Contents I. Introduction
In the past few years, three-dimensional (3-D) microdevices have been achieved through the assembly of two-dimensional (2-D) surface micromachined planar structures. Applications include free-space beam steering reflectors [1], tunable Fabry–Pérot etlons [2], and corner cube reflectors [3], etc. The most commonly used technique is micromachined hinges [4]. To achieve stable 3-D assemblies, multiple hinged flaps are rotated off the substrate surface simultaneously. With special latching components, these flaps will be able to contact and support each other to form the required 3-D structure. Different actuation schemes, such as turbulent fluid flow and dedicated on-chip microactuators (e.g., thermal [5]and electrostatic actuator [6]) have been devised to render the assembly process. However, dedicated microactuators have relatively large footprints. Incorporating microactuators will sacrifice the achievable device density. To overcome this disadvantage, Yi and Liu developed a method to assemble hinged flaps [7] using magnetostatic actuation methods [8]–[11]. A Permalloy (NiFe) piece is electroplated onto each hinged flap. An external magnetic field is applied to generate the actuation. Flaps with Permalloy pieces of different dimensions can be rotated off the substrate surface in a predetermined sequence, so that the assembly of complicated 3-D structures is possible. Since the Permalloy piece is directly attached onto each flap, the additional substrate space for the actuator is then not required. This actuation method is space-efficient and capable of addressing a batch-scale assembly in parallel.