A study of the geometry of microball lens arrays using the novel batch-fabrication technique

Transcription

1 Sensors and Actuators A 122 (2005) A study of the geometry of microball lens arrays using the novel batch-fabrication technique C.H. Chien, C.T. Pan, C.C. Hsieh, C.M. Yang, K.L. Sher Department of Mechanical and Electro-Mechanical Engineering, and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan Received 13 September 2004; received in revised form 15 November 2004; accepted 4 April 2005 Available online 6 June 2005 Abstract The innovative concept of batch-fabrication of microball lenses not only reduces the cost of micro assembly, but also replaces conventional aspheric lenses or costly GRINs without sacrificing performance. The process of production is based on thermal reflow of dual-layer polymer systems to batch-fabricate microball lens arrays. The shape of the microball lens and neck width will affect the application of an optoelectronic product and its fabrication method, such as the alignment of the light source and the size of the holder. Therefore, this study focuses on the influence of the major factors of microball radius and neck width. The results reveal that the interactive relationship between the microball radius and neck width plays an important role in the formation of microball lens and application. In the dual-layer polymer system, the PI layer was used as a holder to prop up the AZ4620 layer, and the height of the PI layer was controlled by the reflow temperature. When the microball neck width is improperly controlled, it results in the microball peeling off from the interface between the two polymers Elsevier B.V. All rights reserved. Keywords: Microball lens; Neck size; Critical aspect ratio; Reflow; Surface tension; Insertion loss; Photoresist; PI 1. Introduction Recently, integrated microlens arrays have been applied in some keys area of various fields such as mobile phone panels, liquid-crystal displays, and personal digital accessories. Microlenses have been used to improve the brightness of illuminations and simplify the light guide module texture. There has been a rapid growth in laptop displays, with 25% increase in light output reported using microlens technology [1]. Micro-optical functions and devices, such as optical efficiency enhancements, focal plane optical concentration, color separation, optical scanning and optical switching, have demonstrated the potential industrial uses. Compact devices and mini-features can be fabricated, when this micromanufacturing technology has been successfully developed. Miniaturization devices will revolutionize many Corresponding author. Tel.: x4239; fax: address: panct@mail.nsysu.edu.tw (C.T. Pan). electro-optical systems by using micro-optics, such as video cameras, videophones, optical switches, optical scanners, and high-definition projection displays [2]. Microlens fabrication methods are needed both for the purposes of higher accuracy and lower cost in the rapid growth of commercial optical devices. Silicon micromachined submounts are now under rapid development for use in hybrid free-space MOEMS (microopto-electro-mechanical systems) chip integration. A submount is used to accommodate various free-space MOEMS chips and reduce the packaging cost through the utilization of minimal active optical alignment. Commercial microball lenses have applications in MOEMS. Micro-optical switches are indispensable in optical communication systems. They offer all-optical networks to enhance light propagation performance and data exchange speed [3]. Microscale Fresnel lenses, refractive lenses, gratings and precise optical mounts have been investigated and characterized for 1 2 and 2 2 optical switches [4]. In addition, Toshiyoshi /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.sna

2 56 C.H. Chien et al. / Sensors and Actuators A 122 (2005) and Fujita [5] have used a surface micromachining technique to fabricate a 2 2 matrix optical switch. They employed a commercially available collimated beam fiber (CBF) that was equipped with a spherical lens. However, these approaches use up most of the process time to align the microoptical components. These procedures are time consuming and imprecise. Furthermore, the size of commercial microball lens components range from several hundred mtosev- eral mm, which necessitates more space to accommodate the components. At microscales, refractive lenses offer several important features compared with diffractive optics: significantly