- Email: [email protected]
Surface ripple changes during Cr film ablation with a double ultrashort laser pulse
ARTICLE IN PRESS
Optics and Lasers in Engineering 46 (2008) 306–310 www.elsevier.com/locate/optlaseng
Surface ripple changes during Cr film ablation with a double ultrashort laser pulse Jaegu Kima,, Suckjoo Nab, Sunghak Choa, Wonseok Changa, Kyunghyun Whanga a
Korea Institute of Machinery & Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305 343, Republic of Korea Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305 701, Republic of Korea
b
Received 28 March 2007; received in revised form 24 November 2007; accepted 2 December 2007
Abstract Surface modification is investigated experimentally by varying the time separation of double femtosecond laser radiation and surface ripples by varying the time separation and polarization direction of double pulses train. Nanometer-sized particles are formed during resolidification of the molten region when the second pulse arrives within 10 ps and the molten material is ejected much after 10 ps. The ripple in the outer region remains oblique to the sum of the vector direction of the two pulses when the time delay is zero. With time delay ranging from 0.5 to 10 ps and different polarization directions of the laser radiation, the ripple generally aligned perpendicular to the polarization direction of the electric field with multiple pulses in the vicinity of ablation threshold is effectively eliminated without fragments at the edge. Furthermore, remnant ripples on irradiated area at higher energies with the same polarization direction are removed by irradiation at a lower energy with each different polarization direction of double pulse. Based on morphological observations for different time delays, possible mechanisms of ripple formations and eliminations are suggested. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ultrashort laser; Thin film ablation; Ripples; Double pulses
1. Introduction Material processing using ultrashort pulse lasers having under a few picosecond time duration has an excellent advantage in terms of processing quality over relatively longpulse lasers. The shorter pulse width reduces the thermal damage to the periphery of the irradiated area because the time required to transfer energy from the electron system to the lattice is longer than the laser pulse. Therefore, many studies have examined the beam property of ultrashort lasers and processing conditions in nano- and micromachining [1–5]. When a sample is irradiated at high pulse, thermal damage occurs below the boiling temperature and much molten particles are spread on the sample. To avoid this phenomenon, the material should be processed using multiple pulses at energy as low as possible. However, near the ablation threshold energy with multiple pulses generate a periodic surface pattern called the laser-induced periodic surface structure (LIPPS) or ripples [6–8]. The formation of Corresponding author. Tel.: +82 42 868 7141; fax: +82 42 868 7149.
E-mail address: [email protected] (J. Kim). 0143-8166/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2007.12.003
ripples perpendicular to the polarization direction of the laser radiation has been attributed to the interference between the incident wave and the surface scattered wave, which results in inhomogeneous energy deposition [6]. The spacing of the ripples is dependent on the wavelength and the incident angle expressed as L ¼ l=ð1 sin yi Þ, although subwavelength spacing has been observed [9,10]. The ripple formation can be applied to nano-patterning or optical devices such as gratings. Otherwise, the ripple should be eliminated to obtain high-quality micromachining. In this study, the surface changes using double pulses of sufficiently high energy to ablate a Cr thin film, and the ripple formation with a time-delayed double-pulse train created by splitting a single pulse and controlling the optical beam path length have been investigated as well as its elimination using pulses with different laser polarization.
2. Experimental setup and methods An amplified Ti:sapphire laser is used operating at a pulse duration of 250 fs full-width at half-maximum
ARTICLE IN PRESS J. Kim et al. / Optics and Lasers in Engineering 46 (2008) 306–310
307
Fig. 1. Scheme of the experimental system and beam intensity profile with rectangular mask.
