Technique of microball lens formation for efficient optical coupling

Transcription

1 Technique of microball lens formation for efficient optical coupling Cheng-Tang Pan, Chi-Hui Chien, and Chi-Chang Hsieh A batch-fabricated microball lens array not only provides accurate coupling distances but also replaces traditional components such as aspheric lenses and expensive graded-index lenses without sacrificing performance and reduces assembly cost. The results of extensive experiments show a critical aspect ratio of 0.3. That is, when the aspect ratio is larger than 0.3, the final shape of a reflowed lens changes to that of a microball rather than of a mushroom. Using a laser with a 633-nm wavelength yields an optimum coupling distance of 8 m with an insertion loss below 1.3 db coupling efficiency, 73% Optical Society of America OCIS codes: , , Introduction Refractive lenses on a microscale exhibit several important properties: significantly less wavelength sensitivity than for diffractive optics necessary for broadband application, the possibility of large numerical apertures, and a high efficiency of light. 1 Several techniques have been applied to the fabrication of refractive microlenses. One way to fabricate refractive microlenses is to melt cylindrical posts of photoresist. 2,3 Surface tension yields photoresist cylinders of spherical shape. Surface tension also leads to relatively short focal lengths for the resultant microlenses. The development of very large-scale integration based processing techniques has permitted coherent arrays of refractive microlenses to be deposited upon silicon substrates by use of a combination of lithography and reactive-ion etching. 4,5 Multimask-level photoresist patterning and sequential reactive-ion etching to form binary optic microlens arrays were performed. A laser writing system for the fabrication of continuous-relief micro-optical elements in photoresist was described by Gale et al. 6 Besides, an analogous microlens array was fabricated by excimer-laser ablation. 7 Micro-optics printing The authors are with the Department of Mechanical and Electro- Mechanical Engineering and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaoshiung 804, Taiwan. C. T. Pan s address is panct@mail.nsysu.edu.tw. Received 4 February 2004; revised manuscript received 18 June 2004; accepted 23 August $ Optical Society of America technology facilitates printing a number of droplets upon a substrate to form a circular microlens array. 8 Microlenses ranging in diameter from 20 mto5mm have been fabricated. The use of deep-x-ray lithography to fabricate micro-optical components is potentially useful for mass production. 9 Lee et al. used a modified Lithographie Galvanik Abformung technique to fabricate microlenses by melting a deepx-ray irradiated pattern in a poly methyl methacrylate substrate. 10 Micro-optical components of any desired shape with smooth and vertical sidewalls, lateral dimensions in the micrometer range, and heights of as much as several hundred micrometers were achieved. Following molding either injection molding or hot embossing, the fabrication of optical components by mass production was achieved. 11,12 However, most of the approaches mentioned above require complicated processes for fabricating microlenses. By hybrid chip integration of free-space microoptoelectromechanical systems, a silicon micromachined submount is now under development. The submount is designed to accommodate various freespace micro-optoelectromechanical system chips and to reduce the packaging cost by use of minimal active optical alignment. A commercial microball lens can be applied in micro-optoelectromechanical systems. The micro-optical switch an indispensable element in optical communication systems is representative of these devices. It offers an all-optical network to increase the speed of data exchange and of propagation of light. 13 Microscale Fresnel lenses, refractive lenses, beam splitters, gratings, and precise optical mounts have been tested and characterized for November 2004 Vol. 43, No. 32 APPLIED OPTICS 5939

