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DOI:10.1007/s00339-007-3930-zAppl. Phys.A 87, 691695(2007)Materials Science & ProcessingAppliedPhysicsAy.hayasakiud.kawamuraHigh-density bump formation on a glass surfaceusing femtosecond laser processing in waterDepartment of Optical Science and Technology, Faculty of Engineering, The University of Tokushima,2-1 Minamijosanjima-cho, Tokushima 770-8506, JapanReceived: 13 November 2006/Accepted:29 January2007Published online: 29 March 2007 Springer-Verlag 2007ABSTRACTMicrometer-sized bumps were formed on a glasssurface using a focused femtosecond laser processing in water.The bumps were formed over a wide ranges of pulse irradiationparameters,including irradiation energyandfocusposition. Thebumps exhibited a wide variety of morphologies and sizes de-pending on the parameters. The use of a liquid, namely heavywater, which returns after breakdown and cavitation bubble for-mation, enabled us to fabricate bumps with high spatial density,which is not possible using a solid coating that is ablated. A de-siredarrangementofbumps onaglass surfacewasfabricatedbytuning the processing time interval to be more than the disap-pearancetimeofabubble, generatedby focusing afemtosecondlaser pulse near the water/glass interface.PACS42.62.Cf; 42.70.Ce; 52.38.Mf; 78.47.+p; 79.20.Ds1IntroductionFemtosecond lasers are powerful tools for micro-and nano-structuring of transparent materials because theycan process with high spatial resolution resulting from mul-tiple photon absorption, and reduced thermal damage due tothe ultra-short interaction time between the laser pulse andthe material, as well as various physical phenomena causedby the ultra-high intensity of the laser pulse 111. Fem-tosecond laser processing is being increasingly applied tothe development of three-dimensional optical and fluidic de-vices7,8,1014.Asthemorphologyoftheprocessedtrans-parentmaterialisrelatedtothethermaleffectsofvaporizationand dissolution due to thermal diffusion, interaction with thehotvapor plume, and a low-energy-density region in the laserpulse, 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. Inparticular, whenafemtosecond laserpulseisfocusednear thesurfaceofatransparentmaterial,adifferenceinthefocuspos-itiongivesrisetoalargedifferenceinthesurfacemorphology.u Fax: +81-88-656-9435, E-mail: hayasakiopt.tokushima-u.ac.jpThe typical surface morphology of glass processed bya tightly-focused femtosecond laser pulse, changes froma cavity to a bump when the focus position changes from theoutside to the inside of the glass. The cavity is surroundedby 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 hundrednanometers to several micrometers is formed by melting theglass surface with the melted glass being pushed up by a mi-croexplosion inside the glass 1520. Due to the ranges offocal position and irradiation pulse energy, the surface melt-ingandtheinternalmicroexplosionoccursimultaneouslyandthe bumps formed are very narrow. Bumps typically exhibitssmallvariation in sizeand structure.In a previous study, we found that a transparent coatingon the glass for decreasing the amount of debris attachedto the glass surface allows bump formation over a slightlywider range of focal positions compared to bare glass, whenthe coating thickness is sufficiently larger than the length ofthe focal volume 19,21. Furthermore, we found that whenthe coating thickness is shorter than the length of the fo-cal volume, that is, when the coating surface is ablated bya single laser pulse focused at the boundary between thetransparent coating and the glass, bumps were produced overa fairly wide range of focus positions compared to usinga thick coating 20. From those investigations, we believethat the amount of coating material ablated in the focal vol-ume, which depends on the coating thickness, affects thestrength of a shielding effect of the plasma generated whenablating the coating. As a result, the size and structure ofthe formed bump can be changed. The transparent coatingmethod has the disadvantage that the spatial density of thebumpsislimitedtoseveralmicrometersbecauseofablationofthetransparent coating. In order to achieve