reduced wavelength sensitivity, the possibility of really large numerical apertures, and a high light efficiency [6]. Several fabrication techniques have been applied to manufacture refractive microlenses, such as melting cylindrical posts of photoresist to produce a refractive microlens [7,8]. Multimask-level photoresist patterning and sequential RIE forming binary optic microlens arrays were observed. Using a laser writing system to fabricate continuous-relief microoptical elements in photoresist was described by Gale et al. [9]. Moreover, a similar microlens array was fabricated by excimer laser ablation [10]. Surface tension causes the photoresist cylinders to transform into a desirable spherical shape. According to very large-scale integration (VLSI)- based processing techniques, coherent arrays of refractive microlenses are constructed on the surface of silicon using a combination of lithography and the reactive ion etching (RIE) technique [11,12]. Micro-optics printing technology can form a circular microlens array by printing a number of droplets on a substrate [13]. The use of deep X-ray lithography to fabricate micro-optical components exhibits great potential for mass production [14]. Using the modified LIGA process to fabricate microlens through melting a deep X-ray-irradiated pattern in the PMMA substrate was investigated [15]. Micro-optical components of any desired shape with smooth and perpendicular walls, sideward dimensions in the micrometer range, and heights up to several hundred micrometers can be achieved along with a growth in the molding process and the mass production of optical components [16,17]. However, most of the above-mentioned approaches require complicated processes to fabricate the microlens. Much research has focused on how to batch-fabricate microlens arrays [18 20], but not how to do the same for microball lenses. Melting glass at the fiber s terminal is a prerequisite in the conventional technique of fabricating microball lenses [21,22]. In addition, Cox et al. [23] used a micro-inkjet printer to form a microlens at the fiber s terminal to improve fiber performance. However, the control of the size and position of the microlens is imprecise. A novel concept is of a silicon-based coupling platform integrated with an electrostatic-driven out-of-plane optical switch. This can reduce the large amount of space required for motion in in-plane optical switches. The method of corner compensation with the help of IntelliCAD software has been applied to Fig. 1. Schematic illustration of dual-layer system for microball lens fabrication: (a) before reflow process; (b) after reflow process. design a self-parking framework and flat-topped mesa. These were fabricated at the same time by the anisotropic wet etching technique. The microball lens array was batch-fabricated using a micro-electro-mechanical systems technique and batch-assembled onto the coupling platform. The design of the optical platform is able to control the distance between the fiber, the microball lens and the vertical mirror. Therefore, it can enhance the coupling efficiency. Furthermore, not only does it provide an accurate coupling distance, but also it reduces the cost of the micro-assemblage [24,25]. On the other hand, Yang et al. [26] have also developed a mathemat- Fig. 2. Three-dimensional schematic drawing and cross-sectional view of coupling platform integrated with microball lens: (a) 3-D schematic drawing; (b) cross-sectional view.

3 C.H. Chien et al. / Sensors and Actuators A 122 (2005) ical model to design and fabricate micro-ball lens arrays by using the reflow process. But in Yang s research, the height of polyimide is not a controlled factor; also, the effects of reflow temperature and other parameters in the dual-layer polymer system are not discussed. Pan et al. [27] has utilized a thicker photoresist (AZ4620) and PI to quickly and neatly fabricate microlenses in mushroom and spherical shapes by use of the reflow process. The microlens measurement results show that using a laser of 633 nm wavelength, the optimum coupling distance is about 8 m with an insertion loss below 1.3 db (with a coupling efficiency of about 73%). But the paper does not discuss the effective parameters needed for batch-fabrication methods further. Therefore, the purpose of this study will focus on the influence factor and Fig. 3. Flow chart of microball lens fabrication: (a) spin coating polyimide and photoresist; (b) lithography; (c) etching polyimide; (d) reflow.

