Injection Compression Molded Microlens Arrays for ... - MDPI

micromachines Article

Injection Compression Molded Microlens Arrays for Hyperspectral Imaging Marcel Roeder 1,2, * ID , Marc Drexler 1 , Thilo Rothermel 1 , Thomas Meissner 1,3 , Thomas Guenther 1 and André Zimmermann 1,2 ID 1

2 3

*

Hahn-Schickard, Allmandring 9B, 70569 Stuttgart, Germany; [email protected] (M.D.); [email protected] (T.R.); [email protected] (T.M.); [email protected] (T.G.); [email protected] (A.Z.) Institute for Micro Integration (IFM), University of Stuttgart, Allmandring 9B, 70569 Stuttgart, Germany Balluff GmbH, Schurwaldstraße 9, 73765 Neuhausen auf den Fildern, Germany Correspondence: [email protected]; Tel.: +49-711-6858-3729

Received: 25 June 2018; Accepted: 17 July 2018; Published: 18 July 2018







Abstract: In this work, a polymer microlens array (MLA) for a hyperspectral imaging (HSI) system is produced by means of ultraprecision milling (UP-milling) and injection compression molding. Due to the large number of over 12,000 microlenses on less than 2 cm2 , the fabrication process is challenging and requires full process control. The study evaluates the process chain and optimizes the single process steps to achieve high quality polymer MLAs. Furthermore, design elements like mounting features are included to facilitate the integration into the final HSI system. The mold insert was produced using ultraprecision milling with a diamond cutting tool. The machining time was optimized to avoid temperature drifts and enable high accuracy. Therefore, single immersions of the diamond tool at a defined angle was used to fabricate each microlens. The MLAs were replicated using injection compression molding. For this process, an injection compression molding tool with moveable frame plate was designed and fabricated. The structured mold insert was used to generate the compression movement, resulting in a homogeneous pressure distribution. The characterization of the MLAs showed high form accuracy of the microlenses and the mounting features. The functionality of the molded optical part could be demonstrated in an HIS system by focusing light spectrums onto a CCD image sensor. Keywords: microlens array; ultraprecision milling; injection compression molding; microstructures; polymer optics; hyperspectral imaging

1. Introduction The applications for polymer optics are growing significantly in recent years. Advantages in the fabrication process and improvements in the material’s properties enable polymer optics to compete with traditional glass lenses. Applications for polymer optics can be found in the fields of medical engineering, automotive, illumination, sensors and measurement systems. Especially the comparably easy and fast fabrication of freeform and micro structured optics are significant advantages compared to traditional glass lenses. Therefore, limitations in the available refraction indices of transparent polymer materials can be compensated by advanced form and optical design. Examples for microstructured optics are Fresnel lenses [1], microprism arrays [2], diffractive optical elements [3], and microlens arrays (MLA) [4]. In this work, a polymer MLA is fabricated. MLAs can be used in color imaging [5], fingerprint identification [6], 3D light field cameras [7] and hyperspectral imaging (HSI) [8]. The fabrication of MLAs can be performed with a multitude of methods. Examples are ultraprecision machining [9], Micromachines 2018, 9, 355; doi:10.3390/mi9070355

www.mdpi.com/journal/micromachines

Micromachines 2018, 9, 355

2 of 14

LIGA (Lithographie Galvanik Abformung) [10], E-beam writing [11], laser machining [12], and electric discharge machining [13]. The biggest challenge for most of the fabrication methods is to achieve optical surface quality with Ra < 10 nm. With E-Beam writing and laser machining, it is possible to create surfaces with surface roughness of about Ra = 10 nm and higher [14]. Sub-10 nm surface roughness remains challenging. The LIGA process however is able to achieve surface qualities of Ra < 10 nm, similar to the ultraprecision machining process [9,10]. The resulting surface quality is also strongly depending on the substrate material which is a limitation of all technologies. When high resolution and surface quality is needed, the machining has to be performed on soft or brittle materials which cannot be used in injection molding. Therefore, a subsequent electroplating process is mandatory to create a solid mold insert. An advantage of these technologies is the machining time which can be very fast, depending on size and shape of the MLA. The fabrication of MLA mold inserts by means of electric discharge machining is still in an early stage, so further improvements are necessary for the technology to compete with the other methods. Especially the resulting surface roughness with > 100 nm is not suitable for optical applications [15]. For this work, the MLA is fabricated using ultraprecision milling (UP-milling) and injection compression molding. The UP-milling is used to produce an optical mold insert that can be transferred into a molding tool. Therefore, a diamond milling tool with a defined cutting edge is used. UP-milling is broadly used for the fabrication of optical mold inserts, especially freeform optics [16] and MLAs [17]. The main advantages of the technology are the resulting optical surface quality with Ra < 10 nm without necessity of a post-treatment as well as very high form accuracy in the micrometer range [18]. However, diamond machining must be performed on non-ferrous materials when no special machining equipment like ultrasonic machining is available. Otherwise, increased tool wear will significantly affect the resulting surface quality and form accuracy. Therefore, nickel–phosphorous (NiP) coatings are usually applied for the fabrication of ultraprecision machined mold insert. Injection molding and hot embossing [19] are commonly used technologies for the replication of polymer optics. Especially injection compression molding is used, when high quality polymer optical components are required [20]. Injection compression molding is a process variant of regular injection molding. Thereby, a compression stamper is used to generate a homogeneous pressure distribution during the molding process. Thus, homogeneous material density within the polymer part is achieved, resulting in components with low birefringence. Furthermore, improved form accuracy is possible. A detailed description of the process can be found in the literature [21]. The MLA produced in this work is applied in a HSI system. Thereby each individual microlens focuses a light spectrum on a photo sensor. HSI has many applications in the fields of medicine [22], agriculture [23], astronomy [24] and food processing [25]. Due to the fact, that a spectrum is obtained for every microlens, detailed information on the investigated object can be acquired with spatial information. The aim of this paper is to demonstrate a successful process chain for the fabrication of polymer MLAs with focus on the mold design, mold insert fabrication by means of UP-milling and replication by injection compression molding. In the first section of the paper the design of the MLA and the molding tool are described as well as the fabrication methods. Subsequently, the results of the ultraprecision milling and the injection compression molding are shown. In the last section of the paper, the results are discussed and reviewed. 2. Materials and Methods 2.1. Microlens Array and Mold Design The MLA fabricated in this work comprises of a structured area of 13 × 5 mm. Every individual microlens has a radius of 1 mm and a spherical shape. The pitch between the microlenses is 127.5 µm, resulting in a total of about 12,000 lenses for the whole MLA. On the polymer MLA, the microlenses are convex with a sag height of 4 µm. Furthermore, the MLA includes mounting features to enable the

Micromachines 2018, 9, 355

3 of 14

Micromachines 2018, 9, x

3 of 14

integration into a HSI system. The microlenses have to be orientated in an angle of 26.56◦ to focus the light the pixels of athe photo The designed part sensor. thickness the polymer is 500 angleonto of 26.56° to focus lightsensor. onto the pixels of a photo Theofdesigned part MLA thickness ofµm. the The design of the MLA is shown in Figure polymer MLA is 500 µm. The design of the1.MLA is shown in Figure 1.

