Microfluidics assisted optics manufacturing technique

The conventional micro/nano-manufacturing techniques can hardly process interior microstructures. The entire fabrication process is complex and requires large-footprint and high-cost equipment. The presented microfluidics assisted optics manufacturing technique is feasible to create the curved surface inside microstructure using various modified materials. The fabrication process is simple. Only small, low-cost devices are needed. In this paper, microfluidics assisted optics manufacturing technique is introduced in detail and compared with the current manufacturing techniques. A diversity of interesting micro-optics, including microlens array and compound eye, are demonstrated. These optical components are all fabricated by the microfluidics assisted manufacturing technique and possess their own outstanding features.


Introduction
Fabricating complex-structured optics including microlens array (MLA) and compound eye has a great challenge due to the requirements of miniature size, high resolution and 3D profile [1][2][3].The traditional optical lens manufacturing process is a meticulous procedure that involves cutting, grinding and polishing to shape and refine the optics [4,5].The primary advantages lie in the manufacturing precision, versatility across various materials, and the ability to tailor optical properties to specific requirements [6,7].During the manufacturing, the profile of the optics could be meticulously controlled, ensuring a high level of accuracy in the final product.Furthermore, this process can be applied to a wide range of materials, including glass, plastic, and crystalline substances, making it adaptable to various industries.It also allows for customization, enabling optical components to meet specific design and performance criteria.However, the traditional optics manufacturing has several disadvantages.The most notable issue is the substantial amount of material waste generated during grinding and polishing, leading to inefficiency and increased costs.The process is time and labor-intensive, requiring skilled operators and a significant investment in time.Moreover, it is not well-suited for complex shapes or intricate optical components, as achieving precision becomes increasingly challenging.In such cases, alternative manufacturing techniques, e.g.laser direct writing and gray-scale lithography, may be more efficient and cost-effective [6][7][8][9][10].
Harnessing surface tension for shaping optics is a widely used technique [11][12][13].It was employed as early as the 1660s to make small lenses for the use as the magnifying lens with molten glass [14].Shaping liquid surface requires the cooperation of intermolecular force of the liquid and environmental factors.Once manipulation of liquid surface tension can be precisely realized, the surface modulation depth, which is related to the optical power, can be well controlled.Besides, mastering liquid curing process to avoid deformation and surface wrinkle is also important.Typically, UV curable resin and thermally curable elastomer are used in the manufacturing.Once a desired profile is realized, the resin is solidified under UV exposure or heating.In this paper, we thoroughly study the advanced micro/nano manufacturing techniques for optics fabrication.We summarize the manufacturing procedure of these techniques.The outstanding features and the weakness are investigated.We focus on the discussion on microfluidics assisted optics Nevertheless, laser direct writing is not as easily scalable for mass production as other techniques like moulding.It can be time-consuming and expensive for large quantities.Voxel-modulation laser scanning technique was demonstrated to speed up the fabrication of compound eye.The inner spherical based was produced by rapid low-precision scanning with large voxels, while the outer ommatidia were fabricated by high-precision small voxel [18].Initial setup costs for laser direct writing equipment and the process can be relatively high, making it less economical for large-scale production.Therefore, the choice between laser direct writing and other lens manufacturing methods depends on factors like the required customization, production volume, material selection, and cost considerations.Laser direct writing is particularly advantageous for applications that demand highly customized and complex microlens designs, rapid prototyping, and small batch production.However, for high-volume production, cost-efficiency, and specific material requirements, alternative methods such as molding or lithography may be more appropriate.

