the transition of the size and the structure of nanobump array as a function of pulse width and fluence. The effect of energy loss on the size and structure change is discussed. 2. Experimental In this experiment, a demagnification system was used to split and correlate femtosecond laser beams, as shown in Fig. 1. A femtosecond laser beam is split by a transmission beam splitter, and correlated on the surface of a material through two convex lenses configuring a demagnification system. Some beams which are not used are dumped between the lenses. The interval of the interference pattern L can be calculated by L = ML/2, where M = f 2/f 1 < 1 is the demagnification factor of the system, and L is the period of the transmission beam splitter. In this experiment, L 1.7 mm was used. This system has advantages in easy alignment and wide interfering region
6556
Y. Nakata et al. / Applied Surface Science 253 (2007) 6555–6557
Fig. 1. Experimental setup. A femtosecond laser beam is split by a transmission beam splitter, and correlated on a surface of a material by a demagnification system.
compared to the beam correlation system using mirror beam splitter [7,10]. A ultra-short laser system, of which the center wavelength was 780 nm, was used. Negative chirped pulse was used at longer pulse width. On the other hand, a gold thin film with the thickness of about 50 nm evaporated on transparent quartz glass was used as a target. All the structures shown in this paper were generated in a single shot, and the processes were carried out at room temperature and without any evacuation. The surface morphology was examined ex situ by scanning electron microscope (SEM). 3. Results Images of nanobump arrays generated at 87 mJ/cm2 and at different pulse width are shown in Fig. 2. Interfering four beams
Fig. 2. Bird’s-eye views of the transition of the nanobump array generated on gold thin film by four interfering femtosecond laser beams, observed using SEM. The femtosecond laser fluence was 87 mJ/cm2. Top left inset illustrates the beam incidence on the film, and top right inset illustrates the formation process of a bump: (a) nanobump array generated at 120 fs, (b) nanobump array generated at 355 fs, (c) nanobump array generated at 741 fs and (d) nanobump array generated at 1220 fs.
Fig. 3. Transition of the height and diameter of nanobump as a function of pulse width at (a) 87 mJ/cm2 and (b) 114 mJ/cm2.
were used, and the top left inset illustrates beam incidence on the film. When the pulse width was shorter value of 120 fs and 355 fs, the shape was conical nanobump and they had almost same size. On the other hand, when the pulse width was elongated to 741 fs, the size was smaller. At 1220 fs, the size was much smaller. The transition of the height and diameter of nanobump as a function of pulse width at 87 mJ/cm2 and at 114 mJ/cm2 is summarized in Fig. 3. When the fluence was 87 mJ/cm2 as in Fig. 2, there are small change under about 350 fs, but they decreased over this pulse width, and no nanobump was generated over 1.6 ps. On the other hand, when the fluence was 114 mJ/cm2, the film was ablated and nanohole array was generated under about 700 fs. The size decreased with the pulse width, as well as the case at 87 mJ/cm2. The nanobump array generated at long pulse width and high fluence had liquid-like structure, as shown in Fig. 4. The pulse width was 2.4 ps, and the fluence was 190 mJ/cm2. The shape is similar to the stop motion of a water drop.
Fig. 4. Nanobump array generated at the pulse width of 2.4 ps, and at the fluence of 190 mJ/cm2.
