A NEW INNOVATIVE METHOD FOR THE FABRICATION OF SMALL LENS ARRAY MOLD INSERTS

Transcription

1 A NEW INNOVATIVE METHOD FOR THE FABRICATION OF SMALL LENS ARRAY MOLD INSERTS Chih-Yuan Chang and Po-Cheng Chen Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Taiwan Abstract This paper proposed a new innovative method for the fabrication of small lens array mold inserts using polymer hot embossing with a small steel ball array. Only one hot embossing step is required, and a small concave lens array pattern is directly fabricated onto a polymer substrate. The polymer substrate with a small concave lens array pattern can be used as a mold insert for rapid replication of the small polymer lens array by a UV molding process. In addition, the diameter and depth of the small concave lens array pattern on the surface of the polymer substrate can be controlled by adjusting the conditions of the hot embossing process. Therefore, various small lens array mold inserts with different dimension can be effectively fabricated with high throughput and low cost. Introduction Nowadays, many small lens arrays have emerged as an essential component in many optoelectronic systems, such as optical fiber switches, charge-coupled device (CCD) cameras, light-emitting diode LED arrays, flat panel displays, and even biomedical instruments. Therefore, its production method is clearly an important issue of concern. So far, many methods have been proposed and developed for fabrication of convex small lens arrays in millimeter or micrometer-scale, such as thermal reflow process [1], laser ablation process [2], inkjet printing process [3], micro dispensing process [4], modified LIGA process [5], gray scale photolithography process [6], and polymer replication process [7-9]. Among these, polymer replication process with a mold insert is regarded as the most suitable for replicating and massproducing the small lens arrays because it offers significant possibilities for reducing the cost of micro-lens array manufacturing when large volumes are required. However, in the polymer replication process, a small lens array mold insert that contains a concave small lens array pattern is required, and its fabrication represents the bulk of the cost. Hence, the development of a low-cost, highefficiency method for the fabrication of various small lens array mold inserts is a key issue for small lens array fabrication. Over the past years, several methods for preparing small lens array mold inserts with different shape have been proposed, such as focused ion beam technology [10], the isotropic etching of the glass master [11], the combination of the thermal reflow technique and electroforming process [12] and ultra-precision diamond ball-end milling [13]. However, most of the fabrication techniques of small lens array mold inserts are complicated, time-consuming and require expensive equipment. Here, we propose a low-cost and direct method for the fabrication of small lens array mold inserts with different dimension. In this method, a rectangular cavity is fabricated on a metal plate using a precision milling process. Next, a thin layer of polymer adhesive was coated in the rectangular cavity. Then, many high precise small steel balls with highly polished surfaces are placed in the cavity to form a closely packed array. When the polymer adhesive cured, a closely packed small steel ball array was fixed in the rectangular cavity. Then, a polymer substrate Poly Methyl Methacrylate or acrylic (PMMA sheet) is placed on top of the small steel ball array, and they are placed in a hot embossing machine. During the hot embossing process period, the small steel ball array is pressed into the PMMA sheet, and after a proper cooling process, the PMMA sheet is removed from the small steel ball array. A small concave lens array pattern is formed on the surface of the PMMA sheet, and a small lens array mold is thus obtained. The effective method only needs one simple step, and a small lens array mold with a concave lens array pattern can be directly fabricated. The diameter and depth of the small concave lens array pattern on the surface of the PMMA sheet can be controlled by adjusting the processing conditions of the hot embossing process. Therefore, small lens array mold inserts with different diameter and depth could be fabricated. These PMMA sheets with a concave lens array pattern can be used as molds for the rapid replication of polymer lens array devices using a UV molding process. In this study, the effects of the hot embossing processing conditions on the shape and dimensions of the fabricated small lens array mold inserts were investigated. After undergoing a UV-molding process, the replication quality of the small polymer lens array device were measured and analyzed. Experimental Setup and method Figure 1 shows the construction of the hot embossing machine used in this study. The machine primarily consists of top and bottom heating plates with cooling system (The maximum working temperature is 250 o C), a commercial hydraulic cylinder with pressure regulator (The maximum applied pressure is 15MPa) and a power supply with control panel. The working temperature, SPE ANTEC Anaheim 2017 / 1922

