Multistep sequential batch assembly of three-dimensional ferromagnetic microstructures with elastic hinges

In this paper, we have developed a process for multistep sequential batch assembly of complex three-dimensional (3-D) ferromagnetic microstructures. The process uses the magnetic torque generated by an external magnetic field perpendicular to the substrate to lift hinged structures. We found that a dimensionless factor that depends on the volume of the magnetic material and the stiffness of the hinges determines the sensitivity of the hinged microstructures to a magnetic field. This factor was used as a criterion in designing a process for sequential batch assembly, i.e., for setting appropriate differences in sensitivity. Using a dimensionless factor in the design of the sequential assembly simplified the assembly process, which requires only placing the structures on a permanent magnet, and which can be used to carry out multistep sequential batch assembly. We fabricated hinged microstructures, which consist of 4.5-mu m-thick electroplated Permalloy plates and 200-nm-thick nickel elastic hinges of various sizes. In an experiment, four plates (600 mu m x 800 mu m) were lifted sequentially and out-of-plane microstructures were assembled in a four-step process. Assembly of more complex out-of-plane microstructures (e.g., regular tetrahedrons; 800 mu m long on one side) was also shown to be feasible using this method of sequential batch assembly.