Fabrication of Random Microlens Array for Laser Beam Homogenization with High Efficiency

micromachines

Article Fabrication of Random Microlens Array for Laser Beam Homogenization with High Eﬃciency

Li Xue 1,2, Yingfei Pang 2, Wenjing Liu 1,2, Liwei Liu 2, Hui Pang 2, Axiu Cao 2,* , Lifang Shi 2,*, Yongqi Fu 1,* and Qiling Deng 2

1 University of Electronic Science and Technology of China, Chengdu 610054, China; [email protected] (L.X.); [email protected] (W.L.) 2 Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China; [email protected] (Y.P.); [email protected] (L.L.); [email protected] (H.P.); [email protected] (Q.D.) * Correspondence: [email protected] (A.C.); [email protected] (L.S.); [email protected] (Y.F.); Tel.: +86-028-8510-1178 (A.C. and L.S.); +86-152-0834-0157 (Y.F.) Received: 25 February 2020; Accepted: 19 March 2020; Published: 24 March 2020

Abstract: The miniaturized and integrated microlens array (MLA) can effectively achieve the beam homogenization, compactness and miniaturization of laser systems. When the high-coherence laser beam is homogenized by means of using the MLA, interference fringes will occur in the homogenized light spot due to the periodicity of the MLA, which seriously affects the uniformity of the homogenized light spot. To solve this problem, a novel random microlens array (rMLA) structure was proposed for the purpose of achieving beam homogenization. The coherence in the homogenization process is suppressed by means of breaking the periodicity of the MLA. The homogenized light spot with a high energy utilization is then obtained accordingly. In the fabrication process, a clever method of combining chemical etching with lithography technology is performed to fabricate a honeycomb rMLA and a rectangular rMLA. The experimental results show that the energy utilization rate of the two types of the rMLAs is about 90%, and the uniformity of the homogenized light spots generated by the honeycomb rMLA and the rectangular rMLA are more than 80% and 85%, respectively. Meanwhile, fully cost-effective fabrication is possible to be realized.

Keywords: chemical etching; random microlens array; beam homogenization; laser

1. Introduction Lasers have been widely applied in many fields, such as medical treatment [1,2], laser illumination [3,4], laser detection [5,6] and satellite communication [7,8]. However, the intensity distribution of a laser is usually in a Gaussian profile, which is always required to be converted into a flat-top profile so as to achieve the desired effect, e.g., laser welding [9,10], an optical system of exposure machine [11], laser drilling [12,13] and laser projection [14,15]. There are some methods to achieve beam homogenization, such as an aspheric lens group [16–18], free-form lens [19–21], diffractive optical element (DOE) [22–24] and microlens array (MLA) [25–27]. For the aspheric lens group and free-form surface lens, the former has a high light energy utilization rate and high beam uniformity, which can be applied to high-power lasers, while the latter is a simple optical system with high design freedom which can be used to effectively achieve high uniformity. However, the above two methods require nanometer surface roughness, which increases the fabrication difficulty. In addition, because of the limitation of the sizes of the corresponding lenses, it is a challenge to realize the miniaturization and integration of the system. The DOE can be used to achieve the miniaturization and integration of the systems, and also has high design freedom. Nevertheless, the DOE is generally only suitable for a single wavelength, and cannot be used to shape the beam in a

Micromachines 2020, 11, 338; doi:10.3390/mi11030338 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 338 2 of 12 broad spectral range. Meanwhile, energy efficiency depends on the number of phase levels of the DOE; the greater the number of phase levels is, the higher the energy efficiency will be, but also the more complex the fabrication. In contrast, the MLA is widely applied to the field of multi-wavelength laser beam homogenization to achieve the miniaturization and integration of the systems, due to its merits of a high energy utilization rate, extremely simple structure and high beam uniformity. Many related research works of beam homogenization by means of using MLA have been reported before. In 2017, Hang et al. [28] used a monolithic periodic MLA to homogenize the laser beam. The collimated laser beam is subdivided into multiple beamlets by the MLA, and then the beamlets are superposed and recombined with each other to suppress the nonuniformity by a focusing lens. The uniformity of the obtained homogenized light spot is 91.1%. The energy utilization rate of the MLA with anti-reflection coating can reach 93%. In order to homogenize the incident laser beam with a certain divergence angle, a method on the basis of two periodic MLAs is proposed by Hwang et al. [29] to obtain a homogenized light spot with high energy efficiency in 2017. However, when high-coherence lasers are shaped by their method, interference between beamlets will also occur due to the periodicity of the MLA, the resulting interference fringe appears in the obtained homogenized light spot and the uniformity of the homogenized light spot will be deteriorated. In order to eliminate the effect of interference on the homogenized light spot, Xianzi et al. [30] prepared a 16-level random phase fresnel lens array by means of setting the phase of sub lenses of the fresnel lens array to a random arrangement in 2019. This broke the coherence between the beamlets, and obtained a homogenized light spot with a uniformity of 83%. However, the fabrication of the 16-level random microlens array (rMLA) is complex. In 2019, Liu et al. [31] designed a continuous profile MLA with sub lenses of the random apertures and arrangements. It realized a breakthrough of the coherence of beamlets so that the uniformity obtained by simulation is able to reach 94.33%. However, they proposed to fabricate the rMLA by 3D printing technology, which is not yet mature. At present, the fabrication technology of continuous rMLA is not yet mature. In contrast, we innovatively combined the traditional lithography exposure technology with chemical etching technology to propose a novel method to fabricate rMLA, which can be used to fabricate high-efficiency, low-cost and large-area rMLA. Meanwhile, in order to further reduce the surface roughness of microlenses, three kinds of chemical solutions were prepared in this paper: the fundamental chemical etching solution (deionized water, hydrofluoric acid solution (HF solution) (40%) and ammonium fluoride particles (NH4F)); the H2SO4 chemical etching solution (deionized water, HF solution (40%) and the H2SO4 (95–98%)); and the HNO3 chemical etching solution (deionized water, HF solution (40%) and the HNO3 (65–68%)), respectively. We compared the etching effects of the three kinds of solutions and obtained a best suitable solution ratio. Adopting the proposed method, we fabricated two types of rMLAs, which were the honeycomb rMLA and the rectangular rMLA, respectively. Moreover, a chromium film layer was plated on the surface of the glass substrate, which masterly avoided side etching and the curl-up of the edge. In addition, the high-quality surface of the microlenses can effectively guarantee the homogenization quality of the beam, and the randomness of the rMLA is able to successfully break the coherence of the laser beam so that the uniformity of the modulated beam can be improved. Section2 describes the beam homogenization model of rMLA, while Section3 demonstrates the fabrication process of the rMLA in detail, Section4 shows experimental results and verifies the validity of our method and Section5 is a summary.

