A Microlens Array with Different Focal Lengths Fabricated by Roll-To-Roll UV Lithography

Abstract

A simple, highly efficient and low cost roll-to-roll (R2R) UV imprinting lithography facility was achieved for fabricating micro-structures. Firstly, a novel microlens array with focuses distributed on a curved surface was designed and analyzed by an optical software ZEMAX. Then an ultra-precision diamond machine was applied to generate the freeform microlens array features on the master mold, and a belt-type polydimethylsiloxane (PDMS) mold with a microlens array pattern was prepared from the machined master mold. The R2R process was employed to replicate the microlens arrays, followed by an evaluation of their profiles and optical properties. Our experiments demonstrate that the applied method is reliable and efficient for producing the polymeric microlens arrays.

1. Introduction

Microlens arrays have been applied in many electro-optical and micro-optical systems, such as flexible light-field smart-phones ¹ , solar concentrators ² , data storage ³ and Integral Imaging 3D displays ⁴ , etc. These microlens arrays are typically composed of microlens cells arranged in a deterministic pattern ⁵ . The function of such arrays is dominated by the spacing between the cells, as well as the geometries of the individual microstructure cells. In previous research, various methods have been proposed for fabricating the microlens arrays, including photoresist thermal reflow ⁶ , non-ionic poly(vinyl chloride)/dibutyl adipate gels ⁷ , overplating in electroforming ⁸ , gray-scale photolithography ⁹ , projection photolithography ¹⁰ , excimer laser micromachining ¹¹ , hot embossing ¹² , wet etching ¹³ , etc. However, these processes are complicated and time consuming. Furthermore, the geometry profiles of the microlens fabricated are hard to precisely control by those methods.

Compared to the present work, two techniques, ultra precise turning and R2R imprint, have a great advantage of making such polymeric microlens array. Single point diamond cutting could generate complicated micro -structures with high form accuracy and good surface finish by the fast tool servo (FTS) technique on a precision lathe ¹⁴ . In addition, R2R UV manufacturing was always an ideal method for replicating the microlens arrays, which has been demonstrated in many researches as a simple, low-cost, and high throughput process for fabricating micro patterns ¹⁵ . Due to its excellent properties, the R2R method has been rapidly developed over these years as a promising alternative to conventional nanolithography processes to fulfill the demands of the semiconductor and flexible electronics industries.

In our study, several process steps were employed to make an innovative design of optical polymeric microlens array. Firstly, a microlens array with different focal lengths was designed and optimized by the commercially available numerical optical modeling software, ZEMAX (ZEMAX Development Corp., WA, USA). Secondly, an ultra-precision diamond lathe was applied to accurately control the surface of the designed microlens lens. Thirdly, to achieve higher quality lenses, lower cost and higher production efficiency, the R2R process was applied to replicate the microlens array. Finally, the profiles and optical properties of the microlens array were measured and compared with the design to demonstrate the microlens array fabricated by the R2R process.

2. Optical Design and Simulation

Microlens arrays with focuses distributed on a spherical surface can be used to fabricate complex microstructures on non- planar surfaces ¹⁶ . Besides, they can also be applied in a large field of viewing optical systems, such as compound eyes, large field Fresnel lenses ¹⁷ , etc. In our design, the focal lengths of the microlens at the center, on the second circumference and on the third circumference were 18 mm, 18.24 mm and 19 mm respectively. The focuses of the microlens array were distributed on a spherical substrate with a radius of 16.5 mm. Figure 1 shows the layout of the focus distribution. An initial design based on the curved focal distribution was input to the ZEMAX software, and then the surface profile and microlens substrate thickness were optimized in order to reduce optical aberrations. The final optimized aspherical microlens array was realized by freeform diamond machining of the ultra-precision diamond lathe. The heights were 31.30 mm, 30.89 mm and 29.65 mm, respectively while the distances of the microlens from center to outside were a = 3.665 and b = 5.687, respectively.