Figure 1. Design of the polymer MLA with the microstructured area and mounting features. features.

For the replication process, an injection molding tool is required. The molding tool is designed For the replication process, an injection molding tool is required. The molding tool is designed as an injection compression molding tool for main axis compression. For the injection compression as an injection compression molding tool for main axis compression. For the injection compression molding process the molding tool is equipped with a moveable frame plate. The frame plate can be molding process the molding tool is equipped with a moveable frame plate. The frame plate can be moved separately from the rest of the molding tool using pneumatic cylinders. Before every molding moved separately from the rest of the molding tool using pneumatic cylinders. Before every molding process, the frame plate is moved into a front position to close the cavity before melt injection. Thus, process, the frame plate is moved into a front position to close the cavity before melt injection. Thus, a compression movement can be performed after the material is injected into the cavity. This mold a compression movement can be performed after the material is injected into the cavity. This mold setup can generate high compression forces even on electrical molding machines. The molding tool setup can generate high compression forces even on electrical molding machines. The molding tool includes two mold inserts, each on one side of the molding tool, which form the shape of the includes two mold inserts, each on one side of the molding tool, which form the shape of the polymer polymer MLA. The mold insert on the fixed side represents the rear part of the MLA, consisting of a MLA. The mold insert on the fixed side represents the rear part of the MLA, consisting of a flat optical flat optical surface. On the moveable side, the mold insert with MLA structure is included. This surface. On the moveable side, the mold insert with MLA structure is included. This mold insert is mold insert is used as the moveable compression stamper during the molding process. More details used as the moveable compression stamper during the molding process. More details on the fabrication on the fabrication of the mold inserts are provided in the following sections. The molding tool of the mold inserts are provided in the following sections. The molding tool consists of a single cavity consists of a single cavity with a runner system through the center of the fixed side. with a runner system through the center of the fixed side. Additionally, the molding tool is equipped with a distance sensor, fixed onto the molding tool, Additionally, the molding tool is equipped with a distance sensor, fixed onto the molding tool, which is connected to the molding machine. In this way, accurate positioning of the molding tool which is connected to the molding machine. In this way, accurate positioning of the molding tool and compression stamper is possible which is mandatory to achieve accurate and repeatable results. and compression stamper is possible which is mandatory to achieve accurate and repeatable results. The distance sensor allows a more accurate positioning compared to the internal measurement The distance sensor allows a more accurate positioning compared to the internal measurement system system of the molding machine. Accurate positioning of the mold parts is a key element when of the molding machine. Accurate positioning of the mold parts is a key element when injection injection compression molding is employed. The molding tool has four separate cooling circuits compression molding is employed. The molding tool has four separate cooling circuits which are which are controlled by individual tempering systems. Each mold side is tempered individually by controlled by individual tempering systems. Each mold side is tempered individually by external external water tempering systems. Furthermore, the compression stamper and the distance sensor water tempering systems. Furthermore, the compression stamper and the distance sensor are equipped are equipped with separate tempering systems. Thus, precise temperature control of the individual with separate tempering systems. Thus, precise temperature control of the individual components is components is possible. A CAD drawing of the molding tool is shown in Figure 2. possible. A CAD drawing of the molding tool is shown in Figure 2.

Micromachines 2018, 9, 355 Micromachines 2018, 9, x

4 of 14 4 of 14

Figure2. 2. CAD CAD drawing drawing of of the theinjection injectioncompression compressionmolding moldingtool toolin inan anexploded explodedview. view. Figure

2.2. Mold Insert Fabrication 2.2. Mold Insert Fabrication The fabrication of the mold inserts is one of the most crucial steps in the fabrication of the The fabrication of the mold inserts is one of the most crucial steps in the fabrication of the polymer polymer MLAs. Inaccuracies and form deviations cannot be compensated in the replication process. MLAs. Inaccuracies and form deviations cannot be compensated in the replication process. The mold The mold inserts are fabricated using a five-axis ultraprecision machine (Freeform 700a, AMETEK inserts are fabricated using a five-axis ultraprecision machine (Freeform 700a, AMETEK Precitech Inc., Precitech Inc., Keene, NH, USA). The machine is equipped with hydrostatic bearing axes, Keene, NH, USA). The machine is equipped with hydrostatic bearing axes, temperature control, and an temperature control, and an air bearing spindle. Due to the fact, that ferrous materials cannot be air bearing spindle. Due to the fact, that ferrous materials cannot be machined by diamond cutting tools, machined by diamond cutting tools, the steel mold inserts are coated with a nickel–phosphorous the steel mold inserts are coated with a nickel–phosphorous coating (SuNiCoat® Optics, CZL Tilburg, coating (SuNiCoat® Optics, CZL Tilburg, Tilburg, The Netherlands). After the coating, the mold Tilburg, The Netherlands). After the coating, the mold insert is premachined by means of standard insert is premachined by means of standard milling and grinding to remove spared material. milling and grinding to remove spared material. Afterwards, a fly-cutting process is used to create a Afterwards, a fly-cutting process is used to create a plane surface on the mold inserts with optical plane surface on the mold inserts with optical quality. For the mold insert on the fixed mold side, this is quality. For the mold insert on the fixed mold side, this is the final processing step. For the the final processing step. For the microstructured mold insert, the fly-cutting process is mandatory microstructured mold insert, the fly-cutting process is mandatory to create a flat and even surface to create a flat and even surface for the subsequent UP-milling process. The fly-cutting tool has a for the subsequent UP-milling process. The fly-cutting tool has a diameter of 25 mm and a diamond diameter of 25 mm and a diamond with a 3-mm cutting edge (Matzdorf GmbH, Nürnberg, Germany). with a 3-mm cutting edge (Matzdorf GmbH, Nürnberg, Germany). The spindle speed was set to The spindle speed was set to 1500 1/min at a feed rate of 20 mm/min. Prior to the machining process, 1500 1/min at a feed rate of 20 mm/min. Prior to the machining process, the fly-cutting tool was set to the fly-cutting tool was set to an exact 90◦ angle to the mold insert surface. All machining processes on an exact 90° angle to the mold insert surface. All machining processes on the ultraprecision machine the ultraprecision machine are performed using isoparaffin as a cooling lubrication sprayed on the are performed using isoparaffin as a cooling lubrication sprayed on the machining area and a machining area and a controlled temperature in the machining chamber of 22 ◦ C ± 0.1 ◦ C. controlled temperature in the machining chamber of 22 °C ± 0.1 °C. The MLA microstructured mold insert is fabricated using UP-milling. Due to the large number The MLA microstructured mold insert is fabricated using UP-milling. Due to the large number of microlenses that have to be machined, machining time is a significant quality factor to avoid of microlenses that have to be machined, machining time is a significant quality factor to avoid temperature drifts. Thus, a milling strategy with minimal machining time for each microlens has to be temperature drifts. Thus, a milling strategy with minimal machining time for each microlens has to chosen. Therefore, a single immersion of the diamond milling tool is applied. A detailed description of be chosen. Therefore, a single immersion of the diamond milling tool is applied. A detailed the milling process is published elsewhere [26]. Using this strategy, the quality of the milling tool is the description of the milling process is published elsewhere [26]. Using this strategy, the quality of the most important factor, since the shape of the diamond exactly represents the resulting microlens shape. milling tool is the most important factor, since the shape of the diamond exactly represents the The milling tool used in this work had a radius of 0.9993 mm with a waviness < 50 nm, measured by resulting microlens shape. The milling tool used in this work had a radius of 0.9993 mm with a the manufacturer. The milling tool was provided by Contour Fine Tooling (Contour Fine Tooling BV, waviness < 50 nm, measured by the manufacturer. The milling tool was provided by Contour Fine Valkenswaard, The Netherlands). A picture of the milling process as well as the diamond cutting tool Tooling (Contour Fine Tooling BV, Valkenswaard, The Netherlands). A picture of the milling is shown in Figure 3. process as well as the diamond cutting tool is shown in Figure 3.