2. Beam Homogenization Model of rMLA The working principle of beam homogenization based on the random microlens array (rMLA) is shown in Figure1a. The condition of interference among the beamlets is broken, and the inﬂuence of interference on beam homogenization is suppressed by the rMLA, as the phases of the beamlets are randomly distributed due to the random apertures, focal lengths and arrangements. According to this principle, the preparation of the two types of rMLAs have been proposed in this paper: one is the honeycomb rMLA, as shown in Figure1b, which can be used to produce a circular homogenized Micromachines 2020, 11, x 3 of 6

According to this principle, the preparation of the two types of rMLAs have been proposed in this paper: one is the honeycomb rMLA, as shown in Figure 1b, which can be used to produce a circular homogenized light spot; and the other is the rectangular rMLA, as shown in Figure 1c, which can be usedMicromachines to produce2020, 11a ,rectangular 338 homogenized light spot. The structural parameters of sub lenses3 of of 12 the two types of rMLAs are shown in Figure 1d, including aperture Di and sag height hi . As can belight seen spot; from and Fig theure other 1d, the is the geo rectangularmetric relationship rMLA, as between shown in the Figure radius1c, of which curvature can be usedR and to producethe sag a rectangular homogenized light spot. The structural parameters of sub lenses of thei two types of height h can be obtained, expressed by Equation 1. According to the relationship between the rMLAs arei shown in Figure1d, including aperture Di and sag height hi. As can be seen from Figure1d, the geometric relationship between the radius of curvature R and the sag height h can be obtained, radius of curvature and the focal length, the focal length ( ifi ) can be calculatedi by Equation 2, expressed by Equation (1). According to the relationship between the radius of curvature and the focal where n is the refractive index. Therefore, the divergence angle of the beam passing through the length, the focal length ( f ) can be calculated by Equation (2), where n is the refractive index. Therefore, sub lenses can be calculatedi by Equation 3. Meanwhile, the divergence angle θi of the entire light the divergence angle of the beam passing through the sub lenses can be calculated by Equation (3). beam after passing through the rMLA is formed by the superposition of randomly distributed sub Meanwhile, the divergence angle θ of the entire light beam after passing through the rMLA is formed lenses, and is determined by the divergi ence angle of the sub lenses with a higher frequency of by the superposition of randomly distributed sub lenses, and is determined by the divergence angle of occurrence. the sub lenses with a higher frequency of occurrence. D22 +4h Dii2 + 4h2 (1) Ri = i i Ri = 8hi (1) 8hi 22+ R D2 4h 2 f = -R i = - iD + i4h (2) f =i n-1i = 8(n-i1)h i (2) i −n 1 −8(n i1)h − − i DD θ = tan-1 1 i i (3) θii= tan − (3) 2 fi 2| fi |

FigureFigure 1. 1. (a()a )Schematic Schematic diagram diagram of of random random microlens microlens array array (rMLA) (rMLA);; (b (b) )structural structural parameters parameters of of microlensmicrolens unit; unit; (c (c) )honeycomb honeycomb rMLA; rMLA; ( (dd)) rectangular rectangular rMLA. rMLA.

3.3. F Fabricationabrication of of random Random microlens Microlens array Array (rMLA (rMLA)) FFabricationabrication of of the the rMLA rMLA is is realized realized by by the the combinati combinationon of of chemical chemical etching etching and and lithography lithography technology.technology. Before Before the the glass glass substrate is chemicallychemically etched,etched, thethe chemical chemical etching etching solution solution needs need tos to be beprepared prepared and and the the substrate substrate needs need to bes to pretreated. be pretreated. The composition The composition and proportion and proportion of the solution of the solutionare super are important, super import whichant will, which significantly will significantly affect the chemical affect the etching chemical rate etching and the rate profile and of the the profilerMLA in of the the experiment, rMLA in the and experiment, further affect and the beamfurther homogeneity affect the as beam well. homogeneity For substrate as pretreatment, well. For substratethe surface pretreatment, of the substrate the surface is necessary of the substrate to be coated is necessary with chromium to be coated film aswith the chromium masking layerfilm as of thechemical masking etching. layer Inof particular,chemical etching. the fabrication In particular, of the rMLA the fabrication requires that of the randomrMLA require microporess that array the is processed on the surface of the chromium film, so that the chemical etching solution is able to react with the glass substrate.

