micromachines

Article Machining of Lenticular Silicon Molds with a Combination of Laser Ablation and Diamond Cutting

Jide Han, Lihua Li * and Wingbun Lee

The State Key Laboratory of Ultraprecision Machining Technology, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China; [email protected] (J.H.); [email protected] (W.L.) * Correspondence: [email protected]; Tel.: +852-6355-5356

 Received: 11 March 2019; Accepted: 14 April 2019; Published: 16 April 2019 

Abstract: Lenticular are widely used in the three-dimensional display industry. Conventional lenticular lens components are made of plastics that have low thermal stability. An alternative is to use glass to replace plastic as the lenticular lens component material. Single crystal silicon is often used as the mold material in the precision glass molding process. It is, however, difficult to fabricate a lenticular lens silicon mold that has a large feature size compared to the critical depth of cut of silicon. In order to solve the problems of machining lenticular lens silicon molds using the conventional diamond cutting method, such as low machining efficiency and severe tool wear, a hybrid machining method that combined laser ablation and diamond cutting was proposed. A feasibility study was performed to investigate the possibility of using this method to fabricate a lenticular lens silicon mold. The influence of the laser parameters and machining parameters on the machining performance was investigated systematically. The experimental results indicated that this hybrid machining method could be a possible method for manufacturing lenticular lens silicon molds or other similar microstructures.

Keywords: hybrid machining; laser ablation; diamond cutting; lenticular lens; single crystal silicon

1. Introduction Autostereoscopic display technology is a glass-free, three-dimensional (3D) technology that allows people to acquire a 3D perception of images without wearing specialized headgear or . There are mainly two types of autostereoscopic display techniques: the type and the lenticular lens type [1]. With the parallax barrier type display, 50% of the light that each eye can observe is blocked by the barrier. Therefore, there is a great light efficiency loss in this type of display. For the lenticular lens type, the 3D effect is realized by the function of lenticular lenses, which can deliver different images to the right and left eyes separately. Light is not blocked, thus the throughput of light in the lenticular lens display is much higher than that of the parallax barrier display. Conventionally, the lenticular lens is made of plastic using the injection molding method. However, the reliability of this kind of display is low because the thermal expansion between the lenticular lens and the liquid crystal display (LCD) panel is different, thus causing a location shift between them [2]. A lenticular lens made of glass is therefore needed for high quality large 3D-LCD panels. Precision glass molding is a promising method in the large-scale industrial production of lenticular lenses. Single crystal silicon is an important mold material used in the precision glass molding industry due to its high thermal stability and available processing methods [3–5]. However, silicon is a brittle material that is relatively difficult to cut using a diamond tool because cracks are easily formed on the machined surfaces during the cutting process [6,7]. By controlling the depth of cut to less than a critical value, crack-free machining can be achieved [8–12]. Since the critical depth of cut of single

Micromachines 2019, 10, 250; doi:10.3390/mi10040250 www.mdpi.com/journal/micromachines Micromachines 2019, 10, 250 2 of 11 crystal silicon is only a few hundred nanometers [13], it is still a rather time consuming process to fabricate microstructures with feature sizes much larger than the value of the critical depth of cut. A lenticular lens is such a microstructure. The depth of the lenticular lens is typically in the order of tens of micrometers [14]. This means that generating this kind of microstructure on a silicon workpiece surface with the ductile region machining method will have extremely low efficiency. It is not difficult to estimate that even creating one groove will require hundreds of passes with diamond cutting. Therefore, the tool wear problem will be very serious due to the long tool workpiece contact time. The poor machinability of silicon leads to low machining efficiency. There are several approaches to improve the machinability of silicon, such as increasing the critical depth of cut through modifying the material property of the cutting region with the ion implantation method [15,16], or using a laser to assist machining in the diamond cutting process [17,18]. These methods, however, can only improve the machining performance of silicon to a limited extent. Nonconventional machining, or hybrid machining methods, are also possible alternatives to the diamond cutting method in generating large feature sized microstructures on brittle materials. Youn et al. [19] demonstrated the possibility of using focused ion beam (FIB) milling to produce microstructures with good surface quality (Ra = 4–30 nm) on glassy carbon though the efficiency is very low. Kim et al. [20] successfully fabricated a lenticular lens fused silica mold, which was then used for polymer replication by a combined method of femtosecond laser milling and CO2 laser polishing. They could achieve a surface roughness of less than 10 nm after the laser polishing process. However, the limitation of this method is that it is difficult to predict the exact profile of the output surface, since the subsequent laser polishing process will change the topography of the surface created by femtosecond laser milling in an uncertain way. Thus, to obtain a designed surface texture within a reasonable form error, extensive trial and error is needed. Other examples of combined machining methods that follow the paradigm of the rough-finish sequential process are laser ablation followed by electrical discharge machining (EDM) [21], femtosecond laser milling followed by FIB finishing [22], and electrochemical discharge machining (ECDM) followed by micro grinding [23]. To the authors’ knowledge, there is no research into the combined machining of laser ablation and diamond cutting, especially for machining brittle materials, such as single crystal silicon. In order to improve the machining efficiency, as well as reduce diamond tool wear in fabricating lenticular lens silicon mold inserts, a new hybrid machining method combining laser ablation and diamond cutting was proposed in this research. This hybrid method also followed the paradigm of the rough-finish process. The laser ablation served as a rough machining process to remove the majority of the material, and the diamond cutting was the finishing process to shape the grooves, created by laser ablation, to their final status.

