Versatile route to gapless microlens arrays using laser-tunable wet-etched curved surfaces

This work reveals a cost-efficient and flexible approach to various microlens arrays on polymers, which is essential to micro-optics elements. An 800-nm femtosecond laser is employed to control the hydrofluoric (HF) acid etching process on silica glasses, and concave microstructures with smooth curved surfaces are produced by this method. Then, the micro-structured glass templates can serve as molds for replicating microlenses on polymers. In this paper, a high-ordered microlens array with over 16,000 hexagonal-shaped lenses is fabricated on poly (dimethyl siloxane) [PDMS], and its perfect light-gathering ability and imaging performance are demonstrated. The flexibility of this method is demonstrated by successful preparation of several concave molds with different patterns which are difficult to be obtained by other methods. This technique provides a new route to small-scaled, smooth and curved surfaces which is widely used in micro-optics, biochemical analysis and superhydrophobic interface. ©2012 Optical Society of America OCIS codes: (140.7090) Ultrafast lasers; (160.5470) Polymers; (220.0220) Optical design and fabrication; (230.3990) Micro-optical devices. References and links 1. H. A. Biebuyck and G. M. Whitesides, “Self-organization of organic liquids on patterned self-assembled monolayers of alkanethiolates on gold,” Langmuir 10(8), 2790–2793 (1994). 2. M. H. Wu, C. Park, and G. M. Whitesides, “Fabrication of arrays of microlenses with controlled profiles using gray-scale microlens projection photolithography,” Langmuir 18(24), 9312–9318 (2002). 3. Q. Peng, Y. Guo, S. Liu, and Z. Cui, “Real-time gray-scale photolithography for fabrication of continuous microstructure,” Opt. Lett. 27(19), 1720–1722 (2002). 4. S. K. Lee, K. C. Lee, and S. S. 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Commun. 282(20), 4119–4123 (2009). #167178 $15.00 USD Received 20 Apr 2012; revised 9 May 2012; accepted 10 May 2012; published 23 May 2012 (C) 2012 OSA 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12939 13. S. S. Mao, F. Quéré, S. Guizard, X. Mao, R. E. Russo, G. Petite, and P. Martin, “Dynamics of femtosecond laser interactions with dielectrics,” Appl. Phys. (Berl.) 79, 1695–1709 (2004). 14. W. G. Roeterdink, L. B. F. Juurlink, O. P. H. Vaughan, J. Dura Diez, M. Bonn, and A. W. Kleyn, “Coulomb explosion in femtosecond laser ablation of Si(111),” Appl. Phys. Lett. 82(23), 4190 (2003). 15. H. Hu, X. Wang, H. Zhai, N. Zhang, and P. Wang, “Generation of multiple stress waves in silica glass in high fluence femtosecond laser ablation,” Appl. Phys. Lett. 97(6), 061117 (2010). 16. M. Budiman, E. M. Hsu, H. K. Haugen, and G. A. Botton, “Cross-sectional study of femtosecond laser bulk modification of crystalline α-quartz,” Appl. Phys., A Mater. Sci. Process. 98(4), 849–853 (2010). 17. T. 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Introduction
Due to their low-cost, comparable transparency and ease of preparation, polymers are widely used to fabricate micro-optics elements such as two-dimensional (2D) refractive microlens arrays (MLAs).Combined with other functional devices, polymer MLAs play a key role in optical communications, high-definition display, biomedical systems and so on.In the past decades, a variety of strategies have been adopted for manufacturing MLAs.In general, the majority of fabrication process for polymer MLAs, including photoresist reflow method [ [1,2]], gray-scale photolithography [3], LIGA process (German acronym for lithography, electroforming, and molding) [4], and so on, requires masks for photolithography, expensive equipments, and complex and elaborate processing.Moreover, it is difficult to obtain desired profiles of the microlenses.Some maskless approaches such as ink-jet process and selfassembled particle monolayers demonstrate the ability to produce semi-spherical polymeric microlenses [5,6], but the sophisticated processes are needed to precisely control the alignment of each lens.
