Laser micromachining has been proved to be a useful tool for the formation of microstructures in semiconductor and optical materials. It is also widely adopted for dicing of light-emitting diode chips. The authors propose a modified laser micromachining setup which enables three-dimensional structures to be formed. A mirror is inserted in the optical path between the focusing optics and the machining plane so that the beam strikes the sample at an oblique angle. By translating and/or rotating the sample as micromachining is carried out, various three-dimensional structures such as a pyramid or a conic section can be obtained. Trenches as small as 10 μm on sapphire have been realized with nanosecond ultraviolet laser pulses. Laser-induced damage, due to resolidification of the ablation melt, accumulates with increasing scans of the beam; it can be removed by chemical and mechanical treatment. © 2009 American Vacuum Society.published_or_final_versio

Choi, HW

Lai, PT

Wang, XH

Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures Processing Measurement and Phenomena

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Title Laser micromachining of optical microstructures with inclinedsidewall profileAuthor(s) Wang, XH; Lai, PT; Choi, HWCitation Journal Of Vacuum Science And Technology B: MicroelectronicsAnd Nanometer Structures, 2009, v. 27 n. 3, p. 1048-1052Issued Date 2009URL http://hdl.handle.net/10722/58802Rights Creative Commons: Attribution 3.0 Hong Kong LicenseLaser micromachining of optical microstructures with inclined sidewallprofileX. H. Wang, P. T. Lai, and H. W. ChoiaDepartment of Electrical and Electronic Engineering, Semiconductor Lighting and Display Laboratory,The University of Hong Kong, Pokfulam Road, Hong KongReceived 29 December 2008; accepted 16 March 2009; published 20 April 2009Laser micromachining has been proved to be a useful tool for the formation of microstructures insemiconductor and optical materials. It is also widely adopted for dicing of light-emitting diodechips. The authors propose a modified laser micromachining setup which enables three-dimensionalstructures to be formed. A mirror is inserted in the optical path between the focusing optics and themachining plane so that the beam strikes the sample at an oblique angle. By translating and/orrotating the sample as micromachining is carried out, various three-dimensional structures such asa pyramid or a conic section can be obtained. Trenches as small as 10 m on sapphire have beenrealized with nanosecond ultraviolet laser pulses. Laser-induced damage, due to resolidification ofthe ablation melt, accumulates with increasing scans of the beam; it can be removed by chemicaland mechanical treatment. © 2009 American Vacuum Society. DOI: 10.1116/1.3117344I. INTRODUCTIONMicromachining with nanosecond laser pulses is a pow-erful tool that is suitable for replacing or complementingtraditional wafer processes such as dicing and etching, aswell as advanced process developments such as laserlift-off,1 laser-assisted machining,2 and rapid prototyping.3Tightly focused nanosecond laser pulses enable microma-chining with much higher precision with dimensions down toseveral micrometers.4–6 For more advanced applications,considerations such as laser wavelength, pulse energy, rep-etition rate, and pulse duration should be seriously taken intoaccount. Drilling and cutting with nanosecond or even fem-tosecond ultraviolet UV laser pulses have been reported toexcel with very small heat-affected zones.5–7 Laser microma-chining is gradually being adopted for gallium nitride GaNbased light-emitting diodes LEDs. Since the epitaxial GaNlayers are typically grown on sapphire, separation of fabri-cated LED dies is commonly achieved by wafer sawing,which is slow and expensive. Using high energy laser pulsesnot only increases the process efficiency but also enables ahigh packing density of chips by virtue of the reduced di-mensions of the scribe lanes.One of the typical configurations of laser micromachiningrelies on a laser scanner head which steers the beam into theincident direction;5 although it is relatively fast, mechanicalvibrations tend to be magnified, resulting in loss of patternresolution. Alternatively, the sample to be micromachinedcan be mounted onto a precision motorized translation mod-ule while the optical beam remains static; this is more suit-able for the processing of optical microstructures. In a con-ventional laser micromachining setup for wafer dicing, thefocused laser beam is incident perpendicularly onto thesample to be processed, so that only two-dimensional pat-terns can be generated and vertical cuts can be obtained.aAuthor to whom correspondence should be addressed; electronic mail:hwchoi@hku.hk1048 J. Vac. Sci. Technol. B 27„3…, May/Jun 2009 1071-1023/2009Projecting the beam at an oblique angle to the sample en-ables three-dimensional