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3.1. Preparation of Chemical Etching Solution In the experiment, ultra-white glass was selected as the substrate material, which has a transmittance of 91.5% in the visible regime. The main component of the selected substrate is SiO2, with a content of 73%. The contained impurities mainly involve Al2O3, Fe2O3, CaO, MgO, Na2O and K2O. Among them, CaO and Na2O occupy a relatively high proportion, which are 11% and 15%, respectively. The other impurities are low, at about 0.15%, which were not considered as key factors in this experiment. During chemical etching, three different chemical etching solutions were fabricated. The compositions of the chemical etching solution included deionized water, hydrofluoric acid solution (HF solution) (40%) and ammonium fluoride particles (NH4F), which was called the fundamental chemical etching solution. The proportion of ingredients was 10:3:1. Meanwhile, the units of deionized water and HF solution were “mL”, while the unit of NH4F was “g”. The HF solution is able to react with various oxides of ultra-white glass [32], and the chemical reaction formulas are expressed as Equations (4)–(6)

SiO + 4HF SiF + 2H O (4) 2 → 4 2 CaO + 2HF CaF + H O (5) → 2 2 Na O + 2HF 2NaF + H O (6) 2 → 2 where SiF4 will continue to react with the generated ﬂuoride, which can be expressed by Equations (7) and (8). Among these, the reaction rate of CaF2 is slow and will be residues

SiF + CaF CaSiF (7) 4 2 → 6 SiF + 2NaF Na SiF (8) 4 → 2 6 where SiF4 will react with the NH4F added in the chemical etching solution, expressed as Equation (9), to accelerate the etching rate of the glass.

SiF + 2NH F (NH ) SiF (9) 4 4 → 4 2 6 In the process of a reaction between the etching solution and glass substrate, the generated impurities CaF2 and Na2SiF6 with low solubility would adhere to the surface of the glass and prevent further etching. Although the chemical etching solution was continuously oscillated during the experiment to prevent impurities from adhering to the glass surface, the surface of the fabricated rMLA is not smooth, with a roughness of 106 nm, as shown in Figure2. Thereby, a lot of stray light is generated during the beam homogenization process, which seriously aﬀects the uniformity of the homogenized light spot. Micromachines 2020, 11, x 5 of 6

FigureFigure 2. The2. The rMLA rMLA fabricated fabricated by the fundamental fundamental chemical chemical etching etching solution. solution.

It was necessary to consider adding other chemical reagents to react with the generated impurities of the fundamental chemical etching to dissolve it in the etching solution. In this paper, the concentrated sulfuric acid (H2SO4) was selected to prepare the H2SO4 chemical etching solution. The components of the H2SO4 chemical etching solution were deionized water, HF solution (40%), and H2SO4 (95–98%). The proportion of ingredients was 5:2:2, and their units were “mL”. The diluted H2SO4 is able to react with the generated fluoride, which can be expressed as Equation 10:

 H2 SO 4 + CaF 2 CaSO 4 + 2HF (10)

As can be seen from Equation 10, H2SO4 is able to react with CaF2. The produced CaSO4 will dissolve to some extent, but the remaining impurities still adhered to the glass surface. Although the chemical etching solution was also oscillated in the experiment, the impurities still hindered any further etching of the glass. The experimental result shows that the surface of the fabricated rMLA is improved compared to the fundamental chemical etching solution. However, the surface of the fabricated rMLA by the H2SO4 chemical etching solution is still not smooth, with a roughness of 64 nm, as shown in Figure 3, which will affect the uniformity of the homogenized light spot.

Figure 3. The rMLA fabricated by the H2SO4 chemical etching solution.

Based on the above analysis, we added the concentrated nitric acid (HNO3) to prepare the HNO3 chemical etching solution in order to improve the surface quality of the fabricated rMLA. The compositions of the etching solution were deionized water, HF solution (40%) and the HNO3 (65–68%). The proportion of ingredients was 5:2:2 and their units were “mL”. The diluted HNO3 is able to react with the generated fluoride, which can be expressed as Equation 11:

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Micromachines 2020, 11, 338 5 of 12 Figure 2. The rMLA fabricated by the fundamental chemical etching solution. It was necessary to consider adding other chemical reagents to react with the generated impurities of theIt fundamental was necessary chemical to consider etching to adding dissolve other it in the chemical etching reagents solution. toIn this react paper, with the the concentrated generated impurities of the fundamental chemical etching to dissolve it in the etching solution. In this paper, sulfuric acid (H2SO4) was selected to prepare the H2SO4 chemical etching solution. The components the concentrated sulfuric acid (H2SO4) was selected to prepare the H2SO4 chemical etching solution. of the H2SO4 chemical etching solution were deionized water, HF solution (40%), and H2SO4 (95–98%). The components of the H2SO4 chemical etching solution were deionized water, HF solution (40%), The proportion of ingredients was 5:2:2, and their units were “mL”. The diluted H2SO4 is able to react withand H the2SO generated4 (95–98%). ﬂuoride, The proportion which can be of expressed ingredients as Equationwas 5:2:2 (10):, and their units were “mL”. The diluted H2SO4 is able to react with the generated fluoride, which can be expressed as Equation 10: H SO + CaF CaSO + 2HF (10) 2 4 2  4 H2 SO 4 + CaF 2→ CaSO 4 + 2HF (10) As can be seen from Equation (10), H SO is able to react with CaF . The produced CaSO will As can be seen from Equation 10, H22SO44 is able to react with CaF22. The produced CaSO44 will dissolve to some extent, but thethe remainingremaining impuritiesimpurities stillstill adheredadhered toto thethe glassglass surface.surface. Although the chemical etching solution waswas also oscillated in the experiment, the impurities still hindered any further etching of the glass.glass. The experimental result show showss that the surface of the fabricatedfabricated rMLArMLA isis improvedimproved comparedcompared toto thethe fundamentalfundamental chemicalchemical etchingetching solution.solution. However, thethe surface of the fabricated rMLA by the H SO chemical etching solution is still not smooth, with a roughness of 64 fabricated rMLA by the H2SO44 chemical etching solution is still not smooth, with a roughness of 64 nm,nm, as shown in Figure3 3,, which which will will a affectﬀect thethe uniformityuniformity ofof thethe homogenizedhomogenized lightlight spot.spot.