(FWHM) with linear polarization, at a maximum power of 500 mW at a repetition rate of 100 kHz, and at a wavelength of 800 nm. For a double pulse, a Michelson interferometer is modified and the pulse energy and polarization are varied using a half-wave plate and polarizer. One of the two pulses is delayed by controlling the optical path length using an accurate motion stage with a resolution of 0.1 mm (Fig. 1). A two-photon excited photodiode is used to determine a time delay of zero for each pulse. For flatting the power distribution of Gaussian profile at the irradiated area, we placed a 2 2-mm rectangular mask in the beam path to study ripple formation. A magnification microscope objective IR lens ( 100, NA 0.8; 20, NA 0.4; Olympus) is used with an XYZ stage for accurate positioning. The beam is incident at an angle of 901 to the sample. To adjust the number of pulses, a mechanical shutter is used and the pulse number is controlled by shutter open time duration. The experiments are performed in air at ambient pressure and temperature. The Cr thin films are fabricated by sputtering with an average thickness of 200 nm on a glass substrate. The workpiece is cleaned with methanol before placing it on the stage. Scanning electron microscopy is used to investigate the surface morphology. 3. Results and discussion The results of ablation of a 200-nm Cr thin film by a single shot using the 100 objective lens and no mask are shown in Fig. 2(a) and (b). Pure melting at the edge region and evaporation at the center region in accordance with the Gaussian’s power distribution at irradiation area are observed in the irradiated area. Nanometer-sized particles are formed in the outer irradiated area where the irradiation energy is below the evaporation temperature. With a time separation between Fig. 2(a) and (b) of up to 10 ps, the particle formation area is similar to that for a time delay of zero (Fig. 2(c)). We interpret this observation as the energy from a foregoing pulse has not enough time to be transferred into the lattice heat. As the delay time
Fig. 2. Surface pattern of a Cr thin film irradiated by a single pulse at pulse energy of 98 nJ by direct focusing without mask. The time delay between (a) and (b) is (c) 0 ps, (d) 10 ps, (e) 30 ps, and (f) 50 ps.
between two pulses increase further, the following pulse eject molten material formed by previous pulse (Fig. 2(f)), resulting in a decrease in the particles formation area. From this experimental result, the volume of lattice melting may be plentiful to flow after about 10 ps from the end of a foregoing pulse, which is in reasonable agreement with the simulation result from a two-temperature model and experimental results for the ablation behavior of other materials [11–13]. Then, the molten volume is pushed away
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Fig. 4. Ablated area as a function of time delay for a Cr thin film by 1000 pulses at a different single pulse energies.
Fig. 3. Surface pattern of a Cr thin film irradiated by 10,000 pulses at a single pulse energy of (a) 30 nJ and vertical polarization or (b) 29 nJ and horizontal polarization with rectangular mask. The time delay between (a) and (b) is (c) 0 ps, (d) 1 ps, (e) 20 ps, and (f) 50 ps.
from the center region by the recoil pressure formed by the following pulse and covered up the particles generated by a foregoing pulse. To study the effect of the time delay and polarization on the surface ripple formation process, the delay time between divided pulses after passing through a rectangular mask to normalize the beam intensity distribution is adjusted and the number of pulses is set to 10,000 at lower energy to form the surface ripple. With a single pulse train blocking another split pulse using the 20 objective lens, a periodic ripple is formed perpendicular to the polarization of the laser beam, as usual (Fig. 3(a) and (b)). Fig. 3(c) shows an oblique ripple pattern in the outer region resulting from the sum of each laser polarization direction for a time delay of zero between pulses Fig. 3(a) and (b). For time delays of 0.5 to about 10 ps, a clearer edge is observed. As the time delay is increased, fragments and ripples at the edge is observed (Fig. 3(e) and (f)), and the ablated area decrease. The changes of surface ripple and ablated area with 1000 pulses is shown in Fig. 4. Ripples in metal generally arise from lattice reordering caused by lattice movement according to the thermal energy distribution and resolidification. The surface
melting process from a steady state to the lattice melting state with ultrashort pulses has been modeled using electron and lattice subsystems [14]. This model represents the free electrons absorb the laser energy and then transfer it to the lattice via electron–lattice coupling. This coupling constant and hot electron diffusion are the dominant factors causing surface melting and ripple formation in metals [15]. It is possible to explain electron and lattice thermal behavior and ripple formation from experimental results. When the initial pulse irradiates the metal sample, the free electrons are excited and form a hot electron gas. Then, the hot electron energy diffuses to the lattice according to the electron–lattice interaction. Finally, the entire system reaches equilibrium. The equilibrium time is expected to be about 10 ps, based on Fig. 2 that the melt ejection by second pulse occurs after 10 ps from previous pulse in agreement with experimental results for fused silica [12]. When the temperature of the equilibrium state is larger than the melting point, the materials form a liquid phase. The molten phase solidifies to form nanometer-sized particles via surface tension during cooling down if no subsequent pulses are incident. The resolidification of droplets also has been reported for other properties of the laser radiation and for different materials properties as well [16,17]. For suitable subsequent pulses, the irradiation and the induced recoil pressure eject the molten material to the surrounding area enlarging the heat-affected zone and spreading the debris. For multiple low-energy pulses, the hot electrons induced by laser radiation may form the periodic oscillations of electron density and a non-uniform energy distribution on the metal surface due to surface electromagnetic waves (SEWs) or plasma waves, such as plasmons [9]. In a few tens of picoseconds, the energy of the hot oscillating electrons diffuses to the surrounding cold lattice. Therefore, the ripple may form as a consequence of the
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Fig. 5. Surface pattern of a Cr thin film irradiated by 10,000 pulses at a single pulse energy of 90 nJ and (a) vertical and (b) horizontal polarization.