Micromachines 2018, 9, 355 Micromachines 2018, 9, x

5 of 14 5 of 14

Figure 3. (a) UP-milling of the mold insert; (b) Cutting edge of the diamond cutting tool. Figure 3. (a) UP-milling of the mold insert; (b) Cutting edge of the diamond cutting tool.

For the UP-milling process of the MLA mold insert, the spindle speed was set to 70,000 min−1. the milling UP-milling process the MLA moldhad insert, the time spindle speed set to the 70,000 min−1 . PriorFor to the process, theof milling spindle a lead of 10 minwas to reach operating Prior to the milling process,Otherwise, the millingspindle spindleelongation had a leaddue timetoofinternal 10 min heating to reachwill the operating temperature of the system. affect the temperature of the system. Otherwise, spindle elongation due to internal heating will affect milling milling process. Although the sag depth of each microlens is only 4 µm in the moldthe inset, the process. Although the sag depth of each microlens is only 4 µm in the mold inset, the diamond tool diamond tool was immersed 9 µm into the material to compensate potential unevenness on the was immersed 9 µm intofeed the material to compensate potential unevenness substrate surface. substrate surface. The rate during the tool immersion was set to on 2.5the m/min, during the The feed rate during the tool immersion was set to 2.5 m/min, during the repositioning of the milling repositioning of the milling tool the feed rate was increased to 50 mm/min to reduce processing time. tool milling the feedtool ratewas was immersed increased to mm/min to the reduce processing time. The milling potential tool was The in 50 a 20° angle to substrate surface to compensate ◦ angle to the substrate surface to compensate potential decentering of the diamond immersed in a 20 decentering of the diamond on the tool. The machining parameters used for the fabrication of the on themold tool. insert The machining MLA are shownparameters in Table 1. used for the fabrication of the MLA mold insert are shown in Table 1. Table 1. Machining parameters for the fabrication of the MLA mold insert by means of UP-milling. Table 1. Machining parameters for the fabrication of the MLA mold insert by means of UP-milling.

Parameter Parameter

Value

Spindle speed (min ) −1 Spindle speed (min Tool immersion (µm)) Tool immersion (µm) Immersion Immersion Feed rate (mm/min) Feed rate (mm/min) Tool positioning Tool positioning Tool angle (◦ ) Tool angle (°)◦ Temperature ( C) Temperature (°C) Tool radius (mm) Tool radius (mm) −1

Value

70,000 70,000 9 9 2.5 2.5 5050 20 20 22 22 0.9993 0.9993

2.3. Injection Compression Molding 2.3. Injection Compression Molding The injection compression molding is performed on an Arburg Allrounder 370A molding machine The injection compression molding is performed onmelt an injection Arburg unit. Allrounder 370A molding (Arburg GmbH & Co. KG, Loßburg, Germany) with vertical As a molding material, machine (Arburg GmbH & Co. KG, Loßburg, Germany) with vertical melt injection unit. As a Zeonex COP 330R (Zeon Cooperation, Tokyo, Japan) was used due to its low autofluorescence and molding material, Zeonex 330Rcompression (Zeon Cooperation, Tokyo,performed Japan) was due fabrication to its low good optical properties. TheCOP injection molding process forused the MLA autofluorescence and good optical properties. The injection compression molding process can be divided into five process steps, which are shown in Figure 4. At the beginning of the process, performed for the MLA fabrication can plate be divided five process steps, which shown Figure the structured mold insert and frame are ininto a backwards position. Then,are the frameinplate of 4. At the beginning of the process, the structured mold insert and frame plate are in a backwards the molding tool is moved into the front position by means of pneumatic cylinders. Afterwards, position. Then, frame Thereby, plate of the molding tool is moved intocavity the front by structured means of the molding toolthe is closed. the frame plate creates a sealed even position though the pneumatic the molding toolThis is closed. Thereby, the frame plate creates a mold insert cylinders. remains in Afterwards, a defined backward position. is important to ensure the complete filling of sealed cavity even though the structured mold insert remains in a defined backward position. This is the cavity. If the mold insert is moved to the front to early, the resulting gap is too small, preventing important ensure the theflow cavity. If the mold insertprocess is moved the injection front to the melt to to completely fillcomplete the cavityfilling due toof high resistance. The third step to is the early, resulting is tooWith small, the melt to completelymovement fill the cavity due high flow of thethe melt into thegap cavity. a preventing defined delay, the compression with thetostructured resistance. The third process step is the injection of the melt into the cavity. With a defined delay, the mold insert is started, moving the insert to a defined position. Thus, a homogeneous pressure within compression movement thestep structured mold insert is started,tool moving the insert to aresulting defined the cavity is created. In with the last of the process, the molding is opened and the position. Thus, a homogeneous pressure within the cavity is created. In the last step of the process, the molding tool is opened and the resulting MLA ejected. The most crucial aspects in the process

Micromachines 2018, 9, 355

6 of 14

Micromachines 2018, 9, x

6 of 14

MLA ejected. The mostofcrucial aspects in the process the positioning of the structured mold insert are the positioning the structured mold insertareprior to injection, delay of the compression prior to injection, delay of the compression movement and final position of the structured mold movement and final position of the structured mold insert. The interplay of these parametersinsert. has to The interplay of these parameters has to be defined carefully to obtain high quality optical components. be defined carefully to obtain high quality optical components.