Figure 3. The rMLA fabricated by the H2SO4 chemical etching solution. Figure 3. The rMLA fabricated by the H2SO4 chemical etching solution.

Based on the above analysis, we addedadded thethe concentratedconcentrated nitric acid acid (HNO(HNO33) to prepare the HNO3 chemicalchemical etching etching solution solution in in order order to to improve improve the the surface surface quality quality of of the the fabricated rMLA rMLA.. The compositions of the etching solution were deionizeddeionized water, HFHF solutionsolution (40%)(40%) andand thethe HNOHNO33 (65–68%).(65–68%). The proportion of ingredientsingredients waswas 5:2:2 andand theirtheir unitsunits werewere “mL”.“mL”. TheThe diluteddiluted HNOHNO33 is able to react with the generated ﬂuoride,fluoride, whichwhich cancan bebe expressedexpressed asas EquationEquation (11):11:

CaF + 2HNO 2HF + Ca(NO ) (11) 2 3 → 3 2

From Equation (11), HNO3 is able to react with CaF2. Meanwhile, the generated Ca(NO3)2 is easily soluble in water. By continuously oscillating the chemical etching solution, the surface of the fabricated rMLA by the HNO3 chemical etching solution can be smooth, with a roughness of 13 nm, which reaches the surface quality of the optical device, as shown in Figure4. Therefore, the HNO 3 chemical etching solution is subsequently selected to carry out the experiment for fabricating the rMLA. Micromachines 2020, 11, x 6 of 6

 CaF2 + 2HNO 3 2HF + Ca(NO 3 )2 (11)

From Equation 11, HNO3 is able to react with CaF2. Meanwhile, the generated Ca(NO3)2 is easily soluble in water. By continuously oscillating the chemical etching solution, the surface of the fabricated rMLA by the HNO3 chemical etching solution can be smooth, with a roughness of 13 nm, which reaches the surface quality of the optical device, as shown in Figure 4. Therefore, the HNO3 Micromachineschemical2020 etching, 11, 338 solution is subsequently selected to carry out the experiment for fabricating the6 of 12 rMLA.

Figure 4. The rMLA fabricated by the HNO3 chemical etching solution. Figure 4. The rMLA fabricated by the HNO3 chemical etching solution. 3.2. Fabrication3.2. Fabrication of rMLA of rMLA TwoTwo types types of rMLAsof rMLAs are are fabricated fabricated inin this this paper, which which are are the the honeycomb honeycomb rMLA rMLA and andthe the rectangularrectangular rMLA, rMLA, respectively. respectively. The The fabrication fabrication requires the the pretreatment pretreatment of the of thesurface surface of the of glass the glass substrate—that is, the random micropores array with different structures needs to be fabricated on substrate—that is, the random micropores array with different structures needs to be fabricated on the the surface of the substrate. surface of the substrate. 3.2.1. Fabrication of Honeycomb rMLA 3.2.1. Fabrication of Honeycomb rMLA For the fabrication of the honeycomb rMLA, a chromium film with a thickness of about 140 nm Forwas thefirst fabrication coated on a of glass the substrate, honeycomb as shown rMLA, in a Figure chromium 5a. Secondly, film with the a glass thickness substrate of about was put 140 nm was firstinto the coated chromium on a glassremoval substrate, solution and as shown soaked infor Figure 10 s to 5etcha. Secondly,a series of random the glass micropores substrate on was the put into thechromium chromium film, removalas shown solution in Figure and5b. Finally, soaked the for preprocessed 10 s to etch substrate a series of was random put into micropores the chemical on the chromiumetching film, solution as shown to fabricate in Figure the honeycomb5b. Finally, rMLA. the preprocessed During the etching substrate process, was the put etching into thesolution chemical etchingwas solution continuo tously fabricate oscillated the honeycomb and the substrate rMLA. was During cleaned the every etching 10 process, min in order the etching to prevent solution the was continuouslydetached oscillatedchromium andfilm thefrom substrate affecting wasthe profile cleaned of the every rMLA. 10 min Meanwhile, in order the to preventtotal etching the time detached was about 25 min. The experimental result observed by an OLYMPUS BX51 microscope is shown in chromium film from affecting the profile of the rMLA. Meanwhile, the total etching time was about Figure 6. As can be seen from the microscopic image of the fabricated honeycomb rMLA, the surface 25 min. The experimental result observed by an OLYMPUS BX51 microscope is shown in Figure6. of the rMLA is composed of irregular microstructures, making the overall profile look like a As canhoneycomb. be seen from Meanwhile, the microscopic the surface imageof the honeyco of the fabricatedmb rMLA honeycombhas quite a high rMLA, filling the ratio, surface which of the rMLAguarantees is composed that the of irregular whole beam microstructures, passing through making the rMLA the is overall able to profile be modulated look like, so a thathoneycomb. the Meanwhile,uniformity the of surface the generated of the honeycomblight filed is further rMLA improved. has quite Moreover, a high filling the surface ratio, which of the honeycomb guarantees that the wholerMLA beamis extremely passing smooth through, with the a roughness rMLA is able of 13 to nm be, as modulated, shown in Figure so that 6, which the uniformity verifies that of the generatedhigh-quality light filed rMLA is furthercan be improved. fabricated by Moreover, our method. the surface However, of the due honeycomb to the irregularity rMLA is of extremely the smooth,structure with parameters a roughness of ofthe 13 honeycomb nm, as shown rMLA, in it Figure is difficult6, which to measure verifies the that sagittal high-quality heights and rMLA the can be fabricatedapertures. by our method. However, due to the irregularity of the structure parameters of the honeycombMicromachines rMLA, 2020, it11 is, x difficult to measure the sagittal heights and the apertures. 7 of 6