controllable fabri-cationofbumpswithahighdensity,itispossibleto useliquidon the transparent material in place of the transparent coatingduringfemtosecondlaser processing,becausetheliquid natu-rally returns after breakdown and bubble formation. Fabrica-tionofcomplexstructuresonasiliconsurfacebyfemtosecondlaserprocessing inwater has beendemonstrated 2224.In this paper, we demonstrate formation of high-densitymicrometer-sized bumps by femtosecond laser processing inwater. In Sect. 2, we describe the experimental setup and692Applied Physics A Materials Science & Processingprocedure. In Sect. 3, we describe the experimental results.We investigated the effects of irradiation parameters, includ-ing energy and focal position, on the morphology and sizeof the bumps. We demonstrated that, by tuning the process-ing time interval to be more than the disappearance time ofa bubble 2528 generated by a femtosecond laser pulse fo-cused near the water/glass interface, we could fabricate a de-siredstructureontheglasssurface, composedofhigh-densitybumps.InSect. 4,weconclude ourstudy.2Experimental setup and procedureThe experimental setup consisted of an amplifiedfemtosecond laser and an optical microscope and is shownin Fig. 1. It was the same as the setup used in our pre-vious work 19,20. The amplified femtosecond laser pro-duced pulses with a peak wavelength of800nm, a durationof 150fs, and a maximum repetition rate of1 kHz. Theirradiation pulse energyEat the sample was controlled byneutraldensityfilters,andisgivenbytheproductoftheenergymeasured before introducing the laser pulse into the opticalmicroscope(Olympus,IX70)andthetransmittance oftheop-tical microscope, including a40objective lens (numericalaperture,NA = 0.65). The transmittance of the microscopewas0.69.The processedareawas observedunder transmittedillumination by a usual charge-coupled device (CCD) imagesensor with the frame rate of30frames/s. The focus positionZof the laser pulse was defined as the distance moved alongthe optical axis by the microscope stage. The zero position(Z = 0) was defined as the position where a structure wasformedontheglasssurfacebyirradiation ofalaserpulsewithnearablation threshold energy.The structure of the sample is also shown in Fig. 1. Thesample was prepared as follows. Four ordinary microscopecover slips (Matsunami) which were subjected to ultrasoniccleaning in ethanol and pure water were prepared. They wereFIGURE 1Experimental setup and the structure of the sample. The spacerglasses were removed when the processing was performeda target glass, a window glass for sandwiching water, andtwo spacer glasses with a thickness of130m. Poly-methylmethacrylate (PMMA)with toluene solventwas used to formwalls on the window glass. After sufficiently evaporating thesolvent the spacer glasses were removed, and a small cham-ber with a side length of1015mmcomposed of the PMMAwalls on the glass was formed. Water was dropped in thesmall chamber and the target glass was fixed on the chamberwith a small amount of the PMMA that was used as a glue.In this experiment, deuterium oxide (heavy water, hereafterreferred to simply as “water”) was used because of its lowlinearabsorptionaroundthewavelengthof800nm.Afterpro-cessing, the target glass was removed from the chamber andsubjected to ultrasonic cleaning in pure water and ethanol.Thesurfacestructureoftheprocessedareawasobservedwithan atomic force microscope (AFM; Digital Instruments, Di-mension3000).3Experimental resultsFigure 2 shows structures processed in water overa range ofZfrom4.0to12.0mwhen the energyEwas2.1J. Figure 2a and b show an AFM image and its corres-ponding profile, whose vertical range is500nm. Figure 2cand d showtop and side views of the processed area observedwith the transmission optical microscope. Figure 2e showsthe diameter and height of the bumps, which were obtainedfrom the AFM observation, and the length of a void, whichFIGURE 2(a) AFM images of the processed area and (b) their profiles.Theirradiation energy was 2.1 J.The vertical range is500 nm.