SURFACE MICROMACHINING FOR MICROELECTROMECHANICAL SYSTEMS

Surface micromachining is characterized by the fabrication of micromec hanical structures from deposited thin films. Originally employed for integrated circuits, films composed of materials such as low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, an d silicon dioxides can be sequentially deposited and selectively remov ed to build or ''machine'' three-dimensional structures whose function ality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 19 60's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectro mechanical systems (MEMS) make a thorough review of surface micromachi ning appropriate at this time. Four central issues of consequence to t he MEMS technologist are: i) the understanding and control of the mate rial properties of microstructural films, such as polycrystalline sili con, ii) the release of the microstructure, for example, by wet etchin g silicon dioxide sacrificial films, followed by its drying and surfac e passivation, iii) the constraints defined by the combination of micr omachining and integrated-circuit technologies when fabricating monoli thic sensor devices, and iv) the methods, materials, and practices use d when packaging the completed device. Last, recent developments of hi nged structures for postrelease assembly, high-aspect-ratio fabricatio n of molded parts from deposited thin films, and the advent of deep an isotropic silicon etching hold promise to extend markedly the capabili ties of surface-micromachining technologies.