Figure 5. Fabrication schematic of honeycomb rMLA: (a) chromium plating; (b) corroded chromium Figure 5. Fabrication schematic of honeycomb rMLA: (a) chromium plating; (b) corroded chromium ﬁlm; and (c) honeycomb rMLA obtained by chemical etching. film; and (c) honeycomb rMLA obtained by chemical etching.

Figure 6. Microscopic image of the honeycomb rMLA.

3.2.2. Fabrication of Rectangular rMLA The fabrication of the rectangular rMLA, which is slightly different from the honeycomb rMLA, requires a mask predesigned and prefabricated. Then, through exposure, development, chromium removal and other processes, the random microporous structure on the mask plate was transferred to the chromium film of the substrate. Finally, chemical etching was performed to generate rMLA. The designed mask is a micropores array of random rectangular arrangement, as shown in Figure 7a. The radiuses of micropores are the same as 1.8 μm, and the distances between adjacent micropores varies randomly from 24 μm to 45 μm. The mask was fabricated by high-precision laser direct writing technology with a fabrication accuracy of ±250 nm. The pattern of the mask plate observed under the microscope is shown in Figure 7b.

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MicromachinesFigure 5.2020 Fabrication, 11, 338 schematic of honeycomb rMLA: (a) chromium plating; (b) corroded chromium7 of 12 film; and (c) honeycomb rMLA obtained by chemical etching.

Figure 6. Microscopic image of the honeycomb rMLA.

3.2.2. Fabrication of R Rectangularectangular rMLA The fabrication of the rectangular rMLA rMLA,, which is slightly different diﬀerent from the honeycomb rMLA rMLA,, requires a maskmask predesignedpredesigned andand prefabricated.prefabricated. Then,Then, throughthrough exposure,exposure, development,development, chromium removal andand otherother processes,processes, thethe random random microporous microporous structure structure on on the the mask mask plate plate was was transferred transferred to theto the chromium chromium ﬁlm film of theof the substrate. substrate. Finally, Finally, chemical chemical etching etching was was performed performed to generateto generate rMLA. rMLA. The designed mask is a microporesmicropores arrayarray of randomrandom rectangularrectangular arrangement, as shown in Figure7 7a.a.The The radiusesradiuses ofof microporesmicroporesare arethe thesame sameas as1.8 1.8 µμm,m, andand thethe distancesdistances betweenbetween adjacent micropores varies randomly from 24 µμmm to 45 µμm.m. The mask was fabricated by high-precisionhigh-precision laser direct writing technology with aa fabricationfabrication accuracyaccuracy of of ±250250 nm. nm. The The pattern of the mask plate ± observed under the microscope is shown in Figure7b. . Micromachinesobserved under 2020, 11the, x microscope is shown in Figure 7b 8 of 6

FigureFigure 7. MaskMask part part pattern: pattern: (a ) (a rectangular) rectangular randomly randomly arranged arranged mask mask data; data;and ( band) microscopy (b) microscopy pattern patternof mask of made mask by made laser by direct laser writing direct writing technology. technology.

Subsequently,Subsequently, the the chromium chromium film ﬁlm,, with with a a thickness thickness of of about about 140 140 nm nm,, was was coated coated on on the the glass glass substrate,substrate, as as shown shown in in Figure Figure 88a.a. The photoresist of the type AZ MIR-703MIR-703 PhotoresistPhotoresist (14(14 cp)cp) (AZ(AZ ElectronicElectronic materials, materials, Somerville, Somerville, MA, MA, USA) USA) was was spin spin-coated-coated on on the the glass substrate substrate with with chromium filmﬁlm at at a a speed speed of of 4000 4000 r/min r/min with with a a time time o off 30 30 s. T Thehe thickness thickness of of the the photoresist photoresist was was about about 700 700 nm, nm, asas shown shown in in Figure 88b.b. The photoresist was then exposedexposed byby thethe exposureexposure machinemachine withwith aa centercenter wavelength of of 365 365 nm. The The exposure exposure method method was was contact contact exposure exposure with with an an exposure exposure time time of of 20 20 s s andand the the ex exposureposure power power of of 3 3 mW, mW, as shown in Figure 88c.c. ByBy thethe developmentdevelopment with developer of AZ 300MIF300MIF (AZ (AZ Electronic Electronic materials, materials, Somerville, Somerville, MA, MA, USA) USA) for for 35 35 s, s, the the microporous microporous structure structure on on the the mask was transferred to the photoresist, as shown in Figure 8d. The substrate was then put into the chromium removal solution for 40 s to transfer the microporous structure on the photoresist to the chromium film to complete the substrate pretreatment, as shown in Figure 8e. The substrate was finally put into the etching solution for etching. The etching solution penetrated from the micropores and contacted with the glass surface to produce a reaction. After the glass substrate was etched for a period of time, micropits were generated on the substrate surface, as shown in Figure 8f. The operation steps in the etching process were consistent with the fabrication of the honeycomb rMLA. During the etching process, the etching solution was continuously vibrated to prevent the falling chromium film from affecting the surface shape of rMLA. The glass substrate was cleaned every 10 min or so. After etching for about 40 min, the rMLA was obtained, as shown in Figure 8g. The experimental result obtained with the microscope is shown in Figure 9a, where it can be clearly seen that the structure is random due to the irregularity of arrangement of microlenses, the surface is smooth with a roughness of 13 nm, the filling ratio is high and the arrangement of rectangular rMLA is basically consistent with the arrangement of Figure 7b. Comparing the profiles data of the microlenses measured with the step profilometer (Stylus Profiler System, Dektak XT, Bruker, Karlsruhe, Germany) with an ideal spherical surface, it can be seen that the profile of the microlenses is almost spherical, and the fitting graph is shown in Figure 9b.