2 pressure and time can be precision controlled. Figure 2 shows a photograph of a PMMA sheet on a closely packed small steel ball array. In this study, high precise steel balls with highly polished surfaces are used. The diameter of the small steel balls was 2mm±0.08μm. The surface roughness of the small steel balls was 0.01μm. This kind of precise steel ball was applied in the ball screw industry. Therefore, the accuracy of these steel balls is high and uniform. The dimension layout of the rectangular cavity was 30mm 30mm 3mm. The dimensions of the PMMA sheets used in this study were 30 mm 30 mm 4 mm, and the glass transition temperature of PMMA was approximately 102 C. embossing machine came up to press the PMMA sheet and the small steel ball array. At the same time, the heating plates were heated to proper working temperature. When the proper working temperature is reached, the higher working pressure is applied. The softened surface of the PMMA sheet was pressed by the small steel ball array under the constant pressure. The small concave lens array pattern was formed on the surface of the PMMA sheet due to the plastic visco-elastic deformation principle. After the pressing time period ended, the heating plates are cooled to 60 o C, while maintaining the applied pressure to prevent uncontrolled shrinkage and distortion. Finally, the bottom heating plate came down, and the PMMA sheet was de-molded from the small steel ball array. A PMMA sheet with a small concave lens array pattern was obtained. Figure 1. Schematic drawing of the hot embossing machine. (a) Material preparation (b) Hot embossing and cooling Figure 2. Photograph of a PMMA sheet on a small steel ball array. Figure 3 shows the fabrication procedure of the PMMA sheet with small concave lens array pattern. First, a PMMA sheet was placed on top of the small steel ball array, and the stack of PMMA sheet and the small steel ball array was fixed on the bottom heating plate of the hot embossing machine. Then, bottom heating plate of the hot (c) De-molding Figure 3 Fabrication procedure of the PMMA sheet with small concave lens array pattern. SPE ANTEC Anaheim 2017 / 1923

3 Results and Discussion To verify the uniformity of the proposed processing, the shape and dimension of the fabricated small concave lens array were measured by a scanning electron microscope (Hitachi S-3000N, Japan) and a surface profiler (Alpha-Step Profilometer, KLA-Tencor). Figure 4 shows a photograph and SEM image of a fabricated PMMA sheet with small concave lens array pattern. In the preliminary test, the processing conditions of working temperature at 110 o C, the applied pressure at 0.4MPa and the pressing time at 2 minutes. It is observed that array of concave lens was successfully fabricated over the whole PMMA sheet. The average diameter of a fabricated small concave lens array was μm with a standard deviation of 2.4μm. The average depth of a fabricated small concave lens array was 600.8μm with a standard deviation of 1.1μm. Figure 6 shows the effect of working temperature on the dimension of fabricated small concave lens array pattern. In this experiment, the working temperature was varied from 80 o C to 125 o C while keeping the applied pressure at 0.4MPa and pressing time at 2 minutes. With the increasing of working temperature, the average diameter and depth of the fabricated small concave lens array are increases dramatically. The minimum average diameter and depth of the embossed small concave lens array were 204.8μm and 20.2μm, respectively, while the maximum values were μm and μm. This is because when the working temperature is below the glass transition temperature of the PMMA material, the surface of PMMA sheet is still rigid. Therefore, the average diameter and depth of the fabricated small concave lens array are still small. On the other hand, when the working temperature is above the glass transition temperature of the PMMA material, the hardness of the PMMA sheet was decreased and the surface of the PMMA sheet was molten. The average diameter and depth of the fabricated small concave lens array were thus increased rapidly. Figure 4 Photograph and SEM image of a fabricated PMMA sheet with small concave lens array pattern. Figure 5 shows the comparison between surface profile of a steel ball and a fabricated small concave lens. The result suggests that the surface profile of a small concave lens was very well fitted with the steel ball. These results indicate good uniformity and controllability of the proposed process. Figure 5 Comparison between a steel ball profile and a small concave lens profile. Figure 6. Effect of working temperature on the dimension of fabricated small concave lens array pattern. Figure 7 shows the effect of applied pressure on the dimension of fabricated concave micro-lens array pattern. In this experiment, the applied pressure was varied from 0.2MPa to 0.8MPa while keeping the working temperature at 95 o C and pressing time at 2 minutes. The measured result suggests that the average diameter and depth of fabricated small concave lens array increased with applied pressure. When a high pressure was applied from the hot embossing machine, the surface of PMMA sheet was easier to deform and expanded to the bigger concave lens array pattern. The average diameter and depth of embossed small concave lens array were thus increased obviously. Figure 8 shows the effect of pressing time on the dimension of fabricated small concave lens array pattern. In this experiment, the pressing time was varied from 1 minute to 4 minutes while keeping the working SPE ANTEC Anaheim 2017 / 1924