2. Materials and Methods

2.1. Experimental Process Figure1 illustrates the process of this hybrid machining method. First, the workpiece material was scanned by a high power nanosecond laser beam. By controlling the laser scanning parameters, grooves with pre-designed shapes were generated on the silicon workpiece surface. Second, the diamond tool was be aligned to the same position of the grooves generated by laser to remove the remaining material and perform finish cutting. Micromachines 2019, 10, 250 3 of 11 Micromachines 20192019,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 3 3 of of 11 11

Figure 1. Illustration of the hybrid machining method. Figure 1. IllustrationIllustration ofof thethe hyhybrid machining method. The flow chart of the hybrid machining process is shown in Figure2. First, the influence of the The flow chart of the hybrid machining process is shown in Figure 2. First, the influence of the laser ablation parameters, such as scan speed, laser power and defocus depth on ablation depth was laserlaser ablationablation parameters,parameters, suchsuch asas scanscan speed,speed, lalaser power and defocus depth on ablation depth was investigated. Second, grooves with pre-designed shapes were generated by the laser ablation method investigated.investigated. Second,Second, groovesgrooves withwith pre-designedpre-designed shapshapes were generated by the laser ablation method withwith the the optimized optimized laser laser ablation ablation parameters. parameters. Third, Third, toolird, tool workpiecetool workpieceworkpiece re-alignment rere-alignment-alignment and diamondandand diamonddiamond cutting experimentscutting experiments were performed. were performed.

Figure 2. Flow chart of the hybrid machining process. FigureFigure 2. 2.Flow Flowchart chart ofof thethe hybridhybrid machining process. process.

2.2.2.2. Laser Laser Ablation Ablation Experiment Experiment TheThe laser laser ablation ablation experiment experiment was was conductedconducted on the homemade laser laser micro-machining micro-machining setup setup developeddeveloped by by the the authors, authors, which which is is shown shown in in FigureFigure3 3.. TheThe laserlaser being used used was was a a nanosecond nanosecond pulsed pulsed laserlaserlaser with withwith a wavelengthaa wavelengthwavelength of ofof 2 22µ µm.µm.m. The TheThepulse pulsepulse durationdurationduration and repetition repetition rate rate of of this this laser laser were were 30 30 ns ns and and 3030 kHz, kHz, respectively. respectively. The The rated rated average average laserlaser powerpower was was 10 10 W W and and the the peak peak power power was was 10 10 kW. kW. The The workpiece material was single crystal silicon. In order to find the optimum laser machining workpieceworkpiece material material was was single single crystal crystal silicon. silicon. In order In toorder find to the find optimum the optimum laser machining laser machining parameters parameters for a designed value of groove depth, the influence of laser scan speed, laser power, and forparameters a designed for value a designed of groove value depth, of groove the influence depth, ofthe laser influence scan speed,of laser laser scan power,speed, andlaser defocus power, and depth defocus depth on ablation depth was investigated. The definition of the defocus depth is illustrated ondefocus ablation depth depth on was ablation investigated. depth was The investigated. definition of Th thee definition defocus depthof the isdefocus illustrated depth in is Figure illustrated4, and inin FigureFigure 4,4, andand refersrefers toto thethe distancedistance betweenbetween thethe workpieceworkpiece surfacesurface andand laserlaser beambeam focalfocal point.point. refers to the distance between the workpiece surface and laser beam focal point.