Micromachines 2020, 11, 338 8 of 12 mask was transferred to the photoresist, as shown in Figure8d. The substrate was then put into the chromium removal solution for 40 s to transfer the microporous structure on the photoresist to the chromium film to complete the substrate pretreatment, as shown in Figure8e. The substrate was finally put into the etching solution for etching. The etching solution penetrated from the micropores and contacted with the glass surface to produce a reaction. After the glass substrate was etched for a period of time, micropits were generated on the substrate surface, as shown in Figure8f. The operation steps in the etching process were consistent with the fabrication of the honeycomb rMLA. During the etching process, the etching solution was continuously vibrated to prevent the falling chromium film from affecting the surface shape of rMLA. The glass substrate was cleaned every 10 min or so. After etching for about 40 min, the rMLA was obtained, as shown in Figure8g. The experimental result obtained with the microscope is shown in Figure9a, where it can be clearly seen that the structure is random due to the irregularity of arrangement of microlenses, the surface is smooth with a roughness of 13 nm, the filling ratio is high and the arrangement of rectangular rMLA is basically consistent with the arrangement of Figure7b. Comparing the profiles data of the microlenses measured with the step profilometer (Stylus Profiler System, Dektak XT, Bruker, Karlsruhe, Germany) with an ideal spherical surface, it can be seen that the profile of the microlenses is almost spherical, and the fitting graph is shownMicromachinesMicromachines in Figure 20 2020209, b.11, 11, x, x 9 9of of 6 6

FigureFigureFigure 8. 8. Fabrication8. Fabrication Fabrication schematic schematic schematic ofof of rectangularrectangular rectangular rMLA: rMLA: ( (a(a)a) )chromium chromiumchromium plating; plating; plating; ( b( (b)b )coated )coated coated photoresist; photoresist; photoresist; (c)(c exposure;()c )exposure; exposure; (d ( )d( development;d) )development; development; ((e e())e chromium)chromium chromium removal; removal; ( (f(f)f) )under underunder etching; etching;etching; and and and ( g( (g)g )rectangular) rectangular rectangular rMLA rMLA rMLA obtainedobtainedobtained by by by chemical chemical chemical etching. etching. etching.

FigureFigureFigure 9. 9. 9. Micrographs MicrographsMicrographs and and and simulation simulation simulation profiles profiles proﬁles:: :( a()a )m microscopic (icroscopica) microscopic image image of imageof rectangular rectangular of rectangular rMLA rMLA; ;and and rMLA; ( b(b) ) andcomparisoncomparison (b) comparison of of microlenses microlenses of microlenses profile profile proﬁleand and ideal ideal and sphere. sphere. ideal sphere.

4.4. Experiments Experiments BasedBased on on the the fabricated fabricated honeycomb honeycomb random random microlens microlens array array ( rMLA)(rMLA) and and rectangular rectangular rMLA, rMLA, anan o opticalptical setup setup was was constructed constructed to to test test the the uniformity uniformity of of the the homogenized homogenized light light spot spots sgenerated generated by by thethe two two types types of of rMLA rMLAs,s ,as as shown shown in in Figure Figure 10a. 10a. TheThe rMLA rMLA was was illuminated illuminated by by a a laser laser beam beam with with a a wavelength wavelength of of 650 650 nm nm and and diameter diameter of of 5 5 mm.mm. After After the the incident incident li lightght was was modulated modulated by by the the rMLA, rMLA, a a homogenized homogenized light light spot spot was was able able to to be be obtainobtaineed.d. The The charges charges coupling coupling device device ( CCD(CCD) )was was then then used used to to collect collect and and record record the the intensity intensity of of thethe homogenized homogenized light light spot, spot, as as shown shown in in Figure Figure 1 100aa. .In In order order to to characterize characterize the the diverge divergencence angle angle of of thethe reconstructed reconstructed light light spot, spot, the the sagittal sagittal height heights sand and the the aperture apertures sof of the the fabricated fabricated rMLA rMLA need neededed to to bebe measured. measured. Subsequently, Subsequently, the the divergence divergence angle angle c couldould be be calculated calculated by by Equations Equations 1 1––3.3. Using Using the the theoreticaltheoretical equations, equations, the the divergence divergence angle angle o of fthe the rectangular rectangular rMLA rMLA c ouldcould be be obtained. obtained. However, However, it it wawas s difficult difficult to to measure measure the the sagittal sagittal height heights s andand the the apertureapertures s duedue to to thethe irregularity irregularity of of the the structurestructure parameters parameters of of the the honeycomb honeycomb rMLA rMLA, ,resulting resulting in in the the divergence divergence angle angle of of the the honeycomb honeycomb rMLArMLA notnot bebeinging directlydirectly calculated calculated by by the the theoretical theoretical Equations Equations 1 1––3.3. Therefore, Therefore, the the geometric geometric calculationcalculation method method wa was s performed performed to to calculated calculated the the divergence divergence angle angle of of the the honeycomb honeycomb rMLA, rMLA, whichwhich w wouldould be be described described as as follows. follows. As As c ouldcould be be seen seen from from Figure Figure 1 100bb, ,the the propagat propagationion distance distance Z, the size of the incident light T and the reconstructed light spot T were able to be measured. Z, the size of the incident light T0 0 and the reconstructed light spot T were able to be measured.

