A BASIC EXPERIMENTAL STUDY OF CAST FILM EXTRUSION PROCESS FOR FABRICATION OF PLASTIC MICROLENS ARRAY DEVICE

Chih-Yuan Chang and Yi-Min Hsieh and Xuan-Hao Hsu

Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Taiwan

Abstract

This paper reports a highly effective method for

fabrication of plastic microlens array device based on a cast film extrusion process. In this method, a thin steel mold with a micro-circular hole array pattern is fabricated by photolithography, and a wet chemical etching process. The thin steel mold was then wrapped onto a metal cylinder to form an embossing roller mold. During the cast film extrusion process operation, the molten polymer film was extruded and immediately pressed against the surface of the embossing roller mold. Under the proper processing conditions, the molten polymer will just partially fill the micro-circular holes in the mold and due to surface tension form a convex lens surface. A continuous plastic film with a microlens array pattern was fabricated. This technique shows great potential for the mass production of large-area plastic microlens array device with high productivity and low cost.

Introduction

In recent years, plastic microlens array devices have been applied increasingly in optoelectronic systems, such as optical fiber switches, charge-coupled device (CCD) cameras, organic light-emitting arrays, liquid crystal displays with backlight modules, and even biomedical instruments with optical sensors. Therefore, the fabrication method of plastic microlens devices is clearly an important issue. Over the past years, many plastic replication processes for microlens array fabrication have been proposed, such as injection molding, hot embossing, and UV molding with a microlens array mold. These technologies are regarded as the most suitable mass-production methods for the fabrication of plastic microlens array devices, because they offer high repeatability with low cost, and versatility in polymers selection. On the other hand, for the cost-effective fabrication of large-area plastic microlens array devices, many roll-to-roll (R2R) manufacturing methods have been developed, such as roll-to-roll hot embossing [1-2] and roll-to-roll UV embossing processes, with a roller mold [3-4]. The R2R manufacturing technology has indeed attracted more and more widespread attention and has achieved sustainable development in industrial application. Here, unlike previous reports, we propose a rapid and effective method for fabrication of large-area plastic microlens array device using cast film extrusion process with a mold of micro-circular hole array. In this

method, a thin steel mold with a micro-circular hole array pattern is fabricated by photolithography and a wet chemical etching process. The thin steel mold was then wrapped onto a metal cylinder in the cast film extrusion system to form an embossing roller mold. During the cast film extrusion process operation, the molten polymer film was extruded and immediately pressed against the surface of the embossing roller mold. Under the proper processing condition to control the die temperature, screw speed and rolling speed of embossing rollers, the molten polymer will just partially fill the micro-circular hole array pattern of the mold. Furthermore, the top hemispherical portion of the microlens is naturally constructed by the surface tension of polymer. A continuous plastic film with a large-area microlens array pattern was fabricated. In this study, a cast film extrusion facility has been constructed and tested. A basic processing condition of the cast film extrusion process was investigated. The web width and thickness of the extrusion films were measured and analyzed. Then, a thin steel mold was wrapped onto a metal cylinder to form an embossing roller mold. Finally, the effects of processing conditions on the shape and quality of the fabricated plastic microlens array devices were investigated. The optical property and surface roughness of the fabricated plastic microlens array devices were also measured and analyzed.

The cast film extrusion facility and process

Figure 1 shows the construction of the cast film extrusion facility in the experiments. The facility mainly consists of a single-screw extruder with a flat T-die, a film embossing system with a cooling unit and a conveying system with highly polished rollers. The single-screw extruder consists of one rotating screw inside a stationary cylindrical barrel. The barrel is often manufactured in sections and features a heating unit. In the cast film extrusion process, the plastic beads are melted and mixed by the screw rotation. Then, the molten polymer travels through a flat die system to adopt its final flat film shape. Immediately after exiting the T-die, the melt curtain enters the film embossing system and the conveying system with highly polished rollers to freeze the film. A roll of plastic film was obtained. A thin steel mold with micro-circular holes array pattern was wrapped onto a metal cylinder in the film embossing system to form an embossing roller mold. During the cast film extrusion operation, the molten polymer film was extruded and immediately pressed against the surface of

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the embossing roller mold. The molten polymer could be controlled and partial filling the micro-circular holes array pattern of the mold. A continuous plastic film with a large-area of microlens array pattern was fabricated. In this study, an optical polycarbonate (PC) raw material (Chi Mei Corporation, Taiwan) was used for the fabrication of optical films with a microlens array pattern. The glass transition temperature (Tg) and refractive index of the polycarbonate material are 145 ℃ and 1.55, respectively. In addition, the width and gap of the flat-film-die are 25cm and 0.2mm, respectively. In this process, the die temperature, screw speed and rolling speed of embossing rollers can be controlled and adjusted.