2 and 2 2 optical switches. 14 Furthermore, Toshiyashi et al. 15 described using surface micromachining to produce 2 2 matrix optical switches. They employed a commercially available collimated beam fiber that was equipped with a spherical lens 1mmin diameter. However, most approaches to micromachining require significant amounts of time for aligning micro-optical components. These procedures are time consuming and lack precision. Furthermore, the sizes of commercial microball lens components range from several hundred micrometers to several millimeters, and thus more space to accommodate the components is required. Many papers have treated batch fabrication of microlens arrays, but there has been discussion of batch fabrication of microball lenses. The conventional method for fabricating microball lenses is to form a high-refractive-index ball lens by melting glass at the fiber s terminal to enhance the efficiency of fiber optical coupling. 19,20 Cox et al. 21 used a micro-inkjet printer to form a microlens at the fiber s terminal to enhance fiber performance. But it is hard to control the size and position of a microlens. Heremens et al. 22 offered a novel method for optimizing microlenses by reflow of multiple layers of photoresist, but they did not give detailed information on reflow temperature. Besides, their experimental results revealed that in forming a microlens it is hard to constrain the shape into a sphere. In this paper we describe an effective process that uses two polymeric layers to achieve fast and neat batch fabrication of a microball lens array. The new method is based on the thermal reflow of two polymeric layers to batch fabricate a microball lens array onto a planar wafer. Therefore the fabrication process not only provides an accurate coupling distance through lithography to enhance coupling efficiency but also reduces the cost of micro-assembly. A three-dimensional schematic drawing of an optical coupling platform integrated with a microball lens to enhance coupling efficiency is shown in Fig. 1 a. A cross-sectional view of two microball lenses between optical fibers at a specific distance is illustrated in Fig. 1 b. Because accurate alignment between fibers and the microball lens is achieved, the light emitted from the input fiber can be precisely transmitted to the opposite fiber or redirected to the other orthogonal fiber. Fig. 1. a Three-dimensional schematic drawing and b crosssectional view of the coupling platform integrated with the microball lens. equilibrium. The induced force tends to pull point B toward the center of the column until the surface tension at points A and B is in equilibrium. Young s model describes a liquid drop on a solid surface as T SA T LS T AL cos, (1) where is the equilibrium contact angle, T AL is the surface tension energy of the liquid, T SA is the surface tension energy of the solid, and T LS is the solid liquid interface energy. A photoresist used as an optical material has some 2. Experimental Method A. Experimental Principle In a dual-layer superstrate system, as shown in Fig. 2, the photoresist labeled M2 and an underlayer polymer labeled M1 are spin coated onto a substrate, followed by exposure and development. When the system is heated afterward in an oven to the temperature T g of photoresist M2, the photoresist begins to show the behavior of a liquid and to exhibit reduced surface energy. The applied reflow temperature should be lower than T g of polymer M1. As the temperature increases, points A and B are not in Fig. 2. Theoretical surface tensions in a dual-layer system for fabrication of microball lenses APPLIED OPTICS Vol. 43, No November 2004

3 defects, and evaporation of solvent during reflow will produce inaccurate experimental results. In this study we ignore the effects of evaporation of solvent on the shape of the microball lens. We and our colleagues are continuing research on ways to specify the loss of photoresist during reflow. B. Experimental Setup As we mentioned in Subsection 2.A, when the materials of photoresist M2 and underlayer polymer M1 are selected properly, a microball lens can be produced successfully. For the study the polymers AZ4620 and SP431 were selected to be M2 and M1, the photoresist and the underlayer polymer, respectively. AZ4620 was purchased from Clariant Company in Japan. Its ingredients include propylene glycol monomethyl ether acetate and naphthoquinone diazide derivative as well as some additives, and its T g is C. SP341 is a polyimidebased polymer PI with a T g of 300 C and was purchased from Toray Company. Polymer SP341 was smoothly spin coated onto the hot-dried wafer, followed by deposition of the AZ4620 layer. Exposure and development built a cylinder of dual layers upon a wafer. The wafer was then put into an oven for reflow. A schematic flow chart of the fabrication of a microball lens is illustrated in Fig. 3. The PI was first smoothly spin coated onto the wafer, followed by deposition of the AZ4620 layer. Exposure and development built a cylinder of dual layers upon a wafer. The wafer was then put into an oven for reflow to produce the microball lens array. The rotational speed of the spin coater determined the layer thickness of the photoresist. The higher the rotational speed, the less the thickness. Speeds of 600, 800, 1000 and 1200 rpm produced thicknesses of 33, 26, 18 and 15 m, respectively. For AZ4620 the exposure dose is 520 mj cm 2. The oven temperature was set at 190 C, 220 C, and 250 C to reflow AZ4620; these temperatures are lower than the T g temperature of the PI material 300 C. An optical microscope, a scanning-electron microscope, and an interferometer were used to measure the profile and the surface roughness of the microlenses, and the radius of a microlens was measured by an image processing unit that produced two- and three-dimensional plots of the microlens. 3. Results and Discussion A. Effects of Primary Material Diameter In the experiment, two layers of polymeric materials were spun onto a substrate. Then a columnlike photoresist was defined through photolithography. The small diameter of the column produced microlenses with small radii of curvature. Figure 4 shows the curvature of a microlens formed at a representative diameter of development. A small-diameter photoresist column is advantageous. Fig. 3. Schematic illustration of the fabrication of microball lenses: a polyimide and photoresist spin coating, b lithography, c polyimide etching, d reflow. B. Effects of the Thickness of the Polyimide-Based Polymer Layer The single layer of photoresist after reflow forms a ball-shaped lens, as illustrated in Fig. 5 a. However, with the PI underneath, the upper photoresist can successfully form a ball lens, as shown in Fig. 5 b. As a further step explained in Ref. 23, Fig. 6 shows that the thicker the PI slower spinning speed in the tested range, the more effective is the assisting PI; thus a microlens with a small radius is produced for all diameters as a consequence. In addition, the density of the photoresist adopted for this study is approximately 1 2 kg m 3, and the diameter of microball lens is less than 50 m. Therefore the effects of gravity on the profile of the microball lens are limited. C. Effects of the Protoresist s Aspect Ratio Figure 7 shows the effects of the aspect ratio H D on the radius of curvature R of the microball lens, where H is the thickness of the photoresist and D is the 10 November 2004 Vol. 43, No. 32 APPLIED OPTICS 5941

