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DOI: 10.1007/s00339-007-3930-z Appl. Phys. A 87, 691695 (2007) Materials Science 42.70.Ce; 52.38.Mf; 78.47.+p; 79.20.Ds 1 Introduction Femtosecond lasers are powerful tools for micro- and nano-structuring of transparent materials because they can process with high spatial resolution resulting from mul- tiple photon absorption, and reduced thermal damage due to the ultra-short interaction time between the laser pulse and the material, as well as various physical phenomena caused by the ultra-high intensity of the laser pulse 111. Fem- tosecond laser processing is being increasingly applied to the development of three-dimensional optical and fluidic de- vices 7, 8, 1014. As the morphology of the processed trans- parent material is related to the thermal effects of vaporization and dissolution due to thermal diffusion, interaction with the hot vapor plume, and a low-energy-density region in the laser pulse, it is highly sensitive to not only the physical proper- ties of the material, but also to the laser irradiation parameters, such as the wavelength, pulse duration, pulse energy, numer- ical aperture of the focused beam, and the focus position. In particular, when a femtosecond laser pulse is focused near the surface of a transparent material, a difference in the focus pos- ition gives rise to a large difference in the surface morphology. a117 Fax: +81-88-656-9435, E-mail: hayasakiopt.tokushima-u.ac.jp The typical surface morphology of glass processed by a tightly-focused femtosecond laser pulse, changes from a cavity to a bump when the focus position changes from the outside to the inside of the glass. The cavity is surrounded by a ring-shaped protrusion and scattered debris. Their size, and the amount of debris strongly depends on the focus pos- ition also. A bump with a diameter from several hundred nanometers to several micrometers is formed by melting the glass surface with the melted glass being pushed up by a mi- croexplosion inside the glass 1520. Due to the ranges of focal position and irradiation pulse energy, the surface melt- ing and the internal microexplosion occur simultaneously and the bumps formed are very narrow. Bumps typically exhibits small variation in size and structure. In a previous study, we found that a transparent coating on the glass for decreasing the amount of debris attached to the glass surface allows bump formation over a slightly wider range of focal positions compared to bare glass, when the coating thickness is sufficiently larger than the length of the focal volume 19, 21. Furthermore, we found that when the coating thickness is shorter than the length of the fo- cal volume, that is, when the coating surface is ablated by a single laser pulse focused at the boundary between the transparent coating and the glass, bumps were produced over a fairly wide range of focus positions compared to using a thick coating 20. From those investigations, we believe that the amount of coating material ablated in the focal vol- ume, which depends on the coating thickness, affects the strength of a shielding effect of the plasma generated when ablating the coating. As a result, the size and structure of the formed bump can be changed. The transparent coating method has the disadvantage that the spatial density of the bumps is limited to several micrometers because of ablation of the transparent coating. In order to achieve controllable fabri- cation of bumps with a high density, it is possible to use liquid on the transparent material in place of the transparent coating during femtosecond laser processing, because the liquid natu- rally returns after breakdown and bubble formation. Fabrica- tion of complex structures on a silicon surface by femtosecond laser processing in water has been demonstrated 2224. In this paper, we demonstrate formation of high-density micrometer-sized bumps by femtosecond laser processing in water. In Sect. 2, we describe the experimental setup and 692 Applied Physics A Materials Science Digital Instruments, Di- mension 3000). 3 Experimental results Figure 2 shows structures processed in water over a range of Z from 4.0 to 12.0 m when the energy E was 2.1 J. Figure 2a and b show an AFM image and its corres- ponding profile, whose vertical range is 500 nm. Figure 2c and d show top and side views of the processed area observed with the transmission optical microscope. Figure 2e shows the diameter and height of the bumps, which were obtained from the AFM observation, and the length of a void, which FIGURE 2 (a) AFM images of the processed area and (b) their profiles. The irradiation energy was 2.1 J. The vertical range is 500 nm. (c)Topand (d) side views observed with a transmission optical microscope. (e) Diameter and height of bumps versus focus position, and the length of voids formed in the glass versus focus position HAYASAKI et al. High-density bump formation on a glass surface using femtosecond laser processing in water 693 was obtained from a side view observation. The bumps were formed on the glass surface over a wide range of Z, from 4.0 to 8.0 m.AsZ increased, the