Figure 4. Process steps of the injection compression molding process. Figure 4. Process steps of the injection compression molding process.

The molding parameters used for the replication of the MLAs are shown in Table 2. The molding parameters used for the replication of the MLAs are shown in Table 2. Table 2. Molding parameters. Table 2. Molding parameters.

Parameter

Value

MeltParameter temperature (°C)

Value 235/250/265/265/255/65

Melt temperature (◦ C) Fixed side 235/250/265/265/255/65 120 Fixed side 120 Mold temperature (°C) Moveable side 120 Mold temperature (◦ C) Moveable side 120 Compression stamper Compression stamper 120120 Holding pressure (bar) 300 Holding pressure (bar) 300 Injection pressure (bar) 1250–1400 Injection 1250–1400 Injectionpressure time (s) (bar) 0.08 Cycle timetime (s) (s) 28 0.08 Injection

Cycle time (s)

28

3. Results 3. Results 3.1. Mold Insert Fabrication

3.1. Two Moldfly-cutting Insert Fabrication processes were performed on the mold inserts for the fixed and moveable mold side. At first, the moldprocesses insert for were the fixed side wasonmachined. machined area of themoveable mold insert is Two fly-cutting performed the mold The inserts for the fixed and mold 10 × 13 to theinsert fact, that thefixed diameter of the fly-cutting tool is largerarea thanofthe side. Atmm. first,Due the mold for the side was machined. The machined themold moldinsert, insert

is 10 × 13 mm. Due to the fact, that the diameter of the fly-cutting tool is larger than the mold insert, a single cutting transit is sufficient to machine the area. The resulting surface represents the backside

Micromachines 2018, 9, 355

Micromachines 2018, 9, x

7 of 14

7 of 14

a single cutting transit of14 Micromachines 2018, 9, x is sufficient to machine the area. The resulting surface represents the backside 7 of theofMLA. Therefore, the quality of the surface is important for the optical performance of the resulting the MLA. Therefore, the quality of the surface is important for the optical performance of the of the MLA. Therefore, the ofathe surface important for performance of the polymer MLA. For the cutting process, cutting depth of 3 µm was The resulting resulting polymer MLA. Forquality the cutting process, a is cutting depth of chosen. 3the µ moptical was chosen. Thesurface resulting resulting polymer MLA. For the cutting process, a cutting depth of 3 µ m was chosen. The resulting roughness was Ra = 12 nm, measured by white light interferometry (WLI, WYKO NT9100a, Veeco surface roughness was Ra = 12 nm, measured by white light interferometry (WLI, WYKO NT9100a, surface roughness was RaPlainview, =NY, 12 USA). nm, measured byinsert white light interferometry (WLI, WYKO NT9100a, Instruments Inc., Plainview, TheUSA). mold and the surface are shown inare Veeco Instruments Inc., NY, The mold insert and measurement the surface measurement Veeco Figure 5.Instruments shown in Figure 5. Inc., Plainview, NY, USA). The mold insert and the surface measurement are shown in Figure 5.

Figure 5. (a) Mold insert with optical surface after fly-cutting process; Surface profile acquired Figure 5. (a) Mold insert with flatflat optical surface after fly-cutting process; (b)(b) Surface profile acquired Figure 5. (a) Mold insert with flat optical surface after fly-cutting process; (b) Surface profile acquired by WLI. by WLI. by WLI.

The second fly-cutting process was performed to create a flat surface for the subsequent The second fly-cutting process was performed to createtoa flat surface forsurface the subsequent The second fly-cutting performed create a previous flat for theUP-milling subsequent UP-milling process. The sameprocess cuttingwas conditions as applied in the fly-cutting process were process. The same cutting conditions as applied in the previous fly-cutting process were used. UP-milling same roughness cutting conditions as applied the previous used. The process. resultingThe surface was similar to theinprior process fly-cutting with Ra =process 12 nm.were The The resulting surface roughness was similarwas to the prior to process with process Ra = 12 nm. The measurement used. The resulting surface roughness similar the prior with Ra = 12 nm. The measurement results are shown in Figure 6. results are shown in Figure 6. measurement results are shown in Figure 6.

Figure 6. (a) WLI Surface measurement of the mold insert after the fly-cutting process, (b) extracted Figure WLI Surface measurement the mold insert after fly-cutting process, extracted Figure 6. 6. (a)(a) WLI Surface measurement ofof the mold insert after thethe fly-cutting process, (b)(b) extracted 2D-profile along AB. 2D-profile along AB. 2D-profile along AB.

For the fabrication of the microlenses machined into the mold insert, an UP-milling process was For thefabrication fabrication ofwas the positioned microlensesinmachined into the insert, anan UP-milling process used. mold insertof a defined angle of 26.56°. Thus, the machining ofwas the For The the the microlenses machined into themold mold insert, UP-milling process used. The mold insert was positioned in a defined angle of 26.56°. Thus, the machining of ◦ individual microlenses be done in ainway that they areoforiented a horizontal line. This isthe an was used. The mold insertcould was positioned a defined angle 26.56 . in Thus, the machining of the individual microlenses could be done in a way that they are oriented in a horizontal line. This is an advantage during thecould milling have to in be amoved (x-axis and z-axis), individual microlenses be process, done in asince wayonly that two they axes are oriented horizontal line. This is the milling only two axes have have tobe bemoved moved (x-axiswhen andz-axis), z-axis), resulting induring reduced positioning errors.since The third (y-axis) only has to be moved another anadvantage advantage during the millingprocess, process, since only axis two axes to (x-axis and resulting in reduced positioning errors. third axis (y-axis) only has to be moved when another line of in microlenses has to be started. TheThe milling is illustrated Figure 7. The milling time resulting reduced positioning errors. The third axisstrategy (y-axis) only has to bein moved when another line line of microlenses has to be started. The milling strategy is illustrated in Figure 7. The milling time an individual was about s. The resulting process timein forFigure the whole insert was of for microlenses has tolens be started. The1milling strategy is illustrated 7. Themold milling time forabout an for individual lens was about 1resulting s. The horizontal resulting time for whole mold insert was4.5 about 4.5an h, since the repositioning after every line for adds towhole thethe machining time. individual lens was about 1 s. The process process time the mold insert was about h, 4.5 h, the repositioning afterhorizontal every horizontal line to the machining since thesince repositioning after every line adds to adds the machining time. time.