interaction between the oscillations of hot electron density or the lattice under the conditions of certain time separation between split pulses. When the material is irradiated by two superposed beams polarized perpendicular to each other, the ripples are aligned in the direction of the superposed field of the laser polarized field (Figs. 3(c) and 4). For delay times of 0.5–10 ps, the subsequent SEWs of the second pulse may interfere with the initial ones induced by the first pulse, resulting in the dispersion and the disturbance of the hot electron oscillation increasing the electron temperature. The energy of the hot electron induced by the first pulse does not diffuse to the lattice within those delay times. When the delay time is much larger than about 20 ps, the second pulse does not affect the initial SEWs because most of their energy has already transferred to the lattice resulting in the melting of the lattice. The interaction of the SEWs following the electron waves generated by the first pulse with the electron waves by the second pulse may also cause the decrease in the area of ablation with increasing pulse separation (Figs. 3(f) and 4) compared to the interaction between the SEWs at subsequent pulses before the diffusion of the electron energy. Therefore, the maximum ablation area may result from the superposition of electron temperature. This is in reasonable agreement with the crater depth obtained double pulse experiment [18]. If the polarization direction of the second pulse is the same direction as the first, a steady ripple may be generated by the SEWs (Figs. 3(a), (b) and 4). The initial electron oscillation as the basis of the SEWs, and the electron diffusion time to the lattice as basis for lattice melting, govern the ripple formation and the ablated area in double pulse laser machining. Compared to the surface irradiated at a high fluence with a single pulse train and the same polarization direction (Fig. 5(a) and (b)), the remnant ripple on the glass substrate and at the edge of the area irradiated at an energy of 90 nJ was eliminated totally with a lower-energy double-pulse train with a total energy of 59 nJ (Fig. 3(d)). Consequently, using a double pulse with a different polarization within about 10 ps, which affects the hot electron oscillating direction, is an optimal condition for micromachining.
4. Conclusions A time-delayed dual femtosecond laser pulse was used to examine surface modification and ripple formation in Cr thin films. A subsequent pulse following an initial high fluence pulse with a delay of at least 10 ps resulted in the ejection of molten material. The ripples in the outer region and on the glass substrate at same polarization direction of the laser radiation or without time delay was eliminated using double pulses with different polarization with time delays ranging from 0.5 to 10 ps and multiple pulses. The energy of the hot electrons caused by the ultrashort pulse is transferred to the lattice in accordance with the electron–lattice coupling strength and results in equilibrium state after about 10 ps. The hot electron oscillation is the initial source of SEWs, and these can be disturbed by a subsequent pulse with different polarization direction when it arrives at the sample before transferring the energy of the hot electrons to the cold lattice. This disturbance of the total energy and electron–lattice energy coupling determines the surface morphology. Furthermore, the total hot electron energy defines the ablated area unless the energy of hot electrons induced by a foregoing pulse is transferred to the cold lattice. This double-pulse method having perpendicular polarization direction should help to develop a subtractive micromachining process that does not produce a ripple pattern with an ablation energy saving way. References [1] Madou MJ. Fundamentals of microfabrication. 2nd ed. Boca Raton, FL: CRC Press; 2001. [2] Furukawa H, Hashida M. Simulation on femto-second laser ablation. Appl Surf Sci 2002:197–8. 114–7. [3] Chowdhury IH, Xu X. Heat transfer in femtosecond laser processing of metal. Num Heat Transf A 2003;44:219–32. [4] Chien CY, Gupta MC. Pulse width effect in ultrafast laser processing of materials. Appl Phys A 2005;81(6):1257–63. [5] Perrie W, Gill M, Robinson G, Fox P, O’Neill W. Femtosecond laser micro-structuring of aluminium under helium. Appl Surf Sci 2004; 230:50–9. [6] Young JF, Preston JS, van Driel HM, Sipe JE. Laser-induced periodic surface structure. II: experiments on Ge, Si, Al, and brass. Phys Rev B 1983;27:1155–72.
