MEMS Rotary Stage with Linear Stiffness Utku Baran, Wyatt O. Davis, Sven Holmström, Dean Brown, Jaibir Sharma, Sertan Kutal Gokce, Hakan Urey Koç University, Istanbul, Turkey Abstract— A comb-actuated rotary MEMS stage with a novel spring structure to achieve linear spring stiffness is presented. The present stages can at resonance rotate ±7deg. and ±4.5deg at vacuum and ambient pressure, respectively. Keywords-component; Comb Drive, Linear MEMS Scanner, Low-frequency Scanning I. INTRODUCTION iniaturized laser beam scanning engines is becoming increasingly important as the core technology in applications as pico projectors, head-worn displays and head-up displays [1]. For a projector engine the scanning motion needed is obtained either by one bidirectional scanner or by a separate fast scanner for the vertical axis and a slow scanner for the horizontal axis [2]. The slow scanner is typically driven quasi-statically with a saw-tooth signal for electromagnetically actuated scanners. This excludes high performing out-of-plane torsion scanners without off-set fingers, since they only operate at resonance frequency [3]. In off-set torsion scanners the scan angle is limited by the thickness of the device layer [4]. A less researched option is the in-plane rotary scanner. The main challenge for these types of devices has been to make them respond in a linear fashion. In this paper, a novel spring design meant to achieve linear spring stiffness is implemented in a comb-actuated in-plane rotary stage. The ultimate design goal of the device is to create an optical scanner through mounting a thin mirror vertically on top of the rotary ring of the stage. It will be driven at 60 Hz, enabling its use as the slow scanner of laser projection system. To avoid harmonic distortions from higher modes and ensure a pure in-plane rotational motion, it is essential that the resonance modes are well engineered. In Section II finite element modeling modeling and design considerations are explained. In Section III device are characterized and finally, in Section IV the paper is concluded and future work is described. II. DEVICE DESIGN AND FABRICATION A. Design and Theory MEMS rotary stage (Figure 1a) which has a 75 device layer thickness and 3mm size in diameter is designed with linear rotational stiffness. In this design configuration, two anchors are placed inside of the rotating ring and two flexures which are attached to each of them at a specific angle with respect to the ring. Angular electrostatic comb actuators are bound by arms to the ring, as illustrated in Figure 1b, to enable the rotational motion. The anchors’ locations are arranged according to optimum point estimation explained in [5], and by optimizing the locations of the anchors and shapes of the flexures, high linearity in rotation is obtained. Figure 1. a) Sketch of the stage b) SEM Picture of the stage ANSYS, Finite-element method (FEM) software, was used to predict the mechanical vibration modes of the structure. The device is modeled to be able to actuate at 60 Hz non-resonant frequency which is the demanded frequency of slow-axis scanners for laser projection systems. While engineering the vibration modes two goals are made: The in-plane rotation mode should be placed at the principal resonance frequency and the other modes should be well separated from the in- plane rotation mode to avoid unwanted coupling. In-plane rotation mode’s resonance frequency is placed as 573 Hz at primary mode and the consecutive modes are found to be above 2000 Hz. Furthermore, again using FEM, static analysis is performed to investigate the linearity by calculating the correspondence of the in-plane rotation resultant moment to the ideal case of full linearity. Corresponding linearity constant was found to be 0.985, where 1 is the ideal scenario. B. Fabrication A similar three-mask fabrication process is used as explained in Arslan et.al. [6]. On top of the Silicon-on insulator wafers, Al is sputtered and patterned by photolithography. Consecutively, using photolithography and deep reactive-ion etch the front-side and back-side device structures are defined and the devices are released through etching of the buried oxide in HF vapor. III. CHARACTERIZATION After fabrication it was observed that the Side etch during deep reaction ion etch of the device features was larger than expected. Since the comb-fingers and flexures 1-2 μm thinner than designed for, the pull-in voltages of the device in turn became much lower than predicted. The maximum achievable voltages for the present devices are much lower than the voltage requirements for full deflection. As a result, high deflections can only be reached at resonance at this point, not M This project is sponsored by Microvision, Seattle, USA. 978-1-4577-0336-2/11/$26.00 c 2011 IEEE 37