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Fabrication of a random convex-lens-shaped microstructure using a laser ablation
Current Applied Physics 12 (2012) 1307e1312
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Fabrication of a random convex-lens-shaped microstructure using a laser ablation Jiwhan Noh a, b, Jae-Hoon Lee a, Suckjoo Na b, * a b
Korea Institute of Machinery and Materials (KIMM), 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Korea Advanced Institute of Science and Technology (KAIST), 371-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2011 Received in revised form 20 February 2012 Accepted 11 March 2012 Available online 28 March 2012
A convex-lens-shaped microstructure with a diameter of 50 mm on a metallic mold substrate was fabricated in this paper. A laser ablation process, in which the laser beam was focused and irradiated on the metallic mold substrate in order to remove a part of the substrate, was used for that. The convexlens-shaped microstructure has not been reported in any studies of microstructure using the laser ablation process. It was proposed that the unbalanced ablation and re-adherence of the melted particles was the processing mechanism of the convex lens shape. The convex-lens-shaped microstructure fabricated in this study is smaller than the focused spot. It was expected that the same convex-lensshaped microstructure can be fabricated even if the focused spot size is increased, so long as the fluence of the laser can be maintained. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of a laser ablation process. The fabricated convex-lensshaped microstructure on the metallic substrate can be used as the mold for the micro lens. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Convex-lens-shaped microstructure Laser ablation processing Metallic mold substrate Micro lens
1. Introduction As the focused laser pulse energy is irradiated on a substrate, a part of the substrate surface is melted or ablated. Particularly, if the energy is larger than the ablation threshold value of the substrate and is then irradiated, a part of the substrate will be removed [1,2]. For example, if a laser with a pulse duration in units of nanoseconds were irradiated on metal, parts of the metal surface would be ablated and the surface would become rougher [3,4]. When a laser with a pulse duration in units of nanoseconds creates a large heat-affected-zone that makes it difficult to create micro sized structures. With the recent development of the ultrashort pulse laser, which has a pulse duration in the femto or picosecond domain, the micro processing technology that uses the laser has been rapidly developing. That is because the short pulse duration of the ultrashort laser has enabled fabrication of the microstructures with a greatly reduced heat-affected-zone [5e10]. As an ultrashort laser is focused and irradiated on a substrate surface, the focused beam is irradiated on the substrate surface. In that case, a micro sized hole in the size of the focused spot is created. As the sample or focused beam is moved using a stage or galvano-type scanner, a micro groove will be created. In other words, fabrication of a micro hole or a groove smaller than the focused spot size is impossible. To fabricate a smaller
* Corresponding author. E-mail addresses: [email protected] (J. Noh), [email protected] (S. Na). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.03.017
microstructure, the smaller focused beam is required. In order to make the smaller focused beam size, the diameter of the input laser beam into focused lens may be increased, a short wavelength laser may be used, or a lens with a short focal length can be used. However, that will create a processing problem as the smaller focused spot size will result in a smaller depth of focus. Therefore, the fabrication of a microstructure beyond the diffraction limit is impossible. Fabricating a microstructure using a focused laser has another problem in that the processing time increases when fabricating a lot of microstructures, because the structures must be fabricated with a focused beam one by one. To overcome the problem, there are ongoing studies of fabricating structures smaller than the focused spot size. When a substrate is irradiated by the energy from using a laser on a level close to the ablation threshold, a type of ripple is generated on the substrate [11]. It is generally a nano size pattern that is proportional to the wavelength of the laser used. The direction of the nano pattern can also be adjusted by using the polarization direction of the laser [12]. The principle of this nano pattern generation was known as the interference between the irradiated laser beam and scattered laser beam [13e15]. However, many experiments have created nano patterns that cannot be explained by that interference theory. Although there are ongoing studies that use a coulomb explosion to explain it, no principle to explain all experimental results has been discovered yet. Since this nano pattern uses the Gaussian laser beam or flat top shaped beam using the shaping element of the Gaussian beam, it is very difficult to fabricate nano patterns with a high aspect ratio. Tsing-Hua fabricated a nano spike
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with a high aspect ratio by irradiating the femtosecond laser on a wafer in the SF6 or Cl2 environment [16]. Although they were able to fabricate a nano spike with high aspect ratio that could not be implemented in a nano ripple, it could only be applied on the silicon wafer and they had to use SF6 or Cl2, which are toxic gases, and thus it had a limited industrial application. In this paper, the picosecond laser is irradiated on a mold material called, NAK80, in the air and we studied the micro/nano pattern that was created on the irradiated area. In the course of studying the micro nano pattern on a mold material, we discovered a convex-lens-shaped microstructure that was smaller than the laser spot. To the best of our knowledge, there has been no report of a convex-lens-shaped microstructure. Since the process proposed in this paper was executed in the air, it can fabricate large structures. It was expected that the same convex-lens-shaped microstructure can be fabricated, even if the focused spot size is increased, so long as the same energy per unit area is irradiated on a substrate. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of the laser ablation processes. Because the picosecond laser is superior to the femtosecond laser in terms of price and stability, this study used the picosecond laser instead of the femtosecond laser, which was widely used in the previous nano ripple studies. Since the convex-lens-shaped microstructure was fabricated on the surface of a mold made of NAK80, the process will enable the injection molding of plastic products with micro patterns at a low cost. This kind of manufactured plastic surface can be used as a micro lens array. 2. Experimental details Fig. 1 shows the conceptual diagram of the experiment. As shown in the figure, a substrate surface is scanned with a focused
laser beam that used a Galvanometer mirror and f-theta lens. If the energy per unit area of the focused laser beam is larger than the ablation threshold of the substrate, ablation, which means “removal of the substrate” will occur. This study focuses on the ablated surface of the substrate, particularly on the microstructure that is smaller than the focused spot. In a typical laser ablation process, a groove in the shape of a reverse Gaussian that has the width of the focused spot size is created when the pulse overlap rate is 90% or more. That is because the TEM00 laser beam has Gaussian distribution and thus the energy distribution on the focused area also has Gaussian distribution. In other words, the energy will be the highest at the center of the laser and it will become smaller as it moves away from the center. As a result, there will be more ablation in the area of high energy and less ablation in the area of low energy. Thus, a reverse Gaussian-shaped groove will be created on the substrate. This study focused on the microstructure that is smaller than the focused spot and not on the reverse Gaussian-shaped groove. Various micro/nano structures, as shown in Fig. 1(a), (b), and (c), were fabricated by adjusting the pulse overlap rate, line overlap rate, laser fluence, and number of scans. As shown in the figure, a unidirectional scan, not a cross pattern scan, was used. The picosecond laser used in the experiments was a diodepumped, mode-locked Nd:YVO4 laser with a pulse width of 12 ps. The maximum repetition rate was 640 kHz and the fundamental wavelength was 1064 nm. The laser was equipped with second harmonic generators to make laser wavelengths of 532 nm. In the experiments, a laser wavelength of 532 nm was used. A half-wave plate and a polarizer were used to control the laser power. Instead of a mechanical shutter, an external TTL signal served as a shutter for the laser pulses. This prevented extra irradiation from the laser pulses onto the specimen that were caused by the time delay of the mechanical shutter at every end point of the laser beam’s path. The
Fig. 1. Conceptual diagram of the experiment.
