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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 1Introduction 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 fl uidic de- vices7,8,1014.Asthemorphologyoftheprocessedtrans- parentmaterialisrelatedtothethermaleffectsofvaporization and dissolution due to thermal diffusion, interaction with the hotvapor plume, and a low-energy-density region in the laser pulse, it is highly sensitive to not only the physical proper- tiesofthematerial,butalsotothelaserirradiationparameters, such as the wavelength, pulse duration, pulse energy, numer- ical aperture of the focused beam, and the focus position. In particular, whenafemtosecond laserpulseisfocusednear the surfaceofatransparentmaterial,adifferenceinthefocuspos- itiongivesrisetoalargedifferenceinthesurfacemorphology. u 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- ingandtheinternalmicroexplosionoccursimultaneouslyand the bumps formed are very narrow. Bumps typically exhibits smallvariation in sizeand 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 suffi ciently 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 bumpsislimitedtoseveralmicrometersbecauseofablationof thetransparent coating. In order to achieve controllable fabri- cationofbumpswithahighdensity,itispossibleto useliquid on the transparent material in place of the transparent coating duringfemtosecondlaser processing,becausetheliquid natu- rally returns after breakdown and bubble formation. Fabrica- tionofcomplexstructuresonasiliconsurfacebyfemtosecond laserprocessing inwater has beendemonstrated 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 692Applied Physics A Materials Science Digital Instruments, Di- mension3000). 3Experimental results Figure 2 shows structures processed in water over a range ofZfrom4.0to12.0mwhen the energyEwas 2.1J. Figure 2a and b show an AFM image and its corres- ponding profi le, whose vertical range is500nm. Figure 2c and d showtop 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 profi les. Theirradiation energy was 2.1 J.The vertical range is500 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 HAYASAKIet al. High-density bump formation on a glass surface using femtosecond laser processing in water693 was obtained from a side view observation. The bumps were formed on the glass surface over a wide range ofZ, from 4.0to8.0m.AsZincreased,theheightanddiameterofthe bumps increased. WhenZwas6.0m, the bump had a max- imumheightof400 nmandadiameterof3.6 m.WhenZwas 8.0m,alowbumpwithaheightof50nmwasformed.When Zwasgreaterthan8.0m,voidswereformedinsidetheglass andno structurewasformedon theglasssurface. The length of the void under the bump also increased as Zincreased. The voids formed whenZwas 4 to12mwere nearlyequalinlength.Undermoredetailed observationinthe side view shown in Fig. 2d, we found that the voids had dif- ferent gray levels whenZwas between 6.0 and8.0m. The darkhueofthevoidsunderthehighbumpsatZ = 3.0mand Z = 6.0mwas darker than those of the voids formed com- pletelyinsidetheglass.Weexpectedthevoidinthehighbump to have lower density than the others, because an internal mi- croexplosiondisplaced theglass material fromthefocal point andformedthehighbump,thuscausingadecreaseindensity. This bump formation phenomenon is the same as that ob- served in our previous study in which glass having a trans- parentpolymercoating wasprocessed.Theprincipleofbump 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 thebump formation in the glass processed in wa- ter occurs over a wider range ofZ, as shown in Fig. 3. The irradiation beam parameters werealmost the sameas our pre- vious experiments (shown in Fig. 3 in 19). The irradiation energy wasE = 0.69J. When processing glass with a poly- mercoating,bumpformation wasobservedwhenZwas1.0 to4.0m20 whereas when processing in water, bump for- mation was observed whenZwas4.0to7.0m. The main 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 nextparagraph. Figure 4 shows AFM images of the processed structures forvariousenergiesEwhenZ = 0.Bumpswereformedwhen Ewas 0.17 to6.9J, and their structures drastically changed depending onE. The diameter and height of the bump in- creased asEincreased to4.1J. WhenEwas4.1J, the diameter was5.1mand the height was1.57m. With fur- ther increase ofE, both dimensions decreased. WhenE 2.1J, there was little debris around the periphery of the bump. Although, whenE 2.1J, debris was distributed around the periphery, and the amount of debris increased as Eincreased. 