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Figure 3. Laser micro-machining setup. FigureFigure 3.3. Laser Laser micro-machining micro-machining setup.setup.

Figure 4. Illustration of the definition of defocus depth. Figure 4. Illustration of the definition of defocus depth. Figure 4. IllustrationIllustration of of the defi definitionnition of defocus depth. There are several methods to produce a groove of a given depth, such as changing laser power, There are several methods to produce a groove of a given depth, such as changing laser power, There are several methods to produce a groove of a given depth, such such as as changing changing laser laser power, power, scanscan speed speed or ordefocus defocus depth. depth. It It is is relatively relatively eaeasiersier to adjust thethe defocusdefocus depth depth by by a anumerically numerically scan speed or defocus depth. It is relatively easier to adjust the defocus depth by a numerically controlledscancontrolled speed 3-degree or 3-degree defocus of of freedom depth.freedom It(DOF) (DOF)is relatively motion motion eaplatformplatformsier to thanadjust itit istheis toto defocusadjust adjust the the depth laser laser powerby power a numerically and and scan scan controlled 3-degree of freedom (DOF) motion platform than it is to adjust the laser power and scan speed.controlledspeed. Therefore, Therefore, 3-degree in ofin this freedomthis experiment, experiment, (DOF) motion differentdifferent platform dedepths thanof laserlaser it is ablation ablationto adjust grooves groovesthe laser was waspower realized realized and byscan by speed. Therefore, in this experiment, different depths of laser ablation grooves was realized by changing changingspeed.changing Therefore, the the defocus defocus in thisdepth. depth. experiment, different depths of laser ablation grooves was realized by changingthe defocusFigureFigure the 5 depth. showsdefocus5 shows the depth.the procedure procedure for for determining determining the defocus depthdepth profile. profile. First, First, the the actual actual depth depth Figure5 shows the procedure for determining the defocus depth profile. First, the actual depth of of ofthe Figurethe designed designed 5 shows lenticular lenticular the procedure lens lens at at for differentdifferent determining positions,positions, the defocusDnn,, waswas depth calculated.calculated. profile. Then Then First, according according the actual to todepththe the relationshipofthe relationshipthe designed designed between lenticular between lenticular the lens the laser atlaserlens di ffablation erentablationat different positions, depthdepth positions, ananDndd, wasdefocus calculated.Dn, wasdepthdepth calculated. obtained Thenobtained according from Thenfrom the totheaccording theexperimental experimental relationship to the result,relationshipbetweenresult, the thethe defocus laserdefocusbetween ablation depth, depth, the depthdlaser nd, nwas, was ablation and calculated. calculated. defocus depth depth TheThe an intervalintervald obtained defocus betweenbetween fromdepth the two twoobtained experimental laser laser scanning scanningfrom result,the paths pathsexperimental the was defocuswas set set toresult,depth, beto be5 thedµm. 5n ,µm. wasdefocus calculated. depth, d Then, was interval calculated. between The two interval laser scanningbetween pathstwo laser was scanning set to be 5pathsµm. was set to be 5 µm.

Figure 5. The procedure to determine defocus depth profile. Figure 5. TheThe procedure procedure to to determine determine defocus depth profile. profile. Figure 5. The procedure to determine defocus depth profile.

