This invention relates to the field of optical mirrors. 
Movable mirrors are frequently used to direct a beam of light or to change the resonant frequency of an optical resonant cavity.  Typically, such mirrors are moved piezo-electrically, and also typically, their inertia prevents their being moved rapidly.  In addition, piezo-electric devices have hysteresis in their performance. 
A type of low inertia movable surface can be created in which electrostatic forces are used to move a thin membrane rapidly.  The first attempt at creating such an electrostatically deformable membrane is described in the patent of Guckel et al. ( U.S. Pat.  No.  4,203,128). 
Guckel et al.  formed a silicon membrane by first exposing one surface of a silicon wafer to boron gas to form a shallow boron doped layer.  The wafer was then etched from the other side until only a thin membrane defined by the boron-doped region remained.  Such a process yielded a membrane which was thick and so was suitable in electro-mechanical applications.  The membrane was too thin, however, to support the high-quality, multilayer, dielectric coatings required for many optical applications. 
Advances in optics, and the widespread use of lasers, has increased the demand for movable mirrors for use in tunable filters and steering laser beams.  This invention provides a low inertia mirror which can be moved rapidly.  In addition, the invention can be used to create devices, such as curved mirrors with a variable focal length, which are not practical using the teachings of the prior art.