Vertical-actuated electrostatic comb drive with in situ capacitive position correction for application in phase shifting diffraction interferometry

• Published in: Journal of microelectromechanical systems Vol. 12; no. 6; pp. 960 - 971
• Main Authors: Lee, A.P., McConaghy, C.F., Sommargren, G., Krulevitch, P., Campbell, E.W.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.12.2003; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Capacitors; Diffraction; Digital signal processors; Electrostatic levitation; Electrostatics; Exact sciences and technology; Fingers; General equipment and techniques; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Interferometers; Interferometry; Levitation; Mechanical instruments, equipment and techniques; Microactuators; Micromechanical devices and systems; Microstructure; Optical design; Optical diffraction; Optical interferometry; Optical sensors; Phase shifters; Phase shifting interferometry; Physics; Transducers; Light interferometry; Index Terms-Comb drive; Interdigital transducers; Capacitive transducer; Position control; Phase shifting interferometry; Levitation; Microactuators; vertical levitation; electrostatic; Surface micromachining; Mirrors; Microelectromechanical device
• ISSN: 1057-7157; 1941-0158
• DOI: 10.1109/JMEMS.2003.820262

This research utilizes the levitation effect of electrostatic comb fingers to design vertical-to-the-substrate actuation for optical phase shifting interferometry applications. For typical polysilicon comb drives with 2 /spl mu/m gaps between the stationary and moving fingers, as well as between the microstructures and the substrate, the equilibrium position is nominally 1-2 /spl mu/m above the stationary comb fingers. This distance is ideal for most phase shifting interferometric applications. A parallel plate capacitor between the suspended mass and the substrate provides in situ position sensing to control the vertical movement, providing a total feedback-controlled system. The travel range of the designed vertical microactuator is 1.2 /spl mu/m. Since the levitation force is not linear to the input voltage, a lock-in amplifier capacitive sensing circuit combined with a digital signal processor enables a linearized travel trajectory with 1.5 nm position control accuracy. A completely packaged micro phase shifter is described in this paper. One application for this microactuator is to provide linear phase shifting in the phase shifting diffraction interferometer (PSDI) developed at LLNL which can perform optical metrology down to 2 /spl Aring/ accuracy.