In recent years, femtosecond (fs) laser direct writing process has become a popular tool to fabricate arbitrary three-dimensional (3D) microstructures with nanometer accuracies.Femtosecond-laser-induced two-photon polymerization (TPP) allows for the formation of microlenses or more complex 3D microstructures in polymers [7][8][9].However, this point-bypoint process suffers from the limitation of low efficiency.Recently, H. Sun and associates have proposed an improved TPP process by the usage of the negative tone resin SU-8 [10,11].Only the surface layer of microstructures, or nanoshells, was photopolymerized by direct laser writing, and the inner portion was solidified by ultra-violet lights.The fabrication efficiency was increased significantly.For example, it needed only 6 minutes to fabricate a microlens structure with a diameter of 80 µm [11], which typically cost several tens of minutes for the previous TPP technique.Despite this, it is still not suitable for fabrication of large-area MLAs with over thousands of lenses.
Replica molding process is an inexpensive and reproducible method to produce MLAs, and could be used for large-scale industrial production.But fabrication of molding templates for replicating microlenses is still a challenge.Herein, we present a novel approach for fast fabrication of large-area concave microlens structures on silica glasses, which can be used as molding templates for the replication of MLAs on polymers.Our approach differs from the classical fs laser direct writing process.It involves ms-time-scaled in situ laser exposures and hydrofluoric (HF) acid wet etch process.The focused fs laser pulses trigger a series of complex material responses and change its physical and chemical properties in the focal spots; the concave curved structures with smooth surfaces then form in those laser-modified spots with the aid of chemical wet etch.We demonstrate the ability to control the size, alignment and shape of the curved structures through the proper control of the laser parameters, chemical etching time and precise arrangement of laser exposure spots.The convex MLAs can be replicated on polymers by the molding processes from the laserfabricated glass molding templates.This technique offers several advantages over previous methodologies of microlens fabrication.First, MLAs can be rapidly and stably fabricated.For example, a glass mold of over 16,000 concave structures with diameters of 80 µm was produced within 3 hours.The fabrication efficiency has been improved by orders of magnitude comparing to the TPP process.Second, the maskless process allows for the scalable shape-controlled fabrication of MLAs.To simplify the expression of this work, we introduce the fabrication of a gapless hexagonal-shaped microlens array in details, which is produced on poly (dimethyl siloxane) [PDMS]; other curved microstructures with different shapes, sizes and alignments are also introduced to demonstrate flexibility of this method.In addition, the fabricated concave molds are also suitable for fabricating microlenses on other polymers.The fabrication procedures of the polymeric MLAs are schematically depicted in Fig. 1, which comprise of two steps of producing molding templates with concave microstructures and replicating the convex microlenses on polymers.To fabricate molding templates, an fs laser beam, which is created by a Ti: sapphire pulsed laser oscillator-amplifier system (wavelength = 800 nm; pulse duration = 30 fs; repetition rate = 1 kHz), is focused onto the surface of a polished silica glass chip by a microscope objective lens (NA = 0.5, Nikon).The laser exposure spots are generated point-by-point.For each spot, the exposure time is controlled by a fast mechanical shutter and the laser power is tuned by a variable attenuator.The optical setup has been reported in other literatures [12].After the laser exposures, the sample is immersed in 5% hydrofluoric (HF) acid solution (diluted by deionized water) at room temperature.To guarantee the uniformity of the microstructures, an ultrasonic bath is used to remove the bubbles generated at liquid-solid interface during the chemical etching process.The fabrication process can be monitored by an optical microscope equipped with a CCD camera.When the smooth concave surfaces are successfully fabricated, the sample is cleaned by deionized water and dried.