J. Noh et al. / Current Applied Physics 12 (2012) 1307e1312
horizontally polarized beam was focused by an f-theta scan lens with a maximum scan area of 58 58 mm2 (Sill Optics S4LFT0101/ 121). The effective focal length of the f-theta lens was 89.7 mm. The working distance was 97 mm. Max beam entrance beam diameter is 6 mm. The optical scan angle is 25.4 . The beam was manipulated over the sample by a two-mirror galvoscanner (RAY LASE SS-15 D2). The fluence was determined by measuring the power at the exit of the f-theta lens and it was then divided by the repetition rate and the irradiated surface area. These values were averages, as the energy distribution of the pulses was Gaussian using the TEM00 mode. The processing material was NAK80 mold steel with a uniform hardness of approximately 40HRc throughout. It never required stress relieving, even after heavy machining. It also had a uniform grain structure without pinholes, inclusion, or hard spots. NAK80 is commonly used for the mold material for plastic lens. For mass production for plastic lens, lens curvature is fabricated on the NAK80 surface. With that NAK80 mold material, plastic lens can be fabricated by injection mold process whit low coat. NAK80 is composed of 93.05% of Fe, 1% of Al, 0.15% of C, 1% of Cu, 1.5% of Mn, 3% of Ni, 3% of Si. The laser was irradiated on the NAK80 material in the air and not in a vacuum or certain gas. 3. Results and discussion Fig. 2(d) shows the conceptual diagram of the defocus laser processing system. The beam reflected from the Galvanometer mirror passes through the f-theta lens and is focused on the sample.
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To test various conditions, this study experimented with the sample in and out of focus. The radius of the input beam that was irradiated to the f-theta lens was 1 mm, the laser wavelength was 532 nm, and the effective focal length of the f-theta lens was 89.7 mm. The working distance of f-theta lens is 97 mm. Since M2 of the picosecond laser was 1.3, the focused optical spot size (1/e2 of maximum intensity value) was about 40 mm. The diameter of the focused spot would be 40 mm if the distance between the sample and f-theta lens were 97 mm. However, it would be 125.1mm if the distance between the sample and f-theta lens were 102 mm, and 240.4 mm if the distance between the sample and f-theta lens were 107 mm. In other words, the laser was irradiated after placing the sample 5 mm and 10 mm off the focus. The average power of the laser was 3.5 W, the repetition rate was 50 kHz, the scan speed was 183 mm/s, and the scan repetition count was 5. A unidirectional scan and not a cross pattern, was used. The pulse duration was 12 ps. Although the same conditions were used in all cases, the fluence value, which is the energy delivered to the substrate, and the intensity value were varied according to the position of the sample. The fluence and intensity were 55.704 [J/cm2] and 464.4 [GW/cm2], respectively, in Fig. 2(a); 5.694 [J/cm2] and 47.4 [GW/ cm2], respectively in Fig. 2(b); and 1.547 [J/cm2] and 12.9 [GW/cm2], respectively, in Fig. 2(c). Since the size of the focused spot was 40mm in Fig. 2(a), 125 mm in Fig. 2(b), and 240 mm in Fig. 2(c), the pulse overlap rates were all different. The pulse overlap rate means the overlapped area between the pulses during laser scanning. The pulse overlap rate was 90.8% in Fig. 2(a), 97.0% in Fig. 2(b), and
Fig. 2. (a) SEM image of the working distance ¼ 121 mm, (b) SEM image of the working distance ¼ 130 mm, (c) SEM image of the working distance ¼ 135 mm, (Common processing Conditions: Laser average power : 3.5 W, repetition rate: 50 kHz, scan speed: 183 mm/s, scan repetition: 5 times, laser wavelength: 532 nm, laser pulse duration: 12 picoseconds), (d) Conceptual diagram of the defocus processing system.