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Micromachines 2019, 10, x FOR PEER REVIEW 5 of 11 2.3. Diamond Cutting Experiment 2.3. Diamond Cutting Experiment After the laser ablation process, grooves with a rough shape were obtained on a silicon workpiece After the laser ablation process, grooves with a rough shape were obtained on a silicon surface. However, the surface quality of the laser-generated grooves was very unsatisfactory. In laser workpiece surface. However, the surface quality of the laser-generated grooves was very ablation, instead of directly removing it from the workpiece surface, the melt-off material piled up unsatisfactory. In laser ablation, instead of directly removing it from the workpiece surface, the melt- on the workpiece surface and subsequently needed to be removed by the diamond cutting process. off material piled up on the workpiece surface and subsequently needed to be removed by the Since the laser ablation experiment was performed on the laser micro-machining equipment, when the diamond cutting process. Since the laser ablation experiment was performed on the laser micro- workpiece was moved to the diamond cutting machine, tool alignment was needed to make the groove machining equipment, when the workpiece was moved to the diamond cutting machine, tool axis comply with the diamond tool travel axis. Subsequently, diamond cutting with different finish alignment was needed to make the groove axis comply with the diamond tool travel axis. depths of cuts was conducted to investigate the influence of the depth of cut on the surface quality of Subsequently, diamond cutting with different finish depths of cuts was conducted to investigate the the machined grooves. influence of the depth of cut on the surface quality of the machined grooves. The diamond cutting experiment was performed on a Nanoform 200 ultra-precision lathe. Figure6 The diamond cutting experiment was performed on a Nanoform 200 ultra-precision lathe. shows the setup of the diamond cutting experiment. A charge-coupled device (CCD) camera was Figure 6 shows the setup of the diamond cutting experiment. A charge-coupled device (CCD) camera mounted on the lathe to assist with the alignment work. The tilt stage was used to tilt the workpiece was mounted on the lathe to assist with the alignment work. The tilt stage was used to tilt the surface to make sure the cutting process could be performed under a constant depth of cut. The Y workpiece surface to make sure the cutting process could be performed under a constant depth of motion stage was used to adjust the position of the workpiece along the vertical direction. cut. The Y motion stage was used to adjust the position of the workpiece along the vertical direction.

Figure 6. Diamond cutting experimental setup.

3. Results and Discussion

3.1. Laser Ablation Experimental Results Figure 77 shows shows aa typicaltypical opticaloptical microscopicmicroscopic imageimage ofof thethe cross-sectionalcross-sectional viewview ofof laserlaser ablatedablated grooves onon a a silicon silicon workpiece workpiece surface. surface. The The laser la powerser power was 4.3was W 4.3 and W the and scan the speed scan was speed 50 mm was/min 50 inmm/min this case. in this The case. workpiece The workpiece surface wassurface adjusted was adjust to beed at theto be focal at the point focal of point the laser of the beam. laser All beam. the groovesAll the grooves in this figurein this were figure generated were generated with the with same the laser same parameters. laser parameters.

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Figure 7. Cross-section view of laser-machined grooves on the silicon workpiece surface. Figure 7. Cross-section view of laser-machined grooves on the silicon workpiece surface. It can be seen from this image that the depths of these laser-generated grooves were almost the same,It which can be means seen from the thisrepeatability image that of the the depths laser ablation of these process laser-generated was reliable, grooves and were thus almostacceptable the same,for our whichwhich purpose. meansmeans Laser thethe repeatability repeatability ablation experiments of of the the laser laser ablationwith ablation changing process process waslaser was reliable, power reliable, andand and thus scan thus acceptable speed acceptable were for ourforconducted. purpose.our purpose. Figure Laser Laser8 ablation shows ablation the experiments experimental experiments with result changing withs thatchanging laser indicated power laser the and power influence scan speedand of scan werelaser speed conducted.power were and Figureconducted.scan speed8 shows Figureon the laser experimental8 showsablation the depth. experimental results A that linear indicated result relationships that the influenceindicated between ofthe laserlaser influence powerablation of and laserdepth scan power speedand laserand on laserscanpower ablationspeed can beon depth.seen. laser A Aablation linear linear relationship relationshipdepth. A linear can between simila relationship laserrly be ablation seen between between depth laser andthe ablation laser laser ablation power depth can depthand be laser seen. and Apowerscan linear speed. can relationship be seen. A can linear similarly relationship be seen can between simila therly be laser seen ablation between depth the andlaser scan ablation speed. depth and scan speed.