Fabrication procedures and equipments
The convex MLAs are replicated on poly (dimethyl siloxane) [PDMS] by the fabricated mold.The liquid PDMS is prepared by mixing Dow Corning Sylgard 184 with catalyst (10:1 ratio) and degassing in vacuum for 30 minutes.The mixture is then poured onto the mold and cured at 70°C for 100 minutes.

Characterization of the microlens arrays
The morphology of the polymeric MLAs is observed by a field-emission scanning electronic microscope (FE-SEM, JEOL JSM-7000F).The samples for SEM observations are pretreated by coating a thin film of Pt atoms with thickness of a few nanometers and used voltage was 20 keV.The cross-sectional and 3D profiles of the microlenses are measured by a laser scanning confocal microscope (LSCM, LASETEC S130).The imaging ability of the MLAs is obtained by the optical microscope system (OM, NIKON CV-100) with tungsten light source.One effect that induces the material modifications is brought by strong shockwaves generated by the laser-matter interactions.With the help of schematic diagrams shown in Fig. 2(d), the process could be simply described.When the fluence of incoming fs laser pulses is higher than the damage threshold of silica glass, the energy of photons deposits into the material because of the nonlinear effects such as multiphoton absorption process [13]; atoms are ionized and Coulomb explosion is then triggered within a couple of picoseconds [14].The explosion-produced plasmas burst out of the focal spot, forming a damage crater on the sample surface as shown in Fig. 2(b).A shockwave is then produced by a strong repelling force induced by the rapid expansion of the ablation plume; the lattices of the surrounding materials are consequently compressed.In the next few nanoseconds, several shockwaves are produced by the repeated compression and release of the materials.The time-resolved shadowgraphs of the multiple shockwaves inside silica glass and ejections of materials have been observed through a pump-probe technique [15].In addition, the damages have been visualized by the cross-sectional transmission electron microscope investigations [16,17].It is demonstrated that the fs-laser-induced damage region inside a silicon chip could range from hundreds of nanometers to micrometers, which depends on the used laser power [17].In our experiment, intense fs laser pulses will produce a micrometer-sized modified region, which is rapidly etched out by HF acid at higher etching speed.After then, the etching speed inside concave structures is identical to that of unexposed materials.The diameter of the structures increases linearly and the depth is constant.

Fabrication of the mold with hexagonal-shaped concave structures
Proper arrangement of the exposure spots is important to fabricate concave structures served as molds.For example, trianglular-aligned spots allow for producing hexagonal-shaped concave structures.Each fabrication process could be observed under an optical microscope equipped with a CCD camera, which is shown in Fig. 3(a).The interspacing between neighboring laser exposure spots is 80 µm, which is precisely controlled by the 3D stage.The utilized laser power, P, is 2.5 mW and the exposure time is 500 ms.Because the energy density is much higher than the breakdown threshold of silica glass (~2 J/cm 2 ), micrometersized ablation-induced craters can be clearly observed as black dots as shown in the left image of Fig. 3(a).The morphology evolution of the laser exposure spots under the HF treatment is given in the rest three figures of Fig. 3(a), which were captured at 15, 35 and 60 minutes, respectively.The micro-craters are expanded and progressively formed circular patterns.Finally, these expanding circular structures "squeeze" with each other and the hexagonal-shaped structures are produced.The SEM image of the fabricated hexagonal microstructures is shown in Fig. 3(b), demonstrating the ability to fabricate high-ordered, uniform and smooth microstructures by this method.Fill factor is an important parameter for high-quality MLAs; close-packed MLAs are preferred in display and lighting systems because they can gather more light and improve the performance of devices.This method enables one-step and fast fabrication of 100% fill-factors concave molds for replicating MLAs, which is difficult to be obtained by other methods.In addition, an array of over 16,000 hexagonal-shaped microstructures is successfully manufactured within 3 hours, indicating the high-efficiency of this method.The 3D and cross-sectional profiles are investigated by the LSCM and the results are shown in Figs.3(c) and 3(d), respectively.The measured value of diameter and depth of the concave structures is 80.7 µm and 6.7 µm, respectively.The depth of the microstructures can be increased using higher laser powers, which will be discussed in the following section.

Control of the size and shape of the concave microstructures
The range of the laser-modified materials is closely associated with the laser parameters such as the laser power, as discussed above, and it will consequently influence the size of the wetetched concave microstructures.To study the power-dependency of the size of concave structures, groups of laser exposure spots are produced on the silica glass chips with different laser powers.The glass chips are then treated by the HF acid solution for 40 minutes.In the experiments, the laser power ranges from 0.3 mW to 5 mW, and it has been confirmed that regular structures will not be obtained by the out-of-range powers.The results of the diameter and the depth of the formed concave structures are shown in Figs.4(a) and 4(b), respectively.The insertions in Fig. 4(a), from left to right, present the OM results produced by 0.5-mW, 1.5-mW, 2.5-mW, 3.5 mW and 4.5-mW exposures, respectively, with the same exposure time of 500 ms.It indicates that the depth and diameter of the concave structures are closely related to the laser power and the values increase with the increase of power.For the gapless MLAs, the diameter, r, of the lens is determined by the interspacing of the neighboring exposure spots, and the sag-height, h, can be increased by higher laser powers.The relationships between laser exposure time and microlens sizes are also investigated by the similar method.Concave structures are produced by different laser exposure times ranging from 10 ms to 3000 ms, and the laser power is 1.5 mW and etching time is 40 minutes.The  (c) and 4(d) demonstrate that both diameter and depth of the structures have weak dependency to the laser exposure time.Although the laser-exposure durations will not significantly tune the final size of the structures, we found that a longer duration is important to fabricate uniform microlenses.As discussed above, the size of concave structures is closely related to the employed laser power, and the inevitable power fluctuation of the output laser pulses will impact on the uniformity of the MLAs.Unstable laser-modified regions caused by the power fluctuation will be abated by larger numbers of pulses in longer exposure times.The usage of power-dependence of the concave structures is a convenient approach so that the microlenses can be individually or collectively controlled, which is difficult and expensive to be achieved by the planer semiconductor processes.For example, a dual-size mold for replicating convex polymer MLAs, which composes of 60-µm and 30-µm circularshaped microstructures, was fabricated.The OM image of the result is given in Fig. 5(a).The employed powers for large and small microlenses are 4.5 mW and 0.7 mW, respectively, and the etching time is 25 minutes.Because the diameters and the depth of the microlenses are different, the replicated convex microlens array has two different focal planes.Additionally, the shape of microlenses can be controlled by precise arrangement of the etching time, laser parameters and alignment of laser-exposure spot.For example, by prolonging the etching process from 25 min to 45 min, the dual-size circular-shaped pattern is transformed into a gapless dual-shape concave mold with rhombus-and octagonal-shaped microstructures, as shown in Fig. 5(b).Moreover, triangular-shaped and rectangular-shaped concave molds with 100% fill-factors are also successfully fabricated by the hexagonal-and rectangular-aligned laser-exposure spot arrays.More complex structures can be produced without expensive photomasks and time-consumed additional processes, demonstrating the flexibility of this method.These complex concave micro-curved structures can serve as molds for reproducible manufacturing of polymeric MLAs, which may have potential applications in various scientific fields.