Micromachines 2020, 11, x 10 of 6

Therefore, the geometric relationship of these four parameters (θ, T0 , T and Z) can be expressed as Equation 12. T-T θ = tan-1 0 (12) 2Z

Figure 10. Experimental measurement: ( (aa)) the the experimental setup; and (b) the diagram of geometricgeometric calculation method.

TheSubsequently, rMLA was the illuminated uniformity byand a energy laser beam utilization with aof wavelengththe homogenized of 650 light nm spot and were diameter tested. of 5The mm. incident After light the incident was an uneven light was light modulated spot with by a thewavelength rMLA, a of homogenized 650 nm, as shown light spotin Figure was able11a. toA becircular obtained. homogenized The charges light coupling spot was device generated (CCD) by was a honeycomb then used torMLA, collect as and shown record in Figure the intensity 11b, and of thea rectangular homogenized homogenized light spot, light as shown spot was in Figure generated 10a. Inby order a rectangular to characterize rMLA, the as divergenceshown in Figure angle of11c the. The reconstructed uniformity of light the spot, light thespot sagittal was calculated heights and by theusing apertures Equation of 13 the, where fabricated the uniformit rMLA neededies of tothe be circular measured. spot Subsequently,and the rectangle the divergencespot were about angle 81% could and be 88%, calculated respectively. by Equations The power (1)–(3). ( P Usingin ) of the theoretical equations, the divergence angle of the rectangular rMLA could be obtained. However, the input beam measured with the power meter was about 3.5 mW, and the output powers ( Pout ) it was diﬃcult to measure the sagittal heights and the apertures due to the irregularity of the structure after passing through the honeycomb rMLA and the rectangular rMLA were all about 3.2 mW. parameters of the honeycomb rMLA, resulting in the divergence angle of the honeycomb rMLA not According to Equation 14, the energy utilization rate was about 90%. Therefore, it can be concluded being directly calculated by the theoretical Equations (1)–(3). Therefore, the geometric calculation that both the honeycomb rMLA and the rectangular rMLA have a quite high energy utilization rate. method was performed to calculated the divergence angle of the honeycomb rMLA, which would be described as follows. As could be seen from Figure 10b, the propagation distance Z, the size of the incident light T0 and the reconstructed light spot T were able to be measured. Therefore, the geometric relationship of these four parameters (θ, T0, T and Z) can be expressed as Equation (12).

1 T T0 θ = tan− − (12) 2Z

The measured T0 was 5 mm and Z was 27 mm, the homogenized light spot size T was calculated based on the MATLAB (version 7.1, MathWorks, Natick, MA, USA) numerical analysis software. At the edge of the spot, the dimension when the energy drops to 1/e2 is recorded as T, which can be obtained by multiplying the number of pixels occupied by the light spot and the pixel size of the CCD. For the honeycomb rMLA, the number of pixels occupied by a circular light spot was 1177 at the central cross section, and the pixel size of the CCD was 7.4 µm. Therefore, the calculated T was about 8.7 mm. On the basis of the geometric calculation method, according to Equation (12), the divergence angle of the circular light spot could be obtained, which was about 7.8◦. For the rectangular rMLA, the aperture and depth of sub lenses in multiple rMLA were measured by the step proﬁlometer (Stylus Proﬁler System, Dektak XT, Bruker, Karlsruhe, Germany), according to Equations (1)–(3), the average value of departure divergence was about 22◦. Meanwhile, the geometric calculation method was also performed to calculated the divergence angle of the rectangular rMLA. According to the experimental Micromachines 2020, 11, 338 10 of 12 measurement, the number of pixels occupied by the rectangular light spot was 2161 at the central cross section and the calculated T was 16 mm. Therefore, according to Equation (12), it could be calculated that the divergence angle of the rectangular light spot was about 23◦, which is basically consistent with the average divergence angle calculated by the theoretical Equations (1)–(3). Subsequently, the uniformity and energy utilization of the homogenized light spot were tested. The incident light was an uneven light spot with a wavelength of 650 nm, as shown in Figure 11a. A circular homogenized light spot was generated by a honeycomb rMLA, as shown in Figure 11b, and a rectangular homogenized light spot was generated by a rectangular rMLA, as shown in Figure 11c. The uniformity of the light spot was calculated by using Equation (13), where the uniformities of the circular spot and the rectangle spot were about 81% and 88%, respectively. The power (Pin) of the input beam measured with the power meter was about 3.5 mW, and the output powers (Pout) after passing through the honeycomb rMLA and the rectangular rMLA were all about 3.2 mW. According to Equation (14), the energy utilization rate was about 90%. Therefore, it can be concluded that both the honeycomb rMLA and the rectangular rMLA have a quite high energy utilization rate. Micromachines 2020, 11, x 11 of 6 uv tu N 2 Micromachines 2020, 11, x X 11 of 6 − RMS = N (Ij- 2 I) /N (13) − (13) RMS=j (Ij -I) /N j N - 2 RMS= (I -I) /N (13) Pjout P =j Pout (14) P= P (14) Pinin Pout P= (14) Pin