4 58 C.H. Chien et al. / Sensors and Actuators A 122 (2005) analyze each process parameter to enhance the performance of the fabricated process and its practicability. 2. Experimental procedure The process is based on the thermal reflow of two polymeric layers to batch-fabricate microball lens arrays onto a wafer. Two polymeric layers were coated in an orderly fashion on to a silicon substrate. The upper layer was a photoresist, which was expected to form the shape of a microball. The lower layer was a polyimide (PI) material, which was expected to form a neck or pedestal to support the microball lens after the thermal reflow process. Once the patterned photoresist is heated up to its glass transition temperature (T g ), it will became liquid-like and flow down. Because the surface will maintain the minimum surface energy, the surface of the photoresist will become spherical. A schematic illustration of microball lens fabrication is shown in Fig. 1. Since polymeric layers are defined through the lithography process, the fabrication process not only offers a precise coupling distance, but also reduces the cost of a miniature assembly. A 3-D schematic drawing of an optical coupling platform integrated with a microball lens to enhance the coupling efficiency is illustrated in Fig. 2a. The cross-sectional view of two microball lenses between optical fibers at an accurate distance is illustrated in Fig. 2b. Due to the benefits of providing an accurate alignment distance between fibers and microball lens, the light emitted from the input fiber can be precisely transmitted to the opposite fiber or redirected to another orthogonal one Lithography process In the study, AZ4620 and PI were selected as the photoresist and under-layer polymer, respectively. AZ4620 was purchased from the Clariant Company in Japan. Its ingredients mainly include propylene glycol monomethyl ether acetate and a naphthoquinone diazide derivative as well as some additives, and its T g is between 175 and 180 C. As regards PI, it is a polyimide-based polymer with a T g of about 300 C, purchased from the Toray Company. The polymer PI was smoothly spin-coated on to a hot-dried wafer followed by AZ4620. The exposure and development were carried out in order to build a dual-layer cylinder on the wafer. The wafer was then put in an oven to reflow. A schematic flow chart showing how to fabricate a microball lens is illustrated in Fig. 3. First, the PI was smoothly spin-coated on to the wafer followed by AZ4620 as shown in Fig. 3a. The exposure and development were carried out in order to build a dual-layer cylinder on the wafer as illustrated in Fig. 3b and c. The wafer was then put in an oven to reflow and so produce a microball lens array (see Fig. 3d). The rotational speed of the spin-coater determined the layer thickness of AZ4620. The higher the rotational speed, the lower was the thickness. Speeds of 600, 800, 1000, and 1200 rpm produce thicknesses of 33, 26, 18, and 15 m, respectively. For AZ4620, the exposure dose is 520 mj/cm Reflow process The photolithography technique was used to define the column shape. The size of the microball lens was brought about through control of the height of AZ4620 (H) and its diameter (D) (see Fig. 1). Oven temperatures were set at 190, 220, and 250 C to reflow AZ4620, which is lower than the T g temperature of the PI material (300 C). Using a different dimension of AZ4620, a different size of microlens can be created as illustrated in Fig. 4. The microscope, SEM and interferometer were used to measure the profile and surface roughness of the microlens, and its radius was measured by an image-processing unit able to illustrate the measured 2-D and 3-D (see Fig. 4) plot of the microlens. The reflow process was used to shape the surface of the microball lens. After the reflow process, the AZ4620 and PI layers melted and become liquid-like in their behavior. The weight of the AZ4620 pressed on to the PI layer, thus the PI layer became the neck as shown in Fig. 1b Linear regression analysis Regression analysis is used to study relationships between measurable variables. Linear regression is used for a special class of relationships, namely those that can be described by straight lines, or by generalizations of straight lines to many dimensions. The results of regression analysis may lead to modification of the original description of a fitted model, and a return to the aggregate analysis after modifying the data or assumptions [28]. Through linear regression analysis, a relationship between the radius of a microball lens and the thickness of PI can be found. An equation can also be derived from this to explain the phenomenon of deformation. The equation can be used to reduce the experimental number and cost. 3. Results and discussions 3.1. Effects of reflow temperatures The oven temperatures were set at 190, 220, and 250 C to reflow AZ4620, which is lower than the T g temperature of the PI material. Its T g is between 175 and 180 C. As regards PI, its T g is about 300 C. Once the thermal energy input goes beyond the threshold, the formation process is started and is then literally controlled by the aspect ratio of the AZ4620 layer. The relationship between the radius of microball lens, PI thickness and different reflow temperatures is shown in Fig. 5. The radius of the microball was also controlled by the reflow temperatures, which were insignificant in shaping the microball lens.