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[7] Ba¨uerle D. Laser processing and chemistry. 3rd ed. Berlin: Springer; 2000. p. 574–86. [8] Mannion PT, Magee J, Coyne E, O’Connor GM, Glynn TJ. The effect of damage accumulation behavior on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air. Appl Surf Sci 2004;233:275–87. [9] Wagner R, Gottmann J, Horn A, Kreutz EW. Subwavelength ripple formation induced by tightly focused femtosecond laser radiation. Appl Surf Sci 2006;252:8576–9. [10] Borowiec A, Haugen HK. Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses. Appl Phys Lett 2003;82(25):4462–4. [11] Cheng C, Xu X. Mechanisms of decomposition of metal during femtosecond laser ablation. Phys Rev B 2005;72:165415. [12] Chowdhury IH, Xu X, Weiner AM. Ultrafast double-pulse ablation of fused silica. Appl Phys Lett 2005;86:151110.
[13] Spyridaki M, Koudoumas E, Tzanetakis P, Fotakis C, Stoian R, Rosenfeld A, et al. Temporal pulse manipulation and ion generation in ultrafast laser ablation of silicon. Appl Phys Lett 2003;83:1474–6. [14] Qiu TQ, Tien CL. Short-pulse laser heating on metals. Int J Heat Mass Transf 1992;35:719–26. [15] Wang J, Guo C. Ultrafast dynamics of femtosecond laser-induced periodic surface pattern formation on metals. Appl Phys Lett 2005; 87:251914. [16] Simon P, Ihlemann J. Ablation of submicron structures on metals and semiconductors by femtosecond UV-laser pulses. Appl Surf Sci 1997;109–110:25–9. [17] Lee SK, Yoon KK, Whang KH, Na SJ. Excimer laser ablation removal of thin chromium films from glass substrates. Surf Coat Technol 1999;113:63–74. [18] Semerok A, Dutouquet C. Ultrashort double pulse laser ablation of metals. Thin Solid Films 2004;453/454:501–5.
Optics and Lasers in Engineering 46 (2008) 306–310 www.elsevier.com/locate/optlaseng
Surface ripple changes during Cr film ablation with a double ultrashort laser pulse Jaegu Kima,, Suckjoo Nab, Sunghak Choa, Wonseok Changa, Kyunghyun Whanga a
Korea Institute of Machinery & Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305 343, Republic of Korea Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305 701, Republic of Korea
b
Received 28 March 2007; received in revised form 24 November 2007; accepted 2 December 2007
Abstract Surface modification is investigated experimentally by varying the time separation of double femtosecond laser radiation and surface ripples by varying the time separation and polarization direction of double pulses train. Nanometer-sized particles are formed during resolidification of the molten region when the second pulse arrives within 10 ps and the molten material is ejected much after 10 ps. The ripple in the outer region remains oblique to the sum of the vector direction of the two pulses when the time delay is zero. With time delay ranging from 0.5 to 10 ps and different polarization directions of the laser radiation, the ripple generally aligned perpendicular to the polarization direction of the electric field with multiple pulses in the vicinity of ablation threshold is effectively eliminated without fragments at the edge. Furthermore, remnant ripples on irradiated area at higher energies with the same polarization direction are removed by irradiation at a lower energy with each different polarization direction of double pulse. Based on morphological observations for different time delays, possible mechanisms of ripple formations and eliminations are suggested. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ultrashort laser; Thin film ablation; Ripples; Double pulses
1. Introduction Material processing using ultrashort pulse lasers having under a few picosecond time duration has an excellent advantage in terms of processing quality over relatively longpulse lasers. The shorter pulse width reduces the thermal damage to the periphery of the irradiated area because the time required to transfer energy from the electron system to the lattice is longer than the laser pulse. Therefore, many studies have examined the beam property of ultrashort lasers and processing conditions in nano- and micromachining [1–5]. When a sample is irradiated at high pulse, thermal damage occurs below the boiling temperature and much molten particles are spread on the sample. To avoid this phenomenon, the material should be processed using multiple pulses at energy as low as possible. However, near the ablation threshold energy with multiple pulses generate a periodic surface pattern called the laser-induced periodic surface structure (LIPPS) or ripples [6–8]. The formation of Corresponding author. Tel.: +82 42 868 7141; fax: +82 42 868 7149.
