High-Reflectivity TwoAxis Vertical Combdrive Microscanners for Subcellular Scale Confocal Imaging Applications

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High-Reflectivity TwoAxis Vertical Combdrive Microscanners for Subcellular Scale Confocal Imaging Applications
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  High-Reflectivity Two-Axis Vertical Combdrive Microscanners for Sub-cellular Scale Confocal Imaging Applications   Karthik Kumar* 1 , Kazunori Hoshino 1 , Hyun-Joon Shin 2 , Rebecca Richards-Kortum 2 , and Xiaojing Zhang 1 1  Microelectronics Research Center and Department of Biomedical Engineering The University of Texas at Austin, Austin, Texas 78758 2  Department of Bioengineering, Rice University, Houston, Texas 77005 *Telephone: +1-512-232-4275, Fax: +1-512-471-0616, E-mail: kkumar@mail.utexas.edu   A BSTRACT  We present 500x700µm metal-coated scanning micromirrors fabricated from bonded SOI-Si wafers with ~90% reflectivity at 633nm. Confocal images with 1 μ m resolution were generated using single axis actuation of ±2.5 °  at 1.87 KHz. I.   I NTRODUCTION   Microscanners are essential components in miniaturization of optical diagnostic equipment such as confocal imaging. Previous work has shown integration of silicon-based microscanners into confocal and other systems for imaging [1-3]. The reflectivity of silicon mirrors at typical imaging wavelengths of 600-1550 nm is usually only ~30%. Low reflectivity places limits on minimal pinhole size and adversely affects depth resolution of confocal imaging systems. In this paper we describe two-axis self-aligned vertical comb-drive microscanners, fabricated from bonded SOI-Si wafers, that provide ~90% reflectivity at 633nm by depositing a thin film of aluminum on the mirror surface. Misalignment tolerance of 2.5 μ m is achieved for the critical backside alignment step. Confocal images with 1 μ m resolution are acquired using the microscanner. II.   D EVICE D ESIGN AND F ABRICATION  Mirror rotation about two axes is achieved by use of self-aligned vertical comb drive actuators [4,5]. Decoupled two-axis rotation is achieved by mounting the mirror and inner stator combs by torsion rods in a frame with gimbals in the orthogonal direction. Mirrors are fabricated with dimensions of 500 μ m x 700 μ m to facilitate illumination at 45º incidence by 500 μ m diameter laser beam, allowing for uncomplicated optical  paths and easy integration into imaging systems. Fabrication begins with protection of the Silicon-on-Insulator (SOI) 30 μ m device layer surface by thermal oxidation. With the front side surface protected by oxide, alignment marks are dry etched into the backside of the wafer. Front side oxide is removed and coarse features of mirror frame and outer stator combs, aligned to the backside alignment marks, are etched into the device layer by Deep Reactive Ion Etching (DRIE). Thermal oxide of ~4800Å thickness is grown on a separate <100> bare silicon wafer, which is then fusion  bonded on top of the SOI wafer. Initial protection of SOI device layer by oxide is important for achieving high yield in the fusion bonding process. After bonding, the top Si wafer is ground to thickness ~30 μ m and  polished to give smooth surface for optical interface. The mirror will be fabricated in this layer. 1 μ m of low temperature oxide (LTO) is deposited on the front side of the wafer. Bond pad features are defined by partially etching the LTO layer down to depth of 0.3 μ m. The exact features of the stator and rotor combs of the microscanner are then defined by etching through the LTO layer. Misalignment tolerance, during the critical backside alignment step, between these features and the coarse features defined in the SOI device layer is given by half the comb gap spacing or 2.5 μ m in our case. After  patterning of LTO is complete, DRIE is used to transfer the comb features of the microscanner to the upper (rotor) layer. This is followed by dry oxide etch to remove LTO oxide above the bond pads and etch the Fig. 1:  (a) SEM image of top surface of mirror. Embedded: close-up view of torsion rods and comb  banks. (b) SEM showing mirror release by backside DRIE of SOI wafer substrate. Embedded: photograph showing robustness of released mirror being moved out of the focal plane of the microscope on actuation by a micro-manipulator probe.   (a) (b) micro mirror comb  bank torsion rods bond  pads backside optical window 120 0-7803-9562-X/06/$20.00 ©2006 IEEE P22 4:00 PM – 6:00 PM  intermediate insulating layer simultaneously. DRIE is used again to trim the coarse features in the SOI device layer to match the features in the upper layer. After this self-alignment step, all features of the microscanner are defined, and backside DRIE is used to release the scanner. The device wafer is bonded to a handle wafer  by photoresist, and backside DRIE of the outline of the microscanner is performed using the alignment marks  previously etched into the backside of the device wafer. The device is soaked in acetone for 12 hours to release device wafer from the handle wafer. Dry oxide etch is  performed on the front and back sides to remove exposed oxide from the mirror surfaces. E-beam evaporation is used to coat a thin film (500-1000 Ǻ ) of aluminum on the mirror surface to improve reflectivity. The non-conformal nature of deposition combined with large step height can be taken advantage of to deposit metal on the mirror surface without electrically connecting the different layers. Images of the finished device are shown in Fig. 1. III.   I MAGING R  ESULTS   We fabricated scanners with and without metal film deposition to test improvement in reflectivity. Scanners with silicon as reflecting surface exhibited 33.4% reflectivity, while scanners with aluminum coating exhibited 89.3% reflectivity at 633nm. Scanner resonant frequencies were measured by application of 50 ± 17.5sin( ω t) Volts to one comb bank on each axis (Fig. 2). Single-axis resonant frequency of 1.87 KHz was measured with maximum deflection angle of ±2.5 ° . Using these scanners in a confocal imaging setup, we imaged the rotor combs of one of our microscanners and a USAF 1951 resolution target (Fig. 3) at 1 μ m resolution. IV.   C ONCLUSIONS   Scanning micromirrors of size 500x700 μ m were fabricated using a comb self-aligned design, with aluminum film coating forming the reflecting surface. High reflectivity of 89.4% was achieved at 633nm as compared to 33.4% for scanners with crystal silicon as reflecting surface. The scanners were incorporated into a laser scanning confocal imaging setup, and depth images of combs and USAF target were acquired at 1 μ m resolution. Potential applications of the high-reflectivity MEMS scanners include two-dimensional in vivo confocal imaging at sub-cellular scale. R  EFERENCES   [1] D.L. Dickensheets, G.S. Kino, “Micromachined scanning confocal optical microscope,” Opt. Lett. 21, 764-766 (1996). [2] H. Xie, Y. Pan, and G.K. Fedder, “Endoscopic optical coherence tomography with new MEMS mirror,” Electr. Lett. 39, 1535-1536 (2003). [3] W. Piyawatanametha, P.R. Patterson, D. Hah, H. Toshiyoshi, and M.C. Wu, “Surface and bulk-micromachined two-dimentional scanner driven by angular vertical comb actuators”, J. Micro. Electr. Mech. Sys. 14(6) 1329-1338, (2005) [4] U. Krishnamoorthy, D. Lee, O. Solgaard, “Self-aligned vertical electrostatic combdrives for micromirror actuation,” J. Micro. Electr. Mech. Sys. 12, p458-464 (2003). [5] D. Lee, O. Solgaard, “Two-axis gimbaled microscanner in double SOI layers actuated by self-aligned vertical electrostatic combdrive,” Proceedings of the Solid-State Sen. Act, p352-355 (Hilton Head, South Carolina (2004).   Fig. 2:  Measured single-axis frequency response of microscanner actuated with 50 ± 17.5sin( ω t) Volts, with resonant freuenc of 1.87 kHz. Fig. 3:  (a) Schematic of confocal imaging setup used to test the microscanners. (b) Depth image of microscanner rotor combs obtained by confocal imaging microscope incorporating our microscanner. (c) Depth image of USAF 1951 resolution target group 6, element 3 (80.63 line  pairs/mm). Field of view is 25 μ m 2  and resolution is 1 μ m. (b) (c) (a) 5 m5 m   (a) 121
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