micromachining. Nevertheless, it isnot viable to mount the translation module at a tilted angle,as it will result in severe beam distortion. In our proposedapproach, a laser beam turning mirror was introduced intothe optical path to achieve a continuously tunable rangeof tilting angle for beam projection while retaining beamquality.II. EXPERIMENTAL DETAILSThe setup for laser micromachining consists of an UVlaser source, beam focusing optics, and an x-y motorizedtranslation stage. The laser source is a third harmonicneodymium-doped yttrium lithium fluoride diode-pumpedsolid state laser manufactured by Spectra Physics. The laseremits at 349 nm, while the pulse repetition rate ranges fromsingle pulse to 5 kHz. At a reference diode current of 3.2 A,the pulse energy is 120 J at a repetition rate of 1 kHz, witha pulse width of around 4 ns. The TEM00 beam allows fortight focusing, offering high spatial resolution. After beamexpansion and collimation through a beam expander, the la-ser beam is reflected 90° by a dielectric laser line mirror andfocused onto the horizontal machining plane to a very tinyspot several micrometers in diameter with a focusing triplet.All optics used are made of UV fused silica and are antire-flection coated. The additional feature of our setup, as illus-trated in the schematic of Fig. 1, is the insertion of an UVmirror at an oblique angle within the optical path betweenthe focusing optics and the machining plane, which serves todeflect the convergent beam to strike the sample at an ob-lique angle with respect to the horizontal working plane. Thesize of the beam at the focal point not only is limited by thecapability of the UV objective lens but is also sensitive to thecoaxiality of the optics. With this modified setup, it is rela-tively easy to optimize and monitor the beam through thetube lens imaged with a charge coupled device camera. Oncethe optical setup is optimized before insertion of the tilting1048/27„3…/1048/5/$25.00 ©2009 American Vacuum Society1049 Wang, Lai, and Choi: Laser micromachining of optical microstructures 1049mirror, the mirror can be inserted without affecting the co-axiality of the laser beam, so that the dimension of the beamspot remains unaffected.The beam can be effectively applied for microsectioningwith nonvertical sidewall profiles. The angle of incidence ofthe deflected laser beam on the wafer is 2, where  asindicated in Fig. 1 is the angle between the plane of themirror to the normal. This angle is readily and precisely con-trolled by mounting the mirror onto a rotation stage; thus, theincident angle can be varied over a wide range. In this ex-periment, we have used an UV objective with a focal lengthof 75 mm based on two considerations. First, the focal lengthshould be long enough to accommodate the mirror in theoptical path. Second, an ideal tool for the fabrication of mi-crostructures should have a very long penetration depth andnegligible lateral dispersion. Nevertheless, an objective lenswith a longer focal length also produces a larger focusedbeam spot. The two parameters are related via the followingequation:d =4M2fD, 1where M2 quantifies the beam quality,  is the wavelength ofthe laser beam, f is the focal length, and D is the diameter ofFIG. 1. Color online Schematic of laser micromachining setup.the incident beam.JVST B - Microelectronics and Nanometer StructuresIII. RESULTS AND DISCUSSIONSAs our scheme of micromachining is targeted at die sepa-ration of GaN LEDs, sapphire wafers are used for testing theresults, as it is the typical substrate for the metal organicchemical vapor deposition growth of GaN. The quality of thecleave can be quantified by the width, depth, linearity, andsidewall roughness of the trench formed by the laser beam.Each of these parameters will be investigated. Since the focallength of the focusing lens f =75 mm is much longer thanthe thickness of the sapphire wafer t=420 m, the depthof the trench mainly depends on the number of micromachin-ing cycles. The number of cycles is controlled by configuringthe translation stage to repeat its linear path over a number oftimes. Since the position repeatability of the stage is betterthan 5 m, increasing the number of cycles should not con-tribute significantly to the width of the feature. Figure 2shows the cross-sectional optical image of a 420 m thicksapphire wafer that has been micromachined with an incidentbeam inclined at 45° or 60°, with scan cycles ranging from 1to 10. These incisions were carried out by setting the laserpulse energy to 54 J at a repetition rate of 2 kHz. Therelationship between the inclined cutting depth and the num-ber of passes of the beam is plotted in Fig. 3. After the firstpass of the beam, a narrow trench with a width of 20 mand a depth of 220 m was formed. Successive scans ofthe beam along the trench results in further deepening andwidening, but the extent was increasing less. The depth ofthe trench depends on the effective penetration of the beam.From the second scan onwards, the beam has to pass throughthe narrow gap before reaching the bottom of the trench forfurther machining. The energy available at this point is at-tenuated, partly due to lateral machining of the channelcausing undesirable widening, absorption, and diffractioneffects. Therefore, the depth of the trench tends to saturateafter multiple scans. For comparison, vertical trenches wereFIG. 