E-mail address: [email protected] (J. Kim). 0143-8166/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2007.12.003
ripples perpendicular to the polarization direction of the laser radiation has been attributed to the interference between the incident wave and the surface scattered wave, which results in inhomogeneous energy deposition [6]. The spacing of the ripples is dependent on the wavelength and the incident angle expressed as L ¼ l=ð1 sin yi Þ, although subwavelength spacing has been observed [9,10]. The ripple formation can be applied to nano-patterning or optical devices such as gratings. Otherwise, the ripple should be eliminated to obtain high-quality micromachining. In this study, the surface changes using double pulses of sufficiently high energy to ablate a Cr thin film, and the ripple formation with a time-delayed double-pulse train created by splitting a single pulse and controlling the optical beam path length have been investigated as well as its elimination using pulses with different laser polarization.
2. Experimental setup and methods An amplified Ti:sapphire laser is used operating at a pulse duration of 250 fs full-width at half-maximum
ARTICLE IN PRESS J. Kim et al. / Optics and Lasers in Engineering 46 (2008) 306–310
307
Fig. 1. Scheme of the experimental system and beam intensity profile with rectangular mask.
(FWHM) with linear polarization, at a maximum power of 500 mW at a repetition rate of 100 kHz, and at a wavelength of 800 nm. For a double pulse, a Michelson interferometer is modified and the pulse energy and polarization are varied using a half-wave plate and polarizer. One of the two pulses is delayed by controlling the optical path length using an accurate motion stage with a resolution of 0.1 mm (Fig. 1). A two-photon excited photodiode is used to determine a time delay of zero for each pulse. For flatting the power distribution of Gaussian profile at the irradiated area, we placed a 2 2-mm rectangular mask in the beam path to study ripple formation. A magnification microscope objective IR lens ( 100, NA 0.8; 20, NA 0.4; Olympus) is used with an XYZ stage for accurate positioning. The beam is incident at an angle of 901 to the sample. To adjust the number of pulses, a mechanical shutter is used and the pulse number is controlled by shutter open time duration. The experiments are performed in air at ambient pressure and temperature. The Cr thin films are fabricated by sputtering with an average thickness of 200 nm on a glass substrate. The workpiece is cleaned with methanol before placing it on the stage. Scanning electron microscopy is used to investigate the surface morphology. 3. Results and discussion The results of ablation of a 200-nm Cr thin film by a single shot using the 100 objective lens and no mask are shown in Fig. 2(a) and (b). Pure melting at the edge region and evaporation at the center region in accordance with the Gaussian’s power distribution at irradiation area are observed in the irradiated area. Nanometer-sized particles are formed in the outer irradiated area where the irradiation energy is below the evaporation temperature. With a time separation between Fig. 2(a) and (b) of up to 10 ps, the particle formation area is similar to that for a time delay of zero (Fig. 2(c)). We interpret this observation as the energy from a foregoing pulse has not enough time to be transferred into the lattice heat. As the delay time
Fig. 2. Surface pattern of a Cr thin film irradiated by a single pulse at pulse energy of 98 nJ by direct focusing without mask. The time delay between (a) and (b) is (c) 0 ps, (d) 10 ps, (e) 30 ps, and (f) 50 ps.
between two pulses increase further, the following pulse eject molten material formed by previous pulse (Fig. 2(f)), resulting in a decrease in the particles formation area. From this experimental result, the volume of lattice melting may be plentiful to flow after about 10 ps from the end of a foregoing pulse, which is in reasonable agreement with the simulation result from a two-temperature model and experimental results for the ablation behavior of other materials [11–13]. Then, the molten volume is pushed away
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Fig. 4. Ablated area as a function of time delay for a Cr thin film by 1000 pulses at a different single pulse energies.