height and diameter of the bumps increased. When Z was 6.0 m, the bump had a max- imum height of 400 nm and a diameter of 3.6 m.WhenZwas 8.0 m, a low bump with a height of 50 nm was formed. When Z was greater than 8.0 m, voids were formed inside the glass and no structure was formed on the glass surface. The length of the void under the bump also increased as Z increased. The voids formed when Z was 4 to 12 m were nearly equal in length. Under more detailed observation in the side view shown in Fig. 2d, we found that the voids had dif- ferent gray levels when Z was between 6.0 and 8.0 m.The dark hue of the voids under the high bumps at Z = 3.0 m and Z = 6.0 m was darker than those of the voids formed com- pletely inside the glass. We expected the void in the high bump to have lower density than the others, because an internal mi- croexplosion displaced the glass material from the focal point and formed the high bump, thus causing a decrease in density. This bump formation phenomenon is the same as that ob- served in our previous study in which glass having a trans- parent polymer coating was processed. The principle of bump formation in that study was based on the suppression of the material emission from the glass surface by a shielding effect of plasma generated by ablation of the polymer and by phys- ical blocking of the polymer. One difference in the present study is that the bump formation in the glass processed in wa- ter occurs over a wider range of Z, as shown in Fig. 3. The irradiation beam parameters were almost the same as our pre- vious experiments (shown in Fig. 3 in 19). The irradiation energy was E = 0.69 J. When processing glass with a poly- mer coating, bump formation was observed when Z was 1.0 to 4.0 m 20 whereas when processing in water, bump for- mation was observed when Z was 4.0 to 7.0 m.Themain reason for the difference is that the physical blocking of wa- ter is weaker than that of the polymer coating. This is further supported by the results for structures processed with high pulse energies, above several microjoules, discussed in the next paragraph. Figure 4 shows AFM images of the processed structures for various energies E when Z = 0. Bumps were formed when E was 0.17 to 6.9 J, and their structures drastically changed depending on E. The diameter and height of the bump in- creased as E increased to 4.1 J.WhenE was 4.1 J,the diameter was 5.1 m and the height was 1.57 m. With fur- ther increase of E, both dimensions decreased. When E 2.1 J, there was little debris around the periphery of the bump. Although, when E 2.1 J, debris was distributed around the periphery, and the amount of debris increased as E increased. The scattered region of the debris is indicated by the squares on the solid lines in Fig. 4. Processing in wa- ter produced more scattered debris around the bump than processing with an applied polymer coating. This further sup- ports the assertion that water had weaker physical blocking than the polymer coating. Most of the debris was not removed by ultrasonic cleaning in water. Therefore, the glass material scattered in the liquid state at the glass/water interface ad- hered to the glass surface and solidified. Figure 5 show bumps arranged in a straight line with high density. The linearly-arranged bumps were processed by ir- FIGURE 3 Diameter and height of bumps versus focus position. E was 0.69 J FIGURE 4 AFM images of the structures processed with (a) E = 0.69 J, (b) E = 2.8 J, (c) E = 4.1 J, (d) E = 4.8 J, (e) E = 5.5 Jand(f) E = 6.9 J. (g) Diameter and height of bump and debris diameter versus irradiation energy radiating the laser pulses at a spatial interval shorter than the diameter of a single bump. In this case, the spatial interval D was set to 2.0 m, under the condition that a single bump with a diameter of 3.6 m and a height of 56 nm was formed when E was 3.5 J and Z was 6.0 m. The structure was processed by scanning the microscope stage so that a single pulse was irradiated at each location, repeated at a repetition rate R of 1Hz. The shape of the linearly-arranged bumps was controlled by changing D, as shown in Fig. 6a and b. When D was 0.8 m, the bumps were smoothly connected, to form a line of bumps. When D was 5.0 m,thatis,whenD was suffi- ciently larger than the bump diameter, the bumps had isolated peaks. 694 Applied Physics A Materials Science & Processing FIGURE 5 AFM observation of linearly-arranged bumps formed under E = 3.5 J, Z = 6.0 m, R = 1 Hz, and D = 2.0 m. (a)and(b) are the pro- files across and along the linearly-arranged bumps. The vertical range of the profiles is 250 nm and its horizontal length is 60 m FIGURE 6 Surface structures formed under various conditions. The same irradiation energy of E = 2.1 J was used. In (a)and(b), Z = 6.0 mand R = 1 Hz, and the pulse irradiation spatial intervals of (a) D = 0.8 mand (b) D = 5.0 m were different. In (c)and(d), R = 1HzandD = 0.5 m, and the focus positions of (c) Z = 6.0 mand(d) Z = 3.0 m were different. In (e)and(f), Z = 6.0 mandD = 0.5 m, and the repetition rates of (e) R = 2Hzand(f) R = 5 Hz were different. The AFM images are 88 m 2 