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98.4% in Fig. 2(c). By varying the fluence and overlap rate, we discovered different micro/nano patterns on the treated surface. The process of fabricating the convex-lens-shaped microstructure shown in Fig. 2(b) is the most promising in terms of the processing speed. The 50mm sized convex-lens-shaped structure in Fig. 2(b) was fabricated with a focused spot diameter of 125 mm That indicates that the convex-lens-shaped microstructure can be created regardless of the focused spot size. The fact that the convexlens-shaped microstructure is randomly created without a uniform interval means that the spot size of the focused laser is not related to the convex-lens-shaped microstructure. Therefore, this kind of convex-lens-shaped microstructure can be fabricated, even if the focused spot size increases, so long as the laser fluence that has been irradiated on the substrate can be maintained. Since the depth of focus also increases when the focused spot size increases, it is more beneficial from the processing viewpoint. Since more and more ultrashort lasers with a high average power are being introduced, the processing speed of the convex-lens-shaped microstructure suggested in this paper is likely to keep increasing. Fig. 3 shows the SEM images of the result from increasing the scan repetition counts under the conditions of Fig. 2(b). Fig. 3(a), (c), (e), and (g) show the scan repetition count of 10, 15, 25, and 100, respectively. Fig. 3(b), (d), (f), and (h) are the magnified SEM images of Fig. 3(a), (c), (e), and (g), respectively. We particularly noted the convex-lens-shaped microstructure of Fig. 2(b) because there has been no report of that micro shape to the best of our knowledge. Therefore, we intended to study the processing mechanism by trying different scan repetition count at the same processing condition. As expected, the number of convex-lens-shaped microstructures shown in Fig. 3(a) is higher than that in Fig. 2(b). As the number of convex-lens-shaped microstructures increased, the nano ripple area decreased by that much. Fig. 3(b) also shows the nano ripple pattern at the top of the convex-lens-shaped microstructure. Most microstructures did not have the nano ripple on the convex-lens-shaped microstructures. They had a surface with small roughness. As shown in Fig. 3 (c), (d), (e), and (f), the number of convex-lens-shaped microstructures increased and the nano ripple area decreased by that much as the number of scans increased. As the number of scans increased, the number of convex-lens-shaped microstructures with a nano pattern increased and the number of convex-lens-shaped microstructures with small roughness decreased. Fig. 3(g) is a case of 100 scans and it shows that the existing convex-lens-shaped microstructures disappeared and that new shape microstructures are created. While Fig. 3(a) through Fig. 3(f) show the convex-lens-shaped microstructures with a diameter of around 50um, Fig. 3(g) and (h) show a microstructure of a different shape with a diameter of around 100um. And the nano ripple patterns shown in Fig. 3(a) and (c), and (e) are hardly shown in Fig. 3(g). However, some nano ripple patterns were observed between the microstructures. Fig. 3(h) is a magnified SEM image of the nano ripple pattern in Fig. 3(g). The micro pattern in the size of around 10 mm as shown in Fig. 2(a), has been widely reported in other studies of laser processing [17]. This 10 mm sized micro pattern is known to be created by the defect of the substrate and multi-reflection from the processed surface. In other words, the ablation threshold value of a section is different from other sections because of the defect in the material, and that causes the ablation to occur sooner or later in the defect section than others when the laser is irradiated. This kind of unbalanced ablation results in the small groove on the surface. When the laser is irradiated on this groove, there will be more reflected beams from the inclined surface of groove. Since the incidence angle of the laser incidence beam is larger on the inclined surface of groove, the reflectivity of both the P wave and S wave increases in accordance with the Fresnel Reflection Principle.
Therefore, the beam reflected from the inclined surface heads toward the deeper position of the groove and a deeper groove is created. There has been no report of the convex-lens-shaped microstructure, as shown in Fig. 2(b) and Fig. 3(a) and (c). However, there are many reports of nano pattern laser ripples that are created on the area that is not shaped like a convex lens [18]. Since this study focuses on the convex-lens-shaped microstructures, this paper intends to discuss the mechanism to create the convex-lens-shaped microstructures. This mechanism can be explained in two ways. The first way is to explain it by using the ablation difference due to the material defect. The material used in this study is NAK80, which is a mold material consisting of many elements. It consists of 93% Fe, 3% Ni, 3% Si, 1.5% Mn, 1% Al, 1% Cu, and 0.15% C. These elements have different ablation thresholds resulting from the laser irradiation. Therefore, grooves can be created on the surface due to the difference of the ablation threshold values. The Fresnel Reflection Principle will create the deeper microstructures when the laser beam is reflected to the groove. If there is a part of very high ablation threshold in the material, the convex-lens-shaped microstructure will be created as shown in Fig. 2(b). Although this ablation difference will accurately explain Fig. 2(a), it does not explain all of the mechanisms of creating the convex lens shape. The second way of creating the convex-lens-shaped microstructure can be explained by the melting particle which is ejected from the substrate. A laser is a type of heat source and it can evaporate and melt parts of the substrate. Since this study uses a laser with a pulse duration of 12 ps, very little of the substrate will be melted. It generally takes 10 ps or longer to transform the electron vibration to the lattice vibration in an iron. Therefore, when a laser with the pulse duration of 12 ps is irradiated on the iron, most of the laser energy is absorbed by the electrons and is not delivered to the lattice. Since the external energy is not delivered to the lattice, there should be no melting of the substrate. However, that is just theory, and some melting is actually observed in most cases even when the picosecond laser is used. Melting is also observed in some laser power domains even with the femtosecond laser. Melting is generated because of heat accumulation when many laser pulses are irradiated on the surface. However, the fact that a laser with the pulse duration of picoseconds or femtoseconds creates less melting than that of one with nanoseconds has been proven in many studies. Therefore, it can be assumed that there will be melting of some elements even with the picosecond laser used in this study. If these elements are melted first, the melted part will be shaped as spherical by the surface tension of melting particle. When the laser is irradiated again on the spherical part, the spherical part will spread wider and become the mask for the laser. Since this material is melted, it is possible that its ablation threshold value is changed. Particularly, if the material is oxidized after being melted, its ablation threshold value will be higher than that of an existing substrate. Because of that, the melted material will spread and take the role of a mask to the laser beam. If the melted particles can settle on the unprocessed area by the shock wave of the laser processing, this theory becomes even more probable. That is because these spherical shaped particles are positioned on the unprocessed area and the laser is irradiated on it, it will spread wider and is very likely to become the mask to the laser. The fact that the roughness of the convex-lens-shaped microstructure is small can mean that these spherical shaped particles are spread by the laser and adhered on the surface. In the typical interaction between the laser and material, it is impossible to fabricate the roughness as small as the convex-lens-shaped microstructure. If the laser continues being irradiated on the convex-lens-shaped microstructure, parts of the structures will be destroyed, as shown in Fig. 3(e) and (f).