Fig. 3. Surface pattern of a Cr thin film irradiated by 10,000 pulses at a single pulse energy of (a) 30 nJ and vertical polarization or (b) 29 nJ and horizontal polarization with rectangular mask. The time delay between (a) and (b) is (c) 0 ps, (d) 1 ps, (e) 20 ps, and (f) 50 ps.
from the center region by the recoil pressure formed by the following pulse and covered up the particles generated by a foregoing pulse. To study the effect of the time delay and polarization on the surface ripple formation process, the delay time between divided pulses after passing through a rectangular mask to normalize the beam intensity distribution is adjusted and the number of pulses is set to 10,000 at lower energy to form the surface ripple. With a single pulse train blocking another split pulse using the 20 objective lens, a periodic ripple is formed perpendicular to the polarization of the laser beam, as usual (Fig. 3(a) and (b)). Fig. 3(c) shows an oblique ripple pattern in the outer region resulting from the sum of each laser polarization direction for a time delay of zero between pulses Fig. 3(a) and (b). For time delays of 0.5 to about 10 ps, a clearer edge is observed. As the time delay is increased, fragments and ripples at the edge is observed (Fig. 3(e) and (f)), and the ablated area decrease. The changes of surface ripple and ablated area with 1000 pulses is shown in Fig. 4. Ripples in metal generally arise from lattice reordering caused by lattice movement according to the thermal energy distribution and resolidification. The surface
melting process from a steady state to the lattice melting state with ultrashort pulses has been modeled using electron and lattice subsystems [14]. This model represents the free electrons absorb the laser energy and then transfer it to the lattice via electron–lattice coupling. This coupling constant and hot electron diffusion are the dominant factors causing surface melting and ripple formation in metals [15]. It is possible to explain electron and lattice thermal behavior and ripple formation from experimental results. When the initial pulse irradiates the metal sample, the free electrons are excited and form a hot electron gas. Then, the hot electron energy diffuses to the lattice according to the electron–lattice interaction. Finally, the entire system reaches equilibrium. The equilibrium time is expected to be about 10 ps, based on Fig. 2 that the melt ejection by second pulse occurs after 10 ps from previous pulse in agreement with experimental results for fused silica [12]. When the temperature of the equilibrium state is larger than the melting point, the materials form a liquid phase. The molten phase solidifies to form nanometer-sized particles via surface tension during cooling down if no subsequent pulses are incident. The resolidification of droplets also has been reported for other properties of the laser radiation and for different materials properties as well [16,17]. For suitable subsequent pulses, the irradiation and the induced recoil pressure eject the molten material to the surrounding area enlarging the heat-affected zone and spreading the debris. For multiple low-energy pulses, the hot electrons induced by laser radiation may form the periodic oscillations of electron density and a non-uniform energy distribution on the metal surface due to surface electromagnetic waves (SEWs) or plasma waves, such as plasmons [9]. In a few tens of picoseconds, the energy of the hot oscillating electrons diffuses to the surrounding cold lattice. Therefore, the ripple may form as a consequence of the
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309
Fig. 5. Surface pattern of a Cr thin film irradiated by 10,000 pulses at a single pulse energy of 90 nJ and (a) vertical and (b) horizontal polarization.
interaction between the oscillations of hot electron density or the lattice under the conditions of certain time separation between split pulses. When the material is irradiated by two superposed beams polarized perpendicular to each other, the ripples are aligned in the direction of the superposed field of the laser polarized field (Figs. 3(c) and 4). For delay times of 0.5–10 ps, the subsequent SEWs of the second pulse may interfere with the initial ones induced by the first pulse, resulting in the dispersion and the disturbance of the hot electron oscillation increasing the electron temperature. The energy of the hot electron induced by the first pulse does not diffuse to the lattice within those delay times. When the delay time is much larger than about 20 ps, the second pulse does not affect the initial SEWs because most of their energy has already transferred to the lattice resulting in the melting of the lattice. The interaction of the SEWs following the electron waves generated by the first pulse with the electron waves by the second pulse may also cause the decrease in the area of ablation with increasing pulse separation (Figs. 3(f) and 4) compared to the interaction between the SEWs at subsequent pulses before the diffusion of the electron energy. Therefore, the maximum ablation area may result from the superposition of electron temperature. This is in reasonable agreement with the crater depth obtained double pulse experiment [18]. If the polarization direction of the second pulse is the same direction as the first, a steady ripple may be generated by the SEWs (Figs. 3(a), (b) and 4). The initial electron oscillation as the basis of the SEWs, and the electron diffusion time to the lattice as basis for lattice melting, govern the ripple formation and the ablated area in double pulse laser machining. Compared to the surface irradiated at a high fluence with a single pulse train and the same polarization direction (Fig. 5(a) and (b)), the remnant ripple on the glass substrate and at the edge of the area irradiated at an energy of 90 nJ was eliminated totally with a lower-energy double-pulse train with a total energy of 59 nJ (Fig. 3(d)). Consequently, using a double pulse with a different polarization within about 10 ps, which affects the hot electron oscillating direction, is an optimal condition for micromachining.