Micromachines 2018, 9, 355

8 of 14

Micromachines 2018, 9, x Micromachines 2018, 9, x

8 of 14 8 of 14

Figure Millingstrategy strategyfor for the fabrication of the Figure 7. 7. Milling themicrolenses. microlenses. Figure 7. Milling strategy forthe thefabrication fabrication of of the microlenses.

The resulting MLA mold insert is shown in Figure 8. The surface roughness of a single The resulting MLA mold insert is shown in Figure The surface roughness of a microlens single The resulting is shown in Figure 8. The 8. surface roughness microlens was MLA Ra = mold 4 nm,insert measured by WLI. The roughness depth was Rt =of27a single nm. For the microlens was Ra = 4 nm, measured by WLI. The roughness depth was Rt = 27 nm. For the was Ra = 4 nm, of measured by WLI. The roughness depth was Rt =was 27 nm. For the evaluation of the evaluation the surface roughness the overall curvature mathematically removed. evaluation of the surface roughness the overall curvature was mathematically removed. Furthermore deviation of the was analyzed a laser autofocus surface roughnessthe theform overall curvature wasmicrolenses mathematically removed.using Furthermore the form probe deviation Furthermore the form deviation of the microlenses was analyzed using a laser autofocus probe (Mitaka MLP-3, Japan). The resulting form deviation of P-V 39.5 nm of themeasurement microlensessystem was analyzed using aTokyo, laser autofocus probe measurement system (Mitaka MLP-3, measurement system (Mitaka MLP-3, Tokyo, Japan). The resulting form deviation of P-V 39.5 nm from the required 1 mm radius is shown in Figure 8d). The distance between the microlenses of 127.5 in Tokyo,from Japan). The resulting form deviation of P-V 39.5 nm from the required 1 mm radius is shown the required 1 mm radius is shown in Figure 8d). The distance between the microlenses of 127.5 µ m, which the diameter, the could be achievedof very accurately and no drift in thediameter, distance could be Figure 8d. Theequals distance microlenses 127.5 µm, which µm, which equals thebetween diameter, could be achieved very accurately and equals no driftthe in the distance could could be measured. achieved very accurately and no drift in the distance could be measured. be measured.

.

.

Figure 8. (a) Mold insert after UP-milling; (b) WLI measurement of the MLA on the mold insert; (c) Surface roughness of a single microlens with Ra = 4 nm, (d) Form deviation of a single microlens from a 1-mm radius measured with Mitaka MLP-3.

Micromachines 2018, 9, x

9 of 14

Figure 8. (a) Mold insert after UP-milling; (b) WLI measurement of the MLA on the mold insert; (c) Micromachines 9, 355 of a single microlens with Ra = 4 nm, (d) Form deviation of a single microlens9 of 14 Surface2018, roughness

from a 1-mm radius measured with Mitaka MLP-3.

3.2. Injection Compression Molding 3.2. Injection Compression Molding The The molding molding process process was was performed performed by by starting starting with with aa regular regular injection injection molding molding process process to to evaluate the process parameters regarding filling behavior in the cavity. Accordingly, no compression evaluate the process parameters regarding filling behavior in the cavity. Accordingly, no movement carried out. Afterwards, the compression movementmovement was included improve compressionwas movement was carried out. Afterwards, the compression was to included to the optical quality of the components. This approach proved to be suitable to establish injection improve the optical quality of the components. This approach proved to be suitable to establish compression molding molding processes, since thesince quality the parts is affected by a multitude of factors. injection compression processes, theof quality of the parts is affected by a multitude of Splitting the process into two steps, simplifies the optimization of the molding parameters. Molding factors. Splitting the process into two steps, simplifies the optimization of the molding parameters. parameters were adjusted a way to accuracy fillingand behavior. Cycle timeCycle was Molding parameters were in adjusted in improve a way toform improve form and accuracy filling behavior. not a relevant factor in this work. One of the most critical aspects of this molding process was the time was not a relevant factor in this work. One of the most critical aspects of this molding process coordination of the moveable structured mold insert thewith injection of the material. Due toDue the was the coordination of the moveable structured moldwith insert the injection of the material. limitation of the part thickness to 500 µm, material flow is limited and the material cooling occurs to the limitation of the part thickness to 500 µm, material flow is limited and the material cooling rapidly. The result a narrow in whichin a sufficient part quality be obtained. occurs rapidly. Thewas result was a process narrow window process window which a sufficient partcould quality could be The process steps of the compression molding process are described in Section 2.3 (Figure 4). Before the obtained. The process steps of the compression molding process are described in Section 2.3 (Figure injection phase, the structured mold insert is positioned in a backward position, about 0.9 mm from the 4). Before the injection phase, the structured mold insert is positioned in a backward position, about end position. Thus, the beginning injection there is ainjection larger cavity faster 0.9 mm from the endatposition. Thus, of at the the material beginning of the material there allowing is a largera cavity filling of the cavity. With 0.7 s delay, the compression movement is started. At this point, the injection allowing a faster filling of the cavity. With 0.7 s delay, the compression movement is started. At this phase already completed, the material injection onlythe takes 0.8 s. Ininjection contrast only to thetakes part thickness, point, isthe injection phase since is already completed, since material 0.8 s. In the replication of the micro structured area is not a limiting factor. Since the microlenses are relatively contrast to the part thickness, the replication of the micro structured area is not a limiting factor. flat with significant ratio,flat the filling of the microlenses structures does represent a Since the no microlenses areaspect relatively with no significant aspect ratio, the filling of not the microlenses challenge combination the application of a compression the micro are structures and doesinnot represent awith challenge and in combination with theforce, application of astructures compression replicated accurately without necessity of adjusting the molding parameters. An injection compression force, the micro structures are replicated accurately without necessity of adjusting the molding molded MLAAn with sprue iscompression shown in Figure 9. For the integration MLAin into the HSI system, parameters. injection molded MLA with sprue ofisthe shown Figure 9. For the the sprue has to be removed mechanically. integration of the MLA into the HSI system, the sprue has to be removed mechanically.