J. Noh et al. / Current Applied Physics 12 (2012) 1307e1312
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Fig. 3. (a) SEM image of the scan repetition count ¼ 10, (c) SEM image of the scan repetition count ¼ 15, (e) SEM image of the scan repetition count ¼ 25, (g) SEM image of the scan repetition count ¼ 100; (b), (d), (f), and (h) are the magnified SEM images of (a), (c), (e), and (g), respectively. (Common processing conditions: Laser average power - 3.5 W, repetition rate - 50 kHz, scan speed - 183 mm/s, scan repetition count - 5, laser wavelength - 532 nm, and laser pulse duration - 12 ps).
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The convex-lens-shaped microstructure, as shown in Fig. 3(d), can be used as the mold of the plastic micro lens. The substrate used in this study is NAK80, which is actually used as the mold material. The convex-lens-shaped microstructure fabricated in this study can be used in injection molding to manufacture a micro plastic lens. Since the mold is convex-lens-shaped, the replicated plastic will be concave-lens-shaped. To manufacture the convex lens shape plastic, the convex-lens-shaped microstructure mold can be used to fabricate the mold in the reverse shape using the electroforming process. The micro convex lens shape can be manufactured on the plastic surface through injection molding using this reverse shape mold. Currently, the photolithography process and mechanical process are used to manufacture the micro lens. The photolithography process requires many steps to manufacture the curvature of a micro lens. That is because it is very difficult to manufacture the curvature of a convex lens in the photolithography process. In the mechanical process which uses the mechanical bite for milling process, it is difficult to manufacture a small micro lens. The process of fabricating a convex-lens-shaped microstructure, as described in this study, is simpler than the conventional methods and will allow for the manufacturing of the micro lens mold at a low cost. Since the surface roughness of the convex-lens-shaped microstructure is small, there will be less scattering on the surface of the micro lens manufactured with it and that will improve the optical performance of the micro lens. Furthermore, the laser process in this study is implemented in the air and not in a vacuum or special gas. That will enable manufacturing on a large area. Because of the characteristics of laser processing, the convex-lens-shaped microstructure can also be manufactured on a 3-dimensional curved surface. Furthermore, the processing speed can be improved since the micro structured fabricated in this study is smaller than the focused spot. The process of using the focused laser beam tends to be slow because each of the micro patterns has to be fabricated with the focused laser beam. However, the process described in this paper enables the manufacturing of microstructures even when the focused spot is enlarged. A large focused spot size means a higher processing speed. If the focused spot size increases, the power of the laser must also increase to maintain the energy per unit area. However, the power of the laser in industrial application keeps increasing through continuous R&D. Therefore, the process proposed in this paper is appropriate for high speed processing. Furthermore, compared to the conventional processes, it is a drier and more eco-friendly process. However, it is not possible to manufacture all types of micro lenses using the laser process proposed in this paper. For example, it is difficult to change the radius of the curvature. Although, the radius of the curvature may be partially changed by varying the laser condition, it is still limited. 4. Conclusion We fabricated a convex-lens-shaped microstructure with a diameter of 50 mm on a metallic substrate. The mechanism of fabricating the convex lens shape can be explained by unbalanced ablation. Ablation unbalance is caused by the difference of ablation threshold values of different components of the material being
processed. This difference enabled the creation of the microstructures that were smaller than the focused spot. Another mechanism is the re-adherence of the melted particles. During the laser ablation process, the melted particles may adhere to the unprocessed area by shock waves. These melted particles may act as the mask to the laser beam to create the convex-lens-shaped microstructure. The convex-lens-shaped micro structured fabricated on a mold substrate that was described in this paper can be used as the mold of the micro lens.
Acknowledgments This research was supported by a grant from Korean government funded research project (Main research project of Korea Institute of Machinery and Materials) and a grant (10SeaHeroB0402-01) from the Plant Technology Advancement Program funded by the Ministry of Land, Transport and Maritime Affairs of the Korean government.