Figure 1

The focuses are distributed on the spherical surface, the SR=16.5 mm, and focal length F=18 mm, F₁=18.24 mm, F₂=19 mm, the heights of the microlens from center to outside are 31.30 mm, 30.89 mm and 29.65 mm, the distances of the circles centers are 3.665 mm, 5.687 mm respectively

3. Experimental

3.1 Imprinting Roll Mold

The fabrication process of the roller mold is shown in Figure 2 . In order to obtain a higher precision surface profile with optical quality, an aluminum male mold of a microlens array was machined by an ultra-precision machine (Nanotech FG 350) based on the design. Then 1:10 weight of PDMS (Sylgard 184, Dow Corning) curing agent and prepolymer was mixed thoroughly in a 50 mL beaker, and the prepolymer mixture was place in atmospheric environment for 2 h to remove air bubbles created during mixing. After that, PDMS was cast onto the machined mold and cured in a 90°C environment for 30 min. After curing, the PDMS soft mold was peeled from the aluminum mold, and cut into the working size, 18 mm wide and 37.7 mm long, by a sharp razor blade, Finally, the soft mold was wrapped onto a copper bar with 12 mm in diameter by double-sided tape fixation for the roll mold.

Figure 2

Imprinting roller mold fabrication process, (a) positive mold was fabricated by nanotech 350FG, (b) PDMS liquid was poured on the mold and cured, (c) the soft mold was obtained, and (d) the roller mold was achieved

3.2 R2R Process for Manufacturing Microlens Arrays

The R2R process inherits the high resolution of the traditional imprint lithography, and also has the advantages of low cost, high-speed and large area. In this paper, an experiment setup for R2R process was developed as shown in Figure 3 .

Figure 3

Experiment setup for R2R process

The device includes the following modules: the tension module to strain the substrate, the coating module for coating the resin onto the substrate, the imprinting module to supply the imprinting force, UV light for operating the replicating process, the adjustment module for tuning the wrap angle between roller mold and poly(ethylene terephthalate) (PET) film.

In this study, two key materials were used for the printing: 1) a high transparency flexible PET film (A4300, TOYOBO, 188 μm thickness, produced by Toyobo Co Ltd) was used for the substrate; 2) an acrylic resin, with good adhesion strength for PET film, low viscosity for easily coating, high transparency and small scattering coefficient, etc. In this work, we chose Tex Year, 1551M2, with optical grade properties, 450 mPa.s viscosity and 1.54 refractive index.

In R2R process, the roll mold was set up in the imprinting module, and the PET film was wrapped on the roller of each device's modules, and then UV resin was coated onto the PET film. Consequently, the coated PET was transported to the imprinting module at a speed of 0.2 m/min driven by the wrap module. At the same time, the resin was cured by an LED UV light in the imprint module. Finally, the microlens arrays were obtained after demolding from the imprinting roller. Figure 4(a) shows the microscopic picture of a R2R fabricated microlens array.

Figure 4

(a) Microscopic picture of the imprinted microlens array, (b) surface profile of a microlens measured by a WYKO profiler (the height of the microlens is 31.302 μm)

In order to validate the microlens profile, a WYKO profiler was employed. Figure 4(b) shows the surface profile of a microlens, which was in good agreement with our design in ZEMAX.

3.3 Optical Properties Testing

The replicated microlens array was measured by an optical setup shown in Figure 5 . The focal length measurement was conducted by capturing the focus of the microlens array and the flat substrate. In the experiment, the tested microlens array was mounted on a high-precision linear translation stage, and a CCD camera was used to monitor the laser light passing through the microlens array. The focal length of the micro-lenses was measured by observing the focal points at their sharpest position.

Figure 5

The system for measuring the microlens array's focal length

The focus of the microlens array captured by the CCD camera is shown in Figure 6 . The measured difference of the focal length of the microlens at the center and second circumference was 0.215 mm, and the difference between second circle and third circle was 0.702 mm. The results showed a good agreement with the theoretical optical design shown in Figure 1 .

Figure 6

The focuses of the microlens array at the focal position of the microlens in third circumference

4. Conclusions

This paper proposed a novel process to fabricate a microlens array focusing on a curved substrate. The microlens array was designed and optimized by ZEMAX and the master mold was machined by an ultra-precision machine tool. In order to reduce costs and increase production efficiency, the R2R process was applied to replicate the microlens array at an imprinting velocity of 0.2 m/min. Finally, the optical properties of imprinted microlens array were measured, which showed a good agreement with our design. The results demonstrated the feasibility of this method to fabricate lower-cost freeform microlens arrays with higher quality. This cost-effective technique has great industrialization potential for fabricating components with micro and nano- scale microstructures.