Results of the replicated hexagonal-shaped microlens array
The fabricated concave mold with hexagonal-shaped structures was subsequently used to replicate convex microlens array on PDMS.The result is investigated by the SEM and the LCSM. Figure 6(a) reveals a SEM image of the top view of the microlenses, demonstrating uniform hexagonal structures.The 3D and cross-section profiles of the microlenses are given in Figs.6(b) and 6(c), respectively.The measured diameter of the microlenses is 79.2 µm, which is close to the interspacing of the adjacent exposure spots.The diameter of the microlenses can be easily changed from a few micrometers to hundreds of micrometers by the proper arrangement of laser-exposure spots and chemical etching time.The sag-height of the microlenses is 6.6 µm.Comparing the mold and replicated MLA, the replication consistency of the diameter and sag-height (depth of the concave mold) is 98.1% and 98.5%, respectively, indicating that the polymeric MLA was faithfully replicated.We should note that the silica mold can also used in the hot embossing and UV-light curing process to fabricated MLAs on other polymers.
The optical parameters of the MLA, including the radius of curvature, R, focal length, f, fnumber, f # , and the numerical aperture, NA, are estimated by equations: where h is the sag-height, r is the radius of the microlenses, and n is the refractive index of the material (n = 1.41 for PDMS).Consequently, the radius of curvature, focal length, fnumber and the numerical aperture are calculated as 122 µm, 298 µm, 3.76, and 0.13, respectively.The obtained f # is smaller than those of previous reports using other methods [18,19], indicating that the MLA fabricated in our work has better light-gathering power.In addition, smaller f-numbers can be obtained by microlenses with larger sag-height, which can be easily fabricated with larger laser powers.The imaging performance of the MLA was studied by the microscope system.A sheet of paper with a word "soft" is placed between the light source and the MLA, and images are visualized through the CCD camera, as shown in

Conclusions
We have presented a low-cost, high-efficient and flexible method to polymer microlens arrays (MLAs) with gapless and complex shapes, which includes the fabrication of concave molds and replication processes.To fabricate molds, a focused femtosecond laser with the average power of a few microwatts was used to expose the surface of silica glasses and concave micro-curved structures were wet-etched by a 5% hydrofluoric acid solution after several minutes.The formation mechanism of the microstructures is associated with the enhancement of the chemical corrosions in the laser-exposure spots where the materials are modified by a series of laser-induced effects such as shockwaves.By proper arrangement of the laser power, etching time and alignment of exposure spots, scalable shape-controlled microstructures with smooth curved surfaces can be fabricated.To demonstrate the flexibility of this method, gapless micro-curved structures with triangular, rectangular and hexagonal shapes, and complex dual-size and dual-shape structures are shown in this work.Subsequently, a convex microlens array with over 16, 000 lenses has been faithfully replicated on PDMS by serving the concave hexagonal-shaped microstructures as a mold.The calculating results of the optical parameters demonstrate the ability of this method to fabricate high-f-numbers microlenses and the observation of imaging performance further indicated the high-quality of the microlens array.Above all, this method has many salient features that make it attractive for micromolding polymers.Firstly, it is a rapid and low-cost approach for micro-sized, curved surfaces on polymers.Secondly, the dissimilar microstructures with different sagheight can be created in one mold and the shapes can be simply controlled individually by precise arrangement of laser exposure spots and the laser power, which could provide a designer of micro-optical or micro-analysis devices with a valuable prototyping tool.