References [1] H.D. lm, K.H. Oh, S.G. Kim, S.H. jeong, Application of Etchant jet for laser micro-machining of metal channels, Int. J. Precis. Eng. Man 10 (2009) 101e105. [2] C.Y. Ho, Y.H. Tsai, C.S. Chen, M.Y. Wen, Ablation of aluminum oxide ceramics using femtosecond laser with multiplepulses, Curr. Appl. Phys 11 (2011) 301e305. [3] Y. Jee, M.F. Becker, R.M. Walser, Laser-induced damage on single-crystal metal surfacesm, J. Opt. Soc. Am. B 5 (1988) 648. [4] M.C. Gower, Industrial applications of laser micromachining, Opt. Express 7 (No.2) (2000) 57. [5] S. Nolte, C. Momma, H. Jacobs, A. Tünnermann, Ablation of metals by ultrashort laser pul, J. Opt. Soc. Am. B 14 (1997) 10. [6] J. Bonse, J.M. Wrobel, J. Krüger, W. Kautek, Ultrashort-pulse laser ablation of indium phosphide in air, Appl. Phys. A 72 (2001) 89e94. [7] A. Rosenfeld, M. Lorenz, R. Stoian, D. Ashkenasi, Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation, Appl. Phys. A 69 (1999) 373e376. [8] J. Güdde, J. Hohlfeld, J.G. Müller, E. Matthias, Damage threshold dependence on electronephonon coupling in Au and Ni films, Appl. Surf. Sci. 40 (1998) 127e129. [9] Y. Bellouard, A. Said, M. Dugan, P. Bado, Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemical etching, Opt. Express 12 (2004) 2120e2129. [10] I.Y. Chung, J.D. Kim, K.H. Kang, Ablation drilling of Invar alloy using ultrashort pulsed laser, Int. J. Precis. Eng. Man 10 (2009) 11e16. [11] M. Huang, F. Zhao, Y. Cheng, N. Xu, Z. Xu, Large area uniform nanostructures fabricated by direct femtosecond laser ablation, Opt. EXPRESS 16 (2008) 19354e19365. [12] S.H. Kim, I.B. Sohn, S.h. Jeong, Parallel ripple formation during femtosecond laser grooving of ceramic, Appl. Phys. A 103 (2011) 1053e1057. [13] D.C. Emmony, R.P. Howson, L.J. Willis, Laser mirror damage in germanium at 10.6 mm, Appl. Phys. Lett. 23 (1973) 598. [14] Z. Guosheng, P.M. Fauchet, A.E. Siegman, Growth of spontaneous periodic surface structures on solids during laser illumination, Phys. Rev. B 26 (1982) 5366. [15] J.E. Young, J.S. Preston, H.M. van Driel, E. Sipe, Laser-induced periodic surface structure. II. Experiments on Ge, Si, Al, and brass, Phys. Rev. B 27 (1983) 1155. [16] T.H. Her, R.J. Finlay, C. Wu, S. Deliwala, E. Mazur, Microstructuring of silicon with femtosecond laser pulses, Appl. Phys. Lett. 73 (1998) 1673. [17] J.H. Noh, J.H. Lee, S.Y. Lee, S.j. Na, Fabrication of random microspikes on mold metal by ultrashort laser ablation for hydrophilic surface, Jpn. J. Appl. Phys. 49 (2010) 106e503. [18] S.H. Kim, I.B. Sohn, S.H. Jeong, Fabrication of uniform nanogrooves on 6H-SiC by femtosecond laser ablation, Appl. Phys. A 102 (2011) 55e59.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Fabrication of a random convex-lens-shaped microstructure using a laser ablation Jiwhan Noh a, b, Jae-Hoon Lee a, Suckjoo Na b, * a b
Korea Institute of Machinery and Materials (KIMM), 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Korea Advanced Institute of Science and Technology (KAIST), 371-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2011 Received in revised form 20 February 2012 Accepted 11 March 2012 Available online 28 March 2012
A convex-lens-shaped microstructure with a diameter of 50 mm on a metallic mold substrate was fabricated in this paper. A laser ablation process, in which the laser beam was focused and irradiated on the metallic mold substrate in order to remove a part of the substrate, was used for that. The convexlens-shaped microstructure has not been reported in any studies of microstructure using the laser ablation process. It was proposed that the unbalanced ablation and re-adherence of the melted particles was the processing mechanism of the convex lens shape. The convex-lens-shaped microstructure fabricated in this study is smaller than the focused spot. It was expected that the same convex-lensshaped microstructure can be fabricated even if the focused spot size is increased, so long as the fluence of the laser can be maintained. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of a laser ablation process. The fabricated convex-lensshaped microstructure on the metallic substrate can be used as the mold for the micro lens. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Convex-lens-shaped microstructure Laser ablation processing Metallic mold substrate Micro lens
1. Introduction As the focused laser pulse energy is irradiated on a substrate, a part of the substrate surface is melted or ablated. Particularly, if the energy is larger than the ablation threshold value of the substrate and is then irradiated, a part of the substrate will be removed [1,2]. For example, if a laser with a pulse duration in units of nanoseconds were irradiated on metal, parts of the metal surface would be ablated and the surface would become rougher [3,4]. When a laser with a pulse duration in units of nanoseconds creates a large heat-affected-zone that makes it difficult to create micro sized structures. With the recent development of the ultrashort pulse laser, which has a pulse duration in the femto or picosecond domain, the micro processing technology that uses the laser has been rapidly developing. That is because the short pulse duration of the ultrashort laser has enabled fabrication of the microstructures with a greatly reduced heat-affected-zone [5e10]. As an ultrashort laser is focused and irradiated on a substrate surface, the focused beam is irradiated on the substrate surface. In that case, a micro sized hole in the size of the focused spot is created. As the sample or focused beam is moved using a stage or galvano-type scanner, a micro groove will be created. In other words, fabrication of a micro hole or a groove smaller than the focused spot size is impossible. To fabricate a smaller
* Corresponding author. E-mail addresses: [email protected] (J. Noh), [email protected] (S. Na). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.03.017
microstructure, the smaller focused beam is required. In order to make the smaller focused beam size, the diameter of the input laser beam into focused lens may be increased, a short wavelength laser may be used, or a lens with a short focal length can be used. However, that will create a processing problem as the smaller focused spot size will result in a smaller depth of focus. Therefore, the fabrication of a microstructure beyond the diffraction limit is impossible. Fabricating a microstructure using a focused laser has another problem in that the processing time increases when fabricating a lot of microstructures, because the structures must be fabricated with a focused beam one by one. To overcome the problem, there are ongoing studies of fabricating structures smaller than the focused spot size. When a substrate is irradiated by the energy from using a laser on a level close to the ablation threshold, a type of ripple is generated on the substrate [11]. It is generally a nano size pattern that is proportional to the wavelength of the laser used. The direction of the nano pattern can also be adjusted by using the polarization direction of the laser [12]. The principle of this nano pattern generation was known as the interference between the irradiated laser beam and scattered laser beam [13e15]. However, many experiments have created nano patterns that cannot be explained by that interference theory. Although there are ongoing studies that use a coulomb explosion to explain it, no principle to explain all experimental results has been discovered yet. Since this nano pattern uses the Gaussian laser beam or flat top shaped beam using the shaping element of the Gaussian beam, it is very difficult to fabricate nano patterns with a high aspect ratio. Tsing-Hua fabricated a nano spike