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 processingwith anapplied polymercoating. Thisfurther sup- ports the assertion that water had weaker physical blocking thanthepolymer coating. Mostofthedebriswasnotremoved by ultrasonic cleaning in water. Therefore, the glass material scattered in the liquid state at the glass/water interface ad- heredto theglasssurfaceand solidifi ed. Figure 5 show bumps arranged in a straight line with high density. The linearly-arranged bumps were processed by ir- FIGURE 3Diameter and height of bumps versus focus position. E was 0.69 J FIGURE 4AFM 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 J and (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 Dwas set to2.0m, under the condition that a single bump with a diameter of3.6mand a height of56nmwas formed whenEwas3.5JandZwas6.0m. The structure was processed by scanning the microscope stage so that a single pulse was irradiated at each location, repeated at a repetition rateRof1Hz. The shape of the linearly-arranged bumps was controlled by changingD, as shown in Fig. 6a and b. WhenDwas 0.8m, the bumps were smoothly connected, to form a line of bumps. WhenDwas5.0m, that is, whenD was suffi - ciently larger than the bump diameter, the bumps had isolated peaks. 694Applied Physics A Materials Science & Processing FIGURE 5AFM observation of linearly-arranged bumps formed under E = 3.5 J, Z = 6.0m, R = 1 Hz, and D = 2.0 m. (a) and (b) are the pro- fi les across and along the linearly-arranged bumps. The vertical range of the profi les is 250 nm and its horizontal length is 60 m FIGURE 6Surface structures formed under various conditions. The same irradiation energy of E = 2.1 J was used. In (a) and (b), Z = 6.0 m and R = 1 Hz, and the pulse irradiation spatial intervals of (a) D = 0.8 m and (b) D =5.0 mwere different. In (c) and (d), R =1Hz and D =0.5 m, and the focus positions of (c) Z = 6.0 m and (d) Z = 3.0 m were different. In (e) and (f), Z = 6.0 m and D = 0.5 m, and the repetition rates of (e) R = 2Hz and (f) R = 5Hz were different. The AFM images are 88 m2 To fabricate bumps with high density,ZandRwere carefully chosen, in addition toEandD. With the irradi- ation conditionsZ = 6.0m,E = 2.1J,D = 0.5m, and R = 1 Hz, a smooth line of bumps with a uniform height was FIGURE 7Bubbles generated on the water/glass interface observed with aCCD 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 fi lled circle and the bars formed, as shown in Fig. 6c. The width and height of the line of the bumps were about4.2mand60nm, respectively. With the irradiation conditionsZ = 3.0m,E = 2.1J,D = 0.5m, 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 ablationoftheformedbumpwhenthefocuspositionwasnear theglasssurface. Selection of the repetition rateRwas also important in forming high-density bumps. Figure 6e and f show AFM im- ages of a structure processed withR = 2and5 Hz, respec- tively. The other conditions (Z = 6.0m,E = 2.1J, and D = 0.5m)werethesame asthosein theexperiment shown in Fig. 6c. This difference depending only onRwas strongly relatedtothedisappearancetimeofthecavitation bubblegen- erated byplasmaformation atthewater/glassinterface. Figure 7ad show the bubble generated at the water/glass interface observed with the CCD image sensor whenE = 4.8JandZ = 0.0m. As the expansion of a bubble is less than10s28,itcannotbecapturedwithanordinaryCCD image sensor. Only the contraction of a bubble was observed, as shown in Fig. 7ac. In Fig. 7d, the circular pattern was thelaser-processed structure,becauseitdidntchangetempo- rally.Theelapsedtimet = 0 wasdefi nedasthetime whenthe bubblewasobserved.ThedisappearancetimeofthebubbleTd HAYASAKIet al. High-density bump formation on a glass surface using femtosecond laser processing in water695 was the time from generation to extinction of the bubble. Be- cause the CCD image sensor with30frames/swas used, the temporalresolution of themeasurement was33ms.Figure7e shows the disappearance timeTdfor the pulse energyE. 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 fi rst 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 of5Hz, becauseTdwas 250mswhenEwas2.1J. 4Conclusions 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-defi ned, high-density bumps, it was important to select appropriate parameters, in- cluding the processing time interval, the irradiation energy, thefocus position, and thespatial interval. A desired arrange- mentofbumpswithhighspatialdensityonaglasssurfacewas 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. ACKNOWLEDGEMENTSThis 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 Scientifi c Research (B) #16360035 from the Ministry of Education, Culture, Sports, Science and Technology. REFERENCES 1 D. Du, X. Liu, G. Korn, J. Squier, G. 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