1. Introduction

In recent years, microlens arrays (MLAs) have been employed in many electro-optical systems, such as flat panel display, micro-scanning system, fiber-coupling and optical communication. For MLA fabrication, many methods have been reported, including thermal reflow [1], gray-scale photolithography [2–3], micro-transfer molding [4–5], reactive ion etching [6], 3-D diffuser lithography [7] and two photon photopolymerization [8]. The thermal reflow method has been widely used due to its simplicity, but the cycle time is still long. To improve the productivity, several replication methods for manufacture of plastic microlens arrays have been used, including injection molding [9], hot embossing [10] and UV-molding [11]. All methods achieve good precision and relatively high throughput, but their efficiency is limited because they are batch-wise processes. To further improve the productivity, we have reported a rolling embossing process for continuous fabrication of UV-cured plastic microlens arrays on glass substrates [12]. The roller used was an aluminum cylinder wrapped with an electroplated nickel sheet bearing an array of microlens-cavity.

In this paper, we present a novel fabrication method for microlens arrays by hybrid extrusion rolling embossing technology. The roller mold is fabricated using dry film resist (DFR). The DFR is patterned by photolithography and then laminated onto a copper roller. The micro-hole array is generated directly on the metallic roller surface by wet etching. This roller mold is employed in the rolling unit under the slit-die on the extruder. During the extrusion rolling embossing process, the extruded hot film is immediately pressed against the surface of roller mold. An array of convex microlenses is generated by partial protrusion of the film into the micro-holes of the mold under the action of rolling pressure and surface tension. The uniformity and optical properties of the microlenses array have been verified with microscope, surface profiler and beam profiler.

2. Fabrication of the roller with micro-hole array using dry film resist

2.1 Fabrication procedure

Figure 1 shows the conformation and photograph of dry film resist (DFR, ETERTEC® HT-115T, Eternal, Taiwan). The steps in the fabrication procedure as shown in Fig. 2 are as follows:

(a) A suitable size DFR sheet is patterned using photolithography with a plastic photomask with an array of printed opaque circles of 200 µm diameter and 400 µm pitch. Remove the cover film.

(b) Upon heating and with a proper laminating pressure, the patterned resist of DFR is laminated onto a copper roller of 88 mm in diameter and 110 mm in length. The substrate film of DFR is then removed.

(c) The resist on roller is developed using 0.85% sodium carbonate solution.

(d) The copper roller is etched using ferric chloride solution.

(e) After the remaining DFR is stripped using 5% NaOH, a copper roller with a micro-hole array is obtained.

 

Fig. 1. Dry film resist (a) schematics showing the conformation (not to the scale) (b) photograph of the roll of DFR

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Fig. 2. Schematic diagrams of microstructure fabrication process on roller using dry film resist

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2.2 Results and discussion on the fabricated roller

After each step in this fabrication process, the roller is observed by an optical microscope (Zoomkop) to monitor the dimensional changes in the diameters of the micro-holes. The results are shown in Table 1. The original diameter of the opaque circles on the photomask is 200 µm. After the DFR is developed, the average diameter of the patterned micro-holes on the resist is 181.1 µm. This difference may be due to the squeeze effect during the laminating step. The measured average diameter of the fabricated micro-holes on the roller is 213.2 µm. The increase in the diameter of the micro-holes is caused by the undercut effect because the ferric chloride etching is isotropic. The diameter of the fabricated micro-holes on the mold is 6.67% larger than that of the opaque circles in the photomask.

To characterize the micro-hole array on the roller mold, a 3-D surface profiler (ET-4000K, Osaka Lab. Ltd., Japan) and a scanning electron microscope (SEM, Hitachi, Japan) are employed. The average depth of the micro-holes is 28.34 µm. The bottom of the etched surface is curvy due to the isotropic wet etching. Figure 3 shows the micrograph, SEM and the 3-D profile images of the fabricated roller surface.

Table 1. The diameter of the micro-holes after each step during roller fabrication process

View Table

 

Fig. 3. Images of the fabricated roller mold with micro-hole arrays using (a) a digital camera (b) SEM (c) 3-D surface profiler

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3. Fabrication of microlens array by hybrid extrusion rolling embossing

3.1 Hybrid extrusion rolling embossing system

A hybrid extrusion rolling embossing facility as shown in Fig. 4 is constructed and used for the fabrication of microlens array. Extruder and slit-die are used to fabricate the plastic films. Polycarbonate (PC) material is used. Its refractive index is 1.586. After the PC film is extruded, it enters the roller micro-embossing unit immediately under the die. As the PC film is rolled through the rollers, an array of convex microlenses is generated (as illustrated in Fig. 5) by partial protrusion of the film into the micro-holes of the mold under the action of rolling pressure and surface tension. The rolling pressure can be controlled by the roller thrust, which is provided by two pneumatic cylinders. They can apply a thrust, ranging from 12 kgf to 48 kgf, on the two ends of the roller shaft.

