1006 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 13, NO. 6, DECEMBER 2004 Two-Axis Single-Crystal Silicon Micromirror Arrays Mehmet R. Dokmeci, Member, IEEE, Ajay Pareek, Member, IEEE, Shivalik Bakshi, Marc Waelti, Clifford D. Fung, Khee Hang Heng, and Carlos H. Mastrangelo, Member, IEEE Abstract—This paper presents the design, fabrication, and testing of a two-axis 320 pixel micromirror array. The mirror platform is constructed entirely of single-crystal silicon (SCS) minimizing residual and thermal stresses. The 14- -thick rectangular silicon platform is coated with a 0.1- -thick metallic (Au) reflector. The mirrors are actuated electrostatically with shaped parallel plate electrodes with 86 gaps. Large area 320-mirror arrays with fabrication yields of 90% per array have been fabricated using a combination of bulk micromachining of SOI wafers, anodic bonding, deep reactive ion etching, and surface micromachining. Several type of micromirror devices have been fabricated with rectangular and triangular electrodes. Triangular electrode devices displayed stable operation within a ( , ) (mechanical) angular range with voltage drives as low as 60 V. [1124] Index Terms—Al etching, deep-reactive ion etching (DRIE), micromirrors, optical switching, silicon on insulator (SOI) wafers. I. INTRODUCTION T HE increased demand for broad-band telecommunication services has sparked much interest in the use of micromir- rors in all optical networks [1]–[4]. Transparent switching sys- tems require arrays of small mirrors that steer optical beams from one input port to any output port with little propagation loss. The passive nature of these systems permits routing of op- tical signals independent of their wavelength, modulation and polarization without expensive high speed signal regeneration optoelectronics. For low loss propagation, mirrors are required to be optically flat and capable of steering the beam in one or two angular di- rections over a fairly large angular range [5]. Micromirror arrays have been implemented using several fabrication technologies and actuation mechanisms. Mirror arrays have been fabricated using surface micromachined polycrystalline silicon [6] or bulk micromachined silicon on insulator (SOI) [7]. A key optical design parameter is the mirror flatness, char- acterized by its radius of curvature (ROC). The Au reflective surface causes bimetallic warping of the mirror. Thicker bulk micromachined mirrors hence are more desirable for this ap- plication. In this paper, we present the design, fabrication, and Manuscript received July 26, 2003; revised May 24, 2004. Subject Editor O. Solgaard. M. R. Dokmeci is with the Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115 USA (e-mail: mehmetd@ece.neu.edu). A. Pareek, S. Bakshi, C. D. Fung, and K. H. Heng were with Corning IntelliSense, Wilmington, MA 01887 USA. M. Waelti was with Corning IntelliSense, Wilmington, MA 01887 USA. He is now with Phonak, AG, Staefa, Switzerland. C. H. Mastrangelo is with Corning Incorporated, Corning, NY 14831 USA (e-mail: mastrangc@corning.com). Digital Object Identifier 10.1109/JMEMS.2004.839812 Fig. 1. Cross section of bulk micromachined mirror. The entire structure is made of SCS. testing of a high yield bulk micromachined mirror array with single-crystal silicon (SCS) flexures. The devices are electro- statically actuated using parallel plate electrodes fabricated on the glass substrates. II. DEVICE STRUCTURE Due to fabrication simplicity and lower power requirements, electrostatic actuation was selected as the actuation method. Fig. 1 shows a schematic cross section of the two-axis mirror device. The device consists of a monolithic layer of SCS an- odically bonded to a glass substrate with metallic electrodes. Ground shields are patterned on the areas under the mirror to minimize actuator drift caused by substrate charging effects. The silicon layer is bulk micromachined to define the actuator gap, mirror plate, gimbal, and suspension springs all from the same single-crystal layer. Electrical connection to the top silicon is made through a silicon-to-metal lead transfer established by overlapping some of the glass metal to the bonding area as seen in Fig. 1. The monolithic construction is advantageous since it minimizes stresses in the structure. The dimension of the mirror, and typical ROC of at least 50 cm is required, are determined based on the acceptable losses of the optical cross-connect for a given optical path [5]. These two parameters dictate the mass of the mirror, and the frequency response determines the switching speed, spring constant and the operating voltage for a desired angular range. The dimen- sions of the mirror are generally in the 0.5–1 mm diameter range, and typical ROC of at least 50 cm are required to mini- mize the insertion losses for the optical system. In order to ob- tain a high ROC with a 0.1- -thick Au reflector, a 14- -thick silicon layer was used for the platform. Several types of torsional flexures have been used including straight and folded beams. Thick and narrow straight flexures have the highest ratio of vertical to angular stiffness; however they tend to be susceptible to bonding stresses as the torsional spring constant is a function of the tension, and their resistance 1057-7157/04$20.00 © 2004 IEEE Authorized licensed use limited to: Northeastern University. Downloaded on May 25, 2009 at 17:02 from IEEE Xplore. Restrictions apply.