Mohammad J. Moghimi

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.

MOEMS deformable mirrors for focus control in vital microscopy

Deformable membrane mirrors are promising MOEMS devices for focus control and aberration correction in vital microscopy, offering high speed focus adjustment in an optical system that can be miniaturized for in vivo use. This paper describes mirrors comprising metalized polymer membranes suspended over three concentric circular electrodes for electrostatic actuation. The membranes are 2-μm thick and 3 mm in diameter, made from the fully cross-linked photoset epoxy SU-8 2002. A layer of SU-8 2025 is used to establish a 30-μm thick air gap between the electrodes and the membrane mirror. The membranes are actuated by applying voltage to each electrode individually to achieve displacement as large as 12 μm while minimizing spherical aberration. Surface deflection is studied using phase-shift interferometry under both static and dynamic excitation. Using the deformable MOEMS mirror for focus control in an optical microscope we demonstrate the ability to adjust the location of the focal plane by 85 μm using an N.A. = 0.75 optical system.

Electrostatic-Pneumatic Membrane Mirror With Positive or Negative Variable Optical Power

A microelectromechanical systems (MEMS) deformable mirror is described that uses a novel actuation scheme to increase the optical focus range. In this method, electrostatic-pneumatic actuation is used to achieve a convex surface curvature of the mirror with negative focus, while direct electrostatic actuation creates a concave surface with positive focus. Mirrors have demonstrated 20-μm total stroke with more than 2-kHz bandwidth. The fabricated device consists of two membranes made of the photoset epoxy SU-8. One membrane serves as the deformable mirror and the other one as the pneumatic actuator. The pneumatic membrane also provides a built-in valve for pressure equalization. The principle of operation, fabrication process, and results are presented.

Deformable mirror with controlled damping for fast focus tracking and scanning

Air flow is the dominant damping mechanism for deformable membrane mirrors that are actuated with electrostatic pressure from a counter electrode in close proximity to the flexible membrane. We use cryogenic deep silicon etching to create through-wafer perforations in the backplate in order to control air damping and achieve high-speed focus control. This paper describes both our design approach and device fabrication details. We show that damping can be controlled by selecting the proper hole pattern, and we present experimental and simulated frequency response measurements for small membrane displacements. Also we measured the 95% settling time of a 4 mm diameter mirror subjected to a 10 μm step deflection to be less than 200 μs.

Miniature non-mechanical zoom camera using deformable MOEMS mirrors

We present a miniature non-mechanical zoom camera using deformable MOEMS mirrors. Bridger Photonics, Inc. (Bridger) in collaboration with Montana State University (MSU), has developed electrostatically actuated deformable MEMS mirrors for use in compact focus control and zoom imaging systems. Applications including microscopy, endomicroscopy, robotic surgery and cell-phone cameras. In comparison to conventional systems, our MEMS-based designs require no mechanically moving parts. Both circular and elliptical membranes are now being manufactured at the wafer level and possess excellent optical surface quality (membrane flatness < λ/4). The mirror diameters range from 1 - 4 mm. For membranes with a 25 μm air gap, the membrane stroke is 10 μm. In terms of the optical design, the mirrors are considered variable power optical elements. A device with 2 mm diameter and 10 μm stroke can vary its optical power over 40 diopters or 0.04mm∧(-1). Equivalently, this corresponds to a focal length ranging from infinity to 25 mm. We have designed and demonstrated a zoom system using two MOEMS elements and exclusively commercial off-the-shelf optical components to achieve an optical zoom of 1.9x with a 15° full field of view. The total optical track length of the system is 36 mm. The design is approximately 30 mm x 30 mm x 20 mm including the optomechanical housing and image sensor. With custom optics, we anticipate achieving form factors that are compatible with incorporation into cell phones.

Improved micro-optoelectromechanical systems deformable mirror for in vivo optical microscopy

Abstract. Micro-optoelectromechanical systems (MOEMS) deformable mirrors are being developed for focus control in miniature optical systems including endoscopic microscopes and small form-factor camera lenses. A new process is described to create membrane mirrors made from the photoset polymer SU-8. The SU-8 also serves as the adhesive layer for wafer bonding, resulting in a simple, low cost fabrication process. The process details and the optical properties of the resulting focus control mirrors, which have a diameter of 2 mm, a stroke in excess of 8 μm and very low residual aberration, are described. Multiple actuation electrodes allow active control of more than 1.4 μm peak-to-peak of wavefront spherical aberration. The MOEMS mirror is demonstrated in a confocal microscope in which it provides focus control during capture of in vivo images.

An improved focus control mirror using SU-8 wafer bonding process

We are developing MEMS deformable mirrors for focus control in miniature optical systems, including endoscopic microscopes and small form-factor camera lenses. This paper describes a new process to create mirrors made from the photoset polymer SU-8. The SU-8 also serves as the adhesive layer for wafer bonding, resulting in a simple, low cost fabrication process. The paper describes the process details and the optical properties of the resulting focus control mirrors, which have a diameter of 2 mm, a stroke in excess of 8 μm and very low residual aberration. Multiple actuation electrodes allow control of more than 0.4 μm peak-peak of spherical aberration.

High speed focus control capability of electrostatic-pneumatic MEMS deformable mirrors

MEMS deformable mirrors are versatile elements for optical focus control. Electrostatic-pneumatic actuation of the mirrors offers relatively large membrane stroke to increase focus range. Moreover, this novel actuation method provides high speed focus control with either positive or negative focus. The speed of focusing is dependent on membrane tension, membrane size, air channel configuration, and the size of the backchamber. A 3 mm diameter mirror with 5 mm diameter actuator membrane achieves 30 kHz bandwidth with electrostatic actuation and 8 kHz bandwidth with pneumatic actuation. The settling time of the step response for both electrostatic and pneumatic actuation is approximately 100μs.

Electrostatic-pneumatic membrane mirror with positive or negative variable optical power

A MEMS deformable mirror is described that uses a novel actuation scheme to increase the optical focus range. In this method electrostatic-pneumatic actuation is used to achieve a convex surface curvature of the mirror, while direct electrostatic actuation creates a concave surface. The fabricated device consists of two membranes made of the photoset epoxy SU-8. One membrane serves as the deformable mirror and the other one as the pneumatic actuator. The pneumatic membrane also provides a built-in valve for pressure equalization. The principle of operation, fabrication process and results are presented.

Electrostatic-pneumatic MEMS deformable mirror for focus control

A novel actuation scheme for MEMS deformable mirrors is presented in this paper. In this method electrostatic actuation is combined with pneumatic actuation to achieve either concave or convex surface curvature and to increase the optical focus range of the mirrors. The fabricated device consists of two membranes made of the photoset epoxy SU-8. One membrane serves as the deformable mirror and the other one as the pneumatic actuator. The principle of the operation, fabrication process and deflection curves are presented.