Design, modeling, fabrication and testing of a high aspect ratio electrostatic torsional MEMS micromirror

• Published in: Journal of micromechanics and microengineering Vol. 16; no. 10; pp. 2147 - 2156
• Main Authors: Joudrey, Kurt, Adams, George G, McGruer, Nicol E
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.10.2006; Institute of Physics
• Subjects: Applied sciences; Circuit properties; Electric, optical and optoelectronic circuits; Electronics; Exact sciences and technology; General equipment and techniques; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Integrated optics. Optical fibers and wave guides; Mechanical engineering. Machine design; Mechanical instruments, equipment and techniques; Micromechanical devices and systems; Optical and optoelectronic circuits; Physics; Precision engineering, watch making; Transducers; Elastic constant; Rotating mirror; Integrated optics; Deformation modulus; Capacitive transducer; Bulk micromachining; Experimental study; Modeling; Dynamic test; Parallel plate capacitor; Young modulus; Finite element method; Optical system; Angular measurement; Electrostatic machine; Surface micromachining; Nickel; Silicon; Electric motor; Dynamic model; Electrostatic actuators; Microelectromechanical device; Micromirrors; High aspect ratio microstructure HARM
• ISSN: 0960-1317; 1361-6439
• DOI: 10.1088/0960-1317/16/10/031

As an essential part of an optical imager project, there was the need for a very high aspect ratio MEMS optical scanning mirror (5 mm X 150 mum clear aperture), capable of a +/-2 deg sweep at 1 kHz with an applied voltage less than 200 V. This paper reports on the design, fabrication, modeling and testing of such an electrostatically actuated MEMS mirror. Fabrication involves using a 1 0 0 n-type double side polished silicon wafer, along with surface and bulk micromachining techniques, to produce a mirror with nickel torsion springs and nickel electrostatic actuators. There were 8 masks and 20 processing steps required. The performance of these devices was measured and found to be within the required specifications. Analysis involved developing models to predict the dynamic behavior of these MEMS micromirrors. A basic parallel plate capacitor model was adjusted with finite element analysis to account for fringing fields. Young's modulus of the electroplated nickel was determined to be 110 GPa from a comparison of the model with the results of dynamic testing, and the residual strain test structures led to a value of 0.0029 for the residual strain. Each of these values is within the rather wide range of published values. Once these values were determined, the model agreed very well with the measurements of the dynamic angular response of the mirror.