FigureFigure 8.8. The influenceinfluence ofof thethe laserlaser powerpower andand scanscan speedspeed onon laserlaser ablationablation depth.depth. Figure 8. The influence of the laser power and scan speed on laser ablation depth. FigureFigure9 9 shows shows the the experimental experimental results results of of the the influence influence of of defocus defocus depth depth on on the the laser laser ablation ablation depth.depth.Figure The 9 laser shows scanscan the speedspeed experimental waswas fixedfixed results toto 400400 of mmmm/mi the/min influencen forfor allall of experiments.experiments. defocus depth UnlikeUnlike on the thethe laser relationshiprelationship ablation betweendepth.between The laserlaser laser ablationablation scan speed depthdepth was andand fixed laserlaser to power,power, 400 mm/mi whichwhichn couldcould for all bebe experiments. depicteddepicted withwith Unlike aa linearlinear the function,function,relationship thethe influencebetweeninfluence laser ofof defocusdefocus ablation depthdepth depth onon andlaserlaser laser ablationablation power, depthdepth whic waswash could aa non-linear non-linear be depicted e effect.ffect. with a linear function, the influence of defocus depth on laser ablation depth was a non-linear effect.

FigureFigure 9.9. The influenceinfluence ofof defocusdefocus depthdepth onon laserlaser ablationablation depth.depth. Figure 9. The influence of defocus depth on laser ablation depth.

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The cross-sectionalcross-sectional profile profile of of the the lenticular lenticular lens lens groove groove was anwas arc. an In arc. this In experiment, this experiment, two grooves two withgrooves diff erentwith depthsdifferent were depths generated were generated in the laser in ablation the laser process. ablation The process. design The arcradius design of arc the radius grooves of wasthe grooves 0.5 mm, was and 0.5 the mm, depths and of the the depths grooves of were the grooves 50 µm and were 80 50µm. µm The and width 80 µm. of theThe grooves width of were the calculatedgrooves were to be calculated 0.436 mm to and be 0.436 0.543 mm,mm and respectively. 0.543 mm, The respectively. width of a The single width laser of ablated a single groove laser wasablated around groove 5 µ m,was which around could 5 µm, be which estimated could from be es Figuretimated7. Thus,from toFigure generate 7. Thus, one to lenticular generate lens one groove,lenticular tens lens or groove, even more tens than or even one hundred more than repetitions one hundred of laser repetitions scanning of passes laser werescanning required. passes For were the lenticularrequired. For lens the groove, lenticular the depths lens groove, were di thefferent depths at diwefferentre different positions at different of the arc. positions In the experimental of the arc. In process,the experimental this was realizedprocess, by this changing was realized the defocus by changing depth. The the procedure defocus depth. to determine The procedure the defocus to depthdetermine profiles the wasdefocus shown depth in Figureprofiles5. was The defocusshown in depth Figure profiles 5. The for defocus the designed depth profiles lenticular for lens the groovedesigned depths lenticular of 50 µlensm andgroove 80 µ mdepths were shownof 50 µm in Figureand 80 10 .µm The were corresponding shown in laserFigure powers 10. The to producecorresponding these two laser grooves powers were to produce 4.3 W and these 5.3 two W, respectively.grooves were 4.3 W and 5.3 W, respectively.

µ µ Figure 10. Defocus depth profiles profiles for lenticular lens groove depths of (a)) 5050 µmm andand ((bb)) 8080 µm.m. The optical microscopic images of the cross-sectional profiles of the laser-ablated grooves are The optical microscopic images of the cross-sectional profiles of the laser-ablated grooves are shown in Figure 11. The red line shows the design lenticular lens profile. The experimental results shown in Figure 11. The red line shows the design lenticular lens profile. The experimental results agreed well with the design profiles. agreed well with the design profiles.