5 C.H. Chien et al. / Sensors and Actuators A 122 (2005) Fig. 5. The relation between radius of microball lens and PI thickness under different reflow temperature. sist plays a significant role in the microball lens fabrication process, where H is the thickness of the AZ4620 layer, and D the diameter of the AZ4620 (or PI) one before the reflow process. Either the smaller diameter (D) will produce a smaller radius, or the thicker (H) photoresist will produce a smaller microball lens. It is considered that the higher surface energy difference drives the formation of the spherical shape. Fig. 7 illustrates that when the applied reflow temperature is higher than the T g of AZ4620, and at a certain ratio of H and D, the cylinder of dual layers becomes mushroom-like in shape, as shown in Fig. 7a. Otherwise, at a ratio of H and D of around 0.3, the cylinder changes Fig. 4. Size of microball lens under different aspect ratio: (a) m; (c) 150 m. m; (b) 3.2. Effects of aspect ratio of dual-layer system In the dual-layer system, the column-like photoresist was defined by photolithography and the thicknesses of the upper layer (AZ4620) and lower layer (PI), which were coated on. Their thicknesses can be controlled through the spin coating rate. As illustrated in Fig. 6, the aspect ratio (H/D) of photore- Fig. 6. Effects of aspect ratio (H/D) on radius of microlens (R/D).

6 60 C.H. Chien et al. / Sensors and Actuators A 122 (2005) Fig. 7. Shape profile of microball lens formation: (a) mushroom-like shape; (b) ball shape. Fig. 8. Reflow characteristics: (a) single layer after reflow; (b) dual layer after reflow. to a ball-like structure, as shown in Fig. 7b. The final reflowed shape changes to a microball array from the micromushroom by changing the ratio of diameter D and height H. When the aspect ratio of the patterned photoresist is larger than 0.3, the reflowed shape will always be a microball shape Effects of reflow process and microball lens surface energy After the reflow process, when AZ4620 shrinks to reduce the surface energy, it will produce shear stress, which changes the shape of the microball. Due to the AZ4620 shrinking to reduce the surface energy, the pressure between the inside and outside of the polymer will force it to tend to equilibrium. A small curvature has a large differential pressure. Therefore, a small radius will have a better shrink capability. When the AZ4620 layer reaches the glass transition temperature, it enters a liquid-like state, and the centripetal force between each molecule attracts them to each other in order to maintain equilibrium. If the centripetal force is lower than the other on the surface, it will cause an imbalance in the surface tension, and produce a dragging force toward the center of column. Therefore, the liquid-like state tends to form the smallest area in order to reduce the surface energy and equilibrium. It becomes spherical. Due to the glass transition temperature of the lower layer being higher than the upper layer, thus the shape of the upper layer will become mushroom-like, which has the smallest surface energy. Between a certain range of D and H of AZ4620 (see Fig. 7), the mushroom will become spherical which means we can control the radius and thickness of the upper layer to change its shape. The result reveals that the microball must maintain a minimal surface energy before solidification. Fig. 9. Microball radius and microball neck size relations.

7 C.H. Chien et al. / Sensors and Actuators A 122 (2005) Effects of PI thickness The contact angle of a single layer of AZ4620 cannot reach up to 90 after reflow to form a ball-shaped lens, as illustrated in Fig. 8a. However, with the PI underneath, the upper AZ4620 can successfully form a microball lens, as shown in Fig. 8b. It is because their T g (s) are different (T g for AZ4620 = C, T g for PI = 300 C). It results in different reflow rates between AZ4620 and PI. Due to the surface tension, the AZ4620 creates a microball profile, while the PI transforms into the neck of a pedestal to support the microball lens Relationship between the microball size and necking phenomenon of microball The different radius of a microball will affect the focal length, and the range of radii of microballs will affect the microball neck width. The relation between the microball radius and microball neck width is illustrated in Fig. 9, in which D ranges from 50 to 160 m and H is a constant 18 m under a 12 h reflow process. There exists a proportional relationship between the microball radius and microball neck width. The results reveal that when the radius of microball lens is increased, then the neck width of the pedestal supporting the Fig. 10. Microball neck size and PI thickness relations: (a) reflow temperature at 190 C; (b) reflow temperature at 220 C; (c) reflow temperature at 250 C.