Figure 1. Cast film extrusion facility.

To verify the feasibility and uniformity of the cast

film extrusion process, the thickness of the polycarbonate film was measured at different locations. As shown in figure 2, four sections of the fabricated polycarbonate film were selected in each processing parameter experiment. In addition, each section had ten different measurement points.

Figure 2. Positions of the measurement points on the polycarbonate film.

To study the various processing parameters of the cast film extrusion process for the fabrication of PC film, three basic processing parameters, i.e., the die temperature, screw speed and rolling speed, were selected. Figure 3 shows the effect of die temperature on the average thickness of fabricated PC film. The measured result suggests that the average thickness of fabricated PC film decreased with die temperature. One possible explanation is that the higher temperature results

in neck-in behavior in the cast film extrusion process. The average thickness of fabricated PC film was thus decreased. In addition, a trend is observed that the edge of the film was thicker than middle of the film. When the die temperature was at 270℃, the thickness of PC films was not uniform. As shown in Figure 4, some Sharkskin defect was observed on the surface of the fabricated PC film. When the die temperature is increased, the surface quality and thickness uniformity of the PC film was improved. When the die temperature was set to 290℃, the thickness uniformity of the middle area (the -width -of the -P2〜 P9 was about 14 mm) of the PC film was excellent. The average thickness of the middle area of the PC film was 0.251 mm, with a standard deviation of 0.015 mm.

Figure 3. The effect of die temperature on the average thickness of fabricated PC film.

Figure 4. Sharkskin defect was observed in the PC film.

Figure 5 shows the effect of screw speed on the average thickness of fabricated PC film. A trend is observed that the average thickness of fabricated PC film increase obviously when the screw speed increases, as higher screw speed results in more output. Under high

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screw speed condition, the surface quality and thickness uniformity of the PC film was poor. One possible explanation is that the higher screw speed or polymer flow rate results in die swell behavior in the cast film extrusion process, the surface quality and thickness uniformity of the PC film was thus poor. This problem can be eliminated by slow speed screw operation with a slower polymer flow stream.

Figure 5. The effect of screw speed on the average thickness of fabricated PC film.

Figure 6 shows the effect of rolling speed on the average thickness of fabricated PC film. The result suggests that increasing the rolling speed results in a small decrease in the average thickness of fabricated PC film. It can be concluded that the rolling speed has little effect on the surface quality and thickness uniformity of the PC film.

Figure 6. The effect of rolling speed on the average thickness of fabricated PC film.

Fabrication of the embossing roller mold with array pattern of micro-circular holes

Figure 7 shows the fabricating procedure of the thin stainless steel mold with micro-circular hole array pattern. In a first step, a thin stainless steel substrate (SUS 304L) with thickness of 0.1mm was cleaned with detergent and distilled water. This substrate was preheated to 100℃with a hot plate. Two pieces of dry film photo-resist (ETERTEC® HT-115, Eternal Materials Co., Ltd., Taiwan) were laminated on both sides of this substrate by using a dry film laminator. The thin stainless steel substrate with dry film photoresist layers was then exposed to UV radiation through two photo-masks with micro-circle array pattern for 15 seconds using a double-sided UV exposure machine. After dipping the substrate in the developer solution (1.2% Na2CO3•H2O) for 30 seconds, the photoresist layers with micro-circular hole array pattern were formed on both sides of the thin stainless steel substrate. The second step was wet chemical etching. The thin stainless steel substrate with photoresist pattern was then soaked in an etchant with the following chemical composition: 10 wt% FeCl3, 5 wt% HCl and 3 wt% HNO3. The wet etching process was carried out at 40℃ for 1 hour. In the finally step, the dry film photoresist layers were stripped using 4% NaOH solution. The thin stainless steel mold with micro-circular hole array pattern was obtained. Figure 8 shows a photograph and an optical micrograph of the thin stainless steel mold. The area of the array pattern was about 13 cm × 25 cm. The diameter and pitch of the micro-circular holes on the mold were 402 µm and 803 µm, respectively. The depth of the micro-circular holes was 99 µm.