µ Figure 11.11. Cross-sectional profilesprofiles of of laser-machined laser-machined grooves grooves with with designed designed depth depth of (ofa) ( 50a) 50m µm and and (b) µ 80(b) 80m. µm. 3.2. Diamond Cutting Experimental Results 3.2. Diamond Cutting Experimental Results Figure 12 shows an image of the tool alignment process captured by CCD camera. After the tool Figure 12 shows an image of the tool alignment process captured by CCD camera. After the tool alignment work was completed, diamond cutting experiments were performed. The diamond cutting alignment work was completed, diamond cutting experiments were performed. The diamond cutting tool served as the shaper of grooves, previously generated by the laser, to give them the final shape. tool served as the shaper of grooves, previously generated by the laser, to give them the final shape. First, rough cutting was conducted to remove the laser deteriorated material that was still attached to First, rough cutting was conducted to remove the laser deteriorated material that was still attached the workpiece surface. The depth of the rough cut was set to be comparative to the laser grooved depth. to the workpiece surface. The depth of the rough cut was set to be comparative to the laser grooved Since the material in the region had been deteriorated by laser scanning and was no longer physically depth. Since the material in the region had been deteriorated by laser scanning and was no longer bonded to the base material, it could be removed easily without inducing large cracks on the machined physically bonded to the base material, it could be removed easily without inducing large cracks on workpiece surface, even when the depth of cut was as large as tens of micrometers. Second, finish the machined workpiece surface, even when the depth of cut was as large as tens of micrometers. cutting was performed to investigate the influence of the depth of cut on groove surface roughness. Second, finish cutting was performed to investigate the influence of the depth of cut on groove surface roughness.

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FigureFigure 12.12. charge-coupledCharge-coupledcharge-coupled devicedevice (CCD)(CCD) cameracamera capturedcaptured image image shows shows the the tool tool alignment alignment process. process.

FigureFigure 13 1313 shows showsshows the thethe groove groovegroove surface surfacesurface profiles profilesprofiles measured measuredmeasured by Zygo by (Newviewby ZygoZygo (Newview 8000,(Newview Zygo Corporation,8000,8000, ZygoZygo Corporation,Middlefield,Corporation, CT, Middlefield,Middlefield, USA) after CT,CT, finish USA)USA) cutting afterafter with finishfinish di ff cutticuttierentngng depths withwith differentdifferent of cuts. For depthsdepths all of ofof the cuts.cuts. grooves, ForFor allall the ofof total thethe grooves,cuttinggrooves, depth thethe totaltotal was cuttingcutting the same. depthdepth For waswas example, thethe same.same. if the ForFor total example,example, finish cutting ifif thethe totatota depthll finishfinish was cutting 4cuttingµm, it depthdepth would waswas need 44 µm,µm, one itpassit wouldwould for a needneed depth oneone of pass cutpass of forfor 4 µ aa m depthdepth and oftwentyof cutcut ofof passes 44 µmµm andforand a twentytwenty depth of passespasses cut of forfor 0.2 aa µ depthdepthm. ofof cutcut ofof 0.20.2 µm.µm.

FigureFigure 13. 13. GrooveGroove surface surface profiles profilesprofiles after after finish finishfinish cu cuttingcuttingtting with with differentdidifferentfferent depthsdepths ofof cut.cut.

Figure 14 shows the relationship between the depth of cut and groove surface roughness. The surface roughness was measured on the bottom of the grooves with a sampling area of 250 µm × 250 µm. It can be seen that the surface roughness decreased with a decreasing depth of cut. When the depth of cut was 4 µm, large cracks frequently formed on the machined groove surface, which deteriorated the machined surface quality. When the depth of cut was 0.2 µm, the machined groove surface appeared to be much smoother with fewer and smaller cracks formed. The surface roughness was thus dramatically reduced compared with those machined with larger depths of cuts.

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FigureFigure 14.14. Relationship betweenbetween depthdepth ofof cutcut andand surfacesurface roughness.roughness.