Figure 11. Homogenization spot effect collected by CCD: (a) the incident laser beam; (b) the circular Figure 11. Homogenization spot eﬀect collected by CCD: (a) the incident laser beam; (b) the circular homogenization spot reconstructed by the honeycomb rMLA; and (c) the rectangular homogenization spot reconstructed by the honeycomb rMLA; and (c) the rectangular homogenization homogenizationFigure 11. Homogenization spot recons tructedspot effect by thecollected rectangular by CCD: rMLA (a) the. incident laser beam; (b) the circular spot reconstructedhomogenization by the spot rectangular reconstructed rMLA. by the honeycomb rMLA; and (c) the rectangular Manyhomogenization incident spot laser recons beamstructed with by the arbitrary rectangular profile rMLAs and. uneven intensity distribution, in Manyaddition incident to those laser mentioned beams above, with arbitrarycan also be proﬁles modulated and by uneven the rMLA intensity to generate distribution, a homogenized in addition to thoselight mentioned Manyspot. Forincident above, instance, laser can utilizing alsobeams be the with modulated fabricated arbitrary rectangular by profile the rMLAs and rMLA, unevento generate the laser intensity beam a homogenized distribution, with the cross light in spot. For instance,spotaddition (Figure utilizing to those 12a) thecanmentioned be fabricated shaped above, into rectangular cana rectangular also be rMLA,modulated homogenized the by laser the light rMLA beam spot to with (Figure generate the 1 cross2 ab ).homogenized spot (Figure 12a) light spot. For instance, utilizing the fabricated rectangular rMLA, the laser beam with the cross can be shaped into a rectangular homogenized light spot (Figure 12b). spot (Figure 12a) can be shaped into a rectangular homogenized light spot (Figure 12b).

Figure 12. Beam homogenization with an arbitrary shape laser beam: (a) free-form incident laser beam; and (b) the reconstructed rectangular homogenization spot. Figure 12.FigureBeam 12. homogenizationBeam homogenization with with an arbitraryan arbitrary shape shape laser laser beam: beam: (a (a)) free-form free-form incident incident laser laser beam; 5. Conclusionbeam; and (b) the reconstructed rectangular homogenization spot. and (b) the reconstructed rectangular homogenization spot. This paper proposed a method of combining chemical etching with lithography technology to 5. Conclusion prepare the random microlens array (rMLA) to break the periodicity of the MLA, suppress the cohereThisnce paper during propose the homogenizationd a method of combining process and chemical obtain etching a homogenized with lithography light spot technology with high to energyprepare utilization. the random Through microlens measurement, array (rMLA) the uniformityto break the of periodicitythe circular oflight the spot MLA was, suppress about 81%, the whilecohere thence rectangular during the light homogenization spot was about process 88%. and The obtain energy a utilization homogenized rate light of the spot homogenized with high lightenergy spots utilization. by the two Through types measurement,of rMLAs was the about uniformity 90%. The of designedthe circular rMLA light can spot be was expected about to81%, be usedwhile in the laser rectangular welding machinelight spots, medicalwas about treatment, 88%. The exposure energy machine utilization lighting rate of system the homogenizeds and other fields.light spots by the two types of rMLAs was about 90%. The designed rMLA can be expected to be used in laser welding machines, medical treatment, exposure machine lighting systems and other Author Contributions: Conceptualization, L.X.; Formal analysis, L.X.; Funding acquisition, Q.D.; Investigation, fields. Y.P. and W.L.; Software, W.L., L.L. and A.C.; Validation, H.P.; Writing—original draft, L.X.; Writing—review & editing,Author Contributions:Y.P., A.C., L.S. and Conceptualization Y.F. All authors, L.X.;have Formal read and analysis, agreed L.X.; to the Funding published acquisition, version of Q.D.; the manuscript.Investigation, Y.P. and W.L.; Software, W.L., L.L. and A.C.; Validation, H.P.; Writing—original draft, L.X.; Writing—review &

editing, Y.P., A.C., L.S. and Y.F. All authors have read and agreed to the published version of the manuscript.

Micromachines 2020, 11, 338 11 of 12

5. Conclusions This paper proposed a method of combining chemical etching with lithography technology to prepare the random microlens array (rMLA) to break the periodicity of the MLA, suppress the coherence during the homogenization process and obtain a homogenized light spot with high energy utilization. Through measurement, the uniformity of the circular light spot was about 81%, while the rectangular light spot was about 88%. The energy utilization rate of the homogenized light spots by the two types of rMLAs was about 90%. The designed rMLA can be expected to be used in laser welding machines, medical treatment, exposure machine lighting systems and other ﬁelds.

Author Contributions: Conceptualization, L.X.; Formal analysis, L.X.; Funding acquisition, Q.D.; Investigation, Y.P. and W.L.; Software, W.L., L.L. and A.C.; Validation, H.P.; Writing—original draft, L.X.; Writing—review & editing, Y.P., A.C., L.S. and Y.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by National Natural Science Foundation of China (Nos. 61605211, 61905251); The Instrument Development of Chinese Academy of Sciences (No. YJKYYQ20180008); The National R&D Program of China (No. 2017YFC0804900); Sichuan Science and Technology Program (No. 2019YJ0014); Youth Innovation Promotion Association, CAS and CAS “Light of West China” Program. The authors thank their colleagues for their discussions and suggestions to this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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