4 Fig. 5. Reflow characteristics: a single layer after reflow, b dual layer after reflow. from that of a micromushroom when the ratio D H changes: When the aspect ratio of the patterned photoresist is larger than 0.3, the reflowed shape will always be that of a microball. The final diameter of a microball lens, however, is little affected by the temperature of the reflow. Hence the eventual formation of a microball lens is considered largely a geometrycontrolled process, whereas the thermal input provides the required energy drive for the process. Fig. 4. Effects of various diameters on microlens formation PI at 1000 rpm, photoresist at 800 rpm, reflow at 220 C. Diameters a 50-, b 100-, c 150- m. diameter of the photoresist or of the PI before reflow Fig. 2. Either the smaller diameter D will produce a smaller radius as discussed in Subsection 3.A, or the thicker H photoresist will produce a smaller microball lens. This is so because of the higher surface energy driving the formation of the ball shape. Notice further that, beyond an aspect ratio of 0.3, the minimum radius of curvature relative to the applied diameter of the dual cylinder will be reached. Figure 8 illustrates that, when the reflow temperature is higher than the temperature T g of the photoresist, and at a ratio H D less than 0.3, the cylinder of the dual layer exhibits a mushroomlike profile Fig. 8 a. Otherwise, at ratio H D of 0.3, the shape of the cylinder becomes ball-like, as shown in Fig. 8 b. The final reflowed shape evolves to that of microball Fig. 6. Effects of PI thickness on the radius of a microball lens APPLIED OPTICS Vol. 43, No November 2004

5 Fig. 8. Shape profiles of microball lens formation: a mushroom shape, b ball shape. Fig. 7. Effects of aspect ratio H D on the radius of a microlens R D at temperatures of a 190 C, b 220 C, and c 250 C. D. Effects of Reflow Temperature To reflow the AZ4620 layer we set the oven temperature at 190 C, 220 C, and 250 C, i.e., lower than the T g temperature of the PI. Figure 9 shows that the radius of curvature of a microball lens is insignificantly affected by the reflow temperature. Once the thermal energy input goes beyond the threshold, the formation process is started and is then controlled literally by the aspect ratio of photoresist layer. Reflow temperatures will affect melting time. In our research the effects of several reflow temperatures on a microball lens were tested and discussed. If the reflow temperature is too high, the photoresist becomes liquidlike, which will cause process failure. If the reflow temperature is too low, however, the processing time will be increased. E. Coupling Efficiency The microball lens described in this study was a polymer-based lens, which still presents many problems, such as the effects of temperature, humidity, and moisture absorption, to be conquered for longdistance application and accurate optical transmis- 10 November 2004 Vol. 43, No. 32 APPLIED OPTICS 5943

6 Fig. 9. Relationship between reflow temperature and radius of a microlens. sion. The preliminary experimental setup of a microlens powermeter was as shown in Fig. 10, which consisted of a powermeter, a high-resolution table, and a PC controller to record the measurement results. It is difficult to set up the measurement system to record on a microscale a three-dimensional microball lens. So far, the measurement concentrates on the power loss that occurs when a laser beam passes through the ball lens. In the future, measurements of the width of the point-spread function or of the Strehl ratio will be explored. Considering the local optical transmission network system, in this study we used inexpensive laser light at 633 nm to test optical coupling and collimate it into a microlens of 40- m diameter. The focal spot on the microball lens is shown in Fig. 11 a the diameter of this spot is 5 m. By moving the stage we measured each coupling position from the microball Fig. 10. Experimental setup of the microlens powermeter. Fig. 11. Result of measuring a microball lens: a Photograph of visible light focused by a microball lens. b Coupling efficiency of a microball lens of 40- m diameter. lens to powermeter. From the shape of the spot size, we can see that the microball lens has the ability to focus a laser beam as a refractive lens does. The coupling efficiency was measured as a function of the variation of the intensity of light relative to the distance between the microball lens and the powermeter. By moving the stage Fig. 10 we measured each coupling position from microball lens to powermeter. Figure 11 b reveals the experimental measurement of coupling efficiency as a function of distance from the microball lens and the powermeter. In this optical coupling measurement the optimum coupling distance is 8 m and the insertion loss is 1.3 db. The maximum coupling efficiency is approximately 73%. The operating temperature for a microball lens should be below 175 C because the value of temperature T g of AZ4620 is between C. Therefore, using this batch-fabrication process not only provides an accurate coupling distance between fibers and the microball lens but also reduces the cost of microassembly. This study shows that inexpensive and batch-microassembled microball lens array processing can be used to replace traditional processing with components such as aspheric lenses and 5944 APPLIED OPTICS Vol. 43, No November 2004