 

Fig. 4. Schematic diagram and photograph showing the hybrid extrusion rolling embossing facility

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Fig. 5. Under proper rolling pressure, PC film is partially protruded in the holes and form the convex lens due to surface tension during rolling embossing

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3.2 The results and discussion on the fabricated microlens arrays

Figure 6 shows the SEM image and the 3-D surface profile of the fabricated microlens array under the processing parameters of 290°Cmelt temperature, 3.5 rpm roller rotation speed and 24 kgf roller thrust. An array of microlenses has been successfully fabricated over the whole PC film. The average diameter of the microlenses is 209.7 µm, which is 3.5 µm (-1.64%) less than that of the micro-holes in the roller mold due to shrinkage of the polymer. The average sag height of the fabricated microlenses is 12.06 µm, which is much less than the depth of copper micro-holes on the roller mold (28.34 µm). The controlled partial protrusion of film into the micro-hole allows the microlens surface quality determined primarily by surface tension, rather than the mold. In addition, simply by adjusting processing parameters, it enables the manufacture of microlens array of specified sag heights within a certain range using the same roller mold. With the melt temperature set at 290°C, the sag heights of the fabricated microlens range between 8.45 µm and 20.38 µm, depending on the rolling pressure.

In order to characterize the uniformity of the fabricated microlens array, the surface profiles of 30 microlenses (randomly selected in a 200×200 microlens array) from a single process run are measured. The average diameter is 210.4 µm with a standard deviation of 0.79 µm (3.75%), while the average sag height is 12.89 µm with a standard deviation of 0.82 µm (6.36%). The relatively high standard deviation in the sag height is primarily caused by the bending deflection of the roller, which is supported from two ends of its shaft. The deflection affects the uniformity of local rolling pressure. The problem can be overcome with cambering the roller, providing a slight convexity in the roller to compensate for the bending deflection.

The substrate used in conventional hot embossing is ready-made films fabricated using similar extruder, die and (flat) rollers. For hot embossing of microlens array, the film has to be reheated, embossed and cooled. Additional thermal stress is induced during the process, degrading its thermal stability. As for injection molding of microlens array, significant flow- and thermal-induced residual stresses are induced in the molded microlens array during filling, packing and cooling the relatively thin moldings. The hybrid extrusion rolling embossing process not only improves productivity of the fabrication but also provides good thermal stability in the fabricated microlens array.

 

Fig. 6. Fabricated microlens array on PC film (a) SEM (b) 3-D Surface profiler

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3.3 The optical property of the molded microlens arrays

The optical property of the fabricated microlens array is measured using a beam profiler. The beam profiler is composed of expanding lenses, an optical attenuator, a filter, a micrometer scale resolution Z-stage, a microscope, a CCD system and a 633 nm wavelength laser light source. Figure 7(a) shows the schematic diagrams of microlens array focal length measurement system. The average focal length is measured to be 721.3µm for the fabricated PC microlens with a diameter of 210.4 µm and a sag height of 12.89 µm. The theoretical radius of curvature (R), focal length (f) and numerical aperture (NA) can be determined based on the geometry of the molded microlens:

$h=R-\sqrt{R^2-{\left(\frac{D}{2}\right)}^2},f=\frac{R}{n-1},\mathrm{and}NA=\frac{D}{2f}$

where D, h and n are the diameter, the sag height and the refractive index of PC, respectively. The calculated radius of curvature is 430.5µm, the focal length is 734.64µm, and the numerical aperture is 0.143. Figure 7(b) shows a portion of the focused light spots observed by this system. The image reveals that the pitch and the intensity of the focused light spots are uniform.

 

Fig. 7. microlens array focal length measurement system

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4. Conclusions

This paper reports a novel and effective method to fabricate microlens arrays on polycarbonate films by hybrid extrusion rolling embossing with a metallic cylinder mold. The roller mold bearing micro-hole array used is fabricated using photolithography using dry film resist. During the extrusion rolling embossing process, the extruded hot film is immediately pressed against the surface of the rollers. Convex microlenses are partially protruded in the micro-holes on the roller mold under the influence of rolling pressure and surface tension. The microlens surface quality is determined primarily by surface tension, rather than the mold. By adjusting the processing parameters, this method enables the manufacture of microlenses array of specified sag heights within a certain range using the same roller mold. The uniformity, profiles and optical properties of the microlenses have been characterized and verified. The technique shows the potential of a cost-effective continuous mass-production process for microlens arrays fabrication.