(c) Topand(d) side views observed with a transmission optical microscope. (e) Diameterand height of bumps versus focus position, and the length of voids formed inthe glass versus focus positionHAYASAKIet al. High-density bump formation on a glass surface using femtosecond laser processing in water693was obtained from a side view observation. The bumps wereformed on the glass surface over a wide range ofZ, from4.0to8.0m.AsZincreased,theheightanddiameterofthebumps increased. WhenZwas6.0m, the bump had a max-imumheightof400 nmandadiameterof3.6 m.WhenZwas8.0m,alowbumpwithaheightof50nmwasformed.WhenZwasgreaterthan8.0m,voidswereformedinsidetheglassandno structurewasformedon theglasssurface.The length of the void under the bump also increased asZincreased. The voids formed whenZwas 4 to12mwerenearlyequalinlength.Undermoredetailed observationintheside view shown in Fig. 2d, we found that the voids had dif-ferent gray levels whenZwas between 6.0 and8.0m. ThedarkhueofthevoidsunderthehighbumpsatZ = 3.0mandZ = 6.0mwas darker than those of the voids formed com-pletelyinsidetheglass.Weexpectedthevoidinthehighbumpto have lower density than the others, because an internal mi-croexplosiondisplaced theglass material fromthefocal pointandformedthehighbump,thuscausingadecreaseindensity.This bump formation phenomenon is the same as that ob-served in our previous study in which glass having a trans-parentpolymercoating wasprocessed.Theprincipleofbumpformation in that study was based on the suppression of thematerial emission from the glass surface by a shielding effectof plasma generated by ablation of the polymer and by phys-ical blocking of the polymer. One difference in the presentstudy is that thebump formation in the glass processed in wa-ter occurs over a wider range ofZ, as shown in Fig. 3. Theirradiation beam parameters werealmost the sameas our pre-vious experiments (shown in Fig. 3 in 19). The irradiationenergy wasE = 0.69J. When processing glass with a poly-mercoating,bumpformation wasobservedwhenZwas1.0to4.0m20 whereas when processing in water, bump for-mation was observed whenZwas4.0to7.0m. The mainreason for the difference is that the physical blocking of wa-ter is weaker than that of the polymer coating. This is furthersupported by the results for structures processed with highpulse energies, above several microjoules, discussed in thenextparagraph.Figure 4 shows AFM images of the processed structuresforvariousenergiesEwhenZ = 0.BumpswereformedwhenEwas 0.17 to6.9J, and their structures drastically changeddepending onE. The diameter and height of the bump in-creased asEincreased to4.1J. WhenEwas4.1J, thediameter 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 thebump. Although, whenE 2.1J, debris was distributedaround the periphery, and the amount of debris increased asEincreased. The scattered region of the debris is indicatedby the squares on the solid lines in Fig. 4. Processing in wa-ter produced more scattered debris around the bump thanprocessingwith anapplied polymercoating. Thisfurther sup-ports the assertion that water had weaker physical blockingthanthepolymer coating. Mostofthedebriswasnotremovedby ultrasonic cleaning in water. Therefore, the glass materialscattered in the liquid state at the glass/water interface ad-heredto theglasssurfaceand solidified.Figure 5 show bumps arranged in a straight line with highdensity. The linearly-arranged bumps were processed by ir-FIGURE 3Diameter and height of bumps versus focus position. E was0.69 JFIGURE 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 versusirradiation energyradiating the laser pulses at a spatial interval shorter than thediameter of a single bump. In this case, the spatial intervalDwas set to2.0m, under the condition that a single bumpwith a diameter of3.6mand a height of56nmwas formedwhenEwas3.5JandZwas6.0m. The structure wasprocessed by scanning the microscope stage so that a singlepulse was irradiated at each location, repeated at a repetitionrateRof1Hz.The shape of the linearly-arranged bumps was controlledby changingD, as shown in Fig. 6a and b. WhenDwas0.8m, the bumps were smoothly connected, to form a lineof bumps. WhenDwas5.0m, that is, whenDwas suffi-ciently larger than the bump diameter, the bumps had isolatedpeaks.694Applied Physics A Materials Science & ProcessingFIGURE 5AFM observation of