Fig. 1 .
Fig. 1.Schematic diagrams of the fabrication procedures: (1) an array of laser-exposed spots is produced on a silica glass by a point-by-point process; (2) concave microstructures with smooth surfaces are formed by chemical wet etching process; (3) and (4) show the replica molding of convex PDMS MLAs.

Fig. 2 .
Fig. 2. (a) The evolution of the diameter (black circles) and depth (blue dots) of the structures versus the wet-etching time.(b) The SEM image of a laser-induced crater.(c) The morphology of the crater after a 30-minutes chemical corrosion.(d) Schematic diagrams of the formation process of the concave structures.To investigate the formation mechanism of the circular-shaped concave structures, morphology evolution of a laser-induced crater during the chemical etching process is studied by measuring the diameter and depth of the structure at different etching times, and the values are plotted in Fig.2(a).The crater was ablated by a 500-ms exposure with the laser power of 2.5 mW, as shown in Fig.2(b).The diameter, D, of the crater is about 4 µm and the depth, d, is about 0.5 µm.When immersed in the 5% HF acid solution, materials inside the crater are rapidly etched out.In the first one minute, the values of D and d have reached to about 7 µm and 3 µm, respectively; the etching velocity at this period ranges from 150 to 180 µm/h in both diameter and depth directions.This value then decreases in the following 30 minutes, and declines to about 30 µm/h at about 30 minutes, as shown in Fig.2(a).At this moment, the measured results of D and d are about 32 µm and 7.4 µm, respectively, and the morphology of the concave structure is shown in Fig.2(c).After 30 minutes, the evolutions of the diameter and depth express different rules, which can be apparently observed in Fig.2(a).The value of D keeps increasing at a constant speed of about 30 µm/h, however the d stops growing and stays at about 7.5 µm.The depth, which is the difference between the bottom of the structure and the sample surface, indicates that the etching speeds in the laser-exposed regions and unexposed regions are identical after a 30-minute chemical treatment.In other words, after

Fig. 3 .
Fig. 3.The results of the hexagonal-shaped mold for replicating PDMS MLAs.(a) The formation process of the concave microstructures during the chemical etching.The laser exposure spots are triangular-packed with an interspacing of 80 µm.The circular-shaped curved surfaces are expanding during the chemical etching process and the patterns begin to "squeeze" with each other at 60 minutes.The scale bar is 100 µm.(b) The SEM result of the hexagonal-shaped mold.(c) and (d) show the 3D and cross-sectional profiles of the mold, respectively.

Fig. 4 .
Fig. 4. The diameter and the depth of the circular concave structures vs. the laser parameters.(a) The power-dependency of the diameter of the structures.Insertions show the OM images of the structures fabricated by 500-ms exposure and 40-min wet etch process.The used laser power, from left to right, is 0.5 mW, 1.5 mW, 2.5 mW, 3.5 mW and 4.5 mW, respectively.The scale bar is 50 µm.(b) The power-dependency of the depth of the structures.(c) The relationship between the diameter and laser exposure time.The OM images of the structures shown in the insertions are produced by 0.5-mW laser pulses, and the exposure time is 10 ms, 100 ms, 500 ms, 1000 ms, 1500 ms and 3000ms, respectively.(d) The relationship between the depth and the laser exposure time.

Fig. 5 .
Fig. 5.The OM images of the concave molds with different shapes.(a) Dual-size mold with circular-shaped structures.(b) Dual-shape mold with octagonal and diamond structures.(c) Triangular-shaped concave mold.(d) Rectangular-shaped concave mold.

#
Fig. 6(d).The equivalent and clear images of the word further demonstrate that the microlenses have uniform profiles and smooth surfaces.

Fig. 6 .
Fig. 6.The results of the replicated microlens array.(a) The SEM image of the microlens array.(b) and (c) present the 3D and cross-sectional profiles of the microlenses.(d) The images of a word, "soft", generated through the fabricated microlens array, which is captured by the optical microscope system equipped with tungsten light source.The insertion shows the magnified images.