Batch-Fabricated Ferromagnetic Microactuators with Silicon Flexures

This manuscript investigates the use of magnetism and ferromagnetic materials in microelectromechanical systems (MEMS) and describes ways to integrate ferromagnetic materials with conventional silicon-based surface micromachining. Surface-microma-chined, batch-fabricated structures that combine plated-nickel and nickel-iron films with polysilicon mechanical flexures, are studied. The microstructures are constructed in a batch-fabrication process that uses electroplating and conventional lithography, materials, and equipment.
We study specifically two prototype magnetically microactuated structures: (1) one consisting of a ferromagnetic plate attached to the substrate via a polysilicon cantilever, and (2) another in which a ferromagnetic plate is held by a pair of beams that a replaced in torsion by magnetic forces. A microactuator consisting of a 400×(47-40)×7μm^rectangular plate of NiFe attached to a 400×(0.9-1.4)×2.25μm^3 polysilicon cantilever beam has been moved more than a length of 1.2 mm, rotated through angles exceeding 180°, and actuated with over 0.185 nN-m of torque. A microactuated mirror made from a 430×130×15μm^3 NiFe plate attached to a pair of 400×2.2×2.2μm^3 polysilicon-torsional beams has been rotated more than 90° out of the plane of the wafer and actuated with a torque greater than 3.0 nN-m. Models of the mechanical and magnetic behavior of each microactuator are developed and used to predict its performance. Experimental results confirm the accuracy of the analytical models. The out-of-plane microactuators have applications as microphotonic elements for optical scanners, displays, and fiber-switching devices.
We also introduce two methods to individually address and control magnetically activated devices: (1) by actuating devices using the magnetic field generated by coils integrated around each device, (2) by using electrostatic forces to clamp selected devices to an insulated ground plane while unclamped devices are freely moved through large out-of-plane excursions by an off-chip magnetic field. A microactuator consisting of a 450×450×5μm^3 plate of NiFe attached to pair of 200×2×2μm^3 polysilicon torsional beams has been rotated more than 45° out of the plane of the wafer by the magnetic field generated by an integrated 10-turn nickel coil carrying a current of 500 mA. Microactuators consisting of 1000×1000×8μm^3 plates of NiFe attached to pairs of 500×15×2μm^3 polysilicon torsional beams have been selectively clamped to the insulated substrate by a 5 V drive while unselected devices are rotated 90° by an external magnetic field of 8 kA/m. Applications for these structures are as addressable micromirrors from an array in a display system or else as individual switches among optical-fiber paths.
Jack William Judy
Timothy D. Sands
Publication date: 
October 31, 1996
Publication type: 
Ph.D. Dissertation
Judy, J. W. (1996). Batch-fabricated Ferromagnetic Microactuators with Silicon Flexures. United States: University of California, Berkeley.

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