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with a high aspect ratio by irradiating the femtosecond laser on a wafer in the SF6 or Cl2 environment [16]. Although they were able to fabricate a nano spike with high aspect ratio that could not be implemented in a nano ripple, it could only be applied on the silicon wafer and they had to use SF6 or Cl2, which are toxic gases, and thus it had a limited industrial application. In this paper, the picosecond laser is irradiated on a mold material called, NAK80, in the air and we studied the micro/nano pattern that was created on the irradiated area. In the course of studying the micro nano pattern on a mold material, we discovered a convex-lens-shaped microstructure that was smaller than the laser spot. To the best of our knowledge, there has been no report of a convex-lens-shaped microstructure. Since the process proposed in this paper was executed in the air, it can fabricate large structures. It was expected that the same convex-lens-shaped microstructure can be fabricated, even if the focused spot size is increased, so long as the same energy per unit area is irradiated on a substrate. Therefore, the method proposed in this paper will improve the low processing speed, which has been the problem of the laser ablation processes. Because the picosecond laser is superior to the femtosecond laser in terms of price and stability, this study used the picosecond laser instead of the femtosecond laser, which was widely used in the previous nano ripple studies. Since the convex-lens-shaped microstructure was fabricated on the surface of a mold made of NAK80, the process will enable the injection molding of plastic products with micro patterns at a low cost. This kind of manufactured plastic surface can be used as a micro lens array. 2. Experimental details Fig. 1 shows the conceptual diagram of the experiment. As shown in the figure, a substrate surface is scanned with a focused
laser beam that used a Galvanometer mirror and f-theta lens. If the energy per unit area of the focused laser beam is larger than the ablation threshold of the substrate, ablation, which means “removal of the substrate” will occur. This study focuses on the ablated surface of the substrate, particularly on the microstructure that is smaller than the focused spot. In a typical laser ablation process, a groove in the shape of a reverse Gaussian that has the width of the focused spot size is created when the pulse overlap rate is 90% or more. That is because the TEM00 laser beam has Gaussian distribution and thus the energy distribution on the focused area also has Gaussian distribution. In other words, the energy will be the highest at the center of the laser and it will become smaller as it moves away from the center. As a result, there will be more ablation in the area of high energy and less ablation in the area of low energy. Thus, a reverse Gaussian-shaped groove will be created on the substrate. This study focused on the microstructure that is smaller than the focused spot and not on the reverse Gaussian-shaped groove. Various micro/nano structures, as shown in Fig. 1(a), (b), and (c), were fabricated by adjusting the pulse overlap rate, line overlap rate, laser fluence, and number of scans. As shown in the figure, a unidirectional scan, not a cross pattern scan, was used. The picosecond laser used in the experiments was a diodepumped, mode-locked Nd:YVO4 laser with a pulse width of 12 ps. The maximum repetition rate was 640 kHz and the fundamental wavelength was 1064 nm. The laser was equipped with second harmonic generators to make laser wavelengths of 532 nm. In the experiments, a laser wavelength of 532 nm was used. A half-wave plate and a polarizer were used to control the laser power. Instead of a mechanical shutter, an external TTL signal served as a shutter for the laser pulses. This prevented extra irradiation from the laser pulses onto the specimen that were caused by the time delay of the mechanical shutter at every end point of the laser beam’s path. The
Fig. 1. Conceptual diagram of the experiment.
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horizontally polarized beam was focused by an f-theta scan lens with a maximum scan area of 58 58 mm2 (Sill Optics S4LFT0101/ 121). The effective focal length of the f-theta lens was 89.7 mm. The working distance was 97 mm. Max beam entrance beam diameter is 6 mm. The optical scan angle is 25.4 . The beam was manipulated over the sample by a two-mirror galvoscanner (RAY LASE SS-15 D2). The fluence was determined by measuring the power at the exit of the f-theta lens and it was then divided by the repetition rate and the irradiated surface area. These values were averages, as the energy distribution of the pulses was Gaussian using the TEM00 mode. The processing material was NAK80 mold steel with a uniform hardness of approximately 40HRc throughout. It never required stress relieving, even after heavy machining. It also had a uniform grain structure without pinholes, inclusion, or hard spots. NAK80 is commonly used for the mold material for plastic lens. For mass production for plastic lens, lens curvature is fabricated on the NAK80 surface. With that NAK80 mold material, plastic lens can be fabricated by injection mold process whit low coat. NAK80 is composed of 93.05% of Fe, 1% of Al, 0.15% of C, 1% of Cu, 1.5% of Mn, 3% of Ni, 3% of Si. The laser was irradiated on the NAK80 material in the air and not in a vacuum or certain gas. 3. Results and discussion Fig. 2(d) shows the conceptual diagram of the defocus laser processing system. The beam reflected from the Galvanometer mirror passes through the f-theta lens and is focused on the sample.
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To test various conditions, this study experimented with the sample in and out of focus. The radius of the input beam that was irradiated to the f-theta lens was 1 mm, the laser wavelength was 532 nm, and the effective focal length of the f-theta lens was 89.7 mm. The working distance of f-theta lens is 97 mm. Since M2 of the picosecond laser was 1.3, the focused optical spot size (1/e2 of maximum intensity value) was about 40 mm. The diameter of the focused spot would be 40 mm if the distance between the sample and f-theta lens were 97 mm. However, it would be 125.1mm if the distance between the sample and f-theta lens were 102 mm, and 240.4 mm if the distance between the sample and f-theta lens were 107 mm. In other words, the laser was irradiated after placing the sample 5 mm and 10 mm off the focus. The average power of the laser was 3.5 W, the repetition rate was 50 kHz, the scan speed was 183 mm/s, and the scan repetition count was 5. A unidirectional scan and not a cross pattern, was used. The pulse duration was 12 ps. Although the same conditions were used in all cases, the fluence value, which is the energy delivered to the substrate, and the intensity value were varied according to the position of the sample. The fluence and intensity were 55.704 [J/cm2] and 464.4 [GW/cm2], respectively, in Fig. 2(a); 5.694 [J/cm2] and 47.4 [GW/ cm2], respectively in Fig. 2(b); and 1.547 [J/cm2] and 12.9 [GW/cm2], respectively, in Fig. 2(c). Since the size of the focused spot was 40mm in Fig. 2(a), 125 mm in Fig. 2(b), and 240 mm in Fig. 2(c), the pulse overlap rates were all different. The pulse overlap rate means the overlapped area between the pulses during laser scanning. The pulse overlap rate was 90.8% in Fig. 2(a), 97.0% in Fig. 2(b), and
Fig. 2. (a) SEM image of the working distance ¼ 121 mm, (b) SEM image of the working distance ¼ 130 mm, (c) SEM image of the working distance ¼ 135 mm, (Common processing Conditions: Laser average power : 3.5 W, repetition rate: 50 kHz, scan speed: 183 mm/s, scan repetition: 5 times, laser wavelength: 532 nm, laser pulse duration: 12 picoseconds), (d) Conceptual diagram of the defocus processing system.
