Surface micromachining is characterized by the fabrication of micromechanical 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, and silicon dioxides can be sequentially deposited and selectively removed to build or "machine" three-dimensional structures whose functionality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 1960's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectromechanical systems (MEMS) make a thorough review of surface micromachining appropriate at this time. Four central issues of consequence to the MEMS technologist are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, (ii) the release of the microstructure, for example, by wet etching silicon dioxide sacrificial films, followed by its drying and surface passivation, (iii) the constraints defined by the combination of micromachining and integrated-circuit technologies when fabricating monolithic sensor devices, and (iv) the methods, materials, and practices used when packaging the completed device. Last, recent developments of hinged structures for postrelease assembly, high-aspect-ratio fabrication of molded parts from deposited thin films, and the advent of deep anisotropic silicon etching hold promise to extend markedly the capabilities of surface-micromachining technologies.

Optical systems are ubiquitous in the present-day societal fabric, from sophisticated fiber-optic telecommunication infrastructure to visual information display, down to mundane chores such as bar-code reading at the supermarket. Most of these existing systems are built from bulk optical components, as they have been for many years. Just as miniaturization and batch-process production have revolutionized electronics, similar advances in optics will certain greatly expand its applications and markets. Production techniques for optical systems that employ the emerging micro-electromechanical systems (MEMS) technologies give promise of achieving this success. Simple micromechanical fabrication techniques are already employed in fiber-optic components to produce what is generally described as silicon-optical-bench systems. New developments, especially those permitting the use of microactuated structures, make substantial increases in system sophistication possible. Surface micromachining, in which microoptical systems are batch-fabricated and placed on top of a silicon wafer, has become a promising approach to this progression. With the demonstration that surface-micromachined elements can be "folded" out from the plane in which they are constructed, an important new degree of design freedom has emerged. This paper examines some of the results obtained and attempts to project possibilities for surface micromachining in future optical systems.