4 temperature at 95 o C and applied pressure at 0.4MPa. The measured result suggests that increase of pressing time results in small increase in the diameter and depth of fabricated small concave lens array. adhesive tape, and then a UV-curable polymer layer with a thickness of 1.5 mm is coated on the polycarbonate. A PMMA sheet with a small concave lens array pattern was used as a mold insert and placed on the polycarbonate substrate with a UV-curable polymer layer. Then, the stack of the mold insert, polycarbonate substrate with UV-curable polymer layer and glass plate was placed into a transparent PET bag, and the vacuum suction process was operated by a vacuum system with a pressure regulator. The vacuum suction pressure is applied to the stack, and the cavity of the small lens array mold insert is filled with UV-curable polymer. When the degree of vacuum reached torr, the UV-curable polymer on the polycarbonate substrate is cured by UV irradiation for 30 seconds. After the vacuum pressure is released and the mold insert is removed from the polycarbonate substrate, a UV-molded small lens array is obtained. Figure 7. Effect of applied pressure on the dimension of fabricated small concave lens array pattern. Figure 9. Schematic drawing of the UV-molding process. Figure 10 shows the three types of UV-molded small lens array produced by various mold inserts with different dimension. Average diameter of these UV-molded small lens arrays were 496.5μm, 796.2μm and μm. Average sag height of these UV-molded small lens arrays were 113.8μm, 310.8μm and 589.6μm. The results show that the deviations of diameter and sag height of the replicated UV-molded small lens arrays from the mold inserts were less than 3%. Figure 8. Effect of pressing time on the dimension of fabricated small concave lens array pattern. Based on the above study, the diameter and depth of the embossed small concave lens array pattern on the surface of PMMA sheet could be changed and controlled by adjusting the processing conditions of the hot embossing process. Therefore, various mold inserts with different concave lens array patterns have been successfully fabricated. To verify the feasibility of the small lens array mold inserts, a UV-molding process was used for the rapid replication of the polymer lens array. Figure 9 shows a schematic drawing of the UV-molding process used in this study. During the UV-molding operation, a polycarbonate substrate is fixed on a glass plate by SPE ANTEC Anaheim 2017 / 1925

5 (a) Average diameter and sag height of the UV-molded small lens array were 496.5μm and 113.8μm. replicated. The deviations of diameter, pitch and height of the UV-molded micro-lens arrays from the molds were less than 3%. These experimental results show that the proposed method can be used for the fabrication of small lens array mold inserts with various dimensions and rapid replication of polymer lens arrays with high productivity and low cost. References (b) Average diameter and sag height of the UV-molded small lens array were 796.2μm and 310.8μm. (c) Average diameter and sag height of the UV-molded small lens array were μm and 589.6μm. Figure 10. Three types of UV-molded small lens array produced by various mold inserts with different dimension. Conclusions 1. C.K. Chung, and Y.Z. Hong, Microsyst. Technol., 13, 523 (2007). 2. K. Naessens, H. Ottevaere, R. Baets, P.V. Daele, and H. Thienpont, Appl. Opt., 42, 6349 (2003). 3. X. Zhu, L. Zhu, H. Chen, M. Yang, and W. Zhang, Optics & Laser Technology, 66, 156 (2015). 4. H.C. Cheng, C.H. Wang, C.F. Huang, Y.K. Shen, Y. Lin, D.Y. Sheu, and Y.H. Hu, Polym. Adv. Technol., 21, 632 (2010). 5. S.K. Lee, K.C. Lee, and S.S. Lee, J. Micromech. Microeng., 12, 334 (2005). 6. K. Totsu, and M. Esashi, J. Vac. Sci. Technol. B, 23(4), 1487 (2005). 7. M.V. Kunnavakkam, F.M. Houlihan, M. Schlax, J.A. Liddle, P. Kolodner, O. Nalamasu, and J.A. Rogers, Appl. Phys. Lett., 82, 1152 (2003). 8. B.K. Lee, D.S. Kim, and T.H. Kwon, Microsyst. Technol., 10, 531 (2004). 9. H. Hocheng, T.T. Wen, and S.Y. Yang, Materials and Manufacturing Processes, 23, 261 (2008). 10. Y. Fu, Microelectron. Eng., 56, 333 (2001). 11. P. Zhang, G. Londe, J. Sung, E. Johnson, M. Lee, and H.J. Cho, Microsyst. Technol., 13, 339(2007). 12. T.H. Lin, S.Y. Hung, H. Yang, and C.K. Chao, J. Micromech. Microeng., 17, 419 (2007). 13. S. Kirchberg, L. Chen, L. Xie, G. Ziegmann, B. Jiang, K. Rickens, and O. Riemer, Microsyst. Technol., 18, 459 (2012). In this paper, an effective method for the fabrication of various micro-lens array molds using the hot embossing process with a polymer substrate and a small steel ball array was demonstrated. The experimental result suggests that the surface profile of the embossed concave micro-lens on the polymer substrate was very well fitted by small steel balls. In addition, the diameter and depth of the micro-lens array molds could be changed and controlled by adjusting the processing conditions of the hot embossing process. That working temperature and applied pressure changes increases showed increased results. While pressing time increase only showed slight increase. After undergoing a vacuum-assisted UVmolding process with the micro-lens array mold, a polymer micro-lens array device could be successfully SPE ANTEC Anaheim 2017 / 1926