Figure MLA before before the the sprue sprue is is removed. removed. Figure 9. 9. Injection Injection compression compression molded molded MLA

The WLI measurements of the micro structures show a homogeneous distribution of the The WLI measurements of the micro structures homogeneous of was the microlenses with no visible deviations (Figure 10a). A show single amicrolens of the distribution molded MLA microlenses with no visible deviations (Figure 10a). A single microlens of the molded MLA was characterized using the Mitaka MLP-3 system (Figure 10b). The measurement shows, that the sag characterized Mitaka MLP-3 system (Figure 10b). from The measurement thatisthe sag height of aboutusing 4 µmthe is fully replicated. The form deviation the aimed 1 = shows, mm radius about height of about 4 µm is fully replicated. The form deviation from the aimed 1 = mm radius is P-V 80 nm for a single lens (Figure 10c). The surface roughness of a single microlens was measured about P-V= 80 nm However, for a single (Figure 10c). Theroughness surface roughness of a single was to be Ra 6 nm. thelens measurement of the depth resulted in Rt microlens = 53 nm. Both measured to be Ra = 6 nm. However, the measurement of the roughness depth resulted in Rt = 53 nm. measurements were performed using WLI. For the evaluation of the surface roughness the overall Both measurements were performed using WLI. For the evaluation of the surface roughness the overall lens radius was mathematically removed. lens radius was mathematically removed.

Micromachines 2018, 9, 355

10 of 14

Micromachines 2018, 9, x

10 of 14

Micromachines 2018, 9, x

10 of 14

Figure 10. (a) WLI measurement of the molded microlenses; (b) SEM image of the molded MLA; (c)

Figure 10. WLI measurement of the molded microlenses; (b)(d) SEM imagemeasurement of the molded Form(a) deviation from a 1 mm radius measured with Mitaka MLP-3; Roughness by MLA; (c) FormWLI deviation a 1 mm radius measured with Mitaka MLP-3; (d) Roughness measurement by with Ra from = 6 nm. WLI with Ra = 6 nm.

Figure the 10. (a) WLI needs measurement the molded microlenses; (b) SEM image of the moldedthe MLA; (c) Since MLA to be of integrated into a hyperspectral imaging system, planarity Formthe deviation fromfeatures a 1 mm on radius MLP-3; (d) Roughness measurement by for between mounting the measured sides andwith the Mitaka micro structured area is an important factor Since MLA needs toabelaser integrated into a hyperspectral WLI with Ra =Using 6 nm. the the application. autofocus probe measurement,imaging the anglesystem, betweenthe theplanarity mountingbetween the mounting features on the area sides and the micro structured areaTo is show an important factor features and the structured was measured to be 0.07° (Figure 11a). the functionality of for the Since the MLA needs to be integrated into a hyperspectral imaging system, the planarity molded MLA,athe partautofocus was integrated intomeasurement, a hyperspectral imaging system. The resulting image onfeatures application. Using laser probe the angle between the mounting between the mounting features on the sides and◦the micro structured area is an important factor for camera chip is shown in Figure 11b. light(Figure spectra generated by the hyperspectral imaging and thethe structured area was ameasured to beThe 0.07 11a). the To show functionality of molded the application. Using laser autofocus probe measurement, angle the between the mounting system are homogeneously distributed throughout the whole area, showing that the molded MLA MLA, the part was integrated into hyperspectral system. The image on the features and the structured areaa was measured toimaging be 0.07° (Figure 11a). Toresulting show the functionality of camera performs as required. A detailed characterization of the hyperspectral imaging system is not part of molded MLA, the part was integrated into a hyperspectral imaging system. The resulting imagesystem on chip is shown in Figure 11b. The light spectra generated by the hyperspectral imaging are this work and will be published elsewhere. the camera chip is shown in Figure 11b. The light spectra generated by the hyperspectral imaging homogeneously distributed throughout the whole area, showing that the molded MLA performs as system are homogeneously distributed the whole area, system showing is that molded MLA required. A detailed characterization of thethroughout hyperspectral imaging notthe part of this work and performs as required. A detailed characterization of the hyperspectral imaging system is not part of will be published elsewhere. this work and will be published elsewhere.

Figure 11. (a) Angle of 0.07◦ between the mounting features and the microstructured area measured with laser autofocus probe system; (b) Image of the resulting light spectra on the camera system using the molded MLA; (c) Close view of the light spectra at position A.

Micromachines 2018, 9, x

11 of 14

Figure 11. (a) Angle of 0.07° between the mounting features and the microstructured area measured Micromachines 2018,autofocus 9, 355 11 of 14 with laser probe system; (b) Image of the resulting light spectra on the camera system

using the molded MLA; (c) Close view of the light spectra at position A.

4. Discussion 4. Discussion The The results results presented presented in in this this work work show show that that to to combination combination of of ultraprecision ultraprecision machining machining and and injection compression molding is a suitable process chain to fabricate polymer MLAs injection compression molding is a suitable process chain to fabricate polymer MLAs with with aa high high number In the the following, following, the the results results as as well well as as each each process process step step is is critically critically reviewed reviewed number of of microlenses. microlenses. In and discussed. and discussed. Fly-cutting Fly-cutting proved proved to to be be aa suitable suitable method method to to prepare prepare the the optical optical mold mold inserts. inserts. It It combines combines fast fast machining with low surface roughness. The resulting surface roughness of Ra = 12 nm corresponds to machining with low surface roughness. The resulting surface roughness of Ra = 12 nm corresponds values found in the literature [27]. Particularly, since the mold inserts are not rotationally symmetric, to values found in the literature [27]. Particularly, since the mold inserts are not rotationally fly-cutting a more suitable method tomethod create atoflat surface ultraprecision diamonddiamond turning. symmetric, is fly-cutting is a more suitable create a flatthen surface then ultraprecision However, accurate positioning of the fly-cutting tool and the mold are ainsert key to obtain turning. However, accurate positioning of the fly-cutting tool and insert the mold are a keyan to optical obtain surface quality. an optical surface quality. The that was was chosen for the of the the MLA proved to to be The milling milling strategy strategy that chosen for the fabrication fabrication of MLA mold mold insert insert proved be suitable to create accurate microlenses with homogeneous dimensions. Due to a single immersion suitable to create accurate microlenses with homogeneous dimensions. Due to a single immersion of of the However, the the milling milling tool, tool, machining machining time time could could be be kept kept at at 4.5 4.5 h h for for 12,000 12,000 microlenses. microlenses. However, the applied applied milling milling strategy strategy also also comes comes with with several several restrictions, restrictions, which which limits limits the the range range of of application. application. The The most most important factor for the process is the quality of the milling tool. The radius of the cutting diamond, important factor for the process is the quality of the milling tool. The radius of the cutting diamond, waviness cutting diamond need to betovery accurate in theinsubmicrometer range. waviness and andpositioning positioningofofthethe cutting diamond need be very accurate the submicrometer Notably, the milling strategy can only applied whenwhen the MLA consists of microlenses withwith the range. Notably, the milling strategy canbe only be applied the MLA consists of microlenses same radius. Furthermore, microlenses with aspheric or free-form shape are not possible. In these the same radius. Furthermore, microlenses with aspheric or free-form shape are not possible. In cases, different milling strategies like spiral immersion are necessary. Changes in the of the these cases, different milling strategies like spiral immersion are necessary. Changes in radius the radius of microlenses and the pitch can be easily adjusted with the proposed strategy. A critical factor in the the microlenses and the pitch can be easily adjusted with the proposed strategy. A critical factor in machining process is tool wear, since defects on the diamond cutting edge willwill inevitably result in the machining process is tool wear, since defects on the diamond cutting edge inevitably result marks in the microlenses. An example of a MLA after milling with awith worna cutting diamond is shown in marks in the microlenses. An example of a MLA after milling worn cutting diamond is in Figure 12. shown in Figure 12.

