Michael Helmbrecht

Fluidic self-assembly of micromirrors onto microactuators using capillary forces

The authors discuss the application of self-assembly techniques for positioning microscopic components onto a substrate in a desired configuration. The basis is a fluidic self-assembly technique in which capillary forces assemble microparts with submicrometer alignment precision. A heat-curable acrylate-based adhesive is used to provide the capillary forces for assembly and is then polymerized in a bath of water at 80/spl deg/C for 16 h with continuous nitrogen bubbling. The application we describe is self-assembly of flat silicon micromirrors; onto surface-micromachined actuators for use in an adaptive-optics mirror array. Photolithography defines shapes of hydrophobic self-assembled monolayers for self-assembly. Mirrors with fill factors up to 95% were assembled. Mirrors 464 /spl mu/m in diameter and assembled onto actuators remain flat to within 6 nm rms. This mirror quality would be difficult to attain without the process decoupling afforded by microassembly. The general self-assembly approach described here can be applied to parts ranging in size from the nanometer to the millimeter scale and to a variety of part and substrate materials.

Micromirrors for Adaptive-Optics Arrays

We present a micromachined mirror array for use in adaptive optics. Piston motions of more than 6 µm as well as tip/tilt motions of 11 mrad (0.65°) are demonstrated. Single-crystal-silicon mirrors are assembled onto electrostatic parallel-plate actuators. The actuators lift off the substrate after microstructure release as a consequence of residual stresses in nickel-polysilicon bimorph flexures. The assembled mirrors provide fill factors of 95%. Peak-to-valley surface variations are smaller than 30 nm over a 464 µm-diameter (vertex-to-vertex) mirror segment.

Fluidic self-assembly of micromirrors onto surface micromachined actuators

We describe the fluidic self-assembly of ultra-flat, single-crystal silicon micromirrors onto a surface micromachined actuator array. Sub-micron precision self-alignment of the mirrors onto the actuator platforms is achieved by pattern matching hydrophobic binding sites on the underside of the mirror and on the platform.

Stroboscopic interferometer with variable magnification to measure dynamics in an adaptive-optics micromirror

Interferometry has proven a powerful tool to measure out-of-plane movements in MEMS with high accuracy, In this paper, we demonstrate a setup for stroboscopic interferometry that combines the precise data registration obtained by combining phase-shifting techniques with the high spatial resolution and aperture that are characteristic for an optical microscope.

MOEMS spatial light modulator development at the Center for Adaptive Optics

The National Science Foundation Center for Adaptive Optics (CfAO) is coordinating a program for the development of spatial light modulators suitable for adaptive optics applications based on micro-optoelectromechanical systems (MOEMS) technology. This collaborative program is being conducted by researchers at several partner institutions including the Berkeley Sensor & Actuator Center, Boston Micromachines, Boston University, Lucent Technologies, the Jet Propulsion Laboratory, and Lawrence Livermore National Laboratory. The goal of this program is to produce MEMS spatial light modulators with several thousand actuators that can be used for high-resolution wavefront control applications that would benefit from low device cost, small system size, and low power requirements. The two primary applications targeted by the CfAO are astronomy and vision science. In this paper, we present an overview of the CfAO MEMS development plan along with details of the current program status.

MEMS: Some Self-Assembly Required

Fluidic self-assembly can provide the vital step in the fabrication of chip-sized optical MEMS for a wide variety of applications. Nature herself provides an inspirational model for fluidic self-assembly, as well as useful guidelines for processes that can be employed to build microsystems. In this review article, we describe some achievements in assembly that have already been demonstrated, including our own research on fluidic self-assembly of micromirrors for an adaptive-optics array. The results reported to date lead us to predict significant growth in the applications to microsystems of fluidic self-assembly techniques.

FEM correlation and shock analysis of a VNC MEMS mirror segment

Microelectromechanical systems (MEMS) are becoming more prevalent in today’s advanced space technologies. The Visible Nulling Coronagraph (VNC) instrument, being developed at the NASA Goddard Space Flight Center, uses a MEMS Mirror to correct wavefront errors. This MEMS Mirror, the Multiple Mirror Array (MMA), is a key component that will enable the VNC instrument to detect Jupiter and ultimately Earth size exoplanets. Like other MEMS devices, the MMA faces several challenges associated with spaceflight. Therefore, Finite Element Analysis (FEA) is being used to predict the behavior of a single MMA segment under different spaceflight-related environments. Finite Element Analysis results are used to guide the MMA design and ensure its survival during launch and mission operations. A Finite Element Model (FEM) has been developed of the MMA using COMSOL. This model has been correlated to static loading on test specimens. The correlation was performed in several steps—simple beam models were correlated initially, followed by increasingly complex and higher fidelity models of the MMA mirror segment. Subsequently, the model has been used to predict the dynamic behavior and stresses of the MMA segment in a representative spaceflight mechanical shock environment. The results of the correlation and the stresses associated with a shock event are presented herein.

Micro-electro-mechanical systems spatial light modulator development

The NSF Center for Adaptive Optics (CfAO) is coordinating a five to ten year program for the development of MEMS-based spatial light modulators suitable for adaptive optics applications. Participants in this multi-disciplinary program include several partner institutions and research collaborators. The goal of this program is to produce MEMS spatial light modulators with several thousand actuators that can be used for high-resolution wavefront control applications and would benefit from low device cost, small system size, and low power requirements. We present an overview of the CfAO MEMS development plan along with details of the current program status. Piston mirror array devices that satisfy minimum application requirements have been developed, and work is continuing to enhance the piston devices, add tip-tilt functionality, extend actuator stroke, create a large array addressing platform, and develop new coating processes.

Operation of a MOEMS Deformable Mirror in Cryo: Challenges and Results

Micro-opto-electro-mechanical systems (MOEMS) Deformable Mirrors (DM) are key components for next generation optical instruments implementing innovative adaptive optics systems, both in existing telescopes and in the future ELTs. Characterizing these components well is critical for next generation instruments. This is done by interferometry, including surface quality measurement in static and dynamical modes, at ambient and in vacuum/cryo. We use a compact cryo-vacuum chamber designed for reaching 10–6 mbar and 160 K in front of our custom Michelson interferometer, which is able to measure performance of the DM at actuator/segment level and at the entire mirror level, with a lateral resolution of 2 µm and a sub-nanometer z-resolution. We tested the PTT 111 DM from Iris AO: an array of single crystalline silicon hexagonal mirrors with a pitch of 606 µm, able to move in tip, tilt, and piston (stroke 5–7 µm, tilt ±5 mrad). The device could be operated successfully from ambient to 160 K. An additional, mainly focus-like, 500 nm deformation of the entire mirror is measured at 160 K; we were able to recover the best flat in cryo by correcting the focus and local tip-tilts on all segments, reaching 12 nm rms. Finally, the goal of these studies is to test DMs in cryo and vacuum conditions as well as to improve their architecture for stable operation in harsh environments.

Cryogenic testing of MOEMS deformable mirror for future optical instrumentation

An Iris AO MOEMS deformable mirror has been successfully operated and tested at 160K. Surface deformation at room temperature and in cryo has been measured and DM architecture contributions analyzed.