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98.4% in Fig. 2(c). By varying the fluence and overlap rate, we discovered different micro/nano patterns on the treated surface. The process of fabricating the convex-lens-shaped microstructure shown in Fig. 2(b) is the most promising in terms of the processing speed. The 50mm sized convex-lens-shaped structure in Fig. 2(b) was fabricated with a focused spot diameter of 125 mm That indicates that the convex-lens-shaped microstructure can be created regardless of the focused spot size. The fact that the convexlens-shaped microstructure is randomly created without a uniform interval means that the spot size of the focused laser is not related to the convex-lens-shaped microstructure. Therefore, this kind of convex-lens-shaped microstructure can be fabricated, even if the focused spot size increases, so long as the laser fluence that has been irradiated on the substrate can be maintained. Since the depth of focus also increases when the focused spot size increases, it is more beneficial from the processing viewpoint. Since more and more ultrashort lasers with a high average power are being introduced, the processing speed of the convex-lens-shaped microstructure suggested in this paper is likely to keep increasing. Fig. 3 shows the SEM images of the result from increasing the scan repetition counts under the conditions of Fig. 2(b). Fig. 3(a), (c), (e), and (g) show the scan repetition count of 10, 15, 25, and 100, respectively. Fig. 3(b), (d), (f), and (h) are the magnified SEM images of Fig. 3(a), (c), (e), and (g), respectively. We particularly noted the convex-lens-shaped microstructure of Fig. 2(b) because there has been no report of that micro shape to the best of our knowledge. Therefore, we intended to study the processing mechanism by trying different scan repetition count at the same processing condition. As expected, the number of convex-lens-shaped microstructures shown in Fig. 3(a) is higher than that in Fig. 2(b). As the number of convex-lens-shaped microstructures increased, the nano ripple area decreased by that much. Fig. 3(b) also shows the nano ripple pattern at the top of the convex-lens-shaped microstructure. Most microstructures did not have the nano ripple on the convex-lens-shaped microstructures. They had a surface with small roughness. As shown in Fig. 3 (c), (d), (e), and (f), the number of convex-lens-shaped microstructures increased and the nano ripple area decreased by that much as the number of scans increased. As the number of scans increased, the number of convex-lens-shaped microstructures with a nano pattern increased and the number of convex-lens-shaped microstructures with small roughness decreased. Fig. 3(g) is a case of 100 scans and it shows that the existing convex-lens-shaped microstructures disappeared and that new shape microstructures are created. While Fig. 3(a) through Fig. 3(f) show the convex-lens-shaped microstructures with a diameter of around 50um, Fig. 3(g) and (h) show a microstructure of a different shape with a diameter of around 100um. And the nano ripple patterns shown in Fig. 3(a) and (c), and (e) are hardly shown in Fig. 3(g). However, some nano ripple patterns were observed between the microstructures. Fig. 3(h) is a magnified SEM image of the nano ripple pattern in Fig. 3(g). The micro pattern in the size of around 10 mm as shown in Fig. 2(a), has been widely reported in other studies of laser processing [17]. This 10 mm sized micro pattern is known to be created by the defect of the substrate and multi-reflection from the processed surface. In other words, the ablation threshold value of a section is different from other sections because of the defect in the material, and that causes the ablation to occur sooner or later in the defect section than others when the laser is irradiated. This kind of unbalanced ablation results in the small groove on the surface. When the laser is irradiated on this groove, there will be more reflected beams from the inclined surface of groove. Since the incidence angle of the laser incidence beam is larger on the inclined surface of groove, the reflectivity of both the P wave and S wave increases in accordance with the Fresnel Reflection Principle.