7 11 for optical switching. Thus so far our study has not further addressed the application of antireflection coating. Indeed, if antireflection coating were applied to microball lenses, the coupling efficiency should be enhanced significantly. The profile of a fabricated microball lens is as shown in Fig. 11 a. In this research we developed software with which to calculate radius. It can be seen that just a little deviation from the microball shape, near the bottom, occurred, as illustrated in Fig. 11 b. There are many factors that contributed to this complicated phenomenon; they include the evaporation of solvent and the effects of gravity, which we intend to take into account in our future studies. Fig. 12. Lens fabricated by our novel batch-fabrication technique: a Surface profile of the fabricated lens. b Deviation of the microball lens from perfect spherical shape. expensive gradient-index lenses without sacrificing performance. A V-type groove not only can provide accuracy of position and location but also can be batch fabricated by lithography. The V-type groove can reduce energy loss in optical coupling and obviate the need for manual assembly. 23 In this research the geometry of the microball lens is not really suitable for producing a uniform antireflection coating on a microball lens s surface. The study focuses on the phenomenon of the photoresist and on ways to control the shape of the lens to enhance the optical coupling efficiency as shown in Fig. F. Surface Profile of the Lens and Degree of Deformation of the Microball Lens The surface profile of the fabricated lens is shown in Fig. 12 a ; the diameter of the microball lens is 31.1 m. We have developed software with which to calculate the radius of a microball lens, as illustrated in Fig. 12 b. This software utilizes two straight lines the lines are connected to the circle s edge, and the perpendicular bisector of the line on the edge of the circle will cross at a point that is the center of a circle. From that point a circle can be drawn to fit the profile of the experimental microball lens. Our drawn circle shows good agreement with the experimental circle. G. Surface Roughness of the Lens In our study the roughness of a microball lens was measured with a WYKO optical measurement system, as illustrated in Fig. 13. Because the microball has a three-dimensional profile, it is hard to measure the surface roughness by atomic-force microscopy. A microball lens was fabricated by thermal reflow. After the reflow the roughness of the microball lens s surface could be decreased to less than 30 nm, as measured with the WYKO machine, which is a noncontact measurement system. Besides, in this study Fig. 13. WYKO optical measurement system. 10 November 2004 Vol. 43, No. 32 APPLIED OPTICS 5945

8 we applied 633-nm laser light to measure the microball s optical properties; the length of the light was four times greater than the microball lens s roughness. Therefore we have shown that the effect of surface roughness on the coupling is limited. 4. Conclusions In this study, an effective method for forming microball lenses by using dual superstrate layers that comprise a photoresist and a PI layer beneath it was investigated. The results show that a reflowed microball is controlled chiefly by the aspect ratio of its photoresist and by the diameter of the primary material when it has been heat treated adequately. A thicker polymer was found to be more advantageous for forming smaller balls, whereas the reflow temperature was of little effect in the range tested. The experimental result reveals that the critical aspect ratio H D to produce a ball is 0.3. That is, when the aspect ratio is larger than 0.3, the final reflowed shape evolves into that of a microball from that of a micromushroom. The experimental result yielded an optimum coupling distance between fiber and microball lens of 8 m and a coupling efficiency of 73%. The insertion loss was about 1.3 db. The operating temperature for a microball lens should be below 175 C because the temperature T g of the polymer AZ4620 is C. The new method is based on the thermal reflow of two polymeric layers to batch fabricate a microball lens array. Therefore the fabrication process not only can provide an accurate coupling distance through lithography to enhance coupling efficiency but also can reduce the cost of microassembly. Furthermore, the sizes of the components of commercial microball lenses range from several hundred micrometers to several millimeters. Thus more space is needed to accommodate the components. In our study we successfully fabricated array-level lenses that can be useful in a variety of commercial applications. The novel method of batch fabrication not only fabricates arrays of microball lenses but also controls the size, position, number, and distribution of lenses. Its application to fiber coupling, fiber optical components, micro-optical benches, and integrated optics is promising. The authors thank Tung-Chuan Wu and Min-Chan Chou of the Mechanical Industry Research Laboratories, Industrial Technology Research Institute of Taiwan, for their guidance and the National Science Council Taiwan for financial support of the project grant numbers NSC E CC3 and NSC E References 1. S. Sinzinger and J. Jahns, Microoptics Wiley-VCH, Weinheim, Germany, 1999, pp Z. D. Popovic, R. A. Sprague, and G. A. N. Connell, Technique for the monolithic fabrication of microlens arrays, Appl. Opt. 27, M. C. Hutley, Optical techniques for the generation of microlens arrays, J. Mod. Opt. 37, M. 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