8 62 C.H. Chien et al. / Sensors and Actuators A 122 (2005) lower than the T g of the polymer. As the temperature rises, Fig. 11. Fruitful result of the microball lens: (a) a microball lens, polyimide pedestal and a missing microball lens; (b) the missing ball in another place. microball lens should also expand in to maintain equilibrium. When the radius of the microball lens and the neck width of the pedestal cannot maintain equilibrium, the microball lens will peel off from the neck. Since the microball size and neck width relate to the reflow temperature in this study, we have used the statistical package of the social science (SPSS 11.5) for linear repression analysis. Through SPSS analysis the equation can be expressed as a dimensionless equation. The relationship between PI thickness (before reflow) and neck width (after reflow) of the microball lens is as demonstrated in Eq. (1): N s = PI t (1) where N s is the neck width ( m) and PI t is the PI thickness ( m). This equation is helpful in showing the relationship between the thickness of the neck and the dimension of the microball lens. The relationship between the PI thickness and that of the neck is demonstrated under different reflow temperatures of 190, 220, and 250 C, respectively. The result shows that increasing the thickness of PI will enlarge the neck width of the pedestal of the microball as illustrated in Fig. 10. Fig. 11 shows the effect of this on the microball lens. The applied reflow temperature should be

9 C.H. Chien et al. / Sensors and Actuators A 122 (2005) [6] S. Sinzinger, J. Jahns, Microoptics, Wiley VCH Verlag GmbH, Weinheim, 1999, pp [7] Z.D. Popovic, R.A. Sprague, G.A.N. Connell, Technique for the monolithic fabrication of microlens arrays, Appl. Opt. 27 (1988) [8] M.C. Hutley, Optical techniques for the generation of microlens arrays, J. Mod. Opt. 37 (1990) [9] M.T. Gale, M. Rossi, J. Pedersen, H. Schutz, Fabrication of continuous-relief micro-optical elements by direct laser writing in photoresists, Opt. Eng. 22 (11) (1994) [10] K. Zimmer, D. Hirsch, F. Bigl, Excimer laser machining for the fabrication of analogous microstructures, Appl. Surf. Sci (1996) [11] M. Stern, T.R. Jay, Dry etching for coherent refractive microlens arrays, Opt. Eng. 33 (11) (1994) [12] M.E. Matamedi, M.P. Griswold, R.E. Knowlden, Silicon microlenses for enhanced optical coupling to silicon focal planes, Proc. SPIE 1544 (1991) [13] W.R. Cox, T. Chen, D. Hayes, Micro-optics fabrication by ink-jet printing, Opt. Photonics News 12 (6) (2001) [14] J. Gottert, J. Mohr, Characterization of micro-optical components fabricated by deep-etch X-ray lithography, SPIE: Micro-Optics II 1506 (1991) [15] S.K. Lee, K.C. Lee, S.S. Lee, A simple method for microlens fabrication by the modified LIGA process, J. Micromech. Microeng. 12 (2002) [16] H. Yang, M.C. Chou, A. Yang, C.K. Mu, R.F. Shyu, Realization of fabricating microlens array in mass production, Proc. SPIE 3739 (1999) [17] H. Yang, C.T. Pan, M.C. Chou, Ultra-fine machining tool/molds by LIGA technology, J. Micromech. Microeng. 11 (2001) [18] D. Daly, R.F. Stevens, M.C. Hutley, N. Davies, The manufacture of microlenses by melting photoresist, Meas. Sci. Technol. 