Y. Nakata et al. / Applied Surface Science 253 (2007) 6555–6557
4. Discussion The top right inset in Fig. 2 illustrates the three-dimensional structure of a nanobump, which has been already known by exfoliating the film and observing it from the backside by using atomic force microscope (AFM) [8]. The structure is hollow, which is thought to be generated by inflation with periodical melting and evaporation caused by the interference pattern of femtosecond laser beams. The arrows represent the exhalation of vapor, and the pressure inside the bump is higher because it is closed. It is recognized that the melting structure is quite little in the case of femtosecond laser processing, compared to nanosecond laser processing. But by using lower fluence and smaller size, thermal process such as melting and evaporation can be dominant process, and characteristic structure such as hollow nanobump can be formed. In the case of femtosecond laser, transfer of energy from laser to material occurs in a quite short time. Electrons are firstly excited by femtosecond laser irradiation and thermalized in about 100 fs, then they have temperature. Then they diffuse with raising the temperature of the material in some ps. When the fluence was 87 mJ/cm2, the size of nanobump decreased at the pulse width longer than 350 fs, which is due to the loss of energy. The loss of energy can be due to the change of the absorbance of laser [11] or heat radiation, or due to the diffusion of high energy electrons from excited region to not-excited region, or heat conduction from excited region to not-heated region or to transparent substrate. There is quite few data of these phenomena between gold film and quartz glass substrate, so it is difficult to speculate about energy loss paths in detail. The loss of energy can be compensated by using higher fluence. For example, the size of nanobump at 87 mJ/cm2 and at 750 fs is almost same at 114 mJ/cm2 and at 2.9 ps. On the other hand, the basic structure is different, which is discussed as follows. At long pulse width of 2.4 ps and high fluence of 190 mJ/ cm2, they had liquid-like structure, as shown in Fig. 4. The mechanism might be similar to that of water drop, in which the viscosity and surface tension play important roles [8]. The motion is frozen as temperature down. Korte et al. supposed that the mechanism is Marangoni convection, but it should not the major mechanism because the effect is just for convection, and moreover the film thickness is too thin to form temperature distribution across the film and to be convected, which is just 50 nm [12]. The different shape should be induced by the different transition of the distribution of temperature. In the case of longer pulse width, the distribution should be relatively uniform in the lateral direction due to heat conduction. If relatively uniform temperature distribution induces such structure, less correlation of femtosecond laser beams can cause the same process. This might be another and easier way to control the liquefaction of the process. In our system shown in Fig. 1, less correlation can be obtained by just inserting
6557
transparent glasses to change the optical paths of suitable beams. As mentioned above, the size is not so different with the pulse width shorter than about 350 fs, as shown Fig. 2 (a and b). Within the pulse width, energy loss processes are not prominent and not effective to size down. Availability of longer pulse results in longer lifetime of optics used in femtosecond laser and correlation system. For example, power density at 300 fs is one third of that at 100 fs. This point of view has infrequently discussed and investigated, but is very important in practical point of view. Longer lifetime results in low photon cost, which is essential to practical use of femtosecond laser, because it is thought to have relatively higher photon cost. In summary, we investigated the transition of the height and diameter of nanobump as a function of fluence and pulse width. The size decrease according to the elongation of pulse width in some ps suggest that the energy loss is thought to be due to diffusion of electron and heat conduction or change of absorbance of laser. A pulse width of 350 fs is enough short to generate nanobump array without loss of energy. A liquidlike structure is thought to be generated with relatively uniform temperature distribution. Acknowledgments The experiments were performed using femtosecond laser system at the Institute for Ionized Gas and Laser research, Kyushu University. The measurement of surface morphology was made using SEM at the Center of Advanced Instrumental Analysis, Kyushu University. This research was partly supported by Iketani science and technology foundation, Amada foundation for metal work technology, Murata science foundation and CASIO science promotion foundation. References [1] S. Iijima, Nature 354 (1991) 56. [2] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [3] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. [4] C.V. Shank, J.E. Bjorkholm, H. Kogelnik, Appl. Phys. Lett. 18 (1971) 395. [5] Y. Nakata, T. Okada, M. Maeda, Appl. Phys. Lett. 81 (2002) 4239. [6] Y. Nakata, T. Okada, M. Maeda, Jpn. J. Appl. Phys. 42 (2003) L379. [7] Y. Nakata, T. Okada, M. Maeda, Appl. Phys. A 77 (2003) 399. [8] Y. Nakata, T. Okada, M. Maeda, Jpn. J. Appl. Phys. 42 (2003) L1452. [9] K.K. Seet, V. Mizeikis, S. Juodkazis, H. Misawa, J. Non-cryst. Sol. 352 (2006) 2390. [10] K. Kawamura, M. Hirano, T. Kamiya, H. Hosono, J. Non-cryst. Sol. 352 (2006) 2347. [11] D.R. Bach, D.E. Casperson, D.W. Forslund, S.J. Gitomer, P.D. Goldstone, A. Hauer, J.F. Kephart, J.M. Kindel, R. Kristal, G.A. Kyrala, K.B. Mitchell, D.B. van Hulsteyn, A.H. Williams, Phys. Rev. Lett. 50 (1983) 2082. [12] F. Korte, J. Koch, B.N. Chichkov, Appl. Phys. A 79 (2004) 879.