Figure 12. 12. MLA MLAon ona test a test substrate with resulting microlenses significant toolofwear of the Figure substrate with resulting microlenses after after significant tool wear the cutting cutting diamond measured diamond measured by WLI.by WLI.

Tool wear can be reduced when the cutting depth is kept at a minimum and cooling lubricant is Tool wear can be reduced when the cutting depth is kept at a minimum and cooling lubricant used. Nevertheless, tool wear remains a factor which is difficult to predict. Due to the fact that the is used. Nevertheless, tool wear remains a factor which is difficult to predict. Due to the fact that individual microlenses need to be positioned very accurately, exchanging the diamond cutting tool the individual microlenses need to be positioned very accurately, exchanging the diamond cutting during the machining process is no valid option. Recalibration of a new cutting tool cannot be tool during the machining process is no valid option. Recalibration of a new cutting tool cannot be performed with the required accuracy. performed with the required accuracy. Compared to other applicable fabrication technologies like e-beam writing and laser machining, Compared to other applicable fabrication technologies like e-beam writing and laser machining, the ultraprecision machining process achieves higher surface quality, comparable to what can be the ultraprecision machining process achieves higher surface quality, comparable to what can be obtained with the LIGA process. Furthermore, the mold insert could be machined directly without

Micromachines 2018, 9, 355

12 of 14

any further post-treatment or electroplating process. Compared to the other technologies, machining time is in the same range [12,28]. Laser machining can produce faster and more microlenses, when the lenses are very small (diameter < 10 µm) [14]. The resulting MLA mold insert fulfilled all the required properties considering form accuracy of the microlenses, surface roughness, and positioning of the microlenses. The resulting form deviation of < 40 nm in the mold insert is very low and could only be achieved due to the high quality and form accuracy of the diamond cutting tool. The resulting surface roughness of an individual microlens of Ra = 4 nm in the mold insert fulfills the requirements for optical applications. It is important to mention that the measurement of the microlens radius does not generate reliable results due to the fact that only a small section of the 1-mm radius is available for the measurement. Small deviations and surface roughness affect the calculation of the resulting radius, leading to high inaccuracy. Therefore, it is advisable to analyze the form deviation from the required radius instead of only measuring the radius. The molded MLA was fabricated by injection compression molding. Due to the required part thickness of 500 µm, the process window was significantly limited. The complete filling of the cavity needed to be fast to avoid the cooling of the material before the cavity was filled. To improve the filling behavior, the cavity was increased during the material injection by keeping the structured mold insert in a backwards position. Due to this strategy, an injection compression molding tool was mandatory to allow the additional movement of the molding tool. Keeping the structured mold insert in a backwards position enabled a fast injection of material and, subsequently, the part could be compressed to the required part thickness. However, the movements of the structured mold insert which is used as a compression stamper is critical and needs to be timed accurately with the injection phase to obtain accurate molded parts. A regular injection molding process was also tested, but entire parts could not be obtained. The resulting surface roughness of Ra = 6 nm complies with the required optical surface quality which is needed for the application. Equally, the measured form deviation of ±40 nm is very good and sufficient for the application in the hyperspectral imaging system. Form deviation doubled compared to the mold insert, however, form deviations in the sub-micrometer range are extremely good. The increased form deviation is most likely a result of shrinkage and warpage during the molding process. Since mounting features are included in the molded part, the angle between these features and the micro structured area is critical. A large angle results in a defocus of the image on the camera chip in the hyperspectral imaging system and, therefore, reduces the resolution of the system. The measured 0.07◦ however does not affect the quality of the system. 5. Conclusions To summarize the results, it can be concluded that the combination of ultraprecision milling and injection compression molding is a suitable process chain for the fabrication of high quality molded MLAs. The MLA fabricated in this work contains over 12,000 individual microlenses. An ultraprecision milling strategy, where the diamond cutting tool is immersed into the material without any further movement, was determined to be the most suitable method for the fabrication of the MLA mold insert. Thereby, accuracy of the microlenses was the focus, as well as short machining time. Due to the milling strategy, the milling tool is the most crucial factor regarding the quality of the mold insert, since it exactly represents the resulting shape of the microlenses. For the replication of the MLAs, injection compression molding was determined to be a suitable method. Due to the fact that mounting features were included at the MLA and the part thickness was only 500 µm, a customized injection compression molding process was mandatory. To obtain complete filling of the cavity, the structured mold insert needed to be in a backwards position prior to the injection phase, otherwise the required dimensions could not be reached. In summary, it can be concluded that an understanding of the fabrication processes—as well as a high-control ultraprecision machining and injection compression molding—are required to achieve high quality MLAs.

Micromachines 2018, 9, 355

13 of 14

Author Contributions: M.R. designed and conducted the experiments, analyzed the resulting parts and wrote the manuscript. M.D. and T.M. supported the design and execution of the experiments. T.R. designed the molding tool and organized the fabrication. T.G. and A.Z. critically revised the manuscript, supervised the project and provided the funding. Funding: Ministry of Finance and Economy of Baden-Wuerttemberg: innBW IDAK. Acknowledgments: The research presented in this paper is supported from the budget of the Ministry of Finance and Economy of Baden-Württemberg (Project innBW IDAK). We would like to thank the funding organization. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12.

13. 14.

15. 16. 17. 18. 19.

