High Speed Focus Control MEMS Mirror With Controlled Air Damping for Vital Microscopy

A high speed focus control microelectromechanical systems (MEMS) mirror with a step response time of 100 μsec and small-displacement bandwidth of 25 kHz is reported for a 3 mm diameter, electrostatically actuated SU-8 membrane mirror. The dominant effect limiting the mirror bandwidth is viscous air damping, and the innovation we describe is the use of a perforated counter-electrode backplate that facilitates air flow underneath the membrane. We have adopted a model, originally developed for a MEMS microphone, to engineer the damping characteristics and design the air hole patterns. Cryogenic deep silicon etching creates through-wafer perforations in the backplate, and fabricated devices achieve wide-bandwidth actuation. The design approach, fabrication process, and dynamic characterization of the MEMS mirrors are shown. Finally, the focus control mirror is used in a confocal microscope for fast axial focus scanning to provide x-z cross-sectioned in vivo images.