CHARGE DISSIPATION IN ELECTROSTATICALLY DRIVEN DEVICES
BACKGROUND OF THE INVENTION
 This invention relates to charge dissipation in electrostatically driven devices, such as micro-electro-mechanical systems ("MEMS") devices.
 In a typical MEMS device, a movable structure that includes a conductive member is resiliently mounted to a substrate. Resilient mounting may be effected by, e.g., one or more micromachined springs, a membrane, a cantilever, or a torsional plate. The substrate includes one or more fixed conductive electrodes that are supported, and are insulated from each other and from the movable conductive member, by dielectric material. Applying a potential difference between the movable conductive member and one or more of the fixed electrodes produces an attractive electrostatic force urging the movable structure toward the fixed electrodes. The resilient mounting of the movable structure provides a restoring force.
 In certain applications, the deflection of a MEMS device's movable structure ideally would be a unique function of the potentials of each of the fixed electrodes and the movable conductive member. If it were, the device could be operated in an open-loop manner; application of predetermined constant voltages to the fixed electrodes with respect to the movable conductive member would produce forces on the movable structure causing it to assume a well-defined position and then remain at that position for as long as the predetermined constant voltages continued to be applied.
 Real dielectrics are imperfect, and when voltages are applied to the fixed electrodes of a MEMS device, charges may move in or on the dielectric that separates the electrodes. Charges can move within the dielectric due to filling or emptying of charge traps in the dielectric, and mobile ions can migrate along the surface of the dielectric. Such moving charges cause a time-dependent electrostatic force on the movable structure, whose position thus changes with time ("drifts") in response to the changing force, even if the voltages applied to the electrodes are held constant. Drift can be a problem in electrostatically driven devices.
 One solution to drift problems would be to provide sensors responsive to the position of the movable structure and feedback electronics, and to adjust the voltages applied to the electrodes so as to maintain the movable structure in a desired position. However, such sensors and feedback electronics can be costly and bulky, and can dissipate far more power than the MEMS device.
 A solution to drift problems arising from mobile charges in the dielectric of certain electrostatically driven devices has been to deposit or grow a thin conductive layer, referred to as a charge-dissipation layer ("CDL"), on top of the dielectric to bleed off surface charge from the dielectric. For instance, U.S. Pat. No. 5,949,944 describes such a CDL for LiNb03 modulators, which like MEMS device are electrostatically driven and may experience problems due to accumulation of surface charges. A prior art CDL typically consists of a thin film of a poor conductor, for example, a thin film of a doped oxide.
 Prior art processes to create CDLs may require elevated temperature processing, and may require lithogra
phy or masking in order to restrict deposition of material to particular areas. The electrical properties of CDLs produced by prior art processes may be difficult to control, and the CDL electrical properties may change due to oxidation or corrosion unless extra processing steps are taken to protect the CDL.
SUMMARY OF THE INVENTION
 Problems of the prior art are addressed by an implanted-ion charge-dissipation structure within the dielectric of an electrostatically driven device. Implanting ions within the dielectric increases the conductivity of the dielectric in the region where the ions are implanted, and such a region of increased conductivity can provide a charge dissipation structure that reduces the effect on device performance of mobile charges within or on the surface of the dielectric. Ion implantation to form a charge dissipation structure within a dielectric can be performed without masking, lithography, or elevated temperatures, the electrical properties of such a charge dissipation structure can be controlled relatively easily, and an implanted ion charge dissipation structure is protected from oxidation and corrosion by the dielectric within which it is formed.
BRIEF DESCRIPTION OF THE DRAWINGS
 The foregoing and other aspects, features, and advantages of the invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:
 FIG. 1 is a schematic diagram illustrating the elements of a MEMS device in which the invention may be employed.
 FIG. 2 is a schematic diagram illustrating the formation of a charge-dissipation structure in accordance with an embodiment of the invention.
 FIG. 3 is a graph illustrating an example of dopant concentration as a function of depth in a charge-dissipation structure in accordance with an embodiment of the invention.
 FIG. 4 is a graph comparing examples of the drift of a prior art MEMS device and the drift of a MEMS device in accordance with an embodiment of the invention.
 FIG. 5 is a graph comparing other examples of the drift of a prior art MEMS device and the drift of MEMS devices in accordance with embodiments of the invention.
 FIG. 6 is a schematic diagram illustrating the elements of another device in which the invention may be employed.
 FIG. 7A is a schematic diagram illustrating the formation of a charge-dissipation structure in accordance with another embodiment of the invention, and FIG. 7B is a schematic diagram illustrating a device having such a charge-dissipation structure.
 FIG. 1 is a schematic diagram illustrating the elements of an electrostatically driven device in which the invention may be employed. The device illustrated is an optical MEMS device, sometimes referred to as a micro