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Figure 7. Fabricating procedure of the thin stainless steel mold with micro-circular holes array pattern.

Figure 8 Photograph and optical micrograph of the thin stainless steel mold with micro-circular hole array. Fabrication of plastic microlens array device

As illustrated by the previous cast film study, a roll of PC film was successfully fabricated using the cast film extrusion process. With optimized processing condition, an excellent thickness uniformity of the middle area was achieved. Next, the thin steel mold with micro-circular hole array was wrapped onto forming roller (Number 2) to form the embossing roller mold for the fabrication of plastic microlens array device. Figure 9 shows the fabrication principle of plastic microlens array device in this process. Under the proper control of processing

conditions, the molten polymer film rolled through the film embossing system with pressing roller (Number 1) and embossing roller mold (Number 2). The molten polymer will just partially fill the micro-circular holes of mold. The top hemispherical portion of the microlens is naturally constructed by the surface tension of polymer. The continuous plastic film with microlens array pattern was fabricated.

Figure 9 Fabrication principle of plastic microlens array device in this process.

To investigate the effect of roller embossing

conditions with respect to the shape and dimensions of the fabricated microlens array pattern, the rolling speed of embossing rollers was varied from 1 RPM to 2 RPM while the die temperature was kept at 290℃ and screw speed at 300 RPM. Figure 10 shows the effect of the rolling speed of embossing rollers on the shape and dimensions of the fabricated microlens array pattern. The result suggests that the average height of the fabricated microlens array decrease with the applied rolling speed of embossing rollers. In addition, the shape of the formed microstructures changed from a cylinder to a pure hemisphere. One possible explanation is that the higher rolling speed, the molten polymer material did not have sufficient time to flow and to fill the micro-hole array cavity of the roller mold. The average height of the fabricated microlens array pattern was thus decreased. When the rolling speed of embossing rollers was at 1.5〜2 rpm, the molten polymer material will just partially fill the micro-circular holes array and form a pure hemisphere microlens array pattern.

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(a) The rolling speed of embossing rollers was 1 RPM

(b) The rolling speed of embossing rollers was 1.5 RPM

(c) The rolling speed of embossing rollers was 2 RPM Figure 10. The effect of the rolling speed of embossing rollers on the shape and dimensions of the fabricated microlens array pattern.

After the cast film extrusion process, the plastic

microlens array device was successfully fabricated. The optical property of the fabricated plastic microlens array

device was further measured using an optical beam profiler system. The optical beam profiler system was composed of expanding lenses, a mirror, a micrometer-scale resolution stage, a CCD camera and a 532 -nm laser light source. Figure 11 shows the part of focused light spot array pattern of the fabricated plastic microlens array device (from Fig.10b) on the PC film. The images reveal that the pitch and the intensity of the focused light spots array pattern are uniform.

Figure 11. The part of focused light spots array pattern of the fabricated plastic microlens array device.

To characterize the surface morphology of the

fabricated plastic microlens array device, the surface roughness was measured by an Atomic Force Microscope (AFM). In the experiment, twenty plastic microlenses (from Fig.10b) on the PC film were picked and measured. The average surface roughness (Ra) of these microlenses was 8.4 nm. The minimum surface roughness (Ra) of a single microlens was 6.6 nm, the AFM image and roughness analysis result was shown in Figure 12. These experimental results suggest the proposed method used in this study could successfully fabricate a plastic microlens array device with good optical property and surface quality for many optoelectronic systems.

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Figure 12. AFM scanning image of a single microlens.

Conclusions

This paper reports a highly effective method for the

fabrication of plastic film with a large area microlens array device. A laboratory-made cast film extrusion facility has been developed and modified. Under the proper processing conditions, the thickness uniformity of a roll of plastic film was good, and the pure hemisphere microlens array pattern has been successfully fabricated. Finally, the optical properties and surface roughness of the fabricated plastic microlens array device were measured and proved satisfactory. These results have shown that the continuous cast film extrusion process could provide an effective way for the mass production of high-performance plastic optical films with high throughput.

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