2. Color online Cross-sectional optical microphotograph of laser mi-cromachined microtrenches at an inclination of 45° at a range of scancycles of between 1 and 10.micromachined with the tilting mirror removed, an example1050 Wang, Lai, and Choi: Laser micromachining of optical microstructures 1050of which is shown in the cross-sectional optical image of Fig.4a. Both the intersecting vertical and oblique cuts wereformed after three passes of the scanning beam. A three-dimensional 3D field-emission scanning-electron micro-scope FE-SEM view of the same structure with the trian-gular section removed is shown in Fig. 4b. It is worthnoting that after the insertion of an additional tilting mirror,the ablation efficiency is reduced, the extent of which scalesinversely with the angle of the incident beam, as is evidentfrom the plot in Fig. 3. This may be attributed topolarization-dependent reflectivity and deformation of thebeam spot.Apart from linear micromachining, it is also possible tocreate three-dimensional microstructures using this modifiedmicromachining scheme. One example is rotary microma-chining for the preparation of conic structures. In this case,the sample is mounted on a motorized rotation stage whilethe beam is focused and incident at a fixed distance R fromthe rotary axis. The sample is micromachined while the stageis rotated. Since the incident beam is tilted at an obliqueangle, an inverted conical structure is drilled through thesapphire substrate after 10 cycles of rotation, as illustrated inFigs. 5a and 5b. The radius of the top brim is determinedby the position of the incident beam with respect to the ro-tary axis, while the radius of the lower brim depends on theincident angle and the depth of the conics and thus the num-ber of rotations of the beam. The top and bottom radii, asmeasured from the plan view optical microphotograph inFig. 5b, are 420 and 140 m, respectively. Since theincident angle of the beam was tuned to 45°, the length of theinclined surface is estimated at 400 m. Deviation of circu-larity mainly stems from microscale wobble of the rotarystage.The formation of inclined profiles with laser microma-chining in semiconductor and optical materials can poten-tially be exploited to create chip geometries which can assistwith light extraction, such as the truncated-inverted-pyramid8FIG. 3. Depth of microtrenches as a function of scan cycles for beam inci-dent angles of 45° and 60°.structure or the inverted conical structure as illustrated inJ. Vac. Sci. Technol. B, Vol. 27, No. 3, May/Jun 2009Fig. 4. The inclined plane or curved surface can be mirrorcoated to act as a reflector in LEDs, enhancing light extrac-tion efficiency and narrowing emission patterns. This tech-nology can also be applied in the fabrication of other devicesfeaturing diversified three-dimensional microstructures, suchas devices and structures for optical communication,9,10 mi-crofluidic chemical channel,11 and medical applications.12The material used in our study—sapphire—is known to bethe second hardest material; repeating the results in mostother materials should be trivial. The sedimentary by-productfrom laser ablation in the form of metal oxide and nitride canbe removed by soaking in diluted 18% HCl solution for 5min, followed by sonication in de-ionized water. The opticalmicrophotographs in Figs. 6a and 6b show the surfaceconditions of a laser-ablated sample before and after thecleaning process, testifying to the effectiveness of damageremoval.After the chemical treatment, the surface morphologies ofthe micromachined samples are examined with atomic forcemicroscopy AFM. 3D images of the AFM scans are shownin Fig. 7. The surface topography of the sapphire surfaceFIG. 4. Color online a Cross-sectional optical image showing an inter-secting vertical 3 cycles and an oblique cut 60°, 5 cycles on sapphire.b FE-SEM image showing the 3D view of the structure in a, with thetriangular column removed.after two machining cycles exhibits a uniform roughness1051 Wang, Lai, and Choi: Laser micromachining of optical microstructures 1051with a rms. value of 150 nm. With more cycles, increasingdensities and dimensions of granules are observed on theAFM image and the rms roughness increased to 218 nmafter 5 cycles. The formation of the larger grains on the sur-face is a result of uneven aggregation and resolidification ofthe melted material. The evacuation rate of the ablated spe-cies declines as the beam reaches deeper into the trench, andFIG. 