Lin, C.-M.; Hsieh, H.-K. Processing optimization of fresnel lenses manufacturing in the injection molding considering birefringence effect. Microsyst. Technol. 2017, 23, 5689–5695. [CrossRef] Brinksmeier, E.; Gläbe, R.; Schönemann, L. Review on diamond-machining processes for the generation of functional surface structures. CIRP J. Manuf. Sci. Technol. 2012, 5, 1–7. [CrossRef] Roeder, M.; Schilling, P.; Hera, D.; Guenther, T.; Zimmermann, A. Influences on the fabrication of diffractive optical elements by injection compression molding. J. Manuf. Mater. Process. 2018, 2. [CrossRef] Lee, B.-K.; Kim, D.S.; Kwon, T.H. Replication of microlens arrays by injection molding. Microsyst. Technol. 2004, 10, 531–535. [CrossRef] Tanida, J.; Shogenji, R.; Kitamura, Y.; Yamada, K.; Miyamoto, M.; Miyatake, S. Color imaging with an integrated compound imaging system. Opt. Express 2003, 11, 2109–2117. [CrossRef] [PubMed] Shogenji, R.; Kitamura, Y.; Yamada, K.; Miyatake, S.; Tanida, J. Bimodal fingerprint capturing system based on compound-eye imaging module. Appl. Opt. 2004, 43, 1355–1359. [CrossRef] [PubMed] Duparré, J.W.; Wippermann, F.C. Micro-optical artificial compound eyes. Bioinspir. Biomim. 2006, 1. [CrossRef] [PubMed] Dombrowski, M.; Catanzaro, B. Spatially Corrected Full-Cubed Hyperspectral Imager. U.S. Patent 7,242,478, 10 July 2007. Scheiding, S.; Yi, A.Y.; Gebhardt, A.; Loose, R.; Li, L.; Risse, S.; Eberhardt, R.; Tünnermann, A. Diamond milling or turning for the fabrication of microlens arrays: Comparing different diamond machining technologies. In Proceedings of the Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IV, San Diego, CA, USA, 14 February 2011. Kim, D.S.; Lee, H.S.; Lee, B.-K.; Yang, S.S.; Kwon, T.H.; Lee, S.S. Replications and analysis of microlens array fabricated by a modified LIGA process. Polym. Eng. Sci. 2006, 46, 416–425. [CrossRef] Yu, W.X.; Yuan, X.-C. Fabrication of refractive microlens in hybrid SiO2 /TiO2 sol-gel glass by electron beam lithography. Opt. Express 2003, 11, 899–903. [CrossRef] [PubMed] Choi, H.-K.; Ahsan, M.S.; Yoo, D.; Sohn, I.-B.; Noh, Y.-C.; Kim, J.-T.; Jung, D.; Kim, J.-H.; Kang, H.-M. Formation of cylindrical micro-lens array on fused silica glass surface using CO2 laser assisted reshaping technique. Opt. Laser Technol. 2015, 75, 63–70. [CrossRef] Takino, H.; Hosaka, T. Shaping of steel mold surface of lens array by electrical discharge machining with single rod electrode. Appl. Opt. 2014, 53, 8002–8005. [CrossRef] [PubMed] Yong, J.; Chen, F.; Yang, Q.; Du, G.; Bian, H.; Zhang, D.; Si, J.; Yun, F.; Hou, X. Rapid fabrication of large-area concave microlens arrays on PDMS by a femtosecond laser. ACS Appl. Mater. Interfaces 2013, 5, 9382–9385. [CrossRef] [PubMed] Takino, H.; Hosaka, T. Shaping of steel mold surface of lens array by electrical discharge machining with spherical ball electrode. Appl. Opt. 2016, 55, 4967–4973. [CrossRef] [PubMed] Fang, F.Z.; Zhang, X.D.; Weckenmann, A.; Zhang, G.X.; Evans, C. Manufacturing and measurement of freeform optics. CIRP Ann. 2013, 62, 823–846. [CrossRef] Holme, N.C.R.; Berg, T.W.; Dinesen, P.G. Diamond Micro-Milling for Array Mastering. In Proceedings of the Laser Beam Shaping IX, San Diego, CA, USA, 17 September 2008. Brinksmeier, E.; Riemer, O.; Osmer, J. Tool path generation for ultraprecision machining of free-form surfaces. Prod. Eng. 2008, 2, 241–246. [CrossRef] Peng, L.; Deng, Y.; Yi, P.; Lai, X. Micro hot embossing of thermoplastic polymers: A review. J. Micromech. Microeng. 2014, 24. [CrossRef]

Micromachines 2018, 9, 355

20. 21. 22. 23.

24. 25.

26.

27. 28.

14 of 14

Sortino, M.; Totis, G.; Kuljanic, E. Comparison of injection molding technologies for the production of micro-optical devices. Procedia Eng. 2014, 69, 1296–1305. [CrossRef] Bäumer, S. Handbook of Plastic Optics; John Wiley & Sons: New Jersey, NJ, USA, 2011. Lu, G.; Fei, B. Medical hyperspectral imaging: A review. J. Biomed. Opt. 2014, 19. [CrossRef] [PubMed] Dale, L.M.; Thewis, A.; Boudry, C.; Rotar, I.; Dardenne, P.; Baeten, V.; Pierna, J.A.F. Hyperspectral imaging applications in agriculture and agro-food product quality and safety control: A review. Appl. Spectrosc. Rev. 2013, 48, 142–159. [CrossRef] Hege, E.K.; O’Connell, D.; Johnson, W.; Basty, S.; Dereniak, E.L. Hyperspectral Imaging for Astronomy and Space Surviellance. In Proceedings of the Imaging Spectrometry IX, San Diego, CA, USA, 7 January 2004. Gowen, A.A.; O’Donnell, C.P.; Cullen, P.J.; Downey, G.; Frias, J.M. Hyperspectral imaging—An emerging process analytical tool for food quality and safety control. Trends Food Sci. Technol. 2007, 18, 590–598. [CrossRef] Roeder, M.; Drexler, M.; Guenther, T.; Zimmermann, A. Evaluation of Ultraprecision Milling Strategies for Microlens Array Mould Inserts for the Replication by Injection-Compression Moulding. In Proceedings of the 18th International Conference & Exhibition, Venice, Italy, 4–8 June 2018. Zhang, S.J.; To, S.; Zhu, Z.W.; Zhang, G.Q. A review of fly cutting applied to surface generation in ultraprecision machining. Int. J. Mach. Tools Manuf. 2016, 103, 13–27. [CrossRef] Kim, C.; Sohn, I.-B.; Lee, Y.J.; Byeon, C.C.; Kim, S.Y.; Park, H.; Lee, H. Fabrication of a fused silica based mold for the microlenticular lens array using a femtosecond laser and a CO2 laser. Opt. Mater. Express 2014, 4, 2233–2240. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).