To fabricate bumps with high density, Z and R were carefully chosen, in addition to E and D. With the irradi- ation conditions Z = 6.0 m, E = 2.1 J, D = 0.5 m,and R = 1Hz, a smooth line of bumps with a uniform height was FIGURE 7 Bubbles generated on the water/glass interface observed with a CCD image sensor, when the elapsed time (a) t = 2/30, (b)8/30, (c)12/30, and (d)13/30 s. (e) The disappearance time of bubbles for the pulse energy. Three measurements at each pulse energy are indicated as the center filled circle and the bars formed, as shown in Fig. 6c. The width and height of the line of the bumps were about 4.2 m and 60 nm, respectively. With the irradiation conditions Z = 3.0 m, E = 2.1 J, D = 0.5 m,andR = 1Hz, many sub-micrometer sized spikes were formed, as shown in Fig. 6d. The irregularly shaped structures were formed as a result of a single bump formed by the previous laser pulse being destroyed by the next laser pulse, because the energy density at the glass surface enabled ablation of the formed bump when the focus position was near the glass surface. Selection of the repetition rate R was also important in forming high-density bumps. Figure 6e and f show AFM im- ages of a structure processed with R = 2 and 5Hz, respec- tively. The other conditions (Z = 6.0 m, E = 2.1 J,and D = 0.5 m) were the same as those in the experiment shown in Fig. 6c. This difference depending only on R was strongly related to the disappearance time of the cavitation bubble gen- erated by plasma formation at the water/glass interface. Figure 7ad show the bubble generated at the water/glass interface observed with the CCD image sensor when E = 4.8 J and Z = 0.0 m. As the expansion of a bubble is less than 10 s 28, it cannot be captured with an ordinary CCD image sensor. Only the contraction of a bubble was observed, as shown in Fig. 7ac. In Fig. 7d, the circular pattern was the laser-processed structure, because it didnt change tempo- rally. The elapsed time t = 0 was defined as the time when the bubble was observed. The disappearance time of the bubble T d HAYASAKI et al. High-density bump formation on a glass surface using femtosecond laser processing in water 695 was the time from generation to extinction of the bubble. Be- cause the CCD image sensor with 30 frames/s was used, the temporal resolution of the measurement was 33 ms. Figure 7e shows the disappearance time T d for the pulse energy E.We believe that the bubble mainly consisted of gaseous hydro- gen, oxygen, and water vapor. The laser irradiation in the presence of bubbles was equivalent to laser irradiation not in water, but in gas. Consequently, when the time interval be- tween irradiated pulses was shorter than the disappearance time of the bubble, the suppression of the material emission from the glass surface by a shielding effect of plasma and by a physical blocking of a covered material became weak, and a single bump formed by the first laser pulse was destroyed by the second laser pulse, resulting in the formation of an ir- regular structure. As shown in Fig. 6f, the irregular structure was formed under the repetition rate of 5Hz, because T d was 250 ms when E was 2.1 J. 4 Conclusions We have demonstrated bump formation on a glass surface using femtosecond laser processing in water. We in- vestigated the effect of the irradiation energy and focus pos- ition of the focused-femtosecond laser pulse on the morph- ology of the bump. Bumps with high spatial density were processed by irradiating the laser pulses with a spatial inter- val between irradiation positions shorter than the diameter of a single bump. In order to form well-defined, high-density bumps, it was important to select appropriate parameters, in- cluding the processing time interval, the irradiation energy, the focus position, and the spatial interval. A desired arrange- ment of bumps with high spatial density on a glass surface was fabricated by tuning the processing time interval to be more than the disappearance time of a cavitation bubble, generated by a femtosecond laser pulse focused near the water/glass interface. ACKNOWLEDGEMENTS This work was supported by The Venture Business Incubation Laboratory of The University of Tokushima, The Asahi Glass Foundation, The Murata Science Foundation, Science and Technology Incubation Program in Advanced Regions, Research for Promot- ing Technological Seeds from the Japan Science and Technology Agency, and Grant-in Aid for Scientific Research (B) #16360035 from the Ministry of Education, Culture, Sports, Science and Technology. REFERENCES 1 D. Du, X. Liu, G. Korn, J. Squier, G. Mourou, Appl. Phys. Lett. 64, 3071 (1994) 2 H. Kumagai, K. Midorikawa, K. Toyoda, S. Nakamura, T. Okamoto, M. Obara, Appl. Phys. Lett. 65, 1850 (1994) 3 B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry, Phys. Rev. Lett. 74, 2248 (1995) 4 D. von der Linde, H. Schler, J. Opt. Soc. Am. B 13, 216 (1996) 5 H. Varel, D. Ashkenasi, A. Rosenfeld, R. Herrmann, F. Noack, E.E.B. Campbell, Appl. Phys. A 62, 293 (1996) 6 K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. 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