1 (1990) [19] Z.D. Popovic, R.A. Sprague, G.A. Neville Connell, Technique for monolithic fabrication of microlens arrays, Appl. Opt. 27 (7) (1988) [20] M.C. Hutley, R.F. Stevens, D. Daly, The manufacture of microlens arrays and fan-out gratings in photoresist, Optical Connection and Switching Networks for Communication and Computing, 1990, pp. 11/1 11/3. [21] V. Russo, G.C. Righini, S. Sottini, S. Trigari, Lens-ended fibers for medical applications: A new fabrication technique, Appl. Opt. 23 (19) (1984) [22] G.D. Khoe, J. Poulissen, H.M. de Vrieze, Efficient coupling of laser diodes to tapered monomode fibers with high-index end, Electron. Lett., 17th 19 (6, March) (1983) [23] W. Royall Cox, C. Guan, D.J. Hayes, D.B. Wallace, Microjet printing of micro-optical interconnects, Int. J. Microcircuits Elect. Packaging 23 (3) (2000) [24] S.C. Shen, C.T. Pan, M.C. Chou, H.P. Chou, Electromagnetic optical switch for optical network communication, J. Magn. Magn. Mater. 239 (2001) [25] C.T. Pan, Silicon-based coupling platform for optical fiber switching in free space, J. Micromech. Microeng. 14 (2004) [26] H. Yang, C.K. Chao, C.P. Lin, S.C. Shen, Micro-ball lens array modeling and fabrication using thermal reflow in two polymer layers, J. Micromech. Microeng. 14 (2004) [27] C.T. Pan, C.H. Chien, C.C. Hsieh, Technique of microball lens formation for efficient optical coupling, Appl. Opt. 43 (32) (2004) [28] N.R. Draper, H. Smith, Applied Regression Analysis, Wiley, New York, Biographies Chi-Hui Chien was born in Kaohsiung, Taiwan, Republic of China, in He received the BE degree from Tamkange College, Taiwan, Republic of China, in 1977 and the MS degree from Auburn University, Alabama, USA, in 1980, both in mechanical engineering, and the PhD degree in theoretical and applied mechanics from University of Illinois at Urbana-Champaign, USA, in He joined National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China, as an associate professor in Currently, he is a full professor in the Department of Mechanical and Electro-Mechanical Engineering in the same university. His current research interests include glass fiber buckling, the study of warpage, fatigue life, and interfacial adhesion of the IC package, and optical method in nano-scale measurement. C.T. Pan was born in Nauto, Taiwan, Republic of China, in He received his engineering degree of master and doctor in 1993 and 1998, respectively, from Power Mechanical Engineering Department of National Tsing Hua University in Hsinchu, Taiwan. He was a researcher in the field of laser machining polymer in the TU Berlin (IWF) in Germany from 1997 to 1998 and a researcher of MEMS Division in the MIRL/ITRI, Hsinchu in Taiwan from 1998 to He joined National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China, as an assistant professor in His current research interests focus on MEMS, NEMS, and LIGA process. Chi-Chang Hsieh received the BE and ME degree from Department of Mechanical and Automation Engineering, Da-Yeh University, Taiwan, ROC, in 1999 and now is the doctoral candidate in Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University. His current researches include MEMS process, applied optical metrology, and interfacial adhesion of the IC package, etc. Cong-Ming Yang was born in Tainan, Taiwan, Republic of China, in He received the BE degree from Department of Mechanical Engineering, NKAS University, Taiwan, ROC, and MS degree from Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University, Taiwan, ROC in His current researches include MEMS processes and interfacial adhesion of the IC package, etc. Kun-Lin Sher was born in Kaohsiung, Republic of China, in He received his engineering degree from Department of Automation Engineering, Kao-Yuan Institute of Technology, Taiwan, ROC in 2003 and now is the master candidate in Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University. His current researches interests focus on MEMS, NEMS process.