This paper presents the fabrication of
an array of high aspect ratio photoresist based
refractive microlenses (ML) using photolithography
and thermal reflow. First, and in order to evaluate
and predict the MLs optical properties and physical
dimensions, finite element analysis was done. These
simulations helped to design the super high resolution
chrome on soda lime glass photomask as well as
the parameters for the lithographic processes. Then,
an array of high aspect ratio structures (length 4.9
mm, width 30 μm and 5 μm spacing between adjacent
structures) with 5 μm thickness were fabricated.
The thermal reflow technique (using a hotplate) was
applied and an array of MLs measuring 5 and 32 μm
at the vertex and radius, respectively, was achieved.
When the photoresist (PR) is heated up above its
glass transition temperature, it melts and the surface
tension effect causes the fabricated microstructure to
obtain the spherical lens profile. The hotplate thermal
reflow is simple and easy to control, thus permitting
the fabrication of smooth and homogeneous
surfaces essential for good quality refractive microlenses.This work was supported by the Portuguese Foundation
for Science and Technology under the projects
FCT/PTDC/EEA-ELC/109936/2009 and FCT/MITPT/
EDAM-SI/0025/2008

Carmo, João Paulo Pereira

Correia, J. H.

Gomes, J. M.

Rocha, R. P.

Universidade do Minho: RepositoriUM

Fabrication of AZ4562 refractive microlenses array for light enhancement on optical microsystems