Therefore, the beam reflected from the inclined surface heads toward the deeper position of the groove and a deeper groove is created. There has been no report of the convex-lens-shaped microstructure, as shown in Fig. 2(b) and Fig. 3(a) and (c). However, there are many reports of nano pattern laser ripples that are created on the area that is not shaped like a convex lens [18]. Since this study focuses on the convex-lens-shaped microstructures, this paper intends to discuss the mechanism to create the convex-lens-shaped microstructures. This mechanism can be explained in two ways. The first way is to explain it by using the ablation difference due to the material defect. The material used in this study is NAK80, which is a mold material consisting of many elements. It consists of 93% Fe, 3% Ni, 3% Si, 1.5% Mn, 1% Al, 1% Cu, and 0.15% C. These elements have different ablation thresholds resulting from the laser irradiation. Therefore, grooves can be created on the surface due to the difference of the ablation threshold values. The Fresnel Reflection Principle will create the deeper microstructures when the laser beam is reflected to the groove. If there is a part of very high ablation threshold in the material, the convex-lens-shaped microstructure will be created as shown in Fig. 2(b). Although this ablation difference will accurately explain Fig. 2(a), it does not explain all of the mechanisms of creating the convex lens shape. The second way of creating the convex-lens-shaped microstructure can be explained by the melting particle which is ejected from the substrate. A laser is a type of heat source and it can evaporate and melt parts of the substrate. Since this study uses a laser with a pulse duration of 12 ps, very little of the substrate will be melted. It generally takes 10 ps or longer to transform the electron vibration to the lattice vibration in an iron. Therefore, when a laser with the pulse duration of 12 ps is irradiated on the iron, most of the laser energy is absorbed by the electrons and is not delivered to the lattice. Since the external energy is not delivered to the lattice, there should be no melting of the substrate. However, that is just theory, and some melting is actually observed in most cases even when the picosecond laser is used. Melting is also observed in some laser power domains even with the femtosecond laser. Melting is generated because of heat accumulation when many laser pulses are irradiated on the surface. However, the fact that a laser with the pulse duration of picoseconds or femtoseconds creates less melting than that of one with nanoseconds has been proven in many studies. Therefore, it can be assumed that there will be melting of some elements even with the picosecond laser used in this study. If these elements are melted first, the melted part will be shaped as spherical by the surface tension of melting particle. When the laser is irradiated again on the spherical part, the spherical part will spread wider and become the mask for the laser. Since this material is melted, it is possible that its ablation threshold value is changed. Particularly, if the material is oxidized after being melted, its ablation threshold value will be higher than that of an existing substrate. Because of that, the melted material will spread and take the role of a mask to the laser beam. If the melted particles can settle on the unprocessed area by the shock wave of the laser processing, this theory becomes even more probable. That is because these spherical shaped particles are positioned on the unprocessed area and the laser is irradiated on it, it will spread wider and is very likely to become the mask to the laser. The fact that the roughness of the convex-lens-shaped microstructure is small can mean that these spherical shaped particles are spread by the laser and adhered on the surface. In the typical interaction between the laser and material, it is impossible to fabricate the roughness as small as the convex-lens-shaped microstructure. If the laser continues being irradiated on the convex-lens-shaped microstructure, parts of the structures will be destroyed, as shown in Fig. 3(e) and (f).
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Fig. 3. (a) SEM image of the scan repetition count ¼ 10, (c) SEM image of the scan repetition count ¼ 15, (e) SEM image of the scan repetition count ¼ 25, (g) SEM image of the scan repetition count ¼ 100; (b), (d), (f), and (h) are the magnified SEM images of (a), (c), (e), and (g), respectively. (Common processing conditions: Laser average power - 3.5 W, repetition rate - 50 kHz, scan speed - 183 mm/s, scan repetition count - 5, laser wavelength - 532 nm, and laser pulse duration - 12 ps).
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The convex-lens-shaped microstructure, as shown in Fig. 3(d), can be used as the mold of the plastic micro lens. The substrate used in this study is NAK80, which is actually used as the mold material. The convex-lens-shaped microstructure fabricated in this study can be used in injection molding to manufacture a micro plastic lens. Since the mold is convex-lens-shaped, the replicated plastic will be concave-lens-shaped. To manufacture the convex lens shape plastic, the convex-lens-shaped microstructure mold can be used to fabricate the mold in the reverse shape using the electroforming process. The micro convex lens shape can be manufactured on the plastic surface through injection molding using this reverse shape mold. Currently, the photolithography process and mechanical process are used to manufacture the micro lens. The photolithography process requires many steps to manufacture the curvature of a micro lens. That is because it is very difficult to manufacture the curvature of a convex lens in the photolithography process. In the mechanical process which uses the mechanical bite for milling process, it is difficult to manufacture a small micro lens. The process of fabricating a convex-lens-shaped microstructure, as described in this study, is simpler than the conventional methods and will allow for the manufacturing of the micro lens mold at a low cost. Since the surface roughness of the convex-lens-shaped microstructure is small, there will be less scattering on the surface of the micro lens manufactured with it and that will improve the optical performance of the micro lens. Furthermore, the laser process in this study is implemented in the air and not in a vacuum or special gas. That will enable manufacturing on a large area. Because of the characteristics of laser processing, the convex-lens-shaped microstructure can also be manufactured on a 3-dimensional curved surface. Furthermore, the processing speed can be improved since the micro structured fabricated in this study is smaller than the focused spot. The process of using the focused laser beam tends to be slow because each of the micro patterns has to be fabricated with the focused laser beam. However, the process described in this paper enables the manufacturing of microstructures even when the focused spot is enlarged. A large focused spot size means a higher processing speed. If the focused spot size increases, the power of the laser must also increase to maintain the energy per unit area. However, the power of the laser in industrial application keeps increasing through continuous R&D. Therefore, the process proposed in this paper is appropriate for high speed processing. Furthermore, compared to the conventional processes, it is a drier and more eco-friendly process. However, it is not possible to manufacture all types of micro lenses using the laser process proposed in this paper. For example, it is difficult to change the radius of the curvature. Although, the radius of the curvature may be partially changed by varying the laser condition, it is still limited. 4. Conclusion We fabricated a convex-lens-shaped microstructure with a diameter of 50 mm on a metallic substrate. The mechanism of fabricating the convex lens shape can be explained by unbalanced ablation. Ablation unbalance is caused by the difference of ablation threshold values of different components of the material being
processed. This difference enabled the creation of the microstructures that were smaller than the focused spot. Another mechanism is the re-adherence of the melted particles. During the laser ablation process, the melted particles may adhere to the unprocessed area by shock waves. These melted particles may act as the mask to the laser beam to create the convex-lens-shaped microstructure. The convex-lens-shaped micro structured fabricated on a mold substrate that was described in this paper can be used as the mold of the micro lens.
Acknowledgments This research was supported by a grant from Korean government funded research project (Main research project of Korea Institute of Machinery and Materials) and a grant (10SeaHeroB0402-01) from the Plant Technology Advancement Program funded by the Ministry of Land, Transport and Maritime Affairs of the Korean government.
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