4. Conclusions A time-delayed dual femtosecond laser pulse was used to examine surface modification and ripple formation in Cr thin films. A subsequent pulse following an initial high fluence pulse with a delay of at least 10 ps resulted in the ejection of molten material. The ripples in the outer region and on the glass substrate at same polarization direction of the laser radiation or without time delay was eliminated using double pulses with different polarization with time delays ranging from 0.5 to 10 ps and multiple pulses. The energy of the hot electrons caused by the ultrashort pulse is transferred to the lattice in accordance with the electron–lattice coupling strength and results in equilibrium state after about 10 ps. The hot electron oscillation is the initial source of SEWs, and these can be disturbed by a subsequent pulse with different polarization direction when it arrives at the sample before transferring the energy of the hot electrons to the cold lattice. This disturbance of the total energy and electron–lattice energy coupling determines the surface morphology. Furthermore, the total hot electron energy defines the ablated area unless the energy of hot electrons induced by a foregoing pulse is transferred to the cold lattice. This double-pulse method having perpendicular polarization direction should help to develop a subtractive micromachining process that does not produce a ripple pattern with an ablation energy saving way. References [1] Madou MJ. Fundamentals of microfabrication. 2nd ed. Boca Raton, FL: CRC Press; 2001. [2] Furukawa H, Hashida M. Simulation on femto-second laser ablation. Appl Surf Sci 2002:197–8. 114–7. [3] Chowdhury IH, Xu X. Heat transfer in femtosecond laser processing of metal. Num Heat Transf A 2003;44:219–32. [4] Chien CY, Gupta MC. Pulse width effect in ultrafast laser processing of materials. Appl Phys A 2005;81(6):1257–63. [5] Perrie W, Gill M, Robinson G, Fox P, O’Neill W. Femtosecond laser micro-structuring of aluminium under helium. Appl Surf Sci 2004; 230:50–9. [6] Young JF, Preston JS, van Driel HM, Sipe JE. Laser-induced periodic surface structure. II: experiments on Ge, Si, Al, and brass. Phys Rev B 1983;27:1155–72.
ARTICLE IN PRESS 310
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[7] Ba¨uerle D. Laser processing and chemistry. 3rd ed. Berlin: Springer; 2000. p. 574–86. [8] Mannion PT, Magee J, Coyne E, O’Connor GM, Glynn TJ. The effect of damage accumulation behavior on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air. Appl Surf Sci 2004;233:275–87. [9] Wagner R, Gottmann J, Horn A, Kreutz EW. Subwavelength ripple formation induced by tightly focused femtosecond laser radiation. Appl Surf Sci 2006;252:8576–9. [10] Borowiec A, Haugen HK. Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses. Appl Phys Lett 2003;82(25):4462–4. [11] Cheng C, Xu X. Mechanisms of decomposition of metal during femtosecond laser ablation. Phys Rev B 2005;72:165415. [12] Chowdhury IH, Xu X, Weiner AM. Ultrafast double-pulse ablation of fused silica. Appl Phys Lett 2005;86:151110.
[13] Spyridaki M, Koudoumas E, Tzanetakis P, Fotakis C, Stoian R, Rosenfeld A, et al. Temporal pulse manipulation and ion generation in ultrafast laser ablation of silicon. Appl Phys Lett 2003;83:1474–6. [14] Qiu TQ, Tien CL. Short-pulse laser heating on metals. Int J Heat Mass Transf 1992;35:719–26. [15] Wang J, Guo C. Ultrafast dynamics of femtosecond laser-induced periodic surface pattern formation on metals. Appl Phys Lett 2005; 87:251914. [16] Simon P, Ihlemann J. Ablation of submicron structures on metals and semiconductors by femtosecond UV-laser pulses. Appl Surf Sci 1997;109–110:25–9. [17] Lee SK, Yoon KK, Whang KH, Na SJ. Excimer laser ablation removal of thin chromium films from glass substrates. Surf Coat Technol 1999;113:63–74. [18] Semerok A, Dutouquet C. Ultrashort double pulse laser ablation of metals. Thin Solid Films 2004;453/454:501–5.