Gray-scale lithography
Grayscale lithography is one kind of photolithographic process that allows for the creation of threedimensional microstructures with varying heights or depths in a single exposure [19][20][21].This technique is used in microfabrication and micro-optics to produce complex structures.The fabrication procedure is illustrated in Fig. 2. In the fabrication, a layer of photoresist is spin coated to the substrate.A grayscale mask is required, which is a special photomask that consists of varying levels of transparency.The mask contains grayscale patterns that define the desired heights or depths of the microstructures.The grayscale mask is placed between a UV light source and the photoresist-coated substrate.During exposure, the light passes through the mask, and the varying levels of transparency result in different light intensities reaching the photoresist.The photoresist undergoes a photochemical reaction based on the intensity of light it receives.The more intense the light, the more the photoresist is exposed and undergoes a change in its solubility.After exposure, the coated substrate is subjected to a development process.The developer solution selectively removes the exposed or unexposed portions of the photoresist.Depending on the specific application, etching or other processing steps may be used to transfer the grayscale pattern from the photoresist to the substrate.This step can create microstructures with varying heights or depths, based on the grayscale pattern.Grayscale lithography allows for the creation of highly customized and complex lens shapes, making it suitable for applications with unique optical requirements.Unlike some other techniques that involve multiple steps, grayscale lithography can produce three-dimensional microstructures, including lenses, in a single exposure, simplifying the manufacturing process.This method offers precise control over the heights and shapes of microstructures, resulting in lenses with accurate and well-defined optical properties.Grayscale lithography is well-suited for small-batch or prototyping production, making it cost-effective for niche or research applications.
The manufacturing process is shown in Fig. 3. Micro-scale droplets are generated from a piezo-actuated ink-jet nozzle and dripped onto a substrate.When the microdroplets are stably on the substrate, the balance of the forces, i.e. gravity and surface tension, applied on the microdroplets would be achieved.Under the action of the forces, mainly attributed to the surface tension, the microdroplets turn into the spherical crown.The shape could be described by the contact angle between the microdroplets and the substrate.The contact angle could be tuned by modifying the wettability of the substrate.As a result, the profile of the microdroplets, corresponding to the focal length of the microlenses, could be controlled.
In addition, the volume of the jetting determines the size of the droplets which is proportional to the aperture of the microlenses.It is worth noting that when the diameter of the droplets is larger than the capillary length,  > 2√  /, where γls is the tension force for the liquid-substrate interface, ρ is the density of the liquid and g is the gravity of earth, the contribution of the gravity could not be neglected and the profile of the droplets could be fitted to an elliptical function [27].Consequently, the shape of the microdroplets could be fixed by UV curing.Microdroplet jetting/dripping technique is a simple way to produce MLAs with controllable focal length and diameter.It requires a drop-on-demand jet nozzle and a high-precision 2D translation stage to generate and position the micro-scale droplets one by one.Nevertheless, it is not very efficient to manufacture a large-area MLA.

Thermal reflow
Thermal reflow technique is a sophisticated way to produce MLAs and has already been employed in the MLA manufacturing industry [28,29].In the MLA fabrication, an array of cylinders is produced by photolithography technique, as shown in Fig. 4.Then, the cylinder array is heated to melt down into spherical caps.After cooling, MLA is formed.The aspect ratio, i.e. the ratio between the height and the diameter, and the spacing of the cylinders are key parameters to affect the curvature, the aperture and the filling factor of the microlenses.High numerical aperture and high filling-factor MLAs of a large area could be produced by using thermal reflow method in a time-saving manner.Since the modulation of the diameter and the curvature of the microlenses is intrinsically related in the fabrication, it is not easy to control the optical parameters individually.