FABRICATION OF AZ4562 REFRACTIVE MICROLENSES ARRAY FOR LIGHT ENHANCEMENT ON OPTICAL MICROSYSTEMS  R. P. Rocha, J. P. Carmo, J. M. Gomes, J. H. Correia  University of Minho; School of Engineering, Department of Industrial Electronics, Guimarães, Portugal  Abstract — This paper presents the fabrication of an array of high aspect ratio photoresist based refractive microlenses (ML) using photolithography and thermal reflow. First, and in order to evaluate and predict the MLs optical properties and physical dimensions, finite element analysis was done. These simulations helped to design the super high resolu-tion chrome on soda lime glass photomask as well as the parameters for the lithographic processes. Then, an array of high aspect ratio structures (length 4.9 mm, width 30 µm and 5 µm spacing between adja-cent structures) with 5 µm thickness were fabricated. The thermal reflow technique (using a hotplate) was applied and an array of MLs measuring 5 and 32 µm at the vertex and radius, respectively, was achieved. When the photoresist (PR) is heated up above its glass transition temperature, it melts and the surface tension effect causes the fabricated microstructure to obtain the spherical lens profile. The hotplate ther-mal reflow is simple and easy to control, thus permit-ting the fabrication of smooth and homogeneous surfaces essential for good quality refractive mi-crolenses.   Keywords : Microlenses array, Photolithography, Photoresist, Thermal Reflow  I - Introduction  Standard microfabrication processes (i.e. photolitho-graphy and photoresist thermal reflow) were used to fabricate an array of refractive microlenses. By using these technologies, it is possible to fabricate three dimensional microstructures that are reproducible to customize high-quality and cost effective optical micro-components. Microlenses (ML) were first fabricated in 1988 by [1] and opened a large number of new applica-tions for such optical structures and at the same time reducing the mechanical and electrical complexity of the existing systems. Basically, refractive MLs are used for collimation, focusing or imaging and are an appeal-ing alternative for applications where miniaturization and alignment simplicity are requirements.  Examples of these applications include imaging [2], biomedical instruments [3], lab-on-a-chip systems [4] and in optical communications [5]. The fabrication processes have been through various changes and improvements along this time period as well as the materials selected to fabricate refractive MLs. The two main materials used are glass and polymers and the current fabrication technologies for the latter are the microjet technique, photoresist reflow, ultraviolet curing, hydrophobic effect, LIGA and the soft replica molding methods [6]. The never stopping improvements observed in the polymeric based material properties and continuous process technology enhancements justify the focus given to this material when fabricating MLs. The main contribution presented in this paper is to report on the fabrication of micro-sized, high aspect ratio arrays of MLs to be used in light acquisition enhancement for image sensors. Photolithographic processes were used to accomplish such optical microdevices in the 35 and 5 µm width and thickness sizes, respectively. The thermal reflow was applied to obtain the actual lens profile based on the surface tension phenomenon [7]. This technique allows the fabrication of high quality spheri-cal and parabolic shaped MLs by heating up the photo-resist (PR) above its glass transition temperature. Once this temperature is reached, the photoresist starts to melt with the surface tension effect causing the fabricated microstructure to gain the desired lens profile. The entire process is relatively simple to perform and the thermal reflow specifically is very well controlled without the need for high-technology equipment guaran-teeing good dimensional control and a smooth homoge-neous surface. Moreover, this is an important topic because it provides the fabrication of MLs arrays in a reproducible way and at low-cost. The actual values of the several process parameters will obviously depend on the desired final size, requirements and applications of the micro-optical device. In the presented case the purpose is having the fabrication being done directly on top of an image sensor and the main contributions are the geometric sizes, the aspect ratio achieved and the quick reflow time when compared to other works [8].  II – Microlenses array design and fabrication  A. Microlenses array design  The ML design started with finite element method (FEM) simulations and in Figure 1 it is possible to see the result. The models used to evaluate and define the MLs characteristics are shown in the following equa-tions:  ...43221  NNNn   (1)  hhrRandnRf2122    (2)  In (1) n is the refractive index for a given wavelength λ given by Cauchy’s empirical equation where N1, N2 and N3 are the photoresist’s constants [9]. The equations representing the focal length f, and radius of curvature R, are shown in (2) [10] assuming their cross-section to be spherical. The simulated dimensions of the microlens are 5 and 32 μm for the sag and width, respectively, for an impinging wavelength λ=580 nm (center of the visible spectrum), where r is half the line segment of the reflowed photoresist interfacing with the substrate and h is the maximum height, or vertex. The light is being irradiated parallel to the surface of the substrate and it is clearly seen that the simulations are in agreement with the theoretical focal length of 48.6 μm.    Figure 1: FEM simulations showing the light concentration for microlenses measuring 5 and 32 μm for the vertex h and width 2r, respectively. B. Microlenses array fabrication  There are several different materials available for fabricating microlenses arrays such as the BCB, PDMS, SU-8/2, AZ9260 and AZ4562 photoresists, for example. These polymers allow the microlenses array fabrication by thermally reflowing the three dimensional structure achieved through the photolithographic process. The selected photoresist was the AZ4562 due to the fabrica-tion requirements, i.e., this positive photoresist is ideal for coating thicknesses above 3–5 μm without having to increase the exposure energy considerably and still providing enough energy down to the substrate of the AZ [11]. The processing steps overview of the ML array fabrication process is presented in Figure 2.  Figure 2: Microlenses array fabrication steps. This process permits the production of an array contain-ing 142 microlenses. The rectangles that compose the mask array measure 4.9 mm in length, width of 30 µm and 5 µm spacing between adjacent rectangles. This setup allows the fabrication of good optical quality in just a few minutes and with high degree of reproducibil-ity of their characteristics. To the author’s knowledge, this is the first time that such a geometry, with the presented dimensions and aspect ratio, is used to fabri-cate an array of MLs. Different sized arrays were de-signed and printed into a 128k dpi super high resolution chrome on soda lima glass 3x3–0.060 inches mask with each array covering an area of 25 mm2, as seen in Figure 3. The rectangles are coated with chromium making them opaque to light and the spacing between rectangles is transparent allowing the photoresist under it to be later exposed to the UV light. The fabrication process of the microlenses array requires several steps and process parameters summarized in Table 1. First, it is necessary to spin coat the AZ4562 at 6000 r.pm. during 20 sec-onds, in a previously cleaned glass substrate, to achieve the desired 5 µm thickness.     