5. Color online Laser micromachined conic section in a plan viewand b 3D view.FIG. 6. Color online a Surface optical microphotograph of a laser-patterned sapphire sample prior to cleaning treatment. b The same sampleafter a cleaning process of soaking in 18% HCl solution for 5 min, followedwith 20 s of sonication.JVST B - Microelectronics and Nanometer Structuresstatistically the density of aggregation is more pronounced atthese deeper sites. After successive scan cycles, the aggrega-tion of the melt during micromachining resulted in unevensidewall roughening. An inclined surface machined with 10cycles exhibits different rms roughnesses at different depths,making it difficult to find a representative value. This findingis corroborated with the FE-SEM images, as shown in Figs.8a–8f. The micromachined surface generally consists of astreaky patterning running along the inclined plane, whichmay be due to wobbling of the stage during linear motion.On top of this pattern are clusters of solids due to melt reso-lidification. The scale of these clusters increases in area uponincreasing scan cycles. Nevertheless, these clusters are typi-cally very thin and easily removed with a chemical and me-chanical post-treatment. Laser micromachining is a nonlinearprocess that begins with the absorption of the laser pulse,followed by material breakdown and ablation. Generally,shorter pulse widths help minimize the heat-affected zoneand produce sharp features. The nanosecond laser pulsesFIG. 7. Color online AFM morphology images of the inclined sapphiresurfaces after laser micromachining for a 2 cycles and b 5 cycles. Thecorresponding rms roughnesses are 150 and 218 nm, respectively.used in our experiment proves its capability for this purpose.1052 Wang, Lai, and Choi: Laser micromachining of optical microstructures 1052As the pulse fluence from the laser far exceeds the criticalfluence, ablation takes place in the very early stages of pulsedeposition within subnanosecond or even shorter time scale,disregarding the specific mechanisms of the nonlinear opticalprocess. The continuing plasma absorption of the remainingpulse and heating of the ions is accompanied by the expul-sion of the ablated vapor or melt driven by the vapor pres-FIG. 8. Low and high magnification FE-SEM images of the inclined micro-machined surfaces after a and b 2 cycles, c and d 5 cycles. and eand f 10 cycles.sure and the recoil of the light pressure. Therefore, the heat-J. Vac. Sci. Technol. B, Vol. 27, No. 3, May/Jun 2009affected zone due to heat diffusion is reduced by the fact thata large amount of the absorbed energy is carried away by theaccompanying vaporization.In summary, a laser micromachining setup for the forma-tion of high-aspect-ratio trenches at oblique angles is devel-oped. The key element of the setup is the insertion of a laserturning mirror between the focusing optics and the plane ofthe sample. The process has been successfully demonstratedon sapphire wafers; it can therefore be adopted for the dicingof GaN LED dies with angled chip profiles, a geometrywhich is beneficial for light extraction. Experiments showthat there is little compromise on ablation efficiency. Laser-induced damaged, in the form of melt resolidification, isreadily removed with a chemical and mechanical treatment.ACKNOWLEDGMENTSThis work was jointly supported by a CERG grant of theResearch Grant Council of Hong Kong Project No. HKU7121/06E and the University Development Fund of the Uni-versity of Hong Kong Nanotechnology Research Institute,Project No. 00600009.1A. Elgawadi, J. Krasinski, G. Gainer, A. Usikov, and V. Dmitriev, J.Appl. Phys. 103, 123512 2008.2Y. G. Tian and Y. C. Shin, J. Am. Ceram. Soc. 89, 3397 2006.3Li Lü, J. Fuh, Y.-S. Wong, Laser-Induced Materials and Processes forRapid Prototyping Kluwer Academic Publishers, Norwell, 2001.4E. Gu, C. W. Jeon, H. W. Choi, G. Rice, M. D. Dawson, E. K. Illy, and M.R. H. Knowles, Thin Solid Films 453–454, 462 2004.5T. Otani, L. Herbst, M. Heglin, S. V. Govorkov, and A. O. Wiessner,Appl. Phys. A: Mater. Sci. Process. 79, 1335 2004.6X. Liu, D. Du, and G. Mourou, IEEE J. Quantum Electron. 33, 17061997.7R. Singh, M. J. Alberts, and S. N. Melkote, Int. J. Mach. Tools Manuf.48, 994 2008.8M. R. Krames et al., Appl. Phys. Lett. 75, 2365 1999.9M. M. Ragheb, H. H. El-Refaei, D. Khalil, and O. A. Omar, J. LightwaveTechnol. 25, 2531 2007.10S. Janz et al., IEEE J. Sel. Top. Quantum Electron. 12, 1402 2006.11J. M. Kohler and M. Zieren, Thermochim. Acta 310, 25 1998.12Y. Torisawa, H. Shiku, T. Yasukawa, M. Nishizawa, and T. Matsue, Bio-materials 26, 2165 2005.

Laser micromachining of optical microstructures with inclined sidewall profile

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Laser micromachining of optical microstructures with inclined sidewall profile

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