linearly-arranged bumps formed underE = 3.5 J, Z = 6.0m, 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 theprofiles is 250 nm and its horizontal length is 60 mFIGURE 6Surface structures formed under various conditions. The sameirradiation energy of E = 2.1 J was used. In (a) and (b), Z = 6.0 m andR = 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, andthe 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 m2To fabricate bumps with high density,ZandRwerecarefully chosen, in addition toEandD. With the irradi-ation conditionsZ = 6.0m,E = 2.1J,D = 0.5m, andR = 1 Hz, a smooth line of bumps with a uniform height wasFIGURE 7Bubbles generated on the water/glass interface observed withaCCD 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 filledcircle and the barsformed, as shown in Fig. 6c. The width and height of theline 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 spikeswere formed, as shown in Fig. 6d. The irregularly shapedstructures were formed as a result of a single bump formedby the previous laser pulse being destroyed by the next laserpulse, because the energy density at the glass surface enabledablationoftheformedbumpwhenthefocuspositionwasneartheglasssurface.Selection of the repetition rateRwas also important informing 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, andD = 0.5m)werethesame asthosein theexperiment shownin Fig. 6c. This difference depending only onRwas stronglyrelatedtothedisappearancetimeofthecavitation bubblegen-erated byplasmaformation atthewater/glassinterface.Figure 7ad show the bubble generated at the water/glassinterface observed with the CCD image sensor whenE =4.8JandZ = 0.0m. As the expansion of a bubble is lessthan10s28,itcannotbecapturedwithanordinaryCCDimage sensor. Only the contraction of a bubble was observed,as shown in Fig. 7ac. In Fig. 7d, the circular pattern wasthelaser-processed structure,becauseitdidntchangetempo-rally.Theelapsedtimet = 0wasdefinedasthetime whenthebubblewasobserved.ThedisappearancetimeofthebubbleTdHAYASAKIet al. High-density bump formation on a glass surface using femtosecond laser processing in water695was the time from generation to extinction of the bubble. Be-cause the CCD image sensor with30frames/swas used, thetemporalresolution of themeasurement was33ms.Figure7eshows the disappearance timeTdfor the pulse energyE. Webelieve that the bubble mainly consisted of gaseous hydro-gen, oxygen, and water vapor. The laser irradiation in thepresence of bubbles was equivalent to laser irradiation not inwater, but in gas. Consequently, when the time interval be-tween irradiated pulses was shorter than the disappearancetime of the bubble, the suppression of the material emissionfrom the glass surface by a shielding effect of plasma and bya physical blocking of a covered material became weak, anda single bump formed by the first laser pulse was destroyedby the second laser pulse, resulting in the formation of an ir-regular structure. As shown in Fig. 6f, the irregular structurewas formed under the repetition rate of5Hz, becauseTdwas 250mswhenEwas2.1J.4ConclusionsWe have demonstrated bump formation on a glasssurface 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 wereprocessed by irradiating the laser pulses with a spatial inter-val between irradiation positions shorter than the diameter ofa single bump. In order to form well-defined, high-densitybumps, it was important to select appropriate parameters, in-cluding the processing time interval, the irradiation energy,thefocus position, and thespatial interval. A desired arrange-mentofbumpswithhighspatialdensityonaglasssurfacewasfabricated by tuning the processing time interval to be morethan the disappearance time of a cavitation bubble, generatedby a femtosecond laser pulse focused near the water/glassinterface.ACKNOWLEDGEMENTSThis work was supported by TheVenture Business Incubation Laboratory of The University of Tokushima,The Asahi Glass Foundation, The Murata Science Foundation, Science andTechnology 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 Ministryof Education, Culture, Sports, Science and Technology.REFERENCES1 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. Cam