Figure 3: In a) a whole picture of the array mask and in b) a detailed zoom-in with the chromium rectangle width (30) and spacing (5) is expressed in µm.  b) a) r h R After the coating, a prebake phase, using a computer controlled hot plate at 100º C for 5 minutes, is necessary to evaporate the solvents present in the photoresist. Next, to obtain the required array-like structure, the mask with the correspondent geometry is placed on top, directly contacting with the coated photoresist and exposed to UV light (365 nm) using a mask aligner. The AZ4562 being under a 134 W exposure during 30 seconds makes the unexposed material insoluble. Afterwards, the developing phase is achieved by either the AZ400K or the AZ351B developers in a 1:4 concen-tration with distilled water. To accomplish it, the sub-strate is immersed into two developer baths for 2 min-utes and 15 seconds each, in a magnetic stirrer plate. This is required to leave just the unexposed photoresist in the substrate. The photolithographic process ends with the structures being rinsed with distilled water and dried out with a nitrogen flow. Finally, to obtain the lens profiles, the thermal reflow technique is applied so the substrate holding the array containing the fabricated structures is placed on a hotplate at 130º C for 5 min-utes.   Table 1: Fabrication Process steps and parameters Process steps Process parameters Spin coating 20 secs. @ 6000 r.p.m. Prebake (hotplate) 5 min. @ 100ºC Exposure 365 nm (mask aligner) 30 secs. in contact mode @ 134 W Developing AZ400K or AZ351B developers in a 1:4 con-centration with distilled water (2*2min.15secs) Cleaning Rinse with distilled water and dry with N2 flow Thermal Reflow (hotplate) 5 min. @ 130ºC   III - Results and Discussion  A. Pre-thermal reflow array  After coating the photoresist one should guarantee that, if not all, at least most of the solvent is cleared off. To evaporate the solvent, a hotplate or an oven could be used. If this is not accomplished the trapped solvent may form bubbles and lift the resist film causing adhe-sion failure. In the process presented in this paper, the hotplate was used due to its better ability to evaporate the solvent as it ramps up to the final selected prebake temperature.  By considerably reducing the existing bulk solvent concentration a better resist profile is achieved as well as a decrease of the developer etch to the unexposed resist. Figure 4 shows SEM images of the fabricated structure after the photolithographic process. It should be noted that the physical dimensions of the array elements, i.e. width and pitch distance, are very close to the mask dimensions.    Figure 4: In a) and b) SEM images of a single element of the array and an overview of the array, respectively.  B. Post-thermal reflow array  The PRs that do not cross-link have a given soften-ing point that can be used to thermally reflow the fabricated three dimensional structures. The AZ4562, being a positive photoresist, is one of these PR and its softening point is around 110 ºC [9]. During the fabrica-tion optimization, it was realized that the best reflow conditions were using a hotplate in a temperature range between 130–140 ºC during 5 to 10 minutes depending on the thermal coefficient and thickness of the substrate. After applying the designated temperature the PR’s viscosity decreases and the consequent flow occurs due to surface tension thus achieving the desired ML pro-files. In Figure 5 are represented SEM images of one lens and a part of the entire ML array showing the obtained consistent result. It is clear from the figures that the thermal reflow process using a hotplate allows obtaining the desired microlens profiles necessary to refract the incident light. a) b)   Figure 5: In a) and b) SEM images of a single microlens an overview of the ML array, respectively.  IV - Conclusions  This paper presented the fabrication process of an array of refractive spherical microlenses for light acquisition enhancement in image sensors. The mi-crolenses design started with the FEM simulations to set and evaluate some parameters needed to fulfill the desired objectives. These objectives included the mi-crolenses’ purpose of enhancing the light capture on optical microsystems. The complete fabrication process was explained and the initial and final structures ob-tained were measured using SEM photographs. The several steps that comprise the photolithographic proc-ess were explained and justified and are flexible enough to be changed according to the desired final size, re-quirements and applications of the micro-optical device. It was also shown that the thermal reflow technique is ideal to obtain lens profiles in polymer based micro-structures due to the surface tension effect. Moreover, the thermal reflow is an easy process to control and the results obtained are very smooth. This is an important requirement for obtaining microlenses with good qual-ity. The direct fabrication of such an array of MLs on top of image sensors allows new approaches and appli-cations for optical microsystems. Acknowledgements  This work was supported by the Portuguese Founda-tion for Science and Technology under the projects FCT/PTDC/EEA-ELC/109936/2009 and FCT/MIT-PT/EDAM-SI/0025/2008. The authors would also like to acknowledge Dr.-Ing Christian Koch from Micro-Chemicals GmbH for the technical support.  References  [1]  Zoran D. Popovic, Robert A. Sprague, and G. A. Neville Connell, "Technique for monolithic fabrication of microlens arrays," Applied Optics, vol. 27, no. 7, pp. 1281-1284, 1988.  [2]  O. Matoba, E. Tajahuerce, B. Javidi, "Three-dimensional object recognition based on multiple perspectives imaging with microlens arrays," in LEOS 2001. 14th Annual Meeting of the IEEE Lasers and Electro-Optics Society. Part vol.2, pp.495-6, Piscataway, New Jersey, USA, 2001.  [3]  K. Carlson, M. Chidley, K. B. Sung, M. Descour, A. Gillenwater, M. Follen, R. Richards-Kortum, "In vivo fiber-optic confocal reflectance microscope with an injection-molded plastic miniature objective lens," Applied Optics, vol. 44, no. 10, pp. 1792-1797, 2005.  [4]  Nikolas Chronis, Gang Liu, Ki-Hun Jeong and Luke Lee, "Tunable liquid-filled microlens array integrated with microfluidic network," Optics Express, vol. 11, no. 19, pp. 2370-2378, 2003.  [5]  H. Hamam, "A two-way optical interconnection network using a single mode fiber array," Optics Communications, vol. 150, no. 1-6, pp. 270-276, 1998.  [6]  Jeng-Rong Ho, Teng-Kai Shih, J.-W. John Cheng, Cheng-Kuo Sung and Chia-Fu Chen, "A novel method for fabrication of self-aligned double microlens arrays," Sensors and Actuators A: Physical, vol. 135, no. 2, p. 465–471, 2007.  [7]  A. Schilling, R. Merz, C. Ossmann and H.P. Herzig, "Surface profiles of reflow microlenses under the influence of surface tension and gravity," Optical Engineering, vol. 39, pp. 2171-2176, 2000.  [8]  V. Lin, H.-C. Wei, H.-T. Hsieh, J.-L. Hsieh and G.-D.J. Su, "Design and fabrication of long-focal-length microlens arrays for Shack–Hartmann wavefront sensors," Micro & Nano Letters, vol. 6, no. 7, p. 523–526, 2011.  [9]  M. GmbH, "http://www.microchemicals.eu/," [Online].  [10] D. Daly, R. F. Stevens, M. C. Hutley and N. Davies, "The manufacture of microlenses by melting photoresist," Measurement Science and Technology, vol. 1, no. 8, p. 759–766, 1990.  [11] C. GmbH, "http://www.clariant.com," [Online].   a) b) 

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Fabrication of AZ4562 refractive microlenses array for light enhancement on optical microsystems

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