The main focus of this paper is on the feasibility of using this hybrid method to machine lenticular Figure 14 shows the relationship between the depth of cut and groove surface roughness. The surface lensroughness microstructures was measured on a siliconon the bottom workpiece of the surface. grooves Since with most a samp of theling material area of 250 is removed µm × 250 in µm. the It laser can ablationbe seen that step, the which surface is a relativelyroughness fast decreased and low with cost a process, decreasing this machiningdepth of cut. method When isthe expected depth of to cut be highlywas 4 µm, efficient, large as cracks well asfrequently cost effective, formed compared on the machined to the conventional groove surface, diamond which cutting deteriorated method. the In thismachined research, surface a preliminary quality. When experimental the depth approach of cut was was 0.2 successfully µm, the machined performed. groove The surface surface appeared quality ofto thebe machinedmuch smoother grooves with was fewer evaluated and tosmaller demonstrate cracks theformed. feasibility The surface of using roughness this hybrid was method thus todramatically machine lenticular reduced lenscompared silicon with mold those inserts machined in terms with of improving larger depths machining of cuts. e fficiency, as well as reducingThe toolmain wear. focus of this paper is on the feasibility of using this hybrid method to machine lenticularHowever, lens microstructures there are limitations on a ofsilicon this research. workpiec Fore surface. example, Since even most though of the the material machined is removed surface qualityin the laser could ablation be improved step, which significantly is a relatively by reducing fast and the low finish cost depthprocess, of this the machining cut, the best method surface is roughnessexpected to of be the highly machined efficient, groove, as well which as cost was effective, in the order compared of 100 nm,to the still conventional could not fulfilldiamond the requirementcutting method. of a practicallyIn this research, useful a high preliminary quality lenticular experimental lens moldapproach insert. was There successfully are several performed. potential methodsThe surface to further quality improve of the machined the machining grooves performance was evaluated of this hybridto demonstrate method, such the feasibility as using a negativeof using rakethis hybrid angle diamond method tool,to machine or further lenticular reducing lens the si finishlicon depthmold ofinserts the cut in toterms below of theimproving critical depth machining of cut ofefficiency, single crystal as well silicon. as reducing Another tool limitation wear. of this method is that, at present, the two machining steps wereHowever, conducted there separately are limitations on two independent of this research. machine For example, systems, whicheven though brings the about machined the problem surface of re-alignmentquality could before be improved the second significantly machining by step reducing was conducted. the finishIn depth the future, of the thiscut,can the bebest solved surface by combiningroughness theof the two machined steps into onegroove, machine which system. was in the order of 100 nm, still could not fulfill the requirementMachining of a highly practically brittle useful materials high quality using conventional lenticular lens methods mold insert. is a long-termThere are several challenge. potential This workmethods was to a pilotfurther research improve study the of machining using hybrid performa methodsnce to machineof this hybrid highly method, brittle materials. such as Furtherusing a worknegative is still rake required angle diamond to make thistool, method or further more reducing applicable the finish for the depth fabrication of the ofcut lenticular to below lenses.the critical 4.depth Conclusions of cut of single crystal silicon. Another limitation of this method is that, at present, the two machining steps were conducted separately on two independent machine systems, which brings aboutIn the this problem research, of a re-alignment hybrid machining before method the second that combinedmachining laser step ablationwas cond anducted. diamond In the cuttingfuture, wasthis proposedcan be solved to machine by combining a lenticular the lenstwo siliconsteps into mold. one A machine feasibility system. study was performed to investigate the feasibilityMachining of highly using thisbrittle method materials to fabricate using conventional a lenticular lens methods silicon is mold. a long-term The influence challenge. of laserThis power,work was laser a pilot scan speedresearch and study defocus of using depth hybrid on the methods laser ablation to machine depth highly was investigated. brittle materials. It was Further found thatwork there is still was required a linear to relationship make this method between more laser applicable power and for laser the ablationfabrication depth, of lenticular and between lenses. laser scan speed and laser ablation depth, whereas, the influence of defocus depth on laser ablation depth was4. Conclusions a non-linear effect. In the diamond cutting process, the machined surface quality was able to be improved significantly by reducing the finish depth of the cut. The experimental results indicate In this research, a hybrid machining method that combined laser ablation and diamond cutting that this could be a feasible method of manufacturing lenticular lens silicon molds or other similar was proposed to machine a lenticular lens silicon mold. A feasibility study was performed to microstructures with improved machining efficiency, as well as reduced tool wear. investigate the feasibility of using this method to fabricate a lenticular lens silicon mold. The influence of laser power, laser scan speed and defocus depth on the laser ablation depth was investigated. It was found that there was a linear relationship between laser power and laser ablation depth, and between laser scan speed and laser ablation depth, whereas, the influence of defocus depth on laser

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Author Contributions: Conceptualization, L.L., W.L.; Methodology, J.H., L.L.; Validation, J.H.; Writing—Original Draft Preparation, J.H.; Writing—Review and Editing, L.L., W.L.; Supervision, W.L. Funding: The authors would like to express their sincere thanks to the Research Committee of Hong Kong Polytechnic University and the Research Grants Council of the Hong Kong Special Administrative Region of the People’s Republic of China (Project code: PolyU RTTR). Conflicts of Interest: The authors declare no conflict of interest.

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