Microfluidics assisted manufacturing
Recently, various cost-effective methods for fabrication MLAs are demonstrated by curving liquid surface with the assistance of the microstructure mold.MLAs with controllable curvature can be realized by driving UV-curable liquid to flow through an air gap under a hydrophobic microhole array and on a hydrophilic substrate [30].Surface tension overcomes viscosity of the liquid to curve the liquid surface under the microholes, as depicted in Fig. 5(a).The surface curvature could be controlled by adjusting the gap between the substrate and the microhole array.
Besides that, when the microhole array mold is gently pressed on the liquid, the liquid surface under the microholes would be curved due to surface tension [31].The schematic diagram is shown in Fig. 5(b).If the liquid is heated, the pressure difference across the liquid-air interface would be changed according to Eotvos Ramsay-Shield relation.The variation of the pressure difference could be harnessed to control the surface curvature.The curved surface with the thermally controlled curvature could be UV cured, forming concave MLAs.MLAs with controllable focal length and aperture can be directly produced in a microhole array mold [32].The fabrication procedure is simple, as illustrated in Fig. 6.
Step 1: The liquid is filled into the microholes.
Step 2: The mold is spun at a high rotation speed.
Step 3: The liquid is solidified under the UV exposure, forming MLAs.During the spin of the mould, partial liquid is spun out of the microholes and the surface of the remaining liquid in the microholes becomes curved due to the thermodynamic equilibrium between three phases, i.e. solid phase (the wall of the microholes), liquid phase (the UVcurable liquid), and the gas phase (air in the microholes).The curvature of the microlenses could be controlled by modifying the wettability of the microhole walls or adjusting the inclined angle of the microhole walls [33].By setting the microhole with different inclined walls, the focal length of the microlenses can be individually modified.As a result, the depth of view of the MLA could be extended because the microlenses on the MLA have a large range of the focal length.Biomimetic compound eyes can also be produced by microfluidics assisted manufacturing technique.The complex structure of the compound eye, specifically the outer MLA on the hemisphere and the internal waveguide structure, could be precisely realized [34].Fig. 8 shows the fabrication procedure.A hemispherical pit mold with an array of microholes and a hemispherical substrate with an array of hollow channels are 3D printed using a projection micro-stereo-lithography 3D printer.The pit mold is then filled with photoresist.After spinning the pit mold, only a portion of photoresist remains in the microholes.Then, the photoresist is UV cured.The hemispherical substrate is placed in the mold and elastomer is used to fill the mold and the substrate.After curing, the hemispherical substrate is taken out.The MLA is formed on the substrate and the internal channels are all filled with the elastomer, forming an apposition compound eye.

Conclusion
A diversity of micro/nano manufacturing techniques have been presented for optics fabrication.These techniques have their own features in aspects of the ability of 3D construction, manufacturing precision, low cost/rapid processing and massive production.Among these advanced techniques, microfluidics assisted manufacturing technique provides a unique way for fabricating optics.High-cost and largefootprint equipment is not utilized.The fabrication procedure is simple.The surface tension plays an important role in the liquid surface shaping, and the wettability could be easily modified.Thus, the optical parameters, e.g.focal length and aperture, of the microlenses could be flexibly controlled.
Although the recently proposed microfluidics assisted optics manufacturing techniques attract lots of interests due to its extraordinary features, it still meets great challenge.For example, since the liquid surface is formed by surface tension in the capillary, the surface is always spherical and in the scale below capillary length.Fabricating lenses with large, free-form profiles is a difficult topic.We believe the microfluidics assisted manufacturing techniques could be well developed and applied to fabricate more interesting optics.

Figure1.
Figure1.Fabrication procedure of laser direct writing.(a) Fabrication of the MLA.(b) Fabrication of the compound eye.

Figure 5 .
Figure 5. Fabrication procedure of the microfluidics assisted manufacturing techniques.(a) Fabrication of the MLA based on lateral flow.(b) Fabrication of the MLA by thermally curving photosensitive gel beneath microholes.

Figure 6 .
Figure 6.Fabrication of MLA with controllable focal length by modifying surface wettability.(a) Fabrication procedure of the concave mold and MLA.(b) SEM images of the concave MLAs fabricated with the surface treatment of 20 s, 60 s and 120 s [32].

Figure 7 .
Figure 7. Fabrication of uniform-aperture multi-focus MLA using microholes with inclined walls.(a) Fabrication procedure of the multi-focus MLA.(b) 3D printed microholes with different inclined walls, residual photoresist after spin coating, and lenslets obtained by replicating the mold [33].

Figure 8 .
Figure 8. Fabrication procedure of the apposition compound eye.